Reduction-Induced Exclusive Activation of the <i>ansa</i>-1,2-Bis(dimethylsilylene)ethane Chain in <i>ansa-</i>Permethyltitanocene Compounds

The ansa-1,2-bis(dimethylsilylene)ethane chain is exclusively activated when the parent ansa-permethyltitanocene compounds are reduced to titanium(II) either by thermolysis of titanium−methyl bonds or by the reduction of the parent titanocene dichloride ansa-[ TiCl2(η5-CH2Me2SiC5Me4)2] (8) with magnesium. The compound [Ti(η2:η5:η5-C5Me4SiMe2CHCHSiMe2C5Me4)] (10), with the double bond back-bonded to titanium(II), is formed by thermolysis of ansa-[TiMe2(η5-CH2Me2SiC5Me4)2] (9) with elimination of methane or by hydrogen transfer from the ansa-chain to easily leaving ethene or 1,4-bis(trimethylsilyl)buta-1,3-diyne π-coordinated to an ansa-titanocene moiety. The paramagnetic compound ansa-[TiIII{η1:η5:η5-C5Me4SiMe2CHCH2SiMe2C5Me4}] (12), with the ansa-chain tethered to titanium, arises from demethanation of transiently formed ansa-[TiIIIMe(η5-CH2Me2SiC5Me4)2] or by the reduction of 8 with magnesium in tetrahydrofuran. The intermediacy of magnesium as a hydrogen transfer agent in the mechanism of the latter reaction is suggested.