RECENT DEVELOPMENTS IN

CHALCOGEN CHEMISTRY: 5

Tristram Chivers

Department of Chemistry,

University of Calgary,

Calgary, Alberta, Canada

Outline • Introduction:

– Macrocycles incorporating P2N2 Rings Bridged by NH, O, Se Groups

(a) Synthetic Approaches

(b) Structures – Host-guest chemistry

• Macrocycles with E–En–E (E = S, Se, Te; n = 0, 1, 2) functionalities

– Formation of E–E (E = S, Se, Te) bonds in acyclic PNP-bridged systems via oxidation

– An oxidative approach to P2N2-supported polychalcogen macrocycles

– A key ditelluride intermediate

Lecture 5: Polychalcogen Rings and Macrocycles

Incorporating P2N2 Scaffolds

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PIII2N2 rings act as building blocks for macrocycles incorporating bridging NH groups Host-guest chemistry – entrapment of halide ions by NH groups

Review: S. G. Calera and D. S. Wright. Dalton Trans., 2010, 39, 5055.

P2N2-Supported Macrocycles with NH Bridges

3

Cyclocondensation Strategy

• Added halide influences size of macrocycle formed

4

+ 4 Et3N

- 4 [Et3NH]Cl

2

2

P

N

P

N

N

P

N

P

N

N P

N

P

N

N

P

N

P

N N

H H

H H

P

N

P

N Cl Cl

P

N

P

N

N H 2 N H 2

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu

t Bu Bu t Bu Bu t Bu Bu t Bu Bu

D. S. Wright et al. Angew. Chem IE, 2008, 47, 1111.

• Reduction in hot toluene → hexameric Se-containing macrocycle (80 % yield )

• P atoms in different oxidation states (PIII/PV)

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Chalcogen-containing Macrocycles Incorporating P2N2 Rings:

A Reductive Strategy

Review: Chivers, Ritch, Robertson, Konu and Tuononen, Acc. Chem. Res., 2010, 43, 1053.

Acyclic Dichalcogenides

Spirocyclic Contact-Ion Pairs

Chalcogen-Chalcogen Bond Formation:

Acyclic PNP-Bridged Systems

• Oxidation of dichalcogeno PNP-bridged monoanions → Dichalcogenides

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R 2 P P R 2

N

E E

L i

T M E D A

R 2 P P R 2

N

E E

R 2 P P R 2 N

E E R 2 P

R 2 P

N

T e

T e

T e

R 2 P

P R 2

N

T e 1 / 2 I 2

- L i I

o r

E = S e , T e ; R = i P r R = t B u

• Alkali-metal derivatives of dichalcogenido PV2N2-Bridged dianions are known

(E = S, Se: Chivers, Krahn, Parvez and Schatte, Inorg. Chem., 2001, 40, 1493) (E = Te: Briand, Chivers and Parvez, Angew. Chem. IE, 2002, 41, 3468)

• Can macrocycles with –E–E– bridges be generated via oxidation?

• Influence of chalcogens?

Dichalcogenido P2N2-Bridged Dianions

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Two-electron oxidation:

By-products:

(a) tBuN(H)E=P(μ-NtBu)2P=E(H)NtBu (E = S, Se) (diprotonated precursors)

(b) A new P-Se compound: NMR: Two 77Se resonances [1J(P,Se) = 524 Hz]

E = S, 35 % E = Se, 53 %

Oxidation of Dichalcogenido P2N2-Bridged Dianions

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• Planar P6Se6 framework

• P2N2 rings perpendicular to plane

• Trigonal crystal system P 63/m

• P atoms are inequivalent

< P-Se-Se-P = 180°

d(Se–Se) = 2.32 Å d(Se1’···Se1) = 3.315 Å

Sulfur analogue Planar P6S6 framework d(S–S) = 2.12 Å d(S1’··S1) = 3.311 Å

A Trimeric P2N2-Bridged Diselenide

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31P NMR (solid state): δ = -67.6 and -59.8

31P NMR (solution): AA’X system δ = -67.3 (s) 1JPSe = 428 Hz,

2JPP = 20 Hz 77Se NMR (solution): δ = 409 (d, 1JPSe = 428 Hz)

• Two P environments in the solid state, cf. XRD • P atoms equivalent in solution

P6Se6 Macrocycle: Solid-State and Solution NMR

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(Se atoms ca. 1.0 Å above and below mean plane)

Is the structure flexible enough to allow a dynamic exchange process? • New dispersion-corrected potential B3LYP-D3 (in ADF) for optimization

• Calculated planar structure in good agreement with experimental values

• Puckered structure 30.5 kJ mol-1 lower in energy

• In solution puckered stucture will undergo conformational exchange

P6Se6 Macrocycle: Conformational Isomers

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• Space-filling model d(Se1’···Se1) = 3.315 Å • Calculated bond orders (Nalewajski-Mrozek) → no Se···Se bonding

• Planar structure in solid state attributed to packing effects

• Air-stable orange solid

• Low solubility

cf. P6S6 Macrocycle

• Moisture-sensitive yellow crystals

• Low solubility

P6Se6 Macrocycle: Intramolecular Se···Se Interactions?

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• Orange crystals of by-product of I2 oxidation obtained from hexane

• XRD: A P2N2-Bridged Tetraselenide

• Tetraselenide is formed by deliberate I2 oxidation of P6Se6 macrocycle i.e. Se2

2- oxidized to Se42- ligands

d(P–Se) = 2.28 Å d(Se1–Se3) = 2.34 Å d(Se3–Se4) = 2.33 Å Se2–Se4–Se3 = 103.4 ° Se1–Se3–Se4 = 103.2 ° Se2–Se4–Se3–Se1 = 94.4(1) °

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A P2N2-Bridged Tetraselenide: Structure

• Direct synthesis via metathesis with Se2Cl2 – 56% yield

• Hexane-soluble, bright orange solid

NMR (same parameters as by-product of I2 oxidation):

31P: δ = -50.8 (s, 1JPSe = 524 Hz, 2JPP = 10 Hz)

77Se: δ = 673.0 (pseudo-t, 2JSeP = 20 Hz), 336.7 (dd, 1JSeP = 524 Hz,

3JSeP = 6 Hz)

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A P2N2-Bridged Tetraselenide: Direct Synthesis

One-electron oxidation with I2

• Formation of central Te-Te unit bridged by two P2N2 rings

• cf. Oxidation of acyclic chalcogen-centred PNP-bridged anions

• N,Te-Chelation of TMEDA-solvated Li+ ions

What about P-Te Systems? A Key Ditelluride Intermediate

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• d(Te1–Te1’) = 2.755(3) Å;

Electronic structure (DFT: PBED3, TZP, ZORA) • Te···Te orbital interaction is insignificant: Bond Order = 0.08

• The tellurium lone pairs available for

σ-donation are not combined

• Structure with a P-Te-Te-P torsion angle of 98.0o is more stable by 7.9 kJ mol-1

Why is the Dianionic Ditelluride Planar?

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Preparation of a new Na2 derivative:

XRD: Symmetrical (N,N’ and Te,Te’) coordination of alkali metal cations cf. Li2 derivative shows unsymmetrical (N,Te and Te,Te’) coordination

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Ditelluro P2N2-Bridged Dianion

Two-electron oxidation with I2

Two resonances (2:1) δ 125Te = 443 (dd, 1JPTe = 1031 Hz, 3JPTe = 41 Hz)

δ 125Te = 362 (t, 2JPTe = 35 Hz)

125Te NMR:

Black crystals (41 %)

A P2N2-Bridged Tritelluride: Synthesis and NMR

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d(P–Te) = 2.53-2.54 Å

d(Te–Te) = 2.72 Å

• Can macrocycles with –E–E– bridges be generated via oxidation? Oxidation provides a versatile route to polychalcogen rings stabilized by P2N2

scaffolds • What is the influence of chalcogens?

E = S, Se: Planar, macrocyclic trimers P6E6 are formed preferentially

E = Se: P2N2-bridged tetraselenide

E = Te: P2N2-bridged tritelluride

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Te

Te

Te

P

P N N

N N

Conclusions - 1

Conclusions- 2

Can the initial oxidation product be isolated for S and Se systems ? Can these dianionic dichalcogenides be used as building blocks to incorporate other main group elements in P2N2-supported macrocycles ?

What is the significance of the dianionic ditelluride ?

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• Planar P6S6 unit

• P2N2 rings perpendicular to plane

• Trigonal crystal system P 63/n

< P-S-S-P = 180o

d(S–S) = 2.121 Å d(S1’···S1) = 3.311 Å

A Trimeric P2N2-Bridged Disulfide

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