Catalytic applications, transition metal

In addition to structure stabilization and catalytic applications, transition metal-binding sites may be designed and exploited for regulatory purposes. For example, a regulatory metal-binding site has been engineered into the active site of trypsin, where metal binding inhibits proteo-... 

In addition to catalytically active transition metal complexes, several stable, electrophilic carbene complexes have been prepared, which can be used to cyclopropanate alkenes (Figure 3.32). These complexes have to be used in stoichiometric quantities to achieve complete conversion of the substrate. Not surprisingly, this type of carbene complex has not attained such broad acceptance by organic chemists as have catalytic cyclopropanations. However, for certain applications the use of stoichiometric amounts of a transition metal carbene complex offers practical advantages such as mild reaction conditions or safer handling. 

It is also possible to form block co-polymers via the living anionic ROP of the phosphorus-bridged ferrocenophanes (Section 12.06.3.3.3). " Diblock co-polymers such as PI-/)-PFP 108 can be prepared by the sequential anionic polymerization of isoprene and ferrocenophane 107 (Scheme 10). These materials yield spherical micelles in -hexanes with an amorphous PFP core and a PI corona. The PI corona can be cross-linked via radical reactions to yield a permanently cross-linked shell, which retains its integrity even in good solvents for both blocks. With PFP block co-polymers the possibility of the coordination of various catalytically active transition metal moieties to the phosphorus centers may prove useful for catalysis and for materials science applications. 

Selective NOx removal is an area where the boundary between sorbent and catalyst tends to disappear. Many different types of sorbents have been investigated for NOx sorption for both cold (near ambient temperature) and hot (200-400 °C) applications. Transition metal oxides appear to be the best. In both temperature ranges, a significant amount of adsorption is achieved usually when assisted by catalytic oxidation of NO to NO2 as an intermediate step. In air near ambient temperature, about 5% of the NO is in the form of NO2. NO2 adsorbs more easily than NO for both chemisorption and physical adsorption. The normal boiling point of NO is -152°C and that of NO2 is 21 °C. Thus, NO2 adsorbs readily near ambient temperature in micropores and mesopores by pore filling. In chemisorption, NO2 readily forms nitrite and nitrate on transition metal oxides. These chemisorbed species can be decomposed or desorbed only at elevated temperatures, e.g., 200-300 °C. Other surface species are also formed, such as NO+ (nitrosyl). These species could be desorbed at near-ambient temperature. The complex chemistry of NO is due to the fact that there is one electron occupying the antibonding orbital of NO, which is empty in most other molecules. 

Polymeric materials containing finefy-divided metal or metal-oxide nanoparticles show unusual properties that could make them e edally useful in many technological applications. Transition metal catalysis is certainly a field that can benefit to a great extent by the introduction of these novel materials. The search for thetic methods to obtain nanoparticles that are stabilized within a polymer matrix, the characterization of these materials, and their application in catalytic reactions are receiving increased attention. 

Among the large number of catalytically active transition metal complexes, Wilkinson s catalyst (Fig. 3), is one of the most important and probably the most widely studied of all known homogenous hydrogenation catalysts. For this reason, the immobilization of this complex and its deri-vates on solid supports and their application in heterogeneous reactions are intensely investigated. 24,46,149 isi... 

The transition metals and their compounds are well-known for their catalytic properties. The works related to catalytic applications involving metallic compounds have been awarded about 15 Nobel prizes between 1901 and 2014. While the idea of catalysis was originally conceived by Berzebus, it was... 

Therefore, finding an appropriate catalytic system—transition metal complex, ligand and reaction conditions—are the key-questions for successful application of this methodology in order to selectively prepare desired products. 

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiCl3 as a catalyst for polymerization of ethylene 116). However, only in recent years as a result of the application of relatively stable organometallic compounds of transition... 

During the last decade N-heterocyclic carbene complexes of transition metals have been developed for catalytic applications for many different or-... 

Certain aspects of catalytic applications of transition metal amidinate complexes have already been summarized in review articles. The "Chemistry of... 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Draganjac M, Rauchfuss TB (1985) Transition metal polysulfides Coordination compounds with purely inorganic chelate ligands. Angew Chem Int Ed Engl 24 742-757 DuBois MR (1989) Catalytic applications of transition metal complexes. Chem Rev 89 1-9 Ansari MA, Ibers JA (1990) Soluble selenides and tellurides. Coord Chem Rev 100 223-266... 

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