Syntheses, Crystal Structures, and Physical Properties of Dinuclear Copper(I) and Tetranuclear Mixed-Valence Copper(I,II) Complexes with Hydroxylated Bipyridyl-Like Ligands

Four copper complexes with hydroxylated bipyridyl-like ligands, namely [Cu2(ophen)2] (1), [Cu4(ophen)4(tp)] (2), [Cu4(obpy)4(tp)] (3), and [Cu4(obpy)4(dpdc)]⋅2H2O (4), (Hophen=2-hydroxy-1,10-phenanthroline, Hobpy=6-hydroxy-2,2′-bipyridine, tp=terephthalate, dpdc=diphenyl-4,4′-dicarboxylate) have been synthesized hydrothermally. X-ray single-crystal structural analyses of these complexes reveal that 1,10-phenanthroline (phen) or 2,2′-bipyridine (bpy) ligands are hydroxylated into ophen or obpy during the reaction, which provides structural evidence for the long-time argued Gillard mechanism. The dinuclear copper(I) complex 1 has three supramolecular isomers in the solid state, in which short copper–copper distances (2.66–2.68 Å) indicate weak metal–metal bonding interactions. Each of the mixed-valence copper(i,ii) complexes 2–4 consists of a pair of [Cu2(ophen)2]+ or [Cu2(obpy)2]+ fragments bridged by a dicarboxylate ligand into a neutral tetranuclear dumbbell structure. Dinuclear 1 is an intermediate in the formation of 2 and can be converted into 2 in the presence of additional copper(II) salt and tp ligands under hydrothermal conditions. In addition to the ophen-centered π→π* excited-state emission, 1 shows strong emissions at ambient temperature, which may be tentatively assigned as an admixture of copper-centered d→s,p and MLCT excited states.