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

Abstract

A series of (imido)­vanadium­(V) alkylidene complexes containing an imidazolidin-2-iminato ligand of the type V­(CHSiMe<sub>3</sub>)­(NR)­(X)­(PMe<sub>3</sub>) (R = 2,6-Me<sub>2</sub>C<sub>6</sub>H<sub>3</sub> (Ar, <b>3a</b>), 1-adamantyl (Ad, <b>3b</b>), C<sub>6</sub>H<sub>5</sub> (<b>3c</b>); X = 1,3-Ar′<sub>2</sub>(CH<sub>2</sub>N)<sub>2</sub>CN; Ar′ = 2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) have been prepared in <i>n</i>-hexane in the presence of PMe<sub>3</sub> from the corresponding dialkyl complexes V­(NR)­(CH<sub>2</sub>SiMe<sub>3</sub>)<sub>2</sub>(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<sup>–1</sup> (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 C<sub>6</sub>H<sub>6</sub> afforded the phenyl complex V­(NAr)­(CH<sub>2</sub>SiMe<sub>3</sub>)­(C<sub>6</sub>H<sub>5</sub>)­(X) (<b>4a</b>) by 1,2-C–H activation via an alkylidene intermediate and the diphenyl complex V­(NAr)­(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>(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

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