Synthesis and catalytic study of open metal site metal–organic frameworks of Cu3(BTC)2 microbelts in selective organic sulfide oxidation

The synthesis of open metal site metal–organic frameworks of Cu₃(BTC)₂ with microbelt morphology and a study of the catalytic oxidation of organic sulfides are reported. This CuBTC was found to be an efficient, selective and waste‐free green heterogeneous co‐catalyst for the green H₂O₂ catalytic oxidation of sulfides. The catalyst can be isolated from the reaction mixture and reused at least five times. Copyright © 2016 John Wiley & Sons, Ltd.     

Asymmetric Allylation of Ketones and Subsequent Tandem Reactions Catalyzed by a Novel Polymer‐Supported Titanium–BINOLate Complex

By using a novel, simple, and convenient synthetic route, enantiopure 6‐ethynyl‐BINOL (BINOL=1,1‐binaphthol) was synthesized and anchored to an azidomethylpolystyrene resin through a copper‐catalyzed alkyne–azide cycloaddition (CuAAC) reaction. The polystyrene (PS)‐supported BINOL ligand was converted into its diisopropoxytitanium derivative in situ and used as a heterogeneous catalyst in the asymmetric allylation of ketones. The catalyst showed good activity and excellent enantioselectivity, typically matching the results obtained in the corresponding homogeneous reaction. The allylation reaction mixture could be submitted to epoxidation by simple treatment with tert‐butyl hydroperoxide (TBHP), and the tandem asymmetric allylation epoxidation process led to a highly enantioenriched epoxy alcohol with two adjacent quaternary centers as a single diastereomer. A tandem asymmetric allylation/Pauson–Khand reaction was also performed, involving simple treatment of the allylation reaction mixture with Co₂(CO)₈/N‐methyl morpholine N‐oxide. This cascade process resulted in the formation of two diastereomeric tricyclic enones in high yields and enantioselectivities.     

Ethylene Epoxidation with Nitrous Oxide over Fe–BTC Metal–Organic Frameworks: A DFT Study

The epoxidation of ethylene with N₂O over the metal‐organic framework Fe–BTC (BTC=1,3,5‐benzentricarboxylate) is investigated by means of density functional calculations. Two reaction paths for the production of ethylene oxide or acetaldehyde are systematically considered in order to assess the efficiency of Fe–BTC for the selective formation of ethylene oxide. The reaction starts with the decomposition of N₂O to form an active surface oxygen atom on the Fe site of Fe–BTC, which subsequently reacts with an ethylene molecule to form an ethyleneoxy intermediate. This intermediate can then be selectively transformed either by 1,2‐hydride shift into the undesired product acetaldehyde or into the desired product ethylene oxide by way of ring closure of the intermediate. The production of ethylene oxide requires an activation energy of 5.1 kcal mol^?1, which is only about one‐third of the activation energy of acetaldehyde formation (14.3 kcal mol^?1). The predicted reaction rate constants for the formation of ethylene oxide in the relevant temperature range are approximately 2–4 orders of magnitude higher than those for acetaldehyde. Altogether, the results suggest that Fe–BTC is a good candidate catalyst for the epoxidation of ethylene by molecular N₂O.     

A Copper‐Based Metal–Organic Framework Acts as a Bifunctional Catalyst for the Homocoupling of Arylboronic Acids and Epoxidation of Olefins

A copper(I)‐based metal–organic framework ({[Cu₂Br₂(pypz)]_n?nH₂O} (Cu—Br–MOF) [pypz=bis[3,5‐dimethyl‐4‐(4’‐pyridyl)pyrazol‐1‐yl] methane] has been synthesized by using an elongated and flexible bridging ligand. The structure analysis reveals that each pypz ligand acts as a tritopic ligand connected to two Cu₂Br₂ dimeric units, forming a one‐dimensional zig–zag chain, and these chains further connected by a Cu₂Br₂ unit, give a two‐dimensional framework on the bc‐plane. In the Cu₂Br₂ dimeric unit, the copper ions are four coordinated, thereby possessing a tetrahedral geometry; this proves to be an excellent heterogeneous catalyst for the aerobic homocoupling of arylboronic acids under mild reaction conditions. This method requires only 3 mol % of catalyst and it does not require any base or oxidant—compared to other conventional (Cu, Pd, Fe, and Au) catalysts—for the transformation of arylboronic acids in very good yields (98 %). The shape and size selectivity of the catalyst in the homocoupling was investigated. The use of the catalyst was further extended to the epoxidation of olefins. Moreover, the catalyst can be easily separated by simple filtration and reused efficiently up to 5 cycles without major loss of reactivity.     

Copper(II) Schiff Base Complex Immobilized on Superparamagnetic Fe3O4@SiO2 as a Magnetically Separable Nanocatalyst for Oxidation of Alkenes and Alcohols

A new heterogeneous catalyst containing a copper(II) Schiff base complex covalently immobilized on the surface of silica‐coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂‐Schiff base‐Cu(II)) was synthesized. Characterization of this catalyst was performed using various techniques. The catalytic potential of the catalyst was investigated for the oxidation of various alkenes (styrene, α‐methylstyrene, cyclooctene, cyclohexene and norbornene) and alcohols (benzyl alcohol, 3‐methoxybenzyl alcohol, 3‐chlorobenzyl alcohol, benzhydrol and n ‐butanol) using tert ‐butyl hydroperoxide as oxidant. The catalytic investigations revealed that Fe₃O₄@SiO₂‐Schiff base‐Cu(II) was especially efficient for the oxidation of norbornene and benzyl alcohol. The results showed that norbornene epoxide and benzoic acid were obtained with 100 and 87% selectivity, respectively. Moreover, simple magnetic recovery from the reaction mixture and reuse for several times with no significant loss in catalytic activity were other advantages of this catalyst     

Manganese(III) porphyrin supported on multi-wall carbon nanotubes: A highly efficient and reusable biomimetic catalyst for epoxidation of alkenes with sodium periodate

In this paper, the biomimetic epoxidation of alkenes catalyzed by tetrakis(p-aminophenyl)porphyrinatomanganese(III) chloride, [Mn(TNH₂PP)Cl], supported on functionalized multi-wall carbon nanotubes, MWCNT, is reported. The catalyst, [Mn(TNH₂PP)Cl-MWCNT], was used as an efficient and heterogeneous catalyst for epoxidation of alkenes with NaIO₄ at room temperature, in the presence of imidazole as an axial ligand. This new heterogenized catalyst was characterized by elemental analysis, FT IR spectroscopy, diffuse reflectance UV–Vis spectrophotometry, scanning electron microscopy and transmission electron microscopy. The biggest advantage of Mn(TNH₂PP)Cl-MWCNT is its high reusability in the oxidation reactions, in which the catalyst was reused several times without significant loss of its catalytic activity.     

Titanium Dioxide‐Grafted Copper Complexes: High‐Performance Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathodes of fuel cells significantly hampers fuel cell performance. Therefore, the development of high‐performance, non‐precious‐metal catalysts as alternatives to noble metal Pt‐based ORR electrocatalysts is highly desirable for the large‐scale commercialization of fuel cells. TiO₂‐grafted copper complexes deposited on multiwalled carbon nanotubes (CNTs) form stable and efficient electrocatalysts for the ORR. The optimized catalyst composite CNTs@TiO₂–ZA–[Cu(phen)(BTC)] shows surprisingly high selectivity for the 4 e^? reduction of O₂ to water (approximately 97 %) in alkaline solution with an onset potential of 0.988 V vs. RHE, and demonstrates superior stability and excellent tolerance for the methanol crossover effect in comparison to a commercial Pt/C catalyst. The copper complexes were grafted onto the surface of TiO₂ through coordination of an imidazole‐containing ligand, zoledronic acid (ZA), which binds to TiO₂ through its bis‐phosphoric acid anchoring group. Rational optimization of the copper catalyst’s ORR performance was achieved by using an electron‐deficient ligand, 5‐nitro‐1,10‐phenanthroline (phen), and bridging benzene‐1,3,5‐tricarboxylate (BTC). This facile approach to the assembly of copper catalysts on TiO₂ with rationally tuned ORR activity will have significant implications for the development of high‐performance, non‐precious‐metal ORR catalysts.     

Graphene oxide nanosheets supported manganese(III) porphyrin: a highly efficient and reusable biomimetic catalyst for epoxidation of alkenes with sodium periodate

Efficient epoxidation of alkenes catalyzed by tetrakis(p-aminophenyl)porphyrinatomanganese(III) chloride, [Mn(TNH₂PP)Cl], supported on graphene oxide nanosheets, is reported. The catalyst, [Mn(TNH₂PP)Cl]@GO, was prepared by covalent attachment of amino groups of porphyrin to carboxylic acid groups of GO. This new heterogenized catalyst was characterized by ICP, FT-IR and diffuse reflectance UV–vis spectroscopies, scanning electron microscopy and transmission electron microscopy. This catalyst was applied as an efficient and reusable catalyst in the epoxidation of alkenes with NaIO₄ at room temperature, in the presence of imidazole as axial ligand. The most noteworthy advantage of [Mn(TNH₂PP)Cl]@GO is its high reusability in the oxidation reactions, in which the catalyst was reused several times without significant loss of its catalytic activity.     

Pickering Emulsion Stabilized by Catalytic Polyoxometalate Nanoparticles: A New Effective Medium for Oxidation Reactions

Decyl‐, dodecyl‐, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW₁₂O₄₀]^3? anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic “Pickering” emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).     

Efficient and recyclable novel Ni‐based metal–organic framework nanostructure as catalyst for the cascade reaction of alcohol oxidation–Knoevenagel condensation

A novel Ni‐based metal–organic framework (Ni‐MOF) with a Schiff base ligand as an organic linker, Ni₃(bdda)₂(OAc)₂?6H₂O (H₂bdda = 4,4′‐[benzene‐1,4‐diylbis(methylylidenenitrilo)]dibenzoic acid), was synthesized and characterized using powder X‐ray powder diffraction, thermogravimetric analysis, Brunauer–Emmett–Teller measurements, inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, elemental analysis and Fourier transform infrared spectroscopy. The synthesized Ni‐MOF exhibited a high catalytic activity in benzyl alcohol oxidation using tert‐butyl hydroperoxide under solvent‐free conditions. Also, the efficiency of the catalyst was investigated in the cascade reaction of oxidation–Knoevanagel condensation under mild conditions. The Ni‐MOF catalyst could be recovered and reused four times without significant reduction in its catalytic activity.     

Metal–Organic Frameworks as Efficient Heterogeneous Catalysts for the Regioselective Ring Opening of Epoxides

An iron‐based metal–organic framework, [Fe(BTC)] (BTC: 1,3,5‐benzenetricarboxylate) is an efficient catalyst in the ring opening of styrene oxide with alcohols and aniline under mild reaction conditions. Out of the various alcohols tested for ring opening of styrene oxide, methanol was found to be the most reactive in terms of percentage conversion and reactivity. The rate of the ring‐opening reaction of styrene oxide decreases as the size of the alcohol is increased, suggesting the location of active sites in micropores. [Fe(BTC)] was a truly heterogeneous catalyst and could be reused without loss of activity. The analogous compound [Cu₃(BTC)₂] was also found to be effective, although with somewhat lower activity than [Fe(BTC)]. The present heterogeneous protocol is compared with a homogeneous catalyst to give an insight into the reaction mechanism.     

Mn(III)‐salan/graphene oxide/magnetite nanocomposite as a highly selective catalyst for aerobic epoxidation of olefins

A nanocomposite was synthesized using carbon‐coated Fe₃O₄ nanoparticle‐decorated reduced graphene oxide as a convenient and efficient supporting material for grafting of a manganese–reduced Schiff base (salan) complex via covalent attachment. The nanocomposite was characterized using X‐ray diffraction, Fourier transform infrared and diffuse reflectance UV–visible spectroscopies, inductively coupled plasma atomic emission spectrometry and scanning electron microscopy. It was evaluated as a catalyst for the aerobic epoxidation of olefins in acetonitrile in combination with a sacrificial co‐reductant (isobutyraldehyde). The catalytic performance of the heterogeneous system of the Mn–salan complex is superior to that of the homogeneous one. The catalyst activity strongly depends on the reaction temperature and nature of the solvent. The epoxide yield increases with the nucleophilic character of the olefin. The nanocomposite performs well as an epoxidation catalyst for electron‐rich and conjugated olefins. It can be recovered from the reaction medium by magnetic decantation and reused, maintaining good catalytic activity. Copyright © 2016 John Wiley & Sons, Ltd.     

Copper‐Containing SiCN Precursor Ceramics (Cu@SiCN) as Selective Hydrocarbon Oxidation Catalysts Using Air as an Oxidant

A molecular approach to metal‐containing ceramics and their application as selective heterogeneous oxidation catalysts is presented. The aminopyridinato copper complex [Cu₂(Ap^TMS)₂] (Ap^TMSH=(4‐methylpyridin‐2‐yl)trimethylsilanylamine) reacts with poly(organosilazanes) via aminopyridine elimination, as shown for the commercially available ceramic precursor HTT 1800. The reaction was studied by ¹H and ¹³C NMR spectroscopy. The liberation of the free, protonated ligand Ap^TMSH is indicative of the copper polycarbosilazane binding. Crosslinking of the copper‐modified poly(organosilazane) and subsequent pyrolysis lead to the copper‐containing ceramics. The copper is reduced to copper metal during the pyrolysis step up to 1000 °C, as observed by solid‐state ⁶⁵Cu NMR spectroscopy, SEM images, and energy‐dispersive spectroscopy (EDS). Powder diffraction experiments verified the presence of crystalline copper. All Cu@SiCN ceramics show catalytic activity towards the oxidation of cycloalkanes using air as oxidant. The selectivity of the reaction increases with increasing copper content. The catalysts are recyclable. This study proves the feasibility of this molecular approach to metal‐containing SiCN precursor ceramics by using silylaminopyridinato complexes. Furthermore, the catalytic results confirm the applicability of this new class of metal‐containing ceramics as catalysts.     

Organophosphate‐accelerated copper‐catalyzed C(sp2)–C(sp) Sonogashira‐type cross couplings

A dramatic acceleration in copper‐catalyzed Sonogashira‐type reactions was observed when an organophosphate was used as additive. The catalyst systems featuring low copper loading (0.5 mol% < Cu < 5 mol%) gave Sonogashira‐type products with a broad scope of aromatic and aliphatic terminal alkynes as well as aryl iodides in good to excellent yields. Among the organophosphate/copper catalytic systems, that of 4 mol% Cu(OTf)₂/10 mol% (R)‐(?)‐1,1′‐binaphthyl‐2,2′‐diyl hydrogenphosphate exhibited the highest catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

A recyclable nanosize aminoarenethiolato copper(I) catalyst for C-C coupling reactions

An aminoarenethiolato copper(I) catalyst was attached to a carbosilane dendritic wedge, which had been prepared via a novel convergent synthetic method. Compared with the unsupported complex, this novel dendritic copper(I) catalyst is more robust towards hydrolysis and oxidation and has increased solubility in common organic solvents. The catalytic activity of the dendritic copper catalyst was tested in the 1,4-addition of Et₂Zn to 2-cyclohexenone. In both polar (Et₂O) and apolar (hexane) solvents excellent activity was observed. The fact that the catalytic copper site remains attached to the nanosize dendritic aminoarenethiolate ligand allows separation of this catalyst by means of nanofiltration.     

Simple Transformation of Hierarchical Hollow Structures by Reduction of Metal–Organic Frameworks and Their Catalytic Activity in the Oxidation of Benzyl Alcohol

Metal–organic framework (MOF)-based derivatives have been found to be promising heterogeneous catalysts for organic transformations. Herein, hollow-structure Cu-MOFs derived by reduction of Cu₃(BTC)₂ (BTC=1,3,5-benzenetricarboxylate; denoted as RCB) were prepared by using hydrazine hydrate as a reducing agent under various conditions. The influence of hydrazine hydrate induced the structure of Cu₃(BTC)₂ and led to dynamic variation in the interior and exterior as well as oxidation states of the Cu ion. The synthesized materials were characterized by SEM, TEM, N₂ sorption isotherms, XRD, and XPS. The product of the catalytic reaction was observed by GC-MS. In addition, the prepared RCBs were found to have excellent catalytic activity and selectivity for benzyl alcohol oxidation when assisted by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO).     

Palladium on magnetic Irish moss: A new nano‐biocatalyst for suzuki type cross‐coupling reactions

A novel heterogeneous magnetic palladium nano‐biocatalyst was designed by utilizing Irish moss, a family of sulfated polysaccharides extracted from algae, as a natural biopolymer. This magnetic Irish moss decorated with palladium (Pd–Fe₃O₄@IM) to form a biomagnetic catalytic system was synthesized and well characterized by FT–IR analysis, X‐ray powder diffraction, field emission scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, atomic absorption spectroscopy and transmission electron microscopy. The catalyst was stable to air and moisture and displayed high catalytic activity in ligand‐free Suzuki–Miyaura cross‐coupling reactions conducted under green chemistry reaction conditions. The aromatic ketones are produced by the cross‐coupling reaction between acid chlorides and aryl boronic acid derivatives in high yields.     

A Water‐Soluble Copper–Polypyridine Complex as a Homogeneous Catalyst for both Photo‐Induced and Electrocatalytic O2 Evolution

The water‐soluble polypyridine copper complex [Cu(F₃TPA)(ClO₄)₂] [ 1 ; F₃TPA=tris(2‐fluoro‐6‐pyridylmethyl)amine] catalyzes water oxidation in a pH 8.5 borate buffer at a relatively low overpotential of 610 mV. Assisted by photosensitizer and an electron acceptor, 1 also exhibits activity as a homogeneous catalyst for photo‐induced O₂ evolution with a maximum turnover frequency (TOF) of (1.58±0.03)×10^?1 s^?1 and a maximum turnover number (TON) of 11.61±0.23. In comparison, the reference [Cu(TPA)(ClO₄)₂] [TPA=tris(2‐pyridylmethyl)amine] displayed almost no activity under either set of conditions, implying the crucial role of the ligand in determining the behavior of the catalyst. Experimental evidence indicate the molecular catalytic nature of 1 , leading to a potentially practical strategy to apply the copper complex in a photoelectrochemical device for water oxidation.     

Catalytic Oxidation of CO and Benzene over Metal Nanoparticles Loaded on Hierarchical MFI Zeolite

In order to obtain highly active catalytic materials for oxidation of carbon monoxide and volatile organic compounds (VOCs), monometallic platinum, copper, and palladium catalysts were prepared by using of two types of ZSM-5 zeolite as supports—parent ZSM-5 and the same one treated by HF and NH₄F buffer solution. The catalyst samples, obtained by loading of platinum, palladium, and copper on ZSM-5 zeolite treated using HF and NH₄F buffer solution, were more active in the reaction of CO and benzene oxidation compared with catalyst samples containing untreated zeolite. The presence of secondary mesoporosity played a positive role in increasing the catalytic activity due to improved reactant diffusion. The only exception was the copper catalysts in the reaction of CO oxidation, in which case the catalyst, based on untreated ZSM-5 zeolite, was more active. In this specific case, the key role is played by the oxidative state of copper species loaded on the ZSM-5 zeolites.     

Catalytic Activity Dependency on Catalyst Components in Aerobic Copper–TEMPO Oxidation

The influence of catalyst components in the copper–TEMPO (2,2,6,6‐tetramethylpiperidine N‐oxide) catalysed aerobic oxidation of alcohols was investigated. The type and amount of base greatly influences reactivity. The bipyridyl ligand concentration had no major influence on catalysis, but excessive amounts led to a decrease in activity for longer reaction times. The kinetic dependency for TEMPO was found to be 1.15, and for copper 2.25, which is an indication of a binuclear catalytic system. Optimised conditions with various allylic and aliphatic alcohols give good to excellent rapid oxidations.