Redox Noninnocent Copper(I) Complex Where Metal Is a Spectator and Ligand Is an Actor in the Glaser Coupling Reaction of Alkynes

An extended trisazo dipyridyl ligand, L, and its copper­(I) complex, [1] + , were synthesized and fully characterized. Complex [1]+ has five coordination geometry satisfied by ligand L. L has a low-lying π* orbital; thus, [1]+ showed very facile multiple ligand-based redox events. Moreover, due to the strong π-acceptor nature of L, the Cu­(II)/Cu­(I) redox potential of [1]+ was anodic (0.62 V). The redox events of both L and [1] + were characterized using various spectroscopic studies and density functional theory (DFT) calculations. Taking advantage of the multiple facile ligand-based reductions in [1] + , the Glaser coupling reaction of terminal alkynes was explored. Various kinds of alkynes were found to be effective when using [1] + as a precatalyst. The mechanism of the reaction was investigated thoroughly by several controlled experiments, isolation, and characterization of the intermediates using various spectroscopic studies as well as by single-crystal X-ray structure determination. These studies showed that the L in [1] + acted not only in the electron transfer events but also as a locus for binding the substrate, breaking and forming bonds, and, finally, releasing the product. Thus, here, the metal mainly acted as a spectator and ligand L acted as an actor.