Thermodynamic and electrochemical behaviour of lanthanide cryptates in N,N-dimethylformamide

Abstract

The stability of 222 (L¹), 221 (L²), and 211 (L³) cryptates of samarium(III), europium(III), and ytterbium(III) and the corresponding complexes with the related monocycle, 22 (L⁴) have been determined in N,N-dimethylformamide by potentiometry using a competitive method with auxiliary alkali cations. Their electrochemical behaviour has also been investigated in the same solvent by polarography and cyclic voltammetry. The results clearly show the low stability of the trivalent cryptates. The electrochemical study of Ln³⁺ ions with increasing amounts of ligand shows two different behaviours: (i) with L³, Ln³⁺ ions are complexed as shown by the decrease of the reduction waves of Ln³⁺ but no stabilization of Ln²⁺ ions is observed; (ii) with L¹, L², and L⁴ the height of the reduction wave of Ln³⁺ is not affected by the ligand concentration and the reduction occurs according to the scheme: Ln³⁺+ e^–⇌ Ln²⁺+ L →[Ln(L)]²⁺. The differences δ between the redox potentials of the free and the complexed lanthanide(III) cations show an important stabilization of the divalent cryptates. Their stability constants have been calculated from the values of δ and the stability constants of the trivalent complexes. While no particular ligand or cation selectivity is observed for the trivalent cryptates, this is not so for the reduced lanthanides which form the strongest complexes with L¹. The results are interpreted by size and solvation considerations.