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

## TL;DR

This paper introduces a new copper complex that uses CO2 to efficiently convert terminal alkynes into carboxylic acids and esters.

## Contribution

The study presents the first structurally characterized dearomatized Cu-CNC complexes used in direct carboxylation via metal-ligand cooperativity.

## Key findings

- Dearomatized Cu-CNC complexes were crystallographically and spectroscopically characterized for the first time.
- The enthalpy and entropy of formation and activation parameters for the complex were experimentally determined.
- New copper intermediates like copper-styrenyl and copper-propiolate were identified through NMR and computational studies.

## 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 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 [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.

This work highlights the first structurally characterized dearomatized Cu-CNC complexes and their application as catalysts for the direct carboxylation of terminal alkynes via metal–ligand cooperativity (MLC).