Anion allosteric effect in the recognition of tetramethylammonium salts by calix[4]arene cone conformers

Specific Encapsulation of Acetylcholine Chloride by a Self-Assembled Molecular Capsule with Sulfonamido Moiety

New molecular capsule 1₂, which encapsulates only acetylcholine chloride in the ion-pair form, has been developed. Cavitand 1 with four sulfonamido moieties on the upper rim of tetraimino-cavitand self-assembled to form a stable molecular capsule in the presence of an acetylcholine chloride guest through eight intermolecular -NH···O═S hydrogen bonds, two from each of the four paired sulfonamide units.

Anion coordination chemistry using O-H groups

This review covers significant advances in the use of O-H groups in anion coordination chemistry. The review focuses on the use of these groups in synthetic anion receptors, as well as more recent developments in transport, self-assembly and catalysis.

Shape-Selective Recognition of Quaternary Ammonium Chloride Ion Pairs

Synthetic receptors that recognize ion pairs are potentially useful for many technical applications, but to date there has been little work on selective recognition of quaternary ammonium (Q⁺) ion pairs. This study measured the affinity of a tetralactam macrocycle for 11 different Q⁺·Cl^- salts in chloroform solution. In each case, NMR spectroscopy was used to determine the association constant ( K_a) and the structure of the associated complex. K_a was found to depend strongly on the molecular shape of Q⁺ and was enhanced when Q⁺ could penetrate the macrocycle cavity and engage in attractive noncovalent interactions with the macrocycle's NH residues and aromatic sidewalls. The highest measured K_a of 7.9 × 10³ M^-1 was obtained when Q⁺ was a p-CN-substituted benzylic trimethylammonium. This high-affinity Q⁺·Cl^- ion pair was used as a template to enhance the synthetic yield of macrocyclization reactions that produce the tetralactam receptor or structurally related derivatives. In addition, a permanently interlocked rotaxane was prepared by capping the end of a noncovalent complex composed of the tetralactam macrocycle threaded by a reactive benzylic cation. The synthetic method provides access to a new family of rotaxanated ion pairs that can likely act as anion sensors, molecular shuttles, or transport molecules.

p-tert-Butylthiacalix[4]arenes functionalized by N-(4'-nitrophenyl)acetamide and N,N-diethylacetamide fragments: synthesis and binding of anionic guests

New p-tert-butylthiacalix[4]arenes, which are mono-, 1,2-di- and tetrasubstituted at the lower rim containing N-(4'-nitrophenyl)acetamide and N,N-diethylacetamide groups in cone and partial cone conformations have been synthesized. Their complexation ability towards a number of tetrabutylammonium salts n-Bu₄NX (X = F^-, Cl^-, Br^-, I^-, CH₃CO₂^-, H₂PO₄^-, NO₃^-) was studied by UV spectroscopy. The effective receptor for the anions studied as well as selective receptors for F^-, CH₃CO₂^- and H₂PO₄^- ions, which based on the synthesized thiacalix[4]arenes, have been obtained. It was shown that p-tert-butylthiacalix[4]arene tetrasubstituted at the lower rim by N-(4'-nitrophenyl)acetamide moieties bonded to the anions studied with association constants within the range of 3.55 × 10³-7.94 × 10⁵ M^-1. Besides, the binding selectivity for F^-, Cl^-, CH₃CO₂^-, and H₂PO₄^- anions against other anions was in the range of 4.1-223.9. Substituting one or two fragments in the macrocycle with N,N-diethylacetamide groups significantly reduces the complexation ability of the receptor. In contrast to the 1,3-disubstituted macrocycle containing two N-(4'-nitrophenyl)acetamide moieties, the 1,2-disubstituted thiacalix[4]arene, which contains only one such fragment and a N,N-diethylacetamide moiety, selectively binds F^- anions.

Limitations and extensions of the lock-and-key principle: differences between gas state, solution and solid state structures

The lock-and-key concept is discussed with respect to necessary extensions. Formation of supramolecular complexes depends not only, and often not even primarily on an optimal geometric fit between host and guest. Induced fit and allosteric interactions have long been known as important modifications. Different binding mechanisms, the medium used and pH effects can exert a major influence on the affinity. Stereoelectronic effects due to lone pair orientation can lead to variation of binding constants by orders of magnitude. Hydrophobic interactions due to high-energy water inside cavities modify the mechanical lock-and-key picture. That optimal affinities are observed if the cavity is only partially filled by the ligand can be in conflict with the lock-and-key principle. In crystals other forces than those between host and guest often dominate, leading to differences between solid state and solution structures. This is exemplified in particular with calixarene complexes, which by X-ray analysis more often than other hosts show guest molecules outside their cavity. In view of this the particular problems with the identification of weak interactions in crystals is discussed.

Heteroditopic receptors for ion-pair recognition

Ion-pair recognition is a new field of research emerging from cation and anion coordination chemistry. Specific types of heteroditopic receptor designs for ion pairs and the complexity of ion-pair binding are discussed to illustrate key concepts such as cooperativity. The importance of this area of research is reflected by the wide variety of potential applications of ion-pair receptors, including applications as membrane transport and salt solubilization agents and sensors.

Lower-rim substituted calixarenes and their applications

This review discusses in detail “calixarenes” since their discovery as by-products of the phenol formaldehyde bakelites till the present scenario wherein calixarene has assumed a new dimension in the field of supramolecular chemistry. Extensive literature exists for calixarenes; but herein we have tried to concentrate on the different lower-rim modified calixarenes with their potential applications. An attempt has also been made to critically evaluate the synthesis procedures for different lower-rim substituted calixarenes.

Ion pair receptors

Compared with simple ion receptors, which are able to bind either a cation or an anion, ion pair receptors bearing both a cation and an anion recognition site offer the promise of binding ion pairs or pairs of ions strongly as the result of direct or indirect cooperative interactions between co-bound ions. This critical review focuses on the recent progress in the design of ion pair receptors and summarizes the various binding modes that have been used to accommodate ion pairs (110 references).

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Synthesis and structure of mono-bridged resorcinarene host: a ditopic receptor for ammonium guests

The synthesis and structural properties of tetramethoxy resorcinarene mono-crown-5 (1) are described. The binding characteristics of 1 toward acetylcholine and tetramethylammonium salts were investigated by 1H NMR titration. It was observed that the cavity of 1 provides a better fit to acetylcholine compared to the smaller tetramethylammonium cation, as acetylcholine is able to interact with both the crown ether moiety and the free hydroxyl groups of receptor 1 simultaneously.

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.

Self-Assembly of a Double Calix[6]arene Pseudorotaxane in Oriented Channels

A synthetic study to disclose the more appropriate manner by which two calix[6]arene units could be connected for the construction of an extended tubular structure was undertaken. As a result, a head-to-tail double calix[6]arene having the structure of an oriented nanotube that is about 2.6 nm long and 1.6 nm wide was prepared and characterized. This molecule is able to act as a wheel-type host and forms a supramolecular complex with an axle-type molecule, derived from 4,4'-bipyridinium (viologen), through very efficient self-assembly in solution. The properties of such a pseudorotaxane-type complex, which is stabilized by a combination of noncovalent interactions, were investigated in solution by UV/Vis absorption spectroscopy and voltammetric methods. These observations provide a clue about the location of the bipyridinium unit along the nanotube. In the solid state, the complex undergoes a further stage of self-assembly, thereby initiating extended oriented tubular structures. Crystallographic studies revealed that the positioning of the viologen dication in this asymmetric wheel is addressed by a complicated pattern of cooperative noncovalent intermolecular interactions that involve only one half of the host, whereas the remaining (more polar) half of the host is exploited to create long-range structural order that leads to a "secondary" structure of extended supramolecular channels that, in turn, self-assemble in the lattice, thus giving rise to a "tertiary" structure of parallel sandwiches of nanotubes.

The Wurtz−Fittig Reaction in the Preparation of C-Silylated Calixarenes

The Wurtz-Fittig reaction of tetraiodo calixarene 3 with Na/Me3SiCl in DME gave a mixture of tetrakis- and tris-silylated calixarenes (6 and 7). Tris(silyl) calixarene 7 was assigned the flattened cone conformation. A model study using p-bromoanisole and p-iodoanisole with Na/Me3SiCl gave the best results with p-bromoanisole in toluene. Attempts to extend this reaction to the tetrabromo calixarene 4 resulted in slow reactions giving mixtures of products. However, the Wurtz-Fittig reaction of the bromo benzyloxycalixarene 5 was faster, giving the debenzylated silyl ether 12.

A 10-Å Spectroscopic Ruler Applied to Short Polyprolines

Fluorescence resonance energy transfer (FRET) from the amino acid tryptophan (Trp) as donor and a 2,3-diazabicyclo[2.2.2]oct-2-ene-labeled asparagine (Dbo) as acceptor in peptides of the general structure Trp-(Pro)n-Dbo-NH2 (n = 1-6) was investigated by steady-state and time-resolved fluorescence, CD, and NMR spectroscopy as well as by molecular dynamics (MD) simulations (GROMOS96 force field). The Trp/Dbo FRET pair is characterized by a very short Förster radius (R0 ca. 9 A), which allowed distance determinations in such short peptides. Water and propylene glycol were investigated as solvents. The peptides were designed to show an early nucleation of the poly(Pro)II (PPII) secondary helix structure for n > or = 2, which was confirmed by their CD spectra. The shortest peptide (n = 1) adopts preferentially the trans conformation about the Trp-Pro bond, as confirmed by NMR spectra. The FRET efficiencies ranged 2-72% and were found to depend sensitively on the peptide length, i.e., the number of intervening proline residues. The analysis of the FRET data at different levels of theory (assuming either a fixed distance or distance distributions according to a wormlike chain or Gaussian model) afforded donor-acceptor distances between ca. 8 A (n = 1) and ca. 16 A (n = 6) in water, which were found to be similar or slightly higher in propylene glycol. The distances afforded by the Trp/Dbo FRET pair were found to be reasonable in comparison to literature data, expectations from the PPII helix structure, and the results from MD simulations. The persistence lengths for the longer peptides were found to lie at 30-70 A in water and 220 +/- 40 A in propylene glycol, suggesting a more rigid PPII helical structure in propylene glycol. A detailed comparison with literature data on FRET in polyprolines demonstrates that the donor-acceptor distances extracted by FRET are correlated with the Förster radii of the employed FRET pairs. This demonstrates the limitations of using FRET as a spectroscopic ruler for short polyprolines, which is presumably due to the breakdown of the point dipole approximation in Förster theory, when the size of the chromophores becomes comparable or larger than the distances under investigation.

Structure of a 4:1:4 Supramolecular Assembly of Neutral TiiiiPO Cavitands and Tetrakis(N-methylpyridinium)porphyrin Iodide

Four tetraphosphorylated TiiiiPO cavitands 1 encapsulate the pyridinium heads of a tetrakis(N-methylpyridinium)porphyrin iodide to form a 4:1:4 (host)4/guest4+/4I- complex. The single-crystal X-ray diffraction analysis shows the arrangement of the four cavities bound to the CH3N+ groups of the porphyrin moiety and the four iodide anions nested between the phenethyl substituents of the hosts. 1H NMR investigations show that the structure is preserved in chloroform solution and underscore the effect of the counteranions.

Ortho-Substituted Catechol Derivatives: The Effect of Intramolecular Hydrogen-Bonding Pathways on Chloride Anion Recognition

This paper reports a series of chloride anion receptors containing two catechol head groups connected through their ortho-positions via a spacer chain. The linking group chosen to attach the spacer chain to the catechol units has a major impact on the anion-binding potential of the receptor. Linking groups that are capable of forming stable six-membered intramolecular hydrogen-bonded rings with the catechol O-H groups significantly inhibit the ability of the catechol units to hydrogen bond to chloride anions. However, where the linking groups are only capable of forming five- or seven-membered intramolecular hydrogen-bonded rings, then anion binding via hydrogen bonding through the catechol O-H groups becomes a possibility. This process is solvent dependent; the presence of competitive solvent (e.g., DMSO-d6) disrupts the intramolecular hydrogen-bonding pattern and enhances anion binding relative to simple unfunctionalized catechol. The most effective receptor is that in which the hydrogen-bonding linker (-CH2CONH-) is most distant from the catechol units and can only form a seven-membered intramolecular hydrogen-bonded ring. In this case, the receptor, which contains two catechol units, is a more effective chloride anion binder than simple unfunctionalized catechol, demonstrating that the two head groups, in combination with the N-H groups in the linker, act cooperatively and enhance the degree of anion binding. In summary, this paper provides insight into the hydrogen-bonding patterns in ortho-functionalized catechols and the impact these have on the potential of the catechol O-H groups to hydrogen bond to a chloride anion.

Anion Partitioning and Ion-Pairing Behavior of Anions in the Extraction of Cesium Salts by 4,5‘ ‘-Bis(tert-octylbenzo)dibenzo-24-crown-8 in 1,2-Dichloroethane

A systematic study of anion partitioning and ion pairing was performed for an extraction of individual cesium salts into 1,2-dichloroethane (1,2-DCE) using 4,5' '-bis(tert-octylbenzo)dibenzo-24-crown-8 as the cesium receptor. Equilibrium constants corresponding to the extraction of ion pairs and dissociated ions, formation of the 1:1 cesium/crown complex (confirmed by electrospray mass spectrometry), and dissociation of the ion pairs in water-saturated 1,2-DCE at 25 degrees C were obtained from equilibrium modeling using the SXLSQI program. The standard Gibbs energy of partitioning between water and water-saturated 1,2-DCE was determined for picrate, permanganate, trifluoromethanesulfonate, methanesulfonate, trifluoroacetate, and acetate anions. The dissociation of the organic-phase complex ion pair [Cs(4,4' '-bis(tert-octylbenzo)dibenzo-24-crown-8]+NO3- observed in the extraction experiments was shown to be consistent with the dissociation constant determined independently by conductance measurements. As attributed to the large effective radius of the complex cation, the evident anion discrimination due to ion pairing in the 1,2-DCE phase was relatively small, by comparison only a tenth of the discrimination exhibited by the anion partitioning. Only chloride and picrate exhibit evidence for significantly greater-than-expected ion-pairing tendency. These results provide insight into the inclusion properties of the clefts formed by opposing arene rings of the crown ether upon encapsulation of the Cs+ ion, whose weak anion recognition likely reflects the preferential inclusion of 1,2-DCE molecules in the clefts. Observed anion extraction selectivity in this system, which may be ascribed predominantly to solvent-induced Hofmeister bias selectivity toward large charge-diffuse anions, was nearly the same whether cesium salts were extracted as dissociated ions or ion pairs.