(N-pyrrolyl)B(C6F5)2 - A new organometallic Lewis acid for the generation of group 4 metallocene cation complexes

Article abstract of DOI:10.1002/(SICI)1521-3765(20000117)6:2<258::AID-CHEM258>3.0.CO;2-E

Treatment of the (C₆F₅)₂BF·OEt₂ (3) complex with N-pyrrolyl lithium gives bis(pentafluorophenyl)(N-pyrrolyl)borane (2), a strong organometallic Lewis acid, which was characterized by X-ray diffraction (B-N bond length: 1.401(5) A). It exhibits a columnar superstructure in the crystal and contains π-stacks of pyrrolyl units. Compound 2 readily abstracts alkyl anions from a variety of alkyl Group 4 metallocene-type complexes and leads to the clean formation of the respective metallocene ions or ion pairs. For example, the treatment of Cp₃ZrCH₃ (9) with 2 transfers a methyl anion to yield the ion pair [Cp₃Zr] +[(C₄H₄N)B (CH₃)(C₆F₅)₂]^- (12). The X-ray crystal structure analysis of 12 shows a close contact between zirconium and the pyrrolyl-β-carbon (2.641(2)A). The borane 2 adds to (butadiene)zirconocene (13) to yield the betaine system [Cp₂Zr]+[(C₄H₆)B(NC₄H₄)(C₆F₅)₂]^- (15). Complex 15 contains a distorted η³-allyl moiety inside the metallacyclic framework and it features an internal Zr⁺···(pyrrolyl)B^- ion pair interaction with a Zr···-pyrrolyl-α-carbon separation of 2.723(3) A (determined by X-ray diffraction). From the dynamic NMR spectra of 15 the bond strength of the internal ion pair interaction was estimated to be ΔG(diss)(≠) (223 K) ?15 kcal mol^-1. Treatment of dimethylzirconocene (16) with 2 yields the metallocene borate salt [Cp₂ZrCH₃]⁺[(C₄H₄N)B(CH₃)(C₆F₅)₂]^- (17), which is an active catalyst for the polymerization of ethene.

Full text of DOI:10.1002/(SICI)1521-3765(20000117)6:2<258::AID-CHEM258>3.0.CO;2-E

FULL PAPER
(N-Pyrrolyl)B(C6F5)2ÐA New Organometallic Lewis Acid for the Generation
of Group 4 Metallocene Cation Complexes
Gerald Kehr, Roland Fröhlich, Birgit Wibbeling, and Gerhard Erker*[a]
Abstract: Treatment of the (C6F5)2BF ´
OEt2 (3) complex with N-pyrrolyl lith-
ium gives bis(pentafluorophenyl)(N-
pyrrolyl)borane (2), a strong organome-
tallic Lewis acid, which was character-
ized by X-ray diffraction (B N bond
length: 1.401(5) Š). It exhibits a colum-
nar superstructure in the crystal and
contains p-stacks of pyrrolyl units. Com-
pound 2 readily abstracts alkyl anions
from a variety of alkyl Group 4 metal-
locene-type complexes and leads to the
clean formation of the respective metal-
locene ions or ion pairs. For example,
the treatment of Cp3ZrCH3 (9) with 2
transfers a methyl anion to yield the ion
pair [Cp3Zr] [(C4H4N)B(CH3)(C6F5)2]
the metallacyclic framework and it fea-
tures an internal Zr ´´´ (pyrrolyl)B ion
‡
‡
(12). The X-ray crystal structure analysis
of 12 shows a close contact between
zirconium and the pyrrolyl-b-carbon
(2.641(2) Š). The borane 2 adds to
(butadiene)zirconocene (13) to yield
pair interaction with a Zr´´´ pyrrolyl-a-
carbon separation of 2.723(3) Š (deter-
mined by X-ray diffraction). From the
dynamic NMR spectra of 15 the
bond strength of the internal ion
pair interaction was estimated to be
‡
the betaine system [Cp2Zr] [(C4H6)B-
ˆ
1
(NC4H4)(C6F5)2] (15). Complex 15 con-
tains a distorted h3-allyl moiety inside
DGdiss (223 K) ꢀ 15 kcalmol .
Treat-
ment of dimethylzirconocene (16) with
2 yields the metallocene borate salt
[Cp2ZrCH3] [(C4H4N)B(CH3)(C6F5)2]
‡
Keywords: alkyl anion abstraction ´
boron ´ ion pairs ´ Lewis acids ´
metallocenes ´ Ziegler catalysts
(17), which is an active catalyst for the
polymerization of ethene.
fluorine- or fluoroalkyl-substituted arene groups at boron.[7]
We have now used, to our knowledge for the first time, a
Lewis acidic abstractor/activator component in this chemistry
that bears a halogen-free nitrogen-hetaryl substituent at
boron, along with two remaining C6F5 substituents. The (N-
pyrrolyl)bis(pentafluorophenyl)borane system (2) appears to
make effective use of the electronegative Group 15 heter-
oatom and at the same time it avoids a pronounced nitrogen ±
boron p-interaction, so that the boron center in compound 2 is
a suitable cation generator in organometallic Group 4 metal
chemistry. In this work, we have characterized compound 2
and examples of its application for the effective formation of a
variety of Group 4 metal-cation systems are described and
discussed.
Introduction
Group 4 metal cations play an important role in organo-
metallic chemistry and in catalysis.[1] Many of these species are
extremely electrophilic and have a pronounced tendency to
find electronic stabilization in the absence of external donor
ligands by the formation of ion pairs.[2] Therefore, the nature
of the counteranion, and hence the mode of cation gener-
ation,[3] is quite essential for controlling the features of such
species.
Among the established methods for the generation of early
transition metal cations, the treatment of a suitable precursor,
usually a metal-alkyl or -hydride complex, with an electro-
philic borane has proven very successful and has found
extensive use and application.[2, 4] In the majority of cases, the
strong organometallic Lewis acid, tris(pentafluorophenyl)-
borane (1),[5] was used.[6] A number of related electrophilic
organoboron derivatives were tested for this application;
most of them were similar to 1 in that they bore various
Results and Discussion
Synthesis and characterization of (N-Pyrrolyl)B(C6F5)2 (2):
The title compound (2) was prepared by the reaction of the
(C6F5)2BF ´ OEt2 complex (3)[8] with N-pyrrolyl lithium (4) in a
1:1 stoichiometry in diethyl ether (Scheme 1). Crystallization
from pentane gave the boron compound 2 in ꢀ50% yield as
colorless crystals. (N-Pyrrolyl)B(C6F5)2 was characterized by
elemental analysis, spectroscopy (see below), and by an X-ray
crystal structure analysis.
[a] Prof. Dr. G. Erker, Dr. G. Kehr, Dr. R. Fröhlich,[‡]
B. Wibbeling[‡]
Organisch-Chemisches Institut der Universität
Corrensstrasse 40, D-48149 Münster (Germany)
Fax : (‡ 49)2518336503
E-mail: erker@uni-muenster.de
‡
[ ] Responsible for the X-ray crystal structure analyses.
258
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Chem. Eur. J. 2000, 6, No. 2

258±266
Scheme 1. Synthesis of 2 and 6.
In the crystal, the boron atom in 2 is bonded to two C6F5
groups and also closely bonded to the NC4H4 ring system (see
Figure 1). The B C(aryl) bond length (B C21/C21*) is
1.582(3) Š. The B N1 bond is relatively long at 1.401(5) Š
(similar to (Me3Si)2N B(C6F5)2: 1.400(3) Š[9]). Both the
boron and the nitrogen center in 2 have trigonal planar
coordination [sum of bond angles: 360.08 (B), 360.08 (N)].
The relative spatial arrangement of the (C4H4N)B(C6F5)2
monomeric units of 2 in the crystal is noteworthyÐcompound
2 adopts a columnar superstructure in the solid state (see
Figure 2). This is characterized by a stack of parallel
C4H4N [B] rings, from which the B(C6F5)2 substituents
laterally protrude. The B(C6F5)2 moieties each exhibit a
chiral two-bladed propeller-like conformation. The relative
B(C6F5)2 conformations are of the same chiral sense in each
individual column; however, they are of opposite relative
chirality in adjacent columns. The vertical separation between
the pyrrolyl rings in the columns of 2 amounts to ꢀ3.9 Š. The
corresponding intermolecular separation between C3/C3*
and the next B atom amounts to 3.98 Š.[10]
The saturated analogue of 2, (N-pyrrolidinyl)B(C6F5)2 (6),
was prepared for a structural, spectroscopic, and chemical
comparison. The compound was obtained by two different
synthetic routes (Scheme 1), namely by treatment of the
precursor (C6F5)2BF(OEt2) (3) with N-pyrrolidinyl lithium (5)
or alternatively by the reaction of N-pyrrolidinylborondi-
chloride (8)[11] with pentafluorophenyllithium (7). Both path-
ways gave 6 in ꢀ50% yield.
Figure 1. Views of the molecular structures of the compounds (N-
pyrrolyl)B(C6F5)2 (2, top) and (N-pyrrolidinyl)B(C6F5)2 (6, bottom) with
selected bond lengths [Š] and angles [8]. Compound 2: B N1 1.401(5),
B C21 1.582(3), N1 C2 1.402(3), C2 C3 1.346(4), C3 C3* 1.433(6), C21-
B-C21* 118.3(3), C21-B-N1 120.8(2), B-N1-C2 126.9(2), C2-N1-C2*
106.3(3). Compound 6: B N1 1.366(3), B C21 1.595(3), B C31 1.586(4),
N1 C2 1.479(3), C2 C3 1.509(4), C3 C4 1.505(6), C4 C5 1.508(4), N1 C5
1.488(3), C21-B-C31 116.4(2), C21-B-N1 121.3(2), C31-B-N1 122.4(2),
B-N1-C2 125.7(2), B1-N1-C5 124.8(2), C2-N1-C5 109.4(2).
In contrast to 2, the X-ray crystal structure analysis of 6 in
the solid state showed isolated molecules (Figure 1). The
B N1 bond in 6 (1.366(3) Š) is markedly shorter than in 2.
The boron atom and the nitrogen center in 6 both have a
trigonal planar coordination geometry (sum of bond angles at
N1: 359.98, at B: 360.18). The five-membered ring in 6 has
adopted a twist conformation so that C3 and C4 are located
above and below the C2-N1-C5 plane (Figure 1). The
observed geometry at N1 and rather short bond lengths
indicate a partial double-bond character of the B N linkage in
6.[12] From a structural point of view, the interaction between
the nitrogen lone pair and the boron is significantly less
pronounced in the pyrrolyl compound 2.
This interpretation is supported by the different chemical
behavior of 2 and 6 (see below) and by differences in some of
their spectroscopic properties. Thus, the 11B NMR resonance
of 2 occurs at d ˆ 40.8 (in [D6]benzene), that is, at a
substantially increased d value than that found for 6 (d ˆ
32.5). This probably indicates a marked increase in the
electrophilic character of the boron center in 2 [the strong
organometallic Lewis acid B(C6F5)3 (1) shows a 11B NMR
resonance at d ˆ 60 in [D6]benzene, while in [D8]tetrahydro-
furan (i.e., as the coordinatively saturated THF adduct) it is
d ˆ 2.5]. The 15N NMR spectra of 2 (d ˆ 188) and 6 (d ˆ
238) are also very different.[13]
Reactions with zirconocene complexes: B(C6F5)3 (1) readily
abstracts alkyl anion equivalents from zirconocene complexes
and related compounds.[2, 4] We started with the treatment of
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259

G. Erker et al.
FULL PAPER
Figure 2. Two projections of the columnar arrangement of the molecules of 2 in the crystal.
(N-pyrrolidinyl)B(C6F5)2 (6) with a variety of these Group 4
metal compounds. At room temperature, no reaction was
observed with the examples selected (see below), while
unspecific decomposition reactions took place at elevated
temperatures. This was very different when the hetarene
analogue (N-pyrrolyl)B(C6F5)2 (2) was employed. Here a very
similar behavior to compound 1 was observed. Clean methyl
abstractions or related (C4H4N)B(C6F5)2 addition reactions
were found. A series of representative examples is described
below.
The X-ray crystal structure analysis of 12 (Figure 3) shows
that a methyl group was transferred from zirconium to boron
during the reaction (B C41 1.621(2) Š). This results in the
formation of
a [(C4H4N)B(C6F5)2(CH3)] anion moiety
[B C(aryl) ˆ 1.662(2) Š and 1.651(2) Š; boron-centered
‡
bond angles between 102.8(2) and 115.0(2)8] and a [Cp3Zr]
cation unit [with the three Cp(centroid)-Zr-Cp(centroid)
angles amounting to 118.0, 118.1, and 116.48]. In the solid
state these two subunits of 12 combine to form a tight ion pair
(schematically depicted by the resonance forms A and B
shown in Scheme 2).
The closest contact between the two parts occurs between
Zr and the pyrrolyl carbon atom C13 (2.641(2) Š, while
Zr´´´ C14 ˆ 3.418(2) Š and Zr´´´ C15 ˆ 4.303(2) Š). This is
ꢀ0.2 ± 0.3 Š greater than a true Zr C s-bond;[15] however,
this interaction is quite strongÐthe Zr C13 separation falls
into the range of Zr C(Cp) bond lengths, which were found to
be between 2.549(2) and 2.660(2) Š in complex 12. It is not
unexpected that the internal pyrrolyl bonding features in 12
are quite different from those usually found in a pyrrol
moiety.[16] For instance, the pyr-
We have already reported that B(C6F5)3 abstracts a methyl
‡
anion from [Cp3ZrCH3] (9) to yield the [Cp3Zr] cation
system (10), which readily reacts with added monodentate
two-electron donors to form a variety of adducts, such as the
d0-metal carbonyl cation in 11 (see Scheme 2).[14] Complex 9
reacts quite similarly with 2: at room temperature a rapid
reaction occurs between 9 and 2 in a 1:1 stoichiometry to
afford 12 (isolated yield > 70%). Crystallization from ben-
zene gave single crystals of 12 that were suitable for an X-ray
crystal structure analysis.
rolyl bond lengths alternate in
the starting material 2: N1 C2
1.402(3) Š, C2 C3 1.346(4),
and C3 C3* 1.433(6) (Fig-
ure 1), whereas the sequence
of bond lengths inside the
(C4H4N) ring of 12 is quite
different: N11 C12 1.336(2) Š,
C12 C13 1.418(2), C13 C14
1.443(2), C14 C15 1.356(2),
Scheme 2. Reaction of 9 with B(C6F5)3 (1) and (N-pyrrolyl)B(C6F5)2 (2).
and C15 N11 1.398(2). The
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Organometallic Lewis Acids
258±266
by dynamic NMR spectroscopy. The 15N NMR resonance of
12 was observed at d ˆ 159 (in [D8]toluene at 298 K;
GHMBC, by polarization transfer from both the B-CH3 and
the pyrrolyl-C2H resonances).
We have recently reported that 1 adds to (butadiene)zirco-
nocene (13) to yield the metallocene-hydrocarbyl betaine
system 14.[18] The dipolar product has served as a single-
component Ziegler catalyst.[19] In complex 14 a lateral
coordination site at the bent metallocene wedge is used to
coordinate a fluorine atom from one of the o-C F bonds of
the C6F5 group at the boron center. The Zr´´´ F C(aryl)
coordination was shown by X-ray diffraction and by varia-
ble-temperature 19F NMR spectroscopy. From the latter, a
ˆ
1
bond dissociation energy (DGdiss† of ꢀ8 kcalmol
was
deduced.[20]
Compound 2 shows an analogous reaction pattern with
[(butadiene)ZrCp2] (13).[21] They react rapidly at ambient
temperature in toluene solution to give the dipolar 1:1
addition product 15 (isolated in 65% yield, Scheme 3).
Single crystals of 15 were obtained from benzene/pentane.
The X-ray crystal structure analysis shows that the borane 2
has cleanly added to a terminal carbon atom of the butadiene
ligand of 13 to form the betaine complex 15 (B C6 1.620(4) Š;
Figure 4). The remaining former butadiene carbon atoms
form a h3-allyl ligand moiety at zirconium (syn-substituted at
C7). Its bonding to Zr is distorted toward a s ± p coordination,
as often observed for [Cp2Zr(allyl)] groups.[22] This is evident
from the observed C C and C Zr bond lengths [C8 C9
1.411(5), C7 C8 1.359(4), C6 C7 1.496(4), C9 Zr 2.402(3),
C8 Zr 2.525(3), and C7 Zr 2.682(3) Š] and angles [C7-C8-C9
122.7(3), C6-C7-C8 125.6(3), B-C6-C7 110.7(2)8]. Since com-
plex 15 is chiral because of the p-allyl coordination, two
diastereotopic C6F5 substituents are present at boron [B C21
1.651(4) Š, B C31 1.657(4) Š, and bond angles at boron
between 106.1(2)8 and 115.2(2)8].
Figure 3. Molecular structure of 12 with an unsystematic atom numbering
scheme. Selected bond lengths [Š] and angles [8]: Zr C13 2.641(2), Zr C14
3.418(2), Zr C15 4.303(2), N11 B 1.605(2), N11 C12 1.336(2), N11 C15
1.398(2), C12 C13 1.418(2), C13 C14 1.443(2), C14 C15 1.356(2), B C21
1.662(2), B C31 1.651(2), B C41 1.621(2); N11-B-C21 102.8(1), N11-B-
C31 112.1(1), N11-B-C41 109.8(1), C21-B-C31 111.9(1), C21-B-C41
115.0(1), C31-B-C41 105.6(1), B-N11-C12 130.3(1), B-N11-C15 121.8(1),
C15-N11-C12 107.3(1), N11-C12-C13 110.9(1), C12-C13-Zr 113.1(1), C12-
C13-C14 104.3(1), Zr-C13-C14 110.2(1), C13-C14-C15 107.4(2), C14-C15-
N11 110.1(2).
N B bond length in 12 is much longer than that in the
precursor 2. The nitrogen coordination sphere is again
trigonal planar (sum of bond angles at N11: 359.48). These
features indicate some iminium ion character of the B ± pyr-
rolyl moiety of the ion pair 12, as illustrated by the mesomeric
formula B in Scheme 2.
The pyrrolyl group is the fourth substituent at the boron
atom in 15. It is again coordinated weakly to the electron-
In solution, complex 12 shows the expected NMR features
(11B NMR signal at d ˆ 6.8 in [D6]benzene at ambient
temperature).
Sharp
1H/
13C NMR resonances of the
three symmetry-equivalent Cp
ligands are observed at d ˆ 4.94
and 113.5, whereas the corre-
sponding C4H4N resonances at
d(1H) ˆ 7.65 (2-/3-H) and 5.58
(3-/4-H) as well as d(13C) ˆ
142.5 and 113.4 are broadened.
This probably indicates the be-
ginning of an exchange of the
respective signals on the NMR
timescale, which is probably
caused by a dissociation/recom-
bination process of the ion pair
structure in solution.[2, 17] Un-
fortunately, as the monitoring
temperature is lowered, the
solubility of 12 rapidly de-
creased in the various solvents
tested and has thus precluded
the investigation of this process
Scheme 3. Reaction of [Cp2Zr(butadiene)] with B(C6F5)3 (1) and (N-pyrrolyl)B(C6F5)2 (2).
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G. Erker et al.
FULL PAPER
equal intensity at 193 K in
[D8]toluene, namely a set of
four o-F (d ˆ 163.2, 162.9,
162.6, 161.9), two p-F (d ˆ
158.4, 157.7), and four m-F
signals (d ˆ 134.1,
133.9,
131.3 and 128.6).
This observed pattern indi-
cates that the rotation about the
B C(aryl) vectors is frozen at
193 K on the 282 MHz 19F
NMR timescale, and that there
are two diastereotopic C6F5
groups at boron. Raising the
temperature leads to coales-
cence of the respective m-F
and o-F pairs of resonances of
each individual C6F5 group,
while the pair of p-F resonances
remains unchanged by the on-
set of the B C(aryl) rotational
process. A Gibbs activation en-
Figure 4. Molecular structure of 15. Selected bond lengths [Š] and angles [8]: Zr C9 2.402(3), Zr C8 2.525(3),
Zr C7 2.682(3), Zr C5 2.723(3), C9 C8 1.411(5), C8 C7 1.359(4), C7 C6 1.496(4), C6 B 1.620(4), B N1
1.574(4), N1 C2 1.342(4), C2 C3 1.381(5), C3 C4 1.375(5), C4 C5 1.393(4), C5 N1 1.395(4), B C21 1.651(4),
B C31 1.657(4); C9-Zr-C8 33.2(1), C9-Zr-C7 56.8(1), C9-Zr-C5 124.9(1), C8-Zr-C7 30.1(1), C8-Zr-C5 93.5(1), C7-
Zr-C5 68.3(1), C9-C8-C7 122.7(3), C8-C7-C6 125.6(3), C7-C6-B 110.7(2), C6-B-N1 107.5(2), C6-B-C21 106.5(2),
C6-B-C31 115.2(2), N1-B-C21 114.0(2), N1-B-C31 106.1(2), C21-B-C31 107.9(2), B-N1-C2 128.4(3), B-N1-C5
123.6(2), N1-C2-C3 110.9(3), C2-C3-C4 106.7(3), C3-C4-C5 107.7(3), C4-C5-N1 108.0(3), C4-C5-Zr 103.3(2), N1-
C5-Zr 116.8(2).
ˆ
ergy of DGrot (223 K) ˆ 9.0 Æ
1
0.5 kcalmol was obtained for
the B C6F5 rotational process
of complex 15.[24]
The H/13C NMR signals of the [(p-allyl)Zr] group in 15
were observed at d ˆ 1.30, 1.92 (9-H/H')/47.0 (C9), 4.85 (8-H)/
121.0 (C8), and 4.14 (7-H)/110.0 (C7) (at 298 K in [D8]toluene,
atom numbering as in Figure 4; C(6)H2 signals at d ˆ 2.85,
2.34/28.3). These signals are almost temperature-invariant in
the applied temperature range. At low temperature [1H NMR:
213 K (600 MHz), 13C NMR: 233 K (150 MHz), both in
1
deficient zirconium center to form a (here internal) ion pair.
However, a comparison of the characteristic bonding features
reveals that the Zr/(C4H4N) [B] interaction in 15 is probably
slightly less pronounced than in 12 (see above). In 15 the
N1 B bond length is 1.574(4) Š. Again, the nitrogen center
exhibits trigonal planar coordination (sum of bond angles at
N1: 358.68).
[D8]toluene] the pyrrolyl group exhibits four H/13C NMR
1
There is a close contact between the pyrrolyl C5 and Zr
[2.723(3) Š; compare with Zr´´´ C4 3.332(3) Š and Zr´´´ N1
3.576(3) Š]. Again, the C C and C N bond order inside the
five-membered heterocycle is very different from a normal
pyrrol unit. It is also distorted towards an iminium-type
mesomeric structure, albeit less pronounced than in complex
12 (see above and Figure 3), and it has a different relative
arrangement because of the close Zr C5 (i.e., a-carbon)
contact in 15. The pertinent bond lengths are 1.395(4) Š
(N1 C5), 1.393(4) Š (C4 C5), 1.375(3) Š (C3 C4),
1.381(5) Š (C2 C3), and 1.342(4) Š (N1 C2). The internal
ion-pair character is supported by the obtained bond angles at
the pyrrolyl-carbon center C5 [116.8(2)8 (N1-C5-Zr), 103.3(2)
(C4-C5-Zr), and 108.0(3) (N1-C5-C4)], these appear to be
typical for a dominating electrostatic interaction[14, 23] without
a pronounced rehybridization at the ring carbon center. A
resonance hybrid description with the formulations A and B
(see Scheme 3) may serve as an approach to illustrate the
bonding features found for complex 15.
methine resonances of the C4H4N nucleus of equal intensity at
d ˆ 7.70, 5.22/145.0, 134.1 (a-CH), and 6.24, 5.98/110.1, 92.0 (b-
CH), as expected from the internal ion pair structure of 15
(Figure 4). Raising of the temperature results in the broad-
ening and pairwise coalescence of the respective resonances.
This dynamic behavior is most likely caused by a reversible
cleavage of the internal ion pair combined with a rotation of
the pyrrolyl ring around the B N vector (via 15a, analogous
to that observed for 14, see Scheme 3). The activation barrier
of this internal pyrrolyl equilibration process may, to a first
approximation, be used to obtain an estimate of the strength
of the interaction between the pyrrolyl-borate anion/zircono-
cene cation ion-pair.[2, 17] From the dynamic NMR spectra of
ˆ
1
15 a value of DGdiss (223 K) ˆ 15.0 Æ 0.3 kcalmol was ob-
tained.
The reaction of dimethylzirconocene (16) with (N-pyrrol-
yl)B(C6F5)2 (2) proceeded rapidly at room temperature
(Scheme 4). A methyl group is transferred from the transition
metal to the boron to yield the organometallic salt
The low-temperature NMR spectra indicate a similar
structure of complex 15 in solution. It shows a 11B NMR
signal at d ˆ 7.3 and a 15N NMR signal at d ˆ 165 (for
details see the Experimental Section). Complex 15 is chiral. It
exhibits the signals of a pair of diastereotopic Cp ligands (1H:
d ˆ 4.70, 4.45; 13C: d ˆ 109.8 , 107.0, both in [D8]toluene). The
19F NMR spectrum exhibits a total of ten C6F5 signals of
‡
[Cp2Zr(CH3)] [(C4H4N)B(CH3)(C6F5)2] (17; isolated yield:
>60%). The product shows broad 1H/13C NMR CH3[B]
resonances at d ˆ 1.10/10.9. The (N-pyrrolyl)B group exhibits
1H NMR signals at d ˆ 7.20 and 5.05 for the 2-/5-H and 3-/4-H
pairs of hydrogens (corresponding 13C NMR signals at d ˆ
139.2 and 99.4). We have no experimental information about
262
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Chem. Eur. J. 2000, 6, No. 2

Organometallic Lewis Acids
258±266
Instruments). Pyrrolidinylborondichloride (8),[11] the bis(pentafluorophen-
yl)boronfluoride ± diethyl ether complex (3),[8] bis(h5-cyclopentadienyl)di-
methylzirconium (16),[28] tris(h5-cyclopentadienyl)methylzirconium (9),[15]
and (butadiene)zirconocene (13)[29] were prepared according to literature
procedures.
Preparation of bis(pentafluorophenyl)(N-pyrrolyl)borane (2): A suspen-
sion of N-pyrrolyllithium (4, 5.3 g, 73 mmol) prepared by deprotonation of
pyrrol with n-butyllithium in diethyl ether (50 mL) was added in small
portions with stirring to a freshly prepared 0.52m solution of complex 3
(140 mL, 73 mmol) at 788C. The reaction mixture was allowed to warm
to room temperature and stirring was continued for 14 h. The solvent was
then changed from diethyl ether to pentane. The precipitate was removed
by filtration and washed with pentane (2 Â 20 mL). The pale yellow
pentane phases were combined, and their volume reduced in vacuo until
the first precipitate was observed and then stored at 88C. The product was
obtained as pale yellow crystals by fractional crystallization from pentane.
Yield: 15.6 g (52%); m.p. 1268C; 1H NMR (200.13 MHz, [D6]benzene,
300 K): d ˆ 6.59 (m, 2H; 2-/5-H), 6.23 (m, 2H; 3-/4-H); 13C NMR
(75.47 MHz, [D6]benzene, 300 K): d ˆ 147.1 (dm, 1J(C,F) ˆ 257 Hz, o-
C6F5), 143.4 (dm, 1J(C,F) ˆ 240 Hz, p-C6F5), 138.1 (dm, 1J(C,F) ˆ 254 Hz,
m-C6F5), 127.7 (C2/5), 118.3 (C3/4), 110.0 (br, ipso-C of C6F5); 11B NMR
(64.21 MHz, [D6]benzene, 300 K): d ˆ 40.8 (n1/2 ˆ 480 Hz); 15N NMR
(50.74 MHz, [D6]benzene, 298 K): d ˆ 187.5; 19F NMR (282.41 MHz,
[D6]benzene, 300 K): d ˆ 130.5 (m, 2F, o-F), 148.4 (t, 1F, p-F), 160.0
(m, 2F, m-F); C16H4NBF10 (411.0): calcd C 46.76, H 0.98, N 3.41; found C
46.58, H 1.25, N 3.30.
Scheme 4. Synthesis of 17, a catalyst for the polymerization of ethene.
the ion pairing in 17.[25] The 11B NMR spectra of 17 shows
signals at d ˆ 6.7; the 15N NMR signal is also in the expected
range at d ˆ 159.
The salt 17 is, as expected, an active catalyst for alkene
polymerization. Polyethylene is formed at the catalyst 17 in
toluene solution at 608C and 2 bar ethene pressure with a
1
moderate activity (ꢀ18 kgmol[Zr] 1 h 1 bar ).
Conclusions
X-ray crystal structure analysis of 2: Formula C16H4NBF10, M ˆ 411.01,
0.6  0.6  0.1 mm, a ˆ 17.112(1), b ˆ 11.791(1), c ˆ 7.677(1) Š, b ˆ
91.75(1)8, Vˆ 1548.2(3) Š3, 1calcd ˆ 1.763 gcm 3, m ˆ 17.14 cm 1, empirical
absorption correction with y-scan data (0.589 ꢁ C ꢁ 0.999), Z ˆ 4, mono-
clinic, space group C2/c (No. 15), l ˆ 1.54178 Š, Tˆ 223 K, w/2q scans,
2440 reflections collected (Æh, Æk, l), [(sinq/l] ˆ 0.53 Š 1, 1155 inde-
We conclude that boron Lewis acids that contain electro-
negative substituents, which are different from the usually
used fluoro- or fluoroalkyl-substituted trisarylboranes, have
an interesting application potential in metallocene cation
chemistry. In the example described here (2), the pyrrol
aromaticity keeps the electron pair at nitrogen electronically
occupied and thus prevents an effective intramolecular
neutralization of the borane Lewis acidity. The remaining
electron-withdrawing inductive effect of the nitrogen center
seems to assist in making the trivalent boron atom in 2 an
effective Lewis acid that features the typical anion abstraction
activity towards carbon centers bound to Group 4 metal-
locenes. The resulting salts, ion pairs, and addition products
show properties similar to those of their often used B(C6F5)3
analogues, although the pyrrolyl-borate moiety appears to
lead to slightly increased ion-pairing energies. This study
indicates that borane derivatives of the aromatic heterocycle
pyrrol, and probably of a great variety of other related
heterocyclic systems as well, have a marked potential as Lewis
acidic compounds for the generation of electrophilic, early
transition metal cations and of active catalyst systems derived
thereof.[26]
pendent and 1064 observed reflections [I ꢂ 2s(I)], 129 parameters, R ˆ
3
0.067, wR2 ˆ 0.191, max./min. residual electron density: 0.32/ 0.44 eŠ
,
hydrogens calculated and riding.
Preparation of bis(pentafluorophenyl)(N-pyrrolidinyl)borane (6)
Method 1: A freshly prepared suspension of pentafluorophenyllithium
(80.0 mmol) [Caution: C6F5Li is potentially explosive and should always be
handled with particular care, and only at low temperature] in pentane
(50 mL) was stirred at 788C, and a solution of compound 8 (6.07 g,
40.0 mmol) in pentane (20 mL) was added over a period of 15 min. The
reaction mixture was allowed to warm to room temperature and stirring
was continued for 14 h. All insoluble material was filtered off, and the solid
material was washed with pentane (50 mL). The combined yellowish
pentane phases were reduced to one half of their original volume and were
stored at 88C. The product was obtained as yellowish crystals by fractional
crystallization from pentane. Yield: 9.1g (55%).
Method 2: A freshly prepared solution of complex 3 (0.48 m, 150 mL,
72.0 mmol) in diethyl ether was stirred at 788C, and a suspension of N-
pyrrolidinyl lithium (5, 5.55 g, 72.0 mmol) in diethyl ether (50 mL) was
added carefully in small portions. The reaction mixture was allowed to
warm to room temperature and stirring was continued for 14 h. The solvent
was changed from diethyl ether to pentane. All insoluble material was
filtered off, and the solid was washed with pentane (2 Â 20 mL). The
combined clear pentane phases were reduced in volume in vacuo until the
first precipitate was observed and were stored at 88C. The product was
obtained as yellowish crystals by fractional crystallization from pentane.
Yield: 14.3 g (34.4 mmol, 48%); m.p. 988C; 1H NMR (200.13 MHz,
[D6]benzene, 300 K): d ˆ 2.88 (m, 4H; 2-/5-H), 1.30 (m, 4H; 3-/4-H);
13C NMR (50.5 MHz, [D6]benzene, 300 K): d ˆ 146.4 (dm, 1J(C,F) ˆ
242 Hz, o-C6F5), 141.9 (dm, 1J(C,F) ˆ 232 Hz, p-C6F5), 137.7 (dm,
1J(C,F) ˆ 253 Hz, m-C6F5), 122.2 (br, ipso-C of C6F5), 50.1 (C2/5), 25.6
Experimental Section
All reactions were carried out in an inert atmosphere (Ar) with Schlenk-
type glassware or in a glovebox. Solvents were dried and distilled under Ar
prior to use. NMR experiments were carried out on a Varian Unity plus600,
Bruker AC200P, or Bruker ARX300 spectrometer. Chemical shifts of the
heteronuclei are given relative to BF3 ´ OEt2 [d(11B) ˆ 0 for X(11B) ˆ
32.084 MHz], to neat nitromethane [d(15N) ˆ 0 for X(15N) ˆ 10.133 MHz],
and to neat CFCl3 [d(19F) ˆ 0 for X(19F) ˆ 94.077 MHz]. The spectral
assignments were mostly based on the results of a combination of APT
(attached proton test), COSY 45 (1H,1H COSY), HETCOR (1H,13C
heteronuclear shift correlation), GCOSY, 1D-TOCSY, GHSQC, and/or
GHMBC experiments.[27] IR spectra were recorded on a Nicolet 5DXC FT-
IR spectrometer; melting points were measured by DSC (DSC 2010, TA
(C3/4); 11B NMR (64.21 MHz, [D6]benzene, 300 K): d ˆ 32.5 (n1/2
ˆ
240 Hz); 15N NMR (50.74 MHz, [D6]benzene, 298 K): d ˆ 238; 19F
NMR (282.41 MHz, [D6]benzene, 300 K): d ˆ 133.0 (m, 2F, o-F),
153.4 (t, 1F, m-F), 162.1 (m, 2F, m-F); C16H8NBF10 (415.0): calcd C
46.30, H 1.94, N 3.37; found C 45.96, H 1.98, N 3.25.
X-ray crystal structure analysis of 6: Formula C16H8NBF10, M ˆ 415.04,
0.3  0.2  0.2 mm, a ˆ 9.766(1), b ˆ 17.707(2), c ˆ 10.456(1) Š, b ˆ
115.86(1)8, Vˆ 1627.1(3) Š3, 1calcd ˆ 1.694 gcm 3, m ˆ 16.32 cm 1, empirical
Chem. Eur. J. 2000, 6, No. 2
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263

G. Erker et al.
FULL PAPER
absorption correction with y-scan data (0.948 ꢁ C ꢁ 0.999), Z ˆ 4, mono-
clinic, space group P21/c (No. 14), l ˆ 1.54178 Š, Tˆ 223 K, w/2q scans,
3502 reflections collected ( h, ‡k, Æl), [(sinq/l] ˆ 0.62 Š 1, 3309 inde-
MolEN or Denzo-SMN, absorption correction for CCD data SORTAV,
structure solution SHELXS-86 and SHELXS-97, structure refinement
SHELXL-97, and graphics SCHAKAL-92.[30] Crystallographic data (ex-
cluding structure factors) for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication nos. CCDC-120975 (2), CCDC-120976 (6),
CCDC-120977 (12), and CCDC-120978 (15). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (fax: (‡44)1223-336-033, e-mail: deposit@
ccdc.cam.ac.uk).
pendent and 2512 observed reflections [I ꢂ 2s(I)], 254 parameters, R ˆ
3
0.058, wR2 ˆ 0.172, max./min. residual electron density 0.31/ 0.36 eŠ
,
hydrogens calculated and riding.
Reaction of (N-pyrrolyl)B(C6F5)2 (2) with Cp3ZrCH3 (9)Ðpreparation of
complex 12: A mixture of compounds 9 (0.13 g, 0.44 mmol) and 2 (0.18 g,
0.44 mmol) was dissolved in toluene (10 mL) at room temperature and
stirred for 1.5 h. Then the solvent was removed in vacuo. Pentane (50 mL)
was added to the remaining solid, and the mixture stirred for 3 h. The solid
product was collected by filtration, washed with pentane (2 Â 10 mL), and
dried in vacuo to afford 12 as a yellow solid. Yield: 0.22 g (71%); 1H NMR
(200.13 MHz, [D6]benzene, 300 K): d ˆ 7.65 (m, 2H; 2-/5-H), 5.58 (m, 2H;
3-/4-H), 4.94 (s, 15H; Cp), 1.28 (brm, 3H; [B]CH3); 13C NMR (125.9 MHz,
[D6]benzene, 298 K): d ˆ 148.7 (dm, 1J(C,F) ˆ 236 Hz, o-C6F5), 142.5 (br,
C2/5), 139.3 (dm, 1J(C,F) ˆ 239 Hz, p-C6F5), 137.6 (dm, 1J(C,F) ˆ 249 Hz,
m-C6F5), 113.5 (Cp), 113.4 (C3/4), 10.9 (br, [B]CH3), ipso-C of C6F5 not
Reaction of the bis(h5-methylcyclopentadienyl)dimethylzirconium with
compound
(32.4 mg, 115.9 mmol) and compound
2:[25]
Bis(h5-methylcyclopentadienyl)dimethylzirconium
2
(47.7 mg, 116.1 mmol) were
dissolved in toluene (2 mL) at room temperature and stirred for 5 min.
The solvent was removed in vacuo, pentane (5 mL) was added to the
remaining precipitate, and the mixture was stirred for 10 min. The yellow
suspension was filtered and the solid washed with pentane (2 Â 5 mL) to
1
give the product as a yellow powder. H NMR (599.9 MHz, C7D8, 298 K):
observed; 11B NMR (64.21 MHz, [D6]benzene, 300 K): d ˆ 6.8 (n1/2
ˆ
d ˆ 7.14 (m, 2H; pyrrolyl H(2,5)), 5.23/4.64 (m, each 2H; Cp(2,5)), 5.20 (m,
2H; pyrrolyl H(3,4)), 4.99/4.89 (m, each 2H; Cp(3,4)), 1.46 (s, 6H; Me),
1.01 (brm, 3H; BMe), 0.15 (s, 3H; ZrMe).
188 Hz); 15N NMR (50.74 MHz, [D8]toluene, 298 K): d ˆ 159.5; 19F
NMR (282.41 MHz, [D6]benzene, 300 K): d ˆ 132.4 (m, 4F, o-F), 159.9
(t, 2F, p-F), 164.1 (m, 4F, m-F); C32H22NBF10Zr (711.0): calcd C 53.94, H
3.11, N 1.97; found C 52.81, H 3.18, N 2.04.
Reaction of the bis(h5-methylcyclopentadienyl)dimethylzirconium with
compound
1:[25]
Bis(h5-methylcyclopentadienyl)dimethylzirconium
(20.0 mg, 71.5 mmol) and compound 1 (36.7 mg, 71.7 mmol) were dissolved
in toluene (2 mL) at room temperature and stirred for 5 min. The solvent
was removed in vacuo, pentane (5 mL) was added to the remaining
precipitate, and the mixture was stirred for 10 min. The suspension was
filtered, and the solid washed with pentane (2 Â 5 mL) to give the product
as a yellowish powder. 1H NMR (599.9 MHz, C7D8, 298 K): d ˆ 5.47/5.04
(m, each 2H; Cp(2,5)), 5.33/5.22 (m, each 2H; Cp(3,4)), 1.44 (s, 6H; Me),
0.20 (s, 3H; ZrMe), 0.06 (brm, 3H; BMe).
X-ray crystal structure analysis of 12: Formula C32H22NBF10Zr´ 2/2C6H6,
M ˆ 790.64, 0.45  0.15  0.10 mm, a ˆ 8.301(1), b ˆ 12.125(1), c ˆ
16.632(1) Š, a ˆ 91.73(1), b ˆ 100.46(1), g ˆ 97.24(1)8, Vˆ 1630.6(3) Š3,
1calcd ˆ 1.610 gcm 3, m ˆ 4.25 cm 1, absorption correction with SORTAV
Å
(0.832 ꢁ Tꢁ 0.959), Z ˆ 2, triclinic, space group P1 (No. 2), l ˆ 0.71073 Š,
Tˆ 198 K, w and y scans, 16865 reflections collected (Æh, Æk, Æl), [(sinq/
l] ˆ 0.67 Š 1, 7991 independent and 6895 observed reflections [I ꢂ 2s(I)],
527 parameters, R ˆ 0.030, wR2 ˆ 0.072, max./min. residual electron density
0.36/ 0.50 eŠ 3, hydrogens calculated and riding. The asymmetric unit
contains two half molecules of benzene.
Reaction of dimethylzirconocene (16) with (C4H4N)B(C6F5)2 (2)Ðprepa-
ration of 17: A mixture of compounds 16 (0.13 g, 0.53 mmol) and 2 (0.24 g,
0.53 mmol) was dissolved in toluene (10 mL) at room temperature and the
mixture was stirred for 5 min. The solvent was removed in vacuo, pentane
(20 mL) was added to the residue, and the mixture was stirred for 10 min.
The solid product was collected by filtration, washed with pentane (2 Â
5 mL), and dried in vacuo. Yield: 0.22 g (62%); m.p. 1958C (decomp);
1H NMR (599.9 MHz, [D8]toluene, 298 K): d ˆ 7.15 (m, 2H; 2-/5-H), 5.14
(m, 2H; 3-/4-H), 5.14 (s, 10H; Cp), 0.99 (brm, 3H; [B]CH3), 0.10 (s, 3H;
[Zr]CH3); 13C NMR (125.9 MHz, [D8]toluene, 298 K): d ˆ 148.6 (dm,
1J(C,F) ˆ 248 Hz, o-C6F5), 139.3 (dm, 1J(C,F) ˆ 250 Hz, p-C6F5), 139.2 (C2/
5), 137.7 (dm, 1J(C,F) ˆ 246 Hz, m-C6F5), 112.5 (Cp), 99.4 (C3/4), 37.3
([Zr]CH3), 10.5 (br, [B]CH3), ipso-C6F5 not observed; 11B NMR
(64.21 MHz, [D6]benzene, 300 K): d ˆ 6.7 (n1/2 ˆ 192 Hz); 15N NMR
(50.74 MHz, [D8]toluene, 298 K): d ˆ 158.5; 19F NMR (282.41 MHz,
[D6]benzene, 300 K): d ˆ 132.5 (m, 4F, o-F), 159.6 (t, 2F, p-F), 163.9
(m, 4F, m-F). C28H20NBF10Zr (661.0): calcd C 50.76, H 3.04, N 2.11; found C
49.68, H 3.14, N 1.96.
Reaction of (butadiene)zirconocene (13) with (C4H4N)B(C6F5)2 (2)Ð
preparation of 15: A ꢀ1:1 mixture of s-cis- and s-trans 13 (0.12 g,
0.44 mmol) and 2 (0.18 g, 0.44 mmol) were dissolved in toluene (10 mL) at
room temperature and the mixture was stirred for 1 h. The solvent was
removed in vacuo, pentane (50 mL) was added to the remaining solid, and
the mixture stirred for 3 h. The product 15 was collected by filtration,
washed with pentane (2 Â 10 mL), and dried in vacuo to give 15 as a yellow
solid. Yield: 0.20 g (65%); m.p. 1768C (decomp); 1H NMR (599.2 MHz,
[D8]toluene, 213 K, atom numbering as in Figure 4): d ˆ 7.70 (s, 1H; 2-H),
6.24 (d, 1H; 3-H), 5.98 (m, 1H; 4-H), 5.22 (s, 1H; 5-H), 4.75 (m, 1H; 8-H),
4.70 (s, 5H; Cp), 4.45 (s, 5H; Cp'), 4.01 (dd, 3J(H,H) ˆ 15.2 Hz, 11.3 Hz,
1H; 7-H), 2.90 (d, 2J(H,H) ˆ 16.2 Hz, 1H; 6-H), 2.52 (dd, 2J(H,H) ˆ
16.2 Hz, 3J(H,H) ˆ 11.3 Hz, 1H; 6-H'), 1.87 (dd, 2J(H,H) ˆ 4.2 Hz,
3J(H,H) ˆ 7.2 Hz, 1H; 9-H), 1.22 (dd, 2J(H,H) ˆ 4.2 Hz, 3J(H,H) ˆ
13.3 Hz, 1H; 9-H'); 13C NMR (150.7 MHz, [D8]toluene, 233 K): d ˆ 148.0
(dm, 1J(C,F) ˆ 234 Hz, o-C6F5), 145.0 (C2), 138.9 (dm, 1J(C,F) ˆ 235 Hz, p-
C6F5), 137.2 (dm, 1J(C,F) ˆ 254 Hz, m-C6F5), 134.1 (C5), 121.0 (C8), 111.0
(C7), 110.1 (C3), 109.8 (Cp), 107.0 (Cp'), 92.0 (C4), 47.2 (C9), 27.8 (br, C6),
ipso-C of C6F5 not observed; 11B NMR (64.21 MHz, [D6]benzene, 300 K):
d ˆ 7.3 (n1/2 ˆ 185 Hz); 15N NMR (50.74 MHz,[D6]benzene, 298 K): d ˆ
165.6; 19F NMR (282.41 MHz, [D8]toluene, 193 K): d ˆ 128.6 (1F, o-F),
131.3 (1F, o-F), 133.9 (1F, o-F), 134.1 (1F, o-F), 157.7 (1F, p-F),
158.4 (1F, p-F), 161.9 (1F, m-F), 162.6 (1F, m-F), 162.9 (1F, m-F),
163.2 (1F, m-F); C30H20NBF10Zr (686.5): calcd C 52.49, H 2.94, N 2.04;
found C 51.84, H 3.27, N 2.16.
Ethene polymerization: A Büchi glass autoclave (1 L) was charged with
toluene (300 mL) and triisobutylaluminum (3 mL). The mixture was
thermostated at 608C and then saturated with ethene at 2 bar for 1 h.
The catalyst 17 (78 mmol) was freshly prepared in toluene (3 mL), and this
solution was then injected by syringe into the reaction chamber. The
reaction was allowed to proceed for 1 h, and then it was quenched by the
addition of aqueous HCl/methanol (1:1 per volume, 15 mL). The resulting
polyethylene was collected by filtration, washed with HCl, methanol,
and acetone, and then dried in vacuo to give 2.9 g of polyethylene, which
1
corresponds to a catalyst activity of a ˆ 18.6 kgPEmol[Zr] 1 h 1 bar
.
X-ray crystal structure analysis of 15: Formula C30H20NBF10Zr, M ˆ 686.5,
0.2  0.2  0.05 mm, a ˆ 13.757(1), b ˆ 11.905(1), c ˆ 16.126(1) Š, b ˆ
93.27(1)8, Vˆ 2636.8(3) Š3, 1calcd ˆ 1.729 gcm 3, m ˆ 5.11 cm 1, absorption
correction with SORTAV (0.905 ꢁ Tꢁ 0.975), Z ˆ 4, monoclinic, space
group P21/n (No. 14), l ˆ 0.71073 Š, Tˆ 198 K, w and y scans, 19770 re-
flections collected (Æh, Æk, Æl), [(sinq/l] ˆ 0.71 Š 1, 7975 independent
and 5063 observed reflections [I ꢂ 2s(I)], 388 parameters, R ˆ 0.055,
wR2 ˆ 0.102, max./min. residual electron density 0.54/ 0.48 eŠ 3, hydro-
gens calculated and riding.
In a second experiment, under otherwise analogous conditions, 1.0 g of
polyethylene was formed at 61 mmol of 17 over a period of 30 min (a ˆ
16.4).
Acknowledgments
Financial support from the Fonds der Chemischen Industrie, the Deutsche
Forschungsgemeinschaft, and the Ministerium für Schule und Weiterbil-
dung, Wissenschaft und Forschung des Landes Nordrhein-Westfalen is
gratefully acknowledged.
All data sets were collected with Enraf Nonius CAD4 or KappaCCD
diffractometers, equipped with sealed-tube or rotating-anode generators.
Programs used: data collection EXPRESS or COLLECT, data reduction
264
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0947-6539/00/0602-0264 $ 17.50+.50/0
Chem. Eur. J. 2000, 6, No. 2

Organometallic Lewis Acids
258±266
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‡
the activation energy of ion pair dissociation in [(Me2C5H3)2ZrCH3] -
ˆ
[CH3B(C6F5)3] as DGdiss (353 K) ꢀ 18.3 kcalmol 1, and that methyl
ˆ
group exchange is markedly slower in this case (DGex (355 K) ˆ
19.7 kcalmol 1).[2a] We have found
a similar value for the ion
ˆ
‡
pair dissociation of [(MeC5H4)2ZrCH3] [CH3B(C6F5)3] ]: DGdiss
1
(353 K) ꢀ 17.3 Æ 0.5 kcalmol at the coalescence temperature of the
MeCp-2,4-H 1H NMR resonances at d ˆ 5.33 and 5.22. For the
‡
corresponding [(MeC5H4)2ZrCH3] [(pyrrolyl)CH3B(C6F5)2] ion pair
the activation energy of ion pair dissociation is definitively higher
ˆ
(DGdiss > 18.5 Æ 0.5 kcalmol 1); however, it could not accurately be
determined as a result of the rapid decomposition of this ion pair at
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[30] Programs used: Data collection: EXPRESS, Nonius, 1994; COL-
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Received: May 31, 1999 [F 1825]
266
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0947-6539/00/0602-0266 $ 17.50+.50/0
Chem. Eur. J. 2000, 6, No. 2