Chromium aryl complexes with N-donor ligands as catalyst precursors for selective ethylene trimerization

Article abstract of DOI:10.1021/om500459k

A series of 8-amino-2-arylquinoline ligands (1-6) were synthesized and reacted with CH₃CrCl₂(thf)₃. Under these conditions a CH bond of the 2-aryl substituent is metalated, leading to organochromium complexes with monoanionic tridentate ligands (8-13). The presence of a chromium-carbon σ bond in these complexes has been established by X-ray analysis. Furthermore, 8-(piperidin-1-yl)quinoline (14) was used as neutral bidentate ligand in addition to an external aryl group, leading to complex 15. Finally, the tris-aryl complex 18 was synthesized, which features a rare five-coordinate chromium(III) metal center. All chromium complexes were tested as catalysts for the selective trimerization of ethylene after activation with methylaluminoxane (MAO). Several of the new catalyst precursors show good behavior for the selective trimerization of ethylene. Although chlorido ligands in the catalyst precursor will be substituted by methyl groups during the activation with MAO, there is a clear difference in the catalytic behavior when the complex contains a methyl (or aryl) group prior to addition of MAO. The mechanism of catalyst activation has been studied in more detail with the tris-aryl complex 18.

Full text of DOI:10.1021/om500459k

Article
pubs.acs.org/Organometallics
Chromium Aryl Complexes with N‑Donor Ligands as Catalyst
Precursors for Selective Ethylene Trimerization
Mathias Ronellenfitsch, Hubert Wadepohl, and Markus Enders*
Institute of Inorganic Chemistry, University of Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
S
* Supporting Information
ABSTRACT: A series of 8-amino-2-arylquinoline ligands (1−
6) were synthesized and reacted with CH3CrCl2(thf)3. Under
these conditions a CH bond of the 2-aryl substituent is
metalated, leading to organochromium complexes with
monoanionic tridentate ligands (8−13). The presence of a
chromium−carbon σ bond in these complexes has been
established by X-ray analysis. Furthermore, 8-(piperidin-1-
yl)quinoline (14) was used as neutral bidentate ligand in
addition to an external aryl group, leading to complex 15.
Finally, the tris-aryl complex 18 was synthesized, which
features a rare five-coordinate chromium(III) metal center. All chromium complexes were tested as catalysts for the selective
trimerization of ethylene after activation with methylaluminoxane (MAO). Several of the new catalyst precursors show good
behavior for the selective trimerization of ethylene. Although chlorido ligands in the catalyst precursor will be substituted by
methyl groups during the activation with MAO, there is a clear difference in the catalytic behavior when the complex contains a
methyl (or aryl) group prior to addition of MAO. The mechanism of catalyst activation has been studied in more detail with the
tris-aryl complex 18.
and tridentate SNS, PNP, and NNN ligands.9 The catalyst
INTRODUCTION
■
precursors become active upon addition of aluminum alkyl
Linear α-olefins (LAO), which are important industrial
reagents, often as methylaluminoxane (MAO). Therefore, it is
products, are predominantly synthesized by the nonselective
obvious that initially chromium alkyl species are formed which
oligomerization of ethylene. Well-established industrial pro-
upon reaction with ethylene somehow enter into the catalytic
cesses lead to α-olefins with distributions of the chain lengths
cycle, which itself operates via metallacycles and Cr(III)/Cr(I)
(e.g., Shell Higher Olefin Process (SHOP) or processes based
or Cr(IV)/Cr(II) redox couples. A small number of chroma-
on the Ziegler Aufbau reaction).1,2 A growing demand for
cycles have been isolated, but they usually do not allow the
uniform α-olefins such as 1-hexene or 1-octene is due to the
catalytic formation of 1-hexene.10−12 From these observations a
increasing production of linear low-density polyethylene
working hypothesis is that chromium-based trimerization
(LLDPE), where such α-olefins are used as comonomers.3
catalyst precursors should enable the formation of mono-
The catalytic systems developed so far predominantly contain
cationic chromium dialkyl (or diaryl) species. Practically all of
chromium as the catalytic center.4 The observation that
the hitherto reported precatalysts fall into that scheme. MAO as
chromium-based ethylene polymerization catalysts may also
activator is able to alkylate the metal center and to abstract an
produce small amounts of 1-hexene5 has culminated in the
anionic ligand (i.e., Cl− or CH3 ) and is therefore a privileged
−
development of the so-called Phillips trimerization catalyst,
cocatalyst. Complexes of the type LnCrCl3 activated with MAO
which was presented in 1991.6 Even in 1977 researchers from
should lead to LnCr(CH3)2 cations, which could form the
Union Carbide proposed a metallacyclic mechanism for
proposed Cr(I) species by reductive elimination. However,
selective ethylene trimerization catalyzed by a combination of
Bercaw et al. have shown that, during the initiation process,
a chromium(III) source and partially hydrolyzed triisobutyla-
reductive elimination is observed only after ethylene insertion
luminum.7 Later it was found that addition of the simple
into the chromium−carbon bonds (in this case into the Cr−
phenyl bond).13 Consequently, the nature of the carbanionic
bidentate ligand dimethoxyethane improved the selectivity for
1-hexene to 74%.8
ligand in the catalyst precursor is able to influence the initiation
From the year 2000 onward there have been many reports
process.
about molecular chromium systems which catalyze the
trimerization or the tetramerization of ethylene with high
Special Issue: Catalytic and Organometallic Chemistry of Earth-
activity and selectivity. Most of the reported ligands are neutral
Abundant Metals
or monoanionic chelate ligands with two or three donor
atoms.2 Examples are bidentate PNP ligands (P coordination
Received: May 2, 2014
and sometimes additional coordination of a phenoxy moiety)
© XXXX American Chemical Society
A
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Some of the tridentate PNP and SNS ligands provide the
opportunity for deprotonation of the secondary amine function
by aluminum alkyls during activation to give an anionic ligand.
Indeed, the best results with these catalyst systems have been
obtained with ligands containing a secondary amine functio-
n.9e,g,h,14 However, the products isolated after treatment of a
SNSCrCl3 complex with various aluminum alkyls did show that
the NH moiety is not deprotonated under these conditions.9f,15
We present here our results on the synthesis and catalytic
properties of catalyst precursors where there is already a
chromium−carbon bond established prior to addition of MAO
as cocatalyst. This is achieved via the use of
order to verify under which conditions a NH or a CH
activation occurs. All chromium complexes used and prepared
in this work are paramagnetic and give relatively badly resolved
1H NMR spectra. Nevertheless in situ NMR spectroscopy was a
valuable tool for monitoring the reaction with 1. In 19F NMR a
signal could only be detected if the aryl did not coordinate to
the chromium center. First, the reaction of 1 with
CH3CrCl2(thf)3 under varying conditions was tested. In
toluene as solvent complexation occurs rapidly at room
temperature and the NH activation product 7 is obtained
(see Scheme 2). In tetrahydrofuran (thf), however, complex-
ation does not take place without heating to about 70 °C.
Stirring the mixture in a closed tube for 1 h at 100 °C
completes the reaction, giving the CH activation product: i.e.,
complex 8. Thus, with CH3CrCl2(thf)3 as precursor the
complexes 7 and 8 were obtained as dark red and brown
solids, respectively, and could be recrystallized and charac-
terized by X-ray analysis. When using p-tolylCrCl2(thf)3 as
chromium precursor and thf as solvent, heating of the reaction
mixture was necessary as well. However, in this case only the
NH activation product 7 was formed, as could be shown by
NMR spectroscopy. Analyzing the reaction mixtures by 19F and
1H NMR spectroscopy revealed that generally under a given
condition either complex 7 or 8 is formed, but not a mixture of
the two compounds.
16,17
18
CH3CrCl2(thf)3
and p-tolylCrCl2(thf)3 as well-known
chromium complexes already containing a chromium−carbon
bond.
RESULTS
■
Synthesis and Characterization of Organochromium
Complexes. The starting point of the work presented here was
a patent describing trimerization catalysts using 2-aminomethyl-
5-arylpyridines as ligand precursors.19 Many of the ligands
described there have a NH group which could probably be
deprotonated by reaction with metal alkyl precursors. However,
upon reaction with CH3CrCl2(thf)3 or with p-tolylCrCl2(thf)3
the NH functionality remains intact while an ortho C−H bond
of the ligand aryl group is activated, leading to complexes with a
new chromium−carbon bond to the resulting tridentate
monoanionic ligands. After activation with MAO these systems
produced 1-hexene with up to 90% selectivity and an activity of
5800 g/((g of Cr) h).
The presence of an intramolecular carbon−chromium bond
in 8 was established from single-crystal X-ray diffraction. A
comparison of the solid-state molecular structures of complexes
7 and 8 substantiates that in the case of 7 the amine function
was deprotonated, resulting in a monoanionic, bidentate ligand
(Figure 1). The two chlorido ligands are located trans to each
We chose a related ligand system where aryl groups are
attached to the 2-position of 8-aminoquinolines. Some
molecules of that type have already been described in the
literature, and we added the new derivatives shown in Scheme
1.
Scheme 1. Ligand Precursors 1−6 Based on 8-
Aminoquinolines with an Aryl Substituent in the 2-Position
Figure 1. Molecular structures of 7 and 8. For clarity, hydrogen atoms
have been omitted and only one of the two disordered molecules of 8
is shown; a figure showing the disorder is given in the Supporting
Information. Selected bond lengths (Å) and angles (deg): 7, Cr−N1
2.1080(15), Cr−N11 1.9575(15), Cr−O41 2.1202(13), Cr−O31
2.0612(12), N11−Cr−N1 81.67(6), N11−Cr−O41 176.78(6); 8,
Cr−N1 2.0159(15), Cr−N11a 2.326(3), Cr−C19 2.0416(19), Cr−O1
2.0414(13), N1−Cr−N11a 75.24(8), C19−Cr−N11a 149.96(8).
The ligands were prepared following a synthetic route
described for a derivative.20 We wanted to evaluate the
reactivity toward organometallic chromium precursors in
Scheme 2. Complexes Obtained by Reaction of 1 with CH3CrCl2(thf)3 and p-tolylCrCl2(thf)3 in Tetrahydrofuran (thf) or
Toluene
B
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other, and the octahedral geometry is completed by two thf
molecules. In 8 the octahedral coordination sphere is distorted,
since the relatively inflexible ligand is coordinating in a
meridionally tridentate fashion. This is reflected by the C−
Cr−N(amino) angle, which shows a value of about 150°
instead of the optimum 180°. The tridentate coordination is the
result of the formation of a chromium−carbon bond
connecting the metal center with the aryl remainder of the
ligand and the two nitrogen atoms acting as neutral donors.
The secondary amine function of the ligand is not
deprotonated, so that the ligand is monoanionic as well.
Consequently, we could show that ligand 1 provides different
reaction pathways during complexation with the chromium
precursors leading to 7 or 8. With CH3CrCl2(thf)3 as precursor,
both complexes can be obtained, depending on the solvent
used. In thf the chromium precursors are stable at room
temperature and reaction with the ligands occurs only at
elevated temperatures. However, at such temperatures the
chromium precursor MeCrCl2(thf)3 or a short living
decomposition product attacks the aromatic CH bond of the
ligand, leaving the NH function unchanged.
We performed DFT calculations in order to estimate the
relative energies and free energies of complexes 7 and 8. The
calculations show that 7 is lower in enthalpy by 70 kJ/mol in
comparison to 8, indicating that the formation of the latter is
kinetically controlled. The calculated Gibbs energy difference of
13 kJ/mol, still indicates a preference for 7. Heating of a
solution of 8 in thf did not lead to a rearrangement with
formation of the thermodynamically favored 7.
Several chromium-based trimerization catalyst precursors
feature a NH moiety at the ligand which could possibly react
with the cocatalyst (i.e., with MAO). However, it is not clear if
such a ligand reactivity is beneficial for a good trimerization
catalyst. Our initial catalytic experiments did not exhibit a clear
advantage having a NH moiety present in the ligand, and so we
decided to eliminate one of the reaction pathways by
synthesizing ligands with a tertiary amine function. Con-
sequently, we should obtain complexes containing a metal−
carbon bond between the ligand and chromium with better
yields, as the system has fewer opportunities for undesired
reactions. Thus, we synthesized the derivatives 2−6, where the
anilinyl remainder is replaced by piperidinyl and pyrrolidinyl
groups, respectively (see Scheme 1 and the Supporting
Information for details).
temperature. The complexes are formed immediately under
these conditions, which can be observed by their precipitation
after a few minutes from a formerly clear solution. By this
procedure the complexes were obtained in isolated yields
ranging from 52 to 85%. Complexes 9, 11, and 13 could be
analyzed by X-ray structure analysis (see Figure 2 for 9). Their
Figure 2. Molecular structures of 9 (left) and 15 (right). Hydrogen
atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (deg): 9, Cr−N1 2.0060(12), Cr−N2 2.4074 (12), Cr−C12
2.0369(14), Cr−O1 2.0871(11), N2−Cr−N1 77.67(4), N2−Cr−C12
157.67(4); 15, Cr−N1 2.051(2), Cr−N2 2.432(2), Cr−C16 2.083(3),
Cr−O1 2.092(2), N1−Cr−N2 77.30(9), N2−Cr−C16 170.97(9).
molecular geometries are all similar to that of the previously
discussed complex 8. In every case the chromium atom is
coordinated by the two neutral donor atoms of the ligand in
addition to a bond to the ortho C atom of the aryl substituent.
This shows that the CH activation of the ligand by the
chromium alkyl precursor is a reliable reaction. As expected,
two chlorido ligands and one THF molecule complete the
distorted-octahedral coordination geometry.
In order to evaluate the influence of the chelating aryl ligand
for the catalytic properties, we replaced it by an external aryl
group. Therefore, ligand 14 was synthesized and reacted with
the chromium precursors p-tolylCrCl2(thf)3, CH3CrCl2(thf)3,
and CrCl3(thf)3 respectively (Scheme 4). The new complexes
15−17 were obtained and could be characterized by elemental
analysis and by X-ray structure analysis (see Figure 2 for 15).
The green complex 16 is formed only after recrystallization
from thf. The initially formed compound is a yellow powder
and does not contain coordinated thf, as could be shown by
elemental analysis. The catalytic experiments were performed
with yellow 16-thf. The geometry of 15 with an external p-tolyl
ligand is very similar to those of complexes 9, 11, and 13 (with
an internal aryl ligand). The angle N2−Cr−C12 of 157.76(4)°
in 9 opens up to 170.97(9)° (N2−Cr−C16) in 15. Not only
the aryl ligand in 15 but also the methyl ligand in 16 (see the
Supporting Information) is located in a position trans to the
tertiary amine group. Therefore, the meridional arrangement of
the two N-donor atoms together with the carbanionic ligand is
favored irrespective of the covalent attachment of the
carbanionic group to the aminoquinoline ligand.
After reaction with CH3CrCl2(thf)3 complexes 9−13 were
obtained (Scheme 3). As mentioned before, 2−6 offer no
opportunity to form an anionic heteroatomic donor, making it
possible to carry out the reaction in toluene and thus at room
Scheme 3. Chromium Complexes 9−13 Synthesized by
Conversion of Ligands 2−6 with CH3CrCl2(thf)3 in Toluene
a
at Room Temperature
Synthesis of the Triphenyl Complex 18. Reaction of 3
equiv of the Grignard reagent p-tolylMgCl with the trichlorido
complex 17 led to the tris-aryl complex 18 in a relatively high
yield of 81%. The elemental analysis already showed that no thf
is present in the solid. Single-crystal X-ray analysis confirmed
the composition (Scheme 5). The chromium center is
coordinated by only five ligand atoms, resulting in a square-
pyramidal coordination geometry. The Cr atom is slightly
displaced (0.356(2) Å) from the plane defined by the ligand
a
Molecular structures of 9, 11, and 13 have been determined by X-ray
analysis (see the Supporting Information).
C
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Scheme 4. Complexes 15−17 Obtained by Use of Ligand 14
a
Scheme 5. Preparation of Complex 18 (Left) and Molecular Structure As Obtained from X-ray Diffraction (Right)
a
Selected bond lengths (Å) and angles (deg): Cr−N1 2.138(3), Cr−N2 2.253(3), Cr−C16 2.073(4), Cr−C23 2.054(4), Cr−C30 2.034(4); N1−
Cr−N2 76.0(1), C23−Cr−C16 89.6(2), C23−Cr−N2 153.3(2), C16−Cr−N1 165.7(2), C30−Cr−C16 101.2(2).
atoms N1−N2−C16−C23 (RMSD 0.120 Å). A number of
molecular structures of tris-aryl or tris-alkyl chromium(III)
complexes have been described, and the majority are six-
coordinate, as for example Ph3Cr(thf)3.21 Five-coordinate
chromium(III) complexes usually feature a square-pyramidal
geometry similar to that of 18. A couple of examples have been
published with five-coordinate tris-aryl complexes.22
2). With 10 the selectivity for 1-hexene in the LAO fraction is
better but the major product is still PE (entry 3). However, the
formation of PE can be suppressed efficiently by keeping the
temperature at 60 °C or below (see entries 3−6). Under these
conditions an overall selectivity toward 1-hexene of over 90%
was observed (entry 6). A remarkable increase in activity could
be achieved by shortening the activation time, which means the
time the chromium complex and MAO are stirred in toluene
before the trimerization is started by the insertion of ethylene.
Starting the reaction by pressurizing the autoclave immediately
after the chromium complex comes in contact with MAO
instead of stirring it for 20 min increased the activity by a factor
of 1.7, while the selectivity stayed almost the same (compare
entries 6 and 7).
The replacement of the piperidyl by a pyrrolidinyl remainder
(complex 13), which means a relatively small change in the ring
size of the cycloamine, results in a catalyst performing with
considerably worse selectivity (compare entries 8 and 13). A
comparison of the results of 9−11 (see entries 11, 12, 8) shows
that the electron-withdrawing trifluoromethyl group seems to
be disadvantageous concerning the selectivity. On the other
hand, complex 12 gives a comparably low activity, even though
it has two electron pushing methyl groups at the aryl remainder
of the ligand at positions ortho and para to the chromium−
carbon bond. If a more electron rich aryl remainder is positive
in general, this effect must be overbalanced by the increased
steric hindrance in this case.
Catalytic Ethylene Trimerization. Complexes 7−13 and
15−18 were tested as catalysts for the selective trimerization of
ethylene to 1-hexene after activation with methylaluminoxane
(MAO) under various conditions (see Table 1 and the
Supporting Information for experimental details). The denoted
α-olefins were identified by their retention times in the gas
chromatograms after reference measurements with the pure
substances. All other signals in the chromatogram that could
not be identified in this way were summarized under “other”.
The amount of a single unidentified product was typically not
more than 2%. Thus, cyclic or branched isomers of LAOs could
not be identified under our conditions. Before we discuss the
performance of the complexes, it must also be mentioned that
the compounds CH3CrCl2(thf)3, p-tolylCrCl2(thf)3, and
CrCl3(thf)3 give oligomerization catalysts under our conditions
with high activities (see entries 19−21). Under the given
reaction conditions only a small amount of polymer and a
broad distribution of LAOs was obtained. In comparison to this
result the ligands used in this report lowered the overall activity
but shifted the selectivity from a range of LAOs toward 1-
hexene.
Ethylene Trimerization with Complexes 15−17 (Bidentate
Ligands). The complexes 15 and 16-thf showed relatively high
activity, comparable to those complexes with tridentate ligands.
This shows that a linkage of the chromium to the ligand via an
alkyl or aryl remainder is not beneficial for the catalytic
Ethylene Trimerization with Complexes 7−13 (Tridentate
Ligands). Complexes 7 and 8 gave unsatisfying results. A larger
amount of polyethylene (PE) was produced in the reaction, and
no selectivity was observed in the LAO fraction (entries 1 and
D
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a
Table 1. Results of the Ethylene Trimerization Experiments
PE
cat. amt
(μmol)
activity (g/((g
of Cr) h))
1-hexene activity
(g/((g of Cr) h))
1-hexene
1-oct
1-dec
1-dodec
other
(wt
entry
complex
T (°C)
p (bar)
(wt %) (wt %) (wt %) (wt %) (wt %)
%)
b
1
8
7.9
10.0
12.7
10.5
11.1
12.5
11.3
8.5
85−95
85−95
90−95
70−75
60−63
50−53
50−55
49−51
49−51
49−51
49−51
49−51
50−55
49−51
49−51
49−51
45−65
50−52
49−51
49−51
45−55
50−58
48−52
50−54
50−53
50−53
48−53
24
24
25
25
25
25
25
25
10
10
25
25
25
25
25
25
25
10
25
10
10
10
10
10
10
10
10
1760
8800
230
1500
13
17
34
49
90
92
90
87
88
74
87
74
56
77
79
78
83
87
30
34
38
54
85
83
76
83
83
9
17
1
8
14
1
7
12
1
10
14
1
53
26
62
39
1
c
2
7
c
3
10
31800
13800
8200
10800
6800
c
4
10
3
3
2
4
c
5
10
7400
3
2
2
2
c
6
10
5600
5200
1
1
0
3
3
7
10
9300
8400
4
2
2
1
1
d
8
d
10
12600
4600
10900
4100
5
3
2
2
1
9
10
10.9
10.1
8.9
3
2
1
4
2
e
10
10
6800
5000
3
2
1
6
14
2
d
11
9
11600
9800
10100
7300
4
3
2
2
d
12
11
7.6
3
2
1
1
19
7
d
13
13
8.9
11400
6300
6400
14
3
11
1
6
6
d
14
12
9.8
4900
1
2
16
2
d
15
15
8.4
14000
13800
26500
10400
27600
40100
27200
3000
11000
10800
22000
9000
13
13
8
0
0
6
d
16
16-thf
16-thf
16-thf
8.1
0
0
6
3
e
17
11.1
12.3
8.8
0
0
4
5
e
18
4
0
0
5
4
19
20
p-tolylCrCl2(THF)3
8200
24
25
25
6
18
18
17
1
13
11
11
0
11
11
7
4
CH3CrCl2(THF)3
10.4
17.1
10.4
10.0
11.4
11.4
11.4
11.4
13500
10500
1700
1
e
21
CrCl3(THF)3
17
2
e
22
6
33
5
e
23
18
22200
15900
26000
30600
18300
18900
13200
19800
25400
15200
6
0
0
4
e
24
16-thf
16-thf
16-thf
16-thf
7
0
0
3
7
f
25
5
1
0
3
15
7
g
26
6
1
0
3
h
27
7
0
0
3
7
a
Conditions: 83 mL of toluene, cocatalyst 2 mL of a 10% solution of MAO in toluene (“MAO 1”) or 1 mL of a 30% solution of MAO in toluene
(“MAO 2”)). The autoclave was pressurized with ethylene immediately after addition of the chromium complex to the toluene MAO mixture,
b
c
d
e
f
g
reaction time 30 min. Activation time 30 min. Activation time 20 min. Reaction time 60 min. 1 mL of MAO 2. 2 mL of MAO 2. 3 mL of MAO
2. 1 mL of MAO 2, 2 mmol of AlMe3. Note that two different activation parameters have been used, differing in amounts and charges of MAO,
h
resulting in different activities (compare entry 9 with 10 and entry 18 with 19).
ethylene trimerization. Nevertheless, a chromium−carbon bond
in the catalyst precursor enhances strongly both activity and
selectivity, as a comparison between complexes 16 and 17
shows (compare entry 18 with 22). This is surprising, since
during activation with MAO the complexes are already
expected to be alkylated. However, in the end all of the
chlorido ligands have to be removed from the complex to get to
the active species. Thus, many reaction steps are necessary in
this process, surely providing many side reactions leading to
inactive compounds. A detailed study of the activation process
of similar catalysts was reported by Bercaw et al.23 They were
able to identify spectroscopically two main products, but these
were not the active catalysts. Considering this, it seems
reasonable that providing a complex that is one step closer to
the active species gives better results. Furthermore, it is
remarkable that 15 and 16-thf gave very similar results both in
activity and selectivity, indicating that the same active species is
formed in both cases. This implies that the initial aryl or alkyl
remainder, respectively, is no longer present in the active
species. It also shows that the coordinated thf molecule in 15
does not influence the catalytic properties, as it is most
probably abstracted by the Lewis acidic cocatalyst.
ligands (9−13), where other LAOs in addition to 1-hexene and
1-octene were also produced. Increasing the amount of MAO
by a factor of 3 doubles the catalytic activity, whereas the
selectivity remains unchanged (see entries 24−26). Addition of
AlMe3 has almost no influence on the catalytic behavior, with
only a slight increase in activity and no changes in selectivity
(see entry 27 with addition of 1 mmol of AlMe3).
Investigation of Catalyst Activation. Bercaw, Labinger,
and co-workers have demonstrated that a triphenyl chromium-
(III) catalyst precursor is active for selective ethylene
trimerization only after abstraction of a carbanionic phenyl
group. They also showed that ethylene inserts into the
chromium−phenyl bonds prior to reductive elimination
under formation of the active catalyst.13 We used complex 18
in order to investigate the activation process. In a first
experiment we heated a solution of 18 in the absence of the
cocatalyst and in the absence of ethylene. At a typical
trimerization reaction temperature (i.e., 50 °C), the complex
decomposes slowly over hours. At 80 °C the decomposition is
1
complete within 60 min, as has been monitored by H NMR
(disappearance of paramagnetically shifted signals and consid-
erable increase of signals in the diamagnetic region (i.e., δ 0−10
ppm)). The reaction mixture was filtered through a short silica
gel column in order to eliminate the decomposed chromium
compounds. Analysis of the solution by GC-MS revealed the
formation of ligand 14 and 4,4′-dimethylbiphenyl as the only
Having a look at the selectivity in the LAO fraction, one can
also notice that considerable amounts of 1-octene (between 4
and 13%; see entries 15−18) but no other LAO is produced.
This distinguishes these complexes from those with tridentate
E
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organic products. The nature of the resulting chromium
compounds could not be clarified. This experiment was
repeated in the presence of ethylene at atmospheric pressure.
Again the mixture needed heating to 80 °C in order to be
complete within 60 min. The analysis of the organic byproducts
now showed a third component, which was identified as 4-
methylstyrene. This molecule must have formed after insertion
of one molecule of ethylene into the chromium−phenyl bond
and subsequent β-elimination. In a third experiment MAO was
added, ethylene was fed into the solution, and the mixture was
stirred for 1 h at 50 °C. The reaction mixture then was
quenched with water, filtered over glass wool, and analyzed by
GC-MS and by GC-FID (FID denotes a flame ionization
detector). The analysis showed that 1-hexene was formed in the
amount we would expect under these conditions, showing that
the catalytically active species must have been formed. It also
revealed that, in contrast to the aforementioned experiments in
the absence of MAO, no biphenyl and no methylstyrene had
formed but instead the product from multiple insertions of
ethylene into the chromium−aryl moiety (23−25, Scheme 6).
The GC-FID analysis showed that the amounts of the ligand 18
and the compounds 23−26 are all of the same order of
magnitude (see Figure S2 in the Supporting Information).
chromium aryl moiety occurs and after β-H elimination this
leads to 22 (a cis or trans isomer is possible).
From these results we propose the activation pathways given
in Scheme 7, which are similar to those suggested by Bercaw,
Labinger, and co-workers for the triphenyl chromium(III)
catalyst with a PNP ligand.
Cocatalysts Other Than MAO. Several catalytic runs with
activators other than MAO were performed (see Table 2), and
the reaction solutions were analyzed by GC. No formation of
any LAO nor polyethene was observed.
Table 2. Attempts To Activate the Complexes 10, 16-thf, and
18 without MAO
complex
activation
conditions
10
596 equiv Me3Al
9 equiv Et3Al
8 equiv Et3Al
a
25 bar, 90 °C, 30 min
atm, room temp, 1 h
atm, room temp, 1 h
atm, 50 °C, 1 h
16-thf
16-thf
18
18
b
atm, 50 °C, 30 min
atm, 50 °C, 30 min
18
240 equiv Me3Al
a
1 equiv of N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate.
b
1 equiv of [H(Et2O)2]+[B(C6H3(CF3)2)4]−.
Scheme 6. Compounds Detected after Trimerization with 18
at Atmospheric Pressure
CONCLUSION
a
■
The reaction of CH3CrCl2(thf)3 or p-tolylCrCl2(thf)3 with
ligands 1−6 leads to CH-activated ligands in most cases.
However, in the presence of a NH functionality also the
deprotonation of the amino group was observed, and therefore
ligands with secondary amines may lead to different species
upon reaction with organometallic chromium precursors. The
chromium aryl ligands in complexes 8−13 are part of a
tridentate ligand, and in order to simplify the system, we
replaced the internal aryl by an external aryl (or methyl
respectively) combined with the bidentate ligand 14. The three
complexes 15−17 should lead to the same active species after
activation with MAO, and indeed the results for the methyl
complex (16) in comparison with those for the p-tolyl complex
(15) are almost identical. However, the trichlorido complex 17
is much worse than 15 or 16 with respect to catalytic activity
and selectivity. This shows that the aryl (or methyl) group in
the catalyst precursor is important for the activation pathway
leading to a good trimerization catalyst.
a
For 22 the trans isomer is also possible.
Some of the identified compounds are a result of β-H
eliminations (19 and 21). In addition to these, we identified
considerable amounts of 1-butyl-4-methylbenzene (20).
Quenching of the reaction mixture in the end with D2O gave
the monodeuterated compound. This shows that 20 is not
formed during the activation but by the hydrolysis of a
corresponding chromium alkyl or aluminum alkyl species. In
addition, we suggest that insertion of 1-hexene into the
Scheme 7. Proposed Activation Pathways for Complex 18 As Concluded from the Analysis of Organic Byproducts after a
Catalytic Trimerization Run under Typical Conditions
F
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could be analyzed by X-ray crystallography. 19F NMR (thf-d8, 376.27
MHz): no signal. 1H NMR (thf-d8, 399.89 MHz): δ 12.83 (bs), 11.70
(bs), −10.86 (bs). Anal. Calcd (found) for C26H22Cl2CrF3N2O: C,
55.93 (54.70); H, 3.97 (4.10); N, 5.02 (5.04). We attribute the low C
value in the elemental analysis to insufficient combustion in the
presence of the CF3 moiety.28
Synthesis of Complex 9. A mixture of 350 mg (1.21 mmol, 1
equiv) of 2-phenyl-8-(piperidin-1-yl)quinoline, 429 mg (1.21 mmol, 1
equiv) of CH3CrCl2(thf)3 and 8 mL of toluene was stirred for 24 h at
room temperature. Then the solvent was removed under reduced
pressure. The obtained solid was first washed with a hexane/thf
mixture (1/1) and afterward with pure hexane before it was dried
under reduced pressure. The product was obtained with a yield of 496
mg (85%) as a brown solid. Diffusion of hexane into a solution of the
complex in thf gave crystals suitable for X-ray analysis. Anal. Calcd
(found) for C24H27Cl2CrN2O: C, 59.79 (59.58); H, 5.64 (5.76); N,
5.81 (5.72).
The tris-aryl complex 18 features a rare five-coordinate
chromium(III) metal center. This complex was used for the
identification of catalyst activation products ,which gave insight
into the mechanism of catalyst activation.
EXPERIMENTAL SECTION
■
General Synthetic Procedures. All manipulations were per-
formed under an argon atmosphere using standard Schlenk techniques
or in an argon-filled drybox. Solvents were purchased anhydrous and
stored over molecular sieves or in a solvent purification system after
degassing. Methylaluminoxane was purchased from Sigma-Aldrich
(MAO 1, 10% in toluene) and from Chemtura Organometallics
GmbH (MAO 2, 21−23% MAO, 7−9% aluminum alkyls).
2,8-Dibromoquinoline,24 8-bromo-2-phenylquinoline,25
CH3CrCl2(thf)3,26 and p-tolylCrCl2(thf)3 were synthesized accord-
27
ing to the literature.
Synthesis of Complex 10. A mixture of 200 mg (0.66 mmol, 1
equiv) of 8-(piperidin-1-yl)-2-p-tolylquinoline, 232 mg (0.66 mmol, 1
equiv) of CH3CrCl2(thf)3, and 4 mL of toluene was stirred for 24 h at
room temperature. Then the solvent was removed under reduced
pressure. The obtained solid was washed three times with thf and dried
under reduced pressure. The product was obtained with a yield of 228
mg (69%) as a brown solid. Anal. Calcd (found) for C25H29Cl2CrN2O:
C, 60.49 (60.35); H, 5.89 (5.55); N, 5.64 (5.72).
Synthesis of Complex 11. A mixture of 200 mg (0.56 mmol, 1
equiv) of 8-(piperidin-1-yl)-2-(4-(trifluoromethyl)phenyl)quinoline,
197 mg (0.56 mmol, 1 equiv) of CH3CrCl2(thf)3, and 4 mLof toluene
was stirred for 24 h at room temperature. Then the solvent was
removed under reduced pressure. The obtained solid was washed with
hexane and recrystallized by diffusion of hexane into a solution of the
solid in thf. In this way crystals suitable for X-ray structure analysis
were obtained. After it was washed with hexane and dried under
reduced pressure, the product was obtained with a yield of 109 mg
Catalytic Ethylene Trimerization. In all cases (except for 16-thf,
where we used the yellow material without coordinated thf) we used
the crystalline material from X-ray analysis for catalytic applications.
The autoclave was heated to 100 °C under vacuum for 30 min, flushed
with argon, and cooled to the desired temperature. The solvent (80
mL of toluene), the MAO solution (MAO 1, 2 mL; MAO 2, 1 mL)
and the chromium complexes were transferred into the autoclave as a
solution or a suspension in toluene. Immediately after this, the
catalysis was started by pressurizing the autoclave with ethene. In some
cases the chromium compound, MAO, and toluene were stirred for a
certain time at room temperature before they were transferred into the
autoclave.
After the reaction time, the autoclave was cooled by the internal
cooling system and depressurized carefully. A 1−3 mL sample of the
reaction mixture was quenched with 5−10 drops of water, filtered, and
diluted with diethyl ether before analyzing it by gas chromatography.
To calculate the amount of 1-hexene, the toluene signal was used as an
internal standard and referenced to measurements with 1-hexene/
toluene mixtures. The 1-octene and 1-dodecene signals were identified
by reference measurements. The 1-decene signal was identified by a
GC/MS measurement. The amounts of LAOs other than 1-hexene
were calculated by referring to the 1-hexene signal under the
assumption that the intensity is proportional to the molecular weight
of the compound due to their structural similarity (FID detector).
The amount of polyethylene was determined by quenching the
reaction mixture with a mixture of 50 mL of methanol and 10 mL of
concentrated hydrochloric acid. After filtration the solid obtained was
dried at 100 °C and weighed afterward.
(31%) as
a brown solid. Anal. Calcd (found) for
(C25H26Cl2CrF3N2O)(C4H8O): C, 55.96 (54.00); H, 5.51 (5.37); N,
4.50 (4.60). We attribute the low C value in the elemental analysis to
insufficient combustion in the presence of the CF3 moiety.28
Synthesis of Complex 12. A mixture of 230 mg (0.73 mmol, 1
equiv) of 2-(3,5-dimethylphenyl)-8-(piperidin-1-yl)quinoline, 257 mg
(0.73 mmol, 1 equiv) of CH3CrCl2(thf)3 and 6 mL of toluene was
stirred for 24 h at room temperature. Then the solvent was removed
under reduced pressure. The obtained solid was washed with thf and
afterward with pentane. After drying under reduced pressure the
product was obtained with a yield of 245 mg (66%). Further
purification by washing with thf at 65 °C gave 192 mg of the very pure
product. Anal. Calcd (found) for C26H31Cl2CrN2O: C, 61.18 (61.01);
H, 6.12 (6.31); N, 5.49 (5.73).
Synthesis of Complex 13. A mixture of 196 mg (0.68 mmol, 1
equiv) of 8-(pyrrolidin-1-yl)-2-p-tolylquinoline, 241 mg (0.68 mmol, 1
equiv) of CH3CrCl2(thf)3, and 5 mL of toluene was stirred for 24 h at
room temperature. Then the solvent was removed by filtration. The
obtained solid was washed with thf and pentane. After drying under
reduced pressure the product was obtained with a yield of 274 mg
(84%). Further purification by washing with thf at 65 °C gave 107 mg
of the pure product. Crystals suitable for X-ray analysis were obtained
by storing a solution of the complex in thf at −30 °C. Anal. Calcd
(found) for C24H27Cl2CrN2O: C, 59.76 (59.18); H, 5.64 (5.53); N,
5.81 (5.75).
Synthesis of Complex 7. A 102 mg portion (280 μmol, 1 equiv)
of N-phenyl-2-(4-(trifluoromethyl)phenyl)quinolin-8-amine and 103
mg (291 μmol, 1.04 equiv) of CH3CrCl2(thf)3 were dissolved in 4 mL
of toluene in a Schlenk tube and stirred for 6 h at room temperature.
Afterward the mixture was cooled to −30 °C overnight before it was
filtered. The obtained red solid was purified by crystallizing it via
diffusion of hexane into a solution of the product in thf. Then the
product was washed three times with hexane and dried under reduced
pressure. It was obtained in a yield of 73 mg (41%). Crystals were
grown by diffusion of hexane into a solution of the complex in thf and
could be analyzed by X-ray crystallography. 19F NMR (thf-d8, 376.27
1
MHz): δ −55.73 ppm. H NMR (thf-d8, 399.89 MHz): δ 22.56 (bs),
20.80 (bs), 16.64 (bs). Anal. Calcd (found) for C30H30Cl2CrF3N2O2:
C, 57.15 (56.09); H, 4.80 (4.89); N, 4.44 (4.65). We attribute the low
C value in the elemental analysis to insufficient combustion in the
presence of the CF3 moiety.28
Synthesis of Complex 15. A solution of 342 mg (3.67 mmol, 1
equiv) of 8-(piperidin-1-yl)quinoline and 693 mg (1.61 mmol, 1
equiv) of p-tolylCrCl2(thf)3 in 5 mL of thf was stirred for 2 h at 60 °C.
After cooling to room temperature the solvent was removed from the
precipitating solid by filtration. The brown product was recrystallized
from thf, giving crystals suitable for X-ray analysis. After drying under
reduced pressure the product was obtained with a a yield of 176 mg
(20%). Anal. Calcd (found) for (C25H31Cl2CrN2O)(C4H8O): C, 61.05
(60.95); H, 6.89 (6.96); N, 4.91 (5.27).
Synthesis of Complex 8. A 123 mg portion (338 μmol, 1 equiv)
of N-phenyl-2-(4-(trifluoromethyl)phenyl)quinolin-8-amine, 128 mg
(361 μmol, 1.1 equiv) of CH3CrCl2(thf)3, and 2 mL of thf were stirred
in a closed Schlenk tube with a Teflon valve at 100 °C for 2 h.
Afterward the mixture was cooled to −30 °C and the solvent was
removed from the precipitating solid by filtration. After it was washed
with hexane and dried under reduced pressure, the product was
obtained with a yield of 17.5 mg (9%) as a brown solid. Crystals were
grown by diffusion of hexane into a solution of the complex in thf and
Synthesis of Complex 16. A mixture of 778 mg (1.61 mmol, 1
equiv) of 8-(piperidin-1-yl)quinoline, 1.30 g (3.67 mmol, 1 equiv) of
G
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CH3CrCl2(thf)3, and 21 mL of toluene was stirred for 24 h at room
temperature. Then the solvent was removed from the precipitated
solid by filtration. The yellow product was washed two times with thf
and dried under reduced pressure to give a yield of 1.03 g (66%). Anal.
Calcd (found) for C15H19Cl2CrN2 (16-C4H8O): C, 51.44 (51.44); H,
5.47 (5.21); N, 8.00 (8.11).
Dissolution of the yellow complex (without coordinated thf) in thf
leads to a green solution which upon cooling to −30 °C gives crystals
suitable for X-ray analysis. The green compound contains one
coordinated thf. The catalytic experiments have been performed
with the yellow complex 16-thf.
Synthesis of Complex 17. A mixture of 850 mg (4.00 mmol, 1
equiv) of 8-(piperidin-1-yl)quinoline and 1.50 mg (4.00 mmol, 1
equiv) of CrCl3(thf)3 in 10 mL of thf was stirred under reflux for 1.5 h.
After cooling to room temperature the solvent was removed from the
precipitating solid by filtration. The gray product so obtained was
washed with thf. After drying under reduced pressure the product was
obtained in a yield of 1.37 g (77%). Crystals suitable for X-ray analysis
were obtained by diffusion of heptane into a solution of the complex in
thf. Anal. Calcd (found) for C19H27Cl2CrN2O: C, 48.83 (48.88); H,
5.46 (5.14); N, 6.33 (7.26).
J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet. Chem.
2004, 689, 3641. (e) Agapie, T. Coord. Chem. Rev. 2011, 255, 861.
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Synthesis of Complex 18. A 3.4 mL portion of a 2 M solution of
p-tolylMgCl in thf (6.80 mmol, 3 equiv) was dropped slowly into a
suspension of 1.00 g of 17 (2.26 mmol, 1 equiv) in 16 ml of thf at 0
°C. After the addition was complete, the mixture was stirred for 2 h at
room temperature. The solvent was removed from the precipitating
solid by filtration. The dark green product was dried under reduced
pressure, giving a yield of 980 mg (81%). Further purification by
recrystallization from toluene gave 465 mg of the very pure product.
Crystals suitable for X-ray analysis were obtained by diffusion of
heptane into a solution of the complex in toluene. Anal. Calcd (found)
for C35H37CrN2: C, 78.18 (78.49); H, 6.94 (7.09); N, 5.21 (5.04).
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Morgan, D. H.; Neveling, A.; Otto, S.; Overett, M.; Slawin, A. M. Z.;
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Chem. Commun. 2005, 620. (d) Overett, M. J.; Blann, K.; Bollmann,
A.; Dixon, J. T.; Hess, F.; Killian, E.; Maumela, H.; Morgan, D. H.;
Neveling, A.; Otto, S. Chem. Commun. 2005, 622. (e) McGuinness, D.
S.; Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann,
A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125,
5272−5273. (f) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.;
Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238.
(g) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving instrumentation and
general experimental procedures, general synthetic procedures,
synthesis and analytical data of ligands 1−6 and 14,
identification of organic byproducts during initiation of catalytic
trimerization, data from X-ray diffraction analysis, the molecular
structure of 8 showing the disordered molecules, and details of
DFT calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
Commun. 2003, 334. (i) Maas, H.; Mihan, S.; Kohn, R.; Seifert, G.;
̈
Tropsch, J. US 6844290 B1, 2005. (j) Mihan, S.; Molnar, F.; Maas, H.;
Prinz, M. WO 03/076367 A2, 2003. (k) Mihan, S.; Maas, H. WO 03/
076368 A1, 2003. (l) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.;
̈
̈
Grimm, S.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39
(23), 4337. (m) Kohn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem.
̈
Commun. 2000, 1927. (n) Kohn, R. D.; Smith, D.; Mahon, M. F.;
̈
AUTHOR INFORMATION
Prinz, M.; Mihan, S.; Kociok-Kohn, G. J. Organomet. Chem. 2003, 638,
̈
■
200.
Corresponding Author
(10) Emrich, R.; Heinemann, O.; Jolly, P. W.; Krueger, C.;
Verhovnik, G. P. J. Organometallics 1997, 16, 1511.
(11) (a) Agapie, T.; Schofer, S. J.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2004, 126, 1304. (b) Agapie, T.; Day, M. W.; Henling, L.
M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2006, 25, 2733.
(c) Agapie, T.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2007,
129, 14281.
(12) Monillas, W. H.; Young, J. F.; Yap, G. P. A.; Theopold, K. H.
Dalton Trans. 2013, 42, 9198.
(13) Schofer, S. J.; Day, M. W.; Henling, L. M.; Labinger, J. A.;
Bercaw, J. E. Organometallics 2006, 25, 2743.
(14) Temple, C. N.; Gambarotta, S.; Korobkov, I.; Duchateau, R.
*M.E.: tel, +49-6221-546247; fax, +49-6221-541616247; e-mail,
markus.enders@uni-heidelberg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the Federal
Ministry of Education and Science (BMBF) under the project
funding reference number 03X3565B. We thank our cooper-
ation partners Basell Polyolefine GmbH (Germany) and the
groups from the Universities of Freiburg and Konstanz (Rolf
Organometallics 2007, 26, 4598.
(15) Temple, C.; Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov,
I.; Duchateau, R. Angew. Chem., Int. Ed. 2006, 45, 7050.
(16) Kurras, E. Monatsber. Dt. Akad. Wiss. 1963, 5, 378.
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Organomet. Chem. 1972, 37, 317.
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1966, 88 (18), 4287.
Mulhaupt and Stefan Mecking, respectively) for fruitful
̈
interactions within our joint research project.
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