Catalytic Hydrogenation of CO 2 to Formates by a Lutidine-Derived Ru–CNC Pincer Complex: Theoretical Insight into the Unrealized Potential

Metal–ligand cooperative properties of a bis-N-heterocyclic carbene ruthenium CNC pincer catalyst and its activity in CO 2 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 H 2 and CO 2 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 H 2 by 1* results in the bis-hydrido complex 2 that is active in hydrogenation of CO 2 . 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 CO 2, resulting in the stable adduct 3 that is not catalytically competent. By an increase in the H 2 /CO 2 ratio, this deactivation path can be effectively suppressed, resulting in a stable and rather high catalytic performance of Ru–CNC.