Tetrahalo- and (mixed-tetrahalo)cuprates of the piperazinium dication.Coordination geometry changes in some CuX42- anions.

Some halo- and (mixed-halo)cuprates of the type (pipzH·HX)2CuY4 (X = Y = Cl, Br, X = Cl, Y = Br), (pipzH·HCl)2CuCl3Br (pipzH·HX = piperazinium monocation hydrohalide), (pipzH2)CuCl4, (pipzH2)CuCl3Br, (pipzH2)CuCl2Br2, (pipzH2)CuBr4, and (pipzH2)Cu2Cl6 (pipzH2 = piperazinium dication) were prepared and characterized by means of far-ir, ir, and near-ir spectroscopy, magnetic moments, and conductivity measurements. For the (pipzH·HX)2CuY4 and (pipzH·HCl)2CuCl3Br complexes, which are thermochromic, an approximately square-planar geometry may be suggested, while for the (pipzH2)CuXmYn (m + n = 4) complexes a distorted tetrahedral geometry may be suggested from the room-temperature spectroscopic properties of these compounds, compared to those of other tetrahalocuprates of known structure. The planar geometry is probably stabilized by extensive N-H?Cl hydrogen-bonding interactions, as is demonstrated by the change in the coordination geometry from square planar to distorted tetrahedral when the compounds are heated at 80-95°C or dissolved in dimethylformamide. In fact a weakening of the hydrogen-bonding network, caused by the increasing disorder due to the thermal motion or by a solvent effect, is responsible for the coordination geometry change. A band at 19 000 cm-1, which appears only in the (pipzH2)Cu2Cl6 complex, is characteristic of Cu2Cl6 2- dimer species. Also for this complex an approximately square-planar geometry is suggested