Multisubstituted C2-symmetric ansa -metallocenes bearing nitrogen heterocycles: Influence of substituents on catalytic properties in propylene polymerization at higher temperatures

Article abstract of DOI:10.1039/d1dt00645b

In this work we systematically studied the effects of modifications of substituents on the performance of the isospecific zirconocene-based catalyst family, Me2Si(2-Alk-4-(N-carbazolyl)Ind)ZrX2 (X = Cl, Me), wherein the progenitor was shown to be particul

Full text of DOI:10.1039/d1dt00645b

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Multisubstituted C2-symmetric ansa-metallocenes
bearing nitrogen heterocycles: influence of substi-
tuents on catalytic properties in propylene
polymerization at higher temperatures†
Cite this: Dalton Trans., 2021, 50,
6170
a
Pavel S. Kulyabin, a Vyatcheslav V. Izmer,a Georgy P. Goryunov,
a
Mikhail I. Sharikov,a Dmitry S. Kononovich,a Dmitry V. Uborsky,
Jo Ann M. Canichb and Alexander Z. Voskoboynikov
*
a
In this work we systematically studied the effects of modifications of substituents on the performance of
the isospecific zirconocene-based catalyst family, Me2Si(2-Alk-4-(N-carbazolyl)Ind)ZrX2 (X = Cl, Me),
wherein the progenitor was shown to be particularly suitable in high-temperature propylene polymeriz-
ation processes. In order to obtain the required zirconocenes, we developed a novel synthetic pathway to
4-(N-carbazolyl)indenes through Pd-catalyzed cyclizations of 2,2’-dibromobiaryls with 4-aminoindenes,
which were synthesized via Buchwald–Hartwig reaction or electrophilic amination of 4-indenyl Grignard
reagents with trimethylsilylmethyl azide. By a number of examples, the anion-promoted rac-to-meso iso-
merization method was shown to work reliably well for preparation of rac-ZrMe2-complexes. Certain zir-
conocenes among the 21 tested in propylene polymerization at 70 and 100 °C under MAO or borate acti-
vation outperformed the parent catalyst in molecular weight capability, regio- or stereoselectivity.
Received 25th February 2021,
Accepted 9th April 2021
DOI: 10.1039/d1dt00645b
rsc.li/dalton
nity. Nonetheless, the potential of those systems was recog-
nized by both academia and polyolefin industry, and an exten-
Introduction
Metallocenes of group 4 metals have been known as catalysts sive effort was aimed at optimizing performance of metallo-
for homogeneous olefin polymerization since the early works cene-based catalysts for propylene polymerization.7 In 1994
by Natta1 and Breslow2 in the 1950s. More than 20 years later, Spaleck et al. reported that zirconocene rac-Me2Si(2-Me-4-Ph-
the discovery of MAO and then boron-based activators made it Ind)2ZrCl2 (I, Fig. 1) upon activation with MAO produced
possible to dramatically enhance the activity of these catalysts highly isotactic polypropylene of high MW which was recog-
up to a factor of 10 000.3,4 Pioneering work by Ewen on the nized as one of the major achievements in single-site metallo-
relationship between metallocene symmetry and polymer cene catalyst design.8 The whole family of C2-symmetric Me2Si-
microstructure pointed out the direction for the design of bridged bis(2-alkyl-4-arylindenyl) metallocenes – which has
complexes depending on the desired polypropylene (PP) pro- grown a lot in size and diversity since then – is now often
perties.5 In 1985, the first indenyl-based ansa-zirconocene was referred to as “Spaleck-type” catalysts. The performance of the
used for isospecific polymerization of propylene by Kaminsky Spaleck-type catalysts is known to deteriorate with increasing
and Brintzinger.6 In comparison with the classic Ziegler–Natta polymerization temperature. Thus, MW capability, stereo- and
catalysts, these early C2-symmetric ansa-metallocene dichlor- regioselectivity drop significantly at temperatures above
ides activated with MAO produced isotactic polypropylene
(iPP) with relatively low molecular weight (MW) and crystalli-
aDepartment of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1/3,
Moscow 119991, Russia. E-mail: voskoboy@med.chem.msu.ru
bBaytown Technology and Engineering Complex, ExxonMobil Chemical Company,
5200 Bayway Drive, Baytown, Texas 77520, USA.
E-mail: jm.canich@exxonmobil.com
†Electronic supplementary information (ESI) available. CCDC 2046384–2046389.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1dt00645b
Fig. 1 Propylene polymerization precatalysts.
6170 | Dalton Trans., 2021, 50, 6170–6180
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60 °C.7,9–11 Catalyst activity should rise exponentially with reac- the indenyls, and conduct propylene polymerization experi-
tion temperature, but due to side deactivation processes, ments at 70 and 100 °C using MAO or [HMe2NPh][B(C6F5)4]
monomer diffusion limitations and decrease of monomer con- (AB) as activators. In addition, we prepared a few 4-(N-azolyl)
centration (in case of solution polymerization) the productivity indenyl metallocenes where the N-azolyl substituents were
vs. temperature dependence curve is bell-shaped with a different from N-carbazolyl to test whether the latter moiety is
maximum at 60–100 °C. Meanwhile, it is beneficial for indus- crucial for the applicability of the respective catalysts at high
trial processes to conduct polymerization in solution at higher polymerization temperatures.
temperatures due to increased throughput, decreased energy
needed for devolatilization, and decreased fouling.12
Modifications of I by introduction of substituents in the
Results and discussion
Syntheses of the zirconocenes
4-phenyl ring are known to efficiently improve performance of
Spaleck-type catalysts in iPP synthesis.13,14 It was shown pre-
viously that, for example, rac-Me2Si(2-Me-4-(3,5-tBu2-Ph)
In our previous paper, we described a simple procedure for
Ind)2ZrCl2 forms the catalyst which displays higher stereo- and
installation of carbazolyl group in indenes based on
Buchwald–Hartwig reaction of 4/7-bromo-2-methylindene19 (1)
regioselectivity in comparison with I. In addition, an ethyl
group in position 2 of the indenyl moieties can further
and N-carbazolyllithium with Pd(OAc)2/tBuMePhos as a cata-
lyst.16 The described procedure worked well for syntheses of
enhance regiocontrol, and large substituents in position 6
were shown to significantly boost MW capability.14 Schöbel
indenes 7–14 and gave the (N-carbazolyl)-substituted indenes
in 38–80% yields (Scheme 1).
et
al.
developed
rac-Me2Si(2-Me-4-(3,5-tBu2-Ph)-7-
MeO-Ind)2HfCl2 with a record breaking MW capability, regio-
and stereoselectivity at low polymerization temperatures.15
These studies prove that modifications of substituents in I can
lead to catalysts with advanced performance in terms of
activity, MW capability, stereo- and regioselectivities. Recently
we reported our results of investigation of 4-(N-azolyl)-substi-
tuted metallocenes II–IV (Fig. 1)16 where we found that cata-
lysts derived from 4-(N-carbazolyl)-substituted zirconocenes, II
or MII, demonstrated high stereoselectivity in combination
with decent molecular weight capability in propylene polymer-
ization at 100 °C.
Soon after that, a further investigation of catalytic perform-
ance of variously substituted in different positions Spaleck-
type metallocenes in propylene polymerization at high temp-
erature11 confirmed that zirconocene II had the best combi-
nation of stereo- and regioselectivity resulting in the highest
melting point of iPP produced at 100 °C among the series of
21 catalysts. Although higher MW capability was demonstrated
by four metallocenes with 6-tBu groups in the indenyls, includ-
ing the Resconi/Nifant’ev catalyst17,18 bearing also MeO-groups
in position 5, none of them was close to II in the crystallinity
of the obtained iPP (i.e. its high melting point (Tm)). This
being said, analogues of II modified with additional substitu-
ents and their polymerization behavior have not been studied
yet.
In the next set of analogues of II, we aimed to replace the
N-carbazolyl with other N-azolyl moieties with similar steric
properties – 2,5-dimethyl-N-pyrrolyl and 2-R-N-indolyl (R = Me,
Ph) which are substituted versions of IV and III, respectively.
Unfortunately, our attempts to synthesize the corresponding
N-azolyl-substituted indenes via cross-couplings of 4/7-bromo-
2-methyl-1H-indene and 2,5-dimethylpyrrole or 2-methylindole
were unsuccessful leading to side products.16 Ivchenko et. al.
described their own attempt to synthesize 2-methyl-4-(2,5-
dimethyl-N-pyrrolyl)-indene by Friedel–Crafts cyclization of
chloroanhydride of 3-(2-(2,5-dimethyl-N-pyrrolyl)phenyl)-2-
methylpropanoic acid, which resulted in isomerization of 2,5-
dimethyl-N-pyrrolyl fragment to 2,4-dimethyl-N-pyrrolyl.20
Therefore, we opted for a synthetic route based on late-
stage construction of the pyrrole core via well-known Paal–
Knorr reaction of 4/7-amino-2-methylindene (15) with preas-
sembled indene skeleton. Besides that, there are examples of
the synthesis of 2-substituted 1-arylindoles from the corres-
ponding anilines via transition metal-catalyzed reactions.21–24
The previously published synthetic pathway to 15 is too labor-
Inspired by the results of systematic studies showing the
impact of structural modifications of Spaleck type catalysts on
their propylene polymerization performance conducted by
some of us in collaboration with Prof. V. Busico group,11,13,14
we decided to investigate whether the same and even more
diverse manipulations with the ligand substitution on II might
have similar effects in the 4-(N-azolyl)-substituted family of
metallocenes, which could lead to new and better performing
catalysts for iPP production, particularly at high polymeriz-
ation temperatures. Thus, we aimed to synthesize a variety of
rac-Me2Si(2-Alk-4-(N-carbazolyl)Ind)2ZrX2 complexes (X = Cl,
Scheme 1 Syntheses of (N-carbazolyl)indenes via Buchwald–Hartwig
Me), optionally further substituted in positions 5, 6, and 7 of cross-couplings.
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and time-consuming,25 so we developed our own method for
The technique developed by Ackermann22 appeared to be
the synthesis of 15 from 1. First, we tried to apply well-devel- the most effective among those we tested for assembly of the
oped Buchwald–Hartwig reaction with various ammonia (2-R-N-indolyl) core via derivatization of 15 giving indenes 25
equivalents.26–29 The only effective one was benzophenone (R = Me) and 26 (R = Ph) in 76 and 78% yields respectively
imine29 which allowed us to obtain indene 15 in 56% yield (Scheme 5).
(Scheme 2). Analogously, indenes 18–20 were synthesized in
42–58% yields (Scheme 2).
In 2003 Nozaki and co-workers described a route to multi-
substituted carbazoles via palladium-catalyzed double
Another method we chose for synthesis of substituted N-arylation of primary amines with 2,2′-dihalobiphenyls.33,34
4/7-aminoindenes was based on electrophilic amination. Having primary amine 15 in hand, we synthesized several 4/7-
Trimethylsilylmethyl azide (TMSMA) is known to be used for (N-carbazolyl)indenes, where the carbazolyl moieties were sym-
preparation of aromatic primary amines from sterically hindered metrically substituted at positions 2, 7, or 3, 6, or 2, 3, 6, 7. In
aryllithium and aryl Grignard reagents.30 Amine 15 was syn- this way, indenes 27–30 were obtained from 15 in 58–65%
thesized via reaction of TMSMA with Grignard reagent prepared yields (Scheme 6). Moreover, this method made it possible to
from 1 by the common method of “entrainment” with 1,2-dibro- synthesize sterically encumbered 4/7-(N-carbazolyl)indenes
moethane. Subsequent quenching with water gave the desired 31–34 (Scheme 6) containing substituents in ortho-position to
product 15 in 51% yield (Scheme 3). Analogously, indenes 18–20 the (N-carbazolyl) moiety, which we previously failed to obtain
and 22 were synthesized in similar or better yields (Scheme 3) in via Buchwald–Hartwig amination of the corresponding
comparison with the Buchwald–Hartwig procedure (Scheme 2) 4/7-bromoindenes. Summarizing the above, 4/7-aminoindenes
without the use of transition metals. Notably, more sterically proved to be very convenient starting materials for various
hindered bromoindenes gave higher yields.
(N-azolyl)indenes, where N-azolyl is optionally substituted
As it was expected, 15 readily reacted with 2,5-hexanedione N-pyrrolyl, N-indolyl or N-carbazolyl.
in the presence of 10 mol% of iodine31 giving the required
Me2Si-bridged bis(indenyl) proligands L1–L22 were pre-
(2,5-dimethyl-N-pyrrolyl)-substituted indene 23 in 53% yield pared from indenes 7–14, 23–34 in good yields by two
(Scheme 4, method A). An alternative procedure with methods using either CuCN35 or N-methylimidazole36 as cata-
32
ZrOCl2(H2O)8 as a catalyst gave 70% yield and was also used lysts for nucleophilic substitution at silicon (Scheme 7,
for synthesis of tert-butyl substituted indene 24 from 18 methods A, B; see ESI for details†). The proligands were sub-
(Scheme 4, method B).
sequently dilithiated with nBuLi in ether and reacted with
ZrCl4(THF)2 to prepare the corresponding zirconocene dichlor-
Scheme 2 Synthesis of aminoindenes via cross-coupling.
Scheme 5 Syntheses of 4-(2-R-N-indolyl)indenes.
Scheme 3 Syntheses of aminoindenes via electrophilic amination.
Scheme 6 Syntheses of (N-carbazolyl)indenes via double arylation of
Scheme 4 Synthesis of 2,5-dimethyl-(N-pyrrolyl)indenes.
6172 | Dalton Trans., 2021, 50, 6170–6180
aminoindenes.
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Scheme 7 Synthesis of proligands and zirconocenes. a Method A: (1) 2.05 equiv. nBuLi, Et2O; (2) 0.1 equiv. CuCN, 0.5 equiv. Me2SiCl2, THF. Method
B: (1) 2.05 equiv. nBuLi, Et2O; (2) 0.01 equiv. N-methylimidazole, 0.5 equiv. Me2SiCl2, Et2O; or, for L3, L4: (1) 1.0 equiv. nBuLi, Et2O; (2) 0.01 equiv.
N-methylimidazole, 1.0 equiv. 2-Me-4-(N-carbazolyl)indenyl-SiMe2Cl, Et2O. b Method C (for M5, M6, M8, M12): 10 equiv., MeMgBr, Et2O-PhMe,
100 °C. Method D (for M7, M9–M11, M13): (1) 10 equiv., MeMgBr, Et2O-PhMe, 100 °C. (2) 0.05 equiv. LiCl, THF, 50 °C. c R6 = H, except for C22. d rac/
meso = 1/1, not tested. e 95% rac.
ides C1–C22 (Scheme 7). Crystallization afforded C1–C4, C6, dimethyl complexes by treatment with excess of MeMgBr in sus-
C11, C12, C16, C18–C20, and C22 as pure rac-zirconocenes, pension in toluene-ether at 100 °C (Scheme 7, methods C and
and C15, C17, and C21 in low yields (0.9–4%) as mixtures of D). After work up, the yields of the pure rac/meso mixtures of
∼95% rac and ∼5% of meso-isomers. Unfortunately, in some the dimethyl complexes M5–M13 were almost quantitative.
cases we were unable to isolate the required rac-complexes Unfortunately, under these conditions, rac/meso mixtures of
even after extensive recrystallizations.
indolyl-substituted complexes M14 and M15 were formed
There are a number of examples of racemoselective synth- together with unidentified and inseparable impurities.
eses of ansa-metallocenes in the literature with utilization of Dimethyl complexes M6 and M12 were prepared from pure rac-
special zirconium reagents37–40 instead of ZrCl4(THF)2, or pro- isomers of C6 and C12 respectively, so no isomerization was
ligand modification.41,42 In the former case, additional steps needed. Racemic forms of M5 and M8 were successfully isolated
are often required to convert the products of racemoselective from mixtures of methylated rac- and meso-complexes by crystal-
reactions to the target metallocene dichlorides. Also, some lization (Scheme 7, C). Mixtures of rac/meso-complexes M7, M9–
success was achieved in photoinduced meso-to-rac isomeriza- M11 and M13 obtained after methylation were treated with LiCl
tion of bridged zirconocenes.43–45 Drawbacks of the method in THF at 50 °C (Scheme 7, D) which expectedly resulted in the
are instability of the complexes under the isomerization con- desired enrichment of the rac-isomers, all of which were suc-
ditions42 and dependence of the achievable photostationary cessfully isolated after subsequent recrystallizations (see ESI for
rac/meso ratios on the ligand substitution resulting in rac/meso details†). Thus, we demonstrated that the method of meso-to-rac
ratios of <1 in some cases.44
isomerization of dimethyl zirconocenes with LiCl previously
In 1999 R. Lin from Albemarle found that rac/meso ratio of developed for preparation of MII (Fig. 1)16 is reliable and can be
silicon-bridged zirconocene dichlorides could be adjusted by used for synthesis of racemic dimethyl metallocenes when solu-
heating rac/meso mixtures with LiCl in DME or THF.46 Later, bility of the corresponding dichlorides is an issue.
the reversible substitution of cyclopentadienyl ligand by Cl−
Thus, we successfully synthesized 21 novel racemic ansa-zir-
anion in zirconocenes was studied by Buck et al.47 The rate of conocenes, which can be combined in four main groups accord-
the anion-promoted meso-to-rac isomerization depends on the ing to the nature and positions of the varied substituents: (1)
solubility of the starting rac/meso mixture in THF, which is very 2-alkyl substitution on the indenyls, (2) substituents on 4-(N-car-
low for the zirconocene dichlorides we had obtained. For this bazolyl), (3) substituents in positions 5, 6, 7 of the indenyls, (4)
reason, complexes C5–C13 were converted to more soluble other N-heteroaryls instead of carbazolyl (Scheme 7).
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Crystal structures of the 4-(N-azolyl)-substituted zirconocenes
N-pyrrolyl and indenyl fragments in C22 are comparable with
those in C16 but much higher than in M6, M7, M10, M12, and
IV (32.25/49.76° in the latter16). Based on the data, we can con-
clude that modification of IV with 2,5-methyls in N-pyrrolyl
giving C22 affects the dihedral angle between heterocycle and
indene much more than the replacement of N-pyrrolyl in IV by
N-carbazolyl in II. Thus, the 2,5-dimethyl-N-pyrrolyl moiety
appears to be closer in steric properties to mesityl rather than
to N-carbazolyl which results in a close proximity of methyl
groups in N-pyrrolyl to the metal center.13 A likely conse-
quence of this is an anomalously large Cl1–Zr–Cl2 angle48 in
C22 (103.59° vs. 96.65 in IV16), similar to the mesityl-substi-
tuted analogue of I as evidenced by the respective structures
calculated with DFT.13 The comparatively large Cl1–Zr–Cl2
angle observed in C16 can also be explained by the larger car-
bazolyl/indenyl dihedral angle in comparison with the other
complexes. Large dihedral angles between the space-demand-
ing heteroaryl substituents and indenyls can also be respon-
sible for larger dihedral angles between the two planes of
cyclopentadienyl rings in C16 and C22 (63.28° and 63.25°) in
comparison with those in III, IV,16 M6, M7, M10 and M12
(58.33–59.99°).
Single crystals suitable for X-ray structure determination were
obtained for rac-zirconocenes C16, C22, M6, M7, M10 and
M12 by recrystallization from toluene/hexane mixture at
−30 °C (Fig. 2, Fig. S1 and Tables S3–S8 in ESI†). Important
angles of the complexes are given in Table 1, which includes
structural parameters for both halves of each complex,
because the structures are not perfectly C2-symmetric due to
incorporation of solvent molecules and/or crystal lattice
effects.
In all of the complexes, the zirconium atoms coordination
polyhedron exhibits
lengths, Zr–Cl in C16 and C22 and Zr–CH3 in M6, M7, M10
and M12, are in narrow ranges, 2.410–2.421 and
a pseudotetrahedral geometry. Bond
Å
2.238–2.266 Å respectively, as well as the distances between the
zirconium atoms and centroids of cyclopentadienyls
(2.251–2.266 Å) and angles Cp–Zr–Cp′, which seem to be
weakly dependent on the ligand structure (Table 1). While in
complexes M6, M7, M10, M12 with substituted carbazolyl moi-
eties, the dihedral angles between the planes of carbazolyl and
indenyl fall in the range 48.15–55.33° (larger angle 68.22° in
M7 is caused by cocrystallized toluene molecule), in C16 they
are much larger (68.65/77.80°). Apparently, the 5-methyl
groups on the indenyls increase the dihedral angle more sig-
nificantly than the substituents in the carbazolyl moieties. The
analogous dihedral angles between the planes of 2,5-dimethyl-
Propylene polymerization
The next goal was to find how these modifications of the proto-
types II–IV translate into propylene polymerization perform-
ance, and to see if, and to what extent, the previously observed
effects of certain structural changes in zirconocene I11,13,14 can
manifest themselves in 4-(N-carbazolyl) family of catalysts.
For propylene polymerization tests at 70 and 100 °C, the
rac-zirconocene dichlorides C1–C4, C6, C10–C12, C15–C22,
and dimethyl complexes M6–M8, M12, M13 were activated by
MAO (molar ratio Al/Zr = 500/1). Complex MII was used as a
reference. As the cationic active species formed from both var-
iants (–ZrCl2 and –ZrMe2) of the precatalysts are believed to
have identical structure, which is responsible for the MW capa-
bility of a catalyst and microstructure of iPP, the differences of
those parameters in pairs of ZrCl2/ZMe2-based catalysts with
the same ligand were neglected. On the other hand, the
activity can be affected, because the dichlorides require
additional step of alkylation by MAO to form the active
species, whereas the dimethyl complexes are already alkylated.
Thus, the lower activity of catalysts formed from the dichlor-
ides in comparison with the reference dimethyl zirconocene
MII under MAO activation is uninformative, whereas higher
activity of the former can be taken into consideration. Besides
Fig. 2 ORTEP style representation of zirconocene M6 with ellipsoids
drawn at 50% probability level. Hydrogen atoms are omitted for clarity.
Table 1 Selected solid state structural parameters of C16, C22, M6, M7, M10, M12
C16
C22
M6
M7
M10
M12
X1–Zr–X2
Cp–Cp’c
99.88a
63.28
68.65/77.80
103.59a
63.25
68.85/82.05
96.83b
59.97
55.33/53.56
97.46b
58.45
48.15/68.22
98.97b
59.41
51.72/59.33
97.66b
59.99
49.42/50.04
Hetar-Indc
a X = Cl. b X = Me. c Dihedral angle.
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that, for the purpose of a more precise comparison, a series of Zr–Cl′ angle in the crystal structure of C22, discussed above,
additional experiments with anilinium borate activator AB can be an indirect proof of protrusion of the 2,5-methyls in the
([HMe2NPh][B(C6F5)4], molar ratio AB/Zr = 1.1/1) werecon- pyrrolyl into the equatorial plane of the catalyst (previously
ducted for dimethyl zirconocenes M5–M10 and M13 also in mentioned for 4-mesitiyl analogue of I13) thus hindering the
comparison with MII.
approach of the monomer to the metal and leading to poor
Zirconocenes with various 4-(N-heteroaryls) instead of 4-(N- activity of both M13 and C22. On the other hand, 4-(2-Ph-N-
carbazolyl), activated by MAO (Table 2). Previously, we demon- indolyl)-substituted C15 could compete with MII quite well
strated that 4-(N-carbazolyl)-substituted zirconocene II outper- being 2-fold more active at 100 °C, slightly less selective and
formed 4-(N-pyrrolyl) and 4-(N-indolyl) analogues in terms of displaying molecular weight capability close to that of the
stereo-, regioregularity and MW of iPP produced.16 None of reference at both temperatures.
those contained a 2,5-disubstituted pyrrole core, which could
Zirconocenes with varied substitution in the 4-(N-carbazolyl)
be an alternative to the N-carbazolyl. However, it appeared that moieties, activated by MAO (Table 3). Having confirmed that
replacement of N-carbazolyl in MII with 2,5-dimethyl-N-pyrro- the 4-(N-carbazolyl) substituent provides higher selectivities in
lyl in M13 made the latter a substantially worse catalyst in all iPP synthesis than the other heterocyclic analogues tested, we
respects, starting from the activity which dropped by an order were interested to examine if modification of the carbazolyl
of magnitude. The effect of additional 6-tBu in C22 on the itself could improve performance of MII.
molecular weight was much more modest than what could be
Minor structural modifications of zirconocene, particularly
expected based on the same modification of I.14 The large Cl– in distant regions of the ligand, should accordingly have a
Table 2 Propylene polymerization using zirconocenes M13, C22, C15 activated by MAOa
Regioerrorsd
Pre-cat.
4-(N-azolyl) (R6)
Tp, °C
Ab
Mw, kDa
Đc
Tm, °C
mmmm, %
[2,1]
[3,1]
MII
Reference
70
100
70
100
70
100
70
158
109
8
10
3
11
198
198
575
309
496
96
672
129
597
286
2.1
1.9
1.6
1.8
2.0
1.9
2.5
1.8
159.1
156.8
153.9
150.7
150.0
147.8
156.4
154.3
98.5
97.7
98.0
—
91.1
—
22
18
13
—
29
—
26
26
7
12
24
—
67
—
10
19
M13
C22
C15
2,5-Me2-pyrrolyl (H)
2,5-Me2-pyrrolyl (6-tBu)
2-Ph-indolyl (H)
97.5
96.8
100
a Polymerization conditions: 0.04 µmol precat. (0.06 µmol for C22), 500 eq. MAO, 4.1 mL toluene, 1.0 mL propylene, reactor quenched at 8 psig
pressure loss or at a maximum time limit of 30 minutes. b Activity in kgPP/(mmolZr h). c Mw/Mn. d Number per 104 monomer units.
Table 3 Propylene polymerization using zirconocenes C6, M6–M8, C10–C12, M12 activated by MAOa
Regioerrorsd
Pre- cat.
Substitution in 4-(N-carbazolyl)
Tp, °C
Ab
Mw, kDa
Đc
Tm, °C
mmmm, %
[2,1]
[3,1]
MII
Reference
70
100
70
100
70
100
70
100
70
100
70
100
70
100
70
158
109
197
104
252
120
174
79
163
92
104
102
20
575
309
358
196
296
171
442
260
557
352
602
240
1006
304
674
293
505
268
2.1
1.9
2.4
1.6
2.3
1.8
2.0
2.0
2.1
1.8
2.2
2.0
1.5
1.4
1.6
1.6
1.7
1.7
159.1
156.8
157.2
154.6
157.1
154.1
156.8
154.1
159.2
155.2
158.0
154.1
159.2
155.1
156.9
152.2
157.6
153.3
98.5
97.7
96.0
96.0
97.7
93.2
98.4
97.6
96.7
98.4
96.9
97.5
—
22
18
28
27
27
26
28
25
25
25
28
24
—
—
35
32
35
38
7
12
7
19
9
12
8
16
7
10
4
13
—
—
2
14
3
C6
4,5-(CH2CH2)
4,5-(CH2CH2)
3,6-Me2
M6
M7
M8
C10
C11
C12
M12
2,7-Me2
2,3;6,7-Bis(CH2)3
3,6-tBu2
22
39
7
117
6
—
2,7-tBu2
97.5
97.7
98.3
97.7
100
70
100
2,7-tBu2
14
a Polymerization conditions: 0.04 µmol precat. (0.025 µmol for C11), 500 eq. MAO, 4.1 mL toluene, 1.0 mL propylene, reactor quenched at 8 psig
pressure loss or at a maximum time limit of 30 minutes. b Activity in kgPP (mmolZr h)−1 c Mw/Mn. d Number per 104 monomer units.
.
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subtle influence on the performance of the respective catalyst.
Zirconocenes with varied substitution in positions 5, 6, and
Indeed, this was observed for the complexes containing small 7 of indenyls, activated by MAO (Table 4). Most of the zircono-
methyl or –CH2– groups in positions 2–7 of the carbazolyl cenes in this group contain structural changes which effects
moiety. One would not expect that in some cases (e.g. C6, M6, on polymerization performance have been thoroughly studied
M7, and C10) those presumably innocent modifications had recently for Spaleck-type catalysts.11,13,14 To our surprise, in
albeit predictably small but negative effect on the stereo- and some cases, those effects were not reproduced for the 4-(N-car-
regioselectivity of the catalysts which produced iPP with bazolyl) catalysts.
slightly larger number of errors and consequently lower Tm (by
1–3 °C vs. that for MII at both polymerization temperatures). additional 6-tBu groups was observed for six 6-tBu/6-H pairs of
M7 also gave iPP with lower Mw at 70 °C than the reference. catalysts in propylene polymerization at 60 °C with an average
For example, the increase of MW capability caused by
Lower selectivity and molecular weight capability of C6/M6, ∼2-fold difference,11 and for five pairs at 100 °C with a ∼2–4
however, is perfectly explained by the slightly different geome- fold difference.14 This effect was accompanied with higher or
try of the 4,5-(CH2CH2)-substituted carbazolyl in comparison nearly the same stereoselectivity and lower regioselectivity that
with the unsubstituted one, caused by the subtending effect of resulted in lower Tm of the obtained iPP. However, modifi-
the –CH2CH2– group. The contraction of the two benzene cation of II with 6-tBu groups giving zirconocene C18 did not
rings makes such angles as N2–C40–C41 (Fig. 2 (M6), Fig. S2†) result in an increase of MW capability of the latter, whereas
in the carbazolyl fragment larger (133.91–134.19° in M6 and the numbers of stereoerrors became higher for polymerization
129.84–130.33° in M7) thus decreasing steric hindrance near at 100 °C, and regioselectivity was lower at both temperatures.
the catalytic site and disencumbering rotation of the carbazo- Furthermore, unlike the rest of the catalysts in Table 4, C18
lyl about the C(Ind)–N bond. This is remarkable evidence of was tested in isohexane under conditions identical to those
the virtually optimal geometry of N-carbazolyl as an aromatic used for II in our previous report.16 Thus, a more direct com-
substituent in position 4 of bis(indenyl) ansa-zirconocene iso- parison with the latter results reveals that C18 produced iPP
specific propylene polymerization catalysts.
with lower Mw than II (617 kDa for C18, and 1073 kDa for II16)
Nevertheless, 2,7-Me2-N-carbazolyl-substituted M8 appeared at 70 °C. At 100 °C II, MII, and C18 gave iPP of similar Mw,
to be comparable with MII in selectivity and was able to with C18 being considerably less stereo- and regioselective.
produce iPP with somewhat higher Mw at 100 °C than the
Similarly, it was previously shown that substitution of I with
reference catalyst. The larger 3,6-tBu2 groups on the carbazolyl 5-Me on indenyls (referred to as “hard lock”) limits the range
gave C11 a ∼1.5-fold advantage over the reference catalyst in of possible values of the indenyl/4-aryl dihedral angle improv-
the molecular weight capability at 70 °C, which however van- ing stereoselectivity and MW capability at the expense of lower
ished at 100 °C. When the positions 2 and 7 in carbazolyl are regioselectivity.14 In contrast with those findings, 5-Me-substi-
occupied by two tBu groups in C12/M12, the regioselectivity tuted C16 demonstrated reduced stereoselectivity and pro-
appears to be lower than that of MII and M8. Besides that, duced iPP with lower Mw and lower Tm.
under MAO activation, a severe drop in activity at 100 °C vs.
70 °C was observed for both catalysts in this pair, and particu- pulled back being a part of 5-membered cycle, displayed an
larly for M12, for which there was a 20-fold decrease. even slightly better stereoselectivity in comparison with MII.
On the contrary, C17 bearing smaller 5-CH2-group that is
Table 4 Propylene polymerization using zirconocenes C16–C21 activated by MAOa
Regioerrorsd
Pre-cat.
Deviation from MII structure
Tp °C
Ab
Mw, kDa
Đc
Tm, °C
mmmm, %
[2,1]
[3,1]
MII
Reference
70
100
70
100
70
100
70
100
70
100
70
100
70
100
158
109
52
575
309
572
197
534
215
617
311
861
493
369
87
2.1
1.9
1.8
1.8
1.8
1.9
2.4
1.9
1.9
1.7
1.8
1.9
1.7
1.7
159.1
156.8
156.2
155.1
158.3
154.9
154.5
152.5
155.6
153.4
158.5
156.4
151.4
150.8
98.5
97.7
94.4
95.3
98.6
98.1
98.6
96.1
97.6
—
22
18
17
18
23
16
21
25
31
—
27
24
—
58
7
12
0
C16
C17
C18e
C19
C20
C21
5-Me
74
14
13
29
21
33
10
—
3
5,6-(CH2CMe2CH2)
127
166
59
63
56
49
68
77
11
6-tBu
5-MeO-6-tBu
7-MeO
99.0
97.8
—
14
—
13
4-(2,3,6,7-Me4-carbazolyl)-5-MeO-6-tBu
1290
497
47
97.9
a Polymerization conditions: 0.04 µmol precat. (0.08 µmol for C18), 500 eq. MAO, 4.1 mL toluene, 1.0 mL propylene, reactor quenched at 8 psig
pressure loss or at a maximum time limit of 30 minutes. b Activity in kgPP (mmolZr h)−1
0.5 mL toluene, 3.6 mL isohexane.
.
c Mw/Mn. d Number per 104 monomer units. e Solvent:
6176 | Dalton Trans., 2021, 50, 6170–6180
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The slightly lower Tm of the iPP is probably caused by a some- cantly, that only low melting and low MW polymers were
what larger number of regioerrors. At 100 °C, C17 also formed at both temperatures in the case of C1. Exactly the
appeared to be ∼1.5-fold active vs. the reference, but produced same effect was observed previously for Spaleck metallo-
lower Mw iPP even at 70 °C despite the 5,6-dialkyl substitution, cenes.14 Substitution of one isopropyl group with methyl sig-
whereas both 5- and 6-Me-substituted analogues of I showed nificantly enhanced performance of C3 in comparison with
an advantage over I in this regard.14
C1. There is substantially less discrepancy in the performance
Rieger’s “ultra-rigid” metallocene Me2Si(2-Me-4-(3,5-tBu2- indicators between C3 and MII than between C3 and C1, that
15
Ph)-7-MeO-Ind)ZrCl2 was demonstrated to have a higher can be explained by chain back skip between the “2-methyl”
stereo- and slightly lower regioselectivity resulting in higher and “2-isopropyl” sites.49 Nevertheless, C3 was still a much
crystallinity of iPP produced at 100 °C in comparison with its worse catalyst than MII. Replacement of 2-methyls in MII with
7-unsubstituted analogue, while the Mw was somewhat lower 2-isobutyls in C2 gave almost the same catalyst performance
(44 vs. 60 kDa, respectively).11 C20, which is the 7-MeO-substi- with a bit different distribution of errors, similar MW capa-
tuted II, did not show any improvement over the reference MII bility and slightly lower Tm for iPP produced at 100 °C which
in selectivity, but the Mw of iPP produced by C20 appeared to probably originates from a somewhat higher number of
be almost 2-fold lower at 70 °C and furthermore 3.5-times regioerrors. However, at 70 °C, the unsymmetrically 2-Me/
lower at 100 °C vs. MII.
2-nBu substituted C4 was able to produce iPP with the same Tm
Meanwhile, the positive effect of Resconi/Nifant’ev and 1.7-fold higher Mw in comparison with MII. At 100 °C the
5-MeO-6-tBu indenyl substitution pattern17,18 on the MW capa- MW capability of C4 became similar to that of MII, whereas
bility of metallocene catalysts in propylene polymerization the stereo- and regioselectivities were slightly lower, resulting
appears to be real for the 4-(N-carbazolyl)-substituted family. in a lower iPP Tm by 2 °C vs. MII.
Thus, at 70 °C, C19 gave iPP with 1.4-fold higher Mw in com-
Selected dimethyl zirconocenes, activated by AB (Table 6).
parison with MII, and with 1.6-fold higher Mw at 100 °C. Activation of dimethyl complexes with AB allows for a more
However, in line with the difference previously observed precise comparison of the respective catalysts including also
between I and Resconi/Nifant’ev catalyst,11,14,18 the regio- evaluation of their activity. In this way, zirconocene M5 with
selectivity of C19 is lower in comparison with MII. C21 con- n-butyl groups in position 2 of indenyls, M6–M10, M12 con-
taining 4-(2,3,6,7-Me4-N-carbazolyl) moiety in addition to the taining variously substituted carbazolyl moieties, and the refer-
5-MeO-6-tBu substitution demonstrated the highest MW capa- ence zirconocene MII were tested in propylene polymerization
bility among the newly prepared zirconocenes outperforming at 70 and 100 °C.
C20 at 70 °C. This advantage of C21 over C19 disappeared in
These experiments confirmed the lower stereo- and regio-
polymerization at 100 °C with both catalysts producing iPP selectivities and MW capability of M6 and M12 in comparison
with almost identical Mw. Crystallinity of iPP produced by C21 with the reference. This being said, these two catalysts
was lower than in the case of C19. Interestingly, C21 was the appeared to be the most active in the series, with M12 exceed-
only catalyst in the study where activity at 100 °C was signifi- ing 5.5 times the activity of MII at 70 °C and 3.7 times at
cantly higher (∼4-fold) than at 70 °C.
100 °C. The behavior of M7–M10 with alkyl-substituted carba-
Zirconocenes with varied substitution in position 2 of inde- zolyl moieties represents an interesting trend. First, M9 with
nyls, activated by MAO (Table 5). Isopropyl groups in position tetramethyl-substituted carbazolyl appeared to generate
2 of indenyls in C1 dramatically deteriorate catalytic perform- slightly more stereo- and regioerrors than both M7 and M8
ance in comparison with MII. Although no regioerrors were bearing 3,6-Me2-N-carbazolyl and 2,7-Me2-N-carbazolyl respect-
found, stereoselectivity and MW capability dropped so signifi- ively, and M10 where positions 2,3 and 6,7 in the carbazolyl
Table 5 Propylene polymerization using zirconocenes C1–C4 activated by MAOa
Regioerrorsd
Pre- cat.
R2, R2′
Tp, °C
Ab
Mw, kDa
Đc
Tm, °C
mmmm, %
[2,1]
[3,1]
MII
Me
70
100
70
100
70
100
70
100
70
100
158
109
3
575
309
47
2.1
1.9
1.5
1.4
1.6
1.6
1.6
1.9
1.6
1.6
159.1
156.8
137.7
105.3
158.6
155.8
155.4
147.1
159.5
154.9
98.5
97.7
85.4
66.3
98.4
97.7
96.0
—
22
18
0
7
12
0
10
6
18
23
—
3
C1e
C2
C3
C4
iPr
3
11
5
iBu
71
55
17
9
37
18
602
251
207
52
957
297
28
19
10
—
23
19
Me, iPr
Me, nBu
97.9
96.8
14
a Polymerization conditions: 0.04 µmol precat. (0.08 µmol for C1), 500 eq. MAO, 4.1 mL toluene, 1.0 mL propylene, reactor quenched at 8 psig
pressure loss or at a maximum time limit of 30 minutes. b Activity in kgPP (mmolZr h)−1
0.5 mL toluene, 3.6 mL isohexane.
.
c Mw/Mn. d Number per 104 monomer units. e Solvent:
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Table 6 Propylene polymerization using zirconocenes M5–M10, and M12 activated with ABa
Regioerrorsd
Pre- cat.
Deviation from MII
Tp, °C
Ab
Mw, kDa
Đc
Tm, °C
mmmm, %
[2,1]
[3,1]
MII
Reference
70
100
70
100
70
100
70
100
70
100
70
100
70
100
70
130
212
77
815
222
867
220
366
131
830
219
956
312
674
205
962
278
284
150
2.3
1.8
1.8
1.8
3.2
2.3
1.9
1.9
2.3
1.8
2.6
2.1
2.0
1.9
3.7
2.4
162.1
155.4
161.1
156.7
156.1
154.2
159.6
156.1
159.3
156.0
157.3
154.5
159.1
157.1
154.3
154.9
98.8
97.3
98.5
96.4
97.1
95.6
99.1
97.5
98.6
98.2
98.1
97.4
98.9
97.9
98.5
97.8
16
14
15
8
5
13
3
12
11
20
5
14
4
13
8
16
4
14
17
24
M5
2-nBu
94
M6
4,5-(CH2)3- carbazolyl
3,6-Me2-carbazolyl
2,7-Me2-carbazolyl
2,3,6,7-Me4-carbazolyl
2,3;6,7-bis-(CH2)3-carb.
2,7-tBu2-carbazolyl
452
559
104
133
113
189
193
294
87
14
11
19
15
21
16
23
19
23
17
20
16
M7
M8
M9
M10
M12
185
717
789
100
a Polymerization conditions: 0.025 μmol precat., 1.1 equiv. [HMe2NPh][B(C6F5)4], 0.5 μmol Al(n-octyl)3, 0.2 mL toluene, 3.9 mL isohexane, 1 mL
propylene; reactor quenched at 8 psig pressure loss or at a maximum time limit of 30 minutes. b Activity in kgPP (mmolZr h)−1
per 104 monomer units.
.
c Mw/Mn. d Number
are occupied with smaller CH2-groups included in 5-mem- (N-carbazolyl)indenes
starting
from
4/7-haloindenes.
bered rings which further reduce their steric effect. In other Buchwald–Hartwig reaction and electrophilic amination of the
words, the four methyl groups in M9 is already “too much”, Grignard reagents derived from the 4/7-haloindenes afforded
whereas 3,6-Me2 (M7), 2,7-Me2 (M8), and 2,3,6,7-bis(CH2)3 4/7-aminoindenes, which were converted to the various 4/7-(N-
(M10) is just enough. The latter two modifications of the car- pyrrolyl)indenes, 4/7-(N-indolyl)indenes and 4/7-(N-carbazolyl)
bazolyl also gave catalysts with better stereoselectivities at indenes. Some of the sterically hindered 4/7-(N-carbazolyl)
100 °C than MII, the same (M8) or even slightly higher (M10) indenes were unavailable by other methods. The developed
Tm and 1.3–1.4-fold higher Mw of iPP produced.
pathway allowed for excellent divergence of the synthetic
It was shown previously that the 2-ethyl homologue of I dis- sequence. In the synthesis of rac-zirconocenes, we extensively
played better regioselectivity than the latter.14 In experiments applied anion-promoted meso-to-rac isomerization of ZrMe2-
with MAO activation, we did not see any improvements in complexes, which use was previously described only once by
regioselectivity of the two zirconocenes containing larger ourselves. A total of 22 novel zirconocenes were prepared (for
primary 2-alkyls in indenyls (2-iBu in C2 and 2-Me/2′-nBu in six of which we determined the crystal structures by X-ray diffr-
C4) over MII. However, M5, the 2-n-butyl-subtitued homologue action analysis) and 21 of them were tested in propylene poly-
of MII, upon activation with AB gave iPP with notably less merizations at 70 and 100 °C in comparison with the reference
regioerrors than the reference catalyst. Together with just catalyst, rac-Me2Si(2-Me-4-(N-carbazolyl)-Ind)2ZrMe2 (MII). In
slightly lower stereoselectivity, this resulted in 1 °C lower Tm of some cases, we managed to achieve better molecular weight
iPP produced at 70 °C and 1 °C higher at 100 °C. At both capability at 100 °C versus the reference at the same level of
polymerization temperatures, the Mw of iPP was about the stereo- and regioselectivity (zirconocenes bearing alkyl-substi-
same for M5 and MII. Noteworthy in 2-Me/2-Et pairs of tuted carbazolyl moieties), or at the expense of regioselectivity
Spaleck complexes, the 2-ethyl-substituted ones produced poly- (5-MeO-6-tBu-indenyl substitution).
mers with lower MW, with the difference being more pro-
nounced at higher temperature.11
Experimental part
Starting compounds 7-bromo-2-methyl-1H-indene (1),50 3,6-
Conclusions
dimethyl-9H-carbazole,51 3,6-di-tert-butyl-9H-carbazole,52 8,9-
In this work, we synthesized a series of analogues of promising dihydro-4H-benzo[def]carbazole,53
4-bromo-2-isopropyl-1H-
zirconocene catalysts for high-temperature propylene polymer- indene (2),54 7-bromo-2-methyl-5-tert-butyl-1H-indene (5),55
ization, rac-Me2Si(2-Me-4-(N-carbazolyl)Ind)2ZrX2 (X = Cl, Me), 7-bromo-4-methoxy-2-methyl-1H-indene (6),56 4-bromo-2,5-
with various structural modifications on the indenyl and car- dimethyl-1H-indene (16),50 7-bromo-5-tert-butyl-6-methoxy-2-
bazolyl moieties and evaluated their effects on the catalytic methyl-1H-indene
performance using activation with MAO or [HMe2NPh]- benzene,58 1-bromo-2-(phenylethynyl)benzene,59
[B(C6F5)4]. We applied a novel approach in the synthesis of 4/7- bromo-4,4′-di-tert-butyl-1,1′-biphenyl,60 were synthesized as
(21),57
1-bromo-2-(methylethynyl)-
2,2′-di-
6178 | Dalton Trans., 2021, 50, 6170–6180
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previously described. Syntheses of 4-bromo-2-butyl-1H-indene
(3), 4-bromo-2-isobutyl-1H-indene (4) and 4-bromo-2,2,6-tri-
methyl-1,2,3,5-tetrahydro-s-indacene (17) was accomplished
according to the previously published procedure50 for syntheses
of various 4-bromoindenes (see ESI†). Syntheses of 2,2′-dibromo-
Notes and references
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4,4′-dimethylbiphenyl,
2,2′-di-bromo-4,4′,5,5′-tetramethyl-
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Propylene homopolymerizations were performed in a parallel
pressure reactor setup with 48 reaction cells (PPR48), fully con-
tained in glovebox under nitrogen. The method was generally
described previously.61 A pre-weighed glass vial (5.0 mL
working volume) insert and disposable stirring paddle were
fitted to each reaction vessel of the reactor. For experiments
with activation by MAO, toluene, a solution of MAO in toluene
(500 equiv.), and liquid propylene (1.0 mL) were added via
syringe. The reactor was then heated to process temperature
(70 °C or 100 °C) while stirring at 800 RPM. A precatalyst solu-
tion (in toluene) was then added via syringe to the reactor at
process conditions. The total amount of solvent added to the
reactor, including that used for MAO and precatalysts solutions
was 4.1 ml. For experiments with activation by AB, isohexane
(3.8 mL,) and liquid propylene (1.0 mL) were added via
syringe, and the reactor was then heated to process tempera-
ture (70 °C or 100 °C) while stirring at 800 RPM. Solutions of
scavenger ((n-octyl)3Al in isohexane), AB (1.1. equiv., in
toluene), and, finally, precatalyst (in toluene) were then added
via syringe to the reactor at process conditions. The total
amount of toluene added to the reactor from precatalyst and
AB solutions was 0.2 ml. Reactor temperature was monitored
and typically maintained within 1 °C. Polymerizations were
halted by addition of approximately 50 psi O2/Ar (5 mol% O2)
gas mixture to the reactors for approximately 30 seconds. The
polymerizations were quenched based on a predetermined
pressure loss of approximately 8 psi or for a maximum of
30 minutes polymerization time. Further work-up and analysis
of the polymer samples are detailed in the ESI.†
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Conflicts of interest
There are no conflicts to declare.
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P. S. Kulyabin, G. P. Goryunov, D. V. Uborsky,
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Acknowledgements
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Crystallography Center of the Institute of Organoelement
Compounds of the Russian Academy of Sciences for help with
the X-ray measurements and refinements.
ation of water or methanol from the respective 1-indanols
or 1-methoxyindanes (see ESI†), which often gives a
mixture of double bond position isomers, 4/7-substituted
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