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.