Tridentate copper ligand influences on heme-peroxo-copper formation and properties: Reduced, superoxo, and μ-peroxo iron/copper complexes

In cytochrome c oxidase synthetic modeling studies, we recently reported a new μ-η:η-peroxo binding mode in the heteronuclear heme/copper complex [(L)Fe-(O )-Cu] (6) which is effected by tridentate copper chelation (J. Am. Chem. Soc. 2004, 126, 12716). To establish fundamental coordination and O-reactivity chemistry, we have studied and describe here (i) the structure and dioxygen reactivity of the copper-free compound (L)Fe (1), (ii) detailed spectroscopic properties of 6 in comparisons with those of known μ-η: η heme-peroxo-copper complexes, (iii) formation of 6 from the reactions of [(L)FeCu] (3) and dioxygen by stopped-flow kinetics, and (iv) reactivities of 6 with CO and PPh. 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 CH-Cl/6% EtCN show that dioxygen reacts with iron(II) first to form a heme-superoxide moiety, [(EtCN)(L)Fe-(O) ⋯Cu(EtCN)] (5), which further reacts with Cu to generate 6. Compared to those properties of a known μ-η:η-heme-peroxo-copper complex, 6 has a significantly diminished resonance Raman ν(O-O) stretching frequency at 747 cm and distinctive visible absorptions at 485, 541, and 572 nm, all of which seem to be characteristics of a μ-η: η-heme-peroxo-copper system. Addition of CO or PPh to 6 yields a bis-CO adduct of 3 or a PPh adduct of 5, the latter with a remaining Fe-(O) moiety. © 2005 American Chemical Society.