Insights into the binding properties of a cuprous ion embedded in the tren cap of a calix[6]arene and supramolecular trapping of an intermediate

A Water Molecule Triggers Guest Exchange at a Mono-Zinc Centre Confined in a Biomimetic Calixarene Pocket: a Model for Understanding Ligand Stability in Zn Proteins

In this study, the ligand exchange mechanism at a biomimetic Zn^II centre, embedded in a pocket mimicking the possible constrains induced by a proteic structure, is explored. The residence time of different guest ligands (dimethylformamide, acetonitrile and ethanol) inside the cavity of a calix[6]arene-based tris(imidazole) tetrahedral zinc complex was probed using 1D EXchange SpectroscopY NMR experiments. A strong dependence of residence time on water content was observed with no exchange occurring under anhydrous conditions, even in the presence of a large excess of guest ligand. These results advocate for an associative exchange mechanism involving the transient exo-coordination of a water molecule, giving rise to 5-coordinate Zn^II intermediates, and inversion of the pyramid at the Zn^II centre. Theoretical modelling by DFT confirmed that the associative mechanism is at stake. These results are particularly relevant in the context of the understanding of kinetic stability/lability in Zn proteins and highlight the key role that a single water molecule can play in catalysing ligand exchange and controlling the lability of Zn^II in proteins.

"Two-Story" Calix[6]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding

A new "two-story" calix[6]arene-based ligand was synthesized, and its coordination chemistry was explored. It presents a tren cap connected to the calixarene small rim through three amido spacers. X-ray diffraction studies of its metal complexes revealed a six-coordinate Zn^II complex with all of the carbonyl groups of the amido arms bound and a five-coordinate Cu^II complex with only one amido arm bound. These dicationic complexes were poorly responsive toward exogenous neutral donors, but the amido arms were readily displaced by small anions or deprotonated with a base to give the corresponding monocationic complexes. Cyclic voltammetry in various solvents showed a reversible wave for the Cu^II/Cu^I couple at very negative potentials, denoting an electron-rich environment. The reversibility of the system was attributed to the amido arms, which can coordinate the metal center in both its +II and +I redox states. The reversibility was lost upon anion binding to Cu. Upon exposure of the Cu^I complex to O₂ at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide Cu^II complex. Such a species was proposed to be responsible for oxygen insertion reactions onto the ligand according to the unusual and selective four-electron oxidative pathway previously described with a "one-story" calix[6]tren ligand.

Straightforward synthesis and catalytic applications of rigid N,O-type calixarene ligands

Here, we report a simple one-step access to new rigid N,O-calixarene ligands which is based on copper-catalized amination at the lower rim. We also present their main group and late transition metal complexes which show superior catalytic activity, in several organic transformations, compared with known metal calixarene complexes.

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.

An induced-fit process through mechanical pivoting of aromatic walls in host-guest chemistry of calix[6]arene aza-cryptands

The per-ipso-nitration of a TMPA-capped calix[6]arene has been achieved. The substitution of the six bulky tBu substituents for nitro groups has a strong impact on the behavior of the ligand during guest recognition. The complexation of the aza cap (by H(+) or Cu(+)) associated with the encapsulation of a guest triggers an induced-fit process leading to the loss of the cone conformation of the host in favor of alternate conformations. Such a "pivoting" response of one or two walls of the calixarene core induces a large mechanical motion of the corresponding aromatic units. This stands in strong contrast with the "breathing" phenomena previously identified with other calix[6]arene-based complexes that expand or shrink the size of their cone as a function of the guest. Because of the covalently attached rigid TMPA cap, three arene units of this new calixarene host have a restricted mobility, which forces it to respond in a different manner to a supramolecular stress.

Coordination of lead(II) in the supramolecular environment provided by a "two-story" calix[6]arene-based N6 ligand

First insights into the coordination properties and host-guest behavior of a "two story" calix[6]aza-cryptand (1) are described. The ligand is constituted of a triazacyclononane (TACN) cap and three pyridine (PY) spacers connected to the calix[6]arene small rim. The resulting N6 donor site coordinates Pb(II) ions to give complexes that are highly stable. X-ray diffraction structures reveal a hemidirected environment for Pb(II) with strong coordination to the TACN cap and weaker bonds with the three PY residues. A guest molecule, either water or EtOH, sitting in the calixarene macrocycle and hydrogen-bonded to the phenoxyl units at the level of the small rim further stabilizes the complexes through electrostatic interactions with the metal center and the calixarene core. In-depth (1)H NMR studies confirm the host-guest behavior of the complexes in solution, with evidence of embedment of neutral guest molecules such as EtOH, BuOH, and N-Me-formamide. Hence, in spite of the presence of a N6 donor, the calixarene macrocycle can be open to guest interaction, giving rise to seven-coordinate dicationic complexes. Noteworthy also is the flexibility of the macrocycle that allows Pb(II) to adopt its preferred hemidirected environment in spite of the three covalent links connecting the calixarene core to the three PY groups. The flexibility of the system is further illustrated by the possible coordination of an exogenous anionic ligand in the exo position. Hence, compared to the previously described "one story" calix[6]aza-cryptands, ligand 1 displays several similar but also new features that are discussed.

Interaction between monomers of two surfactants derived from the [Ru(2,2'-bpy)(3)](2+) complex and α, β and γ-cyclodextrins: formation of [2]- and [3]-pseudorotaxanes

Two new surfactants derived from the tris(2,2'-bipyridine) ruthenium(II) complex, [Ru(2,2'-bpy)(3)](2+), were synthesized and characterized: the double-tailed [Ru(2,2'-bipy)2(4,4'-(C(11)H(23))(2)-2,2'-bipy)](2+) surfactant (RuC11) and the mono-tailed [Ru(2,2'-bipy)(2)(4-(CH(3))-4'-(C(13)H(27))-2,2'-bipy)](2+) surfactant (RuC13). The main characteristic of these species is the presence of an inorganic complex as the polar head of the surfactant with interesting luminescence properties, which were used to study the interaction of these cationic surfactants with α-, β- and γ-cyclodextrins (CD). The results showed the formation of [2]- and [3]-pseudorotaxanes. The binding constant values as well as the stoichiometry of the complexes formed were obtained; the results were confirmed, from a qualitative point of view, with NMR spectra.

Binding of Ru(NH3)5pz2+ to 4-sulfocalix[4]arene sodium salt. Effects of the host-guest interaction on electron transfer processes

The reactions [Ru(NH3)5pz]2++S2O8(2-) and [Ru(NH3)5pz]2++[Co(C2O4)3]3- have been studied in solutions of 4-sulfocalix[4]arene sodium salt. Results show a binding of the ruthenium complex to the calixarene with a 2:1 stoichiometry; that is, a ruthenium molecule binds to two calixarene molecules. This stoichiometry changes when NaCl is added to the medium. Thus, a mixture of 1:1 and 2:1 adducts is found in the presence of 0.1 mol dm(-3) NaCl and only 1:1 adducts when the salt concentration is increased up to 0.3 mol dm(-3). Results show that the binding of the ruthenium complex to the calixarene is due to electrostatic and nonelectrostatic interactions. Kinetic data are interpreted by using the pseudophase model and taking into account the stoichiometry of the ruthenium binding to calixarene. The presence of a supporting electrolyte in the medium produces ion pair formation which exerts an influence on the kinetic rate constants.

Rigidity and/or Flexibility of Calixarenes. Effect of the p-Sulfonatocalix[n]arenes (n = 4, 6, and 8) on the Electron Transfer Process [Ru(NH3)5pz]2+ + Co(C2O4)33-

The reaction [Ru(NH3)5pz]2+ + Co(C2O4)33- has been studied in aqueous solutions of p-sulfonatocalix[n]arene (n = 4, 6, and 8). The results are interpreted by using the pseudophase model. Results show that the rigidity and/or flexibility of the assembled rings have a great effect on the thermodynamics of inclusion of the guest into the host and, therefore, on the kinetics of the electron transfer processes that take place in these media. The obtained results are discussed from the viewpoint of two types of interactions: electrostatic and nonelectrostatic. From surface potential measurements, the guest-host interactions have been demonstrated to be mainly due to nonelectrostatic interactions, although the species are charged. So, the nonelectrostatic contribution to the equilibrium constant in all the calixarenes studied is 1 order of magnitude higher than the electrostatic one (Knel = 144 and 884 mol-1 dm3 for p-sulfonatocalix[n]arene (n = 4 and 6, respectively) and Kel approximately 10 mol-1 dm3). Electrostatic interactions also affect the kinetic results.