Complexation studies of Mn2+, Zn2+ and Cd2+ ions with a series of tetradentate (N4) Schiff base ligands containing pyridine moiety in acetonitrile and nitromethane solutions by a competitive NMR technique using 7Li nucleus as a probe

Lithium-7 NMR spectroscopy was used to investigate the stoichiometry and stability of a Li⁺ complex with N¹,N²-bis(pyridin-2-ylmethylene)ethane-1,2-diamine (L¹), N¹,N³-bis(pyridin-2-ylmethylene)propane-1,3-diamine (L²) and N¹,N⁴-bis(pyridin-2-ylmethylene) butane-1,4-diamine (L³) in acetonitrile (AN) and nitromethane (NM) solutions. A competitive ⁷Li NMR method was also employed to probe the complexation of Mn²⁺, Cd²⁺ and Zn²⁺ ions with L¹, L² and L³ in the same solvents. The formation constants of the resulting complexes were evaluated from computer fitting of the mole ratio data to an equation that relates the observed chemical shifts to the formation constant. In both solvents, the stability of the resulting 1:1 complexes were found to vary in the order Zn²⁺>Cd²⁺>Mn²⁺>Li⁺. In addition, the stability of M–L complexes of M²⁺ with the Schiff base ligands found to vary in the order M²⁺–L¹ > M²⁺–L² > M²⁺–L³.