XAS structural comparisons of reversibly interconvertible oxo- and hydroxo-bridged heme-copper oxidase model compounds

In this study on model compounds for the iron-copper dinuclear center in heme-copper oxidases, we (i) detail the synthesis and reversible acid-base interconversion of μ-oxo and μ-hydroxo complexes [(F₈-TPP)Fe^III-(O^2-)-Cu^II(TMPA) ^- (1) and [(F₈-TPP)Fe^III-(OH^-)-Cu ^II(TMPA)]²⁺ (2) [F₈-TPP = tetrakis(2,6-difluorophenyl)-porhyrinate(2^-), TMPA = tris[(2-pyridylmethyl)amine]; (ii) compare their physical properties; (iii) establish the structure of 2 using XAS (X-ray absorption spectroscopy), a novel application of a three-body two-edge multiple-scattering (MS) analysis of ligand connectivity; and (iv) compare the XAS of 2 with those of 1 and an enzyme preparation. Complex 1 was prepared by reaction of [(TMPA)Cu^II(CH₃CN)]²⁺ (3) and [(F₈-TPP)Fe^III-OH] (4) with triethylamine in acetonitrile (>70% yield). Salts 2-(ClO₄)₂ and 2-(CF₃SO₃)₂ were synthesized (>60% yield) by addition of 3 with 4 in dichloroethane or by protonation of 1 with triflic acid. In a ¹H-NMR spectroscopic titration (298 K) with triflic acid, the pyrrole 65 ppm resonance for 1 progressively converts to one near 70 ppm (71.5 for triflate, 68.5 for perchlorate), diagnostic of 2. The protonation-deprotonation rate is slow on the NMR time scale, the ¹H-NMR spectral properties are consistent with antiferromagnetically coupled high-spin iron(III) and Cu(II) ions (S = 2 ground state), and the interaction is weaker in 2 (2, 5.5 ± 0.1 μB; 1, 5.1 ± 0.1 μB, Evans method). UV-vis spectroscopy was also used to monitor the conversion of 2 (Soret, 410 nm) to 1 (434 nm) using Et₃N. The aqueous pX_a for deprotonation of 2 is estimated as 8 ± 2.5. Both Fe and Cu K-edge XAS was performed on 1, 2, and μ-peroxo complex [{(TMPA)Cu}₂(O₂)]²⁺ (5). The strong MS interaction observed in the EXAFS of 1 is due to the nearly linear Fe-O-Cu moiety. Least-squares refinement of the Cu K-EXAFS of 1 gives Cu⋯Fe = 3.56 ± 0.03 Å, 〈Cu-O-Fe = 176 ± 5°, Cu-O = 1.83 ± 0.02 Å; the Fe K-EXAFS analysis gives Fe-O = 1.72 ± 0.02 Å, Fe⋯Cu = 3.54 ± 0.05 Å, 〈Fe-O-Cu = 172 ± 10°. The intense Fe-Cu (or Cu-Fe) feature is lacking in 2, but the iron-edge spectra do reveal a weaker MS ascribed to the Fe-Cu interaction. The Cu-O(H) and Fe-O(H) bonds are elongated in 2 (1.89 ± 0.02 Å and 1.87 ± 0.02 Å, respectively), with Fe⋯Cu = 3.66 ± 0.03 Å. This protonated complex is bent; 〈Fe-O(H)-Cu = 157 ± 5°. An EXAFS comparison with an enzyme preparation of the quinol oxidase aa₃-600 from Bacillus subtilis supports the notion that μ-OH^- complex 2 may be a good heme-Cu enzyme model for the resting state and/or turnover intermediate.