Active Hydrogenation Catalysts Research Articles

Discovery of Highly Selective Alkyne Semihydrogenation Catalysts Based on First‐Row Transition‐Metallated Porous Organic Polymers

Five different first-row transition metal precursors (V(III), Cr(III), Mn(II), Co(II), Ni(II)) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR-IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high-throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single-site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes.

Cationic pyridyl(benzoazole) ruthenium(II) complexes: Efficient and recyclable catalysts in biphasic hydrogenation of alkenes and alkynes

The synthesis, structural characterization of cationic 2-(2-pyridyl)benzoazole)ruthenium(II) complexes and their applications in biphasic hydrogenations of alkenes is reported. Reactions of 2-(2-pyridyl)benzoimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with [η6-(2-phenoxyethanol)RuCl2]2 produced the corresponding cationic complexes [η6-(2-phenoxyethanol)RuCl(L1)]Cl (1), [η6-(2-phenoxyethanol)RuCl(L2)]Cl (2) and [η6-(2-phenoxyethanol)RuCl(L3)]Cl (3) in good yields. Solid state structures of 1–3 confirmed the bidentate coordination modes of L1–L3 and formation of cationic species through displacement of one chloride ligand from Ru(II) coordination sphere. Complexes 1–3 produced active catalysts for high pressure hydrogenation of alkenes both in methanol and biphasic conditions. Relatively lower activities were observed in the hydrogenation of terminal alkynes giving a mixture of alkane and alkene products. Complexes 1–3 were recyclable under biphasic conditions and retained significant catalytic activities in six cycles. Reaction parameters such as substrate/catalyst ratio, temperature, and aqueous/organic ratio affected the catalytic trends.

Process Intensification for the Continuous Flow Hydrogenation of Ethyl Nicotinate

Here we report a process intensification study for the selective, partial, and full hydrogenation of ethyl nicotinate using a trickle bed reactor for meso-flow transformations (HEL FlowCAT). The process achieved a throughput of 1219 g d–1 (78 g h–1 of product per g of active catalyst) for the partial hydrogenation to ethyl 1,4,5,6-tetrahydropyridine-3-carboxylate, whereas the productivity for the full hydrogenation process reached a 1959 g d–1 of throughput (408 g h–1 of product per g of active catalyst) on this laboratory-scale flow chemistry platform.

Molecular metal catalysts on supports: organometallic chemistry meets surface science.

Recent advances in the synthesis and characterization of small, essentially molecular metal complexes and metal clusters on support surfaces have brought new insights to catalysis and point the way to systematic catalyst design. We summarize recent work unraveling effects of key design variables of site-isolated catalysts: the metal, metal nuclearity, support, and other ligands on the metals, also considering catalysts with separate, complementary functions on supports. The catalysts were synthesized with the goal of structural simplicity and uniformity to facilitate incisive characterization. Thus, they are essentially molecular species bonded to porous supports chosen for their high degree of uniformity; the supports are crystalline aluminosilicates (zeolites) and MgO. The catalytic species are synthesized in reactions of organometallic precursors with the support surfaces; the precursors include M(L)2(acetylacetonate)1-2, with M = Ru, Rh, Ir, or Au and the ligands L = C2H4, CO, or CH3. Os3(CO)12 and Ir4(CO)12 are used as precursors of supported metal clusters, and some such catalysts are made by ship-in-a-bottle syntheses to trap the clusters in zeolite cages. The simplicity and uniformity of the supported catalysts facilitate precise structure determinations, even in reactive atmospheres and during catalysis. The methods of characterizing catalysts in reactive atmospheres include infrared (IR), extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and nuclear magnetic resonance (NMR) spectroscopies, and complementary methods include density functional theory and atomic-resolution aberration-corrected scanning transmission electron microscopy for imaging of individual metal atoms. IR, NMR, XANES, and microscopy data demonstrate the high degrees of uniformity of well-prepared supported species. The characterizations determine the compositions of surface metal complexes and clusters, including the ligands and the metal-support bonding and structure, which identify the supports as ligands with electron-donor properties that influence reactivity and catalysis. Each of the catalyst design variables has been varied independently, illustrated by mononuclear and tetranuclear iridium on zeolite HY and on MgO and by isostructural rhodium and iridium (diethylene or dicarbonyl) complexes on these supports. The data provide examples resolving the roles of the catalyst design variables and place the catalysis science on a firm foundation of organometallic chemistry linked with surface science. Supported molecular catalysts offer the advantages of characterization in the absence of solvents and with surface-science methods that do not require ultrahigh vacuum. Families of supported metal complexes have been made by replacement of ligands with others from the gas phase. Spectroscopically identified catalytic reaction intermediates help to elucidate catalyst performance and guide design. The methods are illustrated for supported complexes and clusters of rhodium, iridium, osmium, and gold used to catalyze reactions of small molecules that facilitate identification of the ligands present during catalysis: alkene dimerization and hydrogenation, H-D exchange in the reaction of H2 with D2, and CO oxidation. The approach is illustrated with the discovery of a highly active and selective MgO-supported rhodium carbonyl dimer catalyst for hydrogenation of 1,3-butadiene to give butenes.

Distinguishing homogeneous from nanoparticle asymmetric iron catalysis

This perspective will examine the use of a wide range of techniques for differentiating homogeneous from nanoparticle asymmetric catalysis as it pertains to two highly active systems developed within our group. The 6,5,6 and 5,5,5-precatalysts, trans-[Fe(NCMe)CO(PPh2C6H4CHNCHPh–)2][BF4]2 and trans-[Fe(CO)Br(PR2CH2CHNCHPh–)2][BF4], respectively, are highly active and selective asymmetric transfer hydrogenation (ATH) catalysts. Here, we will review the series of tests that were undertaken to support the idea that the 6,5,6-precatalyst forms iron nanoparticles (Fe NPs) during catalysis, whereas the 5,5,5-system remains homogeneous. Techniques include the use of NMR and DFT to investigate intermediates and activation steps, reaction profile and induction period analysis, substoichiometric poisoning, electron microscopy imaging, dynamic light scattering (DLS), X-ray photoelectron microscopy (XPS), magnetometry, and multiphasic analysis. We also elaborate on the wider applicability of these and other tests to probe the true nature of an active catalyst, with emphasis on the importance of using a wide range of techniques for insightful mechanistic evaluations.

Mechanism of CO2hydrogenation to formates by homogeneous Ru-PNP pincer catalyst: from a theoretical description to performance optimization

The reaction mechanism of CO2 hydrogenation by pyridine-based Ru-PNP catalyst in the presence of DBU base promoter was studied by means of density functional theory calculations. Three alternative reaction channels promoted by the complexes potentially present under the reaction conditions, namely the dearomatized complex 2 and the products of cooperative CO2 (3) and H2 (4) addition, were analysed. It is shown that the bis-hydrido Ru-PNP complex 4 provides the unique lowest-energy reaction path involving a direct effectively barrierless hydrogenolysis of the polarized complex 5*. The reaction rate in this case is controlled by the CO2 activation by Ru–H that proceeds with a very low barrier of ca. 20 kJ mol−1. The catalytic reaction can be hampered by the formation of a stable formato-complex 5. In this case, the rate is controlled by the H2 insertion into the Ru–OCHO coordination bond, for which a barrier of 65 kJ mol−1 is predicted. The DFT calculations suggest that the preference for the particular route can be controlled by varying the partial pressure of H2 in the reaction mixture. Under H2-rich conditions, the former more facile catalytic path should be preferred. Dedicated kinetic experiments verify these theoretical predictions. The apparent activation energies measured at different H2/CO2 molar ratios are in a perfect agreement with the calculated values. Ru-PNP is a highly active CO2 hydrogenation catalyst allowing reaching turnover frequencies in the order of 106 h−1 at elevated temperatures. Moreover, a minor temperature dependency of the reaction rate attainable in excess H2 points to the possibility of efficient CO2 hydrogenation at near-ambient temperatures.

ChemInform Abstract: Cyclometalated Iridium Complexes as Highly Active Catalysts for the Hydrogenation of Imines.

A robust cyclometalated iridium catalyst has been developed for highly effective hydrogenation of imines. With very low catalyst loading (down to 0.005%), good to excellent yields have been achieved for a range of substrates in a short reaction time under mild conditions, providing an easy, efficient protocol for making amines.

Rhodium Catechol Containing Porous Organic Polymers: Defined Catalysis for Single-Site and Supported Nanoparticulate Materials

A single-site, rhodium(I) catecholate containing porous organic polymer was prepared and utilized as an active catalyst for the hydrogenation of olefins in both liquid-phase and gas-phase reactors. Liquid-phase, batch hydrogenation reactions at 50 psi and ambient temperatures result in the formation of rhodium metal nanoparticles supported within the polymer framework. Surprisingly, the Rh(I) complex is catalytically active and stable for propene hydrogenation at ambient temperatures under gas-phase conditions, where reduction of the Rh(I) centers to Rh(0) nanoparticles requires at least 200–250 °C under a flow of hydrogen gas. After high-temperature treatment, the Rh(0) nanoparticles are active arene hydrogenation catalysts that convert toluene to methylcyclohexadiene at a rate of 9.3 × 10–3 mol g–1 h–1 of rhodium metal at room temperature. Conversely, single-site Rh(I) is an active and stable catalyst for the hydrogenation of propylene (but not toluene) under gas-phase conditions at room temperature.

Aqueous-phase hydrogenation and hydrodeoxygenation of biomass-derived oxygenates with bimetallic catalysts

The reaction rate on a per site basis for aqueous-phase hydrogenation (APH) of propanal, xylose, and furfural was measured over various alumina-supported bimetallic catalysts (Pd–Ni, Pd–Co, Pd–Fe, Ru–Ni, Ru–Co, Ru–Fe, Pt–Ni, Pt–Co, and Pt–Fe) using a high-throughput reactor (HTR). The results in this paper demonstrate that the activity of bimetallic catalysts for hydrogenation of a carbonyl group can be 110 times higher than monometallic catalysts. The addition of Fe to a Pd catalyst increased the activity for hydrogenation of propanal, xylose, and furfural. The Pd1Fe3 catalyst had the highest reaction rate for APH of propanal among all catalysts tested in the HTR. The addition of Fe to the Pd catalyst increased the reaction rate for xylose hydrogenation by a factor of 51, compared to the monometallic Pd catalyst. However, no bimetallic catalyst tested in this study was more active than the monometallic Ru catalyst for hydrogenation of xylose. The Pd1Fe3 catalyst had the highest reaction rate for APH of furfural, which was 9 times higher than the rate of the Pd catalyst. The Pd1Fe3/Zr–P, a bimetallic bifunctional catalyst, was 14 times more active on a per site basis than a Pd/Zr–P catalyst for aqueous-phase hydrodeoxygenation (HDO) of sorbitol in a continuous flow reactor. The addition of Fe to the Pd catalyst increased the rate of C–C cleavage reactions and promoted the conversion of sorbitan and isosorbide in HDO of sorbitol. Pd1Fe3/Zr–P also had a higher yield of gasoline-range products than the Pd/Zr–P catalyst.

Assembly of evenly distributed Au nanoparticles on thiolated reduced graphene oxide as an active and robust catalyst for hydrogenation of 4-nitroarenes

Au/SRG nanohybrid with evenly distributed Au nanoparticles anchored on reduced graphene oxide via covalent thiol linkage shows superior catalytic performance for hydrogenation of 4-nitrophenol.

(Pyridyl)benzoazole ruthenium(ii) and ruthenium(iii) complexes: role of heteroatom and ancillary phosphine ligand in the transfer hydrogenation of ketones

The synthesis and structural characterization of ruthenium complexes supported by 2-(2-pyridyl)benzoazole ligands and their evaluation as catalysts in the transfer hydrogenation of ketones are reported. Reactions of 2-(2-pyridyl)benzoimidazole (L1), 2-(2-pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3) with RuCl3·3H2O produced the corresponding complexes [RuCl3(L1)] (1), [RuCl3(L2)] (2) and [RuCl3(L3)] (3), respectively. Similarly, treatment of L1-L3 with RuCl2(PPh3)2 afforded the corresponding Ru(II) complexes [RuCl2(PPh3)2(L1)] (4), [RuCl2(PPh3)2(L2)] (5) and [RuCl2(PPh3)2(L3)] (6), respectively. Solid state structures of 1 and 2 confirmed the bidentate coordination mode of L1 and L2 to ruthenium. (31)P{(1)H} NMR spectroscopy revealed coordination of two PPh3 ligands trans to each other in an octahedral environment in 4-6 as confirmed by the solid state structure of 6. Complexes 1-6 produced active catalysts in the transfer hydrogenation of ketones in 2-propanol at 82 °C. Ruthenium(II) complexes 4-6, containing the PPh3 ligand, exhibited higher catalytic activities than the corresponding ruthenium(III) compounds 1-3. Complexes 1 and 4 of L1 were more active than the corresponding complexes of L2 and L3. Density functional theoretical calculations showed that dipole moments of 1-6 control their catalytic activities.

A new efficient bis(phosphinite)-ruthenium(II) catalyst system for the asymmetric transfer hydrogenation of aromatic ketones

Many chiral ligands bearing phosphorus donor atoms were developed and applied in the area of metal catalyzed asymmetric catalysis. A new chiral bis(phoshinite) ligand was successfully synthesized from optically active (1S,8S)-3,6-[N-(S)-α-phenylethyl]diaza-1,8-diphenoxymethyl-1,8-octanediol in high yield. A ruthenium(II) complex generated in situ from [Ru(η6-p-cymene)(μ-Cl)Cl]2, bis(phosphinite) ligand and KOH acts as an active catalyst for asymmetric transfer hydrogenation (ATH) of acetophenone derivatives in isopropyl alcohol (IPA) as both solvent and hydrogen source. The catalytic reaction proceeds cleanly to give enantiomerically enriched secondary alcohols up to 90% or 89% ee’s at 50 or 82°C, respectively.

Strategies for Designing Supported Gold–Palladium Bimetallic Catalysts for the Direct Synthesis of Hydrogen Peroxide

Hydrogen peroxide is a widely used chemical but is not very efficient to make in smaller than industrial scale. It is an important commodity chemical used for bleaching, disinfection, and chemical manufacture. At present, manufacturers use an indirect process in which anthraquinones are sequentially hydrogenated and oxidized in a manner that hydrogen and oxygen are never mixed. However, this process is only economic at a very large scale producing a concentrated product. For many years, the identification of a direct process has been a research goal because it could operate at the point of need, producing hydrogen peroxide at the required concentration for its applications. Research on this topic has been ongoing for about 100 years. Until the last 10 years, catalyst design was solely directed at using supported palladium nanoparticles. These catalysts require the use of bromide and acid to arrest peroxide decomposition, since palladium is a very active catalyst for hydrogen peroxide hydrogenation. Recently, chemists have shown that supported gold nanoparticles are active when gold is alloyed with palladium because this leads to a significant synergistic enhancement in activity and importantly selectivity. Crucially, bimetallic gold-based catalysts do not require the addition of bromide and acids, but with carbon dioxide as a diluent its solubility in the reaction media acts as an in situ acid promoter, which represents a greener approach for peroxide synthesis. The gold catalysts can operate under intrinsically safe conditions using dilute hydrogen and oxygen, yet these catalysts are so active that they can generate peroxide at commercially significant rates. The major problem associated with the direct synthesis of hydrogen peroxide concerns the selectivity of hydrogen usage, since in the indirect process this factor has been finely tuned over decades of operation. In this Account, we discuss how the gold-palladium bimetallic catalysts have active sites for the synthesis and hydrogenation of hydrogen peroxide that are different, in contrast to monometallic palladium in which synthesis and hydrogenation operate at the same sites. Through treatment of the support with acids prior to the deposition of the gold-palladium bimetallic particles, we can obtain a catalyst that can make hydrogen peroxide at a very high rate without decomposing or hydrogenating the product. This innovation opens up the way to design improved catalysts for the direct synthesis process, and these possibilities are described in this Account.

Cp*Co(III) Catalysts with Proton-Responsive Ligands for Carbon Dioxide Hydrogenation in Aqueous Media

New water-soluble pentamethylcyclopentadienyl cobalt(III) complexes with proton-responsive 4,4'- and 6,6'-dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP, respectively) ligands have been prepared and were characterized by X-ray crystallography, UV-vis and NMR spectroscopy, and mass spectrometry. These cobalt(III) complexes with proton-responsive ligands predominantly exist in their deprotonated [Cp*Co(DHBP-2H(+))(OH2)] forms with stronger electron-donating properties in neutral and basic solutions, and are active catalysts for CO2 hydrogenation in aqueous bicarbonate media at moderate temperature under a total 4-5 MPa (CO2:H2 1:1) pressure. The cobalt complexes containing 4DHBP ligands ([1-OH2](2+) and [1-Cl](+), where 1 = Cp*Co(4DHBP)) display better thermal stability and exhibit notable catalytic activity for CO2 hydrogenation to formate in contrast to the catalytically inactive nonsubstituted bpy analogues [3-OH2](2+) (3 = Cp*Co(bpy)). While the catalyst Cp*Ir(6DHBP)(OH2)(2+) in which the pendent oxyanion lowers the barrier for H2 heterolysis via proton transfer through a hydrogen-bonding network involving a water molecule is remarkably effective (ACS Catal. 2013, 3, 856-860), cobalt complexes containing 6DHBP ligands ([2-OH2](2+) and [2-Cl](+), 2 = Cp*Co(6DHBP)) exhibit lower TOF and TON for CO2 hydrogenation than those with 4DHBP. The low activity is attributed to thermal instability during the hydrogenation of CO2 as corroborated by DFT calculations.

Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine cobalt alkyl and hydride complexes.

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)CoCH3. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.

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.

Screening of Catalysts for Hydrodeoxygenation of Phenol as a Model Compound for Bio-oil

Four groups of catalysts have been tested for hydrodeoxygenation (HDO) of phenol as a model compound of bio-oil, including oxide catalysts, methanol synthesis catalysts, reduced noble metal catalysts, and reduced non-noble metal catalysts. In total, 23 different catalysts were tested at 100 bar H2 and 275 °C in a batch reactor. The experiments showed that none of the tested oxides or methanol synthesis catalysts had any significant activity for phenol HDO under the given conditions, which were linked to their inability to hydrogenate the aromatic ring of phenol. HDO of phenol over reduced metal catalysts could effectively be described by a kinetic model involving a two-step reaction in which phenol initially was hydrogenated to cyclohexanol and then subsequently deoxygenated to cyclohexane. Among reduced noble metal catalysts, ruthenium, palladium, and platinum were all found to be active, with activity decreasing in that order. Nickel was the only active non-noble metal catalyst. For nickel, the effect of support was also investigated and ZrO2 was found to perform best. Pt/C, Ni/CeO2, and Ni/CeO2-ZrO2 were the most active catalysts for the initial hydrogenation of phenol to cyclohexanol but were not very active for the subsequent deoxygenation step. Overall, the order of activity of the best performing HDO catalysts was as follows: Ni/ZrO2 > Ni-V2O5/ZrO2 > Ni-V2O5/SiO2 > Ru/C > Ni/Al2O3 > Ni/SiO2 ≫ Pd/C > Pt/C. The choice of support influenced the activity significantly. Nickel was found to be practically inactive for HDO of phenol on a carbon support but more active than the carbon-supported noble metal catalysts when supported on ZrO2. This observation indicates that the nickel-based catalysts require a metal oxide as a carrier on which the activation of the phenol for the hydrogenation can take place through heterolytic dissociation of the O–H bond to facilitate the reaction.

Screening of Soluble Rhodium Nanoparticles as Precursor for Highly Active Hydrogenation Catalysts: The Effect of the Stabilizing Agents

We report the preparation of rhodium nanoparticles (NPs) stabilized by 1-octadecanethiol (ODT), polyvinyl alcohol (PVA), and tetraoctylammonium bromide (TOAB), and their application for hydrogenation catalysis. The three metal–ligand systems correspond to different mechanism of NPs stabilization via strong covalent linkage, chemisorbed atoms and electrostatic interactions, respectively. We found a strong effect of the interaction between the stabilizer and the surface of the metal nanoparticle on the catalytic activity. The Rh NPs were studied as soluble nanoparticle catalysts and as precursors for the synthesis of supported catalysts. All catalysts were tested in the hydrogenation of cyclohexene under similar conditions as a model reaction. Generally, Rh–ODT NPs were inactive, Rh–PVA NPs exhibited distinct activities in solution (aqueous biphasic catalysis) and as a supported catalyst, and Rh–TOAB NPs exhibited similar activities in solution and after immobilization. This last result opens the opportunity for the preparation of highly active Rh NP catalysts both in solution and as a heterogeneous catalyst. Additionally, the stability of the nanoparticles depends on the choice of ligand and on the functionalization of the support surface before immobilization. By optimizing the catalyst synthesis and reaction conditions, turnover frequencies as high as 700,000 h−1 where observed for stable and recyclable catalyst.

Synthesis, structural characterisation and catalytic application of dichloro(η 6-p-cymene){diphenyl(3-methyl-2-indolyl)phosphine}ruthenium(II) in the transfer hydrogenation of ketones

The synthesis and investigation of ruthenium complexes containing the relatively unexplored ligand, diphenyl-2-(3-methyl)indolylphosphine, is presented herein. The complexes [RuCl2{PPh2(C9H8N)}3], [Ru2(μ-Cl)3(Cl)(MeCN){PPh2(C9H8N)}4] and [RuCl2(η 6-p-cymeme){PPh2(C9H8N)}] have been studied. Single crystals of the latter two complexes have been prepared and investigated by X-ray crystallography. A detailed examination of [RuCl2(η 6-p-cymeme){PPh2(C9H8N)}] has been carried out. This complex was found to be an active catalyst in the catalytic transfer hydrogenation of ketones. The complex, [RuCl2(η 6-p-cymeme){PPh2(C9H8N)}], was synthesised and fully characterised by spectroscopic, analytical and structural methods. The complex was also found to be an active catalyst in the transfer hydrogenation of ketones.

A Computational Investigation of the Insertion of Carbon Dioxide into Four‐ and Five‐Coordinate Iridium Hydrides

AbstractRecently, it has been demonstrated that the insertion of CO2 into iridium hydrides is a crucial step in the catalytic conversion of CO2 and H2 into formic acid. We and others have elucidated the mechanism by which CO2 inserts into six‐coordinate iridium(III) trihydrides supported by pincer ligands; these complexes are very active catalysts for CO2 hydrogenation. However, it has also been demonstrated that five‐coordinate iridium(III) dihydrides can react with CO2 and catalyze both thermal and electrochemical CO2 hydrogenation. In this work, we study the mechanism of CO2 insertion into pincer‐supported five‐coordinate iridium(III) dihydrides and four‐coordinate iridium(I) hydrides using density functional theory. The mechanisms differ slightly between the two cases. Insertion into the five‐coordinate species is a multistep process involving initial CO2 precoordination, whereas insertion into the four‐coordinate species proceeds via a single step with no prior CO2 coordination. Both of these mechanisms are different from the pathway that was recently proposed for CO2 insertion into six‐coordinate iridium(III) trihydrides. In addition, a complete pathway for catalytic CO2 hydrogenation using a five‐coordinate iridium(III) dihydride has been calculated.