New air stable cationic methallyl Ni complexes bearing imidoyl-indazole carboxylate ligand: Synthesis, characterization and their reactivity towards ethylene

Article abstract of DOI:10.1016/j.molcata.2015.12.020

Three new neutral N,N imidoyl-indazole ligands with a methoxycarbonyl functional group (1-3) and three new air-stable cationic methallyl nickel complexes (4-6) were prepared. These compounds were characterized by NMR, FT-IR and elemental analyses. In addition, compounds 1, 2, 3 and 4 were analyzed using X-ray diffraction. An evaluation of the reactivity of complexes 4-6 toward ethylene was conducted by using 5 equivalents of B(C₆F₅)₃. At 1 bar of ethylene and 20 °C, only complexes 4 and 6 were able to produce butene, showing unusual air stability and dimerizing ethylene even after exposure to air for 48 h. By increasing the pressure to 12 bar at 20 °C, complex 6 showed a catalytic activity of 401 Kg product (mol Ni)^-1 h^-1, producing a low molecular weight polyethylene (26.6 Kg/mol) with almost exclusively methyl branches (2.3 mol%). By increasing the temperature to 60 °C at 12 bar of ethylene, the system 6/B(C₆F₅)₃ increased its catalytic activity to 487 Kg product (mol Ni^-1) h^-1, but produced a significant decrease in the molecular weight of the polymer (1.8 Kg/mol) and a broad distribution of branches.

Full text of DOI:10.1016/j.molcata.2015.12.020

Accepted Manuscript
Title: New air stable cationic methallyl Ni complexes bearing
imidoyl-indazole carboxylate ligand: Synthesis,
characterization and their reactivity towards ethylene
Author: Alan R. Cabrera Ivan Martinez Constantin G.
Daniliuc Griselda B. Galland Cristian O. Salas Rene S. Rojas
PII:
S1381-1169(15)30184-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.12.020
MOLCAA 9723
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
4-12-2015
23-12-2015
27-12-2015
Please cite this article as: Alan R.Cabrera, Ivan Martinez, Constantin G.Daniliuc,
Griselda B.Galland, Cristian O.Salas, Rene S.Rojas, New air stable cationic methallyl
Ni complexes bearing imidoyl-indazole carboxylate ligand: Synthesis, characterization
and their reactivity towards ethylene, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.12.020
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New air stable cationic methallyl Ni complexes bearing imidoyl-
indazole carboxylate ligand: Synthesis, characterization and their
reactivity towards ethylene
Alan R. Cabrera,*,a,d Ivan Martinez,a Constantin G. Daniliuc,b Griselda B. Galland,c Cristian
O. Salas,a Rene S. Rojas*,a
a Nucleus Millennium of Chemical Processes and Catalysis, Faculty of Chemistry, Pontificia
Universidad Católica de Chile, Casilla 306, Santiago, Chile, b Chemisches Institut der
Universität Münster, Corrensstrasse 40, 48149 Münster, Germany. c Instituto de Química,
Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil.d Universidad Bernardo O
Higgins, Departamento de Ciencias Químicas y Biológicas, Laboratorio de
Bionanotecnología, General Gana 1702, Santiago, Chile.
1

Graphical abstract
Highlights
 Design and synthesis of three neutral N,N imidoyl-indazole methoxycarbonyl ligands
 Synthesis and characterization of air-stable [(methallyl)Ni(N,N)]B(Ar’)4 complexes
 Study of ethylene polymerization with Ni-complexes and B(C6F5)3
 Polymerization products strongly depended of the complex and reaction conditions
2

Abstract
Three new neutral N,N imidoyl-indazole ligands with a methoxycarbonyl functional group (1-
3) and three new air-stable cationic methallyl nickel complexes (4-6) were prepared. These
compounds were characterized by NMR, FT-IR and elemental analyses. In addition,
compounds 1, 2, 3 and 4 were analyzed using X-ray diffraction. An evaluation of the
reactivity of complexes 4-6 towards ethylene was conducted by using 5 equivalents of
B(C6F5)3. At 1 bar of ethylene and 20 °C, only complexes 4 and 6 were able to produce
butene, showing unusual air stability and dimerizing ethylene even after exposure to air for 48
hours. By increasing the pressure to 12 bar at 20 °C, complex 6 showed a catalytic activity of
401 Kg product (mol Ni)-1 h-1, producing a low molecular weight polyethylene (26.6 Kg/mol)
with almost exclusively methyl branches (2.3 mol%). By increasing the temperature to 60 °C
at 12 bar of ethylene, the system 6 / B(C6F5)3 increased its catalytic activity to 487 Kg product
(mol Ni-1) h-1, but produced a significant decrease in the molecular weight of the polymer (1.8
Kg/mol) and a broad distribution of branches.
KEYWORDS: Nickel; Ethylene; Dimerization; Polymerization; Imidoyl-indazole.
3

1. Introduction
The development of post-metallocene catalysts based on late transition metals has
allowed access to a variety of branched polyolefins, with different grades and sizes of these
branches.[1–3]
Among these, methyl branches are of great interest because of the improvements in
polymer properties, such as resistance to traction or break elongation.[4,5] There are several
reports on catalytic systems that indicate that they are able to produce polyethylene with
varying branching degrees.[6–8] To date it is not possible to have an absolute control of the
branching size and degree using only ethylene as monomer. It is a challenge to have a catalyst
that allows absolute control of the microstructure of the polyethylene produced having only
methyl branches.
Furthermore, Ni-allyl or Ni-methallyl complexes are a post-metallocene catalyst class
that have been synthesized to meet this challenge.[9] These complexes bear a P,O ; N,O or
N,N chelating ligand (see Fig. 1) and some of these contain in their structure a Lewis base
group such as carboxylate or cyano. This group serves two purposes: Act as an electron-
withdrawing group activating the metal center by a resonance effect and the ability to
coordinate a Lewis acid, thereby increasing the electron deficiency of the metal center,
making it more reactive towards olefins and avoiding by this way the use of a large amount of
cocatalyst.[10–12]
Recently, it was published by our group an efficient Ni-methallyl complex employing
a β-diketimine N,N chelating ligand (Fig. 1c).[13] This complex was remotely activated by
using B(C6F5)3 as Lewis acid, which formed a strong coordination adduct between the cyano
group and the borane. This adduct was active towards the polymerization of ethylene with
high activities for this kind of system. It produced polyethylene with isolated methyl branches
that resemble the polymer structure of that obtained by the copolymerization of ethylene with
propene.[14]
To the extent of our knowledge, in literature only have been reported Ni-allyl or Ni-
methallyl complexes that bear a N,N chelating ligand corresponding to α-diimine or β-diimine
kind of.[15] Of interest is our research on Ni-methallyl complexes with a new type of N,N
4

ligand that exhibits high activities towards -olefins and, in the case of ethylene, are highly
selective to methyl branches.
Herein we report the synthesis and characterization of new air-stable cationic
methallyl nickel complexes bearing an N,N imidoyl-indazole derivative ligand. These
complexes have a methoxycarbonyl functional group at different positions in the indazole
fragment. A study is detailed in this report on the reactivity of these new complexes towards
ethylene, using B(C6F5)3 as a cocatalyst. Also to the position of the methoxycarbonyl group in
the ligand is expected to have a decisive effect on the reactivity of the complexes.
2. Experimental
2.1. Reagents and materials
All manipulations were performed under an inert atmosphere using 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 and pentane were distilled from benzophenone ketyl.
2.2. Synthesis and characterization of imidoyl-indazole carboxylate ligands (1-3) and cationic
methallyl Ni complexes (4-6)
2.2.1. Imidoyl-indazole carboxylate 1
N-(2,6-diisopropylphenyl)acetimidoylchloride [16] (405 mg, 1.72 mmol) was
added dropwise to a solution of methyl-1H-indazole-3-carboxylate (300 mg, 1.72
mmol) in anhydrous toluene (40 ml). The mixture was refluxed for 3 h with vigorous
stirring. The yellow solution was evaporated in vacuum to dryness. Crude product was
purified via silica gel chromatography (4:1 Petroleum ether / ethyl acetate).
1 was
1
isolated as white solid in 85% yield (546 mg, 1.45 mmol). H NMR (400 MHz, CD3Cl,
298 K): δ/ppm = 8.87 (d, J = 8.4Hz, 1H), 8.32 (d, J = 8.4Hz, 1H), 7.55 (t, J = 8.4Hz,
1H), 7.41 (t, J = 8.4Hz, 1H), 7.19 (m, 3H), 4.07 (s, 3H), 2.95 (hept, J = 6.9Hz, 2H),
13
2.54 (s, 3H), 1.21 (d, J = 2.0 Hz, 6H), 1.18 (d, J = 2.0 Hz, 6H). C{1H} NMR (100
MHz, CD3Cl, 298 K): δ/ppm = 162.7, 155.6, 143.3, 140.4, 137.8, 137.0, 129.1, 125.2,
125.0, 124.2, 123.3, 122.0, 116.6, 52.4, 28.4, 23.6, 23.0, 17.3. Elemental analysis (%)
5

C23H27N3O2 (M = 377.48 g/mol): calculated C 73.18, H 7.21, N 11.13; found C 72.86,
H 7.57, N 10.92. HRMS-ESI (C23H28N3O2 [M + H]): Calc: 378.2182; Found:
378.2161. For additional 2D NMR spectrum and assignments data see Supplementary
data.
2.2.2. Imidoyl-indazole carboxylate 2
N-(2,6-diisopropylphenyl)acetimidoylchloride [16] (405 mg, 1.72 mmol) was added
dropwise to a solution of methyl-1H-indazole-5-carboxylate (300 mg, 1.72 mmol) in
anhydrous toluene (40 ml). The mixture was refluxed for 3 h with vigorous stirring. The
yellow solution was evaporated in vacuum to dryness. Crude product was purified via silica
gel chromatography (4:1 Petroleum ether / ethyl acetate). 2 was isolated as white solid in 90%
yield (578 mg, 1.53 mmol). 1H NMR (400 MHz, CD3Cl, 298 K): δ/ppm = 8.86 (d, J = 8.9Hz,
1H), 8.54 (s, 1H), 8.22 (s, 1H), 8.21 (m, 1H), 7.20 (d, J = 7.4Hz, 2H), 7.13 (t, J = 7.4Hz, 1H),
3.95 (s, 3H), 2.94 (hept, J = 6.8Hz, 2H), 2.45 (s, 3H), 1.19 (t, J = 7.4Hz, 12H). 13C{1H} NMR
(100 MHz, CD3Cl, 298 K): δ/ppm = 166.8, 155.0, 143.4, 140.9, 137.6, 137.1, 129.3, 126.1,
125.4, 124.0, 123.6, 123.2, 116.0, 52.2, 28.4, 23.5, 22.9, 17.1. Elemental analysis (%)
C23H27N3O2 (M = 377.48 g/mol): calculated C 73.18, H 7.21, N 11.13; found C 73.31, H 7.35,
N 11.24. HRMS-ESI (C23H28N3O2 [M + H]): Calc: 378.2182; Found: 378.2162. For
additional 2D NMR spectrum and assignments data see Supplementary data.
2.2.3. Imidoyl-indazole carboxylate 3
N-(2,6-diisopropylphenyl)acetimidoylchloride [16] (405 mg, 1.72 mmol) was added dropwise
to a solution of methyl-1H-indazole-6-carboxylate (300 mg, 1.72 mmol) in anhydrous toluene
(40 ml). The mixture was refluxed for 3 h with vigorous stirring. The yellow solution was
evaporated in vacuum to dryness. Crude product was purified via silica gel chromatography
(4:1 Petroleum ether / ethyl acetate). 3 was isolated as white solid in 87% yield (559 mg, 1.48
mmol). 1H NMR (400 MHz, CD3Cl, 298 K): δ/ppm = 9.45 (s, 1H, H14), 8.19 (s, 1H), 8.02 (d,
J = 8.5Hz, 1H), 7.82 (d, J = 8.5Hz, 1H), 7.24 (d, J = 8.1Hz, 2H), 7.15 (m, 1H), 3.91 (s, 3H),
13
2.94 (hept, J = 6.8Hz, 2H), 2.43 (s, 3H), 1.20 (dd, J = 7.0, 2.9Hz, 12H). C{1H} NMR (100
MHz, CD3Cl, 298 K): δ/ppm = 167.3, 155.0, 143.7, 138.8, 137.4, 136.5, 130.4, 128.9, 124.2,
124.0, 123.4, 120.6, 118.4, 52.5, 28.5, 23.7, 23.0, 17.2. Elemental analysis (%) C23H27N3O2
(M = 377.48 g/mol): calculated C 73.18, H 7.21, N 11.13; found C 73.02, H 7.11, N 10.98.
6

HRMS-ESI (C23H28N3O2 [M + H]): Calc: 378.2182; Found: 378.2185. For additional 2D
NMR spectrum and assignments data see Supplementary data.
2.2.4. Cationic methallyl Ni complex 4
Methyl-1-(1-((2,6-diisopropylphenyl)imino)ethyl)-1H-indazole-3-carboxylate (1) (40
mg, 0.105 mmol) and NaBAr’4 (Sodium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (94
mg; 0.105 mmol) were mixed in dichloromethane (20ml). The suspension was stirred
vigorously and a solution of [(η3-CH3C(CH2)2)NiCl]2 [12] (16 mg, 0.053 mmol) in
dichloromethane was added. The mixture was stirred for 2 hours at room temperature. The
resulting solution was filtered through celite and the product was crystallized adding pentane
and storing at -20 °C. 4 was obtained as dark orange crystalline solid in 70 % yield (100 mg;
0.074 mmol). 1H NMR (400 MHz, CD3Cl, 298 K): δ/ppm = 8.10 (d, J = 8.3 Hz, 1H), 7.69 (m,
1H), 7.67 (s, 8H), 7.49 (m, 2H), 7.46 (s, 4H), 7.38 (t, J = 7.7 Hz, 1H), 7.29 (m, 2H), 4.12 (s,
3H), 3.82 (s, 1H), 3.23 (hept, J = 6.8 Hz, 1H), 2.84 (hept, J = 6.8 Hz, 1H), 2.79 (s, 1H), 2.59
(s, 3H), 2.52 (s, 1H), 2.37 (s, 1H), 2.21 (s, 3H), 1.31 (d, J = 6.8 Hz, 6H), 1.17 (d, J = 6.8 Hz,
13
3H), 1.07 (d, J = 6.8 Hz, 3H). C{1H} NMR (100 MHz, CDCl3, 298 K,): δ/ppm = 161.4 (t,
JCF = 49 Hz), 160.7, 159.6, 139.4, 138.8, 134.8, 133.1, 129.0, 127.5, 125.1, 124.8, 124.5,
124.5 (q, JCF = 270 Hz), 117.4, 111.7, 63.2, 61.4, 54.1, 28.9, 28.8, 24.2, 24.0, 23.6, 23.2, 22.5,
17.3. 19F NMR (400 MHz, CDCl3, 298 K): δ/ppm = -62.45. 11B{1H} NMR (400 MHz,
CDCl3, 298 K): δ/ppm = -6.65 (ν1/2 ~ 11 Hz). Elemental analysis (%) C59H46BF24N3NiO2 (M
= 1354.48 g/mol): calculated C 52.32, H 3.42, N 3.10; found C 52.12, H 3.50, N 3.05. For
additional 2D NMR spectrum and assignments data see Supplementary data.
2.2.5. Cationic methallyl Ni complex 5
Methyl-1-(1-((2,6-diisopropylphenyl)imino)ethyl)-1H-indazole-5-carboxylate (2) (40
mg, 0105 mmol) and NaBAr’4 (Sodium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (94
mg; 0.105 mmol) were mixed in dichloromethane (20ml). The suspension was stirred
vigorously and a solution of [(η3-CH3C(CH2)2)NiCl]2 [12] (16 mg, 0.053 mmol) in
dichloromethane was added. The mixture was stirred for 2 hours at room temperature. The
resulting pale orange solution was filtered through celite and the product was crystallized
adding pentane and storing at -20 °C. 5 was obtained as orange solid in 80 % yield (115 mg;
1
0.085 mmol). H NMR (400 MHz, CD3Cl, 298 K): δ/ppm = 8.60 (s, 1H), 8.46 (s, 1H), 8.41
7

(d, J = 9 Hz, 1H), 7.77 (d, J = 9 Hz, 1H), 7.69 (s, 8H), 7.48 (s, 4H), 7.39 (t, J = 7.8 Hz, 1H),
7.31 (d, J = 9 Hz, 2H), 3.99 (s, 3H), 3.50 (s, 1H), 3.20 (hept, J = 6.8 Hz, 1H), 2.85 (hept, J =
6.8 Hz, 1H'), 2.65 (s, 1H), 2.60 (s, 3H), 2.57 (s, 1H), 2.38 (s, 1H), 2.17 (s, 3H), 1.30 (d, J =
13
6.8 Hz, 6H), 1.17 (d, J = 6.8 Hz, 3H), 1.08 (d, J = 6.8 Hz, 3H). C{1H} NMR (100 MHz,
CDCl3, 298 K,): δ/ppm = 164.6, 161.6 (t, JCF = 50 Hz), 160.3, 146.3, 140.0, 139.9, 139.7,
134.8, 134.4, 133.6, 129.0, 128.7, 126.1, 125.0, 124.5 (q, JCF = 270 Hz), 117.4, 111.4, 62.9,
56.8, 52.9, 28.9, 28.7, 24.1, 23.9, 23.5, 23.1, 23.0, 16.4. 19F NMR (400 MHz, CDCl3, 298 K):
δ/ppm = -62.40. 11B{1H} NMR (400 MHz, CDCl3, 298 K): δ/ppm = -6.53 (ν1/2 ~ 11 Hz).
Elemental analysis (%) C59H46BF24N3NiO2 (M = 1354.48 g/mol): calculated C 52.32, H 3.42,
N 3.10; found C 52.39, H 3.53, N 3.15. For additional 2D NMR spectrum and assignments
data see Supplementary data.
2.2.6. Cationic methallyl Ni complex 6
Methyl-1-(1-((2,6-diisopropylphenyl)imino)ethyl)-1H-indazole-6-carboxylate (3) (40
mg, 0105 mmol) and NaBAr’4 (Sodium-tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) (94
mg; 0.105 mmol) were mixed in dichloromethane (20ml). The suspension was stirred
vigorously and a solution of [(η3-CH3C(CH2)2)NiCl]2 [12] (16 mg, 0.053 mmol) in
dichloromethane was added. The mixture was stirred for 2 hours at room temperature. The
resulting orange solution was filtered through celite and the product was crystallized adding
pentane and storing at -20 °C. 6 was obtained as brown orange solid in 75 % yield (108 mg;
1
0.079 mmol). H NMR (400 MHz, CD3Cl, 298 K): δ/ppm = 8.35 (s, 1H), 8.26 (s, 1H), 7.96
(d, J = 8.4 Hz, 1H), 7.64 (d, J = 8.7 Hz, 1H), 7.60 (s, 8H), 7.38 (s, 4H), 7.29 (t, J = 7.8 Hz,
1H), 7.20 (m, 2H), 3.83 (s, 3H), 3.40 (s, 1H), 3.11 (hept, J = 6.8 Hz, 1H), 2.76 (hept, J = 6.8
Hz, 1H), 2.57 (s, 1H), 2.57 (s, 3H), 2.49 (s, 1H), 2.30 (s, 1H), 2.07 (s, 3H), 1.20 (d, J = 6.8
Hz, 6H), 1.08 (d, J = 6.8 Hz, 3H), 0.99 (d, J = 6.8 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3,
298 K,): δ/ppm = 165.7, 161.8 (t, JCF = 50 Hz), 160.5, 145.4, 140.2, 139.9, 139.5, 138.0,
134.9, 134.0, 129.2, 129.2, 127.0, 125.2, 124.7 (q, JCF = 270 Hz), 124.0, 117.6, 113.2, 63.2,
57.1, 53.4, 29.1, 28.9, 24.2, 24.0, 23.6, 23.2, 23.1, 16.6, C23 (n.o.). 19F NMR (400 MHz,
11
CDCl3, 298 K): δ/ppm = -62.37. B{1H} NMR (400 MHz, CDCl3, 298 K): δ/ppm = -6.64
(ν1/2 ~ 11 Hz). Elemental analysis (%) C59H46BF24N3NiO2 (M = 1354.48 g/mol): calculated C
52.32, H 3.42, N 3.10; found C 52.25, H 3.36, N 3.01. For additional 2D NMR spectrum and
assignments data see Supplementary data.
8

2.3. Characterization
NMR spectra were recorded on NMR Bruker AV 400. Chemical shifts are given in
parts per million relatives to TMS [1H and 13C, (SiMe4) = 0] or an external standard
11
[(BF3OEt2) = 0 for B NMR, (CFCl3) = 0 for 19F NMR]. Most NMR assignments were
supported by additional 2D experiments. Elemental analysis data were recorded on a Foss-
Heraeus CHNO-Rapid analyzer. HRMS-ESI-MS experiments were carried out using a
Thermo Scientific Exactive Plus Orbitrap Spectrometer. FT-IR spectra were recorded on a
Bruker Vector-22 Spectrophotometer using KBr pellets. GPC analysis was performed on a
Polymer Laboratories high temperature GPC system (model PL 220) equipped with a
refractive index detector. DSC analysis was performed on a Perkin-Elmer DSC7 calorimeter.
GC(MS) of dimeric products was conducted on a GC Perkin Elmer Clarus 680 with
Phenomenex column model Zebron ZB-5MS and MS detector Clarus SQ8T. Thin layer
chromatography (TLC) was performed using Merck GF-254 type 60 silica gel. Column
chromatography was carried out using Merck silica gel 60 (70–230 mesh). For X-ray crystal
structure analysis, data sets were collected with a Nonius Kappa CCD diffractometer by Dr.
Constantin G. Daniliuc; full details can be found in the independently deposited
crystallography information files (cif), CCDC numbers: 1430776 for compound 1, 1430777
for compound 2, 1430778 for compound 3 and 1430779 for compound 4.
13C NMR spectra of the obtained polymers were measured at 120 °C, on a Varian 300,
operating 75 MHz. Polymer samples were prepared in o-dichlorobenzene and benzene-d6
(20% v/v) in a 5-mm sample tube.
2.4. Polymerization tests
2.4.1. Polymerization at 1 bar of ethylene in a NMR tube with J. Young valve
Polymerizations were carried out in a NMR tube with J. Young valve, loaded inside a
glove-box with the appropriate amount of the precatalyst (4, 5 and 6), the corresponding
Lewis acid (B(C6F5)3, BCl3 or BF3) and CDCl3 as solvent. The tube was sealed with the valve
inside the glove-box and was attached to a Schlenk line with ethylene. Ethylene was fed into
1
the tube to a pressure of 1 bar and H NMR was measured every 30 minutes. After the
corresponding time, the reaction mixture was filtered and stored at -30 °C for GC(MS)
measurements.
9

2.4.2. Polymerization at 12 bar of ethylene in a Parr autoclave reactor
Polymerizations were carried out in a Parr autoclave reactor (100 mL), loaded inside a
glove-box with the appropriate amount of the precatalyst (4 and 6), 5 equivalents of B(C6F5)3
and toluene, such that the final volume of the toluene solution was 30 mL. The reactor was
sealed inside the glove-box and was attached to an ethylene line. Ethylene was fed
continuously into the reactor to a pressure of 12 bar. The pressurized reaction mixture was
stirred at the appropriate temperatures. After 10 min, the ethylene was vented and acetone was
added to quench the polymerization. The precipitated polymer was collected by filtration and
dried overnight.
3. Results and discussion
3.1. Synthesis and characterization of cationic methallyl Ni complexes
The reaction sequence to obtain the cationic Ni complexes started with the synthesis of
the new indazole derivative ligands 1-3, which was accomplished using an addition reaction
between equimolar amounts of the respective imidoyl chloride [16] and the respective methyl-
1H-indazole-carboxylate derivatives as is shown in Scheme 1. Each ligand was isolated as a
pale orange solid in good yield and characterized by elemental analyses, FT-IR, NMR
spectroscopy, mass spectrometry and X-ray crystallography.
1H NMR spectra confirm the formation of one isomer for each ligand and show the
iPr
chemical shifts of the CH3 fragment as a double doublet in the region of 1.20 ppm. In
addition, a septet at 2.94 ppm was observed, that correspond to the HCiPr protons of the aniline
C=N
moiety. At  4.00 ppm a singlet belonging to the CH3
group of the imine is observed.
Moreover, ligands 1-3 were characterized by X-ray crystallography and their structures are
shown in Fig. 2. These ligands exhibit an E configuration in the imine, meanwhile the
diisopropylphenyl groups are orthogonal to the indazole ring plane. For further details of the
characterization, see Supplementary data.
10

These ligands are neutral and capable of coordinate to metal center in a N,N five-
membered ring manner and have a methoxycarbonyl functional group which can modify the
electronic properties of the ligand. This promotes the oligomerization or polymerization
catalysis. A diisopropylphenyl substituent was incorporated to provide steric hindrance
around the nickel center as was reported by Brookhart et al. [3]
The methallyl nickel complexes formation proceeded smoothly upon the direct
reaction of two equivalents of the respective ligands 1-3 with one equivalent of [(η3-
CH3C(CH2)2)NiCl]2 [12] in the presence of one equivalent of NaBAr’4 (Ar’: 3,5-
bis(trifluoromethyl)phenyl).[17] This reaction was carried out at room temperature, and to this
way forming compounds 4-6 as orange solids in 70%, 80% and 86% yield respectively (see
Scheme 1 and Supplementary data).
The selected NMR characterization data of complexes 4-6 are summarized in Table 1
and are consistent with the formation of a single isomer containing a nickel center coordinated
to the respective ligand in a N,N fashion and to a η3-(CH2)2CCH3 as a secondary ligand.
1
Complexes 4-6 exhibit similar H NMR spectra, having distinctive chemical shifts for
CH3OC=O, HCiPr and CH3iPr. The methyl group in methoxy carbonyl substituent is shown as a
singlet at δ 4.12, 3.99 and 3.83 ppm for complex 4, 5 and 6, respectively, confirming the
presence of the functional group. The methine proton of diisopropyl groups substituent are
distinct between each other, giving rise to a pair of septets HCiPr shifts (3.23; 2.84 ppm for 4,
1
3.20 ; 2.85 ppm for 5, 3.11 ; 2.76 ppm for 6). A total of three doublets in the H NMR are
iPr
observed, which correspond to the CH3 shifts (1.31; 1.17; 1.07 ppm for 4, 1.30; 1.17; 1.08
ppm for 5, 1.20; 1.08; 0.99 ppm for 6).
Regarding to the secondary ligand, in all three complexes 4-6, the signals of the allylic
hydrogen protons are well defined and sharp. Each of these signals, exhibit a highly restricted
movement on the NMR time scale (δ 3.82, 2.79, 2.52 and 2.37 ppm for 4; 3.50, 2.65, 2.57 and
2.38 ppm for 5; 3.40, 2.49 and 2.30 ppm for 6).13C NMR characterization display the presence
OC=O
13
of the MeOC=O, N-CCC=N and CH3
carbon atom. For MeOC=O group, the C NMR shifts
are 159.6 ppm (4), 164.6 ppm (5) and 165.7 ppm (6). For N-CCC=N carbon atom, the shifts are
OC=O
13
160.7 ppm (4), 160.3 ppm (5) and 160.5 ppm (6). Finally, for CH3
NMR shifts are 54.1ppm (4), 52.9 ppm (5) and 53.4 ppm (6).
carbon atom, the C
11

11
The B and 19F NMR spectra of complexes 4-6 show characteristic singlets for the
-
B(Ar’)4 counterion in CDCl3, indicating that these counterions are “free” ions and are not or
to a very small extent only ion-pairing with the cationic complex parts (11B: -6.5 ppm (ν1/2
11 Hz); 19F: -62.4 ppm) (For further information, see Supplementary data).
=
The molecular structure of complex 4 was determined by an X-ray diffraction study
and is shown in Fig. 3. The crystal structures of the complex confirm a four coordinated
nickel metal center, which adopts an almost ideal square-planar geometry. The dihedral angle
corresponding to N,N chelate section is -0.4o (N1-C43-C41-N3) and the Ni-N1 ; Ni-N3 bond
lengths are 1.932(5) and 1.934(5) Å, respectively. These distances are in agreement with
similar Ni complexes reported on literature.[13] All Ni-Cmethallyl bond lengths are within the
range of 1.986(5) - 2.070(5) Å, typically for η3-methallyl ligated nickel complex. The
methoxycarbonyl group (CH3OC=O) group is rotated out of indazole plane with a dihedral
angle of -35.0(1)o (N1-C1-C31-O1). In regard to 2,6-diisopropylphenyl substituent, this lies
rotated out of the central ligand frame, staying orthogonal to the indazole plane (dihedral
angle: -83.0(8)o (C8-N3-C11-C16)).
3.2. Reactivity of complexes toward ethylene
In order to study the reactivity of complexes 4-6, ethylene oligomerization reactions
were conducted at NMR scale using chloroform-d as solvent and the obtained products were
analyzed by mass spectrometry. Likewise, the reaction conditions were optimized, as well as
the co-activator used, the temperature, and the ethylene pressure. The selected results are
summarized in Table 2.
1H NMR spectroscopy analysis showed that complexes 4-6 do not act as single
component. The complexes reactivity was then tested with five equivalents of B(C6F5)3 as a
reference coactivator, 1 atm of ethylene pressure and 20 °C as reaction temperature.
Complexes 4 and 6 were active to ethylene dimerization; meanwhile complex 5 does not
exhibit reactivity. This is due to the position of the electron withdrawing methoxycarbonyl
functional group in the indazole ring. The positions activated by electronic delocalization are
the 3 and 6 (complex 4 and 6, respectively) as is shown in Fig. 4, while position 5 is not
activated by conjugation, having only an inductive electron withdrawing effect (complex 5). It
12

failed to generate a sufficiently deficient electron density metal center as to be reactive
towards ethylene. Remarkable is the fact that the complexes proved to be stable in the air,
showing the same activity in the case of complex 4 and 6 even after being exposed for 48
hours.
Coactivator was then evaluated, studying the complex 4 and 6 at 20 °C, 1 atm of
ethylene pressure and five equivalents of three different Lewis acids as coactivators (BF3,
BCl3 and B(C6F5)3), exhibiting the activation of the complexes only by using B(C6F5)3.
1
The analysis of the H NMR of the ethylene dimerization reactions (Scheme 2)
showed that complex 4 exhibit a series of new chemical shifts in the olefinic region at δ 5.87
and 4.95 ppm and in the aliphatic region at δ 1.62 and 1.00 ppm, that suggest the formation of
a terminal olefin (see Fig. 5a). Additionally, it was observed a new shift at δ 5.45 ppm, which
was assigned to the protons of an internal double bond because of an isomerization of the
terminal olefin. Obtaining an isomer distribution of 50% of 1-butene and 50% of 2-butene
with a catalytic activity of 0.5 Kg pol mol-1 h-1 (Entry 1, Table 2).
By contrast, complex 6 only shows the shifts at δ 5.45 ppm, which are assigned to the
protons of an internal double bond as a result of the isomerization of the terminal olefin (see
Fig. 5b), producing 100% of 2-butene with a catalytic activity of 1.2 Kg pol mol-1 h-1 (Entry 2,
Table 2). These results are consistent with the fact that the complex 4 has a higher steric
hindrance around the metal center than the complex 6, caused by the presence of the
methoxycarbonyl group in the insertion area, decreasing the catalytic activity and preventing
the reinsertion of the terminal olefin, favoring the formation of 1-butene.
As system 6/B(C6F5)3 was two times more active toward ethylene dimerization, the
effect of the reaction pressure over this system at 20 °C was evaluated, using 12 bar of
ethylene pressure in chloroform as solvent. Amazingly, the formation of a wax was observed,
with an activity 330 times higher than the reaction at 1 bar of ethylene (401 Kg pol mol-1 h-1,
Entry 3, Table 2). This is considered a high catalytic activity and is twice active than similar
Ni complexes previously reported by our group.[13] The reaction product was characterized
by DSC and GPC, showing a melting point of 119 °C and a molecular weight of 26.6x103
13
g/mol, respectively. Microstructural analysis of this polymer was performed using C NMR
13

spectroscopy, where the assignation of their respective spectra and the nomenclature used
were done according to literature.[6] As shown in Fig. 6a, the polymer obtained at 20 °C and
12 bar of ethylene, is composed mainly by methyl branches (2.3 mol%) and by a small
amount (0.4 mol%) of long branches (n≥6). It was thus possible to obtain a highly active
catalyst capable to produce low molecular weight polyethylene, where the 85% of the total
branches are methyl kind.
Finally, the activity of 6/B(C6F5)3 at 12 bar of ethylene and 60 °C was evaluated,
seeking to determine the influence of temperature on the catalytic activity and the morphology
of the obtained product. It was found that by increasing the reaction temperature, the catalytic
activity increased by 20% (487 Kg pol mol-1 h-1, Entry 4, Table 2). The molecular weight of
the obtained product was 1.8x103 g/mol (Mn) and is completely amorphous, as was observed
from the DSC where it was not possible to measure any thermal transition.
Microestructural analysis of the polymer by 13C NMR showed that the product
morphology changes dramatically with the reaction temperature (see Fig. 6b), increasing nine
times the total branches in the wax and exhibiting a broad distribution of chain lengths in the
branches. This highly branched polymer shown the presence of methyl branches (8.7 mol%)
at 19.90 ppm (1B1), 27.42 ppm (B1), 30.36 ppm (B1), 33.14 ppm (brB1) and 37.47 ppm
(B1). Ethyl branches (2.3 mol%) are confirmed by the presence of resonances at 11.10 ppm
(1B2), 26.51 ppm (2B2) and 39.44 ppm (brB2). Propyl branches (3.2 mol%) are mainly
represented by resonances at 14.65 ppm (1B3) and 36.72 ppm (3B3), butyl branches (1.4
mol%) by resonance at 23.37 ppm (2B4), amyl branches (0.6 mol%) by resonance at 32.65
ppm (3B5) and long branches (6.9 mol%) by resonances at 29.59 ppm (4Bn) and 32.16 ppm
(3Bn). Note that the methyl branches are only the 38% of the total. This catalytic behavior is
consistent with higher reaction temperature, where increasing the rate of chain termination is
due to the raise of the -elimination.[18]
4. Conclusions
Three new neutral N,N chelate ligands derived from methyl-1H-indazole-carboxylate
1-3 were successfully synthesized and characterized. The X-ray single crystal characterization
shown an E configuration in the imine and the diisopropylphenyl groups were orthogonal to
the indazole ring plane. From these ligands, cationic Ni complexes 4-6 bearing also a
14

methallyl ligand were prepared in high yields and characterized by NMR, FT-IR and
elemental analyses. The X-ray diffraction study was performed for complex 4 and confirms
an almost ideal square-planar geometry with a four coordinated nickel metal center.
Complexes 4-6 were activated for ethylene dimerization by 5 equivalents of B(C6F5)3.
These compounds bear an electron withdrawing functional group in different ring positions,
which influenced dramatically in their reactivity. At 1 bar of ethylene and 20 °C, complex 4
was able to produce a mixture of butenes, with a distribution of 50% of 1-butene and 50% of
2-butene. By other hand, complex 5 was not active and complex 6 was the most active,
producing 100% of 2-butene. In addition, it was found that complexes 4 and 6 showed an
unusual stability in air, and were reactive towards ethylene even after exposure to air for 48
hours.
By increasing the pressure to 12 bar at 20 °C, complex 6 showed a catalytic activity of
401 Kg product (mol Ni)-1 h-1, being 330 times higher than the reaction at 1 bar of ethylene.
At the same time, this increase in pressure caused that the catalyst was able to produce
polyethylene with a molecular weight of 26.6x103 g/mol and almost exclusively methyl
branches (2.3 mol%). In addition, by increasing the temperature to 60 °C at 12 bar of ethylene
the system 6 / B(C6F5)3 it increased its catalytic activity to 487 Kg product (mol Ni-1) h-1 and
was capable of generating a wax with a molecular weight of 1.8x103 g/mol and with a broad
distribution of branches, being the methyl only the 38% of total.
It is remarkable that complex 6 is able to generate several products only by changing
the reaction conditions as well as the location of the substituent in the indazole derivative ring
allows us to modulate the catalytic activity of the complexes. These catalytic results
encourage us to continue our research toward the optimization of these types of Ni complexes,
aiming to control of the molecular weight and the branching degree of the polymer obtained.
Acknowledgements
We gratefully acknowledge the financial support of FONDECYT (project N°
1130077) and Millennium Nucleus of Chemical Processes and Catalysis (ICM - 120082). A.
R. Cabrera thanks FONDECYT (Postdoctoral fellowship 3140425).
Appendix A. Supplementary data
Supplementary data associated with this article can be found in the online version, including
spectroscopic and crystallographic details: CCDC-1430776 for 1, CCDC-1430777 for 2,
CCDC-1430778 for 3 and CCDC-1430779 for 4.
15

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17

Fig. 1. Nickel methallyl complexes previously reported.
18

Fig. 2. X-ray crystal structure of the ligand 1 (a), ligand 2 (b) and ligand 3 (c) with thermal
ellipsoids drawn at the 30% probability level.
19

Fig. 3. X-ray crystal structure of 4 with thermal ellipsoids drawn at the 30% probability level.
-
B(ArF)4 counterion was omitted for clarity.
20

Fig. 4. Active positions by electron delocalization in the imidoyl-indazole system.
21

Fig. 5. 1H NMR spectra of: (a) 4/B(C6F5)3/ethylene; (b) 6/B(C6F5)3/ethylene.
22

Fig. 6. 13C NMR spectra of the polymers obtained at: (a) 6/B(C6F5)3/12 bar/20 °C; (b)
6/B(C6F5)3/12 bar/60 °C.
23

Scheme 1. Synthetic route for ligands 1-3 and cationic methallyl Ni complexes 4-6.
24

Scheme 2. Dimerization of ethylene using 4-6/B(C6F5)3 at 1 bar of C2H4.
25

Table 1: Selected NMR parameters of complexes 4-6.a
4
5
6
13C NMR (ppm)
Indazole-NCN-Ar
CHIndazole
160.7
160.3
160.5
127.5
146.3
Methallyl
CH2
63.2, 61.4
28.9, 28.7
22.5
62.9, 56.8
28.9, 28.7
23.0
63.2, 57.1
29.1, 28.9
23.1
HCiPr
Methallyl
CH3
C-N
CH3
17.2
16.4
16.6
1H NMR(ppm)
CHIndazole
-
8.46
3.99
8.26
3.83
OC=O
CH3
4.12
Methallyl
CH2
3.82, 2.79 3.50, 2.65 3.40, 2.49
2.52, 2.37
3.23, 2.84
2.59
2.57, 2.38
3.20, 2.85
2.60
2.30
HCiPr
3.11, 2.76
2.57
C-N
CH3
Methallyl
CH3
2.21
2.17
2.07
11B NMR / 19F NMR (ppm)
-6.7 / -62.5 -6.5 / -62.4 -6.6 / -62.4
a NMR spectra in chloroform-d solution.
26

Table 2: Selected ethylene oligomerization reactions.a
e
f
Entry Complex Pb
Tc Ad
20 0.5
20 1.2
20 401
60 487
Mn
PDI Tm Isomer distributiong / ramificationh
1
2
3
4
1
-
-
-
-
-
-
50% 1-butene ; 50% 2-butene
100% 2-butene
4
6
6
6
1
12
12
26.6 1.8 119 2.3 mol% methyl ; 0.4 mol% larger
1.8 2.3 n.o. 8.7 mol% methyl ; 14.4 mol% larger
a
Oligomerization were carried out with 3.7 μmol of Ni, CHCl3 as solvent and the cocatalyst
was 5 equivalents of B(C6F5)3. Duration of reaction was 1 hour. b Ethylene pressure (in bar). c
d
e
Temperature of reaction (in °C). Catalytic activity (in Kg pol mol-1 h-1). in x103 g/mol. f
h
Melting point in °C as determined by DSC. g Determined by GC-MS(ESI). Determined by
13C NMR spectroscopy at 120 °C, in o-dichlorobenzene and benzene-d6 (20% v/v) solution.
n.o.: not observed.
27