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 pK 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 aa 3-600 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.

No abstract

This study considers how the electronic relaxation rate enhancement effects of strong antiferromagnetic coupling in the Fe III -X-Cu II unit (X ) O 2-, OH -) of complexes [(F 8 -TPP)Fe III -O-Cu II (TMPA)] + (1), and [(F 8 -TPP)Fe III -OH-Cu II (TMPA)] + (2) (F 8 -TPP ) tetrakis(2,6-difluorophenyl)porphyrinate(2-); TMPA ) tris(2pyridylmethyl)amine) are manifested as observable upfield-shifted resonances of the TMPA moiety in the 1 H-NMR spectra. The pyrrole resonances appear at 65 and 69 ppm, respectively, for 1 and 2, consistent with an S ) 2 ground state derived from antiferromagnetic coupling of high-spin Fe III (S ) 5/2) and Cu II (S ) 1/2) through the bridging ligand X. Paramagnetic mononuclear complexes [Co(TMPA)(CH 3 CN)] 2+ (S ) 3/2) (X-ray structure reported) and [Cu(TMPA)(CH 3 CN)] 2+ (S ) 1/2) demonstrate downfield-shifted peaks consistent with a σ contact shift mechanism. Assignments for all complexes were achieved via 1 H-and 2 H-NMR spectroscopy of appropriately synthesized methylated and deuteriated derivatives. In [Cu(TMPA)(CH 3 CN)] 2+ , the observed ligand peaks (298 K) are broad; in 1, however, they are considerably sharper and upfield-shifted to -61, -7, 4.5, -21, and -104 ppm, corresponding to pyridyl 6-H, 5-H, 4-H, and 3-H and aminomethyl -CH 2 -, respectively, of TMPA. The observance of these upfield peaks is a consequence of enhancement of the electronic relaxation rate for Cu II due to antiferromagnetic coupling with the faster relaxing Fe III . This observation represents the prototype of a Fe III -X-Cu II S ) 2 spin state hitherto only theoretically predicted. The attenuation of δ for a particular hydrogen as its distance (through bonds) from Cu II increases correlates with a σ contact shift mechanism. Only one peak per type of pyridyl H is observed (from 220 to 300 K for 1 and from 220 to 270 K for 2), indicating dynamic behavior of the Cu-TMPA moiety in solution. Both 1 and 2 exhibit pseudo-Curie temperature dependence manifested as augmentation of δ in both upfield and downfield directions as temperature is lowered. Linear Curie and anti-Curie plots of the pyrrole and TMPA chemical shifts (from 220 to 300 K for 1 and from 220 to 270 K for 2) imply a predominantly S ) 2 spin state in each case; i.e., the extent of antiferromagnetic coupling is strong.

Institutes of Health Grant GM36308. We thank S. Koch for the X-ray structure determination and for helpful arguments. Supplementary Material Available:Tables of atomic positional and thermal parameters and bond distances and angles for 1 (4 pages); listings of observed and calculated structure factors for 1 ( 1 1 pages). Ordering information is given on any current masthead page. (17) We have recently isolated and determined the structure of (Et,N)-Li(MeOH)3(H20)[Ni(S2-1 ,2-~yclohexane)~], which is reversibly oxidized to a Ni(Il1) species in DMSO at El,, = -0.73 V vs SCE (AEp = 60 mV).