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
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The mechanism of hydride abstraction from rhenium-alkyl complexes R-(Re) ((Re) = (5-C5H5)Re(NO)(PPh3)) by Ph3C+PF6- is probed by study of the equilibrium R-(Re) + Ph3C+ R-(Re)+ + Ph3C (eq v) and the effect of oxygen on the rate and deuterium kinetic isotope effect. Equilibrium constants K5 are determined in CH2Cl2 at 208 K from reversible potential measurement for R = PhCH2 (1, 2.5 10-5), (CH3)2CHCH2 (2, 7.9 10-3), and Ph(CH3)CH (3, 5.0 10-2). When generated electrochemically in separate experiments, R-(Re)+ and Ph3C are stable under the reaction conditions. Upon mixing CH2Cl2 solutions of the reactants in (v), rapid reactions ensue giving Ph3CH and hydride abstraction products derived from R-(Re). Thus, if an electron transfer mechanism is operative, very rapid hydrogen atom exchange must take place between R-(Re)+ and Ph3C to displace the unfavorable equilibria (v) to the right. Nearly diffusion controlled rate constants are found for the reaction between Ph3C and O2, suggesting that Ph3C formed in (v) could be trapped by O2 and diverted from the pathway leading to Ph3CH. 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, kH/kD = 5.4, is found for reactions of PhCH2-(Re) and PhCD2-(Re) under nitrogen but not in the presence of oxygen. This indicates that in the presence of O2 rate control switches from hydrogen atom transfer to electron transfer. In the presence of O2, as much as 70% of the organic product is benzophenone, arising from decomposition of Ph3COOH. It is concluded that hydride transfer from R-(Re) to Ph3C+PF6- most likely takes place by an initial electron transfer followed by hydrogen atom transfer to either Ph3C or Ph3COO, depending upon whether O2 is present. 1987 American Chemical Society.