Supramolecular networking of macrocycles based on exo-coordination: from discrete to continuous frameworks

Macrocyclic ligands typically show high selectivity for specific metal ions and small molecules, and these features make such molecules attractive candidates for nanoscale chemical sensing applications. Crown ethers are macrocyclic structures with polyether linkages where the oxygen donors are often separated by an ethylene unit (-O-CH(2)-CH(2)-O-). Because the oxygen lone pairs in crown-type macrocycles are directed inward, the preorganized macrocyclic cavity tends to form complexes where metals coordinate inside the cavity (endo-coordination). However, sulfur-containing macrocycles often demonstrate metal coordination outside of the cavity (exo-coordination). This coordination behavior results from the different torsion arrangements adopted by the X-CH(2)-CH(2)-X atom sequence (X = O, gauche; X = S, anti) in these molecules. Exo-coordination is synthetically attractive because it would provide a means of connecting macrocyclic building blocks in diverse arrangements. In fact, exo-coordination could allow the construction of more elaborate network assemblies than are possible using conventional endocyclic coordination (which gives metal-in-cavity products). Exo-coordination can also serve as a tool for crystal engineering through the use of diverse controlling factors. Although challenges remain in the development of exo-coordination-based synthetic approaches and, in particular, for the architectural control of supramolecular coordination platforms, we have established several strategies for the rational synthesis of new metallosupramolecules. In this Account, we describe our recent studies of the assembly of metallosupramolecules and coordination polymers based on sulfur-containing macrocycles that employ simple and versatile exo-coordination procedures. Initially, we focus on the unusual topological products such as sandwich (1:2, metal-to-ligand), club sandwich (2:3), and cyclic oligomeric complexes as discrete network systems. The primary structures we achieve in these networked macrocycles are one to three dimensional coordination polymers based on homo- and heteronuclear metal systems. Using crystal engineering methods, we have also investigated variation in the donors, interdonor distances, ligand isomer structures, and the effect of counter anions on the chemical and physical properties of the products. Understanding the relationship between structure and function in these exo-coordination products is an important step in the design of these new supramolecules for practical applications. We investigated the photophysical properties of the exocyclic complexes and a chromogenic macrocycle, which exhibited cation-selective and anion-controlled color change depending on an exo- or endo- ligand binding mode. Overall, we suggest that the exocyclic coordination behavior provides a versatile strategy for the preparation of new molecular networks and materials.