Ruthenium metal carbonyl clusters

It should be stressed that, in this treatment of metal-carbonyl clusters, the number of nonbonding electron pairs allocated to each metal atom [3 pairs for each ruthenium atom of H2Ru8(CO)i8 2 pairs for each rhodium atom of Rhe(CO)i6] is not arbitrary but is chosen with two objectives in mind (o) to reduce the number of electrons formally remaining for skeletal bonding to fewer than the number of orbitals remaining, because only then is it realistic to assume that all these electrons can be accommodated in bonding MO s and (6) to provide a suitable number of electron pairs on each metal atom for metal carbon... 

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... 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

The water-gas-shift reaction catalysed homogeneously in the presence of polynuclear metal carbonyls is of current interest. In some ruthenium systems, the principal species present in basic solutions under reaction conditions of one atmosphere pressure and at 100°C are [HRu3(CO)n] and [H3Ru4(CO)i2] . This has reasonably been taken as evidence to implicate ruthenium clusters as the probable catalysts, although it should be noted that mononuclear systems effectively promote the water-gas-shift reaction. An important finding is that mixed ruthenium-iron carbonyl clusters, e.g.. 

K. Lazar, Z. Schay and L. Guczi, Direct evidence for the correlation between surface carbon and carbon monoxide + hydrogen selectivity on iron and iron-ruthenium catalysts prepared form metal carbonyl clusters, I. Mol. Catal. 17(2-3) (1982) 205-218. 

Ruthenium-Rhodium Bimetallic Catalysis. In seeking to inqprove the ethylene glycol syntheses of Table 1, one possibility that has not been extensively studied until recently (46-49), is the use of mixed metal centers with bimetallic, polymetallic or bridged-metal carbonyl clusters either as catalyst precursors, or generated in situ. 

Other organic processes facilitated by metal carbonyl clusters include a palladium carbonyl catalysed Diels-Alder reaction the selective reduction of aromatic nitro compounds using rhodium and ruthenium phosphine-carbonyls aza- and oxa-carbonylations of allyl phosphates by rhodium carbonyls Michael reactions of alkoxy-alkenones using iron... 

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... 

Syntheses of medium- and high-nuclearity ruthenium and osmium clusters continue to be largely by thermolyses of lower nuclearity precursors, but surface-mediated methods have also been employed the use of inorganic oxides or zeolites in the preparation of metal carbonyl clusters, including pentaosmium and hexaruthenium carbido clusters, and the decaosmium complexes [H50sio(CO)24] and [Osio(/t6-C)(CO)24], has been reviewed. 

Indeed, there is a unity with the field of heterogeneous catalysis. As evidence of this, similar (or identical) Rh (C0)2 sites can be prepared either by CO chemisorption on preformed metal particles [69] or by decomposition of rhodium carbonyl clusters on the oxide surface [62-66]. Further evidence for this can be seen from the observation of metal carbonyl clusters under operating supported metal catalysts. For example, ruthenium catalysts for the conversion of synthesis gas to polymethylene [122] afford mixtures of cluster species at elevated temperatures (120°C) and pressures (1000 atm) [123]. One of these was Ru3(CO)i2 others appear to be ill-characterised. A similar observation has been recently reported for Ru/MgO and Os/MgO synthesis gas conversion catalysts [124]. On this basic support, two anionic clusters were isolated, viz. [Ru5C(C0)i5] and [OsiQC(CO)24] 7 which may be synthesised in solution by thermolysis in basic or reducing media. It is unclear whether these clusters are actually effecting the catalysis. They may instead, as highly stable species, be formed in a side reaction. 

CO2MC] were obtained by metal exchange liom (he same ruthenium-dicobalt precursor and analogous functionalized (cyclopentadienyl)metal carbonyl Related reactions of the selenido-containing cluster RuCo2(/r rSe)... 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

Nakabayashi, M., M. Yamashita, and Y. Saito, Preparation of size-controlled ruthenium metal particles on carbon from hydro-carbonyl cluster complex. Chem. Lett., 1275-1278 (1994). 

Cabeza, J.A., in Braunstein, P., Oro, L.A., Raithby, P.R. (Eds.), Homogeneous Catalysis with Ruthenium Carbonyl Cluster Complexes Metal Clusters in Chemistry, Vol. 2. Wiley-VCH GmbH, Weinheim,... 

However, while ruthenium carbonyl was found to decompose the formate ion in basic media, the rate was slower (<0.1 mmol trimethyl ammonium formate to H2 and C02 per hour) than the rate of the water-gas shift reaction (>0.4 mmol H2/hr) at 5 atm CO and 100 °C. Increasing CO pressure decreased the formate decomposition rate. However, it was observed that increasing the CO pressure from 5 atm CO to 50 atm increased the H2 production rate to 10 mmol/hr. They proposed, in a similar manner to Pettit et al.,34 a mechanism that involved nucleophilic attack by amine (instead of hydroxide). Activation of the metal carbonyl (e.g., Ru3(CO) 2 cluster to Ru(CO)5) was suggested to be favored by dilution, increases in CO pressure, or, in the case of Group VIb metal carbonyl complexes, photolytic promotion. The mechanism is shown below in Scheme 9 ... 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

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