Mechanism of Selective Ammoxidation of Propene to Acrylonitrile on Bismuth Molybdates from Quantum Mechanical Calculations

In order to understand the mechanism for selective ammoxidation of propene to acrylonitrile by bismuth molybdates, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for the various steps involved in converting the allyl-activated intermediate to acrylonitrile over molybdenum oxide (using a Mo_3O_9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH_3 in the feed. We find that the rate-determining step in converting of allyl to acrylonitrile at all feed partial pressures is the second hydrogen abstraction from the nitrogen-bound allyl intermediate (Mo−NH−CH_2−CH═CH_2) to form Mo−NH═CH−CH═CH_2). We find that imido groups (Mo═NH) have two roles: (1) a direct effect on H abstraction barriers, H abstraction by an imido moiety is (~8 kcal/mol) more favorable than abstraction by an oxo moiety (Mo═O), and (2) an indirect effect, the presence of spectator imido groups decreases the H abstraction barriers by an additional ~15 kcal/mol. Therefore, at higher NH_3 pressures (which increases the number of Mo═NH groups), the second H abstraction barrier decreases significantly, in agreement with experimental observations that propene conversion is higher at higher partial pressures of NH_3. At high NH_3 pressures we find that the final hydrogen abstraction has a high barrier [ΔH‡_(fourth-ab) = 31.6 kcal/mol compared to ΔH‡_(second-ab) = 16.4 kcal/mol] due to formation of low Mo oxidation states in the final state. However, we find that reoxidizing the surface prior to the last hydrogen abstraction leads to a significant reduction of this barrier to ΔH‡_(fourth-ab) = 15.9 kcal/mol, so that this step is no longer rate determining. Therefore, we conclude that reoxidation during the reaction is necessary for facile conversion of allyl to acrylonitrile

Mechanism of Ru(II)-Catalyzed Olefin Insertion and C−H Activation from Quantum Chemical Studies

The mechanism of catalytic hydroarylation of olefins by the homogeneous Ru(Tp)(CO)(Ph)(NCCH_3) catalyst recently reported by Gunnoe et al. is characterized using quantum mechanics (density functional theory). The catalytic cycle features two key steps, 1,2-olefin insertion and C−H activation via an unusual mechanism, oxidative hydrogen migration. We find that these two key steps are competitive and that improving the rate of one step is detrimental to the rate of the other. The Ru catalyst has better balance and consequently higher activity than the previously explored Ir-based system

The Mechanism by Which Ionic Liquids Enable Shilov-Type CH Activation in an Oxidizing Medium

Quantum mechanical studies on methane CH activation catalyzed by PtCl_2 in concentrated H_2SO_4 and ionic liquid solution show that the effect of the ionic liquid is to enable Shilov-like chemistry in an oxidizing medium, by solvating the otherwise insoluble PtCl_2(s) in H_2SO_4. Other possible mechanisms have been investigated and discarded

Inaccessibility of β-Hydride Elimination from −OH Functional Groups in Wacker-Type Oxidation

Quantum mechanics calculations (B3LYP and MPW1K density functional theory) on mechanisms relevant to the Wacker process for dehydrogenation of alcohol to ketone show that the commonly accepted mechanism for product formation (β-hydride elimination (BHE) leading to Pd−H formation) is not energetically feasible (36.2 kcal/mol). An alterative pathway involving a five-bodied reductive elimination (RE) leads to an activation enthalpy of 18.8 kcal/mol, which is just half that of the BHE from the −OH group usually assumed for the Wacker process. We find that a water molecule catalyzes both processes, reducing the barrier to 17.2 for RE and 25.0 for BHE, but will not change the relative ordering of the two mechanisms. This suggests that assumptions of BHE mechanisms should be reexamined for cases in which the β atom is not an alkyl group

Thermal decomposition of RDX from reactive molecular dynamics

We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH2N(NO2)]3] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH2N(NO2)]4]. Our simulations show that the equilibrium population of CO and CO2 (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO2 molecules. The equilibrium populations of N2 and H2O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales

Transition state energy decomposition study of acetate-assisted and internal electrophilic substitution C−H bond activation by (acac-O,O)_2Ir(X) complexes (X = CH_3COO, OH)

Chelate-assisted and internal electrophilic substitution type transition states were studied using a DFT-based energy decomposition method. Interaction energies for benzene and methane C−H bond activation by (acac-O,O)_2Ir(X) complexes (X = CH_3COO and OH) were evaluated using the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). A ratio of ~1.5:1 for forward to reverse charge-transfer between (acac-O,O)_2Ir(X) and benzene or methane transition state fragments confirms “ambiphilic” bonding, the result of an interplay between the electrophilic iridium center and the internal base component. This analysis also revealed that polarization effects account for a significant amount of transition state stabilization. The energy penalty to deform reactants into their transition state geometry, distortion energy, was also used to understand the large activation energy difference between six-membered and four-membered acetate-assisted transition states and help explain why these complexes do not activate the methane C−H bond

Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes

The photophysical properties for a series of facial (fac) cyclometalated Ir(III) complexes (fac-Ir(C^N)_3 (C^N = 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), 1-(2,4-difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9′-dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C^N)_2(C^N′) (C^N = ppz or F2ppz and C^N′ = ppy or F2ppy), and fac-Ir(CC′)_3 (C^C′ = 1-phenyl-3-methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77−378 K. Photoluminescent quantum yields (Φ) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy)_3 (Φ = 0.97) is temperature-independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm^−1. The pre-exponential factors for deactivation are large for complexes with C^N ligands (1011−1013 s^−1) and significantly smaller for fac-Ir(pmb)_3 (109 s^−1). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir−N (Ir−C for fac-Ir(pmb)_3) bond rupture leading to a five-coordinate species that has triplet metal-centered (^3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and ^3MC states. The energy level for the ^3MC state is estimated to lie between 21700 and 24000 cm^−1 for the fac-Ir(C^N)_3 complexes and at 28000 cm^−1 for fac-Ir(pmb)_3

Hydrovinylation of Olefins Catalyzed by an Iridium Complex via CH Activation

Olefin dimerizations are typically proposed to proceed via a Cossee−Arlman type migratory mechanism involving relatively electron-rich metal hydrides. We provide experimental evidence and theoretical calculations that show, in contrast, relatively electron-poor O-donor Ir complexes can catalyze the dimerization of olefins via a mechanism that involves olefin CH bond activation and insertion into a metal−vinyl intermediate

Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands

Trends in methane activation have been explored for rhenium-based catalysts in conjunction with bidentate oxygenated ligands of the form (L_1)(L_2)Re(OH)(OH_2) [L_1, L_2 = acac, catechol, glycol]. When placed in acidic media, the equilibrium for this reference catalyst shifts to the protonated forms (L_1)(L_2)Re(OH_2)(OH_2) in almost all cases. In all cases the activation of the reference complex proceeds through a concerted metathesis type transition state, and only one of the 13 reference complexes proceeds with methane activation through a barrier of less than 35 kcal mol-1. Study of the identity complexes (L_1 = L_2) revealed that protonation of the ligand oxygens is unfavorable for acac and catechol, but favorable for glycol; however in only one case is the barrier for methane activation improved by this route. Electron density on the central rhenium is the best predictor for the magnitude of the methane activation barrier; namely, increased electron density (obtained by considering lower oxidation states) on the metal leads to lower barriers. Lower oxidation states form weaker Re−O bonds, which increase lability of the leaving groups and decrease the barrier to proton transfer from methane