Osmium carbonyl clusters reduction

Roberto, D., Cariati, E., Ugo, R., and Psaro, R., Surface-mediated organometallic synthesis High-yield preparations of neutral and anionic osmium carbonyl clusters by controlled reduction of silica-supported [Os(CO)3Cl2]2 and OsClj in the presence of Na2C03 or K2C03. Inorg. Chem. 35,2311 (1996). 

We recently obtained evidence for a step-by-step process (Scheme 5) which explains the origin of the high selectivity observed in the controlled reduction of silica-supported [Os(00)3012)2 to generate different neutral or anionic osmium carbonyl clusters, in the presence of alkali metal carbonates (Schemes 1 and 2). ... 

Fig. 2.56. Reduction reactions of osmium carbonyl clusters [John GR, Johnson BFG, Lewis J (1979) J. Organomet. Chem. 186 C69 Vargas MD, Nicholls JN (1986) Adv. Inorg. Chem. Radio Chem 30 123]...

Metal carbonyl cluster compounds which contain three ruthenium or three osmium atoms in the cluster core are common.1 Potentially useful reagents for syntheses of these compounds are the triruthenium and triosmium dianions [M3(CO)h]2 (M = Ru, Os).2 Therefore, it is desirable to develop good synthetic routes to obtain [M3(CO)11]2- (M = Ru, Os) of high purity in high yields. A method that is particularly useful for generating [M3(CO)n]2 (M = Ru, Os) is the designed stoichiometric reduction of M3(CO)12 (M = Ru, Os) using an electron carrier such as potassium-benzophenone.3... 

Following the isolation of these complexes, all of the mechanistic studies on the carbonylation and reduction reactions of nitroarenes catalysed by Ru3(CO)i2, even in the presence of several promoters, have focused on the reactivity of these or related clusters [157-164]. Moreover, many studies have been also conducted on analogous osmium [165-172] and iron (see paragraph 6.6.) clusters, including insertion reactions of isocyanates, which yield potential intermediates in the carbonylation reaction (Insertion reaction of other cumulenes into the Ru-N bond will not be discussed here. However, see the paragraph of the synthesis of heterocycles later in this chapter). Although not all of the previously mentioned studies were intended to be a basis for a mechanistic understanding of the reactions here discussed, they still contain a lot of information on the possible transformations of amido or imido moieties on a trinuclear cluster. 

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

Large metal clusters have been supported on many metal oxides. For example, the carbido-cluster anion [OsioC(CO)24] has been adsorbed directly onto MgO. This cluster anion can also be prepared in situ on MgO by the Reductive Carbonylation of adsorbed H2OSCI6. The adsorbed osmium cluster anion appears to be ionically bound to the surface. 

Several other polynuclear metal carbonyls were tested as catalysts for the reduction of nitrobenzene to aniline using carbon monoxide and water as the reducing agent. Rhodium, iridium and osmium clusters were found to be very effective and they were far less susceptible to oxidative degradation than [Fe(CO)5]. 3... 

When the reductive carbonylation (1 atm CO or CO -h H2) of H2OSCI6 supported on MgO is performed at 200-250 °C the only surface carbonyl species detected are osmium(II) di- and tricarbonyl species of the type [Os(CO) c OMg 2] x = 2 or 3). These surface intermediates are quite stable under CO or CO -i- H2 and are reduced to give anionic osmium clusters only at temperatures approaching 275 °C. The reduction of the osmium(II) subcarbonyl species and the initiation of cluster growth were found to coincide with the loss of chemically bound water from the MgO surface, which becomes significant only at temperatures exceeding 250 °C. 

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