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

The reaction of dipiperidinoacetylene (pipC Cpip, pip = NC5H10, 1a) with [Cp2Ti(eta(2)-btmsa)] (2) or with [Cp2Zr(eta(2)-btmsa)(py)] (4) (btmsa = bis(trimethylsilyl)acetylene, py = pyridine) afforded the metallacyclopentadienes [Cp2M(C(4)pip(4))] (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*Ti-2) and -zirconocene (Cp*Zr-2) complex fragments can only accommodate one alkyne ligand. Thus, the titanacyclopropene [Cp*2Ti(C(2)pip(2))] (7) was isolated from the reaction of la with [Cp*2Ti(eta(2)-btmsa)] (6) or with [Cp*2TiCl] in the presence of magnesium, whereas the zirconacyclopropenes [Cp*Zr-2(C2X2)] (8a, X = pip; 8b, X = NC5H9-4-Me; 8c, X = NEW were prepared by the reduction of [Cp*2ZrCl2] with magnesium in the presence of la, 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-diaminovinyi isomers [Cp*(eta(6)-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 center dot MgCl2](2), 8b, and 8c were established by X-ray diffraction analyses.