Transition Metals as Catalysts

Laszlo pointed out that solids of fractal dimension D near 2.0 are usually more efficient catalysts than those of D near 3.0. An adsorbed species diffusing across the surface of a catalyst will find a target much more quickly (i.e., catalysis will be more efficient) in space of dimension near 2 than of D near 3. We find, for example, that catalytic activities of variously prepared activated charcoals increase as we go from D = 3.0 to D = 1.9. 

These relative chemisorption strengths enable us to make some simple predictions regarding suitable metal catalysts for specific reactions. For example, a catalyst for the Haber process must chemisorb both N2 and H2, but not too strongly. Since N2 is the less readily bound, we choose Fe, Ru, or Os. The latter two are expensive, so our best choice is iron—usually finely divided, on a suitable refractory support. 

Similarly, we can ask what would be the best catalyst for hydrogenating an olefin such as ethylene. Since olefins (alkenes) are more strongly chemisorbed than hydrogen, we choose a metal that just barely chemisorbs H2—this means Co, Rh, Ir, Ni, Pd, or Pt. In practice, nickel is the least expensive choice. Again, it should be finely divided (maximum surface area) for greatest catalytic efficiency and be dispersed on the internal surfaces of a porous support such as alumina with surface area on the order of 200 m2 g-1.8 

---

In addition to nanocapsules filled with metals and carbides, various exotic carbon materials with hollow structures, such as single-wall (SW) tubes[9,10], bamboo-shaped tubes, and nanochains[l 1], are produced by using transition metals as catalysts. 

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... 

The formation of surface defects of a crystal lattice. It was observed while using crystal compounds of transition metals as catalysts [e.g. as was shown by Arlman (171, 173), for a TiCl3 surface defects appear on the lateral faces of the crystal]. In this case low surface concentration of the propagation centers should be expected, as is illustrated in the case of polymerization by titanium dichloride (158). The observed... 

GA Kovtun. Complexes of Transition Metals as Catalysts of Chain Termination in Oxidation, Doct. Sci. (Chem.) Thesis Dissertation, Institute of General and Inorganic Chemistry, Moscow, 1984, pp. 1 19 [in Russian]. 

The use of dispersed or immobilized transition metals as catalysts for partial hydrogenation reactions of alkynes has been widely studied. Traditionally, alkyne hydrogenations for the preparation of fine chemicals and biologically active compounds were only performed with heterogeneous catalysts [80-82]. Palladium is the most selective metal catalyst for the semihydrogenation of mono-substituted acetylenes and for the transformation of alkynes to ds-alkenes. Commonly, such selectivity is due to stronger chemisorption of the triple bond on the active center. 

OrganometaUic compounds, of transition metals as catalysts for polymerization of vinyl monomers and olefins, 23 263-325 OrganometaUic transformation, molecular analogs, 38 288, 290-291 Organophosphorus substrates, hydrolysis of, effect of cycloamyloses on, 23 235 Organosilicon compounds, hexacoordinated, 34 155... 

You can find more information about transition metals as catalysts at www. brightredbooks.net... 

Carbides of transition metals as catalysts for oxidation reactions... 

Olefinic double-bond isomerization is probably one of the most commonly observed and well-studied reactions that uses transition metals as catalysts [1]. However, prior to our first achievement of asymmetric isomerization of allylamine by optically active Co(I) complex catalysts [2], there were only a few examples of catalytic asymmetric isomerization, and these were characterized by very low asymmetric induction (<4% ee) [3], In 1978 we reported that an enantioselective hydrogen migration of a prochiral allylamine such as AVV-diethylgerany-lamine, (1) or N V-diethylnerylamine (2) gave optically active citronellal ( )-enamine 3 with about 32% ee utilizing Co(I)-DIOP [DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane] complexes as the catalyst (eq 3.1). 

Miller DM, Buettner GR, Aust SD (1990) Transition metals as catalysts of, autoxidation reactions. Free Radical Biol Med 8 95-108... 

Benzyl groups can be removed hydrogenolytically with a transition metal as catalyst. 

Transition metals are active as catalysts because of their capacity to chemisorb atoms, given that the main role of transition metals as catalysts is to atomize gaseous molecules, such as H2, 02, N2, and CO, thereby providing atoms to other reactants and reaction intermediates [27],... 

Edye, L. A., Richards, G. N., and Zheng, G., Transition-metals as catalysts for pyrolysis and gasification of biomass. ACS Symposium Series 1993, 515, 90-101. 

Thus the kinetics of the zeolite catalyzed reactions clearly paralleled closely those of the homogeneous reaction using the same transition metal as catalyst. Again zeolites acted as simple solvents for the carbonylation reaction though perhaps a specific role of the zeolite should not perhaps be entirely excluded. The kinetic similarities strongly suggested nearly identical reaction pathways in the zeolite medium and in solution with identical rate limiting step (24, 27). 

Although this chapter includes the applications of transition metals as catalysts and reagents for C-C bond forming reactions, it is planned from the point of view of the reaction mechanism rather than on the basis of transformation. 

The direct transfer of carbene from diazocompounds to olefins catalyzed by transition metals is the most straightforward synthesis of cyclopropanes [3,4]. Reactions of diazoesters with olefins have been studied using complexes of several transition metals as catalysts. In most cases trans-isomers are preferably obtained, but the selectivity depends on the nature of the complex. In general the highest trans-selectivity is obtained with copper catalysts and it is reduced with palladium and rhodium complexes. Therefore, the rhodium mesotetraphenylporphyrin (RhTPPI) [5] and [(r 5-C5H5)Fe(CO)2(THF)]BF4 [6] are the only catalysts leading to a preference for the cis-isomer in the reaction of ethyl diazoacetate with styrene. 

---