Kinetic Studies of CO₂ Insertion into Metal-Element σ-Bonds

ConspectusDespite the plethora of metal catalyzed reactions for CO₂ utilization that have been developed in academic laboratories, practical systems remain elusive. The understanding of the elementary steps in catalysis is a proven method to improve catalytic performance. In many catalytic cycles for CO₂ utilization, the insertion of CO₂ into a metal-element σ-bond, such as hydrides, alkyls, amides, or hydroxides, is a crucial step. However, despite the many demonstrations of CO₂ insertion, there are a paucity of kinetic studies, and information about the reaction mechanism has been predominantly elucidated from computational investigations. In this Account, kinetic studies on CO₂ insertion into late transition metal-element σ-bonds performed by my group are summarized, along with their implications for catalysis.A common pathway for CO₂ insertion into a metal hydride involves a two-step mechanism. The first step is nucleophilic attack on CO₂ by the hydride to generate an H-bound formate, followed by rearrangement to form an O-bound formate product. Kinetic studies on systems in which both the first and second steps are proposed to be rate-determining, known as inner-sphere and outer-sphere processes, respectively, show that insertion rates increase as (i) the ligand trans to the hydride becomes a stronger donor, (ii) the ancillary ligand becomes more electron-donating, and (iii) the Dimroth-Reichardt parameter of the solvent increases. However, the magnitude of these effects is generally smaller for inner-sphere processes because there is less buildup of charge in the key transition state. For similar reasons, the presence of Lewis acids only increases the rate of outer-sphere processes. These results suggest it may be possible to experimentally differentiate between inner- and outer-sphere processes.The insertion of CO₂ into a metal-alkyl bond results in the formation of a C-C bond, which is important for the generation of fuels from CO₂. For square planar Group 10 complexes, the presence of a strong donor ligand trans to the alkyl group is critical for kinetically promoting insertion. Further, the nucleophilicity of the alkyl ligand directly impacts the rate of CO₂ insertion via an S_E2 mechanism, as does the steric bulk of the complex, and the reaction solvent. In contrast to the relatively slow rates of insertion observed for metal alkyls, CO₂ insertion is rapid for metal hydroxides and amides. Although kinetics trends could be determined for hydroxides, reactions with amides are too fast for quantitative studies.Overall, the rates of insertion correlate with the nucleophilicity of the element in the metal-element σ-bond, so amide > hydroxide > hydride > alkyl. Due to the related pathways for insertion, similar trends in ligand and solvent effects are observed for insertion into different metal-element σ-bonds. Thus, the same strategies can be used to control the rates of insertion across systems. Differences in the magnitude of solvent and ligand effects are caused by variation in the amount of charge build-up on the metal in the rate-determining transition state. Likely, given that CO₂ is related to organic molecules such as aldehydes, ketones, and amides, the results described in this Account are general to a wider range of substrates.