Kinetic study on the reaction of tributylphosphine with methylviologen. Reactivity of the phosphine radical cation intermediate towards nucleophiles

Tributylphosphine, (Bu3P)-P-n (BP), was reacted with 1,1 '-dimethyl-4,4'-bipyridinium (methylviologen; MV2+) in the presence of an alcohol or thiol (RXH; X = O, S) in acetonitrile under an argon atmosphere at 50 degrees C, which resulted in the gradual formation of the one-electron reduced form of the MV2+, MV+. Meanwhile, BP was oxidized to tributylphosphine oxide (BP-O). The increase in the amount of MV+, which was followed spectrophotometrically with BP and RXH being in large excess, did not obey first-order kinetics. The observation, along with the results from product analysis, shows that single-electron transfer (SET) takes place from BP to MV2+ to generate tributylphosphine radical cation BP.+, as well as MV+, and the resulting BP.+ undergoes ionic reaction with RXH and back electron transfer from MV+ in comparable efficiency. A regression analysis of the kinetic data gave the relative value of the second-order rate constant, k(Nu)(rel), for the ionic reaction of BP.+ with RXH. Comparison of the k(Nu)(rel) values thus obtained for reactions with various RXH's shows that the reaction of BP.+ with nucleophile RXH is governed by both steric and electronic factors of RXH. The activation energy E-a of the reaction was found to be significantly large, which is in contrast to previous observations that ionic reactions of ca;bon radical cations with nucleophiles usually have very small values of E-a.