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Mössbauer and DFT Study of the Ferromagnetically Coupled Diiron(IV) Precursor to a Complex with an FeIV2O2 Diamond Core

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journal contribution
posted on 29.04.2009, 00:00 by Marlène Martinho, Genqiang Xue, Adam T. Fiedler, Lawrence Que, Jr., Emile L. Bominaar, Eckard Münck
Recently, we reported the reaction of the (μ-oxo)diiron(III) complex 1 ([FeIII2(μ-O)(μ-O2H3)(L)2]3+, L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) with 1 equiv of H2O2 to yield a diiron(IV) intermediate, 2 (Xue, G.; Fiedler, A. T.; Martinho, M.; Münck, E.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 20615−20). Upon treatment with HClO4, complex 2 converted to a species with an FeIV2(μ-O)2 diamond core that serves as the only synthetic model to date for the diiron(IV) core proposed for intermediate Q of soluble methane monooxygenase. Here we report detailed Mössbauer and density functional theory (DFT) studies of 2. The Mössbauer studies reveal that 2 has distinct FeIV sites, a and b. Studies in applied magnetic fields show that the spins of sites a and b (Sa = Sb = 1) are ferromagnetically coupled to yield a ground multiplet with S = 2. Analysis of the applied field spectra of the exchange-coupled system yields for site b a set of parameters that matches those obtained for the mononuclear [LFeIV(O)(NCMe)]2+ complex, showing that site b (labeled FeO) has a terminal oxo group. Using the zero-field splitting parameters of [LFeIV(O)(NCMe)]2+ for our analysis of 2, we obtained parameters for site a that closely resemble those reported for the nonoxo FeIV complex [(β-BPMCN)FeIV(OH)(OOtBu)]2+, suggesting that a (labeled FeOH) coordinates a hydroxo group. A DFT optimization performed on 2 yielded an Fe−Fe distance of 3.39 Å and an Fe−(μ-O)−Fe angle of 131°, in good agreement with the results of our previous EXAFS study. The DFT calculations reproduce the Mössbauer parameters (A-tensors, electric field gradient, and isomer shift) of 2 quite well, including the observation that the largest components of the electric field gradients of FeO and FeOH are perpendicular. The ferromagnetic behavior of 2 seems puzzling given that the Fe−(μ-O)−Fe angle is large but can be explained by noting that the orbital structures of FeO and FeOH are such that the unpaired electrons at the two sites delocalize into orthogonal orbitals at the bridging oxygen, rationalizing the ferromagnetic behavior of 2. Thus, inequivalent coordinations at FeO and FeOH define magnetic orbitals favorable for ferromagnetic ineractions.