Me 2 C 6 H 3 ], prepared from V(NAd)Cl 2 (L) (1) by reaction with LiMe (2.0 equiv), exhibited remarkable catalytic activities for ethylene dimerization in the presence of MAO affording 1-butene with high selectivity [TOF = 1 120 000−1 530 000 h −1 (311−425 s −1 ), C 4 ′ = 97.1−98.4%], and the catalyst performances (activity, selectivity) were similar to those by the dichloride analogue (1) under the same conditions. The dimethyl complex (2a) reacted with 1.0 equiv of R′OH to yield the mono alkoxide complexes, V(NAd)Me(OR′)(L) [R′ = OC(CF 3 ) 3 (3a), OC(CH 3 )(CF 3 ) 2 (3b), OC(CH 3 ) 3 (3c)], and structures of these complexes (3a−c) and 2a were determined by Xray crystallography. Reactions of 2a with [Ph 3 C][B(C 6 F 5 ) 4 ] in Et 2 O and 3c with B(C 6 F 5 ) 3 in THF afforded the corresponding cationic complexes confirmed by NMR spectra. Both NMR and V K-edge XANES analysis of the toluene or toluene-d 8 solution of 1 and 2a did not show any significant changes in the oxidation state upon addition of MAO, Me 2 AlCl, or Et 2 AlCl (10 equiv). Resonances ascribed to formation of the other vanadium(V) species were observed in the 51 V NMR spectra, and no significant differences in the XANES spectra (V−K pre-edge peaks and edge) were observed from 1 or 2a upon addition of Al cocatalyst. Taking into account these results and others, it is thus suggested that cationic vanadium(V) alkyl/hydride species play a role in this catalysis.
(Imido)vanadium(V) dichloride complexes
containing 2-(2′-benzimidazolyl)-6-methylpyridine
ligand (L) of type V(NR)Cl
2
(L) [R = 1-adamantyl (Ad,
1
), C
6
H
5
(
2
), and 2,6-Me
2
C
6
H
3
(
3
)] have been prepared,
and their structures were determined by X-ray crystallography as distorted
trigonal bipyramidal structures around vanadium. Reactions with ethylene
using
1–3
in the presence of methylaluminoxane
(MAO) afforded a mixture of oligomer and polymers, and the compositions
were affected by the imido ligand employed. By contrast,
1–3
exhibited remarkable catalytic activities for ethylene polymerization
in the presence of Me
2
AlCl; the phenylimido complex (
2
) exhibited the highest activity [80 100 kg-PE/mol-V·h
turn over frequency (TOF, 2 850 000 h
–1
, 792 s
–1
)]. The ethylene copolymerizations with
norbornene afforded ultrahigh-molecular-weight copolymers with uniform
molecular weight distributions and compositions [e.g.,
M
n
= 1.71–2.66 × 10
6
,
M
w
/
M
n
= 2.27–2.53].
On the basis of V nuclear magnetic resonance (
51
V NMR),
electron spin resonance, and V K-edge X-ray absorption near-edge structure
(XANES) spectra of the catalyst solution, the observed difference
in the catalyst performance in the presence of (between) MAO and Me
2
AlCl cocatalyst should be due to the formation of different
catalytically active species with different oxidation states. Apparent
changes in the oxidation state were observed in the (especially in
the NMR and XANES) spectra upon addition of Me
2
AlCl, whereas
no significant changes in the spectra were observed in presence of
MAO.
Bu3 catalyst possessed uniform molecular weight distributions and compositions. Formation of vanadium(III) species could be assumed upon presence of Al i Bu3, especially on the basis of both NMR and ESR spectra (silent) and certain significant changes in the V K-edge XANES (pre-edge and edge region) spectrum, whereas no significant changes in the XANES spectrum were observed from the toluene solution containing 1 upon addition of MAO. It is thus suggested that a difference in the catalyst performances between in the presence of MAO and Al i Bu3 should be due to a formation of different catalytically active species with different oxidation states.
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