Tris(pentafluorophenyl)borane adducts of substituted imidazoles: Conformational features and chemical behavior upon deprotonation

Article abstract of DOI:10.1039/b210030b

Tris(pentafluorophenyl)borane adds to the donor nitrogen atom of 1,4,5-trimethyl- and 1-methyl-4,5-diphenyl-imidazole (5a,b) to yield the corresponding adducts 6a and 6b, respectively. Treatment of 1- methylbenzimidazole with B(C₆F₅)₃ or B(C ₆H₅)₃ gave the related adducts 6c and 7, respectively. All four adducts were characterized by X-ray diffraction. In solution, the compounds 6 exhibit dynamic ¹⁹F NMR spectra, each featuring 15 separate ¹⁹F NMR resonances at low temperature. With increasing temperature a coalescence of the o-, m- and p-F signals of only a pair of -C₆F₅ signals is observed, leaving the set of five resonances of the third -C₆F₅ group unchanged. It required a further increase of the monitoring temperature to eventually observe the coalescence of the respective signals of all three -C₆F ₅ groups. A DFT study revealed no specific intramolecular interactions of the F-C(Ar) substituents with other moieties of the molecules 6; a topological control is thus likely to have caused this remarkably specific dynamic behavior. Deprotonation of the compounds 6a and 6c at carbon atom C2 was achieved by treatment with methyllithium. The expected "Arduengo carbene anions" (8) are, however, not stable under the reaction conditions but rapidly react by an intramolecular nucleophilic aromatic substitution at one of the adjacent -C₆F₅ groups to yield the heterotricyclic products 9. The respective benzimidazole-derived compound 9c was also characterized by an X-ray crystal structure analysis.

Full text of DOI:10.1039/b210030b

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Tris(pentafluorophenyl)borane adducts of substituted imidazoles:
conformational features and chemical behavior upon deprotonation
Dominik Vagedes, Gerhard Erker,* Gerald Kehr,† Klaus Bergander,† Olga Kataeva,‡
Roland Fröhlich,‡ Stefan Grimme§ and Christian Mück-Lichtenfeld§
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, D-48149 Münster,
Germany. E-mail: erker@uni-muenster.de; Fax: ϩ49 (251) 83 36503
Received 15th October 2002, Accepted 5th February 2003
First published as an Advance Article on the web 3rd March 2003
Tris(pentafluorophenyl)borane adds to the donor nitrogen atom of 1,4,5-trimethyl- and 1-methyl-4,5-diphenyl-
imidazole (5a,b) to yield the corresponding adducts 6a and 6b, respectively. Treatment of 1-methylbenzimidazole with
B(C6F5)3 or B(C6H5)3 gave the related adducts 6c and 7, respectively. All four adducts were characterized by X-ray
diffraction. In solution, the compounds 6 exhibit dynamic 19F NMR spectra, each featuring 15 separate 19F NMR
resonances at low temperature. With increasing temperature a coalescence of the o-, m- and p-F signals of only a pair
of –C6F5 signals is observed, leaving the set of five resonances of the third –C6F5 group unchanged. It required a
further increase of the monitoring temperature to eventually observe the coalescence of the respective signals of all
three –C6F5 groups. A DFT study revealed no specific intramolecular interactions of the F–C(Ar) substituents with
other moieties of the molecules 6; a topological control is thus likely to have caused this remarkably specific dynamic
behavior. Deprotonation of the compounds 6a and 6c at carbon atom C2 was achieved by treatment with methyl-
lithium. The expected “Arduengo carbene anions” (8) are, however, not stable under the reaction conditions but
rapidly react by an intramolecular nucleophilic aromatic substitution at one of the adjacent –C6F5 groups to yield the
heterotricyclic products 9. The respective benzimidazole-derived compound 9c was also characterized by an X-ray
crystal structure analysis.
corresponding anionic carbene systems 2 are unstable with
Introduction
regard to rearrangement to their thermodynamically favored
The chemistry of N,NЈ-disubstituted imidazol-2-ylidenes (1)
isomers 3 which takes place by an intermolecular reaction
(Scheme 1) (“Arduengo carbenes”)1 has found much interest in
pathway.5 However, the respective imidazol-2-ylidenes could be
recent years. These systems represent rare examples of stable
shown to be generated as reactive intermediates in the case
of the B(C6F5)3 adducts (4) because here intramolecular
SNAr-fluoride displacement is more rapid than the otherwise
and isolable carbene systems,2 and they have found some
application as ligands in transition metal catalysis.3 Substi-
tution of one of the groups attached to nitrogen by a borane
ubiquitous intermolecular borane transfer.
would result in the generation of the anionic analogues of the
We have now prepared a small series of substituted imidazole
neutral imidazol-2-ylidenes. Such “Arduengo carbene anions”
could be envisaged e.g. as novel, very useful ligand systems in
organotransition metal chemistry.
tris(pentafluorophenyl)borane adducts and found that they
display an unconventional dynamic conformational behavior.
Deprotonation of these systems probably generates the
“anionic Arduengo carbenes” as reactive intermediates that
react by subsequent ring-closure due to a rapid intramolecular
nucleophilic aromatic substitution. An example of the resulting
tricyclic intramolecular borane–imidazole adducts was, for the
first time, characterized by X-ray diffraction, which is also
described in this paper.
Results and discussion
Synthesis and structural characterization
For this study we have prepared the B(C6F5)3 adducts6,7 of
1,4,5-trimethylimidazole (5a), 1-methyl-4,5-diphenylimidazole
(5b) and 1-methylbenzimidazole (5c). For the purpose of struc-
tural comparison, we have also prepared the BPh3 adduct of 5c,
that was obtained crystalline for an X-ray crystal structure
analysis. The reactions of the reagents 5 with tris(penta-
fluorophenyl)borane were carried out in pentane at room tem-
perature (Scheme 2). The clean adducts precipitated from the
reaction mixtures as white solids during a period of ca. 12 h,
after which time the adduct formation was practically complete.
The adducts 6a–c were obtained in >90% yield. Single crys-
tals suited for an X-ray crystal structure analysis were obtained
of the 1,4,5-trimethylimidazole/B(C6F5)3 adduct 6a from
benzene at ambient temperature. Diffusion of pentane vapor
into a toluene solution gave single crystals of the 1-methyl-
benzimidazole/B(C6F5)3 addition compound 6c.
Scheme 1
To the best of our knowledge, only a single simple example
of such a system has been published as yet. Siebert and co-
workers have prepared and described the corresponding BH3
adduct (2a).4 Attempts to synthesize the corresponding BR3
(R = aryl or alkyl) systems has invariably resulted in the form-
ation of their C-bonded isomers (3). In a combined experi-
mental and theoretical study we have recently shown that the
† NMR spectroscopy.
‡ X-Ray crystal structure analyses.
§ Quantum chemical calculations.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
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propeller geometry, but their position and orientation are
quite different from one another. The C31–C36 ring system (see
Fig. 1) has its connecting B–C31 vector oriented almost in the
imidazole plane (dihedral angle C2–N1–B–C31: Ϫ5.5(2)Њ),
whereas the adjacent C21–C26 C6F5 group is oriented gauche to
this plane (along to the connecting N1–B vector). We will
denote the former as the (C6F5)in
group and the latter
plane
the (C6F5)gauche substituent. The corresponding dihedral angle
C2–N1–B–C21 amounts to 116.9(2)Њ. For illustration of this
specific conformational geometry a different view of the struc-
ture of compound 6a is depicted in Fig. 2. The distorted three-
bladed propeller geometry that contains the imidazole ring, the
C31–C36 (C6F5)in plane and the C21–C26 (C6F5)gauche substituents
at the central boron atom is characterized by dihedral angles of
Ϫ62.0(2)Њ (N1–B–C31–C36), 115.7(2)Њ (N1–B–C21–C26) and
116.9(2)Њ (C21–B–N1–C2).
Scheme 2
The X-ray crystal structure analysis of compound 6a shows
that the bulky borane has become attached covalently to the
trimethylimidazole nitrogen atom (N1–B: 1.588(2) Å, see
Fig. 1). The imidazole framework shows the typical bond
delocalization of a five-membered heteroaromatic system
(for details see the legend of Fig. 1).8
Fig. 2 A projection of the structure of compound 6a along the B–N1
vector showing the distorted three-bladed propeller arrangement made
up by the imidazole, the (C6F5)in
(C31–C36) and the (C6F5)gauche
plane
(C21–C26) groups.
The 1-methylbenzimidazole/B(C6F5)3 adduct shows an
analogous structure. Attachment of the Lewis acid to the ring
nitrogen has again only resulted in a marginal alteration of the
typical imidazole structural parameters. The most remarkable
solid state structural feature of 6c is the observed conform-
ational arrangement of the [(aryl)3/heteroaryl)] orientation
around boron (Fig. 3). It is almost identical to that observed of
Fig. 1 A view of the molecular structure of compound 6a. Selected
bond lengths (Å) and angles (Њ): N1–B 1.588(2), N1–C2 1.324(2), N1–
C5 1.397(2), C2–N3 1.322(2), N3–C4 1.386(3), C4–C5 1.355(3), N3–C6
1.459(2), C4–C7 1.482(3), C5–C8 1.492(3), B–C11 1.639(3), B–C21
1.652(2), B–C31 1.651(3); B–N1–C2 125.5(1), B–N1–C5 128.0(2), C2–
N1–C5 106.5(2), N1–C2–N3 110.8(2), C2–N3–C4 108.4(2), N3–C4–C5
106.2(2), C4–C5–N1 108.2(2), N1–B–C11 112.1(1), N1–B–C21
103.6(1), N1–B–C31 108.3(1), C11–B–C21 113.9(1), C11–B–C31
104.3(1), C21–B–C31 114.8(1).
The most noteworthy structural feature about 6a is the
observed conformational arrangement of the four aryl moieties
around the tetravalent boron center.9 Tetraarylborates can
adopt a chiral conformation where three of their planar substi-
tuents are found in an (idealized) C3-symmetric three-bladed
propeller arrangement. The remaining aryl ring cannot partici-
pate in this arrangement and serves as a pivot. Complex 6a
in particular shows a conformational topology that is related
to such an arrangement, only that the (distorted) three-
bladed propeller orientation is made up by two of the C6F5
substituents and the imidazole heteroaryl ring system.
Thus, the third C6F5 group (C11–C16) is taking up the pivot
function. It can be seen from the projection in Fig. 1, that the
N1–B vector is oriented almost coplanar with the C11–C16
plane (dihedral angles N1–B–C11–C12: Ϫ175.9(1)Њ, C2–N1–B–
C11: Ϫ119.9(2)Њ). We will term this substituent at boron the
(C6F5)piv group. The other two C6F5 substituents are part of the
Fig. 3 Projection of the molecular structure of compound 6c. Selected
bond lengths (Å) and angles (Њ): N1–B 1.600(3), N1–C2 1.324(3), N1–
C5 1.402(3), C2–N3 1.337(3), N3–C4 1.384(3), C4–C5 1.395(3), N3–
C10 1.464(3), C4–C6 1.395(3), C5–C9 1.393(3), B–C11 1.641(3), B–C21
1.647(3), B–C31 1.651(3); B–N1–C2 128.2(2), B–N1–C5 125.2(2), C2–
N1–C5 106.4(2), N1–C2–N3 112.0(2), C2–N3–C4 107.7(2), N3–C4–C5
106.3(2), C4–C5–N1 107.6(2), N1–B–C11 110.5(2), N1–B–C21
102.5(2), N1–B–C31 110.9(2), C11–B–C21 116.1(2), C11–B–C31
104.2(2), C21–B–C31 112.9(2).
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
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compound 6a. In 6c again a distorted three-bladed propeller
geometry is made up by the heteroaryl ring and two of the C6F5
planes. Again, the C31–C36 ring is placed with its B–C31 vector
syn-oriented with the (benz)imidazole N1–C2(H) vector
(dihedral angle C31–B–N1–C2 Ϫ4.8(3)Њ) and this (C6F5)in
plane
ring is rotated by 131.3(2)Њ (i.e. N1–B–C31–C32) from the
N1–B vector. The C21–C26 group makes up the (C6F5)gauche
part (C2–N1–B–C21 115.9(2)Њ, N1–B–C21–C22 Ϫ63.2(2)Њ)
that complements the chiral substructure of 6c. The remaining
C11–C16 C6F5 substituent serves as the pivot in the aryl4B-
type structure of 6c.
The B(aryl)3(heteroaryl) systems all seem to adopt this char-
acteristic conformational type in the crystal, regardless of the
presence of C6F5 groups. We have to deduce this from the typi-
cal structure of the 1-methylbenzimidazole/B(C6H5)3 adduct 7
(see Fig. 4). Here, a C6H5 group (C11–C16) serves as the pivot,
whereas the C31–C36 C6H5 substituent is oriented close to “in
plane” (C2–N1–B–C31 Ϫ20.4(2)Њ, N1–B–C31–C32 Ϫ91.7(2)Њ)
and the remaining C21–C26 ring is (C6H5)gauche
.
Fig.
5
Temperature-dependent dynamic 19F NMR spectra of
compound 6c in toluene-d8 solution.
diastereotopic and it makes the ortho- and meta-fluorines on
each of the C6F5 substituents diastereotopic as well. A con-
formation as found of the compounds 6 and 7 in the solid
state would account for the observation of these two sets
of topological diastereotopisms under static conditions in
solution. Thus, we may assume that the compound 6c adopts
similar chiral conformational structures in solution and in the
solid state and that this may probably be characterized by
the specific geometric features as they are depicted in Fig. 3
(see above).
Fig. 4 A view of the molecular structure of the methylbenzimidazole/
B(C6H5)3 adduct 7. Selected bond lengths (Å) and angles (Њ): N1–B
1.623(2), N1–C2 1.324(2), N1–C5 1.403(2), C2–N3 1.334(2), N3–C4
1.383(2), C4–C5 1.396(2), N3–C10 1.456(2), C4–C6 1.388(2), C5–C9
1.391(2), B–C11 1.627(2), B–C21 1.630(2), B–C31 1.637(2); B–N1–C2
126.6(1), B–N1–C5 126.7(1), C2–N1–C5 106.0(1), N1–C2–N3 112.4(2),
C2–N3–C4 107.7(1), N3–C4–C5 106.2(1), C4–C5–N1 107.7(1), N1–B–
C11 105.1(1), N1–B–C21 106.3(1), N1–B–C31 108.0(1), C11–B–C21
114.1(1), C11–B–C31 112.3(1), C21–B–C31 110.6(1).
Upon raising the temperature, four out of the six ortho-
fluorine signals broaden and eventually coalesce to a single
averaged signal of then four-fold intensity. Likewise two of the
para-fluorine NMR signals coalesce and four of the meta-19F
resonances coalesce to a single averaged resonance. Upon
inspection of the series of 19F NMR spectra, we thus notice that
at some intermediate temperature in the investigated temper-
ature range we are principally observing a set of ortho, meta
and para resonances of a relative intensity of two originating
from a pair of conformationally equilibrating C6F5 groups [19F
NMR signals at ca. δ Ϫ133 (ortho), δ Ϫ157 (para) and δ Ϫ164
(meta), see e.g. the spectrum at 313 K in Fig. 5] in addition
to a differentiated set of two ortho (δ Ϫ129, Ϫ130), one para
(δ Ϫ155) and two meta (δ Ϫ161, Ϫ163) 19F NMR resonances of
relative intensity one. These latter five signals correspond to the
third C6F5-group of compound 6c at boron, which is neither
conformationally equilibrating with its pair of neighboring
C6F5 groups nor shows rapid rotation around the B–C bond at
this temperature on the 19F NMR time scale. Thus we have the
curious situation that in compound 6c in this specific temper-
ature range two of the C6F5 groups are exhibiting rapid rotation
around the B–C bonds in conjunction with rapid rotation
around the N–B vector, which leads to pairwise equilibration of
the ortho- and meta-fluorines in each ring and loss of the char-
acteristic diastereotopic differentiation between these two indi-
vidual C6F5 rings, while the third C6F5 ring remains conform-
ationally “locked” in its rotated position along its connecting
B–C vector.
Dynamic behavior in solution
1
The H NMR spectrum of the benzimidazole/B(C6F5)3 adduct
(6c) shows the signal of the “isolated” 2-H proton at δ 7.61. The
corresponding 13C NMR resonance is observed at δ 141.9. In
addition we find the aromatic 4-H (δ 7.64) 5-/6-H (δ 6.88) and
7-H (δ 6.47) signals and the N–CH3 1H NMR resonance (δ 2.18)
at the expected positions. The 19F NMR spectra, though, reveal
a remarkable conformational behavior of the addition com-
pound 6c. The fluorine NMR spectra of this compound were
examined in the temperature range between 243 and 373 K at
563.6 MHz using a toluene-d8 solution. As depicted in Fig. 5 the
fluorine NMR signals of compound 6c are nicely separated into
three sets of resonances corresponding to the ortho-, para- and
meta-F substituents of the three C6F5 groups at the central
boron atom.
Moreover, in the low-temperature regime (243–253 K) 15
different fluorine NMR resonances are observed (see Fig. 5),
which indicates that a chiral tetraarylborate conformation has
become frozen on the 19F NMR time scale. We must assume
that under these conditions rotation around all three C(aryl)–B
bonds is frozen out and the rotation around the benzimid-
azole N–B vector as well. This makes all three C6F5 groups
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
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Fig. 6 DFT-calculated structures of the two conformers of 1-methylbenzimidazole/B(C6F5)3 adduct 6cA and 6cB. Structural parameters are given
in Table 2.
Table 1 Gibbs activation energies (∆G‡ in kcal molϪ1) of the two
conformational equilibration processes observed in the compounds 6
by dynamic 19F NMR spectroscopya
Table 2 Selected calculated bond lengths (Å), angles and dihedral
angles (Њ) of conformers 6cA and 6cB
6cA (Calc.)
6c (X-ray)
6cB (Calc.)
Compound
∆G‡a (T c/K)
∆G‡b (T c/K)
N1–B
C2–N1
1.621
1.322
1.600(3)
1.324(3)
1.627
1.324
6a
6b
6c
10.2 (233)
13.8 (303)
13.0 (273)
17.0 (373)
17.4 (383)
15.9 (343)
C2–N1–B
N1–B–C11
N1–B–C21
N1–B–C31
Θ1 (C2–N1–B–C31)
Θ2 (C2–N1–B–C21)
Θ3 (C2–N1–B–C11)
Φ1 (N1–B–C31–C32)
Φ2 (N1–B–C21–C22)
Φ3 (N1–B–C11–C12)
126.3
111.8
101.3
109.4
Ϫ12.1
109.2
Ϫ126.8
132.1
Ϫ63.5
13.3
128.2(2)
110.5(2)
102.5(2)
110.9(2)
Ϫ4.8(3)
115.9(2)
Ϫ119.8(2)
131.3(2)
Ϫ63.2(2)
6.9(2)
121.9
112.4
100.6
110.2
Ϫ40.9
80.8
Ϫ154.9
160.2
Ϫ88.0
43.2
a ∆G‡ 0.3 kcal molϪ1; for details see the text.
Raising the temperature further eventually results in an over-
coming of the activation barrier of this last remaining restricted
B–C6F5 rotation which leads to complete conformational equi-
libration and thus a joining of the remaining five separate
19F NMR signals into the general coalescence. At the highest
investigated temperature (373 K) we thus have monitored a
single set of three averaged ortho, para and meta C6F5 signals in
a 2 : 1 : 2 intensity ratio (see Fig. 5).
limit our discussion to compound 6c. A detailed conform-
ational search (B3LYP/TZVP) produced two minima 6cA and
6cB which are separated by 3.5 kcal molϪ1, the former being
more stable. Fig. 6 shows a view of the two calculated structures.
Obviously, 6cA corresponds to the conformation found in the
crystal. The agreement between the calculated and solid state
structures is quite good. The largest discrepancies are found for
the dihedral angle Θ1 and Φ3 (about 10Њ) which may be attrib-
uted to crystal packing effects. Conformer 6cB shows almost
local Cs symmetry of the B(C6F5)3 moiety as can be concluded
from Fig. 6. 6cB can be reached from 6cA by a 30Њ rotation
around the B–N vector with a tiny barrier for the back rotation
Line shape analysis of the para 19F NMR resonances10 gave
us the activation energies of the two spectroscopically differen-
tiated dynamic processes that were observed by the dynamic 19
F
NMR spectra of 6c. The lower energy barrier of the two, which
corresponds to a B–N and B–C rotational process equilibrating
two of the C6F5 groups, has a Gibbs activation energy of ∆G‡
a
(273 K) = 13.0 0.3 kcal molϪ1. Overcoming the B–C rotational
barrier of the remaining B–C6F5 unit is characterized by a
slightly higher activation energy of ∆G‡ (343 K) = 15.9 0.3
b
kcal molϪ1
.
The compounds 6a and 6b show the same type of conform-
ational behavior. Each of these compounds exhibits 15 equal
intensity 19F NMR signals at low temperature. Raising the
monitoring temperature in each case results in a coalescence of
the signals of two C6F5 groups combined with pairwise ortho-
and meta-F coalescence (process a), followed by a separated
coalescence of the remaining C6F5 group at distinctly higher
temperature (process b). The Gibbs activation energies of these
processes were also obtained from a line shape analysis of their
respective sets of para 19F NMR signals. The ∆G‡ and ∆G‡
6cB
6cA of 0.2 kcal molϪ1. The calculated structure 6cA did
not reveal any exceptionally close contacts between fluorine
atoms and the imidazole nucleus or its periphery that could
account for any of the unusual dynamic behavior exhibited by
this compound. We have therefore examined a variety of spe-
cific single N–B and B–C rotational processes to check whether
any specific unfavorable interaction could be detected that
would explain why one of the C6F5 rings would not give up so
easily its “left/right” separation whereas the others did. No
specific pathway could, however, be detected that would indi-
cate a necessary raising in energy to account for the observed
behavior.
Due to the enormous complexity of the total conformational
surface of the compounds 6 we did not attempt to calculate the
overall favored internal equilibration pathway that is generally
followed in these unsymmetrical tetraarylborate type com-
pounds, but it seems clear that a highly cooperative mechanism
must be followed here that results in the observed unusual, very
specific conformational features of these molecules.
a
b
values of the compounds 6a and 6b are in a similar order of
magnitude as observed of the benzimidazole/B(C6F5)3 adduct
6c (see Table 1).
Theoretical studies
We have carried out a theoretical study using density functional
theory (DFT) to achieve some mechanistic insight into the con-
formational processes that determine the observed unusual
dynamic behavior of the methylimidazole/B(C6F5)3 adducts 6
that we had monitored by 19F NMR spectroscopy in solution.
We have investigated 6a and 6c which behave very similarly, but
To illustrate this, we have performed a molecular dynamics
simulation of compound 6c at the semiempirical MNDO11
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
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Table 3 19F NMR spectra of the compounds 9a and 9ca
Compound
2Ј-F
3Ј-F
4Ј-F
5Ј-F
o-Fb
m-Fb
p-Fb
9a
9c
Ϫ132.6
Ϫ131.7
Ϫ157.2
Ϫ156.0
Ϫ151.2
Ϫ148.2
Ϫ134.8
Ϫ132.3
Ϫ132.8
Ϫ132.7
Ϫ164.0
Ϫ163.5
Ϫ157.5
Ϫ157.2
a 9a in benzene-d6–THF-d8 (10 : 1), 9c in toluene-d8–THF-d8 (10 : 1) at 563.6 MHz, ambient temperature. b C6F5 groups.
level. After equilibration (50 ps), values for Θ1, Φ1, Φ2 and Φ3
were followed for additional 50 ps. The results shown in Fig. 7
clearly demonstrate the cooperative motion of the three aryl
rings and the propeller as a whole. Although in our MD simu-
lation no full rotation event has been observed, the moderate
twisting motions of the rings (about 10–20Њ amplitude with a
period of about 2 ps) occur fully correlated.
Scheme 3
closed a heterocyclic ring system due to the covalent attachment
of the (aryl)3B moiety through the newly formed carbon–
carbon bond between the adjacent imidazole C2 center and a
boron bound aryl group.
The products 9 show very characteristic NMR spectra,
including their 19F NMR spectra (see Table 3). In addition,
the tricyclic product 9c was characterized by an X-ray crystal
structure analysis (see Fig. 8).
Fig. 7 Dihedral angles Θ1 and Φ1–Φ3 along a 50 ps MNDO trajectory
(NVT ensemble, T = 300 K) for compound 6c. For clarity, the negative
of Φ2 is plotted.
In summary, our calculations did not reveal any evidence for
a specific C–F interaction—be it in the ground state or at some
point along potential rotational pathways—with other groups
of the molecules that could account for the observed differen-
tiation in the rotational behavior of one pair of C6F5 groups
against the remaining single pentafluorophenyl substituent. It
remained as a possible explanation that these observed charac-
teristic conformational features of 6 are due to a general
topochemical control. A natural candidate for the specific
C6F5 group that needed a slightly higher energy to rotate
around the B–C vector would then be the (C6F5)pivot (Φ3) unit.
Unfortunately, much longer MD runs in the nanosecond range
to check this hypothesis are computationally not feasible at
present. Before additional experimental evidence becomes
available to us this seems to be a reasonable assumption to
account for the remarkable observed dynamic behavior of these
compounds.
Fig. 8 Molecular structure of 9c. Selected bond lengths (Å) and angles
(Њ): N1–B 1.590(2), N1–C2 1.345(2), N1–C5 1.386(2), C2–N3 1.340(2),
N3–C4 1.391(2), C4–C5 1.399(3), C2–C36 1.456(3), N3–C10 1.467(2),
C4–C6 1.390(3), C5–C9 1.390(3), B–C11 1.629(3), B–C21 1.646(3),
B–C31 1.625(3); B–N1–C2 112.9(1), B–N1–C5 139.3(2), C2–N1–C5
107.8(2), N1–C2–N3 110.8(2), C2–N3–C4 107.5(2), N3–C4–C5
107.1(2), C4–C5–N1 106.8(2), N1–C2–C36 111.9(2), N3–C2–C36
137.2(2), N1–B–C11 112.0(2), N1–B–C21 106.4(1), N1–B–C31 95.9(1),
C11–B–C21 115.0(1), C11–B–C31 110.0(2), C21–B–C31 115.9(2).
Single crystals of 9c that were suitable for X-ray crystal struc-
ture analysis were obtained by allowing pentane vapor diffuse
into a toluene solution at 4 ЊC. The structural analysis shows
the presence of a planar tricyclic ring system. The benzimid-
azole ring is connected with the C6F4 unit by means of the
newly formed C2–C36 bond (see Fig. 8) and the B–N1 linkage.
The latter amounts to 1.590(2) Å, which is marginally shorter
than the N–B bond found in its precursor, the adduct 6c (see
above). In 9c the remaining C6F5 groups at the boron center are
also found in a rotated conformation, making the B(C6F5)2
moiety a chiral subunit.
Reactions with bases
The compounds 6a and 6c were each treated with methyl-
lithium. In each case the imidazole 2-H proton is selectively
abstracted. We assume that this reaction generates the corre-
sponding “Arduengo carbene anions” (8a, 8c). However, these
compounds are unstable under the applied reaction conditions
due to a rapid intramolecular nucleophilic aromatic substi-
tution reaction.5,12 Attack of the C2 carbanion-type nucleophile
takes place at the ortho C–F moiety of one of the C6F5 groups.
Fluoride is eliminated and lithium fluoride precipitated. The
resulting C–C coupled products (9a, 9c, see Scheme 3) were
isolated in high yield. They represent intramolecular imidazo-
lium borates themselves, only that the N3–B interaction has
Experimental
Reactions with organometallic compounds were carried out in
an inert atmosphere (argon) using Schlenk-type glassware or in
a glovebox. Solvents, including deuterated solvents used for
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
1341

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NMR spectroscopy, were dried and distilled under argon prior
to use. The following instruments were used for physical charac-
terization of the compounds: Bruker AC 200 P NMR spectro-
meter (1H: 200 MHz; 13C: 50 MHz; 11B: 64 MHz) at 300 K and
Varian Unity plus (1H: 600 MHz; 13C: 150 MHz; 19F: 564 MHz)
NMR spectrometer at 298 K; a Nicolet 5 DXC FT–IR spectro-
meter; elemental analysis were carried out with a Foss-Heraeus
CHN-rapid elemental analyzer or a Vario El III micro ele-
mental analyzer; melting points were determined by differential
scanning calorimetry (2010 DSC, Du Pont/STA Instruments).
1,4,5-Trimethylimidazole (5a),13 1-Methyl-4,5-diphenylimid-
azole (5b)14 and tris(pentafluorophenyl)borane6,15 were pre-
pared according to literature procedures.
C-2), 133.5 (C, C-5), 130.6 (CH, o-Ph), 130.5 (CH, o-Ph), 129.3
(C, ipso-C), 129.3 (CH, p-Ph), 128.8 (CH, p-Ph), 128.4 (CH,
m-Ph), 128.2 (CH, m-Ph), 126.0 (C, ipso-C), 120.1 (C, ipso-C),
32.7 (CH3, NCH3). δF (benzene-d6, 564 MHz): Ϫ127.7 (m, 1F,
o-PhF), Ϫ130.5 (m, 1F, o-PhF), Ϫ133.4 (br, 4F, o-PhF), Ϫ155.3
(m, 1F, p-PhF), Ϫ158.0 (m, 2F, p-PhF), Ϫ160.4 (m, 1F, m-PhF),
Ϫ161.1 (m, 1F, m-PhF), Ϫ165.1 (br, 4F, m-PhF). δB (benzene-d6,
64 MHz): Ϫ7.7 (ν = 240 Hz).
½
1-Methylbenzimidazole–tris(pentafluorophenyl)borane adduct
(6c). Reaction of 437 mg (3.31 mmol) 1-methylbenzimidazole
(5c) with 1.69 g (3.31 mmol) tris(pentafluorophenyl)borane in
pentane carried out as described above yielded 1.92 g (90%) of
the adduct 6c as a white solid. mp: 231 ЊC. Anal. Calc. for
C26H8N2BF15 (644.1): C, 48.48; H, 1.25; N, 4.35. Found: C,
48.04; H, 1.33; N, 4.16%. IR (KBr): ν 3000, 1649, 1557, 1517,
1424, 1392, 1284, 1100, 984, 864 and 748 cmϪ1. δH (benzene-d6,
600 MHz): 7.64 (d, 3JHH = 7.6 Hz, 1H, 4-H), 7.61 (s, 1H, 2-H),
Preparations
1-Methylimidazole–borane derivatives 6a–c (general pro-
cedure). A suspension of the respective imidazole in 20 ml
pentane was cooled to 0 ЊC and reacted with an equivalent
amount of tris(pentafluorophenyl)borane. After stiring for 12 h
at room temperature the solid was collected by filtration and the
product was dried in vacuo.
3
6.88 (m, 2H, 5-H and 6-H), 6.47 (d, JHH = 7.3 Hz, 1H, 7-H),
2.18 (s, 3H, NCH3). δC (benzene-d6, 150 MHz): 148.8 (dm,
1
1JCF = 243 Hz, o-PhF), 141.9 (CH, C-2), 140.4 (dm, JCF = 248
Hz, p-PhF), 137.5 (dm, 1JCF = 253 Hz, m-PhF), 135.7 (C, C-7a),
132.5 (C, C-3a), 126.1 (CH, C-6), 125.7 (CH, C-5), 118.9 (C,
ipso-C), 116.3 (CH, C-4), 111.6 (CH, C-7), 30.9 (CH3, NCH3).
δF (benzene-d6, 564 MHz): Ϫ129.5 (m, 1F, o-PhF), Ϫ130.5 (m,
1F, o-PhF), Ϫ134.7 (br, 4F, o-PhF), Ϫ155.7 (m, 1F, p-PhF),
Ϫ157.2 (br, 2F, p-PhF), Ϫ161.5 (m, 1F, m-PhF), Ϫ163.1 (m, 1F,
m-PhF), Ϫ164.1, (br, 4F, m-PhF). δB (benzene-d6, 64 MHz): Ϫ7.9
1,4,5-Trimethylimidazole–tris(pentafluorophenyl)borane
adduct (6a). Reaction of 110 mg (1.00 mmol) 1,4,5-trimethyl-
imidazole (5a) with 512 mg (1.00 mmol) of tris(pentafluoro-
phenyl)borane in pentane carried out as described above
yielded 588 mg (95%) of the adduct 6a as a white solid. mp: 232
ЊC. Anal. Calc. for C24H10N2BF15 (622.1): C, 46.33; H, 1.62; N,
4.50. Found: C, 46.19; H, 1.53; N, 3.17%. IR (KBr): ν 3176,
2935, 1647, 1563, 1524, 1458, 1373, 1283, 1097, 978, 870, 785
and 697 cmϪ1. δH (benzene-d6, 600 MHz): 7.12 (s, 1H, 2-H), 1.81
(s, 3H, NCH3), 1.57 (s, 3H, 4-CH3), 0.99 (s, 3H, 5-CH3).
(ν = 250 Hz).
½
X-Ray crystal structure analysis of 6c: formula C26H8BF15N2,
M = 644.15, colourless crystal 0.60 × 0.50 × 0.20 mm,
a = 9.949(1), b = 15.245(1), c = 16.698(1) Å, β = 100.61(1)Њ,
V = 2489.3(3) Å3, Dc = 1.719 g cmϪ3, µ = 1.80 cmϪ1, empirical
absorption correction via SORTAV (0.900 ≤ T ≤ 0.965), Z = 4,
monoclinic, space group P21/n (no. 14), λ = 0.71073 Å, T =
198 K, ω and φ scans, 15549 reflections collected ( h, k, l ),
[(sinθ)/λ] = 0.66 ÅϪ1, 5909 independent (Rint = 0.053) and 3742
observed reflections [I ≥ 2σ(I )], 398 refined parameters, R =
0.050, wR2 = 0.113, max. residual electron density 0.31 (Ϫ0.24)
e ÅϪ3, hydrogens calculated and refined as riding atoms.
1
δC (benzene-d6, 150 MHz): 148.7 (dm, JCF = 244 Hz, o-PhF),
1
1
140.3 (dm, JCF = 249 Hz, p-PhF), 137.4 (dm, JCF = 249 Hz,
m-PhF), 135.5 (CH, C-2), 131.4 (C, C-4), 125.8 (C, C-5), 119.4
(C, ipso-C), 31.1 (CH3, NCH3), 9.7 (CH3, 4-Me), 6.9 (CH3,
5-Me). δF (benzene-d6, 564 MHz): Ϫ128.2 (m, 1F, o-PhF),
Ϫ129.3 (br, 1F, o-PhF), Ϫ131.0 (m, 1F, o-PhF), Ϫ133.5, Ϫ134.3,
Ϫ139.7 (each br, each 1F, each o-PhF), Ϫ155.7 (m, 1F, p-PhF),
Ϫ156.9, Ϫ157.4 (each br, each 1F, each p-PhF), Ϫ161.4 (m, 1F,
m-PhF), Ϫ163.3 (m, 1F, m-PhF), Ϫ163.6, Ϫ164.0, Ϫ164.5,
Ϫ164.9 (each br, each 1F, each m-PhF). δB (benzene-d6, 64
1-Methylbenzimidazole–triphenylborane adduct (7). Tri-
phenylborane (1.00 g, 4.13 mmol) was added to a suspension
of 1-methylbenzimidazole (5c) (546 mg, 4.13 mmol) in 20 ml
pentane at 0 ЊC. The reaction mixture was allowed to warm to
room temperature and was stirred for another 12 h. The precipi-
tate was collected by filtration to yield a white solid 1.27 g
(82%). mp: 208 ЊC. Anal. Calc. for C26H23N2B (374.3): C, 83.43;
H, 6.19; N, 7.48. Found: C, 82.00; H, 6.26; N, 7.06%. IR (KBr):
ν 3065, 2997, 1539, 1487, 1461, 1425, 1382, 1256, 1187, 1160,
1082, 976, 857 and 747 cmϪ1. δH (benzene-d6, 600 MHz): 7.69
(m, 6H, o-Ph), 7.62 (s, 1H, 2-H), 7.32 (m, 6H, m-Ph), 7.37 (d,
3JHH = 8.5 Hz, 1H, 4-H), 7.22 (m, 3H, p-Ph), 6.79 (m, 1H, 6-H),
6.63 (m, 1H, 5-H), 6.49 (d, 3JHH = 8.3 Hz, 1H, 7-H), 2.03 (s, 3H,
NCH3). δC (benzene-d6, 150 MHz): 153.4 (C, ipso-C), 143.1
(CH, C-2), 137.1 (C, C-7a), 135.2 (CH, o-Ph), 133.4 (C, C-3a),
127.5 (CH, m-Ph), 125.4 (CH, p-Ph), 124.7 (CH, C-6), 124.3
(CH, C-5), 121.5 (CH, C-4), 110.2 (CH, C-7), 30.4 (CH3,
MHz): Ϫ8.7 (ν = 140 Hz).
½
X-Ray crystal structure analysis of 6a: formula C24H10-
BF15N2, M = 622.15, colourless crystal 0.35 × 0.25 × 0.20 mm,
a = 9.791(1), b = 19.826(1), c = 12.529(1) Å, β = 102.24(1)Њ,
V = 2376.8(3) Å3, Dc = 1.739 g cmϪ3, µ = 1.85 cmϪ1, empirical
absorption correction via SORTAV (0.938 ≤ T ≤ 0.964), Z = 4,
monoclinic, space group P21/n (no. 14), λ = 0.71073 Å, T = 198
K, ω and φ scans, 22809 reflections collected ( h, k, l ),
[(sinθ)/λ] = 0.67 ÅϪ1, 5800 independent (Rint = 0.052) and 3695
observed reflections [I ≥ 2σ(I )], 382 refined parameters, R =
0.043, wR2 = 0.093, max. residual electron density 0.25 (Ϫ0.28)
e ÅϪ3, hydrogens calculated and refined as riding atoms.
1-Methyl-4,5-diphenylimidazole–tris(pentafluorophenyl)-
borane adduct (6b). Treatment of 133 mg (568 µmol) 1-methyl-
4,5-diphenylimidazole (5b) with 291 mg (568 µmol) tris(penta-
fluorophenyl)borane in pentane according to the general
procedure yielded 399 mg (94%) of product 6b. mp: 242 ЊC.
Anal. Calc. for C34H14N2BF15 (746.29): C, 54.72; H, 1.89; N,
3.75. Found: C, 54.60; H, 1.68; N, 2.38%. IR (KBr): ν 3165,
1647, 1549, 1475, 1460, 1447, 1374, 1283, 1142, 1099, 983, 918,
807, 766 and 697 cmϪ1. δH (benzene-d6, 600 MHz): 7.55 (s, 1H,
2-H), 6.77 (m, 1H, p-Ph), 6.74 (m, 2H, m-Ph), 6.70 (br, 2H,
o-Ph), 6.68 (m, 1H, p-Ph), 6.58 (m, 2H, m-Ph), 6.53 (m, 2H,
o-Ph), 2.25 (s, 3H, NCH3). δC (benzene-d6, 150 MHz): 148.9
NCH3). δB (benzene-d6, 64 MHz): 0.1 (ν = 470 Hz).
½
X-Ray crystal structure analysis of 7: formula C26H23BN2,
M = 374.27, light yellow crystal 0.25 × 0.25 × 0.10 mm,
a = 20.155(2), b = 14.400(1), c = 17.119(2) Å, β = 123.12(1)Њ,
V = 4161.2(7) Å3, Dc = 1.195 g cmϪ3, µ = 5.27 cmϪ1, empirical
absorption correction via ψ scan data (0.880 ≤ T ≤ 0.949),
Z = 8, monoclinic, space group C2/c (no. 15), λ = 1.54178 Å,
T = 223 K, ω/2θ scans, 8590 reflections collected ( h, k, l ),
[(sinθ)/λ] = 0.62 ÅϪ1, 4241 independent (Rint = 0.033) and 2791
observed reflections [I ≥ 2σ(I )], 264 refined parameters, R =
0.041, wR2 = 0.110, max. residual electron density 0.22 (Ϫ0.17)
e ÅϪ3, hydrogens calculated and refined as riding atoms.
1
1
(dm, JCF = 239 Hz, o-PhF), 140.4 (dm, JCF = 252 Hz, p-PhF),
137.4 (dm, JCF = 245 Hz, m-PhF), 137.4 (C, C-4), 137.3 (CH,
1
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
1342

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Treatment of compound 6a with methyllithium, formation of
the heterocycle 9a. To a mixture of 100 mg (159 µmol) 1,4,5-
trimethylimidazole–tris(pentafluorophenyl)borane adduct (6a)
and 3.50 mg (159 µmol) of methyllithium 2 ml of benzene–
tetrahydrofuran (10 : 1) was added. The resulting reaction mix-
ture was allowed to stir for 12 h, the solvent was removed in
vacuo and benzene was added. A lithium fluoride precipitate
was separated by filtration and removal of the solvent yielded a
light brown solid (85 mg, 89%). mp: 70 ЊC. Anal. Calc. for
C24H9N2BF14 (602.1): C, 47.87; H, 1.51; N, 4.65. Found: C,
47.66; H, 2.81; N, 4.76%. IR (KBr): ν 2965, 1645, 1556, 1518,
1464, 1421, 1378, 1285, 1263, 1096, 1022, 976, 871 and 762
cmϪ1. δH (benzene-d6–tetrahydrofuran-d8 10 : 1, 600 MHz):
2.99 (d, JHF = 2.4 Hz, 3H, NCH3), 1.75 (s, 3H, 5-CH3), 1.45 (s,
3H, 4-CH3). δC (benzene-d6–tetrahydrofuran-d8 10 : 1, 150
MHz): 148.9 (dm, 1JCF = 245 Hz, o-PhF), 145.5 (C, C-2), 140.3
(dm, 1JCF = 251 Hz, p-PhF), 137.6 (dm, 1JCF = 250 Hz, m-PhF),
129.5 (C, C-5), 128.3 (C, C-4), 115.2 (C, ipso-C), 32.2 (CH3, JCF
= 17.2 Hz, NCH3), 8.8 (CH3, 5-CH3), 8.1 (CH3, 4-CH3). The
carbon-atoms C-1Ј to C-6Ј were not detected. δF (benzene-d6–
tetrahydrofuran-d8 10 : 1, 564 MHz): Ϫ132.6 (m, 1F, F-2Ј),
Ϫ132.8 (m, 4F, o-PhF), Ϫ134.8 (m, 1F, F-5Ј), Ϫ151.2 (m, 1F,
F-4Ј), Ϫ157.2 (m, 1F, F-3Ј), Ϫ157.5 (m, 2F, p-PhF), Ϫ164.0 (m,
4F, m-PhF). δB (benzene-d6–tetrahydrofuran-d8 10 : 1, 64 MHz):
See http://www.rsc.org/suppdata/dt/b2/b210030b/ for crystal-
lographic data in CIF or other electronic format.
DFT Calculations
Geometries were optimized with the Turbomole package of
programs22 without any symmetry restrictions. We have used
the B3-LYP hybrid functional23 together a Gaussian AO basis
set of TZVP quality.24 Comparative calculations using the B–P
functional or the MNDO11 semi empirical method gave similar
results.
For the molecular dynamics simulation, a modified version
of the Mopac 7.0 program25 was used.
Acknowledgements
Financial support from the Fonds der Chemischen Indus-
trie and the Deutsche Forschungsgemeinschaft is gratefully
acknowledged.
References
1 A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
1991, 113, 361; A. J. Arduengo III, H. V. R. Dias, R. L. Harlow and
M. Kline, J. Am. Chem. Soc., 1992, 114, 5530; review: A. J.
Arduengo III, Acc. Chem. Res., 1999, 32, 913.
Ϫ8.0 (ν = 130 Hz).
½
Treatment of compound 6c with methyllithium, formation of
the heterocycle 9c. Analogous to the preparation of compound
9a, 176 mg (272 µmol) of the 1-methylbenzimidazole–tris-
(pentafluorophenyl)borane adduct (6c) was reacted with 6.00
mg (272 µmol) of methyllithium in 2 ml benzene–tetra-
hydrofuran (10 : 1). Workup of the suspension yielded in a
brown solid (160 mg, 94%). mp: 61 ЊC. Anal. Calc. for
C26H7N2BF14 (624.1): C, 50.03; H, 1.13; N, 4.49. Found: C,
50.79; H, 3.98; N, 4.05%. IR (KBr): ν 2965, 1645, 1608, 1575,
1550, 1518, 1460, 1423, 1383, 1315, 1291, 1098, 975, 796 and
751 cmϪ1. δH (benzene-d6–tetrahydrofuran-d8 10 : 1, 600 MHz):
7.61 (m, 1H, 4-H), 7.02 (m, 2H, 5-H and 6-H), 6.93 (m, 1H, 7-
H), 3.36 (d, JHF = 3.6 Hz, 3H, NCH3). δC (benzene-d6–tetra-
hydrofuran-d8 10 : 1, 150 MHz): 152.7 (C, C-2), 148.9 (dm,
1JCF = 240 Hz, o-PhF), 140.5 (dm, 1JCF = 251 Hz, p-PhF), 137.7
2 D. Bourissou, O. Guerret, F. P. Gabbaï and G. Bertrand, Chem. Rev.,
2000, 100, 39.
3 J. Huang, E. D. Stevens, S. P. Nolan and J. L. Petersen, J. Am. Chem.
Soc., 1999, 121, 2674; M. Scholl, T. M. Trnka, J. P. Morgan and
R. H. Grubbs, Tetrahedron Lett., 1999, 40, 2247; T. Weskamp,
F. J. Kohl, W. Hieringer, D. Gleich and W. A. Hermann,
Angew. Chem., 1999, 111, 2573; T. Weskamp, F. J. Kohl,
W. Hieringer, D. Gleich and W. A. Hermann, Angew. Chem.,
Int. Ed., 1999, 38, 2416; M. Scholl, S. Ding, C. W. Lee and R. H.
Grubbs, Org. Lett., 1999, 1, 953; L. Jafarpour, A. C. Hillier and
S. P. Nolan, Organometallics, 2002, 21, 442 and references therein.
Reviews: W. A. Herrmann and C. Köcher, Angew. Chem., 1997, 109,
2256; W. A. Herrmann and C. Köcher, Angew. Chem., Int. Ed. Engl.,
1997, 36, 2162; T. Weskamp, P. W. Böhm and W. A. Herrmann,
J. Organomet. Chem., 2000, 600, 12.
4 A. Wacker, H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem., 1998,
843; A. Weiss, H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem.,
2002, 1607; A. Wacker, C. G. Yan, G. Kaltenpoth, A. Ginsberg,
A. M. Arif, R. D. Ernst, H. Pritzkow and W. Siebert, J. Organomet.
Chem., 2002, 641, 195; see also: I. I. Padilla-Martinez, M. de Jeses
Rosalez-Hoz, R. Contreras, S. Kerschl and B. Wrackmeyer,
Chem. Ber., 1994, 127, 343; K. Okuda, R. Suzuki and M. Oda,
J. Chem. Soc., Chem. Commun., 1995, 2069; see for a comparison:
H. Nöth and B. Wrackmeyer, Chem. Ber., 1974, 107, 3070; N. Kuhn,
H. Kotowski, D. Bläser and R. Boese, Z. Naturforsch., Teil B, 1997,
52, 351; I. I. Padilla-Martínez, A. Ariza-Castolo and R. Contreras,
Magn. Reson. Chem., 1993, 31, 189.
1
(dm, JCF = 247 Hz, m-PhF), 137.2 (C, C-7a), 133.6 (C, C-3a),
126.8 (CH, C-6), 125.7 (CH, C-5), 115.4 (C, ipso-C), 114.6 (CH,
C-4), 112.5 (CH, C-7), 32.2 (CH3, JCF = 16.7 Hz, NCH3). The
carbon-atoms C-1Ј to C-6Ј were not detected. δF (benzene-d6–
tetrahydrofuran-d8 10 : 1, 564 MHz): Ϫ131.7 (m, 1F, F-2Ј),
Ϫ132.3 (m, 1F, F-5Ј), Ϫ132.7 (m, 4F, o-PhF), Ϫ148.2 (m, 1F,
F-4Ј), Ϫ156.0 (m, 1F, F-3Ј), Ϫ157.2 (m, 2F, p-PhF), Ϫ163.5 (m,
4F, m-PhF). δB (benzene-d6–tetrahydrofuran-d8 10 : 1, 64 MHz):
Ϫ8.3 (ν = 180 Hz).
½
5 D. Vagedes, G. Kehr, D. König, K. Wedeking, R. Fröhlich, G. Erker,
C. Mück-Lichtenfeld and S. Grimme, Eur. J. Inorg. Chem., 2002,
2015; see also: K. Fujita, S. Hikichi, M. Akita and Y. Moro-oka,
J. Chem. Soc., Dalton Trans., 2000, 1255; X. Zheng and
G. E. Herberich, Organometallics, 2000, 19, 3751; A. Wacker,
H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem., 1999, 789; S. Solé,
H. Gornitzka, O. Guerret and G. Bertrand, J. Am. Chem. Soc., 1998,
120, 9100; I. I. Padilla-Martínez, F. J. Martínez- Martínez,
A. López-Sandoval, K. I. Girón-Castillo, M. A. Brito and
R. Contreras, Eur. J. Inorg. Chem., 1998, 1547; G. A. McGibbon,
C. Heinemann, D. J. Lavorato and H. Schwartz, Angew. Chem.,
1997, 109, 1572; G. A. McGibbon, C. Heinemann, D. J. Lavorato
and H. Schwartz, Angew. Chem., Int. Ed. Engl., 1997, 36, 1478;
G. Maier, J. Endres and H. P. Reisenauer, Angew. Chem., 1997,
109, 1788; G. Maier, J. Endres and H. P. Reisenauer, Angew. Chem.,
Int. Ed. Engl., 1997, 36, 1709; L. Weber, E. Dobbert, H.-G.
Stammler, B. Neumann, R. Boese and D. Bläser, Chem. Ber./Rec,
1997, 130, 705; W. P. Fehlhammer, H. Hoffmeister U. Eckert and
C. Robl, Z. Naturforsch., Teil B, 1993, 48, 1448.
X-Ray crystal structure analysis of 9c: formula C26H7BF14N2,
M = 624.15, yellow crystal 0.25 × 0.20 × 0.10 mm, a = 11.259(3),
b = 11.878(4), c = 17.461(4) Å, β = 99.00(2), V = 2306.4(11) Å3,
Dc = 1.797 g cmϪ3, µ = 16.83 cmϪ1, empirical absorption correc-
tion via ψ scan data (0.678 ≤ T ≤ 0.850), Z = 4, monoclinic,
space group P21/n (no. 14), λ = 1.54178 Å, T = 223 K, ω/2θ
scans, 9465 reflections collected ( h, k, Ϫl ), [(sinθ)/λ] =
0.62 ÅϪ1, 4708 independent (Rint = 0.036) and 3474 observed
reflections [I ≥ 2σ(I )], 389 refined parameters, R = 0.039,
wR2 = 0.103, max. residual electron density 0.24 (Ϫ0.27) e ÅϪ3
hydrogens calculated and refined as riding atoms.
,
Data sets were collected with Enraf-Nonius CAD4 and
Nonius KappaCCD diffractometers, the latter equipped with a
rotating anode generator Nonius FR591. Programs used: data
collection: EXPRESS (Nonius B.V., 1994) and COLLECT
(Nonius B.V., 1998), data reduction: MolEN16 and Denzo-
SMN,17 absorption correction for CCD data: SORTAV,18
structure solution: SHELXS-97,19 structure refinement:
SHELXL-97,20 graphics: SCHAKAL.21
6 A. G. Massey, A. J. Park and F. G. A. Stone, Proc. Chem. Soc.
London, 1963, 212; A. G. Massey and A. J. Park, J. Organomet.
Chem., 1964, 2, 245; A. G. Massey and A. J. Park, Organometallic
Syntheses, ed. R. B. King and J. J. Eisch, Elsevier, New York, 1986,
vol. 3, p. 461.
CCDC reference numbers 181962–181965.
D a l t o n T r a n s . , 2 0 0 3 , 1 3 3 7 – 1 3 4 4
1343

View Article Online
7 For other B(C6F5)3/Lewis-base adduct chemistry, see: e.g.
K. Ishihara, N. Hananki and H. Yamamoto, Synlett, 1993, 577;
K. Ishihara, N. Hananki and H. Yamamoto, Synlett, 1994,
963; T. Nagata, T. Toshihiro, T. Yamada, K. Imagawa and
T. Mukaiyama, Bull. Chem. Soc. Jpn., 1994, 67, 2614; L. Li and
T. J. Marks, Organometallics, 1998, 17, 3996; D. J. Parks, W. E. Piers,
M. Parvez, R. Atencio and M. J. Zaworotko, Organometallics, 1998,
17, 1369; L. H. Doerrer and M. L. H. Green, J. Chem. Soc., Dalton
Trans., 1999, 4325; C. Bergquist and G. Parkin, J. Am. Chem. Soc.,
1999, 121, 6322; C. Bergquist, B. M. Bridgewater, C. J. Harlan,
J. R. Norton, R. A. Friesner and G. Parkin, J. Am. Chem. Soc.,
2000, 122, 10581; R. E. La Pointe, G. R. Roof, K. A. Abboud
and J. Klosin, J. Am. Chem. Soc., 2000, 122, 9560; J. Zhou,
S. J. Lancaster, D. A. Walker, S. Beck, M. Thornton-Pett and
M. Bochmann, J. Am. Chem. Soc., 2001, 123, 223; B. Y. Lee,
G. C. Bazan, J. Vela, J. A. Komon and X. Bu, J. Am. Chem. Soc.,
2001, 123, 5352; D. Vagedes, G. Erker and R. Fröhlich,
J. Organomet. Chem., 2002, 641, 148; D. Vagedes, G. Erker and
R. Fröhlich, J. Organomet. Chem., 2002, 651, 157; J. M. Blackwell,
W. E. Piers, M. Parvez and R. McDonald, Organometallics, 2002,
21, 1400.
L.-L. Wong and A. Seela, Organometallics, 1992, 11, 2660 and
references therein.
11 M. J. S. Dewar and W. Thiel, J. Am. Chem. Soc., 1977, 99, 4899.
12 For a related reaction of a B(C6F5)3/ylide adduct, see: S. Döring,
G. Erker, R. Fröhlich, O. Meyer and K. Bergander, Organometallics,
1998, 17, 2183.
13 E. Schaumann, in Methoden der Organischen Chemie (Houben-
Weyl), 1994, E8c 6.
14 W. L. Collibee and J.-P. Anselme, Tetrahedron Lett., 1985, 26, 1595.
15 J. L. W. Pohlmann and F. E. Brinckmann, Z. Naturforsch., Teil B,
1965, 20, 5; J. Lambert, S. Zhang and S. M. Ciro, Organometallics,
1994, 13, 2430.
16 MolEN: K. Fair, Enraf-Nonius B.V., Delft, 1990.
17 Denzo-SMN: Z. Otwinowski and W. Minor, Methods Enzymol.,
1997, 276, 307–326.
18 SORTAV: R. H. Blessing, Acta Crystallogr., Sect. A, 1995, 51, 33–37;
R. H. Blessing, J. Appl. Crystallogr., 1997, 30, 421–426.
19 SHELXS-97: G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990, 46,
467–473.
20 SHELXL-97: G. M. Sheldrick, Universität Göttingen, 1997.
21 SCHAKAL: E. Keller, Universität Freiburg, 1997.
22 Turbomole 5.5: Universität Karlsruhe, 1997.
23 A. D. Becke, J. Chem. Phys., 1993, 98, 5648; P. J. Stephens,
F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem.,
1994, 98, 11623.
8 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen
and R. Taylor, J. Chem. Soc., Perkin Trans 2, 1987, S 1.
9 See, for a comparison: E. L. Eliel and S. H. Wilen, Stereochemistry
of Organic Compounds, Wiley, New York, 1994, ch. 14-6, pp. 1156–
1160.
10 DNMR3: D. A. Kleier and G. Binsch, Indiana University 1970;
D. A. Kleier and G. Binsch, J. Magn. Reson., 1970, 3, 146; XSINM:
K. Marat, University of Manitoba 1997; M. L. H. Green,
24 A. Schäfer, C. Huber and R. Ahlrichs, J. Chem. Phys., 1994, 100,
5829.
25 J. J. P. Stewart, MOPAC: A General Molecular Orbital Package,
Quant. Chem. Prog. Exch., 1990, 10, 86.
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