Metal ligand cooperativity in the direct carboxylation and esterification of terminal alkynes by Cu-CNC complexes bearing 2,6-lutidine linkers

CO₂ utilization is a significant and emerging field in catalysis, playing a crucial role in reducing atmospheric CO₂ and mitigating climate change. In this work, we report on Cu(i) complexes that utilize atmospheric CO₂ for the direct carboxylation and esterification of terminal alkynes. The Cu(i) complexes bear ligands of the type 2,6-bis(3-alkyl/arylimidazol-2-ylidene) methylpyridine I(R)^{C^N^C}, where R = ⁱPr, Me, 2,6-ⁱPr₂Ph (Dipp), 2,4,6-Me₃Ph (Mes), and 4-CF₃Ph. While copper-catalyzed carboxylation reactions are not unprecedented, this work presents the first example of metal ligand cooperativity (MLC) through a dearomatization-aromatization process used in the direct carboxylation of terminal alkynes. It also presents the first dearomatized Cu-CNC complexes that have been crystallographically and spectroscopically characterized. Further investigation using UV-vis spectroscopy revealed the enthalpy and entropy of formation, as well as the activation parameters for the dearomatized [Cu^I(I(ⁱPr)^{C^N^C})*] complex. This marks the first time such data have been reported for dearomatized-metal-CNC systems. To establish mechanistic details of the reaction, we performed stoichiometric reactions and characterized products with a variety of NMR methods. Combined with supporting computational studies, the work yields several new CNC-supported copper intermediates, including copper-styrenyl, copper-acetylide, and copper-propiolate. While the reactive and labile nature of some of these intermediates precludes their solid-state characterization, DFT-computed structures are consistent with spectroscopic characterization.