Abstracts

The systematic and complete theoretical study of the catalytic cycle of methanol carbonylation catalyzed by [Rh(CO)₂I₂]^- complex was carried out using a gradient-corrected density functional method. The main goals of our study were an exploration of the free energy profile for the entire catalytic cycle of methanol carbonylation and clarification of the detailed mechanism for each elementary step with accurate account for environment effects.

The various possible isomers for the intermediates involved in the catalytic cycle were considered and their relative stability was estimated. The complex [Rh(CO)₂I₂]^- was determined to have preferential cis conformation. The six-coordinated [RhCH₃(CO)₂I₃]^- and [RhCH₃CO(CO)₂I₃]^- species were calculated to exist in a form of trans isomers. These results are in agreement with the experimental data with the exception of complex [RhCH₃(CO)₂I₃]^- which was characterized as a cis isomer by IR and NMR spectroscopy. As all isomers of species [RhCH₃(CO)₂I₃]^- have the similar energies within 8 kJ/mol it is likely that they are easily converted to each other.

The geometrical and energetic parameters of the transition states for the activated reaction steps such as CH₃I oxidative addition, CO migratory insertion and CH₃COI reductive elimination were elucidated. The first elementary step of the catalytic cycle, CH₃I oxidative addition, was calculated to have the highest activation barrier. Its energy is 189 kJ/mol in gas phase and 135 kJ/mol in solution. According to the calculations CH₃I oxidative addition proceed via a back-side S_N2 mechanism. A front-side approach was calculated to have the higher activation barrier of 194 kJ/mol. The CO migratory insertion and CH₃COI reductive elimination in solution, were calculated to proceed with smaller activation barriers of 75 and 73 kJ/mol, respectively. The activation barriers of CO migratory insertion and of CH₃COI reductive elimination 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.

The magnitude of the solvent effect was found to decrease on going from six-fold to five-fold to four-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.

Note. Abstracts are published in author's edition

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