Borohydride, hydride, halide, and carbonyl derivatives of bis(pentamethylcyclopentadienyl)niobium

Article abstract of DOI:10.1021/om00136a022

A synthetic entry to the chemistry of permethylnicobocene compounds is provided via the borohydride derivative (η⁵-C₅Me₅)₂NbBH₄, obtained in 30-50% yield from NbCl₅, LiC₅Me₅, and NaBH₄ in 1,2-dimethoxyethane. Bridge ? terminal hydride exchange occurs with an exceptionally large barrier (ΔG? = 16.4 ± 0.4 kcal·mol^-1 at 388 ± 8 K). A reinvestigation of the same process for Cp₂NbBH₄ reveals, contrary to an earlier report, a comparable barrier (ΔG? = 14.6 ± 0.2 kcal·mol^-1 at 346 ± 3 K). Treatment of Cp*₂NbBH₄ with pyridine under 1 atm H₂ affords Cp*₂NbH₃. When heated with ethylene or carbon monoxide, Cp*₂NbH₃ yields Cp*₂NbH(C₂H₄) or Cp*₂NbH(CO), respectively. Reaction of Cp*₂NbH(CO) with HCl affords Cp*₂NbCl(CO), which is cleanly converted to Cp*₂Nb(CH₃)(CO) with methyllithium. Paramagnetic Cp*₂NbCl₂ is obtained via treatment of Cp*₂NbBH₄ with 3 M hydrochloric acid.