Anion Effect on the Structural Conformation of Tetranuclear Cadmium(II) Complexes

Six tetranuclear cadmium(II) complexes, Na[Cd4(hmt)2(H2O)6(dpa)4(NO3)]·8H2O (1), Na[Cd4(hmt)2(CH3OH)2(H2O)4(dpa)4I]·8H2O (2), Na[Cd4(hmt)2(H2O)6(dpa)4Br]·8H2O (3), Na2[Cd4(hmt)2(H2O)6(dpa)4(SCN)2]·4H2O (4), {Na[Cd4(hmt)2(H2O)6(dpa)4Cl](H2O)4}n (5), and {[Cd4(hmt)3(H2O)5(dpa)4](H2O)10}n (6) (hmt = hexamethylenetetramine, H2dpa = diphenic acid), were synthesized
by reactions of H2dpa, hmt, and CdX2 (X = NO3-, I-, Br-, Cl-, SCN-, and OAc-). All the complexes contain a basic unit [Cd4(dpa)4], in which four dpa ligands in a chelating bis-bidentate coordination mode link two six-coordinate and two seven-coordinate
cadmium(II) centers. The research result demonstrates that the nature of the counterions has an important effect on the assembly
process of neutral and anionic mixed ligands with metal ions. In 1−3, NO3-, I-, or Br- bridges the two seven-coordinate CdII
centers of the unit to form a discrete tetranuclear molecule with a cis-conformation and hmt as a terminal ligand coordinated to each
of the two six-coordinate cadmium(II) centers. Two SCN- in 4 acting as terminal ligands generate a discrete trans-conformational
tetranuclear cluster anion. The hmt in 5 acting as a μ-ligand links cis-conformational tetranuclear units by Cd−N bonds to generate
a one-dimensional chain structure, while the hmt in 6 acting as a μ-ligand links the tetranuclear units by Cd−N bonds to generate
a two-dimensional layer structure composed of a planar-conformation [Cd4(dpa)4] unit. The fluorescent studies in the solid at room
temperature show that the six complexes exhibit strong violet or blue emissions in the range 370−500 nm arising from the charge-transfer transition and that the bridging anions have a significant effect on their fluorescent properties.