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

Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees 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 l-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 Nd3+ and Sm3+ among the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a monotonically declining pattern for (I) except for a jump at Ce3+, and a characteristic peak profile at Nd3+ and Sm3+ 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. (C) 1998 Elsevier Science B.V.