Homogeneous Hydrogenation Catalysts Research Articles

Tailoring rhodium-based metal-organic layers for parahydrogen-induced polarization: achieving 20% polarization of 1H in liquid phase

Abstract Heterogeneous catalysts for parahydrogen-induced polarization (HET-PHIP) would be useful for producing high sensitivity contrasting agents for magnetic resonance imaging (MRI) in liquid phase, as they can be removed by simple filtration. Although homogeneous hydrogenation catalysts are highly efficient for PHIP, their sensitivity decreases when anchored on porous supports due to slow substrate diffusion to the active sites and rapid depolarization within the channels. To address this challenge, we explored two-dimensional (2D) metal-organic layers (MOLs) as supports for active Rh complexes with diverse phosphine ligands and tunable hydrogenation activities, taking advantage of accessible active sites and chemical adaptability of the MOLs. By adjusting the electronic properties of phosphines, TPP-MOL-Rh-dppb (TPP = tris(4-carboxylphenyl)phosphine), featuring a κ2-connected di(phosphine) ligand, generated hyperpolarized styrene achieving over 2400-fold signal enhancement and polarization level of 20% for 1H in methanol-d4 solution. The TPP-MOL-Rh-dppb effectively inherited the high efficiency and pairwise addition of its homogenous catalyst while maintaining the heterogeneity of MOL. This work demonstrates the potential of 2D phosphine-functionalized MOLs as heterogenous solid support for HET-PHIP.

Transition‐Metal‐Catalysed Transfer Hydrogenation Reactions with Glycerol and Carbohydrates as Hydrogen Donors

AbstractCatalytic transfer hydrogenation is a practical approach for reduction of various unsaturated compounds and an alternative to pressurised hydrogenation and traditional metal hydride reagents. Among the various hydrogen donors, biomass such as glycerol and carbohydrates received considerable attention for the reduction reactions due to their non‐toxic, eco‐friendly and renewable nature. The present review addresses the development of homogeneous and heterogeneous catalysts for the transfer hydrogenation of carbonyl compounds, alkenes, alkynes, nitro compounds, CO2 etc. using glycerol and carbohydrates as potential hydrogen donors.

Cyclopentadienone Diphosphine Ruthenium Complex: A Designed Catalyst for the Hydrogenation of Carbon Dioxide to Methanol.

The development of homogeneous metal catalysts for the efficient hydrogenation of carbon dioxide (CO2) into methanol (CH3OH) remains a significant challenge. In this study, a new cyclopentadienone diphosphine ligand (CPDDP ligand) was designed, which could coordinate with ruthenium to form a Ru-CPDDP complex to efficiently catalyze the CO2-to-methanol process using dihydrogen (H2) as the hydrogen resource based on density functional theory (DFT) mechanistic investigation. This process consists of three catalytic cycles, stage I (the hydrogenation of CO2 to HCOOH), stage II (the hydrogenation of HCOOH to HCHO), and stage III (the hydrogenation of HCHO to CH3OH). The calculated free energy barriers for the hydrogen transfer (HT) steps of stage I, stage II, and stage III are 7.5, 14.5, and 3.5 kcal/mol, respectively. The most favorable pathway of the dihydrogen activation (DA) steps of three stages to regenerate catalytic species is proposed to be the formate-assisted DA step with a free energy barrier of 10.4 kcal/mol. The calculated results indicate that the designed Ru-CPDDP and Ru-CPDDPEt complexes could catalyze hydrogenation of CO2 to CH3OH (HCM) under mild conditions and that the transition-metal owning designed CPDDP ligand framework be one kind of promising potential efficient catalysts for HCM.

Catalytic hydrogenation of olefins by a multifunctional molybdenum-sulfur complex

Exploration of molybdenum complexes as homogeneous hydrogenation catalysts has garnered significant attention, but hydrogenation of unactivated olefins under mild conditions are scarce. Here, we report the synthesis of a molybdenum complex, [Cp*Mo(Ph2PC6H4S−CH = CH2)(Py)]+ (2), which exhibits intriguing reactivity toward C2H2 and H2 under ambient pressure. This vinylthioether complex showcases efficient catalytic activity in the hydrogenation of various aromatic and aliphatic alkenes, demonstrating a broad substrate scope without the need for any additives. The catalytic pathway involves an uncommon oxidative addition of H2 to the cationic Mo(II) center, resulting in a Mo(IV) dihydride intermediate. Moreover, complex 2 also shows catalytic activity toward C2H2, leading to the production of polyacetylene and the extension of the vinylthioether ligand into a pendant triene chain.

Sc(OTf)3: An efficient homogeneous catalyst for microwave-assisted transfer hydrogenation of ethyl levulinate to γ-valerolactone under mild conditions

γ-Valerolactone (GVL) is considered as a versatile platform chemical obtained from biomass refinery. In this study, eco-friendly group III metal (Sc, Y, La) triflates were examined as homogeneous catalysts for microwave-assisted transfer hydrogenation of ethyl levulinate (EL) to GVL under mild conditions. Experimental results revealed that the selectivity of GVL has a positive relationship with the Lewis acidity of metal ion, which is related to effective charge density of the metal center (3.23, 2.81, 2.60 for Sc3+, Y3+, La3+, respectively). The stronger Lewis acidity of metal ion, the higher catalytic activity of MPV reduction. Among these catalysts, Sc(OTf)3 exhibited the best catalytic activity, achieving 98.4% EL conversion and 92.4% GVL yield at 140 °C in 3.0 h. Furthermore, Sc(OTf)3 showed high stability and recyclability for at least ten reaction cycles. In addition, a plausible reaction pathway and mechanism were proposed.

Sc(Otf)3: An Efficient Homogeneous Catalyst for Microwave-Assisted Transfer Hydrogenation of Ethyl Levulinate to Γ-Valerolactone Under Mild Conditions

Sc(Otf)3: An Efficient Homogeneous Catalyst for Microwave-Assisted Transfer Hydrogenation of Ethyl Levulinate to Γ-Valerolactone Under Mild Conditions

Utilizing Design of Experiments Approach to Assess Kinetic Parameters for a Mn Homogeneous Hydrogenation Catalyst.

Homogeneous hydrogenation catalysts based on metal complexes provide a diverse and highly tunable tool for the fine chemical industry. To fully unleash their potential, fast and effective methods for the evaluation of catalytic properties are needed. In turn, this requires changes in the experimental approaches to test and evaluate the performance of the catalytic processes. Design of experiment combined with statistical analysis can enable time‐ and resource‐efficient experimentation. In this work, we employ a set of different statistical models to obtain the detailed kinetic description of a highly active homogeneous Mn (I) ketone hydrogenation catalyst as a representative model system. The reaction kinetics were analyzed using a full second order polynomial regression model, two models with eliminated parameters and finally a model which implements “chemical logic”. The coefficients obtained are compared with the corresponding high‐quality kinetic parameters acquired using conventional kinetic experiments. We demonstrate that various kinetic effects can be well captured using different statistical models, providing important insights into the reaction kinetics and mechanism of a complex catalytic reaction.

Both sites must turn over in tandem catalysis: Lessons from one-pot CO2 capture and hydrogenation

By combining capture and catalytic conversion of carbon dioxide to chemicals and fuels in a single process operation, one can potentially increase the efficiency and feasibility of waste carbon utilization. We report here results for CO2 capture using amines supported on high-surface-area silicas, including fumed silica and mesoporous SBA-15, in tandem with a homogeneous ruthenium hydrogenation catalyst in a batch reactor system, and compare these with previous reports for two- and three-phase versions of capture + hydrogenation systems. While diamine- and polyamine-functionalized solid capture agents led to higher methanol productivities than homogeneous amine and homogeneous catalyst combinations, turnover of the amine capture sites is a significant limitation that has not been previously recognized. These results demonstrate that tandem catalysis in these systems has not yet been achieved, despite previous claims to the contrary.

Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions.

In the context of a carbon neutral economy, catalytic CO2 hydrogenation to methanol is one crucial technology for CO2 mitigation providing solutions for manufacturing future fuels, chemicals, and materials. However, most of the presently known catalyst systems are used at temperatures over 220 °C, which limits the theoretical yield of methanol production due to the exothermic nature of this transformation. In this review, we summarize state-of-the-art catalysts, focusing on the rationales behind, for CO2 hydrogenation to methanol at temperatures lower than 170 °C. Both hydrogenation with homogeneous and heterogeneous catalysts is covered. Typically, additives (alcohols, amines or aminoalcohols) are used to transform CO2 into intermediates, which can further be reduced into methanol. In the first part, molecular catalysts are discussed, organized into: (1) monofunctional, (2) M/NH bifunctional, and (3) aromatization-dearomatization bifunctional molecular catalysts. In the second part, heterogeneous catalysts are elaborated, organized into: (1) metal/metal or metal/support, (2) active-site/N or active-site/OH bifunctional heterogeneous catalysts, and (3) cooperation of catalysts and additives in a tandem process via crucial intermediates. Although many insights have been gained in this transformation, in particular for molecular catalysts, the mechanisms in the presence of heterogeneous catalysts remain descriptive and insights unclear.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes.

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Ruthenium triphos complexes [Ru(X(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2; X = H3C-C, N) as catalysts for the conversion of furfuryl acetate to 1,4-pentanediol and cyclopentanol in aqueous medium

The ruthenium complexes [Ru(H3CC(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2 (1, (H3CC(CH2PPh2)3 = triphos) and [Ru(N(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2 (2, N(CH2PPh2)3 = N-triphos) have been evaluated as homogeneous ionic hydrogenation catalysts for the catalytic hydrodeoxygenation of furfuryl alcohol and furfuryl acetate to 1,4-pentanediol and cyclopentanol in aqueous media reaction mixtures. For furfuryl alcohol, only marginal yields of 1,4-pentanediol could be achieved with mass balance deficiencies due to humin formation ranging from 67% to 90%. Attempts to improve the catalytic activity of 2 by enhancing its water solubility by nitrogen protonation and (or) methylation failed. Employing the less self-reactive furfuryl acetate as the substrate substantially diminishes humin formation, yielding up to 43% of 1,4-pentanediol and 19% of cyclopentanol (via Piancatelli rearrangement) with 1 and up to 33% of 1,4-pentanediol and 5% of cyclopentanol with 2. A design of experiments study was used to determine and compare the yield responses of the multiple parallel reaction channels with 1,4-pentanediol, cyclopentanol, and humins as a function of reaction temperature, time, catalyst load, and substrate concentration. This explores the correlations between these parameters and their impact on the reaction outcome and suggests an extremely complex overall reaction cascade of interdependent pathways of both acid- and metal-catalyzed steps with some significant differences emerging between the two catalysts.

Synthesis and characterization of novel tetradentate ruthenium complexes of a pyridine-o-phenylenediamine based chelate ligand

The synthesis of the new complexes [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(Cl)(DMSO)](Cl), [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(MeCN)2](OTf)2 and [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(PhCN)2](OTf)2 is reported. The ligand, N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine, was complexed to ruthenium using the ruthenium precursor, RuCl2(DMSO)4, to yield the intermediate complex [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(Cl)(DMSO)](Cl). The metathesis of DMSO and chloride ligands from [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(Cl)(DMSO)](Cl) was carried out in the appropriate nitrile solvent with 2 equivalents of AgOTf to yield the complexes [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(MeCN)2](OTf)2 and [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(PhCN)2](OTf)2. Both complexes were characterized by single crystal XRD. [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(MeCN)2](OTf)2 and [cis-Ru(N,N′-bis-(2-pyridylmethyl)-o-phenylenediamine)(PhCN)2](OTf)2 both possess the hypothetical characteristics of homogeneous hydrogenation catalysts that are anticipated to be acid- water- and temperature stable, and have the potential to be tested as such.

Selective Hydrogenation of Fatty Nitriles to Primary Fatty Amines: Catalyst Evaluation and Optimization Starting from Octanenitrile

AbstractIn this contribution, an evaluation of the potential of various homogeneous and heterogeneous catalysts for a selective hydrogenation of fatty nitriles toward primary amines is reported exemplified for the conversion of octanenitrile into octane‐1‐amine as a model reaction. When using heterogeneous catalysts such as the ruthenium catalyst Ru/C, the palladium catalyst Pd/C, and the platinum catalyst Pt/Al2O3, low selectivities in the hydrogenation are observed, thus leading to a large portion of secondary and tertiary amine side‐products. For example, when using Ru/C as a heterogeneous catalyst, high conversions of up to 99% are obtained but the selectivity remains low with a percentage of the primary amine being at 60% at the highest. The study further reveals a high potential of homogeneous ruthenium and manganese catalysts. When also taking into account economical considerations with respect to the metal price, in particular, manganese catalysts turn out to be attractive for the desired transformation and their application in the model reaction leads to the desired primary amine product with excellent conversion, selectivity, and high yield.Practical Applications: This work describes an optimized hydrogenation process for transforming fatty nitriles to their corresponding primary amines. In general, fatty amines belong to the most applied fatty acid‐derived compounds in the chemical industry with an annual product volume exceeding 800 000 tons per year in 2011 and are widely required in the chemical industry since such compounds are either directly used in home products such as fabric softeners, dishwashing liquids, car wash detergents, or carpet cleaners or in a broad range of industrial products, for example, lubricating additives, flotation agents, dispersants, emulsifiers, corrosion inhibitors, fungicides, and bactericides, showing additional major applications, e.g., in the detergents industry. Among them primary amines play an important industrial role. However, a major concern of current processes is the lack of selectivity and the formation of secondary and tertiary amines as side‐products. By modifying a recently developed catalytic system based on manganese as economically attractive and environmentally benign metal component an efficient and selective access to fatty amines when starting from the corresponding nitriles is achieved. For example, hydrogenation of octanenitrile leads to a synthesis of octane‐1‐amine with >99% conversion and excellent selectivity with formation of secondary and tertiary amine side‐products being suppressed to an amount of <1%.

Combined CO2 Capture and Hydrogenation to Methanol: Amine Immobilization Enables Easy Recycling of Active Elements.

Amines were immobilized onto solid supports and employed for tandem CO2 capture and conversion to CH3 OH using homogeneous hydrogenation catalysts. The hydrogenation proceeded through the formation of formamide intermediates. After hydrogenation, the immobilized amines were easily filtered and collected to be reused. The catalyst and methanol were recovered from the filtrate. Covalently-attached (to polymer support or silica) amine functionalities displayed the highest recycling potential with almost no leaching under the hydrogenation reaction conditions. Using polyethylenimine grafted onto a solid-silica support, the catalyst and amine were successfully recycled, and CO2 (either pure or from the air) was efficiently captured and converted to CH3 OH over multiple cycles.

A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide.

A highly efficient recyclable system for capture and subsequent conversion of CO2 to formate salts is reported that utilizes aqueous inorganic hydroxide solutions for CO2 capture along with homogeneous pincer catalysts for hydrogenation. The produced aqueous solutions of formate salts are directly utilized, without any purification, in a direct formate fuel cell to produce electricity and regenerate the hydroxide base, achieving an overall carbon-neutral cycle. The catalysts and organic solvent are recycled by employing a biphasic solvent system (2-MTHF/H2O) with no significant decrease in turnover frequency (TOF) over five cycles. Among different hydroxides, NaOH and KOH performed best in tandem CO2 capture and conversion due to their rapid rate of capture, high formate conversion yield, and high catalytic TOF to their corresponding formate salts. Among various catalysts, Ru- and Fe-based PNP complexes were the most active for hydrogenation. The extremely low vapor pressure, nontoxic nature, easy regenerability, and high reactivity of NaOH/KOH toward CO2 make them ideal for scrubbing CO2 even from low-concentration sources-such as ambient air-and converting it to value-added products.

Ruthenium(II)-Arene Complexes of the Water-Soluble Ligand CAP as Catalysts for Homogeneous Transfer Hydrogenations in Aqueous Phase

The neutral Ru(II) complex κP-[RuCl2(η6-p-cymene)(CAP)] (1), and the two ionic complexes κP-[RuCl(η6-p-cymene)(MeCN)(CAP)]PF6 (2) and κP-[RuCl(η6-p-cymene)(CAP)2]PF6 (3), containing the water-soluble phosphine 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP), were tested as catalysts for homogeneous hydrogenation of benzylidene acetone, selectively producing the saturated ketone as product. The catalytic tests were carried out in aqueous phase under transfer hydrogenation conditions, at mild temperatures using sodium formate as hydrogen source. Complex 3, which showed the highest stability under the reaction conditions applied, was also tested for C=N bond reduction from selected cyclic imines. Preliminary NMR studies run under pseudo-catalytic conditions starting from 3 showed the formation of κP-[RuH(η6-p-cymene)(CAP)2]PF6 (4) as the pivotal species in catalysis.

(NHC)CoR2]: pre-catalysts for homogeneous olefin and alkyne hydrogenation

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR2] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH2SiMe3)2] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η2-PhCCPh)2].

The Evolution of Chemical High-Throughput Experimentation To Address Challenging Problems in Pharmaceutical Synthesis.

The structural complexity of pharmaceuticals presents a significant challenge to modern catalysis. Many published methods that work well on simple substrates often fail when attempts are made to apply them to complex drug intermediates. The use of high-throughput experimentation (HTE) techniques offers a means to overcome this fundamental challenge by facilitating the rational exploration of large arrays of catalysts and reaction conditions in a time- and material-efficient manner. Initial forays into the use of HTE in our laboratories for solving chemistry problems centered around screening of chiral precious-metal catalysts for homogeneous asymmetric hydrogenation. The success of these early efforts in developing efficient catalytic steps for late-stage development programs motivated the desire to increase the scope of this approach to encompass other high-value catalytic chemistries. Doing so, however, required significant advances in reactor and workflow design and automation to enable the effective assembly and agitation of arrays of heterogeneous reaction mixtures and retention of volatile solvents under a wide range of temperatures. Associated innovations in high-throughput analytical chemistry techniques greatly increased the efficiency and reliability of these methods. These evolved HTE techniques have been utilized extensively to develop highly innovative catalysis solutions to the most challenging problems in large-scale pharmaceutical synthesis. Starting with Pd- and Cu-catalyzed cross-coupling chemistry, subsequent efforts expanded to other valuable modern synthetic transformations such as chiral phase-transfer catalysis, photoredox catalysis, and C-H functionalization. As our experience and confidence in HTE techniques matured, we envisioned their application beyond problems in process chemistry to address the needs of medicinal chemists. Here the problem of reaction generality is felt most acutely, and HTE approaches should prove broadly enabling. However, the quantities of both time and starting materials available for chemistry troubleshooting in this space generally are severely limited. Adapting to these needs led us to invest in smaller predefined arrays of transformation-specific screening "kits" and push the boundaries of miniaturization in chemistry screening, culminating in the development of "nanoscale" reaction screening carried out in 1536-well plates. Grappling with the problem of generality also inspired the exploration of cheminformatics-driven HTE approaches such as the Chemistry Informer Libraries. These next-generation HTE methods promise to empower chemists to run orders of magnitude more experiments and enable "big data" informatics approaches to reaction design and troubleshooting. With these advances, HTE is poised to revolutionize how chemists across both industry and academia discover new synthetic methods, develop them into tools of broad utility, and apply them to problems of practical significance.

Unsymmetrical Iron P-NH-P' Catalysts for the Asymmetric Pressure Hydrogenation of Aryl Ketones.

The reductive amination of α-dialkylphosphine acetaldehydes with enantiopure β-aminophosphines is a new, versatile route to unsymmetrical tridentate (pincer) ligands P-NH-P'. Four new ligands PR2 CH2 CH2 NHCHR'CHR''PPh2 (R=iPr, Cy, R'=Ph, CH(CH3 )2 , R''=Ph, H) prepared in this way are used to make the iron(II) complexes mer-FeCl2 (CO)(P-NH-P') and mer-FeCl(H)(CO)(P-NH-P'). The hydride complex with the rigid ligand with R'=R''=Ph is an efficient and highly enantioselective homogeneous asymmetric pressure hydrogenation (APH) catalyst. Prochiral aryl ketones are reduced under mild conditions (THF, 0.1 mol % catalyst, 1 mol % KOtBu, 5-10 bar, 50 °C) to the (S)-alcohols, usually in enantiomeric excess (ee) greater than 90 %. DFT calculations provided transition-state structures for the enantiodetermining hydride-transfer step.

Hydrogenation of CO2 in Water Using a Bis(diphosphine) Ni–H Complex

The water-soluble Ni bis(diphosphine) complex [NiL2](BF4)2 (L = 1,2-[bis(dimethoxypropyl)phosphino]ethane and the corresponding hydride, [HNiL2]BF4, were synthesized and characterized. These complexes were specifically designed for CO2 hydrogenation. For HNiL2+, the hydricity (ΔG°H–) was determined to be 23.2(3) kcal/mol in aqueous solution. On the basis of the hydricity of formate, 24.1 kcal/mol, the transfer of a hydride from HNiL2+ to CO2 to produce formate is favorable by 1 kcal/mol. Starting from either NiL22+ or HNiL2+ in water, catalytic hydrogenation of CO2 was observed with NaHCO3 (0.8 M) as the only additive. A maximum turnover frequency of [4.0(5)] × 10–1 h–1 was observed at 80 °C and 34 atm of a 1:1 mixture of CO2 and H2. This report demonstrates the use of a homogeneous first-row transition-metal catalyst for CO2 hydrogenation in water using NaHCO3 as an inexpensive, readily available base.