The reaction of ferrocenyl acetylene with [RuCp(PR3)(CH3CN)2]PF6 (R = Me, Ph, Cy).: Formation of the first allenyl carbene complexes

The reaction of [RuCp(PR3)(CH3CN)(2)]PF6, where R = Ph and Cy, with ferrocenyl acetylene (HCmCFc) affords the first eta (2)-allenyl carbene complexes [CpRu(=C(Fc)-eta (2)-CH=C=CH(Fc))(PR3)]PF6. The new type of C-C bond formation is affected by the strong electronic coupling of the ferrocenyl moiety with the conjugated allenyl carbene unit, which facilitates transient vinylidene formation with subsequent alkyne insertion into the Ru=C double bond. The vinylidene intermediate appears to occur only in the presence of bulky substituents of the phosphine co-ligand. For small R = Me, head-to-tail coupling between two alkyne molecules involving phosphine migration is preferred, giving the more usual eta (3)-allyl carbene complex [CpRu(=C(Fc)-eta (3)-CHC(Fc)CHPMe3]PF6. The X-ray structure of an eta (2)-allenyl carbene complex is presented. (C) 2001 Elsevier Science B.V. All rights reserved.