Ni(II) complexes bearing 2-aryliminobenzimidazole: Synthesis, structure and ethylene oligomerization study

Article abstract of DOI:10.1016/j.inoche.2004.12.021

A series of nickel complexes bearing 2-aryliminobenzimidazole ligands were synthesized. Single-crystal X-ray analysis of complex 2 shows the nickel center adopts a distorted square-pyramidal geometry. Treatment of the complexes with methylaluminoxane (MAO) leads to active catalysts for ethylene oligomerization.

Full text of DOI:10.1016/j.inoche.2004.12.021

Inorganic Chemistry Communications 8 (2005) 246–248
www.elsevier.com/locate/inoche
Ni(II) complexes bearing 2-aryliminobenzimidazole:
synthesis, structure and ethylene oligomerization study
*
Jianlong Du, Li-Jun Li , Yanfeng Li
College of Chemistry & Environmental Science, Hebei University, 88 Wusi Donglu, Baoding 071002, PR China
Received 22 November 2004; accepted 23 December 2004
Abstract
A series of nickel complexes bearing 2-aryliminobenzimidazole ligands were synthesized. Single-crystal X-ray analysis of complex
2 shows the nickel center adopts a distorted square-pyramidal geometry. Treatment of the complexes with methylaluminoxane
(MAO) leads to active catalysts for ethylene oligomerization.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Nickel complex; Schiff base; Ethylene oligomerization; Crystal structure
In recent years there has been increasing interest in
the development of late transition metal based com-
plexes as catalysts for the polymerization and oligomer-
ization of olefins [1,2]. The significant achievement by
Brookhart group showed that Ni(II) and Pd(II) com-
plexes bearing bulky a-diimine ligands convert both eth-
ylene and a-olefins to high molecular mass polymers
with varying and controllable degrees of branching
[3,4]. Moreover, Brookhart and Gibson groups individ-
ually reported highly active polymerization catalysts
based on Fe(II) and Co(II) incorporating 2,6-
bis(imino)pyridyl ligands [5,6]. Modifications to the sub-
stituents of imine group result in dramatic changes to
the productivity and physical properties of the resultant
polyolefins [7,8]. Attracted by academic research and
potential industrial application, nickel complexes con-
taining pyridylimine ligands were explored for ethylene
oligomerization and polymerization [9]. Recently, exten-
sive study have been done with nickel complexes bearing
8-iminoquinoline [10] and related ligands 2-(2-pyr-
idyl)quinoxaline [11] and benzodiazepine [12], these
nickel complex systems performed activity for ethylene
oligomerization. In our current research, the ligand moi-
ety is modified as benzimidazole to form their nickel
complexes, and catalytic behavior of nickel complexes
are investigated.
The synthetic procedure of ligands (L1–L5, 2-arylimi-
nobenzimidazole) and their nickel complexes is generally
shown in Scheme 1. Ligands were prepared as white or
pale yellow solid in good yields by the condensation of
one equivalent of the appropriate aniline with one equiv-
alent of 2-acetylbenzimidazole. Ligands were character-
1
ized by microanalysis, H NMR and IR spectroscopy.
1
Nickel complexes 1–5 were synthesized by dissolving
(1,2-dimethoxyethane)nickel bromide in dichlorome-
thane (Scheme 1), followed by addition of two equiva-
lents of the corresponding ligand. The resultant nickel
complexes were precipitated from the reaction solution.
After washing with diethyl ether and re-crystallized from
ethanol, the complexes were obtained in good yield and
high purity. The complexes were characterized by ele-
mental analysis, and IR spectroscopy.
1
*
The synthetic details of ligands and the complexes are available in
the supplementary material.
Corresponding author. Tel./fax: +86 312 5079628.
E-mail address: llj@mail.hbu.edu.cn (L.-J. Li).
1387-7003/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.12.021

J. Du et al. / Inorganic Chemistry Communications 8 (2005) 246–248
247
H
N
NH2
H
N
N
N
R1
R1
TsOH
+
R1
R1
toluene, reflux
N
O
R2
L
R2
R1
R1
NH
+
N
R2
N
(DME)NiBr2
CH2Cl2
(1) R1=CH3, R2=H
(2) R1=CH3, R2=CH3
(3) R1=i-Pr, R2=H
(4) R1=CH3, R2=Br
(5) R1=Et, R2=H
R1
R1
Ni Br
N
Br -
R2
N
NH
Scheme 1. Synthesis of ligands and complexes.
Fig. 1. Crystal structure of complex 2 with 30% probability displace-
ment ellipsoids, H atoms and non-coordinated EtOH molecular are
˚
omitted for clarity. Selected bond lengths (A) and angles (ꢁ): Ni(1)–
N(4), 2.036(4); Ni(1)–N(1), 2.037(4); Ni(1)–N(6), 2.083(3); Ni(1)–N(3),
2.121(4); Ni(1)–Br(1), 2.3968(11) and N(4)–Ni(1)–N(6), 79.30(14);
N(1)–Ni(1)-N(3), 78.96(14); N(1)–Ni(1)–Br(1), 95.65(10); N(3)–
Ni(1)–Br(1), 125.64(9); N(4)–Ni(1)–Br(1), 96.55(11); N(6)–Ni(1)–
Br(1), 127.60(11).
Fig. 2. Packing view along a-axis of complex 2.
˚
complex, the Ni–N (benzimidazole) [2.036, 2.037 A]
bonds are shorter than the Ni–N (imino) [2.083,
˚
2.121 A] bonds. Center atom Ni forms two five-mem-
2
To confirm its structure, crystal of complex 2 suit-
able for X-ray determination was grown by vapor diffu-
sion of Et2O into the ethanol solution. The crystal
bered rings with two ligands, and the two planes
[Ni(1)–N(1)–N(3), Ni(1)–N(4)–N(6)] make a dihedral
angel of 106.8ꢁ. At the same time, the two 2,4,6-tri-
methyl substituted aryl rings are oriented approximately
perpendicular to the their basal coordination plane, with
an approximately 89.1ꢁ and 92.6ꢁ twist angle about the
C–N bonds. The non-coordinated EtOH solvent mole-
cule is connected to one L2 ligand by H-bonding with
structure of
2 consists of mononuclear complex
[Ni(L2)2Br]+BrÀ and EtOH solvent molecule (Fig. 1
and Fig. 2). The structure reveals electrostatic interac-
tion between cation complex [Ni(L2)2Br]+ and counter-
ion BrÀ (Fig. 2). Each Ni atom locates in an extremely
distorted square-pyramidal environment formed by
two benzimidazole nitrogen atoms, two imino nitrogen
atoms of two ligands and one bromide atom, another
bromide atom acting as a counter ion locating far from
˚
O1Á Á ÁN5 distance of 2.76 A.
Upon treatment with methylaluminoxane (MAO), all
of the complexes are active ethylene oligomerization cat-
alysts. The catalysts activity is listed in Table 1, the
molecular weight distribution of the oligomers are calcu-
lated on the base of GC analysis. Further investigation
of complex 2 is performed at different reaction tempera-
tures. Like the complexes in the 8-iminoquinoline [10]
and 2-(2-pyridyl)quinoxaline [11] systems, the catalytic
activity of complex 2 is sensitive to reaction tempera-
ture. Lower temperature is generally favorable, the best
˚
the center atom Ni(II) [Ni(1)Á Á ÁBr(2) = 6.82 A]. In the
2
ꢀ
Complex 2, [C38H44Br2N6NiO], M = 819.32, triclinic, P1,
˚
T = 293(2) K, a = 11.921(4), b = 12.899(4), c = 14.313(5) A,
3
˚
a = 88.475(5), b = 66.920(5), c = 75.405(6)ꢁ, V = 1952.8(11) A , Z = 2,
Number of collected reflections = 11,166, Number of independed
reflections = 7823, R(F2) = 0.0543, wR = 0.1496. Full crystallographic
detail was deposited at the Cambridge Crystallographic Data Center
with number CCDC 256208.

248
J. Du et al. / Inorganic Chemistry Communications 8 (2005) 246–248
Table 1
Activity and distribution for the oligomerization
Cat.
Temp. (ꢁC)
Activity g/mol h atm
Distribution of oligomers (%)
C4
C6
C8
C10
C12
C14
C16
C18
1
2
2
2
3
4
5
15
0
2.11 · 105
1.45 · 105
1.99 · 105
0.72 · 105
1.95 · 105
2.35 · 105
1.86 · 105
6.34
19.07
13.93
15.84
19.54
17.79
18.72
18.60
16.30
16.14
14.29
14.84
11.54
14.61
13.16
23.32
23.03
24.79
21.08
21.10
24.48
23.09
18.95
17.19
19.15
16.24
18.14
18.79
19.15
11.78
12.99
13.17
7.29
3.78
7.80
6.43
1.88
3.21
3.26
3.58
0.73
3.62
3.20
0.74
1.54
0.28
1.22
0.53
2.07
15
50
15
15
15
18.40
13.77
7.99
11.85
11.87
7.86
12.44
Condition: catalyst 5 lmol, [Al]/[Ni] = 1500, toluene solvent, 1 atm of ethylene, reaction time 0.5 h.
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catalytic temperature is at 15 ꢁC. Steric and electronic
environmental around the central metal affects the cata-
lytic activities and the distribution of oligomers. Accord-
ing to Table 1, the order of their activities are listed as
4 > 1 > 2 > 3 > 5 at the same reaction condition. Com-
plex 4 shows the highest activity of 2.35 · 105 g (ethyl-
ene) molÀ1 (Ni) hÀ1. The oligomers are mixture of
a-olefin and internal olefin in range of C4 to C18. Fur-
ther detail investigation, including varying reaction
conditions and modifying the ligands, is under way.
[4] C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J. Am.
Chem. Soc. 118 (1996) 11664.
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Acknowledgement
[9] (a) S.P. Meneghetti, P.J. Lutz, J. Kress, Organometallics 18
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This work was supported by Foundation of Hebei
University.
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Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/
j.inoche.2004.12.021.
(d) B.Y. Lee, X. Bu, G.C. Bazan, Organometallics 20
(2001) 5425;
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