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 NN^RC^R′ (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.