Homogeneous Hydrogenation Catalysts Research Articles

Rhodium(i) diphenylphosphine complexes supported on porous organic polymers as efficient and recyclable catalysts for alkene hydrogenation

Highly stable, chemoselective and recyclable immobilized Rh(i) homogeneous catalysts for alkene hydrogenation.

The Immobilization of a Transfer Hydrogenation Catalyst on Colloidal Particles

AbstractIn this paper, we report a new synthetic procedure to immobilize a transfer hydrogenation catalyst on the surface of colloidal polystyrene particles. Using supports of colloidal dimensions allows for combining a relatively high surface area for catalyst binding, mobility of the catalyst, and facile recovery by centrifugation or sedimentation. The immobilization procedure relies on the covalent attachment of terpyridine moieties on the particle surface. These immobilized terpyridines are subsequently employed as colloidal ligands, which participate in the formation of an active ruthenium‐based transfer hydrogenation catalyst. The resulting functional colloidal particles successfully catalyze the transfer hydrogenation of acetophenone with 2‐propanol as the hydrogen donor. Thorough analysis of the chemical composition of the colloidal surface led to the determination of the catalyst loading per particle. This enabled us to conduct reference hydrogenations with equal concentrations of the homogeneous transfer hydrogenation catalyst to probe the effect of the immobilization procedure on the catalytic activity. Despite a decrease in transfer hydrogenation activity, full acetophenone conversion is still achievable within 24 h. Preliminary experiments show that the catalytic colloids are recyclable without significant loss of transfer hydrogenation activity.

Metal-Catalysed Transfer Hydrogenation of Ketones.

We highlight recent developments of catalytic transfer hydrogenation of ketones promoted by transition metals, while placing it within its historical context. Since optically active secondary alcohols are important building blocks in fine chemicals synthesis, the focus of this review is devoted to chiral catalyst types which are capable of inducing high stereoselectivities. Ruthenium complexes still represent the largest part of the catalysts, but other metals (e.g. Fe) are rapidly penetrating this field. While homogeneous transfer hydrogenation catalysts in some cases approach enzymatic performance, the interest in heterogeneous catalysts is constantly growing because of their reusability. Despite excellent activity, selectivity and compatibility of metal complexes with a variety of functional groups, no universal catalysts exist. Development of future catalyst systems is directed towards reaching as high as possible activity with low catalyst loadings, using "greener" conditions, and being able to operate under mild conditions and in a highly selective manner for a broad range of substrates.

ChemInform Abstract: Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?

AbstractReview: 100 refs.

Why Does Industry Not Use Immobilized Transition Metal Complexes as Catalysts?

AbstractMuch effort has gone into the immobilization of homogeneous catalysts based on the idea that in this way the catalysts could be not only separated more easily from the product but also reused several times, thus reducing the cost of the catalyst use. So far none of these immobilized catalysts have been used by industry. In this article we critically review the use of immobilized homogeneous catalysts from the point of view of process development for the pharmaceutical and fine chemical industry. The first and foremost question that needs to be answered is: will immobilizing a homogeneous catalyst really lead to lower costs? The answer is thus far always no. This is caused mostly by the fact that homogeneous catalysts are not stable and thus there is little point in immobilizing them. The second reason is the extra added cost that is incurred in immobilizing the catalysts. Other problems are lower rates, sometimes lower selectivities and metal leaching. Three different areas are discussed. The research on immobilized metathesis catalysts is analyzed in detail; in general the immobilized catalysts do not achieve sufficient turnovers to be interesting for industrial use. Very many publications have appeared on immobilized palladium catalysts that were used for CC bond‐forming reactions, such as Suzuki, Heck or Sonogashira reactions. These catalysts are invariably converted to nanoparticles after the first run. Although these catalysts can be reused, there is no reason to use an expensive support based on immobilized ligands. This also does not protect the product from palladium contamination. Even more effort has gone into the immobilization of homogeneous hydrogenation catalysts. Most of these catalysts suffer from the same problems as the other immobilized catalysts: catalyst deactivation, low turnover numbers, and leaching of the metal. In addition, the heterogenization adds complexity to the system, increasing risk and prolonging process development.magnified image

Ruthenium-8-quinolinethiolate-phenylterpyridine versus ruthenium-bipyridine-phenyl-terpyridine complexes as homogeneous water and high temperature stable hydrogenation catalysts for biomass-derived substrates

[(4′-Ph-terpy)(bipy)Ru(L)](OTf)n and [(4′-Ph-terpy)(quS)Ru(L)](OTf)n (n=0 or 1 depending on the charge of L, L=labile ligand, e.g., H2O, CH3CN or OTf, bipy=2,2′-bipyridine, quS=quinoline-8-thiolate) have been evaluated as catalysts for the hydrogenation of the biomass-derivable C6-substrates 2,5-dimethylfuran (obtainable from 5-hydroxymethylfurfural) and 2,5-hexanedione (the hydrolysis product of 2,5-dimethylfuran). Operating in aqueous acidic medium at T=175–225°C the bipy complex is only marginally active, while the quinoline-8-thiolate complex realizes yields of hydrogenated products up to 97% starting from 2,5-hexanedione and up to 66% starting from 2,5-dimethylfuran. The catalyst can also convert the 5-HMF derived acetone 4-(5-methyl-2-furanyl)-3-buten-2-one into 2,5,8-nonatriol, a potentially valuable cross-linker for polymer formulations. On the basis of DFT calculations, the higher activity of the quinoline-8-thiolate complex is proposed to be rooted in a metal–ligand bifunctional mechanism for the heterolytic activation and transfer of dihydrogen to the carbonyl substrate with the hydride-thiol complex [(4′-Ph-terpy)(quSH)Ru(H)]+ as the active catalyst.

Efficient enantioselective hydrogenation of quinolines catalyzed by conjugated microporous polymers with embedded chiral BINAP ligand

Chiral Ir complexes were successfully used in the asymmetric hydrogenation of olefins, ketones, and quinolines. However, almost all the catalytic systems could not tolerate a high catalyst loading because of the formation of an irreversible iridium dimer and trimer during the reaction. It is expected that higher catalytic activity may be achieved if the Ir-complexes were isolated in space. The development of conjugated microporous polymers (CMPs) gives the opportunity for the spatial separation of the complexes. A series of chiral CMPs based on the chiral (R)-BINAP ligand (BINAP-CMPs) with different surface areas were synthesized. The BINAP ligands were separately distributed in the framework and were three times more active than the homogeneous catalyst (TOF 340 h−1VS 100 h−1) for the asymmetric hydrogenation of quinolines.

Iron- and Cobalt-Catalyzed Alkene Hydrogenation: Catalysis with Both Redox-Active and Strong Field Ligands.

The hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my group's efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the interplay of the electronic structure of the base metal compounds and their catalytic performance. First generation, aryl-substituted pyridine(diimine) iron dinitrogen catalysts exhibited high turnover frequencies at low catalyst loadings and hydrogen pressures for the hydrogenation of unactivated terminal and disubstituted alkenes. Exploration of structure-reactivity relationships established smaller aryl substituents and more electron donating ligands resulted in improved performance. Second generation iron and cobalt catalysts where the imine donors were replaced by N-heterocyclic carbenes resulted in dramatically improved activity and enabled hydrogenation of more challenging unactivated, tri- and tetrasubstituted alkenes. Optimized cobalt catalysts have been discovered that are among the most active homogeneous hydrogenation catalysts known. Synthesis of enantiopure, C1 symmetric pyridine(diimine) cobalt complexes have enabled rare examples of highly enantioselective hydrogenation of a family of substituted styrene derivatives. Because improved hydrogenation performance was observed with more electron rich supporting ligands, phosphine cobalt(II) dialkyl complexes were synthesized and found to be active for the diastereoselective hydrogenation of various substituted alkenes. Notably, this class of catalysts was activated by hydroxyl functionality, representing a significant advance in the functional group tolerance of base metal hydrogenation catalysts. Through collaboration with Merck, enantioselective variants of these catalysts were discovered by high throughput experimentation. Catalysts for the hydrogenation of functionalized and essentially unfunctionalized alkenes have been discovered using this approach. Development of reliable, readily accessible cobalt precursors facilitated catalyst discovery and may, along with lessons learned from electronic structure studies, provide fundamental design principles for catalysis with earth abundant transition metals beyond alkene hydrogenation.

Activation and deactivation of a robust immobilized Cp*Ir-transfer hydrogenation catalyst: a multielement in situ X-ray absorption spectroscopy study.

A highly robust immobilized [Cp*IrCl2]2 precatalyst on Wang resin for transfer hydrogenation, which can be recycled up to 30 times, was studied using a novel combination of X-ray absorption spectroscopy (XAS) at Ir L3-edge, Cl K-edge, and K K-edge. These culminate in in situ XAS experiments that link structural changes of the Ir complex with its catalytic activity and its deactivation. Mercury poisoning and "hot filtration" experiments ruled out leached Ir as the active catalyst. Spectroscopic evidence indicates the exchange of one chloride ligand with an alkoxide to generate the active precatalyst. The exchange of the second chloride ligand, however, leads to a potassium alkoxide-iridate species as the deactivated form of this immobilized catalyst. These findings could be widely applicable to the many homogeneous transfer hydrogenation catalysts with Cp*IrCl substructure.

Iron(II) Complexes of the Linear rac-Tetraphos-1 Ligand as Efficient Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and Formic Acid Dehydrogenation

The linear tetraphosphine 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos-1, P4) was used as its rac and meso isomers for the synthesis of both molecularly defined and in situ formed Fe(II) complexes. These were used as precatalysts for sodium bicarbonate hydrogenation to formate and formic acid dehydrogenation to hydrogen and carbon dioxide with moderate to good activities in comparison to those for literature systems based on Fe. Mechanistic details of the reaction pathways were obtained by NMR and HPNMR experiments, highlighting the role of the Fe(II) monohydrido complex [FeH(rac-P4)]+ as a key intermediate. X-ray crystal structures of different complexes bearing rac-P4 were also obtained and are described herein.

Homogeneous Hydrogenation of Fatty Acid Methyl Esters and Natural Oils under Neat Conditions

A series of ruthenium- and iron-based pincer catalysts have been tested for the homogeneous hydrogenation of fatty acid methyl esters to fatty alcohols with turnover numbers (TONs) of up to 1860. These catalysts operate under neat conditions (no solvent) from the gram to the kilogram scale. The required temperature (135 °C) and H2 pressure (300–1000 psig) are much lower than those needed for current industrial processes, which rely on heterogeneous copper chromite catalysts. The fatty alcohol products can be purified through distillation, resulting in low levels of Ru (1 ppm) and P (<25 ppm). The five-coordinate Milstein catalyst and Takasago’s borane-stabilized catalyst are also effective for the direct hydrogenation of coconut oil to fatty alcohols. These results hold the promise of using homogeneous catalysts for ester hydrogenation in an industrial setting.

Heterolytic Cleavage of H2 Revealed by Neutron Single Crystal Diffraction

Synthetic biologically inspired complexes exhibiting reactivity similar to hydrogenase enzymes have provided evidence of hydride transfer to the metal and proton transfer to an amine, but key structural information about the intermediate is not readily discernible with X-rays. The greater sensitivity of neutron to hydrogen makes it ideal for studying the structure and dynamics of catalytic materials. The newly commissioned TOPAZ neutron single crystal diffractometer at the SNS is capable of continuous 3D diffraction space mapping from a small stationary crystal, permitting detailed structural study at atomic resolution. The structure measured on TOPAZ for an Fe-based mononuclear electrocatalyst confirms that reaction of [CpFeN-L)](BARF) (1) with H2 under mild conditions leads to heterolytic cleavage of the H-H bond into a proton and hydride[1]. The precise location of H atoms in [Fe-H···H-N]+ reveals an unconventional H-bonding interaction, where the ferrous hydridic site {Fe(II)-H-} acts as the H-bond acceptor and the nitrogen of the protic pendant amine {L-N-H+} as the H-bond donor. The neutron structure provides clear evidence of a crucial intermediate involving an Fe-H···H-N interaction in the oxidation of H2. The result clarifies the key role of the pendant amine in the iron complex and provides insights into the design of synthetic electrocatalysts sought as cost-effective alternatives to platinum in fuel cells. The reaction is also a critical step in homogeneous catalysts for hydrogenation of C=O and C=N bonds. A preliminary result from TOPAZ measurement shows that 1 undergoes further single-crystal to single-crystal chemical reaction with moisture in the air, leading to a Fe(H2O)+ complex. Abbreviations: Cp = pentafluoropyridylcyclopentadienide; N-L= 1, 5-di(tert-butyl)-3,7-di(benzyl)-1,5-diaza-3,7-diphospha-cyclooctane; BARF = [B[3,5-(CF3)2C6H3]4]–

Homogeneous Hydrogenation Art of Nitrile Butadiene Rubber: A Review

Hydrogenation of nitrile butadiene rubber (NBR) is an important research topic in the field of chemical modification of unsaturated polymers. This review provides an overview of the fundamental data, challenges, and recent advances in the research area of homogeneous hydrogenation of NBR. Besides NBR and its hydrogenation product HNBR, some other important elastomers such as styrene butadiene rubber (SBR), polybutadiene rubber (BR), polystyrene-b-polybutadiene-b-polystyrene (SBS block copolymer), and natural rubber (NR) were also involved for the related hydrogenation reactions and characterization. A brief comparison of hydrogenation catalysts including the heterogeneous and homogeneous catalysts was first conducted and the importance of rhodium (Rh) complex catalysts for homogeneous hydrogenation of elastomers was addressed. This article continued to discuss the reaction pathways for realization of the homogeneous hydrogenation of NBR. This was followed by an examination of the microstructures and characterization techniques of NBR and HNBR. After that the recovery/recycle of precious metal catalysts from the reaction substrate, which have been drawing much attention from industrial manufacturers, was reviewed and highlighted in this article. The applications of HNBR were then summarized. In the end, future research directions were suggested.

High-Activity Iron Catalysts for the Hydrogenation of Hindered, Unfunctionalized Alkenes.

The activity of aryl-substituted bis(imino)pyridine and bis(arylimidazol-2-ylidene)pyridine iron dinitrogen complexes has been evaluated in a series of catalytic olefin hydrogenation reactions. In general, more electron donating chelates with smaller 2,6-aryl substituents produce more active iron hydrogenation catalysts. Establishment of this structure-activity relationship has produced base metal catalysts that exhibit high turnover frequencies for the hydrogenation of unfunctionalized, tri- and tetrasubstituted alkenes, one of the most challenging substrate classes for homogenous hydrogenation catalysts.

Parahydrogen-Induced Polarization in Heterogeneous Hydrogenations over Silica-Immobilized Rh Complexes

The use of heterogeneous catalysts for parahydrogen-induced polarization (PHIP) of nuclear spins opens new horizons for production of hyperpolarized substances. Immobilization of homogeneous hydrogenation catalysts is a promising approach for designing the efficient heterogeneous catalytic systems capable of PHIP generation. Herein, we study the formation of PHIP in the gas-phase and in the liquid-phase hydrogenations of propyne and propylene catalyzed by silica-immobilized Rh complexes synthesized by the ligand-exchange anchoring of the Wilkinson’s complex RhCl(PPh3)3, the binuclear complex Rh2Cl2(C8H12)2 and the cationic complex [Rh(C8H12)2]+[BF4]− to the phosphine-modified silica gel. We consider the stability and the mechanistic aspects of the hydrogenation over the immobilized Wilkinson’s catalyst in terms of PHIP observation. Using a PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) effect, it is found, in particular, that liquid-phase propyne hydrogenation over the immobilized Wilkinsons’s catalyst at 70°C proceeds in a stable regime with a stereoselective cis addition of a hydrogen molecule, while in the gas phase at the same temperature the hydrogenation stereoselectivity is observed only for a short time after the reaction is started, and then the catalyst rapidly loses its activity. The reasons of the catalyst deactivation are discussed based on the literature data, the results of infrared spectroscopy study, and the comparison to the behavior of the immobilized binuclear and cationic Rh complexes. In addition, it is shown that the immobilized Wilkinson’s catalyst is reduced as temperature increases in the range of 90–130°C, as confirmed by X-ray photoelectron spectroscopy.

Remove Rhodium Catalysts from HNBR Solution

Rhodium complex is excellent catalyst for nitrile-butadiene rubber homogeneous hydrogenation with difficulties in its recovery. A new extraction method for recovery noble metal catalysts from hydrogenated nitrile-butadiene rubber solution was investigated. Rhodium metal catalysts can be efficiently, easily removed from hydrogenated nitrile-butadiene rubber solution using amine as ligand and hydrogen peroxide as oxidant. The condition of removing noble metal catalysts from hydrogenated nitrile-butadiene rubber solution was carefully studied, including oxidant, reaction temperature, and the concentration of amine. The removal rate of the rhodium catalyst over 96% with the optimal conditions.1H-NMR characterization showed that there was no change in the structure and nitrile group of hydrogenated nitrile-butadiene rubber after rhodium catalysts were removed.

Bioenergy II: Group 8 Metal Complexes as Homogeneous Ionic Hydrogenation and Hydrogenolysis Catalysts for the Deoxygenation of Biomass to Petrochemicals - Opportunities, Challenges, Strategies and the Story so Far

Biomass is characterized by a high oxygen content (typically ? 40 % w/w). Its conversion into feedstocks that can directly be used in existing petrochemical feed streams will therefore require an efficient and selective deoxygenation chemistry. Ruthenium or iron-based water/acid-tolerant and high-temperature stable metal complexes that are active ionic hydrogenation and hydrogenolysis catalysts maybe ideally suited for this purpose.

Chelating NHC Ruthenium(II) Complexes as Robust Homogeneous Hydrogenation Catalysts

A series of ruthenium(II) complexes have been prepared by using bidentate chelating N-heterocyclic carbene (NHC) ligands that feature different donor groups E (E = olefin, thioether, carboxylate, and NHC). Rigid coordination of all donor sites was concluded from NMR spectroscopy, and the electronic impact of the donor group was evaluated by electrochemical analyses. The chelating donor group had a strong influence on the activity of the metal center in catalyzing direct hydrogenation of styrene. A thioether group or a second NHC donor site essentially deactivates the metal center. Complexes comprising a NHC tethered with an olefin or a carboxylate group showed appreciable activity, though only the carboxylate-functionalized system proved to be a precursor for homogeneous hydrogenation. According to in situ high-pressure NMR analyses, complexes featuring a carboxylate chelating group are remarkably resistant toward reductive elimination even under strongly reducing conditions and may, therefore, be used repeatedly.

Catalytic hydrogenolysis of ethanol organosolv lignin

Abstract The production of ethanol based on lignocellulosic materials will bring about the coproduction of significant amounts of under-utilized lignin. This study examines the potential of conventional heterogeneous and novel homogeneous catalysts for the selective cleavage of the aryl-O-aryl and aryl-O-aliphatic linkages of ethanol organosolv lignin to convert it from a low grade fuel to potential fuel precursors or other value added chemicals. The development of hydrogenolysis conditions that effectively increase the solubility of lignin were initially examined with Ru(Cl)2(PPh3)3 and demonstrated the ability to decrease the molecular weight and enhance the solubility of the lignin polymer. Later studies examined several heterogeneous and homogeneous hydrogenation catalysts at optimized reaction conditions resulting in 96.4% solubility with Ru(Cl)2(PPh3)3, increase in H/C ratio with Raney-Ni, Pt/C and extensive monomer formation with NaBH4/I2. The changes in molecular structure of lignin were followed by size exclusion chromatography, qualitative and quantitative NMR spectroscopy and elemental analysis. These studies demonstrated that aryl-O-aryl and aryl-O-aliphatic linkages could be cleaved and the hydrogenated lignin had a decrease in oxygen functionality and the formation of products with lower oxygen content.

Asymmetric hydrogenation, transfer hydrogenation and hydrosilylation of ketones catalyzed by iron complexes

The conventional homogeneous catalysts for the enantioselective hydrogenation or transfer hydrogenation of ketones are based on platinum metals and, in particular, ruthenium. This method provides valuable enantiopure alcohols for the fine chemical industries. This tutorial review summarizes recent successes in replacing expensive and toxic ruthenium in these catalysts with "greener" iron substitutes including my lab's recent progress in this area using iron complexes containing readily-prepared tetradentate ligands. It will enlighten chemists interested in homogeneous catalysis and asymmetric synthesis.