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 °C to give the complex stability constants ( K s ) and the thermodynamic parameters for the stoichiometric 1:1 complexation of light lanthanoid(III) nitrates (La−Gd) with N -methoxyethylaza-4,7,11,14-tetraoxacyclohexadecane (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 from the thermodynamic point of view. Possessing structural flexibility as compared with lariat ether 15-(2,5-dioxahexyl)-15-methyl-1,4,7,10,13-pentaoxacyclohexadecane (16-crown-5 lariat) ( 2 ), aza-16-crown-5 lariat gave the highest K s for Nd 3+ among the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a characteristic peak profile at Nd 3+ for 1, the relative cation selectivity for Nd 3+ over the neighboring Pr 3+ and Sm 3+ amounted to 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 selectivity of aza-16-crown lariat ( 1 ) is attributed to the entropic loss that is minimized only when a strict size match is materialized between the ionic diameter of the lanthanoids and the relatively flexible three-dimensional cavity induced upon lariat ligation. Therefore, the three-dimensional induced fit plays an important role in the recognition of trivalent lanthanoid ions by lariat ethers with structural flexibility.