Cobalt complexes cyclopentadienyls

Fig. 5. Top X-ray structure of cobalt complex 19a in a shaded ball-and-stick representation. Note that the carbon atoms C1-C4 were part of the original cyclohexadiene ring in compound 16. The isolated 5-membered ring is the cobalt-complexed cyclopentadienyl ligand. Bottom Packing structure of 19a in the crystal showing the close proximity between the Cp ligands and the fullerene moieties...

In the context of pyridin synthesis from alkynes and nitriles homogeneously catalyzed by (cyclopentadienyl)cobalt complexes (Eq. 13), it was found that electron-withdrawing groups on the cyclopentadienyl ring significantly increase the activity [63]. During the screening of various cobalt half-sandwich complexes... 

Yong et al. developed a cobalt-catalyzed [2+2+2] cyclotrimerization of terminal alkynes in good yields in aqueous media (80/20 mixture of water and ethanol) at room temperature. A cyclopentadienyl cobalt complex bearing a pendant phosphine ligand was used as a catalyst (Eq. 4.59). The cyclotrimerization of internal alkynes resulted in lower yields and required an elevated temperature, most likely due to steric interactions. For example, cyclotrimerization of 2,5-dimethyl-3-hexyne gave hexaisopropylbenzene in 51% yield and the reaction of diphenylethyne resulted in a 47% yield of hexaphenylbenzene.112... 

In a later paper by Weiss,68 the methodology was extended to a more complex cyclopropene, and an intermediate cobaltacyclobutene (103) was proposed. In an analogous insertion reaction with nonacarbonyldiiron, a vinylcarbene complex was isolated along with the expected vinylketene complex (see Section VI,B). However, no such vinylcarbene cobalt complex was isolated, even when cyclopentadienyl bis(ethene) cobalt was used in place of dicarbonylcyclopentadienyl cobalt, and the only product isolated was the vinylketene complex 104, represented here in the rf -allylacyl structure. 

This complex is analogous in all ways to the cobalt complex (XXIV), and the proposed structure is supported by its infrared spectrum which shows bands due to the ir-cyclopentadienyl-metal grouping, a methylene group, and conjugated double bonds co-ordinated to the metal atom (at 1451 cm-1). 

More recently, the parent mono- and di-methylenecyclopropane cyclopentadienyl-cobalt -complexes were prepared by ligand exchange reactions583. Reaction of methyl-enecyclopropane (and its tetramethyl derivative) with CpCo(ethylene)2 in pentane affords... 

C. Cyclopentadienyl Cobalt Complexes of OFCOT and the Perfluorobicyclo[3.3.0]octadienediyl Ligand... 

The involvement of the a-elimination reaction in this cycle has been in question following experiments on cyclopentadienyl cobalt complexes, where evidence for olefin insertion for Ziegler-Natta polymerization catalysis has been obtained by labelling experiments, using C2H4 with a deuterated cobalt complex (70) ... 

Sm02C2sH44, Samarium(II), bis(Ti -penta-methylcyclopentadienyl)bis(tetra-hydrofuran)-, 27 155, 28 297 Sm03C42H63, Samarium, tris(2,6-di-tm-butylphenoxo)-, 27 166 Sm2Cl2Si4C44Hg4, Samarium, tetrakis T -l,3-bis(trimethylsilyl)cyclopentadienyl]di-p-chloro-di-, 27 171 SnCjHio, Stannane, trimethyl-, cobalt complex, 29 175, 178,180 SnCljPjC52H4, Stannate(l-), hexachloro-, 3,3,4,5-tetrahydro-l,l,33-tetraphenyl-lfM,2,3-triphospholium, 27 255 SnS4C24H2o, Tin(IV), tetrakis(benzenethio-lato)-,29 18... 

In addition to the release of carbon monoxide, transition metal carbonyl complexes have also found applications in anticancer chemotherapy [212], radiolabelling [213-216], and as photosensitisers. The cobalt complex shown in Fig. 14 a demonstrates a higher anticancer activity than cis-platin in certain mammary tiunor cells Hnes [217,218]. Complexes such as Tc-99m(I)(CO)3(OH2)3 along with hydroxymethyl phosphine derivatives [219] and the cyclopentadienyl complex shown in Fig. 14b have useful radiopharmaceutical applications, in diagnosis, using Tc, as the metal centre, and in therapy based on Re and Re isotopes [220]. 

---