Formation of cobalt(II) complexes with five pyridine oximes in aqueous solution

Abstract

Complex formation equilibria of cobalt(II) with pyridine-2-carboxamidoxime (1), pyridine-2-acetamidoxime (2), pyridine-2-aldoxime (3), 1-(2-pyridinyl)ethanone oxime (4) and 6-methylpyridine-2-aldoxime (5) were studied in 0.1 M NaCl solution at 25°C by potentiometric titrations with use of a glass electrode. Experimental data were analysed with the least-squares computer program SUPERQUAD to determine the complexes formed and their stability constants. With ligands 1−4 Co(II) forms mono, bis and (except for 2) tris complexes of the type and deprotonated/hydrolyzed products of the bis and tris complexes, Co(HL)L+ and Co(HL)2L+. The deprotonated complexes have a low spin structure (t 2 g 6 e g ) and are readily oxidized after the easy loss of the sole e g electron. Their formation with increasing pH involves slow attainment of equilibrium in the pH range 3−5. Only with ligand 3 could studies be continued to pH 8−10 by using very low cobalt(II) ion concentrations. There, CoL2 is formed quantitatively, while complexes Co(HL)L2 and could not be observed. Ligand 1 also forms the complex Co2(HL)2H2L5+ and ligand 2 the complex Co(H2L)3+ with the positively charged ligand (H2L+). Ligand 5 forms the complexes Co(HL)2+, CoL2, Co2L2OH+, and Co2L3OH mainly in the pH range 6−10. The stabilities of these complexes and also their oxidation reactions are reduced by the steric requirements of the 6-methyl groups of the ligand.