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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 &gt;94 kcalmol for homolytic, 1H --&gt; H + 1, vs. approximately 127 kcalmol for heterolytic, 1H --&gt; 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 --&gt; 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.

proceedings of the national academy of sciences of the united states of america

Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo “cubane” complexes containing the Mn <sub>4</sub> O\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\setlength{\oddsidemargin}{-69pt}\begin{document}\begin{equation*}{\mathrm{_{4}^{6+}}}\end{equation*}\end{document} and Mn <sub>4</sub> O\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym} \usepackage{…