Supramolecular Control of Hetero-multinuclear Polytopic Binding of Metal Ions (ZnII, CuI) at a Single Calix[6]arene-Based Scaffold

A Calix[6]arene scaffold was functionalized to provide a tridentate binding site at the small rim and three bidentate chelate sites at the large rim of the cone to generate a heteropolytopic ligand. Its complexation to one equivalent of Zn-II at the small rim yields a funnel complex displaying both host-guest properties and preorganization of the three chelate groups at the large rim. These two aspects allowed the full control of the binding events to regioselectively form dinuclear Zn-II and heteropolynuclear Zn-II/Cu-I complexes. The heteropolynuclear systems all rely on the host-guest relationship thanks to the induced-fit behavior of the calix cavity. With the short guest MeCN, the large rim is preorganized into a trigonal tris-triazole core and accommodates a single Cu-I ion. A long guest breaks this spatial arrangement, and three Cu-I ions can then be bound at the tris-bidentate triazole-dimethylamine site at the large rim. In a noncoordinating solvent however, the tetranuclear complex is submitted to scrambling and the addition of exogenous pi-acceptor ligands is required to control the binding of Cu-I in a well-defined environment. Hindrance selectivity was then induced by the accessibility at the small rim site. Indeed, while CO can stabilize Cu-I at both coordination sites, PPh3 cannot fit into the cavity and forces Cu-I to relocate at the large rim. The resulting well-defined symmetrical tetranuclear complex thus arises from the quite remarkable selective supramolecular assembly of nine partners (1 Zn-II, 3 Cu-I, 1 calixarene, 1 guest alkylamine, 3 PPh3).