The complexation thermodynamics of light lanthanides by crown ethers

This article describes the complexation thermodynamics of light lanthanides by crown ethers, including symmetric crown ethers, less-symmetric crown ethers, unsaturated crown ether, lariat ether (C-pivot and N-pivot), and bis(crown ethers). Calorimetric and spectrophotometric titrations afforded the complex stability constants and thermodynamic parameters of the above ligand-lanthanide bindings, displaying high binding ability and cation selectivity towards different lanthanides. The high cation binding ability and cation selectivity by various crown ethers are discussed from the viewpoint of thermodynamics according to the crown ether's cavity size, electron density, softness, spatial arrangement, additional binding site, and other major factors governing cation-ligand complexation. The compensatory relationship between Delta H and T Delta S was found to be exhibited in the complexation of light lanthanides with crown ethers. The slope (alpha) and intercept (T Delta S-0) of the regression line could be taken as measures of the conformational changes and the extent of desolvation caused upon complexation, respectively. (C) 2000 Elsevier Science S.A. All rights reserved.