Polymeric nickel(II) squarate complexes with imidazole derivatives:: syntheses, crystal structures, spectroscopic and voltammetric studies

Two squarate complexes of nickel(II), [Ni(C4O4)(4-Meim)(2)(H2O)(2)](n) (1) (4-Meim: 4-methylimidazole) and [Ni(C4O4)(his)(2)](n)center dot nH(2)O (2) (his: histamine or 2-(4-imidazolyl)ethylamine), have been synthesized and characterized by X-ray diffraction, IR and UV-Vis techniques. The electrochemical properties of the nickel(II) complexes are investigated by cyclic voltammetry. In both complexes the Ni(II) ions have distorted octahedral geometry. Both structures comprise infinite chains parallel to [010] for 1, [10 (1) over bar] for 2 built up of bis(monodentate) squarate-O-1,O-3 (mu-1,3) C4O42- anions bridging [Ni(4-Meim)(2)(H2O)(2)] (1) and [Ni(his)(2)] (2) units. These chains are held together by hydrogen bonds between the noncoordinated oxygens of the squarate anions and the water molecules, forming a two-dimensional sheet of Ni(II) ions. These sheets are held together by hydrogen bonds between the imidazole (1) or amine (2) hydrogens and noncoordinated squarate oxygens (1 and 2), or water (2) molecules. IR and UV-Vis spectra of both compounds agree with the observed crystal structure. The voltammetric behaviors of 1 and 2 were investigated in DMSO (dimethylsulfoxide) solution by cyclic voltammetry using n-Bu4NClO4 supporting electrolyte. Both complexes exhibit only ligand (C4O42-) centred electroactivity in the potential -0.2-1.4 V versus Ag/AgCl reference electrode. The squarate anion is oxidized in two consecutive steps to the corresponding radical monoanion and neutral form.