Nitrogen Heterocyclic Carbene Research Articles

Nanosilicon Stabilized With Ligands: Effect of High‐Energy Proton Beam on Luminescent Properties

ABSTRACTSilicon nanopowders with nitrogen heterocyclic carbene (NHC) and butyl as stabilizing ligands were synthesized by bottom‐up chemical methods. Transmission electron microscopy (TEM) was used to obtain nanoparticle size distribution with 1.8–2.5‐nm average diameter. Optical characteristics (photoluminescence [PL] and infrared [IR] absorption spectra) of samples were investigated as fabricated and on different steps of irradiation by high‐energy 22.5‐MeV protons. The PL spectral changes are slightly different for two cases, but in general, we can see a decrease in luminescence amplitude with fluence growth up to 4·1014 cm−2, mainly for NHC‐stabilized nanosilicon. Main mechanisms of radiation‐induced changes in nanosilicon sample optical properties are discussed by the joint use of PL and IR spectra analysis.

Greening up organic reactions with caffeine: applications, recent developments, and future directions.

Numerous studies in the field of alkaloid chemistry have provided researchers with valuable insights into their unique properties as catalysts. Among the diverse natural catalysts, caffeine has emerged as a green, expedient, and biodegradable catalyst with high efficiency and applicability. Interest in using caffeine as a catalyst has burgeoned over the past few years with its role in diverse multicomponent reactions. Preparation of its imidazolium salts and further conversion to Nitrogen Heterocyclic Carbene (NHC) ligands and ionic liquids offers new paradigms. Caffeine has also played a multifaceted role as a support material in influencing the structural properties of nanoparticles. We hope that the chemistry of caffeine and its applications for sustainable organic transformations discussed in this review will stimulate new thinking and open new avenues in this field.

Carbene and photocatalyst-catalyzed 3-acylation of indoles for facile access to indole-3-yl aryl ketones.

3-Acyl indoles play important roles in both organic synthesis and diverse types of functional molecules. Herein, a facile nitrogen heterocyclic carbene (NHC) and photocatalyst cooperatively-catalyzed 3-acylation of indoles was disclosed. The reaction proceeded via radical cross-coupling of indole-based aryl radical cations with NHC-bound ketyl radical species, which are less explored in radical NHC catalysis. The reaction exhibits mild reaction conditions, broad substrate scope, and good functional group tolerance. Mechanistic studies support our proposed reaction pathway. The synthesis of structurally diverse analogs of an aldose reductase inhibitor and antibacterial activity investigation further demonstrated the utility of the current acylation reaction.

π-Hole Triel Bonds between TrPhX2 (Tr = B, Al, Ga; X = F, Cl, Br, CH3, OH) and (BH)2(NHC)2 (NHC = Nitrogen-Heterocyclic Carbene)

The π-hole triel bond formed by (BH)2(NHC)2 (NHC denotes nitrogen-heterocyclic carbene) and TrPhX2 (Tr = B, Al, and Ga; X = F, Cl, Br, CH3, and OH) was investigated computationally, with the B=B bond in (BH)2(NHC)2 being the electron donor. A large interaction energy ensures that the complex is quite stable. When the substituent X in the electron acceptor is fixed, the magnitude of the interaction energy varies with the identity of the Tr atom. When Tr is Al or Ga, the interaction energy is stronger than when it is B. With an increase in the electron-withdrawing ability of the substituents, the interaction energy shows distinct changes. When Tr is B or Al, the interaction energy varies as TrPhBr2 > TrPhCl2 > TrPhF2, which is different from the order of their positive electrostatic potentials. When Tr = Ga, the interaction energy hardly changes with an increase in the electronegativity of the halogen atoms. For CH3 and OH substitution, larger interaction energies were obtained, with the interaction energy for the OH substituent being the largest. The main interactions in these systems are a triel bond and an X· ·H hydrogen bond. When the substituents are fixed, the interaction energy of the triel bond increases in the order AlPhX2 < GaPhX2 < BPhX2, which is different from the order of the positive electrostatic potentials on the Tr atom in TrPhX2. When X is a halogen atom, the interaction energy of the triel bond decreases in the order Br > Cl > F, which is opposite to the trend for the positive electrostatic potentials on Tr in TrPhX2. In most complexes, the interaction energy for the hydrogen bond is less than that for the triel bond; there is no hydrogen bond in the methyl-substituted complex. In general, the interaction energy of the hydrogen bonds increases with an increase in the electronegativity of the halogen atoms.

Synthesis and high sensitivity Al3+ recognition of fluorescent probe molecule based on pincer bis‐N‐heterocyclic carbene

Two bis-(Nitrogen-Heterocyclic Carbene, NHC) derivatives, 1, 3-bis[1-(quinolin-8-yl)-imidazolium-3-yl)]-benzene bis-hexafluorophosphate (2a), 2, 5-bis[1-(quinolin-8-yl)-imidazolium-3-yl)]-pyridine bis-hexafluorophosphate (2b), were synthesized, and characterized by 1H NMR and 13C NMR spectra and X-ray single crystal diffraction. The bis-NHC 2a was demonstrated as an excellent fluorescence chemo-sensor molecule for selectively recognition of Al3+ ion in aqueous-organic media. Its pyridine analogue molecule 2b haven't selectivity recognition character to metal ions with the fluorescence enhancement for most metal ions. A 1:1 binding ratio and strong supramolecular interaction between Al3+ ion and the carbonic hydrogen and quinoline nitrogen were suggested by Job's plot analysis and 1HNMR spectroscopy titrations of Al3+ ion and 2a. The detection limit for Al3+ ion was calculated to be 0.32 µM. The practical application in tap water samples has been evaluated, which reveals the potential applications of bis-NHC 2a in complicated environments.

Boosting Palladium-Catalyzed Aryl-Nitro Bond Activation Reaction by Understanding the Electronic, Electrostatic, and Polarization Effect: A Computational Study from a Basic Understanding to Ligand Design.

Although palladium-catalyzed aryl-nitro bond activation reaction has recently gained a lot of interest, it still requires rather harsh conditions. We here systematically explore the substituent effect on oxidative addition steps, known as the rate-determining step, by density functional theory simulations based on a Nakao's nitrogen heterocyclic carbene (NHC) ligand. The key aryl ring on the catalyst, ring A, acts as a π-donor and stabilizes the palladium center of the transition state, and thus an electron-rich ring A is expected to lower the barrier. However, the polarization and electrostatic effects were shown to be more important, although they were often ignored before. These effects originate from through-space interaction with a nitro group in the resting state, and the overall effect is that any polarizable or partly negative group near ortho- or meta-site of ring A is harmful for the reaction. Based on these discoveries, we proposed a list of guidelines for successful ligand developments and designed several new ligands. These ligands exhibit a significantly lower barrier than the reported Nakao's ligand by as large as ∼5 kcal/mol, in both gas phase and solvent with a moderate dipole. These candidates will promote further experimental studies and enhance the ability to improve ligands in a rational and predictive manner.

Highly Efficient Thermally Activated Delayed Fluorescence from Pyrazine-Fused Carbene Au(I) Emitters.

Metal-based thermally activated delayed fluorescence (TADF) is conceived to inherit the advantages of both phosphorescent metal complexes and purely organic TADF compounds for high-performance electroluminescence. Herein a panel of new TADF Au(I) emitters has been designed and synthesized by using carbazole and pyrazine-fused nitrogen-heterocyclic carbene (NHC) as the donor and acceptor ligands, respectively. Single-crystal X-ray structures show linear molecular shape and coplanar arrangement of the donor and acceptor with small dihedral angles of <6.5°. The coplanar orientation and appropriate separation of the HOMO and LUMO in this type of molecules favour the formation of charge-transfer excited state with appreciable oscillator strength. Together with a minor but essential heavy atom effect of Au ion, the complexes in doped films exhibit highly efficient (Φ∼0.9) and short-lived (<1 μs) green emissions via TADF. Computational studies on this class of emitters have been performed to decipher the key reverse intersystem crossing (RISC) pathway. In addition to a small energy splitting between the lowest singlet and triplet excited states (ΔEST ), the spin-orbit coupling (SOC) effect is found to be larger at a specific torsion angle between the donor and acceptor planes which favours the RISC process the most. This work provides an alternative molecular design to TADF Au(I) carbene emitters for OLED application.

Homogeneous and noncovalent immobilization of NHC-Cu catalyzed azide-alkyne cycloaddition reaction

Abstract A series of new pyrene tagged nitrogen heterocyclic carbene copper (NHC-Cu) molecular complexes were synthesized and characterized by 1H- and 13C-NMR, high resolution ESI-mass spectrometry (HR-MS). The corresponding NHC-Cu complex was immobilized onto the surface of multi-walled carbon nanotubes (MWNTs) by using π-π stacking interaction, the noncovalent immobilization catalytic material MWNTs@NHC-Cu were characterized by FT-IR, XRD, XPS, thermogravimetry (TGA) and TEM. The new NHC-Cu complexes were tested both in the homogeneous phase and once immobilized onto multi-walled carbon nanotubes support for the azide-alkyne cycloaddition reaction. Both were active in the two-component click cycloaddition reaction of terminal alkynes and organic azides and three-component cycloaddition reaction that employs various terminal alkynes, NaN3 and organic halides with the advantage of excellent yields, low catalyst dosage, short reaction time. The MWNTs@NHC-Cu catalyst could be separated from the reaction system at the end of the reaction and reused in another catalytic cycle, but with loss of product yield.

Retraction Note to: Mononuclear Cu Complexes Based onNitrogen Heterocyclic Carbene: A Comprehensive Review.

None of the authors have responded to any correspondence from the publisher about this retraction.

Mononuclear Cu Complexes Based on Nitrogen Heterocyclic Carbene: A Comprehensive Review.

During the last decade, organometallic, coordination, and catalytic chemistry of the three-dimensional metals such as copper (Cu) has been greatly affected by the emergence of nitrogen heterocyclic carbene (NHC) complexes. The NHCs, and in particular the mononuclear CuI-based ones, have been proven vastly useful in several applications such as in biosynthesis, catalysis, photochemistry, etc. This review tries to thoroughly describe a series of mononuclear CuI NHC complexes and their subcategories such as heteroleptics, and bidentate and tridentate heteroatom complexes, and give some detailed insights on their development, emergence, and applications. A brief outlook is also disclosed to enable other researchers to further develop a platform for future advances and studies in the field of CuI-based NHCs.

Formation of anionic NHC complexes through the reaction of benzimidazoles with mercury chloride. Subsequent protonation and transmetallation reactions

Anionic nitrogen heterocyclic carbene complexes of C,N-bound mercuramacrocycles {Hg4Cl4(R-Bim)4} (BimH = benzimidazole; R = Ph, Py) were prepared either from the simple reaction of N-substituted benzimidazoles with HgCl2 or from the reaction of N-substituted benzimidazoles with Hg(OAc)2 and NaCl. The former reaction is unique as no additional base is required, although in low reaction yield. Under similar reaction conditions, only N-bound benzimidazole complexes were obtained when HgBr2 or HgI2 instead of HgCl2 was employed. The different reactivity could be due to the hard-soft acid-base property of mercury(II) and halides, that facilitates the cleavage of Hg-Cl bond and promotes the C-metallation of HgCl2 and benzimidazole. Upon protonation of {Hg4Cl4(R-Bim)4} with HBF4⋅OEt2, NH,NR-NHC complexes of [HgCl(R-Bim-H)][BF4] (R = Ph, Py) were obtained. Preliminary study on the transmetallation reaction of {Hg4Cl4(Ph-Bim)4} with PdCl2(CH3CN)2 and LiCl did provide a C,N-bound NHC palladium complex.

Molecular Manipulations of a Utility Nitrogen-Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes.

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium-magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR2 (R=CH2 SiMe3 ) affords [(THF)3 Na(μ-IPr- )MgR2 (THF)] (2) [IPr- =:C{[N(2,6-iPr2 C6 H3 )]2 CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR3 ] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr2 Na2 MgR4 ] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPrMe MgR2 }2 ] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR2 and aIPrMe with the latter isomerizing to the olefinic NHC IPr=CH2 . The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to AuI metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr- )MgR(THF)2 ] (4) and [aIPrMe AuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)3 }+ cation.

Highly Selective Olefin Metathesis with CAAC-Containing Ruthenium Benzylidenes

Several olefin metathesis reactions are studied, namely, jojoba oil oligomerization, methyl oleate self-metathesis, ring-closing metathesis (RCM) to form a nitrogen heterocycle, and 1,5-hexadiene acyclic diene metathesis polymerization (ADMET). The catalyst containing the Bertrand–Grubbs cyclic alkyl amino carbene (CAAC) ligand showed high selectivity by diminishing isomerization reactions; this was especially clear at high temperatures where the more widely used nitrogen heterocyclic carbene (NHC)-based catalysts show side reactions. Experimental and computational studies determined that it is much more difficult to produce ruthenium hydrides with CAAC, a property that can explain the improved observed activity. This finding opens a pathway for the development of even more selective olefin metathesis catalysts for reactions that require harsh conditions.

A new nitrogen heterocyclic carbene containing diiron complex as bio-inspired catalyst for proton reduction and benzene hydroxylation

A new diiron complex (μ-dmedt)[Fe2(CO)5IPr] (2, dmedt = 2,3-butanedithiol, IPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene) was synthesized as analogue of the active site of [Fe-Fe]-H2ases. The sterically bulky nitrogen heterocyclic carbene ligand of IPr coordinated to the iron center was introduced via CO/L substitution reaction in mild conditions. The reactivity of electrocatalytic reduction of proton and catalytic hydroxylation of benzene were explored for the developed diiron complex. Complex 2 undergoes two irreversible reduction events at ca. -2.17 V and −2.50 V and acts as an efficient electrocatalyst for proton reduction to hydrogen with both HOAc and H2O as proton source. The catalytic hydroxylation of benzene by complex 2 achieved phenol yield of 32.4% with almost 100% selectivity. The enhanced catalytic activity compared to the all carbonyl precursor and related derivatives was attributed to the good electron donating ability of introduced IPr ligand.

Synthesis, characterization and a reactivity study of some allyl palladium complexes bearing bidentate hemi-labile carbene or mixed carbene/PPh3 ligands

With the aim at synthesizing novel allyl complexes that can potentially act as catalysts in the Tsuji–Trost catalyzed reaction, we have synthesized and characterized some allyl and 2-Meallyl palladium derivatives with one hemilabile bidentate or two strong mono-coordinating spectator ligands. The hemilabile ligands are constituted by one nitrogen heterocyclic carbene (NHC) fragment acting as the pivot bearing a labile wing with a pyridine nitrogen or sulfur atom as the second stabilizing atom. One of two monodentate ligands is in all cases PPh3 whereas the other is a mono- or partially coordinated hemilabile carbene. The complexes were characterized by standard spectroscopic methods and elemental analysis and in two cases by SC-XRD technique.The reactivity of two selected complexes toward the Tsuji–Trost reaction was tested by stoichiometric allyl amination carried out with piperidine and the results of such a mechanistic investigation integrated by a computational study are also reported in this paper.

Double stabilization of nanocrystalline silicon: a bonus from solvent

Double stabilization of the silicon nanocrystals was observed for the first time by 29Si and 13C MAS NMR spectroscopy. The role of solvent, 1,2-dimethoxyethane (glyme), in formation and stabilization of silicon nanocrystals as well as mechanism of modification of the surface of silicon nanocrystals by nitrogen-heterocyclic carbene (NHC) was studied in this research. It was shown that silicon nanocrystals were stabilized by the products of cleavage of the C–O bonds in ethers and similar compounds. The fact of stabilization of silicon nanoparticles with NHC ligands in glyme was experimentally detected. It was demonstrated that MAS NMR spectroscopy is rather informative for study of the surface of silicon nanoparticles but it needs very pure samples.

Nitrogen heterocyclic carbene containing pentacoordinate iron dicarbonyl as a [Fe]-hydrogenase active site model.

A novel pentacoordinate mono iron dicarbonyl complex bearing a nitrogen heterocyclic carbene ligand was reported as a model of a [Fe]-hydrogenase active site, which exhibits interesting proton coupled CO binding reactivity, electro-catalytic proton reduction and catalytic transfer hydrogenation reactivity.

Traditional and ion-pair halogen-bonded complexes between chlorine and bromine derivatives and a nitrogen-heterocyclic carbene.

A theoretical study of the halogen-bonded complexes (A-X···C) formed between halogenated derivatives (A-X; A = F, Cl, Br, CN, CCH, CF3, CH3, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug'-cc-pVDZ level of theory. Two types of A-X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C-X distances and small binding energies (8-54 kJ·mol(-1)). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C-X distances and large binding energies (148-200 kJ·mol(-1)), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A(-) and (+)[X-CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A(LP) to the σ* orbital of C-X bond is also identified as a significant stabilizing interaction in type-II complexes.

Synthesis and properties of nanosilicon prepared by homogeneous and heterogeneous reduction of tetraethyl orthosilicate

Nanocrystalline silicon was synthesized by reduction of tetraethyl orthosilicate both in homogeneous and heterogeneous processes with surface stabilization by nitrogen heterocyclic carbene (NHC) ligands. The presence of NHC ligands on the surface of silicon nanocrystals was demonstrated by IR spectroscopy. According to transmission electron microscopy data, the size of silicon nanocrystals is in the range of 2.2–8.4 nm. Photoluminescence of the samples is clearly recorded in the blue spectral region.

Role of Electronegative Substituents on the Bond Energies in the Grubbs Metathesis Catalysts for M = Fe, Ru, Os

Coupled cluster theory [CCSD(T)] with the aug-cc-pVDZ/aug-cc-pVDZ-PP basis sets is used to predict the thermodynamic properties of models of the Grubbs catalyst, H2ImM(PH3)(CRR′) and H2ImM(C2H4)(CRR′) for M = Fe, Ru, and Os and CRR′ = CH2, CHF, and CF2. The PH3 and C2H4, imidazolinium carbene (H2Im), and CRR′ bond dissociation energies (BDEs) are reported. Because of low metal-carbene BDEs, the M = Fe complexes are unlikely to form, so they will not be good catalysts for olefin metathesis. The metal–carbene BDE is an important component in metathesis catalyst design and correlates with the singlet–triplet splitting in the carbene. The two metallacyclobutane intermediates (cis and trans to the imidazolinium carbene) formed by reaction of the CRR′═CRR′ with the 14-electron active species (H2ImM(CRR′)) (R and R′ either H or F) are investigated at the same level of theory. The metallacycles cis to the nitrogen heterocyclic carbene are lower in energy than the trans conformer with the exception of four M = Fe metallacycles in the gas phase and in CH2Cl2 solution at 298 K. The olefin π complex for the simplest CH2 ligand plus C2H4 reactant combination is more stable in the gas phase, but in CH2Cl2 solution at 298 K, the cis metallacycles are more stable than the π complexes or the trans metallacycles on the free energy scale as a result of the large dipole moments in the cis metallacycles. The results show that the best energy balance is achieved with M = Ru, a CH2 carbene substituent, and a C2H4 reactant. The energetics for the Grubbs catalysts are shown to differ from those of the Schrock catalysts.