Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo "cubane" complexes containing the Mn4O46+ and Mn4O47+ core types

Marcelino Maneiro, Wolfgang F. Ruettinger, Emilie Bourles, George L. McLendon, G Charles Dismukes

Research output: Contribution to journalArticle

45 Citations (Scopus)

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 languageEnglish
Pages (from-to)3707-3712
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume100
Issue number7
DOIs
Publication statusPublished - Apr 1 2003

Fingerprint

Manganese
Protons
Electrons
Isotopes
Entropy
Catalysis
Water

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 journalArticle

@article{ba3325fe2278433388634b1834b919bf,
title = "Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo {"}cubane{"} complexes containing the Mn4O46+ and Mn4O47+ core types",
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.",
keywords = "Bond-dissociation energy, Hydrogen atom transfer, Kinetic isotope effect, Manganese, Proton transfer",
author = "Marcelino Maneiro and Ruettinger, {Wolfgang F.} and Emilie Bourles and McLendon, {George L.} and Dismukes, {G Charles}",
year = "2003",
month = "4",
day = "1",
doi = "10.1073/pnas.0637229100",
language = "English",
volume = "100",
pages = "3707--3712",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
number = "7",

}

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

UR - http://www.scopus.com/inward/record.url?scp=0037386589&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0037386589&partnerID=8YFLogxK

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 -