To Couple or Not To Couple: The Dilemma of Acetylide Carbons in [(η~5-C_5H_5)_2M(μ-CCR)_2M(η~5-C_5H_5)_2] Complexes (M = Ti, Zr)- A Theoretical Study for R = H, F

摘要

C-C coupling reactions mediated by transition metal complexes form a major area of chemical research. Several sophisti-cated'synthetic steps are brought about by multinuclear transition metal organometallics. We focus our attention here on C-C coupling reactions with diamagneric homodinuclear Ti and Zr complexes. More than 25 years ago Teuben and de Liefde Meijer suggested structures la or 2a (Scheme 1) for their dimer of bis(η~5-cyclopentadienyl)phenylethynyl titanium. An X-ray structure analysis of the related [(η~5-C_5H_4CH_3)_2Ti(μ-PhC_4Ph)Ti(η~5-C_5H_4CH_3)_2] by Sekutowski and Stucky in 1976 showed it to have the C-C coupled georne-try 3b; a result that, along with spectral data, demonstrated that Teuben's compound also has a similar structure (3a). It was also shown that 3b can be obtained from PhCsC-C = CPh. In 1989 Hawthorne and co-workers discovered that trimethylsilyl-substituted acetylene led to the Ti complex 2c: the C-C coupled product 3c was not formed.