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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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journal contribution
posted on 28.12.2016, 16:19 by Suzanne M. Adam, Isaac Garcia-Bosch, Andrew W. Schaefer, Savita K. Sharma, Maxime A. Siegler, Edward I. Solomon, Kenneth D. Karlin
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)­F8FeIII-(O22–)-CuII(DCHIm)4]+, and LS-3DCHIm, [(DCHIm)­F8FeIII-(O22–)-CuII(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, ket = 1.6 M–1 s–1 (−90 °C). The intermediacy of a high-valent [(DCHIm)­F8FeIVO] species is thus implied, and separate experiments show that one-electron reduction-protonation of [(DCHIm)­F8FeIVO] occurs faster (ket2 = 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.