Highly active neutral nickel(II) complexes bearing P,N-chelate ligands: Synthesis, characterization and their application to addition polymerization of norbornene

Article abstract of DOI:10.1002/ejic.200400856

Neutral nickel(II) complexes [[R-NP]NiPh(PPh₃)](1, R = H; 2, R = CH₃) bearing P,N-chelate ligands ([H-NP] = (2-diphenylphosphanyl)benzenamine L¹: [CH₃-NP] = (2-diphenylphosphanyl) N-methylbenzenamine L²

Full text of DOI:10.1002/ejic.200400856

FULL PAPER
Highly Active Neutral Nickel(II) Complexes Bearing P,N-Chelate Ligands:
Synthesis, Characterization and Their Application to Addition Polymerization
of Norbornene
Hai-Yu Wang[a] and Guo-Xin Jin*[a]
Keywords: Nickel / P,N-chelate ligands / Homogeneous catalysts / Polymerization / Polynorbornene
Neutral nickel(II) complexes [[R–NP]NiPh(PPh3)](1, R = H; 2,
R = CH3) bearing P,N-chelate ligands ([H–NP] = (2-diphenyl-
phosphanyl)benzenamine L1; [CH3–NP] = (2-diphenylphos-
phanyl) N-methylbenzenamine L2) have been synthesized
and characterized. The molecular structure of complex 1 has
been confirmed by single-crystal X-ray analyses. After
activation with methylaluminoxane (MAO), catalytic precur-
sors 1 and 2 could polymerize norbornene to afford addition-
type polynorbornene (PNB) with very high activities
(4.43×107 g-PNBmol–1-Nih–1), high molecular weight Mw
(3.07×106 gmol–1) and moderate molecular weight distribu-
tion Mw/Mn. Catalytic activities, polymer yield, Mw and Mw/
Mn of PNB have been investigated under various reaction
conditions.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
tems have also been applied recently to this reaction.[30,31d]
Following our research on neutral nickel complexes,[16,31]
we synthesized neutral nickel(ii) complexes with a chelating
P,N ligands and firstly investigated their catalytic norbor-
nene addition polymerization behaviors. These neutral nick-
el(ii) complexes have very high activities for cyclo-olefin po-
lymerization. This article focuses on the syntheses of neu-
tral nickel(ii) complexes 1 and 2 bearing P,N-chelate ligands
and the polymerization of norbornene upon activation with
methylaluminoxane (MAO). The typical molecular struc-
ture of catalytic precursor 1 was characterized by an X-ray
crystallographic study. To the best our knowledge, neutral
nickel(ii) complexes bearing P,N-chelate ligands for the ad-
dition polymerization of norbornene have been scarcely in-
vestigated.
Introduction
Various late transition metal complexes containing che-
lating ligands act as catalysts for the polymerization and
oligomerization of olefins. Early work on the Shell Higher
Olefin Process (SHOP)[1] utilized neutral Ni catalysts con-
taining P–O chelates. Subsequent work has investigated,
mainly, several classes of ligands: [O-P]–,[2–6] [O-N]–,[7–16]
[N-N]–.[17–23] P,N ligands have attracted increasing recent
attention because of their bonding versatility with a metal
center and the relative ease with which the electronic and
steric properties of the donor atoms can be modified.[24–26]
Braunstein et al.[27] reported a series of nickel complexes,
containing P,N-chelate ligands, and their catalytic ethylene
oligomerization behavior. Liang et al.[28] have synthesized
several metal complexes supported by bidentate di-
arylamido phosphane ligands.
Homo-polymer addition of polynorbornenes is of con-
siderable importance because the products have interesting
and unique properties, including high chemical resistance,
good solubility in organic solvents, good UV resistance, low
dielectric constant, high glass-transition temperature, excel-
lent optical transparency, large refractive index, and low bi-
refringence. However, the addition polymerization of nor-
bornene was much less developed than ROMP (ring-open-
ing metathesis polymerization). In 1993, Deming and No-
vak introduced the first nickel complex for the addition po-
lymerization of norbornene.[29] Several other catalyst sys-
Results and Discussion
Syntheses of Ligands and Complexes
Syntheses of complexes 1 and 2 are outlined in Scheme 1.
(2-Diphenylphosphanyl)benzenamine (ligand L1) was pre-
pared according to literature methods.[36] Ligand L2 [(2-di-
phenylphosphanyl)-N-methylbenzenamine] was synthesized
by monolithiation of the NH2 group of ligand L1 with
nBuLi (1.0 equiv.), followed by the addition of CH3I (1.0
equiv.). After workup, L2 was extracted with hot hexane
and obtained after evaporation of solvent as a white solid.
Ligands L1 and L2 were treated with nBuLi in THF and
then treated with trans-chloro(phenyl)bis(triphenylphos-
phane)nickel(ii) to give complexes 1 and 2, respectively.
Both complexes were purified by recrystallization from tol-
[a] Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Fudan University,
Shanghai, 200433, China
Fax: +86-21-65641740
E-mail: gxjin@fudan.edu.cn
Eur. J. Inorg. Chem. 2005, 1665–1670
DOI: 10.1002/ejic.200400856
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H.-Y. Wang, G.-X. Jin
FULL PAPER
Table 1. Crystal data and summary of data collection and refine-
ment details for 1.
Formula
C42H35NNiP2
674.36
monoclinic
P21/c
16.011(4)
9.916(2)
21.824(5)
3448.2(14)
95.634(3)
4
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
V [Å3]
β [°]
Z
Color
red
Crystal size [mm]
Dcalcd. [g cm–3]
μ [mm–1]
0.20×0.10×0.08
1.299
0.686
θ limits [°]
λ [Å]
F(000)
1.28/26.01
Mo-Kα (0.71073)
1408
No. of obsd. reflections
No. of parameters refined
R1[I Ͼ 2σ(I)]
wR2(all data)
GOF on F2
6745
415
0.0379
0.0806
0.859
Scheme 1. Syntheses of nickel complexes 1 and 2.
1
uene. All compounds were well characterized by H NMR,
FT-IR as well as elemental analysis.
Figure 1. Molecular structure of complex 1. Selected bond lengths
[Å] and angles [°]: Ni(1)–N(1) = 1.892(2), Ni(1)–C(19) = 1.910(2),
Ni(1)–P(1) = 2.1670(9), Ni(1)–P(2) = 2.1999(9), P(1)–C(6) =
1.795(3), N(1)–C(1) = 1.366(3) ; N(1)–Ni(1)–C(19) = 172.80(10),
N(1)–Ni(1)–P(1) = 84.44(7), C(19)–Ni(1)–P(1) = 89.23(8), N(1)–
Ni(1)–P(2) = 95.83(7), C(19)–Ni(1)–P(2) = 90.83(8), P(1)–Ni(1)–
P(2) = 173.97(3).
Single Crystal X-ray Structure Analyses of Complex 1
A single crystal of 1 suitable for X-ray diffraction study
was grown from a concentrated CH2Cl2 solution at –30 °C.
Table 1 summarizes the collection and refinement data of
the analyses. Figure 1 shows the ORTEP diagram of 1.
Complex 1 contains a chelating [P,N] ligand, a tri-
phenylphosphane group (PPh3) and a phenyl group. The
Addition Polymerization of Norbornene
Preliminary experiments indicated that complexes 1and
bulky diphenylphosphane moiety occupies the position 2 can not catalyze ethylene polymerization with or without
trans to PPh3, with a nearly linear P(1)–Ni(1)–P(2) angle of methylaluminoxane (MAO) as co-catalyst at 10 atm of eth-
173.97(3)°, and the phenyl group attached to Ni lies trans ylene. However, after activation with MAO, they could cat-
to N(1), with a C(19)–Ni(1)–N(1) angle of 172.80(10). The alyze the polymerization of norbornene to afford addition-
cis angles at nickel are in the range 84.44–95.83°. Thus, the type polynorbornene (PNB) with high activities (107 g-
nickel center lies perfectly on the square plane defined by PNB mol–1-Nih–1), high molecular weight Mw (106 gmol–1)
the four donor atoms. In addition, the metal ion deviates and moderate molecular weight distributions Mw/Mn (2.65–
from the [P(1), N(1), P(2) and C(19)] plane by ca. 0.0828 Å. 3.93). Complexes 1 and 2 themselves and MAO did not
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Eur. J. Inorg. Chem. 2005, 1665–1670

Highly Active Neutral Nickel (ii) Complexes Bearing P,N-Chelate Ligands
FULL PAPER
Table 2. Addition polymerization of norbornene with nickel complexes 1and 2 activated by methylaluminoxane (MAO).[a]
[c]
[c]
Entry
Complex
Amount of cocat [equiv.]
Polymer [g]
Activity[b]
Mw
Mw/Mn
1
2
3
4
5
6
1
1
1
1
1
1
1
1
2
2
2
–
500
1000
2000
3000
4000
5000
10000
–
Trace
1.05
1.13
1.30
1.12
1.06
1.03
trace
0.942
0.808
trace
trace
–
3.14
3.39
3.90
3.36
3.18
3.09
–
2.83
2.42
–
2.71
2.61
2.57
2.06
2.88
3.07
3.40
3.20
2.72
2.65
2.32
2.58
7
8[d]
9
3000
4000
–
2.46
2.28
3.93
3.81
10
11[d]
12[e]
3000
–
[a] Polymerization conditions: solvent, chlorobenzene; total volume 10 mL; nickel complex (0.2 μmol), norbornene (1.88 g) [norbornene:
nickel (molar) = 100 000]; reaction time 10 min; 30 °C. [b] 107 g-PNBmol–1-Nih–1. [c] Mw (106 gmol–1) and Mw/Mn determined by GPC.
[d] Without co-catalyst MAO. [e] Without nickel complex.
produce polymers under the same conditions (entries 8, 11, Table 3. Influence of the reaction temperature (T) on activities of
complex 1.
12 in Table 2). In general, the steric structure and bulky
[b]
group in the nickel complexes slightly influence the catalytic
activities. Moreover, the addition mechanism of norbor-
nene, when using neutral nickel complexes, has been hy-
pothesized as the insertion of norbornene into the Ni–C
bond.[30i,30l] Thus, the N-methyl group of complex 2 makes
the insertion of norbornene into the Ni–C bond more diffi-
cult than H of complex 1, so complex 2 displays lower cata-
lytic activity than complex 1.
To investigate the reaction parameters affecting addition
polymerization of norbornene, the catalytic precursor 1 was
studied under different reaction conditions. MAO was es-
sential for the polymerization of norbornene catalyzed by 1
and 2. It initiates the polymerization of norbornene and
probably creates an empty site for insertion of the norbor-
nene monomer. Varying the MAO:complex 1 ratio (ex-
pressed here as Al:Ni ratio) had considerable effects on
catalytic activity and Mw. The activity of 1 is almost zero
for Al:Ni = 500 (entry 1, Table 2), but is up to 107 g-
PNBmol–1-Nih–1 when Al:Ni Ͼ 1000. The catalytic activi-
ties of complex 1 increase first and then decrease with in-
creasing Al:Ni ratio; the activity is highest at Al:Ni = 3000.
The Mw of polymers also exhibits remarkable changes with
Al:Ni, and is lowest at Al:Ni = 4000.
Entry
T[°C]
Polymer [g] Activity[a]
Mw
Mw/Mn
1
2
3
4
0
0.309
1.30
0.927
3.90
3.24
2.40
2.80
2.57
1.81
1.37
3.23
2.72
2.67
2.89
30
60
90
1.08
0.799
[a] 107 g-PNBmol–1-Nih–1. [b] Mw (106 gmol–1). Polymerization
conditions: solvent, chlorobenzene; total volume 10 mL, nickel
complex (0.2 μmol), norbornene (1.88 g) [norbornene:nickel(molar)
= 100000]; MAO (0.35 mL,1.7 m) [Al:Ni = 3000]; reaction time
10 min.
The reaction temperature also affects considerably the
catalytic activities and Mw (Table 3). With increasing reac-
tion temperature, the catalytic activities first increase and
then decrease – the highest activity is at 30 °C (3.90×107 g-
PNBmol–1-Nih–1). In contrast, Mw decreased with increas-
ing temperature while Mw/Mn varied irregularly.
Figure 2. Activity (᭡) and yield(᭺) vs. reaction time; 0.2 μmol
complex 1, norbornene:nickel(molar) = 100 000, Al:Ni = 3000,
Vtotal = 10 mL, polymerization at 30 °C.
Figure 2 reveals the effects of reaction time on catalytic
activities and polymer yields. The yields of PNB gradually
increase with increasing reaction time, but the catalytic ac- and then remains nearly constant when the concentration
tivities of complex 1 always decrease. After 1 h, the yield is exceeds 3.0 molL–1.
almost up to 90%, but the activity has reduced to 106 g-
PNBmol–1-Nih–1.
The crystallinity of the resultant polymers was investi-
gated by XRD. Figure 4 shows the XRD diagram of the
The concentration of norbornene also exerts a consider- obtained PNB. Two broad halos at 2θ of 11 and 18° are
able influence on the polymerization reaction (Figure 3). present. This pattern is predominantly intrachain, probably
The catalytic activity of complex 1 increases almost linearly corresponding to a short-range order, or to a pseudo-peri-
with monomer concentration in the range 0.5–2.5 molL–1, odicity arrangement of the bicycle units along the chain.
Eur. J. Inorg. Chem. 2005, 1665–1670
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H.-Y. Wang, G.-X. Jin
FULL PAPER
Conclusions
(2-Diphenylphosphanyl)benzenamine (L1) and (2-di-
phenylphosphanyl)-N-methylbenzenamine (L2) were pre-
pared in good yields for the syntheses of neutral nickel(ii)
complexes 1 and 2. Complex 1 was characterized by single-
crystal X-ray diffraction. Both complexes showed high cata-
lytic activities for the addition polymerization of norbor-
nene after activation with MAO. Catalytic activities of up
to 4.43×107 g-PNBmol–1-Nih–1 and Mw up to
3.07×106 g·mol–1 were observed. Activities, polymer yield
and Mw can be controlled by varying the reaction parame-
ters. PNBs obtained here are amorphous and soluble in ha-
logenated aromatic hydrocarbons. Therefore, neutral Niii
complexes with chelating P,N ligands are, perhaps, a prom-
ising system for the addition polymerization of norbornene.
We are currently working to develop new P-N nickel com-
plexes for the polymerization of norbornene.
Figure 3. Activity vs. norbornene concentration; 0.2 μmol complex
1, Al:Ni = 3000, Vtotal = 10 mL, polymerization at 30 °C for
10 min.
Experimental Section
General Remarks: All manipulations of air- and/or water-sensitive
compounds were performed under dry nitrogen using standard
Schlenk and vacuum-line techniques. Solvents were dried by boil-
ing under reflux with appropriate drying agents and distilled under
nitrogen before use. (2-Diphenylphosphanyl)benzenamine[36] and
trans-[Ni(PPh3)2(Ph)Cl][37] were prepared according to literature
procedures. Norbornene (bicyclo[2.2.1]hept-2-ene, Acros) was puri-
fied by distillation over sodium and used as a chlorobenzene solu-
tion. Methylaluminoxane (MAO) was purchased from Aldrich as
10% weight of a toluene solution and used without further purifi-
cation. Other commercially available reagents were purchased and
used without purification.
This is almost in agreement with reported results.[32,33] No
traces of Bragg reflections, characteristic of crystalline re-
gions, are revealed. The PNB is therefore, non-crystalline.
1H NMR spectra were recorded with a Varian Unity-400 spectrom-
eter. Elemental analyses were performed on an Elementar vario EL
III Analyzer. FT-IR spectra were recorded with a Niclolet-FT-IR-
50X spectrometer. NMR spectroscopic data for PNB were obtained
at ambient temperature with a Bruker AC 500 spectrometer instru-
ments using [D4]-o-chlorobenzene as solvent. Average molecular
weight (Mw) and molecular weight distributions (Mw/Mn) of PNB
products were determined using a PL GPC-220 gel permeation
chromatograph at 150 °C, employing narrow standards calibration,
and equipped with three PL gel columns (sets of PL gel 10 m
MIXED-B LS). Trichlorobenzene was used as solvent at a flow
rate of 1.00 mLmin–1. Differential scanning calorimetric (DSC)
measurements were performed with a Perkin–Elmer Pyris 1 DSC.
The X-ray diffraction (XRD) diagram of the polymer powder was
obtained using a Bruker D4 Endeavor X-ray diffractometer with
monochromatic radiation at a wavelength of 1.54 Å. Scanning was
performed with 2θ ranging from 5 to 60°.
Synthesis of Ligand L2: A solution of ligand L1 (0.555 g, 2.0 mmol)
in THF (20 mL) was cooled to –78 °C and nBuLi was then added
slowly (1.0 equiv., 0.87 mL, 2.3 m, 2.0 mmol). After the solution
was stirred for 1 h at –78 °C, CH3I (1.0 equiv., 0.125 mL, 2.0 mmol)
was slowly added at –78 °C. The reaction mixture was then stirred
for 2 h at –78 °C and warmed to room temperature overnight. The
solution was hydrolyzed with degassed water (5 mL) and extracted
with diethyl ether (3×3 mL). Finally, the organic phase was sepa-
rated, dried over degassed MgSO4, and filtered. After evaporation
of the diethyl ether, the product was isolated as a white solid (yield
0.456 g, 1.56 mmol, 78%). 1H NMR (CDCl3): δ = 2.82 (s, 3 H, N–
Figure 4. XRD diagram of PNB.
1
All polymers obtained showed very similar IR and H
1
NMR spectra. H NMR spectra exhibited no trace of the
double bond that is typical for ROMP polynorbor-
nene.[34,30l] PNB resonances appear at 0.9–2.6 (m, maxima
at 1.3, 1.5, 1.7, 2.4 ppm). A double bond, which is often
seen at 1680–1620 cm–1, was also absent from the IR spec-
tra. The glass transition temperature (Tg) of addition-type
homo-polynorbornene has proved difficult to obtain since
it is, apparently, close to the temperature at which decom-
position tends to set in.[35] Our attempts to determine the
Tg of PNB also failed, and DSC studies did not give an
endothermic signal upon heating to the decomposition tem-
perature (above 450 °C).
1668
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Eur. J. Inorg. Chem. 2005, 1665–1670

Highly Active Neutral Nickel (ii) Complexes Bearing P,N-Chelate Ligands
CH3), 6.65 (m, 2 H, m-H of P), 6.75 (m, 1 H, p-H of P), 7.34–7.23
FULL PAPER
(m, 11 H, Ar–H) ppm.
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Synthesis of Complex 1: A solution of ligand L1 (0.277 g, 1.0 mmol)
in THF (15 mL) was cooled to –78 °C, and nBuLi was added drop-
wise (1.0 equiv., 0.48 mL, 2.3 m, 1.0 mmol). This mixture was al-
lowed to warm to room temperature and then stirred for 2 h to
afford the lithium salt of L1. After evaporation of THF under vac-
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0.449 g). C43H37NP2Ni (688.42): calcd. C 75.02, H 5.42, N, 2.03;
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Polymerization of Norbornene: In a typical procedure (entry 4,
Table 2), nickel complex 1 (0.2 μmol) in chlorobenzene (1.0 mL),
norbornene (1.88 g) in chlorobenzene (3.0 mL) and another 3.0 mL
of fresh chlorobenzene were added to a special polymerization bot-
tle (20 mL) with strong stirrer under nitrogen. After keeping the
mixture at 30 °C for 10 min, MAO (0.35 mL) was added to the
polymerization system via syringe and the reaction was initiated.
Ten minutes later, acidic ethanol (Vethanol:Vconcd.HCl = 20:1) was
added to terminate the reaction. The PNB produced was isolated
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˜
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Acknowledgments
Financial support by the National Nature Science Foundation of
China for Distinguished Young Scholars (29925101, 20274008) and
by the Key Project of Science and Technology of the Education
Ministry of China is gratefully acknowledged.
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Received: October 12, 2004
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