The effect of ligand basicity on the thermal stability of heterodinuclear NiII–ZnII complexes

Four complexes of the nuclear structure NiII–ZnII were prepared with bis-N,N′-(salicylidene)-1,3-propanediamine (LH2), bis-N,N′-(salicylidene)-2,2′-dimethyl-1,3-propanediamine (LDMH2) and the reduced derivatives of these Schiff bases, bis-N,N′-(2-hydroxybenzyl)-1,3-propanediamine (LHH2), bis-N,N′-(2-hydroxybenzyl)-2,2′-dimethyl-1,3-propanediamine (LDMHH2). The complexes were characterized using IR spectroscopy, elemental analysis and thermogravimetric methods. The stoichiometry of the complex molecules were found to be NiL·ZnCl2·(DMF)2, NiLDM·ZnCl2·(DMF)2, NiLH·ZnCl2·(DMF)2 and NiLDMH·ZnCl2·(DMF)2. The molecular models of the complexes prepared with the reduced Schiff bases were determined according to the X-ray diffraction method. It is seen that in these complexes Ni(II) is in octahedral and Zn(II) is in tetrahedral coordination sphere. Ni(II) ion is coordinated between two nitrogen and two oxygen donors of the ligand and oxygen donors of the two DMF molecules. Zn(II) ion on the other hand is coordinated between two oxygen of the organic ligand forming two μ bonds. It also coordinates two Cl ions. The thermogravimetric analysis showed that the complex NiLDMH·ZnCl2·(DMF)2 containing methyl groups is more stable than the other complex NiLH·ZnCl2·(DMF)2 containing reduced Schiff base. The coordinative DMF molecules in NiLDMH·ZnCl2·(DMF)2 were thermally cleaved. However, the cleavage of DMF molecules NiLH·ZnCl2·(DMF)2 resulted in the thermal degradation of the complex. In order to explain the TG data of the ligands were titrated in non-aqueous medium and their basicity strengths were determined. It was found that the basicity of the ligands containing two methyl groups were stronger. It is understood that the two methyl groups increase the negative charge density on nitrogen causing an increase in complex stability.