Competitive 7Li NMR study of the stoichiometry, stability and thermodynamic data for the complexation of Li+, Mn2+, Zn2+ and Cd2+ ions with two asymmetrical branched pentadentate (N5) amines containing pyridine moiety in ionic liquid–acetonitrile mixtures

Lithium-7 NMR spectroscopy was used to investigate the stoichiometry, stability and thermodynamic data of a Li+ complex with two asymmetrical branched amines, 3,6-bis(2-pyridylmethyl)-5-methyl-3,6-diazahexane-1-amine (A1) and 4,7-bis(2-pyridylmethyl)-6-methyl-4,7-diazaheptane-1-amine (A2) in 50–50 and 75–25 wt% acetonitrile (AN)- [BMIM][PF6] (ionic liquid) mixture solution at various temperatures. A competitive 7Li NMR method was also employed to probe the complexation of Mn2+, Zn2+ and Cd2+ ions with A1 and A2 in the same solvent systems. 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. There is an inverse relationship between the complex stability and the amount of ionic liquid in the solvent mixtures. The temperature dependence formation constant was used for the evaluation of the enthalpy and entropy values for the complexation reaction. It was concluded that in all complexes, except for lithium ion, the resulting complex enthalpy is stabilized and the ΔH compensates the TΔS contribution. In the all studied solvent mixtures, the stability of the resulting 1:1 complexes was found to vary in the order Cd2+ > Mn2+ > Zn2+ > Li+.