Structural characterization of nitro-substituted phenoxyiminato nickel complexes; inter-molecular π-π interactions in the solid states and effect of the electron drawing groups on catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2009.11.033

Three phenoxyiminato nickel complexes [(L)Ni(PPh₃)(Ph)] have been prepared by the introduction of the electron drawing nitro substitutes on ortho or para-position of phenoxy and characterized by X-ray crystallography. Experimental and theory calculation suggested that the ortho and para nitro groups may be equally responsible for the small net charge on the central metal atoms because the interplanar angles between o-nitro planes and aromatic rings (28.7-37.3°) are much higher than those between p-nitro planes and aromatic rings (8.2-13.8°). In the solid states of these nickel complexes there exist the short inter-molecular π-π stacking interactions with the distances of 3.5828 A? (ortho-nitro), 3.5844 A? (para-nitro), and 3.0929 A? (ortho- and para-nitro) between two neighboring nitro-phenyl moieties.

Full text of DOI:10.1016/j.jorganchem.2009.11.033

Journal of Organometallic Chemistry 695 (2010) 643–647
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Structural characterization of nitro-substituted phenoxyiminato nickel
complexes; inter-molecular p–p interactions in the solid states and effect
of the electron drawing groups on catalytic activity
*
Dao Zhang, Linhong Weng, Guo-Xin Jin
Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Department of Chemistry, Fudan University, 200433 Shanghai, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three phenoxyiminato nickel complexes [(L)Ni(PPh3)(Ph)] have been prepared by the introduction of the
electron drawing nitro substitutes on ortho or para-position of phenoxy and characterized by X-ray crys-
tallography. Experimental and theory calculation suggested that the ortho and para nitro groups may be
equally responsible for the small net charge on the central metal atoms because the interplanar angles
between o-nitro planes and aromatic rings (28.7–37.3°) are much higher than those between p-nitro
planes and aromatic rings (8.2–13.8°). In the solid states of these nickel complexes there exist the short
Received 4 September 2009
Received in revised form 19 November 2009
Accepted 24 November 2009
Available online 27 November 2009
Dedicated to Prof. Hemult G. Alt on the
occasion of his 65th birthday
inter-molecular
nitro), and 3.0929 Å (ortho- and para-nitro) between two neighboring nitro-phenyl moieties.
Ó 2009 Elsevier B.V. All rights reserved.
p–p stacking interactions with the distances of 3.5828 Å (ortho-nitro), 3.5844 Å (para-
Keywords:
Nickel complex
Nitro-substituted phenoxyiminato ligand
Molecular structure
p–p Interactions
1. Introduction
limited number of reports that describe the incorporation of elec-
tron drawing groups in the ligand ‘‘back-bone” [19–24]. Carlini
et al. reported a Ziegler–Natta-type bis(nitro-substituted pheno-
xyiminato) nickel catalysts for homopolymerization of ethylene
[19,22], styrene [23], norbornene [24] and methyl methacrylate
(MMA) [20,21] with the methylalumoxane (MAO) or bis(1,5-cyclo-
octadiene)nickel(0) as cocatalysts. Herein, we report the introduc-
tion of nitro substitutes on the ortho and para-position of the
phenol of the phenoxyiminato back-bone (Chart 1), as well as the
synthesis and structural characterization of their nickel complexes.
The electron drawing groups adds significantly to the net charge on
nickels and thus has a major effect on catalytic activity of ethylene
polymerization. In the solid states of these nickel complexes there
Olefin polymerization by cationic complexes of d8 metals (late
transition metals) has been studied intensely in the last decade
[1–6]. These catalysts are much more tolerant towards polar re-
agents in general by comparison to their highly oxophilic early
transition metal counterparts. Thus, ethylene and 1-olefins can
be copolymerized with electron-deficient vinyl monomers such
as acrylates [7–10]. Grubbs and co-workers firstly reported a series
of highly active neutral single-component phenoxyiminato based
nickel catalysts for ethylene polymerization and copolymerization
without any cocatalysts [11,12]. This discovery caused consider-
able interest, as it opened the chance to generate high-value prod-
ucts from cheap monomer supplies. The mechanism of ethylene
polymerization by nickel phenoxyiminato catalysts has also been
studied [13]. Until now, most studies still focused on the steric ef-
fect of this kind of neutral Ni(II) complexes, usually on the ortho
position of phenol and imine. The degrees of polymer branching
and thus crystallinity, high polymerization rates, and tolerance to-
ward polar environments can be improved by this way [1–24].
Although the influence of electronic effect on catalytic activity
of phenoxyiminato nickel(II) complexes [18], there have been a
exist the short inter-molecular
two neighboring molecules.
p–p stacking interactions between
2. Results and discussion
The nitro-substituted o-hydroxy-benzaldehyde was used as
starting materials for preparation of the corresponding nitro-
substituted phenol-imine via a straightforward Shiff-base conden-
sation sequence. Ligands HL1–HL3 are light-yellow, yellow and or-
ange–yellow solid which were treated with excess sodium hydride
in THF to afford the corresponding sodium salts which react with
nickel source trans-[NiPhCl(PPh3)2] to give complex 1–3 in 58–
* Corresponding author. Tel.: +86 21 65643776; fax: +86 21 65641740.
E-mail address: gxjin@fudan.edu.cn (G.-X. Jin).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.11.033

644
D. Zhang et al. / Journal of Organometallic Chemistry 695 (2010) 643–647
Table 1
Ph
i-Pr
Ph3P
O
Selected bond lengths (Å) and angles (°).
Ni
1
2
3
Ni–O
Ni–N
Ni–P
Ni–C (Ph)
C(Ph)–Ni–O
C(Ph)–Ni–N
O–Ni–N
C(Ph)–Ni–P
O–Ni–P
N–Ni–P
1.9162(16)
1.9328(18)
2.1819(8)
1.893(3)
170.94(10)
94.31(9)
90.29(7)
90.87(8)
85.07(5)
173.70(6)
1.901(3)
1.901(3)
N
1.942(3)
2.1838(14)
1.896(4)
1.951(3)
2.1748(12)
1.893(4)
i-Pr
R
HL1, 1: R =H,
HL2, 2: R = NO2, X = H
HL3, 3: R = NO2, X = NO2
X = NO2
164.09(16)
95.57(16)
92.12(13)
86.43(13)
88.19(10)
170.81(10)
170.80(17)
93.26(15)
92.38(13)
85.31(12)
89.59(9)
X
Chart 1.
175.51(11)
75% yield. Ligand HL3 bearing two nitro groups is appraised to ex-
hibit enhanced reactivity in certain instances. Complexes 1–3 were
characterized by 1H NMR and elemental analysis.
are almost similar to those [1.9328(18) Å and 1.9162(16) Å, respec-
tively] of 1 with a nitro group in 5-position within the range of
experimental error. Secondly, complex 2 revealed a more twisted
planar structure around nickel center than 1. The trans- bond angle
The crystals of 1–3 suitable for X-ray analysis were obtained as
light-yellow platelet/orange red/dark-red block crystals, respec-
tively, and sealed in the suitable glass capillary. The crystal
structures of these nickel complexes are depicted in Fig. 1. The cor-
responding bond distances and angles are summarized in Table 1.
For complexes 1–3, the unit cell in the solid state contains two
crystallographically independent, but chemically similar molecules
that are related by a non-crystallographic pseudo-inversion center.
And each molecule contains a phenoxyiminato chelating ligand, a
triphenylphosphine and a phenyl group. The Ni atoms are arranged
in a distorted square-planar geometry made up of four atoms of
the ligand (P, O, N, C). The PPh3 groups and the imine nitrogen
atoms are trans to each other.
of C–Ni–O and N–Ni–P for complex
2 [1] are 164.09(16)
[170.80(17)], 170.81(10) [175.51(11)]°, respectively. Thirdly, the
Ni–N bond distance and O–Ni–P bind angle in complex 3 having
two nitro groups in 3- and 5-positions further decrease to
1.9328(18) [1.951(3) for 1 and 1.942(3) Å for 2] and 85.07(5)
[89.59(9) for 1 and 88.19(10)° for 2], which indicated the elec-
tron-withdrawing effect.
The packing diagrams for the isostructural compounds 1–3 are
shown in Fig. 2, and the two molecules have strong inter-molecular
p–p interactions. The p–p interaction type, distances and dihedral
angles of the corresponding two parts were listed in Table 2 in
which the intramolecular dihedral angles of nitro group with phe-
nyl ring or another nitro group were also included. For complex 3,
the nitro group of Ni(1) molecule was almost parallel to phenyl
The most notable is the introduction of electron-withdrawing
nitro groups that have obvious influence to the molecular struc-
tures (see Table 1). Firstly, the Ni–N [1.942(3) Å] and Ni–O
[1.901(3) Å] bond distances of 2 bearing a nitro group in 3-position
Fig. 1. Molecular structures of complexes 1–3. All hydrogen atoms were omitted for clarity.
Fig. 2. The inter-molecular
hydrogen atoms are omitted for clarity.
p–p
stacking interactions between adjacent two phenoxyiminato ligands. The 2,6-iPr2(C6H3) moieties, coordinated phenyl, PPh3 groups, and all

D. Zhang et al. / Journal of Organometallic Chemistry 695 (2010) 643–647
645
Table 2
Analysis of short inter-molecular
p
–
p
-interactions.a.
Compound
p
–p-Interaction type
d
p
(Å)
d (°)
p–p
a
(°)
b (°)
c
(°)
–p
1
2
3
The phenyl ringÁ Á Áphenyl ring
The phenyl ringÁ Á Áphenyl ring
The nitro groupÁ Á Áphenyl ring
3.5928
3.5844
3.0939
13.8
0
8.2
–
47.3
28.7
13.8
–
8.2
–
–
24.3
a
Stacking parameters taken into account are d
(inter-molecular
p–p
-interaction distance), d (angle between two inter-molecular
p–p
p–
p-interaction moieties), a (angle
p
–p
between o-nitro planes and aromatic ring), b (angle between p-nitro plane and aromatic ring),
c (angle between o-nitro planes and p-nitro planes).
respectively (Table 4). Although nickel center in 1 has a more po-
sitive charge, the ortho-nitro groups of 2 are mostly close to the
metal centers and act as more bulky groups to protect the active
metal centers, which is responsible for the similar ethylene poly-
merization activity of 2 [14.7 kg/(mol Ni h atm)] and 1 [11.6 kg/
(mol Ni h atm)]. Crystal structures of 1–3 also supported the result.
The interplanar angles between o-nitro planes and aromatic rings
(28.7–37.3°) are much larger than those between p-nitro planes
and aromatic rings (8.2–13.8°).Compared with the complexes 1
Table 3
Result of polymerization of ethylene.a.
b
c
Entry
Nickel
mol)
T (°C)
Time
(min)
Yield
(g)
Activity
Mv
(
l
1
2
4
5
6
7
8
1 (50)
2 (44)
3 (68)
3 (44)
3 (48)
3 (52)
3 (45)d
60
60
25
2
2
2
30
30
120
60
60
30
30
2.89
3.24
1.62
3.10
0.59
4.21
0.90
11.6
14.7
1.19
7.05
1.23
16.2
4.00
7000
54 000
108 000
39 000
16 000
81 000
42 000
55 À 80
95
60
60
2
2
2
and
2 with mono nitro substitute, the bis(nitro)-substituted
complex 3 has relatively more positive nickel center and the net
charge on Ni is – 0.0360. Thus the ortho and para nitro groups
may be responsible for the small net charge on the central metal
atoms besides bulky effect of ortho position.
a
Polymerization conditions: 100 mL of autoclave, 50 mL of toluene, 10.0 atm of
ethylene pressure.
b
kg/(mol Ni h atm).
Determined with Ubbelohde viscometer in dekalin at 135 0.1 °C.
Using 10 mL of toluene and 50 mL of hexane as polymerization solvent.
c
d
3. Experimental
ring of Ni (1A) (the dihedral angle is only 8.2°), and the average dis-
3.1. General considerations
tance is 3.0939 Å, which indicated that they have strong
p–p inter-
action. However, complex 1 revealed different stacking mode
and the corresponding angle and average distance are 13.8° and
3.5928 Å, respectively (see Table 2), which was similar to complex
p–p
All experiments with metal complexes were carried out under
argon using standard Schlenk and vacuum-line techniques. Sol-
vents were dried by refluxing with appropriate drying agents and
distilled under argon prior to use. The commercially available re-
agents, 2,6-diisopropyl aniline (Aldrich Co.), NaH (60%, Aldrich
Co.) were purchased and used without purification. The starting
materials nitro-substituted 2-hydroxy-benzaldehyde, trans-
[Ni(PPh3)2(Ph)Cl], phenoxyiminato ligand (HL1–HL3), and
[(L)Ni(PPh3)(Ph)] (1–3) were prepared according to the previously
described methods [11,12,14–17]. Methylalumoxane (MAO) was
purchased from Azko Nobel as a 7 wt% in toluene solution. 1H
NMR spectra were recorded on a Bruker DMX 500 spectrometer
in C6D6 or CDCl3. Elemental analysis was performed on an Elemen-
tar Vario EL III analyzer.
2 bearing weak
p–p interaction in solid state. Moreover, for the
three complexes two phenyl rings bearing nitro group of Ni(1)
and Ni(1A) molecule are absolutely parallel to each other.
Nickel complexes 1–3 were tested for ethylene polymerization
and the results were summarized in Table 3. Every polymerization
without any activators such as Al-compounds or Ni(COD)2. As ex-
pected, complexes 1–3 characterized by the presence of electron-
withdrawing nitro groups on the phenoxyiminato ligand and of
bulky isopropyl constituents on the N-aryl moiety, show much
higher activities (Table 3). The position of nitro groups seems much
sensitive to their catalytic behaviors. There is a clear increase in
activity from 1 to 2 and 3 (Table 3, entries 1, 2 and 7). The molec-
ular weights Mv and the number of methyl branches determined
by high-temperature 13C NMR spectra of the solid polymer are sim-
ilar to those of the PEs produced using Grubbs catalysts (5–8
branches per 1000 carbon atoms) under similar conditions.
It has been reported that catalytic activity relied more on the
electronic configuration of the catalyst in the ground state, espe-
cially the net charge on the central metal atom. The electron-with-
drawing nature of nitro, which results in a more electrophilic metal
center in the complexes, may increase the corresponding activity
in the ethylene polymerization. The QEq method calculation indi-
cates the net charge on nickels in the complexes with nitro substi-
tutes in para or ortho positions are 0.1431 (1) and 0.2646 (2),
3.2. [(L1)Ni(PPh3)(Ph)] (1)
Ligand 5-nitro-N(2,6-diisopropylphenyl) phenol-imine (HL1)
was obtained as light-yellow solid in 79% yield. Anal. Calc. for
C19H22N2O3: C, 69.92; H, 6.79; N, 8.58. Found: C, 70.01; H, 6.75;
N, 8.62%. 1H NMR (300 MHz, CDCl3): d 1.20 (d, 12H, CH(CH3)2),
2.93 (septet, 2H, CH(CH3)2), 7.04–7.26 (m, 4H, H–Ar), 8.34–8.38
(m, 3H, CH@N + Hp–ArOH + Hm–ArOH), 14.30 (bs, 1H, OH). Complex
1 was obtained as a yellow solid in 58% yield using a modified lit-
erature method [11]. Anal. Calc. for C43H41N2NiO3P: C, 71.39; H,
5.71; N, 3.87; Found: C, 71.37; H, 5.77; N, 3.89%. 1H NMR
(500 MHz, C6D6): d 0.96 (d, 6H, J = 6.6 Hz), 1.22 (d, 6H, J = 6.6 Hz),
3.89 (sept., 2H, J = 6.6 Hz), 5.91–7.90 (m, 30H), 8.06 (s, 1H).
Table 4
The net charge QEq [e] of Ni, P, N, O atoms.
3.3. [(L2)Ni(PPh3)(Ph)] (2)
1
2
3
Ligand 3-nitro-N-(2,6-diisopropylphenyl) phenol-imine (HL2)
was obtained as orange–yellow solid in 86% yield. 1H NMR
(300 MHz, CDCl3): d 1.20 (d, 12H, CH(CH3)2), 2.96 (septet, 2H,
CH(CH3)2), 7.15–7.26 (m, 4H, Ho–ArOH + Ho–AriPr + Hm–AriPr),
Ni
P
N
O
C
–0.143116
0.665537
–0.48247
–0.589547
–0.061028
–0.264609
0.666157
–0.508289
–0.605737
–0.151963
–0.036006
0.62251
–0.474192
–0.545395
–0.031415
7.21–7.26 (m, 3H, Hp–ArOH + Hm–ArOH), 7.68 (d, 1H, Hp–ArNO ),
2

646
D. Zhang et al. / Journal of Organometallic Chemistry 695 (2010) 643–647
Table 5
Crystallographic data.
1
2
3
Empirical formula
Formula weight
Temperature (K)
Crystal size (mm)
Crystal system
Space group
a (Å)
C43H41N2NiO3P
723.46
293(2)
C43H41N2NiO3P
723.46
293(2)
C43H40N3NiO5P
768.46
293(2)
0.10 Â 0.10 Â 0.08
0.15 Â 0.10 Â 0.08
0.20 Â 0.10 Â 0.05
Triclinic
Triclinic
Triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
9.592(2)
9.452(4)
11.715(3)
13.017(4)
14.951(4)
84.227(4)
67.431(4)
67.147(4)
1936.8(9)
2
b (Å)
c (Å)
12.128(3)
16.091(4)
90.915(4)
97.916(3)
98.491(4)
1832.3(8)
2
1.311
0.615
760
11.548(5)
18.943(9)
97.041(6)
91.700(6)
113.392(6)
1876.4(15)
2
1.280
0.601
760
a
(°)
b (°)
c
(°)
V (Å3)
Z
D
(Mg/m3)
1.318
0.591
804
s
Absorbent coefficient (mmÀ1
F (0 0 0)
)
h range (°)
Number of reflections collected/unique
1.28–25.01
7739/6366
(Rint = 0.0450)
6189/0/418
0.937
0.0604, 0.1386
0.1052, 0.1573
0.978/À0.353
1.09–25.01
7872/6490
(Rint = 0.0255)
6490/0/451
1.053
0.0536, 0.1392
0.0793, 0.1524
0.463/À0.497
1.48–25.01
8179/6740
(Rint = 0.0207)
6740/0/482
0.827
0.0377, 0.0645
0.0730, 0.0712
0.350/À0.199
Number of data/restraints/parameter
Goodness of fit (GOF) on F2
a
Final R indices (I > 2
r
(I))
a
R indices (all data)
Largest difference peak and hole (e ÅÀ3
)
a
R1
=
R
||Fo| – |Fc||/
R
|Fo|. wR2 = [
R
(|Fo|2 À |Fc|2)2/
R .
(Fo2)]1/2
8.16 (d, 1H, Ho–ArNO ), 8.39 (s, 1H, CH@N), 14.96 (bs, 1H, OH). Anal.
3.6. Crystal structure determination
2
Calc. for C19H22N2O3: C, 69.92; H, 6.79; N, 8.58. Found: C, 70.01; H,
6.75; N, 8.62%. Complex 2 was isolated by filtration and washing
with hexane as a dark-red solid in 72% yield. Anal. Calc. for
C43H41N2NiO3P: C, 71.39; H, 5.71; N, 3.87. Found: C, 71.38; H,
The single crystals suitable for X-ray analysis was sealed into a
glass capillary and the intensity data of the single crystal were col-
lected on the CCD Bruker Smart APEX system. Data obtained with
5.74; N, 3.88%. 1H NMR (500 MHz, C6D6):
d
1.08 (d, 6H,
the
x
-2h scan mode were collected on a Bruker SMART 1000 CCD
radiation
J = 6.6 Hz), 1.30 (d, 6H, J = 6.6 Hz), 4.05 (sept., 2H, J = 6.6 Hz),
diffractometer with graphite-monochromated Mo K
a
6.06–7.85 (m, 30H), 8.09 (s, 1H).
(k = 0.71073 Å) at 293 K. The structures were solved using direct
methods, while further refinement with full-matrix least squares
on F2 was obtained with the SHELXTL program package [25,26]. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were introduced in calculated positions with the displace-
ment factors of the host carbon atoms. Crystallographic data are
summarized in Table 5.
3.4. [(L3)Ni(PPh3)(Ph)] (3)
Ligand 3,5-dinitro-N-(2,6-diisopropylphenyl) phenol-dimine
(HL3) was obtained as apricot-yellow solid in 91% yield. Anal. Calc.
for C19H21N3O5: C, 61.45; H, 5.70; N, 11.31. Found: C, 61.41; H,
5.78; N, 11.37%. 1H NMR (300 MHz, CDCl3): d 1.24 (d, 12H,
CH(CH3)2), 2.96 (septet, 2H, CH(CH3)2), 7.29 (d, 2H, Ho–AriPr),7.39
(t, 1H, Hm–AriPr), 8.31 (s, 1H, CH@N), 8.52 (d, 1H, Ho–ArCH@N),
9.06 (d, 1H, Hp–ArCH@N),16.29 (bs, 1H, OH). Complex 3 was isolated
as a dark-red solid in 75% yield. Anal. Calc. for C43H40N3NiO5P: C,
67.21; H, 5.25; N, 5.47. Found: C, 67.17; H, 5.24; N, 5.48%. 1H
NMR (500 MHz, C6D6): d 0.98 (d, 6H, J = 6.6 Hz, CH(CH3)2), 1.16
(d, 6H, J = 6.6 Hz. CH(CH3)2), 3.84 (sept, 2H, J = 6.6 Hz, CH(CH3)2),
6.24 (m, 2H, J = 7.2 Hz, Hm–ArN@), 6.33 (m, 1H, J = 6.9 Hz, Hp–
ArN@), 6.83 (m, 4H, J = 7.6 Hz, H–Ar), 6.93(m, 1H, J = 7.6 Hz, H–
Ar),7.02 (m, 9H, J = 7.4 Hz, H–Ar), 7.40 (d, 1H, JHH = 8.2 Hz, H–Ar),
7.54 (t, 6H, J = 8.6 Hz, H–Ar),7.73 (s, 1H, H–Ar), 8.23 (s, 1H, CH@N).
Supplementary material
CCDC 730606, 730607 and 730608 contain the supplementary
crystallographic data for 1, 2 and 3. These data can be obtained free
of charge from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
This work was supported by the National Science Foundation of
China (20721063, 20771028), Shanghai Science and Technology
Committee (08DZ2270500, 08DJ1400103), Shanghai Leading Aca-
demic Discipline Project (B108) and the National Basic Research
Program of China
3.5. Ethylene polymerization
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ethylene pressure was raised to 10 atm and maintained for a cer-
tain time. The polymerization was terminated by the addition of
methanol and dilutes HCl (10%). The solid polyethylene was fil-
tered, washed with methanol and dried at 40 °C in vacuum.
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