The oxidative addition and migratory 1,1-insertion in the Monsanto and Cativa processes. A density functional study of the catalytic carbonylation of methanol

Abstract

Density functional theory with hybrid B3LYP exchange and correlation functional has been used to investigate the first two catalytic reactions, the oxidative addition and migratory 1,1-insertion of the Monsanto and Cativa processes. One of the main interests has been to study if the previously unidentified trans forms of the active catalytic species [Rh(CO) 2I 2] − ( 1) or [Ir(CO) 2I 2] − ( 2) have any significance in these processes. The oxidative addition has been studied using both cis and trans forms of 1 and 2. We have also studied the oxidative addition of methyl iodide to [Rh(CO) 2I 3] 2− ( 3). In addition, different isomers of dicarbonyls [CH 3Rh(CO) 2I 3] − ( 4) and [CH 3Ir(CO) 2I 3] − ( 5) and tricarbonyl [CH 3Ir(CO) 3I 2] ( 8) has been used in the 1,1-insertion study to see if these could provide new, alternative reaction pathways. The calculated free energies of activation for the oxidative addition of methyl iodide to cis- 1 and cis- 2 are 20.8 and 16.9 kcal/mol, respectively. The corresponding free energy barriers for trans- 1 and trans- 2 are 15.0 and 13.2 kcal/mol, respectively. The oxidative addition is the rate-determining step in the Monsanto process and the reaction with trans- 1 is predicted to accelerate that step. The presence of 3 could enhance the addition even more; the free energy of activation is only 5.6 kcal/mol. For the 1,1-insertions we have found similar activation energies in fac, cis- and mer, trans-structures. In the rhodium system, the free energies of activation are in the order of 20 kcal/mol and in the iridium system 30 kcal/mol. Interestingly, the insertions in mer, cis-dicarbonyls have considerably lower activation energies, half of those calculated for the insertions in mer, trans- and fac, cis-structures. The iodide dissociation from fac, cis- 5 could provide the path to mer, cis- 5 and so significantly enhance the rate of the insertion in the iridium system. The rate of the insertion should also increase if experimentally proposed tricarbonyl 8 is used instead of 5. According to our calculations of different isomers of 8, this seem to be true although the insertion barrier in mer, cis- 5 is calculated to be even lower. Our results are consistent with the experiments and other computational results. These results show that the geometrical arrangement of the ligands has a very large effect on the catalytic activity of the complexes and this suggests possible improvements to these industrially important processes.