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

摘要

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 equivalent of H2O2 to yield a diiron(IV) intermediate, >2 (Xue, Fiedler, Martinho, Münck, and Que, 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 (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 [LFe^IV(O)(NCMe)]²⁺ complex, showing that site >b (labeled FeO) 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 non-oxo Fe^IV complex [(β-BPMCN)Fe^IV(OH)(OO^tBu)]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 degrees, 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 is puzzling given that the Fe-(μ-O)-Fe angle is large but can be explained by noting that the orbital structures of FeO 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. Inequivalent coordinations at Fe_O and Fe_OH define magnetic orbitals favorable for ferromagnetic ineractions.