Synthesis, Characterization and Reactivity of Chromium(VI) Alkylidenes

Chromium(VI) dialkyls are remarkably acidic, yielding a new set of anionic chromium alkylidenes upon deprotonation with KN(TMS)₂. We describe the characterization of three anionic chromium alkylidenes of the type [(ArN)₂Cr(=CHR)(CH₂R]^- (R = ^tBu, SiMe₃, Ph; Ar = 2,6-diisopropylphenyl). NEt₄[(ArN)₂Cr(=CHSiMe₃)(CH₂SiMe₃)] has been structurally characterized by X-ray diffraction. We have also studied the C-H activation of cyclohexane effected by the transient neopentylidene (ArN)₂Cr(=CHCMe₃). This reaction generated a tri-nuclear chromium(IV) complex, namely ‘[Cr(NAr)₂]₃’, as well as the two organic products cyclohexene and neopentane. A probable intermediate in this reaction could be trapped by PPh₃ to form (ArN)₂Cr(PPh₃)₂. The latter undergoes ligand substitution reactions with ethylene and CO to produce p-acid complexes of tetravalent chromium.