DFT Study of the Ethylene Hydroformylation Catalytic Cycle Employing a HRh(PH3)2(CO) Model Catalyst

Abstract

The potential energy hypersurface for ethylene hydroformylation catalyzed by HRh(PH3)2(CO) was mapped out at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory using effective core potentials. Combining the results obtained for each elementary step there are a number of possible pathways for the hydroformylation catalytic cycle, originating from the trans (2a) and cis (2b) isomers of the active catalyst. At both levels of theory employed, a preference was predicted for the pathways originating from the trans isomer of the active catalyst, 2a. The alternative pathways originating from the cis isomer, 2b, were discounted because of the large activation barriers predicted for the two migratory insertion reactions, arising from unfavorable interactions between the equatorial phosphine ligands and the migrating axial ligand. Considering only those reaction paths originating from the trans isomer of the active catalyst, 2a, a strong preference was identified for the oxidative addition of H2 to the unsaturated Rh-acyl complex (6a) on the same side as the ethyl moiety of the acyl ligand, as opposed to the addition on the opposite face (i.e., on the same side as the acyl oxygen). In the final aldehyde reductive elimination step an energetic preference was predicted for the migration of the hydride ligand trans to the CO ligand in the most stable H2 oxidative addition products; however, this migration would lead to the generation of the cis catalyst instead of the trans catalyst. Therefore, either the less electronically favored hydride ligand trans to the PH3 ligand migrates to the acyl carbon, thereby regenerating 2a, or there must be some interconversion between the pathways originating from 2a and 2b. This interconversion would most likely occur at either the η2-olefin adduct (3b) or the CO addition intermediate (5b), since previous research indicates that complexes of this type can undergo facile pseudorotation. For the energetically feasible catalytic cycle, the CO insertion step is predicted to be the rate-determining step with predicted activation barriers of 20.4 and 14.9 kcal/mol, at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory, respectively. The experimental enthalpy of hydroformylation (−28 kcal/mol), corresponding to the energy difference between the end aldehyde product and the constituent reactant species, C2H4, CO, and H2, is overestimated by about 7 kcal/mol (−34.7 kcal/mol) at the B3LYP//B3LYP level. However, recomputing the energies of the species with the CCSD(T) methodology yields a value of −24.4 kcal/mol, which is more in accord with experiment.