Methane oxidation is extremely difficult chemistry to perform in the laboratory. The CÀH bond in CH 4 has the highest bond energy (104 kcal mol À1 ) amongst organic substrates. In nature, the controlled oxidation of organic substrates is mediated by an important class of enzymes known as monooxygenases and dioxygenases, [1] and the methane monooxygenases are unique in their capability to mediate the facile conversion of methane to methanol. [2,3] With a turnover frequency approaching 1 s À1 , 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. [3] 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. [4,5] We show herein that these model tricopper complexes can mediate efficient catalytic oxidation of methane to methanol as well.The oxidation of CH 4 mediated by the tricopper complex [Cu I Cu I Cu I (7-N-Etppz)] 1+ in acetonitrile (ACN), where 7-N-Etppz corresponds to the ligand 3,3'-(1,4-diazepane-1,4diyl)bis[1-(4-ethylpiperazine-1-yl)propan-2-ol], is summarized in Figure 1 A. A single turnover (turnover number; TON = 0.92) is obtained when this Cu I Cu I Cu I complex is activated by excess dioxygen in the presence of excess CH 4 (Figure 1 B). 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 k 1 = 0.065 min À1 (Figure 1 B, inset). If we assume that the kinetics is limited by the dioxygen activation of the Cu I Cu I Cu I cluster with the subsequent O-atom transfer to the substrate molecule being rapid, then k 1 = k 2 ·[O 2 ] 0 , and from the solubility of oxygen in ACN at 25 8C (8.1 mm), [6] we obtain the bimolecular rate constant k 2 of 1.33 10 À1 m À1 s À1 for the dioxygen activation of the Cu I Cu I Cu I 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. [7,8] The process can be made catalytic by adding the appropriate amounts of H 2 O 2 to regenerate the spent catalyst after O-atom transfer from the activated tricopper complex to CH 4 . This multiple-turnover reaction is depicted in Figure 1 C. In these experiments, the [Cu I Cu I Cu I (7-N-Etppz)] 1+ catalyst is activated by O 2 as in the single-turnover experiment described earlier, but the spent catalyst is regenerated by twoelectron ...

Catalysis of alkane oxidation by a tricopper complex. The tricopper complex can mediate efficient conversion of small alkanes to their corresponding alcohols without over oxidation under ambient conditions.

Following recent progress towards understanding the structure of the particulate methane monooxygenase in methanotrophic bacteria, it is now possible to attempt the development of laboratory catalysts for the conversion of methane into MeOH under ambient conditions. To this end, a class of tricopper complexes that are capable of efficiently oxidizing small hydrocarbon substrates at room temperature has recently been developed. In this Minireview, we describe the development of a tricopper cluster to accomplish the catalytic conversion of methane into MeOH, as well as a number of small n‐alkanes into their corresponding alcohols and ketones, with high efficiencies. The properties of this robust catalytic system are discussed.

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