Synthesis and Structural Analysis of (Imido)vanadium(V) Dichloride Complexes Containing Imidazolin-2-iminato- and Imidazolidin-2-iminato Ligands, and their Use as Catalyst Precursors for Ethylene (Co)polymerization

A series
of (imido)­vanadium­(V) dichloride complexes containing
1,3-imidazolin-2-iminato or 1,3-imidazolidin-2-iminato ligands of
the type, V­(NR′)­Cl2(L) [R′ = 2,6-Me2C6H3, L = 1,3-R2(CHN)2CN (1a–c,e)
or 1,3-R2(CH2N)2CN (2a–d), R = tBu (a), 2,6-Me2C6H3 (b), 2,6-iPr2C6H3 (c), C6H5 (d), 2,6-(Ph2CH)2-4-MeC6H2 (e); L = 1,3-(2,6-iPr2C6H3)2(CHN)2CN, R′ = 1-adamantyl (Ad, 3c),
C6H5 (4c); L = 1,3-(2,6-iPr2C6H3)2(CH2N)2CN, R′ = Ad (5c)], were prepared and characterized. The molecular structures of 1a, 2a,c,d, 3c, 4c, and 5c were determined by X-ray crystallography.
All complexes showed high catalytic activity for ethylene polymerization
especially in the presence of Et2AlCl cocatalyst; the 2,6-R2C6H3 analogues (R = Me, iPr; 1b,c, 2b,c) exhibited higher catalytic activities than the tBu analogues (1a, 2a), which display rather unique (small) VNC­(imido)
bond angles in the solid state. A good correlation between the activity
and the 51V NMR chemical shift was found for the (arylimido)­vanadium
precatalysts (1a–c,e, 2a–d, and 4c). These complexes showed
high catalytic activity for the copolymerization of ethylene with
norbornene (NBE), affording ultrahigh molecular weight copolymers
with uniform molecular weight distributions. The activities were affected
by the imido ligand as well as by the substituents in the anionic
ligand, and the 2,6-iPr2C6H3 analogues (especially 2c and 4c) showed the higher activities. The complexes 2c and 4c also showed high activities with efficient comonomer
incorporation for the ethylene copolymerization with 5-ethylidene-2-norbornene
(ENB) in the presence of Et2AlCl; both the comonomer incorporation
and the molecular weight in the resulting polymers were affected by
the comonomer employed (NBE vs ENB).