Study of Complex Formation between Kryptofix 21 with La3+, Y3+ and Ce3+ Cations in Some Binary Mixed Non-Aqueous Solvents Using the Conductometric Method

Abstract

The complexation reactions between La3+, Y3+ and Ce3+ cations with the macrocyclic ligand, kryptofix 21, were studied in methanol-acetonitrile (MeOH-AN) and methanol-methylacetate (MeOHMeOAc) binary mixed solvent solutions at different temperatures using the conductometric method. The conductance data show that in most solvent systems, the kryptofix 21 forms a 1: 1 [M: L] complex with La3+, Y3+ and Ce3+ metal cations, but in the case of Y3+ cation in pure methylacetate, in addition of formation of a 1: 1 [ML] complex, 1: 2 [ML2] and 1: 3 [ML3] complexes are formed in solution. In the case of Ce3+cation, a 1: 1 [ML] and also a 1: 2 [ML2] complexes are formed in this solvent system at all studied temperatures. The electrical conductance data in acetonitrile, show that a 1: 1 [ML] and also a 1: 2 [ML2] complexes are formed between the ligand and La3+ and Ce3+ metal cations at different temperatures. The stability constants of the 1: 1 [ML] complexes were determined using the conductometric data and a computer program, GENPLOT. A non-monotonic relationship was observed between logKf of the 1: 1 complexes with the composition of the binary solvent solutions which was discussed in term of solvent-solvent interactions and also preferential solvation of the metal cations and the ligand in solutions. The selectivity order of the ligand for the metal cations in MeOH–AN and MeOH–MeOAc binary solvent solutions, at 25°C was found to be: Y3+ > La3+ > Ce3+ and La3+ > Y3+ > Ce3+, respectively. The values of the standard thermodynamic quantities (ΔHc° and ΔSc°) for formation of the 1: 1 complexes were obtained from temperature dependence of the stability constans of the complexes and the results show that the thermodynamics of the complexation reactions between kryptofix 21 and La3+, Y3+ and Ce3+ cations, is affected by the nature and composition of the mixed solvents systems.