Synthesis and reactivity of N-heterocycle-B(C6F 5)3 complexes. 4. Competition between pyridine- and pyrrole-type substrates toward B(C6F5)3: Structure and dynamics of 7-B(C6F<sub

Article abstract of DOI:10.1021/ic051285p

Reaction between 7-azaindole and B(C₆F₅)₃ quantitatively yields 7-(C₆F₅)₃B-7-azaindole (4), in which B(C₆F₅)₃ coordinates to the pyridine nitrogen of 7-azai

Full text of DOI:10.1021/ic051285p

Inorg. Chem. 2006, 45, 1683−1692
Synthesis and Reactivity of N-Heterocycle-B(C6F5)3 Complexes. 4.
Competition between Pyridine- and Pyrrole-Type Substrates toward
B(C6F5)3: Structure and Dynamics of 7-B(C6F5)3-7-azaindole and
+
-
[7-Azaindolium] [HOB(C6F5)3]
Francesca Focante,*,†,# Isabella Camurati,# Luigi Resconi,# Simona Guidotti,# Tiziana Beringhelli,*,‡
Giuseppe D’Alfonso,‡ Daniela Donghi,‡ Daniela Maggioni,‡ Pierluigi Mercandelli,*,§ and Angelo Sironi§
Dipartimento di Chimica Organica “A. Mangini”, UniVersita` di Bologna, Viale Risorgimento 4,
I-40136 Bologna, Italy, Basell Polyolefins Italia, Centro Ricerche “G. Natta”, I-44100 Ferrara,
Italy, Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Facolta` di Farmacia,
UniVersita` degli Studi di Milano, Via Venezian 21, I-20133 Milano, Italy, and Dipartimento di
Chimica Strutturale e Stereochimica Inorganica, UniVersita` degli Studi di Milano,
Via Venezian 21, I-20133 Milano, Italy
Received July 29, 2005
Reaction between 7-azaindole and B(C6F5)3 quantitatively yields 7-(C6F5)3B-7-azaindole (4), in which B(C6F5)3
coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a
second equivalent of B(C6F5)3. Reaction of 7-azaindole with H2O−
B(C6F5)3 initially produces [7-azaindolium]+[HOB-
(C6F5)3]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known
complexes (C6F5)3B-pyrrole (1) and (C6F5)3B-indole (2), with formation of free pyrrole or indole, giving the more
stable adduct (C6F5)3B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with
B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid
state and in solution and compared to the structures of other (C6F5)3B-N-heterocycle complexes. Two dynamic
processes have been found in compound 4. Their activation parameters (
J/mol K and 76 (5) kJ/mol,
Hq Sq ) −5 (18) J/mol K) are comparable with those of other (C6F5)3B-based
adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed.
∆ ) 66 (3) kJ/mol, ∆
Hq Sq ) −18 (10)
∆
)
∆
Introduction
B(C6F5)3, in general, shows a very strong affinity as a
Lewis acid for different nitrogen functionalities such as
amines, imines, nitriles, and aromatic N-heterocycles.5,6
Pyrroles and indoles usually do not react with electrophiles
at the nitrogen position because of the lack of the basic lone
pair, which is engaged in the aromatic π-system; however,
when pyrrole and indole are reacted with B(C6F5)3, instan-
taneous and quantitative formation of 1:1 B-N adducts takes
place ((C6F5)3B-5H-pyrrole, 1; (C6F5)3B-3H-indole, 2), likely
resulting from the previous protonation of the pyrrole double
bond by a catalytic amount of H2O-B(C6F5)3 and the
subsequent reaction at the newly formed iminic nitrogen.7
This reaction is shown in Scheme 1.
Pyridine undergoes a range of simple electrophilic addi-
tions involving donation of the nitrogen lone pair to an
electrophile and subsequent generation of the pyridinium salt,
which maintains the aromatic sextet.1,2 In particular, the
reaction with the strong Lewis acid tris(pentafluorophenyl)-
borane, B(C6F5)3, yields the stable adduct (C6F5)3B-ΝC5H5
(3).3,4
* To whom correspondence should be addressed. E-mail:
francesca.focante@basell.com (F.F.); tiziana.beringhelli@unimi.it (T.B.);
pierluigi.mercandelli@unimi.it (P.M.).
† Dipartimento di Chimica Organica “A. Mangini”, Universita` di Bologna.
# Basell Polyolefins Italia, Centro Ricerche “G. Natta”.
‡ Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Facolta`
di Farmacia, Universita` degli Studi di Milano.
§ Dipartimento di Chimica Strutturale e Stereochimica Inorganica,
Universita` degli Studi di Milano.
(3) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
(4) Holtcamp M. W. Int. Patent Application WO 99/64476.
(5) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1 and references therein.
(6) Focante, F.; Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem.
ReV. 2005, in press and references therein.
(1) Joule, J. A.; Mills, K. Heterocyclic Chemistry, IV ed.; Blackwell:
London, 2000; Chapter 5.
(2) Wilson, J. W.; Worral, I. J. Inorg. Nucl. Chem. Lett. 1967, 3, 57.
10.1021/ic051285p CCC: $33.50
Published on Web 01/19/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 4, 2006 1683

Focante et al.
Scheme 1
Scheme 4
Scheme 5
Scheme 6
Scheme 2
Scheme 3
As part of our study of the structural and dynamic features
and reactivity of (C6F5)3B-N-heterocycle complexes, in this
paper, we compare (i) the different reactivities of B(C6F5)3
toward pyridine- and pyrrole/indole-type substrates since both
heterocycles lead to stable adducts with a B-N bond and
(ii) the dynamic processes in these compounds.
behavior can be explained by taking into account the stronger
basicity of pyridine (pKa ) 5.25) with respect to 7-azaindole
(pKa ) 4.59).9 The reaction however is not instantaneous
like the one with complex 2, but it takes several days. A
kinetic study performed on two samples of 4 in CD2Cl2
treated with 1 and 5 equiv of pyridine showed that the rate
of disappearance of 4 is independent of the pyridine
concentration, indicating that the exchange reaction occurs
through a dissociative mechanism (t1/2 ) 15.2 h, see Figure
S1 in Supporting Information) according to literature data
for analogous compounds.5,6,8
Results and Discussion
1. Synthesis and Reactivity. Pyridine, when treated with
1 equiv of B(C6F5)3, instantaneously and quantitatively gives
the Lewis acid-base adduct 3 (Scheme 2).3,4
When adducts 1 or 2 are treated with pyridine, pyrrole
and indole, respectively, are freed, and complex 3 is formed
immediately and quantitatively (Scheme 3, reaction exempli-
fied for complex 2). The mechanism probably follows a
dissociative path, analogous to that of the exchange reactions
occurring with other B(C6F5)3 complexes.5,6,8
The driving force for the reaction shown in Scheme 3 is
the recovery of aromaticity in the five-membered ring of
indole (or pyrrole), while, in the case of pyridine, the
coordination reaction with B(C6F5)3 does not alter the
resonance stabilization in the six-membered ring. Therefore
borane strongly prefers coordination to the nitrogen of
pyridine, and this can be directly observed in the reaction
of B(C6F5)3 with 7-azaindole, where B(C6F5)3 coordinates
exclusively to the pyridinic moiety of the molecule (Scheme
4).
Attempts to react 4 with a second equivalent of B(C6F5)3
to obtain a diborane complex failed, likely because of both
steric and electronic reasons. In fact, the presence of a
positively charged nitrogen, from the coordination with
boron, inductively affects the nucleophilicity of the five-
membered ring. For example, the reaction of 4 with the
strong Brønsted acid H2O-B(C6F5)3,8 aimed at protonating
the pyrrolic ring in the â-position, does not take place. On
the other hand, the 1:1 reaction of 7-azaindole with H2O-
B(C6F5)3 produces the azaindolium salt 5, which slowly
displaces the H2O molecule to form complex 4, while no
protonation of the pyrrole double bond is observed (Scheme
6).10 The experiment was carried out in deuterated dichloro-
1
methane and monitored at different times with H NMR
analyses. Spectra showed instantaneous formation of salt 5
and slow conversion into 4 (67% in 24 h); the reaction did
not evolve further with time.11
Complex 5 is however quite stable in the solid state: a
mixture of 4 and 5 in toluene solution and a solution of 4 in
In addition, it has been observed that coordination of
B(C6F5)3 to pyridine is favored with respect to coordination
with 7-azaindole, in fact the pyridine displaces the latter from
complex 4 to give the pyridine complex 3 (Scheme 5). This
(7) Guidotti, S.; Camurati, I.; Focante, F.; Angellini, L.; Moscardi, G.;
Resconi, L.; Leardini, R.; Nanni, D.; Mercandelli, P.; Sironi, A.;
Beringhelli, T.; Maggioni, D. J. Org. Chem. 2003, 68, 5445.
(8) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.; Friesner,
R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581.
(9) Adler, T. K.; Albert, A. J. Chem. Soc. 1960, 1794.
(10) Indole can be easily protonated at its C-3 with formation of the 3H-
indolium cation, see: Joule, J. A.; Mills, K. Heterocyclic Chemistry,
IV ed.; Blackwell: London, 2000; Chapter 16.
1684 Inorganic Chemistry, Vol. 45, No. 4, 2006

N-Heterocycle-B(C6F5)3 Complexes
Table 1. Assignments of the 19F Resonances of Compound 4 at 233 K
(δ)
ring
A
ortho
para
meta
-127.22 (A2)
-131.37 (A6)
-127.57 (B2)
-137.46 (B6)
-131.64 (C2)
-134.94 (C6)
-153.95 (A4)
-160.86 (A5)
-162.26 (A3)
-161.99 (B3)
-162.30 (B5)
-162.10 (C3)
-163.07 (C5)
B
C
-154.95 (B4)
-153.78 (C4)
Table 2. Dipolar and Scalar Correlation Observed between the
Resonances of Adduct 4
19F-19F COSY 1H-19F COSY 19F-19F NOESY 1H-19F HOESY
A2-B6
B2-A6
B2-C6
H6-A2
H6-C2
H1-C6
A2-B6
A6-B2
C6-B2
A6-C2
H6-A2
H6-C2
H1-C6
H1-B6
occured showing the onset of dynamic processes responsible
for the loss of the H‚‚‚F couplings in the corresponding
proton spectra.
To establish the solution structure of 4, first the 19F
resonances of the three aryl rings (Table 1) have been
assigned by means of [19F-19F] COSY (Figure S6) per-
formed at 233 K; then, the relative orientations of the three
rings and of the heterocyclic moiety have been established
on the basis of the 19F-1H and 19F-19F dipolar interactions,
(Figure S7 and Figure S8) and of the ortho 19F-19F through-
space scalar correlations (Figure S9).
Shortly, four short H‚‚‚F distances and four short inter-
ring distances, F‚‚‚F, characterize the solution structure of
the adduct 4. Table 2 reports the pairs of signals between
which homo- and heteronuclear nOe’s, as well as “through-
space” scalar correlations, have been observed. The solution
structure derived accordingly is shown in Figure 3 and is
substantially identical to the one determined in the solid state
(see below). The plane of B ring is almost coplanar with the
N-B bond and can be regarded as the in-plane substituent
in the complex conformation,12 while the other two adopt a
two-bladed propellerlike conformation.
Figure 1. Variable-temperature 1H NMR spectra of 4 (7.1 T, # marks
toluene). The resonances are labeled according to Figure 3.
nonanhydrous toluene, both kept at -5 °C for a few days,
gave colorless crystals of 5 suitable for X-ray single-crystal
analysis. Crystals of compound 4 were obtained from
anhydrous dichloromethane by cooling the solution for
several months at -25 °C.
2. Solution Structure of Adducts 4 and 5. The 1H NMR
spectrum of adduct 4 in toluene-d8 (Figure 1) shows, at 300
K, six signals that have been assigned by 2D scalar and
dipolar correlation experiments (see Figures S2-S4). When
the temperature is lowered, the resonances shifted to high
field, eventually leading to the inversion of H2 and H3. In
contrast, below 260 K, the two lower field signals (H1 and
H6) shift to lower fields. These shifts, together with the shape
itself of these signals, suggest the occurrence of intramo-
lecular N-H‚‚‚F and C-H‚‚‚F “through-space” couplings.
Indeed a {19F}1H spectrum recorded at 233 K (Figure S5)
shows that, upon fluorine decoupling, the H1 pseudo-doublet
becomes a singlet, and the H6 quartet becomes a doublet.
In the19F spectrum at 300 K, the appearance of fifteen
resonances indicates conformational rigidity on the NMR
time scale (C1 symmetry). When the temperature was
increased (Figure 2), a generalized broadening of the signals
As far as adduct 5 is concerned, even at 183 K the 19F
NMR spectrum in toluene-d8 shows only three signals,
indicating that the ion-pair interaction is not tight enough to
stop the free rotation of the three aromatic rings. Interestingly
proton exchange is observed between H1, H7, and OH. These
two latter protons are in the fast-exchange regime and result
in a single signal in the explored temperature range, while
H1 (that at low temperature moves at low field, likely
involved in a hydrogen bond interaction with the OH group
of the borate moiety) is in slow exchange with them at 223
K as shown in a 2D [1H-1H] EXSY/NOESY experiment
(Figure S10). Similar behavior can be observed in dichloro-
methane at comparable temperatures. However, overcooling
of the sample (below 173 K) results in the H7/OH exchange
1
being slowed enough to observe, in the H spectrum, the
H7 resonance at +16.46 ppm, while the three 19F resonances
(11) Small amounts of compound 5 can also be observed in the solution
of 4 because of the reaction with adventitious water present in the
solvents. The relative amount of the two species remains constant with
time, and no decomposition reactions leading to borane cleavage are
observed for weeks.
appear to be broadened.
(12) Vagedes, D.; Erker, G.; Kehr, G.; Bergander, K.; Kataeva, O.; Fro¨lich,
R.; Grimme, S.; Mu¨ch-Lichtenfeld, C. Dalton Trans. 2003, 1337.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1685

Focante et al.
Figure 2. Variable temperature19F NMR spectra of 4 (7.1 T, toluene-d8). The resonances are labeled according to Figure 3.
Table 3. Activation Parameters Obtained for the Two Dynamic
Processes Observed in Some N-Heterocycle-B(C6F5)3 Adducts
3. Solution Dynamics of Adduct 4. The 19F variable-
temperature spectra suggest the presence of dynamic pro-
cesses, which eventually lead to the dynamic equivalence
of the three perfluorinated rings. [19F-19F] EXSY experi-
ments have been performed at different temperatures and
the analysis of the observed exchange cross-peaks pointed
to two distinct processes.
At 275 K, the main process involves rings B and C (see
the cross-peaks B2-C2, B2-C6, B6-C2, and B6-C6 in
Figure 4a) leading to the exchange of the in-plane location
of two rings. At this temperature, the enantiomerization
process occurs through a ring A flip,13 rotation of rings B
and C around their B-Cipso bonds, together with a small
rotation around the B-N interaction of the heterocyclic
moiety. When the temperature is increased to 283 K,
exchange cross-peaks between the ring A and C resonances
became clearly observable as shown in the meta region by
A5-C3 and A5-C5 exchange cross-peaks14 (Figure 4b),
indicating the onset of wider rotations around the B-N bond.
No exchange cross-peak is detectable between the signals
of the A and B rings up to 305 K.
exchange
process
Ea
(kJ/mol)
∆Hq
(kJ/mol)
∆Sq
(kJ/mol K)
adduct
ln A
4
B-C
A-C
B-C
A-Ba
B-C
A-C
69(3)
78(5)
69.7(6)
75.3(9)
60(1)
28(1)
31(2)
31.6(2)
31.3(3)
29(0.7)
31(1)
66(3)
76(5)
67.1(5)
72.6(9)
58(1)
-18(10)
-5(18)
9.3(2)
7(3)
2
7
-12(5)
72(3)
69(3)
2(20)
a In compound 2, the ring labeled C is the one that adopts the in-plane
conformation.
constants estimated through the analysis of several 2D
[19F-19F] EXSY experiments15 and plotted in Figure 5.
The dynamic behavior of compound 4 is similar to that
of the (C6F5)3B-indole adduct (2),7 and the activation
parameters of the two processes are comparable (see Table
3). Likely, in the two compounds, the rotation hindrance
experienced by the perfluoroaryl and heterocyclic moieties
is similar, being in both cases engaged in four H‚‚‚F
intramolecular interactions.
The relevance of these interactions is supported by the
comparison with the dynamic behavior of other (C6F5)3B-
N-heterocycle adducts. For instance if we compare compound
2 with the (C6F5)3B-anthranil complex (6, Scheme 7) where
the nitrogen is the boron binding site, no interaction can be
established between position 2, occupied by an oxygen atom,
Therefore rotation around B-N bond is not free, likely
hindered by the interactions between the two protons H6 and
H1 with four of the ortho-fluorine atoms. It should be noted
that in the B/C exchange three out of the four H‚‚‚F
interactions have to interchange.
The activation parameters for the B/C and A/C exchanges,
reported in Table 3, have been calculated from the rate
(15) The close contacts between some of the ortho-fluorines causes the
appearance of the J and nOe contributions to the cross-peaks observed
in EXSY experiments. Therefore their values are not reliable for
quantitative measurements, but in a series of experiments at the same
temperature with different mixing times, they can be used qualitatively
to detect exchange processes. For the quantitative volume analysis,
we have taken into account the exchange cross-peaks of the meta
region.
(13) Brocas, J.; Gielen, M.; Willem R. In The Permutational Approach to
Dynamic Stereochemistry; McGraw-Hill University Press: Cambridge,
U.K., 1983.
(14) A3-C3 and A3-C5 cross-peaks are hidden by the stronger cross-peaks
from the B/C exchange.
1686 Inorganic Chemistry, Vol. 45, No. 4, 2006

N-Heterocycle-B(C6F5)3 Complexes
Figure 4. Selected regions of a 2D [19F-19F] EXSY/NOESY experiment
of 4 (7.1 T, τm ) 0.1 s, red peaks ) nOe, black peaks ) exchange, toluene-
d8): (a) ortho region (275 K) and (b) meta region (283 K).
Figure 3. Two views of the solution structure of 4 as derived from 2D
19F-19F and 19F-1H correlation experiments. The experimental dipolar and
scalar interactions are indicated with dotted lines.
and the fluorine atoms of the B(C6F5)3 moiety. Indeed the
variable-temperature 19F spectra of 6 show a single set of
signals at room temperature, and even at 200 K, the exchange
of two rings is fast on the NMR time scale (Figure S11).
The dynamic behavior of pyrrole adducts fit nicely into this
view, while for (C6F5)3B-pyrrole (1), free rotation around
the B-C and B-N bonds is observed at temperatures as
low as 173 K;7 for the N-[(C6F5)3B]-2,4-dimethylpyrrole
adduct (7, Chart 1),7 the interactions between Me2 and the
fluorine B6 (Figure 6) and between 5′ and A2 (data not
shown) make both the in-plane interchange and the B-N
rotation slower. However the smaller activation parameters
indicate that these interactions are less effective than in 4
and 2.
Figure 5. Arrhenius plot of the rate constants for the dynamic processes
observed for compound 4: exchange of B/C rings (0) and exchange of
A/C rings (O).
positive polarization of these hydrogen atoms, it simply
shows the spatial proximity of these nuclei.16
Among these H‚‚‚F interactions, only the one between
N-H and fluorine C6 in complex 4 (see Figure 3) can be
described as a hydrogen bond. For all the others, the
observation of a scalar or dipolar coupling between the ortho-
fluorines and carbon-bound hydrogens does not imply any
Interestingly the activation data found in the present study
are higher than those reported by Erker and co-workers12
for the (C6F5)3B-N-methylbenzimidazole adduct. Smaller
Inorganic Chemistry, Vol. 45, No. 4, 2006 1687

Focante et al.
Figure 7. ORTEP view of the structure of (7-aza-1H-indole)tris-
(pentafluorophenyl)boron (4) with a partial labeling scheme. Thermal
ellipsoids are drawn at the 30% probability level; hydrogen atoms are given
arbitrary radii. Only one of the two independent molecules is shown.
Figure 6. [1H-19F] COSY of 7 (203 K, 7.1 T, JH-F ) 4 Hz, # indicates
toluene).
Scheme 7
Chart 1
activation parameters have also been reported by Piers and
co-workers18 for (C6F5)3B-N-benzylimine adducts where the
barrier for the rotation was attributed to the π-stacking
interaction between the positively polarized benzylic ring and
one of the negatively polarized perfluorinated rings.
4. Solid State Structures of 4 and 5. The molecular
structures of the complex of 7-aza-1H-indole with tris-
(pentafluorophenyl)borane (4) and of the 7-aza-1H-indolium
hydroxotris(pentafluorophenyl)borate ion pair (5) have been
determined in the solid state by X-ray diffraction analysis.
Figures 7 and 8 show an ORTEP view of complexes 4 and
5, respectively. Tables 4 (distances) and 5 (angles) contain
the most relevant bonding parameters for the two compounds.
Details of the inter- and intramolecular hydrogen bonds for
compounds 4 and 5 are reported in Table 6.
Figure 8. ORTEP view of the structure of the 7-aza-1H-indolium
hydroxotris(pentafluorophenyl)borate ion pair (5) with a partial labeling
scheme. Thermal ellipsoids are drawn at the 30% probability level; hydrogen
atoms are given arbitrary radii. Only the major disordered component of
the azaindolium cation is shown.
firmed. The neutral azaindole ligand usually coordinates
through the pyridine nitrogen atom N(7), whereas the anionic
azaindolato ligand binds through the indole nitrogen atom
N(1) or bridges by means of a µ-κN1:κN7 coordination
mode.19 In accordance with these observations, in 4 the
azaindole assumes a κN7 coordination mode.
In this class of compounds, boron-nitrogen bond distances
show a sensible dependence on the electronic and steric
environment, in particular short bond distances usually
correlate with the presence of unhindered strong bases. The
B(1)-N(7) distance in 4 (1.608 Å, mean value) is similar to
the B-N distances found in other pyridine derivatives such
as 3 (1.628 Å),6 the 4-(NMe2)pyridine adduct (1.604 Å), and
The formulation of 4 as an equimolecular adduct between
7-aza-1H-indole and tris(pentafluorophenyl)borane is con-
(16) Bona fide intramolecular hydrogen bonds have been observed in
adducts between B(C6F5)3 and primary and secondary amines.17 The
N-H‚‚‚F-C hydrogen bonds make rotation around the B atom
hindered enough to allow the observation of separate resonances for
the three perfluorinated rings at room temperature or slightly below.
(17) (a) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann, M.;
Bochmann, M. J. Organomet. Chem. 2004, 680, 193. (b) Mountford,
A. J.; Lancaster, S. J.; Coles, S. J.; Horton, P. N.; Hughes, D. L.;
Hursthouse, M. B.; Light, M. E. Inorg. Chem. 2005, 44, 5921.
(18) Blackwell, J. M.; Piers, W. E.; Parvez, M.; McDonald, R. Organo-
metallics 2002, 21, 1400.
(19) (a) Dufour, P.; Dartiguenave, Y.; Dartiguenave, M.; Dufour, N.; Lebuis,
A.-M.; Be´langer-Garie´py, F.; Beauchamp, A.-L. Can. J. Chem. 1990,
68, 193. (b) Dufour, N.; Lebuis, A.-M.; Corbeil, M.-C.; Beauchamp,
A.-L.; Dufour, P.; Dartiguenave, Y.; Dartiguenave, M. Can. J. Chem.
1992, 70, 2914. (c) Wu, Q.; Esteghamatian, M.; Hu, N.-X.; Popovic,
Z.; Enright, G.; Breeze, S. R.; Wang, S. Angew. Chem., Int. Ed. 1999,
38, 985.
1688 Inorganic Chemistry, Vol. 45, No. 4, 2006

N-Heterocycle-B(C6F5)3 Complexes
Table 4. Selected Bond Distances (Å) for 4 and 5a
Table 6. Intra- and Intermolecular Hydrogen-Bond Parameters (Å, deg)
for 4 and 5a
4
D-H
H‚‚‚A
D‚‚‚A
D-H‚‚‚A
I
II
5
4
5
N(1)-C(2)
N(1)-C(7a)
C(2)-C(3)
C(3)-C(3a)
C(3a)-C(4)
C(3a)-C(7a)
C(4)-C(5)
C(5)-C(6)
C(6)-N(7)
N(7)-C(7a)
Xb-B(1)
1.385(3)
1.343(3)
1.321(4)
1.424(4)
1.386(3)
1.407(3)
1.373(3)
1.381(3)
1.350(3)
1.353(3)
1.605(3)
1.658(3)
1.640(3)
1.661(3)
1.382(3)
1.354(3)
1.339(3)
1.416(3)
1.367(3)
1.401(3)
1.379(3)
1.375(3)
1.354(3)
1.357(3)
1.612(3)
1.649(3)
1.643(3)
1.637(3)
1.395(6)
1.339(7)
1.366(7)
1.442(7)
1.376(6)
1.433(5)
1.386(7)
1.378(7)
1.353(6)
1.345(7)
1.500(3)
1.660(3)
1.662(3)
1.675(3)
N(1)-H(N1)‚‚‚F(22)
0.860
0.860
2.310
2.320
3.135(3)
3.095(3)
160.0
150.0
O(1)-H(O1)‚‚‚F(12)
O(1)-H(O1)‚‚‚F(36)
N(1)-H(N1)‚‚‚O(1)
N(1)-H(N1)‚‚‚F(13i)
N(7)-H(N7)‚‚‚O(1)
N(7)-H(N7)‚‚‚F(34ii)
0.840
0.840
0.860
0.860
0.860
0.860
2.193
2.279
2.390
2.367
1.977
2.657
2.769(2)
2.803(2)
2.977(5)
3.098(5)
2.732(4)
3.254(4)
125.7
120.7
124.3
140.6
143.0
126.1
a For each interaction in 4, we report two values related to the two
independent molecules in the asymmetric unit of 4‚1/2CH2Cl2. Only the
major disordered component of the 7-azaindolium cation in 5 is considered.
Symmetry operations: (i) 1 - x, 1 - y, 2 - z; (ii) 1 - x, 1 - y, 1 - z.
B(1)-C(11)
B(1)-C(21)
B(1)-C(31)
a I and II refer to the two independent molecules in the asymmetric unit
of 4‚1/2CH2Cl2. Only the major disordered component of the 7-azaindolium
cation in 5 is considered. b X ) N(7) for 4, and X ) O(1) for 5.
As far as 5 is concerned, the structure of the [B(C6F5)3OH]-
anion is similar to the previously determined ones,22 in
particular the hydrogen atom of the hydroxo group is
involved in a bifurcated hydrogen bond with two ortho-
fluorine atoms of two different pentafluorophenyl rings,
whereas the oxygen atom acts as an hydrogen-bond acceptor
for both the N-H groups of the cation (see Figure 8 and
Table 6). The azaindolium cation is disordered, as in most
of its previously determined structures.23 Disorder is often
observed in the crystals of compounds containing fused five-
and six-member rings, for which the two halves of the
molecule show a near equivalence of the van der Waals and
electrostatic potential envelopes.
As can be seen from Figures 7 and 8 and from the torsion
angles reported in Table 5, in both complexes 4 and 5 the
B(C6F5)3 moiety adopts a very similar conformation. In
particular, one of the three phenyl rings, the so-called in-
plane ring, labeled C(11-16), almost eclipses the B-N or
the B-O bond, whereas the other two exhibit a two-bladed
propeller-like conformation. This very same conformation
can be found in most of the crystal structures of tris(pentafluor-
phenyl)borane derivatives collected in the Cambridge Struc-
tural Database;24 from this evidence, it can be inferred that
this molecular fragment is very rigid, so that the presence
of a high energy barrier to the enantiomerization process of
the chiral moiety can be expected. It must be noted that this
arrangement of the pentafluorophenyl rings does not seem
to be a consequence of the formation of any particular non-
bonded or hydrogen-bonded interaction between the B(C6F5)3
moiety and the rest of the molecule because it can be
observed even in adducts with sterically undemanding
ligands, not bearing any acidic hydrogen atoms.
Table 5. Selected Bond and Torsion Angles (deg) for 4 and 5a
4
I
II
5
C(2)-N(1)-C(7a)
N(1)-C(2)-C(3)
108.5(2)
109.8(2)
107.8(2)
136.5(3)
105.8(2)
117.7(2)
118.2(2)
120.9(2)
122.8(2)
108.0(2)
109.9(2)
107.5(2)
136.2(2)
106.3(2)
117.5(2)
118.9(2)
120.6(2)
122.7(2)
107.9(3)
109.9(4)
107.5(4)
138.0(4)
104.4(5)
117.6(5)
119.6(4)
121.0(4)
120.2(4)
120.2(4)
110.3(4)
128.3(4)
121.5(5)
C(2)-C(3)-C(3a)
C(3)-C(3a)-C(4)
C(3)-C(3a)-C(7a)
C(4)-C(3a)-C(7a)
C(3a)-C(4)-C(5)
C(4)-C(5)-C(6)
C(5)-C(6)-N(7)
C(6)-N(7)-C(7a)
N(1)-C(7a)-C(3a)
N(1)-C(7a)-N(7)
C(3a)-C(7a)-N(7)
C(6)-N(7)-B(1)
115.92(19)
108.1(2)
127.5(2)
115.51(19)
108.4(2)
126.8(2)
124.4(2)
124.8(2)
123.45(19)
120.56(18)
110.62(18)
103.16(18)
110.88(17)
114.92(18)
102.22(18)
115.25(19)
-16.4(3)
173.73(19)
-111.6(2)
64.1(2)
123.10(19)
120.96(19)
111.16(18)
101.44(17)
112.36(18)
116.04(19)
102.80(18)
113.42(19)
-19.1(3)
C(7a)-N(7)-B(1)
Xb-B(1)-C(11)
113.45(17)
100.96(16)
110.19(16)
113.67(17)
105.15(16)
113.65(17)
14.7(3)
X-B(1)-C(21)
X-B(1)-C(31)
C(11)-B(1)-C(21)
C(11)-B(1)-C(31)
C(21)-B(1)-C(31)
Xb-B(1)-C(11)-C(12)
X-B(1)-C(11)-C(16)
X-B(1)-C(21)-C(22)
X-B(1)-C(21)-C(26)
X-B(1)-C(31)-C(32)
X-B(1)-C(31)-C(36)
171.33(19) -169.06(17)
-108.9(3)
64.9(2)
95.9(3)
-71.3(2)
132.6(2)
-45.6(3)
-130.1(2)
-133.7(2)
62.6(3)
56.2(3)
a I and II refer to the two independent molecules in the asymmetric unit
of 4‚1/2CH2Cl2. Only the major disordered component of the 7-azaindolium
cation in 5 is considered. b X ) N(7) for 4, and X ) O(1) for 5.
the 4-[4′-(NMe2)C6H4CtC]pyridine adduct (1.620 Å).20 This
result could be related to a balance between the slightly lower
basicity of 7-azaindole with respect to pyridine9 and the
building up of some attractive NH‚‚‚F hydrogen-bond
interactions between the acidic indole NH proton and the
ortho-fluorine substituents of the borane moiety.
Bond distances within the heterocyclic moiety in 4
compare well to those observed in the free azaindole
molecule21 and are consistent with an extensive delocalization
of the π-electrons in both the rings.
(21) Mean bond distances (Å) for the 7-azaindole molecule are as
follows: N(1)-C(2) ) 1.370(5), N(1)-C(7a) ) 1.361(4), C(2)-C(3)
) 1.342(5), C(3)-C(3a) ) 1.419(5), C(3a)-C(4) ) 1.383(6), C(3a)-
C(7a) ) 1.398(5), C(4)-C(5) ) 1.377(6), C(5)-C(6) ) 1.375(5),
C(6)-N(7) ) 1.337(5), N(7)-C(7a) ) 1.341(4).19a
(22) (a) Bergquist, C.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 6322. (b)
Duchateau, R.; van Santen, R. A.; Yap, G. P. A. Organometallics
2000, 19, 809. (c) Stibrany, R. T.; Brant, P. Acta Crystallogr., Sect.
C 2001, 57, 644. (d) Bergquist, C.; Fillebeen, T.; Morlok, M. M.;
Parkin, G. J. Am. Chem. Soc. 2003, 125, 6189.
(23) (a) Sheldrick, W. S. Z. Naturforsch., B 1982, 37, 653. (b) Poitras, J.;
Leduc, M.; Beauchamp, A.-L. Can. J. Chem. 1993, 71, 549.
(24) CSD, version 5.26; November 2004. Allen, F. H. Acta Crystallogr.,
Sect. B 2002, 58, 380.
(20) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D.; Kakkar,
K. Chem. Mater. 1998, 10, 1355.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1689

Focante et al.
Table 7. Selected F‚‚‚F and F‚‚‚H Distances (Å) for 4a
F(22) F(26) F(32) F(36)
EI-MS spectra were recorded on a VG 7070 EQ spectrometer
operating at 70 eV.
H(N1) H(6)
The samples for NMR studies were dissolved in CD2Cl2 (Aldrich,
99.8% atom D), CDCl3 (Aldrich, 99% atom D), or C7D8 (CIL,
99.5% atom D); all were dried over activated 4 Å molecular sieves
before use. The 1H and 13C NMR spectra obtained using a Bruker
DPX 200 spectrometer were acquired with a 15° pulse and 2 s of
F(12) 4.550(3) 5.618(3) 6.056(3) 2.828(2)
4.617(3) 5.602(3) 5.987(3) 2.852(2)
F(16) 2.733(2) 4.812(3) 3.209(2) 4.519(3)
2.738(2) 4.773(3) 2.940(2) 4.651(3)
3.11
3.08
4.08
4.03
2.18
2.19
4.61
4.74
5.40
5.35
4.91
4.90
4.07
4.08
5.36
5.40
5.75
5.74
2.91
2.79
4.14
4.32
2.70
2.57
F(22)
F(26)
F(32)
F(36)
4.557(3) 5.631(4)
4.373(3) 5.718(3)
2.749(2) 4.868(3)
2.879(2) 4.657(3)
1
delay for H and with a 45° pulse and 6 s of delay for 13C.
NMR dynamic studies in toluene solutions were performed on
a Bruker DRX 300 spectrometer equipped with a QNP probehead
1
(90° H ) 10 µs, 90° 19F ) 10 µs, 90° 13C ) 7 µs); 11B and 15N
NMR spectra were recorded on a Bruker DRX 400 equipped with
a BBI probehead (90° 11B ) 22 µs, 15N ) 29 µs). 19F NMR spectra
were referenced to external CFCl3 (δ ) 0 ppm), and the 11B NMR
spectra were referenced to external Et2O‚BF3 (δ ) 0 ppm), while
15N NMR spectra were referenced to external CH3NO2 (δ ) 0 ppm).
All the NMR data reported here were acquired at 298 K, if not
otherwise specified. The temperature was calibrated with a standard
CH3OH/CD3OD solution for the range of 188-300 K.25
N-[Tris(pentafluorophenyl)borane]-pyridine (3). A solution of
pyridine (0.37 g, 4.68 mmol) in 3 mL of dichloromethane was added
at room temperature to a solution of B(C6F5)3 (2.39 g, 4.67 mmol)
in 15 mL of dichloromethane. A light exothermicity was observed.
After the mixture was stirred for 30 min at room temperature, the
solvent was evaporated under reduced pressure to give a white solid
(quantitative yield). 1H NMR (CD2Cl2): δ 7.06-7.13 (m, 2H, H3
and H5), 7.42-7.52 (m, 1H, H4), 8.51-8.55 (m, 2H, H2 and H6).
13C NMR (CD2Cl2): δ 123.45 (C3 and C5), 135.54 (C4), 149.80
(C2 and C6).
a Close contacts are indicated in bold (the sum of the van der Waals
radii for the F‚‚‚F and F‚‚‚H contacts are 3.0 and 2.7 Å, respectively). For
each interaction, we report two values related to the two independent
molecules in the asymmetric unit of 4‚1/2CH2Cl2. N-H and C-H bond
distances are normalized to 1.009 and 1.083 Å, respectively.31
This conformation eventually implies some short F‚‚‚F
non-bonded contacts between the ortho-fluoro substituents
of the three phenyl rings, and in the case of adduct 4, some
F‚‚‚H contacts involving the hydrogen atoms bound to N(1)
and C(6). A list of these interactions for compound 4, (in
which they are revealed also in solution, at low temperature,
by NMR spectroscopy) is reported in Table 7.
Conclusions
Pyridine removes the borane from the complexes (C6F5)3B-
pyrrole (1) and (C6F5)3B-indole (2) releasing pyrrole or indole
and giving the more stable adduct (C6F5)3B-pyridine (3). The
preference of B(C6F5)3 for the pyridinic moiety is exemplified
in the reaction between the borane and 7-azaindole which
leads to a stable adduct (4), where the borane is coordinated
to the pyridinic nitrogen. The competition between pyridine
and 7-azaindole for the coordination with B(C6F5)3 has been
studied as well.
7-[Tris(pentafluorophenyl)borane]-7-azaindole (4). A solution
of 7-azaindole (0.57 g, 4.7 mmol) in 10 mL of dichloromethane
was added at room temperature to a solution of B(C6F5)3 (2.43 g,
4.7 mmol) in 20 mL of dichloromethane. A light yellow solution
was obtained; after the mixture was stirred for 1 h, the solvent was
removed in a vacuum to give a white solid (quantitative yield).
Anal. Calcd for C25H6BF15N2: C, 47.7; H, 0.960; N, 4.45. Found:
C, 47.64; H, 1.12; N, 3.83. EI-MS: m/z 630.0 (C6F5)3B-7-N2C7H6.
1H NMR (CDCl3): δ 6.73 (dd, 1H, J ) 3.67 Hz, J ) 1.96 Hz,
H3), 7.32 (dd, 1H, J ) 3.67 Hz, J ) 2.45 Hz, H2), 7.38 (dd, 1H,
J ) 7.34 Hz, J ) 6.85 Hz, H5), 8.36 (m, 2H, H4 and H6), 9.02
(bs, 1H, NH). 13C NMR (CD2Cl2): δ 103.59 (C3), 115.76 (C5),
125.10 (C3a), 126.56 (C2), 135.91 (C4), 139.12 (C6), 142.76 (C7a).
For the structural and dynamic studies, generally a colorless
solution of 4 was prepared by dissolving ca. 20-25 mg in an NMR
The B(C6F5)3 moiety in the solid-state structures of
compounds 4 and 5 shows a typical conformation, found by
now in more than 40 adducts, taking into account nitrogen-
containing derivatives only.6
In neutral adducts, this conformation also drives the
arrangement of the heterocyclic moiety with respect to the
borane fragment. This can result in intramolecular contacts
that, as in the case of 4, contribute to the activation barriers
for the rotation of the substituents around boron atom and
make it slow enough to allow the determination through
NMR spectroscopy both of the solution structure and of the
nature and energetics of these dynamic processes.
1
tube containing C7D8. H NMR (300 K, C7D8): δ 8.74 (br s, 1H,
J ) 2.02 Hz, J ) 2.6 Hz, H1), 8.08 (m, 1H, J ) 6.13 Hz, H6),
7.28 (d, 1H, J ) 7.67 Hz, H4), 6.48 (dd, 1H, J ) 6.13 Hz, J )
7.67 Hz, H5), 6.11 (dd, 1H, J ) 3.75 Hz, J ) 2.02 Hz, H2), 5.93
(dd, 1H, J ) 3.75 Hz, J ) 2.6 Hz, H3). 13C NMR (C7D8) (Figure
S12): δ 142.71 (C7′), 138.90 (C6), 135.61 (C4), 126.00 (C2),
125.00 (C3′), 115.43 (C5), 103.42 (C3). 19F NMR chemical shifts
are reported in Table 1. 11B NMR (C7D8): δ -5.17 (br s). 15N
NMR (213 K, C7D8): δ -186 (N7), -251 (N1).
The rate constants for the exchanges of rings B/C and A/C
estimated by volume analysis of 2D 19F EXSY experiments are
the following: B/C k (T) 0.22 (278), 0.36 (283), 1.04 (291), 1.98
(299), 4.00 (305), 4.89 (310), 6.67 s-1 (315 K); A/C k (T) 0.087
(283), 0.31 (291), 0.70 (299), 2.86 s-1 (315 K).
Reaction of 4 with Pyridine. A CD2Cl2 solution of compound
4 (60.6 mg in 1.21 mL, 0.0795 M) was prepared and transferred
under a nitrogen atmosphere into two NMR tubes. Different
Experimental Section
General Procedures and Starting Materials. All operations
were performed under nitrogen using conventional Schlenk-line
techniques. Solvents were purified by degassing with N2 and passing
over activated (8 h, N2 purge, 300 °C) Al2O3, and they were stored
under nitrogen. Indole (Aldrich or Fluka), pyridine, 7-azaindole,
2,5-dimethylpyrrole, anthranil (Aldrich), and B(C6F5)3 (Boulder
Scientific Company) were used as received. Compound H2O-
B(C6F5)3 was synthesized as reported in a previous publication.7
(25) Van Geet, A. L. Anal. Chem. 1970, 42, 679.
1690 Inorganic Chemistry, Vol. 45, No. 4, 2006

N-Heterocycle-B(C6F5)3 Complexes
amounts of pyridine (1 and 5 equiv) were added at low temperature
(193 K) to the two samples. 19F NMR spectra were acquired at
298 K at different times on both the samples to monitor the
exchange reaction. The resulting data are shown in Figure S1.
1F), -155.01 (t, 1F), -155.60 (br s, 1F); meta -162.29 (m, 1F),
-162.69 (br, 3F), -163.41 (br s, 1F), -163.78 (br s, 1F).
N-[Tris(pentafluorophenyl)borane]-2,4-dimethyl-5H-pyr-
role (7). Thirty milligrams of adduct 7 (0.0494 mmol), prepared
according to literature procedures,7 was dissolved in 0.7 mL of C8D7
in the NMR tube. The solution appears to be yellow and clear. 1H
NMR (C8D7): δ 1.28 (s, 3H, CH3 in 4), 1.52 (br t, 3H, CH3 in 2),
4.30 (broad AB system, 2H, H5,H5′), 5.15 (s, 1H, J ) 1.57 Hz,
H3).19F NMR (300 K, C8D7): δ ring A 127.78 (m, ortho), -132.16
(m, ortho), -155.57 (ps-t, para), -161.83 (m, meta), -163.37 (m,
meta); ring B -128.90 (br s, ortho), -142.10 (br s, ortho), -157.32
(br s, para), -163.68 (br s, meta), -164.57 (br s, meta); ring C
-132.49 (br s, ortho), -133.88 (br s, ortho), -156.59 (br s, para),
-163.23 (br s, meta), -164.24 (br s, meta). 11B NMR (C7D8): δ
-8.25 (br s).
Reaction of 7-Azaindole with H2O-B(C6F5)3. In a 5 mm NMR
tube, H2O-B(C6F5)3 (23 mg, 43 µmol) was added at room
temperature to a solution of 7-azaindole (5.2 mg, 43 µmol) in 1
mL of CD2Cl2 to give a light yellow solution. The reaction was
monitored via 1H NMR at different times. Complex 5 was
instantaneously and quantitatively formed, as shown by spectro-
scopic monitoring, and slowly converted into complex 4. According
to the spectra area values, the following percentages on compound
4 with respect to 5 were registered: 20:80 after 30 min, 36:64 after
1.5 h, 51:49 after 3.1 h, 54:46 after 4.7 h, and 67:33 after almost
24 h. The latter value did not change with time, remaining
unchanged even after two weeks. An excess of H2O was added to
the mixture, but the ratio 4/5 was not altered and remained
approximately 2:1. [7-azaindolium]+[HOB(C6F5)3]- (5). 1H NMR
(CD2Cl2): δ 6.84 (d, 1H, J ) 3.52 Hz, H3), 7.43-7.50 (m, 1H,
H5), 7.56 (d, 1H, J ) 3.52 Hz, H2), 7.84 (bs, 2H, OH and NH7),
8.08 (d, 1H, J ) 5.87 Hz, H6 or H4), 8.52 (d, 1H, J ) 7.83 Hz,
H4 or H6), 10.01 (bs, 1H, NH1).1H NMR (220 K, CD2Cl2): δ
6.72 (d, 1H, J ) 3.5 Hz, H3), 7.36 (dd, J ) 7.8, 5.6 Hz, 1H, H5),
7.40 (1H, H2), 7.84 (bs, 2H, OH and NH7), 8.06 (d, 1H, J ) 5.87
Hz, H6), 8.36 (1H, H4), 11.73 (bs, 1H, NH1). The resonances of
H2 and H4, assigned through a 2D NOESY/EXSY experiment, lie
under signals of 4, also present in solution.1H NMR (168 K, CD2-
Cl2): δ 6.63, 7.20, 7.94, 12.73, 16.46; the other resonances are
overlapped by the signals of compound 4. 19F NMR (168 K, CD2-
Cl2): δ -135,94 (6F ortho, ∆ν1/2 ) 208 Hz), -159.32 (3F para,
∆ν1/2 ) 90 Hz), -164.34 (6F meta, ∆ν1/2 ) 72 Hz). 1H NMR (233
K, C7D8): δ 6.09 (d, 1H, J ) 3.20 Hz, H3), 6.25 (bs, 2H, OH and
NH7), 6.44 (dd, 1H, J ) 5.03 Hz, J ) 7.77 Hz, H5), 6.60 (d, 1H,
J ) 3.20 Hz, H2), 7.32 (d, 1H, J ) 7.77 Hz, H4), 7.59 (d, 1H, J
) 5.03 Hz, H6), 11.11 (bs, 1H, NH1). 19F NMR (233 K, C7D8):
δ -135.47 (ps dd, 6F ortho), -158.62 (t, 3F para), -163.89 (m,
6F meta). 11B NMR (C7D8): δ -3.60 (br s).
The rate constants for the exchanges of rings B/C and A/C
estimated by volume analysis of 2D 19F EXSY experiments are as
follow: B/C k (T) 0.160 (235), 0.760 (247), 1.14 (251), 2.58 (259),
5.79 (265), 9.25 s-1 (271 K); A/C k (T) 0.125 (265), 0.302 (271),
0.580 (277), 1.25 (284), 1.82 s-1 (289 K).
X-ray Crystal Structure Analysis. Suitable crystals of 4‚1/
2CH2Cl2 were obtained by cooling a solution of 4 in anhydrous
dichloromethane for several months at -25 °C. Crystals of 5,
instead, grew at -5 °C in few days in either toluene solutions of
4 and 5 or solutions of 4 in nonanhydrous toluene. The crystals
were mounted in air on a glass fiber tip onto a goniometer head.
Single-crystal X-ray diffraction data were collected on a Siemens
SMART CCD area detector diffractometer using graphite-mono-
chromated Mo KR radiation (λ ) 0.71073 Å). Data collection was
performed at room temperature for 4‚1/2CH2Cl2 and at 100 K for
5, to obtain a better data set for the latter disordered phase.
Unit cell parameters were initially obtained from about 100
reflections taken from 45 frames collected in three different ω
regions and eventually refined against about 3000 reflections with
2° e θ e 25°.
A full sphere of reciprocal space was scanned by 0.3° ω step,
collecting 1800 frames (the exposure times were 10 and 40 s for
4‚1/2CH2Cl2 and 5, respectively). Intensity decay was monitored
by re-collecting the initial 50 frames at the end of data collection
and analyzing the duplicate reflections. After integration, an
empirical absorption correction was made on the basis of the
symmetry-equivalent reflection intensities measured (for 4‚1/2CH2-
Cl2, 25909 reflections with an average redundancy of 9.0; for 5,
6674 reflections with an average redundancy of 3.1).26
N-[Tris(pentafluorophenyl)borane]-anthranil (6). A solution
of anthranil (99%, 0.446 g, 3.71 mmol) in 5 mL of toluene was
added at room temperature under a nitrogen atmosphere to a solution
of tris(pentafluorophenyl)borane (99.4%, 1.914 g, 3.72 mmol) in
15 mL of toluene. Exothermicity was not observed. During the
addition, the color of the solution turned from light yellow to dark
yellow. The reaction mixture was stirred for 1 h at room temper-
ature, and then the solvent was evaporated in vacuo to give a yellow
powder (yield 98%). Anal. Calcd for C25H5BF15NO: C, 47.6; H,
0.800; N, 2.22. Found: C, 47.8; H, 0.94; N, 1.98. EI-MS: m/z
Crystal data and data collection parameters are summarized in
Table 8.
The structures were solved by direct methods (SIR9727) and
subsequent Fourier synthesis; they were refined by full-matrix least-
squares on F2 (SHELX9728) using all reflections. Scattering factors
for neutral atoms and anomalous dispersion corrections were taken
from the internal library of SHELX97. Weights were assigned to
1
631.0 (C6F5)3B - NOC7H5. H NMR (C6D6): δ 6.30 (ddd, 1H, J
) 9.00 Hz, J ) 6.65 Hz, J ) 0.78 Hz, H5 or H6), 6.52 (dt, 1H, J
) 9.00 Hz, J ) 1.17 Hz, H4), 6.62 (ddd, 1H, J ) 9.00 Hz, J )
6.65 Hz, J ) 1.17 Hz, H6 or H5), 6.94 (bd, 1H, J ) 9.00 Hz, H7),
7.17 (d, 1H, J ) 0.98 Hz, H3). 13C NMR (C6D6): δ 111.54 (C7),
119.67 (C7a or C3a), 120.89 (C4), 126.59 (C5 or C6), 137.64 (C6
or C5), 149.77 (C3a or C7a), 156.66 (C3).
2
individual observations according to the formula w ) 1/[σ2(Fo ) +
2
(aP)2 + bP], where P ) (Fo + 2Fc2)/3; a and b were chosen to
2
give a flat analysis of variance in terms of Fo . Anisotropic
parameters were assigned to all non-hydrogen atoms except for the
minor disordered component of the 7-azaindolium cation in 5 (see
below).
The variable-temperature spectra were recorded on a solution
prepared by dissolving 39 mg (0.063 mmol) of 6 in 0.6 mL of
C7D8 (0.104 M) at room temperature. The solution appeared to be
beige and transparent. 1H NMR (300 K, C7D8): δ 7.05 (overlapped
to a toluene signal 1H, H7), 6.97 (d, 1H, H3), 6.48 (dd, 1H, H5/6),
6.42 (d, 1H, H4), 6.21 (dd, 1H, H5/6). 19F NMR (188 K, C7D8):
δ ortho -129.74 (br s, 1F), -130.48 (ps d, 1F), -133.11 (ps d,
1F), -134.68 (br s, 1F), -138.52 (br s, 1F); para -154.44 (br s,
(26) Sheldrick, G. M. SADABS; Universita¨t Go¨ttingen: Go¨ttingen, Ger-
many, 1996.
(27) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
(28) Sheldrick, G. M. SHELX97; Universita¨t Go¨ttingen: Go¨ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1691

Focante et al.
Table 8. Summary of Crystal Data, Data Collection, and Structure
Refinement Parameters for 4‚1/2CH2Cl2 and 5
bonding); however, they were eventually placed in idealized position
and refined riding on their parent atom with an isotropic displace-
ment parameter of 1.2 (1.5 for the hydroxo group) times that of
the pertinent parent atom.
The structure of 4‚1/2CH2Cl2 suffers from a disorder of the
dichloromethane solvate molecule. Because it was difficult to refine
a consistent disordered model (possibly because of the large volume,
200 Å3, of the cavity occupied by the solvent), its contribution was
subtracted from the observed structure factor according to the
BYPASS procedure,29 as implemented in PLATON.30
The 7-azaindolium cation in 5 is disordered over two orientations,
in which the five- and six-member rings are exchanged. It was
possible to resolve the two individuals, whose refined partial
occupation factors are 0.647(6) and 0.353(6). Bond distances in
the two disordered components were restrained to be equal within
a tolerance of 0.01 Å. The nitrogen atoms were unambiguously
identified on the basis of their position and their proximity to
suitable hydrogen-bond acceptors.
The final difference electron density map showed no features of
chemical significance, with the largest peaks lying close to the
fluorine atoms of the pentafluorophenyl substituents. Final con-
ventional agreement indices and other structure refinement param-
eters are listed in Table 8.
4‚1/2CH2Cl2
5
formula
fw
C25H6BF15N2‚1/2CH2Cl2 C7H17N2+‚C18HBF15O-
672.59
648.14
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
monoclinic
P21/c (No. 14)
19.249(8)
14.026(5)
18.467(8)
triclinic
P1h (No. 2)
8.094(4)
9.890(4)
15.091(5)
92.86(1)
101.00(1)
96.99(1)
1173.7(8)
2
94.32(1)
4972(3)
8
Z
F(000)
2648
1.797
0.289
colorless
1.1-25.0
-22 e h e 22
-16 e k e 16
-21 e l e 21
0
640
1.834
0.195
colorless
2.1-25.0
-9 e h e 9
-11 e k e 11
-17 e l e 17
0
density (g cm-3
)
)
abs coeff (mm-1
cryst color
θ range (deg°)
index ranges
intensity decay (%)
min. transmission
factors
0.857
0.813
no. of measured
reflns
no. of independent
66355
8770
12353
4136
Acknowledgment. T.B. thanks Prof. Angelo Gavezzotti
for helpful discussion. Thanks are due to the Italian CNR
(ISTM) for providing facilities for inert-atmosphere and low-
temperature experiments.
reflns
a
R
int, Rσ
0.0525, 0.0309
5334
0.0352, 0.0363
3093
no. of reflns
with I > 2σ(I)
no. of data/
8770/0/775
4136/9/435
Supporting Information Available: Twelve figures showing
details of the NMR experiments. This material is available free of
charge via the Internet at http://pubs.acs.org. CCDC 279502 and
279503 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk).
restraints/params
weights (a, b)b
GOF S(F2)c
0.02, 0
1.616
0.0381
0.0870
0.05, 0.4
1.022
0.0344
0.0937
0.230, -0.257
R1(F), I > 2σ(I)d
wR2(F2), all datae
largest diff peak,
0.237, -0.201
hole (e Å-3
)
2
2
2
2
2
a Rint ) ∑|Fo2 - Fmean |/∑|Fo |; Rσ ) ∑|σ(Fo )|/∑|Fo |. b w ) 1/[σ2(Fo )
2
2
2
2
+ (aP)2 + bP], where P ) (Fo + 2Fc )/3. c S ) [∑w(Fo - Fc )2/(n -
p)]1/2, where n is the number of reflections and p is the number of refined
IC051285P
2
2
parameters. d R1(F) ) ∑||Fo| - |Fc||/∑|Fo|. e wR2(F2) ) [∑w(Fo -Fc )2/
4
∑wFo ]1/2
.
(29) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46,
194.
(30) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
(31) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
Some of the hydrogen atoms were clearly found in a difference
Fourier maps (in particular hydrogen atoms involved in hydrogen
1692 Inorganic Chemistry, Vol. 45, No. 4, 2006