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Ligands and Coordination ChemistryBased on Vinyl and Alkynyl Substituted

Pyrazoles

Der Naturwissenschaftlichen Fakultätder Friedrich-Alexander-Universität Erlangen-Nürnberg

zurErlangung des Doktorgrades Dr. rer. nat.

vorgelegt von

Tobias Paulaus Neuendettelsau

Als Dissertation genehmigt von der Naturwissen-schaftlichen Fakultät der Friedrich-Alexander-Universität

Erlangen-Nürnberg

Tag der mündlichen Prüfung: 10. Juni 2015

Vorsitzender des Promotionsorgans: Prof. Dr. Jörn Wilms

Gutachter: Prof. Dr. Nicolai Burzlaff

Prof. Dr. Julien Bachmann

Die vorliegende Arbeit entstand in der Zeit von November 2011 bis Dezember 2014 imDepartment für Chemie und Pharmazie (Lehrstuhl für Anorganische und MetallorganischeChemie) der Friedrich-Alexander-Universität Erlangen-Nürnberg unter der Anleitung vonProf. Dr. Nicolai Burzlaff.

Früher starben die Menschen mit 35 Jahren,heute schimpfen sie bis 95 auf die Chemie.

Carl Heinrich Krauch

Contents

1 State of Knowledge 11.1 Scorpionate Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1 The History of Scorpionate Ligands . . . . . . . . . . . . . . . . . . 21.1.2 Bis(pyrazol-1-yl)acetic Acids . . . . . . . . . . . . . . . . . . . . . . 21.1.3 Bis(pyrazol-1-yl)methane Based Ligands . . . . . . . . . . . . . . . 4

1.2 Polymerizable Bis(pyrazolyl)acetic Acids . . . . . . . . . . . . . . . . . . . 71.2.1 Derivatives of Bis(pyrazolyl)acetic Acids . . . . . . . . . . . . . . . 71.2.2 Solid Phase Immobilization of Bis(3,5-dimethylpyrazol-1-yl)acetic

acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.2.3 Copolymerization Immobilization of Bis(3,5-dimethylpyrazol-1-yl)-

acetic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.2.4 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (Hbdmvpza) . . . . 101.2.5 Immobilization of Hbdmvpza . . . . . . . . . . . . . . . . . . . . . 121.2.6 Metal Complexes of Hbdmvpza Based Copolymers . . . . . . . . . 13

1.3 Ferrocene as Building Block . . . . . . . . . . . . . . . . . . . . . . . . . . 171.3.1 The History of Ferrocene . . . . . . . . . . . . . . . . . . . . . . . . 171.3.2 Substitution Reactions of Ferrocene . . . . . . . . . . . . . . . . . . 181.3.3 Transition Metal Catalyzed Cross-Coupling Reactions with Ferrocene 201.3.4 Negishi Type Coupling Reactions with Ferrocene . . . . . . . . . . 211.3.5 Kumada Type Coupling Reactions with Ferrocene . . . . . . . . . . 211.3.6 Sonogashira Type Coupling Reactions with Ferrocene . . . . . . . . 211.3.7 Redox Properties of Ferrocene and its Derivatives . . . . . . . . . . 241.3.8 Ferrocene Substituted Scorpionate Ligands . . . . . . . . . . . . . . 26

1.4 The Rieske Dioxygenase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301.4.1 Model Complexes of the Rieske Dioxygenase . . . . . . . . . . . . . 33

1.5 Molybdenum Containing Enzymes . . . . . . . . . . . . . . . . . . . . . . . 361.5.1 The Sulfite Oxidase . . . . . . . . . . . . . . . . . . . . . . . . . . . 371.5.2 The DMSO Reductase . . . . . . . . . . . . . . . . . . . . . . . . . 411.5.3 Model Complexes of the DMSO Reductase . . . . . . . . . . . . . . 44

1.6 Coordination Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461.6.1 Building Blocks for Coordination Polymers . . . . . . . . . . . . . . 461.6.2 Polyyne Bridged Coordination Polymers . . . . . . . . . . . . . . . 481.6.3 Pyrazole Based Ligands for Coordination Polymers . . . . . . . . . 50

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Contents

2 Objectives and Aims 53

3 Results and Discussion 573.1 Oxo-Transfer Catalysis by Chelate and Scorpionate Oxomolybdenum Com-

plexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.1.1 Bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1) . . . . . . . . . . 593.1.2 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane (bdmvpzm) (2) . . . . 603.1.3 [MoO2Cl2(bdmvpzm)] (3) . . . . . . . . . . . . . . . . . . . . . . . 613.1.4 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (4) . . . . . . . . . 633.1.5 [MoO2Cl(bdmvpza)] (5) . . . . . . . . . . . . . . . . . . . . . . . . 653.1.6 Copolymers containing bdmvpzm (2) and Hbdmvpza (4) . . . . . . 683.1.7 Molybdenum Containing Copolymers . . . . . . . . . . . . . . . . . 70

3.1.7.1 Treatment of Copolymers with [MoO2Cl2(THF)2] . . . . . 703.1.7.2 Copolymerization of [MoO2Cl2(bdmvpzm)] (3) and

[MoO2Cl(bdmvpza)] (5) . . . . . . . . . . . . . . . . . . . 733.1.8 Oxygen Atom Transfer Catalysis . . . . . . . . . . . . . . . . . . . 74

3.2 4-Ethynyl Substituted Pyrazole Based Ligands . . . . . . . . . . . . . . . . 783.2.1 Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)methane (14) . . 803.2.2 Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane

(bedmpzm) (15) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 813.2.3 Attempted Synthesis of Bis(4-trimethylsilyl-ethynyl-3,5-dimethyl-

pyrazol-1-yl)acetic acid (16) . . . . . . . . . . . . . . . . . . . . . . 823.2.4 [CuI(bedmpzm)] (17) . . . . . . . . . . . . . . . . . . . . . . . . . . 833.2.5 [ZnCl2(bedmpzm)] (18) . . . . . . . . . . . . . . . . . . . . . . . . 843.2.6 [MnCl2(bedmpzm)2] (19) . . . . . . . . . . . . . . . . . . . . . . . . 843.2.7 [CoCl2(bedmpzm)] (20) . . . . . . . . . . . . . . . . . . . . . . . . 853.2.8 [MoO2Cl2(bedmpzm)] (21) . . . . . . . . . . . . . . . . . . . . . . . 863.2.9 4-Iodopyrazole (22) and 4-Iodo-3,5-dimethylpyrazole (23) . . . . . 873.2.10 4-Iodo-1-tritylpyrazole (24) and 4-Iodo-3,5-dimethyl-1-trityl-

pyrazole (25) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 873.2.11 3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26) . . . . . 883.2.12 4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27) . . . . . . . . . . . . 893.2.13 3,5-Dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) . . . . . . . . . 903.2.14 2,2-Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (29) . . . . . 913.2.15 (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazol-

1-yl)methane [HOPhbdmeTMSpzm] (30) . . . . . . . . . . . . . . . 923.2.16 [MoO2Cl2(HOPhbdmeTMSpzm)] (31) . . . . . . . . . . . . . . . . . 93

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3.2.17 (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)me-thane (32) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.2.18 (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpy-razol-1-yl)methane (33) . . . . . . . . . . . . . . . . . . . . . . . . 96

3.2.19 (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)me-thane (34) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

3.2.20 Summary of 4-Ethynyl Substituted Pyrazole Based Ligands . . . . 1003.3 Coordination Polymers of 1,4-Bis(1H -pyrazol-4-yl)butadiynes . . . . . . . 101

3.3.1 3,5-Dimethyl-4-ethynyl-1-tritylpyrazole (35) . . . . . . . . . . . . . 1023.3.2 1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadiyne (36) . . . . . . 1033.3.3 1,4-Bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) . . . . . . . . 1043.3.4 Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethylpyrazol-4-yl)-

butadiyne)) (38/39) . . . . . . . . . . . . . . . . . . . . . . . . . . 1083.3.5 Poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)bu-

tadiyne)) (40) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113.3.6 4-Ethynyl-1-tritylpyrazole (42) . . . . . . . . . . . . . . . . . . . . 1143.3.7 1,4-Bis(1-tritylpyrazol-4-yl)butadiyne (43) . . . . . . . . . . . . . . 1143.3.8 Attempted Synthesis of 1,4-Bis(1H -pyrazol-4-yl)butadiyne (44) . . 115

3.4 Ferrocene Based Models for Rieske Dioxygenases . . . . . . . . . . . . . . . 1183.4.1 Bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45) . . . . . . . . . . 1203.4.2 Bis(4-iodopyrazol-1-yl)methane (46) . . . . . . . . . . . . . . . . . 1203.4.3 Bis(4-iodopyrazol-1-yl)acetic acid (47) and Bis(4-iodo-3,5-dimethyl-

pyrazol-1-yl)acetic acid (48) . . . . . . . . . . . . . . . . . . . . . . 1223.4.4 Methyl Bis(4-iodopyrazol-1-yl)acetate (49) and Methyl Bis(3,5-di-

methyl-4-iodopyrazol-1-yl)acetate (50) . . . . . . . . . . . . . . . . 1233.4.5 Bis(4-ethynylferrocenylpyrazol-1-yl)methane (befcpzm) (51) . . . . 1253.4.6 Methyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate

(mbefcpzac) (52) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1283.4.7 Bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)me-

thane (bepfcdmpzm) (53) . . . . . . . . . . . . . . . . . . . . . . . 1303.4.8 Methyl bis((4-ethynyl-1-ferrocenylphen-4-yl)pyrazol-1-yl)acetate

(mbepfcpzac) (54) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1333.4.9 Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)me-

thane (bdmfctpzm) (55) . . . . . . . . . . . . . . . . . . . . . . . . 1353.4.10 Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-

yl)me-thane (bdmfcmtpzm) (56) . . . . . . . . . . . . . . . . . . . 1373.4.11 Summary of the Ferrocene Based Models for Rieske Dioxygenases . 140

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4 Summary and Outlook 143

5 Zusammenfassung und Ausblick 151

6 Experimental Section 1596.1 General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

6.1.1 Working Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 1606.1.2 Spectroscopic and Analytical Methods . . . . . . . . . . . . . . . . 1606.1.3 Destabilization of Copolymers . . . . . . . . . . . . . . . . . . . . . 1626.1.4 Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

6.2 Oxo-Transfer Catalysis by Chelate and Scorpionate Oxomolybdenum Com-plexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1646.2.1 Synthesis of Bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1) . . . 1646.2.2 Synthesis of Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane

(bdmvpzm) (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1656.2.3 Synthesis of [MoO2Cl2(bdmvpzm)] (3) . . . . . . . . . . . . . . . . 1666.2.4 Synthesis of Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid

(Hbdmvpza) (4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1676.2.5 Synthesis of [MoO2Cl(bdmvpza)] (5) . . . . . . . . . . . . . . . . . 1686.2.6 Copolymerization of bdmvpzm (2) with MMA to form P6 . . . . . 1696.2.7 Copolymerization of bdmvpzm (2) with EGDMA to form P7 . . . . 1706.2.8 Copolymerization of Hbdmvpza (4) with MMA to form P8 . . . . . 1716.2.9 Copolymerization of Hbdmvpza (4) with EGDMA to form P9 . . . 1726.2.10 Synthesis of P6-Mo . . . . . . . . . . . . . . . . . . . . . . . . . . 1736.2.11 Synthesis of P7-Mo . . . . . . . . . . . . . . . . . . . . . . . . . . 1746.2.12 Synthesis of P8-Mo . . . . . . . . . . . . . . . . . . . . . . . . . . 1756.2.13 Synthesis of P9-Mo . . . . . . . . . . . . . . . . . . . . . . . . . . 1766.2.14 Copolymerization of [MoO2Cl2(bdmvpzm)] (3)

with MMA to form P10 . . . . . . . . . . . . . . . . . . . . . . . . 1776.2.15 Copolymerization of [MoO2Cl2(bdmvpzm)] (3)

with EGDMA to form P11 . . . . . . . . . . . . . . . . . . . . . . 1786.2.16 Copolymerization of [MoO2Cl(bdmvpza)] (5)

with MMA to form P12 . . . . . . . . . . . . . . . . . . . . . . . . 1796.2.17 Copolymerization of [MoO2Cl(bdmvpza)] (5)

with EGDMA to form P13 . . . . . . . . . . . . . . . . . . . . . . 1806.3 4-Ethynyl Substituted Bis(pyrazolyl)methane Ligands . . . . . . . . . . . . 181

6.3.1 Synthesis of Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)me-thane (14) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

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6.3.2 Synthesis of Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (bedm-pzm) (15) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

6.3.3 Synthesis of [CuI(bedmpzm)] (17) . . . . . . . . . . . . . . . . . . . 1836.3.4 Synthesis of [ZnCl2(bedmpzm)] (18) . . . . . . . . . . . . . . . . . 1846.3.5 Synthesis of [MnCl2(bedmpzm)] (19) . . . . . . . . . . . . . . . . . 1856.3.6 Synthesis of [CoCl2(bedmpzm)] (20) . . . . . . . . . . . . . . . . . 1866.3.7 Synthesis of [MoO2Cl2(bedmpzm)] (21) . . . . . . . . . . . . . . . . 1876.3.8 Synthesis of 4-Iodopyrazole (22) . . . . . . . . . . . . . . . . . . . . 1886.3.9 Synthesis of 4-Iodo-3,5-dimethylpyrazole (23) . . . . . . . . . . . . 1896.3.10 Synthesis of 4-Iodo-1-tritylpyrazol (24) . . . . . . . . . . . . . . . . 1906.3.11 Synthesis of 4-Iodo-3,5-dimethyl-1-tritylpyrazole (25) . . . . . . . . 1916.3.12 Synthesis of 3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpy-

razole (26) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1926.3.13 Synthesis of 4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27) . . . . . 1936.3.14 Synthesis of 3,5-Dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) . . 1946.3.15 Synthesis of 2,2-Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic

acid (29) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1956.3.16 Synthesis of (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)-

ethynylpyrazol-1-yl)methane [HOPhbdmeTMSpzm] (30) . . . . . . . 1966.3.17 Synthesis of [MoO2Cl2(HOPhbdmeTMSpzm)] (31) . . . . . . . . . . 1976.3.18 Synthesis of (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-ethynylpyrazol-

1-yl)methane (32) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1986.3.19 Synthesis of (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsi-

lyl)ethynylpyrazol-1-yl)methane (33) . . . . . . . . . . . . . . . . . 1996.3.20 Synthesis of (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyra-

zol-1-yl)methane (34) . . . . . . . . . . . . . . . . . . . . . . . . . 2006.4 Coordination Polymers of 1,4-Bis(1H -pyrazol-4-yl)butadiynes . . . . . . . 202

6.4.1 Synthesis of 3,5-Dimethyl-4-ethynyl-1-tritylpyrazole (35) . . . . . . 2026.4.2 Synthesis of 1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)buta-

diyne (36) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2036.4.3 Synthesis of 1,4-Bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) . 2046.4.4 Synthesis of Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethyl-

pyrazol-4-yl)butadiyne)) (38/39) . . . . . . . . . . . . . . . . . . . 2056.4.5 Synthesis of Poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H -py-

razol-4-yl)butadiyne)) (40) . . . . . . . . . . . . . . . . . . . . . . . 2066.4.6 Attempted synthesis of Poly(cobalt(II)bromido-bis(1,4-bis(3,5-dime-

thyl-1H -pyrazol-4-yl)butadiyne)) (41) . . . . . . . . . . . . . . . . 2076.4.7 Synthesis of 4-Ethynyl-1-tritylpyrazole (42) . . . . . . . . . . . . . 208

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6.4.8 Synthesis of 1,4-Bis(1-tritylpyrazol-4-yl)butadiyne (43) . . . . . . . 2096.4.9 Attempted Synthesis of 1,4-Bis(1H -pyrazol-4-yl)butadiyne (44) . . 210

6.5 Ferrocene Based Models for Rieske Dioxygenases . . . . . . . . . . . . . . . 2116.5.1 Synthesis of Bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45) . . . 2116.5.2 Synthesis of Bis(4-iodopyrazol-1-yl)methane (46) . . . . . . . . . . 2126.5.3 Synthesis of Bis(4-iodopyrazol-1-yl)acetic acid (47) . . . . . . . . . 2136.5.4 Synthesis of Bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid (48) . . 2146.5.5 Synthesis of Methyl bis(4-iodopyrazol-1-yl)acetate (49) . . . . . . . 2156.5.6 Synthesis of Methyl bis(4-iodopyrazol-1-yl)acetate (50) . . . . . . . 2166.5.7 Synthesis of Bis(4-ethynylferrocenylpyrazol-1-yl)methane (51) . . . 2176.5.8 Synthesis of Methyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate (52) 2186.5.9 Synthesis of Bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpy-

razol-1-yl)methane (53) . . . . . . . . . . . . . . . . . . . . . . . . 2196.5.10 Synthesis of Methyl bis((4-ethynyl-1-ferrocenylphen-4-yl)pyrazol-1-

yl)acetate (54) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2206.5.11 Synthesis of Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyra-

zol-1-yl)methane (55) . . . . . . . . . . . . . . . . . . . . . . . . . 2216.5.12 Synthesis of Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-

yl)pyrazol-1-yl)methane (56) . . . . . . . . . . . . . . . . . . . . . . 2226.6 Oxygen Atom Transfer Catalysis . . . . . . . . . . . . . . . . . . . . . . . 224

Appendix 225A Details of Structure Determinations . . . . . . . . . . . . . . . . . . . . . . 226B Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231C List of Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

Bibliography 237

Danksagung 251

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1 State of Knowledge

1


1.1 Scorpionate Ligands

1.1.1 The History of Scorpionate Ligands

When S. Trofimenko synthesized first hydro(trispyrazol-1-yl)borates in 1967, he laidthe foundation for a whole new group of ligands. [1] These ligands are of the general form[RR’B(pz)2]− as depicted in figure 1.1. The pyrazole rings can either be unsubstituted orbear sterically more demanding moieties in the ring positions 3, 4 and 5. Such moleculeswith their three donor groups are excellent tripodal ligands and bind to metal centersin a facial κ3 fashion. The resulting complexes are of the general composition [RR’B(µ-pz)2MLn]. [2]

NN

B

NN

R

R'

M

Figure 1.1: Poly(pyrazolyl)borate ligands according to Trofimenko. [1]

The shape of those ligands, especially when coordinated towards a metal center remindedTrofimenko of a scorpion - with the pyrazole rings as the scissors and the third moiety asits sting. Hence he called those ligands scorpionate ligands. If the third donor is of thesame kind as the other two, the term homoscorpionate ligands is used. If it is different,they are called heteroscorpionate ligands. [3] A first heteroscorpionate ligand bearing asulfur donor group, was introduced by Ghosh et al. in 1998. In order to obtain thisN,N,S binding motif, he used a thioether group as third donor moiety. [4]

Despite the widespread applications that were found for these compounds, they still boreone drawback, namely their B-N bond, which is very sensitive towards hydrolysis. [5] Firstmodifications of this concept were the exchange of the boron atom for other elements suchas aluminum, indium, gallium or silicon. Such analogues could also change the charge ofthe resulting ligands. [3] The most prominent variation was however based on the bis- andtris(pyrazolyl)methane ligands, that have been known since 1937. [6–8] From there, Oteroet al. started to synthesize the class of bis(pyrazol-1-yl)acetic acids. [9,10]

1.1.2 Bis(pyrazol-1-yl)acetic Acids

Those bis(pyrazol-1-yl)acetic acids were first published in the late 90s. Otero et al.deprotonated bis(3,5-dimethylpyrazol-1-yl)methane with n-butyllithium and subsequently

2


reacted it with carbon dioxide, to obtain bis(3,5-dimethylpyrazol-1-yl)acetate (bdmpza)(see scheme 1.1). [9,10]

N N

N N

1. n-BuLi2. CO2

THF

N N

N N

CO2- × H2O1/4

4

Li+

Scheme 1.1: Synthesis of bis(pyrazolyl)acetato ligands according to Otero et al. [10]

Following this route, they were able to obtain N,N,O coordinating ligands in addition tothe already known tris(pyrazolyl)methane ligands, which are N,N,N coordinating. How-ever this synthetic pathway only supports pyrazole precursors with substituents in orthoposition. Without them, those positions would be deprotonated when n-butyllithium isapplied.This problem was overcome by Burzlaff et al. in an attempt to simplify the synthesisof Hbdmpza from a three step synthesis under nitrogen atmosphere [10] to a nonsensitiveone pot synthesis in 2001. [11]

N

N

R

R

N

N

R

R

N

NH

R

ROHO

2

Br

Br O

OH

+

1. KOH, K2CO3, TEBA2. HCl

THF

R = H (bpzaH)R = Me (bdmpzaH)

Scheme 1.2: One pot synthesis of bis(pyrazol-1-yl)acetic acids according to Burzlaffet al. [11]

To do so, they deprotonated the desired pyrazole derivatives with potassium carbonateand potassium hydroxide in the presence of a phase transfer catalyst like benzyltriethyl-ammonium chloride and dibromo- or dichloroacetic acid, to obtain the correspondingbis(pyrazol-1-yl)acetic acid after acidification (see scheme 1.2). [11,12]

Complexes of the middle and late transition metals of those ligands were published fromthere on. Apart from their apparent use for coordination chemistry, [11–13] their tripodalfacial N,N,O coordination motif enables them to serve as model system for metalloenzymesthat feature the frequently found “2-His-1-carboxylate facial triade” motif at their activesite (see figure 1.2). [14,15]

3


M

XX

N

X

NO

N N

His His

Glu/Asp O

Figure 1.2: 2-His-1-carboxylate-triade, X = water/substrate.

1.1.3 Bis(pyrazol-1-yl)methane Based Ligands

At the same time, a different approach was pursued byHiggs et al. They published N,N,Oand N,N,S κ3-coordinating bis(pyrazol-1-yl)methane ligands, starting from bis(pyrazol-1-yl)ketone. [16,17]

N

N

R1

R2

N

N

R1

R2

O

N

NH

R1

R2

N

N

R1

R2

N

N

R1

R2

OH

HO

Y

- CO2

Et3N/COCl2THF

R1 = H, Me

R2 = H, Me, i-Pr

CoCl2

R3 R4

R3 = H, Me

R4 = H, Me, i-Pr

Y = OH, SCN

Scheme 1.3: Synthesis of a N,N,O coordinating heteroscorpionate ligand according toCarrano et al. [16,17]

Therefore, they deprotonated the pyrazole with triethylamine in order to subsequentlyreact it with phosgene to obtain the corresponding keto compound (see scheme 1.3). Thisinitial step was first published by Byers et al. By this means, it is possible to turnany pyrazoles into the corresponding keto compounds, allowing it to influence the stericaldemand of the resulting compounds. [18]

The following cobalt catalyzed step was inspired by a reaction, which was published byPeterson and Thé in 1973 and leads to a metal catalyzed rearrangement at the bridgingcarbon atoms of bis(pyrazol-1-yl)ketones with aldehydes or ketones to the correspondingbis(pyrazol-1-yl)methanes and carbon dioxide. [19–21]

The thiocyanate derivative was reacted further with lithium aluminum hydride to reducethe thiocyanate moiety to a thiol, since the direct synthesis of the thiophenol based ligandwas not possible due to the high reactivity of thiols. [17]

For easier handling, the problematic use of phosgene was abandoned by Artaud andBurzlaff and replaced by solid triphosgene. [22,23] Yet Reger et al. could further re-

4


duce the toxicity of the reaction by employing thionyl chloride to create a sulfinyl bridgebetween the pyrazoles. This leads to the formation of a 1,1’-sulfinylbispyrazole, whichcan, under release of sulfur dioxide, be turned over in the same way as the correspondingketo compounds (see scheme 1.4). [24–26]

N

N

N

NS

O

N

NH

N

N

N

N

R

HO

- SO2

THF CoCl2

NaH/SOCl2

R

Scheme 1.4: Modified Peterson reaction according to Reger et al. [24–26]

Inspired by the results of Higgs et al. and Rebek et al. [16,27], the above concepts werefurther used by Elflein et al. in a reaction pathway, which initially aimed at the synthesisof chiral heteroscorpionate ligands. First ligands of this class were published by Tolmanand coworkers [28] and Oter et al. [29] Burzlaff and coworkers were able to find a onepot synthesis for such compounds based on bis(pyrazol-1-yl)methane and could synthesizea whole library of N,N,N and N,N,O as well as N,N,S coordinating ligands. [30]

N

NH2

O

O

OOCCl3Cl3C

R4

O

S1. 2 NaH2. SOCl2

- 2 NaCl

pyridine, - SO2

CoCl2 or pyridine

- CO2

R1

R3

R2N

N

R1

R3

R2N

N

R1

R3

R2

N

N

R1

R3

R2N

N

R1

R3

R2

R4

O

N

N

R1

R3

R2N

N

R1

R3

R2

R4

O

Scheme 1.5: One pot synthesis of bis(pyrazol-1-yl)methane based ligands according toElflein et al. [23]

Originally, camphorpyrazole was reacted with phosgen and later trisphosgen as statedabove. [22] In a next step the aforementioned modified Peterson reaction with salicy-ladehyde was carried out. (see scheme 1.5). [23]

5


However, this reaction pathway led to erratic yields, when sterically demanding hinderedpyrazoles were used. [23] Such problems are known for Peterson rearrangements withsterically hindered pyrazoles. [20] Therefore the synthesis was altered to the final one potsynthesis based on publications of Byers et al. and Reger et al. [18,26] One the one hand,the cobalt catalyst was omitted in favor of pyridine. This led to reproducible yields of upto 70 %. On the other hand, thionyl chloride was used as bridging agent, as was shownbefore (see scheme 1.4). Furthermore, the intermediate sulfinyl bridged species was notisolated. After treating the pyrazole derivatives with sodium hydride and thionyl chloride,the mixture was reacted with salicylaldehyde and pyridine without workup, delivering thedesired ligand yields of up to 60 % (see scheme 1.5). [23]

As mentioned before, this procedure led to the synthesis of a wide range of new chiral andachiral heteroscorpionate ligands, since any desired aldehyde and pyrazole derivatives canbe used in this reaction. [30,31]

This concept was adopted up by Hoffmann et al. Apart from the synthesis of sev-eral new ligands from the mentioned synthesis with aldehydes, it was found that alsoketo bridged compounds can be employed in this reaction pathway without altering thereaction conditions. By doing so, it was possible to obtain (2-pyridinyl)(phenyl)[bis(3-phenylpyrazolyl)]methane from 3-phenylpyrazole and benzoyl pyridine. [31]

6

1.2 Polymerizable Bis(pyrazolyl)acetic Acids


1.2.1 Derivatives of Bis(pyrazolyl)acetic Acids

As mentioned before, bis(pyrazolyl)acetic acids, and especially the bis(3,5-dimethylpyrazol-1-yl)acetic acid (bdmpza), were quickly adopted by Burzlaff et al. for their N,N,Obinding motif, which closely resembles the “2-His-1-carboxylate triade”, which is foundin a broad range of metalloenzymes. [14,15] Furthermore, they were readily available viathe one pot synthesis shown in chapter 1.1.2. Searching for model complexes of iron andzinc enzymes, it was observed, that, due to the high coordination potential of bdmpzacombined with the low sterical demand, complexation of iron(II) and zinc(II) chlorideswith bdmpza led to the formation of bisligand complexes with two equivalents of bdmpzacoordinated to one metal center (see figure 1.3). [12]

Figure 1.3: Molecular structure of the bisligand complex of ZnCl2 with bdmpza accordingto Beck et al. [12]

Of course this double coordination is undesired, since the active site of the resulting modelenzyme is effectively inhibited by the second coordinated ligand. It was expected, that thisbehavior could be overcome by an increase of the sterical demand of bdmpza. Therefore,the methyl substituents of the pyrazole rings, were exchanged for tert-butyl groups, whatindeed led to the required singular coordination. The resulting zinc complex is depicted infigure 1.4. Unfortunately, the increased sterical demand also made the application of theone pot synthesis impossible, thus it was necessary to fall back to the original syntheticroute of Otero et al. (see scheme 1.1). [10,12] Furthermore, it was possible to obtain chiral

7


complexes by using two differently substituted pyrazole moieties. Upon introduction ofthe carboxylate function, a chiral center is formed at the methylene bridge. [32]

Figure 1.4: Molecular structure of the bisligand complex of ZnCl2 with bdtbpza accord-ing to Beck et al. [12]

Nevertheless, this concept could not be extended to all bio relevant transition metals.The reaction with some metals, such as copper, still leads to the formation of bisligandor even dinuclear complexes. Apart from this, those sterically demanding ligand couldinfluence the reactivity of the resulting transition metal centers. [12,33]

1.2.2 Solid Phase Immobilization of bdmpza

In an effort to overcome these drawbacks, Burzlaff and coworkers adapted the conceptof immobilization for bdmpza. In a first attempt, an allyl linker group was introducedat the bridging carbon atom, which allowed for the solid phase fixation of the ligand.The attachment of the ligand to an mercaptopropyl-silica matrix inhibited the formationof bisligand complexes, while keeping the original κ3-N,N,O binding motif intact. Asdepicted in scheme 1.6, it was possible to obtain the corresponding rhenium and man-ganese complexes. The tripodal coordination was thereby kept intact, as was proven byIR spectroscopy. [34]

8


N

N N

NM

OC COCO

OO

N

N N

N

OHO

N

N N

N

CO2H1. LDA, THF2. H2C=CH-CH2Br3. H+

1. KOtBu, THF

2. [MBr(CO)5]

SH

Si

ORO

O

N

N N

NM

OC COCO

OO

Si

ORO

O

S

Scheme 1.6: Solid phase fixation of Mn and Re complexes on a mercaptopropyl-silicamatrix by Hübner et al. [34]

1.2.3 Copolymerization Immobilization of bdmpza

As an alternative to the solid phase fixation presented before, a second linker type withthe capability to be copolymerized in a vinylogous polymerization reaction was developed.This hydroxymethyl linker was introduced at the methine bridge as well. [33]

Apart from the aforementioned prevention of bisligand formation by withdrawing theligand’s free mobility, this procedure provides another advantage. By using techniquesof molecular imprinting, it is possible to create cavities in the resulting polymer. [33,35]

To do so, dummy substrates are bound to the metal center of the complex prior tocopolymerization. After the embedment of the complex in the polymer, those dummysubstrates are removed, leaving stable cavities at the active sites. These cavities aresimilar to the substrate pockets of natural enzymes. [36–42] This technique can however onlyresemble the shape of such a cavity, thus increasing the substrate selectivity, whereas a realenzyme provides a wide range of especially hydrogen bonds, which support the catalyticactivity of the enzyme.The synthesis of this hydroxymethyl substituted bdmpza derivative is depicted in scheme1.7. The vinylogous copolymerization reaction can easily be initialized by a radical starter.For rhenium and manganese complexes, this copolymerization step can be carried eitherprior or after the complexation of metal fragments. Transition metals like copper, whichtend to form bisligand complexes do however still do so, if the complexation is carried out

9


N

N N

N

OHO

N

N N

N

CO2HOH

1. LDA, THF2. (HCHO)n

3. H+ / H2O

1. LDA, THF2. (HCHO)n

3. H+ / H2O

N

N N

N

CO2H

O

O

Scheme 1.7: Synthesis of a copolymerizable bdmpza derivative by Hübner et al. [33]

before the polymerization. However, in any case, the original N,N,O binding motif waskept intact. [33]

1.2.4 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (Hbdmvpza)

Unfortunately, this concept soon reached its limit with respect to metal fragments withbulky substituents as for example [RuCl2(PPh3)3]. Depending on the desired substrate,such bulkier fragments are necessary to create a cavity large enough for catalytic appli-cation.

N

N N

N

1. POCl3, DMF2. H2O

N

N N

N

1. n-BuLi2. CO23. H+

N

N N

NN

N N

N

OO

OHO

"Ph3P=CH2"

Scheme 1.8: Synthesis of a copolymerizable bdmpza derivative by Türkoglu et al. [35]

Due to the influence of the linker group located at the methine bridge on the coordinationsite, complexation of such fragments is inhibited. Furthermore, the low rigidity of the

10


linker group constitutes another drawback. On the one hand, it can cause chirality andthus enantiomers by dynamic interconversions. [33] On the other hand, the linker grouptends to coordinate towards the metal center, thus replacing the carboxylate donor.For these reasons, it was attempted to move the linker group away from the coordinationsite of the ligand. As the most distant is position 4 of the pyrazole rings, a variation witha vinyl group was attempted in this position. [35]

Figure 1.5: Molecular structure of [Mn(bdmvpza)(CO)3]. [35]

As depicted in scheme 1.8, the ligand synthesis started from bis(3,5-dimethylpyrazol-1-yl)methane, which is readily available from 3,5-dimethylpyrazole in a phase transferreaction. [43] The resulting methylene bridged compound is then turned over in a Vilsmeier-Haack formylation to obtain the corresponding bisaldehyde derivative, as reported byPotapov et al. [44] The next step consists of a Wittig reaction to introduce the desiredvinyl linkers. The final N,N,O binding motif is achieved by deprotonation of the methylenebridge with n-butyllithium and subsequent treatment with carbon dioxide followed byaqueous workup.Several transition metal complexes of the resulting Hbdmvpza ligand could be obtainedby deprotonation of the carboxylate moiety with potassium tert-butoxide and the de-sired metal fragment. Since the coordination site is not influenced by the linker groups,the coordination of carbonyl fragments as well as reactions with bulky precursors as[RuCl2(PPh3)3] were possible. The IR spectra recorded from the resulting complexesrevealed almost identical absorption bands as similar κ3-N,N,O coordinated bis(pyrazol-1-yl)acetato complexes, which have been studied in the past. In combination with X-raystructure determination (see figures 1.5 and 1.8), an influence of the linkers on the coor-dination site could be ruled out and the original binding motif could be verified. [11,33,35]

11


1.2.5 Immobilization of Hbdmvpza

As mentioned before, the most important feature of Hbdmvpza is its capability to formcopolymers in a vinylogous copolymerization reaction. For this purpose, methyl methacry-late (MMA) and ethylene glycol dimethacrylate (EGDMA) were used as copolymers. It isalso possible to polymerize the sole ligand to form a homopolymer by the same procedure.The resulting copolymer structures are depicted in figure 1.6. [35]

OO

N N

N

CO2H

N

N N

N

CO2H

N

x y z

OO

N N

N

CO2H

N

N N

N

CO2H

N

x y z

OO

Figure 1.6: Molecular structures of Hbdmvpza copolymers with MMA (top) andEGDMA (bottom) according to Türkoglu et al. [35]

The composition of the resulting compound could be determined by elemental analysis.Since nitrogen is only contained in the ligand moieties, the nitrogen value enables thecalculation of the ligand content of the copolymer. [35]

Such an analysis of the homopolymer led to a value of 3.05 mmol/g polymer. Copoly-merization with MMA on the other hand led to the formation of two different fractions:The first fraction precipitated directly from solution during the polymerization processand was characterized by a high ligand incorporation of 0.746 mmol/g. On the contrary,the second fraction had to be precipitated from the reaction solution with methanol andexposed a significantly lower amount of ligand incorporation of only 0.304 g/mmol. [35]

12


It was assumed, that the highly reactive crosslinking ligand molecules preferably reactedwith each other, as long as their concentration was sufficiently high. Therefore, highlycrosslinked polymers were formed in the early phase of the reaction, which then precipi-tated because of their low solubility. [35,45]

This theory was supported by the results of the reaction with EGDMA, which is acrosslinker itself. In this case, the resulting copolymer directly precipitated from solu-tion with a ligand incorporation of 0.371 mmol/g. Since the feedstock contained ligandmonomers in a concentration of 0.302 mmol/g, both components of the reaction showeda similar reactivity. [35]

The exact polymer structure could not be determined. However, since free radical poly-merization was used in the process, atactic arrangement has to be assumed in both cases.The size distribution for the soluble MMA copolymers could be determined to be between5 × 103 g/mol and 1 × 107 g/mol. Whether these measurements derived from long singlechains or several shorter chains, crosslinked by ligand moieties remained unclear. How-ever, the broad mass distribution range was most likely caused by random crosslinking ofboth of these units. [35]

1.2.6 Metal Complexes of Hbdmvpza Based Copolymers

It was expected, that the copolymerized Hbdmvpza ligand would bond to metal fragmentsin the same κ3-N,N,O fashion, as it was observed for the free ligand. In order to verifythis behavior, the copolymers were reacted with manganese and rhenium pentacarbonylbromides. [35]

N

N N

N

OHO

EG

DM

A /

MM

A

EG

DM

A / M

MA

N

N N

NM

OC COCO

OO

EG

DM

A /

MM

A

EG

DM

A / M

MA

1. KOtBu

2. [MBr(CO)5]

Scheme 1.9: Complexation of manganese and rhenium tricarbonyl fragments into theEGDMA and MMA copolymers. (M = Mn/Re). [35]

To do so, the insoluble polymers were treated with potassium tert-butoxide and chargedwith the corresponding carbonyl compounds, as depicted in scheme 1.9. In order to verifythe coordination motif, the resulting compounds were examined via IR spectroscopy.Therefore, nujol mulls of the four polymers (Mn-EGDMA, Mn-MMA, Re-EGDMA andRe-MMA) were manufactured and analyzed. The two corresponding monomeric carbonyl

13


complexes [Mn(bdmvpza)(CO)3] and [Re(bdmvpza)(CO)3] were used as references for asuccessful N,N,O coordination. [35]

As can be seen in figure 1.7, the spectra of the polymer embedded complexes (figure1.7 b,c,e,f) agreed well with the reference spectra (figure 1.7 a, d), thus confirming thedesired coordination mode. The reaction product of polymethylmethacrylate (PMMA)with [ReBr(CO)5] was used as control experiment and did not exhibit any of the desiredvibrations (figure 1.7 g). [35]

Figure 1.7: IR spectra of (a) [Mn(bdmvpza)(CO)3](THF), (b) Mn-EGDMA (nujol), (c)Mn-EGDMA (nujol), (d) [Re(bdmvpza)(CO)3](THF), (e) Re-MMA (nu-jol), (f) Re-EGDMA (nujol), (g) control experiment with PMMA and[ReBr(CO)5] (nujol). [35]

However, non of the methods employed so far could determine the overall metal contentof the resulting compounds, since it was unlikely, that every coordination site would beoccupied after the complexation. Therefore, atomic absorption spectroscopy (AAS) wasused in the case of manganese containing polymers and inductively coupled plasma atomicemission spectroscopy (ICP-AES) in the case of the rhenium containing polymers.An overview over the results is given in table 1.1. In general, the metal contents of the

14


Polymer Metal/polymer Metal/polymer Occupied ligand sites[mg/g] [mmol/g] [%]

Mn-MMA 25.8 0.470 63Re-MMA 57.9 0.311 42Mn-EGDMA 8.10 0.148 40Re-EGDMA 7.11 0.038 10

Table 1.1: Metal content of the copolymers by Türkoglu et al. [35]

MMA copolymers were considerably higher than those of the corresponding EGDMApolymers. Since EGDMA as a crosslinker leads to a much denser polymer structure,than the MMA polymers, which are crosslinked at the ligand moieties only, it is moredifficult for metal fragments to reach the actual coordination sites. Thus, the lowersterical hindrance of the MMA copolymers leads to higher metal loadings. [35]

Figure 1.8: Molecular structure of [Cu(bdmvpza)2] as shown by Türkoglu et al. [35]

Finally, after having shown, that the original κ3-N,N,O binding motif was kept intactat the polymerized ligand bonding sites, it was necessary to investigate, if the forma-tion of bisligand complexes could be effectively avoided by means of copolymerizationof Hbdmvpza. Since the so far used precursor [MnBr(CO)5] reacts with this ligand tothe 1:1 complex [Mn(bdmvpza)(CO)3], which is depicted in figure 1.5, copper(II) acetatewas used. This salt was known to form bisligand complexes from experiments with bdm-pza and did so when reacted with Hbdmpza under formation of a blue complex (seefigure 1.8). [35]

15


To obtain a 1:1 copper polymer, the MMA copolymer was deprotonated with potassiumtert-butoxide and treated with copper(II) chloride. The resulting compound was filteredoff and washed thoroughly with methanol to obtain the desired product as a green powder(Cu-MMA). [35]

To achieve an insight in the coordination of the copper salt to the polymer, UV/Visspectra of both Cu-MMA and the monomeric bisligand complex [Cu(bdmvpza)2] werecompared, as it is depicted in figure 1.9. The polymer could be analyzed by creatingpolymer pellets of Cu-MMA in a hydraulic press, which were made transparent by a dropof mineral oil.

Figure 1.9: UV/Vis spectra of (a) [Cu(bdmvpza)2] in methanol and (b) Cu-MMA (poly-mer pellet, nujol) by Türkoglu et al. [35]

As can be seen from the figure, a bathochromic shift of the absorption maximum by64 nm was determined for Cu-MMA. These findings agreed well with measurements ofthe corresponding copper polymers of the methacryloxy-substituted ligand, which waspresented above (see chapter 1.2.3). [33,35]

This bathochromic shift strongly indicated the formation of one-sided κ3-N,N,O coordi-nated copper centers and therefore 1:1 complex moieties in the polymer. Nevertheless,other coordination modes like κ2-N,O could not be excluded entirely. [35]

16

1.3 Ferrocene as Building Block


1.3.1 The History of Ferrocene

Ferrocene was first synthesized by T. J. Kealy and P. L. Pauson in 1951 when theywere trying to find a synthesis for fulvalene by reacting cyclopentadienylmagnesium bro-mide with ferric chloride. Instead they obtained an up till then unknown orange com-pound, which they called dicyclopentadienyl iron. The first striking feature of the newsubstance was its remarkable stability which they accounted to resonance structures (seefigure 1.10 (i)). [46] First insight into molecular structure was brought by infrared studiesof G. Wilkinson and R. B. Woodward. From their data, they proposed the correctstructure for the new compound (see figure 1.10 (ii)) and also its final name ferrocene(Fc). [47] The proof for their proposal was first delivered by E. O. Fischer and W. Pfaband later in more detail by Dunitz et al. via X-ray structure analysis. [48]

FeFe Fe2+

(i)(ii)

Figure 1.10: (i) First proposed mesomeric structures of ferrocene by T. J. Kealy and P.L. Pauson from 1951. [46]; (ii) Actual structure of ferrocene by G. Wilkin-son and R. B. Woodward. [47]

Since then, ferrocene became more and more important in organometallic chemistry. Awide range of applications was found during the last 60 years. On the one hand itcan serve as a tunable electron reservoir in ligands for transition metal complexes, sinceits redox properties are strongly influenced by its chemical environment. [49] Heterocyclicferrocene derivatives on the other hand are of special interest due to their remarkablephotophysical, [50] magnetic, [51] and redox properties. [52] Another field with great potentialfor the application of ferrocene derivatives is the medicinal chemistry, especially in the fieldof cancer research in particular breast cancer. The biological activity of well-establisheddrugs can be increased and their broad spectrum enhanced by the addition of ferrocenemoieties. [53]

The fact that ferrocene moieties have such an influence on the reactivity of its deriva-tives, [54] makes it interesting to implement it in different compounds to study the resultingeffects and electronic properties.

17


1.3.2 Substitution Reactions of Ferrocene

In order to synthesize different ferrocene derivatives, it was first necessary to find suitableways to substitute ferrocenes. Unfortunately, direct nitrations are not possible, sinceferrocene would be oxidized simultaneously. [55] The cyclopentadienyl rings are howeveraccessible for a wide range of reactions that are known from classical organic aromaticchemistry. These include acylation, [56] borylation, [56] mercuration, [57] and lithiation. [58]

These pathways and the nucleophilic substitution reactions towards bromoferrocene andferrocenylboronic acid are depicted in scheme 1.10.

Fe

n-BuLi ort-BuLi

Hg(OAc)2LiCl

BBr3

Fe

Fe

Fe

HgCl

Li

BBr2

Fe

Br

Fe

B(OH)2

NBS

Tosylbromide

B(OnBu)3,H2O

H2O

Fe

O

AcetlychlorideAlCl3

Scheme 1.10: Functionalization routes of ferrocene towards bromoferrocene andferrocenylboronic acid. [56–59]

The shown conversions are of special importance because bromo- and iodoferrocene areeducts for a wide range of subsequent reactions. Those compounds can be obtained byseveral synthetic pathways. As can be seen in scheme 1.10, it is possible to react ferrocenewith mercury acetate followed by the addition of a lithium halide like lithium chloride orbromide, to yield the corresponding halomercury derivative as it was shown by Fish andRosenblum Treatment of the monosubstituted species with N -bromosuccinimide (NBS)leads to the formation of bromoferrocene. [57]

An alternative reaction pathway is initial functionalization of ferrocene with n-butylli-thium and N,N,N’,N’-tetramethylethylenediamine or t-butyllithium at low temperatures,thus avoiding the formation of dilithiated products or cross reactions with solvents. Sub-

18


sequent treatment with tosyl bromide leads to the formation of bromoferrocene. [58]

Kamounah et al. on the other hand pursued a different approach. Reacting ferrocenewith boron tribromide yields dibromoborylferrocene, which can easily be hydrolyzed to fer-roceneboronic acid. From there, either a Suzuki coupling reaction with halide-substitutedaromatic compounds or the reaction to iodoferrocene by application of N -iodosuccinimide(NIS) can be carried out (see scheme 1.11). [59]

Fe

B(OH)2

Fe

IN-iodosuccinimide

Scheme 1.11: Iodination of ferrocenylboronic acid. [59]

Starting from bromoferrocene, it was possible to functionalize ferrocene with a range ofdifferent moieties via copper(I) promoted Ullmann reactions. By this way a variety ofN -, O- and S donor groups could be introduced as depicted in scheme 1.12. [60]

Fe

Br

Fe

SPh

Fe

OPh

Fe

CN

Fe

OC(O)CH3

Fe

N3

Fe

NH2

NaOPhCuCl

NaOAcCuCl

NaSPhCuCl

KCNCuCl

NaN3

CuCl

LiAlH4

Scheme 1.12: Functionalization of bromoferrocene via copper(I) promoted Ullmannreactions. [60]

19


1.3.3 Transition Metal Catalyzed Cross-Coupling Reactions withFerrocene

Advances in transition metal catalyzed homo and hetero coupling reactions made it pos-sible to proceed from the above mentioned C-X connected ferrocene derivatives to C-Cbonded aromatic systems. Among them are monodentate building blocks, which featureN donor functions like ferrocenylpyridines, [61] ferrocenylpyrazoles, [62–64] ferrocenylpyrim-idines, [65,66] ferrocenylpyrazine, ferrocenylimidazoles, [67] and ferrocenyltriazoles [63,68,69] (seefigure 1.11).

NFe N

NH

Fe Fe N

N

Fe

N

N Fe

N

N Fe N

N

Fe N

N

Fe N

N

Fe

N

N N

(i)

(iv)

(vii)

(ii) (iii)

(viii) (ix)

(v) (vi)

Figure 1.11: Heteroaryl substituted ferrocene derivatives: (i) ferrocenylpyridine,(ii) ferrocenylpyrazole, (iii) 2-ferrocenyl-4,6-dimethylpyrimidine, (iv) N-ferrocenylpyrazole, (v) N-ferrocenyl-3,5-dimethylpyrazole, (vi) 2-ferrocenyl-pyrazine, (vii) 5-ferrocenylpyrimidine, (viii) 4-ferrocenylpyrimidine, (ix) 1-ferrocenyl-1,2,3-triazole. [61–64,66,68,69]

While 3-ferrocenylpyrazoles were obtained by a different approach starting from acetyl-ferrocene, [70] N donor ferrocene derivatives are generally accessible by copper mediatedUllmann type substitution reactions. [62,63] However, ferrocene triazoles are synthesizedvia the copper(I)-catalyzed Azide-Alkyne Cycloaddition (CuAAC). [68,69] The remainingC-C coupled ferrocene compounds are obtained by Kumada or Negishi coupling reac-tions as will be shown in the next sections.

20


1.3.4 Negishi Type Coupling Reactions with Ferrocene

To perform a Negishi type C-C coupling reaction, it is first necessary to lithiate fer-rocene with t-butyllithium at low temperatures. Subsequent addition of zinc(II) chlo-ride leads to a transmetallation reaction, thus forming ferrocenyl zinc chloride, as shownby Mochida et al. The actual coupling step with an appropriate aryl halide such as5-bromopyrimidine or 4-iodo-1-tritylpyrazole is catalyzed by a palladium catalyst likebis(triphenylphosphine)palladium dichloride. This way, ferrocenylpyrimidine and ferro-cenylpyrazole can be obtained as depicted in scheme 1.13. [64,71]

Fe Fe

Fe

Fe

ZnCl

N

N

N

NTrt

1. t-BuLi2. ZnCl2

[PdCl2(PPh3)2]5-bromopyrimidine

[PdCl2(PPh3)2]4-iodo-1-tritylpyrazole

Scheme 1.13: Reaction pathway of Negishi type coupling reactions with ferrocene to-wards 5-ferrocenylpyrimidine and 4-ferrocenyl-1-tritylpyrazole. [64,71]

1.3.5 Kumada Type Coupling Reactions with Ferrocene

Another applicable cross coupling reaction is the Kumada coupling reaction. It startsfrom bromoferrocene, which has to be transformed into a Grignard reagent in thefirst place. The resulting ferrocenyl magnesium bromide is reacted with an arylbromidein the presence of a nickel(II) catalyst like Dichlorido[bis(1,3-diphenyl-phosphino)pro-pane]nickel(II). The resulting compound is the corresponding ferrocenylarene, as can beseen in scheme 1.14. [71]

1.3.6 Sonogashira Type Coupling Reactions with Ferrocene

The last cross coupling reaction to be considered here is the Sonogashira couplingreaction. This palladium(0) catalyzed reaction was first reported by Sonogashira et al.in 1975 and has found a wide range of applications since then. [72] The reaction in general

21


Fe

Fe

Fe

MgBr

3-bromopyridine[NiCl2(dppp)]



Fe

N

N

N

Scheme 1.14: Reaction pathway of Kumada type coupling reactions with ferrocene to-wards ferrocenylpyridines. [71]

creates a C-C bond between an terminal alkyne and an organic halide. In contrast to theother coupling reactions presented thus far, the two coupling partners are not directlybound by a single bond, but instead there is always at least an ethynyl moiety betweenthem, serving as a linker group.

Fe Fe N

CO2Me

+

N

CO2Me

X

X = Cl / Br

[Pd(PPh3)2Cl2]CuI, NEt3, RT

Scheme 1.15: Reaction pathway of Sonogashira type coupling reactions with ethynyl-ferrocene. [73]

To perform the coupling reaction with a ferrocene derivative and a heterocycle, two dif-ferent educt combinations are feasible. It is possible to start with a ferrocene halide anda heteroarylacetylene and vice versa. However, as stated by Torres et al. the first eductcombination leads to very low yields in the case of ferrocene. Therefore Torres usedethynylferrocene in combination with different haloheterocycles and obtained yields of upto 96 % (see scheme 1.15). [73]

22


Following this procedure a variety of different ferrocenylethynyl substituted heterocycleswere accessible. A selection of them is presented in figure 1.12. [73–75]

N

Fe Fe(i)

N HCl

Fe

(ii)N

Fe Fe

N

N

BN

BN

Bu-nn-Bu

Bu-nn-Bu

R R

(iv)

(iii)

N N

Fe Fe

(v)

Fe Fe

Figure 1.12: Ethynylferrocene substituted heterocycles. (i) 2,6-ferrocenylethynyl-pyridine, (ii) 3,4-Bis(ferrocenylethynyl)-N-phenylpyrrole, (iii) ferrocenylpyrazaboles (R = varying spacer groups, see figure 1.13) (iv) 4-ferrocenylethynyl-pyridine hydrochloride, (v) bis-6,6’-(ferrocenylethynyl)-2,2’-bipyridine. [73–75]

The most interesting compounds for this work are the group of the ferrocenyl pyrazaboles(figure 1.12 (iii)) byMisra et al., which are the only literature known 4-ferrocenylethynyl-pyrazole derivatives. The spacer groups used by this workgroup are mainly phenyl ringsconjugated with ethynyl or vinyl groups. By doing so, they tried to influence the electro-chemical potential of the corresponding ferrocenyl pyrazaboles. [75]

23


1.3.7 Redox Properties of Ferrocene and its Derivatives

Pure ferrocene has a redox potential of +400 mV against standard hydrogen electrode(SHE). [76] However, this potential can be easily influenced by the chemical environmentof the ferrocene moiety, for example by the introduction of different substituents. [52]

Mochida et al. examined the influence of pyrazole substituents on the redox propertiesof ferrocene and found for 4-ferrocenyl-1-tritylpyrazole (see scheme 1.13) a shift of theelectrochemical potential to E1/2 = −0.03 V, which was found as a quasi reversible redoxprocess by cyclovoltammetry. After deprotection of this compound with trifluoroaceticacid they obtained 4-ferrocenylpyrazole with E1/2 = −0.04 V (acetonitrile, vs. Fc/Fc+). [77]

While this may seem to be only a minor adjustment in the electrochemical potential of theresulting ferrocenyl compounds, the fact, that the half wave potential is shifted towardsmore negative values is still noteworthy, since other heteroaryl compounds differ greatlyin this behavior: 4-, 3-, and 2-ferrocenylpyridine for example alter the redox potentialof the ferrocene moieties to E1/2 = +0.206 V, +0.168 V and +0.155 V vs. Fc/Fc+,respectively. [61] 5-Ferrocenylpyrimidine (E1/2 = +0.14 V vs. Fc/Fc+), 4-ferrocenyltriazole(E1/2 = +0.21 V vs. Fc/Fc+) and 4-ferrocenyltetrazole (E1/2 = +0.27 V vs. Fc/Fc+)exhibit higher half wave potentials than pure ferrocene as well. [64]

N

N

BN

BN

Bu-nn-Bu

Bu-nn-Bu

R R

R = a) Fc b) Fc

Fcc) d)

Fc

Fce) Fcf)

g)

Fc

Figure 1.13: Series of ferrocenyl pyrazaboles by Misra et al. [75]

In general, the redox potential tends towards more negative values, if the size of thearomatic backbone, to which the ferrocene moiety is bonded, is increased. The systemFc(CH=CH)n-CO2Me for example is strongly influenced by the value of n. For n = 0,1, 2 or 3 the half wave potentials move from 0.72 V to 0.59 V, 0.49 V and 0.46 V (forn = 3). [78] A similar correlation was found for diferrocenyl compounds, that are bondedby vinyl moieties. [78] With increasing length of the polyvinyl chain (Fc(CH=CH)nFc), the

24


potential difference between the two redox processes of the two iron centers in the cyclicvoltammogram (∆E1/2) decreases. For n = 1, the difference amounts to 170 mV. Forn = 3, this value decreases to ~100 mV. Compounds with more than three vinyl groupsdo not show two distinguishable processes anymore. [78]

In alkynyl bridged compounds of the general form Fc(C≡C)nFc (n = 1,2) redox processesgenerate relatively stable ferrocenium cations. [79] The more electron donating the con-jugated bound substituents of the ferrocene moieties are, the better the correspondingferrocenium cations are stabilized. Electron withdrawing groups on the other hand sta-bilize the neutral form of ferrocene and result in an anodic shift of the redox-wave incomparison to pure ferrocene. [71]

Figure 1.14: Cyclic voltammograms of ferrocenyl pyrazaboles a) - g) by Misra et al.as depicted in figure 1.13. Reaction conditions: dichloromethane, 0.1 m[NBu4][PF6], scan rate 100 mV s−1. [75]

As already mentioned, Misra et al. synthesized and examined a group of ferrocenylpyrazaboles via a Negishi type coupling reaction (figure 1.13 a)) and Sonogashirareactions (figure 1.13 b)-g)). Unfortunately, cylic voltammetry measurements were notcarried out versus Fc/Fc+ but versus an Ag/AgCl electrode instead and can thereforenot be compared to the aforementioned compounds in absolute numbers (see figure 1.14).Nevertheless, they nicely show the relative influence of different linker systems on theoverall redox potential of the corresponding compounds. [75]

25


1.3.8 Ferrocene Substituted Scorpionate Ligands

As mentioned in chapter 1.1.1, tris(pyrazolyl)borate (Tp) ligands, so called scorpionates,have found a wide range of applications throughout inorganic chemistry. Therefore, itis not surprising, that it was attempted, to combine the aforementioned properties offerrocene with those of the Tp ligand system. The first approach in this direction wascarried out by the father of scorpionate chemistry S. Trofimenko himself. [80]

Trofimenko and coworkers started their experiments with the synthesis of a ferrocenylsubstituted pyrazole. This precursors could be obtained from the reaction of acetylfer-rocene with ethyl formiate and hydrazine. From the resulting 3-ferrocenylpyrazole, it waspossible to obtain the corresponding bis(pyrazolyl)borates. [80,81]

NN

B

N NR

RPd(π - CH2CHCH2)

Fe

Fe

Figure 1.15: First transition metal complex of a ferrocene substituted poly(1-pyrazolyl)borate by Trofimenko and coworkers (R = C2H5). [80]

From there, they could obtain first transition metal complexes of the new ligands suchas, for example, the palladium complex depicted in 1.15. These compounds were the firstpoly(1-pyrazolyl)borate complexes, in which the boron backbone is bonded to a transitionmetal center via the nitrogen donors of the pyrazole rings and at the same time to anothertransition metal, in this case a ferrocenyl moiety, via one nitrogen atom and the carbonframework of a pyrazole ring. [81]

However, it was not possible to prepare the corresponding tris(1-pyrazolyl)borates. Thereaction conditions, that are required to synthesize the corresponding unsubstituted Tpligand, [1] led in this case to decomposition of the ferrocenyl substituents. [80]

Another approach was pursued by Jäkle et al. The ferrocenyl substituent of their ligandswas not introduced at the pyrazole rings, but instead at the boron backbone of the ligand.Depending on the desired ligand, mono or di-substituted (dibromoboryl)ferrocenes were

26


reacted with pyrazole and triethylamine, to obtain the corresponding ligand. [82]

This technique on the one hand stabilized the boron nitrogen bond against hydrolysis. Onthe other hand, it was used as a linker for the potential use in a organometallic polymer,as depicted in figure 1.16. Following this procedure, a variety of different ligands andcomplexes could be synthesized. Among them were mono and dinuclear (see figure 1.16)as well as bisligand complexes and derivatives with higher sterical demand. [82,83]

NN

N N

N NB

Fe

M

N N

NN

NN BM

Figure 1.16: Dinuclear complex [Fc(MBpz3)2] with ferrocene acting as linker group ac-cording to Jäkle et al. (M = Li, Tl). [82]

In 2010, Chen and Jordan reported on an altered Tp synthesis, which avoided the hightemperatures, that were involved in the synthetic pathway of Trofimenko. [1] Instead,they performed a Lewis acid catalyzed synthesis at relatively low temperatures of only60 ◦C. [84]

However, first tris(1-pyrazolyl)borates with ferrocene substituted pyrazole moieties wereunknown before 2014. The Lewis acid catalyzed reaction by Chen and Jordan wasadapted by Sirianni et al. to circumvent the heat induced decomposition of the ferrocenylsubstituents. From this basis, they succeeded in the synthesis of the first ferrocenyl-substituted hydrotris(pyrazolyl)borate ligands, as depicted in scheme 1.16. [85]

Tl

N

HN N

R

R = H, Me, iPr

FeN

NB

N

N N3

R RR

FcFe Fe

1. LiBH4,

MeB(OiPr)2(6 mol% vs pz)

Toluene, 100 °C2. TlOAc

Scheme 1.16: Synthesis of TpFc,R ligands as thallium salts according to Sirianni et al. [85]

27


The group also investigated the electrochemical properties of the resulting compoundsby cyclic voltammetry in dichloromethane with [NBu4][PF6] as electrolyte and Cp*2Fe asreference. In order the make the reported values more comparable to other measurementsin this work, they were converted to potentials against Fc/Fc+ (Cp*2Fe, E0 = −0.59 Vvs. Fc/Fc+). [86]

The observed potentials were independent of the second substituents R of the pyrazolemoieties (see scheme 1.16), which was surprising, since one of them contains electron with-drawing (CF3) and one of them electron donating (CH3) groups as second substituents.All three of them displayed a single reversible redox process at a half wave potential ofapproximately +30 mV vs Fc/Fc+ (compare 1.3.7). [85]

The molecular structure of the methyl substituted derivative is depicted in figure 1.17.It shows the high sterical demand of the ligand, which is induced by the three ferrocenylmoieties, all of which are oriented towards the coordination site. This symmetrical isomercould be obtained from the crude mixture by thermal isomerization. [85]

Figure 1.17: Molecular structure of TpFc,MeTl as published by Sirianni et al. [85]

Another ferrocene substituted scorpionate ligand system was developed by Tampier etal. in 2013. They reported on the first bis(pyrazol-1-yl)acetic acid bearing ferrocenylsubstituents. [71]

The synthesis of these new ligands started from ferrocene, which, after a transmetallationwith zinc, was reacted in a Negishi coupling reaction, according to the procedure pub-lished by Mochida et al. (see 1.3.3). [64] After removal of the protecting trityl group, thealready established one pot synthesis was used, to obtain the corresponding bis(pyrazol-1-yl)acetic acid (Hbfcdmpza) (see scheme 1.17). [11,71]

28


N

N N

N

FeFe

N

N N

N

FeFe

R

R

R

R

R = H bfcpzmR = CH3 bfcdmpzm

R R

R R

O

R = H bfcpzkR = CH3 bfcdmpzk

Scheme 1.17: Synthesis of bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)acetic acid(H[bfcdmpza]) according to Tampier et al. [71]

While it was not possible to fully purify this ligand from educt contamination, the groupcould obtain a crystal structure of the corresponding iron(II) complex, which is depictedin figure 1.18. [71]

Along with this compound, the group synthesized a variety of N,N coordinating deriva-tives. All of them could be suitable for future model complexes of Rieske dioxygenases,due to their beneficial redox potential region, which reaches in to the redox potentialregion of Rieske ferredoxins with [2Fe-2S] clusters (see chapter 1.4). [71]

Figure 1.18: Molecular structure of [Fe(bfcdmpza)2] by Tampier et al. [71]

29


1.4 The Rieske Dioxygenase

The aforementioned Rieske clusters are electron reservoirs in enzymes like the Rieskedioxygenases and other oxygenases. These are iron based enzymes, capable of activatingmolecular oxygen and directly insert it into an organic cosubstrate, as was first discoveredby O. Hayaishi in 1955 as he was studying the catechol 1,2-dioxygenase. [87] Since then,mass spectrometry could show, that both atoms of oxygen, that are transferred duringthis cis-hydroxylation, derive from the same molecule of diatomic oxygen, as it is shownin scheme 1.18. [88]

H

H

+ 18O2 + 16O2

H18OH

H18OH

H16OH

H16OH

or

Scheme 1.18: Example for the incorporation of molecular oxygen into organic substratesin reactions catalyzed by Rieske dioxygenases, using a mixture of 16O2and 18O2. [88,89]

Rieske dioxygenases in bacteria are mostly used for the aerobic decomposition of aro-matic substances. However, using molecular oxygen as oxidizing agent for cis-hydroxy-lations is also an interesting option for synthetic chemistry. Oxygenases were used inthis context, before they were even known to exist. In 1952, Peterson and Murraypublished the stereospecific steroid hydroxylation by fungi. [90] This revolutionized thesynthesis of the anti-inflammatory cortisones by shortening the synthesis from 37 stepswith 0.15 % overall yield to a six step synthesis. Subsequently, the price of progesteronedropped from US$ 200 to $ 6 per gram. [89]

In modern synthetic chemistry, similar reactions are carried out by catalytical reactionswith osmium tetroxide and hydrogen peroxide as oxidizing agents. The specific applicationof Rieske dioxygenases or synthetic analogues would however offer a probably cheaperand in any case more environmentally friendly alternative. [91]

There are many Rieske dioxygenases with distinct structural and mechanistic propertiesto be found in nature. In general, they are enzymes of the family of the non-heme irondioxygenases. The iron atom at the active site is coordinated by the aforementioned 2-His-1-carboxylate facial triade (see figure 1.2). [89,92] Depending on their substrates, theyare divided into several groups, as for example naphthalene- or biphenyl-dioxygenases.Furthermore, Rieske dioxygenases are capable to perform a peroxide shunt instead ofactivating oxygen reductively. This is similar to what could be observed for cytochromeP450. [93,94]

Exemplary, the focus will be on the naphthalene dioxygenase. It is a bacterial dioxygenase.

30


Figure 1.19: Part of the protein structure of the naphthalene-1,2-dioxygenase(NDOS, PDB:1NDO) with the Rieske [2Fe-2S] cluster connected viaAsp205. [89,92,93]

In contrast to plant-type iron-sulfur clusters, where the iron-sulfur clusters are coordinatedby four cysteine ligands, they are bound by two histidine and two cysteine donors. [95] Thestructure of the active site of the napthalene-1,2-dioxygenase (NDOS, PDB:1NDO) isdepicted in the figures 1.19 and 1.20. [89,92,93]

The active site of the naphthalene-1,2-dioxygenase contains a 2-Histidine-1-carboxylatemotif, to which an iron(II) center is bound. Molecular oxygen is activated at this site andthe respective substrate, in this case naphthalene, is oxidized. The iron center is therebybonded by the two nitrogen donors of the two histidine moieties and by the carboxylategroup of a aspartic acid in a κ2-O,O fashion. [89,92]

O

OFe2+

OH2NHis

N NHO O

Asp205

HN

NFe3+

HisN S

SFe3+

Cys

Cys~ 3.0 Å ~ 2.7 Å

Figure 1.20: Schematic representation of the naphthalene-1,2-dioxygenase with theRieske [2Fe-2S] cluster connected over Asp205. [89,92,93]

Another aspartic acid (Asp205) serves as bridge to a Rieske [2Fe-2S] ferredoxine cluster.The iron(II) center is oxidized to iron(III) during the catalytic cycle. The restoration

31


Fe3+

S

Fe3+

S

Fe2+ Fe2+

S

Fe3+

S

Fe2+1 e-

O2+

Fe3+

S

Fe3+

S

Fe3+

+

OHOH

H

H

Scheme 1.19: Reaction cycle of naphthalene dioxygenase according to Wackett etal. [89]

of the initial iron(II) species is carried out with electrons from the Rieske cluster. Theelectrons are thereby transferred over the bridging aspartic acid (Asp205). The electronreservoir is then refilled with electrons from NADH. Thus electrons are transported fromNADH over the [2Fe-2S] cluster to the active site and thereby to the molecular oxygen.This transfer chain spreads over 12 Å and is crucial for the catalytic activity. [89,92]

Fe2+O2

Fe3+O

OH Fe3+ +

OH

OH

Fe5+

O

OH

Fe3+ +

OH

OH

A

B

Scheme 1.20: Possible reaction pathways with intermediates of the dioxygenation byRieske dioxygenases. [89]

The reaction cycle depicted in scheme 1.19 was deduced from single turnover experiments.In this cycle, the histidine bonded iron center of the Rieske cluster and the previouslyformed iron(III) center of the active site are reduced to iron(II). The next step is the

32


concerted oxidization of the substrate, as depicted in scheme 1.20. The electrons forthe oxidization are delivered one by the iron(II) center of the active site and one by theRieske cluster. [96]

The redox potential of the Rieske cluster varies greatly, depending on the electrostaticenvironment in the enzyme. The potential ranges from −150 mV to +400 mV againstSHE. [97,98]

1.4.1 Model Complexes of the Rieske Dioxygenase

Model systems for the Rieske dioxygenase are sparse. There are several model complexesof the [2Fe-2S] cluster by Meyer and coworkers. [99–101] One example of them is depictedin figure 1.21. These clusters show a highly tunable reduction potential, depending onthe substituents R. The half wave potentials range from −1.47 V (R = Cl) over −1.56 V(R = H) to −1.94 V (R = tBu) vs Fc/Fc+. [100] However, these complexes can hardly beused in a working model of a Rieske dioxygenase, because the iron centers of the clustercan not be bonded to an additional active site, since they are coordinatively saturated.

SFe

SS

R

R

S

R

R

Fe

S

R

R

S

R

R

(NEt4)2

Figure 1.21: [2Fe-2S] clusters according to Ballmann et al. (R = Cl, H, tBu). [100]

Therefore, functional models are of interest. Iron(II) complexes of the ligands chlorido-3-(dipyridin-2-yl-methyl)-1,5,7-trimethyl-2,4-dioxo-3-azabi-cyclo[3.3.1]nonan-7-carboxylato-iron(II) and bis(di-(2-pyridyl)methyl)benzamid-iron(II) depicted in figure 1.22 work assuch, while not being structural models of Rieske dioxygenases. [102,103]

N

N

N

O

O

OH

O

N

HN

ON

Figure 1.22: Ligands for Rieske dioxygenase models by Oldenburg et al. [102,103]

33


These models however have redox potentials between−2.0 V and−1.5 V vs Fc/Fc+. [102,103]

Thus they are in the same range as the [2Fe-2S] cluster models reported by Ballmannet al. [100] These values are drastically cathodically shifted in comparison to the naturalRieske ferredoxine clusters, which means, they are stronger reducing agents.A new approach was hence started by Burzlaff and coworkers. Ferrocene, with areduction potential of +400 mV versus SHE (see chapter 1.3), [76] offers the opportunity,to create model systems, that reach into the redox potential of natural Rieske clustersof −150 mV to +400 mV against SHE. [97]

As shown before, the electrochemical potential of ferrocene can be influence by its chemicalenvironment. Furthermore, theRieske dioxygenases contain the 2-Histidine-1-carboxylatefacial triade motif at their active sites, which can be resembled by the bdmpza ligand (seechapter 1.1.2). The group attempted to combine these two concepts by substituting thepyrazole rings of the established ligand with ferrocenyl moieties, which should serve aselectron reservoirs, as it is shown in figure 1.23. [71]

N

N N

NFe

OO

Fe

e-

Figure 1.23: Possible electron transfer in a ferrocene based bis(pyrazolyl)acetate modelsystem. [71]

The synthesis of the resulting 4-ferrocenyl substituted bdmpza derivative and the result-ing crystal structure of the corresponding iron(II) complex was already shown in chap-ter 1.3.8. While this compound could not be studied by cyclovoltammetry due to eductcontaminations, the carbonyl and methylene bridged derivatives were analyzed. [71]

The methylene bridged ligand bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)methane (bfcdm-pzm) could easily be obtained by the reaction of the corresponding 4-ferrocenylpyrazole ina base assisted substitution reaction on dichloromethane under phase transfer conditions(see 1.21 top). [71]

The carbonyl bridged derivate bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)ketone (bfcdm-pzk) was accessible from the reaction of aforementioned pyrazoles with triphosgene in thepresence of triethylamine. (see 1.21 bottom). [71]

The cyclic voltammograms of these two compounds revealed each one reversible redox po-tential. The half wave potentials versus Fc/Fc+ were determined to −9 mV for bfcdmpzmand −12 mV for bfcdmpzk (at a scanrate of 0.3 V/s). [71]

34


N

N N

NFe Fe

N

NH

Fe

N

N N

NFe Fe

O

KOH, K2CO3,CH2Cl2,TEBAC

NEt3,triphosgene

Scheme 1.21: Synthesis of bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)methane (bfcdm-pzm) and bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)ketone (bfcdmpzk) ac-cording to Tampier et al. [71]

These values are very promising, since the redox potentials reach into the range of nat-ural Rieske clusters. [97] Especially in combination with a N,N,O binding motif, suchcompounds could serve as interesting model systems for Rieske dioxygenases. [71] There-fore, new ligands based on a similar concept will be presented as part of this work (seechapter 3.4).

35


1.5 Molybdenum Containing Enzymes

Apart from the iron depending metalloproteins, another group of enzymes are of interestfor this work, this is the group of molybdenum containing enzymes. Molybdenum canbe found in multinuclear metal centers of nitrogenases and is therefore crucial in thenitrogen fixation. [104,105] Moreover, they are also common metal centers in the active sitesof mononuclear metalloproteins. Such proteins usually catalyze oxygen-atom-transfer(OAT) reactions and are therefore called oxotransferases, although this name has nointended mechanistical connotation. [106,107] The common structural feature of most ofthese enzymes is an active site based on a Mo=O unit. Hence they are referred to asoxomolybdenum enzymes. [108]

Figure 1.24: Crystal structure of the chicken sulfite oxidase. [109] (PDB:1SOX)

This large family of metalloproteins can be divided in three groups by their reactionmechanisms. Enzymes of the first group are hydroxylases, which are able to catalyzethe oxidative hydroxylation of a broad range of aldehydes and aromatic heterocyclesunder cleavage of a C-H bond. This group is called xanthine oxidase family. [106] The

36


two remaining groups both catalyze proper OAT reactions. One of them is the sulfiteoxidase family, a member of which is depicted in figure 1.24. The other one is the familyof DMSO reductases. These two families can be distinguished by their characteristicUV/vis spectra. [106]

1.5.1 The Sulfite Oxidase

Molybdenum containing metalloproteins are important for the human biochemistry, aswell. Xanthine oxidase, aldehyde oxidoreductase and sulfite oxidase (SO) can be foundin the human body. [110] If the latter is not functional due to a genetic disorder, a diseasecalled molybdenum cofactor deficiency (MoCD) is caused.In the human body, the SO catalyzes the oxidation from sulfite to sulphate, whereatelectrons are transferred to ferricytochrome c (cyt c, see figure 1.25). [106,111,112] If thisdetoxification step is disabled, the high physiological concentration of sulfite leads tosevere symptoms like the dislocation of ocular lenses, mental retardation and attenuatedgrowth of the brain. [113]

SO32− + H2O + 2 (cyt c)ox → SO4

2− + 2 (cyt c)red + 2 H+

Figure 1.25: Reaction equation of the sulfite oxidase. [106,111,112]

Worldwide, there are only about 100 known cases of this disease. [114] It can be causedby two different genetic disorders, which both lead to the inability to produce the ne-cessary molybdenum-cofactor (Mo-co) of the enzyme. This cofactor is common amongthe oxomolybdenum enzymes. It is depicted in figure 1.26. In two thirds of the cases,patients suffer from the inability to form a precursor of Mo-co (MoCD type A), whichis called cyclic pyranopterin monophosphate (cPMP). [114,115] In the remaining patients,the disease is caused by a point mutation in the protein itself, which is called an isolatedsulfite oxidase deficiency, since only the SO itself is affected (MoCD type B). [115–117]

Until today, there is no known cure for this disease, which usually leads to death inearly infancy. [118] Despite of this, a case in 2009 caused a sensation. An infant girl withearly symptoms of MoCD was treated with cPMP, which was experimentally expressedin cultures of Escherichia coli. by Veldmann et al., who were studying the influence ofcPMP supplementation on mice. [118,119] As a result of this treatment, the little girl fullyrecovered.The crystal structure of this important human enzyme is still unknown, only the structureof the heme domain was reported so far. [120] The only known crystal structure of an intactanimal SO was obtained from the chicken liver enzyme and is depicted in figures 1.24 and1.26. [109] Figure 1.24 shows the tertiary structure of the enzyme. It is a homodimer, whose

37


subunits consist of a heme domain the N -terminus and the Mo-co at the larger C-terminaldomain. The two domains are connected over a flexible polypeptide. [109] The active siteof the protein, which is depicted in figure 1.26, contains a molybdenum atom, which iscoordinated in a pseudo square pyramidal geometry by five ligands. The axial position isoccupied by the terminal oxo group. The equatorial positions contain three sulfur atoms,two from the Mo-co, one from a cysteine moiety. The remaining free coordination site isoccupied by a water/hydroxo ligand. [110]

Figure 1.26: Active site with molybdenum-cofactor of the chicken sulfite oxidase. [109](PDB:1SOX)

The mechanism of action of the sulfite oxidase was proposed by Hille and is generallyaccepted by now. It is split in an oxidative and a reductive half reaction. [121–124] In theoxidative half reaction, the metal center is oxidized by cyt c, as already mentioned (seefigure 1.25). This process is divided in two steps, in each of which one intraprotein electrontransfer from molybdenum to the cyt c takes place. The actual oxidation of the substratesulfite happens during the reductive half reaction (see scheme 1.22). Hille proposed anattack of the free electron pair of the sulfite at one of the Mo=O units of the molybdenumcenter. [122]

38


Mo

O

OS

ScysS

SH

OOO

Mo

O

OH2

Scys

(IV)H2O

SO

O

OO

Mo

O

OHn

Scys

(IV)

(V)e-

Mo

O

O

Scys

(VI)

e-, nH+

SO

O

O

Detectable by EPR

S

S

S

S

S

S

Scheme 1.22: Proposed reaction mechanism of the sulfite oxidase by Hille et al. [108,110]

The proposed mechanism was supported by EPR spectroscopy, which was able to de-tect the intermediate molybdenum(V) species. It was deduced from model reactions ofmolybdenum(VI) dioxo compounds with phosphines. [125,126]

MoO(L-NS2)(DMF)

Me2SO

DMF

MoO(L-NS2)(Me2SO)

MoO2(L-NS2)

MoO(L-NS2)(OPPh3)

PPh3

DMF

Ph3PO

Me2S

Scheme 1.23: Catalytic cycle for the reduction of DMSO catalyzed by MoO(L-NS2) withPPh3 as reductant. [127]

These model reactions were carried out with an model complex for oxo molybdenum en-zymes, which was developed by Holm and coworkers in 1985 and contained a 2,6-bis(2,2-diphenyl-2-mercaptoethanyl)pyridine ligand (L-NS2). [125,128] This bulky ligand was neces-sary to prevent the formation of oxo-bridged molybdenum(V) dimers. [125] The correspond-ing molybdenum(VI) complexes performed successfully at the quantitative reduction of(p-C6H4F)2SO and 3-fluoropyridine N -oxide. [125,128]

Furthermore, this system was able to catalyze the oxidation of phosphine substrates. [127]

39


The catalytic cycle is depicted in scheme 1.23. Therein, oxygen was transferred fromdimethyl sulfoxide to a phosphine substrate under the release of dimethyl sulfide. Thesystem proved to be a good functional model of oxo-transferase enzymes, with a highstability with turnover numbers above 500 before decomposition. [127]

Another model system was developed by Heinze and Fischer. In order to overcomethe tendency of oxo molybdenum complexes to form molybdenum(V) dimers, they used acopolymerized ligand system composed of two 2-imino-pyrrolato ligands, which formed abis(chelate) dioxido molybdenum(VI) complex as shown in figure 1.27. [129] The formationof such oxo-bridged dinuclear molybedenum(V) species was found to be a thermodynamicsink in this system via density functional theory computations. [130]

MoN

N

O

N

O

N

O S

iPr

iPrOS

iPr

iPr

Figure 1.27: Polymerized model system by Heinze and Fischer. [129]

This two-point fixation resembles the fixation of the molybdenum center in real oxotrans-ferases like the sulfite oxidase or DMSO reductase, which is therein accomplished by pterindithiolene and cysteinato ligands (compare figure 1.26). [106,129,131–134]

In their study, Heinze and Fischer catalyzed the oxidation of trimethylphosphine. Therequired oxygen atoms for the OAT were taken from water molecules. The protons of thesewater molecules were transferred to the phosphazane base P1-tBu. [135] The anionic molyb-denum species, which is formed by this process is then reduced by [Fe(AcC5H4)2][BF4] torestore the initial catalyst. [129]

In order to find a way to catalyze this reaction under homogeneous conditions, Heinzeet al. studied a similar, yet not solid phase fixated system, concerning its electrochemicalproperties. However, they found that it is impossible to suppress condensation and com-proportionation of the system without an excess of reactive substrate and the absenceof water. Therefore, a full biomimetic catalytic cycle is not possible under homogeneousconditions with this system. [136] However, recent studies of the group have shown, thatligand derivatives with a drastically increased sterical demand, which was achieved bythe introduction of isopropyl groups, could possibly avoid the formation of dimeric com-plexes. [137]

40


1.5.2 The DMSO Reductase

The second group of oxygen-atom-transfer catalyzing oxomolybdenum enzymes consistsof the DMSO reductases (see chapter 1.5). [106] These enzymes are found exclusively ineubacteria, such as E. coli [138], R. sphaeroides [139] and R. capsulatus [140]. They serve asreductases under aerobic conditions and allow for an higher energy yield than fermentationcould deliver. [141] Fermentation is the least effective process to obtain energy, since it isonly yielded by substrate chain phosphorylation. [138]

Figure 1.28: Crystal structure of dimethyl sulfoxide reductase from Rhodobacter capsu-latus. [142] (PDB:1DMS)

While catalyzing the same reactions, the protein structures differ significantly among thesedifferent bacteria. The proteins found in R. sphaeroides and R. capsulatus (see figures1.28 and 1.29) contain the Mo-co (see figure 1.30) as only cofactor. The corresponding E.coli enzyme on the other hand consists of three subunits: the A-subunit contains the Mo-co, a B-subunit with four [4Fe-4S] clusters and a transmembrane C-subunit. The latteris also responsible for the binding and oxidation of menaquinol. In this process, electronsare transferred from the C-subunit to the B-subunit and from there to the Mo-co. [141]

Figure 1.29 shows the active site of the DMSO reductase of R. capsulatus. [142] As can be

41


seen, the molybdenum center is located between two molybdopterin units, which formthe molybdenum cofactors. This common feature of the different enzymes is depicted infigure 1.30 for comparison. [141]

Figure 1.29: Structure of the active site of the dimethyl sulfoxide reductase fromRhodobacter capsulatus. [142] (PDB:1DMS)

So far, only two crystal structures of DMSO reductases could be obtained. On the onehand the crystal structure of the DMSO reductase of R. capsulatus [142,143], which is de-picted in figure 1.28 and on the other hand the X-ray structure of the DMSO reductaseof R. sphaeroides. [144]

N

HN

NH

HN

OOPO3

2-

H2N

O S

S

Mo O

OO

Figure 1.30: Structure of the common molybdenum-cofactor. [141]

A reaction mechanism for the DMSO reductase was proposed by Kisker et al. in 1997(see scheme 1.24). [144–146] As can be seen in the crystal structure of the DMSO reductase(figure 1.29), the molybdenum center is bound by two pterin derivatives. The reactionitself can be divided in two half reactions, as it was already observed for the SO (comparechapter 1.5.1).In the oxidative half cycle, the reduced molybdenum(IV) species of the enzyme bindsthe substrate DMSO. The bonding weakens the sulfur oxygen bond and two electronsare transferred from the molybdenum center to the substrate. Dimethyl sulfide (DMS) isreleased and the molybdenum center remains in an oxidation state of VI. [144–146]

42


Mo

O-ser

SSS

S IV

Mo

ser-O

SSS

S

OSMe2

IV

DMSO

Mo

ser-O

SSS

S

O

VI

DMS

e-

Mo

ser-O

SSS

S

O

V

2

Mo

ser-O

SSS

S

OH

V

+ H+ - H+

e-

OH-

Scheme 1.24: Proposed reaction mechanism of the DMSO reductase. Coordination inthe Mo(IV) and Mo(V) states as observed in the crystal structures of theseforms. [144–146]

In the reductive part of the reaction, a proton and two electrons are transferred to themetal center. A hydroxyl anion is released and the initial molybdenum(IV) speciesis restored. The electron source for this reaction is most likely a water soluble cy-tochrome. [109,144–146]

43


1.5.3 Model Complexes of the DMSO Reductase

In order to mimic the pterin coordination environment of the Mo-co in the DMSO reduc-tase, Fischer et al. studied a trichloro(quinonoid-N(8)H -6,7-dihydropterin)oxomolybde-num(IV) (Mo(IV)OCl3(H+-q-H2Ptr)), which is depicted in figure 1.25. [147]

N

HN

NH

N

H2N

O

Mo ClO

Cl

Cl

N

HN

NH

N

H2N

O

Mo ClO

Cl

Cl

N

HN

NH

N

H2N

O

Mo ClO

Cl

Cl

Scheme 1.25: Structure of trichloro(quinonoid-N(8)H-6,7-dihydropterin)oxomolybde-num(IV) as determined by Fischer et al. via X-ray structure analy-sis. [147]

The reaction of this complex with DMSO was monitored via 13C NMR spectroscopyand mass spectrometry. The authors postulated a change of the oxidation state of themolybdenum center from the chemical shift of the carbonyl carbon atom of the pterinunit. The formation of a not closer specified Mo(VI)(O)2 species was assumed. Duringthis reaction, a release of DMS was observed via mass spectrometry. [147]

While the system was able to transfer oxygen from DMSO to the molybdenum center,the reaction was not carried out catalytically, since the pterin ligand was presumablyoxidized in a second reaction step. Thus the original complex could only be restored bythe addition of new tetrahydropterin to the reaction mixture. [147]

[MoO2Cl2] + pzK

1:1toluene

1:2tolueneN

NMo N

NMo

N

N

O OO

O

Cl

tBu

tBu

tBu

tBu tBu

tBu

Scheme 1.26: Synthesis and structure of DMSO reductase model complexes by Mostet al. [148]

Therefore, Most et al. found a new approach. They employed µ2-pyrazolato ligands with

sterical demanding tBu moieties. [148] The bulky ligands were supposed to avoid bridgingstructures of molybdenum complexes with pyrazolato ligands. [149]

Depending on the stoichiometry, either 1:1 or 1:2 complexes of molybdenum(VI) dichloridedioxide with potassium 3,5-di-tert-butylpyrazolate (pzK) could be prepared, as depicted

44


in scheme 1.26. [148] The resulting compounds [MoO2Cl(µ2-pz)] and [MoO2(µ2-pz)2] showedcatalytic activity similar to DMSO reductases in the presence of triphenylphosphine andDMSO: [148]

PPh3 + (Me)2SO[Mo]−−→ OPPh3 + (Me)2S

The experiments were carried out in deoxygenated DMSO-d6 and analyzed via 31P NMRspectroscopy. The model compounds were applied in a concentration of 10 mol%. Whilethe 1:1 complex was able to oxidize 100 % of the triphenylphosphine within two hours atroom temperature, the 1:2 complex did not exhibit any activity under these conditions.Yet, at a temperature of 80 ◦C, it reached similar activity as the 1:1 complex. Theformation of DMS could be verified via gas chromatography. [148]

Inspired by these results, Mösch-Zanetti and coworkers developed a wide range ofmolybdenum and tungsten containing catalyst in order to investigate the mechanism andgeometry dependent activity regarding OAT reactions. [107,150]

The same reaction was examined in the studies of Hammes et al. The group useddioxo-molybdenum complexes of scorpionate ligands such as bis(3,5-dimethylpyrazol-1-yl)acetate (bdmpza, see also 1.1.2). The kinetics regarding the oxidation of triphenylphos-phine has been studied for several scorpionate complexes. [151] The formation of µ-oxobridged bdmpza oxo-molybdenum complexes was reported by C̆eh and coworkers, whichis, as outlined above, unfavorable for OAT reactions. [152–154] Therefore, a copolymerizablederivative was developed as part of this work (see chapter 3.1).

45


1.6 Coordination Polymers

1.6.1 Building Blocks for Coordination Polymers

In general, polymers are defined as high molecular weight molecules, which consist ofrepeated monomeric units, linked with covalent bonds. [155] Coordination polymers on theother hand are infinite systems of alternating metal ions and organic ligands, linked viacoordination or other weak chemical bonds. [156] If such systems build higher dimensionalorder structures, they are also referred to metal-organic coordination networks or metal-organic frameworks (MOF). [157]

N

N

N N

HN

N

N

N

N

NO

N O

neutral ligands

anionic ligands

O O

OO

O O

O

O

O

O

O

O

O

O

cationic ligands

N

N N

N

N N

N N

N N

Figure 1.31: Selection of typical used organic molecules as organic linkers in coordinationpolymers. [158]

46


To be considered as such, the ligand must be a bridging organic group. At least in onedimension, the metal ions must solely be bridged by this organic ligand. Furthermore,there must be at least one carbon atom between the donor atoms of the ligand. [157]

A selection of common organic linkers for coordination polymers is depicted in figure1.31. [158] As can be seen, neutral ligands and cationic ligands mostly rely on nitrogenheterocycles, while anionic ligands often use carboxylate donors.

linear chain

zigzag chain

ladder

Figure 1.32: Various common 1D, 2D and 3D polymer motifs. [158]

Depending on the number and geometry of the donor functions of the ligand, a vastvariety of different structural motifs can be obtained (see figure 1.32). [158,159] Even 1Dcoordination polymers are known to form unique higher dimensional packing motifs dueto π− π and other intermolecular interactions. [160]

For this work, two groups of ligands for coordination polymers are of special interest: thepolyyne bridged ligands and coordination polymer ligands based on pyrazole moieties.

47


1.6.2 Polyyne Bridged Coordination Polymers

In the recent years, several groups examined coordination polymers of polyyne bridged lig-and systems. [161–170] Schröder and coworkers used among others silver complexes of [1,4-bis(4-pyridyl)butadiyne] (pybut) [171] and [1,4-bis(4-pyridylethynyl)phenylene] (pyphe) [172]

to study the resulting interactions between the polymer chains. [161]

Figure 1.33: Arrangement of infinite chains in {[Ag(pybut)]BF4 × MeCN}∞ (top) and{[Ag(pybut)PO2F2 × MeCN}∞ (bottom). [161]

The arrangements of infinite chains depicted in figure 1.33 strongly depended on theinfluence of the counter ion. Stronger coordinating anions stabilize short cation - cationdistances and the formation of head-to-head ligand placement (see figure 1.33, bottom).Weakly coordinating anions on the other hand promote a different chain arrangementwith the ligands in a head-to-tail orientation and cationic centers separated by over 7 Å(see figure 1.33, top). [161]

In order to create non linear 1D coordination polymers with non linear polymer chains,ligands with angular donor functions are required. Burzlaff and coworkers recentlyreported on a butadiyne bridged ligand, bearing imidazole moieties. This bis(N -methyl-imidazol-2-yl)butadiyne (bmib) ligand was synthesized in a Glaser homo coupling reac-tion from 2-ethynyl-N -methylimidazole. [170]

N

N

N

N

Figure 1.34: Structure of bis(N-methylimidazol-2-yl)butadiyne (bmib) according toBurzlaff and coworkers. [170]

The reaction of this ligand with zinc(II) acetate yielded a 1D coordination polymer.The structure revealed by X-ray structure analysis consisted of alternating dinuclear

48


[Zn2(O2CCH3)4] paddle-wheel units and trinuclear [Zn3(O2CCH3)6] units, as depictedin figure 1.35. [170]

Figure 1.35: Crystal structure of the coordination polymer [Zn5(OAc)10(bmib)2]n ac-cording to Burzlaff and coworkers. [170]

The shape of this structure reminded of the battlements of medieval castles and wastherefore named after it. This alternating paddle-wheel and trinuclear coordination withN,N donor ligands is very rare and has only been reported once before. [170,173]

49


1.6.3 Pyrazole Based Ligands for Coordination Polymers

While most of the nitrogen donor based ligands for coordination polymers rely on pyridinemoieties (see chapter 1.6.1), pyrazole based systems gained more and more attention inthe recent years. [174–179]

N

HN N

NHN

HN N

NH

X

N

HN N

NHO

OH

HN

N

NNH

N

HN

N

HN

NNH

(i)(ii)

(iii) (iv)

(v) (v)

X = H, NO2, NH2, OH, SO3H

Figure 1.36: Selection of known pyrazole based ligands for coordination polymers.(i) [175], (ii) [176], (iii) [177], (iv) [178], (v) [179].

Such ligands were employed by Colombo et al. for the synethesis of isoreticular fami-lies of cobalt and nickel MOFs bearing organic functionalities (see figure 1.36 (i)). Theintroduction of different groups X at the bridging aryl ring enabled the group to finetune pore size, shape, volume and hence the adsorption capacity and selectivity towardspecific guest molecules. By this procedure they were able to achieve a high adsorptionsensitivitiy in gas separation experiments of mixtures of polar and apolar gases. [175]

A similar structure was developed by Heering et al. with cobalt coordination polymersof ligand (iii) (figure 1.36). They also obtained isoreticular structures, which showedpromising results at low-pressure hydrogen storage and the absorption of carbon diox-ide. [177]

Silver(I) sulfate coordination polymers of the bipyrazole ligand (ii) in figure 1.6.1 exposeda high degree of structural diversity depending on both solvent and temperature duringthe crystallization process. [176]

50


N

HN N

NH+ Ag2SO4

120 °C, H2O/EtOH 90 °CH2O/MeCN

120 °C, H2O/MeCN

Scheme 1.27: Crystal structures resulting from the reaction of bispyrazole with silver(I)sulfate at different conditions according to Du et al. [176]

The resulting structures are depicted in scheme 1.27. While a reaction temperature of120 ◦C in ethanol led to the formation of a 2-fold 3D net (scheme 1.27, left), the samereaction in acetonitrile resulted in the formation of a 3D polycatenation with interpene-trating planes (scheme 1.27, right). If the temperature was lowered to 90 ◦C during thereaction in acetonitrile, helix based 2D layers were obtained (scheme 1.27, middle). [176]

Du et al. found one reason for this coordination behavior in the two different possibleconfirmations of the bispyrazole ligand. The metal centers can either be coordinated ina trans or cis fashion to the ligand. While the helical structure consists of purely transcoordinated silver ions, the other two structures consist of different mixtures of cis andtrans conformations. [176]

N

N

R

R

O

O

N

N

R

R

O

OR = H / CH3

Figure 1.37: Structures of 1,4-bis[1-Boc-pyrazol-4-yl]butadiyne and 1,4-bis[1-Boc-3,5-dimethylpyrazol-4-yl]butadiyne (Boc2L). [180–182]

During the course of this thesis,Navarro and coworkers picked up a concept of Vasilevskyet al. for the synthesis of 1,4-bis[1-(1-ethoxyethyl)pyrazol-4-yl]butadiyne. [183] Based on

51


this protocol, it was possible to synthesize first metal organic frameworks based on thisligand as well as the 3,5-dimethyl substituted derivative (Boc2L) (see figure 1.37). [180–182]

Starting from these compounds they could obtain crystal structures of metal organicframeworks containing nickel and cobalt. The tert-butyloxycarbonyl (Boc) protectinggroup was removed in situ during the reaction with the metal salts. Therefore, the struc-tures contain anionic pyrazolate species, allowing a κ2 N,N coordination. The molecularstructure of the respective cobalt(II) oxide MOF is depicted below. [181]

Figure 1.38: Perspective view, down [001], of the doubly interpenetrated networks foundin the crystal structure of Co4O(L)3. The two symmetry-related frameworksare depicted in blue and yellow for the sake of clarity. [181]

As can be seen in figure 1.38, the structure of Co4O(L)3 consists of a two-fold interpene-trated MOF, containing tetrahedraly coordininated Co4O6− centers.Similar to this, one task of this work will be to isolate unprotected 1,4-bis(1H -pyrazol-4-yl)butadiyne ligands suitable for the formation of coordination polymers and to obtainrespective linear coordination polymers of the free ligand. (see chapter 3.3).

52

2 Objectives and Aims

53

2 Objectives and Aims

During the last years, Burzlaff et al. investigated the synthesis and coordination be-havior of heteroscorpionate ligands. In the beginning, bis(pyrazol-1-yl)acetic acids wereused and modified concerning their sterical demand in order to influence their coordi-nation behavior. This was achieved by the introduction of bulkier substituents in thepositions 3 and 5 of the pyrazole rings. [11,12]

The concept was later on extended to bis(pyrazol-1-yl)methane ligands, as was shown inchapter 1.1.3. [22–26] However, all of these concepts disregarded the possibility to introducefunctional groups in position 4 of the pyrazole rings. First attempts in this directionwere carried out by Türkoglu et al. in 2010, when they introduced vinyl groups at thisposition to obtain ligands capable of solid phase fixation via vinylogous polymerizationreactions (see chapter 1.2.4). [35]

The first part of this thesis should be a first catalytic study of polymeric materials de-rived from this ligand. Their reactivities should be evaluated in the context of a modelsystem for the DMSO reductase. The transfer of oxygen from dimethyl sulfoxide totriphenylphosphine should be used as model OAT reaction. Therefore, N,N and N,N,Ocoordinated dioxomolybdenum complexes of the corresponding ligands were to be synthe-sized and polymerized. The influence of the polymerization process on the coordinationmotif as well as the influence of the polymerization on the ability of the resulting lig-and charged material to coordinate metal fragments was to be investigated. The amountof molybdenum incorporation in the respective polymers should be analyzed via atomicabsorption spectroscopy (AAS). Finally, the influence of the solid phase fixation of the cat-alytic species on their catalytic performance concerning the aforementioned OAT reactionshould be determined.Apart from vinyl substituted pyrazole ligands, ethynyl substituted pyrazole based ligandsshould be developed. To do so, 4-ethynyl substituted pyrazoles were to be synthesized,which should then be used in the synthesis of bis(pyrazol-1-yl)acetic acid and bis(pyrazol-1-yl)methane derivatives. Such substituents could then serve as versatile linker groupsin order to add additional functionalities to the resulting ligands. The correspondingone pot syntheses using dibromoacetic acid and phase transfer conditions to obtain thebis(pyrazol-1-yl)acetic acid derivative and thionyl chloride followed by pyridine and analdehyde to obtain bis(pyrazol-1-yl)methane derivatives as established by Burzlaff etal. [11,23] should be used as a synthetic route towards such ligand systems.Starting from 4-ethynyl pyrazoles, bisacetylene bridged ligands suitable for coordinationpolymers should be synthesized via Glaser type coupling reactions. In order to avoidpremature formation of such polymers during the metal catalyzed coupling reactions,bulky protecting groups should be applied. The ability of the resulting ligand to form1D coordination polymers with metal fragments should be verified via X-ray structuredetermination of the resulting materials. Such compounds could prove useful in the field

54

of molecular electronics.The last part of this work should be the investigation of possible model systems forRieskedioxygenases based on bis(4-ethynylpyrazol-1-yl)acetic acid derivatives bearing ferrocenylmoieties (see chapter 1.4). The acetylene function of the ligand precursors should thereinserve as a linker group forClick reactions with ferrocenyl azides. For Sonogashira typecoupling reactions, the corresponding 4-iodopyrazole derivatives should be synthesized.The electrochemical potential of the ferrocenyl moieties in these compounds should beinfluenced by changes in the chemical environment of the ferrocenyl groups. Therefore,the half wave potentials of the resulting model compounds should be determined andoptimized.

55

3 Results and Discussion

57


3.1 Oxo-Transfer Catalysis by Chelate and ScorpionateOxomolybdenum Complexes

In the first part of this thesis, model-systems for DMSO reductases was to be synthe-sized starting from the above mentioned bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid(Hbdmvpza) ligand (see chapter 1.2.4) as well as the N,N coordinating bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane (bdmvpzm). [35]

N

N N

N N

N N

N

OHO

Figure 3.1: Polymerizable ligands for the application in DMSO reductase models.

In order to do so, these ligands were to be prepared from 3,5-dimethylpyrazole and hadto be reacted to the corresponding oxomolybdenum complexes. The same should be donewith the respective copolymers of these ligands.Such compounds should be capable of performing OAT reactions in a similar way, asnatural DMSO reductases do. Thus, such compounds could serve as model systems tostudy the reactivity and mechanism of action of these enzymes. Furthermore, they shouldserve as a benchmark for the influence of the polymer matrix on the catalytic activity ofthe resulting materials.

O

O

O

O

O

OMMA EGDMA

Figure 3.2: Monomers for the copolymerization of bdmvpzm (2) and Hbdmvpza (3),(left: MMA, right: EGDMA).

The co-monomers that should be used for these polymer embedded model systems aredepicted in figure 3.2. On the one hand methyl methacrylate (MMA) and on the otherhand ethylene glycol dimethacrylate (EGDMA) were used. While the former leads tolinear polymer strains, the latter is a crosslinker due to its two polymerization sites. Thisproperty should lead to three dimensional networks with distinct different properties.

58

3.1 Oxo-Transfer Catalysis by Chelate and Scorpionate Oxomolybdenum Complexes

3.1.1 Bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1)[44]

The first step towards a copolymerizable 4-vinyl substituted ligand starts from bis(3,5-dimethylpyrazol-1-yl)methane, which can be easily obtained in a phase transfer reactionof 3,5-dimethylpyrazole in dichloromethane under basic conditions, as shown by Juliá etal. [43]

Starting from there, a Vilsmeier-Haack formylation was carried out, which led tothe formation of the corresponding bisaldehyde derivative. The Vilsmeier reagent washereby prepared by the reaction of phosphorous oxychloride with dimethylformamideat a temperature of 96 ◦C. The latter served as the solvent at the same time. Afteraqueous workup, the desired product is obtained in yields of 26 % referring to bis(3,5-dimethylpyrazol-1-yl)methane.

N

N N

N

OO

N

N N

N

1. POCl3, DMF

2. H2O

1

Scheme 3.1: Synthesis of bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1). [44]

The 1H NMR spectrum of compound 1 reveals the two methyl groups in position 5 ofthe pyrazole with a chemical shift of 2.18 ppm. The corresponding methyl substituents inposition 3 of the pyrazole rings are detected at 2.79 ppm. The protons of the methylenebridge are observed at 6.11 ppm. The success of the formylation reaction can be observedby the signal of the aldehyde proton at 9.93 ppm.The analysis via 13C NMR spectroscopy also shows the methyl substituents as the mostupfield signals with chemical shifts of 10.1 and 12.7 ppm, respectively. The carbon atomof the methylene bridge is found at 59.1 ppm. The signals at 118.8, 146.5 and 151.8 ppmcan be assigned to the pyrazole carbon atoms. The signal of an aldehyde carbon atom at185.0 ppm also confirms the effected synthesis.

59


3.1.2 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane(bdmvpzm) (2)[35]

In the next step, the obtained bisaldehyde 1 was reacted in a Wittig reaction. By doingso, the desired vinyl linker was introduced, which is crucial for the desired copolymeriza-tion capability of the resulting ligand. The synthetic route reported by Türkoglu et al.was employed. [35]

The required ylid [Ph3P+-C−H2 ↔ Ph3P=CH2] was generated in situ from the reaction oftriphenyl-methyl-phosphoniumbromide with potassium tert-butoxide in tetrahydrofuran.After the addition of compound 1, the desired vinyl substituted product 2 is formed. Afterpurification of the crude product via column chromatography to remove triphenylphos-phine oxide, which is generated during the reaction, compound 2 could be obtained inyields of 80 % referring to 1.

N

N N

N

[Ph3P-CH2 Ph3P=CH2]

N

N N

N

OO

1 2

Scheme 3.2: Synthesis of Bis(3,5-dimethyl-4-vinylpyrazol-y-1l)methane (bdmvpzm)(2). [35]

The 1H NMR spectrum of the resulting compound shows signals of the methyl groups atthe C5 and C3 carbon atoms of the pyrazole rings at 2.27 and 2.47 ppm. The introducedvinyl linkers create an AMX system in the spectrum, which proves the successful formationof the desired compound. [184] The terminal protons of the vinyl groups result in a doubletof doublet at 5.14 ppm (Z ) and 5.29 (E). The corresponding coupling constant for thegeminal coupling is calculated to 2JH,H = 1.4 Hz. The 3JH,H(E) amounts to 17.7 Hz andfor 3JH,H(Z ), 11.4 Hz is measured. The non terminal protons of the vinyl groups also splitup in doublets of doublets at a chemical shift of 6.48 ppm. Their coupling constants are3JH,H = 11.5 Hz and 17.9 Hz, respectively. The CS symmetry of compound 2 is reflectedby these results. The singlet at 6.09 ppm can be assigned to the protons of the methylenebridge.Furthermore, 13C NMR spectroscopy was performed. The spectrum shows the methyl sig-nals at 10.2 and 13.6 ppm. The bridging methylene carbon atom is detected at 60.7 ppm.The resonances of the vinyl groups can be found at 112.8 ppm (=CH2) and at 127.4 ppm(-CH=). The remaining signals at 116.5, 138.0 and 146.8 ppm can be assigned to thecarbon atoms of the pyrazole rings.

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3.1.3 [MoO2Cl2(bdmvpzm)] (3)

For the synthesis of a molybdenum(VI)-dioxo complex of the N,N coordinating ligandbdmvpzm (2), the designated metal fragment molybdenum(VI) dichloride dioxide wasfirst dissolved in tetrahydrofuran, to obtain the solvent stabilized [MoO2Cl2(THF)2].To the resulting solution, 2 was added, which led to the precipitation of the desiredN,N’ complex 3 as a yellow-green solid. This procedure followed roughly the synthesisof molybdenum(VI) cis-dioxo complexes with polypyrazolylmethane ligands published bySantos et al. [149]

N

N N

NMo

Cl O

Cl

O

[MoO2Cl2]

THFN

N N

N

2

3

Scheme 3.3: Synthesis of [MoO2Cl2(bdmvpzm)] (3).

The 1H NMR spectrum of complex 3 reveals the two methyl groups at the C5 carbonatoms of the pyrazole rings at 2.15 ppm. The corresponding groups at the C3 carbonatoms have a chemical shift of 2.44 ppm. As for the free ligand, the vinyl protons split upinto doublets of doublets at 5.09 ppm (Z ) and 5.26 (E), respectively. The correspondingcoupling constants 3J amount to 11.6 Hz for the former and 17.9 Hz for the latter. The2J coupling constants were each of 1.2 Hz. The singlet at 6.50 ppm can be assigned tothe protons of the methylene bridge. The remaining non terminal vinyl protons are foundat 6.50 ppm, with coupling constants of 3JH,H = 17.9 Hz and 3JH,H = 11.7 Hz.In the 13C NMR spectrum, the two methyl substituents of the C5 carbon atoms of thepyrazole rings appear at 9.69 ppm, while their counterparts at the C3 carbon atoms appearat 13.5 ppm. The carbon atom of the methylene bridge is detected at 59.0 ppm. Thevinyl linker appears at a chemical shift of 112.3 ppm in the case of the terminal carbonatoms and of 127.6 ppm in the case of the non terminal carbon atoms. The remainingsignals at 115.2, 138.0 and 145.8 ppm can be assigned to the carbon atoms of the pyrazolerings.The complex was analyzed via IR spectroscopy as well. Therefore, the metal precursormolybdenum(VI) dichloride dioxide was used as a reference. In the latter, the two charac-teristic Mo=O bands can be found at ν̃ = 959 (νsym) and 915 cm−1 (νasym). The spectrumof 3 shows these bands as well at ν̃ = 944 (νsym) and 917 cm−1 (νasym). These findingsagree with the literature values of similar compounds published by Santos et al., whichindicate a successful complexation. [149]

61


In addition, the composition of the complex was determined via elemental analysis, whichconfirmed the desired compound. However, it was not possible to verify 3 via massspectroscopy (ESI-MS). Neither the characteristic isotopic pattern of molybdenum, norfragments in the relevant m/z range could be found.However, it was possible to obtain a single crystal suitable for X-ray structure determi-nation by layering a solution of 3 in dichloromethane with n-hexane.

Figure 3.3: Molecular structure of [MoO2Cl2(bdmvpzm)] (3). Thermal ellipsoids aredrawn at the 50 % probability level. Hydrogen atoms have been omitted forclarity.

The resulting molecular structure of 3 is depicted in figure 3.3. The molybdenum(VI)center therein is sixfold coordinated, which can be best described as a distorted octahe-dron. The ligand itself is κ2 N,N coordinated, as it was expected. The bond lengthsMo-Cl1 and Mo-Cl2 of 2.3806(5) and 2.3738(5) Å are almost identical, as are the Mo-N bonds with 2.3575(19) and 2.3591(17) Å and the Mo-O bonds with 1.6982(16) and1.6961(15) Å. While this is a normal bond length for a molybdenum oxygen double bond,the Mo-N bonds are slightly longer than expected (2.2 Å), which can be attributed to thetrans influence of the oxo groups. [151] On the other hand, the O-Mo-N angles differ by 4◦.Furthermore, the axial Cl-Mo-Cl angle is slightly bent with 162.669(18)◦.Both of the vinyl groups are turned out of the pyrazole plane. The torsion angle C12-C13-C16-C17 amounts to 39.163(9)◦ and is directed to the underside of the butterfly shapedmolecule, in which both planes, that are spanned by the pyrazole rings, stand in an angle

62


of 134.80◦. The other vinyl moiety is turned towards the upper side for 38.096(9)◦. Yetboth of the vinyl linkers are directed towards the metal center.

Distances (Å)Mo-Cl1 2.3806(5) Mo-N21 2.3591(17)Mo-Cl2 2.3738(5) N11-N12 1.373(2)Mo-O1 1.6982(16) N21-N22 1.374(2)Mo-O2 1.6961(15) C16-C17 1.322(4)Mo-N11 2.3575(19) C26-C27 1.323(3)

Angles (deg)Cl1-Mo-Cl2 162.669(18) Cl2-Mo-N21 83.76(4)Cl1-Mo-O1 95.14(6) O1-Mo-O2 104.08(8)Cl1-Mo-O2 97.08(5) O1-Mo-N11 168.68(7)Cl1-Mo-N11 82.77(4) O1-Mo-N21 91.16(7)Cl1-Mo-N21 81.61(4) O2-Mo-N11 87.23(7)Cl2-Mo-O1 94.38(6) O2-Mo-N21 164.75(7)Cl2-Mo-O2 94.60(5) C12-C13-C16-C17 39.163(9)N11-Mo-N21 77.53(6) C22-C23-C26-C27 38.096(9)Cl2-Mo-N11 85.06(4)

Table 3.1: Selected interatomic distances (Å) and angles (deg) for compound 3.

In comparison to other complexes containing a MoO2Cl2N2 moiety, no significant differ-ence to the angles or bond lengths could be found. [149,185] A list of selected interatomicdistances and angles is shown in table 3.1.This shows, that there is no influence of the vinyl linker groups on the coordination siteof bdmvpzm (2), as it was already shown before by Türkoglu et al. for the N,N,Ocoordinating bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (Hbdmvpza) (4). [35]

3.1.4 Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (4)[35]

Having obtained a N,N coordinated molybdenum(VI) complex, the next step was tofunctionalize compound 2 further to obtain a κ3 N,N,O coordinating derivative bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid 4, which was first synthesized by Türkoglu etal. in 2010. [35] This ligand is capable off resembling the desired “2-His-1-carboxylate tri-ade” motif, which has a wide range of applications in bioinorganic chemistry (see figure 1.2in chapter 1.1.2). [14,15,35]

The necessary carboxylate function was introduced at the methylene bridge, as depictedin scheme 3.4. Therefore, the original bis(pyrazolyl)acetato ligand synthesis of Otero

63


et al. was employed. [10] The methylene bridge was deprotonated with n-butyllithium intetrahydrofuran at a temperature of −78 ◦C. Subsequent treatment with a dry stream ofcarbon dioxide at −20 ◦C followed by aqueous workup of the reaction mixture led to theformation of the desired carboxylic acid 4 in yields of 68 %.

N

N N

NTHF

OHO

1. n-BuLi2. CO2

N

N N

N

2 4

Scheme 3.4: Synthesis of bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid 4 according toTürkoglu et al. [35]

In the 1H NMR spectrum of Hdmvpza 4, there are two singlets for the methyl substituentsat the pyrazole carbon atoms 3 and 5 at 2.32 and 2.28 ppm, respectively. Furthermore,the AMX system, that could already be found in the spectrum of bdmvpzm (2), can beobserved once again. [184] It is caused by the vinyl moieties and consists of one doubletof doublets caused by the terminal vinyl protons at chemical shifts of 5.24 ppm (Z ) and5.34 ppm (E). The corresponding coupling constant amounts to 2JH,H = 0.8 Hz for thegeminal couplings. The constants 3JH,H(E) and 3JH,H(Z ) were determined to 17.9 Hzand 11.6 Hz. Another doublet of doublets is caused by the non terminal vinyl protonsat 6.45 ppm. The corresponding coupling constants are 3JH,H = 11.7 Hz and 3JH,H =17.9 Hz, respectively. These findings reflect the C S symmetry of compound 4. The mostdownfield signal at 6.45 ppm can be assigned to the proton of the methine bridge.The 13C NMR spectrum shows the methyl signals at chemical shifts of 10.0 and 13.4 ppm.The signal at 70.5 ppm could be assigned to the bridging carbon atom. The terminalcarbon atoms of the vinyl linkers could be observed at 114.8 ppm, whereas the nonterminal carbon atoms of the linkers lead to signals at 128.5 ppm. Furthermore, thecarbon atoms of the pyrazole rings lead to singlets at 115.7 (C4), 137.3 (C5) and 145.1 ppm(C3). The carbon atom of the carboxylate group is detected at 165.4 ppm.

64


3.1.5 [MoO2Cl(bdmvpza)] (5)

After the synthesis of Hbdmpvza (4), it was possible to synthesize the correspondingN,N,O coordinated complex of molybdenum(VI) dichloride dioxide. In the course of thereaction, one chlorido ligand was abstracted to maintain a sixfold coordination. Thesynthetic method, that was employed was based on the synthetic route of compound 3,except of the addition of potassium tert-butoxide to deprotonate the carboxylate donorfunction. A similar complex [MoO2Cl(bdmpza)] was already published by Hammes etal., yet without the additional vinyl linker substituents. [151] While the compound, thatwas obtained by Hammes et al. was a white solid, the vinyl substituted derivative is ofa red-brown color.

[MoO2Cl2]

THF N

N N

NMo

Cl OO

OON

N N

N

OHO

4

5

Scheme 3.5: Synthesis of [MoO2Cl(bdmvpza)] (5).

Nevertheless, the findings that were revealed in the 1H NMR spectrum of 5, agree withthe results, that were found for the [MoO2Cl(bdmpza)] complex of Hammes et al. De-pending on the solvent, the two isomers, that are depicted in figure 3.4 can be found indifferent ratios. [151] The cis isomer is defined as the isomer with the oxo groups cis to theoxygen donor of the ligand while the trans isomer has one oxo group trans to the oxygendonor. [151] Yet it was not possible to separate them from each other for analysis. Thisis not surprising, considering, that the difference in energy between the two isomers forthe very similar [MoO2Cl(bdmpza)] complex was calculated via DFT calculations to only2.58 kcal/mol. [151]

N

N N

NMo

O OCl

OON

N N

NMo

Cl OO

OO

Figure 3.4: Isomers: (left: trans-isomer, right: cis-isomer) of [MoO2Cl(bdmvpza)] (5).

The spectrum implies the loss of the former C S symmetry for one of the isomers. While thetwo isomers could not be separated, they could be distinguished in the 1H NMR spectrum.

65


The methyl protons of the symmetric cis isomer could be detected at 2.17 and 2.26 ppm,respectively. The AMX system caused by the vinyl moieties could be observed as well. [184]

The terminal vinyl protons are detected at 5.12 (Z ) and 5.28 ppm (E). The correspondingcoupling constant amounts to 2JH,H = 1.0 Hz for the geminal couplings. The constants3JH,H(E) and 3JH,H(Z ) were determined to 18.0 Hz and 11.5 Hz. Another doublet ofdoublets is caused by the non terminal vinyl protons at 6.51 ppm. The correspondingcoupling constants are 3JH,H = 11.7 Hz and 3JH,H = 17.9 Hz, respectively. Furthermorethe singlet at 7.27 ppm could be assigned to the methine bridge. In the case of theasymmetric trans isomer, the four methyl groups split up into four singlets at 2.54, 2.61,2.71 and 2.79 ppm. The AMX systems appear as multiplets at 5.41 and 6.57 ppm, whichcould not be resolved. The singlet at 7.05 ppm was assigned to the methine bridge of thetrans isomer.The analysis of the 13C NMR of compound 5 proved easier. Due to the lower timeresolution of 13C NMR spectroscopy, only one set of signals could be observed. Thus, themethyl substituents in position 5 and 3 of the pyrazole rings could be detected at 9.91 and13.5 ppm. The bridging carbon atom was found at 71.3 ppm. Furthermore, the carbonatoms of the vinyl moieties can be assigned to the signals at 112.7 ppm in the case of theterminal atoms, while the non terminal carbon atoms can be detected at 127.4 ppm. Theremaining signals at 115.7, 138.4 and 145.6 ppm are caused by the carbon atoms of thepyrazole rings, whereas the singlet at 165.9 ppm derives from the carboxylate group.While it was not possible to obtain a molecular structure of compound 5, the structureof Hammes et al., which is depicted in figure 3.5, should, [151] apart from the lackingvinyl linkers, closely resemble the structure of compound 5. This similarity was alreadyobserved for the corresponding N,N coordinated complex [MoO2Cl2(bdmvpzm)] (3) andits non vinyl substituted counterpart by Santos et al., [149] as stated in chapter 3.1.3.

νsym(Mo=O) νasym(Mo=O)[MoO2Cl2] 959 915[MoO2Cl2(bdmvpzm)] (3) 944 917[MoO2Cl(bdmvpza)] (5) 942 911[MoO2Cl(bdmpza)] [151] 941 910

Table 3.2: IR data of selected molybdenum(VI) dioxo compounds in cm−1.

Another argument speaking in favor of the isostructurality of the two complexes is deliv-ered by the results of the IR measurements (KBr). As mentioned in 3.1.3, the vibrationalbands of the metal educt [MoO2Cl2] are found at νsym = 959 cm−1 and νasym = 915 cm−1.The corresponding absorptions in compound 5 can be observed at νsym = 942 cm−1 andνasym = 911 cm−1, which shows the successful complexation. The IR absorption bands,that were found by Hammes et al. for [MoO2Cl(bdmpza)], are νsym = 941 cm−1 and

66


νasym = 910 cm−1, [151] which amounts to a difference of only 1 cm−1 in both values incomparison to 5. This clearly speaks for a very similar coordination sphere. A list of thementioned absorption bands for comparison is shown in table 3.2.As can be seen in figure 3.5, the molybdenum(VI) center is sixfold coordinated, as forcompound 3. However, one chlorido ligand was exchanged by the carboxylate donor groupof the ligand. Nevertheless, the coordination motif of a distorted octahedron is preserved.The Mo=O bonds are slightly elongated (1.754(8) and 1.769(7) Å) in comparison tocompound 3 (1.693(3) Å). The trans influence of the oxo group on the Mo-N can beobserved again. The corresponding bond has a length of 2.323(9) Å compared to thebond trans to the chlorido ligand with only 2.212(9) Å. [151]

Figure 3.5: Molecular structure of [MoO2Cl(bdmpza)] by Hammes et al. [151] Thermalellipsoids are drawn at the 50 % probability level. Hydrogen atoms have beenomitted for clarity. The structure is compositionally disordered. The natureof disorder is however indicating that the trans isomer is predominant. [151]

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3.1.6 Copolymers containing bdmvpzm (2) and Hbdmvpza (4)[35]

The two vinyl substituted ligands bdmvpzm (2) and Hbdmvpza (3) were copolymerizedwith two comonomers suitable for vinylogous polymerizations. On the one hand, thelinear polymerizing methyl methacrylate (MMA) and on the other hand the crosslinkingethylene glycol dimethacrylate (EGDMA) were used for this purpose.

OO

N N

N

CO2H

N

N N

N

CO2H

N

x y z

OO

N N

N

CO2H

N

N N

N

CO2H

N

x y z

OO

Figure 3.6: Molecular structures of Hbdmvpza copolymers with MMA (top) andEGDMA (bottom). [35]

The reactions were carried out in dry xylenes at a temperature of 85 ◦C. In order toincrease the surface area of the resulting compounds, xylenes were used as solvent forits porogenic properties. This was necessary to preserve the reactivity of the obtainedcopolymers and ensure access to the embedded coordination site for metal fragments. Thecopolymerization process was initialized by the addition of the radical starter azobisiso-butyronitrile (AIBN).The possible molecular structures, which result from the reactions depicted in scheme3.6, are depicted in figure 3.6. The amount of incorporated ligand moieties embedded inthe copolymers was in a similar magnitude as reported earlier by Gazi Türkoglu. [35,186]

However, in contrast to these earlier results, no first fraction of the MMA copolymers with

68


AIBNMMA/EGDMA

AIBNMMA/EGDMA

N

N N

N

OHO

EG

DM

A /

MM

A

EG

DM

A / M

MA

N

N N

N

EG

DM

A /

MM

A

EG

DM

A / M

MA

xylene, 85°C

xylene, 85°CN

N N

N

OHO

4

N

N N

N

2 P6/P7

P8/P9

Scheme 3.6: Synthesis of copolymers of bdmvpzm (2) and Hbdmvpza (3) with MMA orEGDMA.

an increased amount of ligand incorporation could be obtained as precipitate (comparechapter 1.2.5). The most likely reason for this is the higher starting concentration ofMMA in the reaction feedstock (see table 3.3).

Polymer Ligand Added Ligand/monomer Ligand/polymer Yieldmonomer monomer [mmol/g][a] [mmol/g][b] [%][c]

P6 bdmvpzm MMA 0.294 0.443 28.1P7 bdmvpzm EGDMA 0.267 0.310 95.8P8 Hbdmvpza MMA 0.294 0.624 18.4P9 Hbdmvpza EGDMA 0.265 0.228 96.5

Table 3.3: Incorporation of bdmvpzm 2 and Hbdmvpza 4 in copolymers with MMAand EGDMA. [a] Ratio related to composition feed. [b] Ratio related tocomposition in the final polymer. [c] Weight percent related to total weightof monomers in feed.

As can be seen in table 3.3, the ligand incorporation in the MMA copolymers is stillhigher than for the EGDMA copolymers. As stated in chapter 1.2.5, this is probably dueto the higher reactivity of the crosslinking ligand monomers, in comparison to the MMAmonomers, which therefore react preferentially with each other. [35] Yet, the yields of thecopolymerization with EGDMA were almost quantitative.

69


The values listed in table 3.3 were determined by the % N value of the elemental analysisof the polymers. The calculations were carried out according to the following equation: [35]

mmol Ligand

g Polymer= % N

4 · 14 g ·mol−1 · 10

3.1.7 Molybdenum Containing Copolymers

In general, there are two different possible routes to obtain molybdenum containingcopolymers of the bdmpvz (2) and Hbdmvpza (4) ligands. On the one hand, the pre-viously presented copolymers P6, P7, P8 and P9 can be charged with a molybdenumfragment. On the other hand, the complexes 3 and 5 can be embedded in copolymers.

3.1.7.1 Treatment of Copolymers with [MoO2Cl2(THF)2]

The coordination of molybdenum(VI) dichloride dioxide to the copolymers P6, P7, P8and P9 was carried out in tetrahydrofuran. The N,N,O coordinating polymers P8 andP9 were deprotonated with potassium tert-butoxide prior to the addition of the metalfragment. To compensate for the increased sterical hindrance of the polymer structure,the reaction temperature was raised to 50 ◦C.

EG

DM

A /

MM

A

EG

DM

A / M

MA

EG

DM

A /

MM

A

EG

DM

A / M

MA

THF, 50 °C

THF, 50 °C N

N N

NMo

Cl OO

OON

N N

N

OHO

EG

DM

A /

MM

A

EG

DM

A / M

MA

N

N N

N

EG

DM

A /

MM

A

EG

DM

A / M

MA

N

N N

NMo

Cl O

Cl

O

[MoO2Cl]

1. KOtBu

2. [MoO2Cl2]

P6/P7

P8/P9

P6-Mo/P7-Mo

P8-Mo/P9-Mo

Scheme 3.7: Synthesis of Synthesis of P6-Mo, P7-Mo, P8-Mo and P9-Mo.

It was expected, that the EGDMA polymers would exhibit a lower amount of complex-ated molybdenum, due to the highly crosslinked structure of the compound, which causes

70


a high level of sterical hindrance in these polymers. The resulting metal loaded copoly-mers were analyzed via atomic absorption spectroscopy (AAS) after chemical digestionof the respective samples. The obtained results confirmed these expectations, as listed intable 3.4.As can be seen, the values revealed a higher incorporation of molybdenum in MMA poly-mers. In P8-Mo 78.5 % of the available coordination sites were occupied and 61.2 % inP6-Mo. In contrast, the highly crosslinked EGDMA polymers exhibited low occupationratios of only 14.8 (P7-Mo) and 8.41 % (P9-Mo).

Polymer Ligand Copolymer Ligand incorpo- Metal/polymer Occupancymonomer ration [mmol/g] [mmol/g] [%]

P6-Mo bdmvpzm MMA 0.443 0.271 61.2P7-Mo bdmvpzm EGDMA 0.310 0.0459 14.8P8-Mo Hbdmvpza MMA 0.624 0.490 78.5P9-Mo Hbdmvpza EGDMA 0.228 0.0192 8.41

Table 3.4: Molybdenum content of the polymers after treatment with [MoO2Cl2] as de-termined by AAS.

The question, why in the case of the MMA copolymers the N,N,O coordinating ligandmoieties show higher degrees of occupation, while the opposite is the case for the EGDMApolymers, cannot be easily answered. A possible explanation for these findings is, that inthe relatively flexible MMA copolymers, the influence of the higher coordination potentialof the κ3 coordinating N,N,O binding sites dominates. In the already highly crosslinkedEGDMA polymer however, the additional carboxylate moiety might block access to thecoordination site from one side, thus further increasing the sterical hindrance and therebyhampering the complexation.As mentioned above, Gazi Türkoglu reported, that the κ2 or κ3 coordination modewas not influenced by the polymerization process (see chapter 1.2.6). [35,186] For the Ru-and Mn complexes reported earlier, IR studies strongly indicated an unchanged κ3-N,N,Ocoordination of the respective metal center. [35,186]

In order to confirm, that the same applies to the molybdenum(VI) compounds discussedherein, the N,N,O coordinated were analyzed via IR spectroscopy. Unfortunately, theoxo molybdenum vibrations appear in the spectra in an area, where the polymer itself isabsorbing, as well.The results for P8-Mo are depicted in figure 3.7. For comparison, the spectra of poly-methylmethacrylate (PMMA), the homopolymer of MMA and of the monomeric complex[MoO2Cl(bdmvpza)] (5) were added. The spectrum of compound 5 (figure 3.7 (a)) showsthe two characteristic Mo=O vibrational bands. As discussed in chapter 3.1.5, they ap-pear at ν̃ = 942 cm−1 and ν̃ = 911 cm−1. These vibrations were also found in the spectrum

71


of the corresponding copolymer P8-Mo at ν̃ = 943 cm−1 and ν̃ = 913 cm−1. As the vibra-tions appear at almost identical wave numbers, it can be assumed, that the coordinationmotif is unchanged.

(a)

(b)

(c)

Figure 3.7: IR spectra of (a) [MoO2Cl(bdmvpza)] (5), (b) PMMA and (c) P8-Mo.

A comparison of the three spectra in figure 3.7 shows, that the spectrum of P8-Mo (c)obviously resembles a superposition of the absorptions of the free complex [MoO2Cl-(bdmvpza)] (5 (a) and PMMA (b).The same results were found for the copolymer P9-Mo. There, the vibrations weredetected at ν̃ = 944 cm−1 as a shoulder and ν̃ = 910 cm−1 as a weak band.

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3.1.7.2 Copolymerization of [MoO2Cl2(bdmvpzm)] (3) and[MoO2Cl(bdmvpza)] (5)

The second possibility to obtain molybdenum containing polymers was the copolymeriza-tion of the complexes [MoO2Cl2(bdmvpzm)] (3) and [MoO2Cl(bdmvpza)] (5) with MMAand EGDMA. To do so, the complex monomers were dissolved in acetonitrile and heatedto a temperature of 65 ◦C. As before, the polymerization was started by the addition ofAIBN. The results of the copolymerization process can be seen in table 3.5, as determinedby AAS.

EG

DM

A /

MM

A

EG

DM

A / M

MA

EG

DM

A /

MM

A

EG

DM

A / M

MA

MeCN, 65 °C

N

N N

NMo

Cl OO

OO

N

N N

NMo

Cl O

Cl

O

N

N N

NMo

Cl O

Cl

O

N

N N

NMo

Cl OO

OO

AIBNMMA/EGDMA

MeCN, 65 °C

AIBNMMA/EGDMA

5

3

P12/P13

P10/P11

Scheme 3.8: Copolymerization of [MoO2Cl2(bdmvpzm)] (3) and [MoO2Cl(bdmvpza)](5) with MMA and EGDMA.

As already mentioned, the MMA copolymer bares a lower activity in the polymerizationprocess, since it is not a crosslinker. Therefore, the complex incorporation is comparativelyhigh. The opposite applies to EGDMA. The corresponding polymers expose a low levelof complex incorporation.While a change of the coordination scheme upon polymerization seems unlikely, IR spec-troscopy of the resulting compounds was carried out, nevertheless. Table 3.6 shows thecorresponding IR data. For comparison, the respective absorptions of the monomericcomplexes were added.These results show, that the compounds obtained by polymerization subsequent to com-plexation, exhibit the same absorption characteristics as the aforementioned polymers(see chapter 3.1.7.1). In comparison with the monomeric complexes, only slight shifts of

73


Polymer Complex Copolymer Complex/monomer Metal/polymermonomer monomer [mmol/g][a] [mmol/g]

P10 MoO2Cl2(bdmvpzm) (3) MMA 0.211 0.646P11 MoO2Cl2(bdmvpzm) (3) EGDMA 0.191 0.0334P12 MoO2Cl(bdmvpza) (5) MMA 0.208 0.401P13 MoO2Cl(bdmvpza) (5) EGDMA 0.188 0.0307

Table 3.5: Molybdenum content of the copolymers obtained by copolymerization of [MoO2Cl2-(bdmvpzm)] (3) and [MoO2Cl(bdmvpza)] (5) with MMA and EGDMA as deter-mined by AAS. (a) Ratio related to composition feed.

the wave numbers were observed. This argues for an unchanged coordination motif.P11 however did show the corresponding Mo=O bands only as weak shoulders. Thiswas unexpected, since the metal content of the polymer appeared to be sufficiently high.The most likely explanation is, that the corresponding absorptions were overlaid by theabsorptions of the copolymer. Another explanation could be an unwanted side reaction.Yet, the most relevant side reaction, that could have occurred, is the reaction with tracesof moisture during the polymerization process. This might have led to the formation ofoxido-bridged dimers or oxidodiperoxo molybdenum(VI) centers, linked by µ2-bridgingoxygen atom. Such compounds would however still show strong absorptions in the sameregion, which were not observed either. [187,188]

Polymer ν̃ [cm−1] monomeric complexesP10 947, 915

944, 917 (3)P11 944, 919[a]

P12 941, 911942, 911 (5)

P13 943, 911

Table 3.6: Observed Mo=O vibrational bands of P10, P11, P12 and P13 (KBr). [a] as weakshoulders

3.1.8 Oxygen Atom Transfer Catalysis

The previously presented complexes and copolymers were now applied in an oxygen atomtransfer (OAT) reaction. More specifically the transfer of an oxygen atom from dimethylsulfoxide to triphenylphosphine was attempted. This reaction resembles the catalyticactivity of the DMSO reductase (see chapter 1.5.2). On the one hand, this serves as averification, if such a system can be useful as model for the DMSO reductase. On theother hand, the reaction may serve as a benchmark for the whole concept of the 4-vinylsubstituted bis(pyrazolyl)acetic acids and their reactivity in the copolymerized state.

74


MoN N

OOCl

Cl

MoN N

OOCl

Cl

PPh3

MoN N

OCl

Cl

MoN N

OCl

Cl

O

S

VI

V

IV

PPh3

OPPh3DMSO

DMS

Scheme 3.9: Proposed catalytic cycle and relevant oxidation states for the reductionof DMSO by the presented molybdenum complexes and their copolymers(here: [MoO2Cl2(bdmvpzm)] (3)).

The catalysis was carried out in deoxygenated dimethyl sulfoxide to exclude this alterna-tive source of oxygen atoms. 200 equivalents of triphenylphosphine were dissolved, beforethe catalysts were added. After 24 hours in the case of polymer catalysts and 6 hours inthe case of the monomeric complexes, samples were taken and analyzed via 1H and 31PNMR spectroscopy.

Catalyst ligand/complex monomer copolymerP10 MoO2Cl2(bdmvpzm) (3) MMAP11 MoO2Cl2(bdmvpzm) (3) EGDMAP12 MoO2Cl(bdmvpza) (5) MMAP13 MoO2Cl(bdmvpza) (5) EGDMA

P6-Mo bdmvpzm MMAP7-Mo bdmvpzm EGDMAP8-Mo Hbdmvpza MMAP9-Mo Hbdmvpza EGDMA

Table 3.7: Composition of copolymers employed in catalytic DMSO reduction.

1H NMR spectra were used to monitor the release of dimethyl sulfide, which was foundat a chemical shift of 1.98 ppm. Furthermore, the formation of triphenylphosphine oxidewas monitored via 31P NMR spectroscopy. The yields of the catalytic reactions werecalculated by the integration of the corresponding signals in the 31P NMR spectra.The results of the experiments are shown in table 3.8. For better comparison, the compo-

75


Catalyst t neduct (t0) ncatalyst neduct (t) nproduct (t) y TON TOF[h] [mmol] [µmol] [mmol] [mmol] [%] [10−5 s−1]

- 24 1.50 7.50 1.50 0 0 0 03 6 1.50 7.50 0.660 0.840 56 112 5195 6 1.50 7.50 0.675 0.825 55 110 509

P10 24 1.50 7.50 0.195 1.31 87 174 201P11 24 1.50 7.50 1.49 0.0200 1 2 2.31P12 24 1.50 7.50 0.960 0.540 36 72 83.3P13 24 1.50 7.50 1.14 0.360 24 48 55.6

P6-Mo 24 1.50 7.50 0.165 1.34 89 178 206P7-Mo 24 1.50 7.50 0.435 1.07 71 142 164P8-Mo 24 1.50 7.50 0.840 0.660 44 88 102P9-Mo 24 1.50 7.50 1.38 0.120 8 16 18.5

Table 3.8: Results of the catalytic reduction of dimethyl sulfoxide.

sition of all of the involved copolymers are listed in table 3.7. The measurements display,that all of the employed materials showed OAT activity and thus relevant catalytic activ-ity concerning DMSO reductase models. As control experiment, dimethyl sulfoxide wasstirred with triphenylphosphine without additional catalyst. However, no reaction couldbe observed in this case. The calculations of yields (y), turnover numbers (TON) andturnover frequencies (TOF) were calculated according to the following equations:

y = nproduct(t)nreactant(t0)

· 100%

TON = nproduct(t)ncatalyst

TOF = TONt

In general, all of the EGDMA based copolymers revealed a lower activity than their MMAbased counterparts. This was to expect, since the latter polymers are significantly lesscrosslinked.Furthermore, the copolymers, which were charged with molybdenum subsequent to thepolymerization process show a higher activity than the corresponding polymers, in whichthe finished complexes were embedded. The reason for this correlation presumably liesin the amount of active metal sites. While many metal centers can be blocked duringthe polymerization process, if the final complexes are incorporated, the first mentioned

76


polymers P6-Mo to P9-Mo contain metal fragments only at accessible active sites. Thusit is reasonable, that their relative catalytic activity is higher.The overall results show that the obtained materials are potential models, mimickingoxomolybdenum enzymes as shown above, especially for DMSO reductases. Nevertheless,the transfer of oxygen to triphenylphosphine is only a starting point, since it is no bio-relevant oxygen acceptor. Therefore, further studies have the be conducted on this topic.

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3.2 4-Ethynyl Substituted Pyrazole Based Ligands

The second part of this thesis is about the introduction of ethynyl linkers in position 4 ofthe pyrazole rings of pyrazole based ligands. While first compounds with vinyl groups inthis position have already been reported byGazi Türkoglu, [35,186] this is a new approachto add additional functionality to the well established system of bis(pyrazolyl)acetato orbis(pyrazolyl)methane scorpionate and chelate ligands.Acetylene moieties offer a great potential for coupling reactions like the Sonogashiraor the Glaser coupling reactions as well as Click chemistry reactions like the coppercatalyzed azide-alkyne cycloaddition (CuAAC) or theHuisgen cycloaddition, as depictedin scheme 3.10.

R'

R' Br

R' N3 R' IR'

N

NN

R'

HN

N

HN

N

HN

N

HN

N

HN

N

HN

N

NH

N

Scheme 3.10: Possible reactions of acetylene linker groups.

These reactions provide relatively mild ways to functionalize ligands and the complexesthereof. Examples of such functionalizations could be the introduction of fluorophores,which could be triggered by paramagnetic fluorescence quenching, induced by the coordi-nated metal centers. Furthermore, moieties that are capable of promoting the solubilityof the resulting complexes could be attached easily to overcome solubility issues.In order to obtain such compounds, different reaction pathways were to be used, as will bediscussed in the following chapter. Two general approaches are to be mentioned here. Onthe one hand, the introduction of an ethynyl linker to an already coupled pair of pyrazole

78


moieties and on the other hand the synthesis of ethynyl substituted pyrazole derivatives,which should then be reacted to the corresponding ligands.

79


3.2.1 Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)-methane (14)

In order to obtain an ethynyl substituted bis(pyrazol-1-yl)methane ligand from bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1), Corey-Fuchs conditions were chosen. Thefirst step of this two step reaction is the formation of a dibromovinyl species from analdehyde precursor in a Wittig type reaction. To do so, 57 was treated with tetrabro-momethane in the presence of triphenylphosphine and triethylamine. After purificationvia column chromatography, the product 14 could be obtained as a violet solid in goodyields.

N

N N

NHH

Br

Br

Br

BrCBr4, PPh3

NEt3N

N N

N

OO

1 14

Scheme 3.11: Synthesis of Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)metha-ne (14).

The 1H NMR spectrum of compound 14 shows two singlets corresponding to the methylsubstituents of the pyrazole rings at 2.11 and 2.38 ppm. Compared to the educt, anupfield shift is noticed. The protons of the methylene bridge are detected at 6.01 ppm.The most striking difference in comparison to the educt spectrum is the signal of theproton adjacent to the reaction site. It is shifted from 9.93 ppm to 7.18 ppm in theproduct NMR spectrum.The 13C NMR spectrum is very similar to the educt spectrum as far as the bis(pyrazol-1-yl)methane moiety is concerned, with the methyl groups at 11.6 and 13.2 ppm, thebridging carbon atom at 54.0 ppm and the carbon atoms of the pyrazole rings at chemicalshift of 116.1, 138.8 and 147.3 ppm. The two carbon atoms of the dibromovinyl moietyclearly indicate the product formation with signals at 61.0 and 93.0 ppm.Furthermore, the product 14 could be confirmed by elemental analysis. Due to the highmolecular mass of the bromide residues, this is a conclusive proof for the successful productformation.

80


3.2.2 Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane(bedmpzm) (15)

The second step of the Corey-Fuchs reaction consists of the formation of the desiredethynyl substituted product by treatment of the dibromovinyl precursor 14 with twoequivalents of n-butyllithium. The first equivalent leads, after initial lithiation, to therearrangement to a terminal alkyne. This alkyne reacts with the second equivalent to thecorresponding lithium acetylide, which is protonated to the desired product after aqueousworkup. [189]

Standard conditions for this reaction suggest, as common for lithiation reactions withn-butyllithium, tetrahydrofuran as solvent and a temperature of −78 ◦C. [190] Yet thesereaction conditions lead to decomposition of the substrate. The reaction could not becarried out in tetrahydrofuran at any other temperature up to 0 ◦C either.

The only reaction condition that led to the formation of the desired product 15 in accept-able yields was by using diethyl ether as solvent at a temperature of 0 ◦C. After quenchingof the reaction and removal of the solvent, the crude product could easily be purified bywashing with methanol.

N

NN

N

n-BuLi

Et2O, 0 °CN

N N

NHH

Br

Br

Br

Br

14 15

Scheme 3.12: Synthesis of bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (15).

The successful synthesis was confirmed by 1H NMR spectroscopy. The signals of themethyl groups are therein detected at 2.10 and 2.44 ppm. The protons of the ethynylmoieties are detected at 4.15 ppm and the protons of the methylene bridge appear at achemical shift of 6.13 ppm.

In the 13C NMR spectrum, the methyl groups are observed at 10.5 and 12.3 ppm. Themethylene bridge is detected at 60.7 ppm and the carbon atoms of the pyrazole rings areassigned to the signals at 102.4, 144.2 and 151.1 ppm. The ethynyl moiety that has beenintroduced during the reaction can be found at 75.4 and 81.1 ppm.

The presence of the terminal alkyne could also be observed in the IR spectrum of com-pound 15. There, the alkyne group shows a strong C-H stretch vibration at 3221 cm−1

and a -C≡C- stretch band at 2106 cm−1.

81


3.2.3 Attempted Synthesis of Bis(4-trimethylsilyl-ethynyl-3,5-di-methylpyrazol-1-yl)acetic acid (16)

In an attempt to use the aforementioned Corey-Fuchs reaction to obtain a corre-sponding N,N,O coordinating bis(pyrazol-1-yl)acetic acid from bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)methane (14), a one pot reaction pathway was evaluated.Therefore, the educt 14 was converted into the respective lithium acetylide by two equi-valents of n-butyllithium as mentioned in section 3.2.2. The resulting mixture was chargedwith trimethylsilyl chloride to obtain the trimethylsilyl-ethynyl species in situ. After-wards, another equivalent of n-butyllithium was added and a dry stream of carbon dioxidewas applied. Aqueous workup was supposed to subsequently release the free carboxylicacid.

N

NN

N

n-BuLiEt2O, 0 °C

TMS TMS

1. Trimethylsilyl chloride2. CO23. H2O

CO2H

N

N N

NHH

Br

Br

Br

Br

14

16

Scheme 3.13: Attempted Synthesis of Bis(4-trimethylsilyl-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (16).

However, no solid product could be obtained from this reaction, nor did NMR spectroscopyof the resulting brownish oil reveal any signals indicating a successful synthesis.In further attempts, the synthesis was altered. Using 15 as educt also led to decompo-sition. The same is true for the direct application of three equivalents of n-butyllithiumfrom the beginning or alterations of the solvent to tetrahydrofuran or varying temper-atures. It could potentially be impossible to selectively deprotonate this molecule dueto the number of acidic protons. However, the initial reaction protocol was supposed toovercome this problem. Yet all of the attempted syntheses led to decomposition of thesubstrate.

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3.2.4 [CuI(bedmpzm)] (17)

To evaluate the coordination behavior of 15, a series of transition metal complexes thereofwas synthesized. In a first attempt copper(I) iodide was reacted with 15. After stirring of15 with this metal salt at ambient conditions for 30 minutes, the resulting colorless com-plex [CuI(bedmpzm)] (17) precipitated from solution and could be collected by filtrationin a very good yield of 85 %.

N

NN

N

CuIacetonitrile

Cu

I

N

NN

N

15

17

Scheme 3.14: Synthesis of [CuI(bedmpzm)] (17).

The formation of the complex 17 could be confirmed by the shift of the signal of themethyl groups next to the coordination site from 2.10 ppm in the pure ligand to 2.20 ppmin the metal complex in the 1H NMR spectrum. The other methyl groups on the contrarywere only slightly shifted by 0.01 ppm to 2.45 ppm. The protons of the terminal acetylenesare detected at 4.20 ppm and the methylene bridge appears at 6.23 ppm.Due to the low solubility, it was impossible to obtain a 13C NMR spectrum. However, thecomplex 17 could be confirmed by elemental analysis.Apart from the complex monomer shown in scheme 3.14, a polymeric system would bethinkable as well. In such a structure, the coordinatively unsaturated copper(I) couldbe bonded side-on to the acetylene moieties of another complex molecule. Such a coor-dination mode would ultimately lead to a polymeric structure with the same elementalcomposition as the structure proposed above.However, in this case, an insoluble material has to been assumed, which was not thecase. Furthermore, the ν(C≡C) band in the IR spectrum would be expected at 1900-1960 cm−1 as reported in the case of hydrotris(3-mesitylpyrazolyl)borato-copper(I) alkynecomplexes. [191] Instead, the corresponding vibration was found at 2112 cm−1, which isvery close to the free ligand (15) with 2106 cm−1. These findings strongly argue for theformation of the monomeric complex [CuI(bedmpzm)] (17) with no side-on coordinationto the acetylene moieties as depicted in scheme 3.14.

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3.2.5 [ZnCl2(bedmpzm)] (18)

In a similar fashion, the colorless [ZnCl2(bedmpzm)] (18) complex could be obtained.Once again, acetonitrile was used as solvent and the product precipitated within one hourafter the addition of zinc(II) chloride to the ligand solution and could be collected viafiltration. Also for this compound, the yields were quite high with 79 % referring tobedmpzm (18).

N

NN

N N

NN

N

ZnCl2acetonitrile

Zn

ClCl15

18

Scheme 3.15: Synthesis of [ZnCl2(bedmpzm)] (18).

In the 1H NMR spectrum of compound 18, the methyl groups are detected at 2.10 and2.44 ppm. The protons of the acetylene moieties can be found at 4.15 ppm and themethylene bridge has a chemical shift of 6.14 ppm.As for the copper(I) complex, a 13C NMR spectrum could not be obtained due to thelow solubility of the complex. However, elemental analysis confirmed the desired zinc(II)complex.As observed for the corresponding copper(I) complex [CuI(bedmpzm)] (17), the ν(C≡C)vibration was only shifted slightly in comparison to the free ligand 15 from 2106 cm−1

to 2110 cm−1, arguing against any side-on interaction of metal ions with the acetylenemoieties.

3.2.6 [MnCl2(bedmpzm)2] (19)

A third transition metal complex of 15 that was synthesized was [MnCl2(bedmpzm)2] (19).To do so, equimolar amounts of manganese(II) chloride and 58 were dissolved in acetoni-trile and the resulting solution was stirred for one hour. Once again, the product precip-itated during this time and the resulting colorless powder was collected via filtration inyields of 82 %.In contrast to the previously reported complexes of bedmpzm (15), the reaction led to theformation of a bisligand complex with two equivalents of 15 bonded to the manganese(II)center.Unfortunately, it was impossible to record any NMR spectra, since the compound proofedinsoluble in common solvents. However, due to their paramagnetic nature, they wouldhave been of limited value.

84


N

NN

N

N

NN

N

MnCl2acetonitrile

N

NN

N

Mn2 Cl Cl

15

19

Scheme 3.16: Synthesis of [MnCl2(bedmpzm)2] (19).

Nevertheless it was possible to confirm the composition of [MnCl2(bedmpzm)2] (19) viaelemental analysis hinting at the proposed structure, as it is depicted in scheme 3.16.The IR spectrum once again shows no interaction of the metal ions with the acetylenemoieties since the ν(C≡C) vibrational band remains almost unchanged at 2105 cm−1.

3.2.7 [CoCl2(bedmpzm)] (20)

A cobalt(II) chloride complex of bedmpzm (15) could be obtained by the same procedureas before. Yet precipitation of the deep blue complex took 24 hours. After this time, theproduct could be collected by filtration and was obtained in yields of 80 %.

N

NN

N N

NN

N

CoCl2acetonitrile

Co

ClCl1520

Scheme 3.17: Synthesis of [CoCl2(bedmpzm)] (20).

As for the mangenese(II) complexe (19), no NMR spectra could be recorded for [CoCl2-(bedmpzm)] (20), since the complex was insoluble in common organic solvents. However,due to their paramagnetic nature, they would have been of limited value. The compositionof the complex however could be confirmed by elemental analysis.Once again, the ν(C≡C) vibration was only shifted slightly in comparison to the freeligand 15 from 2106 cm−1 to 2114 cm−1, arguing against any side-on interaction of metalions with the acetylene moieties.

85


3.2.8 [MoO2Cl2(bedmpzm)] (21)

In order to prepare a molybdenum(VI) complex of bedmpzm (15), molybdenum(VI)dichloride dioxide was at first dissolved in dry tetrahydrofuran to obtain the solventstabilized [MoO2Cl2(THF)2]. To the resulting solution was added a solution of bedmpzm(15). The product could be collected by filtration of the precipitate. A yield of 77 %regarding bedmpzm (15) could be obtained.

N

NN

N N

NN

N

[MoO2Cl2]

Mo

Cl ClOO

THF

15

21

Scheme 3.18: Synthesis of [MoO2Cl2(bedmpzm)] (21).

In contrast to the other transition metal complexes of bedmpzm (15) presented above,21 was reasonably well soluble. Thus 1H NMR as well as 13C NMR spectra could bemeasured.In the 1H NMR spectrum, the methyl groups are detected at 2.10 and 2.44 ppm. Theprotons of the acetylene moieties appear with a chemical shift of 4.15 ppm and the protonsof the methylene bridge can be found at 6.14 ppm.The 13C NMR spectrum on the other hand shows the methyl groups at 10.1 and 12.1 ppm.The carbon atom of the methylene bridge is assigned to the signal at 59.5 ppm. At 75.4and 84.1 ppm follow the signals of the carbon atoms of the acetylene moieties. The signalsat 101.2, 143.9 and 149.9 ppm derive from the pyrazole rings.The composition of this complex could as well be confirmed by elemental analysis.Also for this complex, the IR spectrum revealed no interaction of the molybdenum ionswith the acetylene moieties. The ν(C≡C) vibration was only shifted slightly in compar-ison to the free ligand 15 from 2106 cm−1 to 2118 cm−1. Furthermore, the ν(Mo=O)vibrational bands are found at νsym = 948 cm−1 and νasym = 919 cm−1, which arein good agreement with the findings for the corresponding vinyl substituted complex[MoO2Cl2(bdmvpzm)] (3, νsym = 944 cm−1, νasym = 917 cm−1, see chapter 3.1.3).

86


3.2.9 4-Iodopyrazole (22) and 4-Iodo-3,5-dimethylpyrazole (23)[192]

Since the initial aim to obtain an N,N,O coordinating 4-ethynylpyrazole based ligandfailed on the route presented above, another synthetic pathway was attempted. Thus itwas decided to attach the ethynyl substituents to the pyrazoles first and then build upchelate or scorpionate ligands with ethynyl linkers starting from these educts.

NH

N

NH

N

NH

NI

NH

NI

[NH4]2[Ce(NO3)6],I2

[NH4]2[Ce(NO3)6],I2

22

23

Scheme 3.19: Synthesis of 4-iodopyrazole (22) and 4-iodo-3,5-dimethylpyrazole(23). [192]

The synthetic pathway that was chosen, was to carry out an Sonogashira reactionwith 4-iodopyrazoles. To obtain these, pyrazole, respectively 3,5-dimethylpyrazole hadto be iodinated first. This was done in an electrophilic aromatic substitution reaction.The pyrazoles were therefore dissolved in acetonitrile and reacted with elemental iodinein the presence of ceric ammonium nitrate, as it is depicted in scheme 3.19. 4-iodo-3,5-dimethylpyrazole (23) could be obtained in yields of 69 % and 4-iodopyrazole (22) inyields of 83 %. The spectroscopic data agreed with the literature values. [192]

3.2.10 4-Iodo-1-tritylpyrazole (24)[77] and4-Iodo-3,5-dimethyl-1-tritylpyrazole (25)[71]

In order to avoid undesired side reactions, especially electrophilic substitutions, a protec-tion group was introduced. Therefore, triphenylmethylchloride (TrtCl) was applied afterthe deprotonation of the pyrazoles 22 and 23 with sodium hydride (see scheme 3.20).4-Iodo-1-tritylpyrazol (24) could be obtained in yields of 85 % and 4-Iodo-3,5-dimethyl-1-tritylpyrazole (25) in yields of 41 %. The obtained spectroscopic data agreed with theliterature values. [71,77]

The 1H NMR spectrum exhibits the signals of the methyl groups of compound 25 at1.58 and 2.23 ppm. The pyrazole protons of compound 25 are detected at 7.43 and7.69 ppm. A comparison of the signals of the trityl protecting group shows that they are

87


NH

NI

NH

NI

NI

N

NI

N

NaHTrtCl

- NaCl

NaHTrtCl

- NaCl

22

23

24

25

Scheme 3.20: Synthesis of 4-iodo-1-tritylpyrazol (24) [77] and 4-iodo-3,5-dimethyl-1-tritylpyrazole (25). [71]

shifted downfield in the spectrum of the unsubstituted 24 at 7.34 and 7.43 ppm, whilethe corresponding signals of 25 are observed at 7.10 and 7.27 ppm.In the 13C NMR spectrum of the dimethyl substituted 25, the methyl groups are assignedto the signals at 14.6 and 15.7 ppm. The carbon atoms of the pyrazole rings are detectedat 66.9, 142.5 and 147.5 ppm. The remaining signals at 78.9, 127.3, 127.5, 130.3 and142.9 ppm were assigned to the trityl protecting group. The spectrum of 24 is similarand contains the signals of the pyrazole carbon atoms at 55.6, 136.4 and 144.7 ppm. Thetrityl protecting group is observed at chemical shifts of 79.3. 127.8. 127.9. 130.1 and142.7 ppm.

3.2.11 3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26)

As a precursor for the synthesis of 4-ethynyl substituted bis(pyrazol-1-yl)methane ligands,26 was synthesized in a Sonogashira reaction from 3,5-dimethyl-4-iodo-1-tritylpyrazo-le (23) and trimethylsilyl acetylene to introduce the desired triple bond in position 4 ofthe pyrazole ring. The reaction was carried out in a mixture of dimethylformamide andtriethylamine at 60 ◦C in a sealed flask to avoid premature evaporation of the volatileacetylene precursor. Similar reactions have been reported before, using different protect-ing groups. [183,193]

After completion of the reaction, the product was purified via column chromatographyand could be obtained in yields of 71 % referring to 23.

88


NI

N

N

NSi

[PdCl2(PPh3)2]CuI, TMS-acetylene

DMF, NEt3, 60 °C

25 26

Scheme 3.21: Synthesis of 3,5-dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26).

The trimethylsilyl group can be found in the 1H NMR spectrum with a chemical shiftof 0.23 ppm, thus confirming successful product formation. The methyl groups result insignals at 1.57 and 2.26 ppm. The trityl protection group is assigned to two multiplets at6.85-6.95 and 7.00-7.10 ppm.The successful substitution of the iodine can also be observed in the 13C NMR spectrum,in which the newly introduced trimethylsilyl groups cause a signal at 0.23 ppm. Themethyl groups at the pyrazole ring are found at 12.9 and 13.9 ppm. The carbon atomsof the triple bond are assigned to the signals at 97.9 and 98.1 ppm. The tertiary carbonatom of the trityl group has a chemical shift of 78.6 ppm while the carbon atoms of thephenyl rings rise signals at 127.3, 127.5, 130.3 and 142.8 ppm. Finally, the pyrazole ringwas detected at 104.4, 145.4 and 148.1 ppm.

3.2.12 4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27)

Similar to the synthesis of 26, it was possible to obtain the unsubstituted 4-((trimethylsi-lyl)ethynyl)-1-tritylpyrazole (27) via a Sonogashira reaction from 4-iodo-1-tritylpyrazole(24) and trimethylsilyl acetylene. After purification via column chromatography, the com-pound could be obtained in yields of 68 %.

NI

N

N

NSi


DMF, NEt3, 60 °C

24 27

Scheme 3.22: Synthesis of 4-(trimethylsilyl)ethynyl-1-tritylpyrazole (27).

The 1H NMR spectrum of compound 27 exhibits the signal of the protecting trimethylsilylgroup at 0.21 ppm. The trityl group appears as a multiplet at a chemical shift of 7.29 ppm.

89


The remaining signals at 7.56 and 7.77 ppm can be assigned to the protons of the pyrazolering.The 13C NMR spectrum is also very similar to the spectrum of compound 26. However,the TMS group is shifted upfield considerably to a chemical shift of −0.06 ppm. Thesame could be observed for the carbon atoms of the triple bond, which could be detectedwith an upfield shift of about 3 ppm at 95.1 and 96.5 ppm, respectively.

3.2.13 3,5-Dimethyl-4-(trimethylsilyl)ethynylpyrazole (28)

The next step on the way to pyrazole based scorpionate ligands bearing acetylene moietieswas the synthesis of an ethynyl substituted derivative of 3,5-dimethylpyrazole.

N

NSi

N

NHSi

CF3COOH

CH2Cl2

26 28

Scheme 3.23: Initial synthesis of 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28).

Therefore, the trityl protected species 3,5-dimethyl-4-(trimethylsilylethynyl)-1-tritylpyra-zole (26) (see section 3.2.11) was deprotected with trifluoroacetic acid in dichloromethane(see scheme 3.23). After purification via column chromatography, the product could beobtained in yields of 41 %. However, under these conditions the protection group proofedto be unnecessary for the Sonogashira reaction, that is applied to obtain this product,so a more straight forward synthesis was employed.

N

NH

N

NHSiI


DMF, NEt3, 60 °C

23 28

Scheme 3.24: Final synthesis of 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28).

An alternative pathway via a Sonogashira reaction with trimethylsilylacetylene and 3,5-dimethyl-4-iodopyrazole (23) also resulted in 3,5-dimethyl-4-(trimethylsilyl)ethynylpyra-zole (28). Complete turnover could only be achieved, if dimethylformamide was employed

90


as a solvent. Reactions in tetrahydrofuran and triethylamine led to incomplete reactions(see scheme 3.24).Both pathways led to identical products. After purification via column chromatography,the product 28 could be obtained in yields of 71 % referring to 4-iodo-3,5-dimethyl-1-tritylpyrazole (25).

N

NSi

HN

NSi

H

Figure 3.8: Cyclic dimer structure of 3,5-dimethyl-4-(trimethylsilyl)ethynylpyra-zole (28). [194]

The 1H NMR spectrum of compound 28 shows the signal of the trimethysilyl group at0.24 ppm, thus confirming the successful substitution of the iodine. The methyl groupsare detected at 2.31 ppm. These protons do not split in two singlets, as it would beexpected, since the pyrazoles form symmetric dimeric structures via hydrogen bonds asdepicted in figure 3.8. [194] The proton of the secondary amine is assigned to a signal at10.45 ppm.The formation of the product could also be verified via 13C NMR spectroscopy, with thenewly introduced trimethylsilyl group at 0.21 ppm. The methyl groups at the pyrazolering are detected at 11.3 ppm. The sp hybridized carbon atoms are observed at 97.0 and97.7 ppm. The pyrazole carbon atom in position 4 to which they are bonded is detectedat 101.5 ppm. The remaining pyrazole carbon atoms can be found at 147.2 ppm.

3.2.14 2,2-Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (29)

With a suitable ethynyl substituted pyrazole precursor at hand, a first bis(pyrazolyl)aceticacid derivative could be synthesized. The one pot synthesis by Burzlaff et al. wasapplied. [11] Therefore, the pyrazole was reacted with dibromoacetic acid under basic con-ditions in the presence of a phase transfer catalyst, as depicted in scheme 3.25. Afteracidic workup, compound 29 could be obtained as yellowish powder by filtration. Due tothe basic conditions used during the phase transfer reaction, the trimethylsilyl protectinggroups were removed during the process, thus rendering a further deprotection step moot.The 1H NMR spectrum of 29 showed the signals of the methyl groups at 2.23 and2.35 ppm. The existence of the signal of acetylenic protons at 3.19 ppm in combina-tion with the missing signal of the trimethylsilyl group confirmed the deprotection duringthe phase transfer reaction. Furthermore, the proton at the bridging carbon atom was

91


N

N N

N

OHO

N

NHSi2

1. Br2CHCO2H, KOH, K2CO3, BTEAC2. HCl

THF

28 29

Scheme 3.25: Synthesis of 2,2-bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (29).

detected at a chemical shift of 6.82 ppm. The remaining resonance at 9.86 ppm could beassigned to the acidic proton of the carboxylate function.Furthermore, the compound could also be confirmed via 13C NMR spectroscopy. Inthe corresponding spectrum, the methyl groups were found at chemical shifts of 10.6and 12.4 ppm, respectively. The resonance of the bridging carbon atom was observed at81.6 ppm. The carbon atoms of the ethynyl moiety were detected at 103.3 and 124.8 ppm.The signals at 144.7, 146.7 and 151.4 ppm could be assigned to the carbon atoms of thepyrazole rings. The last signal at 166.0 ppm was caused by the carbon atom of thecarboxylate moiety.Despite the fact that it was possible to confirm the compound via mass spectrometry, theelemental analysis showed impurities, which could not be removed. While the originalbis(3,5-dimethylpyrazol-1-yl)acetic acid can be purified by recrystallization from acetone,the additional ethynyl substituents drastically improved the solubility of compound 29,making such procedures ineffective.

3.2.15 (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)-ethynylpyrazol-1-yl)methane [HOPhbdmeTMSpzm] (30)

Starting from 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) it was now also possibleto obtain a first bis(pyrazol-1-yl)methane based heteroscorpionate ligand. For the syn-thesis, the well established one pot synthesis of Elflein et al. was employed, that wasalready mentioned in section 1.1.3. [23]

In a first attempt compound 28 was deprotonated with sodium hydride prior to reactingit with thionyl chloride, to obtain a S=O bridged species in situ. Without further purifica-tion, salicylaldehyde and one equivalent of pyridine were added to the reaction mixture, toobtain the desired ligand (2-hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)-ethynyl-pyrazol-1-yl)methane [HOPhbdmeTMSpzm] (30) (see scheme 3.26).After purification via column chromatography, 30 could be obtained in yields of 45 %referring to 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28).The 1H NMR spectrum of compound 30 shows the protons of the trimethylsilyl groups

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SN N

N NSiSi

NH

NSi2

N N

N NSiSi

OH

O

OH , pyridine

- SO2

NaH,SOCl2THF,0 °C

28

30

Scheme 3.26: Synthesis of (2-hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynyl-pyrazol-1-yl)methane [HOPhbdmeTMSpzm] (30).

at 0.23 ppm. The successful ligand formation is backed by the splitting of the methylsignal. The signals appear in the spectrum at 2.14 and 2.23 ppm, respectively. Thesignals of the aromatic protons are detected as a multiplet at 6.79 ppm and a tripletwith a coupling constant of 3JH,H = 7.21 Hz at 7.15 ppm. Furthermore, the formationof the desired compound is proved by the methine signal at 7.40 ppm. More downfieldat 9.03 ppm appears the broad signal of an acidic proton, which can be assigned to thephenolic hydroxyl group.The desired N,N,O coordinating ligand could also be confirmed by 13C NMR spectroscopy.The signals of the the trimethylsilyl groups are therein shown at 0.15 ppm, followed bythe methyl groups of the pyrazole moieties at 10.6 and 12.6 ppm. The methine carbonatom is detected at 72.9 ppm. Further downfield the carbon atoms of the ethynyl moietiesare assigned to the signals at 96.4 and 98.8 ppm. The pyrazole ring exhibits signals at104.0 ppm for the carbon atom in position 4 and 143.7 ppm respectively 150.8 ppm for thepositions 3 and 5. Furthermore, the carbon atoms of the phenoxy moiety have appearedat 117.8, 120.0, 121.1, 129.2, 131.0 and 154.8 ppm.

3.2.16 [MoO2Cl2(HOPhbdmeTMSpzm)] (31)

After the successful synthesis of ligand 30, it was attempted to obtain a first transitionmetal complex thereof. With molybdenum dichloride dioxide, a molybdenum(VI) metalfragment was chosen. In accordance with the synthesis for similar compounds by San-tos et al., this salt was first dissolved in tetrahydrofuran to form the solvent stabilized[MoO2Cl2(THF)2]. [149] After the addition of ligand 30, the so far colorless solution turned

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yellow and subsequent treatment with n-hexane led to the precipitation of the yellow[MoO2Cl2(HOPhbdmeTMSpzm)] (31) complex. The desired κ3 coordination could how-ever not be achieved by this route, although similar complexes with a menthopyrazolebased ligand exhibited N,N,O coordination without the addition of base to the reactionmixture. [195]

N N

N NSiSi

OH

[MoO2Cl2(THF)2]

N N

N NSiSi

OH

MoO O

Cl

Cl

30

31

Scheme 3.27: Synthesis of [MoO2Cl2(HOPhbdmeTMSpzm)] (31).

In general, two isomers of this complex would be possible. On the one hand, the twochlorido ligands could be in cis position to each other. On the other hand, it would bepossible for them to be in trans position. The later possibility is depicted in scheme 3.27as the first option would lead to an asymmetric complex, resulting in two sets of signalsin the NMR spectra. This was however not observed. Therefore, the exclusive formationof the symmetric trans isomer is assumed.Thus, the methyl groups of the pyrazole rings only result in two instead of four signalsat 2.55 and 2.75 ppm in the 1H NMR spectrum. Also, the trimethyl silyl groups do notsplit up and are detected at 0.23 ppm. The signals of the aryl protons can be observedat 6.86 ppm as a triplet with a coupling constant of 3JH,H = 7.5 Hz and as a secondtriplet at 6.93 ppm with 3JH,H = 4.5 Hz. The remaining two aryl protons split up each

94


into doublets, one at 7.10 ppm with a coupling constant of 3JH,H = 9.0 Hz and one at7.34 ppm with 3JH,H = 6.0 Hz. The remaining singlet at 7.30 ppm can be assigned tothe proton of the methine bridge. The phenolic proton could not be observed. This isprobably caused by deuterium exchange with traces of DCl in the solvent.The 13C NMR spectrum revealed only one set of signals as well. The trimethyl silyl groupis therein detected at −0.05 ppm, which is a 0.20 ppm upfield shift in comparison to thepure ligand 30. The methyl groups on the other hand are barely influenced and leadto signals at 11.0 and 13.9 ppm, respectively. The bridging carbon atom has a chemicalshift of 70.4 ppm. The signals at 93.8 and 100.9 ppm can be assigned to the acetylenegroups. The pyrazole rings are found at chemical shifts of 106.3, 142.8 and 156.7 ppm. Thecarbon atoms of the aryl ring are detected at 120.6, 120.7, 121.8, 128.4 and 132.6 ppm. Asexpected, the carbon atom with the strongest downfield shift is the hydroxyl substitutedone at 157.1 ppm.The successful incorporation of the molybdenum fragment could furthermore be confirmedvia infrared spectroscopy. Therein, the stretching vibrations of the molybdenum oxygenbonds are observable at ν̃ = 941 (νsym) and 921 cm−1 (νasym).Furthermore, the proposed κ2 coordinated complex is also backed by the elemental anal-ysis. There, the carbon value differs by 1.05 %. If the ligand was κ3 coordinated, thedifference amounts to 3.69 %, since one chloride atom would have to be abstracted in thiscase. For these reasons, the κ2 coordinated complex can be assumed.

3.2.17 (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)me-thane (32)

In order to enable ligand 30 to undergo cross coupling or Click reactions, it is necessaryto remove the protecting trimethyl silyl groups. To show that this is easily possible, twodifferent reaction conditions were evaluated.On the one hand, it was attempted to remove the protecting group by the addition ofpotassium fluoride in a mixture of methanol and tetrahydrofuran. On the other hand,potassium carbonate was used. Both routes led to the terminal alkyne with similar yieldsof around 80 % and can therefore be chosen, what can gain importance, if not a freeligand, but instead a complex is to be deprotected.The loss of the trimethyl silyl group can reliably be verified by the non existence of thecorresponding signal in the 1H NMR spectrum of compound 32. The methyl groups arestill found at 2.18 and 2.23 ppm. The additional proton of the terminal alkyne, which isa second evidence for the successful deprotection, is detected at 3.20 ppm. The methineproton is slightly shifted downfield to 7.45 ppm. The protons of the aryl ring split up ina triplet at 7.15 ppm with a coupling constant of 3JH,H = 6.0 Hz. The remaining aryl

95


N N

N N

OH

N N

N N

OH

Si Si

K2CO3 or KFTHF, MeOH

30

32

Scheme 3.28: Synthesis of (2-hydroxyphenyl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (32).

protons result in a multiplet at 6.81 ppm.The 13C NMR spectrum also confirms the absence of the trimethyl silyl group. The mostupfield signals in the spectrum are caused by the methyl groups at 10.5 and 12.5 ppm. Thecarbon atoms of the alkyne moiety are shifted upfield to 75.3 and 81.4 ppm in comparisonto the protected species with 93.8 and 100.9 ppm. The shift of the pyrazole carbon atomsis only slightly altered. Their resonances can be found at 102.7, 144.1 and 150.9 ppm.The same is true for the aryl carbon atoms with chemical shifts between 120 and 130 ppmand 154.8 ppm for the hydroxyl substituted carbon atom.Furthermore, the compound could be verified via mass spectrometry. Therein is found asthe deprotonated species [M-H]− with 100 % intensity at 343.165 m/z.

3.2.18 (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)-ethynylpyrazol-1-yl)methane (33)

Ut was possible to obtain an N,N,N coordinating ligand by the same one pot synthesisas used for the synthesis of [HOPhbdmeTMSpzm] (30, see section 3.2.15). Therefore,the salicylaldehyde that was used for the synthesis of 30 was replaced by 1-methyl-2-imidazolecarboxaldehyde.

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After the deprotonation of 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) by sodiumhydride, the pyrazolate was reacted with thionyl chloride was applied. After subsequentaddition of 1-methyl-2-imidazolecarboxaldehyde and one equivalent of pyridine, the de-sired product (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazol-1-yl)methane (33) was formed and could be isolated via column chromatography.Compound 33 could be obtained in yields of 9 % referring to 3,5-dimethyl-4-((trimethylsi-lyl)ethynyl)pyrazole (28).

SN N

N NSiSi

NH

NSi2

N N

N NSiSi

O

OH , pyridine

- SO2

NaH,SOCl2THF,0 °C

28

33

NN NN

Scheme 3.29: Synthesis of (1-methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)-ethynylpyrazol-1-yl)methane (33).

Product formation could be confirmed via 1H NMR spectroscopy. The signal at 0.22 ppmcould be assigned to the trimethylsilyl groups. The methyl groups of the pyrazole ringscould be detected at 2.17 and 2.23 ppm, respectively. The methyl group of the imidazolemoiety is shifted upfield to 3.33 ppm in comparison to the educt spectrum, in which it isdetected at 3.99 ppm. In the same way, the two protons of the imidazole ring are shiftedupfield and generate two doublets at 6.92 ppm and 7.04 ppm with 3JH,H = 3.0 Hz. Theeduct spectrum of 1-methyl-2-imidazolecarboxaldehyde shows these protons at 7.08 and7.24 ppm. The last signal at 7.48 ppm could be assigned to the methine bridge.The 13C NMR spectrum verified the proposed structure as well. The trimethylsilyl groupscould be detected at 0.14 ppm, followed by the methyl groups of the pyrazole rings at10.7 and 12.6 ppm. The carbon atom of the methine bridge, that was formed during thereaction, shows a signal at 70.4 ppm. The acetylene carbon atoms have chemical shiftsof 96.5 and 98.5 ppm. The pyrazole rings appear at chemical shifts of 104.7, 140.1 and144.4 ppm. These results correlate, just as the values from the 1H NMR, well with thevalues found for compound 30 (see section 3.2.15), indicating that the different third donor

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group does only slightly influence the electronical structure of the pyrazole backbone ofthe ligand. Furthermore, the methyl group of the imidazole moiety is detected at 33.0 ppmand the carbon atoms of the imidazole ring can be found at 123.0, 128.2 and 150.8 ppm.

3.2.19 (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (34)

As already shown for compound 32, it might be necessary to remove the protectingtrimethylsilyl groups, in order to enable further reactions like Click or further Sono-gashira reactions with the acetylene residue.As stated in chapter 3.2.17, two different deprotection pathways were evaluated, whichboth succeeded in similar yields of almost 80 %. On the one hand, the deprotection wascarried out using basic conditions. In this case, potassium carbonate was employed. Onthe other hand the reaction was carried out exploiting the high affinity of silyl moietiestowards halides, especially fluoride. [196] In both cases, the reaction was carried out in amixture of tetrahydrofuran and methanol.

N N

N N

N N

N NSi Si

NN

NN

33

34

K2CO3 or KFTHF, MeOH

Scheme 3.30: Synthesis of (1-methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (34).

The successful deprotection could be monitored via NMR spectroscopy. The 1H NMRspectrum of 34 shows no signal of the trimethyl silyl anymore. The signals of the methyl

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substituents of the pyrazole rings are detected at 2.21 and 2.24 ppm, respectively. Theproton of the terminal alkyne, a further proof for the performed desilylation, is found ata chemical shift of 3.19 ppm. The protons of the methyl group located at the imidazolemoiety are detected at 3.34 ppm. The signals at 6.93 and 7.05 ppm can be assigned to theprotons of the imidazole moiety. Most downfield at 7.54 ppm, the proton of the methinebridge is detected.Similarly, no trimethyl silyl resonance could be found in the 13C NMR spectrum of com-pound 34. Instead, the most upfield signals are the resonances of the methyl substituentsof the pyrazole rings at 10.5 and 12.5 ppm. The methyl substituent of the imidazole moi-ety is detected at 30.0 ppm. The carbon atom of the methine bridge has a chemical shiftof 70.2 ppm. The signals at 75.3 and 81.4 ppm could be assigned to the carbon atomsof the acetylene moiety. In comparison to the trimethyl silyl protected acetylene, thesesignals are shifted upfield by 20 ppm (compare 3.2.18). The carbon atoms of the imidazolering are detected at 103.4, 140.0 and 144.7 ppm. The remaining signals at 123.1, 128.0and 150.9 ppm could be assigned to the pyrazole rings.

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3.2.20 Summary of 4-Ethynyl Substituted Pyrazole Based Ligands

In this chapter, it could be shown, that it is possible to obtain the corresponding chelateand heteroscorpionate ligands based on 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28)in the established one pot syntheses. [11,23] The ligands that could be synthesized by variousprotocols are depicted in scheme 3.31.

HN

N

N N

N N

NN

N N

N N

OH

N

N N

N N

N N

N

OHO

TMS

3234

15 29

28

Scheme 3.31: 4-Ethynylpyrazole based scorpionate ligands.

The implementation of the acetylene moieties allows for a range of reactions to furthermodify the ligands or their complexes. Among these reactions are Glaser, Cadiot-Chodkiewicz or Sonogashira coupling reactions as well as the copper(I)-catalyzedazide-alkyne cycloaddition (CuAAC).The possible applications are manifold. In chapter 3.4, model ligands for Rieske dioxyge-nases will be presented, that are based on these principles. Furthermore, the introductionof fluorophores, which could be switched via paramagnetic fluorescence quenching, in-duced by metal centers coordinated to the ligand, could be an interesting opportunity.Besides, these linkers could serve as an easy way to PEGylate such ligands, thus im-proving the solubility of the resulting compounds via polyethylene glycol polymer (PEG)chains attached to the linker groups, which is a common procedure in pharmaceuticalchemistry. [197]

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3.3 Coordination Polymers of 1,4-Bis(1H-pyrazol-4-yl)butadiynes

3.3 Coordination Polymers of 1,4-Bis(1H-pyrazol-4-yl)-butadiynes

So far, 4-ethynyl substituted pyrazoles were used to obtain ligands by connecting twoequivalents via methylene and methine bridges. Yet, the ethynyl moieties enable theseprecursors to undergo homo coupling reactions. Starting from the trityl protected 4-ethynyl substituted pyrazoles discussed above (see chapter 3.2), ligands suitable for 1Dcoordination polymers should be obtainable via such coupling reactions.A related compound could be obtained by T. Waidmann by performing a Glaser cou-pling reaction with a similarly substituted imidazole derivative. [170] The resulting ligandis depicted in figure 3.9.

N

N

N

N

Figure 3.9: Structure of bis(N-methylimidazol-2-yl)butadiyne (bmib) according to T.Waidmann. [170]

Thus, corresponding compounds based on pyrazole derivatives should be synthesized inthis part of this thesis. As already discussed above (see chapter 1.6.3), Navarro andcoworkers could already succeed in the preparation of first MOFs based on such ligands.For their experiments, they used the Boc protected ligands depicted in figure 3.10, whichwere deprotected in situ during the complexation with metal fragments. [180–182]

N

N

R

R

O

O

N

N

R

R

O

OR = H / CH3

Figure 3.10: Structures of 1,4-bis[1-Boc-pyrazol-4-yl]butadiyne and 1,4-bis[1-Boc-3,5-dimethylpyrazol-4-yl]butadiyne (Boc2L). [180–182]

However, Navarro and coworkers did not isolate the unprotected ligand and could there-fore neither study its properties nor obtain coordination polymers based on the neutral1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne ligand.Therefore, the synthesis using the trityl protecting group instead of the Boc protectinggroup and the properties of 1,4-bis(1H -pyrazol-4-yl)butadiynes as well as their abilitiesto form coordination polymers will be discussed in the following chapter.

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3.3.1 3,5-Dimethyl-4-ethynyl-1-tritylpyrazole (35)

To synthesize the Glaser coupling product of 4-ethynyl-3,5-dimethylpyrazole, it wasnecessary to avoid premature coordination of the pyrazole to the metal catalyst, that isused during this procedure. Therefore, trityl, a bulky protecting group, was employed toshield the nitrogen donor functions of the resulting ligand.For this reason, the synthesis was started from 3,5-dimethyl-4-((trimethylsilyl)ethynyl)-1-tritylpyrazole (26, see chapter 3.2.11). In the first step, the trimethylsilyl group, whichprotects the terminal alkyne function had to be removed, in order to make Glaser typecoupling reactions possible.Therefore, 26 was treated with potassium carbonate in a mixture of tetrahydrofuran andmethanol, to obtain the desired terminal alkyne species in high yields. The crude productwas washed with water and afterwards used without further purification.

N

NSi

K2CO3

THF/MeOH N

N

26 35

Scheme 3.32: Synthesis of 3,5-dimethyl-4-ethynyl-1-tritylpyrazole (35).

The successful deprotection of the alkyne function was confirmed via NMR spectroscopy.In comparison to 26, the trimethylsilyl signal in the 1H NMR spectrum disappeared. Theprotons of the methyl group are shifted upfield to 1.37 and 2.06 ppm, respectively. Thehydrogen atom of the terminal alkyne can be found at 3.02 ppm. More downfield inthe aromatic region, the resonances of the trityl group were observed as a multiplet at6.99 ppm.These findings could also be confirmed via 13C NMR spectroscopy. No signals resultingfrom trimethylsilyl groups could be found therein. The methyl substituents are detectedat 12.9 and 13.8 ppm, respectively. The carbon atoms of the triple bond exhibit an upfieldshift from 97.9 and 98.1 ppm to 78.5 and 81.2 ppm. The signals of the pyrazole ring arefound at 103.3, 145.6 and 148.1 ppm. The trityl protecting group was detected at chemicalshifts of 127.3, 127.5, 130.3 and 142.8 ppm.

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3.3.2 1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadiyne (36)

After the removal of the trimethylsilyl protecting group, the terminal alkyne could nowbe reacted in a Glaser type homo coupling reaction. In order to increase yields, anEglinton coupling reaction was used. The original Glaser protocol uses a catalyticamount of a copper(I) salt, which is oxidized by oxygen to the active catalytic copper(II)species. During the coupling step, the copper(II) ions get reduced and are reoxidized byoxygen again. [198]

The Eglinton coupling however uses stoichiometric amounts of a copper(II) salt, e.g.copper(II)acetate to promote the reaction. In both cases, pyridine is used as base todeprotonate the terminal alkyne. [199]

The reaction goes to completion without the observation of any side products and couldtherefore be purified by thorough washing with water to remove the metal salts.

N

N

N

N

N

N2

MeCN/pyridineCu(OAc)2

35

36

Scheme 3.33: Synthesis of 1,4-bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadiyne (36).

The success of the coupling reaction was confirmed by NMR spectroscopy. The 1H NMRspectrum of the resulting compound contains two singlets at 1.57 and 2.25 ppm, originat-ing from the two methyl substituents of the pyrazole rings. Furthermore, two multipletsat 7.11 and 7.29 ppm represent the trityl protecting groups. No signal of an acetyleneproton could be observed anymore.Furthermore, the 13C NMR spectrum reveals the methyl substituents of the pyrazole rings

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at chemical shifts of 13.2 and 14.4 ppm. The signals at 74.8 and 79.3 ppm can be assignedto the four carbon atoms of the butadiyne bridge. This is a strong sign for the successfulcoupling reaction, since those carbon atoms experienced a strong upfield shift of around20 ppm in comparison to the spectra of the trimethyl silyl protected precursor (see chapter3.2.11) and the terminal alkyne species (see chapter 3.3.1). The remaining signals of thepyrazole rings at 104.0, 147.4 and 149.3 ppm and of the trityl protecting groups at 127.9,128.1, 130.8 and 143.2 ppm persist almost unchanged.

3.3.3 1,4-Bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne (37)

In the last step of the ligand synthesis, the trityl protecting group needs to be removed.This bulky group prevented the nitrogen donor functions of the ligand from polymerizationwith the metal ions involved in the Eglinton coupling reaction shown above (chapter3.3.2).

N

NH

N

HN

N

N

N

N

CF3CO2H CH2Cl2

36

37

Scheme 3.34: Synthesis of 1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne (37).

The deprotection of the pyrazole moieties can be performed under mild conditions. Astoichiometric amount of trifluoroacetic acid in dichloromethane leads to the formationof the unprotected product. This product is capable of forming a polymer structure ofits own, as it is depicted in figure 3.11. This might explain its low solubility in unpolarsolvents. These polymer strands can only be cleaved by polar or even better by proticsolvents, that are capable to form hydrogen bridges on their own, for example methanol.

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n

N

NH

N

NH

N

NH

N

NH

Figure 3.11: Polymer structure of 1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne (37).

Since this is not the case for dichloromethane, in which the reaction is carried out, thedesired product precipitates directly from solution upon its formation and can be collectedvia filtration. The crude product was washed with organic solvents of lower polarity toobtain it in its pure form.The obtained compound was examined via NMR spectroscopy. The methyl substituentsof the pyrazole rings split in the 1H NMR spectrum into two singlets, depending on thesolvent. Whereas deuterated acetone only leads to one singlet at 2.28 ppm, deuterateddimethyl sulfoxide leads to a splitting into two singlets at 2.17 and 2.25 ppm. Further-more, it was possible to detect the signal of the pyrazole NH at 12.6 ppm. This behaviormost likely relates to the ability of dimethyl sulfoxide molecules to bond to the ligandvia hydrogen bridges. While the compound is a hydrogen bond acceptor due to its un-protonated nitrogen atom, it can act as a hydrogen bond donor as well via its second,protonated nitrogen donor, as it is depicted in figure 3.11.13C NMR spectroscopy could only be performed in dimethyl sulfoxide, since the requiredconcentration could not be reached in acetone. As observed in the 1H NMR, the methylgroups split up into two signals at 10.0 and 12.5 ppm. The carbon atoms of the butadiynebridge are detected at 97.5 and 98.3 ppm. Furthermore, the signals at 105.5, 144.1 and150.7 ppm can be assigned to the carbon atoms of the pyrazole rings.Crystals suitable for an X-ray structure determination could be obtained by layering asolution of 37 in methanol with n-hexane. The crystal structure contains two molecules of1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) and one water molecule in the asym-metric unit. The compound crystallized in the monoclinic space group P21/c. Selectedbond lengths and angles are listed in table 3.9.As can be seen in figures 3.13 and 3.12, the two different molecules mostly differ inthe angle of the butadiyne linker. The angles 6 (C4-C3A-C3B) and 6 (C3A-C4-C23) ofthe linear species amount to 179.70(20)◦ and 176.81(18)◦ and result in an almost linearconformation. The corresponding angles of the bent molecule on the other hand differmore from the ideal 180◦ angles with 6 (C1-C2A-C2B) = 176.37(12)◦ and 6 (C2A-C1-C13)= 174.97(18)◦. While this may seem like a minor difference, the significance can be easilyseen in figure 3.12, which provides a side view on the packing motif. Therein, the linearmolecules are colored blue, while the bent species is depicted in yellow. Furthermore, the

105


Figure 3.12: Packing motif of 37 (blue: linear molecule, yellow: bent molecule). Thermalellipsoids are drawn at the 50 % probability level. Hydrogen atoms havebeen omitted for clarity.

butadiyne linkers of both molecules contain discrete triple (in both cases 1.202(2) Å) andsingle bonds (1.377(3) Å and 1.375(3) Å) which show no sign of conjugation. [200]

As can also be seen in figure 3.12, the two different species are arranged in alternatinglayers. The pyrazole rings show parallel-displaced π − π stacking. The distance betweenthe pyrazole rings amounts to 3.5330(21) Å. The linear molecules connect two layers of

Distances (Å)bent molecule linear molecule

C1-C2A 1.202(2) C3A-C4 1.202(2)C2A-C2B 1.377(3) C3A-C3B 1.375(3)C1-C13 1.419(2) C4-C23 1.420(2)C13-C14 1.402(2) C23-C24 1.403(2)N11-N12 1.362(2) N21-N22 1.366(2)N11-O1 2.9097(14) N21-O1 2.9082(18)N12-N22 2.8848(19) N21-N11 4.2482(20)

Angles (deg)bent molecule linear molecule

C1-C2A-C2B 176.37(12) C4-C3A-C3B 179.70(20)C2A-C1-C13 174.97(18) C3A-C4-C23 176.81(18)N11-N12-C14 109.25(13) N21-N22-C24 108.82(13)N11-N12-N22 120.249(104) N21-N22-N12 119.247(104)

Table 3.9: Selected interatomic distances (Å) and angles (deg) for both molecules in the asym-metric unit of compound 37.

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the bent species via hydrogen bridges. Each molecule of the latter is connected to onlyone of the neighboring layers via hydrogen bridges to one of the linear molecules, as itis depicted in figure 3.13. Apart from π stacking, these hydrogen bridges seem to bethe driving force behind the curving of the butadiyne bridge of the bent species. Thiscurvature is necessary to align with the pyrazole moiety of the next linear molecule aswell as with the central water molecule in such a way, that hydrogen bonding becomespossible.This leads to a structure, in which the oxygen atom of the water molecule is surroundedtetrahedrally by four ligand molecules. This is done via hydrogen bridges. These bridgesseem to exist on the hand between O1 and N11 as well as between O1 and N21. Thecorresponding distances amount to 2.9097(14) Å and 2.9082(18) Å, respectively, which isvery well within in the range of typical hydrogen bridges. [201]

Figure 3.13: Molecular structure of 37. Thermal ellipsoids are drawn at the 50 % prob-ability level. Most hydrogen atoms have been omitted for clarity. Only oneof the two disordered proton distributions is depicted.

The distance between the two bonding pyrazole moieties (N12-N22) is even smaller with2.8848(19) Å, which also argues for the existence of a hydrogen bridge between them. [201]

Summing it up, every second ligand molecule accepts one hydrogen bond from the watermolecule and at the same time donates one hydrogen bond to its bonding partner. Thepartner itself then donates one hydrogen bond back to the water molecule, which leadsto the observed tetrahedral structure around the water center, with a 50:50 disorderregarding the water as well as the pyrazole protons.

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3.3.4 Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethylpyrazol-4-yl)butadiyne)) (38/39)

Having obtained the unprotected 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37),the next step was, to synthesize a first coordination polymer thereof. This could beaccomplished by mixing a solution of the ligand in acetone with a solution of cobalt(II)-acetylacetonate, likewise dissolved in acetone. The product immediately precipitated as aninsoluble violet powder. The composition of the obtained compound was first determinedvia elemental analysis. The results suggested, that ligand 37 was deprotonated by acetyl-acetonate, thus enabling it to bind one cobalt atom with each nitrogen donor function.Since one acetylacetonato ligand gets protonated during the reaction, only one such ligandremains at each cobalt center.

N

N

N

NCo

O O

CoO O

n

N

NH

N

HN

acetoneCo(acac)2

- Hacac

37

38

Scheme 3.35: Synthesis and proposed structure of poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethylpyrazol-4-yl)butadiyne)) (38).

These considerations led to the proposed structure of the obtained compound depictedin scheme 3.35. This suggestion however was thus far solely based on the elemental com-position. Therefore, the residue of the reaction was analyzed via 1H NMR spectroscopy.The spectrum exposed the predicted acetylacetone as a tautomeric keto enol equilibriumwith the signals of the prevalent enol form at 2.05 and 5.50 ppm. The keto form wasindicated by singlets at 2.24 and 3.59 ppm. These findings supported the proposed struc-ture presented above. However, a variety of different structures with this composition

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are possible and while very similar coordination modes have been reported for severalcompounds in literature, [202] it is not possible to definitely confirm one of them withoutan X-ray structure determination. As was recently shown by Navarro and Zhu (seechapter 1.6.3), pyrazole based coordination polymers can also result in higher order threedimensional structures. [176,180–182]

While a blue single crystal suitable for X-ray structure determination could be obtainedby slow evaporation of a highly diluted solution of ligand 37 and cobalt(II)acetylacetonatein methanol, the structure which was revealed therein, did not match the composition ofthe aforementioned violet powder. Instead a one dimensional chain of cobalt(II)acetylace-tonate alternating with ligand units and bonded in a κ1 fashion are observed (39). Thedeprotonation of the amine function by one of the acetylacetonato ligands did not occur,as can be seen in figure 3.15, in which one repetition unit of the 1D coordination polymeris depicted.

Figure 3.14: Preliminary molecular structure of two strands of 39. Thermal ellipsoids aredrawn at the 50 % probability level. Hydrogen atoms and water moleculeshave been omitted for clarity.

The compound crystallized in the triclinic crystal system in the space-group P−1. Se-lected bond lengths and angles are listed in table 3.10. The packing motif of 39 is depictedin figure 3.14. The single strands of the polymer are therein layered orthogonally to eachother. The pyrazole rings show parallel-displaced π − π stacking. The distance betweenthe pyrazole rings amounts to 3.4293(76) Å. The linear strands result from trans coordi-nation of the metal centers to the ligand molecules. The structure determination showed,that the cobalt centers in the polymer are sixfold coordinated in a only slightly distortedoctahedron. The angle N1-Co1-O2 amounts to 90.22(4)◦ and the angle between the oxy-gen donors of the acetylacetonato ligands measures 90.48(15)◦. They are both close to

109


Figure 3.15: Preliminary molecular structure of one repetition unit of 39. Thermal el-lipsoids are drawn at the 50 % probability level. Most hydrogen atoms andwater molecules have been omitted for clarity.

the ideal value of 90◦. Furthermore, the coordinative bonds have lengths of 2.150(4) Å(N1-Co1) and 2.042(4) Å (Co1-O1). The bond lengths of the carbon atoms of the bu-tadiyne linker are close to the ideal values for such a system. The triple bonds amount to1.190(6) Å, which is close to the literature value of 1.20 Å. The bonds between the sp2-spcarbon atoms (C4-C6) have lengths of 1.425(6) Å and the single bond between the twosp hybridized carbon atoms C7A and C7B amounts to 1.382(9) Å. Both values agree wellwith the literature values of 1.43 Å and 1.37 Å, respectively. This clearly shows that thereare no conjugation effects but instead two discrete triple bonds in this compound. [200]

Distances (Å)N1-N2 1.352(5) N1-C5 1.320(6)N2-C3 1.336(6) C4-C6 1.425(6)C3-C4 1.367(7) C4-C5 1.409(6)C6-C7 1.190(6) C7A-C7B 1.382(9)N1-Co1 2.150(4) Co1-O1 2.042(4)

Angles (deg)N1-N2-C3 113.0(4) N2-N1-C5 105.2(4)O1-Co1-O2 90.48(15) N1-Co1-O2 90.22(4)


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3.3.5 Poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (40)

Since the reaction of ligand 37 with cobalt(II)acetylacetonate led most likely to deproto-nation of the ligand by the involved acetylacetonato ligands of the metal salt, the metalfragment was changed to cobalt(II)chloride. Deprotonation of the amine function by chlo-ride was not supposed to occur for hydrochloric acid is the stronger acid compared to theprotonated ligand. Instead, a κ1 coordination motif was expected.

N

NH

N

HN

acetone

N

NH

N

HN

Co

Cl

Cln

CoCl2

37

40

Scheme 3.36: Synthesis and proposed structure of poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (40).

Therefore, an equimolar solution of 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37)and cobalt(II)chloride in acetone was stirred at room temperature. In contrast to polymer38, no reaction could be observed after 24 hours. While heating the reaction mixture toreflux for another 24 hours, the desired compound was obtained in good yields as a bluepowder. As stated for compound 38, it is not possible to clarify unambiguously the struc-ture of the obtained polymer without an X-ray structure determination, especially sincethe substance is completely insoluble in common solvents. Elemental analysis howeverstrongly supports the structure, which is presented in scheme 3.36.As discussed above, the ligand was not deprotonated by the very weak base chloride.Thus, only a κ1 coordination of the ligand by a single cobalt dichloride center is possible.A structure similar as the one found in the X-ray analysis of 39 (see chapter 3.3.4)is therefore assumed. Nevertheless, higher dimensional coordination polymer structuresthan the proposed one dimensional chain cannot be ruled out entirely.In an attempt to obtain another cobalt(II)dihalide polymer, the synthesis was repeated

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with the corresponding bromide salt. In this case however, no reaction could be observed,even by continuous stirring under reflux in solvents like acetonitrile.

N

NH

N

HN

acetone/acetonitrile

N

NH

N

HN

Co

Br

Brn

CoBr2

37

41

Scheme 3.37: Attempted synthesis of poly(cobalt(II)bromido-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (41).

A possible explanation for this behavior could be the increased sterical demand of thebromido substituents compared to the chlorido substituents. This could for example leadto an interference with the methyl substituents of the ligand.

Summary

So far, it was possible to obtain the unprotected 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)bu-tadiyne (37) ligand. This ligand tends to form polymers by itself, due to its pyrazolemoieties, which can act at the same time as hydrogen bond donors and acceptors. Thiswas also confirmed by an X-ray structure determination.Based on this ligand 37, the structure of the 1D coordination polymer poly(cobalt(II)-acetylacetonato-bis(1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne)) (39) could be clar-ified, which is the first coordination polymer, in which 37 is incorporated in its neutralform instead of its anionic form. However, this material could only be obtained as acrystal. The direct reaction of 37 with cobalt(II)acetylacetonate led to the formation ofa polymer containing 37 in its deprotonated state 38.Therefore, the synthesis of coordination polymers of copper(II)dihalides was attempted,since the halides, as weaker bases compared to acetylacetonate, should not be able todeprotonate 37. This assumption was confirmed. The corresponding copper(II)chloridebased polymer of 37, poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)bu-

112


tadiyne)) (40), could be obtained. However, the synthesis of the corresponding copper(II)-bromide coordination polymer was not successful so far.

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3.3.6 4-Ethynyl-1-tritylpyrazole (42)

In another approach, the synthesis of 4-ethynyl-1-tritylpyrazole (42) was attempted.Therefore, the previously discussed compound 27 (see chapter 3.2.12) was deprotectedunder basic conditions, to enable it to undergo Glaser type coupling reactions as shownin chapter 3.3.1). Reaction with potassium carbonate in a mixture of tetrahydrofuranand methanol led to the removal of the trimethylsilyl protecting group, while leaving thetrityl group in place, which is useful to avoid the immediate formation of coordinationpolymers in the subsequent Eglinton coupling step. The desired compound 42 couldbe obtained in yields of 73 %.

N

NSi

K2CO3

THF/MeOH N

N

27 42

Scheme 3.38: Synthesis of 4-ethynyl-1-tritylpyrazole (42).

The successful deprotection could be confirmed via 1H NMR spectroscopy. While a newsignal at 3.01 ppm was found and assigned to the terminal proton of the alkyne moiety, nomore resonances of the TMS protecting group could be observed. The remaining signalsof the trityl group (7.13 and 7.19 ppm) as well as the the signals of the pyrazole ringprotons at 7.56 and 7.77 ppm, respectively, remained almost unchanged.The resonances of the alkyne moiety in the 13C NMR spectrum are shifted upfield by20 ppm to 75.4 and 78.3 ppm.

3.3.7 1,4-Bis(1-tritylpyrazol-4-yl)butadiyne (43)

As already shown for the dimethyl substituted compound 36, the next step consisted ofan Eglinton homo coupling step. In this case, the product 43 showed a significantlyhigher solubility in chlorinated solvents than its substituted counterpart 36 and was thusextracted with dichloromethane. Compared to 37 could be obtained as a white powderin yields of 66 %.The 1H NMR spectrum of compound 43 showed signals of the trityl protecting groupat 7.14 and 7.33 ppm as multiplets, while the protons of the pyrazole rings resonate atchemical shifts of 7.58 and 7.78 ppm.In the 13C NMR spectrum, the signals of the butadiyne carbon atoms appear at 72.8 and74.8 ppm, respectively. The trityl protecting group are assigned to the signals at 79.3,

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N

N

N

N

N

N2

MeCN/pyridineCu(OAc)2

42

43

Scheme 3.39: Synthesis of 1,4-bis(1-tritylpyrazol-4-yl)butadiyne (43).

127.9, 128.0, 130.1 and 136.6 ppm. Furthermore, the carbon atoms of the pyrazole ringsare detected at chemical shifts of 100.8, 142.4 and 142.9 ppm.These findings agree well with the signals found for the dimethyl substituted compound36.

3.3.8 Attempted Synthesis of 1,4-Bis(1H-pyrazol-4-yl)butadiyne (44)

While the precursors of 44 could be obtained in generally the same way as the precursors ofthe dimethyl substituted 37, it was not possible to remove the trityl protecting group fromthe nitrogen donor of 43 with trifluoroacetic acid. This was independent of the appliedtemperature or concentration of this reagent. Similar procedures involving hydrochloricacid or acetic acid did not lead to the desired product, either.However, reacting compound 43 with a Lewis acid, in this case boron tribromide, led topromising results, as depicted in scheme 3.40. Yet, no viable method for the purificationof 44 could be found during the course of this thesis.The 1H NMR spectrum of a bright green crude sample, taken during the reaction asreaction control, of compound 44 revealed only one signal, apart from solvent signals: asinglet at 7.31 ppm, which refers to the four protons of the pyrazole rings. This wouldagree nicely with the expected signal for the desired compound.More information could be gained from the 13C NMR spectrum of this sample. Only one

115


N

N

N

N

N

NH

N

HN

BBr3 CH2Cl2

43

44?

Scheme 3.40: Attempted synthesis of 1,4-bis(1H-pyrazol-4-yl)butadiyne (44).

signal typical for an sp hybridized carbon atom could be observed at a chemical shift of82.8 ppm or possibly the sp2 C4 pyrazole carbon atom. At 127.7 and 130.6 ppm appeartwo singlets with relatively high intensities, indicating carbon atoms, which carry protons.Therefore, they were assigned to the carbon atoms C3 and C5 of the pyrazole rings.The remaining two singlets at 127.8 and 145.6 ppm could be caused by the remainingatoms of the butadiyne linker. In this case, the discrete triple bonds of the linker groupwould be deallocated in favor of an cumulene like structure. A set of signals in thisarea would be typical for [5]cumelenes (e.g. tetraferrocenyl[5]cumulene 119.8, 119.9 and140.8 ppm). [203,204] Yet, after quenching of the reaction mixture, this product could notbe isolated.

N

NH

N

HN

44

?N

NCC

N

NC C

-H2

Scheme 3.41: Possible oxidation product of 1,4-bis(1H-pyrazol-4-yl)butadiyne (44).

This hypothesis is supported by the IR spectrum of the crude product. Therein, the initialC≡C band of the educt is accompanied by a second band at 1958 cm−1 in the spectrumof 44 (e.g. tetraferrocenyl[5]cumulene 1973 cm−1). [203–205]

In order to react to a cumelene, molecular hydrogen must have been released and 44 musthave been oxidized during this reaction as depicted in scheme 3.41. The bright green color

116


of the crude sample, that was taken and analyzed during the reaction, might indicate theinvolvement of radical species during the reaction.However, while these are surely interesting findings, there is more investigation needed infuture works to further elucidate this topic.

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3.4 Ferrocene Based Models for Rieske Dioxygenases

Apart from the above presented model systems for DMSO reductases, a model systemfor Rieske dioxygenases was to be synthesized. The most important aspect of thesecompounds are, besides their coordination motif, the electrochemical properties. As statedin chapter 1.3, ferrocene is a compound, that is widely used in this context due to itsdistinct electrochemical properties. [64,66,71,77,206]

N

N N

NFe

OO

Fe

e-

Figure 3.16: Possible electron transfer in a ferrocene based bis(pyrazolyl)acetate modelsystem.

Recent attempts to use ferrocene in combination with scorpionate ligands and more specifi-cally ferrocene substituted bis(pyrazolyl)acetic acids were undertaken by S. Tampier andS. Bleifuss. [71] In order to resemble the active site of a Rieske dioxygenase, ferrocenewas supposed to serve as an electron reservoir, which would transfer electrons towards aniron center, which is κ3 N,N,O coordinated by the ligand. One potential variant of sucha system is depicted in scheme 3.16.

N

N N

N

FeFe

N

N N

N

FeFe

R

R

R

R


R R

R R

O


Figure 3.17: Selected ferrocenyl pyrazole compounds by Tampier et al. [71]

The reaction pathway was based on the work of Mochida et al., who recently reportedon the synthesis of 4-ferrocenyl-1H -pyrazoles. These were obtained via Negishi type cou-pling reactions of 4-iodopyrazoles with ferrocene. [77] Starting from there, the correspond-ing 4-ferrocenylpyrazoles were reacted to bis(pyrazolyl)methanes, -ketones and -aceticacids via phase transfer reactions as already discussed in chapter 1.1.2. [71]

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While the bis(pyrazolyl)methanes and -ketones could be obtained in a pure form, Tam-pier and Bleifuss did not succeed to isolate the desired tripodal bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)acetic acid. However, the electrochemical potentials of the obtainedcompounds was promising. [71]

In this work, similar ligands for Rieske model systems are reported. These ligandshowever do not contain ferrocene, which is directly bonded to the pyrazole rings. Insteadthe ferrocenyl moieties were attached via triazole units (by Click chemistry) and viaacetylene linkers (by Sonogshira coupling reactions) as shown in scheme 3.42.

N

N

Fe

N

N

N

NFe

N3

N

NI Fe

N N

N

+

+Fe

CuSO4Sodium ascorbate

[PdCl2(PPh3)2]CuIDMF, NEt3

Scheme 3.42: Exemplary syntheses for ligands suitable for Rieske model systems.

In order to obtain these ligands, the corresponding ethynyl substituted ligand precursorbis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (15) was reacted with ferrocenyl azides incopper(I)-catalyzed alkyne-azide cycloadditions (scheme 3.42 top). The iodinated ligandprecursors for the Sonogashira coupling reactions (scheme 3.42 bottom) on the otherhand were prepared as shown in the following chapter.

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3.4.1 Bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45)

For the synthesis of acetylene linked Rieske dioxygenase models, it was necessary tofirst synthesize iodo substituted bispyrazolyl precursors. Such compounds could later beemployed in Sonogashira reactions to obtain the desired model systems.The first precursor of this kind was the N,N coordinating bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45). This compound was first synthesized by Potapov et al. from 1,1’-methylenebis(pyrazole) in 2006. [207] From there, the group used iodine and iodic acid ina mixture of acetic acid and sulphuric acid for the iodination. [207]

However, instead of following this procedure, the synthesis presented herein started fromthe above mentioned 4-iodo-3,5-dimethylpyrazole (23, see chapter 3.2.9). Two equiva-lents of 23 were connected to the corresponding bis(pyrazol-1-yl)methane 45 by a phasetransfer reaction in dichloromethane, which served as reactant as well as a solvent. Thereaction was catalyzed by benzyltriethylammoniumchloride (BTEAC) as a phase transfercatalyst. This synthetic route is based on the report of Juliá et al. regarding bispyra-zolylmethane. [43]

N

N N

NII

N

NHI2

KOH, K2CO3,BTEAC

CH2Cl2

2345

Scheme 3.43: Synthesis of bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45).

The spectroscopic data agreed with those reported by Potapov et al. [207] The 1H NMRspectrum shows the methyl substituents of the pyrazole rings at chemical shifts of 2.19and 2.47 ppm respectively. The protons of the introduced methylene bridge are detectedat 6.14 ppm.Furthermore, the 13C NMR spectrum of 45 shows the resonances of the four methylgroups at 12.2 and 14.0 ppm. The iodinated carbon atoms are strongly shifted upfieldto 62.0 ppm. The carbon atom of the methylene bridge is detected at 65.1 ppm and thesignals at 142.1 and 150.4 ppm can be assigned to the remaining carbon atoms of thepyrazole rings.

3.4.2 Bis(4-iodopyrazol-1-yl)methane (46)

While the procedure of Juliá et al. [43] led to good results for the synthesis of bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45) as was discussed above, the same protocolwas not applicable for the synthesis of bis(4-iodopyrazol-1-yl)methane (46) using from

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4-iodopyrazole (22), instead.Surprisingly not the anticipated methylene bridged product, but the corresponding car-boxylic acid 47 was obtained as it is depicted in scheme 3.44. The actually desired productcould only be obtained in yields of 3 % as a side product. The source of the carboxylgroup is most likely the potassium carbonate, which is used as base during the phasetransfer reaction.

N

N N

NII

N

NHI2

KOH, K2CO3,BTEAC

CH2Cl2

N

N N

NII

CO2H

22

46

47

Scheme 3.44: Attempted synthesis of bis(4-iodopyrazol-1-yl)methane (46).

This changed reactivity must be caused by the activation of the pyrazoles by the iodinesubstituent in position 4 of the pyrazole ring.Thus, the synthesis of 46 was carried out in two steps, as proposed by Potapov et al. (seechapter 3.4.1). [207] Yet, the iodination conditions were changed to the milder combinationof iodine and cer(IV)ammoniumnitrate.

[NH4]2[Ce(NO3)6]I2

N

N N

NII

N

N N

N46

Scheme 3.45: Synthesis of bis(4-iodopyrazol-1-yl)methane (46).

By this procedure, the desired bis(4-iodopyrazol-1-yl)methane (46) could be isolated inalmost quantitative yields.The spectroscopic data obtained from this compound agree with the finding of Potapovet al. [207] The 1H NMR spectrum shows one signal for the protons of the methylenebridge at 6.24 ppm. The protons in positions 5 and 3 of the pyrazole rings are detectedat chemical shifts of 7.56, respectively 7.69 ppm.The signal at 58.6 ppm of the 13C NMR spectrum could be assigned to iodine substitutedcarbon atoms of the pyrazole rings. The methylene bridge is detected at 65.3 ppm. Theremaining carbon atoms of the pyrazole moieties lead to signals at 134.0 and 146.0 ppm.

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3.4.3 Bis(4-iodopyrazol-1-yl)acetic acid (47) andBis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid (48)

Having obtained the desired N,N coordinating precursors 45 and 46, the correspondingN,N,O coordinating ligands were to be synthesized. The synthesis was based on the onepot procedure reported by Burzlaff et al. [11] The two educts 4-iodopyrazole 22 and 4-iodo-3,5-dimethylpyrazole 23 were deprotonated with a mixture of potassium carbonateand potassium hydroxide under phase transfer conditions in tetrahydrofuran. As phasetransfer catalyst, BTEAC was used. The reaction with dibromoacetic acid leads to the for-mation of the corresponding carboxylates bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid(48) and bis(4-iodopyrazol-1-yl)acetic acid (47).The desired compounds could be extracted from an aqueous solution after acidic workup.

N

N N

NII

N

NHI2

N

N N

NII

N

NHI2

CO2H

CO2H

KOH, K2CO3,BTEAC, Br2CHCO2H

THF

KOH, K2CO3,BTEAC, Br2CHCO2H

THF

23

2247

48

Scheme 3.46: Synthesis of bis(4-iodopyrazol-1-yl)acetic acid (47) and bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid (48).

The spectrum of compound 48 contains two singlets for the methyl substituents of thepyrazole rings at 2.24 and 2.26 ppm. The proton of the methine bridge can be found at7.00 ppm.The 1H NMR spectrum of compound 47 displays the signals of the methine bridge at achemical shift of 7.22 ppm. The pyrazole protons in position 5 and 3 of the pyrazole ringsare detected at 7.60 and 7.93 ppm, respectively.Furthermore, the products were analyzed via 13C NMR spectroscopy. The spectrum ofcompound 47 shows the iodo substituted carbon atom in position 4 of the pyrazole ringat 59.4 ppm. The bridging carbon atom is detected at 75.3 ppm. The remaining carbonatoms of the pyrazole rings are found at 136.6 and 147.19 ppm. The carbon atom of thecarboxyl group has a chemical shift of 165.6 ppm.Apart from the two additional signals of the methyl substituents at 12.3 and 14.1 ppm,the 13C NMR spectrum of compound 48 agrees with these findings. At 67.1 ppm, the

122


iodo substituted carbon atom is found, followed by the signal of the methine bridge at71.9 ppm. The remaining pyrazole carbon atoms are detected at 143.1 and 151.3 ppmand the carboxyl group at 164.5 ppm, clearly indicating the successful synthesis of thedesired compounds.

3.4.4 Methyl Bis(4-iodopyrazol-1-yl)acetate (49) and MethylBis(3,5-dimethyl-4-iodopyrazol-1-yl)acetate (50)

The obtained N,N,O coordinating iodinated derivatives 47 and 48 were to be esterified inthe next step, since the free carboxylic acids prevented their application in Sonogashirareactions.For the esterification, the corresponding educts were dissolved in methanol. Some dropsof sulfuric acid were added, to provide the required catalytical concentration of protonsfor the reaction, as it is depicted in scheme 3.47. After three days, both products couldbe obtained in yields of 40 - 50 %.

N

N N

NII

N

N N

NII

CO2H

CO2H

N

N N

NII

N

N N

NII

MeOH, H2SO4

MeOH, H2SO4

O O

O O

49

50

47

48

Scheme 3.47: Synthesis of methyl bis(4-iodopyrazol-1-yl)acetate (49)) and methylbis(3,5-dimethyl-4-iodopyrazol-1-yl)acetate (50).

The 1H NMR spectra of both products clearly indicate the success of the esterificationprocesses. The spectrum of 50 contains two singlets of the methyl substituents of thepyrazole rings at 2.21 and 2.23 ppm. The signal of the proton of the methine bridgeremains unchanged as well at 7.01 ppm. However, the singlet at 3.89 ppm can be assignedto the methyl ester.The corresponding 13C NMR spectrum exhibits the signals of the two methyl substituentsof the pyrazole rings at 12.3 and 14.1 ppm. The relevant signal of the methyl ester isdetected at 53.7 ppm. The singlet of the methine bridge carbon atom is found at 74.0 ppm.The four remaining signals can be assigned to the pyrazole carbon atoms at 68.0, 142.7and 150.7 ppm and to the carbon atom of the carboxylate group at 164.5 ppm.

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A similar situation can be found in the 1H NMR spectrum of compound 49. The methylester is therein detected at 3.89 ppm. The remaining protons are found at 7.00 ppm forthe methine group and 7.60 and 7.80 ppm for the remaining protons of the pyrazole rings.In the 13C NMR spectrum of compound 49, the methyl ester group is found at 54.1 ppm.The chemical shift of the methine carbon atom is 74.3 ppm and the carbon atoms of thepyrazole rings are detected at 59.1, 134.5 and 146.2 ppm. The signal at 163.9 ppm canbe assigned to the carboxylate.

124


3.4.5 Bis(4-ethynylferrocenylpyrazol-1-yl)methane (befcpzm) (51)

Using the so far discussed iodinated ligand precursors, it was now possible to synthesizefirst ligands suitable as models for Rieske dioxygenases. In a first attempt, a palladiumcatalyzed Sonogashira reaction was carried out to react ethynylferrocene with bis(4-iodopyrazol-1-yl)methane 46 (see chapter 3.4.3). Bis(triphenylphosphine)palladium di-chloride was hereby used as catalyst and copper(I) iodide as co-catalyst. The resultingcrude product bis(4-ethynylferrocenylpyrazol-1-yl)methane (befcpzm) (51) was purifiedvia column chromatography (see scheme 3.48).

N

N N

N

FeN

N N

NI I

FeFe

+ 2

[PdCl2(PPh3)2]CuI, DMF, NEt360 °C

46

51

Scheme 3.48: Synthesis of befcpzm (51).

The 1H NMR spectrum of compound 51 shows the signals of the β protons of the cy-clopentadienyl rings at 4.22 ppm. The α protons were found at 4.45 ppm and the protonsof the unsubstituted rings appeared at 4.23 ppm. The methylene bridge was detected at6.23 ppm and the remaining protons of the pyrazole rings are observed at 7.66 and 7.78ppm.Two signals in the 13C NMR spectrum could not be assigned unambiguously. They appearat 65.1 and 65.5 ppm, respectively. One of them is caused by the bridging carbon atom,while the other signals belongs to the quaternary carbon atoms of the cyclopentadienylrings. The remaining signals of the ferrocenyl moieties are detected at 68.7, 69.9 and71.2 ppm. The signals at 75.6 and 89.4 ppm could be assigned to the carbon atoms ofthe acetylene linkers. Their anchor point in position 4 of the pyrazole rings is found at achemical shift of 105.9 ppm, while the remaining pyrazole carbon atoms appear at 131.8and 143.2 ppm, respectively.The cyclic voltammogram of befcpzm (51) shows one reversible redox process with an

125


-0.4 -0.2 0.0 0.2-1.6x10-5

-1.2x10-5

-8.0x10-6

-4.0x10-6

0.0

4.0x10-6

8.0x10-6

1.2x10-5

1.6x10-5

I (A)

U (V) vs Fc/Fc+

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s

Figure 3.18: Cyclic voltammogram of befcpzm (51) in acetonitrile under nitrogen atmo-sphere at 25 ◦C. Conditions: 51: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1 mol/L,scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s.

oxidation potential of +170 mV and a reduction potential of +83 mV versus Fc/Fc+.With these values, the half wave potential calculates to +127 mV. However, as reportedby S. Tampier, the lacking methyl substitution and thus missing +I effect tend to leadto higher reduction potentials. [71] Under the applied conditions, there was no electroniccommunication between the ferrocenyl moieties indicated so far.Interestingly, it was not possible to synthesize similar compounds with 3,5-dimethyl sub-stituted pyrazole moieties. This was surprising, since sterical hindrance as the most strik-ing reason for this behavior appears unlikely considering the fact, that it was possibleto directly bind ferrocene rings to 3,5-dimethylpyrazoles by a Negishi coupling reactionutilizing the same catalyst as reported by S. Tampier. [71]

A single crystal suitable for X-ray structure determination could be obtained by slowlyevaporating a solution of 51 in ethyl acetate and n-hexane. The molecular structure isdepicted in figure 3.19. Selected angles and bond lengths are listed in table 3.11. Thestructure has a C2 symmetry axis.The bond lengths of the acetylene linker are close to the ideal values for a unconjugatedsystem with discrete triple and single bonds. The lengths of the triple bond between

126


Figure 3.19: Crystal structure of befcpzm (51). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

C5 and C6 amounts to 1.205(5) Å, which is close to the literature value of 1.20 Å. [200]

The bond lengths between the sp2-sp carbon atoms (C3-C5 and C6-C7) are measuredto be 1.426(6) Å and 1.421(5) Å, respectively. These values agree with the literaturevalue of 1.43 Å for a sp2-sp bond, too. [200] Furthermore, the linker is bent slightly. Theangles between C3-C5-C6 and C5-C6-C7 deviate from the ideal 180◦ by 5.9◦ and 2.0◦,respectively.The torsion angle between the pyrazole ring and the cyclopentadienyl ring 6 (C4-C3-C7-C8) is 39.9(1)◦. Therefore, the tilt between the ferrocenyl moieties and the pyrazole rings islarger than for systems, in which ferrocene is bonded directly to pyrazole in position 4. Forbis(4-ferrocenylpyrazol-1-yl)methane, this angle is 19.81(37)◦ and 6.16(38)◦, respectively(the structure is not C2 symmetric). [195]

Distances (Å)C1-N2 1.447(4) N1-N2 1.354(4)N1-C4 1.326(5) N2-C2 1.351(4)C2-C3 1.384(6) C3-C4 1.416(5)C3-C5 1.426(6) C5-C6 1.205(5)C6-C7 1.421(5)

Angles (deg)N2-C1-N2 110.7(4) C1-N2-N1 118.9(2)C2-C3-C4 104.3(3) C2-N2-N1 112.7(3)C3-C5-C6 174.1(4) C5-C6-C7 178.0(4)C4-C3-C7-C8 39.9(1)


127


3.4.6 Methyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate(mbefcpzac) (52)

Yet to mimic a Rieske dioxygenase, a N,N,O binding motif instead of the so far presentedpresented N,N coordinating ligand would be preferable since it would resemble the 2-His-1-carboxylate triade more closely. [89,92] However, in previous works, it was not possible tosynthesize ferrocenyl substituted bis(pyrazol-1-yl)acetic acids in a useful scale. [71]

By performing a Sonogashira reaction with ethynylferrocene and methyl bis(4-ethynyl-pyrazol-1-yl)acetate (49, see chapter 3.4.6), it was possible to take an important step inthis direction. The esterification of the precursor compound was necessary since Sono-gashira reactions with carboxylic acids are usually not successful. The reaction wascarried out in accordance to the synthesis of compound 51 (see chapter 3.4.5) to obtainmethyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate (mbefcpzac) (52).

N

N N

N

FeN

N N

NI I

FeFe

+ 2


OO

OO

49

52

Scheme 3.49: Synthesis of mbefcpzac (52).

In the 1H NMR spectrum of mbefcpzac (52), the methyl ester group is detected at 3.91ppm. The signal of the unsubstituted cyclopentadienyl rings appears at 4.24 ppm and theβ protons of the substituted rings appear as a shoulder at 4.23 ppm. The correspondingα protons are found at 4.46 ppm. The proton of the methine bridge has a chemicalshift of 6.99 ppm. The remaining pyrazole protons are detected at 7.70 and 7.88 ppm,respectively.The 13C NMR confirms the product formation as well. The methyl ester is detected

128


at 54.0 ppm. The signal at 74.6 ppm is assigned to the methine bridge. The ferrocenemoieties are detected at 68.8, 69.9, 71.3 and 75.4 ppm. The presence of the triple bonds isconfirmed by signals at 82.8 and 106.2 ppm. The pyrazole rings are assigned to the signalsat 132.3, 143.4 and 146.8 ppm. The carboxylate carbon atom is found at 164.00 ppm.

-0.4 -0.2 0.0 0.2-4.0x10-6

-3.0x10-6

-2.0x10-6

-1.0x10-6

0.0

1.0x10-6

2.0x10-6

3.0x10-6

4.0x10-6

I (A)

U (V) vs Fc/Fc+

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s

Figure 3.20: Cyclic voltammogram of mbefcpzac 52 in acetonitrile under nitrogen atmo-sphere at 25 ◦C. Conditions: 52: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1 mol/L,scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s. Rippled noise dueto low solubility of 52.

The cyclic voltammogram of mbefcpzac (52) once again shows one reversible redox pro-cess with an oxidation potential of +176 mV and a reduction potential of +83 mV. Thehalf wave potential therefore calculates to +130 mV versus Fc/Fc+. In comparison tobefcpzm (51), which features the same ferrocenyl moieties, yet does not have the estergroup, the difference of the half wave potential only amounts to 3 mV (see chapter 3.4.5).This shows that there is only a minor influence of the ester group on the overall potential.As shown previously, electron withdrawing groups at the bridging carbon atom, especiallyketo functions, can have a way more significant influence. [71]

As stated for befcpzm (51, see chapter 3.4.5), the missing electron donating methyl sub-stituents at the pyrazole rings have an undesired influence on the measured potentialby leaving the pyrazole moieties more electron deficient than comparable 3,5-dimethyl

129


substituted compounds. However, as already discussed for compound 51 the attemptedsynthesis of the respective substituted species was not successful so far.

3.4.7 Bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)methane (bepfcdmpzm) (53)

In order to decrease the half wave potential further towards values within the rangeof Rieske clusters, the ferrocenyl substituent was modified by a moiety with a largeraromatic linker. This should on the one hand lower the redox potential of the result-ing products and on the other hand enable the synthesis with 3,5-dimethyl substitutedpyrazole rings by placing the sterically demanding ferrocenyl unit further away from theactive site of the coupling reaction. One ferrocenyl precursor that, in comparison toethynylferrocene, has been shown to influence the potential in such a way, is 1-ferrocenyl-4-ethynylbenzene, as was previously published for pyrazabole compounds by Mobin andcoworkers. [208,209] This precursor was synthesized by a diazotization reaction starting from4-ethynylaniline and subsequent reaction with ferrocene. [210]

N

N N

N

FeN

N N

NI I + 2


Fe Fe

45

53

Scheme 3.50: Synthesis of befcpdmpzm (53).

The actual product synthesis was again carried out via a palladium catalyzed Sono-gashira reaction in dimethylformamide, yet this time with the 3,5-dimethyl substitutedbis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45) as coupling partner. If only two equiv-alents of 1-ferrocenyl-4-ethynylbenzene were used in the reaction, the prevalent productwas the mono substituted species ((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)-(4-iodopyrazol-1-yl)methane. Yet, raising the ratio to four equivalents led to the for-

130


mation of the desired product bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)methane (bepfcdmpzm) (53). Purification of the crude product via column chro-matography showed that larger quantities of the Glaser product of the ferrocenyl pre-cursor was formed, although the reaction was carried out under careful exclusion of oxygen,which is usually needed to reoxidize the active copper species during Glaser couplingreactions.

-0.4 -0.2 0.0 0.2

-8.0x10-6

-6.0x10-6

-4.0x10-6

-2.0x10-6

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5 0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s

I (A)

U (V) vs Fc/Fc+

Figure 3.21: Cyclic voltammogram of befcpdmpzm (53) in acetonitrile under nitrogenatmosphere at 25 ◦C. Conditions: 53: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1mol/L, scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s.

The successful product formation could be confirmed by NMR spectroscopy as well as bymass spectrometry. In the 1H NMR spectrum, the methyl groups of the pyrazole rings aredetected at 2.31 and 2.57 ppm, respectively. The proton signals of the cyclopentadienylrings are shifted to 4.35 ppm for the β protons and 4.67 ppm for the α protons. The signalof the unsubstituted cyclopentadienyl rings is found at 4.05 ppm. With a chemical shiftof 6.09 ppm, the methylene bridge can be observed. The signals of the phenylene ringsappear as two doublets at 7.39 and 7.44 ppm with coupling constants of 8.29 and 8.48 Hz,respectively. These differ from the educt spectrum, in which these protons appear as onesinglet at 7.42 ppm, thus confirming the successful synthesis.The 13C NMR spectrum shows the methyl groups at 10.8 and 12.6 ppm. The carbon atom

131


of the methylene bridge can be found at 61.0 ppm. The signals at 66.5, 69.3, 69.7 and80.2 ppm derive from the cyclopentadienyl rings. The acetylene bridges are detected at84.3 and 93.5 ppm and the phenylene rings at 120.8, 125.8, 131.3 and 139.4 ppm. Finally,the carbon atoms of the pyrazole rings are assigned to the signals at 103.7, 143.2 and150.6 ppm.The cyclic voltammogram of befcpdmpzm (53) shows one reversible redox process as forthe previous compounds, as depicted in figure 3.21. The oxidation potential is measuredto be +47 mV and the corresponding reduction potential to be −30 mV. Based on thesevalues the half wave potential can be calculated to +9 mV. The enlarged aromatic systemof the ferrocenyl moieties in combination with the +I-effect of the methyl substituents ofthe pyrazole rings led to a overall change of the half wave potential of −117 mV. Thisraises hope towards an adjustable system, where the potential of the ferrocenyl moietiescan be fine tuned to negative values.

132


3.4.8 Methyl bis((4-ethynyl-1-ferrocenylphen-4-yl)pyrazol-1-yl)acetate (mbepfcpzac) (54)

The same precursor was now to be incorporated into a N,N,O coordinating ligand as well.Therefore, once again a Sonogashira reaction was carried out. As for the synthesis ofmbefcpzac (52), methyl bis(4-ethynylpyrazol-1-yl)acetate (49) was used, to obtain thedesired N,N,O scorpionate ligand methyl bis((4-ethynyl-(1-ferrocenylphen-4-yl)pyrazol-1-yl)acetate (mbepfcpzac) (54). An excess of 1-ferrocenyl-4-ethynylbenzene was againnecessary to avoid the formation of the mono substituted product.

N

N N

N

FeN

N N

NI I + 2


OO

OO

Fe Fe

49

54

Scheme 3.51: Synthesis of mbepfcpzac (54).

The 1H NMR of mbepfcpzac (54) shows a singlet of the ester methyl group at 3.93 ppm.The cyclopentadienyl rings show basically the same chemical shifts as found for compound53 with 4.05 ppm for the protons of the unsubstituted rings, 4.33 ppm for the β protons ofthe substituted rings and 4.66 ppm for the corresponding α protons. The hydrogen atomof the methine bridge is detected at 7.02 ppm. The two doublets at 7.40 and 7.44 ppmwith coupling constants of 8.10 Hz and 8.48 Hz derive from the protons of the phenylenerings. Further downfield, the protons of the pyrazole rings are assigned to the signals at7.75 and 7.95 ppm.The 13C NMR spectrum of 54 shows the signal of the methyl ester at 30.0 ppm. Themethine bridge is detected at 54.1 ppm. The ferrocene moieties are assigned to thesignals at 66.5, 68.5, 69.3 and 69.7 ppm. The acetylene linkers were observed at 82.9 and

133


105.9 ppm and the phenylene moieties were observed at chemical shifts of 125.8, 131.4,132.6 and 135.7 ppm. The remaining signals at 120.1, 139.9, 143.4 and 156.7 ppm wereassigned to the pyrazole carbon atoms and the carboxylate, respectively.

-0.4 -0.2 0.0 0.2

-8.0x10-6

-6.0x10-6

-4.0x10-6

-2.0x10-6

0.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

I (A)

U (V) vs Fc/Fc+

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s

Figure 3.22: Cyclic voltammogram of mbepfcpzac (54) in acetonitrile under nitrogenatmosphere at 25 ◦C. Conditions: 54: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1mol/L, scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s.

The cyclic voltammogram of mbepfcpzac (54) shows one reversible redox potential withan oxidation potential of +68 mV and a reduction potential of −15 mV. With thesevalues the half wave pontential calculates to +27 mV versus Fc/Fc+. Considering, thatthe influence of the additional methyl carboxylate group is only a minor one, as was shownfor compounds 51 and 52, most of the potential difference to 53 derives from the absenceof the methyl substituents at the pyrazole rings. However, the difference only calculatesto 17 mV in this case, showing that for this system the methyl groups are only of minorimportance for the optimization of the half wave potential.

134


3.4.9 Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (bdmfctpzm) (55)

Another approach to access ferrocenyl based chelate ligands relied on Click chemistryreactions instead of Sonogashira coupling reactions. Having obtained bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15) as shown in chapter 3.2.2, it was possibleto synthesize a first model complex for Rieske dioxygenases containing a triazole linkergroup. To do so, a copper(I) catalyzed alkyne-azide cycloaddition was carried out. There-fore, copper(II) sulfate was employed, which was in situ reduced by sodium ascorbate toform the active copper(I) species. The necessary ferrocene azide educt was synthesizedin one step from bromoferrocene according to Plazuk et al. [211] The purification of thecrude Click-reaction product however was not fully successful even after column chro-matography. Nevertheless, the formation of bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (bdmfctpzm) (55) could be verified by NMR spectroscopy andmass spectrometry and the properties of the ferrocene moieties could be determined viacyclic voltammetry (see figure 3.23).

N

N N

N

N

N N N N

N FeFe

Fe

N3

N

N N

N+

CuSO4sodium ascorbate

15

55

Scheme 3.52: Synthesis of fcbdmtpz (55).

Compared to bedmpzm (15) the signals of the methyl groups in 55 are slightly shifteddown field to 2.37 and 2.73 ppm, respectively. The ferrocenyl protons split up into threesignals. The unsubstituted cyclopentadienyl rings show a chemical shift of 4.23 ppmwhereas the protons of the substituted rings can be found at 4.28 for the β and 4.87 ppmfor the α protons. This confirms the product formation since the corresponding signalsof azidoferrocene are located at 4.05 and 4.29 ppm. The methylene bridge is detected at6.25 ppm. Another proof for the successful product formation is the presence of a triazoleproton peak at 7.69 ppm.

135


The 13C NMR spectrum of compound 55 shows the signals of the methyl groups at 11.0and 13.4 ppm. The bridging methylene group can be found at 60.8 ppm. The ferrocenylmoieties are detected at 62.1, 66.7, 70.1 and 93.7 ppm. The carbon atoms of the triazolerings exhibit chemical shifts of 109.7 and 119.7 ppm. Finally, the signals of the pyrazolerings are found at 139.0, 141.1 and 146.8 ppm.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

1.0x10-5

1.5x10-5

I (A)

U (V) vs Fc/Fc+

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s *

*

Figure 3.23: Cyclic voltammogram of bdmfctpzm (55) in acetonitrile under nitrogenatmosphere at 25 ◦C. Conditions: 55: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1mol/L, scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s; * interalferrocene standard.

The cyclic voltammogram features one reversible redox process. The oxidation potentialof compound 55 could be determined to +201 mV and the reduction potential to +144 mVversus Fc/Fc+. Therefore the half wave potential of the ferrocene moieties lies with 173 mVversus Fc/Fc+ significantly higher then the one observed for pure ferrocene. Those valuesagree well with comparable compounds in which ferrocene is attached to triazole rings. [68]

A lower potential would however be desirable considering the potential application asa model complex for Rieske dioxygenases (see 1.4). Under the applied conditions, noelectrochemical communication between the two ferrocenyl moieties could be observed.

136


3.4.10 Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (bdmfcmtpzm) (56)

The second model compound, which was accessible from 15 via a Click reaction, was de-rived from azidomethylferrocene. The additional methylene bridge between the ferrocenemoieties and the triazole rings was used to lower the influence of the triazoles on thepotential of the ferrocenes, thus leading to a more desirable potential compared to com-pound 55. The necessary precursor azidomethylferrocene was synthesized from ferrocenein two steps via the ferrocenemethanol intermediate. [212,213]

The reaction was carried out under similar conditions as the synthesis of 55, using cop-per(II) sulfate and sodium ascorbate to create the active copper(I) in situ as typicalfor CuAAC reactions. After purification via column chromatography, bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (bdmfcmtpzm) (56) could beobtained as an orange powder.

N

N N

N

N

N N N N

N

FeN

N N

N+

CuSO4sodium ascorbate

N3

FeFe

15

56

Scheme 3.53: Synthesis of bdmfcmtpzm (56).

The NMR spectra of bdmfcmtpzm (56) show similar chemical shifts as observed forbdmfctpzm (55). Successful product formation can be confirmed due to the existence of asignal for the triazole protons in the 1HNMR spectrum at 7.37 ppm. The methylene bridgeat the ferrocenyl moieties is detected at 5.30 ppm. The signals of the cyclopentadienylrings are shifted to 4.16 ppm for the unsubstituted rings and 4.27 ppm for the α protonsas well as 4.20 ppm for the β protons of the substituted rings.The 13C NMR spectrum contains a signal for the methylene bridge of the ferrocenylmoieties at 49.9 ppm. The cyclopentadienyl carbon atoms can be found at 68.7, 68.8,68.9 and 81.1 ppm, followed by the signals of the triazole rings at 109.9 and 119.7 ppm,

137


which give strong evidence of the success of the synthesis.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3-2.0x10-5

-1.5x10-5

-1.0x10-5

-5.0x10-6

0.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

2.5x10-5

I (A)

U (V) vs Fc/Fc+

0.1 V/s 0.2 V/s 0.3 V/s 0.4 V/s 0.5 V/s

Figure 3.24: Cyclic voltammogram of bdmfcmtpzm (56) in acetonitrile under nitrogenatmosphere at 25 ◦C. Conditions: 56: 5 × 10−4 mol/L, [NBu4][PF6]: 0.1mol/L, scan rates: 0.1 V/s, 0.2 V/s, 0.3 V/s, 0.4 V/s, 0.5 V/s.

The cyclic voltammogram of compound 56 shows a reversible redox process. The oxida-tion potential lies at +60 mV and the reduction potential at −9 mV versus Fc/Fc+. Thehalf wave potential of the process can therefore be calculated to +26 mV. The influence ofthe methylene bridge decreased the potential by 147 mV, providing a way more beneficialredox potential concerning the application as a model for Rieske dioxygenases. As adrawback, the conjugation towards the binding site of the ligand is interrupted by theadditional methylene bridge.A single crystal suitable for X-ray structure determination could be obtained by layeringa solution of compound 56 in ethyl acetate with n-hexane. Selected bond lengths andangles are listed in table 3.12. The molecular structure is depicted in 3.25. The com-pound crystallized in the triclinic space-group P−1. It was found that the triazole ringsare slightly twisted out of plane compared to the pyrazole rings with dihedral angles of18.172(1005)◦ and 3.77(102)◦. The angles between the triazole units and the ferrocenylmoieties amount to 112.2(5)◦ and 115.5(7)◦. The dihedral angle between the triazolerings and the link to the ferrocenyl units are 6 (N14-N15-C18-C30) = 55.344(740)◦ and

138


Figure 3.25: Crystal structure of bdmfcmtpzm (56). Thermal ellipsoids are drawn atthe 50 % probability level. Hydrogen atoms have been omitted for clarity.

6 (N24-N25-C28-C40) 50.995(908)◦, respectively. However, as the NMR spectra indicatefree rotation of these groups, those angles are probably caused by crystal packing effects.The angle at the methylene bridge between the pyrazoles amounts to 6 (N12-C1-N22) =112.3(5)◦.

Distances (Å)C1-N12 1.451(7) C1-N22 1.449(8)N11-N12 1.357(7) N21-N22 1.368(7)N11-C12 1.344(8) N12-C14 1.360(8)N21-C22 1.323(8) N22-C24 1.355(9)N15-C18 1.491(7) N25-C28 1.459(9)C18-C30 1.487(10) C28-C40 1.492(10)

Angles (deg)N12-C1-N22 112.3(5) C1-N22-N21 117.9(5)C1-N12-N11 118.2(5) N15-C18-C30 112.2(5)N25-C28-C40 115.5(7) N13-C16-C13-C14 18.172(1005)N23-C26-C23-C24 3.77(102) N14-N15-C18-C30 55.344 (740)N24-N25-C28-C40 50.995(908)


A comparison to the molecular structure of bis(4-ferrocenyl-3,5-dimethylpyrazol-1-yl)-methane reported by Tampier and Bleifuss shows very similar bond lengths and anglesas far as the bispyrazolyl backbone of the ligands is concerned. [71] Therefore, the additionaltriazole linker has most likely no significant influence on the coordination properties ofthis N,N chelate ligand.

139


3.4.11 Summary of the Ferrocene Based Models for RieskeDioxygenases

As was stated in the last section, optimization of the half wave potential for the examinedcompounds bepfcdmpzm (53) and mbepfcpzac (54) by the addition of the +I-effect ofmethyl groups in pyrazole position 3 and 5, seems to be of minor importance for thissystem.

Compound Eox in mV Ered in mV E1/2 in mVbefcpzm (51) +170 +83 +127mbefcpzac (52) +176 +83 +130

bepfcdmpzm (53) +47 −30 +9mbepfcpzac (54) +68 −15 +27bdmfctpzm (55) +201 +144 +173bdmfcmtpzm (56) +60 −9 +26

bfcdmpzm [71] +33 −50 −9bfcpzm [71] +42 +42 0

bfcdmpzk [71] +37 −56 −9bfcpzk [71] +103 +4 +54

Table 3.13: Summary of the electrochemical properties of the synthesized ferrocenyl derivativesand selected known ferrocenyl pyrazolyl compounds [71] (arithmetic means of 0.1-0.5 V/s, versus Fc/Fc+).

Taking into account, that the methyl carboxylate moiety at the bridging carbon atomonly leads to a slight shift of the potential by 3 mV for the compounds befcpzm (51) andmbefcpzac (52), the estimated change caused by the methyl groups calculates to only15 mV.

N

N N

N

FeFe

N

N N

N

FeFe

R

R

R

R


R R

R R

O


Figure 3.26: Ferrocenylpyrazole based ligands according to S. Tampier. [71]

As can be seen in table 3.13, this influence is much stronger for keto based ligands asbfcdmpzk and bfcpzk (for structures see figure 3.26). There, the presence of the methylgroups shifts the potential by 63 mV to more negative values. [71] However, without such

140


strong electron withdrawing bridging groups, this point seems to be only of minor impor-tance.Thus the choice of the ferrocenyl moiety in the selected system is the more importanttask for the synthesis of such model complexes. As can be seen in table 3.13, the halfwave potentials of the ligands that could be obtained so far range from +173 to +9 mV. Itseems very likely that these values can be improved further towards lower redox potentialsby a careful choice of ferrocenyl precursors.

N

NHI

R

R

N

NI

R

R

NN

I

R

R

NN

IR

R

N

NI

R

R

N

NI

R

R

O

N

NI

R

R

NN

I

R

R

NN

IR

RBH

K+

N

N

R

R

(i) (ii)

(iii)

Rx

Fe

Scheme 3.54: Possible further ligands based on the presented synthetic route; conditions:(i) CHCl3/Base, (ii) KBH4, (iii) COCl2.

In contrast to the attempts of Tampier et al. to obtain N,N,O coordinating scorpionateligands based on 4-ferrocenylpyrazole (see figure 3.26), [71] such compounds should now beaccessible. Based on the presented results, the use of the ester groups to avoid unwantedinterference of the carboxylate functions allows Sonogashira reactions to design variousscorpionate ligands.Following this synthetic concept, a variation of the ligand backbone might be possibleas well, as depicted in scheme 3.54. A ligand precursor is thereby synthesized as thecorresponding 4-iodo-1H -pyrazolyl derivative and then subsequently reacted in a Sono-gashira reaction with any desired ethynyl-ferrocene moiety. Several routes to accessvarious tripodal ligands are well established scorpionate chemistry. [1,23] E.g. the applica-tion of the one pot synthesis reported by Elflein et al. for the reaction of pyrazoles withaldehydes to obtain even more binding motifs should be possible as well. [23]

141

4 Summary and Outlook

143


In order to overcome one of the most important drawbacks of bis(pyrazol-1-yl)acetic acids,namely their tendency to form bisligand complexes, they were modified following variousconcepts, so far. Apart from an increase of the sterical demand, vinyl linkers were in-troduced, to make the ligand capable to undergo copolymerization reactions. The mostrecent ligand of this type is the bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (Hbd-mvpza), which was first reported on in 2010. [12,33–35] So far, complexes of this ligand andits copolymers were not employed in catalytic studies in order to compare their reactivityin their copolymerized state.

N

N N

NMo

Cl O

Cl

O

N

N N

NMo

Cl OO

OO

3 5

Figure 4.1: DMSO reductase model compounds 3 and 5.

Therefore, two complexes based on this ligand system were synthesized (see figure 4.1),one with a N,N (3) and one with a N,N,O coordination motif (5). These oxomolybdenumcompounds showed good reactivity concerning the oxygen atom transfer (OAT) reactionfrom dimethyl sulfoxide to triphenylphosphine. Thus, these complexes might serve asfunctional model complexes for DMSO reductases.




In order to investigate the properties of the corresponding copolymers, both complexeswere copolymerized with MMA and EGDMA (P10 - P13), respectively. Apart from this,copolymers of the two ligands with MMA and EGDMA were synthesized and subsequentlyloaded with molybdenum fragments (P6-Mo - P9-Mo). All of the obtained molybdenumcontaining polymers were analyzed via atomic absorption spectroscopy concerning the

144

molybdenum contents. It was found, that, in the case of the copolymerized complexes P10- P13, the amount of incorporated complex fragments depends strongly on the appliedcopolymer. The reactivity of MMA during the polymerization reaction was found to belower than the reactivity of the complex molecules, due to not being a crosslinker. Thecomplex incorporation of up to 0.646 mmol/g was therefore rather high. The crosslinkingEGDMA monomer on the other hand led to copolymers with metal contents of only upto 0.0334 mmol/g.When the polymers of the ligands were charged with the molybdenum fragments (P6-Mo - P9-Mo), the properties of the two copolymers also led to different results. Thehighly crosslinked EGDMA copolymers reached occupation levels of up to 14.8 % whilethe less crosslinked MMA copolymers contained metal fragments in up to 78.5 % of theavailable binding sites. The amount of binding sites was thereby determined by thenitrogen value of the elemental analysis of the respective copolymers. The coordinationmotif of the incorporated complex moieties were investigated via IR spectroscopy andremained unchanged in either case when compared to the free complexes.The results of the catalytic DMSO reduction of the obtained compounds are summarizedin table 4.2. As can be seen, all of the applied catalysts were able to catalyze the reductionof dimethyl sulfoxide under oxidation of triphenylphosphine.

Catalyst t [h] yield of OPPh3 [%] TON TOF [10−5 s−1]3 6 56 112 5195 6 55 110 509

P10 24 87 174 201P11 24 1 2 2.31P12 24 36 72 83.3P13 24 24 48 55.6

P6-Mo 24 89 178 206P7-Mo 24 71 142 164P8-Mo 24 44 88 102P9-Mo 24 8 16 18.5

Table 4.2: Results of catalytic DMSO reduction (ncatalyst = 7.50 µmol, 0.5 mol%;nPPh = 1.50 mmol).

In general, the MMA copolymers exposed higher catalytic activities than the correspond-ing EGDMA derivatives, which is caused by the better access to the metal centers. Itwas also obvious that in the latter, the polymers which were charged with metal frag-ments subsequently to the polymerization process showed a significantly higher activity.Their counterparts, for which the final complexes were copolymerized probably contain a

145


certain amount of metal sites, which were locked in and therefore blocked from substrateaccess during the polymerization. Since the EGDMA polymers are highly crosslinked incomparison to the MMA copolymers, the effect there is more striking (compare P11 andP7-Mo).In future experiments, such effects could be avoided by the application of molecular im-printing. Therefore, a dummy substrate can be bound to the active site prior to thecopolymerization process, which is removed afterwards. By this way, a cavity might becreated, which would possibly keep the active site accessible for substrates in catalyticstudies.The second part of this thesis dealt with the synthesis of ligands bearing an acetyleneinstead of a vinyl linker group in position 4 of the pyrazole rings. As such, a Hbdmpzaderivative as well as bis(pyrazolyl)methane derivatives were synthesized (see scheme 4.1).

HN

N

N N

N N

NN

N N

N N

OH

N

N N

N N

N N

N

OHO

TMS

15

28

29

3234

Scheme 4.1: 4-Ethynylpyrazole based scorpionate ligands.

Apart from bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15), which wasinitially synthesized under Corey-Fuchs conditions starting from bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1), these ligands were synthesized starting from 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28). This compound was synthesized from 3,5-dime-thylpyrazole in four steps. Starting from there, 15 could also be synthesized under phasetransfer conditions. The well established one pot synthesis of bispyrazolylacetic acids by

146

Burzlaff et al. was used in the synthesis of the Hbdmpza derivative 29. [11] Due tothe basic conditions of this reaction, the trimethylsilyl groups are removed during thesynthesis without an additional step.Bis(4-ethynylpyrazol-1-yl)methane ligands on the other hand were also obtained followinga one pot synthesis procedure established by Elflein et al. [23] Following this protocol,basically any desired aldehyde can be used as bridging group thus altering the coordinationmotif. In this thesis, salicylaldehyde as well as 1-methyl-2-imidazolecarboxaldehyde wereused to obtain the corresponding N,N,O and N,N,N coordinating ligands 30 and 33. Theprotecting trimethylsilyl group was kept in place during the reaction and could be removedin an additional step under either basic conditions or the application of potassium fluoridein methanol.The introduction of acetylene linkers allows for a range of reactions to further modify theligands or complexes thereof as depicted in scheme 4.2. Included are coupling reactionslike the Glaser homo coupling reaction or the Cadiot-Chodkiewicz hetero couplingreaction. Furthermore, modifications via Click chemistry (CuAAC) or Sonogashirareactions are feasible.

R'

R' Br

R' N3 R' IR'

N

NN

R'

HN

N

HN

N

HN

N

HN

N

HN

N

HN

N

NH

N

Scheme 4.2: Possible reactions of the acetylene linker groups of 3,5-dimethyl-4-ethynyl-pyrazole.

Possible applications are manifold. Besides the herein presented modifications to serveas model complexes for Rieske dioxygenases, the introduction of fluorophores could bebeneficial. The fluorescence could be switched via paramagnetic fluorescence quenching

147


induced by metal centers coordinated to the ligand. Another possible application is thePEGylation of the ligand. Thereby, polyethylene glycol polymer (PEG) chains are at-tached to the ligand molecules to improve their solubility. This can be achieved by themethods mentioned above, since a wide range of PEG derivatives are commercially avail-able. This process is well established in pharmaceutical chemistry, since water solubilitycan be greatly improved without increasing the toxicity of the corresponding drugs. [197]

Starting from 3,5-dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26), the N,N coor-dinating ligand 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) could be obtained,which is suitable to synthesize 1D coordination polymers. Therefore, the trimethylsilylprotecting group was removed to allow a Glaser homo coupling reaction of the obtainedcompound. A trityl protecting group was applied for this purpose. This group protectsthe nitrogen donors from building such polymers with the metal catalysts used during thecoupling reaction.

Figure 4.2: Molecular structure poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (39)

The deprotected ligand readily forms coordination polymers with metal salts. As such,cobalt acetylacetonato and cobalt chloride coordination polymers could be obtained.It was possible to determine the structure of the cobalt(II)acetylacetonato polymer (38)via X-ray structure determination as it is depicted in figure 4.2.Furthermore, in order to design model systems for Rieske dioxygenases several ligandsbearing ferrocenyl moieties were synthesized. These model systems were on the one handbased on the above mentioned bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (bedm-pzm) (15) and on the other hand on 4-iodo substituted bis(pyrazolyl)methanes (45, 46)and -acetic acid (49). These ligand backbones were reacted with several ferroncenylprecursors in copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) or Sonogashira

148

N

N N

N

N

N N N N

N

FeFe

N

N N

N

OO

Fe Fe

56

54

Figure 4.3: Examples of Rieske dioxygenase models presented in this thesis.

reactions.The influence of different linker groups on the electrochemical potential of the resultingcompounds was investigated via cyclic voltammetry. This procedure revealed for each ofthe examined compounds one reversible redox potential. The obtained half wave poten-tials are summarized in table 4.3. In order to mimic the behavior of ferredoxin clusters,a potential of −150 up to +400 mV vs. SHE would be desirable, what equates to −550to 0 mV vs. the ferrocene/ferrocenium couple. As can be seen in table 4.3, the mostpromising candidates are those with an additional phenyl group in the linker moiety. Incombination with the N,N,O binding motif of bis(pyrazolyl)acetic acids, which mimicsthe natural 2-His-1-carboxylate triad binding motif of Rieske dioxygenases, these lig-ands are promising candidates for model complexes. However, it was shown, that thepotential of such compounds can be tuned over a relatively wide potential range, whichleaves room for future optimizations. Due to the versatility of this concept, differentferrocenyl derivatives can be introduced as electron donors easily.

Compound Eox in mV Ered in mV E1/2 in mVbefcpzm (51) +170 +83 +127mbefcpzac (52) +176 +83 +130


Table 4.3: Summary of the electrochemical properties of the synthesized ferrocenylderivatives (versus Fc/Fc+).

149


For the studies conducted herein, the carboxylate donor function was protected by aesterification. In future studies, iron(II) centers need to be coordinated subsequent tosaponification of the ester group, to create structural and functional models of a Rieskecenter.

150

5 Zusammenfassung und Ausblick

151


Bis(pyrazol-1-yl)essigsäure Liganden wurde in den letzten 15 Jahren auf verschiedensteWeise modifiziert, um das Problem der Bisligandkomplexbildung zu vermeiden. Abgese-hen von der Erhöhung des sterischen Anspruchs wurden z.B. Vinyllinker eingeführt, dieden Liganden befähigen, Vinylgruppen-basierte Copolymerisationsreaktionen einzugehen.Der jüngste Ligand dieser Art ist Hbdmvpza, welcher 2010 publiziert wurde. [12,33–35] Bisherwurden Komplexe dieses Liganden und seiner Copolymere nicht in katalytischen Studieneingesetzt, bei denen ihre jeweilige Reaktivität im copolymerisierten Zustand verglichenwurde.

N

N N

NMo

Cl O

Cl

O

N

N N

NMo

Cl OO

OO

3 5

Abbildung 5.1: Modellverbindungen 3 und 5 für DMSO-Reduktasen.

Aus diesem Grund wurden auf Grundlage dieses Ligandsystems zwei Komplexe syn-thetisiert (siehe Abbildung 5.1): einer von ihnen mit einem N,N (3) und einer mit einemN,N,O Koordinationsmotiv (5). Diese Oxomolybdän Verbindungen zeigten gute Reak-tivität im Bezug auf die Sauerstofftransfer (OAT) Reaktion von Dimethylsulfoxid aufTriphenylphosphan. Daher eignen sie sich für den Einsatz als Modelsysteme für DMSO-Reduktasen.

Katalysator Ligand/Komplex-Monomer CopolymerP10 MoO2Cl2(bdmvpzm) (3) MMAP11 MoO2Cl2(bdmvpzm) (3) EGDMAP12 MoO2Cl(bdmvpza) (5) MMAP13 MoO2Cl(bdmvpza) (5) EGDMA


Tabelle 5.1: Zusammensetzung der Copolymere für die katalytische DMSO-Reduktion.

Um die Eigenschaften der jeweiligen Copolymere zu überprüfen wurden beide Komplexejeweils mit MMA und EGDMA copolymerisiert (P10 - P13). Darüber hinaus wurden diemetallfreien Liganden ebenfalls mit MMA und EGDMA polymerisiert und nachträglichmit Molbydän beladen (P6-Mo - P9-Mo). Alle erhaltenen molybdänhaltigen Polymere

152

wurden, nachdem sie chemisch aufgeschlossen wurden, mittels Atomabsorptionsspektros-kopie bezüglich des Molybdängehaltes analysiert. Die Ergebnisse zeigten, dass im Fall dercopolymerisierten Komplexe P10 - P13 die Menge an eingebetteten Komplexfragmentenstark vom eingesetzten Copolymer abhängig ist. Die Reaktivität von MMA im Bezugauf die Polymersationsreaktion erwies sich als niedriger als die der Komplexmoleküle,da es sich bei MMA um keinen Quervernetzer handelt. Dadurch erklärt sich der hoheGehalt an Komplexfragmenten im Copolymer von bis zu 0.646 mmol/g. Im Gegensatzdazu führte der Einsatz des quervernetzenden EGDMA nur zu Metallgehalten von bis zu0.0334 mmol/g.

Katalysator t [h] Ausbeute an OPPh3 [%] TON TOF [10-5 s-1]3 6 56 112 5195 6 55 110 509

P10 24 87 174 201P11 24 1 2 2.31P12 24 36 72 83.3P13 24 24 48 55.6

P6-Mo 24 89 178 206P7-Mo 24 71 142 164P8-Mo 24 44 88 102P9-Mo 24 8 16 18.5

Tabelle 5.2: Ergebnisse der katalytischen DMSO Reduktion (ncatalyst = 7.50 µmol,0.5 mol%; nPPh = 1.50 mmol).

Wurden die Copolymere der freien Liganden nachträglich mit Molybdän beladen (P6-Mo- P9-Mo), führten die unterschiedlichen Eigenschaften der beiden Copolymere ebenfallszu voneinandner abweichenden Ergebnissen. Die stark quervernetzten EGDMA Copoly-mere erreichten einen Besetzungsgrad von bis zu 14.8 %, während die weit weniger querver-netzten MMA Copolymere an bis zu 78.5 % der verfügbaren Bindungsstellen Metallfrag-mente enthielten. Die Anzahl der verfügbaren Bindungsstellen wurden dabei über denStickstoffgehalt der Elementaranalyse der jeweiligen Polymere bestimmt. Das Koordina-tionsmotiv der eingebetteten Metallfragmente wurde mittels IR-Spektroskopie untersuchtund erwies sich in beiden Fällen als unverändert im Vergleich zu den freien Komplexen.Die Ergebnisse der katalytischen Reduktion von DMSO durch die erhaltenen Verbindun-gen sind in Tabelle 5.2 zusammengefasst. Es ist ersichtlich, dass alle verwendeten Kataly-satoren Aktivität im Bezug auf die katalytische Reduktion von Dimethylsulfoxid unterOxidation von Triphenylphosphan zeigten.Im Allgemeinen zeigten die MMA basierten Copolymere eine höhere katalytische Aktiv-ität als die entsprechenden EGDMA Polymere, da diese leichter einen Zugang zu den

153


Metallfragmenten erlauben. Weiterhin wurde klar, dass bei letzteren die Polymere, dienach der Polymerisation mit Metallfragmenten beladen wurden, eine erhöhte Aktivitätzeigten. Deren Gegenstücke, bei denen der bereits koordinierte Komplex copolymerisiertwurde, enthielten vermutlich eine gewisse Anzahl an Metallzentren, zu denen der Zu-gang für Substrate durch den Polyermisationsvorgang blockiert wurde. Dieser Effekt isthier besonders stark, da EGDMA Polymere im Vergleich zu MMA-Polymeren wesentlichstärker quervernetzt sind (vergleiche P11 und P7-Mo).In zukünftigen Versuchen könnte dieser Effekt durch molecular imprinting vermieden wer-den. Hierbei wird ein Dummy-Substrat vor der Polymerisation an das reaktive Zentrumgebunden und anschließend wieder entfernt. Auf diese Weise wird ein Hohlraum erzeugt,der dazu führt, dass das aktive Zentrum des Komplexes für Substratmoleküle zugänglichbleibt.Der zweite Teil dieser Arbeit beschäftigte sich mit der Synthese von Liganden, welche miteiner Acetylengruppe anstelle der Vinylgruppe an Position 4 des Pyrazolrings substituiertsind. Als solche wurde ein Hbdmpza-Derivat sowie Bis(pyrazol-1-yl)methan Ligandendargestellt (siehe Schema 5.1).

HN

N

N N

N N

NN

N N

N N

OH

N

N N

N N

N N

N

OHO

TMS

15

28

29

3234

Schema 5.1: 4-Ethinylpyrazol-basierte Skorpionatliganden.

Mit Ausnahme von Bis((4-ethinyl)-3,5-dimethylpyrazol-1-yl)methan (bedmpzm) (15), wel-ches unter Corey-Fuchs Bedingungen ausgehend von Bis(3,5-dimethyl-4-formylpyrazol-

154

1-yl)methan (1) synthetisiert wurde, wurden diese Liganden ausgehend von 3,5-Dimethyl-4-(trimethylsilyl)ethinylpyrazol (28) dargestellt. Diese Verbindung wurde ausgehend von3,5-Dimethylpyrazol in vier Schritten erhalten. Mit diesem Edukt konnte 15 ebenfallsunter Phasentransferbedingungen dargestellt werden. Für die Synthese der entsprechen-den Bis(pyrazolyl)essigsäure 29 wurde die bekannte Eintopfsynthese für Bispyrazoly-lessigsäuren angewendet, die von Burzlaff et al. 2001 etabliert wurde. [11] Aufgrundder basischen Reaktionsbedingungen dieser Reaktion, wurden die Trimethylsilylschutz-gruppen ohne weitere Reaktionsschritte im Lauf der Synthese entfernt.Die Bis(4-ethinylpyrazol-1-yl)methan Liganden wurden ebenfalls mittels einer Eintopfsyn-these erhalten. In diesem Fall mittels der Eintopfsynthese für enantionmerenreine Het-eroskorpionat Liganden, die bereits vielfach im Arbeitskreis Anwendung fand. [23] Mittelsdieser Reaktion können prinzipiell verschiedenste Aldehyde als Brückengruppen einge-setzt und damit das Koordinationsmotiv beliebig angepasst werden. Im Rahmen dieserArbeit wurden Salicylaldehyd sowie 1-Methyl-2-imidazol-carboxaldehyd verwendet um dieentsprechendenN,N,O undN,N,N koordinierenden Liganden darzustellen. Die Trimethyl-silylschutzgruppen blieben hierbei intakt und konnten in einem weiteren Schritt unterVerwendung einer Base oder durch Kaliumfluorid entfernt werden.

R'

R' Br

R' N3 R' IR'

N

NN

R'

HN

N

HN

N

HN

N

HN

N

HN

N

HN

N

NH

N

Schema 5.2: Mögliche Reaktionen der Acetylen-Linker-Gruppen für 3,5-Dimethyl-4-ethinylpyrazol.

Die Einführung von Acetylen-Linkern erlaubt eine ganze Reihe von Reaktionen, mit derenHilfe die Liganden oder deren Komplexe weiter modifiziert werden können, wie in Schema

155


5.2 dargestellt. Unter diesen sind Reaktionen wie die Glaser Homokupplungsreak-tion, sowie die Cadiot-Chodkiewicz-Reaktion als Heterokupplungsreaktion. Darüberhinaus können weitere Schritte in Form von Click-Chemie-Reaktionen (CuAAC) oderSonogashira-Kupplungen durchgeführt werden.Die dadurch enstehenden Anwendungsmöglichkeiten sind vielfältig. Abgesehen von denhier vorgestellten Modellliganden für Rieske Dioxygenasen (s.u.), wäre die Einführungvon Fluoreszensgruppen ein lohnendes Ziel. Die Fluoreszens könnte hierbei durch para-magnetisches Fluoreszensquenching, ausgelöst von an das aktive Zentrum des Ligandenkoordinierten Metallen, gesteuert werden. Eine weitere Anwendungsmöglichkeit bestehtin der PEGylierung des Liganden. Hierbei werden Polyethylenglycol-Polymer-Stränge(PEG) mit dem Liganden verbunden um ihre Löslichkeit zu verbessern. Dies kanndurch die oben genannten Methoden durchgeführt werden, da eine breite Auswahl anPEG-Derivaten kommerziell verfügbar ist. Dieser Prozess wird insbesondere in der phar-mazeutischen Chemie häufig angewendet, um die Wasserlöslichkeit von Substanzen starkzu erhöhen, ohne dabei die Toxizität der jeweiligen Verbindungen zu erhöhen. [197]

Abbildung 5.2: Molekülstruktur von Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiin)) (39).

Ausgehend von 3,5-Dimethyl-4-(trimethylsilyl)ethinyl-1-tritylpyrazol (26), welches ein Zwis-chenprodukt bei der Synthese oben genannter Liganden ist, war es darüber hinaus möglich,einen Ligand mit Anwendungsmöglichkeiten für 1D-Koordinationspolymere darzustellen.Dafür wurde die Trimethylsilylschutzgruppe entfernt, um eineGlaser Homokupplung zuermöglichen. Eine Tritylschutzgruppe wurde hierbei eingesetzt, um die vorzeitige Bildungvon Polymeren mit dem Metallkatalysator der Kupplungsreaktion zu verhindern.Der entschützte Ligand bildete leicht Koordinationspolymere mit Metallsalzen, nach-dem die Tritylschutzgruppen entfernt wurden. Es konnten Koordinationspolymere von

156

N

N N

N

N

N N N N

N

FeFe

N

N N

N

OO

Fe Fe

56

54

Abbildung 5.3: Auswahl von im Rahmen dieser Arbeit dargestellten Rieske-Dioxygenase-Modellen.

Kobalt(II)acetylacetonat und Kobalt(II)chlorid erhalten werden. Die Struktur des Ko-balt(II)acetylacetonat Polymers konnte mittels Röntgenstrukturanalyse aufgeklärt wer-den, wie in Schema 5.2 dargestellt.

Im letzten Teil der Arbeit wurden mit dem Ziel Modellkomplexe fürRieske-Dioxygenasenaufzubauen Ferrocenyl substituierte Liganden synthetisiert. Diese Modellsysteme wurdeneinerseits ausgehend vom oben genannten Bis(4-ethinyl-3,5-dimethylpyrazol-1-yl)methan(bedmpzm) (15) und andererseits ausgehend von 4-Iodo-substituierten Bis(pyrazol-1-yl)methanen (45, 46) und -essigsäuren (49) dargestellt. Diese Liganden wurden mitverschiedenen Ferrocenylvorstufen in CuAAC oder Sonogashira Reaktionen umgesetzt.Der Einfluss verschiedener Linkergruppen auf das elektrochemische Potential der dabeierhaltenen Verbindungen wurde mittels zyklischer Voltammetrie bestimmt. Dabei wurdefür alle untersuchten Substanzen ein reversibles Redoxpotential beobachtet. Die darauserrechneten Halbwellenpotentiale sind in Tabelle 5.3 zusammengefasst. Um das Verhal-

Verbindung Eox in mV Ered in mV E1/2 in mVbefcpzm (51) +170 +83 +127mbefcpzac (52) +176 +83 +130


Tabelle 5.3: Zusammenfassung der elektrochemischen Eigenschaften der dargestelltenFerrocenylverbindungen (gegen Fc/Fc+).

157


ten natürlicher Ferredoxincluster nachbilden zu können, wird ein Potential zwischen −150bis +400 mV gegen SHE benötigt, was einem Potential von −550 bis 0 mV gegen dasFerrocen/Ferrocenium-Redoxpaar entspricht. Wie aus Tabelle 5.3 ersichtlich, erwiesensich die Verbindungen mit zusätzlichen Phenylresten in der Linkereinheit als die vielver-sprechendsten Kandidaten. In Kombination mit dem N,N,O Koordinationsmotiv derBis(pyrazolyl)essigsäuren, welches die natürlich vorkommende 2-His-1-carboxylat Triadeder Rieske Dioxygenasen nachzubilden vermag, stellen derartige Verbindungen einenvielversprechenden Ausgangspunkt für Rieske-Modellsysteme dar. Es wurde weiterhingezeigt, dass das elektrochemische Potential solcher Verbindungen über einen relativ bre-iten Potentialbereich justierbar ist, was Raum für weitere Optimierungen lässt. Aufgrundder Vielseitigkeit dieser Herangehensweise, können weitere Ferrocenyleinheiten leicht alsElektronendonoren eingeführt werden.Für die hier durchgeführten Untersuchungen wurden die Carboxylatgruppen der Ligandendurch Veresterung geschützt. In zukünftigen Versuchen müssen diese verseift und anEisen(II)-Zentren koordiniert werden, um strukturelle und funktionelle Modellsystemefür Rieske-Zentren darstellen zu können.

158

6 Experimental Section

159


6.1 General Remarks

6.1.1 Working Techniques

All air sensitive compounds were prepared under dry nitrogen atmosphere using con-ventional Schlenk techniques. Purchased solvents (p.a. grade, < 50 ppm H2O) weredegassed prior to use and stored under nitrogen atmosphere.

6.1.2 Spectroscopic and Analytical Methods

NMR SpectraThe 1H, 13C and 31P NMR spectra were recorded using a Bruker DPX300 AVANCE anda Bruker DRX400 WB spectrometer. The calibrations of the spectra were carried outon the signal of the deuterated solvent, tetramethylsilane or H3PO4. Multiplicities aremarked as follows:

s singlet

d doublet

t triplet

m multiplet

Infrared SpectroscopyInfrared spectra were recorded with an Excalibur FTS-3500 in CaF2 cuvettes (0.2 mm) oras KBr pellets. The latter were prepared using a Perkin-Elmer hydraulic press (10 t/cm2).Relative absorption intensities were marked as follows:

vs very strong

s strong

m medium

w weak

br broad

Mass SpectraESI-MS spectra were recorded with a Bruker Daltonics maXis ultrahigh resolution ESI-TOF mass spectrometer with a resolution of at least 40.000 FWHM. Peaks were identifiedusing simulated isotopic patterns created within the Bruker Data Analysis software.

160

6.1 General Remarks

Elemental AnalysisElemental analyses were determined with an EURO EA 3000 (Euro Vector) and EA 1108(Carlo Erba) instrument (σ = ± 1 % of the measured content).

Polymer analysisThe amount of incorporated ligand moieties in the polymers was determined by the nitro-gen content (% N) of the elemental analysis of the respective polymer using the followingequation:

mmol Ligand

g Polymer= % N

4 · 14 g ·mol−1 · 10

Atomic absorption spectrometry (AAS)Atomic absorption spectrometry (AAS) was carried out using a Perkin-Elmer 5100 F-AASwith AS-90 sample automation. Method of calibration was standard addition. Wave-length/spectral band width in nm: 313.3 nm.

Sample Preparation for AASThe samples of the according copolymers (50 mg) were suspended in H2SO4 (p.a., conc.,3.00 mL). The suspension was heated for 2 h at 140 ◦C in a 25 or 50 mL volumetric flask.The black solution was cooled down to room temperature and treated with H2O2 (35 wt.%solution in water). The mixture was heated to 140 ◦C for 12 h to give a clear colourlesssolution. After cooling to room temperature, further H2O2 (1.00 mL) was added andstirring was continued for 12 h at 140 ◦C. The solution was cooled to room temperatureagain and diluted to 25.0 respectivley 50.0 mL with nitric acid (2 wt.% solution in water)and analyzed by AAS.

Cyclic VoltammetryCyclic voltammetry experiments were carried out using an AUTOLAB PGSTAT 100. Athree electrode cell with a gold disk working electrode, a platinum wire counter electrodeand a silver wire as pseudo-reference electrode was used. Cyclic voltammetry was per-formed in acetonitrile or dichloromethane containing 0.1 m [n-Bu4N]PF6 as supportingelectrolyte. All solutions were deoxygenated with nitrogen before each experiment and ablanket of nitrogen was used to cover the solution during the experiment. The potentialvalues (E) were calculated using the following equation: E = (Epc + Epa)/2, where Epc

and Epa correspond to the cathodic and anodic peak potentials, respectively. Potentialsare referenced to the ferrocene/ferrocenium (Fc/Fc+) couple, which was used as internalstandard. [214]

161


X-ray Structure DeterminationX-ray structure determinations were carried out with a Bruker-Nonius Kappa-CCD diffrac-tometer and a Agilent Super Nova S2 CCD diffractometer. Single crystals were mountedwith Paratone-N, glue or perfluorated oil on a glass fiber. The structures were solvedby using direct methods and refined with full-matrix least-squares against F 2 (SHELX-97). [215] A weighting scheme was applied in the last steps of the refinement with w =1/[σ2(F 2

o ) + (aP )2 + bP ] and P = [2Fc2 + max(F 2o , 0)]/3. Most hydrogen atoms were

included in their calculated positions and refined in a riding model.

6.1.3 Destabilization of Copolymers

In order to destabilize EGDMA and MMA, they were washed each three times with 5 %sodium hydroxide solution in order to remove the stabilizer. Afterwards, they were driedrigorously with sodium sulfate. The destabilized copolymers were stored at −30 ◦C.

6.1.4 Chemicals

The following chemicals were used as purchased without further purification:

- azobisisobutyronitrile

- benzyltriethylammonium chloride

- bis(triphenylphosphine)palladiumdichloride

- cer(IV) ammoniumnitrate

- cobalt(II) bromide

- cobalt(II) chloride

- cobalt(II) acetylacetonate

- copper(II) acetate

- copper(I) iodide

- copper(II) sulfate

- dibromoacetic acid

- dichloroacetic acid

- 3,5-dimethylpyrazole

- 4-ethynylaniline

- ethynylferrocene

- ethylene glycol dimethacrylate

- ferrocene

- iodine

- manganese(II) chloride

- 1-methyl-2-imidazolecarboxaldehyde

- methyl methacrylate

- molybdenum(VI) dichloride dioxide

- n-butyllithium

- paraformaldehyde

- phosphorous oxychloride

- potassium fluoride

- potassium tert-butoxide

162

6.1 General Remarks

- pyrazole

- salicylaldehyde

- sodium ascorbate

- sodium azide

- sodium hydride

- sodium nitrite

- thionyl chloride

- triethylamine

- trifluoroacetic acid

- triphenylmethyl-phosphoniumbromide

- trimethylphosphane

- trimethylsilylacetylene

- trityl chloride

- zinc(II) chloride

The following chemicals were synthesized according to literature procedures.

- azidomethylferrocene [212,213]

- bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1) [44]

- bis(3,5-dimethylpyrazol-1-yl)methane [43]

- bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (4) [35]

- bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane (bdmvpzm) (2) [35]

- ferrocene azide [211]

- 1-ferrocenyl-4-ethynylbenzene [208,209]

- 4-iodopyrazole (22) [192]

- 4-iodo-3,5-dimethylpyrazole (23) [192]

- 4-iodo-3,5-dimethyl-1-tritylpyrazole (25) [71]

- 4-iodo-1-tritylpyrazole (24) [71]

163


6.2 Oxo-Transfer Catalysis by Chelate and ScorpionateOxomolybdenum Complexes

6.2.1 Synthesis of Bis(3,5-dimethyl-4-formylpyrazol-1-yl)me-thane (1)[44]

A solution of bis(3,5-dimethylpyrazol-1-yl)methane (8.96 g, 43.9 mmol) in dimethylfor-mamide (100 mL) was heated to a temperature of 96 ◦C. Within one hour, phosphorusoxychloride (14.9 g, 96.6 mmol) was slowly added. The resulting solution was stirred overnight at this temperature. After this time, the heating was removed and the solution waschilled to 0 ◦C under vigorous stirring. Subsequently, the reaction mixture was pouredinto ice water, what led to the formation of a precipitate. The latter was filtered off andwashed thoroughly with water to remove impurities. Finally, the product was dried in adesiccator.

N

N N

N

O O

1C11H16N4

MW: 204.28 g/mol

Yield: 2.88 (11.1 mmol, 26 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.41 (s, 6 H, C5-CH3), 2.79 (s, 6 H, C3-CH3),6.11 (s, 2 H, -CH2-), 9.93 (s, 2 H, -CHO) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.1 (C5-CH3), 12.7 (C3-CH3), 59.1 (-CH2-),118.8 (C4), 146.5 (C5), 151.8 (C3), 185.0 (-CHO) ppm.

164


6.2.2 Synthesis of Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane(bdmvpzm) (2)[35]

A suspension of triphenylmethyl-phosphonium bromide (19.8 g, 55.4 mmol) in tetrahy-drofuran was treated with potassium tert-butoxide (5.80 g, 51.6 mmol). The resultingmixture was stirred for one hour at room temperature. After this time, it was reactedwith bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1) (4.80 g, 18.4 mmol). The solu-tion was heated to a temperature of 60 ◦C and stirred for two hours. After this time, theheating was removed and the stirring was continued overnight at room temperature. Sub-sequently all solids were filtered off and all volatiles were removed in vacuo. The remainingresidue was dissolved in dichloromethane and purified via column chromatography (silica,n-pentane:ethyl acetate 7:3 v/v).

N

N N

N

2C15H20N4

MW: 256.35 g/mol

Yield: 3.78 g (14.8 mmol, 80 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.27 (s, 6 H, C5-CH3), 2.47 (s, 6 H, C3-CH3),5.14 (dd, 2JH,H = 1.4 Hz, 3JH,H = 11.6 Hz, 2 H, (Z )-H2C=), 5.29 (dd, 2JH,H = 1.4 Hz,3JH,H = 17.9 Hz, 2 H, (E)-H2C=), 6.09 (s, 2 H, -CH2-), 6.48 (dd, 3JH,H = 17.9 Hz, 3JH,H

= 11.5 Hz, 2 H, -HC=) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.2 (C5-CH3), 13.6 (C3-CH3), 60.7 (-CH2-),112.8 (=CH2), 116.5 (C4), 127.4 (-HC=), 138.0 (C5), 146.8 (C3) ppm.

165


6.2.3 Synthesis of [MoO2Cl2(bdmvpzm)] (3)

A solution of molybdenum(VI) dichloride dioxide (0.388 g, 1.95 mmol) in tetrahydrofuranwas treated with bdmvpzm (1) (0.500 g 1.95 mmol). The resulting solution was stirredfor one hour at room temperature, whereupon a yellow precipitate was formed. Thisprecipitate was collected via filtration and dried in vacuo.Crystals suitable for an X-ray structure determination could be obtained by layering asolution of 3 in dichloromethane with n-hexane.

N

N N

NMo

Cl O

Cl

O

3C15H20Cl2MoN4O2MW: 455.21 g/mol

Yield: 0.604 g (1.33 mmol, 68 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.15 (s, 6 H, C5-CH3), 2.44 (s, 6 H, C3-CH3),5.09 (dd, 2JH,H = 1.2 Hz, 3JH,H = 11.6 Hz, 2 H, (Z )-H2C=), 5.26 (dd, 2JH,H = 1.2 Hz,3JH,H = 17.9 Hz, 2 H, (E-H2C=), 6.12 (s, 2 H, -CH2-), 6.50 (dd, 3JH,H = 17.9 Hz, 3JH,H

= 11.7 Hz, 2 H, -HC=) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 9.69 (C5-CH3), 13.5 (C3-CH3), 59.0 (-CH2-),112.3 (=CH2), 115.2 (C4), 127.6 (-HC=), 138.0 (C5), 145.8 (C3) ppm.

Elemental analysis of C15H20Cl2MoN4O2 (455.21 g/mol): calcd. C 39.58, H 4.43,N 12.31; found C 39.83, H 4.30, N 12.53 %.

IR (KBr): ν̃ = 944 (s, νsym(Mo=O)), 917 (s, νasym(Mo=O)) cm−1.

166


6.2.4 Synthesis of Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid(Hbdmvpza) (4)[35]

A solution of bdmvpzm (2) (3.75 g, 14.6 mmol) in tetrahydrofuran was cooled to −80 ◦C.At this temperature, n-butyllithium (1.6 m solution in n-hexane, 9.58 mL, 15.3 mmol)was added slowly under vigorous stirring. The resulting solution was allowed to warm toa temperature of −20 ◦C over the next four hours. After this time, a dry stream of carbondioxide was passed through the reaction mixture for one hour. Subsequently, the vesselwas allowed to slowly reach room temperature and stirred over night. Afterwards, allvolatiles were removed and the remaining residue was dissolved in water (250 mL). Theresulting aqueous phase was washed with diethyl ether (2 × 50 mL) in order to removeimpurities. After acidification to an pH value of 2 with diluted hydrochloric acid, thesolution was extracted with diethyl ether (2 × 100 mL) and the combined organic phaseswere dried (sodium sulfate). The solvent was removed in vacuo and the product was driedin high vacuum.

N

N N

N

OHO

4C16H20N4O2

MW: 300.36 g/mol

Yield: 2.96 g (9.87 mmol, 68 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.28 (s, 6 H, C5-CH3), 2.32 (s, 6 H, C3-CH3),5.24 (dd, 2JH,H = 0.8 Hz, 3JH,H = 11.6 Hz, 2 H, (Z )-H2C=), 5.34 (dd, 2JH,H = 0.8 Hz,3JH,H = 17.9 Hz, 2 H, (E)-H2C=), 6.45 (dd, 3JH,H = 17.9 Hz, 3JH,H = 11.7 Hz, 2 H,-HC=), 6.83 (s, 1 H, CbridgeH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.0 (C5-CH3), 13.4 (C3-CH3), 70.5 (Cbridge),114.8 (H2C=), 115.7 (C4), 128.5 (-HC=), 137.3 (C5), 145.1 (C3), 165.4 (CO2

−) ppm.

167


6.2.5 Synthesis of [MoO2Cl(bdmvpza)] (5)

A solution of Hbdmvpza (4) (100 mg, 0.333 mmol) in tetrahydrofuran was charged withpotassium tert-butoxide (0.0374 mg, 0.333 mmol). To the resulting mixture, molybde-num(VI) dichloride dioxide (66.0 mg, 0.333 mmol) was added. Subsequently, the reactionwas stirred over night and filtrated afterwards. All volatiles were removed and the re-maining residue was dissolved in dichloromethane. The desired product was obtained byprecipitation from this solution with diethyl ether and subsequent collection via filtration.It was washed thoroughly with diethyl ether and dried in vacuum.

N

N N

NMo

Cl OO

OO

5C16H19ClMoN4O4MW: 462.76 g/mol

Yield: 76.8 mg (0.166 mmol, 50 %)

1H NMR (symmetric cis-isomer, 300 MHz, DMSO-d6, 25 ◦C): δ = 2.17 (s, 6 H, C5-CH3), 2.26 (s, 6 H, C3-CH3), 5.12 (dd, 2JH,H = 1.0 Hz, 3JH,H = 11.5 Hz, 2 H, (Z )-H2C=),5.28 (dd, 2JH,H = 1.0 Hz, 3JH,H = 18.0 Hz, 2 H, (E-H2C=), 6.51 (dd, 3JH,H = 17.9 Hz,3JH,H = 11.7 Hz, 2 H, -HC=), 7.27 (s, 1 H, -CbridgeH-) ppm.

1H NMR (asymmetric trans-isomer, 300 MHz, DMSO-d6, 25 ◦C): δ = 2.54 (s, 3 H,C5-CH3 or C5’-CH3), 2.61 (s, 3 H, C5-CH3 or C5’-CH3), 2.71 (s, 3 H, C3-CH3 or C3’-CH3),2.79 (s, 3 H, C3-CH3or C3’-CH3), 5.41 (m, AMX system, coupling not resolved, 4 H,H2C=), 6.57 (m, AMX system, coupling not resolved, 2 H, -HC=), 7.05 (s, 1 H, CbridgeH)ppm.

13C NMR (75.5 MHz, DMSO-d6, 25 ◦C): δ = 9.91 (C5-CH3), 13.5 (C3-CH3), 71.3(Cbridge), 112.7 (=CH2), 115.7 (C4), 127.4 (-HC=), 138.4 (C5), 145.6 (C3), 165.9 (CO2

−)ppm.

Elemental analysis of C16H19ClMoN4O4 (462.76 g/mol): calcd. C 41.53, H 4.14, N12.11; found C 41.87, H 4.17, N 10.59 %.


168


6.2.6 Copolymerization of bdmvpzm (2) with MMA to form P6

A solution of bdmvpzm (2) (85.0 mg, 0.332 mmol) in dry xylenes (10 mL) was charged withmethyl methacrylate (MMA) (1.10 mL, 11.9 mmol). The resulting mixture was heated toa temperature of 80 ◦C. Azobisisobutyronitrile (AIBN) (20.0 mg, 0.120 mmol) was addedto start the copolymerization. The reaction was stirred for five hours and subsequentlypoured in a mixture of methanol (300 mL) and diluted hydrochloric acid (3.00 mL). Theresulting white precipitate was collected by filtration and washed thoroughly with drymethanol and dried in vacuo.

N

N N

NMMA MMA

P6

Yield: 0.310 g (28 %)

Elemental analysis: C 61.34, H 8.02, N 2.34 %.

Incorporation: 0.443 mmol/g.

169


6.2.7 Copolymerization of bdmvpzm (2) with EGDMA to form P7

A solution of bdmvpzm (2) (85.0 mg, 0.332 mmol) in dry xylenes (10 mL) was chargedwith ethylene glycol dimethacrylate (EGDMA) (1.10 mL, 5.85 mmol). The resultingmixture was heated to a temperature of 80 ◦C. Azobisisobutyronitrile (AIBN) (20.0 mg,0.120 mmol) were added to start the copolymerization. The reaction was stirred forfive hours. Subsequently, the resulting white precipitate was collected by filtration andwashed thoroughly with dry methanol and dried in vacuo.

N

N N

NEGDMA EGDMA

P7

Yield: 1.19 g (96 %)



170


6.2.8 Copolymerization of Hbdmvpza (4) with MMA to form P8

A solution of Hbdmvpza (4) (100 mg, 0.333 mmol) in dry xylenes (10 mL) was chargedwith methyl methacrylate (MMA) (1.10 mL, 11.9 mmol). The resulting mixture washeated to a temperature of 80 ◦C. Azobisisobutyronitrile (AIBN) (20.0 mg, 0.120 mmol)was added to start the copolymerization. The reaction was stirred for five hours andsubsequently poured in a mixture of methanol (300 mL) and diluted hydrochloric acid(3.00 mL). The resulting white precipitate was collected by filtration and washed thor-oughly with dry methanol and was dried in vacuo.

N

N N

NMMA MMA

OHO

P8

Yield: 0.205 g (18 %)



171


6.2.9 Copolymerization of Hbdmvpza (4) with EGDMA to form P9

A solution of Hbdmvpza (4) (100.0 mg, 0.333 mmol) in dry xylene (10 mL) was chargedwith ethylene glycol dimethacrylate (EGDMA) (1.10 mL, 5.85 mmol). The resultingmixture was heated to a temperature of 80 ◦C. Azobisisobutyronitrile (AIBN) (20.0 mg,0.120 mmol) was added to start the copolymerization. The reaction was stirred forfive hours. Subsequently, the resulting white precipitate was collected by filtration andwashed thoroughly with dry methanol and was dried in vacuo.

N

N N

NEGDMA EGDMA

OHO

P9

Yield: 1.21 g (97 %)



172


6.2.10 Synthesis of P6-Mo

A suspension of P6 (0.300 mg, ligand content 0.441 mmol/g, equals 0.132 mmol) intetrahydrofuran was treated with molybdenum(VI) dichloride dioxide (26.4 mg, 0.132 mmol).The resulting mixture was stirred for 24 hours at a temperature of 50 ◦C. Subsequently,all volatiles were removed and the residue was washed thoroughly with dry methanol.Afterwards, the polymer was dried in vacuo.

MMA MMAN

N N

NMo

Cl O

Cl

O

P6-Mo

AAS: Mo content = 0.271 mmol/g.

173



A suspension of P7 (0.300 mg, ligand content 0.310 mmol/g, equals 0.0930 mmol) intetrahydrofuran was treated with molybdenum(VI) dichloride dioxide (18.6 mg, 0.0930 mmol).The resulting mixture was stirred for 24 hours at a temperature of 50 ◦C. Subsequently,all volatiles were removed and the residue was washed thoroughly with dry methanol.Afterwards, the polymer was dried in vacuo.

EGDMA EGDMAN

N N

NMo

Cl O

Cl

O

P7-Mo


174



The copolymer P8 (500 mg, ligand content 0.624 g/mol, equals 0.312 mmol) was sus-pended in dry tetrahydrofuran. Potassium tert-butoxide (35.0 mg, 0.312 mmol) wasadded and the resulting mixture was stirred for one hour at a temperature of 50 ◦C.Subsequently, molybdenum(VI) dichloride dioxide (62.0 mg, 0.312 mmol) was added tothe suspension. After 24 hours of stirring, the polymer was collected by filtration, washedthoroughly with methanol and dried in vacuo.

MMA MMAN

N N

NMo

Cl OO

OO

P8-Mo


175



The copolymer P9 (500 mg, ligand content 0.228 g/mol, equals 0.114 mmol) was sus-pended in dry tetrahydrofuran. Potassium tert-butoxide (12.8 mg, 0.114 mmol) wasadded and the resulting mixture was stirred for one hour at a temperature of 50 ◦C.Subsequently, molybdenum(VI) dichloride dioxide (22.7 mg, 0.114 mmol) was added tothe suspension. After 24 hours of stirring, the polymer was collected by filtration, washedthoroughly with methanol and dried in vacuo.

EGDMA EGDMAN

N N

NMo

Cl OO

OO

P9-Mo


176


6.2.14 Copolymerization of [MoO2Cl2(bdmvpzm)] (3) with MMA toform P10

A solution of [MoO2Cl2(bdmvpzm)] (3) (0.100 g, 0.220 mmol) in a mixture of acetonitrile(6.00 mL) and xylenes (2.00 mL) was treated with MMA (1.00 mL, 9.39 mmol). Theresulting solution was heated to a temperature of 65 ◦C and AIBN (15.0 mg, 0.0913 mmol)was added. Subsequently, the solution was stirred for 24 hours. All volatiles were removedin vacuo and the residue was washed with dry methanol. The obtained material was driedin vacuum.

MMA MMAN

N N

NMo

Cl O

Cl

O

P10

Yield: 311 mg (30 %)



177


6.2.15 Copolymerization of [MoO2Cl2(bdmvpzm)] (3) with EGDMAto form P11

A solution of [MoO2Cl2(bdmvpzm)] (3) (0.100 g, 0.220 mmol) in a mixture of acetonitrile(6.00 mL) and xylene (2.00 mL) was treated with EGDMA (1.00 mL, 5.30 mmol). Theresulting solution was heated to a temperature of 65 ◦C and AIBN (40.0 mg, 0.240 mmol)was added. Subsequently, the solution was stirred for 24 hours. All volatiles were removedin vacuo and the residue was washed with dry methanol. The obtained material was driedin vacuum.

EGDMA EGDMAN

N N

NMo

Cl O

Cl

O

P11

Yield: 539 mg (47 %)


IR (KBr): ν̃ = 944 (w, νsym(Mo=O)), 919 (w, νasym(Mo=O)) cm−1.

178


6.2.16 Copolymerization of [MoO2Cl(bdmvpza)] (5) with MMA toform P12

A solution of [MoO2Cl(bdmvpza)] (5) (0.100 g, 0.216 mmol) in a mixture of acetonitrile(6.00 mL) and xylenes (2.00 mL) was treated with MMA (1.00 mL, 9.39 mmol). Theresulting solution was heated to a temperature of 65 ◦C and AIBN (15.0 mg, 0.0913 mmol)was added. Subsequently, the solution was stirred for 24 hours. All volatiles were removedin vacuo and the residue was washed with dry methanol. The obtained material was driedin vacuum.

MMA MMAN

N N

NMo

Cl OO

OO

P12

Yield: 281.0 mg (27 %)

AAS: Mo content = 0.401 mmol g−1.


179


6.2.17 Copolymerization of [MoO2Cl(bdmvpza)] (5) with EGDMAto form P13

A solution of [MoO2Cl(bdmvpza)] (5) (0.100 g, 0.216 mmol) in a mixture of acetonitrile(6.00 mL) and xylenes (2.00 mL) was treated with EGDMA (1.00 mL, 5.30 mmol). Theresulting solution was heated to a temperature of 65 ◦C and AIBN (15.0 mg, 0.0913 mmol)was added. Subsequently, the solution was stirred for 24 hours. All volatiles were removedin vacuo and the residue was washed with dry methanol. The obtained material was driedin vacuum.

EGDMA EGDMAN

N N

NMo

Cl OO

OO

P13

Yield: 516 mg (45 %)

AAS: Mo content = 0.0307 mmol g−1.


180

6.3 4-Ethynyl Substituted Bis(pyrazolyl)methane Ligands


6.3.1 Synthesis of Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)methane (14)

Tertrabromomethane (5.08 g, 15.32 mmol) was dissolved in dichloromethane at a tem-perature of 0 ◦C. Triphenylphosphine (8.04 g, 30.6 mmol) was slowly added. Subse-quently, bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1.00 g, 3.83 mmol) and triethy-lamine (5.00 mL, 35.8 mmol) were given to the reaction mixture. The cooling was removedand the solution was stirred for 72 h. After that time, the solvent was removed in vacuoand the residue was purified via column chromatography (silica, n-pentane:ethyl acetate7:3 v/v).

N

N N

NHH

Br

Br

Br

Br

14C15H16Br4N4

MW: 571.94 g/mol

Yield: 1.84 g (3.22 mmol, 84.1 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.11 (s, 6 H, C3-CH3), 2.38 (s, 6 H, C5-CH3),6.01 (s, 2 H, -CH2-), 7.18 (s, 2 H, Br2C=C-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 11.6 (C3-CH3), 13.2 (C5-CH3), 54.0 (-CH2-),61.0 (C-Br2), 93.0 (H-C=), 116.1 (C4), 138.8 (C5), 147.3 (C3) ppm.

Elemental analysis of C15H16Br4N4 (571.93 g/mol): calcd. C 31.50, H 2.82, N 9.80;found C 31.35, H 2.58, N 9.95 %.

181


6.3.2 Synthesis of Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane(bedmpzm) (15)

A solution of bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)methane (14) (0.300 g,0.525 mmol) in diethyl ether (50 mL) was cooled to 0 ◦C. N -butyllithium (2.30 ml,3.68 mmol, 1.60 m in n-hexane) was added slowly and the resulting yellow solution wasstirred for 20 minutes. The reaction was quenched by addition of a satured solution ofammonium chloride (10 mL). The solution was stirred for additional 30 minutes until itwas extracted with diethyl ether (3 × 20 mL). The combined organic phases were driedover sodium sulfate and the solvent was removed in vacuo. The remaining residue waswashed with methanol to remove impurities.

N

N N

N

15C15H16N4

MW: 252.32 g/mol

Yield: 92.0 mg (0.365 mmol, 69.5 %)

1H NMR (300 MHz, DMSO-d6, 25 ◦C): δ = 2.10 (s, 6 H, C3-CH3), 2.44 (s, 6 H, C5-CH3),4.15 (s, 2 H, C≡C-H), 6.13 (s, 2 H, -CH2-) ppm.

13C NMR (75.5 MHz, DMSO-d6, 25 ◦C): δ = 10.5 (C3-CH3), 12.3 (C5-CH3), 60.7 (-CH2-), 75.4 (C≡C), 81.1 (C≡C), 102.4 (C4), 144.2 (C5), 151.1 (C3) ppm.

Elemental analysis of C15H16N4 (252.32 g/mol): calcd. C 71.40, H 6.39, N 22.21; foundC 70.97, H 6.02, N 22.50 %.

IR (KBr): ν̃ = 3221 (s, C≡C-H), 2106 (m, C≡C), 1555 (m, C=N) cm−1.

182


6.3.3 Synthesis of [CuI(bedmpzm)] (17)

A solution of bis((4-ethynyl)-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15) (0.150 g,0.595 mmol) and copper iodide (0.113 g, 0.595 mmol) in acetonitrile (15 mL) was stirredfor 30 minutes at room temperature. During this time, a white precipitate was formed,which was filtered off and dried in vacuo.

N

N N

NCu

I

17C15H16N4CuI

MW: 442.77 g/mol

Yield: 0.224 g (0.505 mmol, 85 %)


A 13C NMR spectrum could not be collected due to low solubility.

Elemental analysis of C15H16N4CuI (442.77 g/mol): calcd. C 40.69, H 3.64, N 12.65;found C 40.76, H 3.49, N 12.43 %.


183


6.3.4 Synthesis of [ZnCl2(bedmpzm)] (18)

A solution of bis((4-ethynyl)-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15) (0.150 g,0.594 mmol) and zinc(II) chloride (0.0810 g, 0.594 mmol) in acetonitrile (15 mL) wasstirred for one hour until the formation of a white precipitate occured, which was filteredoff and dried in vacuo.

N

N N

NZn

ClCl

18C15H16N4ZnCl2

MW: 388.60 g/mol


A 13C NMR spectrum could not be collected due to low solubility.

Elemental analysis of C15H16N4ZnCl2 (388.60 g/mol): calcd. C 46.36, H 4.15, N 14.42;found C 46.27, H 4.00, N 14.15 %.

IR (KBr): ν̃ = 3244 (C≡C-H), 2110 (C≡C), 1549 (m, C=N) cm−1.

184


6.3.5 Synthesis of [MnCl2(bedmpzm)] (19)

A solution of bis((4-ethynyl)-3,5-dimethylpyrazolyl)methane (bedmpzm) (15) (0.150 g,0.594 mmol) and manganese(II) chloride (0.0750 g, 0.594 mmol) in acetonitrile (15 mL)was stirred for one hour at room temperature. The formed precipitate was filtered off,washed with diethyl ether (3 × 5 mL) and dried in vacuo.

N

N N

N

Cl Cl

N

N N

N

Mn

19C30H32N8MnCl2

MW: 630.48 g/mol

Yield: 0.154 g (0.244 mmol, 82 %)

Elemental analysis of C30H32N8MnCl2 (630.48 g/mol): calcd. C 57.15, H 5.12, N 17.77;found C 57.26, H 4.88, N 17.49.


185


6.3.6 Synthesis of [CoCl2(bedmpzm)] (20)

A solution of bis((4-ethynyl)-3,5-dimethylpyrazolyl)methane (bedmpzm) (15) (0.150 g,0.594 mmol) and cobalt(II) chloride (0.141 g, 0.594 mmol) in acetonitrile (15 mL) wasstirred over night. The resulting deep blue precipitate was filtered off and dried in vacuo.

N

N N

NCo

ClCl

20C15H16N4CoCl2

MW: 382.15 g/mol

Yield: 0.182 g (0.475 mmol, 80 %)

Elemental analysis of C15H16N4CoCl2 (382.15 g/mol): calcd. C 47.14, H 4.22, N 14.66;found C 47.29, H 4.33, N 14.65 %.

IR (KBr): ν̃ = 3265 (s, C≡C-H), 2114 (w, C≡C), 1551 (m, C=N) cm−1.

186


6.3.7 Synthesis of [MoO2Cl2(bedmpzm)] (21)

A solution of molybdenum(VI) dichloride dioxide (0.317 g, 1.60 mmol) in tetrahydro-furan was treated with bis((4-ethynyl)-3,5-dimethylpyrazolyl)methane (bedmpzm) (15)(0.402 g, 1.60 mmol). After stirring for one hour, the resulting precipiate was filtered offand dried in vacuo.

N

N N

NMo

ClClOO

21C15H16N4O2MoCl2MW: 451.81 g/mol

Yield: 470 mg (1.23 mmol, 77 %)


13C NMR (75.5 MHz, DMSO-d6, 25 ◦C): δ = 10.1 (C3-CH3), 12.1 (C5-CH3), 59.5 (CH2),75.4 (C≡C), 84.1 (C≡C), 101.2 (C4), 143.9 (C3), 149.9 (C5) ppm.

Elemental analysis of C15H16N4O2MoCl2 (451.18 g/mol): calcd. C 39.93, H 3.75, N12.42; found C 40.33, H 3.22, N 11.61 %.

IR (KBr): ν̃ = 3274 (s, C≡C-H), 2118 (w, C≡C), 1557 (m, C=N), 948 (s, νsym(Mo=O)),919 (vs, νasym(Mo=O)) cm−1.

187


6.3.8 Synthesis of 4-Iodopyrazole (22)

To a solution of pyrazole (22.9 g, 336 mmol) and iodine (50.1 g, 201 mmol) in acetonitrile(250 mL) was slowly added cer(IV)ammoniumnitrate (92.0 g, 186 mmol). After threehours of stirring, the volatiles were removed and the remaining residue was dissolved inethyl acetate (250 mL) and water (250 mL). A saturated solution of sodium thiosulfatewas slowly added until the red color disappeared. The organic phase was separated andwashed with brine (2 × 250 mL), dried over sodium sulfate and the solvent removed invacuo.

N

NHI

22C3H3IN2

MW: 193.98 g/mol

Yield: 53.0 g (273 mmol, 81 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 7.65 (s, 2 H, CH) ppm.

188


6.3.9 Synthesis of 4-Iodo-3,5-dimethylpyrazole (23)

To a solution of 3,5-dimethylpyrazole (32.3 g, 336 mmol) and iodine (50.1 g, 201 mmol)in acetonitrile (250 mL) was slowly added cer(IV)ammoniumnitrate (92.0 g, 186 mmol).After three hours of stirring, the volatiles were removed and the remaining residue wasdissolved in ethyl acetate (250 mL) and water (250 mL). A saturated solution of sodiumthiosulfate was slowly added until the red color disappeared. The organic phase wasseparated and washed with brine (2 × 250 mL), dried over sodium sulfate and the solventremoved in vacuo.

N

NHI

23C5H7IN2

MW: 222.03 g/mol

Yield: 52.0 g (234 mmol, 69.6 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.20 (s, 6 H, CH3) ppm.

189


6.3.10 Synthesis of 4-Iodo-1-tritylpyrazol (24)[77]

Sodium hydride (5.52 g, 138 mmol, 60 % in petrol ether) was added slowly to a solution of4-iodopyrazole (22) (19.4 g, 100 mmol) in dry THF. After the addition of trityl chloride(27.9 g, 100 mmol), the reaction was stirred for three days. Afterwards, all volatiles wereremoved and the colorless residue was washed with water (3 × 50 mL) and diethyl ether(3 × 50 mL).

N

NI

24C22H17IN2

MW: 436.30 g/mol

Yield: 45.0 g (96.6 mmol, 41.3 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 7.15 (m, 6 H, Trt), 7.34 (m, 9 H, Trt), 7.43 (s,1 H, C5-H), 7.69 (s, 1 H, C3-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 55.6 (C4), 79.3 (C -(C6H5)3), 127.8 (Trt),127.9 (Trt), 130.1 (Trt), 136.4 (C5), 142.7 (Trt), 144.7 (C3) ppm.

190


6.3.11 Synthesis of 4-Iodo-3,5-dimethyl-1-tritylpyrazole (25)[71]

Sodium hydride (13.1 g, 327 mmol, 60 % in petrol ether) was added slowly to a solutionof 4-iodo-3,5-dimethylpyrazole (23) (51.7 g, 233 mmol) in dry THF. After the additionof trityl chloride (65.2 g, 233 mmol), the reaction was stirred for three days. Afterwards,all volatiles were removed and the colorless residue was washed with water (3 × 50 mL)and diethyl ether (3 × 50 mL).

N

NI

25C24H21IN2

MW: 464.34 g/mol

Yield: 37.1 g (85.0 mmol, 85 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 1.58 (s, 3 H, CH3), 2.23 (s, 3 H, CH3), 7.10(m, 6 H, Trt), 7.27 (m, 9 H, Trt) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 14.6 (C5-CH3), 15.7 (C3-CH3), 66.9 (C4),78.9 (C -(C6H5)3), 127.3 (Trt), 127.5 (Trt), 130.3 (Trt), 142.5 (C5), 142.9 (Trt), 147.5 (C3)ppm.

191


6.3.12 Synthesis of 3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpy-razole (26)

A solution of 4-iodo-3,5-dimethyl-1-tritylpyrazole (25) (15.0 g, 32.0 mmol) in dimethyl-formamide (150 mL) was treated with copper(I) iodide (1.22 g, 6.40 mmol), bis(triphenyl-phosphine)palladium dichloride (2.27 g, 3.23 mmol), triethyl amine (22.2 mL, 160 mmol)and trimethylsilylacetylene (4.88 mL, 35.2 mmol). The solution was stirred for 24 hoursat a temperature of 60 ◦C. Subsequently, volatiles were removed under vacuo and theremaining residue was dissolved in dichloromethane and water (2:1 v/v, 300 mL). Af-ter the phases were separated, the aqueous phase was extracted with dichloromethane(3 × 75 mL) and the combined organic phases were dried (sodium sulfate). The sol-vent was removed and the crude product was purified via column chromatography (silica,n-hexane:ethyl acetate 7:3 v/v).

N

NSi

26C29H30N2Si

MW: 434.66 g/mol

Yield: 9.93 g (22.8 mmol, 71.3 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.23 (s, 9 H, Si(CH3)3), 1.57 (s, 3 H, C5-CH 3),2.26 (s, 3 H, C3-CH 3), 6.85-6.95 (m, 6 H, Trt), 7.00-7.10 (m, 9 H, Trt) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 0.23 (Si(CH3)3), 12.9 (C5-CH3), 13.9 (C3-CH3), 78.6 (C -(C6H5)3), 97.9 (C≡C), 98.1 (C≡C), 104.4 (C4), 127.3 (Trt), 127.5 (Trt),130.3 (Trt), 142.8 (Trt), 145.4 (C5), 148.1 (C3) ppm.

Elemental analysis of C29H30N2Si (434.66 g/mol): calcd. C 80.14, N 6.45, H 6.96; foundC 79.55, N 6.27, H 6.93 %.

ESI MS: m/z (%) = 243.11 (46) [Trt]+, 457.21 (80) [M+Na]+, 891.42 (100) [M2+Na]+.

IR (KBr): ν̃ = 2152 (s, C≡C), 1551 (m, C=N) 1250 (s, Si-CH3), 861 (vs, Si-CH3) cm−1.

192


6.3.13 Synthesis of 4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27)

A solution of 4-iodo-1-tritylpyrazole (24) (5.00 g, 11.5 mmol) in dimethylformamide(75 mL) was treated with copper(I) iodide (0.436 g, 2.29 mmol), bis(triphenylphosphine)-palladium dichloride (0.807 g, 1.15 mmol), triethyl amine (8.00 mL, 57.3 mmol) andtrimethylsilylacetylene (1.75 mL, 12.6 mmol). The solution was stirred for 24 hours at atemperature of 60 ◦C. Subsequently, volatiles were removed under vacuo and the remainingresidue was dissolved in dichloromethane and water (2:1 v/v, 350 mL). After the phaseswere separated, the aqueous phase was extracted with dichloromethane (3 × 50 mL) andthe combined organic phases were dried (sodium sulfate). The solvent was removed andthe crude product was purified via column chromatography (silica, n-hexane:ethyl acetate9:1 v/v).

N

NSi

27C27H26N2Si

MW: 406.60 g/mol

Yield: 3.18 g (7.82 mmol, 68 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.21 (s, 9 H, Si(CH3)3), 7.13 (m, 6 H, Trt),7.29 (m, 9 H, Trt), 7.56 (s, 1 H, C3-H), 7.77 (s, 1 H, C5-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = −0.06 (Si(CH3)3), 79.0 (C -(C6H5)3), 95.1(C≡C), 96.5 (C≡C), 101.9 (C4), 127.7 (Trt), 127.8 (Trt), 130.0 (Trt), 135.6 (Trt), 142.5(C5), 142.5 (C3) ppm.

Elemental analysis of C27H26N2Si (406.60 g/mol): calcd. C 79.76, H 6.89, N 6.45; foundC 79.84, H 6.39, N 6.90 %.

ESI MS:m/z (%) = 243.12 (60) [Trt]+, 333.14 (18) [M−Si(CH3)3]+, 429.18 (82) [M+Na]+,739.32 [M+(M−Si(CH3)3)]+, 835.36 (100) [2M+Na]+.

IR (KBr): ν̃ = 2164 (s, C≡C), 1545 (w, C=N), 1248 (m, Si-CH3), 864 (s, Si-CH3) cm−1.

193


6.3.14 Synthesis of 3,5-Dimethyl-4-(trimethylsilyl)ethynylpyra-zole (28)

A solution of 3,5-dimethyl-4-(trimethylsily)ethynyl-1-tritylpyrazole (26) (9.93 g,22.8 mmol) in dichloromethane (300 mL) and water (6 mL) was treated with trifluoroaceticacid (3.87 mL, 50.6 mmol). The reaction was stirred for 24 hours at room temperature.Subsequently, the solution was neutralized with a saturated solution of sodium carbonate.The organic phase was washed with water (2 × 40 mL) and dried (sodium sulfate). Allvolatiles were removed and the residue was purified via column chromatography (silica,chloroform to remove impurities, then chloroform:acetone 9:1 v/v).

Yield: 1.78 g (9.25 mmol, 41 %)

Alternative Synthesis: A solution of 4-iodopyrazole (23) (5.00 g, 22.5 mmol) indimethylformamide (70 mL) was treated with bis(triphenylphosphine)palladium dichlo-ride (1.58 g, 0.0275 mmol), cooper(I) iodide (0.857 g, 4.50 mmol), triethyl amine (15.6 mL,113 mmol) and trimethylsilylacetylene (4.68 mL, 33.8 mmol). The resulting mixture wasstirred for 24 hours at a temperature of 60 ◦C. After this time, all volatiles were distilledoff and the residue was purified via column chromatography (silica, n-hexane:ethyl acetate6:4 v/v).

N

NHSi

28C10H16N2Si

MW: 192.34 g/mol

Yield: 3.41 g (17.7 mmol, 79 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.24 (s, 9 H, Si(CH3)3), 2.31 (s, 6 H, CH3),10.45 (s, 1 H, NH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 0.21 (Si(CH3)3), 11.3 (C3- and C5-CH3), 97.0(C≡C), 97.7 (C≡C), 101.5 (C4), 147.2 (C3 and C5) ppm.

Elemental analysis of C10H16N2Si (192.34 g/mol): calcd. 62.45, N 14.57, H 8.38; found62.55, N 14.20, H 8.24 %.

ESI MS: m/z (%) = 193.11 (100) [MH]+.

194


IR (KBr): ν̃ = 2155 (vs, C≡C), 1557 (w, C=N), 1250 (s, Si-CH3), 867 (vs, Si-CH3) cm−1.

6.3.15 Synthesis of 2,2-Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (29)

3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26) (1.78 g, 9.25 mmol) was dis-solved in tetrahydrofuran (300 mL). Potassium hydroxide (1.55 g, 27.6 mmol), potassiumcarbonate (3.81 g, 27.6 mmol), benzyltriethylammonium chloride (0.210 g,0.922 mmol) and dibromoacetic acid (1.01 g, 4.64 mmol) were added to the solution.The resulting reaction mixture was heated to reflux at 75 ◦C and stirred for three days.Subsequently, all volatiles were removed in vacuo. The remaining residue was dissolvedin water (50 mL) and the pH value of the solution was adjusted to a value of 3. It was ex-tracted with diethyl ether (3 × 50 mL), the combined organic phases were dried (sodiumsulfate) and the solvent was removed in vacuo.

N

N N

N

OHO

29C16H16N4O2

MW: 296.33 g/mol

Yield: 0.590 g (1.99 mmol, 43.0 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.23 (s, 6 H, C5-CH 3), 2.35 (s, 6 H, C3-CH 3),3.19 (s, 2 H, C≡CH ), 6.82 (s, 1 H, CbridgeH), 9.86 (s, 1 H, CO2H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.6 (C5-CH3), 12.4 (C3-CH3), 81.6 (Cbridge),103.3 (C≡C), 124.8 (C≡C), 144.7 (C4), 146.7 (C5), 151.4 (C3), 166.0 (CO2H) ppm.

ESI MS: m/z (%) = 333.1 (60) [M+K]+, 172.1 (90) [C7H7N2+K]+.

IR (KBr): ν̃ = 3288 (vs, C≡C-H), 2122 (m, C≡C), 1748 (vs, CO2H), 1562 (s, C=N) cm−1.

195


6.3.16 Synthesis of (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethyl-silyl)ethynylpyrazol-1-yl)methane [HOPhbdmeTMSpzm] (30)

Sodium hydride (0.370 g, 9.24 mmol, 60 % in petrol ether) was suspended in dry tetrahy-drofuran (40 mL) at a temperature of 0 ◦C. The mixture was treated with 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) (1.71 g, 8.91 mmol) and allowed to stir at thistemperature for 30 minutes. After this time, thionyl chloride (0.346 mL, 4.75 mmol) wasadded slowly and the solution was stirred for additional 30 minutes. Then the coolingwas removed and the mixture was treated with salicylaldehyde (1.13 g, 9.24 mmol) andpyridine (0.747 mL, 9.24 mmol). The resulting solution was heated to reflux for 16 hours.Subsequently, excess aldehyde and pyridine were distilled off in a vacuum distillation at105 ◦C. The remaining residue was redissolved in dichloromethane and washed with wa-ter (3 × 50 mL). The organic phase was dried (sodium sulfate) and the volatiles wereremoved in vacuo. The crude product was purified via column chromatography (silica,n-hexane:ethyl acetate 8:2 v/v) to obtain 30 as a colorless solid.

N N

N NSiSi

OH

30C27H36N4OSi2

MW: 488.78 g/mol

Yield: 0.408 g (0.835 mmol, 19 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.23 (s, 18 H, Si(CH3)3), 2.14 (s, 6 H, C3-CH3),2.23 (s, 6 H, C5-CH3), 6.79 (m, 3 H, CAr-H), 7.15 (t, 1 H, 3JH,H = 7.21 Hz, CAr-H), 7.40(s, 1 H, CbridgeH), 9.03 (s, 1 H, OH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 0.15 (Si(CH3)3), 10.6 (C5-CH3), 12.6 (C3-CH3), 72.9 (Cbridge), 96.4 (C≡C), 98.8 (C≡C), 104.0 (C4), 117.8 (Car), 120.0 (Car), 121.1(Car), 129.2 (Car), 131.0 (Car), 143.7 (C5), 150.8 (C3), 154.8 (Car-OH) ppm.

Elemental analysis of C27H36N4OSi2 (488.78 g/mol): calcd. C 66.35, N 11.46, H 7.42;found C 66.37, N 11.08, H 7.64 %.

ESI MS: m/z (%) = 511.23 (100) [M+Na]+, 999.47 (90) [2M+Na]+.

196


IR (KBr): ν̃ = 3143 (br, OH), 2156 (s, C≡C), 1559 (m, C=N), 1249 (s, Si-CH3), 859 (vs,Si-CH3) cm−1.

6.3.17 Synthesis of [MoO2Cl2(HOPhbdmeTMSpzm)] (31)

Molybdenum(VI) dichloride dioxide (29.2 mg, 0.147 mmol) was dissolved in tetrahydro-furan (10 mL). The resulting solution was stirred for ten minutes at room temperature.Subsequently, [HOPhbdmeTMSpzm] (30) (70.0 mg, 0.147 mmol) was added. The reactionmixture turned yellow. After one hour, the solution was filtered and the filtrate wastreated with n-hexane (15 mL). The desired complex precipitated after ten minutes whilestirring and was collected by filtration.

N N

N NSiSi

OH

MoO O

Cl

Cl

31C27H36Cl2MoN4O3Si2MW: 687.64 g/mol

Yield: 27.3 mg (0.0397 mmol, 27 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.23 (s, 18 H, Si(CH3)3), 2.55 (s, 6 H, C3-CH 3),2.75 (s, 6 H, C5-CH 3), 6.85 (t, 3JH,H = 7.5 Hz, 1 H, Car-H), 6.93 (t, 3JH,H = 4.5 Hz, 1 H,Car-H), 7.09 (d, 3JH,H = 9.0 Hz, 1 H, Car-H), 7.30 (s, 1 H, CbridgeH), 7.33 (d, 3JH,H =6.0 Hz, 1 H, CAr-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = −0.05 (Si(CH3)3), 11.0 (C3-CH3), 13.9 (C5-CH3), 70.4 (Cbridge), 93.8 (C≡C), 100.9 (C≡C), 106.3 (C4), 120.6 (Car), 120.7 (Car), 121.8(Car), 128.4 (Car), 132.6 (Car), 142.8 (C3), 156.7 (C5), 157.1 (Car-OH) ppm.

Elemental analysis of C27H36Cl2MoN4O3Si2 (687.64 g/mol): calcd. C 47.16, H 5.28, N8.15; found C 46.11, H 5.27, N 7.41 %.

IR (KBr): ν̃ = 3440 (br, OH), 2160 (s, C≡C), 1559 (w, C=N), 1249 (s, Si-CH3), 941 (m,νsym(Mo=O)), 921 (vs, νasym(Mo=O)), 866 (vs, Si-CH3) cm−1.

197


6.3.18 Synthesis of (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-ethynyl-pyrazol-1-yl)methane (32)

Procedure (i) [HOPhbdmeTMSpzm] (30) (100 mg, 0.210 mmol) was dissolved in a mix-ture of tetrahydrofuran (5 mL) and methanol (10 mL). The resulting solution was treatedwith potassium fluoride (244 mg, 4.20 mmol) and stirred for six hours at room tempera-ture. Afterwards, all volatiles were removed in vacuo and the residue was suspended in asaturated solution of sodium carbonate (10 mL). The aqueous suspension was extractedwith diethyl ether (3 × 30 mL) and the combined organic phases were dried (sodiumsulfate). The solvent was removed and the product was obtained as a white solid.

Yield: 56.4 mg (0.164 mmol, 78 %)

N N

N N

OH

32C21H20N4O

MW: 344.42 g/mol

Procedure (ii) [HOPhbdmeTMSpzm] (30) (50.0 mg, 0.105 mmol) was dissolved in amixture of tetrahydrofuran (5 mL) and methanol (10 mL). The resulting solution wastreated with potassium carbonate (145 mg, 1.05 mmol) and stirred over night at roomtemperature. After this time, all volatiles were removed in vacuo and the residue wasredissolved in dichloromethane (30 mL) and the resulting solution was washed with water(3 × 30 mL). Subsequently, the organic phase was dried over sodium sulfate and thesolvent was removed.

Yield: 27.4 mg (0.0796 mmol, 76 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.18 (s, 6 H, C3-CH 3), 2.23 (s, 6 H, C5-CH 3),3.20 (s, 2 H, C≡C-H), 6.81 (m, 3 H, Car-H), 7.15 (t, 1 H, 3JH,H = 6.0 Hz, Car-H), 7.45 (s,1 H, Cbridge) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.5 (C3-CH3), 12.5 (C5-CH3), 75.3 (C≡C),81.4 (C≡C), 102.7 (C4), 119.8 (Car), 119.9 (Car), 121.0 (Car), 125.5 (Car), 130.9 (Car),

198


144.1 (C3), 150.9 (C5), 154.8 (Car-OH) ppm. Cbridge could not be resolved.

ESI MS: m/z (%) = 367.15 (100) [M+Na]+, 711.32 (26) [2M+Na]+.

IR (KBr): ν̃ = 3430 (br, OH), 3291 (vs, C≡C-H), 2114 (m, C≡C), 1559 (m, C=N) cm−1.

6.3.19 Synthesis of (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazol-1-yl)methane (33)

Sodium hydride (0.370 g, 9.24 mmol, 60 % in petrol ether) was suspended in dry tetrahy-drofuran (40 mL) at a temperature of 0 ◦C. The mixture was treated with 3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazole (28) (1.71 g, 8.91 mmol) and allowed to stir at thistemperature for 30 minutes. After this time, thionyl chloride (0.346 mL, 4.75 mmol) wasadded slowly and the solution was stirred for additional 30 minutes. Then the cooling wasremoved and the mixture was treated with 1-methyl-2-imidazolecarboxaldehyde (1.02 g,9.24 mmol) and pyridine (0.747 mL, 9.24 mmol). The resulting solution was heated toreflux for 16 hours. Subsequently, excess aldehyde and pyridine were distilled off in avacuum distillation at 105 ◦C. The remaining residue was redissolved in dichloromethane(50 mL) and washed with water (3 × 50 mL). The organic phase was dried (sodium sul-fate) and the volatiles were removed in vacuo. The crude product was purified via columnchromatography (silica, n-hexane:ethyl acetate 1:9 v/v) to obtain 33 as a brownish solid.

N N

N NSiSi

NN

33C25H36N6Si2

MW: 476.78 g/mol

Yield: 0.369 g (0.774 mmol, 17 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 0.22 (s, 18 H, Si(CH3)3), 2.17 (s, 6 H, C3-CH3)2.23 (s, 6 H, C5-CH3), 3.33 (s, 3 H, CIm-CH3), 6.93 (d, 1 H, 3JH,H = 0.9 Hz), 7.05 (d, 1H, 3JH,H = 1.0 Hz), 7.48 (s, 1 H, CbridgeH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 0.14 (Si(CH3)3), 10.7 (C5-CH3), 12.6 (C3-CH3), 33.0 (Nimid-CH3), 70.5 (Cbridge), 97.5 (C≡C), 95.5 (C≡C), 104.7 (C4), 123.0 (Cimid),

199


128.2 (Cimid), 140.1 (C5), 144.4 (C3), 150.8 (Cimid) ppm.

Elemental analysis of C25H36N6Si2 (476.78 g/mol): calcd. C 62.98, N 17.63, H 7.61;found C 62.64, N 16.84, H 7.65 %.

ESI MS: m/z (%) = 975 (100) [2M+Na]+, 477 (100) [M+H]+.

IR (KBr): ν̃ = 2156 (s, C≡C), 1559 (w, C=N), 1250 (Si-CH3), 861 (vs, Si-CH3) cm−1.

6.3.20 Synthesis of (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (34)

Procedure (i) (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynyl-pyrazol-1-yl)methane (33) (100 mg, 0.210 mmol) was dissolved in a mixture of tetrahydro-furan (5 mL) and methanol (10 mL). The resulting solution was treated with potassiumfluoride (244 mg, 4.20 mmol) and stirred for six hours at room temperature. Afterwards,all volatiles were removed in vacuo and the residue was suspended in a saturated solutionof sodium carbonate (10 mL). The aqueous suspension was extracted with diethyl ether(3 × 30 mL) and the combined organic phases were dried (sodium sulfate). The solventwas removed and the product was obtained as a white solid.

Yield: 50.9 mg (0.153 mmol, 73 %)

N N

N N

NN

34C19H20N6

MW: 332.41 g/mol

Procedure (ii) (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynyl-pyrazol-1-yl)methane (33) (40.0 mg, 0.0839 mmol) was dissolved in a mixture of tetrahy-drofuran (5 mL) and methanol (10 mL). The resulting solution was treated with potassiumcarbonate (116 mg, 0.839 mmol) and stirred over night at room temperature. After thistime, all volatiles were removed in vacuo and the residue was redissolved in dichloro-

200


methane (30 mL) and the resulting solution was washed with water (3 × 30 mL). Subse-quently, the organic phase was dried (sodium sulfate) and the solvent was removed.

Yield: 22.0 mg (0.0663 mmol, 79 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.21 (s, 6 H, C3-CH 3), 2.24 (s, 6 H, C5-CH 3),3.19 (s, 2 H, C≡C-H), 3.34 (s, 3 H, Nimid-CH 3), 6.93 (s, 1 H, Cimid-H), 7.05 (s, 1 H,Cimid-H), 7.54 (s, 1 H, CbridgeH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.5 (C3-CH3), 12.5 (C5-CH3), 30.0 (Nimid-CH3), 70.2 (Cbridge), 75.3 (C≡C), 81.4 (C≡C), 103.4 (C4), 123.1 (Cimid), 128.0 (Cimid),140.0 (C3), 144.7 (C5), 150.9 (Cimid) ppm.

ESI MS: m/z (%) = 355.16 (100) [M+Na]+, 687.34 (45) [2M+Na]+.


201



6.4.1 Synthesis of 3,5-Dimethyl-4-ethynyl-1-tritylpyrazole (35)

A solution of 3,5-dimethyl-4-(trimethylsilylethynyl)-1-tritylpyrazole (59) (4.00 g,9.20 mmol) in a mixture of tetrahydrofuran and methanol (1:1 v/v) was treated withpotassium carbonate (6.36 g, 46.0 mmol) and stirred over night. Subsequently the solventwas removed in vacuo and the remaining residue was dissolved in dichloromethane (100mL) and water (100 mL). The organic phase was washed with water two more times anddried (sodium sulfate). Finally the solvent was removed in vacuo. The product was usedwithout further purification

N

N

35C26H22N2

MW: 362.48 g/mol

Yield: 2.95 g (8.14 mmol, 89 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 1.37 (s, 3 H, C5-CH3), 2.06 (s, 3 H, C3-CH3),3.02 (s, 1 H, C≡C-H), 6.99 (m, 15 H, Trt) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 12.9 (C5-CH3), 13.8 (C3-CH3), 78.5 (C≡C),81.2 (C≡C), 103.3 (C4), 127.3 (Trt), 127.5 (Trt), 130.3 (Trt), 142.8 (Trt), 145.6 (C3),148.1 (C5) ppm. (C -(C6H5)3) could not be resolved.

Elemental analysis of C26H22N2 (362.48 g/mol): calcd. C 86.15, N 7.73, H 6.12; foundC 85.60, N 7.41, H 6.10 %.

202


6.4.2 Synthesis of 1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadi-yne (36)

To a solution of 4-ethynyl-3,5-dimethyl-1-tritylpyrazole (35) (6.85 g, 18.9 mmol) in a3:1 (v/v) mixture of acetonitrile and pyridine (150 mL) was added copper(II) acetate(2.52 g, 12.6 mmol) and the resulting mixture was heated to reflux for two hours. Afterthis time, the reaction was cooled down to ambient temperature and stirred for another24 hours. Subsequently it was extracted with ethyl acetate (200 mL) and the organic phasewas washed thoroughly with water until the disappearance of the blue copper color. Thesolvent was removed, the residue was washed with acetone to remove remaining impuritiesand the product was dried in vacuo.

N

N

N

N

36C52H42N4

MW: 722.94 g/mol

Yield: 4.70 g (13.0 mmol, 69 %)

1H NMR (300 MHz, CD2Cl2, 25 ◦C): δ = 1.57 (s, 3 H, C5-CH3), 2.25 (s, 3 H, C3-CH3),7.11 (m, 12 H, Trt), 7.29 (m, 18 H, Trt) ppm.

13C NMR (75.5 MHz, CD2Cl2, 25 ◦C): δ = 13.2 (C5-CH3), 14.4 (C3-CH3), 74.8 (C≡C),79.3 (C≡C), 104.0 (C4), 127.9 (Trt), 128.1 (Trt), 130.8 (Trt), 143.22 (Trt), 147.4 (C5),149.3 (C3) ppm. (C -(C6H5)3) could not be resolved.

ESI MS: m/z (%) = 745.33 (100) [M+Na]+, 1468.67 (55) [2M+Na]+.

IR (KBr): ν̃ = 2144 (w, C≡C), 1560 (w, C=N) cm−1.

203


6.4.3 Synthesis of 1,4-Bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne(37)

1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadiyne (36) (1.12 g, 1.55 mmol) was dis-solved in a mixture of water and dichloromethane (1:50 v/v, 102 mL) and treated withtrifluoroacetic acid (0.263 mL, 3.41 mmol). The resulting solution was stirred for 24 hoursat ambient temperature. The product forms a precipitate, which is filtered off and washedthoroughly with water and chloroform before it is dried in vacuo. Crystals suitable for anX-ray structure determination could be obtained by layering a solution of 37 in methanolwith n-hexane.

N

NH

N

HN

37C14H14N4

MW: 238.29 g/mol

Yield: 0.218 g (0.915 mmol, 59 %)

1H NMR (300 MHz, acetone-d6, 25 ◦C): δ = 2.28 (s, 6 H, C5-CH3 and C3-CH3) ppm.

1H NMR (300 MHz, DMSO-d6, 25 ◦C): δ = 2.17 (s, 3 H, C5-CH3), 2.25 (s, 3 H, C3-CH3),12.62 (s, 1 H, NH) ppm.

13C NMR (75.5 MHz, DMSO-d6, 25 ◦C): δ = 10.0 (C5-CH3), 12.5 (C3-CH3), 97.5 (C≡C),98.3 (C≡C), 105.5 (C4), 144.1 (C5), 150.7 (C3) ppm.


ESI MS: m/z (%) = 239.13 (100) [MH]+, 261.11 (30) [M+Na]+.

IR (KBr): ν̃ = 3172 (m, N-H), 2152 (w, C≡C), 1550 (w, C=N), 1243 (vs, C-C) cm−1.

204


6.4.4 Synthesis of Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethylpyrazol-4-yl)butadiyne)) (38/39)

A solution of 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) (38.0 mg, 0.159 mmol)in acetone (20 mL) was added to a solution of cobalt(II) acetylacetonate (40.9 mg,0.159 mmol) in acetone (20 mL). A violet solid immediately precipitated and was collectedby filtration. The crude product was thoroughly washed with acetone and subsequentlydried in vacuum. Crystals suitable for an X-ray structure determination could be obtainedby slow evaporation of a highly diluted solution of 37 and cobalt(II) acetylacetonate (39).

N

N

N

NCo

O O

CoO O

n

38(C24H26Co2N4O4)nMW: 552.36 g/mol

Yield: 35.1 mg (63.5 µmol, 40 %)

Elemental analysis of C24H26Co2N4O4 (552.36): calcd. C 52.00, N 10.11, H 5.09; foundC 52.01, N 10.01, H 4.70 %.

IR (KBr): ν̃ = 2143 (m, C≡C), 1580 (s, acetylacetonate), 1519 (vs, acetylacetonate),1428 (s, acetylacetonate), 1266 (m, acetylacetonate) cm−1

205


6.4.5 Synthesis of Poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (40)

A solution of 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) (38.0 mg, 0.159 mmol)in acetone (20 mL) was added to a solution of cobalt(II) chloride (37.9 mg, 0.159 mmol) inacetone (20 mL). The resulting solution was stirred under reflux for 24 hours. During thistime, a blue precipitate was formed, which was subsequently collected via filtration. Thecrude product was thoroughly washed with acetone and subsequently dried in vacuum.The material was obtained as a microcrystalline powder, which crystallized containingacetone.

N

NH

N

HN

Co

Cl

Cln

xO

0.5

40(C14H14Cl2CoN4)nMW: 368.13 g/mol

Yield: 51.2 mg (0.139 mmol, 87 %)

Elemental analysis of C14H14CoN4 (× 0.5 eq. acetone) (368.13): calcd. C 46.87,N 14.11, H 4.31; found C 46.90, N 14.00, H 4.43 %.

IR (KBr): ν̃ = 3258 (s, N-H), 2151 (m, C≡C), 1567 (s, C=N) cm−1.

206


6.4.6 Attempted synthesis of Poly(cobalt(II)bromido-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (41)

A solution of 1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37) (38.0 mg, 0.159 mmol)in acetone (20 mL) was added to a solution of cobalt(II) bromide (52.0 mg, 0.159 mmol)in acetone (20 mL). The resulting solution was stirred under reflux for 24 hours. Afterthis time, no precipitate was formed.

N

NH

N

HN

Co

Br

Brn

41(C14H14Br2CoN4)nMW: 457.04 g/mol

In an alternative attempt, acetone was exchanged by acetontrile and the resulting solutionstirred for 24 hours under reflux. As before, no precipitate was formed and no hints for asuccessful reaction could be found.

207


6.4.7 Synthesis of 4-Ethynyl-1-tritylpyrazole (42)

4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27) (1.00 g, 2.46 mmol) was dissolved in amixture of tetrahydrofuran and methanol (1:2 v/v, 50 mL) and treated with potassiumfluoride (2.86 g, 49.2 mmol). The resulting solution was stirred for 48 hours at roomtemperature. After this time, all volatiles were removed and the residue was suspended ina saturated solution of sodium bicarbonate. The suspension was extracted with diethylether (3 × 50 mL), the combined organic phases were dried (sodium sulfate) and thesolvent was removed in vacuo.

N

N

42C24H18N2

MW: 334.42 g/mol

Yield: 0.600 g (1.80 mmol, 73 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 3.01 (s, 1 H, C≡C-H), 7.17 (m, 6 H, Trt), 7.35(m, 9 H, Trt), 7.59 (s, 1 H, C3-H), 7.60 (s, 1 H, C5-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 75.4 (C≡C), 78.3 (C≡C), 79.1 (Trt), 100.8(C4), 127.8 (Trt), 127.9 (Trt), 130.1 (Trt), 135.9 (Trt), 142.5 (C3), 142.6 (C5) ppm.


ESI MS: m/z (%) = 243.12 (100) [Trt]+, 357.14 (100) [M+Na]+, 691.28 (72) [2M+Na]+.

IR (KBr): ν̃ = 3284 (m, cch), 2165 (w, C≡C), 1544 (w, C=N) cm−1.

208


6.4.8 Synthesis of 1,4-Bis(1-tritylpyrazol-4-yl)butadiyne (43)

To a solution of 4-ethynyl-1-tritylpyrazole (42) (1.29 g, 3.85 mmol) in a 3:1 v/v mixtureof acetonitrile and pyridine (160 mL) was added copper(II) acetate (1.92 g, 9.63 mmol).The resulting suspension was heated to reflux (85 ◦C) for two hours. After this time,the reaction was allowed to cool to room temperature and the stirring was continued for24 hours. All volatiles were removed and the residue was extracted with dichloromethane(100 mL) in an ultrasonic bath. The organic phase was filtrated, washed with water(4 × 50 mL) and dried (sodium sulfate).

N

N

N

N

43C48H34N4

MW: 666.83 g/mol

Yield: 0.847 g (1.27 mmol, 66 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 7.14 (m, 6 H, Trt), 7.33 (m, 6 H, Trt), 7.58 (s,1 H, C5-H), 7.78 (s, 1 H, C3-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 72.8 (C≡C), 74.8 (C≡C), 79.3 (C -Trt3), 100.8(C4), 127.9 (Trt), 128.0 (Trt), 130.1 (Trt), 136.6 (Trt), 142.4 (C5), 142.9 (C3) ppm.

Elemental analysis of C48H34N4 (666.83 g/mol): calcd. C 86.46, N 8.10, H 5.02; foundC 84.91, N 8.10, H 5.02 %.

ESI MS: m/z (%) = 689.26 (100) [M+Na]+, 1356.54 (10) [2M+Na]+.

IR (KBr): ν̃ = 2151 (w, C≡C), 1539 (s, C=N) cm−1.

209


6.4.9 Attempted Synthesis of 1,4-Bis(1H-pyrazol-4-yl)butadiyne(44)

A mixture of 1,4-bis(1-tritylpyrazol-4-yl)butadiyne (43) (200 mg, 0.300 mmol) in dichloro-methane (10 mL) was treated with boron tribromide (0.301 mg, 1.20 mmol). The resultingsolution was stirred for 24 hours at room temperature. Subsequently, all volatiles wereremoved in vacuo and the remaining residue was extracted with chloroform.

N

NH

N

HN

44C10H6N4

MW: 182.19 g/mol

1H NMR (300 MHz, CD2Cl2, 25 ◦C): δ = 7.31 (s, 4 H) ppm.

13C NMR (75.5 MHz, CD2Cl2, 25 ◦C): δ = 82.8, 127.7, 127.8, 130.6, 145.6 ppm.

IR (KBr): ν̃ = 2208 (w), 1958 (w) cm−1.

210



6.5.1 Synthesis of Bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45)

To a solution of 4-iodo-3,5-dimethylpyrazole (23) (5.00 g, 22.5 mmol) in dichloromethane(250 mL) were added potassium hydroxide (4.55 g, 81.1 mmol), potassium carbonate(11.2 g, 81.1 mmol) and benzyltriethylammonium chloride (0.462 g, 2.03 mmol). Theresulting suspension was heated to reflux for 48 hours. After this time, the solids werefiltered off and the remaining solution was washed with water (3 × 250 mL) before it wasdried (sodium sulfate). The solvent was removed in vacuo.

N

N N

NI I

45C11H14I2N4

MW: 456.06 g/mol

Yield: 2.65 g (5.81 mmol, 52 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.19 (s, 6 H, C3-CH3), 2.47 (s, 6 H, C5-CH3),6.14 (s, 2 H, -CH2-) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 12.2 (C5-CH3), 14.0 (C3-CH3), 62.0 (C4),65.1 (-CH2-), 142.1 (C5), 150.4 (C3) ppm.

Elemental analysis of C11H14I2N4 (456.06 g/mol): calcd. C 28.97, N 12.29, H 3.09;found C 30.39, N 12.39, H 3.24 %.

ESI MS: m/z (%) = 478.20 (100) [M+Na]+, 934.85 (35) [2M+Na]+.

IR (KBr): ν̃ = 1539 (s, C=N) cm−1.

211


6.5.2 Synthesis of Bis(4-iodopyrazol-1-yl)methane (46)

Attempted synthesis To a solution of 4-iodopyrazole (22) (10.0 g, 51.6 mmol) indichloromethane (250 mL) were added potassium hydroxide (10.5 g, 186 mmol), potassiumcarbonate (25.7 g, 186 mmol) and benzyltriethylammonium chloride (1.18 g, 5.16 mmol).The resulting suspension was heated to reflux for 24 hours. After this time, all volatileswere removed and the residue was taken up in ethyl acetate (250 mL) and water (250 mL).The phases were seperated and the organic phase was washed with water (2 × 250 mL)before it was dried (sodium sulfate). The solvent was removed in vacuo. The remainingresidue was thoroughly washed with acetone.While the desired product bis(4-iodopyrazol-1-yl)methane (46) could only be obtained intraces, the acetone phase contained significant amounts of bis(4-iodopyrazol-1-yl)aceticacid (47).

Yield: 0.270 g (0.675 mmol, 3 %)

Successful synthesis To a solution of bis(pyrazol-1-yl)methane (10.0 g, 67.2 mmol) andiodine (20.4 g, 80.4 mmol) in acetonitrile (150 mL) was slowly added cer(IV)ammonium-nitrate (36.8 g, 33.6 mmol). After vigorous stirring for three hours, all volatiles wereremoved in vacuo and the remaining residues were dissolved in ethyl acetate (150 mL)and water (150 mL). Subsequently, a saturated solution of sodium thiosulfate was slowlyadded until the iodine color disappeared. The phases were separated and the organicphase was washed with brine (2 × 150 mL) and dried (sodium sulfate). The solvent wasremoved in vacuo and the obtained compound was dried in vacuum.

N

N N

NII

46C7H6I2N4

MW: 399.96 g/mol

Yield: 21.0 g (52.4 mmol, 78 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 6.24 (s, 2 H, -CH2-), 7.56 (s, 2 H, C3H), 7.69(s, 2 H, C5H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 58.6 (C4), 65.3 (-CH2-), 134.0 (C3), 147.1(C5) ppm.

Elemental analysis of C7H6I2N4 (399.96 g/mol): calcd. C 21.02, N 14.01, H 1.51; found

212


C 20.61, N 13.41, H 1.51 %.

ESI MS: m/z (%) = 398.86 (100) [M-H]−.

IR (KBr): ν̃ = 1513 (m, C=N) cm−1.

6.5.3 Synthesis of Bis(4-iodopyrazol-1-yl)acetic acid (47)

A solution of 4-iodopyrazole (22) (4.80 g, 24.1 mmol) in tetrahydrofuran (150 mL) wastreated with potassium hydroxide (3.97 g, 74.8 mmol), potassium carbonate (10.3 g,74.8 mmol), benzyltriethylammonium chloride (0.593 g, 2.50 mmol) and dibromoaceticacid (2.72 g, 12.5 mmol). The suspension was heated to reflux and stirred over night.Afterwards, all volatiles were removed in vacuo and the remaining residue was redis-solved in water (100 mL). The resulting solution was neutralized with half concentratedhydrochloric acid and extracted with diethyl ether (3 × 100 mL) to remove impurities.Subsequently, the aqueous phase was acidified to pH 1 and once again extracted withdiethyl ether (3 × 100 mL). The combined organic phases were dried (sodium sulfate)and the solvent was removed in vacuo.

N

N N

NII

O OH

47C8H6I2N4O2

MW: 443.97 g/mol

Yield: 2.76 g (6.22 mmol, 50 %)

1H NMR (300 MHz, CD3CN, 25 ◦C): δ = 7.22 (s, 1 H, CbridgeH), 7.60 (s, 2 H, C5-H),7.93 (s, 2 H, C3-H) ppm.

13C NMR (75.5 MHz, CD3CN, 25 ◦C): δ = 59.4 (C4), 75.3 (Cbridge), 136.6 (C5), 147.2(C3), 165.6 (-CO2H) ppm.

Elemental analysis of C8H6I2N4O2 (443.97 g/mol): calcd. C 21.64, N 12.62, H 1.36;found C 22.41, N 12.75, H 1.53 %.

ESI MS: m/z (%) = 398.86 (80) [M-CO2H]−, 442.85 (100) [M-H]−.

IR (KBr): ν̃ = 1719 (s, CO2H), 1517 (m, C=N) cm−1.

213


6.5.4 Synthesis of Bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid(48)

A solution of 4-iodo-3,5-dimethylpyrazole (23) (15.0 g, 67.5 mmol) in tetrahydrofuran(300 mL) was treated with potassium hydroxide (13.7 g, 243 mmol), potassium carbonate(33.6 g, 243 mmol), benzyltriethylammonium chloride (1.39 g, 6.09 mmol) and dibro-moacetic acid (7.38 g, 33.9 mmol). The suspension was heated to reflux and stirred overnight. Afterwards, all volatiles were removed in vacuo and the remaining residue wasredissolved in water (100 mL). The resulting solution was extracted with diethyl ether (3× 100 mL) to remove impurities. Subsequently, the aqueous phase was acidified to pH 2and once again extracted with diethyl ether (3 × 100 mL). The combined organic phaseswere dried (sodium sulfate) and the solvent was removed in vacuo. The product was usedwithout further purification.

N

N N

NI I

OHO

48C12H14I2N4O2

MW: 500.07 g/mol

Yield: 10.2 g (20.4 mmol, 60 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.24 (s, 6 H, C3-CH3), 2.26 (s, 6 H, C5-CH3)7.00 (s, 1 H, CbridgeH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 12.3 (C3-CH3), 14.1 (C5-CH3), 67.1 (C4),71.9 (Cbridge), 143.1 (C3), 151.3 (C5), 164.5 (CO2H) ppm.

214


6.5.5 Synthesis of Methyl bis(4-iodopyrazol-1-yl)acetate (49)

A solution of bis(4-iodopyrazol-1-yl)acetic acid (60) (1.00 g, 2.25 mmol) in methanol(100 mL) was treated with concentrated sulfuric acid (0.160 mL, 2.86 mmol). The reactionmixture was stirred for 72 hours at room temperature. After this time, the solvent wasdistilled off and the residue redissolved in diethyl ether (100 mL). The solution was washedwith a saturated solution of sodium bicarbonate (3 × 100 mL) and dried afterwards(sodium sulfate). Subsequently all volatiles were removed in vacuo.

N

N N

NII

O O

49C9H8I2N4O2

MW: 458.00 g/mol

Yield: 426 mg (0.930 mmol, 41 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 3.89 (s, 3 H, CH3), 7.00 (s, 1 H, CbridgeH), 7.60(s, 2 H, C3-H), 7.80 (s, 2 H, C5-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 54.1 (CH3), 59.1 (C4), 74.3 (Cbridge), 134.5(C3), 146.2 (C5), 163.9 (CO2) ppm.


ESI MS: m/z (%) = 480.86 (100) [M+Na]+, 938.73 (10) [2M+Na]+.

IR (KBr): ν̃ = 1756 (vs, CO2H), 1513 (m, C=N) cm−1.

215


6.5.6 Synthesis of Methyl bis(4-iodopyrazol-1-yl)acetate (50)

A solution of bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid (48) (1.18 g, 2.36 mmol)was dissolved in methanol (150 mL) and treated with concentrated hydrochloric acid(1.00 mL). The resulting mixture was stirred for 72 hours at room temperature. Subse-quently all volatiles were removed in vacuo and the remaining residue was redissolved indiethyl ether (100 mL) and washed with a saturated solution of sodium bicarbonate (3 ×100 mL). The organic phase was dried (sodium sulfate) and the solvent was distilled off.

N

N N

NI I

OO

50C13H16I2N4O2

MW: 514.10 g/mol

Yield: 0.622 g (1.21 mmol, 51 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.21 (s, 6 H, C3-CH3), 2.23 (s, 6 H, C5-CH3)3.89 (s, 3 H, CO2-CH3), 7.01 (s, 1 H, CbridgeH) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 12.3 (C3-CH3), 14.1 (C5-CH3), 53.7 (CO2-CH3), 68.0 (C4), 74.0 (Cbridge), 142.7 (C3), 150.7 (C5), 164.5 (CO2) ppm.


ESI MS: m/z (%) = 536.93 (100) [M+Na]+, 1050.87 (45) [2M+Na]+.

IR (KBr): ν̃ = 1764 (vs, CO2H), 1550 (m, C=N) cm−1.

216


6.5.7 Synthesis of Bis(4-ethynylferrocenylpyrazol-1-yl)methane (51)

A solution of bis(4-iodopyrazol-1-yl)methane (46) (100 mg, 0.275 mmol) in tetrahydro-furan (50 mL) was treated with bis(triphenylphosphine)palladium dichloride (19.3 mg,0.0275 mmol), copper(I) iodide (5.24 mg, 0.0275 mmol) and ethynylferrocene (116 mg,0.550 mmol). After the addition of triethylamine (10 mL), the solution was heated to60 ◦C and stirred at this temperature for 48 hours. After this time, all volatiles weredistilled off and the remaining residue was purified via column chromatography (silica,n-hexane:ethyl acetate 9:1 v/v). The second colored fraction yielded 51 as an orange pow-der after removal of the solvent. Crystals suitable for an X-ray structure determinationcould be obtained by slowly evaporating a solution of 51 in ethyl acetate and n-hexane.

N N

N N

FeFe

51C31H24Fe2N4

MW: 564.25 g/mol

Yield: 90.0 mg (0.160 mmol, 58 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 4.22 (s, 4 H, C5H4), 4.23 (s, 10 H, Cp), 4.45(s, 4 H, C5H4), 6.23 (s, 2 H, -CH2-), 7.66 (s, 2 H, C3), 7.78 (s, 2 H, C5) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 65.1 (CFc-C≡C or -CH2-), 65.5 (CFc-C≡C or-CH2-), 68.7 (C5H4), 69.9 (Cp), 71.2 (C5H4), 75.6 (C≡C), 89.4 (C≡C), 105.9 (C4), 131.8(C5), 143.2 (C3) ppm.

Elemental analysis of C31H24Fe2N4 (564.25 g/mol): calcd. C 65.99, N 9.93, H 4.29;found C 66.19, N 9.58, H 4.33 %.

ESI MS: m/z (%) = 564.07 (25) [M]+, 565.08 (22) [MH]+, 587.06 (100) [M+Na]+.

IR (KBr): ν̃ = 2225 (w, C≡C), 1764 (vs, CO2H), 1563 (m, C=N) cm−1.

217


6.5.8 Synthesis of Methyl bis(4-ethynylferrocenylpyrazol-1-yl)aceta-te (52)

A solution of methyl bis(4-iodopyrazol-1-yl)acetate (49) (119 mg, 0.238 mmol) in tetrahy-drofuran (20 mL) was treated with bis(triphenylphosphine)palladium dichloride (16.7 mg,0.0238 mmol), copper(I) iodide (4.53 mg, 0.0238 mmol) and ethynylferrocene (100 mg,0.476 mmol). After the addition of triethylamine (5 mL), the solution was heated to60 ◦C and stirred at this temperature for 24 hours. After this time, all volatiles weredistilled off and the remaining residue was purified via column chromatography (silica,n-hexane:ethyl acetate 7:3 v/v). The second colored fraction yielded 51 as an orangepowder after removal of the solvent.

N N

N N

FeFe

OO

52C33H26Fe2N4O2

MW: 622.29 g/mol

Yield: 44.0 mg (70.7 µmol, 30 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 3.91 (s, 3 H, -CO2CH3), 4.23 (s, 4 H, C5H4),4.24 (s, 10 H, Cp), 4.46 (s, 4 H, C5H4), 6.99 (s, 1 H, CbridgeH), 7.70 (s, 2 H, C3H), 7.88(s, 2 H, C5H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 54.0 (CO2-CH3), 68.8 (C5H4), 69.9 (Cp),71.3 (C5H4), 74.6 (Cbridge), 75.4 (CFc-C≡C), 82.8 (C≡C), 106.2 (C≡C), 132.3 (C4), 143.4(C3), 146.8 (C5), 164.0 (CO2) ppm.

Elemental analysis of C33H26Fe2N4O2 (622.29 g/mol): calcd. C 63.69, N 9.00, H 4.21;C 61.71, N 8.93, H 4.26 %.

ESI MS: m/z (%) = 622.07 (20) [M]+, 623.08 (15) [MH]+, 645.06 (100) [M+Na]+, 661.04(50) [M+K]+.

IR (KBr): ν̃ = 2225 (w, C≡C), 1566 (m, C=N) cm−1.

218


6.5.9 Synthesis of Bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)methane (53)

A solution of bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15) (160 mg,0.439 mmol) and 4-(ferrocenyl)-phenylacetylene (400 mg, 1.76 mmol) in a mixture ofdimethylformamide (30 mL) and triethylamine (10 mL) was treated with bis(triphenyl-phosphine)palladium dichloride (30.8 mg, 0.0439 mmol) and copper(I) iodide (8.36 mg,0.0439 mmol). The reaction was heated to 65 ◦C and stirred for 24 hours. After thistime, all volatiles were distilled off and the remaining residue was purified via columnchromatography (silica, 1. n-hexane:ethyl acetate 8:2 v/v, 2. n-hexane:ethyl acetate 7:3v/v). The second colored fraction yielded 61 as a brownish oil after removal of the sol-vent. By precipitation from dichloromethane with n-pentane, the desired product couldbe obtained as an orange powder.

N

NN

N

FeFe

53C47H40Fe2N4

MW: 772.55 g/mol

Yield: 39.0 mg (51.0 µmol, 5.80 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.31 (s, 6 H, C3-CH3), 2.57 (s, 6 H, C5-CH3),4.05 (s, 10 H, Cp), 4.35 (s, 4 H, C5H4), 4.67 (s, 4 H, C5H4), 6.09 (s, 2 H, -CH2-), 7.39 (d,2 H, 3JH,H = 8.29 Hz, phenylene), 7.44 (d, 2 H, 3JH,H = 8.48 Hz, phenylene) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.8 (C3-CH3), 12.6 (C5-CH3), 61.0 (-CH2-),66.5 (C5H4), 69.3 (C5H4), 69.7 (Cp), 80.2 (CFc-C6H4), 84.3 (C≡C), 93.5 (C≡C), 103.7(C4), 120.8 (phenylene), 125.8 (phenylene), 131.3 (phenylene), 139.4 (phenylene), 143.2(C3), 150.6 (C5) ppm.

Elemental analysis of C47H40N4Fe2 (772.54): calcd. C 73.07, H 5.22, N 7.25; found C71.18, H 4.98, N 5.82 g/mol.

ESI MS:m/z (%) = 772.19 (24) [M]+, 773.20 (21) [MH]+, 795.18 (100) [M+Na]+, 1568.38(17) [2M+Na]+.

219


IR (CH2Cl2): ν̃ = 2213 (w, C≡C), 1523 (m, cn) cm−1.

6.5.10 Synthesis of Methyl bis((4-ethynyl-1-ferrocenylphen-4-yl)pyrazol-1-yl)acetate (54)

A solution of methyl bis(4-iodopyrazol-1-yl)acetate (49) (150 mg, 0.341 mmol) and 4-(ferrocenyl)-phenylacetylene (292 mg, 1.02 mmol) in a mixture of dimethylformamide(30 mL) and triethylamine (10 mL) was treated with bis(triphenylphosphine)palladiumdichloride (23.9 mg, 0.0341 mmol) and copper(I) iodide (6.49 mg, 0.0341 mmol). Thereaction was heated to 65 ◦C and stirred for 24 hours. After this time, all volatiles weredistilled off and the remaining residue was purified via column chromatography (silica, 1.n-hexane:ethyl acetate 8:2 v/v, 2. n-hexane:ethyl acetate 7:3 v/v). The second coloredfraction yielded 62 as a brownish oil after removal of the solvent. By precipitation fromdichloromethane with n-pentane, the desired product could be obtained as an orangepowder.

N

N N

N

OO

Fe Fe

54C45H34Fe2N4O2

MW: 774.48 g/mol

Yield: 95.3 mg (0.123 mmol, 24 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 3.93 (s, 3 H, CO2-CH3), 4.05 (s, 10 H, Cp), 4.36(s, 4 H, C5H4), 4.66 (s, 4 H, C5H4), 7.02 (s, 1 H, -CH-), 7.40 (d, 2 H, 3JH,H = 8.10 Hz,phenylene), 7.44 (d, 2 H, 3JH,H = 8.48 Hz, phenylene), 7.75 (s, 2 H, C3-H), 7.95 (s, 2 H,C5-H) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 30.0 (CO2-CH3), 54.1 (-CH-), 66.5 (C5H4),68.5 (CFc-C6H4), 69.3 (C5H4), 69.7 (Cp), 82.9 (C≡C), 105.9 (C≡C), 120.1 (C4), 125.8(phenylene), 131.4 (phenylene), 132.6 (phenylene), 135.7 (phenylene), 139.9 (C3), 143.4(C5), 156.7 (CO2) ppm.

Elemental analysis of C45H34Fe2N4O2 (774.48 g/mol): calcd. C 69.79, H 4.43, N 7.23;

220


found C 67.92, H 4.24, N 6.30.

ESI MS:m/z (%) = 774.14 (31) [M]+, 775.14 (17) [MH]+, 797.13 (100) [M+Na]+, 1572.27(11) [2M+Na]+.

IR (KBr): ν̃ = 2143 (w, C≡C), 1717 (vs, CO2H), 1522 (w, C=N) cm−1.

6.5.11 Synthesis of Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (55)

Bis(4-ethynylpyrazol-1-yl)methane (15) (50.0 mg, 0.198 mmol) and ferrocene azide(90.0 mg, 0.396 mmol) were dissolved in tetrahydrofuran (30 mL). To this mixture wasadded a solution of copper sulfate (24.8 mg, 9.92 µmol) and sodium ascorbate (78.6 mg,0.396 mmol) in water (30 mL). The resulting mixture was stirred for 72 hours. After thistime, the solution was extracted with ethyl acetate (3 × 50 mL), the combined organicphases were dried (sodium sulfate) and the solvent removed in vacuo. The crude productwas purified via column chromatography (silica, n-hexane:ethyl acetate 7:3 v/v).

N

N N

N

N

N N N N

N FeFe

55C35H34Fe2N10

MW: 706.42 g/mol

Yield: 34.0 mg (48.1 µmol, 24 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.37 (s, 6 H, C3-CH3), 2.73 (s, 6 H, C5-CH3),4.23 (s, 10 H, Cp), 4.28 (s, 4 H, C5H4), 4.87 (s, 4 H, C5H4), 6.25 (s, 2 H, -CH2-), 7.69 (s,2 H, triazole) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 11.0 (C3-CH3), 13.4 (C5-CH3), 60.8 (-CH2-),62.1 (C5H4), 66.7 (C5H4), 70.1 (Cp), 93.7 (CFc-triazole), 109.7 (triazole), 119.7 (triazole),139.0 (Cpz), 141.1 (Cpz), 146.8 (Cpz) ppm.

ESI MS: m/z (%) = 707.17 (30) [MH]+.

IR (KBr): ν̃ = 1526 (s, C=N) cm−1.

221


6.5.12 Synthesis of Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (56)

Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (15) (0.100 g, 0.396 mmol) and azido-methyl ferrocene (0.210 g, 0.871 mmol) were dissolved in a mixture of dichloromethane(10 mL) and methanol (25 mL). To this mixture were added solutions of copper sulfate(19.8 mg, 7.92 µmol) in water (1.5 mL) and sodium ascorbate (62.8 mg, 0.317 mmol) inwater (1.5 mL). The resulting mixture was stirred for 72 hours. After this time, the solventwas removed and the remaining residue was redissolved in water and dichloromethane (1:1v/v, 50 mL). The aqueous phase was extracted with dichloromethane (3 × 50 mL) andthe combined organic phases were again washed with water (1 × 50 mL). Afterwards, theywere dried (sodium sulfate) and the solvent was removed in vacuo. The crude productwas purified via column chromatography (silica, 1. n-hexane:ethyl acetate 7:3 v/v, 2.n-hexane:ethyl acetate 3:7 v/v). The fourth colored fraction yielded the desired productas yellow powder after removal of the solvent. Crystals suitable for an X-ray structuredetermination could be obtained by layering a solution of 56 in ethyl acetate with n-hexane.

N

N N

N

N

N N N N

N

FeFe

56C37H38Fe2N10

MW: 734.47 g/mol

Yield: 69.0 mg (93.8 µmol, 24 %)

1H NMR (300 MHz, CDCl3, 25 ◦C): δ = 2.25 (s, 6 H, C3-CH3), 2.60 (s, 6 H, C5-CH3),4.16 (s, 10 H, Cp), 4.20 (s, 4 H, C5H4), 4.27 (s, 4 H, C5H4), 5.30 (s, 4 H, CFc-CH2), 6.15(s, 2 H, -CH2-), 7.37 (s, 2 H, triazole) ppm.

13C NMR (75.5 MHz, CDCl3, 25 ◦C): δ = 10.8 (C3-CH3), 13.1 (C5-CH3), 49.9 (CFc-CH2), 60.8 (-CH2-), 68.7 (C5H4), 68.8 (Cp), 68.9 (C5H4), 81.1 (CFc-CH2), 109.9 (triazole),119.7 (triazole), 138.7 (Cpz), 140.8 (Cpz), 146.7 (Cpz) ppm.

Elemental analysis of C37H38N10Fe2 (734.47 g/mol): calcd. C 60.42, H 5.34, N 19.04;found C 59.54, H 5.36, N 18.42 %.

222


ESI MS: m/z (%) = 735.21 (10) [MH]+, 757.19 [M+Na]+, 1492.40 (30) [2M+Na]+.

IR (KBr): ν̃ = 1522 (m, C=N) cm−1.

223


6.6 Oxygen Atom Transfer CatalysisAn evacuated Schlenk tube was flushed with nitrogen and charged with triphenylphos-phine (0.393 g, 1.50 mmol, 200 eq.) and degased dimethyl sulfoxide (10.0 mL). Thecatalyst (0.05 mol%) was added to the resulting solution and the mixture was stirred for6 or 24 hours. The samples were analyzed by 1H and 31P NMR spectroscopy. Calculationof yields was done via integration of the OPPh3 signal of the 31P NMR spectra.

Catalyst t neduct (t0) ncatalyst neduct (t) nproduct (t) y TON TOF[h] [mmol] [µmol] [mmol] [mmol] [%] [10−5 s−1]

- 24 1.50 7.50 1.50 0 0 0 03 6 1.50 7.50 0.173 0.220 56 112 5195 6 1.50 7.50 0.177 0.216 55 110 509

P10 24 1.50 7.50 0.0511 0.342 87 174 201P11 24 1.50 7.50 0.3895 0.0944 1 2 2.31P12 24 1.50 7.50 0.252 0.142 36 72 83.3P13 24 1.50 7.50 0.299 0.0944 24 48 55.6

P6-Mo 24 1.50 7.50 0.0433 0.350 89 178 206P7-Mo 24 1.50 7.50 0.114 0.279 71 142 164P8-Mo 24 1.50 7.50 0.220 0.173 44 88 102P9-Mo 24 1.50 7.50 0.362 0.0315 8 16 18.5

Table 6.1: Results of the catalytic reduction of dimethyl sulfoxide.

The results of the catalytic studies are depicted in table 6.1. For reference, the compositionof the deployed catalysts is depicted in table 6.2.




224

Appendix

225

Appendix

A Details of Structure Determinations

[MoO2Cl2(bdmvpzm)] (3)Empirical formula C15H20Cl2MoN4O2

Formula mass [g mol−1] 455.19Crystal color/habit yellow blockCrystal system monoclinicSpace group P21/a

a [Å] 13.6299(8)b [Å] 15.0051(15)c [Å] 8.9003(10)α [◦] 90β [◦] 90.026(9)γ [◦] 90V [Å3] 1820.3(3)θ [◦] 6 to 20h −18 to 18k −20 to 20l −12 to 12F (000), Z 920, 4µ(Mo-Kα) [mm−1] 0.71069Crystal size [mm] 0.16× 0.11× 0.10Dcalcd. [g cm−1], T [K] 1.661, 150Reflections collected 58192Independent reflections 5066Obs. reflections, I > 2σI 4720Parameter 222Weight parameter a 0.0369Weight parameter b 0.0869R1 (observed) 0.0214R1 (overall) 0.0255wR2 (observed) 0.0624wR2 (overall) 0.0647Diff. hole / peak [eÅ] −1.254 / 0.372

Table A.1: Details for the structure determination of [MoO2Cl2(bdmvpzm)] (3).

226


1,4-Bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37)Empirical formula 2(C14H14N4) × H2OFormula mass [g mol−1] 494.60Crystal color/habit colorless blockCrystal system monoclinicSpace group P21/c

a [Å] 12.7184(14)b [Å] 7.1608(7)c [Å] 14.5737(6)α [◦] 90β [◦] 106.123(6)γ [◦] 90V [Å3] 1275.08(19)θ [◦] 6 to 20h −16 to 16k −9 to 9l −18 to 18F (000), Z 524, 2µ(Mo-Kα) [mm−1] 0.71073Crystal size [mm] 0.40× 0.34× 0.10Dcalcd. [g cm−1], T [K] 1.288, 150Reflections collected 24081Independent reflections 2916Obs. reflections, I > 2σI 2373Parameter 180Weight parameter a 0.0689Weight parameter b 0.5837R1 (observed) 0.0483R1 (overall) 0.0624wR2 (observed) 0.1390wR2 (overall) 0.1483Diff. hole / peak [eÅ] −0.322 / 0.091

Table A.2: Details for the structure determination of 1,4-Bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne (37).

227

Appendix

Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-di-methyl-1H -pyrazol-4-yl)butadiyne)) (38)

Empirical formula 2(C12H14Co0.5N2O2) × 3 H2OFormula mass [g mol−1] 545.46Crystal color/habit blue plateCrystal system triclinicSpace group P − 1a [Å] 8.743(3)b [Å] 13.092(3)c [Å] 13.163(9)α [◦] 90.25(6)β [◦] 97.29(3)γ [◦] 97.42(4)V [Å3] 1481.6(12)θ [◦] 2.948 to 27.409h −11 to 11k −16 to 16l −17 to 17F (000), Z 571, 2µ(Mo-Kα) [mm−1] 0.71073Crystal size [mm] 0.40× 0.34× 0.10Dcalcd. [g cm−1], T [K] 1.2225, 150Reflections collected 40883Independent reflections 6782Obs. reflections, I > 2σI 3793Parameter 347Weight parameter a 0.1040Weight parameter b 1.6398R1 (observed) 0.0731R1 (overall) 0.1372wR2 (observed) 0.1836wR2 (overall) 0.2336Diff. hole / peak [eÅ] −0.6583 / 0.9840

Table A.3: Details for the preliminary structure determination of poly(cobalt(II)acetyl-acetonato-bis(1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)butadiyne)) (38).

228


Bis(4-ethynylferrocenylpyrazol-1-yl)-methane (51)

Empirical formula C31H24Fe2N4

Formula mass [g mol−1] 564.24Crystal color/habit brown blockCrystal system I b a 2Space group orthorhombica [Å] 13.7913(3)b [Å] 18.9209(4)c [Å] 9.25359(18)α [◦] 90β [◦] 90γ [◦] 90V [Å3] 2414.67(9)θ [◦] 3.966 to 73.536h −15 to 16k −21 to 23l −11 to 11F (000), Z 1160, 4µ(Cu-Kα) [mm−1] 1.54184Crystal size [mm] 0.4959× 0.2257× 0.2257Dcalcd. [g cm−1], T [K] 1.552, 150Reflections collected 3941Independent reflections 1916Obs. reflections, I > 2σI 1834Parameter 168Weight parameter a 0.1000Weight parameter b 0R1 (observed) 0.0302R1 (overall) 0.0320wR2 (observed) 0.0867wR2 (overall) 0.0897Diff. hole / peak [eÅ] −0.349 / 0.478

Table A.4: Details for the structure determination of bis(4-ethynylferrocenylpyrazol-1-yl)methane (51).

229

Appendix

Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-tri-azol-4-yl)pyrazol-1-yl)methane (56)

Empirical formula 2(C37H38Fe2N10) × H2OFormula mass [g mol−1] 1486.96Crystal color/habit yellow blockCrystal system triclinicSpace group P − 1a [Å] 7.630(2)b [Å] 10.4620(19)c [Å] 21.610(6)α [◦] 92.53(2)β [◦] 98.591(18)γ [◦] 91.61(2)V [Å3] 1703.0(7)θ [◦] 2.73 to 25.19h −9 to 9k −12 to 12l −25 to 25F (000), Z 774, 1µ(Mo-Kα) [mm−1] 0.71073Crystal size [mm] 0.18× 0.13× 0.12Dcalcd. [g cm−1], T [K] 1.450, 150Reflections collected 36705Independent reflections 6114Obs. reflections, I > 2σI 4006Parameter 455Weight parameter a 0.0715Weight parameter b 10.2974R1 (observed) 0.0854R1 (overall) 0.1347wR2 (observed) 0.1979wR2 (overall) 0.2332Diff. hole / peak [eÅ] −1.209 / 0.12

Table A.5: Details for the structure determination of bis(3,5-dimethyl-4-(1-methyl-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (56).

230

B Abbreviations

B Abbreviations

∆ Heat

δ Chemical shift in ppm

ν̃ Wavenumber

n-BuLi n-Butyllithium

iPr iso-Propyl-

tBu tert-Butyl-

J Scalar coupling constant in Hz

AAS Atomic absorption spectroscopy

AIBN Azobisisobutyronitrile

bdmfcmtpzm Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)-pyrazol-1-yl)methane

bdmfctpzm Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)-pyrazol-1-yl)methane

bdmpza Bis(3,5-dimethylpyrazol-1-yl)acetate

bdmvpzm Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane

bdtbpzm Bis(3,5-tert-butylpyrazol-1-yl)methane

bedmpzm Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane

befcpzm Bis(4-ethynylferrocenylpyrazol-1-yl)methane

bepfcdmpzm Bis((4-ethynyl-1-ferrocenylphen-4-yl)-3,5-dimethylpyrazol-1-yl)methane

bmip Bis(N -methylimidazol-2-yl)butadiyne

Boc tert-Butyloxycarbonyl

br Broad

BTEAC Benzyltriethylammonium chloride

231

Appendix

Bu Butyl

calcd. Calculated

conc. Concentrated

cPMP Cyclic pyranopterin monophosphate

CuAAC Copper(I)-catalyzed azide-alkyne cycloaddition

cyt c Ferricytochrome c

d Doublet

DMF N,N -Dimethylformamid

DMS Dimethyl sulfide

DMSO Dimethyl sulfoxide

E1/2 Half wave potential

EA Elemental Analysis

EGDMA Ethylene glycol dimethacrylate

ESI Electrospray ionization

FAB Fast atom bombardment

Fc Ferrocene/ferrocenyl-

fig. Figure

Hbdmvpza Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acidic acid

HOPhbdmeTMSpzm (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethy-nylpyrazol-1-yl)methane

Hz Hertz

ICP-AES Inductively coupled plasma atomic emission spectroscopy

IR Infrared spectroscopy

K[pz] Potassium 3,5-di-tert-butylpyrazolate

m Multiplet

232

B Abbreviations

m/z Ratio of mass to charge

mbefcpzac Methyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate

mbepfcpzac Methyl bis((4-ethynyl-1-ferrocenylphen-4-yl)pyrazol-1-yl)-acetate

MMA Methyl methacrylate

Mo-co Molybdenum cofactor

MoCD Molybdenum cofactor deficiency

MOF Metal organic framework

MS Mass spectrometry

NBS N -bromosuccinimide

NMR Nuclear magnetic resonance

OAT Oxygen atom transfer

PEG Polyethylene glycol

PMMA Poly(methyl methacrylate)

ppm Parts per million

Ptr Pterin

pybut [1,4-Bis(4-pyridyl)butadiyne]

pyphe [1,4-Bis(4-pyridylethynyl)phenylene]

pz Pyrazolyl-

RT Room temperature

S Substrate

s Singlet

SHE Standard hydrogen electrode

SO Sulfite oxidase

t Triplet

233

Appendix

THF Tetrahydrofuran

TOF Turnover frequency

TON Turnover number

Trt Triphenylmethyl-/Trityl-

y Yield

234

C List of Compounds

C List of Compounds

- Bis(3,5-dimethyl-4-formylpyrazol-1-yl)methane (1)- Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)methane (bdmvpzm) (2)- [MoO2Cl2(bdmvpzm)] (3)- Bis(3,5-dimethyl-4-vinylpyrazol-1-yl)acetic acid (Hbdmvpza) (4)- [MoO2Cl(bdmvpza)] (5)- MMA copolymer of 2 (P6)- EGDMA copolymer of 2 (P7)- MMA copolymer of 4 (P8)- EGDMA copolymer of 4 (P9)- Molybdenum(VI) containing MMA copolymer (P6-Mo)- Molybdenum(VI) containing EGDMA copolymer (P7-Mo)- Molybdenum(VI) containing MMA copolymer (P8-Mo)- Molybdenum(VI) containing EGDMA copolymer (P9-Mo)- MMA copolymer of complex 3 (P10)- EGDMA copolymer of complex 3 (P11)- MMA copolymer of complex 5 (P12)- EGDMA copolymer of complex 5 (P13)- Bis(4-(2,2-dibromovinyl)-3,5-dimethylpyrazol-1-yl)methane (14)- Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)methane (bedmpzm) (15)- Bis(4-trimethylsilyl-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (16)- [CuI(bedmpzm)] (17)- [ZnCl2(bedmpzm)] (18)- [MnCl2(bedmpzm)] (19)- [CoCl2(bedmpzm)] (20)- [MoO2Cl2(bedmpzm)] (21)- 4-Iodopyrazole (22)- 4-Iodo-3,5-dimethylpyrazole (23)- 4-Iodo-1-tritylpyrazol (24)- 4-Iodo-3,5-dimethyl-1-tritylpyrazole (25)- 3,5-Dimethyl-4-(trimethylsilyl)ethynyl-1-tritylpyrazole (26)- 4-(Trimethylsilyl)ethynyl-1-tritylpyrazole (27)- 3,5-Dimethyl-4-(trimethylsilyl)ethynylpyrazole (28)

235

Appendix

- 2,2-Bis(4-ethynyl-3,5-dimethylpyrazol-1-yl)acetic acid (29)- (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazol-1-yl)me-thane [HOPhbdmeTMSpzm] (30)

- [MoO2Cl2(HOPhbdmeTMSpzm)] (31)- (2-Hydroxyphenyl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (32)- (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-(trimethylsilyl)ethynylpyrazol-1-yl)me-thane (33)

- (1-Methylimidazol-2-yl)-bis(3,5-dimethyl-4-ethynylpyrazol-1-yl)methane (34)- 3,5-Dimethyl-4-ethynyl-1-tritylpyrazole (35)- 1,4-Bis(3,5-dimethyl-1-tritylpyrazole-4-yl)butadiyne (36)- 1,4-Bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne (37)- Poly(cobalt(II)acetylacetonato-bis(1,4-bis(3,5-dimethylpyrazol-4-yl)buta-diyne)) (38/39)

- Poly(cobalt(II)chlorido-bis(1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne)) (40)- Poly(cobalt(II)bromido-bis(1,4-bis(3,5-dimethyl-1H -pyrazol-4-yl)butadiyne)) (41)- 4-Ethynyl-1-tritylpyrazole (42)- 1,4-Bis(1-tritylpyrazol-4-yl)butadiyne (43)- 1,4-Bis(1H -pyrazol-4-yl)butadiyne (44)- Bis(4-iodo-3,5-dimethylpyrazol-1-yl)methane (45)- Bis(4-iodopyrazol-1-yl)methane (46)- Bis(4-iodopyrazol-1-yl)acetic acid (47)- Bis(4-iodo-3,5-dimethylpyrazol-1-yl)acetic acid (48)- Methyl bis(4-iodopyrazol-1-yl)acetate (49)- Methyl bis(4-iodopyrazol-1-yl)acetate (50)- Bis(4-ethynylferrocenylpyrazol-1-yl)methane (51)- Methyl bis(4-ethynylferrocenylpyrazol-1-yl)acetate (52)- Bis((4-ethynyl-(1-(ferrocenyl)-phen-4-yl))-3,5-dimethylpyrazol-1-yl)methane (53)- Methyl bis(4-ethynyl-(1-(ferrocenyl)-phen-4-yl)pyrazol-1-yl)acetate (54)- Bis(3,5-dimethyl-4-(1-ferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (55)- Bis(3,5-dimethyl-4-(1-methylferrocenyl-1,2,3-triazol-4-yl)pyrazol-1-yl)methane (56)

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Danksagung

251

Danksagung

Mein besonderer Dank gilt meinem Doktorvater Prof. Dr. Nicolai Burzlaff für dieAufnahme in seine Arbeitsgruppe, den großen akademischen Freiraum zur Bearbeitungder interessanten Themenstellung, seine Unterstützung während der Promotion und dieangenehme Atmosphäre im Arbeitskreis sowie für die Empfehlung für die Graduate SchoolMolecular Science.

Bei Prof. Dr. Dr. hc. mult. Rudi van Eldik und seiner Arbeitsgruppe möchte ichmich für den freundlichen Empfang am Lehrstuhl und die stetige Versorgung mit Kaffeebedanken. Ebenso danke ich Herrn Prof. Dr. Sjoerd Harder dafür, dass sich daspositive Arbeitsklima auch nach seiner Übernahme des Lehrstuhls nicht verändert hat.

Natürlich wäre so eine Arbeit nicht ohne die Mithilfe einer Vielzahl von Mitarbeitern desDepartments möglich. Deshalb möchte ich mich bei Dr. Achim Zahl und JochenSchmidt für die Messung unzähliger NMR-Spektren und bei Jochen zusätzlich für dieMessung meiner AAS-Spektren bedanken. Für die Aufnahme der Massenspektren dankeich besonders Dr. Oliver Tröppner und dem Masse-Max Dürr. Außerdem dankeich Christina Wronna für die Durchführung der Elementaranalysen. Den Mitarbeiternder Werkstatt Peter Igel und Manfred Weller, dem Schreiner Wilfried Hof-mann und dem Elektriker Uwe Reißer danke ich für die Hilfe bei allerlei Problemenmit der Technik und dem Mobiliar sowie der häufigen Unterstützung bei einer Vielzahlmerkwürdiger Sonderanfertigungen für Promotionshüte. Natürlich möchte ich mich auchbei unserem Glasbläser Ronny Wiefel, bei Christl Hofmann für die Aufsicht überden Sondermüll sowie bei unseren Magazinmännern Roman Kania und Guido Grimmbedanken.

Mein besonderer Dank gilt den akademischen Räten Dr. Carlos Dücker-Benfer(einfach dafür, dass du bist wie du bist), Dr. Christian Färber (ich werde zeitlebensbei jeder größeren und kleineren Ansammlung von Kartons an dich denken) und Dr.Jörg Sutter (immer der rechte Spruch zur rechten Zeit). Bleibt wie ihr seid! Ohneeuch würde so einiges fehlen. An dieser Stelle danke ich natürlich auch der heimlichenChefin des Lehrstuhls: Ursula Palmer. Danke, dass du dich so gewissenhaft um alleskümmerst und vor allem immer ein offenes Ohr hast.

Meinen Kollegen aus dem Arbeitskreis gilt mein ganz besonderer Dank. Zum einen denAltvorderen Stefan Tampier, Gazi Türkoglu, Fatima Tepedino (alles für denSchäferhund), Tom Godau (auf dass wir bald auf dich anstoßen können), AndreasBeyer (der wohl größte Fugger am ganzen Ballermann) und natürlich Sascha Blei-fuß (was soll man da sagen... einfach für alles. Halt die Stellung an der Uni!). Natür-lich möchte ich mich auch ganz herzlich bei den nicht ganz so alten Kollegen bedanken.

252

LieberNico Fritsch (der König der Düse), lieber Philipp Rodehutskors (eines Tageskomme ich mal vorbei auf der Hallig), lieber Frank Strinitz (bewahr dir deinen gesun-den Appetit), lieber Thomas Waidmann (danke für die gemeinsamen Jahre im Labor!),liebe Susy Spörler (danke für gelegentliche Blicke über den kulinarischen Tellerrand),liebe Eva Heinze (danke, dass du immer ein paar Kapazitäten für mich frei hattest)und liebe Julia Nils Stuber (ja, tief drinnen bist auch du ein Burzlaff), ich denke,man kann ohne Übertreibung sagen, ihr wart der beste Arbeitskreis, den man sich nurwünschen kann und ich hoffe, dass wir uns nicht so schnell aus den Augen verlieren wer-den. Danke für alles, ihr Zöpfe! Natürlich möchte ich mich auch bei allen neuen Burzlaffsbedanken: Marleen Mayer, Stephan Pflock und Lisa Müller, ich wünsche euchviel Glück und alles Gute für eure Promotion und ärgert die Susy nicht zu sehr!

Ebenso gilt mein Dank allen Mitarbeitern aus dem Arbeitskreis Harder. Insbesondereauch an Harmen Zijstra, Johanne Penafiel und Julia Intemann, ich hoffe, dasswir noch das ein oder andere Bier zusammen trinken werden. Darüber hinaus möchte ichmich bei meinen Mitarbeiter- und Bachelorstudenten danken, die mich im Laufe der Zeitunterstützt haben.

Zuletzt gilt jedoch mein größter Dank meinen Eltern, die mich während meiner Studienzeitstets unterstützt haben und immer für mich da waren und natürlich meiner FreundinJani, ohne deren große Unterstützung und Verständnis in stressigen Phasen das allesnicht möglich gewesen wäre.

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