Syntheses, structures, and luminescent properties of Zn(II) and Cd(II) complexes: 3-D supramolecules based on 2,6-bis(imino)pyridine ligands constructed by hydrogen bonding interactions

Six 5-coordinate 2,6-bis(imino)pyridine metal complexes, [2,6-(ArN=CMe)₂C₅H₃NMCl₂ · nCH₃CN] (Ar = 4-MeC₆H₄, M = Zn, n = 0.5, Zn1, M = Cd, n = 1, Cd1; Ar = 2,6-Et₂C₆H₃, M = Zn, n = 0.5, Zn2, M = Cd, n = 0.5, Cd2; Ar = 2,4,6-Me₃C₆H₂, M = Zn, n = 1, Zn3, M = Cd, n = 1, Cd3), were synthesized in acetonitrile by the reactions of the corresponding bis(imino)pyridines with ZnCl₂ or CdCl₂ · 2.5H₂O, respectively. The structures of Zn1-Zn3 and Cd1-Cd3 were determined by the single-crystal X-ray diffraction. In all complexes, the ligand is tridentate with further coordination by two chlorides, resulting in a distorted trigonal bipyramid. All complexes self-assemble through hydrogen bonding interactions to form a 3-D supramolecular structure. At 298 K in dichloromethane, all complexes have blue luminescent emissions at 405-465 nm, which can be attributed to ligand-centered π^* → π transitions. The zinc and cadmium centers play a key role in enhancing fluorescent emission of the ligands.