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, <b>L</b>, and its copper(I) complex, [<b>1</b>]^<b>+</b>, were synthesized and fully characterized. Complex [<b>1</b>]⁺ has five coordination geometry satisfied by ligand <b>L</b>. <b>L</b> has a low-lying π* orbital; thus, [<b>1</b>]⁺ showed very facile multiple ligand-based redox events. Moreover, due to the strong π-acceptor nature of <b>L</b>, the Cu(II)/Cu(I) redox potential of [<b>1</b>]⁺ was anodic (0.62 V). The redox events of both <b>L</b> and [<b>1</b>]^<b>+</b> were characterized using various spectroscopic studies and density functional theory (DFT) calculations. Taking advantage of the multiple facile ligand-based reductions in [<b>1</b>]^<b>+</b>, the Glaser coupling reaction of terminal alkynes was explored. Various kinds of alkynes were found to be effective when using [<b>1</b>]^<b>+</b> 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 <b>L</b> in [<b>1</b>]^<b>+</b> 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 <b>L</b> acted as an actor.