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

Abstract

Monochloroacetato (MCA) cyclic diamine (piperazine (pipz); N-methylpiperazine (mpipz); 1,4-diazacycloheptane (dach)) complexes of nickel(II), zinc(II) and cadmium(II) were synthesized. A thermal investigation was carried out and the stereochemical changes which occurred during thermal decomposition were studied. The complexes were characterized with the help of elemental and thermal analyses and IR spectral and magnetic moment data. They were found to have the compositions: [Ni(pipz)(MCA) 2(H 2O) 2], [NiL 2(MCA) 2]·2H 2O (L =mpipz or dach), [Zn(pipz)(MCA) 2]·H 2O, [Zn(mpipz)(MCA) 2]·2H 2O, (Zn(dach)(MCA) 2], [Cd(pipz) 2(MCA) 2]·2H 2O and [CdL 2(MCA) 2] (L = mpipz or dach). Attempts to prepare N, N'-dimethylpiperazine complexes failed. Configurational and conformational changes were studied using thermal analysis and IR spectral and magnetic moment (in the case of nickel(II) complexes) data. All the complexes of nickel(II) and cadmium(II) appeared to possess an octahedral structure, whereas those of zinc(II) appeared to be tetrahedral. Thermodynamic parameters such as activation energy E a, enthalpy change Δ H and entropy change Δ S for the dehydration 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 obtained by plotting E a> vs. Δ S shows that a system with a higher entropy change Δ S will require less energy E a for decomposition.