Versatile Chelating Behavior of Benzil Bis(thiosemicarbazone) in Zinc, Cadmium, and Nickel Complexes

Reactions of benzil bis(thiosemicarbazone), LH₆, with M(NO₃)₂·nH₂O (M = Zn, Cd, and Ni), in the presence of LiOH·H₂O, show the versatile behavior of this molecule. The structure of the ligand, with the thiosemicarbazone moieties on opposite sides of the carbon backbone, changes to form complexes by acting as a chelating molecule. Complexes of these metal ions with empirical formula [MLH₄] were obtained, although they show different molecular structures depending on their coordinating preferences. The zinc complex is the first example of a crystalline coordination polymer in which a bis(thiosemicarbazone) acts as bridging ligand, through a nitrogen atom, giving a 1D polymeric structure. The coordination sphere is formed by the imine nitrogen and sulfur atoms, and the remaining position, in a square-based pyramid, is occupied by an amine group of another ligand. The cadmium derivative shows the same geometry around the metal ion but consists of a dinuclear structure with sulfur atoms acting as a bridge between the metal ions. However, in the nickel complex LH₆ acts as a N₂S₂ ligand yielding a planar structure for the nickel atom. The ligand and its complexes have been characterized by X-ray crystallography, microanalysis, mass spectrometry, IR, ¹H, and ¹³C NMR spectroscopies and for the cadmium complex by ¹¹³Cd NMR in solution and in the solid state.