Ethylene oligomerizations by diazene bridged Ni(II) catalysts derived from pyrazole-scaffold-based binucleating ligands with alkyl and aryl pendant arms

Article abstract of DOI:10.1007/s10562-011-0618-y

The coordination and organometallic chemistry of a series of diazene (N₂)-bridged Ni(II) catalysts derived from pyrazole-scaffold-based ligands bearing alkyl and aryl pendent arms was investigated. Binucleating ligands were obtained as products of the condensation reaction between 3,5-dichloroformyl-1H-pyrazole and aliphatic/aromatic primary/secondary amines under anhydrous conditions. The Ni(II) catalysts were activated with ethyl aluminum sesquichloride (EASC) to oligomerize the ethylene mainly into C4, C6, C8, and C10 fractions with activities up to 1.2 and 0.5 × 10⁶ g (mol-Ni)^-1 bar^-1 h^-1 at 30 and 50 °C, respectively. All catalysts were found to be electrochemically active in the working potential range of -2 to +2 V. A change in the potential of Ni(II) was provoked by the N₄ donor bridging ligands, increasing the ethylene oligomerization activity.

Full text of DOI:10.1007/s10562-011-0618-y

Catal Lett (2011) 141:1219–1227
DOI 10.1007/s10562-011-0618-y
Ethylene Oligomerizations by Diazene Bridged Ni(II) Catalysts
Derived from Pyrazole-Scaffold-Based Binucleating Ligands
with Alkyl and Aryl Pendant Arms
•
Srinivasa Budagumpi Renjith P. Johnson
•
•
Hongsuk Suh Chang-Sik Ha Il Kim
•
Received: 10 March 2011 / Accepted: 26 April 2011 / Published online: 7 May 2011
Ó Springer Science+Business Media, LLC 2011
Abstract The coordination and organometallic chemistry
of a series of diazene (N2)-bridged Ni(II) catalysts derived
from pyrazole-scaffold-based ligands bearing alkyl and
aryl pendent arms was investigated. Binucleating ligands
were obtained as products of the condensation reaction
between 3,5-dichloroformyl-1H-pyrazole and aliphatic/
aromatic primary/secondary amines under anhydrous con-
ditions. The Ni(II) catalysts were activated with ethyl
aluminum sesquichloride (EASC) to oligomerize the eth-
ylene mainly into C4, C6, C8, and C10 fractions with
activities up to 1.2 and 0.5 9 106 g (mol-Ni)-1 bar-1 h-1
at 30 and 50 °C, respectively. All catalysts were found to
be electrochemically active in the working potential range
of -2 to ?2 V. A change in the potential of Ni(II) was
provoked by the N4 donor bridging ligands, increasing the
ethylene oligomerization activity.
Keywords Binuclear ꢀ Nickel complexes ꢀ Pyrazole
ligand ꢀ Oligomerization ꢀ Transition metal
1 Introduction
Catalysts containing two or more metal centers are fre-
quently employed in the hope of obtaining synergistic
effects of the two active metal centers and the possible
occurrence of tandem reactions promoted by these two
centers [1]. The activity of such compounds depends on the
nature of the ligand, the metal, the chelating atoms, and the
metal–metal distance, which affect the metal–metal inter-
actions internally. Compounds encompassing a bimetallic
core may demonstrate explicit reactivity patterns for the
conversion of olefin monomers into oligomers/polymers,
which arise from a subtle interplay between the metals and
the ligands. Currently, there is great interest in both bridged
and non-bridged bimetallic late-first-row transition-metal
coordination catalysts for the oligomerization/polymeriza-
tion of olefins with definite electronic and steric modula-
tions [2–11].
S. Budagumpi ꢀ R. P. Johnson ꢀ C.-S. Ha ꢀ I. Kim (&)
Department of Polymer Science and Engineering, The WCU
Center for Synthetic Polymer Bioconjugate Hybrid Materials,
Pusan National University, Busan 609-735, Republic of Korea
e-mail: ilkim@pusan.ac.kr
The catalytic oligomerization of ethylene is a major
source of higher olefins, which are versatile chemical
intermediates for a wide variety of industrial and consumer
products. In particular, nickel-based coordination com-
pounds are known to be effective catalysts for a number of
important industrial processes. For example, they are used
in the production of low-carbon a-olefins, which are used
as comonomers for the production of oil additives, fatty
acids, sulfur-free fuels, linear low-density polyethylenes,
plasticizers, and lubricants. The oxidation state of the
nickel ions appears to play a crucial role in the catalytic
performance in these reactions. For instance, in the ethyl-
ene oligomerization catalyzed by Ni-containing molecular
S. Budagumpi
e-mail: sinu@pusan.ac.kr
R. P. Johnson
e-mail: rpj@pusan.ac.kr
C.-S. Ha
e-mail: csha@pusan.ac.kr
H. Suh
Chemistry Institute for Functional Materials, Pusan National
University, Pusan 609-735, Korea
e-mail: hsuh@pusan.ac.kr
123

1220
S. Budagumpi et al.
sieves, it is considered that Ni2? ions in high-coordination
unsaturated environments are the active sites [12]. Con-
versely, there are many arguments supporting the idea that
low-valent nickel ions, most likely Ni1? species, are the
active sites for ethylene oligomerization [13–15].
recorded in a KBr (ligands)/CsI (catalysts) disc matrix using
a Shimadzu FTIR spectrometer (IRPrestige-21) in the range
4000–200 cm-1. The electronic spectra of the complexes
were recorded on a Shimadzu UV-1650PC spectropho-
tometer in the range 1000–200 nm. Cyclic voltammetric
studies were performed at room temperature in chloroform
under O2-free conditions by using a Kosentech Model
CV-104 (S. Korea). Fast atom bombardment mass spectra
(FAB-MS) were obtained from the Korea Basic Science
Institute on a JEOL JMS AX 505 spectrometer, operated at
3 kV accelerating voltage, equipped with a JEOL MS-FAB
10 D FAB gun operated at a 10 mA emission current, pro-
ducing a beam of 6 keV xenon. Nitrobenzyl alcohol was
used as matrix. Data acquisition and processing were
accomplished using JEOL Complement software. Ther-
mogravimetric (TG) and differential thermal analysis
(DTA) measurements of the catalysts were recorded on a
DuPont 951 thermogravimetric analyzer under a nitrogen
atmosphere, keeping the final temperature at 800 °C and
using a heating rate of 10 °C min-1. The oligomers were
analyzed by gas chromatography (HP6890), performed
using a J&W Scientific DB608 column (30 m 9 0.53 mm)
with a flame ionization detector (FID). The injector and
detector temperatures were kept constant at 250 °C. The
program set the initial temperature to 30 °C (held for 2 min)
and finishing temperature to 250 °C (held for 10 min), with
Apart from the oxidation state of the nickel ion, the
nature of the substituents attached to the ligand core and
the oligomerization temperature also play a significant role
in the formation of active sites, which is determined
by the interplay of catalysts and alkyl-aluminum-based
co-catalysts such as ethyl aluminum sesquichloride (EASC)
and methyl aluminoxane (MAO). Although it has been
reported that many Ni(II) and Co(II) complexes containing
N,O- and N,N-bidentate ligands are efficient olefin oligo-
merization/polymerization precatalysts, the catalytic per-
formance of complexes with a bimetallic Ni(II) core for
ethylene oligomerization has hardly been studied. Here, we
attempt to correlate the oligomerization activities of a series
of bridged binuclear Ni(II) catalysts bearing different alkyl
and aryl pendent arms as a function of temperature.
2 Experimental
2.1 Materials and Synthesis
All reactions were performed under a purified nitrogen
atmosphere using standard Schlenk techniques. Ethylene of
polymerization grade (SK Co., Korea) was purified by
passing it through columns of Fisher RIDOXTM catalyst
and the molecular sieves 5A and 13X. Organic solvents
(ethanol, methanol, and diethyl ether) were distilled over
CaH2 and stored over molecular sieves (4A). Toluene was
distilled over Na/benzophenone under nitrogen and stored
over molecular sieves (4A). All chemicals (1H-pyrazole-
3,5-dicarboxylic acid, thionyl chloride, nickel(II) bromide
ethylene glycol dimethyl ether (NiBr2ꢀDME), and all
amines used in the present study) 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.
a heating rate of 10 °C min-1
.
2.3 Ethylene Oligomerizations
Ethylene oligomerizations were performed at 1.3 bar in a
250-mL round-bottomed flask, kept at constant temperature
by a thermostat and 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 catalyst, the reactor was charged with tol-
uene (80 mL) by using a syringe, and immersed in a con-
stant-temperature bath that had previously been set to the
desired temperature. When the reactor had equilibrated
with the bath temperature, nitrogen gas was removed in
vacuum and ethylene was introduced into the reactor.
When no more absorption of ethylene into the toluene was
observed, the required amount of co-catalyst was injected
into the reactor, resulting in the start of oligomerization.
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 personal computer through an A/D converter. The olig-
omerization was quenched by the addition of methanol
containing HCl (5 vol.%) 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.
2.2 Physical Methods
1H and 13C NMR spectra were recorded in CDCl3 solvent
on a Varian Gemini-2000 300-MHz spectrometer at room
temperature using tetramethylsilane (TMS) as the internal
reference. Analytical thin-layer chromatography (TLC) was
carried out using aluminum plates precoated with Merck
0.25-mm silica gel 60F containing the fluorescent indicator
UV254. All ligands were purified by using a Combi-Flash
(Companion) auto-column machine. Elemental analysis
was carried out using a Vario EL analyzer. IR spectra were
123

Binuclear Catalysts for Ethylene Oligomerizations
2.4 Preparation of the Ligands
1221
2.4.1 Preparation of 1H-Pyrazole-3,5-
bis(isopropyl)carboxamide (HL1)
1H-Pyrazole-3,5-dicarboxylic acid (1.56 g, 0.01 mol) was
heated at reflux with thionyl chloride (10 mL) in a 50-mL
Schlenk flask for 4 h at 115 °C under a nitrogen atmo-
sphere. Excess thionyl chloride was doubly trapped under
reduced pressure in a Dewar flask kept in liquid nitrogen.
The resultant white pasty solid was cooled in an ice bath
for 15 min, and an ethanolic solution of isopropyl amine
(1.18 g, 0.02 mol) was then added and the mixture was
heated at reflux for a further 4 h, whereupon a clear col-
orless solution was produced. Diethyl ether was added to
the obtained reaction mixture, affording a colorless solid
(HL1), which was separated by filtration and purified using
hexane ethyl acetate mixture (7:3 v/v). The product was
dried in vacuum at room temperature and stored in a vac-
uum dessicator. The reactions involved in the preparation
of the ligands are outlined in Fig. 1. Yield: 68%. Elemental
analysis: Found (calculated) for C11H18N4O2: C 55.2
(55.4), H 7.5 (7.6), N 24.1 (23.5). FTIR (KBr disc) cm-1
:
Fig. 1 Synthetic route for the preparation of binucleating ligands
3320 m(N–H, amide, and pyrazole ring), 2920 m(C–H,
alkyl), 1643 m([C=O, amide), 1567 m([C=N, pyrazole
ring), and 1055 m(N–N, pyrazole ring). H NMR (CDCl3)
ppm: 9.2 (s, 1H, pyrazole ring –NH proton), 8.1 (s, 2H,
amide protons), 7.0 (s, 1H, pyrazole aromatic proton), 3.1
(sept, 2H, methylene proton), and 0.9 (d, 12H, methyl
proton). 13C NMR (CDCl3) ppm: 161 ([C=O, amide), 109,
116 (aromatic pyrazole), 47 (methylene), and 24 (methyl).
UV–Vis (CHCl3) nm: 300 (p–p* of C=N) and 311 (n–p* of
N and O).
1
2.4.3 Preparation of 1H-Pyrazole-3,5-bis(20,60-
dimethylaniline)carboxamide (HL3)
HL3 was prepared according to the procedure detailed in
Sect. 2.4.1, except that 2,6-dimethyl aniline (2.42 g,
0.02 mol) was added instead of isopropyl amine. Yield:
73.6%. Elemental analysis: Found (calculated) for
C21H22N4O2: C 69.2 (69.6), H 6.5 (6.1), N 14.6 (15.5).
FTIR (KBr disc) cm-1: &3330 m(N–H, amide and pyrazole
ring) 2922, 2879 m(C–H, alkyl and aryl), 1648 m([C=O,
amide), 1533 m([C=N, pyrazole ring), and 1044 m(N–N,
2.4.2 Preparation of 1H-Pyrazole-3,5-
bis(diisopropyl)carboxamide (HL2)
1
pyrazole ring). H NMR (CDCl3) ppm: 10.2 (s, 1H, pyra-
HL2 was prepared according to the procedure detailed in
Sect. 2.4.1, except that diisopropyl amine (2.02 g,
0.02 mol) was added instead of isopropyl amine. Yield:
61%. Elemental analysis: Found (calculated) for
C17H30N4O2: C 63.8 (63.3), H 10.0 (9.4), N 17.1 (17.4).
FTIR (KBr disc) cm-1: &3300 m(N–H, pyrazole ring),
2922 m(C–H, alkyl), 1647 m([C=O, amide), 1491 m([C=N,
pyrazole ring), and &1055 m(N–N, pyrazole ring). 1H
NMR (CDCl3) ppm: 11.5 (s, 1H, pyrazole ring –NH pro-
ton), 7.9 (s, 1H, pyrazole aromatic proton), 3.3 (sept, 2H,
methylene proton), and 1.4 (d, 24H, methyl proton). 13C
NMR (CDCl3) ppm: 178 ([C=O, amide), 109, 100 (aro-
matic pyrazole), 62 (methylene), and 45 (methyl). UV–Vis
(CHCl3) nm: 304 (p–p* of C=N) and 312 (n–p* of N
and O).
zole ring –NH proton), 9.6 (s, 2H, amide protons), 7.1
(s, 1H, pyrazole aromatic proton), 6.7–7.0 (m, 6H, aro-
matic), and 2.4 (s, 12H, methyl). 13C NMR (CDCl3) ppm:
138 ([C=O, amide), 131, 129, 118 (aniline aromatic
region), 102, 105 (pyrazole aromatic region), and 20
(methyl). UV–Vis (CHCl3) nm: 291 (p–p* of C=N) and
319 (n–p* of N and O).
2.4.4 Preparation of 1H-Pyrazole-3,5-bis(20,60-
diethylaniline)carboxamide (HL4)
HL4 was prepared according to the procedure detailed in
Sect. 2.4.1, except that 2,6-diethyl aniline (2.98 g, 0.02 mol)
was added instead of isopropyl amine. Yield: 72.1%. Ele-
mental analysis: Found (calculated) for C25H30N4O2: C 71.2
123

1222
S. Budagumpi et al.
(71.7), H 8.3 (7.2), N 13.1 (13.4). FTIR (KBr disc) cm-1
:
Ni2L2Br3: Yield: 73.2%. Elemental analysis: Found
(calculated) for C17H29N4O2Ni2Br3: C 30.5 (30.1), H 4.6
(4.3), N 8.6 (8.3). FTIR (KBr disc) cm-1: 3177 m(N–H,
amide), 3020 m(C–H, alkyl), 1647 m([C=O, amide), 1420
m([C=N, pyrazole ring), 1410 m(N–N, pyrazole ring), and
482 m(M–N). UV–Vis (Toluene) nm: 308 (p–p* of C=N),
&320 (n–p* of N and O), and &470 (d–d). FAB mass
spectrum, m/z 597 [M? –Br], 322 [M? –Ni2Br3].
Ni2L3Br3: Yield: 73.2%. Elemental analysis: Found
(calculated) for C21H21N4O2Ni2Br3: C 35.7 (35.1), H 3.2
(3.0), N 7.6 (7.8). FTIR (KBr disc) cm-1: 3190 m(N–H,
amide), 3006 m(C–H, alkyl and aryl), 1638 m([C=O,
amide), 1434 ([C=N, pyrazole ring), 1391 m(N–N, pyra-
zole ring), and 456 m(M–N). UV–Vis (Toluene) nm: 300
(p–p* of C=N), 315 (n–p* of N and O), and &460 (d–d).
FAB mass spectrum, m/z 637 [M? –Br], 477 [M? –3Br],
362 [M? –Ni2Br3].
Ni2L4Br3: Yield: 73.2%. Elemental analysis: Found
(calculated) for C25H29N4O2Ni2Br3: C 40.1 (38.8), H 3.5
(3.8), N 7.6 (7.2). FTIR (KBr disc) cm-1: 3177 m(N–H,
amide), 3012 m(C–H, alkyl and aryl), 1637 m([C=O,
amide), 1417 ([C=N, pyrazole ring), 1413 m(N–N, pyra-
zole ring), and 455 m(M–N). UV–Vis (Toluene) nm: &325
(p–p* of C=N and n–p* of N and O), and &460 (d–d).
FAB mass spectrum, m/z 693 [M? –Br], 534 [M? –3Br],
418 [M? –Ni2Br3].
Ni2L5Br3: Yield: 73.2%. Elemental analysis: Found
(calculated) for C29H37N4O2Ni2Br3: C 41.2 (41.9), H 4.5
(4.5), N 6.6 (6.7). FTIR (KBr disc) cm-1: 3200 m(N–H,
amide), 2926 m(C–H, alkyl and aryl), 1642 m([C=O,
amide), 1472 ([C=N, pyrazole ring), 1355 m(N–N, pyra-
zole ring), and 471 m(M–N). UV–Vis (Toluene) nm: 312
(p–p* of C=N), &320 (n–p* of N and O), and &470
(d–d). FAB mass spectrum, m/z 751 [M? –Br], 590
[M? –3Br], 474 [M? –Ni2Br3].
3387 m(N–H, amide and pyrazole ring), 2916, 2881 m(C–H,
alkyl and aryl), 1635 m([C=O, amide), 1458 m([C=N, pyr-
azole ring), and 1047 m(N–N, pyrazole ring). 1H NMR
(CDCl3) ppm: 10.1 (s, 1H, pyrazole ring –NH proton), 7.9
(s, 2H, amide protons), 7.3 (s, 1H, pyrazole aromatic pro-
ton), 6.9–7.2 (m, aromatic protons), 2.9 (q, 4H, methylene),
and 1.1 (t, 6H, methyl). 13C NMR (CDCl3) ppm: 158
([C=O, amide), 132, 128, 121 (aniline aromatic region),
100, 97 (pyrazole aromatic region), 36 (methylene), and 23
(methyl). UV–Vis (CHCl3) nm: 303 (p–p* of C=N) and 311
(n–p* of N and O).
2.4.5 Preparation of 1H-Pyrazole-3,5-bis(20,60-
diisopropylaniline)carboxamide (HL5)
HL5 was prepared according to the procedure detailed in
Sect. 2.4.1, except that 2,6-diisopropyl aniline (3.54 g,
0.02 mol) was added instead of isopropyl amine. Yield:
58.7%. Elemental analysis: Found (calculated) for
C29H38N4O2: C 71.7 (73.4), H 8.5 (8.1), N 9.9 (11.8). FTIR
(KBr disc) cm-1: 3375 m(N–H, amide and pyrazole ring),
2935, 2889 m(C–H, alkyl and aryl), 1645 m([C=O, amide),
1477 m([C=N, pyrazole ring), and 1052 m(N–N, pyrazole
1
ring). H NMR (CDCl3) ppm: 11.9 (s, 1H, pyrazole ring
–NH proton), 11.2 (s, 2H, amide protons), 7.8 (s, 1H,
pyrazole aromatic proton), 7.2–7.6 (m, aromatic protons),
3.1 (sept, 4H, methylene), and 1.1 (d, 24H, methyl). 13C
NMR (CDCl3) ppm: 112 ([C=O, amide), 106, 103, 100
(aniline aromatic region), 94, 91 (pyrazole aromatic
region), 44 (methylene), and 11 (methyl). UV–Vis (CHCl3)
nm: 297 (p–p* of C=N) and 306 (n–p* of N and O).
2.5 Preparation of the Catalysts
For preparing a representative catalyst (Ni2L1Br3), a solu-
tion of ligand (2 mmol) in methanol (15 mL) was stirred
for 30 min, after which a methanolic solution of NiBr2
ethylene glycol dimethyl ether (NiBr2ꢀDME) (4 mmol) was
added dropwise over a period of 30 min. The reaction
mixture was heated at reflux for 8 h and then cooled to
room temperature; the resultant mixture was precipitated in
ether and filtered. The filtered solids were washed with
methanol and ether to remove traces of metal salt, and they
were finally dried and stored in vacuum. Yield: 73.2%.
Elemental analysis: Found (calculated) for C11H17N4O2-
Ni2Br3: C 23.3 (22.2), H 2.5 (2.9), N 11.1 (9.4). FTIR (KBr
disc) cm-1: 3397 m(N–H, amide), 2986 m(C–H, alkyl), 1648
m([C=O, amide), 1528 m([C=N, pyrazole ring), 1405
m(N–N, pyrazole ring), and 469 m(M–N). UV–Vis (Tolu-
ene) nm: 302 (p–p* of C=N), 310 (n–p* of N and O), and
&460 (d–d). FAB mass spectrum, m/z 513 [M? –Br], 353
[M? –3Br], 238 [M? –Ni2Br3].
3 Results and Discussion
3.1 General Characterization and Electrochemistry
of Ligands and Catalysts
The elemental analyses of all the compounds are consistent
with the proposed structures with 2:1 metal–ligand stoi-
chiometry (Fig. 2). All the synthesized compounds are
insoluble in water, sparingly soluble in common organic
solvents such as hexane, pentane, toluene, and dioxane, and
completely soluble in THF, CHCl3, DMF, and DMSO.
Except for HL1, all the compounds are stable and non-
hygroscopic at room temperature. Ligand HL1 is hygro-
scopic and handled with care in the reactions.
On the basis of a comparison of the IR spectra of the
ligands and respective Ni(II) catalysts, it is possible to
123

Binuclear Catalysts for Ethylene Oligomerizations
1223
Fig. 2 Proposed structures of
the complexes
determine the coordination mode of the ligands. At first
glance, all the ligands show characteristic medium-inten-
(N–H) in the coordination. All the Ni(II) catalyst spectra
show a d–d transition band in the region 450–515 nm,
indicating that the catalysts are four-coordinated Ni(II)
systems [21, 22].
sity bands in the regions 1643–1651 and 3320–3350 cm-1
,
which are attributed to amide carbonyl and N–H stretching
frequencies, respectively. Upon metallation, the former
band is unchanged, whereas the latter is shifted to higher
energy by 20–25 cm-1, indicating the involvement of the
–NH groups present in the side arms without deprotona-
tion. Another point about the IR spectra that deserves
comment is the appearance of pyrazole ring vibrations in
the region 1567–1574 cm-1. However, this particular band
shifted to a lower-energy region in the catalyst spectra,
suggesting the classical endogenous diazine-bridging of the
heterocyclic ring system [16, 17]. Two non-ligand bands
appeared in all the spectra of the Ni(II) catalysts at around
450–485 and 275 cm-1, because these arise purely from
Ni–N and Ni–Br stretching modes [18].
Thermogravimetric analysis was carried out on all five
catalysts under a nitrogen atmosphere at a heating rate of
10 °C min-1. All the catalysts exhibit similar thermal
behavior. The TGA displays two obvious steps in the
weight loss of the catalysts. The first corresponds to the
loss of three bromide ions (two coordinated and one
bridged) as HBr in the range 220–275 °C (the weight loss
observed is consistent with the theoretical values). The
same is further confirmed by an exothermic DTA signal at
around 255 °C in all cases. The second weight loss occurs
above 500 °C, and is attributed to the decomposition of the
organic part of the catalyst. In the case of Ni2L3Br3, an
additional weight loss is observed in the range 90–110 °C,
which is attributed to the presence of two crystal-held
water molecules. Except for Ni2L3Br3, all the catalysts
appear to have excellent thermal stability, as no clear
weight loss is observed below 220 °C.
The acquisition of meaningful 1H and 13C NMR spectra
of the ligands supports their respective structures. All the
ligands show distinguished singlets in their 1H NMR
spectra in the regions 9.3–11.2 and 8.5–9.8 ppm, which are
ascribed to the resonance of the pyrazole ring –NH and
amide protons of the ligands, respectively [19, 20]. The
aromatic and methyl/ethyl/isopropyl signals of all the
The molecular weight of the catalysts produced by each
ligand was analyzed by FAB mass spectrometry, and the
results were found to be consistent with the mass of the
catalysts. Furthermore, the fragmentation pattern of
the catalysts initially followed the sequential elimination of
bromides, then the ligand part.
ligands are found in comparable ranges of their 1H and 13
C
NMR spectra.
The UV–Vis spectra of the free ligands show two sharp
discernible bands in the ranges 291–304 and 306–319 nm;
these absorptions are consistent with p–p* and n–p*
transitions arising from the[C=N and N–H functionalities,
respectively [21, 22]. The Ni(II) derivatives were also
subjected to UV–Vis absorption studies; however, due to
their poor solubility in toluene, it was not possible to cal-
culate their molar extension coefficients, so instead,
absorption maxima are given at the end of the character-
ization part for each catalyst. Upon metallation, these
bands showed a red shift, indicating the involvement of
pyrazolyl diazine ([C=N) and amide nitrogen atoms
A number of studies have focused on the electrochem-
ical behavior of metal complexes containing two or more
chemically equivalent electroactive sites. For the reversible
sequential transfer of two electrons, DEp in the cyclic
voltammograms (CVs) would be 42 mV [23]. In contrast,
cyclic voltammograms of molecules with multiple, non-
interacting redox centers would be similar to those of the
corresponding species with a single center, and DEp should
be 58 mV [24]. However, the cyclic voltammograms of the
present catalysts show no indication of mixed-valence
metals. Representative cyclic voltammograms of Ni2L4Br3
123

1224
S. Budagumpi et al.
Oxidation: NiðIIÞ; NiðIIÞ ꢁ eꢁ ! NiðIIIÞ; NiðIIÞ
in DMF solutions are shown in Fig. 3. The CVs of the free
ligands and their Ni(II) complexes were measured, scan-
ning between -2 and ?2 V at scan rates of 25, 50, 100,
150, 200, 250, and 300 mV min-1. Under similar experi-
mental conditions, electrochemical studies of the free
ligand systems using cyclic voltammetry showed no elec-
trochemical activity within the scanned potential window.
Therefore, the electrochemical waves observed in the
cyclic voltammograms of the catalysts are due purely to the
metal.
ꢁ eꢁ ! NiðIIIÞ; NiðIIIÞ
Reduction: NiðIIIÞ; NiðIIIÞ þ eꢁ ! NiðIIÞ; NiðIIIÞ
+ eꢁ ! NiðIIÞ; NiðIIÞ
Although the chemical environments around the two
Ni(II) ions are the same, the overall two-electron exchanges
appear as separate one-electron processes, indicating that
oxidation/reduction at one site has a steric and electronic
influence on the other site, and hence induces a positive/
negative shift in the oxidation/reduction potential of the
metal ion. 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.
The CVs of all the catalysts show one-electron quasi-
reversible redox behavior for each Ni(II) ion. The oxidation
potentials for the Ni(II) catalysts are in the range 50–150
and 950–1100 mV at a scan rate of 25 mV s-1, and shift to
higher potentials as the scan rate is increased. Corre-
spondingly, two reduction waves are observed in the
reverse scan, which can be attributed to the reduction of
one Ni(III) per wave. The reduction potentials are in the
region 100–250 and 950–1050 mV at a scan rate of
25 mV s-1, and shift towards more negative potentials
with increasing scan rate. Overall, the difference between
the anodic and cathodic peak potentials (DEp) is more than
58 mV; hence, the redox system is designated as a one-
electron quasi-reversible process for each Ni(II) ion. The
redox processes are not coupled with any chemical reac-
tion; this is evident from the E1/2 value that is constant for
different scan rates in all the catalyst voltammograms. The
oxidation and corresponding reduction reactions obey the
diagnostic criteria, i.e., Ipc/Ipa is almost constant but not
unity for a quasi-reversible electron transfer [25]. Assign-
ments for the stepwise nickel-based redox processes of the
catalysts are shown below.
3.2 Analysis of Active Species by the UV–Vis Spectral
Technique
Because structurally diverse active species or aluminum
intermediates were formed, we contemplated which struc-
tural components might be essential for an active Ni(II)
species in the olefin oligomerizations in the presence of
highly active alkyl aluminum compounds, i.e., EASC. We
conducted a thought experiment to identify the Ni(II)
active species formed when catalysts interact with a
pyrophoric alkyl aluminum compound and those that might
be required for a compound to show oligomerization
activity and to permit the formation of ethylene oligomers.
Absorption spectra of all the Ni(II) catalysts were obtained
in the presence of the co-catalyst EASC as a function of
Fig. 4 UV–Vis spectra of Ni2L5Br3 in presence of EASC (Ni/Al =
300) and ethylene monomer as a function of time
Fig. 3 Cyclic voltammograms of Ni2L4Br3 catalyst at different scan
rates
123

Binuclear Catalysts for Ethylene Oligomerizations
1225
Table 1 Results of ethylene oligomerizations by Ni2L1–5Br3 cata-
lysts combined with ethyl aluminum sesquichloride (EASC)
time under the conditions for ethylene oligomerization;
scans were taken before the addition of EASC and every
5 min after EASC addition for 30 min.
Entry Catalyst Temp. Rp 9 10-6 Distribution of olefinsb
a
(°C)
(%)
In the visible range, the Ni(II) catalyst absorption is very
weak between 450 and 485 nm because of its poor solu-
bility in toluene. This could be attributed to the four-
coordinated Ni(II) ion under the present ligand field. As
soon as EASC is added to the toluene solution of the
sparingly soluble Ni(II) catalyst, a vigorous reaction
commences and the solution becomes homogeneous. The
addition of EASC in an appropriate molecular ratio to the
catalyst induces a new d–d transition band; this shifts to
lower energy with the formation of a four-coordinated
Ni(II) species. A regular shift to lower energy is observed
in all the Ni(II) catalysts because of the formation of the
ethyl nickel intermediate, in which the ethyl group induces
an electropositive character on the metal ion. Between 485
and 515 nm the absorption increases as a function of time,
leading to absorption maxima, because of the formation of
a greater number of ethyl substituted intermediates as time
progresses. Correspondingly, the maxima due to p–p* and
n–p* transitions also suffer a gradual shift to lower ener-
gies because of the formation of ethyl-substituted inter-
mediates. It is pertinent to note the green–black coloration
of the reaction medium, followed by the precipitation of
black solid particles, and a decrease in the intensity of the
band at 485–515 nm. These changes are accompanied by a
rapid deactivation of the catalytic system [26–28]. Simul-
taneously, a band appears in the region 700–720 nm, which
is due to the formation of the active species. The UV–Vis
spectra of Ni2L5Br3 in the presence of EASC (Ni/Al 300:1)
and ethylene as a function of time are shown in Fig. 4. On
the basis of this, it can be assumed that the four-coordi-
nated active species is formed during the oligomerization,
and is responsible for the formation of the obtained olig-
omers with defined yield.
C4 a-C4 C6 C8 C10
1
2
3
4
5
6
7
8
9
10
Ni2L1Br3 30
0.66
0.75
1.02
1.22
1.24
0.32
0.40
0.48
0.49
0.45
95.9 2.0
79.9 50.8
4.1 0.0 0.0
7.4 5.7 7.0
Ni2L2Br3 30
Ni2L3Br3 30
Ni2L4Br3 30
Ni2L5Br3 30
Ni2L1Br3 50
Ni2L2Br3 50
Ni2L3Br3 50
Ni2L4Br3 50
Ni2L5Br3 50
44.3 0.0 50.3 3.9 1.5
68.1 13.4 25.4 4.4 2.0
78.5 12.8 11.3 6.1 4.0
79.6 18.9 12.4 4.3 3.7
77.1 78.1 17.2 3.0 2.7
93.3 8.8
93.2 13.3
88.3 3.6
0.8 3.4 2.5
2.0 1.9 2.9
3.7 4.1 3.9
Conditions:catalyst = 5 lmol, [EASC]/[Ni] = 300, toluene = 80 mL,
PC H = 1.3 bar, and reaction time = 30 min
2
4
a
Rate of polymerization over
(mol-Ni)-1 bar-1 h-1
a polymerization period in g
b
Determined by GC analysis
All our attempts to carry out similar characterizations of
the Ni(II) active species formed during oligomerization
using cyclic voltammetric techniques were unsuccessful
because of the deposition of solid samples on the electrodes.
3.3 Ethylene Oligomerizations
The catalyst activity is comparable to some Ni(II) coordi-
nation catalysts with N-donor ligands under similar con-
ditions. Previously, Brookhart reported that catalysts with
only one substituent on the N-ring for the a-diimine do not
yield polymers but instead produce oligomers [29]. The
products are mainly butenes and hexenes, with small/
Fig. 5 Ethylene consumption
rate curves of Ni2L1-5Br3
catalysts at 30 °C (a) and 50 °C
(b). Oligomerization conditions:
catalyst = 5 lmol, [EASC]/
[Ni] = 300, toluene = 80 mL,
PC H = 1.3 bar, and reaction
2
4
time = 30 min
123

1226
S. Budagumpi et al.
moderate amounts of higher oligomers up to C10. Similar
outcomes have been reported in many studies involving
atom-bridged active bimetallic cores for olefin oligomer-
izations [3, 30, 31]. The oligomer distribution pattern is
consistent with a nickel hydride species being the active
center, where fast elimination of b-hydride limits the
products to mostly butenes and hexanes; however, in a few
cases higher olefins are also observed in trace amounts. For
these binuclear nickel systems, a coordination–insertion
mechanism seems likely, followed by b-hydride elimina-
tion (chain transfer) [29].
way to produce the desired products. The effects of various
steric substituents on the catalysts’ activities were investi-
gated by simple ortho substitution of the aromatic amine
ligand. The overall activities of all the catalysts at 30 and
50 °C were compared (Table 1). The catalysts oligomerize
the ethylene into C4, C6, C8, and C10 fractions with
activities up to 1.2 and 0.5 9 106 g(mol-Ni)-1 bar-1 h-1 at
30 and 50 °C, respectively (Fig. 5a, b). The mechanism of
oligomerization is depicted in Fig. 6. When chain transfer
occurs from species V/VI, only a-olefins can be produced.
Internal olefins (2-alkenes) can be produced only when
chain transfer occurs from secondary alkyl agnostic species
VII after consuming an ethylene molecule [32].
After combination of the Ni(II) catalysts with EASC, we
found them all to be highly active in ethylene oligomer-
izations under the given conditions (Fig. 5; Table 1). In
order to increase the oligomerization activity of the cata-
lysts, the ligand design was tuned sterically in a significant
Catalyst systems composed of electron-deficient nickel
centers such as Ni2L2Br3 (compared with other aliphatic
amine ligands) and Ni2L5Br3 (compared with other
Fig. 6 Proposed mechanism for
ethylene oligomerization by
diazene bridged Ni(II) catalysts
combined with EASC
123

Binuclear Catalysts for Ethylene Oligomerizations
1227
Acknowledgment We thank the World Class University Program
(No. R32-2008-000-10174-0) and Brain Korea-21 (BK-21) for grants
that supported this work.
aromatic amine ligands) were found to be the most active,
while catalyst activity was found to be lower in relatively
electron-rich metal centers such as Ni2L1Br3, Ni2L3Br3,
and Ni2L4Br3. However, the catalyst Ni2L2Br3 showed
remarkable activity towards the conversion of ethylene,
especially to the a-C4 fraction, and produced 50.84 and
76.44% of the a-C4 fraction out of its 751.4 and
403.5 kg mol-1 yields at 30 and 50 °C, respectively. The
catalysts Ni2L3Br3 and Ni2L4Br3 revealed that the reduc-
tion of steric methyl/ethyl at the ortho-aryl position resul-
ted in a decrease in their activities, and the steric
environment around the central metal affected their cata-
lytic activities because of the relatively poorer electron-
donor methyl/ethyl substituents compared to the isopropyl
group. In the case of the catalysts derived from aliphatic
amines, Ni2L2Br3 bears two additional isopropyl groups on
either side of the tetratopic ligand, which increase the
electron deficiency on the nickel center, ultimately causing
a higher yield of oligomers at both the temperatures used.
These results are rather better than those reported for
bridged binuclear catalysts in the literature [3].
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In this article, we have presented the synthesis and spec-
troscopic and structural characterization of a new family of
pyrazole-scaffold-based binuclear Ni(II) complexes suit-
able for ethylene oligomerizations into C4, C6, C8, and C10
fractions, with activities up to 1.2 and 0.5 9 106 g(mol-
Ni)-1 bar-1 h-1 at 30 and 50 °C, respectively. The ligands
in the present study were synthesized by the action of
3,5-dichloroformyl-1H-pyrazole on aliphatic and aromatic
primary/secondary amines under anhydrous conditions.
Indeed, N2-coordination of the heterocyclic ring to the
Ni(II) ions offers an exceptional opportunity to develop and
increase the scope of diazine-bridged binuclear Ni(II)
complexes. Moreover, it has been shown that heterocyclic
diazine-bridged Ni(II) complexes can also serve as ethylene
oligomerizing agents, in which the two metal centers are
connected by a pyrazolyl-diazine module. The electro-
chemical study of the catalysts using cyclic voltammetry in
dimethyl formamide at a concentration of 10-3 M indicated
the quasi-reversible one-electron redox behavior of each
Ni(II) ion. It is assumed that the change in the potential of
Ni(II) is provoked by the bridging ligands, increasing the
ethylene oligomerization activity.
31. Jie S, Zhang D, Zhang T, Sun W-H, Chen J, Ren Q, Liu D, Zheng
G, Chen W (2005) J Organomet Chem 690:1739
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123