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

Abstract 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 × 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+