Ni(ii) and Fe(ii) complexes based on bis(imino)aryl pincer ligands: Synthesis, structural characterization and catalytic activities

Article abstract of DOI:10.1039/c2dt30778b

Bis(imino)aryl NCN pincer Ni(ii) complexes 2,6-(ArNCH)₂C ₆H₃NiBr (1: Ar = 2,6-Me₂C₆H ₃; 2: Ar = 2,6-Et₂C₆H₃; 3: Ar = 2,6-ⁱPr₂C₆

Full text of DOI:10.1039/c2dt30778b

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Ni(II) and Fe(II) Complexes Based on Bis(imino)aryl Pincer Ligands:
Synthesis, Structural Characterization and Catalytic Activities
Jingshun Zhang,a Wei Gao,a,* Xiaomei Lang,b Qiaolin Wu,a Lei Zhang,a and Ying Mua,*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
Bis(imino)aryl NCN pincer Ni(II) complexes 2,6-(ArN=CH)2C6H3NiBr (1: Ar = 2,6-Me2C6H3; 2: Ar = 2,6-Et2C6H3; 3: Ar =
2,6-iPr2C6H3) were prepared via oxidative-addition of Ni(0)(Ph3P)4 with bis(N-aryl)-2-bromoisophthalaldimine. These nickel
complexes were well characterized by NMR and elemental analyses. Their solid molecular structure were established by X-ray
10 diffraction analyses. The nickel metal centers adopt distorted square planar geometries with the bromine atoms as one coordinate
ligand. The NCN pincer Fe(II) complexes 2,6-(ArN=CH)2C6H3Fe(ꢀ-Cl)2Li(THF)2 (4: Ar = 2,6-Me2C6H3; 5: Ar = 2,6-Et2C6H3; 6:
Ar = 2,6-iPr2C6H3) were synthesized by lithium salt metathesis reactions of the ligand lithium salts with FeCl2. X-ray structure
analyses of 4 and 5 revealed that the Fe(II) complexes are hetero-dinuclear with the iron atoms in trigonal bipyramidal
environments. When activated with MAO, the nickel complexes are active for norbornene vinyl polymerization but are inert for
15 butadiene polymerization. The Fe(II) complexes show moderate activities in butadiene polymerization when activated with
alkylaluminum affording the cis-1,4 enriched polymer.
substituents on the catalytic behaviour of the metal complexes
were well inspected.8
Introduction
The bis(imino)aryl NCN pincer ligands, which have similar
In the past decades, late transition metal complexes have
backbone to the bis(imine)pyridine, have been explored and
attracted considerable attentions in the olefin polymerization
50 widely investigated in the coordination chemistry and
20 area for their low electrophilicity and more heteroatom
organometallic chemistry. The complexes based on such
tolerance.1 Ever since the discovery of highly active Ni and
ligands exhibit some unique properties in balancing the
Pd olefin polymerization catslysts bearing bidentate α-diimine
stability vs reactivity via the ligand modification and variation
ligands by Brookhart and co-workers in 1995,2 a large amount
of late-transition-metal based catalysts were synthesized and
25 investigated. It is revealed that the bulky substituents of the
ligands in the axial position play an important role in
11
of the metal center.9 The Pd,10,11 Pt,11 Rh11b,12 and Ir
55 complexes supported by such ligands were synthesized and
investigated in catalysis on Heck-coupling and C–C bond
formation etc reactions and material science. The lanthanide
compelxes and chromium complexes bearing NCN pincer
ligands, we have reported early, show high activities and high
60 selectivities in dienes polymerization.13 More recently we
have reported the pincer aluminium and zinc complexes which
show moderate activitites and good controllabilities in lactide
polymerization.14 In despite of some Ni(II) complexes based
on the NCN pincer ligand have been reported,15 their
65 molecular-structure or catalytic behaviors were less explored.
Moreover the Fe(II) complexes with NCN pincer ligands are
not reported yet. Their structure and catalytic activities are
also unknown so far. Herein we report the synthesis and
structural characterization of the NCN pincer Ni(II) and Fe(II)
70 complexes. The catalytic behavior of these complexes towards
norbornene and butadiene polymerization were also presented.
suppressing the β-H elimination, and therefore high molecular
weight polymer were produced.2 Three years later, Brookhart
and Gibson reported respectively the Fe(II) and Co(II)
30 complexes supported by bis(imine)pyridine ligands, which
show high activities in ethylene polymerization.3 The
bis(imino)pyridine Ni(II) complexes show moderate to low
5
activities in ethylene oligomerization/polymerization,4, but
6
exhibit high activities in norbornene vinyl polymerization
35 and butadiene polymerization.5 In these bis(imino)pyridine-
based complexes, the metal centers were firmly chelated by
the tridentate ligands. In addition, the dihedral angles between
the two N-aryl moieties are relatively smaller than those in the
bidentate
α-diimine analogous and the metal centers were
40 efficiently protected by the two N-aryl moieties during the
activation and polymerization. Inspired by the pioneering
works of Brookhart and Gibson, some tridentate ligands with
similar frameworks were extensively explored.7 Substituents
with different steric and electronic effects were used and the
45 influence of the steric and electronic properties of the
Results and discussion
Synthesis and characterization of nickel complexes
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Br
Ni
Ar
Ar
Ar
Ar
N
N
N
Br
N
Ni(0)(Ph3P)4
toluene 100 ¡ãC
1
2
3
Ar = 2,6-Me2-C6H3
Ar = 2,6-Et2-C6H3
Ar = 2,6-iPr2-C6H3
Transmetallation, transcyclometallation and oxidative-
addition are the main strategies for preparing pincer
complexes.13a The oxidative-addition is the most convenient
method and was widely used in the syntheses of the Pd, Pt, Ru
and Ni pincer complexes. The nickel complexes were
synthesized via the direct oxidative-addition of Ni(0)(Ph3P)4
with the corresponding bromine-substituted ligands in toluene
at 100 ºC. After evaporations of the toluene in vacuum, the
residues were washed with hot hexane to remove the PPh3.
5
Figure 3. Perspective view of complex 3 with thermal ellipsoids drawn at
35 30% probability level. Hydrogens and uncoordinated solvents are omitted
for clarity.
10 The products were obtained as orange powder in nearly
quantative yields. All the nickel complexes are high soluble in
toluene and dichloromethane. These complexes are very stable
at room temperature and no apparent decomposition was
shows two sets of doublets at 1.15 ppm and 1.36 ppm for non-
equivalent isopropyl methyl groups, probably due to the
restricted rotation about the N-Caryl bonds arising from the
40 coordination of the ligand to the nickel metal center.16
found when open to the air for several days.
The solid molecular structure of complexes 1–3 were
established by X-ray diffraction analysis. The molecular
structures were depicted in Figures 1–3 respectively.
Crystallographic data and selected bond distances and angles
45 are given in Table 1 and 2. All the complexes 1–3 are
isostructural, adopting a distorted square planar geometry
around the nickel atom. The monoanionic NCN-pincer ligand
1
15
Complexes 1–3 were characterized with H NMR and 13C
{1H} NMR spectroscopy. The 1H NMR spectra of these
complexes show similar topology at low field and for all the
complexes a singlet was observed within
δ 7.89～7.90 ppm
assignable to the CH=N proton which shifting to upfield
20 when compared to that in the corresponding free ligands. At
high field, complexe 1 and 2 show typical resonances for the
methyl and ethyl groups attached to the ligands respectively.
While in contrast to the free ligands, in which only one set of
doublet for isopropyl methyl group was found, complex 3
coordinates to the central metal ion in a κC:κN:κN′ tridentate
mode with the two N-aryl rings being perpendicular to the
50 NCN plane. The nickel atoms are essentially coplanar with the
NCN plane. The Ni–C bond lengths, 1.827(7) Å in 1, 1.804(6)
Å in 2 and 1.823(4) Å in 3 fall in a reasonable range for the
linkage between nickel atom and a carbon atom.15a The C–
Nimine bond lengths ranging from 1.278(3) Å to 1.2898(4) Å
55 are comparable to those in the analogous rhodium and
platinum complexes.10 The bond-length of Ni–Br of 2.3345(13)
Å in 1, 2.3486(10) Å in 2, and 2.3549(10) Å in 3 are slightly
shorter than that of 2.60 Å (av.) for Ni–Br in the literatures.15a
Synthesis and characterizaiton of pincer Fe(II) complexes
25
Figure 1. Perspective view of complex 1 with thermal ellipsoids drawn at
30% probability level. Hydrogens and uncoordinated Ph3P are omitted for
clarity.
THF
THF
Li
Ar
Ar
Cl
Cl
N
Br
N
Ar
Ar
N
Fe
N
nBuLi
FeCl2, THF
4
Ar = 2,6-Me2-C6H3
Ar = 2,6-Et2-C6H3
Ar = 2,6-iPr2-C6H3
5
6
60
Attempts to synthesize the pincer Fe(II) complexes via
oxidative-addition of the ligand with low value iron
precursors, such as Fe(CO)5 or Fe(0)(Me3P)4, in toluene failed.
Unidentified mixture with gray metal powder were obtained.
65 Alternatively, the lithium salt elimination reaction was used to
synthesize the pincer Fe(II) complexes. The ligand lithium
salts were prepared in THF at low temperature by addition of
30 Figure 2. Perspective view of complex 2 with thermal ellipsoids drawn at
30% probability level. Hydrogens and uncoordinated solvents are omitted
for clarity.
2
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nBuLi to the ligands in THF solution. Reactions of the in-situ
2.181(2)
Å
are slightly shorter than those in the
prepared ligand lithium salts with FeCl2 in THF afford the
corresponding NCN pincer Fe(II) complexes in moderate
yields. No informative NMR spectra were obtained due to the
paramagnetic nature of these compelxes. While these Fe(II)
complexes were well characterized with IR, elemental
analyses, and X-ray crystallography analysis as well. The
molecular structure of 4, and 5, were shown in Figures 4–5
respectively.17 It can be seen that both the pincer Fe(II)
bis(imino)pyridine derivatives.3a,c All the iron complexes 4–6
are paramagnetic. The complex 4 and 5 exhibited a magnetic
moment (determined with Evans method)20 of 3.11 and 2.66 ꢀB
30 respectively, which are comparable to the spin-only value for
5
an S = 1 state (2.83
ꢀB). But, complex 6 is high spin species
with a magnetic moment of 4.80 ꢀB, which in good agreement
with the spin-only value for high spin d6 (Fe2+) centers.7c, 21a,
21b
The exact reason for the difference in magnetic moments
10 complexes are chlorine-bridged hetero-dinuclear. The Fe(II)
atoms are in a trigonal bipyramidal environment and two
additional THF ligands complementing the coordinated sphere
of the lithium atoms. The Fe(II) atoms are essentially coplanar
with the NCN plane and the two N-aryl rings are oriented
15 basically orthogonal to the coordination plane. The Fe–Cl
bond lengths (2.3972 Å (av) in 4 and 2.3967(16) Å in 5) are
similar to that of 2.375(2)–2.417(3) Å in the [Fe(ꢀ-Cl)Ar′]2
35 was unknown yet, but we think the repulsions of the bulky
moieties in the two N-aryl group may weaken the
coordination of the N atoms to the Fe(II) center and afford
smaller splitting energy which favors a high spin state. A
similar phenomenon in which the electronic environment of
40 the complexes varied with the bulkiness of the auxiliary
aromatic groups was also observed in bis(imino)pyridine Fe(II)
complexes.21c
(Ar′
=
C6H3-2,6-(C6H3-2,6-iPr2)2).18 The Fe–Cipso bond
distances (2.037(4) Å in 4 and 2.058(9) Å in 5) are comparable
20 to that of 2.0317(13) Å in [CpFe(C6H3-2,6-iPr2],18 but slightly
longer than that of 1.950(18)–1.979(3) Å in FeCl(PMe3)3(Ar-
CH=N-R).19 The N–Fe bond lengths of 2.170(2) Å and
Figure 5. Perspective view of complex 5 with thermal ellipsoids drawn at
45 30% probability level. Hydrogens are omitted for clarity.
Figure 4. Perspective view of complex 4 with thermal ellipsoids drawn at
25 30% probability level. Hydrogens are omitted for clarity.
Table 1 Crystal Data and Structural Refinements Details for 1, 2, 3, 4 and 5
1ꢁ1/3 PPh3ꢁ0.1CH2Cl2
2
3
4
5
Formula
C30H28BrN2Ni(PPh3)0.3 C29H31BrCl2N2Ni
C33H41BrCl2N2Ni
C32H39Cl2FeLiN2O2
C36H47Cl2FeLiN2O2
(CH2Cl2)0.1
Fw
Cryst. Syst.
Space group
a (Å)
b (Å)
c (Å)
α (deg)
572.73
Rhombohedral
R-3
26.515(7)
26.515(7)
20.941(6)
90.000
617.08
Trigonal
P4(3)2(1)2
13.639(2)
13.639(2)
16.039(4)
90.000
675.20
Monoclinic
Cc
11.756(5)
21.816(11)
12.863(7)
90.000
617.34
Monoclinic
P2(1)/c
9.7112(9)
21.3935(19)
15.7623(14)
90.000
673.45
Monoclinic
C2/c
10.4769(19)
17.050(3)
20.291(4)
90.000
β (deg)
90.000
90.000
93.458(19)
93.5040(10)
98.088(3)
γ (deg)
120.000
12750(6)
18
1.343
5283
90.000
2983.6(10)
4
1.374
1264
90.000
3293(3)
4
1.362
1400
90.000
3268.6(5)
4
1.255
1296
90.000
3588.5
4
1.247
1424
V (Å3)
Z
Dcalcd (g .cm−3)
F(000)
θ range for data
collection
No. of reflns
Unique
3.05 ≤θ≤ 27.48
2.99 ≤ θ ≤27.47
3.17 ≤θ≤ 27.48
1.61 ≤θ≤ 26.06
2.03 ≤θ≤ 25.06
39703
6542
0.949
0.0824
0.2038
0.1423
29441
3414
1.019
0.0478
0.1245
0.0640
15969
7129
1.081
0.0435
0.0904
0.0457
18242
6436
1.068
0.0639
0.1909
0.0252
9154
3167
1.075
0.0726
0.1816
0.0743
Goodness-of-fit on F2
R1 (I > 2σ)
Rw (I > 2σ)
Rint
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Table 2 Selected bond lengths (Å) and angles (deg) for 1, 2, 3, 4, and 5.
catalyst activity and the molecular-weight of the resultant
polymers. As shown in Table 3. catalytic activity of
40 precatalyst 3 increased rapidly first with increases in Al:Ni
ratios, and then remained steady after the Al:Ni molar ratio
reached about 2000:1. However, the molecular weights of the
polymers gradually decreased with increasing in Al:Ni ratios.
Catalyst activity and molecular weights for the resultant
45 polymers were also dramatically influenced by the
polymerization temperature. When the polymerization
temperature increased from 15 oC to 45 oC, both catalyst
activities and the molecular-weights of the polymer gradually
increased. While further increase of the temperature resulted
50 in dramatically decrease of the activity probably due the
deactivation of the active species at high temperature. The
FT-IR spectra of the resulted polymer show the characteristic
peak of polynorbornene in the range of 940 cm-1 to 943 cm-1
which can be assigned to the vinyl addition polymer. No
55 detectable absorption peaks at 960 cm-1 was found suggesting
that the norbornene polymerization initiated by the nickel or
ferrous complexs and MAO were performed in vinyl addition
manner.24
1
Ni(1)−C(1)
Ni(1)−N(1)
Ni(1)−N(2)
1.827(7)
1.923(6)
1.918(6)
Ni(1)−Br(1)
N(2)−Ni(1)−N(1)
C(1)−Ni(1)−Br(1)
2
2.3345(13)
162.6(3)
176.2(2)
Ni(1)−C(1)
Ni(1)−N(1)
1.840(6)
1.947(3)
N(1)−Ni(1)−N(1A)
C(1)−Ni(1)−Br(1)
161.88(7)
180.000(15)
Ni(1)−Br(1) 2.3486(10)
3
Ni(1)−C(1)
Ni(1)−N(1)
Ni(1)−N(2)
1.823(4)
1.938(4)
1.945(4)
Ni(1)−Br(1)
N(2)−Ni(1)−N(1)
C(1)−Ni(1)−Br(1)
4
2.3549(10)
161.13(15)
174.25(13)
Fe(1)−C(1)
Fe(1)−N(1)
Fe(1)−N(2)
2.037(4)
2.438(3)
2.445(3)
Fe(1)−Cl(2)
N(2)−Fe(1)−N(1)
Cl(1)−Fe(1)−Cl(2)
2.3733(10)
147.54(11)
99.11(4)
Fe(1)−Cl(1) 2.3811(11)
5
Fe(1)−C(1)
Fe(1)−N(1)
2.058(9)
2.463(5)
N(2)−Fe(1)−N(1)
Cl(1)−Fe(1)−Cl(2)
146.94(11)
98.45(9)
Fe(1)−Cl(1) 2.3967(16)
Polymerization of norbornene
Polymerization of butadiene
The nickel and iron complexes were tested to initiate the
norbornene polymerization. The selective data were
summarized in Table 3. The results showed that, activated
with MAO, the nickel complexes are moderate active towards
the vinylic polymerization of norbornene at room temperature
and conversions of 32.6%–57.2% were obtained within 60
5
60
Polybutadiene is the most widely used synthetic rubbers.25
The most efficient catalysts for butadiene polymerization are
homogeneous Ziegler-Natta catalyst systems composed of
complexes of various transition metals such as Ti,25, 26d V,26
Cr,27 Co,28 and Nd,29 and co-activators aluminum alkyls or
10 min (entry 1–3). Under the activation with MAO, the Fe(II)
complexes are less active when compared to the nickel
analogues. Complex 4 and 5 show very low activities and
almost no polymer was obtained in these systems.
Interestingly, complex 6 with bulky 2,6-diisopropyl group
15 exhibits moderate activity (conversion of 70% in 60 min),
affroding high molecular weight polymer. The are few reports
about the vinyl polymerization of norbornene with Fe(II)
complexes. The bis(imino)pyridine Fe(II) complexes show
varied activities and remarkably, the activities is highly
20 sensitive to the substitutes of the N-aryl moieties in ligands.22
Some polynuclear carboxylate Fe(II) complexes were also
reported to show poor activities in norbornene
polymerization.23 It is worth to note that, in the case of the
nickel-based catalytic system, the ortho substituents of the
25 ligands show strong influences on both of the activities and
the molecular-weights of the resulted polymers. With increase
in the size of the ortho substituents, the activities increased
alongside dramatic decrease in the molecular weight of the
resultant polymers. This might be ascribed probably to the
30 differences in the activated-efficiency in the activation and
the differences in the stabilities of the resulted active species
during the polymerization. Complexes with bulky substituents
at the axial position are more stable and can survive for long
time. Complex 3 was used to study the effect of reaction
35 conditions on vinyl polymerization of norbornene by
changing the Al:Ni molar ratios, and reaction temperature.
Variation of the ratio of MAO show significant effects on
65 the aluminum alkyl chloride. Some nickel and ferrous
complexes were also reported for the butadiene
polymerization affording cis-1,4 enriched polybutadiene.30
The complexes 1–6 were tested for butadiene polymerizations.
The preliminary polymerization data are summarized in Table
70 4. It is worth to note that under the activation of MAO or
alkylaluminum the nickel complexes (1–3) show very low
activities in butadiene polymerization and almost no polymer
were obtained. In contrast, under the same conditions, the
pincer Fe(II) complexes (4–6) show moderate activities for
75 butadiene polymerization. Up to 85.2% conversion was
obtained with 6/AlMe3 at room temperature in 30 minutes
affording the polybutadiene with 75.3% cis-1,4 unit. The
activities and molecular-weight of the polymer show similar
dependence on the ortho bulkyness as described in the
80 polymerization of norbornene with nickel complexes. With
complex 4, 71.1% conversion were achieved affording the
polymer with high molecular weight and broad PDI. While
when 6 was used, high conversion was observed and the
molecular weight of the resultant polymer is relatively low
85 with narrow PDI (2.95). The activities also show strong
dependence on the kind of co-activator used. When the AlEt3
and Al(iBu)3 were used low activities were observed. It is
worth to note that, in all cases, the cis-1,4 selectivity of this
system did not show strong dependence on the ligands and co-
90 activators, and varied in a narrow range between 71.0% to
75.5%.
4
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Table 3. Addition polymerization of norbornene with nickel and ferrous complexes activated by methylaluminoxane(MAO)a
entry
1
2
3
4
5
6
7
8
Cat
Al/M
500
500
500
500
500
500
500
500
500
250
1000
1500
2000
3000
500
500
500
Temp, oC
15
Yield, b
32.6
42.2
57.2
–
trace
70.0
45.8
64.6
65.1
30.8
65.9
69.7
75.0
75.1
63.6
69.2
40.9
%
Mv,c ×105
13.5
7.8
4.3
–
Time，min
1
2
3
4
5
6
3
3
3
3
3
3
3
3
3
3
3
60
60
60
60
60
60
30
90
120
60
60
60
60
60
60
60
60
15
15
15
15
15
15
15
15
15
15
15
15
15
30
45
60
–
–
3.0
4.8
5.7
–
2.9
3.3
3.1
3.2
6.2
7.5
3.1
9
10
11
12
13
14
15
16
17
a Polymerization conditions: toluene 10 mL; catalyst, 10µmol; Norbornene/Cat = 2000; b Isolated yield. c Obtained by capillary viscosimetry using the
Mark–Houwink coefficients a = 0.56, K = 7.78 × 10−4 dl/g.
5
Table 4. Summary of butadiene polymerization catalyzed by pincer Fe(II) complexesa
b
entry
cat
[BD]/[Fe]
Tp
(°C)
time
(min)
Conv
(%)
Mn
(×104)
PDI
microstructurec (%)
cis-1,4
1,2-
27.4
26.8
26.5
24.6
24.5
28.9
27.0
1
2
3
4
5
6
6
6
6
6
500
500
500
1000
2000
500
20
20
20
20
20
20
20
30
30
30
60
60
71.1
78.9
85.2
83.2
75.8
40.2
44.6
86.0
116
5.03
7.15
2.95
2.91
2.89
2.62
5.67
72.6
73.2
73.5
75.4
75.5
71.1
73.0
5.54
16.5
29.1
17.6
64.7
4
5
6d
7e
35
180
500
a The polymerization reactions: toluene (10 mL), cat 20 ꢀmol, Cat:AlMe3 = 25. b Determined by gel permeation chromatography (GPC) with respect to
a polystyrene standard. c Determined by NMR spectrum and IR. dAlEt3 was used. eAl(iBu)3 was used.
suitable for X-ray determination was obtained by
Experimental
recrystallization from mixed hexane and dichlormethane
1
10 General considerations
solution. H NMR (300 MHz, CDCl3):
δ 2.37 (s, CH3, 12H),
7.01–7.05 (mult, 6H), 7.20 (t, JH–H = 7.0 Hz, 1H, p-Ni-C6H3),
35 7.41 (d, JH–H = 7.0 Hz, 2H, m-Ni-C6H3), 7.90 (s, 2H, HC=N)
All manipulations involving air and moisture-sensitive
compounds were carried out under an atmosphere of dried and
purified nitrogen using standard Schlenk and vacuum-line
techniques. Toluene and hexane were dried over sodium metal
15 and distilled under nitrogen. 2,6-dimethylaniline, 2,6-
diethylaniline (90%) and 2,6-diisopropylaniline were bought
from Aldrich Chemical Co. and used without further
purification. Elemental analyses were performed on a Varian
EL microanalyzer. NMR spectra were carried out on Varian
20 300 Hz instrument at room temperature in CDCl3 solution for
ligands and complexes.
ppm. 13C NMR (75 MHz, CDCl3):
δ 14.52 (s, 2C CH3),
119.60, 121.56, 121.89, 122.95, 125.80, 139.23, 168.25(s, 2C,
HC=N) ppm. Anal. Calcd for C24H23BrN2Ni: C, 60.30; H,
4.85; N, 5.86. Found: C, 60.35; H, 4.88; N, 5.74.
40 Bis(N-2,6-diethylphenyl)isophthalaldimin-2-yl
nickel
bromide (2) Using the same work-up procedure described for
the preparation of 1, treatment of bis(N-2,6-diethylphenyl)-2-
bromoisophthalaldimine (0.47 g, 1.00 mmol) with Ni(Ph3P)4
yielded complex 2 as orange powder. (0.50 g, 94.6%). 1H
45 NMR (300 MHz, CDCl3):
δ 1.19 (t, JH–H = 7.5 Hz, 12H,
CH2CH3), 2.54 (q, JH–H = 7.5 Hz, 8H, CH2CH3), 7.03 (d, JH–H
= 7.2 Hz, 4H, m-N-C6H3), 7.13–7.19 (m, 2H, p-N-C6H3), 7.20
(t, JH–H = 7.0 Hz, 1H, p-Ni-C6H3), 7.42 (d, JH–H = 7.2 Hz, 2H,
m-Ni-C6H3), 7.90 (s, 2H, HC=N) ppm. 13C NMR (75 MHz,
Synthesis of the complexes
Bis(N-2,6-dimethylphenyl)isophthalaldimin-2-yl
nickel
bromide (1) To a solution of bis(N-2,6-dimethylphenyl)-2-
25 bromoisophthalaldimine (0.42 g, 1.00 mmol) in 20 mL toluene
was added Ni(Ph3P)4 (1.10 g, 1.00 mmol), and the reaction
mixture was stirred overnight at 100 °C. The solvent was
evaporated to dryness under reduced pressure. The resulted
residue was washed with hot hexane (50 mL) and filtered over
30 Celite to yield orange powder (0.47 g, 98%). The crystal
50 CDCl3):
δ 14.59 (s, 4C, CH2CH3), 25.37 (s, 4C, CH2CH3),
124.62, 125.76, 126.57, 127.18, 136.36, 144.09, 173.06 ppm.
Anal. Calcd for C28H31BrN2Ni: C, 62.96; H, 5.85; N, 5.24.
Found: C, 63.02; H, 5.83; N, 5.20.
55 Bis(N-2,6-diisopropylphenyl)isophthalaldimi-n-2-yl nickel
bromide (3) Using the same work-up procedure described for
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the
preparation
of
1,
treatment
of
bis(N-2,6-
1160w, 1103w, 1045m, 933w, 883w, 844w, 798w, 775m,
755m, 613w, 520m, 512w.
diisopropylphenyl)-2-bromoisophthalaldimine (0.53 g, 1.00
mmol) with Ni(Ph3P)4 yielded complex 3 as orange powder.
General procedure for butadiene polymerization
1
(0.56, 95.5%). H NMR (300 MHz, CDCl3):
δ 1.15 (d, JH–H =
60 In a typical polymerization experiment butadiene solution of
toluene and 500 mmol alkylaluminum were added into a 25
mL flask. Then 25 ꢀmol complex 6 was added to initiate the
5
7.2 Hz, 12H, CH(CH3)2), 1.36 (d, JH–H = 7.2 Hz, 12H;
CH(CH3)2), 3.38 (sept, JH–H = 7.2 Hz, 4H, CH(CH3)2), 7.06
(m, 2H, p-N-C6H3), 7.09 (m, 4H, m-N-C6H3), 7.20 (t, JH–H
=
polymerization. After
a designated time, methanol was
7.2 Hz, 1H, p-Ni-C6H3), 7.45 (d, JH–H = 7.2 Hz, 2H, m-Ni-
C6H3), 7.89 (s, 2H, HC=N) ppm. 13C NMR (75 MHz, CDCl3):
injected into the system to quench the polymerization. The
65 mixture was poured into a large quantity of methanol to
precipitate the white solids. Filtered and dried under vacuum
at 40 °C for 24 h polybutadiene was given at a constant
weight.
10 δ 22.57 (s, 4C, CH(CH3)2), 24.36 (s, 4C, CH(CH3)2), 28.75 (s,
4C, CH(CH3)2), 122.80, 124.28, 126.27, 127.17, 140.86,
143.80, 146.47, 172.15 ppm. Anal. Calcd for C32H39BrN2Ni:
C, 65.11; H, 6.66; N, 4.75. Found: C, 65.07; H, 6.68; N, 4.70.
General procedure for norbornene polymerization
[Bis(N-2,6-dimethylphenyl)isophthalaldimin-2-yl]Fe(µ-
15 Cl)2Li(THF)2 (4) A hexane solution of BuLi (0.66 mL, 1.05
70
A 0.5 ml aliquot of a fresh catalyst solution (toluene, 10
ꢀmol catalyst), 4.0 ml of a solution of norbornene (20 mmol)
in toluene and 10.5 ml toluene were added under inert gas
atmosphere into a Schlenk flask with a mechanical stirrer. The
reaction was started by the addition of 5 ml of a MAO
n
mmol) was added dropwise to a THF (20 ml) solution of
bis(N-2,6-dimethylphenyl)-2-bromoisophthalaldimine (0.42 g,
1.00 mmol) at –78°C. The mixture was stirred for 4 hours
before the addition of FeCl2 (0.14 g, 1.10 mmol). The mixture
20 was allowed to warm to ambient temperature gradually and
stirred overnight. The solvent was removed under reduce
pressure and the residue was treated with toluene, after
evaporation of the toluene to dryness, the product was
obtained as purple powder (0.43 g, 70%). Crystals of 4
25 suitable for X-ray structural determination was grown in
75 solution (5 mmol MAO in toluene). The total reaction volume
was 20 ml, which was achieved by variation of the amount of
toluene if necessary. After 10 min, acidic ethanol was injected
into the Schlenk flask to end the reaction and the reaction
mixture was poured into 200 ml of acidic ethanol. The
80 polymer was isolated by filtration, washed with acetone and
dried under vacuum at 80 ºC for 24 h.
THF/hexane
mixed
solution.
Anal.
Calcd
for
C32H39Cl2FeLiN2O2: C, 62.26; H, 6.37; N, 4.54. Found: C,
62.14; H, 6.31; N, 4.47. IR (KBr) ν (cm–1) 3420s, 1639w,
1473w, 1380w, 1349w, 1253w, 1203w, 1149w, 1091m, 852w,
30 802w,767s, 690w, 609w.
X-ray crystallography
Single crystals of 1, 2, 3, 4, and 5 suitable for X-ray structural
analysis were obtained from CH2Cl2/hexane or THF/hexane
85 mixed solution. Diffraction data were collected at 293K with a
Bruker SMART-CCD diffractometer equipped with graphite-
monochromated Mo-Kα radiation (λ = 0.71073Å). Diffraction
data of 1, and 3 were collected at 293 K on a Rigaku R-AXIS
RAPID IP diffractometer equipped with graphite-
[Bis(N-2,6-diethylphenyl)isophthalaldimin-2-yl]Fe(µ-
Cl)2Li(THF)2 (5) Following the same procedure described for
the preparation of 4, treatment of bis(N-2,6-diethylphenyl)-2-
n
bromoisophthalaldimine (0.47 g, 1.00 mmol) with BuLi (1.6
35 M in hexane, 0.66 mL, 1.05 mmol) and then in situ adding 90 monochromated Mo-Kα radiation (λ = 0.71073 Å). Details of
FeCl2 (0.14 g, 1.10 mmol) yielded complex 5 as purple
powder (0.48 g, 72%). Crystals of 5 suitable for X-ray
structural determination was grown in THF/hexane mixed
solution. Anal. Calcd for C36H47Cl2FeLiN2O2: C, 64.20; H,
the crystal data, data collections, and structure refinements are
summarized in Table 2. The structures were solved by direct
methods and refined by full-matrix least-squares on F2. All
non-hydrogen atoms were refined anisotropically. Hydrogen
40 7.03; N, 4.16. Found: C, 64.11; H, 7.08; N, 4.19. IR (KBr) ν 95 atoms were introduced in calculated positions with the
(cm–1) 3421s, 2966m, 2942m, 2873m, 1635s, 1550m, 1454m,
1380m, 1373m, 1334m, 1230m, 1184m, 1164m, 1103m,
1049m, 871m, 798m, 759w, 690w, 505m.
displacement factors of the host carbon atoms. All
calculations were performed using the SHELXTL31
crystallographic software packages.
45 [Bis(N-2,6-diisopropylphenyl)isophthalaldimin-2-yl]Fe(µ-
Cl)2Li(THF)2 (6) Following the same procedure described for
Conclusions
100 In conclusion, A series of Ni(II) and Fe(II) complexes based
on bis(imino)aryl [NCN]- ligands were synthesized and
characterized. With MAO as cocatalyst, the nickel complexes
(1–3) and iron complex (6) show moderate activities towards
vinyl polymerization of norbornene, while the iron complexe
105 with less bulky substituents show very low activities. Under
the activation of alkylaluminum, the Fe(II) complexes show
moderate activities in butadiene polymerization affording cis-
1,4 enriched polymer.
the
preparation
of
4,
treatment
of
bis(N-2,6-
diisopropylphenyl)-2-bromoisophthalaldimine (0.53 g, 1.00
n
mmol) with BuLi (1.6 M in hexane, 0.66 mL, 1.05 mmol) and
50 then in situ adding FeCl2 (0.14 g, 1.10 mmol) yielded complex
6 as purple powder (0.49 g, 68%). Crystals of 6 suitable for
X-ray structural determination was grown in THF/hexane
mixed solution. Anal. Calcd for C40H55Cl2FeLiN2O2: C, 65.85;
H, 7.60; N, 3.84. Found: C, 65.78; H, 7.57; N, 3.75. IR (KBr)
55 ν (cm–1) 3421s, 3062w, 2962s, 2869m, 1623s, 1550s, 1461m,
1434m, 1384m, 1361m, 1326m, 1253w, 1226w, 1180m,
6
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135
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For Graphic Abstract use only
Ni(II) and Fe(II) Complexes Based on Bis(imino)aryl Pincer
Ligands: Synthesis, Structural Characterization and
Catalytic Activities
5
Jingshun Zhang, Wei Gao, Xiaomei Lang, Qiaolin Wu, Lei
Zhang, and Ying Mu
Bis(imino)aryl [NCN]– Ni(II) and Fe(II) complexes are
synthesized and investigate as catalysts for norbornene and
butadiene polymerization in the presence of MAO or
10 alkylaluminum.
THF
THF
Li
Br
Ar
Ar
Ar
Cl
Cl
Ar
N
Ni
N
Ar
Ar
N
Br
N
nBuLi
N
Fe
N
Ni(0)(Ph3P)4
toluene 100 ¡ãC
FeCl2, THF
8
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]