Kinetics and thermodynamics of formation and electron-transfer reactions of Cu–O2 and Cu2–O2 complexes

Abstract

The kinetics and thermodynamics of formation of Cu(II)-superoxo (Cu-O2) complexes by the reaction of Cu(I) complexes with dioxygen (O2) and the reduction of Cu(II)-superoxo complexes to dinuclear Cu-peroxo complexes are discussed. In the former case, electron transfer from a Cu(I) complex to O2 occurs concomitantly with binding of O2•− to the corresponding Cu(II) species. This is defined as an inner-sphere Cu(II) ion-coupled electron transfer process. Electron transfer from another Cu(I) complex to preformed Cu(II)-superoxo complexes also occurs concomitantly with binding of the the Cu(II)-peroxo species with the Cu(II) species to produce the dinuclear Cu-peroxo (Cu2-O2) complexes. The kinetics and thermodynamics of outer-sphere electron-transfer reduction of Cu2-O2 complexes are also been discussed in light of the Marcus theory of outer-sphere electron transfer.

Mechanistic study of a manganese porphyrin catalyst for on-demand production of chlorine dioxide in water

“…The outcome is higher and more persistent chlorine dioxide production at pH 5.00 vs. 6.90. This constitutes another example of the importance of PCET rates and how they dictate the underlying reaction mechanism, a hallmark of Professor Fukuzumi's research and career in this very fundamental and central area of research in chemistry [23][24][25]. Our results parallel and complement many of the observations noted for iron(III) and mercury(II) mediated chlorite decomposition in aqueous medium via redox Fenton type chemistry [10,[26][27][28].…”

Peroxo and Superoxo Moieties Bound to Copper Ion: Electron-Transfer Equilibrium with a Small Reorganization Energy

“…45a,46 With copper ion in a tetradentate tris(2-pyridylmethyl)-amine (tmpa) environment, the Cu(II) to Cu(I) reorganization energy is smaller, λ = 1.29 eV, also using ferrocenyl redox reagents, where both Cu(II) and Cu(I) tmpa complexes tend to be pentacoordinate with a solvent molecule as fifth ligand. 47 …”

“…51 In a calculation by Fukuzumi, based on thermodynamic data available, a Cu I −O 2 species where dioxygen is hypothetically bound to copper(I) prior to electrontransfer), the internal (inner-sphere) electron-transfer from the copper(I) to the O 2 to give a cupric-superoxide product (that species which is observed) gives a calculated λ (total) value of 1.74 eV. 47b A related calculation indicates that free dioxygen binds to a porphryinate-cobalt(II) complex to give the Co(III)- superoxide species with λ = 1.89 eV. 47b An interesting finding and analysis by Roth and Klinman 52 is that for glucose oxidase, reduction of O 2 to give superoxide is rate-limiting but is made to be quite fast because of an enzyme adaption which lowers the reorganization energy by ~0.8 eV, via generation of a positive charge from His protonation.…”

Transition Metal Complexes and the Activation of Dioxygen

“…Most recently, Rolff et al [340] describe work showing (a) that both the (μ-η 2 :η 2 -peroxo)-and bis(μ-oxo)dicopper cores are capable of hydroxylating phenolates like in tyrosinase in both stoichiometric and catalytic reactions, (b) that reactions of phenols with these cores often yield radical coupling products, pointing to the importance of proton 'management' by the enzyme, and (c) how the orbital interactions involved in arene hydroxylation reactions of both cores underly their reactivity. Halvagar et al [266] describe work with focus on revealing the factors that favor the formation of either or both of these cores in oxygenation reactions, while Fukuzumi and Karlin [341] summarize efforts to reveal the kinetics and thermodynamics of the formation of dicopper-oxygen species and their mechanisms of reduction by hydride and electron-transfer reagents. Herein, rather than duplicating the discussions presented in these (and earlier) review articles, we focus on selected pertinent recent work published in 2012 and 2013.…”

“…Another noteworthy target is the (μ-oxo)-dicopper(II) unit, which has been proposed recently on the basis of spectroscopy and theory to be the active oxidant in Cu-doped zeolite catalysts that perform the same reaction as pMMO [336,337]. Many studies of synthetic dicopper-oxygen intermediates have aimed to address the aforementioned issues (as well as others), and these have been summarized in extensive reviews, to which readers interested in work appearing prior to early 2012 are pointed [261][262][263][264][265][266][338][339][340][341]. Most recently, Rolff et al [340] describe work showing (a) that both the (μ-η 2 :η 2 -peroxo)-and bis(μ-oxo)dicopper cores are capable of hydroxylating phenolates like in tyrosinase in both stoichiometric and catalytic reactions, (b) that reactions of phenols with these cores often yield radical coupling products, pointing to the importance of proton 'management' by the enzyme, and (c) how the orbital interactions involved in arene hydroxylation reactions of both cores underly their reactivity.…”