Thermal investigation and stereochemical studies of some cyclic ligand complexes of nickel(II), copper(II), zinc(II) and cadmium(II) in the solid state. Part 2

Abstract

N-methylmorpholine (Mmorph) and N,Nt́-dimethylpiperazine (DMP) complexes of nickel(II), copper(II), zinc(II) and cadmium(II) were synthesized. Thermal investigations were carried out and the stereochemical changes that occur during the thermal decomposition reactions were studied. The complexes were found to have the following compositions [Ni(Mmorph)Br 2] · H 2O, [Cu(Mmorph)Br 2] · H 2O, [Zn(Mmorph) 2Br 2], [Ni(DMP) 2(H 2O) 2Br 2], [Cu(DMP)Br 2], [2ZnBr 2 · 3DMP], and [Cd(DMP)Br 2]. Attempts to prepare a methylmorpholine complex of CdBr 2 failed. Some intermediate complexes were isolated using a temperature-arrest technique (pyrolysis). The characterization and study of the conformational and configurational changes were carried out using elemental and thermal analyses, IR spectral data and magnetic moment measurements. Most of the complexes of methylmorpholine and dimethylpiperazine appeared to be tetrahedral; however, [Zn(Mmorph) 2Br 2], [Ni(DMP) 2(H 2O) 2Br 2] and [2ZnBr 2 · 3DMP] are octahedral. The thermodynamic parameters such as activation energy E* a, enthalpy change Δ H, and entropy change Δ S were evaluated for the dehydration steps and decomposition reactions of the complexes using some standard methods and their values compared. The stability trend of the complexes with respect to activation energy is compared. A linear correlation obtained by plotting E* a against Δ S values showed that a system with a higher entropy change Δ S will require less energy E* a for its thermal decomposition.