Synthesis and characterization of multiferrocenyl-substituted group 4 metallocene complexes.

The reaction of different metallocene fragments [Cp(2)M] (Cp=η(5)-cyclopentadienyl, M=Ti, Zr) with diferrocenylacetylene and 1,4-diferrocenylbuta-1,3-diyne is described. The titanocene complexes form the highly strained three- and five-membered ring systems [Cp(2)Ti(η(2)-FcC(2)Fc)] (1) and [Cp(2)Ti(η(4)-FcC(4)Fc)](2) (Fc=[Fe(η(5)-C(5)H(4))(η(5)-C(5)H(5))]) by addition of the appropriate alkyne or diyne to Cp(2)Ti. Zirconocene precursors react with diferrocenyl- and ferrocenylphenylacetylene under C-C bond coupling to yield the metallacyclopentadienes [Cp(2)Zr(C(4)Fc(4))](3) and [Cp(2)Zr(C(4)Fc(2)Ph(2))](5), respectively. The exchange of the zirconocene unit in 3 by hydrogen atoms opens the route to the super-crowded ferrocenyl-substituted compound tetraferrocenylbutadiene (4). On the other hand, the reaction of 1,4-diferrocenylbuta-1,3-diyne with zirconocene complexes afforded a cleavage of the central C-C bond, and thus, dinuclear [{Cp(2)Zr(µ-η(1):η(2)-C≡CFc)}(2)] (6) that consists of two zirconocene acetylide groups was formed. Most of the complexes were characterized by single-crystal X-ray crystallography, showing attractive multinuclear molecules. The redox properties of 3, 5, and 6 were studied by cyclic voltammetry. Upon oxidation to 3(n+), 5(n+), and 6(n+) (n=1-3), decomposition occured with in situ formation of new species. The follow-up products from 3 and 5 possess two or four reversible redox events pointing to butadiene-based molecules. However, the dinuclear complex 6 afforded ethynylferrocene under the measurement conditions.