Synthesis, Characterization, and Reactivity of Chromium(VI) Alkylidenes

Chromium(VI) dialkyls are remarkably acidic, yielding a set of anionic chromium alkylidenes upon deprotonation with KN(TMS)(2). We describe the characterization of three anionic chromium alkylidenes of the type [(ArN)(2)Cr(=CHR)(CH2R](-) (R = Bu-t, SiMe3, Ph; Ar = 2,6-diisopropylphenyl). NEt4[(ArN)(2)Cr(=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)(2)Cr(=CHCMe3). This reaction generated a trinuclear chromium(IV) complex as well as the two organic products cyclohexene and neopentane. A probable intermediate in this reaction could be trapped by PPh3 to form (ArN)(2)Cr(PPh3)(2). The latter undergoes ligand substitution reactions with ethylene and CO to produce pi-acid complexes of tetravalent chromium and, in the case of CO, a rare terminal imido complex of Cr(II).