THERMODYNAMIC AND ELECTROCHEMICAL STUDIES OF LANTHANIDE AND ALKALINE-EARTH-METAL CRYPTATES IN DIMETHYL-SULFOXIDE

The thermodynamic functions (Delta G degrees, Delta H degrees and Delt a S degrees) for the complexation of Eu2+ and alkaline-earth-metal ion s (Ca2+, Sr2+ and Ba2+) by the cryptands 3,16,21-pentaoxa-1,10-diazabi cyclo[8.8.5]tricosane and 6,21.24-hexaoxa-1,10-diazabicyclo[8.8.8]hexa cosane have been determined in dimethyl sulfoxide (dmso) using calorim etry and potentiometry at 25 degrees C in a 0.1 mol dm(-3) ionic mediu m. These investigations, supported by other electrochemical measuremen ts. showed that. in contrast to the literature, the lanthanide(iii) io ns do not exhibit any tendency towards complexation by these cryptands in dmso. The stability constants of the divalent europium cryptates ( determined using Naf as a competing ion and a selective glass electrod e) and their enthalpies of formation are compatible with those of Sr2, the alkaline-earth-metal ion having similar ionic radius. The effect of the ion charge and solvent donicity on the complexation reactions is discussed and, in particular. the relative importance of the ion- a nd ligand-solvent interactions in the stabilisation of cryptates is sh own to rationalise the apparent anomalous stability sequence of alkali - and alkaline-earth-metal cryptates in water and in dmso.