Electrochemical studies on molecular recognition of anions: Complex formation between xylylenyl bis-iminoimidazolinium and dicarboxylates in nitrobenzene and water

Abstract

This paper reports studies of complex formation between the synthetic dicarboxylate receptor para-xylylenylbis-(iminoimidazolinium) ( 1 2+) and dicarboxylates in electrolyte-containing nitrobenzene and water using cyclic voltammetry resulting from charge transfer across a liquid|liquid interface. A series of dicarboxylate—tetrabutylammonium salts was prepared and investigated systematically. The receptor 1 2+ shows modest selectivity in binding glutarate in nitrobenzene over longer dicarboxylates. The interactions between 1 2+ and glutarate are very weak in the aqueous phase, and the association constant for the binding reaction is 190 ± 100 M −1. The association constants between 1 2+ and glutarate in electrolyte-containing nitrobenzene saturated with an aqueous phase and in electrolyte-containing nitrobenzene that has not been saturated with an aqueous phase are 7.46 ± 0.39 × 10 4 and 2.12 ± 0.28 × 10 5 M −1, respectively. 1 2+ transfers across the water|nitrobenzene interface reversibly. However, it does not facilitate the transfer of shorter chain dicarboxylates, as crown ethers do for alkali metals. The failure to observe facilitated transfer is not due to a lack of complexation between glutarate and 1 2+. Rather it arises because at potentials sufficiently negative (water phase vs. nitrobenzene phase) to drive the anionic dicarboxylate into the receptor-enriched nitrobenzene, the cationic receptor has already been driven in the opposite direction, depleting it from the organic side of the interface.