Synthesis, structure, magnetic and catalytic properties of new dinuclear chromium(iii) complexes with oxazoline alcoholate ligands

Article abstract of DOI:10.1039/b809814j

The dinuclear chromium complexes [Cr₂(N,O)₃Cl ₃] (6) (N,O = 4,4-dimethyl-2-oxazolylmethanolate), [Cr ₂(N,O^Me2)₂(EtOH)₂Cl₄] (7) and [Cr₂(N,O^Me2)

Full text of DOI:10.1039/b809814j

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structure, magnetic and catalytic properties of new dinuclear
chromium(III) complexes with oxazoline alcoholate ligands†
Suyun Jie,a,b Roberto Pattacini,a Guillaume Rogez,c Claudia Loose,d Jens Kortusd and Pierre Braunstein*a
Received 10th June 2008, Accepted 29th August 2008
First published as an Advance Article on the web 22nd October 2008
DOI: 10.1039/b809814j
The dinuclear chromium complexes [Cr2(N,O)3Cl3] (6) (N,O = 4,4-dimethyl-2-oxazolylmethanolate),
[Cr2(N,OMe2)2(EtOH)2Cl4] (7) and [Cr2(N,OMe2)2(H2O)2Cl4] (8) (N,OMe2 = 4,4-dimethyl-2-
oxazolyldimethylmethanolate) have been prepared and characterized, including by single-crystal X-ray
diffraction. Complex 6 is unsymmetrical, with two chloride ligands terminally bound to one Cr atom,
whereas 7 and 8 (in 8·C4H8O) which contain two molecules of coordinated ethanol or water,
respectively, are centrosymmetric. These chromium complexes are paramagnetic, and the magnetic
properties of 6 and 7 in the solid state correspond to antiferromagnetic behaviour, which was confirmed
by DFT calculations of their electronic structures. Complexes 6–8 were evaluated in the catalytic
oligomerization and/or polymerization of ethylene with different aluminium-based cocatalysts, and
MMAO proved to be the most effective one. In the presence of MMAO, the influence of different
reaction parameters, such as the Al/Cr molar ratio, reaction temperature and ethylene pressure, was
investigated. Complex 7 showed the highest activity for ethylene polymerization at both 1 atm and 10
atm of ethylene pressure, up to 620 000 g mol-1(Cr) h-1 in the latter case.
nickel complexes containing the chelating oxazoline alcohol
ligands (N,OH) are also active for ethylene oligomerization with
Introduction
MAO or AlEtCl2 as cocatalyst.8 In contrast, cobalt complexes
with NOP- or NOPON-type ligands were found to have moderate
activity for ethylene oligomerization.9 It appeared to us interesting
to prepare and study chromium complexes containing such NOP-
or NOPON-type ligands for the catalytic oligomerization and/or
polymerization of ethylene. Although our initial objective was
hampered by P–O bond cleavage during the preparation or re-
crystallization of the complexes, the resulting dinuclear chromium
complexes with anionic N,O oxazoline alcoholate are of great
interest and we could subsequently prepare them directly from the
corresponding oxazoline alcohol.
While the Cr/SiO2 Phillips catalyst, including its gas-phase Union
Carbide UNIPOL version, is used to produce more than one-third
of the polyethylene sold worldwide,1 significant improvements
in activities for non-metallocene homogeneous transition-metal
olefin oligomerization and polymerization catalysts have recently
been achieved.2 In recent years, Cp-free chromium complexes have
attracted much attention as efficient and selective homogeneous
catalysts, with the expectation that variation of their ligands
could further improve their performances.3 In particular, various
diphosphinoamine ligands with different substituents on both
the N and P atoms have led to highly active and selective
chromium precatalysts for the trimerization4 and, more recently,
tetramerization5 of ethylene.
Transition metal complexes with chelating P,N bidentate or
N,P,N tridentate ligands containing oxazoline heterocycles, of e.g.
Ru(II), Ni(II) and Pd(II), are efficient precatalysts in a number
of reactions.6 Nickel complexes bearing bidentate NOP-type or
tridentate NOPON-type ligands (see below) are very active for
the dimerisation of ethylene in the presence of small amounts of
cocatalyst (2 or 6 equiv. of AlEtCl2).7 Hexacoordinated dinuclear
Results and discussion
Synthesis and characterization
The oxazoline alcohols
oxazolylmethanoxy)diphenylphosphine ligand 3 (NOP)11 and
1
and 2,10 the (4,4-dimethyl-2-
the bis(oxazolinyl)phenylphosphonite ligands 4 (NOPON) and
12
5 (NOPONMe2
)
were prepared according to the literature
(Scheme 1).
aLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070, Strasbourg
Cedex, France. E-mail: braunstein@chimie.u-strasbg.fr
The reactions of [CrCl3(THF)3] with ligand 3 (NOP) or ligand
4 (NOPON) in dichloromethane produced the same green powder
and the corresponding green single crystals (see below) were
analyzed by X-ray diffraction.† This indicated that cleavage of P–O
bond(s) had taken place to yield a new, unsymmetrical dinuclear
complex 6 [eqn (1)]. The latter was then prepared in high yield
in a more rational way by reaction of the oxazoline alcoholate
bKey Laboratory of Engineering Plastics and Beijing National Laboratory for
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, China
cIPCMS-UMR 7504, Groupe des Mate´riaux Inorganiques, 23 rue du Loess,
B.P. 43, F-67034, Strasbourg Cedex 2, France
dInstitut fu¨r Theoretische Physik, TU Bergakademie Freiberg, Leipziger Str.
23, D-09599, Freiberg, Germany
derived from 1 with [CrCl3(THF)3] in THF [eqn (1)]. The infrared
-1
=
n(C N) stretching bands of complex 6 at 1653, 1646 and 1624 cm
† CCDC reference numbers 690755–690757. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b809814j
=
correspond to three different C N bonds, and these values are
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Addition of NaH to the mixture containing the oxazoline
alcohol 2 and [CrCl3(THF)3] in THF resulted in the disappearance
of the initial green precipitate and a dramatic color change from
green to purple. After filtration and concentration of the filtrate
under reduced pressure, the residue was treated with diethyl ether
to give a gray–purple powder and a blue–purple solution. A purple
solid was obtained after placing the blue–purple solution at a
lower temperature; it was redissolved in dichloromethane and slow
diffusion of n-pentane into this solution afforded purple crystals
of 8·THF [eqn (3)]. An X-ray structure determination established
the presence of a water molecule coordinated to each chromium
centre in the dinuclear complex 8. The infrared absorptions at
3260 and 1644 cm-1 correspond to the stretching vibration bands
=
of n(O–H) from water and n(C N) respectively.
Scheme 1 Synthesis of the ligands.
shifted to lower wavenumbers when compared to that for the free
oxazoline alcohol 1 (1673 cm-1).
(3)
The single-crystal X-ray structures of complexes 6–8 in the solid
state are shown in Fig. 1–3 and selected bond lengths and angles
are collected in Tables 1–3.
(1)
A similar behaviour was observed in the reaction between ligand
5 (NOPONMe2) and [CrCl3(THF)3] with the scission of the P–O
bonds. The purple crystals obtained by layering a CH2Cl2–EtOH
solution containing 2–5 vol% EtOH with n-pentane were shown
by X-ray diffraction to correspond to a centrosymmetric dinuclear
complex 7, in which an ethanol molecule is coordinated to each
chromium centre [eqn (2)]. Details of the crystal structure are
given below. An alternative synthesis consisting in the reaction of
the oxazoline alcohol 2 with [CrCl3(THF)3] in THF afforded a
green powder, of which the green solution in the CH2Cl2–EtOH
mixture, layered with n-pentane, yielded purple crystals of 7 after
a few days. The infrared stretching vibration bands for n(O–H)
Fig. 1 ORTEP view of the molecular structure of complex 6 with thermal
ellipsoids drawn at the 50% probability level. H atoms omitted for clarity.
Complex 6 (Fig. 1) crystallized in the non-centrosymmetric
space group P212121. Its asymmetric dinuclear molecular structure
shows three alcoholate ligands, one chelating on Cr1 and two on
Cr2. Their oxygen atoms (O1, O3 and O5) further symmetrically
bridge the metal centres and define the face shared by the two
distorted coordination octahedra. The aminoalcoholates thus
=
from ethanol and n(C N) for complex 7 were clearly observed at
3142 and 1645 cm-1 respectively (shift from 1655 cm-1 for the free
oxazoline alcohol 2).
1
2
feature a m-k (O):k (N,O) bonding mode, rather rare in chromium
complexes.13 Dichromium compounds bridged by three alcoholate
ligands are also rather rare.14 The coordination around the metal
centres is completed by two terminal chlorides for Cr1 and one for
Cr2. To minimize the steric repulsion between the methyl groups
in the a position to the nitrogen, the two O, Cr, N chelation planes
on Cr2 form a dihedral angle of 84.23(8)◦. The six Cr–O bonds,
three Cr–N bonds and three Cr–Cl bonds have similar lengths,
(2)
98 | Dalton Trans., 2009, 97–105
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◦
˚
Table 1 Selected bond lengths (A) and bond angles ( ) in complex 6
Bond lengths
Cr1–O1
Cr1–O3
Cr1–O5
Cr1–N1
Cr1–Cl1
Cr1–Cl2
N1–C2
2.008(2)
2.032(2)
2.005(2)
2.044(3)
2.3066(8)
2.3003(9)
1.278(4)
1.274(4)
Cr2–O1
Cr2–O3
Cr2–O5
Cr2–N2
Cr2–N3
Cr2–Cl3
N3–C14
Cr1–Cr2
1.999(2)
1.978(2)
1.999(2)
2.056(3)
2.056(3)
2.3091(8)
1.282(4)
2.7388(6)
N2–C8
Bond angles
O1–Cr1–O3
O1–Cr1–O5
O3–Cr1–O5
N1–Cr1–O1
Cl1–Cr1–Cl2
Cr1–O1–Cr2
Cr1–O3–Cr2
77.80(7)
78.13(7)
77.74(8)
81.308)
92.75(3)
86.24(7)
86.13(8)
O1–Cr2–O3
O1–Cr2–O5
N2–Cr2–O3
N3–Cr2–O5
Cr1–O5–Cr2
79.27(8)
78.45(9)
78.46(7)
81.75(9)
86.32(7)
Fig. 2 ORTEP view of the molecular structure of complex 7 with thermal
ellipsoids drawn at the 50% probability level. H atoms omitted for clarity,
except the OH protons. Selected bond distances and angles are reported in
Table 2. Symmetry operations generating equivalent atoms (¢): -x, -y, -z.
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) in complex 7.
Symmetry operations generating equivalent atoms (¢): -x, -y, -z
Bond lengths
Cr1–N1
Cr1–O1
Cr1–O1¢
Cr1–O3
2.039(2)
1.974(1)
1.984(1)
2.049(2)
Cr1–Cl1
Cr1–Cl2
N1–C2
2.2916(5)
2.3393(6)
1.271(2)
Cr1 ◊ ◊ ◊ Cr1¢
3.0867(5)
Bond angles
O1–Cr1–O1¢
O1–Cr1–N1
O1¢–Cr1–N1
Cl1–Cr1–O1
77.52(5)
80.11(6)
157.55(6)
175.58(4)
Cl1–Cr1–Cl2
Cl2–Cr1–O3
Cr1–O1–Cr1¢
92.68(2)
175.50(4)
102.48(5)
◦
˚
Table 3 Selected bond lengths (A) and bond angles ( ) in complex 8 in
8·THF. Symmetry operations generating equivalent atoms (¢): -x, -y, -z
Fig. 3 ORTEP plot of the molecular structure of centrosymmetric
molecule A of complex 8 in 8·C4H8O with thermal ellipsoids drawn
at the 50% probability level. H atoms omitted for clarity, except the
Bond lengths
◦
˚
water protons. Selected bond distances (A) and angles ( ) are reported
in Table 3. Independent molecule B shows analogous bond lengths and
angles. Symmetry operations generating equivalent atoms (¢): -x, -y, -z.
Cr1A–N1A
Cr1A–O1A
Cr1A–O1A¢
Cr1A–O3WA
Cr1A–Cl1A
Cr1A–Cl2A
N1A–C2A
2.044(2)
1.966(2)
1.986(2)
2.037(2)
2.286(1)
2.352(1)
1.273(4)
3.0894(6)
Cr1B–N1B
Cr1B–O1B
Cr1B–O1B¢
Cr1B–O3WB
Cr1B–Cl1B
Cr1B–Cl2B
N1B–C2B
2.042(3)
1.967(2)
1.983(2)
2.051(2)
2.2890(9)
2.3473(8)
1.267(4)
3.0876(6)
respectively, which are in good agreement with the values typically
found for Cr(III) species. The geometrical constrains due to the
chelating and bridging coordination modes [mean chelation N–
Cr–O angle: 81.4(1)◦, mean O–Cr–O angle: 78.47(9)◦] result, as
mentioned above, in a significantly distorted octahedral geometry
for both metal centres [with the largest deviation of the axial angle
from the ideal 180◦ being for N1–Cr1–O5, 157.74(9)◦]. The three
Cr1A ◊ ◊ ◊ Cr1A¢
Cr1B ◊ ◊ ◊ Cr1B¢
Bond angles
O1A–Cr1A–O1A¢
O1A–Cr1A–N1A
Cl2A–Cr1A–O3wA
Cl1A–Cr1A–Cl2A
Cr1A–O1A–Cr1A¢
77.16(9)
80.28(9)
178.05(8)
92.39(4)
102.84(9)
O1B–Cr1B–O1B¢
O1B–Cr1B–N1B
Cl2B–Cr1B–O3wB
Cl1B—Cr1B–Cl2B
Cr1B–O1B–Cr1B¢
77.16(9)
80.25(9)
178.30(8)
93.29(3)
102.84(9)
=
C N bonds in the oxazoline heterocycles retain a typical double-
˚
bond character with an average bond length of 1.278(4) A. The
˚
Cr–Cr distance of 2.7388(6) A is short enough to be consistent with
a bonding interaction between the metals. Chromium complexes
with short Cr–Cr contacts have been reported in the literature,15
15g
˚
the shortest bond being 1.749(2) A.
In view of the structural similarity between 7 and 8 (see Tables 2
and 3), only the first will be described in some detail.
In the dinuclear centrosymmetric crystal structure of 7, each
metal centre is N,O chelated by an alcoholate ligand. The
oxygen atoms further bridge the chromium atoms in such a
way that a planar Cr2(m-O)2 group is formed. Two mutually cis
At variance with the chelates featured in 6, the N,O ligands in
complexes 7 and 8 contain two methyl groups on the carbon atom
a to the alcoholate oxygen atom. These dinuclear, centrosymmetric
complexes (Fig. 2 and 3) are similar, differing only in the nature of
the coordinated solvent molecules (ethanol in 7 and water in 8).
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terminal chlorides and an ethanol molecule complete the distorted
octahedral coordination of each metal, the two octahedra sharing
one edge. The smallest angles at the metals are, as in the case
of 6, the chelation and the O–Cr–O angles. The Cr–N and
Cr–O bond distances are close to those found in 6 and similar to
those found in mononuclear and dinuclear Cr(III) aminoalcoholate
and picolinate complexes.16,17 The Cr1–Cl2 bond distance is
only slightly longer than Cr1–Cl1, while the Cr–Cr distance of
interaction, in agreement with the negative sign of the Weiss
temperature (-181(1) and -41(1) K for 6 and 7 respectively).
The fitting of the c = f(T) curves (Fig. 5) was done by using the
following spin Hamiltonian where all parameters have their usual
19
meaning and the spin operator S is defined as S = SCr1 + SCr2
:
˚
3.0867(6) A is longer than that in 6. The C2–N1 bond in the
oxazoline ring has the typical double-bond character with a
˚
length of 1.271(2) A. The O1, Cr1, N1, C1, C2, C3, C4, O2,
Cl1 atoms together with their centrosymmetric counterparts are
almost coplanar [maximum deviation from the mean plane defined
˚
by the aforementioned atoms: 0.061(1) A (Cl1)], as a result of the
equatorial chelation of the alcoholates (if the solvent molecules
and Cl2 are considered as axial ligands). In both structures two
intramolecular centrosymmetric hydrogen bonds are established
between the solvents and the axial chlorides (H ◊ ◊ ◊ O distances:
˚
˚
2.31(3) A in 7 and 2.29(4) A for molecule B in 8).
Magnetic properties
The magnetic properties of 6 and 7 in the solid state were
investigated in the temperature range of 300–1.8 K with an applied
field of 5 kOe. The Curie constants C = 4.2(1) emu K mol-1 and
C = 3.8(1) emu K mol-1 for 6 and 7 respectively were determined
from a fit of the 1/c versus T curve to the Curie–Weiss law in the
high-temperature region (150–300 K) (see Fig. 4). They are in very
good agreement with the expected value for two Cr(III) ions.18
Fig. 5 Experimental c = f(T) curves for 6 (᭺) and 7 (ꢀ) and best fits (full
lines) (see text). Insert : zoom on the low-temperature region.
To reproduce the data satisfactorily we had to consider a certain
amount r of paramagnetic impurity (Simpur = 3/2).
The fits leads to the following values: J = -43.6(1) cm-1, g =
1.95(1)and r = 0.9(1)% (R = 8.6 ¥ 10-4) for6 and J = -16.4(1) cm-1,
g = 1.98(1) and r = 0.3(1)% (R = 2.8 ¥ 10-5) for 7.20 The nature and
magnitude of the magnetic interaction is perfectly in line with the
previously published magneto-structural correlations in hydroxo-
or alkoxo-bridged Cr(III) binuclear complexes.16,21 For instance, the
structural features of the core of complex 7 are very similar to those
of the core of [Cr(pic)2OH]2 (pic = 2-carboxypyridine)16 (Cr–O
˚
bond lengths range from 1.974(1) to 1.984(1) A in 7 and from
˚
1.934 to 1.980 A in [Cr(pic)2OH]2; the Cr–Cr distance is 3.0867(5)
˚
˚
A in 7 and 2.999 A in [Cr(pic)2OH]2 and the Cr–O–Cr angle is
102.48(5)◦ in 7 and ranges from 99.0 to 101.6◦ in [Cr(pic)2OH]2)
and the magnetic coupling constants are thus logically very similar
(J = -16.4 cm-1 for 7 and J = -16.04 cm-1 for [Cr(pic)2OH]2).16
The magnitude of the antiferromagnetic coupling in 6 deserves
a comment. The Cr–O–Cr bridges are more bent in 6 (mean
value 86.2(7)◦) than in 7 but the Cr–Cr distance is much shorter.
Therefore, the fact that a Cr–O–Cr angle close to 90◦ would
tend to reinforce the ferromagnetic contribution to the overall
exchange coupling22 is counter-balanced by the shortness of the
Cr–Cr distance which leads to a direct overlap of the magnetic
orbitals, bringing in that way a relatively strong antiferromagnetic
contribution.
Fig. 4 Experimental 1/c = f(T) curves for 6 (᭺) and 7 (ꢀ) and best fits
(full lines) using the Curie–Weiss law above 150 K.
For both compounds, the room temperature value for the cT
product (2.6 and 3.3 emu K mol-1 for 6 and 7) is below the expected
value for two uncoupled Cr(III) ions (3.75 emu K mol-1 considering
g = 2.0). The cT product decreases continuously down to almost
0 emu K mol-1 upon decreasing the temperature down to 1.8 K,
indicating the occurrence of an intramolecular antiferromagnetic
In order to estimate the magnetic exchange coupling in these
two complexes from first-principles calculations and to clarify
the influence of the different number of bridging ions (three O
100 | Dalton Trans., 2009, 97–105
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Table 4 Energy difference (hartree) between the ferromagnetic and
for complex 7 with larger Cr–Cr distance. However, the number
of bridging ligands may also play a role, since a larger number
of bridges may lead to an increased antiferromagnetic coupling.
This would influence the total J value in the correct direction,
because complex 6 with three bridging atoms shows stronger
antiferromagnetic coupling. Nevertheless, this increase may not be
as strong as one might expect due to a decreased electronic density
at each bridge in the case of three bridging ligands compared to
only two bridging ligands. It seems that there exists a mixture
of several effects which result in an overall antiferromagnetic
coupling.
symmetry-broken antiferromagnetic DFT solution for a model Cr-dimer
˚
as a function of the Cr–Cr distance (A) (DE < 0 = antiferromagnetic
behaviour, DE > 0 = ferromagnetic behaviour)
˚
Cr–Cr/A
1.5
2
2.5
2.74
3
3.08
DE/hartree
-0.156
0.067
0.151
0.162
0.189
0.192
bridges in 6 and only two in 7) we performed density functional
theory (DFT)23 calculations at the all-electron level with the
Gaussian-orbital-based NRLMOL code24 using the Perdew–
Burke–Ernzerhof (PBE) generalized-gradient approximation for
the exchange and correlation functional.25
The DFT calculations of the electronic structures of 6 and
7 confirmed the experimental results. Both molecules show an
overall antiferromagnetic behaviour, however the magnitude of
the exchange coupling obtained from DFT is larger compared to
the experimental values (J = -57 cm-1 for 6 and J = -37 cm-1 for 7),
which is usually the case for J values obtained from PBE.26 This
can be attributed to a general failure of DFT because standard
DFT functionals are not localizing the d-states strongly enough.
In order to elucidate in detail the influence of the different Cr–
Cr distance (shorter Cr–Cr distance in 6) on the overall coupling
strength, we carried out a series of model calculations based on a
Cr(III) dimer varying the Cr–Cr distances.
As shown in Table 4 the ferromagnetic coupling increases with
increasing distance. This behaviour can be explained by the Hay–
Thibeault–Hoffmann model.27 It describes weakly interacting
metal centres in terms of pairwise interaction of molecular orbitals.
On the basis of configuration interaction of molecular orbitals, one
finds for small interactions that the difference of squares of orbital
energies becomes a measure of the singlet–triplet energy splitting.
If the orbitals carrying the spins have the same energy, then
antiferromagnetic interactions vanish and ferromagnetic coupling
is predicted. By increasing the Cr–Cr distance the interaction of the
two ions and therefore the splitting of the d-orbitals is reduced,
which leads consequently to a reduced singlet–triplet splitting.
Therefore, the ferromagnetic interaction becomes stronger. This
behaviour suggest a smaller overall antiferromagnetic coupling
Catalytic oligomerization and/or polymerization of ethylene
Complexes 6–8 were evaluated as precatalysts in the oligomer-
ization and/or polymerization of ethylene in order to examine
the influence of coordinated ligands and solvent molecules on
the catalytic performances of the complexes. Different reaction
conditions, such as the nature of the cocatalyst, the Al/Cr
molar ratio, the reaction temperature and ethylene pressure, were
investigated and the catalytic results were summarized in Table 5.
Under most of the conditions employed, each complex yielded
predominantly polyethylene and 1-butene as the main oligomeric
products with negligible amounts of C6 oligomers and no C8+
oligomers were detected.
Various aluminium-based cocatalysts were used with complex
6 to identify the most effective one at 20 ◦C and 1 atm of ethylene
pressure. When triethylaluminium or diethylaluminium chloride
was used as cocatalyst only 1-butene was observed but the activity
for ethylene oligomerization was low or very low respectively.
In the presence of MAO or MMAO, complex 6 led mostly to
polyethylene with a small amount of butenes with moderate
activities. Very small amount of 1-butene were formed with MAO
(Ao < 100 g mol-1(Cr) h-1) but a higher activity for ethylene
dimerization was found with MMAO, although both MAO and
MMAO led to comparable activities for ethylene polymerization.
In view of our interest for ethylene oligomerization products,
MMAO was therefore chosen in the subsequent investigations.
Table 5 Ethylene oligomerization and/or polymerization with precatalysts 6–8a
Oligomers
Polymers
Co-catalyst
Al/Cr
Tb/◦C
Pc/atm
Ao/g mol-1(Cr) h-1
Ap/g mol-1(Cr) h-1
Tmd/◦C
6
6
6
6
6
6
6
6
6
6
7
8
6
7
8
AlEt3
Et2AlCl
MAO
500
200
500
100
300
20
20
20
20
20
20
20
0
1
1
1
1
1
1
1
1
1
1
1
1
10
10
10
3080
310
<100
350
700
960
2720
1630
3120
2280
1960
1650
36 000
24 700
23 700
—
—
—
—
14 000
6800
12 500
15 400
16 100
27300
5100
n.d.
n.d.
n.d.
n.d.
n.d.
135.6
n.d.
n.d.
135.6
134.6
134.9
134.0
132.0
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
500
1000
1000
1000
1000
1000
1000
1000
1000
1000
40
60
0
1400
37 900
23 600
140 000
620 000
234 000
0
25
25
25
a General conditions: 5.0 mmol of precatalyst; 30 mL of toluene at 1 atm; 100 mL of toluene at 10 atm; reaction time = 30 min. b Reaction temperature.
c Ethylene pressure. d Determined by DSC.
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On treatment with MMAO, both oligomerization and polymer-
ization activities of complex 6 increased along with increasing the
Al/Cr molar ratio from 100, 300, 500 to 1000 and the best catalytic
result was obtained with a Al/Cr molar ratio of 1000. The reaction
temperature also significantly affected the catalytic activity. Lower
temperatures were more favorable for ethylene polymerization and
the polymerization activity greatly decreased when the reaction
temperature was increased. When the temperature was increased
in the presence of aluminium-based cocatalysts and MMAO was
found to be the most effective cocatalyst. On treatment with
MMAO, complexes 6–8 mainly produced linear polyethylenes.
Complex 7 was the most active for ethylene polymerization at both
1 and 10 atm of ethylene pressure, with activities up to 620 000 g
mol-1(Cr) h-1 in the latter case.
Experimental
◦
from 0 to 40 C, the oligomerization activity went up gradually;
however, at 60 ◦C, the amount of both oligomers and polymer
declined.
General considerations
Complexes 6–8 were also investigated at 0 ◦C in the presence
of MMAO (Al/Cr = 1000). Under similar conditions, these
three complexes showed comparable oligomerization activities,
whereas complex 7 produced mostly polyethylene. The difference
of catalytic properties between 6 and 7 can be ascribed to different
structures and the introduction of the gem-dimethyl groups on
the carbon atom a to the oxazoline ring for 7. The presence of
different solvent molecules (ethanol or water) coordinated to the
chromium centres resulted in different catalytic activities between
7 and 8.
The catalytic experiments performed under 10 atm of ethylene
pressure indicated that ethylene pressure significantly affected the
catalytic behaviour (Table 5). Each complex gave much higher
catalytic activity and complex 7 was most active for ethylene
polymerization among these three precatalysts. The highest poly-
merization activity reached was 620 000 g mol-1(Cr) h-1.
The melting points (Tm) of the polyethylenes obtained with
complexes 6–8 were determined by differential scanning calorime-
try analysis (DSC). For the different precatalysts, the melting
points at 1 atm were very similar (6–8: 135.6, 135.6 and 134.6 ◦C
respectively) and the increase in ethylene pressure led to a slightly
lower melting points of the polyethylenes (range from 132.0 to
134.9 ◦C). The melting points around 135 ◦C were consistent with
linear polyethylenes.1c All the PE samples obtained showed intense
bands at ca. 2918 cm-1 (nasCH2) and 2850 cm-1 (nsCH2) in the FT-
IR spectra. The peaks around 1473–1464 cm-1 and 730–718 cm-1
were assigned, respectively, to scissoring and rocking vibrations of
sequential methylene groups in the solid state. Additional bands
include a weak peak at ca. 1641 cm-1 attributed to stretching
All manipulations were carried out under an inert-gas atmosphere
using standard Schlenk techniques. IR spectra in the range 4000–
650 cm-1 were recorded on a Thermo Nicolet 6700 instrument,
equippedwithSMART Orbit DiamondATR accessory. Elemental
C, H, N analyses were performed on a Flash EA 1112 microan-
alyzer. GC analyses were performed with a Varian CP-3800 gas
chromatograph equipped with a flame ionization detector and a
30 m (0.2 mm i.d., 0.25 mm film thickness) CP-Sil 5 CB capillary
column. The yield of oligomers was calculated by referencing to
the mass of solvent on the basis of the prerequisite that the mass of
each fraction was approximately proportional to its integrated area
in the GC trace. DSC traces and melting points of polyethylenes
were obtained from the second scanning run on a Perkin-Elmer
DSC-7 instrument, which was initially calibrated for the melting
point of an indium standard at a heating rate of 10 ◦C min-1.
All solvents were dried and distilled using common techniques
unless otherwise stated. The compounds 4,4-dimethyl-2-oxazolyl-
methanol (1),29 4,4-dimethyl-2-oxazolyldimethyl methanol
(2),4 (4,4-dimethyl-2-oxazolylmethoxy)diphenylphosphine (3),30
bis(4,4-dimethyl-2-oxazolylmethoxy)phenylphosphine (NOPON,
4),
bis(4,4-dimethyl-2-oxazolyldimethylmethoxy)phenylphos-
phine (NOPONMe2, 5)31 and [CrCl3(THF)3]32 were prepared
according to the literature methods. Methylaluminoxane (MAO,
1.46 mol L-1 in toluene) and modified-methylaluminoxane
(MMAO-3A, 7% aluminium in heptane solution) were purchased
from Akzo Corp (USA). Trimethylaluminium (2 mol L-1 in
toluene) and diethylaluminium chloride (Et2AlCl, 1 mol L-1 in
hexane) were purchased from Acros Chemicals. Other chemicals
were commercially available and used without further purification
unless otherwise stated.
=
vibration absorption band of C C double bond, a weak peak
at 1386 cm-1 due to the methyl symmetrical deformation mode
and a weak peak at 909 cm-1 corresponds to the methyl rocking
band due to the chain-terminating methyl groups.28
Synthesis of [Cr2(N,O)3Cl3] (6)
Method A. To a stirred solution of the phosphinito-oxazoline
ligand 3 (0.504 g, 1.61 mmol) in 15 mL of dichloromethane was
added under nitrogen 0.8 equiv. of solid [CrCl3(THF)3] (0.482 g,
1.29 mmol) and the colour of the solution immediately changed
from colourless to green. The reaction mixture was stirred for
16 h at room temperature. After evaporation of the solvent under
reduced pressure, the residue was washed with diethyl ether (3 ¥
15 mL) and pentane (15 mL) and dried under vacuum to afford
a green powder (0.650 g). This green powder was recrystallized to
give green crystals of 6 (0.258 g, 67% yield).
Conclusion
The dinuclear chromium(III) complexes 6–8, containing anionic,
N,O-oxazoline-alcoholate ligands, have been prepared by different
methods and characterized by infrared spectroscopy and elemen-
tal analysis. Their structures in the solid state were established
by single-crystal X-ray diffraction analysis.† Complex 6 has
an unsymmetrical dinuclear structure, whereas 7 and 8, which
contain coordinated ethanol or water molecules respectively,
are centrosymmetric molecules. The magnetic properties of the
paramagnetic, Cr(III) complexes 6 and 7 show intramolecular
antiferromagnetic interactions which were confirmed by DFT cal-
culations of their electronic structures. Complexes 6–8 were used as
precatalysts in the dimerization and/or polymerization of ethylene
Method B. To a stirred solution of the bis(oxazoline)-
phenylphosphonite NOPON ligand 4 (0.275 g, 0.76 mmol) in
20 mL of dichloromethane was added under nitrogen 0.8 equiv.
of solid [CrCl3(THF)3] (0.254 g, 0.68 mmol), and the colour of
the solution changed to blue–green and then green. The reaction
102 | Dalton Trans., 2009, 97–105
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mixture was stirred for 15 h at room temperature. After the solvent
was evaporated under reduced pressure, the residue was washed
with diethyl ether (2 ¥ 30 mL) and dried in vacuum to yield a green
powder (0.315 g), which was recrystallized to give green crystals
of 6 (0.111 g, 55% yield).
addition of diethyl ether produced a precipitate and a blue–purple
solution. The precipitate was filtered, washed with diethyl ether
and dried under vacuum to give a gray–purple powder (0.261 g,
88% yield). The blue–purple solution was placed in the fridge
overnight and a purple solid was obtained, which was redissolved
in dichloromethane and layering of the solution with n-pentane
afforded purple crystals of 8·THF suitable for X-ray diffraction.
Method C. To a solution of the oxazoline alcohol 1 (0.215 g,
1.66 mmol) in 30 mL of THF was added NaH (0.100 g, 2.5 mmol,
60% in oil) and the mixture was stirred at room temperature for 4 h.
After removing the excess NaH by filtration, solid [CrCl3(THF)3]
(0.370 g, 0.99 mmol) was added to the solution. The reaction
mixture was stirred at room temperature for 18 h. The NaCl
formed was removed by filtration and the solution was evaporated
under reduced pressure, the solid residue washed with diethyl ether
and dried under vacuum to give a green powder (0.277 g, 93%
yield). The powder obtained was dissolved in dichloromethane
and the solution was layered with n-pentane for several days to
FT-IR: 3260 cm-1 (n(O–H) from H2O), 1644 cm-1 (n(C N)). Anal.
=
calcd for C16H32Cl4Cr2N2O6 (594.24): C, 32.34; H, 5.43; N, 4.71.
Found: C, 32.39; H, 5.84; N, 4.38.
Oligomerization and polymerization of ethylene
At 1 atm of ethylene pressure. A flame-dried three-neck
flask was loaded with the precatalyst then evacuated and filled
three times with nitrogen. Then ethylene was charged together
with freshly distilled toluene and stirred for 10 min under 1 atm
of ethylene pressure. The reaction temperature was controlled
by the water bath and a certain quantity of cocatalyst was
injected via a syringe. The reaction mixture was stirred for
30 min and the mixture was cooled with an ice bath. Then an
aliquot amount of the solution was collected with a syringe and
quenched by the addition of 5% aqueous hydrogen chloride for
GC analysis. The contents and distribution of oligomers were
determined by GC. The residual solution was quenched with
5% hydrochloric acid ethanol. The precipitated polymers were
collected by filtration, washed with ethanol, dried under vacuum
at 60 ◦C to constant weight.
give green crystals of 6. FT-IR: 1653, 1646, 1624 cm-1 (n(C N)).
=
Anal. calcd for C18H30Cl3Cr2N2O6 (594.8): C, 36.35; H, 5.08; N,
7.06. Found: C, 36.64; H, 5.43; N, 6.64.
Synthesis of [Cr2(N,OMe2)2(EtOH)2Cl4] (7)
Method A. To a solution of the bis(oxazoline)phenylphos-
phonite NOPONMe2 ligand 5 (0.194 g, 0.46 mmol) in 15 mL of
dichloromethane, was added under nitrogen 0.9 equiv. of solid
[CrCl3(THF)3] (0.158 g, 0.42 mmol) and the colour of the solution
changed from purple to green. The reaction mixture was stirred
for 18 h at room temperature. After the dichloromethane was
evaporated under reduced pressure, the residue was washed with
diethyl ether (3 ¥ 20 mL) and dried under vacuum to yield a gray–
green powder (0.155 g) which was recrystallized to give purple
crystals of 7 (0.074 g, 54% yield).
At 10 atm of ethylene pressure. A 250 mL stainless steel
autoclave reactor equipped with a mechanical stirrer and a
temperature controller was heated in vacuo for at least 2 h above
80 ◦C, allowed to cool to the required reaction temperature under
ethylene atmosphere, and then charged with toluene, the desired
amount of cocatalyst, and a toluene solution of precatalyst; the
total volume of the reaction mixture was 100 mL. At the reaction
temperature, the reactor was sealed and pressurized to 10 atm with
ethylene, and the ethylene pressure was maintained by feeding of
ethylene. After the reaction was carried out for 30 min, the pressure
was released and a small amount of the reaction solution was
collected and 5% aqueous hydrogen chloride was added for GC
analysis. The residual reaction solution was quenched with 5%
hydrochloric acid in ethanol, the polymers were filtered, washed
with ethanol and dried under vacuum at 60 ◦C to constant weight.
Method B. The oxazoline alcohol 2 (0.252 g, 1.60 mmol) and
solid [CrCl3(THF)3] (0.375 g, 1.00 mmol) were mixed in a Schlenk
tube and 30 mL of THF were added. The mixture was stirred at
room temperature for 16 h, affording a green precipitate and a
green solution. After partial evaporation of the THF, diethyl ether
was added and the product was collected by filtration, washed
with diethyl ether and dried under vacuum, to give a green powder
(0.360 g). This green powder could be transformed into purple
crystals of 7 (0.165 g, 51% yield).
The powders obtained by these two methods were dissolved in
a mixture of dichloromethane–ethanol (2–5 vol% ethanol) which
was layered with n-pentane for several days to produce 7 as purple
crystals. FT-IR: 3142 cm-1 (n(O–H) from ethanol), 1645 cm-1
Magnetic measurements
=
(n(C N)). Anal. calcd for C20H40Cl4Cr2N2O6 (650.35): C, 36.94;
Magnetic measurements were performed using a Quantum Design
MPMS-XL SQUID magnetometer. The susceptibility measure-
ment was performed in the 300–1.8 K temperature range with an
applied field of 5 kOe. Magnetization measurements at different
fields at a given temperature confirm the absence of ferromagnetic
impurities. Data were corrected for the sample holder and
diamagnetism was estimated from Pascal constants.
H, 6.20; N, 4.31. Found: C, 36.55; H, 6.27; N, 4.05.
Synthesis of [Cr2(N,OMe2)2(H2O)2Cl4] (8)
The oxazoline alcohol 2 (0.252 g, 1.60 mmol) and solid
[CrCl3(THF)3] (0.375 g, 1.00 mmol) were mixed in a Schlenk
tube and 30 mL of THF were added. The mixture was stirred at
room temperature for 15 h to form a green precipitate and a green
solution. NaH (0.06 g, 1.5 mmol, 60% in oil) was then added and
some bubbles were produced and the green precipitate disappeared
gradually to generate a purple solution. The mixture was stirred
at room temperature for another 3 h and filtered. The filtrate was
concentrated under reduced pressure to a very small volume, and
X-Ray structure determinations
Diffraction data of complexes 6, 7 and 8 in 8·THF were
collected on a Kappa CCD diffractometer using graphite-
33
˚
monochromatized Mo Ka radiation (l
=
0.71073 A).
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Table 6 Crystallographic data for complexes 6, 7 and 8·C4H8O
6
7
8·C4H8O
Formula
C18H30Cl3Cr2N3O6
594.80
C20H40Cl4Cr2N2O6
650.34
Monoclinic
C16H32Cl4Cr2N2O6·C4H8O
Formula weight
Crystal system
Space group
666.34
Orthorhombic
P212121
8.7526(2)
14.4126(5)
19.5866(6)
90.00
90.00
90.00
2470.8(1)
4
Green
Triclinic
¯
P1
C2/c
˚
a/A
19.4810(7)
9.5507(3)
16.9528(7)
90.00
113.479(2)
90.00
2893.0(2)
4
Purple
10.3470(4)
10.8770(4)
14.7240(7)
98.425(2)
102.112(3)
109.096(2)
1489.0(1)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Color
Violet
Crystal size/mm
0.06 ¥ 0.05 ¥ 0.05
0.1 ¥ 0.08 ¥ 0.08
0.10 ¥ 0.05 ¥ 0.05
D
calc/g cm-3
1.599
1.493
1.486
m/mm-1
1.242
27.49
1224
18 007
4813
296
0.0545
0.0364
0.0696
0.0736
1.011
0.264, -0.491
0.003(17)
1.156
30.03
1352
12 332
3321
163
0.0434
0.0374
0.0949
0.1022
1.066
0.417, -0.734
—
1.127
27.42
692
12 878
4481
340
0.0492
0.0478
0.1074
0.1249
1.023
0.385, -0.644
—
Max. 2q
F(000)
Measured refl.
Observed refl. [I > 2s(I)]
Parameters
Rint
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
wR2 (all data)
S
Largest diff. peak, hole/e A
Flack parameter
-3
˚
Crystallographic and experimental details are summarized in
Table 6. The structures were solved by direct methods (SHELXS-
97)34 and refined by full-matrix least-squares procedures (based on
F2, SHELXL-97) with anisotropic thermal parameters for all the
non-hydrogen atoms. The hydrogen atoms were introduced into
the geometrically calculated positions (SHELXS-97 procedures)
and refined riding on the corresponding parent atoms, with refined
torsion angles for the methyl groups. The water and ethanol
protons in 7 and 8·THF, respectively, were found in the DF maps
and refined unconstrained. Crystallographic data for all of the
structures in this paper can be found in the ESI.†
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Acknowledgements
We thank the French Embassy in Beijing for a doctoral grant
to S. J. to perform research in Strasbourg in the context of the
Sino-French Joint Catalysis Laboratory, the CNRS, the Ministe`re
de la Recherche (Paris) and the Institut Franc¸ais du Pe´trole
(IFP) for support. We are grateful to Dr Lydia Brelot (Institut
de Chimie, Strasbourg) for crystal structure determinations,
to Professor Wen-Hua Sun (Institute of Chemistry, Chinese
Academy of Sciences, Beijing) for his kind cooperation along
with some works done in his group. J. K. and C. L. thank the
German DAAD and the DFG for financial support within the
SPP 1137: Molecular Magnetism program, and the computer
resources, technical expertise and assistance provided by the ZIH
TU Dresden.
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