Ruthenium oriented

Oxide catalysts, in general, show a smaller degree of activity toward carbon monoxide and hydrogen than the metal catalysts. High pressures and temperatures are required for conversion which is the result of surface reactions. Whereas the high hydrogenating power of cobalt, nickel, and ruthenium orient the hydrogenation of carbon monoxide almost entirely toward hydrocarbons, and the less active iron also produces some alcohols oxide catalysts favor the formation of alcohols. 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

Ruthenium complexes containing this ligand are able to reduce a variety of double bonds with e.e. above 95%. In order to achieve high enantioselectivity, the reactant must show a strong preference for a specific orientation when complexed with the catalyst. This ordinarily requires the presence of a functional group that can coordinate with the metal. The ruthenium-BINAP catalyst has been used successfully with unsaturated amides,23 allylic and homoallylic alcohols,24 and unsaturated carboxylic acids.25... 

Fig. 5.3. (a) View of (S)-BINAP-ruthenium complex showing the chiral environment, (b) Relationship of reactant to chiral environment showing preferred orientation. The binaphthyl rings are omitted for clarity. Adapted from J. 

Various X-ray crystal structures of metal-ligand complexes provided evidence of the geometry of the complexes in the solid state, even though the structure of these complexes may differ in solution. The hrst crystal structure of a bis(oxazoline)-metal complex was determined in 1994 by Brown and co-workers. " This group crystallized and elucidated the structure of V,V-bis-[2-((45)-(methyl)-l,3-oxazoli-nyl)]methane-bi(ri ethene)rhodium(I), 18a, as depicted in Figure 9.3. The key features of this crystal structure include the C2-axis of symmetry, the axial positions of the methyl groups and the orientation of the ethene molecules, orthogonal to the complexation square plane. In 1995, Woodward and co-workers were able to crystallize and determine the structure of benzylbis(oxazoline) with ruthenium... 

Li et al. [47] fabricated SAMs of ruthenium phthalocyanine (RuPc) on a silver substrate precoated with an SAM of 4-mercaptopyridine (PySH) or l,4-bis[2-(4-pyridyl)ethenyl]-benzene (BPENB). SERS spectroscopy was used to explore the structure and orientation of the self-assembled films, and they successfully observed Raman bands due to vibrational modes of the pigment molecules in the composite films in the SERS spectra. 

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