Further insights into the spectroscopic properties, electronic structure, and kinetics of formation of the heme-peroxo-copper complex [(F 8TPP)FeIII-(O22-)-Cu II(TMPA)]+

Article abstract of DOI:10.1021/ic061726k

In the further development and understanding of heme-copper O ₂-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F₈TPP)Fe ^III-(O₂^2-)-Cu^II(TMPA)](ClO ₄) (3), formed by addition of O₂ to a 1:1 mixture of the porphyrinate-iron(II) complex (F₈TPP)Fe^II (1a) {F ₈TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)Cu^I(MeCN)](ClO₄) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t_1/2 (RT; MeCN) ≈ 20 min; λ_max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O₂ = 1:1:1}, MALDI-TOF mass spectrometry {¹⁶O₂, m/z 1239 [(3 + MeCN)⁺]; ¹⁸O₂, m/z 1243}, and resonance Raman spectroscopy {ν_(O-O) = 808 cm^-1; Δ ¹⁶O₂/¹⁸O₂ = 46 cm^-1; Δ¹⁶O₂/^16/18O₂ = 23 cm ^-1}. Consistent with a μ-η²:η¹ bridging peroxide ligand, two metal-O stretching frequencies are observed {ν_(Fe-O) = 533 cm^-1, ν_(Fe-O-Cu) = 511 cm^-1}, and supporting normal coordinate analysis is presented. ²H and ¹⁹F NMR spectroscopies reveal that 3 is high-spin {also μ_B = 5.1 ± 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized μ-oxo complex [(F₈TPP)Fe ^III-O-Cu^II(TMPA)]⁺ (4) (known S = 2 system, antiferromagnetically coupled high-spin Fe^III and Cu^II). Moessbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; δ = 0.57 mm/s, |ΔE_Q| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and Moessbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F₈TPP)Fe^III-(O₂ ^-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of μ-peroxo complex 3 yields μ-oxo complex 4 with concomitant release of ～0.5 mol O₂ per mol 3. Characterization of the reaction 1a/1b + O₂ → 2 → 3 → 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.