Anion binding and transport by steroid-based receptors

The steroid nucleus is well-established as a scaffold for anion receptors. The bile acids are especially useful, providing inexpensive starting points with helpful substitution patterns. This article describes developments since an earlier review in 2003. Included are podand and cyclic structures, uncharged and positive receptors, and various arrays of H-bond donor and other binding functionality. Applications have been found in anion sensing, selective extractions, transport across bilayer membranes, and the discovery of antibiotics.     

Non-covalent synthesis of ionic and molecular complexes of benzoic acid and substituted 2-aminopyrimidines by varying aryl/alkyl substituents and their supramolecular chemistry

The non-covalent synthesis of ionic and molecular complexes of substituted 2-aminopyrimidines with benzoic acid in crystalline solid phase is reported. The nature of supramolecular architecture varied with the substituents in the 2-aminopyrimidine ring. Two complexes have been synthesised from two differently substituted 2-aminopyrimidines and benzoic acid. In one case, proton transfer takes place and an ionic organic salt is formed, whereas in the other, there is no proton transfer and a hydrogen-bonded molecular complex is formed.     

Anion sensing by Phenazine-based urea/thiourea receptors

The novel colorimetric receptors 2,3-bis-N-(9,10-diaza-anthracen-1-yl)-N′-phenylurea and 2,3-bis-N-(9,10-diaza-anthracen-1-yl)-N′-phenylthiourea have been prepared by the reaction of 2,3-diaminophenazine with phenylisocyanate and phenylisothiocyanate, respectively, in quantitative yields. The interaction and colorimetric sensing properties of receptor = 2 and 3 with different anions were investigated by naked eye, UV-vis and fluorescence spectroscopy in DMSO. The receptors effectively and selectively recognized biologically important F⁻, CH₃COO⁻, H₂P in the presence of other anions, such as Cl⁻, Br⁻, I⁻ and HS in DMSO.     

Anion Control in the Self-Assembly of a Cage Coordination Complex

An anion is encapsulated in the center of the new cage compound [Ni₆(atu)₈X]X₃ (X=Cl—for the structure see picture—or Br; Hatu=amidinothiourea). A combination of Lewis acid–base and hydrogen-bonding interactions cause the square-planar [Ni(Hatu)₂]²⁺ units, after deprotonation, to assemble to form this compound. A remarkable feature is the anion dependence of the cage formation; nitrate, acetate, and perchlorate are unsuitable as templates.     

A [2]catenane containing an upper-rim functionalized calix[4]arene for anion recognition

A novel [2]catenane is synthesized in good yield from an upper-rim functionalized calix[4]arene macrobicycle and a nitro-substituted isophthalamide threading component in the presence of a chloride anion template via a ring-closing metathesis (RCM) reaction. After halide anion template removal the resulting catenane host exhibits a high degree of selectivity for chloride in competitive organic solvent mixtures.     

Host-guest assembly of pyridinium-conjugated calix[4]arene via cation-π interaction

25-(4-Pyridiniumbutoxy)-26,27,28-trihydroxycalix[4]arene bromide 1 was designed to have both host and guest units in one molecule, and was assembled to become an oligomer via a cation-π interaction. X-ray diffraction (XRD) crystallographic study of 1 revealed that it oriented in a one-dimensional structure. Titrimetric ¹H NMR analysis and electrospray ionization mass spectrometry (ESI-MS) analysis indicated that 1 formed an oligomer in solution, and the restraint of the segmental motion would lead to the stabilization of the cation-π interaction compared with a bi-molecular complex composed of 25,26,27,28-tetrahydroxycalix[4]arene and N-butylpyridinium bromide.     

Effective receptors for fluoride and acetate ions: synthesis and binding study of pyrrole- and cystine-based cyclopeptido-mimetics

Two conformationally constrained pseudo-cyclopeptides (1, 2) consisting of pyrrole-, pyridine-, and cystine-moieties were designed and synthesized as neutral receptors for anionic guests. The anion recognition abilities of these two receptors were examined photometrically in acetonitrile solution. The UV-vis study revealed that the [1+1] receptor (1) formed 1:1 complexes with anions, whereas the [2+2] receptor (2) led to 1:2 mode binding with anions. Both receptors displayed good affinity and selectivity for fluoride and acetate ions.     

How to improve guanidinium cations for oxoanion binding in aqueous solution?: The design of artificial peptide receptors

Guanidinium cations are a prominent recognition motif for oxoanion binding both in Nature and in abiotic systems. However, simple ion pairing based on alkyl guanidinium cations is not strong enough to achieve an efficient complexation in aqueous solvents. Nature uses the less polar microenvironment of a protein to shield the ion pair from the solvent thereby increasing complex stability. For artificial supramolecular systems other ways to improve the binding affinity of guanidium cations have to be found. We will describe herein our use of modified acylguanidinium cations with additional H-bond donor sites to achieve oxoanion binding in aqueous solvents. The thermodynamic characterization of such systems is described as well as some applications from the field of bioorganic chemistry (e.g. artificial receptors for anionic biomolecules).     

Molecular recognition and fluorescent sensing of urethane in water

Molecular recognition and fluorescent sensing of Group 2A carcinogen-urethane was achieved in aqueous solution with endo-functionalized molecular tubes. The syn-configured molecular tube was found to be a good fluorescent sensor for urethane in water (concentration range:6.2-60 μmol/L) and in beer (concentration range:22.9-60 μmol/L).     

Cucurbit[10]uril-based chemistry

With the biggest cavity in the cucurbit[n]urils (CB[n]s) family, CB[10] has shown its unique molecular recognition properties. This review gives a brief summary of the research progresses in the CB[10]-based chemistry, involving its purification and applications in fields such as molecular recognition and molecular assembly.     

Recognition of a dicarboxylic acid with dipicolyl urea in solution and in solid phases: intramolecular hydrogen bond inhibiting both pyridine nitrogens from binding carboxyl groups

Recognition of a dicarboxylic acid in solution as well as in the solid phase by the pyridyl urea based pseudoditopic receptor 1 has been studied. The X-ray structures of both the receptor and its complex with 1,4-phenylenediacetic acid are also presented. Intramolecular hydrogen bonding inhibits both the pyridine ring nitrogens from forming hydrogen bonds with the carboxyl group and force the receptor to behave in a monotopic manner, using the syn urea amide moiety to bind carboxyl group of a dicarboxylic acid to form a 2:1 complex. Binding of receptor 1 with a monocarboxylic acid is also compared.     

Amide based receptors for anions

This review article illustrates the contribution of amide based receptors to the development of anion complexing agents. Amides are incorporated into a wide variety of systems that can be divided into two broad categories; organic and inorganic. The first section is separated into cyclic and acyclic systems built on a solely organic framework. The second section is comprised of metal containing systems such as metallocenes, [Ru(bipy)₃]²⁺ based complexes, porphyrins and other metallo-based receptors. Where appropriate, the results of solution binding studies and sensing outputs are summarized.     

Macrocyclic anion receptors based on directed hydrogen bonding interactions

Natural anion receptors use charge-neutral dipoles to bind small anions with high affinities and selectivities. A convergent and rigid display of hydrogen bond donors such as amide, thiourea and urea functional groups in macrocyclic scaffolds would be one of the most efficient ways to create synthetic anion receptors that mimic natural ones. In this article, we present examples of natural anion receptors and discuss the synthesis of neutral macrocyclic receptors and their anion binding properties.     

Steroids as organising elements in anion receptors

The steroid nucleus has proved useful in various areas of supramolecular chemistry, acting as a building-block for extended, well-defined molecular architectures, and a scaffold for preorganised arrays of functionality. This article discusses its applications in the area of anion recognition, where cholic acid (2) has been especially valuable. The three secondary hydroxyl groups on 2 are nicely arranged for H-bond donation to a single anion, and may be converted into stronger neutral or positively-charged H-bond donors. Macrocyclic and acyclic receptors derived from 2 have been used to bind inorganic anions, carboxylates and nucleic acids.     

Cucurbituril anchored silica gel

A cucurbit[6]uril anchored silica gel is synthesized via reaction of perallyloxycucurbit[6]uril and mercaptopropyl functionalized silica gel and fully characterized by various spectroscopic methods; the amount of accessible host molecules attached on silica surface is quantified by fluorescence spectroscopy using FITC-spermine as a guest molecule.     

Stable supramolecular complex of porphyrin macroring with pyridyl and fullerenyl ligands

Binding constants and thermodynamic parameters for 1:1 complexation of a porphyrin macroring, self-assembled from three trisporphyrinatozinc through imidazole-Zn coordination on the terminal porphyrins, with several multidentate pyridyl and fullerenyl ligands were examined. In benzonitrile, the ligand having one fullerenyl and two coordinative pyridyl moieties surprisingly afforded the highest affinity. The thermodynamic data of the complexation indicated that an unusual and fairly large positive entropic change, which may be attributed to extensive desolvation of the solutes especially from the large cavity of porphyrin macroring and fullerene surface, significantly contributed to the enhancement of the binding constant.     

Anion and ion-pair receptor chemistry: highlights from 2000 and 2001

This review article highlights advances made in abiotic anion coordination chemistry in 2000 and 2001. The structure of this review is that similar to the previous reviews in this series that covered 1997, 1998 and 1999 [1 and 2]. The review also includes examples of ion-pair receptors. The first section examines anion receptors that do not contain metal ions. This is followed by a review of metal containing anion receptors in which the metal can function as: (i) a coordination site for the anion; (ii) an agent withdrawing electron density from the receptor; (iii) an organisational element in the receptor; (iv) a sensor; and (v) a co-bound guest in ion-pair receptor. Examples of the role of anions in directing the self-assembly of complex molecular architectures are presented in the final section.     

Effect of Alkylpyrazinium Counterions on the Host-guest Recognition Based on Dimethoxypillar[5]arenes

In this work, we synthesized n-octylpyrazinium bromide(G-Br) and n-octylpyrazinium hexafluorophosphate(G-PF₆) as model guests and studied their host-guest complexation with 1,4-dimethoxypillar[5]arene(DMP5A). Effect of alkylpyrazinium counterions on the host-guest recognition was investigated. Based on the ¹H NOESY spectra, the binding site of DMP5A with G-PF₆ is the same as that of DMP5A with G-Br. However, G-PF₆ forms a stronger complex with DMP5A than G-Br, owing to that hexafluorophosphate forms weaker doubly inonic H-bonds with ammonium cation than bromide ion in chloroform, which leads to some aggregates that could be dissociated with the addition of DMP5A.