Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O4(6+) and Mn4O4(7+) core types

The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn(4)O(4)(O(2)PPh(2))(6), 1, and [Mn(4)O(4)(O(2)PPh(2))(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 kcalmol for homolytic, 1H --> H + 1, vs. approximately 127 kcalmol for heterolytic, 1H --> H(-) + 1(+), dissociation of the OH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (deltaH = -1.2 kcalmol and deltaS= -32 calmol.K; 25-6.7 degrees C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no HD kinetic isotope effect (k(H)k(D) = 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H + pzH --> 1H(2) + 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.