Chemical interaction between alkaline earth metal ions and the benzoate or 2,6-naphthalenedicarboxylate ion in acetonitrile and alcohols

The chemical interactions of alkaline earth metal ions (M2+ = Ca2+, Ba2+) with the benzoate (L−) and the 2,6-naphthalenedicarboxylate (L2−) ions have been examined by means of UV-visible spectroscopy in acetonitrile (MeCN) and several alcohols. In MeCN, the absorption band at around 220 nm of L− (1.0 × 10−3 mol dm−3) firstly has decreased and almost disappeared in the presence of 5.0 × 10−4 mol dm−3 M2+ and then suddenly recovered at 1.0 × 10−3 mol dm−3 Ca2+ or 5.0 × 10−2 mol dm−3 Ba2+. The successive phenomena should be caused by the ML2 precipitation and the re-dissolution with the formation of the cationic species of ML+ from ML2. The specific chemical interaction between L− and Ba2+ can take place not only in MeCN but also amphiprotic solvents, such as, ethanol, 1-propanol, 2-propanol, and 1-butanol. However, the addition of ≥10% H2O or ≥50% MeOH to MeCN has inhibited the precipitation between L− and Ba2+. Between the L2− and Ba2+ ions, both precipitation and re-dissolution have been observed in binary mixed solvents, MeCN-ROH of 50-100% MeOH and of 70% EtOH. The solubility products (pKsp) and the coordination or “reverse” coordination constants (log K1 and log K2) between L− or L2− and Ba2+ have been evaluated in MeCN, the alcohols, and the binary mixtures of MeCN-H2O or MeCN-MeOH. Where, K1 = [BaL+] / ([Ba2+] [L−]) and K2 = [Ba2L2+] / ([Ba2+]2 [L2−]) for the chemical equilibria, Ba2+ + L− ⇆ ML+ and 2Ba2+ + L2− ⇆ Ba2L2+, respectively.