COMPLEXATION THERMODYNAMICS OF CROWN-ETHERS .3. 12-CROWN-4 TO 36-CROWN-12 - FROM RIGID TO FLEXIBLE LIGAND

Complex stability constants and thermodynamic parameters have been determined for the complexation of alkali metal ions with large-sized crown ethers, i.e. 24-crown-8 to 36-crown-12, in methanol at 25-degrees-C. Using the present and reported data, the complexation behaviour of 12-crown-4 through 36-crown-12 is discussed comparatively and globally from the thermodynamic point of view. The complex stability sequence for each cation as a function of ring size shows a characteristic two-peak profile with a global maximum and a smaller local maximum at a larger ring size, both of which may be interpreted in terms of the size-fit concept. Thermodynamically, the complexation of alkali metal ions with the crown ethers is absolutely enthalpy-driven in methanol, but the cation selectivity determined by the size-fit relationship is shown to be mostly entropy-governed. The large, but ring size insensitive, enthalpy and entropy changes for large crown ethers are rationalized by the mutually resembling cation-ligand interaction, degree of desolvation, and substantial structural freezing upon encapsulating complexation forming a highly flexible 3D cavity. However, the size-fit concept still appears to play a subsidiary role even in the complexation by larger crown ethers, although its control becomes fairly loose.