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 (ΔAH† = -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 (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 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.
| Original language | English |
|---|---|
| Pages (from-to) | 3707-3712 |
| Number of pages | 6 |
| Journal | Proceedings of the National Academy of Sciences of the United States of America |
| Volume | 100 |
| Issue number | 7 |
| DOIs | |
| Publication status | Published - Apr 1 2003 |
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Keywords
- Bond-dissociation energy
- Hydrogen atom transfer
- Kinetic isotope effect
- Manganese
- Proton transfer
ASJC Scopus subject areas
- Genetics
- General
Cite this
Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O46+ and Mn4O47+ core types. / Maneiro, Marcelino; Ruettinger, Wolfgang F.; Bourles, Emilie; McLendon, George L.; Dismukes, G Charles.
In: Proceedings of the National Academy of Sciences of the United States of America, Vol. 100, No. 7, 01.04.2003, p. 3707-3712.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O46+ and Mn4O47+ core types
AU - Maneiro, Marcelino
AU - Ruettinger, Wolfgang F.
AU - Bourles, Emilie
AU - McLendon, George L.
AU - Dismukes, G Charles
PY - 2003/4/1
Y1 - 2003/4/1
N2 - 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 (ΔAH† = -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 (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 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.
AB - 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 (ΔAH† = -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 (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 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.
KW - Bond-dissociation energy
KW - Hydrogen atom transfer
KW - Kinetic isotope effect
KW - Manganese
KW - Proton transfer
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U2 - 10.1073/pnas.0637229100
DO - 10.1073/pnas.0637229100
M3 - Article
VL - 100
SP - 3707
EP - 3712
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
SN - 0027-8424
IS - 7
ER -