Anion coordination and anion-templated assembly: Highlights from 2002 to 2004

This review article highlights advances made in abiotic anion coordination chemistry from 2002 to 2004. The structure of this review is that similar to the previous reviews in this series that covered 1997–2001 [P.A. Gale, Coord. Chem. Rev. 199 (2000) 181; P.A. Gale, Coord. Chem. Rev. 213 (2001) 79; P.A. Gale, Coord. Chem. Rev. 240 (2003) 191]. The first section examines anion receptors that do not contain metal ions. This is followed by a review of metal containing anion receptors and finally the role of anions in directing the self-assembly of complex molecular architectures are presented in the final section.     

Synthesis of the anionic fluororeceptors based on thiourea and amide groups and recognition property for α,ω-dicarboxylate

Three new neutral receptors (1, 2 and 3) containing thiourea and amide groups were synthesized by simple steps in good yields. The binding properties for anions of 1, 2 and 3 were examined by UV-vis, fluorescence, and ¹H NMR spectroscopy. Receptors 1, 2 and 3 all had a better adipate anion selectivity by comparison with other dicarboxylate anions. The association constants of 1, 2 and 3 with adipate were higher as compared to other anions (malonate, succinate, glutarate). In particular, a distinct color change was observed from light yellow to orange-red upon addition of adipate to the solution of 1 in DMSO. The UV-vis and fluorescence data indicate that a 1:1 stoichiometry complex is formed between compound 1, 2 or 3 and dicarboxylate anions through hydrogen bonding interactions.     

Synthesis, structure and the binding properties of the amide-based anion receptors derived from 1H-indole-7-amine

Indole-7-amine was investigated as an alternative to aniline in construction of amide-based anion receptors. Replacement of aniline with indolamine introduces additional binding site—indolyl NH, which can enhance anion binding for more than five times. The molecular modelling of indole-containing receptors revealed the correlation between their conformational preferences and their affinity towards anions.     

Visible-Light-Switchable Tellurium-Based Chalcogen Bonding: Photocontrolled Anion Binding and Anion Abstraction Catalysis

Exploring new noncovalent bonding motifs with reversibly tunable binding affinity is of fundamental importance in manipulating the properties and functions of supramolecular self-assembly systems and materials. Herein, for the first time, we demonstrate a unique visible-light-switchable telluro-triazole/triazolium-based chalcogen bonding (ChB) system in which the Te moieties are connected by azobenzene cores. The binding strengths between these azo-derived ChB receptors and the halide anions (Cl⁻, Br⁻) could be reversibly regulated upon irradiation by visible light of different wavelengths. The cis-bidentate ChB receptors exhibit enhanced halide anion binding ability compared to the trans-monodentate receptors. In particular, the telluro-triazolium-based ChB receptor can achieve both high and significantly photoswitchable binding affinities for halide anions, which enable it to serve as an efficient photocontrolled organocatalyst for ChB-assisted halide abstraction in a Friedel–Crafts alkylation benchmark reaction.     

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.     

脲类受体对阴离子的结合、识别和分离

在阴离子受体中,脲基因其作为双氧键给予体,能与许多阴离子结合,成为一类重要的、有效的受体单元,同时该类受体的功能化为阴离子结合行为的检测提供了方便.本文综述了近年来单脲、二脲、多脲及金属辅助的脲类受体的设计合成及对阴离子的结合、识别和分离方面的研究进展.     

A New Anion Receptor with a Glycoluril Molecular Scaffold

The rational design and synthesis of a new anion receptor containing a glycoluril molecular scaffold are reported. This new receptor utilizes four amide hydrogen bonds arranged at the corner of the glycoluril unit. This new anion receptor binds spherically shaped halide ions in a 1:1 stoichiometry and has a high affinity for fluoride.     

Synthesis and evaluation of fluorimetric and colorimetric chemosensors for anions based on (oligo)thienyl-thiosemicarbazones

A family of heterocyclic thiosemicarbazone dyes (3a–d) containing thienyl groups has been synthesized, characterized, and their chromo-fluorogenic response in acetonitrile in the presence of selected anions was studied. Acetonitrile solutions of 3a–d show absorption bands in the 338–425 nm range, which are modulated by the groups attached to the thiosemicarbazone moiety. The fluoride, chloride, bromide, iodide, dihydrogen phosphate, hydrogen sulfate, nitrate, acetate, and cyanide anions were used in the recognition studies. Only sensing features were observed for fluoride, cyanide, acetate, and dihydrogen phosphate anions. Two different chromogenic responses were found, (i) a small shift of the absorption band due to coordination of the anions with the thiourea protons and (ii) the appearance of a new red-shifted band due to deprotonation of the receptor. For the latter process changes in the color solutions from pale-yellow to orange-red were observed. Fluorescence studies showed a different emission behavior according to the number of thienyl rings in the π-conjugated bridges. Stability constants for the two processes (complex formation+deprotonation) for receptors 3a–d in the presence of fluoride and acetate anions were determined from spectrophotometric titrations using the HypSpec program. The interaction of 3d with fluoride was studied through ¹H NMR titrations. Semiempirical calculations to evaluate the hydrogen-donating ability of the receptors were also performed.     

Anion Receptors Containing ‐NH Binding Sites: Hydrogen‐Bond Formation or Neat Proton Transfer?

When the amide‐containing receptor 1 ⁺ is in a solution of dimethyl sulfoxide (DMSO) in the presence of basic anions (CH₃COO^?, F^?, H₂PO₄^?), it undergoes deprotonation of the ‐NH fragment to give the corresponding zwitterion, which can be isolated as a crystalline solid. In the presence of less basic anions (Cl^?, Br^?, NO₃^?), 1 ⁺ establishes true hydrogen‐bond interactions of decreasing intensity. The less acidic receptor 2 ⁺ undergoes neat proton transfer with only the more basic anions CH₃COO^? and F^?, and establishes hydrogen‐bond interactions with H₂PO₄^?. An empirical criterion for discerning neutralisation and hydrogen bonding, based on UV/Vis and ¹H NMR spectra, is proposed.     

Anion recognition by bisimidazolium and bisbenzimidazolium cholapods

Bile acid-based acyclic receptors bearing two imidazole and benzimidazole moieties have been synthesized. Anion binding studies using ¹H NMR revealed that imidazolium receptor exhibits high selectivity for chloride ion while benzimidazolium receptor showed selectivity for Y-shaped acetate ion through hydrogen bond interactions involving imidazolium C-2 and acetyl methylene hydrogens.     

Solution and Solid State Studies of Urea Derivatives of DITIPIRAM Acting as Powerful Anion Receptors

Herein, we present the synthesis and anion binding studies of a family of homologous molecular receptors 4–7 based on a DITIPIRAM (8-propyldithieno-[3,2-b:2′,3′-e]-pyridine-3,5-di-amine) platform decorated with various urea para-phenyl substituents (NO₂, F, CF₃, and Me). Solution, X-ray, and DFT studies reveal that the presented host–guest system offers a convergent array of four urea NH hydrogen bond donors to anions allowing the formation of remarkably stable complexes with carboxylates (acetate, benzoate) and chloride anions in solution, even in competitive solvent mixtures such as DMSO-d₆/H₂O 99.5/0.5 (v/v) and DMSO-d₃/MeOH-d₃ 9:1 (v/v). The most effective derivatives among the series turned out to be receptors 5 and 6 containing electron-withdrawing F- and -CF₃ para-substituents, respectively.     

Anion recognition by a disiloxane-1,3-diol in organic solvents

To explore the anion recognition ability of silanol derivatives, ESI-MS and ¹H NMR titrations of a 1,3-disiloxanediol 1 with anions were studied in CDCl₃ and MeCN-d₃. The results indicate that 1 showed strong binding to anions such as AcO⁻ and halides.     

Anion recognition by a family of quinoline-functionalised bis-amide hosts in solid state and in solution

The interaction between a series of bidentate, amide-functionalised 8-oxyquinoline receptors and coordinating anions is investigated. Anion recognition properties and conformations were studied by solid-state structures, Hartree–Fock calculations and solution-phase ¹H NMR investigations. Our findings suggest that the amide-oxyquinoline motif coordinates anions and water with a well-defined, consistent geometry involving multiple hydrogen bonds. Solution studies of the neutral receptors reveal the unprotonated oxyquinoline ring to indirectly contribute to anion binding by the adjacent amide NH groups despite being unable to directly participate through hydrogen bonding.     

Diamidopyrazoles: A New Class of Anion Receptors

The synthesis of a series of 3,5-diamidopyrazoles is reported. The anion binding properties of these systems were examined via ¹H NMR, UV/Vis and ITC titration techniques. Target compound 2a acts as a selective receptor for phosphate and sulfate anions in DMSO, whereas N-methylated 2b and the unmethylated species 4d show no appreciable binding affinity. Insights into the binding events occurring in solution came from the solid state structures of compounds 2a and 4a, 4b, and 4d, which were deduced from single crystal X-ray diffraction analyses.     

Electrochemical Bromination of Substituted Thiophenes in Batch and Continuous Flow

An electrochemical bromination of thiophenes from ammonium bromide is developed both in batch and continuous-flow conditions. Substituted thiophenes possessing 2-hydrogens selectively afford a series of 2-bromothiophenes while 2-substituted benzothiophenes generate 3-bromobenzo[b]thiophenes. Based on the cyclic voltammetric studies and literature, a bromine cation involved mechanism was proposed.     

Anion recognition with porphyrin-bottomed tetraurea receptors

Three tetraurea-appended porphyrin-derived anion receptors were synthesized and were found to efficiently encapsulate various anions with high association constants of up to 1.1×10⁸ L/mol.     

Aramidocalix[4]arenes as new anion receptors

Calix[4]arenes bearing aromatic amide (aramid) moieties at the upper rim display interesting recognition properties toward anions and in particular versus planar trigonal nitrate and Y-shaped benzoate. Molecular modeling and DFT calculations indicate that the high affinity displayed by aramidocalix[4]arenes 3 and 4 for nitrate anion is likely due to the planar arrangement of the NH groups, which form an unusual six-hydrogen-bond scheme with nitrate anion.     

Photochemical- and pH-switching Properties of a New Photoelastic Ligand Based Upon Azobenzene. Basicity and Anion Binding

Abstract

The synthesis of a new cylindrical macrotricyclic ligand (L) based upon two 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane rings connected via two azobenzene pillars is described. The ligands displays photoelastic properties: light absorption at 366 nm produces trans→cis isomerization of the azobenzene moieties with formation of trans-cis (L _E-Z ) and cis-cis (L _Z-Z ) isomers. Recovery of the thermodynamically more stable trans-trans (L _E-E ) isomer can be obtained by thermal back-isomerization or visible light (430 nm) absorption. The isomeric composition of L solution depends upon pH: for pH < 5.5 the L _E-E form is the unique species in solution, while increasing amounts of Z species are formed at increasing pH. The protonation constants of L _E-E have been determined in 0.1 mol dm^?3 Me₄NNO₃ solution at 298.1 ± 0.1 K. The protonated species of L form stable inclusion complexes with anionic species such as [Co(CN)₆]^3?.