Further Insights into the Spectroscopic Properties, Electronic Structure, and Kinetics of Formation of the Heme−Peroxo−Copper Complex [(F8TPP)FeIII−(O2 2-)−CuII(TMPA)]

• Published in: Inorganic chemistry 2007-05, Vol.46 (10), p.3889-3902
• Main Authors: Ghiladi, Reza A, Chufán, Eduardo E, del Río, Diego, Solomon, Edward I, Krebs, Carsten, Huynh, Boi Hanh, Huang, Hong-wei, Moënne-Loccoz, Pierre, Kaderli, Susan, Honecker, Marcus, Zuberbühler, Andreas D, Marzilli, Lisa, Cotter, Robert J, Karlin, Kenneth D
• Format: Article
• Language: English
• Subjects: Cold Temperature; Copper - chemistry; Deuterium - chemistry; Electrochemistry; Fluorine Radioisotopes - chemistry; Heme - chemistry; Iron Compounds - chemistry; Kinetics; Magnetic Resonance Spectroscopy; Organometallic Compounds - chemistry; Oxygen - chemistry; Peroxides - chemistry; Solvents; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; Spectroscopy, Mossbauer; Spectrum Analysis, Raman
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic061726k

In the further development and understanding of heme−copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII−(O2 2-)−CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate−iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(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; λmax = 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; Δ16O2/18O2 = 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 μB = 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)FeIII−O−CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). Mössbauer 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 Mössbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII−(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 + O 2 → 2 → 3 → 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.