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
- Publication date
- Publisher
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>CN; 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