Nickel complexes bearing [N,N] 2-pyridylbenzamidine ligands: Syntheses, characterizations, and catalytic properties for ethylene oligomerization

Article abstract of DOI:10.1016/j.poly.2009.02.023

A series of nickel(II) complexes with different steric and electronic substituted 2-pyridylbenzamidine ligands, [2,6-iPr₂C₆H₃-N{double bond, long}C(R₁)-NH-(5-R₃, 6-R₂)Py]NiBr₂ (R

Full text of DOI:10.1016/j.poly.2009.02.023

Polyhedron 28 (2009) 1386–1392
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Nickel complexes bearing [N,N] 2-pyridylbenzamidine ligands: Syntheses,
characterizations, and catalytic properties for ethylene oligomerization
a
a
Feng-Shou Liu a,b, Hai-Yang Gao a, , Ke-Ming Song , Ling Zhang , Fang-Ming Zhu a,b, Qing Wu a,b,
*
*
a DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China
b PCFM Lab, OFCM Institute, Sun Yat-Sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of nickel(II) complexes with different steric and electronic substituted 2-pyridylbenzamidine
ligands, [2,6-iPr2C6H3–N@C(R1)–NH–(5-R3, 6-R2)Py]NiBr2 (R1 = Ph, R2 = H, R3 = H, 1; R1 = Ph, R2 = H,
R3 = NO2, 2; R1 = 4-CH3OC6H4, R2 = H, R3 = H, 3; R1 = 4-CH3C6H4, R2 = Me, R3 = H, 4), have been synthesized
in high yield and the solid state structures of 1, 2 and 4 have been crystallographically characterized. Acti-
vated with methyaluminoxane (MAO), 1–4 showed moderate turnover frequency (TOF) for ethylene olig-
omerization in dichloromethane. The influences of the ligand structure on catalytic properties were
Received 28 December 2008
Accepted 24 February 2009
Available online 5 March 2009
Keywords:
2-Pyridylbenzamidine ligand
Nickel complex
Catalysis
Ethylene oligomerization
studied. Complex
2 with an electron-withdrawing group revealed the highest TOF of up to
11.7 Â 103 mol ethylene/(mol Ni h). Moreover, the reaction temperature and Al/Ni molar ratio were also
examined in detail.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
pyridylformamidines and pyridylamidines(VI) were synthesized
and investigated in metal–metal interactions [48], they have rarely
The development of late transition metal catalysts for ethylene
oligomerization and polymerization has received much attention
in both academic and industrial areas in the past decades [1,2].
The representative is famous SHOP-type [P,O] nickel catalyst which
been applied in olefin polymerization.
In this contribution, syntheses of a series of novel [N,N] 2-pyr-
idylbenzamidine nickel(II) complexes was reported. Molecular
structures of the nickel complexes were also characterized by
X-ray single-crystal analyses. Furthermore, ethylene oligomeriza-
tion catalyzed by these nickel complexes after activation with
MAO was investigated.
can produce linear
Brookhart and co-workers reported that
a
-olefins without a cocatalyst [3–6]. Since
-diimine nickel com-
a
plexes [7–10], and pyridinediimine iron complexes [11,12] were
highly efficient for ethylene polymerization and/or oligomerization
in the presence of MAO, various nickel complexes with [N,N]
[13–19], [N,O] [20–26], [N,P] [27–31], [P,P] [32–35] ligands were
explored as olefin polymerization and/or oligomerization catalyst
precursors.
2. Experimental
2.1. General procedures
Recently, nickel complexes bearing a [N,N] six-membered che-
lating ring have been prepared and applied in olefin polymeriza-
tion [36–47]. It is noteworthy that catalytic products in ethylene
polymerization are much dependent on catalyst structure. For
example, for b-diimine nickel complexes (Scheme 1, I) [36], the
products were polyethylenes, while for b-diketiminate [39–42]
and anilido-imine nickel complexes(II) [43,44], both polyethylenes
and oligomers were found. As to nickel complexes bearing quin-
olylimine(III) [45], b-pyridylimine(IV) [46], and dipyridylamine(V)
ligands [47], only ethylene oligomers were obtained. Although
All manipulations were performed under nitrogen atmosphere
using glove box and Schlenk techniques. Polymerization-grade
ethylene and extra-pure-grade nitrogen were further purified be-
fore feeding into the reactor by passing through a DC-IB gas puri-
fication instrument.
2.2. Materials
Tetrahydrofuran, hexane and toluene were refluxed over metal-
lic sodium for 24 h. Dichloromethane was refluxed over phospho-
rus pentoxide for 6 h, and then distilled under nitrogen
atmosphere before being used. 2,6-Diisopropylaniline and 2,6-
dimethylaniline were purchased from Aldrich Chemical Co. and
distilled under reduced pressure before being used. p-Toluoyl
* Corresponding authors. Address: DSAPM Lab, Institute of Polymer Science,
School of Chemistry and Chemical Engineering, Sun Yat-Sen (Zhongshan) Univer-
sity, Guangzhou 510275, China.
E-mail address: ceswuq@mail.sysu.edu.cn (Q. Wu).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.02.023

F.-S. Liu et al. / Polyhedron 28 (2009) 1386–1392
1387
2.5.1. Synthesis of 2,6-iPr2C6H3–N@C(Ph)–NH–Py (L1)
R
Benzoyl chloride (1.1 mL, 10 mmol) and triethylamine (1.6 mL,
11 mmol) were added to a vigorously stirred solution of 2,6-diiso-
propylaniline (1.9 mL, 10 mmol) in CH2Cl2 (45 mL). After 3 h, the
precipitate of (C2H5)3N Á HCl was filtrated, and white solid powder
benzamide was obtained by evaporating the solvent. Then excess
of thionyl chloride (2.0 mL, 28 mmol) was added to the benzamide
and the reaction mixtures were stirred for 2 h at 80 °C. The remain-
der thionyl chloride was distilled off under reduced pressure, to
give the imidoyl chloride as yellow and slowly solidifying oil.
Successively, toluene (40 mL), triethylamine (1.6 mL, 11 mmol)
and 2-aminopyridine (0.94 g, 10 mmol) were added to the reaction
system. The mixtures were heated to reflux for 24 h under the pro-
tection of nitrogen atmosphere. (C2H5)3N Á HCl was removed by fil-
tration, and toluene was evaporated from the filtrate. After
recrystallization of the product from ethanol, L1 was obtained as
light yellow crystal in 48% yield (1.72 g, 4.83 mmol). Melting point:
156 °C. 1H NMR (300 MHz, CDCl3), d (ppm) [an isomer (E-syn and E-
anti) ratio of 1.2:1] major isomer: 13.31(s, 1H, NH), 8.25 (br, 1H,
R
R
N
N
NH
N
N
N
Ar
Ar
Ar
R
Ar
Ar
I
II
III
R1
H
N
N
R
N
N
N
N
N
N
Ar
Ar
R2
R3
IV
V
VI
Scheme 1. Types of [N,N] six-membered ring ligands.
chloride, p-anisoyl chloride, 2-amino-6-methylpyridine and 2-ami-
no-5-nitropyridine were purchased from Aldrich Chemical Co. and
used as received. Benzoyl chloride was purchased from Guangzhou
Chemical Reagent Factory and used without further purification. 2-
Aminopyridine was purchased from Shanghai Chemical Reagent
Factory and used after recrystallization from ethanol. Methylalu-
minoxane (MAO) was prepared by partial hydrolysis of trimethyl-
aluminum (TMA) in toluene at 0–60 °C with Al2(SO4)3 Á 18H2O as
the water source. The initial [H2O]/[Al] in molar ratio was 1.3.
pyridyl
3.26 (m, 2H, CH(CH3)2), 1.14 (d, 12H, CH(CH3)2). Minor isomer:
13.31 (s, 1H, NH), 8.25 (br, 1H, pyridyl -H), 7.66–6.92 (m, 11H,
a-H), 7.66–6.92 (m, 11H, pyridyl and phenyl protons),
a
pyridyl and phenyl protons), 3.04 (m, 2H, CH(CH3)2), 0.94 (d,
12H, CH(CH3)2). 13C NMR (75 MHz, CDCl3), d (ppm) major isomer:
162.79, 144.99, 144.56, 137.27, 136.04, 134.41, 129.12, 128.80,
128.29, 127.47, 127.10, 123.30, 117.47, 113.26, 28.65, 25.19. Major
isomer: 160.53, 144.99, 144.56, 137.27, 136.04, 134.41, 129.12,
128.80, 128.29, 127.47, 127.10, 123.30, 117.47, 113.26, 28.27,
21.83. EI-MS (m/z): 358 [M]+; 264 [MÀC5H5N2]+. Elemental Anal.
Calc. for C24H27N3: C, 80.63; H, 7.61; N, 11.75. Found: C, 80.47;
H, 7.59; N, 11.64%.
2.3. Measurement
1H and 13C NMR spectra were recorded on a Varian Mercury-
Plus 300 MHz NMR spectrometer and referenced versus TMS as
standard. Elemental analyses were determined with a Vario EL Ser-
ies Elemental Analyzer from Elementar. The GC–MS data were re-
corded with a Finnigan Voyager GC-8000 Top Series GC–MS
System with DB-5MS GC column. The following temperature pro-
gram of the oven was adopted: keeping 40 °C for 2 min, increasing
the temperature by 5 °C/min heating to 110 °C, and then by a
15 °C/min heating until 250 °C which was kept constant for a fur-
ther 2 min.
2.5.2. Synthesis of 2,6-iPr2C6H3–N@C(Ph)–NH–(5-NO2–Py) (L2)
L2 was prepared according to the method described for L1,
the mixtures were purified by column chromatography on silica
gel using petroleum ether/ethyl acetate (10/1) as eluent, and
then recrystallized from ethanol in 31% yield. Melting point:
140 °C. 1H NMR (300 MHz, CDCl3): d (ppm) [an isomer ratio of
2.3:1] major isomer: 13.12 (s, 1H, NH), 9.13 (br, 1H, pyridyl
a
-H), 8.38 (br, 1H, pyridyl
phenyl protons), 3.13 (m, 2H, CH(CH3)2), 1.13 (d, 12H, CH(CH3)2).
Minor isomer: 13.12 (s, 1H, NH), 9.13 (br, 1H, pyridyl -H), 8.38
(br, 1H, pyridyl -H), 7.40–7.06 (m, 9H, pyridyl and phenyl pro-
c-H), 7.40–7.06 (m, 9H, pyridyl and
a
2.4. X-ray structure determination
c
tons), 2.94 (m, 2H, CH(CH3)2), 0.92 (d, 12H, CH(CH3)2). 13C NMR
(75 MHz, CDCl3), d (ppm) major isomer: 163.38, 144.08, 142.95,
138.56, 134.92, 132.01, 130.18, 129.04, 128.54, 128.50, 127.99,
122.70, 123.65, 122.35, 28.49, 25.12. Minor isomer: 163.05,
144.08, 142.95, 138.56, 134.92, 132.01, 130.18, 129.04, 128.54,
128.50, 127.99, 122.70, 123.65, 122.35, 28.39, 21.79. EI-MS (m/
z): 403 [M]+; 264 [MÀC5H4N2O2]+. Elemental Anal. Calc. for
C24H26N4O2: C, 71.62; H, 6.51; N, 13.92. Found: C, 71.40; H,
6.61; N, 14.08%.
The X-ray diffraction data of single crystals were obtained with
the
x
À2h scan mode on a Bruker SMART 1000 CCD diffractiometer
with graphite-monochromated Mo K
a
radiation (k = 0.71073 Å) at
173 K. The structure was solved using direct methods, and further
refinement with full-matrix least squares on F2 was obtained with
the SHELXTL program package. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were introduced in calculated
positions with the displacement factors of the host carbon atoms.
2.5. Ethylene oligomerization
2.5.3. Synthesis of 2,6-iPr2C6H3–N@C(4-CH3OC6H4)–NH–Py (L3)
L3 was prepared according to the method described for L1 in
46% yield. Melting point: 169 °C. 1H NMR (300 MHz, CDCl3): d
(ppm) [an isomer ratio of 1.3:1] major isomer: 13.25 (s, 1H, NH),
Ethylene oligomerization reactions were performed in a 50 mL
glass flask equipped with a magnetic stirrer, and continuous feed
of ethylene was used. A typical reaction was performed by adding
proper amount of MAO solid and solvent (19 mL) into the reactor
under ethylene atmosphere. The catalyst in dichloromethane solu-
8.23 (br, 1H, pyridyl
a-H), 7.64 (br, 1H, pyridyl c-H), 7.38–6.68
(m, 9H, pyridyl and phenyl protons), 3.72 (s, 3H, OCH3), 3.24 (m,
2H, CH(CH3)2), 1.13 (d, 12H, CH(CH3)2). Minor isomer: 13.25 (s,
tion (1 mL, Ni = 5 lmol) was injected into the reactor via syringe.
1H, NH), 8.23 (br, 1H, pyridyl
a-H), 7.64 (br, 1H, pyridyl c-H),
Ethylene was continuously fed in order to maintain the ethylene
pressure at 0.5 atm. After 30 min, the reaction was terminated by
addition of cold acidic ethanol (ethanol–HCl, 95:5). The organic
layer was analyzed quickly by gas chromatography (GC) for deter-
mining the composition and mass distribution of the oligomers.
7.38–6.68 (m, 9H, pyridyl and phenyl protons), 3.72 (s, 3H,
OCH3), 3.02 (m, 2H, CH(CH3)2), 0.94 (d, 12H, CH(CH3)2). 13C NMR
(75 MHz, CDCl3), d (ppm) major isomer: 162.82, 144.97, 144.47,
137.19, 134.72, 130.50, 128.56, 126.98, 123.56, 122.41, 117.20,
113.67, 113.21, 112.85, 55.15, 28.65, 25.12. Minor isomer:

1388
F.-S. Liu et al. / Polyhedron 28 (2009) 1386–1392
160.25, 144.97, 144.47, 137.19, 134.72, 130.50, 128.56, 126.98,
123.56, 122.41, 117.20, 113.67, 113.21, 112.85, 55.15, 28.25,
21.94. EI-MS (m/z): 388 [M]+; 294 [MÀC5H4N2]+. Elemental Anal.
Calc. for C25H29N3O: C, 77.48; H, 7.54; N, 10.84. Found: C, 77.55;
H, 7.64; N, 10.76%.
R1
Cl
SOCl2
R1COCl
NHCOR1
N
NH2
H
N
R1
H
R1
2.5.4. Synthesis of 2,6-iPr2C6H3–N@C(4-CH3C6H4)–NH–(6-CH3–Py)
(L4)
R3
NH2
N
N
N
N
(DME)NiBr2
R2
R3
N
N
L4 was prepared according to the method described for L1. The
mixtures were purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (10/1) as eluent, and then
recrystallized from ethanol in 63% yield. Melting point: 119 °C.
1H NMR (300 MHz, CDCl3): d (ppm) [an isomer ratio of 1.5:1] major
isomer: 13.35 (s, 1H, NH), 7.56–6.80 (m, 10H, pyridyl and phenyl
protons), 3.30 (m, 2H, CH(CH3)2), 2.45 (s, 3H, methyl protons on
pyridyl), 2.50 (s, 3H, CH3), 1.15 (d, 12H, CH(CH3)2). Minor isomer:
13.35 (s, 1H, NH), 7.56–6.80 (m, 10H, pyridyl and phenyl protons),
3.04 (m, 2H, CH(CH3)2), 2.45 (s, 3H, methyl protons on pyridyl),
2.50 (s, 3H, CH3), 0.99 (d, 12H, CH(CH3)2). 13C NMR (75 MHz,
CDCl3), d (ppm) major isomer: 162.39, 153.65, 152.72, 144.70,
139.00, 137.50, 134.56, 133.31, 128.76, 128.08, 126.97, 123.20,
119.41, 116.62, 28.53, 25.43, 24.29, 21.81. Minor isomer: 160.50,
153.65, 152.72, 144.70, 139.00, 137.50, 134.56, 133.31, 128.76,
128.08, 126.97, 123.20, 119.41, 116.62, 28.23, 24.29, 21.81, 21.36.
EI-MS (m/z): 386 [M]+; 278 [MÀC6H7N2]+. Elemental Anal. Calc.
for C26H31N3: C, 81.00; H, 8.10; N, 10.90. Found: C, 80.93; H,
8.05; N, 10.91%.
R3
Ni
R2
Br
toulene/Et3N
R2
Br
1-4
L1-4
1:R1=Ph, R2=H, R3=H;
2:R1=Ph, R2=H, R3=NO2; 4: R1=4-CH3C6H4, R2=Me, R3=H;
3: R1=4-CH3OC6H4, R2=H, R3=H;
Scheme 2. Preparation of nickel complexes 1–4.
3. Results and discussion
3.1. Ligand and nickel complex syntheses
Four new 2-pyridylbenzamidine ligands with various steric and
electronic substituents were synthesized following a classical
route for N,N-disubstituted amidines [49,50]. The synthetic proce-
dures are outlined in Scheme 2, and the particulars of each step
were detailed in the Section 2. Benzamide could be easily available
from 2,6-diisopropylaniline and substituted benzoyl chloride in
high yield with triethylamine as the precipitator. Imidoyl chloride
could be produced by the dehydration reaction between benzam-
ide and excess of thionyl chloride. After distilling the residue of
thionyl chloride, the 2-pyridylbenzamidine ligands were obtained
by the reaction between the quantitative 2-aminopyridine deriva-
tives and imidoyl chlorides in refluxing toluene. Pure L1 and L3
were obtained as light yellow crystals by recrystallization in etha-
nol, while L2 and L4 were purified using silica chromatography and
then recrystallized in ethanol. All of these ligands were proved by
1H and 13C NMR, MS and elemental analyses. A single crystal of L2
grown from ethanol solution also assuredly confirmed the ligand
structure.
2.5.5. Synthesis of [2,6-iPr2C6H3–N@C(Ph)–NH–Py]NiBr2 (1)
L1 (358 mg, 1.0 mmol) was dissolved in 40 mL anhydrous
dichloromethane under nitrogen atmosphere. (DME)NiBr2
(308 mg, 1.0 mmol) was added under room temperature. The light
yellow solution changed into purple quickly, and the solution was
stirred for another 12 h. Then the reaction mixture was filtrated
and 20 mL hexane was added. After being filtrated from Celite,
the solid was washed by hexane (2 Â 10 mL). Drying in vacuum,
532 mg of complex 1 was obtained in 92% yields. Decomposition
temperature: 289 °C. FAB+-MS: m/z: 575, 576, 577, 578 [M]+;
495, 496, 497 [MÀBr]+; 415, 416, 417 [MÀ2Br]2+; 264, 265
[L1ÀC5H5N2]+. Elemental Anal. Calc. for C24H27Br2N3Ni: C, 50.05;
H, 4.72; N, 7.30. Found: C, 49.72; H, 4.83; N, 7.16%.
The nickel complexes 1–4 were synthesized by mixing (DME)-
NiBr2 and corresponding ligand in anhydrous dichloromethane in
high yield. Because of paramagnetism, nickel complexes 1–4 were
2.5.6. Synthesis of [2,6-iPr2C6H3–N@C(Ph)–NH–(5-NO2–Py)]NiBr2 (2)
Complex 2 was prepared according to the method described for
1 in 77% yields as brown solid. Decomposition temperature:
277 °C. FAB+-MS: m/z: 619, 620, 621, 622 [M]+; 539, 541, 542
[MÀBr]+; 460, 461, 462 [MÀ2Br]2+; 264, 265 [L2ÀC5H4N2O2]+. Ele-
mental Anal. Calc. for C24H26Br2N4NiO2: C, 46.42; H, 4.22; N, 9.02.
Found: C, 46.15; H, 4.13; N, 8.89%.
2.5.7. Synthesis of [2,6-iPr2C6H3–N@C(4-CH3OC6H4)–NH–Py]NiBr2 (3)
Complex 3 was prepared according to the method described for
1 in 89% yields as purple solid. Decomposition temperature:
302 °C. FAB+-MS: m/z: 605, 606, 607, 608 [M]+; 525, 526, 527
[MÀBr]+; 445, 446, 447 [MÀ2Br]2+; 294, 295 [L3ÀC5H4N2]+. Ele-
mental Anal. Calc. for C25H29Br2N3NiO: C, 49.55; H, 4.82; N, 6.93.
Found: C, 49.34; H, 4.91; N, 6.86%.
2.5.8. Synthesis of [2,6-iPr2C6H3–N@C(4-CH3C6H4)–NH–(6-CH3–
Py)]NiBr2 (4)
Complex 4 was prepared according to the method described for
1 in 76% yields as purple solid. Decomposition temperature:
305 °C. FAB+-MS: m/z: 603, 604, 605, 606, 607 [M]+; 523, 524,
525, 526 [MÀBr]+; 443, 444, 445 [MÀ2Br]2+
;
278, 279
Fig. 1. Molecular structure of L2 depicted with 30% thermal ellipsoids and with
hydrogen atoms omitted. Selected bond lengths (Å): C(3)–N(3) 1.371(2), C(6)–N(3)
1.393(2), C(6)–N(4) 1.276(2), C(13)–N(4) 1.416(2).
[L4ÀC6H7N2]+. Elemental Anal. Calc. for C26H31Br2N3Ni: C, 51.70;
H, 5.17; N, 6.96. Found: C, 51.51; H, 5.34; N, 6.83%.

F.-S. Liu et al. / Polyhedron 28 (2009) 1386–1392
1389
Fig. 4. Molecular structure of 4 depicted with 30% thermal ellipsoids and with
hydrogen atoms omitted. Selected bond lengths (Å) and angles (°): C(5)–N(2)
1.399(4), C(6)–N(2) 1.368(3), C(6)–N(3) 1.299(3), C(14)–N(3) 1.458(3), Ni(1)–N(1)
1.997(2), Ni(1)–N(3) 1.989(2), Ni(1)–Br(1) 2.3997(5), Ni(1)–Br(2) 2.3483(5), N(1)–
Ni(1)–N(3) 95.8(1), N(1)–Ni(1)–Br(1) 100.79(7), N(3)–Ni(1)–Br(2) 104.98(7), N(3)–
Ni(1)–Br(1) 114.28(7), N(3)–Ni(1)–Br(2) 104.98(7), Br(1)–Ni(1)–Br(2) 126.25(1).
Fig. 2. Molecular structure of 1 depicted with 30% thermal ellipsoids and with
hydrogen atoms omitted. Selected bond lengths (Å) and angles (°): C(1)–N(3)
1.402(5), C(6)–N(3) 1.375(4), C(6)–N(2) 1.291(4), C(13)–N(2) 1.449(4), Ni(1)–N(1)
1.985(3), Ni(1)–N(2) 1.974(3), Ni(1)–Br(1) 2.3911(7), Ni(1)–Br(2) 2.3263(7), N(1)–
Ni(1)–N(2) 92.6(1), N(1)–Ni(1)–Br(1) 101.21(9), N(1)–Ni(1)–Br(2) 110.30(9), N(2)–
Ni(1)–Br(1) 105.14(9), N(2)–Ni(1)–Br(2) 115.22(9), Br(1)–Ni(1)–Br(2) 126.38(3).
lengths and bond angles, respectively. The data collection and
structure refinement parameters are summarized in Table 1.
Similar to the b-diimine nickel complexes [36], all of the nickel
complexes reveal a distorted tetrahedral geometry around the
nickel metal center and the [N,N] six-membered chelating rings
exhibit a boat conformation with metal atom. The nickel atom is
surrounded by a pyridyl nitrogen atom, an imine N atom, and
two terminal Br atoms. In addition, there is no intra- or inter-
molecular hydrogen bonding (NiÁÁÁH) between the hydrogen on
the amidine nitrogen atom and nickel metal atom. Bearing the sim-
ilar steric hindrance of ligand, the Ni–Npyridine and Ni–Nimine bond
lengths of 1 and 3 are nearly the same. In contrast, the Ni–Npyridine
and Ni–Nimine bond lengths of 4 are 1.997(2) and 1.989(2) Å,
respectively, which are longer than those of 1 and 3. The result
indicates that the introduction of methyl on the ortho position of
pyridyl decreases the bonding ability between pyridine, imine
and nickel metal. Moreover, the N–Ni–N bite angles of 1 and 3
are 92.6(1)° and 92.7(1)°, respectively, which are smaller than that
of 4 (95.8(1)°), well in accordance with those observed for nickel
complexes bearing bulky substituents oriented in a more open
environment [46–47,51].
Fig. 3. Molecular structure of 3 depicted with 30% thermal ellipsoids and with
hydrogen atoms omitted. Selected bond lengths (Å) and angles (°): C(1)–N(3)
1.397(4), C(6)–N(3) 1.370(4), C(6)–N(2) 1.298(4), C(14)–N(2) 1.453(5), Ni(1)–N(1)
1.984(3), Ni(1)–N(2) 1.968(3), Ni(1)–Br(1) 2.4006(7), Ni(1)–Br(2) 2.3342(7), N(1)–
Ni(1)–N(2) 92.7(1), N(1)–Ni(1)–Br(1) 99.90(10), N(1)–Ni(1)–Br(2) 112.70(9), N(2)–
Ni(1)–Br(1) 103.89(9), N(2)–Ni(1)–Br(2) 118.01(9), Br(1)–Ni(1)–Br(2) 123.94(3).
3.3. Ethylene oligmerization
not characterized by 1H NMR but were characterized by MS,
elemental analysis, and X-ray crystallography.
Ethylene oligomerizations were carried out with the nickel
complexes in the presence of MAO in order to study their catalytic
properties. Initially, toluene was used as the reaction media for
ethylene oligomerization, and consumption of ethylene was ob-
served. However, it was surprising that the obtained liquid mix-
tures were complicated and only small amount of ethylene
oligomers was detected by GC–MS (Fig. 5). According to the
GC–MS analysis, peak A and B can be assigned to butene and hex-
ene, respectively, while peaks C–G can be attributed to alkylated
toluene and the collected peaks H should be ascribed to ditolylme-
thanes. The existence of oligomers of ethylene, alkylated toluene
and ditolylmethanes in the products means that there are multiple
catalytic processes in the system, involving ethylene oligomeriza-
tion and toluene alkylations by either ethylene or the oligomers
by a Friedel–Crafts process [52–55] and disproportionation [56].
Very recently, nickel complexes bearing [N,P] five-membered che-
lating ring were reported to catalyze reactions with ethylene pro-
ducing oligomers and alkylated toluene [57].
3.2. Molecular structure
A previous reference [48] reported that the N,N-phenylpyridyl-
benzamidine forms a dimer in the solid state which results from
the presence of weak inter-molecular hydrogen bond involving
the hydrogen atom in the amidine nitrogen atom and the pyridyl
groups. In contrast, the X-ray structure of ligand L2 (Fig. 1) shows
that the 2-pyridylbenzamidine ligand exhibits a monomer struc-
ture, and adopts E-syn rather than E-anti geometry. In addition,
the bond lengths of N(4)–C(6) and N(3)–C(6) are 1.276(2) and
1.393(2) Å, indicative of double and single bonds, respectively.
Nickel complexes suitable for X-ray diffraction determination
were grown from dichloromethane/hexane or tetrahydrofuran/
hexane at ambient temperature under a nitrogen atmosphere.
ORTEP diagrams are given in Figs. 2–4 along with selected bond

1390
F.-S. Liu et al. / Polyhedron 28 (2009) 1386–1392
Table 1
Crystallographic data for L2, 1, 3 and 4.
Compound
L2
1
3
4
Formula
C24H25N4O2
401.48
C24H27Br2N3Ni
576.02
0.09 Â 0.07 Â 0.05
monoclinic
P2(1)/n
9.4357(12)
18.215(2)
14.0164(18)
90
101.021(3)
90
C25H29Br2N3NiO
606.04
0.10 Â 0.08 Â 0.06
monoclinic
P2(1)/n
9.3676(12)
19.349(2)
13.9412(19)
90
98.471(2)
90
C26H31Br2N3Ni
604.07
0.10 Â 0.08 Â 0.05
monoclinic
P2(1)/n
9.7549(10)
18.1652(18)
14.5490(14)
90
91.968(2)
90
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
0.12 Â 0.09 Â 0.01
triclinic
ꢀ
P1
8.2857(15)
11.834(2)
12.549(2)
110.520(3)
107.437(3)
90.894(3)
2
1089.4(3)
1.224
426
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Z
4
4
4
Volume (Å3)
Dcalc (g cmÀ3
F(000)
2364.6(5)
1.618
1160
2499.3(6)
1.611
1224
2576.6(4)
1.557
1224
)
l
(mmÀ1
)
0.080
4.216
3.996
3.873
No. of observed reflections
No. of unique reflections
Rint
8533
4230
0.0263
11162
5031
0.0347
12445
5357
0.0472
12028
5024
0.0224
Data/parameters
Goodness of fit
4230/271
1.038
5031/271
1.099
5357/289
0.986
5024/289
1.029
R1/wR2 (I > 2
R1/wR2 (all data)
r
(I))
0.0449, 0.1166
0.0732, 0.1348
0.0344, 0.0760
0.0653, 0.0993
0.0398, 0.0824
0.0805, 0.0975
0.0267, 0.0672
0.0388, 0.0732
reactions are summarized in Table 2. It was shown that four
complexes exhibited moderate catalytic activities (TOF ca. 103–
104 mol ethylene/mol Ni h). To evaluate the effect of the catalyst
structure on TOF, 1–4 complexes were screened under the same
conditions of T = 20 °C and Al/Ni = 600. In comparison to 1, 4 with
methyl on the ortho position of pyridine, and 3 containing electron-
donating methoxy on the para position of the phenyl exhibited a
lower TOF. Meanwhile, 2 containing electron-withdrawing nitro
group on the meso position of pyridine showed the highest TOF
up to 10.5 Â 103 mol ethylene/(mol Ni h). A reasonable explanation
for the TOF enhancement is that the introduction of an electron-
withdrawing group would generate a more electrophilic nickel
center, resulting in reduction of activation energy for ethylene
insertion and favoring ethylene monomer coordination and inser-
tion [9,58].
C7H8
D
CH2Cl2
C
H
FG
E
B
A
0
2
4
6
8
10
12
14
16
18
20
H
Retention Time (min)
C2H5
(C2H4)nH
(n=1, 2, 3, 4)
C, D, F, G
Furthermore, the structure of the complexes also affects the dis-
tribution of oligomers. GC–MS analyses confirmed that the oligo-
mers obtained by 1, 3 and 4/MAO at 20 °C mainly contained
butene and hexene. Besides butene and hexene, the products ob-
tained by 2/MAO also contained higher oligomers such as octene
and decene (Fig. 6).
C2H5
A
B
E
Fig. 5. GC–MS analysis of the products obtained from 2/MAO in toluene at 0 °C.
In order to obtain the onefold oligomers, dichloromethane was
used as solvent instead of toluene and the results of the catalytic
Compared with other nickel complexes with [N,N] six-mem-
bered chelating ring, it revealed that complexes 1–4 show higher
Table 2
The data of ethylene oligomerizations with 1–4/MAO in CH2Cl2.
Run
Complex
Al/Ni
T (°C)
TOFa
Distribution of oligomers (mol%)
C4
C6
C8
C10
b
1
2
3
4
5
6
7
8
1
2
3
4
2
2
2
2
600
600
600
600
600
600
400
800
1200
600
20
20
20
20
À10
0
20
20
20
0
4.5
10.5
2.3
3.6
3.2
7.9
8.1
11.7
9.8
2.1–5.6
55.7
31.8
76.2
69.6
17.8
19.9
34.7
28.3
35.6
44.3
38.7
23.8
20.4
26.3
30.1
42.5
34.9
49.2
–
–
4.1
–
25.4
–
–
46.2
42.4
15.6
27.7
12.0
–
9.7
7.6
7.2
9.1
3.2
9
2
1–4
10c
alkylated toluene and ditolylmethanes
Reaction conditions: Ni complex, 5
l
mol; tp, 0.5 h; CH2Cl2, 20 mL; ethylene pressure, 0.5 atm.
a
103 mol ethylene/(mol Ni h).
b
Not detected or in trace.
c
Toluene was used instead of CH2Cl2 as solvent.

F.-S. Liu et al. / Polyhedron 28 (2009) 1386–1392
1391
complex 2 exhibited the highest TOF and obtained higher order
oligomers. Besides, the reaction temperature and Al/Ni molar ratio
also proved to affect the catalytic properties.
CH Cl
2
2
Supplementary data
CCDC 713921, 713922, 713923 and 713924 contains the sup-
plementary crystallographic data for L2, 1, 3 and 4, respectively.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
C8
C4
C6
C10
0
2
4
6
8
10
12
14
16
Retention Time (min)
Fig. 6. GC–MS analysis of ethylene oligomers obtained by 2/MAO in CH2Cl2 at 0 °C
(run 6 in Table 2).
Acknowledgments
The financial Supports by NSFC (Projects 20604034, 20674097
and 20734004), the Science Foundation of Guangdong Province
(Project 8251027501000018 and 06300069) and the Ministry of
Education of China (Foundation for Ph.D. Training) are gratefully
acknowledged.
activity than that of quinolylimine nickel complex(III) [45]. It is
supposed that the hydrogen atom on the amidine nitrogen may
lose in the presence of MAO. In this case, the neutral amidine li-
gand transfers to anionic ligand, resulting in a stronger strength
of the Ni–Nimine bond. As a result, it may stabilize the transition
state for ethylene insertion, and increase the activity of the ethyl-
ene oliogmerization [26,19,40]. However, they are one or two or-
ders of magnitude lower than those of b-pyridylimine(IV) [46]
and dipyridylamine(V) nickel complexes [47]. Moreover, the car-
bon lengths of oligomers produced by 1–4/MAO are shorter than
those by dipyridylamine(V) nickel complexes. Considering the
hydrogen atom on the amidine nitrogen atom is being replaced
by one bulky alkyl substituent in dipyridylamine(V) nickel com-
plexes, there is increased steric hindrance in the axial position in
the dipyridylamine(V) nickel complexes, which could protect the
active species, thus enhancing the catalytic activity and obtaining
higher oligomers in the process of oligomerizaion.
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