Heme-copper/dioxygen adduct formation relevant to cytochrome c oxidase: Spectroscopic characterization of [(⁶L)Fe^III-(O ₂^2-)-Cu^II]⁺

In the further development and understanding of heme-copper dioxygen reactivity relevant to cytochrome c oxidase O₂-reduction chemistry, we describe a high-spin, five-coordinate dioxygen (peroxo) adduct of an iron(II)-copper(I) complex, [(⁶L)Fe^IICu ^I](BArF₂₀) (1), where ⁶L is a tetraarylporphyrinate with a tethered tris(2-pyridylmethyl)amine chelate for copper. Reaction of 1 with O₂ in MeCN affords a remarkably stable [t_1/2 (rt; MeCN)≈60 min] adduct, [(⁶L)Fe ^III-(O₂^2-)-Cu^II]⁺ (2) [EPR silent; λ_max = 418 (Soret), 561 nm], formulated as a peroxo complex based on manometry (1:O₂ = 1:1; spectrophotometric titration, -40°C, MeCN), mass spectrometry {MALDI-TOF-MS: ¹⁶O₂, m/z 1191 ([(⁶L)Fe^III-( ¹⁶O₂^2-)-Cu^II]⁺); ¹⁸O₂, m/z 1195}, and resonance Raman spectroscopy (νv_(O-O) = 788 cm^-1; Δ¹⁶O ₂/¹⁸O₂ = 44 cm^-1; Δ¹⁶O₂/^16/18O₂ = 22 cm ^-1). ¹H and ²H NMR spectroscopy (-40°C, MeCN) reveals that 2 is the first heme-copper peroxo complex which is high-spin, with downfield-shifted pyrrole resonances (δ_pyrrole = 75 ppm, s, br) and upfield shifted peaks at δ = -22, -35, and -40 ppm, similar to the pattern observed for the μ-oxo complex [(⁶L)Fe ^III-O-Cu^II](BAr^F) (3) (known S = 2 system, antiferromagnetically coupled high-spin Fe^III and Cu^II). The corresponding magnetic moment measurement (Evans method, CD₃CN, -40°C) also confirms the S = 2 spin state, with μ_B = 4.9. Structural insights were obtained from X-ray absorption spectroscopy, showing Fe-O (1.83 Å) and Cu-O (1.882 Å) bonds, and an Fe...Cu distance of 3.35(2) Å, suggestive of a μ-1,2-peroxo ligand present in 2. The reaction of 2 with cobaltocene gives 3, differing from the observed full reduction seen with other heme-Cu peroxo complexes. Finally, thermal decomposition of 2 yields 3, with concomitant release of 0.5 mol O₂ per mol 2, as confirmed quantitatively by an alkaline pyrogallol dioxygen scavenging solution.