A quantum mechanics/molecular mechanics study of the steric influence of the PR3 spectator ligands on the energetics of ethylene insertion into the Rh–H bond of HRh(PR3)2(CO)(η2-CH2CH2)

Abstract

Details of a quantum mechanics/molecular mechanics (QM/MM) study aimed at determining how the substituents of the spectator phosphine ligands influence the energetics of the key step in olefin hydroformylation, the insertion of an olefin into the Rh–H bond of HRh(PR₃)₂(CO)(η²-olefin), are discussed. For ethylene there are a total of three insertion pathways originating from the two η²-ethylene isomers, with either a bis-equatorial arrangement (ee) or a mixed equatorial, axial arrangement (ea) of the two PR₃ ligands. The energetics, kinetic and thermodynamic, of these three pathways were computed for a variety of phosphine substituents (PR₃; R = Me, ^tBu, Ph, meta-PhSO₃⁻, and para-PhSO₃⁻) using the ONIOM QM/MM approach. These calculations predict that two reaction channels, one originating from the more stable ea η²-ethlyene adduct and the other from the less stable ee η²-ethylene adduct, will be operative for the ethylene insertion reaction with the PMe₃ ligand system, although the latter will be preferred thermodynamically over the former. In the case of the aryl phosphine ligands, a clear energetic preference, kinetically and thermodynamically, was found for ethylene insertion to proceed from the least stable ee ethylene adduct versus the paths originating from the more stable ea ethylene adduct.