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-η2:η2-peroxo binding mode in
the heteronuclear heme/copper complex [(2L)FeIII−(O22-)−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 CH2Cl2/6% 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 CuI 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.