Computational Study on a Transfer Hydrogenation Catalysed by a Ru(II) Bis-Pyrazolyl Pyridine Complex

Density functional calculations were performed to investigate the Ru-catalysed transfer hydrogenation of acetophenone to 1-phenylethanol using isopropanol as the reducing agent. The catalyst is [Ru(bpp)Cl-2(PPh3)], where bpp is a tridentate bis-pyrazolyl pyridine ligand. We studied an inner sphere monohydridic mechanism where the active Ru centre binds, in addition to bpp, one of the original ligands as a spectator, which can be either chloride or triphenylphosphine. In both cases, catalyst initiation appears to be feasible (with triphenylphosphine being the preferred spectator ligand) and the catalytic cycle is efficient with a small energetic span. For which one of the cases the calculated energetic span is smaller depends on the treatment of entropy in solution. Since pathways involving dissociation of charged ligands are not preferred, the product is formed by a fast proton transfer within the Ru coordination sphere, from a coordinated isopropanol to the product alcoholate. Subsequently, both product release and catalyst regeneration can be accomplished by the dissociation of the charge neutral product alcohol. The beta-hydride transfer from a coordinated isopropanolate anion to ruthenium is slightly different depending on whether there is a chloride or phosphine present trans to the alcoholate. In both cases, this step contains the transition state with the highest Gibbs energy.

Automated summary

Density functional calculations were used to study the Ru-catalyzed transfer hydrogenation of acetophenone to 1-phenylethanol using isopropanol as the reducing agent. The study revealed that both chloride and triphenylphosphine ligands can initiate the catalytic cycle efficiently, with triphenylphosphine being preferred. Proton transfer within the Ru coordination sphere plays a crucial role in the formation of the product, while product release and catalyst regeneration occur through the dissociation of the neutral product alcohol. The transition state involving beta-hydride transfer from a coordinated isopropanolate anion to ruthenium has the highest Gibbs energy.