Bimetallic catalysts complexes

In an effort to constrain the flexibility in this type of bimetallic catalyst, complex 46 was synthesised which contains imidazolyl-imine ligands attached directly to the 1,2-phenylene scaffold via the imine nitrogen (Scheme 17) [91]. This arrangement significantly reduces the degree of conformational freedom for complex 46, which... 

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... 

A bimetallic catalyst can be obtained by the reaction of tetrabutyltin with Rh/Si02 catalyst. The partial hydrogenolysis leads to the Rhs[Sn(n-C4H9)2]/ Si02 surface organometallic complexes, which proved to be fully selective in the hydrogenation of unsaturated aldehydes into the corresponding unsaturated alcohols.318... 

Hsu et al. [15] applied a bimetallic catalyst comprising rhodium and ruthenium for the hydrogenation to combine the high selectivity of the rhodium complex with the lower cost of the ruthenium complex. When the amount of each metal is identical, the catalytic activity of the bimetallic complex catalyst system was similar to that of the single rhodium-complex catalyst, containing... 

Reactions involving bimetallic catalysts, either homo-dinuclear or hetero-bimetallic complexes, and chemzymes were highlighted by Steinhagen and Helmchen96c in 1996. Some examples are discussed in Chapter 2. Among these examples, Shibasaki s reports have been of particular significance.97 Shibasaki s catalyst is illustrated as 130, which consists of one central metal M1 (La+3, Ba+2, or A1+3), three other metal ions (M2)+ [(M2)+ can be Li+, Na+, or K+], and three bidentated ligands, such as (R)- or (iS )-BINOL. The catalyst exhibits both Lewis acidic properties because of the existence of central metal and the Lewis basic properties because of the presence of the outer metal ions. 

The surface structure and characteristics (density and acidity) of the hydroxyl groups presented in Fig. 13.21 (using CrystalMaker 2.1.1 software) give very useful information to understand the reactivity of the surface of the particles, particularly when adsorption of another complex is desired to synthesize a bimetallic catalyst, or to control the interaction with an oxide carrier (the deposition step). The isoelectric point calculated with the model (5.9) is in fair agreement with the experimental value (4.3). 

A special application of bimetallic ruthenium complexes was found in the olefin metathesis reaction vide infra) The two metal centers were closely attached to one another through /r-halide anions. The labile assembly was the key feature to the formation of highly active catalysts. 

Supported bimetallic catalysts have gained unquestionable importance in subjects such as refining, petrochemistry and fine chemistry since their earliest use in the 1950s [1, 2]. The catalytic behavior of such a system is influenced by the size of the metal particles and by the interactions among them and with the support and other catalyst components. The second metal may influence the first metal through electronic interactions or by modifying the architecture of the active site. Very often, the interactions between the two metals are complex and largely unknown, and consequently the preparation procedure critically influences the nature of the catalytic system obtained. 

In this chapter, SOMC/M will be used to study the reactivity of organometallic complexes with the surface of supported metals. In 1984, Travers [31] and Margit-falvi [32] simultaneously described this application of SOMC for the preparation of bimetallic catalysts. 

The use of carbonyl complexes has enabled proper study of the role of rhenium in Pt-Re bimetallic catalysts used in the reforming of naphtha [57-60] and tailoring of the preparation of Pt-Ru bimetallic particles. Pt-Ru systems are of interest in developing electrodes for fuel cell applications [61]. 

Our design of bimetallic catalysts based on crown-complexed alkaline-earth metal ions, for use in reactions of ester and activated amides endowed with a distal carboxylate anchoring group, is based on the mechanistic hypothesis outlined in Scheme 5.3. Such hypothesis critically rests on the finding that in EtOH solution... 

In the presence of a large excess of EtO ion, the bimetallic catalyst is fully saturated with EtO as shown by structure I in Scheme 5.3. Incremental additions of a carboxylate substrate would cause the gradual conversion of I into the 1 1 productive complex II, but further additions would yield the unproductive complex III. As expected from this mechanism a bell-shaped profile is observed in a plot of initial rate versus substrate concentration related to the catalyzed ethanolysis of 16 (Figure 5.5). The fairly good quality of the fit supports the validity of Scheme 5.3. Further confirmation comes from the finding that benzoate anions behave as competitive inhibitors of the reaction. Since the reaction product of the ethanolysis of 16 is also a benzoate anion, product inhibition is expected. Indeed, only four to five turnovers are seen in the ethanolysis of 16 before product inhibition shuts down the reaction. The first two turnovers are shown graphically in Figure 5.6. 

The catalytic asymmetric nitroaldol reaction was extended to a direct catalytic asymmetric nitro-Mannich-type reaction promoted by hetero-bimetallic catalysts (Scheme 2) [53-55] or by EtjNBOX-Cu complexes [56]. These topics are reviewed in Chap. 28.2. 

Trost et al. [11] reported another impressive example of bimetallic catalysts in which a Zn-Zn homobimetallic complex (17, Scheme 7) serves as an effective catalyst for direct aldol reactions [11-13]. The proposed structure of the catalyst was verified by mass spectrometry and the best ratio of Et2Zn and the ligand. The chemical yield was moderate in the reaction of methyl ketones (1) (Scheme 7, top) [11,12], but a highly atom-economic system was achieved when a-hydroxylated ketones (10) were used as a substrate (Scheme 7, bottom) [13]. Excellent diastereo- and enantioselectivity were obtained under mild conditions. In contrast to the case of Shibasaki s heteropolymetallic catalyst, syn-1,2-diols (syn-11) were obtained as the major diastereomers. 

The transfer of a nitrene from PhI=NTos as a commercial source can also be catalyzed by a bimetallic iron complex which exhibits moderate activity, as reported by Avenier and Latour (Scheme 9.12) [24]. The iron complex 10 is better suited for aliphatic substrates where better yields were reported compared with copper catalyst systems applied to the same reactions. 

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