Effect of Nucleophilicity on the Kinetics of CO₂ Insertion into Pincer-Supported Nickel Complexes

The insertion of CO₂ into metal-element σ-bonds such as M-H, M-OH, and M-NH₂ is often proposed to be a key step in catalytic CO₂ conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO₂ insertion or comparing insertion into L_nM-E (E = H, OH, or NH₂) complexes with the same ancillary ligands. Here, we use a rapid mixing stopped-flow instrument to measure the rate of CO₂ insertion into (^tBuPCP)Ni(OH) (^tBuPCP = 2,6-C₆H₃(CH₂P^tBu₂)₂) and show that it proceeds at a faster rate than insertion into the corresponding metal hydride. We also demonstrate that the rate of CO₂ insertion into (^tBuPCP)Ni(OH) can generally be increased by using solvents with higher acceptor numbers or by modifying the ligand scaffold to reduce steric bulk or increase electron donation to the metal center. We also attempt to measure the rate of CO₂ insertion into (^tBuPCP)Ni(NH₂); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO₂ insertion into analogous L_nM-E (E = H, OH, or NH₂) systems and provides quantitative experimental evidence that the rate of insertion into a metal-element σ-bond correlates with the nucleophilicity of E.