Models for Diferrous Forms of Iron-Oxo Proteins. Structure and Properties of [Fe₂BPMP(O₂CR)₂]BPh₄ Complexes

A series of bimetallic complexes, [M^IIM′^IIBPMP(O₂CR)₂]X₂ where BPMP is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol, has been synthesized to serve as models for the diferrous forms of iron-oxo centers in proteins. Complex 1 (M = M′ = Fe, R = C₂H₅, X = BPh₄, solvate = 0.8 CH₂Cl₂) has been characterized by X-ray diffraction methods as having a (μ-phenoxo)bis(μ-carboxylato)diiron core. 1 crystallizes in the triclinic space group P1 with cell constants: a = 12.607 (6) Å, b = 15.113 (13) Å, c = 16.601 (6) Å, α = 81.42 (6)°, β = 88.88 (4)°, Γ = 67.89 (5)°, Z = 2, V = 2879.4 ų. From 11 192 reflections (of 13865 where/(obsd) > σ(I)) collected at 175 K, the structure was solved by the Patterson method and refined anisotropically to R = 0.058 and R_w = 0.074. The metal centers in 1 have distinct six-coordinate environments but have similar structural parameters. They have been characterized as high-spin Fe(II) centers by electronic spectral, NMR, Mössbauer, and EPR methods with the help of the analogous heterobimetallic complexes such as the Fe^IIZn^II and Fe^IIGa^III derivatives. Most interestingly, 1 and 2 (M = M′ = Fe, R = Ph, X = BPh₄) exhibit low field EPR signals near g = 16, similar to those reported for deoxyhemerythrin azide, reduced methane monooxygenase and reduced ribonucleotide reductase. The signal for 1 has an intensity that is enhanced in parallel mode (B₁ ‖ B), a characteristic of integer spin systems, and has a temperature dependence indicative of a ground-state transition. Analysis of EPR spectra shows that the two iron sites of 1 are ferromagnetically coupled. Depending on the sign of the zero-field splitting parameters D_i of the individual Fe(II) sites, both a weak and a strong coupling scheme are compatible with the data. Similar but significantly less intense signals are observed for analogous Fe^IIZn^II or Fe^IIGa^III complexes, as expected for the S = 2 centers in these complexes.