Reactivity of a Silica-Supported Mo Alkylidene Catalyst toward Alkanes: A DFT Study on the Metathesis of Propane

The metathesis of alkanes is a process in which a given alkane is transformed into higher and lower homologues. Here, we carried out DFT calculations in order to get insights into the most favorable reaction pathway for the metathesis of propane into mainly ethane and butane catalyzed by a silica-supported molybdenum alkylidene bearing an imido ligand at 150 °C. The overall catalytic process is divided into two stages, precursor activation and catalytic cycle, and both of them consist of the same types of reactions, (i) ligand exchange, (ii) proton transfer between two α-carbons, and (iii) ligand rearrangement, which in turn consists of several steps, such as β-H elimination, alkene cross-metathesis, and alkene insertion. Our results suggest that the formal ligand exchange reaction with propane proceeds through a dissociative mechanism with the formation of a high-energy molybdenum alkylidyne species. The calculated energetics at 150 °C indicates that the active species is a molybdenum propylidene species that is formed with an overall Gibbs activation barrier of 39.4 kcal mol^–1. The catalytic cycle to the main products (ethane and butane) has an energy span of 43 kcal mol^–1, whereas the cycle for the production of minor products (methane and pentane) has a much higher energy span, in agreement with experiments. These data suggest that the catalytic cycle is the rate-determining stage in the whole process and thus the precursor activation should be faster. The results obtained here help to rationalize the chemical reactivity of supported molybdenum alkylidene catalysts toward alkanes

Reactivity of a Silica-Supported Mo Alkylidene Catalyst toward Alkanes: A DFT Study on the Metathesis of Propane

The metathesis of alkanes is a process in which a given alkane is transformed into higher and lower homologues. Here, we carried out DFT calculations in order to get insights into the most favorable reaction pathway for the metathesis of propane into mainly ethane and butane catalyzed by a silica-supported molybdenum alkylidene bearing an imido ligand at 150 °C. The overall catalytic process is divided into two stages, precursor activation and catalytic cycle, and both of them consist of the same types of reactions, (i) ligand exchange, (ii) proton transfer between two α-carbons, and (iii) ligand rearrangement, which in turn consists of several steps, such as β-H elimination, alkene cross-metathesis, and alkene insertion. Our results suggest that the formal ligand exchange reaction with propane proceeds through a dissociative mechanism with the formation of a high-energy molybdenum alkylidyne species. The calculated energetics at 150 °C indicates that the active species is a molybdenum propylidene species that is formed with an overall Gibbs activation barrier of 39.4 kcal mol^–1. The catalytic cycle to the main products (ethane and butane) has an energy span of 43 kcal mol^–1, whereas the cycle for the production of minor products (methane and pentane) has a much higher energy span, in agreement with experiments. These data suggest that the catalytic cycle is the rate-determining stage in the whole process and thus the precursor activation should be faster. The results obtained here help to rationalize the chemical reactivity of supported molybdenum alkylidene catalysts toward alkanes

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 <i>one-site</i> 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

Adsorption of Nitrate and Bicarbonate on Fe-(Hydr)oxide

In this work, we used density functional theory calculations to study the resulting complexes of adsorption and of inner- and outer-sphere adsorption-like of bicarbonate and nitrate over Fe-(hydr)­oxide surfaces using acidic, neutral, and basic simulated pH conditions. High-spin states that follow the 5<i>N</i> + 1 (<i>N</i> is the number of Fe atoms, each having five unpaired electrons) rule are preferred. Monodentate mononuclear (MM₁) surface complexes are shown to lead to the most favorable thermodynamic adsorption for both bicarbonate and nitrate with −63.91 and −28.25 kJ/mol, respectively, under neutral conditions. Our results suggest that four types of regular and charged-assisted hydrogen bonds are involved in the adsorption process; all of them can be classified as closed-shell (long-range or ionic). The formal charges induce unusually short and strong hydrogen bonds. The ability of high multiplicity states of Fe clusters to adsorb oxyanions in solvated environments arises from orbital interactions: the 4s virtual orbitals in Fe have a large affinity for the 2p-type electron pairs of oxygens

A Detailed Look at the Reaction Mechanisms of Substituted Carbenes with Water

Two competitive reaction mechanisms for the gas-phase chemical transformation of singlet chlorocarbene into chloromethanol in the presence of one and two water molecules are examined in detail. An analysis of bond orders and bond order derivatives as well as of properties of bond critical points in the electron densities along the intrinsic reaction coordinates (IRCs for intermediates → transition state (TS) → products) suggests that, from the perspective of bond breaking/formation, both reactions should be considered to be highly nonsynchronous, concerted processes. Both transition states are early, resembling the intermediates, yielding rate constants whose magnitudes are mostly influenced by structural changes and to a lesser degree by bond breaking/formation. For the case of one water molecule, most of the energy in the reactants region of the IRC is used for structural changes, while the transition state region encompasses the majority of electron activity, except for the formation of the C–O bond, which extends well into the products region. In the case of two water molecules, very little electron flux and comparatively less work required for structural changes is noticed in the reactants region, leading to an earlier transition state and therefore to a smaller activation energy and to a larger rate constant. This, together with evidence gathered from other sources, allows us to provide plausible explanations for the observed difference in rate constants