Mechanistic insight into the isomerization of allyl alcohol catalyzed by the Co(II)-PNP catalyst: Crucial role of spectator ligand

Abstract

Pincer complexes based on first-row transition metals have a wide range of applications in the field of homogeneous catalysis. However, the variable coordination geometry and electronic structure bring about complicated influencing factors, which restricts the deep understanding of the catalytic mechanisms. Herein, a detailed theoretical study based on the density functional theory (DFT) was conducted to illuminate the mechanistic preference of the isomerization of allyl alcohol catalyzed by the Co(II) pincer catalyst developed by de Vries et al. (Chem. Eur. J., 25 (2019) 7820–7825). Three different coordination geometries, two spin states (doublet and quartet) and corresponding pathways were taken into consideration. The calculation results suggest that the Co(II)-PNP catalyst prefers the inner-sphere migratory insertion mechanism at doublet state instead of metal-ligand cooperation (MLC) mechanism, which is consistent with the experimental results. Although the catalyst bears the NH functional ligands, the π−basic spectator ligands lead to the changeable coordination geometries, resulting in the emergence of spin crossover process with high energy barrier. Based on these findings, we designed a series of model catalysts and evaluated their catalytic activity on a theoretical level. The results indicate that π−acidic ligands and rigid tetradentate ligands can effectively avoid the occurrence of spin crossover processes and ensure that the Co(II) catalyst follows the MLC mechanism at low spin state. In addition, the activity of the catalyst can be precisely adjusted by changing the ligand substituents electronic effect. These findings and predictions are expected to provide guidance for future catalyst design.