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

• Improved synthetic route to a POCNP-type proligand, m -C 6 H 4 (OP t Bu 2 )(NHP t Bu 2 ). • Synthesis of a new nickel hydride complex bearing the hybrid pincer ligand. • Systematic comparison of three pincer systems for CO 2 capture from air. • Systematic comparison of three pincer systems for ΔG of the CO 2 insertion reaction. 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-C 6 H 3 (XP t Bu 2 )(YP t Bu 2 )}NiH. Of the three nickel hydrides investigated herein, the PNCNP-pincer complex (X = Y = NH) is the most reactive one towards CO 2 , whether it is for the rate of CO 2 capture from air or for the thermodynamic favorability of CO 2 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 -C 6 H 4 (OP t Bu 2 )(NHP t Bu 2 ), is also developed. The PNCNP-, POCNP- and POCOP-pincer supported nickel hydride complexes can capture CO 2 from air and from nickel formate complexes bearing a different pincer ligand. The reactions are kinetically and thermodynamically more favorable when the backbone linkers have more NH groups.