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)FeIII−(O2-)−CuII(TMPA)]+
(1) and
[(F8-TPP)FeIII−(OH-)−CuII(TMPA)]2+
(2) [F8-TPP =
tetrakis(2,6-difluorophenyl)porphyrinate(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)CuII(CH3CN)]2+
(3) and [(F8-TPP)FeIII−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 pKa 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.