Solvate-supported proton transport in zeolites.

Solvate-supported proton transport in zeolite H-ZSM-5 was studied by means of complex impedance spectroscopy. The zeolite shows enhanced proton mobility in the presence of NH3 and H2O that depends on the concentration of the solvate molecule, temperature (298-773 K), and the SiO2/Al2O3 ratio of the zeolite (30-1000). In general, proton conductivity in H-ZSM-5 is most effectively supported in the presence of NH3 and H2O at high concentrations, low temperatures, and low SiO2/Al2O3 ratios (< or = 80). For the aluminum-rich samples desorption measurements reflect different transport mechanisms that depend on the respective temperature range. Up to about 393 K a Grotthus-like proton transport mechanism is assumed, whereas at higher temperatures (393-473 K) vehiclelike transport seems to dominate. The activation energies for NH4+ and H3O+ vehicle conductivity depend on the SiO2/Al2O3 ratio, and the values are in the range of 49-59 and 39-49 kJ mol-1, respectively, and thus significantly lower than those for "pure" proton conduction in solvate-free samples.     

Synthesis and study of calix[6]cryptamides: A new class of heteroditopic receptors that display versatile host-guest properties toward neutral species and organic associated ion-pair salts

The synthesis of a new family of molecular receptors, namely the calix[6]cryptamides, was achieved through an original [1+1] macrocyclization step that consists of a peptide-coupling reaction between tripodal triscarboxylic acids and a calix[6]trisamine subunit. Several C3- or C3v-symmetrical calix[6]arene-based compounds capped by a trisamido cryptand unit on the narrow rim have been obtained, with the more flexible partners leading to the best yields. These calix[6]cryptamides exhibit two favorably positioned binding sites for the complexation of organic-associated ion pairs in close proximity: a well-defined calix[6]arene cavity suitable for the inclusion of ammonium ions and a cryptamide unit for the coordination of anions. We demonstrate one example, chiral calix[6]cryptamide 12, that constitutes a heteroditopic receptor capable of cooperatively binding both a primary ammonium ion and its chloride counterion, thanks to a combination of polarization and induced-fit effects. In addition, the hydrophobic calixarene cavity of 12 can strongly bind neutral guests through hydrogen bonding and is capable of discriminating between different enantiomers. All these versatile host-guest properties differ greatly from those observed in the parent calix[6]azacryptands.     

The cooperative effect in ion-pair recognition by a ditopic hemicryptophane host

The heteroditopic hemicryptophane 1 , which bears a tripodal anion binding site and a cation recognition site in the molecular cavity, proved to be an efficient ion‐pair receptor. The hemicryptophane host binds anions selectively depending on shape and hydrogen‐bond‐accepting ability. It forms an inclusion complex with the Me₄N⁺ ion, which can simultaneously bind anionic species to provide anion@[ 1? Me₄N⁺] complexes. The increased affinity of [ 1? Me₄N⁺] for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the hemicryptophane cavity, thus allowing the formation of the contact ion pair. Density functional theory calculations were performed to analyze the Coulomb interactions of the ion pairs, which compete with the ion‐dipole ones, that originate in the ion–hemicryptophane contacts.     

A versatile tripodal host with cylindrical conformation: solvatomorphism, inclusion behavior, and separation of guests

Tripodal host 2,4,6-tris(1-phenyl-1H-tetrazolylsulfanylmethyl)mesitylene (TPTM) has been synthesized through a facile procedure. As expected, it adopts an all-syn cylindrical configuration, thereby delimiting an inner cavity. To explore the solvatomorphism and inclusion behavior of TPTM, a series of organic and inorganic species were employed as guests to afford 17 inclusion compounds (1, 2, 3 a-3 f, 4 a-4 i) that can be classified into four distinct forms (forms I-IV), under similar conditions. These compounds were characterized by single-crystal and powder X-ray diffraction, and (1)H NMR studies. In compound 1 with form I, one foot of a TPTM molecule inserts into the cavity of an opposite TPTM molecule to form a dimeric "hand-shake" motif with one acetonitrile molecule occupying the void. Compound 2 with form II contains three types of capsule-shaped dimers, each of which holds a CH(2)Cl(2) molecule as the guest. In compounds 3 a-3 f with form III, each pair of TPTM molecules interdigitates to form a capsule-shaped dimeric unit accommodating a guest molecule in the endo-cavity. In compounds 4 a-4 i with form IV, each TPTM molecule makes contact with three nearby TPTM molecules in a "self-including" manner to generate a graphite-like organic layer, and through further superposition to form open hexagonal channels. From the experimental and theoretical results, the intrinsic properties of guest molecules, such as size, shape, and self-interaction, can be regarded as the main factors leading to these solvatomorphism phenomena and the subtle inclusion behavior of TPTM. Thermogravimetric analyses show that the encapsulated guest molecules in these compounds can be evacuated at relatively high temperatures, and this demonstrates the outstanding inclusion capability of TPTM. In addition, for compound 4 a with benzene molecules in the channels, reversible exchange of toluene and separation of xylene isomers on single crystals have been observed.     

Fluorescent hydroxyflavone-zeolite nanoparticles: ship-in-a-bottle synthesis and photophysical properties.

3-Hydroxyflavone (3-OHF) was incorporated in zeolite micropores by ship-in-a-bottle synthesis. This strategy consists of constructing the molecule by reaction of small precursors within the cavity. 3-OHF molecules exhibit excited-state intramolecular proton transfer (ESIPT) and a tautomeric equilibrium between the 3-OHF-excited structures N* and T*exists. This equilibrium is strongly affected by the protic nature and polarity of the surrounding medium. The textural and spectroscopic characterization of the dye-loaded zeolite colloids enabled the study of the correlation between the optical properties of the dye and the zeolite micropore environment.     

Encapsulation of small polar guests in molecular apple peels

Three aromatic oligoamides have been prepared that have alternating 1,6-diaminopyridine and 1,6-pyridinedicarboxylic acid units at the center of the sequence and two 8-amino-2-quinolinecarboxylic acid units at each extremity. The three oligomers differ in the number--3, 5, or 7-of pyridine units in the sequence. They were designed to adopt helically folded conformations in solution and in the solid state. The sequence of monomers was chosen so that the diameter of the helix is larger in the center than at each extremity, and hence they resemble helically wrapped apple peels. According to modeling studies, the pyridine units were expected to define a polar hollow within the helix that is large enough to accommodate small polar guests, whereas the quinoline units at each end of the oligomeric sequences were expected to completely cap the hollow and transform the helix cavities into a closed shell that may act as a capsule. Crystallographic studies demonstrate that the oligomers do fold into helices that define a cavity isolated from the surrounding medium in the solid state. Depending on the number of pyridine rings, one or two water molecules are bound within the capsules. The crystal structure of a capsule fragment shows that MeOH can also be hosted by the largest oligomer. Solution NMR studies confirm that binding of water also occurs in solution with the same stoichiometry as observed in the solid state. The capsules have distinct signals depending on whether they are empty, half-full, or full, and these species are in slow exchange on the NMR timescale at low temperature. Indeed, the binding and release of water molecules requires a significant conformational distortion of the helix that slows down these processes. The addition of small polar molecules such as methanol, hydrazine, hydrogen peroxide, or formic acid to the largest capsule leads to the observation of new sets of NMR signals of the capsules that were assigned to complexes with these guests. However, water appears to be the preferred guest.