Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo “cubane” complexes containing the Mn4O
and Mn4O
core types
- Marcelino Maneiro*,
- Wolfgang F. Ruettinger,
- Emilie Bourles,
- George L. McLendon, and
- G. Charles Dismukes†
-
Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved February 4, 2003 (received for review November 29, 2002)
Abstract
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 O—H bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (ΔH‡ = −1.2 kcal/mol and ΔS‡ = −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 (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 → 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 N—H 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 O—H bond cleavage step during catalysis of photosynthetic water oxidation.
Footnotes
-
↵ * On leave from: Departamento de Química Inorgánica, Facultade de Química, Universidade de Santiago de Compostela, E15706, Spain.
-
↵ † To whom correspondence should be addressed. E-mail: dismukes{at}princeton.edu.
-
This paper was submitted directly (Track II) to the PNAS office.
- Abbreviations:
- pcet,
- proton-coupled electron transfer;
- PSII-WOC,
- photosystem-II water-oxidizing complex;
- KIE,
- kinetic isotope effect;
- pzH,
- phenothiazine;
- pzH+•,
- the cation radical;
- pz+,
- the cation and pz• the neutral radical;
- Ph2PO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{_{2}^{-}}}\end{equation*}\end{document},
- diphenylphosphinate;
- pzD,
- mono-deuterio-pzH;
- BDE,
- bond-dissociation enthalpy
- Copyright © 2003, The National Academy of Sciences
