Ni(II) complexes with ligands derived from phenylpyridine, active for selective dimerization and trimerization of ethylene

Article abstract of DOI:10.1016/j.jorganchem.2012.08.005

An electrophilic substitution-carbonylation reaction on phenylpyridine based on the concept of 'umpolung' was used to prepare a series of pyridine based carbonyl compounds and bispyridine derivatives. The key intermediate which enhances this reaction is a base aggregate formed by the association of BuLi with lithium 2-dimethylaminoethanolate (LiDMAE) which is stabilized in nonpolar solvents. The presence of polar chelating amides that are used as acyl donors was found to collapse the superbase aggregates liberating nucleophilic 'free' BuLi. These nucleophiles lead a classical nucleophilic reaction to introduce butyl tails on the pre-ligand molecules. Pyridine carbonyl compounds produced by these electrophilic substitution-carbonylation reactions, on treatment with 2,6-diisopropylaniline and (DME)NiBr₂ in glacial acetic acid at reflux temperature, gave Ni(II) complexes in good yields in a one pot protocol. These complexes are active toward ethylene, producing selective dimerization and trimerization products.

Full text of DOI:10.1016/j.jorganchem.2012.08.005

Journal of Organometallic Chemistry 718 (2012) 8e13
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ni(II) complexes with ligands derived from phenylpyridine, active for selective
dimerization and trimerization of ethylene
Deepak Chandran a, Kyeong Mi Lee a, Hyuk Chul Chang a, Ga Young Song a, Ji-Eun Lee b, Hongsuk Suh c,
,
Il Kim a
*
a The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735,
Republic of Korea
b Central Instrumentation Facility, Gyeongsang National University, Jinju, Republic of Korea
c Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
An electrophilic substitutionecarbonylation reaction on phenylpyridine based on the concept of
‘umpolung’ was used to prepare a series of pyridine based carbonyl compounds and bispyridine deriv-
atives. The key intermediate which enhances this reaction is a base aggregate formed by the association
of BuLi with lithium 2-dimethylaminoethanolate (LiDMAE) which is stabilized in nonpolar solvents. The
presence of polar chelating amides that are used as acyl donors was found to collapse the superbase
aggregates liberating nucleophilic ‘free’ BuLi. These nucleophiles lead a classical nucleophilic reaction to
introduce butyl tails on the pre-ligand molecules. Pyridine carbonyl compounds produced by these
electrophilic substitutionecarbonylation reactions, on treatment with 2,6-diisopropylaniline and (DME)
NiBr2 in glacial acetic acid at reflux temperature, gave Ni(II) complexes in good yields in a one pot
protocol. These complexes are active toward ethylene, producing selective dimerization and trimeriza-
tion products.
Received 19 June 2012
Received in revised form
17 July 2012
Accepted 2 August 2012
Keywords:
Catalysis
Ethylene oligomerization
Transition metal chemistry
Ni(II) complex
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
and [N,O] chelating ligands based on ‘ligand oriented catalyst design
concept’, yielding industrially important ethylene oligomers [6,7].
Nitrogen chelating ligands having [N Nˇ ], [N Nˇ Nˇ ] and [N ˇ N ˇ N ˇ N]
structures are well known for their ability to coordinate with
various transition elements giving complexes which are active
catalysts in CeC bond forming reactions [1]. [N Nˇ ] and [N ˇ N ˇ N]
complexes bearing various transition elements such as Ni, Pd, Al, Co,
Fe and Cr are widely accepted catalysts in ethylene polymerization/
oligomerization reactions [1e4]. Designing the ligands of these
complexes in a reasonable way, they were found to produce
industrially important oligomers [5]. Complexes used in catalytic
purposes are preferred to have completely inert and non-labile
ligands for their stability. Realization of these characteristics has
been accomplished only for a few non-cyclopentadienyl systems
and remains a vibrant area of research. To achieve new ligand
structures and to finely tune them for better catalytic activity, a good
understanding in the successful design and synthesis of organo-
metallic complexes is required. In this background, we have inves-
tigated in the recent years for new types of complexes having [N,N]
The famous [N ˇ N] structures,
a-diimines, with Ni and Pd
complexes have paved dramatic changes in the field of single site
non-metallocene catalysts mediated olefin polymerization/oligo-
merization reactions [3]. After identifying heterocyclic compounds as
the efficient candidates providing nitrogen chelating ligands,
growing demand for such compounds has made chemists to design
efficient and selective methods to introduce functionalities on such
heterocyclic compounds [8e16]. In pyridine derivatives, the nitrogen
atom polarizes the ring p-system, resulting in a decreased electron
density on carbon atoms that make electrophilic substitution difficult
on the rings. After Seebach and Corey have developed the concept of
‘umpolung’ [17,18], a large number of investigations have been con-
ducted on various electrophilic/nucleophilic reactions, enabling this
concept potential to employ on pyridine based compounds. Most
efficient electrophilic substitution reported on pyridines involves the
use of lithiated pyridines as intermediates [19]. Halopyridines are
classically used as a potential substrate, because they can be con-
verted to lithiated pyridines [19]; however, the p-deficiency of pyri-
dine ring favors nucleophilic attack rather than lithiation. This leads
to investigate on a non-nucleophilic and strongly basic reagents that
can act as lithiating agents for pyridines. Fort and coworkers have
* Corresponding author. Tel.: þ82 51510 2399; fax: þ82 51512 8563.
E-mail address: ilkim@pusan.ac.kr (I. Kim).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.08.005

D. Chandran et al. / Journal of Organometallic Chemistry 718 (2012) 8e13
9
modified strongly nucleophilic n-BuLi by associating with lithium
chelating 2-dimethylaminoethanol (DMAE) in order to effectively
conduct a regioselective a-lithiation on pyridine rings directly [20].
a nitrogen cold stream (ꢀ100 ꢁC). The data were corrected for
Lorentz and polarization effects (SAINT), and semi-empirical
absorption corrections based on equivalent reflections were
applied (SADABS). The structure was solved by direct methods and
refined by full-matrix least-squares on F2 (SHELXTL). All the non-
hydrogen atoms were refined anisotropically, and hydrogen
atoms were added to their geometrically ideal positions.
Association with DMAE considerably increases the basicity/nucleo-
philicity ratio of BuLi by enhancing basicity through complexation
and inhibiting nucleophilicity through the formation of sterically
hindered aggregates. The key intermediate which enhances this
reaction is a base aggregate formed by the association of BuLi with
lithium 2-dimethylaminoethanolate (LiDMAE) which is stabilized in
nonpolar solvents. It has been found that the presence of chelating
polar solvents influences the stability of this super-base aggregate,
resulting in changing the reaction pathways and hence such solvents
can be used as trapping solvents to produce new compounds based
on pyridine [20]. It is appropriate to explore the potential of such
reactions to produce different pyridine based ligands.
In this regard, an attempt to synthesize various pre-ligands
based on phenylpyridine using reactions mediated by BuLi-
LiDMAE superbase has been made. A new method to preparare
pyridine-imine ligands based nickel complexes with [N,N] chela-
tion structure using an electrophilic substitutionecarbonylation
reaction on phenylpyridine is explained. Certain interesting
observations during the ligand synthesis, which are useful for
future design and development of organometallic complexes based
on ligand-oriented design are also described.
2.4. Oligomerization procedure
Ethylene oligomerizations were performed in a 250 mL glass
reactor equipped with septum adapter and a magnetic stir bar. The
required amount of catalyst was added to the reactor and kept
under vacuum for 30 min and then purged with N2. The reactor was
then charged with toluene (80 mL) under N2 and then pressurized
with ethylene (1.3 bar) at the desired temperature with stirring. The
oligomerization was started by the addition of cocatalyst. The
ethylene flow to the reactor was monitored by a mass flow meter
from the rate of consumption, measured by a hotwire flow meter
(model 5850
D from Brooks Instrument Div.) connected to
a personal computer through an A/D converter. After the given
reaction time the reactor was cooled to 0 ꢁC and samples for olig-
omer analysis were collected from the reactor by passing a 10 mL of
this cold mixture through a silica column to remove Al species.
Oligomers were analyzed by gas chromatography.
2. Experimental
2.5. Synthesis of pre-ligands
2.1. General procedures
To a cold (0 ꢁC) solution of dimethylaminoethanol (1 mL,
10 mmol) in n-hexane (25 mL) a solution 8 mL (20 mmol) of n-BuLi
(2.5 M in hexane) was added dropwise over a period of 10 min.
After stirring for 30 min, 2-phenylpyridine (0.6 mL, 4 mmol) in
n-hexane was added at 0 ꢁC and was stirred for another 30 min.
After the temperature was brought down to ꢀ78 ꢁC. an acylating
agent [4 mmol of N,N-dimethylformamide (DMF; 0.3 mL), N,N-di-
methylacetamide (DMAc; 0.4 mL) or N,N-dimethylbenzamide
(DMB; 0.6 g)] was added slowly. After keeping the solution
at ꢀ78 ꢁC for 1 h, it was warmed to room temperature overnight.
The reaction mixture was again cooled to -5-0 ꢁC and carefully
hydrolyzed with chilled water. The aqueous layer was extracted
with diethyl ether, dried over MgSO4 and then removed solvents
under vacuum to get the crude product. The reactions using DMF,
DMAc and DMB as the acyl donor gave products 1, 4 and 7, product
2, 5 and 7, and product 3, 6 and 7, respectively, in high purity, after
purifying using column chromatography (n-hexane/ethyl acetate).
All reactions and operations were performed under a purified
nitrogen atmosphere using standard glove box and Schlenk tech-
niques. Polymerization grade of ethylene (SK Co., Korea) was
purified by passing it through columns of Fisher RIDOXÔ catalyst
and molecular sieve 5 A/13X. All solvents used for synthesis and
oligomerization experiment were purified according to the stan-
ꢀ
ꢀ
dard procedure and stored over molecular sieves (4 A) under
nitrogen condition. Methylalumoxane (MAO) (8.4 wt.% total Al
solution in toluene) was donated by LG Chemicals, Korea and was
used without purification. All other reagents were purchased from
Aldrich Chemical Co. and were used without further purification.
2.2. Characterizations
UVevis spectra were recorded on a Shimadzu UV-1650 PC
spectrometer in toluene at room temperature under nitrogen
atmosphere. Fourier transform infrared (FTIR) spectra were recor-
ded on a Shimadzu IRPrestige-21 spectrometer by making CsI
pellets of the samples. Fast atom bombardment mass spectroscopy
(FAB-MS) was determined using a JMS-700 (JEOL Instrument).
Electron impact mass spectroscopy (EI-MS) was determined using
a JMS-600 (JEOL Instrument). Elemental analyses were carried out
using a Vario EL analyzer. 1H NMR (300 MHz) and 13C NMR
(75 MHz) spectra were recorded on a Varian Gemini 2000 spec-
trometer and the chemical shifts were reported in parts per million
relative to internal (CH3)4Si (1H, 13C) standard. FAB-MS spectra of
the complexes were recorded on a JMS-AX505WA/HP5890 Series II
GCemass spectrometer. Oligomerization products were analyzed
by a 7890A GC System (Agilent Technologies) with a J&W Scientific
30 m column with 0.250 mm inner diameter.
2.5.1. (2-Phenylpyridine-6-yl)-methanone (1)
Yield ¼ 0.07 g (12.2%). 1H NMR (300 MHz, CDCl3):
d
10.01 (s, 1H,
190.2,
CHO), 8.16e7.11 (m, 8H, Ar); 13C NMR (75 MHz, CDCl3):
d
158.6, 153.5, 141.7, 134.2, 128.4, 127.5, 123.3, 118.4. MS (EIþ): 182.
2.5.2. Methyl(2-phenylpyridine-6-yl)-methanone (2)
Yield ¼ 0.01 g (13.4%). 1H NMR (300 MHz, CDCl3):
d
8.10e7.31
197.1,
(m, 8H, Ar), 2.45 (s, 3H, Me); 13C NMR (75 MHz, CDCl3):
d
169.4,154.4,139.9,136.0,129.9,127.6,124.1,122.2, 24.4. MS (EIþ): 197.
2.5.3. Phenyl(2-phenylpyridine-6-yl)-methanone (3)
Yield ¼ 0.16 g (15.6%). 1H NMR (300 MHz, CDCl3):
d 8.74e7.18
d 200.7, 156.0, 153.9, 138.8,
(m, 13H, Ar), 13C NMR (75 MHz, CDCl3):
137.7, 136.9, 136.7, 133.0, 132.9, 129.4, 127.8, 122.9. MS (EIþ): 259.
2.3. Crystallographic studies
2.5.4. 4-Butyl-6-phenyl-pyridine-2-carbaldehyde (4)
The crystal was picked up with Paraton N oil and mounted on
Yield ¼ 0.76 g (78.2%). 1H NMR (300 MHz, CDCl3):
d 9.84 (s, 1H,
a Bruker SMART CCD diffractometer equipped with a graphite-
CHO), 7.79e7.10 (m, 7H, Ar), 2.78 (t, 2H, CH2), 1.50 (qn, 2H, CH2),
monochromated Mo K
a
(
l
¼ 0.71073 A) radiation source and
1.02 (sx, 2H, CH2), 0.91 (t, 3H, CH3); 13C NMR (75 MHz, CDCl3):
ꢀ

10
D. Chandran et al. / Journal of Organometallic Chemistry 718 (2012) 8e13
d
196.7, 158.1, 155.8, 145.8, 145.3, 131.4, 129.1, 126.7, 116.3, 37.0, 33.5,
56.06; H, 4.88; N, 4.45. IR (CsI, cmꢀ1) 3421, 3182, 1616, 1585, 1550,
22.6, 13.9. MS (EIþ): 238.
1500, 1462, 1388, 1292, 1253, 1188, 1118, 1060, 1033, 960, 802, 736,
354, 282. MS (FABþ): 476 [M-(2Br þ H)].
2.5.5. 1-(4-Butyl-6-phenyl-pyridin-2-yl)-ethanone (5)
Yield ¼ 0.79 g (77.4%). 1H NMR (300 MHz, CDCl3):
d
8.16e6.98
2.6.4. [(4-butyl-6-phenyl-pyridin-2-ylmethylene)-(2,6-diisopropyl-
phenyl)-amine]NiBr2 (11)
(m, 7H, Ar), 2.71 (t, 2H, CH2), 2.40 (S, 3H, CH3), 1.52 (qn, 2H, CH2),
1.01 (sx, 2H, CH2), 0.92 (t, 3H, CH3); 13C NMR (75 MHz, CDCl3):
The complex was obtained as pink crystals in 57.6% yield. Anal.
Calcd for C28H34Br2N2Ni: C, 54.50; H, 5.55; N, 4.54. Found: C, 54.96;
H, 5.08; N, 4.85. IR (CsI, cmꢀ1) 3422, 3180, 1618, 1581, 1551, 1500,
1462, 1388, 1292, 1253, 1188, 1118, 1060, 1033, 960, 802, 736, 354,
282. MS (FABþ): 457 [M ꢀ 2Br].
d
202.7,163.7,162.4,156.8,139.2,130.5,129.2,128.1,127.9, 36.8, 33.9,
25.2, 22.4, 13.9. MS (EIþ): 253.
2.5.6. (4-Butyl-6-phenyl-pyridin-2-yl)-phenyl-methanone (6)
Yield ¼ 0.95 g (75.1%). 1H NMR (300 MHz, CDCl3):
d 8.06e7.08
(m, 12H, Ar), 3.0 (t, 2H, CH2), 1.52 (qn, 2H, CH2), 0.99 (sx, 2H,
2.6.5. [1-(4-butyl-6-phenyl-pyridin-2-yl)-ethylidene]-(2,6-
diisopropyl-phenyl)-amine]NiBr2 (12)
CH2), 0.91 (t, 3H, CH3); 13C NMR (75 MHz, CDCl3):
d 200.7, 155.1,
153.8, 139.3, 138.6, 137.8, 134.8, 133.4, 130.9, 129.0, 128.3, 125.7.
121.6, 36.5, 33.8, 22.2, 13.9. MS (EIþ): 315.
The complex was obtained as pink crystals in 58.9% yield. Anal.
Calcd for C29H36Br2N2Ni: C, 55.19; H, 5.75; N, 4.44. Found: C, 55.06;
H, 5.28; N, 4.65. IR (CsI, cmꢀ1) 3421, 3182, 1616, 1585, 1550, 1500,
1462, 1388, 1292, 1253, 1188, 1118, 1060, 1033, 960, 802, 736, 354,
282. MS (FABþ): 551 [M ꢀ Br].
2.5.7. 6,60-diphenyl-[2,20]bipyridinyl (7)
Yield (N,N-dimethylformamide reaction) ¼ 0.02 g (3.4%), Yield
(DMAc reaction) ¼ 0.02 g (3.0%), and Yield (N,N-dimethylbenza-
mide reaction) ¼ 0.02 g (2.8%). 1H NMR (300 MHz, CDCl3):
2.6.6. [[(4-butyl-6-phenyl-pyridin-2-yl)-phenyl-methylene]-(2,6-
diisopropyl-phenyl)-amine]NiBr2 (13)
d
8.62e7.08 (m, 16H, Ar), 13C NMR (75 MHz, CDCl3):
d 160.7, 155.1,
138.1, 135.8, 129.5, 129.0, 126.3, 124.0, 119.0. MS (EIþ): 308.
The complex was obtained as pink crystals in 59.0% yield. Anal.
Calcd for C34H38Br2N2Ni: C, 58.91; H, 5.53; N, 4.04. Found: C, 58.29;
H, 5.36; N, 4.81. IR (CsI, cmꢀ1) 3424, 3179, 1619, 1580, 1549, 1498,
1461, 1388, 1291, 1254, 1187, 1118, 1061, 1036, 959, 801, 735, 354,
286. MS (FABþ): 613 [M ꢀ Br], 532 [M-(2Br þ H)].
2.6. Synthesis of complexes
Complexes 8e13 were synthesized in a similar manner using
nickel(II) bromide ethylene glycol dimethyl ether complex [(DME)
NiBr2]. A suspension of (DME)NiBr2 (1 mmol), 2-phenyl pyridine-6-
aldehydes or ketones 1e6 (1 mmol) and 2,6-diisopropyl aniline
(1 mmol) in glacial acetic acid (10 mL) was refluxed for 4 h. The
precipitate was filtered and washed with diethyl ether (5 mL ꢂ 3).
The residue collected were redissolved in dichloromethane and
concentrated to saturated solution, and then reprecipitated using
diethyl ether, filtered and washed with diethyl ether (5 mL ꢂ 3). The
solid residue was dried under vacuum overnight at 40 ꢁC.
2.6.7. [6,60-diphenyl-[2,20]bipyridinyl]NiBr2 (14)
The complex was obtained as a reddish yellow crystals in 58.6%
yield. Anal. Calcd for C22H16Br2N2Ni: C, 50.15; H, 3.06; N, 5.32.
Found: C, 49.86; H, 3.21; N, 4.66. IR (CsI, cmꢀ1) 3423, 3179, 1614,
1585, 1550, 1503, 1459, 1389, 1291, 1253, 1188, 1120, 1062, 1033, 961,
802, 735, 355, 281. MS (FABþ): 366 [M ꢀ 2Br].
3. Results and discussion
Complex 14 was synthesized by refluxing compounds
7
(1 mmol) with (DME)NiBr2 (1 mmol) in glacial acetic acid (10 mL)
for 4 h. The precipitate was filtered and washed with diethyl ether
(5 mL ꢂ 3). The residue collected were redissolved in dichloro-
methane and concentrated to saturated solution and then repre-
cipitated using diethyl ether, filtered and washed with diethyl ether
(5 mL ꢂ 3). The solid residue was dried under vacuum overnight at
40 ꢁC.
An attempt to synthesize various pre-ligands based on phenyl-
pyridine, using reactions mediated by BuLi-LiDMAE superbase has
been made. This superbase can easily a-lithiate phenylpyridine. The
treatment of the lithiated product with an N,N-dialkyl substituted
amides of suitable monocarboxylic acids, which can act as an acyl
donor, results in
a-acylation of phenylpyridine. DMF, DMAc and
DMB are employed as the acylating reagents. Finally phenyl-
pyridine based acyl products were obtained by hydrolysis of macro-
complexes with water.
2.6.1. N-((2-phenylpyridine-6-yl)methylene)-2,6-
diisopropylanilineNiBr2 (8)
As these acylating agents can act as chelating agents as well,
they are able to deviate the reaction mechanism to produce
unexpected products based on phenylpyridine. Various products
obtained from one reaction could be purified using column chro-
matographic method. A lengthy silica flash column was employed
for the separation, with n-hexane/ethyl acetate as eluent. Three
classes of compounds were separated as indicated in Scheme 1. The
compounds 1e3 were obtained in 12.2e15.6% yield and
compounds 4e6 in 75.4e78.2% yield. The compounds 4e6 show
aliphatic signals assignable to a butyl tail in the molecule. A third
product, compound 7 was obtained in all the three reactions with
varying yield (2.8e3.4%).
Compounds 1e3 are formed according to an expected mecha-
nism: i.e. a reaction mediated by highly basic BuLi-LiDMAE aggre-
gate. However, the chelating amides that are used as acyl donors
can also act as trapping agents. Due to their polar presence, the
superbase aggregate can collapse to liberate nucleophilic, free BuLi.
At this condition there is a high probability for compounds 1e3 to
The complex was obtained as brown crystals in 56.0% yield.
Anal. Calcd for C24H26Br2N2Ni: C, 51.38; H, 4.67; N, 4.99. Found: C,
49.99; H, 4.56; N, 5.01. IR (CsI, cmꢀ1) 3398, 3066, 1620, 1566, 1543,
11,497, 1458, 1419, 1303, 1126, 1038, 1010, 810, 764, 443, 328, 278.
MS (FABþ): 480 [M ꢀ Br].
2.6.2. N-((2-phenylpyridine-6-yl)methylmethylene)-2,6-
diisopropylanilineNiBr2 (9)
The complex was obtained as a brown crystals in 56.3% yield.
Anal. Calcd for C25H28Br2N2Ni: C, 52.22; H, 4.91; N, 4.87. Found: C,
51.39; H, 5.08; N, 4.65. IR (CsI, cmꢀ1) 3420, 2959, 1636, 1558, 1521,
1506, 1458, 1409, 1382, 1340, 1319, 1192, 1126, 1039, 1012, 813, 765,
444, 327, 280. MS (FABþ): 495 [M ꢀ Br].
2.6.3. N-((2-phenylpyridine-6-yl)phenylmethylene)-2,6-
diisopropylanilineNiBr2 (10)
The complex was obtained as a brown crystals in 58.6% yield.
Anal. Calcd for C30H30Br2N2Ni: C, 56.56; H, 4.75; N, 4.40. Found: C,
undergo the classical nucleophilic reaction. Such
a reaction

D. Chandran et al. / Journal of Organometallic Chemistry 718 (2012) 8e13
11
center coordinates to the neutral ligand through two sp2 hybridized
nitrogen atoms. The remaining two coordinations are fulfilled by
bonding with two bromide ions. Previous report showed that the Ni
complexes of this general type are dimeric when steric interactions
permit the monomeric halves LNiX2 (where L is ligand molecule) to
come sufficiently close [22]. It may be the steric properties of the
ligand molecule that prevents the close approach of monomeric
halves. The angles, the four coordinations in the tetrahedron
making with one another, range from 80.9(7) to 131.8(5)o, deviating
largely from the standard tetrahedral angle 109.5ꢁ indicating
a severe distortion. The bond length between central Ni atom and
the nitrogen from pyridine ring and that between Ni atom and
imine nitrogen are non-identical. That between Ni and pyridyl
nitrogen is found slightly shorter than the other (see Fig. 1 and
Table 1). From the plane passing through the atoms N2, C16 and
ꢀ
C11, the other nitrogen atom N1 deviates by a distance 0.203 A.
These deviations from standard tetrahedral angle and the non-
planar arrangement of the ligands with metal center can be
resulted from the steric demands within the ligand molecule to
orient toward most stable conformations. The torsional angle
between the planes that is formed between N1-C11 and N2-C16
ꢁ
ꢀ
is ꢀ16.1 . Accordingly the N2 deviates 0.194 A from the plane where
the pyridine ring (mean: N1, C7, C8, C9, C10, C11) lies. The plane of
phenyl ring (C23, C24, C25, C26, C27, C28) that is attached to the
imine nitrogen N2 is located nearly vertical to the plane of pyridine
ring (N1, C7, C8, C9, C10, C11). The methylinic phenyl ring (C17, C18,
C19, C20, C21, C22) makes 53.04ꢁ through one direction with the
plane of pyridine ring. The plane that runs through the third phenyl
ring (C1, C2, C3, C4, C5, C6) in the molecule that is directly attached
to the pyridine ring makes an angle of 42.22ꢁ with the plane that
passes through all atoms in pyridine ring. It indicates that all the
conformation possibilities have been utilized to orient different
groups in three dimensional planes to get a structure with
minimum strain.
Pink colored crystals of complex 14 were obtained at conditions
that are similar for complex 13. Selected bond angles and bond
lengths of this molecule from its crystallographic measurements
are displayed in Table 1 and ORTEP diagram is depicted in Fig. 2. The
metal complex exists as monomeric unit with a metal to ligand
ratio of 1:1. The coordination geometry of the complex 14 is similar
to that of previously reported symmetrical phenanthroline or
bipyridine Ni complexes [22]. The Ni center is in tetrahedral
geometry being four coordinated by the two nitrogen donors of the
bidentate ligand and two bromide ions. The distortions from the
standard tetrahedral structure are very clearly observed from the
bond angles that Ni makes with its four coordination sites. The
bond angle values 83.19(19), 118.69(11), 100.64(11), 98.18(11),
116.49(10) and 130.45(4)o indicates the deviation from the stan-
dard tetrahedral angle value of 109.5ꢁ. The dihedral angle of the
NeNieN and BreNieBr planes is 75.6ꢁ indicating slightly flattened
Scheme 1. Synthesis of ligands 1e7.
introduces a butyl tail at the para position of compounds 1e3
producing compounds 4e6. It has also been reported that, when
polar trapping agents such as THF is used in these reactions; there is
a chance for dimerization of pyridine derivatives to yield bipyridine
derivatives [20]. The structure of all these compounds was inten-
sively analyzed using different NMR spectroscopy, elemental
analysis and EI-Mass spectra.
For the phenylpyridine carbonyl compounds 1e6, the synthesis
of imino-pyridyl ligands was attempted through classical imine
reaction. However, this protocol was not effective since a major
portion of the product was decomposed during the separation
steps by column chromatography. Thus, a one-pot synthesis is
advantageous to synthesize metal complexes in high yield [15,21].
A one-pot synthetic technique has been adopted to prepare the
desired Ni complexes 8e13 from compounds 1e6 (Scheme 2). The
phenylpyridine carbonyl compounds 1e6 on treatment with 2,6-
diisopropylaniline and (DME)NiBr2 in glacial acetic acid gave Ni
complexes 8e13 in good yields. Complex 14 was prepared from 7 by
refluxing with (DME)NiBr2 in glacial acetic acid. All complexes were
characterized by FAB-MS, elemental analysis and IR spectroscopy.
Among the complexes 8e13, those complexes with methylinic
phenyl groups easily yielded crystals from their dichloromethane
solution on vapor deposition of n-pentane. More idea about their
structure was obtained from the crystallographic information of
their solid state structure.
ꢀ
tetrahedral. The two NieN bond distances are 2.00(4) and 2.02(4) A
ꢀ
which are very close to a typical value of 2 A. The NieBr bond
lengths 2.36(11) and 2.37(10) A in the molecule are also around the
standard value of 2.3 A. The two pyridyl rings constituting the
ꢀ
ꢀ
bipyridyl ligand backbone are not located in a single plane. The two
planes in which each of this pyridyl units are located are separated
from each other by an angle of 21.76ꢁ. The plane passing through
one pyridyl ring (mean: C1, C2, C3, C4, C5, N1) makes an angle of
45.71ꢁ with the plane passing through the phenyl ring (mean: C6,
C7, C8, C9, C10, C11) attached to it. The angle between plane passing
through the other pyridyl ring (mean: C12, C13, C14, C15, C16, N2)
and that passing through the phenyl ring attached to it (mean: C17,
C18, C19, C20, C21, C22) is 36.55ꢁ.
Deep orange-brown crystals of imino-pyridyl Ni complex 13
suitable for solid state structure measurements were obtained by
vapor deposition of n-pentane to a CH2Cl2 solution of the complex.
The solid state structure was obtained by single crystal XRD
measurements. Selected bond angles and bond lengths are dis-
played in Table 1 and ORTEP diagram is depicted in Fig. 1. The
complex crystallized as monomeric unit with a tetrahedral Ni metal
Ethylene oligomerizations have been performed with all the
Ni(II) complexes at moderate conditions. Due to the open structure

12
D. Chandran et al. / Journal of Organometallic Chemistry 718 (2012) 8e13
Scheme 2. Synthesis of complexes 8e14.
of the metal center, which exists as a common feature in all of these
complexes, when activated with ethylaluminum sequichloride
(EASC), yielded oligomers as summarized in Table 2. All the
experiments have been conducted in chlorobenzene at 30 ꢁC and
1.3 bar ethylene pressure for 30 min. It is interesting to note that the
complexes give dimerization and trimerization products exclu-
sively regardless of structure. Complexes 8e13 can be divided into
two categories. Complexes 8e10 represent an (imino)pyridine
structure with varying methylenic substituents: eH (complex 8),
-Me (complex 9), and ePh (complex 10). Complexes 11e13 are
characterized by the same structure with an n-butyl substituent at
the para-position of the core pyridine ring. The activity of these two
sets of complexes toward ethylene is significantly influenced by the
methylenic substituents. Complexes 8 and 11 with methylenic
hydrogen show low activity: i.e. 16.0 and 16.3 ꢂ 106 g-oligomer/
mol-Ni h bar, respectively. Complexes 10 and 13 with methylenic
phenyl substituent exhibit high activity: i.e. 34.9 and 35.7 ꢂ 106 g-
oligomer/mol-Ni h bar, respectively, while complexes 9 and 12 with
methylenic methyl substituent display intermediate activity:
i.e. 25.2 and 26.5 ꢂ 106 g-oligomer/mol-Ni h bar, respectively.
Comparing the two complexes in different categories, the
Table 1
ꢀ
Selected bond lengths (A) and bond angles (o) For complexes 13 and 14.
Bond angles (o)
ꢀ
Bond lengths (A)
Complex 13
Ni(1)eN(1) 2.0049(18)
Ni(1)eN(2) 2.0138(16)
Ni(1)eBr(1) 2.3581(4)
Ni(1)eBr(2) 2.3536(4)
N(2)eC(16) 1.287(3)
N(2)eC(23) 1.448(2)
C(11)eC(16) 1.490(3)
N(1)eNi(1)eN(2) 80.99(7)
N(1)eNi(1)eBr(2) 131.87(5)
N(2)eNi(1)eBr(2) 104.33(5)
N(1)eNi(1)eBr(1) 93.50(4)
N(2)eNi(1)eBr(1) 112.98(4)
Br(2)eNi(1)eBr(1) 125.161(17)
N(1)eC(11)eC(16) 114.32(18)
N(2)eC(16)eC(11) 115.36(18)
Complex 14
Ni(1)eN(1) 2.009(4)
Ni(1)eN(2) 2.021(4)
Ni(1)eBr(1) 2.3719(10)
Ni(1)eBr(2) 2.3597(11)
N(1)eC(1) 1.350(6)
N(2)eC(12) 1.361(6)
N(1)eNi(1)eN(2) 83.19(7)
N(1)eNi(1)eBr(2) 118.69(11)
N(2)eNi(1)eBr(2) 100.64(11)
N(1)eNi(1)eBr(1) 98.18(11)
N(2)eNi(1)eBr(1) 116.49(10)
Br(2)eNi(1)eBr(1) 130.45(4)
N(1)eC(1)eC(12) 115.2(4)
N(2)eC(12)eC(1) 115.2(4)
Fig. 1. ORTEP diagram of the solid-state molecular structure of complex 13. Thermal
ellipsoids are drawn at the 50% probability level.

D. Chandran et al. / Journal of Organometallic Chemistry 718 (2012) 8e13
13
4. Conclusions
A series of [N Nˇ ] type bidentate Ni(II) complexes based on
phenylpyridine have been synthesized using a direct method to
activate the a-CH in pyridine. This method has found to be a very
efficient to synthesize pyridine related ligands in that various metal
complexes with different structures are produced in a single reac-
tion in good yields. Solid state structure obtained from single
crystal XRD measurement sheds light on the details of three
dimensional arrangements of atoms and groups in the complex
molecules. The distortions from the standard tetrahedral structure
are very clearly observed from the bond angles that Ni makes with
its four coordination sites. All these complexes showed high activity
toward ethylene when activated with EASC, yielding exclusively
1-butene (81e88%) and some amounts of 1-hexene (3e9%)
together with negligible internal olefins.
Fig. 2. ORTEP diagram of the solid-state molecular structure of complex 14. Thermal
ellipsoids are drawn at the 50% probability level.
Acknowledgments
This work was supported by grants-in-aid for the World Class
University Program (No. R32-2008-000-10174-0) and the National
Core Research Center Program from MEST (No. R15-2006-022-
01001-0), and the Brain Korea 21 program (BK-21).
complexes with n-butyl substituents (complexes 11e13) show
higher activity than the corresponding complexes with no n-butyl
substitution (complexes 8e10).
Complex 14 comes under a different class of ligand having
bipyridine architecture when compared with other complexes of
(imino)pyridine structure. This again provides an open metal center
leading to oligomerization of ethylene when activated with EASC in
ethylene atmosphere. This complex shows only a moderate activity
of 0.2 ꢂ 106 g-oligomer/mol-Ni h bar toward ethylene.
Appendix A. Supplementary material
CCDC 756871 and 758995 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
All the complexes produced 1-butene (81e88%) as a major
product together with 1-hexene (3e9%) as a minor product,
demonstrating they tend to favor chain termination over propa-
gation. It was difficult to find a specific trend according to the
structure of the complexes. In general nickel catalysts tend to yield
internal olefins in ethylene oligomerization reactions [23,24];
however, the complexes of this study produces only negligible
amount of internal olefins. These results demonstrate that (i)
Appendix B. Supplementary material
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2012.08.005.
References
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b-H elimination after ethylene insertion, followed by
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A detailed investigation on the properties of these complexes in
ethylene oligomerization reactions based on ion-pair concept along
with a DFT investigation on the background of these results are
currently ongoing and will be reported elsewhere in due course.
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¼
¼
C6
C4
d
e
d
e
a
¼
SC4i
¼
a
¼
SC6i
¼
1
2
3
4
5
6
7
8
9
16.0
25.2
34.9
16.3
26.5
35.7
0.2
86
83
82
86
84
81
88
4
3
7
5
2
7
7
6
8
7
5
9
8
3
4
6
4
4
5
4
1
10
11
12
13
14
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a
Oligomerization conditions: catalyst
¼
2.5
m
mol, solvent
¼
80 mL,
time ¼ 30 min, and [EASC]/[Ni] ¼ 150.
b
Average rate of oligomerization in Rp ꢂ 10ꢀ6 (g-oligomer)(mol-Ni)ꢀ1hꢀ1 barꢀ1
.
c
d
e
Determined by GC.
a
-olefin.
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Sum of internal olefins other than
a
-olefin.