N-heterocyclic Carbene Research Articles

4-Halomethyl-Substituted Imidazolium Salts: A Versatile Platform for the Synthesis of Functionalized NHC Precursors.

N,N'-Diarylimidazolium salts containing haloalkyl functional groups that are reactive with various nucleophiles are considered to be promising reagents for the preparation of functionalized N-heterocyclic carbene (NHC) ligands, which are in demand in catalysis, materials science, and biomedical research. Recently, 4-chloromethyl-functionalized N,N'-diarylimidazolium salts became readily available via the condensation of N,N'-diaryl-2-methyl-1,4-diaza-1,3-butadienes with ethyl orthoformate and Me3SiCl, but these compounds were found to have insufficient reactivity in reactions with many nucleophiles. These chloromethyl salts were studied as precursors in the synthesis of bromo- and iodomethyl-functionalized imidazolium salts by halide anion exchange. The 4-ICH2-functionalized products were found to be unstable, whereas a series of novel 4-bromomethyl functionalized N,N'-diarylimidazolium salts were obtained in good yields. These bromomethyl-functionalized imidazolium salts were found to be significantly more reactive towards various N, O and S nucleophiles than the chloromethyl counterparts and enabled the preparation of previously inaccessible heteroatom-functionalized imidazolium salts, some of which were successfully used as NHC proligands in the preparation of Pd/NHC and Au/NHC complexes.

A Strongly Reducing sp2 Carbon‐Conjugated Covalent Organic Framework Formed by N‐Heterocyclic Carbene Dimerization

AbstractCovalent organic frameworks linked by carbon‐carbon double bonds (C=C COFs) are an emerging class of crystalline, porous, and conjugated polymeric materials with potential applications in organic electronics, photocatalysis, and energy storage. Despite the rapidly growing interest in sp2 carbon‐conjugated COFs, only a small number of closely related condensation reactions have been successfully employed for their synthesis to date. Herein, we report the first example of a C=C COF, CORN‐COF‐1 (CORN=Cornell University), prepared by N‐heterocyclic carbene (NHC) dimerization. In‐depth characterization reveals that CORN‐COF‐1 possesses a two‐dimensional layered structure and hexagonal guest‐accessible pores decorated with a high density of strongly reducing tetraazafulvalene linkages. Exposure of CORN‐COF‐1 to tetracyanoethylene (TCNE, E1/2=0.13 V and −0.87 V vs. SCE) oxidizes the COF and encapsulates the radical anion TCNE⋅− and the dianion TCNE2− as guest molecules, as confirmed by spectroscopic and magnetic analysis. Notably, the reactive TCNE⋅− radical anion, which generally dimerizes in the solid state, is uniquely stabilized within the pores of CORN‐COF‐1. Overall, our findings broaden the toolbox of reactions available for the synthesis of redox‐active C=C COFs, paving the way for the design of novel materials.

Enantio- and Diastereoselective Desymmetrization of 1,1'-Biaryl-2,6-Dicarbaldehydes by Copper-Catalyzed 1,2-Addition of Silicon Nucleophiles.

A desymmetrizing 1,2-addition of silicon nucleophiles to biaryl derivatives containing an 2,6-dicarbaldehyde-1-yl unit is reported. The reaction is catalyzed by copper with a triazolium-ion-derived N-heterocyclic carbene as the chiral ligand and makes use of an Si-B reagent as the silicon pronucleophile. The practical methodology furnishes axially chiral aromatic carbaldehydes decorated with a centrally chiral α-hydroxysilane moiety in moderate to good yields and with high enantio- as well as excellent diastereoselectivities. The silicon nucleophile always attacks at the diastereotopic face of either carbonyl group away from the ortho substituent on the phenyl ring at C1 of the 2,6-dicarbaldehyde-1-yl fragment. The resulting axially and centrally chiral products can be further converted into valuable biaryl compounds with hardly any erosion of the enantiomeric excess.

A Crystalline Mesoionic Diazasilole Featuring Low-Valent Silicon.

A 1,4,2-diazasilole containing a low-valent silicon atom has been synthesized employing a bulky imino N-heterocyclic carbene ligand. This molecular structure is characterized by a mesoionic C2N2Si five-membered ring, notable for its delocalized π electrons, intrinsic charge-separated zwitterionic properties, and a distinctly nucleophilic silicon center, culminating in 6π aromaticity. This compound manifests either mesoionic silylene or silylone characteristics upon coordination with transition metals. Demonstrating extraordinary versatility, this compound engages in diverse reactions such as coordination with iron or iridium, oxidation by S8, intramolecular ring saturation under the coordination influence of iridium, silicon atom transfer facilitated by Ph2Se2, ring contraction induced by Ph2Te2, and skeletal rearrangement triggered by Et3N•HCl. These reactions culminate in the formation of a variety of unprecedented silicon-based heterocycles, which are typically formidable to achieve using conventional methods. This study unveils previously unexplored facets of low-valent 1,4,2-diazasilole, positioning it as a promising foundational building block for future innovations in unique silicon compounds.

Formic acid dehydrogenation using Ruthenium-POP pincer complexes in ionic liquids

Formic acid is one of the most promising candidates for the long-term storage of hydrogen in liquid form. Herein, we present a new collection of ruthenium pincer complexes of the general formula [RuHCl(POP)(PPh3)] using commercially available or easy-to-synthesize tridentate xantphos-type POP pincer ligands. We applied these complexes in the dehydrogenation of formic acid to CO2 and H2 using the ionic liquid BMIM OAc (1-butyl-3-methylimidazolium acetate) as solvent under mild, reflux-free conditions. The best performing catalyst with respect to maximum turnover frequency, the literature-known complex [RuHCl(xantphos)(PPh3)] Ru-1, produced a maximum turnover frequency of 4525 h−1 with 74% conversion after 10 min at 90 °C and complete conversion (> 98%) occurring within 3 h. On the other hand, the best overall performing catalyst, the novel complex [RuHCl(iPr-dbfphos)(PPh3)] Ru-2, facilitated full conversion within 1 h leading to an overall turnover frequency of 1009 h−1. Moreover, catalytic activity was observed at temperatures as low as 60 °C. Only CO2 and H2 are observed in the gas phase, with no CO detected. High-resolution mass spectrometry suggests the presence of N-heterocyclic carbene complexes in the reaction mixture.

Engineering the Coordination Environment in the Silver(I)- and Ruthenium(II)-N-Heterocyclic Carbene Complexes in Instigating the Electrocatalytic Hydrogen Evolution Reaction.

The quest for cost-efficient and high-performance electrocatalysts towards electrocatalytic water splitting is a key and an interdisciplinary area of study. Considerable progress is being driven by developments in the field of energy research. In a fundamental study, we have synthesized NHC precursors (6 and 7) and corresponding metal-NHC complexes of silver(I)- (8 and 9) and ruthenium(II)- (10 and 11) of a N-heterocyclic carbene-based ligand type incorporating coumarins. These NHC precursors and metal-NHC complexes were characterized through various analytical and spectral techniques. The silver(I)-NHC complexes 8 and 9 displayed a linear coordination geometry with a center of inversion, which is evidenced by the single-crystal X-ray diffraction technique. Both the series of complexes were assessed for their efficacies in the hydrogen evolution reaction (HER). The results demonstrated that attributed to its peculiar coordination geometry, high electrical conductivity the silver(I)- and ruthenium(II)-NHC complexes exhibited exemplary electrocatalytic activity. Activities of the hydrogen evolution reaction on two differently modified electrode substrates with metal-NHC complexes have been studied. To attain the benchmark HER current density of 10 mA cm-2, in 1.0 M KOH, an overpotential of -375 to -527 mV vs RHE was required for the metal-NHC complexes. Based on the analysis of the Tafel slope values, the rate-determining step was the adsorption of hydrogen as investigated in the potential window. The molecular electrocatalyst 10 presented a superior stability and maintained the electrocatalytic activity for a duration of 18 h with complex 8 and 24 h with respect to complex 10 in 1.0 M KOH. Apace with these studies, hydrogen oxidation studies were examined in 0.5 M H2SO4 by a substantial current density at the platinum ring electrode. This research offers feasible guidance for developing organometallic-based molecular electrocatalysts with good electrocatalytic performance.

Pd/IPrBIDEA‐Catalyzed Synthesis of Pyrroles via Cascade C−C/C−F Cleavage/Annulation of gem‐Difluorocyclopropanes

AbstractA regioselective cascade C−C/C−F cleavage/annulation of gem‐F2CPs with enaminones catalyzed by Pd/IPrBIDEA is reported. The NHC ligand IPrBIDEA plays a pivotal role in enhancing the reaction efficiency. With this protocol, a variety of fully substituted pyrroles were obtained in good to excellent yields. Furthermore, this approach facilitates the modification of some bioactive scaffolds.

Photoluminescence of Tetra(benzimidazole)phenylethene Derivatives and Their Carbene Metallacycles in Aggregates and Langmuir-Blodgett Films.

Molecular structure, external stimuli, and environmental factors all have a strong effect on the internal molecular rotation and vibration of aggregate-induced emission (AIE) luminogens. Here, we report the AIE effect for several newly synthesized amphiphilic tetra(benzimidazole)phenylethene (TBiPE) derivatives and their binuclear N-heterocyclic carbene silver (NHC-Ag) metallacycles in solutions, aggregates, and Langmuir-Blodgett (LB) films. Monolayer behaviors and microscopic images indicate that introducing alkyl chains and binuclear NHC-Ag metallacycles can facilitate the formation of a well-defined insoluble monomolecular layer at the air-water interface. Absorption and luminescence spectral features suggest that both the TBiPEs' cyclization structure and binuclear NHC-Ag metallacycles can provide additional driving forces to restrict the internal molecular motion of the tetraphenylethene (TPE) unit, thus enhancing the AIE effects. Further, external stimuli and environmental factors such as poor solvent addition and LB film deposition also play important roles in the photoluminescent intensity, maximum wavelength, and lifetime. These internal and external factors can result in around 30-40 nm blue-shift for the maximum luminescence wavelength and 2-3 times shorter for the luminescent lifetime. These phenomena can be attributed to the reason that the nonradiative energy transfer efficiency is weakened because of the enhanced hydrophobic interaction and metal-carbene coordination as well as the closely packed arrangement of AIEgens in the LB films; consequently, the radiation energy transfer efficiency increased.

Catalytic Behavior of NHC–Silver Complexes in the Carboxylation of Terminal Alkynes with CO2

A number of N-heterocyclic carbene–silver compounds (NHCs)AgX were tested in the direct carboxylation of terminal alkynes using carbon dioxide as the C1 carbon feedstock. The reactions proceed at a pressure of 1 atm of CO2 at room temperature, in the presence of Cs2CO3, and using silver–NHC complexes as catalysts. Thus, phenylacetylene and several alkynes are converted to the corresponding propiolic acids in good to high conversions. The activity of the catalysts is strongly influenced by the substituents on the NHC backbone and the nature of the counterion. Specifically, the most active compound exhibits iodide as the counterion and is stabilized by a benzimidazole derivative. After 24 h of reaction, a quantitative conversion is obtained utilizing DMF as the solvent and phenylacetylene as the substrate.

Understanding the Role of NHC Ditopic Ligand Substituents in the Molecular Diversity and Emissive Properties of Silver Complexes.

Silver bis(carbene) complexes featuring ditopic N-heterocyclic carbene (NHC) ligands have been synthesized which enable the assembly of supramolecular architectures via reaction with silver triflate. Through a systematic exploration of crystal structures and emissive properties, this study investigates the impact of the substituents on the NHC ditopic ligands [R-Im-2-Z-py], where R = Me, benzyl (Bz), or 2-naphthylmethyl (NaphCH2), and Z = H or Cl in the structural framework and emissive properties observed in the silver bis(carbene) or polynuclear species. Remarkably diverse structural motifs emerge in both of them, predominantly influenced by the choice of R wingtip and second by the Z substituent. Also the emissive quantum yield of the complexes is mostly governed by the selection of the R wingtip and further modulated by the Z substituent. Several of the mono and polynuclear complexes exhibit complex emissive profiles, including the observation of dual emission phenomena. Particularly noteworthy is the complex [Ag(NHC)]2]OTf (R = Bz, Z = Cl), which demonstrates an exceptionally intense single blue emission with a remarkable solid-state quantum yield (Φ) of 24%.

Ruthenium(II) N-heterocyclic carbene polymer based sensors for detection of predatory drugs like ketamine and scopolamine.

The current demand in the field of abusive drug research is to have a highly sensitive and rapid method for detecting ketamine and scopolamine, which are frequently used in drug-facilitated sexual assaults administered via beverages and food. We report here the first electrochemical sensors of N-heterocyclic carbene (NHC) coordinated ruthenium organometallic 1-dimensional polymers and their multi-walled carbon nano tube (MWCNT)-based composite for detecting ketamine and scopolamine. The preparation of ruthenium NHC organometallics and the MWCNT composite as sensors offers significant advantages for electrochemical applications, with enhanced sensitivities of 121.979 and 3.273 μA μM-1 cm-2 for ketamine and scopolamine, respectively, and selectivity in sensing applications. The complex-carbon composite sensor has a low limit of detection i.e., 0.194 and 3.18 nM for ketamine and scopolamine identification, respectively. Alongside, the selectivity of the composite sensor was evaluated in the presence of other blood constituents, and the study evidenced remarkable discernment towards the title drugs. Furthermore, real-time analyses using the composite sensor demonstrated quantitative identification of ketamine and scopolamine. Therefore, this innovation has the potential to provide valuable tools for forensic investigations and address the urgent need for real-time detection of date rape drugs. This contribution demonstrates a robust proof-of-concept that emphasizes the importance of creating non-enzymatic, environmentally friendly, and cost-effective sensors for on-site sensing applications as point-of-care devices.

Anodic Desulfurization of Heterocyclic Thiones - A Synthesis to Imidazoles and Analogues.

An electrochemical desulfurization of 2-mercapto-imidazoles to the corresponding imidazole is established. This novel anodic transformation is bromide-mediated and easy to conduct in the simplest electrochemical setup, consisting of an undivided cell, carbon electrodes, and constant current electrolysis. The method proved successful in 14 diverse examples of imidazoles and triazoles with up to a 97% yield. The scalability was proven in the multigram synthesis of a technically relevant N-heterocyclic carbene (NHC) ligand precursor.

Atomically Precise Ternary Cluster: Polyoxometalate Cluster Sandwiched by Gold Clusters Protected by N‐Heterocyclic Carbenes

AbstractCoinage metal (Au, Ag, Cu) cluster and polyoxometalate (POM) cluster represent two types of subnanometer “artificial atoms” with significant potential in catalysis, sensing, and nanomedicine. While composite clusters combining Ag/Cu clusters with POM have achieved considerable success, the assembly of gold clusters with POM is still lagging. Herein, we first designedly synthesized two cluster structural units: an Au3O cluster stabilized by diverse N‐heterocyclic carbene (NHC) ligands and an amine‐terminated POM linker. The subsequent reaction involved amine substitution in the POM linker for the central O atom in the Au3O cluster, resulting in the first ternary composite cluster—a POM cluster sandwiched by two Au clusters protected by NHCs. Single‐crystal X‐ray diffraction and other characteristic methods characterized their atomically precise structures. Furthermore, altering the NHC ligands decreased the number of gold atoms in the sandwich structures, accompanying the different protonated degrees of amine ligand in the terminal end of the POM linker. These composite clusters showed excellent performances in catalytic H2O2 conversion through the synergistic effect between gold clusters and POM clusters. This work opens a new avenue to functional composite metal clusters and would promote their enhanced catalysis applications through intercluster synergistic interactions within composite systems.

Dipicolinate‐Ruthenium‐NHC/trpy Complexes: Pincer Ligand Induced Hemilability of Dipicolinate and Enhancement of Cytotoxic Property

AbstractA ruthenium(II) aquo complex [Ru(dipic)(PPh3)2(OH2)], (1 A), bearing O,N,O‐coordinated 2,6‐dipicolinate was synthesized via reaction of [Ru(PPh3)3Cl2] with 2,6‐dipicolinic acid. Crystal structure of 1 A was determined. Utilizing 1 A as a synthon, three new complexes were synthesized. Reaction of 1 A with a bidentate NHC‐ligand, viz. 1‐methyl‐3‐(2′‐pyridyl)imidazole (IMeCN), afforded [Ru(dipic)(IMeCN)(PPh3)], (2 A). Similar reactions of 1 A with two pincer ligands, viz. 2,6‐bis(1′‐methyl imidazolyl)pyridine (IMeCNC) and 2,2′;6′,2“‐terpyridine (trpy), respectively yielded [Ru(dipic)(IMeCNC)(PPh3)], (3 A) and [Ru(dipic)(trpy)(PPh3)], (3 B). Structure of 2 A was optimized by DFT method. Structures of 3 A and 3 B were determined by X‐ray crystallography. In both 3 A and 3B the 2,6‐dipicolinate ligand was found to be N,O‐coordinated with one carboxy end remaining unbound to the metal center. Anticancer property of 2 A, 3 A and 3 B was studied and compared. Cytotoxicity of 3 A, studied against three selected cancer cell lines, viz. chronic myelogenous leukemia cell line (K562), hepatocellular carcinoma cell line (HepG2) and breast adenocarcinoma cell line (MCF7), was found to be the most promising. It displayed the best activity against K562, while it did not have any appreciable effect on its normal counterpart, viz. the human peripheral blood mononuclear cells (hPBMC). The comparative cytotoxicity studies indicated that presence of the C,N,C‐coordinated pincer NHC ligand in 3 A, together with presence of the free carboxylate end of N,O‐coordinated dipic ligand, are presumably responsible for its superior cytotoxic behavior.

Birth of organocatalysis by N-heterocyclic carbenes

Birth of organocatalysis by N-heterocyclic carbenes

The Role of Spacer Length in Macrocyclization Reactions Under Confinement

We studied the influence of the distance of olefin metathesis catalysts from the inner surface of a mesoporous support on macrocyclization and Z‐selectivity under confinement. For these purposes, the cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) catalysts [Mo(N‐(2‐tBu‐C6H4)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)Br+ B(ArF)4‐] Mo2, [Mo(N‐(2‐tBu‐C6H4)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)OTf+ B(ArF)4‐] Mo3, [Mo(N‐(2,6‐Me2‐C6H3)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)Br+ B(ArF)4‐] Mo5 and [Mo(N‐(2,6‐iPr2‐C6H3)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)+Br B(ArF)4‐] Mo7 (B(ArF)4 = tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate), all containing a trimethoxysilylpropyl tether, were selectively immobilized inside the mesopores of SBA‐15. Under confinement, both macro(mono)cyclization (MMC) and Z‐selectivity were higher than in solution but lower than with catalysts directly bound to the surface of the mesoporous supports. These findings are in agreement with existing theoretical models on substrate and product distribution in mesopores, which suggest that the highest substrate concentration is found at the pore wall and that it increases with decreasing pore diameter.

The new pincer-type NHCs obtained by synthesizing Ag(I)-NHC complexes with various tails containing hydroxyl or acetate derivatives: Structural properties and in vitro antibacterial activities

This article discusses the synthesis, chemistry, and uses of transition metal complexes containing functionalized N-Heterocyclic carbenes (NHCs). NHCs were obtained by coupling functional group compounds containing ethyl acetate, propyl acetate, and ethanol to the 5,6-dimethylbenzimidazole compound. Thermogravimetric analysis (TGA), FT-IR, and solution (1H and 13C) NMR spectroscopy were used to characterize the elements in these compounds, and Rietveld analysis using X-ray powder diffraction (XPD) and ICP (Inductively Coupled Plasma) data was used to determine their structures. Major uses include catalyst modification and immobilization using a functional group to immobilize catalysts on a 5,6-dimethylbenzimidazole derivative support. The effects of the drug ampicillin were compared with the antimicrobial activities of 3a-c and 4a-c determined by the minimum inhibitory concentration (MIC; μg/ml). The MIC value of the compound bis(1,3-bis(3-ethoxy-3-oxopropyl)-5,6-dimethyl-1H-3λ4-benzo[d]imidazol-2-ylidene)silver(I)hexafluorophosphate (4b) was measured as 0.25 ± 2.05 μg/mL. This value indicates that this complex has strong antibacterial abilities.

UiO-67 MOF-Encapsulated NHC-Based Single-Site Copper Catalyst and Its Application in Regioselective Borylation of Terminal Alkynes.

N-Heterocyclic carbenes (NHCs) act as versatile ligand backbones due to their strong σ-donation and π-acceptance properties. However, the encapsulation of NHC-coinage metal complexes in a metal-organic framework (MOF) to utilize them in organic catalysis is rare. In this work, an NHC-coordinated CuBr (NHC = Bn2Im; 1,3-dibenzyl-imidazol-2-ylidene) complex was encapsulated in UiO-67 MOF ((Bn2Im)2CuBr@UiO-67) and further utilized toward the regioselective protoboration of terminal alkynes. (Bn2Im)2CuBr@UiO-67 was found to show superior catalytic performance in aiding the protoboration of terminal alkynes, with a very high turnover frequency (TOF) of 14333.3 h-1, much higher than those of many other reported copper-based heterogeneous catalysts. Our catalyst also retained excellent catalytic efficiency for up to five cycles for the above-mentioned process. The newly synthesized (Bn2Im)2CuBr@UiO-67 and the recovered catalyst post-catalysis were characterized using various analytical techniques, including powder X-ray diffraction (PXRD), IR spectroscopy, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS).

Cross-Coupling of NHC/CAAC-Based Carbodicarbene: Synthesis of Electron-Deficient Diradicaloids.

Herein, we report nickel(0)-catalyzed cross-coupling reactions of NHC/CAAC-based carbodicarbene (NHC = N-heterocyclic carbene and CAAC = cyclic(alkyl)(amino)carbene) with different aryl chlorides, bromides, and iodides. The resulting aryl-substituted cationic carbodicarbene derivatives are prone to one-electron oxidation yielding radical-dications, which, depending on the aryl motif employed, follow different modes of radical-radical dimerization and constitute an entry point to carbon/nitrogen- and nitrogen/nitrogen-centered diradicaloids. Subsequently, this coupling strategy was strategically applied to the synthesis of p-phenylene- and p,p'-biphenylene-bridged carbon/carbon-centered electron-deficient diradicaloids. The employed π-conjugated spacer plays a crucial role in determining the triplet population at room temperature by modulation of the singlet-triplet gap: EPR inactive for p-phenylene vs EPR active for p,p'-biphenylene. Nearly two decades after the disclosure of carbodicarbenes as donor-stabilized atomic carbon equivalents by Tonner and Frenking in 2007, we demonstrate their cross-couplings with a series of aryl halides/dihalides and, based on this, developed a modular methodology for the systematic synthesis of various electron-deficient diradicaloids.

Electrocatalytic CO2 reduction to HCO2H by protic NHC-Ir complexes

In electrochemical CO2-conversion strategies, various transition metal-based molecular electrocatalysts are employed to reduce CO2 into products like CO and HCO2H, with H2 as a competitive side product. However, achieving selectivity towards HCO2H remains challenging, and suitable catalysts for the same are limited. Herein we report a normal and an abnormal protic-NHC-based Cp*Ir(III)-half sandwich complexes for catalytic CO2 electroreduction in an aqueous acetonitrile solvent. Both the catalysts predominantly produced HCO2H as the CO2-reduced product at an applied potential of –2.66 V vs Fc+/Fc with 5 % H2O as the proton source; however, the normal protic-NHC-bound complex achieved a Faradic efficiency (FE) of 86±4 %, while the complex with the abnormal protic-NHC ligand furnished FE up to 72±4 %. The protic proton of the protic NHC ligand in these complexes was proposed to participate in a proton relay process, facilitating generation of the crucial Ir–H intermediate, which reacts with CO2 to produce HCO2H through stabilization of the Ir–OCHO intermediate.