Other Bimetallic Catalysts

A bimetallic catalyst prepared from BINOL and lithium aluminum hydride has been found to result in useful asymmetric induction in the Pudovik reaction [17]. The (f )-ALB catalyst 64 (10 mol %) facilitates the addition of dimethyl phosphite to a variety of electron-rich and electron-poor aryl aldehydes in high yield with induction in the range 71-90 % ee. The nature of the solvent is important in this reaction—the induction for addition to benzaldehyde dropped from 85 % ee to 65 % ee when the solvent was changed from toluene to dichloromethane. Aluminum seems to be a key to the success of this reaction, because reaction with benzaldehyde was not as successful with other bimetallic catalysts. BINOL catalysts with lanthanum and potassium gave only 2 % ee, a catalyst with lanthanum and sodium gave a low 32 % ee, and a catalyst with lanthanum and lithium gave only a 28 % ee [18]. Aliphatic aldehydes were not successfully hydrophosphonylated with dimethyl phosphite by catalyst 64 (Sch. 9). Induction was low (3-24 % ee) for unbranched and branched substrates. a,/3-Unsaturated aldehydes were, however, reported to work nearly as well as aryl aldehydes with four examples in the range 55-89 % ee. The failure of aliphatic aldehydes with this catalyst can be overcome by reduction of the product obtained from reactions with a,)3-unsaturated aldehydes. As illustrated by the reduction of 67 with palladium on carbon, this can be done without epimerization of the a-hydroxy phos-phonate. 

For the catalyst containing osmium alone on silica, the osmium clusters behave as if they are more electron deficient than pure metallic osmium that is, there appear to be more unfilled d states to accommodate the electron transitions from the 2pin core level of the absorbing atom. In the silica-supported osmium-copper clusters, however, the osmium atoms appear to be less electron deficient than they are in the pure osmium clusters dispersed on silica. The presence of the copper thus appears to decrease the number of unfilled d states associated with the osmium atoms. This observation is the first that we have made regarding the electronic interaction between the components of a bimetallic cluster catalyst. Further studies of such interactions are currently in progress on other bimetallic catalysts. 

In the presence of the sodium-containing heterobimetallic catalyst (R)-LSB (10 mol%), the reaction of enone 52 with TBHP (2 equiv) was found to give the desired epoxide with 83% ee and in 92% yield [56]. Unfortunately LSB as well as other bimetallic catalysts were not useful for many other enones. Interestingly, in marked contrast to LSB an alkali metal free lanthanoid BINOL complex, which was prepared from Ln(0- -Pr)3 and (R)-BINOL or a derivative thereof (1 or 1.25 molar equiv) in the presence of MS 4A (Scheme 17), was found to be applicable to a range of enone substrates. Regarding enones with an aryl-substitu-ent in the a-keto position, the most effective catalytic system was revealed when using a lanthanum-(.R/)-3-hydroxymethyl-BINOL complex La-51 (l-5mol%) and cumene hydroperoxide (CMHP) as oxidant. The asymmetric epoxidation proceeded with excellent enantioselectivities (ees between 85 and 94%) and yields up to 95%. 

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

These bimetallic systems can be considered as a new class of bimetallic catalysts, the properties of which cannot be interpreted by geometric (ensemble) or electronic (ligand) effects. Synergetic effects reported for other bimetallic catalysts which, like Pt-Mo, incorporate in the same cluster the noble metal atoms and easily oxidized metal atoms, could probably be interpreted by similar dual-site mechanisms. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

It is known that the addition of Pt errhances the activity of hyam catalysts while sacrificing selectivity [4-5]. Bimetallic catalysts were prepared and tested to see if one could improve the activity of these catalysts while maintaining the good selectivity. As expected, Catalyst A with an 8% Pd + 2% Pt loading showed an increased activity of 57.3 compared to 32.7 for Catalyst A with a 10% Pd loading (Table 10.2). On the other hand, the selectivity of the bimetallic catalyst was... 

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. 

Comparison of the Cu K-edge EXAFS signals for the monometallic Cu/Si02 and the bimetallic Ru-Cu/Si02 catalyst, on the other hand, provides clear evidence for the proximity of ruthenium to copper atoms in the latter. This is seen in the different shape of the measured EXAFS signal and the distorted inverse transform of the first coordination shell. Note that the intensity of the latter is weaker for the bimetallic catalyst, while the region between k=8 and k=15 A-1 has become more important, which points to the presence of a scattering atom heavier than copper in the first coordination shell. The reduced intensity in the Cu Fourier transform of the bimetallic catalyst is indicative of a lower coordination of the copper, which is characteristic of surface atoms. 

The first published patent on LDHs as catalyst precursors used high supersaturation conditions. Mg/Al-COs, Ni/Al-CO3, Co/Mn/Al-C03, Co/Mn/Mg/Al - CO3, and Ni/Cr/Al - CO3 LDHs, as well as other bimetallic and multimetallic LDHs were synthesized by adding a solution containing the... 

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. 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

Other bimetallic systems have been investigated. An Fe-Co-based catalyst [266] exhibits high activity (achieving complete acetic acid conversion at 400 °C), high H2 selectivity and good stability. 

It was noted earlier that EXAFS is a result of two fundamental processes (a) K- (or L-) absorption of an X-ray photon which is the photoelectric effect and (b) an effective diffraction of the electron so emitted. In the case of an isolated absorbing atom (absorber) one sees only the characteristic rise in absorbance, // (= In /(,//), at the energy corresponding to the edge and an exponential decrease thereafter. When the absorber is surrounded by other atoms, // exhibits undulations sometimes up to 2000 eV beyond the edge. Undulations starting 30 eV beyond the edge constitute the EXAFS. As an example, the EXAFS of a bimetallic catalyst is shown in Fig. 2.12. 

Catalyst type. Raney nickel is the preferred catalyst for preparative purposes. Other Ni catalysts are suitable [1,4], Bimetallic and noble metal catalysts have been studied [6,14],... 

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