Selectivity Effects on N, N, N′-Cobalt Catalyzed Ethylene Dimerization/Trimerization Dictated through Choice of Aluminoxane Cocatalyst

Article abstract of DOI:10.1021/acs.organomet.8b00924

The cobalt(II) chloride complexes, [2-(C₇H₄N₂H)-8-(ArN)C₁₀H₈N]CoCl₂ (Ar = 2,6-Me₂C₆H₃ Co1; 2,6-Et₂C₆H₃ Co2; 2,6-i-Pr_2<

Full text of DOI:10.1021/acs.organomet.8b00924

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Selectivity Effects on N,N,N′‑Cobalt Catalyzed Ethylene
Dimerization/Trimerization Dictated through Choice of
Aluminoxane Cocatalyst
Yongfeng Huang,† , ‡R ,§andi Zhang, Tongling Liang, Xinquan Hu,* Gregory A. Solan,*
,‡
‡,§
‡
,†
,‡,∥
and Wen-Hua Sun*
†
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
‡
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
§
CAS Research/Education Center for Excellence in Molecular Sciences and International School, University of Chinese Academy of
Sciences, Beijing 100049, China
∥
Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, U.K.
*
S Supporting Information
ABSTRACT: The cobalt(II) chloride complexes, [2-(C H N H)-8-
7
4
2
(
ArN)C H N]CoCl (Ar = 2,6-Me C H Co1; 2,6-Et C H Co2; 2,6-
10 8 2 2 6 3 2 6 3
i-Pr C H Co3; 2,4,6-Me C H Co4; 2,6-Et -4-MeC H Co5; 2,4,6-t-
2
6
3
3
6
2
2
6
2
Bu C H Co6), have each been prepared by a one-pot template
3
6
2
reaction of 2-benzoimidazolyl-5,6,7-trihydroquinolin-8-one with the
corresponding aniline in the presence of cobalt dichloride. The
molecular structures of the methanol adducts, Co1(HOMe) and
Co4(HOMe), reveal distorted octahedral geometries that self-assemble
to form networks based on NH···Cl and OH···Cl intermolecular hydrogen bonding interactions. On activation with
methylaluminoxane (MAO), all six cobalt complexes catalyzed ethylene dimerization with a high selectivity for 1-butene. By
marked contrast, with modified methylaluminoxane (MMAO), products the result of ethylene dimerization and trimerization
were observed with a bias toward the C products (up to 49% 1-hexene). In general, the MMAO-promoted oligomerizations
6
5
−1
−1
display higher catalytic activities with mesityl-containing Co4 the stand-out performer [7.60 × 10 g·mol (Co) h at 50 °C].
INTRODUCTION
dimers and trimers (Chart 1). Moreover, the nature of the
■
benzimidazole N-R substituent can influence the catalytic
Linear α-olefins such as 1-butene and 1-hexene are employed,
among other things, as comonomers in the manufacture of
linear low-density polyethylene (LLDPE). Selective routes to
such short chain α-olefins via the selective dimerization and
trimerization of ethylene are known and indeed operated
industrially. With regard to dimerization, the AlphaButol
process makes use of a homogeneous catalyst based on a
8b
8c
performance with alkylated precatalysts of type A8 or B
a
proving less active than their protio-C counterpart. On the
other hand, substitution of the NH unit in C for an O atom in
9
D leads to an unusual switch between oligomerization and
polymerization as a consequence of the temperature employed.
In recent years, we have found that the introduction of
controlled amounts of strain into a bis(imino)pyridine can
have an effect on the performance of an ethylene polymer-
ization catalyst. For example, sterically enhanced examples of
1
combination of titanium tetrabutoxide and triethylaluminum,
while trimerization technology largely involves chromium
catalysts some of which were originally disclosed by Union
10
2
3
4
cyclohexyl-fused E on suitable activation function effectively
at 60 °C, generating polyethylene waxes, while the
corresponding bis(imino)pyridine-cobalt catalyst is much less
productive under such high temperature operating condi-
Carbide, Chevron-Phillips, and BP. In addition, other first
row transition metal catalysts have been investigated for their
effectiveness and selectivity in both transformations, though
the use of cobalt is less well developed.
5
6
11
tions. To explore whether a similar structural modification
would have beneficial effects on an oligomerization catalyst, we
report six examples of benzimidazole-containing F in which the
steric and electronic properties of the N-aryl group have been
systematically varied. An in-depth study is then conducted to
Catalysts composed of bis(arylimino)pyridine-iron and
cobalt complexes have shown promise for their development
in this field as their less sterically demanding derivatives display
a preference for ethylene oligomerization over polymer-
7
,8
ization.
Indeed, alternative N,N,N-cobalt catalysts have
emerged that favor the formation of shorter chain oligomers.
Among them, cobalt precatalysts based on benzimidazole-
substituted pyridylimines have shown a predilection toward
Received: December 19, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00924
Organometallics XXXX, XXX, XXX−XXX

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Article
Chart 1. Developments in Unsymmetrical N,N,N′-Chelating Ligands for Cobalt(II) Chlorides
determine the effects of the precatalyst structure, type and
amount of cocatalyst, and temperature on the selectivity of the
ethylene oligomerization. Full synthetic and characterization
details are additionally presented for the complexes.
RESULTS AND DISCUSSION
Synthesis and Characterization of Co1−Co6. The
cobalt(II) chloride complexes, [2-(C H N H)-8-(ArN)C H -
■
7
4
2
10
8
N]CoCl (Ar = 2,6-Me C H Co1; 2,6-Et C H Co2; 2,6-i-
2
2
6
3
2
6
3
Pr C H Co3; 2,4,6-Me C H Co4; 2,6-Et -4-MeC H Co5;
2
6
3
3
6
2
2
6
2
2
6
,4,6-t-Bu C H Co6), were obtained in reasonable yield (49−
3
6
2
4%) by templating 2-benzoimidazol-6,7-dihydroquinolin-8
(
5H)-one (1) with the corresponding aniline in the presence
of cobalt dichloride in acetic acid at reflux (Scheme 1).
Figure 1. ORTEP representation of Co1(HOMe) with the thermal
ellipsoids set at the 30% probability level. Hydrogen atoms have been
omitted for clarity.
Scheme 1. Template Synthetic Route to Co1−Co6
Attempts to prepare the free ligands themselves by an acid
catalyzed Schiff base condensation reaction of ketone 1 with
the appropriate aniline proved unsuccessful. It is worthy of
note that related one-pot template approaches have been
applied elsewhere to synthesize similar species. All six cobalt
complexes have been characterized by IR spectroscopy as well
as by elemental analysis. In addition, the adducts Co1(HOMe)
and Co4(HOMe) have been the subject of single crystal X-ray
diffraction studies.
Single crystals of Co1(HOMe) and Co4(HOMe) suitable
for the X-ray determinations were grown by layering diethyl
ether onto a solution of the corresponding complex in a
methanol/dichloromethane mixture. Perspective views are
shown in Figures 1 and 2; selected bond lengths and angles
are tabulated in Table 1. The structures of Co1(HOMe) and
Co4(HOMe) are similar and will be discussed together. Each
cobalt center is surrounded by two chlorides, one oxygen
bound methanol, and three nitrogen atoms belonging to the
chelating 2-benzoimidazolyl-8-arylimino-5,6,7-trihydroquino-
Figure 2. ORTEP representation of Co4(HOMe) with the thermal
ellipsoids set at the 30% probability level. Hydrogen atoms have been
omitted for clarity.
12
line (aryl = 2,6-Me Ph Co1; 2,4,6-Me Ph Co4) to complete
2
3
a geometry that can be best described as distorted
13
octahedral. The methanol ligand occupies a site trans to
Cl(1) [O(1)−Co(1)−Cl(1) 167.57(5)° Co1, 166.49(5)°
Co4], while Cl(2) is trans to the central nitrogen donor of
the mer-configured tridentate ligand [N(2)−Co(1)−Cl(2)
178.49(4)° Co1, 177.25(5)° Co4]; similar N,N,N-CoCl -
2
(HOMe) adducts have been structurally characterized
8c
previously. Of the three cobalt−nitrogen distances, the one
involving the central pyridine is the shortest [Co(1)−N(2)
2.0895(15) Å Co1, 2.0853(17) Å Co4] while the exterior
nitrogen donors show some variation. In particular, the Co−
B
DOI: 10.1021/acs.organomet.8b00924
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Article
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Co1(HOMe) and Co4(HOMe)
Co1
Co4
bond lengths
Co(1)−N(1)
Co(1)−N(2)
Co(1)−N(3)
Co(1)−O(1)
Co(1)−Cl(1)
Co(1)−Cl(2)
N(1)−C(1)
N(3)−C(7)
N(4)−C(1)
2.1907(15)
2.1842(17)
2.0853(17)
2.2471(17)
2.2682(18)
2.5576(7)
2.2850(6)
1.320(3)
2.0895(15)
2.2532(15)
2.2145(16)
2.5600(9)
2.2876(6)
1.323(2)
1.285(2)
1.355(2)
1.280(3)
1.356(3)
bond angles
N(1)−Co(1)−N(2)
N(1)−Co(1)−N(3)
N(2)−Co(1)−N(3)
N(1)−Co(1)−Cl(2)
N(2)−Co(1)−Cl(2)
N(3)−Co(1)−Cl(2)
N(1)−Co(1)−Cl(1)
N(2)−Co(1)−Cl(1)
N(3)−Co(1)−Cl(1)
Cl(2)−Co(1)−Cl(1)
O(1)−Co(1)−Cl(1)
O(1)−Co(1)−Cl(2)
O(1)−Co(1)−N(1)
O(1)−Co(1)−N(2)
O(1)−Co(1)−N(3)
75.40(6)
149.50(5)
74.31(6)
105.77(4)
178.49(4)
104.46(4)
87.36(4)
85.31(5)
93.67(4)
95.68(3)
167.57(5)
95.10(5)
83.77(6)
84.05(6)
89.62(6)
75.43(6)
149.65(6)
74.43(6)
104.69(5)
177.25(5)
105.24(5)
87.86(5)
86.47(5)
93.76(5)
96.28(2)
166.49(5)
95.64(5)
82.99(7)
81.63(7)
89.20(7)
Figure 3. NH···Cl and OH···Cl hydrogen-bonding interactions
between neighboring molecules of Co1(HOMe).
characteristic of bound imine-nitrogen atoms; no absorption
bands corresponding to complexed CO groups or to free
ketone 1 could be detected. In addition, the elemental analysis
data confirmed complexes of composition LCoCl2.
Ethylene Oligomerization. To explore the role of the
cocatalyst, two separate oligomerization studies were per-
formed with each precatalyst using either methylaluminoxane
(
MAO) or modified methylaluminoxane (MMAO) as the
activator. Typically, the oligomerization runs were performed
in toluene at 10 atm C H over 30 min and any selectivity for
2
4
dimerization, trimerization as well as specific α-olefins, assessed
using gas chromatography (GC); the results of the two studies
are gathered in Tables 2 and 3.
Nbenzimidazole distance is noticeably shorter [Co(1)−N(1)
.1907(15) Å Co1, 2.1842(17) Å Co4] than the Co−N
2
imine
distance [Co(1)−N(3) 2.2532(15) Å Co1, 2.2471(17) Å
Co4], reflecting the superior donor properties of the
benzimidazole-nitrogen. The Co−Cl bond distances also
display some differences with Co(1)−Cl(1) (2.5600(9) Å
Co1, 2.5576(7) Å Co4) notably longer than Co(1)−Cl(2)
Catalytic Evaluation Using MAO. To allow an optimization
of the conditions, Co4 was selected as the test precatalyst and
the effect of temperature, Al:Co molar ratio, run time, and
pressure were all investigated (entries 1−12, Table 2). First,
with the Al:Co molar ratio fixed at 1000, the run temperature
was increased from 30 to 60 °C and the activity in each case
measured after 30 min (entries 1−4, Table 2). The optimum
(
2.2876(6) Å Co1, 2.2850(6) Å Co4), an observation that can
be plausibly attributed to the variation in trans-donor. Within
each 5-membered chelate ring, the bite angles show some
modest variation with that involving Nbenzimidazole [N(1)−
Co(1)−N(2) 75.41(6)° Co1, 75.40(7)° Co4] slightly larger
5
−1
−1
activity of 1.23 × 10 g·mol (Co)·h for Co4/MAO was
observed at 40 °C with a 100% selectivity for ethylene
dimerization (C ) with 1-butene being the major component
4
than that with N
[N(2)−Co(1)−N(3) 74.31(6)° Co1,
(91%). Indeed, all runs resulted in solely C formation with the
imine
4
7
4.43(6)° Co4], which presumably derives from the
selectivity for 1-butene decreasing as the temperature was
raised.
constraints of the 5-membered imidazole ring. The imine
bond lengths [C(7)−N(3) 1.285(2) Å Co1, 1.280(3) Å Co4]
are typical of this functionality while the N-aryl group is
inclined almost orthogonally with respect to the imine vector.
Second, with the temperature maintained at 40 °C, the
Al:Co molar ratio for Co4/MAO was increased from 500 to
2000 and the activity determined (entries 2, 5−7, Table 2). On
comparison of the four runs, a molar ratio of 1000 resulted in a
3
In each structure, the sp -hybridized C(9) atom belonging to
5
−1
−1
the trihydroquinoline unit is folded away from the plane of the
neighboring unsaturated heterocycle with the result that it
adopts a confirmation cis to the coordinated methanol. Due to
the presence of both OH and NH groups within Co1(HOMe)
and Co4(HOMe), an extensive intermolecular hydrogen
bonding network is a feature of both structures (Figures 3
and S1). In particular, Cl(1) undergoes both Cl···HO and Cl···
HN interactions with two different neighboring molecules
peak activity of 1.23 × 10 g·mol (Co)·h . Once again, only
products derived from ethylene dimerization could be detected
with the selectivity for 1-butene reducing as the Al:Co molar
was raised (e.g., α-C : 100% Al:Co = 500; 78% Al:Co = 2000).
4
These decreases in 1-butene content with Al:Co ratio (entries
2, 5−7, Table 2) or reaction temperature can be attributed to
the greater thermodynamic stability of internal olefins (e.g., 2-
butene).
(
N(4)···Cl(1) 3.127 Å Co1; 3.140 Å Co4; O(1)···Cl(1) 3.131
Third, with the temperature and Al:Co molar ratio fixed at
40 °C and 1000, respectively, the runs using Co4/MAO were
performed over 5, 15, 45, and 60 min. The highest activity of
3.17 × 10 g·mol (Co)·h was observed after 5 min with 1-
butene being the only product identifiable. However, this
Å, 3.187 Å) leading to a two-dimensional array within the
crystal structure.
5
−1
−1
The IR spectra of Co1−Co6 reveal imine stretching
−
1
frequencies in the range 1625−1618 cm , which are
C
DOI: 10.1021/acs.organomet.8b00924
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Article
a
Table 2. Ethylene Oligomerization Using MAO as Cocatalyst
c
oligomer distribution (%)
b
entry
precatalyst
T [°C]
Al:Co
t [min]
activity
C /∑C
4
α-C4
1
2
3
4
5
6
7
8
9
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co1
Co2
Co3
Co5
Co6
30
40
50
60
40
40
40
40
40
40
40
40
40
40
40
40
40
1000
1000
1000
1000
500
1500
2000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
30
30
30
30
30
30
30
5
15
45
60
30
30
30
30
30
0.49
1.23
0.90
0.81
0.43
0.61
0.44
3.17
1.52
1.06
0.88
0.46
0.36
0.53
0.39
0.77
0.24
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
94.1
90.8
85.1
61.1
100
84.2
78.0
100
100
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
80.5
71.2
91.9
100
d
100
86.8
90.6
94.4
30
a
b
5
−1
−1
c
General conditions: 3 μmol of precatalyst, 10 atm C H , 100 mL of toluene. × 10 g·mol (Co)·h . Determined by GC, and ∑C signifies the
2
4
d
total amounts of oligomers. 5 atm C H .
2
4
a
Table 3. Ethylene Oligomerization Using MMAO as Cocatalyst
c
oligomer distribution (%)
b
entry
precatalyst
T [°C]
Al:Co
t [min]
activity
C /∑C
4
α-C4
C /∑C
6
α-C6
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co1
Co2
Co3
Co5
Co6
30
40
50
60
70
50
50
50
50
50
50
50
50
50
50
50
50
50
50
1000
1000
1000
1000
1000
500
1500
2000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
30
30
30
30
30
30
30
30
5
15
45
60
30
30
30
30
30
30
30
4.58
4.96
7.60
4.50
3.59
2.15
7.14
6.73
26.3
9.59
5.91
5.23
6.00
5.12
4.76
3.55
5.28
5.31
5.64
34.0
17.3
48.5
21.4
12.9
30.4
15.0
6.3
52.3
64.8
70.4
66.1
61.9
85.9
61.6
59.1
58.0
55.7
68.7
71.6
63.7
75.4
57.7
62.3
47.3
56.2
62.8
66.0
82.7
51.5
78.6
87.1
69.6
85.0
93.7
83.5
88.2
91.8
93.0
84.0
55.7
90.2
68.8
91.0
87.4
82.9
35.0
40.3
34.6
35.1
35.7
35.3
35.3
35.5
35.5
35.6
35.5
36.0
34.9
38.1
36.4
29.8
49.2
49.2
37.3
16.5
11.8
8.2
1
1
1
1
1
1
1
1
1
1
7.0
d
e
16.0
44.3
9.8
31.2
9.0
12.6
17.1
a
b
5
−1
−1
c
General conditions: 3 μmol of precatalyst, 10 atm C H , 100 mL of toluene. 10 g·mol (Co)·h . Determined by GC, and ∑C signifies the
2
4
d
e
total amounts of oligomers. 5 atm C H . 100 mL of 1,2-xylene as solvent with the conditions otherwise the same as in entry 3.
2
4
selectivity for 1-butene gradually decreased over time with only
1.2% selectivity noted after 60 min. With the pressure
reduced to 5 bar C H , no loss in selectivity for C products
was observed (92% 1-butene) but the activity dropped by
more than half (entry 12, Table 2). Overall, these results
highlight the importance of reaction temperature, Al:Co ratio,
and reaction time on influencing both the selectivity for 1-
butene and the catalytic activity.
Co6. It would appear that both electronic and steric hindrance
effects play a combined role on affecting performance with
mesityl-containing Co4 the most active and the 2,4,6-tri-t-butyl
Co6 the least. In particular, para-methyl Co4 and Co5 are
more active than para-H Co1, Co2 and Co3 highlighting the
importance of the electron donating para-substituent. On the
other hand, the most sterically encumbered Co6 likely impedes
ethylene coordination, leading to the lowest activity. It is
noteworthy that the current systems generally display higher
productivities when compared with other N,N,N-cobalt
7
2
4
4
With the optimized conditions established as temp = 40 °C,
Al:Co = 1000, PC2H4 = 10 atm, the remaining five cobalt
precatalysts were similarly screened. In terms of activity (range:
8
,9
5
−1
−1
1
.23−0.24 × 10 g·mol (Co)·h ), the six precatalysts were
oligomerization precatalysts such as A−D (Chart 1).
Nevertheless, all precatalysts, regardless of N-aryl substitution,
found to fall in the order: Co4 > Co5 > Co2 > Co3 ∼ Co1 >
D
DOI: 10.1021/acs.organomet.8b00924
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Article
gave solely C products with selectivities for 1-butene of
Co6 were additionally screened (entries 3, 15−19, Table 3).
4
greater than 86.7% and indeed 100% for Co1 and Co2.
Catalytic Evaluation Using MMAO. As with the MAO
study, Co4 was again used as the test precatalyst to establish
the optimized conditions for the runs this time using MMAO
as the cocatalyst. The effect of temperature was first explored
with the Al:Co ratio set at 1000 (entries 1−5, Table 3). On
increasing the temperature from 30 to 70 °C, the highest
Inspection of the tabulated data reveals activities in the range
5
−1
−1
7.60−4.76 × 10 g·mol (Co)·h , with their relative values
falling in the order: Co4 > Co6 > Co2 > Co5 > Co3 > Co1.
When compared with the results obtained with MAO, these
MMAO-promoted systems not only are more active but also
reveal some variations in relative performance with the most
sterically hindered system Co6 now notably toward the top of
the order rather than the bottom. Nevertheless, all precatalysts
form predominantly products the result of ethylene trimeriza-
tion rather than dimerization. In terms of the 1-hexene
selectivity, some variation is observed between precatalysts
5
−1
−1
activity of 7.60 × 10 g·mol (Co)·h was observed at 50 °C
entry 3, Table 3), which when compared with the MAO study
(
represents not only a higher operating temperature but also a
significantly higher activity (entry 2, Table 2). However, this
higher activity is at the expense of poorer selectivity with
with Co3 and Co5 affording 49% α-C selectivity while Co2
6
products now the result of dimerization (C ) and trimerization
only 30%. Compared with previously reported N,N,N-cobalt
oligomerization catalysts (e.g., A−D, Chart 1), much higher
overall catalytic activity is again a feature of the current
systems. Moreover, unlike that observed with A−D, ethylene
trimerization products have emerged as a major component of
the oligomerization distribution. When compared with
chromium-based catalysts bearing similar benzimidazole-
containing multidentate ligands, the Co/MMAO-type catalysts
described in this work show higher selectivity toward
trimerization of ethylene and no evidence of polymeric
fractions, a byproduct that was a feature of these previous
4
(
C ) evident (C /∑C = 48.5%, C /∑C = 51.5%). Moreover,
6
4
6
as the temperature is raised, the bias toward the C products
6
becomes more pronounced (70 °C: C /∑C = 12.9%, C /∑C
4
6
=
87.1%). With regard to the C products, the selectivity
6
toward 1-hexene remains relatively constant (ca. 35%) between
5
0 and 70 °C, while, at 40 °C, a greater content of 1-hexene is
apparent (up to 40%).
With the temperature of the run retained at 50 °C, the Al:Co
molar ratio was increased from 500 to 2000 (entries 3, 6−8).
Examination of the data reveals the activity of Co4/MMAO
5
−1
−1
14
reaches its highest value of 7.60 × 10 g·mol (Co)·h with
an Al:Co molar ratio of 1000 with a gradual drop in the level as
the ratio is increased to 2000 (entries 3, 6−8, Table 3). In
studies. On the other hand, when compared to the well-
documented diphosphinoamine(PNP)-type chromium cata-
lysts, the selectivity of the current catalysts for 1-hexene is
15
terms of the oligomer distribution, the preference for C
6
noticeably less.
In this investigation, the C /C composition of short chain
products becomes more evident at higher molar ratios,
reaching a maximum of 93.7% at a ratio of 2000. However,
the selectivity for 1-hexene remains constant at around 36% for
all the Al:Co ratios, highlighting the capacity of these cobalt
catalysts to mediate isomerization to give 2-hexene as the
4
6
alkenes can be influenced by the alkylaluminum activator
employed. With MAO, ethylene dimerization was achieved,
while, with MMAO, both dimers and trimers were observed. It
would appear that the rapid β-H elimination step that occurs
under the activation with MAO is influenced by the structural
features of the aluminoxane employed. With MMAO, it would
seem plausible that the steric properties of the isobutyl groups
belonging to this cocatalyst are somehow subtly affecting the
active site, leading to competitive ethylene trimerization.
major C product.
6
The effect of time on activity of Co4/MMAO shows similar
trends to that seen with MAO but with markedly higher
activities observed over short run times. Indeed, the highest
5
−1
−1
activity of 26.3 × 10 g·mol (Co)·h of the entire study was
seen after 5 min, which equates to an 8-fold increase when
compared to that seen with MAO after the same time period.
As the time elapses, the activity slowly decreases, reaching its
CONCLUSIONS
■
5
−1
−1
lowest value of 5.23 × 10 g·mol (Co)·h after 60 min,
Six examples of 2-benzoimidazolyl-8-arylimino-5,6,7-trihydro-
quinoline-cobalt(II) chloride complexes (Co1−Co6) have
been successfully synthesized and fully characterized including
in two cases by single crystal X-ray diffraction. On activation
with either MAO or MMAO, the current systems exhibited
higher catalytic activities for ethylene oligomerization when
compared with previously reported N,N,N-cobalt precatalysts
which is still higher than that seen with MAO after 5 min (3.17
10 g·mol (Co)·h , entry 8, Table 3). On the other hand,
the bias toward the C products increases with time with the
5
−1
−1
×
6
amount of C constituting 93.0% of the mixture after 60 min;
6
the 1-hexene content however remains constant at around 36%
over the various reaction times. Lowering the pressure from 10
to 5 atm C H , under otherwise identical conditions, saw the
8
,9
such as A−D (Chart 1). With MAO as cocatalyst, all the
2
4
5
−1
−1
5
activity drop from 7.60 to 6.00 × 10 g·mol (Co)·h with an
increased preference for C products at lower pressure (entries
complexes were effective (activities up to 1.23 × 10 g·
−
1
−1
mol (Co)·h for Co4) for selective ethylene dimerization
6
3
and 13, Table 3). It is worth highlighting that no evidence of
(C ) in which 1-butene accounted for the major isomer. By
4
octenes could be detected in the GC. Indeed, we ran the
oligomerizations in two different aromatic solvents, namely,
toluene and 1,2-xylene, to check for any solvent overlap (entry
stark contrast, with MMAO as cocatalyst, products the result of
ethylene dimerization (C ) and trimerization (C ) were
4
6
obtained with the C hydrocarbon constituting the major
6
3
, Table 3 vs entry 14, Table 3). In both of these runs, no
fraction with selectivities for 1-hexene up to 49% (with Co3
and Co5). Moreover, these MMAO-promoted oligomeriza-
peaks corresponding to octenes could be observed. Notably, a
slightly lower activity was observed in xylene when compared
to toluene, but the product distribution was nevertheless
similar with hexenes being the major component (more details
show in Figures S2 and S3).
5
tions displayed a relatively higher activity (up to 7.60 × 10 g·
−
1
−1
mol (Co)·h for Co4) despite the loss of selectivity for
dimerization. It is unclear as to the origin of these variations in
the C /C product distribution that derive from the type of
4
6
Using the optimal conditions established for Co4/MMAO
aluminoxane employed and hence will be the subject of further
investigations.
(
temp = 50 °C, Al:Co = 1000, PC2H4 = 10 atm), Co1−Co3 and
E
DOI: 10.1021/acs.organomet.8b00924
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
1
957 (s), 2904 (w), 2868 (m), 1626 (ν
463 (w), 1434 (m), 1393 (m), 1357 (m), 1324 (m), 1297 (m), 1266
, m), 1595 (s), 1489 (w),
EXPERIMENTAL SECTION
CN
■
General Considerations. All manipulations of air- and/or
moisture-sensitive operations were undertaken in a nitrogen
atmosphere using standard Schlenk techniques. Toluene was refluxed
over sodium and distilled under nitrogen immediately prior to use.
Methylaluminoxane (MAO, 1.46 M solution in toluene) and modified
methylaluminoxane (MMAO, 1.93 M in n-heptane) were purchased
from Akzo Nobel Corp. High-purity ethylene was purchased from
Beijing Yanshan Petrochemical Co. and used as received. Other
reagents were purchased from Aldrich, Acros, or local suppliers. NMR
spectra were recorded on a Bruker DMX 400 MHz instrument at
ambient temperature using TMS as an internal standard. IR spectra
were recorded on a PerkinElmer System 2000 FT-IR spectrometer.
GC analyses were performed with a Varian CP-3800 gas chromato-
graph (Beijing, China) equipped with a flame ionization detector and
a 30 m (0.2 mm i.d., 0.25 mm film thickness) CP-Sil 5 CB column.
The compound 2-(1H-benzoimidazol-2-yl)-6,7-dihydroquinolin-
(
(
8
m), 1238 (m), 1208 (s), 1148 (m), 1105 (m), 836 (s), 751 (s), 689
w), 662 (m). Anal. Calcd for C H Cl CoN : C, 64.15, H, 6.65, N,
.80. Found: C, 64.48, H, 6.39, N, 8.72%.
X-ray Crystallographic Studies. Single crystals of Co1(HOMe)
3
4
42
2
4
and Co4(HOMe) suitable for the X-ray diffraction studies were
grown by slow diffusion of n-heptane into a solution of the
corresponding complex in a mixture of methanol and dichloro-
methane at room temperature. Data collection for both X-ray
determinations was carried out on a Rigaku MM007-HF Saturn 724
+
CCD diffractometer with confocal mirror monochromated Mo−Kα
radiation (λ = 0.71073 Å). Cell parameters were obtained by global
refinement of the positions of all collected reflections. Intensities were
corrected for Lorentz and polarization effects and empirical
absorption. The structures were solved by direct methods and refined
by full-matrix least-squares on F . All hydrogen atoms were placed in
calculated positions. Structure solution and refinement were
2
16
8
(5H)-one (1) was prepared as described in the literature.
Synthesis of [2-(C H N H)-8-(ArN)C H N]CoCl . Ar = 2,6-
17
performed by using the SHELXL-97 package. Details of the X-ray
structure determination and refinements are provided in Table 4.
7
4
2
10
8
2
Me C H (Co1). A suspension of 1 (0.158 g, 0.60 mmol), 2,6-
2
6 3
dimethylaniline (0.087 g, 0.72 mmol), and cobalt dichloride (0.074 g,
.57 mmol) in glacial acetic acid (10 mL) was stirred at reflux for 6 h,
affording a brown precipitate. On cooling to room temperature, the
precipitate was collected by filtration, washed with diethyl ether (3 ×
Table 4. Crystal Data and Structure Refinement Details for
Co1(HOMe) and Co4(HOMe)
0
Co1-(HOMe)
Co4-(HOMe)
2
0 mL) and then dried under reduced pressure, yielding Co1 as a
−
1
formula
C H Cl CoN O
C H Cl CoN O
brown solid (0.160 g, 57%). FT-IR (KBr, cm ): 3031 (w), 2968 (w),
25 26
2
4
26 28
2
4
formula weight
T (K)
528.33
173
542.35
173(2)
1
1
625 (ν
, m), 1591 (m), 1488 (w), 1462 (s), 1434 (w), 1423 (w),
CN
407 (w), 1377 (w), 1352 (w), 1318 (m), 1295 (m), 1263 (m), 1230
(
8
m), 1198 (m), 1106 (m), 1069 (m), 1036 (m), 984 (m), 926 (m),
67 (m), 834 (m), 773 (s), 745 (s), 696 (m), 665 (m). Anal. Calcd
for C H Cl CoN : C, 58.08, H, 4.47, N, 11.29. Found: C, 58.46, H,
wavelength (Å)
crystal system
space group
a (Å)
b (Å)
c (Å)
0.71073
monoclinic
0.71073
monoclinic
24
22
2
4
P2 /c
P2 /c
1
1
4
.29, N, 10.98%.
6.8781(14)
20.363(4)
16.998(3)
90
93.68(3)
90
2375.7(8)
4
1.474
0.973
1088
4−54.966
−6 ≤ h ≤ 8
7.0575(2)
20.0029(6)
17.2584(5)
90
95.216(3)
90
2426.29(12)
4
1.482
Ar = 2,6-Et C H (Co2). Using a similar procedure and molar ratios
2
6 3
as that described for Co1, Co2 was obtained as a brown powder
(
−1
0.190 g, 64%). FT-IR (KBr, cm ): 3142 (w), 3114 (w), 3064 (w),
958 (m), 2930 (m), 2872 (m), 1624 (ν , m), 1590 (s), 1453 (s),
α (deg)
β (deg)
2
1
1
(
(
CN
405 (m), 1374 (m), 1323 (m), 1298 (m), 1261 (m), 1240 (m),
223 (m), 1186 (m), 1145 (m), 1113 (w), 1093 (m), 1057 (m), 1040
γ (deg)
volume (Å )
3
m), 1012 (w), 865 (m), 803 (m), 772 (s), 747 (s), 686 (m), 660
m). Anal. Calcd for C H Cl CoN : C, 59.56, H, 5.00, N, 10.68.
Z
2
6
26
2
4
Dcalc (g cm−3)
μ (mm−1)
F(000)
Found: C, 59.87, H, 4.79, N, 10.36%.
Ar = 2,6-i-Pr C H (Co3). Using a similar procedure and molar
0.955
1120
2
6 3
ratios as that described for Co1, Co3 was obtained as a brown powder
(
−
1
0.180 g, 57%). FT-IR (KBr, cm ): 3099 (w), 3068 (w), 2962 (s),
928 (m), 2867 (m), 1618 (ν , m), 1590 (m), 1513 (w), 1462
θ range (deg)
limiting indices
4.072−54.958
−9 ≤ h ≤ 8
−24 ≤ k ≤ 25
−22 ≤ l ≤ 22
16612
5556 (0.0414)
5556/0/315
1.026
2
CN
(
m), 1442 (m), 1410 (w), 1383 (m), 1359 (m), 1324 (m), 1300 (m),
−
−
26 ≤ k ≤ 24
20 ≤ l ≤ 22
1
271 (m), 1252 (w), 1219 (m), 1184 (m), 1116 (m), 1047 (m), 929
(
(
m), 838 (m), 801.0 (m), 774 (s), 747 (s), 698 (m), 665 (m), 654
w). Anal. Calcd for C H Cl CoN : C, 60.88, H, 5.47, N, 10.14.
no. of rflns collected
15414
2
8
30
2
4
no. unique rflns [R(int)]
data/restraints/parameters
goodness of fit on F2
5416 (0.0254)
5416/0/305
0.607
Found: C, 61.26, H, 5.25, N, 9.89%.
Ar = 2,6-Et -4-MeC H (Co4). Using a similar procedure and molar
2
6 2
ratios as that described for Co1, Co4 was obtained as a brown powder
(
−
1
0.180 g, 62%). FT-IR (KBr, cm ): 3058 (m), 2988 (w), 2948 (w),
final R indices [I > 2σ(I)]
R1 = 0.0322
R1 = 0.0465
2
1
914 (w), 2863 (w), 1626 (ν
376 (w), 1349 (m), 1319 (m), 1296 (m), 1269 (m), 1234 (m), 1214
, m), 1592 (s), 1461 (s), 1410 (w),
CN
wR
R1 = 0.0358
2
= 0.0729
wR = 0.1202
2
R indices (all data)
R1 = 0.0535
(
(
1
m), 1149 (m), 1037 (m), 858 (s), 834 (m), 765 (s), 747 (s), 709
m), 666 (w). Anal. Calcd for C H Cl CoN : C, 58.84, H, 4.74, N,
wR = 0.0766
wR = 0.1272
2
2
2
5
24
2
4
largest diff peak and hole (e Å−3
)
0.34 and −0.24
and −0.30
0.98. Found: C, 59.11, H, 4.49, N, 10.71%.
Ar = 2,6-Et -4-MeC H (Co5). Using a similar procedure and molar
2
6 2
Oligomerization of Ethylene. General Procedure. The ethylene
oligomerizations were carried out in a 250 mL stainless steel autoclave
equipped with an ethylene pressure control system, a mechanical
stirrer, and a temperature controller. The autoclave was evacuated and
refilled with ethylene three times. Toluene (50 mL), the desired
amount of cocatalyst (MAO or MMAO), and a solution of the
precatalyst (Co1−Co6, 3 μmol) in toluene (50 mL) were successively
added by syringe under an ethylene atmosphere, taking the total
volume of solvent to 100 mL. When the desired reaction temperature
was reached, the stirring was commenced and the ethylene pressure
−
1
F
DOI: 10.1021/acs.organomet.8b00924
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
increased to 10 atm and maintained at this level by a constant feed of
ethylene. After 30 min, the reaction was stopped by cooling the
reactor in an ice bath and the excess pressure then slowly released. A
small amount of this cooled reaction solution was collected (ca. 1
mL) and 5% aqueous hydrogen chloride added to terminate the
reaction. A sample of this mixture (0.02 μL) was then immediately
injected into the GC instrument to determine the distribution of
oligomers obtained. The mass of C and C was calculated based on
chromium, molybdenum or tungsten and a ligand containing at least
one phosphorous, arsenic or antimony atom bound to at least one
(hetero)hydrocarbyl group. U.S. Patent US6800702B2, 2002.
(3) (a) Dohring, A.; Jensen, V. R.; Jolly, P. W.; Thiel, W.; Weber, J.
C. Phosphinoalkyl-Substituted Cyclopentadienyl Chromium Catalysts
for the Oligomerization of Ethylene. In Organometallic Catalysts and
Olefin Polymerization; Springer: Berlin, 2001; pp 127−136.
(b) Gibson, V. C.; Redshaw, C.; Solan, G. A. Bis(imino)pyridines:
Surprisingly Reactive Ligands and a Gateway to New Families of
Catalysts. Chem. Rev. 2007, 107, 1745−1776. (c) Wang, Z.; Solan, G.
A.; Zhang, W.-J.; Sun, W.-H. Carbocyclic-fused N,N,N-pincer ligands
as ring-strain adjustable supports for iron and cobalt catalysts in
ethylene oligo-/polymerization. Coord. Chem. Rev. 2018, 363, 92−
4
6
the ratio of the corresponding peaks to the toluene peak in the gas
chromatogram. Since the volume of the toluene (n) is fixed, the
amount of butenes A (n ) and hexenes B (n ) can be determined (see
1
2
Tables S1 and S2). Prior to the catalytic evaluations, the retention
times for 1-alkenes (1-butene and 1-hexene) and internal alkenes (2-
butene, 2-hexene, and 3-hexene) were established. The catalytic
activity was based on the mass of all oligomers fractions obtained.
1
(
08.
4) (a) Broene, R. D.; Brookhart, M.; Lamanna, W. M.; Volpe, A. F.,
Jr Cobalt-Catalyzed Dimerization of r-Olefins to Give Linear α-Olefin
Products. J. Am. Chem. Soc. 2005, 127, 17194−17195. (b) Thiele, D.;
de Souza, R. F. Oligomerization of Ethylene Catalyzed by Iron and
Cobalt in Organoaluminate Dialkylimidazolium Ionic Liquids. Catal.
Lett. 2010, 138, 50−55. (c) Zhang, N.; Wang, J.; Huo, H.; Chen, L.;
Shi, W.; Li, C.; Wang, J. Iron, cobalt and nickel complexes bearing
hyperbranched iminopyridyl ligands: Synthesis, characterization and
evaluation as ethylene oligomerization catalysts. Inorg. Chim. Acta
2018, 469, 209−216. (d) Xu, Z.; Chada, J. P.; Xu, L.; Zhao, D.;
Rosenfeld, D. C.; Rogers, J. L.; Hermans, I.; Mavrikakis, M.; Huber,
G. W. Ethylene Dimerization and Oligomerization to 1-Butene and
Higher Olefins with Chromium-Promoted Cobalt on Carbon
Catalyst. ACS Catal. 2018, 8, 2488−2497. (e) Caovilla, M.; Thiele,
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00924.
■
*
S
The approach to determining the catalytic activity and
relative percentage of the products (Tables S1 and S2),
the hydrogen-bonding interactions in the crystal
structure (Figure S1), and the gas chromatograms
(
Figures S2−S36) (PDF)
Accession Codes
D.; de Souza, R. F.; Gregorio, J. R.; Bernardo-Gusmao, K. Cobalt-β-
́
̃
CCDC 1871156 and 1871157 contain the supplementary
crystallographic 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.
diimine complexes for ethylene oligomerization. Catal. Commun.
2017, 101, 85−88. (f) Li, C.; Wang, F.; Lin, Z.; Zhang, N.; Wang, J.
Cobalt complexes based on hyperbranched salicylaldimine ligands as
catalyst precursors for ethylene oligomerization. Appl. Organomet.
Chem. 2017, 31, No. e3756. (g) Haghverdi, M.; Tadjarodi, A.; Bahri-
Laleh, N.; Haghighi, M. N. Cobalt complexes based on 2-(1H-
benzimidazol-2yl)-phenol derivatives: Preparation, spectral studies,
DFT calculations and catalytic behavior toward ethylene oligomeriza-
tion. J. Coord. Chem. 2017, 70, 1800−1814.
AUTHOR INFORMATION
Corresponding Authors
E-mail: xinquan@zjut.edu.cn (X.H.).
E-mail: whsun@iccas.ac.cn. Tel: +86-10-62557955. Fax: +86-
0-62618239 (W.-H.S).
■
*
*
1
*
(5) (a) Britovsek, G. J. P.; Gibson, V. C.; McTavish, S. J.; Solan, G.
A.; White, A. J. P.; Williams, D. J.; Britovsek, G. J. P.; Kimberley, B. S.;
Maddox, P. J. Novel olefin polymerization catalysts based on iron and
cobalt. Chem. Commun. 1998, 849−850. (b) Britovsek, G. J. P.;
Mastroianni, S.; Solan, G. A. S.; Baugh, P. D.; Redshaw, C.; Gibson, V.
C.; White, A. J. P.; Williams, D. J. M.; Elsegood, R. J. Oligomerisation
of Ethylene by Bis(imino)pyridyliron and -cobalt complexes. Chem. -
Eur. J. 2000, 6, 2221−2231. (c) Small, B. L.; Brookhart, M. Iron-
Based Catalysts with Exceptionally High Activities and Selectivities for
Oligomerization of Ethylene to Linear α-Olefins. J. Am. Chem. Soc.
E-mail: gas8@leicester.ac.uk. Tel: +44-116-2522096 (G.A.S.).
ORCID
Wen-Hua Sun: 0000-0002-6614-9284
Notes
The authors declare no competing financial interest.
1
998, 120, 7143−7144.
ACKNOWLEDGMENTS
(6) (a) Small, B. L.; Rios, R.; Fernandez, E. R.; Carney, M. J.
Oligomerization of Ethylene Using New Iron Catalysts Bearing
Pendant Donor Modified α-Diimine Ligands. Organometallics 2007,
■
This work was supported by the National Natural Science
Foundation of China (No. 21871275). G.A.S. thanks the
Chinese Academy of Sciences for a President’s International
Fellowship for Visiting Scientists.
2
6, 1744−1749. (b) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci,
F.; Zanobini, F.; Laschi, F.; Sommazzi, A. Oligomerisation of Ethylene
to Linear α-Olefins by new C - and C -Symmetric [2,6-Bis(imino)-
s
1
pyridyl]iron and -cobalt Dichloride Complexes. Eur. J. Inorg. Chem.
003, 2003, 1620−1631. (c) Small, B. L. Tridentate Cobalt Catalysts
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DOI: 10.1021/acs.organomet.8b00924
Organometallics XXXX, XXX, XXX−XXX