In cytochrome c oxidase synthetic modeling studies, we recently reported a new μ-η2:η2-peroxo binding mode in the heteronuclear heme/copper complex [(2L)FeIII-(O 22-)-CuII]+ (6) which is effected by tridentate copper chelation (J. Am. Chem. Soc. 2004, 126, 12716). To establish fundamental coordination and O2-reactivity chemistry, we have studied and describe here (i) the structure and dioxygen reactivity of the copper-free compound (2L)FeII (1), (ii) detailed spectroscopic properties of 6 in comparisons with those of known μ-η2: η1 heme-peroxo-copper complexes, (iii) formation of 6 from the reactions of [(2L)FeIICuI]+ (3) and dioxygen by stopped-flow kinetics, and (iv) reactivities of 6 with CO and PPh3. In the absence of copper, 1 serves as a myoglobin model compound possessing a pyridine-bound five-coordinate iron(II)-porphyrinate which undergoes reversible dioxygen binding. Oxygenation of 3 below -60°C generates the heme-peroxo-copper complex 6 with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) to yield an S = 2 spin system. Stopped-flow kinetics in CH2-Cl2/6% EtCN show that dioxygen reacts with iron(II) first to form a heme-superoxide moiety, [(EtCN)(2L)FeIII-(O2-) ⋯CuI(EtCN)]+ (5), which further reacts with Cu I to generate 6. Compared to those properties of a known μ-η2:η1-heme-peroxo-copper complex, 6 has a significantly diminished resonance Raman ν(O-O) stretching frequency at 747 cm-1 and distinctive visible absorptions at 485, 541, and 572 nm, all of which seem to be characteristics of a μ-η2: η2-heme-peroxo-copper system. Addition of CO or PPh3 to 6 yields a bis-CO adduct of 3 or a PPh3 adduct of 5, the latter with a remaining FeIII-(O2-) moiety.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry