Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO_2 Hydrogenation to Methanol and Catalyst Inhibition

摘要

The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)-hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equilibrium is approximately temperature-independent; relative to H-2, it is more favored at elevated temperatures. We also measure the hydricity of (PNP)Ru-H in THE and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.