Synthesis, structure, and catalytic ethylene oligomerization of nickel(II) and cobalt(II) complexes with symmetrical and unsymmetrical 2,9-diaryl-1,10-phenanthroline ligands

Article abstract of DOI:10.1016/j.ica.2008.03.018

A series of nickel(II) and cobalt(II) complexes, NiX₂L (X = Cl, Br; 1-6) and CoCl₂L (7-9), with 2,9-diaryl-1,10-phenanthroline ligands (L¹-L³) have been synthesized and characterized by elemental analysis, UV-Vi

Full text of DOI:10.1016/j.ica.2008.03.018

Inorganica Chimica Acta 362 (2009) 89–96
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Inorganica Chimica Acta
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Synthesis, structure, and catalytic ethylene oligomerization of nickel(II) and
cobalt(II) complexes with symmetrical and unsymmetrical
2,9-diaryl-1,10-phenanthroline ligands
b
c
a,
Peiju Yang a, Yue Yang a,d, Cui Zhang a,d, Xiao-Juan Yang a, , Huai-Ming Hu , Yici Gao , Biao Wu
*
*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b Department of Chemistry, Northwest University, Xi’an 710069, China
c College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
d Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 April 2007
Received in revised form 28 February 2008
Accepted 5 March 2008
Available online 18 March 2008
A series of nickel(II) and cobalt(II) complexes, NiX2L (X = Cl, Br; 1–6) and CoCl2L (7–9), with 2,9-diaryl-
1,10-phenanthroline ligands (L1–L3) have been synthesized and characterized by elemental analysis,
UV–Vis, IR spectroscopy, and X-ray crystal structural study (for 1, 4–7, 9). The solid-state structures of
1, 5–7 and 9 show four-coordinate, slightly flattened tetrahedral geometry at the Ni(II) or Co(II) center,
while 4 is five-coordinated (square-pyramidal), containing a THF molecule as an auxiliary ligand. The title
complexes (1–9) display good catalytic activities in ethylene oligomerization when activated with meth-
ylaluminoxane (MAO). While the Co(II) precatalysts produce primarily C4 isomers, the Ni(II) complexes
give ethylene dimers and trimers at normal pressure. The activities and yields of linear a-olefins increase
with increasing ethylene pressure for the Ni(II) complexes, leading to more high-molar-mass products
(C8–C18). Complex 6 displays the best catalytic activity among the complexes studied (up to 1518 kg/
mol[Ni] h at 10 atm).
Keywords:
Nickel(II)
Cobalt(II)
1,10-Phenanthroline
Ethylene oligomerization
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
has been a focus to explore novel polyolefin materials via the de-
sign and synthesis of late transition metal catalysts with a deliber-
Linear a-olefins in the range of C4–C20 are important industrial
materials and are widely used for the preparation of detergents,
plasticizers and as co-monomers for linear low-density polyethyl-
ene (LLDPE) [1–4]. One of the major processes for the production of
linear a-olefins is the oligomerization of ethylene, for example, the
SHOP process [5]. Late transition metal complexes are supposed to
have a strong propensity for undergoing b-hydrogen elimination
process on the central metal, which would facilitate ethylene olig-
omerization [6]. Particularly, aryl-substituted a-diimine Ni(II) and
Pd(II) complexes [7], and 2,6-bis(imino)pyridyl Fe(II) and Co(II)
complexes [8,9] have been found to have high catalytic activities
for the polymerization and oligomerization of ethylene, which
has inspired much research interest in the late transition metal
catalysis systems. Extensive studies have proved that the produc-
tivity and physical properties of polyolefins are strongly affected
by the coordination environment around the metal atom, such as
the coordination atom, the ligand backbone, as well as the substi-
tute groups on the ligands of the precatalysts [10,11]. Therefore, it
ate tuning of the ligand backbone.
1,10-Phenanthroline and its derivatives are well-established li-
gands for late transition metal coordination chemistry because
their steric and electronic environment can be conveniently tai-
lored by varying the substituents [12]. The 2,9-diaryl-1,10-phen-
anthrolines and their complexes, due to their rich chemical and
physical properties, have been frequently used in the field of
molecular biology and supramolecular self-assembly (e.g., the cat-
enates) [13–15]. However, as typical a-diimines, late transition
metal complexes of bidentate 1,10-phenanthrolines, have rarely
been reported for catalytic ethylene oligomerization/polymeriza-
tion [16,17], although some work of complexes with bis(imino)-
or mono(imino)-1,10-phenanthroline ligands in
a tridentate
(N^N^N) coordination fashion has appeared recently [18–24]. In
the present work, we have studied a series of nickel(II) and cobal-
t(II) dihalide complexes with bidentate (N^N) 1,10-phenanthroline
ligands bearing symmetrical or unsymmetrical aryl substituents on
the 2,9-positions. Such ligands may provide sufficient protecting
shield for the transition metal center, thus benefiting olefin oligo-
merization to produce high-molar oligomers. Herein, we report
the synthesis and crystal structure of these novel Ni(II) and Co(II)
* Corresponding authors. Tel./fax: +86 931 4968286 (B. Wu).
E-mail addresses: yangxj@lzb.ac.cn (X.-J. Yang), wubiao@lzb.ac.cn (B. Wu).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.018

90
P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
complexes, as well as their catalytic activities for ethylene oligo-
merization with the assistance of methylaluminoxane (MAO).
nanthroline had a significant effect on the yield of 2-phenyl-1,10-
phenanthroline, which increased gradually when the molar ratio
of phenyllithium to 1,10-phenanthroline was decreased from 4:1
to 1:1, reaching a maximum of 52% when 1,10-phenanthroline
was reacted with a slight excess of phenyllithium. Since 2-phe-
nyl-1,10-phenanthroline [29] is an important starting material
for the synthesis of the unsymmetrical ligand L2, this procedure
proved to be a facile, one-pot way to both L1 and the intermediate
monophenyl 1,10-phenanthroline. In contrast, when 1,10-phenan-
throline was reacted with 4 equiv. of naphthyllithium at room
temperature, only 2,9-bis(1-naphthyl)-1,10-phenanthroline (L3)
was isolated, with neglectable monoarylated phenanthroline. The
yield of the 2,9-substituted-1,10-phenanthrolines was dependent
on the steric bulk of the substituents (L1: 50%; L3: 31.5%). The
2,9-diaryl-1,10-phenanthrolines (L1–L3) were characterized with
elemental analysis, 1H NMR, ESI-MS and IR spectroscopy.
2. Results and discussion
2.1. Syntheses
Ligands L1–L3 were prepared by the modified literature proce-
dures [25–28]. According to the method utilized by Sauvage [26],
a two-step synthetic approach can be applied for the preparation
of 2,9-substituted phenanthrolines: 1,10-phenanthroline is first re-
acted with a slightly excess of the aryllithium reagent (RLi) at room
temperature, and the procedure is then repeated once using the
monoarylated phenanthroline as the reactant. This method was
adopted in our work to prepare the unsymmetrical ligand L2, 2-
phenyl-9-naphthyl-1,10-phenanthroline, where 2-phenyl-1,10-
phenanthroline was reacted with naphthyllithium to afford the
product. For the preparation of the symmetrical 2,9-disubstituted
1,10-phenanthrolines (L1 and L3), an alternative one-step synthesis
was employed, as shown in Scheme 1. In this modified procedure
1,10-phenanthroline was reacted directly with 4 equiv. aryllithium
reagents at room temperature, followed by hydrolysis and oxida-
tion with activated MnO2 to give the desired diarylated ligands.
The crude products could be purified by column chromatography
and subsequent recrystallization. It should be noted that in the
reaction of 1,10-phenanthroline with phenyl lithium, both 2,9-di-
phenyl-1,10-phenanthroline (L1, 50%) and the monoarylated prod-
uct, 2-phenyl-1,10-phenanthroline (39%), were obtained. The two
compounds, with a total yield of 89%, could be well separated by
column chromatography using ethyl acetate/petroleum ether as
eluent. Changing the stoichiometry of phenyllithium and 1,10-phe-
The nickel(II) and cobalt(II) complexes 1–9 were prepared by
the reaction of the ligands L1–L3 with NiCl2 ꢀ 6H2O, NiBr2 ꢀ 6H2O,
or CoCl2 ꢀ 6H2O in THF (Scheme 1), followed by recrystallization
from CH2Cl2 or CH2Cl2/CH3CN. They were characterized by elemen-
tal analysis, UV–Vis and IR spectroscopy, and the solid-state struc-
tures of 1, 4–7, and 9 were confirmed by X-ray diffraction analysis.
2.2. X-ray crystal structures
2.2.1. Nickel(II) complexes
Dark-red single crystals of the Ni(II) complexes 1, 5, 6 could be
obtained through slow evaporation of a mixed solution of dichloro-
methane/acetonitrile (50:50). Growth of single crystals of com-
plexes 3 and 4, which contain the unsymmetrical ligand L2, is
very difficult, as many attempts to crystallize them have been
unsuccessful. Finally, pale green crystals of complex 4 suitable
for X-ray analysis were obtained from its solution in CH2Cl2/CH3CN
in the presence of THF, and it turned out that the solid-state struc-
ture of 4 incorporates a THF molecule as an auxiliary ligand (vide
infra).
X-ray structural studies show that all of the Ni(II) complexes
have a metal to phenanthroline ratio of 1:1. The coordination
geometry of the three complexes (1, 5, and 6) that contain sym-
metrical ligands is similar to one another, with the nickel center
being four-coordinated by the two nitrogen donors of the biden-
tate ligand and two halide ions (Figs. 1–3). Complex 1 contains a
NiCl2L1 molecule in an asymmetrical unit, while there are two
independent halves of NiCl2L3 molecules in 5, and half NiBr2L3
molecule in 6 per asymmetrical unit. The coordination environ-
ment around the Ni(II) ion in these three compounds is slightly
flattened tetrahedral, and the dihedral angle of the N–Ni–N and
X–Ni–X (X = Cl or Br) planes is 78.0°, 78.3/79.8° and 78.3° for 1,
5, and 6, respectively. Similar coordination geometry has also been
N
N
i)
N
N
N
N
N
R1
iii)
R1
R1
ii)
iii)
LMX2
N
1
1:
L , M = Ni, X = Cl
R1
R2
2: L1, M = Ni, X = Br
3: L2, M = Ni, X = Cl
4: L2, M = Ni, X = Br
L1: R1 = R2 = Ph
L2: R1 = Ph,
3
5:
6:
L , M = Ni, X = Cl
3
L , M = Ni, X = Br
R2 = 1-naphthyl
R = R = 1-naphthyl
7: L1, M = Co, X = Cl
8: L2, M = Co, X = Cl
9: L3, M = Co, X = Cl
1
2
L3:
Scheme 1. Synthesis of the ligands (L1–L3) and complexes 1–9. Reagents and co-
nditions: (i) (a) R1Li, diethyl ether/toluene; (b) H2O; (c) MnO2. (ii) (a) R2Li, diethyl
ether/toluene; (b) H2O; (c) MnO2. (iii) MX2 ꢀ 6H2O, THF.
Fig. 1. The molecular structure of complex 1 with 30% probability ellipsoids. Hy-
drogen atoms are omitted for clarity.

P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
91
described as distorted square pyramidal, with the atoms N1, Br1,
Br2 and O1 of the THF molecule defining the basal plane and atom
N2 occupying the apical site. The four basal atoms are efficiently
coplanar (mean deviation of N1, Br1, Br2, O1: 0.007 Å), and the
nickel atom is located slightly above this tetragonal plane by
0.291 Å, leading to the distortion of the bond angles between the
apical and basal atoms from 90°. Furthermore, the Ni–N2 bond is
bent to N1 and Br2 to adapt to the chelating coordination of N1
and N2. As a result, the N2–Ni–N1 (81.4(1)°) and N2–Ni–Br2
(90.82(9)°) angles are more acute than the N2–Ni–Br1
(101.59(9)°) and N2–Ni–O1 (115.0(1)°) angles. The two Ni–N bond
lengths are slightly different, with the axial Ni–N2 being longer
than the equatorial Ni–N1 by about 0.015 Å, while the two Ni–Br
bonds differ by 0.07 Å.
Fig. 2. The molecular structure of 5, showing the two independent molecules.
The Ni–N bond distances in these complexes (1, 4–6) fall into
the range of 1.998–2.060 Å (Table 1), which is normal for nickel(II)
phenanthroline and bipyridine complexes [32]. Although each of 1,
5 or 6 has two identical substitute groups on the phenanthroline
backbone, the molecule of complex 1 is not symmetrical, while
the molecules of 5 and 6 have a two-fold axis bisecting the
N–Ni–N and X–Ni–X angles. The dihedral angle between the phe-
nanthroline moiety and the attached phenyl or naphthyl substituent
is 28.1° and 43.4° for 1, 64.9/53.3° (two independent molecules) for
5 and 57.9° for 6. In the asymmetrical complex 4, the two dihedral
angles of phenanthroline-phenyl and phenanthroline-naphthyl are
50.6° and 67.7°, respectively.
2.2.2. Cobalt(II) complexes
Fig. 3. The molecular structure of 6.
The structures of two cobalt(II) complexes (7, 9) with the sym-
metrical diphenyl- or dinaphthyl-ligands (L1 and L3) have been
determined, which are similar to the nickel(II) analogs (1, 5) with
the corresponding ligands. The coordination geometry about the
Co(II) center is distorted tetrahedral, with the dihedral angle of
the N–Co–N and Cl–Co–Cl planes being 77.9° and 79.4/80.4°,
respectively. In an asymmetrical unit, complex 7 contains a CoCl2L1
molecule, while 9 has two independent halves of CoCl2L3 mole-
cules (Figs. 5 and 6). The Co–N bond distances in the complexes
(7, 9) are in the range of 2.049–2.237 Å. The dihedral angle be-
tween the phenanthroline moiety and the attached phenyl or
naphthyl substituent is 28.0°, 39.8° for 7, and 52.7/68.0° for 9.
There is no significant intermolecular p–p stacking in these
complexes.
observed in some copper(II) complexes [30,31]. There exist minor
differences of the N–Ni–N bite angle (1, 84.42(7)°; 5, 83.0(2)/
83.6(2)°; 6, 83.8(3)°).
The molecular structure of complex 4 bearing unsymmetrical li-
gand (L2) is different from those with symmetrical phenanthro-
lines. As mentioned above, this compound could be crystallized
only in the presence of THF, and a THF molecule was found to par-
ticipate in the coordination with Ni(II) center. This ‘‘extra” ligand
might be critical in the stabilization of the solid-state structure,
as there is a rather strong intramolecular C–Hꢀ ꢀ ꢀp interaction be-
tween the THF (C32H32B) and the naphthyl moiety (C19–C28)
(Hꢀ ꢀ ꢀCg distance: 2.21 Å; Cꢀ ꢀ ꢀCg: 3.14 Å; Hꢀ ꢀ ꢀplane: 2.17 Å; C–
Hꢀ ꢀ ꢀCg angle: 160.0°; Cg is the centroid of the naphthyl moiety).
As shown in Fig. 4, the nickel atom in compound 4 is five-coordi-
nated by the bidentate ligand L2, two bromide ions and a THF mol-
ecule. The coordination configuration around nickel(II) is best
2.3. Spectroscopic properties
The UV–Vis spectra of the Ni(II) and Co(II) complexes were
studied in dichloromethane with various concentrations. In the
Fig. 4. The molecular structure of 4, showing the square pyramidal, five-coordinate
Ni(II) center.
Fig. 5. The molecular structure of 7.

92
P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
Fig. 6. The structure of 9, showing the two independent molecules.
Table 1
Selected bond lengths (Å) and bond angles (°) for 1, 4–7 and 9
NiCl2L1 (1)
Ni–N(1) 2.028(2)
Ni–N(2) 2.008(2)
Ni–Cl(1) 2.223(1)
Ni–Cl(2) 2.2308(8)
N(2)–Ni–N(1)84.42(7)
N(2)–Ni–Cl(1)114.90(6)
N(1)–Ni–Cl(2)112.43(6)
N(1)–Ni–Cl(1) 99.80(5)
N(2)–Ni–Cl(2) 96.65(6)
Cl(1)–Ni–Cl(2)136.86(3)
NiBr2L2(THF) (4)
Ni–N(1) 2.045(3)
Br(1)–Ni 2.4920(7)
Ni–O(1) 2.077(3)
N(1)–Ni–N(2) 81.4(1)
Ni–N(2) 2.060(3)
Br(2)–Ni 2.5649(7)
N(1)–Ni–Br(1) 89.61(9)
N(2)–Ni–Br(1) 101.59(9)
O(1)–Ni–Br(2) 87.6(1)
N(2)–Ni–Br(2) 90.82(9)
O(1)–Ni–Br(1) 90.3(1)
N(1)–Ni–Br(2) 88.74(9)
N(1)–Ni–O(1) 163.3(1)
N(2)–Ni–O(1) 115.0(1)
Br(1)–Ni–Br(2) 167.09(3)
NiCl2L3 (5)
Ni(1)–N(1) 1.998(4)
Ni(2)–N(2) 2.020(4)
N(1)–Ni(1)–N(1)a 83.0(2)
Ni(1)–Cl(1) 2.212(1)
Ni(2)–Cl(2) 2.205(1)
N(2)–Ni(2)–N(2)b 83.6(2)
N(2)–Ni(2)–Cl(2) 101.1(1)
Cl(1)–Ni(1)–Cl(1)a 127.62(9)
N(1)–Ni(1)–Cl(1) 103.0(1)
N(1)–Ni(1)–Cl(1)a 115.8(1)
Cl(2)–Ni(2)–Cl(2)b 130.0(1)
N(2)–Ni(2)–Cl(2)b 115.9(1)
NiBr2L3 (6)
CoCl2L1 (7)
Ni–N 2.028(5)
Ni–Br 2.3459(8)
N–Ni–Nc 83.8(3)
N–Ni–Br 101.31(12)
N–Ni–Brc 116.19(11)
Br(1)–Ni–Br(1)c 129.29(5)
Co(1)–N(2) 2.049(2)
Co(1)–N(1) 2.070(2)
Co(1)–Cl(1) 2.2370(8)
Co(1)–Cl(2) 2.2395(9)
N(2)–Co(1)–N(1) 83.21(7)
N(2)–Co(1)–Cl(1) 118.25(6)
N(1)–Co(1)–Cl(1) 103.60(6)
N(2)–Co(1)–Cl(2) 101.19(6)
N(1)–Co(1)–Cl(2) 116.90(6)
Cl(1)–Co(1)–Cl(2) 126.11(3)
CoCl2L3 (9)
Co(1)–N(1) 2.066(3)
Co(1)–Cl(1) 2.222(1)
N(1)–Co(1)–N(1)d 80.5(2)
Co(2)–N(2) 2.078(3)
N(2)–Co(2)–Cl(2) 106.81(7)
N(1)–Co(1)–Cl(1)d 119.29(8)
N(2)–Co(2)–Cl(2)e 119.71(7)
N(1)–Co(1)–Cl(1) 107.77(8)
N(2)–Co(2)–N(2)e 117.28(6)
Cl(2)–Co(2)–Cl(2)e 117.63(7)
Co(2)–Cl(2) 2.196(1)
Cl(1)–Co(1)–Cl(1)d 81.5(2)
Symmetry code:
a
(ꢂ1 ꢂ x, ꢂy, z).
b
(ꢂx, ꢂy, z).
c
(1 ꢂ x, 1 ꢂ y, z).
d
(ꢂx, ꢂy, z).
e
(ꢂx, 1 ꢂ y, z).
UV region, the ligands displayed typical ligand-centered p–p* tran-
sitions at ꢁ260 and 308 nm for L1, and ꢁ230 and 290 nm for L2 and
L3, respectively (Fig. 7a). Upon coordination with metal ions, there
are minor changes of these bands.
absorptions could be attributed to the 4A2 ? 4T1(P) transition, cor-
responding to a tetrahedral cobalt(II) center [33]. Compared to the
ligand-centered absorption bands, the molar absorption coeffi-
cients of the visible transitions are much lower. The MLCT band
of 7–9 could not be resolved, probably due to overlap with the
higher-energy d–d transition. On the other hand, the d–d transi-
tions of the Ni(II) complexes in the visible region were not
observed.
The visible spectra of the complexes were obtained at higher
concentration (10ꢂ3 M) and Fig. 7b presents the absorption bands
of complexes NiCl2L3 (5), NiBr2L3 (6) and CoCl2L3 (9). In all cases,
the Ni(II) complexes (1–6) showed a broad peak with the maxi-
mum absorption at 500–520 nm and e value around 100 Mꢂ1 cmꢂ1
,
which is assigned to the MLCT charge transfer of the complexes.
The cobalt(II) complexes 7–9, however, showed very different
absorption bands from the nickel(II) compounds: their absorptions
appeared in much longer wavelengths. All the three Co(II) com-
plexes exhibited very similar absorption patterns, with a sharp
peak at ꢁ575 nm (e ꢁ 300 Mꢂ1 cmꢂ1) and a relatively broad band
at ꢁ675 nm accompanied by a shoulder. These long wavelength
2.4. Oligomerization of ethylene
In the present work, complexes 1–9 are active for ethylene olig-
omerization using MAO as cocatalyst. It should be noted that
although complex 4 crystallized as a THF adduct in the solid-state
structure, samples used in the ethylene oligomerization experi-
ments were obtained from CH2Cl2 and have the composition of

P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
93
0.8
0.6
0.4
0.2
0.0
0.6
a
b
L1
5
6
9
L2
L3
0.4
0.2
0.0
400
500
600
700
800
200
300
400
Wavelength (nm)
Wavelength (nm)
Fig. 7. The UV–Vis spectra of (a) ligands L1–L3 (concd. 1 ꢃ 10ꢂ5 M) and (b) complexes 5, 6 and 9 (concd. 1 ꢃ 10ꢂ3 M).
NiX2L as confirmed by elemental analysis. All the experiments
were repeated three times and the mean values were taken as
the final results. The results of ethylene oligomerization at 1 atm
are listed in Table 2. As analyzed by GC and GC–MS, ethylene di-
mers and trimers are the main oligomeric products for the nicke-
l(II) precatalysts (1–6), while the cobalt(II) complexes (7–9) lead
to mainly C4 isomers. The activities of Ni(II) complexes are slightly
higher than those of the Co(II) analogs.
Under normal pressure (1 atm), the nickel(II) complexes bear-
ing different steric-demanding ligands and different halide ions
showed slightly different catalytic activities for ethylene oligomer-
ization to yield isomers of the short-chain dimers and trimers (Ta-
ble 2, run 1–6). Sterically bulkier catalysts resulted in higher
activities, i.e., the activity increased gradually from the diphenyl-
substituted catalysts 1 and 2 to the phenyl/naphthyl (3, 4), and
then to the dinaphthyl-substituted compounds (5, 6) under the
same conditions. The properties of the oligomer products also dis-
played some differences: the complexes (5 and 6) containing
dinaphthyl groups (Table 2, run 5 and 6) led to a higher yield of
ethylene trimers (C6 about 29–45%) than the less bulky compounds
1–4 (C6 about 17–22%) under the identical reaction conditions (run
1–4). Furthermore, the catalytic activities of the dibromides are
better than those of the dichloride analogs. The cobalt(II) com-
plexes (7–9) exhibited slightly lower activities than the nickel(II)
analogs; however, the distribution of the ethylene oligomers is sig-
nificantly different, with C4 isomers as the only products.
2.4.1. Effects of Al/M molar ratio, oligomerization temperature and
time
It is well known that the catalytic performance of a given cata-
lyst is greatly dependent on the oligomerization conditions. To
probe the effects of reaction parameters on the ethylene oligomer-
ization behavior, the precatalyst 6 was investigated under varying
conditions, such as the amount of MAO (i.e., the molar ratio of Al/
M), the reaction temperature and reaction time. The catalytic activ-
ity of complex 6 increased rapidly when the Al/Ni ratio was in-
creased from 250 to 1000 (Table 2, run 6–8), but there were no
significant changes with larger Al/Ni ratios (from 1000 to 2000,
run 6 and 9, 10). However, at Al/Ni = 250 (Table 2, run 7) the pre-
catalyst led to a better yield of linear a-olefins (78%) and ethylene
dimers (C4 about 96%) than at Al/Ni = 500–2000 (C4 about 55–73%,
linear a-olefins about 8–26%) at 15 °C (run 6 and 8–10).
The catalytic lifetime of complex 6 was investigated with Al/
Ni = 1000 and reaction temperature at 15 °C (Table 2, run 6 and
11–15) in the range of 15–90 min. The precatalyst showed a rela-
tively stable activity, only slightly decreasing when the reaction
time exceeded 60 min, and was still active when prolonged to
90 min. On the other hand, the temperature factor was tested for
complex 6 ranging from 0 to 60 °C with Al/Ni = 1000 (run 6 and
16–19). The catalytic activity increased when the temperature
was raised from 0 to 15 °C and then to 30 °C, but it remained nearly
constant at further elevated temperature, up to 60 °C. The yield of
the linear a-olefins showed a slight increase (run 6 and 16–19)
with the increasing temperature. However, the detailed mecha-
nism for the increase is not yet clear. In general, the selectivity
for linear a-olefins is low for the nickel(II) precatalysts (Table 2)
due to their ability to isomerize a-olefins [34].
Table 2
Ethylene oligomerization with 1–9/MAO at 1 atm
Run Complex Al/M
Time
T
Activity
Product
(mol/mol) (min) (°C) (kg/mol* h) distribution (wt%)
C4
C6
Linear
a-olefins
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1
2
3
4
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
7
8
9
1000
1000
1000
1000
1000
1000
250
30
30
30
30
30
30
30
30
30
30
15
45
60
75
90
30
30
30
30
30
30
30
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
0
58
72.5
86
90
94
136
21
82
112
118
121
108
103
98
93
85
149
134
144
39
82
79
78
83
71
58
96
73
60
60
61
63
60
60
61
62
73
74
76
>99
>99
>99
18 24
21
22 10
17 37
29 19
42 22
8
4
78
8
500
27
1500
2000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
40 13
40 26
39 16
37 20
40 14
40 14
39 17
38 22
27 27
25 32
24 32
2.4.2. Effects of ethylene pressure on the catalytic behavior
The ethylene oligomerization and polymerization behaviors of
precatalysts 1–9 were investigated under 10 atm of ethylene
(Table 3). At this pressure, the activities of the nickel(II) dibromide
complexes were significantly higher than those of the dichloride
analogs (Table 3, run 23–28), which is in contrast to the minor dif-
ferences between the bromides and chlorides at 1 atm (Table 2). It
can be seen that the catalytic activities of the Ni(II) and Co(II) com-
plexes were considerably enhanced at higher pressure, and the oli-
gomer distribution was also impacted, as GC–MS analysis revealed
30
45
60
15
15
15
–
–
–
74
43
64
42
40
General conditions: 10 lmol precatalyst; 40 mL toluene as solvent.

94
P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
Table 3
recorded with an HP 8453 spectrometer. Oligomerization activity
measurements and oligomer distribution analysis were carried
out on an Agilent 6890N/5973N GC/MS spectrometer (Hp-1 ms
column) and Sp-2100 detectors.
Ethylene oligomerization with 1–9/MAO at different pressures
Run
Complex
Pressure
(atm)
Activity
(kg/mol[M] h)
Product distribution (wt%)
C4
C6
C P 8
Linear
a-olefins
4.2. Synthesis of the ligands L1–L3
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
1
2
3
4
5
6
6
6
6
7
8
9
9
9
9
10
10
10
10
10
10
7.5
5
2.5
10
10
10
7.5
5
257
766
235
747
233
1518
652
294
113
39
33
61
48
38
35
46
28
33
38
27
24
35
36
44
44
51
22
–
21
12
28
27
30
11
28
16
5
–
–
–
–
25
26
27
34
28
40
45
45
41
59
13
28
14
14
22
4.2.1. 2,9-Diphenyl-1,10-phenanthroline (L1) and 2-phenyl-1,10-
phenanthroline
A solution of bromobenzene (18.84 g, 120 mmol) in 20 mL of
dry diethyl ether was added dropwise under nitrogen to a suspen-
sion of lithium (2.48 g, 360 mmol) in 150 mL of diethyl ether at
0 °C. The solution was allowed to stir for 1 h, and was slowly added
to a suspension of 1,10-phenanthroline (5.40 g, 30 mmol) in 80 mL
of toluene at 0 °C. The mixture first became yellow and then dark
violet, which was stirred overnight at room temperature. After
addition of water (60 mL) the organic layer was separated, and
the aqueous phase was extracted three times with dichlorometh-
ane (3 ꢃ 15 mL). The combined organic extracts were stirred with
110 g of activated MnO2 for 2 h. After filtration, the solution was
dried over MgSO4, and the solvent removed by rotary evaporation
to give a solid of the mixture of the desired di- and monoarylated
1,10-phenanthroline. The two products were then separated by
column chromatography on silica gel using ethyl acetate/petro-
leum ether (2:1, v/v) as the eluent. The disubstituted 2,9-diphe-
nyl-1,10-phenanthroline was eluted first (yield 50%), followed by
the 2-phenyl-1,10-phenanthroline (39%). Both products were
recrystallized from toluene. Total yield: 89% (based on 1,10-phe-
nanthroline). The characterization of the two compounds showed
identical results to the literature reports [29,35].
73
>99
>99
>99
>99
>99
>99
104
118
111
58
–
–
–
–
–
–
2.5
46
–
General conditions: 10 lmol precatalyst; 100 mL toluene as solvent; Al/M = 1000;
60 min.
an increase of the yield of linear a-olefins and ethylene trimers. The
best catalytic activities of the complexes at 10 atm were obtained
for complex 6 (1518 kg/mol[Ni] h). It is noteworthy that higher
carbon-number olefins (C8–C18) were also produced together with
ethylene dimers and trimers at 10 atm pressure for the Ni(II) com-
plexes, while the Co(II) analogs gave only dimers at higher pres-
sure, the same as at normal pressure. Finally, the precatalysts 6
and 9 were chosen to test the change of the catalytic activity with
the ethylene pressure (run 28–30, 34–37). The activities of the two
compounds increased gradually with increasing ethylene pressure.
4.2.2. 2-Phenyl-9-(1-naphthyl)-1,10-phenanthroline (L2)
Similar to the above procedure, 1-bromonaphthalene (4.97 g,
24 mmol) was reacted with lithium (0.48 g, 69.5 mmol) in diethyl
ether. Then the aryllithium agent was reacted with 2-phenyl-
1,10-phenanthroline (1.53 g, 6 mmol) in 55 mL of toluene. After
hydrolysis with 30 mL of water and extraction with CH2Cl2, the or-
ganic extracts were stirred with 50 g of activated MnO2. The sol-
vent was removed and the resulting yellow solid was purified by
column chromatography (ethyl acetate/petroleum ether = 1:4),
and the product was recrystallized from ethyl acetate/petroleum
ether. Yield: 1.45 g (68.0%). Mp: 172–173 °C. ESI-MS: m/z 357
[M+H]+. 1H NMR (CDCl3, 400 MHz, ppm): d = 8.86 (d, J = 9.2 Hz,
1H, naphthyl-H2), 8.85 (d, J = 8.4 Hz, 2H, H4, H7), 8.32–8.36 (m,
2H, Ph-o-H), 8.13 (d, J = 8.4 Hz, 1H, H3), 8.01 (d, J = 8.0 Hz, 1H,
H8), 7.92–7.96 (m, 3H, naphthyl-H4,5,8), 7.85 (s, 2H, H5, H6),
7.54–7.61 (m, 3H, Ph-m-H + naphthyl-H3), 7.40–7.46 (m, 3H, Ph-
p-H + naphthyl-H6,7). 13C NMR (CDCl3, 100 MHz, ppm): d = 158.9,
156.8, 146.2, 146.0, 139.4, 138.3, 136.7, 136.2, 134.2, 131.6,
129.4, 129.3, 128.7, 128.3, 127.7, 127.4, 126.6, 126.4, 126.1,
125.9, 125.8, 125.3, 124.4, 119.9. IR (KBr, m/cmꢂ1): 3034, 1586,
1577, 1539, 1505, 1483, 852, 779, 696, 517. Anal. Calc. for
C28H18N2: C, 87.93; H, 4.74; N, 7.32. Found: C, 88.31; H, 4.61; N,
7.03%.
3. Conclusions
The synthesis, structure, and catalytic ethylene oligomerization
properties of six nickel(II) halide and three cobalt(II) chloride com-
plexes with symmetrical and unsymmetrical 2,9-disubstituted
phenanthroline ligands are reported. All complexes exhibit good
catalytic activities for ethylene oligomerization, with Ni(II) com-
plexes being slightly better than the Co(II) ones. At normal pres-
sure, the major products of the Ni(II) complexes are ethylene
dimers and trimers, while the Co(II) analogs give only dimers at
either normal or higher pressure. The ethylene oligomerization
activities are influenced by the Al/M molar ratio, reaction time,
and ethylene pressure, and complex 6 displays the best catalytic
activity among the series of complexes (up to 1518 kg/mol[Ni] h
at 10 atm).
4. Experimental
4.1. General considerations
All manipulations were carried out under an atmosphere of
nitrogen using standard Schlenk vacuum line techniques. Solvents
were refluxed over an appropriate drying agent and distilled under
nitrogen prior to use. MAO solution (1.65 M) in toluene was pur-
chased from Albemarle Corp (USA). 1,10-Phenanthroline, 1-bromo-
naphthalene, bromobenzene, the nickel(II) and cobalt(II) salts, and
other chemicals were commercially available and were used with-
out further purification. 1H NMR spectra were obtained on a Mer-
cury plus-400 spectrometer with TMS as the internal standard. IR
spectra were recorded as KBr pellets with an HP5890 II GC/
NEXUS-870 spectrometer. ESI-MS spectra were measured on a
Waters ZQ 4000 instrument. Element analyses were performed
with an Elementar VarioEL instrument. Electronic spectra were
4.2.3. 2,9-Bis(1-naphthyl)-1,10-phenanthroline (L3)
The same reaction and work-ups employed for L1 and L2
were followed for L3, with 1-bromonaphthalene (28.98 g,
140 mmol), lithium (2.48 g, 360 mmol), 1,10-phenanthroline
(5.40 g, 30 mmol), and activated MnO2 (120 g) being used in the
preparation. The product was purified by column chromatography
(ethyl acetate/petroleum ether = 4:1) and recrystallized from ethyl
acetate/petroleum ether. Yield: 4.08 g (31.5%). Mp: 209–210 °C.
ESI-MS: m/z 433.6 [M+H]+. 1H NMR (CDCl3, 400 MHz, ppm):
d = 8.61 (d, J = 6.8 Hz, 2H, H4, H7), 8.39 (d, J = 8.0 Hz, 2H, naph-
thyl-H2), 7.99 (d, J = 8.8 Hz, 2H, naphthyl-H4), 7.91 (s, 2H, H5,

P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
95
H6), 7.88 (d, J = 6.4 Hz, 6H, H3, H8 + naphthyl-H5,8), 7.56 (t, J = 7.6,
8.0 Hz, 2H, naphthyl-H3), 7.46 (m, 4H, naphthyl-H6,7). 13C NMR
(CDCl3, 100 MHz, ppm): d = 159.1, 146.2, 138.6, 136.1, 134.0,
131.5, 129.1, 128.6, 128.2, 127.4, 126.4, 126.2, 126.1, 125.6,
125.2, 124.7. IR (KBr, m/cmꢂ1): 3041, 1618, 1582, 1541, 1509,
1499, 1152, 797, 776, 746, 731, 706, 667, 651, 625, 607, 584,
558, 507, 494, 495, 442, 415. Anal. Calc. for C32H20N2: C, 88.86;
H, 4.66; N, 6.48. Found: C, 88.69; H, 4.75; N, 6.51%.
705, 663, 519. Anal. Calc. for C28H18N2Br2Ni: C, 55.96; H, 3.02; N,
4.66. Found: C, 55.78; H, 3.25; N, 4.58%. ESI-MS: 521.3 [MꢂBr]+.
4.3.5. NiCl2L3 (5)
Yield: 46.4 mg (82.6%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3054,
1625, 1588, 1558, 1515, 1495, 1365, 1152, 879, 861, 802, 777,
665, 517. Anal. Calc. for C32H20N2Cl2Ni: C, 68.37; H, 3.59; N, 4.98.
Found: C, 67.98; H, 3.32; N, 4.76%. ESI-MS: 525.4 [MꢂCl]+.
4.3. Synthesis of the complexes 1–9
4.3.6. NiBr2L3 (6)
Yield: 49.9 mg (76.7%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3055,
1625, 1587, 1557, 1514, 1494, 1156, 873, 797, 778, 664, 515. Anal.
Calc. for C32H20N2Br2Ni: C, 59.04; H, 3.10; N, 4.30. Found: C, 58.88;
H, 2.96; N, 4.15%. ESI-MS: 571.4 [MꢂBr]+.
The title complexes were prepared by reaction of nickel or co-
balt dichloride or dibromide with the corresponding ligands. The
typical synthesis is described below for 1 and 7, and complexes
2–6 and 8, 9 were synthesized following similar procedures.
4.3.1. NiCl2L1 (1)
4.3.7. CoCl2L1 (7)
The ligand2,9-diphenyl-1,10-phenanthroline (33.2 mg, 0.1 mmol)
and NiCl2 ꢀ 6H2O (23.9 mg, 0.1 mmol) in THF (10 mL) were refluxed
for 8 h to obtain a pink precipitate, which was filtered and washed
with ethanol (3 ꢃ 5 mL) and diethyl ether (3 ꢃ 5 mL). The product
was recrystallized from CH2Cl2. Yield: 40.2 mg (87%). Mp > 300 °C.
IR (KBr, m/cmꢂ1): 3045, 1622, 1585, 1551, 1508, 1487, 1422, 1361,
1155, 869, 775, 740, 703, 660, 521, 439. Anal. Calc. for C24H16N2Cl2Ni:
C, 62.39; H, 3.49; N, 6.06. Found: C, 62.15; H, 3.46; N, 5.86%. ESI-MS:
425.4 [MꢂCl]+.
The ligand 2,9-diphenyl-1,10-phenanthroline(33.2 mg, 0.1 mmol)
and CoCl2 ꢀ 6H2O (23.8 mg, 0.1 mmol) in THF (10 mL) were refluxed
for 8 h to obtain a blue precipitate, which was filtered and washed
with ethanol (3 ꢃ 5 mL) and diethyl ether (3 ꢃ 5 mL). The product
was then recrystallized from CH2Cl2/CH3CN. Yield: 37.2 mg
(80.5%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3046, 1620, 1586, 1576,
1549, 1506, 1487, 1445, 1419, 1358, 1276, 1179, 1156, 1121, 1021,
907, 869, 851, 776, 759, 741, 703, 658. Anal. Calc. for C24H16N2Cl2Co:
C, 62.36; H, 3.49; N, 6.06. Found: C, 62.59; H, 3.25; N, 6.26%. ESI-MS:
426.4 [MꢂCl]+.
4.3.2. NiBr2L1 (2)
Yield: 45.1 mg (82.1%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3059,
1622, 1588, 1553, 1512, 1488, 1152, 862, 776, 702, 661, 601. Anal.
Calc. for C24H16N2Br2Ni: C, 52.32; H, 2.93; N, 5.09. Found: C, 52.18;
H, 2.78; N, 4.92%. ESI-MS: 471.4 [MꢂBr]+.
4.3.8. CoCl2L2 (8)
Yield: 39.1 mg (76.4%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3444,
3058, 2922, 1623, 1587, 1555, 1512, 1496, 1424, 1395, 1363,
1343, 1298, 1245, 1188, 1152, 1132, 1103, 1062, 1023, 975, 907,
868, 800, 779, 759, 665, 612. Anal. Calc. for C28H18N2Cl2Co: C,
65.65; H, 3.54; N, 5.47. Found: C, 65.27; H, 3.75; N, 5.61%. ESI-
MS: 476.5 [MꢂCl]+.
4.3.3. NiCl2L2 (3)
Yield: 29.3 mg (60.2%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3057,
1625, 1587, 1556, 1514, 1488, 1152, 804, 781, 705, 663, 519. Anal.
Calc. for C28H18N2Cl2Ni: C, 65.68; H, 3.54; N, 5.47. Found: C, 65.47;
H, 3.50; N, 5.22%. ESI-MS: 475.4 [MꢂCl]+.
4.3.9. CoCl2L3 (9)
Yield: 39.2 mg (69.7%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3426,
3057, 2923, 1623, 1586, 1552, 1511, 1490, 1420, 1395, 1361,
1240, 1187, 1150, 1023, 986, 907, 873, 803, 779, 757, 661, 606,
550, 433. Anal. Calc. for C32H20N2Cl2Co: C, 68.35; H, 3.58; N, 4.98.
Found: C, 68.62; H, 3.41; N, 4.79%. ESI-MS: 526.6 [MꢂCl]+.
4.3.4. NiBr2L2 (4)
Yield: 33.6 mg (58.4%). Mp > 300 °C. IR (KBr, m/cmꢂ1): 3057,
1625, 1587, 1555, 1513, 1499, 1487, 1364, 1154, 876, 803, 781,
Table 4
Crystallographic data for compounds 1, 4–7, and 9
1
4
5
6
7
9
Formula
Fw
C24H16Cl2N2Ni
462.00
C32H26Br2N2NiO
673.08
C32H20Cl2N2Ni
562.11
C32H20Br2N2Ni
651.04
C24H16Cl2CoN2
462.22
C32H20Cl2CoN2
562.33
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
7.808(2)
13.836(3)
19.122(4)
90
monoclinic
P21/n
9.634(2)
17.400(4)
15.805(3)
90
orthorhombic
Pba2
15.784(3)
17.441(4)
9.354(2)
90
orthorhombic
P21212
15.981(3)
8.728(2)
9.402(2)
90
monoclinic
P21/n
7.918(2)
13.770(2)
19.148(3)
90
orthorhombic
Pba2
15.422(3)
17.208(3)
9.678(2)
90
a (°)
b (°)
101.47(3)
90
2024.4(7)
4
1.516
944
98.93(3)
90
2617.3(9)
4
1.708
1352
90
90
2575.1(9)
4
1.450
90
90
1311.5(5)
2
1.649
101.262(8)
90
2047.5(6)
4
1.499
940
90
90
2568.3(9)
4
1.454
c (°)
V (Å3)
Z
Dcalc (g cmꢂ3
F(000)
)
1152
0.986
648
3.811
1148
0.901
l (mmꢂ1
)
1.235
3.825
1.112
h Range (°)
Reflection collected (Rint
Independent reflections
1.83–27.34
17879 (0.0379)
4396
1.75–27.48
24504 (0.0501)
6000
1.74–28.08
15833 (0.0910)
5082
2.17–28.30
8347 (0.0520)
3185
1.83–26.81
11913 (0.0337)
4334
1.77–28.60
15836 (0.0445)
5643
)
Observed reflections [I > 2r(I)]
R1; wR2 [I > 2r(I)]
3458
4039
2168
1704
3067
2938
0.0310; 0.0828
0.0441; 0.0866
1.094
0.0440; 0.1128
0.0706; 0.1227
0.977
0.0398; 0.0719
0.1299; 0.0854
0.794
0.0464; 0.1010
0.1115; 0.1198
1.007
0.0374; 0.0845
0.0605; 0.0956
1.027
0.0352; 0.0787
0.0834; 0.0870
0.809
R1; wR2 (all data)
Goodness-of-fit (F2)

96
P. Yang et al. / Inorganica Chimica Acta 362 (2009) 89–96
4.4. X-ray crystal structure determination
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2008.03.018.
Diffraction data for the complexes were collected with a Rigaku
RAXIS-RAPID IP diffractometer (1, 4) or a Bruker SMART CCD area
detector (5–7, 9) at room temperature (293 K) with graphite-
monochromated MoKa radiation (k = 0.71073 Å). An empirical
absorption correction using SADABS [36] was applied for all the data.
The structures were solved by direct methods using the SHELXS pro-
gram. All non-hydrogen atoms were refined anisotropically by full-
matrix least-squares on F2 by the use of the program SHELXL [37].
The hydrogen atoms were included in idealized geometric posi-
tions with thermal parameters equivalent to 1.2 times those of
the atom to which they were attached. Crystallographic data for
the complexes are listed in Table 4.
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4.5. General procedure for ethylene oligomerization
4.5.1. Oligomerization at 1 atm
In a typical experiment, the complex (10 lmol) was added to a
glass reactor, which was evacuated and filled three times with
nitrogen and twice with ethylene. Freshly distilled toluene
(40 mL) was added to the reactor, followed by methylaluminoxane
(MAO) toluene solution via a syringe. The reaction mixture was
stirred under 1 atm ethylene for a given period while the temper-
ature was maintained by a water bath. A small amount of the reac-
tion mixture was taken out with a syringe and quenched by
ice-cooled 10% HCl solution, and the product was analyzed by GC
and GC–MS. Then the reaction was terminated by adding HCl/EtOH
solution, and no polymers were found. The yield of oligomers was
calculated by referencing with the mass of the used solvent based
on the prerequisite that the mass of each fraction is approximately
proportional to its integrated areas in the GC trace [38].
4.5.2. High-pressure oligomerization
A 1 L stainless-steel autoclave equipped with an electronically
controlled stirrer was evacuated and filled three times with
nitrogen and twice with ethylene. Freshly distilled toluene
(100 mL) was added. After the temperature reached a certain point,
MAO was injected and then a toluene solution (20 mL) of the cat-
alyst was added. The reactor pressure was kept constant through-
out the oligomerization process by manually controlled addition of
ethylene. After 30 min the reactor was cooled in an ice bath, the
extra ethylene was vented off and the reaction was quenched by
10% HCl. Quantitative GC analysis of the product was performed
immediately after the termination of the reaction.
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Acknowledgment
We thank the ‘‘Bairen Jihua” program of Chinese Academy of
the Sciences for financial support.
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: Area-Detector Absorption Correction,
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Appendix A. Supplementary material
[37] G.M. Sheldrick, SHELXS-97, SHELXL-97, Programs for Crystal Structure Analysis,
University of Göttingen, Germany, 1997.
CCDC 620348–620351, 631891 and 631892 contain the supple-
[38] R.L. Grob, Modern Practice of Gas Chromatography, second ed., 1985.
mentary crystallographic data for 1, 4–7 and 9. These data can be