Cobalt cluster compounds

Similar chemistry occurs with R3SnCo(CO)4 if the reaction is performed photochemi-cally in hexane however, when conducted thermally in polar solvents, cobalt fluorides, R3SnF and cobalt cluster compounds are formed342. In the case of an attempted insertion into a Rh—Sn bond under photochemical conditions, ligand exchange chemistry occurred (equation 146)343. 

Few chemical reactions of lactone-carbonylcobalt complexes have been reported. Hydrogenation 24) of butenolactone-heptacarbonyldi-cobalt, Co2(CO)g(HC2H), gives y-butyrolactone, butyric acid, and dodecacarbonyltetracobalt. Treatment of the same lactone complex with sulfuric acid in warm acetone 50, 125) gives a cobalt cluster compound of formula Co3(CO)gCCH=CHCOOH. The use of methyl alcohol in place of acetone in the latter reaction yields Cog(CO)gCCH=CHCOOMe. 

Furthermore, the redox system of the cell could be influenced by the cobalt cluster. Compound 31 has been shown to form disulfides when treated with thiols such as glutathione or cysteine [124]. The activity of glutathione reductase is a very sensitive indicator for the redox state within cells. It has been shown that glutathione reductase activity was not influenced by 34, 31, ASS, or the alkyne ligand of 31 [109]. A mechanism of action based on changing the redox state of the cell can therefore be excluded. 

Cobalt cluster compounds have also been prepared from tri- and tetrahalides under phase transfer conditions according to equation 9.20 [49]. 

The cobalt cluster compounds which have been prepared by this method are recorded in Table 9.9 in which R refers to the substituent in equation 9.20. 

In a one-pot process for the preparation of the complexes from cobalt(II) nitrate, which is converted into the tetracarbonyl anion by the standard procedure [9], higher yields of (2) are claimed (R = Cl, 42% R = Br, 36% R = H, 30%) using cetyltrimethylammonium bromide as the catalyst. It is known that the cluster compounds are unstable under basic conditions and it was noted that, for example, in the preparation of the chloro compound, extended reaction times (4.5 hours) resulted in the total decomposition of the product [10]. 

Cobalt offers many possibilities of cluster-core-geometry, but the chemistry of cobalt clusters is limited, again due to the weakness of first row metal-metal bonds and their susceptibihty to nucleophilic cleavage. Only in case of the methinyl tricobalt enneacarbonyls has a singular chemistry been developed, and therefore these compounds will be treated under a separate heading. 

Heterometallic cobalt clusters are known with several other metal atoms (see later chapters). Here the ones with iron diould be mentioned. From mixtures of iron and cobalt carbonyls and chalcogen compounds the clusters EFeCo2(CO)9 with E = S, Se,... 

The spacer units in 3.60 are assembled from polyphosphazenes that bear p-bromophc-noxy side groups via a lithiation reaction, and treatment with a diorganochlorophosphine to give 3.62. The chemistry is summarized in reaction sequence (45).107 Polymer 3.62 coordinates to a variety of metallo species,108 including osmium cluster compounds and cobalt carbonyl hydroformylation catalysts. When used as a polymeric hydroformylation catalyst, this latter species proved how stable the polyphosphazene backbone is under the drastic conditions often needed for these types of reactions. The weakest bonds in the molecule proved to be those between the phosphine phosphorus atoms and the aromatic spacer groups. 

A related effect is that silicon-transition-metal compounds, once formed, undergo Si-M bond cleavage when dissolved in polar solvents (252,262,300,305,306,310). The products are often complex, but have been shown to include a cluster compound in one case involving cobalt (252). 

The chemistry of silicon-cobalt clusters is more complicated because of the ready elimination of CO from proximate Co(CO)4 groups (208) to give Co2(CO)7or Co CO)9units. This elimination is generally easier for silicon-cobalt compounds than for corresponding germanium and tin... 

Cluster Compounds.—Following the recent108 preparation of [Rh6C(CO)5]2 , the first carbido-cluster of the cobalt subgroup, two related species have now been obtained (Scheme 10).109 Preliminary single-crystal X-ray studies were carried out... 

Our group has used a combined metathesis-PKR for the synthesis of tricyclic compounds in one step. The process starts from pure cobalt complexed dienynes 52. The cobalt cluster acts first as a protecting group to avoid undesired enyne metathesis processes. The methodology allows the formation of tricyclic [6.5.5] (53) and [7.5.5] (54) structures including, in some examples, oxygen or nitrogen. Tricycles 53 are obtained in a total stereoselective manner, while compounds 54 are formed as mixtures of two diastereomers (Scheme 17) [110]. 

Silicon forms a variety of compounds containing clusters of transition metal atoms that exhibit a wide range of structures. DisUane reacts with Co2(CO)g to give the cobalt cluster (38) in which the silicon is coordinated to five Co atoms (equation 48). The Si-- Si distance of 2.817 A is too long for a bond (normally about 2.35 A) and the Si-Co distances average 2.336 A. ... 

---