Ion aggregations of alkali or alkaline earth metal ions with 2,6-naphthalenedicarboxylate in binary acetonitrile-water and acetonitrile-methanol media

The ion aggregations of alkali metal (Li+, Na+) and alkaline earth metal (Mg2+, Ca2+) with 2,6-naphthalenedicarboxylate ions (L2-) have been examined by means of UV-visible spectroscopy in the binary mixtures of acetonitrile (MeCN) and H2O or methanol (MeOH). The formation of precipitates between Li+ or Na+ and L2- (1.0 x 10(-4) mol dm(-3)) has been observed under the wide Li+ or Na+ concentration range in 2.0% (v/v) H2O-MeCN. With increasing of H2O or MeOH content, more Li+ or Na+ ions are required to form precipitates. The precipitation extent of Na2L approaches to that of Li2L either in 10% H2O-MeCN or 20% MeOH-MeCN. In the mixtures of MeCN containing 5.0 - 20% H2O, the absorbance of L2- firstly decreases and then recovers with increasing concentration of magnesium ions, which is caused by the formation of precipitates (MgL) and the re-dissolution of "reverse-coordinated" species of Mg2L2+ from MgL. No apparent ion aggregation behaviors of Mg2+ and L2- can be observed in 20% H2O and 50% MeOH. The precipitation and re-dissolution occur between calcium ion and L2- in 10% - 30% H2O and 20% - 50% MeOH. The precipitation of L2- with M2+ is in the order of Ba2+ > Ca2+ > Mg2+, and reversely, the "reverse" coordination ability increases in the order Ba2+ < Ca2+ < Mg2+. The solubility products (pK(sp)) and the "reverse" coordination constants (log K-3 and log K-2) have been evaluated in all the systems. (C) 2021 Elsevier B.V. All rights reserved.