COMPLEXATION THERMODYNAMICS OF CROWN-ETHERS - 6 - CALORIMETRIC TITRATION OF CATION COMPLEXATION WITH SOME AZACROWN ETHERS

Complex stability constants (K-s) and thermodynamic parameters (Delta H degrees and Delta S degrees) were determined in methanol at 25 degre es C for the stoichiometric 1:1 complexation of some alkali metal ions (Na+, K+, and Cs+) with N-phenylaza-15-crown-5 (1), N-octylaza-15-cro wn-5 (2), and N-benzylaza-15-crown-5 to N-benzylaza-24-crown-8 (3-6). The N-substituent was shown to have a drastic effect; upon complex sta bility of the aza-15-crown-5 series (1-3). The aza-15-crown-5 2 posses sing the least-bulky substituent afforded the most stable complexes wi th all cations examined and the highest Na+/K+ selectivity up to 13. T he larger-sized azacrown ethers 4-6 gave the highest complex stabiliti es for the size-matched cations; i.e., 4 showed the highest K+/Cs+ sel ectivity up to 40, while 6 showed the highest Cs+/K+ selectivity of 10 . These results indicate that the N-substituent plays crucial roles in addition to the size-fit concept in the complexation of azacrown ethe rs with a nondonating side-chain. Thermodynamically, the complexation of alkali metal ions with azacrown ethers is enthalpy-driven, but the cation selectivity is mostly entropy-governed in methanol.