Formation Of N-heterocyclic Carbenes Research Articles

N-Heterocyclic Carbene Formation in the Ionic Liquid [EMIM+][OAc-]: Elucidating Solvation Effects with Reactive Molecular Dynamics Simulations.

Recent experimental and theoretical work has debated whether N-heterocyclic carbenes (NHCs) are natively present in imidazolium-based ionic liquids (ILs) such as 1-ethyl-3-methylimidazolium acetate ([EMIM+][OAc-]) at room temperature. Because NHCs are powerful catalysts, determining their presence within imidazolium-based ILs is important, but experimental characterization is difficult due to the transient nature of the carbene species. Because the carbene formation reaction involves acid-base neutralization of two ions, ion solvation will largely dominate the reaction free energy and thus must be considered in any quantum chemical investigation of the reaction. To computationally study the NHC formation reaction, we develop physics-based, neural network reactive force fields to enable free energy calculations for the reaction in bulk [EMIM+][OAc-]. Our force field explicitly captures the formation of NHC and acetic acid by deprotonation of a EMIM+ molecule by acetate and in addition describes the dimerization of acetic acid and acetate. Using umbrella sampling, we compute reaction free energy profiles within the bulk IL and at the liquid/vapor interface to understand the influence of the environment on ion solvation and reaction free energies. Compared to reaction of the EMIM+/OAc- dimer in the gas phase, the bulk environment destabilizes formation of the NHC as expected due to the large ion solvation energies. Our simulations reveal a preference for the product acetic acid to share its proton with an acetate in solution and at the interface. We predict NHC content in bulk [EMIM+][OAc-] to be on the order of parts-per-million (ppm) levels, with order-of-magnitude enhancement of NHC concentration at the liquid/vapor interface. The interfacial enhancement of NHC content is due to both poorer solvation of the ionic reactants and solvophobic stabilization of the neutral NHC molecule at the liquid/vapor interface.

Thermal stability and decomposition mechanism of dicationic imidazolium-based ionic liquids with carboxylate anions

This work presents the synthesis, thermal stability, and decomposition mechanisms of the dicationic ionic liquids (DILs) [Bis(CnMIM)][CmCOO]2 (in which n = 4 and 10, and m = 3 and 6). Short-term thermal stability was determined using conventional thermogravimetric analysis, while long-term thermal stability was studied through the kinetic analysis of thermal decomposition, using isothermal and isoconversional methodologies to determine the kinetic triplet (activation energy[Ea], pre-exponential factor [A], and kinetic model [f(α)]). For the isothermal methodology, several kinetic models were applied to determine which best fit the experimental data. In the isoconversional approach, the methods of Friedman, Osawa-Flynn-Wall and Kissinger-Akahira-Sunose were used, together with the Ea values. When the Ea values did not vary with the variation in α, the compensation effect was used to calculate lnA and f(α). [Bis(C4MIM)][C6COO]2 was found to be the stablest of the DILs analyzed, and there was a relationship between thermal stability and molar mass in both the isothermal and isoconversional methodologies, in which an increase in the cation's spacer chain led to a decrease in stability. Additionally, the results showed that there was more than one reaction occurring in the decomposition process. The thermal decomposition mechanism of the DILs was determined by analyzing the thermal decomposition residue, using electrospray ionization mass spectroscopy, nuclear magnetic resonance, and thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy. The results indicated that [Bis(C4MIM)][C3COO]2 and [Bis(C10MIM)][C3COO]2 decompose mainly via nucleophilic substitution reactions, but also through formation of N-heterocyclic carbenes and elimination reactions.

Synthesis and chiroptical properties of organometallic complexes of helicenic N-heterocyclic carbenes.

Novel [4, 6]helicenes (4a,b) bearing a fused imidazolium unit have been prepared from [4, 6]helicene-2,3-di-n-propyl-amines 3a,b. The in situ formation of N-heterocyclic carbene (NHC) derivatives followed by their complexation to iridium(I) or rhodium(I) gave access to complexes 1a, 1'a, and 1b, containing mono-coordinated helicene-NHC, chloro and COD (COD = 1,5-cyclooctadiene) ligands. Ir and Rh complexes 1a and 1'a were characterized by X-ray crystallography. HPLC and NMR analyses showed that Ir(I) complex 1b existed as a mixture of two diastereomeric complexes corresponding to enantiomeric pairs M-(-)/P-(+)-1b1 and M-(-)/P-(+)-1b2 which differ by the position of COD through space. The chiroptical properties (electronic circular dichroism and optical rotation) of the four stereoisomers were measured. These complexes were also tested as catalysts in a transfer hydrogenation reaction.

Triazolium-Based Ionic Liquids: A Novel Class of Cellulose Solvents

We present first results on triazolium-based ionic liquids (ILs) as a novel class of nonderivatizing solvents for cellulose. Despite their chemical similarity to imidazolium cations, the 1,2,3-triazolium cation lacks the isolated ring proton, leading to reduced formation of N-heterocyclic carbenes (NHCs) and therefore to lower reactivity and less unwanted side reactions. We synthesized six ILs based on 1,2,3-triazolium and 1,2,4-triazolium cations. The acetates are room-temperature ionic liquids and dissolve a similar amount of cellulose as the corresponding imidazolium salt. From NMR spectroscopy of the solution, we rule out polymer degradation. The cellulose solubility rises with increasing anion basicity, while being almost independent of the cation. We perform molecular dynamics simulations and compute enthalpies of solvation, which quantitatively fit the experimental solubilities. Trajectory analysis reveals strong hydrogen bonds between acetate anions and cellulose hydroxyl groups, while the cations do not form strong hydrogen bonds with cellulose. Thus, the simulations provide an atomistic explanation of our experimental observations.

Restricting shuttling in bis(imidazolium)…pillar[5]arene rotaxanes using metal coordination.

Metal coordination to a series of bis (imidazolium)…pillar[5]arene [2]rotaxanes through the formation of metal-carbene bonds facilitates a new strategy to restrict the shuttling motion in [2]rotaxanes. Whereas the pillar[5]arene macrocycle rapidly shuttles along the full length of the bis (imidazolium) rod for the parent [2]rotaxane, Ag(i) coordination to the imidazolium groups through the formation of N-heterocyclic carbenes leads to restricted motion, effectively confining the shuttling motion of the [2]rotaxane. The Ag(i) coordinated [2]rotaxanes can be reacted further, either removing the Ag-carbene species to recreate the parent [2]rotaxane, or reaction with more bulky Pd(ii) species to further restrict the shuttling motion through steric inhibition.

Is the formation of N-heterocyclic carbenes (NHCs) a feasible mechanism for the distillation of imidazolium ionic liquids?

We describe the synthesis of two tetrachloroindate ionic liquids used as probes to study the involvement of NHCs (N-heterocyclic carbenes) in the distillation of imidazolium derivatives. Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS), electrospray ionization mass spectrometry (ESI-MS), atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS) and laser-induced acoustic desorption (LIAD) were used to depict the possibility of the involvement of NHCs during the distillation process. Each type of imidazolium derivative showed a particular mechanism of distillation, pointing firmly to the dependence of both the cation and the anion natures to distil as ion pairs or NHCs. Ionic liquid 1-n-butyl-3-methylimidazolium tetrachloroindate (1a) exhibited a preference to distil as ion pairs, whereas 3,3'-(ethane-1,2-diyl)bis(1-methyimidazolium)bis-tetrachloroindate (1b) may react with the Lewis acid anion, affording a bidentate NHC complex to distil. Thermodynamics, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses of the ionic liquid 1a were also conducted and helped understand the preference for ion pairs instead of NHCs. The performed theoretical calculations did not forwent the possibility of NHC formation; however, they clearly indicated the high stability of the anions (Lewis acids in nature) and also indicated that the possible reaction between NHC and the anion is not favoured. The calculated thermodynamic values were in accordance with the features observed by MS and indicated ion pairs as the feasible species for the distillation of imidazolium-based ionic liquids.

The influence of substituent field and resonance effects on the ease of N-heterocyclic carbene formation from imidazolium rings.

Using a set of twelve selected substituents, the influence of substituent properties on the ease of deprotonation of imidazolium cations and mesoionic imidazolium-4-olates measured by the CREF index has been investigated. Significant correlations between CREF values and the Swain and Lupton field (F) and resonance (R) substituent constants have been found. In all cases the field effect has the greatest influence but resonance effects are also significant.

Evidence for the spontaneous formation of N-heterocyclic carbenes in imidazolium based ionic liquids.

We present a study of the reactions of aldehydes in ionic liquids which gives evidence for the spontaneous formation of N-heterocyclic carbenes in ionic liquids based on 1,3-dialkyl substituted imidazolium cations from the lack of a deuterium isotope effect on the reaction of these ionic liquids with aldehydes.

N-heterocyclic carbene formation induced fluorescent and colorimetric sensing of fluoride using perimidinium derivatives.

In this study, two perimidinium derivatives (1 and 2) were designed, synthesized, and developed as efficient fluorescent and colorimetric chemodosisensors for F(-) in DMSO or more competitive media (DMSO containing 10 % water). In the presence of F(-) , the yellow and non-fluorescent solution of 1/2 became colourless and exhibited strong blue fluorescence. This unique spectroscopic behaviour of 1/2 towards F(-) was attributed to the formation of N-heterocyclic carbene deprotonated by F(-) , which immediately reacted with water to give a colourless and fluorescent carbinol. Interestingly, it was found that this carbinol intermediate was unstable and further underwent a redox disproportionation to generate two other optically changed compounds. All the proposed mechanisms for the sensing process have been carefully confirmed by experiments.

Formation of N-Heterocyclic Carbene–Boryl Radicals through Electrochemical and Photochemical Cleavage of the B–S bond in N-Heterocyclic Carbene–Boryl Sulfides

The B-S bond in N-heterocyclic carbene (NHC)-boryl sulfides can be cleaved homolytically to NHC-boryl or NHC-thioboryl and thiyl radicals using light, either directly around 300 nm or with a sensitizer at a longer wavelength (>340 nm). In contrast, the electrochemical reductive cleavage of the B-S bond is difficult. This easy photolytic cleavage makes the NHC-boryl sulfides good type I photopolymerization initiators for the polymerization of acrylates under air.

O-Glycosidation reactions promoted by in situ generated silver N-heterocyclic carbenes in ionic liquids

We herein report O-glycosidation reactions promoted via silver N-heterocyclic carbene complexes formed in situ in ionic liquids. Seven different room temperature ionic liquids were screened for the glycosidation reaction of 4-nitrophenol with tetra- O-acetyl-α- d-galactopyranosyl bromide. Good to excellent yields were obtained using Ag–NHC complexes derived from imidazolium halide salts to promote the glycosidation reaction, whereas yields considered moderate to low were obtained without use of the silver carbene complex. Anion metathesis of the ionic liquids with inexpensive alkylammonium halides also resulted in silver N-heterocyclic carbene formation and subsequent O-glycosidation in the presence of silver carbonate. Effective utility of this methodology has been demonstrated with biologically relevant acceptors (including flavones and steroids) where O-β-glycoside products were obtained selectively in moderate to good yields. We have also demonstrated that the Ag–NHC complex is a superior promoter to traditionally used silver carbonate for the glycosidation of polyphenolic acceptors. The ionic liquids used in the study could be recycled three times without apparent loss in activity.

Carbenes in ionic liquids

The chemistry of 1,3-dialkylimidazolium-based ionic liquids (ILs) can easily be linked to that of N-heterocyclic carbenes (NHCs) in the presence of sufficiently basic counteranions. B3LYP/6-31+G*, B3LYP/aug-cc-pVTZ and MP2/6-311+G** studies show that increasing the basicity of the anionic component the relative stability of the ion pair and that of the hydrogen bonded complex of the corresponding free acid and NHC itself can be shifted toward the formation of NHC. In the case of the acetate anion, the ion pair and the NHC-acetic acid complex have similar stability. Photoelectron spectroscopic studies show that the vapor of EMIM-acetate is dominated by the NHC-acetic acid complex. The mass spectrum of the same compound shows the presence of both acetic acid and 1-ethyl-3-methylimidazolium-2-ylidene, in agreement with the low pressure during the MS experiment, which facilitates dissociation. The possibility of systematic and simple variation of the NHC content of the ILs facilitates the extension of carbene chemistry in ionic liquids.

Concomitant Formation of N-Heterocyclic Carbene−Copper Complexes within a Supramolecular Network in the Self-Assembly of Imidazolium Dicarboxylate with Metal Ions

A new building block containing an imidazolium salt was synthesized and used for the construction of supramolecular networks with metal ions. We discovered the concomitant formation of the N-heterocyclic carbene-copper complex (CN = 2) in the self-assembly of imidazolium dicarboxylates and copper nitrates in N,N-dimethylformamide under heating. The proton in the 2 position of the imidazolium salt was abstracted, and Cu(II) was reduced to Cu(I) during the self-assembly process.

Alkene Hydroformylation Catalyzed by Rhodium Complexes in Ionic Liquids: Detection of Transient Carbene Species

D/H exchange reactions at the imidazolium cation of an ionic liquid were observed in hydroformylation reactions of 1-octene catalyzed by [Rh(acac)(CO)2](phosphine ligand) dissolved in 1-n-butyl-3-methylimidazolium-d3 N-bis(trifluoromethanesulfonyl)imidate at 75 °C under 5 atm (constant pressure) of CO/H2 (1:1). This D/H exchange occurs preferentially at the C2 position and occurs essentially in the presence of the phosphine ligand and the rhodium precursors, and it is more pronounced in the presence of bases, even a very weak one such as methanol. The presence of phosphine is also essential for the hydroformylation reaction. This result suggests the formation of N-heterocyclic carbenes (NHCs) in the biphasic hydroformylation reactions promoted by Rh complexes in imidazolium ionic liquids. These carbene species are easily displaced by the other species present in the ionic liquid, and they do not cause any significant changes in the catalytic activity or selectivity of the hydroformylation reaction.

Nanoclusters in Ionic Liquids: Evidence for N-Heterocyclic Carbene Formation from Imidazolium-Based Ionic Liquids Detected by 2H NMR

The mystery of how 1,3-substituted imidazolium-based ionic liquids (ILs) can provide high stabilization for transition-metal(0) nanoclusters, that is, in the absence of the usual strongly coordinating anions, has been probed. 2H NMR product and kinetic studies of 1,3-substituted imidazolium ILs under D2 reveal that nanocluster-catalyzed H/D exchange occurs at the 2- (as well as at the 4-, 5-, and 8-) C-H positions of the imidazolium cation. The results (i) provide compelling evidence that N-heterocyclic carbene formation and ligation of nanoclusters is occurring in ILs; and (ii) argue that N-heterocyclic carbenes merit further investigation as heretofore unappreciated stabilizers of transition-metal nanoclusters.