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
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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, 3a), 1-adamantyl (Ad, 3b), C6H5 (3c); X = 1,3-Ar′2(CH2N)2CN; Ar′ = 2,6-iPr2C6H3) have been prepared in n-hexane in the presence of PMe3 from the corresponding dialkyl complexes V(NR)(CH2SiMe3)2(X) (2a–c). These alkylidene complexes (3a–c) exhibit catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene: the phenylimido analogue (3c) 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 2a with C6H6 afforded the phenyl complex V(NAr)(CH2SiMe3)(C6H5)(X) (4a) 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 3a–c for ROMP, the reactivity of 2a–c for formation of these alkylidenes by α-hydrogen elimination, and the reactivity of 2a–c toward benzene were highly affected by the nature of the imido ligand.
