Enhanced Durability and Recyclability of Copper(II) Catalysts in Chan-Evans-Lam Coupling Reactions.

Abstract

Copper-catalyzed Chan-Evans-Lam (CEL) coupling has emerged as a powerful method for CN bond formation under mild conditions. In this study, a novel monodentate ligand, 4-((2,3,5,6-tetramethylbenzyl)amino)benzoic acid (TMABH), was rationally designed to construct copper paddlewheel-based architectures for CEL catalysis. The NH linkage in TMABH was deliberately incorporated as it acts as Lewis basic sites, facilitating proton transfer and stabilizing catalytic intermediates during CN coupling. Two distinct copper materials were synthesized: (i) a discrete dinuclear paddlewheel complex, [(Cu₂(TMAB)₄(DMF)₂)]·4DMF (referred here as Cu^II-0D), and (ii) a coordination polymer, {[(Cu₂(TMAB)₄(4,4'-bipyridine)₂)]}_n (Cu^II-1D), enabling a direct comparison of structural effects on catalytic performance. Both catalysts were comprehensively characterized using single-crystal X-ray diffraction, powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and CHN analyses. Catalytic evaluations revealed excellent activity across a range of arylboronic acids and Aniline in the presence of atmospheric oxygen, without requiring external oxidants. Cu^II-1D material serves as a robust heterogeneous catalyst with excellent recyclability, whereas its Cu^II-0D counterparts lose structural integrity after the first cycle, thereby limiting their reusability. This is the first report comparing the catalytic performance of discrete and polymeric copper paddlewheel frameworks derived from the same ligand, providing a clear structure-activity relationship. Furthermore, the study highlights the potential of copper paddlewheel-based architectures as robust, recyclable, and scalable heterogeneous catalysts for sustainable CN bond formation.