Thermal investigation and stereochemical studies of some cyclic diamine complexes of nickel(II), zinc(II) and cadmium(II) in the solid state. part II.

Abstract

Acetate-cyclic diamine (piperazine (pipz), N-methylpiperazine (mpipz) and 1,4-di-azacycloheptane (dach)) complexes of nickel(II), zinc(II) and cadmium(II) were synthesized. The thermal behaviour of these complexes was studied using thermogravimetry (TG) and differential thermal analysis (DTA) and the stereochemical changes occurring during thermal decomposition were investigated. The complexes were found to have the compositions [Ni(pipz)(OAc) 2]·4H 2O, [Ni(mpipz) 2 (OAc) 2]·2H 2O, [Ni(dach) 2](AcO) 2·2H 2O, [ML (OAc) 2]·H 2O (M = Zn or Cd and L = pipz), [Zn(mpipz) 2(OAc) 2]·2H 2O, [Zn(dach)(OAc) 2 ] ·H 2O, [Cd(mpipz)(OAc) 2]·H 2O and [Cd(dach)(OAc) 2]·0.5H 2O. Attempts to prepare N, N'-dimethylpiperazine complexes failed. The characterization and study of configurational and conformational changes were carried out with the aid of elemental and thermal analyses, IR spectra and magnetic moment and conductivity measurements. The nickel complexes of pipz and mpipz, the zinc complex of mpipz and all the cadmium complexes appeared to be octahedral. The dach complex of nickel was square planar and the pipz and dach complexes of zinc were tetrahedral. The thermodynamic parameters, such as the activation energy E a ★, the enthalpy change Δ H and the entropy change Δ S, for the dehydration steps and decomposition reactions of the complexes were evaluated using some standard methods. The order of stability of the complexes (with respect to E a ★) follows the trend pipz > mpipz > dach. A linear correlation was found between E a ★ and Δ S for the decomposition reactions of the complexes.