Single-crystal X-ray diffraction and solution studies of anion-π interactions in N-(pentafluorobenzyl)pyridinium salts

Article abstract of DOI:10.1002/ejoc.201301336

A solid-state structural study on anion-π interaction in various N-(pentafluorobenzyl)pyridinium salts accompanied by NMR spectroscopic investigations is presented. The crystal structures of 1a-1d reveal different kinds of contacts with anions, including anion-π interactions. In particular, the solid-state structure of 1b-I₃ shows distinct evidence of anion-π interactions. Attempts to study anion-π interactions in solution were not successful, but their presence in solution could not be ruled out. The interactions of N-(pentafluorobenzyl)pyridinium salts are investigated in the solid state as well as in solution. The triiodide salt shows distinct evidence of anion-π interactions with both the pyridinium and the pentafluorophenyl unit. Solution investigations to prove the attractive nature of the noncovalent force failed, but they did not rule out the presence of the interaction. Copyright

Full text of DOI:10.1002/ejoc.201301336

FULL PAPER
DOI: 10.1002/ejoc.201301336
Single-Crystal X-ray Diffraction and Solution Studies of Anion–π Interactions
in N-(Pentafluorobenzyl)pyridinium Salts
Michael Giese,[a] Markus Albrecht,*[a] Tatjana Repenko,[a] Johannes Sackmann,[a]
Arto Valkonen,[b] and Kari Rissanen*[b]
Keywords: Anion–π interactions / Pyridinium salts / Triiodides / Noncovalent interactions / Structure elucidation
A solid-state structural study on anion–π interaction in vari- solid-state structure of 1b-I3 shows distinct evidence of
ous N-(pentafluorobenzyl)pyridinium salts accompanied by
NMR spectroscopic investigations is presented. The crystal
structures of 1a–1d reveal different kinds of contacts with
anions, including anion–π interactions. In particular, the
anion–π interactions. Attempts to study anion–π interactions
in solution were not successful, but their presence in solution
could not be ruled out.
open question.[8] In 2008, we started our work on anion–π
interactions in (pentafluorophenyl)ammonium and -phos-
phonium salts.[9] Our first crystallographic results revealed
a versatile positional variation of the anion above the C6F5
unit. To describe the observed binding motifs, the hapticity
(η) nomenclature for cation–π interactions was transferred
to systems with anion–π interactions. Further investigations
showed that the position of the anion above an electron-
deficient arene can be controlled by directing substituents
in the molecular skeleton through the utilization of CH–
anion and NH–anion interactions.[10] Moreover, the effect
of the electron density of the π-system on the interaction
with anions was extensively studied. The attraction in
highly fluorinated systems (four to five fluorine atoms) is
switched to repulsion in arenes with a low degree of
fluorination (two to three fluorine atoms).[11] Crystallo-
graphic studies on the role of the anions in anion–π interac-
tions do not show dependence on the geometry of the
anion.[12] However, serendipity led to the remarkable stabili-
zation of the labile tetraiodide dianion (I42–) by anion–π
interactions.[13] Our first attempts to clarify the relevance of
anion–π interactions in solution by studying (penta-
fluorobenzyl)phosphonium salts did not show any evidence
for this intermolecular force in chloroform.[14]
Introduction
Anions play a crucial role in biological and chemical sys-
tems.[1] A large number of enzymes and enzymatic cofactors
are negatively charged,[2] and diseases such as cystic fibrosis
are caused by a malfunction of anion channels.[3] This
shows the high relevance of selective anion receptors. Com-
mon anion receptors are based on electrostatic attractions,
hydrogen bonds, or hydrophobic effects.[4]
In recent years, a new type of noncovalent interaction
has gained considerable attention in the field of anion re-
cognition, namely, the anion–π interaction.[5] In the 1990s,
Schneider et al.[6] reported the first experimental evidence
for the attractive interaction of the negatively charged parts
of a molecule with polarized π-systems. Following the com-
putational studies of Mascal, Alkorta and Deyà[7a,7b] sup-
ported the attractive nature and explained it with the inter-
action of an anion with the π-system of an electron-de-
ficient arene. This intermolecular force is suggested to be
appropriate for the design of superior anion receptors and,
therefore, the interest in anion–π interactions has grown
rapidly within the last 20 years.[5]
Since the report by Schneider et al.,[6] numerous theoreti-
cal and structural studies have been performed to show the
relevance of anion–π interactions.[7] In contrast, the role of
this noncovalent interaction in solution has remained an
The present study reports the experimental evidence of
anion–π interactions in crystals of N-(pentafluorobenzyl)-
pyridinium salts. Additional solution NMR spectroscopic
investigations have been performed.
[a] Institut für Organische Chemie, RWTH Aachen,
Landoltweg 1, 52074 Aachen, Germany
E-mail: markus.albrecht@oc.rwth-aachen.de
http://www.oc.rwth-aachen.de/akalbrecht/
[b] Department of Chemistry, Nanoscience Center, University of
Jyväskylä,
Results and Discussion
Survontie 9, 40014 Jyväskylä, Finland
E-mail: kari.t.rissanen@jyu.fi
On the basis of previous results on the directing effects
of substituents on the position of anions above electron-
poor arenes, a series of N-(pentafluorobenzyl)pyridinium
salts (Br, I, PF6, and BF4) was synthesized. In these salts,
https://www.jyu.fi/kemia/tutkimus/orgaaninen/en/research/
rissanen
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301336.
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M. Albrecht, K. Rissanen et al.
FULL PAPER
Scheme 1. Synthesis of the N-(pentafluorobenzyl)pyridinium salts.
the anion should be located above the π-system by fixation arenes (pentafluorophenyl and pyridinium units). The
through CH–anion interactions of the pyridinium moiety bromide anion is in close contact with the pentafluoro-
or encapsulated in a cradle of anion–π interactions with phenyl group [C1–3···Br 3.922(3), 3.478(3), 3.726(3) Å; η3-
both electron-deficient arenes (see Scheme 1). A series of type] and the pyridinium unit [C2,5,6···Br 3.951(3), 3.824(3),
solid-state structures could be obtained and showed anion– 3.662(3) Å, N1···Br 3.750(2) Å; η4-type]. However, the
π as well as CH–anion interactions. Additionally, NMR ti- shortest anion–π contact was found with the pyridinium
tration experiments with n-tetrabutylammonium salts were unit of an N-(pentafluorobenzyl)pyridinium cation averted
performed to investigate the interplay of the interactions in from the zigzag strand [C5···Br 3.395(3) Å, N1···Br
solution.
3.803(2) Å; η2-type]. Additional CH–anion interactions
were found between the benzyl protons and neighboring
bromide anions [CH···Br 2.985 Å; x, y – 1, z symmetry op-
eration].
Synthesis of Pyridinium Compounds
The N-(pentafluorobenzyl)pyridinium halides 1a, 1b, and
2a were prepared by a nucleophilic substitution of pyridine
or 4-tert-butylpyridine with the corresponding penta-
fluorobenzyl halide. By serendipity, the iodide was oxidized
in air during the crystallization, and the triiodide salt of the
N-(pentafluorobenzyl)pyridinium compound (1b-I3) was
obtained. The tetrafluoroborate (1c and 2b) and hexa-
fluorophosphate (1d) salts were obtained by salt metathesis
from saturated aqueous solutions.
All compounds – except 1b-I3 – were fully characterized
by standard analytical methods, and representative solid-
state structures were obtained.
Solid-State Studies
Owing to the complexity of the interplay of noncovalent
interactions in the crystal lattice, the overall packing will
not be described. The focus will be on the relevant interac-
tions of adjacent ions. Relevant anion–π contacts and ar-
ene–arene interactions will be discussed in detail. It should
be noted that the crystal structures of the non-fluorinated
analogues of the compounds described in the following are
well known.[15] However, these structures show exclusively
CH–anion interactions and give no hint of anion–π interac-
tions with neither the phenyl nor the pyridinium unit.
Figure 1. (a) Side and (b) top views of the asymmetric unit in the
crystal structure of 1a·H2O illustrating the close contact between
the anions and cations; (c) view of the molecular packing; black:
carbon, white: hydrogen, green: fluorine, blue: nitrogen, red: oxy-
gen, dark red: bromine.
N-(Pentafluorobenzyl)pyridinium Bromide and Iodide (1a
and 1b)
A closely related structure is observed for 1b, which crys-
tallizes in the triclinic space group P1 and shows two anions
¯
Crystals of 1a·H2O were obtained by the diffusion of and two cations per asymmetric unit (Figure 2). The se-
ethyl acetate into a solution of 1a in dimethyl sulfoxide lected anion–cation pairs (1b-A and 1b-B) possess similar
(DMSO). The cocrystallized water molecule networks the structural features, and both anions show short CH–anion
anions to form a zigzag strand [Figure 1c; O···Br 3.401(2), distances [CH···I: 1b-A: 2.920, 2.946, 3.068 Å (all from
3.503(2) Å]. This strand is bordered by electron-deficient neighboring cations), 1b-B: 2.907, 3.025 Å (latter from
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Anion–π Interactions in N-(Pentafluorobenzyl)pyridinium Salts
neighboring cation of A)] as well as close anion–π contacts
to the pentafluorophenyl unit [1b-A: C5–6···I 4.005(3),
3.852(3) Å, C2–4···I 3.802(3), 3.629(3), 4.081(3) Å to I of B;
1b-B: C2···I 3.953(3) Å]. The crystal packing reveals the T-
stacking of the pentafluorophenyl moieties of neighboring
cations (CF···aryl 3.259 Å; see Figure S1). There is a sym-
metry-related π···π stacking interaction between the pyrid-
inium rings of adjacent cations of A with a shortest C···C
distance of 3.374(4) Å [center···center 3.60 Å; –x + 2, –y,
–z symmetry operation].
Figure 3. (a and c) Side and (b and d) top views of the solid-state
structures of 1c (selected 1c-A pair) and 1d; black: carbon, white:
hydrogen, green: fluorine, blue: nitrogen, orange: phosphorus, pink:
boron.
simultaneous anion–π interactions, whereby both electron-
deficient arenes face the hexafluorophosphate anion in
parallel. The C6F5 and C5H5N rings span (ЄC6F5–C6H5N
65.93°) an electron-deficient cradle for the hexafluorophos-
phate ion that leads to attractive anion–π interactions
[C6F5: C···F 2.823(3)–3.563(3) Å, CgAr···F 3.27 Å; C5H4N:
C···F 3.152(3)–3.461(3) Å]. The anion is slightly shifted
away from the centers of the π-systems. A wider view of
the crystal packing shows that there are additional anion–
π contacts with the pyridinium unit of an adjacent cation
[C/N···F 3.065(2)–3.717(3) Å] and several CH–anion inter-
actions [C–H···F 2.445–2.649 Å].
Figure 2. (a and b) Two views of the selected anion–cation pair of
1b-A and (c) the asymmetric unit of 1b showing the assignment of
the ion pairs; black: carbon, white: hydrogen, green: fluorine, blue:
nitrogen, violet: iodine.
N-(Pentafluorobenzyl)pyridinium Tetrafluoroborate and
Hexafluorophosphate (1c and 1d)
All relevant anion–π distances of the previously de-
scribed structures are summarized in Table S2.
The crystal structure of 1c shows obvious similarities to
As already mentioned in the introduction, an additional
that of 1b. Like 1b, compound 1c crystallizes in the triclinic N-(pentafluorobenzyl)pyridinium salt structure was found.
¯
space group P1 with two anions and two cations per asym- The triiodide 1b-I3 was obtained by crystallization of 1b in
metric unit (Figure 3). In the selected anion–cation pairs, methanol under an oxidative atmosphere. It crystallized in
¯
both anions are fixed by CH–anion interactions [C–H···F: the triclinic space group P1 with two halves of a triiodide
1c-A: 2.582, 2.620 Å; 1c-B: 2.504 Å] above the π-system ion in the asymmetric unit (central iodine atoms in special
[C2–3···F: 1c-A: 3.066(3), 3.205(3) Å, 1c-B: 2.996(3), positions) and shows a remarkable feature (Figure 4). The
3.331(3) Å; CgAr···F: 1c-A: 3.34 Å, 1c-B: 3.65 Å; Cg is the crystal structure reveals that two N-(pentafluorobenzyl)-
centroid of the aryl ring] and show η2-type anion–π con- pyridinium cations create an electron-deficient box around
tacts. Anion–π η2-type interactions from the anion of B to one triiodide anion. This leads to close contacts between
the pyridinium moiety of the adjacent A pair were also ob- the central iodine atom and the pentafluorophenyl [C···I
served in the structure of 1c [N···F 3.103(3) Å, C6···F 3.765(4)–4.003(4) Å; CgAr···I 3.63 Å] ring as well as to the
3.080(3) Å; –x + 1, –y + 1, –z + 2 symmetry operation]. pyridinium units [C/N···I 3.858(4)–4.106(5) Å; CgAr···I
Additional CH–anion interactions with the benzyl protons 3.74 Å]. The second triiodide anion shows anion–π contacts
[1c-A: C–H···F 2.469, 2.395 Å; 1c-B: C–H···F 2.259, between the terminal iodine atoms and the pyridinium unit
2.293 Å] are present in the solid-state structure, and almost [C/N···I 3.657(3)–4.196(4) Å, CgAr···I 3.69 Å; –x + 1, –y,
all pyridyl protons show very weak CH–anion contacts –z + 1 symmetry operation]. Additionally, interactions be-
[C–H···F 2.400–2.564 Å].
tween the central [C2–3···I 3.885(4), 4.003(4) Å, CgAr···I
Crystals of 1d were obtained by diffusion of diethyl ether 4.32 Å; η2-type] and terminal iodine atoms [C1,6···I
into a solution of 1d in N,N-dimethylformamide (DMF). 3.608(4), 3.910(4) Å] with the pentafluorophenyl groups are
The salt crystallizes in the orthorhombic space group Pbca. observed. No CH···I contacts were found in the structure
In contrast to the previously described structures, 1d shows of 1b-I3.
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M. Albrecht, K. Rissanen et al.
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used owing to the lower acidity of their protons and their
higher air stability. Owing to the low solubility of N-(penta-
fluorobenzyl)pyridinium tetrafluoroborate as well as hexa-
fluorophosphate in chloroform, the corresponding 4-tert-
butylpyridinium salts were synthesized. Only the tetra-
fluoroborate 2b showed an appropriate solubility in chloro-
form for NMR spectroscopic measurements. To determine
the differential binding constants for the 4-tert-butyl-N-
(pentafluorobenzyl)pyridinium cation, 2b was titrated with
tetrabutylammonium chloride, bromide, iodide, tetra-
fluoroborate, and hexafluorophosphate. The investigated
solution studies are summarized in Scheme 2.
Initial studies on the stoichiometry of the investigated
anion–receptor complexes (guest: receptor) by the method
of Job[16] revealed significant shifts for the benzyl and pyr-
idyl protons in the 1H NMR spectra and for the ortho-,
meta-, and para-fluorine atoms in the 19F NMR spectra
(Figure 5). Interestingly, the stoichiometry of the complexes
varies in dependence of the signal monitored in the NMR
spectra. Although the titrations with tetrabutylammonium
tetrafluoroborate and hexafluorophosphate do not show
systematic development during the measurement, the
stoichiometry for the halides can be determined. The benzyl
and ortho-fluorine signals show 1:1 complexes for all three
halides. The pyridyl proton signal reveals a 1:1 complex for
the chloride and bromide and a 1:2 stoichiometry for the
iodide. The meta and para 19F signals show a 2:1 complex
for the chloride and a 1:1 complex for the iodide. The data
for the bromide titration does not show a systematic behav-
ior. However, the differences in the complex stoichiometries
might be attributed to the rather flat Job curves and should
not be overemphasized (see Table S3).
Figure 4. (a and b) Two different views of the encapsulated triiod-
ide anion in the solid-state structure of 1b-I3 and (c) the second
triiodide anion in the unit cell; black: carbon, white: hydrogen,
green: fluorine, blue: nitrogen, violet: iodine.
The presented series of N-(pentafluorobenzyl)pyridinium
salts demonstrate the interactions between bromide, iodide,
tetrafluoroborate, and hexafluorophosphate anions and
electron-deficient pentafluorobenzyl and pyridinium units.
The strongest structural evidence for the attractive nature
of anion–π interactions within this series is found in the
solid-state structure of 1b-I3. The C6F5 unit and the C5NH5
–
moiety show short distances to the linear I3 anion. Struc-
tural hints of anion–π interactions in the solid state are nu-
merous,[7] but their role in solution remains an open ques-
tion.[5]
To ascertain the differential association constants, a
0.1 m stock solution of 2b and a 0.4 m solution of the tetra-
butylammonium salts were prepared. The solutions were
mixed in different ratios and filled to a standard volume of
0.7 mL to guarantee a fixed receptor concentration.
Solution Studies
As already mentioned in the introduction, several
The signals of the pyridyl protons in the NMR spectra
attempts have been made to investigate the relevance of used to produce the titration curves show a downfield shift
anion–π interaction in solution.[8,14] However, irrefutable upon the addition of halides and an upfield shift with tetra-
evidence is still missing. Competing noncovalent interac- fluoroborate and hexafluorophosphate anions. Further-
tions and solvation effects make it difficult to obtain dis- more, the shifts for the halides are significantly stronger
tinct evidence for anion–π interactions in solution.
than for the less-coordinating anions. A comparison of the
Earlier studies with (pentafluorobenzyl)phosphonium different signals followed during the NMR spectroscopic
salts showed that CH–anion interactions cover the attrac- measurements shows that there is a strong upfield shift for
tion between anions and electron-deficient π-systems. In the the ortho-fluorine signal, whereas the signals of the benzyl
present study, N-(pentafluorobenzyl)pyridinium salts were and pyridyl protons as well as that of the para-fluorine
Scheme 2. Investigated equilibrium in solution.
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Anion–π Interactions in N-(Pentafluorobenzyl)pyridinium Salts
Figure 5. (a) Job plots for the pyridinium complexes with various anions (Cl, Br, I, PF6, and BF4 added as tetrabutylammonium salt).
(b) Job plots for the titration of 2b with nBu4NBr in CDCl3. (c) Selected 1H NMR spectra for the titration of 2b with nBu4NBr in CDCl3.
(d) Selected 19F NMR spectra for the titration of 2b with nBu4NBr in CDCl3.
atom shift downfield. For the meta-fluorine signal, a sys-
Assuming 1:1 complexes, the association constants vary
tematic shift could not be observed. The determined bind- from 236 to 704, whereby they decrease in the series from
ing constants under consideration of the complex stoichio- chloride to bromide to iodide. A comparison of the binding
metries are summarized in Table 1.
constants determined by monitoring the benzylic proton
signal with those by monitoring the ortho-fluorine signal
reveals a significant difference. Moreover, the binding con-
stants obtained from the ortho-fluorine signals are closer
together than those for the benzyl protons. However, owing
to the direct interaction of the anion with the electron-de-
ficient C6F5 unit, the results derived from the fluorine sig-
nal should be more expressive in anion–π studies. Neverthe-
less, it is difficult to determine the role of anion–π interac-
tions in solution from the observed results.
Table 1. Differential binding constants Ka for the 1:1 complexes
of pyridinium salts with various anions (Cl, Br, and I added as
tetrabutylammonium salt). The binding constants were determined
1
by H/19F NMR spectrometric analyses in CDCl3. Errors are esti-
mated to be lower than 20%.
Cl
Br
I
Hbenzyl
Fortho
704
388
518
352
325
236
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M. Albrecht, K. Rissanen et al.
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synthesis of the N-(pentafluorobenzyl)pyridinium halides was per-
formed neat; therefore, equimolar amounts of pyridine or 4-tert-
butylpyridine were mixed together with the corresponding penta-
fluorobenzyl halide (bromide or iodide) at 50 °C, and the mixture
Conclusions
We observed anion–π interactions in the solid-state struc-
tures of N-(pentafluorobenzyl)pyridinium salts. Thereby,
the C6F5 unit as well as the pyridinium group is involved in was stirred until a colorless solid was obtained. The resulting solids
were ground and dried under vacuum to remove the excesses start-
ing materials.
anion–π contacts. In most solid-state structures, the anion is
embedded in a number of CH–anion and anion–π contacts.
However, only the crystal structures of 1b-I3 and 1d show
a cooperative anion–π interaction between the two electron-
deficient arenes and the anion located between them. The
most remarkable structure is the triiodide 1b-I3, which en-
capsulates the anion in an electron-deficient channel. As
this structure was observed by serendipity, future studies
will focus on the trapping of polyhalides in electron-de-
ficient cavities (e.g., to stabilize labile anionic species such
as the tetraiodide dianion).
Although the performed NMR spectroscopic measure-
ments do not give distinct evidence of the interactions of
anions with electron-deficient arenes, the present study re-
veals that the calculated association constants strongly de-
pend on the signal monitored during the analysis. This ob-
servation raises the question of whether the differences are
related to the observation of different binding events. Fur-
ther studies should reveal which signals are the most reli-
able for the investigation of weak forces such as anion–π
interactions in solution by NMR spectroscopy.
1a: Light yellow solid; yield 1.30 g (3.8 mmol, quant.). M.p. 169 °C.
1H NMR (MeOD, 300 MHz): δ = 9.06 (br. d, J = 5.5 Hz, 2 H,
Hpyr), 8.68 (tt, J = 7.9, 1.3 Hz, 1 H, Hpyr), 8.18 (m, 2 H, Hpyr), 6.10
(s, 2 H, Hbenzyl) ppm. 19F NMR (MeOD, 300 MHz): δ = –142.68
(m, 2 F, Fo), –153.10 (m, 1 F, Fp), –163.21 (m, 2 F, Fm) ppm. MS
(ESI): m/z (%) = 260.1 (70) [M]+, 598.9 (100) [M2Br]+. IR (KBr):
ν = 3122 (w), 3019 (w), 2954 (w), 1875 (w), 2733 (w), 2384 (w),
˜
2172 (w), 2084 (w), 1860 (w), 1662 (w), 1629 (m), 1579 (w), 1513
(vs), 1481 (s), 1436 (m), 1378 (w), 1308 (m), 1278 (w), 1211 (m),
1164 (m), 1128 (m), 1107 (m), 1083 (w), 1030 (s), 969 (s), 926 (s),
826 (w), 775 (m), 746 (w), 727 (m), 687 (s) cm–1. C12H7BrF5N
(338.97): calcd. C 42.38, H 2.07, N 4.12; found C 42.37, H 2.09, N
4.16.
1b: Yellow solid; yield 300 mg (0.8 mmol, 80%). M.p. 200 °C. 1H
NMR (MeOD, 300 MHz): δ = 9.09 (br. d, J = 5.7 Hz, 2 H, Hpyr),
8.69 (tt, J = 7.8, 1.5 Hz, 1 H, Hpyr), 8.19 (m, 2 H, Hpyr), 6.11 (s, 2
H, Hbenzyl) ppm. 19F NMR (MeOD, 300 MHz): δ = –142.53 (m, 2
F, Fo), –153.09 (m, 1 F, Fp), –163.14 (m, 2 F, Fm) ppm. MS (ESI):
m/z (%) = 260.8 (100) [M]+, 181.3 (97) C7H2F5+. IR (KBr): ν =
˜
3123 (w), 3047 (m), 3021 (w), 2956 (m), 2873 (w), 2375 (w), 2082
(w), 1991 (w), 1849 (w), 1751 (w), 1662 (w), 1629 (m), 1579 (w),
1512 (vs), 1481 (vs), 1435 (m), 1371 (m), 1306 (m), 1277 (w), 1211
(m), 1162 (m), 1127 (s), 1104 (m), 1079 (w), 1055 (w), 1027 (s), 968
(s), 924 (s), 864 (w), 827 (w), 772 (m), 745 (w), 723 (m), 683 (vs)
cm–1. C12H7F5IN (386.95): calcd. C 37.23, H 1.82, N 3.62; found
C 37.03, H 1.77, N 3.55.
Experimental Section
General: Solvents were used after distillation, and commercially
available reagents were used as received without further purifica-
tion. The 1H and 19F NMR spectroscopic data were measured with
a Varian Mercury 300 or an Inova 400 spectrometer with samples
in deuterated solvents. Mass spectra were recorded with a Finnigan
SSQ 7000 or Thermo Deca XP spectrometer by using EI (70 eV)
or ESI techniques. Infrared spectra were obtained with a Perkin–
Elmer FTIR spectrometer. The samples were measured in KBr
(4000–650 cm–1). Elemental analysis was performed with a CHN-
O-Rapid Vario EL instrument from Heraeus. The melting points
were obtained by using a Büchi B-540 instrument. Single-crystal
X-ray data were collected at 123(2) K by using a Bruker-Nonius
KappaCCD diffractometer with an APEX-II detector and graph-
ite-monochromated Mo-Kα (λ = 0.71073 Å) radiation. The COL-
LECT[17] software was used for the data collection (θ and ω scans),
and DENZO-SMN[18] was used for the processing. The structures
were solved by direct methods with SIR2004[19] and refined by full-
matrix least-squares methods with the WinGX software,[20] which
utilizes the SHELXL-97 module.[21] Lorentzian polarization and
multiscan absorption corrections (SADABS)[22] were applied to all
data. All C–H hydrogen atom positions were calculated and refined
as riding atoms with 1.2 times the thermal parameters of the C
atoms. H atoms from water molecules in 1a were found from the
electron-density map and restrained (by DFIX; s = 0.02) to a dis-
tance of 0.84 Å from the O atom with thermal parameters 1.5 times
that of the O atom. CCDC-936265 (for 1a-H2O), -936266 (for 1b-
I3), -936267 (for 1b), -936268 (for 1c), and -936269 (for 1d) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2a: White solid; yield 300 mg (0.8 mmol, 98%). M.p. 192 °C. 1H
NMR (CDCl3, 300 MHz): δ = 9.57 (d, J = 6.8 Hz, 2 H, Hpyr), 8.08
(d, J = 6.8 Hz, 2 H, Hpyr), 6.56 (s, 2 H, Hbenzyl), 1.42 (s, 3 H,
CH3) ppm. 19F NMR (CDCl3, 300 MHz): δ = –139.07 (m, 2 F, Fo),
–148.99 (m, 1 F, Fp), –158.95 (m, 2 F, Fm) ppm. MS (EI, 70 eV):
m/z (%) = 316.1 (34) [M]+. IR (KBr): ν = 3125 (w), 2941 (m), 2877
˜
(w), 2709 (w), 2380 (w), 1986 (w), 1848 (w), 1641 (m), 1561 (w),
1507 (vs), 1472 (s), 1387 (m), 1304 (w), 1275 (w), 1241 (w), 1187
(m), 1126 (s), 1025 (s), 967 (s), 936 (s), 841 (m), 767 (w), 714 (w),
669 (m) cm–1. C16H15BrF5N (396.19): calcd. C 48.50, H 3.82, N
3.54; found C 48.62, H 3.63, N 3.51.
N-(Pentafluorobenzyl)pyridinium Tetrafluoroborate and Hexafluoro-
phosphate (1c and 1d) and 4-tert-Butyl-N-(pentafluorobenzyl)pyrid-
inium Tetrafluoroborate (2b): The salts 1c, 1d, and 2b were prepared
by salt metathesis by using saturated aqueous solutions of N-
(pentafluorobenzyl)pyridinium bromide (1a) or 4-tert-butyl-N-
(pentafluorobenzyl)pyridinium bromide (2a). To these solutions, a
saturated solution of the corresponding ammonium salt (tetrafluo-
roborate or hexafluorophosphate) was added. The precipitated so-
lid was collected by filtration and dried in vacuo.
1c: White solid; yield 90 mg (0.3 mmol, 90%). M.p. 166 °C. 1H
NMR (MeOD, 300 MHz): δ = 9.04 (br. d, 2 H, Hpyr), 8.68 (tt, J =
7.8, 1.2 Hz, 1 H, Hpyr), 8.16 (m, 2 H, Hpyr), 6.07 (s, 2 H, Hbenzyl
)
ppm. 19F NMR (MeOD, 300 MHz): δ = –142.83 (m, 2 F, Fo),
–153.14 (m, 1 F, Fp), –154.79 (s, 4 F), –163.29 (m, 2 F, Fm) ppm.
+
MS (EI, 70 eV): m/z (%) = 260.5 (100) [M]+, 181.1 (98) C7H2F5
.
IR (KBr): ν = 3139 (w), 3097 (w), 3032 (w), 2324 (w), 2089 (w),
˜
N-(Pentafluorobenzyl)pyridinium Bromide and Iodide (1a and 1b)
and 4-tert-Butyl-N-(pentafluorobenzyl)pyridinium Bromide (2a): The
1663 (w), 1633 (w), 1516 (vs), 1487 (s), 1460 (w), 1439 (w), 1376
(w), 1308 (w), 1289 (w), 1220 (m), 1175 (m), 1130 (m), 1038 (vs,
2440
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 2435–2442

Anion–π Interactions in N-(Pentafluorobenzyl)pyridinium Salts
br), 969 (s), 925 (s), 869 (w), 829 (w), 779 (m), 748 (w), 726 (w),
687 (s) cm–1. C12H7BF5F4N (347.05): calcd. C 41.54, H 2.03, N
4.04; found C 41.18, H 1.71, N 4.09.
1d: White solid; yield 110 mg (0.3 mmol, 91%). M.p. 179 °C. 1H
NMR ([D6]DMSO, 300 MHz): δ = 9.06 (br. d, J = 5.7 Hz, 2 H,
Dcalcd. = 1.779 Mgm–3, μ = 0.195 mm–1, F(000) = 688, 7818 col-
lected reflections (θmax = 25.25°) of which 4672 independent (Rint
= 0.0250) and 3463 with IϾ2σ(I), Tmax = 0.9846, Tmin = 0.9528,
full-matrix least squares on F2 with 0 restraints and 415 parameters,
GOF = 1.030, R1 = 0.0417 [IϾ2σ(I)], wR2 (all data) = 0.0941,
largest peak/hole = 0.283/–0.240 e– Å–3.
H
pyr), 8.65 (tt, J = 7.8, 1.5 Hz, 1 H, Hpyr), 8.17 (m, 2 H, Hpyr), 6.07
(s, 2 H, Hbenzyl) ppm. 19F NMR ([D6]DMSO, 300 MHz): δ = –70.19
(d, J = 710.6 Hz, PF6), –140.17 (m, 2 F, Fo), –151.99 (m, 1 F, Fp),
–161.50 (m, 2 F, Fm) ppm. MS (EI, 70 eV): m/z (%) = 260.1 (100)
1d: Colorless blocks from DMF/Et2O, C12H7F11NP (405.16), crys-
tal size 0.30ϫ0.25ϫ0.20 mm, orthorhombic, space group Pbca
(no. 61), a = 6.84290(10), b = 12.6254(2), c = 31.8647(4) Å, V =
2752.93(7) Å3, Z = 8, Dcalcd. = 1.955 Mgm–3, μ = 0.331 mm–1,
F(000) = 1600, 4557 collected reflections (θmax = 25.25°) of which
[M]+, 181.1 (89) C7H2F5+. IR (KBr): ν = 3145 (w), 3102 (w), 1659
˜
(w), 1627 (w), 1511 (m), 1451 (w), 1396 (w), 1357 (w), 1310 (w),
1278 (w), 1226 (w), 1176 (w), 1130 (m), 1112 (w), 1026 (m), 970
(m), 917 (m), 878 (w), 824 (vs), 778 (m), 738 (m), 690 (m) cm–1.
C12H7F5F6NP (405.01): calcd. C 35.57, H 35.41, N 3.46; found C
35.41, H 1.92, N 3.48.
2475 independent (Rint = 0.0136) and 2150 with IϾ2σ(I), Tmax
=
0.9368, Tmin = 0.9073, full-matrix least squares on F2 with 0 re-
straints and 226 parameters, GOF = 1.029, R1 = 0.0330 [IϾ2σ(I)],
wR2 (all data) = 0.0792, largest peak/hole = 0.287/–0.302 e– Å–3.
2b: White solid; yield 86 mg (0.2 mmol, 85%). M.p. 150 °C. 1H
NMR (CDCl3, 400 MHz): δ = 8.77 (d, J = 7.0 Hz, 2 H, Hpyr), 8.00
(d, J = 7.0 Hz, 2 H, Hpyr), 5.96 (s, 2 H, Hbenzyl), 1.41 (s, 3 H,
CH3) ppm. 19F NMR (CDCl3, 400 MHz): δ = –140.22 (m, 2 F, Fo),
–148.67 (m, 1 F, Fp), –152.38 (s, 4 F, BF4), –158.95 (m, 2 F, Fm)
Supporting Information (see footnote on the first page of this arti-
cle): Crystal stacking in 1b, anion–π distances in the previously
described structures, data for titration experiments, 1H and 19F
NMR spectra.
ppm. MS (EI, 70 eV): m/z (%) = 316.1 (34) [M]+. IR (KBr): ν =
˜
Acknowledgments
3327 (w), 3146 (w), 3083 (w), 2972 (w), 2088 (w), 1980 (w), 1646
(m), 1572 (w), 1509 (vs), 1471 (m), 1364 (w), 1305 (w), 1280 (w),
1183 (w), 1120 (m), 1034 (vs), 964 (m), 916 (m), 817 (w), 762 (w),
734 (w), 693 (w), 657 (w) cm–1. C16H15BF5F4N (403.09): calcd. C
47.67, H 3.75, N 3.47; found C 47.58, H 3.74, N 4.33.
This work was supported by the Fonds der Chemischen Industrie
(M. A.) and the Academy of Finland (K. R.: project nos 122350,
140718, 265328, and 263256).
Crystal Data
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–0.293 e– Å–3.
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¯
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9.4367(2), b = 11.6827(2), c = 12.6011(3) Å, α = 103.4940(10), β =
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¯
0.13ϫ0.12ϫ0.07 mm, triclinic, space group P1 (no. 2), a =
9.2487(2), b = 9.3202(2), c = 10.4842(3) Å, α = 98.063(2), β =
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¯
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Eur. J. Org. Chem. 2014, 2435–2442
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2014, 2435–2442