Complex formation of alkali and alkaline earth metal ions with cryptands containing thiourea units as a part of macrocyclic framework in aqueous methanol Original Research Article

The stability constants (Ks) of the complexes of alkali and alkaline earth metal ions with new type of the cryptands containing one or two thiourea moieties in one of the bridges were determined by means of pH-metric measurements in 95% aqueous methanol at 25 °C. Cryptands studied do not show any regular alteration of complexes stability depending on the mutual relation of cryptand cavity and cation sizes. In all cases, they form the most stable complexes with K+ along the series of alkali metal ions and with cations of Ba2+ or Sr2+ in the series of alkaline earth ions independently of variations of their structure. The log Ks values for K+, Sr2+ and Ba2+ vary in limits 3.51–5.90, 2.29–7.05 and 2.35–7.51, respectively, depending on the cryptands structure. The complexes stability of the studied cryptands increases in the order Li+ < Na+ (Cs+) < Cs+ (Na+) < Rb+ < K+ and Mg2+ < Ca2+ < Sr2+ (Ba2+) < Ba2+ (Sr2+). However, cryptands containing at least one oxygen atom between the nitrogen bridgehead and group of thiourea form considerably more stable complexes with respect to cryptands in which thiourea group connected with nitrogen bridgeheads via ethylenic chain. The origins of the cryptands complexation behavior are discussed in terms of ligands and complexes structural features.