Methanol Carbonylation Catalyzed by the Anion of the Complex Dicarbonyldiiodorhodium(I). A Density Functional Study of the Catalytic Cycle

The potential energy profile of the full catalytic cycle of methanol carbonylation catalyzed by [Rh(CO) 2 I 2 ] - complex was explored computationally using a gradient-corrected density functional method. The equilibrium structures of all isomers of the intermediates involved in the catalytic process have been calculated. The transition states of CH 3 I oxidative addition, the CO migratory insertion, and the CH 3 COI reductive elimination were also located. The rate-determining step of the reaction, CH 3 I oxidative addition, was found to proceed via a back-side S N 2 mechanism. The activation barrier of CO migratory insertion is calculated lower than that of CH 3 I reductive elimination; this finding confirms the hypothesis that the unstable nature of the complex [RhCH 3 (CO) 2 I 3 ] - is mainly due to its fast decomposition into the acyl species. The trans conformers of the six-coordinated intermediates [RhCH 3 (CO) 2 I 3 ] - and [Rh(CH 3 CO)(CO) 2 I 3 ] - are more stable than their cis conformers. The activation barriers of CO migratory insertion into the Rh−CH 3 bond of [RhCH 3 (CO) 2 I 3 ] - and of CH 3 COI reductive elimination from [Rh(CH 3 CO)(CO) 2 I 3 ] - are higher for the trans isomers than those of the corresponding cis isomers. Therefore, the lowest-energy path is determined by the corresponding cis dicarbonyl species which have to be accessed by a ligand rearrangement. Solvent effects of the intermediates were calculated to increase from 6-fold to 5-fold to 4-fold coordinated complexes. While the solvent effects on the transition states are in general similar to those of the six-coordinated complexes, they affect oxidative addition and the reductive elimination steps in a crucial way.