Synthesis, characterization and ethylene oligomerization studies of nickel complexes bearing novel bis-α-diimine ligands

Article abstract of DOI:10.1007/s10562-013-1123-2

Sterically modulated, diazine bridged bis-α-diimine Ni(II) complexes have been synthesized to provide a versatile set of catalytic systems for ethylene oligomerization in combination with methyl aluminoxane (MAO) as cocatalyst. All the dinickel catalysts

Full text of DOI:10.1007/s10562-013-1123-2

Catal Lett (2014) 144:181–191
DOI 10.1007/s10562-013-1123-2
Synthesis, Characterization and Ethylene Oligomerization Studies
of Nickel Complexes Bearing Novel Bis-a-diimine Ligands
•
Sandeep P. Netalkar Priya P. Netalkar
•
•
M. P. Sathisha Srinivasa Budagumpi
•
Vidyanand K. Revankar
Received: 1 July 2013 / Accepted: 26 September 2013 / Published online: 12 October 2013
Ó Springer Science+Business Media New York 2013
Abstract Sterically modulated, diazine bridged bis-a-
diimine Ni(II) complexes have been synthesized to provide
a versatile set of catalytic systems for ethylene oligomer-
ization in combination with methyl aluminoxane (MAO) as
cocatalyst. All the dinickel catalysts efficiently oligomerize
ethylene to produce C4–C20 fractions at activities of up to
1,123 kg-oligomer mol-Ni-1 bar-1 h-1 at 30 °C with
Al(MAO)/Ni ratio at 300. The change in potential of metal
center induced by substituents, as evidenced by cyclic
voltammetric measurements, influences the oligomeriza-
tion activity. Complexes C1 and C3 produces traces of
polyethylene at 30 °C, while at higher temperature all the
complexes yielded oligomers only.
1 Introduction
The oligomerization of ethylene is the major industrial
process for the production of linear a-olefins, which are
important reactants used in the preparation of detergents,
lubricants, plasticizers, oil field chemicals, and monomers
for copolymerization. Current industrial processes employ
catalysts including either alkylaluminum compounds or a
combination of alkylaluminum compounds and early tran-
sition metal compounds, specially, nickel(II) complexes
together with bidentate monoanionic [P, O] ligands which
have evidentially been proved as ideal catalysts for ethylene
oligomerization (the SHOP process). But since the pio-
neering work by Brookhart’s group on a-diimino nickel
complexes for high molecular weight polyethylene, interest
in these a-diimine systems based complexes and various
derivatives thereof have arose extensively in both academic
and industrial research [1–8]. These a-diimine ligands form
cationic complexes with late transition metal halides, which
are the active catalysts in the poly/oligomerization of eth-
ylene and other a-olefins to form high molecular weight
polymers. Further, their higher tolerance of polar functional
groups in the monomer has been recognized as a key
advantage over early transition metal catalyst systems [9–
17]. The key feature of these catalytic systems is the sym-
metrical presence of bulky ortho-aryl substituents at the
imine-nitrogen atoms, which effectively block the axial
coordination sites and prevents b-hydrogen elimination, and
thus retarding the rate of chain termination [4–7, 17–20].
The geometry and 3-dimensional orientation of a-diimine
ligand systems is also of fundamental importance as it
defines the environment around the metal centers. There-
fore, both electronic and steric environments of ligands are
crucial in stabilizing the late transition metal ions and
controlling their activity in olefin poly/oligomerizations.
Keywords Binucleating ligands ꢀ Ethylene
oligomerization ꢀ Methylaluminoxane ꢀ Square planar
Ni(II) complex ꢀ Crystal structures
Dedicated to our teacher, Professor S.T. Nandibewoor on the occasion
of his 60th birthday.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-013-1123-2) contains supplementary
material, which is available to authorized users.
S. P. Netalkar ꢀ P. P. Netalkar ꢀ V. K. Revankar (&)
Department of Chemistry, Karnatak University, Pavate Nagar,
Dharwad 580 003, Karnataka, India
e-mail: vkrevankar@rediffmail.com
M. P. Sathisha
P.G. Department of Chemistry, P.C. Jabin Science College,
Hubli 580 031, Karnataka, India
S. Budagumpi
Department of Chemistry, Faculty of Science, Universiti
Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
123

182
S. P. Netalkar et al.
In more recent years much effort has been devoted
effects and electron densities at the metal centers
(Scheme 1) and their behaviour towards oligomerization of
ethylene with methyl aluminoxane (MAO) as co-catalyst.
towards designing new ligand frameworks for improved
catalyst performance leading to intense research focusing
on discovering unique as well as more efficient catalytic
systems and processes benefiting from cooperative effects
between adjacent active metal centers in multinuclear
complexes [21–35]. In the perspective, multinuclear group-
X metal complexes serve as efficient catalysts in olefin
poly/oligomerizations due to the electronic interactions
found in the form of cooperative effects between metal
centers. The same electronic effects cannot be expected in
their mononuclear counterparts [36–39]. Steric modula-
tions in the ligands of multinuclear group-X metal com-
plexes affect ethylene polymerization behavior not only
from the viewpoint of yield but also product distribution.
Subsequently, there are few works on modified a-diimine
ligand systems, however, only few reports showed the use
of bis-a-diimine ligands in the formation of active group-X
metal complexes for ethylene poly/oligomerizations [40–
43]. Bridged and non bridged binuclear Ni(II) complexes
and bimetallic palladium(II) complexes have been studied
for ethylene poly/oligomerization and have been found to
oligomerize ethylene to higher olefins with remarkably
high activity and selectivity [37–39, 44, 45]. Focused in
developing alternative nickel complexes as unique and
efficient catalytic systems for ethylene activation, benefit-
ing from cooperative effects between adjacent active metal
centers, in this work, we report the synthesis, character-
ization and oligomerization behaviour of three new bis-a-
diimine Ni(II) complexes of the type [Ni2(bis-a-diimi-
ne)Cl4], which are sterically modulated by varying the
substitutions at the 2,6-aryl position. These bis-a-diimine
ligands are derived from simple azine scaffold-based
ketimines, in order to study the influence of different steric
2 Experimental
2.1 General Considerations
All manipulations involving air-sensitive materials were
performed under a purified nitrogen atmosphere using
Schlenk techniques. Dichloromethane was distilled from
calcium hydride; toluene and hexane were distilled from
sodium. All reagents used in this study were purchased
from Sigma-Aldrich and used without further purification.
MAO solution (10 %) in toluene was purchased from
Sigma-Aldrich to be used directly without any treatment.
Metal and chloride contents were estimated according to
standard methods [46]. The 1H and 13C NMR spectra were
recorded on a Bruker 500 MHz and 100 MHz spectrometer
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 4,000–400 cm-1. The
electronic spectra were measured on a Hitachi 150-20
spectrophotometer over the range of 1,000–200 nm. TG
and DTA measurements were recorded in nitrogen atmo-
sphere on a Universal V2.4F TA instrument using a final
temperature of 1,000 °C and heating rate of 20 °C/min. All
of the olefin estimations were monitored by gas chroma-
tography equipped with FID detector using an HP-5
column.
2.2 Synthesis of Ligands (L1–L3)
A 50 ml round bottom flask was charged with (2.73 g,
22.7 mmol) 2,6-dimethylaniline, (1.96 g, 22.7 mmol)/
(3.4 g, 22.7 mmol) 2,6-diethylaniline/(4.03 g, 22.7 mmol)
2,6-diisopropyllaniline, 2,3-butanedione, ethanol (20 mL)
and glacial acetic acid (0.4 mL). The pale yellow solution
was stirred for 36 h at room temperature without isolation
of the product. The completeness of the reaction was
monitored by TLC by the consumption of reactants. Then,
hydrazine hydrate (0.55 mL, 11 mmol) in 10 mL ethanol
was added drop wise with stirring. After few hours yellow
solid began to separate. The mixture was further stirred
overnight. The yellow precipitate was isolated by filtration,
washed with 2 9 10 mL ethanol, and dried in vacuum.
Slow evaporation of ethanolic solution yielded yellow
colored crystals of L1 and L3 suitable for X-ray diffraction
analysis. Schematic representation for the synthesis of
ligands and numbering pattern for NMR assignment is
given in Scheme 2.
Scheme 1 Catalyst precursor under investigation for ethylene olig-
omerization. The part enclosed under rectangle represents Brook-
hart’s type core
123

Synthesis, Characterization of Nickel Complexes
183
Scheme 2 Synthetic route for
the preparation of sterically
tuned bis-a-diimine ligands
IR cm-1: 1632, (C=N), 2963, (CH3), H NMR (CDCl3,
ppm): 1.186 (dd, 24H, J=4.5, 7 Hz, CH(CH3)2), 2.052 (s,
methyl, 6H, C10H3 and C11H3), 2.259 (s, 6H, methyl,
C7H3 and C14H3), 2.691 (sep, 4H, J=7 Hz, CH(CH3)),
7.186 (d, 4H, J=7.5 Hz, C3H, C5H, C17H and C19H),
7.116 (t, 2H, J=7.5 Hz, C4H and C18H). 13C NMR
(CDCl3, ppm): 165.91(–C8N1 and –C13N4), 156.74
(–C9N2 and –C12N3), 122.96 (C5H, C3H, C17H and
C19H), 123.73 (C4H and C18H), 135.20 (C2H, C6H,
C16H and C20H), 146.08 (C1H and C15H), 28.31
(CH(CH3)2), 23.11 (CH(CH3)2), 16.33 (–C7H3 and –C14H3),
12.88 (C10H3 and C11H3). kmax (nm): 256, 370. m/z: 486.
1
2.2.1 2,6-Dimethyl-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine (L1)
Yield: 70% Anal. Calc. for C24H30N4 (%): C, 76.97; H,
8.07; N, 14.96. Found (%): C, 76.95; H, 8.06; N, 14.99. IR
cm-1: 1628, (C=N), 2963, (CH3). 1H NMR (CDCl3, ppm):
2.008 (s, 12H, Benz–CH3), 1.992 (s, 6H, methyl, C10H3
and C11H3), 2.204 (s, 6H, methyl, C7H3 and C14H3), 7.06
(d, 4H, J=7.5 Hz, C3H, C5H, C17H and C19H), 6.93 (t,
2H, J=7.5 Hz, C4H and C18H). 13C NMR (CDCl3, ppm):
166.04 (–C8N1 and –C13N4), 156.19 (–C9N2 and
–C12N3), 123.10 (C4H and C18H), 124.83(C5H, C3H,
C17H and C19H), 127.89 (C2H, C6H, C16H and C20H),
148.33(C1H and C15H), 17.87 (Benz–CH3), 15.65 (–C7H3
and –C14H3), 12.85 (C10H3 and C11H3). kmax (nm): 255,
370. m/z: 374.
2.3 Preparation of Complexes
A 50 mL round bottom flask was charged with 0.55 g
(2.5 mmol) NiCl2(DME), 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 atmosphere. The resulting red/
brown mixture was stirred at room temperature over night.
Then the mixture was filtered to remove the unreacted
NiCl2(DME) to give a clear red/brown solution which
yielded a red/brown solid by removing the dichlorometh-
ane under reduced pressure. The solid product was washed
with 3 9 10 mL hexane, and then dried in vacuum.
Schematic representation for the synthesis of complexes
and numbering pattern for NMR assignment is given in
Scheme 3.
2.2.2 2,6-Diethyl-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine (L2)
Yield, 60 %. Anal. Calc. for C28H38N4 (%): C, 78.10; H,
8.89; N, 13.01. Found (%): C, 78.13; H, 8.87; N, 13.0. IR
cm-1: 1630, (C=N), 2963, (CH3). 1H NMR (CDCl3, ppm):
1.170 ( t, 12H, J=7.5 Hz, CH2–CH3), 2.035 (s, 6H, methyl,
C10H3 and C11H3), 2.239 (s, 6H, methyl, C7H3 and
C14H3), 2.367 (q, 8H, J=7.5 Hz, CH2–CH3), 7.127 (d, 4H,
J=7.5 Hz, C3H, C5H, C17H and C19H), 7.050 (t, 2H,
J=7.5 Hz, C4H and C18H). 13C NMR (CDCl3, ppm):
165.82 (–C8N1 and –C13N4), 156.43 (–C9N2 and
–C12N3), 123.48 (C5H, C3H, C17H and C19H), 125.98
(C4H and C18H), 130.63 (C2H, C6H, C16H and C20H),
147.39 (C1H and C15H), 24.62 (CH2–CH3), 16.02 (–C7H3
and –C14H3), 13.67 (CH2–CH3), 12.80 (C10H3 and
C11H3). kmax (nm): 256, 376. m/z: 430.
2.3.1 2,6-Dimethyl-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine dinickel(II) chloride (C1)
Yield: 64 %. Anal. Calc. for C24H30Cl4N4Ni2 (%): C,
45.49; H, 4.77; N, 8.84; Ni, 18.52; Cl, 22.38. Found (%): C,
45.51; H, 4.75; N, 8.84; Ni, 18.56; Cl, 22.34. FTIR cm-1
:
1
2.2.3 2,6-Diisopropyl-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine (L3)
1611, (C=N), 2963, (CH3). H NMR (CDCl3, ppm): 1.988
(s, 12H, Benz–CH3), 2.263 (s, 6H, methyl, C10H3 and
C11H3), 2.361 (s, 6H, methyl, C7H3 and C14H3), 7.095,
(d, J=7.5 Hz, 4H, C3H, C5H, C17H and C19H), 6.992, (t,
J=7.5 Hz, 2H, C4H and C18H). kmax (nm): 255, 370, 427.
ESI mass, m/z: 632.2 [M], 596.3 [M–Cl]
Yield, 75 %. Anal. Calc. for C32H46N4 (%): C, 78.96; H,
9.53; N, 11.51. Found (%): C, 78.91; H, 9.57; N, 11.52.
123

184
S. P. Netalkar et al.
Scheme 3 Synthetic routes for
the preparation of sterically
tuned dinickel(II) complexes
derived from bis-a-diimine
ligands
2.3.2 2,6-Diethyl-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine dinickel(II) chloride (C2)
MAO was injected into the reactor and oligomerization
commenced. The oligomerization rate was determined
every 0.01 s from the rate of ethylene consumption, mea-
sured by a hotwire flow meter connected to a computer
through an A/D converter. Oligomerization was quenched
by the addition of methanol containing hydrochloric acid
(5 v/v %), the resulting mixture was passed through an
alumina column to remove cocatalyst species and the
obtained oligomers were analyzed by GC. The rate of
ethylene consumption was directly obtained from the
computerized experimental set up and is expressed as in
terms of gram-product mol-Ni-1 h-1 bar-1 [48, 49].
Yield: 70%, Anal. Calc. for C28H38Cl4N4Ni2 (%): C, 48.75;
H, 5.55; N, 8.12; Ni, 17.02; Cl, 20.56. Found (%): C, 48.73;
H, 5.54; N, 8.14; Ni, 17.04; Cl, 20.56. FTIR cm-1: 1619,
(C=N), 2963, (CH3). 1H NMR (CDCl3, ppm): 1.16 (t,
J=7.5 Hz, 12H, CH2-CH3), 2.129 (s, 6H, methyl, C10H3
and C11H3), 2.291 (s, methyl, 6H, C7H3 and C14H3):
2.342, (m, 8H, CH2-CH3), 7.129, (d, J=7.5 Hz, 4H, C3H,
C5H, C17H and C19H), 7.092, (t, J=7.5 Hz, 2H, C4H and
C18H). kmax (nm): 255, 370, 417. ESI mass m/z: 689.9
[M], 654.4 [M–Cl].
3 Results and Discussion
2.3.3 2,6-Diisopropyll-phenyl-{1-methyl-2-[(1-methyl-2-
methylimino-propylidene)-hydrazono]-propylidene}-
amine dinickel(II) chloride (C3)
3.1 General Characterization and Electrochemistry
of the Bis-a-diimine Ligands and Their Ni(II)
Complexes
Yield: 70% Anal. Calc. for C32H46Cl4N4Ni2 (%): C, 51.55;
H, 6.22; Cl, 19.01; N, 7.51; Ni, 15.74. Found (%): C, 51.53;
H, 6.20; Cl, 19.03; N, 7.54; Ni, 15.73. IR cm-1: 1620,
(C=N), 2963, (CH3), 1H NMR (CDCl3, ppm): 1.15 (d,
J=7 Hz , CH(CH3)2), 2.022 (s, 6H, methyl, C10H3 and
C11H3), 2.229 (s, 6H, methyl, C7H3 and C14H3):, 2.660,
(m, 4H, CH(CH3)), 7.105, (d, J=7.5 Hz, 4H, C3H, C5H,
C17H and C19H), 7.162, (t, J=7.5 Hz, 2H, C4H and
C18H). kmax (nm): 255, 370, 426. m/z: 745.9 [M], 710.4
[M–Cl].
The elemental analyses of all the complexes C1–C3 are
consistent with the proposed structures with 2:1 metal–
ligand stoichiometry. Comparing the FTIR spectra of bis-
a-diimine ligands and their respective Ni(II) complexes
gives an idea about the coordination mode of the ligands to
the Ni(II) metal ion. The FTIR spectra of all three ligands,
L1–L3 exhibit characteristic intense broad band at around
1,630 cm-1, which can be assigned to azomethine func-
tionality. Upon metallation, this band is shifted to lower
frequency by 10–20 cm-1 in the spectras of corresponding
Ni(II) complexes C1–C3, indicating the involvement of
azomethine nitrogen atoms in the coordination. The methyl
and aromatic C–H stretching frequencies are observed at
around 2,960 and 3,000 cm-1 in all the compounds.
2.4 Olefin Oligomerization Activity
Ethylene oligomerization experiments were performed in a
250 mL round bottom flask equipped with a magnetic
stirrer and a thermometer [47–49]. After addition of the
nickel complex (C1–C3), the reactor was charged with
toluene (60 mL) via syringe and immersed in a thermostat
to acquire constant temperature previously set to a tem-
perature of 30 °C. When the reactor was equilibrated to the
thermostat temperature, ethylene gas was pressurized into
the reactor (1.3 bar) after removing the prefilled nitrogen
gas under reduced pressure. When no more absorption of
ethylene into toluene was observed, a prescribed amount of
The acquisition of 1H and 13C NMR spectra of the
1
ligands supports their respective structures. The H NMR
spectra of bis-a-diimine ligands L1–L3 consisted of a
series of well resolved peaks. The numbering scheme for
the assignment of carbons and corresponding protons is
given in Scheme 2, detailed chemical shift values for all
the carbon and protons are given in experimental section.
The protons of the two methyl groups attached to the azine
fragment in the ligands L1–L3 resonated as singlets in the
123

Synthesis, Characterization of Nickel Complexes
185
range 1.992–2.052 and 2.204–2.259 ppm respectively. The
methyl/ethyl/isopropyl signals of all the ligands are found
in expected ranges in their 1H NMR spectra and are
detailed in experimental section. The aromatic signals in all
the ligands were observed in the expected region, ranging
7.06–7.186 and 6.93–7.116 respectively.
spectra of complexes, C1–C3 show molecular ion peak
with m/z values at 632.2, 689.9 and 745.9 respectively.
These values are in good agreement with the proposed
composition for the complexes. Further, the complexes
C1–C3 also show the fragmentation peak corresponding to
[M–Cl] at 596.3, 654.4 and 710.4 respectively.
The 13C NMR spectral analyses agreed with the 1H
NMR spectral data in demonstrating the architecture of the
azine-scaffold-based binucleating ligands L1–L3. In the
13C NMR spectra of ligands L1–L3, the signal observed
around *166 ppm is due to azomethine carbon (linkage
between aniline and the one side of 2,3-butanedione), at the
lowest downfield position of the carbon atoms. Another
signal around *156 ppm in the spectra of ligands was
assigned to the azomethine carbon (linkage between two
2,3-butanedione molecules). The appearance of these two
signals additionally confirms the successful formation of
the bis-a-diimine ligands.
Thermal studies of dinickel(II) complexes were under-
taken in order to study their thermal stability and as sup-
portive data for the proposed molecular formulae. The
dinickel(II) complexes C1–C3 are hygroscopic and on
exposure to atmosphere absorb moisture which is observed
in the thermogram of these complexes as weight loss cor-
responding to water molecules at 65–75 °C. This is further
confirmed by the appearance of endothermic signal at the
same temperature range for complexes C1–C3. The weight
loss of around 55–60 % for complexes C1–C3, in the
region of 150–450 °C corresponds to simultaneous
decomposition of ligands and elimination of four coordi-
nated chlorine atoms as HCl. The elimination of chlorine
atoms is an exothermic process and as expected, is
observed as exothermic DTA signal in the range
250–270 °C. The plateau obtained after heating complexes
above 500 °C accounts for the formation of stable metal
oxides.
1
The H NMR of the complexes, C1–C3 showed down-
field shifts of signals compared to respective free ligands
L1–L3. The two methyl groups attached to the azine frag-
ment in the complex C1–C3 resonated as singlets in the
range 2.022–2.263 and 2.229–2.361 ppm respectively,
showing shifts to downfield region and suggesting the
coordination through azomethine nitrogen. The methyl/
ethyl/isopropyl signals of all the complexes have again been
shifted to downfield region as expected and are detailed in
experimental section. The aromatic signals in all the com-
plexes were observed in the region 6.992–7.102 ppm.
Electronic spectra of all the compounds L1–L3 and C1–
C3 were recorded in their dichloromethane solution over
the scan range 200–800 nm. Electronic spectral data of bis-
a-diimine ligands L1–L3 are very similar to each other due
to their structural similarity. The electronic absorption
spectra of all ligands exhibit a strong band at around
250 nm assignable to p ? p* and weak band around
380 nm corresponds to n ? p* transitions associated with
azomethine linkage [50, 51]. The electronic absorption
spectral data of the dinickel(II) square planar complexes
C1–C3 are also similar since they differ only in substitu-
ents on phenyl ring at ortho-aryl position and these do not
alter the coordination mode of the ligands. The spectral
profiles below 380 nm are similar and are ligand centered
transitions (intraligand p–p* and n–p*) of benzene and
non-bonding electrons present on the nitrogen of the azo-
methine group in the Schiff base complexes. Since these
complexes are diamagnetic, the bands at around
The cyclic voltammetric experiments of the free ligands
and their Ni(II) complexes were performed at room tem-
perature in DMSO under O2 free conditions in the potential
range -1.0 to ?1.0 V, using a glassy carbon working
electrode (0.082 cm2), a platinum counter electrode and an
Ag/Ag? reference electrode. The electrochemical studies
of the free ligands showed no electrochemical response
within the similar working potential range. Therefore, the
observed electrochemical responses in the cyclic voltam-
mograms (CVs) of the complexes C1–C3 are purely due to
the presence of metal centers. CVs of the complexes are
shown in Fig. 2.
A one-electron quasi-reversible redox behavior for each
Ni(II) ion is observed in the CVs of the complexes C1–C3.
The oxidation potentials for all the Ni(II) catalysts are in
the range -0.15 to 0.05 and 0.35 to 0.5 V at a scan rate of
0.1 V s-1 differing only slightly from each other, and shift
to higher potentials with an increase in scan rate. Corre-
spondingly, in the reverse scan, two reduction waves
appear which can be attributed to the reduction of one
Ni(III) per wave. The reduction potentials are in the region
0.025 to 0.25 and -0.05 to -0.25 mV, and with increasing
scan rate shifts towards more negative potentials. For the
complex C3, the separation between the anodic and
cathodic peak potentials, DEp = Epa - Epc was found to
be 200 and 350 mV. For C2, DEp was 80 and 300 mV and
for C1, DEp was found to be 75 and 275 mV. Overall, the
difference between the anodic and cathodic peak potentials
(DEp) is more than 58 mV. This suggests one electron
1
1
390–445 nm could be assigned to A1g ? B1g transition
consistent with square-planar nickel(II) complexes [50,
51]. UV–Vis spectra are shown in Fig. 1.
The mass spectra of bis-a-diimine ligands, L1–L3 show
molecular ion peaks at 374, 430 and 486 respectively,
corresponding to their molecular weights. The ESI mass
123

186
S. P. Netalkar et al.
Fig. 1 UV–Vis spectra of ligands L1–L3 and complexes C1–C3
This redox mechanism can be explained in terms of the
flexibility and size of the coordination cavity in these
catalysts, in addition to the geometric requirements and the
size of the nickel ions in different oxidation states.
3.2 Crystallographic Studies of L1 and L3
The X-ray diffraction data were collected using a Bruker
SMART APEX2 CCD area-detector diffractometer. The
structures were 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 anisotropi-
cally. Structure solution and refinement were performed
using SHELXL-97 package [55].
Fig. 2 Cyclic voltammograms of complexes C1–C3, at scan rate
0.1 Vs-1
transfer quasi-reversible redox process assignable to the
Ni(II)/Ni(III) couple, for each Ni(II) ion. The oxidation and
corresponding reduction reactions obey the diagnostic cri-
teria, i.e., Ipc/Ipa is almost constant but not unity for a
quasi-reversible electron transfer [52–54]. The stepwise
nickel-based redox processes of the catalysts can be
assigned as shown in Eqs. 1 and 2.
The single crystal X-ray structure of bis-a-diimine ligand
L1 [C36H45N6] was unambiguously determined at 296 K and
was shown to be monoclinic space group P 1 21/n 1, with
˚
a = 18.4500(6) A, b = 6.1307(2) A, c = 30.3580(10) A,
˚
˚
3
˚
b = 94.183(2)°, volume = 3424.69(19) A . Details of the
data collection and refinements are given in Table 1.
Selected bond lengths, torsion angles and bond angles are
given in Tables 3, 4 and 5 respectively (in Supplementary
data). A perspective view of the ligand is shown in Fig. 3
showing 50 % probability ellipsoids. The asymmetric unit of
the compound crystal shows two independent ligand mole-
Oxidation: NiðIIÞ; NiðIIÞ ꢁ eꢁ ! NiðIIIÞ; NiðIIÞ
ð1Þ
ꢁ eꢁ ! NiðIIIÞ; NiðIIIÞ
Reduction: NiðIIIÞ; NiðIIIÞ þ eꢁ ! NiðIIÞ; NiðIIIÞ
ð2Þ
˚
cules A and B. The C–C bond distances (1.38 A) and C–C–C
þ eꢁ ! NiðIIÞ; NiðIIÞ
bond angles (120°) of pendent phenyl rings are comparable
with similar reported structures [37–39]. [N2A=C11A,
N3A=C14A and C15A=N4A [1.275(4), 1.274(4) and
Although the two Ni(II) ions in complexes, C1–C3 are
in the same chemical environments, the overall two-
electron transfer process appear as separate one-electron
processes, indicating that oxidation/reduction at one site is
influenced by the steric and electronic factors on the other
site, and hence induces a positive/negative shift in the
oxidation/reduction potential of the metal ion. The shift in
CVs of complexes C1–C3 is due to the perturbation on the
nickel center caused by substituents at 2,6-positions. These
result strongly supports the influence of these substituents
on nickel center and hence the oligomerization activity.
˚
1.270(4) A respectively] bond distances in molecule A are
comparable with reported structures [37–39] and show slight
deviation with respect to molecule B where N=C
[N2B=C11B, N1B=C10A and C6B=N1B [1.275(4),
˚
1.271(4) and 1.419(4) A respectively] agrees well with the
values for double bond character, confirming the formation
of bis-imine bonds [37–39]. Bond angles C6A–N1A–
C10A = 121.0(3)°, C11A–N2A–N3A = 117.7(3)° C15A–
N4A–C23A 121.0(3)° and C24A–C23A–N4A = 119.6(4)°
123

Synthesis, Characterization of Nickel Complexes
187
are comparable with the reported values for analogous
compound [37–39] and corresponding angles in the mole-
cule B are also in agreement with the literature values. The
torsion angles [72.17°] between the two planes [C10A–C11–
N2–H9A2] and [C1A–C2A–C3A–C4A–C5A–C6A], sug-
gests that the phenyl rings of the molecule are almost per-
pendicular to the azine fragment, due to which the ligand is
non-planar in nature. The torsion angle 172.3(3) exhibited by
N1A–C10A–C11A–N2A and -8.0(5) by C9A–C10A–
C11A–N2A indicates that N1A and N2A atom are trans, and
N2A and C9A are cis to each other., 166.9(3)° by N3A–
C14A–C15A–N4A and -13.8(5) N3A–C14A–C15A–C16
A indicates that N3A and N4A atom are trans to each other
and N3A and C16A are cis to each other.
Table 1 Crystal data and structure refinement details for bis-a-dii-
mine ligands L1 and L3
Crystal Data
L1
L3
Empirical formula
Formula mass
C36H45N6
561.78
C32H46N4
490.76
Crystal size (mm)
Crystal system
Space group
0.45 9 0.32 9 0.21 0.40 9 0.35 9 0.35
Monoclinic
Triclinic
P-1
P2 /n
1
˚
a(A)
18.4500(6)
6.1307(2)
30.3580(10)
90
10.0113(4)
12.5354(5)
13.4582(5)
66.076(2)
84.287(3)
82.065(3)
1527.29 (10)
2
˚
b(A)
˚
c(A)
a(°)
b(°)
94.183(2)
90
c(°)
Volume [pm3 9 106]
The single crystal X-ray structure of bis-a-diimine ligand
L3 was unambiguously determined at 296 K and was shown
˚
to be triclinic space group P-1, with a = 10.0113(4) A,
3424.69 (19)
4
Z
Dcalc (g cm-3
)
1.090
1.067
˚
˚
b = 12.5354(5) A, c = 13.4582(5) A, b = 84.287(3)°,
3
volume = 1527.29(10) A . Details of the data collection
l (cm-1) (Mo Ka)
0.71073
296 (2)
0.063
˚
and refinements are given in Table 1. Selected bond lengths,
torsion angles and bond angles are given in Tables 3, 4 and 5
respectively. A perspective view of the ligand is shown in
Fig. 4 showing 50 % probability ellipsoids. In the compound
Temperature (K)
296 (2)
Index range
h
-16 ? 16
-5 ? 5
-11 ? 12
-15 ? 15
16 ? 16
k
˚
crystal, the C–C bond distances (1.38 A) and C–C–C bond
l
-27 ? 27
22771
angles (120°) of pendent phenyl rings are in accordance with
the values reported in literature [37–39]. The bond distances
of N4–C8, N3–C7, N2–C6 and N1–C5 [1.271(3), 1.279(3),
Reflections collected
Unique reflections
22161
2263 (0.0308)
5497 (0.0582)
(Rint
)
˚
1.274(3), and 1.274(3) A respectively] agrees well with the
Reflections with
2776
2713
Fo [ 4r(Fo)
Number of parameters 380
values for double bond character, confirming the formation
of bis-imine bonds [37–39]. The torsion angle 167.41(13)8
exhibited by N1–C5–C6–N2 and -12.33(13)°by N1–C5–
C6–C17 indicates that N1 and N2 atom are trans, and N1 and
C17 are cis to each other. Similarly, the torsion angle for N4–
C7–C8–N3 = 173.15(2)° clearly shows that N4 and N3 are
trans to each other. A closelook at the torsion angles [69.78°]
337
R1
wR2 (all data)
0.0537
0.0734
0.1687
0.2467
Max. and min.
residual density ((e
0.216, -0.180
0.389, -0.236
pm-3) 9 10-6
)
Fig. 3 ORTEP projection of bis-a-diimine ligand L1 showing 50 % probability ellipsoids
123

188
S. P. Netalkar et al.
Fig. 4 ORTEP projection of
bis-a-diimine ligand L3
showing 50 % probability
ellipsoids
Table 2 Results of ethylene oligomerization with dinickel(II) complexes C1–C3 combined with MAO at 30 and 50 8C
Entry
Catalyst
Temperature (°C)
Activity Rap
Distribution of olefins (%)b
C4
a-C4
C6
C8
C10
C12–20
Polymer
1
2
3
4
5
6
C1
C2
C3
C1
C2
C3
30
30
30
50
50
50
662.2
889.6
1,123.4
371.7
439.7
530.0
76.3
70.3
85.1
77.2
67.8
78.4
87.0
28.4
60.4
45.5
13.7
24.7
12.2
15.6
4.7
6.3
8.9
4.1
4.8
5.8
6.1
2.1
3.2
3.1
3.1
4.0
2.4
3.1
–
Wax
3.0
Trace
Trace
13.5
20.2
12.2
Wax
2.2
–
–
–
Wax
Conditions: catalyst = 5 lmol; [MAO]/[Ni] = 300; toluene solvent = 60 mL; PC2H4 = 1:3 bar; reaction time = 30 min
a
Rate of oligomerization in kg-oligomer mol-Ni-1 bar-1 h-1
b
Determined by GC using calibration curves with standard solutions
between the two planes [C9, C10, C13, C14, C15, C29] and
[C8, C7, N3, N2 C6, C5, N1], suggests that the phenyl rings
of the molecule are perpendicular to the azine fragment, due
to which the ligand is non-planar in nature.
3.3 Ethylene Oligomerizations
Numerous attempts have been made in Schiff-base derived
dinickel(II) and their alkyl/aryl-substituted counterparts to
obtain exceptionally good activity in olefin oligomeriza-
tion. This is with the hope that the dinuclear catalysts
would be more tolerant to simple monomers as well as
functionalized monomers, which is a highly desirable
property due to the cooperative effect that exists between
Fig. 5 Ethylene consumption rate curves of dinickel(II) complexes
C1–C3 at 30 and 50 °C. oligomerization conditions: com-
the metal centers. Especially, when high molecular weight
polymers are targeted, the nature of the cooperative effect
between metal centers, steric control, nature of the mono-
mer and cocatalyst become crucial. In this perspective, we
have designed a series of sterically tuned dinickel(II)
complexes, which mimic the Brookhart catalyst for ethyl-
ene oligomerization. For ethylene oligomerization
plex = 5 lmol, [MAO]/[Ni] = 300, toluene = 60 mL, Pethylene
1.3 bar, and reaction time = 30 min
=
applications, good catalytic performances of dinickel(II)
complexes have been recorded as a means of imposing
close spatial confinement on multi-nickel centers have also
123

Synthesis, Characterization of Nickel Complexes
189
been suggested. In the case of present dinickel(II) systems
C1–C3, maximum steric hindrance offered by the bis-a-
diimine ligand systems will not allow the monomer units to
enter from axial positions, by which polymer chain ter-
mination occurs. The monomer obstruction is offered suf-
ficiently by the steric substituents present on 2,6-positions
of bis-anilines, whereas on the other side of the metal
center equal monomer obstruction does not occur, which
cannot prohibit the axial ethylene insertion that results in
the formation of oligomers with only traces of polymer.
However, in combination with quantitative amount of
MAO, the synthesized dinickel(II) complexes C1–C3,
selectively afforded dimerization to tetramerization of
ethylene along with small amounts of higher olefin pro-
ducts and trace amounts of polyethylene. The distribution
of oligomers is given in Table 2. All the three dinickel(II)
complexes give high 1-butene content as nearly 75 % of
the total C4 content formed. Apart from butane contents, in
all the cases a considerable amount of higher olefins like
C6, C8, C10 and C12–20 contents were observed with nearly
12–20, 4–10, 2–4 and 2–3 % compositions, respectively.
Among three test samples, dinickel(II) complexes C1 and
C3, produce the traces of polyethylene which was hard to
characterize using spectral technique. From the ethylene
Fig. 6 Proposed mechanism for
ethylene oligomerization by
dinickel(II) complexes C1–C3
in combination with MAO
123

190
S. P. Netalkar et al.
Acknowledgments The authors thank USIC, Karnatak University,
Dharwad, for providing spectral facilities. Recording of NMR and IR
spectra from IIT-Madras and IIT-Bombay is gratefully acknowledged.
One of the authors (Sandeep P. Netalkar) is thankful to Department of
Science & Technology for providing financial assistance under
INSPIRE fellowship program.
consumption profile [Fig. 5] it is evident that all the di-
nickel(II) complexes were highly active in the initial
10–15 min, however, become inactive which is attributed
to the deactivation of active sites. The oligomer distribu-
tion pattern is consistent with a nickel alkyl species being
the active center, where fast elimination of b-hydride limits
the formation of higher olefins. Based on this observation,
a preliminary mechanism of ethylene oligomerization is
suggested in Fig. 6.
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