Active Catalytic Species Research Articles

Thiolate Bridged Gold(I)–NHC Catalysts: New Approach for Catalyst Design and its Application to Trapping Catalytic Intermediates

New dinuclear N-heterocyclic gold complexes with bridging thiolate ligands have been designed as catalytic precursors with desired properties such as stability, recyclability and that do not require additives. The dinuclear compound [(AuNHC)2 (μ-SC6 F5 )]OTf could slowly release the active catalytic species [Au(NHC)]+ and the precursor [Au(SC6 F5 )(NHC)] in solution, which means that both species would remain stable throughout the catalytic cycle and the pre-catalyst could easily be recovered. The properties exhibited by the complexes have been taken advantage of to gain new insights on the gold-catalyzed hydroalkoxylation of alkynes, with the aim of clarifying all the steps of the catalytic cycle, together with the characterization of intermediates and final products. Isolation and characterization of the pure final spiroketals and the thermodynamic intermediate have been achieved for the first time. Moreover, the kinetic intermediate has also been detected for the first time.

Imidazolium Salts of Ruthenium Anionic Cyclopentadienone Complexes: Ion Pair for Bifunctional Catalysis in Ionic Liquids

The reactivity of the dinuclear complex dicarbonyl[η4‐3,4‐bis(4‐methoxyphenyl)‐2,5‐diphenylcyclopenta‐2,4‐dienone] (2) with imidazolium salts has been studied using a library of variously functionalized imidazolium salts, and leading to a class of ruthenium anionic complexes of the type [dicarbonyl{η4‐3,4‐bis(4‐methoxyphenyl)‐2,5‐diphenylcyclopenta‐2,4‐dienone} (halide)Ru][1,3‐disubsittuted‐imidazolium] (4) in the form of ion pair with imidazolium. The reaction is clean, general and quantitative and the complexes formed are stable to air and moisture both in the solid state and in solution. The ionic complexes 4 show affinity for ionic liquids and represent stable precursors of a catalytic active species, likely similar to those generated from Shvo catalyst, a well‐established homogeneous bifunctional catalyst for hydrogen transfer. Our approach allows the use of green ionic liquids (IL) as solvents, providing better control of catalytic reactions involving 4, and easier catalyst recycle. IL‐supported ionic pair catalysts exhibit interesting catalytic activity (up to > 99 % of conversion) in the transfer hydrogenation of a model compound such as 4‐fluoroacetophenone. The IL‐supported redox catalysts can be also recycled exploiting the biphasic nature of the system. The synthetic method here discussed represents a novelty within the field of IL‐supportation and is potentially useful for the heterogenization of the catalysts.

N2 catalyzes p-cresol transformation

Gaseous nitrogen is a bit of a chemical wallflower. While high temperatures and pressures such as those used in the Haber-Bosch process can convince the compound to react, gaseous N2 rarely acts as an active catalytic species. Now a group of international researchers has found that adding N2 gas to a ruthenium-based catalyst speeds up the hydrodeoxygenation of p-cresol into toluene by over 400%; it’s a reaction that could have applications in converting bio-oils into useful chemicals (Nat. Catal. 2019, DOI: 10.1038/s41929-019-0368-6). Led by Jun Li from Tsinghua University and Edman Tsang and Dermot O’Hare from the University of Oxford, the researchers stumbled across this chemistry when trying to pressurize their reactor with an inert gas, O’Hare says. They added gaseous N2 to a solid-state titanium oxide–supported Ru catalyst, with hydrogen gas and p-cresol, and found that the Ru surface activates the N2 to form either N2H or N2H2. These

Task-Specific Properties and Prospects of Ionic Liquids in Cross-Coupling Reactions.

Ionic liquids (ILs) are considered as highly useful materials for potential diverse uses such as greener and more convenient alternatives to volatile organic solvents, reagents, additives, ligands and co-solvents. Thermal stability, negligible vapor pressure and high polarity with ionic environments have possibly conferred some unique physico-chemical properties and a wider electrochemical window on ILs. More importantly, these properties are tuneable, depending on variations in alkyl chains and counter-anions. On the other hand, various transition-metal-catalyzed cross-coupling reactions constitute an important backbone of contemporary organic synthesis. A vast number of C-C and C-heteroatom cross-coupling reactions are reported in the presence of ILs, often showing better performance. The influence of IL on the action of a given catalyst or on the course of a reaction can be relatively complex, and is not understood well enough to be able to draw succinct conclusions. However, there are a few reports in the literature that help understand the role of actual and active catalytic species stabilized in an IL environment. Stabilization, which can be either helpful or detrimental to catalysis depends on specific circumstances. This review article is aimed primarily at summarizing the various applications of ILs during the past decade, focusing as far as possible on the task-specific properties of ILs in transition-metal-catalyzed C-C and C-heteroatom cross-coupling reactions. Several successful achievements and noteworthy progress in this field of research leads to the sensible conclusion that future prospects in this field of research are not only bright but promise new horizons.

Aerobic Tetrazine‐Catalyzed Oxidative Nitroso‐Diels‐Alder Reaction of N‐Arylhydroxylamines with Dienecarbamates: Access to Functionalized 1,6‐Dihydro‐1,2‐oxazines

AbstractA new organocatalytic and environmentally friendly aerobic oxidative‐catalyzed nitroso‐Diels‐Alder (NDA) reaction of N‐arylhydroxylamines with dienecarbamates has been developed. 3,6‐Bis(2,2,2‐trifluoroethoxy)‐s‐tetrazine was found to be an effective catalyst to selectively oxidize various N‐arylhydroxylamines in the presence of molecular oxygen, but in the absence of photostimulation, as a terminal oxidant. In addition, active catalytic species can be generated in situ from commercially available 3,6‐dichlorotetrazine.

Highly selective 1-butanol obtained from ethanol catalyzed by mixed metal oxides: Reaction optimization and catalyst structure behavior

Synthesis and characterization of a copper mixed metal oxide obtained from hydrotalcite precursor as well as catalytic runs for ethanol conversion to 1-butanol are described. Applying the surface response model, the reaction was optimized reaching an ethanol conversion of 79.6% into gaseous phase (69% of yield) and condensed phase (31% of yield) products using a batch reactor at 350 °C for 5 h. The main product of the condensed phase was 1-butanol with 25.4% yield and 32% selectivity, these results being among the higher ones reported for this reaction in the literature. Recycling catalyst experiments demonstrated that, for at least four cycles, the ethanol conversion remains almost constant but the 1-butanol yield and selectivity both decreased. XRD, EPR, surface area measurements and acidity/basicity experiments carried out after the first and fourth catalyst recycling cycles show major modifications in the initial mixed metal oxide structure and copper oxidation state indicating that the active catalytic species increases during the first catalytic run.

MnIII Porphyrins: Catalytic Coupling of Epoxides with CO2 under Mild Conditions and Mechanistic Considerations

AbstractA series of 5,10,15,20‐tetrakis(2,3‐dichlorophenyl)porphyrinate complexes of manganese(III) [MnIII(T2,3DCPP)X] with six different axial ligands (X=NO3−, AcO−, IO3−, Br−, Cl−, HO−) were investigated as catalysts in the cycloaddition reactions of CO2 and styrene oxide (SO), under mild conditions, i. e., atmospheric pressure and 60 °C. [MnIIIT(2,3DCPP)IO3] showed the best catalytic performance, selectively producing the respective cyclic carbonate from diverse epoxides using tetrabutylammonium bromide as a nucleophile source. Mechanistic considerations were inferred from electronic spectra and spectrophotometric titrations, showing that there are a series of equilibriums involved in the formation of the catalytic active species. Stability constants for the proposed equilibrium models were determined using SQUAD software. A catalytic cycle has been proposed based on those observations.

Taming heterogeneous rhenium catalysis for the production of biomass-derived chemicals

Rhenium is one of important components for heterogeneous catalysts, which has been recently used for the catalytic reactions related to the production of biomass-derived chemicals such as deoxydehydration of vicinal OH groups, C-O hydrogenolysis, and hydrogenation of carboxylic acids, and so on. Suitable oxidation state of Re as a catalytic active species is strongly dependent on the catalytic reactions. The control of the oxidation state of Re species on the catalysts is crucial on the catalyst development.

Amorphous Cobalt Iron Borate Grown on Carbon Paper as a Precatalyst for Water Oxidation.

The key to improving water oxidation is to develop efficient and earth-abundant catalysts for the oxygen evolution reaction (OER). Herein, a new amorphous cobalt iron borate supported on 3D carbon paper integrated electrode is reported as a precatalyst for the OER, which was synthesized by using a one-pot hydrothermal method. An optimum OER activity was obtained at 25 % Fe doping by screening the compositions of the Co and Fe. The best synthesized catalyst needs an overpotential of 227 mV to deliver a current density of 10 mA cm-2 and also exhibits a long-term durability of 24 h. Impressively, we find that CoFe oxyhydroxide was formed in situ in the OER process, which serves as the real catalytic active species for OER. Moreover, the direct conversion from CoFe borate to CoFe oxyhydroxide is reported for the first time in metal borate OER catalysts. The discovery in this work, that is, that metal borate as a precursor can efficiently catalyze the OER in alkaline media, significantly widens the family of OER catalysts.

Activation, Deactivation and Reversibility Phenomena in Homogeneous Catalysis: A Showcase Based on the Chemistry of Rhodium/Phosphine Catalysts

In the present work, the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, has been used to showcase how the in situ generation of precatalysts, the conversion of precatalysts into the actually active species, as well as the reaction of the catalyst itself with other components in the reaction medium (substrates, solvents, additives) can lead to a number of deactivation phenomena and thus impact the efficiency of a catalytic process. Such phenomena may go unnoticed or may be overlooked, thus preventing the full understanding of the catalytic process which is a prerequisite for its optimization. Based on recent findings both from others and the authors’ laboratory concerning the chemistry of rhodium/diphosphine complexes, some guidelines are provided for the optimal generation of the catalytic active species from a suitable rhodium precursor and the diphosphine of interest; for the choice of the best solvent to prevent aggregation of coordinatively unsaturated metal fragments and sequestration of the active metal through too strong metal–solvent interactions; for preventing catalyst poisoning due to irreversible reaction with the product of the catalytic process or impurities present in the substrate.

Facile Fabrication of Amorphous Ni−P Supported on a 3D Biocarbon Skeleton as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

AbstractTransition metals with intrinsic electrocatalytic activity usually suffer from low anodic instability when used as electrocatalysts for the oxygen evolution reaction (OER). This work draws inspiration from stainless metals to synthesize highly anticorrosive amorphous Ni−P via electroless deposition. Holly leaves with three‐dimensional micro/nanoarchitecture were firstly calcined at 800 °C in nitrogen to obtain self‐supporting biocarbon as the substrate for eletroless deposition. After depositing Ni−P for 30 min, the as‐obtained Ni−P/carbonized leaf (Ni−P/CL) composite showed an excellent electrocatalytic performance for the OER. Benefiting from the high electronic conductivity and catalytic activity of Ni−P, and the efficient mass transport behavior in the three‐dimensional architecture, a low overpotential of 300 mV was required to reach a current density of 40 mA cm−2. The anticorrosive amorphous and P‐doping feature of Ni−P can prevent catalytic active species from leaching out; and to some extent, can protect the biocarbon substrate from corrosion during the OER process. The current retained 85.7 % of the initial value after performing at 1.6 V (vs. RHE) for 12 h.

Synergy between Ionic Liquids and CuCl2 in Gas–Liquid Phase Reactions of Acetylene Hydrochlorination

We studied the acetylene hydrochlorination in gas–liquid phase reactions using ionic liquids (IL) as the reaction media and CuCl2 as the catalyst. The Cu-IL catalyst showed strong synergy between the IL and the Cu(II) active catalytic species. For [PrMIm]Cl, the Cu-IL catalyst exhibited significant enhancement of the catalytic activity in comparison with the CuCl2 catalyst supported on activated carbon and the IL alone as the catalyst. We have also performed DFT calculations of the reaction process, which provides a good explanation of our experimental results and for the synergetic effect. Our result suggests that ILs may be used to improve the activity of other metallic catalysts for the hydrochlorination reaction of acetylene.

Concentration-induced structural diversity and catalytic activity of BF3/n-BuOH complexes for n-decene polymerization

Abstract The BF3/n-BuOH complexes were investigated as active species in catalyzing n-decene polymerization reaction. The structures of BF3/n-BuOH complexes were characterized not only by modern spectrum but also by calculation at theoretical level. The results confirmed that BF3/n-BuOH complexes changed from BF3·(n-BuOH)2 complexes to BF3·n-BuOH complexes with the mass fraction of BF3 increasing. These two complexes have different catalytic activity, but BF3·n-BuOH was superior. The highest n-decene conversion could reach 99% and the most excellent selectivity of n-decene trimer and tetramer could reach up to 80% yield by a series of controlled conditions. This work can help to understand the catalytic active species of n-decene polymerization and provide support for industrialization of poly-alpha-olefins (PAOs).

In Situ-Generated Supported Potassium Lactate: Stable Catalysis for Vapor-Phase Dehydration of Lactic Acid to Acrylic Acid.

We have studied unsupported, silica gel- and amorphous silica–alumina-supported catalysts derived from K salts for the vapor-phase dehydration of lactic acid (LA) to acrylic acid (AA). A catalytic study shows that the supported catalysts improve the activity and selectivity for the production of AA and decrease the selectivity for the production of propionic acid (PA). The silica–alumina-supported catalysts remain fairly stable in the catalytic performance during 90 h of reaction. The IR spectroscopic characterization combined with the catalytic study demonstrates that potassium lactate (C3H5KO3) in situ generated from LA and a K salt is an important reaction intermediate for the production of AA and the catalytic stability is associated with the chemical stability of C3H5KO3 and the activity for the regeneration of C3H5KO3 in the catalytic cycle. On silica–alumina, C3H5KO3 is well stabilized and smoothly regenerated during the reaction, leading to the good catalytic stability. This work suggests for the first time that lactate salt acts as the true catalytic active species for the dehydration of LA to AA. We also propose a predominant reaction pathway for the vapor-phase dehydration of LA to AA with K salt catalyst systems.

Porphyrin and Phthalocyanine Graphene Assemblies As Electrocatalysts for CO2 Reduction

Electrochemical CO2 conversion is in the global spotlight as one of the most promising ‘clean’ approach for the conversion of atmospheric and marine CO2 into clean and renewable liquid fuels as well as offering a potential approach to the amelioration of a major greenhouse gas. In particular, carbon monoxide (CO) produced by electrochemical CO2 reduction is a principal industrial feedstock for the production of hydrocarbons and bulk oxygenated products. Consequently, the development of a highly energy efficient and selective heterogeneous catalyst for the conversion of CO2 to CO is of major interest in energy and environmental science research and industry. It is challenging to split CO2 due to the high activation energy required in the reduction process. In order to resolve this, a variety of catalysts have been investigated to reduce the activation energy including precious metals, non-precious metals, transition metal oxides, transition metal chalcogenides and metal complexes. In the presence of molecular catalysts, specifically, CO2 reduction typically proceeds by a proton-coupled electron transfer reaction, resulting in lower overpotentials due to the stabilization of the metal-carbon dioxide complex. Depending on the process under the CO2 atmosphere, a variety of gaseous or liquid products can be generated at various potentials. However, the competitive hydrogen evolution reaction (HER) can concomitantly occur at a similar potential to the electrochemical CO2 reduction reaction.Therefore, selectivity is a vital property in a practical electrocatalyst for this process. Metal complexes (e.g. metalloporphyrins and metallophthalocyanines) have been widely studied as homogeneous catalysts for electrochemical CO2 reduction due to their high selectivity, low cost, and ease of preparation on a large scale. However, the use of the metal complexes as homogeneous catalysts is not ideal for practical applications since the catalytic activities are influenced by a number of factors including (i) diffusion of the catalyst into the diffusion layer adjacent tothe electrode surface, limiting the number of active catalytic species, (ii) catalyst deactivation processes such as dimerization and aggregation and (iii) the use of volatile and flammable organic solvents like dimethylformamide and acetonitrile, which is not ideal.[1,2] In addition, unlike heterogeneous catalysts, it is not easy to separate or reuse the homogeneous catalysts following the reaction, one of the most important requirements in commercial use. For these reasons, there have been a number of studies in which the molecular complexes have been immobilised and used as heterogeneous catalysts for the electrochemical CO2 reduction.[3-5] Although the majority of metal complexes have demonstrated improved catalytic performance through immobilisation, most of the catalysts for the production of CO operate at high overpotentials and show poor catalytic durability of less than 5 h, resulting in a low energy efficiency for the electrochemical CO2 reduction catalysis. Therefore, a stable, efficient metal complex-based heterogeneous catalyst for the electrocatalytic reduction of CO2 at low overpotentials has yet to be realised. We have developed simple and facile self-assembly methods for the fabrication of heterogeneous composite electrocatalysts for CO2 reduction to CO using reduced graphene oxide with both metalloporphyrins and metallophthalocyanines by π-π stacking and electrostatic interactions.[6,7] The resulting electrocatalysts possess a number of advantages including (1) facile and rapid electron transfer, (2) easy electrolyte and CO2 accessibility, (3) structural robustness, (4) excellent faradaic efficiencies (>96%) at overpotentials as low as 280 mV, and (5) superior long-term stability (up to 50 h) under the electrochemical reduction conditions. In contrast to previous work, these catalysts exhibited excellent electrocatalytic CO2 reduction performance and stability. Costentin, C.; Robert, M.; Savéant, J.-M. Curr. Opin. Electrochem. 2017, 2, 26–31.Bullock, M.; Das, A.; Appel, A. Chem. Eur. J. 2017, 23, 7626–7641.Zhang, X.; Wu, Z.; Zhang, X.; Li, L.; Li, Y.; Xu, H.; Li, X.; Yu, X.; Zhang, Z.; Liang, Y.; Wang H. Nat. Commun. 2017, 8, 14675.Tatin, A.; Comminges, C.; Kokoh, B.; Costentin, C.; Robert, M.; Savéant, J.-M. Proc. Nat. Acad. Sci. 2016, 113, 5526–5529.Hu, X.; Rønne, M.; Pedersen, S.; Skrydstrup, T.; Daasbjerg, K. Angew. Chem. Int. Ed. 2017, 56, 6468–6472.Choi, J.; Wagner, P.; Jalili, R.; Kim, J.; MacFarlane, D. R.; Wallace, G. G.; Officer, D. L. Adv. Ener. Mater. 2018, 8, 1801280, doi.org/10.1002/aenm.201801280.Choi, J.; Kim, J.; Wagner, P.; Gambhir, S.; Jalili, R.; Byun, S.; Sayyar, S.; Lee, Y. M.; MacFarlane, D. R.; Wallace, G. G.; Officer, D. L. Ener. Environ. Sci., 2018, submitted for publication.

Formation of XPhos-Ligated Palladium(0) Complexes and Reactivity in Oxidative Additions.

Understanding the nature of the intermediate species operating within a palladium catalytic cycle is crucial for developing efficient cross-coupling reactions. Even though the XPhos/Pd(OAc)2 catalytic system has found numerous applications, the nature of the active catalytic species remains elusive. A Pd0 complex ligated to XPhos has been detected and characterized in situ for the first time using cyclic voltammetry and NMR techniques. In the presence of XPhos, Pd(OAc)2 initially associates with the ligand to form a complex in solution, which has been characterized as PdII (OAc)2 (XPhos). This PdII center is then reduced to the Pd0 (XPhos)2 species by an intramolecular process. This study also sheds light on the formation of PdI -PdI dimers. Finally, a kinetic study probes a dissociative mechanism for the oxidative addition with aryl halides involving Pd0 (XPhos) as the reactive species in equilibrium with the unreactive Pd0 (XPhos)2 . Remarkably, the reportedly poorly reactive PhCl reacts at room temperature in the oxidative addition, which confirms the crucial role of the XPhos ligand in the activation of aryl chlorides.

Role of Alkaline-Earth Metal-Catalyst: A Theoretical Study of Pyridines Hydroboration.

Density functional theory (DFT) calculations have been performed to investigate the mechanism of alkaline-earth-metal-catalyzed hydroboration of pyridines with borane. In this reaction, the active catalytic species is considered to be an alkaline earth metal hydride complex when the corresponding alkaline earth metal is used as the catalyst. The theoretical results reveal that initiation of the catalytic cycle is hydride transfer to generate a magnesium hydride complex when β-diimine alkylmagnesium is used as a pre-catalyst. The magnesium hydride complex can undergo coordination of the pyridine reactant followed by hydride transfer to form a dearomatized magnesium pyridine intermediate. Coordination of borane and hydride transfer from borohydride to magnesium then give the hydroboration product and regenerate the active magnesium hydride catalyst. The rate-determining step of the catalytic cycle is hydride transfer to pyridine with a free energy barrier of 29.7 kcal/mol. Other alkaline earth metal complexes, including calcium and strontium complexes, were also considered. The DFT calculations show that the corresponding activation free energies for the rate-determining step of this reaction with calcium and strontium catalysts are much lower than with the magnesium catalyst. Therefore, calcium and strontium complexes can be used as the catalyst for the reaction, which could allow mild reaction conditions.

Graphitic Carbon Nitride Microspheres Supported α-FeO(OH) Hybrids for Visible Light Photodegradation of MO

Graphitic carbon nitride(g-C3N4) microspheres supported α-FeO(OH) hybrids[α-FeO(OH)/g-C3N4] were prepared by means of a self-assembly method in deionized water. By UV-visible diffuse reflectance spectroscopy, it has been confirmed that α-FeO(OH)/g-C3N4 has a wider absorption range than g-C3N4. The feature of α-FeO(OH)/g-C3N4 can be attributed to the efficient separation of the electron-hole pairs with photoluminescence spectra. The degradation rate of methyl orange(MO) is up to 99% under the optimal conditions of 110 min, initial concentration of 30 mg/L, an α-FeO(OH)/g-C3N4 dosage of 15 mg as well as visible light. The mechanism for this photocatalytic reaction was proposed, with hydroxyl radicals being a major active catalytic species.

OX-1 Metal–Organic Framework Nanosheets as Robust Hosts for Highly Active Catalytic Palladium Species

A catalytic system based on OX-1 metal–organic framework nanosheets is reported, incorporating catalytically active palladium (Pd) species. The Pd@OX-1 guest@host system is rapidly synthesized via a one-step single-pot supramolecular assembly, with the possibility of controlling the Pd loading. The structures of the resulting framework and of the active Pd species before and after catalytic reactions are studied in detail using a wide variety of techniques including synchrotron radiation infrared spectroscopy, inelastic neutron scattering, and X-ray absorption spectroscopy. Crystals of the resulting Pd@OX-1 composite material contain predominantly atomic and small cluster Pd species, which selectively reside on benzene rings of the benzenedicarboxylate (BDC) linkers. The composites are shown to efficiently catalyze the Suzuki coupling and Heck arylation reactions under a variety of conditions. Pd@OX-1 further shows potential to be recycled for at least five cycles of each reaction as well as an ability to recapture active Pd species during both catalytic reactions.

N- and S-doped porous carbon decorated with in-situ synthesized Co–Ni bimetallic sulfides particles: A cathode catalyst of rechargeable Zn-air batteries

A novel 3D composites comprising in-situ formed CoNi bimetallic sulfides particles (CoNiS@NSPC) embedded in N, S-doped porous carbon was facilely constructed for the air cathode catalyst for Zn-air batteries. The synergistic interplay between N, S doped carbon matrix, the porous structure and the highly catalytic active cobalt nickel sulfides species contributes greatly to the excellent bifunctional catalytic activity both in the ORR and OER in 0.1 M KOH solution for the CoNiS@NSPC catalyst. The ORR measurement results showed that the half-wave potential (0.82 V vs. RHE), current density (5.23 mA cm−2 at 0.2 V vs. RHE) of our sample were very close to that of Pt/C (0.82 V, 5.17 mA cm−2), meanwhile its stability and methanol tolerance were superior to Pt/C. The OER measurement results showed that the CoNiS@NSPC catalyst exhibited a small overpotential (0.47 V vs. RHE) at 10 mA cm−2, close to that of RuO2 (0.39 V). Importantly, compared to the mixture of Pt/C and RuO2 cathodes, the rechargeable Zn-air battery based on CoNiS@NSPC cathode exhibits smaller discharge/charge overpotential (0.85 V), higher power density (51.6 mW cm−2), higher voltaic efficiency (58.5%) at 10 mA cm−2 and an improved cycling stability over 3600 min.