Mechanistic insights and comparative analysis of Ru(ii)-NNC pincer complexes with anionic-, protic-, and classical-NHCs for transfer hydrogenation of ketones

Abstract

A comparison of catalytic activity of a series of ruthenium(ii)-NNC pincer type complexes bearing unsymmetrical pincer ligands NNRCR′ (R = Me, OMe and R′ = H, Me) with protic-NHC (R′ = H

1-4) and classical NHC (R′ = Me

5-7) in transfer hydrogenation of ketones is reported. The new Ru-NNC complexes (3-7) have been characterized by multinuclear NMR, and HRMS, and structures of 3 and 6 have been determined using single crystal X-ray diffraction technique. Complexes 1-7 are assessed as catalysts for the transformation of various ketones to their corresponding alcohols. Notably, under optimized conditions, maximum TON values up to 9900 and TOF 550 h−1 were achieved using protic-NHC complexes 1 and 3. Control experiments with CuI as a phosphine scavenger or with excess PPh3 revealed that the phosphine dissociation enhanced the catalytic activity. The slightly high catalytic activity of 1 and 3 in the presence of catalytic amounts of strong bases is attributed to the deprotonation of NH functionality that facilitates phosphine dissociation. Mechanistic investigations using mass and NMR analyses reveal that complexes 1 and 3 are converted to their anionic-NHC forms 1′ and 3′, respectively, and remain deprotonated during the catalytic cycle. NMR tube experiments with 1 and 1′ support that the 2-propanol failed to protonate the anionic NHC complex 1′ under catalytic conditions. Computational studies using DFT are carried out to investigate the differences between the protic-, anionic-, and classical-NHC forms in the TH of ketones. DFT studies reveal that the TH catalysis using these complexes follows an inner-sphere mechanism as the protonation of anionic-NHC required for an outer-sphere mechanism involves a high energy transition state. The proposed mechanism, based on experimental and theoretical studies, suggests that phosphine dissociation is the rate-determining step (RDS), and the anionic-NHC complex was slightly more active than other complexes due to the comparatively smaller dissociation energy required for phosphine dissociation. © 2023 The Royal Society of Chemistry.