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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.

The reaction of rac-(ebthi)Zr(η2-Me3SiC2SiMe3) (1) [ebthi = 1,2-ethylene-1,1‘-bis(η5-tetrahydroindenyl)] with an excess of ethylene at room temperature leads to the corresponding zirconacyclopentane 3, which was isolated as stable yellow crystals and characterized by an X-ray crystal structure analysis. At 203 K complex 1 reacts with only 1 equiv of ethylene to form the zirconacyclopentene 2, possibly an intermediate in the reaction from 1 to 3. Upon activation with tris(pentafluorophenyl)borane under an ethylene atmosphere, complex 3 was shown to be an effective catalyst for ethylene polymerization. The reaction of 2 equiv of styrene with the alkyne complex 1 leads to the unsymmetrically substituted zirconacyclopentane 6.

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The reduction of rac-(EBTHI)ZrCl2 [EBTHI = 1,2-ethylene-1,1‘-bis(η5-tetrahydroindenyl)] with equimolar amounts of magnesium in the presence of Me3SiC2SiMe3 in THF at room temperature gave the first zirconocene−alkyne complex containing no additional ligands, rac-(EBTHI)Zr(η2-Me3SiC2SiMe3), 2. With pyridine and (S)-(−)-nicotine complex 2 gave the ligand-containing complexes rac-(EBTHI)Zr(L)(η2-Me3SiC2SiMe3) (L = pyridine, 3; L = (S)-(−)-nicotine, 4). In the reaction of 2 with acetone the insertion product rac-(EBTHI)(ZrOCMe2C(SiMe3)C(SiMe3), 5, was formed. Complex 2 reacted with 2 equiv of PhNCHPh with substitution of the alkyne to form rac-(EBTHI)ZrNPhCHPhCHPhNPh, 6. Compounds 2 and 6 were characterized by an X-ray structure analysis.

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Depending on the nature of the metal and the ring size, lactams react with titanocene and zircocene complexes of bis‐(trimethylsilyl)acetylene Cp2Ti(Me3SiC2SiMe3) and Cp2Zr‐(L)(Me3SiC2SiMe3) (L = Py, THF) to yield different products. In the reaction of Cp2Zr(Py)(Me3SiC2SiMe3) with N‐methyl‐€‐caprolactam a simple ligand exchange reaction occurs and the complex Cp2Zr(Me3SiC2SiMe3)[OH2)5] (1) was isolated. With β‐propiolactam the alkenyl‐amido complex Cp2)(SiMe3)][–NH2] (2) was obtained, which indicates that an agostic metal‐hydrogen interaction has taken place. The reaction of Cp2Ti(Me3‐SiC2SiMe3) with €‐caprolactam gives after elimination of the alkyne and molecular hydrogen the first early transition metal complex with a deprotonated coordinated €‐caprolactam in a η2‐amidate bonding fashion (3). The obtained complexes were characterized by NMR spectra (1, 2) and crystal structure analysis (1–3) and discussed as elemental steps in anionic ring‐opening polymerization of lactams catalyzed by metallocene‐alkyne complexes.

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