Synthesis of a self-assembled molecular capsule that traps pyridine molecules by a combination of hydrogen bonding and copper(II) coordination.

Molecule and molecular assemblies with cavities of different size and shape to encapsulate guest molecules have been synthesized in view of their potential use as selective hosts for anion sensing, catalysis, selective recognition, and separation of guest molecules.[1] Rebek, Jr. and co-workers recently demonstrated that the binding of a reagent inside a capsular cavity boosts the rate of a Diels–Alder reaction.[2] The selfassembly of relatively small molecules, through hydrogen bonding, or metal–ligand interactions, proved to be very useful in forming large capsular cavities.[1] While several capsular cavities have been synthesized by using organic frameworks (e.g. resorcinarene, calixarene),[1] coordination complexes with a redox stable metal center,[1] and a few with a redox active metal center,[3] there is no report of a capsular cavity with the redox-active metal center having an available binding site inside the capsule.[4] The synthesis of a capsular cavity with an available coordination site at redox active metal centers inside the pocket can, in principle, facilitate the study of the reactivity of the bound guest molecule inside a cavity. In our effort to synthesize a capsular cavity with a redox center, we have synthesized a self-assembled capsule of an octameric CuII coordination complex by using an easy to synthesize ligand (Scheme 1). This capsule has four guest pyridine molecules trapped inside the cavity (Figure 1), in which two of the pyridine molecules are held with a combination of hydrogen bonds and CuII coordination. The octameric CuII complex (Figure 1) was synthesized from the tetradentate deprotonated ligand (L2 , Scheme 1) and the CuII salt, Cu(ClO4)2·6H2O. Because of the presence of an amine hydrogen atom, an imidazole N H group (a H-bond donor), and carboxylate oxygen atoms (a H-bond acceptor) in