Synthesis and structure of titanocene complexes with eta(2)-coordinated internal ferrocenylacetylenes

The reduction of (eta(5)-C5H5-Me-n(n))(2)TiCl2 (n = 0, 4, and 5) complexes by magnesium metal in tetrahydrofuran and in the presence of [(trimethylsil yl)ethynyl]ferrocene (2) or [(phenyl)ethynyl]ferrocene (3) affords the (eta (5)-C5H5-nMen)(2)Ti(eta(2)-FcC equivalent to CR) complexes [Fe = (eta(5)-C5 H5)Fe(eta(5)-C5H4), R = SiMe3 (4-6) and Ph (7-9)]. Crystal structures of 5 and 9 show a titanacyclopropene-like mode of coordination of the acetylenes 2 and 3. Bonding of the acetylenes to the titanocene unit results in a rem arkable downfield shift of C-13 NMR resonances of the acetylenic carbon ato ms and in a large red shift of the v(C equivalent to C) wavenumbers. Testin g the complexes 6 and 9 toward head-to-tail dimerization of HC equivalent t o CSiMe3 showed that compound 9 induces dimerization to give exclusively 2, 4-bis(trimethylsilyl)but-1-en-3-yne (10), whereas complex 6 is inactive.