Synthesis of highly branched oligoethylenes by air-stable N,N-indazole derivate methallyl Ni(II) complexes

Article abstract of DOI:10.1016/j.ica.2020.119761

Two new neutral N,N-indazole ligands bearing the carbonitrile functional group (L1 and L2) and two new air-stable cationic nickel methallyl complexes (C1 and C2) were prepared. These compounds were fully characterized by NMR, FT-IR and elemental analysis.

Full text of DOI:10.1016/j.ica.2020.119761

InorganicaChimicaActa510(2020)119761
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Research paper
Synthesis of highly branched oligoethylenes by air-stable N,N-indazole
derivate methallyl Ni(II) complexes
Claudia Arayaa, Iván Martínez-díazb, Sebastián A. Correaa,b, Constantin G. Daniliucc,
René S. Rojasa,⁎
a Departamento de Química Inorgánica, Facultad de Química y de Farmacia, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Macul, Santiago, Chile
b Departamento de Ciencias Químicas y Biológicas, Universidad Bernardo O`Higgins, General Gana 1702, Santiago, Chile
c Organisch-Chemisches Institut, Westfälische Wilhelm-Universität, Corrensstrasse 40, 48149 Münster, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ni complexes
Air-stable
Remote activation
Hyperbranched structure
Two new neutral N,N-indazole ligands bearing the carbonitrile functional group (L1 and L2) and two new air-
stable cationic nickel methallyl complexes (C1 and C2) were prepared. These compounds were fully char-
acterized by NMR, FT-IR and elemental analysis. In addition, compounds L1, L2 and C2 were analyzed using X-
ray diffraction. The reactivity of complexes C1 and C2 toward ethylene was studied by using 5 equivalents of B
(C6F5)3. At 12.5 bar of ethylene and 20 °C, complexes C1 and C2 could produce oligoethylenes with a moderate
activity ∼ 700 kg of PE (mol of Ni)-1h−1. By increasing the temperature to 60 °C, the catalytic activity increased
to ∼ 103 kg of PE (mol of Ni)-1 −1, and oligoethylenes were produced. The microstructure studied by 13C NMR
h
revealed a broad distribution of branches, including methyl, ethyl, propyl, butyl and long branches. Remarkably,
complexes C1 and C2 showed the presence sec-butyl branches, which are characteristic of hyperbranched mi-
crostructures. The GPC analysis result is consistent with the use of low-molecular-weight materials, and the PD1
was ∼ 1, which may indicate the presence of living behavior.
1. Introduction
group-IV tandem polymerization processes [13] and tandem catalyst
systems based on late transition metals [14,15]. A Pd-based catalyst can
In 1995, a real breakthrough in the development of late transition
metal catalysts to efficiently polymerize ethylene was introduced by
Brookhart and coworkers [1]. Since then, a wide variety of nickel-based
initiators has been studied for the polymerization of high-molecular-
weight-polyethylene [2–5]. A key feature of nickel-based catalysts is
their ability to polymerize ethylene via the “chain-walking mechanism”,
which enables the production of attractive branched polymeric mate-
rials [6–8]. High-molecular-weight materials are interesting due to
their mechanical and rheological properties, and active academic and
industrial studies have focused on these materials. However, this con-
cept has been shifted to another class of undeveloped materials: low-Mn
highly branched polyethylenes (HBPE), which can be used as synthetic
lubricants, interface active agents or viscosity modifiers and functional
additives [9–11].
undergo extensive “chain-walking” during polymerization and produce
HBPE materials [4,10]. However, they are self-limited due to their low
activity. Meanwhile, nickel-based catalysts are more attractive in terms
of abundant availability of the metal. Among all well-known nickel
catalysts, Ni-methallyl complexes with different coordination modes,
i.e., P,O; N,O or N,N (Chart 1), have been studied. Bazan et al. reported
the initiator (Chart 1a), which could produce almost exclusively 1-bu-
tene [16]. Lee et al. replaced the diphenylphosphino group of initiator
1a for the alkylideneamino group, and the initiator (Chart 1b) could
produce polyethylene [17]. These examples show the ligand-induced
changes in the polymerization processes. Particularly, for 1b, the chain
transfer reaction is dramatically reduced, which favors the polymer
growth.
In this context, our research group has focused on the synthesis of
N,N nickel complexes. Trofymchuck et al. synthetized β-diketoenami-
nate nickel complexes (Chart 1c) that were active toward ethylene
polymerization, and they obtained high-molecular-weight branched
materials in good activity [18]. Cabrera et al. reported nickel complexes
bearing indazole derivates (chart 1d) that were active to ethylene
Group IV (PAO - polyalphaolefins) accounts for the largest product
segment of the global synthetic lubricant market, and production is
expected to reach US 33.8 Bn by 2023 [12]. The large global demand
makes active research of the development of new catalysts that can
produce HBPE materials necessary. Their synthesis can be achieved by
⁎ Corresponding author.
E-mail address: rrojasg@uc.cl (R.S. Rojas).
https://doi.org/10.1016/j.ica.2020.119761
Received 4 April 2020; Received in revised form 11 May 2020; Accepted 11 May 2020
Availableonline13May2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

C. Araya, et al.
InorganicaChimicaActa510(2020)119761
Chart 1. Selected Ni-methallyl complexes.
polymerization [19]. Interestingly, the branching pattern shows the
presence of almost exclusively methyl branches; such branching control
mediated by a catalyst is interesting and opens the opportunity to de-
velop a new N,N-indazole ligand, which enables us to produce nickel
catalysts that can control the branching content and access novel
branching architectures.
Here, we report the synthesis and structural characterization of new
indazole derivates with the carbonitrile functional group in the ligand
backbone. This functional group enables the remote activation of the
metal center by Lewis acid as tris(pentafluorophenyl)borane, B(C6F5)3.
Upon polymerization reactions, hyperbranched low-molecular-weight
materials were obtained due to the presence of sec-butyl branches,
poured into a brine solution and extracted with diethyl ether. The or-
ganic layer was dried over Na2SO4, and the solvent was removed in
vacuum to afford a brown oil, which corresponded to 1a in a 67% yield.
1H NMR (400 MHz, CDCl3, 300 K): δ/ppm = 9.59 (s, 1H), 7.42 (d,
J = 2.9 Hz, 5H), 7.18 (t, J = 7.3 Hz, 1H), 7.07 (d, J = 7.8 Hz, 1H), 6.74
(d, J = 8.0 Hz, 1H), 6.53 (dd, J = 22.7, 16.3 Hz, 2H). 13C NMR
(101 MHz, CDCl3, 300 K): δ/ppm = 180.29, 133.24, 131.43, 129.25,
128.79, 128.46, 127.32, 127.20, 116.95, 116.87, 115.76.
2.2.2. Synthesis of 3-phenyl-1H-indazole (1b)
1a was added to a solution of Cu(OAc)2 (0.38 g, 2.09 mmol) in
DMSO. The mixture was heated at 85 °C for 3 h, subsequently poured
into brine and extracted with EtOAc. The solvent was concentrated in
vacuum. After purification by column chromatography (n-hexane/
EtOAc 20:1), a yellow solid was obtained in 47.5% yield. 1H NMR
(400 MHz, CDCl3, 300 K): δ/ppm = 12.06 (s, 1H), 8.05 (t, J = 7.4 Hz,
3H), 7.57 (t, J = 7.2 Hz, 2H), 7.48 (t, J = 7.1 Hz, 1H), 7.34 (t,
J = 7.5 Hz, 1H), 7.25 – 7.15 (m, 2H). 13C NMR (101 MHz, CDCl3,
300 K): δ/ppm = 145.71, 141.75, 133.63, 129.01, 128.24, 127.86,
126.79, 121.34, 121.06, 120.98, 110.41. Anal Calc. for C13H10N2: C,
80.39: H, 5.19: N, 14.42. Found: C, 80.38: H, 5.20: N, 14.40.
which are indicative of
a branch-on-branch motif. This unique
branching motif has not been reported in nickel methallyl complexes.
2. Experimental
2.1. General remarks
All manipulations were performed in an inert atmosphere using the
standard glovebox and Schlenk-line techniques. All reagents were used
as received from Aldrich unless otherwise specified. Ethylene was
purchased from Matheson Tri-Gas (research grade, 99.99% pure).
Toluene, THF, hexane, and pentane were distilled from benzophenone
ketyl, and NEt3 was dried over KOH. The starting compound N-(2-cia-
nophenyl)benzomidoyl [20] and [(η3-CH3C(CH2)2)NiCl]2 [21] were
synthesized according to the literature procedures. The NMR spectra
were obtained using Bruker Avance 400 spectrometers. The chemical
shifts are given in parts per million relative to TMS (1H and 13C,
δ(SiMe4) 0) or an external standard (δ(BF3•OEt2) 0 for 11B, δ(C6H5CF3)
0, and for 19F NMR. Most NMR assignments were supported by addi-
tional 2D experiments. FT-IR spectra were acquired on a Shimadzu
IRTracer-100 spectrophotometer using KBr pellets. An elemental ana-
lysis (C,H,N) was performed on an Elementar Vario EL III Analyzer. X-
ray diffraction was used to analyze the crystal structures. The data were
collected with a Bruker D8 Venture CMOS or a Bruker APEX II CCD
diffractometer and provided by Dr. Constantin G. Daniliuc; complete
crystallographic details can be found in the independently recorded
crystallographic information files. All polymerization reactions were
performed in a Parr autoclave reactor as described below. The polymers
(waxes) were dried overnight in vacuum, and the polymerization ac-
tivities were calculated from the mass of the obtained product. The
polymers were characterized by GPC analysis at 135 °C in o-di-
chlorobenzene (in a Polymer Laboratories, high-temperature chroma-
tograph, PI-GPC 200). The 1H NMR spectra of the polymers were ob-
tained in solution (C6D6).
2.2.3. Synthesis 1-((N-(2-cyanophenyl)-1-phenylimine)indazole (L1)
N-(2-cyanophenyl)benzomidoyl chloride (0.500 g, 2.08 mmol) was
added dropwise to a solution of 1H-indazole (0.250 mg, 2.08 mmol) in
anhydrous toluene using triethylamine as the catalyst. The mixture was
refluxed for 36 h with vigorous stirring. The yellow solution was re-
fluxed to dryness. The crude was purified via silica gel chromatography
(4:1 hexane/ethyl acetate), and L1 was collected as a pale yellow solid
in 60% yield. 1H NMR (400 MHz, CDCl3, 298 K): δ/ppm = 8.98 (s, 1H),
7.68 (d, J = 8.9 Hz, 2H), 7.53 (d, J = 9.0 Hz, 1H), 7.38 (m, 6H), 7.27
(m, 1H), 7.09 (m, 2H), 6.84 (d, J = 8.0 Hz, 1H). 13C NMR (101 MHz,
CDCl3, 298 K): δ/ppm = 150.92, 133.32, 133.01, 131.02, 130.31,
129.79, 128.62, 128.54, 128.34, 124.26, 124.01, 123.67, 122.39,
121.83, 121.24, 119.11, 117.24, 105.16. FT-IR (KBr): ν/cm−1 = 2223
(C^N). Anal Calc. for C21H14N4: C, 78.24: H, 4.38: N, 17.38. Found: C,
78.28: H, 4.32: N, 17.42.
2.2.4. Synthesis
indazole (L2)
of
3-phenyl-1((N-(2-cyanophenyl)-1-phenylimine)
To a solution of 3-phenyl-1H-indazole (0.12 g, 0.63 mmol) in dry
toluene, N-(2-cianophenyl)benzomidoyl chloride (0.15 g, 0.63 mmol)
was added. The mixture was refluxed at 80 °C for 24 h. L2 was obtained
as analytically pure yellow crystals after the column chromatography
purification (ethyl acetate/hexanes 98/2) in a 68.8% yield. 1H NMR
(400 MHz, CDCl3, 300 K): δ/ppm = 8.82 (d, J = 8.0 Hz, 1H), 8.07 (d,
J = 7.9 Hz, 1H), 7.88 (d, J = 6.4 Hz, 2H), 7.63 (t, J = 7.0 Hz, 1H), 7.53
(d, J = 7.7 Hz, 1H), 7.50 – 7.40 (m, 6H), 7.40–7.30 (m, 4H), 7.02 (t,
J = 7.5 Hz, 1H), 6.82 (d, J = 7.9 Hz, 1H). 13C NMR (101 MHz, CDCl3,
300 K): δ/ppm = 157.80, 152.08, 148.87, 141.32, 132.99, 132.80,
132.23, 131.50, 130.15, 129.88, 129.08, 128.80, 128.14, 127.83,
124.90, 124.53, 123.21, 122.36, 121.23, 117.81, 116.78, 105.60. FT-IR
(KBr): ν/cm−1 = 2217 (C^N). Anal Calc. for C27H18N4: C, 81.39: H,
4.55: N, 14.06. Found: C, 81.38: H, 4.55: N, 14.07.
2.2. Synthesis of ligands
2.2.1. Synthesis of 2-[imino(phenyl)methyl]aniline (1a)
To a cooled solution of 2-aminobenzonitrile solution in THF (1.22 g,
0.01 mol), phenylmagnesium bromide (11 mL, 0.03 mol, 3 M) was
added dropwise. Then, the reaction mixture was stirred for 3 h at
−10 °C. To remove the unreacted Grignard reagent, the mixture was
2

C. Araya, et al.
InorganicaChimicaActa510(2020)119761
2.3. Synthesis of complexes
in a good yield, and they were fully characterized by NMR spectro-
scopy, FT-IR, elemental analysis and X-ray crystallography.
2.3.1. Synthesis of (1-((N-(2-cyanophenyl)-1-phenylimino)indazole)(η3-
methylallyl)niquel(II) tetrafluoroborate (C1)
The elemental analysis is consistent with the ligands in Scheme 2.
However, the regiochemistry can be modulated by the reaction condi-
tions to obtain one of the N-1 or N-2 substituted indazole regioisomers.
We use a nondeprotonating base as NEt3, which is a well-known N-2
directed [23]. 1H, 13C NMR and FT-IR analysis are consistent with the
formation of a single isomer for compounds L1 and L2. For example,
only one peak for the nitrile 13C NMR chemical shift was observed for
each ligand at ∼ 117.8 ppm, and a stretching band appears at 2221 and
2225 cm−1 in the FT-IR. However, these results cannot help us distin-
guish the two regioisomers.
An X-ray diffraction study was performed to identify the chemical
structure for both ligands; single crystals for L1 and L2 were grown in
ether/n-hexane (1:3). Fig. 1 shows the structure for both ligands. As
expected, an N-2 substituted indazole was obtained for L1 due to the
use of Et3N as a nondeprotonating base. However, L2 shows a N-1
substituted indazole probably due to the steric hindrance around the 3-
phenyl moiety, which makes the N-1 attack more prone to nucleophilic
attack. These ligands exhibit an E configuration in the imine; mean-
while, the nitrile group is in plane for L1, and the L2 nitrile group is
orthogonal to the indazole plane. For further details of the X-ray char-
acterization, see Supplementary data.
L1 (645 mg, 2.0 mmol) and NaBF4 (109 mg, 0.99 mmol) were mixed
in dichloromethane (10 mL). The suspension was vigorously stirred,
and a solution of [(η3-CH3C(CH2)2)NiCl]2 (300 mg, 0.99 mmol) in di-
chloromethane was added. The mixture was stirred for 2 h at room
temperature. The resulting solution was filtered through celite, and a
red crystalline solid was obtained by adding pentane and storing at
−30 °C in 75% yield. 1H NMR (400 MHz, CDCl3, 298 K): δ/ppm = 8.17
(s, 1H), 7.63 (m, 2H), 7.59 (m, 1H), 7.41 (m, 6H), 7.33 (m, 2H), 7.08
(m, 1H), 3.46 (s, 1H), 3.18 (s, 1H), 2.72 (s, 1H), 2.65 (s, 1H), 2.28 (s,
3H). 13C NMR (400 MHz, CDCl3, 298 K): δ/ppm = 163.6, 152.5, 148.5,
136.1, 134.7, 134.1, 133.5, 131.7, 129.8, 128.5, 127.0, 123.5, 123.4,
122.6, 115.7, 106.9, 60.1, 23.3. 19F NMR (400 MHz, CDCl3, 298 K): δ/
ppm = m, −157.22. 11B NMR (400 MHz, CDCl3, 298 K): δ/ppm = s,
−1.23. FT-IR (KBr): ν/cm−1 = 2221 (C^N). Anal Calc. for
C
25H24BF4N4Ni: C, 57.09: H, 4.60: N, 10.65. Found: C, 56.92: H, 4.55:
N, 11.07.
2.3.2. Synthesis
of
(3-phenyl-1((N-(2-cyanophenyl)-1-phenylimino)
indazole)(η3-methylallyl)niquel(II) tetrafluoroborate (C2)
The methallyl nickel complexes were prepared by adding two
equivalent of the respective ligand with one equivalent of [(η3-
CH3C(CH2)2)NiCl]2 in the presence of one equivalent of NaBF4. The
target complexes C1 and C2 are red and brown solids in 75 and 76%
yields, respectively (Scheme 3). 1H and 13C NMR spectra in CDCl3 for
both complexes are consistent with the formation of a single isomer
containing a nickel center ligated in an N,N fashion and η3-(CH2)2Me.
Complex C1 features four separate resonances for the syn and anti
allylic hydrogen atoms at δ 2.65, 2.72 (Hsyn) and 3.18, 3.40 (Hanti) ppm.
Meanwhile, complex C2 shows syn and anti allylic hydrogen atoms at δ
2.30, 2.36 (Hsyn) and 2.43, 2.50 (Hanti) ppm. The resonances of anti
allylic hydrogen atoms appeared as broadened signals compared to the
syn allylic hydrogen atoms, which suggests a fluxional behavior of the
L2 (797 mg, 2.0 mmol) and NaBF4 (109 mg, 0.99 mmol) were mixed
in dichloromethane (10 mL). The suspension was vigorously stirred,
and a solution of [(η3-CH3C(CH2)2)NiCl]2 (300 mg, 0.99 mmol) in di-
chloromethane was added. The mixture was stirred for 2 h at room
temperature. The resulting solution was filtered through celite and
dried in vacuum. The solid was washed with cooled dichloromethane,
and recrystallization occurred in pentane at −30 °C overnight, which
yielded a brown solid in 76%. 1H NMR (400 MHz, CDCl3, 300 K): δ/
ppm = 7.98 (d, J = 7.2 Hz), 7.85 (d, J = 7.1 Hz), 7.69 – 7.58 (m, 4H),
7.58 – 7.49 (m, 4H), 7.49 – 7.41 (m, 2H), 7.34 – 7.27 (m, 2H), 7.18 (t,
J = 7.7 Hz, 1H), 5.87 (d, J = 7.9 Hz, 1H), 2.50 (s,1H), 2.43 (s,1H), 2.36
(s,1H), 2.30 (s,1H), 2.12 (s, 3H). 13C NMR (101 MHz, CDCl3, 300 K): δ/
ppm = 161.75, 158.63, 149.93, 139.22, 134.65, 132.36, 132.10,
132.01, 131.19, 130.23, 130.05, 129.20, 128.89, 127.08, 126.88,
126.77, 126.49, 126.03, 125.86, 123.53, 112.22, 107.16, 58.24, 57.64,
53.59, 53.26, 22.88. FT-IR (KBr): ν/cm−1 = 2225 (C^N). Anal Calc.
for C31H28BF4N4Ni: C, 61.84: H, 4.69: N, 9.31. Found: C, 61.38: H, 4.55:
N, 9.87.
-
allyl moiety on the NMR time scale. [24,25] BF4 counter ion NMR
singlets in CDCl3 are consistent with “free” ions and are not an ion-
pairing with the cationic complex parts.
(
11B: −0.68 ppm, ν1/
2 = 11 Hz; 19F: −151.77 ppm) [19].
The molecular structure for C2 was determined by an X-ray dif-
fraction study and is shown in Fig. 2. The crystal structure of the
complex confirms a four-coordinated nickel metal center, which adopts
an almost ideal square-planar geometry. The methallyl group was found
disordered over two positions with a ratio of 81:19. Only the main
disordered part of this methallyl group is discussed below. The dihedral
angle corresponding to the N,N chelate section is −2.6° (N1-C41-C43-
N3). The N3-Ni1-N1, N3-Ni1-C43 N1-Ni1-C41 and C43-Ni1-C41 angles
were 82.9(1), 101.4(2), 104.3(2) and 71.5(2)o, respectively. The Ni1-
N1, Ni1-N3, Ni1-C42, Ni1-N43 and Ni1-C41 bond distances were
1.924(3), 1.923(3), 1.982(5), 2.011(5) and 2.020(4) Å, respectively.
The C37-N4 bond distance of 1.144(7) indicate the presence of a triple
bond [18,26,27]. Additionally, the cyanophenyl substituent is ortho-
gonal to the indazole plane (dihedral angle: 80.5(8)o (C8-N3-C31-
C36)), and the CN functionality is free to coordinate the Lewis acid.
Additional X-ray details have been reported in the Supplementary data.
3. Results and discussion
3.1. Synthesis and characterization of cationic methallyl Ni complexes
Scheme 1 shows the reaction pathway of the formation of the 3-
substituted 1H-indazole. The reaction proceeds via the nucleophilic
attack of phenylmagnesium bromide of the nitrile functional group of o-
aminobenzonitrile. The resulting o-aminoaryl NeH ketimine (1a) can
react with a catalytic amount of Cu(OAc)2, which facilitated NeN bond
formation to yield the desired 3-phenyl-1H-indazole (1b) [22].
Scheme 2 shows the reaction path to ligands L1 and L2. The ligands
were synthesized by the reaction of N-(2-cianophenyl)benzomidoyl
chloride with 1H-indazole and 3-phenyl-1H-indazole in the presence of
Et3N in toluene at reflux. Each ligand was isolated as a pale-yellow solid
Scheme 1. Synthesis of 3-phenyl-1H-indazole. i)
THF, ii) Cu(OAc)2, DMSO.
3

C. Araya, et al.
InorganicaChimicaActa510(2020)119761
Scheme 2. Synthetic route for ligands L1 and L2.
Fig. 1. a) X-ray crystal structure of ligands L1 and b) L2. The thermal ellipsoids are set at 30% probability.
Scheme 3. Synthetic route for complexes C1 and C2.
4

C. Araya, et al.
InorganicaChimicaActa510(2020)119761
conjugated with the nickel center. This interaction removes electron
density from the nickel core and makes it more prone to ethylene co-
ordination. Despite the cationic nature of these complexes, they proved
to be air-stable, which shows the same NMR pattern and ethylene oli-
gomerization activity after being exposed to ambient atmosphere for
48h and 7 days.
3.3. Oligomer characterization
Viscous materials were recovered from the reactor. The micro-
structural analysis of oligoethylenes by 13C NMR showed the presence
of methyl, ethyl, propyl, butyl, sec-butyl and long branches for all
samples (Table 2). Interestingly, sec-butyl moieties were found (Fig. 3),
which indicates that the microstructure is hyperbranched, since the
presence of sec-butyl is indicative of branch-on-branch moieties. It is
well-known that the branching densities and microstructure of oligo/
polyethylenes are greatly affected by the behavior of Ni or Pd agostic
alkyl intermediates [32,33]. These species can undergo “chain-walking”
via β-hydride elimination, olefin rotation and reinsertion. NMR studies
have shown that the barrier to a 1,2 shift in Ni agostic intermediates is
ca. 14 Kcal/mol, while for Pd agostic intermediates, the 1,2-chain-
walking is lower (9-10 kcal/mol), which implies that many 1,2-shifts
occur prior to insertion and yield hyperbranched structures [34]. To the
best of our knowledge, such microstructure has not been reported in
other methallyl nickel complexes reported in the literature.
Fig. 2. X-ray crystal structure of C2 with thermal ellipsoids at the 30% prob-
ability. Only the main disordered parts of methallyl and 2-cyanophenyl groups
are shown.
3.2. Reactivity of complexes toward ethylene
Methallyl nickel complexes require a coactivator to generate the
active species. For this purpose, we select tris(pentafluorophenyl)
borane (B(C6F5)3) because it easily coordinates with the carbonitrile
functional group [5,18,19,23–25]. Complexes C1 and C2 were reacted
with 1 equivalent of (B(C6F5)3) in CDCl3 solution at room temperature.
The NMR characterization by 11B and 19F is characteristic for the
Lewis acid adduct formation (in situ). In the 19F NMR spectrum, three
signals at -134.6, -156.2 and -163.6 ppm indicate the attachment of the
(B(C6F5)3) Lewis acid. The separation of the resonances of the meta and
para fluorine substituents by Δm,p = 7.4 ppm indicates the presence of
a distinctly changed tetracoordinated boron compared to tricoordinated
(B(C6F5)3), Δm,p = 20.1 ppm, but it is similar to that of the pyridine
adduct (C6F5)3B-NC5H5 with Δm,p = 7.2 ppm [29]. The 11B NMR
resonance at -10.2 ppm also confirms a tetracoordinated boron [28,30].
A series of reference experiment showed that C1 and C2 that were
activated with one equivalent of (B(C6F5)3) were not active toward
ethylene (entry 1, Table 1). This result was expected due to the slow
initiation rates in methallyl nickel complexes, even when they were
activated with a strong Lewis acid as (B(C6F5)3) [24,31]. Another fact to
consider is that this adduct is “labile”, which implies that there is the
dissociation of nitrile from borane, followed by recoordination [18,26].
Thus, the complex reactivity was tested with five equivalents of (B
(C6F5)3) as the coactivator at 12.5 bar and 20 and 60 °C (Table 1).
As expected, under these conditions, the kinetics ethylene oligo-
merization is accelerated for complexes C1 and C2. These enhanced
activities are possible through the acid-base Lewis interaction between
boron atom and cyano group in the ligand backbone, which is
The presence of phenyl ring in the C1 (Fig. 2) indazole framework
does not have a large effect on the activity and microstructure pattern;
this relative insensibility is due to its poor steric hindrance around the
nickel center. Finally, the GPC analysis reveals a low-molecular-weight
oligomer with a polydispersity index ∼1, which suggests that this
catalyst may exhibit a living behavior [35,36].
4. Conclusion
In summary, we prepared two new N,N-indazole ligands (L1 and
L2), which were characterized by NMR spectroscopy, FT-IR, elemental
analysis and X-ray diffraction [37]. In addition, two new organome-
tallic species were isolated and characterized: The complexes C1 and
C2. Complex C2 was fully characterized by 1H, 13C, 11B and 19F NMR,
FT-IR, elemental analysis and X-ray diffraction [37].
A series of oligomerization reactions was performed, which shows
that complexes C1 and C2 produced hyperbranched low-molecular-
weight oligoethylenes. Interestingly, the microstructure pattern studied
by 13C NMR spectroscopy revealed the presence of sec-butyl moieties,
which have not been found in nickel methallyl complexes. Thus, the
design of new N,N-indazole derivates will allow one to obtain a new
catalyst that can generate oli/polyethylenes with interesting micro-
structures.
The GPC analysis reveals that these systems can behave as living
oligomerization catalysts. Future work will be directed to the study of
the living behavior and theoretical properties of the oligomerization
mechanism.
Table 1
Selected oligomerization reactions.
Entry*
Complex
T (°C)
activitya
Mwb
Mnb
PDIc
5. Author statement
1
2
3
4
5
C1, C2
C1
C2
C1
C2
20, 60
20
20
60
60
0
–
–
–
690
711
1.102
1.234
906
757
915
1011
856
743
857
928
1.05
1.01
1.07
1.08
These authors contributed equally.
Declaration of Competing Interest
a Activity is presented in units of kg of PE (mol of Ni)-1h−1 b Molecular weights
.
are presented in units of g mol−1. Molecular weights were determined by GPC. c
Polydispersity index. * Entry 1, complexes C1 and C2 were activated with one
equivalent of B(C6F5)3. Entries 2–5, complexes C1 and C2 were activated with
five equivalents of B(C6F5)3.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
5

C. Araya, et al.
InorganicaChimicaActa510(2020)119761
Table 2
Microstructure distribution.
Entry
Complex
Branches/1000Ca
Methylb
Ethylb
Propylc
Butyld
Sec-butyle
Longf
1
2
3
4
5
C1, C2
C1
C2
C1
C2
–
–
–
7
9
7
9
–
4
7
5
8
–
6
11
6
10
–
7
9
7
8
–
74
89
70
87
57
47
55
45
19
17
20
20
a The degree of branching was calculated form the 1H NMR intensity ratios of the methyl groups (and corrected for the saturated end groups vs. the overall integral).
b Percentages of different branch lengths can be calculated from the relative intensity ratios of the methyl (1B1, 1B2) or methine (C4+) signals of the respective
branches in the 13C NMR spectra.
Fig. 3. 13C NMR spectrum of hyperbranched polyethylene obtained with C1. (Table 1, entry 2).
Acknowledgment
[7] T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich, R.H. Grubbs,
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synthesis, properties, functionalization, and applications, Polym. Chem. 3 (2012)
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This work was supported by FONDECYT project no. 1161091,
CONICYT (grant. No. 21140118) and FONDECYT (postdoctoral fel-
lowship 3160480) of Chile.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119761.
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