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

Formation of a Metallomacrocycle Possessing Nitrogen Donors Assembled in the Cavity and Unique Water Clusters

2,2'-Bipyridyl (bpy) is widely used as a chelating unit for metal complexation but is not usually considered as a hydrogen-bonding unit. This is because the metal-free bpy units are usually in a transoid conformation, and the two nitrogen lone pairs are pointed to the opposite sides. We now report a metallomacrocycle whose three metal-free bpy units are in a cisoid conformation and are fixed in the cavity. The complexation of nickel(II) only at the salen units of the triangular bpytrisalen ligand produced this rigid and planar macrocycle. Its cavity is surrounded by hydrogen-bond acceptors (N of bpy and O of salen), and it was found that unique pentagonal prism clusters of water molecules templated by the cavity were formed in the crystal. This study has not only increased the variation of the synthetic methodologies of multinuclear complexes but has also provided the structural platform on which multiple bpy units exert hydrogen-bonding functions.

Chemoselective guest-triggered shaping of a polynuclear CuII calix[6]complex into a molecular host

A new calix[6]arene scaffold bearing a tris-imidazole binding site at the small rim and three tetradentate aza ligands at the large rim was synthesized. The system binds three Cu^II ions at the large rim sites and is unable to bind a fourth one, which remains in solution. The charge repulsion between the complexes, together with the flexibility of the scaffold, disorganizes the small rim site for binding and prevents its use for host-guest studies. Although the presence of MeCN or DMF guests does not alter this state, the addition of a heptylamine guest, which further displays Brønsted basicity, restores its receptor ability by stabilizing the extra Cu^II ion at the tris-imidazole site with concomitant guest encapsulation and binding of an exo hydroxo ligand. This chemoselective nuclearity switch yields a tetranuclear complex in which the guest backbone is preorganized in front of three potentially reactive Cu(ii) complexes, reminiscent of polynuclear Cu^II enzyme active sites.

Mimicking the Regulation Step of Fe-Monooxygenases: Allosteric Modulation of FeIV -Oxo Formation by Guest Binding in a Dinuclear ZnII -FeII Calix[6]arene-Based Funnel Complex

A heteroditopic ligand associated with a calix[6]arene scaffold bearing a tris(imidazole) coordinating site at its small rim and an amine/pyridine ligand at its large rim has been prepared, and its regioselective coordination to Zn^II at the small rim and Fe^II in the amine/pyridine ligand has been achieved. The heterodinuclear complex obtained displays an overall cone conformation capped by the tris(imidazole)Zn^II moiety and bears a non-heme Fe^II complex at its base. Each of the metal centers exhibits one labile position, allowing the coordination inside the cavity of a guest alkylamine at Zn^II and the generation of reaction intermediates (Fe^III (OOH) and Fe^IV O) at the large rim. A dependence between the chain length of the encapsulated alkylamine and the distribution of Fe^III (OOH) intermediates and Fe^III (OMe) is observed. In addition, it is shown that the generation of the Fe^IV O intermediate is enhanced by addition of the alkylamine guest. Hence, this supramolecular system gathers the three levels of reactivity control encountered in oxidoreductases: i) control of the Fe^II redox properties through its first coordination sphere, allowing us to generate high valent reactive species; ii) control of guest binding through a hydrophobic funnel that drives its alkyl chain next to the reactive iron complex, thus mimicking the binding pocket of natural systems; iii) guest-modulated reactivity of the Fe^II center towards oxidants.

Biomimetic cavity-based metal complexes

The biomimetic association of a metal ion with a cavity allows selective recognition, unusual redox properties and new reactivity patterns.

A versatile strategy for appending a single functional group to a multifunctional host through host-guest covalent-capture

Mono-functionalization of a molecular host is a key step for the development of various efficient systems ranging from supramolecular fluorescent probes to supramolecular catalysts. The presence of several identical reactive groups on the host makes its selective mono-functionalization a challenge. We propose a general two-step strategy to achieve this, based on the receptor properties of the host. A guest bearing two orthogonal functions is first reacted with the host presenting itself a reactive function that is complementary to one of those of the guest. As a result, the host is selectively mono-functionalized by the covalent capture of the guest, which inhibits further reaction of the host. The second function that was present on the guest and which is now covalently linked to the host can be activated in the second step for the grafting of various objects. As a proof of concept, the strategy is described on a calix[6]arene scaffold presenting three identical reactive units. Using Huisgen thermal azide-alkyne cycloaddition for the host-guest covalent-capture step, three examples of post-functionalization are described, allowing cavities bearing a single redox tag, fluorescent probe or polydentate ligand through esterification, Schiff base formation or nucleophilic substitution to be obtained.