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A conformational study of phospha(III)- and phospha(V)-guanidine compounds

A conformational study of phospha(III)- and phospha(V)-guanidine compounds
A conformational study of phospha(III)- and phospha(V)-guanidine compounds

Spectroscopic, crystallographic, and computational studies of the substituent distribution about the "NCN" unit in a series of phospha(III)- and phospha(V)-guanidines, R2PC{NR′}{NHR′} and R2P(E)C-{NR′}{NHR′} (R = Ph, Cy; R′ = iPr, Cy; E = S, Se), are reported. In the phosphorus(III) systems, the P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be the Esyn-(α) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E syn-(α) and Zanti in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the α- or β-conformer relative to rotation about the P-C amidine bond; however, DFT calculations indicate a low-energy structure for the N,N′-dimethyl model complex in the β-conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the Esyn-(β) and Zsyn-(β) isomers in solution (∼3:2 ratio), whereas for the P-dicyclohexyl analogues, the latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement about the imine double bond) is the dominant form. Intramolecular E⋯HN (E = S, Se) interactions are observed in solution for the Zsyn-(β) configuration of both P-substituted species, characterized by JseH coupling in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the infrared spectrum. An X-ray crystallographic analysis of representative Ph2P(E)- and Cy2P(E)- substituted species shows exclusively the Esyn-(β) configuration for the P-diphenyl substituted compounds and the ZSyn-(β) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents. Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the observed preferences in substituent orientation, suggesting that steric factors play an important role in determining the subtle energetic differences at work in these systems.

0002-7863
13879-13893
Mansfield, Natalie E.
4d6b9a06-2b2e-4ba9-916e-8f8ff27c2ac5
Grundy, Joanna
0bc72187-8dce-41fc-b809-93a6adbe0980
Coles, Martyn P.
4cb5ee08-daf9-4b58-9627-61732e0a818b
Avent, Anthony G.
6fe5d51d-5f31-454f-938a-ec91c840f2ce
Hitchcock, Peter B.
4c0fe381-c3e8-452c-a284-fe14acd3b748
Mansfield, Natalie E.
4d6b9a06-2b2e-4ba9-916e-8f8ff27c2ac5
Grundy, Joanna
0bc72187-8dce-41fc-b809-93a6adbe0980
Coles, Martyn P.
4cb5ee08-daf9-4b58-9627-61732e0a818b
Avent, Anthony G.
6fe5d51d-5f31-454f-938a-ec91c840f2ce
Hitchcock, Peter B.
4c0fe381-c3e8-452c-a284-fe14acd3b748

Mansfield, Natalie E., Grundy, Joanna, Coles, Martyn P., Avent, Anthony G. and Hitchcock, Peter B. (2006) A conformational study of phospha(III)- and phospha(V)-guanidine compounds. Journal of the American Chemical Society, 128 (42), 13879-13893. (doi:10.1021/ja064212t).

Record type: Article

Abstract

Spectroscopic, crystallographic, and computational studies of the substituent distribution about the "NCN" unit in a series of phospha(III)- and phospha(V)-guanidines, R2PC{NR′}{NHR′} and R2P(E)C-{NR′}{NHR′} (R = Ph, Cy; R′ = iPr, Cy; E = S, Se), are reported. In the phosphorus(III) systems, the P-diphenyl substituted compounds are observed as only one isomer, shown by NMR spectroscopy to be the Esyn-(α) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E syn-(α) and Zanti in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the α- or β-conformer relative to rotation about the P-C amidine bond; however, DFT calculations indicate a low-energy structure for the N,N′-dimethyl model complex in the β-conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the Esyn-(β) and Zsyn-(β) isomers in solution (∼3:2 ratio), whereas for the P-dicyclohexyl analogues, the latter configuration (in which the nitrogen substituents are in a more sterically unfavorably cisoid arrangement about the imine double bond) is the dominant form. Intramolecular E⋯HN (E = S, Se) interactions are observed in solution for the Zsyn-(β) configuration of both P-substituted species, characterized by JseH coupling in the NMR spectrum for the P(V)-seleno compounds and a bathochromic shift of the NH absorption in the infrared spectrum. An X-ray crystallographic analysis of representative Ph2P(E)- and Cy2P(E)- substituted species shows exclusively the Esyn-(β) configuration for the P-diphenyl substituted compounds and the ZSyn-(β) form for the P-dicyclohexyl derivatives, independent of the chalcogen and the nitrogen substituents. Results from a DFT analysis of model compounds fail to identify a compelling electronic argument for the observed preferences in substituent orientation, suggesting that steric factors play an important role in determining the subtle energetic differences at work in these systems.

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More information

e-pub ahead of print date: 3 October 2006
Published date: 25 October 2006

Identifiers

Local EPrints ID: 426201
URI: https://eprints.soton.ac.uk/id/eprint/426201
ISSN: 0002-7863
PURE UUID: 0f17a87c-0691-491b-8a4c-d968d126b0cc

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Date deposited: 16 Nov 2018 17:30
Last modified: 16 Nov 2018 17:30

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Contributors

Author: Natalie E. Mansfield
Author: Joanna Grundy
Author: Martyn P. Coles
Author: Anthony G. Avent
Author: Peter B. Hitchcock

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