Diimidazolium Salts Research Articles

Stirring or milling? First synthesis of Rh(I)-(di-N-heterocyclic carbene) complexes both in solution and in a ball mill

An environment-friendly, convenient, fast and solvent-free mechanochemical approach have been accomplished for the synthesis of several diimidazolium salts and the bridging dinuclear rhodium(I)–N-heterocyclic carbene complexes of the type [{RhCl(cod)}2(μ-di-NHC)] derived from them. The compounds were synthesized also by the classical solvent method and the results of the two approaches were compared. A systematic study of both the mechanochemical and the solvent syntheses has also been carried out to determine the effects of various factors influencing the reactions. This is the first report on the mechanochemical synthesis of poly-NHC metal complexes as well as NHC–Rh complexes in ball mill.

Task-Specific Organic Salts and Ionic Liquids Binary Mixtures: A Combination to Obtain 5-Hydroxymethylfurfural From Carbohydrates.

The increase in energy demand and depletion of fossil fuels are among major issues of modern society. Valorization and transformation of raw materials in products of industrial value represent a challenge. This justifies the growing interest of scientific research toward the identification of suitable media and methodologies able to pursue above goals, paying attention to matter of sustainability. On this subject, we studied sulfonic-acid functionalized diimidazolium salts as catalysts for the conversion of fructose and sucrose to 5-hydroxymethylfurfural (5-HMF) in an ionic liquid mixture. In general, using these salts allowed us to obtain 5-HMF in good yields from both substrates in mild conditions. Indeed, at 60°C and in the presence of 20 mol% of catalyst, 5-HMF yields of 60 and 30% were obtained from fructose and sucrose, respectively. The catalytic system was recycled and used up to six times observing no appreciable loss in yield for the first four cycles. Moreover, we gathered mechanistic information by in situ 1H NMR monitoring the dehydration of fructose. To dissect the role of acidity on the reaction, we determined the Hammett acidity function of each salt. Comparison of these results with yields and reactivity observed in the presence of related monocationic salts and with a dicationic salt bearing only one sulfonic acid group, allowed stating that the reactivity observed is the result of the combined action of acidity and structural features of the catalysts. Overall, the approach proposed here could contribute to pave the way to increase sustainability in the raw material valorization processes.

Ionic liquids gels: Soft materials for environmental remediation

HypothesisNanostructured sorbents and, in particular, supramolecular gels are emerging as efficient materials for the removal of toxic contaminants from water, like industrial dyes. It is also known that ionic liquids can dissolve significant amounts of dyes. Consequently, supramolecular ionic liquids gels could be highly efficient sorbents for dyes removal. This would also contribute to overcome the drawbacks associated with dye removal by liquid–liquid extraction with neat ionic liquids which would require large volumes of extractant and a more difficult separation of the phases. ExperimentsHerein we employed novel supramolecular ionic liquid gels based on diimidazolium salts bearing naturally occurring or biomass derived anions, to adsorb cationic and anionic dyes from wastewaters. We also carried out a detailed investigation of thermal, structural, morphological and rheological features of our gels to identify which of them are key in designing better sorbents for environmental remediation. FindingsThe most effective gels showed fast and thorough removal of cationic dyes like Rhodamine B. These gels could also be reused up to 20 times without any loss in removal efficiency. Overall, our ionic gels outperform most of gel-based sorbents systems so far reported in literature.

Insights into the anion effect on the self assembly of perylene bisimide diimidazolium salts

We studied how changing the anion affects the self-assembly of three diimidazolium salts bearing a perylene bisimide spacer. In particular we used [BF4−] and [NTf2−] based salts together with a mixed salt bearing both [I−] and [BF4−] anions. Thermal as well as concentration and temperature dependent spectroscopic investigations revealed that changing the anion impacts on solid state and solution aggregation, leading to articulate thermotropic behaviors and contrasting solvent effects, respectively. We also studied the electrochemical behavior of the salts to verify if changing the anion alters the redox properties of the salts. Finally, different anions also lead to very different aggregate morphologies, as evidenced by SEM and TEM measurements.

Formation of Dinuclear Iridium Complexes by NHC-Supported C–H Bond Activation

The formation of dinuclear iridium complexes by NHC-supported aryl-CH bond activation reactions is reported. Complex IrH5(PiPr3)2 (1) reacts with 1,3-bis(3-methylimidazolium)benzene bis(tetrafluoroborate) ([C6H4(HImMe)2][BF4]2) to give the dinuclear species {[IrH2(PiPr3)2]2[μ-C6H2(ImMe)2]} (2), with a core formed by five fused rings. Complex 1 also reacts with 1,1′-diphenyl-3,3′-alkylenediimidazolium dihalides ([(CH2)n(HImC6H5)2]X2; n = 2, 3, 4). The reactions lead to the dinuclear complexes {[IrH2(PiPr3)2]2[μ-(CH2)n(ImC6H4)2]} (n = 2 (3), 3 (4), 4 (5). These compounds can be described as two six-coordinate d6-iridium dihydride metal fragments containing an orthometalated phenylimidazolylidene, which are connected through an alkylene linker. In contrast to the diimidazolium salts, 1,1′-diphenyl-3,3′-methylenedibenzimidazolium dibromide ([CH2(HBzImC6H5)2]Br2) reacts with 1 to give a 1:1 mixture of the mononuclear complexes IrH2Br{κ-N-(HBzImC6H5)}(PiPr3)2 (6) and IrH2{κ2-C,C-(MeBzImC6H4)}(PiPr3)2 (7). The l...

The anion impact on the self-assembly of naphthalene diimide diimidazolium salts

Self-assembly behavior of naphthalene diimide diimidazolium salts was analyzed as a function of their anions. Changes in the anion nature significantly impact the properties of aggregates.

The effects of structural changes on the anti-microbial and anti-proliferative activities of diimidazolium salts

Anti-microbial and anti-proliferative activities of diimidazolium salts have been analyzed as a function of the main changes in their structural features.

Alcohol-Induced C-N Bond Cleavage of Cyclometalated N-Heterocyclic Carbene Ligands with a Methylene-Linked Pendant Imidazolium Ring.

Reaction of the pentamethylcyclopentadienyl rhodium iodide dimer [Cp*RhI2 ]2 with 1,1'-diphenyl-3,3'-methylenediimidazolium diiodide in non-alcohol solvents, in the presence of base, led to the formation of bis-carbene complex [Cp*Rh(bis-NHC)I]I (bis-NHC=1,1'-diphenyl-4,4'-methylenediimidazoline-5,5'-diylidene). In contrast, when employing alcohols as the solvent in the same reaction, cleavage of a methylene C-N bond is observed, affording ether-functionalized (cyclometalated) carbene ligands coordinated to the metal center and the concomitant formation of complexes with a coordinated imidazole ligand. Studies employing other 1,1'-diimidazolium salts indicate that the cyclometalation step is a prerequisite for the activation/scission of the C-N bond and, based on additional experimental data, a SN 2 mechanism for the reaction is tentatively proposed.

Insights into the Formation and Structures of Molecular Gels by Diimidazolium Salt Gelators in Ionic Liquids or "Normal" Solvents.

Insights are provided into the properties of molecular gels formed by diimidazolium salts both in "normal" solvents and ionic liquids. These materials can be interesting for applications in green and sustainable chemistry in which ionic liquids play a significant role, like catalysis and energy. In particular, two positional isomers of a diimidazolium cation have been examined with a wide range of anions for their ability to form gel phases. In particular, di-, tri-, and tetravalent anions bearing aliphatic or aromatic spacers were paired with the divalent cations. The properties of the organo- and ionogels formed have been analyzed by means of several different techniques, including calorimetry, rheology, resonance light scattering, UV/Vis absorption, polarizing optical microscopy, and powder X-ray diffraction measurements. The investigations performed enabled us to obtain a wide range of conductive materials characterized by a high thermal stability and a low corrosiveness of the gelator (organogels) or of both gelator and solvent (ionogels). The information gained should be useful in the broader quest to identify and promote their applications.

Anti-bacterial screening of water soluble carbonyl diimidazolium salts and its derivatives

We have examined the antibacterial screening against Gram positive and Gram negative pathogens with nearly eight different counter anions containing carbonyl diimidazolium salts. We have carried out antibactericidal studies under well diffusion and micro dilution as well as docking techniques for carbonyl diimidazolium salts. Counter anion Br− and CF3SO3− containing carbonyl diimidazolium salt with benzyl substituted compounds showed better inhibitory activities than the others. We have described the hydrogen bonding and binding energy between the proteins and our carbonyl diimidazolium salts under computer assisted docking analysis.

Self-assembly of fluorescent diimidazolium salts: tailor properties of the aggregates changing alkyl chain features

Self assembly of fluorescent diimidazolium NDI salts showed properties of aggregates changing with alkyl chain length, with an odd–even effect.

Functionalised diimidazolium salts: the anion effect on the catalytic ability

How is it possible to catalyze and simultaneously control the outcome of a reaction? Employing task specific ionic liquids and changing their anions.

Organic salts and aromatic substrates in two-component gel phase formation: the study of properties and release processes.

To identify gel phases able to act as confined reaction media or materials for the removal of organic pollutants, we studied two-component gel phases formed by naphthalenedisulfonate diimidazolium salts in the presence of some organic guests, in 1-propanol solution. Guests differing in π-surface area, bulkiness and electronic properties were taken into account. Soft materials obtained were investigated for their thermal stability, self-repairing ability and morphology. Furthermore, two-component gel phase formation was studied using resonance light scattering (RLS) measurements. Guest release processes from the gel phase were also studied. These processes were monitored as a function of time using both UV-vis and RLS measurements and considering important parameters such as the gelator concentration, the nature of extraction solvent and the extension of contact surface area between solvent and gel phase. Data collected shed light on the properties of the two-component gels and could represent a useful tool to better plan the application of these soft materials.

π-Conjugated diimidazolium salts: rigid structure to obtain organized materials.

Phenylene ethynylene based diimidazolium salts differing in the alkyl chain length borne on the imidazolium ion and anion nature were synthesized. Their properties were studied both in solution and in the solid state. Salts obtained were able to aggregate in organic solvent solution. Aggregate formation was studied by performing concentration dependent measurements using UV-vis, fluorescence and Resonance Light Scattering. Furthermore, features of the aggregates were also investigated in the solid state by means of fluorescence and Scanning Electron Microscopy measurements. Finally, Density Functional Theory calculations were performed to obtain insights into the interaction geometry in the salts investigated. Data collected evidence that aggregation processes are affected by a combined action of different factors derived from the nature of the salt and solvent. The above features also influence the morphology of the aggregates as well as the ability of their thin films to give blue emission. On the whole, information gained could represent a useful starting point for applications of these salts in the optoelectronic field among others.

Di-, tri-, and tetranuclear copper(I) complexes of phenanthroline-linked dicarbene ligands.

A family of 2,9-di(3-R-1H-imidazolium-1-yl)-1,10-phenanthroline iodides and hexafluorophosphates (R = allyl, benzyl, mesityl, picolyl) were synthesized from 2,9-diiodophenanthroline and imidazole or N-substituted imidazoles. Simple reactions of these diimidazolium salts with copper powder at room temperature have afforded a series of multinuclear copper(I)-NHC complexes in good yields. The structures vary depending on the N substituents and counterions. [Cu4(L1)2(MeCN)4](PF6)4 (R = allyl) exhibits a zigzag Cu4 chain with two terminal [Cu(NHC)(MeCN)2] and two internal [Cu(phen)(NHC)] moieties. [Cu3(L2)2](PF6)3 (R = benzyl) contains a strictly linear Cu3 framework with two [Cu(NHC)2] units and a [Cu(phen)2] located at the center. Both complexes [Cu3(L4)2](PF6)3 (R = mesityl) and [Cu3(L5)2](PF6)3 (R = picolyl) consist of a triangular Cu3 core in which two copper(I) ions are surrounded by a phen and a NHC group and the third copper(I) is coordinated by two NHC groups. [Cu3(L3)2](PF6)3 derived from 2,9-di(3-benzyl-1H-benzimidazolium-1-yl)-1,10-phenanthroline hexafluorophosphate can undergo transannulation of the benzimidazolylidene ring giving [Cu2(L3')2](PF6)2. The decomposition process might involve solvent-induced rearrangement of the ligand and hydrolysis of carbene moieties. Treatment of 2,9-di(3-mesityl-1H-benzimidazolium-1-yl)-1,10-phenanthroline iodide with copper powder generated dinuclear complex [Cu2(L4)2][Cu2(μ-I)2I2] (R = mesityl) possessing a macrocyclic cation and [Cu2(μ-I)2I2](2-) anion. Tetranuclear complex [Cu4(L5)2(μ-I)2](CuI3) (R = picolyl) has been obtained from its diimidazolium iodide and copper powder. All Cu(I) complexes have been characterized by X-ray single diffraction analysis, elemental analysis, and NMR spectra. Their redox behavior and fluorescent properties have also been studied.

Two‐Component Hydrogels Formed by Cyclodextrins and Dicationic Imidazolium Salts

AbstractIn the framework of our recent interest in studying the gelling behaviour of imidazolium salts, we took into account the possibility of obtaining two‐component hydrogels by combining cyclodextrins and dicationic imidazolium salts. In particular, we used diimidazolium salts differing in the alkyl chain length and in the size, shape, and coordination ability of the anion, namely the 3,3′‐di‐n‐decyl‐ and 3,3′‐di‐n‐dodecyl‐1,1′‐(1,4‐phenylenedimethylene)diimidazolium dibromide, and 3,3′‐di‐n‐dodecyl‐1,1′‐(1,4‐phenylenedimethylene)diimidazolium ditetrafluoroborate and dihexafluorophosphate. As far as cyclodextrins are concerned, to gain information on the effects of different sized cyclic oligosaccharides, we used both α‐ and β‐cyclodextrins. The interaction between the organic salts and the oligosaccharides was investigated by using polarimetry, competitive fluorescence measurements, and 1H NMR spectroscopic analysis. In the case of gel formation, properties of the obtained soft materials were studied by determining their thermodynamic stability, analyzing their optical properties by means of UV/Vis spectroscopy, and studying their morphology by using scanning electron microscopy. The gelation process was studied by resonance light scattering measurements and this approach has allowed us to gather information on the size of the aggregates characterizing the gel phases. Finally, the soft materials were also investigated with respect to their ability to self‐repair after the action of external stimuli, such as magnetic stirring or ultrasound irradiation. Taken together, the data collected demonstrate that the properties of these two‐component gels can be controlled by changing the nature of the cyclodextrin, the salts, or, more simply, by changing the host/guest ratio.

The Gelling Ability of Some Diimidazolium Salts: Effect of Isomeric Substitution of the Cation and Anion

AbstractThe gelling ability of some geminal imidazolium salts was investigated both in organic solvents and in water solution. Organic salts differing either in the cation or anion structure were taken into account. In particular, the effects on the gel‐phase formation of isomeric substitution on the cation or anion as well as of the use of mono‐ or dianions were evaluated. As far as the cation structure is concerned, isomeric cations, such as 3,3′‐di‐n‐octyl‐1,1′‐(1,4‐phenylenedimethylene)diimidazolium and 3,3′‐di‐n‐octyl‐1,1′‐(1,3‐phenylenedimethylene)diimidazolium, were used. On the other hand, in addition to the bromide anion, isomeric dianions, such as the 1,5‐ and 2,6‐naphthalenedisulfonate anions, were also examined. After preliminary gelation tests, different factors affecting the obtained gel phases, such as the nature of the solvent, organogelator concentrations, and action of external stimuli, were analyzed. The gel‐phase formation was also studied as a function of time, by using resonance light scattering measurements. Gel morphologies were analyzed by scanning electron microscopy. To further support the understanding of the different behavior shown by the isomeric cations, some representative ion pairs were analyzed by DFT‐based investigations. The collected data underline the significant role played by isomeric substitution of both cation and anion structures in determining the gelling capability of the investigated salts, as well as the properties of the gel phases. Finally, DFT investigations were helpful in the identification of the structural features affecting the self‐assembly.

Geminal Imidazolium Salts: A New Class of Gelators

The gelling behavior of some geminal diimidazolium salts was investigated in solvents differing in polarity and hydrogen bond donor ability. The used salts, namely the 3,3'-di-n-decyl-1,1'(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(decim)(2)][Br](2) (1), the 3,3'-di-n-dodecyl-1,1'(1,4-phenylenedimethylene)diimidazolium dibromide [p-Xyl-(dodecim)(2)][Br](2) (2), and the 3,3'-di-n-dodecyl-1,1'(1,4-phenylenedimethylene)diimidazolium ditetrafluoroborate [p-Xyl-(dodecim)(2)][BF(4)](2) (3), differ in the alkyl chain length and in the anion properties, such as size, shape, and coordination ability. In all cases in which gelation process was observed, the obtained gels were characterized by gel melting temperature determination, resonance light scattering, and UV-vis measurements. On the whole, the investigation allowed to get information about both the thermodynamic stability and the features of the aggregates characterizing the soft materials at the equilibrium. Data collected by us point out that the used organic salts are able to behave as both hydro- and organogelators. In particular, bromide salts formed hydrogels in the presence of α-cyclodextrin allowing to hypothesize that the gelation process is favored by the formation of supramolecular assemblies. To verify this hypothesis, 1D and 2D (1)H NMR measurements were carried out. Both the alkyl chain length and the anion ability to reticulate the three-dimensional network proved to be determinant factors in affecting the gelation process as well as the features of the gel phases. Finally, with the future aim to use the obtained gels as reaction media, the effect of a guest molecule such as the UV-vis active probe Nile Red was studied.

Mesomorphic and ion conducting properties of dialkyl(1,4-phenylene)diimidazolium salts

A new family of dialkyl(1,4-phenylene)diimidazolium salts has been synthesized. Salts containing triflate and bis(trifluoromethanesulfonyl)imide anions and different alkyl chains have been prepared. The molecular structures of didodecyl(1,4-phenylene)diimidazolium triflate and bis(trifluoromethylsulfonyl)imide salts were determined by single crystal X-ray diffraction, and the mesomorphic and ionic conducting properties of all these compounds were investigated and are reported here.

Smooth C(alkyl)–H bond activation in rhodium complexes comprising abnormal carbene ligands

Rhodation of trimethylene-bridged diimidazolium salts induces the intramolecular activation of an alkane-type C-H bond and yields mono- and dimetallic complexes containing a formally monoanionic C,C,C-tridentate dicarbene ligand bound to each rhodium centre. Mechanistic investigation of the C(alkyl)-H bond activation revealed a significant rate enhancement when the carbene ligands are bound to the rhodium centre via C4 (instantaneous activation) as compared to C2-bound carbene homologues (activation incomplete after 2 days). The slow C-H activation in normal C2-bound carbene complexes allowed intermediates to be isolated and suggests a critical role of acetate in mediating the bond activation process. Computational modelling supported by spectroscopic analyses indicate that halide dissociation as well as formation of the agostic intermediate is substantially favoured with C4-bound carbenes. It is these processes that discriminate the C4- and C2-bound systems rather than the subsequent C-H bond activation, where the computed barriers are very similar in each case. The tridentate dicarbene ligand undergoes selective H/D exchange at the C5 position of the C4-bound carbene exclusively. A mechanism has been proposed for this process, which is based on the electronic separation of the abnormal carbene ligand into a cationic N-C-N amidinium unit and a metalla-allyl type M-C-C fragment.