Silica-supported Tantalum Hydride Research Articles

Mechanistic Insights into Alkane Metathesis Catalyzed by Silica-Supported Tantalum Hydrides: A DFT Study.

Alkane metathesis transforms small alkanes into their higher and lower homologues. The reaction is catalyzed by either supported d0 metal hydrides (M = Ta, W) or d0 alkyl alkylidene complexes (M = Ta, Mo, W, Re). For the silica-supported tantalum hydrides, several reaction mechanisms have been proposed. We performed DFT-D3 calculations to analyze the viability of the proposed pathways and compare them with alkane hydrogenolysis, which is a competitive process observed at the early stages of the reaction. The results show that the reaction mechanisms for alkane metathesis and for alkane hydrogenolysis present similar energetics, and this is consistent with the fact that the process taking place depends on the concentrations of the initial reactants. Overall, a modified version of the so-called one-site mechanism that involves alkyl alkylidene intermediates appears to be more likely and consistent with experiments. According to this proposal, tantalum hydrides are precursors of the alkyl alkylidene active species. During precursor activation, H2 is released and this allows alkane hydrogenolysis to occur. In contrast, the catalytic cycle implies only the reaction with alkane molecules in excess and does not form H2. Thus, the activity for alkane hydrogenolysis decreases. The catalytic cycle proposed here implies three stages: (i) β-H elimination from the alkyl ligand, liberating ethene, (ii) alkene cross-metathesis, allowing olefin substituent exchange, and (iii) formation of the final products and alkyl alkylidene regeneration by olefin insertion and three successive 1,2-CH insertions to the alkylidene followed by α abstraction. These results relate the reactivity of silica-supported hydrides with that of the alkyl alkylidene complexes, the other common catalyst for alkane metathesis.

Hydrazine N-N Bond Cleavage over Silica-Supported Tantalum-Hydrides.

Hydrazine reacts with silica-supported tantalum-hydrides [(≡SiO)2TaHx] (x = 1, 3), 1, under mild conditions (100 °C). The IR in situ monitoring of the reaction with N2H4 or (15)N2H4, and the solid-state MAS NMR spectra of the fully (15)N labeled compounds (CP (15)N, (1)H-(15)N HETCOR, (1)H-(1)H double-quantum, and (1)H-(1)H triple-quantum spectra) were used to identify stable intermediates and products. DFT calculations were used for determining the reaction pathway and calculating the (15)N and (1)H NMR chemical shifts. Combining the experimental and computational studies led to the following results. At room temperature, only hydrazine adducts, 1-N2H4, are formed. Upon heating at 100 °C, the hydrazine adducts are converted to several species among which [(≡SiO)2Ta(═NH)(NH2)], 2, [(≡SiO)2TaH(NH2)2], 3, and [(≡SiO)2TaH2(NH-NH2)], 4, were identified. The final product 2 is also formed in the reaction of N2 with the same silica-supported tantalum-hydride complexes, and the species identified as 3 and 4 had been previously suggested by DFT studies as intermediates on the reaction pathway for N-N cleavage in N2. The present computational studies (cluster models with M06 functional complemented by selected calculations with periodic calculations) show that 2 is formed via 3 and 4, with either N2 or N2H4. This strengthens the previous proposal of the existence of 3 and 4 as intermediates in the reaction of N2 with the tantalum-hydrides. However, the reaction of N2 does not imply the formation of N2H4 or its hydrazido monoanionic or dianionic ligand as an intermediate. For this reason, this study informs both on the similarities and differences of the reaction pathways involving N2 and N2H4 with tantalum-hydrides.

Mechanistic aspects of dinitrogen cleavage and hydrogenation to produce ammonia in catalysis and organometallic chemistry: relevance of metal hydride bonds and dihydrogen

Dinitrogen cleavage and hydrogenation by transition-metal centers to produce ammonia is central in industry and in Nature. After an introductory section on the thermodynamic and kinetic challenges linked to N2 splitting, this tutorial review discusses three major classes of transition-metal systems (homogeneous, heterogeneous and biological) capable of achieving dissociation and hydrogenation of dinitrogen. Molecular complexes, solid-state Haber-Bosch catalytic systems, silica-supported tantalum hydrides and nitrogenase will be discussed. Emphasis is focused on the reaction mechanisms operating in the process of dissociation and hydrogenation of dinitrogen, and in particular on the key role played by metal hydride bonds and by dihydrogen in such reactions.

Successive Heterolytic Cleavages of H2Achieve N2Splitting on Silica-Supported Tantalum Hydrides: A DFT Proposed Mechanism

DFT(B3PW91) calculations have been carried out to propose a pathway for the N(2) cleavage by H(2) in the presence of silica-supported tantalum hydride complexes [(≡SiO)(2)TaH(x)] that forms [(≡SiO)(2)Ta(NH)(NH(2))] (Science 2007, 317, 1056). The calculations, performed on the cluster models {μ-O[(HO)(2)SiO](2)}TaH(1) and {μ-O[(HO)(2)SiO](2)}TaH(3), labelled as (≡SiO)(2)TaH(x) (x = 1, 3), show that the direct hydride transfers to coordinated N-based ligands in (≡SiO)(2)TaH(η(2)-N(2)) and (≡SiO)(2)TaH(η(2)-HNNH) have high energy barrier barriers. These high energy barriers are due in part to a lack of energetically accessible empty orbitals in the negatively charged N-based ligands. It is shown that a succession of proton transfers and reduction steps (hydride transfer or 2 electron reduction by way of dihydride reductive coupling) to the nitrogen-based ligands leads to more energetically accessible pathways. These proton transfers, which occur by way of heterolytic activation of H(2), increase the electrophilicity of the resulting ligand (diazenido, N(2)H(-), and hydrazido, NHNH(2)(-), respectively) that can thus accept a hydride with a moderate energy barrier. In the case of (≡SiO)(2)TaH(η(2)-HNNH), the H(2) molecule that is adding across the Ta-N bond is released after the hydride transfer step by heterolytic elimination from (≡SiO)(2)TaH(NH(2))(2), suggesting that dihydrogen has a key role in assisting the final steps of the reaction without itself being consumed in the process. This partly accounts for the experimental observation that the addition of H(2) is needed to convert an intermediate, identified as a diazenido complex [(≡SiO)(2)TaH(η(2)-HNNH)] from its ν(N-H) stretching frequency of 3400 cm(-1), to the final product. Throughout the proposed mechanism, the tantalum remains in its preferred high oxidation state and avoids redox-type reactions, which are more energetically demanding.

Poisoning Experiments Aimed at Discriminating Active and Less-Active Sites of Silica-Supported Tantalum Hydride for Alkane Metathesis

Only 50% of the silica-supported tantalum hydride sites are active in the metathesis of propane. Indeed, more than 45% of the tantalum hydride can be eliminated by a selective oxygen poisoning of inactive sites with no significant decrease in the global turnover. Conversely, cyclopentane induces no such selective poisoning. Hence, the active tantalum hydride sites that show greater resistance to oxygen poisoning correspond to the ν Ta-H bands of higher wavenumbers, particularly that at 1860 cm -1 . These active tantalum hydride sites should correspond to tris- or monohydride species relatively far from silica surface oxygen atoms.

Metathesis of Alkanes and Related Reactions

The transformation of alkanes remains a difficult challenge because of the relative inertness of the C-H and C-C bonds. The rewards for asserting synthetic control over unfunctionalized, saturated hydrocarbons are considerable, however, because converting short alkanes into longer chain analogues is usually a value-adding process. Alkane metathesis is a novel catalytic and direct transformation of two molecules of a given alkane into its lower and higher homologues; moreover, the process proceeds at relatively low temperature (ambient conditions or higher). It was discovered through the use of a silica-supported tantalum hydride, ([triple bond]SiO)(2)TaH, a multifunctional catalyst with a single site of action. This reaction completes the story of the metathesis reactions discovered over the past 40 years: olefin metathesis, alkyne metathesis, and ene-yne cyclizations. In this Account, we examine the fundamental mechanistic aspects of alkane metathesis as well as the novel reactions that have been derived from its study. The silica-supported tantalum hydride catalyst was developed as the result of systematic and meticulous studies of the interaction between oxide supports and organometallic complexes, a field of study denoted surface organometallic chemistry (SOMC). A careful examination of this surface-supported tantalum hydride led to the later discovery of alumina-supported tungsten hydride, W(H)(3)/Al(2)O(3), which proved to be an even better catalyst for alkane metathesis. Supported tantalum and tungsten hydrides are highly unsaturated, electron-deficient species that are very reactive toward the C-H and C-C bonds of alkanes. They show a great versatility in various other reactions, such as cross-metathesis between methane and alkanes, cross-metathesis between toluene and ethane, or even methane nonoxidative coupling. Moreover, tungsten hydride exhibits a specific ability in the transformation of isobutane into 2,3-dimethylbutane as well as in the metathesis of olefins or the selective transformation of ethylene into propylene. Alkane metathesis represents a powerful tool for making progress in a variety of areas, perhaps most notably in the petroleum and petrochemical fields. Modern civilization is currently confronting a host of problems that relate to energy production and its effects on the environment, and judicious application of alkane metathesis to the processing of fuels such as crude oil and natural gas may well afford solutions to these difficulties.

Non-Oxidative Coupling Reaction of Methane to Ethane and Hydrogen Catalyzed by the Silica-Supported Tantalum Hydride: (≡SiO)2Ta−H

Silica-supported tantalum hydride, (SiO)2Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl.

BREAKING UP IS HARD TO DO

THE NARCISSISTIC EMBRACE of two nitrogen atoms makes N 2 one of the most unreactive diatomic molecules in the natural world. Now,French researchers have found new way to split up this stalwart, self-centered couple, using single tantalum atom as the activator ( Science 2007,317,1056). Chemists Jean-Marie Basset, Alessandra Quadrelli, and Mostafa Taoufik at the University of Lyon and their colleagues in Lyon and at the University of Montpellier report the stoichiometric cleavage and subsequent hydrogenation of N 2 on silica-supported tantalum hydride,using N 2 and H 2 gas as starting materials. Much of the activation to make ammonia in the past has relied on multiple-metal-atom surface, Basset says. We can activate dinitrogen molecule with just one metal atom. This new mechanism for splitting triply bonded N 2 is a significant and exciting advance, comments Paul J. Chirik, chemist at Cornell University who works on dinitrogen functionalization. Findings such as these ...

Primary Products and Mechanistic Considerations in Alkane Metathesis

Alkane metathesis, a reaction catalyzed by the silica-supported tantalum hydride [(SiO)2Ta-H], 1, which transforms acyclic alkanes into their higher and lower homologues, was reported in 1997. New studies conducted in a continuous flow reactor in the case of propane indicate that, by varying the contact time, hydrogen and olefins are primary products. This crucial observation, as well as the known properties of tantalum alkyls to perform alpha-H or beta-H eliminations, supports the proposition of a new mechanism involving metallacyclobutane intermediates just like in olefin metathesis. The observed selectivities for linear and branched Cn+1 and Cn+2 products as well as the linear/branched ratio can be well-explained on the basis of the minimization of steric interactions between 1,2- or 1,3-substituents in the various tantallacyclobutane intermediates or during their formation. Hydrogen plays a specific role in the cleavage of metal alkyls to complete the catalytic cycle.

Cross-metathesis between ethane and toluene catalyzed by [(SiO)2TaH]: the first example of a cross-metathesis reaction between an alkane and an aromatic

The silica-supported tantalum hydride [([triple bond, length as m-dash]SiO)(2)TaH] catalyzes at moderate temperature (150-250 degree C) the cross-metathesis reaction between toluene and ethane, to form mainly ethylbenzene and xylenes.

Ethane hydrogenolysis on silica-supported tantalum hydride. Quantum-chemical study

The structures of the (H4Si2O5)O2Ta(R, RR"R″, RR‴) clusters (R, R", R″ = H, Me, Et, Pr; R‴ = CH2), which model the basic structural units of the catalytic cycle of ethane hydrogenolysis on silica-supported tantalum hydride, were studied by the density functional theory (B3LYP) and the perturbation theory (MP2). The possible structure of active sites was proposed based on comparison of experimental results with calculated data. Ethane hydrogenolysis and metathesis proceed by a mechanism involving the formation of ethylene π-complexes and carbenium derivatives of tantalum as intermediates.

Activation of ethane in the metathesis reaction on silica-supported tantalum hydride: a quantum-chemical study

The structures of the (H4Si2O5)O2TaR and (H4Si2O5)O2TaRH2 clusters (R = H, Me, or Et), which model the main structural units of the catalytic cycle of the ethane metathesis on silica-supported tantalum hydride, were studied using the density functional theory at the B3LYP level. Even at high temperatures, activation of ethane cannot proceed via a two-step mechanism involving the oxidative addition of ethane and reductive elimination of hydrogen to form pentavalent tantalum compounds.

Synthesis, Characterization, and Reactivity of the Highly Unsaturated Silica-Supported Trisiloxy Tantalum: (⋮SiO)3Ta(III)

The silica-supported tantalum hydride, (⋮SiO)2Ta(III)H, is transformed under hydrogen by thermal treatment up to 500 °C into a new well-defined surface species, the trisiloxy tantalum (⋮SiO)3Ta(III). The Ta LIII-edge EXAFS gives a first coordination sphere of three oxygen atoms at 1.908 A. The mechanism of formation of (⋮SiO)3Ta(III) has been studied by IR: it involves a hydride transfer from tantalum to silicon and a siloxy transfer from silicon to tantalum. This surface reconstruction occurs by opening of the ⋮Si−O−Si⋮ bridge of silica probably in the vicinity of the tantalum hydride. (⋮SiO)3Ta(III) reacts with pyridine and PMe3 to give 1:1 adducts. It also reacts with ethanol and water by an oxidation process to give molecular hydrogen and the surface compounds (⋮SiO)3Ta(V)(OEt)2 and (⋮SiO)3Ta(V)(OH)2. It still reacts with molecular oxygen to give (⋮SiO)3Ta(V)(O) by an unknown mechanism. Although (⋮SiO)3Ta(III) exhibits an extreme electron deficiency and redox properties due to its d2 configuration, t...

Synthesis, Characterization, and Properties of Silica-Supported Trimethylphosphine Disiloxy Tantalum Hydride, (⋮SiO)2TaH(PMe3)

The silica-supported tantalum hydride (⋮SiO)2TaH reacts with PMe3 to form a new well-defined surface species, the trimethylphosphine disiloxy tantalum hydride (⋮SiO)2TaH(PMe3). The Ta LIII-edge EXAFS gives a first coordination sphere of ca. two oxygen atoms at 1.955 Å and one phosphorus atom at 2.615 Å. The presence of the phosphine on the metal center cancels the catalytic activity of the tantalum hydride toward metathesis of acyclic alkanes but affords the oligomerization of ethylene into 1-butene and higher oligomers.

Low-Temperature Hydrogenolysis of Alkanes Catalyzed by a Silica-Supported Tantalum Hydride Complex, and Evidence for a Mechanistic Switch from Group IV to Group V Metal Surface Hydride Complexes

C-C-Bindungsspaltung nach einem neuartigen Mechanismus (wahrscheinlich über σ-Bindungsmetathese) ist möglich mit einem Tantalhydrid auf Kieselgel, (≡SiO)2Ta−H 1 (siehe Schema). Dieser Komplex, mit dem auch Alkanmetathesen durchgeführt werden können, katalysiert die Spaltung von C-C-Bindungen in Alkanen unter milden Bedingungen.

Low-Temperature Hydrogenolysis of Alkanes Catalyzed by a Silica-Supported Tantalum Hydride Complex, and Evidence for a Mechanistic Switch from Group IV to Group V Metal Surface Hydride Complexes

Low-Temperature Hydrogenolysis of Alkanes Catalyzed by a Silica-Supported Tantalum Hydride Complex, and Evidence for a Mechanistic Switch from Group IV to Group V Metal Surface Hydride Complexes

H/D exchange between CH4 and CD4 catalysed by a silica supported tantalum hydride, (SiO)2Ta–H

The silica supported tantalum hydride (SiO)2Ta–H 1, catalyses the H/D exchange reaction between CH4 and CD4 at 150 °C producing the statistical distribution of all methane isotopomers.

Metathesis of Alkanes: Evidence for Degenerate Metathesis of Ethane over a Silica-Supported Tantalum Hydride Prepared by Surface Organometallic Chemistry.

The mutual exchange of methyl groups of ethane molecules is catalyzed by a silica-supported tantalum hydride surface complex (see the schematic representation of the transition states). This process, which resembles degenerate olefin metathesis, is five times faster than the productive alkane metathesis.

Metathesis of Alkanes: Evidence for Degenerate Metathesis of Ethane over a Silica-Supported Tantalum Hydride Prepared by Surface Organometallic Chemistry

Metathesis of Alkanes: Evidence for Degenerate Metathesis of Ethane over a Silica-Supported Tantalum Hydride Prepared by Surface Organometallic Chemistry

Activation and functionalisation of the C–H bonds of methane and higher alkanes by a silica-supported tantalum hydride complex

Silica-supported tantalum hydride activates at low temperature the C–H and the C–D bonds of cyclooctane and CD4, respectively, to form the corresponding cyclooctyl and perdeuteriomethyl-tantalum surface complexes; these complexes are transformed under molecular oxygen into the corresponding tantalum-alkoxy derivatives which with acetic acid give rise to the corresponding alkylacetates.