Di-, tri-, and tetra(pentafluorophenyl) derivatives for oligotopic anion-π interactions

Article abstract of DOI:10.1021/ic4008087

The present study describes a series of pentafluorobenzyl ammonium salts with two, three, or four C₆F₅ units in order to investigate simultaneous interactions of several perfluorinated arenes with anions in the crystalline state. Most of the structures show multiple anion-π contacts. However, only 6·2HI reveals an effective encapsulation of the iodide ion by the aromatic units. For comparison, the structure of 4b is investigated because it offers two π-systems with inverse charge distribution to a bromide anion. Only the electron-deficient π-system of the pentafluorophenyl group interacts with the anion.

Full text of DOI:10.1021/ic4008087

Article
pubs.acs.org/IC
Di‑, Tri‑, and Tetra(pentafluorophenyl) Derivatives for Oligotopic
Anion−π Interactions
Michael Giese,† Markus Albrecht,*,† Simon Steike,† Anton Ackermann,† Arto Valkonen,‡
and Kari Rissanen‡
†Institut fur Organische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
̈
‡Nanoscience Center, Department of Chemistry, University of Jyvaskyla, P.O. Box 35, FIN-40014, Jyvaskyla, Finland
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The present study describes a series of pentafluorobenzyl ammonium salts with
two, three, or four C6F5 units in order to investigate simultaneous interactions of several
perfluorinated arenes with anions in the crystalline state. Most of the structures show multiple
anion−π contacts. However, only 6·2HI reveals an effective encapsulation of the iodide ion by
the aromatic units. For comparison, the structure of 4b is investigated because it offers two π-
systems with inverse charge distribution to a bromide anion. Only the electron-deficient π-system
of the pentafluorophenyl group interacts with the anion.
deficiency, and the net attractive anion−π interaction turns into
a repulsive force. Additionally, the effect of the size and shape
of the anion have been investigated in the crystals of
pentafluorophenyl ammonium cations with halides, nitrate,
tetrafluoroborate, and hexafluoroborate.10 However, because of
its electrostatic nature, the geometry of the anions seems not to
be relevant in the solid-state structures of the investigated
anion−π complexes.
INTRODUCTION
■
Supramolecular chemistry, the chemistry of the noncovalent
(or reversible) bond, deals with molecular self-assembly and
recognition processes driven by weak intermolecular inter-
actions.1 These may vary dramatically in strength, from strong
electrostatic to weakly dispersive attraction.2 Because of their
crucial role in biological and chemical processes, interactions of
arenes like π−π stacking or cation−π interactions have
attracted considerable attention.3 Intuitively, the interaction
between anions and aromatics should be repulsive. However,
anion−π interactions recently were identified for appropriate
systems to be an attractive force. Computational as well as
crystallographic results prove the existence of this new kind of
intermolecular interaction.4 Especially in the field of anion
recognition and sensing, the anion−π interaction promises new
applications and the development of novel anion receptors.
Nevertheless, the relevance of anion−π interactions in solution5
or in the gas phase6 is debatable.
̀
In 2005, Frontera, Deya, and co-workers reported additive
behavior of anion−π interactions between trifluoro-1,3,5-
triazine (TFZ) and chloride anions (Figure 1).11 They
calculated the binding energies of the complexes of chloride
with one, two, or three TFZ units in chloroform and water, as
well as in the gas phase. Their results show a strict dependence
of the binding energy on the number of involved electron-
deficient π-systems. In 2008, Ghosh et al. described the crystal
In 2008, we started our work on anion−π interactions in the
solid state utilizing pentafluorophenyl ammonium and
phosphonium salts7 as electron-deficient π-acceptors. First
results showed the appropriateness of pentafluorophenyl
groups for anion−π investigations in the solid state and
revealed a large variation of the anion position with respect to
the electron-deficient C6F5 unit. Our studies showed that the
position strongly depends on the directing substituents close to
the π-system.8 Moreover, the dependence of anion−π
interactions on the fluorination degree of the phenyl group
proved to be important.9 Successive reduction of the number of
fluorine atoms at the arene moiety reduced the electron
̀
Figure 1. System of Frontera, Deya, and co-workers (A) to investigate
the additivity of anion−π interactions and the Ghosh derivative (B).
Received: April 4, 2013
© XXXX American Chemical Society
A
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−150.8 (m, 2F, Fpara), −162.6 (m, 4F, Fmeta). MS (EI, 70 eV): m/z
239.2 (100, [M]2+, C20H20F10N22+). IR (KBr): ν (cm−1) 3387 (m),
3029 (s), 2975 (s), 2657 (s), 2321 (s), 2156 (s), 2156 (s), 2081 (s),
2001 (s), 1657 (m), 1527 (w), 1501 (w), 1426 (m), 1396 (s), 1369
(s), 1314 (m), 1233 (s), 1076 (w), 969 (w), 908 (m), 870 (m), 815
(s), 771 (s), 725 (s), 687 (m). Anal. Calcd for C20H20F10N2O4S·2H2O
(574.10): C 39.35, H 3.96, N 4.59. Found: C 39.38, H 4.13, N 4.56.
Tetramethyl Bis(pentafluorobenzyl)propylenediammonium Di-
bromide 3. A solution of 200 mg (2.0 equiv, 0.77 mmol, 116 μL)
of pentafluorobenzyl bromide in 10 mL of acetonitrile was mixed with
44 mg (1.0 equiv, 0.38 mmol, 64 μL) of tetramethyl propylene
diamine. The mixture was stirred for 20 h at ambient temperature. The
precipitating white solid was filtered off and washed with ethyl acetate.
The remaining white solid was dried in a vacuum. Yield: 210 mg
structure of tris(pentafluorobenzyl-2-aminoethyl)ammonium
chloride and bromide.12 Unfortunately, the anion is fixed by
strong NH−anion interactions in the cap of the ammonium salt
and does not show η6-anion−π interaction with the central
anion. Similar results were obtained for the corresponding urea
compounds.13 Inspired by these studies, we started our
investigations of interactions of oligo pentafluorophenyl
derivatives with anions in the solid state. The present paper
reports investigations of concerted anion−π interactions of the
pentafluorophenyl groups with various anions in the solid state.
EXPERIMENTAL SECTION
■
1
Commercially available reagents were used as received, and solvents
were distilled and used without further purification. The 1H (300
MHz) and 19F (282 MHz) NMR spectra were recorded on a Varian
Mercury 300 spectrometer in deuterated solvents. Mass spectrometric
data were recorded on a Finnigan SSQ 7000 and Thermo Deca XP as
electron ionization (EI, 70 eV) or electrospray ionization (ESI).
Infrared spectra were taken on a PerkinElmer Spektrum 100 FTIR
spectrometer. All samples were measured in KBr (4000−650 cm−1).
The elemental analysis was performed on a CHN-O-Rapid Vario EL
colorless solid (0.3 mmol, 84%). mp: 200 °C. H NMR (MeOD, 300
MHz): δ 4.84 (s, 4H, Hbenzyl), 3.65 (m, 4H, CH2), 3.27 (s, 12H, CH3),
2.60 (m, 2H, CH2). 19F NMR (CDCl3, 300 MHz): δ −137.80 (m, 4F,
Fortho), −150.72 (m, 2F, Fpara), −162.50 (m, 4F, Fmeta). MS (EI, 70
+
eV): m/z 571.1 (100, [MBr]+, C21H22BrF10N2 ). IR (KBr): ν (cm−1)
3401 (m), 3322 (m), 3207 (w), 3013 (w), 2981 (w), 2660 (w), 2323
(w), 2198 (w), 2182 (w), 2120 (w), 1992 (w), 1659 (m), 1527 (s),
1505 (vs), 1470 (s), 1440 (m), 1425 (m), 1380 (m), 1364 (m), 1312
(m), 1233 (w), 1184 (w), 1137 (s), 1075 (w), 1043 (s), 1028 (s), 977
(s), 960 (vs), 922 (m), 903 (m), 853 (s), 773 (w), 754 (m), 719 (w),
692 (w), 677 (w). Anal. Calcd for C21H22Br2F10N2·H2O (650.00): C
37.63, H 3.61, N 4.18. Found: C 37.62, H 3.63, N 4.21.
from Heraeus, and melting points were obtained on a Buchi B-540.
̈
Synthesis of Di-, Tri-, and Tetra(pentafluorophenyl)-
ammonium Salts. Dimethyl Bis(pentafluorobenzyl)ammonium
Bromide 1. Equimolar amounts of pentafluorobenzyl bromide (400
mg, 1.54 mmol, 230 μL) and dimethyl amine (40% aqueous solution,
180 mg, 1.54 mmol, 180 μL) were dissolved in 10 mL of acetonitrile
and stirred for 48 h at ambient temperature. The solvent was removed,
and the remaining white solid was twice recrystallized from methanol.
We were not able to separate both salts quantitatively, but during the
crystallizations we isolated crystals of 1 as well as of H2N(CH3)2Br.
1-Pentafluorobenzylammonium-4-benzylammonium-bicyclo-
[2.2.2]octanedibromide 4b. To a solution of 200 mg (1.0 equiv, 0.54
mmol) of pentafluorobenzyl-1,4-diazoniabicyclo[2.2.2]octanbromide
in 5 mL of ethanol was added 91 mg (1.0 equiv, 0.54 mmol, 64 μL) of
benzyl bromide, and the solution was stirred for 24 h at ambient
temperature. The precipitating white solid was filtered off and dried in
a vacuum. Yield: 282 mg colorless solid (0.5 mmol, 97%). mp: 252 °C,
1
Yield of 1: 367 mg colorless solid (0.6 mmol, 40%). mp: 213 °C. H
1
under decomposition. H NMR (MeOD, 300 MHz): δ 7.61 (m, 5H,
NMR (MeOD, 300 MHz): δ 4.74 (s, 4H, Hbenzyl), 3.08 (s, 6H, CH3),
Haryl), 5.04 (s, 2H, Hbenzyl), 4.89 (s, 2H, Hbenz), 4.05 (m, 6H, CH2),
3.12 (m, 6H, CH2). 19F NMR (MeOD, 300 MHz): δ −137.49 (m, 2F,
Fortho), −149.72 (m, 1F, Fpara), −162.06 (m, 2F, Fmeta). MS (ESI): m/z
465.33 (90, [MBr]+, C20H21F5N2Br). IR (KBr): ν (cm−1) 3638 (w),
3478 (w), 3413 (m), 3350 (m), 2997 (m), 2902 (w), 2085 (w), 1661
(m), 1584 (w), 1527 (s), 1508 (vs), 1458 (m), 1426 (m), 1396 (m),
1356 (w), 1310 (m), 1213 (w), 1139 (s), 1095 (m), 1055 (s), 1004
(m), 963 (vs), 905 (m), 855 (vs), 772 (s), 710 (s), 659 (m). Anal.
Calcd for C20H21F5N2Br·H2O (543.00): C 42.73, H 4.12, N 4.98, N
4.18. Found: C 42.83, H 4.45, N 4.77.
+
2.60 (s, 6H, H2N(CH3)2 ). 19F NMR (MeOD, 300 MHz): δ −137.35
(m, 4F, Fortho), −150.77 (m, 2F, Fpara), −162.55 (m, 4F, Fmeta). MS
(ESI): m/z 406.2 (100, [M]+, C16H10F10N+). IR (KBr): ν (cm−1) 3448
(w), 3010 (w), 2773 (w), 2437 (w), 1740 (w), 1658 (m), 1506 (vs),
1425 (m), 1392 (m), 1310 (m), 1261 (w), 1227 (w), 1135 (vs), 1060
(m), 1042 (m), 979 (vs), 958 (vs), 918 (s), 904 (m), 870 (s), 802
(m), 748 (w), 693 (w), 671 (m). Anal. Calcd for
C16H10F10NBr·NH2(CH3)2Br (612.14): C 35.22, H 2.96, N 4.58.
Found: C 35.66, H 2.39, N 4.28.
Tetramethyl Bis(pentafluorobenzyl)ethylenediammonium Dibro-
mide 2a. To begin, 2.0 equiv of pentafluorobenzyl bromide (400 mg,
1.54 mmol, 230 μL) was dissolved in 10 mL of acetonitrile; 1.0 equiv
of TMEDA (89 mg, 0.77 mmol, 115 μL) was added. After the mixture
was stirred for 48 h at ambient temperature, the precipitated white
solid was filtered off and washed with ethyl acetate. The remaining
white solid was dried in a vacuum. Yield: 392 mg colorless solid (0.6
1,4,7-Tris(pentafluorobenzyl)-1,4,7-triazacyclononane 5. 1,4,7-
Triazacyclononanhydrochloride was obtained by the procedure
described in the literature.14 To a solution of 300 mg (1.0 equiv,
1.26 mmol) of 1,4,7-triazacyclononane hydrochloride in 5 mL of dry
toluene was added 282 mg (4.0 equiv, 5.03 mmol) of potassium
hydroxide. Then a solution of 985 mg (3.0 equiv, 3.77 mmol, 572 μL)
of pentafluorobenzyl bromide dissolved in 1 mL of dry toluene was
slowly added. The suspension was heated for 7 h to 110 °C and then
cooled to ambient temperature. Then 71 mg (1.0 equiv, 1.26 mmol) of
potassium hydroxide and 328 mg (1.0 equiv, 1.26 mmol, 190 μL) of
pentafluorobenzyl bromide were added, and the mixture was stirred
for an additional 12 h at 110 °C. After the mixture was cooled to room
temperature, the precipitating salts were filtered off and the solvent
was slowly removed. Yield: 574 mg colorless solid (0.9 mmol, 68%).
1
mmol, 80%). mp: 205 °C. H NMR (MeOD, 300 MHz): δ 4.80 (s,
4H, Hbenzyl), 4.20 (s, 4H, CH2), 3.19 (s, 12H, CH3). 19F NMR
(MeOD, 300 MHz): δ −136.45 (m, 2F, Fortho), −146.86 (m, 1F, Fpara),
−159.68 (m, 2F, Fmeta). MS (EI, 70 eV): m/z 559.0 (100, [M + Br]+,
+
C20H20BrF10N2 ). IR (KBr): ν (cm−1) 3007 (w), 2954 (w), 2049 (w),
1982 (w), 1663 (m), 1508 (vs), 1476 (m), 1443 (m), 1403 (w), 1381
(w), 1357 (w), 1331 (w), 1308 (m), 1234 (w), 1135 (s), 1035 (s), 969
(vs), 924 (w), 904 (m), 892 (m), 876 (m), 816 (m), 766 (w), 721
(m), 682 (m). Anal. Calcd for C20H20BrClF10N2 (628.06): C 37.64, H
3.16, N 4.39. Found: C 37.60, H 3.25, N 4.38.
Tetramethyl Bis(pentafluorobenzyl)ethylenediammonium Sul-
fate 2b. To a solution of 200 mg (1.0 equiv, 0.31 mmol) of
tetramethyl bis(pentafluorobenzyl)ethylendiammonium dibromide
(2a) in 30 mL of water was added a solution of 97 mg (1.0 equiv,
0.31 mmol) of silver(I) sulfate in 20 mL of water. The precipitating
white solid was filtered off, and the solvent was removed under
reduced pressure. The remaining white solid was dried in a vacuum.
Yield: 167 mg colorless solid (0.3 mmol, 90%). mp: 193 °C. 1H NMR
(D2O, 300 MHz): δ 4.93 (s, 4H, Hbenzyl), 4.43 (s, 4H, CH2), 3.28 (s,
12H, CH3). 19F NMR (CDCl3, 300 MHz): δ −137.3 (m, 4F, Fortho),
1
mp: 90 °C, under decomposition. H NMR (CDCl3, 300 MHz): δ
3.71 (s, 6H, Hbenzyl), 2.70 (s, 12H, CH2). 19F NMR (CDCl3, 300
MHz): δ −142.35 (m, 2F, Fmeta), −155.57 (m, 1F, Fpara), −162.38 (m,
+
2F, Fortho). MS (EI, 70 eV): m/z 669.3 (6, [M]+, C27H18F15N3 ), 181.1
+
(100, C7H2F5 ). IR (KBr): ν (cm−1) 2930 (w), 2870 (w), 2827 (w),
2795 (w), 1656 (w), 1497 (s), 1454 (m), 1364 (m), 1315 (m), 1298
(m), 1241 (w), 1126 (s), 1019 (s), 992 (s), 962 (s), 935 (s), 904 (m),
877 (w), 797 (w), 741 (w), 669 (w). Anal. Calcd for
C27H18F15N3·H2O (669.43): C 47.17, H 2.93, N 6.11. Found: C
47.49, H 3.04, N 6.00.
Tetrakis(pentafluorobenzyl)ethylene Diamine 6. To begin, 46 mg
(1.0 equiv, 0.81 mmol, 60 μL) of ethylene diamine, 458 mg (4.25
B
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Scheme 1. Synthesis of the Pentafluorobenzyl Ammonium Salts
equiv, 3.43 mmol) of potassium carbonate, and 870 mg (4.25 equiv,
3.43 mmol, 505 μL) of pentafluorobenzyl bromide were dissolved in
30 mL of acetonitrile and stirred for 48 h at ambient temperature. The
precipitating salts were filtered off, and the solvent was removed under
reduced pressure. The remaining white solid was recrystallized from
methanol and dried in a vacuum. Yield: 108 mg colorless solid (0.1
correction was applied on all data, and absorption effects were
corrected with the multiscan method (SADABS15f). All C−H
hydrogen positions were calculated and refined as riding atoms.
Hydrogen atoms bonded to O (in 2b, 3, and 4b) or N (6·2HI) were
found from Fourier maps and restrained (by DFIX, s = 0.02) to a
distance of 0.84 Å from O or 0.91 Å from N atoms. Thermal
parameters of hydrogen atoms were set to 1.2 times those of the
parent C and N atoms or 1.5 times those of parent O atoms. In 2a, the
disordered positions for Br− and Cl− were constrained to be equal
(EXYZ) and the anitsotropic displacement parameters were equalized
(EADP) for Br− and Cl−. Anisotropic displacement parameters
restraints (SIMU, s1 = 0.01, s2 = 0.02; DELU, s = 0.01; ISOR, s = 0.01)
were also applied for few atoms in structures 2a, 2b, 4b, and 5-N-
Me+I−. The bond distances in disordered solvents were restrained to
be more equal in 2b (SADI, s = 0.02). Crystallographic data and
parameters for nine structures (Table S1) and for a second structure
related to 2a (Table S2) are summarized in the Supporting
Information.
1
mmol, 25%). mp: 116 °C, under decomposition. H NMR (CDCl3,
300 MHz): δ 3.61 (s, 8H, Hbenzyl), 2.51 (s, 4H, CH2). 19F NMR
(CDCl3, 300 MHz): δ −142.9 (m, 4F, Fortho), 154.3 (m, 2F, Fpara),
162.0 (m, 4F, Fmeta). MS (EI, 70 eV): m/z 390.1 (100, C15H6F10N+).
IR (KBr): ν (cm−1) 2987 (w), 2925 (w), 2850 (w), 2103 (w), 1657
(m), 1497 (vs), 1428 (w), 1377 (w), 1335 (m), 1297 (m), 1236 (m),
1127 (s), 1013 (s), 964 (m), 930 (m), 914 (m), 850 (m), 802 (w),
763 (m), 740 (m), 691 (m), 664 (w). Anal. Calcd for
C30H12F20N2·2H2O (780.40): C 44.13, H 1.98, N 3.43, Found: C
44.26, H 1.80, N 3.18.
Crystallography. Single-crystal X-ray data were collected at
123.0(1) K using a Bruker-Nonius KappaCCD diffractometer with
an APEX-II detector and graphite-monochromatized Mo Kα (λ =
0.71073 Å) radiation. COLLECT15a software was used for the data
collection (θ and ω scans) and DENZO-SMN15b for the processing.
The structures were solved by direct methods with SIR200415c and
refined by full-matrix least-squares methods with WinGX software,15d
which utilizes the SHELXL-97 module.15e Lorentzian polarization
RESULTS AND DISCUSSION
■
Synthesis of Ammonium Compounds. The bis-
(pentafluorobenzyl)ammonium bromides 1, 2a, and 3 were
obtained by reaction of the corresponding pentafluorobenzyl
bromide with diethyl amine or the corresponding tetramethyl
C
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amines via nucleophilic substitution (Scheme 1). Compound
2b is available by salt metathesis from 2a and silver sulfate in
aqueous solution. Compounds 4a and 4b were synthesized by
reacting 1-(pentafluorobenzylammonium)-4-aza-1-
azoniabicyclo[2.2.2]octane bromide with equimolar amounts
of pentafluorobenzyl and benzyl bromide, respectively. The
tris(pentafluorobenzyl)triaminocyclononane (5) was synthe-
sized from 1,4,7-triaminocyclononane14 in toluene in the
presence of potassium hydroxide. Crystals of 5 were obtained
by diffusion of HI into a solution of 5 in methanol. During the
crystallization process, 5 was methylated and the crystal
structure of the corresponding ammonium iodide (5-N-
Me+I−) was obtained.
The tetrakis(pentafluorobenzyl)ethylene amine (6) was
obtained by reacting pentafluorobenzyl bromide with ethylene
diamine in the presence of potassium carbonate in acetonitrile
(Scheme 1). The corresponding hydroiodide (6·2HI) was
crystallized by diffusion of HI into a solution of 6 in
chloroform.
Figure 2. (a) View of one ion pair as observed in the crystal structure
of 1. (b) Solid-state structure of 2a showing the anion−π and lone-
pair−π interactions. (c) Representative part of the crystal structure of
2b. The sulfate anions show exclusively CH−anion binding to the alkyl
groups and hydrogen bonding to the cocrystallized water (disordered
solvent molecules were omitted for clarity). (d) Molecular packing of
3 reveals that one bromide anion is interacting with two
pentafluorobenzyl units of different dications (gray, carbon; light
blue, hydrogen; dark blue, nitrogen; light green, fluorine; dark red,
bromide; light red, oxygen; yellow, sulfur).
All isolated compounds except 5-N-Me+I− and 6·2HI are
characterized by standard analytical methods (1H and 19F
NMR, IR, MS, and elemental analysis). For 1−5 , as well as for
5-N-Me+I−, 6, and 6·2HI, suitable crystals for X-ray analysis
could be obtained.
Solid-State Studies. Crystals are the result of the interplay
of inter- and intramolecular forces, which may have a significant
influence on how the molecules or complexes pack into a
crystal lattice. In the following, the focus will be the analysis of
the relevant interactions between the π-systems and anions
present in the crystal. In systems with multiple electron-
deficient arenes, the anion−π interaction can occur as
simultaneous interactions of two π-systems by paneling the
anion (Scheme 2, I) or as two separate anion−π interactions
(separated interactions, Scheme 2, II).
Compound 2a cocrystallizes with one molecule of DMSO
from a DMSO/diethyl ether mixture. Because one of the two
electron-deficient moieties shows lone-pair−π interactions with
the oxygen atom of the cocrystallized DMSO (C···O = 3.325−
3.602 Å; center···O = 3.170 Å), only the second pentafluor-
ophenyl group is able to interact with anions. A closer look at
the packing of 2a reveals that one bromide is sandwiched
between two pentafluorophenyl groups of adjacent cations.
Hydrogen-bonding interaction of the methyl groups directs the
anion between the π-systems (CH···Br = 2.986 Å). However,
the anion is shifted off center (C2−4···Br = 3.527, 3.724, 3.782
Å; C3−5···Br = 3.608, 3.620, 3.771 Å) because of the
cocrystallized solvent molecule showing an η3 type anion−π
interaction with both arenes. The second anion does not show
interactions with electron-deficient moieties and is surrounded
exclusively by CH−anion contacts (CH···Br = 2.677−3.343 Å).
A wider view of the crystal packing reveals the T-shaped
orientation of the perfluorinated arenes (CF···center = 3.046
Å). A second solid-state structure related to 2a was obtained by
a substitution reaction of TMEDA and pentafluorobenzyl
chloride and bromide. The solid-state structure was obtained by
crystallization from DMSO/diethyl ether and cocrystallizes
with one molecule of DMSO. The crystal structure reveals
features similar to those of the bromide structure with
disordered (1:1) bromide/chloride content over two equal
positions (1, 0.6Br−/0.4Cl−; 2, 0.4Br−/0.6Cl−; see Supporting
Information, Table S2).
So far, not one of the described structures shows
simultaneous interaction of two pentafluorophenyl groups in
order to capture the anion in an electron-deficient cavity.
Because the described ammonium salts have the opportunity to
interact with two anions and/or lone pairs of cocrystallized
solvent molecules, we synthesized tetramethyl bis-
(pentafluorobenzyl)ethyleneammonium sulfate (2b) by salt
metathesis. Unfortunately, 2b crystallizes with two molecules of
water and disordered solvent molecules (additional water,
methanol, and ethyl acetate). The pentafluorophenyl units of
2b do not show anion−π interactions with the sulfate. The
sulfate anions of adjacent ion pairs are bridged by water
Scheme 2. Binding Motifs for the Interaction of Electron-
Deficient Arenes with Anions
Di- and Tetramethyl Bis(pentafluorobenzyl)ammonium
Salts (1−3). Crystals of dimethyl bis(pentafluorobenzyl)-
ammonium bromide (1) were obtained from a mixture of
methanol and diethyl ether. Compound 1 (Figure 2)
crystallizes in the monoclinic space group P21/c with two ion
pairs (A and B) per asymmetric unit. In both ion pairs, the
anion is not centered above a π-system. However, the bromide
is fixed in both cases close to two carbon atoms of the electron-
deficient arenes (η2-binding motif: A, C2···Br = 3.624 Å, C3···Br
= 3.528 Å; B, C4···Br = 3.558 Å, C5···Br = 3.417 Å). The second
electron-deficient arene unit of the cation in A shows an
additional short (η1 type) contact to a neighboring bromide
(C3···Br = 3.459 Å). Moreover, the anions are embedded in
various CH−anion interactions with the protons of the methyl
or the benzyl group (CH···Br = 2.683−3.025 Å).
D
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molecules (HOH···OSO3 = 1.920−1.930 Å) and embedded in
various CH−anion interactions from the benzyl or alkyl groups
(CH···OSO3 = 2.322−2.718 Å). One of the two π-systems
interacts with the oxygen atom of a cocrystallized disordered
solvent molecule (H2O, ethyl acetate; for H2O, C···O = 3.210−
3.373 Å; center···O = 2.992 Å).
The solid-state structure of 4b was obtained from ethanol and
supports this suggestion. One molecule of ethanol cocrystallizes
per asymmetric unit. The two methyl groups direct the bromide
(CH···Br = 2.891, 2.866 Å) above the center of the
pentafluorobenzyl unit (η6-binding motif: C···Br = 3.791−
3.864 Å; center···Br = 3.571 Å). The anion also accepts CH−
anion interactions from the neighboring cation (CH···Br =
2.792, 2.883 Å; CHbenzyl···Br = 3.009 Å). The second anion
shows a hydrogen bridge to the OH group of the cocrystallized
ethanol (OH···Br = 2.469 Å). The 1,4-diazabicyclo[2.2.2]-
octane unit of the cation binds the second anion by CH−anion
interactions (CH···Br = 2.659, 2.822 Å), and the anion forms
similar contacts to two neighboring cations (CH···Br = 2.832,
2.882, 2.839 Å; CHaryl···Br = 2.851 Å). A wider look at the
crystal packing of 4b reveals that each cation is surrounded by
six anions, whereby only one shows anion−π interactions. This
crystal structure is an important argument in the discussion of
anion−π interactions in the solid state because it clearly shows
that the position above an electron-deficient arene is favored for
an anion while the electron-rich benzyl group is not able to
interact.
N,N,N-Tris(pentafluorobenzyl)triaminocyclononane (5)
and Its Methylated Ammonium Salt (5-N-Me+I−). Crystals
of 5 could be obtained from toluene by slow evaporation of the
solvent. The pentafluorobenzyl groups of 5 show inter- and
intramolecular aryl−aryl contacts. Whereas for the intra-
molecular interactions the arenes are oriented in a T-shaped
fashion (CF···center = 2.933 Å), the intermolecular interactions
show a shifted face-to-face (center···center = 3.551 Å;
plane···plane = 3.276 Å) as well as a T-shaped orientation of
the electron-deficient C6F5 moieties (CF···center = 3.475 Å).
The methylated ammonium iodide 5-N-Me+I− could be
obtained by diffusing HI into a solution of 5 in methanol.
Compound 5 crystallizes in the orthorhombic space group Pbca
with disordered water molecules (Figure 4). The anion is fixed
above the C6F5 unit by CH−anion interactions (CH···I =
3.289, 3.127 Å) and shows short distances to three carbon
Again, no simultaneous intramolecular anion−π interactions
of the pentafluorobenzyl groups with one anion are observed in
the solid state. Therefore, the spacer distance between the two
electron-deficient arenes was elongated in order to create a
more flexible backbone and allow wrapping around the anion.
In 3, the pentafluorobenzyl groups are linked by a propylene
chain. The longer chain enables a higher adaptability and
should be more appropriate for “intramolecular” anion−π
interactions in the sense of I. However, the solid-state structure
of 3 shows that one bromide anion is fixed by the methyl
groups (CH···Br = 2.923, 3.178 Å) above the π-system in an η3-
binding motif (C4−6···Br = 3.880, 3.615, 3.642 Å). The
molecular packing shows that a second pentafluorophenyl
unit is close to the anion. The distances of 4.081 and 4.102 Å
are a little too long for showing attractive anion−π interactions.
A second bromide anion is surrounded by several CH−anion
interactions (CH···Br = 2.828−2.993 Å). Anions are connected
via the cocrystallized water molecule (HOH···Br = 2.415, 2.423
Å).
A more rigid linker between the two pentafluorobenzyl units
was reported earlier.8 The 1,4-bis(pentafluorobenzyl-
ammonium)-1,4-bisazoniabicyclo[2.2.2]octane dibromide (4a)
crystallized with three molecules of methanol per asymmetric
unit. The oxygen atom of the methanol interacts with the
electron-deficient arene (C···OHCH3 = 3.503−4.078 Å;
center···OHCH3 = 3.537 Å) and forces the anion away from
the position above the π-system. Only an η2-anion−π
interaction with one of two pentafluorobenzyl moieties
(C5/6···Br = 3.416, 3.411 Å) is observed in the crystal structure.
Starting from this and inspired by our study on the dependence
of the anion position above arenes with various electron
densities,9 we were interested in investigating the direct
competition of an electron-rich and an electron-deficient
arene in the solid state. Therefore, the bis(pentafluorobenzyl)-
ammonium derivative 4b with a pentafluorobenzyl as well as a
benzyl group was synthesized (Figure 3). While the electron-
deficient C6F5 unit should interact attractively with an anion,
the benzyl group should be repulsive with respect to anions.
Figure 4. Two representative views of 5 showing the intramolecular
π−π interactions (a) as well as a view of the crystalline packing
exhibiting the intermolecular π−π interactions (b). The interactions of
the iodide in the solid-state structure of 5-N-Me+I− with the
surrounding pentafluorophenyl units are shown in (c) and (d).
(Disordered water molecules were omitted for clarity; gray, carbon;
light blue, hydrogen; dark blue, nitrogen; light green, fluorine; violet,
iodide.)
Figure 3. Two representative views of the ammonium cation in 4b
showing the asymmetric unit (a) as well as all anionic neighbors of the
cation (b); cocrystallized ethanol molecule was omitted for clarity
(gray, carbon; light blue, hydrogen; dark blue, nitrogen; light green,
fluorine; dark red, bromide; light red, oxygen).
E
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Article
atoms of the electron-deficient arene (η3, C4−6···I = 4.138,
3.982, 4.040 Å; center···I = 3.942 Å). The remaining
pentafluorobenzyl units of the ion pair are oriented away
from the iodide. However, the pentafluorobenzyl groups of
neighboring cations exhibit further anion−π contacts (η2,
C2/3···I = 3.969, 3.865 Å; center···I = 4.175 Å; η2, C5/6···I =
4.029, 3.960 Å; center···I = 4.480 Å).
interactions with only one anion is possible. Because of the
insufficient preorganization and rigidity of the backbone, an
efficient electron-deficient cavity could not be observed.
CONCLUSION
■
This study shows examples for both failure and success of a
concept. On the basis of previous results, a series of solid-state
structures of pentafluorobenzyl ammonium salts with two,
three, or four electron-deficient arenes was presented. Not one
of the structures creates a perfect electron-deficient cavity to
shield anions effectively from the surroundings. In the solid-
state structure of 6·2HI, three sides around the anion are
paneled by C6F5 units. Therefore, 6 is the most promising
anion−π capsule. Further investigations on more rigid scaffolds
have to be performed to find an ideal π-acceptor for anion−π
investigations in the crystal as well as in solution. An interesting
structure described within this work is the solid-state structure
of 4b because it directly provides an electron-deficient as well as
an electron-rich moiety. In the solid state, only the attractive
interactions between bromide and the pentafluorobenzyl group
are observed.
Tetrakis(pentafluorobenzyl)ethylene Amine (6) and Its
Hydroiodide (6·2HI). Compound 6 crystallizes in the
monoclinic space group P21/c from chloroform. Various π−π
contacts can be observed in the crystalline packing of the
tetrakis(pentafluorobenzyl)ethylene amine (6). The most
prominent arene−arene interaction is the intramolecular
edge-to-face interaction of adjacent C6F5 units (CF···center =
3.196 Å; ∠ = 83.71°).
After HI is diffused into a solution of 6 in chloroform, the
corresponding hydroiodide 6·2HI was obtained with one
molecule of chloroform per asymmetric unit (Figure 5). One
The results of the present study are a further step toward
understanding the nature of anion−π interactions as a basis for
the development of superior anion receptors.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal data for 1, 2a, 2b, 3, 4b, 5, 5-N-Me+I−, 6, and 6·2HI,
and crystallographic data for the structure related to 2a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: markus.albrecht@oc.rwth-aachen.de.
■
Figure 5. Part of the crystal structure of 6 indicating the
intramolecular π−π interactions (a) and the intermolecular π−π
interactions (b). The representative parts of the crystal structure (c
and d) show the inter- and intramolecular interactions of the arenes
and their anion−π contacts as observed in the solid-state structure of
6·2HI. (Disordered water molecules were omitted for clarity; gray,
carbon; light blue, hydrogen; dark blue, nitrogen; light green, fluorine;
dark green, chlorine; violet, iodide.)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Academy of Finland (K.R.,
project numbers 130629, 122350, 140718).
■
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