Guest-Triggered Zn^II Translocation and Supramolecular Nuclearity Control in Calix[6]arene-Based Complexes

Two new polytopic ligands based on a calix[6]­arene scaffold were synthesized. The truncated cone-shaped calixarene was functionalized at its small rim by a tris-imidazole site, aimed at generating a tetrahedral Zn^II complex, where a fourth labile site inside the cavity is accessible through the funnel provided by its large rim. Tridentate aza ligands (either two or three) were then grafted at this large rim (the entrance of the cavity). Zn^II coordination studies, monitored by ¹H NMR spectroscopy, showed unprecedented behavior in this family of heteropolytopic ligands. Indeed, it gives access to complexes of various nuclearities in acetonitrile, where zinc binding is under the supramolecular control of the guest. It is first shown that, in the absence of a good guest donor (a primary amine), Zn^II binding is favored at the large rim where two tridentate nitrogenous groups can form an octahedral complex. The addition of a long guest such as heptylamine induces the quantitative translocation of the Zn^II ion from the large rim octahedral (O_h) site to the small rim tetrahedral (T_d) site provided by the trisimidazole core and the guest ligand. With 2 equiv of Zn^II, well-defined dinuclear complexes were obtained and isolated, with one Zn^II ion bound at each rim. Interestingly, it is shown that the binding mode at the large rim is under the supramolecular control of the guest bound at the small rim (with short guests, the O_h environment is obtained at the large rim, whereas long guests disrupt this core through an induced-fit process); the partially included and dangling alkyl chain opens the large rim (entrance of the cavity) and pushes apart the tridentate moieties. As a result, a guest-induced switch of Zn^II binding mode occurs and frees one of the tridentate groups from coordination, allowing further extension of the complex nuclearity.