Effect of Ph on the Stability of Coordination Compounds of Co(iii) With Diaminoethanol Ligands in Non-Aqueous Solutions

Abstract

Inner complex compound of cobalt(III) with diaminoethanol, [Co(DetmHdetm)], and polynuclear complex compound 2Co-Ni with diaminoethanol, [Ni(CoDetmHdetm)2](NO3)2, were synthesized. Stabilities of the obtained compounds were investigated in non-aqueous (dimethylformamide) solutions at different values of pH (from acid to alkaline). Methods of potentiometric titration and electron absorption spectra were used to determine the range of pH values where the compounds do not decompose. As a result of these studies, we found that at the upper value of pH=7, the coordination surrounding of the metals (Co(III), Ni(II)) does not change. When the pH value becomes lower than 7 (with adding HCl acide), the inner complex of Co(III) begins to decompose. The ligand H2detm is replaced by Cland partial protonization of diethanolamine occurs. Also, transformation of N,Ncis isomer of inner complex compound of cobalt(III) to N,N-trans isomer takes place, thus reducing the symmetry of the complex. As for the polynuclear complex compound 2Co-Ni, when pH value becomes lower than 7, molecules of solvent begin to react with the products of partially destroyed complex. As a result, new complex compound is formed with the increase of coordination number of nickel(II) from 4 to 6. However, in alkaline solutions where pH values are up to 10, the heterometal complex compound of 2Co-Ni remains stable. Only partial hydrolysis of the solvent occurs in these conditions. The schemes of transformations of [Co(DetmHdetm)] and [Ni(CoDetmHdetm)2](NO3)2 occuring in non-aqueous (dimethylformamide) solutions at different values of pH (from acid to alkaline) are presented in the paper. The results of this study can be used for the preparation of precursor solution for the technology of catalytic materials production. Evidently, the range of pH from 7 to 10 must be maintained in order to avoid the contamination of the final products by the decomposed wastes of complexes and solvent.