A Semiflexible Tetrahydrazone Macrocycle for Binding of Pyrophosphate and Smaller Anions.

Macrocyclization has proven to be a useful design strategy in the development of efficient anion receptors. In addition to the ring size, the overall preorganization due to structural rigidity is key. To explore this in the context of developing an efficient pyrophosphate receptor, three macrocycles featuring a 26-membered interior ring size and similar H-bonding motifs have been synthesized, and their anion binding ability has been investigated. Computational studies and nuclear magnetic resonance (NMR) data showed different degrees of preorganization as a result of differences in flexibility. The interaction of the three macrocycles with chloride, dihydrogen phosphate, and dihydrogen pyrophosphate was investigated in solution by NMR and ultraviolet-visible spectroscopy and in the solid state by X-ray crystallography. The tetrahydrazone-based macrocycle featuring intermediate flexibility exhibited the best affinity for all three anions investigated. Our results suggest that in addition to the proper preorganization of binding groups in a macrocycle a certain degree of flexibility is also required for an optimal affinity with the target guest.

A 4-state acid-base controlled molecular switch based on a host-guest system.

A novel ZnII funnel complex that presents three phenol functions within a calix[6]arene macrocycle is described. Host-guest studies, monitored by 1H NMR spectroscopy, evaluate the impact of the replacement of three anisole moieties present in a previously described system with phenols. It is now shown that the dicationic complex is responsive to anions, whereas deprotonation of one phenol unit completely inhibits any hosting response. These properties, combined with those of the corresponding protonated ligand, allow us to obtain different molecular switches, and one of them shows guest embedment changes between four different host states, thus giving rise to a rare case of a triple molecular switch.

Tuning the Fluorescence Through Reorientation of the Axle in Calix[6]arene-Based Pseudorotaxanes.

This work describes a calix[6]arene-based wheel that binds, in non-polar media, a stilbazolium salt to yield a mixture of pseudorotaxane orientational isomers. The isomer's abundance ratio evolves with time and can be reversibly tuned by adjusting the temperature. The spectroscopic properties, and notably the emission spectrum, of the bound guest depend on its orientation inside the non-palindromic wheel, suggesting such a system as a switch with spectroscopic readout.

"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 ZnII complex with all of the carbonyl groups of the amido arms bound and a five-coordinate CuII 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 CuII/CuI 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 CuI complex to O2 at low temperature, a green species was obtained with a UV-vis signature typical of an end-on superoxide CuII 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.

Immobilization of Monolayers Incorporating Cu Funnel Complexes onto Gold Electrodes. Application to the Selective Electrochemical Recognition of Primary Alkylamines in Water.

The immobilization of a copper calix[6]azacryptand funnel complex on gold-modified electrodes is reported. Two different methodologies are described. One is based on alkyne-terminated thiol self-assembled monolayers. The other relies on the electrografting of a calix[4]arene platform bearing diazonium functionalities at its large rim and carboxylic functions at its small rim, which is post-functionalized with alkyne moieties. In both cases, the CuAAC electroclick methodology proved to be the method of choice for grafting the calix[6]azacryptand onto the monolayers. The surface-immobilized complex was fully characterized by surface spectroscopies and electrochemistry in organic and aqueous solvents. The Cu complex displays a well-defined quasi-reversible system in cyclic voltammetry associated with the Cu(II)/Cu(I) redox process. Remarkably, this redox process triggers a powerful selective detection of primary alkylamines in water at a micromolar level, based on a cavitary recognition process.

Supramolecular assistance for the selective demethylation of calixarene-based receptors.

The selective demethylation of methoxy groups of several multifunctionalized 1,3,5-trimethoxycalix[6]arene-based receptors has been achieved. It is shown in this study that the best reagent is trimethylsilyl iodide (TMSI) and that the conformation adopted by the calixarene core is crucial for both the selectivity and the efficiency of the process. A key feature appears to be the "in" or "out" orientation of the methoxy substituents relative to the macrocyclic cavity. If projected inward, the reaction is slow and not selective. If projected outward, the reaction is fast and selective. A strategy that consists of exploiting the host-guest properties of the receptors to change their conformation and to permit their selective demethylation has been developed. Four cases of such a supramolecular assistance are reported, demonstrating the efficiency of the strategy. Such an allosteric control is highly reminiscent of biological processes allowing selective transformation of multifunctional molecules.