Ruthenium Carbene Complexes Research Articles

Formation of Ruthenium Carbenes by gem‐Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans‐Hydrogenation

Insights into the mechanism of the unusual trans-hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para-hydrogen (p-H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans-reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter-ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over-reduction, which frequently interfere with regular alkyne trans-hydrogenation.

Formation of Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-Hydrogenation.

Insights into the mechanism of the unusual trans‐hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para‐hydrogen (p‐H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans‐reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter‐ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over‐reduction, which frequently interfere with regular alkyne trans‐hydrogenation.

Selective [2+2] Cycloaddition Reactions of Isocyanates and Thioisocyanates across the M=C Bond in a Ruthenium Carbene Complex

The reactivity of the (carbene)ruthenium complex [(η6-p-cymene)Ru=CRR′] with the unsymmetrical methandiide ligand [CRR′=C(Ph2PS)(SO2Ph)] towards different unsaturated organic substrates was examined. While no transformation was observed with a series of different ketones (no Wittig-type reactivity), treatment with isocyanates as well as thioisocyanates resulted in selective [2+2] cycloaddition reactions across the Ru=C double bond. The products were isolated in high yields and characterized in solution as well as in the solid state. The addition reactions were found to proceed selectively according to the HSAB principle with the soft ruthenium center preferring the softer donor atoms. Hence, isocyanate addition occurred with the C=N bond, while the thioisocyanates added to the metal–carbon bond with the C=S bond. Density functional theory studies on the reaction mechanism confirmed the observed selectivity and the formed complexes as kinetically as well as thermodynamically favoured products.

Net charge effects in N-heterocyclic carbene–ruthenium complexes with similar oxidation states and coordination geometries

A chelating benzimidazolylidene–carboxylate ligand (1) was transferred from [Ag(1)] to [RuCl2(dmso)4] to afford trans(C)-[Ru(1)2(bpy)] (2), but all attempts to incorporate 1 into cis(Cl)-[RuCl2(bpy)2] were unsuccessful. Alternatively, reaction of [RuCl(1)(η6-cymene)] (3) with bpy and AgOTf successfully produced [Ru(1)(bpy)2][OTf] (4). Methylation of 2 and 4 with MeOTf yielded trans(C)-[Ru(1-Me)2(bpy)][OTf]2 (5) and [Ru(1-Me)(bpy)2][OTf]2 (6), accompanied by a +2 and +1 increase in net charge, respectively. Cyclic voltammetry indicated that the increase in Ru(II)/Ru(III) oxidation potential from 2 to 5 was twice the increase from 4 to 6. UV–Vis spectroscopy also revealed that transitions in 5 occurred at higher energy than in 2 by double the difference between 6 and 4. Crystallographic analysis demonstrated that the coordination environment of Ru did not differ significantly between 2 and 5, suggesting that the observed shifts in oxidation potentials and absorption wavelengths are not due to changes in coordination chemistry or formal oxidation state. Collectively, these results show that 5 and 6 feature Ru centers more electron deficient than those in 2 and 4, respectively, by amounts linearly proportional to the difference in net charge.

N-chelate ruthenium carbene complexes in olefin metathesis and isomerization

The catalytic activity of N-chelate ruthenium carbene complexes in the metathesis of hex-1-ene has been studied in comparison to the second generation Grubbs catalyst.

ChemInform Abstract: Benzimidazolin‐2‐ylidene N‐Heterocyclic Carbene Complexes of Ruthenium as a Simple Catalyst for the N‐Alkylation of Amines Using Alcohols and Diols.

AbstractSimple air and moisture stable ruthenium complexes are synthesized from readily available benzannulated N‐heterocyclic carbene ligands and applied as catalysts for the alkylation of various amines using alcohols under solvent‐free conditions.

An attractive route to transamidation catalysis: Facile synthesis of new o-aryloxide-N-heterocyclic carbene ruthenium(II) complexes containing trans triphenylphosphine donors

Well-defined robust ruthenium(II) complexes 3a–d bearing o-aryloxide-N-heterocyclic carbene ligands with different wingtip substituents (3a (R=Me), 3b (R=Ph), 3c (R=iPr) and 3d (R=Mes)) in the imidazole ring were synthesized in good yields by the reaction of imidazolium proligands with metal precursor [RuHCl(CO)(PPh3)3] by transmetallation from the corresponding silver carbene complexes. All the Ru(II)–NHC complexes have been characterized by elemental analyses, spectroscopic methods as well as ESI mass spectrometry. The molecular structure of the complex 3a was identified by means of single-crystal X-ray diffraction analysis, which revealed that the complexes possess a distorted octahedral geometry. In order to explore the catalytic potential of the synthesized complexes, all the four [Ru–NHC] complexes [3a–d] were tested as catalysts for transamidation of carboxamides with amines. Notably, the complex 3a was found to be very efficient and versatile catalyst toward transamidation of a wide range of amides with amines.

ChemInform Abstract: Supported Ruthenium—Carbene Catalyst on Ionic Magnetic Nanoparticles for Olefin Metathesis.

The Grubbs–Hoveyda ruthenium–carbene complex has been covalently immobilized on ionic magnetic nanoparticles utilizing an imidazolium salt linker. The supported catalyst exhibited excellent catalytic activity for ring-closing metathesis (RCM) and cross-metathesis (CM) in the presence of less than 1 mol % of ruthenium. The catalysts can easily be recovered magnetically and reused up to seven times with minimal leaching of ruthenium species.

A new type of ruthenium carbene complexes with N-chelating ligand

New ruthenium carbene complexes containing a six-membered N-chelating fragment were synthesized. A catalytic activity of these complexes in the olefin metathesis reactions was demonstrated.

Benzimidazolin-2-ylidene N-heterocyclic carbene complexes of ruthenium as a simple catalyst for the N-alkylation of amines using alcohols and diols

Simple non-chelating ruthenium benzimidazolin-2-ylidene complexes as efficient N-alkylation catalysts using alcohols and diols following a hydrogen borrowing strategy.

Cyclic alkyl amino carbene (CAAC) ruthenium complexes as remarkably active catalysts for ethenolysis.

An expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100,000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340,000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial-scale production of linear α-olefins (LAOs) and other terminal-olefin products.

Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis

AbstractAn expanded family of ruthenium‐based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed‐oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100 000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340 000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross‐metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial‐scale production of linear α‐olefins (LAOs) and other terminal‐olefin products.

The effect of silica-coating on catalyst recyclability in ionic magnetic nanoparticle-supported Grubbs–Hoveyda catalysts for ring-closing metathesis

The Grubbs–Hoveyda type ruthenium–carbene complexes have been covalently immobilized on both silica coated and uncoated magnetic nanoparticles utilizing an imidazolium salt linker. These MNP-supported catalysts showed comparable catalytic activity to the homogeneous imidazolium salt-tagged ruthenium catalyst in the ring-closing metathesis (RCM) of dienes, and could effectively catalyze RCM reactions in the presence of only 0.85 mol% Ru. The MNP-supported ruthenium catalysts could be easily recovered using an external magnet. The recyclability of the silica-coated MNP-supported catalysts was superior to silica uncoated MNP-supported catalysts allowing reuse 14 times, due to the increased compatibility of the silica-coated MNPs with the reaction medium.

Supported ruthenium-carbene catalyst on ionic magnetic nanoparticles for olefin metathesis.

The Grubbs-Hoveyda ruthenium-carbene complex has been covalently immobilized on ionic magnetic nanoparticles utilizing an imidazolium salt linker. The supported catalyst exhibited excellent catalytic activity for ring-closing metathesis (RCM) and cross-metathesis (CM) in the presence of less than 1 mol % of ruthenium. The catalysts can easily be recovered magnetically and reused up to seven times with minimal leaching of ruthenium species.

Cationic bis-N-heterocyclic carbene (NHC) ruthenium complex: structure and application as latent catalyst in olefin metathesis.

An unexpected cationic bis-N-heterocyclic carbene (NHC) benzylidene ether based ruthenium complex (2 a) was prepared through the double incorporation of an unsymmetrical unsaturated N-heterocyclic carbene (U2 -NHC) ligand that bore an N-substituted cyclododecyl side chain. The isolation and full characterization (including X-ray diffraction studies) of key synthetic intermediates along with theoretical calculations allowed us to understand the mechanism of the overall cationization process. Finally, the newly developed complex 2 a displayed interesting latent behavior during ring-closing metathesis, which could be "switched on" under acidic conditions.

Ring-opening and double-metallation reactions of the N-Heterocyclic carbene ligand in Cp∗(IXy)Ru (IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene) complexes. Access to an anionic fischer-type carbene complex of ruthenium

An N-heterocylic carbene (NHC) ring-opening reaction was observed upon treatment of the silyl complex Cp∗(IXy-H)(H)RuSiH2Mes [1; IXy=1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H=1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp∗=η5-C5Me5] with LiCH2SiMe3. This reaction results in formation of a novel, anionic Fischer-type carbene complex Cp∗(H)Ru{κ2-CHN(Xyl)CH[Si(CH2SiMe3)Mes]N(Xyl)Li} (4). This ring-opening reaction demonstrates a new pathway for NHC degradation via the cooperation of ruthenium, silicon, and lithium. Additionally, treatment of the dinitrogen complex Cp∗(IXy-H)Ru(N2) with LiCH2SiMe3 led to the doubly-metallated complex [Cp∗(IXy-2H)Ru]Li (5), which exhibits a solid-state polymeric structure and metallation of both xylyl groups of the IXy ligand. Finally, removal of a hydride from 4, achieved with two equiv of B(C6F5)3, led to C–H activation of the Cp∗ ligand and formation of an unusual, formally dianionic η5-Me4C5CH2B(C6F5)3 ligand in a ruthenium carbene complex [η5-Me5C5CH2B(C6F5)3](H)Ru{κ3-CHN(Xyl)CH[SiH(CH2SiMe3)Mes]N(Xyl)} (7).

Methandiide as a non-innocent ligand in carbene complexes: from the electronic structure to bond activation reactions and cooperative catalysis.

The synthesis of a ruthenium carbene complex based on a sulfonyl-substituted methandiide and its application in bond activation reactions and cooperative catalysis is reported. In the complex, the metal-carbon interaction can be tuned between a Ru-C single bond with additional electrostatic interactions and a Ru=C double bond, thus allowing the control of the stability and reactivity of the complex. Hence, activation of polar and non-polar bonds (O-H, H-H) as well as dehydrogenation reactions become possible. In these reactions the carbene acts as a non-innocent ligand supporting the bond activation as nucleophilic center in the 1,2-addition across the metal-carbon double bond. This metal-ligand cooperativity can be applied in the catalytic transfer hydrogenation for the reduction of ketones. This concept opens new ways for the application of carbene complexes in catalysis.

Synthesis, structure and catalytic study of oxygen chelated ruthenium (II) carbene complex

New oxygen chelated ruthenium carbene complex containing carbonyl oxygen and ether oxygen has been developed. The X-ray structure of the complex showed that the carbonyl oxygen of the amide and the terminal oxygen of the benzylidene ether are both coordinated to the metal to give an octahedral structure. The catalytic activities of this new complex for olefin metathesis reactions were investigated and it exhibited excellent performances for the ring-closing metathesis (RCM) of diethyl diallymalonate at 30°C and even at 0°C. The initiation rate of the catalyst was higher than that for the Hoveyda catalyst ((H2IMes)(Cl)2Ru=C(H)(C6H4-2-OiPr)) and it was also active for cross metathesis (CM).

Illustrating Catalysis with Interlocking Building Blocks: A Ruthenium Carbene Complex for Olefin Metathesis Reactions

Although the generally accepted mechanism for olefin metathesis reactions is simple and elegant, it seems hard for students to actually visualize and fully understand the metathesis polymerization mechanisms. A unique activity is described that uses interlocking building blocks equipped with magnets to illustrate the concept of a ruthenium carbene complex for olefin metathesis, including cross metathesis, ring-opening metathesis, and acyclic diene metathesis polymerization. The block model of a ruthenium carbene complex was constructed by mimicking a real molecule [Ru{C(H)Ph}Cl2(PCy3)2], known as the first-generation Grubbs’ catalyst. The block models of reactant olefins were constructed from magnets linking two, 2 × 2 blocks. The ruthenium complex model can produce new olefin models by the successive disassembly and assembly of bonding in the reactant olefin models. Students enjoyed constructing the block models and using them to understand the catalytic olefin metathesis reactions.

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