Reduction-Induced Cyclization and Redox Reactions of Fully Methylated Titanocene Dichlorides Bearing Pendant Alkenyldimethylsilyl Groups, [TiCl₂{η⁵-C₅Me₄(SiMe₂R)}₂] (R = Vinyl and Allyl)

Reduction of titanocene dichlorides bearing (ω-alkenyl)dimethylsilyl substituents [TiCl2{η5-C5Me5(SiMe2R)}2], where R = CHCH2 (3) and CH2CHCH2 (4), affords highly reactive Ti(II) intermediates, which immediately undergo intramolecular reactions with the pendant double bonds in a way dependent on the length of the alkenyl chain. The reduction of 4 with magnesium at 60 °C affords cleanly the cyclopentadienyl-ring-tethered titanacyclopentane, [Ti(η1:η1:η:5η5-C5Me4SiMe2CH(Ti)CH(Me)CH(Me)CH(Ti)SiMe2C5Me4] (5), which can be opened by HCl to an ansa-titanocene dichloride with a six-membered saturated bridging chain, [TiCl2{η:5η5-C5Me4SiMe2CH2CH(Me)CH(Me)CH2SiMe2C5Me4)2}] (6). An analogous reaction of 3 can be accomplished at low temperatures and only to some extent, providing a mixture of titanacyclopentane complex [Ti(η1:η1:η:5η5-C5Me4SiMe2CH(Ti)CH2CH2CH(Ti)SiMe2C5Me4] (10) and the fluxional η2-alkene complex [Ti(η5-C5Me4SiMe2CHCH2)(η2:η5-C5Me4SiMe2CHCH2)] (9). At 60 °C, the reduction of 3 yields a mixture of ansa-η2-alkene complex [Ti{η2:η5:η5-C5Me4SiMe2CH2CHCHCH2SiMe2C5Me4}] (7) and the product of a hydrogen loss, the doubly tethered, paramagnetic η3-allyl Ti(III) complex [Ti{η3:η5:η5-C5Me4SiMe2CHCHCHCH2SiMe2C5Me4}] (8). When the reduction of 3 with magnesium is carried out in the presence of bis(trimethylsilyl)ethyne as the proton acceptor, 8 is obtained as the sole isolated product. Complex 8 reacts with protic reagents such as HCl and methanol under protonation/decomplexation of the allyl system to give the d1 paramagnetic complexes [TiX{η:5η5-C5Me4SiMe2CH2CHCHCH2SiMe2C5Me4}] (X = Cl, 12; OMe, 13). Oxidation of 8 with PbCl2 yields η1-alkenyl complex [TiCl{η1:η:5η5-C5Me4SiMe2CH(Ti)CHCHCH2SiMe2C5Me4}] (11). A similar oxidation of 12 to ansa-titanocene dichloride [TiCl2{η:5η5-C5Me4SiMe2CHCHCHCH2SiMe2C5Me4}] (14) containing a symmetrical unsaturated bridging chain requires a stronger oxidizing agent, AgCl. Compounds 5, 7, 8, and 10−14 have been characterized by X-ray crystallography.