Allyldimethylsilylcyclopentadienyl Hafnium Complexes: Formation and Trapping of Hafnocenium Cations

The mixed silyl-stannyl-cyclopentadiene C5H4[SiMe2(CH2CHCH2)][Sn(nBu)3] was synthesized by reaction of the lithium salt Li[C5H4{SiMe2(CH2CHCH2)}] with the corresponding
chlorotrialkyltin and then reacted with HfCl4 to prepare the monocyclopentadienyl complex
[Hf{η5-C5H4SiMe2(CH2CHCH2)}Cl3·DME] (DME = dimethoxyethane). Reaction of this
compound with Na(C5H5) or Li(C5H5) afforded low yields of the dichloro hafnocene [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHCH2)}Cl2], which was also isolated more efficiently by transmetalation of the lithium salt Li[C5H4{SiMe2(CH2CHCH2)}] with [Hf(η5-C5H5)Cl3·DME].
Alkylation of the mono- and dicyclopentadienyl complexes with MgCl(CH2Ph) or LiMe gave
the tribenzyl monocyclopentadienyl [Hf{η5-C5H4SiMe2(CH2CHCH2)}(CH2Ph)3] or the dialkyl
hafnocene derivatives [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHCH2)}R2] (R = Me, CH2Ph). The
reaction of the dibenzyl hafnocene with the Lewis acid B(C6F5)3 was monitored by NMR
spectroscopy at variable temperature. Formation of the hafnocenium cationic species [Hf(η5-C5H5){η5-C5H4SiMe2(CH2-η2-CHCH2)}(CH2Ph)]+ was initially observed, which then
slowly transformed into the metallacyclic alkyl [Hf(η5-C5H5){η5-C5H4SiMe2CH2CH(CH2Ph)-η1-CH2}]+ cation, resulting from the benzyl migratory insertion of the allylic alkene moiety.
Reaction of these intermediate cationic species with KOtBu gave the neutral dicyclopentadienyl hafnocene compounds [Hf(η5-C5H5){η5-C5H4SiMe2(CH2CHCH2)}(CH2Ph)(OtBu)] and
[Hf(η5-C5H5){η5-C5H4SiMe2[CH2CH(CH2Ph)-η1-CH2]}(OtBu)], which were characterized by
elemental analyses and NMR spectroscopy.