N‐Heterocyclic Imine‐Supported Bimetallic Cu(II) Catalyst for Azide‐Alkyne Cycloaddition: Solvent‐free, Reductant‐free, ppm‐level Catalysis to Access 1,4‐Disubstituted Triazoles

In this article, we present a unique bimetallic paddle wheel copper(II) complex with the molecular formula [Cu2 C42 H54 N6 O8 ]. Several characterization tools have been employed to analyze this complex including single-crystal X-ray diffraction, HRMS, FTIR, and UV-vis spectroscopy. This copper(II) complex excels admirably as a catalytic system in parts-per-million level (ppm) loading for the azide-alkyne 'click' reaction under solvent-free conditions, allowing for the quantitative conversion of numerous 1,4-disubstituted 1,2,3-triazole. The specially designed coordinated ligand (perimidin-2-imine) in the Cu(II) complex accelerates the reaction rate significantly during the oxidative homocoupling reaction (OHC) and acts as a base for Cu-coordinated alkyne deprotonation. It has been demonstrated that the catalyst loading of 2.5 ppm is adequate to catalyze the cycloaddition of benzyl azide to phenylacetylene, leading to the extremely high turnover number of 120000 and a turnover frequency of 5000 h-1 . Synergistic evidence from stoichiometric reactions and experimental results provides insights into the plausible mechanism for the reaction. Each copper atom contributes to the outcome of the proposed reaction, one by bonding to the acetylide and the other by activating the azide as part of a bimetallic synergistic pathway.