Abstract
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, [Ti-II{eta(5)-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 degrees C, while under vacuum at 140 degrees C it liberated hydrogen to give the asymmetrical doubly tucked-in titanocene [Ti-II{eta(3):eta(4)-C5Me2(SiMe3)(CH2)(2)}{eta(5)-C5Me4(SiMe3)}] (3).
The mono- and dimethyl derivatives of 1, the complexes [(TiMe)-Me-III{eta(5)-C5Me4(SiMe3)}(2)] (5) and [(TiMe2)-Me-IV{eta(5)-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 [Ti-III{eta(5):eta(1)-C5Me4(SiMe2CH2)}{eta(5)-C5Me4(SiMe3)}] (4) was obtained from 5, and compound 6 afforded [Ti-II{eta(6):eta(1)-C5Me3(CH2)(SiMe2CH2)}{eta(5)-C5Me4(SiMe3)}] (7) at only 90 degrees 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 eta(3):eta(4)-allyldiene and eta(6)-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.