Dihydrogen Pressure Research Articles

Grafting Copper Atoms and Nanoparticles on Double-Walled Carbon Nanotubes: Application to Catalytic Synthesis of Propargylamine.

The decoration of carbon nanotubes (CNTs) by metal nanoparticles (NPs) combines the advantages of a high specific surface material with catalytic properties of metal nanocrystals. Little work has been devoted to the decoration of CNTs with copper NPs, and no evidence of copper atomic decoration of CNTs has shown up until now. Herein, we demonstrate that the strong acidic oxidation of double-walled CNTs (dwCNTs) is very efficient for the decoration of the carbon surface by copper NPs and atoms. This treatment severely degraded the CNT walls and generated a large amount of disordered sp3 carbon. This amorphous carbon film bears many chemically active functions like carboxyl and hydroxyl ones. In such conditions, the CNT walls behave as very efficient ligands for the stabilization of copper obtained by the thermolysis of the mesityl precursor in organic solution under mild dihydrogen pressure. In addition to copper NPs, we evidenced the presence of a regular coverage with copper atoms over the dwCNTs. This nanocomposite catalyzes the quantitative synthesis of propargylamines via one A3-type coupling reaction. Five consecutive catalytic cycles with 100% yield could be performed with no loss of activity, and the combination of Cu supported on dwCNTs allows a facile recycling of the catalytic material.

RhNPs supported on N-functionalized mesoporous silica: effect on catalyst stabilization and catalytic activity.

Amine and nicotinamide groups grafted on ordered mesoporous silica (OMS) were investigated as stabilizers for RhNPs used as catalysts in the hydrogenation of several substrates, including carbonyl and aryl groups. Supported RhNPs on functionalized OMS were prepared by controlled decomposition of an organometallic precursor of rhodium under dihydrogen pressure. The resulting materials were characterized thoroughly by spectroscopic and physical techniques (FTIR, TGA, BET, SEM, TEM, EDX, XPS) to confirm the formation of spherical rhodium nanoparticles with a narrow size distribution supported on the silica surface. The use of nicotinamide functionalized OMS as a support afforded small RhNPs (2.3 ± 0.3 nm), and their size and shape were maintained after the catalyzed acetophenone hydrogenation. In contrast, amine-functionalized OMS formed RhNP aggregates after the catalytic reaction. The supported RhNPs could selectively reduce alkenyl, carbonyl, aryl and heteroaryl groups and were active in the reductive amination of phenol and morpholine, using a low concentration of the precious metal (0.07-0.18 mol%).

Facilitating hydrogen atom migration via a dense phase on palladium islands to a surrounding silver surface

The migration of species across interfaces can crucially affect the performance of heterogeneous catalysts. A key concept in using bimetallic catalysts for hydrogenation is that the active metal supplies hydrogen atoms to the host metal, where selective hydrogenation can then occur. Herein, we demonstrate that, following dihydrogen dissociation on palladium islands, hydrogen atoms migrate from palladium to silver, to which they are generally less strongly bound. This migration is driven by the population of weakly bound states on the palladium at high hydrogen atom coverages which are nearly isoenergetic with binding sites on the silver. The rate of hydrogen atom migration depends on the palladium-silver interface length, with smaller palladium islands more efficiently supplying hydrogen atoms to the silver. This study demonstrates that hydrogen atoms can migrate from a more strongly binding metal to a more weakly binding surface under special conditions, such as high dihydrogen pressure.

Pyridine‐Stabilized Rhodium Nanoparticles in Ionic Liquids as Selective Hydrogenation and Transfer Hydrogenation Catalysts

Rhodium nanoparticles (RhNPs) stabilized with pyridine‐based ligands in the ionic liquid [BMIM][BF4] (RhNPs‐I to III) were synthesized from the organometallic precursor [Rh(µ‐OMe)COD]2 under dihydrogen pressure. The pyridine‐stabilized RhNPs showed smaller size compared to the ligand free RhNPs‐V and presented higher activity and selectivity in the hydrogenation of acetophenone to 1‐phenylethanol. In the case of pyridine‐capped RhNPs‐I, the system was reused for several runs without loss of activity and selectivity. Nitrobenzene was reduced to aniline with dihydrogen in the presence of RhNPs‐I with moderate activity. When the hydrogen source was formic acid‐Et3N azeotrope (transfer hydrogenation) the reaction was completed within minutes with high selectivity. Under transfer hydrogenation conditions, it was possible to apply the catalytic system RhNPs‐I in multistep processes for the generation of substituted arylic amines through the reductive N‐alkylation of nitrobenzene and benzaldehyde; and the synthesis of substituted pyrroles through the nitroarene reduction/Paal‐Knorr condensation.

Catalytic Chemoselective and Stereoselective Semihydrogenation of Alkynes to E-Alkenes Using the Combination of Pd Catalyst and ZnI2

An efficient E-selective semihydrogenation of internal alkynes was developed under low dihydrogen pressure and low reaction temperature from commercially available reagents: Cl2Pd(PPh3)2, Zn0, and ZnI2. Kinetic studies and control experiments underline the significant role of ZnI2 in this process under H2 atmosphere, establishing that the transformation involves syn-hydrogenation followed by isomerization. This simple and easy-to-handle system provides a route to E-alkenes under mild conditions.

P-Stereogenic Phosphines for the Stabilisation of Metal Nanoparticles. A Surface State Study

Palladium and ruthenium nanoparticles have been prepared following the organometallic precursor decomposition methodology, under dihydrogen pressure and in the presence of borane protected P-stereogenic phosphines. NMR (Nuclear Magnetic Resonance) monitoring of the corresponding syntheses has permitted to determine the optimal metal/ligand ratio for leading to small and well-dispersed nanoparticles. Exchange ligand reactions of the as-prepared materials have proven the strong interaction of the phosphines with the metal surface; only oxidative treatment using hydrogen peroxide could release the phosphine-based stabiliser from the metal surface. Pd and Ru nanoparticles have been evaluated in hydrogenation reactions, confirming the robustness of the stabilisers, which selectively permitted the hydrogenation of exocyclic C=C bonds, preventing the coordination of the aromatic rings and as a result, their hydrogenation.

Extending the Scope of the B(C6 F5 )3 -Catalyzed C=N Bond Reduction: Hydrogenation of Oxime Ethers and Hydrazones.

The B(C6 F5 )3 -catalyzed hydrogenation is applied to aldoxime triisopropylsilyl ethers and hydrazones bearing an easily removable phthaloyl protective group. The CN reduction of aldehyde-derived substrates (oxime ethers and hydrazones) is enabled by using 1,4-dioxane as the solvent known to participate as the Lewis-basic component in FLP-type heterolytic dihydrogen splitting. More basic ketone-derived hydrazones act as Lewis bases themselves in the FLP-type dihydrogen activation and are therefore successfully hydrogenated in nondonating toluene. The difference in reactivity between aldehyde- and ketone-derived substrates is also reflected in the required catalyst loading and dihydrogen pressure.

ChemInform Abstract: B(C6F5)3‐Catalyzed Hydrogenation of Oxime Ethers Without Cleavage of the N—O Bond.

The hydrogenation of oximes and oxime ethers is usually hampered by N-O bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C6F5)3 is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.

Formation of a C-C double bond from two aliphatic carbons. Multiple C-H activations in an iridium pincer complex.

The search for novel, atom-economic methods for the formation of C-C bonds is of crucial importance in synthetic chemistry. Especially attractive are reactions where C-C bonds are formed through C-H activation, but the coupling of unactivated, alkane-type Csp3 -H bonds remains an unsolved challenge. Here, we report iridium-mediated intramolecular coupling reactions involving up to four unactivated Csp3 -H bonds to give carbon-carbon double bonds under the extrusion of dihydrogen. The reaction described herein is completely reversible and the direction can be controlled by altering the reaction conditions. With a hydrogen acceptor present a C-C double bond is formed, while reacting under dihydrogen pressure leads to the reverse process, with some of the steps representing net Csp3 -Csp3 bond cleavage. Mechanistic investigations revealed a conceptually-novel overall reactivity pattern where insertion or deinsertion of an Ir carbene moiety, formed via double C-H activation, into an Ir-C bond is responsible for the key C-C bond formation and cleavage steps.

B(C6F5)3-catalyzed hydrogenation of oxime ethers without cleavage of the N-O bond.

The hydrogenation of oximes and oxime ethers is usually hampered by N-O bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C6F5)3 is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.

Heterolytic activation of dihydrogen by platinum and palladium complexes

Wide bite angle diphosphine ligands were used to prepare [(diphosphine)M(2-(diphenylphosphino)pyridine)](2+) complexes (M = Pd, Pt). Except for the ligand with the largest bite angle, 2-(diphenylphosphino)pyridine coordinates in a bidentate mode leading to bis-chelate complexes. In the case of Xantphos (9,9-dimethyl-4,5-bis(diphenylphosphino)-xanthene, βn = 111°) two types of complexes are formed, in which 2-(diphenylphosphino)pyridine coordinates in a mono- or bidentate fashion, respectively. The crystal structures of three of the Pt complexes were determined. The X-ray crystal structure of [(Xantphos)-Pt(2-(diphenylphosphino)pyridine)](2+) shows that Xantphos coordinates in a tridentate P,O,P fashion. Under dihydrogen pressure, the pyridyl moiety in the platinum complexes can de-coordinate to provide a vacant coordination site at the metal center. Furthermore it can act as an internal base to assist the heterolytic cleavage of dihydrogen. The reaction yields a platinum hydride with a protonated pyridine moiety in close proximity to one another. The structure as well as the reactivity of the complexes towards dihydrogen is governed by the steric requirements of the diphosphines. The crystal structure of [(dppf)PtH(2-(diphenylphosphino)pyridinium)](OTf)2 has been determined. Palladium complexes containing DPEphos or Xantphos decompose under dihydrogen pressure. In the case of dppf slow heterolytic splitting of dihydrogen occurs to form the hydride complex [(dppf)PdH(2-(diphenylphosphino)pyridinium)](OTf)2 which contains a protonated 2-(diphenylphosphino)pyridine ligand. In solution, this compound slowly undergoes P-C bond cleavage of the 2-(diphenylphosphino)pyridine ligand to form [(dppf)Pd(PHPh2)(η(1)-C5H4NH)](OTf)2. When the 6-methyl-2-pyridyldiphenylphosphine ligand is used, the reaction of the palladium complex with dihydrogen is very fast and the hydride complex immediately rearranges to the diphenylphosphino compound resulting from P-C bond cleavage.

Copper nanoparticles generated in situ in imidazolium based ionic liquids

Copper nanoparticles (CuNPs) have been successfully synthesized in imidazolium based ionic liquid (IL) under mild conditions. In this self-ordered medium, particles with narrow size distribution can be formed. No ligand is required to stabilize the suspension, which can be directly used to coat technological substrates. Several parameters are examined: the nature of IL, stirring, dihydrogen pressure and temperature. Under optimal conditions, well dispersed, zero-valent CuNPs of 4.5nm with narrow size distribution are obtained. These stable suspension of Cu(0)NPs is a promising starting material to form conformal copper seed layers in high aspect ratio interconnect structures.

Application of a Supramolecular‐Ligand Library for the Automated Search for Catalysts for the Asymmetric Hydrogenation of Industrially Relevant Substrates

A procedure is described for the automated screening and lead optimization of a supramolecular-ligand library for the rhodium-catalyzed asymmetric hydrogenation of five challenging substrates relevant to industry. Each catalyst is (self-) assembled from two urea-functionalized ligands and a transition-metal center through hydrogen-bonding interactions. The modular ligand structure consists of three distinctive fragments: the urea binding motif, the spacer, and the ligand backbone, which carries the phosphorus donor atom. The building blocks for the ligand synthesis are widely available on a commercial basis, thus enabling access to a large number of ligands of high structural diversity. The simple synthetic steps enabled the scale-up of the ligand synthesis to multigram quantities. For the catalyst screening, a library of twelve new chiral ligands was prepared that comprised substantial variation in electronic and steric properties. The automated procedures employed ensured the fast catalyst assembly, screening, and direct acquisition of samples for analysis. It appeared that the most selective catalyst was different for every substrate investigated and that small variations in the building blocks had a major impact on the catalyst performance. For two substrates, a catalyst was found that provided the product with outstanding enantioselectivity. The subsequent automated optimization of these two leads showed that an increase of catalyst loading, dihydrogen pressure, and temperature had a positive effect on the catalyst activity without affecting the catalyst selectivity.

Ultrasmall iron nanoparticles: Effect of size reduction on anisotropy and magnetization

Stable iron nanoparticles have been synthesised by the decomposition of {Fe(N[Si(CH3)3]2)2}2 under dihydrogen pressure. Those conditions lead to a system of monodisperse and metallic nanoparticles which diameter is less than 2nm and stabilized by HN[Si(CH3)3]2. The magnetization is found to be MS=1.92μB∕at., i.e., 10% lower than the bulk value. The Mössbauer spectrum is fitted by two contributions of metallic iron. The magnetic anisotropy energy constant increases up to 5.2×105J∕m3, i.e., ten times the bulk one.

From inconsistent results to high speed hydrosilylation

After noting that the presence of dihydrogen, generated in situ from the partial hydrolysis of a silane with residual water, significantly enhances the rate of the rhodium-catalysed hydrosilylation of acetophenone, we developed a high speed hydrosilylation reaction under dihydrogen pressure.

Synthesis and catalytic activity of perfluoroalkylated pyridine–palladium(II) complex toward olefin hydrogenation in scCO 2 and conventional organic solvents

The perfluoroalkylated pyridine, ( o-COOCH 2(CF 2) 7CF 3-NC 5H 4), has been synthesized in two step from nicotinic acid and then the title compound, [Pd( o-COOCH 2-(CF 2) 7CF 3-NC 5H 4) 2(OAc) 2], obtained by reaction with Pd(OAc) 2. The catalytic activity of the Pd(II) complex examined for hydrogenation of styrene, 1-octene, t-2-octene and cyclohexene for both in supercritical carbon dioxide and in organic solvents. The effect of temperature and dihydrogen pressure, as well as the influence of the palladium and substrate concentrations have been studied ( T = 320–353 K, p(H 2) 6–30 bar, [Pd] = (1.12–2.62) × 10 −5 M, [Styrene] = (8.70–34.8) × 10 −3 M). The experimentally determined rate law is r = k cat [Pd] a [Styrene] b p(H 2) c ( a = 1, b = 1 and c = 0.14) where k cat = 4.4 × 10 −2 (mol −1 l s −1) at 320 K. Activation parameters have been calculated at 320 K. Supercritical carbon dioxide is a more effective, green reaction medium for olefin hydrogenation compared with conventional organic solvents.

Isotopic transient analysis of ammonia synthesis over Ru/MgO catalysts promoted by cesium, barium, or lanthanum

The global and intrinsic kinetics of ammonia synthesis over ruthenium supported on MgO (∼2 wt% Ru) and promoted with Cs, Ba, or La were examined at 3 atm and various dihydrogen pressures with steady-state and isotopic transient measurements. Steady-state, global measurements revealed that Cs–Ru/MgO was strongly inhibited by dihydrogen whereas Ba–Ru/MgO and La–Ru/MgO were weakly inhibited by the reactant. However, the isotopic transient measurements showed that the intrinsic turnover frequency was a weak positive function of dihydrogen pressure regardless of the added base. Promotion by these compounds appeared to be electronic in nature because their presence increased the intrinsic turnover frequency of an active site, with Cs being the most effective. However, the coverage of nitrogen-containing intermediates was greater on Ba- and La-promoted catalysts than on Cs–Ru/MgO at 673 K and stoichiometric conditions. Therefore, the global activity of a promoted catalyst is a competition between inhibition by dihydrogen and enhancement of dinitrogen dissociation. Furthermore, promotion with Cs, Ba, and La introduced a new and highly active class of sites to the Ru/MgO system.

Ruthenium dihydrogen complexes with wide bite angle diphosphines.

The wide bite angle diphosphines homoxantphos (10,11-dihydro-4,5,-bis(diphenylphosphino)dibenzo[b,f]oxepine), sixantphos (4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin), and thixantphos (2,8-dimethyl-4,6-bis(diphenylphosphino)phenoxathiin) were used to prepare cis[MH(2)(diphosphine)(2)] complexes (1a-f) by reaction of [Ru(cod)(cot)] (cod = cyclo-octa-1,5-diene, cot = cyclo-octa-1,3,5-triene) with 2 equiv of the diphosphine under dihydrogen pressure. The electronic properties of the thixantphos ligand were varied. Complexes 1a-f can be protonated with HBF(4) or CF(3)COOH to yield hydrido(dihydrogen) complexes cis[MH(H(2))(diphosphine)(2)](+) (2a-f), which were characterized by VT (variable temperature) NMR and T(1) measurements. These complexes show fast hydrogen atom exchange between the eta(2)-H(2) and the terminal hydride at all temperatures studied. They are thermally unstable toward dihydrogen loss yielding the cationic monohydride complexes cis[MH(diphosphine)(2)](+) (3a-f). Coordination of the eta(2)-H(2) is dominated by sigma --> d donation, and hence, the H-H distance is hardly influenced by the electronic properties of the ligands.

Electrochemical storage of energy in carbon nanotubes and nanostructured carbons

Possibilities of electrochemical energy conversion using carbon nanotubes and related materials in various systems, such as lithium batteries, supercapacitors, hydrogen storage, are considered. It is shown that for these applications the electrochemical properties of multiwalled (MWNTs) and single walled (SWNTs) nanotubes are essentially dominated by their mesoporous character. During lithium insertion into nanotubular materials a high irreversible capacity C irr (from 460 to 1080 mAh/g) has been observed after the first cycle with a tendency to further decomposition of electrolyte with cycling. Penetration of solvated lithium ions in the accessible mesopores is at the origin of this phenomenon; an almost linear dependence has been found between the mesopore volume and C irr. Reversible capacity for lithium insertion C rev ranged between 220 and 780 mAh/g; however, a great divergence (hysteresis) between insertion and extraction characteristics was observed independently on the kind of nanotubes and oxygen content. Amount of lithium stored by electrostatic attraction is negligible in comparison to real redox reactions which for thermodynamic reasons present linear variation of potential, especially during deinsertion (pseudocapacitive effects). During positive polarization, i.e., removal of lithium, resistivity of the electrode also gradually increases. Due to the open network of mesopores formed by the nanotubes entanglement, and consequently an easily accessible electrode–electrolyte interface, nanotubular materials are quite adapted for supercapacitor electrodes in various electrolytic solutions. High values of capacitance (80 F/g) have been obtained in 6 M KOH for materials with a surface area of only ca. 430 m 2/g. Capacitance values have been enhanced either by additional oxygenated functionalisation of nanotubes (130 F/g) or by conducting polypyrrole (PPy) electrodeposition where the maximum values reached 170 F/g. The next domain of energy storage in the carbon nanostructures is the accumulation of hydrogen by the electrochemical decomposition of aqueous alkaline medium on a negatively polarized carbon electrode in ambient conditions. For SWNTs only moderate values (below 0.5 wt.% of H 2) have been found, while for activated carbons with highly developed surface area of 1500 m 2/g, the amount of reversibly sorbed hydrogen was ca. 2 wt.%, noticeably larger than under dihydrogen pressure (only 0.4 wt.% for the same material at 70 bar and 273 K). The enhancement observed for the activated carbon is interpreted by the formation of nascent hydrogen during water reduction which penetrates easily in the available carbon nanopores. The values obtained by this method are comparable to those of metallic alloys, such as LaNi 5 for example.

Kinetics of the hydrogenation of 1-octene catalyzed by [Ni( o-MeO-dppp)(OAc) 2

The kinetics of the nickel-catalyzed hydrogenation of 1-octene have been studied using nickel(II) acetate in combination with the ligand 1,3-bis(di( o-methoxyphenyl)phosphanyl)propane ( o-MeO-dppp). The effects of temperature and dihydrogen pressure, as well as the influence of the nickel and substrate concentrations have been studied ( T=30–60°C, p(H 2)=30–60 bar, [Ni]=0.35–2.81 mM, [1-octene]=0.98–3.91 M, ligand/Ni=1.1 in a mixture of dichloromethane and methanol). The kinetic study results in the surprisingly simple rate law r= k cat[Ni][1-octene] p(H 2). The kinetic data are consistent with a mechanism in which the terminating hydrogenolysis step of the nickel–alkyl complex is the rate-determining step of the catalytic cycle with all preceding steps at equilibrium. The first-order dependence on the substrate is confirmed by the results with other substrates, as the hydrogenation rate strongly depends on the nature of the substrate.