The synthesis and characterisation of R3PXCN [R = 2,4,6-(CH3O)3C6H2, 2,6-(CH3O)2C6H3, NCCH2CH2, C6H11 or PhCH2; X = Br or I] in the solid state and in solution

Abstract

The compounds R₃PXCN [R = 2,4,6-(CH₃O)₃C₆H₂, 2,6-(CH₃O)₂C₆H₃, NCCH₂CH₂, C₆H₁₁ or PhCH₂; X = Br or I] have been synthesized and characterised by elemental analysis, X-ray powder diffraction and infrared and ³¹P-{H} NMR spectroscopy. In the solid state and in solution the compounds exist as mixtures of the ionic compounds [R₃PCN]X and [R₃PX]CN with no evidence for the species R₃PX₂ or R₃P(CN)₂. Infrared studies indicate the presence of both covalently bound and ionic cyanide. The ³¹P-{H} NMR studies show that in CDCl₃ the compounds exist predominantly as [R₃PCN]X, although a minor component due to [R₃PX]CN is also observed in ca. 10∶1 ratios. These novel compounds are discussed with reference to the analogous R₃PX₂ compounds. An X-ray crystallographic study of [{2,4,6-(CH₃O)₃C₆H₂}₃PCN]I reveals an ionic structure with no long cation–anion interactions. In contrast, the crystal structure of [2,4,6-(CH₃O)₃C₆H₂]₃PI₂ represents a further example of the molecular four-co-ordinate charge-transfer structure R₃P–I–I; d(I–I) for this compound, 3.3394(5) Å, is rather short considering the very high basicity of the parent tertiary phosphine [cf.d(I–I) for PhMe₂PI₂ = 3.408(5) Å].