THE FIRST TITANACYCLIC 5-MEMBERED CUMULENE - SYNTHESIS, STRUCTURE, AND REACTIVITY

The first five-membered titanacyclic cumulene Cp(2)Ti-C(tBu)=C=C=C(tBu ) (5) was prepared by treatment of the titanocene generator Cp(2)Ti(Me (3)SiC(2)SiMe(3)) with the di-tert-butylbutadiyne in a molecular one-t o-one ratio. The reaction of 5 with ''Cp(2)Zr'' or of the analogous co mplex Cp(2)ZrC(tBu)=C=C=C(tBu) with ''Cp(2)Ti'' leads to cleavage of t he central C-C bond in the metallacyclocumulene complexes to afford th e same product, the heterodimetallic sigma,pi-alkynyl-bridged titanoce ne-zirconocene complex (1):eta(2)-C=CtBu)Cp(2)Ti(mu-eta(1):eta(2)-C=Ct Bu) (6), in which each alkynyl group is sigma-bonded to one and pi-bon ded to the other metal as shown by X-ray diffraction studies. In solut ion the two alkynyl groups become equivalent. The coupling reaction of ''Cp(2)Ti'' with bis(trimethylsilyl)butadiyne in a one-to-two molecul ar ratio leads to the unsymmetrically substituted titanacyclopentadien e i-C(SiMe(3))=C(C=CSiMe(3))-C(SiMe(3))=(C=CSiMe(3)) (7).