Evaluation of Bis(phosphine) Ligands for Ethylene Oligomerization: Discovery of Alkyl Phosphines as Effective Ligands for Ethylene Tri- And Tetramerization

Article abstract of DOI:10.1021/acs.organomet.9b00721

Fifty-three bis(phosphines) were evaluated as ligands for chromium-catalyzed ethylene tetramerization in a high-throughput reactor. Selected ligands previously reported in the literature gave high activities with expected selectivities when evaluated under our reactor conditions. While the majority of ligands evaluated gave low activity catalysts that produced mostly high-density polyethylene (HDPE), alkyl phosphines were unexpectedly identified as a promising ligand class. In particular, the MeDuPhos ligand led to an active catalyst that produced 81.8 wt % α-olefins (50.0 wt % 1-octene, 31.8 wt % 1-hexene) and 3.5 wt % HDPE, approaching the selectivity of the state-of-the-art i-Pr-PNP ligand.

Full text of DOI:10.1021/acs.organomet.9b00721

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Article
Evaluation of Bis(phosphine) Ligands for Ethylene Oligomerization:
Discovery of Alkyl Phosphines as Effective Ligands for Ethylene Tri-
and Tetramerization
Scott D. Boelter, Dan R. Davies, Kara A. Milbrandt, David R. Wilson, Molly Wiltzius, Mari S. Rosen,*
and Jerzy Klosin*
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ABSTRACT: Fifty-three bis(phosphines) were evaluated as
ligands for chromium-catalyzed ethylene tetramerization in a
high-throughput reactor. Selected ligands previously reported in
the literature gave high activities with expected selectivities when
evaluated under our reactor conditions. While the majority of
ligands evaluated gave low activity catalysts that produced mostly
high-density polyethylene (HDPE), alkyl phosphines were
unexpectedly identified as a promising ligand class. In particular, the MeDuPhos ligand led to an active catalyst that produced
81.8 wt % α-olefins (50.0 wt % 1-octene, 31.8 wt % 1-hexene) and 3.5 wt % HDPE, approaching the selectivity of the state-of-the-art
i-Pr-PNP ligand.
byproduct is of particular concern, as it is not soluble at the
relatively low temperatures (<100 °C) used in the ethylene
tetramerization process and therefore can lead to significant
reactor fouling.
INTRODUCTION
■
The selective oligomerization of ethylene to linear α-olefin
(LAO) products is an active area of research due to the
extensive use of these short-chain α-olefins as comonomers
with ethylene in the production of linear low-density
polyethylene (LLDPE).1 Linear α-olefins used in LLDPE
production are conventionally obtained from the nonselective
oligomerization of ethylene, which yields Schulz−Flory or
Poisson distributions.2 However, market demand is increas-
ingly rendering the supply of short-chain LAOs from
traditional sources inadequate.1a
Selective oligomerization technologies have emerged to
answer the market demand for short-chain LAOs. Ethylene
trimerization for the production of 1-hexene has been known
since 1967, and the process to produce 1-hexene based on
selective ethylene trimerization has been commercialized.3
However, it was not until 2004 that catalysts capable of the
selective tetramerization of ethylene to 1-octene were reported
by researchers from Sasol Technology.4 This first report
described a Cr-based system with a bis(diphenylphosphino)-
amine (PNP), bis(diphenylphosphino)hydrazine (PNNP), or
a 1,2-bis(diphenylphosphino)ethane (dppe) ligand. Sasol’s
best example, utilizing bis(diphenylphosphino)isopropylamine
(i-Pr-PNP, 1), yielded 68.3 wt % 1-octene at 653 psi of
ethylene and 45 °C.4 The other major products of the catalysis
were 1-hexene (16.9 wt %); cyclic C6 products (methyl-
cyclopentane and methylenecyclopentane; 5.0 wt %); C10−
C18 α-olefins (tri- and tetramer products of ethylene with 1-
hexene and 1-octene; 8.7 wt %); and high-density polyethylene
(HDPE; 1.1 wt %) (Scheme 1). While all non-α-olefin
products are undesirable, the high-density polyethylene
Since Sasol’s initial report of the PNP/Cr tetramerization
system, the search for tetramerization ligands that lead to
catalysts with improved 1-octene selectivity and fewer
undesirable byproducts has been an active area of research in
both industrial and academic settings. Several chemical and oil
companies in addition to Sasol, such as SK Energy,5 Chevron
Phillips Chemical,6 ExxonMobil,7 and Shell,8 have been active
in this area. Sasol has published several reports4,9,10 describing
the tetramerization capabilities of i-Pr-PNP derivatives.
Interestingly, small changes in ligand structure have significant
impact on both the activity and selectivity of the resulting
chromium catalysts. For instance, replacing the i-Pr-N bridge
in ligand 1 (Figure 1) with a Me-N bridge lowered the octene
selectivity by 15.5 wt % and increased the amount of HDPE
produced by a factor of 5.10a Furthermore, simply changing the
i-Pr-N bridge to a CH2 bridge (ligand 7, Figure 1, Table 1)
resulted in a ligand that no longer led to a competent
tetramerization catalyst, but rather resulted in a low-activity
catalyst that produced a Schulz−Flory distribution of LAOs.10b
Outside the PNP ligand family, a variety of two- and three-
carbon-bridged, diphenyl phosphine-based ligands have proven
Special Issue: Organometallic Chemistry at Various
Length Scales
Received: October 24, 2019
https://dx.doi.org/10.1021/acs.organomet.9b00721
© XXXX American Chemical Society
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A

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Scheme 1. Major Reaction Products of Chromium-Catalyzed Ethylene Tetramerization
use of (S,S)-chiraphos (3, Figure 1) as advantageous over i-Pr-
PNP due to greater catalyst longevity.5 Despite the reported
high activities and low amounts of HDPE formed, this catalyst
and similar derivatives, such as the catalyst using the Ph2PC(t-
Bu)CHPPh2 (5, Figure 1) ligand described by the Shi group
(Table 1), did not reach the 1-octene selectivities of the
chromium catalyst with i-Pr-PNP (1).11
Whether a catalyst favors the formation of 1-octene or 1-
hexene is known to be influenced by the steric bulk of the
ligand, with less sterically encumbering catalysts leading to
higher 1-octene selectivities.10f Along these lines, among
ligands with three-carbon bridges between the phosphines,
the selectivity of the resulting catalysts appears to be somewhat
dependent on the rigidity of the bridging group. When a rigid
linker, such as in 1,8-bis(diphenylphosphino)naphthalene (8,
Figure 1, Table 1), is employed, a switch to an ethylene
trimerization catalyst is observed, presumably because ligand
steric constraints prevent the formation of the larger
metallacycle intermediate that leads to 1-octene.10b In contrast,
the more flexible 1,3-bis(diphenylphosphino)propane ligand
(9, Figure 1, Table 1) results in the production of more 1-
octene than 1-hexene.10b
Figure 1. Structures of selected ligands previously studied for
chromium-catalyzed ethylene tetramerization.
Table 1. Literature Values for Chromium-Catalyzed
Ethylene Tetramerization with Selected Ligands
b
selectivity
a
activity
ligand #
(g product/g Cr h)
1-octene
1-hexene
HDPE
While most of the ligands described for use in tetrameriza-
tion catalysis employ two diphenylphosphine groups to
coordinate to the Cr center, there are a few examples of
ligands with other coordinating groups that lead to active
tetramerization catalysts. These ligands typically lead to lower
activity catalysts that produce higher levels of HDPE. The
most successful of these ligands is a phosphine-amidine ligand
from Chevron that led to a catalyst that exhibited not only
moderate activity and relatively low HDPE formation, but also
relatively low octene selectivity.6,12 Other ligands with N-
coordinating groups gave catalysts with low activity and led to
high HDPE levels.7,13
Given the greater success of bis(phosphine)-based ligands
over other ligand families for ethylene tetramerization, we
chose to focus our search for new tetramerization catalysts
within this ligand class. This paper describes the evaluation of
53 bis(phosphine) ligands with varying steric and electronic
attributes, as it has been demonstrated that small changes in
ligand structure can lead to significant changes in the activity
and selectivity of the resulting catalysts.4 Unexpectedly, we
discovered that some alkyl phosphines can lead to very active
Cr-based catalysts that exhibit high selectivities toward 1-
octene and 1-hexene.
c
110a
1 950 000
144 000
1 929 000
303 000
1 926 000
2 240 000
21 000
69.5
59.3
59.2
46.7
40.4
56.8
16.8
15.7
31.4
7.3
39.8
13.0
0.9
4.9
0.7
5.2
0.1
0.9
,
d
,
210b
e
35b
,
d
,
410b
511
f
,
d
d
d
d
610b
710b
810b
910b
,
,
,
,
Schulz−Flory Distribution of LAOs
31.4
30.3
70 000
13 000
61.9
8.6
4.0
24.3
a
Activities are highly conditions-dependent, making comparisons
b
across different reports difficult. Selectivity is dependent on both
temperature and pressure. At higher pressures and lower temper-
atures, 1-octene selectivity is higher. Selectivities are given for three of
the five major reactor products (1-octene, 1-hexene, and HDPE). The
two other major reaction products are C10−C18 α-olefins and cyclic
c
products (methylcyclopentane and methylenecyclopentane). Condi-
tions: 2.5 μmol Cr(acac)3, 3 μmol ligand, MMAO-3A:Cr = 300:1, 60
d
°C, 653 psi ethylene, 100 mL methylcyclohexane. Conditions: 5−30
μmol Cr, MMAO-3A:Cr = 500:1, 60 °C, 725 psi ethylene, 100 mL
e
methylcyclohexane. Conditions: 2.5 μmol catalyst, MMAO-12A:Cr =
300:1, 45 °C, 435 psi ethylene, 150 mL methylcyclohexane.
f
Conditions: 1.0 μmol precatalyst, MMAO-3A:Cr = 500:1, 40 °C,
580 psi ethylene, 30 mL methylcyclohexane.
capable of giving active tetramerization systems with
chromium. Sasol has described the use of dppe (2), 1,3-
bis(diphenylphosphino)propane (9, dppp), and 1,2-bis-
(diphenylphosphino)ethene (4) as ligands for chromium-
catalyzed ethylene tetramerization.10b SK Energy described the
RESULTS AND DISCUSSION
■
Bidentate diphenylphosphine-based ligands were selected as
the main focus of this study due to the already established
success of such compounds in ethylene tetramerization
B
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catalysis and the commercial availability of a diverse range of
ligands in this class. A series of bidentate alkyl phosphines was
also included in this study to augment the diversity of ligand
electronic parameters. The 53 ligands thus selected were
assessed in a high-throughput reactor (HTR) that has 24, 14
mL reactors and is housed in a nitrogen-filled drybox. Uniform
conditions of 0.05 μmol of CrCl3(THF)3 and 0.06 μmol of
ligand (1.2 equiv relative to Cr) were used, with 50 μmol of
MMAO-3A (Al:Cr ratio of 1000:1) as the activator.
Precatalyst solutions were prepared by mixing a 1.0 mM
solution of bis(phosphine) ligand and a 1.0 mM solution of
CrCl3(THF)3, both in chlorobenzene for 30 min. Active
catalysts were then generated in situ by adding precatalyst
solutions to a MMAO-3A methylcyclohexane solution (50
mM) at ambient temperature and pressure. Oligomerization
reactions were carried out at an ethylene pressure of 500 psi
and a temperature of 60 °C for 30 min. Methylcyclohexane (4
mL total volume) was the primary reaction solvent in this
study. In order to achieve high oligomerization activities, it is
imperative to employ rigorously purified solvents and reagents
as we found that, despite the use of a relatively high level of
MMAO-3A, small amounts of impurities in solvents and gas
feeds can dramatically lower activity or even completely shut
down this catalytic process.
The catalyst loading used here allowed for the evaluation of
both high activity catalysts and lower activity catalysts, as
enough product was made even by the lower activity catalysts
for meaningful analysis. Liquid products from these reactions
were analyzed by gas chromatography while the mass of the
HDPE byproduct15 was measured gravimetrically. The state-
of-the-art i-Pr-PNP (1) ligand was included in every screened
ligand library as a reference against which all other ligands were
compared (Tables 2−3, Figures 1−4). While the majority of
ligands in this investigation had not been previously studied in
the context of ethylene oligomerization, some ligands reported
in the literature to give active oligomerization catalysts were
included in this study. Importantly, not only did these ligands
lead to active catalysts under our conditions, they also gave
similar selectivities to those provided in literature reports, thus
validating our experimental setup. For example, the state-of-
the-art i-Pr-PNP (1) ligand led to a catalyst with a high activity
of 492 000 g product/g Cr h with a selectivity of 62.0 wt % 1-
octene, 24.9 wt % 1-hexene, and 2.2 wt % HDPE (compare
with literature values in Table 1: 69.5 wt % 1-octene, 16.8 wt %
1-hexene, and 0.9 wt % HDPE). The known oligomerization
catalysts with two-carbon bridges in the bis(phosphine)
backbone (Table 2; ligands 2−4 and 6) also performed
similarly to those described in the literature reports. Selectivity
trends in newly evaluated ligands with two-carbon bridges
followed the steric trends reported in the literature,10f in which
bulkier ligands led to greater 1-hexene formation and less 1-
octene. While dppe (2) leads to a catalyst that primarily
tetramerizes ethylene, giving 53.7 wt % 1-octene under our
conditions, ligand 12, in which an o-methyl substituent was
added to two of the four dppe phenyl groups, gives a catalyst
that primarily trimerizes ethylene, giving 52.7 wt % 1-hexene
with an activity about half that of dppe (2) (Table 2). Ligand
11, which has two o-methoxy substituents appended to the
dppe framework, led to an excellent ethylene trimerization
catalyst. With an activity of 529 000 g product/g Cr h and with
selectivities of 97.2 wt % 1-hexene and 0.8 wt % HDPE, under
our conditions it gives superior 1-hexene selectivity and
Table 2. High-Throughout Ethylene Oligomerization Using
Selected Bis(phosphine) Ligands
abc
,
,
selectivity
(wt%)
product
a b
ab
,
,
yield
activity
(g product/g Cr h) 1-octene 1-hexene HDPE
ligand #
(mg)
1
2
3
4
639.4
206.7
726.7
274.5
590.1
1270.8
688.6
78.4
492 000
159 000
559 000
211 000
454 000
978 000
529 000
60 000
7000
111 000
111 000
157 000
46 000
6000
62.0
53.7
52.4
45.4
55.6
4.7
0.4
8.0
2.0
0.3
7.1
1.2
9.9
5.1
1.3
41.5
3.2
35.7
33.9
0.0
24.9
23.5
35.2
13.0
22.3
90.4
97.2
52.7
2.5
16.3
12.3
0.2
67.6
11.8
1.3
12.7
67.2
12.9
48.1
26.8
78.3
79.6
31.8
2.2
8.6
3.2
11.0
5.5
0.6
6
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
0.8
35.3
92.4
81.1
46.2
97.8
16.5
69.0
94.3
7.1
24.6
10.1
1.5
67.9
19.0
14.8
3.5
8.6
144.4
144.3
204.7
60.3
8.8
8.5
150.3
28.2
79.2
453.2
10.7
40.0
6000
114 000
22 000
61 000
349 000
8000
31 000
90 000
185 000
0.0
1.2
50.0
116.6
240.2
a
b
Averaged over two replicates. Product yield and activity and
selectivity calculations include all major reaction products: 1-octene,
1-hexene, HDPE, cyclic C6 products (methylcyclopentane and
methylenecyclopentane), and C10−C18 α-olefin compounds. Sample
c
calculations are provided in the Supporting Information. Selectivities
for the main products of interest (1-octene, 1-hexene, and HDPE) are
given here. The amounts of the cyclic products (methylcyclopentane
and methylenecyclopentane) and higher α-olefin products (C10−
C18) were quantified to obtain complete mass balance. Complete
information on reaction products can be found in the Supporting
Information.
comparable HDPE formation to the o-OMe PNP ligand
(ligand 10) developed by British Petroleum.16
Incorporating additional coordinating groups in ligands that
lead to successful oligomerization catalysts tends to greatly
diminish their activity and lead to the formation of high levels
of HDPE. For instance, replacing the i-Pr-N bridge in i-Pr-PNP
(1) with a 2-pyridyl-N bridge (29, Table 3, Figure 4) leads to a
16-fold decrease in activity and a switch in selectivity to a
polymerization catalyst that yields 87.7 wt % HDPE. Given
that the PNP derivative with a p-tolyl-N bridge is known to
give a successful ethylene tetramerization catalyst,10d it must be
the inclusion of a pyridyl group and not the alkyl-to-aryl switch
that causes the activity and selectivity change. Similarly,
changing the phenyl groups in dppe (2) to 2-pyridyl groups
(ligand 13) leads to a catastrophic 24-fold decrease in catalytic
activity with a selectivity of 92.4 wt % HDPE. The most
surprising ligand class to emerge from this study was that of the
alkyl phosphines. Given that most successful tetramerization
catalysts have relatively electron-withdrawing (i.e., phenyl)
substituents on the phosphorus atoms, this ligand class was not
expected to yield very productive catalysts based on previous
studies.10b However, this class of ligands has unexpectedly
C
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phosphine ligands is as expected:10f the dimethylphosphine-
based ligand 22 gives the highest 1-octene selectivity, while the
bulkier methyl/t-butyl-phosphine ligand 25 gives the highest 1-
hexene selectivity.
Table 3. High-Throughout Ethylene Oligomerization Using
Selected Bis(phosphine) Ligands
a bc
,
,
selectivity
(wt %)
product
ab
The most promising alkyl phosphine evaluated in this study
is MeDuPhos (27). Comprising two five-membered phospha-
cycles known as phospholanes, the MeDuPhos ligand (27) is
structurally distinct from ligands used in conventional
diphenylphosphine-based ethylene oligomerization catalysts.
While under the conditions used here MeDuPhos (27) led to a
catalyst with about half the activity of the catalyst formed from
the benchmark i-Pr-PNP (1), the selectivity for the desired
products was impressive. At 50.0 wt % 1-octene, 31.8 wt % 1-
hexene, and 3.5 wt % HDPE, the combined selectivity of useful
products 1-octene and 1-hexene (81.8 wt %) of this catalyst
approaches that of i-Pr-PNP (1) (86.9 wt % 1-octene + 1-
hexene). Furthermore, the 3.5 wt % HDPE observed for this
catalyst is one of the lowest levels of HDPE produced by a
catalyst outside the PNP family.
ab
,
,
yield
activity
(g product/g Cr h) 1-octene 1-hexene HDPE
ligand #
(mg)
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
60.5
39.4
21.2
282.2
668.4
78.1
43.0
46.0
25.3
8.0
11.8
16.3
18.0
80.8
93.4
13.3
16.9
5.9
46 000
30 000
16 000
217 000
514 000
60 000
33 000
35 000
19 000
6000
37.3
2.8
21.6
52.2
53.4
0.5
1.5
0.5
0.7
2.3
2.3
2.2
0.5
27.1
0.3
0.4
3.1
0.7
0.0
6.2
9.0
0.7
1.3
5.6
1.4
3.1
0.9
0.5
0.0
0.0
6.7
5.8
23.7
87.7
51.1
7.8
11.1
26.6
35.0
18.4
1.7
20.3
11.6
5.4
2.5
2.8
1.1
6.5
0.7
54.4
8.2
2.6
6.3
8.8
9.4
1.9
1.7
2.3
77.2
85.5
71.4
82.2
85.1
88.9
77.6
96.2
49.3
96.4
41.3
70.1
92.6
88.5
56.0
40.3
94.8
91.7
51.7
68.5
70.3
95.3
89.9
91.7
29.8
9000
13 000
14 000
62 000
72 000
10 000
13 000
5000
CONCLUSION
■
The high-throughput evaluation of 53 bidentate phosphine
ligands led to the identification of a novel class of phosphine
ligands that give active ethylene tetramerization catalysts with
chromium. While selectivity trends reported in the literature in
which increased steric bulk shifted selectivity from 1-octene to
1-hexene were observed for some of the newly evaluated
ligands, most of the diphenylphosphine-based ligands studied
here led to low activity catalysts that produce high levels of
HDPE. In contrast, select bidentate alkyl phosphines were
identified as active oligomerization catalysts with chromium.
While diethylphosphine-based ligands (20 and 23) led to high
catalytic activities unlike their dimethyl- and diisopropylphos-
phine analogues (19, 21, 22, and 24), they favored 1-hexene
over 1-octene production. Out of all alkyl phosphine ligands
screened, the MeDuPhos ligand (27) led to a catalyst that
unexpectedly exhibited the best combination of activity and 1-
octene selectivity. This ligand produced a catalyst with good
activityabout half the activity of the benchmark i-Pr-PNP
ligandand excellent selectivity: 50.0 wt % 1-octene, 31.8 wt
% 1-hexene, as well as one of the lowest levels of high-density
polyethylene (3.5 wt %) outside of the PNP ligand family.
Structurally, MeDuPhos (27) is unique from ligands used in
known ethylene oligomerization catalysts, most of which are
based on a diphenylphosphine motif. The discovery of alkyl
phosphines, and specifically the MeDuPhos ligand (27),
enabled by high-throughput research, as effective ligands for
ethylene tetramerization will open new avenues into ethylene
tetramerization research. The preparation and ethylene
oligomerization evaluation of chromium complexes with
bis(phospholanes) and related ligands will be the focus of
our next contribution.17
7.6
6000
20.3
54.2
9.4
9.9
17.2
8.2
10.4
5.7
8.3
6.8
28.0
16 000
42 000
7000
8000
13 000
6000
8000
4000
6000
5000
14.5
9.2
10.6
1.0
1.6
2.7
22 000
67.3
a
b
Averaged over two replicates. Product yield and activity and
selectivity calculations include all major reaction products: 1-octene,
1-hexene, HDPE, cyclic C6 products (methylcyclopentane and
methylenecyclopentane), and C10−C18 α-olefin compounds. Sample
c
calculations are provided in the Supporting Information. Selectivities
for the main products of interest (1-octene, 1-hexene, and HDPE) are
given here. The amounts of the cyclic products (methylcyclopentane
and methylenecyclopentane) and higher α-olefin products (C10−
C18) were quantified to obtain complete mass balance. Complete
information on reaction products can be found in the Supporting
Information.
provided the most promising leads for further study. While
ligands containing dimethylphosphino (19,10b 22) and
diisopropylphosphino (21, 24)10b fragments led to low activity
catalysts as reported previously, we found that analogous
ligands containing diethylphosphino groups (20 and 23) are
considerably more active and exhibit 1-octene selectivities of
41.5 and 33.9 wt %, respectively, which is respectable especially
in light of the lower levels of HDPE produced (7.1 and 1.5 wt
%, respectively). Thus, the diethylphosphino group appears to
be an advantaged motif among these two-carbon-bridged
ligands for Cr-catalyzed ethylene tetramerization. Given that
the electronic differences between methyl, ethyl, and isopropyl
substituents are relatively small, we can hypothesize that there
are steric reasons for these different catalytic behaviors. The
selectivity trend across these phenylene-bridged bidentate alkyl
EXPERIMENTAL SECTION
■
General Methods. All solvents and reagents were used as
received from commercial sources unless otherwise noted. Bis-
(phosphines) 2−6, 11−15, 17−21, 25−29, 31, 33−36, 38−52, 54,
55, 57, and bis(dichlorophosphino)benzene were purchased from
Strem. Bis(phosphines) 16, 30, 32, 53, and 56 were purchased from
Sigma-Aldrich. Commercial ligands were used without purification.
For an indication of the level of purity of commercial ligands, all
purchased compounds were evaluated by 1H and 31P NMR
spectroscopy (see SI for NMR spectra). Bis(phosphines) 1,4 10,18
D
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Figure 2. Bis(phosphine) ligands screened in this study.14
24,10b and 3719 were prepared using modified literature procedures.
Ligands 22 and 23 were made in an analogous fashion to ligand 24.10b
All syntheses and manipulations were carried out in a nitrogen-filled
glovebox unless otherwise noted. Tetrahydrofuran and hexanes were
dried and degassed via a solvent system that employed A2 alumina
and Q5 reactant (finely divided copper on alumina). Deuterated
solvents were purchased from Cambridge Isotopes. C6D6 was dried
over Na/K and CDCl3 was dried over activated 4 Å molecular sieves
before use. NMR spectra were recorded on Varian MR-400 or
VNMRS-500 MHz NMR spectrometers. Chemical shifts are
referenced to residual protons in the deuterated solvent for 1H
NMR spectra and 31P NMR spectra are referenced according to the
E
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Article
Figure 3. Ethylene oligomerization performance of bis(phosphine) ligands 1−4, 6, and 10−27.
Figure 4. Ethylene oligomerization performance of bis(phosphine) ligands 28−57.
IUPAC-recommended unified scale method.20 High resolution mass
spectrometry (HRMS) analyses were recorded on an Agilent
Technologies 6230 TOF LC-MS coupled with a 1290 Infinity UPLC.
High-Throughput Experiments. All catalytic evaluations were
performed using a high-throughput reactor housed in a nitrogen-filled
glovebox in which reactions are performed in 24 individual reactor
cells in a stainless-steel block fitted with glass inserts. All 24 reactors
share a common headspace. Efficient mixing is achieved through the
use of magnetic stir bars. The ethylene feed and the methylcyclohex-
ane solvent were passed through purification columns containing A2
alumina (activated under nitrogen for 8 h at 300 °C) and Q5 reactant
(Q5 reactant activated under 5% H2 in argon for 1 h and then under
nitrogen for 7 h at 200 °C). Chlorobenzene and nonane were purified
by passing them through activated alumina. The methylcyclohexane,
chlorobenzene, and nonane were stored over activated 3 Å molecular
sieves (3 Å molecular sieves are activated under nitrogen for 4 h at
250 °C).
Each ligand was evaluated in duplicate in the high-throughput
reactor. Furthermore, i-Pr-PNP (1) was included in each library as a
positive control. Consistent activity and selectivity of this standard
from run to run allowed for the catalysis results to be compared across
high-throughput runs.
After 30 min, the reactions were terminated by stopping the
ethylene feed and cooling to room temperature. The reactor was
slowly vented at room temperature in order to limit loss of low-boiling
analytes such as 1-hexene. A liquid sample was removed from each
reaction for GC analysis, and the remainder of the liquid present was
removed in vacuo on a Savant SC250EX SpeedVac Concentrator
(Thermo Fisher). The glass tubes were then weighed to determine
the amount of solids present. The weight of the residual solids from
the control tube containing no catalyst solution was subtracted from
the weight of the total residual solids in each reaction vial to yield the
amount of polymer produced in each reaction.
Synthesis of N-(Diphenylphosphino)-N-(1-methylethyl)-P,P-di-
phenyl-phosphinous amide (1).
Precatalyst solutions were prepared in 8 mL vials by mixing the
ligand and CrCl3(THF)3 in situ in chlorobenzene as follows: 120 μL
of a 1.0 mM solution of CrCl3(THF)3 in chlorobenzene was
combined with 144 μL of a 1.0 mM solution of the ligand in
chlorobenzene in a 1:1.2 ratio in 8 mL vials. The samples were mixed
for 30 min on a shaker prior to addition to the glass inserts.
Each glass insert was equipped with a magnetic stir bar, weighed,
and charged with a predetermined amount of methylcyclohexane
solvent via a liquid handler such that the final volume would be 4 mL.
The liquid handler then delivered a solution of 10 wt % nonane (the
GC internal standard in methylcyclohexane) for a total of 50 mg of
nonane in each glass insert. 250 μL of an MMAO-3A solution (200
mM in methylcyclohexane, 50 μmol final loading), followed by 110
μL of precatalyst solution (0.05 μmol Cr final loading), were then
added to the glass inserts via an Eppendorf pipet. One tube, used as a
control, was charged as described above, except that precatalyst
solution was not added. The glass tubes were inserted into the reactor
that was subsequently sealed. The cells were pressurized with 100 psi
of ethylene and heated to 60 °C, as monitored by five thermocouples
in the stainless-steel block. Once the reaction temperature was
reached, the cells were pressurized up to 500 psi with ethylene.
A glass jar containing 24.0 mL of chlorodiphenylphosphine
(129.7 mmol, 2.2 equiv) and 67.5 mL of triethylamine (480.8
mmol, 8.2 equiv) dissolved in 50 mL of dichloromethane was
cooled to −35 °C in a glovebox freezer. To this chilled solution
was added 5.0 mL of isopropylamine (58.4 mmol, 1 equiv)
slowly over the course of 15 min, causing the formation of a
white precipitate. The resulting slurry was stirred for 24 h
before it was filtered through a disposable filter and
concentrated under reduced pressure giving an off-white
solid. This off-white solid was recrystallized by dissolving it
in 600 mL of hot ethanol, filtering, and allowing the solution to
cool to room temperature. This yielded a first crop of white
crystals, which were collected by vacuum filtration and washed
on the filter with 20 mL of ethanol. The filtrate was placed in
the freezer overnight, causing the precipitation of a second
F
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crop of white crystals, which were collected by vacuum
filtration and washed on the filter with 20 mL of ethanol. The
two batches of crystalline material were combined and dried
further in vacuo, giving 19.44 g of a white crystalline solid
The reaction mixture was allowed to warm to room
temperature and was stirred overnight. The reaction mixture
was then filtered and the solvent was removed in vacuo. The
crude product was recrystallized from hot hexanes, giving 0.6 g
1
1
(yield = 77.9%). H NMR (400 MHz, CDCl3) δ 7.44−7.30
of a pale yellow solid (yield = 55%). H NMR (500 MHz,
(m, 20H), 3.96−3.61 (m, 1H), 1.19 (d, J = 6.5 Hz, 6H). 31P
NMR (162 MHz, CDCl3) δ 48.80.
CDCl3) δ 7.44−7.33 (m, 2H), 7.33−7.24 (m, 2H), 1.85−1.55
(m, 8H), 1.06−0.87 (m, 12H). 13C NMR (126 MHz, CDCl3)
δ 144.79−144.56 (m), 129.70 (t, J = 2.8 Hz), 128.15, 19.72 (t,
J = 3.4 Hz), 9.67 (t, J = 6.8 Hz). 31P NMR (202 MHz, CDCl3)
δ −28.74. HRMS (ESI-Q-TOF) m/z [M + H]+ Calcd for
C14H35P2 255.1432. Found: 255.1425.
Synthesis of 1,1′-(1,2-Phenylene)bis[1,1-bis(1-methylethyl)-
phosphine] (24).
Synthesis of N-[Bis(2-methoxyphenyl)phosphino]-P,P-bis(2-me-
thoxyphenyl)-N-methyl-phosphinous amide (10).
Fifteen mL of 2-methoxyphenylmagnesium bromide (15
mmol, 4 equiv, 1.0 M in THF) was added slowly to a cold
(−78 °C) solution of 0.7 mL of PBr3 (7.50 mmol, 2 equiv)
dissolved in 85 mL of THF. The reaction mixture was stirred
for 1.5 h before being warmed to room temperature and stirred
for 18 h. 5.8 mL of triethylamine (41.3 mmol, 11 equiv) was
then added via syringe followed by addition of 0.253 g of
methylamine hydrochloride (3.75 mmol, 1 equiv) as a solid.
The resulting white slurry was stirred for 24 h before it was
concentrated under reduced pressure. Methanol (20 mL) was
then added to the residue, which was stirred for 30 min. The
resulting sold was collected via filtration, using nitrogen to
push the solution through the filter to keep the atmosphere
oxygen-free, and was dried in vacuo, giving 1.09 g of a white
solid (yield = 56.0%). 1H NMR (400 MHz, CDCl3) δ 7.33 (t, J
= 7.5 Hz, 4H), 7.14−7.05 (m, 4H), 6.87 (ddd, J = 8.8, 5.1, 2.1
Hz, 8H), 3.60 (s, 12H), 2.45 (s, 3H). 31P NMR (162 MHz,
CDCl3) δ 51.57.
To a stirring solution of 1.5 g of 1,2-bis(dichlorophosphino)-
benzene (5.36 mmol, 1 equiv) in 30 mL of THF at −20 °C
was added 11.0 mL of a THF solution of isopropylmagnesium
chloride (21.98 mmol, 4.1 equiv, 2 M) in a dropwise fashion.
The reaction mixture was allowed to warm to room
temperature and was stirred overnight. The reaction mixture
was then filtered, and the solvent was removed in vacuo. The
crude product was recrystallized from hot hexanes, giving 1.2 g
1
of an off-white solid (yield = 72.5%). H NMR (400 MHz,
CDCl3) δ 7.52−7.45 (m, illl2H), 7.33−7.26 (m, 2H), 2.10
(hept, J = 7.0 Hz, 4H), 1.12 (q, J = 7.1 Hz, 12H), 0.88 (dd, J =
11.4, 6.9 Hz, 12H). 13C NMR (101 MHz, CDCl3) δ 144.83−
143.95 (m), 132.46 (t, J = 2.1 Hz), 127.68, 24.91 (t, J = 5.5
Hz), 20.24 (br.), 19.45 (br.). 31P NMR (162 MHz, CDCl3) δ
−4.77. The 31P NMR chemical shift observed here is different
than that reported previously for this bis(phosphine), whereas
the 1H and 13C spectra are consistent with the reported
data.10b HRMS (ESI-Q-TOF) m/z [M + H]+ Calcd for
Synthesis of 1,1′-(1,2-Phenylene)bis[1,1-dimethylphosphine]
(22).
C18H33P2 311.2058. Found: 311.2045.
General Quantitative Gas Chromatography (GC) Informa-
tion. Quantitative GC analysis was performed using an Agilent 7890
Series instrument equipped with an Agilent DB-5MS column (30 m ×
32 μm). The inlet was held at 300 °C, and the oven temperature was
held at 70 °C for 8 min, followed by a 50 °C/min ramp to 300 °C,
where the temperature was held for 3.4 min for a total run time of 16
min.
Samples for GC analysis were prepared by quenching 75 μL of the
reaction mixture with 25 μL of methanol. The response factors were
determined for 1-octene, 1-hexene, methylcyclopentane, and methyl-
enecyclopentane via calibration using a standard solution with known
concentrations. The response factors used for the C10 to C18
fractions were determined using the terminal olefin of the same
carbon length (e.g., 1-decene for the C10 fraction). The
concentrations of the reaction products were reported by the GC
on a g/g nonane basis because nonane was included as an internal
standard in the reaction tubes (50 mg nonane per 4 mL of reaction
mixture). The amounts of the various reaction products produced
were calculated relative to the amount of nonane added to the
reaction mixture. For details, see the Supporting Information.
To a stirring solution of 1.2 g of 1,2-bis(dichlorophosphino)-
benzene (4.29 mmol, 1 equiv) in 30 mL of THF at −20 °C
was added 5.79 mL of a THF solution of methylmagnesium
bromide (17.4 mmol, 4.05 equiv, 3 M) in a dropwise fashion.
The reaction mixture was allowed to warm to room
temperature and was stirred overnight. The reaction mixture
was then filtered and the solvent was removed in vacuo. The
crude product was recrystallized from hot hexanes, giving 0.58
1
g of a pale yellow solid (yield = 53.2%). H NMR (500 MHz,
CDCl3) δ 7.53−7.38 (m, 2H), 7.37−7.27 (m, 2H), 1.32 (s,
12H). 31P NMR (162 MHz, CDCl3) δ −54.7. HRMS (ESI-Q-
TOF) m/z [M + H]+ Calcd for C10H17P2 199.0698. Found:
199.0806.
Synthesis of 1,1′-(1,2-Phenylene)bis[1,1-diethylphosphine] (23).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00721.
To a stirring solution of 1.2 g of 1,2-bis(dichlorophosphino)-
benzene (4.29 mmol, 1 equiv) in 30 mL of THF at −20 °C
was added 8.7 mL of a THF solution of ethylmagnesium
chloride (17.4 mmol, 4.05 equiv, 2 M) in a dropwise fashion.
Analytical details for ethylene oligomerization experi-
ments, NMR spectra for all ligands studied, raw data for
G
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(8) De Boer, E. J. M.; van der Heijden, H.; On, Q. A.; Smit, J. P.; van
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ethylene oligomerization studies, sample activity and
selectivity calculations (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
(9) (a) Blann, K.; Bollmann, A.; Dixon, J. T.; Neveling, A.; Morgan,
D. H.; Maumela, H.; Killian, E.; Hess, F. M.; Otto, S.; Pepler, L.
Tetramerization of Olefins. US Patent US7,511,183B2, March 31,
2009 (Sasol Technology). (b) Dixon, J. T.; Killian, E.; Bollmann, A.;
Walsh, R. N.; Overett, M. J.; Blann, K.; Morgan, D. H.
Oligomerisation of Olefinic Compounds in an Aliphatic Medium.
US Patent US7,964,763B2, June 21, 2011 (Sasol Technology).
(c) Mokhadinyana, M. S.; Maumela, M. C.; Mogorosi, M. M.;
Overett, M. J.; Van Den Berg, J.-A.; Janse van Rensburg, W.; Blann, K.
Tetramerization of Ethylene. International Patent Application WO
2014/181250A1, November 13, 2014 (Sasol Technology).
Mari S. Rosen − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States;
orcid.org/0000-0002-1153-3051; Email: msrosen@
dow.com
Jerzy Klosin − Corporate R&D, The Dow Chemical Company,
Midland, Michigan 48667, United States; orcid.org/0000-
0002-9045-7308; Email: jklosin@dow.com
Authors
(10) (a) Blann, K.; Bollmann, A.; de Bod, H.; Dixon, J. T.; Killian,
E.; Nongodlwana, P.; Maumela, M. C.; Maumela, H.; McConnell, A.
E.; Morgan, D. H.; Overett, M. J.; Pretorius, M.; Kuhlmann, S.;
Wasserscheid, P. Ethylene Tetramerisation: Subtle Effects Exhibited
by N-Substituted Diphosphinoamine Ligands. J. Catal. 2007, 249,
244−249. (b) Overett, M. J.; Blann, K.; Bollmann, A.; de Villiers, R.;
Dixon, J. T.; Killian, E.; Maumela, M. C.; Maumela, H.; McGuinness,
D. S.; Morgan, D. H.; Rucklidge, A.; Slawin, A. M. Z. Carbon-Bridged
Diphosphine Ligands for Chromium-Catalysed Ethylene Tetramerisa-
tion and Trimerisation Reactions. J. Mol. Catal. A: Chem. 2008, 283,
114−119. (c) Kuhlmann, S.; Blann, K.; Bollmann, A.; Dixon, J. T.;
Killian, E.; Maumela, M. C.; Maumela, H.; Morgan, D. H.; Pretorius,
M.; Taccardi, N.; Wasserscheid, P. N-Substituted Diphosphino-
amines: Toward Rational Ligand Design for the Efficient Tetrame-
rization of Ethylene. J. Catal. 2007, 245, 279−284. (d) Killian, E.;
Blann, K.; Bollmann, A.; Dixon, J. T.; Kuhlmann, S.; Maumela, M. C.;
Maumela, H.; Morgan, D. H.; Nongodlwana, P.; Overett, M. J.;
Pretorius, M.; Hofener, K.; Wasserscheid, P. The Use of Bis-
(diphenylphosphino)amines with N-Aryl Functionalities in Selective
Ethylene Tri- and Tetramerisation. J. Mol. Catal. A: Chem. 2007, 270,
214−218. (e) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon, J. T.;
Hess, F.; Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.; Otto,
S. Ethylene Trimerisation and Tetramerisation Catalysts with Polar-
Substituted Diphosphinoamine Ligands. Chem. Commun. 2005, 622−
624. (f) Cloete, N.; Visser, H. G.; Engelbrecht, I.; Overett, M. J.;
Gabrielli, W. F.; Roodt, A. Ethylene Tri- and Tetramerization: A
Steric Parameter Selectivity Switch from X-ray Crystallography and
Computational Analysis. Inorg. Chem. 2013, 52, 2268−2270.
(11) Zhang, J.; Wang, X.; Zhang, X.; Wu, W.; Zhang, G.; Xu, S.; Shi,
M. Switchable Ethylene Tri-/Tetramerization with High Activity:
Subtle Effect Presented by Backbone-Substituent of Carbon-Bridged
Diphosphine Ligands. ACS Catal. 2013, 3, 2311−2317.
Scott D. Boelter − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States
Dan R. Davies − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States
Kara A. Milbrandt − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States
David R. Wilson − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States
Molly Wiltzius − Corporate R&D, The Dow Chemical
Company, Midland, Michigan 48667, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00721
Notes
The authors declare no competing financial interest.
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