Isolation of Monomeric Mn<sup>III/II</sup>−OH and Mn<sup>III</sup>−O Complexes from Water:  Evaluation of O−H Bond Dissociation Energies

Gupta, Rajeev; MacBeth, Cora E.; Young, Victor G.; Borovik, A. S.

doi:10.1021/ja016741x

Cited by 82 publications

(66 citation statements)

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“…Borovik has disclosed a compound with the backbone structure 4, which features an unusual non-bridging terminal hydroxo ligand, and has measured its pK a and redox properties ( Figure 7) [64][65][66]. In a preliminary study of 4 and its isomers, the Mn IV oxo was observed to abstract the hydrogen atom from TEMPO-H, cyclohexadiene, dihydroanthracene, 1,2-diphenyl hydrazine, and 2,4,6-tri-tertbutyl phenol.…”

Section: Metal-oxo Complexes For C-h Bond Oxidationmentioning

confidence: 99%

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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Proton-coupled electron transfers (PCETs) are unconventional redox processes in which both protons and electrons are exchanged, often in a concerted elementary step. While PCET is now recognized to play a central a role in biological redox catalysis and inorganic energy conversion technologies, its applications in organic synthesis are only beginning to be explored. In this chapter we aim to highlight the origins, development and evolution of PCET processes most relevant to applications in organic synthesis. Particular emphasis is given to the ability of PCET to serve as a non-classical mechanism for homolytic bond activation that is complimentary to more traditional hydrogen atom transfer processes, enabling the direct generation of valuable organic radical intermediates directly from their native functional group precursors under comparatively mild catalytic conditions. The synthetically advantageous features of PCET reactivity are described in detail, along with examples from the literature describing the PCET activation of common organic functional groups.

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Section: Metal-oxo Complexes For C-h Bond Oxidationmentioning

confidence: 99%

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Miller

Tarantino

Knowles

2016

Topics in Current Chemistry Collections

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“…Only a limited number of model Mn complexes having mononuclear (10,11) or binuclear manganese-oxo cores have contributed to understanding the kinetic steps in pcet chemistry (3,12). These and related studies led to different results with some systems following a pathway of initial H abstraction (3,13,14) [with large kinetic isotope effect (KIE) in the range of 30-40 (15)], in contrast to other cases that exhibited an electrochemical step followed by a proton-transfer step with minimal H͞D isotope effect (1,16).…”

mentioning

confidence: 99%

Kinetics of proton-coupled electron-transfer reactions to the manganese-oxo “cubane” complexes containing the Mn ₄ 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 ₄ O\documentclass[12pt]{minimal}\usepackage{amsma

Maneiro

Ruettinger

Bourlès

et al. 2003

Proc. Natl. Acad. Sci. U.S.A.

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The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn 4O4(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 kcal͞mol for homolytic, 1H 3 H ؉ 1, vs. Ϸ127 kcal͞ mol for heterolytic, 1H 3 H ؊ ؉ 1 ؉ , dissociation of the OOH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (⌬H ‡ ‫؍‬ ؊1.2 kcal͞mol and ⌬S ‡ ‫؍‬ ؊32 cal͞mol⅐K; 25-6.7°C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no H͞D kinetic isotope effect (k H͞kD ‫؍‬ 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H ؉ pzH 3 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 NOH 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 OOH bond cleavage step during catalysis of photosynthetic water oxidation.bond-dissociation energy ͉ manganese ͉ hydrogen atom transfer ͉ proton transfer ͉ kinetic isotope effect A tom-and ion-transfer reactions comprise a major class of chemical reactions found in biological systems, notably metabolic pathways (1-5). Reactions involving the net transfer of a hydrogen atom may be viewed in terms of two coupling limits with intermediate cases implicit (6): movement of a H atom between a donor͞acceptor pair vs. independent proton͞electron-transfer steps involving different electron and proton sites. As a class these are referred to as protoncoupled electron-transfer (pcet) processes (7).An example arises in the case of the water-oxidizing͞O 2 -evolving complex of photosynthesis [photosystem-II wateroxidizing complex (PSII-WOC)], which catalyzes the extraction of four electrons and four protons from two water molecules mediated by an inorganic core, Mn 4 O x Ca 1 Cl y , and a tyrosyl radical (8,9). Only a limited number of model Mn complexes having mononuclear (10, 11) or binuclear manganese-oxo cores have contributed to understanding the kinetic steps in pcet chemistry (3, 12). These and related studies led to different results with some systems following a pathway of initial H abstraction (3,13,14) [with large kinetic isotope effect (KIE) in the range of 30-40 (15)], in contrast to other cases that exhibited an electrochemical step followed by a proton-transfer step with minimal H͞D isotope effect (1, 16). Herein we report kinetics parameters for the initial pcet reaction between ...

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“…This HBDE is precisely the thermodynamic value necessary for Babcock's H-atom abstraction model (Gardner & Mayer 1995;Caudle & Pecoraro 1997). More recent work on a single set of compounds suggests that going higher than Mn IV leads to a large increase in the HBDE (Gupta et al 2002). Nonetheless, these studies demonstrated that PCET was thermodynamically important in the water-oxidation process.…”

Section: K1mentioning

confidence: 93%

Using small molecule complexes to elucidate features of photosynthetic water oxidation

Meelich

Zaleski

Pecoraro

2007

Phil. Trans. R. Soc. B

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The molecular oxygen produced in photosynthesis is generated via water oxidation at a manganesecalcium cluster called the oxygen-evolving complex (OEC). While studies in biophysics, biochemistry, and structural and molecular biology are well known to provide deeper insight into the structure and workings of this system, it is often less appreciated that biomimetic modelling provides the foundation for interpreting photosynthetic reactions. The synthesis and characterization of small model complexes, which either mimic structural features of the OEC or are capable of providing insight into the mechanism of O 2 evolution, have become a vital contributor to this scientific field. Our group has contributed to these findings in recent years through synthesis of model complexes, spectroscopic characterization of these systems and probing the reactivity in the context of water oxidation. In this article we describe how models have made significant contributions ranging from understanding the structure of the water-oxidation centre (e.g. contributions to defining a tetrameric Mn 3 Ca-cluster with a dangler Mn) to the ability to discriminate between different mechanistic proposals (e.g. showing that the Babcock scheme for water oxidation is unlikely).

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Isolation of Monomeric Mn^III/II−OH and Mn^III−O Complexes from Water: Evaluation of O−H Bond Dissociation Energies

Cited by 82 publications

References 20 publications

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Using small molecule complexes to elucidate features of photosynthetic water oxidation

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Isolation of Monomeric MnIII/II−OH and MnIII−O Complexes from Water: Evaluation of O−H Bond Dissociation Energies

Cited by 82 publications

References 20 publications

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

Using small molecule complexes to elucidate features of photosynthetic water oxidation

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Isolation of Monomeric Mn^III/II−OH and Mn^III−O Complexes from Water: Evaluation of O−H Bond Dissociation Energies