Critical Aspects of Heme–Peroxo–Cu Complex Structure and Nature of Proton Source Dictate Metal–Operoxo Breakage versus Reductive O–O Cleavage Chemistry

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• 作者：Suzanne M. Adam ; Isaac Garcia-Bosch ; Andrew W. Schaefer ; Savita K. Sharma ; Maxime A. Siegler ; Edward I. Solomon ; Kenneth D. Karlin
• 刊名：Journal of the American Chemical Society
• 出版年：2017
• 出版时间：January 11, 2017
• 年：2017
• 卷：139
• 期：1
• 页码：472-481
• 全文大小：696K
• ISSN：1520-5126

文摘

The 4H⁺/4e^– reduction of O₂ to water, a key fuel-cell reaction also carried out in biology by oxidase enzymes, includes the critical O–O bond reductive cleavage step. Mechanistic investigations on active-site model compounds, which are synthesized by rational design to incorporate systematic variations, can focus on and resolve answers to fundamental questions, including protonation and/or H-bonding aspects, which accompany electron transfer. Here, we describe the nature and comparative reactivity of two low-spin heme–peroxo–Cu complexes, LS-4DCHIm, [(DCHIm)F₈Fe^III-(O₂^2–)-Cu^II(DCHIm)₄]⁺, and LS-3DCHIm, [(DCHIm)F₈Fe^III-(O₂^2–)-Cu^II(DCHIm)₃]⁺ (F₈ = tetrakis(2,6-difluorophenyl)-porphyrinate; DCHIm = 1,5-dicyclohexylimidazole), toward different proton (4-nitrophenol and [DMF·H⁺](CF₃SO₃^–)) (DMF = dimethyl-formamide) or electron (decamethylferrocene (Fc*)) sources. Spectroscopic reactivity studies show that differences in structure and electronic properties of LS-3DCHIm and LS-4DCHIm lead to significant differences in behavior. LS-3DCHIm is resistant to reduction, is unreactive toward weakly acidic 4-NO₂–phenol, and stronger acids cleave the metal–O bonds, releasing H₂O₂. By contrast, LS-4DCHIm forms an adduct with 4-NO₂–phenol, which includes an H-bond to the peroxo O-atom distal to Fe (resonance Raman (rR) spectroscopy and DFT). With addition of Fc* (2 equiv overall required), O–O reductive cleavage occurs, giving water, Fe(III), and Cu(II) products; however, a kinetic study reveals a one-electron rate-determining process, k_et = 1.6 M^–1 s^–1 (−90 °C). The intermediacy of a high-valent [(DCHIm)F₈Fe^IV═O] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)F₈Fe^IV═O] occurs faster (k_et2 = 5.0 M^–1 s^–1), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.