Ion-pair Receptor Research Articles

Extending the family of heteroditopic calix[4]diquinone receptors for cooperative AND ion-pair recognition

New heteroditopic calix[4]diquinone receptors with different anion binding units and calix[4]diquinone scaffolds have been synthesised. Ion-pair binding studies using UV/visible and 1H NMR spectroscopies reveal that receptors 1, 2 and 4 are cooperative AND ion-pair receptors, which display little affinity for ‘free’ ions but enhanced binding of the ion-pairs NaCl, NH4Cl and KCl. The more preorganised receptor 2 binds the ion-pairs more weakly than receptor 1. Varying the nature of the calix[4]diquinone scaffold appears to have little effect on ion-pair binding, although the calix[4]diquinone framework is more conformationally flexible than the tert-butylcalix[4]diquinone one and can be synthesised using the milder oxidant chlorine dioxide.

Ion-Pair Recognition by Metal - Salophen and Metal - Salen Complexes

The development of heteroditopic receptor systems that can simultaneously bind cationic and anionic species is one of the most challenging research topics in supramolecular chemistry, attracting the attention of a large number of research groups worldwide. Such an interest is due especially to the fact that the overall receptor–ion-pair complex is neutral and this can be advantageous in many situations, such as salt solubilization and extraction, and membrane-transport applications. Receptors designed for ion-pair complexation are molecules comprising well-known anion-binding motifs and familiar cation-binding sites. An important family of compounds that can use metal Lewis-acidic centres for anion recognition and that can be easily derivatized to introduce an additional binding site for the cation is metal–salophen and metal–salen complexes. This short review shows that the high versatility of salen and salophen ligands and of the corresponding metal complexes allows, through simple modifications of the basic skeleton, the obtention of highly efficient receptors for ion pairs.

Multichannel HSO4− recognition promoted by a bound cation within a ferrocene-based ion pair receptor

A ferrocene-based ion pair receptor is shown only to recognise HSO(4)(-) anions in the presence of a cobound Pb(2+) or Zn(2+) cation guest species through a perturbation of the redox potential of the ferrocene unit and a remarkable enhancement of the fluorescence.

ChemInform Abstract: Ion Pair Receptor Based on Anion—π Interaction.

AbstractNovel azacrown (VI) is synthesized via a one‐pot reaction between triazine (I) and benzaldehyde (II), followed by condensation with diamine (IV) and reduction of the intermediate bisimine.

Computational investigation of a new ion-pair receptor for calix[4]pyrrole

Theoretical studies of a new ion-pair receptor, meso-octamethylcalix[4]pyrrole (OMCP), and its interactions with the halide anions F(-), Cl(-), and Br(-) and the cesium halides CsF, CsCl, and CsBr have been performed. Geometries, binding energies, and binding enthalpies were evaluated with the restricted hybrid Becke three-parameter exchange functional (B3LYP) method using the 6-31+G(d) basis set and relativistic effective core potentials. The optimized geometric structures were used to perform natural bond orbital (NBO) analysis. The two typical types of hydrogen bonds, N-H…X(-) and C-H…X(-), were investigated. The results indicate that hydrogen bonding interactions are dominant, and that the halide anions (F(-), Cl(-), and Br(-)) offer lone pair electrons to the σ*(N-H) or σ*(C-H) antibonding orbitals of OMCP. In addition, electrostatic interactions between the lone pair electrons of the halide anion and the LP* orbitals of Cs(+) as well as cation-π interactions between the metal ion and π-orbitals of the pyrrole rings have important roles to play in the Cs(+)•OMCP•X(-) complexes. The current study further demonstrates that this easy-to-make OMCP host compound functions as not only an anion receptor but also an ion-pair receptor.

Calix[4]Pyrrole-Calix[4]Arene-Crown-5 Conjugate with Flexible Linkers as a Model for a Selective Ion-Pair Container

Crowning glory: A diametrically linked calix[4]pyrrole–calix[4]arene–crown-5 pseudo dimer tethered by alkyl chains has been synthesized and characterized as a ditopic ion-pair receptor.

Second Generation Calix[6]trenamides – Highly Selective Graftable Receptors for Neutral Guests and Contact Ion Pairs

AbstractWith the aim of designing graftable calix[6]tren‐based receptors, calix[6]trenamides decorated with an appended functional arm were synthesized through [1+1] macrocyclization reactions between calix[6]trisamines and NTA‐lysine derivatives. NMR spectroscopy and ESI‐MS studies showed that one of these calix[6]trenamides, 5, behaves as a unique heteroditopic receptor towards charged or neutral species. Notably, it can selectively bind contact ammonium fluoride salts through a highly cooperative process. In addition, acid–base control of its binding properties was clearly evidenced. This second generation of graftable receptors opens new perspectives in the design of sensors for charged or neutral species.

A Simple but Effective Dual Redox and Fluorescent Ion Pair Receptor Based on a Ferrocene−Imidazopyrene Dyad

The ferrocene-imidazopyrene dyad, bearing the imidazole ring as the only receptor site, acts as a redox and optical molecular sensor for ion pairs, exhibiting an easily detectable signal change in the redox potential of the ferrocene/ferrocinium redox couple and in the emission spectrum. Perturbation of the emission spectrum follows the order Pb(2+) > Hg(2+) > Zn(2+) for cations and H(2)PO(4)(-) > AcO(-) for anions.

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 Me(4)N(+) ion, which can simultaneously bind anionic species to provide anion@[1⋅Me(4)N(+)] complexes. The increased affinity of [1⋅Me(4)N(+)] 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.

Ion pair receptors based on anion–π interaction

A novel type of ditopic ion pair receptors based on anion-π interaction is reported. Oxacalix[2]arene[2]triazine azacrowns were synthesized efficiently from a one-pot reaction between 2,4-dichloro-4-methoxytriazine and 3,5-dihydroxybenzaldehyde followed by condensation with a diamine and reduction of bisimine; they acted as selective ditopic receptors to recognize ion pairs.

ChemInform Abstract: Ion Pair Receptors

AbstractReview: 110 refs.

Anion and cation binding by a new indole/pyridine/amine-based ion-pair receptor

A new ion-pair receptor bis(3-bromoindol-2-ylmethyl)(2-pyridylmethyl)amine ( 1) was synthesized and studied for its anion and cation binding behavior using ESI-MS and 1H NMR spectroscopy. Among halides, 1 exhibits the strongest binding with Cl − to form a 1:1 adduct ( K a = 1042 ± 21 in CD 3CN). Among alkali metal ions, Li + and Na + showed the strongest binding in the formation of a 1·M + complex. The simultaneous binding of Cl − and Li + to 1 was confirmed by 1H NMR titration of a 1:1 mixture of 1 and Cl − with LiPF 6 in 83:17 v/v mixture of CDCl 3 and DMSO- d 6. DFT-optimized structures of 1·Cl −, 1·Li +, and 1·Li +·Cl − are consistent with the chemical shift changes observed in 1H NMR studies.

A Calix[4]arene Strapped Calix[4]pyrrole: An Ion-Pair Receptor Displaying Three Different Cesium Cation Recognition Modes

An ion-pair receptor, the calix[4]pyrrole-calix[4]arene pseudodimer 2, bearing a strong anion-recognition site but not a weak cation-recognition site, has been synthesized and characterized by standard spectroscopic means and via single-crystal X-ray diffraction analysis. In 10% CD(3)OD in CDCl(3) (v/v), this new receptor binds neither the Cs(+) cation nor the F(-) anion when exposed to these species in the presence of other counterions; however, it forms a stable 1:1 solvent-separated CsF complex when exposed to these two ions in concert with one another in this same solvent mixture. In contrast to what is seen in the case of a previously reported crown ether "strapped" calixarene-calixpyrrole ion-pair receptor 1 (J. Am. Chem. Soc. 2008, 130, 13162-13166), where Cs(+) cation recognition takes place within the crown, in 2.CsF cation recognition takes place within the receptor cavity itself, as inferred from both single-crystal X-ray diffraction analyses and (1)H NMR spectroscopic studies. This binding mode is supported by calculations carried out using the MMFF94 force field model. In 10% CD(3)OD in CDCl(3) (v/v), receptor 2 shows selectivity for CsF over the Cs(+) salts of Cl(-), Br(-), and NO(3)(-) but will bind these other cesium salts in the absence of fluoride, both in solution and in the solid state. In the case of CsCl, an unprecedented 2:2 complex is observed in the solid state that is characterized by two different ion-pair binding modes. One of these consists of a contact ion pair with the cesium cation and chloride anion both being bound within the central binding pocket and in direct contact with one another. The other mode involves a chloride anion bound to the pyrrole NH protons of a calixpyrrole subunit and a cesium cation sandwiched between two cone shaped calix[4]pyrroles originating from separate receptor units. In contrast to what is seen for CsF and CsCl, single-crystal X-ray structural analyses and (1)H NMR spectroscopic studies reveal that receptor 2 forms a 1:1 complex with CsNO(3), with the ions bound in the form of a contact ion pair. Thus, depending on the counteranion, receptor 2 is able to stabilize three different ion-pair binding modes with Cs(+), namely solvent-bridged, contact, and host-separated.

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).

Ion Pairs and C60: Simultaneous Guests in Supramolecular Nanotubes

Unlikely bedfellows: Naphthalenediimide-based supramolecular nanotubes act as size-selective receptors for ion pairs, as demonstrated by competition experiments with C60 (see picture). Fullerene forms mixed complexes inside the cavity of the nanotubes in the presence of the anion and ammonium ions of the appropriate size.

A Ditopic Ion‐Pair Receptor Based on Stacked Nucleobase Quartets

AbstractReichen Sie bitte das Salz! Die Ergebnisse hochleistungsfähiger Rechnungen sprechen dafür, dass der Stapelkomplex aus einem Guanin‐ und einem Adeninquartett (G4A4) in wässriger Lösung als wirksamer ditoper Rezeptor für NaCl fungieren kann (siehe Bild; Na+, Cl− gelb, O rot, N blau, C schwarz, H weiß).magnified image

Crown-6-calix[4]arene-Capped Calix[4]pyrrole: An Ion-Pair Receptor for Solvent-Separated CsF Ions

An ion-pair receptor, 1, containing both cation- and anion-recognizing sites, has been synthesized and characterized. Single-crystal X-ray diffraction structural studies and (1)H NMR spectroscopic analyses confirmed that 1 forms stable 1:1 complexes with CsF in solution and in the solid state in spite of the large separation enforced between the receptor-bound anion and cation. In 9:1 CDCl3/CD3OD, binding of fluoride anion within the calix[4]pyrrole core of 1 was not observed in the absence of a cobound cesium cation; however, it was seen in this solvent mixture under conditions where a Cs(+) cation was bound to the crown ether-strapped calix[4]arene subunit.

Calix[6]tris(thio)ureas: Heteroditopic Receptors for the Cooperative Binding of Organic Ion Pairs

The straightforward syntheses of C3v symmetrical calix[6]trisureas and -thiourea have been achieved. NMR studies have shown that these flexible compounds possess a major cone conformation. While these neutral hosts can strongly bind anions such as AcO(-) or HSO4(-) through induced fit processes, they can also behave as unique heteroditopic receptors for organic ion pairs with a remarkable positive cooperativity in the complexation process, the anion acting as an allosteric effector.

Calix[4]pyrrole: A New Ion-Pair Receptor As Demonstrated by Liquid−Liquid Extraction

Solvent-extraction studies provide confirming evidence that meso-octamethylcalix[4]pyrrole acts as an ion-pair receptor for cesium chloride and cesium bromide in nitrobenzene solution. The stoichiometry of the interaction under extraction conditions from water to nitrobenzene was determined from plots of the cesium distribution ratios vs cesium salt and receptor concentration, indicating the formation of an ion-paired 1:1:1 cesium:calix[4]pyrrole:halide complex. The extraction results were modeled to evaluate the equilibria inherent to the solvent-extraction system, with either chloride or bromide. The binding energy between the halide anion and the calix[4]pyrrole was found to be about 7 kJ/mol larger for cesium chloride than for the cesium bromide. The ion-pairing free energies between the calix[4]pyrrole-halide complex and the cesium cation are nearly the same within experimental uncertainty for either halide, consistent with a structural model in which the Cs+ cation resides in the calix bowl. These results are unexpected since nitrobenzene is a polar solvent that generally leads to dissociated complexes in the organic phase when used as a diluent in extraction studies of univalent ions. Control studies involving nitrate revealed no evidence of ion pairing for CsNO3 under conditions identical to those where it is observed for CsCl and CsBr.

An ion pair receptor showing remarkable enhancement of anion-binding strengths in the presence of alkali metal cations

An ion pair receptor was prepared by coupling of a diazacrown ether and a rigid biindole scaffold bearing hydrogen bond donors of two indole NHs. The former serves as the cation-binding site and the latter functions as the anion-binding site. The anion-binding affinities to the receptor, determined by 1H NMR titration experiments in 10% (v/v) DMSO- d 6/CD 3CN at 24 ± 1 °C, have been greatly improved when an alkali metal cation binds to the adjacent diazacrown ether. For example, the association constant between chloride and receptor alone is 7 M −1, but the magnitudes increase into 120 M −1, 14,000 M −1, and 6200 M −1 in the presence of lithium, sodium, and potassium ions, respectively. The enhanced binding affinities must be attributed to electrostatic interactions by possibly forming contact ion pairs.