Bio-inspired Calix[6]Arene–Zinc Funnel Complexes

Abstract

A bio-inspired supramolecular system is presented. A calix[6]arene possessing three imidazolyl arms on alternate phenolic positions binds a zinc ion. The resulting complex contains a hydrophobic pocket, which has a flattened conic shape. The system behaves as a selective molecular funnel for neutral guests that bind the metal centre. The exceptional stability of these tetrahedral dicationic complexes is exemplified by the acetaldehyde ternary adduct that was analysed by X-ray crystallography. The ligand is deeply buried in the heart of the calixarene cavity, pointing its methyl group selectively towards the centre of one of the aromatic walls, thereby establishing a stabilizing CH/π interaction. Protic guests undergo hydrogen bonding with the phenolic oxygens of the calixarene structure. The selectivity of the binding in the cavity is based on both the affinity of the donor atom of the guest ligand for the zinc ion and the relative host–guest geometries. The helical shape of the tris-imidazolyl groups binding the metal centre is the base of the chirality of the system. The twisted calix[6]arene structure of the zinc funnel complexes is shown to provide a new example of a cavity suitable for host–guest chiral induction.

Abstract

Formation of helical zinc complexes.

Keywords:

• Zinc
• Calixarene
• Biomimetic
• Host–guest
• Coordination
• N-ligand

Notes

^†Studies related to chiral induction from guest to host in supramolecular assemblies are still rare. See [10–13]

^‡Although we previously reported a similar phenomenon with Cu(I) complexes, we were never able to observe it with an organic guest such as a nitrile. See [24,25].

The Future of Supramolecular ChemistrySupramolecular chemistry concerns the reversible assembly of discrete entities through the establishment of multiple weak interactions between the different components. As these phenomena are fundamental in the biological world, nature has been a major source of inspiration for chemists involved in the supramolecular field. Two main routes have now been opened. One concerns the construction of new edifices through auto-assembly. These can be nanosized and are aimed at becoming the base for innovating technologies. The other is the control of recognition processes. Early work with Pedersen crown ethers mimicking valinomycin has allowed the selective recognition of a simple cation. This has been developed thereafter for either charged or neutral molecules, often of biological interest such as phosphates, ammoniums ions, and so on. The next step, as in natural systems, is to associate a catalytic activity to these recognition processes. This belongs to the field of biomimetic chemistry, in which Breslow has been a pioneer with his cyclodextrin-based models of hydrolytic enzymes. Yet another sophistication in this field is the introduction of a metal ion as a central actor in the supramolecular system. Indeed, catalysis by transition metal ions is often encountered in natural systems. Bioinorganic chemistry comprises two aspects: the study of biological systems and their modelling by coordination compounds. In contrast to bioorganic chemistry, however, little has been done in the supramolecular field. Biomimetic inorganic chemistry is mainly focused on mimicking the first sphere coordination environment of the metal ion. Little information is available concerning the influence, or even the control, that the microenvironment provided by a protein can have on the reactivity of the metal. Exploration of this aspect involves two major steps. The first is the control of metal ion binding, including recognition events, through the establishment of weak but decisive interactions between the metal–ligand adduct and its environment. The second is the implementation of a catalytic process that can be as efficient and selective as the biological model. Whereas the first aspect is beginning to emerge, the second, which is by far the most challenging, still belongs to the future. This is a tremendously exciting aspect of Supramolecular Chemistry.Olivia Reinaud was born in 1960 in France. She studied Physics and Chemistry first at Orsay University (Paris XI), then at Pierre et Marie Curie University (Paris VI) and finally at the Ecole Supérieure de Physique et Chimie Industrielles de la ville de Paris (ESPCI). Her PhD thesis concerned the development of new synthetic routes to quinonoid compounds of biological interest, using bio-inspired copper catalysed oxidation methodologies in the group of Drs M. Maumy and P. Capdevielle. She then spent a year as a postdoc fellow in the lab of Dr D. Mansuy at René Descartes University (Paris V) and studied the biochemical aspects of oxidative processes related to the metabolism of linoleic acid by leucocytes. Rejoining her former lab at the ESPCI as a CNRS researcher, she then developed novel biomimetic copper-catalysed processes. In order to complete her experience, she took a 2-year sabbatical and worked at Delaware University (USA) with Professor K. Theopold on the stepwise activation of dioxygen by cobalt trispyrazolyl borate complexes. Upon her return to France, she started a new project associating organic, inorganic and supramolecular chemistry with some “bio-inspiration”. After 4 years at the Ecole Nationale Supérieure de Chimie de Paris (ENSCP), she ended up back in her postdoc lab, at René Descartes University. She switched her CNRS position for a Professor position in 2001. Her current research interest now lies in the field entitled “Supramolecular Bio-Inorganic Chemistry”, mainly dealing with biomimetic metal complexes based on calixarene derivatives.