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, 3a), 1-adamantyl (Ad, 3b), C6H5 (3c); X = 1,3-Ar′2(CH2N)2CN; 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.

本研究合成了一系列含咪唑烷-2-亚氨基配体的（亚胺基）钒(V)亚烷基配合物，其通式为V(CHSiMe3)(NR)(X)(PMe3)（其中R为2,6-二甲基苯基(Ar, 3a)、1-金刚烷基(Ad, 3b)或苯基(3c)；X为1,3-Ar'₂(CH₂N)₂C=N，Ar'为2,6-二异丙基苯基）；该合成反应以正己烷为溶剂，在三甲基膦(PMe3)存在下，由相应的二烷基配合物V(NR)(CH₂SiMe3)₂(X) (2a–c) 制备得到。上述亚烷基配合物(3a–c)对降冰片烯的开环易位聚合(ring-opening metathesis polymerization, ROMP)展现出催化活性：其中苯基亚胺基类似物(3c)在80 ℃下表现出优异的催化性能，例如转换频率(TOF)值达84800 h⁻¹，反应5分钟即可实现7070次催化循环，所得聚合物分子量较高且分子量分布均一。将（芳基亚胺基）钒(V)二烷基配合物2a与苯(C6H6)反应，经由亚烷基中间体发生1,2-碳氢键活化，生成苯基配合物V(NAr)(CH₂SiMe3)(C6H5)(X) (4a)，最终产物为二苯基配合物V(NAr)(C6H5)₂(X)。配合物3a–c的开环易位聚合催化活性、2a–c经由α-氢消除生成亚烷基配合物的反应活性，以及2a–c与苯的反应活性，均受亚胺基配体的结构性质显著影响。