(Arylimido)vanadium(V) dichloride
complexes containing anionic N-heterocyclic carbene
(NHC) ligands that contain weakly
coordinating B(C6F5)3 moieties (WCA-NHC)
of the type [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3, Ar′ =
2,6-iPr2C6H3) showed significant catalytic activity for ethylene polymerization
in the presence of Al cocatalysts (MAO and AliBu3); the activity by the 5–MAO
catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min–1) is the highest among those reported using the other
(imido)vanadium(V) complexes in the presence of MAO, and the 5–AliBu3 catalyst
showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200
min–1). The V K-edge X-ray absorption near-edge
structure (XANES) analyses (in toluene) strongly suggest the formation
of certain vanadium(III) species by reduction with AliBu3 accompanying structural changes; the
EXAFS analysis suggests the presence of the arylimido ligand and one
V–Cl bond (2.34 ± 0.04 Å), which is longer than those
[2.1901(8)–2.2462(8) Å] in the reported (imido)vanadium(V)
complexes. The XANES analysis of [V(NAr)Cl2(OAr)] strongly
suggests the formation of the other vanadium(III) species by reduction
with Me2AlCl or Et2AlCl, and the EXAFS analysis
suggests the presence of the arylimido ligand and two V–Cl
bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant
changes in both the pre-edge peak(s) and the edge peak when these
complexes were treated with MAO, suggesting that the basic geometry
and the high oxidation state were preserved under these conditions.
Mechanistic studies
through Ti K-edge XANES and EXAFS spectra of
the catalyst solution for 1-hexene polymerization using Cp*TiX2(O-2,6-
i
Pr2C6H3) [X = Cl (1), Me (2)]–MAO
catalysts and for syndiospecific styrene polymerization using (
t
BuC5H4)TiCl2(O-2,6-
i
Pr2C6H3) (3)–MAO catalyst have been explored.
Significant changes in the XANES spectrum (low energy shift in the
edge peak, in addition to decrease in intensities of two pre-edge
peaks) were observed when styrene (200 equiv) was added into a toluene
solution containing 3 and MAO, strongly suggesting that
the complex 3 [Ti(IV)] was reduced to Ti(III) by addition
of styrene under these conditions. The EXAFS analysis suggests that
the Ti–O bond (corresponding to coordination of phenoxide)
was preserved along with dissociation of Ti–Cl bonds by treating
with MAO. These XAS analysis data thus suggest that neutral Ti(III)
species, (
t
BuC5H4)Ti(R)(OAr), play a role as the active species. The DFT-based computational
analysis on the syndiospecific styrene insertion reaction also revealed
that the neutral Ti(III) catalyst exhibits the lower activation energy
than the others, strongly supporting the above mechanism. In contrast,
no significant changes (in the oxidation state, basic structure) in
the XANES spectra were observed when the toluene solution containing 1 (or 2) was added MAO and 1-hexene, whereas
preservation of the Ti–O bond along with dissociation of the
Ti–Cl by treating with MAO was suggested through the EXAFS
analysis.
Living polymerizations of internal
alkynes (2-hexyne, 3-hexyne,
4-methyl-2-pentyne, and 1-phenyl-1-propyne) have been demonstrated
at 50 °C by (arylimido)niobium(V)–alkylidene complex catalysts,
Nb(CHSiMe3)(NAr)[OC(CF3)3](PMe3)2 [Ar = 2,6-Me2C6H3 (4a), 2-MeC6H4 (4c), and 2,6-Cl2C6H3 (4d)], in the presence of PMe3, which plays an essential
role to proceed the living polymerization without certain catalyst
deactivation. The living nature of 4a was preserved even
at 80 °C in the 2-hexyne polymerization under optimized conditions,
and 4c and 4d showed higher activity than 4a. The living polymerizations of 4-methyl-2-pentyne by 4c,d and of 3-hexyne by 4c have
also been achieved in the presence of PMe3 at 50 °C,
and the effect of the internal alkynes toward the activity by 4c is in the order 2-hexyne > 4-methy-2-pentyne > 3-hexyne
> 4-octyne ≫ 5-decyne (negligible). The living polymerization
of 1-phenyl-1-propyne has also been demonstrated by 4d (at 25 and 50 °C) in the presence of PMe3.
V K-edge XANES (XANES = X-ray Absorption Near Edge Structure) spectra of the reaction solution of V(NAr)Cl2(OAr) (1, Ar = 2,6-Me2C6H3) with halogenated Al alkyls (Me2AlCl, Et2AlCl, EtAlCl2, 50 equiv) in toluene showed low energy shifts (2.6–3.6 eV on the basis of inflection point in the photon energy) in the edge absorption accompanying slight shift to low photon energy in the pre-edge peak (λmax values); a similar spectrum was observed when the reaction of 1 with Me2AlCl was conducted in n-hexane. These results strongly suggest a formation of similar vanadium(III) species irrespective of kind of Al alkyls and solvent (toluene or n-hexane). Significant low-energy shifts in the edge absorption accompanied with diminishing the strong pre-edge absorption were also observed when VOCl3 or VO(OiPr)3 was treated with Me2AlCl (10 equiv) in toluene, clearly indicating a formation of low oxidation state vanadium species accompanied with certain structural changes (from tetrahedral to octahedral) in solution.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.