Models for Iron-Oxo Proteins. MÖssbauer and EPR Study of an Antiferromagnetically Coupled Fe^IIINi^II Complex

The bimetallic complex [Fe^IIINi^II,BPMP(OPr)₂](BPh₄)₂, where BPMP is the anion of 2,6-bis[(bis(2-pyridylmethyl) amino]methyl]-4-methylphenol, has been synthesized and its structure determined by X-ray diffraction methods as having a (μ-phenoxo)bis(μ-carboxylato) core. The complex crystallizes in the triclinic space group P1 with cell constants: a = 13.607 (3) Å, b = 13.700 (3) Å, c = 25.251 (7) Å, α = 77.29 (2)°,β = 78.25 (2)°, Γ = 61.73 (2)°, Z = 2, V= 4017 (4) ų. The metal centers have distinct six-coordinate N₃O₃ environments and are separated by 3.378 (8) Å, similar to related complexes in this series. We have studied the complex with EPR and Mössbauer spectroscopy and magnetic susceptibility. All three techniques establish that the electronic ground state of the complex has spin S = 3/2. For T < 10 K the EPR spectra arc dominated by signals of the S = 3/2 multiplet. At higher temperatures, an additional resonance appears. It is centered at g = 4.2 and belongs to an excited multiplet with S = 5/2. We have studied the low-temperature Mössbauer spectra of the complex in external fields up to 6.0 T. Analysis of the well-resolved spectra yields D = 0.7 cm^-1 and E/D = 0.32 for the zero-field splitting parameters of the S = 3/2 multiplet. Spectra taken in external fields H < 0.5 T reveal that D and E/D are distributed; a simple Gaussian distribution of E/D values fits the data quite well. The Mössbauer spectra show that the Fe^III site is high-spin. It follows that the dinuclear complex consists of a ferric ion (S₁ = 5/2) which is antiferromagnetically coupled to a high-spin (S₂ = 1) Ni^II. Analysis of the temperature dependence of the g = 4.2 EPR signal yields J = +24 (3) cm^-1 (J = JS₁-S₂); the susceptibility study agrees with this result. Analysis of the ⁵⁷Fe magnetic hyperfine interaction with a spin coupling model yields A = -29.6 (2) MHz; this compares well with A(Fe^III) = -29.5(2) MHz which we obtained here for the Fe^III site of the isostructural FemZnn complex.