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Methane oxidation is extremely difficult chemistry to perform in the laboratory. The C H bond in CH4 has the highest bond energy (104 kcalmol ) amongst organic substrates. In nature, the controlled oxidation of organic substrates is mediated by an important class of enzymes known as monooxygenases and dioxygenases, and the methane monooxygenases are unique in their capability to mediate the facile conversion of methane to methanol. With a turnover frequency approaching 1 s , the particulate methane monooxygenase (pMMO) is the most efficient methane oxidizer discovered to date. Given the current interest in developing a laboratory catalyst suitable for the conversion of methane to methanol on an industrial scale, there is strong impetus to understand how pMMO works and to develop functional biomimetics of this enzyme. pMMO is a complex membrane protein consisting of three subunits (PmoA, PmoB, and PmoC) and many copper cofactors. Inspired by the proposal that the catalytic site might be a tricopper cluster, we have recently developed a series of tricopper complexes that are capable of supporting facile catalytic oxidation of hydrocarbons. We show herein that these model tricopper complexes can mediate efficient catalytic oxidation of methane to methanol as well. The oxidation of CH4 mediated by the tricopper complex [CuCuCu(7-N-Etppz)] in acetonitrile (ACN), where 7-NEtppz corresponds to the ligand 3,3’-(1,4-diazepane-1,4diyl)bis[1-(4-ethylpiperazine-1-yl)propan-2-ol], is summarized in Figure 1A. A single turnover (turnover number; TON= 0.92) is obtained when this CuCuCu complex is activated by excess dioxygen in the presence of excess CH4 (Figure 1B). The reaction is complete within ten minutes, clearly indicating that the oxidation is very rapid. In accordance with the single turnover, the kinetics of the overall process is pseudo first-order with respect to the concentration of the fully reduced tricopper complex with a rate constant k1= 0.065 min 1 (Figure 1B, inset). If we assume that the kinetics is limited by the dioxygen activation of the CuCuCu cluster with the subsequent O-atom transfer to the substrate molecule being rapid, then k1=k2·[O2]0, and from the solubility of oxygen in ACN at 25 8C (8.1 mm), we obtain the bimolecular rate constant k2 of 1.33 10 m 1 s 1 for the dioxygen activation of the CuCuCu cluster. This second-order rate constant is similar to values that we have previously determined for the dioxygen activation of other model tricopper clusters at room temperature. The process can be made catalytic by adding the appropriate amounts of H2O2 to regenerate the spent catalyst after O-atom transfer from the activated tricopper complex to CH4. This multiple-turnover reaction is depicted in Figure 1C. In these experiments, the [CuCuCu(7-N-Etppz)] catalyst is activated by O2 as in the single-turnover experiment described earlier, but the spent catalyst is regenerated by twoelectron reduction by a molecule of H2O2 (Figure 2A). Because the effective turnover number (TON), or the total equivalent of products formed over the time course of the experiment, peaks at approximately six when the turnover is initiated with 20 equivalents of H2O2, it is evident that abortive cycling begins to kick in when the steady-state concentration of the H2O2 concentration exceeds approximately ten equivalents. When the steady-state H2O2 concentration is above this level, reductive abortion of the activated catalyst becomes competitive with the O-atom transfer to methane to produce methanol. In this case, the rate of O-atom transfer is limited by the relatively low solubility of CH4 in ACN under ambient conditions of temperature and pressure (Figure 2B). The [CuCuCu(7-N-Etppz)] complex also mediates the catalytic oxidation of normal C2–C6 alkanes (data not shown) [*] Prof. Dr. S. I. Chan, Y.-J. Lu, Dr. P. Nagababu, Dr. S. Maji, M.-C. Hung, P. D. Minh, J. C.-H. Lai, K. Y. Ng, Prof. Dr. S. S.-F. Yu Institute of Chemistry, Academia Sinica Nankang, Taipei 11529 (Taiwan) E-mail: sunneychan@yahoo.com sfyu@gate.sinica.edu.tw |
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