Chemoselective Hydrazine‐mediated Transfer Hydrogenation of Nitroarenes by Co3O4 Nanoparticles Immobilized on an Al/Si‐mixed Oxide Support

Cobalt oxide nanoparticles (size 2 to 3.5 nm) were successfully impregnated on an alumina–silica (mixed oxide) support through an experimentally viable and easily reproducible protocol. The prepared material was well characterized by XRD, HR‐TEM, BET surface area, EDX and XPS analyses. Porous alumina–silica having a high surface area served as a protective heterogeneous support on which the well‐dispersed Co₃O₄ nanoparticles served as an active catalytic species for the hydrazine‐mediated transfer hydrogenation of nitroarenes. About 2 mol % of the active catalyst in ethanol at 60 °C was adequate for a successful conversion. Moreover, transfer hydrogenation of nitroarenes by the catalyst was found to take place chemoselectively in the presence of other labile functional groups such as halide, alkene, nitrile, carbonyl, and ester. This inexpensive catalyst was also able to catalyze the reaction on a gram scale reaction and found to be robust and recyclable up to eight runs.     

Tuning the Metal–Support Interaction by Structural Recognition of Cobalt‐Based Catalyst Precursors

Controlling the nature and size of cobalt(II) polynuclear precursors on γ‐alumina and silica‐alumina supports represents a challenge for the synthesis of optimal cobalt‐based heterogeneous catalysts. By density functional theory (DFT) calculations, we show how after drying the interaction of cobalt(II) precursor on γ‐alumina is driven by a structural recognition phenomenon, leading to the formation of an epitaxial Co(OH)₂ precipitate involving a Co–Al hydrotalcite‐like interface. On a silica‐alumina surface, this phenomenon is prevented due to the passivation effect of silica domains. This finding opens new routes to tune the metal–support interaction at the synthesis step of heterogeneous catalysts.     

Silica‐Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well‐defined silica‐supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo‐based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off‐cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica‐supported Mo and W imido analogues.     

Cyclisation versus 1,1‐Carboboration: Reactions of B(C6F5)3 with Propargyl Amides

A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C₆F₅)₃ were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N‐substituted propargyl amides with B(C₆F₅)₃ led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C₆F₅)₃ mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species.     

CO2‐Sourced α‐Alkylidene Cyclic Carbonates: A Step Forward in the Quest for Functional Regioregular Poly(urethane)s and Poly(carbonate)s

Described is a robust platform for the synthesis of a large diversity of novel functional CO₂‐sourced polymers by exploiting the regiocontrolled ring‐opening of α‐alkylidene carbonates by various nucleophiles. The reactivity of α‐alkylidene carbonates is dictated by the exocyclic olefinic group. The polyaddition of CO₂‐sourced bis(α‐alkylidene carbonate)s (bis‐αCCs) with primary and secondary diamines provides novel regioregular functional poly(urethane)s. The reactivity of bis‐αCCs is also exploited for producing new poly(β‐oxo‐carbonate)s by organocatalyzed polyaddition with a diol. This synthesis platform provides new functional variants of world‐class leading polymer families (polyurethanes, polycarbonates) and valorizes CO₂ as a chemical feedstock.     

XPS characterisation of carbon‐coated alumina support

Hybrid carbon–alumina supports, synthesised by pyrolysis of grafted 4,4′‐methylenebis‐(phenylisocyanate) moiety on the alumina surface, were characterised by X‐ray photoelectron spectroscopy. The recorded Al 2p and C 1s envelopes showed asymmetry that decreased with an increase in carbon loading. In all experimental Al 2p envelopes, the high‐energy individual components at 75.3–75.9 eV were present along with the low‐energy component at 74.0 eV typical for Al₂O₃. In the case of the C 1s envelope, the component around 284.3–284.4 eV and three high‐energy individual components at 285.9–286.0, 288.0–288.3 and 290.1–290.6 eV were observed. The presence of the high‐energy Al 2p components can be explained considering the occurrence of a steady‐state charging of the different parts of insulating alumina supports. The component around 284.3–284.4 eV in C 1s envelopes can be attributed to carbon, which constitutes the coating and, hence, ensures surface conductivity. The component around 285.9–286.0 eV is connected with carbon in carbonaceous surface species, which do not form the conducting layer on the alumina support. Carbonaceous surface species associated with C? O, C?O and O?C? O groups in carbon coating can be also identified due to the presence of corresponding components in XPS spectra at 285.9–286.0, 288.0–288.3 and 290.1–290.6 eV. Copyright © 2006 John Wiley & Sons, Ltd.     

Tripod Immobilization of Triphenylphosphane on a Silica‐Gel Surface to Enable Selective Mono‐Ligation to Palladium: Application to Suzuki–Miyaura Cross‐Coupling Reactions with Chloroarenes

A silica‐supported triphenylphosphane (Silica‐3p‐TPP) with a Ph₃P‐type core, immobilized on a silica surface, was synthesized and characterized by nitrogen‐absorption measurements and solid‐state NMR spectroscopy. The tripodal immobilization constrains the mobility of the phosphane molecule and causes the lone pair on the phosphorus atom to face in the direction perpendicular to the support, resulting in the selective formation of a 1:1 metal–phosphane species that is free from unfavorable steric repulsions caused by the silica surface. Heterogeneous Pd catalysts created in this manner enabled room‐temperature Suzuki–Miyaura cross‐coupling reactions with unactivated chloroarenes, despite the moderate electronic and steric nature of the Ph₃P‐based ligands. These catalysts also showed potential in reactions with more challenging substrates under mild conditions. Tripodally immobilized and well‐dispersed phosphanes on the silica surface were crucial for high catalytic activity.     

Organocatalyzed Enantioselective Synthesis of 2‐Amino‐4H‐chromene Derivatives

Optically active 2‐amino‐5‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene‐3‐carboxylates, 2‐amino‐5‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene‐3‐carbonitriles, and 2‐amino‐8‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene‐3‐carbonitriles were synthesized. Using cinchona alkaloid‐derived bifunctional catalysts, the corresponding 2‐amino‐4H‐chromene derivatives were obtained in high yields and moderate to high ee values (up to 82% ee) from the tandem Michael addition–cyclization reaction between 1,3‐cyclohexanediones or 1,2‐cyclohexanediones and (E )‐3‐aryl‐2‐cyanoacrylate or alkylidene malononitrile derivatives.     

Catalytic Oxo/Imido Heterometathesis by a Well‐Defined Silica‐Supported Titanium Imido Complex

Grafting Ti(=NtBu)(Me₂Pyr)₂(py)₂ (Me₂Pyr= 2,5‐dimethylpyrrolyl, py=pyridine) onto the surface of silica partially dehydroxylated at 700 °C gives the well‐defined silica‐supported Ti imido complex (≡SiO)Ti(=NtBu)(Me₂Pyr)(py)₂, which is fully characterized by IR and solid‐state NMR spectroscopy as well as elemental and mass balance analyses. While stoichiometric imido‐transfer reactivity is typical for Ti imides, the obtained surface complex is unique in that it enables catalytic transformations involving Ti imido and oxo intermediates. In particular, it efficiently catalyzes imidation of carbonyl compounds with N‐sulfinylamines by oxo/imido heterometathesis.     

Cyclohexene Hydroconversion Using Monometallic and Bimetallic Catalysts Supported on γ‐Alumina

The hydroconversion of cyclohexene (CHE) using monometallic catalysts containing 0.35wt% of Pt, Pd, Ir or Re on a γ‐alumina support, as well as bimetallic catalysts containing combinations of 0.35wt% Pt with 0.35wt% of either Pd, Ir or Re on γ‐alumina, were investigated in a plug flow‐type fixed‐bed reactor. The Cyclohexene (CHE) feed was injected continuously with a rate of 8.33 × 10^?3mole h^?1 on 0.2 g of catalyst using a simultaneous hydrogen gas flow of 20 cm³ min^?1 throughout a broad reaction temperature range of 50–400 °C. The dispersion of the metals in the catalysts was determined via H₂ or CO chemisorption. The activities of the monometallic catalysts were found to be in the order: Pd > Pt > Ir > Re, whereas those of the bimetallic catalysts were in the order: PtPd > PtIr > PtRe. Cyclohexene hydrogenation and dehydrogenation reactions using the current mono‐ and bimetallic catalysts were kinetically investigated applying the absolute reaction rate theory, whereby reaction rate constant, activation energy, enthalpy and entropy of activation were computed to explain surface variations on these catalysts.     

An All‐Alkynyl Protected 74‐Nuclei Silver(I)–Copper(I)‐Oxo Nanocluster: Oxo‐Induced Hierarchical Bimetal Aggregation and Anisotropic Surface Ligand Orientation

The hardness of oxo ions (O^2?) means that coinage‐metal (Cu, Ag, Au) clusters supported by oxo ions (O^2?) are rare. Herein, a novel μ₄‐oxo supported all‐alkynyl‐protected silver(I)–copper(I) nanocluster [Ag_74?xCu_xO₁₂(PhC≡C)₅₀] ( NC‐1 , avg. x=37.9) is characterized. NC‐1 is the highest nuclearity silver–copper heterometallic cluster and contains an unprecedented twelve interstitial μ₄‐oxo ions. The oxo ions originate from the reduction of nitrate ions by NaBH₄. The oxo ions induce the hierarchical aggregation of Cu^I and Ag^I ions in the cluster, forming the unique regioselective distribution of two different metal ions. The anisotropic ligand coverage on the surface is caused by the jigsaw‐puzzle‐like cluster packing incorporating rare intermolecular C?H???metal agostic interactions and solvent molecules. This work not only reveals a new category of high‐nuclearity coinage‐metal clusters but shows the special clustering effect of oxo ions in the assembly of coinage‐metal clusters.     

Spectroscopically distinct sites present in methyltrioxorhenium grafted onto silica-alumina, and their abilities to initiate olefin metathesis

Deposition of CH3ReO3 onto the surface of dehydrated, amorphous silica-alumina generates a highly active, supported catalyst for the metathesis of olefins. However, silica-alumina with a high (10 wt %) Re loading is no more active than silica-alumina with low (1 wt %) loading, while CH3ReO3 on silica is completely inactive. Catalysts prepared by grafting CH3ReO3 on silica-alumina contain two types of spectroscopically distinct sites. The more strongly bound sites are responsible for olefin metathesis activity and are formed preferentially at low Re loadings (< or =1 wt %). They are created by two Lewis acid/base interactions: (1) the coordination of an oxo ligand to an Al center of the support and (2) interaction of one of the adjacent bridging oxygens (AlOSi) with the Re center. At higher Re loadings (1-10 wt %), CH3ReO3 also interacts with surface silanols by H-bonding. This gives rise to highly mobile sites, most of which can be observed by 13C solid-state NMR even without magic-angle spinning. Their formation can be prevented by capping the surface hydroxyl groups with hexamethyldisilazane prior to grafting CH3ReO3, resulting in a metathesis catalyst that is more selective, more robust, and more efficient in terms of Re use.     

Oxorhenium(V)‐ and Tricarbonylrhenium(I) Complexes with Substituted Pyrazoles as Products of the Degradation of Hydrotrispyrazolylborates

Hydrotris(3, 5‐dimethylpyrazol‐1‐yl)borate and hydrotris(3‐phenylpyrazol‐1‐yl)borate decompose during reactions with [ReOCl₃(PPh₃)₂] and [NEt₄]₂[Re(CO)₃Br₃], respectively. The generated pyrazole ligands form complexes with the rhenium(V) oxo and the rhenium(I ) tricarbonyl cores. X‐ray crystal structures of the oxo‐bridged dimer [Cl(PPh₃)(O)Re(μ‐O)(μ‐Me₂pz)₂Re(O)(HMe₂pz)Cl] ( 1 ) and [Re(CO)₃(HPhpz)₂(Phpz)] ( 2 ) (HMe₂pz = 3, 5‐dimethylpyrazole, HPhpz = 3‐phenylpyrazole) show that the substituted pyrazoles can readily deprotonate and act as monodentate or bridging anionic ligands. Re‐N bond lengths between 2.09 and 2.14Å have been observed for the bridging and between 2.12 and 2.23Å for the terminal pyrazole ligands.     

ESIMS Studies and Calculations on Alkali‐Metal Adduct Ions of Ruthenium Olefin Metathesis Catalysts and Their Catalytic Activity in Metathesis Reactions

Electrospray ionization mass spectrometry (ESIMS) and subsequent tandem mass spectrometry (MS/MS) analyses were used to study some important metathesis reactions with the first‐generation ruthenium catalyst 1 , focusing on the ruthenium complex intermediates in the catalytic cycle. In situ cationization with alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) using a microreactor coupled directly to the ESI ion source allowed mass spectrometric detection and characterization of the ruthenium species present in solution and particularly the catalytically active monophosphine–ruthenium intermediates present in equilibrium with the respective bisphosphine–ruthenium species in solution. Moreover, the intrinsic catalytic activity of the cationized monophosphine–ruthenium complex 1 a ?K⁺ was directly demonstrated by gas‐phase reactions with 1‐butene or ethene to give the propylidene Ru species 3 a ?K⁺ and the methylidene Ru species 4 a ?K⁺, respectively. Ring‐closing metathesis (RCM) reactions of 1,6‐heptadiene ( 5 ), 1,7‐octadiene ( 6 ) and 1,8‐nonadiene ( 7 ) were studied in the presence of KCl and the ruthenium alkylidene intermediates 8 , 9 , and 10 , respectively, were detected as cationized monophosphine and bisphosphine ruthenium complexes. Acyclic diene metathesis (ADMET) polymerization of 1,9‐decadiene ( 14 ) and ring‐opening metathesis polymerization (ROMP) of cyclooctene ( 18 ) were studied analogously, and the expected ruthenium alkylidene intermediates were directly intercepted from reaction solution and characterized unambiguously by their isotopic patterns and ESIMS/MS. ADMET polymerization was not observed for 1,5‐hexadiene ( 22 ), but the formation of the intramolecularly stabilized monophosphine ruthenium complex 23 a was seen. The ratio of the signal intensities of the respective with potassium cationized monophosphine and bisphosphine alkylidene Ru species varied from [I _4a ]/[I ₄ ]=0.02 to [I _23a ]/[I ₂₃ ]=10.2 and proved to be a sensitive and quantitative probe for intramolecular π‐complex formation of the monophosphine–ruthenium species and of double bonds in the alkylidene chain. MS/MS spectra revealed the intrinsic metathesis catalytic activity of the potassium adduct ions of the ruthenium alkylidene intermediates 8 a , 9 a , 10 a , 15 a , and 19 a , but not 23 a by elimination of the respective cycloalkene in the second step of RCM. Computations were performed to provide information about the structures of the alkali metal adduct ions of catalyst 1 and the influence of the alkali metal ions on the energy profile in the catalytic cycle of the metathesis reaction.     

Performance of the Cr[CH(SiMe3)2]3/SiO2 catalyst for ethylene polymerization compared with the performance of the Phillips catalyst

The catalysis of a silica‐supported chromium system {Cr[CH(SiMe₃)₂]₃/SiO₂} was compared with a silica‐supported chromium oxide catalyst, the Phillips catalyst (CrO₃/SiO₂). This catalyst was prepared by the calcining of the typical silica support used for the Phillips catalyst at 600 °C and by the support of tris[bis(trimethylsilyl)methyl]chromium(III) {Cr[CH(SiMe₃)₂]₃} on the silica. In the slurry‐phase polymerization, this catalyst conducted the polymerization of ethylene at a high activity without organoaluminum compounds as cocatalysts or scavengers. The activity per Cr was about 6–7 times higher than that of the Phillips catalyst. Upon the introduction of hydrogen to the system, the molecular weight of polyethylene did not change with the Phillips catalyst, but it decreased with the Cr[CH(SiMe₃)₂]₃/SiO₂ catalyst. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 413–419, 2003     

The Unexpected Mechanism Underlying the High‐Valent Mono‐Oxo‐Rhenium(V) Hydride Catalyzed Hydrosilylation of CN Functionalities: Insights from a DFT Study

In this study, we theoretically investigated the mechanism underlying the high‐valent mono‐oxo‐rhenium(V) hydride Re(O)HCl₂(PPh₃)₂ ( 1 ) catalyzed hydrosilylation of C?N functionalities. Our results suggest that an ionic S_N2‐Si outer‐sphere pathway involving the heterolytic cleavage of the Si?H bond competes with the hydride pathway involving the C?N bond inserted into the Re?H bond for the rhenium hydride ( 1 ) catalyzed hydrosilylation of the less steric C?N functionalities (phenylmethanimine, PhCH=NH, and N‐phenylbenzylideneimine, PhCH=NPh). The rate‐determining free‐energy barriers for the ionic outer‐sphere pathway are calculated to be ～28.1 and 27.6 kcal mol^?1, respectively. These values are slightly more favorable than those obtained for the hydride pathway (by ～1–3 kcal mol^?1), whereas for the large steric C?N functionality of N,1,1‐tri(phenyl)methanimine (PhCPh=NPh), the ionic outer‐sphere pathway (33.1 kcal mol^?1) is more favorable than the hydride pathway by as much as 11.5 kcal mol^?1. Along the ionic outer‐sphere pathway, neither the multiply bonded oxo ligand nor the inherent hydride moiety participate in the activation of the Si?H bond.     

Enhancement of C−H Oxidizing Ability in Co–O2 Complexes through an Isolated Heterobimetallic Oxo Intermediate

The characterization of intermediates formed through the reaction of transition‐metal complexes with dioxygen (O₂) is important for understanding oxidation in biological and synthetic processes. Here, the reaction of the diketiminate‐supported cobalt(I) complex L^tBuCo with O₂ gives a rare example of a side‐on dioxygen complex of cobalt. Structural, spectroscopic, and computational data are most consistent with its assignment as a cobalt(III)–peroxo complex. Treatment of L^tBuCo(O₂) with low‐valent Fe and Co diketiminate complexes affords isolable oxo species with M₂O₂ “diamond” cores, including the first example of a crystallographically characterized heterobimetallic bis(μ‐oxo) complex of two transition metals. The bimetallic species are capable of cleaving C−H bonds in the supporting ligands, and kinetic studies show that the Fe/Co heterobimetallic species activates C−H bonds much more rapidly than the Co/Co homobimetallic analogue. Thus heterobimetallic oxo intermediates provide a promising route for enhancing the rates of oxidation reactions.     

Polymorphs and pseudo‐polymorphs of μ‐oxo‐bis{[N,N′‐bis(salicylidene)­propane‐1,3‐diamine]oxorhenium(V)}

Two new polymorphs of the title compound, μ‐oxo‐bis{oxo{2,2′‐[propane‐1,3‐diyl­bis­(nitrilo­methyl­idyne)]­diphen­olato}rhenium(V)}, [Re₂O(C₁₇H₁₆N₂O₃)₂], are reported, containing either a conformation other than the one already known in the literature or a disorder involving both the new and the previously reported conformations. Four pseudo‐polymorphs of the title compound are also reported, containing four chloro­form, two chloro­form, two disordered di­chloro­methane or two water solvate mol­ecules accompany­ing each Re complex molecule. Only in the hydrate does the Re complex adopt the old conformation. In all six structures, the complex molecule is located on a crystallographic inversion centre. Independent of the conformation, all ReV ions display the same, somewhat distorted, octahedral coordination. In all the solvates, hydrogen bonds are donated from the solvent to the O atoms bonded to Re, either of the C—H?O or O—H?O type, although the actual position of the solvent mol­ecule can vary. Only in the hydrate is a two‐dimensional hydrogen‐bonded network found; isolated clusters are formed in all the other solvates.     

Perhydrocarbyl ReVII complexes: comparison of molecular and surface complexes

The molecular complex [Re(=CtBu)(=CHtBu)(CH2tBu)2] 1 reacts with a silica partially dehydroxylated at 700 degrees C to give syn-2, [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], as a single isomer according to mass-balance analysis, IR, and solid-state NMR spectroscopy. 1D and 2D solid-state NMR (HETCOR and long-range HETCOR) on a 13C-labeled-2 has allowed us to observe the chemical shifts of all carbons (including those that are not labeled) and ascertain their assignments. Moreover, EXAFS data are consistent with the presence of two carbons at a relatively short distance (1.79 A), which cannot be deconvoluted, but which are consistent with the presence of alkylidene and alkylidyne carbons along with two other first neighbors at a longer distance (2.01 A), the alkyl carbon and the O atom by which the Re is attached to the surface. Moreover, the data also suggest the presence of a siloxane bridge of the silica surface at 2.4 A in the coordination sphere of the Re center. Thermal and photochemical treatment allow us to observe the anti isomer, which was also fully characterized by 1D and 2D solid-state NMR. This behavior parallels the reactivity of molecular Re complexes, and their respective 1H and 13C chemical shifts match those of the corresponding molecular analogues syn- and anti-2m and n. Finally, the grafting of 1 onto silica involves the reaction of both the alkyl and the alkylidene ligand with an equiprobability, leaving the alkylidyne as a spectator ligand. Noteworthy is the formation of 2 [(=SiO)Re(=CtBu)(=CHtBu)(CH2tBu)], rather than the corresponding trisneopentyl-neopentylidyne Re complex, monografted on silica, [(=SiO)Re(=CtBu)(CH2tBu)3], which would have been expected from the reactivity of 1 with various molecular Br?nsted acids and which also suggests that a proximal siloxane bridge forces the alpha-H abstraction process, leading to syn-2a.     

Concurrent Stabilization of π‐Donor and π‐Acceptor Ligands in Aromatized and Dearomatized Pincer [(PNN)Re(CO)(O)2] Complexes

Aromatized cationic [(PNN)Re(π acid)(O)₂]⁺ ( 1 ) and dearomatized neutral [(PNN*)Re(π acid)(O)₂] ( 2 ) complexes (where π acid=CO ( a ), tBuNC ( b ), or (2,6‐Me₂)PhNC ( c )), possessing both π‐donor and π‐acceptor ligands, have been synthesized and fully characterized. Reaction of [(PNN)Re(O)₂]⁺ ( 4 ) with lithiumhexamethyldisilazide (LiHMDS) yield the dearomatized [(PNN*)Re(O)₂] ( 3 ). Complexes 1 and 2 are prepared from the reaction of 4 and 3 , respectively, with CO or isocyanides. Single‐crystal X‐ray structures of 1 a and 1 b show the expected trans‐dioxo structure, in which the oxo ligands occupy the axial positions and the π‐acidic ligand occupies the equatorial plane in an overall octahedral geometry about the rhenium(V) center. DFT studies revealed the stability of complexes 1 and 2 arises from a π‐backbonding interaction between the d_xy orbital of rhenium, the π orbital of the oxo ligands, and the π* orbital of CO/isocyanide.