Transition metals complexes, catalytic properties

T. Yamamoto, T. Murauyama, Z.-H. Zhou, T. Ito, T. Fukuda, Y. Yoneda, F. Begum, T. Ikeda, S. Sasaki, H. Takezoe, A. Fukuda, and K. Kubota, -ir-Conjugated poly(pyridine-2,5-diyl), poly(2,2 -bipyridine-5,5 -diyl), and their alkyl derivatives. Preparation, linear structure, function as a ligand to form their transition metal complexes, catalytic reactions, //-type electrically conducting properties, optical properties, and alignment on substrates, J. Am. Chem. Soc., 116 4832-4845,... 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

Non-phosphine type ligands are studied time by time with the aim to obtain water-soluble transition metal complexes with catalytic properties. However, with the exception of a few specific reaction types (e.g. oxidations) these catalysts cannot cope with tertiary phosphines - with the ligands on Figure 20 this has been found once again. 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

Transition metal complexes with metal-carbon -bonds are key intermediates in many important industrial processes, in biochemical reactions, organic synthesis, and processes involving aliphatic radicals. Of special interest are those complexes, which are short-lived intermediates in catalytic processes. However due to the high reactivity of the latter complexes, the study of their properties is difficult as their steady state concentration is in most cases far below the detection limit. 

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond ... 

In spite of some declining industrial interest, the last 5 years have seen an unusual academic interest in the catalytic properties of the metal carbonyls. This has been part of a wider surge of interest in the organometallic chemistry of the transition metals and its application to homogeneous catalysis. Reactions such as Ziegler polymerization, the Oxo reaction, and the Wacker process are but a few of the many reactions of unsaturated molecules catalyzed in the coordination sphere of transition metal complexes (20). These coordination catalyses have much in common, and the study of one is often pertinent to the study of the others. 

Hence, there are strong grounds to state that compounds with the tetra-nuclear cluster magnesium core formed in Mg-RH films have catalytic properties similar to those of transition metal complexes. The catalytic activity of organomagnesium clusters can be very high. 

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

There will undoubtedly be many new catalysts described in the near future. In particular, we anticipate that later transition-metal complexes will play a much larger role in this type of chemistry. Although bis-Cp complexes of Groups 3 and 4, the lanthanides, and the actinides have shown exceptional activity and thus far have dominated reported investigations on catalytic homodehydrocoupling, it is most unlikely that they are unique in this property. It is to be expected that other classes of complexes, such as mono-Cp and -rj -alkyl complexes of these elements, will also be active. Other directions for future evolution are the development of catalysts that are air stable, that are tolerant of more functionalities on substrates, and that are more easily manipulated. 

Arylamines are commonplace. They are part of molecules with medicinally important properties, of molecules with structurally interesting properties, of materials with important electronic properties, and of transition metal complexes with catalytic activity. An aryl-nitrogen linkage is present in nitrogen heterocydes such as indoles [1, 2] and benzopyr-azoles, conjugated polymers such as polyanilines [3-9], and readily oxidizable triarylamines used in electronic applications [10-13]. The ability of aryl halides and triflates to form arylamines allows a single group to be used as a synthetic intermediate in aromatic carbon-... 

The vast majority of N-heterocyclic carbenes are based on 5-membered ring systems. It was found that sterically demanding substituents on the NHC are not only beneficial for the stability of the NHC, but also for its catalytic properties. Arguably, the most important and most often employed N-heterocyclic carbenes are imidazol-2-ylidenes IMes and IPr and the imidazolidin-2-ylidenes SIMes and SIPr (Fig. 3). The reactivity of the corresponding transition metal complexes is described in detail in the following sections. 

Silanetriols have been useful building blocks for the preparation of three-dimensional metallasiloxanes. The presence of a metal in the metallasi-loxane framework not only makes these compoimds thermally stable but also improves their catalytic properties. Similarly, silica surfaces act as hosts for numerous transition metal complexes, which are known to catalyze a variety of organic transformations (43). 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

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