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 mu-eta(2):eta(2)-peroxo binding mode in the heteronuclear heme/copper complex [(L-2)Fe-lll-(O-2(2-))-Cu-ll](+) (6) which is effected by tridentate copper chelation (J. Am. Chem. Soc. 2004, 126, 12716). To establish fundamental coordination and O-2-reactivity chemistry, we have studied and describe here (i) the structure and dioxygen reactivity of the copper-free compound (2L)Fell (1), (ii) detailed spectroscopic properties of 6 in comparisons with those of known mu-eta(2):eta(1): heme-peroxo-copper complexes, (iii) formation of 6 from the reactions of [(L-2)(FeCul)-Cu-ll](+) (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(ll)-porphyrinate which undergoes reversible dioxygen binding. Oxygenation of 3 below -60 degrees C generates the heme-peroxo-copper complex 6 with strong antiferromagnetic coupling between high-spin iron(lll) and copper(ll) to yield an S = 2 spin system. Stopped-flow kinetics in CH2Cl2/6% EtCN show that dioxygen reacts with iron(ll) first to form a heme-superoxide moiety, [(EtCN)(L-2) Fe-lll(O-2)Cu-...(l)(EtCN)](+) (5), which further reacts with Cu-l to generate 6. Compared to those properties of a known mu-eta(2):eta(1)-heme-peroxo-copper complex, 6 has a significantly diminished resonance Raman nu(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 mu-eta(2):eta(2)-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 Fe-lll-(O-2(-)) moiety.