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 E_syn-(α) configuration. In contrast, the corresponding P-dicyclohexyl derivatives exist as a mixture of E_syn-(α) and Z_anti 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 E_syn-(β) and Z_syn-(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 Z_syn-(β) configuration of both P-substituted species, characterized by J_SeH 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 E_syn-(β) configuration for the P-diphenyl substituted compounds and the Z_syn-(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.