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 nzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-d iene l-diene (dibenzo-18-crown-6) (2). Data analyses, assuming 1 : 1 s toichiometry were successfully applied to all the light lanthanoid-cro wn ether combinations employed. Using the present and reported data, t he complexation behaviors of (2) and 1,4,7,10,13,16-hexaoxacyclooctade cane (18-crown-6) (1) are compared from the thermodynamic point of vie w. 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 lanth anoid showed a monotonically declining pattern for (I) except for a ju mp at Ce3+, and a characteristic peak profile at Nd3+ and Sm3+ for (2) . Thermodynamically, the complexation of light lanthanoid nitrates wit h 18-crown-6 is mainly enthalpy-driven in acetonitrile, but the comple xation with (2) is chiefly entropy-driven. (C) 1998 Elsevier Science B .V.