Ion pair symmetrization in metallocenium cations partnered with diborane derived borate counteranions

Article abstract of DOI:10.1016/j.jorganchem.2007.05.023

The thermodynamics of ion pair symmetrization in a series of metallocenium species generated from Cp₂^″ ZrMe₂ (Cp^″ = 1, 2 - Me₂ C₅ H₃) were studied using a variety of solution dyn

Full text of DOI:10.1016/j.jorganchem.2007.05.023

Journal of Organometallic Chemistry 692 (2007) 4661–4668
www.elsevier.com/locate/jorganchem
Ion pair symmetrization in metallocenium cations partnered with
diborane derived borate counteranions
*
Lee D. Henderson, Warren E. Piers
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
Received 30 March 2007; received in revised form 14 May 2007; accepted 14 May 2007
Available online 24 May 2007
Dedicated with great respect to Prof. Gerhard Erker, a hero of metallocene chemistry.
Abstract
The thermodynamics of ion pair symmetrization in a series of metallocenium species generated from Cp00ZrMe2 ðCp00 ¼ 1; 2-Me2C5H3Þ
2
were studied using a variety of solution dynamic techniques including line broadening, 2D-EXSY, and 1D-DPFGSE-NOE. Ion pairs
were generated by methide abstraction using the corresponding trityl salts [1-A] to yield ½Cp020ZrMeꢀþ½Aꢀꢁ (A = {C6F4-1,2-[B(C6F5)2]2-
(l-O(C6F5))}ꢁ, 2-O(C6F5); {C6F4-1,2-[B(C6F5)2]2(l-OPh)}ꢁ, 2-OPh; {C6F4-1,2-[B(C6F5)2]2(l-OMe)}ꢁ, 2-OMe; and [B(C6F5)4]ꢁ,
2-B(C6F5)4). The observed activation parameters were interpreted on the basis of a solvent-assisted mechanism of ion pair symmetrization.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Metallocenes; Ion pair symmetrization; Weakly coordinating anions; Fluoroarylborates
1. Introduction
has occupied the efforts of several groups over the past dec-
ade. Many studies have made use of NMR techniques to
extract the kinetic and thermodynamic parameters of ion
pair dynamics by using metallocenium or non-metalloce-
nium cations that incorporate diastereotopic groups whose
chemical exchange is observable on the NMR time scale.
Pioneering studies by Marks et al. [3] utilized the group 4
tetramethyl metallocenes shown in Scheme 1 to identify
two key processes in systems activated by the strong orga-
nometallic Lewis acid B(C6F5)3 [4] and related species. In
these systems, the anion partner is a relatively strongly
bound methyl borate anion, [MeB(C6F5)3]ꢁ. As shown in
the scheme, ‘‘borane dissociation/recombination’’ per-
mutes not only the diastereotopic groups on the Cp rings,
but also the M–Me/BMe groups, while ‘‘ion pair symmetri-
zation’’ (ips) exchanges the former but not the latter. It is
thus possible to observe these processes separately using
this probe. Brintzinger [5] has also utilized various systems
with similar symmetry properties to study these processes,
while subsequently others have engaged in various studies
to probe the kinetic and thermodynamic properties of
Metallocenium ion pairs formed from neutral dialkyl
derivatives Cp2MR2 and an alkide abstractor are the active
catalysts in olefin polymerization systems of significant
industrial importance [1]. For this reason, their structural
and dynamic properties have been studied extensively,
since these features have direct impact on catalyst activity
and selectivity [2]. A key determinant in the behavior of
these catalysts is the nature of the cation–anion interaction,
the strength of which is dictated by a number of integrated
factors including the coordinating properties of the anion,
the structure and symmetry of the cation, the steric/elec-
tronic properties of the growing polymer chain and the
polarity of the solvent medium.
Deconvoluting these variables to develop a detailed
understanding of the dynamic properties of these ion pairs
*
Corresponding author.
E-mail address: wpiers@ucalgary.ca (W.E. Piers).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.023

4662
L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
Scheme 1.
metallocenium ion pairs from both experimental [6–10] and
computational [11] perspectives.
2. Results and discussion
The nature of the ips process in particular has engen-
dered extensive discussion, since activation entropies range
from moderately positive to moderately negative values,
depending on the structure of the metallocene, the concen-
tration regime the experiments have been performed in,
and the nature of the anion. Mechanistic postulates have
therefore also spanned a range of suggestions, including
those that are dissociative in character and ones that
involve ion pair aggregates and are thus associative in nat-
ure. Since full dissociation of the anion is expected to be
highly energetic, more recently solvent molecules have been
proposed to play a significant role in the exchange process
[2b,11l,11m].
All the ion pairs studied were generated by methide
abstraction protocols using the [CPh3]+ cation, affording
ion pairs with weakly coordinating borate counteranions
and Ph3CMe as the by-product [15]. The production of
ion pairs by this method, as opposed to methide abstrac-
tion by borane (e.g. B(C6F5)3), gives ion pairs that permute
diastereotopic Cp methyl groups only by the ips pathway
and avoids complications associated with deconvoluting
the borane dissociation (dr pathway) (Scheme 1). Fewer
solution dynamic studies have been reported for metalloce-
nium ions paired with perfluoroarylborate counteranions,
since they are more difficult to produce cleanly and are sub-
ject to more facile decomposition [2a]. Furthermore, these
ion pairs are typically less soluble in non-polar media such
as toluene than ones with methylborate counteranions,
leading to samples forming oily clathrate-like residues if
the concentration is too high ([Zr] ’ 10 mM).
The majority of above mentioned studies consider ion
pairs with the more coordinating (and stabilizing) [MeB-
(C6F5)3]ꢁ counteranion. Measurements on ion pairs con-
taining more weakly coordinating anions such as
[B(C6F5)4]ꢁ have proven more difficult [2a,5c,12]. We have
prepared a family of borate anions [13] based on the chelat-
ing perfluoroaryl borane C6F4-1,2-[B(C6F5)2]2 [14], that
can be conveniently prepared as their trityl salts and used
to activate metallocenes via methide abstraction. In this
contribution, we describe ion pair symmetrization studies
A
series of trityl borate salts ([1-A]) including
[CPh3]+{C6F4-1,2-[B(C6F5)2]2(l-O(C6F5))}ꢁ ([1-O(C6F5)]),
[CPh3]+{C6F4-1,2-[B(C6F5)2]2(l-OPh)}ꢁ ([1-OPh]), [CPh3]+
{C6F4-1,2-[B(C6F5)2]2(l-OMe)}ꢁ ([1-OMe]), and [CPh3]+
[B(C6F5)4]ꢁ ([1-B(C6F5)4]), were utilized to produce ion
pairs of the general form ½Cp020ZrMeꢀþ½Aꢀꢁ, [2-A], in arene
solvents such as toluene or bromobenzene (Scheme 2).
In all cases a slight excess of trityl salt was utilized to avoid
unwanted dimeric [(L2MMe)2(l-Me)]+ metallocenium
fragments, which are readily identifiable by the upfield
for a series of ion pairs generated from Cp00ZrMe2
2
ðCp00 ¼ 1; 2-Me2C5H3Þ and trityl salts of the {C6F4-1,2-
[B(C6F5)2]2(l-OR)}ꢁ (R = Me, C6H5, C6F5) anions and
the [B(C6F5)4]ꢁ borate anion. Using a combination of solu-
tion dynamics techniques including coalescence determina-
tion, line broadening, 2D-EXSY, and 1D-DPFGSE-NOE
we report the rate constants and activation parameters in
these systems.
1
shifted resonance for the bridging methyl group in the H
NMR spectrum [16]. (Rate enhancements of anion symme-
2þ
trization, potentially via ½Cp020Zrꢀ ½Aꢀ2ꢁ species, were not
observed in the presence of excess trityl borate salt.)

L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
4663
Scheme 2.
Spectroscopic investigations were conducted on samples at
concentrations between 2.5 and 10 mM in zirconium. The
samples of [2-A] examined were found to be stable over
several hours at room temperature and up to a week at
low temperature (ꢁ40 °C). The decomposition products
that resulted have not been identified but appear to be
derived only from the cation [1,17], since the anions remain
intact as monitored by 19F NMR spectroscopy.
that our results were the same, within experimental error,
as those reported in the literature (see Table S-2, support-
ing information). A complete listing of collected rate con-
stants can be found in the supporting information.
A combination of the dynamic NMR methods available
were employed for each ion pair symmetization process and
the results obtained for the compounds [2-OPh] and [2-
OMe] are shown in Fig. 1, which displays overlaid Eyring
plots for each of the methods used (line broadening, 2D-
EXSY, and 1D-DPFGSE-NOE). The plots demonstrate
the excellent agreement for each of the rate constants deter-
mined by each method. These results indicate that the meth-
ods are compatible and provide internal reproducibility and
confidence in the data; use of several methods also allowed
us to extend the temperature range of collectable rate data
due to the increased sensitivity of 2D-EXSY and 1D-
DPFGSE-NOE measurements at lower temperatures.
The activation parameters for ion pair symmetrization
measured in this way for ion pairs 2 are comparable to those
measured in other borate stabilized metallocenium ions
[5c,12], although considerable variation in the DSꢀ values
seems to be the norm in these systems. For example, Brintz-
inger et al. report values between ꢁ7.2 5 and 24.4 6 eu
for [B(C6F5)4]ꢁ partnered with various metallocenium ions
[5c], while Marks et al. report values of 10 1 eu for the
The rates and activation parameters for ion pair symme-
trization were determined in toluene-d8 unless stated other-
wise using coalescence measurements [18], line broadening
[18], 2D-EXSY [19], and 1D-DPFGSE-NOE [20]; data for
the ion pairs studied are shown in Table 1. Rate data were
determined over an average temperature range of 30 °C
with typical data sets collected over a temperature range
of ꢁ30 to ꢁ70 °C, except for [2-O(C6F5)] which exhibited
observable exchange above room temperature. A broader
range of temperature was restricted by increasing solvent
viscosity at lower temperatures and by the coalescence
point of the ion pairs in question at higher temperatures.
Rate constants at various temperatures were not deter-
mined for the ion pair [2-B(C6F5)4] since the instability of
the ion pair and its poor solubility in toluene-d8 precluded
meaningful data collection. Finally, we tested the accuracy
of our measurements by repeating the measurements made
by Marks et al. for ½Cp00ZrCH3ꢀþ½H3CBðC6F5Þ ꢀꢁ, which
½Cp00ZrCH3ꢀþ salts of [B(C6F4TBS)4]ꢁ (TBS = tert-butyl-
2
3
2
exhibited a positive DSꢀ of 13 2 eu [3e], and those made
by Brintzinger et al. on the ansa derivative [Me2Si(C5H4)2-
ZrCH3]+[H3CB(C6F5)3]ꢁ, which exhibited a substantial
negative DSꢀ of ꢁ19 2 eu [5c]. In both cases we found
dimethylsilyl) and [B(C6F4TIPS)4]ꢁ (TIPS = triisopropylsi-
lyl) [12c]. For ion pairs 2, we observe slightly negative DSꢀ
values ranging between ꢁ1 and ꢁ15 eu (Table 1). Brintzin-
ger et al. have interpreted negative DSꢀ values as implicating
Table 1
Activation parameters for ion pair symmetrization in ion pairs 2
Ion pair
DGꢀ298 (kcal molꢁ1
)
DHꢀ (kcal molꢁ1
)
DSꢀ (eu)
Tc (°C)
Methods
[2-O(C6F5)]
[2-OPh]
[2-OMe]
[2-B(C6F5)4]b
a
18.6 1.7
13.1 0.6
13.3 0.8
13 3c
16.2 1.7
9.8 0.4
8.8 0.5
ꢁ8
ꢁ11
ꢁ15
3
2
2
102
ꢁ5
7
1
Tca, LB(2)
1
1
3
Tca, LB(2), EXSY, DPFGSE-NOE
Tca, LB, EXSY, DPFGSE-NOE
Tca
7
Tc = coalescence measurement.
b
c
10% C6D5Br added for solubility.
DGꢀcoal.
.

4664
L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
Fig. 1. Overlaid Eyring plots for ion pair symmetrization in [2-OPh] (a,
top) and [2-OMe] (b, bottom) using line broadening, 2D-EXSY, and 1D-
DPFGSE-NOE measurements.
the involvement of ion pair aggregates in the mechanism of
ion pair symmetrization, and while there is substantial
evidence that such aggregates exist in more ionized systems
[12a,21] such as the borate stabilized metallocenium species
discussed here, it is generally acknowledged that such
aggregates do not play a significant role in the low concen-
tration regimes we are operating in here [12a]. Consistent
with this, the rate of ion pair symmetrization for 2-OMe
was independent of added trityl borate salt 1-OMe (0.5–
2 mM) or increasing metallocene concentration (2.5–
10 mM) of [2-OMe] (Fig. 2a and b). The absence of anion
symmetrization rate enhancements under these conditions
suggests unimolecular exchange processes, which rules out
a bimolecular (in metallocene) associative ion pair symme-
trization mechanism.
Fig. 2. Anion symmetrization rate constants in [2-OMe] as a function of
(a) added [1-OMe]; (b) increasing concentration of [2-OMe]; (c) added
C6D5Br.
depending on the size of the cation or the anion, and the
nature of the cation/anion interactions. The picture pre-
sented assumes that symmetrization of the diastereotopic
groups from the solvent separated ion pair is rapid com-
pared to ion displacement. Consistent with this mechanism
are observations of accelerated rates in more polar solvents
equipped with donor groups. Marks et al. observed signif-
icantly enhanced rates of symmetrization in more polar sol-
vents [3c] (along with more negative DSꢀ values), and we
also observe this, as the observed rate of symmetrization
for 2-OMe increases as C6D5Br is added to the sample
(increasing amounts of C6D5Br from 0 to 2.0 M, Fig. 2c).
In terms of DHꢀ values, those found for compounds 2 are
similar to each other with the exception of 2-O(C6F5), which
has an enthalpy of activation that is somewhat higher than
This being said, the negative DSꢀ values in these and
other systems, and indeed the wide variation in this param-
eter in comparable systems, requires some explanation. A
two-step mechanism involving solvent displacement of
the anion to form a solvent separated ion pair [22,23]
(Scheme 3) is one possibility to account for the observa-
tions. Here, the displacement of the anion by a solvent mol-
ecule may assume associative or dissociative character

L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
4665
Scheme 3.
the others in the series. Values for DHꢀ in this range are usu-
ally associated with more coordinating anions, such as the
methylborate anion [MeB(C6F5)3]ꢁ. The origin of this
increase in the enthalpic barrier for this anion is not clear,
but it may be due to stronger coordination of the anion
via the –OC6F5 fluorines [24], which may be sterically acces-
sible to the metallocenium fragment. 1H, 19F-1D-NOE dif-
ference NMR experiments [12a,25] were suggestive of such
interactions, via detectable NOE enhancements of the Cp–
H, Cp–Me and Zr–Me resonances upon irradiation of the
meta and para-O(C6F5) fluorine’s of the {C6F4-1,2-
[B(C6F5)2]2(l-O(C6F5))}ꢁ anion, but firm conclusions
regarding the nature of the cation/anion interaction were
not derivable from these experiments. Unfortunately, all
attempts to crystallize 2-O(C6F5) met with failure.
trometer equipped with a pulsed field gradient probe. All
metallocene samples were prepared in situ.
3.2. Solution dynamic techniques
Coalescence measurements where used to determine
DGꢀcoal. from Eq. (1), where Tc is the temperature at coales-
cence and dv is the peak separation (in Hz) at the slow
exchange limit
DGzcoal: ¼ ð1:912 ꢂ 10ꢁ2ÞTc½9:972 þ logðTc=dvÞꢀ
ð1Þ
In the case were DHꢀ and DSꢀ values were determined,
DGꢀcoal. was verified using Eq. (2)
DGzcoal: ¼ DHz ꢁ TcDSz
ð2Þ
Peak widths at half height from variable temperature line
broadening experiments [18] were determined using GNMR
software [28]. Rate constants were evaluated from Eq. (3),
where W* is the peak width (in Hz) at half height of the
exchanging system and W0 is the peak width (in Hz) at half
height in the absence of exchange (slow exchange limit)
3. Experimental
3.1. General considerations
All compounds were prepared and handled under air-
and moisture-free conditions using vacuum line techniques
or a glovebox. Toluene-d8 was dried over sodium and dis-
tilled prior to use. Bromobenzene-d5, dichloromethane-d2
and dichloromethane were dried over CaH2 and distilled
prior to use. All other solvents were dried and purified by
passing through activated alumina and Q5 columns.
[CPh3]+{C6F4-1,2-[B(C6F5)2]2(l-OMe)}ꢁ [CPh3]+{C6F4-
k ¼ pðW ꢃ ꢁ W 0Þ
ð3Þ
Quantitative analysis of variable temperature 2D-EXSY
[19] data was performed by measuring peak volumes with
AURALIA software [29]. Data with t1 and t2 dimensions of
256 and 1024 real points respectively, were collected using
a relaxation delay (D1) 5 times the T1 of the resonances in
question for eight scans of each FID. Mixing times were
chosen between 0.05 and 0.75 s at various temperatures
such that the diagonal-to-cross peak ratio was approxi-
mately 4:1. t1 and t2 dimensions were zero-filled to 2048
and 2048 points respectively. Rate constants for an equally
populated system were evaluated utilizing Eq. (4), where sm
1,2-[B(C6F5)2](l-O(C6F5))}ꢁ [13], and Cp00ZrMe2 [26] were
2
prepared from literature procedures. [CPh3]+[B(C6F5)4]ꢁ
was generously donated by Nova Chemicals Ltd. Elemental
analyses were performed by the instrumentation lab at the
University of Calgary. Routine 1H NMR and line broaden-
ing experiments were carried out on either a Bruker DRX-
1
P
P
400 or Bruker AMX2-300 spectrometer. H NMR spectra
is the mixing time and r = Int(diagonal)/ Int(cross).
were referenced relative to TMS at 0 ppm. 19F NMR were
referenced relative to external CFCl3. All variable temper-
ature experiments were calibrated with methanol or ethyl-
ene glycol [27]. 1D-DPFGSE NOE and 2D-EXSY NMR
experiments were performed on a Bruker DRX-400 spec-
k ¼ ð1=smÞ ln½ðr þ 1Þ=ðr ꢁ 1Þꢀ
ð4Þ
Quantitative analysis of variable temperature 1D-
DPFGSE NOE [20] data was performed by measuring
peak intensities with Bruker 1D WINNMR software [30].

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L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
Typical spectra (8–12) were collected with mixing times
between 0.025 and 1.50 s with a relaxation delay (D1) set
to 5 times the T1 of the resonances in question. Rate con-
stants were determined from fitting of the integrated inten-
sities to Eqs. (5) and (6) using iterative least squares fitting
with an Excel spreadsheet. With an equally populated sys-
tem, I1 and I2 are integrated intensities of excitation and
chemical exchange peaks, respectively, I0 is one-half of
the integrated intensity of Me at time = 0, and R1 and R2
are residual relaxation coefficients:
of 0.4 mL. A 0.1 equivalency of activator was added in
excess to prevent the formation of dimeric species (except
for concentration studies). Note: Determinations of the
T1 relaxation times for each exchanging signal were
determined prior to the collection of anion symmetriza-
tion rate constants by the various methods outlined
above.
3.5.1. [Cp020ZrMe]þfC6F 4-1; 2-[B(C6F 5)2]2(l-O(C6F 5))gꢁ
([2-O(C6F5)])
1H NMR (toluene-d8, 333 K): d 5.87 (m, 2H, Cp–H),
5.71 (m, 2H, Cp–H), 5.58 (m, 2H, Cp–H), 1.89 (s, 6H,
Cp–Me), 1.48 (s, 6H, Cp–Me), 0.14 (s, 3H, Zr–Me). Tc
was determined to be 375.15 K, and dv was determined to
be 165.4 Hz. Exchange rate constants were determined
for the Cp–Me signals. Line broadening – [Zr] = 5.0 mM:
line widths determined between 305.5 and 382.3 K, W0
was determined below 280 K.
I1 ¼ I0ð1 þ eꢁ2ktÞðeðꢁR tÞÞ
ð5Þ
ð6Þ
1
I2 ¼ I0ð1 þ eꢁ2ktÞðeðꢁR tÞÞ
2
3.3. Error analysis
For all kinetic experiments the estimated error values
are reported after the rate constants. These values were
established upon examination of potential sources of
experimental error including temperature accuracy, inte-
gration accuracy, mass and concentration accuracy,
NMR sensitivity, and sample purity. Propagation of errors
has led to the total estimated error. This method is benefi-
cial for determining error from potential systematic sources
of error such as temperature calibration of the NMR
probe. The key values utilized for these studies are as fol-
lows: temperature – 2.5% error, line broadening measure-
ments – 8% error, 2D-EXSY measurements – 1% error,
and 1D-DPFGSE-NOE measurements – 1% error. Errors
reported for the energetic parameters (DGꢀ, DHꢀ, and
DSꢀ) were also determined through error propagation.
3.5.2. [Cp020ZrMe]þfC6F 4-1; 2-[B(C6F 5)2]2(l-OPh)gꢁ
([2-OPh])
1H NMR (toluene-d8, 237 K): d 5.82 (m, 2H, Cp–H), 5.71
(m, 2H, Cp–H), 5.42 (m, 2H, Cp–H), 1.94 (s, 6H, Cp–Me),
1.45 (s, 6H, Cp–Me), 0.20 (s, 3H, Zr–Me). Tc was determined
to be 268.45 K, and dv was determined to be 207.7 Hz.
Exchange rate constants were determined for the a-Cp–H
and Cp–Me signals (mixing times sm given in parentheses).
Line broadening – [Zr] = 2.5 mM and [Zr] = 5.0 mM: line
widths determined between 221.6 and 250.9 K, W0 deter-
mined below 216 K. 2D-EXSY – [Zr] = 5.0 mM: T =
211.1 K (0.400 s), 216.9 K (0.225 s), 223.6 K (0.075 s), T =
230.4 K (0.060 s), T = 234.2 K (0.050 s), T = 237.1 K
(0.060 s). 1D-DPFGSE NOE – [Zr] = 2.5 mM: T = 212.0,
217.3, 222.7, 228.0, 233.4, 238.7 K.
3.4. Synthesis of [CPh3]+{C6F4-1,2-[B(C6F5)2]2-
(l-OPh)}ꢁ
3.5.3. [Cp020ZrMe]þfC6F 4-1; 2-[B(C6F 5)2]2(u-OMe)gꢁ
([2-OMe])
A solution of trityl phenoxide (0.030 g, 0.089 mol) in
dichloromethane (5 mL) was added to the diborane C6F4-
1,2-{B(C6F5)2}2 (0.075 g, 0.089 mol) and stirred overnight
at room temperature. Orange crystals were formed upon
layering this solution with hexanes (20 mL) and cooling
to ꢁ40 °C. The product was isolated upon filtration and
washed three times with cold hexanes, then dried under
1H NMR (toluene-d8, 227 K): d 5.78 (m, 2H, Cp–H),
5.70 (m, 2H, Cp–H), 5.40 (m, 2H, Cp–H), 3.65 (s, 3H, l-
OMe), 1.93 (s, 6H, Cp–Me), 1.43 (s, 6H, Cp–Me), 0.25
(s, 3H, Zr–Me). Tc was determined to be 279.80 K, and
dv was determined to be 221.2 Hz. Exchange rate constants
were determined for the a-Cp–H and Cp–Me signals (mix-
ing times sm given in parentheses). Line broadening –
[Zr] = 10.0 mM: line widths determined between 211.0
and 245.2 K, W0 determined at 205.3 K. 2D-EXSY –
[Zr] = 6.6 mM: T = 210.1 K (0.350 s), T = 215.7 K
(0.175 s), 221.7 K (0.125 s), 227.5 K (0.075 s), 236.1 K
(0.050 s). 1D-DPFGSE NOE – [Zr] = 6.6 mM: T = 215.9,
221.7, 230.4, 236.1 K.
1
reduced pressure. (0.095 g, 76%). H NMR (CD2Cl2): d
7.09 (m, 9H, –CPh3), 6.73 (m, 6H, –CPh3). 19F NMR
(CD2Cl2): d ꢁ130.1 (8F, o-F), ꢁ138.6 (2F, C6F4–),
ꢁ159.9 (4F, p-F), ꢁ163.7 (2F, C6F4–), ꢁ166.5 (8F, m-F).
Anal. Calc. for C55H20B2F24O Æ CH2Cl2: C, 53.40; H,
1.76. Found: C, 53.56; H, 1.47.
3.5. General procedure for activation
Solutions of zirconocene ion pairs ([Zr] = 2.5–10 mM)
were prepared by transferring, with a microliter syringe,
stock solutions containing the dimethyl zirconocene com-
plex, [Zr] = 32.51 mmol/L, to stock solutions containing
the activator, c = 20–30 mmol/L, directly in an NMR
tube. Deuterated solvent was added to a final volume
3.6. Additive concentration studies with the ion pair
[2-OMe]
(a) Metallocene concentration: Anion symmetrization
rate constants, determined by 1D-DPFGSE NOE NMR
spectroscopy, were obtained for the ion pair [2-OMe] at

L.D. Henderson, W.E. Piers / Journal of Organometallic Chemistry 692 (2007) 4661–4668
4667
(c) C.L. Beswick, T.J. Marks, J. Am. Chem. Soc. 122 (2000) 10358;
(d) L. Li, C.L. Stern, T.J. Marks, Organometallics 19 (2000) 3332;
(e) C.L. Beswick, T.J. Marks, Organometallics 18 (1999) 2410;
(f) P.A. Deck, C.L. Beswick, T.J. Marks, J. Am. Chem. Soc. 120
(1998) 1772;
(g) L. Li, T.J. Marks, Organometallics 17 (1998) 3996;
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10015;
the following concentrations [Zr] = 2.5, 5.0, 10.0 mM.
T = 212.0, 218.4, 229.8, 235.5, 241.9 K.
(b) [1-OMe] concentration: Anion symmetrization rate
constants, determined by 1D-DPFGSE NOE NMR spec-
troscopy, were obtained for the ion pair [2-OMe] in the
presence of excess trityl salt [1-OMe] at the following
concentrations: [Zr] = 5.0, 0.5, 1.0 and 2.0 mM excess
[1-OMe]; T = 212.0, 218.4, 229.8, 235.5, 241.9 K.
(c) C6D5Br concentration: Anion symmetrization rate
constants, determined by 1D-DPFGSE NOE NMR spec-
troscopy, were obtained for the ion pair [2-OMe] in the
presence of C6D5Br in the following concentrations:
[Zr] = 5.0, 0.5, 1.0, 2.0 mM C6D5Br; T = 212.0, 218.4,
229.8, 235.5, 241.9 K.
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3.6.1. [Cp020ZrMe]þ[B(C6F 5)4]ꢁ ([2-B(C6F5)4])
Addition of the metallocene solution to the activator
solution was done at 195 K. The solution was slowly
1
warmed in the NMR probe. H NMR (90% toluene-d8/
10% C6D5Br, 230 K): d 5.80 (m, 2H, Cp–H), 5.73 (m,
2H, Cp–H), 5.52 (m, 2H, Cp–H), 1.95 (s, 6H, Cp–Me),
1.49 (s, 6H, Cp–Me), 0.09 (s, 3H, Zr–Me). Note: Com-
pound oils out over about 1 hour below 273 K and is not
stable above room temperature. Tc was determined to be
279.92 K, and dv was taken as an average of all other deter-
minations of dv, ꢄ186 Hz.
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(2003) 7641;
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(2002) 451;
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Am. Chem. Soc. 123 (2001) 11193;
Acknowledgements
Funding for this work came from the Natural Sciences
and Engineering Research Council of Canada in the form
of a Discovery Grant (to W.E.P.) and Postgraduate Fel-
lowship support (to L.D.H.).
(b) C.R. Landis, K.A. Rosaaen, J. Uddin, J. Am. Chem. Soc. 124
(2002) 12062;
(c) C.R. Landis, K.A. Rosaaen, D.R. Sillars, J. Am. Chem. Soc. 125
(2003) 1710;
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[8] (a) A. Al-Humydi, J.C. Garrison, W.J. Youngs, S. Collins, Organo-
metallics 24 (2005) 193;
Appendix A. Supplementary material
(b) M. Mohammed, M. Nele, A. Al-Humydi, S. Xin, R.A. Stapleton,
S. Collins, J. Am. Chem. Soc. 125 (2003) 7930.
[9] (a) V. Busico, V. Van Axel Castelli, P. Aprea, R. Cipullo, A. Segre,
G. Talarico, M. Vacatello, J. Am. Chem. Soc. 125 (2003) 5451;
(b) V. Busico, R. Cipullo, W.P. Kretschmer, G. Talarico, M.
Vacatello, V. Van Axel Castelli, Angew. Chem., Int. Ed. 41 (2002)
505.
[10] (a) J.C. Yoder, J.E. Bercaw, J. Am. Chem. Soc. 124 (2002) 2548;
(b) G.M. Wilmes, J.L. Polse, R.M. Waymouth, Macromolecules 35
(2002) 6766.
Examples of each solution dynamic method, a listing of
all rate data collected, and experimental for compꢁarison
studies to the ion pairs ½Cp00ZrMeꢀþ½MeBðC6F5Þ ꢀ and
2
3
[Me2SiCp2ZrMe]+[MeB(C6F5)3]ꢁ. Supplementary data
associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.jorganchem.2007.05.023.
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