Mössbauer and DFT study of the ferromagnetically coupled diiron(IV) precursor to a complex with an Fe ^IV ₂O ₂ diamond core

Recently, we reported the reaction of the (μ-oxo)diiron(III) complex 1 ([Fe ^III ₂(μ -O)(μ -O ₂H ₃)(L) ₂] ³⁺, L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl) amine) with 1 equiv of H ₂O ₂ 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 HClO ₄, complex 2 converted to a species with an Fe ^iv ₂(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-(μO)-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.