Cationic and neutral [4]-cumulenes C=C=C=C=C with five cumulated carbons and three to four ferrocenyl termini

Starting from 1,1-diferrocenyl-1-methoxypropyne, the first air-stable C-5-cumulenium salt (Fc)(2)C=C=C=C-C(Fc)+BF4- (Fc = ferrocenyl) can be synthesized by a metalation, iodination, cross-coupling, and dehydration sequence. Attempted nucleophilic attack of ferrocenyl carbanion affords only radical decomposition products and none of the desired tetraferrocenyl-C-5-cumulene (Fc)(2)C=C=C=C=C(Fc)(2). In contrast, reaction with phenyllithium yields the nucleophilic addition product (Fc)(2)C=C=C(R)CrCFe (R = C6H5) with 100% regioselectivity. Starting from diferrocenyl ketone, the first air-stable protonated C-5-cumulenium salt (Fc)(2)CCH=C=C=C(Fc)(2)+BF4- is prepared by stepwise substitution of propyne with diferrocenyl(methoxy)methyl groups and subsequent 2-fold dehydration. Addition of methoxide affords (Fc)(2)C=C=C(R)CH=C(Fc)(2) (R = OCH3); attempted conversion to tetraferrocenyl-C-5-cumulene (Fc)2C=C=C=C=C(Fc)2 by thermal elimination of methanol in vacuo fails but yields an unusual cyclobutene dimer with eight ferrocenyl substituents. Finally, deprotonation of (Fc)(2)CCH=C=C=C(Fc)(2)+BF4- with "super base" n-BuLi/t-BuOK gives access to tetraferrocenyl-C-5-cumulene (Fc)(2)C=C=C=C=C(Fc)(2), one of the very few C-5-cumulenes known. IR, Raman, UV-vis, MS, NMR, Mossbauer spectroscopy, cyclic voltammetry, and X-ray crystallography are used for a detailed characterization of these metallocenylcumulene compounds.