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

The insertion of CO into metal-element σ-bonds such as M-H, M-OH, and M-NH is often proposed to be a key step in catalytic CO conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO insertion or comparing insertion into LM-E (E = H, OH, or NH) complexes with the same ancillary ligands. Here, we use a rapid mixing stopped-flow instrument to measure the rate of CO insertion into (PCP)Ni(OH) (PCP = 2,6-CH(CHPBu)) and show that it proceeds at a faster rate than insertion into the corresponding metal hydride. We also demonstrate that the rate of CO insertion into (PCP)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 CO insertion into (PCP)Ni(NH); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO insertion into analogous LM-E (E = H, OH, or NH) systems and provides quantitative experimental evidence that the rate of insertion into a metal-element σ-bond correlates with the nucleophilicity of E.