Effect of the Trimethylsilyl Substituent on the Reactivity of Permethyltitanocene

The presence of a trimethylsilyl substituent in place of one of the methyl groups of each of the
cyclopentadienyl ligands of decamethyltitanocene enhances the thermal stability of the resulting complex,
[TiII{η5-C5Me4(SiMe3)}2] (1), and controls the products formed in thermolysis of its methyl derivatives.
Titanocene 1 was found to be stable in toluene solution up to 90 °C, while under vacuum at 140 °C it
liberated hydrogen to give the asymmetrical doubly tucked-in titanocene [TiII{η3:η4-C5Me2(SiMe3)(CH2)2}{η5-C5Me4(SiMe3)}] (3). The mono- and dimethyl derivatives of 1, the complexes [TiIIIMe{η5-C5Me4(SiMe3)}2] (5) and [TiIVMe2{η5-C5Me4(SiMe3)}2] (6), undergo thermolysis at lower temperature than
do the corresponding permethyltitanocene derivatives and eliminate hydrogen from their trimethylsilyl
group. Thus, the known [TiIII{η5:η1-C5Me4(SiMe2CH2)}{η5-C5Me4(SiMe3)}] (4) was obtained from 5,
and compound 6 afforded [TiII{η6:η1-C5Me3(CH2)(SiMe2CH2)}{η5-C5Me4(SiMe3)}] (7) at only 90 °C,
both with liberation of methane. Crystal structures of 3, 5, and 7 were determined. DFT calculations
for titanocene 1 revealed that the metal−cyclopentadienyl bonding is accomplished via a three-center−four-electron orbital interaction. An auxiliary long-range Si−C bond interaction with the Ti center
was also established, providing a reason for the enhanced thermal stability of 1. The molecular
orbitals participating in the exo methylene−titanium bonds for 3 and 7 are also three-centered and are
compatible with the assignment of their activated ligands to η3:η4-allyldiene and η6-fulvene structures,
respectively. Qualitatively, the much higher thermal stability of 3 and 7 compared to that of 1 is due to
the exploitation of four d orbitals in the bonding molecular orbitals for 3 and 7 versus only two d orbitals
for 1.