Three-coordinate metal centers in extended transition metal oxides

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure.     

Ultrasound-assisted synthesis of nanostructured transition metal oxides

Ultrasound pretreatment of aqueous solutions of cetyltrimethylammonium bromide as a structure- directing agent has been applied to prepare nanostructured mesoporous Mn, Fe, and Ni oxides. After removal of the template by triple extraction with a water–ethanol solution of sodium acetate or ammonium chloride, air-calcined samples of oxide materials prepared in such a way possess surface areas of about 300–450 m²/g and are thermally stable up to 300°C.     

Oxidation of dodecane on transition metal oxides

The catalytic oxidation of dodecane on individual and mixed vanadium and molybdenum oxides is studied. Products of the oxidation of alkane are studied qualitatively and quantitatively. The activities of the samples of the catalysts with various ratios of vanadium and molybdenum oxides are compared. One possible scheme for the activation of reagents on a catalyst is given.     

The gel route to transition metal oxides

The so-called “sol-gel” process offers new approaches to the synthesis of transition metal oxides. Based on inorganic polymerization from molecular precursors, it leads to highly condensed species or colloids. These colloids are actually two-phase systems in which small oxide particles are dispersed in a liquid medium. A very large interface separates both phases and interfacial phenomena, at the oxide-water interface, lead to new features in the physics and chemistry of transition metal oxides. Ordered aggregation of oxide particles may occur, giving rise to colloidal crystals or anisotropic tactoids in which the mean distance between particles can be of about 0, 1 μm. This distance can be decreased leading to ordered solid aggregates. Transition metal oxide gels exhibit the physical properties of both phases, i.e., electronic properties arising from electron hopping through the mixed valence oxide network and ionic properties arising from proton diffusion through the liquid phase. Electronic and ionic properties appear to be strongly related through the very large interface. Large coatings can be easily deposited from colloidal solutions and transition metal oxide gels should be very useful for making microionic devices.     

The oxidation of toluene on transition metal oxides

The oxidation of toluene on pure vanadium and molybdenum oxides was found to follow independent paths; it was benzene ring oxidation on V₂O₅ and side chain oxidation on MoO₃. On mixed xV₂O₅ · yMoO₃ oxides, the main reaction was the addition at the double bond preferably positioned meta rather than one-electron oxidation.     

Electron beam induced changes in transition metal oxides

Electron beam induced changes in maximal valence transition metal oxides V(2)O(5), M(o)O(3) and TiO(2) (anatase) were studied by means of electron energy-loss spectroscopy and electron diffraction in transmission electron microscopy. For V(2)O(5), the observed chemical shifts of the L-edge reveal the reduction of V(5+) to V(2+). The structure changes from orthorhombic V(2)O(5) to cubic VO. MoO(3) can be reduced to a phase with an oxidation state less than that in MoO(2). No notable structural or electronic change in TiO(2) (anatase) is observed. The different behaviours of the studied oxides under an electron beam are discussed with respect to bonding energy and lattice structure.     

Recent developments in transition metal diaryl chemistry

This Perspective discusses the synthesis and reactivity of low-coordinate transition metal diaryl complexes. The development of sterically demanding ligands, in particular m-terphenyls is facilitating the isolation of complexes featuring two-coordinate open-shell transition metal centres. These coordinatively unsaturated complexes are now being investigated for their reaction chemistry, which is not only contributing to the elucidation of the structure and bonding within these systems but is also leading to the formation of novel compounds.     

Continuous chirality measures in transition metal chemistry

The definition of the continuous chirality measure(CCM) is provided and its applications are summarized in this tutorial review, with special emphasis on the field of transition metal complexes. The CCM approach, developed in recent years, provides a quantitative parameter that evaluates the degree of chirality of a given molecule. Many quantitative structural correlations with chirality have been identified for most of the important families of metal complexes. Our recent research has shown that one can associate the chirality measures with, e.g., enantioselectivity in asymmetric catalysis. We also explore a fragment approach to chirality in which we investigate which part of a molecule is responsible for the chirality-associated properties of a given family of compounds.     

Oxidative desulfurization of benzene fraction on transition metal oxides

It is established that molecular oxygen is able to oxidize thiophene selectively in a mixture with benzene on V₂O₅ · MoO₃. The introduction of thiophene inhibits the oxidation of benzene. It is shown that the conversion of thiophene during operation of the catalyst is reduced at first and then increases until it reaches its initial value.     

Polymer flame inhibition in presence of transition metal oxides

Summary Transition metal oxides like Fe₂O₃, Ni₂O₃, Co₂O₃ and MnO₂ suppress the combustion of polystyrene. The effect has been explained on the basis of condensed-phase and gas-phase reactions.With 2 figures and 1 table