A case study of solid state inhibition lock opening of pyridine N-oxide in dicarboxylic acid recognition

An ethylene-linked mixed pyridine diamide receptor 1 is synthesised to study the binding with dicarboxylic acid in solid phase. In ¹H NMR, the amide proton adjacent to pyridine N-oxide shows almost negligible shift, which suggests the non-participation of the amide proton for acid binding in solution. However, in solid phase (X-ray, IR), we have found significant role of this amide proton in binding.     

Bis-amidopyrrolyl receptors based on anthracene and carbazole

A new set of diamide receptors containing anthracene and carbazole bridging subunits and either pyrrole or phenyl substituents were synthesised. The four systems produced in this way were shown to bind representative anions in DMSO-d ₆ solution and in the solid state. A higher relative affinity for two test oxoanions, namely dihydrogen phosphate and benzoate, over chloride anion was seen in solution, with the anions in question being studied in the form of their respective tetrabutylammonium salts. However, the specifics of the anion recognition process were seen to depend on structure, with the pyrrole-containing systems displaying higher relative affinities than their corresponding phenyl-containing congeners, and the carbazole receptors proving more effective than the anthracene analogues. Such observations provide support for the notion that both the carbazole NH and the pyrrolic NH protons play an important role in stabilising the receptor-bound anions in solution. Structural analyses of several anion complexes of the diamidopyrrole carbazole receptor reveal that this is not necessarily the case in the solid state; specifically, the pyrrole NH protons are seen to interact with the amide oxygen of another molecule. The net result is an extended one-dimensional coordination polymer.     

Selective detection of F− by chromogenic tetrazole receptor

A chromogenic anion host 4, containing two amide functionalities linked to azo dye and tetrazole rings, was synthesised and its complexes with various anions were investigated. The results show that receptor 4 can recognise selectively biologically important F^?  ion. The binding affinity for F^?  was investigated by naked-eye colour change, UV–vis and ¹H NMR spectroscopy. Addition of F^?  ion in CH₃CN and Dimethyl sulfoxide to receptor 4 causes a change in colour of the solution from colourless to yellow. The stoichiometry for host:anion is 1:1. Furthermore, receptor 4 was used as an ion carrier in ion-selective membrane electrodes. Selectivity of this membrane was studied towards various anions in water solution. Binding behaviour of receptor 4 towards several anions (Cl^? , F^? , Br^? , I^? ) has been investigated using density functional theory calculations.     

Halide Anion Mediated Dimerization of a meso‐Unsubstituted N‐Confused Porphyrin

The new N‐confused porphyrin (NCP) derivatives, meso‐unsubstituted β‐alkyl‐3‐oxo N‐confused porphyrin (3‐oxo‐NCP) and related macrocycles, were synthesized from appropriate pyrrolic precursors by a [3+1]‐type condensation reaction. 3‐Oxo‐NCP forms a self‐assembled dimer in dichloromethane that is stabilized by complementary hydrogen‐bonding interactions arising from the peripheral amide‐like moieties. The protonated form of 3‐oxo‐NCP was observed to bind halide anions (F^?, Cl^?) through the outer NH and the inner pyrrolic NH groups, thus affording a dimer in dichloromethane. The structure of the chloride‐bridged dimer in the solid state was determined by X‐ray diffraction analysis.     

An exclusive fluoride receptor: fluoride-induced proton transfer to a quinoline-based thiourea

A new quinoline-based tripodal thiourea has been synthesized, which exclusively binds fluoride anion in DMSO, showing no affinity for other anions including chloride, bromide, iodide, perchlorate, nitrate, and hydrogen sulfate. As investigated by ¹H NMR, the receptor forms both 1:1 and 1:2 complexes yielding binding constants of 2.32(3) (in log β₁) and 4.39(4) (in log β₂), respectively. The quinoline groups are protonated by fluoride-induced proton transfer from the solution to the host molecule. The 1:2 binding is due to the interactions of one fluoride with NH binding sites of urea sites and another fluoride with secondary ⁺NH binding sites within the tripodal pocket. The formation of both 1:1 and 1:2 complexes has been confirmed by theoretical calculations based on density functional theory (DFT).     

1,3‐Alternate Tetraamido‐Azacalix[4]arenes as Selective Anion Receptors

Six tetraaza[1.1.1.1]cyclophane derivatives bearing peripheral amide groups were prepared according to two distinct synthetic strategies that depend on the connection pattern between the aryl units. NMR experiments combined with the X‐ray structures of two tetraamide derivatives 4 b and 10 show that these cavitands adopt a 1,3‐alternate conformation both in solution and in the solid state. Consequently, the four amide groups of the aza[1.1.1.1]‐m,m,m,m‐cyclophane isomer 10 can contribute to the same recognition process towards neutral water molecules or anion guests. NMR experiments, mass spectrometry analyses and single‐crystal X‐ray structures confirm the anion‐binding ability of this receptor. Absorption spectrophotometric titrations in nonpolar solvents provided evidence for the selectivity of 10 to chloride anions in the halide series, with a corresponding association constant K_a reaching 2.5×10⁶ m ^?1.     

Crystallographic and Solution Anion Binding Studies of Bis-amidofurans and Thiophenes

A variety of furan and thiophene amide and thioamide cleft type anion receptors have been synthesised and crystallographically characterised. Unlike 2,5-diamidopyrrole anions, analogous 2,5-diamidofurans and thiophenes do not interlock in the solid state. The anion binding properties of these receptors have been investigated in DMSO/0.5% water solution using ¹H NMR titration techniques. Solution studies and solid-state evidence suggests that the thiophene receptors may utilise a thiophene CH hydrogen atom for hydrogen bond formation to anions with a 2,4-diamidothiophene showing similar anion binding affinities to a 2,5-diamidopyrrole.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

A Multi-armed Neutral Receptor for α,ω-Dicarboxylate Anions

A multi-armed neutral anion receptor (1) bearing multiple amide and thiourea binding sites was synthesized. Receptor 1 forms 1:2 complexes with dicarboxylate anions, and the sensitivity for recognition of dicarboxylate depends strongly on the chain length of these dicarboxylate anions. Addition of the anions caused a considerable change in the absorbance and fluorescent intensity of the host solution and a consequent visible color change.     

Tight and Selective Caging of Chloride Ions by a Pseudopeptidic Host

The selective molecular recognition of chloride versus similar anions is a continuous challenge in supramolecular chemistry. We have designed and prepared a simple pseudopeptidic cage ( 1 a ) that defines a cavity suitable for the tight encapsulation of chloride. The interaction of the protonated form of 1 a with different inorganic anions was studied in solution by ¹H NMR spectroscopy and ESI‐MS, and in the solid state by X‐ray diffraction. The solution binding data showed that the association constants of 1 a to chloride are more than two orders of magnitude higher than to any other tested inorganic anion. Remarkably, 1 a displayed a high selectivity for chloride over other closely related halides such as bromide (selectivity=111), iodide (selectivity=719), and fluoride (selectivity >1000). Binding experiments (¹H NMR spectroscopy and ESI‐MS) suggested that 1 a has a high‐affinity (inner) binding site and an additional low‐affinity (external) binding site. The supramolecular complexes with F^?, Cl^?, and Br^? have been also characterized by the X‐ray diffraction of the corresponding [ 1 a? nHX] crystalline salts. The structural data show that the chloride anion is tightly encapsulated within the host, in a binding site defined by a very symmetric array of electrostatic H‐bonds. For the fluoride salt, the size of the cage cavity is too large and is occupied by a water molecule, which fits inside the cage efficiently competing with F^?. In the case of the bigger bromide, the mismatch of the anion inside the cage caused a geometrical distortion of the host and thus a large energetic penalty for the interaction. This minimalistic pseudopeptidic host represents a unique example of the construction of a simple well‐defined binding pocket that allows the highly selective molecular recognition of a challenging substrate.     

Salicylaldehyde-indole-2-acylhydrazone: a simple,colorimetric and absorption ratiometric chemosensor for acetate ion

A simple anion receptor (i.e. salicylaldehyde-indole-2-acylhydrazone) was synthesised and its recognition properties were investigated by naked-eye observation, UV–vis titration spectra, ¹H NMR spectroscopy and DFT calculations. The obtained results indicated that this receptor could realise the selective colorimetric sensing and absorption ratiometric response towards AcO^?  in CH₃CN–DMSO medium, by virtue of threefold intermolecular hydrogen bonding interactions formed with phenolic OH, indole NH and amide NH.     

Investigating the imidazolium–anion interaction through the anion-templated construction of interpenetrated and interlocked assemblies

The interaction between imidazolium cations and coordinating anions is investigated through the anion‐templated assembly of interpenetrated and interlocked structures. The orientation of the imidazolium motif with respect to anion binding, and hence the hydrogen bond donor arrangement, was varied in acyclic receptors, interpenetrated assemblies, and the first mono‐imidazolium interlocked systems. Their anion recognition properties and co‐conformations were studied by solution‐phase ¹H NMR investigations, solid‐state structures, molecular dynamics simulations, and density functional theory calculations. Our findings suggest that the imidazolium‐anion binding interaction is dominated by electrostatics with hydrogen‐bonding contributions having weak orientational dependence.     

Cobalt(III) Supramolecuar Complex: Synthesis,Crystal Structure and Anions Recognition Properties of 6,7-Dinitroquinoxaline-2,3-dione

A 3D supramolecular complex [Co(III)(Phen)₂Cl₂][DNQ]Cl·H₂O (DNQ = 6,7-dinitroquinoxaline-2,3-dione) has been synthesized and determined by single-crystal X-ray analysis. Anion binding studies carried out using ¹H NMR and UV-visible revealed that a genuine hydrogen bond interaction between anion receptor 2 and Cl^?  can be detected, but fluoride is deprotonating the anion receptor 2. X-ray analysis results show that it is not possible for the two amide nitrogen atoms of anion receptor to form hydrogen bonds to a chloride ion in the presence of small aliquots of water.     

Utilizing a pH‐Sensitive Dye in the Selective Fluorescent Recognition of Sulfate

A receptor containing amidopyrrole binding subunits and free amino groups, conjugated to a naphthalimide dye, has been designed and synthesized. The intrinsic selectivity of the binding motif for phosphate present in DMSO completely disappears in 10 % DMSO aqueous buffer at pH 3.6, at which the receptor is protonated. The electrostatic interactions between the receptor and an anion start to dominate, thus leading to selectivity for sulfate. The ability of the HSO₄^? anion to transfer the proton to the amino group during the recognition event suppresses the photoinduced electron transfer (PET) on the dye, resulting in a selective turn‐on fluorescent response. The choice of pH of the solution for sensing is dictated by the pK_a value of the dye.     

Synthesis and characterization of a tripodal amide ligand and its binding with anions of different dimensionality

Synthesis and crystal structure of a tren-based amide, L(1), N,N',N'-tris[(2-amino-ethyl)-3-nitro-benzamide] is reported. The crystallographic results show intramolecular hydrogen bonding and aromatic pi...pi stacking among tripodal arms which prevent opening of the receptor cavity. Intermolecular hydrogen bonding in L1 generates the sheetlike network in the solid state. The structural aspects of binding halides (1 and 2), nitrate (3), perchlorate (4), and hexafluorosilicate (5) with the protonated L1 are examined crystallographically. Protonation at the apical nitrogen of L1 in the presence of anions shows a structural transformation from sheet to bilayer network. Anion binding with multiple receptor units is observed via amide N-H...anion and aryl C-H...anion hydrogen-bonding interactions in all the complexes. The aryl group having nitro functionality that contributes to anion binding in complexes 1-5 through CH...anion interactions (either para or meta to nitro C-H) is noteworthy. These studies also show higher anion coordination of chloride (8) and hexafluorosilicate (14) with L1H+.     

Pentafluorophenyl salicylamine receptors in anion–π interaction studies

A crystal structure analysis confirms the appropriateness of pentafluorophenyl salicylamine (1a) as a π-acceptor for anion–π interactions. Crystals of 1a·HCl show that the OH-group fixes the anion in a η²-type binding motif above the electron-deficient arene. Attempts to find some relevance for this weak intermolecular force in solution failed. Stronger CH–, NH– and OH–anion interactions are dominant over the weak anion–π interactions. Due to the hydrogen bonding, the non-fluorinated receptor exhibits the highest binding constants within this series.     

Anion‐Induced Shuttling of a Naphthalimide Triazolium Rotaxane

The anion‐templated synthesis of a rotaxane structure, incorporating the new naphthalimide triazolium motif, is described and the interlocked host shown to exhibit selective, uni‐directional, anion‐induced shuttling. Initial pseudorotaxane investigations demonstrate the ability of a naphthalimide triazolium threading component to form interpenetrated assemblies with counter‐anion‐dependent co‐conformations. ¹H NMR studies reveal that the shuttling behaviour of the analogous rotaxane host system is controlled by selective anion binding and by the nature of the solvent conditions. Complete macrocycle translocation only occurs upon the recognition of the smaller halide anions (chloride and bromide). The rotaxane solid‐state crystal structure in the presence of chloride is in agreement with the solution‐phase co‐conformation. The sensitivity of the axle naphthalimide absorbance band to the position of the macrocycle component within the interlocked structure enabled the molecular motion to be observed by UV/Vis spectroscopy, and the chloride‐induced shuttling of the rotaxane was reversed upon silver hexafluorophosphate addition.     

Multiple Multidentate Halogen Bonding in Solution,in the Solid State,and in the (Calculated) Gas Phase

The binding properties of neutral halogen‐bond donors (XB donors) bearing two multidentate Lewis acidic motifs toward halides were investigated. Employing polyfluorinated and polyiodinated terphenyl and quaterphenyl derivatives as anion receptors, we obtained X‐ray crystallographic data of the adducts of three structurally related XB donors with tetraalkylammonium chloride, bromide, and iodide. The stability of these XB complexes in solution was determined by isothermal titration calorimetry (ITC), and the results were compared to X‐ray analyses as well as to calculated binding patterns in the gas phase. Density functional theory (DFT) calculations on the gas‐phase complexes indicated that the experimentally observed distortion of the XB donors during multiple multidentate binding can be reproduced in 1:1 complexes with halides, whereas adducts with two halides show a symmetric binding pattern in the gas phase that is markedly different from the solid state structures. Overall, this study demonstrates the limitations in the transferability of binding data between solid state, solution, and gas phase in the study of complex multidentate XB donors.     

含硫脲和酰胺单元的新型杯［4］芳烃中性氟离子受体

Two-armed neutral anion receptors (4,5), calix[4]arenes beating thiourea and amide binding sites, were prepared and examined their anion-binding ability by the UV-vis spectra. The results of non-linear curve fitting and Job plot indicate that 4 or 5 forms 1：1 stoichiometry complex with fluoride by hydrogen bonding interactions. Receptors 4 and 5 have an excellent selectivity for fluoride but have no binding ability with acetate, dihydrogen phosphate and the halogen anions (Cl^-,Br^-,I^-).     

Effect of Hydrogen Binding on Selective Recognition of Halide Anions

The interplay of molecular rigidity enforced by interior or exterior hydrogen bonding and affinity for binding halide anions is described to demonstrate the effect of intramolecular hydrogen bonding in anion recognition process. To this end pyridine‐2,6‐dicarboxamides 1 and 2 , and aromatic oligoamides 3 and 4 containing intramolecular hydrogen bonds were explored for their ability in associating with tetrabutylammonium halide (Cl^?, Br^?, and I^?). Binding constants in chloroform solution were calculated using nonlinear curve‐fitting method based on ¹H NMR titration experiments. The trimeric amide 3 , which adopts a crescent conformation as revealed by single‐crystal X‐ray diffraction analysis, strongly binds chloride anion with binding constant as high as 379 L·mol^?1. This is more than 6 times greater than the binding constant for the control receptor 2 with a backbone that is only partially rigidified. The comparative data provided supportive information for rationalizing the observed affinity difference in binding halide anions in terms of local preorganization effected by interior or exterior hydrogen bonding.