Titanocene and Zirconocene Complexes with Diaminoacetylenes: Formation of Unusual Metallacycles and Fulvene Complexes

The reaction of dipiperidinoacetylene (pipCCpip, pip = NC<sub>5</sub>H<sub>10</sub>, <b>1a</b>) with [Cp<sub>2</sub>Ti­(η<sup>2</sup>-btmsa)] (<b>2</b>) or with [Cp<sub>2</sub>Zr­(η<sup>2</sup>-btmsa)­(py)] (<b>4</b>) (btmsa = bis­(trimethylsilyl)­acetylene, py = pyridine) afforded the metallacyclopentadienes [Cp<sub>2</sub>M­(C<sub>4</sub>pip<sub>4</sub>)] (<b>3</b>, M = Ti; <b>5</b>, M = Zr), which in the solid state exhibit twisted five-membered metallacycles with an unusual half-chair conformation. In contrast, the sterically more demanding decamethyltitanocene (Cp*<sub>2</sub>Ti) and -zirconocene (Cp*<sub>2</sub>Zr) complex fragments can only accommodate one alkyne ligand. Thus, the titanacyclopropene [Cp*<sub>2</sub>Ti­(C<sub>2</sub>pip<sub>2</sub>)] (<b>7</b>) was isolated from the reaction of <b>1a</b> with [Cp*<sub>2</sub>Ti­(η<sup>2</sup>-btmsa)] (<b>6</b>) or with [Cp*<sub>2</sub>TiCl] in the presence of magnesium, whereas the zirconacyclopropenes [Cp*<sub>2</sub>Zr­(C<sub>2</sub>X<sub>2</sub>)] (<b>8a</b>, X = pip; <b>8b</b>, X = NC<sub>5</sub>H<sub>9</sub>-4-Me; <b>8c</b>, X = NEt<sub>2</sub>) were prepared by the reduction of [Cp*<sub>2</sub>ZrCl<sub>2</sub>] with magnesium in the presence of <b>1a</b>, bis­(4-methylpiperidino)­acetylene (<b>1b</b>), and bis­(diethylamino)­acetylene (<b>1c</b>), respectively. NMR studies showed that complexes <b>8</b> are in equilibrium with their tucked-in tetramethylpentafulvene–diaminovinyl isomers [Cp*­(η<sup>6</sup>-C<sub>5</sub>Me<sub>4</sub>CH<sub>2</sub>)­Zr­(CX=CHX)] (<b>9</b>) in solution, which are formed by intramolecular C–H-bond activation and hydrogen transfer from one Cp* methyl group to the alkyne ligand. Thermodynamic and kinetic parameters were derived by variable-temperature NMR spectroscopy and DFT experiments. The molecular structures of <b>3</b>, <b>5</b>, <b>7</b>, <b>8a</b>, [<b>8a</b>·MgCl<sub>2</sub>]<sub>2</sub>, <b>8b</b>, and <b>8c</b> were established by X-ray diffraction analyses.