Electrostatic and Non‐covalent Interactions in Dicationic Imidazolium–Sulfonium Salts with Mixed Anions

A series of thioether‐functionalised imidazolium salts have been prepared and characterized. Subsequent reaction of the thioether‐functionalised imidazolium salts with iodomethane affords imidazolium–sulfonium salts composed of doubly charged cations and two different anions. Imidazolium–sulfonium salts containing a single anion type are obtained either by a solvent extraction method or by anion exchange. The imidazolium–sulfonium salts undergo a methyl‐transfer reaction on exposure to water, giving rise to a new, singly charged imidazolium salt with iodide introduced at the 2‐position of the imidazolium ring. Crystal structures of some of the imidazolium–sulfonium salts were determined by X‐ray crystallography providing the topology of the interactions between the dications and the anions. Electrospray ionization mass spectrometry and quantum‐chemical calculations were used to rationalise the relative strength of these interactions.     

Parallel synthesis of tri- and tetrasubstituted ureas from carbamoyl imidazolium salts

A method for producing tri- and tetrasubstituted ureas from carbamoyl imidazolium salts is presented. Carbamoyl imidazolium salts are prepared from the reaction of N,N carbonyldiimidazole (CDI) with secondary amines, followed by alkylation with iodomethane. These stable salts can be stored for extended periods and are effective electrophilic carbamoylation reagents. Primary and secondary amines add to carbamoyl imidazolium salts at room temperature to give tri- and tetrasubstituted ureas in excellent yields. This reaction was used to synthesize ureas using both liquid-liquid extraction and solid-phase extraction (cation exchange) purification techniques. Liquid-liquid extraction affords the product ureas more cleanly than cationic exchange. A series of urea compounds were synthesized using parallel synthesis techniques in high yields and with suitable purity for routine in vitro biological tests. These studies validate the utility of carbamoyl imidazolium salts as useful building blocks for combinatorial library synthesis.     

On the involvement of NHC carbenes in catalytic reactions by iridium complexes, nanoparticle and bulk metal dispersed in imidazolium ionic liquids

D/H exchange reactions at C2, C4 and C5 of the imidazolium cation were observed in catalytic hydrogenation reactions promoted by classical Ir(I) colloid precursors and [Ir(0)](n) nanoparticles dispersed in deuterated imidazolium ionic liquids indicating the participation of carbene species in this media. However, no D/H exchange reaction was observed in cyclohexene hydrogenation promoted by iridium bulk metal dispersed in the ionic liquid [BMI]-d(3).NTf(2). The D/H labeling experiments suggest that the ionic liquids interact with the metal centers preferentially as aggregates rather than isolated ions.     

Synthesis of novel spiro imidazolium salts as chiral ionic liquids

A library of 13 novel chiral spiro imidazolium salts has been synthesized. The effects of N-substituents and counteranions on the melting point of spiro bis(imidazolium) salts are studied in efforts toward the development of room temperature chiral ionic liquids.     

Estimation of the catalytic activity of onium chlorides and bromides in the alkaline hydrolysis of amino acids esters

Relations were investigated between the structure of a series of onium salts, rates of alkaline hydrolysis catalyzed by the salts of 4-nitrophenyl N-benzyloxycarbonylglycinate in a two-phase liquid-liquid system, and the value of the standard exchange enthalpy of anions in ion pairs with the catalyst cation calculated by the semiempiric PM3 method. The catalytic activity of ammonium, phosphonium, pyridinium, imidazolium, and benzimidazolium salts in a wide range of cation structures varies in parallel with the enthalpy of formation of the active form of the catalyst by the exchange of the anion with a hydroxide ion.     

Thermal degradation studies of alkyl-imidazolium salts and their application in nanocomposites

Increasing the thermal stability of organically-modified layered silicates is one of the key points in the successful technical application of polymer-layered silicate nanocomposites on the industrial scale. To circumvent the detrimental effect of the lower thermal stability of alkyl ammonium-treated montmorillonite, a series of alkyl-imidazolium molten salts were prepared and characterized by elemental analysis, thermogravimetry (TGA) and thermal desorption mass spectroscopy (TDMS). The effect of counter ion, alkyl chain length and structural isomerism on the thermal stability of the imidazolium salts was investigated. Alkyl-imidazolium-treated montmorillonite clays were prepared by ion exchange of the imidazolium salts with Na-montmorillonite. These organically-modified clays were characterized by X-ray diffraction (XRD), TDMS and thermogravimetry coupled with Fourier transform infrared spectroscopy (TGA-FTIR), and compared to the conventional quaternary alkyl ammonium montmorillonite. Results indicate that the counter ion has an effect on the thermal stability of the imidazolium salts, and that imidazolium salts with PF₆⁻, N(SO₂CF₃)₂⁻ and BF₄⁻ anions are thermally more stable than the halide salts. A relationship was observed between the chain length of the alkyl group and the thermo-oxidative stability; as the chain length increased from propyl, butyl, decyl, hexadecyl, octadecyl to eicosyl, the stability decreased. The results also show that the imidazolium-treated montmorillonite has greater thermal stability compared to the imidazolium halide. Analysis of the decomposition products by FTIR provides an insight about the decomposition products which are water, carbon dioxide and hydrocarbons.     

Direct Correlation Between the H/D Exchange Reaction and Conformational Changes of the Cation in Imidazolium-Based Ionic Liquid–D2O Mixtures

We present a study on the effects of deuterated water on the conformational equilibria of the imidazolium cation in aqueous mixtures of imidazolium-based ionic liquids at room temperature. We provide spectroscopic evidence that the conformational dynamics of the imidazolium cation in D₂O are directly related to the H/D exchange reaction of the C–H group at position 2 on the imidazolium ring. The relation is supported by comparing Raman spectra obtained from solutions prepared with H₂O and D₂O.     

Mechanisms of acid-base catalysis of beta-elimination reactions in systems activated by a pyridine ring

Beta-elimination reactions from 1 (in quinuclidine/quinuclidinium chloride, imidazole/imidazolium, and acetate/acetic acid buffers) and from 2 (in imidazole/imidazolium and acetate/acetic acid buffers) with formation of 4-vinylpyridine and 2-vinylpyridine, respectively, were studied. The results of a kinetic study of acid-base catalysis and H/D exchange are consistent with NH(+), the protonated substrate, as the species that undergoes carbon deprotonation with an E1cb mechanism. The comparison with previously studied reactions in acetohydroxamate/acetohydroxamic acid buffer confirms this assignment. The high proton activating factor, PAF, value observed (PAF = 1.2 x 10 (6) with isomer 1 in quinuclidine/quinuclidinium buffer) can be explained with the high stability by the resonance of the intermediate carbanion.     

Azolium-linked cyclophanes: effects of structure, solvent, and counteranions on solution conformation behavior

This paper describes the synthesis, structural characterization, and solution behavior of some xylyl-linked imidazolium and benzimidazolium cyclophanes decorated with alkyl or alkoxy groups. The addition of alkyl/alkoxy chains to the cyclophanes allows for studies in chlorinated solvents, whereas previous solution studies of azolium cyclophanes have generally required highly polar solvents. The azolium cyclophanes may exist in a syn/syn conformation (azolium rings mutually syn, arene rings mutually syn) or a syn/anti conformation (azolium rings mutually syn, arene rings mutually anti). The preferred conformation is significantly affected by (i) binding of bromide (ion pairing) to the protons on the imidazolium or benzimidazolium rings, which occurs in solutions of bromide salts of the cyclophanes in chlorinated solvents, and (ii) the addition of alkoxy groups to the benzimidazolium cyclophanes. These structural modifications have also led to cyclophanes that adopt conformations not previously identified for similar azolium cyclophane analogues. Detailed (1)H NMR studies for one cyclophane identified binding of bromide at two independent sites within the cyclophane.     

Synthesis and characterization of novel imidazolium-based ionic discotic liquid crystals with a triphenylene moiety

Two novel triphenylene-tethered imidazolium salts were synthesized either by the quaternization of 1-methylimidazole with an ω-bromo-substituted triphenylene or by the quaternization of a triphenylene-substituted imidazole with methyl iodide. Their chemical structures were determined by ¹H NMR, ¹³C NMR, IR, UV spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. These triphenylene-based imidazolium salts with bromide or iodide as counterion show columnar mesophase properties over a wide temperature range.     

Ionic interaction induced novel ordered columnar mesophases in asymmetric discotic triphenylene salts

Asymmetric triphenylene imidazolium salts with different spacer lengths were successfully synthesized through quarternization of ω-bromo-substituted triphenylenes with 1-methyl imidazole. The asymmetry in ω-bromo-substituted triphenylenes tended to destroy liquid crystallinity in the sample. However, highly ordered columnar mesophases with a lamellar microphase segregation were induced by ionic interactions among the imidazolium salts, and the lamellar morphology was visualized by transmission electron microscopy. On the basis of an X-ray diffraction study on shear oriented samples, a novel rectangular columnar phase with a plane group of pm was observed for a triphenylene imidazolium salt with a spacer length of C₁₁, while an oblique columnar phase was determined for a triphenylene imidazolium salt with a C₈ spacer. Due to the asymmetric molecular shape and ionic interactions in the triphenylene imidazolium salts, the columnar liquid crystalline phase was extended to below room temperature (c. -20°C) for samples with spacer lengths of C₈ and C₁₁.     

Reaction of Selectfluor (F-TEDA-BF4) with chloromethylated-DABCO monocation salts (X = BF4, NTf2) and other nitrogen bases (Et3N; piperidine; basic ionic liquid); unexpected formation of symmetrical [N−H−N] trication salts

Selectfluor reacts with N-chloromethylated DABCO monocation BF₄ or NTf₂ salts in MeCN (rt to 80 °C) to give symmetrical [N−H−N]⁺ trication salts. The same dimeric adducts are formed via the reaction of Selectfluor with Et₃N, piperidine, or a basic-IL (imidazolium with an alkyl-piperidine tether). The resulting stable salts were studied by multinuclear NMR, ¹⁵N/¹H HMBC, electrospray-MS, and by chemical reactivity. This hitherto unreported reactivity behavior contrasts the well documented ‘transfer fluorination’ by Selectfluor to quinuclidine and the quinuclidinic nitrogen of cinchona alkaloids.     

Supercritical CO2-ionic liquid mixtures for modification of organoclays

The use of supercritical CO(2) as solvent in the modification of montmorillonite by imidazolium and phosphonium ionic liquids bearing long alkyl chains (C(18)) known for their excellent thermal stability is described. The objective is to combine the environmentally friendly character of ionic liquids and supercritical carbon dioxide for the organophilic treatment of lamellar silicates. Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new surfactants for cationic exchange of layered silicates. Then, the synthesized phosphonium (MMT-P) or imidazolium (MMT-I) modified montmorillonites, cationically exchanged under supercritical carbon dioxide with or without co-solvent, have been analyzed by thermogravimetric analysis (TGA) and X-ray diffraction (XRD) and compared to montmorillonites treated by conventional cationic exchange.     

Abnormal ligand binding and reversible ring hydrogenation in the reaction of imidazolium salts with IrH(5)(PPh(3))(2)

We show that imidazolium salts do not always give normal or even aromatic carbenes on metalation, and the chemistry of these ligands can be much more complicated than previously thought. N,N'-disubstituted imidazolium salts of type [(2-py)(CH(2))(n)(C(3)H(3)N(2))R]BF(4) react with IrH(5)(PPh(3))(2) to give N,C-chelated products (n = 0, 1; 2-py = 2-pyridyl; C(3)H(3)N(2) = imidazolium; R = mesityl, n-butyl, i-propyl, methyl). Depending on the circumstances, three types of kinetic products can be formed: in one, the imidazole metalation site is the normal C2 as expected; in another, the metalation occurs at the abnormal C4 site; and in the third, C4 metalation is accompanied by hydrogenation of the imidazolium ring. The bonding mode is confirmed by structural studies, and spectroscopic criteria can also distinguish the cases. Initial hydrogen transfer can take place from the metal to the carbene to give the imidazolium ring hydrogenation product, as shown by isotope labeling; this hydrogen transfer proves reversible on reflux when the abnormal aromatic carbene is obtained as final product. Care may therefore be needed in the future in verifying the structure(s) formed in cases where a catalyst is generated in situ from imidazolium salt and metal precursor.     

Designing ionic liquids with boron cluster anions: alkylpyridinium and imidazolium [nido-C(2)B(9)H(11)] and [closo-CB(11)H(12)] carborane salts

A range of new alkylpyridinium and imidazolium carborane salts with [nido-C(2)B(9)H(12)](-), [closo-CB(11)H(12)](-), and [RC(2)B(11)H(11)](-) (R = methyl or butyl) anions have been prepared and characterized by physical and thermal methods, including the solid state structures of five of the salts determined by single crystal X-ray diffraction. The tendency of the salts to form low-melting ionic liquids has been assessed; all the salts studied with [nido-C(2)B(9)H(12)](-) anions melted below 100 degrees C and, significantly, have melting points that are 25-85 degrees C lower than those of the corresponding [closo-CB(11)H(12)](-) analogs, demonstrating that a wider range of boron-rich ionic liquid materials can be readily accessed.     

Anellated N-heterocyclic carbenes: 1,3-Dineopentyl-benzimidazol-2-ylidene,structural aspects of C-protonated precursor salts and an AgCl complex

Access to 1,3-dineopentyl-benzimidazol-2-ylidene (1) by deprotonation of various benzimidazolium salts was studied. [Ag(1)Cl] was prepared from the corresponding benzimidazolium chloride. X-ray crystal structure analyses of benzo-, naphtho- and quinoxalino-anellated imidazolium salts displayed neither significant changes nor systematic trends of bond lengths and angles within the five-membered ring. Consistently downfield shifts of the ¹³C¹H(2) nuclei reflect the influence of electron-withdrawing anellation.     

N‐Heterocyclic Carbene Boranes as Reactive Oxygen Species‐Responsive Materials: Application to the Two‐Photon Imaging of Hypochlorous Acid in Living Cells and Tissues

N‐Heterocyclic carbene (NHC) boranes undergo oxidative hydrolysis to give imidazolium salts with excellent kinetic selectivity for HOCl over other reactive oxygen species (ROS), including peroxides and peroxynitrite. Selectivity for HOCl results from the electrophilic oxidation mechanism of NHC boranes, which stands in contrast to the nucleophilic oxidation mechanism of arylboronic acids with ROS. The change in polarity that accompanies the conversion of NHC boranes to imidazolium salts can control the formation of emissive excimers, forming the basis for the design of the first fluorescence probe for ROS based on the oxidation of B?H bonds. Two‐photon microscope (TPM) ratiometric imaging of HOCl in living cells and tissues is demonstrated.     

Syntheses, structures and antimicrobial activities of bis(imino)acenaphthene (BIAN) imidazolium salts

The syntheses of four new bis(imino)acenaphthene (BIAN) imidazolium chlorides are reported, three of which have been structurally characterized. The synthesis of a new, structurally authenticated BIAN ligand is also described. We report the results of the use of these BIAN imidazolium salts as antimicrobials against the pathogens S. aureus, B. subtilis, E. coli and P. aeruginosa. The antimicrobial efficacies were particularly high for the N-(2,6-diisopropylphenyl)- and N-(mesityl)-substituted BIAN imidazolium salts (MIC values < 0.6 μg/mL).     

分子间作用力对咪唑盐介晶相性质的影响

通过对咪唑环1位(N1)取代烷基、3位(N3)取代基及阴离子的修饰合成了一系列具有近晶A (SmA)相的咪唑类离子液晶. 利用差示扫描量热法、单晶衍射、小角度X射线衍射等手段研究了咪唑盐的介晶相温度范围、介晶态的结构, 并测量了部分咪唑盐的各向异性导电率. 结果表明, 咪唑环N1取代烷基、N3取代基及咪唑盐的阴离子会改变分子间范德华力和氢键, 从而对咪唑盐的介晶相性质产生影响. 此外, 当乙烯基引入到咪唑环N3位置时, 咪唑盐相邻的层结构之间形成π-π堆积作用, 不仅有利于介晶态的形成, 同时使氟硼酸类离子液晶具有最大的层间距和最小的各向异性导电率. 这一结果表明, 调控离子液晶的性质时必须综合考虑各种分子间作用力的影响.     

Syntheses and photochromic studies of dithienylethene-containing imidazolium derivatives and their reactivity towards nucleophiles

A series of dithienylethene-containing imidazolium salts with various substituents on the 2-position of the imidazolium ring has been synthesized. The photochromic properties of these compounds have been studied, and the closed forms are found to be solvatochromic due to the donor-acceptor interaction with the solvent molecules. The closed form of the imidazolium salt shows a much higher affinity towards nucleophiles over the open form of the salt. A reaction pathway has been proposed to account for this reactivity difference based on the structure-property relationship, and the possible structure of the reaction product is discussed.