Ethylene polymerization catalyzed by pyrene-tagged iron complexes: The positive effect of π-conjugation and immobilization on multiwalled carbon nanotubes

Article abstract of DOI:10.1002/cctc.201301063

2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-[1-(pyren-1-ylimino)ethyl] pyridine (L1) and 2,6-bis[1-(pyren-1-ylimino)ethyl]pyridine (L2) were synthesized and used to prepare the corresponding Fe complexes Fe1 and Fe2 from FeCl₂. All new compounds

Full text of DOI:10.1002/cctc.201301063

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DOI: 10.1002/cctc.201301063
Ethylene Polymerization Catalyzed by Pyrene-Tagged Iron
Complexes: The Positive Effect of p-Conjugation and
Immobilization on Multiwalled Carbon Nanotubes
Liping Zhang,[a, b] Wenjuan Zhang,[b] Philippe Serp,[a] Wen-Hua Sun,*[b] and Jꢀrꢁme Durand*[a]
2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-[1-(pyren-1-ylimino)-
ethyl]pyridine (L1) and 2,6-bis[1-(pyren-1-ylimino)ethyl]pyridine
(L2) were synthesized and used to prepare the corresponding
Fe complexes Fe1 and Fe2 from FeCl2. All new compounds
were characterized, and single-crystal XRD analysis of Fe1 was
performed. The p-conjugated pyrenyl substituent enhanced
the catalytic activity of the Fe complexes, which display high
activity in ethylene polymerization up to 107 gPE molFeÀ1 hÀ1
(PE=polyethylene). These Fe complexes were easily immobi-
lized on multiwalled carbon nanotubes (MWCNTs) through
noncovalent p–p interactions. The supported precatalysts
showed better ethylene polymerization activity than their ho-
mogeneous counterparts to produce well-dispersed MWCNT/
PE composite materials.
Introduction
Multiwalled carbon nanotubes (MWCNTs) exhibit a number of
attractive features, such as good electronic and thermal con-
ductivity, which have led to important applications in various
fields of chemical and material sciences over the last few
years.[1] In spite of the growing interest generated by carbon
nanotubes (CNTs) as a support in catalytic applications,[2] re-
ports on the immobilization of discrete single-site transition-
metal-based catalysts remain scarce.[3–9] Among these reports,
some mention the beneficial influence of carbon nanomaterials
on the performances of early-transition-metal-based olefin
polymerization catalysts.[4] In particular, a combination of steric
and electronic effects can result in an increased molecular
weight for polyethylene (PE) produced by MWCNT/metallo-
cene,[5] MWCNT/half-titanocene,[6] or MWCNT/Ziegler–Natta[7]
catalytic systems. Recently, we reported the positive influence
of CNTs or graphene immobilized Ni-based catalysts on ethyl-
ene polymerization.[8] Nanoparticle-supported catalysts effi-
ciently produced PE nanocomposites and allowed good levels
of dispersion and/or improved physical properties.[7,9]
for ethylene activation have attracted increased attention,
both in academic and industrial research.[12] Subsequently, the
2,6-bis(imino)pyridine ligand framework has become the sub-
ject of extensive investigations.[13] The corresponding catalysts
highlighted the important role of the synergy between steric
and electronic effects of the ligands and the catalytically active
metal center. Immobilization of such iron catalysts on various
inorganic supports has been carried out, resulting most of the
time in a reduced catalytic activity compared to the unsup-
ported system[14] with only few examples where high activity is
maintained after immobilization.[15]
If we consider their good electron conductivity and bulki-
ness, CNTs can be a good candidate as macroligands to control
the electronic and steric influences on the reactivity of CNT-im-
mobilized late-transition-metal precatalysts in ethylene poly-
merization. Moreover, the good thermal conductivity of
MWCNTs provides thermostability to the linked molecular spe-
cies; therefore, the CNT-immobilized complex precatalysts
maintain, during the highly exothermic polymerization reac-
tion, a better temperature parameter and hence enhanced ac-
tivities. The anchoring of molecular species through p–p inter-
actions between the graphitic surface and a polyaromatic
group[16] allows the stacking of conjugated molecules on the
intact CNT surface, which maintains its conductive proper-
ties.[17] Moreover, the straightforward p–p interactions afforded
efficient immobilization to avoid common covalent functionali-
zation that requires preliminary chemical modification of the
CNT surface.[18] To perform noncovalent immobilization of com-
plexes on CNTs, a pyrene motif is seen as a valuable and ele-
gant approach.[2d,3g,k,n]
At the end of last century, a series of iron and cobalt com-
plexes with bulky 2,6-bis(imino)-pyridine ligands applied to
olefin polymerization have been discovered by Brookhart[10]
and Gibson[11] Since then, the late transition metal complexes
[a] L. Zhang, Prof. P. Serp, Dr. J. Durand
Laboratoire de Chimie de Coordination UPR CNRS 8241
composante ENSIACET
Universitꢀ de Toulouse
4 allꢀe Emile Monso-CS 44362, 31030 Toulouse Cedex 4 (France)
Fax: (+33)534323596
E-mail: jerome.durand@ensiacet.fr
[b] L. Zhang, Dr. W. Zhang, Prof. W.-H. Sun
Key laboratory of Engineering Plastics
and Beijing National Laboratory for Molecular Science
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (China)
Although CNT-supported Fe complexes have already been
reported for other catalytic reactions,[19] there is, to the best of
our knowledge, no report that deals with ethylene polymeri-
zation by CNT-immobilized Fe precatalysts. Pyrene-tagged
2,6-bis(imino)pyridine derivatives were synthesized and used
E-mail: whsun@iccas.ac.cn
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to form their Fe complexes, which were immobilized onto
MWCNTs through p stacking. All these Fe complexes showed
high activities, in both homogenous and heterogeneous man-
ners, towards ethylene polymerization, in which the CNT-im-
mobilized precatalysts can exhibit even higher activity. Herein
the synthesis and characterization of these complexes and
their immobilization on MWCNTs are reported as well as their
performances in ethylene polymerization.
Figure 1. ORTEP representation of the molecular structure of Fe1. Thermal
ellipsoids are shown at 30% probability. H atoms have been omitted for
clarity. Selected bond lengths [ꢃ] and angles [8]: Fe1ÀN1 2.296(5), Fe1ÀN2
2.083(4), Fe1ÀN3 2.247(5), Fe1ÀCl1 2.2723(18), Fe1ÀCl2 2.3000(19), N1ÀFe1À
N2 73.28(19), N1ÀFe1ÀN3 146.22(19), N2ÀFe1ÀN3 73.05(17), Cl1ÀFe1ÀCl2
115.82(7).
Results and Discussion
Synthesis and characterization of ligands and Fe complexes
The pyrene-tagged 2,6-bis(arylimino)pyridine derivatives 2-[1-
(2,6-diisopropylphenylimino)ethyl]-6-[1-(pyren-1-ylimino)ethyl]-
pyridine (L1) and 2,6-bis[1-(pyren-1-ylimino)ethyl]pyridine (L2)
were synthesized according to a procedure reported previous-
ly[13a,h] (Scheme 1). L1 and L2 reacted individually with a molar
equivalent of FeCl2 in THF to afford the corresponding Fe com-
plexes Fe1 and Fe2 in high yields (Scheme 1). All organic and
complex compounds are air stable, however, the solutions of
the Fe complexes turn slowly from blue to yellow in air, which
indicates the oxidation of iron(II) to iron(III). All compounds
were characterized. In comparison to the IR spectra of L1 and
L2, the C=N stretching vibrations in the spectra of Fe1 and
Fe2 are shifted to lower frequencies (1586 vs. 1639 and
1638 cmÀ1), which indicates effective coordination between
the imino N atoms and the Fe center. The molecular structure
of Fe1 was further confirmed by single-crystal XRD analysis
(Figure 1).
The coordination geometry of the Fe center could be de-
scribed as distorted square-pyramidal, with the basal plane
composed of N1, N2, N3, and Cl1. The Fe center deviates from
the basal plane by 0.6574 ꢃ, and Cl2 deviates by 2.9465 ꢃ on
the same side. The equatorial plane formed by N1, N2, and N3
is almost perpendicular to the imino-pyrene ring (86.18) and to
the 2,6-diisopropylphenyl group (79.18). Two Cl locate on the
different sites from the basal plane at a distance of 1.6508 ꢃ
for Cl1 and 2.1830 ꢃ for Cl2. With regard to the bond lengths
around the Fe center, Fe1ÀCl2 (2.3000(19) ꢃ) is longer than
Fe1ÀCl1 (2.2723(18) ꢃ); the Fe1ÀN1 bond length is similar to
that of Fe1ÀN3 but longer than that of Fe1ÀN2 (ꢀ0.2 ꢃ). This
is partly because N1 and N3 belong to imino groups and N2
belongs to the pyridine ring. A similar geometry was also ob-
served for analogous tridentate iron(II) complexes.[10b,11b,13e–g,20]
The immobilization of Fe1 and Fe2 on MWCNTs was then
performed under a N2 atmosphere by stirring a CH2Cl2 solution
that contained the appropriate complex and a suspension of
the purified CNTs (Scheme 2). The suspension was collected by
filtration and washed with toluene and CH2Cl2 several times
until the filtrate was colorless. X-ray photoelectron spectrosco-
py (XPS) confirmed the presence of only FeII species on the
MWCNT surface without peaks that correspond to Fe0, indica-
tive of the stability of the Fe complexes during the anchoring
process. As a result of the deviation within semiquantitative
analysis by XPS, the exact amount of Fe immobilized on the
MWCNTs was determined by using inductively coupled plasma
mass spectrometry (ICP-MS; 2658 mgFe kgÀ1 for Fe1 and
2909 mgFe kgÀ1 for Fe2).
Scheme 2. Schematic representation of the immobilization of Fe1 on
Scheme 1. Synthetic procedure for L1, L2, Fe1, and Fe2.
MWCNTs.
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Table 1. Ethylene polymerization with Fe1 and Fe2 precatalysts.[a]
[d]
Entry
Catalyst
Cocatalyst
Al/Fe
T [8C]
t [min]
PE [g]
Activity[b]
Tm[c] [8C]
Mw[d] [kgmolÀ1
]
Mw/Mn
1
2
3
4
5
6
7
8
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe2
Fe2
Fe2
Fe2
MAO
1000
1000
1500
2000
2500
3000
2000
2000
2000
2000
2000
2000
2000
1500
2000
2500
2000
2000
30
30
30
30
30
30
40
50
30
30
30
30
30
30
30
30
40
50
30
30
30
30
30
30
30
30
5
10
15
45
60
30
30
30
30
30
5.7
10.5
15.9
19.8
17.3
15.3
16.7
13.8
11.4
13.4
15.2
21.3
22.8
23.7
28.9
22.3
15.5
13.6
2.3
4.2
6.4
7.9
7.0
6.1
6.7
5.5
27.4
16.1
12.2
5.7
4.6
9.5
11.5
8.9
6.2
128.7
124.6
124.7
124.4
124.2
125.3
122.1
127.6
122.4
123.8
124.0
125.8
125.1
120.1
119.9
119.5
122.0
122.7
108
80.3
10.2
9.30
19.8
30.9
n.d.
22.4
75.6
2.40
7.50
8.20
66.5
32.2
2.9
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
21.6
16.3
30.4
39.5
n.d.
38.1
112
9
4.32
14.1
12.4
71.2
38.8
3.57
2.71
3.38
4.39
6.12
10
11
12
13
14
15
16
17
18
3.0
4.1
3.3
5.1
5.4
[a] Reaction conditions: 5 mmol of Fe; 10 atm ethylene; 100 mL toluene. [b] 106 gPE molFeÀ1 hÀ1. [c] Determined by DSC. [d] Determined by GPC.
Ethylene polymerization by the Fe complexes
using Fe1, the Mw and Mw/Mn values of the PEs obtained by
using Fe2 are less sensitive to the polymerization temperature
(Figure 2). The narrower molecular polydispersity of the ob-
tained PEs can probably be attributed to the symmetric struc-
ture of the Fe2 precatalyst.
To both optimize the polymerization parameters and evaluate
the pyrene-induced influence of the ligands, precatalyst Fe1
was explored under various conditions under 10 atm of ethyl-
ene (Table 1). With regard to the different efficiency of cocata-
lysts, methylaluminoxane (MAO) and modified methylalumi-
noxane (MMAO) were tested (Table 1, Entries 1–2), and MMAO
was the best activator (Table 1, Entry 2), which was further
used in the catalytic systems. If we varied the Al/Fe molar ratio
from 1000 to 3000 (Table 1, Entries 2–6), the best activity was
achieved at Al/Fe 2000 (Table 1, Entry 4). The molecular weight
(Mw) and polydispersity (Mw/Mn) values of the obtained PE
gradually increased along with the molar Al/Fe ratio (Table 1,
Entries 3–5). The catalytic activity decreased slightly if the reac-
tion temperature was elevated from 30 to 508C (Table 1, En-
tries 4, 7, 8), due to the lower concentration of ethylene at
a higher temperature. Surprisingly, the molecular weights and
the molecular polydispersity of PE were slightly increased at
higher temperatures (Mw from 30.4–127.6 kgmolÀ1 and Mw/Mn
from 19.8–75.6). The higher molecular weight of the obtained
PE provided information on stable intermediates during poly-
merization; therefore, the catalytic system has better thermo-
stability at an elevated reaction temperature. With regard to
the lifetime of the active species (Table 1, Entries 4, 9–12), the
highest value of 2.74ꢄ107 gPE molFeÀ1 hÀ1 was observed within
5 min (Table 1, Entry 9), and the polymerization was almost fin-
Figure 2. GPC curves of PE produced by Fe2 at different temperatures (En-
tries 15, 17, and 18 in Table 1).
According to the observed data, the current complex preca-
talysts generally showed higher activities than their analog-
s,[10a,11,21] which illustrates the positive influence of the pyrene-
tagged ligands on the Fe complexes. PEs with lower molecular
weights were reported by using Fe complexes that bear lig-
ands that lack ortho-substituted aryl groups;[10b,c,11d,13g] al-
though there is no substituent on 1-aminopyrene, the current
precatalysts produced PEs with higher molecular weights than
their analogues. The pyrene motif played an important role as
a bulky aryl group, which was confirmed by the replacement
of the 2,6-diisopropylphenyl moiety that forms Fe1 by
a pyrene fragment within Fe2 to result in a higher activity in
ethylene polymerization.
ished after 30 min (catalytic activity of 7.9ꢄ106 gPE molFeÀ1 hÀ1
Table 1, Entry 4).
;
Subsequently, the precatalyst Fe2 was also explored by
using MMAO as the cocatalyst (Table 1, Entries 14–18), which
confirmed the optimal Al/Fe molar ratio of 2000 at 308C
(Table 1, Entry 15). Meanwhile, the activity was 1.15ꢄ
107 gPE molFeÀ1 hÀ1 (Table 1, Entry 15), which makes Fe2 approxi-
mately 1.5 times more active than Fe1 (Table 1, Entry 4). In ad-
dition, compared to the properties of the PEs obtained by
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Polymerization using the MWCNT-supported Fe precatalysts
the supported systems, which evidences that the MWCNTs are
not able to stabilize the active species at higher temperatures
because of weak p–p interactions. Moreover, the Mw and
Mw/Mn values are also influenced by the temperature: higher
molecular weights and broader distributions are observed at
elevated reaction temperatures for MWCNT-Fe1 (Figure 3).
Subsequently, the polymerization behavior of MWCNT-support-
ed Fe precatalysts was conducted in the presence of MMAO.
The influence of the reaction parameters, which include the Al/
Fe molar ratio and reaction temperature, was studied by using
the MWCNT-Fe1/MMAO system. Upon increasing the Al/Fe
ratio from 1500 to 4000 (Table 2, Entries 1–5), the activity in-
creased sharply to reach a maximum at Al/Fe=3000. This opti-
mum ratio for the MWCNT-supported Fe precatalyst is signifi-
cantly higher than that of the corresponding homogenous cat-
alyst (vide supra). The high MMAO consumption can be attrib-
uted to the interaction force between the CNTs and the Fe
complex that requires a larger amount of cocatalyst for activa-
tion and/or because some of the cocatalyst is adsorbed on the
CNT surface. On the other hand, the best catalytic activity was
reached by the MWCNT-Fe1 system after 30 min (9.11ꢄ
106 gPE molFeÀ1 hÀ1), which was not only higher than that of the
homogeneous Fe1 system (Table 2, Entry 3 vs. Table 1, Entry 4)
but also evidenced a beneficial influence of the MWCNT to
result in a better stability. Charge transfer between the CNTs
and various adsorbed species was reported in the literature,[22]
which includes an example of immobilized precatalysts in eth-
ylene polymerization.[9c]
Figure 3. GPC traces of PE produced by MWCNT-supported Fe1 at different
temperatures (Entries 3, 6, and 7 in Table 2).
In the experiments with various Al/Fe molar ratios, the PEs
obtained by the MWCNT-Fe1 system showed adaptable molec-
ular weights (Mw =5.07–27.6 kgmolÀ1), with narrower molecu-
lar polydispersity (Mw/Mn =2.2–12.1) than that of the homoge-
neous system. However, the best activity for MWCNT-Fe2 was
8.35ꢄ106 gPE molFeÀ1 hÀ1 (Table 2, Entry 9), which was lower
than that of its corresponding homogeneous counterpart
(Table 1, Entry 15). Two pyrene rings are present within the
framework of Fe2, both of which could interact with the
MWCNTs surface to result in a too high steric hindrance
around the Fe center and, therefore, lead to a lower activity
compared to its unsupported counterpart.
Interestingly, the Mw and Mw/Mn values for the PE obtained
by using MWCNT-Fe2 decrease slightly at higher reaction tem-
peratures because two pyrene groups are available to interact
with the MWCNTs. One advantage was the production of PEs
with a narrow polydispersity (Figure 4). This phenomenon is
opposite to the trend observed for MWCNT-Fe1.
During the polymerization process, PE nanocomposites were
formed by the direct growth of the polymeric chains from the
MWCNTs surface as a result of its interaction with the catalyti-
cally active species. Surface-initiated polymerization processes
have been reported in some cases as an efficient way to dis-
perse CNTs into PE matrices homogeneously.[6–9]
The optimum molar ratio of Al/Fe (3000) was used to investi-
gate the influence of the reaction temperature (Table 2, En-
tries 3, 6, and 7 for MWCNT-Fe1 and Entries 9, 11, and 12 for
MWCNT-Fe2). The elevated reaction temperature resulted in
a lower catalytic activity, even more pronounced in the case of
The polymeric materials obtained by using MWCNT-support-
ed Fe catalysts were analyzed by SEM and TEM, which evi-
denced a good level of dispersion inside the PE matrix of the
individually separated MWCNTs (Figure 5a). If some of the
Table 2. Ethylene polymerization with MWCNT-Fex precatalysts/MMAO.[a]
[d]
Entry
Catalyst
Al/Fe
T [8C]
t [min]
PE [g]
Activity[b]
Tm[c] [8C]
Mw[d] [kgmolÀ1
]
Mw/Mn
1
2
3
4
5
6
7
8
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe1
MWCNT/Fe2
MWCNT/Fe2
MWCNT/Fe2
MWCNT/Fe2
MWCNT/Fe2
1500
2500
3000
3500
4000
3000
3000
2500
3000
4000
3000
3000
30
30
30
30
30
40
50
30
30
30
40
50
30
30
30
30
30
30
30
30
30
30
30
30
3.8
5.4
10.8
6.7
5.1
6.2
3.21
4.57
9.11
5.65
4.30
5.23
2.03
5.70
8.35
4.14
4.64
1.98
124.4
124.0
126.4
124.8
123.7
127.5
127.9
122.4
122.4
122.9
121.0
122.2
18.8
6.59
27.6
5.07
18.6
55.5
67.9
11.5
8.32
8.76
4.13
3.88
12.1
3.50
12.6
2.20
11.2
23.6
27.2
8.48
5.61
6.29
2.99
2.38
2.4
7.41
10.86
5.38
6.03
2.58
9
10
11
12
[a] Reaction conditions: 0.05 g of MWCNT-Fex; 10 atm ethylene; 100 mL toluene. [b] 106 gPE molFeÀ1 hÀ1. [c] Determined by DSC. [d] Determined by GPC.
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plasma mass spectrometry. The catalytic screening of these
heterogenized systems evidenced in one case a significant in-
crease of the productivity compared to the analogous unsup-
ported system, which has one pyrene moiety, whereas in an-
other case, a lower production of polyethylene with a narrower
polydispersity was obtained as a result of the presence of two
pyrene moieties for interaction. This evidences the noninno-
cent role of the MWCNTs in the polymerization process. Finally,
the resultant materials show a good dispersion of the MWCNTs
into the polyethylene matrix, which results from the surface-in-
itiated polymerization reaction.
Experimental Section
Figure 4. GPC traces of PE produced by MWCNT-Fe2 at different tempera-
tures (Table 2, Entries 9, 11, and 12).
General considerations
All manipulations of air- and/or moisture-sensitive compounds
were performed under a N2 atmosphere by using standard Schlenk
and glovebox techniques. Methylaluminoxane (MAO, 1.46m solu-
tion in toluene) and modified methylaluminoxane (MMAO, 1.93m
in heptane, 3 A) were purchased from Akzo Nobel Corp.
High-purity ethylene was obtained from Beijing Yansan Petrochem-
ical Co. FeCl2, 1-aminopyrene, and 2,6-diacetylpyridine were pur-
chased from Alfa Aesar. Multiwalled carbon nanotubes (MWCNT;
98% purity, 2% Fe catalyst, XRD: d002 =0.3405nm, BET: 177 m2 gÀ1
)
were synthesized by chemical vapor deposition and purified ac-
cording to previously reported procedures.[24] NMR spectra were re-
corded by using a Brucker AV 400 MHz instrument at ambient tem-
perature using TMS as an internal standard. IR spectra were record-
ed by using a PerkinElmer spectra one spectrometer. Molecular
weights (Mw) and the molecular weight distribution (Mw/Mn) of PEs
were determined by gel-permeation chromatography (GPC) by
using a PL-GPC220 instrument at 1508C with 1,2,4-trichloroben-
zene as the solvent. Differential scanning calorimetry (DSC) traces
and the melting point of polyethylene were obtained from the
second scanning run by using a PerkinElmer DSC-7 at a heating
rate of 108CminÀ1. X-ray photoelectron spectroscopy (XPS) meas-
urements were performed by using a K-alpha Thermo Scientific in-
strument operating with an AlKa source (1486.6 eV). The binding
energies were corrected with respect to the C1s core level fixed at
284.4 eV. ICP-MS measurements were performed at Antellis. SEM
images were obtained on a JEOL JSM 6700 Field Emission Gun
scanning electron microscope.
Figure 5. a) SEM images of MWCNT/PE composites; b) TEM images of the
MWCNT/PE composites after partial removal of PE.
polymeric material is removed from the samples by dissolu-
tion, a particular lamellar structure is observed that shows
a regular dispersion of small quantities of polymeric material
along the MWCNTs (Figure 5b), similar to that observed for the
crystallization of PE around CNTs that act as nucleating
agents.[23]
Synthesis and characterization
Conclusions
Synthesis of 2-[1-(2,6-diisopropylphenylimino)ethyl]-6-[1-
(pyren-1-ylimino)ethyl]pyridine (L1)
Two Fe complexes based on a 2,6-bis(imino)pyridyl framework
that contain a pyrene moiety have been synthesized and char-
acterized. Activated by modified methylaluminoxane, both Fe
complex precatalysts promote ethylene polymerization with
high activities, comparable to that of the most efficient sys-
tems described in the literature,[19a,c,d] which evidences a benefi-
cial influence of the polyaromatic pyrene substituent(s) intro-
duced on the ligands. By using p–p stacking interactions, cata-
lysts supported on multiwalled carbon nanotubes (MWCNTs)
are produced efficiently by impregnating the Fe complexes on
the MWCNTs. Efficient anchoring is confirmed by IR spectrosco-
py, X-ray photoelectron spectroscopy, and inductively coupled
The monoamine 2-[1-(2,6-diisopropylphenylimino)ethyl]-6-acetyl-
pyridine (A) was synthesized by the condensation reaction of
2,6-diacetylpyridine with 2,6-diisopropylaniline according to a previ-
ously reported procedure.[25] A mixture of A (0.97 g, 3 mmol), 1-
aminopyrene (1.5 equiv., 0.98 g, 4.5 mmol), and a catalytic amount
of p-toluenesulfonic acid (0.1 g) in toluene (50 mL) was heated to
reflux for 10 h. After solvent evaporation, the crude product was
purified by column chromatography on silica gel with petroleum
ether/ethyl acetate (50:1 v/v) as the eluent to afford L1 as a yellow
powder in 68% yield. IR (KBr): n˜ =3029 (m), 1639 (s), 1597 (w),
1454 (m), 1362 (m), 1228 (m), 1105 (s), 840 (m), 765 cmÀ1 (m);
3
1H NMR (400 MHz, [D6]acetone, TMS): d=8.72 (dd, 1H, J=4.5 Hz,
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4J=0.8 Hz, pyridine), 8.59 (dd, 1H, J=4.4 Hz, Py), 8.33 (d, 1H, J=
4.1 Hz, pyrene), 8.03- 8.28 (m, 8H, pyrene, Ar), 7.60 (d, 1H, J=
4.1 Hz, Ar), 7.22 (d, 2H, J=4.1, Ar), 7.11 (t, 1H, J=4.5 Hz, Ar), 2.83–
2.91 (m, 2H, CH), 2.48 (s, 3H, CH3), 2.33( s, 3H, CH3), 1.16–1.22 ppm
(m, 12H, CH(CH3)2); 13C NMR (100 MHz, [D6]acetone, TMS): d=
165.4, 164.9, 148.2, 137.9, 137.4, 135.5, 131.8, 127.5, 126.9, 126.3,
125.9, 124.8, 124.6, 123.7, 123.6, 122.9, 122.8, 122.7, 122.6, 122.3,
22.6, 22.1, 16.5, 16.1 ppm.
3
Table 3. Crystal data and structure refinement for Fe1.
Empirical formula
C37H35Cl2 FeN3
Formula weight
T [K]
648.43
173(2)
Wavelength [ꢃ]
Crystal system
Space group
a [ꢃ]
b [ꢃ]
c [ꢃ]
0.71073
Tetragonal
I4(1)/a
23.447(3)
23.447(3)
29.697(6)
90
Synthesis of 2,6-bis[1-(pyren-1-ylimino)ethyl]pyridine (L2)
a [8]
A mixture of 2,6-diacetylpyridine (0.97 g, 2 mmol), 1-aminopyrene
(3 equiv., 1.27 g, 6 mmol), and a catalytic amount of p-toluenesul-
fonic acid (0.1 g) in toluene (50 mL) was heated to reflux for 3 d.
After solvent evaporation, the crude product was purified by
column chromatography on silica gel with petroleum ether/ethyl
acetate (50:1 v/v) as the eluent to afford L2 as a yellow powder in
52% yield. IR (KBr): n˜ =3045 (m), 1638 (s), 1621 (m), 1566 (m), 1449
(w), 1362 (s), 1226 (s), 1119 (m), 848(s), 758 (s), 711 cmÀ1 (m);
1H NMR (400 MHz, CDCl3, TMS): d=8.72 (d, 2H, J=3.8 Hz, Py), 8.20
(d, 2H, J=4.0 Hz, Py), 8.14–8.17 (m, 4H, Ar), 8.05–8.09 (m, 3H, Py
H, Ar H), 8.04–8.07 (m, 6H, Ar), 8.03 (d, 2H, J=3.8 Hz, Ar), 7.39 (d,
2H, J=4.0 Hz, Ar), 2.36 ppm (s, 6H, CH3); 13C NMR (100 MHz, CDCl3,
TMS): d=168.8, 155.5, 145.6, 137.2, 131.7, 131.5, 127.9, 127.4,
126.9, 126.1, 125.9, 125.4, 124.8, 124.6, 122.9, 122.8, 116.6,
17.0 ppm.
b [8]
90
90
16326(5)
16, 1.055
1.345
5408
0.17ꢄ 0.03ꢄ0.03
1.11–25.32
À13ꢁhꢁ28
À15ꢁkꢁ28
À35ꢁlꢁ24
15655
7443, [R(int)=0.0623]
416
99.8%
g [8]
V [ꢃ3]
Z, Dcalcd [gcmÀ3
]
m [mmÀ1
F(000)
]
Crystal size [mm]
q range [8]
Limiting indices
Reflections collected
Independent reflections
Number of parameters
Completeness to q [%]
Goodness of fit on F2
Final R indices [I>2s(I)]
R indices (all data)
1.012
R1=0.0933, wR2=0.2243
R1=0.1547, wR2=0.2551
0.384 and À0.263
Max./min. D1[a] [eꢃÀ3
]
Synthesis of Fe1 and Fe2
The Fe complexes were prepared by the reaction of FeCl2 with L1
or L2 (1 equiv.) in THF at RT for 12 h (Scheme 1). The obtained pre-
cipitate was collected by filtration, washed with diethyl ether, and
dried under reduced pressure. Fe1: Yield: 88%; IR (KBr): n˜ =1621
(w), 1586 (s), 1463 (m), 1436 (m), 1371 (s), 1104 (m), 845 (s), 799
(m), 713 cmÀ1 (m); elemental analysis calcd (%) for C37H35Cl2FeN3
(647.16): C 68.53, H 5.44, N 6.48; found C 68.83, H 5.43, N 6.38.
Fe2: Yield: 82%; IR (KBr): n˜ =1623 (w), 1586 (s), 1504 (w), 1486 (w),
1369 (m), 1265(m), 1186 (w), 843 (s), 710 cmÀ1 (m).
were performed by using the SHELXL-97 package.[26] Details of the
X-ray structure determinations and refinements are provided in
Table 3. CCDC 951611 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Procedure for ethylene polymerization
Homogeneous polymerization
Immobilization of Fe complexes on MWCNTs
Ethylene polymerization at 10 atm ethylene pressure was per-
formed by using a 250 mL stainless-steel autoclave equipped with
a mechanical stirrer and a temperature controller. Toluene, the de-
sired amount of Fe precatalyst, and a toluene solution of the coca-
talyst (to achieve a total volume of 100 mL) were added to the re-
actor in this order under an ethylene atmosphere. When the de-
sired reaction temperature was reached, the ethylene pressure was
increased to 10 atm, and maintained at this level by a constant
feed of ethylene. After the desired reaction time, the reaction was
quenched by addition of acidic ethanol. The precipitated polymer
was washed with ethanol and water several times and dried in
vacuum.
Prior to use, the MWCNTs were purified and cut. Under an N2 at-
mosphere, CH2Cl2 (100 mL) was added to a mixture of MWCNTs
(3 g) and Fe1 or Fe2 (0.15 g). The resulting suspension was stirred
for 6 h at RT. After filtration, the black powder was fully washed by
toluene and CH2Cl2 and then dried under vacuum for 2 d. The
amount of Fe complex anchored on the MWCNT (2658 mgFekgÀ1
for Fe1 and 2909 mgFekgÀ1 for Fe2) was determined by ICP-MS
taking into account the amount of Fe present (residual catalyst) in
the unfunctionalized samples.
X-ray crystallographic studies
Crystals of Fe1 suitable for XRD analysis were obtained by layering
diethyl ether onto a solution of Fe1 in methanol at RT on a Rigaku
Saturn724+ CCD. With graphite-monochromated MoKa radiation
(l=0.71073 ꢃ) at 173(2) K, cell parameters were obtained by
global refinement of the positions of all collected reflections. Inten-
sities were corrected for Lorentz and polarization effects and em-
pirical absorption. The structures were solved by direct methods
and refined by full-matrix least squares on F2. All H atoms were
placed in calculated positions. Structure solution and refinement
Polymerization using the MWCNT-supported Fe complexes
Heterogeneous polymerizations were performed by first adding
MWCNT-supported catalyst (0.05 g) into the reactor. Then toluene
and the desired amount of cocatalyst (to achieve a total volume of
100 mL) were added to the reactor under an ethylene atmosphere.
The following procedure was the same as that used for homogene-
ous polymerization.
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Acknowledgements
This work was supported by NSFC No. 20911130359 and ANR No.
ANR-09-BLAN-0384.
Keywords: carbon nanotubes
polymerization · supported catalysts
· immobilization · iron ·
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Received: December 11, 2013
Published online on March 5, 2014
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