Mechanistic study of indium extraction by crown ether based on ion pair recognition

In this study, a novel route for selective separation and extraction of indium from a KBr bearing system based on ion pair recognition using benzo 18-crown-6 (B18C6) as the receptor has been proposed. The single crystal of the complex formed between B18C6 and indium in the presence of KBr, [K(B18C6)][InBr 4 ], was obtained by the slow evaporation method. The single crystal X-ray diffraction analysis revealed that the stacking of the [K(B18C6)][InBr 4 ] molecules were mainly sustained by the [K···Br] and multiple [C−H···Br] interactions. The intra- and intermolecular interactions were analyzed through the density functional theory (DFT) calculations such as IGM and Hirshfeld based on the single crystal structure of [K(B18C6)][InBr 4 ]. The liquid-liquid extraction experiments were further carried out and results showed that the presence of KBr could obviously enhance the indium extraction process. B18C6 exhibited excellent extraction ability and selectivity toward indium in a complex system under the optimal conditions. In addition, the reaction mechanism of the indium extraction process was studied by 13 C NMR titrations . The experimental and the theoretical analysis results generally indicate that the presence of KBr could facilitate the formation of the [K(B18C6)] + cation fragment and the [InBr 4 ] − anion fragment. The generated cation and anion fragments further associate with each other in 1:1 stoichiometry to create the complex of [K(B18C6)][InBr 4 ], achieving the efficient and selective extraction of indium. • A novel route for indium extraction based on ion pair recognition was proposed. • Single crystal of the complex formed between B18C6 and indium was obtained. • DFT calculation and single crystal structure analysis were carried out. • Extractability of B18C6 to indium was evaluated by solvent extraction experiments. • The complexation mechanism was clarified by means of 13 C NMR titrations.