Models for Iron-Oxo Proteins. MÖssbauer and EPR Study of an Antiferromagnetically Coupled FeIIINiII Complex

T. R. Holman, Larry Que, C. Juarez-Garcia, M. P. Hendrich, E. Münck

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Abstract

The bimetallic complex [FeIIINiII,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) Å3. 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 FeIII 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) NiII. 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(FeIII) = -29.5(2) MHz which we obtained here for the FeIII site of the isostructural FemZnn complex.

Original languageEnglish (US)
Pages (from-to)7611-7618
Number of pages8
JournalJournal of the American Chemical Society
Volume112
Issue number21
DOIs
StatePublished - Jan 1990

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