Cobalt, mixed-metal clusters

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)]. 

A number of iron-cobalt mixed metal cluster compounds have been obtained by sequential addition of [Co2(CO)g] and [Fe2(CO)g] to [Cp Fe (C0)2-C = C-C=C-H] (77) (Scheme 24). Their formation involves addition of a metal fragment to the C = C bonds, reorganisation of the cluster framework, transfer of metal fragments, valence isomerisation of the C4(H) linkage and 1,2-H shift of the C4H ligand. 

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. ... 

Several mixed-metal clusters containing platinum and cobalt are known and some of them have been employed as methanol homologation catalysts.1 Among them, the title compound2 was first prepared unambiguously from the reaction of dichloro[l,2-ethanediylbis(diphenylphosphine)]platinum with sodium tetracarbonylcobaltate, Na[CO(CO)4]. The compound also may be prepared by the reaction of [l,2-ethanediylbis(diphenyl-phosphine)]bis(phenylethynyl)pIatinum with Co COJg.1... 

Chini et at. synthesized a series of mixed-metal clusters that contain various combinations of the metals within the cobalt triad (113). The... 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

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). 

Iron-cobalt bonds have also been studied within Fe-Co heterotrimetallics featuring FejCo " and FeCOj " cores [225]. The mixed-metal clusters were synthesized using a novel strategy of metal atom substitution (Co) into a preexisting Fej " core, [(L )Fe3(p-Cl)]2. The addition of 2 and Sequiv. of... 

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