Ferracyclopentadiene complexes

A study of optically active substituted analogues of complex 8.15, shows that they undergo rapid thermal isomerization. It is very reasonably suggested that the isomerization occurs by an intramolecular interchange of the roles of the two iron atoms, via an intermediate such as 8.16 [60]. 

Another product formed in the reaction between diphenylacetylene and Fe3(CO)i2 is the complex 8.17 [54], whose structure has been shown by 

C6H5CH=C(C6H5)(C6H5)C=CHCgH5 and reduced products 

X-ray analysis [61]. The X-ray studies show that in the complex 8.17, the unco-ordinated portion of the benzene ring has alternating C=C distances. 

None the less, the benzene ring seems to retain aromatic properties [62]. It is interesting to contrast the related complexes bis-7r-indenyliron [63, 64] and r-cyclopentadienyl- r-indenyl iron [65, 66] where some stabilization of the Kekule structure of the benzene system of the Tc-inden ligand is suggested, since there is loss of aromaticity of the ring. Thus the uncoordinated diene part of the benzene ring in 8.17 is readily hydrogenated. 

---

Both iron- and nickel-mediated cyclooligomerization processes have been used in direct routes to pyrrole derivatives. Preformed ferracyclopentadiene complexes can be converted in moderate yields into 2,3,4,5-tetraphenyl-... 

Thiophenes are desulfurized by chromium and iron atoms. The role of complex intermediates is uncertain but with iron the final product appears to be a ferracyclopentadiene derivative (18) ... 

The reaction of two alkynes in the presence of pentacarbonyliron affords via a [2 + 2 + 1]-cycloaddition tricarbonyl(ri4-cyclopentadienone)iron complexes (Scheme 1.6) [5, 21-23]. An initial ligand exchange of two carbon monoxide ligands by two alkynes generating a tricarbonyl[bis(ri2-alkyne)]iron complex followed by an oxidative cyclization generates an intermediate ferracyclopentadiene. Insertion of carbon monoxide and subsequent reductive elimination lead to the tricarbonyl(T 4-cyclopentadienone)iron complex. These cyclopentadienone-iron complexes are fairly stable but can be demetallated to their corresponding free ligands (see Section 1.2.2). The [2 + 2 + l]-cycloaddition requires stoichiometric amounts of iron as the final 18-electron cyclopentadienone complex is stable under the reaction conditions. 

Despite the rich catalytic alkyne cyclization chemistry of Fe , there are few mononuclear ferracyclopentadienes, perhaps because of their high reactivity e.g., reduction of FeCl2[P(CH3)3]2 with Na-Hg in 2-butyne gives the blue, highly volatile and thermally unstable [(CH3)3P]4Fe[C4(CH3)4] and the cyclotrimer hexamethylbenzene . A similar blue entity IV arises when the 4-coordinate complex (dad)2Fe is exposed to dimethyl butynedioate [dad = ethanedialbis(cyclohexylimine)] . 

Reaction of the vinylcyclopropane (145) with Fea(CO)9 yields (146), which on heating isomerizes to (147) and finally to (148) both (146) and (147) are fluxional. 6,6-Dicyclopropylfulvene reacts with Fe2(CO) to yield (149), characterized crystallographically. l,2-Dimethyl-4,4-dicyanotriafulvene reacts with Fes(CO)i2 to yield the ferracyclopentadiene derivative (150) reactions with l,2-diphenyl-4,4-diacetyltriafulvene yields only a tricarbonylferracyclopentene complex. - The complex (3,5-dimethylaceheptylene)Fe3(CO)8 has structure (151) the related Mn2(CO)e complex has structure (152). Activated... 

---