Mono- and dinuclear cobalt complexes with chelating or bridging bidentate P,N phosphino- and phosphinito-oxazoline ligands: Synthesis, structures and catalytic ethylene oligomerisation

Article abstract of DOI:10.1039/b706818b

Cobalt(ii) complexes of the type [CoCl₂(P,N)], where P,N represents a heterobidentate phosphino- or phosphinito-oxazoline-type ligand, have been synthesised and characterised by infrared spectroscopy and elemental analysis. Their molecular structures were established by single-crystal X-ray diffraction in the solid state. Whereas the phosphino-oxazoline complex [CoCl₂{Ph₂PCH₂ox^Me2}] (Ph ₂PCH₂ox^Me2 = 2-[(diphenylphosphanyl)-methyl]-4, 4-dimethyl-4,5-dihydro-oxazole) (8) and the phosphinito-oxazoline complexes [CoCl₂{Ph₂POCH₂ox^Me2}] (Ph ₂POCH₂ox^Me2 = 1-[4,4-dimethyl-2{1- oxy(diphenylphosphino)-1-methyl}]-4,5-dihydro-oxazole) (9) and [CoCl ₂{Ph₂POCMe₂ox^Me2}] (Ph ₂POCMe₂ox^Me2 = 1-[4,4-dimethyl-2- [1-oxy(diphenylphosphino)-1-methylethyl]]-4,5-dihydrooxazole) (10) are mononuclear, the phosphino-oxazoline complexes [CoCl₂{μ-i-Pr ₂PCH₂ox}]₂ (i-Pr₂PCH₂ox = 2-[(diisopropyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) (6) and [CoCl ₂{-Ph₂PCH₂ox}]₂ (Ph ₂PCH₂ox = 2-[(diphenyl-phosphanyl)-methyl]-4,5-dihydro- oxazole) (7) are dinuclear compounds and contain two bridging phosphino-oxazoline ligands which form a 10-membered ring. In the course of this work, the zwitterionic complex [CoCl₃{Ph₂PCH ₂C(O)OCH₂CMe₂NH₃] (11) was obtained and characterised by X-ray diffraction in which the oxazoline ring has been opened. Air-oxidation of the phosphine function of the mononuclear P,N chelate complex 8 yielded the blue N,O-bridged, centrosymmetric dinuclear complex [CoCl₂{μ-OPPh₂CH₂CNCMe₂CH ₂O}]₂ (12) which contains a 12-membered ring. All these complexes are paramagnetic and their magnetic moments in solution were measured by the Evans method. Complexes 6-10 were evaluated in the catalytic oligomerisation of ethylene with AlEtCl₂ or methylaluminoxane (MAO) as cocatalysts and provided moderate activities. In the presence of AlEtCl ₂ (6-14 equiv.), the selectivities for ethylene dimers were higher than 92% and complex 8 showed the highest turnover frequency with 14 equiv. of AlEtCl₂. When MAO was used as cocatalyst, the catalytic activities were similar to those with AlEtCl₂ but significant amounts of C ₆-C₁₂ oligomers were produced. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706818b

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Mono- and dinuclear cobalt complexes with chelating or bridging bidentate
P,N phosphino- and phosphinito-oxazoline ligands: synthesis, structures and
catalytic ethylene oligomerisation†‡
Suyun Jie,a,b Magno Agostinho,a Anthony Kermagoret,a Catherine S. J. Cazina and Pierre Braunstein*a
Received 8th May 2007, Accepted 29th June 2007
First published as an Advance Article on the web 13th August 2007
DOI: 10.1039/b706818b
Cobalt(II) complexes of the type [CoCl2(P,N)], where P,N represents a heterobidentate phosphino- or
phosphinito-oxazoline-type ligand, have been synthesised and characterised by infrared spectroscopy
and elemental analysis. Their molecular structures were established by single-crystal X-ray diffraction
in the solid state. Whereas the phosphino-oxazoline complex [CoCl2{Ph2PCH2oxMe2}]
(Ph2PCH2oxMe2 = 2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-4,5-dihydro-oxazole) (8) and the
phosphinito-oxazoline complexes [CoCl2{Ph2POCH2oxMe2}] (Ph2POCH2oxMe2
=
1-[4,4-dimethyl-2{1-oxy(diphenylphosphino)-1-methyl}]-4,5-dihydro-oxazole) (9) and
[CoCl2{Ph2POCMe2oxMe2}] (Ph2POCMe2oxMe2 = 1-[4,4-dimethyl-2-
[1-oxy(diphenylphosphino)-1-methylethyl]]-4,5-dihydrooxazole) (10) are mononuclear, the
phosphino-oxazoline complexes [CoCl2{l-i-Pr2PCH2ox}]2 (i-Pr2PCH2ox =
2-[(diisopropyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) (6) and [CoCl2{l-Ph2PCH2ox}]2
(Ph2PCH2ox = 2-[(diphenyl-phosphanyl)-methyl]-4,5-dihydro-oxazole) (7) are dinuclear compounds
and contain two bridging phosphino-oxazoline ligands which form a 10-membered ring. In the course
of this work, the zwitterionic complex [CoCl3{Ph2PCH2C(O)OCH2CMe2NH3] (11) was obtained and
characterised by X-ray diffraction in which the oxazoline ring has been opened. Air-oxidation of the
phosphine function of the mononuclear P,N chelate complex 8 yielded the blue N,O-bridged,
=
centrosymmetric dinuclear complex [CoCl2{l-OPPh2CH2C NCMe2CH2O}]2 (12) which contains a
12-membered ring. All these complexes are paramagnetic and their magnetic moments in solution were
measured by the Evans method. Complexes 6–10 were evaluated in the catalytic oligomerisation of
ethylene with AlEtCl2 or methylaluminoxane (MAO) as cocatalysts and provided moderate activities.
In the presence of AlEtCl2 (6–14 equiv.), the selectivities for ethylene dimers were higher than 92% and
complex 8 showed the highest turnover frequency with 14 equiv. of AlEtCl2. When MAO was used as
cocatalyst, the catalytic activities were similar to those with AlEtCl2 but significant amounts of C6–C12
oligomers were produced.
with important applications in homogeneous catalysis.3,4 This
Introduction
combination can also provide the possibility to fine-tune steric and
electronic requirements by appropriate chemical modifications
and even relatively minor variations may lead to complete changes
in the coordination properties and bonding mode of the ligand, as
will be illustrated in the present work.
Heteroatom-containing ligands are of increasing interest in co-
ordination and organometallic chemistry owing to the properties
they confer to their metal complexes and also to their ability to
act as hemilabile ligands.1 Nitrogen- and phosphorus-based donor
ligands are among the most used in molecular chemistry2 and when
nitrogen as a hard donor and phosphorus as a soft donor are
associated within a functional ligand, they can act cooperatively
to stabilize metal ions in optimum oxidation states and generate
systems of considerable chemical and structural diversity and
Chelating P,N ligands containing N-heterocycles, such as
pyridyl- and oxazolyl-, are receiving considerable atten-
tion.1c–e,1g,3,4d In particular, their nickel complexes are active
catalyst precursors in the catalytic oligomerisation of ethylene,4d,5
which is of major industrial interest.6 Much fewer P,N-cobalt
complexes have been reported as catalyst precursors for ethylene
oligomerisation, even when P,N-containing tridentate ligands are
considered.7 We have recently found that phosphino-oxazoline and
phosphinito-oxazoline nickel complexes I8a and II,8b respectively,
are very active for the dimerisation of ethylene in the presence
of small amounts of a cocatalyst (2 or 6 equiv. of AlEtCl2). The
following phosphino-oxazoline (1–3) and phosphinito-oxazoline
(4–5) ligands have now been selected because of the easy
modification of their stereoelectronic properties with the aim of
aLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070, Strasbourg
Ce´dex, France. E-mail: braunstein@chimie.u-strasbg.fr
bKey Laboratory of Engineering Plastics and Beijing National Laboratory for
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100080, China
† The HTML version of this article has been enhanced with colour images.
‡ CCDC reference numbers 646279–646285. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b706818b
4472 | Dalton Trans., 2007, 4472–4482
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investigating their cobalt complexes and comparing them with the
corresponding nickel complexes. Ligand tuning can be done by
variation of the substituents on P (1 R = i-Pr and 2 R = Ph), by
introducing substituents on the heterocycle (2 R1 = H and 3 R1 =
Me) or on the P-heterocycle bridge (4 R2 = H and 5 R2 = Me).
This can also be done by introduction of an oxygen atom (going
from a phosphine to a phosphinite) which changes the ring size of
the chelate and the electronic density of the metal centre (3 and 4).
Phosphinite ligands are readily available from reactions between
alcohols and chlorophosphines and find interesting applications
in homogeneous catalysis.9
(2)
The diphenylphosphino group was introduced by reaction of the
deprotonated alcohol with PPh2Cl (eqn (2)).13
The cobalt complexes were readily obtained by reaction of
the corresponding ligands with anhydrous cobalt dichloride in
THF at room temperature. When ligands 1 and 2 were used, the
complexes 6 and 7 precipitated from the solution and were isolated
by filtration and washed with diethyl ether to give air-stable bright-
blue powders (eqn (3)).
(3)
Since ligands 1–5 always behaved as chelates in their Ni(II) or
Pd(II) complexes,8,10a–c,13,16 it was unexpected that 6 and 7 would be
dinuclear complexes with bridging phosphino-oxazoline ligands,
as was established by X-ray diffraction (see below). In contrast,
when ligands 3–5 were used, purple or blue chelated complexes
were obtained (8–10) which are satisfactorily soluble in THF
(eqn (4) and (5)).
Results and discussion
Synthesis and characterization of the complexes
The phosphino-oxazoline ligands 1–3 were prepared from the
corresponding oxazolines according to the literature methods
(eqn (1)).10
(4)
(1)
(5)
Lithiation of the oxazolines in THF was carried out with 1.0 equiv.
of n-BuLi at −78 ◦C in order to avoid metallation on the
oxazoline heterocycle,11 followed by the addition of SiClMe3 at low
temperature to form the N-silyl derivatives. Finally, 1.0 equiv. of
chlorophosphine (PR2Cl) was added and the solution was allowed
to warm to room temperature overnight. After treatment and
purification, the phosphino-oxazolines were isolated in high purity
and good yield.
The phosphinito-oxazoline ligands 4 and 5 were prepared from
the oxazoline alcohols, which were readily synthesised by refluxing
glycolic acid or 2-hydroxy-2-methylpropionic acid with 2-amino-
2-methyl-1-propanol in xylene until no further formation of water
was observed in a Dean–Stark trap (eqn (2)).12
=
The infrared m(C N) stretching vibration bands shifted from
1660–1668 cm−1 (free ligands)8b,10a,13 to 1612–1643 cm−1 (coordi-
nated ligands) and the peak intensity became weaker, confirming
coordination of the ligands to cobalt. All the complexes were
paramagnetic and their magnetic moment was measured in
CD2Cl2 solution by the Evans method.14 Magnetic moments in
the range 3.75–3.94 lB were found for complexes 8–10, typical for
the presence of three unpaired electrons on a cobalt(II) high spin
system (S = 3/2) and values of 5.30 and 5.42 lB per complex for
6 and 7, respectively.15 The structures of 6 and 7 in the solid state
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◦
˚
metric dinuclear structure with a distorted tetrahedral coordi-
nation geometry around the metal centres. The cobalt atoms
are coordinated by the nitrogen atom from one ligand and the
phosphorus from another ligand, which results in a ten-membered
ring structure (Fig. 1 and 2). The ten-membered dimetallocycles
display a double boat-like conformation (Fig. 1–3). The Cl1–Co–
Cl2 angle is 116.44(3)◦ in 6 and 118.25(3)◦ in 7 (Table 1). Only
the structure of 7 will be discussed in some detail because of its
similarity with that of 6. The P–Co–N angle (109.27(7)◦) in 7 is
nearly what is expected for a regular tetrahedral coordination,
which results from the coordinated P and N atoms originating
from two separate ligands (this P–Co–N angle is much smaller,
84.65(7)◦, in the chelate complex 8, see below). The Co–N bond
Table 1 Selected bond lengths (A) and bond angles ( ) in complexes 6–8
6
7
8
Co–N
Co–Cl2
Co–Cl1
Co–P
N–C2
C1–C2
P–C1
2.015(2)
2.2466(8)
2.2385(9)
2.4147(9)
1.278(4)
1.479(4)
1.863(3)
1.996(2)
2.2281(8)
2.2306(8)
2.4176(8)
1.281(3)
1.487(4)
1.851(3)
2.003(2)
2.2177(9)
2.2245(8)
2.3981(8)
1.280(3)
1.493(4)
1.856(3)
N–Co–Cl1
N–Co–Cl2
Cl1–Co–Cl2
N–Co–P
Cl1–Co–P
Cl2–Co–P
C2–N–Co
N–C2–C1
C2–C1–P
C1–P–Co
101.88(8)
113.70(7)
116.44(3)
115.30(8)
111.17(3)
99.03(3)
135.0(2)
127.0(3)
114.4(2)
111.49(1)
102.44(7)
116.44(6)
118.25(3)
109.27(7)
107.41(3)
102.69(3)
134.23(2)
125.7(2)
112.15(7)
119.13(7)
113.88(4)
84.65(7)
112.57(3)
111.04(3)
120.36(2)
126.4(2)
109.9(2)
97.61(9)
˚
˚
length (1.996(2) A) is about 0.42 A shorter than that of the Co–
˚
P bond (2.4176(8) A), while the two Co–Cl distances are very
similar. The C2–N bond in the oxazoline heterocycle has a typical
˚
114.81(2)
115.07(9)
double-bond character with a length of 1.281(3) A. Structuraly
characterised precedents for the exceptional bridging rather
than chelating bonding mode of a phosphino-oxazoline ligand
are [{(OC)4Fe(l-PCH2ox-P, N )}2CoCl2] (III)17 and [(OC)3Fe(l-
PCH2ox-P, N )2Cu]BF4 (IV).18
were determined by single crystal X-ray diffraction analysis (Fig. 1
and 2). Selected bond lengths and angles are collected in Table 1.
Fig. 3 Conformation of the ten-membered dimetallocyclic skeleton of
the dinuclear phosphino-oxazoline cobalt complex 7.
Fig. 1 ORTEP view of the molecular structure of complex 6 with
thermal ellipsoids drawn at the 50% probability level. Symmetry operations
generating equivalent atoms (ꢀ): −x, −y, −z.
The phosphino-oxazoline ligand 3 with two gem-methyl groups
on the oxazoline heterocycle formed the chelated mononuclear
complex 8 (eqn (4)) and the coordination geometry around the
cobalt centre was established by X-ray diffraction to be distorted
tetrahedral (Fig. 4).
This structure shows similar characteristics to that of
the five-membered P,N-chelate cobalt(II) complex bearing the
2-pyridylbis(diphenylphosphino)methane ligand.19 Most bond
lengths in 8 are similar to those observed for the dinuclear cobalt
complexes 6 and 7 and the corresponding dinuclear nickel complex
I.8a The N–Co–P bond angle (84.65(7)◦) in 8 is larger than that in
the dinuclear nickel complex I (80.83(7)◦) but much smaller than
those in 6 or 7 (Table 1). The C1, C2, N, P and Co atoms in the
chelate ring are nearly coplanar and the maximum deviation out
Fig. 2 ORTEP view of the molecular structure of complex 7 with
thermal ellipsoids drawn at the 50% probability level. Symmetry operations
generating equivalent atoms (ꢀ): −x, −y, −z.
Whereas chelation of ligands 1 and 2 to CoCl2 was antici-
pated by analogy with their behaviour in Ru(II)10d and Pd(II)
complexes,10a,c,16 complexes 6 and 7 possess in fact a centrosym-
4474 | Dalton Trans., 2007, 4472–4482
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Fig. 4 ORTEP views of the molecular structure of complex 8 with thermal ellipsoids drawn at the 50% probability level.
˚
of the mean plane defined by these atoms is 0.082 A for C1 and
the two chlorine atoms are located on either side of this plane. In
addition, this plane and the oxazoline ring are almost coplanar
with a dihedral angle of only 2.0(1)◦ (Fig. 4b). The two methyl
groups on C4 may provide new steric hindrance compared to the
situation in 6 and 7. The geometry around the phosphorus atom is
also distorted tetrahedralwith aC1–P–Co bond angle of97.61(9)◦,
which is much smaller than those in 6 and 7.
Complex 8 was less stable in solution than 6 or 7. In a Schlenk
tube containing a solution of this complex in CH2Cl2 we observed
the formation of a few blue crystals upon slow layer diffusion
with n-pentane. These were identified by X-ray diffraction which
established that this product resulted from the cleavage of the
=
C N double bond of the oxazoline heterocycle. This must have
been caused by the presence of adventitious water and the small
quantity of product did not allow us to perform all the usual
analyses. In this ring-opened complex 11, the nitrogen atom is not
coordinated but quaternised to form an ammonium salt (Fig. 5).
Fig. 5 ORTEP view of the molecular structure of ring-opened complex
11 (obtained from 8) with thermal ellipsoids drawn at the 50% probability
◦
˚
level. Selected bond lengths (A) and angles ( ): Co–Cl1 = 2.2590(2),
Co–Cl2 = 2.2406(1), Co–Cl3 = 2.2496(1), Co–P = 2.3885(2), O2–C2 =
1.210(6), N–C4 = 1.535(6); Cl2–Co–Cl1 = 110.24(6), Cl2–Co–Cl3 =
115.90(6), Cl3–Co–Cl1 = 106.71(6), Cl1–Co–P = 113.61(6), Cl2–Co–P =
100.83(6), Cl3–Co–P = 109.72(6).
fusion of n-pentane the blue N,O-bridged dinuclear complex
=
[CoCl2{l-OPPh2CH2C NCMe2CH2O}]2 (12) (Fig. 6).
This transformation results in a zwitterionic complex in which
the cobalt atom exhibits a slightly distorted tetrahedral geometry
with the ester phosphine acting as a monodentate ligand through
the phosphorus atom. The nitrogen atom lies far away from the
˚
cobalt centre with a Co · · · N separation of 4.039 A. With an
˚
average length of 2.2497 A, the Co–Cl bonds are a little longer
than in 8 whereas the Co–P bond length is almost unchanged. The
C2–O2 bond points away from the cobalt centre and its length is
˚
typical for a carbonyl group (1.210(6) A). Intramolecular hydrogen
In the centrosymmetric structure of 12, the cobalt atoms also
adopt a distorted tetrahedral coordination geometry, with coor-
dination of a nitrogen atom from one ligand and an oxygen atom
from another ligand to form a twelve-membered mesocycle. The
corresponding bond lengths and angles are very similar to those
in 8. This dinuclear structure is obviously preferred to the alter-
native six-membered ring chelation in a mononuclear complex.
bonding is observed in the structure of 11 between two N–H
˚
protons and two Cl ligands: NH1 · · · Cl1 (dH · · · Cl = 2.70 A with
N–H–Cl = 142◦) and NH3–Cl2 (dH · · · Cl = 2.52 A with N–H–Cl =
˚
150◦).
Accidental air-oxidation of the phosphine function of the
mononuclear P,N complex 8 in CH2Cl2 yielded after slow dif-
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Table 2 Selected bond lengths (A) and bond angles ( ) in complexes 9
and 10
9
10
Co–N
Co–Cl1
Co–Cl2
Co–P
N–C2
C1–C2
C1–O2
P–O2
2.0073(2)
2.2242(7)
2.2139(7)
2.3648(7)
1.272(3)
1.500(3)
1.441(3)
1.6300(2)
2.006(2)
2.2340(8)
2.2227(7)
2.3281(7)
1.275(3)
1.531(4)
1.452(3)
1.6288(2)
N–Co–Cl1
N–Co–Cl2
Cl1–Co–Cl2
N–Co–P
Cl1–Co–P
Cl2–Co–P
C2–N–Co
N–C2–C1
O2–C1–C2
C1–O2–P
O2–P–Co
115.06(6)
112.83(6)
113.28(3)
93.24(6)
113.57(3)
107.01(3)
125.32(2)
127.53(2)
112.24(2)
118.43(1)
107.65(6)
110.31(7)
117.09(7)
117.38(3)
91.21(6)
110.92(3)
106.62(3)
127.28(2)
128.6(2)
Fig. 6 ORTEP view of the molecular structure of oxidized complex 12
(obtained from 8) with thermal ellipsoids drawn at the 50% probability
◦
˚
level. Selected bond lengths (A) and angles ( ): Co–O2 = 1.910(4),
Co–N2 = 2.033(3), Co–Cl1 = 2.2421(1), Co–Cl2 = 2.2599(1), P–O2 =
1.427(4), N2–C2 = 1.284(5); O2–Co–N2 = 106.24(2), O2–Co–Cl1 =
115.09(1), N2–Co–Cl1 = 110.48(1), O2–Co–Cl2 = 103.31(1), N2–Co–
Cl2 = 112.68(10), Cl1–Co–Cl2 = 108.93(5). Symmetry operations gener-
ating equivalent atoms (ꢀ): −x, −y, −z.
109.6(2)
124.62(2)
109.26(7)
In both complexes the cobalt centre has a distorted tetrahedral
coordination geometry. Because these complexes have very similar
geometries and structural parameters, only 10 will be described in
some detail in order to compare it with the corresponding nickel
complex II.8b The six-membered chelate ring shows a boat-like
conformation, similar to that of the nickel complex II and the bond
distances and angles are similar in the Co(II) and Ni(II) complexes.
The bond angles around the cobalt centre are in the range of
91.21(6)◦ (N–Co–P) to 117.38(3)◦ (Cl1–Co–Cl2). The two Co–Cl
bond distances are similar and the Co–P bond is much longer than
Co–N, which resembles the situation in the phosphino-oxazoline
complexes 6–8.
The X-ray structures of the phosphinito-oxazoline complexes
9 and 10 (eqn (5)) are shown in Fig. 7 and 8 and selected bond
lengths and angles are given in Table 2.
Catalytic oligomerisation of ethylene
The phosphino- (6–8) and phosphinito-oxazoline (9 and 10)
cobalt(II) dichloride complexes have been evaluated as precatalysts
for oligomerisation of ethylene in the presence of AlEtCl2 or MAO
as cocatalysts. The influence of the steric and electronic properties
of the ligands and of the Al/Co molar ratio on the catalytic
properties was examined (Tables 3 and 4). Even though they
showed moderate catalytic activities for ethylene oligomerisation,
it is interesting to compare them with their corresponding nickel
complexes and other cobalt-based precatalysts in order to gain
a better insight into the influence of different metal centres and
ligands on the catalytic behaviour.
Fig. 7 ORTEP view of the molecular structure of complex 9 with thermal
ellipsoids drawn at the 50% probability level.
Use of AlEtCl2 as cocatalyst
When 2 equiv. of cocatalyst (AlEtCl2) were added, very low
activities were observed. However, in the presence of 6, 10 or
14 equiv. of AlEtCl2, complexes 6–10 displayed moderate activities
for the selective dimerisation of ethylene (C4 fraction > 92%)
(Table 3, Fig. 9 and 10). Different quantities of cocatalyst led to
the same trend of catalytic activities and selectivities for 1-butene
for each complex. When the amount of AlEtCl2 was increased
from 6 to 14 equiv, the turnover frequencies (TOF) also increased,
whereas the selectivities for 1-butene decreased slightly.
Fig. 8 ORTEP view of the molecular structure of complex 10 with
thermal ellipsoids drawn at the 50% probability level.
4476 | Dalton Trans., 2007, 4472–4482
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Table 3 Ethylene dimerisation with precatalysts 6–10 using AlEtCl2 as cocatalysta
Selectivity (%)
Productivity/gC gCo−1 h−1
TOF/molC molCo h−1
a-Olefin (C4) (%)
−1
AlEtCl2 (equiv.)
C4
C6
C8
2 H4
2 H4
6
6
10
14
6
10
14
2
95
93
92
98
94
94
97
96
95
93
99
95
93
99
97
95
4
7
8
2
5
5
3
4
5
7
1
5
6
1
2
4
<1
<1
<1
0
1
1
700
2100
2800
1000
1200
1600
<100
1800
2500
3700
250
1200
1700
180
380
560
1500
4400
5800
2100
2400
3300
<200
3800
5300
7700
540
2400
3600
370
800
1200
52
46
43
46
41
37
57
31
23
19
71
54
48
75
73
70
6
6
7
7
7
8
0
8
6
<1
<1
<1
0
<1
<1
0
8
10
14
6
10
14
6
8
9
9
9
10
10
10
10
14
<1
1
a Conditions: T = 25 ◦C, 10 bar of C2H4, 35 min, 4 × 10−2 mmol of Co, solvent = 15 mL of toluene. No C10 oligomers were detected.
Table 4 Ethylene oligomerisation with precatalysts 6–10 using MAO as cocatalysta
Selectivity (%)
−1
MAO (equiv.)
C4
C6
C8
C10
C12+
Productivity/gC gCo−1 h−1
TOF/molC molCo h−1
a-Olefin (C4) (%)
2 H4
2 H4
6
100
200
400
100
200
400
100
200
400
100
200
400
100
200
400
54
37
48
36
22
26
32
29
21
29
23
23
33
26
22
21
27
19
25
29
29
28
32
26
24
31
26
19
27
27
16
22
18
18
26
26
23
27
27
21
27
23
21
26
26
8
14
12
14
19
17
13
11
23
15
17
21
10
15
19
1
<1
3
7
3
2
4
1
3
11
2
7
17
6
6
1700
3100
3000
1100
3200
2900
1900
3100
2300
800
2400
1900
300
1000
1300
3600
6600
6300
2300
6800
6200
4000
6500
4900
1600
5100
4100
600
46
46
44
25
34
43
31
38
43
47
46
47
55
50
50
6
6
7
7
7
8
8
8
9
9
9
10
10
10
2100
2700
a Conditions: T = 25 ◦C, 10 bar of C2H4, 35 min, 10−2 mmol of Co, solvent = 20 mL of toluene.
Fig. 9 Catalytic activities of precatalysts 6–10 in the oligomerisation of
Fig. 10 Selectivity of precatalysts 6–10 for 1-butene, within the C4
ethylene using AlEtCl2 as cocatalyst.
fraction, using AlEtCl2 as cocatalyst.
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Under similar conditions, the dinuclear phosphino-oxazoline
complex 6 showed a slightly higher TOF and selectivity for 1-
butene than 7. For example, in the presence of 14 equiv. of
AlEtCl2, 6 gave a TOF of 5800 molC2 molCo h−1 and a
−1
H
4
selectivity of 43% for 1-butene. For comparison, 7 led to a TOF
of 3300 molC molCo h−1 and a selectivity of 37% for 1-butene.
−1
H
2
4
The difference of catalytic properties between 6 and 7 can only
be ascribed to the different stereoelectronic properties of the
phosphorus substituents.
In the presence of 6, 10 or 14 equiv. of AlEtCl2, the mononuclear
phosphino-oxazoline complex 8 was more active than the dinu-
clear complexes 6 and 7 but slightly less selective for 1-butene. The
introduction of the gem-methyl groups in the oxazoline heterocycle
resulted for 8 in higher turnover frequencies than for 7. In contrast
to the corresponding dinuclear nickel complex I, complex 8 in the
presence of 14 equiv. of AlEtCl2 was about five times less active
but more selective for 1-butene (8, 19%; I, 3%).8a Complex 8 needs
Fig. 11 Catalytic activities of precatalysts 6–10 in the oligomerisation of
ethylene using MAO as cocatalyst.
to be treated with only 6 equiv. of AlEtCl2 in order to lead to a
−1
TOF of 3800 molC molCo h−1 whereas under such conditions
H
2
4
complex I is completely inactive.
When compared with the phosphine complex 8, the phosphinite
complex 9 showed lower catalytic activities and higher selectivities
for 1-butene, which illustrates the effect of a different environment
around the phosphorus atom and a different ring size of the
metal chelate. The substituents on the carbon atom a to the
oxazoline ring remarkably influenced the catalytic activities of
the phosphinite cobalt complexes. When 10 or 14 equiv. of
AlEtCl2 were used, complex 10 with two methyl groups on
the carbon atom a to the oxazoline ring was 2/3 less active
than 9 whereas 10 displayed higher selectivities for 1-butene.
In contrast, the corresponding nickel complex II provided high
TOF of 49500 molC molNi h−1 for ethylene dimerisation and
−1
H
2
4
Fig. 12 Selectivity of precatalysts 6–10 for 1-butene within the C4 fraction
using MAO as cocatalyst.
trimerization in the presence of 6 equiv. of AlEtCl2 despite lower
selectivity for 1-butene (8%).8b
When a blank experiment was run with CoCl2 in the presence
of 10 equiv. of AlEtCl2, no catalytic activity was found, which
confirms the major role played by the ligands and indicates that
they actually remain coordinated on the cobalt centre during
catalysis.
to 200 and 400 equivalents. The fractions of the different C6
oligomers are shown in Table 5. Depending on the nature of
reinsertion of 1-butene or 2-butene (1,2- or 2,1-insertion) and
further b-H elimination, various C6 products can be formed.15d,20
1-Hexene was formed by a chain growth mechanism and its Co-
catalyzed isomerization resulted in the formation of 2-hexene and
3-hexene.20b
Use of MAO as cocatalyst
Complexes 6–10 afforded TOF values between 600 and
Bianchini and co-workers21 have reported that upon activation
by MAO, tetrahedral cobalt(II) dichloride precursors with 6-
substituted-2-(imino)pyridyl ligands catalyzed the selective con-
6800 molC2 molCo h−1 (Table 4, Fig. 11 and 12). Note that
−1
H
4
complex I decomposed in the presence of MAO.8a With 100–
400 equiv. of MAO, the catalytic activities and selectivities for
1-butene were comparable to those obtained with AlEtCl2 as
cocatalyst. Similarly, the phosphine complexes 6–8 showed higher
activities than phosphinite complexes 9 and 10. The turnover
version of ethylene to a-olefins with turnover frequencies as
−1
high as 1.5 × 106 molC
molCo h−1 but the most productive
H
2
4
systems have been described by Gibson and co-workers15d with
Co(II) catalysts containing CF3 substituents on the ring of the
bis(imino)pyridine ligand. In contrast, pentacoordinated imino-
phenanthroline Co(II) complexes were only moderately active for
the oligomerisation of ethylene.22 Dinuclear tetrahedral imino-
pyridyl Co(II) complexes and pentacoordinated imino-pyridyl,
phenolate-bridged Co(II) complexes displayed low activities in the
presence of MAO.23 Cobalt(II) complexes bearing P-containing
ligands, such as bis(2-diphenylphosphinoethyl)methylamine7b and
pyrazolyliminophosphorane7c ligands, were also found to be mod-
erately active for this reaction in the presence of ethylaluminoxane
(EAO) or MAO.
frequencies were in the range of 600–4000 molC2 molCo h−1
−1
H
4
−1
with 100 equiv. of MAO and 2700–6300 molC molCo h−1 with
H
2
4
400 equiv. of MAO. The best results were obtained with 7 in the
presence of 200 equiv. of MAO, which afforded a TOF as high as
−1
6800 molC molCo h−1.
H
2
4
When activated by MAO, these cobalt(II) precatalysts produced
C4–C12 olefins. When 100 equiv. of MAO was used, the C4
fractions were larger than C6 for each complex. In contrast to
the diisopropylphosphine complex 6, complexes 7–10 gave more
C6 than C4 products when the amount of MAO was increased
4478 | Dalton Trans., 2007, 4472–4482
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phino)-1-methyl}]-4,5-dihydrooxazole (4)13 and 1-[4,4-dimethyl-2-
{1-oxy(diphenylphosphino)-1-methylethyl}]-4,5-dihydrooxazole
(5)8b,d were prepared according to the literature procedures. Other
chemicals were commercially available and used without further
purification unless otherwise stated. IR spectra in the range
4000–400 cm−1 were recorded on a Bruker IFS28FT instrument.
For all complexes whose magnetic moments were determined by
the Evans method,14 a CH3NO2–CD2Cl2 solution (20 : 80 v/v)
was used as the reference and a solution of the paramagnetic
complex 0.055–0.10 M in CD2Cl2 was prepared (a diamagnetic
correction was introduced for the solvent). Gas chromatographic
analyses were performed on an Thermoquest GC8000 Top Series
gas chromatograph using an HP Pona column (50 m, 0.2 mm
diameter, 0.5 lm film thickness).
Table 5 Catalytic data for complexes 6–10 and distribution of the C6
alkenes in the oligomerisation of ethylene with MAO as cocatalysta
Selectivity (mass%)
C6 from
C6 from
b
MAO (equiv.) 1-Hexene Linear C6
1-butenec
2-butened
6
6
6
7
7
7
8
8
8
9
9
9
100
200
400
100
200
400
100
200
400
100
200
400
29
26
24
13
22
26
22
23
23
33
27
27
33
34
28
63
62
63
81
66
62
70
65
62
58
60
58
62
59
59
1
2
2
0
1
1
1
2
3
0
2
2
0
0
2
7
10
11
6
11
11
7
10
12
9
11
13
5
Syntheses
10 100
10 200
10 400
8
10
Preparation of (2-[(diisopropylphosphanyl)-methyl]-4,5-dihydro-
oxazole)cobalt dichloride (6). To a solution of 2-[(diisopropyl-
phosphanyl)-methyl]-4,5-dihydro-oxazole (1) (0.263 g, 1.31 mmol)
in THF (15 mL) CoCl2 (0.153 g, 1.18 mmol) was added. The
reaction mixture was stirred for 15 h at room temperature.
The resulting product precipitated from the solution and was
isolated by filtration, washed with diethyl ether (2 × 15 mL)
and dried under vacuum to give a bright-blue powder (0.198 g,
◦
a Conditions: T = 25 C, 10 bar of C2H4, 35 min, 10−2 mmol of Co,
solvent = 20 mL of toluene. b Sum of 3-cis-hexene, 3-trans-hexene, 2-cis-
hexene, and 2-trans-hexene. c Corresponds to 2-ethyl-1-butene. d Sum of 3-
methyl-1-pentene, 3-methyl-2-cis-pentene, and 3-methyl-2-trans- pentene.
Conclusion
90% yield). FT-IR (KBr disc): 1640 cm−1 (m(C N)). Magnetic
=
The cobalt(II) complexes 6–10 have been synthesised and char-
acterised by single-crystal X-ray diffraction. Unexpectedly, com-
plexes 6 and 7 with the less bulky ligands have a centrosymmetric
dinuclear structure, in contrast to the mononuclear complexes
8–10 which are of the more usual five-membered and six-
membered chelate type. The coordination geometry around cobalt
can be described as a distorted tetrahedral for each complex.
The magnetic moments of these paramagnetic complexes were
measured in their D2-dichloromethane solutions by the Evans
method.
Complexes 6–10 showed moderate catalytic activities as pre-
catalysts for the oligomerisation of ethylene in the presence of
MAO or AlEtCl2. With the latter, all of the complexes displayed
selectivities higher than 92% for ethylene dimerisation and the
highest TOF were obtained with complex 8 under the same
conditions. Although these cobalt(II) complexes were less active
than the corresponding nickel complexes I and II,8 much higher
selectivities for 1-butene were observed. When MAO was used as
catalyst, complexes 6–10 provided similar catalytic activities and
selectivites for 1-butene. In contrast to AlEtCl2, which afforded
almost exclusively C4 products, the use of MAO as cocatalyst gave
a C4–C12 distribution of oligomers.
moment in solution (Evans method): 5.30 lB. Anal. Calcd for
C20H40Cl4Co2N2O2P2: C, 36.28; H, 6.09; N, 4.23. Found: C, 35.78;
H, 6.15; N, 4.18%.
Preparation of (2-[(diphenylphosphanyl)-methyl]-4,5-dihydro-
oxazole)cobalt dichloride (7). The ligand 2-[(diphenylphos-
phanyl)-methyl]-4,5-dihydro-oxazole (2) (0.573 g, 2.12 mmol) and
0.8 equiv. of anhydrous CoCl2 (0.221 g, 1.70 mmol) were mixed in
a Schlenk tube and then degassed THF (15 mL) was added. The
reaction mixture was stirred for 18 h at room temperature. The
resultant blue precipitate was collected by filtration, washed with
diethyl ether (2 × 15 mL) and dried under vacuum to give a bright-
blue powder (0.442 g, 65% yield). FT-IR (KBr disc): 1643 cm−1
=
(m(C N)). Magnetic moment in solution (Evans method): 5.42 lB.
Anal. Calcd for C32H32Cl4Co2N2O2P2: C, 48.15; H, 4.04; N, 3.51.
Found: C, 47.83; H, 4.31; N, 3.18%.
Preparation of (2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-
4,5-dihydro-oxazole)cobalt dichloride (8). To a solution of the lig-
and 2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-4,5-dihydro-
oxazole (3) (0.350 g, 1.18 mmol) in THF (20 mL), CoCl2 (0.138 g,
1.06 mmol) was added. The reaction mixture was stirred for 15 h
at room temperature to yield a blue-purple solution. The solvent
was removed in vacuo and the residue was washed with diethyl
ether (2 × 15 mL) and pentane (15 mL) and dried under vacuum
to lead to a purple powder (0.420 g, 92% yield). FT-IR (KBr
Experimental
disc): 1615 cm−1 (m(C N)). Magnetic moment in solution (Evans
General considerations
=
method): 3.94 lB. Anal. Calcd for C18H20Cl2CoNOP: C, 50.61; H,
All solvents were dried and distilled using common techniques
unless otherwise stated. All manipulations were carried out under
an inert-gas atmosphere using standard Schlenk techniques. The
4.72; N, 3.28. Found: C, 50.23; H, 4.79; N, 3.17%.
Preparation of (1-[4,4-dimethyl-2{1-oxy(diphenylphosphino)-1-
methyl}]-4,5- dihydro-oxazole)cobalt dichloride (9). To a solu-
tion of 1-[4,4-dimethyl-2{1-oxy(diphenylphosphino)-1-methyl}]-
4,5-dihydro-oxazole (4) (0.441 g, 1.41 mmol) in THF (15 mL),
cobalt dichloride (0.171 g, 1.32 mmol) was added and the
compounds
2-[(diisopropyl-phosphanyl)-methyl]-4,5-dihydro-
oxazole (1),10b,c 2-[(diphenylphosphanyl)-methyl]-4,5-dihydro-
oxazole (2),10a 2-[(diphenylphosphanyl)-methyl]-4,4-dimethyl-4,5-
dihydrooxazole (3),10a 1-[4,4-dimethyl-2{1-oxy(diphenylphos-
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colourless solution immediately became dark blue. The reaction
mixture was stirred for 15 h at room temperature, the solvent
removed under reduced pressure and the residue washed with
diethyl ether (2 × 15 mL). The product was collected by filtration
and dried under vacuum (blue-purple powder, 0.535 g, 92% yield).
X-Ray structure determinations
Single crystals of complexes 6–10 suitable for X-ray diffraction
analysis were obtained by slow layer-diffusion of n-pentane into
their corresponding dichloromethane solutions under nitrogen
atmosphere. Blue crystals of 11 were obtained from the same
Schlenk tube as 8 (purple) by slow layer-diffusion of n-pentane
into a CH2Cl2 solution of 8. Blue crystals of 12 were obtained
independently by slow layer-diffusion of n-pentane into a CH2Cl2
solution of 8. Diffraction data were collected on a Kappa CCD
diffractometer using graphite-monochromated Mo-Ka radiation
IR (KBr disc): 1639 cm−1 (m(C N)). Magnetic moment in solution
=
(Evans method): 3.92 lB. Anal. Calcd for C18H20Cl2CoNO2P: C,
48.78; H, 4.55; N, 3.16. Found: C, 48.35; H, 4.61; N, 3.00%.
Preparation of (1-[4,4-dimethyl-2-[1-oxy(diphenylphosphino)-
1-methylethyl]]- 4,5-dihydro-oxazole)cobalt dichloride (10). To
a solution of 1-[4,4-dimethyl-2-{1-oxy(diphenylphosphino)-1-
methylethyl}]-4,5-dihydro-oxazole (5) (0.410 g, 1.20 mmol) in
THF (20 mL), cobalt dichloride (0.094 g, 0.724 mmol) was added
and the colourless solution rapidly became dark blue. The reaction
mixture was stirred for 15 h at room temperature, the solvent
removed in vacuo and the residue washed with diethyl ether (2 ×
20 mL) and pentane (20 mL). The product was dried in vacuo and
obtained as a dark-blue powder (0.300 g, 88% yield). IR (KBr
˚
(k = 0.71073 A). The relevant data are summarized in Table 6.
Data were collected using U scans and the structures were
solved by direct methods using the SHELX-97 software.24,25 The
refinements were conducted by full-matrix least squares on F2.
No absorption correction was used. All non-hydrogen atoms were
refined anisotropically with H atoms mathematically introduced
as fixed contributors (SHELXL procedure).
disc): 1612 cm−1 (m(C N)). Magnetic moment in solution (Evans
=
Acknowledgements
method): 3.75 lB. Anal. Calcd for C20H24Cl2CoNO2P: C, 50.98; H,
5.13; N, 2.97. Found: C, 50.51; H, 5.08; N, 2.82%.
We thank the French Embassy in Beijing for a doctoral grant
to S. J. to perform research in Strasbourg in the context of the
Franco-Chinese Laboratory for Catalysis, and to the CNRS, the
Ministe`re de la Recherche (Paris) and the Institut Franc¸ais du
Pe´trole (IFP) for support. We are grateful to Professor Wen-Hua
Sun, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100080, China for his kind cooperation, to Dr A. DeCian and
Professor R. Welter (ULP Strasbourg) for the crystal structure
determinations and to M. Mermillon-Fournier for experimental
assistance.
Oligomerisation of ethylene
All catalytic reactions were carried out in a magnetically stirred
(900 rpm) 100 mL stainless steel autoclave. The interior of the
autoclave was protected from corrosion with a glass container. All
catalytic tests were started at 25 ◦C, and no cooling of the reactor
was done during the reaction. In the catalytic experiments with
AlEtCl2, 4.0 × 10−2 mmol of cobalt complex was used. Depending
on the amount of cocatalyst added (3, 5, or 7 mL of cocatalyst
solution in toluene for 6, 10 or 14 equiv. of AlEtCl2, respectively),
12, 10 or 8 mL toluene were added to dissolve the precatalyst so
that the total volume of all solutions was 15 mL. When MAO (1, 2,
or 4 mL of cocatalyst solution in toluene for 100, 200 or 400 equiv.
of MAO, respectively) was used as cocatalyst, 10−2 mmol of cobalt
complex was dissolved in 19, 18 or 16 mL of toluene, respectively,
so that the total volume of all solutions was 20 mL. After injection
of the catalyst solution under a constant low flow of ethylene,
the reactor was brought to working pressure and continuously
fed with ethylene, using a reserve bottle placed on a balance to
allow continuous monitoring of the ethylene uptake. The modest
temperature increase observed (from ca. 25 to 30 ◦C) resulted
solely from the exothermicity of the reaction. The oligomerisation
products and remaining ethylene were only collected from the
reactor at the end of the catalytic experiment. The gaseous and
liquid products were analysed separately and the combined results
are given in Tables 4 and 5. At the end of each test, the reactor
was cooled to 10 ◦C before the gaseous phase was transferred
into a 10 L polyethylene tank filled with water. An aliquot of this
gaseous phase was transferred into a Schlenk flask, previously
evacuated, for GC analysis. The products in the reactor were
hydrolyzed in situ by addition of ethanol (10 mL), transferred to a
Schlenk flask and separated from the metal complexes by trap-to-
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