Computational study on benzotriazole systems and synergic Na, Mg mixed compounds

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Iron-Oxo Catalysts for CH Bond Cleavage : insights from modeling

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In Silico Screening of Iron-Oxo Catalysts for CH Bond Cleavage

International audienceInspired by oxidation enzymes such as P450 and TauD, several groups have based their research on the iron−oxo moiety in the field of alkanes partial oxidation. Still, the controlled cleavage and oxidation of the aliphatic C−H bond remains a prized goal in chemistry. We present here a computational methodology to predict the comparative reactivity of iron−oxo complexes for this process from linear relations based on the sole electronic structure of the reactant state. The efficient correlation of the C−H activation barrier to a simple but intuitive molecular orbital descriptor enables the design of ligands that permit low barrier C−H abstraction steps and the fast screening of novel potential complexes. The activation of the catalyst by a multidentate effect is also evidenced. We anticipate this study to improve the rational design of hydrocarbon oxidation catalysts

Synthesis and structural characterisation of mixed alkalimetal-magnesium mixed ligand alkyl-amido ate complexes

Using two different reaction methodologies, two alkali metal–magnesium alkyl bis(amide) complexes were synthesised. First the lithium magnesiate LiMg{μ-N(SiMe3)2}2(tBu) (1) was prepared by combining equimolar quantities of tBuLi and Mg{N(SiMe3)2}2 in hydrocarbon solvent. An X-ray crystallographic study revealed that the asymmetric unit of 1 has a dinuclear arrangement, based on a planar Li–N–Mg–N four-membered ring. As a result of the presence of intermolecular agostic interactions between the Li centre of one asymmetric unit and a methyl group which is resident on the terminal tert-butyl group of another, 1 is polymeric in the solid-state. Second the sodium magnesiate NaMg{μ-N(SiMe3)2}2(tBu) · (OEt2) (2) was prepared by reacting two molar equivalents of Na{N(SiMe3)2} with one molar equivalent of tBuMgCl in hydrocarbon/diethyl ether solution. X-ray crystallographic analysis revealed that the asymmetric unit of 2 consisted of a dinuclear molecular arrangement. As expected it is not polymeric due to the coordination of the Lewis basic ether. Stabilizing intramolecular agostic NaC bonds are observed (where C is a methyl group resident on a Si atom)

Nízkoteplotní selektivní oxidace metanu na vzdálených binukleárních kationtových centrech zeolitů

Highly active oxygen capable to selectively oxidize methane to methanol at low temperature can be prepared in transition-metal cation exchanged zeolites. Here we show that the alpha-oxygen stabilized by the negative charges of two framework aluminum atoms can be prepared by the dissociation of nitrous oxide over distant binuclear cation structures (M(II) ... M (II), M = cobalt, nickel, and iron) accommodated in two adjacent 6-rings forming cationic sites in the ferrierite zeolite. This alpha-oxygen species is analogous to that known only for iron exchanged zeolites. In contrast to divalent iron cations, only binuclear divalent cobalt cationic structures and not isolated divalent cobalt cations are active. Created methoxy moieties are easily protonated to yield methanol, formaldehyde, and formic acid which are desorbed to the gas phase without the aid of water vapor while previous studies showed that highly stable methoxy groups were formed on isolated iron cations in iron exchanged ZSM-5 zeolites.Vysoce aktivní kyslík dostupný pro selektivní oxidaci metanu na metanol při nízkých teplotách může být připraven iontovou výměnou v zeolitech

Computationally Exploring Confinement Effects in the Methane-to-Methanol Conversion Over Iron-Oxo Centers in Zeolites

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Štěpení kyslíku na vzdálených dvoujaderných Fe centrech v zeolitech. Efekt lokálního uspořádání a mřížkové topologie

Activation of dioxygen is of extreme importance due to its potential for transformation of methane to valuable products and applications in other selective oxidation reactions. Distant binuclear cationic Fe(II) centers in Fe-ferrierite were shown to split dioxygen at room temperature to form a pair of very active oxygen species (i.e., alpha-oxygens) and subsequently oxidize methane to methanol at room temperature as well. Our study reveals that the activity in splitting dioxygen represents a general property of the distant binuclear cationic Fe(II) centers stabilized in the aluminosilicate matrix. Computational models of the ferrierite, beta, A, and mordenite zeolites with various Al sitings in the rings forming the cationic sites were investigated by periodic DFT calculations including molecular dynamics simulations. The results reveal that the Fe(II) sites stabilized in various zeolite matrices can split dioxygen if the two cationic sites forming the distant binuclear Fe(II) centers (i) face each other, (ii) are parallel, and (iii) are axial, and (iv) the Fe center dot center dot center dot Fe distance lies in a narrow range from ca. 7 to ca. 8 A (ca. 7-ca. 10 A for the distance between the two rings (forming the corresponding cationic sites) in empty zeolites since this distance is equal to or larger than the Fe center dot center dot center dot Fe distances). Our study opens the possibility of developing Fe-zeolite-based systems for the dioxygen activation employed for direct oxidations using various zeolite matrices.Aktivace kyslíku je extrémně důležitá z důvodu potenciálu pro transformaci metanu na vhodné produkty a aplikace v jiných oxidačních reakcích. Vzdálená dvoujaderná Fe(II) centra v Fe-FER vykázala štěpení kyslíku při pokojové teplotě do formy páru aktivních kyslíkových druhů a následnou oxidaci metanu na metanol při pokojové teplotě

Stoichiometrically-controlled reactivity and supramolecular storage of butylmagnesiate anions

Toluene is metallated by DABCO-activated disodium tetrabutylmagnesiate, but not by DABCO-activated monosodium tributylmagnesiate; this distinction is rationalised by DFT calculations on model systems, and the crystal structure of the main non-metallated product, which shows interstitial MgBu42- dianions within a polycationic network, is reported

Radical Reactions Affecting Polar Groups in Threonine Peptide Ions

Peptide cation-radicals containing the threonine residue undergo radical-induced dissociations upon collisional activation and photon absorption in the 210–400 nm range. Peptide cation-radicals containing a radical defect at the <i>N</i>-terminal residue, [^•Ala-Thr-Ala-Arg+H]⁺, were generated by electron transfer dissociation (ETD) of peptide dications and characterized by UV–vis photodissociation action spectroscopy combined with time-dependent density functional theory (TD-DFT) calculations of absorption spectra, including thermal vibronic band broadening. The action spectrum of [^•Ala-Thr-Ala-Arg+H]⁺ ions was indicative of the canonical structure of an <i>N</i>-terminally deaminated radical whereas isomeric structures differing in the position of the radical defect and amide bond geometry were excluded. This indicated that exothermic electron transfer to threonine peptide ions did not induce radical isomerizations in the fragment cation-radicals. Several isomeric structures, ion–molecule complexes, and transition states for isomerizations and dissociations were generated and analyzed by DFT and Møller–Plesset perturbational ab initio calculations to aid interpretation of the major dissociations by loss of water, hydroxyl radical, C₃H₆NO^•, C₃H₇NO, and backbone cleavages. Born–Oppenheimer molecular dynamics (BOMD) in combination with DFT gradient geometry optimizations and intrinsic reaction coordinate analysis were used to search for low-energy cation-radical conformers and transition states. BOMD was also employed to analyze the reaction trajectory for loss of water from ion–molecule complexes