An electrochemical investigation of the effect of macrocycle ring size on the binding of di- and trivalent lanthanide cations by 12-crown-4, 15-crown-5, 4-tert-butylbenzo-15-crown-5, and dibenzo-30-crown-10 in propylene carbonate

Abstract

The complexation in anhydrous proplene carbonate of the lanthanide ions by various macrocylic polyethers featuring from four to ten oxygen atoms has been investigated by a competitive potentiometric technique with lead(II) or thallium(I) as auxilliary ions. The stability of the complexes appears to depend primarily on the relative sizes of the metal ions and of the internal cavity of each macrocycle. It depends also on the rigidity of the ligands and is influenced by solvation effects. The small ligands 12-crown-4 (1) and 15-crown-5 (2) exhibit a similar behavior: they form 1:2 lanthanide complexes, the stability of which decreases with decreasing ionic radius despite the higher charge density of the metal ions. The presence of an electron-withdrawing phenyl group, as in 4-tert-butylbenzo-15-crown-5 (3), leads to a strong reduction of the stability constants. A maximum stability of 1:1 complexes of 3 is found at Nd(III) while a marked minimum is observed at Gd(III) in the complexation curve of dibenzo-30-crown-10 (5). Divalent samarium and ytterbium are more strongly coordinated than the corresponding trivalent ions by the crown ethers 1-5. The larger divalent ions fit better into the internal cavity of 5 and they form stable 1:2 sandwich complexes with 1 and 3. Themore » properties of the complexes described in the present work are completely different from those reported so far in the case of noncylic ligands.« less