Effect of backbone linker (O vs NH) on the ability of pincer-supported nickel hydrides to reduce CO2

The reactivity of nickel hydrides supported by a pincer ligand often can be tuned by modifying the ligand at the three donor sites. In this study, ligand modification is made at the backbone, more specifically the linkers X and Y in {2,6-C₆H₃(XP^tBu₂)(YP^tBu₂)}NiH. Of the three nickel hydrides investigated herein, the PNCNP-pincer complex (X = Y = NH) is the most reactive one towards CO₂, whether it is for the rate of CO₂ capture from air or for the thermodynamic favorability of CO₂ insertion into the Ni–H bond. The POCOP-pincer complex (X = Y = O) is the least reactive hydride whereas the POCNP-pincer complex (X = O, Y = NH) is ranked in the middle. To have access to the hybrid pincer complex, an improved synthetic method for the proligand, m-C₆H₄(OP^tBu₂)(NHP^tBu₂), is also developed.