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 ...

Reaction of Fe(CO)2(NO)2 and sparteine/tetramethylethylenediamine (TMEDA) in tetrahydrofuran afforded the electron paramagnetic resonance (EPR)-silent, neutral {Fe(NO)2}10 dinitrosyliron complexes (DNICs) [(sparteine)Fe(NO)2] (1) and [(TMEDA)Fe(NO)2] (2), respectively. The stable and isolable anionic {Fe(NO)2}9 DNIC [(S(CH2)3S)Fe(NO)2]- (4), with a bidentate alkylthiolate coordinated to a {Fe(NO)(2)} motif, was prepared by the reaction of [S(CH2)3S]2- and the cationic {Fe(NO)2}9 [(sparteine)Fe(NO)2]+ (3) obtained from the reaction of complex 1 and [NO][BF4] in CH(3)CN. Transformation from the neutral complex 1 to the anionic complex 4 was verified via the cationic complex 3. Here complex 3 acts as an {Fe(NO)2}-donor reagent in the presence of thiolates. The EPR spectra of complexes 3 and 4 exhibit an isotropic signal with g = 2.032 and 2.031 at 298 K, respectively, the characteristic g value of {Fe(NO)2}9 DNICs. On the basis of N-O/Fe-N(O) bond lengths of the single-crystal X-ray structures of the {Fe(NO)2}9/{Fe(NO)2}10 DNICs, the oxidation level of the {Fe(NO)2} core of DNICs can be unambiguously assigned. The mean N-O distances falling in the range of 1.214(6)-1.189(4) A and the Fe-N(O) bond distances in the range of 1.650(7)-1.638(3) A are assigned as the neutral {Fe(NO)(2)}(10) DNICs. In contrast, the mean N-O bond distances ranging from 1.178(3) to 1.160(6) A and the mean Fe-N(O) bond distances ranging from 1.695(3) to 1.661(4) A are assigned as the anionic/neutral/cationic {Fe(NO)2}9 DNICs. In addition, an EPR spectrum in combination with the IR nu(NO) (the relative position of the nu(NO) stretching frequencies and their difference Deltanu(NO)) spectrum may serve as an efficient tool for discrimination of the existence of the anionic/cationic/neutral {Fe(NO)2}9 DNICs and the neutral {Fe(NO)2}10 DNICs.

Highlights  Subunit B (PmoB) of particulate methane monooxygenase (pMMO) is expressed in E. coli.  PmoB and its variants/mutants are expressed in the membranes as Cu I proteins.  The PmoB of pMMO contains a Cu I sponge with high reduction potentials for the Cu sites.  The PmoB proteins show evidence of a dinuclear copper site.  The PmoB-enriched E. coli membranes produce H 2 O 2 .

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