Models for Iron-Oxo Proteins. Structures and Properties of Fe^IIFe^III, Zn^IIFe^III, and Fe^IIGa^III Complexes with (μ-Phenoxo)bis(μ-carboxylato)dimetal Cores

A series of bimetallic complexes, [M^IIM'^IIIBPMP(O₂CR)₂]X₂ where BPMP is the anion of 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol, has been synthesized to provide models for binuclear metal-oxo centers in proteins (1, M = M' = Fe, R = C₂H₅, X = BPh₄; 2, M = M' = Fe, R = C₆H₅, X = PF₆; 3, M = Fe, M' = Ga, R = C₂H₅, X = BPh₄; 4, M = Zn, M' = Fe, R = CH₃, X = BPh₄; 5, M = Zn, M' = Fe, R = C₂H₅, X = BPh₄). The complexes 1' (1·CH₃COCH₃·0.5CH₃CN) and 4' (4·CH₃CN) have been characterized by X-ray diffraction methods as having (μ-phenoxo)bis(μ-carboxylato)dimetal cores. Both 1' and 4' crystallize in the triclinic space group P1̅ with the following unit cell parameters: for 1', a = 13.489 (9) Å, b = 13.514 (6) Å, c = 25.258 (15) Å, α = 77.23 (4)°, β = 77.89 (5)°, γ = 61.43 (4)°, and Z = 2; for 4', a = 12.914 (4) Å, b = 14.991 (3) Å, c = 20.736 (6) Å, α = 101.66 (2)°, β = 106.84 (2)°, γ = 100.19 (2)°, and Z = 2. The metal centers in the mixed valence complex 1' are ordered as indicated by the differences in the Fe-O bond lengths; these match well with Fe^III-O and Fe^II-O bond lengths in complexes of established structure and valence. The metal centers in 4' exhibit differences in Fe-O and Zn-O bond lengths, but the crystallographic analysis suggests the presence of some disorder in the metal occupancies in this case. The Fe^IIFe^IIIcomplexes have been characterized by electronic spectral, Mossbauer, EPR, NMR, and electrochemical methods. The assignment of these properties to the individual iron centers is facilitated by the availability of the analogous heterobimetallic complexes wherein one of the paramagnetic centers in the mixed valence complex is replaced with a diamagnetic center of corresponding charge, i.e., Zn^IIfor Fe^IIor Ga^IIIfor Fe^IIIThus, 1 exhibits electronic spectral features near 385, 554, and 1350 nm, assigned to Fe^II-to-pyridine, phenolate-to-Fe^III, and intervalence charge-transfer transitions, respectively. Its NMR spectrum exhibits sharp, isotropically shifted resonances, which number half of that expected for a valence-trapped species; this indicates that electron transfer between the metal centers is fast on the NMR time scale under ambient conditions. Its Mossbauer spectrum at temperatures below 200 K, on the other hand, shows features indicative of trapped valences. Taken together, the NMR, Mossbauer, and electronic spectral data classify 1 as a class II mixed valence complex in the Robin-Day scheme. In contrast, the valences appear trapped in the solid state, even at room temperature, as indicated by crystallographic and Mössbauer data. 1 exhibits reversible one-electron waves at +692 and-10 mV versus SCE, corresponding to the Fe^IIIFe^III/Fe^IIFe^IIIand Fe^IIFe^III/Fe^IIFe^IIcouples, respectively. The high potential of the First wave, which is also observed for the corresponding Fe^IIGa^III-bis(propionate) and Mn^IIMn^III-bis(acetate) complexes, emphasizes the point that the BPMP/bis(carboxylate) ligand combination appears to stabilize a total metal charge of +5. Lastly, 1 exhibits a broad EPR signal centered near g = 1.6, which is not observed in either the analogous Zn^IIFe^III or Fe^IIGa^IIIcomplexes. This signal is similar to those observed for the mixed valence forms of binuclear iron-oxo proteins.