Chelating N-heterocyclic Carbene Research Articles

A novel chelating bidentate NHC ligand based on the cationic (η5-cyclopentadienyl)(η6-arene)iron(II) motif

We here present the synthesis of a novel chelating N-heterocyclic carbene (NHC) ligand with the two NHC moieties bound to a cationic (η5-cyclopentadienyl)(η6-arene)iron(II) center and its coordination to palladium(II). Spectroscopic and spectrometric characterization, together with X-ray structural analyses, proves the molecular structure of these new organometallic compounds.

Triphosphiranes as phosphinidene-transfer agents - synthesis of regular and chelating NHC phosphinidene adducts.

In this contribution we describe the general use of aryl-substituted triphosphiranes (Ar3P3; Ar = Mes, Dip, Tip) as phosphinidene transfer reagents towards N-heterocyclic carbenes (NHCs) to give a library of twelve N-heterocyclic carbene phosphinidene adducts of the type ArPNHC (NHCPs), in which the NHCs have varying steric profiles, allowing a systematic evaluation of their structural and NMR-spectroscopic properties. In the next series of experiments we utilized 1,3- and 1,4-phenylene bridged bis-NHCs to access a new class of chelating bis(NHCP)s, of which three derivatives could be structurally characterized. The 1,4-phenylene derivatives were shown to be susceptible to P-CNHC bond cleavage when irradiated with an LED (396 nm), providing a rare example of phosphinidene release from NHCPs. The coordination chemistry of 1,3-phenylene bridged bis(NHCP)s towards GeCl2(dioxane) and GaI3 was investigated and revealed the formation of ion-separated cationic complexes, with significant charge transfer from the ligand to the metal center according to NBO analyses.

Synthesis, structures, and photophysical properties of two Cu(I) complexes supported by N-heterocyclic carbene and phosphine ligands

Abstract Two new cationic four-coordinate Cu(I) complexes supported by different chelating N-heterocyclic carbene ligands and the diphosphine ligand bis[2-(diphenylphosphino)phenyl]ether (POP) have been synthesized. The chemical structures of both complexes have been characterized by 1H NMR, 13C NMR, 31P NMR, and mass spectroscopy, and the crystal structure of one complex has been determined by single-crystal X-ray diffraction. Results of theoretical calculations indicate that the lowest energy electronic transitions of these complexes are mainly the metal-to-ligand charge transfer and ligand-to-ligand charge transfer transitions. The complexes in solid state show intense emissions with high photoluminescence quantum yields. The photophysical behavior at 298 and 77K shows that emissions of these complexes at room temperature are thermally activated delayed fluorescence mixed with phosphorescence.

Ruthenium complexes with chelating carboxylate-NHC ligands as efficient catalysts for C[sbnd]H arylation in water

Ruthenium(ii) complexes with chelating N-heterocyclic carbene (NHC) ligands were studied in the arylation of phenyl group in 2-phenylpyridine and 1-phenylpyrazole with aryl chlorides in water. Complexes with NHC-ligands containing a hemilabile coordinating N-carboxylatomethyl group enable fast and selective ortho-CH-diarylation in the absence of carboxylate-assisting additives.

Transition-metal complexes bearing chelating NHC Ligands. Catalytic activity in cross coupling reactions via C[sbnd]H activation

In recent years, the direct functionalization of CH bonds has become an important tool for CC or C–heteroatom cross-coupling reactions. In these reactions, homogeneous catalysts based on transition metals have demonstrated good selectivity for direct CH activation. This process is not only simple but easily leads to molecules and products of high added value through greener and more energy efficient processes. In this context, metallacyclic complexes have been very successful in CH functionalization reactions, especially with the use of transition metal compounds bearing at least one N-heterocyclic carbene (NHC). Thus, herein we highlight some of the most recent advances on the catalytic CC or C–heteroatom (B, N and O) bond formation via CH activation, using metallacyclic transition metal NHC complexes.

Ruthenium(II) Complexes with Chelating N-Heterocyclic Carbenes and a Ruthenate(II) Complex as Catalysts for the Anti-Markovnikov Hydroaminations of Styrene

Ruthenium(II) Complexes with Chelating N-Heterocyclic Carbenes and a Ruthenate(II) Complex as Catalysts for the Anti-Markovnikov Hydroaminations of Styrene

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

Transmetalation from Magnesium–NHCs—Convenient Synthesis of Chelating π-Acidic NHC Complexes

The synthesis of chelating N-heterocyclic carbene (NHC) complexes with considerable π-acceptor properties can be a challenging task. This is due to the dimerization of free carbene ligands, the moisture sensitivity of reaction intermediates or reagents, and challenges associated with the workup procedure. Herein, we report a general route using transmetalation from magnesium–NHCs. Notably, this route gives access to transition-metal complexes in quantitative conversion without the formation of byproducts. It therefore produces transition-metal complexes outperforming the conventional routes based on free or lithium-coordinated carbene, silver complexes, or in situ metalation in dimethyl sulfoxide (DMSO). We therefore propose transmetalation from magnesium–NHCs as a convenient and general route to obtain NHC complexes.

Catalytic activity of chiral chelating N-heterocyclic carbene palladium complexes towards asymmetric allylic alkylation

The catalytic activity of a series of chiral heteroaryl coordinated chelating N-heterocyclic carbene (NHC) palladium complexes towards asymmetric allylic alkylation (AAA) were presented here. The effects of different N-substituents, NHC backbones and chelate rings on the catalytic activity and the enantioselectivity of the alkylation of (E)-1,3-diarylallyl acetates with dialkyl malonate were investigated. The results showed that, under the optimized conditions, complexes 3a, 3b, and 3i carrying the pyridinyl-coordinated five-membered chelate ring showed high catalytic activity and chiral induction efficiency. The corresponding alkylated products were obtained in high yields with moderate ee. Furthermore, it was found that the substituents of (E)-1,3-diarylallyl acetates and the type of the nucleophile affect the results as well.

Self-assembly of dinuclear Pd(ii)/Pt(ii) metallacyclic receptors incorporating N-heterocyclic carbene complexes as corners

We report herein the self-assembly of a series of new square and rectangular-shaped dinuclear M2L2 metallacycles (M = Pd(ii)/Pt(ii)), receptors self-assembled in water from four different N-monoalkyl-4,4'-bipyridinium derivatives as ligands and square-planar Pd(ii) and Pt(ii) metal centers having the chelating N-heterocyclic carbene 1,1'-di(methyl)-3,3'-methylene-4-diimidazolin-2,2'-diylidene. The concentration-dependent Pd2L2 metallacycles were successfully obtained and characterized by means of NMR experiments in aqueous media. Due to the strong trans effect exerted by the carbene ligands, the synthesis of the Pt2L2 receptors was achieved as well by self-assembly of the components at room temperature in a few hours, in clear contraposition to the harsh reaction conditions usually required for the labilization of other kinetically inert Pt(ii)-N(pyridine) bonds. X-ray diffraction studies of suitable single crystals of two of the obtained receptors offered additional information on the structure of the obtained supramolecules, whose ability as receptors has been explored by the preparation and study of the corresponding inclusion complexes in water with 1,5-dihydroxynaphthalene as the model substrate.

Highly β-Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z-Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts.

It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote exo-cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst.

Osmium(II) Complexes Bearing Chelating N-Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of OsII-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[OsII-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [OsII-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an OsII-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the OsII-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from Myrothecium verrucaria while achieving high potential electroenzymatic oxygen reduction at E = +0.45 V vs Ag/AgCl at pH 6.5.

Catalytic activity of chelating N-heterocyclic carbene palladium complexes towards selective phosphorylation of coumarins

The catalytic activity of a series of heteroaryl coordinated chelating N-heterocyclic carbene (NHC) palladium complexes bearing different N-substituents, NHC backbones and chelating rings towards the phosphorylation of coumarins, and the relationship between the activity and the structure are presented. The results showed that the chelating NHC palladium complex Ia–b bearing the most σ-donating backbone and a more rigid five-membered chelating ring showed the highest efficiency. Using AgNO3 as oxidant, a wide range of coumarins reacted with different dialkyl phosphite or diphenylphosphine oxide smoothly in CH3CN, producing the target product in moderate to high yields.

Theoretical investigation on the electronic structures and phosphorescent properties of a series of cyclometalated platinum(II) complexes with different substituted N-heterocyclic carbene ligands

ABSTRACTA series of substituted platinum(II) complexes with a chelating N-heterocyclic carbene (NHC) ligand and a bidentate monoanionic auxiliary ligand (acetylacetone) have been investigated by using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, to explore their electronic structures, absorption and emission properties, and phosphorescence quantum efficiency. The influence of different substituted groups on photophysical properties of complexes studied has been detailedly analyzed. The lowest energy absorption and emission wavelengths calculated are comparable to the available experimental values. In addition, ionization potential (IP), electron affinities (EA), and reorganization energy (λ) were obtained to evaluate the charge transfer and balance properties between hole and electron. The calculated results also show that, due to a lower , larger3MLCT contribution, and higher value, the complex 5 owns possibly the largest kr value among these complexes. The theoretical studies could provide useful information for the candidated phosphorescent platinum(II) material for use in the organic light-emitting diodes.

Homoleptic chelating N-heterocyclic carbene complexes of palladium immobilized within the pores of SBA-15/IL (NHC–Pd@SBA-15/IL) as heterogeneous catalyst for Hiyama reaction

In this work, SBA-15 was selected as a support for the homoleptic chelating N-heterocyclic carbene complexes of palladium. Here, Pd2+ ions were coordinated to N-heterocyclic carbene (NHC–Pd) which also contain ionic liquid. Then, this NHC–Pd@SBA-15/IL catalyst (NHC–Pd active part of the catalyst) was successfully applied in the catalysis of Hiyama coupling reaction with lower mol% Pd rather to previously reported literatures. The role of TBAF and Cs2CO3 in the decrease of Pd mol% was crucial. Recyclability test showed a successful reusability for 5 runs. Simultaneous application of TBAF and NaOH did destroy the silica structure of SBA-15 in the first run.

N-Alkylation and N,C-Dialkylation of Amines with Alcohols in the Presence of Ruthenium Catalysts with Chelating N-Heterocyclic Carbene Ligands

A series of new benzimidazolium salts and ruthenium(II) complexes containing chelating N-heterocyclic carbenes (NHCs) functionalized with a benzylic group and an acetal group were prepared. All of the synthesized compounds were characterized by elemental analysis and NMR spectroscopy, and the molecular structures of 2c and 2d were determined by X-ray crystallography. All of the complexes were tested in the alkylation of cyclic amine derivatives with alcohols and showed excellent activity in this reaction. Cyclic amines were alkylated with primary and heteroaromatic alcohols. The Ru–NHC complexes also catalyzed N,C3-dialkylation of cyclic amines.

Effects of Phosphine–Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes

Nickel(II) complexes containing chelating N-heterocyclic carbene–phosphine ligands ([NiL2](BPh4)2, for which L = [MeIm(CH2)2PR2], MeIm = 1-methylimidizolylidene, and R = Ph or Et) have been synthesized for the purpose of studying how this class of ligand affects the electrochemical and thermodynamic properties compared to the nickel bis-diphosphine analogues. The nickel complexes were synthesized and then characterized by X-ray crystallography, electrochemical methods, and thermodynamic studies, including DFT calculations. On the basis of the reduction potentials (E°), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex of the phenyl-substituted phosphine–carbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis(diphenylphosphino)propane. The free energy for addition of H2 is presented, and the implications for catalysis are discussed. These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal complexes.

Blue phosphorescent nitrile containing C^C* cyclometalated NHC platinum(ii) complexes

Since C^C* cyclometalated Pt(II) complexes with N-heterocyclic carbene (NHC) ligands have been identified as potential emitter materials in organic light-emitting devices (OLEDs), very promising results regarding quantum yields, colour and stability have been presented. Herein, we report on four nitrile substituted complexes with a chelating NHC ligand (1-(4-cyanophenyl)-3-isopropyl-1H-benzo[d]imidazole or 4-(tert-butyl)-1-(4-cyanophenyl)-3-methyl-1H-imidazole) and a bidentate monoanionic auxiliary ligand (acetylacetone or dimesitoylmethane). The complexes have been fully characterized including extensive 2D NMR studies (COSY, HSQC, HMBC, NOESY, (195)Pt NMR), three of them also by solid-state structures. Photophysical measurements in amorphous PMMA films and pure emitter films at room temperature reveal the impact of the mesityl groups in the auxiliary ligand, which led to a significant increase of the quantum yield, while the decay lifetimes decreased. The electron withdrawing nitrile groups shift the emission towards blue colour coordinates.

Highly Z-Selective and Enantioselective Ring-Opening/Cross-Metathesis Catalyzed by a Resolved Stereogenic-at-Ru Complex

The synthesis of a ruthenium complex that catalyzes Z-selective (up to 98% Z) asymmetric ring-opening/cross-metathesis with high enantioselectivity (up to 95% ee) is reported. The synthesis of the catalyst features the resolution of a chelating N-heterocyclic carbene complex by ligand substitution with a chiral carboxylate.

A bulky bis N-heterocyclic carbene complex of iron with coordination number of six: [CpFe(CO)(CH2(ImtBu)2)]I (where ImtBu=3-tert-butyl-1H-imidazol-1-yl-2(3H)-ylidene)

The crystal structure of the title compound, [CpFe(CO)(CH2(ImtBu)2)]I (where ImtBu = 3-tert-butyl-1H-imidazol-1-yl-2(3H)-ylidene) (1), has been studied by X-ray diffraction. A piano-stool geometry is observed with binding of the Cp ring in three coordination sites, the chelating bis N-heterocyclic carbene (NHC) ligand in two sites, and the CO ligand. This molecule is a cation and the iodide provides a counter-anion that is not coordinated. This structure is similar to other bidentate chelating NHC complexes in the literature, but the combination of t-butyl groups in a pseudo-six-coordinate complex makes the title compound more sterically crowded than its literature analogs. This complex is remarkably stable, with attempts to displace the CO ligand proving unsuccessful and leading to no reaction or decomposition under forcing conditions.