Heterogeneous Metathesis Research Articles

Non-Antibiotic Antimony-Based Antimicrobials.

A series of the eight novel organoantimony(V) cyanoximates of Sb(C6H5)4L composition was synthesized using the high-yield heterogeneous metathesis reaction between solid AgL (or TlL) and Sb(C6H5)4Br in CH3CN at room temperature. Cyanoximes L were specially selected from a large group of 48 known compounds of this subclass of oximes on the basis of their water solubility and history of prior biological activity. The synthesized compounds are well soluble in organic solvents and were studied using a variety of conventional spectroscopic and physical methods. The crystal structures of all reported organometallic compounds were determined and revealed the formation of the distorted trigonal bipyramidal environment of the Sb atom and monodentate axial binding of acido-ligands via the O atom of the oxime group. The compounds are thermally stable in the solid state and in solution molecular compounds. For the first time, this specially designed series of organoantimony(V) compounds is investigated as potential non-antibiotic antimicrobial agents against three bacterial and two fungal human pathogens known for their increasing antimicrobial resistance. Bacterial pathogens included Gram-negative Escherichia coli and Pseudomonas aeruginosa, and Gram-positive Staphylococcus aureus. Fungal pathogens included Cryptococcus neoformans and Candida albicans. The cyanoximates alone showed no antimicrobial impact, and the incorporation of the SbPh4 group enabled the antimicrobial effect. Overall, the new antimony compounds showed a strong potential as both broad- and narrow-spectrum antimicrobials against selected bacterial and fundal pathogens and provide insights for further synthetic modifications of the compounds to increase their activities.

Synthesis of heterogeneous metathesis catalysts for the development of sustainable processes


 In recent decades, it has been tried to develop processes that involve the use of catalysts that can be recovered from the reaction medium, to make them green and sustainable. Approximately, 90 % of all processes in chemistry and petrochemicals use heterogeneous catalysts. In this sense, the supported heterogeneous catalysts present advantages in the purification of the products and the reduction of costs. In the present work, the synthesis of a new heterogeneous supported catalyst, highly active in the metathesis reaction, based on a ruthenium-alkylidene complex, was carried out. The new catalyst was obtained by anchoring of dichloro[1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene](benzylidene)(tricyclohexylphosphine)ruthenium (II) (second generation Grubbs) in various silylated polymeric gels, based on 2-norbornene-5,6-dicarboxylic anhydride (NDA) and cis-cyclooctene (CO) cross-linked with various percentages of 3-aminopropyltriethoxysilane (APTES) (5%, 15%, 100%). The new heterogeneous catalyst was successfully used in the depolymerization reaction via metathesis of natural rubber from Oaxaca (HNO), using 1-octene as CTA; observing the formation of liquid oligomers, achieving a decrease in molecular weight from 137.941 g/mol to 297 g/mol; proving, in this way, its high effectiveness.

Dendritic polarizing agents for DNP SENS.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an effective method to significantly improve solid-state NMR investigation of solid surfaces. The presence of unpaired electrons (polarizing agents) is crucial for DNP, but it has drawbacks such as leading to faster nuclear spin relaxation, or even reaction with the substrate under investigation. The latter can be a particular problem for heterogeneous catalysts. Here, we present a series of carbosilane-based dendritic polarizing agents, in which the bulky dendrimer can reduce the interaction between the solid surface and the free radical. We thereby preserve long nuclear T'2 of the surface species, and even successfully enhance a reactive heterogeneous metathesis catalyst.

ChemInform Abstract: Opportunities of Immobilized Homogeneous Metathesis Complexes as Prominent Heterogeneous Catalysts

AbstractReview: [122 refs.

Opportunities of Immobilized Homogeneous Metathesis Complexes as Prominent Heterogeneous Catalysts

AbstractA plethora of strategies for immobilizing well‐defined metathesis catalysts on solid supports have been developed over the past decades. Whereas most reviews on classical heterogeneous metathesis catalyst designs mainly discuss covalent binding approaches, we here extended this topic with non‐covalent surface science methods, a research area that gained interest recently. Within this context, the main focus was set on the immobilization of ruthenium alkylidenes or so‐called Grubbs complexes, as these catalysts are known for their high activity, stability and commercial viability. Besides giving an overview of different immobilization strategies, the wide diversity of supported Grubbs catalysts also allows for discussing implications of attaching Grubbs catalysts that go beyond the traditional benefits of heterogeneous catalysis, such as easy catalyst separation. More specifically, this Review will summarize several unique opportunities of immobilized homogeneous catalysts in liquid‐phase metathesis transformations in terms of impact on activity, stability, and product selectivity, as to highlight the economic potential, but without ignoring possible shortcomings and limitations.

Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS.

Obtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction-based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid-state NMR spectroscopy (SENS) towards surface species in heterogeneous alkene metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon-carbon bond distance in metallacycles, which are the cycloaddition intermediates in the alkene metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous metathesis catalysts.

Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS

AbstractObtaining detailed structural information of reaction intermediates remains a key challenge in heterogeneous catalysis because of the amorphous nature of the support and/or the support interface that prohibits the use of diffraction‐based techniques. Combining isotopic labeling and dynamic nuclear polarization (DNP) increases the sensitivity of surface enhanced solid‐state NMR spectroscopy (SENS) towards surface species in heterogeneous alkene metathesis catalysts; this in turn allows direct determination of the bond connectivity and measurement of the carbon–carbon bond distance in metallacycles, which are the cycloaddition intermediates in the alkene metathesis catalytic cycle. Furthermore, this approach makes possible the understanding of the slow initiation and deactivation steps in these heterogeneous metathesis catalysts.

Bulky aryloxide ligand stabilizes a heterogeneous metathesis catalyst.

The reaction of [W(=O)(=CHCMe2Ph)(dAdPO)2], containing bulky 2,6-diadamantyl aryloxide ligands, with partially dehydroxylated silica selectively yields a well-defined silica-supported alkylidene complex, [(≡SiO)W(=O)(=CHCMe2Ph)(dAdPO)]. This fully characterized material is a very active and stable alkene metathesis catalyst, thus allowing loadings as low as 50 ppm in the metathesis of internal alkenes. [(≡SiO)W(=O)(=CHCMe2Ph)(dAdPO)] also efficiently catalyzes the homocoupling of terminal alkenes, with turnover numbers exceeding 75,000 when ethylene is constantly removed to avoid the formation of the less reactive square-based pyramidal metallacycle resting state.

Bulky Aryloxide Ligand Stabilizes a Heterogeneous Metathesis Catalyst

AbstractThe reaction of [W(O)(CHCMe2Ph)(dAdPO)2], containing bulky 2,6‐diadamantyl aryloxide ligands, with partially dehydroxylated silica selectively yields a well‐defined silica‐supported alkylidene complex, [(SiO)W(O)(CHCMe2Ph)(dAdPO)]. This fully characterized material is a very active and stable alkene metathesis catalyst, thus allowing loadings as low as 50 ppm in the metathesis of internal alkenes. [(SiO)W(O)(CHCMe2Ph)(dAdPO)] also efficiently catalyzes the homocoupling of terminal alkenes, with turnover numbers exceeding 75 000 when ethylene is constantly removed to avoid the formation of the less reactive square‐based pyramidal metallacycle resting state.

A Well-Defined Silica-Supported Tungsten Oxo Alkylidene Is a Highly Active Alkene Metathesis Catalyst

Grafting (ArO)2W(═O)(═CHtBu) (ArO = 2,6-mesitylphenoxide) on partially dehydroxylated silica forms mostly [(≡SiO)W(═O)(═CHtBu)(OAr)] along with minor amounts of [(≡SiO)W(═O)(CH2tBu)(OAr)2] (20%), both fully characterized by elemental analysis and IR and NMR spectroscopies. The well-defined oxo alkylidene surface complex [(≡SiO)W(═O)(═CHtBu)OAr] is among the most active heterogeneous metathesis catalysts reported to date in the self-metathesis of cis-4-nonene and ethyl oleate, in sharp contrast to the classical heterogeneous catalysts based on WO3/SiO2.

A non-hydrolytic sol–gel route to highly active MoO3–SiO2–Al2O3 metathesis catalysts

MoO3-based materials are known to be appropriate catalysts for the heterogeneous metathesis of light olefins. Classical preparation methods involve the deposition of a Mo oxide phase on the surface of preformed support via impregnation, grafting or thermal spreading. An alternative sol–gel approach for the elaboration of Mo-based catalysts is presented in this article. Mesoporous ternary Si–Al–Mo mixed oxides are prepared in one step, via non-hydrolytic condensation of chloride precursors in non-aqueous media. After calcination, effective catalysts with very good textures and highly dispersed surface molybdenum species are obtained. The Si/Al ratio influences both the texture and the acidity of the materials, which significantly affects the propene self-metathesis activity. The activity also increases with the MoO3 content. The best catalysts with optimized composition significantly outperform the catalysts prepared by other methods.

Homogeneous vs Heterogeneous Polymerization Catalysis Revealed by Single-Particle Fluorescence Microscopy

A high-sensitivity and high-resolution single-particle fluorescence microscopy technique differentiated between homogeneous and heterogeneous metathesis polymerization catalysis by imaging the location of the early stages of polymerization. By imaging single polymers and single crystals of Grubbs II, polymerization catalysis was revealed to be solely homogeneous rather than heterogeneous or both.

Thermal Spreading As an Alternative for the Wet Impregnation Method: Advantages and Downsides in the Preparation of MoO3/SiO2−Al2O3 Metathesis Catalysts

Silica alumina-supported MoO3 catalysts are classically prepared via impregnation of the support with a molybdenum salt solution, usually ammonium heptamolybdate, and subsequent drying and calcination (three steps). The downsides of such a route for the synthesis of heterogeneous metathesis catalysts are linked to the limited control on the nature of the Mop, stabilized at the surface, to the uneven distribution of the deposit in the pores of the support, and to the build up of inactive species that find their origin in the wet step of the preparation. In opposition, the direct thermal spreading of molybdenum oxide onto the support is a straightforward (one step) method involving no wet stage. It allows the conversion of bulk MoO3 crystals to amorphous molybdate species dispersed at the surface of the silica alumina support. This contribution shows that the catalysts obtained via both methods exhibit similar performances in the self-metathesis of propene to butene and ethene. However, based on XRD, XPS, Raman spectroscopy, ICP-AES, N-2 physisorption, TEM, and MAS-NMR spectroscopy, it is shown that the origin of active and inactive species in the two systems is different. Whereas the activity of wet-made catalysts is limited by the formation of bulky MoO3 crystals and of aluminum molybdate, the performances of dry-made catalysts are limited by the incomplete spreading of MoO3 nanocrystallites.

Genesis of active and inactive species during the preparation of MoO 3/SiO 2–Al 2O 3 metathesis catalysts via wet impregnation

The wet impregnation of ammonium heptamolybdate onto silica–alumina is used to prepare MoO 3/SiO 2–Al 2O 3 heterogeneous metathesis catalysts. The preparation is inspected in details in conjunction with physico-chemical characterization tools with the aim to identify the parameters that dictate the genesis of active and inactive metathesis species. The effects of the MoO 3 loading and of the calcination temperature are systematically explored. The samples are characterized by N 2-physisorption, ICP-AES, XRD, Raman, 27Al MAS-NMR and XPS and evaluated in the metathesis of propene to butene and ethene. Particular attention is brought to the interaction of the mesoporous silica–alumina support with the active component, to the decomposition of the precursor salt and to the location of the molybdenum oxide phase with respect to the pores of the support. It is shown that the temperature of calcination influences markedly the performances of the catalyst. High temperature treatments are necessary to decompose efficiently the Mo salt precursor. In the metathesis of propylene, the performances are levelling off when the MoO 3 loading is increased above ∼8 wt.%. This effect is correlated to the build up of MoO 3 crystals and of Al 2(MoO 4) 3 at relatively high loading.

Synthesis of energetic compounds via the metathesis reaction of 4-methylenespiro[2,3]hexane

Transformations of methylenespiro[2,3]hexane (MSH) on a heterogeneous rhenium-alumina metathesis (disproportionation) catalyst were studied. It was found that, owing to a significant difference in the stability of carbenic complexes of methylene and disubstituted carbenes, MSH undergoes isomerization to 4-methylspiro[2,3]hex-4-ene followed by their cometathesis yielding bis(spiro[2,3]hexylidene-4). The feasibility of selective cometathesis of MSH and dicyclobutylidene on the rhenium-alumina catalyst resulting in the formation of 4-cyclobutylidenespiro[2,3]hexane was shown.

Homogeneous metathesis for the production of propene from butene

The cross-metathesis of 1-butene and 2-butene is described for the production of propene and 2-pentene using Phobcat, a homogeneous Grubbs first generation-type catalyst bearing 9-cyclohexyl-9-phosphabicyclo-[3.3.1]-nonane as ligand. In a closed system at 20 bar and 50 °C, the reaction mixture composition at different 1-butene:2-butene feed ratios is in fair agreement with the calculated composition at equilibrium based on thermodynamic data. Compared to heterogeneous metathesis over a WO3/SiO2 catalyst, the homogeneous reaction exhibits better reaction control and selectivity to the desired products.

Molecular understanding of Re-based heterogeneous olefin metathesis catalysts

Molecular understanding of Re-based heterogeneous olefin metathesis catalysts

Tin(IV) Cyanoximates: Synthesis, Characterization, and Cytotoxicity

In recent years, numerous organotin(IV) derivatives have exhibited remarkable cytotoxicity against several types of cancer. However, the properties of the cyanoxime-containing organotin(IV) complexes are unknown. Previously, it has been shown that cyanoximes displayed an interesting spectrum of biological activity ranging from growth-regulation to antimicrobial and pesticide detoxification actions. The work presented here attempts to combine the useful properties of both groups of compounds and investigate the likely antiproliferating activity of the new substances. A series of 19 organotin(IV) complexes, with nine different cyanoxime ligands, were anaerobically prepared by means of the heterogeneous metathesis reaction between the respective organotin(IV) halides (Cl, Br) and ML (M=Ag, Tl; L=cyanoximate anion), using an ultrasound in the CH3CN at room temperature. The compounds were characterized using spectroscopic methods (UV-visible, IR, 1H,13C NMR, 119Sn Mössbauer) and X-ray analysis. The crystal structures of the complexes revealed the formation of two types of tin(IV) cyanoximates: mononuclear five-coordinated compounds of R4-xSnLx composition (R=Me, Et, n-Bu, Ph; x=1, 2; L=cyanoximate anion), and the tetranuclear R8Sn4(OH)2O2L2 species (R=n-Bu, Ph). The latter complex contains a planar [Sn4(OH)2O2]2- core, consisting of three adjacent rhombs with bridging oxo and hydroxo groups. The tin(IV) atoms are five-coordinated and have distorted trigonal-pyramidal surrounding. This is the first instance when the organic anions were found to act as monodentate O-bound planar oxime ligands. All of the compounds were studied in vitro for antiproliferating activity, using human cervical cancer HeLa and WiDR colon cancer cell lines; cisplatin was used as a positive control substance. The two dibutyltin(IV) cyanoximates showed cytotoxicity similar and greater to that of cisplatin.

A new stable Hoveyda–Grubbs catalyst with mixed anionic ligands

The synthesis and characterisation of a new highly active Hoveyda–Grubbs 2nd generation type catalyst is described. Substitution of one chloride ligand with a partially fluorinated trialkoxysilyl substituted carboxylate leads to the stable monocarboxylate ruthenium catalyst ( 3). This catalyst represents the first example of a stable and isolable mono-chloride exchanged carboxylate complex suitable for both homogeneous and heterogeneous metathesis. The reactivity of the new catalyst was tested in representative metathesis reactions and offers an activity comparable to the parent dichloride system ( 1).

Preparation and olefin-metathesis activity of cyclopentylidene-oxo initiator sites on a molybdenum carbide surface

A low surface area planar sample of bulk β-Mo 2C can be activated for olefin-metathesis reactions through the dissociative chemisorption of cyclopentanone to yield surface alkylidene-oxo species. This methodology allows the formation of a sufficiently high coverage of active sites to permit investigation of heterogeneous metathesis by surface science techniques. Reflectance absorbance infrared spectroscopy measurements were used to isolate η 1 and η 2-adsorbed cyclopentanone. The η 2-state was found to be the direct precursor for alkylidene-oxo formation. Olefin-metathesis reactions were carried out using propene and cyclopentene. The surface chemistry of cyclopentanone and cyclopentylidene on β-Mo 2C is compared to analogous organometallic systems.