Transition metal clusters reduction reactions

For mixed lanthanide-transition metal clusters, Yukawa et al. have synthesized an octahedral [SmNi6] cluster by the reaction of Sm3+ and [Ni(pro)2] in nonaque-ous medium [66-68]. The six [Ni(pro)2] ligands use 12 carboxylate oxygen atoms to coordinate to the Sm3+ ion, which is located at the center of an octahedral cage formed by six nickel atoms. The coordination polyhedron of the central Sm3+ ion may be best described as an icosahedron. The [SmNir, core is stable in solution but the crystal is unstable in air. The cyclic voltammogram shows one reduction step from Sm3+ to Sm2+ and six oxidation steps due to the Ni2+ ions. Later, similar [LaNis] and CjdNif> clusters were also prepared. 

There has been considerable recent research interest in the activation of carbon monoxide en route to more complex organic molecules. Among the various reactions that have been investigated and/or newly discovered, the transition metal catalyzed reduction of CO to hydrocarbons (Fischer-Tropsch synthesis) has enjoyed particular attention (l- ). Whereas most of the successful efforts in this area have been directed toward the development of heterogeneous catalysts, there are relatively few homogeneous systems. Among these, two are based on clusters (10,11) and others are stoichiometric in metal (12-17). In this report we detail the synthesis and catalytic chemistry of polystyrene ( ) supported... 

To the best of our knowledge, this is the first report of a dealkylation of tliis type mediated by a transition metal cluster carbonyl. The formation of this hydride could explain in part the large amount of amine 57 formed during the catalytic reactions. This compound has in fact been considered a key intermediate in the catalytic hydrogenation of nitrobenzene to aniline [97]. However, as has been discussed in Chapters 4 and 6, the participation of hydride derivatives in this type of reduction of the nitro to amino group is doubtful. 

We have found that the main group metal and metalloid reductants mocury, bismuth, and antimony are highly effective " in reducing WCIe or M0CI5 at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. BorosUicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit sohd-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products. 

The kinetics of oxidation and reduction of [4Fe-4S] proteins by transition metal complexes and by other electron-transfer proteins have been studied. These reactions do not correlate with their redox potentials.782 The charge on the cluster is distributed on the surface of HiPIP through the hydrogen bond network, and so affects the electrostatic interaction between protein and redox agents such as ferricyanide, Co111 and Mnin complexes.782 783 In some cases, limiting kinetics were observed, showing the presence of association prior to electron transfer.783... 

The conversion of dinitrogen to ammonia is one of the important processes of chemistry. Whereas the technical ammonia synthesis requires high temperature and pressure (1), this reaction proceeds at room temperature and ambient pressure in nature, mediated by the enzyme nitrogenase (2). There is evidence that N2 is bound and reduced at the iron-molybdenum cofactor (FeMoco), a unique Fe/Mo/S cluster present in the MoFe protein of nitrogenase. Although detailed structural information on nitrogenase has been available for some time (3), the mechanism of N2 reduction by this enzyme is still unclear at the molecular level. Nevertheless, it is possible to bind and reduce dinitrogen at simple mono- and binuclear transition-metal systems which allow to obtain mechanistic information on elemental steps involved... 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

Hieber and his school have carried out a systematic investigation of the transition metal carhonyl anions and derivative hydrides 169), and have synthesized many carbonyl anion and hydride clusters, particularly of the first row transition metals. The preparations of these clusters were all based on the reduction of a carbonyl compound. More recently a number of heteronuclear carbonyl anion and hydride clusters have been produced by condensation reactions. 

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