The reaction of tertiary phosphines with (Ph2Se2I2)2—the influence of steric and electronic effects

Abstract

The reaction of (Ph₂Se₂I₂)₂ with a wide variety of tertiary phosphines possessing different steric and electronic properties has been studied, leading in most cases to R₃PSe(Ph)I adducts; [R₃P = (p-CH₃C₆H₄)₃P (1), (m-CH₃C₆H₄)₃P (2), (o-OCH₃C₆H₄)₃P (4), Ph₂MeP (6), Me₂PhP (7), Me₃P (8), Cy₃P (9)]. All of the products formed were characterised by elemental analysis, Raman and multinuclear NMR spectroscopy. Both steric and electronic factors are important in determining the structural motif (CT vs. ionic) observed in the solid-state. In general, highly basic phosphines result in a lengthening of the Se–I interaction, and a preference for an ionic structure. The reaction with (o-CH₃C₆H₄)₃P does not yield a stable R₃PSe(Ph)I adduct, and instead (o-CH₃C₆H₄)₃PI₂ (3) is formed. The unusually long P–I bond, [2.5523(12) Å], and short I–I bond, [3.0724(4) Å], exhibited by 3 is a result of the high steric requirements of this phosphine. The similarly bulky (o-SCH₃C₆H₄)₃P yields a mixture of (o-SCH₃C₆H₄)₃PSe(Ph)I (5a) and [(o-SCH₃C₆H₄)₃PSePh]I₃ (5b). The crystal structures of (m-CH₃C₆H₄)₃PSe(Ph)I, 2, (o-CH₃C₆H₄)₃PI₂, 3, [(o-OCH₃C₆H₄)₃PSePh]I.CH₂Cl₂, 4, [(o-SCH₃C₆H₄)₃PSePh]I₃, 5b, two pseudo-polymorphs of Ph₂MePSe(Ph)I, 6a/6b, and [Me₃PSe(Ph)I]₂·CH₂Cl₂, 8, are reported. The R₃PSe(Ph)I adducts formed exhibit one of four types of behaviour. Type I products, (such as 2) are CT in the solid-state and display fluxionality in solution. Type II products (such as 6a/6b) lie close to the CT/ionic structural borderline, displaying long Se–I bonds, and are more appropriately classified as [R₃PSePh] (acceptor)/I⁻ (donor) CT complexes. Type II complexes ionise in solution to [R₃PSePh]I. Type III products, such as 8, are ionic in solution, but frequently show cation–anion, or cation–solvent interactions in the solid-state, although these interactions are weak and the linear P–Se–I motif is lost. Type IV products (such as 4) are ionic and feature bulky phosphines. They display no short cation–anion interactions in the solid-state.