Three new disubstituted benzo-15-crown-5 derivatives (3-5) have been synthesized from 4',5'-bis-(bromomethyl)benzo-15-crown-5 (2) and the corresponding alkanols in the presence of Na2S2, and their complexation thermodynamics with light lanthanoid(III) nitrates (La-Gd) have been studied in anhydrous acetonitrile at 25 °C. Plots of K(S) against the reciprocal ionic diameter of lanthanoid exhibited monotonically declining pattern for the parent benzo-15-crown-5 (1) and 3 but showed a characteristic peak at Ce3+ for 4 and 5. It is interesting to note that the simple extension, of the alkyl side chains in 4 and 5 can alter the cation selectivity profiles of 1 and 3. Possessing two 2-oxapropyl groups, 3 gave a comparable K(S) for La3+ but a significantly, decreased K(S) for Ce3+ compared with the corresponding values for 1, thus exhibiting an exceptionally high La3+/Ce3+ selectivity of 11. Thermodynamically, the complexation of lanthanoid perchlorates with 1 is absolutely entropy-driven in acetonitrile, while the complexation of lanthanoid nitrates with 3-5 is primarily driven by exothermic enthalpy changes with accompanying moderate entropic gain or small entropic loss.
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