Homogeneous Metal Catalysts Research Articles

Electrochemical utilization of methanol and methanol-d4 as a C1 source to access (deuterated) 2,3-dihydroquinazolin-4(1H)-one

Herein, an electrocatalytic protocol for the synthesis of 2,3-dihydroquinazolin-4(1H)-one has been disclosed. Methanol is activated and utilized as the C1 source to cyclize with 2-aminobenzamides. This cyclization reaction proceeds conveniently (room temperature and air atmosphere) without any homogeneous metal catalysts, external oxidants, or bases. A wide variety of N,N-disubstituted 2,3-dihydroquinazolin-4(1H)-ones are obtained via this approach. Moreover, when methanol-d4 is used, a deuterated methylene motif is incorporated into the N-heterocycles, providing an efficient approach to the deuterated N-heterocycles.

Formic Acid as a Potential On-Board Hydrogen Storage Method: Development of Homogeneous Noble Metal Catalysts for Dehydrogenation Reactions.

Hydrogen can be used as an energy carrier for renewable energy to overcome the deficiency of its intrinsically intermittent supply. One of the most promising application of hydrogen energy is on-board hydrogen fuel cells. However, the lack of a safe, efficient, convenient, and low-cost storage and transportation method for hydrogen limits their application. The feasibility of mainstream hydrogen storage techniques for application in vehicles is briefly discussed in this Review. Formic acid (FA), which can reversibly be converted into hydrogen and carbon dioxide through catalysis, has significant potential for practical application. Historic developments and recent examples of homogeneous noble metal catalysts for FA dehydrogenation are covered, and the catalysts are classified based on their ligand types. The Review primarily focuses on the structure-function relationship between the ligands and their reactivity and aims to provide suggestions for designing new and efficient catalysts for H2 generation from FA.

A universal high-efficient and reusable “on–off” switch for the on-demand hydrogen evolution

Although a few “on–off” approaches such as the pH-adjusting, temperature-adjusting, and controlling by light upon photo-catalysts have been reported, the “on–off” control of the on-broad and on-demand hydrogen production still faces many challenges. Herein, we first report the usage of Zn2+/EDTA-2Na system as an unprecedented, universal, high-efficient, and reusable “on–off” switch for the on-demand H2 evolution upon the hydrolysis of ammonia borane, sodium borohydride and dimethyl aminoborane. This “on–off” switch has been demonstrated to be efficient and reusable in various heterogeneous and homogeneous metal catalysts for on-demand H2 evolution. The detailed characterizations have confirmed that Zn2+ ions attached and located at the surface of catalyst, covered its active sites on the surface, and turned off the H2 production. The H2 generation was switched on by the addition of EDTA-2Na, due to its strong coordination ability with Zn2+ ions. In addition, the Zn2+/EDTA-2Na switch has been successfully reused several times without any activity loss in the “on–off” control of H2 evolution. This switch is of great importance in the practical application for highly efficient, on-site and on-demand H2 evolution under ambient conditions.

Development of Group 4 Metal Complexes Bearing Fused-Ring Amido-Trihydroquinoline Ligands with Improved High-Temperature Catalytic Performance toward Olefin (Co)polymerization

The development of homogeneous metal catalysts with high activity and high thermal stability is vital for the synthesis of polyolefin elastomers (POEs) in solution-phase olefin polymerization processes. In this contribution, the stoichiometric reactions of 8-(2,6-(R1)2-4-R2-anilide)-5,6,7-trihydroquinoline (1–3; 1, R1 = iPr, R2 = H; 2, R1 = Me, R2 = H; 3, R1 = Me, R2 = Me) with MMe4 (M = Hf, Zr) afforded metal complexes 1-HfMe3, 2-HfMe3, 3-HfMe3, and 1-ZrMe3 in high yields. Treatment of ligand 1 with Ti(NMe2)4 resulted in the formation of 1-Ti(NMe2)3, which reacted with SiMe2Cl2 to form 1-TiCl3. 1-TiMe3 was obtained by alkylation of 1-TiCl3 with MeMgBr. All metal complexes were characterized by 1H and 13C NMR spectroscopy, and the molecular structures of complexes 1-HfMe3, 2-HfMe3, 1-ZrMe3, and 1-TiMe3 were determined by single-crystal X-ray diffraction, revealing an approximate trigonal-bipyramidal geometry around the metal center in all of the structures. The complexes showed extremely high activity toward ethylene polymerization (up to 13860 kg of PE (mol of M)−1 h–1) and ethylene/1-octene copolymerization (up to 49000 kg of PE (mol of M)−1 h–1) at elevated temperatures (up to 140 °C). The catalytic properties were highly dependent on the appropriate matching of the metal and cocatalyst. In the presence of [Ph3C][B(C6F5)4], the activity of metal complexes with the same ligand was in the order Hf > Zr > Ti; with B(C6F5)3 as the cocatalyst, this order followed Zr > Ti > Hf; using MAO as the cocatalyst, the Ti complex was highly active, while the Hf and Zr complexes were inactive. The Hf and Zr complexes showed both high-molecular-weight capability and high 1-octene incorporation ability. Therefore, high-molecular-weight polyethylene homopolymers and ethylene/1-octene elastomers were successfully prepared, and the 1-octene incorporations of copolymers could be readily tuned from 1.3 to 43.5 mol % depending on different catalysts and polymerization conditions.

Synthesis of quinazolin-4(3H)-ones via electrochemical decarboxylative cyclization of α‑keto acids with 2-aminobenzamides

Herein, an environmentally benign electrochemical protocol has been disclosed for the synthesis of quinazolin-4(3H)-one derivatives from readily available α‑keto acids and 2-aminobenzamides. This decarboxylative cyclization process proceeds conveniently in the absence of any homogeneous metal catalysts, bases, or external oxidants. This protocol also features CO2 by-products, mild reaction conditions (room temperature and air atmosphere), and a wide variety of substrate scope, including an array of 2,3-disubstituted quinazolinone products.

Homogeneous Metal Catalysts with Inorganic Ligands: Probing Ligand Effects in Lewis Acid Catalyzed Direct Amide Bond Formation

Inorganic clusters have large potential in the development of effective and robust catalysts due to their tunable electronic and structural properties and the ability to bind and stabilize catalytic metals. However, they have been rarely used as ligands in homogeneous metal catalysis, and the effect of the ligand structure on the catalyst’s reactivity has been scarcely investigated. By using well-defined and soluble inorganic clusters such as polyoxometalates (POMs) as representative inorganic ligands for a Hf(IV) Lewis acid metal, we illustrate how the interplay between the dielectric constant of the medium and the ligand structure can be used to convert a poorly active Hf-Keggin 2:2 complex ((Et2NH2)8[Hf(μ-O)(H2O)(PW11O39)]2) into an effective catalyst for a water-tolerant and atom-economic direct amide bond formation. By studying a model reaction between phenylacetic acid and benzylamine, direct catalytic amide formation was observed only in polar aprotic solvents, with yields inversely related to the dielectric constant of the solvents. More interestingly, while a clear improvement was observed for the Hf-Keggin catalyst upon changing the medium from dimethyl sulfoxide (ε = 46.7) to N-methyl-2-pyrrolidone (ε = 32.2), changing the dielectric constant had a minimal effect on the reactivity of the Hf-Wells–Dawson 2:2 complex ((Me2NH2)14[Hf(μ-O)(H2O)(α2-P2W17O61)]2), which gave quantitative yields in both solvents. Detailed mechanistic and spectroscopic analyses revealed that the dielectric constant of the medium plays a key role in providing the optimal balance between formation and stability of the monomeric catalytically active Hf-POM 1:1 species, thereby enabling efficient amide bond formation.

Recent Advances in Catalytic Hydrosilylations: Developments beyond Traditional Platinum Catalysts.

Hydrosilylation reactions, which allow the addition of Si−H to C=C/C≡C bonds, are typically catalyzed by homogeneous noble metal catalysts (Pt, Rh, Ir, and Ru). Although excellent activity and selectivity can be obtained, the price, purification, and metal residues of these precious catalysts are problems in the silicone industry. Thus, a strong interest in more sustainable catalysts and for more economic processes exists. In this respect, recently disclosed hydrosilylations using catalysts based on earth‐abundant transition metals, for example, Fe, Co, Ni, and Mn, and heterogeneous catalysts (supported nanoparticles and single‐atom sites) are noteworthy. This minireview describes the recent advances in this field.

Recent Advances in Catalytic Hydrosilylations: Developments beyond Traditional Platinum Catalysts

AbstractHydrosilylation reactions, which allow the addition of Si−H to C=C/C≡C bonds, are typically catalyzed by homogeneous noble metal catalysts (Pt, Rh, Ir, and Ru). Although excellent activity and selectivity can be obtained, the price, purification, and metal residues of these precious catalysts are problems in the silicone industry. Thus, a strong interest in more sustainable catalysts and for more economic processes exists. In this respect, recently disclosed hydrosilylations using catalysts based on earth‐abundant transition metals, for example, Fe, Co, Ni, and Mn, and heterogeneous catalysts (supported nanoparticles and single‐atom sites) are noteworthy. This minireview describes the recent advances in this field.

Surface Coordination Chemistry of Atomically Dispersed Metal Catalysts.

Atomically dispersed metal catalysts (ADCs), as an emerging class of heterogeneous catalysts, have been widely investigated during the past two decades. The atomic dispersion nature of the catalytic metal centers makes them an ideal system for bridging homogeneous and heterogeneous metal catalysts. The recent rapid development of new synthetic strategies has led to the explosive growth of ADCs with a wide spectrum of metal atoms dispersed on supports of different chemical compositions and natures. The availability of diverse ADCs creates a powerful materials platform for investigating mechanisms of complicated heterogeneous catalysis at the atomic levels. Considering most dispersed metal atoms on ADCs are coordinated by the donors from supports, this review will demonstrate how the surface coordination chemistry plays an important role in determining the catalytic performance of ADCs. This review will start from the link between coordination chemistry and heterogeneous catalysis. After the brief description on the advantages and limitations of common structure characterization methods in determining the coordination structure of ADCs, the surface coordination chemistry of ADCs on different types of supports will be discussed. We will mainly illustrate how the local and vicinal coordination species on different support systems act together with the dispersed catalytic metal center to determine the catalytic activity, selectivity, and stability of ADCs. The dynamic coordination structure change of ADCs in catalysis will be highlighted. At the end of the review, personal perspectives on the further development of the field of ADCs will be provided.

On the relative influence of the hydrodynamics of lab-scale set-ups and the membrane materials on the rejection of homogeneous metal catalysts in solvent resistant nanofiltration

ABSTRACT Solvent resistant nanofiltration is a promising process for homogeneous metal catalyst recovery. However, catalyst rejections are determined using various set-ups with different hydrodynamics, which questions the comparison of the results. The analysis of the membrane performance is another issue even when comparing membranes made of the same material. This study compares the hydrodynamics of three experimental set-ups. Rejections from 57.8 to 80.8% between cross-flow and dead-end SRNF are observed with a Rh catalyst (MW = 258 g·mol−1) in toluene. A high throughput screening system is used to analyze the performance of a set of membranes in terms of permeate fluxes and catalyst rejections.

POMs nanofibers for the oxidation of 5-HMF with O<sub>2</sub>

<p indent=0mm>2,5-Diformylfuran (DFF) is a versatile compound that can be used as a precursor in the synthesis of various value-added products, which can be obtained from aerobic oxidation of 5-hydroxymethylfurfural (HMF). However, aerobic oxidation of 5-HMF gives rise to several pathways and products containing DFF, 2,5-furandicarboxylic acid (FDCA), or maleic anhydride (MA). To date, both homogeneous and heterogeneous metal catalysts have been explored for the oxidation of HMF to DFF with various oxidants. It was found that materials containing vanadium showed higher efficiency with conversions ranging from 84% to 100% and DFF yields of 82%−99.9% at 120−140°C for <sc>3−11 h</sc> in dimethyl sulfoxide (DMSO). Selective oxidation of 5-HMF depended on the nature of the catalysts and supports. Polyoxometalates (POMs) are widely applied in catalysis especially in catalytic transformation of biomass. Especially, POMs containing vanadium had been found to be potential for oxidation of HMF to produce a series of products including DFF and MA. The diversity in structure and components allows POMs to be most active in acidic and redox catalysis. Compared to their homogeneous catalysis, heterogeneous systems showed more favorable in easy separation and reuse, introduction of special active sites belonging to supports, enhancement in surface area. Under such circumstances, heterogeneous POM catalysts are developed using microporous, mesoporous and macroporous SiO₂, TiO₂ and ZrO₂ as supports. Compared to powdered mesoporous materials, electrospun nanofibers are good candidates because of their high surface-to-volume ratios. Herein, trifunctional H₅PMo₁₀V₂O₄₀/meso-ZrO₂(f) had been synthesized using combined electrospinning and surfactant pore-forming technology, which were characterized by IR, ³¹P MAS NMR, XRD, SEM, TEM, and N₂ adsorption-desorption measurement. Combination of IR, ³¹P MAS NMR, XRD, BET surface areas, SEM and TEM could confirm that POM molecules were loaded on ZrO₂ nanofibers <sc>(200−400 nm)</sc> with mesoporous structure. It was found that decorating ZrO₂ nanofibers by H₅PMo₁₀V₂O₄₀ generated enhanced catalytic activity by emerging their unique individual properties of redox ability, Brønsted acidity, Lewis acidity, and nanofiber structure with higher surface area. H₅PMo₁₀V₂O₄₀/meso-ZrO₂(f) materials were evaluated in aerobic oxidation of 5-HMF to DFF. The conversion of HMF followed the order: HPMoV/meso-ZrO₂(5-f) <sc>(0.17 mmol g⁻¹,</sc> 60.1%) < HPMoV/meso-ZrO₂(14-f) <sc>(0.22 mmol g⁻¹,</sc> 79.8%) < meso-ZrO₂(f)<sc>(0.36 mmol g⁻¹,</sc> 92.3%) < HPMoV/meso-ZrO₂(23-f)<sc>(0.41 mmol g⁻¹,</sc> 94.2%) < HPMoV/meso-ZrO₂(28-f) <sc>(0.47 mmol g⁻¹,</sc> 95.6%) < HPMoV/meso-ZrO₂(35-f) <sc>(0.51 mmol g⁻¹,</sc> 96.4%) < HPMoV <sc>(2.66 mmol g⁻¹,</sc> 99.9%), which followed the increase of HPMoV loading hence the Brønsted acidity. The DFF yields depended on their Lewis acidity as HPMoV/meso-ZrO₂(35-f) (B/L=0.04:1, yield=74.6%) < HPMoV/meso-ZrO₂(28-f) (B/L=16.8:100, yield=80.2%) < HPMoV/meso-ZrO₂(23-f) (B/L=0.11:1, yield=89.9%) < HPMoV/meso-ZrO₂(14-f) (B/L=0.71:1, yield =74.5%) < HPMoV/meso-ZrO₂(5-f) (Lewis acidity/Brønsted basicity = 1.1:1, yield=56.8%). Therefore, balancing the L/B ratio could control the DFF generation. As results, H₅PMo₁₀V₂O₄₀/meso-ZrO₂(23) nanofiber (23, represented the POM amount) was found to be most active in aerobic oxidation of 5-HMF to give 89.9% yield of DFF at 96.4% conversion in DMSO at 120°C for <sc>7 h.</sc> The mechanism study showed that the generated O₂ was the active site in this aerobic oxidation being catalyzed by H₅PMo₁₀V₂O₄₀. Moreover, H₅PMo₁₀V₂O₄₀/meso-ZrO₂(f) showed good stability and duration for being reused at least ten times without leaching of POMs from ZrO₂ nanofibers.

Cooperative Use of Brønsted Acids and Metal Catalysts in Tandem Isomerization Reactions of Olefins

AbstractDual catalysis constitutes a growing field of the organic synthesis. It is a useful methodology, which allows for a discovery of new and more efficient transformations. This concept article describes tandem reactions, which comprise of an isomerization of the double bond and following transformation, whereby a combination of a homogeneous metal catalyst and Brønsted acid is used. The aim is to highlight the versatility of this catalyst combination.

Heterogeneously Catalyzed Synthesis of Imidazolones via Cycloisomerizations of Propargylic Ureas Using Ag and Au/Al SBA-15 Systems

The synthesis of imidazolones through the cycloisomerization of ureas, specifically propargylureas, has gained attention due to the large availability of starting materials. However, this type of synthesis normally requires the utilization of strong bases, such as NaOH, expensive homogeneous metal catalysts, such as Ag-, Au-, and Ru-based systems, or toxic and hazardous chemicals. Herein, a study of different synthetic routes for the preparation of imidazolones through the cycloisomerization of propargylic ureas under fast, mild, and environmentally friendly conditions with heterogeneous catalysis was undertaken. First, the synthesis were carried out under mild conditions using toluene and acetonitrile as solvents. Silver and gold nanoparticles supported on AlSBA-15 were used as heterogeneous catalysts. The catalysts were prepared by mechanochemical and microwave-assisted techniques. Sequentially, a range of solvents was replaced by the greener ethanol. Finally, all obtained results were combined in order to carry out the reaction using only water as solvent and promoter of the reaction. Aiming to expedite the procedure, the synthesis were carried out under conventional and microwave irradiation.

Ionic Liquid Immobilized on Graphene‐Oxide‐Containing Palladium Metal Ions as an Efficient Catalyst for the Alkoxy, Amino, and Phenoxy Carbonylation Reactions

AbstractThe Immobilization of homogeneous metal catalysts onto the solid surface is an important challenge in the field of catalysis because these types of catalysts provide the homogeneous nature as well as reusability of catalysts. In this report, novel Palladium ion containing ionic liquid immobilized onto graphene oxide (Pd@GOIL) and characterized using various analytical techniques like FT‐IR, XPS, TGA, SEM, TEM, BET and EXAFS analysis. This synthesized material acts as a highly efficient catalyst for the alkoxycarbonylation, aminocarbonylation and phenoxycarbonylation reactions from easily available aryl iodides. Effect of various reaction parameters like CO pressure, base, solvent, the effect of time and temperature using Pd@GOIL catalyst were examined. This catalyst also tolerates the wide variety of substrates with the effective electronic factor. Specifically, this catalyst shows excellent catalytic performance towards the esterification and amidation without the need of any ligands, co‐catalysts and additives. Additionally, the catalyst shows superior recyclability up to eight recycle runs without discernible activity and selectivity deterioration.

Ligand libraries for high throughput screening of homogeneous catalysts.

To accelerate the discovery of new or improved homogeneous catalysts, research groups in industry and academia have embraced high throughput experimentation (HTE). Such methodologies consist of preparing and testing large numbers of catalysts in parallel. Homogeneous metal catalysts are very well-suited for HTE, since in many cases, they can be prepared by simply mixing a metal precursor and a ligand. However, an HTE program requires a large set of chemically diverse ligands, i.e. a ligand library. In this review, we describe five different approaches for assembling ligand libraries based on an extensive survey of the literature. These approaches are based on commercial ligands, modular ligands, mixtures of ligands, supramolecular ligands or ligands prepared via the tools of biochemistry.

Low‐Temperature Hydrogenation of Carbon Dioxide to Methanol with a Homogeneous Cobalt Catalyst

Herein we describe the first homogeneous non-noble metal catalyst for the hydrogenation of CO2 to methanol. The catalyst is formed in situ from [Co(acac)3 ], Triphos, and HNTf2 and enables the reaction to be performed at 100 °C without a decrease in activity. Kinetic studies suggest an inner-sphere mechanism, and in situ NMR and MS experiments reveal the formation of the active catalyst through slow removal of the acetylacetonate ligands.

Recent XAS studies into Homogeneous metal catalyst in fine chemical and pharmaceutical syntheses.

A brief review of studies using X-ray Absorption Spectroscopy (XAS) to investigate homogeneous catalytic reactions in fine chemical and pharmaceutical context since 2010 is presented. The advantages of the techniques over traditional lab-based analytical tools, particularly when NMR spectroscopy fails to deliver mechanistic insights, are summarised using these examples. A discussion on the current limitations of the techniques and challenges in the near future is also included.Graphical abstractA minireview of recent developments in application of X-ray Absorption Spectroscopy as an effective mechanistic tool to synthetic catalytic reactions relevant to fine chemical and pharmaceutical syntheses.

Application of a Taylor-Couette Reactor in Homogeneous Catalysis

New developments of synthesis routes in homogeneous catalysis often require a precise control of the reaction conditions in order to achieve optimal results. A precise control of the flow- and reactor behaviour can improve the performance of a catalytic system, e.g. by suppressing side- and consecutive reactions. The Taylor-Couette reactor (TCR) allows a precise control of the flow conditions combining the benefits of continuously stirred tank reactors (CSTR) and plug-flow reactors (PFR). A ribbed Taylor Couette reactor for the application in homogeneous catalysis was developed and tested in the continuous hydroamination of the renewable β-myrcene in miniplant-scale. The application of the atom-efficient hydroamination to terpenes allows the continuous production of terpenyl amines based on renewable resources. Catalyst recycling of the homogeneous transition metal catalyst was achieved by means of Thermomorphic Multicomponent Solvent (TMS)-systems. TMS-systems enable the realisation of the reaction under homogeneous conditions at elevated temperatures, but also allow a biphasic phase separation after the reaction. This way the homogeneous catalyst can be separated from the product phase and used again. Yields of the desired terpenyl amines of up to 87 % could be generated in a homogeneous toluene solution on a laboratory scale. The application of the TMS-system acetonitrile/n-heptane yielded 86 %, the TMS-system N,N-dimethylformamide/n-heptane generated 45 % of the desired hydroamination products. Both systems were then transferred into a continuously operated miniplant and realised using a Taylor-Couette reactor. The reactor performance was then validated and compared to the laboratory batch experiments. In the continuous miniplant experiments, the Taylor-Couette reactor could reproduce and even surpass the results of the batch experiments.

Facile construction of symmetric biaryls using (BeDABCO)2Pd2Cl6 as an efficient and highly active catalyst under microwave irradiation

An efficient catalytic system using (BeDABCO)2Pd2Cl6 (BeDABCO, benzyl‐1,4‐diazabicyclo[2.2.2]octane) was developed for the homo‐coupling reaction of various aryl halides. Due to the combination of ionic homogeneous metal catalyst and microwave irradiation, symmetric biaryls were produced in excellent yields and short reaction times in N‐methyl‐2‐pyrrolidone at 120 °C. BeDABCO as an efficient ligand and also a quaternary ammonium salt had an efficient stabilizing effect on the Pd(0) species in this coupling reaction. Copyright © 2015 John Wiley &amp; Sons, Ltd.

ChemInform Abstract: Oxidative Functionalization of Alkanes under Dioxygen in the Presence of Homogeneous Noble Metal Catalysts

AbstractReview: 132 refs.