COMPLEXATION THERMODYNAMICS OF BIS(CROWN ETHER)S .4. CALORIMETRIC TITRATION OF INTRAMOLECULAR SANDWICH COMPLEXATION OF THALLIUM AND SODIUM-IONS WITH BIS(15-CROWN-5)S AND BIS(12-CROWN-4)S - ENTHALPY ENTROPY COMPENSATION

Calorimetric titrations were performed in methanol-water (8:2) at 25°C to give the complex stability constants and thermodynamic parameters for the complexation reactions of thallium ion with a series of bis(benzo-15-crown-5)s 1-3 and bis(15-crown-5) 6, and of sodium ion with a series of bis(12-crown-4)s 7-11. Both quantities are critical functions of the chain length (n) of the bridging methylene. The polymethylene-bridged bis(crown ether) 1 and its hydroxy analogue 3 with the same n gave quite similar stability constants and the maximum stabilities were obtained at n = 5, while the dioxo derivative 2 and the previously reported bis(crown ether) Schiff base 4 appeared to afford the maximum stability constants at larger n of 8-10. Thermodynamically the formation of the intramolecular sandwich complexes is evidently enthalpy driven for most bis(15-crown-5)s, but the profile of stability change is well accounted for in terms of the entropy change. Partial structural freezing induced by introducing a rigid cyclopentanediyl or xylylene moiety in the bridge dramatically enhances the complex stability of bis(crown ether)s 6-10 primarily through the less negative or even positive entropy change upon complexation. The ΔH-TΔS plot to testify to the enthalpy-entropy compensation effect gave an excellent straight line with a slope of 1.03 and an intercept of 4.6, indicating that the intramolecular sandwich complexation of bis(crown ether)s accompanies substantial conformational changes and extensive desolvation, both of which are inferred to be comparable to those for the antibiotic ionophores. © 1990 American Chemical Society.