Carbon clusters transferability

Lanthanide isopropoxides, usually written Ln(OPr )3, but more likely to be oxo-centred clusters Ln50(0Pr )i3, are used, not just as starting materials for the synthesis of catalysts such as the naphthoxides but also as catalysts in their own right. They have been used in the Meerwein-Ponndorf-Verley reaction, where carbonyl compounds are reduced to alcohols, recent studies having shown that the reaction takes place exclusively by a carbon-to-carbon hydrogen transfer. 

The model has be, n shown to have good transferability when applied to a variety of crystal structures. This can be seen from Fig. 3 and Tables II and III, where the energies, vibrational and elastic properties for different coordinated crystalline structures obtained from this model are compared with first-principles LDA calculations and experiments. Applications in molecular-dynamics study of the liquid and amorphous phases of carbon as well as the structures of carbon clusters indicate that the potential does a good job of describing carbon systems over a wide range of bonding environments. These applications are reviewed in Section IV. 

As mentioned, metal-cluster interaction implies charge transfer to the clusters. However such a charge density appears to remain localized in a sector of the sphere and not to extend into the whole carbon cluster, thus not affecting directly the energy of the cluster frontier molecular orbitals. That explains, for instance, the fact that the compounds (Et3P2)2M( 7 -C6o) for M = Ni, Pd, and Pt have all the same reduction potential in spite of the different capabilities of... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Reduction to Alcohols. The organosilane-mediated reduction of ketones to secondary alcohols has been shown to occur under a wide variety of conditions. Only those reactions that are of high yield and of a more practical nature are mentioned here. As with aldehydes, ketones do not normally react spontaneously with organosilicon hydrides to form alcohols. The exceptional behavior of some organocobalt cluster complex carbonyl compounds was noted previously. Introduction of acids or other electrophilic species that are capable of coordination with the carbonyl oxygen enables reduction to occur by transfer of silyl hydride to the polarized carbonyl carbon (Eq. 2). This permits facile, chemoselective reduction of many ketones to alcohols. 

One approach to promoting the kinetics of hydrogen transfer to bound carbon monoxide is based on maximizing the difference in polarity of the carbon (eg. 6+) and hydrogen (eg. 6-) involved (Jt). This strategy leads naturally to a bimolecular approach, based upon MCO and M H. The additional degree of freedom which follows from employing two different transition metals is noteworthy as an alternative to cluster activation or catalysis. 

Two important conclusions can be drawn from the simunary of the symmetry analysis of Ar/CO collisions in Table 6. First, no SIKIE is predicted for C substitution because the symmetry of the system is independent of the isotope of carbon involved. Second, because the predicted a based symmetry restrictions for Ar COj cluster formation are identical to those predicted for (002)2, dependence of the magnitude of observed 0 SIKIE on the conditions of CO2 formation is expected. However, the e/f parity label state propensities for El-produced COJ, inferred from 0 SIKIE in (COj) formation, are not sufficient to predict the magnitude of 0 SIKIE in Ar-COj formation because, for above the threshold for Ar formation, COj ions are also produced by the charge-transfer reaction,... 

---