Ruthenium N-heterocyclic Carbene Research Articles

On the viability of ruthenium (II) N-heterocyclic carbene complexes as dye-sensitized solar cell (DSSCs): A theoretical study

Changes in the ground state and excited state properties of the [Ru(bpy)3]2+ complex induced by functionalization of bpy ligands with N-heterocyclic carbene ligand (imidazolylidene, benzimidazolylidene and 1,2,3-triazolylidene) and substituted methyl wingtip group in protonated and deprotonated forms were studied theoretically using DFT and TDDFT methods. The absorption band is broader and features approximately a 26nm more bathochromic tail than the [Ru(bpy)3]2+. Our calculations show that red shift for 1,2,3-triazolylidene ligand in the protonated series is only due to the affect of the energies of LUMOs which is localized on the substituted ligands. Consistent with previously observed trends in absorption spectra of Ru (II) complexes, a blue shift is observed for the deprotonated series. The effect originates from interplay of positive and negative solvatochromism in 1–3 and 1a–3a complexes respectively. This results in more delocalized character of the electron transition orbitals in the deprotonated complexes. We performed an NTO analysis based on the calculated transition density matrices to analyze the calculated singlet states. The calculated triplet excited state can either be weakly optically allowed 3MLCT or optically forbidden Ru 3d–d transitions. The ability to reduce the HOMO–LUMO gap by the carbene ligand is expected to be beneficial for photovoltaic applications.

Concurrent Cross Metathesis and Enzymatic Oxidation: Enabling Off‐Equilibrium Transformations

Criss-cross catalysis: H. Zhao, J. F. Hartwig and co-workers have combined homogeneous alkene metathesis and biocatalysis in a concurrent fashion. A ruthenium–N-heterocyclic carbene (NHC) complex provides an equilibrating mixture of cross metathesis products. The selective simultaneous epoxidation by cytochrome P450 BM3 enables product yields well above the hypothetical two-step process.

Synthesis, characterization, and transfer hydrogenation of Ru(II)-N-heterocyclic carbene complexes

A new series of ruthenium(II) N-heterocyclic carbene complexes [RuL1,2,3(p-cymene)Cl2] (3a–c) (where L is a N-heterocyclic carbene), have been synthesized via transmetalation. The new ruthenium(II)-NHC complexes were applied to transfer hydrogenation of acetophenone derivatives and aldehydes using 2-propanol as a hydrogen source and KOH as a co-catalyst. The results show that the corresponding alcohols could be obtained in good yield with high catalyst activity (up to 100%) under mild conditions. [RuL1(p-cymene)Cl2] (3a) is much more active than the other complexes in transfer hydrogenation. Reactions, catalyzed by 3a–c, showed the highest reaction rates and yields of alcohol when the substrates bear more electron-withdrawing substituents. All new compounds were characterized by IR, elemental analysis, LC–MS (ESI), and NMR spectroscopy.

Donor functionalized ruthenium N-heterocyclic carbene complexes in alcohol oxidation reactions

N-Pyridyl, N'-amido functionalized imidazolium bromides were obtained in high yields as an N-heterocyclic carbene (NHC) precursor and used as bidentate or a pincer ligands to obtain ruthenium complexes via a silver NHC transmetallation route. The incorporation of a phenyl group as an amido-N substituent (R = Ph) results in a bidentate coordination mode through the C(NHC) and N(pyridyl) donors, whereas in its absence (R = H) a pincer coordination mode was observed through the N(pyridyl)^C(NHC)^O(amido) donors. The ruthenium complex featuring a pincer type NCO coordination mode with a protic NH function adjacent to the coordinating O(amido) atom was found to efficiently catalyse the oxidation of activated alcohols effecting quantitative conversions within 30 minutes. However the oxidation of deactivated alcohols required longer reaction times to effect the quantitative transformation.

Synthesis of N-Heterocyclic Carbene Ruthenium Complex and Application as Catalyst for Suzuki Coupling Reaction

Synthesis of N-Heterocyclic Carbene Ruthenium Complex and Application as Catalyst for Suzuki Coupling Reaction

Abnormal N-Heterocyclic Carbene-Phosphine Ruthenium(II) Complexes as Active Catalysts for Transfer Hydrogenation

The bifunctional phosphine–abnormal N-heterocyclic carbene ruthenium(II) complex RuBr(OAc)(PPh3)(P–aNHC) (1) has been synthesized in high yield by reaction of Ru(OAc)2(PPh3)2 with a phosphine imidazolium bromide (P–NHC × HBr) and characterized by X-ray diffraction. This compound shows high catalytic activity for the transfer hydrogenation of ketones to alcohols in 2-propanol. Rate and efficiency of 1 can be enhanced by the addition of ethylenediamine or benzylamine, affording TOFs up to 140 000 h–1. Reaction of 1 with ethylenediamine leads to the Ru carbene/amine complex [Ru(OAc)(PPh3)(P–aNHC)(H2NCH2CH2NH2)]Br (2), which displays the same activity of the in situ generated species.

An efficient N-heterocyclic carbene based ruthenium-catalyst: Application towards the synthesis of esters and amides

A highly stable benzimidazolylidene based N-heterocyclic carbene (NHC) ruthenium catalyst was prepared starting with readily accessible starting materials. Under inert gas atmosphere and in air the catalyst showed high activity for the direct synthesis of esters from primary alcohols and of amides from primary alcohols and amines. Di-, tri-, and oligo-amides were obtained by using specific starting materials.

Stereoselective Ring‐Opening Metathesis Polymerization of 7‐tert‐Butoxy‐bicyclo[2,2,1]hepta‐2,5‐diene by NHC–Ruthenium Catalysts

AbstractRu complexes having N‐heterocyclic carbene (NHC) ligands, differing in the bulkiness of N‐aryl substituents and in the backbone substitution and/or configuration, are utilized in the ring‐opening metathesis polymerization (ROMP) of 7‐tert‐butoxybicyclo[2,2,1]hepta‐2,5‐diene (7‐TBONBD). The activity, regioselectivity, and stereoselectivity of the catalysts are investigated and rationalized considering the influence of the NHC ligand architectures of Ru complexes on the outcome of the polymerization. The reactivity order is influenced by bulky substituents at the nitrogen of the NHC ligand and by NHC‐backbone substitution. All of the catalysts give rise to living polymerizations and show high selectivity, producing highly syndiotactic polymers with 100% of anti‐units and, surprisingly, an elevated cis‐content (up to 88%).magnified image

Evaluation of arene ruthenium(ii) N-heterocyclic carbene complexes as organometallics interacting with thiol and selenol containing biomolecules

Metal complexes with N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry and are now increasingly considered for the development of new chemical tools and metal based drugs. Ruthenium complexes of the type (p-cymene)(NHC)RuCl(2) interacted with biologically relevant thiols and selenols, which resulted in the inhibition of enzymes such as thioredoxin reductase or cathepsin B. Pronounced antiproliferative effects could be obtained provided that an appropriate cellular uptake was achieved. Inhibition of tumor cell growth was accompanied by a perturbation of metabolic parameters such as cellular respiration.

Catalytic Asymmetric Hydrogenation of Naphthalenes

The catalytic asymmetric hydrogenation of heteroarenes has been intensively studied during the last decade. Nowadays, various heteroaromatics, for example, indoles, pyrroles, and quinolines, can be reduced to the corresponding chiral heterocycles with high stereoselectivity through asymmetric catalysis. Glorius and co-workers recently found that a chiral N-heterocyclic carbene–ruthenium catalyst allowed the site-selective hydrogenation of the carbocyclic rings of some quinoxalines, producing chiral 5,6,7,8-tetrahydroquinoxalines with up to 88% ee. However, to the best of our knowledge, the catalytic asymmetric hydrogenation of aromatics containing no heteroatoms remains unexplored. Herein, we report the first successful enantioselective hydrogenation of carbocyclic arenes, naphthalenes, through asymmetric catalysis. Previously, we had developed the highly enantioselective hydrogenation of N-Boc indoles with a ruthenium catalyst generated from [{RuCl2(p-cymene)}2] and the chiral bisphosphine ligand, PhTrap. During the course of this study, we attempted the reduction of 2-naphthylindole 1 with the PhTrap–ruthenium catalyst (Scheme 1). To our surprise, none of the expected product 2 was obtained. The hydrogenation was accompanied by the partial reduction of the naphthalene ring, yielding tetrahydronaphthylindoline 3 with 90% ee. This observation suggested that the ruthenium complex was capable of reducing carbocyclic aromatic rings. In this context, we began to study the catalytic asymmetric hydrogenation of naphthalenes. In our initial attempt, a solution of dimethyl naphthalene2,6-dicarboxylate (4a) in 1,4-dioxane was stirred at 60 8C under hydrogen gas (50 atm) in the presence of [RuCl(p-cymene){(S,S)-(R,R)-PhTrap}]Cl (6) catalyst. Although formation of the hydrogenation product 5a was observed, the desired reaction was very sluggish and the enantiomeric excess of 5a was only 22% ee (Table 1, entry 1). Both the

Synthesis of ruthenium(II) N-heterocyclic carbene complexes and their catalytic activities in transfer hydrogenation of ketones

Three RuCl2(η6-arene, η1-carbene) and two RuCl2(NHC)(arene) complexes have been prepared by the reaction of bis(1,3-dialkylperhydrobenzimidazol-2-ylidene) (1) and bis(1,3-dialkyl-4-methylzimidazolin-2-ylidene) (3) with [RuCl2(arene)]2 in toluene and characterized by elemental analysis, 1H NMR, 13C NMR and IR spectroscopy. The catalytic activities of these complexes were examined in the transfer hydrogenation of aromatic ketones using 2-propanol as hydrogen source.

ChemInform Abstract: A Computational Perspective of Olefins Metathesis Catalyzed by N‐Heterocyclic Carbene Ruthenium (Pre)catalysts.

AbstractReview: 93 refs.

Ruthenium NHC Catalyzed Highly Asymmetric Hydrogenation of Benzofurans

H2-O-T! Aromatic O-heterocycles are a challenging substrates for asymmetric hydrogenation (H2). An in situ formed chiral N-heterocyclic carbene (NHC) ruthenium complex allows the high yielding, completely regioselective, and highly asymmetric hydrogenation of substituted benzofurans at room Temperature, giving valuable 2,3-dihydrobenzofurans (see scheme).

Synthesis and characterization of new ruthenium N-heterocyclic carbene Hoveyda II-type complexes. Study of reactivity in ring closing metathesis reactions

This manuscript describes the synthesis and structural study of new second generation Hoveyda-Grubbs catalysts: 1,3-dimesityl-acenaphthylenyl-4,5-imidazolin-2-ylidene (BIAN-NHC) ruthenium isopropoxybenzylidene dichloride and 1,3-bis(2,6-dimethylphenyl)-2,3-dihydro-1H-imidazole Cl(2)Ru(=CH-o-O-i-PrC(6)H(4)) . The electrochemical and catalytic behavior of these new complexes was compared with the conventional NHC carbene Hoveyda II IMes-type complexes and for ring closing metathesis reactions.

Dehydrogenative Synthesis of Imines from Alcohols and Amines Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex

A new method for the direct synthesis of imines from alcohols and amines is described where hydrogen gas is liberated. The reaction is catalyzed by the ruthenium N-heterocyclic carbene complex [RuCl2(IiPr)(p-cymene)] in the presence of the ligand DABCO and molecular sieves. The imination can be applied to a variety of primary alcohols and amines and can be combined with a subsequent addition reaction. A deuterium labeling experiment indicates that the catalytically active species is a ruthenium dihydride. The reaction is believed to proceed by initial dehydrogenation of the alcohol to the aldehyde, which stays coordinated to ruthenium. Nucleophilic attack of the amine affords the hemiaminal, which is released from ruthenium and converted into the imine.

Thermally Stable, Latent Olefin Metathesis Catalysts

Highly thermally stable N-aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium catalysts were designed and synthesized for latent olefin metathesis. These catalysts showed excellent latent behavior toward metathesis reactions, whereby the complexes were inactive at ambient temperature and initiated at elevated temperatures, a challenging property to achieve with second generation catalysts. A sterically hindered N-tert-butyl substituent on the NHC ligand of the ruthenium complex was found to induce latent behavior toward cross-metathesis reactions, and exchange of the chloride ligands for iodide ligands was necessary to attain latent behavior during ring-opening metathesis polymerization (ROMP). Iodide-based catalysts showed no reactivity toward ROMP of norbornene-derived monomers at 25 °C, and upon heating to 85 °C gave complete conversion of monomer to polymer in less than 2 hours. All of the complexes were very stable to air, moisture, and elevated temperatures up to at least 90 °C, and exhibited a long catalyst lifetime in solution at elevated temperatures.

Polyethylene Oligomer Supported Olefin Metathesis Catalysts

Polyethylene (PE) oligomer supported N-heterocyclic carbene ruthenium complex 1 as a recyclable ring-closing metathesis (RCM) catalyst was described. The polymeric complex 1 exhibited thermomorphic solubility, ­being insoluble in toluene at 25 ˚C and soluble at 65 ˚C. The homogeneous RCM with 1 was performed in toluene at 65 ˚C, and cooling to 25 ˚C led to precipitation of 1. The reaction system afforded quantitative products with high recyclability.

Dehydrogenative Coupling of Primary Alcohols To Form Esters Catalyzed by a Ruthenium N-Heterocyclic Carbene Complex

The ruthenium complex [RuCl2(IiPr)(p-cymene)] catalyzes the direct condensation of primary alcohols into esters and lactones with the release of hydrogen gas. The reaction is most effective with linear aliphatic alcohols and 1,4-diols and is believed to proceed with a ruthenium dihydride as the catalytically active species.

Iridium NHC Based Catalysts for Transfer Hydrogenation Processes Using Glycerol as Solvent and Hydrogen Donor

A series of iridium and ruthenium N-heterocyclic carbene based catalysts of general formula [IrI2(AcO)(bis-NHC)] or [Ru(η6-arene)(NHC)CO3] have been tested in the reduction of several organic carbonyl compounds using glycerol as solvent and hydrogen donor, by the transfer hydrogenation methodology. The Ir(III) complexes with a chelating bis-NHC ligand and sulfonate groups were the most efficient, due to their solubility in the reaction media and to the strong electron-donor properties of the bis-carbene ligands. The same two catalysts were moderately active in the reduction of olefins and alkynes and, more remarkably, show excellent chemoselectivity in the reduction of the alkenic double bond of α,β-unsaturated ketones, a valuable process for which glycerol had never been used before.

Probing Intermetallic Coupling in Dinuclear N-Heterocyclic Carbene Ruthenium(II) Complexes

A series of bimetallic N-heterocyclic carbene (NHC) ruthenium(II) complexes were synthesized, which comprise two [RuCl(2)(cymene)(NHC)] units that are interlinked via the NHC nitrogens by alkyl chains of different length. Electrochemical characterization revealed two mutually dependent oxidation processes for the complex with a methylene linker, indicating moderate intramolecular electronic coupling of the two metal centers (class II system). The degree of coupling decreases rapidly upon increasing the number of CH(2) units in the linker and provides essentially decoupled class I species when propylene or butylene linkers are used. Electrochemical analyses combined with structural investigations suggest a through-bond electronic coupling. Replacement of the alkyl linker with a p-phenylene group afforded cyclometalated complexes, which were considerably less stable. The electronic coupling in the methylene-linked complex and the relatively robust NHC-ruthenium bond may provide access to species that are switchable on the molecular scale.