Ru Hydride Research Articles

Unpredicted coordination of hydrogen chloride in organoruthenium(II) pyrazolone complex: Synthesis, spectral and structural characterization

Synthesis of novel Ru(II) carbonyl and Ru(II) hydride complexes is reported from the reaction of 4-(2-hydroxybenzyl)-3-methyl-5-oxo-2,5-dihydro-pyrazole-1-carbothioic acid ethyl amide with RuHCl(CO)(PPh3)3. Structural evaluations were done by FT-IR, UV-Vis, 1H NMR and mass spectrometry. The true nature of the Ru(II) complexes was further confirmed by single crystal X-ray diffraction studies and further validated by DFT studies. The electrochemistry of the synthesized complexes was studied using cyclic voltammetry.

“Greening” Ruthenium–Arene Catalyst Precursors with N-Heterocyclic Carbene Ligands Derived from Caffeine and Theophylline

Two ruthenium–arene complexes with the generic formula [RuCl2(p-cymene)(NHC)] were readily obtained from natural, abundant, and renewable methylxanthine alkaloids used as N-heterocyclic carbene (NHC) precursors. After optimization, the known compound Ru-1 featuring the 1,3,7,9-tetramethylxanthine-8-ylidene ligand was isolated in three steps and 72% overall yield starting from caffeine. Four steps were needed to synthesize complex Ru-2, which sported the bulkier 7-isobutyl-1,3,9-trimethylxanthine-8-ylidene ligand, in 41% overall yield starting from theophylline. Multinuclear NMR analysis confirmed the presence of p-cymene and an NHC ligand within the coordination sphere of both products. Yet, they exhibited a significantly different dynamic behavior in solution at room temperature, due to the larger size of the isobutyl chain compared to a methyl group, which severely restricted the rotation of the arene ligand and caused a loss of symmetry. Complexes Ru-1 and Ru-2 were both highly active catalyst precursors for the transfer hydrogenation of unsaturated substrates with isopropanol under basic conditions (11 examples). They were also efficient promoters for the oxidation of alkenes with sodium periodate. Last but not least, they triggered the synthesis of vinyl esters from 1-hexyne and benzoic acid. For this later reaction, the product distribution varied markedly with the nature of the NHC ligand. Altogether, this study demonstrated that the combination of a labile p-cymene ligand and an ancillary NHC ligand derived from caffeine or theophylline provided an efficient and sustainable access to a wealth of highly active Ru–hydride, −oxo, or −alkyne species that were able to catalyze three miscellaneous organic transformations.

Researches on Photofunctional and Photocatalytic Chemistry of Metal Complexes as Core Materials

We found the following new photochemical reactions and photophysical properties of transition metal complexes: (1) photochemical formation of Ru(II) hydride complexes, (2) photochemical ligand substitution reactions of the Re(I)diiminetricarbonyl complexes, and (3) control of photophysical and electrochemical properties of metal complexes by introducing p-p interaction between ligands. Application of these reactions and photophysical properties of transition metal complexes enabled us to synthesize new photofunctional metal-complex oligomers and polymers， and to develop efficient， durable， and highly selective photocatalysts for hydride reduction of NAD(P)+ models and CO2 reduction. We also succeeded construction of new photofunctional hybrids of the metal-complex photocatalysts with various solid materials， i.e. semiconductor-particle photocatalysts， p-type semiconductor electrodes， and mesoporous organosilica， which can drive photocatalytic reduction of CO2 via Z-scheme electron transfer， photocatalytic CO2 reduction by using water as a reductant and visible light as energy， and lightenergy accumulation for photocatalytic CO2 reduction， respectively.

Arene Coordination Induces Migration of a Hydride to a Ru-Bound Carbene of a Pincer Ligand

The synthesis of a new Ru complex (P2C=)RuH(OTf) (11), which contains a carbene/bis(phosphine) pincer ligand is described. Abstraction of the triflate from 11 using Na[BAr4F] results in the formation of an unsaturated, solvent-stabilized Ru hydride species in CD2Cl2. In a toluene/C6H4F2 solvent mixture, the reaction takes a different course and results in the migration of the hydride to the carbene ligand and concomitant binding of the toluene molecule to Ru in a hexahapto coordination mode.

Asymmetric Induction with a Chiral Amine Catalyzed by a Ru-PNP Pincer Complex: Insight from Theoretical Investigation.

In this paper, the mechanism of asymmetric amination of a racemic alcohol with Ellman's sulfinamide and the origin of diastereoselectivity catalyzed by a Ru-PNP pincer complex were studied using density functional theory (DFT). The mechanism involves dehydrogenation of the racemic alcohol, C-N coupling, and hydrogen transfer from the catalyst to the in situ formed imine. The calculated results indicate that both the alcohol dehydrogenation and imine hydrogenation are stepwise. The hydride transfer from a Ru hydride complex to the imine is shown to be the chirality-determining step in the whole catalytic cycle. It was found that the diastereoselectivity mainly stems from the hydrogen bonding interactions between the oxygen atom of the sulfinyl moiety and the hydrogen atom of the NH group of the ligand.

Facile Synthesis of Magnetically Recoverable Pd and Ru Catalysts for 4-Nitrophenol Reduction: Identifying Key Factors.

This paper reports the development of robust Pd- and Ru-containing magnetically recoverable catalysts in a one-pot procedure using commercially available, branched polyethyleneimine (PEI) as capping and reducing agent. For both catalytic metals, ∼3 nm nanoparticles (NPs) are stabilized in the PEI shell of magnetite NPs, whose aggregation allows for prompt magnetic separation. The catalyst properties were studied in a model reaction of 4-nitrophenol hydrogenation to 4-aminophenol with NaBH4. A similar catalytic NP size allowed us to decouple the NP size impact on the catalytic performance from other parameters and to follow the influence of the catalytic metal type and amount as well as the PEI amount on the catalytic activity. The best catalytic performances, the 1.2 min–1 rate constant and the 433.2 min–1 turnover frequency, are obtained for the Ru-containing catalyst. This is discussed in terms of stability of Ru hydride facilitating the surface-hydrogen transfer and the presence of Ru4+ species on the Ru NP surface facilitating the nitro group adsorption, both leading to an increased catalyst efficiency. High catalytic activity as well as the high stability of the catalyst performance in five consecutive catalytic cycles after magnetic separation makes this catalyst promising for nitroarene hydrogenation reactions.

Reactivity of Ru(II) and V(III) complexes of diazafluorene derivatives towards B–H bonds

We report the synthesis and characterization of Ru(II) and V(III) 4,5-diazafluorene derivatives, and their respective reactivity towards B–H bonds. A Ru(II) hydride dinitrogen complex reacts with HBpin to liberate the labile N2 ligand resulting in a dihydridoborate complex. Upon addition of either further HBpin or ClBpin a ligand-based borylation occurs. The first trialkylhydridoborate complex of vanadium was prepared by the addition of 3 eq. of K[HBEt3] to a V(III) chloride precursor supported by a 3,6--bis(mesityl) substituted 4,5-diazafluorene derivative, and features a (μ-H)B((μ-H)CHMe)2(Et) type of coordination mode for the [HBEt3]- ligand.

Metathesis and Decomposition of Fischer Carbenes of Cyclometalated Z-Selective Ruthenium Metathesis Catalysts

The addition of vinyl ethers to Z-selective, cyclometalated ruthenium metathesis catalysts generates Fischer carbene complexes. Although Fischer carbenes are usually thought to be metathesis inactive, we show that Fischer carbenes are metathesis active under certain circumstances. These species were found to decompose facilely to Ru hydride complexes, as identified by both experiment and computation. Since vinyl ethers are often used to quench metathesis reactions implementing Ru-based metathesis catalysts, their decomposition to hydrides can have a deleterious effect on the desired stereochemistry of the olefin product.

In situ Generated Ruthenium Catalyst Systems Bearing Diverse N-Heterocyclic Carbene Precursors for Atom-Economic Amide Synthesis from Alcohols and Amines.

The transition-metal-catalyzed direct synthesis of amides from alcohols and amines is herein demonstrated as a highly environmentally benign and atom-economic process. Among various catalyst systems, in situ generated N-heterocyclic carbene (NHC)-based ruthenium (Ru) halide catalyst systems have been proven to be active for this transformation. However, these existing catalyst systems usually require an additional ligand to achieve satisfactory results. In this work, through extensive screening of a diverse variety of NHC precursors, we discovered an active in situ catalyst system for efficient amide synthesis without any additional ligand. Notably, this catalyst system was found to be insensitive to the electronic effects of the substrates, and various electron-deficient substrates, which were not highly reactive with our previous catalyst systems, could be employed to afford the corresponding amides efficiently. Furthermore, mechanistic investigations were performed to provide a rationale for the high activity of the optimized catalyst system. NMR-scale reactions indicated that the rapid formation of a Ru hydride intermediate (signal at δ=-7.8 ppm in the 1 H NMR spectrum) after the addition of the alcohol substrate should be pivotal in establishing the high catalyst activity. Besides, HRMS analysis provided possible structures of the in situ generated catalyst system.

The Mechanism of Acceptorless Amine Double Dehydrogenation by N,N,N-Amide Ruthenium(II) Hydrides: A Combined Experimental and Computational Study

A detailed mechanistic analysis of the acceptorless double dehydrogenation of primary amines to form nitriles by HRu(bMepi)(PPh3)2 (1, bMepi = 1,3-bis(6′-methyl-2′-pyridylimino)isoindolate) is presented. The presence of the ortho-CH3 substituents on bMepi is critical for amine dehydrogenation, and no catalysis was observed with HRu(bpi)(PPh3)2 (1-bpi, bpi = 1,3-bis(2′-pyridylimino)isoindolate). Outer-sphere, inner-sphere, and hemilabile pathways were evaluated through ligand substitution and kinetic studies, catalyst modifications, and computational analysis. We propose an inner-sphere mechanism in which a Ru–hydride is protonated by coordinated amine followed by H2 release, which forms a Ru–amido intermediate. The stability of Ru–amido species was evaluated through NBO, AIM, and NCI analyses, revealing steric pressure as well as weak noncovalent interactions between the coordinated amido nitrogen atom and the ortho-alkyl substituents, and these interactions impact the overall thermodynamic profile for am...

Experimental and Theoretical Study of CO2 Insertion into Ruthenium Hydride Complexes.

The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298 K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.

Direct Hydrogenation of Biobased Carboxylic Acids Mediated by a Nitrogen-centered Tridentate Phosphine Ligand.

A novel nitrogen-centered tridentate ligand was identified from a series of multidentate ligands and applied for the direct hydrogenation of 9 biogenic acids into alcohols, lactones and esters with high yields. Comparison of substrates and ruthenium precursors suggested that the Ru(II) hydride cationic species was more active to transform acids than the corresponding lactone or esters.

Ruthenium‐Catalyzed Hydroformylation of Alkenes by using Carbon Dioxide as the Carbon Monoxide Source in the Presence of Ionic Liquids

AbstractThe reaction of [BMI⋅Cl] (BMI=1‐butyl‐3‐methylimidazolium) or [BMMI⋅Cl] (BMMI=3‐butyl‐1,2‐dimethylimidazolium) with Ru3(CO)12 generates Ru–hydride–carbonyl–carbene species in situ that are efficient catalysts for a reverse water gas shift/hydroformylation/hydrogenation cascade reaction. The addition of H3PO4 increased the catalytic activity of the first step (i.e., the hydrogenation of CO2 to CO). Under the optimized reaction conditions [120 °C and 6.0 MPa CO2/H2 (1:1) for 17 h], cyclohexene and 2,2‐disubstituted alkenes were easily functionalized to alcohols through sequential hydroformylation/carbonyl reduction.

Imino Glycals via Ruthenium-Catalyzed RCM and Isomerization

<i>N</i>-Allyl-<i>N</i>-homoallylamines were converted in one step into cyclic enamides via a ruthenium-catalyzed assisted tandem catalytic ring-closing metathesis–isomerization sequence. The sequence relies on the in situ transformation of a metathesis active Ru–carbene into an isomerization active Ru–hydride by addition of hydroxide as a chemical trigger.

Synthesis and Reactivity of Ruthenium Hydride Complexes Containing a Tripodal Aminophosphine Ligand

Ru complexes containing a tris(aminophosphine) ligand (N(NP)3) have been prepared and their reactivity examined. The complex [(N((CH2)2NHPiPr2)2(κ2N,P-((CH2)2NHPiPr2))RuCl][Cl] (1) is transformed to [(N((CH2)2NHPiPr2)2((CH2)2NPHiPr2))RuCl]Cl (2) on standing in solution. Deprotonation of the phosphonium center in 2 followed by anion exchange generates the Ru hydride complex [(N((CH2)2NHPiPr2)((CH2)2NPiPr2)(CH2CHNHPiPr2))RuH][BPh4] (3), which can bind acetonitrile to give [(N((CH2)2NHPiPr2)2((CH2)2NPiPr2))Ru(CH3CN)][BPh4] (5). Protonation of 3 and 5 with either a 1 M solution of HCl in diethyl ether or NEt3HCl yields [(N((CH2)2NHPiPr2)2((CH2)2NPHiPr2))RuCl][BPh4] (6). Reaction of 3 with H2 gives the Ru hydride complex [(N((CH2)2NHPiPr2)2(κ2N,P((CH2)2NHPiPr2))RuH][BPh4] (8), which is deprotonated with KN(SiMe3)2 to give [(N((CH2)2NHPiPr2)2)((CH2)2NPiPr2)RuH] (9). This latter species reacts with H2 to generate [(N((CH2)2NHPiPr2)3)Ru(H)2] (10). The hydride species 3 is also shown to react with CO2, N2O, phenylacetylene, and 1-pentyne to give [(N((CH2)2NHPiPr2)2((CH2)2NP(CO2)iPr2))Ru][BPh4] (11), [(N((CH2)2NHPiPr2)2((CH2)2NP(O)iPr2))Ru][BPh4] (12), and [(N((CH2)2NHPiPr2)2((CH2)2NP(R)iPr2))Ru][BPh4] (R = C8H6 (13), C5H10 (14)), in which the substrate is bound to P and the metal center. In contrast, 9 does not react with N2O and alkyne but reacts with CO2 to give CO2 insertion into the N–P bond, yielding (N((CH2)2NHPiPr2)2((CH2)2N(CO2)PiPr2)RuH (15).

Special Issue: Frontiers in Catalysis Research—The Catalysis Research Center of Technische Universität München

Special Issue: Frontiers in Catalysis Research—The Catalysis Research Center of Technische Universität München

Ruthenium‐Catalyzed Hydrogenation of Oxygen‐Functionalized Aromatic Compounds in Water

AbstractThe application of a ruthenium complex, bearing a sulfonated bis‐N‐heterocyclic carbene (NHC) ligand as catalyst precursor for the hydrogenation of aromatic compounds is reported. The reaction proceeds under mild conditions in aqueous phase. The treatment of the Ru complex with 40 bar H2 at 60 °C in water (0.1 M KOH solution) leads to the formation of a catalytically active species, which can be stored and used in catalysis experiments. The catalyst is responsible for the hydrogenation of functionalized aromatic substrates exhibiting endo‐ and exo‐CO bonds, which are relevant products in biomass conversion. Acetophenone is quantitatively reduced to 1‐cyclohexyl ethanol with a selectivity of 100 %. Various other oxygen‐functionalized aromatic substrates were also hydrogenated in moderate to quantitative conversions. To elucidate the nature of the catalytically active species, NMR, UV/vis, and ESI‐MS experiments were undertaken, showing the presence of a mononuclear Ru hydride complex. TEM measurements of a sample of the catalyst solution did not indicate the presence of nanoparticles. Mechanistic investigations point towards a homogenous mechanism.

Base-Free, Acceptorless, and Chemoselective Alcohol Dehydrogenation Catalyzed by an Amide-Derived NNN-Ruthenium(II) Hydride Complex

The bmpi (1,3-bis(6′-methyl-2′-pyridylimino)isoindoline) pincer Ru(II) hydride complex catalyzes base-free, acceptorless, and chemoselective dehydrogenation of alcohols with liberation of dihydrogen under moderate (<120 °C) conditions. Primary alcohols and diols are converted to ester and lactone products with high conversion efficiencies. The catalyst system is remarkably selective for the oxidation of secondary alcohols in the presence of primary alcohols.

Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

Despite the fundamental importance of the hydricity of a transition metal hydride (ΔG(H–)°(MH) for the reaction M–H → M+ + H–) in a range of reactions important in catalysis and solar energy storage, ours (J. Am. Chem. Soc.2009, 131, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru(II) hydride complexes previously studied in water. For [Ru(η(6)-C6Me6)(bpy)H]+ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru(η(6)-C6Me6)(bpy)H]+ pKa = 22.5 ± 0.1 and the [Ru(η(6)-C6Me6)(bpy)(NCCH3)(1/0)](2+/0) electrochemical potential (−1.22 V vs Fc+/Fc). For [Ru(tpy)(bpy)H]+ (tpy = 2,2′:6′,2″-terpyridine) we utilized organic hydride ion acceptors (A+) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined K for the hydride transfer reaction, S + MH+ + A+ → M(S)2+ + AH (S = CD3CN, MH+ = [Ru(tpy)(bpy)H]+), by 1H NMR measurements. Equilibration of initially 7 mM solutions was slow--on the time scale of a day or more. When E°(H+/H–) is taken as 79.6 kcal/mol vs Fc+/Fc as a reference, the hydricities of [Ru(η(6)-C6Me6)(bpy)H]+ and [Ru(tpy)(bpy)H]+ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The pKa estimated for [Ru(tpy)(bpy)H]+ in acetonitrile is 32 ± 3. UV–vis spectroscopic studies of [Ru(η(6)-C6Me6)(bpy)]0 and [Ru(tpy)(bpy)]0 indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water.

Aerobic oxidation of alcohols to aldehydes and ketones using ruthenium(III)/Et3N catalyst

A simple, efficient method for oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones has been developed. Using RuCl3/Et3N as catalyst, the oxidation of benzyl alcohol with oxygen could be achieved with 332 h−1 turnover frequency in the absence of solvent. The influence of versatile N‐containing additives on the catalytic efficiency has been discussed. The presence of minor water would substantially promote the catalytic efficiency, and its role in catalysis has been investigated in detail. The insensitive Hammett correlations of the substituted benzyl alcohols, the normal substrate isotope effect (kH/kD = 3.5 at 335 K), and the linear relationship between O2 pressure and turnover frequency imply that the reoxidation of the Ru(III) hydride intermediate to the active species shares the rate‐determining step with the hydride transfer in the catalytic cycle. Copyright © 2011 John Wiley &amp; Sons, Ltd.