Coordination between alkali metal or alkaline earth metal ions and 1,4- or 2,3-naphthalenedicarboxylate ion in binary acetonitrile-water and acetonitrile-methanol media

Abstract

In binary mixed solvents, acetonitrile-water (MeCN-H2O) and acetonitrile-methanol (MeCN-MeOH), specific interactions between alkali metal (M+=Li+, Na+) or alkaline earth metal ions (Mg2+, Ca2+, Ba2+) and the 1,4- or 2,3-naphthalenedicarboxylate ion [(n-Bu4N+)2L2−] have been thoroughly examined by UV–visible spectroscopy. With increasing LiClO4 concentration, tetrabutylammonium 1,4-naphthalenedicarboxylate (1.0×10−4moldm−3) causes the precipitation of Li2L in 2.0–15% (v/v) MeOH-MeCN, however, the precipitates are apt to re-dissolve completely to form the “reverse-coordinated” species, Li3L+, with the further addition of an excess amount of LiClO4. NaClO4 causes similar precipitation and re-dissolution of the precipitates in similar conditions. The formation of ML and M2L2+ species of Mg2+, Ca2+, and Ba2+ have been observed up to15 or 20% (v/v) H2O-MeCN. Even in sole EtOH, the ML and M2L2+ species have been clearly observed for Mg2+ and Ba2+ but not for Ca2+. Contrastingly, the precipitates of Li2L and Na2L of 2,3-naphthalenedicarboxylate (1.0×10−4moldm−3) would not re-dissolve in the presence of 1.0moldm−3 LiClO4 or NaClO4 in 2.0 or 5.0% (v/v) H2O-MeCN. Because of the higher solubility of MgL in H2O-MeCN solvents, the Mg2L+ species can be observed only in 2.0 or 5.0% (v/v) H2O-MeCN. Between the L2− ion and Ca2+ or Ba2+, the precipitation and the successive re-dissolution of precipitates have been observed obviously in a narrow range around 25 or 30% (v/v) H2O. The precipitates of BaL and “reverse-coordinated” species of Ba2L2+ of the 2,3-derivative have been observed in sole MeOH or EtOH. Generally speaking, the solubility products (pKsp) and the “reverse” coordination constants (log K3 or log K2) both for 1,4- and 2,3-derivatives decrease with increasing contents of H2O or MeOH in MeCN; where, 3M++L2−⇆M3L+, K3=[M3L+]/([M+]3 [L2−]) or 2M2++L2−⇆M2L2+, K2=[M2L2+]/([M2+]2 [L2−]).