molecules
Article
Replacement of the Common Chromium Source CrCl3(thf)3
with Well-Defined [CrCl2(µ-Cl)(thf)2]2
Dong Geun Lee 1,† , Jun Won Baek 1,† , Jung Hyun Lee 1 , Hyun Ju Lee 1 , Yeong Hyun Seo 1 , Junseong Lee 2 ,
Chong Gu Lee 3 and Bun Yeoul Lee 1, *
1
2
3
*
†
Citation: Lee, D.G.; Baek, J.W.; Lee,
J.H.; Lee, H.J.; Seo, Y.H.; Lee, J.; Lee,
C.G.; Lee, B.Y. Replacement of the
Common Chromium Source
CrCl3 (thf)3 with Well-Defined
[CrCl2 (µ-Cl)(thf)2 ]2 . Molecules 2021,
26, 1167. https://doi.org/10.3390/
molecules26041167
Academic Editors: Maurizio Peruzzini
Department of Molecular Science and Technology, Ajou University, 206 Worldcup-ro, Yeongtong-gu,
Suwon 16499, Korea; itl1096@ajou.ac.kr (D.G.L.); btw91@ajou.ac.kr (J.W.B.); moa0811@hanmail.net (J.H.L.);
hjulee4639@ajou.ac.kr (H.J.L.); tdg0730@ajou.ac.kr (Y.H.S.)
Department of Chemistry, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Korea;
leespy@chonnam.ac.kr
Precious Catalysts Inc. 201, Duryu-gil, Angangeup, Gyeongju 38029, Korea; cglee@s-pci.com
Correspondence: bunyeoul@ajou.ac.kr; Tel.: +82-031-219-1844
These authors contributed equally to this work.
Abstract: CrCl3 (thf)3 is a common starting material in the synthesis of organometallic and coordination compounds of Cr. Deposited as an irregular solid with no possibility of recrystallization,
it is not a purity guaranteed chemical, causing problems in some cases. In this work, we disclose a
well-defined form of the THF adduct of CrCl3 ([CrCl2 (µ-Cl)(thf)2 ]2 ), a crystalline solid, that enables
structure determination by X-ray crystallography. The EA data and XRD pattern of the bulk agreed
with the revealed structure. Moreover, its preparation procedure is facile: evacuation of CrCl3 ·6H2 O
at 100 ◦ C, treatment with 6 equivalents of Me3 SiCl in a minimal amount of THF, and crystallization
from CH2 Cl2 . The ethylene tetramerization catalyst [iPrN{P(C6 H4 -p-Si(nBu)3 )2 }2 CrCl2 ]+ [B(C6 F5 )4 ]−
prepared using well-defined [CrCl2 (µ-Cl)(thf)2 ]2 as a starting material exhibited a reliably high
activity (6600 kg/g-Cr/h; 1-octene selectivity at 40 ◦ C, 75%), while that of the one prepared using
the impure CrCl3 (thf)3 was inconsistent and relatively low (~3000 kg/g-Cr/h). By using welldefined [CrCl2 (µ-Cl)(thf)2 ]2 as a Cr source, single crystals of [(CH3 CN)4 CrCl2 ]+ [B(C6 F5 )4 ]− and
[{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2 were obtained, allowing structure determination by X-ray crystallography, which had been unsuccessful when the previously known CrCl3 (thf)3 was used as the
Cr source.
Keywords: chromium compound; CrCl3 (thf)3 ; [CrCl2 (µ-Cl)(thf)2 ]2 ; ethylene tetramerization catalyst
and Luca Gonsalvi
Received: 29 January 2021
Accepted: 19 February 2021
Published: 22 February 2021
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conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
1. Introduction
CrCl3 (thf)3 has been widely used as a starting material for the synthesis of organometallic and coordination compounds of Cr [1–10]. Its preparation method was reported 60 years
ago; anhydrous CrCl3 , which is insoluble in organic solvents as well as in water, was
extracted with THF by the aid of Zn dust using Soxhlet apparatus [11–13]. However, anhydrous CrCl3 is prepared by hazardous and not-facile process (treatment of Cr2 O3 ·xH2 O
with CCl4 at 630 ◦ C leads to the generation of extremely toxic phosgene gas), and thus, is
expensive [14,15]. Another method was reported in which CrCl3 ·6H2 O was reacted with
excess thionyl chloride (S(O)Cl2 ) to generate anhydrous CrCl3 . In this process, a different
form of hygroscopic CrCl3 was generated, from which CrCl3 (thf)3 could be immediately
obtained after adding THF [16–18]. The most attractive and convenient method is reacting
CrCl3 ·6H2 O with excess Me3 SiCl in THF, resulting in the deposition of CrCl3 (thf)3 along
with the generation of byproducts, HCl and Me3 SiOSiMe3 (CrCl3 ·6H2 O + 12 Me3 SiCl →
CrCl3 (thf)3 + 12 HCl + 6 Me3 SiOSiMe3 ) [19]. Excess Me3 SiCl (~60 equiv./Cr) needs to be
added to obtain a completely anhydrous form of CrCl3 (thf)3 [20]. Otherwise (e.g., when
25 equiv. of Me3 SiCl/Cr was added according to the literature), hydrated CrCl3 (thf)2 (H2 O)
is obtained, which was erroneously sold as CrCl3 (thf)3 in the past [20,21].
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(e.g., when 25 equiv. of Me3SiCl/Cr was added according to the literature), hydrated
Sasol disclosed a catalyst system composed of a Cr source, iPrN(PPh )2 ligand, and
CrCl3(thf)2(H2O) is obtained, which was erroneously sold as CrCl3(thf)3 in the 2past
[20,21].
methylaluminoxane (MAO), which produces mainly 1-octene through ethylene tetramerSasol disclosed a catalyst system composed of a Cr source, iPrN(PPh2)2 ligand, and
ization
[22–24]. CrCl (thf)3 has
been actively used as a component of the ethylene tetramermethylaluminoxane3(MAO),
which produces mainly 1-octene through ethylene tetramerization
catalyst
system
[25–34].
also developed
a very efficient
catalysttetramfor ethyization [22–24]. CrCl3(thf)3 has beenWe
actively
used as a component
of the ethylene
lene
tetramerization
(Scheme
1)
[35–37].
The
catalyst
shows
extremely
high
activity
erization catalyst system [25–34]. We also developed a very efficient catalyst for ethylene
(~5000
kg/g-Cr/h)
and
is
advantageous
over
other
reported
systems
in
that
it
works
tetramerization (Scheme 1) [35–37]. The catalyst shows extremely high activity (~5000
with
an
inexpensive
activator,
iBu
Al,
avoiding
the
use
of
expensive
MAO
in
excess
kg/g-Cr/h) and is advantageous over3 other reported systems in that it works with an in(A1/Cr
= 300–500)
[38–43].
the
the studies,
weinhad
a problem
reproexpensive
activator,
iBu3Al,During
avoiding
thecourse
use ofof
expensive
MAO
excess
(A1/Cr =in
300–
ducing
such anDuring
extremely
high activity
and eventually
thatinthe
Cr source CrCl
(thf)3
500) [38–43].
the course
of the studies,
we had afound
problem
reproducing
such 3an
used
in the preparation
cause of this
problem.
structure
of CrCl
3(thf)3 used
extremely
high activitywas
andthe
eventually
found
that theAlthough
Cr sourcethe
CrCl
in the
3 (thf)3
was
revealed by
crystallography,
using
a single the
crystal
selected
from3(thf)
the 3batch
in the
preparation
wasX-ray
the cause
of this problem.
Although
structure
of CrCl
was reSoxhlet
process, it does not
redeposit
good-quality
when
redissolved
vealedextraction
by X-ray crystallography,
using
a singleascrystal
selectedcrystals
from the
batch
in the
process, it does
not redeposit
as good-quality
crystals
when
inSoxhlet
CH2 Clextraction
elucidated
by X-ray
crystallography
does
not redissolved
guarantee the
2 [44,45]. Structure
2Clthe
2 [44,45].
in CHof
elucidated by
crystallography
doesCrnot
guarantee
purity
bulk Structure
and we questioned
theX-ray
purity
of the common
source
CrCl3the
(thf)3 .
3(thf)
of was
the bulk
and we
the purity
the common
Cr source
Inpurity
fact, it
reported
asquestioned
an “irregular
violet of
solid”
which may
meanCrCl
that
it is3. In
not a
fact,CrCl
it was
reported
as
an
“irregular
violet
solid”
which
may
mean
that
it
is
not
a
pure
pure
(thf)
but
a
mixture
containing
other
forms
of
THF
adduct
of
CrCl
(e.g.,
µ2-Cl
3
3
3
3 but a mixture
CrCl3(thf)
containing
other forms
of THF
bridged
multinuclear
species)
[46]. Elemental
analysis
(EA) adduct
data forof
CrCrCl
and3Cl(e.g.,
haveμ2-Cl
been rebridged
multinuclear
species)
[46].
Elemental
analysis
(EA) dataCrCl
for Cr
and Cl have been
ported,
but
common carbon
and
hydrogen
data
were missing.
3 (thf)3 is paramagnetic;
1 H and
13 C3(thf)
3 is paramagreported,
but common
carbon
hydrogen
data were
missing.
CrCl
thus,
it cannot
be subjected
to and
structural
analysis
using
NMR
spectroscopy.
1H and 13C NMR spectrosnetic;
thus,
it
cannot
be
subjected
to
structural
analysis
using
We also previously found that the composition (as well as the structure) of the commercial
copy. of
We(2-ethylhexanoate)
also previously found
that the composition (as well as the structure) of the comsource
3 Cr, another type of important paramagnetic Cr(III) complex
mercial source of (2-ethylhexanoate)3Cr, another type of important paramagnetic Cr(III)
that is commercially used as a component in the Phillips ethylene trimerization catalyst [47].
complex that is commercially used as a component in the Phillips ethylene trimerization
is erroneous [21,48]. By correcting the structure to (2-ethylhexanoate)2 CrOH, the catalytic
catalyst [47]. is erroneous [21,48]. By correcting the structure to (2-ethylhexanoate)2CrOH,
performance was consistent and, moreover, significantly improved.
the catalytic performance was consistent and, moreover, significantly improved.
Si(nBu)3
(nBu)3Si
CrCl3(thf)3
[(CH3CN)4CrCl2]+[B(C6F5)4]-
+
[(CH3CN)4Ag]+[B(C6F5)4]-
P
ligand
Cr+
N
P
AgCl
[B(C6F5)4]-
(nBu)3Si
Cl
Cl
Si(nBu)3
Scheme 1. Preparation of extremely active ethylene tetramerization catalyst.
Scheme 1. Preparation of extremely active ethylene tetramerization catalyst.
2. Results and Discussion
2. Results and Discussion
CrCl3(thf)3 was commercially available from Aldrich (St. Louis, MO, United States),
(thf)3 was
available
from
MO, USA), but
3·6H2O with excess
butCrCl
we 3prepared
it commercially
to ensure complete
dryness
byAldrich
reacting(St.
CrClLouis,
we
prepared
to ensure
dryness
by reacting
CrCl3 ·6H
Me3 SiCl
2 O3with
3SiCl (60 it
Me
equiv.)
[20]. complete
We modified
the procedure
as follows:
CrCl
·6H2Oexcess
was evacu(60
equiv.)
modified
procedure
as follows:
evacuated6Hfor
~6 h
3 ·6H
2 O was [iPrN{P(C
4-pated
for [20].
~6 hWebefore
the the
treatment
with
Me3SiCl.CrCl
The
catalyst
−
before
the3)treatment
with6FMe
The catalyst
[iPrN{P(C
CrCl3,2which
]+[B(Cwas
2}2CrCl2]+[B(C
5)4]3–SiCl.
Si(nBu)
, prepared
according
to Scheme
1 using CrCl
6 H4 -p-Si(nBu)
3 )2 }32(thf)
6 F5 )4 ] ,
prepared
to Scheme
1 using
CrCl
was prepared
via the evacuapreparedaccording
via the evacuation
step,
provided
extremely
high activity
(~5000 kg/g-Cr/h).
In
3 (thf)3 , which
tion
step, provided
high either
activity
kg/g-Cr/h).
contrast,
the
3(thf)3 or
contrast,
the catalystextremely
prepared using
the(~5000
commercial
source ofIn
CrCl
thecatalyst
one
preparedusing
according
thecommercial
reported method
evacuation
step
did not exhibit
prepared
eithertothe
sourcewithout
of CrClthe
prepared
according
3 (thf)
3 or the one
a high activity
(~3000
kg/g-Cr/h),
and there was
some
of methylcyclohexanetosuch
the reported
method
without
the evacuation
step
didamount
not exhibit
such a high activity
insoluble
fractionand
when
ligand
iPrN{P(C
was reacted with
3)2}2
(~3000
kg/g-Cr/h),
there was
some
amount6Hof4-p-Si(nBu)
methylcyclohexane-insoluble
fraction
+[B(C6F5)4]–. The insoluble fraction was negligible when CrCl3(thf)
−
3
4
2
3, which
[(CH
CN)
CrCl
]
when ligand iPrN{P(C6 H4 -p-Si(nBu)3 )2 }2 was reacted with [(CH3 CN)4 CrCl2 ]+ [B(C
6 F5 )4 ] .
was
prepared
via
the
evacuation
step,
was
used
as
the
starting
material.
The insoluble fraction was negligible when CrCl3 (thf)3 , which was prepared via the evacu-
ation step, was used as the starting material.
We investigated what happened in the evacuation step. When the crystalline form
of CrCl3 ·6H2 O (10 g, 37.5 mmol) was evacuated at 100 ◦ C, the color gradually changed
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We investigated what happened in the evacuation step. When the crystalline form of
CrCl3dark
·6H2Ogreen
(10 g,to
37.5
mmol)
waslight
evacuated
100finally
°C, thelight
colorpurple
gradually
changed
from
from
light
green,
gray, at
and
with
gradual
loss of
dark green
to light
light gray,
and finally
light purple
weight. to
weight.
Finally,
an green,
amorphous
powder
was obtained,
andwith
the gradual
weight loss
loss of
converged
Finally,
obtained,
and the
weight
loss converged
to 36% (6.39time
36%
(6.39ang amorphous
remaining).powder
Furtherwas
weight
reduction
was
minimal
when the evacuation
g remaining).
weight reduction
was minimal
when
evacuation
time component
was inwas
increased.Further
It was confirmed
with litmus
paper that
thethe
removed
volatile
creased.
It was
confirmed
with litmus
paper that
the removed
component
was not the
was
not pure
water
but acidic,
presumably
containing
HCl.volatile
From these
observations,
pure waterCr
butcompound
acidic, presumably
containing
HCl. Fromas
these
observations,
the remainremaining
was tentatively
considered
CrCl
2 (OH)(H2 O)2 ; 34% weight
2(OH)(H2O)2; 34% weight loss is exing
Cr
compound
was
tentatively
considered
as
CrCl
loss is expected for the transformation of CrCl3 ·6H2 O to CrCl2 (OH)(H2 O)2 (Scheme 2).
3·6H2O to CrCl2(OH)(H2O)2 (Scheme 2). The structure
pected
for the of
transformation
CrClreported
The
structure
CrCl3 ·6H2 Oofwas
to be [CrCl2 (H2 O)4 ]Cl·2H2 O, and the action
of CrCl3·6H2O was reported to be [CrCl2(H2O)4]Cl·2H2O,−and the action of heat under vacof heat under vacuum could liberate
outer-sphere
Cl (as HCl with a proton in an inneruum could liberate outer-sphere Cl− (as HCl with a proton in an inner-sphere water) and
sphere water) and three water molecules (two from the outer sphere and one from the
three water molecules (two from the outer sphere and one from the inner sphere), conseinner sphere), consequently leaving CrCl2 (OH)(H2 O)2 (possibly, [CrCl2 (H2 O)2 (µ-OH)]2
quently leaving CrCl2(OH)(H2O)2 (possibly, [CrCl2(H2O)2(μ-OH)]2 adopting an octahedral
adopting an octahedral structure) in the reaction pot.
structure) in the reaction pot.
CrCl3.6H2O
evacuation at 100 °C
CrCl2(OH)(H2O)2 + 6 Me3SiCl
THF
CrCl2(OH)(H2O)2 + HCl + 3 H2O
(tentatively assigned)
CrCl3(thf)x + 5 HCl + 3 Me3SiOSiMe3
Scheme 2. Modified preparation method for THF adduct of CrCl3.
Scheme 2. Modified preparation method for THF adduct of CrCl3 .
2O)2, after dissolving in THF, with Me3SiCl resulted in the
Treatment of
Treatment
of CrCl
CrCl2(OH)(H
2 (OH)(H2 O)2 , after dissolving in THF, with Me3 SiCl resulted in
precipitation of a violet solid, which was confirmed by infrared spectroscopy to be comthe precipitation of a violet solid, which was confirmed by infrared spectroscopy to be
pletely anhydrous. Because a large portion of H2O in CrCl3·6H2O was removed at the
completely anhydrous. Because a large portion of H2 O in CrCl3 ·6H2 O was removed at the
evacuation step, 6.0 equiv. Me3SiCl (equivalent amount that needed for the transformation
evacuation step, 6.0 equiv. Me3 SiCl (equivalent amount that needed for the transformation
of CrCl2(OH)(H2O)2 to CrCl3(thf)x, Scheme 2) was added instead of 60 equiv./Cr, and acof CrCl2 (OH)(H2 O)2 to CrCl3 (thf)x , Scheme 2) was added instead of 60 equiv./Cr, and
cordingly, the amount of THF could be minimized (36 mL/10 g—CrCl3·6H2O). The color
accordingly, the amount of THF could be minimized (36 mL/10 g—CrCl3 ·6H2 O). The
and shape of the precipitated solid (irregular violet solid) were almost identical to that of
color
and shape source
of the precipitated
(irregular
violetwithout
solid) were
almost identical
the commercial
of CrCl3(thf)3 solid
and the
one prepared
the evacuation
step. to
that
of
the
commercial
source
of
CrCl
(thf)
and
the
one
prepared
without
the
evacuation
3
3
However, as aforementioned, the catalyst performance was significantly better when usstep.
However,
as aforementioned, the catalyst performance was significantly better
3(thf)3 prepared via the evacuation step. The product of the reaction between
ing CrCl
when
using
CrCl
prepared via the evacuation step. The product of the reaction
+
3 (thf)
6F5)43]− and CrCl3(thf)3 (i.e., [(CH3CN)4CrCl2]+[B(C6F5)4]−) differed de[(CH3CN)4Ag] [B(C
+ [B(C F ) ]− and CrCl (thf) (i.e., [(CH CN) CrCl ]+ [B(C F ) ]− )
between
[(CH
CN)
Ag]
3 source
4
5 4 3. Although [(CH
3
3
45)4]− was
2
6 5 4
33
6F
pending on the
of CrCl63(thf)
CN)4CrCl2]+[B(C
isolated
+
−
differed
depending
on 3the
of CrCl
3 (thf)3 . Although
3 CN)4 CrCl
2 ] [B(C6 F5 )4 ]
by precipitation
in CH
CNsource
at −30 °C,
the precipitate
was not[(CH
composed
of good-quality
◦
was
isolated
byX-ray
precipitation
in CH3could
CN atnot
−30be C,
thefor
precipitate
was notWe
composed
crystals;
thus,
crystallography
used
characterization.
cross- of
good-quality
crystals; thus,
X-ray
crystallography
could
not
be used
4CrCl
2]+[B(C
6F5)4]–for
checked the tentatively
assigned
structure
of [(CH3CN)
by characterization.
counting the
+
We
crosschecked
tentatively
assigned
structure
[(CH
CrCl
F5 )4 ]− by
number
of CH3CNthe
molecules
per each
Cr atom
throughofthe
analysis
integration
3 CN)4of
2 ] [B(C6values
1
in the H-NMR
spectraofrecorded
THF-d8per
with
9-methylanthracene
as the
an internal
counting
the number
CH3 CNusing
molecules
each
Cr atom through
analysis of
3(thf)3 prepared
standard. For
the product
obtainedspectra
using CrCl
via the
evacuation
step,
the
integration
values
in the 1 H-NMR
recorded
using THF-d
with
9-methylanthracene
8
number
of CH3standard.
CN per Cr was
good
agreement
with the
expected
4 (4.1, 3.8,via
or the
as
an internal
For in
the
product
obtained
using
CrCl3value
(thf)3ofprepared
4.0). In contrast,
fromper
4 (6.5,
6.1, 5.3,
or 6.2)agreement
when either
the the
commerevacuation
step, the
thenumber
numberdeviated
of CH3 CN
Cr was
in good
with
expected
3(thf)
3 or
cial source
of CrCl
one prepared
without
the evacuation
step6.1,
was5.3,
used.
value
of 4 (4.1,
3.8, or
4.0).
Inthe
contrast,
the number
deviated
from 4 (6.5,
or 6.2) when
The
most
striking
difference
between
the
two
samples
was
that
good-quality
either the commercial source of CrCl3 (thf)3 or the one prepared without the crystals
evacuation
werewas
deposited
step
used. in a CH2Cl2 solution of CrCl3(thf)3 that was prepared via the evacuation
step,The
whereas
an irregular
solid was
deposited
when
eitherwas
the that
commercial
sourcecrystals
of
most striking
difference
between
the two
samples
good-quality
3(thf)3 or the one prepared without the evacuation step was dissolved in CH2Cl2 (FigCrCl
were deposited in a CH2 Cl2 solution of CrCl3 (thf)3 that was prepared via the evacuation
ure 1).
X-ray crystallography
studies
that
the deposited
were source
a Cl- of
step,
whereas
an irregular solid
was revealed
deposited
when
either the crystals
commercial
bridged dinuclear complex [CrCl2(μ-Cl)(thf)2]2 (Figure 2). The Cr atom adopted an octaCrCl3 (thf)3 or the one prepared without the evacuation step was dissolved in CH2 Cl2
hedral structure, with two THFs being in the cis-position. The bond distances between Cr
(Figure 1). X-ray crystallography studies revealed that the deposited crystals were a
and terminal-Cl (2.277 and 2.289 Å) were shorter than that between Cr and bridge-Cl
Cl-bridged dinuclear complex [CrCl2 (µ-Cl)(thf)2 ]2 (Figure 2). The Cr atom adopted an
(2.362 Å) as well as the Cr–Cl distances (3.299, 3.318, and 3.352 Å) observed for meroctahedral structure, with two THFs being in the cis-position. The bond distances between
CrCl3(thf)3, whose structure has been previously revealed using a single crystal obtained
Cr and terminal-Cl (2.277 and 2.289 Å) were shorter than that between Cr and bridge-Cl
(2.362 Å) as well as the Cr–Cl distances (3.299, 3.318, and 3.352 Å) observed for merCrCl3 (thf)3 , whose structure has been previously revealed using a single crystal obtained
from the reaction pot of Cr(CH2 SiMe3 )4 and HCl in THF (CCDC# 983659) [20]. The distance
between Cr and OTHF situated trans to terminal-Cl was greater than that between Cr and
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from the reaction pot of Cr(CH2SiMe3)4 and HCl in THF (CCDC# 983659) [20]. The distance
THF situated
2SiMe
3)4terminal-Cl
from theCr
reaction
of Cr(CHtrans
and HCl in THF
(CCDC#than
983659)
The distance
between
and Opot
to
was greater
that [20].
between
Cr and
OTHF
situated trans
to bridge-Cl (2.045 vs. 2.018 Å).THF
The OTHF atom situated trans to
THF situated
Cr and
OTHF
situated trans
terminal-Cl
wasOgreater
than
that between
and
Obetween
trans
to bridge-Cl
(2.045tovs.
2.018 Å). The
atom
situated
trans to Cr
termiterminal-Cl
adopted perfect trigonal planar geometry (sum
of bond angle around O1 atom,
OTHF◦ situated
to bridge-Cl
(2.045 vs.
2.018 Å).(sum
The O
atom
situated
trans
to
terminal-Cl
adoptedtrans
perfect
trigonal planar
geometry
ofTHF
bond
angle
around
O1
atom,
2 -hybridization. The other
360.0
), adopted
indicatingπ-donation
π-donation
from
O togeometry
Cr
through
adopting
spangle
nal-Cl
trigonal
planar
of bond
around The
O1 other
atom,
360.0°),
indicatingperfect
from
O to Cr
through(sum
adopting
sp2-hybridization.
THF
O
atom
situated
transtotobridge-Cl
bridge-Cl
deviated
from
a perfect
trigonal
planar
2-hybridization.
THF atom
indicating
π-donation
from O to
Cr
through
adopting
sptrigonal
Thegeometry
other
O360.0°),
situated
trans
deviated
from
a
perfect
planar
geometry
◦ ).
(sum
bond
anglearound
around
O1
atom,352.8°).
352.8
OTHF of
atom
situated
trans toO1
bridge-Cl
deviated
from a perfect trigonal planar geometry
(sum
of
bond
angle
atom,
(sum of bond angle around O1 atom, 352.8°).
2(μ-Cl)(thf)2]2 (a) and irregular solid known as
Figure 1.
1. Macroscopic
views
of crystals
of [CrCl
Figure
Macroscopic
views
of crystals
of [CrCl
2 (µ-Cl)(thf)2 ]2 (a) and irregular solid known as
3(thf)
3Macroscopic
CrCl
(b).
2(μ-Cl)(thf)2]2 (a) and irregular solid known as
Figure
1.
views
of
crystals
of
[CrCl
CrCl3 (thf)3 (b).
CrCl3(thf)3 (b).
Figure 2. X-ray structure of [CrCl2(μ-Cl)(thf)2]2. Selected bond distances (Å) and angles (°): Cr-Cl1,
2.2773(12);
Cr-Cl2,
2.3622(12);
Cr-Cl3,
2.2885(11);
Cr-O1, 2.045(3);
Cr-O2, 2.018(3);
C-O1-C,
2(μ-Cl)(thf)
2]2. Selected
Figure 2. X-ray
structure
of [CrCl
bond distances
(Å) and angles
(°):109.5(3);
Cr-Cl1,
Figure 2. X-ray structure of [CrCl2 (µ-Cl)(thf)2 ]2 . Selected bond distances (Å) and angles (◦ ): Cr-Cl1,
C-O1-Cr,
125.5(2);
125.0(2);
108.7(3);
C-O2-Cr,
C-O2-Cr,
122.6(2).
2.2773(12);
Cr-Cl2,C-O1-Cr,
2.3622(12);
Cr-Cl3,C-O2-C,
2.2885(11);
Cr-O1,
2.045(3);121.5(2);
Cr-O2, and
2.018(3);
C-O1-C,
109.5(3);
2.2773(12);
Cr-Cl2, C-O1-Cr,
2.3622(12);
Cr-Cl3,
2.2885(11);
Cr-O1,
2.045(3);
Cr-O2,
C-O1-C,
C-O1-Cr, 125.5(2);
125.0(2);
C-O2-C,
108.7(3);
C-O2-Cr,
121.5(2);
and2.018(3);
C-O2-Cr,
122.6(2).109.5(3);
C-O1-Cr,
125.5(2);
C-O1-Cr, 125.0(2);
108.7(3);
C-O2-Cr,
121.5(2);
andThe
C-O2-Cr,
122.6(2).
X-ray
crystallography
data doC-O2-C,
not guarantee
the
purity of
the bulk.
EA data
of C
X-ray crystallography
data the
do not
guarantee
the purity
the bulk.
The EA[CrCl
data 2of
were roughly
in agreement with
values
calculated
for the of
revealed
structure
(μ-C
crystallography
do
guarantee
thefor
purity
of thewithin
bulk.
The
EA
data
of C
2(μwereX-ray
roughly
in agreement
with
the not
values
calculated
thealways
revealed
structure
[CrCl
2]2 (31.8%),
Cl)(thf)
althoughdata
the measured
values
were
not
the required
were
roughly
in
agreement
with
the
values
calculated
for
the
revealed
structure
[CrCl
Cl)(thf)
]2 (31.8%),
although
the measured
values30.5%,
were 30.2%,
not always
range
of231.8%
± 0.4%
(30.7%, 30.9%,
30.4%, 31.0%,
30.4%,within
30.9%,the
andrequired
31.2%). 2 (µCl)(thf)
]
(31.8%),
although
the
measured
values
were
not
always
within
the
required
2 231.8%
range
of
0.4%
(30.7%,
30.9%, 30.4%,
31.0%,the
30.5%,
30.2%, 30.4%,
and
31.2%).
3(thf)
3 or
In
contrast,
the ±EA
data
of C measured
for either
commercial
source30.9%,
of CrCl
range
of prepared
31.8%
0.4%
(30.7%,
30.9%, 30.4%,
31.0%,
30.2%,source
30.4%,
andcal3(thf)
331.2%).
In contrast,
the±EA
data
ofthe
C measured
for
either
the30.5%,
commercial
CrCl
or
the
one
without
evacuation
step
deviated
substantially
fromof30.9%,
the
value
In
contrast,
the
EA
data
of Cthe
measured
forfluctuated
either
the commercial
of CrCl
3 (thf)cal3 or the
the
one for
prepared
without
evacuation
step
deviated
substantially
from
the
value
3(thf)
3 (38.5%).
culated
CrCl
The data
severely
by source
variation
in sampling
one
prepared
without
evacuation
step
deviated
from the
value
calculated
3(thf)34.1%,
3the
culated
for CrCl
(38.5%).
Theand
data
fluctuated
severely
by variation
in sampling
points
(32.4%,
26.5%,
38.1%,
33.8%
for a substantially
bottle
of purchased
sample;
34.2%,
for
CrCl37.1%,
(38.5%).
The
data
fluctuated
severely
variation
in sampling
points
(32.4%,
points
(32.4%,
26.5%,and
34.1%,
38.1%,
and 33.8%
for aofby
bottle
of purchased
sample;
34.2%,
36.7%,
34.1%
for
another
bottle
purchased
sample;
31.3%,
30.5%,
3 (thf)3 35.3%,
26.5%,
38.1%,
andfor
33.8%
for another
a bottle
of purchased
34.2%,
36.7%,
37.1%,
36.7%,34.1%,
37.1%, and
35.3%,
and
34.1%
for
bottle
of purchased
sample;
31.3%,
30.5%,
37.0%,
33.6%,
33.9%
the sample
prepared
without
thesample;
evacuation
step).
Before
37.0%,and
33.6%,
andfor
33.9%
for the
sample
preparedsample;
without31.3%,
the evacuation
step). 33.6%,
Before and
35.3%,
34.1%
another
bottle
of purchased
30.5%, 37.0%,
33.9% for the sample prepared without the evacuation step). Before recrystallization, the
sample prepared via the evacuation step also showed fluctuating C content by varying
the sampling point, and the C content agreed neither with the structure of CrCl3 (thf)3 nor
Molecules
2021,
26, 1167
Molecules 2021,
26, x FOR
PEER
REVIEW
5 of 11
5 of 11
recrystallization,
the sample
prepared
via the evacuation step also showed fluctuating C
the structure
of [CrCl
2 (µ-Cl)(thf)2 ]2 (35.3%, 34.3%, 32.5%, 34.2%, and 32.4%). Though the
content by varying
the
sampling
point,
and
the C content
agreed neither
with
structure
carbon contents and the XRD pattern
are inconsistent
with
thethe
formula
of the commercial
of CrCl3(thf)3 nor the structure of [CrCl2(μ-Cl)(thf)2]2 (35.3%, 34.3%, 32.5%, 34.2%, and
source CrCl3 (THF)3 , the Cr content measured by ICP-OES (Inductively Coupled Plasma
32.4%). Though the carbon contents and the XRD pattern are inconsistent with the formula
Optical Emission Spectroscopy) and the Cl content measured by gravimetric analysis after
of the commercial source CrCl3(THF)3, the Cr content measured by ICP-OES (Inductively
treatment of AgNO3 to aqueous solution of CrCl3 (THF)3 were not severely deviated from
Coupled Plasma Optical Emission
Spectroscopy) and the Cl content measured by gravithe values calculated for the formula CrCl3 (THF)3 (Calcd. Cr 13.88, Cl 28.38. Found: Cr
metric analysis after treatment of AgNO3 to aqueous solution
of CrCl3(THF)3 were not
14.04, Cl 29.09%). The Cr content measured for the prepared [CrCl (µ-Cl)(thf)2 ]2 agreed
severely deviated from the values calculated for the formula CrCl3(THF)3 (Calcd. Cr213.88,
accurately
with
calculated
while
the measured
Cl content
was
slightly deviated
2(μCl 28.38. Found:
Cr 14.04,
Cl the
29.09%).
The Crvalue
content
measured
for the prepared
[CrCl
(Calcd.
Cr
17.19,
Cl
35.15.
Found:
Cr
17.18,
Cl
34.58%).
Cl)(thf)2]2 agreed accurately with the calculated value while the measured Cl content was
X-ray
(XRD)
pattern
observed for bulk [CrCl2 (µ-Cl)(thf)2 ]2
slightly deviatedThe
(Calcd.
Cr powder
17.19, Cl diffraction
35.15. Found:
Cr 17.18,
Cl 34.58%).
isolated
via
the
recrystallization
procedure
agreed
well[CrCl
with2(μ-Cl)(thf)
single-crystal
X-ray crystal2] 2
The X-ray powder diffraction (XRD) pattern observed for bulk
lography
data
(cif
file)
(Figure
3a
vs.
Figure
3b),
indicating
that
the
majority
of deposited
isolated via the recrystallization procedure agreed well with single-crystal X-ray crystalsolid
structure
contrast,
the XRD
pattern of the purchased
2 (µ-Cl)(thf)
2 ]2 . In
lography data
(cifhad
file)a (Figure
3a of
vs.[CrCl
Figure
3b), indicating
that
the majority
of deposited
or the2(μ-Cl)(thf)
one prepared
the reported
method
the evacuation step did
2]2. Inby
solid had a CrCl
structure
of3 [CrCl
contrast,
the XRD
patternwithout
of the purchased
3 (thf)
notthe
agree
single-crystal
X-raymethod
crystallography
data
of mer-CrCl
(thf)3 (Figure 3c vs.
CrCl3(thf)3 or
one with
prepared
by the reported
without the
evacuation
step3 did
Figure
3e,f). ThisX-ray
observation,
along with
themer-CrCl
mismatched
data,
uncertainty
3(thf)3 EA
not agree with
single-crystal
crystallography
data of
(Figure
3ccreates
vs.
structurealong
and composition
of the purchased
(thf)
as
well
as
of
the one preFigure 3e,f).about
This the
observation,
with the mismatched
EA data,CrCl
creates
uncertainty
3
3
about the structure
composition
of theThe
purchased
CrCl3(thf)
as well
as ofpot
theofone
pared byand
the reported
method.
solid deposited
in3 the
reaction
CrCl2 (OH)(H2 O)2
prepared by
solid
deposited
in ◦the
reaction
potsignals
of
andthe
Me3reported
SiCl (6 eq)method.
showed The
a large
signal
at 2θ = 11.6
along
with the
observed for
2O)
2 and(thf)
3SiCl
CrCl2(OH)(H
Me
(6
eq)
showed
a
large
signal
at
2θ
=
11.6°
along
with
the
the
CrCl
samples
(Figure
3d).
The
electron
paramagnetic
resonance
(EPR)
spectrum
3
3
3
3
signals observed
for
the
CrCl
(thf)
samples
(Figure
3d).
The
electron
paramagnetic
resoof [CrCl2 (µ-Cl)(thf)2 ]2 showed a strong signal at g = 1.98, while relatively weak signals
2 showed
nance (EPR)were
spectrum
of [CrCl
a strong signal at g = 1.98, while relobserved
for2(μ-Cl)(thf)
the CrCl32](thf)
3 samples; in addition, the signal patterns were com3(thf)3 samples; in addition, the signal patatively weak
signals
were
observed
for
the
CrCl
pletely different for the two samples (Figure 4). In thermogravimetry analysis (TGA) of
terns were completely
thestages
two samples
(Figure loss
4). Inwere
thermogravimetry
analCrCl3 (thf)3different
, roughlyfor
four
of the weight
observed (30%,
50%, 60%, and 84%
3(thf)3, roughly
ysis (TGA) of
CrCl
four
stages
of
the
weight
loss
were
observed
(30%,
50%,
◦
◦
◦
◦
at 170 C, 215 C, 300 C, and 1000 C, respectively), while three stages
of the weight loss
60%, and 84%
at roughly
170 °C, 215
°C, 300for
°C,[CrCl
and 1000
°C, respectively),
whileand
three
stages
of ◦ C, 310 ◦ C, and
were
observed
(µ-Cl)(thf)
]
(35%,
53%,
73%
at
220
2
2 2
the weight loss were
roughly observed for [CrCl2(μ-Cl)(thf)2]2 (35%, 53%, and 73% at 220
1000 ◦ C, respectively).
°C, 310 °C, and 1000 °C, respectively).
Figure 3. X-ray
powder
diffraction
(XRD)
patterns(XRD)
of [CrCl
2(μ-Cl)(thf)2]2 calculated from cif file (a)
Figure
3. X-ray
powder
diffraction
patterns
of [CrCl2 (µ-Cl)(thf)2 ]2 calculated from cif file (a)
and measured (b) and those of CrCl3(thf)3 calculated from cif file (c), obtained from the reaction
and measured (b) and those of CrCl3 (thf)3 calculated from cif file (c), obtained from the reaction
‘CrCl2(OH)(H2O)2 + Me3SiCl (6 eq)’ (d), obtained from the reaction ‘CrCl3·6H2O + Me3SiCl (60 eq)’
‘CrCl
2 O)2 + Me3 SiCl (6 eq)’ (d), obtained from the reaction ‘CrCl3 ·6H2 O + Me3 SiCl (60 eq)’
(e), and purchased2 (OH)(H
(f).
(e), and purchased (f).
Molecules 2021, 26, 1167
Molecules
Molecules2021,
2021,26,
26,xxFOR
FORPEER
PEERREVIEW
REVIEW
6 of 11
66ofof1111
2(μ-Cl)(thf)2]2 and the commercial CrCl3(thf)3.
Figure
2(μ-Cl)(thf)
2]22 ]
3(thf)
3.
Figure4.
EPRspectra
spectraofof
[CrCl
and
thethe
commercial
CrCl
Figure
4.4.EPR
EPR
spectra
of[CrCl
[CrCl
commercial
CrCl
2 (µ-Cl)(thf)
2 and
3 (thf)
3.
3CN)CN)
4CrClCrCl
2]+[B(C]6+F[B(C
5)4]– have
Many
single
crystals
ofof[(CH
unMany
attempts
togrow
grow
single
crystals
of
[(CH
F5 )4 been
]−
have
3CN)
Manyattempts
attemptstoto
grow
single
crystals
[(CH
been
un-been
3 4CrCl
4 2]+[B(C
2 6F5)4]–6have
+
–
+
3CN)4Ag] [B(C
6F5)4] –was −
3(thf)3 or
successful
when
[(CH
reacted
with
the
purchased
CrCl
+
unsuccessful
when
[(CH
was reacted
with
the purchased
CrCl
(thf)3
3CN)
4Ag]
6F5)46
3 3
successful when
[(CH
[B(C[B(C
] Fwas
with the
purchased
CrCl3(thf)
or
3 CN)
4 Ag]
5 )4 ] reacted
3·6H2O and excess Me3SiCl (60 eq) without
the
one
prepared
through
the
reaction
ofofCrCl
3·6H
2O 2
the
one
prepared
through
the
reaction
CrCl
and
excess
Me3Me
SiCl
(60 eq)
or
the
one
prepared
through
the
reaction
of
CrCl
O and
excess
(60 without
eq) without
3 ·6H
3 SiCl
the
single
crystals
suitable
for
X-ray
crystallogtheevacuation
evacuationstep
step(Scheme
[38].
However,
single
crystals
suitable
forfor
X-ray
crystallogthe
evacuation
step
(Scheme1)1)
1)[38].
[38].However,
However,
single
crystals
suitable
X-ray
crystallogra2(μ-Cl)(thf)2]2
raphy
could
be
obtained
when
[CrCl
was
reacted
with
+ [B(C
−
2(μ-Cl)(thf)
2]2 [(CH
raphy
be when
obtained
when 2[CrCl
was3 CN)
reacted
with
phy
could could
be obtained
[CrCl2 (µ-Cl)(thf)
]2 was
reacted with
Ag]
4
6 F5 )4 ] .
3CN)4Ag]+[B(C
6F5)4]−.− The structure determined by X-ray crystallography is shown in
[(CH
+
3CN)4Ag]determined
[(CH
[B(C6F5)4] . The
determined by X-ray
crystallography
is shown
in
The
structure
bystructure
X-ray crystallography
is shown
in Figure 5.
By reacting
3CN)
4CrCl2]+[B(C
6F5)4]− − with a iPrN{P(C6H4-pFigure
reacting
well-defined
[(CH
+[B(C
+ [B(C
− with
3CN)
4CrCl
2]iPrN{P(C
6F5)4] Hwith
6H4-p-[37],
Figure 5.5. By
By[(CH
reacting
well-defined
[(CH
a
iPrN{P(C
well-defined
CN)
CrCl
]
F
)
]
a
-p-Si(nBu)
)
ligand
3
4
2
6 5 4
6 4
3 2
4-pSi(nBu)3)32)2 ligand
[37],
aa high-quality
Cr
complex
H
+ [B(C F[iPrN{P(C
− was6produced,
6H4-p[37],
high-quality
Cr
complex
[iPrN{P(C
aSi(nBu)
high-qualityligand
Cr+ complex
[iPrN{P(C
H
-p-Si(nBu)
)
}
CrCl
]
)
]
6
4
3
2
2
2
6
5
4
3)2}2CrCl2] [B(C
6F5)4]– –was produced, which showed extremely high activity in the
Si(nBu)
+
3)2}2CrCl2] [B(C6F5)4] was produced, which showed extremely high activity in the
Si(nBu)showed
which
extremely high activity in the ethylene tetramerization reaction (activethylene
tetramerization reaction
6600
selectivity:
ethylene
reaction(activity:
(activity:
6600kg/g-Cr/h;
kg/g-Cr/h;
selectivity:1-octene—74.7%,
1-octene—74.7%,
ity: 6600 tetramerization
kg/g-Cr/h; selectivity:
1-octene—74.7%,
1-hexene—7.5%,
cyclic-C6—3.8%,
1-hexene—7.5%,
cyclic-C6—3.8%,
C10+—13.8%,
PE—0.2%;
reaction
conditions:
34
1-hexene—7.5%,
cyclic-C6—3.8%,
C10+—13.8%,
PE—0.2%;
reaction
conditions:
34 bar
bar
C10+—13.8%,
PE—0.2%;
reaction
conditions:
34
bar
ethylene,
235
mL
methylcyclohexane,
3Al, 40 °C, 30
ethylene,
235
mL
methylcyclohexane,
0.45
mmol
Cr
catalyst,
90
mmol
(iBu)
ethylene, 235 mL methylcyclohexane, 0.45 mmol
Cr catalyst, 90 mmol (iBu)3Al, 40 °C, 30
◦
0.45
mmol Cr
Cr catalyst, 90
mmol (iBu)
min).
The Cr complex [iPrN{P(C6 H4 -p3 Al, 40 3C,
6H4-p-Si(nBu)
2]+[B(C6F5)4]− is highly soluble in
min).
[iPrN{P(C
)2}230
CrCl
4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− is highly soluble in
min).The
The Crcomplex
complex
+ [B(C[iPrN{P(C
− is6H
Si(nBu)
)
}
CrCl
]
F
)
]
highly
soluble
in
methylcyclohexane,
which is advan3 2 2
2
6 5 4
methylcyclohexane,
methylcyclohexane,which
whichisisadvantageous
advantageousininhandling
handlingand
andoperating
operatingthe
thetetramerization
tetramerization
tageous
in
handling
and
operating
the
tetramerization
reaction,
and
there
is no way for
reaction,
precursor
reaction,and
andthere
thereisisno
noway
wayfor
forpurification,
purification,necessitating
necessitatingaahigh-quality
high-qualityCr
Cr
precursor−
+
purification,
necessitating
Cr precursor
CrCl
)4 ] as
3 CN)64H
2 ] [B(C
4CrCl
2]+[B(C6F5)4]− a
4-p-Si(nBu)
3)62F
[(CH
ashigh-quality
well as high-quality
ligand[(CH
iPrN{P(C
}25 to
3CN)4CrCl2]+[B(C6F5)4]− as well as high-quality ligand iPrN{P(C6H4-p-Si(nBu)3)2}2 to
[(CH3CN)
well
as consistently
high-qualityhigh
ligand
iPrN{P(C6 H4 -p-Si(nBu)3 )2 }2 to ensure consistently high activity.
ensure
activity.
ensure consistently high activity.
Figure
4CrCl2]+[B(C6F5)4]−. Selected bond distances (Å) and angles
Figure5.5.X-ray
X-raystructure
structureofof[(CH
[(CH3CN)
3CN)4CrCl2]+[B(C6F5)4]−. Selected bond distances (Å) and angles
+
−
Figure
5. X-ray structure
of [(CH3 CN)
bond
distances (Å) and angles
4 CrCl
2 ] [B(CCr-N2-C,
6 F5 )4 ] . Selected
(°):
2.2804(9);
(°):Cr-N1,
Cr-N1,2.025(3);
2.025(3);Cr-N2,
Cr-N2,2.018(3);
2.018(3);Cr-Cl1,
Cr-Cl1,
2.2804(9);
Cr-N2-C,178.8(3);
178.8(3);and
andCr-N1-C,
Cr-N1-C,175.5(3).
175.5(3).
◦
( ): Cr-N1, 2.025(3); Cr-N2, 2.018(3); Cr-Cl1, 2.2804(9); Cr-N2-C, 178.8(3); and Cr-N1-C, 175.5(3).
Molecules 2021, 26, 1167
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Molecules 2021, 26, x FOR PEER REVIEW
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In previous studies, we encountered a similar problem possibly caused by the use
−the use of
In previous
studies,
we 3encountered
a similar
problem
caused2 )by
of impure
CrCl
(thf)3 . In the
reaction
of Li+possibly
[Et2 Al(N(iPr)
2 ] and CrCl3 (thf)3 , solids
3(thf)3. In the reaction of Li+[Et2Al(N(iPr)2)2]− and CrCl3(thf)3, solids were isoimpure CrCl
were isolated; however, their structure could not be determined by X-ray crystallography.
lated; however,
their structure
could not considered
be determined
by[Et
X-ray
crystallography. The
The product
was tentatively
to be
2 Al(N(iPr)2 )2 ]CrCl2 [48]. However, sin2Al(N(iPr)2)2]CrCl2 [48]. However, single crysproduct wasgle
tentatively
considered
to
be
[Et
crystals were obtained when well-defined [CrCl2 (µ-Cl)(thf)2 ]2 replaced the impure
tals were obtained
when
well-defined [CrCl2(μ-Cl)(thf)2]2 replaced the impure CrCl3(thf)3,
CrCl3 (thf)
3 , enabling structure determination by X-ray crystallography. The complex isoenabling structure determination by X-ray+ crystallography. The
complex isolated from the
lated from the reaction of Li [Et2 Al(N(iPr)2 )2 ]− and CrCl3 (thf)3 was not a Cr(III) complex
reaction of Li+[Et2Al(N(iPr)2)2]− and CrCl3(thf)3 was not a Cr(III) complex
[Et Al(N(iPr) ) ]CrCl2 but a Cr(II) complex [{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2 (Figure 6).
[Et2Al(N(iPr)2)22]CrCl2 but 2a 2Cr(II) complex
[{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2 (Figure 6).
Figure 6. X-ray structure of [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2. Selected bond distances (Å) and angles
Figure 6. X-ray structure of [{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2 . Selected bond distances (Å) and angles (◦ ): Cr-Cl1, 2.4212;
(°): Cr-Cl1, 2.4212; Cr-N1, 2.141(6); Cr-N2, 2.119(6); Al-N1, 1.925(6); Al-N2, 1.953(6); Al-Cl2,
Cr-N1, 2.141(6);2.163(3);
Cr-N2, Cr--Cr’,
2.119(6);3.578(2);
Al-N1,N1-Cr-N2,
1.925(6); Al-N2,
1.953(6); Al-Cl2, 2.163(3); Cr–Cr’, 3.578(2); N1-Cr-N2, 84.3(2);
84.3(2); Cl-Cr-Cl’, 83.96(8); N1-Al-N2, 95.0(3); and angle beCl-Cr-Cl’, 83.96(8);
N1-Al-N2,
95.0(3);
and
angle
between
N1-Cr-N2
and Cl-Cr-Cl’ planes, 25.1(1).
tween N1-Cr-N2 and Cl-Cr-Cl’ planes, 25.1(1).
3. Materials3.and
Methods
Materials
and Methods
General Remarks
3.1. General 3.1.
Remarks
All manipulations
were performed
an inert atmosphere
using a standard
All manipulations
were in
performed
in an inert atmosphere
usingglove
a standard glove box
box and Schlenk
techniques.
CH2Cl2 and
were
overstirred
CaH2 and
transferred
and Schlenk
techniques.
CHCH
and
CHstirred
over
CaH2 and transferred to
2 Cl32CN
3 CN were
to a reservoir
under vacuum.
Toluene, hexane,
and
THF were
distilled
from
benzophea reservoir
under vacuum.
Toluene,
hexane,
and THF
were
distilled
from benzophenone
none ketyl. ketyl.
EAs were
out at the
Center, Ajou
University.
CW X-band
EAscarried
were carried
outAnalytical
at the Analytical
Center,
Ajou University.
CW X-band EPR
EPR spectra
were collected
at room
temperature
on an on
EMX
spectrometer
spectra
were collected
at room
temperature
an plus
EMX6/1
plus
6/1 spectrometer (Bruker,
(Bruker, Billerica,
MA,MA,
United
States)
with
experimentalparameters
parameters of
of 11 mW
Billerica,
USA)
with
experimental
mWmicrowave
microwave power, 10 G
power, 10 Gmodulation
modulation amplitude,
and
3
scans
at
KBSI
Western
Seoul
Center
Korea.
HP- HP-XRD data
amplitude, and 3 scans at KBSI Western Seoul Center Korea.
XRD data were
obtained
on
a
D/max-2500V/PC
(Rigaku,
Akishima,
Japan)
equipped
with
were obtained on a D/max-2500V/PC (Rigaku, Akishima, Japan) equipped with a Cu Kα
a Cu Kα radiation
source
(λ =(λ
0.15418
nm).nm).
radiation
source
= 0.15418
2 ]2
3.2. [CrCl2(μ-Cl)(thf)
3.2. [CrCl
2 (µ-Cl)(thf)2 ]2
A Schlenk flask
containing
crystalline
solid
CrCl3·6H2O
(10.0CrCl
g, 37.5
mmol) was imA Schlenk flask
containing
crystalline
solid
3 ·6H2 O (10.0 g, 37.5 mmol) was
mersed in an
oil
bath
at
a
temperature
of
40
°C
and
then
evacuated
under
full
vacuum under
for
immersed in an oil bath at a temperature of 40 ◦ C and then evacuated
full vacuum
1 h. Under evacuation, the bath temperature was raised to 100 °C over an hour and◦then
for 1 h. Under evacuation, the bath temperature was raised to 100 C over an hour and
maintained at 100 °C for 4 h. As volatiles
were removed, the crystalline solid changed to
then maintained at 100 ◦ C for 4 h. As volatiles were removed, the crystalline solid changed
an amorphous powder and the color of the solid gradually changed from dark green to
to an amorphous powder and the color of the solid gradually changed from dark green to
light green, light gray, and finally light purple. The weight reduced from 10.0 to 6.39 g by
light green, light gray, and finally light purple. The weight reduced from 10.0 to 6.39 g by
the removal of volatiles. Cold THF (32 g, −30 °C) was added◦ to dissolve the remaining
the removal of volatiles. Cold THF (32 g, −30 C) was added to dissolve the remaining
solid. By dissolution, heat was generated, and a dark-purple solution was obtained. Some
solid. was
By dissolution,
heatthen,
was generated,
and
dark-purple
insoluble portion
filtered off, and
Me3SiCl (24.5
g, a225
mmol) wassolution
added towas
the obtained. Some
insoluble
portion
was
filtered
off,
and
then,
Me
SiCl
(24.5
g,
225
mmol)
was added to the
3 a purple solid. The solid
filtrate. Stirring the solution overnight led to the deposition of
Stirring
the
solution
overnight
thehexane
deposition
of aThe
purple
solid. The solid was
was isolatedfiltrate.
via filtration
and
washed
with
THF (10 led
mL)toand
(10 mL).
isolated
isolated
via filtration
andvial
washed
withsize)
THFand
(10CH
mL)
and hexane (10 mL). The isolated solid
2Cl2 (53 g) was added to
solid (6.60 g)
was placed
in a large
(~70 mL
(6.60 g) was placed in a large vial (~70 mL size) and CH2 Cl2 (53 g) was added to dissolve
the solid. When the vial was placed inside a closed chamber (~250 mL size) containing
methylcyclohexane (~15 mL), CH2 Cl2 was slowly evaporated and purple crystals were
deposited within 24 h, which were isolated by decantation (4.55 g, 69%).
Molecules 2021, 26, 1167
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3.3. [(CH3 CN)4 CrCl2 ]+ [B(C6 F5 )4 ]−
A solution of [(CH3 CN)4 Ag]+ [B(C6 F5 )4 ]− (7.789 g, 8.189 mmol) in acetonitrile (17.3 g)
was added to a suspension of [CrCl2 (µ-Cl)(thf)2 ]2 (2.478 g, 8.189 mmol) in acetonitrile
(22.6 g). After stirring overnight at 60 ◦ C, precipitated AgCl was removed by filtration and an olive-green solution was obtained. The solvent was removed using a vacuum line to obtain an olive-green solid (7.71 g). The isolated solid (20.0 mg) and 9methylanthracene (20.0 mg) were dissolved in THF-d8 , and the 1 H NMR spectrum of
the solution was recorded to count the number of CH3 CN molecules per each Cr atom,
which was 4.1. The yield was 97% based on the formula [(CH3 CN)4.1 CrCl2 ]+ [B(C6 F5 )4 ]− .
Single crystals suitable for X-ray crystallography were grown in CH2 Cl2 ; a vial containing [(CH3 CN)4 CrCl2 ]+ [B(C6 F5 )4 ]− (100 mg) in CH2 Cl2 (1.0 g) was placed inside a closed
chamber containing methylcyclohexane to deposit crystals.
3.4. [iPrN{P(C6 H4 -p-Si(nBu)3 )2 }2 CrCl2 ]+ [B(C6 F5 )4 ]−
A solution of iPrN{P(C6 H4 -p-Si(nBu)3 )2 }2 (9.50 g, 7.78 mmol) in CH2 Cl2 (120 g) was
added dropwise to a solution of [(CH3 CN)4.1 CrCl2 ]+ [B(C6 F5 )4 ]− (7.55 g, 7.781 mmol) in
CH2 Cl2 (46 g). Upon addition, the color of the solution changed immediately from olivegreen to bluish-green. After stirring for 2.5 h, the solvent was completely removed using
a vacuum line. The residue was dissolved in a minimal amount of methylcyclohexane
(~5 mL), and the solvent was removed using a vacuum line. This procedure was repeated
once more to remove any residual CH3 CN and CH2 Cl2 completely. The residue (15.7 g,
100%) was dissolved in methylcyclohexane (141.3 g) to obtain a 10 wt% solution, which
was used for ethylene tetramerization.
3.5. [{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2
[CrCl2 (µ-Cl)(thf)2 ]2 (0.312 g, 1.03 mmol) was added to a solution of Li+ [Et2 Al(N(iPr)2 )2 ]−
(0.302 g, 1.03 mmol) in toluene (4.5 mL) at −30 ◦ C. The mixture was stirred for 8 h at room
temperature. The color of the solution changed from dark brown to deep greenish-blue,
and LiCl precipitated. The solvent was removed using a vacuum line, and the residue
was dissolved in hexane (30 mL). After removal of LiCl via filtration, the filtrate was
concentrated to ~10 mL and allowed to stand at −30 ◦ C for 2 days. Blue crystals were
deposited (0.178 g, 45%). A second crop of blue crystals was obtained from the mother
liquor (0.028 g, 7%).
3.6. X-ray Crystallography
Specimens of suitable quality and size were selected, mounted, and centered in the
X-ray beam using a video camera. Reflection data were collected at 100 K on an APEX
II CCD area diffractometer (Bruker, Billerica, MA, USA) using graphite-monochromated
Mo K–α radiation (λ = 0.7107 Å). The hemisphere of the reflection data was collected as
ϕ and ω scan frames at 0.5◦ per frame and an exposure time of 10 s per frame. The cell
parameters were determined and refined using the SMART program. Data reduction was
performed using the SAINT software. The data were corrected for Lorentz and polarization
effects. An empirical absorption correction was applied using the SADABS program. The
structure was solved by direct methods and refined by the full matrix least-squares method
using the SHELXTL package and olex2 program with anisotropic thermal parameters for
all non-hydrogen atoms.
Crystallographic data for [CrCl2 (µ-Cl)(thf)2 ]2 (CCDC# 2044805) that were used in all
calculations are as follows: C8 H15.5 Cl3 CrO2 , M = 302.05, triclinic, a = 6.8070(8), b = 9.3170(11),
c = 10.3192(13) Å, α = 108.070(5)◦ , β = 96.448(5)◦ , γ = 94.719(5), V = 613.47(13) Å3 , space
group P-1, Z = 2, and 2285 unique (R(int) = 0.0505). The final wR2 was 0.1251 (I > 2σ(I)).
Crystallographic data for [(CH3 CN)4 CrCl2 ]+ [B(C6 F5 )4 ]− ·2(CH2 Cl2 ) (CCDC# 2044806)
that were used in all calculations are as follows: C34.5 H17 BCl7 CrF20 N4 , M = 1178.48, monoclinic, a = 12.9472(4), b = 8.2694(3), c = 21.7555(8) Å, β = 99.8523(18)◦ , V = 2294.91(14)
Molecules 2021, 26, 1167
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Å3 , space group P2/c, Z = 2, and 4409 unique (R(int) = 0.0285). The final wR2 was 0.1594
(I > 2σ(I)).
Crystallographic data for [{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2 (CCDC# 2044807) that were
used in all calculations are as follows: C28 H66 Al2 Cl4 Cr2 N4 , M = 758.60, monoclinic,
a = 10.2937(2), b = 12.8707(3), c = 14.7815(3) Å, β = 96.5009(18)◦ , V = 1945.77(7) Å3 , space
group P21 /n, Z = 2, and 3675 unique (R(int) = 0. 1527). The final wR2 was 0.0909 (I > 2σ(I)).
4. Conclusions
The commonly used CrCl3 (thf)3 is impure obtaining as an irregular solid with no
possibility of recrystallization. Another form of the THF adduct of CrCl3 (i.e., [CrCl2 (µCl)(thf)2 ]2 ) was facilely prepared in this work. The structure of [CrCl2 (µ-Cl)(thf)2 ]2 determined by X-ray crystallography agreed well with the XRD patterns and the C content
of the bulk. Using well-defined [CrCl2 (µ-Cl)(thf)2 ]2 instead of the impure CrCl3 (thf)3 as
a starting material, the activity of the ethylene tetramerization catalyst [iPrN{P(C6 H4 -pSi(nBu)3 )2 }2 CrCl2 ]+ [B(C6 F5 )4 ]− was consistent and extremely high (6600 kg/g-Cr/h), and
some Cr complexes (e.g., [(CH3 CN)4 CrCl2 ]+ [B(C6 F5 )4 ]− and [{Et(Cl)Al(N(iPr)2 )2 }Cr(µ-Cl)]2 )
whose structure determination had failed previously could be isolated as high-quality crystals, enabling structure determination by X-ray crystallography. We strongly suggest the
use of well-defined [CrCl2 (µ-Cl)(thf)2 ]2 instead of the impure CrCl3 (thf)3 as a Cr source in
the synthesis of organometallic and coordination compounds of Cr.
5. Patents
A patent was applied on this study (Ajou University, Chromium Compound and
Method for Preparing the Same. Kr 10-2020-0052494, 29 April 2020).
Author Contributions: Investigation, D.G.L., J.W.B., J.H.L., H.J.L. and Y.H.S.; Formal analysis, J.L.;
Validation, C.G.L.; Conceptualization, funding acquisition, supervision and writing, B.Y.L. All authors
have read and agreed to the published version of the manuscript.
Funding: C1 Gas Refinery Program (2019M3D3A1A01069100); Priority Research Centers Program
(2019R1A6A1A11051471).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Conflicts of Interest: Ajou University (inventors: D. G. Lee, J. W. Baek, and B. Y. Lee) have applied
for a patent covering this research.
Sample Availability: Samples of [CrCl2 (µ-Cl)(thf)2 ]2 are available from the Precious Catalysts Inc.;
cglee@s-pci.com (C.G.L.).
References
1.
2.
3.
4.
5.
6.
Ambrose, K.; Murphy, J.N.; Kozak, C.M. Chromium Diamino-bis(phenolate) Complexes as Catalysts for the Ring-Opening
Copolymerization of Cyclohexene Oxide and Carbon Dioxide. Inorg. Chem. 2020, 59, 15375–15383. [CrossRef] [PubMed]
Jiménez, J.-R.; Poncet, M.; Doistau, B.; Besnard, C.; Piguet, C. Luminescent polypyridyl heteroleptic CrIII complexes with high
quantum yields and long excited state lifetimes. Dalton Trans. 2020, 49, 13528–13532. [CrossRef]
Baek, J.W.; Hyun, Y.B.; Lee, H.J.; Lee, J.C.; Bae, S.M.; Seo, Y.H.; Lee, D.G.; Lee, B.Y. Selective Trimerization of α-Olefins with
Immobilized Chromium Catalyst for Lubricant Base Oils. Catalysts 2020, 10, 990. [CrossRef]
Song, T.; Tao, X.; Tong, X.; Liu, N.; Gao, W.; Mu, X.; Mu, Y. Synthesis and characterization of chromium complexes 2Me4 CpC6 H4 CH2 (R)NHCrCl2 and their catalytic properties in ethylene homo- and co-polymerization. Dalton Trans. 2019,
48, 4912–4920. [CrossRef] [PubMed]
Sydora, O.L.; Hart, R.T.; Eckert, N.A.; Martinez Baez, E.; Clark, A.E.; Benmore, C.J. A homoleptic chromium(iii) carboxylate.
Dalton Trans. 2018, 47, 4790–4793. [CrossRef]
Olafsen, B.E.; Crescenzo, G.V.; Moisey, L.P.; Patrick, B.O.; Smith, K.M. Photolytic Reactivity of Organometallic Chromium
Bipyridine Complexes. Inorg. Chem. 2018, 57, 9611–9621. [CrossRef]
Molecules 2021, 26, 1167
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
10 of 11
Ojelere, O.; Graf, D.; Mathur, S. Molecularly Engineered Lithium–Chromium Alkoxide for Selective Synthesis of LiCrO2 and
Li2 CrO4 Nanomaterials. Inorganics 2019, 7, 22. [CrossRef]
Förster, C.; Dorn, M.; Reuter, T.; Otto, S.; Davarci, G.; Reich, T.; Carrella, L.; Rentschler, E.; Heinze, K. Ddpd as Expanded
Terpyridine: Dramatic Effects of Symmetry and Electronic Properties in First Row Transition Metal Complexes. Inorganics 2018, 6,
86. [CrossRef]
Zhao, R.; Ma, J.; Zhang, H.; Huang, J. Synthesis and Characterization of Constrained Geometry Oxygen and Sulphur Functionalized Cyclopentadienylchromium Complexes and Their Use in Catalysis for Olefin Polymerization. Molecules 2017, 22, 856.
[CrossRef]
Pei, L.; Tang, Y.; Gao, H. Homo- and Copolymerization of Ethylene and Norbornene with Anilido–Imine Chromium Catalysts.
Polymers 2016, 8, 69. [CrossRef]
Herwig, W.; Zeiss, H. Notes: Chromium Trichloride Tetrahydrofuranate. J. Org. Chem. 1958, 23, 1404. [CrossRef]
Mastalir, M.; Glatz, M.; Stöger, B.; Weil, M.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Synthesis, characterization and reactivity of
vanadium, chromium, and manganese PNP pincer complexes. Inorg. Chim. Acta 2017, 455, 707–714. [CrossRef]
Köhn, R.D.; Coxon, A.G.N.; Hawkins, C.R.; Smith, D.; Mihan, S.; Schuhen, K.; Schiendorfer, M.; Kociok-Köhn, G. Selective
trimerisation and polymerisation of ethylene: Halogenated chromium triazacyclohexane complexes as probes for an internal
‘halogen effect’. Polyhedron 2014, 84, 3–13. [CrossRef]
Heisig, G.B.; Hedin, H. Anhydrous Chromium(III) Chloride. Inorg. Synth. 1946, 2, 193–196. [CrossRef]
Vavoulis, A.; Austin, T.E.; Tyree, S.Y., Jr. Anhydrous Chromium(III) Chloride. Inorg. Synth. 1960, 6, 129–132. [CrossRef]
Shamir, J. New synthesis of chromium trichloride tetrahydrofuranate. Inorg. Chim. Acta 1989, 156, 163–164. [CrossRef]
Pray, A.R. Anhydrous metal chlorides. Inorg. Synth. 1957, 5, 153–156.
House, D.A.; Wang, J.; Nieuwenhuyzen, M. The synthesis and X-ray structure of trans-[CrCl2 (nPrNH2 )4 ]BF4 ·H2 O and the
thermal and Hg2+ -assisted chloride release kinetics from some trans-[CrCl2 (N)4 ]+ complexes. Inorg. Chim. Acta 1995, 237, 37–46.
[CrossRef]
So, J.H.; Boudjouk, P. A convenient synthesis of solvated and unsolvated anhydrous metal chlorides via dehydration of metal
chloride hydrates with trimethylchlorosilane. Inorg. Chem. 1990, 29, 1592–1593. [CrossRef]
Jeon, J.Y.; Park, J.H.; Park, D.S.; Park, S.Y.; Lee, C.S.; Go, M.J.; Lee, J.; Lee, B.Y. Concerning the chromium precursor CrCl3 (thf)3 .
Inorg. Chem. Commun. 2014, 44, 148–150. [CrossRef]
Sydora, O.L.; Knudsen, R.D.; Baralt, E.J. Preparation of an Olefin Oligomerization Catalyst. U.S. Patent 0,150,642A1, 13 June 2013.
Bollmann, A.; Blann, K.; Dixon, J.T.; Hess, F.M.; Killian, E.; Maumela, H.; McGuinness, D.S.; Morgan, D.H.; Neveling, A.;
Otto, S.; et al. Ethylene Tetramerization: A New Route to Produce 1-Octene in Exceptionally High Selectivities. J. Am. Chem. Soc.
2004, 126, 14712–14713. [CrossRef] [PubMed]
Overett, M.J.; Blann, K.; Bollmann, A.; Dixon, J.T.; Haasbroek, D.; Killian, E.; Maumela, H.; McGuinness, D.S.; Morgan, D.H.
Mechanistic Investigations of the Ethylene Tetramerisation Reaction. J. Am. Chem. Soc. 2005, 127, 10723–10730. [CrossRef]
Kuhlmann, S.; Blann, K.; Bollmann, A.; Dixon, J.T.; Killian, E.; Maumela, M.C.; Maumela, H.; Morgan, D.H.; Prétorius, M.;
Taccardi, N.; et al. N-substituted diphosphinoamines: Toward rational ligand design for the efficient tetramerization of ethylene.
J. Catal. 2007, 245, 279–284. [CrossRef]
Boelter, S.D.; Davies, D.R.; Margl, P.; Milbrandt, K.A.; Mort, D.; Vanchura, B.A.; Wilson, D.R.; Wiltzius, M.; Rosen, M.S.; Klosin, J.
Phospholane-Based Ligands for Chromium-Catalyzed Ethylene Tri- and Tetramerization. Organometallics 2020, 39, 976–987.
[CrossRef]
Hirscher, N.A.; Perez Sierra, D.; Agapie, T. Robust Chromium Precursors for Catalysis: Isolation and Structure of a SingleComponent Ethylene Tetramerization Precatalyst. J. Am. Chem. Soc. 2019, 141, 6022–6029. [CrossRef] [PubMed]
Lee, H.; Hong, S.H. Polyhedral oligomeric silsesquioxane-conjugated bis(diphenylphosphino)amine ligand for chromium(III)
catalyzed ethylene trimerization and tetramerization. Appl. Catal. A. Gen. 2018, 560, 21–27. [CrossRef]
Newland, R.J.; Smith, A.; Smith, D.M.; Fey, N.; Hanton, M.J.; Mansell, S.M. Accessing Alkyl- and Alkenylcyclopentanes from
Cr-Catalyzed Ethylene Oligomerization Using 2-Phosphinophosphinine Ligands. Organometallics 2018, 37, 1062–1073. [CrossRef]
Nobbs, J.D.; Tomov, A.K.; Young, C.T.; White, A.J.P.; Britovsek, G.J.P. From alternating to selective distributions in chromiumcatalysed ethylene oligomerisation with asymmetric BIMA ligands. Catal. Sci. Technol. 2018, 8, 1314–1321. [CrossRef]
Alam, F.; Zhang, L.; Wei, W.; Wang, J.; Chen, Y.; Dong, C.; Jiang, T. Catalytic Systems Based on Chromium(III) SilylatedDiphosphinoamines for Selective Ethylene Tri-/Tetramerization. ACS Catal. 2018, 8, 10836–10845. [CrossRef]
Höhne, M.; Peulecke, N.; Konieczny, K.; Müller, B.H.; Rosenthal, U. Chromium-Catalyzed Highly Selective Oligomerization of
Ethene to 1-Hexene with N,N-Bis[chloro(aryl)phosphino]amine Ligands. ChemCatChem 2017, 9, 2467–2472. [CrossRef]
Lifschitz, A.M.; Hirscher, N.A.; Lee, H.B.; Buss, J.A.; Agapie, T. Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced
Isomerization of PNP to PPN Ligands. Organometallics 2017, 36, 1640–1648. [CrossRef]
Tomov, A.K.; Nobbs, J.D.; Chirinos, J.J.; Saini, P.K.; Malinowski, R.; Ho, S.K.Y.; Young, C.T.; McGuinness, D.S.; White, A.J.P.;
Elsegood, M.R.J.; et al. Alternating α-Olefin Distributions via Single and Double Insertions in Chromium-Catalyzed Ethylene
Oligomerization. Organometallics 2017, 36, 510–522. [CrossRef]
Zhang, L.; Meng, X.; Chen, Y.; Cao, C.; Jiang, T. Chromium-Based Ethylene Tetramerization Catalysts Supported by SiliconBridged Diphosphine Ligands: Further Combination of High Activity and Selectivity. ChemCatChem 2017, 9, 76–79. [CrossRef]
Molecules 2021, 26, 1167
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
11 of 11
Kim, E.H.; Lee, H.M.; Jeong, M.S.; Ryu, J.Y.; Lee, J.; Lee, B.Y. Methylaluminoxane-Free Chromium Catalytic System for Ethylene
Tetramerization. ACS Omega 2017, 2, 765–773. [CrossRef]
Kim, T.H.; Lee, H.M.; Park, H.S.; Kim, S.D.; Kwon, S.J.; Tahara, A.; Nagashima, H.; Lee, B.Y. MAO-free and extremely active
catalytic system for ethylene tetramerization. Appl. Organomet. Chem. 2019, 33, e4829. [CrossRef]
Park, H.S.; Kim, T.H.; Baek, J.W.; Lee, H.J.; Kim, T.J.; Ryu, J.Y.; Lee, J.; Lee, B.Y. Extremely Active Ethylene Tetramerization Catalyst
Avoiding the Use of Methylaluminoxane: [iPrN{P(C6 H4 -p-SiR3 )2 }2 CrCl2 ]+ [B(C6 F5 )4 ]− . ChemCatChem 2019, 11, 4351–4359.
[CrossRef]
Ewart, S.W.; Kolthammer, B.W.S.; Smith, D.M.; Hanton, M.J.; Dixon, J.T.; Morgan, D.H.; Debod, H.; Gabrielli, W.F.; Evans, S.J.
Oligomerization of Olefinic Compounds in the Presence of an Activated Oligomerization Catalyst. U.S. Patent WO
2,010,092,554A1, 19 August 2010.
Stennett, T.E.; Haddow, M.F.; Wass, D.F. Avoiding MAO: Alternative Activation Methods in Selective Ethylene Oligomerization.
Organometallics 2012, 31, 6960–6965. [CrossRef]
McGuinness, D.S.; Rucklidge, A.J.; Tooze, R.P.; Slawin, A.M.Z. Cocatalyst Influence in Selective Oligomerization: Effect on
Activity, Catalyst Stability, and 1-Hexene/1-Octene Selectivity in the Ethylene Trimerization and Tetramerization Reaction.
Organometallics 2007, 26, 2561–2569. [CrossRef]
McGuinness, D.S.; Overett, M.; Tooze, R.P.; Blann, K.; Dixon, J.T.; Slawin, A.M.Z. Ethylene Tri- and Tetramerization with Borate
Cocatalysts: Effects on Activity, Selectivity, and Catalyst Degradation Pathways. Organometallics 2007, 26, 1108–1111. [CrossRef]
Hirscher, N.A.; Labinger, J.A.; Agapie, T. Isotopic labelling in ethylene oligomerization: Addressing the issue of 1-octene vs.
1-hexene selectivity. Dalton Trans. 2019, 48, 40–44. [CrossRef]
Hirscher, N.A.; Agapie, T. Stoichiometrically Activated Catalysts for Ethylene Tetramerization using Diphosphinoamine-Ligated
Cr Tris(hydrocarbyl) Complexes. Organometallics 2017, 36, 4107–4110. [CrossRef]
Albert Cotton, F.; Duraj, S.A.; Powell, G.L.; Roth, W.J. Comparative structural studies of the first row early transition metal(III)
chloride tetrahydrofuran solvates. Inorg. Chim. Acta 1986, 113, 81–85. [CrossRef]
Köhn, R.D.; Coxon, A.G.N.; Chunawat, S.; Heron, C.; Mihan, S.; Lyall, C.L.; Reeksting, S.B.; Kociok-Köhn, G. Triazacyclohexane
chromium triflate complexes as precursors for the catalytic selective olefin trimerisation and its investigation by mass spectrometry.
Polyhedron 2020, 185, 114572. [CrossRef]
Kern, R.J. Tetrahydrofuran complexes of transition metal chlorides. J. Inorg. Nucl. Chem. 1962, 24, 1105–1109. [CrossRef]
Dixon, J.T.; Green, M.J.; Hess, F.M.; Morgan, D.H. Advances in selective ethylene trimerization—A critical overview. J. Organomet.
Chem. 2004, 689, 3641–3668. [CrossRef]
Jeon, J.Y.; Park, D.S.; Lee, D.H.; Eo, S.C.; Park, S.Y.; Jeong, M.S.; Kang, Y.Y.; Lee, J.; Lee, B.Y. A chromium precursor for the Phillips
ethylene trimerization catalyst: (2-ethylhexanoate)2 CrOH. Dalton Trans. 2015, 44, 11004–11012. [CrossRef]