The Transient Titanocene(II): Direct Synthesis from Solvated Titanium(II) Chloride and Cyclopentadienylsodium and Ensuing Interception with Diphenylacetylene as 1,1‐Bis(cyclopentadienyl)‐2,3,4,5‐tetraphenyltitanacyclopentadiene

For the first time the unstable titanocene(II) has been directly synthesized by the Wilkinson metallocene approach, namely the interaction of a THF‐soluble form of titanium(II) chloride with two equivalents of cyclopentadienylsodium in THF solution at 0°–25 °C. Because of the transient existence of the titanocene(II) thereby obtained, it could only be chemically trapped in high yield as 1,1‐bis(cyclopentadienyl)‐2,3,4,5‐tetraphenyltitanacyclopentadiene by two equivalents of diphenylacetylene, if the acetylene was added at 25 °C, without removal of the by‐product LiCl and NaCl. If the addition of the acetylene was delayed, in order to filter off the LiCl and NaCl from the reaction mixture, then no trace of the titanacyclopentadiene derivative was found upon hydrolytic workup. Instead, a significant portion of the acetylene was found to have undergone hydrotitanation. This finding is clear evidence that the titanocene(II) had undergone a precedented rearrangement to a known dimer having the structure of a titanocene(III) hydride with a fulvalene bridge between the titanium centers. We suggest that the LiCl and NaCl present in the unfiltered reaction mixture form a dichloro complex with titanocene(II) and thereby retard its dimerizing rearrangement to the titanocene(III) hydride. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)