title: Critical Aspects of Heme-Peroxo-Cu Complex Structure and Nature of Proton Source Dictate Metal-O(peroxo) Breakage versus Reductive O-O Cleavage Chemistry.
abstract: The 4H+/4e-reduction of O2to 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-1s-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-1s-1), consistent with the overall postulated mechanism. The importance of the H-bonding interaction as a prerequisite for reductive cleavage is highlighted.
authors.affiliation: Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Suzanne M
authors.initials: SM
authors.lastname: Adam
authors.affiliation: Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Isaac
authors.initials: I
authors.lastname: Garcia-Bosch
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Schaefer
authors.affiliation: Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Savita K
authors.initials: SK
authors.lastname: Sharma
authors.affiliation: Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Maxime A
authors.initials: MA
authors.lastname: Siegler
authors.affiliation: Department of Chemistry, Stanford University , Stanford, California 94305, United States.
authors.forename: Edward I
authors.initials: EI
authors.lastname: Solomon
authors.affiliation: Johns Hopkins University , Baltimore, Maryland 21218, United States.
authors.forename: Kenneth D
authors.identifier: 0000-0002-5675-7040
authors.initials: KD
authors.lastname: Karlin
mesh_terms: D003300:Copper / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000032:analysis; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory
chemical_list: D005290:Ferric Compounds; D009942:Organometallic Compounds; D011522:Protons; D006418:Heme; D003300:Copper; D006861:Hydrogen Peroxide; D010100:Oxygen