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.