Effect of Halide Anions on the Electroreduction of CO2 to C2H4: A Density Functional Theory Study**

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C₂₊) products for the electrochemical reduction of CO₂ over copper (Cu) catalysts. However, the mechanism behind the increased yield of C₂₊ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO₂ to C₂H₄ on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C−C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO-CO intermediate. We then propose a free-energy pathway of C₂H₄ formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C₂H₄ in the order of I⁻>Cl⁻>Br⁻>F⁻. Lastly, the obtained results are rationalized through Bader charge analysis.     

Hydrogen-Bonding Assembly Meets Anion Coordination Chemistry: Framework Shaping and Polarity Tuning for Xenon/Krypton Separation

Hybrid hydrogen-bonded (H-bonded) frameworks built from charged components or metallotectons offer diverse guest-framework interactions for target-specific separations. We present here a study to systematically explore the coordination chemistry of monovalent halide anions, i.e., F⁻, Cl⁻, Br⁻, and I⁻, with the aim to develop hybrid H-bond synthons that enable the controllable construction of microporous H-bonded frameworks exhibiting fine-tunable surface polarity within the adaptive cavities for realistic xenon/krypton (Xe/Kr) separation. The spherical halide anions, especially Cl⁻, Br⁻, and I⁻, are found to readily participate in the charge-assisted H-bonding assembly with well-defined coordination behaviors, resulting in robust frameworks bearing open halide anions within the distinctive 1D pore channels. The activated frameworks show preferential binding towards Xe (IAST Xe/Kr selectivity ca. 10.5) because of the enhanced polarizability and the pore confinement effect. Specifically, dynamic column Xe/Kr separation with a record-high separation factor (SF=7.0) among H-bonded frameworks was achieved, facilitating an efficient Xe/Kr separation in dilute, CO₂-containing gas streams exactly mimicking the off-gas of spent nuclear fuel (SNF) reprocessing.     

Hydrogenation of carbon monoxide in the presence of homogeneous ruthenium catalysts: effects of onium halides as promoters

The direct formation of ethylene glycol and ethanol from synthesis gas in the presence of a homogeneous ruthenium carbonyl catalyst is promoted by onium halides, such as ammonium, phosphonium and iminium halides. The catalytic activities for ethylene glycol and ethanol formation are dependent on the nature of the halides, and increase in the order I⁻ < Br⁻ < Cl⁻ and Cl⁻ < I⁻ ≤ Br⁻, respectively. The ruthenium catalyst in conjunction with (Ph₃P)₂NCl shows the highest activity for ethylene glycol formation. The catalytic activities are dependent on the electron-accepting abilities of the solvents. A moderate electron-accepting ability of the solvent is important for oxygenate formation.     

Dual Spectroscopic Responses of Pyridinium Hemicyanine Dyes to Anions

The absorption and fluorescence spectroscopic responses of three pyridinium hemicyanine dyes to anions F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, HSO₄⁻ and OAc⁻ were investigated. At lower concentrations of OAc⁻ (less than 1 equiv.), both the absorption and the fluorescence intensities of 1 – 3 were more effectively changed than F⁻ at identical concentrations. At higher concentrations of OAc⁻ (more than 1 equiv.), the interaction was opposite for each compound. ¹H NMR results indicated the interaction between 1 , 2 or 3 and F⁻ proceeded through hydrogen bonding. The results showed that these dyes are promising to develop dual fluorescence and chromogenic chemosensors toward F⁻ and OAc⁻ according to the subtle difference in the affinity of F⁻ and OAc⁻.     

A Turn‐On Fluorescence Chemosensor for Cyanide in Aqueous Media Based on a Nucleophilic Addition Reaction

We synthesized a new cyanide (CN ⁻) chemosensor CX based on a nucleophilic addition reaction prompted by cyanide ion, which could be used for highly selective and sensitive fluorescence turn‐on detection of cyanide in aqueous media. The CX showed selective fluorescence recognition for CN ⁻, the miscellaneous competitive anions (F⁻, Cl⁻, Br⁻, I⁻, AcO ⁻, H₂PO₄⁻, HSO₄⁻, ClO₄⁻, S^{2 −}, PO₄³⁻, CO₃²⁻ and SCN ⁻) did not lead to any significant interference. The detection limit of the sensor towards CN ⁻ is 1.15 × 10⁻⁷ mol•L⁻¹. The sensor has been successfully applied to estimate the cyanide ion in seeds of cherries. Test strips based on CX were fabricated, which could be used as a convenient and efficient CN ⁻ test kit to detect CN ⁻ in aqueous solution for “in‐the‐field” measurement.     

Fluorescent-based Solid Sensor for HSO4− in Water

In this report, we have shown that the encapsulation of the terbium 2-methylimidazole-4,5-dicarboxylic acid complex into inorganic host tetraethoxysilance is considered to be an efficient way for the design of anion sensors. Strong green emission still can be observed when it disperses in pure water. It was found that the luminescence of hybrid material was selectively turned off rapidly (1 s) by hydrogen sulfate compared with the addition of different anions such as F⁻, Cl⁻, Br⁻ and I⁻. Thin film was successfully prepared and also could be a promising tool for recognizing HSO₄⁻.     

Concentration Dependence of Ion Pairing in Imidazolium-Based Ionic Liquid Solutions

Infrared vibrational spectroscopy was used to probe concentration-dependent ion pair dissociation of imidazolium-based ionic liquids with three different halide anions (I⁻, Br⁻, and Cl⁻) in deuterated chloroform. Dissociation of the ion pairs at low concentrations of ionic liquids was found to be the easiest for ionic liquid with Cl⁻ anion, the most electronegative anion among the three investigated. This anomalous trend of ion pair dissociation was explained in terms of varying interaction strength between the solvent (CDCl₃) and the anions investigated.     

Construction of Ternary Iodine–Bromine–Chlorine Octahalides

A series of five ternary octanuclear iodine-bromine-chlorine interhalides, [I₂Br₂Cl₄]²⁻ ( 1 ), [I₃BrCl₄]²⁻ ( 2 ), [I₄Br₂Cl₂]²⁻ ( 3 ), [I₂Br₄Cl₂]²⁻ ( 4 ) and [I₃Br₃Cl₂]²⁻ ( 5 ), have been rationally constructed in two steps. Firstly, addition of a dihalogen (ICl or IBr) to the triaminocyclopropenium chloride salt [C₃(NEt₂)₃]Cl forms the corresponding trihalide salt with [ICl₂]⁻ or [BrICl]⁻ anions, respectively. Secondly, addition of a half-equivalent of a second dihalogen, followed by crystallization at low temperature, gives the corresponding octahalide: addition of Br₂ and IBr to [ICl₂]⁻ gives 1 and 2 , respectively, whereas addition of I₂, Br₂ and IBr to [BrICl]⁻ gives 3 , 4 and 5 , respectively. The five octahalides were characterized by X-ray crystallography and far–IR spectroscopy.     

基于酰腙和酚羟基的阴离子受体的合成及其在含水介质中的阴离子比色识别性能

设计合成了一系列基于羟基和氨基的酰腙类受体分子.利用紫外-可见吸收光谱及1HNMR考察了其与F-,Cl-,Br-,I-,CH3COO-,H2PO4?,HSO4?,ClO?4等阴离子的作用.结果表明,该类受体分子在DMSO溶液中能较好地比色识别F-,CH3COO-,H2PO4?,其中受体2在含水介质中[V(DMSO)∶V(H2O)=7∶3]能选择性比色识别CH3COO-.1HNMR滴定实验研究了受体分子与阴离子之间的作用,结果表明受体分子与阴离子之间以氢键作用方式相结合.     

A new colorimetric and fluorescent chemosensor based on thiacalix[4]arene for fluoride ions

A new thiacalix[4]arene based fluorescent chemosensor thiacalix[4]arene-N-(quinolin-8-yl)acetamide (TCAN8QA) has been synthesized. TCAN8QA has been found to exhibit highly selective behavior for F⁻ ions among all other anions, that is, Cl⁻, Br⁻, I⁻, PO₄⁻³, OH⁻, H₂PO₄⁻, and CH₃COO⁻ in the absorption spectra as well as in the emission spectra. Red shift and quenching in emission spectra constituting the signature for fluoride detection are due to photoinduced charge transfer (PCT) which can be attributed to deprotonation of acidic NH proton in the presence of fluoride ions.     

A Bidentate Antimony Pnictogen Bonding Host System

A bidentate pnictogen bonding host-system based on 1,8-diethynylanthracene was synthesized by a selective tin-antimony exchange reaction and investigated regarding its ability to act as a Lewis acidic host component for the complexation of Lewis basic or anionic guests. In this work, the novel C≡C−Sb(C₂F₅)₂ unit was established to study the potential of antimony(III) sites as representatives for the scarcely explored pnictogen bonding donors. The capability of this partly fluorinated host system was investigated towards halide anions (Cl⁻, Br⁻, I⁻), dimethyl chalcogenides Me₂Y (Y=O, S, Se, Te), and nitrogen heterocycles (pyridine, pyrimidine). Insights into the adduct formation behavior as well as the bonding situation of such E⋅⋅⋅Sb−C^F moieties were obtained in solution by means of NMR spectroscopy, in the solid state by X-ray diffraction, by elemental analyses, and by computational methods (DFT, QTAIM, IQA), respectively.     

Halide anions driven self-assembly of haloperfluoroarenes: Formation of one-dimensional non-covalent copolymers

The supramolecular organization in six solid assemblies involving iodo- and bromoperfluoroarene derivatives is described. Single crystal X-ray analyses show that the formation of the supramolecular architectures is controlled by I⁻?Br–Ar_F, I⁻?I–Ar_F, Br⁻?I–Ar_F, and Cl⁻?I–Ar_F halogen bondings thus proving the X⁻?X′–Ar_F supramolecular synthon, where X can be the same as or different from X′, is particularly robust. In five of the described architectures halide anions form two halogen bondings and form infinite chains wherein dihaloperfluoroarenes, which function as bidentate electron acceptors, and halide anions, which function as bidentate electron donors, alternate. This behaviour shows halide anions have a fair tendency to work as bidentate halogen bonding acceptors.     

Raman Spectroscopic Study on Alkyl Chain Conformation in 1‐Butyl‐3‐methylimidazolium Ionic Liquids and their Aqueous Mixtures

Ionic liquids of 1‐butyl‐3‐methylimidazolium ([BMIM]) cation with different anions (Cl^?, Br^?, I^?, and BF₄^?), and their aqueous mixtures were investigated by using Raman spectroscopy and dispersion‐included density functional theory (DFT). The characteristic Raman bands at 600 and 624 cm^?1 for two isomers of the butyl chain in the imidazolium cation showed significant changes in intensity for different anions as well as in aqueous solutions. The area ratio of these two bands followed the order I^?>Br^?>Cl^?>BF₄^? (in terms of the anion X in [BMIM]X), indicating that the butyl chain of [BMIM]I tends to adopt the trans conformation. The butyl chain was found to adopt the gauche conformation upon dilution, irrespective of the anion type. The Raman bands in the butyl C?H stretch region for [BMIM]X (X=Cl^?, Br^?, and I^?) blueshifted significantly with the increase in the water concentration, whereas that for [BMIM]BF₄ changed very little upon dilution. The blueshift in the C?H stretch region upon dilution also followed the order: [BMIM]I>[BMIM]Br>[BMIM]Cl>[BMIM]BF₄, the same order as the above trans conformation preference of the butyl chain in pure imidazolium ionic liquids, which suggested that the cation‐anion interaction plays a role in determining the conformation of the chain.     

Estimation of the specific adsorption of I−, Br−, and Cl− ions in systems (Tl-Ga)/[N-methylformamide + 0.1m M KA + 0.1(1 − m) M KClO4] (KA is KCl, KBr, and KI) based on the analysis of two-dimensional pressure curves obtained by integration of differential capacitance

By means of an ac bridge, the differential capacitance vs. potential curves are measured in systems (Tl-Ga)/[N-MF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[N-MF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[N-MF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] for the following fractions m of the surfaceactive anion: 0, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. Based on the analysis of curves of two-dimensional pressure found by integrating the differential capacitance, it is shown that the data on the specific adsorption of anions I^?, Br^?, and Cl^? in the mentioned systems can be quantitatively described by the Frumkin isotherm. The main adsorption parameters of I^?, Br^?, and Cl^? anions at the (Tl-Ga)/N-MF interface are determined. It is found that on the (Tl-Ga)/N-MF interface, the same as on the (In-Ga)/N-MF interface, the adsorption energy of ions increases in the sequence Cl^? < Br^? < I^?, in contrast to the Ga/N-MF interface, where the energy increases in the reverse sequence: I^? ≈ Br^? < I^?. For all halide ions (Hal^?), the adsorption energy and the energy of metal-Halinteraction increase in the sequence (Tl-Ga) < (In-Ga) < Ga.     

The Application of Cryptand 22 as a Bifunctional Stationary Phase for Ion Chromatography

Poly (styrene/divinyl benzene) with cryptand 22 as an anchoring group was synthesized and applied as a bifunctional packing material for the separation of both cations and anions. At pH < 2, the resin can be protonated and applied as an anion exchanger for the separation of anions; with water as eluent, inorganic anions such as F^?, Cl^?, Br^?, NO₃^?”, I^? were well separated. After deprotonation at pH> 10, the resin became a cation exchanger and successfully separated alkali metal ions such as Li⁺, K⁺ and Cs⁺ with methanol as eluent. The effects of solvents, flow rate and temperature on the separation of various ions were also investigated.     

Low-voltage thin-layer electrophoresis of inorganic anions on silica gel-G and titanium (IV) tungstate layers: separation of coexisting F−, Cl−, Br− and I−, I−, IO3− and IO4−, Fe(CN)64− and Fe(CN)63−

The electrophoretic behavior of twenty anions has been studied on silica gel-G, titanium (IV) tungstate and silica gel-G- titanium (IV) tungstate admixture layers using 0.1 M solutions of oxalic acid, citric acid, tartaric acid, succinic acid and acetic acid as background electrolyte. The mechanism of migration is explained in terms of adsorption and the solubility of various sodium or potassium salts of the anions in water. Titanium (IV) tungstate behaves only as an adsorbent and not as an ion exchanger. Being a cation exchanger, there is no exchange phenomenon occurring with anions. The migration of halides increase linearly with an increase in the bare ion radii of these ions. Differential migration of the anions on silica gel-G layers led to binary, ternary and quaternary separations of similar anions such as F⁻ – Cl⁻ – Br⁻ – I⁻, I⁻ – IO₃⁻ – IO₄⁻, BrO₃⁻ – IO₃⁻ and Fe(CN)₆³⁻ – Fe(CN)₆⁴⁻. The two cyanoferrate ions are separated from industrial waste water and from fixer and bleach solutions. The migration of anions has also been found to be in accordance with their lyotropic numbers.

     

2, 2′-Dihydroxyazobenzene-based fluorescent system for the colorimetric ‘turn-on’ sensing of cyanide

2, 2′-Dihydroxyazobenzene (DHAB) demonstrated high sensitivity and low selectivity toward three anions: CN⁻, CO₃²⁻, and HCO₃⁻. In the presence of Cu(II), complex DHAB-Cu(II) could give rise to enhanced fluorescence intensity by about 45-fold at 590 nm and visible red-to-reddish orange color change upon the addition of cyanide by utilizing an indirect method, while no changes were observed in the presence of other anions, including F⁻, Cl⁻, Br⁻, I⁻, H₂PO₄⁻, CH₃COO⁻, NO₃⁻, CO₃²⁻ and HCO₃⁻, and SO₄²⁻, making the DHAB-Cu(II) complex a selective and sensitive cyanide chemosensor.     

Allosteric anion recognition by metal complexation of tris(bipyridine-imidazolium) ligand

A new tripodal imidazolium ligand 2 comprising three 2,2′-bipyridine-imidazolium subunits connected through a mesityl spacer has been synthesized and 1:1 complexes of 2 with Fe(II) and Ru(II) ions have been prepared. ¹H NMR spectroscopy including NOE analysis and molecular modeling study established that the complexes exist as a pseudocryptand type twisted structure in solution. The Fe(II) and Ru(II) complexes show strong 1:1 binding of Cl⁻, Br⁻, and I⁻ anions in MeCN-d₃ with a large enhancement of the guest selectivity (Cl⁻ > Br⁻ > I⁻) upon metal complexation.     

Anion exchange in trimethyl‐ and triphenyltin complexes with chromogenic ligands: solution equilibria and colorimetric anion sensing

A series of organotin(IV) compounds R₃Sn(A) where R = Me or Ph and A is a chromogenic nitrophenolate ligand were prepared and studied as possible colorimetric sensors for anions (F⁻, Cl⁻, Br⁻, AcO⁻, H₂PO₄⁻). Equilibrium constants for a complete set of reactions between R₃Sn(A) with A = 2‐amino‐4‐nitrophenolate (ANP) or 4‐nitrophenolate and anions (X⁻) involving formation of complexes R₃Sn(A)(X)⁻ and substitution products R₃Sn(X) and R₃Sn(X)₂⁻ were determined by UV‐vis and ¹H NMR titrations in MeCN and DMSO. The binding selectivity was AcO⁻ > F⁻ > H₂PO₄⁻ > Cl⁻ ≫ Br⁻ in both solvents and both for R = Me and Ph with higher affinity for R = Ph. Compounds with A = ANP were found to have the optimum properties as anion sensors allowing optical detection of F⁻, AcO⁻ and H₂PO₄⁻ anions in the 5–100 µM range by appearance of an intense absorption band of free ANP resulting from its substitution with the analyte. Selectivity and affinity of anion interactions with R₃Sn(ANP) are similar to those for thiourea receptors, but the organotin receptor produces a much larger naked eye detected optical signal, operates equally well in nonpolar and polar solvents and tolerates the presence of up to 20% vol. of water in DMSO. Copyright © 2011 John Wiley & Sons, Ltd.     

‘Naked-eye’ detection of F− ions by two novel colorimetric receptors

Two novel colorimetric receptors, S1 and S2, linked to one or two nitrophenylurea groups were synthesized in good yields, characterized and their chromogenic properties investigated towards various anions (F^?, Cl^?, Br^?, I^?, NO₃^?, AcO^?, ClO₄^?, HSO₄^?, and H₂PO₄^?) by UV–vis and ¹H NMR techniques. The receptors, effectively and selectively, recognized and distinguished the biologically important F^? from other anions such as Cl^?, Br^?, I^? etc. in CH₃CN. This selectivity could be easily observed by the naked eye, indicating that receptors S1 and S2 are potential colorimetric sensors for fluoride ion.