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

Abstract

Recently, we reported the reaction of the (μ-oxo)diiron(III) complex 1 ([Fe^III₂(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 Fe^IV₂(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 Fe^IV sites, a and b. Studies in applied magnetic fields show that the spins of sites a and b (S_a = S_b = 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 [LFe^IV(O)(NCMe)]²⁺ complex, showing that site b (labeled Fe_O) has a terminal oxo group. Using the zero-field splitting parameters of [LFe^IV(O)(NCMe)]²⁺ for our analysis of 2, we obtained parameters for site a that closely resemble those reported for the nonoxo Fe^IV complex [(β-BPMCN)Fe^IV(OH)(OO^tBu)]²⁺, suggesting that a (labeled Fe_OH) 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 Fe_O and Fe_OH 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 Fe_O and Fe_OH 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 Fe_O and Fe_OH define magnetic orbitals favorable for ferromagnetic ineractions.