N-heterocyclic carbene as a promising metal-free electrocatalyst with high efficiency for nitrogen reduction to ammonia

Electrocatalytic nitrogen reduction reaction(NRR)at ambient conditions holds great promise for sustainably synthesizing ammonia(NH3),while developing highly-efficient,long-term stable,and inexpensive catalysts to activate the inert N≡N bond is a key scientific issue.In this work,on the basis of the concept"N-heterocyclic carbenes(NHCs)",we propose a carbon decorated graphitic-carbon nitride(C/g-C3N4)as novel metal-free NRR electrocatalyst by means of density functional theory(DFT)computations.Our results reveal that the introduced C atom in g-C3N4 surface can be regarded as NHCs and catalytic sites for activating N≡N bond,and are stabilized by the g-C3N4 substrate due to sterically disfavored dimerization.Especially,this NHCs-based heterogeneous catalysis can efficiently reduce the activated N2 molecule to NH3 with a low overpotential of 0.05 V via an enzymatic mechanism.Our work is the first report of NHCs-based electrocatalyst for N2 fixation,thus opening an alternative avenue for advancing sustainable NH3 production.

Recent advances in heterogeneous catalytic hydrogenation and dehydrogenation of N-heterocycles

The selective hydrogenation of quinolines to 1,2,3,4-tetrahydroquinolines(py-THQ) and its derivatives has attracted a considerable amount of attention as they show great versatility in many pharmaceuticals, agrochemicals, and fine chemicals. Over the past few decades, great breakthroughs have been achieved in the controlled synthesis of efficient heterogeneous catalysts used for the selective hydrogenation of functionalized quinoline compounds, which allow one to correlate the structure-property relationships. In this review, we will summarize the recent significant progress achieved in this field covering the synthetic strategies, microstructural and chemical features, catalytic performance, and internal relationships. State-of-the-art noble metal-based single(Pd, Pt, Ru, Rh, Ir and Au) and bi/multi-metallic catalysts(RuCu, AuPd, and PdNi) are first introduced, followed by a summary of earth-abundant metal-based catalysts(Co, Fe, Ni, and Cu). Finally, the dehydrogenation of N-heterocycles is introduced to form a reversible hydrogenation/dehydrogenation system for H2 storage, which can be employed in a liquid organic hydrogen system. Furthermore, the reaction mechanism and future research direction in these areas are also discussed. This review will deepen our understanding of the catalytic transformation of N-heterocycles and provide guidance for researchers on the rational design of catalysts.

Operationally Simple Enantioselective Silane Reduction of Ketones by the [Ir(OMe)(cod)]2/Azolium Catalytic System

An operationally simple protocol was designed for the enantioselective silane reduction (ESR) of ketones using air- and moisture-stable [Ir(OMe)(cod)]2 (cod = 1,5-cyclooctadiene) (3) as a metal catalyst precursor. This reaction was driven by chiral hydroxyamide-functionalized azolium salt 2. The catalytic ESR reaction could be performed under benchtop conditions at room temperature. Treatment of 2 with 3 in THF yielded the monodentate IrCl(NHC)(cod) (NHC = N-heterocyclic carbene) complex 4 in 93% yield, herein the anionic methoxy ligand of 3 serves as an internal base that deprotonates the azolium ring of 2. The well-defined Ir complex 4 catalyzed the ESR reaction of propiophenone (6) with (EtO)2MeSiH using the pre-mixing reaction procedure. Based on this success, the catalytic ESR reaction was designed and implemented using an in situ-generated NHC/Ir catalyst derived from 2 and 3. Thus, a wide variety of aryl ketones could be reduced to the corresponding optically active alcohols in moderate to excellent stereoselectivities at room temperature without temperature control. Since the high catalytic activity of 3 was observed, we next evaluated several other transition metal catalyst precursors for the catalytic ESR reaction under the influence of 2. This evaluation revealed that Ir(acac)(cod) (acac = acetylacetonate) (28) and [IrCl(cod)]2 (5) can be successfully used as metal catalyst precursors in the ESR reaction.

Two Amine-tethered Imidazolium NHC Ni(Ⅱ) Complexes: Synthesis, Structure and Catalytic Activity

Nickel（II） complexes Ni（L1）2 （1） and Ni（L2）2·CH3CN·H2O （2） （L1Cl = 2-（3-benzyl- 2,3-dihydroimidazlo-1-yl）-N-（pyridin-2-yl）acetamide, chloride salt, L2Cl = 2-（3-naphthalene-2- ylmethyl）-2,3-dihydro-imidazlo-1-yl）-N-phenylacetamide, chloride salt） with amide-functionalized imidazolium NHC ligand were synthesized and determined by single-crystal X-ray diffraction. Complex 1 crystallizes in the monoclinic system, space group P21/cwith a = 9.2561（19）, b = 15.138（3）, c = 20.750（4） A, β = 90.216（3）°, V = 2907.3（10） A^3 and Z = 4. Complex 2 crystallizes in the orthorhombic system, space group Pbca with a = 15.5469（16）, b = 15.3521（15）, c = 31.177（3） A, V = 7441.2（13） ？3 and Z = 8. They are stable in the air and formed via intermolecular weak interactions, including C-H···π contacts and C–H···O hydrogen bonds. Complex 1 was applied in Suzuki coupling reaction through the investigation of reaction conditions under the optimal choice conditions： 80 ℃, K3PO4, 3% mol catalysts, 6% mol PPh3 and toluene as solvent system could afford 96% yield in 5 h.

Highly active binuclear Cu(Ⅱ) catalyst bearing an unsymmetrical bipyridine-pyrazole-amine ligand for the azide-alkyne cycloaddition reaction

Ligands containing NH groups often show special characteristics.In this paper,a well-defined dinuclear Cu（II） complex bearing an unsymmetrical bipyridine-pyrazole-amine ligand was synthesized by the condensation of N–H to release H2O.Using sodium L-ascorbate as a reductant,the binuclear complex showed excellent activity in 1,3-dipolar cycloaddition reactions between alkynes and azides to obtain 1,4-disubstituted triazoles in 95%–99% isolated yields.

Synthesis of thiopyrano[4″,3″:4′,5′]pyrido[3′,2′:4,5]furo[3,2-d]pyrimidines

Reactions of the 6-hydroxy-thiopyrano[3,4-c]pyridine-5-carbonitrile derivative 1 withα-halo-carbonyl compounds gave the ortho-substituted intermediates 2a-c which were converted into furo[2,3-b]thiopyrano[4,3-d]pyridines 3a-c by fusion of a furan moiety under basic conditions.Further cyclization of 3a-c led to a fusion of a pyrimidine ring,yielding the tetracyclic products 6,7 and 8.In addition,condensation of 6 with various aromatic aldehydes afforded the corresponding imines 9a,b.Mannich reaction of 7 gave products 10a,b.

Dialkyl imidazolium acetate ionosilica as efficient and recyclable organocatalyst for cyanosilylation reactions of ketones

We report new heterogeneous organocatalyst based on silica hybrid supported N-heterocyclic carbene(NHC-)species.The organocatalyst is formed from an imidazolium iodide based ionosilica material,followed by iodide/acetate anion exchange.The imidazolium acetate generates the organocatalytic carbene via partial deprotonation of the imidazolium ring in situ.As monitored via EDX,solid state NMR and ion chromatography measurements,the iodide/acetate exchange involving the imidazolium ionosilica material took place only in small extent.Despite the fact that the exchanged material contains only a very small amount of acetate,we observed good catalytic activity and recyclability in cyanosilylation reactions of ketones with trimethylsilyl cyanide.The versatility of the catalyst was highlighted via reaction with several substrates,yielding the corresponding cyanohydrins in good yields.In recycling experiments,the material showed decreasing catalytic activity starting from the third reaction cycle,but high catalytic activity can be regenerated via another acetate treatment.Our work is important as it highlights the possibility to combine carbene chemistry and silica,which are antagonistic at a first glance.We show that imidazolium acetate based ionosilicas are therefore heterogeneous'proto-carbenes',and that there is no need to form strongly basic silica supported NHCs to obtain heterogeneous NHC-organocatalysts.This work therefore opens the route towards heterogeneous and re-usable NHC-organocatalysts from supported ionic liquid imidazolium acetates.

A New Palladium Complex Containing the Mixture of Carbene and Phosphine Ligands: Synthesis, Crystal Structure and Spectral FT-IR, NMR and UV-Vis Researches

In this paper,a new Pd-based complex that contains A^-heterocyclic carbene(NHC)and triphenylphosphine(PPh?)ligands was synthesized.The c/5-(NHC)PdBr2(PPh3)complex has been prepared by the substitution of 3?chloropyridine ligand in(NHC)PdBr2(3-chloropyridine)complex with triphenylphosphine.This complex has been characterized by using 1^H NMR,^31P{^1H}NMR,^13C{^1H}NMR,FTIR,UV-Vis spectroscopy and elemental analysis techniques.The molecular and crystal structures of the cz5-(NHC)PdBr2(PPh3)complex were determined by singlecrystal X-ray diffraction method.

Synthesis, Characterization, and Evaluation of Antitumor Potential in MCF-7 Cells of Ruthenium-Derived Compounds

To synthesize, characterize and evaluate the antitumor potential derived from ruthenium compounds was generated in this study, from the precursor K[RuCl4(bipy)] a route in a simple and reproducible synthesis for a novel compound of coordinating Ru+3 with bipy and L-trip. The spectroscopic characterization in the middle infrared region (FTIR) shows the interactions between Ru-(L-trip), evidenced by the displacement of the carboxylate ion band for higher energies, and also by the displacements of aliphatic amine bands, suggesting that bidentate coordination of the L-trip ligand occurred. Analysis of the results obtained with thermoanalytical techniques showed that the minimum formula of the compound, [RuCl2(bipy)(L-trip)]1/2H2O. Evaluation of the antitumor potential of precursor K[RuCl4(bipy)] showed the toxic effects on MCF-7 cell line, but did not show selectivity and not reached PBMC cells to the same extent. The evaluation of the antitumor potential of the newly synthesized compound, [RuCl2(bipy)(L-trip)], demonstrated that the insertion of an L-tryptophan molecule into the precursor coordination sphere made it selective when compared to PBMC cells, for MCF-7 type tumor cells.

N-Heterocyclic carbene-catalyzed enantioselective(dynamic) kinetic resolutions and desymmetrizations

N-heterocyclic carbene-catalyzed enantioselective kinetic resolutions,dynamic kinetic resolutions,and desymmetrization reactions are systematically reviewed.The content is organized according to the activation modes involved in these transformations.Future advances within this highly active research field are discussed from our perspectives on the topic.

Electrocatalytic hydrogen evolution from water at low overpotentials with cobalt complexes supported by redox-active bipyridyl-NHC donors

Three cobalt complexes bearing tunable,redox-active bipyridyl N-heterocyclic carbene(NHC)-based ligands have been studied for electrocatalytic hydrogen evolution from aqueous solutions.The effect of structural modifications to the ligand framework is investigated across the catalyst series,which includes a non-macrocyclic derivative(1-Co)and 16-(2-Co)and 15-(3-Co)membered macrocycles.A structure-activity relationship is demonstrated,in which the macrocyclic complexes have greater activity compared to their non-macrocyclic counterpart with the most rigid catalyst,supported by the 15-membered macrocycle,performing best overall.Indeed,3-Co catalyzes H2 evolution from aqueous pH 4 acetate buffer with a Faradaic efficiency of 97%at a low overpotential of 330 mV.Mechanistic studies are consistent with formation of a cobalt-hydride species that is subsequently protonated to evolve H2 via a heterolytic pathway.

Direct and Indirect Electro-Oxidative Intramolecular C–H Aminations

The ubiquity of N-heterocycles in marketed drugs makes the development of metal-free methodologies for constructing C–N bonds of considerable importance.As an environmentally friendly method,electro-oxidative intramolecular C–H amination has emerged as a powerful platform for synthesizing nitrogen-containing heterocycles under metal-and external oxidant-free conditions.In this minireview,the main achievements in this direction since 2020 are summarized,with an emphasis on the substrate scope and mechanistic aspects.The reactions are classifi ed into two categories:direct and indirect electro-oxidative intramolecular C–H aminations.

Synthesis and Crystal Structure of an Indenylnickel(II) NHC Complex (Ind)NiBr(1,3-bisbenzylimidazol-2-ylidene)

The complex （Ind）NiBr（1,3-bisbenzylimidazol-2-ylidene） （Ind = indenyl） 1 has been prepared in ca. 65% yield via the reaction of （Ind）2Ni with an equivalent of 1,3-bis-benzylimidazolium bromide in THF/CH2Cl2 at 45℃, supported by elemental analysis, NMR spectroscopy and X-ray crystal determination. The crystal belongs to the triclinic system, space group Pi with a = 7.072（1）, b = 11.264（2）, c = 14.541（3）°A, α = 102.21（3）,β = 93.44（3）,γ = 90.81（3）°, V = 1129.7（4）°A^3, Z = 2, Mr = 502.08, Dc = 1.476 g/cm^3, F（000） = 512, μ= 2.642 mm^-1, R = 0.0490 and wR = 0.1137 for 3913 observed reflections （I〉 2σ（I））. The center nickel atom is coordinated by a bromide ligand, a carbene carbon and three carbon atoms of the five-membered indenyl group to form either a highly distorted square pyramid or a highly distorted square plane.

[(NHC)CrCl(μ-Cl)(THF)]_2 and(NHC)_2CrCl_2(NHC=1,3-Diisopropyl-4,5-dimethylimidazole-2-ylidene):Syntheses,Structures,and Polymerization Activities

The preparation of divalent chromium N-heterocyclic carbene（NHC,1,3-diisopropyl4,5-dimethylimidazole-2-ylidene） compounds is reported.The reaction of 1：1 molar ratio of NHC with CrCl2 led to an isolation of [（NHC）CrCl（μ-Cl）（THF）]2（1）,while that of 2：1 ratio resulted in the formation of（NHC）2CrCl2（2）.1 can be considered as an intermediate in the formation of 2 and further converted into 2 by the addition of another equiv.of NHC.The reaction of 2 with CpNa afforded an ion pair compound [（NHC）2CrCp]＋[Cp]-（3）,indicating a strong coordination ability of NHC supplanting one of the ionic Cr-Cp bonding.In combination of methylalumoxane（MAO） as cocatalyst 1 and 2 both are active for catalyzing ethylene polymerization.

Palladium/Imadazolium Salt Mediated Cyclisations for the Synthesis of Heterocyclic Compounds

The use of intramolecular reactions involving palladium/imidazolium salts to synthesize hetero-cyclic compounds is described. Reactivity of phenyl, ethyl and methyl substituents leading to isolation of various isomeric products is also illustrated. Rearrangement of phenyl intermediates to furnish benzoxazoles is also mentioned.

Synthesis and Structure of Ferrocenol Esters

N-Heterocyclic carbene-catalyzed oxidative esterification of ferroceneboronic acid by aromatic and heteroaromatic aldehydes affords the new ferrocenol hetaryl and aryl esters 1 - 4, 6 - 8 and 9 - 11. The reaction takes place under mild conditions. The X-ray crystal structure of ferrocenyl esters 3, 6, 11 was determined.

Synthesis and crystal structure of bis (1,3-diisopropyl-4,5-dimethylimidazolium) hexatungstate

The title compound bis（1,3-diisopropyl-4,5-dimethylimidazolium） hexatungstate,[C11H21N2]2[W6O19],was synthesized and characterized by single-crystal X-ray diffraction,IR and elemental analysis.It crystallizes in the monoclinic system,space group P21/n with a=1.112 84（14）,b=1.283 84（17）,c=1.293 28（17）nm,β=96.410（2）°,V=1.836 2（4）nm^3,C22H42N4O19W6,Mr=1 769.70,Dc=3.201 g/cm3,F（000）=1 596,Z=2,μ（Mo Kα）=18.797 mm-1,the final R=0.031 3,wR=0.059 8for observed reflections（I〉2σ（I））.

Synthesis, Characterization of Ruthenium Compounds and Studies of Biological Effects in MCF-7 Tumors Cell Lines

This work presents the synthesis and characterization of compounds derived from the ruthenium transition metal with the nitrogenous ligand 4-aminopy- ridine (4-ampy). The synthesized compounds were characterized by FTIRmed spectroscopy and TG-DTA thermal analysis. For the cytotoxic evaluation of ruthenium compounds, a 66.0 μM aqueous solution containing the complex and the study of data observed in the biological assessment was performed using variance (ANOVA) analysis, followed by Tukey’s multiple comparisons test. Differences between treatments were considered significant when the p-value was less than 0.05 (p < 0.05). TG/DSC thermal analysis for the first complex suggests a stoichiometry of [Ru(Cl)3(4-ampy)(H2O)2]·1/2H2O, which, due to the low solubility in an aqueous medium, was modified to increase its solubility for biological tests. The analysis of the spectra in the medium infrared region (FTIR) for the complex [Ru(Cl)3(4-ampy)(H2O)2]·1/2H2O, shows displacements of the bands observed at 1625 - 1566 cm﹣1 ν(C=C) e (C=N), indicating that coordination to the metallic center occurred by this group. Band displacements were observed in the modified Ru (III) complex, which suggests the presence of the 4-ampy ligand and the coordination by the groups ν(C=C) and (C=N) after the modification. In recent years, researchers worldwide have concentrated on obtaining, developing, and modifying drugs used as chemotherapeutic agents. The evaluation of the cell viability of the modified Ru (III) compound demonstrated cytotoxic effects in the MCF-7 cell line (15.33% ± DP 2.7) but did not affect normal cells (PBMC), which reflects the potential for possible applications.

Chemoselective Transfer Hydrogenation of Nitroarenes with Ammonia Borane Catalyzed by Copper N-Heterocyclic Carbene Complexes

Herein,we present a method for the homogeneous hydrogenation of nitroarenes to produce anilines using low catalyst loading(1 mo%)of copper N-heterocyclic carbene complexes as the catalyst and ammonia borane as the source of hydrogen.A wide range of nitroarenes,featuring diverse functional groups,were selectively transformed into their corresponding primary aromatic amines with high yields.This process can be readily scaled up and exhibits compatibility with various sensitive functional groups,including halogen,trifluoromethyl,aminomethyl,alkenyl,cyano,ester,amide,and hydroxyl.Notably,this catalytic methodology finds application in the synthesis of essential drug compounds.Mechanistic investigations suggest that the in-situ-generated Cu-H species may serve as active intermediates,with reduction pathways involving species such as azobenzene,1,2-diphenylhydrazine,nitrosobenzene,and N-phenylhydroxylamine.

Recent advances in the synthesis and application of N-heterocyclic carbene-based molecular cages

N-Heterocyclic carbene(NHC)-based cages have emerged as a prominent and dynamic research area within the research field of chemistry. Leveraging the distinctive electronic and steric properties of NHC ligands, the design, synthesis, and application of these corresponding cages have garnered substantial scholarly interest. In recent years, we have witnessed the successful fabrication of diverse NHC-based cages through a range of synthetic methodologies, which hold significant potential for applications in molecular recognition and catalysis. In this review, we delve into the foundational synthetic strategies that underlie the creation of NHC-based cages, employing approaches encompassing metal–NHC chelation, coordination assembly,and covalent bonding. Additionally, we compile the diverse applications of these cages within catalytic processes and molecular recognition. Lastly, we shed light on the current limitations of synthesis and outline future trends in the development of NHCbased cages.