COMPLEXATION THERMODYNAMICS OF RARE EARTH(III) WITH CROWN-ETHERS .1. CALORIMETRIC TITRATION OF COMPLEXATION OF LIGHT LANTHANOID NITRATES WITH 15-CROWN-5, 16-CROWN-5, AND 16-CROWN-5 LARIAT 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 15-crown-5 (1), less-symmetrical 16-crown-5 (2 ), and the related lariat ether 15-(2,5-dioxahexyl)-15-methyl-16-crown -5 (3). These structurally related crown-5 derivatives gave stoichiome tric 1:1 complexes with light lanthanoids, displaying strikingly diffe rent cation selectivity profiles. Thus, the complex stability as a fun ction of reciprocal ionic diameter of lanthanoid showed a monotonicall y declining pattern for 1, a unique profile for 2 characterized by a s udden jump of K(s) at Nd and a subsequent plateau, and a relatively fl at pattern for 3. Thermodynamically, the complexation is absolutely en thalpy-driven, while the cation selectivity is evidently entropy-gover ned. The unique complexation behavior of 2 is attributed to the entrop ic loss that is minimized only when a strict size match is materialize d between the cavity of 2 and the ionic diameter of the lanthanoids, i .e., Nd-Gd. On the other hand, the poor cation selectivity for 3 is as cribed to the adjustable three-dimensional cavity induced upon lariat ligation, making the operation of strict size fitting difficult.