Preparation and unusual decomposition of silica-supported tetraphenylphosphonium bromide

Abstract

Amorphous silica gel grafted with a directly bound tetraarylphosphonium salt has been prepared and has been characterised by ¹³C CP MAS NMR, DRIFTS and elemental microanalysis. Thermal analysis of this material reveals a strikingly different decomposition pattern to its unsupported analogue: free tetraphenylphosphonium bromide obeys pseudo-zeroth-order kinetics, with E_a= 204 ± 16 kJ mol^–1, ΔH‡= 206 ± 16 kJ mol^–1 and ΔS‡=–87 ± 25 J K^–1 mol^–1, whereas the supported reagent shows a first-order decay, with E_a= 77 ± 22 kJ mol^–1, ΔH‡= 72 ± 22 kJ mol^–1 and ΔS‡=–197 ± 34 J K^–1 mol^–1. Evolved-gas mass spectrometry shows that the supported tetraarylphosphonium salt shows a greater propensity to eliminate triphenylphosphine than would be expected from purely statistical considerations. This material shows ¹³C CP MAS NMR and decomposition characteristics more typical of the solution state than the solid phase.