N-heterocyclic Carbene Copper Research Articles

Microwave-Assisted Synthesis of Symmetrical 1,4-Disubstituted Bis-1H-1,2,3-triazoles Using Copper N-Heterocyclic Carbene Catalysts

Bis-triazoles separated by a symmetrical linking group are joined at C4 of each triazole or at N1 of each triazole. Preparation of a series of bis-1H-1,2,3-triazoles derived from o-bis(azidomethyl)benzene and an alkyne is reported with use of copper N-heterocyclic carbene catalysis with microwave-assisted heating in an aqueous solvent. The products were symmetrical N1–N1′-bis-1H-1,2,3-triazoles. Additional syntheses utilized dialkynes and organic azides to prepare symmetrical C4–C4′-bis-1H-1,2,3-triazoles. Pure products were often obtained directly when water was used as the solvent with microwave-assisted heating. Results are given for experiments using conventional heating or no heating. Sonication results are given for a reaction where microwave-assisted heating was unsatisfactory.

Synthesis and Characterization of Symmetrical N-Heterocyclic Carbene Copper(II) Complexes-An Investigation of the Influence of Pyridinyl Substituents.

Three new tridentate copper(II) N-heterocyclic carbene (NHC) complexes have been obtained and characterized with symmetrical C-4 substitutions on their pendent pyridine rings. Substitutions including methyl (Me), methoxy (OMe), and chloro (Cl) groups, which extend the library pincer Cu-NHC complexes under investigation, modify the impact of pyridinyl basicity on NCN pincer complexes. Both ligand precursors and copper(II) complexes are characterized using a range of techniques, including nuclear magnetic resonance (NMR) spectroscopy for 1H, 13C, 31P, and 19F nuclei, electrospray ionization mass spectrometry (ESI-MS), X-ray crystallography, cyclic voltammetry, and UV-Vis spectroscopy. The pyridine substitutions lead to minimal changes to bond lengths and angles in the X-ray crystal structures of these related complexes; there is a pronounced impact on the electrochemical behavior of both the ligand precursors and copper complexes in the solution. The substitution in the pyridinyl units of these complexes show an impact on the catalytic reactivity of these complexes as applied to a model C-N bond-forming reaction (CEL cross-coupling) under well-established conditions; however, this observation does not correlate to the expected change in basicity in these ligands.

Synthesis, luminescence properties and computation studies of two-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing quinoline derivatives as ligands

Here, we report a series of two-coordinate N-heterocyclic carbene (NHC) copper(I) complexes having monodenate NHC 1,3-bis(2,6- diisopropylphenyl)imidazol-2-ylidene) (IPr) as a σ donor and quinoline and its derivatives as π-chromophore ligands, [Cu(IPr)(Quinoline)]BF4 (P1), [Cu(IPr)(Chloroquinoline)]BF4 (P2), [Cu(IPr)(Phenylquinoline)]BF4 (P3), and [Cu(IPr)(Aminoquinoline)]BF4 (P4), (Chloroquinoline = 2,4-dichloroquinoline, Phenylquinoline = 2-phenylquinoline, Aminoquinoline = 2,4-diaminequinoline). The X-ray diffraction studies of P1 and P3 revealed the linear coordination around the copper atom. P1 and P2 exhibit the typical metal-to-ligand charge transfer (1MLCT) absorption bands in the lowest-lying energy region, while P3 and P4 show the mixed characters of ligand-to-ligand charge transfer (1LLCT) and π→π* transition. The emission wavelengths of all NHCCu(I) complexes can be fine-tuned to cover the spectral range of 419–543 nm with the main 3(π-π) emission origin in the solid state. The differences for photophysical properties of NHCCu(I) complexes were rationalized using density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations.

Three water-soluble copper(II) N-heterocyclic carbene complexes: toward copper-catalyzed ketone reduction under sustainable conditions.

A series of tridentate copper(II) N-heterocyclic carbene (NHC) complexes with imidazole, benzimidazole, and 5,6-dimethylbenzimidazole azole rings were synthesized and comprehensively characterized via X-ray crystallography, ESI-MS, cyclic voltammetry, and UV-Vis and EPR spectroscopic studies. These complexes were then utilized for the optimization of ketone reduction under sustainable conditions using 2-acetylpyridine and phenylsilane. The relationships between product formation, temperature, reaction time, and catalyst loading for the hydrogenation reactions are covered in detail. Reduction of eighteen different aliphatic, cyclic, and aromatic ketones were demonstrated, which were compatible to produce the corresponding products in moderate to good yields. These systems were used to develop related DNA-hybrid catalytic systems, but only supported weak enantioselectivity. Further thermodynamic experiments showed Cu-NHC complexes did not demonstrate specific binding to DNA, which is consistent with their limited selectivity.

Synthesis and photophysical properties of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes and their application to photoluminescent light-emitting diodes and anti-counterfeiting

Here, a series of dinuclear N-heterocyclic carbene (NHC) copper(I) complexes having 3,3′-(1,4-phenylenebis(methylene))bis(1-(pyridin-2-yl)-1H-imidazolylidene as bis-NHC ligand and bis[(2-diphenylphosphino)phenyl]ether (POP) as auxiliary ligand have been successfully prepared, and their photophysical properites were investigaged experimentally and theocitcally. The resulting complexes all exhibited intense green to yellow emission that originated from the thermally activated delayed fluorescence (TADF) with a high photoluminescence quantum yield of up to 0.67 and longer excited-state lifetimes on the microsecond time scale in the solid state. Green and yellow light-emitting diode (LED) devices based on Cu(I) complexes have successfully achieved good color rendering indices. Moreover, the anti-counterfeiting patterns and QR codes made of Cu(I) complexes have been applied to clothing, banknotes, books and glass plates with excellent anti-counterfeiting effects.

Luminescent four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes bearing the pyridyl-imidazolylidene ligand

The synthesis, characterization and photophysical properties of a series of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes are reported. These NHC-Cu(I) complexes contain two bidentate ligands, the R-substituted pyridyl-imidazolylidene-type ligand and diphosphine ligand bis[(2-diphenylphosphino)phenyl] ether (POP). The photophysical properties of these complexes can be adjusted depending on the nature of substituents at 4-position of the phenyl ring in the NHC ligand. The metal-to-ligand charge transfer (MLCT) absorption band can be observed for all complexes at 348–362 nm, and the emission wavelengths can be fine-tuned to cover the spectral range of 543–555 nm with the higher photoluminescence efficiency of up to 66.2% in the solid state. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations were employed to support potophysical properties of the NHC-Cu(I) complexes.

Highly Diastereoselective Synthesis of C-Glycosides from Glycal Anomers.

This Letter presents a highly diastereoselective synthesis of C-hydroxymethine glycosides from glycal anomers using a chiral N-heterocyclic carbene-copper catalyst. The high diastereoselectivity was synergistically controlled by the stereocenter of the substrate and chirality of the N-heterocyclic carbene-copper complex without being interrupted by the stereochemistry of C5 and the anomeric position. This approach enables the production of a diverse array of C-hydroxymethine glycosides using synthetically versatile glycals and various functionalized aldehydes.

Synthesis and luminescence properties of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with different bisphosphine ligands

Synthesis and luminescence properties of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with different bisphosphine ligands

Copper-Mediated Fluoroalkylation of Aryl Bromides and Chlorides Enabled by Directing Groups.

This report describes the reactions between N-heterocyclic carbene copper(I) fluoroalkyl complexes and aryl halides bearing ortho-directing groups. Pyridine, pyrazole, oxazoline, imine, and ester directing groups are shown to dramatically enhance the reactivity of aryl bromides and chlorides with (IPr)CuI-fluoroalkyl complexes (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; fluoroalkyl = difluoromethyl and pentafluoroethyl) to afford aryl-fluoroalkyl coupling products. This approach is leveraged to achieve the Cu-catalyzed directed fluoroalkylation of a series of aryl bromide substrates.

Development of a Graphene Oxide-Supported N-Heterocyclic Carbene Copper(I) Complex as a Heterogeneous Catalyst for the Selective N-Monoalkylation of Amines

A new N-heterocyclic carbene (NHC) copper(I) complex supported on graphene oxide (GO-NHC-Cu) was synthesised and thoroughly characterised by various instrumental techniques such as FT-IR, FT-Raman, PXRD, XPS, FESEM, EDX, HRTEM, TGA and ICP-OES. The catalytic activity of the supported complex was explored in the N-alkylation of anilines with alcohols under solvent-free and aerobic conditions to afford monoalkylated products in good to excellent yields (20 products, 83–96%). All products were isolated and characterised by 1H and 13C{1H} NMR spectroscopy. The catalyst was recuperated from the reaction mixture by simple filtration and reused for up to five successive cycles with insignificant loss in the catalytic activity. The control experiments showed that the reaction proceeded in aerobic conditions. The green chemistry metrics for the reaction were found to be fairly close to the ideal values: carbon efficiency (95.9%), E-factor (0.15), atom economy (92.14%), process mass intensity (1.15) and reaction mass efficiency (86.80%). The air stability, selectivity, recyclability of the catalyst, and the high yields of the products under solvent-free conditions are some of the salient features of the reported methodology.

Convenient preparation of copper(I) N-heterocyclic carbene complexes under mild conditions

This work describes a new method for the preparation of copper N-heterocyclic carbene (NHC) complexes, which is conducted under air with mild conditions, using bench-stable precursors and a weak base (NEt3). Under optimized conditions, the complexes were prepared in short reaction times of 1 h, giving moderate to good yields after purification by column chromatography. Commonly utilized NHC–HCl salts were found to be applicable reacting with CuCl or CuI giving [(NHC)CuX]-type complexes (X = Cl, I). This study adds to the growing literature in the preparation of catalytically relevant transition metal NHC complexes under mild and operationally simple conditions.

Reactivity and Stability of a Ring-Expanded N-Heterocyclic Carbene Copper(I) Boryl Imidinate

Frustrated Lewis pairs (FLPs) have evolved from a revolutionary concept to widely applied catalysts. We recently reported the ring-expanded N-heterocyclic carbene supported copper(I) boryliminomethanide, (6-Dipp)CuC(=NtBu)Bpin and noted it reacted with heterocumulenes in a fashion reminiscent of FLPs. We thus set out to explore its reactivity with a range of other substrates known to react with FLPs. This was undertaken by a series of synthetic studies using NMR spectroscopy, mass spectrometry, IR spectroscopy, and single crystal X-ray crystallography. (6-Dipp)CuC(=NtBu)Bpin was investigated for its reactivity towards water, hydrogen, and phenylacetylene. Its solution stability was also explored. Upon heating, (6-Dipp)CuC(=NtBu)Bpin decomposed to (6-Dipp)CuCN, which was characterised by SC-XRD and NMR spectroscopy, and pinBtBu. Although no reaction was observed with hydrogen, (6-Dipp)CuC(=NtBu)Bpin reacted with water to form (6-Dipp)CuC(=N(H)tBu)B(OH)pin, which was structurally characterised. In contrast to its FLP-reminiscent heterolytic cleavage reactivity towards water, (6-Dipp)CuC(=NtBu)Bpin acted as a Brønsted base towards phenyl acetylene generating (6-Dipp)CuCCPh, which was characterised by SC-XRD, IR, and NMR spectroscopy, and HC(=NtBu)Bpin

N-Heterocyclic carbene copper complex catalyzed Chan-Evans-Lam reactions of arylboronic acids with azoles and amines

An efficient protocol of Chan-Evans-Lam reactions involving arylboronic acids and azole and amine derivatives was developed based on a series of N-heterocyclic carbene copper (Cu-NHC) complexes as the precatalysts. The relationship between the catalyst structure and catalytic properties was investigated in detail. The results demonstrated that the modification of NHC backbone with acenaphthyl group exhibited significant beneficial effect on the coupling efficiencies. Nonetheless, it appeared that more bulk on the N-aryl moieties of NHC ligands impeded the reactions. The protocol also features mild reaction conditions, such as low catalyst loading (0.1–1 mol%), non-toxic solvent and air as the sole oxidant, which highlights the usefulness of Cu-NHC as the precatalyst for this reaction.

Luminescence color tuning of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with imidazolylidene ligand functionalized by thiazole/benzoxazole moiety

Here, we report a series of the four-coordinate N-heterocyclic carbene (NHC) copper(I) complexes with imidazolylidene and POP ligands, [Cu(BenzThiazolyl-Im)(POP)]PF6 (P1), [Cu(BenzThiazolyl-BenIm)(POP)]PF6 (P2), [Cu(BenzBenzoxazolyl-Im)(POP)]PF6 (P3), and [Cu(BenzBenzoxazolyl-BenIm)(POP)]PF6 (P4), (BenzThiazolyl-Im=3-benzyl-1-(thiazol-2-yl)-1H-imidazolylidene, BenzThiazolyl-BenIm=3-benzyl-1-(thiazol-2-yl)-1H-Benzoimidazolylidene BenzBenzoxazolyl-Im=3-benzyl-1-(benzoxazol-2-yl)-1H-imidazolylidene, BenzBenzoxazolyl-BenIm=3-benzyl-1-(benzocazol-2-yl)-1H-imidazolylidene, POP=bis[2-diphenylphosphino]-phenyl)ether). Thiazole/benzoxazole moiety were functionalized onto imidazolylidene ligand to tune the photophysical properties of the corresponding complexes. P1 and P2 exhibit the metal-to-ligand charge transfer (MLCT) absorption band with the onset at 406–412 nm, while P3 and P4 have similar MLCT band with the onset at 380–389 nm. The emission wavelengths of all NHCCu(I) complexes can be fine-tuned to cover the spectral range of 517–551 nm with the phosphorescent origin in PMMA films. P2 show the highest photoluminescence efficiency of 40.9% with the red-shifted emission wavelength at 551 nm, whereas P4 has the lowest photoluminescence efficiency of 1.80% with the blue-shifted emission wavelength at 517 nm. Density functional theory (DFT) and time dependent density functional theory (TDDFT) calculations were employed to rationalize the differences for photophysical properties of the NHCCu(I) complexes.

Highly Active and Stable Bis Imidazolium-Based Copper N-heterocyclic Carbene Modified Graphene Oxide for O-arylation and N-arylation Reactions in Water

Highly Active and Stable Bis Imidazolium-Based Copper N-heterocyclic Carbene Modified Graphene Oxide for O-arylation and N-arylation Reactions in Water

Copper(II) NHC Catalyst for the Formation of Phenol from Arylboronic Acid.

Arylboronic acids are commonly used in modern organic chemistry to form new C-C and C-heteroatom bonds. These activated organic synthons show reactivity with heteroatoms in a range of substrates under ambient oxidative conditions. This broad reactivity has limited their use in protic, renewable solvents like water, ethanol, and methanol. Here, we report our efforts to study and optimize the activation of arylboronic acids by a copper(II) N-heterocyclic carbene (NHC) complex in aqueous solution and in a range of alcohols to generate phenol and aryl ethers, respectively. The optimized reactivity showcases the ability to make targeted C-O bonds, but also identifies conditions where water and alcohol activation could be limiting for C-C and C-heteroatom bond-forming reactions. This copper(II) complex shows strong reactivity toward arylboronic acid activation in aqueous medium at ambient temperature. The relationship between product formation and temperature and catalyst loading are described. Additionally, the effects of buffer, pH, base, and co-solvent are explored with respect to phenol and ether generation reactions. Characterization of the new copper(II) NCN-pincer complex by X-ray crystallography, HR-MS, cyclic voltammetry, FT-IR and UV-Vis spectral studies is reported.

Helicenic N-heterocyclic carbene copper(I) complex displaying circularly polarized blue fluorescence.

Enantiopure copper(I) chloride complexes bearing a monodentate N-(carbo[6]helicenyl)-NHC ligand have been prepared and characterized experimentally and computationally. Their high stability enables the stereochemistry to be probed by X-ray crystallography and NMR spectroscopy. The resolved enantiomeric complexes emit circularly polarized blue fluorescence with glum ∼1.3 × 10-3 in solution. The photophysical and chiroptical properties of these systems, with their helicene-centred origin, are similar to those of the organic helicene-benzimidazole precursor proligand, although the reverse axial chirality configuration is preferentially observed for the complex compared to the ligand.

Immobilization of vitamin B1 on the magnetic dialdehyde starch as an efficient carbene-type support for the copper complexation and its catalytic activity examination

Since the starch biopolymer is an available and inexpensive matrix with modifiable functionality and stabilization capability for metal ions, in this report, we oxidized it to dialdehyde form for the further functionalization with vitamin B1 as a green σ-donor and π-acceptor carbene type ligand. Immobilization of vitamin B1 on this biopolymer was done through imine bond formation between NH2 groups of aminopyrimidine segment of vitamin B1 and aldehyde functional groups of starch oxide. Thiazolium heterocycle part in this biomolecule provided a carbene type precursor for the metal complexation. After the magnetization process by using of Fe3O4 nanoparticles that lead to quick and facile magnetic separation and metal catalyst recycling, copper ions immobilized on the magnetic support (5.9 wt% Cu, 0.93 mmol/g). The prepared copper N-heterocyclic carbene complex (Fe3O4@DAS@VB1@CuCl nanocomposite) was characterized by FT-IR, SEM, EDX, XRD, VSM, TGA and ICP-OES analysis and then its catalytic activity investigated in azidonation of arylboronic acids and also one-pot coupling reaction of the synthesized aryl azides with phenylacetylene. 1,4-Diaryl 1,2,3-triazoles were obtained in excellent yields (≥90%) at proper reaction times (30–200 min). The magnetic catalyst was recovered by a magnetic field and reused in azidation reaction up to 7 cycle.

Highly ordered mesoporous hybrid silica functionalized with ionic liquid framework supported copper and its application in the oxidation of alcohols

A highly ordered organic-inorganic hybrid nanomaterial containing copper N-heterocyclic carbene complex (Cu-NHC@Pyrm-OMS) was synthesized and characterized using various techniques including FTIR, MAS NMR, XRD, TGA, SEM, and TEM. Cu-NHC@Pyrm-OMS nanomaterial is highly efficient heterogeneous system towards the selective oxidation of primary and secondary alcohols to corresponding aldehydes and ketones under mild conditions. Moreover, the supported copper nanocatalyst exhibited outstanding stability and could be reused at least ten times, remaining almost unchanged from initial activity. This work has focused on sustainable and green chemistry that use recoverable nanocatalyst, clean oxidant and aqueous media.

Synthetic Access to Ring-Expanded N-Heterocyclic Carbene (RE-NHC) Copper Complexes and Their Performance in Click Chemistry

The facile syntheses of ring-expanded N-heterocyclic carbene (RE-NHC) copper(I) halide complexes are reported. The method makes use of a weak inorganic base in a green solvent. The reaction times can be greatly reduced by use of this weak-base route under microwave irradiation. The easy access to these complexes permits an evaluation of the catalytic activity and reaction profiling of [Cu(RE-NHC)X] complexes in the Huisgen 1,3-cycloaddition reaction.