Mixed-Valent Compounds

DOI: 10.1002/anie.200502287

(Pr4N)4[Ag3Fe2(ECN)12]—Anionic NetworkStructures with Mutual Interpenetration**

Stefan Gerber, Henriette Gr ger, J�rgen Ensling,Philipp G�tlich, and Harald Krautscheid*

Dedicated to Professor Dieter Fenske

The synthesis and characterization of compounds withextended network structures has become a fast growingfield of research.[1] In some cases these compounds showporous structures with large cavities, in others the formationof the cavities is prevented by interpenetration of thenetworks. 1998 Batten and Robson reviewed[2] networkstructures with mutual interpenetration, mostly formed byself-aggregation. Following this concept, it is possible, forexample, to synthesize heteronuclear polymeric complexes, ifSCN� ions are coordinating to metal ions with different Lewisacidities.[3] In keeping with Pearson-s HSAB principle,[4] theSCN� ion acts as an ambidentate ligand; it coordinates tohard Lewis acids mainly through the N atom and to soft Lewisacids through the S atom. The central FeII and FeIII atoms inthe homoleptic complexes (Et4N)4[Fe(NCS)6]

[5] and (Me4N)3[Fe(NCS)6],

[6] respectively, are coordinated octahedrally bysix thiocyanato-N ligands, whereas in K[Ag(SCN)2], K4-[Ag2(SCN)6], K3[Ag(SCN)4], and Rb2[Ag(SCN)3]

[7] terminalor bridging thiocyanato ligands are bound tetrahedrallythrough S atoms. If SCN� ions are offered Fe2+, Fe3+, andAg+ ions at the same time, the two coordination motives arecombined in such a way that Fe(NCS)6 octahedra andAg(SCN)4 tetrahedra are linked, as the structures of(Et4N)2[Ag2Fe(SCN)6], (Bu4N)4[Ag2Fe2(SCN)12],

[5] and(Me3NPh)6[Ag6Fe3(SCN)18]

[8] show.

Here we report on the heteronuclear, mixed-valentcoordination compounds 1 and 2, which contain Fe2+, Fe3+,and Ag+ ions and are formed according to Equation (1) innitromethane and crystallized by addition of diethyl ether.The partial reduction of the Fe3+ ions is due to the reducingeffect of thiocyanate, which is oxidized to (SCN)2 andsubsequent products.[9]

2 ðPr4NÞ3½FeðNCEÞ6� þ 3AgECNMeNO2����!

ðPr4NÞ4½Ag3Fe2ðECNÞ12� þ 2 ðPr4NÞSCNþ 1=2 ðSCNÞ21 : E ¼ S

2 : E ¼ Se

ð1Þ

The X-ray structure analyses show that 1 and 2 areisostructural and crystallize in the space group I4̄2d.[10] Asexpected, Ag+ ions are coordinated tetrahedrally to SCN�

and SeCN� ions through S and Se atoms, respectively, andFe2+ and Fe3+ ions are coordinated octahedrally through theN atoms of SCN� and SeCN� (Figure 1a). Two 1,3-m2 bridgingligands link single Fe and Ag atoms in such a way that each Featom is surrounded by three Ag atoms in a trigonal-planararrangement with Ag···Fe distances of 560.5 and 560.8 pm in 1and 568.3 and 570.6 pm in 2. Each Ag atom is connected totwo Fe atoms by two ECN� ligands each (Fe···Ag···Fe> 1748).As shown in Figure 1b (Ag···Fe drawn as dotted lines), thisconnectivity results in a three-dimensional Fe2Ag3 network,whose topology corresponds to the (10,3)-b network ofaThSi2 (Figure 1c) and shows the highest possible tetragonalsymmetry for this type of network.[2, 11]

In this way channels are formed with a width of 1.2 nm@2.7 nm in direction of the crystallographic a and b axes. Thesecavities are partially occupied according to the interpenetra-tion of three of these (10,3)-b [Ag3Fe2(ECN)12]

4� nets, asshown in Figure 2. The Pr4N

+ ions are located in theremaining cavities. After crystallization 1 is hardly solublein nitromethane. The Pr4N

+ ions could not be exchanged forMe4N

+ ions. Similar three-dimensional networks with mutualinterpenetration are found in [Ag(H2NC12H24NH2)2]

+ and[Ag2(pz)3]

2+ ions (pz= pyrazine).[12]

Complexometric titrations show that 1 is a mixed-valentcompound, whereas all Fe atoms occupy equivalent positionsaccording to the X-ray structure analysis. Therefore FeII andFeIII centers are undistinguishable by crystallographic means.The Fe�N bond lengths (210.1–212.2(2) pm in 1 and 209.9–211.1(4) pm in 2) are between those observed in (Me4N)3[FeIII(NCS)6]

[6] (204.8 pm) and (Et4N)4[FeII(NCS)6]

[5]

(217.6 pm). The thermal ellipsoids of the N atoms arealmost spherical and do not indicate differing Fe–liganddistances (Figure 1a). Differential thermal analysis/thermo-gravimetric analysis of 1 shows a phase transition at 161 8C(endothermic) and a thermal stability up to 250 8C underN2.

[13] Single crystals of 1 show no significant electricconductivity. Numerous complexes of SCN� are known withFeII and also with FeIII central atoms. In contrast, only a fewiron complexes of SeCN� have been characterized by X-raystructure analysis, in many of which, however, also otherligands are present.[14] Only one mixed-valent iron complexwith SCN� and SeCN� ligands has been reported, but itscrystal structure was not described.[15]

[*] S. Gerber, Prof. Dr. H. KrautscheidInstitut f$r Anorganische ChemieUniversit+t LeipzigJohannisallee 29, 04103 Leipzig (Germany)Fax: (+49)341-973-6199E-mail: krautscheid@rz.uni-leipzig.de

H. Gr>gerInstitut f$r Anorganische ChemieUniversit+t Karlsruhe (TH) (Germany)

Dr. J. Ensling, Prof. Dr. P. G$tlichInstitut f$r Anorganische und Analytische ChemieUniversit+t Mainz (Germany)

[**] This work was supported by the Fonds der Chemischen Industrie.We thank Dr. M. T. Kelemen (Physikalisches Institut, Universit+tKarlsruhe) for measuring the magnetic susceptibility, Dr. P. Scheer(Forschungszentrum Karlsruhe) for the thermochemical measure-ments. H.K. thanks Prof. D. Fenske (Universit+t Karlsruhe) for hisinvaluable support. E=S, Se.

Supporting information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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The IR spectrum of 1 shows intense vibrational resonan-ces at 2112 (s) and 2069 (vs) cm�1, which can be attributed tothe stretching vibrations nCN of the SCN� ligands. Because ofthe 1,3-m bridging mode of the SCN� ligands, these absorp-tions are slightly shifted compared to those of the mono-nuclear complexes with terminal N-bonded SCN� ligands.[16]

For comparison, the nCN frequencies are 2069 cm�1 in (Pr4N)3[Fe(NCS)6], 2085 cm

�1 in (Et4N)4[Fe(NCS)6] and 2125, 2076,and 2042 cm�1 in (Bu4N)4[Ag2Fe2(SCN)12].

[5]

The almost-black crystals of 1, which appear red in thinlayers, show a very strong absorption at 525 nm in the UV/Visspectrum, which is interpreted as a ligand-to-metal charge-transfer (LMCT) transition.[17] The peak is broadened com-

pared to that of the starting compound (Pr4N)3[Fe(NCS)6](lmax= 480 nm), and like the absorption of(Bu4N)4[Ag2Fe3(SCN)12] (lmax= 530 nm) it is shifted tolower energy as a result of the coordination of the ligand Satoms to Ag+.[5] The high intensity of the LMCT absorptionprevents the identification of an intervalence charge-transfer(IVCT) absorption.

At room temperature the magnetic moment of 1 is c=

2.88 @ 10�2 emumol�1 (after subtraction of the diamagneticincrements).[18, 19] Between 100 K and 300 K the magneticsusceptibility follows approximately Curie-s law with a Curieconstant of C= 8.35. If one assumes meff(Fe

3+)= 5.92 mB as thespin-only value for the high-spin Fe3+ ions, the Fe2+ ionscontribute meff(Fe

2+)= 5.64 mB. The magnetic moment forhigh-spin Fe2+ complexes in octahedral ligand fields istypically between 5.1 and 5.7 mB.

[19] Below 100 K the cTfunction drops off slightly. Apparently at these temperaturesthere are no significant exchange interactions between the Feions, which have separations of 1121 pm within and 1104 pmbetween two networks. According to the structure analysis,the Fe2+ and Fe3+ ions occupy equivalent positions, whereastheMFssbauer spectra can be interpreted as the superpositionof signals of Fe2+ and Fe3+ (Figure 3). At room temperature,130 K, and 4.2 K the spectra show quadrupole doublets withtemperature-dependent splittings ranging from 0.38 mms�1 atroom temperature to 0.65 mms�1 at 4.2 K. This signal with anisomeric shift of 1.15 to 1.27 mms�1 can be assigned to high-spin Fe2+. The comparatively small quadrupole splitting is aresult of the almost equal contributions of the valenceelectrons and the lattice to the electric field gradient withopposite signs. The contribution of the valence electrons isresponsible for the temperature dependence of the quadru-pole splitting. The strongly broadened signal with an isomericshift of about 0.5 mms�1 indicates Fe3+ in a high-spin state. Atroom temperature both portions of the peak area are almostequal, but shift with decreasing temperature in favor of the

Figure 1. a),b) Fragments of the structure of the three-dimensionalpolymeric [Ag3Fe2(SCN)12]

4� ion in 1 (thermal ellipsoids with 70%probability). For clarity, Ag···Fe distances are marked with dashed lines.Distances [pm] and angles [8] in 1 [2]: Fe–N 210.1–212.2(2) [209.9–211.1(4)]; Ag–S 260.99–262.08(8) [Ag–Se 270.52–271.22(6)], C–N115.5–116.3(3) [116.3–116.9(5)], C–S 163.3–163.7(3) [C–Se 178.0–178.3(4)]; N-Fe-N 83.14–93.74(8), 171.4–175.5(1) [83.3–94.3, 171.4–176.5(2)], S-Ag-S 91.18–119.39(4) [Se-Ag-Se 88.65–121.38(2)], S-C-N177.7–178.6(3) [Se-C-N 178.1–179.5(4)]. c) (10,3)-b Network of a-ThSi2and of the Ag and Fe atoms in 1 and 2, respectively; the Fe atomsoccupy the positions of the Si atoms (linking position), between themare the Ag atoms. A section of the [Ag3Fe2(SCN)12]

4� network includingthe tetragonal unit cell is shown (view approximately along the a axis).

Figure 2. Interpenetration of three (10,3)-b nets in the structures of theanions of 1 and 2 ; only the Ag···Fe lines are shown.

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Fe2+ high-spin component. Probably spin relaxation leads tothe strong broadening of the Fe3+ signal, since the relaxationtimes are comparable with the typical time window for the57Fe MFssbauer experiment.

The broad intervalence bands of [(CN)5Fe-ECN-Fe(CN)5]

6� (E=S, Se) at l= 1300 nm with half widths of3550 and 3090 cm�1 show that exchange interactions throughbridging SCN� and SeCN� ligands, respectively, are possi-ble.[15] These complexes can be assigned to class II in the senseof the Robin Day classification[20] with localized oxidationstates, since the nCN frequencies for the FeII(CN)5 andFeIII(CN)5 fragments are distinguishable. The Fe ions in[(CN)5Fe(m-tz)Fe(CN)5]

5� (m-tz= 1,2,4,5-tetrazine), however,cannot be distinguished by either IR or MFssbauer spectros-copy, and can therefore be assigned to class III, which is alsosupported by a minor solvent dependence of the significantlysharper intervalence charge-transfer band at 2495 cm�1.[21]

Because in 1 the Fe2+ and the Fe3+ ions have similar chemicalenvironments, the magnetic features correspond to those ofthe isolated complexes, and these ions also give rise to distinctsignals in the MFssbauer spectra, 1 should be considered aRobin Day class II mixed-valent compound.

Experimental Section1: Equimolar quantities of (Pr4N)3[Fe(NCS)6] (550 mg, 0.57 mmol)(prepared according to reference [22] and recrystallized fromCH2Cl2/hexane), AgNO3 (96 mg), and (Pr4N)SCN (139 mg) were stirred innitromethane (5 mL) at room temperature. After 10 h the red–blacksolution was filtered and 15 mL of diethyl ether was condensed intothe solution. After one week, black crystals of 1, which in thin layers

appear red, were isolated, washed with nitrome-thane and diethyl ether, and dried in vacuum. Yield:50 mg (0.027 mmol, 14%); elemental analysis calcd(%) for C60H112N16Ag3Fe2S12: C 38.4, H 6.7, N 11.9,Fe 5.9; found: C 38.3, H 6.3, N 12.0, Fe 5.9.

Coordination compound 2 was obtained as darkblue crystals in small yield by reaction of AgSeCNwith Fe(ClO4)3 and (Pr4N)SeCN.

The amount of Fe2+ and Fe3+ in 1 was deter-mined by complexometric titration under N2 with0.01m EDTA solution with 5-sulfosalicylic acid asindicator: Compound 1 (85 mg, 0.0453 mmol) wasstirred with an excess of AgNO3 in water acidifiedwith HNO3. Insoluble AgSCN was filtered off, andthe resulting solution was titrated with EDTAsolution to determine the Fe3+ content. The pointof equivalence was reached after addition of EDTAsolution (4.55 mL). After oxidation of the Fe2+ ionswith added H2O2, the titration was continued untilthe second point of equivalence is reached after theaddition of EDTA solution (4.40 mL).

Received: June 30, 2005Published online: November 4, 2005

.Keywords: coordination polymers ·interpenetrating networks · Moessbauerspectroscopy · selenocyanates · thiocyanates

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tometer,Mm= 1877.7 gmol�1, crystal size 0.14 @ 0.18 @ 0.22 mm3,tetragonal, space group I4̄2d (no. 122), a= 1960.65(7) c=2218.1(1) pm, V= 8526.6(6) @ 106 pm3, Z= 4, T= 200 K, 1calcd=

1.463 gcm�3, m= 1.347 mm�1, l= 71.073 pm (MoKa), Tmin=

0.775, Tmax= 0.840, 22659 measured, 4652 independent reflec-tions, Rint= 0.036, 4285 with I> 2s(I), 201 parameters, propylgroups disordered, H atoms in idealized positions, R1 (observedreflections)= 0.029, wR2 (all data)= 0.060, Flack parameter x=�0.04(2), max./min. residual electron density peaks 0.35/�0.47 e/106 pm3; 2 (C60H112N16Ag3Fe2Se12): STOE IPDS diffrac-tometer, Mm= 2440.5 gmol�1, crystal size 0.12@ 0.14 @ 0.35 mm,

Figure 3. M>ssbauer spectra of 1 at 293, 130, and 4.2 K. IS= isomeric shift, QS=quadru-pole splitting, hs=high spin.

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7789Angew. Chem. Int. Ed. 2005, 44, 7787 –7790 � 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

tetragonal, space group I4̄2d (no. 122), a= 1986.21(8) c=2262.09(10) pm, V= 8924.0(6) @ 106 pm3, Z= 4, T= 213 K,1calcd= 1.816 gcm�3, m= 3.123 mm�1, l= 56.087 pm (AgKa),Tmin= 0.5521, Tmax= 0.7134, 40179 measured, 4880 independentreflections, Rint= 0.0465, 4501 with I> 2s(I), 183 parameters,propyl groups disordered, H atoms in idealized positionen, R1

(observed reflections)= 0.030, wR2 (all data)= 0.069, Flackparameter x= 0.01(2), max./min. residual electron densitypeaks 0.74/�0.47 e/106 pm3; structure solution and refinement:SHELXS-97 (Sheldrick, 1990) and SHELXL-97 (Sheldrick,1997). Graphical presentation: Diamond2 (Brandenburg,1999), Schakal92 (Keller, 1992). CCDC 193054 (1) and CCDC-193055 (2) contain the supplementary crystallographic data forthis paper. These data can be obtained free of charge from theCambridge Crystallographic Data Centre via www.ccdc.cam.a-c.uk/data_request/cif.

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[13] Differential thermal analysis/thermogravimetry: Netzsch STA409C, heating rate 5 Kmin�1, N2 atmosphere.

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