N-heterocyclic Carbene Research Articles

Heavy Alkaline Earth Cyclic (Alkyl)(Amino)Carbene Complexes Supported by Aryl-Silyl Amides.

A series of 8 trigonal planar, heavy alkaline earth (AE = Ca-Ba) metal complexes containing cyclic (alkyl)(amino)carbene (CAAC) ligands were prepared from AE bis(amide) species. Complexation can be achieved by first generating the free carbene in situ or by direct addition of the free carbene, with the former route giving rise to unexpected mixed-amide AE complexes. The frontier molecular orbitals of the highly equatorial, 3-coordinate AE-CAAC species were also probed computationally, revealing the lowest unoccupied molecular orbital (LUMO) consisting predominantly of the π* system located on the carbene ligand.

Tuning the Steric and Electronic Properties of Hemilabile NHC ligands for Gold(I/III) Catalyzed Oxyarylation of Ethylene: A Computational Study.

Mechanistic studies on 1,2-oxyarylation of ethylene promoted by gold catalysts bearing hemilabile N-Heterocyclic Carbene (NHC^X) ligands were conducted by DFT calculations, exploring the whole catalytic cycle. After highest energy transition state (TS) barriers were located for NHC^N gold catalyst, and experimental results with different iodoarenes and alcohols rationalized, the study was extended to modified NHC^X catalysts, to observe how electronic and steric effects could affect the rate determining step TS. Electronic effects were investigated on NHC^X (X=H, N, O, P, and S), whereas steric effects emerged when comparing catalysts with different N-R groups (R=Dipp, Mes, tBu and Me). Finally, we suggest a different catalyst design based on N-aryl N-o-donor-aryl NHC, with different donors and NHC backbones to search for better performing systems.

N-heterocyclic carbene switchable radical or benzyne meditated arylation of thiols using DMF/KOtBu

A mild, metal and photoredox-free direct arylation of alkyl and aryl thiols with aryl iodides using a DMF/KOtBu system has been developed. In the absence of an N-heterocyclic carbene (NHC) additive, the reaction proceeds via a benzyne intermediate and was suitable for the substitution of thiols with phenyl, while the presence of an NHC or phenanthroline derivative improves regioselectivity for the reaction of substituted aryl iodides via a radical pathway. This protocol features inexpensive materials and good substrate scope, and could be useful in the arylation of aryl and alkyl thiols.

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.

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 Strongly Reducing sp2 Carbon-Conjugated Covalent Organic Framework Formed by N-Heterocyclic Carbene Dimerization.

Covalent 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.

Generation and Application of Sulfur Ylides in Light‐Mediated Transformations

AbstractSulfur ylides are essential reagents in various organic reactions like epoxidation and cyclopropanation. While traditionally employed in ionic chemistry, recent advancements in visible‐light‐driven photocatalysis reveal their potential as sources of radicals or free carbenes. The photochemical or photocatalytic generation of reactive intermediates using sulfur ylides expands the utility of these reagents in modern organic synthesis, offering sustainable solutions to emerging challenges. Despite early demonstrations in the 1960s, the past years have witnessed a surge in photochemical methods utilizing sulfur ylides as precursors. The use of redox‐active photocatalysts has facilitated numerous radical reactions. This review consolidates key findings in sulfur ylide chemistry, particularly focusing on photo‐mediated reactions, to stimulate further research interest in this dynamic field.

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.

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.

Synthesis, Characterization, and Catalysis of Planar Chiral Ferrocene‐based N‐Heterocyclic Carbene Ligands and Its Metal Complexes

AbstractThis study presents the development of a novel planar chiral ferrocene‐based NHC ligand and its corresponding metal complexes. The key unit of the imidazolium backbone, planar chiral ortho‐isopropyl ferrocenyl amine, was synthesized starting from (S)‐Ugi's amine. Imidazolium was obtained through the sequential condensation of glyoxal with ferrocenyl amine, followed by the ring‐closure of 1,3‐diferrocenyldiimine with chloromethylethyl ether. Chiral imidazolylidene NHC (IFcPr) metal complexes were prepared by treating imidazolium with silver oxide or base followed by transmetalation with appropriate metal salts. The steric and electronic properties of the IFcPr ligand were assessed using %Vbur and TEP, respectively. These assessments indicated that the ligand was as bulky as the adamantyl NHC ligand and exhibited strong donor ability similar to that of a triazolylidene NHC ligand. The catalytic performance of the copper and palladium complexes was evaluated in the asymmetric borylation of ethyl cinnamate and Suzuki–Miyaura coupling reactions, respectively. The IFcPr−Cu complex catalyzed the borylation of ethyl cinnamate with bis(pinacolato) diboron, followed by oxidation to yield the hydroxy ester with 58 % yield and 30 % ee. In contrast, the IFcPr−Pd‐PEPPSI complex provided the axial chiral binaphthyl compound with 59 % yield and 51 % ee at a low catalyst loading (0.1 mol %, TON=590).

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%.

Asymmetric construction of axial and planar chirality with N-heterocyclic carbene (NHC) organocatalysis

Asymmetric construction of axial and planar chirality with N-heterocyclic carbene (NHC) organocatalysis

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.

Two-Coordinate Gold(I) Complex with Rotational Freedom: A Platform Integrating Thermally Activated Delayed Fluorescence, Polymorphism, and Linearly Polarized Luminescence.

Two-coordinate coinage metal complexes have been exploited for various applications. Herein, a new donor-metal-acceptor (D-M-A) complex PZI-Au-TOT, using bulky pyrazine-fused N-heterocyclic carbene (PZI) and trioxytriphenylamine (TOT) ligands, was synthesized. PZI-Au-TOT displays decent thermally activated delayed fluorescence (TADF) with a quantum yield of 93 % in doped film. The crystals of PZI-Au-TOT show simultaneous TADF, polymorphism, and linearly polarized luminescence (LPL). The polymorph-dependent emission properties with widely varied peaks from 560 to 655 nm are attributed to different packing modes in terms of isolated monomers, discrete π-π stacked dimers or dimer PLUS. Two well-defined microcrystals of PZI-Au-TOT exhibit linearly polarized thermally activated delayed fluorescence with a degree of polarization up to 0.64. This work demonstrates that the molecular rotational flexibility of D-M-A type complexes endows an integration of multiple functions into one complex through manipulation of supramolecular aggregation. This type of complexes is expected to serve as a versatile platform for the fabrication of crystal materials for advanced photonic applications.

The Role of Spacer Length in Macrocyclization Reactions Under Confinement

AbstractWe 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.

IPr*F - Highly Hindered, Fluorinated N-Heterocyclic Carbenes.

The introduction of fluorine atom has attracted considerable interest in molecular design owing to the high electronegativity and the resulting polarization of carbon-fluorine bonds. Simultaneously, sterically-hindered N-heterocyclic carbenes (NHCs) have received major interest due to high stabilization of the reactive metal centers, which has paved the way for the synthesis of stable and reactive organometallic compounds with broad applications in main group chemistry, inorganic synthesis and transition-metal-catalysis. Herein, we report the first class of sterically-hindered, fluorinated N-heterocyclic carbenes. These ligands feature variable fluorine substitution at the N-aromatic wingtip, permitting to rationally vary steric and electronic characteristics of the carbene center imparted by the fluorine atom. An efficient, one-pot synthesis of fluorinated IPr*F ligands is presented, enabling broad access of academic and industrial researchers to the fluorinated ligands. The evaluation of steric, electron-donating and π-accepting properties as well as coordination chemistry to Au(I), Rh(I) and Se is presented. Considering the unique properties of carbon-fluorine bonds, we anticipate that this novel class of fluorinated carbene ligands will find widespread application in stabilizing reactive metal centers.