Solvent effects on thermodynamic and electrochemical parameters: lanthanide cryptates in acetonitrile

Abstract

Complexation of the three reducible lanthanide(III) cations Sm³⁺, Eu³⁺, and Yb³⁺, by the cryptands 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane [222 (L¹)], 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane [221 (L²)],4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]icosane[211 (L³)], and the related monocycle 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane [22 (L⁴)], has been investigated in acetonitrile, at different temperatures by a competitive potentiometric method with Ag⁺ as auxiliary cation. The results show that the high stability of lanthanide(III) cryptates and of the corresponding complexes with L⁴ is of enthalpic origin. The electrochemical behaviour of these cryptates and of the corresponding complexes of the crown ether 1,4,7,10,13,16-hexaoxacyclo-octadecane [18C6 (L⁵)] has also been studied in the same solvent by polarography and cyclic voltammetry. The lanthanides(III) and their complexes are reduced in two well separated steps, except for L³ cryptates. The Eu^3+/2+ couple and all [LnL]^3+/2+ couples behave reversibly. The effect of chloride anion has been studied and the formation of stable mixed-ligand complexes has been demonstrated. The differences between the redox potentials of the uncomplexed and of the complexed cations do not show a systematic stabilization of the bivalent cryptates with all ligands. Stabilities of europium(II) cryptates have been calculated from the latter values and from the stability constants of the corresponding trivalent cryptates. Predominance of solvent effects is evidenced for lanthanide(III) cryptates, whereas size effects are dominant for lanthanide(II) complexes as demonstrated by an important macrobicyclic effect.