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

Abstract

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.