PREPARATION AND UNUSUAL DECOMPOSITION OF SILICA-SUPPORTED TETRAPHENYLPHOSPHONIUM BROMIDE

Amorphous silica gel grafted with a directly bound tetraarylphosphoniu m salt has been prepared and has been characterised by C-13 CP MAS NMR , DRIFTS and elemental microanalysis. Thermal analysis of this materia l reveals a strikingly different decomposition pattern to its unsuppor ted analogue: free tetraphenylphosphonium bromide obeys pseudo-zeroth- order kinetics, with E(a) = 204 +/- 16 kJ mol(-1), Delta H double dagg er = 206 +/- 16 kJ mol(-1) and Delta S double dagger = -87 +/- 25 J K- 1 mol(-1), whereas the supported reagent shows a first-order decay, wi th E(a) = 77 +/- 22 kJ mol(-1), Delta H double dagger = 72 +/- 22 kJ m ol(-1) and Delta S double dagger = -197 +/- 34 J K-1 mol(-1). Evolved- gas mass spectrometry shows that the supported tetraarylphosphonium sa lt shows a greater propensity to eliminate triphenylphosphine than wou ld be expected from purely statistical considerations. This material s hows C-13 CP MAS NMR and decomposition characteristics more typical of the solution state than the solid phase.