Poly(N-Heterocyclic Carbene)-Capped Alloy and Core-Shell AuAg Bimetallic Nanoparticles.

N-Heterocyclic carbene (NHC)-stabilized metal nanoparticles (NPs) have recently attracted considerable attention. While most efforts in the field have been devoted to the development of NHC-tethered monometallic NPs and enhancing their stabilities under various conditions, their bimetallic counterparts are rare in the literature. Herein, we demonstrate that the covalent immobilization of Au and Ag atoms on polymerized NHCs is a powerful method to access bimetallic AuAg NPs. In addition, we show that while AuAg alloy NPs are often obtained via this method, the use of bimetallic polymeric substrates with lower Ag content, relative to Au, results in the formation of core-shell NPs with Au core and Ag shell. Application of these nanomaterials for oxygen reduction reaction is demonstrated with all materials exhibiting electrocatalytic activity. This work demonstrates for the first time that while bimetallic poly(NHC-metal)s are viable substrates to access NHC-stabilized bimetallic NPs, careful adjustment of metal content in the polymeric substrates can finetune the microstructure of the resulting NPs, i.e. alloy vs. core-shell.

Enhancing the Machining Performance of Nomex Honeycomb Composites Using Rotary Ultrasonic Machining: A Finite Element Analysis Approach.

Nomex honeycomb composites (NHCs) are commonly used in various industrial sectors such as aerospace and automotive sectors due to their excellent material properties. However, when machining this type of structure, problems can arise due to significant cutting forces and unwanted cell vibrations. In order to remedy these shortcomings, this study proposes to integrate RUM (rotary ultrasonic machining) technology, which consists of applying ultrasonic vibrations along the axis of rotation of the cutter. To fully understand the milling process by ultrasonic vibrations of the NHC structure, a 3D numerical finite element model is developed using Abaqus/Explicit software. The results of the comparative analysis between the components of the simulated cutting forces and those from the experiment indicate a close agreement between the developed model and the experimental results. Based on the developed numerical model, this study comprehensively analyzes the influence of the ultrasonic vibration amplitude on various aspects, such as stress distribution in the cutting zone, chip size, the quality of the machined surface and the components of the cutting force. Ultimately, the results demonstrate that the application of ultrasonic vibrations leads to a reduction of up to 50% in the components of the cutting force, as well as an improvement in the quality of the machined surface and a reduction in the size of chips.

Recent Advances in NHC-Catalyzed Chemoselective Activation of Carbonyl Compounds.

N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.

Visible Light-Mediated Monofluoromethylation/Acylation of Olefins by Dual Organo-Catalysis.

Monofluoromethyl (CH2F) motifs exhibit unique bioactivities and are considered privileged units in drug discovery. The radical monofluoromethylative difunctionalization of alkenes stands out as an appealing approach to access CH2F-containing compounds. However, this strategy remains largely underdeveloped, particularly under metal-free conditions. In this study, we report on visible light-mediated three-component monofluoromethylation/acylation of styrene derivatives employing NHC and organic photocatalyst dual catalysis. A diverse array of α-aryl-ß-monofluoromethyl ketones was successfully synthesized with excellent functional group tolerance and selectivity. The mild and metal-free CH2F radical generation strategy from NaSO2CFH2 holds potential for further applications in fluoroalkyl radical chemistry.

Wingtip-Flexible N-Heterocyclic Carbenes: Unsymmetrical Connection between IMes and IPr.

IMes (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and IPr (IPr=1,3- bis(2,6-diisopropylphenyl)imidazol-2-ylidene) represent by far the most frequently used N-heterocyclic carbene ligands in homogeneous catalysis, however, despite numerous advantages, these ligands are limited by the lack of steric flexibility of catalytic pockets. We report a new class of unique unsymmetrical N-heterocyclic carbene ligands that are characterized by freely-rotatable N-aromatic wingtips in the imidazol-2-ylidene architecture. The combination of rotatable N-CH2 Ar bond with conformationally-fixed N-Ar linkage results in a highly modular ligand topology, entering the range of geometries inaccessible to IMes and IPr. These ligands are highly reactive in Cu(I)-catalyzed ß-hydroboration, an archetypal borylcupration process that has had a transformative impact on the synthesis of boron-containing compounds. The most reactive Cu(I)-NHC in this class has been commercialized in collaboration with MilliporeSigma to enable broad access of the synthetic chemistry community. The ligands gradually cover %Vbur geometries ranging from 37.3 % to 52.7 %, with the latter representing the largest %Vbur described for an IPr analogue, while retaining full flexibility of N-wingtip. Considering the modular access to novel geometrical space in N-heterocyclic carbene catalysis, we anticipate that this concept will enable new opportunities in organic synthesis, drug discovery and stabilization of reactive metal centers.

Divergent Acyl and Decarbonylative Liebeskind-Srogl Cross-Coupling of Thioesters by Cu-Cofactor and Pd-NHC (NHC = N-Heterocyclic Carbene) Catalysis.

Transition-metal-catalyzed cross-coupling reactions of thioesters by selective acyl C(O)-S cleavage have emerged as a powerful platform for the preparation of complex molecules. Herein, we report divergent Liebeskind-Srogl cross-coupling of thioesters by Pd-NHC (NHC = N-heterocyclic carbene) catalysis. The reaction provides straightforward access to functionalized ketones by highly selective C(acyl)-S cleavage under mild conditions. Most crucially, the conditions enable direct functionalization of a range of complex pharmaceuticals decorated with a palette of sensitive functional groups, providing attractive products for medicinal chemistry programs. Furthermore, decarbonylative Liebeskind-Srogl cross-coupling by C(acyl)-S/C(aryl)-C(O) cleavage is reported. Cu metal cofactor directs the reaction pathway to acyl or decarbonylative pathway. This reactivity is applicable to complex pharmaceuticals. The reaction represents the mildest decarbonylative Suzuki cross-coupling discovered to date. The Cu-directed divergent acyl and decarbonylative cross-coupling of thioesters opens up chemical space in complex molecule synthesis.

Toxicity evolution and control for the UV/H2O2 degradation of nitrogen-containing heterocyclic compounds: SDZ and PMM.

This study aimed to achieve toxicity control of sulfadiazine (SDZ) and pirimiphos-methyl (PMM) via the UV/H2O2 process by optimizing the reaction parameters. The results show that both drugs had a good degradation effect under the following parameters: a H2O2 molar ratio of 1:200, and neutral conditions. SDZ and PMM could be degraded by more than 99% within 3 min, respectively. In the Daphnia magna acute toxicity assay and Vibrio fischeri inhibition assay, both SDZ and PMM exhibited a phenomenon of increasing toxicity. Additionally, through the use of density functional theory (DFT) calculation and HPLC-QTOF-MS, 21 transformation products (TPs) were identified, and the principal degradation pathways were proposed. The toxicity of the TPs was determined by comparing the QSAR prediction results with toxicity test data. As a result, under the higher UV light intensity (2300 µW/cm2) and neutral conditions, SDZ showed highest toxicity, whereas PMM showed lowest toxicity under the lowest UV light intensity (450 µW/cm2) and neutral conditions. Four main toxic TPs were identified, and their yields could be reduced by adjusting the reaction parameters. Therefore, the selection of appropriate reaction parameters could reduce the production of toxic TPs and ensure the safety of water environment.

Structure-Activity Relationships in NHC-Silver Complexes as Antimicrobial Agents.

Silver has a long history of antimicrobial activity and received an increasing interest in last decades owing to the rise in antimicrobial resistance. The major drawback is the limited duration of its antimicrobial activity. The broad-spectrum silver containing antimicrobial agents are well represented by N-heterocyclic carbenes (NHCs) silver complexes. Due to their stability, this class of complexes can release the active Ag+ cations in prolonged time. Moreover, the properties of NHC can be tuned introducing alkyl moieties on N-heterocycle to provide a range of versatile structures with different stability and lipophilicity. This review presents designed Ag complexes and their biological activity against Gram-positive, Gram-negative bacteria and fungal strains. In particular, the structure-activity relationships underlining the major requirements to increase the capability to induce microorganism death are highlighted here. Moreover, some examples of encapsulation of silver-NHC complexes in polymer-based supramolecular aggregates are reported. The targeted delivery of silver complexes to the infected sites will be the most promising goal for the future.

Highly Efficient Circularly Polarized Luminescence from Chiral Au13 Clusters Stabilized by Enantiopure Monodentate NHC Ligands.

Chiral Au nanoclusters have promising application prospects in chiral sensing, asymmetric catalysis, and chiroptics. However, enantiopure superatomic homogold clusters with crystallographic structures emitting bright circularly polarized luminescence (CPL) remain challenging. In this study, we designed chiral N-heterocyclic carbenes (NHCs), and for the first time enantioselectively synthesized a pair of monovalent cationic superatomic Au13 clusters. This new enantiomeric pair of clusters has a quasi-C2 symmetric core and exhibited CPL with an unprecedent solution-state quantum yield (QY) of 61 % among those of the atomically precise Au nanoclusters. DFT calculations provided insights into the circular dichroism behavior, and revealed the origin of CPL from superatomic Au clusters. This work opens a new avenue for developing novel homochiral nanoclusters using chiral NHC ligands and provides fundamental understanding of the origin of the chiroptics of metal clusters.

Ag-NHC Complexes in the π-Activation of Alkynes.

Silver-NHC (NHC = N-heterocyclic carbene) complexes play a special role in the field of transition-metal complexes due to (1) their prominent biological activity, and (2) their critical role as transfer reagents for the synthesis of metal-NHC complexes by transmetalation. However, the application of silver-NHCs in catalysis is underdeveloped, particularly when compared to their group 11 counterparts, gold-NHCs (Au-NHC) and copper-NHCs (Cu-NHC). In this Special Issue on Featured Reviews in Organometallic Chemistry, we present a comprehensive overview of the application of silver-NHC complexes in the p-activation of alkynes. The functionalization of alkynes is one of the most important processes in chemistry, and it is at the bedrock of organic synthesis. Recent studies show the significant promise of silver-NHC complexes as unique and highly selective catalysts in this class of reactions. The review covers p-activation reactions catalyzed by Ag-NHCs since 2005 (the first example of p-activation in catalysis by Ag-NHCs) through December 2022. The review focuses on the structure of NHC ligands and p-functionalization methods, covering the following broadly defined topics: (1) intramolecular cyclizations; (2) CO2 fixation; and (3) hydrofunctionalization reactions. By discussing the role of Ag-NHC complexes in the p-functionalization of alkynes, the reader is provided with an overview of this important area of research and the role of Ag-NHCs to promote reactions that are beyond other group 11 metal-NHC complexes.

Recent advances in sustainable N-heterocyclic carbene-Pd(II)-pyridine (PEPPSI) catalysts: A review.

Various catalysts in homogeneous or heterogeneous catalysis deploy unconventional reaction pathways by lowering the activation energy (AE) barrier, controlling the selectivity, and creating environmental impact, thereby bringing about economic viability. Hence, the study of these methodologies is of immense interest. To develop a new chemistry, there is much scope for the invention of brilliant candidates that could effectively catalyze diverse reaction methodologies. The palladium-catalyzed reactions motivate interesting applications on various organic transformations under mild reaction conditions. Although phosphorous designed ligands or catalysts have been used, despite their expensiveness, sensitivity and other properties, there is the necessity of developing even better cross-coupling ligands or catalysts such as N-heterocyclic carbene (NHC)-based palladium complexes. These palladium-NHCs (Pd-NHC) are novel and universal nucleophilic entities that have come into light as the most successful class of catalysts in organometallic chemistry. In the same class, a specific category of palladium-NHCs such as palladium-pyridine enhanced pre-catalyst preparation by the stabilization initiation (palladium-PEPPSI) complexes, are emerging as versatile alternatives to phosphine containing palladium complexes for various cross-coupling reactions due to their excellent catalytic activity. Further to mention that NHCs are recently extensively used as ancillary ligands in organometallic chemistry, which includes industrial-related catalytic transformations due to strong σ-donors to metal centres. Apart from this, many NHC-metal complexes are the fascinating consideration in material science as probable metallo-pharmaceuticals. The current review offers a brief exploration of palladium-PEPPSI complexes over the past few years. Further, the synthesis of a variety of these types of catalysts, their applications in Suzuki-Miyaura, Buchwald-Hartwig, Sonogashira, Negishi couplings direct C2-arylation, O-C(O) cleavage, α-arylation/alkylation of carbonyl compounds and trans-amidation reactions via cross-coupling methodologies, which are covered. Additionally, reported recent developments on reusable heterogeneous PdPEPPSI complexes and their catalytic applications are being covered. Finally, the chiral Pd complexes and their asymmetric transformations are discussed.

Cobalt-N-Heterocyclic Carbene Complexes in Catalysis.

Cobalt-NHC complexes have emerged as an attractive class of 3d transition metal catalysts for a broad range of chemical processes, including cross-coupling, hydrogenation, hydrofunctionalization and cycloaddition reactions. Herein, we present a comprehensive review of catalytic methods utilizing cobalt-NHC complexes with a focus on catalyst structure, the role of the NHC ligand, properties of the catalytic system, mechanism and synthetic utility. The survey clearly suggests that the recent emergence of well-defined cobalt-NHC catalysts may have a tremendous utility in the design and application of catalytic reactions using more abundant 3d transition metals.

Functionalized N-Heterocyclic Carbene Monolayers on Gold for Surface-Initiated Polymerizations.

Although N-heterocyclic carbenes (NHCs) are superior to thiol adsorbates in that they form remarkably stable bonds with gold, the generation of NHC-based self-assembled monolayers (SAMs) typically requires a strong base and an inert atmosphere, which limits the utility of such films in many applications. Herein, we report the development and use of bench-stable NHC adsorbates, benzimidazolium methanesulfonates, for the direct formation of NHC films on gold surfaces under an ambient atmosphere at room temperature without the need for extraordinary precautions. The generated NHC SAMs were fully characterized using ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and contact angle measurements, and they were compared to analogous SAMs generated from an NHC bicarbonate adsorbate. Based on these findings, a unique radical initiator α,ω-bidentate azo-terminated NHC adsorbate, NHC15AZO[OMs], was designed and synthesized for the preparation of SAMs on gold surfaces with both NHC headgroups bound to the surface. The adsorbate molecules in NHC15AZO SAMs can exist in a hairpin or a linear conformation depending on the concentration of the adsorbate solution used to prepare the SAM. These conformations were studied by a combination of ellipsometry, XPS, PM-IRRAS, and scanning electron microscopy using gold nanoparticles (AuNPs) as a tag material. Moreover, the potential utility of these unique radical-initiating NHC films as surface-initiated polymerization platforms was demonstrated by controlling the thickness of polystyrene brush films grown from azo-terminated NHC monolayer surfaces simply by adjusting the reaction time of the photoinitiated radical polymer growth process.

A CO2 -Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion.

Free N-heterocyclic carbenes (NHCs) are generally prepared by treatment of imidazolium precursors with strong alkali reagents, which usually produces inactive NHC dimers. This treatment would destroy porous supports and thus make supported NHC catalysts difficult to recovery and reuse. Herein, we report the first stable CO2 -masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline covalent organic framework (COF). The stable NHC-CO2 moieties in the COF-NHC-CO2 could be transformed in situ into isolated NHCs by heating, which exhibit superior catalytic performances in hydrosilylation and N-formylation reactions with CO2 . The NHC sites can reversibly form NHC-CO2 and thus can be easily recycled and reused while maintaining excellent catalytic activity. Density functional theory calculations revealed that NHC sites can be fully exposed after removal of CO2 -masks and rapidly react with silanes, which endows COF-NHC with high catalytic activity.

Green synthesis of selenium based N-heterocyclic carbene compounds; structural, in-vitro anticancer and molecular docking studies.

Benzimidazolium salts (3-6) were synthesized as stable N-Heterocyclic Carbene (NHC) precursors and their selenium-NHC compounds/Selenones (7-10) were prepared using water as a solvent. Characterization of each of the synthesized compounds was carried out by various analytical and spectroscopic (FT-IR, 1H-, 13C NMR) methods. X-ray crystallographic analyses of single crystals obtained for salts 3 and 5 were carried out. Synthesized salts and their Se-NHCs were tested in-vitro for their anticancer potential against Cervical Cancer Cell line from Henrietta Lacks (HeLa), Breast cancer cell line (MDA-MB-231), Adenocarcinoma cell line (A549) and human normal endothelial cell line (EA.hy926). MTT assay was used for analysis and compared with standard drug 5-flourouracil. Benzimidazolium salts (3-6) and their selenium counter parts (7-10) were found potent anticancer agents. Salt 3-5 were found to be potent anticancer against HeLa with IC50 values 0.072, 0.017 and 0.241 µM, respectively, which are less than standard drug (4.9 µM). The Se-NHCs (7-10) had also shown significant anticancer potential against HeLa with IC50 values less than standard drug. Salts 3, 4 against EA.hy926, compounds 3,5,6, and 10 against MDA-MB-321, and compounds 4, 10 against A-549 cell line were found more potent anticancer agents with IC50 values less than standard drug. Molecular docking for (7-10) showed their good anti-angiogenic potential having low binding energy and significant inhibition constant values with VEGFA (vascular endothelial growth factor), EGF (human epidermal growth factor), COX1 (cyclooxygenase-1) and HIF (hypoxia inducible factor).

Transamidation of Amides and Amidation of Esters by Selective N-C(O)/O-C(O) Cleavage Mediated by Air- and Moisture-Stable Half-Sandwich Nickel(II)-NHC Complexes.

The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N-C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)-NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N-C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stable Ni(II)-NHC complexes in catalytic amide bond-forming protocols by unconventional C(acyl)-N and C(acyl)-O bond cleavage reactions.

Hydrogen-Bonding Assisted Catalytic Kinetic Resolution of Acyclic ß-Hydroxy Amides.

Enantioenriched acyclic α-substituted ß-hydroxy amides are valuable compounds in chemical, material and medicinal sciences, but their enantioselective synthesis remains challenging. A catalytic kinetic resolution (KR) of such amides with selectivity factor(s) up to >200 is developed via enantioselective acylation of primary alcohol with N-heterocyclic carbene. An enhanced selectivity for the catalytic KR process is realized using cyclic tertiary amine as base additive. Diastereomeric transition state models for the process are proposed to rationalize the origin of enantioselectivity.

Potential of Gold Candidates against Human Colon Cancer.

Development of novel metallodrugs with pharmacological profile plays a significant role in modern medicinal chemistry and drug design. Metal complexes have shown remarkable clinical results in current cancer therapy. Gold complexes have attained attention due to their high antiproliferative potential. Gold-based drugs are used for the treatment of rheumatoid arthritis. Gold-containing compounds with selective and specific targets are capable to assuage the symptoms of a range of human diseases. Gold (I) species with labile ligands (such as Cl in TEPAuCl) interact with isolated DNA; therefore, this biomolecule has been considered as a target for gold drugs. Gold (I) has a high affinity towards sulfur and selenium. Due to this, gold (I) drugs readily interact with cysteine or selenocysteine residue of the enzyme to form protein-gold(I) thiolate or protein-gold (I) selenolate complexes that lead to inhibition of the enzyme activity. Au(III) compounds due to their square-planner geometriesthe same as found in cisplatin, represent a good source for the development of anti-tumor agents. This article aims to review the most important applications of gold products in the treatment of human colon cancer and to analyze the complex interplay between gold and the human body.

Donor-Stabilized Monocarba-Bridged Bis(cyclopentadienyl)alanes.

Five monocarba-bridged bis(cyclopentadienyl)aluminum halide NHC and thione complexes and one monocarba-bridged bis(cyclopentadienyl)phosphanylalane NHC complex are reported. The former were synthesized by transmetalation of a C[1]magnesocenophane with the corresponding aluminum(III) chloride and aluminum(III) bromide donor adducts. The phosphanylalane complex was obtained by a subsequent functionalization of the corresponding bromoalane with lithium diphenylphosphide. All complexes were characterized in solution by multinuclear NMR spectroscopy and in the solid state by single crystal X-ray diffraction. Bonding energies of the NHC and thione ligands to the aluminum centres were estimated by DFT calculations.

Mild and Efficient Synthesis of Diverse Organo-AuI -L Complexes in Green Solvents.

An exceptionally mild and efficient method was developed for the preparation of (hetero)aryl-AuI -L complexes using ethanol or water as the reaction medium at room temperature and Ar-B(triol)K boronates as the transmetalation partner. The reaction does not need an exogeneous base or other additives, and quantitative yields can be achieved through a simple filtration as the only required purification method, which obviates considerable waste associated with alternative workup methods. A broad reaction scope was demonstrated with respect to both the L and (hetero)aryl ligands on product Au complexes. Despite the polar reaction medium, large polycyclic aromatic hydrocarbon units can be incorporated on the Au complexes in very good to excellent yields. The approach was demonstrated for the chemoselective manipulation of orthogonally protected aryl boronates to afford a new class of N-heterocyclic carbene-Au-aryl complexes. A mechanistic rationale was proposed.