Thermodynamic and electrochemical studies of lanthanide and alkaline-earth cryptates in dimethyl sulfoxide.

The thermodynamic functions (Delta G degrees, Delta H degrees and Delta S degrees) for the complexation of Eu2+ and alkaline-earth-metal ions (Ca2+, Sr2+ and Ba2+) by the cryptands 4.7.13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane and 4,7,13,16,21.24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane have been determined in dimethyl sulfoxide (dmso) using calorimetry and potentiometry at 25 degrees C in a 0.1 mol dm(-3) ionic medium. These investigations, supported by other electrochemical measurements. showed that. in contrast to the literature, the lanthanide(iii) ions 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 electrode) 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- and ligand-solvent interactions in the stabilisation of cryptates is shown to rationalise the apparent anomalous stability sequence of alkali- and alkaline-earth-metal cryptates in water and in dmso.