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

Recently, we reported the reaction of the (mu-oxo)diiron(III) complex 1 ([Fe-2(III)(mu-O)(mu-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.; Munck, 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-2(IV)(mu-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 Mossbauer and density functional theory (DFT) studies of 2. The Mossbauer 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 [LFeIV(O)(NCMe)](2+) complex, showing that site b (labeled Fe-O) 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 Fe-IV complex [(beta-BPMCN)Fe-IV(OH)((OOBu)-Bu-t)](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 angstrom and an Fe-(mu-O)-Fe angle of 131 degrees, in good agreement with the results of our previous EXAFS study. The DFT calculations reproduce the Mossbauer 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 Fe-OH are perpendicular. The ferromagnetic behavior of 2 seems puzzling given that the Fe-(mu-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.