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

Marlène Martinho, Genqiang Xue, Adam T. Fiedler, Lawrence Que, Emile L. Bominaar, Eckard Münck

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Abstract

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.

Original languageEnglish (US)
Pages (from-to)5823-5830
Number of pages8
JournalJournal of the American Chemical Society
Volume131
Issue number16
DOIs
StatePublished - Apr 29 2009

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