Mechanism of CO2 hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: from a theoretical description to performance optimization

Abstract

The reaction mechanism of CO₂ hydrogenation by pyridine-based Ru-PNP catalyst in the presence of DBU base promoter was studied by means of density functional theory calculations. Three alternative reaction channels promoted by the complexes potentially present under the reaction conditions, namely the dearomatized complex 2 and the products of cooperative CO₂ (3) and H₂ (4) addition, were analysed. It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path involving a direct effectively barrierless hydrogenolysis of the polarized complex 5*. The reaction rate in this case is controlled by the CO₂ activation by Ru–H that proceeds with a very low barrier of ca. 20 kJ mol⁻¹. The catalytic reaction can be hampered by the formation of a stable formato-complex 5. In this case, the rate is controlled by the H₂ insertion into the Ru–OCHO coordination bond, for which a barrier of 65 kJ mol⁻¹ is predicted. The DFT calculations suggest that the preference for the particular route can be controlled by varying the partial pressure of H₂ in the reaction mixture. Under H₂-rich conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments verify these theoretical predictions. The apparent activation energies measured at different H₂/CO₂ molar ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO₂ hydrogenation catalyst allowing reaching turnover frequencies in the order of 10⁶ h⁻¹ at elevated temperatures. Moreover, a minor temperature dependency of the reaction rate attainable in excess H₂ points to the possibility of efficient CO₂ hydrogenation at near-ambient temperatures.