Specific coordination phenomena of alkaline earth metal ions with aromatic sulfonate ions in alcohols and binary solvents of acetonitrile–alcohols

Abstract

The specific interactions between alkaline earth metal (Mg2+, Ca2+, or Ba2+) and p-toluenesulfonate (L−), 1,5-naphthalenedisulfonate (L2−), or 1,3,6-naphthalenetrisulfonate (L3−) ions (from the tetraethylammonium salt of L−, L2−, or L3−) have been examined by means of UV–visible spectroscopy in primary alcohols (from methanol to hexanol) as well as in the binary acetonitrile–alcohols (MeCN–MeOH, MeCN–EtOH), ethanol–methanol (EtOH–MeOH) and methanol–water (MeOH–H2O) solvents. The precipitation of non-charged species (e.g. ML0) and the successive re-dissolution of the precipitates, with increasing concentration of M(ClO4)2, have revealed the formation of cationic charged species or “reverse coordinated” species, M2L2+, even in the protic media as well as in the aprotic solvent MeCN. The solubility products (Ksp) and the “reverse coordination” constants (2M2++L2−⇆M2L2+, K2(−2)=[M2L2+]/[M2+]2 [L2−]) have been evaluated. In ethanol, both phenomena of the precipitation of ML0 and the successive re-dissolution to produce M2L2+ are observed for Ca2+ or Ba2+, but not for Mg2+. In butanol, the interaction between Mg2+ and the L2− causes the complete precipitation of MgL0 (pKsp=10.39) and also the successive re-dissolution of Mg2L2+ (log K2(−2)=8.08). Even in methanol, the interaction between Ba2+ and L2− results in precipitation (log Ksp=8.28) and the “reverse coordinated” species, Ba2L2+ (log K2(−2)=5.58). The interaction of Ba2+ with L− or L3− causes no precipitation in methanol; however, in all the other alcohols, it results both in precipitation (BaL2 or Ba3L2) and the “reverse coordinated” species, BaL+ or Ba2L+. The formulation for the formation constants (K2(−3)) for M2+ and L3− is newly presented and the constants (2Ba2++L3−⇆Ba2L+, K2(−3)=[Ba2L+]/[Ba2+]2 [L3−]) are evaluated in ethanol and propanol as well as in the binary EtOH–MeOH solvents, up to 70% (v/v) MeOH. The donicities toward M2+ of the media have been related to the pKsp and “reverse coordination” constants for L−, L2− and L3−.