MOLECULAR DESIGN OF CROWN-ETHERS .13. COMPLEXATION THERMODYNAMICS OF LIGHT LANTHANOID NITRATES WITH AZA-16-CROWN-5 LARIAT IN ACETONITRILE -ENHANCED SELECTIVITY FOR ND3+

Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees C to give the complex stability constants (K-s) and the thermodynamic parameters for the stoichiometric 1:1 complexation of li ght lanthanoid(III) nitrates (La-Gd) with -methoxyethylaza-4,7,11,14-t etraoxacyclohexadecane (aza-16-crown-5 lariat) (1). Using the present and reported data, the complexation behavior of C- and N-pivot lariats with 16-crown-5 skeletons is discussed comparatively and globally fro m the thermodynamic point of view. Possessing structural flexibility a s compared with lariat ether xyl)-15-methyl-1,4,7,10,13-pentaoxacycloh exadecane (16-crown-5 lariat) (2), aza-16-crown-5 lariat gave the high est K-s for Nd3+ among the light lanthanoid nitrates. The complex stab ility sequence as a function of reciprocal ionic diameter of lanthanoi d showed a characteristic peak profile at Nd3+ for 1, th, relative cat ion selectivity for Nd3+ over the neighboring Pr3+ and Sm3+ amounted t o 9. Thermodynamically, the complexation of light lanthanoid nitrates with the lariat ethers is mainly enthalpy-driven in acetonitrile, but the cation selectivity is entropy-governed. The high cation selectivit y of aza-16-crown lariat (1) is attributed to the entropic loss that i s minimized only when a strict size match is materialized between the ionic diameter of the lanthanoids and the relatively flexible three-di mensional cavity induced upon lariat ligation. Therefore, the three-di mensional induced fit plays an important role in the recognition of tr ivalent lanthanoid ions by lariat ethers with structural flexibility.