Synthesis and characterization of ether-imine-furfural [ONO] nickel(II) complexes and their application in oligomerization of ethylene

Article abstract of DOI:10.1016/j.apcata.2016.06.019

A series of new Ni(II) complexes of general formula {NiCl₂(L)}₂ [Ni1, L?=?5-methyl-2-(C₄H₃[Formula presented])C₂H₄OPh; Ni2, L?=?5-Methyl-2-(C₄H₃[Formula presented])Ph-

Full text of DOI:10.1016/j.apcata.2016.06.019

Applied Catalysis A: General 523 (2016) 247–254
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis and characterization of ether-imine-furfural [ONO]
nickel(II) complexes and their application in oligomerization of
ethylene
J.L.S. Milani, A.C.A. Casagrande, P.R. Livotto, Osvaldo L. Casagrande Jr.∗
Laboratory of Molecular Catalysis, Instituto de Química, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonc¸ alves, 9500, RS, 90501-970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2016
Received in revised form 23 May 2016
Accepted 12 June 2016
Available online 14 June 2016
A
series of new Ni(II) complexes of general formula {NiCl2(L)}2 [Ni1, L = 5-methyl-2-(C4H3O-2-
CH N)C2H4OPh; Ni2, L = 5-Methyl-2-(C4H3O-2-CH N)Ph-2-OPh; Ni3, L = 2-(C4H3O-2-CH N)Ph-2-OPh;
Ni4, L = 5-Methyl-2-(C4H3O-2-CH N)CH2Ph-2-OMe were prepared and characterized by IR spectroscopy,
elemental analysis, high-resolution mass spectrometry (HRMS), and X-ray photoelectron spectroscopy
(XPS). XPS data suggest no interaction of the pendant ether/furfural donor groups with the nickel
metal center. Density functional theory studies indicate the formation of nickel dimeric species
with ether-imine-furfural acting as a monodentate ligand. All nickel precatalysts, activated with
polymethylaluminoxane-improved performance (PMAO-IP), exhibited moderate to good activities for
ethylene oligomerization [TOF = 14.7–57.3 × 103 mol(ethylene) mol(Ni)−1 h−1] with high selectivities for
production of 1-butene (63.2–83.2 wt.%). Activation of Ni2 using different types of cocatalysts (PMAO-
IP, DMAO, MAO or EASC) led to formation of active oligomerization systems with a significant impact
on the activity and selectivity. The use of MAO instead of PMAO-IP gave a slight improvement in
the TOF [69.3 × 103 mol(ethylene) mol(Ni)−1 h−1] with lower selectivity for 1-butene. Higher activity
was obtained using EASC as cocatalyst [206.1 × 103 mol(ethylene) mol(Ni)−1 h−1] along with a drastic
reduction in the selectivity for 1-butene (10.4 wt.%). Under optimized conditions [[Ni] = 5 ␮mol, 30 ◦C,
oligomerization time = 20 min, 20 bar ethylene, MAO as cocatalyst (500 equiv)], precatalyst Ni2 led to
TOF = 63.2 × 103 mol(C2H4)·(mol(Ni)−1 h−1) and 75.2 wt.% selectivity for 1-butene.
Keywords:
Nickel catalysts
Ethylene oligomerization
1-Butene
X-ray photoelectron spectroscopy
DFT calculations
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
mainly by a growing appreciation of the role played by the mul-
tidentate ancillary ligand in influencing catalytic performance.
The oligomerization of ethylene is one of the most important
industrial processes to obtain linear ␣-olefins (LAOs), which are
important commodity chemical for preparing detergents, lubri-
cants, plasticizers, and oil field chemicals, or used as co-monomers
(C4–C8) for the production of linear low-density polyethylene
(LLDPE) [1–12]. Several different LAO processes exist, for exam-
ple, the shell higher olefin process (SHOP), which generates a
range of ␣-olefins from C4 (1-butene) to C20+, which use nickel
complexes as catalyst [13–15]. Over the past decades, consider-
able research effort has been devoted to the design of new nickel
complexes ligated by bidentate [2,16–55] and tridentate ligands
[56–69] owing to their good to excellent performance towards
the production of ␣-olefins. These developments have been driven
For instance, Auffrant reported a new class of nickel catalysts
based on iminephosphorane–phenoxide ligands which upon acti-
vation with Et2AlCl ([Al]/[Ni] = 22.5) showed high TOF (up to 72,000
mol(C2H4) mol(Ni)−1 h−1), giving mostly butenes with high selec-
tivity for 1-butene (93.0 wt.%) [70] (Chart 1). Sun et al. described the
synthesis of nickel complexes bearing 2-(benzimidazol-2-yl)-1,10-
phenanthrolines; upon activation with diethylaluminum chloride
(Et2AlCl), high catalytic activity of up to 1.27 × 107 g mol(Ni)−1 h−1
and high selectivity for 1-butene (90.5 wt.%) could be achieved
[71]. Olivier-Bourbigou et al. disclosed a new class of nickel
complexes based on imine-imidazole ligands bearing a pendant
donor group that are able to oligomerize ethylene in presence of
EtAlCl2 or methylaluminoxane (MAO), producing mostly dimers
and trimers [72]. More recently, Ojwach et al. reported the use
of a (amino)pyridine ligands in the synthesis of the mononuclear
nickel complex [NiBr2(L)2] which were effective precatalysts lead-
ing to mostly ethylene dimers (C4), in addition to trimmers (C6)
∗
Corresponding author.
E-mail address: osvaldo.casagrande@ufrgs.br (O.L. Casagrande Jr.).
http://dx.doi.org/10.1016/j.apcata.2016.06.019
0926-860X/© 2016 Elsevier B.V. All rights reserved.

248
J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
(MAO) (Witco, 5.21 wt.% Al solution in toluene, 20% of TMA),
H
polymethylaluminoxane-improved performance (PMAO-IP) (Akzo
Nobel, 13.0 wt.% Al solution in toluene), and trimethyl aluminum
(TMA) (Sigma-Aldrich, 2 M in toluene) were used as received. EASC
(Akzo Nobel) was used with the previous dilution (2.1 wt.% Al
solution in toluene). TMA-depleted MAO (DMAO) was prepared
by removing all volatiles under moderate heating (40 ◦C) for 8 h.
Infrared spectra were performed on neat products using a FT-IR
Bruker Alpha Spectrometer operating in the ATR mode. High-
resolution mass spectra (HRMS) of nickel precatalysts (Ni1–Ni4)
were obtained by electrospray ionization (ESI) in the positive
mode in CH3OH solutions using a Waters Micromass® Q-Tof spec-
trometer. Elemental analyses were performed by the Analytical
Central Service of the Institute of Chemistry-USP (Brazil) and are
the average of two independent determinations. Quantitative gas
chromatographic analysis of ethylene oligomerization products
was performed on a Agilent 7890A instrument equipped with
a Petrocol HD capillary column (methyl silicone, 100 m length,
0.25 mm i.d. and film thickness of 0.5 ␮m) (36 ◦C for 15 min, then
heating at 5 ◦C min−1 until 250 ◦C); cyclohexane was used as
internal standard.
N
Ph
N
P
2
R
N
N
Cl
N
O
L
Ni
Ni
Br
Cl
Auffrant et al.70
Sun et al.71
O
N
N
N
H
N
Ni
N
O
N
H
Ni
Br
N
Cl
Cl
O
Olivier-Bourbigou et al.72
Ojwach et al.73
Chart 1. Examples of nickel complexes based on neutral ligands applied in ethylene
oligomerization.
2.2. Synthesis of the Ni(II) complexes
and tetramers (C8) when activated with ethylaluminum dichloride
(EtAlCl2) or methylaluminoxane (MAO) [73].
[Ni{5-methyl-2-(C4H3O-2-CH N)C2H4OPh}Cl2] (Ni1). A solu-
tion of NiCl2(dme) (0.19 g, 0.83 mmol) in THF (10 mL) was added
a solution of L1 (0.20 g, 0.92 mmol) in THF (10 mL) and the result-
ing solution was stirred for 24 h at room temperature. The solvent
was removed, and the resulting brown solid residue was washed
with Et2O (2 × 10 mL) and cooled THF (1 × 10 mL). Complex Ni1 was
obtained as a pale brown solid (0.35 g, 91%).
[Ni{5-methyl-2-(C4H3O-2-CH N)Ph-2-OPh}Cl2] (Ni2). This
compound was prepared according to the method described for Ni1
using Ni(dme)Cl2 (0.11 g, 0.54 mmol) and L2 (0.16 g, 0.59 mmol).
Complex Ni2 was obtained as a pale brown solid (0.23 g, 83%).
[Ni{2-(C4H3O-2-CH N)Ph-2-OPh}Cl2] (Ni3). This compound
was prepared according to the method described for 1 using
Ni(dme)Cl2 (0.13 g, 0.57 mmol) and L3 (0.16 g, 0.61 mmol). Com-
plex Ni3 was obtained as a pale pink solid (0.23 g, 79%).
Previously, our group has reported several families of highly
active Ni(II) complexes containing bi and tridentate ligands for
ethylene oligomerization [74–82]. Our results have shown that the
electronic and/or steric features of the ligands play an important
role in selectivity and activity towards production of 1-butene.
Among the class of ligands synthesized in our laboratory, the
ether-imine-furfural ligands have been previously utilized in the
synthesis of Cr(III) complexes and use of them in ethylene oligomer-
ization. These complexes when activated with methylaluminoxane
(MAO), exhibited moderate to high activities for ethylene oligomer-
+
ization producing oligomers in the range C4–C12 with good
selectivity for ␣-olefins [82].
In this contribution, we report the synthesis and characteriza-
tion of Ni(II) complexes bearing ether-imine-furfural ligands and
their catalytic behavior for ethylene oligomerization. We discuss
the performance of these nickel catalysts, evaluating the influ-
ence of experimental parameters (nature of the cocatalyst, [Al]/[Ni]
molar ratio, type of solvent, and reaction temperature) on the cat-
alyst activity and selectivity.
[Ni{5-Methyl-2-(C4H3O-2-CH N)CH2Ph-2-OMe}Cl2]
(Ni4).
This compound was prepared according to the method described
for Ni1 using Ni(dme)Cl2 (0.21 g, 0.95 mmol) and L4 (0.23 g,
1.00 mmol). Complex Ni4 was obtained as a brown solid (0.37 g,
84%).
2. Experimental
2.3. General oligomerization procedure
2.1. General procedures
Ethylene oligomerization reactions were performed in a 100 mL
double-walled stainless Parr reactor equipped with mechanical
stirring, internal temperature control and continuous feed of ethy-
lene. The Parr reactor was dried in an oven at 120 ◦C for 5 h prior
to each run, and then cooled under vacuum for 30 min. A typi-
cal reaction was performed by introducing toluene (30 mL) and
the proper amount of cocatalyst into the reactor under an ethy-
lene atmosphere. After 20 min, the toluene catalyst solution (10 mL,
[Ni] = 10 ␮mol) was injected into the reactor under a stream of ethy-
lene and then the reactor was immediately pressurized. Ethylene
was continuously fed in order to maintain the desired ethylene
pressure. After 15 min, the reaction was stopped by cooling the
system to −40 ◦C and depressurizing. An exact amount of cyclo-
hexane was introduced (as an internal standard) and the mixture
was analyzed by quantitative GLC.
All manipulations involving air- and/or moisture-sensitive
compounds were carried out in an MBraun glovebox or
under dry argon using standard Schlenk techniques. Solvents
were dried from the appropriate drying agents under argon
before use. NiCl2(dme) was purchased from Sigma-Aldrich
and used as received. The ether-imine-furfural pro-ligands N-
((5-methylfuran-2-yl)methylene)-2-phenoxyethanamine
N-((5-methylfuran-2-yl)methylene)-2-phenoxybenzenamine
(L1),
(L2),
(L3),
N-((furan-2-yl)methylene)-2-phenoxybenzenamine
and 2-methoxyphenyl-N-((5-methylfuran-2-
yl)methylene)methanamine (L4) were prepared by literature
procedures [82]. Ethylene (White Martins Co.) and argon were
deoxygenated and dried through BTS columns (BASF) and
activated molecular sieves prior to use. Methylaluminoxane

J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
249
Table 2
R
BE of investigated for nickel complexes (Ni1–Ni4) in the Ni 2p, Ofurfural 1s, Oether 1s
O
regions.
Z
Species
Binding Energy (eV)
Ofurfural (1s)
R
N
NiCl2(dme)
O
N
Cl
2
Oether(1s)
Ni(2p)
THF, r.t, 24 h
Ni
Ni1
Ni2
Ni3
Ni4
531.88
531.93
532.23
531.71
533.49
533.39
533.60
533.60
857.53
856.53
855.90
855.06
L1-L4
Cl
Z
O
Ni3: R = H, Z =
Ni4: R = Me, Z =
Ni1: R = Me, Z =
Ni2: R = Me, Z =
Ph
Ph
O
DGDZVP2 basis set [86–88]. This functional are known by their good
accuracy for thermodynamics and structural properties [89]. All
structures were obtained from full unconstrained geometry opti-
mizations, followed by frequency calculations to ensure the nature
of the stationary points.
Ph
O
O
Scheme 1. Synthesis of nickel precatalysts bearing ether-imine-furfural ligands.
3. Results and discussion
2.4. X-ray photoelectron spectroscopy (XPS)
3.1. Synthesis and characterization of Ni(II) complexes bearing
ether-imine-furfural ligands
X-ray photoelectron spectroscopy was performed in an
Omicron-SPHERA station using Al K ␣ radiation (1486.6 eV). The
anode was operated at 225 W (15 kV, 15 mA). Spectra were taken
at room temperature with a 50 eV pass energy. The samples were
mounted on an adhesive copper tape as thin films. Samples were
prepared in a glove box, transferred under nitrogen atmosphere,
and then evacuated at 10–6 Torr by a turbomolecular pump in an
introduction chamber for 90 min. During data collection, the ion
pumped mass chamber was maintained at 5 < 10–9 Torr. O 1s, and
Ni 2p regions were recorded with a higher resolution (pass energy
of 20 eV). The detection angle of the photoelectrons with respect
to the sample surface (take-off angle) was fixed at 53◦ for all mea-
surements. The C 1s signal from adventitious carbon at 285 eV was
used as an internal energy reference. All spectra were fitted assum-
ing a Shirley background. Lines were fitted by 70% Gaussian + 30%
Lorentzian functions with set values of full width at half maximum
for each line.
The ether-imine-furfural pro-ligands (L1-L4) were readily syn-
thesized by Schiff base condensations between the corresponding
primary amines and furan-2-carboxaldehyde in refluxing ethanol
[82]. The reaction of NiCl2(dme) with 1.1 equiv. L1–L4 in THF at
room temperature yielded the corresponding nickel precatalysts
(Ni1–Ni4), which were isolated in good yields (79–91%) (Scheme 1).
The nickel complexes were isolated as brown (Ni1, Ni2
and Ni4) or pink (Ni3) paramagnetic and moisture-sensitive
solids which show solubility at room temperature in chloroform,
dichloromethane, and THF. The identity of Ni1–Ni4 was established
on the basis of elemental analysis, IR spectroscopy, and ESI-HRMS
as presented in Table 1. Elemental analyses of complexes Ni1–Ni4
are in agreement with the formation of nickel complexes of general
formula [(L)NiCl2], and ESI-HRMS results indicated the formation
of [M−Cl]+ ions. The IR spectra of the nickel complexes showed the
absorption bands at 1625–1659 cm−1 corresponding to the coordi-
nated HC N unit of the pro-ligands (L1–L4) (Figs. S1–S4) [90,91].
Attempts to recrystallize the precatalysts Ni1–Ni4 using different
crystallization procedures resulted in amorphous materials, unfor-
tunately not suitable for a single crystal X-ray diffraction analysis.
Thus, we decided to explore the structural nature of these nickel
complexes using X-ray photoelectron spectroscopy (XPS) and DFT
methods. Table 2 shows the binding energies (BE) extracted from
the high-resolution spectra from the following regions: Ni 2p,
Ofurfural 1s, Oether 1s.
2.5. DFT calculations
All quantum chemical calculations were performed with the
Gaussian 09 program package [83]. Electronic structure calcu-
lations of all studied species were performed using Density
Functional Theory as provided by the B3LYP hybrid functional
formed by the three parameter fit of the exchange-correlation
potential suggested by Becke [84] and the gradient corrected cor-
relation functional of Lee, Yang and Parr [85] with the all electrons
Table 1
Elemental analysis, ISI-HRMS and IR data for nickel complexes Ni1-Ni4.
Complex
Elemental Analysis
Calculated
ESI-HRMS (CH3OH, m/z)
IR (KBr, cm−1
)
Found
C
N
H
C
N
H
Ni1
46.86
4.21
3.90
46.78
4.12
3.67
322.0144 g [M−Cl]+
3111 (w), 3087 (w), 3064 (w), 3041 (w), 2921 (w), 1673 (m), 1649 (s), 1602
(m), 1535 (m), 1520 (m), 1502 (m), 1459 (w), 1435 (w), 1369 (w), 1307
(w), 1241 (s), 1184 (w), 1113 (w), 1089 (w), 1051 (w), 1017 (m), 947 (w),
904 (w), 828 (w), 799 (w), 756 (s), 699 (m), 647 (w), 610 (w).
3361 (s, broad), 1625 (m), 1587 (m), 1502 (m), 1487 (m), 1459 (w), 1388
(w), 1335 (w), 1255 (w), 1255 (m), 1217 (m), 1165 (w), 1074 (w), 1017
(m), 880 (w), 795 (w), 747 (m), 695 (w).
3387 (s, broad), 1625 (m), 1592 (m), 1544 (w), 1502 (m), 1487 (m), 1464
(w), 1398 (w), 1335 (w), 1259 (w), 1217 (m), 1160 (w), 1074 (w), 1022 (w),
880 (w), 799 (w), 747 (m), 695 (w).
Ni2
Ni3
Ni4
53.13
51.97
46.86
3.72
3.34
4.21
3.44
3.57
3.90
52.67
51.45
46.61
3.55
3.11
4.05
3.21
3.23
3.55
370.0146 g [M−Cl]+
355.9985 g [M−Cl]+
322.0142 g [M−Cl]+
3367 (s, broad), 1659 (m), 1588 (m), 1551 (w), 1492 (m), 1459 (w), 1372
(w), 1293 (w), 1256 (m), 1220 (m), 1168 (w), 1028 (w), 970 (w), 1029 (w),
880 (w), 800 (w), 776 (m), 697 (w).

250
J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
The higher BE energy values found for Ni1-Ni3 in comparison
with Ni4 indicate lowest electron density on the nickel coordina-
tion sphere which can be tentatively attributed to the different
coordination mode of L1–L3 as compared to L4 (Fig. 1) [92]. In this
case, the lowest BE energy value found in Ni4 (855.06 eV) suggest
that the electronic density around the Ni metal center is affected by
methoxy group, which its donor character drives electron density
up to Ni atom. On the other hand, the similar BE energies found for
Oether/Ofurfural 1s in L2 (531.72 eV; 533.62 eV) and L3 (531.68 eV;
533.48 eV) (Figs. S1 and S2) compared those ones found for Ni2
(531.93 ev; 533.39 eV) and Ni3 (532.23 eV; 533.60 eV) strongly sug-
gest no noticeable interaction of these oxygen donor atoms with the
nickel metal center (Fig. 2) [93].
The coordination mode of ether-imine-furfural ligand (L4) to
nickel complex (Ni4) was further investigate using DFT methods.
Initial DFT theoretical study carried out to generate a monomeric
stable structure with L4 acting as a tridentate ligand failed. The
most stable structure was achieved for a nickel dimeric species,
which was found to be 24.6 kJ/mol lower in energy than the
monomeric one. In this case, the optimized geometry of Ni4 showed
the formation of a chloro-bridged dimer complex with the nickel
atom containing one N-coordinated ether-imine-furfural and three
coordinated chloride atoms (one terminal and two bridged) as pre-
sented in Fig. 3. The monodentate coordination mode of similar
furfural-imine ligands has been observed for palladium complexes
[90,91].
The Ni(1) O(2) distance is 2.954 Å which is longer than the sum
of covalent radii (1.80 Å) [94], but shorter than the sum of van der
Waals radii (3.55 Å) [95], suggesting that any Ni Oether interac-
tion must be weak. Furthermore, the presence of an interaction
between the oxygen atoms of the furfural moieties and the hydro-
gens from the CH2 unit [H(7)· · ·O(1) and H(23)· · ·O(3) ∼ 2.245 Å;
the sum of the van der Waals radii is ∼2.60 Å) contributes to keep
the furfural group in opposite side, and thus preventing any inter-
action thereof with the nickel metal center [96]. The Ni(1)· · ·Ni(2)
distance of 3.323 Å indicates no direct bonding between the two
nickel atoms as already observed for others chloro-bridged nickel
dimers complexes [97–100].
3.2. Ethylene oligomerization studies
Precatalysts Ni1–Ni4 were initially tested in ethylene oligomer-
ization using polymethylaluminoxane-improved performance
(PMAO-IP) as cocatalyst in toluene at 30 ◦C, and 20 bar constant
ethylene pressure. All experiments were at least duplicated, yield-
ing reproducible results within 10%. Oligomerization results are
summarized in Table 3. Upon activation with PMAO-IP, all the
nickel complexes generated active systems for the production of
short-chain olefins in the C4–C6 range with turnover frequencies
(TOFs) varying from 14.7 to 57.3 × 103 mol(ethylene) mol(Ni)−1 h−1
as presented in Fig. 4. The distinguished catalytic performances of
these precatalysts may be tentatively associated with the interac-
tion of the oxygen-donor groups to the metal center after activation
with the cocatalyst. Thus, we speculate that the formation of the
cationic nickel intermediate species facilitate the coordination of
these oxygen-donor groups onto the metal center to form a more
stable catalytic species like that reported for palladium complexes
[101–104].
Furthermore, in order to verify the influence of the ether-imine-
furfural ligand on the activity, a blank oligomerization reaction
was carried out under the same conditions. Thus, upon activa-
tion with methylaluminoxane (300 equiv), NiCl2(dme) showed
very low activity in ethylene oligomerization (TOF = 1.70 × 103 (mol
ethylene) (mol Ni)−1 h−1 at 30 ◦C), producing mostly 1-butene
(77.0 wt%). Among the catalytic systems studied herein, the
Ni2/PMAO-IP system showed the highest activity of up to 57,300
Fig. 1. Ni 2p XPS spectra of nickel precatalysts.

J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
251
Table 3
Ethylene oligomerization with Ni1-Ni4 catalytic systems.a
entry
cat
Cocatal.
[Al]/[Ni]
Olig. (g)
TOFb (× 103)
Selectivity (wt%)
C4 (␣-C4)
C6 (␣-C6)
1
2
3
Ni1
Ni2
Ni3
Ni4
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
PMAO-IP
PMAO-IP
PMAO-IP
PMAO-IP
PMAO-IP
MAO
MAO
MAO
MAO
DMAO
MAO/TMA
EASC
EASC
MAO
500
500
500
500
500
125
250
500
1000
500
500
500
25
2.00
5.30
1.80
1.30
2.90
3.20
3.50
6.40
7.10
3.90
0.60
19.1
5.5
22.0
57.3
19.2
14.7
63.2
34.4
38.0
69.3
76.5
41.4
6.7
95.3 (81.9)
91.4 (69.2)
99.9 (80.7)
93.0 (89.5)
94.4 (79.7)
100 (80.1)
100 (81.6)
90.2 (65.3)
70.4 (63.6)
93.9 (76.3)
77.0 (67.0)
87.7 (11.9)
74.0 (52.8)
97.8 (83.9)
85.4 (53.9)
4.7 (28.5)
8.6 (18.7)
0.01 (25.0)
7.0 (23.8)
5.6 (25.3)
–
4
5c
6
7
8
9
10
11d
12
13
14e
15f
–
9.8 (16.2)
29.6 (29.9)
6.1 (23.8)
23.0 (25.3)
12.3 (3.6)
26.0 (26.0)
2.2 (28.5)
14.6 (8.0)
206.1
59.2
56.1
550.4
500
500
5.20
51.5
MAO
a
Reaction conditions: toluene = 40 mL, [Ni] = 10 ␮mol, oligomerization time = 15 min, P(ethylene) = 20 bar (kept constant), T = 30 ◦C. The results shown are representative
of at least duplicated experiments.
b
Mol of ethylene converted (mol of Ni)−1 h1 as determined by quantitative GLC.
[Ni] = 5 ␮mols.
c
d
MAO/TMA (1:1).
e
T = 50 ◦C.
f
CH2Cl2 as solvent.
mol(ethylene) mol(Ni)−1 h−1 which is comparable to those others
nickel precatalysts [38,73–82,104].
[41.1 × 103 mol (ethylene) mol(Ni)−1 h−1] suggesting the benefi-
cial role of TMA for this catalyst system. However, increasing the
amount of free TMA to MAO [500 equiv; TMA/MAO = (1:1)] dras-
Precatalyst
Ni1,
when
activated
with
500
equiv
of PMAO-IP, was found to give moderate activity
[22.0 × 103 mol(ethylene) mol(Ni)−1 h−1] along with good selec-
tivity towards 1-butene (78 wt.%). As observed in Fig. 4, the
presence of a more rigid phenyl moiety, as in Ni2 led to a much
higher activity as compared to the counterpart Ni1; however, a
lower selectivity for 1-butene was found using Ni2 (63.8 wt.%).
Decreasing the steric hindrance on the furfural unit by substitution
of the hydrogen by a methyl group at the 5-position (precatalyst
Ni3) caused a reduction in activity by a factor of 3, which is a fact
with no straightforward explanation
When the length of the spacer between the central imine and
the ether donor group was increased by one carbon (compare
complexes Ni4 and Ni2), the activity decreased almost 4 times
[14.7 × 103 mol(ethylene) mol(Ni)−1 h−1] which can be associated
with a more difficult coordination of the OMe group onto the metal
center to deliver a stable 6-membered-ring nickel complex. As
shown in Table 3, the precatalysts Ni1-Ni4 were highly selective
resulting in the formation of predominantly C4 (91.4–99.9 wt.%)
and just small amounts of C6 (hexenes) oligomers. On the other
hand, the selectivity for 1-butene varied from 63.2 to 83.2 wt.%,
suggesting that the ligand structure influences on the product dis-
tribution within the C4 fraction.
Subsequently, the influence of the reaction parameters on the
catalytic performance of Ni2 was investigated (Table 3, entries
5–15). The oligomerization reaction with Ni2/PMAO-IP was bet-
ter conducted with low Ni catalyst concentration in the reactor
(ca. 5 ␮mol) at a setting temperature of 30 ◦C (entry 5). In fact,
under such oligomerization conditions, precatalyst Ni2 provided
to possess high activity [63.2 × 103 mol (ethylene) mol(Ni)−1 h−1],
and higher selectivity for 1-butene (63.2–75.2 wt.%). This effect
can be associated with an easier solubilization of the precatalyst
in toluene solution; however, a decrease of excessive exotherms
during the oligomerization reaction, which would induce catalyst
decay, cannot be ruled out.
tically reduced the activity (6.7 × 103 mol(ethylene) mol(Ni)−1 h−1
,
entry 13). Furthermore, the activation of Ni2 with neat trimethyla-
luminum (TMA) did not produce oligomers in significant amounts,
presumably due to its low Lewis acidity. These results establish
that the TMA/MAO ratio needs to be suitably adjusted to produce
a catalyst system of high activity and long kinetic lifetime. Fur-
thermore, the TMA content has a direct impact on the selectivity.
Thus, increasing of the TMA in the medium lowered affected the
selectivity for 1-butene as shown in Fig. 5.
Activation of the nickel precatalyst Ni2 with 500 equiv of
ethylaluminum sesquichloride (Et3Al2Cl3, EASC) leads to a signifi-
cant increase in activity [206.1 × 103 mol(ethylene) mol(Ni)−1 h−1
]
along with a drastic reduction in the selectivity for 1-butene
(10.4 wt.%). This pronounced decrease in the selectivity can be
associated with exothermicity, so that the reaction with Ni2/EASC
performed at an initial temperature of 30 ◦C rapidly rose to
55–57 ◦C, despite the thermostating fluid around the reactor. In this
case, better selectivity for 1-butene (39.1 wt.%) was obtained using
a lower [Al]/[Ni] molar ratio (25 equiv) at a setting temperature of
30 ◦C (entry 13); under such conditions, the exothermicity could be
limited to 10–15 ◦C.
The aforementioned results show that the nature of the cocat-
alyst plays an important role on the activity and on the selectivity
of these catalyst systems which could be associated with the dif-
ference of Lewis acidity well as the possibility of these cocatalysts
act as ligands attached to the nickel center as already observed in
studies using XAS spectroscopy [105,106].
The influence of the MAO loading on the catalyst behavior was
also studied. When activated with 125 equiv of MAO, precatalyst
Ni2 gave a lower activity [34.4 × 103 mol(ethylene) mol(Ni)−1 h−1
,
entry 6], which was enhanced by the systematic increase of MAO
from 250 to 1000 equiv [76.5 × 103 mol(ethylene) mol(Ni)−1 h−1
,
entry 9]. In all cases, the use of different MAO amount had substan-
tial impact on the selectivity for ␣-C4 varying from 44.8 wt.% (1000
equiv of MAO) to 81.6 wt.% (250 equiv of MAO).
The use of MAO instead of PMAO-IP led to a slight improvement
in the TOF [69.3 × 103 mol(ethylene) mol(Ni)−1 h−1]. When Ni2
was activated with trimethylaluminum (TMA)-depleted methyla-
luminoxane (DMAO) (500 equiv), a lower activity was obtained
The oligomerization temperature also affected the observed
catalytic activities. Elevating the temperature from 30 ◦C
to 50 ◦C led to
a slight reduction in TOF from 69.3 to

252
J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
Fig. 3. DFT-optimized geometry (B3LYP functional and dgdzvp2 basis set) of Ni4.
100
80
60
40
20
0
60
50
40
30
20
10
0
Ni1
Ni2
Ni3
Ni4
Fig. 4. Influence of the nature of nickel precatalyst on TOF and selectivity for 1-
butene (T = 30 ◦C, 20 bar, time = 15 min, [Al]/[Ni] = 250).
80
200
70
60
150
50
100
40
30
50
20
0
10
DMAO
PMAO-IP
MAO
MAO/TMA
EASC
Fig. 5. Influence of the cocatalyst type on TOF and selectivity for 1-butene using Ni2
precatalyst (T = 30 ◦C, 20 bar, time = 15 min, [Al]/[Ni] = 500).
56.1 × 103 mol(ethylene) mol(Ni)−1 h−1 which may be attributable
to a partial decomposition of the active catalytic sites and lower
ethylene solubility at higher temperatures. However, it should be
pointed out that even at higher temperature (50 ◦C), the Ni2/MAO
still operates with high activity and good selectivity for 1-C4
production (82.8 wt.%) when compared to nickel precatalysts
bearing bi- and tridentate ligands [74,76,77,80].
Fig. 2. O 1s XPS spectra of nickel precatalysts.

J.L.S. Milani et al. / Applied Catalysis A: General 523 (2016) 247–254
253
The activation of Ni2 with MAO (500 equiv) in dichloromethane,
led to a much higher TOF [550.4 × 103 mol(C2H4) mol(Ni)−1 h−1].
Although the C4 fraction remains almost unchanged (85.4 wt.%),
the selectivity for 1-butene was drastically reduced, attaining only
46.0 wt.% with a concomitant production of larger amounts of
internal butenes (54.0 wt%) and hexenes (14.6 wt%). This could be
attributed to the exothermicity of the oligomerization reaction, and
higher chain transfer and isomerization rates, which are facilitated
by high degree of dissociation of cationic active Ni-complex ion in
polar solvents [107].
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In conclusion, we have synthesized and characterized a series
of new nickel(II) complexes supported by ether-imine-furfural
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nickel metal center which can be associated with the lower Lewis
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ether-imine-furfural acting as a monodentate ligand. On activa-
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catalytic activity for ethylene oligomerization and selectivities for
1-butene varying from 63.2 to 83.2 wt.%. The distinguished catalytic
performances of these precatalysts may be tentatively associated
with the interaction of the oxygen-donor groups to the metal cen-
ter after activation with the cocatalyst. The use of different types
of cocatalysts has a significant impact on the activity and selectiv-
ity. For instance, the use of MAO instead of PMAO-IP led to a slight
improvement in the TOF with lower selectivity for 1-butene. On the
other hand, the activation of Ni2 with trimethylaluminum (TMA)-
depleted methylaluminoxane led to a lower activity suggesting the
beneficial role of TMA for this catalyst system. Furthermore, the
TMA/MAO ratio needs to be suitably adjusted to produce a catalyst
system of high activity and long kinetic lifetime. Higher activity was
obtained using EASC as cocatalyst; however the 1-butene selectiv-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.06.
019.
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