Neutral Anion Receptors Research Articles

Neutral MOF Anion Receptor: Radical-Promoted Precise Anion Recognition.

Precise ion recognition plays a key role in the anionic decontamination in water. However, the established anionic recognition based on neutral or cationic anion receptor is still restricted by the inherent limitation, such as narrow application scope in organic solvent rather than water for neutral anion receptor and poor selectivity due to non-directional electrostatic interaction for cationic anion receptor. Herein, for the first time, a neutral metal-organic framework (MOF) anion receptor is shown, enabling precise anion recognition, for example, the presence of a variety of 1000-fold competitive anions does not affect the selective adsorption of the target anion at all. A radical-dominating anion-recognition mechanism is proposed for rationalizing the efficacy of the neutral MOF.

Unfurling Anion-π Interactions Involving Graphynes.

Ever since the inception of anion-π interactions, their nature and functional relevance have intrigued researchers. We address the twin challenge of elucidation of the role of extended conjugation and design of all-carbon neutral anion receptors by computations on the anion-π complexes of the halide ions with graphynes. Leveraging on the extended π-conjugation effects, we unfurl the functional relevance of graphynes as anion receptors using descriptors such as electrostatic potential, quadrupole moments, molecular polarizabilities and binding energies. Further, employing natural energy decomposition analysis, we assert that anion-π interactions are not merely dominated by electrostatic interactions. The polarization components do indeed play a crucial role in governing the binding of the anions to the graphynes.

A Highly Efficient Neutral Anion Receptor in Polar Environments by Synergy of Anion−π Interactions and Hydrogen Bonding

Herein, it is shown how anion recognition in highly polar solvents by neutral metal-free receptors is feasible when multiple hydrogen bonding and anion-π interactions are suitably combined. A neutral aromatic molecular tweezer functionalized with azo groups is shown to merge these two kinds of interactions in a unique system and its efficiency as an anion catcher in water is evaluated using first-principles quantum methods. Theoretical calculations unequivocally prove the high thermodynamic stability in water of a model anion, bromide, captured within the tweezer's cavity. Thus, static calculations indicate anion-tweezer interaction energies within the range of covalent or ionic bonds and stability constants in water of more than 10 orders of magnitude. First-principles molecular dynamics calculations also corroborate the stability through the time of the anion-tweezer complex in water. It shows that the anion is always found within the tweezer's cavity due to the combination of the tweezer-anion interactions plus a hydrogen bond between the anion and a water molecule that is inside the tweezer's cavity.

Size-Selective Urea-Containing Metal-Organic Frameworks as Receptors for Anions.

Anion recognition by neutral hosts that function in aqueous solution is an emerging area of interest in supramolecular chemistry. The design of neutral architectures for anion recognition still remains a challenge. Among neutral anion receptor systems, urea and its derivatives are considered as "privileged groups" in supramolecular anion recognition, since they have two proximate polarized N-H bonds exploitable for anion recognition. Despite promising advancements in urea-based structures, the strong hydrogen bond drives detrimental self-association. Therefore, immobilizing urea fragments onto the rigid structures of a metal-organic framework (MOF) would prevent this self-association and promote hydrogen-bond-accepting substrate recognition. With this aim, we have synthesized two new urea-containing metal-organic frameworks, namely [Zn(bpdc)(L2)]n·nDMF (TMU-67) and [Zn2(bdc)2(L2)2]n·2nDMF (TMU-68) (bpdc = biphenyl-4,4'-dicarboxylate; bdc = terephthalate; L2 = 1,3-bis(pyridin-4-yl)urea), and we have assessed their recognition ability toward different anions in water. The two MOFs show good water stability and anion affinity, with a particular selectivity toward dihydrogen arsenate for TMU-67 and toward fluoride for TMU-68. Crystal structure characterizations reveal 3-fold and 2-fold interpenetrated 3D networks for TMU-67 and TMU-68, respectively, where all single interpenetrated networks are hydrogen bonded to each other in both cases. Despite the absence of self-quenching, the N-H urea bonds are tightly hydrogen bonded to the oxygen atoms of the dicarboxylate ligands and cannot be directly involved in the recognition process. The good performance in anion sensing and selectivity of the two MOFs can be ascribed to the network interpenetration that, shaping the void, creates monodimensional channels, decorated by exposed oxygen atom sites selective for arsenate sensing in TMU-67 and isolated cavities, covered by phenyl groups selective for fluoride recognition in TMU-68.

Benzenebistriazole-strapped calix[4]pyrrole: a neutral anion receptor with CH and NH donor groups that exhibits high sulfate binding affinity and selectivity in aqueous solutions.

A calix[4]pyrrole strapped by benzenebistriazole has been prepared as an artificial anion binding receptor. This neutral anion receptor shows high sulfate binding affinity and selectivity in an aqueous solution. In solid state, the receptor binds the sulfate anion in a chair-like 3D cavity via multiple N-H and C-H hydrogen bonds.

Removal of phosphate in presence of interfering sulphate and arsenate by a tripodal thiourea receptor by precipitation through crystallization in semi-aqueous medium

A cyanuric acid platform based tripodal thiourea receptor, L shows encapsulation of phosphate, sulphate and arsenate anions in semi-aqueous medium. Solution phase 1H NMR showed selective recognition of phosphate anion in presence of various anions (e.g. Cl−, Br−, I−, NO3−, HSO4−, CH3CO2− and H2PO4− as TBA salts) in DMSO-d6 at 298 K. L showed partial removal and separation of phosphates from semi-aqueous solution in the presence of interfering arsenate and sulphate via precipitation through crystallization. In the absence of phosphate, L is found to remove/separate arsenate from semi-aqueous medium in presence of sulphate which is the first report of partial removal of arsenate from semi-aqueous medium by a neutral anion receptor. In absence of phosphate and arsenate anions, L could partially remove sulphate anions from mixture of other anions from semi-aqueous solution. All the extracted complexes were characterized by NMR, PXRD, TEM-EDX and Single crystal X-Ray diffraction analysis.

Linear Neutral Receptors for Anions: Synthesis, Structure and Applications

A selective digest of linear anion receptors based on different aromatic skeletons is presented. Since the structures of anions vary from one to another, different strategies have been developed over recent years in order to bind anions efficiently and selectively. Rigidity, number of hydrogen bond donors, steric hindrance, and special preorganization of linear receptors are analyzed to shed light on the rational design of anion receptors.1 Introduction2 1,3- and 1,2-Benzene Derivatives3 1,3- and 5,7-Azulene Derivatives4 1,8-Naphthalene Derivatives5 1,8-Anthracene Derivatives6 2,6-Pyridine Derivatives7 2,5-Pyrrole Derivatives8 Diamidoarenodipyrrole Derivatives9 Carbazole Derivatives10 DITIPIRAM Derivatives11 Conclusion

Progressive Cation Triggered Anion Binding by Electron-Rich Scaffold: Case Study of a Neutral Tripodal Naphthyl Thiourea Receptor

A neutral tripodal electron-rich anion receptor L, decorated from a tren building block, has been deliberately synthesized from acetonitrile for thorough investigation of host–guest binding aptitude in the solid state especially. The naphthyl-based tris-thiourea scaffold, without the presence of any π-acidic or electron-withdrawing aryl terminals, has been established as a potential system that can efficiently bind anions of varied dimensionality consistently triggered by the size of the countercation, as regularly observed from single crystal X-ray analysis, and well supported by solution state NMR binding studies. The newness of this report is the systematic and consistent solid state anion binding within the inner tripodal receptor cleft of highly electron-rich naphthyl terminals. Although the receptor L was first synthesized in the 19th century, no solid state binding studies have been reported until now. However, numerous tren-based urea/thiourea ligands with several terminal functionalizations havin...

Investigating the effects of structure on sulfate recognition by neutral dipeptide receptors

A small library of neutral peptide-based anion receptors was synthesised, where changes were made to the scaffold structure to investigate the effect these structural features have on the anion binding ability of these receptors. These changes included shortening the peptide side chain lengths, increasing the number of electron withdrawing substituents present on the squaramide phenyl substituents and increasing the length and flexibility of the peptide backbone. An effort was also made to increase the aqueous solubility of these receptors by functionalising the N-terminus of the peptide with a hydrophilic moiety. All the receptors displayed strong affinity and selectivity for sulfate in 20% v/v H2O/DMSO-d6 and a 5-fold increase in the affinity of the thiourea receptors was observed upon shortening the side chains by one methylene unit. Overall, the squaramide derivatives displayed much stronger association, in this competitive media, than the thiourea based receptors.

Bambusurils Bearing Nitro Groups and Their Further Modifications

Bambusurils are recently developed neutral anion receptors that show a high affinity towards many inorganic anions, not only in organic solvents but also in water. However, the number of water‐soluble bambusurils and also those bearing functional groups is very limited. In this paper we report the synthesis of four‐ and six‐membered bambusurils containing eight and twelve nitro groups. All the nitro groups on the bambusuril portals could be transformed into amino functions, which provided the macrocycles with water solubility and allowed their further modification. For example, we have demonstrated the conversion of amino groups on the bambusurils into the corresponding urea‐functionalized bambusuril derivatives. We also report the first example of a bambusuril bearing only two functional groups, which was prepared by the condensation of two different glycolurils.

Small-Molecule Uncoupling Protein Mimics: Synthetic Anion Receptors as Fatty Acid-Activated Proton Transporters.

Uncoupling proteins (UCPs) regulate energy expenditure in living cells by inducing proton leakage across the mitochondrial inner membrane, thereby uncoupling adenosine diphosphate phosphorylation from nutrient oxidation. The proton transport activity of UCP1 and UCP2 requires activation by fatty acids. We report here the first examples of synthetic neutral anion receptors performing this biologically important fatty acid-activated function in phospholipid bilayers. We have shown that a tripodal thiourea possesses poor H+/OH- transport activity without fatty acids, but in the presence of long-chain fatty acids is "switched on" as a proton transporter with an activity close to that of a commonly used protonophore. The fatty acid-enhanced proton transport was also observed for other hydrogen and halogen bond-based synthetic anion transporters. We propose that these compounds induce proton permeability by catalyzing transbilayer movement ("flip-flop") of anionic forms of fatty acids, so allowing the fatty acids to complete a proton transport cycle. Several lines of evidence have been provided to support such a fatty acid cycling mechanism. Our findings open up new applications of anion receptor chemistry and provide important clues for understanding biological activities of synthetic anion transporters and potentially the uncoupling mechanism of naturally occurring membrane proteins.

Anion Binding Inside a Bambus[6]uril Macrocycle in Chloroform

Bambus[6]urils are a class of neutral anion receptors with outstanding binding properties in various solvents. The host-guest associations of bambusurils in aqueous environments have been investigated in detail, but their behavior in nonpolar solvents has not yet been reported. This paper presents isothermal calorimetry and 1 H NMR data on the binding of dodecabenzylbambus[6]uril (Bn12 BU[6]) to seventeen anions in chloroform. Under these conditions, anion inclusion in the macrocycle appears to be enthalpy-driven, yielding complexes with stability constants of up to 1010 m-1 . Perchlorate was optimally sized for inclusion in Bn12 BU[6], while larger anions exhibited significantly lower affinity for the macrocycle. In addition, complexes of bambusuril and anions can be differentiated by NMR analysis based on the unique chemical shifts of characteristic peaks in their spectra. This finding was used to perform quantitative and qualitative analysis of multiple anion mixtures.

Enantiomeric recognition of carboxylic anions by a library of neutral receptors derived from α-amino acids and o-phenylenediamine

A library of eight neutral anion receptors consisting of α-amino acid esters attached to o-phenylenediamine by urea groups was synthesized and analysed in terms of capacity for chiral recognition of carboxylates. The NMR titrations revealed that the association constants of complexes consisting of a chiral guest and a chiral host are two orders of magnitude lower than those of achiral partners. Diverse substituents in the receptor structure modify both the affinities and enantioselectivities.

Sulfate-selective recognition by using neutral dipeptide anion receptors in aqueous solution.

The synthesis of six small peptide anion receptors based on thiourea and squaramide recognition moieties is described. These new receptors bind to tetrahedral sulfate anions with remarkable affinity and selectivity in aqueous solution as shown by NMR spectroscopy. Molecular modelling suggests that selectivity is mediated by a hydrogen bond network incorporating the amide backbone protons in a manner similar to that found in the sulfate-binding protein.

Ion-Based Materials Derived from Positively and Negatively Charged Chloride Complexes of π-Conjugated Molecules

Oriented salts from planar charged species were prepared by combining positively and negatively charged receptor-anion complexes with similar geometries using dicationic and electronically neutral π-conjugated receptors. Phenylene- or pyrimidine-bridged bis(imidazolium) dicationic anion receptors formed monocationic receptor-Cl(-) complexes that were accompanied by a free Cl(-). This free Cl(-) was subsequently captured by pyrrole-based neutral anion receptors to form negatively charged receptor-Cl(-) complexes. The ion pair of the resulting positively and negatively charged planar receptor-Cl(-) complexes could produce a supramolecular octane gel, adopting a lamellar self-organized structure in its xerogel state. On the other hand, the solid-state ion pairs had hexagonal columnar mesophases, which formed via alternate stacking of the positively and negatively charged planar receptor-Cl(-) complexes. By use of the flash-photolysis time-resolved microwave conductivity technique, the one-dimensional charge-carrier transporting property, with a mobility of 0.05 cm(2) V(-1) s(-1), was determined for the newly prepared solid-state ion pairs.

ChemInform Abstract: Ion‐Based Materials Comprising Planar Charged Species

AbstractReview: 126 refs.

Pyrrole-Strapped Calix[4]pyrroles Bearing Pyrene Moiety: Synthesis and Anion Binding Property

E-mail: chhlee@kangwon.ac.kr Received October 2, 2012, Accepted October 14, 2012Key Words : Strapped-calix[4]pyrrole, Fluorophore, Anion binding, Pyrene, QuenchingThe design and synthesis of neutral anion receptors withhigh affinity and selectivity for a variety of analyte anionshave been a big challenge in supramolecular chemistry. Theuse of hydrogen bonding interactions as recognition motifsfor specific anions has been studied extensively because ofthe key role of anions in biology as well as in environmentalconcern.

Ion-based materials comprising planar charged species

Materials comprising planar units have the propensity to form stacking assemblies with nanoscale architectures and intriguing properties. In this feature article, some recent examples of ion-based materials comprising positively and negatively charged planar species are described. Various ion-based solid and soft materials can be formed by planar charged species alone or with the aid of other components. It is more challenging to prepare planar anions than planar cations; however, the noncovalent association of electronically neutral planar anion receptors and anions is an efficient strategy for the preparation of planar anions. Suitable candidates include dipyrrolyldiketone boron complexes, which exhibit the formation of various receptor-anion complexes and can be used to fabricate advanced materials in combination with countercations. Ion-based materials consisting of planar charged components have potential as electrically conductive materials, resulting from the ordered arrangement of the planar charged species.

“Unclosed Cryptands”: A Point of Departure for Developing Potent Neutral Anion Receptors

Six macrocyclic lariat-type compounds, representing a new class of anion receptors, were synthesized in a simple approach. We identified the optimal macroring size and the position of the hydrogen bond donating center in the lariat arm offering the best affinities toward chloride and carboxylate anions. The anion-binding properties of such systems were investigated by applying (1)H NMR titrations in DMSO/water and methanol/DMSO mixtures.

Synthesis and evaluation of neutral anion receptors based on acylhydrazide-appended calix[4]arenes

A new class of neutral receptors based upon acylhydrazide-appended calix[4]arenes was synthesised and evaluated for recognition of anions. Detailed NMR and single-crystal X-ray analyses of one of the synthesised compounds reveal that anion recognition in such derivatives is achieved through cooperative hydrogen bond interactions. The presence of three centred NH–O and two OH–O hydrogen bonds at the lower rim of the synthesised calixarene architecture apparently helps the molecular scaffold to retain cone conformation to enable deployment of intermolecular hydrogen bonds for selective recognition of HSO4 − ion in preference to F−, Cl−, Br−, I−, ClO4 −, AcO− and PF6 − ions.