Vanadium(V) Complexes Containing Unsymmetrical N-Heterocyclic Carbene Ligands: Highly Efficient Synthesis and Catalytic Behavior towards Ethylene/Propylene Copolymerization

The highly efficient method has been developed for the synthesis of NHC·VOCl_(3) containing symmetrical or unsymmetrical Nheterocyclic carbene(NHC) ligands by the transmetallation reaction of NHC·AgCl with VOCl_(3).The total isolated yield of VOCl_(3)[1,3-(2,4,6-Me_(3)C_(6)H_(2))_(2)(NCH=)_(2)C:](V4') reached 86% by transmetallation reaction,which is much higher than that(48%) by direct coordination method.This methodology has also been used to synthesize the novel vanadium complexes containing unsymmetrical NHC ligands of VOCl_(3)[PhCH_(2)NCH=CHNR)C:](V5',R=2,4,6-Me_(3)C_(6)H_(2);V6',R=2,4-Me_(2)-6-Ph-C_(6)H_(2);V7',R=2,6-^(i)Pr_(2)-C_(6)H_(3)) with high yield,which could not be obtained by direct coordination method.The catalytic activity and copolymerization ability would be improved by introducing unsymmetrical NHC ligands due to their less steric bulky effect.The vanadium complex V5' containing unsymmetrical NHC ligand exhibits higher catalytic activity(3.7×10^(5)g_(copolymer)·mol^(-1) of V·h^(-1)) than that of V4' containing symmetrical NHC ligand.Moreover,the higher propylene incorporation ratio(45.6 mol%) in the copolymers of ethylene with propylene could be obtained by using V5' than that(39.9%) by using V4'.The results would provide a highly efficient strategy for the synthesis of early transition metal complexes containing versitile NHC ligands,affording the catalyst with both high catalytic activity and copolymerization ability for the synthesis of high performance polyolefin elastomers.

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.

Bio-inspired tetracarbene compounds as a new family of energy saving catalysts

Efficient use of energy is a pressing issue.Improvement of chemical processes is currently an important target for energy efficiency.Although chemical processes are independent of pathways with respect to the final energy output or consumption,the application of catalysts can reduce involved activation energies considerably and therefore save large amounts of energy.

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.

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.

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.

Photoluminescent nickel(Ⅱ)carbene complexes with ligand-to-ligand charge-transfer excited states

While nickel(II)complexes have been widely used as catalysts for carbon-carbon coupling reactions,the exploration of their photophysical and photochemical properties is still in the infancy.Here,a series of square-planar Ni(II)complexes[(diNHC)NiX2]bearing chelating benzimidazole-based bis(N-heterocyclic carbene)ligands and varying anionic coligands(1,X=Cl;2,X=Br;3,X=I)are synthesized and structurally characterized.In solid state,both 1 and 2 exhibit orange-red photoluminescence under ambient conditions.The photophysical and electrochemical measurements along with density functional theory(DFT)calculations reveal that the low-energy emissions can be attributed to singlet excited states with ligand-to-ligand charge-transfer(LLCT)character.This work suggests that strong-field N-heterocyclic carbene ligands play a crucial role to achieve the luminescence of Ni(II)complexes.

An Ultralong Low-Temperature Phosphorescent Pentagonal-Prismatic Organometallic Cylinder Featuring Pentaphenylpyrrole-N-Heterocyclic Carbenes

A pentagonal-prismatic cylinder[Ag5(L)2](PF6)5 obtained from the pentaphenylpyrrole-bridged pentaimidazolium salt H5-L(PF6)5 and AgI ions was determined by X-ray diffraction analyses.The target organometallic cylinder not only exhibited enhanced fluorescence emission in dilute solution at room temperature but also showed an improved phosphorescence ratio compared with the free precursor and maintained a long lifetime(1.39 s)in the solid state at 77 K.Furthermore,the experimental results and DFT calculations confirmed that the formation of the organometallic cylinder promoted intersystem crossing.Meanwhile,the frontier orbitals of[Ag5(L)2](PF6)5 showed the main contribution of building block PPP as the luminescence source of[Ag5(L)2](PF6)5 by a modest heavy-atom effect.These results provide a strategy for constructing enhanced phosphorescent emission and long lifetime organometallic supramolecular phosphorescent materials.

N-Heterocyclic carbene catalyzed C-acylation reaction for access to linear aminoenones

An N-heterocyclic carbene(NHC)-catalyzed carbonyl nucleophilic substitution reaction between 1-cyclopropylcarbaldehydes and N-sulfonyl imines is developed for access to linearβ-aminoenone products.Theβ-aminoenones containing cyclopropyl fragments can be afforded in moderate to excellent yields under mild conditions.The reaction features excellent trans-diastereoselectivities and the desired aminoenone products are all afforded as Z-isomers.

Superatomic Aub clusters ligated by different N-heterocyclic carbenes and their ligand-dependent catalysis, photoluminescence, and proton sensitivity

We report herein a class of superatomic AU13 clusters stabilized by different N-heterocyclic carbenes(NHCs).The clusters show diverse metal surface structures,properties and functions as exemplified by:(1)the first anionic AU13 cluster[Au1g(NHC-1)sBr6],which has bulky NHC-1 ligands that lead to a rather open metal surface contributing to its high catalytic activity;(2)the tricationic cluster[Au13(NHC-2)sBr2]+which has bidentate,benzyl-rich NHC-2 ligands that make it ultra-stable and highly-luminescent,suitable for bio-imaging;and(3)by bearing two pyridyl groups on NHC-3,the dicationic cluster[Au:a(NHC-3)gClg]f+exhibits reversible and stable visible absorption and solubility responses to protonation/deprotonation cycles,making it a potential pH sensor(NHC-1=1,3-disopropylbenzimidazolin-ylidene;NHC-2=1,3-bis(1-benzyl-1H-benzimidazol-1-ium-yl)propane;NHC-3=1,3-bis(icolyI)benzimidazolin-ylidene).The study nicely demonstrates the importance of ligands in designing metal nanoclusters with desired functionalities.

Paddlewheel dirhodium(II)complexes with N-heterocyclic carbene or phosphine ligand:New reactivity and selectivity

The dirhodium(II)-catalyzed organic reactions,including asymmetric transformation,are significantly benefitted from the discovery of simple and efficient protocols to introduce various bridging ligands,e.g.carboxylic acids,amide and phosphoric acids.Ligands lying at axial position of dirhodium(II)complexes could strongly affect the activity and selectivity of these metal complexes involved reactions.This review summarizes the newly explored reactivity of dirhodium(II)complexes with N-heterocyclic carbenes or phosphines as the axial ligands.The asymmetric tries are also highlighted,where their stereo-induction were solely from the axial ligand of dirhodium(II)complexes.In the last part,the reactions catalyzed by inherent chiral ortho-metalated dirhodium(II)/phosphine complexes are also discussed.

氮杂环卡宾-铁配合物催化碳-碳偶联反应的研究进展

作为传统贵金属催化剂的理想替代品,铁具有来源广泛、毒性低和环境兼容性好等特点,目前被用于多种环境友好的催化过程。近年来,氮杂环卡宾(NHC)由于其独特的空间和电子性质,常作为配体,不仅能够被应用于贵金属(如钯、镍和铑等)催化,在铁催化中也得到了广泛的应用。本文综述了氮杂环卡宾及铁配合物催化剂的特点,并重点讨论了近十年其催化的碳-碳偶联包括芳基-芳基交叉偶联、芳基-烷基交叉偶联、烷基-烷基交叉偶联等反应中的研究进展。

Carbene-catalyzed enantioselective seleno-Michael addition as access to antimicrobial active Se-containing heterocycles

Organoseleniums exhibit a diverse set of biological activities that are pivotal for drug discovery and are widely explored in synthetic chemistry and material science.While many transformations have been developed for non-enantioselective C–Se bond formations,the catalyst-controlled stereoselective preparation of chiral organoseleniums continues to be of considerable challenge.In particular,there are limited studies on the enantioselective seleno-Michael addition reactions to access chiral selenium functional molecules.Here,we disclose a carbene-catalyzed highly enantioselective nucleophilic C–Se bond construction through formal[3+3]annulations between selenocarboxamides and bromoenals,affording seleno-thiazinone products with good yields and excellent enantioselectivities.The choice of a weak inorganic base was pivotal to suppressing the unproductive racemization and decomposition of the selenium products.Notably,the catalytically generated chiral selenium-containing heterocyclic products feature remarkable antimicrobial activities that could serve as promising lead scaffolds for further agrochemical development.

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.

N-heterocyclic carbene-catalyzed arene formation reactions

N-heterocyclic carbene-catalyzed reactions for the formation of aromatic compounds are reviewed.The reactions are subdivided into 4 types based on their activation modes.A brief summary on the achievements and the challenges remaining in Nheterocyclic carbene(NHC)-catalyzed arene construction processes is provided at the end of this review.An outlook into the future research direction within this field is also given based on our own opinion and knowledge on the trends of the development of NHC organocatalysis.

N-heterocyclic carbene-catalyzed radical reactions

While N-heterocyclic carbene(NHC)catalyzed electron-pair-transfer processes have been developed into an important tool for synthetically important bond formations during the past decades,the corresponding radical reactions via NHC catalysis have only received growing attention in the past six years.Taking into account the advantages NHC-catalyzed radical reactions might bring,such as creating new activation modes that were previously unobtainable,it is worthwhile to provide a conceptual understanding of this emerging area.Therefore,herein we give an overview of NHC-catalyzed radical reactions via different synthetic techniques.

Synthesis of a Dichlorosilaimine Coordinated by an N-Heterocyclic Carbene from ArN（SiMe3）SiHCl2

NHC-stabilized dichlorosilaimine ArN=Si（IiPr）Cl2 （2） （Ar=2,6-iPr2C6H3, IiPr=1,3-diisopropyl-4,5-dimeth- ylimidazol-2-ylidene） and disilane LSiHCl-SiCl2L （L=ArNSiMe3） （3） could be synthesized from the reaction of aminodichlorosilane ArN（SiMe3）SiHCl2 （1） with IiPr under different conditions. This represents the first route to the generation of donor-stabilized dichlorosilaimine from substituted chlorosilane in the presence of an NHC via dehydrosilylation.

N-Heterocyclic Carbene-Palladium Complex Catalyzed Oxidative Carbonylation of Amines to Ureas

Palladium carbene shows high efficiency without any promoter on oxidative carbonylation of amines to ureas and a new type of palladium carbene complex containing both an aniline and an NHC ligands was found to be the active species for the reaction.

Recent advances in N-heterocyclic carbene-catalyzed radical reactions

The N-heterocyclic carbene(NHC)-catalyzed reactions involving two-electron reaction pathway have been well-established.However,the development of NHC-catalyzed radical reactions is still in its infancy in terms of reaction types and enantioselectivity.In the past decade,several elegant NHC-catalyzed radical reactions have been developed,including NHC-catalyzed oxidation of aldehydes to esters,reductive coupling reactions using Breslow intermediate as SET reductant and NHC-catalyzed reactions via radical homoenolates,dienoaltes and trienolates.This review summarizes the recent advances in NHC-catalyzed reactions involving radical intermediates.

Stereoselective Ring-opening Polymerization of rac-Lactide by Bulky Chiral and Achiral N-heterocyclic Carbenes

Despite the extraordinary success has been achieved in metal catalyst-promoted stereoselective ring-opening polymerization（ROP） of rac-lactide（rac-LA）, well-controlled stereoselective rac-LA ROP by organic catalyst still remains a scientific challenge. Here we report our investigations into organocatalytic stereoselective ROP of rac-LA by utilizing novel bulky chiral and achiral N-heterocyclic carbenes（NHC）, 1,3-bis-（1′-naphthylethyl）imidazolin-2-ylidene. The effect of polymerization conditions（e.g. solvent, temperature, alcohol initiator） on ROP behavior by these bulky NHCs has been fully studied, leading to the formation of isotactic-rich stereoblock polylactide（Pi = 0.81） under optimized conditions with high activity（Conv. = 98% in 30 min） and narrow molecular weight dispersity（D = 1.05）.