Catalytic Hydrogenation of CO2 to Formates by a Lutidine-Derived Ru–CNC Pincer Complex: Theoretical Insight into the Unrealized Potential

Abstract

Metal–ligand cooperative properties of a bis-N-heterocyclic carbene ruthenium CNC pincer catalyst and its activity in CO2 hydrogenation to formates were studied by DFT calculations complemented by NMR spectroscopy and kinetic measurements. The dearomatized Ru–CNC* pincer (1*) is significantly more reactive toward metal–ligand cooperative activation of H2 and CO2 than the structurally related phosphine-based Ru–PNP complex. The enhanced reactivity of Ru–CNC* stems from the combination of electronic properties of this system and the reduced geometric constraints imposed onto the Ru center by the large and flexible CNC chelate. Heterolytic dissociation of H2 by 1* results in the bis-hydrido complex 2 that is active in hydrogenation of CO2. However, under commonly applied reaction conditions, the catalyst rapidly deactivates via metal–ligand cooperative paths. The transient formation of the dearomatized complex Ru–CNC* (1*) in the course of the reaction leads to the irreversible cooperative activation of CO2,...