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Effect of the Trimethylsilyl Substituent on the Reactivity of Permethyltitanocene

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journal contribution
posted on 07.05.2020, 10:01 by Jiří Pinkas, Lenka Lukešová, Róbert Gyepes, Ivana Císařová, Peter Lönnecke, Jiří Kubišta, Michal Horáček, Karel Mach
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, [TiII5-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 [TiII34-C5Me2(SiMe3)(CH2)2}{η5-C5Me4(SiMe3)}] (3). The mono- and dimethyl derivatives of 1, the complexes [TiIIIMe{η5-C5Me4(SiMe3)}2] (5) and [TiIVMe25-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 [TiIII51-C5Me4(SiMe2CH2)}{η5-C5Me4(SiMe3)}] (4) was obtained from 5, and compound 6 afforded [TiII61-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 η34-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.