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

Abstract

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 LnM–E (E = H, OH, 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(CH2PtBu2)2) and show that it proceeds at a faster rate in comparison to 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 inse...