New mechanistic probes of hydride abstraction from rhenium-alkyl complexes (.eta.5-C5H5)Re(NO)(PPh3)(R) by Ph3C+ PF6-. Evidence for initial electron transfer

The mechanism of hydride abstraction from rhenium-alkyl complexes R-(Re) ((Re) = (eta/sup 5/-C/sub 5/H/sub 5/)Re(NO)(PPh/sub 3/)) by Ph/sub 3/C/sup +/PF/sub 6//sup -/ is probed by study of the equilibrium R-(Re) + Ph/sub 3/C/sup +/ in equilibrium R-(Re)/sup .+/ (eq v) and the effect of oxygen on the rate and deuterium kinetic isotope effect. Equilibrium constants K/sub 5/ are determined in CH/sub 2/Cl/sub 2/ at 208 K from reversible potential measurement for R = PhCH/sub 2/ (1, 2.5 x 10/sup -5/), (CH/sub 3/)/sub 2/CHCH/sub 2/ (2, 7.9 x 10/sup -3/), and Ph(CH/sub 3/)CH (3, 5.0 x 10/sup -2/). When generated electrochemically in separate experiments, R-(Re)/sup .+/ and Ph/sub 3/C/sup ./ are stable under the reaction conditions. Upon mixing CH/sub 2/Cl/sub 2/ solutions of the reactants in (v), rapid reactions ensue giving Ph/sub 3/CH and hydride abstraction products derived from R-(Re). Nearly diffusion controlled rate constants are found for the reaction between Ph/sub 3/C/sup ./ and O/sub 2/. Rate enhancements of ca. an order of magnitude are observed when reactions are carried out in the presence of oxygen, while the rhenium products are essentially unchanged. A deuterium kinetic isotope effect, k/sub H//k/sub D/ = 5.4, is found for reactions of PhCH/sub 2/-(Re)more » and PhCD/sub 2/-(Re) under nitrogen but not in the presence of oxygen. In the presence of O/sub 2/, as much as 70% of the organic product is benzophenone, arising from decomposition of Ph/sub 3/COOH.« less