Syntheses, structures, and luminescent properties of copper(II) complexes based on 2,6-bis(imino)pyridyl ligands

A series of five-coordinated 2,6-bis(imino)pyridyl Cu(II) complexes, [2,6-(ArNCMe)₂C₅H₃NCuCl₂· nCH₃CN] (Ar = 4-MeC₆H₄, n = 0.5, Cu1; Ar = 2,6-Et₂C₆H₃, n = 1, Cu2; Ar = 2,4,6-Me ₃C₆H₂, n = 1, Cu3; Ar = 2,6-Me ₂C₆H₃, n = 0, Cu4; Ar = 2-MeC₆H ₄, n = 1, Cu5), were synthesized in acetonitrile by reactions of the corresponding bis(imino)pyridine ligands with CuCl₂·2H ₂O, respectively. The structures of five complexes were determined by single-crystal X-ray diffraction. In all complexes, the metal center is tridentately chelated by ligand and further coordinated by two chlorine atoms, resulting in distorted trigonal-bipyramidal geometry for Cu1, Cu3, Cu4, and Cu5, respectively, and approximately square-pyramidal geometry for Cu2. At 298 K in dichloromethane solution, all complexes exhibit blue-green luminescent emissions at about 478-499 nm, which could be attributed to ligand-centered π* → π transitions. However, fluorescent emission of complexes Cu1-Cu5 is significantly weaker than those of ligands, which could be attributed to fluorophore-quencher interactions via paramagnetic Cu(II) center.