Synthesis of (Imido)vanadium(V) Alkyl and Alkylidene Complexes Containing Imidazolidin-2-iminato Ligands: Effect of Imido Ligand on ROMP and 1,2-C–H Bond Activation of Benzene

A series of (imido)­vanadium­(V) alkylidene complexes containing an imidazolidin-2-iminato ligand of the type V­(CHSiMe₃)­(NR)­(X)­(PMe₃) (R = 2,6-Me₂C₆H₃ (Ar, <b>3a</b>), 1-adamantyl (Ad, <b>3b</b>), C₆H₅ (<b>3c</b>); X = 1,3-Ar′₂(CH₂N)₂CN; Ar′ = 2,6-^<i>i</i>Pr₂C₆H₃) have been prepared in <i>n</i>-hexane in the presence of PMe₃ from the corresponding dialkyl complexes V­(NR)­(CH₂SiMe₃)₂(X) (<b>2a</b>–<b>c</b>). These alkylidene complexes (<b>3a</b>–<b>c</b>) exhibit catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene: the phenylimido analogue (<b>3c</b>) exhibits remarkable activity at 80 °C (e.g. a TOF value of 84800 h^–1 (7070 turnovers after 5 min)), affording high-molecular-weight polymers with uniform molecular weight distributions. The reaction of the (arylimido)­vanadium­(V) dialkyl complex <b>2a</b> with C₆H₆ afforded the phenyl complex V­(NAr)­(CH₂SiMe₃)­(C₆H₅)­(X) (<b>4a</b>) by 1,2-C–H activation via an alkylidene intermediate and the diphenyl complex V­(NAr)­(C₆H₅)₂(X) as the final product. The activity by <b>3a</b>–<b>c</b> for ROMP, the reactivity of <b>2a</b>–<b>c</b> for formation of these alkylidenes by α-hydrogen elimination, and the reactivity of <b>2a</b>–<b>c</b> toward benzene were highly affected by the nature of the imido ligand