Low-temperature kinetic NMR studies on the insertion of a single olefin molecule into a Zr-C bond: Assessing the counterion-solvent interplay

Article abstract of DOI:10.1002/anie.201105122

Sticky counterions: Low-temperature kinetic NMR studies were performed to determine ΔH^≠ and ΔS^≠ values for the insertion of a single 2-methyl-1-heptene molecule into a Zr-C bond of [Cp ₂Zr(I·²-CH₂NMePh)][X] (1a: X ^-=MeB(C₆F₅)₃^-, 1b: B(C₆F₅)₄^-) in [D₈]toluene and a 1:1 mixture of [D₈]toluene and [D₅]chlorobenzene. Both activation parameters critically depend on the interplay of the counterion and the solvent. Copyright

Full text of DOI:10.1002/anie.201105122

Communications
DOI: 10.1002/anie.201105122
NMR Spectroscopy
Low-Temperature Kinetic NMR Studies on the Insertion of a Single
ꢀ
Olefin Molecule into a Zr C Bond: Assessing the Counterion–Solvent
Interplay**
Luca Rocchigiani, Gianluca Ciancaleoni, Cristiano Zuccaccia,* and Alceo Macchioni*
The insertion of an olefin molecule into a metal–carbon bond
is the fundamental step of Ziegler–Natta olefin polymeri-
zation that occurs, in homogeneous phase, through the initial
association of an olefin molecule with a metal cation of the
catalytic ion pair.[1–5] Not surprisingly, the insertion reaction is
Scheme 1. Structures of b-agostic and h2-aziridinium (P=polymeryl
strongly affected by the counterion and solvent,[6–8] the
chain; R1 and R2 =alkyl or aryl groups).
interplay of which determines the type of ion pair. “Sticky”
counterions and low polar solvents favor inner sphere ion
pairs (ISIPs), where the counterion is located in the first
coordination sphere of the catalyst, whereas weakly coordi-
nating counterions and more polar and coordinating solvents
lead to outer sphere ion pairs (OSIPs), where the counterion
is located in the second coordination sphere.[9,10]
low to moderately high polarity. Furthermore, because of the
presence of a chelating N-arm, ion pair 1 undergoes a single
ꢀ
insertion into the Zr C bond by the selection of a hindered
olefin, leading to a stable ion pair 3 featuring a five-
membered azametallacycle (Scheme 2).[28] Finally, counting
of the active sites[12,29,30] is avoided because an insertion of a
single olefin molecule occurs for all zirconium centers.
In many studies the kinetic parameters of the olefin
insertion into the metal–carbon bond are determined.[11–15]
Nevertheless, to the best of our knowledge, the activation
enthalpy (DH°) and entropy (DS°) values have never been
reported for the same catalytic system featuring the two most
ꢀ
frequently used counterions, that is, the “sticky” MeB(C6F5)3
ꢀ
and the weakly coordinating B(C6F5)4 anion. This is likely
due to the difficulties of synthesizing catalytically relevant ion
pairs that are stable in the presence of these counterions,
possibly in solvents of different polarity, for which the
insertion of an olefin molecule occurs with a rate that can
be conveniently followed by standard analytic tech-
niques.[12,16–19]
ꢀ
During our studies on the effect of the chain length on the
self-aggregation tendency of zirconocenium ion pairs,[20–21] we
synthesized zirconaaziridinium[22,23] ion pairs of the general
formula ([Cp2Zr(h2-CH2-NR1R2)][X] (1) which have some
remarkable requisites to be used as good models for inves-
Scheme 2. Olefin insertion into the Zr C bond.
Taking advantage of these rather unique peculiarities, a
low-temperature NMR kinetic study on the single insertion of
2-methyl-1-heptene (2)[31] into the Zr C bond of 1a and 1b
ꢀ
ꢀ
tigating the insertion of a single olefin molecule into a Zr C
(Scheme 2) was performed in [D8]toluene (er297K = 2.27) and a
[D8]toluene/[D5]chlorobenzene mixture (1:1 in volume, er297K
ꢁ 4.00). This led to the unprecedented evaluation of how DH°
and DS° values are affected by a change of the counterion and
solvent.
bond. Particularly, these models resemble the b-agostic-
stabilized structure that is central to olefin polymerization
(Scheme 1)[3,24–27] with the advantage that the chelation of the
nitrogen imparts a larger long-term stability to ion pairs with
both MeB(C6F5)3ꢀ and B(C6F5)4ꢀ counterions in solvents with
Olefin 2 reacted with 1 through a reversible,[32] 1,2-
regioselective single-insertion reaction affording two diaste-
reoisomers because of the presence of the nitrogen and
carbon stereogenic centers in 3 (Scheme 2). The regioselec-
tivity of the reaction was established by the observation in the
13C NMR spectrum of CH2-resonances at 49.4 and 52.0 ppm,
for the RR/SS and RS/SR isomers of 3, respectively, which are
assigned to two carbon atoms directly bound to zirconium
(see the Supporting Information). The discrimination
between RR/SS and RS/SR diastereoisomers was achieved
by the observation of selective NOE interactions of NMe
protons with the methyl moiety bound to the stereogenic
[*] Dr. L. Rocchigiani, Dr. G. Ciancaleoni, Dr. C. Zuccaccia,
Prof. Dr. A. Macchioni
Dipartimento di Chimica, Universitꢀ degli studi di Perugia
Via Elce di sotto, 8, 06123 Perugia (Italy)
E-mail: alceo@unipg.it
[**] This work was supported by grants from the Ministero dell’Istru-
zione, dell’Universitꢀ e della Ricerca through the PRIN 2009 LR88XR
program. We thank Prof. Giuseppe Cardaci for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105122.
11752
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Angew. Chem. Int. Ed. 2011, 50, 11752 –11755

of the consumptions of 1a and 1b were found to be of first
order both for zirconium and the olefin [Eq. (1)].
d½1ꢂ
ð1Þ
n ¼ ꢀ
¼ k2½1ꢂ½2ꢂ ¼ kobs½1ꢂ
dt
Values of second-order rate constants at different temper-
atures (k2 in mꢀ1 sꢀ1), obtained by dividing pseudo first-order
rate constants (kobs in sꢀ1) by the concentration of the olefin,
are reported in Table 1.
[a]
Table 1: Second-order rate constant k2 (mꢀ1 sꢀ1
)
as a function of
temperature T (K) and activation parameters DG° (kcalmolꢀ1),[b] DH°
(kcalmolꢀ1),[a] and DS° (calmolꢀ1 Kꢀ1
)
for the reaction shown in
[a]
Scheme 2.
Entry
T
k2·103
DG°
DH°
DS°
1aQ3a [D8]toluene
1
2
3
4
217.3
222.5
227.4
233.5
1.5ꢃ0.2
15.4ꢃ0.1 8.5ꢃ0.5
ꢀ32ꢃ2
2.1ꢃ0.1
3.4ꢃ0.2
6.0ꢃ0.6
15.6ꢃ0.1
Figure 1. Section of the 1H NOESY NMR spectrum of 3b ([D8]toluene/
[D5]chlorobenzene, T=297 K) showing the selective dipolar interac-
tions allowing to discriminate the RR/SS and RS/SR diastereoisomers.
15.8ꢃ0.1
15.9ꢃ0.1
1bQ3b [D8]toluene
5
6
7
8
217.3
222.5
227.4
233.5 108ꢃ8
52ꢃ6
64ꢃ2
75ꢃ5
13.9ꢃ0.1 4.0ꢃ0.5
ꢀ45ꢃ2
14.1ꢃ0.1
14.4ꢃ0.1
carbon atom (Me3, Figure 1) and with the first CH2 moiety of
the alkyl chain (H4, Figure 1). A complete NMR character-
ization is reported in the Supporting Information.
The equilibrium distribution of diastereoisomers is 50:50
at 298 K for both 3a and 3b whereas enrichment in the RR/SS
diastereoisomers is observed at lower temperature. The
abundance ratio of the two diastereoisomers does not
change in, at least, 12 h below 240 K and is independent of
the excess of olefin.
Kinetic investigations were performed in the presence of a
large excess (> 15 equiv) of 2 (pseudo first-order) over the
217.3–239.0 K range of temperature by monitoring the
disappearance of the Cp resonances of 1a and 1b in the
1H NMR spectrum (Figure 2). Under such conditions the
reaction shown in Scheme 2 went to completion. The kinetics
14.6ꢃ0.1
1aQ3a [D8]toluene/[D5]chlorobenzene
9
222.5
227.4
233.5
239.0
0.371ꢃ0.002 16.4ꢃ0.1 10.9ꢃ0.4 ꢀ25ꢃ3
10
11
12
0.747ꢃ0.006 16.4ꢃ0.1
1.28ꢃ0.09
2.28ꢃ0.05
16.6ꢃ0.1
16.8ꢃ0.1
1bQ3b [D8]toluene/[D5]chlorobenzene
0.363ꢃ0.006 16.4ꢃ0.1 10ꢃ1
13
14
15
16
222.5
227.4
233.5
239.0
ꢀ27ꢃ6
0.84ꢃ0.03
1.3ꢃ0.2
2.1ꢃ0.2
16.4ꢃ0.1
16.6ꢃ0.2
16.8ꢃ0.1
[a] Confidence intervals presented at the 67% level. [b] Confidence
intervals from standard propagation of errors.
In [D8]toluene, olefin insertion occurs about 20–30 times
faster in 1b than in 1a corresponding to a DDG° value of
about ꢀ1.3/ꢀ1.5 kcalmolꢀ1 (Table 1, entries 1–8). This result
is consistent with the few available literature data.[33–35]
Activation parameters were estimated by the dependence of
the k2 values on the temperature (Table 1, Figure 3a). In the
case of 1a, the DH° and DS° values (8.5 ꢃ 0.5 kcalmolꢀ1 and
ꢀ32 ꢃ 2 calmolꢀ1 Kꢀ1) nicely confirm these values obtained
by Landis and co-workers in the classical paper on the
polymerization of 1-hexene catalyzed by [(EBI)ZrMe][MeB-
(C6F5)3] (DH°initiation = 11.2 ꢃ 1.5 kcalmolꢀ1, DH°
=
propagation
6.4 ꢃ 1.5 kcalmolꢀ1,
DS°initiation = ꢀ24 ꢃ 5 calmolꢀ1 Kꢀ1,
DS°propagation = ꢀ33 ꢃ 5 calmolꢀ1 Kꢀ1).[12] The DH° and DS°
parameters for 1b (4.0 ꢃ 0.5 kcalmolꢀ1 and ꢀ45 ꢃ 2 calm-
olꢀ1 Kꢀ1) are relevant because they are the first reported
values for the insertion of an olefin molecule into a
ꢀ
zirconocene ion pair with a B(C6F5)4 counterion. The value
of DH° is about 4.5 kcalmolꢀ1 lower for 1b than for 1a. In line
with the associative nature of the reaction, DS° is negative but
its absolute value is higher in the case of 1b by about
13 calmolꢀ1 Kꢀ1.
Figure 2. a) Dependence of the 1H NMR spectrum on the time for the
insertion of 2 in 1b ([D8]toluene/[D5]chlorobenzene, 222.5 K); b) mono-
exponential decay of the intensity of 1a in toluene (222.5 K).
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Communications
constants when the solvent is changed from [D8]toluene to a
[D8]toluene/[D5]chlorobenzene mixture is because of the
increase in DH° (DDH° = 1.4 and 6.0 kcalmolꢀ1 for 1a and
1b, respectively). The DS° values in [D8]toluene/
[D5]chlorobenzene are less negative than that of 1b in
[D8]toluene but not too different in the case of 1a. Besides
the attenuation of the cation–anion interactions, the polar
solvent also specifically interacts with the metal center,
causing a transformation of 1a and 1b ISIPs into the
[Cp2Zr(h2-CH2-NMePh)(solvent)][X] OSIPs. This finding
ꢀ
was verified in the case of MeB(C6F5)3 because the 1H
ꢀ
resonance of the B Me protons shifts from 0.4 to 1.3 ppm,
when the solvent is changed from [D8]toluene to
a
[D8]toluene/[D5]chlorobenzene mixture and the DdmF/pF
changes from 4.3 to 2.8 ppm at 239 K.[43] The anion does not
affect the activation parameters because it is relegated away
from the metal center both in the ground and transition states.
The olefin must displace the solvent molecule from the first
ꢀ
coordination sphere to insert into the Zr C bond and this
costs around 10 kcalmolꢀ1 from the enthalpic point of view.
For this process, DS° is small and negative (DS° = ꢀ26 cal
molꢀ1 Kꢀ1) because of the balance between olefin coordina-
tion and solvent decoordination. The decoordination of a
solvent molecule does not completely compensate the loss of
entropy because of the association of an olefin molecule. This
is likely due to 1) the higher entropic content of 2, with
respect to chlorobenzene, as a consequence of the presence of
the flexible alkyl chain and 2) the more accentuated loss of
entropy of 2 that, once coordinated, is rapidly “trapped” into
ꢀ
Figure 3. Eyring plots for the insertion of 2 into the Zr C bond of 1a
and 1b in [D8]toluene (a) and a mixture of [D8]toluene and
[D5]chlorobenzene (b).
A coherent picture emerges: to reach the transition state,
a considerable amount of energy is required to remove the
“sticky” MeB(C6F5)3 counterion from the first coordination
ꢀ
ꢀ
sphere, but this enthalpic cost is attenuated by an increased
mobility of the anion, passing from ISIP to OSIP, that partly
compensates the entropic loss related to the association of the
olefin. A smaller amount of energy is required to displace the
the Zr C bond. Consistently, the expulsion of a solvent
molecule and the coordination and insertion of ethene, a
smaller olefin having an intrinsically lower entropic content,
in rac-Me2Si-(2-Me-4Ph-1-indenyl)2ZrCl2, leads to a total
recovery of entropy (DS° = 4 ꢃ 4 calmolꢀ1 Kꢀ1).[44]
ꢀ
B(C6F5)4 anion from the first coordination sphere but,
ꢀ
because of the fact that the degrees of mobility of B(C6F5)4
In conclusion, this study led to the determination of
accurate DH° and DS° values for the insertion of a single
olefin molecule into a Zr C bond, occurring in a system
are similar in ISIPs and OSIPs, the recovery of entropy is
smaller and approaches the typical value of DS° of an
associative bimolecular process occurring in low polar
solvents (around ꢀ50 calmolꢀ1 Kꢀ1).[36–39] Notably, almost
ꢀ
resembling olefin polymerization catalysts. The separation of
the enthalpic and entropic contributions provides a clear
picture of the counterion/solvent interplay. The displacement
of the counterion from the first coordination sphere, as a
consequence of olefin association, is enthalpically much more
expensive for a “sticky” counterion such as MeB(C6F5)3ꢀ with
respect to B(C6F5)4ꢀ, in low-polar and non-coordinating
solvent. The use of a slightly more polar and coordinating
solvent (a toluene/chlorobenzene mixture) causes an increase
of the DH° value because of the competition of the solvent
(specifically, chlorobenzene) with the olefin for the occupancy
of a coordination position. The DS° value associated with the
displacement of the counterion and solvent from the first
coordination sphere has been experimentally evaluated. This
process partially compensates the loss of entropy associated
with the occurrence of a bimolecular association. Interest-
ingly, the compensation is negligible for a weakly interacting
counterion, such as B(C6F5)4ꢀ, whereas it increases for the
the same recovery of entropy (DS° = ꢀ44 ꢃ 5 calmolꢀ1 Kꢀ1
)
was observed for the sole olefin polymerization catalyst with
ꢀ
the MeB(C6F5)3 counterion in the second coordination
sphere.[29]
The insertion of 2 in 1a and 1b is around 3–5 and 50–100
times slower, respectively, in [D8]toluene/[D5]chlorobenzene
than in [D8]toluene (DDG° = 0.5–1.0 and 2.2–2.5 kcalmolꢀ1
for 1a and 1b, respectively) and the anion has no effect on the
insertion rate (Table 1, entries 9–16). Although it could
appear counterintuitive that a reduced insertion rate is
observed by increasing the polarity of the reaction medium
in a system, the reactivity of which is inhibited by ionic
interactions, this is not unprecedented and has already been
reported for some metallocene catalysts,[34] CGC[40] (Con-
strained Geometry Catalysts), and non-metallocene[41] olefin
polymerization catalysts. The Eyring analysis[42] of k2 values as
a function of temperature (Figure 3b) provides a more
detailed picture: 1) the counterion does not affect both DH°
and DS° values as clearly shown by the almost coincidence of
the two straight lines in Figure 3b and 2) the reduction of rate
ꢀ
“sticky” MeB(C6F5)3 counterion. A similar recovery is
observed for the displacement of a chlorobenzene molecule
from the first to the second coordination sphere. We believe
that having determined an experimental scale of entropy
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ꢀ
recovery [solvent > MeB(C6F5)3ꢀ > B(C6F5)4 ] it is important
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reactivity of olefin polymerization catalysts.
Received: July 21, 2011
Published online: October 11, 2011
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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