Halide, hydride, alkyl, and dinitrogen complexes of bis(pentamethylcyclopentadienyl)hafnium

Article abstract of DOI:10.1021/om00120a017

Bis(pentamethylcyclopentadienyl)hafnium dichloride (Cp*₂HfCl₂, Cp* = η⁵-C₅Me₅) is obtained by treatment of HfCl₄ with lithium pentamethylcyclopentadienide in refluxing xylenes. The dioodide derivative is obtained from Cp*₂HfCl₂ with BI₃. The dihydrido derivative is obtained via treatment of Cp*₂HfCl₂ with n-butyllithium under an atmosphere of H₂. Treatment of Cp*₂HfH₂ with alkyl halides (RX) affords either Cp*₂HfX₂ or Cp*₂HfHX and alkane depending on the nature of RX. The reaction of phenyllithium and Cp*₂HfH₂ provides a novel route to the phenyl hydride derivative Cp*₂HfH(Ph). The variance of the chemical shift of the hydride with X for complexes of the type Cp*₂HfH(X) (X = hydride, alkyl, alkenyl, aryl, halide, alkoxide, amide) is discussed in terms of paramagnetic shielding. The dinitrogen complex {Cp*₂HfN₂}₂N₂ is obtained in modest yield by Na/K reduction of Cp*₂HfI₂ and is isostructural with the zirconium analogue. Cp*₂HfH₂ forms an adduct with CO. Under high pressures of CO, reductive elimination of H₂ and generation of Cp*₂Hf(CO)₂ is observed. Treatment of Cp*₂Hf(CO)₂ with H₂ (3 atm) at 125°C cleanly affords Cp*₂HfH(OCH₃) and 1 equiv of CO; Cp*₂Hf(CO)₂ with Cp*₂MH₂ (M = Zr, Hf) under H₂ yields cis-(Cp*₂HfH)(Cp*₂MH)(μ-OCH=CHO). Warming of Cp*₂HfH₂(CO) under H₂ yields Cp*₂HfH-(OCH₃), cis-(Cp*₂HfH)₂(μ-OCH=CHO), trans-(Cp*₂HfH)₂(μ-OCH=CHO), and (Cp*₂HfH)₂(μ-OCH₂CH₂C). The relative yields of the individual products depend critically on reaction conditions, and the complexity of these reactions has thwarted all attempts to elucidate their mechanisms. Treatment of Cp*₂HfH₂ with olefins of the formula CH₂=CRR′ or tert-butylacetylene gives alkyl hydrides Cp*HfH(CH₂CHRR′) or the alkenyl hydride Cp*₂HfH(CH=CHCMe₃), which are very resistant to reductive elimination. Ethylene induces loss of alkane (CH₃CHRRR′) or CH₂=CHCMe₃; however, with formation of the hafnacyclopentane derivative Cp*₂Hf(CH₂CH₂CH₂CH ₂). Hydrogenation of the alkyl hydride complexes or Cp*₂HfH(CH₂= CHCMe₃) affords Cp*₂HfH₂ and CH₃CHRR′ or CH₃CH₂CMe₃.