Anion Recognition Research Articles

Ring‐fused Dipyrrolyldiketone Difluoroboron Complexes for Pioneering Exploration of Photothermal Effect

AbstractDipyrrolyldiketone difluoroboron complexes (BONEPYs) reported by Maeda were mainly applied for the selective recognition of anions. Herein, innovative introduction of the ring‐fused segment in BONEPY system resulted in a twisted structure by X‐ray crystallographic analysis and enhanced the spectral bathochromic shift performance. BONEPY‐nanoparticles adjusted the energy release process of the excited state, endowed high photothermal effect, and could be degraded and discharged out of the body potentially. BONEPY‐nanoparticles are the novel phototherapeutic reagent with improved safety and low side effect.

A new two-dimensional cadmium(II) coordination polymer based on Cd6(CHADC)6 clusters (H2CHADC is cyclohexane-1,2-dicarboxylic acid): synthesis, structure and properties.

The highly effective recognition and detection of metal ions and anions in water has attracted much attention with respect to environmental safety. Herein, a novel Cd-based coordination polymer, poly[[4,4'-bis(2-methylimidazol-1-yl)biphenyl]bis(cyclohexane-1,2-dicarboxylato)dicadmium(II)], [Cd2(C8H10O4)2(C20H18N4)]n or [Cd(CHADC)(4,4'-BMIBP)0.5]n, (I), has been synthesized employing cis-cyclohexane-1,2-dicarboxylic acid (H2CHADC) and 4,4'-bis(2-methyl-1H-imidazol-1-yl)biphenyl (4,4'-BMIBP). Single-crystal X-ray analysis reveals that (I) presents a 6-connected hxl two-dimensional layer based on Cd6(CHADC)6 clusters with the point symbol (36·46·53). Furthermore, (I) has been characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and fluorescence spectroscopy, and exhibits good stability and excellent photoluminescence properties. Coordination polymer (I) was chosen as a fluorescent probe to sense different target analytes and shows an obvious selective recognition response to Fe3+ cations and Cr2O72-/CrO42- anions through luminescence-quenching effects in aqueous solution. The sensing mechanism was investigated and showed that the detection mechanism was resonance energy transfer between (I) and the Fe3+, Cr2O72- and CrO42- ions.

Achieving Selectivity for Phosphate over Pyrophosphate in Ethanol with Iron(III)-Based Fluorescent Probes.

Two iron(III)-based molecular receptors employing 1,2-hydroxypyridinone ligands were developed for phosphate recognition and fluorescence sensing via indicator displacement assay (IDA). The tetra- and pentadentate ligands enable anion recognition by the iron(III) center via its remaining one or two open coordination sites. Weak protective coordination of fluorescein at those sites prevents the formation of μ-oxo dimers in aerated solutions. Its rapid and selective displacement by inorganic phosphate results in a 20-fold increase in the fluorescence of the indicator. Both receptors exhibit high affinity for inorganic phosphate and high selectivity over common competing anions, including halides, acetate, carbonate, and, remarkably, pyrophosphate as well as arsenate. Coordination of phosphate to the iron(III) center was confirmed by ATR-IR and 31P NMR spectroscopy.

Anion binding studies with anthraimidazoledione-based positional isomers: A comprehensive analysis of different strategies for improved selectivity

In the present work, we have reported the design of three different positional isomers of anthraimidazoledione-based charge transfer probes and their anion-binding properties under various conditions. In the acetonitrile medium, the meta isomer showed ratiometric optical response towards basic anions, such as F−, CN−, AcO− and H2PO4−. Based on the differences in the hydrogen bonding ability of these anions, we observed distinguishable output signal, particularly in fluorescence. Though meta and para isomers showed effective interaction with anions, the response was relatively weak for the ortho isomer. We suspected that the presence intramolecular hydrogen bond between pyridine nitrogen and imidazole –NH group might be responsible for such poor performance. Further, we have employed two different strategies to improve the selectivity towards anions. In the first case, selective recognition of anions was achieved using suitable metal ions (Cu2+, Zn2+, Ca2+, and Al3+) as masking agents. On the contrary, we have varied the water content (0, 10, 30 and 50% v/v) in the acetonitrile-water mixture in the second case. The anions with relatively large hydration enthalpy showed no detectable interaction with probe in presence of water. Finally, we used the present system to detect cyanide ion in various natural water samples (tap, pond, and seawater). Also, low-cost reusable paper strips were developed as an alternative method for rapid, on-location detection of CN− ions.

Structurally Forced Ion Binding Affinity: Tetraurea‐Based Macrocycle as a Receptor for Ion Pairs

AbstractA urea‐based complexation site represents a building block customarily used in the design of receptors for anion recognition. Here we report a cyclization driven change of complexation preferences, leading to a urea‐based macrocyclic receptor with selectivity for heavier alkali metal cations. Moreover, thorough 1H and 19F NMR, HRMS and X‐ray diffraction studies of the complexes revealed an inherent capability of the macrocycle to interact with selected ion pairs.

Towards Anion Recognition and Precipitation with Water-Soluble 1,2,4-Selenodiazolium Salts: Combined Structural and Theoretical Study

The synthesis and structural characterization of a series of supramolecular complexes of bicyclic cationic pyridine-fused 1,2,4-selenodiazoles with various anions is reported. The binding of trifluoroacetate, tetrachloroaurate, tetraphenylborate, perrhenate, and pertechnetate anions in the solid state is regarded. All the anions interact with selenodiazolium cations exclusively via a pair of “chelating” Se⋯O and H⋯O non-covalent interactions, which make them an attractive, novel, non-classical supramolecular recognition unit or a synthon. Trifluoroacetate salts were conveniently generated via novel oxidation reaction of 2,2′-dipyridyl diselenide with bis(trifluoroacetoxy)iodo)benzene in the presence of corresponding nitriles. Isolation and structural characterization of transient 2-pyridylselenyl trifluoroacetate was achieved. X-ray analysis has demonstrated that the latter forms dimers in the solid state featuring very short and strong Se⋯O and Se⋯N ChB contacts. 1,2,4-Selenodiazolium trifluoroacetates or halides show good solubility in water. In contrast, (AuCl4)−, (ReO4)−, or (TcO4)− derivatives immediately precipitate from aqueous solutions. Structural features of these supramolecular complexes in the solid state are discussed. The nature and energies of the non-covalent interactions in novel assembles were studied by the theoretical methods. To the best of our knowledge, this is the first study that regards perrhenate and pertechnetate as acceptors in ChB interactions. The results presented here will be useful for further developments in anion recognition and precipitation involving cationic 1,2,4-selenodiazoles.

Anion Recognition by Benzoxaborole.

The binding types (H-bonding or coordinate) and stability constants for complexes of 11 mono- and di-anions with benzoxaborole (1) were determined by 1H and 11B NMR titrations in DMSO or MeCN. Compared to phenylboronic acid (PBA), 1 is a stronger Lewis acid and a poorer H-bond donor with only one B-OH group, which is expected therefore to recognize anions mostly through the coordinate bonding. This is the case however only with F-, HPO42-, and PhPO32- anions, which are coordinately bonded to 1, and partially with SO42-, which forms only the H-bonded complex with PBA, but both H-bonded and coordinate complexes with 1. The majority of tested anions (AcO-, PhPO3H-, (PhO)2PO2-, Cl-, and Br-) form H-bonded complexes with both 1 and PBA, whereas H2PO4- changes the binding mode from coordinate for PBA to H-bonded for 1. The preferable binding type for each anion is confirmed by calculations of DFT-optimized structures of the anion complexes of 1. The preferable binding type can be rationalized considering the effects of the steric hindrance, more significant for the coordinate bonding, and of increased anion basicity, which is favorable for both binding types, but enhances the strength of coordinate bonding more significantly than the strength of H-bonding.

Multiple fluorescence color transitions mediated by anion-π interactions and C-F covalent bond formation

The emission changes of fluorescent dyes under the influence of environmental changes or interaction with analytes are the basis for designing ratiometric fluorescent probes and logic gates. However, it is rare that only one external stimulus induces continuous fluorescent color changes in a fluorescent dye. In this paper, we report a cage-like molecule formed by two benzene rings and three imidazolium salts which produces continuous fluorescence wavelength changes when interacted with fluoride ions. Fluoride ions are first bound to the center of the cage under the action of anion-π interaction, and the (C−H)+···F– type ionic hydrogen bonds induce the blue-shift of fluorescence. The subsequent formation of C-F covalent bonds with fluoride ions makes the fluorophore wavelength continue to blue-shift, and finally obtains continuous multiple fluorescence changes caused by a single external stimulus. According to the fluorescence wavelength and intensity, six different fluorescence signal channels can be obtained, which can be encoded as six numbers from 0 to 5. We expect that this reaction process can find applications in quantitative anion recognition and molecular counters.

Redox-Switchable Chalcogen Bonding for Anion Recognition and Sensing.

Inspired by the success of its related sigma-hole congener halogen bonding (XB), chalcogen bonding (ChB) is emerging as a powerful noncovalent interaction with a plethora of applications in supramolecular chemistry and beyond. Despite its increasing importance, the judicious modulation of ChB donor strength remains a formidable challenge. Herein, we present, for the first time, the reversible and large-scale modulation of ChB potency by electrochemical redox control. This is exemplified by both the switching-ON of anion recognition via ChB oxidative activation of a novel bis(ferrocenyltellurotriazole) anion host and switching-OFF reductive ChB deactivation of anion binding potency with a telluroviologen receptor. The direct linking of the redox-active center and ChB receptor donor sites enables strong coupling, which is reflected by up to a remarkable 3 orders of magnitude modulation of anion binding strength. This is demonstrated through large voltammetric perturbations of the respective receptor ferrocene and viologen redox couples, enabling, for the first time, ChB-mediated electrochemical anion sensing. The sensors not only display significant anion-binding-induced electrochemical responses in competitive aqueous-organic solvent systems but can compete with, or even outperform similar, highly potent XB and HB sensors. These observations serve to highlight a unique (redox) tunability of ChB and pave the way for further exploration of the reversible (redox) modulation of ChB in a wide range of applications, including anion sensors as well as molecular switches and machines.

A highly selective and sensitive salamo-salen-salamo hybrid fluorometic chemosensor for identification of Zn2+ and the continuous recognition of phosphate anions

A salamo-salen-salamo hybrid fluorescent chemical sensor (H4L) was synthesized and characterized. It exhibits high selectivity and sensitivity to Zn2+ in physiological pH range. Meanwhile, its zinc(II) complex (L-Zn2+) continuously responses phosphate anions in DMF/H2O (v/v, 9:1) solution. Moreover, the identification processes are explored using characterization methods such as UV-absorption spectra, IR spectra and ESI-MS spectrum. In addition, the coordination mechanism of H2PO4− and Zn2+ were successfully exploited to make the chemical sensor reproducible. In short, the sensors H4L and L-Zn2+ will be promising detection devices for Zn2+ and phosphate anions.

Benzothiazole azo derivatives as colorimetric probes for optical recognition of different metal ions and anions

In this study, we report on novel optical chemosensors containing benzothiazole moeity, namely, 2‐(6‐chloro‐benzothiazol‐2‐yl azo)‐4‐methyl‐phenol (CBAMP) and 1‐(6‐chloro‐benzothiazol‐2‐yl azo)‐naphthalen‐2‐ol (CBAN), which were synthesized and characterized by Fourier transform infrared spectroscopy (FT‐IR), 1H‐NMR, 13C‐NMR, and mass spectrometry. The solvatochromic behavior was explored in different solvents of various polarities to explore the active fluorescent tautomer, and the photophysical parameters have been measured. The naked eyes, as well as ultraviolet–visible (UV–Vis) and fluorescence spectroscopy, were used to study the colorimetric and optical sensing properties of CBAMP and CBAN toward various metal ions and anions. These chemosensors have a strong detecting ability, with excellent sensitivity and selectivity for certain of the metal ions investigated, as well as CO32− and CN− over other anions. The Benesi–Hildebrand and Job's plots were used to determine the binding constants and stoichiometry of the formed metal ions–sensor complexes, respectively. Co2+ and Cu2+ ions have lower detection limits in CBAMP and CBAN than other metal ions (6.4 × 10−7 and 8.4 × 10−7 M, respectively). Furthermore, fluorescent imaging investigations for Co2+ and Cu2+ ions in living cells reveal CBAMP and CBAN's promise for biological chemosensing.

A highly selective superphane for ReO4− recognition and extraction

Highly selective anion recognition and extraction is challenging and yet critical for removal of pollutants from the environment and the effective recovery of valuable chemicals from low-content (at sub-ppm or ppb level) sources. In this paper, we detail the gram-scale synthesis of a superphane 2 , an anion receptor that selectively binds ReO 4 − . Superphane 2 can extract perrhenate from solid mixtures containing traces of ReO 4 − anion (as low as 200 ppb) and aqueous media with near 100% selectivity over large excesses of competing anions. Meanwhile, up to 99.99% of ReO 4 − can be separated from complex simulated aqueous waste streams containing ppm-level perrhenate via either liquid-liquid extraction or simple column adsorption. Importantly, after extraction or adsorption, superphane 2 can be recycled and reused by simple treatment with aqueous NaHCO 3 . • A superphane for highly selective recognition of ReO 4 − is synthesized in gram scale • Over 99.99% of ReO 4 − can be separated from complex simulated waste streams • Rapid removal of ppb-level ReO 4 − is achieved via extraction or column adsorption • The superphane for ReO 4 − extraction/adsorption can be recycled and reused Extraction of low concentrations of rhenium anions from waste streams is important industrially. Zhou et al. present the facile and gram-scale synthesis of a superphane via dynamic imine chemistry coupled with NaBH 4 reduction, which can separate perrhenate from low-content mixtures and simulated Hanford waste streams with exceptionally high efficiency and selectivity, along with good recyclability and reusability.

Pyrene functionalized oxacalix[4]arene architecture as dual readout sensor for expeditious recognition of cyanide anion.

A pyrene functionalized oxacalix[4]arene architecture (DPOC) was utilized as a fluorescence probe for selective recognition of cyanide ions. The receptor DPOC shows excellent selectivity towards cyanide ion with a red shift of 108nm in absorption band along with a significant change in colour from light yellow to pink. The fluorescence titration experiments further confirm the lower limit of detection as 1.7µM with no significant influences of competing anions. 1H-NMR titration experiments support the deprotonation phenomena, as the -NH proton disappears upon successive addition of cyanide ions. The DFT calculation also indicates a certain increment of -NH bond length upon interaction with cyanide ions. The spectral properties as well as colour of DPOC-CN- system may be reversed upon the addition of Ag+/ Cu2+ ions up to 5 consecutive cycles. Moreover, DPOC coated "test strips" were prepared for visual detection of cyanide ions.

Luminescence Cd(II) coordination compounds based on a semi-rigid tricarboxylic acid ligand for identifying metal cations, inorganic anions and organic solvents

Highly effective recognition and detection of metal ions, anions and organic small molecules from water have attracted remarkable attention for environmental safety. Herein, a novel Cd-based coordination polymer [Cd(HL)(4,4′-bipy)(H2O)]n (1) and a metal–organic framework {[Cd3(L)2(bip)2(H2O)3]·5H2O}n (2) have been synthesized by employing a semi-rigid tricarboxylic acid ligand (H3L) and different N-containing ligands under hydrothermal conditions, where H3L = 3-[(1-carboxynaphthalen-2-yl)oxy] phthalic acid, 4,4′-bipy = 4,4′-bipyridine, bip = 1,4-di(pyridine-4-yl) benzene. Single crystal X-ray analysis reveals that 1 presents a 4-connected 2D network structure with a Schläfli symbol of (44·62),whilst 2 possesses a (3,4,4)-connected 3D framework with a Schläfli symbol of (4.82)2(42.82.102)(8.104.12). Furthermore, 1 and 2 have been characterized by elemental analyses, FT-IR, powder X-ray diffraction, thermogravimetric analyses, fluorescence and UV spectra, exhibiting good stability and excellent photoluminescence properties. 1 and 2 have been chosen as fluorescent probes to sense different target analytes, showing an obvious selective recognition response to Co2+, Ag+, Cr2O72-, CrO42-, nitrobenzene and nitromethane through luminescence quenching effects in aqueous solutions. The possible fluorescence quenching mechanisms are studied in detail, revealing that fluorescence quenching is mainly related to weak interactions, competitive energy absorption and electron transfer.

Fluorescence Sensors Based on Hydroxycarbazole for the Determination of Neurodegeneration-Related Halide Anions.

The environmental presence of anions of natural origin or anthropogenic origin is gradually increasing. As a tool to tackle this problem, carbazole derivatives are an attractive gateway to the development of luminescent chemosensors. Considering the different mechanisms proposed for anion recognition, the fluorescence properties and anion-binding response of several newly synthesised carbazole derivatives were studied. Potential anion sensors were designed so that they combined the native fluorescence of carbazole with the presence of hydrogen bonding donor groups in critical positions for anion recognition. These compounds were synthesised by a feasible and non-expensive procedure using palladium-promoted cyclodehydrogenation of suitable diarylamine under microwave irradiation. In comparison to the other carbazole derivatives studied, 1-hydroxycarbazole proved to be useful as a fluorescent sensor for anions, as it was able to sensitively recognise fluoride and chloride anions by establishing hydrogen bond interactions through the hydrogen atoms on the pyrrolic nitrogen and the hydroxy group. Solvent effects and excited-state proton transfer (ESPT) of the carbazole derivatives are described to discard the role of the anions as Brönsted bases on the observed fluorescence behaviour of the sensors. The anion–sensor interaction was confirmed by 1H-NMR. Molecular modelling was employed to propose a mode of recognition of the sensor in terms of complex stability and interatomic distances. 1-hydroxycarbazole was employed for the quantitation of fluoride and chloride anions in commercially available medicinal spring water and mouthwash samples.

Preorganized Homochiral Pyrrole-Based Receptors That Display Enantioselective Anion Binding.

Herein, a new scaffold for anion recognition based on a tripodal tris(pyrrolamide) motif is presented. The receptors were able to bind to a variety of anions with high affinity. Using density functional theory methods, the three-dimensional geometry of the receptor-anion complex was calculated. These calculations show that the receptors bind anions via a preorganized cavity of amide and pyrrole hydrogen bond donor groups. Based on these findings, homochiral tris(pyrrolamide) receptors were prepared, which produced as much as a 1.6-fold greater affinity for (S)-(+)-mandelate over (R)-(-)-mandelate, demonstrating the ability to differentiate between these enantiomeric anions. The interaction of (S)-(+)-mandelate and (R)-(-)-mandelate within the homochiral receptor was examined by solution NMR spectroscopy and density functional theory calculations. These findings indicate that the preorganized positioning of the pyrrole groups and subsequent sterics allows to differentiate between the stereoisomeric anions.

Macrocyclic Polyoxometalates: Selective Polyanion Binding and Ultrahigh Proton Conduction.

The rational development of an anion templation strategy for the construction of macrocycles has been historically limited to small anions, but large polyoxoanions can offer unmatched structural diversity and ample binding sites. Here we report the formation of a {Mo22 Fe8 } macrocycle by using the Preyssler anion, [NaP5 W30 O110 ]14- ({P5 W30 }), as a supramolecular template. The {Mo22 Fe8 } macrocycle displays selective anion binding behavior in solution. In the solid state, the 1 : 2 host-guest complex, {P5 W30 }2 ⊂{Mo22 Fe8 }, transports protons more effectively, through an extended hydrogen-bonding network, than a related 1 : 1 complex where the guest is completely encapsulated. The results highlight the great potential this anion templation approach has in producing macrocyclic systems for selective anion recognition and proton conduction purposes.

Macrocyclic Polyoxometalates: Selective Polyanion Binding and Ultrahigh Proton Conduction

The rational development of an anion templation strategy for the construction of macrocycles has been historically limited to small anions, but large polyoxoanions can offer unmatched structural diversity and ample binding sites. Here we report the formation of a {Mo22Fe8} macrocycle by using the Preyssler anion, [NaP5W30O110]14− ({P5W30}), as a supramolecular template. The {Mo22Fe8} macrocycle displays selective anion binding behavior in solution. In the solid state, the 1 : 2 host–guest complex, {P5W30}2⊂{Mo22Fe8}, transports protons more effectively, through an extended hydrogen-bonding network, than a related 1 : 1 complex where the guest is completely encapsulated. The results highlight the great potential this anion templation approach has in producing macrocyclic systems for selective anion recognition and proton conduction purposes.

Anion Recognition by a Pincer-Type Host Constructed from Two Polyamide Macrocyclic Frameworks Jointed by a Photo-Addressable Azobenzene Switch.

A sterically crowded light-responsive host 1 was synthetized with a 93% yield by applying a post-functionalization protocol utilizing the double amidation of 4,4′-azodibenzoyl dichloride with a readily available 26-membered macrocyclic amine. X-ray structures of two hydrates of trans-1 demonstrate a very different alignment of the azobenzene linkage, which is involved in T-shape or parallel-displaced π⋯π stacking interactions with the pyridine-2,6-dicarboxamide moieties from the macrocyclic backbone. Despite the rigidity of the macrocyclic framework, which generates a large steric hindrance around the azobenzene chromophore, the host 1 retains the ability to undergo a reversible cis⟷trans isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal cis→trans back-isomerization (ΔG0 = 106.5 kJ∙mol−1, t½ = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. 1H NMR titration experiments in DMSO-d6/0.5% water solution reveal that trans-1 exhibits a strong preference for dihydrogenphosphate (H2PO4−) over other anions (Cl−, MeCO2−, and PhCO2−), whereas the photogenerated metastable cis-1 shows lower affinity for the H2PO4− anion.

Exploring the Structure and Complexation Dynamics of Azide Anion Recognition by Calix[4]pyrroles in Solution.

The structure and anion recognition dynamics between calix[4]pyrroles and azide (N3-) anions in the form of its TBA+ and Na+ salts were investigated in dimethyl sulfoxide solutions by Fourier transform infrared (FTIR) spectroscopy and ultrafast IR spectroscopy. Vibrational energy redistribution of the N3- anion in the complex is accelerated through hydrogen bonding interactions with the N-H proton of the receptor. Rotational dynamics of the bound N3- is greatly restricted, demonstrating a distinct countercation effect. The detailed binding modes of N3- with the receptor were further evaluated by the density functional theoretical (DFT) calculations and nuclear magnetic resonance (NMR) spectroscopy. All of these measurements support the notion that the calix[4]pyrroles are capable of capturing the azide anion in solution. However, the calix[4]pyrroles may not necessarily undergo a conformational change to a cone-like geometry when they bind to the azide anion in the solution.