Correlation between Key Steps and Hydricity in CO2 Hydrogenation Catalysed by Non-Noble Metal PNP-Pincer Complexes

Abstract

Transition metal-catalysed homogeneous hydrogenation of CO2 to formate or formic acid has emerged as an appealing strategy for the reduction of CO2 into value-added chemicals. Since the state-of-the-art catalysts in this realm are primarily based on expensive precious metals and require demanding reaction conditions, the design and development of economically viable non-noble metal catalysts are in great demand. Herein, we exploit the thermodynamic correlation between the crucial reaction steps of CO2 hydrogenation, that is, base-promoted H2-splitting and hydride transfer to CO2 as a guide to estimate the catalytic efficiency of non-noble metal complexes possessing a ligand backbone containing a secondary amine as an “internal base”. A set of three non-noble metal complexes, one bearing tri-coordinated PNP-pincer (1Mn) and the other two based on tetra-coordinated PNPN-pincer (2Mn and 3Fe), have been investigated in this study. The computational mechanistic investigation establishes the role of the “internal” amine base in heterolytically splitting the metal-bound H2, a critical step for CO2 hydrogenation. Furthermore, the thermodynamic correlation between the hydricity (ΔGH−°) of the in situ generated metal-hydride species and the free energy barrier of the two crucial steps could provide an optimal hydricity value for efficient catalytic activity. Based on the computational estimation of the optimal hydricity value, the tri-coordinated PNP-pincer complex 1Mn appears to be the most efficient among the three, with the other two tetra-coordinated PNPN-pincer complexes, 2Mn and 3Fe, showing promising hydricity values. Overall, this study demonstrates how the crucial thermodynamic and kinetic parameters for pincer-based complexes possessing an “internal base” can be correlated for the prediction of novel non-noble metal-based catalysts for CO2 hydrogenation.