Bis-imidazolium Salt Research Articles

Base-catalyzed Effective C2-Amidation of Azolium Salts Using Isocyanates under Mild Conditions.

An unprecedented base-catalyzed hydroarylation of isocyanates with azolium salts was developed, which follows a simple reaction pathway and provided facile access to diverse C2-amidated azolium salts under mild conditions. Importantly, this methodology can also be applied for the successive C2-amidation of a bisimidazolium salt with two different isocyanates to provide the corresponding unsymmetrically substituted bisamide derivatives. Notably, the obtained amidated salts can also serve as a prominent carbene surrogate for the synthesis of metal-NHC complexes.

Photoinduced E to Z isomerization of tetraphenylethylene derivatives within organometallic supramolecular assemblies

Isolation of E-1,2-bis(4-bromophenyl)-1,2-diphenyl-ethylene from the E/Z isomer mixture obtained by a McMurry coupling reaction and reaction of this isomer with imidazole followed by N-alkylation with nBuBr and anion exchange yielded the bisimidazolium tetraphenylethylene (TPE) derivative H2-E-1(PF6)2. The reaction of H2-E-1(PF6)2 with Ag2O yielded the di-nuclear metallarectangle [Ag2(E-1)2](PF6)2 where the two bis-NHC donors E-1 bridge two silver atoms. Irradiation of [Ag2(E-1)2](PF6)2 leads to E/Z isomerization of the di-NHC ligand and formation of Z-1 in the mononuclear complex [Ag(Z-1)]PF6. Demetallation of the di-NHC ligand with NH4Cl/NH4PF6 yielded bisimidazolium salt H2-Z-1(PF6)2. The unique isomerization of the E-TPE derivative into its Z-isomer via metal complex formation/irradiation/demetallation cannot be achieved by irradiation of the individual imidazolium salt. The emissive properties of the TPE complexes [Ag2(E-1)2](PF6)2 and [Ag(Z-1)]PF6 have been investigated.

Imidazolium salts and [Pt(cod)2]: from NHC hydrido complexes to the unprecedented olefinic tetrahedral cluster [Pt4(μ-H)(cod)4]BF4.

Whereas the bis(imine)imidazolium salt 1·Cl is a potential precursor to a NimineCNHCNimine pincer-type ligand, it reacted with [Pt(cod)2] to give the PtII NHC hydrido complex 3, in which chloride coordination and CNHCNimine chelation is preferred over pincer formation. Unexpectedly, reaction of 1·BF4 with [Pt(cod)2] afforded the unprecedented, 56 CVE tetrahedral cluster [Pt4(μ-H)(cod)4]BF4 (7), which was fully characterized. Imidazolium salts are ubiquitous sources of the much studied NHC ligands and not expected to lead to the formation of metal clusters.

Effect of titanium oxide nanoparticles on the dielectric properties and ionic conductivity of a new smectic bis-imidazolium salt with dodecyl sulfate anion and cyanobiphenyl mesogenic groups

A new bis-imidazolium salt with two cyanobiphenyl groups and dodecyl sulfate counterion (BIC) was prepared by the metathesis reaction of the bromide salt with sodium dodecyl sulfate. The ILC shows at room-temperature a stable smectic A phase, confirmed by differential scanning calorimetry, polarized optical microscopy, and powder X-ray diffraction investigations. The ILC was doped with TiO2 nanoparticles, having a high specific area, in concentrations of 0.1%, 0.2%, 1% and 2%.Broadband Dielectric Spectroscopy spectra have been registered in the (10−1–107) Hz frequency range, and (250–330) K temperature domain.The study shows that at constant frequency, the conductivity increases with the temperature and dopant concentration. The characteristic times of the observed relaxation processes showed a temperature variation according to the Vogel-Fulcher-Tammann law. Higher values of characteristic times were calculated at the increase of the thickness of the sample. On the other hand, the increase of the concentration of TiO2 nanoparticles leads to a decrease of the characteristic relaxation times.In order to detect whether the obtained high permittivity increase is due to the Maxwell-Wagner or Electrode Polarization (EP) type phenomena, studies were made on samples of the same concentration, but different thicknesses, and EP was assigned as the main cause of this dielectric constant variation.

New Epoxy Thermosets Derived from a Bisimidazolium Ionic Liquid Monomer: An Experimental and Modeling Investigation

In this study, a novel polymerizable ionic liquid monomer (ILM) based on a bisimidazolium salt was first synthesized in order to substitute bisphenol A diglycidyl ether (DGEBA) as a starting material, avoiding the use of highly toxic Bis-imidazolium and carcinogenic bisphenol A and epichlorohydrin products. From ionic liquid such a new epoxy monomer, an epoxy network was prepared from the copolymerization between the bisimidazolium salt and isophorone diamine (IPD). Subsequently, the properties of this novel ionic liquid based polymer network have been investigated in terms of polyaddition reaction kinetics as well as the architecture-properties, i.e., thermal stability, surface properties, and the mechanical performances. In addition, for the first time, molecular dynamics simulations were combined with experimental measurements in order to confirm the experimental data as well to be able to predict the physical behaviors from their architecture, molecular scale structuration, and thermomechanical properties of the resulting network. Finally, the substitution of DGEBA by this ILM led to a thermosetting polymer with high thermal stability (up to 450 degrees C), hydrophobic behavior (21 mJ m(-2)), and promising mechanical performances (1.7 GPa) including a shape memory behavior.

Dinuclear Gold(I) Complexes Bearing N,N'-Allyl-Bridged Bisimidazolylidene Ligands.

A novel N,N’‐allyl‐bridged bisimidazolium salt and a novel dinuclear Ag(I) and a Au(I) NHC complex are reported. Both metallacyclic complexes have a twisted structural shape due to the rigid allylic system and form two different isomers relating to the position of the double bonds. The allyl‐group shows photoisomerisation, but no reactivity towards bases for the additional coordination of Pd(II).

Central N-heterocyclic carbene moieties in protic pincer-type bis(pyrazole) ligands: Perturbation on steric and electronic properties of ruthenium center

Chelation-assisted C–H bond cleavage of an N,N’-bis(pyrazol-3-ylmethyl)imidazolium salt with RuCl2(PPh3)3 and subsequent anion exchange afforded the cationic pincer-type ruthenium complex [RuCl(PPh3)2(5NCN-H2)]PF6 (3) bearing a five-membered N-heterocyclic carbene (NHC) furnished with two protic pyrazole arms (5NCN-H2). On the other hand, the uncharged dichlorido complex [RuCl2(PPh3)(6NCN-H2)] (4) with a six-membered carbene (6NCN-H2) was obtained by a similar treatment of an N,N′-bis(pyrazol-3-ylmethyl)perimidinium cation. X-ray analysis of 3 and 4 suggested that the different outcome is ascribed to the larger twist of the 6NCN-H2 pincer ligand. Cyclic voltammetry showed that these NHC-centered pincer complexes are oxidized more easily than the pyridine-centered analogue in line with the stronger electron-donating property of the central NHC unit in the pincer ligands. In contrast, the CO stretching frequency of the carbonyl complex [Ru(CO)(PPh3)2(5NCN)] (5) derived from the five-membered NHC complex 3 is unexpectedly higher than that of the pyridine analogue. Computational studies revealed that effective π-back donation to the carbonyl ligand in the NHC-centered pincer complex 5 is retarded by the twist of the pincer plane with six-membered chelation, which allows the d electrons of the metal to spread over the pyrazole π-orbitals.

Ultra-high pressure direct syntheses of bis(imidazolium-3-yl)alkane dichlorides

The ultra-high pressure Menshutkin reaction of N-substituted imidazoles and α,ω-dichloroalkanes provides access to a wide variety of bis(imidazoliumyl)alkane dichlorides in an efficient, one-pot process. Although the substitution reactions of dichloromethane at ambient pressure can be exceedingly slow, at 1.9 GPa, syntheses are usually quantitative in under 24 h. Reactions involving congested imidazole starting materials, that otherwise terminate after formation of the mono-imidazolium salt, are driven to the bis-imidazolium salt at 1.9 GPa. The rates of reaction increase with α,ω-dichloroalkane chain length, decrease with increasing bulk and imidazole N-substituent length, and the reactions of the more hindered imidazoles are more sensitive to the pressure-rate enhancement.

Macrometallocycle binuclear NHC silver(I) complex with bridging azobenzene: Synthesis, structure and recognition for hydrogen sulfate

One bis-imidazolium salt 2,2′-di[2’’-(N-methyl-imidazoliumyl)ethoxyl]azobenzene hexafluorophosphate (LH2·(PF6)2) and its macrometallocycle binuclear N-heterocyclic carbene silver(I) complex [(LAg)2](PF6)2 were prepared and characterized. The structure of [(LAg)2](PF6)2 was determined by X-ray single crystal diffraction, 1H NMR and 13C NMR spectroscopy. In complex [(LAg)2](PF6)2, one 34-membered macrometallocycle was formed by two biscarbene ligand L and two silver(I) ions. Additionally, the selective recognition of hydrogen sulfate using [(LAg)2](PF6)2 as a chemosensor was investigated on the basis of fluorescence titrations, ultraviolet titrations, 1H NMR titrations, HRMS and IR spectra. Complex [(LAg)2](PF6)2 was demonstrated to be an effective chemosensor for hydrogen sulfate.

Dielectric properties of a bisimidazolium salt with dodecyl sulfate anion doped with carbon nanotubes.

A new bisimidazolium salt with dodecyl sulfate as counterion has been designed and prepared. This salt shows a SmA phase that is stable at room temperature. The new ionic liquid crystal (ILC) was characterized by 1H NMR, 13C NMR and IR spectroscopy. Its liquid crystalline properties were analyzed by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) studies. The dielectric spectra of the ILC doped with different concentrations of carbon nanotubes (CNT) were recorded over a wide frequency and temperature range of 10−1 to 107 Hz and 293–338 K, respectively. The values of the activation energy were found in the range of 0.46–0.61 eV; the characteristic time was obtained by fitting the spectra of the dielectric loss with the Havriliak–Negami functions. As a result of doping the ILC with CNT, the electric conductivity increases significantly. Ionic conductivity is dominant and it was indirectly observed through the electrode polarization (EP) effect. The very high dielectric permittivity values and the decrease of the electric conductivity at low frequencies confirm the presence of EP.

Experimental and Theoretical Mechanistic Investigation on the Catalytic CO2 Hydrogenation to Formate by a Carboxylate-Functionalized Bis(N-heterocyclic carbene) Zwitterionic Iridium(I) Compound

The bis-imidazolium salt, 1,1-bis(N-methylimidazolium) acetate bromide, is a convenient precursor for the synthesis of zwitterionic iridium(I) [Ir(cod){(MeIm)2CHCOO}] and cationic iridium(III) [IrH(cod){(MeIm)2CHCOO}]+ compounds (MeIm = 3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized bis(NHC) ligand. The [Ir(cod){(MeIm)2CHCOO}] compound catalyzes the hydrogenation of CO2 to formate in water using NEt3 as base, reaching turnover numbers of approximately 1500. Reactivity studies have shown that activation of the catalyst precursor involves the reaction with H2 in a multistep process that under catalytic conditions results in the formation of a dihydrido iridium(III) octahedral [IrH2(H2O){(MeIm)2CHCOO}] species stabilized by the κ3-C,C′,O coordination of the ligand. DFT studies on the mechanism were carried out to elucidate two possible roles of the base. In the first one, NEt3 neutralizes only the produced formic acid, whereas in the second it assists the proton transfer in heterolytic cleavage of the H2 molecule. Although this base-involved mechanism is more favorable in that it exhibits a lower energy span for the overall reaction, the energy barrier obtained from kinetic experiments suggests that both mechanisms could be operative under the experimental reaction conditions.

Straightforward Preparation Method for Complexes Bearing a Bidentate N-Heterocyclic Carbene To Introduce Undergraduate Students to Research Methodology

A laboratory experiment for students in advanced inorganic chemistry is described. In this experiment, students prepare two metal complexes with a potentially bidentate-carbene ligand. The complexes are synthesized by reaction of a bis-imidazolium salt with silver(I) oxide or palladium(II) acetate. Silver and palladium complexes are binuclear and mononuclear species, respectively, in which the carbene ligand behaves in a different coordination mode. The reaction conditions are straightforward and the complexes air-stable species, even though the palladium compound must be prepared using air-free techniques. Students must also assign the signals in the 1H and 13C-{1H} NMR spectra. The complexes showed a fluxional behavior in solution as can be deduced from their 1H NMR spectra. The students are given the 3D structures of the complexes as an aid to understand this dynamic behavior. The students should also use the FAB–mass spectrum of the silver complex and the concept of isotopic pattern in order to discri...

Cubane Arrives on the Cucurbituril Scene.

Cubane, an intriguing chemical curiosity first studied in the early 1960s, has become a valuable structural motif and has recently been involved in the structures of a great number of prospective compounds. The first dicationic supramolecular guest 5 is prepared and derived from a 1,4-disubstituted cubane moiety, and its binding behavior toward cucurbit[n]urils (CBn) and cyclodextrins (CD) is studied. The bisimidazolium salt 5 forms 1:1 inclusion complexes with CB7, CB8, and β-CD with the respective association constants (6.7 ± 0.5) × 1011 M-1, (1.5 ± 0.2) × 109 M-1, and <102 M-1 in water. The solid-state structures of the 5@CB7 and 5@CB8 complexes are also reported.

Selective recognition of m-dinitrobenzene based on NHC silver(I) macrometallocycle

The bis-imidazolium salt 1,4-bis[N-(9′-anthracenyl-methyl)imidazoliumyl]butane hexafluorophosphate (L1H2·(PF6)2) and its N-heterocyclic carbene (NHC) silver(I) macrometallocycle [(L1Ag)2](PF6)2 (1) have been prepared and characterized. 18-Membered macrometallocycle of 1 is formed via two ligands (L1) and two silver(I) ions. The selective recognition of m-dinitrobenzene (DNB) using 1 as a host was studied on the basis of fluorescence titrations, 1H NMR titrations, HRMS spectra and IR spectra. The results indicate that 1 can distinguish effectively m-dinitrobenzene with other aromatic compounds.

Silver(I), nickel(II) N-heterocyclic carbene complexes based on bidentate bis-imidazolium salt with a quinoxaline linker: syntheses, structures, and characterization

Quinoxaline-bridged bidentate bis-imidazolium dicarbene ligand 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) hexafluorophosphate salt H2L·2PF6 (3) was prepared by a two-step reaction based on 2,3-bis(imidazol-1-yl)quinoxaline (1). First, the 2,3-bis(imidazol-1-yl)quinoxaline reacted with CH3I resulting in the 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) iodide salt H2L·2I (2), then through anion exchange reactions with NH4PF6 in water produced the desired bis-imidazolium bidentate ligand H2L·2PF6 (3). Reaction of the bidentate bis-imidazolium ligands H2L·2PF6 (3) with Ag2O in acetonitrile gave the macrocyclic binuclear silver(I) carbene complex [Ag2(L)2]·2PF6·CH3CN (4). Nickel carbene complex [Ni(L)PPh3Cl]·PF6·2DMSO (5) was obtained via transmetalation of 4 with Ni(PPh3)2Cl2 in DMSO. The bidentate carbene ligand is a chelating ligand in 5, while bridging in 4. The imidazolium ligand H2L·2PF6 (3) and transition metal carbene complexes 4 and 5 have been fully characterized by elemental analysis, NMR, ESI-MS spectroscopy, and X-ray diffraction analyses. Furthermore, the UV and luminescent properties of 3–5 were also studied.

Ionic iron(III) complex of aryloxo-linked bis(imidazolium) cations: efficient and recyclable catalyst for aryl Grignard cross-coupling of alkyl chlorides

A novel phenol-linked bis(imidazolium) salt, H3LCl2 (L = O-4-C(CH3)3-C6H2-2,6-di[CH2{C(NCHCHNAr)}]2, Ar = 2,6-diisopropylphenyl, 1), was designed and used to prepare an ionic iron(III) complex [H2L][FeCl4] (2). Complex 2 was a highly efficient catalyst for aryl Grignard cross-coupling of alkyl chlorides bearing β-hydrogens. Furthermore, complex 2 was reusable and could be reused in at least eight times without significant loss in catalytic activity.

Silver(I)-N-heterocyclic carbene complexes of nitrile-functionalized imidazol-2-ylidene ligands as anticancer agents

Reactions of symmetrically and non-symmetrically substituted nitrile-functionalized imidazolium salts (1–3) with silver(I) oxide in methanol at room temperature afforded complexes (4–6) of the type [NHC-Ag-NHC]PF6, (NHC: imidazol-2-ylidene). All reported compounds have been characterized by spectral (1H, 13C NMR and FTIR) and elemental analysis. The structure of bis-imidazolium salt 3 and silver complex 4 was unambiguously elucidated by the single-crystal X-ray diffraction method. The effect of substitutions on the anticancer activity of compounds 1–6 has been investigated by in vitro cytotoxicity studies against human colorectal (HCT 116) cancer cell line, using the MTT assay method. All three silver complexes (4–6) displayed promising anticancer activity with IC50 values of 6.0±0.2, 14.0±0.6 and 4.0±0.2μM, while imidazolium salts, 1–3, showed least (>200μM) to moderate (20.3±0.2 and 95.0±2μM) anticancer potential, respectively. Bis-imidazolium salt 3 and binuclear complex 6 displayed good activity against human breast (MCF-7) cancer line with IC50 values of 82.4±2.5 and 0.9±0.4μM, respectively.

Synthesis of a Pillar[5]arene with Both Hydroxyl and Methoxycarbonyl‐Methoxy Groups and Its Host‐Guest Complexation with a Bis(imidazolium) Salt

AbstractBy the introduction of methoxycarbonyl‐methoxy groups and hydroxyl groups into a pillar structure, a pillararene entirely with two types of functional groups was successfully prepared, which can form a stable 1:1 complex with a bis(imidazolium) salt in CHCl3/acetone solution (V:V=1:1).

Two Different, Metal-Dependent Coordination Modes of a Dicarbene Ligand

The bisimidazolium salt H2-1(PF6)2 featuring a bridging 1,4-phenylene group reacts with 0.5 equiv of [PdCl(allyl)]2 in the presence of Cs2CO3 to give the dinuclear complex [2](PF6)2, whereas the reaction of the same bisimidazolium salt with 0.5 equiv of [Ir(Cl)2(Cp*)]2 yields the mononuclear orthometalated complex [3]PF6 with one remaining imidazolium unit. The unreacted imidazolium group in complex [3]PF6, however, can also be metalated with RhIII to yield the doubly orthometalated heterobimetallic complex [4]. In complex [4], each MIII center (M = IrIII and RhIII) is coordinated by one NHC unit and orthometalates the central aryl ring of the ligand.

Synthesis, crystal structures, in vitro anticancer, and in vivo acute oral toxicity studies of bis-imidazolium/benzimidazolium salts and respective dinuclear Ag(I)-N-heterocyclic carbene complexes

The synthesis, spectral (FT-IR and NMR), and structural studies of 1,1′-methylene linked 3,3′-2-cyanobenzyl bis-imidazolium salt (L1) and respective dinuclear Ag(I)-NHC complex (C1) are reported. The structures of both compounds were established through single-crystal X-ray diffraction. C1 has a short Ag–Ag separation of 3.16 Å. Both L1 and C1 were tested for potential against leukemia (k562) cell line. For comparison, para-xylyl linked bis-benzimidazolium salts (L2–L4) and their dinuclear Ag(I)-NHC complexes (C2–C4) were synthesized and tested against the same cell line (K562). The IC50 values proved that L2–L4 and C2–C4 are many fold more active than L1 and C1. The mechanism of action and structure activity relationship are discussed. In vivo oral acute toxicity study (sighting study) was carried out which depicts that 2000 mg/kg dose of selected compounds is an appropriate and safe dose for conducting main study on animals.