Metal Ligand Cooperativity in the Direct Carboxylation and Esterification of Terminal Alkynes by Cu-CNC Complexes Bearing 2,6-Lutidine Linkers

Abstract

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 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 [CuI(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.