Dimerization of titanacyclocumulenes to titanium substituted radialenes: synthesis, stability and reactions of five-membered titanacyclocumulenes with a coupling of two 1,4-diphenyl-1,3-butadiyne between two titanocene molecules to radialene-like fused titanacyclopentadiene compounds

The reactions of the excellent titanocene source Cp2Ti(eta(2)-Me3SiC2SiMe3) with different 1,4-substituted 1,3-butadiynes RC=C-C=CR give, by substitution of the acetylene in a one to one complexation of titanocene and the diyne, different five-membered titanacyclocumulenes Cp2Ti[eta(2)-(1-2-3-4)-RC4,R]. While these complexes are very stable for R = Bu-t, the metallacyclocumulene with R = Ph is unstable in solution and stabilizes by dimerization to dinuclear isomers: a fused titanacyclopentadiene-titanacyclopentene complex 1 and a compound 2 consisting of two fused titanacyclopentadiene ring systems and thus possessing a titanium substituted radialene structure. With the monomethyl-cyclopentadienyl complex (H4MeC5)(2)Ti(eta(2)-Me3SiC2SiMe3) in the reaction with PhC=C-C=CPh the titanium substituted radialene 7 was isolated in a low yield among some other as yet unidentified complexes. Complexes 2 and 7 were investigated by an X-ray crystal structure analysis. (C) 1999 Elsevier Science S.A. All rights reserved.