1-Hexene Homopolymerization and Copolymerization with Polar ω-Halo-α-alkenes by (Benz)imidazolin-2-iminato Scandium and Yttrium Complexes

Article abstract of DOI:10.1021/acs.organomet.0c00563

The synthesis and characterization of (benz)imidazolin-2-iminato scandium and yttrium complexes (Sc1-Sc3, Y1, and Y2) are reported herein. All of the complexes were isolated in high yields, and from single-crystal X-ray diffraction analysis, extremely sho

Full text of DOI:10.1021/acs.organomet.0c00563

pubs.acs.org/Organometallics
Article
1‑Hexene Homopolymerization and Copolymerization with Polar
ω‑Halo-α-alkenes by (Benz)imidazolin-2-iminato Scandium and
Yttrium Complexes
Wenpeng Zhao, Beibei Wang, Bo Dong, Heng Liu, Yanming Hu,* Moris S. Eisen,*
and Xuequan Zhang*
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ABSTRACT: The synthesis and characterization of (benz)imidazolin-2-iminato scandium and yttrium complexes (Sc1−Sc3, Y1,
and Y2) are reported herein. All of the complexes were isolated in high yields, and from single-crystal X-ray diffraction analysis,
extremely short N−Mt bonds were revealed, indicating the highly nucleophilic nature of the ligands. In the presence of
[Ph3C][B(C6F5)4] and Al(i-Bu)3, scandium complexes showed medium to high activities for 1-hexene polymerization, revealing the
trend Sc1 > Sc3 > Sc2. More importantly, the scandium complex Sc1 was active in the copolymerization of 1-hexene with polar ω-
halo-α-alkenes, demonstrating the good heteroatom tolerance of the cationic lanthanide metals that is caused by the strong electron-
donating ability of these ligands. The influence of different types of comonomers on the polymerization behaviors and the
microstructure of the resultant copolymer products were investigated in detail.
olefins with polar monomers remains a great challenge. The
imidazolin-2-iminato ancillary ligand is different from other
nitrogen-containing ligands, possessing two limiting resonance
structures (Scheme 1) and can act as a 2σ,4π-electron donor
INTRODUCTION
■
Poly(1-hexene) is a unique elastomer that bears outstanding
fatigue resistance and long-term stability against hydrolysis and
oxidation and currently finds widespread applications in the
fields of biomedical materials,1,2 impact modifiers,3 etc.
However, similarly to other hydrophobic hydrocarbon
polymers, poly(1-hexene) has a very poor surface property
owing to the lack of functional groups, which results in inferior
performances with respect to adhesive ability, printability, and
compatibility with reinforcing inorganic fillers. Although
transition-metal and rare-earth-metal-catalyzed coordination
copolymerization of olefins with polar monomers is a
straightforward method to prepare functionalized polyolefins,
with improved surface properties,4−6 a very few catalytic
systems have been reported for the preparation of function-
alized poly(1-hexene)s to date.7
Scheme 1. Resonance Structures of Imidazolin-2-iminato
Ligands
Organolanthanides have been extensively evaluated as
precatalysts for the homopolymerization of 1-hexene and its
copolymerization with other α-olefins in the past few
decades,6,8−19 and these studies have mainly focused on the
improvement of the catalytic activity and/or stereoselectivity of
the precatalysts.10,11,20−24 Because of the easy deactivation of
the cationic active species by heteroatoms, as a result of the
high electrophilicity of organolanthanides, the development of
lanthanide complexes for the effective copolymerization of
Received: August 25, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00563
© XXXX American Chemical Society
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that can resemble a cyclopentadienyl moiety. Moreover, the
imidazolyl ring can stabilize the positive charge efficiently,
which thus endows the ligand with a high nucleophilicity and
consequently a stronger electron-donating ability than other
nitrogen-containing ligands. Indeed, some unprecedented
bond properties and catalytic behaviors have been presented
in previously reported imidazolin-2-iminato f-element com-
plexes. For instance, imidazolin-2-iminato lanthanides display
exceptionally short metal−nitrogen bonds, some of which are
the shortest ever observed so far.25−29 Imidazolin-2-iminato
actinide complexes display remarkably reduced electrophilicity
at the metal center and high catalytic activities in transforming
oxygen-containing substrates.30−35 In this study, a series of
(benz)imidazolin-2-iminato scandium and yttrium complexes
have been synthesized and subsequently employed for 1-
hexene homopolymerization and copolymerization with polar
ω-halo-α-alkene monomers. As a result, these complexes
demonstrated good tolerance to halogen atoms and performed
with high catalytic activities in promoting the copolymerization
process. Our detailed results are presented herein.
sets of chemically inequivalent isopropyl groups (1.33 and 1.20
ppm for Sc2 and 1.28 and 1.19 ppm for Y2) are observed,
which is similar to the case for the previously reported
bis(imidazolin-2-iminato) scandium chloride complex.36 On
the other hand, in the cases of Sc1 and Sc2, the singlet
resonances at −0.32 and −0.56 ppm are characteristic of the
neosilyl methylene protons, and in the cases of Y1 and Y2,
doublet signals at −0.93 and −0.85 ppm with coupling
constants of 2.7 and 3.5 Hz are observed due to the presence
of 89Y−H coupling, which is consistent with the previously
reported yttrium−neosilyl analogues.28,37,38
Single crystals of complexes Sc1 and Sc2 were obtained by
slow crystallization from their toluene solutions at −35 °C, and
the structures are shown in Figures 1 and 2, respectively.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Complexes Sc1−
Sc3, Y1, and Y2. The mono- and bis(imidazolin-2-iminato)-
ligated scandium and yttrium complexes, Sc1, Sc2, Y1, and Y2
were prepared in high yields by treatment of the imidazolin-2-
imine ligand L1 with 1 or 1/2 equiv of the corresponding
complex Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y) at room
temperature (Scheme 2) and subsequent recrystallization
Figure 1. ORTEP drawing of complex Sc1 with thermal displacement
parameters at 50% probability. Hydrogen atoms and solvent
molecules are omitted for clarity. Bond lengths (Å): Sc01−N1
1.9477(16), Sc01−C32 2.225(2), Sc01−C28 2.252(7), Sc01−O1
2.1545(15), N1−C1 1.273(2), N2−C1 1.404(2), N3−C1 1.394(2).
Bond angles (deg): C1−N1−Sc01 179.15(16), N1−Sc01−C32
118.82(10), N1−Sc01−C28 116.0(2), N1−Sc01−O1 106.66(6),
N1−C1−N2 128.38(17), N1−C1−N3 129.08(17), N3−C1−N2
102.55(15), C32−Sc01−C28 106.7(3), O1−Sc01−C32 96.83(8),
O1−Sc01−C28 109.97(19).
Scheme 2. Synthesis of Complexes Sc1−Sc3, Y1, and Y2
Figure 2. ORTEP drawing of complex Sc2 with thermal displacement
parameters at 50% probability. Hydrogen atoms and solvent
molecules are omitted for clarity. Bond lengths (Å): Sc1−N1
1.990(2), Sc1−N4 1.990(3), Sc1−O1 2.167(2), Sc1−C59 2.225(3),
N1−C1 1.264(4), N2−C1 1.417(4), N3−C1 1.409(4), N4−C28
1.262(4), N5−C28 1.413(4), N6−C28 1.410(4). Bond angles (deg):
N1−Sc1−N4 127.27(10), N1−Sc1−O1 103.35(10), N4−Sc1−O1
102.44(10), N1−Sc1−C59 108.45(12), N4−Sc1−C59 110.03(12),
O1−Sc1−C59 101.91(11), C1−N1−Sc1 166.2(2), C28−N4−Sc1
175.8(2), N1−C1−N3 130.3(3), N1−C1−N2 128.4(3), N3−C1−
N2 101.3(2), N4−C28−N6 129.6(3), N4−C28−N5 128.7(3), N6−
C28−N5 101.8(2).
from their saturated toluene solutions at −35 °C. The
1
structures of these complexes were identified by H NMR
spectroscopy. The two doublet resonances at 1.45 and 1.22
ppm in Sc1 and at 1.41 and 1.22 ppm in Y1, respectively, are
attributed to the diasterotopic isopropyl CH3 groups in the
corresponding mono(imidazolin-2-iminato) complexes. For
the bis(imidazolin-2-iminato complexes) Sc2 and Y2, two
B
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Although Sc1 has been previously reported by Tamm,25 its
crystal structure is shown here to better compare its difference
with the bis(imidazolin-2-iminato) scandium complex Sc2. In
Sc2, the central metal is coordinated by two imidazolin-2-
iminato ligands, one neosilyl group, and one tetrahydrofuran
molecule, displaying a distorted-tetrahedral geometry. Similarly
to other imidazolin-2-iminato lanthanide complexes, Sc2 has
very short Sc−N bond lengths, which are slightly longer than
that of the mono(imidazolin-2-iminato) complex Sc1 (ca.
1.990 Å vs 1.9477(16) Å) but shorter than those of the
bis(imidazolin-2-iminato scandium chloride complex
(2.003(2) Å).36 It is noted that, except for imidazolin-2-
iminato-based complexes, only one example of a β-
diketiminato scandium complex has been reported to have
an Sc−N bond length shorter than 2 Å.39,40 Such an
exceptionally short Sc−N bonds in Sc1 and Sc2 suggest that
the imidazolin-2-imine ligand possesses a strong electron-
donating capacity, as expected. Moreover, in Sc1 and Sc2, the
ligand coordinated to the metal in an approximately linear
fashion, with C−N−Sc bond angles of 179.15(16)° in Sc1 and
166.2(2) and 175.8(2)° in Sc2; the slight deviation from
linearity in the latter might be due to steric repulsion of the
two imidazolin-2-iminato ligands.
for the isopropyl groups at 1.25, 1.14, 1.01, and 0.83 ppm
because of the chemically inequivalent N-methylene sub-
stituents, which indicates that the Dipp group is not free to
rotate. In Sc3, the two metal centers adopt different
coordination geometries. Sc(1) is coordinated by two iminato
groups, one neosilyl group, and one tetrahydrofuran molecule,
adopting a distorted-tetrahedral geometry, while Sc(2) is
coordinated by two iminato groups, two N-methylene groups,
and one neosilyl group, displaying a distorted-square-pyramidal
geometry. The difference in coordination geometries leads to
different bond properties between the two metals and the
ligand. Sc(1) bonds with iminato nitrogen atoms N(1) and
N(4) with much shorter bond lengths (2.002(3) and 2.006(3)
Å) than does Sc(2) (2.251(3) and 2.254(3) Å). Additionally,
in comparison to other imidazolin-2-iminato-bridged dimeric
complexes in which a parallelogram is formed for the center
four-membered ring,26,41−43 the present four-membered ring
shows a rare kite geometry. Furthermore, on comparison with
other intramolecular C−H activated complexes, such as β-
diketiminato-ligated scandium complexes with Sc−C bond
lengths of 2.251(4) Å39 and 2.214(5) Å,44 Sc3 has longer Sc−
C bond lengths (2.304(4) and 2.312(4) Å).
1-Hexene Homopolymerization and Copolymeriza-
tion with ω-Halo-α-alkenes. Complexes Sc1−Sc3, Y1, and
Y2 were initially examined for 1-hexene homopolymerization,
and the results were summarized in Table 1. In the presence of
2.0 equiv of [Ph3C][B(C6F5)4] and 10 equiv of Al(i-Bu)3, the
scandium complexes Sc1−Sc3 gave poly(1-hexene)s in 97.6%,
20.2%, and 34.5% yields, respectively. Such a high catalytic
activity in Sc1 can be explained by steric reasons, as revealed
by the buried volume (%Vbur) in the steric crowding maps
(Figure 4). The %Vbur value of Sc1 is much lower than those of
Sc2 and Sc3 (23.2% vs 45.3% and 64.1% (46.0%)), which
favors the coordination of an incoming monomer to the active
species and consequently leads to higher activity. For Sc2,
although it displays a relatively smaller buried volume, its
activity is still less than that of Sc3. This result may be ascribed
to the cooperative effect in Sc3 caused by the proximity of two
scandium centers (ca. 3.150 Å). During the catalytic process,
the presence of the second cationic metal center helps to
coordinate the 1-hexene monomer and thus results in a higher
probability for the monomer to be enchained.45 Reducing the
amount of [Ph3C][B(C6F5)4] resulted in a decrease in catalytic
activity, and in the absence of [Ph3C][B(C6F5)4], the
complexes could not induce polymerization. Using the borate
compounds [PhNMe2H][B(C6F5)4] and B(C6F5)3 as activa-
tors resulted in much lower activities, demonstrating that
[Ph3C][B(C6F5)4] was an effective cocatalyst for the present
system. It is of note, under identical conditions, that both
yttrium complexes, Y1 and Y2, were completely inactive
toward 1-hexene polymerization, even in the presence of other
types of cocatalysts, including MAO, AlEt3, and AlMe3,
perhaps due to the inherent nature of Y complexes, which
were also reported to be inactive for olefin polymerizations.17
13C NMR analyses of the resultant poly(1-hexene)s from Sc1−
Sc3-mediated systems indicated atactic structures (Figures S9
and S10).
Treating benzimidazolin-2-imine ligand L2 with a stoichio-
metric amount of Sc(CH2SiMe3)3(THF)2 at room temper-
ature gave the dimeric complex Sc3. Colorless crystals were
obtained by slow recrystallization from its saturated toluene
solution at −35 °C. As shown in Figure 3, to our surprise,
intramolecular C−H activation occurrs at the N-methyl
groups, as evidenced by the presence of two sets of doublets
Figure 3. ORTEP drawing of complex Sc3 with thermal displacement
parameters at 50% probability. Hydrogen atoms, isopropyl groups on
2,6-diisopropylbenzyl, methyl groups on neosilyl, and methylenes on
the tetrahedron have been omitted for clarity. Bond lengths (Å):
Sc1−N1 2.002(3), Sc1−N4 2.006(3), Sc1−O1 2.132(3), Sc1−C45
2.221(5), Sc2−C20 2.304(4), Sc2−C40 2.312(4), Sc2−C49
2.215(5), Sc2−N1 2.251(3), Sc2−N4 2.254(3), N1−C1 1.302(5),
N2−C1 1.354(5), N3−C1 1.400(5), N2−C20 1.471(5), N4−C21
1.299(5), N5−C21 1.404(5), N6−C21 1.359(5), N6−C40 1.467(5).
Bond angles (deg): N1−Sc1−N4 90.54(13), N1−Sc1−O1
117.82(12), N4−Sc1−O1 117.43(12), N1−Sc1−C45 117.10(15),
N4−Sc1−C45 112.00(15), C1−N1−Sc1 152.3(3), N1−C1−N2
125.7(3), N1−C1−N3 127.3(3), N2−C1−N3 107.0(3), N1−Sc2−
N4 78.42(12), N1−Sc2−C20 77.76(13), C49−Sc2−C20 105.90(18),
C20−Sc2−C40 104.21(15), C20−Sc2−C40 104.21(15).
The influences of several polymerization parameters,
including Al/Sc ratio, temperature, and monomer feed ratio,
were further investigated using Sc1 as a precatalyst. While the
increment of Al/Sc ratio from 5 to 20 hardly affected the
activity, the molecular weight of the resulting polymer
C
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a
Table 1. Polymerization of 1-Hexene under Different Conditions
b
c
d
d
entry
Cat.
[Cat.]/[B]/[Al]
borate
[Hex]/[Cat.]
T (°C)
yield (g)
0.82
conversn (%)
97.6
10−4Mn
Mw/Mn
1
2
3
4
5
6
7
8
Sc1
Y1
Sc2
Y2
1/2/10
1/2/10
1/2/10
1/2/10
1/2/10
1/0/10
1/1/10
1/2/10
1/2/10
1/2/5
1/2/20
1/2/30
1/2/10
1/2/10
1/2/10
1/2/10
1/2/10
A
A
A
A
A
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
500
20
20
20
20
20
20
20
20
20
20
20
20
0
4.4
nd
3.9
nd
8.5
nd
0.9
2.3
nd
8.7
3.5
1.0
6.1
3.7
2.6
3.0
4.9
2.5
nd
3.4
nd
3.3
nd
6.9
3.1
nd
2.0
4.5
3.0
1.8
3.0
3.9
2.4
2.7
e
nd
20.2
nd
34.5
nd
27.3
57.1
nd
95.0
92.8
60.7
83.6
76.5
32.1
100
83.2
0.17
0.29
Sc3
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
Sc1
A
B
C
A
A
A
A
A
A
A
A
0.23
0.48
9
10
11
12
13
14
15
16
17
0.80
0.78
0.51
0.70
0.64
0.27
0.42
1.39
40
60
20
2000
20
a
b
Conditions: Sc or Y (10 umol), Al(i-Bu)3, 2 mL of chlorobenzene, 12 h. Activator: A, [Ph3C][B(C6F5)4]; B, [PhNMe2H][B(C6F5)4]; C,
c
d
B(C6F5)3. Calculated by [polymer yield (g)/total monomer (g)] × 100. Determined by GPC in THF at 40 °C against a polystyrene standard.
e
nd: not determined.
coordinated ligand to bridge two metals,41,47,48 especially for
the cationic metal centers. Therefore, during the polymer-
ization, when the cationic active species is formed in the
presence of a borate compound, it has some tendency to
coordinate with another molecule of the complex to form a
multimetallic mode and therefore results in a mixture of
monometallic and multimetallic species. Such a multisite
nature gives rise to different chain propagation rates for the 1-
hexene monomer and eventually affords poly(1-hexene)s with
broad molecular weight distributions.
The scandium complex Sc1 was subsequently investigated
for the copolymerization of 1-hexene with polar monomers.
Initial attempts with the choice of 6-hydroxy-1-hexene as a
comonomer proved to be infeasible, even in the premasked
status by Al(i-Bu)3. ω-Halogen-containing 1-olefins were next
examined in 1-hexene copolymerizations due to their relatively
weak deactivation effects on cationic active species.49 Differ-
ently from other heteroatoms, after halogens coordinate to the
cationic metal center, the formed dormant species can be
reactivated due to the much lower metal−halogen bond energy
Figure 4. Steric crowding maps from buried volume calculations for
complexes Sc1−Sc3.46
(Scheme 3). This rule, on the other hand, also implies that the
more electronegative fluoride and chloride groups will exhibit
higher binding energies to the metal center in comparison to
decreased from 8.7 × 104 to 1.0 × 104, indicating the presence
of a chain transfer to the aluminum compound.
Scheme 3. Enchainment Mechanism for ω-Halo-α-alkenes
Similarly to the previously reported scandium complexes,
the reaction temperature had a remarkable influence on the
catalytic performance.12 Polymerizations at 20 °C gave the
highest activity, and decreasing and increasing the temperature
had negative effects on the polymer yields. Moreover, because
of the facilitated chain transfer at high temperature, poly(1-
hexene)s with obviously reduced molecular weights were
obtained.
In comparison with previously reported systems, the present
scandium complexes sometimes afforded poly(1-hexene)s with
broad molecular weight distributions.9−12,17 These results were
probably due to the unique feature of the imidazolin-2-iminato
ligand. Due to its strong electron-donating ability, in addition
to its κ1-coordination mode, it could also act as a μ2-
D
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a
Table 2. Copolymerization of 1-Hexene with ω-Halo-α-alkenes
b
c
d
d
entry
comon
yield (g)
conversn (%)
incorp cont (%)
10−3Mn
Mw/Mn
18
19
20
21
6-bromo-1-hexene
6-chloro-1-hexene
6-iodo-1-hexene
6-hydroxy-1-hexene
6-bromo-1-hexene
4-bromo-1-butene
8-bromo-1-octene
10-bromo-1-decene
0.54
0.45
0.62
58.6
50.0
68.9
0.96
0.83
1.21
1.7
1.6
2.0
1.2
1.2
1.3
e
22
0.51
0.55
0.68
0.74
50.8
60.6
72.7
77.9
1.62
0.04
0.96
1.22
1.4
1.7
2.2
2.0
1.2
1.2
1.4
1.5
23
24
25
a
Conditions unless specified otherwise: Sc1 (10 μmol), [Ph3C][B(C6F5)4], (20 μmol), Al(i-Bu)3, (100 μmol), 2 mL of chlorobenzene, 25 °C, 12h,
b
c
[mon]:[Sc1] = 1000, [comon]:[1-hexene] = 5%. Calculated by [polymer yield (g)/total monomer (g)] × 100. Comonomer incorporation (mol
d
e
%) established by 1H NMR spectra. Determined by means of gel permeation chromatography (GPC) against polystyrene standards. [como]:[1-
hexene] = 10%.
bromide and iodide and thus might be difficult to release
during the reactivation step to regenerate the active cationic
species. Such speculation is indeed verified by the results in
Sc1 mediating the 1-hexene copolymerizations (Table 2). The
reactivity decreased in the order 6-iodo-1-hexene > 6-bromo-1-
hexene > 6-chloro-1-hexene: i.e., the copolymer yields
decreased from 68.9% to 58.6% and 50.0% under identical
conditions. This order is consistent with those in early-
transition-metal complexes reported by Li and Powell.50,51 In
addition, different halogen groups also had a remarkable
influence on the comonomer incorporation in the resulting
copolymers. ω-Iodo-1-hexene was proved to be the most
efficient comonomer in comparison to its bromo- and chloro-
containing counterparts; the incorporation content was higher
than those of the other two (1.21% vs 0.96% and 0.83%) when
5% comonomer was in the feed.
1
Figure 5. H NMR spectra of poly(1-hexene) (bottom) and poly(1-
For ω-halogen-α-alkenes of different chain lengths, in
hexene-co-6-bromo-1-hexene) (top).
addition to the types of different halogen atoms, their
copolymerization performances were also influenced by two
more factors: (1) the distance between halogen and the double
the successful incorporation of halogen-containing comono-
bond and (2) the backbiting effect of the halogen atom in the
mers. Moreover, the signals at 4.6−4.8 and 5.3−5.4 ppm
last inserted comonomer. For the comonomers with a short
ascribed to the olefinic end groups of the homopolymer almost
completely disappeared in the copolymer, implying that the
presence of a ω-halo-α-alkene in the polymerization system
altered the chain-termination pathways significantly. Generally,
there are usually three different types of chain transfer
pathways in the olefin polymerization system: that is, chain
transfer to the aluminum compound, β-H transfer to the metal,
and β-H transfer to the monomer. For the present 1-hexene
−(CH2)n− length between the double bond and the halogen
atom such as the 4-bromo-1-butene monomer, the strong
electron-withdrawing effect of halogen will significantly reduce
the electron density of the double bond and thus render them
less coordinative to be inserted later; moreover, even though
such a comonomer is enchained successfully, the generated
intermediate was prone to be backbitten by the bromine atom
to form a five- or six-membered ring (Scheme 3), which is
energetically unfavorable to be broken. Therefore, in the case
of 4-bromo-1-butene, only 0.04% of 4-bromo-1-butene was
incorporated into the copolymer (entry 22, Table 2). In
addition, the molecular weight of the obtained poly(1-hexene-
co-4-bromo-1-butene) was almost the same as those of other
copolymers, indicating that continuous chain propagation after
4-bromo-1-butene insertion was almost infeasible. For ω-
halogen-α-alkenes with longer methylene spacers, however, the
formation of a larger ring was more difficult due to the lower
entropy.50 As a result, a comonomer with a longer spacer
length afforded a copolymer with higher comonomer
incorporation in a higher yield, which was consistent with
previous reports.52
polymerization system, chain transfer to the aluminum
compounds can be easily identified from the results shown
in Table 1, entries 10−12, in which remarkably decreased
molecular weights were obtained upon increasing the
concentration of Al(i-Bu)3. The last two pathways can be
deduced from the presence of olefinic end groups for the
obtained poly(1-hexene)s. More quantitative evidence of these
two β-H transfer pathways is derived from a plot of the
reciprocal degree of polymerization (1/PN) versus the
reciprocal monomer concentration (1/[M]) (eqs 1−4, Figure
6),53,54 in which the y-axis intercepts yield a ratio of the rate
constants for β-H transfer to monomer over that for chain
growth, ktr,M/kp (0.000802). Therefore, for 1-hexene homo-
polymerization, all three types of chain transfer pathways were
present (Scheme 4), in which chain transfer to AlR3 resulted in
poly(1-hexene)s with saturated chain ends, whereas β-H
transfers to metal and monomer gave rise to poly(1-hexene)s
with olefinic end groups. For 1-hexene/ω-halogen-α-alkene
1
The obtained copolymers were characterized by H NMR
spectroscopy along with poly(1-hexene) for comparison
(Figure 5). The signal at 3.5 ppm was attributed to the
methylene proton adjacent to the halogen groups, indicating
E
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aluminum, monomer, and metal, respectively (see details in the
Supporting Information).
CONCLUSION
■
Mono- and bis((benz)imidazolin-2-iminato) scandium and
yttrium complexes were facilely synthesized and isolated in
high yields in the present study. Due to the highly nucleophilic,
strongly basic properties of the ligands, these complexes
exhibited very short N−Mt bonds and almost linear C−N−Mt
bonds, which showed a striking difference from other nitrogen-
containing ligands. Moreover, intramolecular C−H activation
between the methyl group on the benzimidazolium ring and
the scandium metal center was observed for complex Sc3,
which further resulted in a binuclear structure. In the presence
of [Ph3C][B(C6F5)4] and Al(i-Bu)3, the scandium complexes
exhibited medium to high catalytic activities for 1-hexene
polymerization, and the activity decreased in the order Sc1 >
Sc3 > Sc2, whereas their yttrium counterparts were completely
inactive under identical conditions. Moreover, Sc1 could
successfully initiate the copolymerization of 1-hexene with ω-
halo-α-alkenes, and the catalytic activity was dependent on the
type of halogen group and the distances between the halogen
atom and double bonds of the ω-halogen-containing
comonomers. Moreover, different from 1-hexene homopol-
ymers, the copolymers possessed saturated chain structures
probably due to the suppression of β-H transfers.
Figure 6. Plot of the reciprocal degree of polymerization (1/PN)
versus the reciprocal monomer concentration (1/[M]).
copolymerization systems, however, probably due to the
chelation of halogens to the cationic metal center, β-H
transfers were significantly suppressed, which resulted in the
chain transfer to aluminum as the sole transfer process, and no
olefinic end groups were observed in the resulting copolymers.
Rtr
Rp
1
PN
=
Rtr,Al + Rtr,M + Rtr,Mt
=
=
=
Rp
(1)
(2)
(3)
EXPERIMENTAL SECTION
■
ktr,Al[C*][Al] + ktr,M[C*][M] + ktr,Mt[C*]
kp[C*][M]
General Procedures. All manipulations of air- and moisture-
sensitive materials were performed on a high vacuum line or in a
glovebox with a medium-capacity recirculator (1−2 ppm oxygen). All
of the solvents were dried by sodium and degassed by three freeze−
pump−thaw cycles prior to use (hexane, toluene, benzene-d6).
Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y) and imidazolin-2-imines were
synthesized according to previous reports.30,35,55 1-Hexene (Alfa
Aesar, 97%) was dried over CaH2 and distilled under reduced
pressure. [Ph3C][B(C6F5)4], [PhNHMe2][B(C6F5)4], and B(C6F5)3
were purchased from Innochem (Beijing) Technology and used as
received.
ktr,Al
kp
ktr,M
kp
ktr,Mt
kp
[Al]
[M]
1
[M]
×
+
+
×
i
y
k
ktr,Mt
kp
ktr,M
j
z
1
[M]
tr,Al×[Al]
j
j
z
z
=
+
×
+
j
z
j
j
z
z
kp
kp
(4)
k
{
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded on a Varian Unity spectrometer in CDCl3 at ambient
temperature using tetramethylsilane as an internal standard. Elemental
where PN is the mean degree of polymerization, Rtr is the rate
of chain transfer, Rp is the rate of chain propagation, and ktr,Al
tr,M, and ktr,Mt are the rate constants for chain transfer to
,
k
Scheme 4. Chain Transfer Pathways That Might Be Present in 1-Hexene Homopolymerization and Copolymerization with ω-
Halogen-α-alkene
F
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Article
analysis was performed using a Vario EL spectrophotometer. The
number-average molecular weights (Mn) and molecular weight
distributions (Mw/Mn) of polymers were measured at 30 °C using a
gel permeation chromatography (GPC) instrument equipped with a
Waters 515 HPLC pump, four columns (HMW 7 THF, HMW 6E
THF × 2, HMW 2THF), and a Waters 2414 refractive index detector.
Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0
mL min−1 against polystyrene for calibration.
mL), affording a red solution at room temperature. The mixture was
stirred for 10 min. Then, of 1-hexene (0.84 g, 10 mmol) and Al(iBu)3
(100 μmol) were placed in the flask, and polymerization was initiated.
After 12 h, the mixture was poured into a large quantity of methanol
(50 mL) containing a small amount of hydrochloric acid to terminate
the polymerization. The precipitated polymer was washed with
methanol and dried under vacuum at 70 °C overnight.
X-ray Crystallographic Studies. Crystals for X-ray analyses were
obtained as described in the Experimental Section. Data collections
were performed at lower than room temperature on a Bruker SMART
APEX diffractometer with a CCD area detector using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The
determination of crystal class and unit cell parameters was carried
out using the SMART program package. The raw frame data were
processed using SAINT and SADABS to yield the reflection data file.
The structures were solved using the SHELXTL program. Refinement
was performed on F2 anisotropically for all non-hydrogen atoms by
the full-matrix least-squares method. The hydrogen atoms were placed
at calculated positions and were included in the structure calculation
without further refinement of the parameters. Details of X-ray
structure determinations and refinements are provided in Table S1 in
the Supporting Information. CCDC 1973915 (for Sc1), 1973914 (for
Sc2), and 1973916 (for Sc3) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre.
Synthesis of Complex Sc1. A solution of the scandium precursor
Sc(CH2SiMe3)3(THF)2 (0.3 g, 0.66 mmol) in 2 mL of toluene was
reacted with 1.0 equiv of the ligand in 3 mL of toluene. The reaction
mixture was stirred at room temperature for 12 h, and after partial
removal of the solvent, recrystallization from the concentrated toluene
solution afforded the target complex as off-white crystals. These
crystals are good for single crystal X-ray spectroscopy analysis. Yield:
1
0.50 g (87.0%). H NMR (400 MHz, benzene-d6): δ 7.24−7.06 (m,
6H, HAr), 5.89 (s, 2H, CH), 3.35 (br, 4H, HTHF), 3.32−3.22 (sept,
2H, CH(CH3)2), 1.45 (d, 12H, CH(CH3)2), 1.22 (d, 12H,
CH(CH3)2), 1.10 (br, 4H, HTHF), 0.22 (s, 18H, Si(CH3)3), −0.32
(s, 4H, CH2Si). The spectrum agrees with previous literature data.25
Synthesis of Complex Y1. Complex Y1 was prepared using a
method similar to that for Sc1 by using Y(CH2SiMe3)3(THF)2 as the
1
precursor. H NMR (400 MHz, benzene-d6): δ 7.26−6.98 (m, 6H,
HAr), 5.83 (s, 2H, CH), 3.58−3.02 (m, 12H, CH(CH3)2 and HTHF),
1.41 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.30 (s, 8H, HTHF), 1.22 (d, J =
2
89
6.9 Hz, 12H, CH(CH3)2), 0.28 (s, 18H, Si(CH3)), −0.93 (d, J
=
Y,H
2.7 Hz, 4H, CH2Si). The spectrum agrees with previous literature.27
Synthesis of Complex Sc2. Complex Sc2 was prepared using a
method similar to that for Sc1 by using 2 equiv of the imidazolin-2-
imine ligand. Its single crystal was obtained by slow crystallization
from toluene solution at −35 °C. 1H NMR (400 MHz, benzene-d6): δ
7.22−7.05 (m, 12H, HAr), 5.82 (s, 4H, CH), 3.45 (hept, J = 6.9 Hz,
4H, CH(CH3)2), 3.18 (hept, J = 6.9 Hz, 4H, CH(CH3)2), 2.81 (t, J =
7.0 Hz, 4H, HTHF), 1.33 (d, J = 6.9 Hz, 12H, CH(CH3)2), 1.20 (m,
36H, CH(CH3)2), 1.05 (t, J = 7.0 Hz, 4H, HTHF), 0.21 (s, 9H,
Si(CH3)3), −0.56 (s, 2H, CH2Si). 13C NMR (126 MHz, C6D6): δ
148.27, 147.75, 136.87, 128.01, 123.78, 123.36, 112.91, 70.63, 28.34,
28.21, 27.31, 25.08, 24.49, 23.61, 23.42, 4.35.
Synthesis of Complex Y2. Complex Y2 was prepared using a
method similar to that for Y1 by using 2 equiv of imidazolin-2-imine
ligand. 1H NMR (500 MHz, benzene-d6): δ 7.25−7.11 (m, 4H, HAr),
7.11−7.03 (m, 8H, HAr), 5.83 (s, 4H, CH), 3.40 (hept, J = 6.9 Hz,
4H, CH(CH3)2), 3.18 (hept, J = 6.9 Hz, 4H, CH(CH3)2), 2.70 (t, J =
6.5 Hz, 4H, HTHF), 1.28 (d, J = 7.0 Hz, 12H, CH(CH3)), 1.19 (m,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00563.
■
sı
NMR spectra and crystallographic data for complexes
Sc1−Sc3 (PDF)
Accession Codes
CCDC 1973914−1973916 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
36H, CH(CH3)), 1.05 (t, J = 7.0 Hz, 4H, HTHF), 0.19 (s, 9H,
2
Si(CH3)3), −0.85 (d, J
= 3.5 Hz, 2H, CH2Si). 13C NMR (126
89Y,H
MHz, C6D6): δ 149.13, 148.62, 137.63, 137.40, 137.30, 124.38,
123.98, 113.20, 70.94, 32.30, 29.09, 28.96, 26.15, 25.98, 25.79, 25.36,
25.02, 24.37, 24.30, 23.94, 23.39, 14.69, 5.30, 0.36. Anal. Calcd for
C62H91N6OSiY: C, 70.69; H, 8.71; N, 7.98. Found: C, 70.50, H, 8.39,
N, 8.16.
Yanming Hu − Division of Energy Materials, Dalian Institute
of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, People’s Republic of China; Phone: +86-0431-
85262305; Email: ymhu@dicp.ac.cn
Moris S. Eisen − Schulich Faculty of Chemistry, Technion-
Israel Institute of Technology, Haifa City 32000, Israel;
orcid.org/0000-0001-8915-0256; Phone: +972-4-
8292680; Email: chmoris@tx.technion.ac.il
Xuequan Zhang − Key Laboratory of Rubber-Plastics,
Ministry of Education/Shandong Provincial Key Laboratory
of Rubber-plastics, Qingdao University of Science &
Technology, Qingdao 266042, People’s Republic of China;
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences, Changchun 130022, Jilin, People’s Republic of
China; orcid.org/0000-0003-2487-9438; Phone: +86-
0431-85262306; Email: xqzhang@ciac.ac.cn, tel
Synthesis of Complex Sc3. Complex Sc3 was prepared using a
method similar to that for Sc1 by using 1 equivalent of the
benzimidazolin-2-imine ligand. Its single crystal was obtained by slow
crystallization from toluene solution at −35 °C. 1H NMR (400 MHz,
benzene-d6): δ 7.23 (d, J = 7.8 Hz, 2H, HAr), 7.08−6.97 (m, 6H, HAr),
6.98−6.93 (m, 2H, HAr), 6.80 (t, J = 7.6 Hz, 2H, HAr), 6.28 (d, J = 7.8
Hz, 2H, HAr), 3.24−3.14 (m, 4H, CH2 and CH(CH3)2), 3.05−3.02
(d, 2H, CH2), 2.73−2.59 (m, 6H, CH(CH3)2 and HTHF), 1.25 (d, J =
6.8 Hz, 6H, CH(CH3)2), 1.14 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.05−
0.96 (m, 10H, CH(CH3)2 and HTHF), 0.83 (d, J = 6.8 Hz, 6H,
CH(CH3)2), 0.39 (s, 9H, Si(CH3)3), 0.36 (s, 9H, Si(CH3)3), 0.11 (s,
2H, CH2Si), −0.27 (s, 2H, CH2Si). 13C NMR (126 MHz, C6D6): δ
154.81, 150.57, 149.99, 135.84, 133.90, 132.87, 129.59, 125.07,
124.25, 123.18, 120.66, 110.27, 108.88, 70.86, 45.98, 29.16, 28.86,
25.44, 25.03, 24.89, 4.32, 0.36.
Authors
Wenpeng Zhao − Key Laboratory of Rubber-Plastics, Ministry
of Education/Shandong Provincial Key Laboratory of
Rubber-plastics, Qingdao University of Science & Technology,
Qingdao 266042, People’s Republic of China; Changchun
General Procedure for 1-Hexene Polymerization. A typical
polymerization procedure was as follows: in a glovebox, [Ph3C][B-
(C6F5)4] (20 μmol) in chlorobenzene (1.0 mL) was added to a
solution of the scandium complex (10 μmol) in chlorobenzene (1.0
G
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Polymerization of 1-Hexene and Copolymerization of Ethylene with
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Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, Jilin, People’s Republic of China
Beibei Wang − Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, Jilin,
People’s Republic of China
Bo Dong − Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun 130022, Jilin, People’s
Republic of China
Heng Liu − Key Laboratory of Rubber-Plastics, Ministry of
Education/Shandong Provincial Key Laboratory of Rubber-
plastics, Qingdao University of Science & Technology,
Qingdao 266042, People’s Republic of China; Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, Jilin, People’s Republic of China;
orcid.org/0000-0001-6064-6610
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00563
Notes
The authors declare no competing financial interest.
(17) Pan, Y.; Zhao, A.; Li, Y.; Li, W.; So, Y.-M.; Yan, X.; He, G.
Bis(oxazoline)-derived N-heterocyclic carbene ligated rare-earth
metal complexes: synthesis, structure, and polymerization perform-
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ACKNOWLEDGMENTS
■
This work was supported by the Jilin Provincial science and
technology development program (20190103122JH), the Joint
Funds of the National Natural Science Foundation of China
(U1862206), the National Natural Science Foundation of
China (no. 21801236), and the East-West Cooperation Project
of Ningxia Key R&D Plan (2019BFH02018).
(19) Song, T.; Liu, N.; Tong, X.; Li, F.; Mu, X.; Mu, Y. Half-
sandwich rare-earth metal complexes bearing a C5Me4-C6H4-o-
CH2NMe2 ligand: synthesis, characterization and catalytic properties
for isoprene, 1-hexene and MMA polymerization. Dalton Trans. 2019,
48, 17840−17851.
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https://dx.doi.org/10.1021/acs.organomet.0c00563
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