Kinetics of proton-coupled electron-transfer reactions

The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn4O4(O2PPh2)6, 1, and [Mn4O4(O2PPh2)6] , 1 , with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1 . Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE >94 kcal mol for homolytic, 1H 3 H 1, vs. 127 kcal mol for heterolytic, 1H 3 H 1 , dissociation of the OOH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction ( H‡ 1.2 kcal mol and S‡ 32 cal mol K; 25–6.7°C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no H D kinetic isotope effect (kH kD 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H pzH 3 1H2 pz•). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no NOH bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1 provides a concrete example of two-electron pcet that is hypothesized for the OOH bond cleavage step during catalysis of photosynthetic water oxidation.