Sterically modulated binuclear bis-α-diimine Pd(II) complexes: Synthesis, characterization, DFT studies and catalytic behavior towards ethylene oligomerization

Article abstract of DOI:10.1016/j.molstruc.2014.07.034

A series of Pd(II) complexes of bis-α-diimine derivatives with substituent at 2,6-positions of aniline increasing sterically from Me < Et < iPr groups have been synthesized to act as a versatile set of catalytic precursors for oligomerization of ethylene

Full text of DOI:10.1016/j.molstruc.2014.07.034

Journal of Molecular Structure 1075 (2014) 559–565
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Sterically modulated binuclear bis-a-diimine Pd(II) complexes:
Synthesis, characterization, DFT studies and catalytic behavior towards
ethylene oligomerization
Sandeep P. Netalkar a, Srinivasa Budagumpi b, Hassan H. Abdallah c, Priya P. Netalkar a,
Vidyanand K. Revankar a,
⇑
a Department of Chemistry, Karnatak University, Pavate Nagar, Dharwad 580 003, Karnataka, India
b Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Bangalore 562 112, Karnataka, India
c Department of Chemistry, Education College, Salahaddin University, Erbil, Iraq
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
All the Pd(II) complexes efficiently oligomerize ethylene to produce C4AC20 fractions at activities of up to
1218.4 kg-oligomer mol-PdÀ1 barÀ1 hÀ1 at 30 °C in combination with EASC.
ꢀ
Three new Pd(II) complexes were
synthesized and characterized by
spectroscopic methods and their
structures optimized by DFT
calculations.
ꢀ
ꢀ
Pd(II) complexes in combination with
EASC showed better performance
towards oligomerization of ethylene
than MAO.
All the Pd(II) complexes oligomerize
ethylene to C4AC20 fractions at
activities of up to 1218.4 kg-oligomer
mol-PdÀ1 barÀ1 hÀ1 at 30 °C with
EASC.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2014
Received in revised form 12 July 2014
Accepted 14 July 2014
Available online 18 July 2014
A series of Pd(II) complexes of bis-a-diimine derivatives with substituent at 2,6-positions of aniline
increasing sterically from Me < Et < iPr groups have been synthesized to act as a versatile set of catalytic
precursors for oligomerization of ethylene in combination with aluminum activators. Treatment of the
bimetallic Pd(II) complexes with ethylaluminum sesquichloride (EASC) or Methylalluminooxane (MAO)
in toluene generates active cationic catalyst in situ that oligomerize ethylene to a-olefins. All the Pd(II)
complexes efficiently oligomerize ethylene to produce C4AC20 fractions at activities of up to
1218.4 kg-oligomer mol-PdÀ1 barÀ1 hÀ1 at 30 °C in combination with EASC.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Binucleating bis-a-diimine ligands
Ethylene oligomerization
Square planar Pd(II) complex
DFT studies
X-ray diffraction
Introduction
⇑
Corresponding author. Tel.: +91 9900485652.
E-mail address: vkrevankar@rediffmail.com (V.K. Revankar).
The development and application of late transition metal com-
plexes for olefin poly/oligomerization has seen impressive progress
http://dx.doi.org/10.1016/j.molstruc.2014.07.034
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

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S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
over the last decade [1–3]. Most thoroughly studied candidates
among others are the cationic Ni(II) and Pd(II) aryl-substituted
Experimental
a
-diimine complexes bearing bulky ortho-substituents [4,5]. The
General considerations
presence or absence of steric bulk at the axial positions of d8
square-planar complexes to inhibit chain transfer has provided a
general trunk in this area to design a library of compounds suitable
for olefin oligo/polymerization reactions. From the successive work
All the reagents used in this study were purchased from Sigma-
Aldrich and used without further purification. The 1H-NMR and
13C-NMR spectra were recorded on a Bruker AV400 II spectrometer
at 400 MHz and 100 MHz respectively in DMSO-d6 or CDCl3 at
room temperature using TMS as internal reference. IR spectra were
recorded in a KBr disc matrix using an Impact-410 Nicolet (USA)
FTIR spectrometer over the range of 4000–400 cmÀ1. The electronic
spectra were measured on a Jasco V670 spectrophotometer over
the range of 800–200 nm. All of the olefin estimations were mon-
itored by gas chromatography equipped with FID detector using an
HP-5 column.
carried out on Pd(II) and Ni(II) complexes of
a-diimine ligands, it
appears that bulky substituents near the active metal site hinder
the chain transfer process or associative displacement, which
allows the polymerization to proceed smoothly and afford a
high-molecular-weight material [2,6–8]. On the other hand, a less
bulky environment or a combination of hetero-donor bidentate
coordination generally activates the oligomerization of olefins
[9–14]. In general, for the metal complexes oriented for ethylene
oligomerization, ligand design which facilitates b-hydrogen trans-
The ligands L1AL3 were synthesized according to our previ-
ously reported procedure [26]. Schematic representation for the
syntheses of complexes and numbering pattern for NMR assign-
ment is given in Scheme 1.
fer is preferred. The demand for the production of linear a-olefins
(oligomeric fractions containing C4AC20+ range) is ever increasing
mainly because of their application as precursor and many com-
mercial products like synthesize linear low-density polyethylene
and in the production of synthetic lubricants, plasticizers, surfac-
General method for the preparation of Complexes
tants and detergents. These even numbered carbon
a-olefins are
produced industrially but mostly via non-selective oligomerization
of ethylene, except for Shell Higher Olefin Process (SHOP) [15],
which employs nickel complexes as catalysts, relies on b-hydride
elimination for production of lower oligomeric fractions of ethyl-
ene [16]. Therefore there is much scope and demand for the devel-
opment of new catalysts for selective oligomerization of ethylene
free from polymers both from the academic and industrial pur-
suits. With the knowledge and understanding of the process and
mechanism, comes a quest for designing and development of
new generation of oligomerization catalyst with high turnover
number, better efficiency and selectivity. In recent years, quite a
few number of transition metal complexes with excellent activity,
selectivity, living behavior and stability have been reported [17]. In
particular, nickel, palladium, iron and chromium based systems
have been reported for ethylene oligomerization but the most
selective and active are the ones based on palladium and nickel.
From the view point of increasing the efficiency, one of the eas-
iest way would be to make use of more than one active metal i.e.,
insertion of second metal center in addition to one which is known
to actively influence the catalytic process, to the suitably designed
ligand framework, with the expectation to enhance the activity due
to the combined influence of synergistic effects of the active metal
centers and of possible occurrence of tandem reactions promoted
by two or more active metal centers [18–21]. This expectation
has brought the bi- or oligonuclear systems into scene mainly
because of the general interest in cooperative effects that may
occur between the metal centers. An increasing number of binu-
clear precatalysts based on binucleating ligand scaffolds have
therefore been developed for olefin polymerization/oligomeriza-
tion [18,19,22–25] and their number continue to grow.
A
50 mL round bottom flask was charged with 0.72 g
(2.5 mmol) PdCl2(COD), to which a solution of 0.44 g (1.18 mmol)
of L1, 0. 508 g (1.18 mmol) of L2, 0.58 g (1.18 mmol) of L3, in
30 mL dichloromethane was added under an inert nitrogen atmo-
sphere. The resulting reddish orange mixture was stirred at room
temperature overnight. The mixture was filtered and removal of
solvent under reduced pressure resulted orange solid. The solid
was washed with ethanol several times and dried in vacuum.
2,6-Dimethyl-phenyl-{1-methyl-2-[(1-methyl-2-methylimino-
propylidene)-hydrazono]-propylidene}-amine dipalladium(II) chloride
(C1)
Yield: 58.2%. Anal. Calc. for C24H30Cl4N4Pd2 (%): C, 39.5; H, 4.1;
N, 7.7; Found (%): C, 39.4; H, 4.1; N, 7.9. IR (cmÀ1):
m
(CAH): 2860
and 2933;
m
(C@N):1590. 1H-NMR (DMSO-d6, ppm): 2.00 (s, 12H,
Benz CH3), 1.98 (s, 6H, methyl, C10H3 and C11H3), 2.25 (s, 6H,
methyl, C7H3 and C14H3), 7.08 (d, J = 7.5 Hz, 4H, C3H, C5H, C17H
and C19H), 6.99 (t, J = 7.5 Hz, 2H, C4H and C18H). kmax (nm):
290, 312 and 420. ESI mass, (m/z): 729.04 [M], 693.05 [MACl].
2,6-Diethyl-phenyl-{1-methyl-2-[(1-methyl-2-methylimino-
propylidene)-hydrazono]-propylidene}-amine dipalladium (II)
chloride (C2)
Yield: 56.6%, Anal. Calc. for C28H38Cl4N4Pd2 (%): C, 42.8; H, 4.9;
N, 7.1; Found (%): C, 42.5; H, 4.8; N, 7.4. IR (cmÀ1):
m
(CAH): 2866
and 2920;
m
(C@N):1588. 1H-NMR (DMSO-d6, ppm): 1.06 (t,
J = 7.5 Hz, 12H, CH2ACH3), 1.92 (s, 6H, methyl, C10H3 and
C11H3), 2.24 (s, methyl, 6H, C7H3 and C14H3), 2.28 (m, 8H, CH2-
ACH3), 7.08 (d, J = 7.5 Hz, 4H, C3H, C5H, C17H and C19H), 6.98 (t,
J = 7.5 Hz, 2H, C4H and C18H). kmax (nm): 290, 310 and 410. ESI
mass (m/z): 785.01 [M], 749.01 [MACl].
Aiming at higher efficiency and alternative chain propagation
mechanisms, we recently reported a binucleting version of these
intensively studied
bridged by azine spacers featuring appended imine functionalities
with bulky aryl substituent’s at the wingtips making them sym-
a-diimine ligands [26]. These ligands are
2,6-Diisopropyll-phenyl-{1-methyl-2-[(1-methyl-2-methylimino-
propylidene)-hydrazono]-propylidene}-amine dipalladium(II) chloride
(C3)
metrical. These bis-
a-diimine ligands retains all the essential fea-
Yield: 70.5% Anal. Calc. for C32H46Cl4N4Pd2 (%): C, 45.7; H, 5.5;
N, 6.7; Found (%): C, 45.5; H, 5.4; N, 6.9; IR (cmÀ1):
m(CAH):
tures of the -diimine ligands and in addition provides an
a
2868 and 2928;
m
(C@N):1591. 1H NMR (DMSO-d6, ppm): 1.13 (d,
additional adjacent compartment to accommodate another metal,
making them exactly the bimetallic mimic of the originally devel-
oped and most thoroughly studied
plexes. In this paper, we report new binuclear Pd(II) complexes
based on our earlier reported bis-
oligomerization.
J = 7 Hz, CH(CH3)2), 1.93 (s, 6H, methyl, C10H3 and C11H3), 2.12
(s, 6H, methyl, C7H3 and C14H3), 2.49 (m, 4H, CH(CH3)2), 7.11 (d,
J = 7.5 Hz, 4H, C3H, C5H, C17H and C19H), 7.05 (t, J = 7.5 Hz, 2H,
C4H and C18H). kmax (nm): 285, 304 and 415. ESI mass (m/z):
841.05 [M], 805.05 [MACl].
a-diimine based metal com-
a-diimine ligands for ethylene

S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
561
14
11
R
13
12
10
7
2
3
R
R
R
R
8
9
17
19
16
CH2Cl2
1
15
4
18
+PdCl2(COD)
N
N
N
N
N
N
N
N
2
1Cl
Cl
Pd
4
3
Stirr, 24h
20
R
R
Pd
5
6 R
Cl
Cl
C1 : R = Me
C2 : R = Et
C3 : R = iPr
R = Me
Et
iPr
Scheme 1. Synthetic routes for the preparation of sterically tuned dipalladium(II) complexes derived from bis-a-diimine ligands.
Results and discussion
The easily varied steric properties of
a
-diimine ligands are an
important feature of the palladium
presented in this paper.
a-diimine catalyst systems
Infrared spectral studies
All the complexes (C1AC3) show a sharp intense band centered
around 1590 cmÀ1 which can be assigned to azomethine function-
ality which appear in all the ligands around 1630 cmÀ1. The shift in
azomethine stretching band in complexes is an evidence for
involvement of azomethine nitrogen in coordination to the
palladium complexes.
1H NMR spectral studies
Fig. 1. UV–Vis spectra of the complexes C1AC3.
The protons of the two methyl groups attached to the azine
fragment in the ligands resonated as singlets in the range 1.99–
2.05 and 2.20–2.25 ppm respectively. Comparably, these protons
have shown shifts in their respective complexes suggesting the
coordination through azomethine nitrogen. The aromatic signals
in the ligands were observed in the expected region, ranging from
7.06 to 7.18 and 6.93 to 7.11 ppm respectively and have shown
slight shifts in their respective complexes. The methyl/ethyl/
isopropyl signals of all the complexes have been observed in the
expected region and are detailed in experimental section.
solved by direct methods, and refined by full-matrix least-square
on F2. Each hydrogen atom was placed in a calculated position,
and refined using a riding model. All non-hydrogen atoms were
refined anisotropically. Structure solution and refinement were
performed using SHELXL-97 package [28].
A single crystal X-ray structure of bis-a-diimine ligand, L2
[C28H38N4] crystallized in the orthorhombic system with space
group, Pccn. Summaries of the crystallographic data, bond lengths
and bond angles of L2 are tabulated in the Tables 1–3 respectively.
A perspective view of the ligand showing 50% probability ellipsoids
Mass and electronic spectral studies
with the atom labelling scheme is shown in Fig. 3. The bis-a-dii-
mine azine crystallizes with a half-molecule in the asymmetric
unit. The asymmetric unit of the compound sits on a crystallo-
graphically required center of symmetry. The C@NAN@C linkage
adopt a (1E, 2E) configuration, as required by the crystallographi-
cally imposed symmetry. The bond distances of N1AC11 = 1.264
(3) Å agrees well with the values for double bond character, con-
firming the formation of bis-imine bonds [29,30]. The phenyl rings
on either sides of azine fragment are almost perpendicular to each
other due to which the ligand is non-planar in nature. The torsion
angle À175.7(2)° exhibited by N1AC11AC13AN2 and 7.1(4)° by
N1AC11AC13AN2 indicates that N1 and N2 are trans to each other
and N1 and C14 are cis to each other. In the extended crystal struc-
The free ligands exhibited intense absorption bands at wave-
length shorter than 300 nm and less intense bands around
345 nm assigned to intramolecular p–p
*and n–p* transitions orig-
inating from phenyl rings and non-bonding electrons present on
the nitrogen of the azomethine group. The complexes C1AC3 exhi-
bit weaker absorptions with maxima around 425 nm assigned to
1A1g ? 1B1g transition consistent with square-planar palladium
(II) complexes [27]. These observations along with diamagnetic
properties collectively indicate a square-planar geometry around
the Pd centers. The electronic spectra of complexes are given in
Fig. 1.
The ESI mass spectra of C1, C2 and C3 show molecular ion peaks
at m/z: 729, 785 and 841 respectively. Further molecular fragmen-
tation is consistent with various fragments of the complexes. Frag-
mentation peaks due to elimination of Cl is observed at 693, 749
and 805 in the ESI-MS spectrum of all the complexes. These values
are in good agreement with the proposed composition for the
complexes demonstrating the binuclearity of all the Pd complexes.
ESI-Mass spectrum of Complex C3 is given in Fig. 2.
ture, the molecule is stabilized by intermolecular CHÁ Á Á
p stacking
interactions between C5AH5Á Á ÁC3 and C3AH5Á Á ÁC5 with a contact
distance of 2.793(3) Å forming a 2D zig-zag chain structure.
Computational studies using DFT method
Due to the difficulty in crystallizing the synthesized complexes,
the theoretical calculations were used to predict the best and sta-
ble configuration, and to produce many theoretical parameters
such as the dipole moment, electronic energy and molecular orbi-
tals, HOMO and LUMO. Density Functional Theory, (DFT) was used
to build the structure and calculate those parameters in the gas
X-ray diffraction studies
The X-ray diffraction data were collected using a Bruker SMART
APEX2 CCD area-detector diffractometer. The structures were

562
S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
Fig. 2. ESI-Mass spectrum of Complex C3.
Table 1
Table 2
Crystal data and structure refinement details for bis-
a
-diimine ligands L2.
Selected bond distances (Å) of L2.
Crystal data
L2
Atoms
Bond distances (Å)
Empirical formula
Formula weight
Temperature
Wavelength (Å)
Crystal system, space group
a (Å)
C
28H38N4
N(1)AC(11)
N(1)AC(1)
N(2)AC(13)
N(2)AN(2)
C(1)AC(2)
C(1)AC(6)
C(13)AC(14)
C(13)AC(11)
C(2)AC(3)
C(2)AC(9)
C(11)AC(12)
C(3)AC(4)
C(6)AC(5)
C(6)AC(7)
C(4)AC(5)
C(9)AC(10)
1.263(3)
1.419(3)
1.278(4)
1.376(4)
1.389(4)
1.400(4)
1.482(4)
1.490(4)
1.393(4)
1.479(4)
1.488(4)
1.363(5)
1.392(4)
1.504(5)
1.341(5)
1.381(7)
430.62
296(2) K
0.71073
Orthorhombic, Pccn
12.0282(6)
26.8308(17)
8.3154(5)
90
90
90
2683.6(3)
4
1.066
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalc (g cmÀ3
)
Absorption coefficient
F(000)
0.063
936
Crystal size (mm)
h range for data collection (°)
Limiting indices
0.35 Â 0.30 Â 0.25
1.52–28.25
À14 6 h 6 15,
À35 6 k 6 24,
À11 6 l 6 11
17361
3277 (0.0396)
3277/0/145
0.966
Table 3
Selected bond angles (°) of L2.
No. of reflections collected
Atoms
Bond angles (°)
No. of independent reflections (Rint
No. of data/restraints/parameters
Goodness-of-fit (GOF) on F2
)
C(11)AN(1)AC(1)
C(13)AN(2)AN(2)
C(2)AC(1)AC(6)
C(2)AC(1)AN(1)
C(6)AC(1)AN(1)
N(2)AC(13)AC(14)
N(2)AC(13)AC(11)
C(14)AC(13)AC(11)
C(3)AC(2)AC(1)
C(3)AC(2)AC(9)
N(1)AC(11)AC(12)
N(1)AC(11)AC(13)
C(4)AC(3)AC(2)
C(1)AC(6)AC(5)
121.3(2)
116.9(3)
121.1(2)
120.3(2)
118.5(3)
125.0(2)
115.8(2)
119.2(2)
118.2(3)
120.0(3)
125.4(2)
116.6(2)
120.8(3)
117.4(3)
R1, wR2 [(I > 2
R1, wR2 (all data)
Largest difference in peak and hole (eÅÀ3
r(I))]
0.0789, 0.2475
0.1373, 0.3116
0.437 and À0.236
)
phase using Becke-Lee-Yange-Parr functional (B3LYP) method.
Gaussian 09 [31] was used to perform all the theoretical
calculations. The structures of the complexes were built by using
Gauss View program. 6-31G (d,p) basis set was used for the
atoms, C, H, N and Cl, while Pd was described with LANL2DZ basis
set. B3LYP, as one of the density functional methods, was used to
optimize the starting structures. The optimized structures of the
three complexes are quite close and in a C1 symmetry group
(Fig. 4).
The substituted phenyl group is perpendicular on the surface of
the five membered ring formed by the palladium and two nitrogen
atoms in which the palladium forms a square planar configuration.

S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
563
Fig. 3. ORTEP projection of bis-a-diimine ligand L2 showing 50% probability ellipsoids.
Fig. 4. The optimized structure of the synthesized complexes (a) C1, (b) C2, and (c) C3: Hydrogens have been omitted for clarity.
Table 4
Ethylene oligomerization
Bond distances (Å), bonds angles (°) and some physical properties for the optimized
structure of the complexes C1AC3.
Ethylene oligomerization was performed in a 250 mL round
bottom flask equipped with a magnetic stirrer and a thermometer.
High-dilution techniques were adopted to reduce the monomer
mass transport effect. After the addition of the complexes, the reac-
tor was charged with toluene (80 mL) via a syringe and immersed
in a constant temperature bath previously set to a desired temper-
ature. When the reactor was equilibrated to the bath temperature,
ethylene was pressurized into the reactor (1.3 bar) after removing
the nitrogen gas under vacuum. When no more absorption of eth-
ylene into toluene was observed, a prescribed amount of cocatalyst
was injected into the reactor and oligomerization commenced. The
oligomerization rate was determined every 0.01 s from the rate of
consumption, and measured by a hotwire flow meter (model 5850
D from Brooks Instrument Div.) connected to a PC through an A/D
converter. Oligomerization was quenched by the addition of meth-
anol containing HCl (5 v/v%) after cooling. The resulting mixture
was passed through an alumina column to remove all aluminum
containing species and the obtained oligomers were analyzed by
gas chromatography. The rate of ethylene consumption was
directly obtained from the computerized experimental apparatus
and is expressed as in terms of g-product mol-PdÀ1 hÀ1 barÀ1. In
order to make a convincing assessment of the effects of catalyst
structure on catalytic activity and oligomer distribution, all data
were collected under similar conditions.
Among the factors that govern the catalyst performance is the
interaction of the olefin with the metal, the metal-alkyl bond sta-
bility, the influence of electronic effect exerted by ligands and their
substituents and the steric effects, being most crucial. It has been
well documented that catalysts containing bulky substituents near
the active metal center produce high molecular weight polymers;
this is consistent with the established facts that the bulky ortho-
substituents effectively block the axial coordination sites and
retard b-hydrogen elimination. On the contrary, catalysts lacking
steric bulk around the metal center are prone to b-hydrogen elim-
C1
C2
C3
PdACl
PdAN
NAN
C@N
CACH3
ClAPdACl
PdANAN
PdANAC (phenyl)
PdANACAC (phenyl)
PdANANAPd
CANANAC
2.31
2.09
1.37
1.30
1.50
91.3
121.4
123.4
92.4
97.8
127.7
2.31
2.09
1.37
1.30
1.50
91.2
121.5
123.3
91.7
2.32
2.09
1.37
1.30
1.50
90.9
121.0
123.5
89.8
98.1
94.8
127.5
11.575
0.0974
128.1
11.586
0.0977
Dipole moment (Debye) 11.694
Band gap (eV)
Electronic energy (e.u.)
0.0973
À3245.538524 À3402.666983 À3559.803950
The dihedral angles (PdANANAPd) are 97.8°, 98.1° and 94.8° in the
three complexes respectively. The common bond distances, angles,
dihedral angles and some physical properties for the optimized
structures of the complexes C1AC3 are listed in Table 4. The opti-
mized structures of the studied complexes were used to calculate
the energies of the molecular orbitals (MOs). Besides the HOMO
and LUMO, orbitals namely HOMO-2 through LUMO+2 are also
considered for all the three complexes. The HUMO LUMO gaps
for the three complexes are very close, around 0.1 eV. The molecu-
lar density distribution through the MO is reflecting the contribu-
tions of each atom in the molecular orbital. The localization of the
density for the orbitals HOMO-2 through LUMO+2 for complexes
C1AC3 is summarized in Fig. 5. The highest contribution for the
HOMO molecular orbital for the three complexes is attributed to
the lone pairs on the chloride ions. In the LUMO orbital, in addition
to the localized density from the palladium centre and the two
chloride ions, there is some contribution from other atoms as
shown in Fig. 5.

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S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
Table 5
Results of ethylene oligomerization with dipalladium(II) complexes C1AC3 combined with MAO at 30oC and 50 °C.
a
Entry
Catalyst
Cocatalyst(Al/Pd)
Temp (°C)
Activity Rp
Distribution of olefins (%)b
C4
C6
9.6
11.3
13.6
10.2
10.8
9.9
C8
C10
C12–20
1
2
3
4
5
6
7
8
9
C1
C2
C3
C1
C2
C3
C1
C2
C3
EASC(300)
EASC(300)
EASC(300)
EASC(300)
EASC(300)
EASC(300)
MAO(300)
MAO(300)
MAO(300)
30
30
30
50
50
50
30
30
30
531.2
806.8
1218.4
294.6
383.6
576.5
101.1
414.6
849.5
83.5
80.4
61.8
88.4
85.8
83.7
84.7
75.3
71.2
6.1
5.3
9.4
0.8
1.8
1.4
3.4
6.8
2.3
0.3
1.4
7.6
0.6
1.6
4.8
1.9
5.3
6.5
0.5
1.6
7.6
0.0
0.0
0.4
2.1
1.3
3.4
7.9
11.3
16.6
Results of ethylene oligomerization with dipalladium(II) complexes C1AC3 catalysts combined with EASC/MAO. Conditions: catalyst = 5 mmol; Toluene solvent = 80 mL;
P
C2H4 = 1:3 bar; reaction time = 30 min.
a
Rate of polymerization in Kg-oligomer mol-PdÀ1 barÀ1 hÀ1
.
b
Determined by GC using calibration curves with standard solutions.
Fig. 5. Density distribution for the HOMO and LUMO molecular orbitals for the complexes (a) C1, (b) C2, and (c) C3.
ination interacting with the aluminum cocatalyst ending up with
the oligomers. In the case of present bipalladium(II) systems
C1AC3, maximum steric hindrance offered by the bis-a-diimine
quantity of tetramerization and traces of higher olefin fractions
in combination with EASC as cocatalysts. All the Pd (II) complexes
showed high selectivity for butene fraction accounting nearly
about 75% of the total oligomer content on an average. Apart from
C4 composition, 10–15% of the total contents produced also con-
sisted of C6 and C8 products. Higher olefin contents were also
detected in comparatively trace quantity with all the Pd catalysts.
Percentage yield of the ethylene oligomers and their distributions
are also given in Table 5. The oligomerization reaction carried
out at higher temperature of 50 °C produced a sharp decrease in
activity. This surveillance was also observed in the case of Ni(II)
complexes reported earlier [26]. At 50 °C, the quantity of the C4
fraction increases to some extent, but the formation of higher ole-
fins is also reduced, as expected indicating the reduced lifetime of
active centers in the complexes. The oligomerization rate curves
are illustrated in Fig. 6.
ligand systems on the either side of 2,6-substituted aniline, will
hinder the monomer entrance from axial sites, by which polymer
chain termination occurs. The monomer obstruction is offered suf-
ficiently by the series of sterically tuned substituents, ranging from
Me < Et < iPr, present on 2,6-positions of bis-anilines, whereas on
the other side of the metal center, near the azine fragment, equal
monomer obstruction does not occur, which cannot prohibit the
axial ethylene insertion resulting in the formation of desired olig-
omers. Caruthers reported that catalyst-cocatalyst interaction is
merely structural in nature than electronic [32]. Hence although
MAO has widely been used as an alkylating agent, its long poly-
meric structure might not be able to interact sufficiently with the
catalyst bearing bulky substituents resulting in generation of lower
number of active cationic catalyst. This might be one of the main
reasons for the observed higher activity of all the Pd(II) complexes
in EASC medium over MAO.
The type of aluminum cocatalyst used, has a remarkable effect
on ethylene oligomerization experiments and the amount of alu-
minum cocatalysts affects the molecular weight distributions
[33]. In the present experiments, lower activity observed using
MAO as cocatalyst, prompted us to perform the experiments with
All the binuclear Pd (II) complexes selectively afforded dimer-
ization and trimerization of ethylene accompanied by a small

S.P. Netalkar et al. / Journal of Molecular Structure 1075 (2014) 559–565
565
– Bangalore is gratefully acknowledged. One of the authors (San-
deep P. Netalkar) is thankful to Department of Science & Technol-
ogy for providing financial assistance under INSPIRE fellowship
program.
Appendix A. Supplementary material
CCDC 978823 contains the supplementary crystallographic data
for L2. This data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.
07.034.
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Acknowledgments
The authors thank USIC, Karnatak University, Dharwad, for pro-
viding spectral facilities. Recording of NMR and IR spectra from IISc