Synthesis, characterization, and norbornene polymerization of η3-benzylnickel(II) complexes of N-heterocyclic carbenes

Article abstract of DOI:10.1016/j.jorganchem.2008.03.026

Neutral η¹-benzylnickel carbene complexes, [Ni(η¹-CH₂C₆H₅)(IiPr)(PMe₃)(Cl)] (3) (IiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) and [Ni(η¹-CH₂C₆H₅)(SIiPr)(PMe₃)(Cl)] (4) (SIiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene), were prepared by the reaction between [Ni(η³-CH₂C₆H₅)(PMe₃)(Cl)] and an equivalent amount of the corresponding free N-heterocyclic carbene. The preparation of η³-benzylnickel carbene complexes, [Ni(η³-CH₂C₆H₅)(IiPr)(Cl)] (5) and [Ni(η³-CH₂C₆H₅)(SIiPr)(Cl)] (6) were carried out by the abstraction of PMe₃ from 3 and 4 by the treatment of B(C₆F₅)₃. The treatment of AgX on 5 and 6 produced the anion-exchanged complexes, [Ni(η³-CH₂C₆H₅)(NHC)(X)] (7, NHC = IiPr, X = O₂CCF₃; 8, NHC = IiPr, X = O₃SCF₃; 9, NHC = SIiPr, X = O₂CCF₃; 10, NHC = SIiPr, X = O₃SCF₃). The solid state structures of 3 and 10 were determined by X-ray crystallography. The η³-benzyl complexes of IiPr (5, 7, and 8) alone, in the absence of any activators such as borate and MAO, showed good catalytic activity towards the vinyl-type norbornene polymerization. The catalyst was thermally robust and the activity increases as the temperature rises to 130 °C.

Full text of DOI:10.1016/j.jorganchem.2008.03.026

Journal of Organometallic Chemistry 693 (2008) 2171–2176
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, and norbornene polymerization
of g3-benzylnickel(II) complexes of N-heterocyclic carbenes
b,
Sujith S a, Eun Kyung Noh a, Bun Yeoul Lee a, , Jin Wook Han
*
*
a Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea
b Department of Chemistry, Hanyang University, Seoul 133-791, South Korea
a r t i c l e i n f o
a b s t r a c t
Neutral g1-benzylnickel carbene complexes, [Ni(g1-CH2C6H5)(IiPr)(PMe3)(Cl)] (3) (IiPr = 1,3-bis-(2,
6-diisopropylphenyl)imidazol-2-ylidene) and [Ni(g1-CH2C6H5)(SIiPr)(PMe3)(Cl)] (4) (SIiPr = 1,3-bis-(2,
6-diisopropylphenyl)imidazolin-2-ylidene), were prepared by the reaction between [Ni(g3-
CH2C6H5)(PMe3)(Cl)] and an equivalent amount of the corresponding free N-heterocyclic carbene. The
preparation of g3-benzylnickel carbene complexes, [Ni(g3-CH2C6H5)(IiPr)(Cl)] (5) and [Ni(g3-
CH2C6H5)(SIiPr)(Cl)] (6) were carried out by the abstraction of PMe3 from 3 and 4 by the treatment of
Article history:
Received 3 January 2008
Received in revised form 10 March 2008
Accepted 21 March 2008
Available online 31 March 2008
Keywords:
B(C6F5)3. The treatment of AgX on
5 and 6
produced the anion-exchanged complexes, [Ni(g3-
Carbene ligand
CH2C6H5)(NHC)(X)] (7, NHC = IiPr, X = O2CCF3; 8, NHC = IiPr, X = O3SCF3; 9, NHC = SIiPr, X = O2CCF3; 10,
NHC = SIiPr, X = O3SCF3). The solid state structures of 3 and 10 were determined by X-ray crystallography.
The g3-benzyl complexes of IiPr (5, 7, and 8) alone, in the absence of any activators such as borate and
MAO, showed good catalytic activity towards the vinyl-type norbornene polymerization. The catalyst
was thermally robust and the activity increases as the temperature rises to 130 °C.
Ó 2008 Elsevier B.V. All rights reserved.
g3-Benzylnickel complex
Polymerization catalysis
1. Introduction
ylaluminoxane (MAO), B(C6F5)3, or [B(C6F5)4]ꢀ. The g3-benzylnickel
complexes reported herein show activity for norbornene polymer-
Since the discovery of stable N-heterocyclic carbenes (NHCs)
[1], much attention has been focused on these compounds as ancil-
lary ligands for a number of transition metal complexes. NHCs are
versatile ligands that share many of the coordination properties of
phosphanes [2]. They are more potent r-donors than the phos-
phanes with a reduced p-back bonding ability. As a consequence,
NHC complexes are more stable towards dissociation or degrada-
tion. There have been some reports of NHC metal complexes uti-
lized as catalyst precursors for olefin polymerizations [3–9]. In
this study, we report the preparation, characterization, and olefin
polymerization reactivity of g3-benzylnickel complexes of the
NHC ligands, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene
(IiPr) and 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene
(SIiPr). The g3-benzyl fragment is chosen because g3-benzyl com-
plexes have displayed much faster rates of initiation in olefin poly-
merizations than the more frequently used g3-allyl complexes
[10–13]. Even though some nickel complexes, such as the shell
higher olefin process (SHOP) catalyst [14–17] and the salicylaldi-
mine-based neutral nickel complexes [18–22], can be used as an
‘‘activator free” catalyst in olefin oligomerization or polymeriza-
tion, most of the nickel complexes require activators such as meth-
ization without the requirement of any activator.
2. Experimental
2.1. General
All the manipulations were carried out under an inert atmo-
sphere using standard glove box and Schlenk techniques. Toluene,
pentane, and C6D6 were distilled from benzophenone ketyl. The 1H
(400 MHz), 13C (100 MHz), 31P (162 MHz), and 19F (376 MHz) spec-
tra were recorded on a Varian Mercury Plus 400. The elemental
analyses were carried out at the Inter-University Center Natural
Science Facilities, Seoul National University. Gel permeation chro-
matograms (GPC) were obtained at 140 °C in trichlorobenzene
using the Waters Model 150-C+ GPC and the data were analyzed
using a polystyrene analyzing curve.
2.2. Synthesis of g1- and g3-benzylnickel carbene complexes
2.2.1. Complex 3
The carbene, 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene
(IiPr) [23] (0.30 g, 0.77 mmol) and [g3-(CH2C6H5)NiCl(PMe3)] [24]
(0.20 g, 0.77 mmol) were placed in a vial and toluene (2.5 mL)
was added. The reaction mixture was stirred for 2 h at room
* Corresponding authors. Tel.: +82 31 2191844; fax: +82 31 219 2394 (B.Y. Lee).
E-mail address: bunyeoul@ajou.ac.kr (B.Y. Lee).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.03.026

2172
S. Sujith et al. / Journal of Organometallic Chemistry 693 (2008) 2171–2176
temperature. All volatiles were removed using a vacuum pump.
The remaining oily residue was dissolved in pentane (2 mL).
Recrystallization from the pentane gave 3 as a yellowish brown
colored solid. The yield was 0.47 g (94%). 1H NMR (400 MHz,
for C34H45ClN2Ni: C, 70.91; H, 7.88; N, 4.86. Found: C, 70.51; H,
8.49; N, 4.82%.
2.2.5. Synthesis of complex 7
2
C6D6, 25 °C): d = 0.51 (d, JPH = 8.4 Hz, 9H, PMe3), 0.99 (br, 6H,
To a vial containing compound 5 (0.02 g, 0.03 mmol) and
CF3COOAg (0.007 g, 0.03 mmol) was added C6H6 (0.5 mL). The vial
was covered with aluminium foil and the reaction mixture was
stirred for 8 h at room temperature. The precipitates of AgCl were
removed by filtration over Celite. The recrystallization from pen-
tane produced a red colored crystalline compound 7. Yield was
0.016 g (80%). 1H NMR (400 MHz, C6D6, 25 °C): d = 0.98 (s, 2 H, ben-
zyl-CH2), 1.00 (d, J = 6.8 Hz, 12H, iPr–CH3), 1.39 (d, J = 6.8 Hz, 12H,
iPr–CH3), 2.88 (septet, J = 6.8 Hz, 4H, iPr–CH), 6.32 (d, J = 7.6 Hz, 2H,
benzyl-orthoH), 6.53 (s, 2H, carbene-CH), 6.95 (t, J = 7.6 Hz, 2H,
benzyl-metaH), 7.11 (t, J = 7.6 Hz, 1H, benzyl-paraH), 7.22 (d,
J = 8.4 Hz, 4H), 7.31 (t, J = 8.4 Hz, 2H) ppm. 13C{1H} NMR
(100 MHz, C6D6, 25 °C): d = 18.28, 22.81, 22.94, 26.33, 28.89,
113.62, 124.44, 124.85, 127.56, 130.37, 133.38, 135.99, 145.99,
180.80 ppm. 19F{1H} NMR (376 MHz, C6D6, 25 °C): d = ꢀ74.36 ppm.
Anal. Calc. for C36H43F3N2NiO2: C, 66.38; H, 6.65; N, 4.30. Found: C,
65.91; H, 7.24; N, 4.26%.
iPr–CH3), 1.11 (br, 6H, iPr–CH3), 1.23 (br, 6H, iPr–CH3), 1.50 (d,
3JPH = 9.6 Hz, 2H, benzyl-CH2), 1.70 (br, 6H, iPr–CH3), 2.85 (br, 2H,
iPr–CH), 3.80 (br, 2H, iPr–CH), 6.61 (s, 2H, carbene-CH), 6.87 (d,
J = 7.2 Hz, 3H), 6.98–7.08 (m, 3H), 7.26 (t, J = 8.0 Hz, 3H), 7.36 (d,
J = 6.0 Hz, 2H) ppm. 13C{1H} NMR (100 MHz, C6D6, 25 °C): d = 3.88
1
2
(d, JPC = 24.9 Hz), 12.63 (d, JPC = 23.5), 22.23, 23.67, 26.85, 29.20,
123.25, 123.48, 123.86, 124.75, 126.99, 129.38, 129.99, 137.06,
2
146.10, 148.29, 149.50, 188.96 (d, JPC = 109.9 Hz) ppm. 31P{1H}
NMR (162 MHz, C6D6, 25 °C): d = ꢀ1.40 ppm. Anal. Calc. for
C37H52ClN2NiP: C, 68.37; H, 8.06; N, 4.31. Found: C, 68.11; H,
8.28; N, 4.27%.
2.2.2. Synthesis of complex 4
The complex was synthesized by the same conditions and pro-
cedure as for 3, using 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-
ylidene (SIiPr) [25] (0.10 g, 0.26 mmol). Yield was 0.14 g (84%). 1H
2
NMR (400 MHz, C6D6, 25 °C): d = 0.46 (d, JPH = 8.4 Hz, 9H, PMe3),
1.09 (d, J = 6.8 Hz, 6H, iPr–CH3), 1.20 (d, J = 6.8 Hz, 6H, iPr–CH3),
1.31 (d, J = 6.8 Hz, 6H, iPr–CH3), 1.53 (d, JPH = 9.6 Hz, 2H, benzyl-
2.2.6. Synthesis of complex 8
3
To a vial containing compound 5 (0.012 g, 0.02 mmol) and
CF3SO3Ag (0.006 g, 0.02 mmol) was added C6H6 (0.5 mL). The vial
was covered with aluminium foil and the reaction mixture was
stirred for 3 h at room temperature. The AgCl precipitates were re-
moved by filtration over Celite. Recrystallization from pentane
gave a red colored crystalline compound 8. Yield was 0.012 g
(89%). 1H NMR (400 MHz, C6D6, 25 °C): d = 0.93 (s, 2H, benzyl-
CH2), 1.01 (d, J = 6.8 Hz, 12H, iPr–CH3), 1.50 (d, J = 6.8 Hz, 12H,
iPr–CH3), 3.01 (septet, J = 6.8 Hz, 4H, iPr–CH), 6.51 (s, 2H, car-
bene-CH), 6.52 (d, J = 7.6 Hz, 2H, benzyl-orthoH), 7.01 (t,
J = 7.6 Hz, 2H, benzyl-metaH), 7.07 (t, J = 7.6 Hz, 1H, benzyl-paraH),
7.23 (d, J = 8.0 Hz, 4H), 7.31 (t, J = 6.8 Hz, 2H) ppm. 13C{1H} NMR
(100 MHz, C6D6, 25 °C): d = 16.58, 23.10, 26.36, 29.06, 116.25,
124.61, 125.34, 126.17, 129.08, 130.53, 134.54, 135.96, 145.88,
176.96 ppm. 19F{1H} NMR (376 MHz, C6D6, 25 °C): d = ꢀ77.04 ppm.
Anal. Calc. for C35H43F3N2NiO3S: C, 61.15; H, 6.30; N, 4.07. Found:
C, 60.73; H, 6.89; N, 4.02%.
CH2), 1.77 (d, J = 6.8 Hz, 6H, iPr–CH3), 3.14 (septet, J = 6.4 Hz, 2H,
iPr–CH), 3.49 (t, J = 8.4 Hz, 2H, carbene-CH2), 3.70 (t, J = 8.4 Hz,
2H, carbene-CH2), 4.09 (septet, J = 6.4 Hz, 2H, iPr–CH), 6.74- 6.88
(m, 3H), 7.00 (d, J = 7.6 Hz, 2H), 7.15–7.27 (m, 4H), 7.30 (d,
J = 7.2, 2H) ppm. 13C{1H} NMR (100 MHz, C6D6, 25 °C): d = 3.59 (d,
2
1JPC = 24.2 Hz), 12.53 (d, JPC = 23.5), 23.27, 24.56, 27.36, 29.17,
53.77, 123.19, 123.86, 125.18, 126.86, 129.20, 129.46, 137.45,
2
147.07, 149.49, 217.07 (d, JPC = 103.9 Hz) ppm. 31P{1H} NMR
(162 MHz, C6D6, 25 °C): d = ꢀ1.47 ppm. Anal. Calc. for
C37H54ClN2NiP: C, 68.16; H, 8.35; N, 4.30. Found: C, 68.14; H,
8.36; N, 4.23%.
2.2.3. Synthesis of complex 5
To a vial containing 3 (0.23 g, 0.35 mmol) and B(C6F5)3(0.18 g,
0.35 mmol) was added toluene (2.5 mL) at room temperature.
The reaction mixture was stirred for 2 h. The white precipitates
of B(C6F5) ꢁ PMe3 were removed by filtration over Celite. Finally,
the solvent was removed to give the product as a violet colored
powder. Yield was 0.21 g (99%). 1H NMR (400 MHz, C6D6, 25 °C):
d = 1.02 (s, 2H, benzyl-CH2), 1.05 (d, J = 6.8 Hz, 12H, iPr–CH3),
1.46 (d, J = 6.8 Hz, 12H, iPr–CH3), 3.14 (septet, J = 6.8 Hz, 4H,
iPr–CH), 6.17 (d, J = 7.6 Hz, 2H, benzyl-ortho H), 6.61 (s, 2H, car-
bene-CH), 6.89 (t, J = 7.6 Hz, 2H, benzyl-meta H), 7.20 (t,
J = 7.6 Hz, 1H, benzyl-paraH), 7.25 (d, J = 8.0 Hz, 4H), 7.33 (t,
J = 8.0 Hz, 2H) ppm. 13C{1H} NMR (100 MHz, C6D6, 25 °C):
d = 19.53, 23.14, 26.58, 29.02, 114.49, 115.87, 124.22, 124.78,
127.23, 130.19, 133.48, 136.74, 146.75, 183.49 ppm. Anal. Calc.
for C34H43ClN2Ni: C, 71.16; H, 7.55; N, 4.88. Found: C, 70.82; H,
8.15; N, 4.85%.
2.2.7. Synthesis of complex 9
The complex was synthesized according to same conditions and
procedure as for 7 using 6. Yield was 78%. 1H NMR (400 MHz, C6D6,
25 °C): d = 1.13 (d, J = 6.8 Hz, 12H, iPr–CH3), 1.34 (s, 2H, benzyl-
CH2), 1.48 (d, J = 6.8 Hz, 12 H, iPr–CH3), 3.22 (septet, J = 6.8 Hz,
4H, iPr–CH), 3.36 (s, 4H, CH2), 6.22 (d, J = 7.6 Hz, 2H, benzyl-
orthoH), 6.92 (t, J = 7.6 Hz, 3H, benzyl-meta and paraH), 7.21 (d,
J = 8.0 Hz, 4H), 7.31 (t, J = 8.0 Hz, 2H) ppm. 13C{1H} NMR
(100 MHz, C6D6, 25 °C): d = 20.14, 23.58, 26.84, 28.94, 54.06,
114.27, 116.74, 124.95, 127.48, 129.59, 133.09, 136.13, 146.95,
211.62 ppm. 19F{1H} NMR (376 MHz, C6D6, 25 °C): d = ꢀ74.03 ppm.
Anal. Calc. for C36H45F3N2NiO2: C, 66.17; H, 6.94; F, 8.72; N, 4.29.
Found: C, 65.74; H, 7.48; N, 4.30%.
2.2.4. Synthesis of complex 6
The complex was synthesized according to the same conditions
and procedure as for 5, using 4 as a starting material. The product
was obtained as a violet colored powder. Yield was 94%. 1H NMR
(400 MHz, C6D6, 25 °C): d = 1.05 (s, 2H, benzyl-CH2), 1.16 (d,
J = 6.8 Hz, 12H, iPr–CH3), 1.53 (d, J = 6.8 Hz, 12H, iPr–CH3), 3.50
(septet, J = 6.8 Hz, 4H, iPr–CH), 3.52 (s, 4H, CH2), 6.14 (d,
J = 7.2 Hz, 2H, benzyl-orthoH), 6.85 (t, J = 7.2 Hz, 2H, benzyl-me-
taH), 7.20 (t, J = 7.6 Hz, 1H, benzyl-paraH), 7.23 (d, J = 6.8 Hz, 4H),
7.30 (t, J = 6.8 Hz, 2H) ppm. 13C{1H} NMR (100 MHz, C6D6, 25 °C):
d = 20.16, 24.00, 27.08, 28.97, 53.83, 115.14, 117.45, 124.68,
127.44, 129.43, 133.19, 137.21, 147.88, 214.91 ppm. Anal. Calc.
2.2.8. Synthesis of complex 10
The complex was synthesized according to same conditions and
procedure as for 8 using 6. Yield was 86%. 1H NMR (400 MHz, C6D6,
25 °C): d = 1.13 (d, J = 6.4 Hz, 12H, iPr–CH3), 1.21 (s, 2H, benzyl-
CH2), 1.53 (d, J = 6.4 Hz, 12H, iPr–CH3), 3.23 (septet, J = 6.4 Hz,
4H, iPr–CH), 3.31 (s, 4H, CH2), 6.23 (d, J = 7.6 Hz, 2H, benzyl-
orthoH), 7.03 (t, J = 7.6 Hz, 3H, benzyl-meta and paraH), 7.24 (d,
J = 7.6 Hz, 4H), 7.34 (t, J = 7.6 Hz, 2H) ppm. 13C{1H} NMR
(100 MHz, C6D6, 25 °C): d = 19.28, 23.96, 26.69, 29.06, 54.42,
116.15, 117.14, 125.32, 126.17, 129.76, 134.50, 135.99, 146.56,
208.14 ppm. 19F{1H} NMR (376 MHz, C6D6, 25 °C): d = ꢀ77.48 ppm.

S. Sujith et al. / Journal of Organometallic Chemistry 693 (2008) 2171–2176
2173
Anal. Calc. for C35H45F3N2NiO3S: C, 60.97; H, 6.58; N, 4.06. Found:
C, 60.63; H, 7.11; N, 4.06%.
(k = 0.7107 Å) was used for all structures. The structures were
solved by the direct methods using the SHELX-96 program and
least-squares refinement using the SHELXL-PLUS (5.1) software pack-
age. All non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were included in the calculated positions. The
crystal data and refinement results are summarized in Table 1.
2.3. General procedure for norbornene polymerization
To a vial containing 5 g of norbornene solution in toluene
(30 wt%) was added the catalyst inside the glove box. The solution
was vigorously stirred. When the stirring was ceased, the viscous
solution was taken out from the glove box and poured into a flask
containing acetone. The white precipitates were collected by filtra-
tion and dried under a vacuum. The polymerization at 130 °C was
carried out using a bomb reactor. The 1H NMR spectra were ob-
tained at 100 °C in C2D2Cl4. The 13C-CPMAS spectrum was mea-
sured at room temperature, with a pulse angle of 90°, and a
relaxation delay of three seconds on a Bruker 500.
3. Results and discussion
3.1. Syntheses and characterization of g1- and g3-benzylnickel carbene
complexes
The g1-benzylnickel(II) complexes [Ni(g1-CH2C6H5)(PMe3)
(Cl)(IiPr)] IiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene
(3) and [Ni(g1-CH2C6H5)(PMe3)(Cl)(SIiPr)] (4) (SIiPr = 1,3-bis-(2,6-
diisopropylphenyl)imidazolin-2-ylidene) are prepared in high
yields by the reaction between [Ni(g3-CH2C6H5)(PMe3)(Cl)] and
the corresponding free N-heterocyclic carbene. The complexes
are fully characterized by the 1H, 13C, and 31P NMR. In the 1H
NMR spectra, the PMe3 signal is observed at 0.44–0.54 ppm as a
doublet by coupling with phosphorous (2JPH = 9.6 Hz), and the
g1-benzyl-CH2 signal is observed at 1.48–1.55 ppm as a doublet
by coupling with phosphorous (3JPH = 9.6 Hz). In the 13C NMR
spectra, the g1-benzyl carbon signal is observed at 3.4–4.0 ppm
as a doublet by coupling with phosphorous (2JPC = 25.0 Hz). In
addition, the carbene carbon signals are observed at 189.0 ppm
(2JPC = 109.9 Hz) and at 217.0 ppm (2JPC = 109.9 Hz) as a doublet
for complexes 3 and 4, respectively. B(C6F5)3 has been commonly
2.4. X-ray crystallography
Crystals of 3 and 10 were mounted in thin-walled glass capillar-
ies and sealed under argon. The data sets were collected on a Bru-
ker Smart CCD detector single diffractometer. Mo Ka radiation
Table 1
Crystallographic data and structure refinement for 3 and 10a
3
10
Molecular formula
Formula weight
Size (mm3)
Crystal system
Space group
a (Å)
C
36H52ClN3NiP
C35H45F3N2NiO3S
689.50
0.19 ꢂ 0. 13 ꢂ 0.11
Monoclinic
Cc
16.400(13)
12.571(10)
16.321(13)
98.768(15)
3325(5)
650.93
0.20 ꢂ 0.17 ꢂ 0.12
Monoclinic
C2/c
34.6025(18)
10.6917(5)
20.5602(11)
104.3370(10)
7369.5(6)
8
used as
a PMe3-abstracting reagent to yield phosphine-free
g3-benzyl complexes which show fast initiation in olefin polymer-
izations [10–13]. Addition of an equivalent amount of B(C6F5)3 to 3
and 4 generates the desired g3-benzyl complexes 5 and 6 in nearly
quantitative yields (Scheme 1).
b (Å)
c (Å)
The 1H and 13C NMR spectra strongly support the formation of
the PMe3-free g3-benzyl complexes. The methyl signal of PMe3 dis-
appears completely and the characteristic upfield-shifted g3-ben-
zyl signals [10–13] are observed at 6.15–6.18 ppm (doublet,
ortho) and 6.85–6.88 ppm (triplet, meta). The upfield-shifted ben-
zyl-CH2 signals are observed at 1.02 ppm and 1.05 ppm for com-
plexes 5 and 6, respectively. The carbene carbon signals are
observed as a singlet almost in the same region as observed for
the g1-benzyl complexes 3 and 4 (183.5 ppm and 214.9 ppm for
compounds 5 and 6, respectively). Preparation of related g3-allyl
IiPr Ni(II) complexes by the reaction between Ni(COD)2 and allyl
chloride in the presence of corresponding free NHC was reported
b (°)
Volume (Å3)
Z
4
l (mmꢀ1
)
0.669
0.701
Number of data collected
Number of unique data
Number of data/restraints/
parameters
37439
9146
9146/0/391
15949
8161
8161/2/415
Final indices
[I > 2r(I)]
GOF
R1 = 0.0491
wR2 = 0.1226
0.996
R1 = 0.0824
wR2 = 0.1894
0.880
a
Data collected at 233(2) K with Mo Ka radiation (k(Ka) = 0.7107 Å),
P
P
P
P
2
R(F) = kFoj ꢀ jFck/
jFoj with Fo > 2.0r(I), Rw = [
[w(Fo ꢀ Fc2)2]/ [w(Fo)2]2]1/2
with Fo > 2.0r(I).
iPr
iPr
A
N
A
N
iPr
iPr
A
N
A
N
iPr
Cl
Ni
+
Me3P
iPr
Cl Ni
iPr
iPr
Ph
-A-A-
1 -CH=CH-
2 -CH2-CH2-
-A-A-
3 -CH=CH-
4 -CH2-CH2-
PMe3
iPr
iPr
A
N
A
N
iPr
iPr
iPr
A
N
A
N
iPr
iPr
AgX
iPr
Cl
B(C6F5)3
Ni
Ni
-A-A-
X
X
-A-A-
5 -CH=CH-
6 -CH2-CH2-
7
8
9
-CH=CH- CF3CO2
-CH=CH- CF3SO3
-CH2-CH2- CF3CO2
10 -CH2-CH2- CF3SO3
Scheme 1. Syntheses of Ni-complexes.

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S. Sujith et al. / Journal of Organometallic Chemistry 693 (2008) 2171–2176
[26,27]. The addition of AgX (X = CF3COO, CF3SO3) to a solution of
complexes 5 or 6 in toluene gives the corresponding anion-ex-
changed complexes 7–10 in good yields (78–90%). In case of the
reactions with AgO3SCF3, the displacement of Cl anion by the
CF3SO3 anion is completed within 3 h. In contrast, the reaction with
AgO2CCF3 is rather slow. The reaction is not completed after 8 h
and trace amounts of the original material (5–8%) still remain.
The pure products are obtained from the reaction mixture in
78–80% yields by recrystallization. The structure of complexes
7–10 are in agreement with the 1H, 13C, and 19F NMR spectra.
Characteristic g3-benzyl signals are observed at 6.22–6.52 ppm
(doublet, ortho) and 6.92–7.03 ppm (triplet, meta). In the 13C
NMR spectra, the carbene carbon signals are observed at 178.0–
180.8 ppm for complexes 7–8 and at 208.1–211.6 ppm for
complexes 9–10, which are slightly upfield-shifted from the
chemical shifts observed for the corresponding chloride complexes
5–6. The single signal is observed at the range of ꢀ74.0 to
ꢀ77.5 ppm in the 19F NMR spectra of 7–10.
3.2. X-ray crystallographic studies
Fig. 2. ORTEP diagram of complex 10 (30% probability level). Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Ni–C(1), 1.858 (8); Ni–
C(34), 1.917 (9); Ni–C(29), 2.112 (8); Ni–C(28), 2.327 (8); Ni–O(1), 1.950 (6); O(1)–
S, 1.436 (6); C(29)–C(34), 1.445 (11); C(28)–C(29), 1.383(13); C(1)–N(2), 1.342 (9);
C(1)–N(1), 1.352 (10); C(1)–Ni–O(1), 103.5 (3); C(28)–Ni–(O1), 99.5 (3); C(34) –Ni–
C(1), 88.2 (3); C(34)–Ni–C(28), 68.6 (3); Ni–C(34)–C(29), 76.4 (5); Ni–C(29)–C(28),
80.5 (5); S–O(1)–Ni, 144.0 (4); Ni–C(1) –N(2), 129.6 (5); Ni–C(1)–N(1), 123.6 (5);
C(1)–N(2)–C(16), 125.9(6); C(1)–N(1) –C(4), 123.2 (6).
The single crystals of 3 suitable for X-ray crystallography are
obtained from a pentane solution at ꢀ15 °C. The ORTEP drawing
shows the square planar geometry around the Ni center with a
trans-relationship between the Cl and the benzyl ligands (Fig. 1).
The Ni-C(1) bond length (1.909(3) Å) is comparable to those
observed for other Ni(II) carbene complexes (for trans-dichlor-
obis-(1,3-dicyclohexylimidazol-2-ylidene)nickel(II),
1.911(2) Å
[28]; for [(tBuCCmeth)NiCl(PMe3)] (tBuCCmeth = 1,10-methylene-3,30-
di-tert-butyldiimidazole-2,20-diylidene), 1.942(4) Å) [29]. The
imidazolylidene ring is situated perpendicularly to the square
plane of the nickel (dihedral angle of N(1)–C(1)–Ni–Cl, 91.3°).
The single crystals of 10 suitable for X-ray crystallography are
obtained from a solution in a cosolvent of pentane and toluene.
Fig. 2 shows the structure of 10. The Ni centre is best described
as a distorted square planar with a trans-relationship between
the OSO2CF3 ligand and the benzyl-CH2 fragment (angles of
O(1)–Ni–C(34), 166.7(3)°). The Ni–C(29) and Ni–C(28) distances
(2.112(8) and 2.327(8) Å, respectively) indicates the chemical
bonding between these two atoms, supporting the g3-hapticity of
the benzyl ligand. By changing the hapticity from g1 to g3, the
H2C–Cipso distance is contracted (1.445(11) and 1.503(5) Å for 10
and 3, respectively) while the Cipso–Cortho distance is almost invari-
able (1.383(13) and 1.380(5) for 10 and 3, respectively). By forming
the g3-bonding, the Ni–C(34)–C(29) angle (76.4(5)°) becomes
acute and severely deviated from the angle of the ideal sp3 carbon
atom (109.5°), which is highly comparable with the observations
found in other g3-benzyl complexes of Ni [10–13] and Rh [30].
The imidazolin-2-ylidene ring is not situated as perpendicularly
to the square plane as is observed for 3 but is rather tilted (dihedral
angle of N(1)–C(1)–Ni–O(1), 120.3°).
3.3. Polymerization studies
The reactivity of the g3-benzyl complexes 5–10 for ethylene
polymerization or oligomerization is tested by adding ethylene
gas to a sealable NMR cell containing a C6D6 solution of complexes
5–10 and the reaction is monitored by the 1H NMR spectroscopy. A
mixture of cis- and trans-2-butene is formed slowly, with the rate
being so slow that it takes more than one day to transform all the
gaseous ethylene to butene (turnover frequency, less than 0.5 hꢀ1).
Norbornene can be polymerized via the vinyl-addition polymer-
ization [31]. Recently, Fink et al. reported that in the polymer pre-
pared using rac-[iPr(Ind)2ZrCl2]/MAO, the chain is grown through
an unexpected 2-exo,70-syn linkage through the r-bond metathesis
[32,33]. Complexes 5–10 are tested for norbornene polymerization
without the addition of any extra activators. There have been a
number of publications regarding the norbornene polymerization
with nickel complexes activated with MAO or B(C6F5)3 [34–38],
but the use of neutral nickel complexes as a catalyst without the
addition of any activator has not been reported. The imidazoliny-
lidene g3-benzyl complexes 6, 9, and 10 show negligible activity
toward norbornene polymerization. In contrast, the imidazolylid-
ene g3-benzyl complexes 5, 7, and 8 exhibit good activity (Table
2). When the complexes are added to the norbornene solution in
toluene (30 wt%) with [norbornene]/[Ni] ratio of 5000 and the
Fig. 1. ORTEP diagram of complex 3 (30% probability level). Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Ni–C(1), 1.909 (3); Ni–
C(31), 1.966 (4); Ni–Cl, 2.258 (10); Ni–P, 2.195 (12); N(2)–C(16), 1.451 (4); N(1)–
C(4), 1.446 (4); C(1)–N(2), 1.364 (4); C(1)–N(1), 1.360 (4); N(1)–C(1)–N(2), 103.10
(3); C(1)–Ni–Cl, 89.72 (10); C(1)–Ni–C(31), 93.56(15); C(1)–Ni–P, 164.56 (10); Cl–
Ni–P, 87.84 (4); Ni–C(1)–N(2), 122.2 (2); Ni–C(1)–N(1), 134.2 (2); Ni–C(31)–C(32),
115.5 (3); P–Ni–C(31), 92.79 (13).

S. Sujith et al. / Journal of Organometallic Chemistry 693 (2008) 2171–2176
2175
Table 2
Norbornene polymerization resultsa
b
b
Entry
Catalyst
T (°C)
Time (min)
Yield (g)
Activity (ꢂ10ꢀ3 g/mol h)
Mw ꢂ 10ꢀ3
Mw/Mn
1
2
3
4
5
6
7
8
5
7
8
8
8
8
8
25
25
25
50
80
100
130
25
120
150
120
90
60
35
0.26
0.17
0.32
0.30
0.42
0.51
0.35
Trace
40.6
21.4
50.0
410
323
341
229
226
156
120
2.4
2.5
2.6
3.2
3.3
2.9
3.1
62.5
131.3
274.8
328.0
20
120
6, 9, 10
a
Polymerization conditions: 5 g norbornene solution in toluene (30 wt%), 3.2 lmol complex, [monomer]/[catalyst] = 5000.
Determined by GPC in 1,2,4-trichlorobenzene at 140 °C using polystyrene standards.
b
solution is stirred, a very viscous solution is obtained within 2.5 h
Even though the determinations of the absolute molecular
weights are limited [41], we can routinely measure the relative
molecular weights on GPC using a polystyrene standard curve
and compare the data with the previously reported ones. The
molecular weights (Mw) of the polymers obtained by 5, 7, and 8
at room temperature (410, 323, and 341 ꢂ 103, respectively) are
smaller than that of the polymer obtained either by Ni(acac)2/
MAO catalyst (3.8 ꢂ 106) [31] or by B(C6F5)3-activated zwitterionic
catalyst (633 ꢂ 103) [40]. Narrow molecular distribution
(MWD = 2.4–3.4) indicate the presence of a single active species
in the polymerization solution. The molecular weight decreases
with increase in temperature (entries 3–7), which is a general
trend observed in the polymerization process.
and the stirring is ceased. Among the three complexes, trif-
luoromethanesulfonato complex 8 shows the highest activity at
room temperature (entries 1–3). Complex 8 is thermally robust
and the activity increases as the temperature climbs to 130 °C (en-
tries 4–7). The activity at 130 °C is 6.5 times higher than that ob-
served at 25 °C and it reaches 3.3 ꢂ 105 g/molNi ꢁ h. The
polynorbornenes prepared using 6, 9, and 10 are soluble in toluene,
trichlorobenzene, or dichloroethane at a high temperature. The 1
H
NMR spectrum of the obtained polynorbornene in C2D2Cl4 con-
firms that it is obtained exclusively by the vinyl-type polymeriza-
tion. Arndt et al. proposed three types of polynorbornene based on
the investigation of the solid state 13C-CPMAS NMR spectra [39].
Three broad signals are observed in the 13C-CPMAS spectra of
Type-1, which is observed for the polymers obtained using the cat-
alytic system of [(Ni(acac)2, Ni(2-ethylhexanoate)2, or Ni(COD)2)/
MAO] or [(Cp2ZrCl2 or Me2C(Cp)(Flu)ZrCl2)/MAO]. The CPMAS-type
II spectrum is closely related to type I spectrum. The type-II spec-
trum is observed for the polymers produced by a catalytic system
of {rac-[Me2Si(Ind)2]ZrCl2/MAO}. In type-III spectra, the number of
signals is much more and it is observed for the polymers produced
by the palladium complexes and the metallocene complexes, rac-
[C2H4(IndH4)2]ZrCl2 and meso-[Me2Si(Ind)2]ZrCl2. The 13C-CPMAS
spectrum of the polynorbornene produced by 8 looks like a combi-
nation of type-I and type-III as was observed on the polymer ob-
tained with a zwitterionic nickel complex (Fig. 3) [40]. Broad
signals at 30, 40, and 50 ppm, which are typical type-I signals,
are overlapped with the signals at 35 and 58 ppm, which are typ-
ical type-III signals.
4. Conclusions
Some g3-benzyl nickel complexes of NHC, [Ni(g3-CH2C6H5)-
(NHC)(X)] (NHC = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-yli-
dene or 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene; X = Cl,
O2CCF3, O3SCF3) are prepared and characterized in this study.
While the imidazolinylidene complexes are inactive, the imidazol-
ylidene-based g3-benzyl complexes exhibit good activity for
norbornene homopolymerization without the requirement of any
activator. Moreover, the trifluoromethanesulfonato complex gives
the highest activity. It is thermally robust and the activity increases
as the temperature rises to 130 °C. The solid state 13C-CPMAS spec-
trum reveals some unusual pattern.
5. Supplementary material
CCDC 649337 and 649338 contain the supplementary crystallo-
graphic 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.
Acknowledgement
This work was supported by Grant No. R01-2006-000-10271-0
from the Basic Research Program of the Korea Science & Engineer-
ing Foundation.
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