A Density Functional Theory Study of the Direct Conversion of Methane to Acetic acid by RhCl3.

Iordan Hristov, and Tom Ziegler*
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary, Alberta, Canada T2N 1N4

In the last years it was reported that dioxygen can directly functionalize  methane at low temperatures with RhCl3 as the catalyst and I- as the promoter. The main products are acetic acid and methanol. The active form of the catalyst is considered to be [Rh(CO)2I2]-. Here we have proposed a mechanism for this process and investigated it theoretically with DFT. Methane can be activated by [Rh(CO)2I2]- either by an oxidative addition or by a s-bond metathesis mechanism to a Rh-CH3 complex (a). A migratory insertion of CO is very favorable at this stage and a Rh-COCH3 complex (b) can be formed. The hydrolysis of a and b will produce methanol  and acetic  acid  respectively and form [HRh(CO)2I]- which itself can catalyze the same process of activation and functionalization  with even lower barriers. When this happens it will lead to the formation of [H2Rh(CO)2]- that can easily by oxidized by oxygen to the dihydroxy complex. After a ligand exchange with  I- the catalytic  cycle is completed.