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

An\nextended trisazo dipyridyl ligand, <b>L</b>, and its copper(I)\ncomplex, [<b>1</b>]<sup><b>+</b></sup>, were synthesized\nand fully characterized. Complex [<b>1</b>]<sup>+</sup> has\nfive coordination geometry satisfied by ligand <b>L</b>. <b>L</b> has a low-lying π* orbital; thus, [<b>1</b>]<sup>+</sup> showed very facile multiple ligand-based redox events. Moreover,\ndue to the strong π-acceptor nature of <b>L</b>, the Cu(II)/Cu(I)\nredox potential of [<b>1</b>]<sup>+</sup> was anodic (0.62 V).\nThe redox events of both <b>L</b> and [<b>1</b>]<sup><b>+</b></sup> were characterized using various spectroscopic studies\nand density functional theory (DFT) calculations. Taking advantage\nof the multiple facile ligand-based reductions in [<b>1</b>]<sup><b>+</b></sup>, the Glaser coupling reaction of terminal alkynes\nwas explored. Various kinds of alkynes were found to be effective\nwhen using [<b>1</b>]<sup><b>+</b></sup> as a precatalyst.\nThe mechanism of the reaction was investigated thoroughly by several\ncontrolled experiments, isolation, and characterization of the intermediates\nusing various spectroscopic studies as well as by single-crystal X-ray\nstructure determination. These studies showed that the <b>L</b> in [<b>1</b>]<sup><b>+</b></sup> acted not only in the\nelectron transfer events but also as a locus for binding the substrate,\nbreaking and forming bonds, and, finally, releasing the product. Thus,\nhere, the metal mainly acted as a spectator and ligand <b>L</b> acted as an actor.