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

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Abstract

The potential energy profile of the full catalytic cycle of methanol carbonylation catalyzed by [Rh(CO)2I2]- 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 CH3I oxidative addition, the CO migratory insertion, and the CH3COI reductive elimination were also located. The rate-determining step of the reaction, CH3I oxidative addition, was found to proceed via a back-side SN2 mechanism. The activation barrier of CO migratory insertion is calculated lower than that of CH3I reductive elimination; this finding confirms the hypothesis that the unstable nature of the complex [RhCH3(CO)2I3]- is mainly due to its fast decomposition into the acyl species. The trans conformers of the six-coordinated intermediates [RhCH3(CO)2I3]- and [Rh(CH3CO)(CO)2I3]- are more stable than their cis conformers. The activation barriers of CO migratory insertion into the Rh−CH3 bond of [RhCH3(CO)2I3]- and of CH3COI reductive elimination from [Rh(CH3CO)(CO)2I3]- 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.

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