Complexation Thermodynamics of Crown Ethers. 6.¹,² Calorimetric Titration of Cation Complexation with Some Azacrown Ethers

Complex stability constants (Ks) and thermodynamic parameters (ΔH° and ΔS°) were determined in methanol at 25 °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-crown-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 stability of the aza-15-crown-5 series (1−3). The aza-15-crown-5 2 possessing the least-bulky substituent afforded the most stable complexes with all cations examined and the highest Na+/K+ selectivity up to 13. The larger-sized azacrown ethers 4−6 gave the highest complex stabilities for the size-matched cations; i.e., 4 showed the highest K+/Cs+ selectivity 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 ethers 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.