Novel catalytic system for ethylene oligomerization: An iron(III) complex with an anionic N,N,N ligand

Article abstract of DOI:10.1021/om200197s

We report a novel iron(III)-based catalytic system formed by the addition of an anionic N,N,N ligand to an iron(III) precursor. The complex has been characterized by FT-IR, mass spectrometry, and X-ray diffraction. This precatalyst proved to form a stable, active species for the selective oligomerization of ethylene. Up to 66 wt % of butenes was obtained with a selectivity of 98% in 1-butene.

Full text of DOI:10.1021/om200197s

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Novel Catalytic System for Ethylene Oligomerization: An Iron(III)
Complex with an Anionic N,N,N Ligand
Adrien Boudier,† Pierre-Alain R. Breuil,*,† Lionel Magna,† Claudine Rangheard,† Jꢀerꢀemie Ponthus,†
Hꢀelꢁene Olivier-Bourbigou,*,† and Pierre Braunstein‡
†IFP Energies nouvelles, Rond-point de l'ꢀechangeur de Solaize, 69360 Solaize, France
‡Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universitꢀe de Strasbourg, 4 rue Blaise Pascal,
F-67081 Strasbourg Cedex, France
S Supporting Information
b
ABSTRACT: We report a novel iron(III)-based catalytic system formed by the addition of an anionic
N,N,N ligand to an iron(III) precursor. The complex has been characterized by FT-IR, mass
spectrometry, and X-ray diffraction. This precatalyst proved to form a stable, active species for the
selective oligomerization of ethylene. Up to 66 wt % of butenes was obtained with a selectivity of 98%
in 1-butene.
inear R-olefins (LAOs) are of considerable importance in the
Scheme 1. Synthesis of the Iron(III) Complex 1
L
chemical and petrochemical industries. They represent an
expanding market with a total demand of 4.2 million tons in
2006. The demand for LAO is growing faster in the C4ꢀC10
range than in the C12þ range. Light LAOs (C4ꢀC8) are mainly
used as comonomers in the copolymerization of ethylene to
produce high-density polyethylene and linear low-density poly-
ethylene. Whereas several processes lead from short (C4ꢀC8) to
full range (C6ꢀC30) distributions in LAOs,1 such as the Shell
Higher Olefin Process (SHOP), the Gulftene process (Chevron-
Phillips), the Ethyl process (Ineos), and the Idemitsu process,
some processes are highly selective for 1-butene (Alphabutol2)
or 1-hexene formation (Alphahexol1c and Phillips3 processes).
Although good iron catalysts for ethylene polymerization are
known,4 the initial discoveries of Brookhart5 and Gibson6
triggered considerable interest in iron(II) complexes with neutral
tridentate N,N,N ligands as precursors to highly active and
selective catalysts for ethylene oligomerization.7 The exact
mechanism of olefin polymerization/oligomerization and the
determination of whether the active species formed by treatment
of the catalyst precursor with methylaluminoxane (MAO) is an
iron(II) or an iron(III) species remain under discussion.8 The
few catalytic systems reported so far using iron(III) precursors
led to good activities but modest selectivities for light C4ꢀC12
LAO (SchulzꢀFlory constant ca. 0.6).9 Mostly neutral tridentate
N,N,N ligand-based systems have been reported, but a larger
diversity of ligands is crucial if one wishes to optimize an iron-
based oligomerization process. Anionic ligands associated with
iron(II)8b,10 or iron(III)11 precursors have rarely been reported,
and they afforded inactive or poorly active systems, suggesting
that an electron-rich central donor group might be detrimental to
catalytic activity.8b We disclose here a novel, well-defined
iron(III)-based catalytic system obtained by treatment of an
anionic ligand with an iron(III) precursor. This remarkably stable
catalyst precursor affords interesting selectivity in the oligomer-
ization of ethylene to short-chain linear R-olefins.
Synthesis and Characterization of an Iron Complex Bearing
2-Methyl-2,4-bis(pyridin-2-yl)-1,2-dihydro-1,10-phenanthro-
line. Ligand L, previously reported by our group,12 was unexpect-
edly obtained by the condensation of 2 equiv of 2-acetylpyridine
with1 equivof8-aminoquinoline. LigandL was deprotonatedwith
n-BuLi, and an in situ reaction with an equimolar THF solution of
FeCl3 afforded the iron(III) complex 1 in excellent yield (>95%)
(Scheme 1).
This complex shows good stability in the solid state and in
solution, and crystals suitable for X-ray diffraction were grown by
diffusion of pentane into a chlorobenzene solution under an inert
atmosphere at room temperature (Figure 1). The shift of the
νCdN absorption band of the imino group to lower wavenumbers
in the FT-IR spectrum, which becomes weaker in comparison
with that of the free ligand (1630 to 1603 cmꢀ1, see the
Received: March 3, 2011
Published: April 18, 2011
r
2011 American Chemical Society
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Table 1. Iron-Catalyzed Oligomerization of Ethylenea
oligomer distribnc,d
C4 (1-C4)e C6 (1-C6)e C8þ PE
f
entry cocatalyst (amt, equiv) activityb
1
2
3
MAO (200)
MAO (500)
2.16
2.13
63 (>97) 18 (>90)
61 (>95) 19 (>87)
66 (>98) 16 (>93)
7
6
7
12
14
12
MAO/TMA (200/20) 1.34
a Reaction conditions: toluene (50 mL), Fe (20 μmol), ethylene
pressure 30 bar, 80 °C, reaction time 2 h. b In units of 105 g of products
(mol of Fe)ꢀ1 hꢀ1 estimated over the steady period of ethylene
c
consumption. Determined by GC. d In units of wt % among all the
products formed. e In units of wt % in the Cn fraction. f Fraction C8 and
higher oligomers.
Figure 1. ORTEP of the iron(III) complex 1 in 1 C6H5Cl (H omitted
3
for clarity). Selected bond lengths (Å) and angles (deg): Fe(1)ꢀN(1) =
2.167(3), Fe(1)ꢀCl(1) = 2.2588(9), Fe(1)ꢀN(2) = 1.950(2), Fe(1)ꢀ
Cl(2) = 2.2399(8), Fe(1)ꢀN(3) = 2.158(3); N(1)ꢀFe(1)ꢀN(3) =
148.56(9), N(1)ꢀFe(1)ꢀCl(1) = 93.55(7), N(1)ꢀFe(1)ꢀN(2) =
77.42(10), N(3)ꢀFe(1)ꢀCl(1) = 97.25(7), N(1)ꢀFe(1)ꢀCl(2) =
104.75(7), N(2)ꢀFe(1)ꢀCl(1) = 141.27(8), N(3)ꢀFe(1)ꢀCl(2) =
99.95(7), Cl(2)ꢀFe(1)ꢀCl(1) = 107.60(3), N(2)ꢀFe(1)ꢀCl(2) =
111.12(8).
Scheme 2. Iron Complexes Tested in Ethylene
Oligomerization
Supporting Information),7d,9a confirms the effective coordina-
tion of the anionic ligand on the iron center. The disappearance
of the NꢀH band of the free ligand at 3386 cmꢀ1 after
deprotonation and complexation to the iron(III) precursor
support the anionic character of the ligand. In 1, the metal is
pentacoordinated with a pseudo-square-pyramidal coordination
geometry (Figure 1). The methyl (C13) and the uncoordinated
pyridyl groups are almost perpendicular to the plane defined by
the iron atom and the three coordinated nitrogen atoms (N1, N2,
and N3). The metrical parameters confirm the þIII oxidation
state of the metal, the short FeꢀN2 bond distance of 1.950(2) Å
being indicative of a covalent bond.8b,13 The molecular formula is
confirmed by ESI-FT/MS with the exact mass measurement of
the M•þ ion (mass accuracy 0.04 ppm; see the Supporting
Information).
For comparison, other iron(II) or iron(III) complexes bearing
the dihydro-1,10-phenanthroline ligand (in 2 and 3), or its
deprotonated form (in 4), have been evaluated (Scheme 2),
but none of them displayed activity toward ethylene oligomer-
ization. These results reveal that for the ligand family studied
both the anionic character of the ligand (complexes 2 and 3 vs
complex 1) and the þIII oxidation state of the metal center
(complexes 2 and 4 vs complex 1) are key parameters for
obtaining an active catalyst.
In the course of our investigations on the synthesis of 1, we
obtained some dark violet crystals by slow diffusion of Et2O into
a MeCN solution of the reaction mixture. An X-ray diffraction
Ethylene Oligomerization by Iron(III) Catalyst. The opti-
mized procedure for conducting the ethylene oligomerization
experiments is to add the MAO as activator at room temperature
under ethylene to the reactor containing the complex solution.
No exotherm was observed at a precatalyst concentration of
0.4 mM. Then the temperature was increased to 80 °C with an
adjustment to the desired pressure (30 bar; see the Supporting
Information). The precatalyst 1 shows a good and stable activity
in the presence of MAO at the molar ratio Al/Fe = 200 (Table 1,
entry 1). Ethylene consumption was steady over 2 h with an
activity of 2.16 ꢁ 105 g of products (mol of Fe)ꢀ1 hꢀ1. Short-
chain oligomers were obtained, up to 63 wt % of butenes with a
selectivity in 1-butene of >97 wt % (Table 1, entry 1). Increasing
the MAO concentration to reach the ratio Al/Fe = 500 led to a
similar activity and a slight increase in polymer formation (14 wt
%, Table 1, entry 2). With the alkylating agent trimethylalumi-
num (TMA) and diethylaluminum chloride (DEAC), tested at
the ratio Al/Fe = 200, no consumption of ethylene was observed.
Thus, MAO is crucial for activating the precatalyst. When
additional TMA was used with MAO in the molar ratio MAO/
TMA/Fe = 200/20/1, the activity slightly decreased and up to
66 wt % of butenes was produced with a fraction of 1-butene
of >98 wt %.
analysis revealed for 5 Et2O the unexpected structure shown in
3
Figure 2. Two oxo bridges connect two dinuclear units which
result from a CꢀC coupling reaction between two phenanthro-
line moieties at the para position of their central ring. Each
Fe(III) metal center carries a terminal chloride ligand and is
pentacoordinated. A symmetry axis passing through the oxygen
atoms relates the halves of the molecule (for details, see the
Supporting Information). Although the quantity of crystals
obtained was too low to perform the usual analyses and the
experimental conditions required to obtain this complex are not
yet clearly identified, we believe that its structure deserves
attention and conveys an important caveat about the importance
of carefully establishing the nature and structure of complexes
used as precatalysts.
In conclusion, we have presented here the first catalytic system
with a fully characterized iron(III) precursor bearing an anionic
nitrogen donor ligand for the highly selective oligomerization of
ethylene to short-chain oligomers (C4ꢀC6). The catalyst ex-
hibits high activity (up to 2.16 ꢁ 105 g (mol of Fe)ꢀ1 hꢀ1) and
high stability with time, and it requires only a modest amount of
MAO (Al/Fe = 200). Up to 66 wt % of butenes was obtained with
a selectivity of >98 wt % in 1-butene. The synergistic influence of
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Figure 2. View of the structure of complex 5 in 5 Et2O (H omitted for
3
clarity). Selected bond distances (Å) and angles (deg): Fe(1)ꢀO(1) =
1.780(3), Fe(2)ꢀO(1) = 1.787(3), Fe(1)ꢀN(2) = 1.978(3), Fe(2)ꢀ
N(5) = 1.968(3), Fe(1)ꢀN(1) = 2.178(3), Fe(2)ꢀN(4) = 2.187(4),
Fe(1)ꢀN(3) = 2.180(3), Fe(2)ꢀN(6) = 2.194(3), Fe(1)ꢀCl(1) =
2.2496(14), Fe(2)ꢀCl(2) = 2.2489(14); Fe(1)ꢀO(1)ꢀFe(2) =
139.27(16).
the anionic character of the ligand and the þIII oxidation state of
the iron precursor on the catalytic activity have been established.
We are currently optimizing such systems and investigating the
potential of other iron(III) complexes bearing anionic ligands for
the catalytic oligomerization of ethylene.
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, and tables giving
b
general considerations, complex syntheses, IR spectra of the
iron(III) complexes 1ꢀ5, mass spectra analyses, the ethylene
oligomerization procedure, and details concerning 5 and CIF
files giving the crystal structures of 1 and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) Rangheard, C.; Proriol, D.; Olivier-Bourbigou, H.; Braunstein,
P. Dalton Trans. 2009, 770.
(13) Balogh-Hergovich, E.; Speier, G.; Reglier, M.; Giorgi, M.;
Kuzmann, E.; Vertes, A. Eur. J. Inorg. Chem. 2003, 1735.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: pierre-alain.breuil@ifpen.fr (P.-A.R.B.); helene.
olivier-bourbigou@ifpen.fr (H.O.-B.).
’ ACKNOWLEDGMENT
We thank IFP Energies nouvelles for the permission to publish
these results and for their support and Dr. E. Jeanneau (Lyon,
France) and Dr. Roberto Pattacini (Strasbourg, France) for
determining the X-ray crystal structures of 1 and 5, respectively.
’ REFERENCES
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