Chromium Complexes Supported by Phenyl Ether-Pyrazolyl [N,O] Ligands as Catalysts for the Oligo- and Polymerization of Ethylene

Article abstract of DOI:10.1002/aoc.5984

A new series of Cr (III) complexes [Cr{1-(3-phenoxypropyl)-1H-pyrazole}Cl₃]₂ (Cr1), [Cr{1-(3-phenoxypropyl)-3,5-dimethyl-1H-pyrazole}Cl₃]₂ (Cr2), and [Cr{1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole}Cl₃]₂ (Cr3) have been synthesized and characterized by elemental analysis, high-resolution mass spectrometry (HRMS) and IR spectroscopy. Upon activation with methylaluminoxane (MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in ethylene oligomerization [TOF = 17,900–29,200 mol (ethylene)·mol (Cr)^?1·h^?1 at 80 °C] with Schultz-Flory distribution of oligomers (K = 0.54–0.66) and production of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under identical oligomerization conditions, Cr1/MAO behaved as a polymerization catalyst generating predominantly polyethylene (63.7 wt%). The amount of 1-butene is the largest component in the liquid fraction suggesting that these precatalysts operate via a Cossee-Arlman mechanism. The catalytic activities, selectivity and product distribution are quite sensitive to the R-group at the 3- and 5-position of the pyrazolyl ring. Based on the electronic and steric effects of R- substituents, it is possible to stablish a trend of activity: Cr2(Pz^Me2) > Cr3(Pz^Ph) > Cr1(Pz). Moreover, the effect of oligomerization parameters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined.

Full text of DOI:10.1002/aoc.5984

Received: 26 June 2020
DOI: 10.1002/aoc.5984
Revised: 28 July 2020
Accepted: 4 August 2020
F U L L P A P E R
Chromium Complexes Supported by Phenyl Ether-Pyrazolyl
[N,O] Ligands as Catalysts for the Oligo- and Polymerization
of Ethylene
Jorge L.S. Milani | Osvaldo L. Casagrande Jr
Laboratory of Molecular Catalysis,
A new series of Cr (III) complexes [Cr{1-(3-phenoxypropyl)-1H-pyrazole}Cl3]2
(Cr1), [Cr{1-(3-phenoxypropyl)-3,5-dimethyl-1H-pyrazole}Cl3]2 (Cr2), and [Cr
{1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole}Cl3]2 (Cr3) have been synthesized
and characterized by elemental analysis, high-resolution mass spectrometry
(HRMS) and IR spectroscopy. Upon activation with methylaluminoxane
(MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in eth-
ylene oligomerization [TOF = 17,900–29,200 mol (ethylene)Ámol (Cr)−1Áh−1 at
Instituto de Química, Universidade
Federal do Rio Grande do Sul, Av. Bento
Gonçalves, 9500, Porto Alegre, RS,
91501-970, Brazil
Correspondence
Osvaldo L. Casagrande Jr., Laboratory of
Molecular Catalysis, Instituto de Química,
Universidade Federal do Rio Grande do
Sul, Avenida Bento Gonçalves, 9500, RS,
90501-970, Brazil.
ꢀ
80 C] with Schultz-Flory distribution of oligomers (K = 0.54–0.66) and pro-
duction of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under
identical oligomerization conditions, Cr1/MAO behaved as a polymerization
catalyst generating predominantly polyethylene (63.7 wt%). The amount of
1-butene is the largest component in the liquid fraction suggesting that these
precatalysts operate via a Cossee-Arlman mechanism. The catalytic activities,
selectivity and product distribution are quite sensitive to the R-group at the 3-
and 5-position of the pyrazolyl ring. Based on the electronic and steric effects
of R- substituents, it is possible to stablish a trend of activity: Cr2
(PzMe2) > Cr3(PzPh) > Cr1(Pz). Moreover, the effect of oligomerization param-
eters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and
on the product distribution were examined.
Email: osvaldo.casagrande@ufrgs.br
Funding information
FAPERGS/PRONEX, Grant/Award
Number: 16/2551-0000; INCT/CNPq-
Catálise, Grant/Award Number:
465454/2014-3
K E Y W O R D S
bidentate ligands, chromium complexes, ethylene oligomerization, phenyl ether-pyrazolyl
ligands, α-Olefins
1 | INTRODUCTION
Schulz-Flory distributions manufactured by Shell, Ineos,
SABIC, and Chevron-Phillips.[9–12] Among all the
Catalytic oligomerization of ethylene is an important
industrial process in view of the importance of linear
α-olefins (LAO) as versatile intermediates for many
industrially substances such as linear low-density poly-
ethylene (LLDPE), plasticizers, surfactants for detergents,
base stock, additives for synthetic lubricants, and alco-
hols for plasticizers.[1–8] LAOs are generally produced
with a broad range of olefins from C4 to C20 featured by
transition-metal-based catalysts, chromium catalysts have
proven to be the most promising candidates for selective
ethylene oligomerization.[13–16] In this way, significant
efforts have been dedicated to the synthesis of new fami-
lies of ligands based on a wide variety of donor-group
combinations aiming to generate more efficient chro-
mium catalyst systems that are capable of selectively
forming α-olefins with high productivities.[17–81] In
Appl Organomet Chem. 2020;e5984.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5984

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MILANI AND CASAGRANDE
particular, chromium catalysts stabilized by pyrazolyl-
based tridentate ligands have been successfully
synthetized and their catalytic application in ethylene
oligomerization explored in details.[51,81–91] Selected
examples are presented in Chart 1. For instance, Tosoh
Corporation reported chromium complex bearing tris(3,-
5-dimethyl-1-pyrazolyl)methane ligand that, once acti-
vated with methylalumoxane (MAO), generate highly
active oligomerization/polymerization species 526.0 x 103
(mol ethylene) (mol Cr)−1Áh−1 with selectivity for
1-hexene >99.5%.[81] In 2007, Casagrande, Carpentier
and co-workers reported the synthesis of chromium (III)
complexes of general formula [CrCl3(NZN)] based on
pyrazolyl tridentate ligands NZN (Z = N, O, S). Once acti-
vated with MAO, these precatalysts formed active cata-
lysts for Schulz-Flory oligomerization to LAOs and low
molecular weight polymer, with TOFs varying from 3.4
to 131.0 x 103 (mol ethylene) (mol Cr)−1Áh−1 at 80 ꢀ C.[83]
Braunstein, Hor and co-workers reported a series of
active Cr (III) complexes with [NON] heteroscorpionate
ligands derived from bis (pyrazol-1-yl)methane.[84] Acti-
vation by MAO resulted in highly selective toward
trimerization (total C6 selectivities >98.9%) to 1-hexene
(>99.6%) with activity of 16,200 g/(g of Cr)Áh. In 2010,
Mountford et al. described the synthesis and structural
characterization of Cr (II) and Cr (III) complexes con-
taining N2N’ and N2O donor heteroscorpionate ligands
with secondary or tertiary amine side arms and phenyl
ether, phenol or phenolate side arms.[87] Upon activation
with MAO, all new chromium complexes are more active
27.5–116.0 x 103 (mol ethylene) (mol Cr)−1Áh−1 than Cr
{HC (pzMe2)3}Cl3 (24.6 x 103 (mol ethylene) (mol
Cr)−1Áh−1) but invariably produce a width Schultz-Flory
distribution of oligomers. Whereas most complexes dis-
play a typical K value between 0.59 and 0.67, with a
K value of 0.37 Cr{(pzMe2)2CHArOMe}Cl3 (Chart 1)
shows a significantly higher selectivity for 1-hexene
(52.4%) and 1-octene (34.7%).
however, to the best of our knowledge, there appear to be
no known examples of chromium complexes bearing
pyrazolyl-based bidentate ligands. Furthermore, the use
of these bidentate ligands could provide an additional
coordination site which can be useful to adjust the elec-
tronic and steric properties of the complexes towards
selective tri- and tetramerization of ethylene. Thus,
inspired by these works, and considering our results
based on nickel complexes bearing phenyl ether-
pyrazolyl bidentate ligand applied in selective
dimerization of ethylene,[92] we have decided to carry out
the synthesis of Cr (III) complexes, and investigate their
catalytic performance in ethylene oligomerization. This
paper will discuss the oligomerization results for these
new systems, evaluating the role of the ligand, and the
experimental parameters on the activity and selectivity.
2 | EXPERIMENTAL
2.1 | General considerations
All manipulations involving air- and/or moisture-sensitive
compounds were carried out in an MBraun glovebox or
under dry argon using standard Schlenk techniques. Tolu-
ene, THF, and hexane were dried over a Braun MB-SPS-
800 solvent purification system. Other solvents were dried
from the appropriate drying agents under argon before
use. [CrCl3(THF)3] was purchased from Sigma-Aldrich
and used as received. The ligands 1-(3-phenoxypropyl)-
1H-pyrazole) (L1), 3,5-dimethyl-1-(3-phenoxypropyl)-1H-
pyrazole (L2) and 1-(3-phenoxypropyl)-3-phenyl-1H-
pyrazole, (L3) were prepared following the literature pro-
cedures.[92] Ethylene (White Martins Co.) and argon were
deoxygenated and dried through BTS columns (BASF) and
activated molecular sieves prior to use. MAO (Witco,
5.21 wt.% Al solution in toluene, 20% TMA),
polymethylaluminoxane-improved performance (PMAO-
IP) (AkzoNobel, 13.0 wt.% Al solution in toluene), EASC
(Akzo Nobel) (2.1 wt.% Al solution in toluene) and
trimethylaluminum (TMA) (Sigma-Aldrich, 2 M in
As alluded to above, the tridentate pyrazolyl ligands
have been successfully used for the synthesis of active
chromium catalysts towards production of α-olefins;
CHART 1 Examples of chromium complexes for
oligomerization catalysis based on pyrazolyl ligands

MILANI AND CASAGRANDE
3 of 10
toluene) were used as received. Infrared spectra were per-
formed on neat products using a FT-IR Bruker Alpha
Spectrometer operating in the ATR mode. Elemental ana-
lyses were performed on a Perkin Elmer 2,400 Series II by
the Analytical Central Service of the Institute of
Chemistry-UFRGS (Brazil) and is the average of two inde-
pendent determinations. High-resolution mass spectra of
Cr (III) complexes (Cr1-Cr3) were obtained by
electrospray ionization (ESI) in the positive mode in meth-
anol or acetonitrile solutions using a Micromass Waters®
Q-Tof spectrometer. Quantitative gas chromatographic
analysis of ethylene oligomerization products was per-
formed on a Agilent 7890A instrument equipped with a
Petrocol HD capillary column (methyl silicone, 100 ꢀm
length, 0.25 mm i.d. and film thickness of 0.5 μm) (36 C
for 15 min, then heating at 5 ꢀC min−1 until 250 ꢀC); cyclo-
hexane was used as internal standard. Melting tempera-
tures were determined by differential scanning
calorimetry (DSC) with a Thermal Analysis Instruments
(0.240 g, 0.65 mmol) in THF (15 ml) to give Cr2 as a pur-
ple solid (0.250 g, 86%). IR (ATR, cm−1): ν 3,062(w), 3,039
(w), 2,933(m), 2,875 (w), 1,637 (m), 1,617 (m), 1,599 (m),
1,588 (w), 1,586 (w), 1,493 (m), 1,472 (m), 1,443 (w),
1,430 (w), 1,395 (w), 1,384 (w), 1,304 (w), 1,292 (m), 1,241
(m), 1,214 (w), 1,200 (w), 1,172 (w), 1,150 (w), 1,081 (w),
1,042 (w), 992 (m), 940 (w), 758 (m), 693 (w). ESI-HRMS
(CH3OH, m/z): 352.0197
g
[M-Cl]+ (calc. For
C14H18Cl2CrN2O: 352.0201 g). Anal. Calcd. for
C28H36Cl6Cr2N4O2: C: 43.26; H: 4.67; N: 7.21. Found: C:
43.12; H: 4.17; N: 6.88.
2.2.3 | [Cr{1-(3-phenoxypropyl)-3-phenyl-
1H-pyrazole}Cl3]2 (Cr3)
This complex was prepared as described above for Cr1,
starting from L3 (0.220 g, 0.79 mmol) and [CrCl3(THF)3]
(0.270 g, 0.72 mmol) in THF (15 mL) to give Cr3 as a pur-
ple solid (0.270 g, 84%). IR (ATR, cm−1): ν 3,240 (m),
2,973 (w), 2,901 (w), 2,872 (w), 1,600 (m), 1,584 (w), 1,546
(w), 1,517 (w), 1,492 (m), 1,467 (w), 1,458 (w), 1,450 (w),
1,339 (w), 1,299 (w), 1,243 (m), 1,227 (m), 1,171 (w),
1,097 (w), 1,079 (w), 1,040 (m), 1,011 (m), 919 (w),
854 (s), 755 (m), 688 (m), 512 (w). ESI-HRMS (CH3OH,
m/z): 400.0198 g [M-Cl]+ (calc. For C18H18Cl2CrN2O:
400.0201 g). Anal. Calcd. for C36H36Cl6Cr2N4O2: C: 49.51;
H: 4.15; N: 6.41. Found: C: 48.78; H: 4.18; N: 6.13.
−1
ꢀ
DSC-Q20 using a heating rate of 10 C min after twice
previous heating to 180 ꢀC and cooling to 40 ꢀC at
10 ꢀC min−1
.
2.2 | Synthesis of the Cr (III) complexes
2.2.1 | [Cr{1-(3-phenoxypropyl)-1H-
pyrazole}Cl3]2 (Cr1)
A solution of L1 (0.200 g, 0.98 mmol) in THF (10 ml) was
added dropwise over a period of 10 min to a solution of
[CrCl3(THF)3] (0.330 g, 0.88 mmol) in THF (15 ml) at
room temperature under constant stirring. The resulting
solution was allowed to stir for 3 hr at room temperature
and then the solvent was removed under vacuum. The
solid residue was washed with diethyl ether (3 x 10 ml) to
give, after drying, Cr1 as a purple solid (0.290 g, 90%). IR
(ATR, cm−1): ν 3,274 (s), 3,119 (w), 3,037 (w), 2,957 (w),
2,875 (w), 1,626 (m), 1,599 (s), 1,586 (m), 1,493 (s), 1,472
(m), 1,455 (w), 1,421 (m), 1,293 (w), 1,266 (w), 1,241 (s),
1,173 (m), 1,152 (w), 1,104 (m), 1,081 (m), 1,050 (m),
944 (w), 908 (w), 887 (w), 864 (w), 783 (m), 756 (s),
693 (m), 609 (m), 514 (m). ESI-HRMS (CH3OH, m/z):
2.3 | Ethylene oligomerization
All ethylene oligomerization tests were performed in a
100 ml double-walled stainless Parr reactor equipped
with mechanical stirring, internal temperature control
and continuous feed of ethylene. The Parr reactor was
dried in an oven at 120 ꢀC for 5 hr prior to each run, and
then placed under vacuum for 30 min. A typical reaction
was performed by introducing toluene (30 ml) and the
proper amount of cocatalyst into the reactor under an
ethylene atmosphere. After 20 min, the toluene catalyst
solution (10 ml, [Cr] = 10 μmol) was injected into the
reactor under a stream of ethylene and then the reactor
was immediately pressurized. Ethylene was continuously
fed in order to maintain the ethylene pressure. After the
desired time, the reaction was stopped by cooling the sys-
323.9883
[M-Cl]+
(calc.
For
C12H14Cl2CrN2O:
323.9888 g). Anal. Calcd. for C24H28Cl6Cr2N4O2: C: 39.97,
H: 3.91, N: 7.77. Found: C: 39.52, H: 3.87, N: 7.13.
ꢀ
tem to −60 C and depressurizing. An exact amount of
cyclohexane was introduced (as an internal standard)
and the mixture was analyzed by quantitative GLC. The
polymer was separated by filtration, washed with small
portions of acidic ethanol, then ethanol and water, and
the resulting material was dried in a vacuum oven at
60 ꢀC for 12 hr.
2.2.2 | [Cr{1-(3-phenoxypropyl)-
3,5-dimethyl-1H-pyrazole}Cl3]2 (Cr2)
This complex was prepared as described above for Cr1,
starting from L2 (0.200 g, 0.72 mmol) and [CrCl3(THF)3]

4 of 10
MILANI AND CASAGRANDE
3 | RESULTS AND DISCUSSION
3.2 | Ethylene Oligomerization studies
3.1 | Synthesis and characterization of
chromium complexes bearing phenyl
ether-pyrazolyl ligands
Ethylene oligomerization was investigated using chro-
mium complexes Cr1-Cr3 in toluene at 80 ꢀC, 20 bar con-
stant ethylene pressure, and using MAO containing
20 wt.% AlMe3 as cocatalyst. Relevant data are compiled
in Table 1. All experiments were at least duplicated, the
error in terms of activity and product selectivity values
was estimated to be approximately 5%. Initial studies,
The phenyl ether-pyrazolyl ligands (L1-L3) were readily
synthesized by reaction of 3-bromopropyl phenyl ether
with sodium pyrazolate in DMF/THF in moderate to
good yields (60–85%).[92] The ligands obtained were fully
characterized by analytical and spectroscopic methods.
The reaction between CrCl3(THF)3 and 1.1 equiv of
L1-L3 in THF at room temperature afforded the chro-
mium complexes Cr1-Cr3 as purple solids in good yields
(84–90%) as shown in Scheme 1. All chromium com-
plexes, which are air- and moisture-sensitive, show mod-
erate solubility at room temperature in dichloromethane
and THF, and are readily soluble in acetonitrile. The
identity of these complexes was established on the basis
of combustion elemental analysis, IR spectroscopy and
high-resolution mass spectrometry (HRMS). Upon reac-
tion with CrCl3(THF)3, either [CrCl3(N,O)(THF)] or the
dimer [Cr(N,O)Cl3]2 were expected, both of which would
maintain the favored hexacoordination of chromium
(III)[14,24,27,28,45,46]; however, the elemental analyses of
complexes Cr1-Cr3 confirmed the formation of dimeric
species of general formula [CrCl2(N,O)(μ-Cl)]2. Further-
more, the ESI-HRMS results indicated the formation of
[M-Cl]+ ions for all chromium complexes (Figures S1-S3).
ꢀ
carried out at 80 C with [Al]/[Cr] = 300, showed that
these catalyst systems were moderately active, affording a
nonselective distribution of α-olefins with turnover fre-
quencies (TOFs) in the range of 6,500–29,200 mol (eth-
ylene)Ámol (Cr)−1Áh−1 over 15-min runs as presented in
Figure 1. In addition, analyses of the data showed that
the activity and product distribution were dependent on
the R substituent at the 3- and 5-position of the
pyrazolyl ring.
The highest activity among the three chromium cata-
lysts screened was reached with Cr2 [TOF = 29,200 × 103
(mol ethylene) (mol Cr)−1 h−1] which is ca 4.5 times
more active than Cr1 [TOF = 6,500 (mol ethylene) (mol
Cr)−1 h−1]. The activity data found for this class of chro-
mium precatalysts are comparable to those for other
chromium complexes stabilized by pyrazolyl-based
ligands.[38,84]
To compare the steric properties of Cr1-Cr3, topo-
graphic steric maps were generated using the SambVca
2.1 software, and metrical data from the optimized struc-
tures (Figure 2). The results are compiled assuming a
3.50 Å for the sphere radius, omitted hydrogen atoms
and scaled bond radii by 1.17, as recommended.[93,94] Par-
ticularly, it was found that Cr1 and Cr2 have similar per-
cent buried volume (%VBur) [%VBur = 64.3 for Cr1; %
VBur = 66.8 for Cr2] suggesting that the presence methyl
groups at the 3- and 5-position of the pyrazolyl ring does
not cause a significant increase in steric protection
around the Cr metal center. In this respect, the higher
activity of Cr2 can be tentatively associated with the
higher electron-donating ability of 3,5-dimethyl-pyrazolyl
unit when compared with unsubstituted pyrazolyl, thus
generating a more stable catalytic species.[95] These
results are in line with previously reported using MAO-
activated
metal
complexes
bearing
pyrazolyl
ligands.[38,92,96]
As presented in Figure 2, the presence of sterically
demanding Ph group in Cr3 resulted in a higher percent
buried volume (%VBur = 70.0) with respect to Cr1. Con-
sidering the comparable electron-withdrawing ability of
PzPh and Pz, we guess that the catalytic activity of Cr3 is
not driven by electronic effects of the phenyl
group.[95,97–99] Thus, the presence of the phenyl group at
3-position of the pyrazolyl ring might be able to promote
SCHEME 1 Synthesis of chromium (III) complexes supported
by phenyl ether-pyrazolyl ligands

MILANI AND CASAGRANDE
5 of 10
TABLE 1 Ethylene Oligomerization with Cr1-Cr3 precatalyst systemsa
Oligomer distribution (wt.%)c
Total
product
(wt.%) (wt.%) (mg)
Time
TOFb
C4
(α-C4)
C6
(α-C6)
C8
(α-C8)
C10
(α-C10)
Olig.
PE
Entry Cat. (min) [Al/Cr] (103Áh−1
)
C12+
1
2
3
Cr1
Cr2
Cr3
15
15
15
300
300
300
6.5
25.3
(93.8)
22.6
(93.0)
21.7
(82.9)
16.1
(91.4)
14.3
36.3
97.2
93.3
63.7
1,240
2,100
1,340
29.2
17.9
72.2
(97.4)
19.9
(92.9)
5.6
(100)
2.3 (100)
2.8
0.0
3.2
43.4
30.5
16.0
6.7
(96.4)
(95.3)
(91.6)
6.9
(90.0)
4
Cr3
Cr3
Cr3
Cr3
Cr3
Cr3
15
15
15
5
1,000
1,500
300
24.8
97.2
8.1
12.3
(92.6)
17.8
(93.2)
20.0
(91.9)
16.1
(93.6)
33.8
46.1
28.7
3.2
68.6
76.5
49.1
94.8
94.1
37.4
31.4
23.5
50.9
5.2
2,503
8,870
1,140
1,280
1,680
2,700
5
13.6
(93.4)
15.3
(93.6)
13.7
(93.9)
11.3
(91.2)
6d
7
16.8
(96.7)
20.9
(96.9)
18.6
(98.9)
15.0
(85.6)
300
52.2
11.3
13.5
68.3
(94.2)
19.6
(90.1)
6.3
(89.2)
2.7
(75.7)
8
30
15
300
54.9
(96.4)
22.1
(89.2)
11.3
(83.0)
4.1
(100.0)
7.6
5.9
9e
300
9.1
14.9
16.2
14.2
45.6
62.6
(89.6)
(91.9)
(90.6)
(91.6)
aReaction conditions unless specified otherwise: toluene = 40 ml, [Cr] = 10 μmol, oligomerization time = 15 min, P (ethylene) = 20 bar (kept
constant), T = 80 ꢀC, MAO. The results shown are representative of at least duplicated experiments, yielding reproducible results within
5%.
bMol of ethylene converted (mol of Cr)−1Áh−1 as determined by quantitative GLC.
cCn, percentage of olefin with n carbon atoms in oligomers; α-Cn, percentage of terminal alkene in the Cn fraction, as determined by quanti-
tative GLC.
dPMAO-IP as cocatalyst.
eT = 100 ꢀC.
a better steric protection around the chromium metal
center, and thus improve the catalyst performance (entry
1vs 3). Based on these results, it is possible to establish a
trend of activity: Cr2(PzMe2) > Cr3(PzPh) > Cr1(Pz),
which is similar to that one found for nickel complexes
bearing the same class of ligands.[92]
associated to formation of different active species, and in
particular the catalytically active oxidation states of chro-
mium (I, II, and III).[71,81,100–103]
The pyrazolyl-type moiety has also a significant
impact on the product distribution in the liquid fraction
as can be seen in Figure 3. For instance, Cr1 precatalyst
led to a more balanced distribution of oligomers [α-C4
(41.3 mol%), α-C6 (24.4 mol %), and α-C8 (15.7 mol%)]
while those containing more sterically demanding
pyrazolyl group (Cr2-Cr3) are centered in lighter
α-olefins fractions with a considerable amount of
1-butene (58.0–82.3 mol %). However, enriched fraction
in α-C6 (26.9 mol %) is obtained using Cr3.
The oligomer distributions obtained by the
Cr1 − Cr3/MAO systems follow a Schulz−Flory distribu-
tion as presented in Figure 4. The amount of 1-butene is
the largest component in the liquid fraction suggesting
that these precatalysts operate via a Cossee-Arlman
Except for Cr1, all other two chromium precatalysts
Cr2 and Cr3 activated with MAO afforded mostly oligo-
mers (93.3–97.2 wt.% of total products) ranging from C4
+
to C12 with a distinct distribution for α-olefins and pro-
duction of polymer varying from 2.8 to 6.7 wt.%. Contra-
sting result was found using Cr1 where the replacement
of the methyl on the pyrazolyl rings by hydrogen
decreases significantly the selectivity and favors the for-
mation of PE which corresponds to 63.7 wt.% of the total
amount of products. A similar behavior was observed in
CrCl3[PzRCH2NCH2Ph].[86] In this case, such selectivity
switch (oligomers vs polymer) can be tentatively

6 of 10
MILANI AND CASAGRANDE
the level of polymer formation increased substantially
[from 6.7 to 23.5–31.4 wt.%] as well as the K values
(K1000 = 0.91; K1500 = 0.85) (Figure S11).
Activation of Cr3 with PMAO-IP instead of MAO
resulted in slightly improvement in activity (8,100 (mol
ethylene) (mol Cr)−1 h−1). The total mass of products
(oligomers + polymer) is quite similar in both cases
(compare entries 3 and 6) suggesting no occurrence of
deactivation pathway. The production of
a larger
amount of polymer (50.9 wt.%) compared to that one
using MAO (6.7 wt.%) strongly suggest that the amount
of “free” TMA present in the reaction medium is not
enough to generate reduced catalytically active chro-
mium species (CrI/CrII), which are assumed to be
responsible for the nonselective oligomerization behav-
ior. However, we cannot rule out the possibility that
the absence of “free” TMA could also decrease the
chain-transfer rate of the growing chain from the cata-
lyst to the aluminum, and thus resulting in a formation
of higher quantity of polyethylene.[27,108,109] The role of
Al activators on catalytic performance of chromium cat-
alysts in homogeneous ethylene oligomerization has
been recently studied by Tromp, Brückner and co-
workers.[110,111] With regards to product distribution
within the liquid fraction, Cr3/PMAO-IP catalytic sys-
tem leads to a more balanced distribution of oligomers,
with a drastic decreases in α-C4 (30.20 mol %) and
increase in α-C8 (16.80 mol %) and α-C10 (10.80 mol %)
fractions (Figures S9 and S10).
FIGURE 1 Influence of the nature of chromium precatalyst
Cr1-Cr3 on TOF and selectivity for oligomers (T = 80 ꢀC, 20 bar,
time = 15 min, [Al]/[Cr] = 300)
mechanism.[16,104] The K values calculated from a plot of
log [mol%] vs carbon number was higher for Cr1
(KC4-C10 = 0.82) than for Cr2 ( KC4-C10 = 0.54) and Cr3
(KC4-C10 = 0.66) as expected taking into account the
higher amounts of heavier olefins and polymer produced
by Cr1.
The DSC curve of the polymer produced by Cr1
showed a single melting transition at 131.5 ꢀC, and a rela-
tively high crystallinity (70%). Conversely, the polymers
produced by Cr2-Cr3 displayed two endothermic peaks
at in the range of 63.4–125.4 ꢀC suggesting the production
of linear oligoethylenes with low molecular weight
(Figures S4-S6). Similar results were observed with other
catalytic systems based on chromium complexes bearing
bi- and tridentate ligands.[37,38,105–107]
To establish the optimal oligomerization conditions,
precatalyst Cr3 was selected as test precatalyst varying
the oligomerization parameters such as [Al]/[Cr] molar
ratio, cocatalyst type, reaction temperature and run time.
The results are collected in Table 1 (entries 4–9). The pre-
catalyst Cr3 was found to be inactive upon activating
with different cocatalysts such as aluminum
sesquichloride (EASC), TMA and triethylaluminum
(TEA) presumably due to the low Lewis acidity of these
activators.
The optimal operating temperature for this class of
precatalysts is 80 ꢀC. At 100 ꢀC the precatalyst still
operating but with slightly lower activity (13,500 mol
(ethylene)Ámol (Cr)−1Áh−1), probably due to the partial
deactivation of the active species ꢀand the lower solubil-
ity of ethylene in toluene at 100 C. At same time, Cr3
produced a significant amount of polymer (1.69 g,
62.6 wt. % of the total mass of products) with activity of
676 kg of PE/mol[Cr]Áh. This result suggests that differ-
ent temperature stabilities exist for the catalytic species
involved in ethylene oligomerization and polymeriza-
tion reactions, respectively. Thus, we guess that higher
temperature favored the catalytic species involved in
polymerization reactions; however, we cannot rule out
that other catalytically active species are being gener-
ꢀ
ated at 100 C. At this temperature, the production of
The [Al]/[Cr] molar ratio was adjusted from 300 to
1,500 (entries 3, 4–5, Table 1). Increasing the
[Al]/[Cr] ratio from 300 to 1,500 equiv had a beneficial
effect on the catalytic activity (TOF = 97,200 (mol ethyl-
ene) (mol Cr)−1 h−1) with a significant decrease in the
production of 1-butene (from 58.03 to 18.4–30.2 mol %),
1-butene dropped to 18.07 mol % with a significant
improvement in the formation of the α-C8 (from 10.16
to 16.33 mol %) and α-C10 (from 3.44 to 11.56 mol %)
fractions. A plot of log (mol %) versus carbon number
shows a Schulz-Flory distribution with a K value of
0.91. (Figures S10 and S11).
+
and production of enriched fractions in C12 (from 1.5 to
17.3–31.7 mol %) (Figures S7-S8 and S10). Furthermore,
The product distribution and productivity were moni-
tored at different time intervals (5, 15 and 30 min) with

MILANI AND CASAGRANDE
7 of 10
FIGURE 2 Steric maps of chromium complexes Cr1-Cr3 bearing phenyl ether-pyrazolyl ligands
15 min, it was not observed a significant variation in total
mass of products (oligomers and polymer), which are
comparable considering an average error of 5% as pres-
ented in Figure 5.
When the oligomerization time was prolonged, the
total mass of products increased (from 1.28 to 1.68 g)
indicating that Cr3 had a minimal catalytic lifetime of
30 min. It is noteworthy to mention that the ratio
between the mass of oligomers/polymer does not
change significantly suggesting that the nature of the
active species remains unchanged along of 30 min. Fur-
thermore, no significant variation in the selectivities for
α-olefins was observed over
a 5–30 min period
(92.6–94.8 wt.%), indicating that parallel chain isomeri-
zation processes are a minor pathway in these oligomer-
ization reactions (Figures S12 and S13). On the other
hand, the reaction time has some influence on the
oligomer distribution. Thus, on extending the reaction
time seems to favor the production of higher amounts
of 1-hexene and 1-octene and reduction of the amount
of 1-butene.
FIGURE 3 Selectivity of Cr1-Cr3/MAO oligomerization
systems for α-olefins (T = 80 ꢀC, 20 bar of ethylene, time = 15 min,
[Al]/[Cr] = 300)
[Al]/[Cr] molar ratio of 300 and the temperature at 80 ꢀC
for investigating the lifetime of the active species. (entries
3, 7 and 8). On prolonging the reaction time from 5 to
FIGURE 4 Oligomers distribution
produced by Cr1-Cr3/MAO
oligomerization systems (C4-C10)
(80 ꢀC, 20 bar, [Al/[Cr] = 300).
(a) Schulz−Flory α-olefin distribution in
molar percentage and (b) plot of the log
(mol%) of vs carbon number

8 of 10
MILANI AND CASAGRANDE
(CrI/CrII), which is assumed to be responsible for the
nonselective oligomerization behavior.
ACKNOWLEDGMENTS
This work was supported in part by FAPERGS/PRONEX
(Grant/Award Number: 16/2551-0000) and INCT/CNPq-
Catálise (Grant/Award Number 465454/2014-3). This
paper is dedicated to Professor P.H. Dixneuf for his out-
standing contribution to the field of Organometallic
Chemistry and Catalysis.
AUTHOR CONTRIBUTIONS
OSVALDO CASAGRANDE: Conceptualization; data
curation; formal analysis; investigation; methodology;
supervision. Jorge Milani: Conceptualization; formal
analysis; methodology.
FIGURE 5 Monitoring of selectivities and productivities as a
function of time in the ethylene oligomerization reaction
ꢀ
promoted by the Cr3/MAO system (T = 80 C, 20 bar of ethylene,
[MAO]/[Cr] = 300)
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able from the corresponding author upon reasonable
request.
4 | CONCLUSIONS
A new class of chromium complexes bearing a phenyl
ether-pyrazolyl bidentate ligands have been reported.
When activated with MAO, these catalyst systems show
moderate activity in ethylene oligomerization affording
Schulz−Flory olefin distribution. Variations in electronic
and steric effects on the pyrazolyl group can significantly
affect the activity and selectivity, as well as the product
distribution (oligomers versus polymers). Thus, the
higher electron-donating ability of 3,5-dimethyl-pyrazolyl
unit afforded the highest activity among the three chro-
mium catalysts screened. The contrasting catalytic behav-
ior of Cr1 that acts as ethylene polymerization catalyst
and Cr2-Cr3 that afforded mostly oligomers can be asso-
ciate to the influence of pyrazolyl unit on the formation
of different active species, and in particular the catalyti-
cally active oxidation states of chromium. Furthermore,
the Cr1 precatalyst led to a more balanced distribution of
oligomers while those containing more sterically
demanding pyrazolyl group (Cr2-Cr3) are centered in
lighter α-olefins fractions. Oligomerization studies involv-
ing Cr3 showed that the use of higher MAO loading posi-
tively affected the catalytic activity with a significant
decrease in the production of 1-butene; however the pres-
ence of higher amounts of MAO increased substantially
ORCID
Osvaldo L. Casagrande Jr https://orcid.org/0000-0002-
9855-6159
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