Hydrogen-Bonded Analogues of Cavitands We are grateful to Mr. R. Kauppinen for his assistance with NMR measurements. This work was supported by the Finnish Academy

Naphthalene-functionalized resorcinarene as selective, fluorescent self-quenching sensor for kynurenic acid

Naphthalene-functionalized resorcinarene selectively binds kynurenic acid in the presence of excess tryptophan in aqueous media, highlighting the potential of functionalized resorcinarenes as sensory recognition elements for biomolecular analytes.

High-affinity and selective detection of pyrophosphate in water by a resorcinarene salt receptor

Pyrophosphate (PPi) is a byproduct of DNA and RNA synthesis, and abnormal levels are indicative of disease. We report the high-affinity binding of PPi in water by N-alkyl ammonium resorcinarene chloride receptors. Experimental analysis using ¹H and ³¹P NMR, isothermal titration calorimetry, mass spectrometry, and UV-vis spectroscopy all support exceptional selectivity of these systems for PPi in water. The measured affinity of K₁ = 1.60 × 10⁷ M^-1 for PPi is three orders of magnitude larger than that observed for binding to another phosphate, ATP. This exceptional anion-binding affinity in water is explored through a detailed density functional theory computational study. These systems provide a promising avenue for the development of future innovative medical diagnostic tools.

Recognition of Viologen Derivatives in Water by N-Alkyl Ammonium Resorcinarene Chlorides

Three water-soluble N-alkyl ammonium resorcinarene chlorides decorated with terminal hydroxyl groups at the lower rims were synthesized and characterized. The receptors were decorated at the upper rim with either terminal hydroxyl, rigid cyclohexyl, or flexible benzyl groups. The binding affinities of these receptors toward three viologen derivatives, two of which possess an acetylmethyl group attached to one of the pyridine nitrogens, in water were investigated via ¹H NMR spectroscopy, fluorescence spectroscopy, and isothermal titration calorimetry (ITC). ITC quantification of the binding process gave association constants of up to 10³ M^-1. Analyses reveal a spontaneous binding process which are all exothermic and are both enthalpy and entropy driven.

Anion-Exchange Properties of Trifluoroacetate and Triflate Salts of N-Alkylammonium Resorcinarenes

The synthesis of N-benzyl- and N-cyclohexylammonium resorcinarene trifluoroacetate (TFA) and triflate (OTf) salt receptors was investigated. Solid-state analysis by single-crystal X-ray diffraction revealed that the N-alkylammonium resorcinarene salts (NARSs) with different upper substituents had different cavity sizes and different affinities for anions. Anion-exchange experiments by mixing equimolar amounts of N-benzylammonium resorcinarene trifluoroacetate and N-cyclohexylammonium resorcinarene triflate, as well as N-benzylammonium resorcinarene triflate and N-cyclohexylammonium resorcinarene trifluoroacetate showed that the NARS with flexible benzyl groups preferred the larger OTf anion, whereas the rigid cyclohexyl groups preferred the smaller TFA anions. The anion-exchange processes were confirmed in the solid state by single-crystal and powder X-ray diffraction experiments and in the gas phase by electrospray ionization mass spectrometry.

N-Alkyl Ammonium Resorcinarene Salts as High-Affinity Tetravalent Chloride Receptors

N-Alkyl ammonium resorcinarene salts (NARYs, Y=triflate, picrate, nitrate, trifluoroacetates and NARBr) as tetravalent receptors, are shown to have a strong affinity for chlorides. The high affinity for chlorides was confirmed from a multitude of exchange experiments in solution (NMR and UV/Vis), gas phase (mass spectrometry), and solid-state (X-ray crystallography). A new tetra-iodide resorcinarene salt (NARI) was isolated and fully characterized from exchange experiments in the solid-state. Competition experiments with a known monovalent bis-urea receptor (5) with strong affinity for chloride, reveals these receptors to have a much higher affinity for the first two chlorides, a similar affinity as 5 for the third chloride, and lower affinity for the fourth chloride. The receptors affinity toward chloride follows the trend K1 ≫K2 ≫K3 ≈5>K4, with Ka =5011 m(-1) for 5 in 9:1 CDCl3/[D6]DMSO.

Cooperative Binding of Divalent Diamides by N-Alkyl Ammonium Resorcinarene Chlorides

N-Alkyl ammonium resorcinarene chlorides, stabilized by an intricate array of hydrogen bonds leading to a cavitand-like structure, bind amides. The molecular recognition occurs through intermolecular hydrogen bonds between the carbonyl oxygen and the amide hydrogen of the guests and the cation-anion circular hydrogen-bonded seam of the hosts, as well as through CH⋅⋅⋅π interactions. The N-alkyl ammonium resorcinarene chlorides cooperatively bind a series of di-acetamides of varying spacer lengths ranging from three to seven carbons. Titration data fit either a 1:1 or 2:1 binding isotherm depending on the spacer lengths. Considering all the guests possess similar binding motifs, the first binding constants were similar (K1:10(2)  M(-1)) for each host. The second binding constant was found to depend on the upper rim substituent of the host and the spacer length of the guests, with the optimum binding observed with the six-carbon spacer (K2:10(3)  M(-2)). Short spacer lengths increase steric hindrance, whereas longer spacer lengths increase flexibility thus reducing cooperativity. The host with the rigid cyclohexyl upper rim showed stronger binding than the host with flexible benzyl arms. The cooperative binding of these divalent guests was studied in solution through (1)H NMR titration studies and supplemented by diffusion-ordered spectroscopy (DOSY), X-ray crystallography, and mass spectrometry.

A Halogen-Bonded Dimeric Resorcinarene Capsule

Iodine (I2) acts as a bifunctional halogen-bond donor connecting two macrocyclic molecules of the bowl-shaped halogen-bond acceptor, N-cyclohexyl ammonium resorcinarene chloride 1, to form the dimeric capsule [(1,4-dioxane)3@1(2)(I2)2]. The dimeric capsule is constructed solely through halogen bonds and has a single cavity (V=511 Å(3)) large enough to encapsulate three 1,4-dioxane guest molecules.

Dimeric resorcinarene salt capsules with very tight encapsulation of anions and guest molecules

The N-cyclohexyl ammonium resorcinarene triflate forms two structurally different capsules with cavity volumes of 341 and 679 ų encapsulating either four or eight guest molecules, with very high packing coefficients of 0.74 and 0.67.

N-Alkyl ammonium resorcinarene salts: multivalent halogen-bonded deep-cavity cavitands

N-Alkyl ammonium resorcinarene halides together with 1,4-diiodotetrafluorobenzene form a deep-cavity cavitand-like structure held together by moderate halogen bonds. Each resorcinarene salt⋯(IC₆F₄I)₄ assembly acts as a pocket for the next assembly resulting in a polymeric herringbone and 3-D polymeric egg-crate-like network.

Recognition of N-alkyl and N-aryl acetamides by N-alkyl ammonium resorcinarene chlorides

N-alkyl ammonium resorcinarene chlorides are stabilized by an intricate array of intra- and intermolecular hydrogen bonds that leads to cavitand-like structures. Depending on the upper-rim substituents, self-inclusion was observed in solution and in the solid state. The self-inclusion can be disrupted at higher temperatures, whereas in the presence of small guests the self-included dimers spontaneously reorganize to 1:1 host-guest complexes. These host compounds show an interesting ability to bind a series of N-alkyl acetamide guests through intermolecular hydrogen bonds involving the carbonyl oxygen (C=O) atoms and the amide (NH) groups of the guests, the chloride anions (Cl(-)) and ammonium (NH2(+)) cations of the hosts, and also through CH⋅⋅⋅π interactions between the hosts and guests. The self-included and host-guest complexes were studied by single-crystal X-ray diffraction, NMR titration, and mass spectrometry.

Halogen bonded analogues of deep cavity cavitands

A combination of N-alkyl ammonium resorcinarene halides as acceptors and CCl₃Br as the donor yielded halogen bonded analogues of deep cavity cavitands with host–guest properties.

The complex formation of tetracyclohexylammonium C1-resorcinarene with various guests - an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry study

The complex formation of a tetraammonium C1-resorcinarene (R+4HCl) was studied using electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Although R+4HCl easily loses its counter ions in the ESI process, a neutral self-assembled structure with an intramolecular circular hydrogen-bonded 16-membered -N(+)-H ... X(-) ... H-N(+)- array with ammonium ion as the charge-giving species was observed in the gas phase. In addition to chloride, several other counter ions were also studied. The size and basicity of the counter ion as well as the size of the charge-giving cation strongly affected the gas-phase stability of the self-assembled system. H/D exchange experiments showed that the ammonium substituents in the apical position of R affect the hydrogen-bonding system in the resorcinarene. The complexation of the saturated dicarboxylic acids was found to depend on the length of the carbon chain. The rigidity of the molecular skeleton of the acid improved the complexation considerably. The orientation and position of the carboxylic groups also had an effect on the complexation and consequently enabled stereochemical differentiation of the acids. Mass spectrometric observations were supported by theoretical calculations.

An electrospray ionization Fourier transform ion cyclotron resonance mass spectrometric study of the gas-phase stabilities and fragmentation of N-alkylammonium resorcarenes

Five N-alkylated ammonium resorcarene derivatives, differing either in the nitrogen atom or in the lower rim alkyl chain, were studied using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The main purpose was to clarify the fragmentation and the relative stabilities of the protonated molecules by collision-induced dissociation (CID) experiments. The results indicate that fragmentation of the upper rim alkyl amino substituents at the 2-position occurs readily. Variation in the gas-phase stabilities of these molecules was also observed. In addition, the proposed fragmentation pathways are depicted.

Alkoxy-, Acyloxy-, and Bromomethylation of Resorcinarenes

Reaction of resorcinarene octols with tris-hydroxymethylmethylamine (TRIS), formaldehyde, and alcohols results in tetraalkoxymethylation of the resorcinol rings. Harsh acylation of aminomethylated resorcinarenes with acid anhydrides leads to the complete acylation of eight hydroxyls and substitution of the amino versus acyloxy groups. Acyloxymethylated resorcinarene 6b can be transformed into a tetrabromomethylated derivative 7 through the reaction with HBr in acetic acid.

Dendronized molecular knots: selective synthesis of various generations, enantiomer separation, circular dichroism

For the first time, knot molecules (of the amide type) are synthesized, which bear one to three dendritic units of various generations at their periphery. They were obtained through two different routes: i) attachment of dendritic wedges to new mono-, di- and trihydroxy functionalized dodecaamide knots that have been obtained by selective debenzylation of oligobenzyloxy substituted knots, or ii) cyclization of already dendron substituted pyridine-2,6-dicarbonyl dichlorides with an "extended diamine" to directly yield the "tri-dendroknots". The derivatization of knot molecules by functional substituents and even large dendritic units is an important advance in the synthesis and property variation of molecular knots. This holds true in particular for substitution of the pyridine units of the knots, whereas the isophthalic acid units seem not to tolerate larger substitutents, as reflected in lower knot yields. These syntheses also demonstrate knots to be accessible indirectly by substitution of the corresponding mono-, di- and tri-functionalized knot skeleton. An advantage of dendritic "decoration" is the control of solubility and chromatographic behaviour of the molecular knots (knotanes). Suggestions are made about the threading mechanism by supramolecular template effects leading to the formation of amide-based molecular knots. The topological chirality of the new "dendroknots" is shown by efficient enantioseparations (separation factor alpha between 1.22 and 1.48). For this purpose (commercially unavailable) chiral column material of the Chiralpak type was used, in which the chiral component is covalently bonded to the silica gel support. The racemate splittings provide additional evidence for the knotted structure, as all other conceivable isomers such as macromonocyclic or catenated dodecaamides would not be chiral. The pure enantiomers obtained exhibit pronounced Cotton effects in their circular dichroism spectra. By comparison with the unsubstituted knot, the absolute configuration (Lambda, Delta) of all new knots is derived.

Complexation of small molecules by open-ended resorcarene hosts

[structure: see text] Sterically hindered tetraaminomethylated resorcarenes form inclusion complexes in CDCl(3) with acetonitrile and acetaldehyde, which are kinetically stable on the NMR time scale at 233 K.

Resorcarenes in the boat conformation as building blocks for hydrogen-bonded assemblies including two ammonium cations

Crystal structures are reported for various co-crystals of rccc-resorcarenes with triethylammonium chloride. Usually, two molecules of a C2v-symmetric tetraester 2 in the boat conformation are linked through four hydrogen-bonded chloride anions to give dimeric assemblies. Two of the chloride anions may be replaced by four hydrogen-bonded ethanol molecules in an otherwise similar structure. These assemblies, which consist of six or eight components, posses voluminous, negatively charged chambers in which two triethylammionium cations, 3+, are included as guests by strong electrostatic and hydrogen-bonding interactions. The host-guest N-H...Cl hydrogen bonds were clearly detected at 173 K. These are the first examples of hydrogen-bonded, solid-state capsules trapping two ions of the same charge in close proximity. In the 1:2 complex with 3+ Cl-, the molecule of the parent resorcarene 1 also adopts a boat conformation whose cavity is considerably extended by four hydrogen-bonded chloride anions. The pocket formed in this way again includes two 3+ ions as a result of electrostatic and hydrogen bonding host-guest interactions. All these structures show that the boat conformers of resorcarenes can be used as a novel motif for the construction of hydrogen-bonded assemblies capable of molecular inclusion and encapsulation.