Synthesis of and ethylene oligomerization with binuclear palladium catalysts having sterically modulated bis-imine ligands with methylene spacer

Article abstract of DOI:10.1016/j.jorganchem.2011.03.003

Sterically modulated bis-imine ligands (L¹-L³) were prepared by reacting 4,4′-methylene bis-(2,6-dialkyl aniline) and antipyrine-4-carboxaldehyde in a 1:2 stoichiometric ratio. The reactions of L¹-L³ with dichlo

Full text of DOI:10.1016/j.jorganchem.2011.03.003

Journal of Organometallic Chemistry 696 (2011) 1887e1894
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Journal of Organometallic Chemistry
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Synthesis of and ethylene oligomerization with binuclear palladium catalysts
having sterically modulated bis-imine ligands with methylene spacer
,
Srinivasa Budagumpi a, Yinshan Liu a, Hongsuk Suh b, Il Kim a
*
a The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Division of Chemical Engineering, Pusan National University, Pusan 609-735, Republic of Korea
b Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Pusan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Sterically modulated bis-imine ligands (L1eL3) were prepared by reacting 4,40-methylene bis-(2,6-dialkyl
aniline) and antipyrine-4-carboxaldehyde in a 1:2 stoichiometric ratio. The reactions of L1eL3 with
dichloro(cycloocta-1.5-diene)palladium(II) [PdCl2(cod)] yield the corresponding binuclear palladium
complexes with the general formula Pd2Cl4L (L ¼ L1, L2, and L3). The binucleating ligands bind to the
palladium ion via the lone pair on the imine nitrogen and amide oxygen atoms, resulting in a square-
planar geometry around the metal center. All the palladium catalysts efficiently oligomerize ethylene to
produce C4eC20 fractions at activities of up to 1308 kg-oligomer mol-Pdꢀ1 barꢀ1 hꢀ1 at 30 ꢁC in
combination with ethylaluminum sesquichloride. The formation of active sites by the change in geom-
etry of the metal complexes could be traced using spectroscopic and electrochemical techniques.
Ó 2011 Elsevier B.V. All rights reserved.
Article history:
Received 12 January 2011
Received in revised form
23 February 2011
Accepted 1 March 2011
Keywords:
Binuclear
Bis-imine ligand
Catalysis
Ethylene oligomerization
Palladium
Transition metal chemistry
1. Introduction
comonomers in the production of polyethylene. C4eC8 linear a-
olefins are used in the synthesis of linear aldehydes via hydro-
In the past decade, a new generation of oligomerization catalyst
precursors has emerged with excellent activity, selectivity, living
behavior, and stability [1e3]. In particular, ethylene oligomeriza-
tion systems have been reported for nickel, iron, and chromium, but
palladium-based catalysts constitute the most numerous, active,
and selective group. Essentially, non-metallocene bis-imine based
catalysts comprising of a bimetallic core separated by aliphatic or
aromatic spacers were employed to achieve synergistic effects
found in metallocene catalysts [4,5]. The ligand design of such
systems is of fundamental importance, because the ligand structure
defines the environment around the metal centers. Broadly
speaking, there are two different types of bimetallic cores: a remote
donor bimetallic core with aliphatic or aromatic spacers and an
atom- or bond-bridged core. The former is of greater interest
because the distance between the metals can be tuned using
extended pi-bond conjugations through the spacers. Bimetallic
palladium catalysts offer diverse and difficult challenges with
regards to the design of new efficient catalyst systems for ethylene
formylation reactions. They have also been employed in the
production of poly-
surfactants. Higher linear
for aqueous detergent formulations and hydrophobes in oil-soluble
surfactants. At present, even-carbon-numbered -olefins are
a
-olefin synthetic lubricant base stock and
a
-olefins are used in making surfactants
a
produced industrially, but mostly via non-selective oligomerization
of ethylene [6].
The rapid expansion of this area, stimulated by the promising
catalytic activity of palladium complexes, resulted in considerable
contributions to polyethylene production. Antipyrene-4-carbox-
aldehyde is of particular interest due to the feasibility and flexibility
of ligand design, which allows the introduction of sterically and
electronically demanding features [7,8]. Phenyl and alkyl groups
attached adjacent to functional chromophores such as amide
carbonyls and aldehyde groups in the antipyrine ring make the
derived ligand systems active for ethylene oligomerization. The
required degree of hindrance can be obtained using the alkyl and
phenyl groups, allowing chain termination after the formation of
the desired oligomers. In order to achieve variable steric hindrance
in the ligand systems, a series of bis-amines with different aliphatic
groups can be used [9]. The most notable feature of the catalysts
formed by these combinations is the presence of fixed methyl and
phenyl substituents on one side of the metal and variable aliphatic
substitutions on the other side, which makes these ligands very
oligomerization. Linear a-olefins have a wide range of applications
[6]. The lower carbon numbers are overwhelmingly used as
* Corresponding author. Tel.: þ82 51 5102466; fax: þ82 51 5137720.
E-mail address: ilkim@pusan.ac.kr (I. Kim).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.003

1888
S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
suitable for ethylene oligomerization [4,10]. In this regard, a series
of simple bis-imine ligands derived from 4,40-methylene bis-(2,6-
dialkyl aniline) and antipyrine-4-carboxaldehyde were studied for
the preparation of palladium catalysts, resulting in bidentate liga-
tion and a square-planar Pd coordination. As a continuation of our
research into these catalysis, we herein report interesting results
regarding ethylene oligomerization with a series of sterically
modulated palladium catalysts.
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
ethylene into toluene was observed, a prescribed amount of co-
catalyst was injected into the reactor and oligomerization
commenced. The oligomerization rate was determined every 0.01 s
from the rate of consumption, 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 methanol 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 oligo-
mers were analyzed by gas chromatography. The rate of ethylene
consumption was directly obtained from the computerized exper-
imental apparatus and is expressed as in terms of g-product mol-
Pdꢀ1 hꢀ1 barꢀ1. 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.
2. Experimental
2.1. Materials for the preparation of catalysts
All reactions were performed under a purified nitrogen atmo-
sphere using the standard Schlenk technique. Polymerization-grade
ethylene (SK Co., Korea) was purified by passing through columns of
Fisher RIDOXÔ catalyst and molecular sieves 5A/13X Organic
solvents, namely, ethanol, methanol, and diethyl ether, were distilled
over CaH2 and stored over molecular sieves. Toluene was distilled
over Na/benzophenone under nitrogen and stored over molecular
sieves. The reagents antipyrine-4-carboxaldehyde, dichloro(cyclo-
octa-1.5-diene)palladium(II) [PdCl2(cod)], 4,40-methylene bis-(2,6-
dimethyl aniline), 4,40-methylene bis-(2,6-diethyl aniline),
4,40-methylene bis-(2,6-diisopropyl aniline), and glacial acetic acid
were purchased from Aldrich Chem. Co. and used without further
purification. Unless noted otherwise, all reagents were purchased
from commercial sources and used as received.
2.4. Detection of the active species
2.4.1. 1H NMR spectral technique
Alkyl-palladium active species were detected by in-situ 1H NMR
spectral technique under ethylene oligomerization conditions.
Palladium catalyst (2.5 mmol) was introduced into an NMR tube
under a nitrogen atmosphere and deuterated chloroform (0.7 mL)
was added. The tube was saturated with ethylene gas, followed by
the addition of ethylaluminum sesquichloride (EASC) (Al/Pd ¼
300). The 1H NMR spectra of the reaction mixture was recorded as
a function of time. The changes observed were correlated with
a blank EASC spectrum as a control under similar conditions.
2.2. Physical methods
1H and 13C NMR spectra were recorded in CDCl3 solvent on
a Varian Gemimi-2000 300 MHz spectrometer at room tempera-
ture using TMS as an internal reference. Analytical thin layer
chromatography (TLC) was carried out using Merck 0.25 mm silica
gel 60F pre-coated aluminum plates with fluorescent indicator
UV254. All ligands were purified using a Combi-Flash (Companion)
auto-column machine. Elemental analysis was carried out using
a Vario EL analyzer. Infrared (IR) spectra were recorded as KBr discs
(ligands) or CsI (catalysts) disc matrix using a Shimadzu IRPrestige-
21 over a 4000e200 cmꢀ1 range. The electronic spectra of the
complexes were recorded on a Shimadzu UV-1650PC spectropho-
tometer in the range of 1000e200 nm. Cyclic voltammetry (CV)
studies were performed at room temperature in chloroform under
O2 free conditions using Kosentech Model CV-104. Mass spectra of
catalysts were recorded using positive fast atom bombardment
(FAB) methods on a JEOL JMS-700, HP 5890 Series II spectrometer.
Thermogravimetric (TG) and differential thermal analysis (DTA) of
the catalysts were performed under a nitrogen atmosphere on
a DuPont 951 maintaining the final temperature at 800 ꢁC and
a heating rate of 10 ꢁC/min under a nitrogen atmosphere. The
oligomers were analyzed by gas chromatography (HPe6890) using
a J&W Scientific DB608 column (30 mm 90.53 mm) equipped with
a FID detector. The injector and detector temperature was kept
constant at 250 ꢁC. The initial temperature was set at 30 ꢁC (hold
2 min) and finishing temperature at 250 ꢁC (hold 10 min) with
a heating rate of 10 ꢁC/min.
2.4.2. Electronic spectral technique
For the detection of alkyl-palladium active species formed
during oligomerization, UVevisible spectra of the catalysts were
recorded under oligomerization conditions. Palladium catalyst
(2.5 mmol) was added to a cuvette containing 1 mL of toluene in an
inert atmosphere, and then saturated with ethylene gas (1.3 bar).
EASC (Al/Pd ¼ 300) was injected into the system to initiate oligo-
merization and the UVevisible spectra of the reaction mixture were
recorded over time.
2.5. Syntheses
2.5.1. Preparation of the ligands (L1eL3)
A methanolic solution of antipyrine-4-carboxaldehyde (0.432 g,
2 mmol) was added to a methanolic solution of 4,40-methylene bis-
(2,6-dimethyl aniline) (0.254 g, 1 mmol) in a 50 mL Schlenk flask
under a nitrogen atmosphere. A catalytic amount of formic acid was
added and the reaction mixture was kept at refluxing temperature
for 4 h, and then cooled to room temperature. A pale yellow solid
was separated by filtration and purified by column chromatography
using hexane/ethyl acetate as eluant (8:2 v/v). The product (L1) was
dried under vacuum at room temperature and stored in a vacuum
desiccator. The reactions involved in the preparation of the ligands
are outlined in Fig. 1. Yield: 46.3%. m.p.: 109e111 ꢁC. Elemental
analysis: Found (calculated) for C41H42N6O2: C 75.4 (75.7), H 6.9
2.3. Ethylene oligomerization
(6.5), N 12.1 (12.9). FTIR (KBr disc) cmꢀ1: 3384
ring), 2922 (CeH, alkyl), 1676 (>C]O, amide), 1599
imine), and 1076
(NeN, antipyrine ring). 1H NMR (CDCl3) ppm: 8.4
n
(NeC, antipyrine
(>C]N,
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 addition of the catalyst, the reactor was
charged with toluene (80 mL) via a syringe and immersed in a
constant temperature bath previously set to a desired temperature.
n
n
n
n
(s, 2H, azomethine protons), 7.8e7.4 (m, 10H, aromatic protons),
6.9 (s, 4H, aromatic protons), 3.6 (s, 2H, methylene spacer protons),
3.1 (s, 6H, antipyrine NeMe protons), 2.4 (s, 6H, antipyrine CeMe
protons), and 1.0 (s, 12H, methyl protons). 13C NMR (CDCl3) ppm:

S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
1889
Fig. 1. Preparation of sterically modulated ligands and Pd catalysts.
168 (>C]O amide), 149 (>C]N imine), 121, 119, 116, 109 (aromatic
region), 77 (antipyrine NeMe methyl), 53 (antipyrine CeMe
(2 mmol) in methanol (15 mL) over a period of 30 min. The reaction
mixture was refluxed for 4e5 h, cooled to room temperature, and
then the resultant mixture was precipitated in diethyl ether. The
filtered solids were washed with methanol and ether to remove
traces of metal salt and finally dried and stored under vacuum.
Yield: 79.7%. m.p.: >300 ꢁC. Elemental analysis: Found (calculated)
for C41H42N6O2Pd2Cl4: C 49.4 (49.0), H 4.7 (4.2), N 8.1 (8.4). FTIR
methyl), and 24 (methyl). UVevis (MeOH) nm: 278 (
pe
p* of C]N)
and 303 (nep* of N and O).
L2 was similarly prepared using 4,40-methylene bis-(2,6-diethyl
aniline) (0.31 g, 1 mmol). Yield: 41.4%. m.p.: 172e174 ꢁC. Elemental
analysis: Found (calculated) for C45H50N6O2: C 75.9 (76.5), H 7.4
(7.1), N 12.2 (11.9). FTIR (KBr disc) cmꢀ1: 3397
n
(NeC, antipyrine
(>C]O, amide), 1601 (>C]N,
(NeN, antipyrine ring). 1H NMR (CDCl3) ppm: 8.1
(KBr disc) cmꢀ1: w3380
alkyl), 1653 (>C]O, amide), 1610
antipyrine ring), and 422
n
(NeC, antipyrine ring), 2920
(>C]N, imine), 1070
n
(CeH,
(NeN,
ring), 2916
n
(CeH, alkyl), 1681
n
n
n
n
n
imine), and 1092
n
n
(PdeN). 1H NMR (CDCl3) ppm: 8.5 (s, 2H,
(s, 2H, azomethine protons), 7.6e7.0 (m, 10H, aromatic protons),
6.6 (s, 4H, aromatic protons), 3.9 (s, 2H, methylene spacer protons),
3.2 (s, 6H, antipyrine NeMe protons), 1.9 (s, 6H, antipyrine CeMe
protons), 1.4 (q, 8H, methylene protons), and 0.7 (t, 12H, methyl
protons). 13C NMR (CDCl3) ppm: 171 (>C]O amide), 148 (>C]N
imine), 124, 122, 116, 112 (aromatic region), 79 (antipyrine NeMe
methyl), 43 (antipyrine CeMe methyl), 40 (methylene), and 21
azomethine protons), 7.7e7.0 (m, 10H, aromatic protons), 6.9 (s, 4H,
aromatic protons), 4.1 (s, 2H, methylene spacer protons), 3.0 (s, 6H,
antipyrine NeMe protons), 2.4 (s, 6H, antipyrine CeMe protons),
and 0.9 (s, 12H, methyl protons). 13C NMR (CDCl3) ppm: 184 (>C]O
amide), 153 (>C]N imine), 120, 116, 111, 109 (aromatic region), 75
(antipyrine NeMe methyl), 50 (antipyrine CeMe methyl), and 24
(methyl). UVevis (MeOH) nm: 290 (pe
p* of C]N) and 310 (nep*
(methyl). UVevis (MeOH) nm: w270 (
p
e
p* of C]N) and w300
of N and O). FAB mass (m/z): 935 (Mþ ꢀ Cl).
(nep* of N and O).
Pd2Cl4L2: Yield: 79.7%. m.p.: >300 ꢁC. Elemental analysis: Found
(calculated) for C45H50N6O2Pd2Cl4: C 50.4 (50.9), H 4.9 (4.8), N 8.2
L3 was similarly prepared using 4,40-methylene bis-(2,6-diiso-
propyl aniline) (0.366 g, 1 mmol). Yield: 38.1%. m.p.: 198 ꢁC.
Elemental analysis: Found (calculated) for C49H58N6O2: C 77.6
(7.9). FTIR (KBr disc) cmꢀ1: w3390
n
(NeC, antipyrine ring), 2920
(>C]O, amide), 1610 (>C]N, imine), w1090
(NeN, antipyrine ring), and 434
(PdeN). 1H NMR (CDCl3) ppm:
n
(CeH, alkyl), 1658
n
n
(77.2), H 7.4 (7.7), N 11.6 (11.0). FTIR (KBr disc) cmꢀ1: 3371
n
(NeC,
(>C]O, amide), 1604
(NeN, antipyrine ring). 1H NMR (CDCl3)
n
n
antipyrine ring), 2920
(>C]N, imine), and 1047
n
(CeH, alkyl), 1680
n
n
n
8.0 (s, 2H, azomethine protons), 7.6e7.1 (m, 10H, aromatic protons),
6.8 (s, 4H, aromatic protons), 4.2 (s, 2H, methylene spacer protons),
3.1 (s, 6H, antipyrine NeMe protons), 1.9 (s, 6H, antipyrine CeMe
protons), 1.8 (q, 8H, methylene protons), and 0.9 (t, 12H, methyl
protons). 13C NMR (CDCl3) ppm: 180 (>C]O amide), 151 (>C]N
imine), 126, 122, 119, 111 (aromatic region), 82 (antipyrine NeMe
methyl), 42 (antipyrine CeMe methyl), 40 (methylene), and 27
ppm: 8.3 (s, 2H, azomethine protons), 7.8e7.1 (m, 10H, aromatic
protons), 6.8 (s, 4H, aromatic protons), 4.1 (s, 2H, methylene spacer
protons), 3.5 (s, 6H, antipyrine NeMe protons), 2.0 (s, 6H, antipy-
rine C-Me protons), 2.4 (sept, 4H, isopropyl protons) and, 0.8 (d,
24H, methyl protons). 13C NMR (CDCl3) ppm: 176 (>C]O amide),
152 (>C]N imine), 125, 121, 118, 110 (aromatic region), 79 (anti-
pyrine NeMe methyl), 48 (antipyrine CeMe methyl), 44 (iso-
propyl), 40 (methylene), and 31 (methyl). UVevis (MeOH) nm:
(methyl). UVevis (MeOH) nm: w290 (pe
p* of C]N) and 305
(nep* of N and O). FAB mass (m/z): 991 (Mþ ꢀ Cl).
Pd2Cl4L3: Yield: 79.7%. m.p.: >300 ꢁC. Elemental analysis: Found
(calculated) for C49H58N6O2Pd2Cl4: C 52.8 (52.3), H 5.4 (5.0), N 7.2
w275 (pe
p* of C]N) and w300 (nep* of N and O).
(7.6). FTIR (KBr disc) cmꢀ1: w3370
n
(NeC, antipyrine ring), 2922
(>C]O, amide),1611 (>C]N, imine), w1040
(NeN, antipyrine ring), and 436
(s, 2H, azomethine protons), 7.7e7.2 (m, 10H, aromatic protons), 6.9
n
2.5.2. Preparation of binuclear palladium complexes
(CeH, alkyl),1648
n
n
n
For the preparation of Pd2Cl4L1, a methanolic solution of PdCl2
(cod) (4 mmol) was added drop wise to a solution of L1 ligand
n
(PdeN). 1H NMR (CDCl3) ppm: 8.3

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S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
(s, 4H, aromatic protons), 4.2 (s, 2H, methylene spacer protons),
3.3 (s, 6H, antipyrine NeMe protons), 2.0 (s, 6H, antipyrine CeMe
protons), 2.4 (sept, 4H, isopropyl protons), and 0.9 (d, 24H, methyl
protons). 13C NMR (CDCl3) ppm: 183 (>C]O amide), 156 (>C]N
imine), 129, 120, 118, 103 (aromatic region), 72 (antipyrine NeMe
methyl), 43 (antipyrine CeMe methyl), 38 (isopropyl), 31 (methy-
can be ascribed to the tertiary nitrogen, alkyl CeH, diazine NeN,
and PdeN stretching frequencies, respectively.
Ligands and their Pd complexes have been characterized with
the aid of 1H NMR over a range of 0e14 ppm. An aldehydic proton of
antipyrine-4-carboxaldehyde can be observed as a singlet at
9.8 ppm. The peak is shifted up-field to around 8.2 ppm in ligands,
indicating the formation of azomethine linkage between the
anilines and antipyrine-4-carboxaldehyde. However, in the Pd
complex, the peak shifts down-field by 0.2e0.4 ppm, demon-
strating the involvement of azomethine nitrogen in the Pd coor-
dination. The decrease in electron density surrounding the
azomethine proton after lone-pair donation from nitrogen atom
transfers electron density to electropositive Pd center. This obser-
vation is also supported by 13C NMR spectra of the compounds, in
which azomethine carbon resonance shifts down-field in the
complexes. The involvement of the amide carbonyl in the coordi-
nation is again confirmed by 13C NMR spectra of the complexes,
where amide carbonyl carbon resonance shifts to a down-field
region. In addition, 1H and 13C NMR spectra of the compounds show
a set of peaks in the region 7e8, 0.7e4.2 and 104e145, 21e75 ppm
ascribed to the aromatic and aliphatic resonances, respectively. The
NMR studies allow us to propose a structure for the complexes
prepared. Pd complexes are NMR active only if they possess
a square-planar geometry, hence it is assumed that, the Pd
complexes synthesized possess such geometry with dsp2 hybrid-
ization. The four corners of the plane are covered by imine nitrogen,
amide oxygen and two labile chlorides in cis-fashion on either side
of the methylene spacer.
lene), and 24 (methyl). UVevis (MeOH) nm: w280 (
p
epþ* of C]N)
and w295 (nep* of N and O). FAB mass (m/z): 1047 (M ꢀ Cl).
2.5.3. Preparation of palladium complexes by in-situ method
The disconcertingly low yields of the ligands prompted us to
attempt to prepare the catalysts by an in-situ method. For example,
4,40-methylene bis-(2,6-dialkyl aniline) (1 mmol), antipyrine-4-
carboxaldehyde (2 mmol) and PdCl2 (cod) were added to 25 mL
glacial acetic acid in a 50 mL Schlenk flask under a nitrogen
atmosphere. The reaction mixture was refluxed over a period of
12 h and cooled to room temperature. The resultant mixture was
precipitated in diethyl ether. The filtered solids were washed with
methanol and ether to remove traces of metal salt and finally dried
and stored under vacuum. These catalysts were independently
characterized as previously described; the data obtained were
consistent with the structure of the catalysts prepared by the
conventional method. However, the yield of the catalysts was
considerably improved 30e40%.
3. Results and discussion
3.1. General characterizations and electrochemistry of the
synthesized compounds
NMR active Pd complexes with square-planar geometry, are
diamagnetic in nature [15e18], thus magnetic exchange interac-
tions through methylene spacer can be ruled-out. However, elec-
tronic interactions between both the Pd centers are possible as an
extended pi-electron conjugation can be found through the spacer.
These complexes may act as efficient ethylene oligomerization
catalysts due to these electronic interactions found in the form of
cooperative effects between Pd centers. The same effect cannot be
expected in their mononuclear counterparts.
All the synthesized compounds have been thoroughly charac-
terized by a wide range of spectro-analytical techniques to support
the proposed structures. Our failure to obtain single crystals of the
complexes (using different methods/solvents) may be attributed to
the high amorphous nature of the complexes. The ligands prepared
and their respective palladium complexes are NMR active, which
allows for accurate structural characterization despite the lack of
single-crystal data.
The ease in which the steric and electronic properties of the
complexes are adjusted is an important feature of the group-X
metal complexes when used as olefin polymerization catalysts
[11e13]. In the conventional synthesis of ligands, catalytic amounts
of formic acid are used to achieve an acidic pH in the reaction
mixture, which facilitates the reaction of the aldehyde. A series of
bis-anilines were used in order to tune the steric properties of the
ligands while maintaining the antipyrine core. The ligands have
been show to ligate through the imine (azomethine) nitrogen and
the amide carbonyl oxygen atoms to form stable six-membered
chelated rings. The ligand systems are designed in such a way that,
after coordination to Pd centers, loss of labile chlorides is pre-
vented. The aforementioned chloride elimination plays a significant
role in ethylene oligo/polymerizations due to replacement of said
anions with alkyl groups of aluminum co-catalysts. Dichloro Pd
centers were chosen in the present work, as these derivatives
possess dsp2 hybridization and are NMR active, allowing a deeper
insight into their structures.
The UVevisible spectra of the compounds were recorded at room
temperature in methanol solution at 10ꢀ3 M concentration. The
electronic spectra of the ligands show two distinct bands at 275 and
305 nm. These can be assigned to the intra ligand nep* and nep*
transitions of the amide carbonyl and azomethine functionalities.
These bands suffered red shifts of 10e15 nm in the corresponding Pd
complexes, indicating coordination of the azomethine nitrogen and
amide carbonyl. UVevisible spectra of the Pd complexes are char-
acterized by intense bands around 450 nm (3
-170 l cmꢀ1 molꢀ1),
assigned to the ded transitions of the PdII ion. This observation is
consistent with values for Pd complexes reported in literature
[15e18]. In case of Pd2Cl4L3, a weak band is observed around 415
(3
-140 l cmꢀ1 molꢀ1) nm corresponding to the ded transition of the
metal ion. The electronic and diamagnetic properties collectively
indicate a square-planar geometry around the Pd centers.
The FAB mass spectra of Pd2Cl4L1, Pd2Cl4L2, and Pd2Cl4L3 show
intense peaks for Mþ ꢀ Cl at m/z 935, 991, and 1047, respectively.
Apart from this, the spectra show other peaks at regular intervals
after the elimination of one and two chlorides at 36 and 72
molecular weight losses with exact isotopic patterns. Further
molecular fragmentation is consistent with various fragments of
the complexes, with the appropriate isotopes of both Pd and Cl. The
spectra indicate that all the Pd complexes are binuclear and
monomeric in nature.
All synthesized compounds were characterized by IR spectros-
copy. Selected IR absorption bands and their corresponding
assignments are given in the experimental section. Ligands
L1eL3show two distinct bands around 1680 and 1600 cmꢀ1
,
attributed to amide carbonyl and imine stretching frequencies,
respectively [14]. The spectra of the complexes, the amide and
imine bands underwent negative and positive shifts respectively,
indicating involvement in the coordination with Pd center. A few
other important bands at around 3380, 2920, 1080, and 430 cmꢀ1
The thermal degradation of the Pd complexes was studied in the
temperature range of 30e800 ꢁC under a nitrogen atmosphere. As
expected, there is no weight loss below 100 ꢁC, indicating the
absence of coordinated or lattice celled solvent molecules. Weight

S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
1891
Fig. 2. Cyclic voltammograms of Pd2Cl4L1 (a), Pd2Cl4L2 (b), and Pd2Cl4L3 (c) at different scan rates.
loss is observed in two significant steps for all the Pd complexes.
Weight losses of 14.1%, 13.6%, and 12.9% were observed for Pd2Cl4L1,
Pd2Cl4L2, and Pd2Cl4L3, respectively, at 230e265 ꢁC due to the
elimination of chlorine atoms. This was further supported by the
appearance of an exothermic peak at around 245 ꢁC in the DTA
signal. In the second step, 60.1%, 62.1%, and 62.9% weight loss was
observed for the said series of Pd catalysts, respectively, at
410e460 ꢁC, indicating the degradation of the ligand moiety.
Finally, the graph leveled off due to the formation of the stable Pd
oxide. The data obtained from thermal analysis can be correlated to
the proposed structures of the Pd complexes.
characterization of Pd complexes having different substituent
groups was carried out in dimethylformamide (DMF) solution at
a concentration of 10ꢀ3 M. Tetrabutylammonium perchlorate
(0.1 M) was used as supporting electrolyte and the scans were
recorded under a nitrogen atmosphere, which was created by
purging pure nitrogen gas through DMF solution. In the presence of
aluminum co-catalysts, immediate precipitation and high adsorp-
tion at the electrode surface occurred on applying the potential.
Thus, interpretation was difficult for the cyclic voltammograms
obtained under conditions similar to those during ethylene oligo-
merization. It is reported in the literature [20] that, for the
The electrochemical behavior of metal complexes containing
two or more chemically equivalent electro-active sites has been the
subject of a number of studies in olefin oligo/polymerizations [19].
Steric modulation around the Pd centers may change the orienta-
tion of the catalyst structure and hence the potential at the metal
center, which could severely impact oligomerization activity. CV
measurements allow us to monitor this effect. The electrochemical
reversible sequential transfer of two electrons, the DEp in the cyclic
voltammograms would be 42 mV. In contrast, cyclic voltammo-
grams for molecules with multiple, non-interacting redox centers
will be similar to those of the corresponding species with a single
centre and
DEp should be 58 mV [21]. However, the cyclic vol-
tammograms of the present Pd complexes show no indication of
mixed valence Pd ions.

1892
S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
Fig. 3. Proposed one electron transfer redox behavior of the Pd complexes.
The cyclic voltammograms of the free ligands and their Pd
isopropyl derivatized catalyst is higher than those of the other two
catalysts, since the reduced tetrahedral Pd centers may be the
active species for the conversion of ethylene into its oligomers [22].
complexes were scanned in the potential range of þ2000
to ꢀ2000 mV at different scan rates viz., 25, 50, 100, 150, 200, 250,
and 300 mV/s. Cyclic voltammograms of Pd2Cl4L1, Pd2Cl4L2, and
Pd2Cl4L3 are shown in Fig. 2aec. In order to clarify the uncertainty
regarding Pd versus ligand reduction, we examined the potentially
innocent ligands L1eL3 in the same potential range with same scan
rates, displaying no significant redox waves. This leads us to
conclude that in all cases the redox processes are Pd based.
3.2. Ethylene oligomerization
The Pd complexes prepared act as efficient ethylene oligomeri-
zation catalysts as illustrated by the oligomerization rate curves
(Fig. 4). The activity shown by Pd catalysts is explained in two
different aspects.
The use of multi-nucleating ligands as a means of imposing
close spatial confinement on multi-metal centers has been widely
documented for a variety of catalysts over a range of metal-medi-
ated processes [23e25]. For oligo/polymerization applications,
good catalytic performances have been observed and cooperative
effects by the neighboring active metal centers have also been
suggested [26]. Brookhart and coworkers reported that the cata-
Pd2Cl4L1 shows one electron transfer quasi-reversible redox
behavior at the reported ligand field of L1. The reduction potential of
Pd2Cl4L1 is in the range ꢀ296 to ꢀ405 mV and the corresponding
oxidation potential is in the range ꢀ498 to ꢀ198 mV over the
corresponding scan rates. The difference between cathodic and
anodic peak potentials
DEp is found w95 mV for all scan rates.
Similarly, the ratio of cathodic to anodic peak current (Ipa/Ipc) is
found to be less than one at all scan rates, hence the system is
ascribed to one electron transfer quasi-reversible process. Pd2Cl4L2
shows a reduction peak in the range ꢀ392 to ꢀ441 mV and the
corresponding oxidation potential in the range ꢀ355 to ꢀ309 mV at
lysts with only one substituent on the N-aryl rings for
a-diimine
ligands produce only oligomers [27]. Steric modulations in the
methylene spacer bridged ligands affects ethylene oligomerization
not only from the viewpoint of yield but also product distribution.
In case of the Brookhart catalyst, the group X metal is sufficiently
hindered by four isopropyl groups present at the 2,6-positions of
the anilines attached to a central acenapthaquinone core. This
different scan rates. Peak-to-peak potential difference DEp is found
at around 85 mV for all scan rates, however, the ratio of cathodic to
anodic peak current (Ipa/Ipc) is less than one, indicating one elec-
tron transfer quasi-reversible redox process for the system. Peak
potential difference is found to be less than the Pd2Cl4L1 system,
perhaps due to the presence of the more sterically hindered bulky
ethyl group.
Interestingly, Pd2Cl4L3 system shows one electron transfer
reversible redox behavior in the working potential range and
noticeably the redox peaks shifted to positive voltages in this
particular case. The reduction potential for this electro-active
system is shown in the range 1021e1037 mV and the corresponding
oxidation potential found in the range 961e982 mV. The difference
between cathodic and anodic peak potentials
DEp is found at
around 58 mV for all scan rates. Eventually, the ratio of cathodic to
anodic peak current (Ipa/Ipc) is almost one, indicating one electron
transfer reversible redox process for the system. The reported Pd
complexes bearing different alkyl groups on either side of the
anilines show different electrochemical behaviors, as a conse-
quence of increased steric hindrance as the size of the substituent
increases. The Pd complex with a methyl substitution shows more
peak-to-peak difference in potential whereas the ethyl substituted
complex shows less peak-to-peak difference. The isopropyl deriv-
ative lies almost in the range of a reversible process. This change in
the potential of the Pd centers disturbs the geometry around the
metal center (as shown in Fig. 3). The reduced Pd center assumes
a tetrahedral geometry and in the reverse scan, the oxidized centers
revert to the original square-planar geometry. As a result of this
change in geometry and oxidation sate of the Pd centers, significant
variations are expected in their ethylene oligomerization activity
and in the resulting oligomer distributions (vide infra). It is also
expected from the CV studies that the catalytic activity of the
Fig. 4. Plots of rate of ethylene consumption versus time of ethylene oligomerization
with sterically modulated catalysts: (a) Pd2Cl4L1 at 30 ꢁC, (b) Pd2Cl4L2 at 30 ꢁC, (c)
Pd2Cl4L3 at 30 ꢁC, (d) Pd2Cl4L1 at 50 ꢁC, (e) Pd2Cl4L2 at 50 ꢁC, and (f) Pd2Cl4L3 at 50 ꢁC.
Conditions: catalyst
¼
5
mmol; [EASC]/[Pd]
¼
300; toluene solvent
¼
80 mL;
PC
2 H4
¼ 1:3 bar; reaction time ¼ 30 min.

S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
1893
Table 1
Results of ethylene oligomerization with Pd2Cl4L1, Pd2Cl4L2, and Pd2Cl4L3 catalysts
combined with (EASC). Conditions: catalyst ¼ 5 mol; [EASC]/[Pd] ¼ 300; toluene
m
solvent ¼ 80 mL; PC
¼ 1:3 bar; reaction time ¼ 30 min.
2H4
Distribution of olefinsb (in %)
C4 -C4 C6 C8 C10 C12e20
331.2 72.9 96.1 13.6 4.6 5.3 3.6
806.8 75.3 78.8 7.3 7.3 8.8 1.3
1308.4 79.6 44.3 11.3 5.1 1.9 2.1
a
Entry Catalyst
Temp. Rp
(ꢁC)
a
1
2
3
4
5
6
7
8
Pd2Cl4L1 30
Pd2Cl4L2 30
Pd2Cl4L3 30
PdCl2L3
30
920.1 91.1 33.2
142.6 88.8 13.7
6.4 2.2 0.3 0.0
8.3 2.9 0.0 0.0
Pd2Cl4L1 50
Pd2Cl4L2 50
Pd2Cl4L3 50
263.6 87.8 14.3 10.1 2.1 0.0 0.0
431.6 81.3 12.1
118.4 92.7 12.0
9.3 3.3 1.8 4.3
6.6 0.7 0.0 0.0
PdCl2L3
50
a
Rate of polymerization in kg-oligomer mol-Pdꢀ1 barꢀ1 hꢀ1
Determined by GC using calibration curves with standard solutions.
.
b
maximum hindrance offered by the ligand system will not allow
the monomer units to enter from axial positions, by which chain
termination occurs after polymer formation. However, in the
present group X systems, a similar obstruction is offered by the
substituents present on di-anilines, whereas the antipyrine unit has
a planar phenyl ring, which cannot prohibit the axial monomer
insertion that results in the formation of desired oligomers.
In combination with EASC, all complexes selectively afforded
dimerization and trimerization of ethylene along with small
amounts of tetramerized product and trace amounts higher olefins.
The product distribution is summarized in Table 1. All the three Pd
complexes give high butene content as nearly 75% of the total
oligomer content. 96, 79, and 44% of total C4 content formed by
Pd2Cl4L1, Pd2Cl4L2, and Pd2Cl4L3, respectively, are found to be 1-
butene. Pd2Cl4L1 produces the greatest level of 1-butene. Aside
from C4 composition, 5%e10% of the total oligomers produced
consist of C6 and C8 products. Finally traces of higher olefin content
are also detected with all the Pd catalysts.
Yield of the ethylene oligomers and their distributions are also
given in Table 1. An increase in the temperature of oligomerization
increases to 50 ꢁC produced a sharp (3e4 fold) decrease in activity.
The decrease in activity is attributed to the deactivation of active
sites and the increase in the entropy of monomer units. At 50 ꢁC, the
amount of the C4 fraction increases to some extent but the selec-
tivity toward 1-butene decreases sharply, demonstrating the acti-
vation of various isomerization reactions at high temperature
together with oligomerization. The formation of higher olefins is
also reduced, as expected.
Fig. 5. 1H NMR spectra of the mixture of Pd2Cl4L1, EASC and ethylene, recorded at
different reaction periods.
However, important conclusions can be drawn from this spectral
observation, since the peaks assigned to Pd complex can be
observed, regardless of the complexity of the peaks. After 15 min,
the peak intensities are decreased nearly by five times, indicating
that the amount of unreacted Pd catalyst decreases as a function
time and interestingly, no additional peaks are observed. The
decrease of peak intensity in the presence of same amount of
mixture indicates the formation of an NMR inactive species due to
the change of the geometry of Pd complex. Most probably, this
“NMR unresponsive” compound is the active Pd species possessing
tetrahedral geometry. Presumably it can be formed by the reaction
of the Pd complex and EASC in the presence of ethylene monomer.
For example, a diethyl palladium species with tetrahedral geometry
having sp3 hybridization can be formed. The feasibility of the
formation of tetrahedral geometry can also be postulated from the
cyclic voltammetry results, especially for Pd2Cl4L1.
In order to collect more evidence on the formation of active sites,
UVevisible spectra of the Pd catalysts were recorded under ethylene
oligomerization conditions over the range of 1000 to 200 nm as
a function of time. The ded transitions originating from square-
planar Pd centers are found at around 450 nm (see Experimental).
Addition of EASC to the Pd catalyst in toluene, leads to a decrease in
ded transition and a new absorption at w800 nm is observed due to
the formation of the tetrahedral Pd species. If the square-planar
symmetry is maintained, the ded transition should be observed in
higher energy regions. On the other hand, in case of tetrahedral
intermediates, the plane of symmetry can be totally ignored. This
observation is consistent with previous studies [28,29].
The oligomerizations were also carried out with mononuclear
complex: PdCl2L3 in similar conditions. Comparing with the results
obtained by Pd2Cl4L3, it gives lower activity and produces lower
level of 1-butene. In addition, the activity decreases about 8 fold by
the increase of the temperature from 30 to 50 ꢁC. Although it is too
premature to conclude with the data collected here, all of these
results may be come from the synergistic effects of the binuclear
complexes.
3.3. Detection of active species
As all the Pd catalysts are NMR active, a deeper insight can be
gained as regards the nature of active species responsible for the
oligomerization. 1H NMR spectra of the catalysts were recorded
under ethylene oligomerization conditions over ꢀ2e14 ppm as
a function of time. The representative spectra of Pd2Cl4L1 are shown
in Fig. 5. It is rather difficult to obtain the spectrum of the reaction
mixture immediately after the addition of EASC, hence the first scan
is observed after 5 min. The spectrum is quite complicated due to
the vigorous reaction that occurs between the Pd catalyst and EASC.
4. Conclusions
The synthesis and results of spectroscopic and structural char-
acterizations of a new family of sterically modulated binuclear Pd
complexes suitable for ethylene oligomerization were presented.

1894
S. Budagumpi et al. / Journal of Organometallic Chemistry 696 (2011) 1887e1894
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)
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aniline) and antipyrine-4-carboxaldehyde in low yields; however,
all the Pd complexes were successfully prepared by a one-pot
method in good yield. The electrochemical study of the catalysts
using CV in DMF solution indicated the presence of single electron
transfer redox behavior, leading to a change in the geometry of the
Pd complex from square planar to tetrahedral and vice versa.
The change from the original NMR active square-planar geometry
to the NMR inactive tetrahedral geometry under the oligomeriza-
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Acknowledgments
This work was supported by grants-in-aid for the World Class
University Program (No. R32e2008e000e10174e0) and the
National Core Research Center Program from MEST (No. R15e
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