Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O6+4 and Mn4O7+4 core types

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 H + 1, vs. 127 kcal/mol for heterolytic, 1H H + 1+, dissociation of the OH 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 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 NH 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 OH bond cleavage step during catalysis of photosynthetic water oxidation.