From a historical point of view “From Silicon to Titanium and Back to Silicon”, the genesis of the titanocene complex of bis(trimethylsilyl)acetylene (1) is explained. Similarities to the known 1-silacyclopropenes led us to describe the bonding in 1 in terms of a 1-titanacyclopropene. Complex 1 is a useful source of the highly reactive bis(cyclopentadienyl)titanium(II), which undergoes both stoichiometric and catalytic reactions that bear a striking similarity to some of the reactions of the isolobal silicon analogues.

A thorough investigation of the reactions of diynes, R(C&tbd1;C)(2)R, and polyynes, R(C&tbd1;C)(n)()R, with titanocene and zirconocene was performed using the metallocene sources Cp(2)M(L)(eta(2)-Me(3)SiC&tbd1;CSiMe(3)) (M = Ti, L = -; M = Zr, L = THF, pyridine). The conversions show an array of different products generated in complexation, coupling, and cleavage reactions. These results include remarkable structures (e.g., five-membered metallacyclocumulenes) and reactions (e.g., C-C single-bond cleavage). In addition, the first C-C single-bond metathesis in homogeneous solution was discovered. The presented findings have been supported by theoretical studies.

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The first five-membered titanacyclic cumulene C p , T i -c (tBu)=C=C=k(tBu) (5) was prepared by treatment of the titanocene generator Cp2Ti(Me3SiC2SiMe3) with the di-tertbutylbutadiyne in a molecular one-to-one ratio. The reaction of 5 with "CpzZr" or of the analogous complex Cp2-krC(tBu)=C=C=k(tBu) with "Cp,Ti" leads to cleavage of the central C-C bond in the metallacyclocumulene complexes to afford the same product, the heterodimetallic u,n-alkynyl-bridged titanocene-zirconocene complex CpzZr(p-q' :q2-In cyclizations of unsaturated compounds using the metallocene fragments "Cp2M" (M = Ti or Zr) ['], which are frequently employed preparatively, little is known about reactions with 1 ,Cdisubstituted 1 ,3-butadiyne~[~I.Recently, we have found that two equivalents of the titanocene complex Cp2Ti(Me3SiC2SiMe3)[31, an excellent titanocene source, react with one equivalent of disubstituted butadiynes to give dinuclear complexes of type 114] or Z5]. The obtained types of complexes are well-known[6], but we have found a new way of preparation in which, depending on different substituents, the central C-C single bond of the starting butadiyne is cleaved. R" R"With the complex Cp2Zr(THF)(Me3SiC2SiMe3)[7a] as zirconocene synthon the cleavage reaction to complexes of type l[7bl is more favoured, but when Cp2Zr(Py)(Me3SiC2-SiMe3) (Py = pyridine)[sal was used five-(3) and sevenmembered (4) zirconacyclic cumulene complexes were formed, depending as well on the substituentsLsb1. However, C=CtBu)Cp,Ti(p-q' :q2-C=CtBu) (6), in which each alkynyl group is a-bonded to one and rr-bonded to the other metal as shown by X-ray diffraction studies, In solution the two alkynyl groups become equivalent. The coupling reaction of "Cp2Ti" with bis(trimethylsily1)butadiyne in a one-to-two molecular ratio leads to the unsymmetrically substituted tita-analogous cumulene complexes of titanocene have been unknown so far.In the present paper we report on the synthesis, spectral characteristics, and some chemical properties of the first five-membered metallacyclic cumulene complex of titanocene which can act as an intermediate in the cleavage reaction of diynes. Results and DiscussionThe reaction of Cp2Ti(Me3SiC2SiMe3) with equimolar amounts of tBuC=C-C=CtBu in hexane at room temperature affords green solutions from which the metallacyclic cumulene 5 can be isolated by cooling to -40 "C. Also small amounts of the dinuclear complex 2a are formed after a reaction time of I to 2 days.