Complexation thermodynamics of rare earth (III) with crown ethers. 3 . Calorimetric titration of light lanthanoid (III) nitrates with dibenzo-18-crown-6 in acetonitrile

Abstract

Calorimetric titrations have been performed in anhydrous acetonitrile at 25°C to give the complex stability constants ( K s) and the thermodynamic quantities for the complexation of light lanthanoid (III) nitrates (La–Gd) with 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene (dibenzo-18-crown-6) ( 2). Data analyses, assuming 1 : 1 stoichiometry were successfully applied to all the light lanthanoid–crown ether combinations employed. Using the present and reported data, the complexation behaviors of ( 2 ) and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) ( 1) are compared from the thermodynamic point of view. The rigid structure of ( 2), as compared with ( 1), gave the higher K s for Nd 3+ and Sm 3+ among the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a monotonically declining pattern for ( 1) except for a jump at Ce 3+, and a characteristic peak profile at Nd 3+ and Sm 3+ for ( 2). Thermodynamically, the complexation of light lanthanoid nitrates with 18-crown-6 is mainly enthalpy-driven in acetonitrile, but the complexation with ( 2) is chiefly entropy-driven.