Synthesis,characterization, and catalytic activity of half-sandwich ruthenium complexes with pyridine/phenylene bridged NHC = E (NHC = N-heterocyclic carbene,E = S,Se) ligands

Three half-sandwichruthenium(II) complexes with pyridine/phenylene bridged NHC = E (NHC = N-heterocyclic carbene, E = S, Se) ligands [Ru(p-cymene)L](PF₆)_1–2 ( 1a–1c , L = ligand) were synthesized and characterized. All ruthenium complexes were fully characterized by ¹H and ¹³C NMR spectra, mass spectrometry, and single-crystalX-ray diffraction methods. Moreover, the half-sandwich ruthenium complexes with NHC = E ligands showed highly catalytic activities towards to the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of R–NO₂ to R–NH₂ at 353 K in water.     

4-Vinylbenzyl-substituted silver(I) N-heterocyclic carbene complexes and ruthenium(II) N-heterocyclic carbene complexes: synthesis and transfer hydrogenation of ketones

4-Vinylbenzyl-substituted Ag(I) N-heterocyclic carbene (NHC) complexes and Ru(II) NHC complexes have been synthesized. The Ag(I) complexes were synthesized from the imidazolium salts and Ag₂O in dichloromethane at room temperature. The Ru(II) complexes were prepared from Ag(I) NHC complexes by transmetallation. The six 4-Vinylbenzyl-substituted Ag(I) NHC complexes and six 4-Vinylbenzyl-substituted Ru(II) NHC complexes have been characterized by spectroscopic techniques and elemental analyses. The Ru(II) NHC complexes show catalytic activity for the transfer hydrogenation of ketones.     

Ruthenium Nanoparticles on Nano‐Level‐Controlled Carbon Supports as Highly Effective Catalysts for Arene Hydrogenation

The reaction of three types of carbon nanofibers (CNFs; platelet: CNF‐P, tubular: CNF‐T, herringbone: CNF‐H) with [Ru₃(CO)₁₂] in toluene heated at reflux provided the corresponding CNF‐supported ruthenium nanoparticles, Ru/CNFs (Ru content=1.1–3.8 wt %). TEM studies of these Ru/CNFs revealed that size‐controlled Ru nanoparticles (2–4 nm) exist on the CNFs, and that their location was dependent on the surface nanostructures of the CNFs: on the edge of the graphite layers (CNF‐P), in the tubes and on the surface (CNF‐T), and between the layers and on the edge (CNF‐H). Among these Ru/CNFs, Ru/CNF‐P showed excellent catalytic activity towards hydrogenation of toluene with high reproducibility; the reaction proceeded without leaching of the Ru species, and the catalyst was reusable. The total turnover number of the five recycling experiments for toluene hydrogenation reached over 180 000 (mol toluene) (mol Ru)^?1. Ru/CNF‐P was also effective for the hydrogenation of functionalized benzene derivatives and pyridine. Hydrogenolysis of benzylic C? O and C? N bonds has not yet been observed. Use of poly(ethylene glycol)s (PEGs) as a solvent made possible the biphasic catalytic hydrogenation of toluene. After the reaction, the methylcyclohexane formed was separated by decantation without contamination of the ruthenium species and PEG. The insoluble PEG phase containing all of the Ru/CNF was recoverable and reusable as the catalyst without loss of activity.     

A facile route to ruthenium–carbene complexes and their application in furfural hydrogenation

A number of new N‐heterocyclic carbene (NHC) ligands were synthesized via a multicomponent reaction, wherein an aldehyde or ketone, a primary amine and an α‐acidic isocyanide were reacted, giving the corresponding 2H‐2‐imidazolines. These were easily alkylated with an alkyl halide at position N‐3, yielding the final NHC precursors, that were then complexed with Ru in situ. The resulting complexes are shown to be active and selective catalysts for the transfer hydrogenation of furfural to furfurol, using isopropanol as the hydrogen source. Importantly, the carbene ligand remains coordinated to the ruthenium center throughout the reaction. Copyright © 2009 John Wiley & Sons, Ltd.     

In situ preparation of ultrafine Ru nanocatalyst supported on nitrogen-doped layered double hydroxide by nitrogen glow discharge plasma for catalytic hydrogenation of N-ethylcarbazole

Ultrafine Ru nanoparticles (RuNPs) supported on nitrogen-doped layered double hydroxide (Ru/LDH) were in situ prepared by nitrogen glow discharge plasma (nGDP) without adding any chemical reducing agents or stabilizers. The as-synthesized Ru/LDH catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. During treatment with nGDP, the reduction of Ru³⁺ and nitrogen doping were carried out simultaneously. The resulting RuNPs has a narrow particle size distribution of 1.41–2.61 nm, an ultrafine average particle size of 1.86 nm, and were uniformly dispersed on nitrogen-doped LDH. The complexation of RuNPs and O/N-containing functional groups on LDH improve the catalytic activity and stability of Ru/LDH. The catalyst exhibited excellent properties for the hydrogenation reaction of N-ethylcarbazole (NEC). The conversion of NEC and the selectivity of 12H-NEC were 100% and 99.06% for 1 hr at 120°C and 6 MPa H₂, respectively. The mass hydrogen storage capacity was 5.78 wt%. The apparent activation energy was 35.78 kJ/mol.     

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

A new series of ruthenium(II) N-heterocyclic carbene complexes [RuL^1,2,3(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.     

Transfer hydrogenation of ketones catalysed by half‐sandwich (η6‐p‐cymene) ruthenium(II) complexes incorporating benzoylhydrazone ligands

Neutral half‐sandwich η⁶‐p ‐cymene ruthenium(II) complexes of general formula [Ru(η⁶‐p ‐cymene)Cl(L)] (HL = monobasic O, N bidendate benzoylhydrazone ligand) have been synthesized from the reaction of [Ru(η⁶‐p ‐cymene)(μ‐Cl)Cl]₂ with acetophenone benzoylhydrazone ligands. All the complexes have been characterized using analytical and spectroscopic (Fourier transform infrared, UV–visible, ¹H NMR, ¹³C NMR) techniques. The molecular structures of three of the complexes have been determined using single‐crystal X‐ray diffraction, indicating a pseudo‐octahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene complexes were explored as catalysts for transfer hydrogenation of a wide range of aromatic, cyclic and aliphatic ketones with 2‐propanol using 0.1 mol% catalyst loading, and conversions of up to 100% were obtained. Further, the influence of other variables on the transfer hydrogenation reaction, such as base, temperature, catalyst loading and substrate scope, was also investigated.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Influence of amines on the size control of in situ synthesized ruthenium nanoparticles in imidazolium ionic liquids

Very stable suspensions of small sized (c.a. 1.2 nm) and homogeneously dispersed ruthenium nanoparticles (RuNPs) were obtained by decomposition, under H(2), of (η(4)-1,5-cyclooctadiene)(η(6)-1,3,5-cyclooctatriene)ruthenium(0), [Ru(COD)(COT)], in various imidazolium derived ionic liquids (ILs: [RMIm][NTf(2)] (R = C(n)H(2n+1) where n = 2; 4; 6; 8; 10) and in the presence of amines as ligands (1-octylamine, 1-hexadecylamine). These nanoparticles were compared to others stabilized either in pure ILs or by the same ligands in THF. NMR experiments ((13)C solution and DOSY) demonstrate that the amines are coordinated to the surface of the RuNPs. These RuNPs were investigated for the hydrogenation of aromatics and have shown a high level of recyclability (up to 10 cycles) with neither loss of activity nor significant agglomeration.     

Assembled Porphyrin-Based Multinuclear Ruthenium(II)-NNNN Complex Catalysts for Transfer Hydrogenation of Ketones

Multinuclear porphyrin-based ruthenium(II)-NNNN complexes were efficiently assembled by means of coordinatively unsaturated 16-electron mononuclear ruthenium(II)-pyrazolyl-imidazolyl-pyridine complex, zinc(II) meso-tetra(4-pyridyl)-porphyrin (ZnTPyP), and 4,4′-linked bipyridines. The resultant multinuclear (Ru₄ and Ru₈) porphyrin-based ruthenium(II)-NNNN complexes exhibited exceptionally high catalytic activity at as low as 0.008 mol % Ru loading for the transfer hydrogenation reaction of ketones in refluxing 2-propanol, reaching up to 99 % yields and 5.7×10⁶ h⁻¹ TOFs.     

Efficient transfer hydrogenation of ketones in the presence of ruthenium N-heterocyclic carbene catalysts

Novel ruthenium carbene complexes have been in situ generated and tested for the transfer hydrogenation of ketones. Applying Ru(cod)(methylallyl)₂ in the presence of imidazolium salts in 2-propanol and sodium-2-propanolate as base, turnover frequencies up to 346 h⁻¹ have been obtained for reduction of acetophenone. A comparative study involving ruthenium carbene and ruthenium phosphine complexes demonstrated the higher activity of ruthenium carbene complexes.     

Understanding the catalytic activity of single‐chain polymeric nanoparticles in water

The structuring role of benzene‐1,3,5‐tricarboxamide (BTA) groups for the catalytic activity of single chain polymeric nanoparticles in water was investigated in the transfer hydrogenation of ketones. To this end, a set of segmented, amphiphilic copolymers was prepared, which comprised oligo(ethylene glycol) side chains to impart water solubility, BTA and/or lauryl side chains to induce hydrophobicity and diphenylphosphinostyrene (SDP) units in the middle part as a ligand to bind a ruthenium catalyst. All copolymers were obtained by reversible addition‐fragmentation chain transfer (RAFT) polymerization and showed low dispersities (M_w/M_n = 1.23–1.38) and controlled molecular weights (M_n = 44–28 kDa). A combination of circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) showed that all copolymers fold into a single chain polymeric nanoparticles (SCPNs) as a result of the helical self‐assembly of the pendant BTA units and/or hydrophilic–hydrophobic phase separation. To create catalytic sites, RuCl₂(PPh₃)₃ was incorporated into the copolymers. The Cotton effects of the copolymers before and after Ru(II) loading were identical, indicating that the helical self‐assembly of the BTA units and the complexation of SDP ligands and Ru(II) occurs in an orthogonal manner. DLS revealed that after Ru(II) loading, SDP‐bearing copolymers retained their single chain character in water, while copolymers lacking SDP units clustered into larger aggregates. The Ru(II) loaded SCPNs were tested in the transfer hydrogenation of cyclohexanone. This study reveals that BTA induced stack formation is not crucial for SCPN formation and catalytic activity; SDP‐bearing copolymers folded by Ru(II) complexation and hydrophobic pendants suffice to provide hydrophobic, isolated reaction pockets around Ru(II) complexes. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 12–20     

Synthesis,characterization, and reactivity of ruthenium(II) complexes containing η6-arene-η1-pyrazole ligands

New ruthenium(II) complexes containing η⁶-arene-η¹-pyrazole ligands were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of dichloro-3,5-dimethyl-1-(pentamethylbenzyl)-pyrazole–ruthenium(II), [Ru]L_3b, was determined by X-ray diffraction studies. These complexes were applied in the transfer hydrogenation of acetophenone by isopropanol in the presence of potassium hydroxide. The activities of the catalysts were monitored by NMR.     

Synthesis and structural chemistry of arene-ruthenium half-sandwich complexes bearing an oxazolinyl-carbene ligand

Reaction of a series of directly connected oxazoline-imidazolium salts with silver(I) oxide and subsequent transmetallation with [Ru(p-cymene)Cl₂]₂ and anion exchange with KPF₆ cleanly gave the corresponding 2-oxazolinyl-(N-mesityl)imidazolidene(chloro)ruthenium(II) half-sandwich complexes [RuCl(oxcarb)(p-cymene)]PF₆, two derivatives of which were characterized by X-ray diffraction. Abstraction of the chloro ligand furnished the dicationic aqua complexes [Ru(H₂O)(oxcarb)(p-cymene)](PF₆)₂ which possess a similar coordination geometry. The syntheses were found to be highly diastereoselective, since only one diastereoisomer could be observed in all ruthenium complexes upon reaction of the chiral enantiopure oxazoline-imidazolium salts. Their potential as transfer hydrogenation and Lewis acid catalysts has been probed.     

Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer hydrogenation of carbonyl compounds

A ruthenium (Ru) catalyst supported on magnetic nanoparticles (NiFe₂O₄) has been successfully synthesized and used for hydrogenation of alkynes at room temperature as well as transfer hydrogenation of a number of carbonyl compounds under microwave irradiation conditions. The catalyst shows excellent selectivity toward the desired products with very high yield even after five repeated uses.     

Ruthenium(II) complexes derived from 2‐phenylthiazoline‐4‐carboxylic acid: structure and catalytic activity for transfer hydrogenation reaction

Piano‐stool ([(p‐cymene)Ru(thz)Cl], 2 ) and six‐coordinated ([Ru(thz)₂(PPh₃)₂], 3 ) ruthenium complexes derived from 2‐phenylthiazoline‐4‐carboxylic acid (Hthz, 1 ) were synthesized for the first time, and fully characterized using conventional methods. Also, the molecular structure of complex 3 was determined using X‐ray analysis. These complexes were evaluated as catalysts for transfer hydrogenation of carbonyl compounds in the presence of isopropyl alcohol and KO^tBu. Complex 2 was found to be more active than 3 in transfer hydrogenation. Copyright © 2016 John Wiley & Sons, Ltd.     

Transfer Hydrogenation of Acetophenone Catalyzed by in situ Generated 2,6-Bis(5-thioxo-4,5-dihydro-1,2,4-triazole- 3-yl)pyridine-ruthenium(II) Complexes

2,6-Bis(5-thioxo-4,5-dihydro-1,2,4-triazole-3-yl)pyridines (3, 4) were used for the first time as ligand in ruthenium catalyzed transfer hydrogenation of acetophenone. The in situ prepared three-component system Ru(II)/tridentate triamine ligands (3a—3d, 4a—4d) and KOH catalysed the transfer hydrogenation reaction of acetophenone in good yields under mild conditions.     

RuCl2(BISBI)[(R,R)-DPEN]催化苯乙酮不对称加氢反应研究

报道了配合物RuCl₂(BISBI)[(R,R)-DPEN] [BISBI＝2,2'-二(二苯膦亚甲基)-1,1'-联苯, DPEN＝1,2-二苯基乙二胺]的合成和表征, 并研究了其在苯乙酮不对称加氢反应中的催化性能. 考察了底物/催化剂物质的量比、碱浓度、反应温度和氢气压力等对催化活性和对映选择性的影响, 在苯乙酮/KOH/催化剂的物质的量比为30000∶250∶1, 氢气压力为2 MPa, 反应温度为35 ℃时, 苯乙酮的转化率和生成α-苯乙醇的对映选择性分别达到了100%和65% ee.     

α‐Amino acid: an effective ligand for asymmetric catalysis of transfer hydrogenation of ketones

Ruthenium complexes, prepared by mixing the potassium salt of α‐amino acids and [RuCl₂(arene)]₂, acted as catalysts for the asymmetric transfer hydrogenation of ketones from 2‐propanol in the presence of KOH. For example, the transfer hydrogenation of acetophenone from 2‐propanol was catalyzed effectively by the ruthenium complex prepared from potassium L ‐prolinate and [RuCl₂(p‐cymene)]₂ to give (R)‐1‐phenylethanol in 72% yield with 81% ee. The yields and enantioselectivities of the product were influenced strongly by the structure of the α‐amino acidate ligand, arene ligand, and substrate, by the amount of additional base, and by the concentration of the substrates. The best enantiomeric excesses of the products was 92%, when 1‐tetralone was subjected to this reaction using a prolinated ruthenium complex bearing p‐cymene. Furthermore, the potassium salts of dipeptides were tested as a ligand for this transfer hydrogenation. Copyright © 2001 John Wiley & Sons, Ltd.     

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)‐Catalyzed Hydrogenation of Carbon Dioxide

The reactivity difference between the hydrogenation of CO₂ catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me₂PCH₂CH₂PMe₂), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me₂PCH₂CH₂CH₂PMe₂), together with a more electron‐withdrawing diphosphine ligand, PN^MeP (Me₂PCH₂N^MeCH₂PMe₂), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO₂ catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis–trans isomerization of ruthenium dihydride complex, CO₂ insertion into the Ru?H bond, and H₂ insertion into the ruthenium formate ion. Of these steps, CO₂ insertion into the Ru?H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO₂ catalyzed by ruthenium complexes of dmpe and dmpp, cis–trans isomerization of ruthenium dihydride complex has a similar barrier to that of H₂ insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN^MePRu complex, cis–trans isomerization of the ruthenium dihydride complex has a lower barrier than H₂ insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis–trans isomerization and H₂ insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition‐metal‐catalyzed CO₂ transformation.