Study of the competitive binding of mixed alkali and alkaline earth metal ions with dibenzo-30-crown-10

Abstract

Dibenzo-30-crown-10 (DB30C10) has been used for the extraction of potassium ion into organic solution (CH₂Cl₂) from aqueous potassium picrate generated from the in situ reaction between picric acid and KOH. The extraction equilibrium constant (K_e) was found to be 2.9 × 10⁶. The high value of K_e is ascribed to the facile transport of the K⁺-complex from the aqueous to the organic phase as a result of ion-pair interaction between [K-DB30C10]⁺ and the picrate counter ion. The crystal structure of [K-DB30C10][picrate] shows evidence of such interactions (C-H...π, π...π and C-H...O) between [K-DB30C10]⁺ and the counter ion. MMFF94 molecular mechanics/Monte Carlo conformational search yielded five low energy conformations (5 kcal/mol energy window) of which one of the conformations was similar to the conformation of the complex in the above crystal, indicating that the crystal formation is thermodynamically controlled. The combination of DB30C10 and the picrate anion was employed to determine the selectivity of K⁺ extraction from an equimolar mixture of Na⁺, K⁺, Mg²⁺ and Ca²⁺; the molar percentage of the metal ions in the extract was found to be 7.8%, 82.9%, 7.9% and 1.5%, respectively. When the same experiment was conducted with highly concentrated salt bitterns, the corresponding values were 7.5%, 75.6%, 16.3% and 0.5%, respectively. The hierarchy of selectivity was qualitatively reproduced by molecular mechanics calculations when experimental cation hydration data was taken into account.