Complexation of Metal Ions with Azacrown Ethers Bearing an 8-Hydroxyquinoline Side Arm

Thermodynamic quantities (log, K, ΔH, and TΔS) for theinteractions of six azacrown ethers each bearing an 8-hydroxyquinoline (CHQ)side arm (1-6) with Na+, K+, Ba2+, and Cu2+ were determined by calorimetrictitration in methanol solution at 25°C. The results indicate that theseligands form stable complexes with the cations studied. Ligands 1 and 3 thathave CHQ attached through position 7 (next to the OH group) show highselectivity for Cu2+ (log K values of 8.12 and 9.44, respectively) over Na+,K+, and Ba2+ by more than four orders of magnitude. On the other hand,ligands 2 and 4 that have CHQ attached through position 2 (next to thequinoline nitrogen group) form more stable complexes with Na+, K+, and Ba2+,but less stable complexes with Cu2+, than ligands 1 and 3. All ligandsinteract more strongly with K+ than with Na+. The K+/Na+ selectivity forligands 4 and 5 is about 1.5 log K units. All complexation reactions displaynegative enthalpy changes. In most cases the entropy changes are alsonegative, indicating that formation of the complexes is enthalpy driven. 1HNMR spectral experiments demonstrate coordination of the cations by alldonor atoms of the ligands including those of the CHQ arm. In all cases, theOH signal is observed in the 1H NMR spectra, suggesting that thecomplexation with the cations does not involve deprotonation of the CHQgroups in the ligands.