A Conformational Study of Phospha(III)- and Phospha(V)-guanidine Compounds

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. ISSN 0002-7863

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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-() configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of Esyn-() and Zanti in solution. Spectroscopic techniques are unable to determine whether the latter isomer exists as the - or -conformer relative to rotation about the P-Camidine bond; however, DFT calculations indicate a low-energy structure for the N,N'-dimethyl model complex in the -conformation. In their oxidized sulfo and seleno forms, the P-diphenyl compounds are present as an interconverting equilibrium mixture of the Esyn-() and Zsyn-() 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 HN (E = S, Se) interactions are observed in solution for the Zsyn-() 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-() configuration for the P-diphenyl substituted compounds and the Zsyn-() 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

Item Type: Article
Additional Information: MPC directed the work and is the corresponding author. The paper describes the first detailed solid- and solution-state studies of phosphaguanidine compounds, with emphasis on the configurational distribution of the nitrogen substituents. The results will prove beneficial in future applications of these compounds as ligands in coordination chemistry.
Schools and Departments: School of Life Sciences > Chemistry
Depositing User: Natalie Emma Mansfield
Date Deposited: 06 Feb 2012 19:50
Last Modified: 19 Mar 2013 13:58
URI: http://sro.sussex.ac.uk/id/eprint/22425
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