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

Abstract

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 ...

Functional models for the oxygen-evolving complex of photosystem II

“…In the double pivot mechanism, O 2 is evolved by O-O bond formation across a face of the cube and subsequent loss of O 2 freeing two of the manganese atoms to pivot outward and open the cube (Figure 3, top). This hypothesis is supported by gas phase experiments illustrating the formation of O 2 from Mn 4 O 4 cuboidal complexes [40], as well as reduction of similar Mn 4 O 4 complexes to form Mn 4 O 2 and two water molecules [41,42]. The reduction reactions imply that the reverse oxidation reaction is possible.…”

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

“…9 is thought to feature two Mn III and two Mn IV sites, and 10 to have one Mn III and three Mn IV sites. Movement between these oxidation states comes with reactive consequence: 9 is thought to undergo PCET with pzH 11 to form the pz radical, where 10 is observed to undergo hydride transfer with 11 (net one proton two electron PCET) to form the pz cation [83]. The original assessment of the hydroxo species was based on the capacity for 9 to engage para -cyanophenol in PCET, giving a lower limit of the O-H BDFE as ~94 kcal mol −1 ; inclusion of the potential necessary to move from 9 to 10 furnishes an approximate hydride affinity of ~127 kcal mol −1 [84].…”

Theory of chemical bonds in metalloenzymes. XV. Local singlet and triplet diradical mechanisms for radical coupling reactions in the oxygen evolution complex

“…The BS DFT (UB3LYP) calculations of the model complexes (Mn 4 O 4 and [L 6 Mn 4 O 4 ] 1+ (L 6 (Ph 2 PO 2 ) 6 )]) for CaMn 4 O 4 cluster in PSII are also performed to elucidate their electronic and spin structures; charge and spin populations are utilized for the purpose. Present computational results of manganese oxide clusters, together with those of other groups115–143, are discussed, emphasizing underlying basic concepts, and chemical pictures, explanations, and correspondences to experimental results4–75. Finally, implications of the comprehensive guiding principles for theoretical illumination of oxygen evolution are discussed in relation to oxygenation reactions in p450 and MMO.…”

“…In this series of articles1–3, we have performed theoretical studies of the nature of chemical bonds in metalloenzymes from the viewpoint of protein‐assisted confined states of magnetic inorganic complexes that have strongly correlated electron systems. Past decades structure and reactivity of manganese oxide enzymes have been accepted both experimental4–75 and theoretical interest1–3, 76–144. The nature of chemical bonds of these active sites has indeed been investigated with both experimental and theoretical methods: for example, X‐ray diffraction and various spectroscopic observations, and broken‐symmetry (BS) molecular orbitals (MO) and Kohn‐Sham (KS) density functional (DFT) calculations.…”