Synthesis and Structure of Titanocene Complexes with η<SUP>2</SUP>-Coordinated Internal Ferrocenylacetylenes

The reduction of (η⁵-C<INF>5</INF>H<INF>5</INF><INF>-</INF><INF>n</INF><INF></INF>Me<INF>n</INF><INF></INF>)<INF>2</INF>TiCl<INF>2</INF> (n = 0, 4, and 5) complexes by magnesium metal in tetrahydrofuran and in the presence of [(trimethylsilyl)ethynyl]ferrocene (<BO>2</BO>) or [(phenyl)ethynyl]ferrocene (<BO>3</BO>) affords the (η⁵-C<INF>5</INF>H<INF>5</INF><INF>-</INF><INF>n</INF><INF></INF>Me<INF>n</INF><INF></INF>)<INF>2</INF>Ti(η²-FcC&tbd1;CR) complexes [Fc = (η⁵-C<INF>5</INF>H<INF>5</INF>)Fe(η⁵-C<INF>5</INF>H<INF>4</INF>), R = SiMe<INF>3</INF> (<BO>4</BO>−<BO>6</BO>) and Ph (<BO>7</BO>−<BO>9</BO>)]. Crystal structures of <BO>5</BO> and <BO>9</BO> show a titanacyclopropene-like mode of coordination of the acetylenes <BO>2</BO> and <BO>3.</BO> Bonding of the acetylenes to the titanocene unit results in a remarkable downfield shift of ¹³C NMR resonances of the acetylenic carbon atoms and in a large red shift of the ν(C&tbd1;C) wavenumbers. Testing the complexes <BO>6</BO> and <BO>9</BO> toward head-to-tail dimerization of HC&tbd1;CSiMe<INF>3</INF> showed that compound <BO>9</BO> induces dimerization to give exclusively 2,4-bis(trimethylsilyl)but-1-en-3-yne (<BO>10</BO>), whereas complex <BO>6</BO> is inactive.