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

Abstract

The thermodynamic functions (ΔG°, ΔH° and ΔS°) 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 °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 Na⁺ 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.