Complexation thermodynamics of crown ethers. Part 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 °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, 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.