Anion exchange in alkyl-zirconocene borate ion pairs. Are solvated alkyl-zirconocene cations relevant intermediates?

Article abstract of DOI:10.1021/ja003531w

Ion pairs of the type Cp^x₂ZrMe⁺···A ^- containing various ansa-zirconocene methyl cations in contact with Me-B(C₆F₅)₃^- or B(C₆F₅)₄^- anions have been studied with regard to their anion exchange kinetics by 2D-NMR methods in benzene or toluene solutions. The results - acceleration of anion exchange by added Li⁺···Me-B(C₆F₅) ₃^-, substantial nonproductive exchange between added and Zr-bound Me-B(C₆F₅)₃^- anions, an increase of exchange rates at increased zirconocene concentrations, and the exclusively entropic origin of this rate increase - all indicate that anion exchange occurs by way of ion quadruples or higher ionic aggregates, rather than via dissociation to solvent-separated ions. These findings imply that solvent-separated (i.e. anion-free) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing Cp^x₂ZrMe⁺···A ^- ion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of α-olefins.

Full text of DOI:10.1021/ja003531w

J. Am. Chem. Soc. 2001, 123, 1483-1489
1483
Anion Exchange in Alkyl-Zirconocene Borate Ion Pairs. Are
Solvated Alkyl-Zirconocene Cations Relevant Intermediates?
Stefan Beck, Susanna Lieber, Frank Schaper, Armin Geyer, and
Hans-Herbert Brintzinger*
Contribution from the Fachbereich Chemie, UniVersita¨t Konstanz, D-78457 Konstanz, Germany
ReceiVed September 28, 2000. ReVised Manuscript ReceiVed NoVember 30, 2000
Abstract: Ion pairs of the type Cpx ZrMe+‚‚‚A- containing various ansa-zirconocene methyl cations in contact
2
-
-
with Me-B(C6F5)3 or B(C6F5)4 anions have been studied with regard to their anion exchange kinetics by
2D-NMR methods in benzene or toluene solutions. The resultssacceleration of anion exchange by added
Li+‚‚‚Me-B(C6F5)3-, substantial nonproductive exchange between added and Zr-bound Me-B(C6F5)3- anions,
an increase of exchange rates at increased zirconocene concentrations, and the exclusively entropic origin of
this rate increasesall indicate that anion exchange occurs by way of ion quadruples or higher ionic aggregates,
rather than via dissociation to solvent-separated ions. These findings imply that solvent-separated (i.e. anion-
free) alkyl zirconocene cations are unlikely to be relevant intermediates in reaction systems containing
Cpx ZrMe+‚‚‚A- ion pairs and, hence, also in zirconocene-based catalyst systems for the polymerization of
2
R-olefins.
Introduction
Several lines of reasoning raise the question, however,
whether this notion is indeed correct: Many reactions, where
neutral ligands such as phosphines, amines, or ethers displace
Alkyl zirconocene cations Cpx ZrR+, where Cpx Zr is a
2
2
substituted and/or bridged zirconocene unit and R is a methyl
or a polymeryl group, are assumed to be the active species in
metallocene-catalyzed olefin polymerizations.1 They are thought
to arise by dissociation of a weakly coordinating anion A- from
an ion pair Cp*2ZrR+‚‚‚A-, which is generated when a strong
Lewis acid such as B(C6F5)3, Ph3C+ B(C6F5)4-, or MAO
interacts with a zirconocene dialkyl complex.2
the anion A- from an ion pair Cpx ZrR+‚‚‚A-, recently studied
2
in our laboratories, have all been found to proceed by associative
rather than by dissociative mechanisms.5 In line with this, recent
DFT calculations by Ziegler and co-workers place solvated
Cpx ZrR+ and A- ions at energies which are ca. 160 kJ/mol
2
above those of the contact ion pairs Cpx ZrR+‚‚‚A- in toluene
2
solution,6,7 i.e., substantially higher than the activation energies
of 80-100 kJ/mol reported for anion side exchange reactions.4
These considerations have lead us to reinvestigate the reaction
paths for the dynamic symmetrization of these ion pairs. Here
we report results of kinetic studies by 2D-NMR methods on
anion exchange dynamics in a series of ansa-zirconocene contact
ion pairs, generated from the ring-bridged zirconocene dimethyl
complexes Me4C2(C5H4)2ZrMe2 (1), Me2Si(C5H4)2ZrMe2 (2),
rac-Me2Si(Ind)2ZrMe2 (3), rac-Me2Si(2-Me-Ind)2ZrMe2 (4),
rac-Me2Si(2-Me-Benz[e]Ind)2ZrMe2 (5), and rac-Me2Si(2-Me-
4-tBu-C5H4)2ZrMe2 (6), which cast substantial doubt on the
notion that anion-free, solvated alkyl zirconocene cations are
relevant intermediates in these reaction systems.
So far, however, free alkyl zirconocene cations Cpx ZrR+
2
have been observed only by mass spectrometry in the gas phase.3
The notion that Cpx ZrR+ cations occur as intermediates in
2
solutions containing ion pairs of the type Cpx ZrR+‚‚‚A- is
2
based mainly if not exclusively on the postulate that an exchange
of the anion A- from one side of the zirconocene complex to
the other, which causes a dynamic symmetrization of the NMR
spectra of these ion pairs in hydrocarbon solutions, proceeds
by way of a dissociative mechanism.2a,4 Variations in the
activation energies for this anion exchange process have thus
been proposed to measure variations in the strengths of cation-
anion bonds in the alkyl zirconocene ion pairs considered,4 since
the release of the anion from its coordinative contact with the
zirconocene cation would be a prerequisite for such a dissocia-
tive anion side exchange to occur.
Results and Discussion
(1) Review: Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325.
Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255.
1. Exchange Mechanisms in ansa-Zirconocene Ion Pairs.
The zirconocene contact ion pairs 1A-6A were generated in
(2) (a) Review: Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100,
1391. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. Ewen, J. A.; Elder, M. J. Eur. Patent Appl. 0,427,697, 1991;
U.S. Patent 5,561,092, 1996. (c) Chien, J. C. W.; Tsai, W.-M.; Rausch, M.
D. J. Am. Chem. Soc. 1991, 113, 8570. Ewen, J. A.; Elder, M. J. Eur. Patent
Appl. 0,426,637, 1991.
(3) Richardson, D. E.; Alameddin, G. N.; Ryan, M. F.; Hayes, T.; Eyler,
J. R.; Siedle, A. R. J. Am. Chem. Soc. 1996, 118, 11244. Feichtinger, D.;
Plattner, D. A.; Chen, P. J. Am. Chem. Soc. 1998, 120, 7125.
(4) (a) Deck, P. A.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 6128.
Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120,
1772. (b) Siedle, A. R.; Newmark, R. A. J. Organomet. Chem. 1995, 497,
119. Siedle, A. R.; Hanggi, B.; Newmark, R. A.; Mann, K. R.; Wilson, T.
Macromol. Chem. Symp. 1995, 89, 299.
situ by reaction of about 1.1 equiv8 of B(C6F5)3 with the
2b
respective dimethyl complexes 1-6 in C6D6 solutions at
(5) Beck, S.; Prosenc, M. H.; Brintzinger, H. H. J. Mol. Catal. A: Chem.
1998, 128, 41. Schaper, F.; Brintzinger, H. H. Unpublished results.
(6) Kumar Vanka; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Organo-
metallics 2000, 19, 1841.
(7) Somewhat lower energies have been calculated for species in which
a toluene molecule is coordinated to the Zr center of a Cpx ZrMe+ cation
2
(ref 6). To which degree a species of this type might contribute to anion
side exchange would depend, however, on the activation energy and on
the stereospecificity of its formation from and reconversion to the respective
contact ion pair.
10.1021/ja003531w CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/30/2001

1484 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Beck et al.
Table 1. Apparent First-Order Rate Constants kApp (s-1) for Anion
Exchange Reactions of Me-B(C6F5)3--Containing Ion-Pairs
1A-6A,a Determined from EXSY Signals in C6D6 Solution at
300 K
kapp
[LiMeB-
[Zr], (C6F5)3],
ZrMe/
MeBc
Zr-MeB/
Li-MeBd
complexa mmol/L mmol/L ligand sidesb
e
e
e
e
1A
1A
1A
1A
2A
2A
2A
2A
3A
3A
3A
4A
4A
4A
4A
5A
5A
5A
5A
6A
6A
6A
6A
4
11.7
19
4
1.2 ( 0.1
2.9 ( 0.1
3.5 ( 0.1
2.8 ( 0.1
0.8 ( 0.2
1.6 ( 0.2
3.3 ( 0.2
2.0 ( 0.1
0.6 ( 0.1
1.7 ( 0.2
2.4 ( 0.5
-
-
-
-
4
4
7.3 ( 0.3
4
0.7 ( 0.1
1.6 ( 0.2
3.2 ( 0.2
11.7
19
4
0.9 ( 0.1 7.2 ( 0.3
e
4
-
e
e
11.7
19
3
5
10
4
-
-
0.36 ( 0.05 -e
0.44 ( 0.05 -e
e
0.75 ( 0.1
0.7 ( 0.05
0.3 ( 0.05
0.36 ( 0.05
0.42 ( 0.05
0.4 ( 0.05
30 ( 1
-
-
0
0
0
0
0
0
0
e
4
4
4
5.2 ( 0.2
0.8 ( 0.05
65 ( 1
2
3.5
4.8
4
Figure 1. 2D-NOESY spectrum of a solution containing Me2Si(C5H4)2-
ZrMe2 and B(C6F5)3 in a ratio of 1:1.1 at a zirconocene concentration
[Zr] of 19 mmol/L, T ) 303 K and τm ) 0.1 s.
5
10
20
4
40 ( 1.3
52 ( 1.2
Scheme 1
41 ( 2
0
a Generated in situ by reaction of the corresponding zirconocene
dimethyl complex with 1.1 equiv of B(C6F5)3. b For equivalent protons
on either side of the ligand framework. c For the terminal and bridging
-
.
Me groups. d For Zr-bound and “free” MeB(C6F5)3 e Not sufficiently
resolved.
1).2a Exchange signals between terminal and bridging Zr-CH3
groups, however, can arise only from borane exchange. For
complexes 1A, 3A, and 4A, it was not possible to distinguish
between these two processes, since their Zr-Me and Zr‚‚‚µ-Me-B
1H NMR signals were not sufficiently resolved. For complex
2A, however, the rate of borane exchange was easily measurable
and found to be practically equal to the rate with which protons
on either side of the ansa-zirconocene ligand framework are
dynamically symmetrized (Table 1). In this case, anion exchange
does not appear to contribute significantly to the exchange
between ligand protons. For complexes 5A and 6A, on the other
hand, cross signals due to borane exchange were not detectable
at all at 300 K, even at increased mixing times of τm ) 500
ms. In these two cases, anion exchange appears to be the
dominating process. Complexes 5A and 6A show, interestingly,
quite different exchange rates, 5A being the contact ion pair
with the slowest and 6A that with the fastest side exchange.
To corroborate these mechanistic assignments, exchange rates
between pairs of ligand proton sites were investigated, again at
[Zr] ) 10 mmol/L and T ) 300 K, for the B(C6F5)3 adducts
1A-6A at [B]/[Zr] ratios increasing from 1.1:1 to 3:1. The
apparent first-order rate constants kapp thus obtained (Figure 2)
clearly indicate a direct participation of free B(C6F5)3 in the
exchange process for complexes 1A-3A, presumably by attack
of free borane at the terminal Zr-Me group. Ligand side
exchange in complexes 4A-6A, on the other hand, appears to
be entirely unaffected by excess borane. No exchange signals
between terminal and bridging methyl groups are apparent for
these complexes either, even in the presence of a 3-fold borane
excess.
zirconocene concentrations of [Zr] ) 10 mmol/L and T ) 300
1
K. In their H NMR spectra, separate signals are observed for
the previously homotopic protons on both sides of the ansa-
zirconocene ligand framework (Scheme 1). Exchange between
these proton sites was monitored in the temperature regime of
slow exchange by 2D-NOESY NMR methods. Apparent rate
constants were calculated from intensity ratios r of diagonal
and cross signals for pairs of previously homotopic protons (cf.
Figure 1) according to the equation kapp ) (1/τm) ln[(r + 1)/
(r - 1)], where τm is the mixing time employed and r )
∑I(diag)/∑I(cross).9
For zirconocene contact ion pairs containing the anion Me-
B(C6F5)3-, exchange signals between protons on either side of
the ligand framework arise either from anion exchange, i.e.,
side change of the entire anion, or from borane exchange, i.e.,
exchange of the borane between both Zr-Me groups (Scheme
These results are in line with the observation that borane
exchange is the dominant process for complex 2A but insig-
nificant for 5A and 6A; they imply that also complexes 1A and
(8) A slight excess of borane was chosen to avoid formation of binuclear
cations.
(9) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935.

Anion Exchange in Alkyl-Zirconocene Borate Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1485
Scheme 2
In complexes 4A, 5A, and 6A anion exchange is accelerated
by addition of Li+‚‚‚Me-B(C6F5)3- by factors of 1.2-1.5 (Table
1). These rather congested complexes thus appear to be more
resilient to associative attack by Me-B(C6F5)3-. Even for these
complexes, however, the rates of exchange between “free” and
Zr-bound Me-B(C6F5)3- anions are almost twice as high as their
side exchange rates, indicating again substantial nonproductive
anion exchange.
3. Effects of Zirconocene Concentrations on Side Ex-
change Rates. To clarify whether the exchange reactions
considered proceed unimolecularly, as had been assumed so far,4
we have determined the dependence of exchange rates on
zirconocene concentrations and, in doing so, made the un-
expected observation that these side exchange rates are signifi-
cantly dependent on zirconocene concentrations for all contact
ion pairs studied.
Depending on their solubilities, the B(C6F5)3 adducts 1A-
6A were investigated at zirconocene concentrations in the range
of [Zr] ) 2-20 mmol/L. All solutions were measured at T )
300 K and at a [B]/[Zr] ratio of 1.1:1. The apparent pseudo-
first-order rate constants kapp for the side-exchange reaction,
derived from 2D-NOESY data (Table 1), clearly increase with
increasing Zr concentrations. Plots of ln(kapp) versus ln([Zr])
reveal broken reaction orders of n ) 1.4-1.8 with regard to Zr
concentrations, with exponents n rather close to 2 (n ) 1.7-
1.8) for systems 1A-3A, while somewhat smaller values (n )
1.3-1.4) pertain for systems 4A-6A.
For an interpretation of these fractional reaction orders one
has to take into account that ion quadruples (and probably also
more highly aggregated species) are present in concentration-
dependent equilibria with the monodispersed contact ion pairs
at the zirconocene concentrations considered here (Scheme
2).11,12 If these ion quadruples have higher intrinsic exchange
rates than the monomeric ion pairs, the observed rates would
be expected to increase with [Zr]2 at low zirconocene concentra-
tions, but would approach proportionality to [Zr] as ion
quadruples become the dominant species in the monomer-dimer
equilibrium. The observed fractional reaction order with regard
to [Zr] would thus be compatible with the view that anion
exchange occurs predominantly in dimeric (or higher) aggregates
of the contact ion pairs considered.
Figure 2. Ligand side exchange rate constants kapp (in s-1) for
complexes 1A-6A at different [B]/[Zr] ratios (T ) 300 K, [Zr] ) 10
mmol/L).
3A, like 2A, undergo dynamic symmetrization predominantly
by way of borane exchange, while 4A is dynamically sym-
metrized, like 5A and 6A, predominantly by anion exchange.
2. Exchange between Contact Ion Pairs 1A-6A and
“Free” Me-B(C6F5)3-. To probe the mechanisms of this anion
exchange, it would be desirable to determine the effects of
-
excess Me-B(C6F5)3 anion on the rates of these exchange
-
reactions. “Free” Me-B(C6F5)3 anions cannot be maintained
in any significant concentration in a nonpolar solvent such as
C6D6, however, as they will be associated here with their
1
-
countercations. The H NMR shift of a Me-B(C6F5)3 anion
can be regarded as a measure of its “freedom”: In contact ion
pairs with a strong µ-Me coordination, e.g., in (C5H5)2ZrMe+‚
‚‚µ-Me-B(C6F5)3-, its H NMR signal appears at ca. 0.1 ppm;
1
1H NMR signals at ca. 1.3-1.4 ppm are found, on the other
hand, in “outer-sphere” ion pairs with (C5H5)2ZrMe(PMe3)+
countercations, which are likely to be associated only with the
aromatic residues of the Me-B(C6F5)3- anion.2a,5 This counter-
cation is not sufficiently inert, however, and tends to undergo
side reactions with some of the contact ion pairs considered
here. For this reason we have chosen the ion pair Li+‚‚‚
1
Me-B(C6F5)3-, which gives rise to an H NMR signal with an
intermediate shift of ca. 0.8 ppm, as a source of “free”
-
Me-B(C6F5)3 anions.
Addition of 1 equiv of Li+‚‚‚Me-B(C6F5)3- to C6D6 solutions
of contact ion pairs 1A-6A gave the rate changes summarized
in Table 1: For complexes 1A and 2A, a 2-3-fold increase in
the rate of ligand proton side exchange was induced by
Li+‚‚‚Me-B(C6F5)3-, while borane exchange was found to
continue at about the same rate.
In addition, it was found that Li+‚‚‚Me-B(C6F5)3- exchanges
with the Zr-bound Me-B(C6F5)3- anions of complexes 1A and
2A at a rate that is about 3-fold greater than that of the side
exchange occurring in the respective contact ion pair by itself
(Table 1). Apparently, associative attack of Me-B(C6F5)3- at a
Zr center occurs rather frequently under these conditions, but
predominantly in an unproductive manner, i.e., at the Zr-anion
side of the contact ion pair, and only occasionally so that attack
from the ZrMe side results in side exchange.
That contact ion pairs such as 2A are now susceptible to anion
exchange, to which they were inert by themselves, might be
due to the presence of the Lewis-acidic Li+ cation,10 which could
either increase electrophilicity at the Zr center by interaction
with the terminal Zr-Me group or s in the case of nonproductive
anion-anion exchange s direct the incoming anion into the
vicinity of the leaving anion by coordinating both of these
entities.
In a polynuclear species, intermolecular interactions between
ion pairs will certainly be facilitated. The anion exchange
occurring in dimers of the ion pairs 4A-6A can be envisioned
to proceed by way of intermolecular Zr+-fluoroaryl interactions,
for which substantial precedence has been reported.13 Anion
exchange via a transition state of the type represented in Scheme
3 would first yield F-coordinated contact ion pairs, which can
(11) Beck, S.; Geyer, A.; Brintzinger, H. H. Chem. Commun. 1999, 2477.
(12) Changes in ionic strength of these solutions, which might be argued
to affect the observed exchange rates, are tantamount to changes in the
degree of aggregation. Keeping ionic strengths constant by addition of
“inert” salts is likely to cause added complexity due to formation of mixed
ion aggregates in these solutions.
(13) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910. Yang,
X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840.
(10) Staley, R. H.; Beauchamp, J. L. J. Am. Chem. Soc. 1975, 97, 5920.

1486 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Beck et al.
Table 3. Selected Bond Distances (pm) and Angles (deg) for
Scheme 3
Complex 6A and Two Related Contact Ion Pairs
1Aa
6A
7Ab
Zr-µ-CH3
H3C-µ-B
251.6(8)
167.8(12)
225.8(9)
171.5(5)
91.8(3)
255.0(8)
168.8(13)
229.4(8)
162.7(6)
92.1(4)
254.9(3)
166.3(5)
225.2(4)
161.8(2)
92.0(1)
Zr-CH3
Zr-µ-CH3-B
C-Zr-µ-CH3
a From ref 16. b (1,2-Me2C5H3)2ZrMe-µ-MeB(C6F5)3, from ref 4a.
sponding dimethyl complexes by reaction with Ph3C+ B(C6F5)4-,2c
rather than with B(C6F5)3, undergo ligand side exchange, under
otherwise comparable conditions ([Zr] ) 1.0 mmol/L; [B]/[Zr])
1.1:1), about 5000 times faster than the corresponding Me-
-
Table 2. Activation Parameters for Side Exchange of the
Me-B(C6F5)3--Containing Ion-Pairs 1A-6A and of the
B(C6F5)4--Containing Ion-Pairs 1B-3B and 5B, Determined from
Apparent First-Order Rate Constants Measured at 300, 305, and 310
K in d6-Benzene Solution, and at 252, 257, and 260 K in
d8-Toluene Solution, Respectively.
B(C6F5)3 ion pairs, i.e., too fast for 2D-NMR measurements
at room temperature. Measurements at temperatures of T ) 252,
257, and 260 K in d8-toluene solutions gave, from Eyring plots
of the apparent first-order rate constants kapp, the ∆Hq and ∆Sq
values listed in Table 2.
Remarkably, we find ∆Hq values for complexes 1B-3B and
5B which are close to those found above for the respective ion
complex
[Zr] mmol/L
∆Hq, kJ/mol
∆Sq, J/(mol‚K)
1A
2A
3A
4A
5A
6A
6A
10
10
10
10
5
5
10
50 ( 4
48 ( 2
40 ( 8
53 ( 9
81 ( 4
58 ( 4
62 ( 3
-76 ( 1
-81 ( 8
-108 ( 10
-40 ( 3
17 ( 4
-48 ( 5
-9 ( 5
-
pairs with Me-B(C6F5)3 anions. Enthalpic contributions, i.e.,
a reduced strength of the cation-anion bond, are thus apparently
not the cause for the higher rates of exchange in B(C6F5)4- ion
pairs. ∆Sq values, however, are more positive by ca. 50-100
J/(mol‚K) for these complexes than for the corresponding species
with Me-B(C6F5)3 anions.14
-
1B
2B
3B
5B
1
1
1
1
57 ( 3
50 ( 8
67 ( 8
85 ( 9
8 ( 8
-30 ( 20
27 ( 15
These observations are strikingly similar to those associated
with a concentration increase of the Me-B(C6F5)3- -containing
ion pairs and lead us to propose that the B(C6F5)4- -containing
102 ( 25
-
ion pairs 1B-3B and 5B behave as their Me-B(C6F5)3
-
containing analogues would if their concentrations were even
higher than those used here. Apparently, the more polar ion
pairs 1B-3B and 5B form increased proportions of rapidly
exchanging ion quadruples or higher aggregates already at
then isomerize to a methyl-borate coordinated geometry by an
intramolecular, presumably facile rearrangement. Even though
such a process would be formally unimolecular with respect to
the ion quadruple, it would be of the associative type since the
coordination number of the Zr center is increased in the
exchange transition state.
4. Temperature Dependence of Exchange Rates. Activation
parameters for these side-exchange reactions were determined
by measuring their rates at temperatures of T ) 300, 305, 310,
and 315 K in C6D6 solutions at a [B]/[Zr] ratio of 1.1:1 and by
analyzing Eyring plots for the apparent first-order rate constants
kapp (Table 2).14
relatively low concentrations in the nonpolar solvent used. This
-
notion is supported by the observation that the B(C6F5)4
-
containing ion pairs tend to form a separate oily phase already
at concentrations which are about an order of magnitude lower
than those at which their Me-B(C6F5)3- -containing analogues
begin to separate from their hydrocarbon solutions. These
observations indicate that even a particularly weakly coordinat-
ing anion such as B(C6F5)4- might exchange predominantly by
an associative mechanism.
While activation entropies for these broken-order reactions
are to be interpreted with caution, the effect of changing
zirconocene concentrations, which was determined for complex
6A, is quite illuminating: ∆Hq is practically unaffected by a
concentration change; ∆Sq values, however, are more positive
by ca. 35-40 J/(mol‚K) at [Zr] ) 10 mmol/L than at [Zr] ) 5
mmol/L.14 The data given in Table 2 indicate that the accelerat-
ing effect of increased zirconocene concentrations originates
solely from less unfavorable entropy changes along the reaction
coordinate. This is in line with the notion that more highly
aggregated species, such as ion quadruples, are involved in these
exchange reactions: Anion-exchange transition states such as
that represented in Scheme 3 can obviously be reached from a
preformed ion quadruple with very little further preordering.
6. Crystal Structure of rac-Me2Si(2-Me-4-tBu-C5H2)2Zr-
Me+‚‚‚µ-MeB(C6F5)3-. Whereas a number of crystal structures
have been reported for ion pairs of the type Cp*2ZrMe+‚‚‚
- 2a,15,16
µ-Me-B(C6F5)3
,
chiral representatives of this class of
complexes have not been structurally characterized so far.
Attempts to obtain crystals of ion pairs of the racemic complexes
3, 4, and 5 failed due to the formation of oily precipitates. The
B(C6F5)3 adduct of rac-Me2Si(2-Me-4-tBu-C5H2)2ZrMe2, how-
ever, was isolated in the form of single crystals. Although the
crystals and the resulting X-ray diffraction data were not of
particularly high quality, the essential bond lengths and bond
angles of complex 6A lie within the range of values reported
-
before for other µ-MeB(C6F5)3 contact ion pairs (Figure 3,
Table 3).2a,15,16
-
5. Dynamics of ansa-Zirconocene Ion Pairs with B(C6F5)4
A crystal-packing representation of complex 6A shows the
aggregation of contact ion pairs to rhombus-shaped ion qua-
druples (Figure 4, left). In a packing representation of the
Anions. Ion pairs 1B-3B and 5B, generated from the corre-
(14) Eyring-plot analysis of second-order rate constants ksec, derived from
k
app by division through [Zr], has no effect on the values of ∆Hq and gives
∆Sq values which are more positive than those obtained from kapp by about
(15) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Malik, K. M.
A. Organometallics 1994, 13, 2235.
(16) Beck, S.; Prosenc, M. H.; Brintzinger, H. H.; Goretzki, R.; Herfert,
N.; Fink, G. J. Mol. Catal. A: Chem. 1996, 111, 67.
40 J/(mol‚K) throughout. This does not affect the comparison of ∆Sq values
-
at different zirconocene concentrations or of contact ion pairs with B(C6F5)4
and with Me-B(C6F5)3 anions.
-

Anion Exchange in Alkyl-Zirconocene Borate Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1487
anion-exchange transition states similar to that represented in
Scheme 3 might be accessible from ion-quadruples without
much further preordering.
Conclusions
The weight of the evidence described above s in particular
the acceleration of the dynamic symmetrization of zirconocene
ion pairs by added Li+‚‚‚Me-B(C6F5)3-, the nonproductive anion
exchange which accompanies this rate increase, the increase of
exchange rates with increasing concentrations of the zirconocene
ion pairs, and, finally, the exclusively entropic nature of these
concentration-dependent rate changes s indicates that anion
exchange in nonpolar solutions of these ion pairs proceeds
predominantly by way of ion quadruples or higher order ionic
aggregates. As long as there is no conclusive evidence that this
anion exchange occurs s at least in parts s by way of dissocia-
tive, unimolecular reaction paths, it should be seriously ques-
tioned whether solvated zirconocene alkyl cations of the type
Figure 3. ORTEP/PLUTO drawing of the molecular structure of the
contact ion pair rac-Me2Si(2-Me-4-tBu-C2H2)2ZrMe+‚‚‚µ-MeB(C6F5)3
-
Cpx ZrMe+ are capable of existing, free of a supporting ligand,
2
(6A). H atoms were omitted for clarity.
even as short-lived reaction intermediates in reaction systems
containing Cpx ZrMe+‚‚‚A- ion pairs in hydrocarbon solu-
previously determined crystal structure of complex 1A,16
aggregation of contact ion pairs to ion quadruples is similarly
evident (Figure 4, right). Several relatively short intermolecular
Zr-F distances of ca. 450-550 pm indicate dipolar interactions
of C-F units with a cationic Zr center.
Solid-state structures such as those of 1A or 6A are a priori
likely to contain rhombi of their constituent ions and should
thus not be overinterpreted with respect to possible interactions
in solution. Nonetheless, they illustrate possible proximity
relations in an ion quadruple and let it appear plausible that
2
tions.17
If anion-free Cpx Zr(alkyl)+ cations are indeed unavailable
2
as intermediates in these reaction systems, their implication as
active species of zirconocene-based catalysts for the polymer-
ization of R-olefins would appear inappropriate. Instead,
substitution reactions in which the anion is displaced from these
ion pairs by nucleophilic attack of an olefin substrate (or a
competing entity) would have to be studied with regard to their
thermodynamics and kinetics to establish useful structure-
Figure 4. Schakal drawings of the crystal packing of complexes 6A (left) and 1A 16 (right), projected along the unit-cell c and b axes, respectively.
Black F atoms are within less than 600 pm from the shaded Zr centers. Circled plus and minus signs indicate the approximate location of cationic
and anionic charges, respectively.

1488 J. Am. Chem. Soc., Vol. 123, No. 7, 2001
Beck et al.
glovebox. The NMR tubes were stoppered and immediately transferred
to the spectrometer.
property relations for metallocene-based polymerization cata-
lysts.
Me4C2(C5H4)2ZrMe+-µ-Me-B(C6F5)3- (1A). 1H NMR (600.1 MHz,
C6D6, 7.15 ppm, 300 K) δ 6.09 (pq, 2H, â-Cp, Zr-Me-side), 5.89 (pq,
2H, â-Cp, Me-B-side), 5.58 (pq, 2H, R-Cp, Me-B-side), 5.05 (pq, 2H,
R-Cp, Zr-Me-side), 0.74 (s, 6H, Me4C2, Me-B-side), 0.59 (s, 6H, Me4C2,
Zr-Me-side), 0.38 (s, 3H, Zr-Me), 0.32 (br, 3H, Me-B).
Experimental Section
All reactions were performed under argon with Schlenk-line
techniques or under nitrogen in a glovebox. Solvents were dried prior
to use by refluxing over and distillation from sodium. Deuterated
solvents were dried over 4 Å molecular sieves. Zirconocene dimethyl
complexes (1-5) were synthesized according to ref 18, B(C6F5)3
according to ref 19. [Ph3C][B(C6F5)4] was obtained as a gift from BASF
AG, Me2Si(2-Me-ind)2ZrCl2 from Axiva GmbH. Other chemicals were
purchased from commercial suppliers and used without further purifica-
tion. Exchange NMR spectra were recorded on a Bruker Avance DRX
600 (600 MHz) instrument.
rac-Me2Si(2-Me-Ind)2ZrMe2 (4). In 80 mL of toluene, 476 mg (1
mmol) of rac-Me2Si(2-Me-Ind)2ZrCl2 was dissolved in a double Schlenk
vessel and cooled to -15 °C. After addition of 2.1 equiv of a solution
of MeMgCl in ether the mixture was allowed to warm to room
temperature and stirred overnight, while the color turned from orange
to light yellow and a white solid precipitated. The solvent was removed
in vacuo and the residue extracted three times with toluene. Crystal-
lization from pentane at -80 °C afforded yellow needles in 70% yield.
1H NMR (250 MHz, C6D6, 7.15 ppm, 300 K) δ 7.16 (d, 2H, Ind-7),
6.77 (t, 2H, Ind-6), 7.39 (t, 2H, Ind-5), 7.42 (d, 2H, Ind-4), 6.59 (s,
2H, Ind-3), 1.89 (s, 6H, 2-Me), 0.73 (s, 6H, Me2Si), -0.87 (s, 6H,
Zr-Me).13C NMR (150 MHz; C6D6, 128 ppm; 300 K) δ 134.4, 129.7
(Ind-3a, Ind-7a), 125.8 (Ind-4), 125.7 (Ind-5), 124.5 (Ind-7), 123.6 (Ind-
6), 115.8 (Ind-3), 79.0 (Ind-1), 36.1 (Zr-Me), 18.0 (2-Me), 2.4 (Me2-
Si).
Exchange rates were determined from R-Cp-H, â-Cp, and Me4C2
signals (mixing times τm given in parentheses): T ) 300 K, [Zr]:[B]
) 1:1.1; [Zr] ) 3.95, 11.65, and 19.05 mM (200 ms); T ) 300K, [Zr]
) 10 mM, [Zr]:[B] ) 1:1.1, 2, and 3 (50 ms); T ) 300, 305, 310, and
315 K, [Zr]) 10 mM, [Zr]:[B] ) 1:1.1 (200 ms).
Me2Si(C5H4)2ZrMe+-µ-Me-B(C6F5)3- (2A). 1H NMR (600.1 MHz,
C6D6, 7.15 ppm, 300 K) δ 6.34 (pq, 2H, â-Cp Zr-Me-side), 6.24 (pq,
2H, â-Cp Me-B-side), 5.30 (pq, 2H,R-Cp Me-B-side), 4.89 (pq, 2H,
R-Cp Zr-Me-side), 0.47 (br, 3H, Me-B), 0.32 (s, 3H, Zr-Me), -0.03
(s, 3H, Me2Si Me-B-side), -0.21 (s, 3H, Me2Si Zr-Me-side).
Exchange rate constants were determined for the R-Cp, â-Cp, and
Me2Si signals (mixing times τm given in parentheses): T ) 300 K,
[Zr]:[B] ) 1:1.1, [Zr] ) 19.05, 11.65 (100 ms) and 3.95 mM (150ms);
T ) 300 K, [Zr] ) 10 mM, [Zr]:[B] ) 1:1.1, 1:2 (150 ms), and 1:3
(100 ms); T ) 303, 308, 313, and 318 K, [Zr] ) 10 mM, [Zr]:[B] )
1:1.1 (150 ms).
rac-Me2Si(Ind)2ZrMe+-µ-Me-B(C6F5)
(3A). 1H NMR (600.1
-
3
MHz, C6D6, 7.15 ppm, 300 K) δ 7.50 (d, 1H, Ind-H), 7.03 (m, 2H,
Ind-H), 6.89 (d, 1H, Ind-H), 6.69 (m, 1H, Ind-H), 6.63 (m, 1H, Ind-
H), 6.57 (d, 1H, â-C5H2 Zr-Me-side), 6.29 (m, 2H, Ind-H), 6.22 (d,
1H, â-C5H2 Me-B-side), 5.66 (d, 1H, R-C5H2 Me-B-side), 4.97 (d, 1H,
R-C5H2 Zr-Me-side), 0.34 (s, 3H, Me2Si), 0.20 (s, 3H, Me2Si), -0.44
(br, 3H, Me-B), -0.51 (s, 3H, Zr-Me).
rac-Me2Si(2-Me-4-tBu-C5H2)2ZrMe2 (6). To a solution of 245 mg
(0.5 mmol) of rac-Me2Si(2-Me-4-tBu-C5H2)2ZrCl2 in ether at 0 °C was
added, under exclusion of light, 1.2 mL (1.2 mmol) of a 1 M solution
of MeMgCl in ether. After stirring the reaction mixture overnight and
removal of the solvent by evaporation, the residue was extracted three
times with pentane. Concentration of the solution to about half of its
volume in vacuo and cooling to -80 °C afforded complex 6 in the
form of white needles. Yield 120 mg (55%). 1H NMR (250 MHz, C6D6,
300 K) δ 0.88 (bs, 3H, B-Me), 6.44 (d, 2H, (C5H2)), 5.33 (d, 2H,
(C5H2)), 1.85 (s, 6H, (2-CH3)), 1.37 (s, 18H, (4-C(CH3)3)), 0.34 (s,
6H, (CH3)2Si), 0.14 (s, 6H, ZrCH3). Anal. Found: C, 64.3; H, 9.3.
Calcd: C, 64.4; H, 9.0.
Li+ MeB(C6F5)3-. In a glovebox, 1.1 mg (50 µmol) of solid MeLi,
dried by evaporation of an etheral solution and coevaporation with
pentane, were mixed with 28.5 mg (55 µmol) of tris(pentafluorophenyl)-
borane dissolved in 2.3 mL of benzene. The mixture was stirred for 1
h until a clear solution was obtained. Caution: A 100-mg sample of
dry LiMeB(C6F5)3 underwent detonation upon contact with a spatula.
LiMeB(C6F5)3 should thus be prepared in small portions only and its
solutions should not be allowed to evaporate to dryness.
Exchange rate constants were determined for the R-C5H2 and Me2-
Si signals (mixing times τm are given in parentheses): T ) 300 K,
[Zr]:[B] ) 1:1.1, [Zr] ) 3.95, 11.65, and 19.05 mM (200 ms); T )
300 K, [Zr] ) 10 mM, [Zr]:[B] ) 1:1.1, 1:2, and 1:3 (200 ms); [Zr] )
10 mM, [Zr]:[B] ) 1:1.1, T ) 300 (200 ms), 305 (75ms), 310 (50
ms), and 315 K (25 ms).
rac-Me2Si(2-Me-Ind)2ZrMe+-µ-Me-B(C6F5)3- (4A). 1H NMR (600.1
MHz, C6D6, 7.15 ppm, 300 K) δ 6.93 (d, 1H, Ind-7), 6.55 (t, 1H, Ind-
6), 7.04 (t, 1H, Ind-5), 7.55 (d, 1H, Ind-4), 7.0 (d, 1H, Ind-7′), 6.36 (d,
1H, Ind-6′), 6,21 (t, 1H, Ind-5′), 7.06 (d, 1H, Ind-4′), 6.12 (s, 1H, C5H-
3′), 6.39 (s, 1H, C5H-3′), 1.70 (s, 3H, MeC5H-2), 1.44 (s, 3H, MeC5H-
3′), 0.44 (s, 3H, Me2Si-1-side), 0.49 (s, 3H, Me2Si-1′-side), -0.26 (br,
3H, Me-B), -0.29 (s, 3H, Zr-Me).
Exchange rate constants were determined for the R-C5H and 2-Me
signals (mixing times τm given in parentheses): T ) 300 K, [Zr]:[B]
) 1:1.1, [Zr] ) 3, 5, and 10 mM (300 ms); T ) 300 K, [Zr] ) 10
mM, [Zr]:[B] ) 1:1.1, 1:2, and 1:3 (300 ms); [Zr] ) 10 mM, [Zr]:[B]
) 1:1.1, T ) 300 (200 ms), 305 (75 ms), 310 (50 ms), and 315 K (25
ms).
rac-Me2Si(2-Me-Benz[e]Ind)2ZrMe+-µ-Me-B(C6F5)3 (5A). 1H
-
NMR Measurements. All exchange reactions were monitored by
NOESY experiments, using mixing times τm ) 50-500 ms, which
were chosen so as to obtain a diagonal-to-cross signal ratio of about
4/1. Apparent first-order rate constants kapp were evaluated from the
signal intensities of at least two different proton positions of the
NMR (600.1 MHz, C6D6, 7.15 ppm, 300 K) δ 7.86 (d, 1H, Benz-9),
7.68 (d, 1H, Benz-9′), 7.61 (t, 1H, Benz-8), 7.49 (d, 1H, Benz-6), 7.29
(t, 1H, Benz-7), 7.16 (nr, 1H, Benz-6′), 7.12 (1H, Benz-8′), 7.08 (1H,
Benz-7′), 7.04 (1H, Ind-5), 6.97 (1H, Ind-4), 6.97 (1H, Ind-1), 6.95
(1H, Ind-5′), 6.74 (1H, Ind-4′), 6.73 (1H, Ind-1), 1.87 (s, 3H, 2-Me),
1.65 (s, 3H, 2-Me′), 0.564 (s, 3H, Me2Si), 0.558 (s, 3H, Me2Si), -0.34
(br, 3H, Me-B), -0.41 (s, 3H, Zr-Me).
Exchange rate constants were determined for the H-9,9′ (7.86/7.68
ppm), H-6,6′ (7.49/ 7.16 ppm), and 2-Me signals (mixing times τm given
in parentheses): T ) 300 K, [Zr]:[B] ) 1:1.1, [Zr] ) 2, 3.6, and 4.76
mM (400 ms); T ) 300, 305, 310, and 315 K, [Zr] ) 5 mM, [Zr]:[B]
) 1:1.1 (400 ms).
molecule studied and calculated according to the equation kapp
)
(1/τm) ln[(r + 1)/(r - 1)] with r ) ∑I(diag)/∑I(cross).9
Solutions of the zirconocene ion pairs were prepared by transferring,
with an Eppendorf pipet, stock solutions containing the zirconocene
dimethyl complexes 1-4 and 6 at zirconocene concentrations [Zr] )
40 mmol/L and complex 5 at [Zr] ) 10 mmol/L and solutions
containing the activator components, B(C6F5)3 at c ) 40 mmol/L and
Ph3C+B(C6F5)4 at c ) 5 mmol/L, directly to an NMR tube in the
-
rac-Me2Si(2-Me-4-tBu-C5H2)2ZrMe+-µ-MeB(C6F5)3- (6A). 1H NMR
(600.1 MHz, C6D6, 7.15 ppm, 300 K) δ 6.21, 6.10 (bs, 2, â-C5H2);
5.26, 4.96 (bs, 2, R-C5H2); 1.69, 1.4 (bs, 6, CH3C5H2); 0.96, 0.72 (bs,
18, (CH3)3CC5H2); 0.7 (bs, 3, BCH3); 0.63 (s, 3, ZrCH3); 0.12 (bs, 3,
(CH3)2Si), -0.03 (bs, 3, (CH3)2Si).
Exchange rate constants were determined for the R-Cp-H, â-Cp-H,
and Me2Si signals (mixing times τm given in parentheses): T ) 300
K, [Zr]:[B] ) 1:1.1, [Zr] ) 5, 10, and 20 mM (10, 15, and 35 ms); T
(17) In halocarbon solvents, higher dielectric constants and action of the
halocarbons as stabilizing ligands might favor formation of anion-free alkyl
zirconocene cations.
(18) Wiesenfeldt, H.; Reinmuth, A.; Barsties, E.; Evertz, K.; Brintzinger,
H. H. J. Organomet. Chem. 1989, 369, 359.
(19) Pohlmann, J. L. W.; Brinckman, F. E.; Tesi, G.; Donadio, R. E. Z.
Naturforsch. 1965, 20b, 1. Pohlmann, J. L. W.; Brinckman, F. E. Z.
Naturforsch. 1965, 20b, 5.

Anion Exchange in Alkyl-Zirconocene Borate Ion Pairs
J. Am. Chem. Soc., Vol. 123, No. 7, 2001 1489
-
-
) 300 K, [Zr] ) 10 mM, [Zr]:[B] ) 1:1.1, 2, and 3 (15 ms); T ) 300,
305, 310, and 315 K, [Zr]) 5, 10 mM, [Zr]:[B] ) 1:1.1 (15 and 5
ms).
B(C6F5)3 (250 ms); Me2Si(C5H4)2ZrMe+-µ-Me-B(C6F5)3 (250 ms);
rac-Me2Si(Ind)2ZrMe+-µ-Me-B(C6F5)3 (250 ms); rac-Me2Si(2-Me-
-
Ind)2ZrMe+-µ-Me-B(C6F5)3 (250 ms); rac-Me2Si(2-Me-Benz[e]-
-
[Me4C2(C5H4)2ZrMe]+ B(C6F5)4 (1B). 1H NMR (600.1 MHz,
Ind)2ZrMe+-µ-Me-B(C6F5)3 (350 ms); and rac-Me2Si(2-Me-4-tBu-
-
-
-
C7D8, 2.09 ppm, 252 K) δ 6.10 (br, 2H, â-Cp-H), 5.36 (br, 2H, R-Cp-
H), 4.88 (br, 2H, R-Cp-H), 4.88 (br, 2H, â-Cp-H), 0.74 (s, 6H, Me4C2),
0.87 (s, 6H, Me4C2), 0.26 (s, 3H, Zr-Me), 2.00 (s, 3H, Me-CPh3).
Exchange rate constants were determined for the R-Cp-H, â-Cp-H
,and Me4C2 signals (0.74/0.87 ppm; mixing times τm given in
parentheses): [Zr]:[B] ) 1:1.1, [Zr] ) 1 mM, T ) 252 (100 ms), 257
(50 ms), and 260 K (10 ms).
C5H2)2ZrMe+-µ-MeB(C6F5)3 (20 ms).
Crystal Structure Determination. Crystallization of rac-Me2Si(2-
Me-4-tBu-C5H2)2ZrMe+-µ-MeB(C6F5)3 by slow evaporation of a
-
benzene solution in a glovebox gave yellow monoclinic prisms with
approximate size of 0.4 × 0.4 × 0.4 mm. Crystal data were collected
in the θ-range of 2-27° at 236 K, using a Siemens P4 diffractometer
and Mo KR radiation (λ ) 0.71073 Å): space group P21/n, a )
14.784(2) Å, b ) 20.149(4) Å, c ) 18.810(5) Å, â ) 100.70(2), Z )
4, V ) 5505(2) Å3, Dcalcd ) 1.436 g/cm3, µ ) 0.311 mm-1, and
F(000))2444. A total of 12560 reflections were collected, of which
10816 were independent and 6320 reflections had I > 2σ(I). The
structure was solved by direct methods and refined with the SHELXL
93 program package.20 All non-hydrogen atoms were refined aniso-
tropically. The positions of the hydrogen atoms were calculated and
refined with fixed isotropic U using the riding model technique. The
final agreement factors were R(F) ) 9.65% and Rw(F2) ) 23.0% (w-1
[Me2Si(C5H4)2ZrMe]+ B(C6F5)4- (2B). 1H NMR (600.1 MHz, C7D8,
2.09 ppm, 252 K) δ 6.30 (br, 2H, â-Cp-H), 5.19 (br, 2H, R-Cp-H),
5.10 (br, 2H, R-Cp-H), 4.66 (br, 2H, â-Cp-H), 0.17 (s, 3H, Me2Si ),
-0.17 (s, 3H, Me2Si), 0.17 (s, 3H, Zr-Me), 2.00 (s, 3H, Me-CPh3).
Exchange rate constants were determined for the R-Cp-H, â-Cp-H,
and Me2Si signals (0.17/-0.17 ppm; mixing times τm given in
parentheses): [Zr]:[B] ) 1:1.1, [Zr] ) 1 mM, T ) 252 (150 ms), 257
(100 ms), and 260 K (50 ms).
[rac-Me2Si(Ind)2ZrMe]+ B(C6F5)
(3B). H NMR (600.1 MHz,
-
1
4
C 7D8, 2.09 ppm, 252 K) δ 5.19 (d, 2H, R-C2H5), 4.85 (d, 2H, R-C2H5),
0.47 (s, 3H, Me2Si), -0.16 (s, 3H, Me2Si), -0.78 (s, 3H, Zr-Me), 2.00
(s, 3H, Me-CPh3).
) σ2(F2) + (0.0879P)2 + 50.35P, with P ) (F2 + 2F2calc)). The
obs
Goodness-of-Fit was 1.052. Crystallographic data for the structure
obtained have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication CCDC 134808. Copies can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax +44 1223 336033; e-mail deposit@
ccdc.cam.ac.uk).
Exchange rate constants were determined for R-C5H2 and Me2Si
signals (mixing times τm given in parentheses): [Zr]:[B] ) 1:1.1; [Zr]
) 1 mM; T ) 252, 257, and 260 K (100 ms).
[rac-Me2Si(2-Me-Benz[e]Ind)2ZrMe]+ B(C6F5)4 (5B). H NMR
(600.1 MHz, C7D8, 2.09 ppm, 252 K) δ 1.69 (s, 3H, 2-Me), 1.58 (s,
3H, 2-Me), 0.75 (s, 3H, Me2Si ), 0.53 (s, 3H, Me2Si), -0.66 (s, 3H,
Zr-Me), 2.00 (s, 3H, Me-CPh3).
-
1
Acknowledgment. We thank Ms. Anke Friemel for excellent
technical assistance with 2D-NMR measurements and Dr. Dmitri
Babushkin for valuable comments and suggestions. For gifts
of chemicals we thank Dr. Ju¨rgen Suhm, Kunststoff-Laborato-
rium, BASF AG, and Dr. Carsten Bingel, Axiva Research and
Technologies GmbH. Support of this work by BMBF, BASF
AG, Fonds der Chemischen Industrie and funds of the University
of Konstanz is gratefully acknowledged.
Exchange rate constants were determined for 2-Me and Me2Si signals
(mixing times τm in parentheses): [Zr]:[B] ) 1:1.1, [Zr] ) 1 mM, T
) 252 (150 ms) and 257 and 260 K (100 ms).
-
[rac-Me2Si(2-Me-4-tBu-C5H2)2ZrMe]+ B(C6F5)4 (6B). Exchange
rates were too fast, even at 240 K, for separate signals to be observable.
Exchange Rates with Added Li+ MeB(C6F5)3-. Rate constants were
determined for the symmetrization of the ligand framework and for
exchange between zirconium-bound MeB(C6F5)3- and Li+MeB(C6F5)3
-
JA003531W
at [Zr]:[B] ) 1:1.1, [Zr] ) 4 mM, [Li+MeB(C6F5)3-] ) 4 mM; T )300
K, mixing times τm in parentheses: Me4C2(C5H4)2ZrMe+-µ-Me-
(20) Sheldrick, G. M. SHELXS-86, SHELXL-93; University of Go¨ttingen,
1993.