Synthesis and Properties of 5-Chloro-8-hydroxyquinoline-Substituted Azacrown Ethers:  A New Family of Highly Metal Ion-Selective Lariat Ethers

New 5-chloro-8-hydroxyquinoline (CHQ)-substituted aza-18-crown-6 (4), diaza-18-crown-6 (1), diaza-21-crown-7 (2), and diaza-24-crown-8 (3) ligands, where CHQ was attached through the 7-position, and aza-18-crown-6 (11) and diaza-18-crown-6 (10) macrocycles, where CHQ was attached through the 2-position, were prepared. Thermodynamic quantities for complexation of these CHQ-substituted macrocycles with alkali, alkaline earth, and transition metal ions were determined in absolute methanol at 25.0 °C by calorimetric titration. Two isomers, 1 and 10, which are different only in the attachment positions of the CHQ to the parent macroring, exhibit remarkable differences in their affinities toward the metal ions. Compound 1 forms very stable complexes with Mg2+, Ca2+, Cu2+, and Ni2+ (log K = 6.82, 5.31, 10.1, and 11.4, respectively), but not with the alkali metal ions. Ligand 10 displays strong complexation with K+ and Ba2+ (log K = 6.61 and 12.2, respectively) but not with Mg2+ or Cu2+. The new macrocycles and their complexes have been characterized by means of UV−visible and 1H NMR spectra and X-ray crystallography. New peaks in the UV spectrum of the Mg2+−1 complex could allow an analytical determination of Mg2+ in very dilute solutions in the presence of other alkali and alkaline earth metal cations. 1H NMR spectral and X-ray crystallographic studies indicate that ligand 10 forms a cryptate-like structure when coordinated with K+ and Ba2+, which induces an efficient overlap of the two hydroxyquinoline rings. Such overlapping forms a pseudo second macroring that results in a significant increase in both complex stability and cation selectivity.