Development of phosphine and diimine stabilized copper(I) catalysts for efficient azide–alkyne cycloaddition

The synthesis of a small family of copper(I) complexes bound to diimine and phosphine ligands, and their characterization, spectroscopic properties and application as pre-catalyst for azide–alkyne cycloaddition reaction are reported. Reaction of bis-(N-aryl)glyoxaldimine (p-R-C₆H₄NC(H)(H)CNC₆H₄-R-p; R = OCH₃, CH₃ and Cl; abbreviated as L-R) with [Cu(PPh₃)₂(NO₃)] in refluxing methanol affords a group of three mixed-ligand copper(I) complexes of type [Cu(PPh₃)₂(L-R)]NO₃, depicted respectively as complex 1 (R = OCH₃), 2 (R = CH₃) and 3 (R = Cl). Crystal structures of 1–3 have been determined by X-ray diffraction analysis. In each complex, the copper(I) center is nested in a nearly tetrahedral N₂P₂ coordination environment. Complexes 1–3 exhibit two intense absorptions spanning over the visible and ultraviolet regions. Origin of these absorptions was probed with DFT and TDDFT calculations, which revealed that the lower energy absorption at 346–380 nm is due to an allowed copper-to-diimine charge-transfer transition, and the second absorption near 270 nm is largely due to an intra-phosphine charge-transfer transition. These complexes also show prominent emission spectra while excited at 320 nm. These copper(I) complexes are found to serve as efficient precursor for catalytic azide–alkyne cycloaddition reaction under relatively mild condition, and offering a broad substrate scope.