NNNO-Heteroscorpionate nickel (II) and cobalt (II) complexes for ethylene oligomerization: the unprecedented formation of odd carbon number olefins

Article abstract of DOI:10.1002/aoc.5873

The unprecedented observation of odd carbon number olefins is reported during nickel- catalyzed ethylene oligomerization. Two complexes based on Co (II) and Ni (II) with novel tetradentate heteroscorpionate ligand have been synthesized and fully characterized. These complexes showed the ability to oligomerize ethylene upon activation with various organoaluminum compounds (Et₂AlCl, Et₃Al₂Cl₃, EtAlCl₂, MMAO). Ni (II) based catalytic systems were sufficiently more active (up to 1900 kg·mol (Ni)^?1·h^?1·atm^?1) than Co (II) analogs and have been found to be strongly dependent on the activator composition. The use of PPh₃ as an additive to catalytic systems resulted in the increase of activity up to 4,150 kg·mol (Ni)^?1·h^?1·atm^?1 and in the alteration of selectivity. All Ni (II) based systems activated with EtAlCl₂ produce up to 5 mol. percent of odd carbon number olefins; two probable mechanisms for their formation are suggested – metathesis and β-alkyl elimination.

Full text of DOI:10.1002/aoc.5873

Received: 2 April 2020
DOI: 10.1002/aoc.5873
Revised: 21 May 2020
Accepted: 23 May 2020
F U L L P A P E R
NNNO-Heteroscorpionate nickel (II) and cobalt
(II) complexes for ethylene oligomerization: the
unprecedented formation of odd carbon number olefins
Sergey V. Zubkevich1
Anatoliy S. Kayda1
Dmitry N. Zarubin2
| Vladislav A. Tuskaev1,2
| Victor N. Khrustalev3,4
| Boris M. Bulychev1
| Svetlana Ch. Gagieva1
| Alexander A. Pavlov2
|
|
1Department of Chemistry, Lomonosov
Moscow State University, Leninskie Gory
11, Moscow, 119991, Russia
2Russian Academy of Sciences, A.N.
Nesmeyanov Institute of Organoelement
Compounds, Vavilova st. 28, Moscow,
119991, Russia
3Peoples' Friendship University of Russia
(RUDN University), Miklukho-Maklay St.,
6, Moscow, 117198, Russian Federation
4Russian Academy of Sciences, Zelinsky
Institute of Organic Chemistry, Leninsky
Prospekt 47, Moscow, 119991, Russia
The unprecedented observation of odd carbon number olefins is reported dur-
ing nickel- catalyzed ethylene oligomerization. Two complexes based on Co
(II) and Ni (II) with novel tetradentate heteroscorpionate ligand have been
synthesized and fully characterized. These complexes showed the ability to oli-
gomerize ethylene upon activation with various organoaluminum compounds
(Et2AlCl, Et3Al2Cl3, EtAlCl2, MMAO). Ni (II) based catalytic systems were suf-
ficiently more active (up to 1900 kgꢀmol (Ni)−1ꢀh−1ꢀatm−1) than Co (II) analogs
and have been found to be strongly dependent on the activator composition.
The use of PPh3 as an additive to catalytic systems resulted in the increase of
activity up to 4,150 kgꢀmol (Ni)−1ꢀh−1ꢀatm−1 and in the alteration of selectivity.
All Ni (II) based systems activated with EtAlCl2 produce up to 5 mol. % of odd
carbon number olefins; two probable mechanisms for their formation are
suggested – metathesis and β-alkyl elimination.
Correspondence
Sergey V. Zubkevich, Lomonosov Moscow
State University, Department of
Chemistry, Leninskie Gory 11, Moscow
119991, Russia.
K E Y W O R D S
Email: zubkevich.sergey@gmail.com
cobalt, ethylene oligomerization, heteroscorpionate, nickel, odd carbon number olefins
Funding information
Council of the President of the Russian
Federation, Grant/Award Number: МК-
199.2019.3; RUDN University, Grant/
Award Number: Program “5-100”; Russian
Foundation for Basic Research, Grant/
Award Number: 18-33-20091; Government
of the Russian Federation: АААА-
А16-116053110012-5
1 | INTRODUCTION
use of heteroscorpionate complexes of transition,[5,6] s-
block[7,8] and p-block[9,10] metals as catalysts for lactide
The transition metal complexes with heteroscorpionate
ligands have been intensively studied recently because of
their ability to mediate various catalytic reactions.[1] The
catalytic properties of corresponding chromium-based
complexes, which selectively trimerize ethylene to
1-hexene, have been studied in sufficient detail.[2–4] The
and ε-caprolactone ring-opening polymerization has also
attracted scientific attention in the last decade. At the
same time, the heteroscorpionate complexes of the iron
triad metals, unlike other N-donor ligands,[11,12] have
been studied very superficially in the olefin coupling
catalysis. Casagrande et al.[13] investigated nickel
Appl Organomet Chem. 2020;e5873.
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© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5873

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ZUBKEVICH ET AL.
complexes with boron-containing scorpionate ligands
(Figure 1, а) in ethylene oligomerization. Several MOF
based on nickel with scorpionate-like coordination are
active in ethylene dimerization.[14–16]
purity grade (Linde gas) were dried by purging through a
Super Clean™ Gas Filters. Unless otherwise noted, all
reagents were purchased from Sigma-Aldrich and used
without further purification. Et2AlCl, EtAlCl2 were sup-
plied as 1 M solutions in hexane. Et3Al2Cl3 was supplied
as 1 M solution in toluene, MMAO (MMAO-12) was sup-
plied as 7% wt. solution in toluene. Toluene, THF and
diethyl ether were distilled over Na/benzophenone. Dic-
hloromethane was refluxed over CaH2 and distilled prior
to use. CDCl3, CD3CN and CD3OD were stored under
inert atmosphere. 8-methoxyquinoline-2-carbaldehyde
was prepared close to the method described in litera-
ture.[26] Bis(3,5-dimethylpyrazolyl)methanone was pre-
pared according to literature procedure[27] and
recrystallized from THF. 2-[bis(3,5-dimethylpyrazol-1-yl)
methyl]-8-methoxy-quinoline L was prepared similar to
Earlier, we have obtained a number of nickel
(II) complexes with heteroscorpionate ligands (Figure 1,
b-g), which showed moderate activities towards ethylene
oligomerization with predominant formation of
butenes.[17] However, nickel halide complexes with tri-
dentate heteroscorpionates easily undergo isomeriza-
tion[18] yielding ionic complexes [NiL2]+2[NiHal4]−2
,
which are inactive in oligo- /polymerization of ethylene.
So thermodynamically stable molecular complexes
of nickel halides with heteroscorpionates[19] or
scorpionates[20] are obtained only when bulky substitu-
ents are introduced. In this study, we have designed
novel tetradentate NNNO-heteroscorpionate ligand with
8-methoxyquinoline pendant arm, and synthesized
monomeric molecular complexes based on Ni (II) and Co
(II) halides with it. The obtained complexes have been
studied for their ability to catalyze ethylene oligomeriza-
tion upon activation with Et2AlCl, Et3Al2Cl3, EtAlCl2 or
MMAO. Quite unexpectedly, Ni (II) based systems, acti-
vated with EtAlCl2, produced up to 5 mol. % of odd car-
bon number olefins. Previously several catalytic systems
based on Fe (II) and Cr (III), that exhibit such properties,
were described in literature.[21–24] Two mechanisms have
been introduced to describe the formation of odd-carbon
number olefins: metathesis[21,23] and transfer to
aluminum.[25]
1
Higgs and Carrano.[28] It has been characterized by H
and 13C NMR and X-ray diffraction analyses. Synthetic
details and spectra are given in supplementary informa-
tion. Complexes were prepared by the direct interaction
of CoCl2 or Ni (DME)Br2 with the ligand in THF.
2.2 | Physical and analytical
measurements
NMR spectra were recorded on Bruker Avance-400 and
600 FT-spectrometers (400.13 and 600.22 MHz). Chemi-
cal shifts are reported in ppm and were determined by
reference to the residual solvent peaks. All coupling con-
stants are given in Hertz. IR spectra were recorded on a
Magna-IR 750 spectrophotometer. Elemental analysis
was performed by the microanalytical laboratory at A. N.
Nesmeyanov Institute of Organoelement Compounds.
Mass spectra under atmospheric pressure electrospray
(ESI) were recorded in the full scan mass of positive and
negative ions on the dynamic tandem mass spectrometer
Finnigan LCQ Advantage (USA), equipped with a mass
analyzer oktapol ion trap pump MS Surveyor, Surveyor
2 | EXPERIMENTAL
2.1 | Materials and methods
All manipulations with air-sensitive materials were per-
formed with rigorous exclusion of oxygen and moisture
in oven-dried Schlenk glassware using a dual manifold
Schlenk line. Argon (grade 4.8) and ethylene of special-
autosampler,
nitrogen
generator
Schmidlin-Lab
FIGURE 1 Examples of and Ni(II) scorpionate[13] and heteroscorpionate[17] complexes used as precatalysts in ethylene oligomerization

ZUBKEVICH ET AL.
3 of 14
(Germany) and a system for collecting and processing the
data using the X Calibur program (version 1.3, Finnigan).
Transfer capillary temperature of 150 ꢁC, voltage field
between the needle and counter electrode 4.5 kV. Sam-
ples were introduced into the ion source with the input
syringe acetonitrile at a 50 ml/min flow rate through the
5 mL Reodyne injector loop. High-resolution MALDI
mass spectra were registered on a Bruker ULTRAFLEX II
TOF/TOF instrument with 2,5- dihydroxybenzoic acid
(DHB) as the matrix.
(579.94) C, 43.5; H, 4; N, 10.1; O 2.8; Br, 27.6; Ni, 12.1.
Found: C, 43.2; H, 4.16; N, 10.37; Ni 11.97. MALDI-TOF:
m/z (%) 500 [M –Br, 100%].
2.4 | DFT calculations
All quantum chemical DFT calculations were carried out
in ORCA package v. 3.0.3. X-ray structures of the com-
plexes were used as an initial structure for geometry opti-
mization. It was performed with non-hybrid PBE
functional[29,30] and using def2-TZVP basis sets.[31] After
geometry optimization tensors of hyperfine interactions
for hydrogen and carbon nuclei were calculated using
hybrid PBE0 functional[32] and the same basis sets
def2-TZVP, but with addition primitives with higher
order of exponent to better description of electron density
around a nucleus.
2.3 | Synthesis and characterization
2.3.1 | Synthesis of LCoCl2 (L = 2-[bis
(3,5-dimethylpyrazol-1-yl)methyl]-
8-methoxyquinoline) (1)
2-[bis(3,5-dimethylpyrazol-1-yl)methyl]-
Observed chemical shifts of the nuclei of the para-
magnetic complexes were divided into diamagnetic
8-methoxyquinoline L (250 mg, 0.69 mmol) was dissolved
in 20 ml of anhydrous THF and added to suspension of
CoCl2 (90 mg, 0.69 mmol) in anhydrous THF (20 ml).
The mixture was refluxed until complete dissolution and
then 30 min more. The resulting solution was concen-
trated and anhydrous diethyl ether was added. The pre-
cipitate was filtered off washed with anhydrous Et2O
(2 × 10 ml) and dried in vacuo. Yield: 246 mg (72.4%). 1H
NMR (600 MHz, Methanol-d4) δ 122.55 (s, 1H), 110.97 (s,
1H), 41.90 (s, 6H), 41.43 (s, 2H), 30.03 (s, 1H), 28.34 (s,
1H), 23.96 (s, 3H), −6.85 (s, 1H), −19.91 (s, 1H), −73.42
(s, 6H). Anal. Calcd. for C21H23Cl2CoN5O (491.28) C,
51.3; H, 4.7; N, 14.3; O 3.3; Cl, 14.4; Co, 12. Found: C,
51.5; H, 4.52; N, 13.74; Co 11.8. ESI-MS: m/z (%)
440 [M – CH3, − Cl, 16%], 405 [M+ − CH3, − 2Cl, 7%].
(δDIA), contact (δCS) and pseudocontact (δPCS
)
contributions:
δOBS = δDIA + δCS + δPCS
ð1Þ
As the diamagnetic contribution, the chemical shifts in
1
the H NMR spectrum of an initial ligand was used. The
contact contribution to the chemical shifts was calculated
by Equation 2:
SðS + 1ÞμB
ꢀ
δCS
=
ꢀgꢀAiso
ð2Þ
3kTgN μN
(Aiso is the DFT-calculated isotropic hyperfine coupling
ꢀ
constant, g is the DFT-calculated rotationally averaged
electronic g-value, gN is the nuclear g-value, μB and μN
are the Bohr and nuclear magnetons, respectively, and
kT is the thermal energy), and the pseudocontact contri-
bution, by Equation 3:
2.3.2 | Synthesis of LNiBr2 (L = 2-[bis
(3,5-dimethylpyrazol-1-yl)methyl]-
8-methoxyquinoline) (2)
Â
À
ÁÃ
1
2-[bis(3,5-dimethylpyrazol-1-yl)methyl]-
δPCS
=
ΔXax 3COS2θ−1
ð3Þ
12πr3
8-methoxyquinoline L (188 mg, 0.52 mmol) was dissolved
in 20 ml of anhydrous THF. The resulting solution was
added dropwise to solution of Ni (DME)Br2 (160 mg,
0.52 mmol) in 20 ml of anhydrous THF. The solution was
stirred for 2 hr and concentrated to 3 ml. The complex
crystallized from this solution as orange powder in
10 min. It was filtered off and washed with anhydrous
Et2O (2 × 10 ml) and dried in vacuo. Yield: 205 mg
(74.8%). 1H NMR (600 MHz, Methanol-d4) δ 56.85 (s,
2H), 43.94 (s, 1H), 38.03 (s, 1H), 21.59 (s, 1H), 15.82 (s,
3H), 14.39 (s, 1H), 10.17 (s, 1H), −3.16 (s, 6H), −9.53 (s,
6H), −11.05 (s, 1H). Anal. Calcd. for C21H23Br2N5NiO
(Δχax is the axial anisotropy of the magnetic susceptibility
tensor). The polar coordinates r and θ of the nuclei were
taken from the DFT-optimized geometry of the
complexes.
The value of Δχax was estimated by fitting the chemi-
cal shifts in the H NMR spectrum measured experimen-
1
tally to those calculated with Equation 4:
Â
À
ÁÃ
1
δOBS = δDIA + δCS
+
ΔXax 3COS2θ−1
ð4Þ
12πr3

4 of 14
ZUBKEVICH ET AL.
2.5 | X-ray crystal structure
determination
Alicat's Flow Vision™ software. GC analysis was per-
formed on Chromatec-Crystall 5000.2, equipped with
flame ionization detector and capillary column Restek
Rt®-Alumina BOND/MAPD (length 50 m, inner diameter
0.53 mm). Initial temperature - 70 ꢁC, 5 min, heating rate
- 10 ꢁC/min, up to 200 ꢁC. GC–MS analysis was per-
formed on GC–MS scectrometer Trace GC Ultra – DSQII,
capillary column TR-5MS (length 30 m, inner diameter
0.25 mm, phase thickness 0.25 μm). Initial temperature -
70 ꢁC, 2 min, heating rate - 15 ꢁC/min, up to 280 ꢁC.
Oligomerization of 1-Hexene was performed in a
450-ml reactor (Parr Instrument Co.) equipped with a
magnetic stirrer and inlets for loading components of cat-
alytic systems. Before the experiment the reactor was
The single-crystal X-ray diffraction data for L and 1 was
collected on the ‘Belok’ beamline of the Kurchatov Syn-
chrotron Radiation Source (National Research Center
‘Kurchatov Institute’, Moscow, Russian Federation) using
a Rayonix SX165 CCD detector. In total, 720 frames were
collected with an oscillation range of 1.0ꢁ in the φ scan-
ning mode using two different orientations for each crys-
tal. The semi-empirical correction for absorption was
applied using the Scala program.[33] The data were inde-
xed and integrated using the utility iMOSFLM from the
CCP4 software suite.[34] For details, see SI, Table S2. The
structure was determined by direct methods and refined
by full-matrix least square technique on F2 in anisotropic
approximation for non-hydrogen atoms. The hydrogen
atoms were placed in calculated positions and refined
within the riding model with fixed isotropic displacement
parameters [Uiso(H) = 1.5Ueq(C) for the methyl groups
and 1.2Ueq(C) for the other groups]. All calculations were
carried out using the SHELXTL program suite.[35]
ꢁ
heated up to 100 C and then vacuumed for 10 minutes
to remove residual moisture. Further, it was cooled to
ꢁ
30 C filled with argon and solid catalyst was injected as
suspension in toluene (40 ml). Then necessary amount of
co-catalyst solution dissolved in additional toluene
(10 ml) was loaded in the reactor. Then 10 ml of
1-hexene were injected in the reactor via syringe and the
system was pressurized with additional 3 atm. of argon.
The reaction was conducted for 30 min. After the end of
the process, the reaction solution was quickly cooled
down and then quenched with 15 ml of isopropanol and
10 ml of 5% dilute hydrochloric acid. The organic phase
was further dried over Na2SO4 and the organic com-
pounds were characterized with GC–MS to determine the
composition and molecular mass distribution.
Crystallographic data for 1 and ligand L has been
deposited with the Cambridge Crystallographic Data
Center, CCDC 1915444 and CCDC 1915445. The supple-
mentary crystallographic data can be obtained free of
charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Oligomerization of Ethylene was performed in a
450 ml reactor (Parr Instrument Co.) equipped with a
magnetic stirrer and inlets for loading components of cat-
alytic systems and ethylene at a total ethylene and tolu-
ene vapors pressure of 4 atm.ꢁBefore the experiment the
reactor was heated up to 100 C and then vacuumed for
10 min to remove residual moisture. Further, it was
3 | RESULTS AND DISCUSSION
3.2 | Synthesis and characterization
ꢁ
cooled to 30 C filled with argon and solid catalyst was
To
obtain
the
desired
NNNO-tetradentate
injected as suspension in toluene/chlorobenzene (40 ml).
Then necessary amount of co-catalyst solution dissolved
in additional toluene/chlorobenzene (10 ml) was loaded
in the reactor. The reactor was flushed with ethylene to
remove argon atmosphere. Oligomerization was initiated
by pressurization of 3 additional atm. of ethylene in the
reactor. The pressure of ethylene was maintained con-
stant during oligomerization. After the end of the pro-
cess, the reaction solution was quickly cooled down and
then quenched with 15 ml of isopropanol and 10 ml of
5% dilute hydrochloric acid. The organic phase was fur-
ther dried over Na2SO4 and the organic compounds were
characterized with GC and GC–MS to determine the
composition and molecular mass distribution. Samples
with desiccant were stored in sealed flasks in a refrigera-
tor. The consumption of ethylene was measured using
Alicat Scientific massflow detector M-500SCCM-D and
heteroscorpionate ligand, at first, 8-methoxyquinoline-
2-carbaldehyde has been prepared from 8-methoxy-
2-methylquinoline close to literature procedure.[26] Then,
2-[bis(3,5-dimethylpyrazol-1-yl)methyl]-8-methoxyquino
line L has been prepared from 8-methoxyquinoline-
2-carbaldehyde and bis(3,5-dimethylpyrazol-1-yl)met-
hanone following the synthetic procedure adopted from
Higgs and Carrano.[28] (Scheme 1) It was characterized
1
using H and 13C NMR (SI, Figures S2,S3) (annotations
were proposed based on[17,36]), IR (SI, Figures S4, S5) and
elemental analysis.
Co (II) 1 and Ni (II) 2 monomeric complexes of gen-
eral composition LMHal2 have been synthesized by inter-
action of the ligand L with CoCl2 or Ni (DME)Br2,
correspondingly (Scheme 1). They have been character-
ized by IR-, mass-, 1H NMR-spectroscopies and elemental
analysis.

ZUBKEVICH ET AL.
5 of 14
SCHEME 1 Synthesis of 2-[bis(3,5-dimethylpyrazol-1-yl)methyl]-8-methoxyquinoline L and complexes 1 and 2
According to ESI-MS mass spectra, complexes 1–2 do
not yield the corresponding molecular ions. For Co
(II) complex 1 the meaningful fragments were obtained,
attributed to the loss of chlorine atom and –CH3 group
(m/z = 440) and to the loss of two chlorine atoms and –
CH3 group (m/z = 405) (SI, Figure S8). Complex 2 under
the same conditions undergoes complete destruction and
can be analyzed only using MALDI-TOF mass spectrom-
etry yielding a fragment with m/z = 500, attributed to the
loss of the bromine atom (SI, Figure S13).
The structures of ligand L and cobalt (II) complex
1 were unambiguously established by X-ray diffraction
study and are shown in Figure 2 along with the atomic
numbering scheme. The selected bond lengths and angles
are given in Supporting Information, Table S3. Complex
1 is mononuclear and contains six-coordinated cobalt
atom: three coordination sites are occupied by the three
nitrogen donors of the heteroscorpionate ligand, one is
occupied by the oxygen donor of the methoxy-quinoline
fragment, and the other two are occupied by two hal-
ogenido ligands, which complete a distorted octahedral
coordination environment. Hence, the organic ligand is
tetra-dentate and form the three six-membered and one
five-membered chelate rings. The six-membered chelate
rings are non-planar and adopt a boat conformation, with
the Co1 cobalt and C1 carbon atoms deviating from the
mean plane passed through the other atoms of the ring.
The five-membered chelate ring is in usual envelope con-
formation, with the Co1 cobalt atom out of the mean
plane passed through the other atoms of the ring. The
distortion of the ideal octahedron is directed toward the
square bipyramid, in which the pyrazolyl N1 and quino-
linyl N5 nitrogen atoms, methoxy-O1 oxygen atom, and
chlorido-Cl1 ligand constitute the basal plane, whereas
the pyrazolyl N3 and chlorido-Cl2 ligand occupy the api-
cal positions. Because of that, the apical Co1 Cl2 (2.3897
(7) Å) and Co1 N3 (2.2363(19) Å) bonds are longer than
the corresponding basal Co1 Cl1 (2.3159(8) Å) and
Co1 N1 (2.1292(18) Å) bonds (SI, Table S3). The long
Co1 O1 (2.4196(17)) distance together with the narrow
O1 Co1 N5 (69.58(7)ꢁ) intrachelate bite angle is deter-
mined by the rigid geometry of the scorpionate fragment.
Nevertheless, the basal N1 Co1 N5 (91.11(7)ꢁ) bond
angle is significant larger than the apical N1 Co1 N3
(81.09(7)ꢁ) and N3 Co1 N5 (76.48(7)ꢁ) bond angles
indicating the more flexibility of an apical position com-
pared to a basal one. This fact is also confirmed by the
more deviation of the apical Cl2 Co1 O1 (82.25(4)ꢁ)
bond angle from the ideal octahedral value of 90ꢁ than
the basal Cl1 Co1 O1 (93.65(5)ꢁ) bond angle. The
Cl2ap Co1 N3ap bond angle is equal to 169.07(5)ꢁ.
Crystal packing of 1 is stacking along the crystallo-
graphic b axis (SI, Figure S14). The stacks are linked to
each other by weak C HꢀꢀꢀCl (SI, Table S4) hydrogen
bonding interactions into three-dimensional framework.
It is interesting to point out that the geometry of
ligand L almost does not change upon the complexation
(Figure 2; SI, Table S3). So, the bond lengths at the cen-
tral C1 carbon atom are equal to 1.470(2) Å (C1 N2),
1.444(2) Å (C1 N4) and 1.530(3) Å (C1 C2) for L and
1.452(3) Å (C1 N2), 1.458(3) Å (C1 N4) and 1.522(3) Å
(C1 C12) for 1. Moreover, a sum of the bond angles at
FIGURE 2 Molecular
structure representations of
ligand L and complex 1 in 50%
thermal ellipsoids

6 of 14
ZUBKEVICH ET AL.
the C1 carbon atom is 333.4(5) and 332.9(5)ꢁ for ligand
L and complex 1, respectively.
toluene medium. All experimental results for catalytic
systems based on 1 and 2 are listed in Table 1.
The use of tetradentate NNNO-ligand makes it possi-
ble to obtain the cobalt complex 1 with four coordination
bonds to a metal center in the crystalline state. However,
Complex 1 was moderately active from 60 to
130 kgoligomerꢀmol[Co]−1ꢀh−1ꢀatm−1, but only in combina-
tion with Et2AlCl or Et3Al2Cl3. When other
organoaluminum compounds (OAC) such as MMAO,
Al(i-Bu)3, AlEt3 or AlMe3 have been employed, it showed
no activity. The mixture of butenes and hexenes is
formed during ethylene oligomerization with the α-olefin
share (average of 1-butene and 1-hexene) > 70%. The
increase of ethylene pressure in the reactor (up to
5.77 atm.) and temperature in the range from 30 to 40 ꢁС
has a little effect on the catalytic properties of the system
(Table 1, entries 2 vs 6; 9–11). The decrease of the tem-
perature below 30 ꢁC significantly reduces catalytic activ-
ity (Table 1, entry 8). The use of 1 equiv. of PPh3, that
usually significantly alters the catalytic properties of
nickel based systems,[17,38–52] results only in the slight
activity increase (Table 1, entries 1 vs 3 and 2 vs 4). More-
over, no change in the distribution of oligomers when the
PPh3 was used in the combination with Et3Al2Cl3, sug-
gests that triphenylphosphine is not involved in the cata-
lytic process.
1
analysis of H NMR spectra of complexes 1 and 2 dis-
solved in methanol-d4 revealed that they possess similar
structures in solution, that are different from the ones in
solid. Ni (II) and Co (II) compounds are of paramagnetic
1
nature, so the signals of protons in the H NMR spectra
are significantly broadened and have large chemical
shifts. (SI, Figures S9, S12) The DFT calculations have
been used to assign the signals in the spectra (see experi-
mental part for more details). The list of calculated and
corresponding experimental chemical shift values of pro-
tons in complexes 1 and 2 is given in supporting informa-
tion (SI, Table S1). The correlations between
experimental and calculated chemical shifts (Figure 3) of
ligand protons are fitted by linear function with a good
R2-factor (R2 > 0.95) for both complexes except for sig-
nals of –OCH3 group (marked with red dot). Such differ-
ence between theoretical and experimental values
suppose that these complexes have only tridentate coordi-
nation of the ligand in solution. Indeed, according to X-
ray data the Co1 O1 bond is rather long (2.4196(17) Å)
and weak, so it easily breaks upon dissolution in polar
solvent. Nevertheless, the steric hindrance of the ligand
does not allow these complexes to isomerize into the
ionic form ([ML2]+2[MHal4]−2), typical for nickel
halides,[18,37] that does not exhibit catalytic activity in
ethylene oligomerization.[17]
The composition of the activator has a notable effect
both on activity and selectivity of the studied systems.
Thus, the use of Et3Al2Cl3, that has higher Lewis
acidity,[53] instead of Et2AlCl, with the same Co/Al
molar ratio results in growth of hexenes share from 4 to
18% (Table 1, entries 1 vs 2). This trend becomes even
more noticeable at the elevated pressure of ethylene
(Table 1, entries 2 vs 5). At the same time, an increase
in the amount of Et3Al2Cl3 in the system to 600 equiv.,
despite increasing its activity, significantly reduces the
share of hexenes to 9% (Table 1, entries 5 vs 6 and 7).
The optimal composition of the system that allows to
achieve the highest activity and comparable selectivity
for α-olefins (Table 1, entries 2 vs 9, 5 vs 6) is
[Co]/[Al] = 1/300. However, catalytic system 1/Et3Al2Cl3
3.3 | Oligomerization of ethylene
3.3.1 | Oligomerization of ethylene in
toluene medium
ꢁ
Cobalt (II) 1 and nickel (II) 2 complexes exhibited moder-
showed low thermic stability, becoming inactive at 50 C
ate catalytic activity in ethylene oligomerization in
(Table 1, entry 12).
FIGURE 3 Comparison of
experimental and calculated 1H
chemical shifts for complexes 1 and 2

ZUBKEVICH ET AL.
7 of 14
TABLE 1
Ethylene oligomerization with 1–2/OAC in toluene[a]
Oligomer distributionc, %
Entry
1
Precatalyst
Cocatalyst, [Al]/[M]
Et2AlCl, 150
T, ꢁC
30
30
30
30
30
30
30
25
30
35
40
50
30
30
Activityb
63
C4
C6
4.1
C8
α-C4
84.9
79.2
6.4
α-C6
67.3
74
1
1
1
1
1
1
1
1
1
1
1
1
2
2
95.9
81.7
93.6
86
-
2
Et3Al2Cl3, 150
Et2AlCl, 150
106
79
18.3
6.4
-
3d
4d
5e
6e
7e
8
-
4.6
Et3Al2Cl3, 150
Et3Al2Cl3, 150
Et3Al2Cl3, 300
Et3Al2Cl3, 600
Et3Al2Cl3, 300
Et3Al2Cl3, 300
Et3Al2Cl3, 300
Et3Al2Cl3, 300
Et3Al2Cl3, 300
Et2AlCl, 150
116
62
14
-
80.7
70.4
84.2
85.7
67
74.3
66.8
80.2
79.6
63.2
69
59.5
83.3
90.4
72.7
75.8
64.8
83.8
n.d.
96
40.5
16.7
9.6
-
89
-
90
-
70
23.7
24.2
35.2
16.2
n.d.
4
-
9
120
107
129
0
-
73
10
11
12
13
14
-
70.5
71.6
n.d.
72.4
47.7
67.3
66.5
n.d.
37.7
25.9
-
n.d.
-
60
Et3Al2Cl3, 150
324
89.8
9.5
0.6
aOligomerization has been carried out in 50 mL of toluene at constant excessive ethylene pressure – 42 psi (2.9 atm), time: 30 min,
1ꢀ10−5 mol of corresponding complex.
bkgoligomerꢀmol[M]−1ꢀh−1ꢀatm−1 (M = Ni, Co. The activities were calculated from the total consumption of ethylene (1.0 L ethylene = 1.18 g
product)).
cDetermined by GC (Cn – share of each oligomer fraction; α-Cn – share of α-isomers in their fraction).
dAdditive: Ph3P 1 equiv.
eExcessive pressure of ethylene – 86 psi (5.77 atm).
The activity of systems based on complex 2 (Table 1,
entry 13) was equal to 1 (Table 1, entry 1), when Et2AlCl
was employed, but increased more than five times to
324 kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1, when activator was
substituted for Et3Al2Cl3 (Table 1, entry 14). The same
pattern was observed by us earlier,[17] when similar com-
plex with tridentate ligand without –OMe group was
used. The major products of the ethylene oligomerization
with the 2/OAC are butene isomers.
It should be noted, that Ni (II) and Co (II) complexes
show the ability not only to oligomerize ethylene, but
also to perform Friedel-Crafts alkylation of toluene with
the resulting oligomers.[54] Usually this process is unfin-
ished, since the oligomerization reaction predominates
over the alkylation reaction, although there are reverse
examples when the toluene alkylation is quantitative.[55]
To prevent this side reaction in our study, the catalytic
properties of systems based on complexes 1 and 2 were
studied in the chlorobenzene medium.
and MMAO, have been introduced in this set of experi-
ments. The results of the catalytic experiments are shown
in Table 2.
The activity of catalytic systems based on Co
(II) complex 1 in chlorobenzene medium has increased
to 140–200 kgoligomerꢀmol[Co]−1ꢀh−1ꢀatm−1 (Table 2,
entries 1, 2) due to greater solubility of the complex 1 in
more polar solvent. However, the shares of 1-butene and
1-hexene in the reaction products decreased significantly
compared to experiments in toluene (α-С4 < 45%,
α-С6 < 35%). The use of EtAlCl2 as co-catalyst does not
increase the catalytic activity, compared to Et2AlCl
(Table 2, entries 3 vs 1).
The activity of catalytic systems based on complex 2 is
significantly higher than of systems with complex
1 (Table 2, entries 2 vs 7, 3 vs 11). It increases from 163 to
1912 kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1 (Table 2, entries
5,7,11) in a rowof activators: Et2AlCl< Et3Al2Cl3 < EtAlCl2
according to the increase of their Lewis acidity.[56] The
use of MMAO also affords the active system, but with the
minimal activity – 89 kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1
(Table 2, entry 9). Such effect of the activator composi-
tion on activity was previously observed in.[17,51,57–60] The
use of 1 mol. Equiv. of PPh3 increases the activity from
2 (EtAlCl2, Table 2, entries 11,12) to 12 times (MMAO,
Table 2, entries 9,10) with the maximum value for system
3.3.2 | Oligomerization of ethylene in
chlorobenzene
In addition to the previously used Et2AlCl and Et3Al2Cl3
two new activators: EtAlCl2 (with higher Lewis acidity)

8 of 14
ZUBKEVICH ET AL.

ZUBKEVICH ET AL.
9 of 14
2/300EtAlCl2/PPh3–4,173 kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1
(Table 2, entry 12). However, the increase of PPh3
amount immediately results in lowering of the activity.
Thus, even 1 extra mol. Equiv. of PPh3 decreases the cata-
lytic activity by 17% to 3,457 kgoligomerꢀmol
[Ni]−1ꢀh−1ꢀatm−1 (Table 2, entry 13).
The graph displaying oligomer distributions for sys-
tems based on complexes 1 and 2 is shown on Figure 4.
The obtained oligomers are highly-branched, because
numerous isomers have been observed on GC–MS curves
(Figure 5; SI, Figures S15–S26). When Et2AlCl was used
as the activator, the resulting systems produced only C4
and C6 fractions with traces of octenes both for com-
plexes 1 and 2 (Figure 4; Table 2, entries 1,5,6). On the
other hand, when activators with higher Lewis acidity
were employed, oligomer mixtures with broader distribu-
tions up to C20 were obtained. The share of obtained
hexenes was higher than share of butenes in some experi-
ments (Table 2, entries 3, 5, 8, 11–13). That fact cannot
be ascribed just to the higher volatility of the last. The
maximum hexene/butene ratio is observed for the system
2/300Et3Al2Cl3/PPh3 (Table 2, entry 8) and reaches
71.3/15. This difference, apparently, arises from the high
reaction rate of 1-butene and ethylene co-dimerization,
since the probable reaction mechanism, described below,
does not imply the formation of metallocyclic intermedi-
ates (see paragraph 3.3.4).
FIGURE 5 Part of GC–MS total ion current chromatogram of
entry 8, Table 2, system 2/300 Et3Al2Cl3/Ph3P
for the description of such distribution. This fact is attrib-
uted to a large number of ongoing co-oligomerization
reactions between various olefins obtained during the
catalytic process.
Upon a more detailed consideration of the analysis
results of oligomer mixtures compositions, the formation
of alkyltoluenes in the experiments 2, 7, 8, 10 should be
noted. Their formation is explained by the use of MMAO
and Et3Al2Cl3 toluene solutions as activators. In this
regard, the 2/MMAO system deserves special attention,
because, despite its low activity, it efficiently alkylates tol-
uene with the obtained oligomers (29.7% of
alkyltoluenes).
It was also found that for catalytic systems based on
complex 1 the share of С12 was higher than the share of
С10 (Table 2, entry 2) and even than the one of С8
(Table 2, entry 3), which in this case indicates a signifi-
cantly high reaction rate of 1-hexene dimerization. More
equal distributions of oligomers are observed for systems
based on complex 2. Moreover, we were not able to use
any of the previously proposed mathematical models[61]
3.3.3 | Formation of odd carbon number
olefins
The most intriguing results have been observed using 2/
EtAlCl2 and 2/EtAlCl2/PPh3 catalytic systems. In addi-
tion to expected ethylene oligomers with an even number
of carbon atoms (С4–C20), olefins with an odd number of
carbon atoms (C7–C15) have been obtained (See
Figure 6A; Table 2, entries 11–13). The С7 and C11 frac-
tions prevail among odd carbon number olefins, with the
total amount of such products about 5%. Mass spectra of
odd carbon number olefins are given in supporting infor-
mation (SI, Figures S27-S41).
A more detailed analysis was ꢁcarried out for the frac-
tion with a boiling point >140 C (Figure 6B) isolated
from the oligomer mixture produced in entry 13 (Table 2).
The results showed that the heaviest odd carbon number
olefin (C19) is shorter by only one carbon atom than the
heaviest even carbon number olefin (C20). The use of
1-hexene instead of ethylene significantly alters the distri-
bution pattern of the reaction products. Thus, the oligo-
merization of 1-hexene with the system 2/EtAlCl2
FIGURE 4 Distribution of ethylene oligomers obtained with
catalytic systems based on complexes 1 and 2

10 of 14
ZUBKEVICH ET AL.
FIGURE 6 (A) compositions of
oligomer mixtures obtained with
systems based on 2/EtAlCl2 (Table 2,
entries 11–13); (B) composition of
oligomer fraction with b.p. > 140 ꢁC for
2/300 EtAlCl2/2 PPh3 (Table 2, entry 13)
(Figure 7) results in 6.75% of hexene dimers, 0.86% of tri-
mers and a minor amount of tetramers (not possible to
identify single peaks due to overlap). The bulk of the
reaction mixture (up to 90%) consists of the monomer,
which has undergone significant isomerization (SI,
Figure S42).
Besides the above-mentioned components, insignifi-
cant amounts of C7 and C10 isomers are present in the
reaction mixture as well as C8 isomers and their co-
oligomerization products with 1-hexene – C14, C20 and
C26 (the mass spectra of these compounds are presented
in SI (Figures S43–S50)).
Previously, the formation off odd carbon number olig-
omers during ethylene oligomerization was observed for
chromium-containing systems activated with MAO.[21,22]
Initially, the formation of such products was explained
by a mechanism presupposing the formation of a chro-
mium carbene complex and further olefin metathesis.[21]
Later a mechanism, involving chain-transfer to alumi-
num was introduced,[25] but in our case it is not likely
due to the absence of methyl groups in the activator.
Other reports[23,24] showed that formation of odd carbon
number olefins can proceed with Fe (II) bisiminopyridine
complexes. A mechanism explaining these experimental
facts suggests three simultaneous processes: oligomeriza-
tion, isomerization and metathesis catalyzed by iron
bisiminopyridine complex. Later several theoretical and
computational studies[62,63] established that iron catalysts
have a high potential in the reaction of olefin metathesis.
We cannot suggest a reliable mechanism of formation
of odd carbon number olefins during ethylene oligomeri-
zation and unexpected products (С7, C8, C10, C14, C20,
C26) during 1-hexene oligomerization using the available
data. Based on the distribution of reaction products, it
could be assumed that there is a probability of the follow-
ing processes:
1). Cross-metathesis and ethylenolysis reactions
(Figure 8) catalyzed by Ni complex 2 in the presence of
EtAlCl2. According to[64] the equilibrium shift to the
ethylenolysis reaction occurs at elevated ethylene pres-
sure (in our experiments pethylene ≥ 2.9 atm.). If the
ethylenolysis reaction really occurs, the formation of two
terminal odd carbon number oligomers is quite natural
during the reaction of an even carbon number olefin mol-
ecule with inner double bond and ethylene, although
=
=
unexpected. For example, for C15=: C2 + 2-C16
=
C3= + 1-C15=, followed by subsequent isomerization.
2). β-alkyl elimination,[65] as an alternative mecha-
nism of chain termination. Cleavage of β-CH3 gives prod-
ucts that differ by only one carbon atom, considering the
case of ethylene oligomerization. However, this mecha-
nism much worse describes the distribution of products
formed during the oligomerization of 1-hexene, because
it requires the preferable β-C2H5 elimination.
FIGURE 7 Composition of the oligomer mixture obtained
during 1-hexene oligomerization with 2/300 EtAlCl2
FIGURE 8 Schematic representation of equilibrium between
cross-metathesis and ethylenolysis

ZUBKEVICH ET AL.
11 of 14
FIGURE 9 Part of 13C NMR (600 MHz,
CDCl3) spectrum of oligomer mixture obtained
with 2/300 EtAlCl2/2 PPh3 (Table 2, run 13)
The possible formation of odd carbon number olefins,
as a result of side-reactions in the presence of the
EtAlCl2, is not supported neither by literature[57,66–82] nor
by experimental data (SI, Figure S51).
possible that –OMe group can play a role of oscillating
donor during the catalytic process in less polar medium
(toluene, chlorobenzene), but the additional experiments
are required to determine the coordination mode of the
ligand during the catalytic process and the active species.
Earlier, we have considered the probable mechanism
of formation of hexene isomers using catalytic systems
based on NNN-nickel (II) complexes[43] and showed that
it doesn't differ from the generally accepted model. How-
ever, the 13C NMR spectrum of oligomer mixture
obtained with 2/300 EtAlCl2/2 PPh3 (Figure 9) shows the
presence of intensive signals that belong to cis-3-methyl-
2-pentene (137.91 ppm and 118.09 ppm), trans-3-methyl-
2-pentene (137.69 ppm and 116.97 ppm) and 3-methyl-
1-pentene (144.77 ppm and 112.54 ppm). In addition, the
signals of 1-butene (140.67 ppm and 113.24 ppm), cis-
2-butene (124.72 ppm), trans-2-butene (125.96 ppm), cis-
2-hexene (131.02 ppm) and trans-2-hexene (131.56 ppm)
3.3.4 | The mechanistic aspects of
ethylene oligomerization
In accordance with the most common point of view,[83,84]
ethylene oligomerization with catalytic system based on
nickel and cobalt complexes proceeds via the Cossee–
Arlman coordination–migration mechanism.[85,86] Taking
into account the tetradentate nature of the ligand, the
active species are probably generated through complete
substitution of halide ligands on the metal center. How-
1
ever, the H NMR spectra of methanol solutions of these
complexes revealed only tridentate coordination. It is
SCHEME 2 Proposed mechanism for the
formation of butene and hexene isomers, during
ethylene oligomerization

12 of 14
ZUBKEVICH ET AL.
are clearly visible in the spectrum. The assignments of
these signals have been proposed based on.[43,87]
mechanisms for the formation of these products have
been proposed: ethylenolysis of β-olefins and β-alkyl
elimination during oligomerization. The presence of
branched oligomers (in particular hexenes) in the oligo-
mer mixture is most likely the result of chain migration
and subsequent ethylene insertion processes, which is in
accordance with the Cossee mechanism.
The formation of 3-methyl-2-pentene may indicate
that the oligomerization process proceeds according to
the metallocyclic mechanism.[88–90] In this case,
branched hexene isomers, such as 3-methyl-2-pentene,
3-methyl-1-pentene, 2-ethyl-1-butene, are formed by the
co-dimerization of ethylene and butene through the for-
mation of metallocycle. Thus, the co-dimerization prod-
uct obtained from ethylene and 1-butene will be 2-ethyl-
1-butene, which signals (153.36 ppm and 106.76 ppm) are
absent in the 13C spectrum. Therefore, we proposed a
mechanism for the formation of cis−/trans-3-methyl-
2-pentene and 3-methyl-1-pentene according to the
Cossee mechanism (Scheme 2).
According to this mechanism, at the first stage of the
process, two ethylene molecules sequentially insert into
the nickel hydride complex to form LNi+n-Bu. Further
proceeds the formation either of 1-butene via β-H elimi-
nation, or of LNi+sec-Bu via chain-walking. Also the con-
secutive chain growth can occur, resulting in the
formation of linear hexenes upon their elimination from
LNi+n-Hex. At the same time, the formation of 1-butene
and 1-hexene can be considered reversible, since they can
easily reinsert to metal center and undergo further trans-
formations. Starting from LNi+sec-Bu either cis−/trans-
2-butenes or isomers of 3-methylpentene are formed, the
latter via consecutive chain-walking and insertion steps.
The high speed of chain-walking rearrangements is
worth mentioning – the signals corresponding to internal
olefins are dominating the 13C NMR spectrum of the olig-
omer mixture.
ACKNOWLEDGMENTS
This work was supported by Russian Foundation for
Basic Research (Project 18-33-20091). NMR measure-
ments were funded by the Council of the President of the
Russian Federation (Project МК-199.2019.3). X-ray analy-
sis was supported by the RUDN University Program
“5-100”. Elemental analysis was performed with the
financial support from Ministry of Science and Higher
Education of the Russian Federation using the equip-
ment of Center for molecular composition studies of
INEOS RAS. IR measurements were financed through
the Government of the Russian Federation budget sup-
port (reg. № АААА-А16-116053110012-5).
ORCID
Sergey V. Zubkevich https://orcid.org/0000-0001-5371-
0901
Vladislav A. Tuskaev https://orcid.org/0000-0001-7182-
462X
Svetlana Ch. Gagieva https://orcid.org/0000-0003-4654-
9974
Anatoliy S. Kayda https://orcid.org/0000-0002-3500-
8550
Victor N. Khrustalev https://orcid.org/0000-0001-8806-
2975
Alexander A. Pavlov https://orcid.org/0000-0002-4612-
0169
4 | CONCLUSIONS
Dmitry N. Zarubin https://orcid.org/0000-0003-2626-
8024
Boris M. Bulychev https://orcid.org/0000-0003-3999-
3137
In this work, we have obtained and characterized neutral
monoligated Ni (II) and Co (II) complexes with the new
NNNO-heteroscorpionate ligand. The first two, activated
by Et2AlCl, Et3Al2Cl3 or EtAlCl2, catalyze ethylene oligo-
merization. The activity of the nickel-based catalytic sys-
tem is significantly higher and reaches 1900
kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1 when EtAlCl2 is used. The
addition of triphenylphosphine to this system increases
catalytic activity by 2–12 times. The maximum value of
activity was obtained for system 2/EtAlCl2/PPh3–4,173
kgoligomerꢀmol[Ni]−1ꢀh−1ꢀatm−1. The products of ethylene
oligomerization reaction consisted of branched olefins
with chain lengths ranging from C4 to C20. Catalytic olig-
omerization of ethylene with the 2/EtAlCl2 and 2/
EtAlCl2/PPh3 systems revealed the formation of oligo-
mers with an odd number of carbon atoms, the amount
of which is ꢂ5% of entire oligomer mixture. Two possible
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How to cite this article: Zubkevich SV,
Tuskaev VA, Gagieva SC, et al.
NNNO-Heteroscorpionate nickel (II) and cobalt
(II) complexes for ethylene oligomerization: the
unprecedented formation of odd carbon number
olefins. Appl Organomet Chem. 2020;e5873. https://
doi.org/10.1002/aoc.5873