Potentiometric determination of novel complexes of selected lanthanide ions with N,N\u2019-bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine

Abstract

The stability constants of the complexes formed in the N,N’-bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine (H2L) and La(III), Eu(III), Gd(III), Ho(III), and Lu(III) ion systems were determined in solution with the potentiometric method. The pH-metric titrations were performed in dimethyl sulfoxide/water (v:v, 30:70) mixture at 25.0 °C in 0.1 M LiNO3 ionic strength. The tests were performed for systems with Ln(III) to H2L 1:2 and 1:3 molar ratio but only data of the systems with the metal/ligand ratio 1:2 were taken into calculation. The molar ratio 1:1 was not studied because of the high coordination numbers of the lanthanide ions, and inadequate donor atoms of the ligand. Computer analysis (HYPERQUAD software) of potentiometric data indicated that in solution the lanthanide (Ln) complexes exist as LnL2, Ln(HL)2, and Ln(H2L)2 forms, depending on pH unlike to the solid state where only one form of Ln(H2L)2 occurs. Formation constants increase with decreasing size of the Ln(III) ions. Moreover, complex formation in the Ln3+/H2L systems in solution was performed using UV–Vis spectrophotometric titration.