Oxygen early transition metal oxides

The early transition-metal-oxide clusters do not have metal-metal bonds, but the metals are linked by oxygen atoms that can also occupy non-bridging terminal positions. They are the heteropolyacids and their salts, the heteropolymetallates. These clnsters have been extensively developed in France by Sonchay s school now very active, in the United States by Pope and in Germany by Muller. - They have many kinds of structures, but the most known ones in oxidation catalysis are Keggin structures (XM i204o) in which the central atom X = Si, Ge, ... 

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

Heterolytic oxygen transfer processes can be divided into two categories based on the nature of the active oxidant an oxometal or a peroxometal species (Fig. 4.10). Catalysis by early transition metals (Mo, W, Re, V, Ti, Zr) generally involves high-valent peroxometal complexes whereas later transition metals (Ru, Os), particularly first row elements (Cr, Mn, Fe) mediate oxygen transfer via oxometal species. Some elements, e.g. vanadium, can operate via either mechanism, depending on the substrate. Although the pathways outlined in Fig. 4.10... 

Quahtative information on the d bandwidth in perovskite can be acquired without carrying out these mathematical operations. This is accomplished simply by evaluating the p—d orbital interactions for some of the special points in the BZ. For now, the main focus is on the tt interactions between the metal t g and oxygen p orbitals. From Figure 3.5, it is expected that the Fermi level will lie in one of the t2g-block bands for n jiVOs oxides if B is an early transition metal with six or fewer d electrons. 

The early transition metals (V, Nb, Ta, Mo, W) in their highest oxidation states are able to form metal-oxygen clnster anions, commonly referred to as polyoxoanions or polyoxometalatesd In the gigantic family, the Keggin... 

Only a small minority of organometallic reactions have cleared the hurdle to become catalytic reality in other words, catalyst reactivation under process conditions is a relatively rare case. As a matter of fact, the famous Wacker/Hoechst ethylene oxidation achieved verification as an industrial process only because the problem of palladium reactivation, Pd° Pd", could be solved (cf. Section 2.4.1). Academic research has payed relatively little attention to this pivotal aspect of catalysis. However, a number of useful metal-mediated reactions wind up in thermodynamically stable bonding situations which are difficult to reactivate. Examples are the early transition metals when they extrude oxygen from ketones to form C-C-coupled products and stable metal oxides cf. the McMurry (Ti) and the Kagan (Sm) coupling reactions. Only co-reactants of similar oxophilicity (and price ) are suitable to establish catalytic cycles (cf. Section 3.2.12). In difficult cases, electrochemical procedures should receive more attention because expensive chemicals could thus be avoided. Without going into details here, it is the basic, often inorganic, chemistry of a catalytic metal, its redox and coordination chemistry, that warrant detailed study to help achieve catalytic versions. 

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