Ion‐Pair Recognition by a Heteroditopic Triazole‐Containing Receptor

A new heteroditopic calix[4]diquinone triazole containing receptor capable of recognising both cations and anions through Lewis base and C? H hydrogen‐bonding modes, respectively, of the triazole motif has been prepared. This ion‐pair receptor cooperatively binds halide/monovalent‐cation combinations in an aqueous mixture, with selectivity trends being established by ¹H NMR and UV/Vis spectroscopy. Cation binding by the calix[4]diquinone oxygen and triazole nitrogen donors enhances the strength of the halide complexation at the isophthalamide recognition site of the receptor. Conversely, anions bound in the receptor’s isophthalamide cavity enhance cation recognition. ¹H NMR investigations in solution suggest that the receptor’s triazole motifs are capable of coordinating simultaneously to both cation and anion guest species. Solid‐state X‐ray crystallographic structural analysis of a variety of receptor ion‐pair adducts further demonstrates the dual cation–anion binding role of the triazole group.     

Janus‐Like Squaramide‐Based Hosts: Dual Mode of Binding and Conformational Transitions Driven by Ion‐Pair Recognition

New tripodal squaramide‐based hosts have been synthesised and structurally characterised by spectroscopic methods. In 2.5 % (v/v) [D₆]DMSO in CDCl₃, compound 4 formed dimeric assemblies [log K_dim=3.68(8)] as demonstrated by ¹H NMR spectroscopy and UV dilution experiments. AFM and SEM analyses revealed the formation of a network of bundled fibres, which indicates a preferential mechanism for aggregation. These C₃‐symmetric tripodal hosts exhibited two different and mutually exclusive modes of binding, each one easily accessible by simultaneous reorientation of the squaramide groups. In the first, a convergent disposition of the NH squaramide protons allowed the formation of an array of N? H???X^? hydrogen bonds with anions. In the second mode, reorientation of carbonyl squaramide groups allowed multiple C?O???H interactions with ammonium cations. The titration of 4 with different tetraalkylammonium iodides persistently showed the formation of 1:1 complexes, as well as 1:2 and 1:3 complexes. The corresponding stoichiometries and binding affinities of the complexes were evaluated by multi‐regression analysis. The formation of high‐order complexes, supported by ROESY, NOESY and mass spectrometry experiments, has been attributed to the insertion of NR₄I ion pairs between the carbonyl and NH protons of the squaramide groups located in adjacent arms of 4 . The observed effects reflect the induction of significant conformational changes in the hosts, mainly in relation to the relative orientation of the squaramide groups adapting their geometries to incoming ion‐pair complementary substrates. The results presented herein identify and fully describe two different modes of ion‐pair recognition aimed at directing conformational transitions in the host, therefore establishing a base for controlling more elaborate movements of molecular devices through ion‐pair recognition.     

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.     

Ion‐Pair Complexation with a Cavitand Receptor

The capability of resorcinarenes to bind anions within the alkyl feet at the lower rim has been exploited as the starting point for developing a new cavitand able to engulf contact ion pairs of primary ammonium salts in chlorinated solvents with association constants (K_ass) in the range of 10³–10⁴ M ^?1. Methylene bridges were introduced into the upper rim to freeze the resorcinarene in the cone conformation with the four H_down protons converging in the lower pocket, thereby maximizing the CH–anion interactions responsible for the anion binding. Four additional phosphate moieties were introduced into the lower rim in close proximity to the anionic site to provide hydrogen‐bonding‐acceptor P?O groups and promote cation complexation at the bottom of the cavitand. The binding ability of the synthesized ligands was analyzed by ¹H NMR spectroscopy and, when possible, by isothermal titration calorimetry (ITC); the data were in agreement when complementary techniques were used.     

Co‐Conformational Isomerism in a Neutral Ion‐Paired Supramolecular System

Two different counter‐ion‐free host–guest complexes have been prepared and isolated. These compounds were formed from two equally and opposite doubly‐charged species, the viologen guests 1 a ²⁺ and 1 b ²⁺ and the anti‐disulfodibenzo[24]crown‐8 [ DSDB24C8] ^2? host, which gave rise to the 1:1 neutral complexes [ 1 a?DSDB24C8 ] and [ 1 b?DSDB24C8 ]. These species are held together by hydrogen bonding and π stacking, as well as strong electrostatic interactions. The investigation of these neutral ion‐paired supramolecular systems in solution and in the solid state allowed us to establish their co‐conformational preferences. Compound [ 1 a?DSDB24C8 ], with small methyl groups as substituents on the viologen unit, may adopt three different geometries, 1) an exo nonthreaded, 2) a partially threaded, and 3) a threaded arrangement, depending on the relative spatial orientation between the host and guest: The partially‐threaded structure is preferred in solution and in the solid state. The presence of bulky tert‐butylbenzyl groups in the viologen moiety in compound [ 1 b?DSDB24C8 ] restricts the possible geometrical arrangements to one: The exo nonthreaded arrangement. This structure was confirmed in the solid state by X‐ray crystallography. The stability of the neutral complexes in solution was determined by UV/Vis spectrophotometry. The stoichiometry of the complexes was established by continuous variation experiments, and overall equilibrium constants and ΔG° values were determined on the basis of dilution experiments. The results observed are a consequence of only the intrinsic stability of the complexes as there are no additional contributions from counter ions.     

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.     

Host–Guest Chemistry of a Water‐Soluble Pillar[5]arene: Evidence for an Ionic‐Exchange Recognition Process and Different Complexation Modes

The complexation of an anionic guest by a cationic water‐soluble pillararene is reported. Isothermal titration calorimetry (ITC), ¹H NMR, ¹H and ¹⁹F DOSY, and STD NMR experiments were performed to characterize the complex formed under aqueous neutral conditions. The results of ITC and ¹H NMR analyses showed the inclusion of the guest inside the cavity of the pillar[5]arene, with the binding constant and thermodynamic parameters influenced by the counter ion of the macrocycle. NMR diffusion experiments showed that although a fraction of the counter ions are expelled from the host cavity by exchange with the guest, a complex with both counter ions and the guest inside the pillararene is formed. The results also showed that at higher concentrations of guest in solution, in addition to the inclusion of one guest molecule in the cavity, the pillararene can also form an external complex with a second guest molecule.     

A Series of Amino Acid Functionalized Tripodal Hexaamide Anion Receptors: Ion‐Pair‐Assisted Capped‐Cleft Formation by a Pentafluorophenyl‐Functionalized Amide

A new series of tris(2‐aminoethyl)amine (tren)‐based L ‐alanine amino acid backboned tripodal hexaamide receptors (L1–L5) with various attached moieties based on electron‐withdrawing fluoro groups and lipophilicity have been synthesized and characterized. Detailed binding studies of L1–L5 with different anions, such as halides (F^?, Cl^?, Br^?, and I^?) and oxyanions (AcO^?, BzO^? (Bz=benzoyl), NO₃^?, H₂PO₄^?, and HSO₄^?), have been carried out by isothermal titration calorimetric (ITC) experiments in acetonitrile/dimethylsulfoxide (99.5:0.5 v/v) at 298 K. ITC titration experiments have clearly shown that receptors L1–L4 invariably form 1:1 complexes with Cl^?, AcO^?, BzO^?, and HSO₄^?, whereas L5 forms a 1:1 complex only with AcO^?. In the case of Br^?, I^?, and NO₃^?, no appreciable heat change is observed owing to weak interactions between these anions and receptors; this is further confirmed by ¹H NMR spectroscopy. The ITC binding studies of F^? and H₂PO₄^? do not fit well for a 1:1 binding model. Furthermore, ITC binding studies also revealed slightly higher selectivity of this series of receptors towards AcO^? over Cl^?, BzO^?, and HSO₄^?. Solid‐state structural evidence for the recognition of Cl^? by this new category of receptor was confirmed by single‐crystal X‐ray structural analysis of the complex of tetrabutylammonium chloride (TBACl) and L1. Single‐crystal X‐ray diffraction clearly showed that the pentafluorophenyl‐functionalized amide receptor (L1) encapsulated Cl^? in its cavity by hydrogen bonds from amides, and the cavity of L1 was capped with a TBA cation through hydrogen bonding and ion‐pair interactions to form a capped‐cleft orientation. To understand the role of the cationic counterpart in solution‐state Cl^? binding processes with this series of receptors (L1–L4), a detailed Cl^? binding study was carried out with three different tetraalkylammonium (Me₄N⁺, Et₄N⁺, and Bu₄N⁺) salts of Cl^?. The binding affinities of these receptors with different tetralkylammonium salts of Cl^? gave binding constants with the TBA cation in the following order: butyl>ethyl>methyl. This study further supports the role of the TBA countercation in ion‐pair recognition by this series of receptors.     

Molecular Recognition: Influence of Para‐Versus‐Meta‐Substituted Phenyl Moieties on the Swelling Degree of Macroscopic Polymeric Gels in Aqueous Cyclodextrin Solutions

Free radical terpolymerization of (N,N)‐dimethylacrylamide, ethylene‐glycol‐dimethacrylate and N‐(p‐ or m‐ethyl‐phenyl)acrylamide leads to para‐ and meta‐ethyl‐phenyl‐modified hydrophilic polymer networks. Polymeric networks of different molar ratios are prepared in special molds to give water swellable disc‐ shaped samples. The swelling behavior in water and aqueous cyclodextrin (CD) solution of the obtained samples is described while a distinctive differentiation between the para‐ and meta‐ethyl‐phenyl containing networks in CD solution can be found.

     

Synthesis and Molecular Recognition of Water‐Soluble S6‐Corona[3]arene[3]pyridazines

We report the efficient and scalable synthesis and molecular‐recognition properties of novel and water‐soluble S₆‐corona[3]arene[3]pyridazines. The synthesis comprises a one‐pot nucleophilic aromatic substitution reaction between diesters of 2,5‐dimercaptoterephthalate and 3,6‐dichlorotetrazine followed by the inverse electron‐demand Diels–Alder reaction of the tetrazine moieties with an enamine and exhaustive saponification of esters. The resulting S₆‐corona[3]arene[3]pyridazines, which adopt a 1,3,5‐alternate conformation in the crystalline state, are able to selectively form stable 1:1 complexes with dicationic guest species in water with association constants ranging from (1.10±0.06)×10³ M ^?1 to (1.18±0.06)×10⁵ M ^?1. The easy availability, large cavity size, strong and selective binding power render the water‐soluble S₆‐corona[3]arene[3]pyridazines useful macrocyclic hosts in various disciplines of supramolecular chemistry.     

Complexation Between (O‐Methyl)6‐2,6‐Helic[6]arene and Tertiary Ammonium Salts: Acid/Base‐ or Chloride‐Ion‐Responsive Host–Guest Systems and Synthesis of [2]Rotaxane

Complexation between (O ‐methyl)₆‐2,6‐helic[6]arene and a series of tertiary ammonium salts was described. It was found that the macrocycle could form stable complexes with the tested aromatic and aliphatic tertiary ammonium salts, which were evidenced by ¹H NMR spectra, ESI mass spectra, and DFT calculations. In particular, the binding and release process of the guests in the complexes could be efficiently controlled by acid/base or chloride ions, which represents the first acid/base‐ and chloride‐ion‐responsive host–guest systems based on macrocyclic arenes and protonated tertiary ammonium salts. Moreover, the first 2,6‐helic[6]arene‐based [2]rotaxane was also synthesized from the condensation between the host–guest complex and isocyanate.     

A Light‐Controlled Release System Based on Molecular Recognition of Cyclodextrins

This Communication describes a new light‐controlled release system based on molecular recognition of cyclodextrins. Azobenzene (Azo) residue is employed as a photoresponsive guest residue because it can switch the partner from α‐cyclodextrin (αCD) to β‐cyclodextrin (βCD) by irradiation with UV light. Poly(sodium acrylate)s possessing αCD, βCD, and Azo residues (pAαCD, pAβCD, and pAAzo, respectively) are mixed in aqueous solutions to form aggregates through the formation of inclusion complexes of Azo with αCD and/or βCD. A chemical cargo, 1‐pyrenemethylammonium chloride (PyMA), is contained in the aggregates, and its release behavior is investigated by dialysis experiments under UV irradiation. These data indicate that the amount of PyMA released for the pAαCD/pAβCD/pAAzo ternary mixture is approximately three times as high as those for the pAαCD/pAAzo and pAβCD/pAAzo binary mixtures because of the light‐controlled rearrangement of inclusion complexes.

     

Recognition of 1,3‐Butadiene by a Porous Coordination Polymer

The separation of 1,3‐butadiene from C₄ hydrocarbon mixtures is imperative for the production of synthetic rubbers, and there is a need for a more economical separation method, such as a pressure swing adsorption process. With regard to adsorbents that enable C₄ gas separation, [Zn(NO₂ip)(dpe)]_n (SD‐65; NO₂ip=5‐nitroisophthalate, dpe=1,2‐di(4‐pyridyl)ethylene) is a promising porous material because of its structural flexibility and restricted voids, which provide unique guest‐responsive accommodation. The 1,3‐butadiene‐selective sorption profile of SD‐65 was elucidated by adsorption isotherms, in situ PXRD, and SSNMR studies and was further investigated by multigas separation and adsorption–desorption‐cycle experiments for its application to separation technology.     

Small‐Molecule Anion Recognition by a Shape‐Responsive Bowl‐Type Dodecavanadate

A dodecavanadate, [V₁₂O₃₂]⁴⁻, is an inorganic bowl‐type host with a cavity entrance with a diameter of 4.4 Å in the optimized structure. Linear, bent, and trigonal planar anions are tested as guest anions and the formation of host–guest complexes, [V₁₂O₃₂(X)]⁵⁻ (X=CN⁻, OCN⁻, NO₂⁻, NO₃⁻, HCO₂⁻, and CH₃CO₂⁻), were confirmed by X‐ray crystallographic analyses and a ⁵¹V NMR spectroscopy study. The degree of distortion of the bowl from a regular to an oval shape depends on the type of guest anion. In ⁵¹V NMR spectroscopy, all chemical shifts of the host–guest complexes are clearly shifted after guest incorporation. The incorporation reaction rates for OCN⁻, NO₂⁻, HCO₂⁻, and CH₃CO₂⁻ are much larger than those of NO₃⁻ and halides. The incorporated nonspherical molecular anions in the dodecavanadate host are easily dissociated or exchanged for other anions, whereas spherical halides in the host are preserved without dissociation, even in the presence of the tested anions.     

Characterization of Zn‐Containing Metal–Organic Frameworks by Solid‐State 67Zn NMR Spectroscopy and Computational Modeling

Metal–organic frameworks (MOFs) are an extremely important class of porous materials with many applications. The metal centers in many important MOFs are zinc cations. However, their Zn environments have not been characterized directly by ⁶⁷Zn solid‐state NMR (SSNMR) spectroscopy. This is because ⁶⁷Zn (I=5/2) is unreceptive with many unfavorable NMR characteristics, leading to very low sensitivity. In this work, we report, for the first time, a ⁶⁷Zn natural abundance SSNMR spectroscopic study of several representative zeolitic imidazolate frameworks (ZIFs) and MOFs at an ultrahigh magnetic field of 21.1 T. Our work demonstrates that ⁶⁷Zn magic‐angle spinning (MAS) NMR spectra are highly sensitive to the local Zn environment and can differentiate non‐equivalent Zn sites. The ⁶⁷Zn NMR parameters can be predicted by theoretical calculations. Through the study of MOF‐5 desolvation, we show that with the aid of computational modeling, ⁶⁷Zn NMR spectroscopy can provide valuable structural information on the MOF systems with structures that are not well described. Using ZIF‐8 as an example, we further demonstrate that ⁶⁷Zn NMR spectroscopy is highly sensitive to the guest molecules present inside the cavities. Our work also shows that a combination of ⁶⁷Zn NMR data and molecular dynamics simulation can reveal detailed information on the distribution and the dynamics of the guest species. The present work establishes ⁶⁷Zn SSNMR spectroscopy as a new tool complementary to X‐ray diffraction for solving outstanding structural problems and for determining the structures of many new MOFs yet to come.