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 carbon ylation catalyzed by [Rh(CO)(2)I-2](-) complex was explored computationally using a gradient-corrected density functional method. The equilibrium stru ctures of all isomers of the intermediates involved in the catalytic proces s have been calculated. The transition states of CH3I oxidative addition, t he CO migratory insertion, and the CH3COI reductive elimination were also l ocated: The rate-determining step of the reaction, CH3I oxidative addition, was found td proceed via a back-side S(N)2 mechanism. The activation barri er bf CO migratory insertion is calculated; thesis that the lower than that of CH3I reductive elimination; this finding confirms the hypo unstable nat ure of the complex [RhCH3(CO)(2)I-3](-) is mainly due to its fast decomposi tion into the acyl species. The trans conformers of the six-coordinated int ermediates [RhCH3(CO)(2)I-3](-) and [Rh(CH3CO)(CO)(2)I-3](-) are more stabl e than their cis conformers. The activation barriers of CO migratory insert ion into the Rh-CH3 bond of [RhCH3(CO)(2)I-3](-) and of CH3COI reductive el imination from [Rh(CH3CO)(CO)(2)I-3](-) are higher for the trans isomers th an those of the corresponding cis isomers, Therefore, the lowest-energy pat h is determined by the corresponding cis dicarbonyl species which have to b e accessed by a ligand rearrangement. Solvent effects of the intermediates were calculated to increase from 6-fold to 5-fold to 4-fold coordinated com plexes. While the solvent effects on the transition states are in general s imilar to those of the six-coordinated complexes, they affect oxidative add ition and the reductive elimination steps in a crucial way.