MODELS FOR DIFERROUS FORMS OF IRON-OXO PROTEINS - STRUCTURE AND PROPERTIES OF [FE2BPMP(O2CR)2]BPH4 COMPLEXES

A series of bimetallic complexes, [MIIM′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 Å3. 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 Rw = 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 FeIIZnII and FeIIGaIII 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 Di 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 FeIIZnII or FeIIGaIII complexes, as expected for the S = 2 centers in these complexes. © 1990, American Chemical Society. All rights reserved.