Selective ethylene oligomerization with chromium complexes bearing pyridine-phosphine ligands: Influence of ligand structure on catalytic behavior

Article abstract of DOI:10.1021/om5003683

Chromium complexes bearing a series of pyridine-phosphine ligands have been synthesized and examined for their catalytic behavior in ethylene oligomerization. The choice of solvent, toluene versus methylcyclohexane, shows a pronounced influence on the catalytic activity for all these complexes. Variations of the ligand system have been introduced by modifying the phosphine substituents affecting ligand bite angles and flexibility. It has been demonstrated that minor differences in the ligand structure can result in remarkable changes not only in catalytic activity but also in selectivity toward α-olefins versus polyethylene and distribution of oligomeric products. Ligand PyCH₂N(Me)PⁱPr₂, in combination with CrCl₃(THF)₃ afforded selective ethylene tri- and tetramerization, giving 1-hexene and 1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE.

Full text of DOI:10.1021/om5003683

Article
pubs.acs.org/Organometallics
Selective Ethylene Oligomerization with Chromium Complexes
Bearing Pyridine−Phosphine Ligands: Influence of Ligand Structure
on Catalytic Behavior
Yun Yang,†,‡ Joanna Gurnham,§ Boping Liu,‡ Robbert Duchateau,*,† Sandro Gambarotta,*,§
and Ilia Korobkov∥
†Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, Eindhoven 5600 MB, The
Netherlands
‡State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai
200237, China
§Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
∥X-ray Core facility, Faculty of Science, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
S
* Supporting Information
ABSTRACT: Chromium complexes bearing a series of pyridine−
phosphine ligands have been synthesized and examined for their
catalytic behavior in ethylene oligomerization. The choice of solvent,
toluene versus methylcyclohexane, shows a pronounced influence on
the catalytic activity for all these complexes. Variations of the ligand
system have been introduced by modifying the phosphine
substituents affecting ligand bite angles and flexibility. It has been
demonstrated that minor differences in the ligand structure can
result in remarkable changes not only in catalytic activity but also in
selectivity toward α-olefins versus polyethylene and distribution of
oligomeric products. Ligand PyCH2N(Me)PiPr2, in combination
with CrCl3(THF)3 afforded selective ethylene tri- and tetrameriza-
tion, giving 1-hexene and 1-octene with good overall selectivity and
high purity, albeit with the presence of small amounts of PE.
desired 1-octene.7−23 Typical examples are the BP Cr-PNPOMe
INTRODUCTION
■
trimerization system,8,24,25 the Sasol Cr-PNP tetramerization
1-Hexene and 1-octene are utilized as comonomers for the
system,9,26−28 and the Cr-PNPN trimerization system
production of linear low density polyethylene (LLDPE). Their
developed by Rosenthal and co-workers.10,29−32 The Cr-
rapidly growing market demand in recent years has stimulated
PNNP system recently reported by Gambarotta and co-workers
research for finding new selective ethylene trimerization and
was found to be capable of producing 91% 1-octene with 9% 1-
tetramerization catalysts.1−6
hexene as the only side product, which reiterates the viability of
The majority of the currently existing ethylene trimerization
P, N-based ligands to drive the reaction toward selective
and tetramerization systems are based on chromium complexes
oligomerization.17 Albeit in the absence of a P donor,
bearing a wide variety of ancillary ligands.1,3,4 As a result of
chromium complexes bearing aminodipyridine ligands (Cr-
ready interconversion between mono-, di-, and trivalent
NNN) afford pure 1-octene or 1-hexene, depending on the
oxidation states of chromium, the same precursor might
steric bulk of the ancillary ligand, alongside significant amounts
promote state-dependent selective or nonselective ethylene
of PE wax.33 Similarly, removing the ortho-methoxy substituents
oligomerization as well as ethylene polymerization as a function
from the phenyl group of Cr-PNP complexes shifted the
of the reaction conditions (i.e., cocatalyst, solvent, temper-
catalytic selectivity from ethylene trimerization (ca. 90% 1-
hexene) into tetramerization (up to 70% 1-octene).8,9 Although
ature). This versatility in catalytic performance makes
chromium catalysts excellent candidates for investigating factors
it is not clear whether the switch in the C6/C8 product ratio is
that influence catalytic behavior.
The nature of the ancillary ligand also plays a fundamental
role in determining the performance of the catalysts. In general,
multidentate ligands containing P and N donors have a strong
Special Issue: Catalytic and Organometallic Chemistry of Earth-
Abundant Metals
potential to facilitate the selective ethylene oligomerization
Received: April 8, 2014
affording 1-hexene with high selectivity but also the highly
© XXXX American Chemical Society
A
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mediated by ligand sterics, coordination of the pendant
methoxy group to chromium, or both,8,24,34,35 this finding
demonstrates that even subtle ligand modifications can
significantly influence the catalytic behavior.
Given the versatility of the Cr-PNNP and Cr-NNN systems
as catalysts for 1-octene production, in the present study, we
have attempted to synthesize ligand 1, which is assembled from
fragments of both the PNNP and the aminodipyridine ligand
(Scheme 1), hoping that it would provide high 1-octene
suitable for a single crystal X-ray diffraction analysis. Elemental
analysis pointed to the postulated octahedral structure
consisting of a chromium atom bonded by one ligand, one
THF, and three chlorines: [PyN(Me)PPh2]CrCl3(THF) (1a).
The proposed structure of 1a is supported by the X-ray crystal
structure determination of 2a (vide infra).
Ethylene oligomerization reactions were carried out in
methylcyclohexane using 500 equiv of methylaluminoxane
(MAO) as activator. Since the catalytic behavior of complex 1a
and a mixture of CrCl3(THF)3 plus ligand 1 in a 1:1 ratio were
found to be identical, extensive oligomerization runs were
conducted with in situ generated catalysts (Table 1). Upon
activation, CrCl3(THF)3/1 afforded fairly good activity, which
is comparable to the highest activity reported in a similar
chromium-catalyzed selective ethylene oligomerization system
(1322 g/mmol Cr/h, 80 °C, 40 bar of ethylene).17 C6 and C8
fractions predominated the liquid oligomeric products with an
overall selectivity exceeding 90%. The remaining 10% of liquid
products ranged from C10 to higher oligomers, characteristic for
a statistical product distribution. Besides oligomers, relatively
small quantities (5 wt %) of polyethylene were also obtained.
The simultaneous formation of mainly C6/C8 oligomers and
minor amounts of a statistical distribution of ≥C10 oligomers
and polyethylene suggests the pressence of multiple active
species possibly bearing chromium in different oxidation
states.36,37
Scheme 1. Design of Ligand 1
The purity of 1-hexene was found to be surprisingly low and
over half of the C6 fraction consisted of methylcyclopentane
and methylenecyclopentane in a 1:1 ratio. These cyclic C6
compounds are inferred to arise from a chromium cyclo-
pentylmethyl hydride species, which is formed upon the
rearrangement of the chromacycloheptane intermediate. This
postulated mechanism was first proposed for their formation in
the Sasol Cr-PNP ethylene tetramerization system, suggesting
that the methylcyclopentane and methylenecyclopentane are
produced by a reductive elimination and β-hydrogen transfer
from the chromium cyclopentylmethyl hydride species,
respectively (mechanism A, Scheme 2); or by disproportiona-
tion from two chromium cyclopentylmethyl hydride species
(mechanism B, Scheme 2).38 A recent DFT study by Budzelaar
proved the accessibility of these cyclic C6 products and
proposed an alternative mechanism that the methylcyclopen-
tane and methylenecyclopentane arise from an ethylene
insertion into the Cr−H bond of chromium cyclopentylmethyl
hydride species followed by hydrogen transfer to or from the
selectivity. To investigate the correlation between the catalytic
performance and the ligand structure, the skeleton of ligand 1
was modified, giving a series of ligands with different phosphine
moieties or P−Cr−N bite angles. The corresponding
chromium adducts of the various ligands were activated with
different cocatalysts and subsequently examined for their
ethylene oligomerization behavior.
RESULTS AND DISCUSSION
■
Ligand PyN(Me)PPh2 (1) was readily prepared by the reaction
of 2-(methylamino)pyridine and chlorodiphenylphosphine.
Complexation of 1 with CrCl3(THF)3 was conducted in
dichloromethane, and the ligation was indicated by an instant
color change from purple to blue upon mixing in dichloro-
methane. The resulting complex 1a was isolated as a blue
powder by removing the solvent in vacuo. Attempts to
recrystallize complex 1a from dichloromethane/petroleum
ether resulted in microcrystalline material, unfortunately not
a
Table 1. Ethylene Oligomerization Tests on Cr/Ligand 1
entry
cocatalyst (equiv)
MAO (500)
PE (g)
PE (wt %)
LAO (mL)
activity (g/(mmol Cr·h))
C6 (1-C6) (%)
C8 (1-C8) (%)
b
1
1.04
0.26
0.56
1.38
0.18
0.47
0.18
0.18
15.64
0.37
3.80
49
5
1.49
7.18
7.04
12.43
11.18
4.23
8.32
0.21
0
422
1158
1105
2023
1612
685
52.0 (62)
60.8 (39)
57.2 (40)
58.5 (44)
60.4 (41)
63.3 (37)
59.3 (39)
27.5 (77)
31.3 (76)
30.1 (73)
31.3 (73)
30.3 (72)
29.0 (72)
30.5 (70)
2
MAO (500)
c
3
d
MAO (500)
10
14
2
4
MAO (500)
5
6
7
MMAO (500)
DMAO (500)
14
3
DMAO (500)/TIBA (100)
MAO (500)
1207
66
e
8
e
53
100
56
100
9
DMAO (500)/DEAC (50)
MAO (500)
3127
132
f
10
0.40
0
f
11
DMAO (500)/DEAC (50)
761
a
b
Conditions: CrCl3(THF)3 = 10 μmol, ligand 1 = 10 μmol, methylcyclohexane=100 mL, 60 °C, 30 bar of ethylene, 30 min. Toluene.
c
d
e
f
CrCl3(THF)3 = 10 μmol, ligand 1 = 20 μmol. 50 bar of ethylene. Cr(acac)3. Cr(EH)3.
B
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Scheme 2. Postulated Mechanisms A, B, and C for the
Formation of Cyclic C6 Products
Scheme 3. Further Reaction Routes from
Chromacycloheptane Species
a
selectivity does not improve much, due to a reduced overall C6
selectivity. Increasing the ethylene pressure from 30 to 50 bar
doubled the catalytic activity. The production of both
polyethylene and oligomers increased under higher ethylene
pressure, albeit that PE formation was influenced more
strongly. On the other hand, the C6/C8 ratio was
unaffected.41,42 Increasing the ligand/Cr ratio did not affect
the catalytic behavior with respect to ethylene oligomerization;
however, it doubled the amount of PE. Hence, the in situ
generated catalytic active species for ethylene oligomerization is
inferred to comprise chromium and ligand 1 in a 1:1 ratio.
Changing the cocatalyst did not significantly influence the
distribution of oligomers, implying the generation of identical
active species for selective oligomerization. Varied weight
percentages of PE indicate that the activation procedure affects
the generation of the active species for oligomerization and
polymerization to different extents. Modified MAO (MMAO)
efficiently retards PE formation and drives the reaction toward
oligomerization with improved activity, probably as a result of
the better reducing capacity of MMAO compared with MAO.
Using dried MAO (DMAO) as cocatalyst led to a drop in
oligomer yield, which might be a result of insufficient reduction
of the trivalent chromium precursor. Addition of triisobutyla-
luminum (TIBA) in DMAO gave results similar to MMAO in
terms of inhibiting ethylene polymerization,.43,44
Attempts to improve the catalytic activity for ethylene
oligomerization by using more soluble chromium sources
appeared to be unsuccessful. Upon activation with MAO,
Cr(acac)3/1 only produced small amounts of oligomers and
PE, which might imply that chlorine is needed to produce an
active catalyst for selective ethylene oligmerization. Introduc-
tion of chlorine by using a combination of DMAO and
diethylaluminum chloride (DEAC) as an alternative cocatalyst
for Cr(acac)3/1, did restore the catalytic activity but
unexpectedly shifted ethylene oligomerization toward ethylene
polymerization. Given the absence of oligomeric products in
the Cr(acac)3/1/MAO/DEAC system, the in situ generated
active species is expected to differ from the one resulting from
CrCl3(THF)3/1/MAO, although both combinations contain
a
Transition states are exhibited in the dotted rectangles.
chromium cyclopentylmethyl ethyl species to form the cyclic
products (mechanism C, Scheme 2).39 Given the rather low 1-
hexene purity in our system, we speculate that at the stage of
the chromacycloheptane, the rearrangement step affording the
chromium cyclopentylmethyl hydride species should be very
competitive to the formation of 1-hexene (either concerted or
stepwise routes) and further ring growth (Scheme 3).
Moreover, the main side products (around 30%) in the C8
fraction detected in our system are two C8 compounds in a 1:1
ratio, which are probably methylcycloheptane and methyl-
enecycloheptane (these have also been observed in the Sasol
Cr-PNP ethylene tetramerization system38) generated in a
similar mechanism as for the formation of cyclic C6 products.
Subsequently, the influence of reaction conditions on the
catalytic performance of the CrCl3(THF)3/1 system was
investigated (Table 1). Toluene as solvent negatively affected
the catalytic performance, leading to an activity that
corresponds to approximately 40% of the original activity
obtained in methylcyclohexane and a significant increase of PE
formation. The retarding effect of toluene is possibly due to the
formation of inert Cr(I)−η6-toluene complexes.40 Although 1-
hexene purity is enhanced in toluene, the actual 1-hexene
C
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chlorine. Using another chlorine-free compound, Cr(EH)3
(chromium 2-ethylhexanoate), as the chromiun precusor
resulted in similar selectivity as that obtained for the
Cr(acac)3/1/MAO/DEAC system but with a much lower
ethylene polymerization activity.
In an attempt to improve the catalytic selectivity as well as to
obtain insight into the factors affecting the catalytic behavior,
ligand 1 was modified. Thus, a series of P,N-based ligands with
varying substitutions on the phosphine moiety and different P−
Cr−N bite angles has been synthesized (Scheme 4).
Subsequently, the catalytic behavior of their corresponding in
situ-prepared chromium complexes has been examined.
chelated by the bidentate ligand 2 via the phosphine-P and the
pyridine-N atoms. The distortion mainly arises from the acute
P1−Cr1−N1 bite angle (76°). Chlorides and THF occupy the
other four vacant sites in the octahedral coordinating sphere.
The THF is located trans to the phosphorus donor. In an
analogous octahedral complex [Py(CH2)2N(Me)PPh2]-
CrCl3THF, the THF also resides trans to the phosphine
functionality.18 The ortho-methyl substitutions in the two o-
tolyl groups are oriented opposite to each other. This might be
attributed to steric factors as was revealed by DFT calculations
(B3LYP/TZVP level) showing a 2.4 kcal/mol difference in
energy between the most and least stable conformations.
A small change in the ligand structure by replacing the
phenyl groups in the phosphine moiety with o-tolyl groups led
to a remarkable change in catalytic behavior (Table 2, entries 1
and 2). The catalytic activity of 2a in toluene was found to be 5-
fold higher than that in methylcyclohexane, whereas 1a is
almost 3 times more active in methylcyclohexane than in
toluene. The positive and negative effects of methycyclohexane
on the catalytic activity of 1a and 2a are confusing and possibly
result from a combined contribution of solvent effects and a
shift in the equilibirum of multiple active species. Intrestingly,
increased steric bulk on the phosphine moiety was found to
give rise to improved purity of both 1-hexene and 1-octene.
Another interesting phenomenon obseved for 2a is the very
long induction period for the reaction conducted in toluene.
Unlike other chromium complexes in the present study, which
invariably resulted in an instant exotherm after catalyst
injection, 2a consumes ethylene very slowly in the first 30
min of the reaction. The acceleration of ethylene uptake only
starts when the temperature has reached a certain point
(around 80 °C). Further conducted oligomerizations per-
formed in toluene at varying temperatures using 2a have
revealed that this catalyst shows a strong dependence of its
catalytic behavior on reaction temperature. Selective formation
of C6 and C8 oligomers was only observed at elevated
temperatures (Table 2, entries 1 and 4). However, when the
reaction temperature of an uncontrolled reaction exceeded 105
°C, a rapid decay of the catalyst was indicated by the gradual
decline of temperature. Reactions carried out under isothermal
conditions (80 °C) led to nonselective oligomerization and
higher percentages of PE, although a rather long catalyst
lifetime was accomplished, with the rate of ethylene uptake
remaining unchanged even after 150 min (Table 2, entry 3).
Different catalytic outcomes under varying temperatures
suggest that the active species responsible for PE formation is
generated at a relatively low (≤80 °C) temperature, while the
active species responsible for selective ethylene oligomerization
is formed only at elevated temperature. A similar correlation
between temperature and selectivity has also been reported for
the Cr-NNN tetramerization system, in which 1-octene is only
produced when the reactor temperature is uncontrolled and
allowed to raise above 100 °C.33
Scheme 4. Synthesis of Ligands 4−6
To investigate the steric and electronic effect determined by
the phosphine moiety, the phenyl substituents on P in 1 were
replaced by o-tolyl and isopropyl groups, affording ligands 2
and 3, respectively. Their corresponding chromium complexes
([PyN(Me)PR2]CrCl3THF; R = o-tolyl (2a), isopropyl (3a))
were obtained from the reaction of 2 and 3 with CrCl3(THF)3
in THF. In the case of 2a, crystals suitable for single crystal X-
ray structure determination were grown by slow vapor diffusion
of petroleum ether into a THF solution.
The crystal structure of 2a (Figure 1) is in agreement with
the earlier proposed structure of 1a featuring a distorted
octahedral geometry, in which the trivalent chromium is
Upon activation with MAO in toluene, 3a also afforded C6
and C8 with good overall selectivity (Table 2, entries 5 and 6).
Using methylcyclohexane as solvent led to a drop of activity
and switched the selectivity slightly from C6 to C8. In general,
the oligomer distribution and reactivities of 2a and 3a show the
same trend and differ from the catalytic behavior of 1a.
Compounds 2a and 3a both exhibit higher activity in toluene
than in methylcyclohexane and an improved purity of short
chain linear α-olefins, although the purity is still far from being
satisfactory. The fact that the catalytic behavior of 2a resembles
Figure 1. Partial thermal ellipsoid drawing of 2a at 50% probability.
Selected bond lengths (Å) and angles (deg): Cr1−P1 2.4298(6),
Cr1−N1 2.0977(9), Cr1−Cl1 2.3019(8), Cr1−Cl2 2.3022(3), Cr1−
Cl3 2.3086(2), Cr1−O1 2.0547(1); P1−Cr1−N1 75.84(35), O1−
Cr1−N1 91.93(26), P1−Cr1−Cl2 99.42(5), P1−Cr1−O1
167.47(14), Cl2−Cr1−N1 175.03(29), Cl3−Cr1−Cl1 174.65(1).
D
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a
Table 2. Ethylene Oligomerization Tests on 2a−6a
g
entry
catalyst
solvent
PE (g)
PE (wt %)
LAO (mL)
activity (g/(mmol Cr·h))
C6 (1-C6) (%)
C8 (1-C8) (%)
b
1
2a
2a
2a
2a
3a
3a
4a
4a
4a
4a
5a
5a
6a
6a
toluene
MCH
1.68
0.26
2.24
1.24
0.63
0.64
2.19
1.03
2.38
0.40
4.63
1.80
0.96
0.31
9
12
18
8
23.47
2.62
2180
415
68.1 (92)
68.1 (85)
38.7 (70)
77.8 (90)
65.5 (73)
59.7 (68)
46.3 (>99)
35.5 (63)
32.4 (59)
23.2 (84)
21.2 (85)
27.6 (93)
16.3 (62)
12.5 (87)
31.9 (90)
39.2 (90)
31.0 (>99)
57.9 (88)
60.9 (93)
52.7 (72)
2
c
3
d
toluene
toluene
toluene
MCH
13.87
18.95
9.15
1660
2945
1394
491
4
5
6
7
8
9
26
20
80
82
53
100
48
70
12
2.61
toluene
MCH
12.12
0.36
2177
256
e
9
MCH
0.72
577
f
10
MCH
0.48
150
11
12
13
14
toluene
MCH
927
2.81
0.58
3.18
756
57.9 (95)
30.0 (65)
toluene
MCH
276
50.7 (>99)
29.5 (>99)
500
71.5 (98)
26.7 (>99)
a
b
c
Reaction conditions: catalyst = 10 μmol, 500 equiv of MAO, solvent = 100 mL, 60 °C, ethylene (30 bar), 30 min. 50 min, 60−110 °C. 150 min,
d
e
f
g
80 °C. 80−110 °C. 50 bar. 500 equiv of DMAO and 50 equiv of TEA. MCH = methylcyclohexane.
more that of 3a, containing aliphatic isopropyl substitution on
phosphorus, than that of 1a, containing aromatic phenyl
groups, is puzzling and indicates that electronic effects can be
ruled out.
All bond lengths and bond angles are within the expected
range.
As expected, variations in the bite angle and flexibility of the
ancillary ligand do indeed affect the catalytic performance
significantly (Table 2). Upon activation with MAO in toluene,
4a afforded a statistical distribution of oligomers with
enrichment in C6 and C8 oligomers alongside significant
amounts of PE (Table 2, entry 7). The C6 and C8 fraction
existed of nearly pure 1-hexene and 1-octene (>99%).
Methylcyclohexane as solvent led to a dramatic drop in activity
as well as in purity of 1-hexene and 1-octene. Although 51% of
the liquid products consisted of 1-octene (58% overall C8
selectivity with an 88% purity of 1-octene), the production of
oligomers is rather low compared with the solid (PE) fraction
(Table 2, entry 8). Attempts to avoid PE formation by using a
stronger reducing agent, triethylaluminum (TEA), indeed
resulted in a decreased formation of unwanted polymer
(Table 2, entry 10). However, a simultaneous decline in the
selectivity for C6 and C8 was also observed. In a related Cr-
aminophosphine system, a similar reduction of the PE
formation has been reported, albeit that in this case the
selectivity switched from C8 toward C6 while the overall C6 and
C8 selectivity remained unchanged.18 Similar to 1a, a higher
ethylene pressure increased the catalytic activity of 4a, with
only a slight influence on the product distribution (Table 2,
entry 9). Although 4a produces a large amount of PE, over 50%
of 1-octene selectivity for the liquid fraction indicates the
existence of an active species for selective ethylene tetrameriza-
tion, which is intriguing. To seek a highly selective ethylene
tetramerization catalyst, ligand 4 was utilized in combination
with more chromium precursors. The first attempt was to
complex ligand 4 with methyl chromium dichloride, resulting in
the microcrystalline green material 4b. Regrettably, the crystal
shape of 4b was not suitable for undertaking a single crystal X-
ray diffraction analysis. However, on the ground of elemental
analytical and chemical degradation data, we propose a
structure with the same octahedral geometry and group
distribution as 4a. Complex 4b showed a similar trend as 4a:
high activity for nonselective behavior in toluene and an
increased selectivity for 1-hexene and 1-octene fractions when
tested in methylcyclohexene. The activity of complex 4b was
slightly higher than its trichloro analogue 4a, likely due to the
For the PNP type of ligands, introduction of a CH2 unit in
one of the P−N bonds shifted the dominant oligomer fraction
from 1-octene to 1-hexene, which shows that varying the ligand
bite angle and flexibility might dramatically impact the product
distribution.9,19 Encouraged by this finding, introduction of a
carbon linkage between the pyridine and the aminophosphine
fragments was endeavored for 1−3, affording the analogous
ligands 4−6 (Scheme 4). These ligands were then reacted with
CrCl3(THF)3 to afford corresponding chromium complexes.
The crystals of 4a were suitable for X-ray diffraction analysis,
and the results confirmed the anticipated ligand coordination
(Figure 2). The crystal structure of 4a displays the typical
Figure 2. Drawing of 4a with thermal ellipsoids drawn at 50%
probability. Select bond lengths (Å) and angles (deg) of 4a: Cr1−P1
2.432(2), Cr1−N1 2.164(6), Cr−Cl1 2.317(2), Cr1−Cl3 2.301(2),
Cr1−O1 2.121(5); P1−Cr1−N1 92.15(16), P1−Cr1−Cl3 89.78(8),
N1−Cr1−Cl3 87.09(15), N1−Cr1−Cl1 83.37(15), P1−Cr1−Cl2
88.80(8), O1−Cr1−Cl3 91.72(14).
octahedral geometry of Cr(III). A six-membered chelate is
formed with the bidentate ligand 4, which occupies two
coordination sites around chromium, binding through the
nitrogen of the pyridine moiety and the phosphorus atom.
Three chlorine atoms occupy three meridional coordination
sites, while the last coordination site is occupied by one
molecule of THF trans with respect to the phosphorus atom.
E
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Scheme 5. Catalytic Behaviors of Ligands 1−6 in Combination with CrCl3(THF)3 upon Activation with MAO in
a
Methylcyclohexane
a
The purity of 1-hexene and 1-octene is shown in parentheses. Activity in g/(mmol Cr·h).
increased solubility in aliphatic solvents as introduced to the
Cr-alkyl function.
This observation indicates a possible correlation between the
P−Cr−N bite angle and selectivity. In the case of the o-tolyl-
substituted 2a and 5a, a similar trend in PE formation is
observed with a larger bite angle and flexibility of the ligand
resulting in an increased PE production. For 3a and 6a,
containing more basic alkylphosphine moieties, the increased
bite angle results in higher overall selectivity for 1-hexene and
1-octene, better purity of α-olefin, and less PE. Variations of the
ligand systems proved that for the phosphine substituents the
P−Cr−N bite angle and ligand flexibility play a crucial role in
controlling the catalytic performance of the catalysts.
The catalytic behavior of 5a is different from that of its
closely related complexes 4a and 2a. When activated with MAO
in toluene, complex 5a produced polyethylene as the only
product with moderate activity (Table 2, entry 11). In
methylcyclohexane, PE was still the major product, but
oligomers were formed as well (Table 2, entry 12). Complex
6a showed promising catalytic behavior upon activation with
MAO in methylcyclohexane (Table 2, entry 14). Despite the
presence of PE, 1-hexene and 1-octene were obtained in very
good purity and dominate the liquid fractions with an overall
selectivity exceeding 98%. Toluene negatively affected the
catalytic activity and selectivity (Table 2, entry 13). Besides a
partial loss of C6 and C8 selectivity in the liquid phase, the
productivity of oligomers in toluene also exhibits a significant
drop compared with that in methylcyclohexane, which possibly
arises from the formation of Cr(I)−η6-toluene complexes. The
yield of PE was less affected by solvent, implying that PE is
generated from chromium species with higher oxidation
states.11,45−47
To avoid complications resulting from possible interaction
between toluene and the Cr(I) species, the catalytic outcomes
of complexes 1a−6a in methylcyclohexane were selected and
summarized for further discussion. As it can be seen in Scheme
5, even a minor difference in the ligand structure can give rise
to a noticeably different catalytic behavior. Introduction of a
methylene bridge in the ligand backbone switches the
selectivity of 1a from 61% of C6 and 31% of C8 to 35% of
C6 and 58% of C8 with a significant increase in the PE fraction
for 4a. By addition of one CH2 spacer between the pyridine and
the aminophosphine fragments in 4a, the resultant complex
[Py(CH2)2N(Me)PPh2]CrCl3THF (Cr-PNPy) has recently
been reported to produce C8 with further enhanced selectivity
of 75%.18 Complexes 4a and Cr-PNPy behave similarly in
terms of activity. Furthermore, 1a shows a much higher overall
productivity as well as an improved oligomer/PE ratio than 4a.
CONCLUSIONS
■
In the present study, chromium complexes bearing a series of
pyridine−phosphine ligands have been synthesized and have
shown varying catalytic behavior under typical reaction
conditions for ethylene oligomerization.
The solvent choice has a pronounced influence on the
catalytic activity as well as on the PE/oligomers ratio. The
preference for aliphatic or aromatic surroundings is dependent
on the ligand system. A possible reason could be that the
interactions between solvent and active site are affected by the
differences in steric bulk of the ancillary ligands, thereby leading
to a shift in the equilibrium of active species responsible for
ethylene oligomerization and polymerization. On the other
hand, the selectivity of oligomer products is less affected by
solvents.
Variations of the ligand structure have demonstrated that a
dramatic change in catalytic behavior can be obtained upon a
subtle modification in the ligand skeleton. For chromium
complexes chelated by ligands 1−3, steric bulk in the
phosphine moiety plays a more pronounced role than the
electronic effect in controlling catalytic activity, PE/oligomer
ratio, and the purity of 1-hexene and 1-octene. Introduction of
one CH2 unit between the pyridine and the aminophosphine
moiety increases the flexibility of the ligand scaffold, resulting in
a less distorted octahedral geometry with a P−Cr−N bite angle
F
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Article
1
was dried in vacuo, yielding a white solid (2.3 g, 6.5 mmol, 65%). H
close to 90°. According to the reaction conditions adopted, 4a−
6a enable nonselective oligomerization, selective tri- and
tetramerization, and polymerization of ethylene. The diversity
in catalytic outcomes of 4a−6a might be attributed to the
formation of various active species facilitated by chromium
complexes bearing more flexible ligands 4−6. Nevertheless, 6a
bearing the PyCH2N(Me)PiPr2 ligand shows potential for
being a promising selective ethylene oligomerization catalyst,
producing 1-hexene and 1-octene with high overall selectivity
and very good purity, although thus far the formation of small
amounts of PE is very persistent.
NMR (400 MHz, C6D6) δ: 8.14 (d, 2H), 7.70 (t, 4H), 6.99−6.92 (m,
6H), 6.55 (d, 2H), 6.29 (3, 2H). 31P NMR (400 MHz, C6D6) δ: 52.03
(s). 13C NMR (500 MHz, CDCl3) δ: 160.1, 149.1, 139.1, 136.4, 132.2,
128.6, 128.3, 121.9, 62.5, 37.4.
PyCH2N(Me)P(o-tolyl)2 (5). Ligand 5 was synthesized similarly to
ligand 1, using methyl(2-pyridyl)methylamine (1.25 mL, 10 mmol)
and chlorodi(o-tolyl)phosphine (2.49 g, 10 mmol). By evaporation of
the solvent in vacuo, the product was collected as a red oil (3.0 g, 9
1
mmol, 90%). H NMR (400 MHz, CDCl3) δ: 8.52 (d, 1H), 7.60 (t,
1H), 7.28−7.12 (m, 10H), 4.44 (d, 2H), 2.51 (d, 3H), 2.36 (s, 6H).
31P NMR (400 MHz, CDCl3) δ: 55.48 (s). 13C NMR (500 MHz,
CDCl3) δ: 160.3, 148.9, 141.2, 137.5, 136.4, 130.9, 130.3, 128.6, 125.8,
122.1, 63.6, 38.0, 21.1.
EXPERIMENTAL SECTION
PyCH2N(Me)PiPr2 (6). Ligand 6 was synthesized similarly to ligand
1, using methyl(2-pyridyl)methylamine (1.25 mL, 10 mmol) and
chlorodiisopropylphosphine (1.6 mL,10 mmol). By evaporation of the
solvent in vacuo, the product was collected as a brown oil (2.1 g, 8.8
■
General Procedures. All manipulations for air or moisture
sensitive materials were carried out under inert atmosphere in a
glovebox or using Schlenk techniques. Dry solvents were obtained by
passing them through a column purification system. Starting materials
for ligand synthesis were purchased from Sigma-Aldrich and used as
received. MAO (10 wt % in toluene solution) was purchased from
Sigma-Aldrich and used as received. Dried MAO (DMAO) was
prepared by pumping off all the volatile compounds of MAO at 40 °C
for 6 h in vacuo, leaving a free-flowing white powder. All NMR spectra
were recorded on Varian Mercury 400 or 500 MHz spectrometers at
25 °C.
1
mmol, 88%). H NMR (400 MHz, CDCl3) δ: 8.50 (d, 1H), 7.64 (t,
1H), 7.43 (d, 1H), 7.12 (t, 1H), 4.28 (d, 2H), 2.52 (d, 3H), 1.92 (m,
2H), 1.13−1.05 (m, 12 H). 31P NMR (400 MHz, CDCl3) δ: 89.99 (s).
13C NMR (500 MHz, CDCl3) δ: 160.6, 148.8, 136.3, 121.8, 64.1, 36.9,
26.7, 19.6, 19.2.
Complex Synthesis. [PyN(Me)PPh2]CrCl3(THF) (1a). A suspen-
sion of CrCl3(THF)3 (0.375 g, 1 mmol) in CH2Cl2 (15 mL) was
treated with ligand 1 (0.292 g, 1 mmol). The suspension was stirred
overnight at room temperature. A small amount of insoluble material
was removed by filtration, and the filtrate was evaporated in vacuo,
affording 1a as a pale blue powder (0.484 g, 0.93 mmol, 93%).
Attempts to recrystallize 1a from dichloromethane/petroleum ether
resulted in microcrystalline material, unfortunately not suitable for a
single X-ray diffraction analysis. Anal. Calcd (found) for
C22H25Cl3CrN2OP: C 50.54 (50.94), H 4.82 (4.85), N 5.36 (5.38).
Ligand Synthesis. PyN(Me)PPh2 (1). A 2.5 M hexane solution of
n-BuLi (10 mmol, 4 mL) was added dropwise to a stirred diethyl
ether/petroleum ether (35 mL/20 mL) solution of 2-(methylamino)-
pyridine (10 mmol, 1.0 mL) at −78 °C. After 30 min of stirring, the
reaction mixture was allowed to warm to room temperature and was
stirred for additional 3 h. PClPh2 (10 mmol, 1.8 mL) was then added
dropwise at 0 °C, and the reaction mixture was stirred overnight at
room temperature. The resulting suspension was filtered and washed
twice with diethyl ether (2 × 15 mL). The solvent was evaporated
under reduced pressure until precipitation occurred. The supernatant
was kept in the freezer (−30 °C), and the product was filtered and
[PyN(Me)P(o-tolyl)2]CrCl3(THF)2 (2a).
A suspension of
CrCl3(THF)3 (0.188 g, 0.50 mmol) in THF (10 mL) was treated
with ligand 2 (0.160 g, 0.50 mmol). The suspension was stirred
overnight at room temperature. A small amount of insoluble material
was removed by filtration, and the resulting solution was concentrated.
Green crystals of 2a (0.198 g, 0.36 mmol, 72%) were grown by vapor
diffusion of petroleum ether into the THF solution at room
temperature. Anal. calcd (found) for C24H29Cl3CrN2OP: C 53.99
(53.31), H 5.99 (6.00), N 4.50 (4.97).
1
dried in vacuo, yielding a beige solid (2.4 g, 8 mmol, 80%). H NMR
(400 MHz, CD2Cl2) δ: 8.22 (d, 1H), 7.55 (t, 1H), 7.45−7.38 (m,
11H), 6.76 (t, 1H), 2.92 (d, 3H). 31P NMR (400 MHz, CD2Cl2) δ:
50.82 (s). 13C NMR data were consistent with that reported in the
literature.
PyN(Me)P(o-tolyl)2 (2). Ligand 2 was synthesized similarly to ligand
1, using chlorodi(o-tolyl)phosphine (2.49 g, 10 mmol). Recrystalliza-
tion from diethyl ether/petroleum ether (10 mL/2 mL) gave colorless
crystals (1.8 g, 5.6 mmol, 56%). 1H NMR (400 MHz, CDCl3) δ: 8.27
(d, 1H), 7.51 (t, 1H), 7.38−7.11 (m, 9H), 6.74 (t, 1H), 2.88 (d, 3H),
2.29 (s, 6H). 31P NMR (400 MHz, CDCl3) δ: 40.38 (s). 13C NMR
(500 MHz, CDCl3) δ: 161.3, 147.7, 141.8, 137.0, 134.4, 130.7, 130.5,
129.1, 126.0, 114.4, 110.3, 34.7, 20.9.
[PyN(Me)PiPr2]CrCl3(THF) (3a). A suspension of CrCl3(THF)3
(0.375 g, 1 mmol) in THF (15 mL) was treated with ligand 3
(0.224 g, 1 mmol). The suspension was stirred overnight at 50 °C. A
small amount of insoluble material was removed by filtration, and the
resulting solution was concentrated. Dark green crystals of 3a (0.356 g,
0.78 mmol, 78%) were grown by vapor diffusion of petroleum ether
into THF solution at room temperature. Anal. calcd (found) for
C16H29Cl3CrN2OP: C 42.26 (42.26), H 6.43 (6.40), N 6.16 (6.15).
[PyCH2N(Me)PPh2]CrCl3(THF) (4a). A suspension of CrCl3(THF)3
(0.375 g, 1 mmol) in THF (15 mL) was treated with ligand 4 (0.306 g,
1 mmol). The suspension was stirred overnight at room temperature.
A small amount of insoluble material was removed by filtration, and
the resulting solution was concentrated and layered with petroleum
ether affording 4a at room temperature as dark green crystals (0.342 g,
0.64 mmol, 64%). Anal. Calcd (found) for C23H27Cl3CrN2OP: C
51.46 (50.86), H 5.07 (5.07), N 5.22 (5.15).
PyN(Me)PiPr2 (3). Ligand 3 was synthesized similarly to ligand 1,
using chlorodiisopropylphosphine (1.6 mL, 10 mmol). After removal
of the solvent in vacuo, the product was collected by distillation under
reduced pressure (0.08 mbar, 50 °C) as a colorless oil (2.1 g, 9.2
mmol, 92%). 1H NMR (400 MHz, CD2Cl2) δ: 8.14 (d, 1H), 7.44 (m,
2H), 6.64 (t, 1H), 3.06 (d, 3H), 2.18 (m, 2H), 1.16 (q, 6H), 1.03 (q,
6H). 31P NMR (400 MHz, CD2Cl2) δ: 71.13 (s). 13C NMR (500
MHz, CD2Cl2) δ: 147.0, 136.0, 113.3, 112.1, 111.0, 26.3, 19.2, 19.0.
PyCH2N(Me)PPh2 (4). A 2.5 M hexane solution of n-BuLi (10 mmol,
4 mL) was added dropwise to a stirred diethyl ether solution (35 mL)
of dipyridyl amine (1.7 g, 10 mmol) at −78 °C. The formed yellow
suspension was allowed to warm to room temperature and was stirred
for another 2 h. PPh2Cl (1.8 mL, 10 mmol) in diethyl ether (9 mL)
was then added dropwise to the suspension at −78 °C. After stirring at
−78 °C for 30 min, the suspension was slowly warmed to room
temperature and stirred for 48 h. The suspension was filtered and
washed with diethyl ether. The solvent of the filtrate was removed in
vacuo, yielding a yellow solid. The solid was redissolved in petroleum
ether (20 mL)/diethyl ether (5 mL), and the resulting orange
suspension was filtered and washed with petroleum ether. The product
[PyCH2N(Me)PPh2]MeCrCl2(THF) (4b).
A suspension of
MeCrCl2(THF)3 (0.18 g, 0.5 mmol) in toluene (5 mL) was treated
with ligand 4 (0.15 g, 0.5 mmol). The suspension turned dark green
immediately, and stirring was continued for additional 12 h. The
insoluble complex was collected by centrifugation and redissolved in
THF (5 mL). The olive green solution was layered with heptane (2
mL). The formed crystalline mass was washed with cold hexane (2 × 2
mL) and dried in vacuo (0.19 g, 0.37 mmol, 74%). Anal. calcd (found)
for C24H30Cl2CrN2OP: C 41.68 (41.44), H 5.95 (5.87), N 9.72 (9.66).
[PyCH2N(Me)P(o-tolyl)2]CrCl3(THF) (5a). A suspension of
CrCl3(THF)3 (0.375 g, 1 mmol) in THF (15 mL) was treated with
ligand 5 (0.334 g, 1 mmol). The suspension was stirred overnight at 50
G
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Organometallics
Article
°C. A small amount of insoluble material was removed by filtration,
and the filtrate was dried in vacuo, affording 5a as a green powder
(0.318 g, 0.56 mmol, 56%). Anal. calcd (found) for
C25H31Cl3CrN2OP: C 53.16 (54.43), H 5.53 (5.83), N 4.96 (5.49).
[PyCH2N(Me)PiPr2]CrCl3(THF) (6a). A suspension of CrCl3(THF)3
(0.375 g, 1 mmol) in THF (15 mL) was treated with ligand 6 (0.238 g,
1 mmol). The suspension was stirred overnight at room temperature.
A small amount of insoluble material was removed by filtration, and
the filtrate was dried in vacuo, affording 6a as green powder (0.401 g,
0.86 mmol, 86%). Anal. calcd (found) for C17H31Cl3CrN2OP: C 43.56
(42.15), H 6.67 (6.86), N 5.98 (5.05).
was made to refine THF solvent molecule atomic positions with
anisotropic displacement coefficients. However, this attempt required
introduction of significant amount of displacement and geometry
constrains. For that reason THF molecule was refined with isotropic
thermal displacement parameters set. All non-hydrogen atoms of the
target compound were refined anisotropically. Final Flack parameter
value due to chiral space group was refined to −0.015(18).
For all the compounds, all hydrogen atoms positions were
calculated based on the geometry of bearing non-hydrogen atoms.
All hydrogen atoms were treated as idealized contributions during the
refinement. All scattering factors are contained in several versions of
the SHELXTL program library, with the latest version used being
v.6.12.50 Crystallographic data and selected data collection parameters
are reported in Tables 1.
Ethylene Oligomerization. All of the ethylene oligomerization
experiments were performed in a 200 mL steel Buchi autoclave. The
̈
autoclave was heated in an oven overnight at 150 °C before each run.
After evacuation and rinsing with argon three times, the solvent
(volume of solvent = total volume (100 mL) − volume of catalyst
solution − volume of cocatalyst solution) was charged in the
preheated autoclave. Then cocatalyst was injected, and the solvent
was saturated with ethylene by pressurizing to 30 bar. After 30 min of
stirring, the autoclave was temporarily vented to allow the injection of
the catalyst solution. The autoclave was repressurized to the desired
pressure, and the pressure was maintained throughout the run. The
temperature of the autoclave was controlled by a thermostat bath.
After 30 min, the reaction was quenched by cooling to 0 °C,
depressurizing, and injection of a mixture of ethanol and diluted
hydrochloric acid. The polymer was separated by filtration and dried
overnight at 60 °C under reduced pressure before mass determination.
The oligomers were analyzed by GC-FID for oligomer composition
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for 2a and 4a in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: r.duchateau@tue.nl (Robbert Duchateau).
*E-mail: sgambaro@uottawa.ca. (Sandro Gambarotta).
1
Notes
and by H NMR spectroscopy for activity.
The authors declare no competing financial interest.
X-ray Crystallography. Data collection results for compounds 2a
and 4a represent the best data sets obtained in several trials for each
sample. The crystals were mounted on thin glass fibers using paraffin
oil. Prior to data collection, crystals were cooled to 200.15 K. Data
were collected on a Bruker AXS KAPPA single crystal diffractometer
equipped with a sealed Mo tube source (wavelength 0.71073 Å) APEX
II CCD detector. Raw data collection and processing were performed
with APEX II software package from BRUKER AXS.48 Diffraction data
for 2a and 4a samples were collected with a sequence of 0.5° ω scans
at 0, 120°, and 240° in φ. Initial unit cell parameters were determined
from 60 data frames with 0.3° ω scan each, collected at the different
sections of the Ewald sphere. Semiempirical absorption corrections
based on equivalent reflections were applied.49 Systematic absences in
the diffraction data set and unit-cell parameters were consistent with
monoclinic P21/n (No. 14, alternative settings) for compound 2a and
orthorhombic P212121 (No. 19) for 4a. Solutions in the centrosym-
metric space group for 2a compound yielded chemically reasonable
and computationally stable results of refinement. However, data for
the complex 4a suggested noncentrosymmetric space group for the
structural solution and model refinement. The structures were solved
by direct methods, completed with difference Fourier synthesis, and
refined with full-matrix least-squares procedures based on F2.
Diffraction data for the crystal of the complex 2a were collected to
0.75 Å resolution; however due to small crystal size and weak
diffraction it was discovered that both R(int) and R(σ) exceed 35% for
the data below 1.00 Å resolution. Based on R(σ) value, data were
truncated to 0.95 Å resolution for refinement. Asymmetric unit for this
crystallographic model of 2a consists of one target complex molecule
and one THF crystallization solvent molecule. Both molecules are
located in the general positions. Occupancy of atomic positions for
THF solvent molecules was initially allowed to refine in order to
achieve acceptable values for thermal motion parameters. After initial
refinement, occupancy of THF molecule was restrained at 50%. All
non-hydrogen atoms in the structure were refined anisotropically.
Asymmetric unit of 4a contains one chiral compound molecule and
one THF solvent molecule. Both parts of the structural model are
located in the general positions. Location of strong residual electron
density peak next to oxygen atom of the THF molecule suggested
oxygen positional disorder. Atomic position was split in two, and
occupancies of the both positions were restrained to 50%, providing
acceptable values of thermal motion parameters. Initially the attempt
ACKNOWLEDGMENTS
■
We’d like to thank the Chinese Scholarship Council, the
Natural Science Foundation of China (No. 21004020 and No.
21174037), and the Eindhoven University of Technology for
financial support.
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dx.doi.org/10.1021/om5003683 | Organometallics XXXX, XXX, XXX−XXX