Bimetallic transition metal catalysts

There has been great interest in the preparation of bimetallic transition metal cluster complexes containing palladium.899-902 Bimetallic palladium-ruthenium clusters have been shown to be good precursors to supported bimetallic catalysts.903,904... 

It is also interesting to note that the decomposition of pseudobinary intermetallic compounds (LaNi4Fe and CeNis-jCo ) leads to the formation of bimetallic transition metal particles which display different selectivities than the catalysts derived from the related binary intermetallic compound (Paul-Boncour et al. 1991, France and Wallace 1988). The characterization of the bimetallic particles by XRD, Mossbauer spectroscopy and magnetism indicated that their composition was close to that of the starting alloy. 

The original discovery of this type of catalysts is,of course, due to K. Ziegler. G. Natta having contributed ingenious applications, particularly to stereospecifle polymerization. However we use the expression Ziegler-Natta catalysts" in order to differentiate the bimetallic transition metal—aluminum alkyl catalysts from the monometallic aluminum alkyl catalyst of the Aufbau" reaction, also due to Ziegler. 

Heterogeneous catalysis by metals has been of long-standing interest, with bimetallic catalysts a particular focus.Transition metal carbonyls have... 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

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. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

The subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

In conclusion it is necessary to note the considerable change in chemical activity occurring on transformation from the alkoxides into oxocomplexes. An example is the synthesis of a bimetallic Bi-Ti complex. The complex formation of 2 isopropoxides occurs only in the presence of water (h = 0.2-0.7), which leads to the formation of Bi-oxoisopropoxide, which then reacts with Ti(OPr )4 already at room temperature providing BiTi20(0Pr% [447] (see also Chapter 8). Teyssie et al. [760] have proposed a large group of alkoxides of 3d-transition metals, and also those of Zn, Al, and Mo as highly effective selective catalysts for polymerization of lactones, isocyanates, and so on. 

Metalametallations of alkenes and alkynes are useful methods for the construction of 1,2-dimetala-alkanes and 1,2-dimetala-l-alkenes, which react subsequently with suitable electrophiles to form substituted alkanes and alkenes. Metalametallation is carried out usually with bimetallic reagents of the type R Si-M R, or R Sn-M R in which M = B, Al, Mg, Cu, Zn, Si or Sn. Some metalametallations proceed without catalysts Cu, Ag and Pd compounds are good catalysts. The metalametallation with bimetallic compounds, such as Si-B, Si-Mg, Si-Al, Si-Zn, Si-Sn, Si-Si, Sn-Al or Sn—Sn bonds, catalysed by transition metal complexes, is explained by the oxidative addition of the bimetallic compounds to form 478, and insertion of alkene generates 479. Finally 1,2-dimetallic compounds 480 are formed by reductive elimination. Dimetallation of alkynes proceeds similarly to give 481. Dimetallation is syn addition. 

A number of models of the active centres in Ziegler Natta catalysts have been postulated. The diversity of these models arises from the multitude of products found to be formed or believed to be formed in the reaction of the catalyst precursor with the activator [e.g. schemes (4) to (8) and (12)]. The proposed active centres fall into either of two general categories those containing monometallic species with the central transition metal atom (e.g. Ti), and those containing bimetallic species with the central transition metal atom linked via bridges with the metal atom originating from the activator (e.g. Al). 

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