Structural aspects of halides with cryptands

Abstract

Anion coordination chemistry is a growing field within the inorganic and organic communities [1,2]. Findings compiled over the last several decades show that anions form complexes with receptors (ligands) via hydrogen bonding interactions as opposed to the coordinate covalent bonds operable in transition metal coordination. As binding and structural data accumulate, more informed strategies are being devised to achieve selective binding of anions of different topologies. Nonetheless, in terms of corollaries with transition metal ions, the spherical anions, namely the halides, present the simplest of topologies and the closest structural analogies. In fact, the first synthetic anion receptors were the halide binding bicyclic receptors, 1, known as katapinands, and reported by Park and Simmons in 1968 [3] (Figure 1). Several years later, crystallographic studies confirmed the encapsulation of chloride in H 212+ (n = 9) [4]. Receptors for anions vary widely, and are in many instances either the same or modified versions of the ligands that bind transition metal ions [5-20]. For example, polyamine macrocycles can bind transition metal ions, while polyammonium macrocycles bind anions. In many instances the same ligand can bind either a transition metal ion [21-24] or an anion [25-29]. That being the case, it is appealing to refer to anion receptors as ligands. However, traditionally the commonly understood definition of ligand has been a Lewis base capable of forming a coordinate-covalent bond with a metal ion. Since for anions, the "ligand" actually behaves as a Lewis acid, the definition needs to be expanded to include both Lewis acid and Lewis base behavior. Over the last several years we have explored anion binding by systematically varying ligands with similar frameworks with respect to hydrogen bond donor, charge, and dimensionality [30-42]. We have examined polyammonium [30-38], polyamide [39-41], and polythioamide ligands [42]; monocycles and bicycles; and neutral and charged ligands. Crystallographic findings show that bicyclic ligands or cryptands, as coined first by Lehn [43], more frequently bind anions via encapsulation, while monocycles often do not. This chapter will compare structural effects of a series of cryptands, including some reported by us and some by others, ranging in size from very small and capable of encapsulating just the smallest of halides, to a size large enough to hold multiple species within the cavity. These ligands include the tiny "octaazacryptand," 2; larger flexible azacryptands with aliphatic spacers, 3 and 4; more "rigid" azacryptands with aromatic spacers, 5 and 6; and as corollaries to the rigid azacryptands, the amidocryptands, 7 and 8 (Figure 2). Since this chapter focuses on encapsulated anions, most of the structures shown will include only the ligand and encapsulated anion. © 2005 Springer. Printed in the Netherlands.