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

The reaction of dipiperidinoacetylene (pipCCpip, pip = NC5H10, 1a) with [Cp2Ti­(η2-btmsa)] (2) or with [Cp2Zr­(η2-btmsa)­(py)] (4) (btmsa = bis­(trimethylsilyl)­acetylene, py = pyridine) afforded the metallacyclopentadienes [Cp2M­(C4pip4)] (3, M = Ti; 5, 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*2Ti) and -zirconocene (Cp*2Zr) complex fragments can only accommodate one alkyne ligand. Thus, the titanacyclopropene [Cp*2Ti­(C2pip2)] (7) was isolated from the reaction of 1a with [Cp*2Ti­(η2-btmsa)] (6) or with [Cp*2TiCl] in the presence of magnesium, whereas the zirconacyclopropenes [Cp*2Zr­(C2X2)] (8a, X = pip; 8b, X = NC5H9-4-Me; 8c, X = NEt2) were prepared by the reduction of [Cp*2ZrCl2] with magnesium in the presence of 1a, bis­(4-methylpiperidino)­acetylene (1b), and bis­(diethylamino)­acetylene (1c), respectively. NMR studies showed that complexes 8 are in equilibrium with their tucked-in tetramethylpentafulvene–diaminovinyl isomers [Cp*­(η6-C5Me4CH2)­Zr­(CX=CHX)] (9) 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 3, 5, 7, 8a, [8a·MgCl2]2, 8b, and 8c were established by X-ray diffraction analyses.