N-(Pyridin-2-yl)picolinamide tetranickel clusters: Synthesis, structure and ethylene oligomerization

Article abstract of DOI:10.1016/j.poly.2009.06.079

A series of N-(pyridin-2-yl)picolinamide derivatives was synthesized and characterized. Tetranickel complexes were obtained by stoichiometric reaction of NiBr₂ and corresponding ligands, and characterized by elemental and spectroscopic analysis. Moreover, the coordination pattern of complex 3a was confirmed by single-crystal X-ray diffraction. In the structure, two ligands linked two nickel atoms to form a unit, and two units were bridged via μ₃-OMe and μ₂-Br to form a tetranickel cluster. These Ni(II) complexes were investigated in ethylene oligomerization and found to exhibit remarkable catalytic activities upon activation with MAO. Reaction conditions as well as ligand environment significantly affected the catalytic performance of the nickel complexes; the highest activity could be achieved to be 2.7 × 10⁶ g mol^-1 Ni h^-1.

Full text of DOI:10.1016/j.poly.2009.06.079

Polyhedron 29 (2010) 564–568
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
N-(Pyridin-2-yl)picolinamide tetranickel clusters: Synthesis, structure
and ethylene oligomerization
a,*
a
a,b
Kefeng Wang , Miao Shen , Wen-Hua Sun
a
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 July 2009
A series of N-(pyridin-2-yl)picolinamide derivatives was synthesized and characterized. Tetranickel com-
plexes were obtained by stoichiometric reaction of NiBr and corresponding ligands, and characterized by
2
elemental and spectroscopic analysis. Moreover, the coordination pattern of complex 3a was confirmed
by single-crystal X-ray diffraction. In the structure, two ligands linked two nickel atoms to form a unit,
Keywords:
Tetranickel cluster
N-(Pyridin-2-yl)picolinamides
Ethylene oligomerization
and two units were bridged via
3 2
l -OMe and l -Br to form a tetranickel cluster. These Ni(II) complexes
were investigated in ethylene oligomerization and found to exhibit remarkable catalytic activities upon
activation with MAO. Reaction conditions as well as ligand environment significantly affected the cata-
lytic performance of the nickel complexes; the highest activity could be achieved to be
6
À1
À1
2
.7 Â 10 g mol Ni h
.
Ó 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
In recent years, rapid progress has been made in the field of eth-
2. Experimental
2.1. General considerations
ylene polymerization and oligomerization based on complexes, as
illustrated by recent reviews [1]. Besides the well-explored SHOP
catalyst [2], a substantive breakthrough had been made using
diimino nickel complexes by the group of Brookhart [3]. Moreover,
numerous Ni complexes ligated by N^O [4], P^N [5], P^O [6], N^N
All manipulations of air and/or moisture-sensitive compounds
were carried out under an atmosphere of nitrogen using standard
Schlenk techniques. Solvents were refluxed over an appropriate
drying agent, distilled and degassed before using. MAO solution
À1
[
7], N^P^N [8], P^N^P [9], P^N^N [10], and N^N^N [11] have been
(1.46 mol l ) in toluene was purchased from Lanzhou Research
synthesized and afford interesting results for ethylene oligomeri-
zation or/and polymerization. Nickel complexes coordinated by
amide have not been widely reported, however, such complexes
could exhibit great interesting catalytic behaviour in ethylene
reactivity [4c,11k,12]. Though nickel clusters were widely re-
searched and attracted much attention [13], there is no report of
such nickel clusters for ethylene oligomerization. During our inves-
tigations of nickel complexes bearing N-(pyridin-2-yl)picolinamide
derivatives, tetranickel clusters were obtained and showed good
catalytic activity for ethylene oligomerization. Herein, the synthe-
sis and characterization of these novel tetranickel clusters are re-
ported along with their catalytic performance in ethylene
oligomerization.
Center of PetroChina. High purity ethylene was purchased from
Beijing Yansan Petrochemical Co. and used as received. Other re-
agents were purchased from Aldrich, Acros or local supplier. Ele-
mental analyses were performed on
a
Flash EA 1112
microanalyzer. IR spectra were recorded on a Perkin–Elmer System
2000 FT-IR spectrometer using KBr disc in the range of 4000–
À1
1
13
400 cm . The H and C NMR spectra were recorded on a Bruker
DMX-300 instrument with TMS as the internal standard. GC anal-
ysis was performed with a VARIAN CP-3800 gas chromatograph
equipped with a flame ionization detector and a 30 m (0.25 mm
i.d., 0.25
lm film thickness) CP-Sil 5 CB column.
2.2. Synthesis of ligands 1–3
2
.2.1. N-(Pyridin-2-yl)picolinamide (1)
mixture of pyridin-2-amine (15 mmol), picolinic acid
15 mmol), and trichlorophosphine (5 mmol) in xylene (30 mL)
A
(
was refluxed for 24 h. After solvent evaporation, the product was
purified by column chromatography on silica gel with petroleum
ether/ethyl acetate (8/1 v/v) as eluent to afford the product as a
*
Corresponding author. Tel.: +86 10 62659707; fax: +86 10 62618239.
E-mail address: sunwh@iccas.ac.cn (K. Wang).
0
277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.06.079

K. Wang et al. / Polyhedron 29 (2010) 564–568
565
À1
white powder in 82% yield. Mp: 107–108 °C. FT-IR (KBr, cm ):
then injected into the reactor using a syringe. As the prescribed
temperature was reached, the reactor was pressurized and main-
tained at desired pressure by constant feeding of ethylene. After
the reaction mixture was stirred for the desired period of time,
the reaction was stopped by cooling the reactor on an ice bath be-
fore the excess pressure was released. A small amount of the reac-
tion solution was collected and terminated by the addition of 10%
aqueous hydrogen chloride, and then this mixture was analyzed by
gas chromatography (GC) to determine the distribution of oligo-
mers obtained.
3
(
349 (m), 1682 (s), 1573 (s), 1527 (s), 1434 (m), 1303 (m), 1146
m), 1094 (m), 1039 (m), 997 (m). 1H NMR (CDCl
, 300 MHz): d
3
1
8
1
0.55 (s, 1H, NH), 8.63 (d, J = 4.5 Hz, 1H), 8.42 (d, J = 8.3 Hz, 1H),
.36 (d, J = 4.5 Hz, 1H), 8.29 (d, J = 7.8 Hz, 1H), 7.91 (t, J = 7.7 Hz,
H), 7.76 (t, J = 7.9 Hz, 1H), 7.49 (m, 1H), 7.08 (t, J = 5.7 Hz, 1H).
1
3
C NMR (CDCl
37.7, 126.9, 122.5, 120.0, 114.0. Anal. Calc. for C11
6.32; H, 4.55; N, 21.09. Found: C, 66.21; H, 4.73; N, 20.98%.
3
, 75 MHz): d 162.7, 151.3, 149.4, 148.4, 138.4,
1
6
9 3
H N
O: C,
2.2.2. N-(3,5-Dibromopyridin-2-yl)picolinamide (2)
Using the same procedure as for the synthesis of 1, 2 was ob-
2.5. X-ray crystallographic studies
tained as a white powder in 77% yield. Mp: 162–163 °C. FT-IR
À1
(
KBr, cm ): 3276 (m), 1722 (s), 1563 (s), 1487 (s), 1442 (m),
Single-crystal X-ray studies for compounds 3a were carried out
on a Rigaku R-AXIS Rapid IP diffractometer with graphite-mono-
chromated Mo Ka radiation (k = 0.71073 Å) at 296(2) K. Cell
1
1
3
1
7
1
279 (m), 1205 (m), 1102 (s), 869 (m).
H
NMR (CDCl
3
,
00 MHz): d 10.90 (s, 1H, NH), 8.66 (d, J = 4.5 Hz, 1H), 8.55 (s,
H), 8.34 (d, J = 7.8 Hz, 1H), 8.07 (s, 1H), 7.93 (t, J = 7.7 Hz, 1H),
parameters were obtained by global refinement of the positions
of all collected reflections. Intensities were corrected for Lorentz
and polarization effects and empirical absorption. The structures
were solved by direct methods and refined by full-matrix least-
.53 (t, J = 5.6 Hz, 1H). 13C NMR (CDCl
, 75 MHz): d 161.4, 149.4,
48.5, 148.4, 147.6, 143.1, 137.9, 127.2, 123.1, 114.9, 111.3. Anal.
Br O: C, 37.01; H, 1.98; N, 11.77. Found: C,
7.18; H, 2.12; N, 11.61%.
3
Calc. for C11
H
7
2 3
N
2
3
squares on F . All non-hydrogen atoms were refined anisotropi-
cally. All hydrogen atoms were placed in calculated positions.
Structure solution and refinement were performed by using the
SHELXL-97 package [14]. Crystal data and processing parameters
for 3a are summarized in Table 1.
2
.2.3. N-(3-Chloro-5-(trifluoromethyl)pyridin-2-yl)picolinamide (3)
Using the same procedure as for the synthesis of 1, 2 was ob-
tained as a white powder in 74% yield. Mp: 104–105 °C. FT-IR
À1
(
KBr, cm ): 3312 (s), 3283 (m), 3064 (m), 1725 (s), 1572 (s),
1
501 (s), 1435 (m), 1393 (m), 1321 (m), 1237 (m), 1127 (s), 893
3. Results and discussion
1
(
m). H NMR (CDCl
3
, 300 MHz): d 11.29 (s, 1H, NH), 8.73 (s, 1H),
.68 (d, J = 4.4 Hz, 1H), 8.36 (d, J=7.8 Hz, 1H), 7.99–7.94 (m, 2H),
.56 (t, J = 5.8 Hz, 1H). 13C NMR (CDCl
, 75 MHz): d 161.3, 150.7,
49.1, 148.4, 144.3, 138.0, 134.9, 127.4, 123.6, 120.6. Anal. Calcd
ClF O: C, 47.78; H, 2.34; N, 13.93. Found: C, 47.60; H,
.40; N, 13.78%.
8
7
1
3.1. Synthesis and characterization of the ligands and complexes
3
Ligands 1–3 could be synthesized by reacting picolinic acid with
the corresponding pyridin-2-amine in the presence of trichloro-
phosphine (Scheme 1). The N-(pyridin-2-yl)picolinamides 1–3
for C12
2
H
7
3 3
N
1
were obtained in high yields and characterized by IR, EA, H and
1
3
2.3. Synthesis of complexes 1a–3a
C NMR spectroscopy. In the IR spectra the strong and sharp band
À1
of 1682–1725 cm can be ascribed to the stretching vibration of
1
To a mixture of ligand 1 (0.5 mmol) and NiBr
2
(0.5 mmol) was
C@O, while d 10.55–11.29 in the low field of H NMR could be as-
added methanol (5 mL) at room temperature. The reaction mixture
was refluxed for 6 h and 30 mL diethyl ether was added to precip-
itate the complex. The precipitate was collected by filtration and
washed with diethyl ether, followed by drying under vacuum.
The desired complex 1a was obtained as a green powder in 86%
cribed to N–H of amide.
The nickel complexes 1a–3a were synthesized by refluxing a
2
methanol solution of NiBr and the corresponding ligand. After
adding diethyl ether, the complexes were precipitated. The result-
ing nickel complexes were collected by filtration and washed with
diethyl ether, followed by drying under vacuum. They showed high
stability in both solution and solid state, and were characterized by
IR spectroscopic and elemental analysis. In comparison with the IR
spectra of the ligands, the stretching vibrations of C@O in the com-
plexes were shifted toward lower frequency with reduced inten-
sity, indicating an effective coordination between ligand and
metal. Moreover, the unambiguous molecular structure of 3a was
confirmed by single-crystal X-ray crystallography.
À1
yield. FT-IR (KBr, cm ): 3308(m), 1645 (s), 1575 (s), 1485 (s),
1
C
440 (s), 1342 (m), 1267 (m), 1162 (m), 1022 (m). Anal. Calc. for
: C, 44.22; H, 3.07; N, 13.45. Found: C, 44.50;
46 2 4 6
H38Br N12Ni O
H, 3.10; N, 13.71%.
In a manner similar to that described for 1a, 2a was prepared as
À1
a green solid in 78% yield. FT-IR(KBr, cm ): 3224(m), 1677 (s),
1
(
585 (s), 1559 (m), 1506 (s), 1444 (m), 1216 (m), 1059 (m), 742
m). Anal. Calc. for C46 : C, 29.38; H, 1.61; N, 8.94.
Found: C, 29.04; H, 1.95; N, 8.42%.
4 6
H30Br10N12Ni O
In a manner similar to that described for 1a, 3a was prepared as
a green solid in 82% yield. FT-IR(KBr, cm ): 3229 (m), 1680 (s),
3.2. Crystal structure
À1
1
1
C
3
614 (m), 1587 (m), 1514 (s), 1391 (m), 1324 (m), 1166 (m),
145 (m), 1098 (m), 753 (m). Anal. Calc. for
Cl : C, 36.19; H, 1.82; N, 10.13. Found: C,
6.50; H, 2.11; N, 9.81%.
Crystals of 3a were grown by slow diffusion of diethyl ether into
the solution of 3a in methanol at room temperature. The molecular
structure is shown in Figs. 1 and 2, and selected bond lengths and
angles are listed in Table 2. The structure of complex 3a can be de-
scribed as a tetranickel cluster of four nickel atoms and four li-
gands, and the tetranickel complex can be regarded as a dimer in
50
H30Br
2
4 12 4 6
F N12Ni O
2.4. Procedure for oligomerization of ethylene
which two units were bridged via
3 2
l -OMe and l -Br patterns.
High-pressure ethylene oligomerization was performed in a
For example, Br(1) is linked with Ni(1) and Ni(2A) while O(3) is
connected with Ni(1), Ni(2) and Ni(2A). Br(1A) and O(3A) atoms
are linked with nickel atoms in a similar fashion.
In one unit (Fig. 2), there are two nickel atoms coordinating
with two ligands in different modes. Ni(1) is coordinated with
Br(1), N(3), N(4), N(5), O(1) and O(3) while Ni(2) is coordinated
stainless steel autoclave (0.5 L capacity) equipped with gas ballast
through a solenoid valve for continuous feeding of ethylene at con-
stant pressure. A 100 mL amount of toluene containing the catalyst
precursor was transferred to the fully dried reactor under a nitro-
gen atmosphere. The required amount of cocatalyst (MAO) was

5
66
K. Wang et al. / Polyhedron 29 (2010) 564–568
Table 1
Summary of crystallographic data for 3a.
3
a
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
50 2 4 12 4 8
H30Br Cl F N12Ni O
1691.32
monoclinic
C2/c
22.262(5)
14.206(3)
19.039(4)
90
98.65(3)
90
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
5953(2)
4
1.887
Z
D
l
À3
calcd (g cm
)
À1
(mm
)
2.869
T (K)
F(0 0 0)
h Range (°)
Number of reflections collected
Number of unique reflections
Goodness-of-fit (GOF) on F2
R
293(2)
3344
1.71–27.48
6782
6782
0.914
0.0617
Fig. 1. Molecular structure of 3a, with thermal ellipsoids at the 30% probability
level. Hydrogen atoms and solvent are omitted for clarity.
R1
R1
O
PCl3
N
xylene
R2
N
OH
+
N
reflux
R2
HN
O
N
NH2
1
2
1
2
3
: R =H, R =H;
1
2
: R =Br, R =Br;
1
2
: R =CF , R =Cl.
3
NiBr2
reflux
MeOH
R2
2
R1
O
N
R
N
N
O
N
Ni
N
Fig. 2. One coordination unit of 3a.
N
O
R1
Br
Br
N
Ni
Ni
Ni
Table 2
N
O
O
Selected bond lengths (Å) and angles (°) for 3a.
R1
N
N
N
R1
Bond length
N
2
Br(1)–Ni(2)#1
Br(1)–Ni(1)
Ni(2)–O(3)
Ni(1)–O(1)
Ni(1)–N(4)
N(2)–C(6)
2.529(1)
2.646(2)
2.051(5)
1.971(5)
2.033(7)
1.262(10)
Ni(1)–N(5)
Ni(1)–O(3)
Ni(2)#1–O(3)
Ni(1)–N(3)
O(1)–C(6)
2.045(7)
2.142(5)
2.050(5)
2.161(7)
1.281(9)
2.018(5)
R O
R2
1
2
1
2
1
2
1
a: R =H, R =H; 2a: R =Br, R =Br; 3a: R =CF , R =Cl.
3
O(1)–Ni(2)
Scheme 1. Synthesis of ligands and complexes.
Bond angle
Ni(2)#1–Br(1)–Ni(1)
O(1)–Ni(1)–N(4)
O(1)–Ni(1)–N(5)
N(4)–Ni(1)–N(5)
O(1)–Ni(1)–O(3)
N(4)–Ni(1)–O(3)
N(5)–Ni(1)–O(3)
82.14(5)
175.3(3)
94.1(2)
81.4(3)
78.9(2)
99.3(2)
87.2(2)
O(1)–Ni(1)–N(3)
N(4)–Ni(1)–N(3)
N(5)–Ni(1)–N(3)
O(3)–Ni(1)–N(3)
O(1)–Ni(1)–Br(1)
N(4)–Ni(1)–Br(1)
N(5)–Ni(1)–Br(1)
83.1(2)
99.0(3)
99.2(3)
161.3(2)
87.6(2)
96.5(2)
168.5(2)
with N(1), N(6), O(1) and O(3). Both O(1) and O(3) are connected
with two nickel atoms, forming 4-membered chelating rings. The
N(2)–C(6) bond distance is 1.262(10) Å, displaying typical C@N
double bond character. Similar isomerization from a –CO–NH–
fragment to a –C@N– fragment has also observed in other amide
systems [4c,15]. The pyridine plane of N(1)–C(1)–C(2)–C(3)–
C(4)–C(5) is almost coplanar with the pyridine plane formed by
N(3)–C(7)–C(8)–C(9)–C(10)–C(12), forming the dihedral angle of
3.3. Ethylene oligomerization by 1a–3a
1
.6°. However, another ligand is distorted due to coordination,
and the dihedral angle between pyridine ring of N(4)–C(13)–
C(14)–C(15)–C(16)–C(17) and pyridine ring of N(6)–C(19)–C(20)–
C(21)–C(22)–C(23) is 50.9°.
All of the nickel complexes were systematically investigated for
their catalytic behaviour towards ethylene oligomerization. When
activated with MAO, these tetranickel clusters showed good

K. Wang et al. / Polyhedron 29 (2010) 564–568
567
Table 3
a
Ethylene Oligomerization with 1a–3a/MAO.
Entry
Cat.
Al/Ni
T (°C)
P (atm)
Oligomer distribution(%)b
Activityc
C
4
C
6
1-C
4
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
2a
3a
500
1000
1500
1000
1000
1000
1000
1000
20
20
20
40
60
20
20
20
30
30
30
30
30
20
30
30
98.9
96.8
98.6
97.7
95.6
94.7
95.8
96.6
1.1
3.2
1.4
2.3
4.4
5.3
4.2
2.4
78.2
96.1
82.9
64.0
46.4
65.2
92.8
94.8
6.6
27.0
15.2
10.3
7.5
9.3
19.6
16.1
a
General conditions: 5
Determined by GC.
lmol complex; 30 min; 100 ml Toluene.
b
c
5
À1
À1
1
0 g mol (cat.) h .
activities as well as high selectivity for
a
-C
4
. Firstly, complex 1a
conditions exhibit
performance.
a remarkable influence on the catalytic
was selected to investigate the influence of the reaction conditions
and the results are summarized in Table 3. It can be concluded that
both of the catalytic activity and selectivity were significantly af-
fected with varying reaction parameters.
5
. Supplementary data
CCDC 729794 contains the supplementary crystallographic data
For complex 1a, different Al/Ni ratios played an important role
in the catalytic properties of the system. For example, increasing
the Al/Ni molar ratio from 500 to 1000 (Table 3, entry 1–2), led
for 3a. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
6
À1
À1
to higher activity (2.7 Â 10 g mol (cat.) h , entry 2). Further
increasing the Al/Ni molar ratio above 1000, led to lower activity
(Table 3, entry 3), possibly due to impurities in the cocatalyst lead
to degradation of the active catalyst [3b].
Acknowledgments
The effect of temperature was also examined. Elevation of the
temperature from 20 to 60 °C (entry 4–5) led to a significant de-
This work was supported by NSFC No. 20674089. We are grate-
ful to Dr. Carl Redshaw (School of Chemistry, University of East An-
glia, UK) for the English improvement.
5
À1
À1
crease in activity (7.5 Â 10 g mol (cat.) h , entry 5), probably
due to decomposition of the active species and lower ethylene sol-
ubility at higher temperatures. Moreover, higher temperature also
4
resulted in a sharp decrease of selectivity for 1-C , which could be
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Ethylene oligomerization of complex 1a was also screened un-
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6
(
(
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(
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. Conclusions
(
2
(
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A series of N-(pyridin-2-yl)picolinamide derivatives have been
synthesized, and on reaction of the ligands with NiBr , the corre-
sponding nickel complexes were obtained and characterized. X-
ray crystallographic analyses revealed that four nickel atoms were
coordinated to four ligands, which together with
bridges leads to the formation of tetranickel clusters. These com-
plexes exhibited good activities in ethylene oligomerization with
C
2
2
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(
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3 2
l -OMe and l -Br
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4
as the main product. The activity is as high as
(
6
À1
À1
.7 Â 10 g mol (cat.) h . Both the substituents and catalytic

5
68
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