Molecular design of crown ethers. 17. Complexation thermodynamics of lightlanthanoid nitrates with N-benzylaza-21-crown-7 in acetonitrile

Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees C to give the complex stability constants (K-S) and the thermodyna mic parameters (Delta G(0), Delta H-0 and Delta S-0) for the complexation o f light lanthanoid(III) nitrates (La-Gd) with N-benzylaza-21-crown-7 3. Dat a analyses, assuming 1:1 stoichiometry, were successfully applied to all li ght lanthanoid-azacrown ether combinations employed. Using the present and previous data on 15- to 21- membered N-benzylazacrown ethers 1-3, the effec t of ring size upon complexation behavior was discussed comparatively and g lobally from the thermodynamic point of view. The complexation behaviors ar e analyzed in terms of the size-fit concept, N-substituent coordination num bers, and lanthanoid's surface charge density. Thermodynamically the comple xation of light lanthanoids with azacrown ethers is enthalpy-driven, while the cation selectivity is generally entropy-driven in acetonitrile.