Understanding the Individual and Combined Effects of Solvent and Lewis Acid on CO₂ Insertion into a Metal Hydride

The insertion of CO₂ into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO₂ conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO₂, is important in both organic synthesis and strategies for hydrogen storage using organic liquids. There are however few experimental studies probing the mechanism of these reactions and identifying the effects of specific variables such as Lewis acid (LA) additives or solvent, which have been shown to significantly impact catalytic performance. In this study, we use a rapid mixing stopped-flow instrument to study the kinetics of CO₂ insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF₆ (tpy = 2,2':6',2″-terpyridine, bpy = 2,2'-bipyridine) in various solvents, both in the presence and in the absence of a LA. We show that LAs can increase the observed rate of this reaction and determine the first quantitative trends for the rate enhancement observed for CO₂ insertion in the presence of cationic LAs, Li⁺ ≫ Na⁺ > K⁺ > Rb⁺. Furthermore, we show that the rate enhancement observed with LAs is solvent dependent. Specifically, as the acceptor number (AN) of the solvent increases, the effect of the LA becomes smaller. Last, we demonstrate that there is a significant solvent effect on CO₂ insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO₂ insertion, this work shows that the best model for the rate of insertion is based on the Dimroth-Reichardt E_T(30) value of the solvent, a parameter that better accounts for specific solute/solvent interactions.