Diatomic [CuO]+ and its role in the spin-selective hydrogen- and oxygen-atom transfers in the thermal activation of methane.
The activation of methane and its subsequent conversion into more valuable feedstocks at ambient conditions is regarded as one of the major challenges in contemporary catalysis. In this context, two different transformations are of particular interest. The first one concerns the oxidative coupling of methane (OCM) to the C2 hydrocarbons ethane and ethylene using metal oxide based catalysts in heterogeneous catalysis; 2] the second process is the selective oxidation of methane to methanol, which is performed in nature by the methane monoxygenase (MMO) metalloenzymes. Soluble MMO (sMMO) contains a well-characterized doubly oxygenbridged di-iron cluster; in contrast, the reactivity of particulate MMO (pMMO), after a long controversy about the nature of its active site, has been shown to depend on copper. A useful approach to investigate model systems for oxygen-containing catalysts takes advantage of state-of-theart gas-phase experiments conducted in a mass spectrometer, in conjunction with computational studies; this combined experimental and theoretical approach provides insight into the elementary steps of these reactions at a molecular level and, thus, permits us to unravel detailed mechanistic aspects. For example, the efficient gas-phase activation of methane at room temperature has been demonstrated to be brought about by a variety of systems, including transitionand maingroup-metal oxides as well as some selected nonmetal oxides and mixed metal/nonmetal oxides; based on these studies, a rather detailed understanding of the intriguing mechanistic aspects has been arrived at. With respect to biological relevance, it was demonstrated twenty years ago that bare [FeO] is capable of activating methane at room temperature. The now well-established concept of two-state reactivity (TSR), which also proved important in describing the mechanisms of metalloenzymemediated reactions, is in fact based on a detailed analysis of the gas-phase reactions of this simple, diatomic reagent [FeO]. Yet, only recently has a complete description of the gas-phase conversion of methane to methanol by [FeO] been achieved; this elucidation was based on advanced gas-phase spectroscopy combined with rather high-level calculations. Further, while the detailed nature of the active copper oxide species in pMMO had been under debate for quite some time, 4, 12] bare [CuO] was predicted a decade ago to be a suitable, if not extremely powerful, candidate to mediate the methane to methanol conversion. 14] However, no gasphase experiments with bare [CuO] have been reported to date. The ligated cation [Cu(O)(phen)] (phen = 1,10-phenanthroline) brings about activation of small hydrocarbons, that is, propane or butane, but it is not powerful enough to attack the thermodynamically strong and kinetically inert C H bond of methane. Owing to the relatively low dissociation energy D0(Cu + O) = 130 kJ mol , it proved rather difficult to produce sufficient amounts of [CuO] to probe its reactivity in bond-activation processes, and various attempts to generate this cationic metal oxide by, for example, electrospray ionization mass spectrometry failed. 15] Thus, [CuO] is to date the only bare transition-metal oxide cation of the first row whose reactivity towards methane has not been experimentally investigated. Herein we present our results on 1) the successful formation of gaseous [CuO] and 2) its reactivity towards methane at thermal conditions. Briefly, [CuO] is generated by laser desorption/ionization from isotopically pure copper Cu targets, suitable for the laser-vaporization/ionization source of an FT-ICR mass spectrometer in the presence of a He/N2O plasma (for details about the instrumental setup, see the Experimental Section). As shown in Figure 1, [CuO] brings about efficient activation of methane at room temperature both by hydrogen abstraction [Eq. (1)] and by oxygenatom transfer [Eq. (2)]. Furthermore, the open-shell product cation [CuOH]C itself also homolytically cleaves the C H bond of a second methane molecule, thus giving rise to the formation of a closed-shell water complex [Eq. (3)].