A Novel <i>C</i>₃<i>_v</i>-Symmetrical Calix[6](aza)cryptand with a Remarkably High and Selective Affinity for Small Ammoniums

The three-step synthesis of a calix[6]arene capped with a TAC unit is presented. The novel C3v-symmetrical calix[6](aza)cryptand displayed an exceptionally high affinity for small ammoniums. NMR and X-ray diffraction analyses demonstrated the formation of endo-complexes. These complexes are stabilized thanks to (i) hydrogen bonding to both the aza cap and one phenolic unit of the calixarene and to (ii) cationic and CH−π interactions between the ammonium and the aromatic walls of the host. Combined extraction and competitive binding experiments yielded the free energies of bindings ΔG° in chloroform. The values are the highest ever obtained with a calixarene-type host. Calix[6]TAC displayed the best affinity for EtNH3+. Comparison with other small ammoniums emphasizes the high selectivity of the recognition process

Optically Pure Calix[6]tris-ammoniums: Syntheses and Host−Guest Properties toward Neutral Guests

Optically pure calix[6]arenes bearing chiral amino arms 4, 7, and 10 have been synthesized in high yields from the known symmetrically 1,3,5-trismethylated calix[6]arene. For both compounds 7 and 10, the key step consists of an efficient selective alkylation on the narrow rim of the calix[6]arene with Ba(OH)2 as the base. All of these chiral calix[6]tris-amines possess a similar flattened cone conformation with the cavity occupied by the methoxy groups. In contrast to 4 and 7, upon protonation, the enantiopure calix[6]arene 10 can switch to the opposite flattened cone conformation through self-assembly of its ammonium arms in an ion-paired cap which closes the cavity. As shown by NMR host−guest studies and an X-ray structure, the obtained polarized host (10·3H+) behaves as a remarkable endo-receptor for small polar neutral molecules. Thanks to the tris-cationic chiral binding site of 10·3H+, it was shown that the guests experience a chiral environment upon inclusion. Finally, the first example of enantioselective molecular recognition inside the cavity of a calix[6]arene has been evidenced with a racemic 1,2-diol guest

A Novel <i>C</i>₃<i>_v</i>-Symmetrical Calix[6](aza)cryptand with a Remarkably High and Selective Affinity for Small Ammoniums

The three-step synthesis of a calix[6]arene capped with a TAC unit is presented. The novel C3v-symmetrical calix[6](aza)cryptand displayed an exceptionally high affinity for small ammoniums. NMR and X-ray diffraction analyses demonstrated the formation of endo-complexes. These complexes are stabilized thanks to (i) hydrogen bonding to both the aza cap and one phenolic unit of the calixarene and to (ii) cationic and CH−π interactions between the ammonium and the aromatic walls of the host. Combined extraction and competitive binding experiments yielded the free energies of bindings ΔG° in chloroform. The values are the highest ever obtained with a calixarene-type host. Calix[6]TAC displayed the best affinity for EtNH3+. Comparison with other small ammoniums emphasizes the high selectivity of the recognition process

Optically Pure Calix[6]tris-ammoniums: Syntheses and Host−Guest Properties toward Neutral Guests

Optically pure calix[6]arenes bearing chiral amino arms 4, 7, and 10 have been synthesized in high yields from the known symmetrically 1,3,5-trismethylated calix[6]arene. For both compounds 7 and 10, the key step consists of an efficient selective alkylation on the narrow rim of the calix[6]arene with Ba(OH)2 as the base. All of these chiral calix[6]tris-amines possess a similar flattened cone conformation with the cavity occupied by the methoxy groups. In contrast to 4 and 7, upon protonation, the enantiopure calix[6]arene 10 can switch to the opposite flattened cone conformation through self-assembly of its ammonium arms in an ion-paired cap which closes the cavity. As shown by NMR host−guest studies and an X-ray structure, the obtained polarized host (10·3H+) behaves as a remarkable endo-receptor for small polar neutral molecules. Thanks to the tris-cationic chiral binding site of 10·3H+, it was shown that the guests experience a chiral environment upon inclusion. Finally, the first example of enantioselective molecular recognition inside the cavity of a calix[6]arene has been evidenced with a racemic 1,2-diol guest

Polarizing a Hydrophobic Cavity for the Efficient Binding of Organic Guests: The Case of Calix[6]tren, a Highly Efficient and Versatile Receptor for Neutral or Cationic Species

The host−guest properties of calix[6]tren 1 have been evaluated. The receptor is based on a calix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents a concave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavity and opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules in chloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H+, behaved as a remarkable receptor for small organic molecules. The complexation is driven by a strong charge−dipole interaction and hydrogen bonds between the polar guest and the tetracationic cap of the calixarene. Finally, coordination of Zn2+ to the tren core led to the asymmetrization of calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizes the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. The resulting combination of charge−dipole, hydrogen bonding, CH−π, and van der Waals interactions highly stabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of the polarization, offering either a basic (1), a highly charged and acidic (1.4H+), or a coordination (1.Zn2+) site. As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simple addition of protons, bases, or metal ions

Polarizing a Hydrophobic Cavity for the Efficient Binding of Organic Guests: The Case of Calix[6]tren, a Highly Efficient and Versatile Receptor for Neutral or Cationic Species

The host−guest properties of calix[6]tren 1 have been evaluated. The receptor is based on a calix[6]arene that is covalently capped at the narrow rim by a tren unit. As a result, the system presents a concave hydrophobic cavity with, at its bottom, a grid-like nitrogenous core. Despite its well-defined cavity and opening to the outside at the large rim, 1 did not behave as a good receptor for neutral molecules in chloroform. However, it exhibited efficient endo-complexation of ammonium guests. By contrast, the per-protonated host, 1.4H+, behaved as a remarkable receptor for small organic molecules. The complexation is driven by a strong charge−dipole interaction and hydrogen bonds between the polar guest and the tetracationic cap of the calixarene. Finally, coordination of Zn2+ to the tren core led to the asymmetrization of calixarene cavity and to the strong but selective endo-binding of neutral ligands. This study emphasizes the efficiency of a receptor presenting a concave hydrophobic cavity that is polarized at its bottom. The resulting combination of charge−dipole, hydrogen bonding, CH−π, and van der Waals interactions highly stabilizes the supramolecular architectures. Also, importantly, the tren cap allows the tuning of the polarization, offering either a basic (1), a highly charged and acidic (1.4H+), or a coordination (1.Zn2+) site. As a result, the system proved to be highly versatile, tunable, and interconvertible in solution by simple addition of protons, bases, or metal ions

Easy and Selective Method for the Synthesis of Various Mono-<i>O</i>-functionalized Calix[4]arenes: De-<i>O</i>-functionalization Using TiCl₄

An efficient and selective method for the monofunctionalization of p-tert-butylcalix[4]arene is described. A mono-de-O-functionalization of disubstituted p-tert-butylcalix[4]arenes using titanium tetrachloride was developed to synthesize a series of monosubstituted p-tert-butylcalix[4]arenes with the pendant functions being ethoxycarbonylmethyloxy, 3-ethoxycarbonylpropyloxy, cyanomethyloxy, 3-cyanopropyloxy, 4-bromobutyloxy, 3-hydroxypropyloxy, propyloxy, 2-methylpropyloxy, 3-butynyloxy, and 3-cyanopropyloxy groups. The reaction mechanism of the formation of 5,11,17,23-tetra-tert-butyl-26,27,28-trihydroxy-25-(3-ethoxycarbonylpropyloxy) calix[4]arene was studied by 1H NMR and GC/mass spectroscopy monitoring. Reaction of TiCl4 with the disubstituted p-tert-butylcalix[4]arene produced the corresponding dioxocalix[4]arene titanium dichloride complex, which undergoes elimination of ethyl 4-chlorobutyrate, leading to a trioxocalix[4]arene titanium dichloride complex and to monosubstituted calix[4]arene after hydrolysis. These two complexes were also synthesized, isolated, and fully characterized

Photophysical and Electrochemical Study of New Luminescent and Redox-Active Tetrazine Derivatives Grafted on Gold Nanoparticles

New tetrazine derivatives able to be grafted on gold surfaces have been synthesized with either disulfide or thioctic anchoring units, and their electrochemical and photophysical properties are investigated in solution. In parallel, gold nanoparticles have been electrodeposited on indium tin oxide surfaces and then functionalized by the tetrazine compounds. The modified surfaces have been characterized by X-ray photoelectron spectroscopy with a focus on the influence of the deposition potential and immersion time. Tetrazines remain fully electroactive and emissive once grafted. Electrochemical characterization shows that monolayers of tetrazine are formed with high values of the electron transfer rate with the thioctic anchoring group. The photophysical study allows us to estimate the optimal deposition time and shows a gradual bleaching under continuous excitation with a rate depending on the excitation power. The monolayers grafted through the thioctic unit show the best performances from the electrochemical and photophysical point of view, suggesting a better organization than with the disulfide unit

<i>Ipso</i>-Nitration of Calix[6]azacryptands: Intriguing Effect of the Small Rim Capping Pattern on the Large Rim Substitution Selectivity

The <i>ipso</i>-nitration of calix[6]­arene-based molecular receptors is a important synthetic pathway for the elaboration of more sophisticated systems. This reaction has been studied for a variety of capped calixarenes, and a general trend for the regioselective nitration of three aromatic units out of six in moderate to high yield has been observed. This selectivity is, in part, attributed to the electronic connection between the protonated cap at the small rim and the reactive sites at the large rim. In addition, this work highlights the fact that subtle conformational properties can drastically influence the outcome of this reaction

Synthesis of Novel Tantalacalixarene Complexes: First Example of Intramolecular CH Activation of Monodepleted Aromatic Ring

The alkylation of (dichlorotantala)-p-tert-butylcalix[4]arenes 1 and 3 by two equivalents of NpMgCl (Np = neopentyl) affords the corresponding unreported (bisneopentyltantala)-p-tert-butylcalix[4]arenes 4 and 5. These complexes can undergo dealkylation by modest increase of the temperature to yield 6 and 7. The characterization and structural conformation of the complexes 6 and 7 were determined by 1H, 13C, NOESY, and COSY spectroscopic studies and single-crystal X-ray diffraction. 7 exhibits unprecedented use of an intramolecular C–H activation of an aromatic ring and simultaneously releases of one equivalent of neopentane. DFT calculation supports a mechanism in which the metal passes through a transition state involving a coplanar arrangement with its three ligands (CAr, HAr, CNp)