Evidence for a SN2-type pathway in the exchange of phosphines at a [PhSe]+ centre

Abstract

A range of thio- and seleno-phosphonium cationic complexes [RE(PR′₃)]⁺[X]⁻ (R = Me, Ph; E = S, Se; X = GaCl₄, SbF₆) have been synthesised and structurally characterised. Reaction of [PhSPPh₃][GaCl₄] and [PhSePPh₃][GaCl₄] with P^tBu₃ results in the ready transfer of the “RS⁺” and “RSe⁺” fragments from PPh₃ to the stronger electron donor P^tBu₃. NMR experiments combined with an Eyring analysis on the corresponding degenerate phosphine exchange reaction allowed the thermodynamic values for the phosphine exchange reaction of the sulfur cation (ΔH^‡ 18.7 ± 12.0 kJ mol⁻¹; ΔS^‡ −99.3 ± 36.3 J mol⁻¹ K⁻¹) to be compared with the corresponding values (ΔH^‡ 2.4 ± 1.1 kJ mol⁻¹ and ΔS^‡ −58.1 ± 5.0 J mol⁻¹ K⁻¹) for the [PhSePPh₃]⁺ system. Importantly, the large negative entropy of activation and linear dependence on the rate of exchange are compatible with an S_N2-type exchange process. This conclusion is supported by DFT calculations which confirm that the phosphine exchange process occurs via an associative mechanism. The rate of exchange was found to increase from sulfur to selenium and those with aryl substituents underwent exchange faster than those with alkyl substituents.