Low Catalyst Loading Research Articles

Ultrathin Microporous Transport Layers: Implications for Low Catalyst Loadings, Thin Membranes, and High Current Density Operation for Proton Exchange Membrane Electrolysis

AbstractPorous transport layers (PTL) and their surface properties have the potential to improve the performance of proton exchange membrane water electrolyzers (PEMWE), which is imperative to reduce feedstock costs and lead to their widespread implementation. This work introduces a novel generation of titanium microporous layers (MPLs) with ultra‐low thicknesses of ≈20 µm which reduces raw material costs. They also feature advanced interfacial properties tailored to maximize catalyst utilization at low Ir‐loadings. The bulk morphology and surface properties of the hierarchically structured PTLs are assessed by X‐ray tomographic microscopy. The low surface roughness of the MPL allows the use of thinner membranes since it minimizes possible deformations in the membrane. Cells containing the MPLs outperformed those containing state‐of‐the‐art commercially available PTL materials by up to 100 mV at 7 A cm−2 in combination with low‐loaded catalyst‐coated membranes of 0.4 mgIr cm−2. Hydrogen crossover is also reduced, especially at low current densities, leading to a larger turndown ratio which can enable more cost‐effective operating strategies. Finally, these rationally designed MPLs also lead to high catalyst utilization by overcoming the naturally occurring high in‐plane resistance of low‐loaded catalyst layers.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.

Anthraquinone-Modified Silica Nanoparticles as Heterogeneous Photocatalyst for the Oxidative Hydroxylation of Arylboronic Acids.

In this work, the synthesis and characterization of a heterogeneous photocatalyst based on spherical silica nanoparticles superficially modified with anthraquinone 2-carboxylic acid (AQ-COOH) are presented. The nanomaterial was characterized by TEM, SEM, FT-IR, diffuse reflectance, fluorescence, NMR, DLS, XRD and XPS. These analyses confirm the covalent linking of AQ-COOH with the NH2 functionality in the nanomaterial and, more importantly, the photocatalyst retains its photophysical properties once bound. The heterogeneous photocatalyst was successfully employed in the aerobic hydroxylation of arylboronic acids to phenols under sustainable reaction conditions. Phenols were obtained in high yields (up to 100 %) with low catalyst loading (3.5 mol %), reaching TOF values of 3.7 h-1 . Using 2-propanol as solvent at room temperature, the visible light photocatalysis produced H2 O2 as a key intermediate to promote the aerobic hydroxylation of arylboronic acids. The heterogeneous photocatalyst was reused at least 5 times, without modification of the nanomaterial structure and morphology. This simple heterogeneous system showed great catalytic activity under sustainable reaction conditions.

Amphiphilic Janus dendrimer-stabilized Au, Ag and Pd nanoparticles for the reduction of 4-nitrophenol and Suzuki-Miyaura cross-coupling reactions

Dendrimers with symmetrical 3D structures can be designed with various interior or exterior functional groups, which has been regarded as good stabilizers for various metal nanoparticles in catalysis. Nevertheless, Janus dendrimers with asymmetric structures are hardly researched to load metal nanoparticle catalysts. Here, an amphiphilic Janus dendrimer is used to stabilize metal (Au, Ag and Pd) nanoparticles, whose catalytic activities are also investigated. Au, Ag and Pd nanoparticles have the average diameters of 2.6 ± 2.0 nm, 5.8 ± 3.0 nm and 2.0 ± 1.0 nm, respectively, and show clear lattice fringes. Au and Pd nanoparticles are observed to have significantly higher catalytic performance than that of Ag nanoparticles in the 4-nitrophenol reduction, and Pd nanoparticles possess the highest catalytic activity (TOF: 2250 h−1). In addition, Pd nanoparticles are also highly active in the Suzuki-Miyaura reactions with low catalyst loading (50 ppm) and high TOF (2475 h−1) and show good applicability to tolerate various substituents.

4‐Aryl‐1H‐isochromenes Synthesis from Triol Derivatives through Brønsted Acid‐Catalyzed Pinacol Rearrangement/Intramolecular Cyclization Sequence

Abstract4‐Aryl‐1H‐isochromene derivatives have been successfully synthesized by Brønsted acid‐catalyzed pinacol rearrangement/intramolecular cyclization of the substituted 1‐(2‐(hydroxymethyl)phenyl)‐2‐arylethane‐1,2‐diols. The reaction generally proceeded with low catalyst loading of trifluoromethanesulfonic acid (5 mol%) for short reaction times (10‐20 min) to furnish the desired 4‐aryl‐1H‐isochromenes which were subsequently hydrogenated to 4‐aryl‐isochromans with up to 90 % yield. Moreover, oxidation of the resulting 4‐aryl‐isochroman provided the desired 4‐aryl‐isochroman‐1‐one in good yield.

CuAAC-Based Synthesis, Copper-Catalyzed Aldehyde-Forming Hydrolytic Fission and Antiproliferative Evaluation of Novel Ferrocenoylamino-Substituted Triazole-Tethered Quinine-Chalcone Hybrids.

A series of novel triazole-tethered ferrocenoylamino-substituted cinchona-chalcone hybrids along with two representative benzoylamino-substituted reference compounds were prepared by three methods of CuAAC chemistry. In line with the limited success or complete failure of attempted conversions with low catalyst loadings, by means of DFT modeling studies, we demonstrated that a substantial part of the Cu(I) ions can be chelated and thus trapped in the aroylamino-substituted cinchona fragment and all of the accessible coordinating sites of the chalcone residues. Accordingly, increased amounts of catalysts were used to achieve acceptable yields; however, the cycloadditions with para-azidochalcones were accompanied by partial or complete aldehyde-forming hydrolytic fission of the enone C=C bond in a substituent-, solvent- and copper load-dependent manner. The experienced hydrolytic stability of the hybrids obtained by cycloadditions with ortho-azidochalcones was interpreted in terms of relative energetics, DFT reactivity indices and MO analysis of simplified models of two isomer copper-enone complexes. The novel hybrids were evaluated on HeLa, MDA-MB-231 and A2780 cell lines and showed substantial activity at low-to-submicromolar concentrations. An organometallic model carrying 3,4,5-trimethoxyphenyl residue in the enone part with a para-disubstituted benzene ring in the central skeletal region was identified as the most potent antiproliferative lead, characterized by submicromolar IC50 values measured on the three investigated cells. The biological assays also disclosed that this ferrocenoylamino-containing lead compound displays a ca. two- to five-fold more substantial antiproliferative effect than its benzoylamino-substituted counterpart.

Boronic Acid/Palladium Hybrid Catalysis for Regioselective O-Allylation of Carbohydrates.

Novel imidazole-containing boronic acid and palladium hybrid catalysis for regioselective O-allylation of carbohydrates has been developed. This catalytic process enables the introduction of a useful allyl functional group into the equatorial hydroxy group of cis-1,2-diols of various carbohydrates with low catalyst loading and excellent regioselectivities. This is the first report on hybrid catalysis in combination with a Lewis base-containing boronic acid and a transition metal complex.

NHC‐Catalyzed C3‐Acylation of 2‐Oxindoles by Selective Aerobic Oxidation of Aryl Aldehydes

AbstractWe here report an effective NHC‐catalyzed acylation of 2‐oxindoles using aldehydes with the aid of aerobic oxidation to prepare 3‐alkylideneoxindoles ‐ a biologically active moiety. This protocol is the first report on the use of oxidized Breslow intermediate for the carbon nucleophile and applies to a wide variety of aryl aldehydes, cyclohexanal, and metallocene‐aldehyde. Lower catalyst loading, milder base, greener oxidant, easily available substrates, and ambient reaction temperature exemplify this as a sustainable methodology.

A Highly Chemically Robust 3D Interpenetrated MOF Heterogeneous Catalyst for the Synthesis of Hantzsch 1,4-Dihydropyridines and Drug Molecules.

Metal-organic frameworks (MOFs) have attracted immense attention as efficient heterogeneous catalysts over other solid catalysts, however, their chemical environment instability often limits their catalytic potential. Herein, utilizing a flexible unexplored tetra-acid ligand and employing the mixed ligand approach, a 3D interpenetrated robust framework is strategically developed, IITKGP-51 (IITKGP stands for Indian Institute of Technology Kharagpur), which retained its crystallinity over a wide range of pH solution (4-12). Having ample open metal sites (OMSs), IITKGP-51 is explored as a heterogeneous catalyst in one-pot Hantzsch condensation reaction, with low catalyst loading for a broad range of substrates. The synthesis of drug molecules remains one of the most significant and emergent areas of organic and medicinal chemistry. Considering such practical utility, biologically important Nemadipine B and Nifedipine drug molecules (calcium channel protein inhibitor) are synthesized for the first time by using this catalyst and fully characterized via SC-XRD and other spectroscopic methods. This report inaugurates the usage of a MOF material as a catalyst for the synthesis of drug molecules.

Palladium-based pseudohomogeneous catalyst for highly selective aerobic oxidation of benzylic alcohols to aldehydes

This study presents a novel class of pseudohomogeneous catalysts (PHC) based on carbon quantum dots functionalized with terpyridine ligands (CQDs-Tpy) to immobilize and stabilize palladium nanoparticles (Pd NPs). Extensive characterization techniques clearly confirmed the successful stabilization of Pd NPs on CQDs-Tpy. The effectiveness of the catalyst was demonstrated in the selective aerobic oxidation of primary and secondary of benzylic alcohols to aldehydes in the absence of additives and phase transfer catalyst (PTC). Remarkably, the reactions predominantly yielded aldehydes without further oxidation to carboxylic acids. By employing low catalyst loadings (0.13 mol%), high conversions (up to 89%) and excellent selectivity (> 99%) of the aldehyde derivatives were achieved. Moreover, the CQDs-Tpy/Pd NPs catalyst displayed suitable catalytic activity and recyclability, offering potential economic advantages. This promising approach opens up new opportunities in the field of catalysis for designing subnanometric metal-based PHCs.

Stereoselective Synthesis of Either Exo- or Endo-3-Azabicyclo[3.1.0]hexane-6-carboxylates by Dirhodium(II)-Catalyzed Cyclopropanation with Ethyl Diazoacetate under Low Catalyst Loadings.

Although cyclopropanation with donor/acceptor carbenes can be conducted under low catalyst loadings (<0.001 mol %), such low loading has not been generally effective for other classes of carbenes such as acceptor carbenes. In this current study, we demonstrate that ethyl diazoacetate can be effectively used in the cyclopropanation of N-Boc-2,5-dihydropyrrole with dirhodium(II) catalyst loadings of 0.005 mol %. By appropriate choice of catalyst and hydrolysis conditions, either the exo- or endo-3-azabicyclo[3.1.0]hexanes can be formed cleanly with high levels of diastereoselectivity with no chromatographic purification.

Installation of an organocatalyst into a protein scaffold creates an artificial Stetterase.

Using a protein scaffold covalently functionalised with a thiamine-inspired N-heterocyclic carbene (NHC), we created an artificial Stetterase (ArtiSt) which catalyses a stereoselective, intramolecular Stetter reaction. We demonstrate that ArtiSt functions under ambient conditions with low catalyst loading. Furthermore, activity can be increased >20 fold by altering the protein scaffold.

Unravelling a bench-stable zinc-amide compound as highly active multitasking catalyst for radical-mediated selective alk(en)ylation of unactivated carbocycles under mild conditions.

The direct functionalization of unactivated organic moieties via C-C bond formation has long fascinated synthetic chemists. Although base-metal systems are steadily emerging in this area, achieving multitasking activity in a single catalyst to execute several such functionalizations under mild conditions is challenging. To address this, we herein report an effective protocol for the selective C-alk(en)ylation of indene/fluorene with alcohol as a green alkylating agent employing a naturally abundant and eco-friendly zinc-derived compound, for the first time. Notably, this study unveils the unique potential of a bench-stable Zn compound bearing an amidated imidazolium salt towards C-C bond-forming reactions utilizing an array of alcohols, ranging from aliphatic to aromatic and, attractively, even secondary alcohols. Moreover, this readily scalable protocol, which proceeds via an underdeveloped radical-mediated borrowing hydrogen protocol (an aldehyde is generated from an alcohol, and subsequent condensation with indene/fluorene provides the corresponding alkenylated products) established based on a range of control experiments, works effortlessly under mild conditions using a low catalyst loading. Notably, this approach affords remarkable selectivity towards alkylated or alkenylated products with a high level of functional group tolerance and chemoselectivity. Crucially, the catalytic activity of these Zn compounds can be attributed to their hydrogen atom transfer (HAT) capability, while their selectivity towards different products can be understood in terms of employed reaction conditions. Lastly, the synthetic utility of obtained products was showcased by their late-stage functionalization to access unsymmetrical 9,9-disubstituted fluorenes, which are potentially useful for various optoelectronic applications.

Enhancing isoprene polymerization with high activity and adjustable monomer enchainment using cyclooctyl-fused iminopyridine iron precatalysts.

In this study, a series of structurally rigid cyclooctyl-fused iminopyridine iron complexes, [L2FeCl][FeCl4] and [2L3Fe][Cl][3FeCl4], was synthesized via a one-pot method and investigated as precatalysts in conjunction with methylaluminoxane for isoprene (Ip) polymerization. Combined characterization through FTIR analysis, elemental analysis and single crystal XRD analysis fully verified the structure of these complexes. The most active iron complex, FeH, exhibited a trisligated nature, with its cation adopting an octahedral geometry around the metal center. In contrast, all the other iron complexes (Fe2Me, Fe2Et, Fe2iPr, Fe3Me, Fe2Et,Me) displayed bisligated configurations, with distorted trigonal bipyramidal geometry of cations. During isoprene polymerization, the extent of steric hindrance of the ligand framework exerted a significant impact on catalytic performance. The FeH precatalyst with less steric hindrance demonstrated excellent performance, producing high molecular weight polyisoprenes with conversions exceeding 99% for 4000 equiv. of monomer. Even at very low catalyst loadings, as low as 0.0025 mol% (Fe/Ip), the polymerization of isoprene could proceed smoothly with an exceptionally high activity of 4.0 × 106 gPI (molFe, h)-1. Moreover, this precatalyst exhibited good thermal stability, maintaining high activity levels (typically 105 gPI (molFe, h)-1) across a broad temperature range from -20 °C to 100 °C. Additionally, by adjusting steric substituents and the reaction temperature, the 1,4/3,4 regioselectivity could be modulated from 9/91 to 69/31 while maintaining a high stereoselectivity of cis-1,4 structures (cis/trans: >99/1).

2.2]Paracyclophane-based coumarins: effective organo-photocatalysts for light-induced desulfonylation processes.

Herein, we demonstrate for the first time that coumarins derived from [2.2]paracyclophane (pCp) can act as effective organo-photocatalysts and promote the reductive cleavage of sulfonamides under light-irradiation. In the presence of these original compounds, photodesulfonylation reactions occur under mild conditions at low catalyst loadings in the presence of Hantzsch ester. Theoretical and experimental investigations are described, which elucidate the reaction mechanism and the nature of the active species involved in the photocatalytic process. This proof-of-concept study paves the way for further application of pCps in the field of photocatalysis.

Synergy of redox-activity and hemilability in thioamidato cobalt(III) complexes for the chemoselective reduction of nitroarenes to anilines: catalytic and mechanistic investigation.

Reduction of nitro-compounds to amines is one of the most often employed and challenging catalytic processes in the fine and bulk chemical industry. Herein, we present two series of mononuclear homoleptic and heteroleptic Co(III) complexes, i.e., [Co(LNS)3] and [Co(LNS)2L1L2]x+, respectively (x = 0 or 1, LNS = pyrimidine- or pyridine-thioamidato, L1/L2 = thioamidato, phosphine or pyridine), which successfully catalyze the transformation of nitroarenes to anilines by methylhydrazine. The catalytic reaction can be accomplished for a range of electronically and sterically diverse nitroarenes, using mild experimental conditions and low catalyst loadings, resulting in the corresponding anilines in high yields, with high chemoselectivity, and no side-products. Electronic and steric properties of the ligands play pivotal role in the catalytic efficacy of the respective complexes. In particular, complexes bearing ligands of high hemilability/lability and being capable of stabilizing lower metal oxidation-states exhibit the highest catalytic activity. Mechanistic investigations suggest the participation of the Co(III) complexes in two parallel reaction pathways: (a) coordination-induced activation of methylhydrazine and (b) reduction of nitroarenes to anilines by methylhydrazine, through the formation of Co(I) and Co-hydride intermediates.

A highly active and chemoselective homobimetallic ruthenium catalyst for one-pot reductive amination in water

A bimetallic Ru(ii) complex efficiently catalyzes reductive amination of diverse substrates in water with the highest reported TONs and TOFs. Experimental and DFT calculations point to a cooperative effect for the increased efficiency at low catalyst loading (0.05 mol%).

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Enhancing the photocatalytic upcycling of polystyrene to benzoic acid: a combined computational-experimental approach for acridinium catalyst design.

Converting polystyrene into value-added oxygenated aromatic compounds is an attractive end-of-life upcycling strategy. However, identification of appropriate catalysts often involves laborious and time-consuming empirical screening. Herein, after demonstrating the feasibility of using acridinium salts for upcycling polystyrene into benzoic acid by photoredox catalysis for the first time, we applied low-cost descriptor-based combinatorial in silico screening to predict the photocatalytic performance of a family of potential candidates. Through this approach, we identified a non-intuitive fluorinated acridinium catalyst that outperforms other candidates for converting polystyrene to benzoic acid in useful yields at low catalyst loadings (≤5 mol%). In addition, this catalyst also proved effective with real-life polystyrene waste containing dyes and additives. Our study underscores the potential of computer-aided catalyst design for valorizing polymeric waste into essential chemical feedstock for a more sustainable future.

Direct C-H difluoroallylation of α,β-unsaturated amides and aryl amides by rhodium catalysis.

Herein, we report the first example of Rh-catalyzed C(sp2)-H difluoroallylation of α,β-unsaturated amides with 3-bromo-3,3-difluoropropene under mild conditions. The features of this protocol for difluoroallylation are unprecedented vinylic substrates, ethanol as a green solvent, good functional group and air compatibility, and scale-up synthesis with low catalyst loading.