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The first synthetic complex with a [FeIV2(μ-O)2] diamond core

Recent research from the group of Professor Lawrence Que.

In a paper that appears in the December 26, 2007 issue of Proceedings of the National Academy of Sciences USA (Proc. Natl. Acad. Sci. USA 2007, 104, 20713-20718), Prof. Larry Que, postdoctoral associate Genqiang Xue, and coworkers report the characterization of the first example of a synthetic diiron(IV) complex with an [Fe2(μ-O)2] diamond core. This synthetic precedent lends credence to the postulation of such a core structure for intermediate Q, the key species responsible for the hydroxylation of methane in the catalytic cycle of soluble methane monooxygenase (MMO) [1]. The first crystal structure of an iron(IV)-oxo complex was reported only in 2003 [2], and the title complex may be considered as a head-to-tail dimer of such complexes.

To achieve this goal, Dr. Xue and graduate student Dong Wang performed low temperature bulk electrolysis on [FeIIIFeIV(μ-O)2(L)2]3+ (1), a complex previously characterized crystallographically to have the [Fe2(μ-O)2] diamond core structure but with an iron(III)iron(IV) oxidation state [3]. One-electron oxidation of 1 at 0.9 V vs ferrocene resulted in the near-quantitative formation of new complex 2, as determined by coulometry. Figure 1 shows the conversion of 1 to 2 as followed by spectroelectrochemistry with the appearance of at least two isosbestic points. Further characterization of 2 established the presence of the [FeIV2(μ-O)2] diamond core, specifically by the use of Mössbauer spectroscopy (by graduate student Raymond De Hont and Prof. Eckard Münck of Carnegie Mellon University), resonance Raman spectroscopy (by Drs. Xue and Xiaopeng Shan) and EXAFS analysis (by Dr. Adam Fiedler).

Figure 1. Electrochemical oxidation of an [FeIIIFeIV(μ-O)2]3+ precursor (green dashed line) to the [FeIVFeIV(μ-O)2]4+ complex (red solid line).

With the diiron(IV) complex in hand, we have an unprecedented opportunity to compare the reactivity properties of three different iron(IV)-oxo core units that are supported by the same ligand, namely [FeIIIFeIV(μ-O)2] (1), [FeIV2(μ-O)2] (2) and terminal FeIV=O (3) units. Kinetic results showed that the rates of hydrogen-atom abstraction increased in the order: 1 < 2 < 3, with 3 100-fold more reactive than 2. This surprising observation raises the possibility that the [FeIV2(μ-O)2] diamond core of Q may need to convert to a ring-opened form to break the strong C-H bond of methane.

[1] Shu, L., Nesheim, J. C., Kauffmann, K., Münck, E., Lipscomb, J. D., & Que, L., Jr. (1997) Science 275, 515-518.

[2] Rohde, J.-U., In, J.-H., Lim, M. H., Brennessel, W. W., Bukowski, M. R., Stubna, A., Münck, E., Nam, W., & Que, L., Jr. (2003) Science 299, 1037-1039.

[3] Hsu, H.-F., Dong, Y., Shu, L., Young, V. G., Jr., & Que, L., Jr. (1999) J. Am. Chem. Soc. 121, 5230-5237.

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