The reaction of tertiary phosphines with (Ph2Se 2I2)2 - The influence of steric and electronic effects

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 R 3PSe(Ph)I adducts; [R3P = (p-CH3C 6H4)3P (1), (m-CH3C 6H4)3P (2), (o-OCH3C 6H4)3P (4), Ph2MeP (6), Me 2PhP (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-CH3C 6H4)3PI2 (3) is formed. The unusually long P-I bond, [2.5523(12) ], and short I-I bond, [3.0724(4) ], 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-SCH3C6H 4)3PSePh]I3 (5b). The crystal structures of (m-CH3C6H4)3PSe(Ph)I, 2, (o-CH 3C6H4)3PI2, 3, [(o-OCH3C6H4)3PSePh]I.CH 2Cl2, 4, [(o-SCH3C6H 4)3PSePh]I3, 5b, two pseudo-polymorphs of Ph2MePSe(Ph)I, 6a/6b, and [Me3PSe(Ph)I] 2·CH2Cl2, 8, are reported. The R 3PSe(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. © The Royal Society of Chemistry 2006.