623 research outputs found
Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts
Selective, direct conversion of methane to methanol might seem an impossible task since the CâH bond energy of methane is 105 kcal mol^(â1) compared to the CâH bond energy for methanol of 94. We show here that the Catalytica catalyst is successful because the methanol is protected as methyl bisulfate, which is substantially less reactive than methanol toward the catalyst. This analysis suggests a limiting performance for systems that operate by this type of protection that is well above the Catalytica system
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory
We have studied the magnetic structure of the high
symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = â156.8 K and J_O = â68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging âOâPâOâ unit. In contrast, J_O results from a direct overlap of 3d_(x^2 â y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in VâO bond length as a function of strain along a, we found that the VâO bonds of Vâ(OPO)_(2)âV are covalent and rigid, whereas the bonds of Vâ(O)_(2)âV are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2â300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride
Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes
Quantum mechanics (B3LYP density functional theory) combined with solvation (PoissonâBoltzmann polarizable continuum solvent model) was used to investigate six mechanisms for the aerobic oxidation of alcohols catalyzed by (NHC)Pd(carboxylate)_2(H_2O) complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Of these, we find that âreductive β-hydride eliminationâ, in which the β-hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provides the lowest-energy route and explains the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pK_a. The traditional β-hydride elimination mechanism cannot be responsible for the experimentally observed kinetic parameters, which we find could result from the subsequent reductive elimination of acetic acid, which yields a slightly higher calculated activation barrier. Reversible β-hydride elimination may provide a mechanism for the racemization of chiral alcohols, which would undermine attempts at an enantioselective oxidation. Competition among these pathways can be influenced by changing the electronic properties of the carboxylate and substrate
Iridium complexes bearing a PNP ligand, favoring facile C(sp^3)âH bond cleavage
Hydrogen iodide is lost upon reaction of PNP with IrI_3, where PNP = 2,6-bis-(di-t-butylphosphinomethyl)pyridine to give crystallographically characterized Ir(PNP)*(I)_2, which reacts with H_2 to give Ir(PNP)(H)(I)_2. Ir(PNP)(Cl)_3 is relatively inert towards the intramolecular CâH activation of the tert-butyl's of the PNP ligand
The para-substituent effect and pH-dependence of the organometallic BaeyerâVilliger oxidation of rheniumâcarbon bonds
We studied the BaeyerâVilliger (BV) type oxidation of phenylrhenium trioxide (PTO) by H2O2 in the aqueous phase using Quantum Mechanics (density functional theory with the M06 functional) focusing on how the solution pH and the para-substituent affect the Gibbs free energy surfaces. For both PTO and MTO (methylrhenium trioxide) cases, we find that for pH > 1 the BV pathway having OHâ as the leaving group is lower in energy than the one involving simultaneous protonation of hydroxide. We also find that during this organometallic BV oxidation, the migrating phenyl is a nucleophile so that substituting functional groups in the para-position of phenyl with increased electron-donating character lowers the migration barrier, just as in organic BV reactions. However, this substituent effect also pushes electron density to Re, impeding HOOâ coordination and slowing down the reaction. This is in direct contrast to the organic analog, in which para-substitution has an insignificant influence on 1,2-addition of peracids. Due to the competition of the two opposing effects and the dependence of the resting state on pH and concentration, the reaction rate of the organometallic BV oxidation is surprisingly unaffected by para-substitution
A homolytic oxy-functionalization mechanism: intermolecular hydrocarbyl migration from MâR to vanadate oxo
A new mechanism for generating CâO bonds from metal-hydrocarbyls involving homolytic, intermolecular migration of the hydrocarbyl group to a vanadium oxo is reported. Responsible for the CâO bond in phenol formed by the reaction of OVCl_3 with HgPh_2, it may provide air-regenerable metal oxos a role in aerobic alkane oxidations
- âŚ