Kinetic studies of CO2 insertion into transition metal hydrides provide evidence about the nature of the rate-determining transition state.

Carbon dioxide (CO 2 ) is an appealing feedstock for the sustainable preparation of a variety of carbon-based commodity chemicals because of its high abundance, low cost, and nontoxicity. The high kinetic and thermodynamic stability of CO 2 , however, means that there are currently only a limited number of practical catalytic systems for the conversion of CO 2 into more valuable chemicals, and continued research in this area is required. One promising approach for the eventual transformation of CO 2 is to initially insert the molecule into transition-metal−element σ bonds such as M−H, M−OR, M−NR 2 , and M−CR 3 bonds to form products of the type M−OC(O)E (E = H, OR, NR 2 , or CR 3 ). CO 2 insertion has been demonstrated in numerous stoichiometric reactions involving transition-metal complexes, but in cases where insertion results in the formation of strong M−O bonds, the products are often too stable to undergo further transformations. Group 9 and 10 transition-metal complexes (M = Ni, Pd, Pt, Co, Rh, or Ir) form relatively weak M−O bonds, and as a consequence, a number of group 9 and 10 transition-metal catalysts in which CO 2 insertion is proposed as an elementary step in catalysis have been developed. In this Award Article, we summarize group 9 and 10 transition-metal complexes in which CO 2 insertion into a metal−element σ bond to form a M−OC(O)E-type product has been observed. Mechanistic similarities and differences are highlighted by comparing CO 2 insertion reactions in different types of group 9 and 10 metal−element σ bonds, and a general trend for predicting the ratedetermining step of the insertion process is described based on the nucleophilicity of the element in the σ bond. Although we focus on stoichiometric reactivity, the relevance of CO 2 insertion to catalytic reactions is also emphasized throughout the paper.

The insertion of CO 2 into a metal hydride bond to form a metal formate is a key elementary step in many catalytic cycles for CO 2 conversion. Similarly, the microscopic reverse reaction, the decarboxylation of a metal formate to form a metal hydride and CO 2 , 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 CO 2 insertion into the cationic ruthenium hydride [Ru(tpy)bpy)H]PF 6 (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 CO 2 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 CO 2 insertion in the absence of a LA. Although the AN of the solvent has been previously used to predict the rate of CO 2 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.