Vinyl polymerization of norbornene catalyzed by 2-Py-amidine Ni(II) complex/methylaluminoxane systems

Article abstract of DOI:10.1002/aoc.3183

Two 2-Py-amidine ligands (2-Py-NH-C(Ph) N-Ar, Ar = 2,6-Me₂C ₆H₃ and 2,6-ⁱPr₂C₆H ₃) and the corresponding Ni(II) complexes (1 and 2) were synthesized and characterized using eleme

Full text of DOI:10.1002/aoc.3183

Full Paper
Received: 15 February 2014
Revised: 20 April 2014
Accepted: 3 June 2014
Published online in Wiley Online Library: 29 June 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3183
Vinyl polymerization of norbornene catalyzed by
2-Py-amidine Ni(II) complex/methylaluminoxane
systems
Yong-Ping Niu*, Xiao-Wei Wang, Zhi-Wu Zheng, Xiao-Fei Ma, Jun-Qing Cai,
Jun-Kai Zhang* and Jin-Lian Cheng
Two 2-Py-amidine ligands (2-Py―NH―C(Ph) N―Ar, Ar = 2,6-Me2C6H3 and 2,6-iPr2C6H3) and the corresponding Ni(II)
complexes (1 and 2) were synthesized and characterized using elemental analysis and FT-IR, UV–visible, 1H NMR and 13C
NMR spectroscopies. X-ray crystal structures indicate that the chelate ring conformation of the less bulky complex 1 is relatively
1
planar compared with that of the bulky complex 2. Paramagnetic H NMR and 13C NMR studies show that, in solution, the
time-average structures of complexes 1 and 2 have mirror symmetry. Both complexes 1 and 2 were used as catalyst precursors
for norbornene polymerization with methylaluminoxane as a co-catalyst. The effects of Al/Ni ratio, temperature and structure
of precursors on the catalytic performance were investigated. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: 2-Py-amidine ligands; N-(2-pyridyl)amidine Ni(II) complex; norbornene; vinyl polymerization; chelate ring conformation
metal complexes, even those bearing very simple ligands, have
Introduction
been found to be active in norbornene polymerization upon activa-
tion.[1,2] Among these active catalysts, the catalytic performances of
Through the choice of catalysts, norbornene can be polymerized
via cationic or radical polymerization, ring-opening metathesis
polymerization (ROMP) and vinyl addition polymerization (Fig. 1).[1,2]
Due to the excellent properties of the saturated norbornene (co)
polymers produced, the vinyl addition polymerization of norbornene
has been extensively studied and will continue to be an attractive
field of research.[2]
Ni(II) systems have been found to be most heavily affected by
ligand types and substituents.[2]
Square-planar nickel complexes with bulky diphosphinoamine
ligands (Ar2PN(Me)PAr2, Ar = 2-C6H4(iPr)), upon activation, can
polymerize ethylene with an activity of 105 g (mol Ni)À1 hÀ1 [15]
.
Nickel catalysts with less bulky diphosphinoamine ligands
(Ph2PN(R)PPh2) bearing N-pendant donor groups (R) were found
For olefin addition polymerization, a reasonable hypothesis is
that the bulkier steric hindrance induces more effective ion
separation between the cationic active species and their counter-
anions, and, at the same time, provides better protection to the
donor atoms of ligands from Lewis acidic compounds present in
polymerization systems, leading to increased catalytic activity.[3]
For late-transition-metal-catalyzed α-olefin polymerization, another
important function of the ‘axial’ steric hindrance is to hinder the
formation of the Hβ-agostic species, which usually cause the chain
transfer of the propagating active species through Hβ-elimination
reaction to form low-molecular-weight olefin by-products
(oligomers) and less active H―metal (Ni, Pd, etc.) species.[4–9]
Compared with α-olefin monomers, a marked advantage of
norbornene is that its insertion (exo- and/or endo-) into the
growing polymer chain does not provide active species bearing
available coplanar β-hydrogen atoms.[10,11] Moreover, the ring
strain of norbornene makes it a particularly active monomer for
vinyl addition polymerization.[12] In addition, the propagating
active species in norbornene polymerization can be stabilized by
a perfectly positioned γ-hydrogen atom from the penultimately
enchained monomer via a γ-agostic interaction.[13,14] Based on
the above observations, norbornene is considered as an ideal
monomer for checking the real contributions of ligands and substit-
uents to the derived ‘active’ species. Nearly all first-row transition
to oligomerize ethylene with activities of 105 g (mol Ni)À1 hÀ1 [16]
.
Nickel catalysts bearing diphosphinoamine ligands (Ph2PN(R)
PPh2, R = ―C6H4-p-OMe, ―S―CHMePh) were found to polymer-
ize norbornene with activities of 105 g (mol Ni)À1 hÀ1 [17,18]
.
Cámpora’s and Pringle’s groups showed that, upon activation,
the analogous four-membered nickel complexes with small bite
angle diphosphinomethane ligands display higher activity for
ethylene polymerization and produce higher molecular weight
products than those with wide bite angle diphosphines based on
longer linkage backbones.[19,20] Further studies by Hofmann and
co-workers showed that increasing the steric bulk of P-substituents
of ligands increases the molecular weights of ethylene polymers,
but decreases the catalytic activity.[21] There is still a lack of reports
on the catalytic performance of diphosphinomethane-based nickel
catalysts for norbornene polymerization, but for nickel catalytic
*
Correspondence to: Yong-Ping Niu and Jun-Kai Zhang, School of Chemical
Engineering and Pharmaceutics, Henan University of Science and Technology,
Luoyang 471023, China. E-mail: yongpingniu@gmail.com (Y.-P. Niu);
zhjunkai@gmail.com (J.-K. Zhang)
School of Chemical Engineering and Pharmaceutics, Henan University of
Science and Technology, Luoyang 471023, China
Appl. Organometal. Chem. 2014, 28, 688–695
Copyright © 2014 John Wiley & Sons, Ltd.

Norbornene polymerization catalyzed by 2-Py-amidine Ni(II)/MAO systems
into the chelate ring of catalyst precursor would also decrease
the performance of the derived active species.[26,27] However,
the variation of electronic and steric effects of different ligands
makes it difficult to evaluate and distinguish their contribution
to the catalytic performance.[37] To our knowledge, a reasonable
explanation is still unavailable for the observed difference
between their catalytic behaviors towards ethylene (α-olefin)
and norbornene.
In the work reported here, we synthesized and characterized
two Ni(II) complexes, 1 and 2, bearing 2-Py-amidine ligands
(2-Py―NH―C(Ph) N―Ar, Ar= 2,6-Me2C6H3 (L1), 2,6-iPr2C6H3 (L2)).
The X-ray crystal structure shows that the chelate ring conforma-
tion of the less bulky complex 1 is rather planar, compared with
that of the known analogous 2 having bulkier steric hindrance.[38]
This marked geometric difference between the two complexes
offered us the opportunity to study the influence of conjugate
effect and steric hindrance on the catalytic performance.
Figure 1. Three different types of norbornene polymerization, depending
on the choice of catalyst.[2]
systems with diphosphine ligands ((Ph)2P(CH2)nP(Ph)2, n = 2, 3, 4, 5),
an increase of the linkage length was found to result in a steady
increase of catalytic activities from 103 to 105 g (mol Ni)À1 hÀ1 [2,22]
.
The (bis)imine-based ligands have attracted increasing attention,
because they are relatively easily available and allow easier modifi-
cation of the steric and electronic effects of the polymerization
active species (Fig. 2). The ring conformations of five-membered
nickel chelates, such as α-diimine- and anilinotropone-based nickel
complexes (Fig. 2, left), are usually planar.[7–9] Upon activation,
these sterically open nickel chelates, bearing axial bulky steric
hindrances, have been shown to be highly active and long-lived
catalysts towards ethylene polymerization.[23,24] For comparison,
the ring conformations of six-membered nickel chelates, conju-
gated or not, are usually non-planar (Fig. 2, middle).[25–28] The steric
substituents of the chelated ligands are always pushed out from
the equatorial plane (N1–Ni–N2) and usually take a tilted and
deviated direction. Upon activation, these sterically encumbered
nickel chelates generally show smaller catalytic activities towards
ethylene (α-olefin) polymerization.[26,27] However, they have been
shown to be more active catalysts for norbornene polymerization,
Results and Discussion
Synthesis and Properties of 2-Py-amidine Ligands
The synthetic routes to the ligands and the complexes are shown
in Scheme 1. In order to minimize the formation of bis(imino)
organics (Ar―N C(Ph)―N(Py)―C(Ph) N―Ar), ligands were prepared
by dropwise addition of N-(2,6-dialkylphenyl)benzimidoyl chlo-
ride into a solution of 2-aminopyridine–triethylamine (1:1.2) at
around 0 °C. ‘Pure’ ligands, the elemental analysis data of which
agree with the calculated values, were obtained by recrystalliza-
tion from methanol.
The 1H NMR spectra of both ligands suggest an equilibrium
among the tautomers in solution (Scheme 2).[39,40] Due to the
existence of isomers, most of the aromatic signals in the spectra
are difficult to assign because of the overlapping of the peaks.[38]
In the alkyl region of the spectrum of L1 (Fig. S1), the four signals
at 2.21, 2.16, 2.09 and 1.73 ppm, which are distinguishable from
each other despite overlapping, can be identified as the methyl
groups of different isomers. Correspondingly, the four signals at
8.68, 8.28, 8.23 and 8.17 ppm, of which the latter two signals
are significantly overlapped, are attributed to the H-6-Py protons
of different isomers. For the four H-6-Py signals, only the one at
8.28 ppm exhibits doublet peaks with a visible coupling constant
of 9.8 Hz, and the other three appear as broad singlets because of
the conformation changes of isomers. The observed integral ratio
of all the H-6-Py protons against all the methyl protons is exactly
1:6, which is consistent with the calculated value. Moreover, it
compared with the five-membered ring nickel chelates.[29–31]
A
few literature reports have described the catalytic behaviors of
four-membered nickel chelates based on amidinate ligands
(Fig. 2, right).[32–35] Upon activation by methylaluminoxane
(MAO), these amidinate-based nickel chelates were reported to
give high-molecular-weight polynorbornenes with activities up
to 107 g (mol Ni)À1 hÀ1. However, in contrast, even those bearing
bulky N-2,6-diisopropylaniline groups were found to produce
only ethylene oligomers.[35]
These previous reports indicate that the bite angle and steric
bulk of ligands, which affect the geometry around the nickel
center of active species, play important roles in catalytic perfor-
mance. It is generally accepted that the resting state of the Ni
(II) catalysts is a square-planar olefin complex, bearing ligand
and propagating chain (Fig. 2).[23,24] Increasing the axial steric
hindrance would increase the catalytic activities in both ethylene
(α-olefin) and norbornene polymerization.[2,23,24,26–28,36] It has
also been found that incorporation of a conjugation structure
Figure 2. Conceptual view of the resting state of four-, five- and six-
membered ring nickel catalysts (counter-anion and conjugated structures
of ligands are omitted for clarity).
Scheme 1. Synthesis of N-(2-pyridyl)benzimidamide ligands and corre-
sponding Ni(II) complexes.
Appl. Organometal. Chem. 2014, 28, 688–695
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Y.-P. Niu et al.
ORTEP diagrams, selected bond distances and angles of the com-
plex 2 are shown in Fig. 4. The crystal structure reveals that the Ni
(II) center of complex 1 is coordinated in a tetrahedral geometry,
similar to that of complex 2. The N1–Ni–N2 bite angle observed
in complex 1 is 92.39(8)°, which is very close to that of complex 2
(92.5(2)°) and well consistent with that of other reported Ni(II)
chelates.[25–28] The Br1–Ni–Br2 angle of complex 1 is 107.066(19)°,
which is quite small compared with that of a β-diimine Ni(II) com-
plex (118.54(5)°),[25] a β-iminoamine Ni(II) complex (120.26(3)°)[28]
and complex 2 (125.62(5)°). Between complexes 1 and 2, the most
marked difference in their molecular geometries is their chelate ring
conformations. The chelate ring of the less bulky complex 1 is
relatively planar, with a N1–Ni–N3–N2 dihedral angle of 170.32°
(Fig. 3, right). For comparison, the corresponding dihedral angles
are 144.53° for the bulky analogous 2 and 139.44° for the
β-iminoamine Ni(II) complex, bearing a less bulky 2,6-dimethylphenyl
moiety.[28] In the sense of axial steric hindrance, the observed
tilt and deviation of the aniline arms from the equatorial
plane are quite large in both complexes 1 and 2. For the less
bulky complex 1, the dihedral angles from plane (Ni–N1–C1)
and plane (N1–Ni–N2) to the aniline plane are 78.60° and
72.94°, respectively. The corresponding dihedral angles found
in the bulky complex 2 are 72.54° and 59.77°, and those
found in the β-iminoamine Ni(II) complex are 88.85° and
79.60°,[28] respectively. These angles are usually found to be
very close to 90° for α-diimine- and anilinotropone-based Ni
(II) complexes.[7–9]
Scheme 2. Possible isomers and tautomers of asymmetric
benzimidamide ligands in solution.
seems that there is a correspondence relationship between each
methyl signal and each H-6-Py signal, 2.21/8.28, 2.16/8.23, 2.09/
8.68, 1.73/8.17 ppm. The observations indicate that there are four
pairs of tautomers, which are consistent with the full possible
number of isomers of ligand L1. The results of the 13C NMR mea-
surements of L1 are also consistent with the existence of the four
pairs of tautomers. Four resonances of C-2-Py carbon atoms can
be correspondingly observed at 153.58 (151.83) and 160.82
(162.94) ppm (see Fig. S2). However, at present, it is still difficult
to unequivocally assign these methyl signals and H-6-Py signals
to a specific isomer according to the 1H NMR and 13C NMR data,
because of the complicating influence of the isomerization on
the chemical shifts of protons. According to the profile of the
H-6-Py resonances (8.64, 8.26, 8.21 and 8.15 ppm), similar confor-
mational isomerization of the pyridine moiety can also be ob-
served for ligand L2 (see Fig. S3). The spectrum of ligand L2
presents mainly two methine-iPr resonances, respectively at
3.25 (m, J = 16.3 Hz) and 3.04 (m, J = 15.4 Hz) ppm, which indicates
that the isomerization of the bulkier aniline moiety is difficult. The
observed integral ratios of these resonances, H-6-Py/methines/
methyls, are very close to 1:2:12, which is consistent with the cal-
culated value for ligand L2.
The solution properties of complex 1, as well as those of the
known complex 2, were studied using paramagnetic NMR spec-
troscopy. The presence of paramagnetic Ni(II) results in relatively
poor signal-to-noise ratios of NMR spectra and the loss of cou-
pling multiplicities of protons (see Figs S4 and S5). As marked
in the spectra, all the resonances are assigned to specific protons
or carbon atoms according to the paramagnetic influences of Ni
(II) nuclei on them.[41]
In both 1H NMR spectra (Fig. S4), the resonances of the HN
proton might be too weak to be observed because of the D/H
exchange, or else they might be out of the detection region. The
paramagnetic influence of Ni(II) nuclei on the remote phenyl ring
should be weaker than that on the other rings. Hence, these slightly
shifted resonances, at 9.88 ppm (2H), 9.55 ppm (2H) and 7.41 ppm
(1H), are respectively assigned to H-o-phenyl, H-m-phenyl and
H-p-phenyl protons of complex 1. For complex 2, these phenyl
proton resonances can be respectively observed at 11.70 ppm
(2H), 9.90 ppm (2H) and 7.41 ppm (1H). The two methyl groups
of complex 1, exhibiting only one resonance, can be easily iden-
tified at 29.29 ppm (6H) according to the integral ratios to the
other resonances. This broad resonance should be a time-aver-
aged influence of methyl rotation (in the magnetic field of the
paramagnetic S = 1 Ni(II) center) on the chemical shifts of the
three attached protons. On the other hand, complex 2 exhibits
two methyl resonances of equal intensity. Both of them are only
slightly downfield-shifted, because the four methyl groups are
not directly attached to the aniline ring. The broader resonance
at 6.61 ppm (6H) is attributed to the two methyl groups posi-
tioned closer to the paramagnetic Ni(II) center (above the aniline
ring), whereas the other sharper resonance at 4.15 ppm (6H) is
attributed to the two methyl groups positioned below the
aniline ring. These observations indicate that the rotation of
the isopropyl groups should be to some extent hindered. The
weaker and broader signal at 19.65 ppm (2H), which is compara-
ble to that of the methyl signal of complex 1, is correspondingly
Synthesis, Properties and Structure of 2-Py-amidine Ni(II)
Complexes
The coordination of the ligands (L1 and L2) to Ni(II) was
performed in dichloromethane (DCM) at room temperature.
Filtration of the reaction mixture gave a clear purple filtrate.
Crystals of complexes were obtained by precipitating the filtrate
with n-hexane. This conventional method is less effective due
to the low solubility of complexes in DCM, which led to difficul-
ties in isolating most of the complexes from the residue. Occa-
sionally, when we washed the used glass filters, the complexes
were found to be very soluble and stable in acetone. Layering
acetone filtrates of complexes 1 and 2 with n-hexane gave single
crystals, which are suitable for X-ray diffraction studies. Moreover, af-
ter the crystallization, the purple solutions were faded and no obvi-
ous indications of decomposition were found for both complexes.
The ORTEP diagrams, selected bond distances and angles of
complex 1 are shown in Fig. 3. The result of the X-ray structure of
complex 2 is in agreement with the literature, for which the single
crystals were grown from DCM–n-hexane.[38] For comparison, the
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Appl. Organometal. Chem. 2014, 28, 688–695

Norbornene polymerization catalyzed by 2-Py-amidine Ni(II)/MAO systems
of 1 and at 25.01 ppm (1H) in the spectrum
of 2. The assignments of the H-x-pyridine
signals are thought to be consistent
with the sequence of their line widths,
H-5-pyridine > H-6-pyridine > H-4-pyri-
dine > H-3-pyridine, indicating the con-
formational changes of complex
molecules. The above observations sug-
gest that the solution structures of both
complexes are similar to, but not exactly
the same as, their solid-state structures
which are composed of mutual mirror
conformers (1/1). The exact coincidence
of the resonances of the paired protons,
H-o-phenyl (2H), H-m-phenyl (2H), H-m-
aniline (2H), methyl (6H) and methine
(2H), should indicate a time-averaged
structure of mirror symmetry in solution.
That said, in addition to the rotation of
Figure 3. ORTEP view of complex 1 is drawn with thermal ellipsoids at the 30% probability level. Hy-
drogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Ni(1)–N(1), 1.9761(19);
Ni(1)–N(3), 1.987(2); Ni(1)–Br(2), 2.3804(7); Ni(1)–Br(1), 2.3841(6); N(1)–Ni(1)–N(3), 92.39(8); N(1)–Ni(1)–Br
(2), 105.52(6); N(3)–Ni(1)–Br(2), 105.70(6); N(1)–Ni(1)–Br(1), 135.00(6); N(3)–Ni(1)–Br(1), 107.50(6); Br(2)–
Ni(1)–Br(1), 107.066(19); C(1)–N(1)–Ni(1), 117.81(14); C(20)–N(3)–Ni(1), 116.20(17). Selected torsion an-
gles (°): N(3)–Ni(1)–N(1)–C(1), 168.28(16); N(1)–Ni(1)–N(3)–C(20), À172.52(17).
the aniline and phenyl moieties,[6,28]
a
‘boat-to-boat’ conversion of the chelate
ring should also be involved.[42]
The results of 13C NMR analysis con-
firm those of the 1H NMR analysis. Under
the influence of the same magnetic field,
the distributions of carbon resonances
of both complexes are found to be simi-
lar to those of their attached protons
(Fig. S5). The assignments of the carbon
signals can therefore be accomplished
by referring to those of the protons. As
marked in the spectrum of 1, all the car-
bon signals of complex 1 are clearly ob-
served. The signals of 1-aniline
(737.12 ppm), o-aniline (377.21 ppm), 1-
Figure 4. ORTEP view of complex 2 is drawn with thermal ellipsoids at the 30% probability level.
Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Ni(1)–N(1), 1.969
(5); Ni(1)–N(3), 1.978(6); Ni(1)–Br(2), 2.3229(13); Ni(1)–Br(1), 2.3915(13); N(1)–Ni(1)–N(3), 92.5(2); N(1)–Ni
(1)–Br(2), 116.09(15); N(3)–Ni(1)–Br(2), 110.47(16); N(1)–Ni(1)–Br(1), 105.40(15); N(3)–Ni(1)–Br(1), 100.72
(16); Br(2)–Ni(1)–Br(1), 125.62(5); C(1)–N(1)–Ni(1), 120.6(4); C(24)–N(3)–Ni(1), 121.0(5). Selected torsion
angles (°): N(3)–Ni(1)–N(1)–C(1), 143.7(4); N(1)–Ni(1)–N(3)–C(24), À156.7(5).
phenyl (241.40 ppm), amidine (À80.53
ppm) and 2-pyridine (À131.44 ppm) car-
bon atoms can be respectively assigned,
assigned to both methine protons. Since the rotation of the iso-
propyl groups is somewhat restricted, the relatively broader
methine resonances, indicating relatively larger variations of
the proton position, should also include the contributions from
the conformational changes of the chelate ring. The left 2H sig-
nal, at 26.75 ppm for complex 1 and at 22.91 ppm for complex
2, can be assigned to the H-m-aniline proton. Considering the influ-
ences of the conformational changes, the H-p-aniline proton
should produce a broader resonance than any H-x-pyridine pro-
tons. Therefore, the broader signals, observed at À11.53 ppm (1H)
for complex 1 and at À7.53 ppm (1H) for complex 2, are attributed
to this specific proton. The last four 1H signals should be correlated
with the four pyridine protons. The resonances of these directly at-
tached protons are strongly shifted, due to their closer relationship
with the paramagnetic metal site. In the low-field region of the spec-
trum of 1, the two signals at 72.95 and 65.93 ppm can be respectively
assigned to the H-6-Py and H-5-Py protons, due to their positions
closerto the metal. Correspondingly, the resonances of these two
protons can be respectively observed at 70.39 and 66.09 ppm in
the spectrum of 2. The 1H signals in the up-field region of the spec-
tra, observed at À18.37 ppm in the spectrum of 1 and at À21.21
ppm in the spectrum of 2, are attributed to the H-3-pyridine proton,
due to its orientation with respect to the metal. The resonances of H-
4-pyridine protons are observed at 30.13 ppm (1H) in the spectrum
according to the shifting trends of the other carbon atoms which
are adjacent or in a similar position to them. Despite doubling the
data-scanning time, the signal-to-noise ratio of the spectrum of 2,
as observed in the corresponding proton spectra, is much poorer
than that of the spectrum of 1. The observed resonances in the
spectrum of 2 can be respectively assigned to specific carbon
atoms, according to their integrated intensities and to the assign-
ments of complex 1. Considering the steric effect of aniline moie-
ties, the conformational changes of complex 2 should be a slower
process than that of complex 1. As a result, the signals of these
strongly shifted carbon atoms, 6-Py, 5-Py, 4-Py, 2-Py, 1-An, 1-Ph,
amidine, would be broadened beyond detection.
Norbornene Polymerization
Norbornene polymerization experiments were carried out in
toluene using complexes 1 and 2 as catalyst precursors and
MAO as a co-catalyst (Table 1). The 1/MAO and 2/MAO systems
were found to be efficient catalysts for norbornene polymeriza-
tion, showing an activity of 106 g (mol Ni)À1 hÀ1. The unimodal
molecular weight distributions (MWDs) of polymerization prod-
ucts indicate a single-site behavior of the catalysts. At a constant
temperature of 10 °C, the catalytic activity was observed to
increase with increasing Al/Ni ratio (entries 1, 2, 5 and 3). Optimal
Appl. Organometal. Chem. 2014, 28, 688–695
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Y.-P. Niu et al.
efficiency of increasing MAO is found at an Al/Ni ratio of around
800, beyond which a decline of the catalytic activity is observed
(entry 3). At a constant Al/Ni ratio of 800, the catalytic activity
increases first and then decreases gradually with increasing
temperature (entries 4–8, 11–15). These results are consistent
with the opposing effects of increasing temperature on the
polymerization rate and on catalyst decomposition.[43] The
molecular weight (Mw) of the polymer products decreases and
the MWD (entries 4–7, 11–14) broadens with an increase of
polymerization temperature. In a shorter run time of 1 min (entry 9),
1/MAO catalyst shows an active level of 107 g (mol Ni)À1 hÀ1 and
provides only 17.4% polymer product produced in a longer run
time of 30 min (entry 5). The activity of 1/MAO is found to be
dramatically decreased with a decrease of the initial concentration
of norbornene (entry 10). These observations, obtained from
entries 10 and 9, agree with the total conversion rate of about
50% for a 30 min run (entry 5), and indicate the dependence of
the catalytic activity on the monomer concentration.
are soluble in common solvents such as toluene, xylene, cyclohexane
and chlorobenzene.
Under the same conditions (entries 4–8), the less bulky 1/MAO cat-
alyst shows 2.5 to 3.5 times higher activities than the bulky 2/MAO
catalyst (entries 11–15). The less bulky 1/MAO catalyst is also found
to yield higher molecular weight polymers with a slightly narrower
MWD (entries 4–8). However, in the literature,[4–9,23–31,36–38,44,45]
bulkier catalysts bearing 2,6-diisopropylphenyl substituent were
usually found to be more productive and to provide higher
molecular weight polymers because of a better steric protection
of ligands to their active centers. On the basis of structural and
conformational studies of 1 and 2, the steric hindrances from
the imine N-substituent would force the chelate ring of real
active species to take a non-planar conformation, especially in
the case of the 2/MAO catalyst. From this point of view, the Ni
(II) centers of the 2/MAO catalyst should be more cationic than
those of the 1/MAO catalyst, because the latter has a stronger
charge delocalization onto ligand L1.[26,27] In that respect, the
2/MAO catalyst should be more attractive (active) to norbornene
than the 1/MAO catalyst. Moreover, the steric hindrance from the
bulky N-substituent would also distort the square-planar geometry
of the cationic active species slightly towards the less sterically
demanding tetrahedral arrangement. As a result, the vacant site
of the active species of the 2/MAO catalyst might also be more
easily accessible to norbornene than that of the 1/MAO catalyst.
Another effect of increasing steric hindrance is that it would also
result in the tilt and deviation of the aniline arms from the
equatorial plane and decrease their function as axial hindrances. How-
ever, this effect would not cause chain transfer through β-H elimina-
tion in norbornene polymerization. Moreover, despite not properly
positioned in the axial direction, the bulkier N-aniline groups of
ligand L2 are still thought to provide generally better (or at least
not worse) protection to the active species of 2/MAO catalysts.
Since the literature and the above considerations all indicate
that the 2/MAO catalyst should be more active than the 1/MAO
catalyst, the discrepancy in the present results is thought to be
Additional blank experiments show that when used alone
under polymerization conditions, both MAO and complexes 1
and 2 are completely inactive towards norbornene, indicating
that the initial active species in the two catalysts are the Ni(II)-
1
methylated products of complexes with MAO.[6] In the H NMR
and FT-IR spectra of the obtained polymers (Table 1, entries 6
and 13), the proton resonance at around 0.87 ppm and the IR
absorption at 1376 cmÀ1 can be assigned to the methyl group
from MAO (see Figs S12–S14). The absence of the characteristic
proton resonances at 5.15 (cis) and 5.31 (trans) ppm, which are
related to the ROMP mechanism, indicate a vinyl addition
polymerization pathway of norbornene.[29,44] The absence of the
characteristic IR absorptions of double bonds at about 960 (cis)
and 735 (trans) cmÀ1 can also be taken as evidence against the
ROMP mechanism.[18] The observation of the characteristic
absorption at 941 cmÀ1, which can be assigned to the intact
bicyclic structure of norbornene,[45] also agrees with a vinyl addi-
tion polymerization pathway. The obtained norbornene polymers
Table 1. Influence of temperature and Al/Ni ratio on norbornene polymerizationsa
Activity (10À6 g (mol Ni)À1 hÀ1
)
Mw (10À6 g molÀ1
)
Mw/Mn
c
c
Entry
Catalyst
Al/Ni
Tp (°C)
CR (%)b
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
400
600
1000
800
800
800
800
800
800
800
800
800
800
800
800
10
10
10
0
5.0
7.9
0.38
0.60
3.38
2.10
3.83
2.00
1.41
0.90
20.0
0.20
0.91
1.11
0.80
0.53
0.34
2
3
42.7
27.9
50.7
26.5
18.7
11.9
8.8
4
5.13
3.99
3.72
2.58
1.68
2.00
2.19
2.17
5
10
20
30
40
10
10
0
6
7
8
9
10
11
12
13
14
15
2.7
12.1
14.7
10.6
7.0
4.06
3.23
2.51
2.02.
2.08
2.31
2.76
2.78
10
20
30
40
4.5
aPolymerization conditions: toluene, 20 ml; reaction time, 30 min (except entry 9, 1 min); norbornene, 40 mmol (except entry 10, 20 mmol).
bConversion rate.
cMw and Mw/Mn were determined using gel permeation chromatography with standard polystyrene as the reference and with 1,2,4-
trichlorobenzene as the eluent at 150 °C.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 688–695

Norbornene polymerization catalyzed by 2-Py-amidine Ni(II)/MAO systems
correlated with the two opposing effects of increasing steric hin-
drance on the catalytic activities. That is, in addition to providing
better protection, the increase of steric hindrance can also lead to
difficulties in the coordination process and/or in the insertion
process of monomer. Surrounded by the ligand, the propagating
chain end and the MAO counter-anion, the geometry around the
Ni(II) center of the real active species should be very bulky, despite
the 2-Py-amidine ligands bearing only one imine N-substituent. It is
hard to imagine that the chelate ring conformational changes of
active species could occur at a fast rate which is comparable to that
of the precursors, especially after the aluminium metallization of
the ring N―H group by the MAO co-catalyst at initial status.
Therefore, the present results are thought to indicate that the
active species of the 1/MAO and 2/MAO catalysts have different
abilities in adjusting their conformation to facilitate the insertion
process of the coordinated monomer (norbornene). The relatively
low activities of the 2/MAO catalyst should imply that its active
species experience a relatively long idle period of time during chain
propagation compared with the 1/MAO catalyst. The present
results demonstrate that increasing steric hindrance can also
decrease the catalytic activities of norbornene polymerization,
indicating that the negative factors of increasing steric hindrance
come to dominate in the six-membered ring catalytic systems.
The understandings agree with and explain their catalytic behav-
iors in α-olefin polymerization that oligomers would always be
present as by-products in addition to high-molecular-weight
products.[26] The above understandings are also consistent with
the observations by Wu and co-workers that the 2/MAO and
analogous catalysts bearing bulkier 2,6-diisopropylphenyl substituent
produce only ethylene oligomers with alkylation products of
toluene (solvent) as by-products.[38]
On the basis of the above discussion, the low activities of α-
diimine-based Ni(II) catalysts for norbornene homopolymerization
point to the geometric features of their active species. Their
appropriate bite angle and sterically open nature would allow the
perfectly positioned γ-hydrogen atom from the penultimately
enchained monomer to form relatively stable Hγ-agostic spe-
cies,[13,14] competing with the chain propagation process and
decreasing the catalytic activities. In contrast, the norbornene
polymerization catalyzed by six- and four-membered ring nickel
catalysts are thought to be less affected by this competition,
because of the sterically more open and encumbered nature of
their active species. Further work is currently underway to study if
the five-membered ring nickel chelates with sterically encumbered
nature, upon activation, can be highly active catalysts for the
homo- and copolymerization of norbornene.
Table 2. Crystal data and structure refinement for 2-Py-amidine Ni(II) complexes
Empirical formula
C20H19Br2N3Ni (1)
C24H27Br2N3Ni (2)
Formula weight
519.91
296(2)
576.02
296(2)
Temperature (K)
Wavelength (Å)
0.71073
0.71073
Crystal size (mm3)
0.35 × 0.27 × 0.21
Triclinic
0.34 × 0.32 × 0.27
Monoclinic
P2(1)/n
Crystal system
space group
P-1
a (Å)
8.578(2)
9.476(3)
b (Å)
9.280(2)
18.314(6)
c (Å)
13.936(4)
14.175(4)
α (°)
72.593(3)
90
β (°)
79.753(3)
100.865(4)
90
γ (°)
72.898(3)
Volume (Å3)
1006.8(5)
2416.1(13)
4
Z
2
Calculated density (Mg mÀ3
)
1.715
1.584
Absorption coefficient (mmÀ1
)
4.941
4.126
F(000)
516
1160
θ range for data collection (°)
Limiting indices
2.38–25.50
À10 ≤ h ≤ 10
À11 ≤ k ≤ 11
À16 ≤ l ≤ 16
7530/3725 (R(int) = 0.0158)
99.4
2.39–25.50
À11 ≤ h ≤ 11
À22 ≤ k ≤ 22
À17 ≤ l ≤ 17
17 448/4485 (R(int) = 0.0594)
99.8
Reflections collected/unique
Completeness to θ =25.50 (%)
Absorption correction
None
None
Max. and min. transmission
Refinement method
0.4235 and 0.2767
Full-matrix least-squares on F2
3725/0/236
1.023
0.4021 and 0.3344
Full-matrix least-squares on F2
4485/0/275
1.081
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I > 2σ(I))
R1 = 0.0247
wR2 = 0.0565
R1 = 0.0334
wR2 = 0.0596
0.392 and À0.389
R1 = 0.0562
wR2 = 0.1505
R1 = 0.0942
wR2 = 0.1709
1.019 and À0.553
R indices (all data)
Largest diff. peak and hole (e ÅÀ3
)
Appl. Organometal. Chem. 2014, 28, 688–695
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Y.-P. Niu et al.
and cooled to a temperature below 0 °C. Afterwards, 30 ml of a
toluene solution of N-(2,6-dimethylphenyl)benzimidoyl chloride
(2.44 g, 10 mmol) was added dropwise to the stirred solution.
The mixture was allowed to warm to room temperature, stirred
overnight and refluxed for 6 h. After cooling, the solvents and
the volatile substances were removed under reduced pressure
and the residue was extracted with a total of 150 ml of chloro-
form. After recrystallization from methanol, ligand L1 was finally
obtained as a light-yellow powder.
Conclusions
Two 2-Py-amidine ligands (L1 and L2) and the corresponding Ni
(II) complexes (1 and 2) were synthesized and characterized.
The results show that the chelate ring conformation of the less
bulky 1 is relatively planar compared with that of the bulky
analogous 2. Both complexes 1 and 2 were used as catalyst precursors
with MAO as a co-catalyst for norbornene polymerization. The results
show that 2-Py-amidine Ni(II)/MAO catalysts yield vinyl addition poly-
mers with an activity of 106 g (mol Ni)À1 hÀ1. The obtained products
Yield 78%; m.p. 150–151 °C. Anal. Calcd for C20H19N3 (%): C,
are high-molecular-weight polymers (2.51 × 106–5.13 × 106 gmolÀ1
)
1
79.70; H, 6.35; N, 13.94. Found (%): C, 79.66; H, 6.33; N, 13.92. H
with unimodal MWDs (Mw/Mn = 1.68–2.78), indicating a single-
site behavior of the catalysts. Under the same conditions, the
less bulky 1/MAO catalyst shows 2.5 to 3.5 times higher activities
than the bulky 2/MAO catalyst. The present observations sug-
gest that the negative factor of increasing steric hindrance
comes to dominate in the six-membered ring catalytic systems.
NMR (400 MHz, CDCl3, δ, ppm): 13.01 (s, 1H, NH), 8.68 (brs), 8.28
(d, J = 9.8Hz), 8.23 (brs) and 8.17 (brs) (1H, 6-Py), 7.85–6.60 (m,
11H, H-3,4,5-Py and H-Ar), 2.22, 2.17, 2.09 and 1.73 (s, 6H, Me-An).
13C NMR (400 MHz, CDCl3, isomer and isomer, δ, ppm): 19.05
(Me-An), 113.85 and 118.18 (3-Py), 117.91 and 122.08 (5-Py),
122.75 and 127.31 (p-An), 126.49 and 128.55 (o-An), 127.80 (o-Ph),
128.45 (overlapped, m-Ph and m-An), 128.71 (overlapped, p-Ph
and m-An), 129.71 and 130.03 (1-Ph), 134.67 and 135.46 (4-Py),
137.62 and 137.97 (6-Py), 145.45 (C-amidine), 148.03 and 136.89
Experimental
(1-An), 153.58 (151.83) and 160.82 (162.94) (2-Py). IR (KBr, νmax
,
cmÀ1): 3227 (N―H, s), 3016 (Ar―H, s), 2945 (C―H, Me, Vas), 2845
(C―H, Me, Vs), 1652 (C N, amidine, s), 1576 (Ar-ring, s), 1525 (Ar-ring,
s), 1461 (N―H, bending), 1434 (C N, Py, s), 1308 (C―NH, s), 1225
(Ar―H, in plane bending), 1084 (Ar―H, in plane bending), 773
(764) (Ar―H, deformation, 3 adjacent H out of plane), 701 (Ar-ring,
out of ring plane bending). UV–visible (CH2Cl2, λ, nm (ε, molÀ1
cmÀ1)): 237 (11 780), 266.5 (7587), 285.5 (7644), 323 (6603).
General Remarks
All necessary manipulations involving air- and moisture-sensitive
compounds were performed using standard Schlenk techniques
under nitrogen atmosphere in a vacuum-line. Pyridin-2-amine,
2,6-dimethylaniline and 2,6-diisopropylaniline were purchased
from Aldrich. [Ni(DME)Br2] was prepared according to literature
procedures.[46] MAO solid was prepared by the controlled hydro-
lysis of trimethylamine in toluene at 0–60 °C with ground Al2(SO4)
3 · 18H2O powder as the water source (H2O/Al = 1.3). Norbornene
was purchased from Acros, purified by drying over sodium and
distillation, and used as toluene solution (4.0 mol LÀ1). Hexane and
toluene were thoroughly dried over phosphorus pentoxide and
sodium, respectively. Acetone was distilled and dried over 4 Å
molecular sieve. Other commercially available reagents were pur-
chased and used as received. N-(2,6-Dialkylphenyl)benzanilide and
N-(2,6-dialkylphenyl)benzimidoyl chloride were prepared according
to published procedures.[47,48] The previously studied ligand L2
(2-Py―NH―C(Ph) N―2,6-iPr2C6H3) and the corresponding Ni(II)
complex 2 were prepared for comparison, and their characterization
data are consistent with the literature (see supporting information).[38]
1H NMR spectra were recorded with a Bruker Avance III 400MHz
NMR spectrometer using tetramethylsilane or the residual solvent
signal as the reference for chemical shifts. Chemical shift values
(δ) are reported in ppm, and coupling constants (J) are reported
in Hz. Multiplicities are given in parentheses: s, singlet; d, doublet;
t, triplet; m, multiplet; br, broad; brs, broad singlet. Elemental
analysis was carried out with a Thermo Scientific Flash 2000
Organic Elemental Analyzer. Infrared spectra were recorded with
a Nicolet 6700 FT-IR spectrometer using KBr pellets. UV spectra
were recorded with a PerkinElmer UV–visible spectrophotometer
(Lambda 35) at 25 °C in CH2Cl2. Gel permeation chromatography
analyses of the molecular weight and MWD of the polymers were per-
formed using a PL-GPC 220 instrument with standard polystyrene as
the reference and with 1,2,4-trichlorobenzene as the eluent at 150 °C.
(N-(2-Pyridyl)-N′-(2,6-dimethylphenyl)benzimidamide)NiBr2 (1)
To a stirred solution of L1 (0.75 g, 2.5 mmol) in dry methylene
chloride (20 ml), Ni(DME)Br2 (2.5 mmol, 0.77 g) was added. After
stirring at ambient temperature for two days, a purple slurry
was formed. Methylene chloride was then evaporated under
reduced pressure. The obtained residue was re-dissolved in
25 ml of dry acetone. The resultant dark-purple solution was filtered
to get rid of possible insoluble impurities. Layering the acetone
filtrates with 125 ml of n-hexane gave dark-purple crystals.
Yield (1.09 g) 84%. Anal. Found (%): C, 46.16; H, 3.67; N, 8.06.
Calcd for C20H19Br2N3Ni (%): C, 46.21; H, 3.68; N, 8.08. 1H NMR
(400 MHz, acetone-d6, δ, ppm): 72.95 (brs, 1H, 6-Py), 65.93 (brs,
1H, 5-Py), 30.13 (brs, 1H, 4-Py), 29.29 (brs, 6H, methyls), 26.75
(brs, 2H, m-An), 9.88 (brs, 2H, o-Ph), 9.55 (brs, 2H, m-Ph), 7.41
(brs, 1H, p-Ph), À11.53 (brs, 1H, p-An), À18.37 (brs, 1H, 3-Py).
13C NMR (400 MHz, acetone-d6, δ, ppm): 855.65 (1C, 6-Py),
737.12 (1C, 1-An), 677.11 (1C, 5-Py), 377.21 (2C, o-An), 313.96
(1C, 4-Py), 241.40 (1C, 1-Ph), 147.73 (2C, Me), 130.67 (2C, o-Ph),
103.45 (2C, m-Ph), 95.29 (1C, p-Ph), 56.65 (1C, p-An), À5.17 (2C, m-
An), À48.92 (1C, 3-Py), À80.53 (1C, amidine), À131.44 (1C, 2-Py).
IR (KBr, νmax, cmÀ1): 3205 (N―H, s), 3027 (Ar―H, s), 2960 (C―H,
Me, Vas), 2867 (C―H, Me, Vs), 1629 (C N, chelate ring, s), 1608
(Ar-ring, s), 1518 (Ar-ring, s), 1480 (C N, Py, s), 777 (Ar―H, deforma-
tion, 3 adjacent H out of plane), 707 (Ar-ring, out of ring plane
bending), 3282, 3205, 1629, 1608, 1583, 1518, 1480, 1357, 1308,
1196, 1161, 777, 707, 522. UV–visible (CH2Cl2, λ, nm (ε, molÀ1
cmÀ1)): 246.5 (3178), 289 (2502).
Synthesis of Ligands and Complexes
X-ray Diffraction Measurements
N-(2-Pyridyl)-N′-(2,6-dimethylphenyl)benzimidamide (L1)
The crystal data for complexes were collected using a Bruker SMART
APEX II CCD diffractometer with graphite-monochromatized Mo Κα
radiation (λ = 0.71073 Å) at 296(2) K. The crystal structure was solved
In a round-bottom flask, 2-aminopyridine (0.942 g, 10 mmol) and
triethylamine (1.4 mL, 12 mmol) were dissolved in 30 ml of THF
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Appl. Organometal. Chem. 2014, 28, 688–695

Norbornene polymerization catalyzed by 2-Py-amidine Ni(II)/MAO systems
by direct methods with SHELXS-97.[49] A full-matrix least-squares
refinement on F2 was carried out using SHELXL-97.[50] The final
agreement factor values are R1 = 0.0247, wR2 = 0.0565 for complex
1 and R1 = 0.0562, wR2 = 0.1505 for complex 2. Crystal data
and structure refinement details for complexes are summarized
in Table 2.
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Supporting Information
Additional supporting information may be found in the online
version of this article at the publisher’s web-site.
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