Rhodium mixed-metal clusters

In mixed metal clusters, redistributions can occur in the mass spectrometer so that the highest observed peak arises from such a product. This is clearly exemplified by the heterometallic dodecacarbonyls containing metals of the cobalt triad. The thermal stability of this series of clusters decreases as the rhodium content increases and facile redistributions result in the formation of tetranuclear species containing less rhodium the very thermally stable Q I CO) does not rearrange184. ... 

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

Independent discovery of the silylformylation of alkynes was reported by the Matsuda and Ojima groups. The general reaction involves addition of both CO and tertiary hydrosilane to an alkyne to yield silyl alkenals, catalyzed by rhodium or rhodium-cobalt mixed metal clusters [Eq. (46)]. 

The reaction of H2Os3(CO)10 with Rh(C2H4)2(MeCOCHCOMe) yields the mixed metal cluster RhOsjH2(MeCOCHCOMe)(CO)10 whose structure is shown in Figure 15. One of the acetyl-acetonate oxygen atoms, 0(3), unsymmetrically bridges Os(3) and Rh, while the other, 0(2), is coordinated only to rhodium so that this ligand can be considered as a five-electron donor.509... 

The cluster [CpRhFe2(CO)6(n3-Te)2] (213) exists in solution as an equilibrating mixture of two isomers, one in which the rhodium atom is in a basal site of the Fe2RhTe2 square pyramid, and another in which it occupies the apical site.131 In several other mixed-metal clusters that also exhibit a... 

Rhodium also forms an unusually large number of clusters containing encapsulated hetero atoms as, for example, the [Rhn(S)2(CO)32]3 species whose skeletal structure is shown in Fig. 16-15, and a number that contain carbon atoms. It also forms both smaller, for example, Rh CO), and larger, for example, Rh H CO) clusters that have no encapsulated atoms. The Rh12 species has a very novel structure in which two Rl octahedra are linked together to form an Rh6 octahedron between them. Finally, rhodium also forms many mixed metal clusters, especially with plati-... 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

The process is catalyzed by a number of cobalt and rhodium clusters 361). Anionic clusters such as [FeCo3(CO),2] and [Coj(CO),5] show greater catalytic activity than would be expected from neutral clusters of the same size 361). Other mixed-metal clusters, such as Zo2C04(CO),5 362) and Pt3Co2(CO)9(PPhj)j 363), have also been reported to be active for the stereospecific dimerization of norbomadiene to give 81. In the presence of BF3 Et20, Pt3Co2(CO)9(PPhj)3 converts norbomadiene into 81 with 100% yield and 100% selectivity (20 °C, 30 min, CT 31). 

Polynuclear anionic metal carbonyl compounds are usually prepared by reduction reactions of metal carbonyls M(CO) with such reducing agents as the alkali metals, NaBH4 in ethers, hydrocarbons, liquid ammonia, and similar solvents [see, for example, reactions (2.54), (2.55), (2.84), (2.89), (2.95>-(2.102), and (2.108)-(2.113)]. In alkali medium the metal carbonyls may be reduced by certain solvents (e.g., alcohols) or by the CO ligand itself, and in the presence of Lewis bases the carbonyls disproportionate to give anionic clusters. The mixed metal clusters containing platinum and rhodium are formed by reduction reactions of chloro complexes... 

The rhodium compound [Rh6(/i-AsBu2)2(iW4-AsBu )(/i-CO)2(CO)9] is probably the only example of a Rh cluster in which the Rhg framework is a pentagonal pyramid. Recently, the mixed metal cluster [Co6Ni2(C2)(CO)i6] having the structure of a biprism sharing a rectangular face (the carbon-carbon distance is 149.4 pm) was prepared.An important class of clusters are alkylidyne compounds of the type [Co3CR(CO)9]. 

In addition to these homometallic (rhodium) clusters, several hetero-metallic clusters of the type [M M CO o]2, where M and M1 are each different metals selected from the Co, Rh, Ir triad (jc = 1-11), have been described and claimed to be useful catalysts in the reaction between carbon monoxide and hydrogen to produce oxygenated products (68, 69). These complexes can be prepared from the heterometallic dodecacar-bonyl complexes, [MuM (CO)12] (M, M1 = Co, Rh, or Ir y = 1-3), by simply mixing the appropriate dodecacarbonyl species in THF under nitrogen and then adding water (70). They can be isolated by adding a suitable cation e.g., Al3+, Mg2+, Ca2+, etc. 

One of the most important links between alkylidyne and alkyne compounds is that one of the first synthetic routes for cobalt al-kylidynes involved alkynes as reagents (264-268). In later studies, several other synthetic routes to cobalt (269-280), rhodium (281, 282), iron (283-285), molybdenum (286, 287), ruthenium (288-292), osmium (293, 294), nickel (295, 296), and some mixed-metal (165, 297-302) clusters have been developed. Reagents employed include carbynes (166, 277, 280), alkali metals (269), carbon disulfide (275), dithioesters (276, 282), RCC13, and acids (281, 282). 

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