Strategies for using host-guest chemistry in the extractive separations of ionic guests

Host-guest chemistry has led to a new paradigm in extractive separations, generating new possibilities for efficient separations of ionic species to meet the challenging needs of industry. This account describes the approach the authors have recently undertaken, recent results, and future directions toward highly selective separations of anions based on host-guest chemistry principles. The material presented deals mainly with the genesis and discovery of new extractive systems, illustrating the potential of particular chemical concepts with examples of practical application. Major questions of interest concern the role of anions in extractive processes and factors underlying the recognition and transport of anions. Theoretical efforts explore the technique of molecular-design itself as embodied in the evolving HostDesigner program. Design calculations are capable of generating ranked candidate multifunctional ion receptors based on hydrogen-bond-donor groups having O-H and N-H donor functionalities. Efforts to synthesize candidate receptors together with studies of molecular structure and the thermodynamics of binding and transport provide a complete picture for understanding structure-function relationships and feedback for further molecular modeling. Extraction data are evaluated in a thermochemical context in which the solvent matrix, including use of anion-solvating lipophilic alcohols, plays a pivotal role. Applications are envisioned for the solution of many types of separations needs, and examples are taken mainly from the authors' own research as applied to treatment of radioactive wastes for disposal.