Density functional theory study of the direct conversion of methane to acetic acid by RhCl3

It has recently been reported by Sen et al. that dioxygen can functionalize methane directly at low temperatures with RhCL3 as the catalyst and I- as the promoter. The main products are acetic acid and methanol, with formic acid as a side product. The active form of the catalyst is considered to be [Rh(CO)(2)I-2](-). We propose here a mechanism for the Sen process and investigated it theoretically with DFT. The proposed mechanism is as follows. In the first step, a methane C-H bond is activated by [Rh(CO)(2)I-2](-) either through an oxidative-addition process or by a Q-bond metathesis mechanism, leading in both cases to a Rh-CH3 complex (a). In the next step a facile insertion of CO into the Rh-CH3 bond leads to a RhCOCH3 complex (b). Finally, the hydrolysis of a and b produces methanol and acetic acid, respectively, and forms [Rh(CO)(2)IH](-) (c). The oxidation of c by 02 leads to the peroxo complex [(HOO)Rh(CO)(2)I](-), which can react with another c to yield two hydroxo complexes of the form [(HO)Rh(CO)(2)I](-). Substitution of OH- by I- finally regenerates the [Rh(CO)(2)I-2](-) catalyst.