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 det ermined for the complexation of alkali metal ions with large-sized cro wn 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 1 2-crown-4 through 36-crown-12 is discussed comparatively and globally from the thermodynamic point of view. The complex stability sequence f or each cation as a function of ring size shows a characteristic two-p eak profile with a global maximum and a smaller local maximum at a lar ger ring size, both of which may be interpreted in terms of the size-f it concept. Thermodynamically, the complexation of alkali metal ions w ith the crown ethers is absolutely enthalpy-driven in methanol, but th e cation selectivity determined by the size-fit relationship is shown to be mostly entropy-governed. The large, but ring size insensitive, e nthalpy and entropy changes for large crown ethers are rationalized by the mutually resembling cation-ligand interaction, degree of desolvat ion, and substantial structural freezing upon encapsulating complexati on forming a highly flexible 3D cavity. However, the size-fit concept still appears to play a subsidiary role even in the complexation by la rger crown ethers, although its control becomes fairly loose.