1H and13C NMR studies of cationic intermediates formed upon activation of "oscillating" catalyst (2-PhInd) 2ZrCl2 with MAO, MMAO, and AlMe3/[CPh 3]+[B(C6F5)

Article abstract of DOI:10.1021/om0610694

The conformationally dynamic unbridged metallocene (2-PhInd) ₂ZrCl₂ (1-C1) was activated with methylaluminoxane (MAO), modified methylaluminoxane (MMAO), and AlMe₃/[CPh₃] ⁺[B(C₆F₅

Full text of DOI:10.1021/om0610694

1536
Organometallics 2007, 26, 1536-1540
13
1H and C NMR Studies of Cationic Intermediates Formed upon
Activation of “Oscillating” Catalyst (2-PhInd)2ZrCl2 with MAO,
MMAO, and AlMe3/[CPh3]+[B(C6F5)4]-
Oleg Y. Lyakin,† Konstantin P. Bryliakov,† Nina V. Semikolenova,† Artem Y. Lebedev,‡
Alexander Z. Voskoboynikov,‡ Vladimir A. Zakharov,† and Evgenii P. Talsi*,†
G. K. BoreskoV Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, 630090,
NoVosibirsk, Russian Federation, and Moscow State UniVersity, LomonosoVsky Prospect 1, 113000,
Moscow, Russian Federation
ReceiVed NoVember 22, 2006
The conformationally dynamic unbridged metallocene (2-PhInd)2ZrCl2 (1-Cl) was activated with
methylaluminoxane (MAO), modified methylaluminoxane (MMAO), and AlMe3/[CPh3]+[B(C6F5)4]-. The
following ion pairs were characterized by 1H and 13C NMR: [(2-PhInd)2Zr(µ-Me)2AlMe2]+[Me-MAO]-
(III), [(2-PhInd)2Zr(µ-Me)2AlMe2]+[B(C6F5)4]- (III′), [(2-PhInd)2Zr(µ-Me)2AlMeiBu]+[Me-MAO]-
(IIIMeiBu), [(2-PhInd)2Zr(µ-Me)2AliBu2]+[Me-MAO]- (IIIiBuiBu), and [(2-PhInd)2ZrMe+‚‚‚Me-MAO-] (IV).
In the temperature range -50 to 20 °C, the rotation of indenyl ligands of complexes III, III′, and IIIMeiBu
is faster than the evaluated rate of propylene insertion, whereas for IIIiBuiBu the rate of indenyl ligand
rotation is comparable to or slower than the rate of propylene insertion. The ion pair IIIiBuiBu demonstrates
the fundamental possibility of the existence of intermediates with the “locked” conformation of 2-PhInd
ligands in 1-Cl/MAO or 1-Cl/MMAO systems. For the catalytic system 1-Cl/MAO, both outer sphere
ion pair III and inner sphere ion pair IV are present in the reaction solution at high [Al]MAO/[Zr] ratios.
For the catalytic system 1-Cl/MMAO, the concentration of inner sphere ion pairs of type IV is much
smaller (below the NMR detection limit), and only outer sphere ion pairs IIIMeiBu and IIIiBuiBu are observed.
counterions. For some counterions, their association with [(2-
Ar-indenyl)2ZrP]+ leads to a locked conformation (the ligand
rotation is restricted), leading to long isotactic sequences.
To provide more evidence for the accessible isomers and their
conformational dynamics, direct observation of the active species
would be highly informative. However, detection of the
propagating species [(2-Ar-indenyl)2ZrP]+ (P ) polymeryl) is
complicated by rapid chain transfer and â-hydride elimination.5
Nevertheless, one can expect that the important information on
the dynamics of cationic intermediates could be derived from
the study of the conformational behavior of the initiating active
species of the type [(2-Ar-indenyl)2ZrMe]+. Recently, the
conformationally dynamic unbridged metallocene bis(2-phe-
nylindenyl)dimethylzirconium (2-PhInd)2ZrMe2 (1-Me) has been
activated with trispentafluorophenylborane (B(C6F5)3) or trityl
tetrakispentafluorophenylborate ([Ph3C]+[B(C6F5)4]-) to yield
ion pairs [(2-PhInd)2ZrMe]+[MeB(C6F5)3]- (2a) and [(2-
PhInd)2ZrMe]+[B(C6F5)4]- (2b), respectively. NMR analysis of
metallocene cations of 2a and 2b provides no evidence that the
association of the anions with the (2-PhInd)2ZrMe+ cations leads
to locked conformations at temperatures above -40 °C.5
However, in the case of MAO as activator, ion pairs different
from 2a and 2b are expected: heterobinuclear ion pair [(2-
PhInd)2Zr(µ-Me)2AlMe2]+[Me-MAO]- (III) and contact ion
pair [(2-PhInd)2ZrMe+‚‚‚Me-MAO-] (IV).6-9 It is important
Introduction
The so-called “oscillating” catalysts (bis(2-Ar-indenyl)zir-
conocenes), which produce elastomeric polypropylenes, are an
intriguing class of olefin polymerization catalysts.1-3 Apparently,
formation of stereoblock elastomeric plastics is a result of the
dynamic character of the active species, the cation of general
formula [(2-Ar-indenyl)2ZrP]+ (Ar ) aryl; P ) polymeryl).1-4
According to the hypothesis of Waymouth, the active species
“oscillates” between a “rac-like” (isotactic-selective) and a
“meso-like” (nonstereoselective) conformation at a frequency
intermediate between those of monomer insertion and chain
transfer.1,2,5 Alternatively, Busico has proposed that the achiral
meso-isomer can be disregarded as a source of the atactic
stereosequences, and the generation of isotactic and atactic
stereosequences is a result of the interconversion of chiral rac-
isomers at a rate competitive with propylene insertion.4 For one
of the rac-isomers, the rate of indenyl ring rotation is faster
than propylene insertion (atactic block is formed); for another
rac-isomer, this rotation is slower than propylene insertion
(isotactic block is formed). The key point of this hypothesis is
the assumption that isomers differ by the nature of the
* Corresponding author. Fax: +7 383 3308056. E-mail: talsi@catalysis.ru.
† G. K. Boreskov Institute of Catalysis.
‡ Moscow State University.
(1) Coates, G. W.; Waymouth, R. M. Science 1995, 267, 217.
(2) Lin, S.; Waymouth, R. M. Acc. Chem. Res. 2002, 35, 765.
(3) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000,
100, 1253.
(4) Busico, V.; Castelli, V. V. A.; Aprea, P.; Cipullo, R.; Segre, A.;
Talarico, G.; Vacatello, M. J. Am. Chem. Soc. 2003, 125, 5451.
(5) Lincoln, A. L.; Wilmes, G. M.; Waymouth, R. M. Organometallics
2005, 24, 5828.
(6) Tritto, I.; Donetti, R.; Sacchi, M. C.; Locatelli, P.; Zannoni, G.
Macromolecules 1997, 30, 1247.
(7) Babushkin, D. E.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E.
P. Macromol. Chem. Phys. 2000, 201, 558.
(8) Bryliakov, K. P.; Semikolenova, N. V.; Yudaev, D. V.; Zakharov,
V. A.; Brintzinger, H.-H.; Ystenes, M.; Rytter, E.; Talsi, E. P. J. Organomet.
Chem. 2003, 683, 92.
10.1021/om0610694 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/17/2007

ActiVation of “Oscillating” Catalyst (2-PhInd)2ZrCl2
Organometallics, Vol. 26, No. 6, 2007 1537
bis[2-(3,5-di-tert-butylphenyl)indenyl]dimethylzirconium, in-
denyl ligands rotate at a rate of 94 700 s-1 at -78 °C.11
The addition of a relatively small amount of MAO ([Al]MAO
/
[Zr] ) 10) to the solution of 1-Cl in toluene leads mainly to
the monomethylation of the starting complex with the formation
of complex (2-PhInd)2ZrClMe (1-ClMe). At 20 °C this complex
exhibits two signals for the Cp protons at δ 6.19 (d, 2H, JHH
)
1.7 Hz) and 6.11 (d, 2H, JHH ) 1.7 Hz) due to nonequivalence
of η5-ligands, and a signal at δ -0.57 (s, 3H) for Zr-CH3 protons
(Figure 1b, Table 1).12 1-ClMe was prepared quantitatively in
situ upon interaction of 1-Cl with a 10-fold excess of AlMe3.
In the temperature range -50 to 50 °C, the positions of two Cp
signals of the 1-ClMe only slightly change. In the case of the
locked conformation of ligands, four peaks for the Cp protons
of 1-ClMe should be observed. Thus, rapid rotation of indenyl
ligands takes place in complex 1-ClMe in the temperature range
considered.
Noteworthy, besides the peaks of 1-ClMe, the 1H NMR
spectrum of Figure 1b exhibits weaker peaks of complex II at
δ 6.29 (d, 2H, JHH ) 1.6 Hz) and 5.94 (d, 2H, JHH ) 1.6 Hz)
for the Cp protons and a signal at δ -0.53 (s, 3H) for Zr-CH3
protons. II was not observed upon interaction of 1-Cl with
AlMe3. Interaction of zirconocenes (L2ZrMe2) with MAO at
low [Al]MAO/[Zr] ratios is known to give homobinuclear ion
pairs [L2ZrMe(µ-Me)L2ZrMe]+[Me-MAO]-.6,7 For the catalytic
system Cp2TiCl2/MAO at low [Al]MAO/[Ti] ratios, homobi-
nuclear ion pairs of the type [Cp2TiMe(µ-Cl)Cp2TiCl]+[Me-
MAO]- (II1), [Cp2TiMe(µ-Cl)Cp2TiMe]+[Me-MAO]- (II2), and
[Cp2TiMe(µ-Me)Cp2TiMe]+[Me-MAO]- (II3) can be observed.9
On the basis of relative integral intensities of Cp and Zr-CH3
peaks, II could be assigned to the ion pair [(2-PhInd)2ZrMe-
(µ-Cl)(2-PhInd)2ZrMe]+[Me-MAO]-. Further studies are needed
to verify this assumption.
Figure 1. 1H NMR spectra of 1-Cl (a) and 1-Cl/MAO samples
(b, c): Al:Zr ) 10 (b), Al:Zr ) 100 (c) ([1-Cl] ) 0.02 M (a),
[1-Cl] ) 0.04 M (b), [1-Cl] ) 8 × 10-3 M (c), toluene-d8, 20 °C).
In conditions approaching those of real polymerization
([Al]MAO/[Zr] > 100), heterobinuclear ion pair [(2-PhInd)2Zr-
(µ-Me)2AlMe2]+[Me-MAO]- (III) and contact ion pair [(2-
PhInd)2ZrMe+‚‚‚Me-MAO-] (IV) became the predominant
species in solution (Figures 1c and 2). The proportion of III
increases with the increase of the [Al]MAO/[Zr] ratio, and at
[Al]MAO/[Zr] ≈ 500, complex III prevailed in the reaction
Figure 2. 1H NMR spectra of the 1-Cl/MAO sample at 20 °C (a)
and 50 °C (b) ([1-Cl] ) 8 × 10-3 M, Al:Zr )100, toluene-d8).
to study the conformational behavior of III and IV. It was shown
previously that 1-Me and (2-PhInd)2ZrCl2 (1-Cl), when activated
with modified MAO (MMAO) containing isobutylaluminum
groups, yielded higher tacticity polypropylenes than 1-Me and
1-Cl activated by MAO (containing only methylaluminum
groups).10 It is interesting to compare the conformational
behavior of cationic intermediates formed upon activation of
1-Cl with MAO and MMAO. In this work, we have undertaken
1
solution. The H NMR spectrum of III (toluene-d8, 20 °C)
displays a singlet (4H) at δ 6.39 for the Cp protons and singlets
at δ -0.58 (6H) and -0.97 (6H) for Al-Me and Zr-Me-Al
protons, respectively (Figure 1c, Table 1). The 13C NMR
spectrum of III exhibits a sharp peak of the Zr-Me-Al carbon
at δ 41.20 (J1CH ) 113 Hz) (see Supporting Information). The
1H NMR spectrum of III is similar to that of [(2-PhInd)2Zr(µ-
Me)2AlMe2]+[B(C6F5)4]- (III′) prepared by interaction of 1-Cl
with AlMe3/[Ph3C]+[B(C6F5)4]- ([Zr]:[Al]:[B] ) 1:50:1.2) in
toluene-d8 at 20 °C (Table 1). In the temperature range -50 to
50 °C, no decoalescence of the resonances of III and III′ Cp
protons was observed. We conclude that indenyl ligands of III
and III′ rapidly rotate in the temperature range -50 to 50 °C.
In contrast to the outer sphere ion pair III exhibiting relatively
1
a H and 13C NMR study of ion pairs formed in the catalytic
systems 1-Cl/MAO, 1-Cl/MMAO, and 1-Cl/AlMe3/[CPh3]+-
[B(C6F5)4]- in order to evaluate the possibility of the formation
of intermediates with a locked conformation of the 2-PhInd
ligands.
Results and Discussion
Catalytic Systems 1-Cl/MAO and 1-Cl/AlMe3/[Ph3C]+-
[B(C6F5)4]-. The 1H NMR spectrum of 1-Cl exhibits a singlet
(4H) at δ ∼6.4 ppm for the cyclopentadienyl (Cp) protons in
the temperature range -50 to 50 °C (Figure 1a). This observa-
tion is consistent with the known rapid rotation of the indenyl
ligands in this temperature range. Even for the bulkier precatalyst
1
sharp and well-observed NMR signals, the H and 13C NMR
resonances of the contact ion pair IV are broadened due to direct
interaction of (2-PhInd)2ZrMe+ with nonuniform [Me-MAO]-
counterions (Figure 2). At 20 °C, IV exhibits broadened signals
from Cp protons at about δ 6.1 and 5.9 and broadened signals
from Zr-CH3 protons at about δ -1.4 and -1.9 (Figure 2a).
(9) Bryliakov, K. P.; Talsi, E. P.; Bochmann, M. Organometallics 2004,
23, 149.
(10) Wilmes, G. M.; Polse, J. L.; Waymouth, R. M. Macromolecules
2002, 35, 6766.
(11) Wilmes, G. M.; France, M. B.; Lynch, S. R.; Waymouth, R. M.
Organometallics 2004, 23, 2405.
(12) Table 1 incorporates also the 1H NMR data for ion pairs formed in
the (SBI)ZrCl2/MAO system (SBI ) rac-Me2Si(Ind)2) for comparison.

1538 Organometallics, Vol. 26, No. 6, 2007
Lyakin et al.
1
Table 1. Selected H NMR Chemical Shifts (ppm, 20 °C, toluene-d8) for Complexes Formed upon Activation of (2-PhInd)2ZrCl2
and rac-Me2Si(Ind)2ZrCl2 with MAO and AlMe3/[CPh3]+[B(C6F5)4]-
no.
species
Cp
Zr-Me
µ-Me
Al-Me
1
2
3
4
5
6
7
8
9
(2-PhInd)2ZrCl2 (1-Cl)
(2-PhInd)2ZrClMe (1-ClMe)
6.37
6.19a, 6.11a
6.29b, 5.94b
6.39
-0.57
[(2-PhInd)2ZrMe(µ-Cl)(2-PhInd)2ZrMe]+[Me-MAO]- (II)
[(2-PhInd)2Zr(µ-Me)2AlMe2]+[Me-MAO]- (III)
[(2-PhInd)2Zr(µ-Me)2AlMe2]+[B(C6F5)4]- (III′)
[(2-PhInd)2ZrMe+‚‚‚Me-MAO-] (IV)d
-0.53
-0.97c
-1.01
-0.58
-0.67
6.45
6.1, 5.9
-1.4e, -1.9e
-1.6j
[rac-Me2Si(Ind)2Zr(µ-Me)2AlMe2]+[Me-MAO]-
[rac-Me2Si(Ind)2Zr(µ-Me)2AlMe2]+[B(C6F5)4]-
[rac-Me2Si(Ind)2ZrMe+‚‚‚Me-MAO-]d
6.27f, 5.14f
6.10g, 5.00g
5.8h, 5.2i
-1.28
-1.39
-0.62
-0.72
b
c
d
e
a Doublet with JHH ) 1.7 Hz. Doublet with JHH ) 1.6 Hz. Respective 13C peak of µ-Me carbons is at 41.20 (q, J1CH ) 113 Hz). Broad signals. ∆ν1/2
f
g
h
i
j
≈ 30 Hz. Doublet with JHH ) 2.8 Hz. Doublet with JHH ) 3.0 Hz. ∆ν1/2 ≈ 20 Hz. ∆ν1/2 ≈ 45 Hz. ∆ν1/2 ≈ 60 Hz.
1
Table 2. Selected H NMR Chemical Shifts, δ, for Complexes Formed upon Activation of (2-PhInd)2ZrCl2 with MMAO in
Toluene-d8 and rac-Me2Si(Ind)2ZrCl2 with MAO/AliBu3 in Benzene-d6
no.
1
species
T, °C
Cp
µ-Me
Al-Me
[(2-PhInd)2Zr(µ-Me)2AlMeiBu]+[Me-MAO]- (IIIMeiBu
)
-10
0
10
20
-10
0
10
20
20
6.43, 6.31
6.45, 6.33
6.47, 6.35
6.49, 6.37
6.38, 6.34a
6.40, 6.37b
6.40c
-0.95, -1.08
-0.92, -1.05
-0.90, -1.03
-0.88, -1.00
-1.00
-0.53
-0.53
-0.54
-0.54
2
[(2-PhInd)2Zr(µ-Me)2AliBu2]+[Me-MAO]- (IIIiBuiBu
)
-0.98
-0.95
6.42d
6.35, 6.18
5.15, 5.05
6.33
-0.93
3
4
[rac-Me2Si(Ind)2Zr(µ-Me)2AlMeiBu]+[Me-MAO]-e
[rac-Me2Si(Ind)2Zr(µ-Me)2AliBu2]+[Me-MAO]-e
-1.23, -1.37
-0.55
20
-1.28
5.17
b
c
d
e
a Distance between peaks is about 9.0 Hz. Distance between peaks is about 6.5 Hz. ∆ν1/2 ≈ 8 Hz. ∆ν1/2 ≈ 7 Hz. NMR data from ref 16.
Such a complex picture shows that species IV with various [Me-
MAO]- counterions displays resonances with different chemical
shifts and the exchange of various [Me-MAO]- counterions in
IV is slow on the NMR time scale. Several distinct types of
[Me-MAO]- anions in contact with a zirconocenium cation were
recently observed by 13C NMR in the catalytic system (MeC5H4)2-
ZrMe2/MAO.13 With the increase of temperature up to 50 °C,
the signal of Cp protons of IV at δ 6.1 ppm noticeably increases
at the expense of that at δ 5.9 (Figure 2b). It is tempting to
ascribe this effect to increased mobility of indenyl ligands with
increasing temperature. However, other explanations are pos-
sible. One of them is the change of the nature of [Me-MAO]-
counterions. It was shown previously that the size of oligomeric
molecules of MAO reversibly decreases with increasing tem-
perature.14 Thus, for IV (as well as for III) there is no evidence
in favor of the locked conformation of indenyl ligands in the
temperature range -50 to 50 °C.
shown that with increasing [AliBu3]/Zr ratio from 14 to 100
([Al]MAO/[Zr] ≈ 600) first ion pairs [(SBI)Zr(µ-Me)2AlMe2]+-
[Me-MAO]- and [(SBI)Zr(µ-Me)2AlMeiBu]+[Me-MAO]- then
ion pairs [(SBI)Zr(µ-Me)2AlMeiBu]+[Me-MAO]- and [(SBI)-
Zr(µ-Me)2AliBu2]+[Me-MAO]- became the predominant spe-
1
cies in solution (Table 2).16 They have also shown that the H
NMR signals assigned in ref 15 to ion pair [(SBI)Zr(µ-
Me)2AlMeiBu]+[Me-MAO]- actually belong to ion pair [(SBI)-
Zr(µ-Me)2AliBu2]+[Me-MAO]-. These results helped us to
1
assign the H NMR peaks of similar heterobinuclear ion pairs
in the 1-Cl/MMAO system.
It was found that at relatively low [Al]MMAO/Zr ratios of 30-
70 ([Zr] ) 8 × 10-3 M), mainly monomethylated complex
1-ClMe is present in the reaction solution. With the increase
of the [Al]MMAO/Zr ratio, the concentration of mixed heterobi-
nuclear ion pairs [(2-PhInd)2Zr(µ-Me)2AlMeiBu]+[Me-MAO]-
(IIIMeiBu) and [(2-PhInd)2Zr(µ-Me)2AliBu2]+[Me-MAO]- (III-
1
iBuiBu) increases.17 Figure 3 shows the H NMR spectra of the
As mentioned above, the replacement of MAO by MMAO
as activator results in the increase of the tacticity of polypro-
pylenes produced with 1-Me and 1-Cl catalysts.10 Thus, it is
reasonable to search the intermediates with locked conformation
of 2-PhInd ligands in 1-Cl/MMAO system.
catalytic system 1-Cl/MMAO in toluene-d8 at various temper-
atures ([Zr] ) 6 × 10-3 M, [Al]MMAO/[Zr] ) 200). Resonances
of Cp and Zr-Me-Al protons of IIIMeiBu and IIIiBuiBu are marked
in Figure 3. Their spectral parameters are collected in Table 2
in comparison with those for similar species in the (SBI)ZrCl2/
Catalytic System 1-Cl/MMAO. Recently, on the basis of
the analysis of the 1H NMR spectra of the (SBI)ZrCl2/MMAO
system (SBI ) rac-Me2Si(Ind)2), we have assumed that when
(SBI)ZrCl2 is reacted with MMAO, the mixed heterobinuclear
cation [(SBI)Zr(µ-Me)2AlMeiBu]+ is formed, together with its
methyl congener [(SBI)Zr(µ-Me)2AlMe2]+.15 Then, on the basis
1
MAO/AliBu3 system.16 Besides the H NMR resonances of
1
IIIMeiBu and IIIiBuiBu, the H NMR spectra of Figure 3 show
two broad and intense peaks in the Cp region that could be
assigned to neutral complex (2-PhInd)2ZrClR, where R is
1
of the detailed H and 13C NMR study of (SBI)ZrCl2/MAO/
(15) Bryliakov, K. P.; Semikolenova, N. V.; Panchenko, V. N.; Zakharov,
V. A.; Brintzinger, H.-H.; Talsi, E. P. Macromol. Chem. Phys. 2006, 207,
327.
AliBu3 system in benzene-d6, Babushkin and Brintzinger have
(16) Babushkin, D. E.; Brintzinger H.-H. Chem.-Eur. J. 2007, accepted.
(17) For simplicity, we use the same notation of the counter anion [Me-
MAO]- for MAO- and MMAO-based systems. Apparently, for these two
systems, the counter anions are different. The counter anion incorporates
only Me groups in the case of MAO and both Me and Bu groups in the
case of MMAO.
(13) Babushkin, D. E.; Naundorf, C.; Brintzinger, H.-H. Dalton Trans.
2006, 4539.
(14) Babushkin, D. E.; Semikolenova, N. V.; Panchenko, V. N.; Sobolev,
A. P.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 1997, 198,
3845.
i

ActiVation of “Oscillating” Catalyst (2-PhInd)2ZrCl2
Organometallics, Vol. 26, No. 6, 2007 1539
Figure 4. Proposed structures of IIIMeiBu and IIIiBuiBu
.
Figure 3. 1H NMR spectra of the 1-Cl/MMAO sample at differing
temperatures: -10 °C (a), 0 °C (b), +10 °C (c), +20 °C (d) ([1-
Cl] ) 6 × 10-3 M, Al:Zr ) 200, toluene-d8).
iBu or Me. The ligand R of (2-PhInd)2ZrClR exchanges rapidly
(on the NMR time scale) at room temperature. The relative
concentration of (2-PhInd)2ZrClR decreased with an increase
of the [Al]MMAO/[Zr] ratio up to 400.
It is very important that IIIiBuiBu exhibits two peaks from Cp
protons at low temperatures (Figures 3a, b) that coalesce at
higher temperatures (Figures 3c,d, Table 2). The distance
between unperturbed Cp peaks of IIIiBuiBu at low temperature
is about 10 Hz. To fulfill the conditions of rapid exchange, the
characteristic time of indenyl ligand rotation should be about
10-2 s at 20 °C. This time is comparable with the characteristic
time of propylene insertion (10-2 to 10-3 s).11 Thus, IIIiBuiBu is
the first example of an intermediate with restricted rotation of
indenyl ligands at room temperature found for the “oscillating”
catalyst.18 For less sterically hindered intermediate IIIMeiBu, no
decoalescence of Cp peaks at low temperature was observed.
For this intermediate, the rotation of indenyl ligands remains
fast even at -30 °C (Figures 3 and 4).
The coalescence of Cp peaks of IIIiBuiBu is reversible: when
the sample of Figure 3d was cooled again to -10 °C, the
observed spectrum closely resembled that of Figure 3a. The only
difference was the slightly lower intensity of the 1H NMR peaks.
This is caused by the decrease of the concentration of Zr(IV)
species due to partial reduction of Zr(IV) to Zr(III). In Figure
5 one can see the EPR spectrum recorded 2 h after onset of the
reaction of 1-Cl with MMAO at room temperature ([Zr] ) 6 ×
10-3 M, [Al]MMAO/[Zr] ) 200). The major peak at g ) 1.968
and the minor one at g ) 1.983 are in the typical range reported
for zirconium(III) species.19,20 The integration shows that up to
40% of the starting Zr(IV) complex is converted into Zr(III)
Figure 5. EPR spectrum of the 1-Cl/MMAO sample recorded 2 h
after mixing of reagents at 20 °C ([1-Cl] ) 6 × 10-3 M, Al:Zr )
200, toluene-d8).
species upon interaction with MMAO for 2 h at room temper-
ature. The nature of these species is still unclear.
Tight ion pairs of the type IV were not observed in the
catalytic system 1-Cl/MMAO. Similarly, species of the type
IV did not arise in the catalytic system (SBI)ZrCl2/MMAO.16
Thus, concentration of tight ion pairs is much lower for the
1-Cl/MMAO system than for the 1-Cl/MAO analogue. An EPR-
spin probe study has shown that the oligomeric molecules of
MMAO are bulkier than oligomeric molecules of MAO.15 The
bulkiness of MMAO could prevent the tight contact of the [Me-
MAO]- counterion with zirconium and thus disfavor formation
of ion pairs of the type IV.
Our polymerization study confirms the higher tacticity of
polypropylene produced by the 1-Cl/MMAO system than by
the 1-Cl/MAO analogue, previously reported by Waymouth and
co-workers.10 The polymers produced by the 1-Cl/MMAO
system display higher isotacticity ([mmmm] ) 30% by 13C
NMR) than those produced by the 1-Cl/MAO system ([mmmm]
) 18%) (Table 3).
Mechanism of the “Oscillating Catalysts” Performance.
As mentioned, rotation of indenyl ligands in the active
intermediate should be slow as compared to the rate of propylene
insertion for the formation of long isotactic stereosequences.
In this work we have shown that restricted indenyl ligand
rotation could be observed for the mixed heterobinuclear ion
pair IIIiBuiBu. The proposed active intermediate of the 1-Cl/
MAO system seems to be the ion pair [(2-PhInd)2ZrP]+[Me-
MAO]- (V) with the vacant coordination site of zirconium
(18) Very recently, the restricted rotation of ligands at 20 °C was reported
for unbridged amine-functionalized zirconocene [1-(p-Me2NC6H4)-3,4-
Me2C5H2]2ZrMe2 activated with MAO. Kim, S. K.; Kim, H. K.; Lee, M.
H.; Yoon, S. W.; Do, Y. Angew. Chem., Int. Ed. 2006, 45, 6163.
(19) Fryzuk, M. D.; Jafarpour, L.; Rettig, S. J. Organometallics 1999,
18, 4050.
(20) Williams, G. M.; Schwartz, J. J. Am. Chem. Soc. 1982, 104, 1122.

1540 Organometallics, Vol. 26, No. 6, 2007
Lyakin et al.
a
Table 3. Propylene Polymerizations with (2-PhInd)2ZrCl2
from Aldrich. Methylaluminoxane (MAO) was obtained from
Crompton GmbH (Bergkamen) as a toluene solution (total Al
concentration 1.8 M, Al as AlMe3 0.5 M). MMAO (MAO, modified
with TIBA) was purchased from AKZO as a heptane solution (total
Al content 7.1 wt %).
entry
activator
PP yield, g
productivityb
[mmmm]c
1
2
MAO
MMAO
3.1
2.7
25.8
22.5
18
30
a Polymerization conditions: [Zr] ) 13.5 µmol/L, 150 mL of toluene,
P(C3H6) ) 6 bar, polymerization temperature 30 °C, time 10 min,
1H and 13C NMR spectra were recorded at 250.130 and 62.89
MHz, on a Bruker DPX-250 MHz NMR spectrometer. Typical
AlMAO:Zr ) 2500. In kg of PP/(mol of Zr bar min). Determined by 13
C
b
c
1
operating conditions for H NMR measurements: spectral width
NMR.
5 kHz; spectrum accumulation frequency 0.5-0.2 Hz; number
of transients 32-64; ca. 30° pulse at 2 µs. Typical operating
conditions for 13C NMR measurements: spectral width 20 kHz;
spectrum accumulation frequency 0.2-0.1 Hz; 100-2000 tran-
sients; 45° pulse at 5 µs. 13C,1H-correlations were established by
using standard Bruker HXCOBI pulse programs. For calculations
of 1H and 13C chemical shifts, the resonances of the methyl group
of the toluene-d8 solvent were taken as 2.09 and 20.40 ppm,
respectively.
EPR spectra were measured on a Bruker ER-200D spectrometer
at 9.3 GHz, modulation frequency 100 kHz, modulation amplitude
1 G. Measurements were performed in glass tubes (d ) 5 mm). A
periclase crystal (MgO) with impurities of Mn2+ and Cr3+, which
served as a side reference, was placed into the second compartment
of the dual cavity. EPR spectra were quantified by double
integration with TEMPO toluene solution as standard.
Preparation of MAO Samples. MAO samples used for NMR
studies were prepared from commercial MAO by removal of the
solvent and of most of the trimethylaluminum content in vacuo at
20 °C for 2 h. Thus prepared dry polymeric MAO contained 40 wt
% of total Al and ca. 3 wt % of residual AlMe3. 13C-enriched MAO
(MAO-13C) was prepared by ligand exchange of 99% 13C-enriched
AlMe3 (30 mol % of total Me groups) and solid MAO (70 mol %
of total Me groups) in toluene solution followed by subsequent
removal of volatiles under vacuum at room temperature to give a
sample of MAO-13C (65-70% 13C) with desired AlMe3 content
(polymeric MAO with total Al content of 40% and Al as residual
AlMe3 ca. 5 wt %). A more detailed description is presented in
ref 7.
occupied by AlMe3 or [Me-MAO]-. Waymouth et al. have
obtained practically amorphous polypropylene with the 1-Cl/
MAO system rich in “free” AlMe3.21 Thus, species V with a
vacant coordination site of zirconium occupied by AlMe3 can
be responsible for the atactic stereosequences, and species V
with a vacant coordination site occupied by [Me-MAO]- for
the isotactic stereosequences. This is possible if the indenyl
ligand rotation is fast in the first case and slow in the second
one. Apparently, steric restrictions for the indenyl ligand rotation
are smaller for AlMe3 than for [Me-MAO]-. For the insertion
of propylene, AlMe3 or [Me-MAO]- ligands in V should be
replaced by the monomer molecule. Then, the corresponding
coordination site is again occupied by AlMe3 or [Me-MAO]-
before the next insertion. The switch from atactic to isotactic
stereoblock formation occurs when AlMe3 in V is replaced by
[Me-MAO]-.
The concentration of inner sphere ion pairs IV for the 1-Cl/
MMAO system is lower than for the 1-Cl/MAO counterpart,
whereas the former system produces more isotactic polypro-
pylene than the latter one (Table 3). These data evidence
indirectly against the involvement of species V with inner sphere
coordination of [Me-MAO]- in the formation of the long
isotactic stereosequences by the catalytic system 1-Cl/MMAO.
In the case of MMAO, ion pairs of the type V with vacant
coordination sites occupied by AliBuMe2 and AliBu2Me can be
responsible for the atactic and isotactic stereosequences, re-
spectively. Further studies are needed to verify this model.
Propylene Polymerization Studies. Polymerization was per-
formed in a 1.0 L steel reactor, equipped with magnetic stirrer, a
water jacket for temperature control, and automatic computer-
controlled system for propylene feed.
Conclusions
(2-PhInd)2ZrCl2 (0.0011 g, 2.0 × 10-6 mol) was introduced into
the autoclave in a vacuum-sealed glass ampule. The reactor was
evacuated at 80 °C, cooled to 20 °C, and then charged with 150
mL of toluene solution containing the required amount of the
activator (MAO, MMAO). The reaction mixture was warmed to
30 °C, saturated with propylene to a total pressure of 6 bar. The
reaction was started by breaking the ampule with the complex.
During the reaction time (10 min) propylene pressure, stirring speed,
and temperature were kept constant. After 10 min the reactor was
vented and the polymer was isolated by filtration and dried to
constant weight at ambient temperature.
The study of conformational behavior of ion pairs formed
upon activation of “oscillating” catalyst (2-PhInd)2ZrCl2 with
MAO, MMAO, and AlMe3/[Ph3C]+[B(C6F5)4]- has shown that
for the ion pair [(2-PhInd)2Zr(µ-Me)2AliBu2]+[Me-MAO]- the
rate of indenyl ligand rotation at 20 °C is comparable with the
rate of propylene insertion. This demonstrates the fundamental
possibility of the existence of rac-intermediates with the
restricted rotation of 2-arylindenyl ligands for the (2-ArInd)2ZrCl2/
MAO catalysts. The interconversion between rac-intermediates
with locked and unlocked conformations of the indenyl ligands
(the hypothesis of Busico) seems to be a plausible explanation
of the “oscillating catalysts” performance.
Acknowledgment. This work was supported by the Russian
Fund of Basic Research, grant 06-03-32700.
Experimental Section
Supporting Information Available: 13C NMR spectrum of
1-Cl/MAO-13C sample and 13C NMR spectra of polypropylenes
synthesized by 1-Cl/MAO and 1-Cl/MMAO catalytic systems. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
Toluene was dried over molecular sieves (4 Å), purified by
refluxing over sodium metal, and distilled under dry nitrogen.
Toluene-d8 was dried over molecular sieves (4 Å), degassed in
vacuo, and stored under dry argon. All experiments were carried
out in sealed high-vacuum systems using breakseal techniques. (2-
PhInd)2ZrCl2,1 (SBI)ZrCl2,22 and [CPh3]+[B(C6F5)4]- 23 were syn-
thesized as described. Trimethylaluminum (AlMe3) was purchased
OM0610694
(22) Herrmann, W. A.; Rohrmann, J.; Herdtweck, E.; Spaleck, W.;
Winter, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 1511.
(23) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434,
C1.
(21) Petoff, J. L.; Myers, C. L.; Waymouth, R. M. Organometallics 1999,
32, 7984.