Controlling the position of anions relative to a pentafluorophenyl group

Article abstract of DOI:10.1039/c2nj40089h

The position of an anion above an electron-deficient arene can be controlled by the geometry of appended directing groups. Here a series of ammonium substituted pentafluorophenyl derivatives is investigated. The presented results are one step on the way to find the ideal structural features for an effective and superior receptor for anion-π studies.

Full text of DOI:10.1039/c2nj40089h

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PAPER
Controlling the position of anions relative to a pentafluorophenyl groupw
Michael Giese,a Markus Albrecht,*a Katharina Wiemer,a Arto Valkonenb and
Kari Rissanenb
Received (in Montpellier, France) 7th February 2012, Accepted 12th March 2012
DOI: 10.1039/c2nj40089h
The position of an anion above an electron-deficient arene can be controlled by the geometry of
appended directing groups. Here a series of ammonium substituted pentafluorophenyl derivatives
is investigated. The presented results are one step on the way to find the ideal structural features
for an effective and superior receptor for anion–p studies.
Introduction
Supramolecular chemistry deals with non-covalent bonds,1
which vary dramatically in their strength.2 Both strong electro-
static attraction as well as weakly dispersive repulsion belong to
supramolecular interactions. Even aromatic interactions like p–p
stacking or cation–p interactions are in the focus of intense
studies due to their relevance in biological and chemical
processes.3 Recently the interaction between anions and electron-
deficient arenes was identified as an attractive force based on
computational as well as crystallographic results.4 Especially in
the field of anion recognition and sensing this new interaction
Fig. 1 Two different views of the crystal structure of 3a (green: chloride,
promises new applications and the development of novel anion
black: carbon, light-green: fluorine, blue: nitrogen, white: hydrogen).7
receptors. However, the relevance of anion–p interactions in
solution5 is debatable and gas phase6 studies are rare.
Anion–p interactions are relatively weak non-covalent inter-
We started our work on anion–p interactions in 2008 by
actions in solution and in the solid state and the central fixation
studying pentafluorophenyl ammonium and phosphonium
of an anion above a p-system can easily be disturbed through
salts in the solid state.7 First results showed that the position
packing effects. Substituents can support the position of an anion
of an anion above the electron-deficient C6F5-unit is flexible
above the C6F5 unit by CH– or NH–anion bridges.8,11
and can be controlled by directing substituents.8 Further
An already reported crystal structure of pentafluoroanilinium
studies were able to prove the dependence of anion–p inter-
chloride7 showed the strong directing effect of the ammonium
actions on the fluorination degree of the phenyl group.9
group on the position of the anion in the crystal lattice. The
Through successive reduction of the number of fluorine atoms
hydrogen atoms point away from the aromatic system and thereby
at the arene the electron-density was enhanced and the attractive
the anion is fixed between four pentafluoroanilinium cations
anion–p interaction turns into a repulsive force. Moreover, we
surrounded exclusively by NH–anion interactions (Fig. 1).
studied the effect of the anion geometry on the interaction
of pentafluorophenyl ammonium cations with halides,
nitrates, tetrafluoroborates and hexafluoroborates.10 Due to
its strict electrostatic nature, the anion–p interaction shows no
dependence on the geometry of the studied anion in the
solid state.
Results and discussion
This observation inspired us to use the directing force of the
ammonium group to study the influence of the spacer length
between the ammonium group and the electron-deficient C6F5
unit. The length of the spacer is varied by stepwise insertion of
CH2-groups. By lengthening the spacer between the ammonium
group and the arene the anion should be forced to move closer
to the centre of the pentafluorophenyl system (Scheme 1).
a Institut fur Organische Chemie, RWTH Aachen, Landoltweg 1,
¨
52074 Aachen, Germany. E-mail: markus.albrecht@oc.rwth-aachen.de
Fax: +49 241 809 2385; Tel: +49 241 809 4678
b Department of Chemistry, Nanoscience Center,
University of Jyvaskyla, Survontie 9, 40014 Jyvaskyla, Finland.
¨ ¨
E-mail: kari.t.rissanen@jyu.fi; Fax: +358 14 260 2672;
Tel: +358 50 562 3721
w CCDC reference numbers 866136 and 866137. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2nj40089h
¨
¨
Synthesis of the pentafluorophenylammonium chlorides
While pentafluoroaniline is commercially available, the two
ammonium chlorides containing a pentafluorophenyl group
c
1368 New J. Chem., 2012, 36, 1368–1372
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Solid state investigations
As previously already mentioned hydrogen bond bridges from
the ammonium groups to the chloride anions (NHÁ Á ÁCl = 2.843,
3.130 (anions close to the p-system), 2.311, 2.164 and 2.275 A
(anions in the far periphery)) are the dominating interaction in
the crystal lattice of 3a.7 The ammonium groups direct the anions
away from the electron-deficient unit and bridge adjacent anions
to each other. The fourth anion within the hydrogen bond
distance from the ammonium group and also closest to an aryl
ring is not hydrogen bonded to the ammonium group of the
same molecule, because of the small NHÁ Á ÁCl angle (1101) and
long NHÁ Á ÁCl distance (2.84 A). Only two distances to the ring
are within the sum of the van der Waals (vdW) radii of the
involved atoms (C1/C6Á Á ÁCl = 3.40 and 3.41 A) indicating
anion–p contacts. Due to the fact that the anion is not positioned
directly above the p-system (having a long distance to the
center of the arene (centreÁ Á ÁCl = 4.06 A)) but rather over the
outer periphery and due to the presence of strong NHÁ Á ÁCl
interactions, no ‘‘intramolecular’’ anion–p interaction can be
found in the solid state structure of 3a.
Scheme 1 Concept for studying the effect of the spacer length on the
anion position in the solid state.
Also 3b shows a strong dominance of hydrogen bridges
(NHÁ Á ÁCl = 2.22(2), 2.28(2) and 2.48(2) A) in the crystalline
state and the adjacent anions are bridged by ammonium
groups. As in 3a there is a fourth anion within hydrogen bond
distance from ammonium, but again with a rather small
NHÁ Á ÁCl angle [116(2)1] and long NHÁ Á ÁCl distance [2.68(2)
A]. A closer view at an ion pair shows that the anion is fixed
(by NHÁ Á ÁCl interaction) closer to the arene compared to 3a.
Due to the longer alkyl chain between the directing ammonium
group and the pentafluorophenyl moiety the distance to the
centre of the arene (centreÁ Á ÁCl = 4.37 A) is slightly longer in
respect to 3a. Like in 3a, in 3b only two carbon–anion distances
are close to the sum of the vdW radii of the atoms (C1/C6Á Á ÁCl
= 3.897(3) and 3.663(3) A). Consequently, neither 3a nor 3b
structures match hard criteria for anion–p interactions in the
solid state (Fig. 2).12
Scheme 2 Synthesis of 2,3,4,5,6-(pentafluoro)benzylammonium
chloride 3b.
(3b and 3c) have to be synthesized. For 2,3,4,5,6-(pentafluoro)-
benzylammonium chloride (3b) the Schiff base was generated
from pentafluorobenzaldehyde (4) and benzylamine (5). After
rearrangement of 6 to 7 in triethylamine, the 2,3,4,5,6-(penta-
fluoro)benzylammonium salt was obtained by adding HCl
(Scheme 2).
The 2-(pentafluorophenyl)ethylammonium chloride 3c was
generated from 2,3,4,5,6-(pentafluoro)benzylbromide (8). Sub-
stitution of the bromide with potassium cyanide leads to
2-(pentafluorophenyl)acetonitrile (9) and following reduc-
tion by lithium aluminium hydride gives the corresponding
amine (10). By addition of HCl the final 2-(pentafluorophenyl)-
ethylammonium chloride 3c was obtained (Scheme 3). For all
three compounds crystal structures could be obtained.
In contrast to that, the crystal structure of 3c shows a
chloride anion located above the p-system. The distance to
the centre of the arene from the anion is 3.49 A (3.51 A for the
second molecule in the asymmetric unit) the closest one in this
series of solid state structures. The ammonium group fixes
the anion [NHÁ Á ÁCl = 2.27(2)/2.29(3) A] above the p-system.
Fig. 2 Comparison of the crystal structures of 3a, 3b and 3c
(top: side-view; bottom: top-view; green: chloride, black: carbon,
light-green: fluorine, blue: nitrogen, white: hydrogen).
Scheme 3 Synthesis of 2-(pentafluorophenyl)ethylammonium chloride 3c.
c
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Table 1 Summary of relevant NH/CÁ Á ÁClÀ binding distances [A] of
3a, 3b and 3c
were obtained on a Buchi B-540. Single crystal X-ray data of 3b
¨
and 3c were collected at 123(2) K using a Bruker-Nonius
KappaCCD diffractometer with an APEX-II detector and
graphite monochromatized Mo-Ka (l = 0.71073 A) radiation.
COLLECT13a software was used for the data collection (y and
o scans) and DENZO-SMN13b for the processing. The structures
were solved by direct methods with SIR200413c and refined by
full-matrix least-squares methods with WinGX-software,13d
which utilizes the SHELXL-97 module.13e Lorentzian polariza-
tion and multi-scan absorption (SADABS13f) corrections were
applied on all data. All C–H hydrogen positions were calculated
and refined as riding atom model. Ammonium H-atoms were
found from Fourier maps and fixed (by DFIX) to a distance of
0.91 A from the N atom. Thermal parameters for all H-atoms
were set to 1.2 times that of the C/N atom parameter. Compound
3c was found to be a racemic twin in the non-centrosymmetric
polar space group and TWIN/BASF refinement was applied.
Disorders in anisotropic parameters of aromatic carbons of
3c were restrained by SIMU (s = 0.01) and ISOR (s = 0.01)
in final refinement cycles.
3a7
3b
3c
NHÁ Á ÁClÀ
C1Á Á ÁClÀ
2.84
3.40
4.28
5.02
5.02
4.29
3.41
4.06
2.89
2.22(2)
3.897(3)
4.777(3)
5.356(3)
5.170(3)
4.374(3)
3.663(3)
4.37
2.27(2)/2.29(3)
3.711(6)/3.734(6)
3.359(6)/3.378(7)
3.419(7)/3.428(7)
3.800(7)/3.805(8)
4.126(11)/4.105(11)
4.015(10)/4.112(11)
3.49/3.51
C2Á Á ÁClÀ
C3Á Á ÁClÀ
C4Á Á ÁClÀ
C5Á Á ÁClÀ
C6Á Á ÁClÀ
CentrearylÁ Á ÁClÀ
PlanearylÁ Á ÁClÀ
3.29
3.31/3.33
The anion is not placed above the center of the arene. This is due
to the available void within the crystal packing, while the flexible
spacer allows the shifting of the anion towards the rim of the
aromatic unit. Three carbon–chloride distances are within the
range of the vdW radii [C1–3Á Á ÁCl = 3.711(6), 3.359(6) and
3.419(7) A and C1–3Á Á ÁCl = 3.734(6), 3.378(7) and 3.428(7) A
for the second molecule in the asymmetric unit]. So that at least
the interaction between the pentafluorophenyl unit and the anion
can be described as Z3 anion–p interaction, although there are
two additional NHÁ Á ÁCl contacts found from the ammonium
group [2.24(3)/2.24(2) and 2.31(3)/2.24(2) A]. The following table
summarizes the measured distances in the solid state structures
of 3a, 3b and 3c (Table 1).
Synthesis of compounds
N-(2,3,4,5,6-Pentafluorobenzylidene)benzylamine 6. Equimolar
amounts of pentafluorobenzaldehyde (1.00 g, 5.1 mmol) and
benzylamine (0.55 g, 5.1 mmol) were heated to 60 1C for 5 h. The
obtained light-yellow solid was crushed and dried in vacuo. Yield:
1.37 g of light-yellow solid (4.8 mmol, 94%). Mp: 85 1C.
1H NMR (CDCl3, 300 MHz): d (ppm) = 8.49 (s, 1H, CHN),
7.26–7.34 (m, 5H, Haryl), 4.91 (s, 2H, Hbenzyl). 19F NMR (CDCl3,
300 MHz): d (ppm) = À142.62 (m, 2F, Fortho), À150.94 (m, 1F,
The comparison of the three solid state structures reveals the
obvious influence of the spacer length on the position of the
anion in the crystalline lattice. While in 3a and 3b the linkers
are too short to fix the anion above the p-system, in 3c the
anion is located above the pentafluorophenyl unit and inter-
acts with three carbon atoms. Moreover, the results show that
a central binding of the anion above the electron-deficient
moiety requires a more rigid receptor backbone.
F
para), À161.92 (m, 2F, Fmeta). MS (EI, 70 eV): m/z (%) = 285.0
(43, [M]+, C14H8F5N+), 91.1 (100, C7H7+). IR (KBr) n (cmÀ1
)
= 3033 (w), 2901 (w), 2856 (w), 2652 (w), 2160 (w), 1826 (w),
1652 (m), 1555 (w), 1521 (s), 1491 (vs), 1453 (m), 1420 (w), 1377
(m), 1337 (w), 1311 (m), 1206 (w), 1180 (w), 1135 (m), 1078 (w),
1037 (w), 978 (vs), 926 (s), 857 (w), 820 (w), 789 (m), 750 (s),
701 (s), 681 (m). C14H8F5N (285.21): C 58.96, H 2.83, N 4.91
(%); found: C 58.78, H 2.95, N 4.96 (%).
Conclusions
The presented results show that it is possible to predict the relative
position of an anion in respect to an electron-deficient arene and
that in a connecting chain to the cation at least four s-bonds
between the directing hydrogen atom and the pentafluorophenyl
group are necessary to fix an anion above the p-system. The
chosen linker is too flexible for a fixation of the anion above the
centre of the electron-deficient arene and allows shifting towards
the rim. Probably, the presented results will help to design a
superior receptor to study anion–p interactions in the solid phase,
in solution and in the gas phase.
N-(Benzylidene)-2,3,4,5,6-(pentafluoro)benzylamine 7. 1.37 g
of compound 6 were dissolved in 10 mL of triethylamine and
stirred for 16 h. The solvent was removed under reduced
pressure and the obtained solid was dried in vacuo. Yield:
1
1.23 g yellow solid (4.3 mmol, 90%). Mp: 55 1C. H NMR
(CDCl3, 300 MHz): d (ppm) = 8.32 (s, 1H, CHN), 7.20–7.36
(m, 5H, Haryl), 4.83 (s, 2H, Hbenzyl). 19F NMR (CDCl3,
300 MHz): d (ppm) = À143.31 (m, 2F, Fortho), À155.37
(m, 1F, Fpara), À162.34 (m, 2F, Fmeta). MS (EI, 70 eV): m/z
(%) = 285.2 (91, [M]+, C14H8F5N+), 91.1 (100, C7H7+). IR
(KBr) n (cmÀ1) = 3030 (w), 2983 (w), 2873 (w), 2642 (w), 2182
(w), 2077 (w), 1974 (w), 1910 (w), 1826 (w), 1637 (m), 1579 (w),
1521 (s), 1497 (vs), 1450 (m), 1425 (w), 1382 (m), 1340 (m),
1299 (m), 1220 (m), 1124 (s), 1050 (s), 1003 (s), 983 (s), 955 (s),
927 (s), 851 (w), 791 (w), 759 (s), 693 (s), 673 (m). C14H8F5N
0.5 H2O (285.21): C 57.15, H 3.08, N 4.76 (%); found: C 56.97,
H 3.27, N 4.78 (%).
Experimental
Commercially available reagents were used as received. Sol-
vents were distilled and used without further purification. The
1H (300 MHz) and the 19F (282 MHz) NMR spectra were
taken on Varian Mercury 300 in deuteric chloroform, DMSO
or methanol. The mass spectrometric data were taken on
Finnigan SSQ 7000 and Thermo Deca XP as EI (70 eV) or
ESI. The infrared spectra were obtained on a PerkinElmer
FTIR spectrometer Spektrum 100. The samples were measured
in KBr (4000–650 cmÀ1). The elemental analyses were done on
a CHN-O-Rapid Vario EL from Heraeus. The melting points
2,3,4,5,6-(Pentafluoro)benzylammonium chloride 3b. To a
solution of 1.2 g of compound 7 in 10 mL of diethylether
c
1370 New J. Chem., 2012, 36, 1368–1372
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was added 10 mL aqueous HCl (36%). The aqueous layer was
separated and the solvent was removed. After recrystallisation
from acetone a yellow solid was obtained. The solid was dried
in vacuo. Yield: 0.8 g yellow solid (3.4 mmol, 78%). Mp: 289 1C.
1H NMR (MeOD, 300 MHz): d (ppm) = 7.44 (s, 3H, NH),
4.30 (s, 2H, Hbenzyl). 19F NMR (MeOD, 300 MHz): d (ppm) =
À143.22 (m, 2F, Fortho), À154.41 (m, 1F, Fpara), À164.14
(m, 2F, Fpara). MS (EI, 70 eV): m/z (%) = 197.0 (26, [M]+,
C7H4F5N+), 177.1 (100, C7H3F4N+). IR (KBr) n (cmÀ1) =
2877 (m), 2664 (w), 2628 (w), 2494 (w), 2400 (w), 2074 (w),
2000 (w), 1659 (m), 1593 (m), 1562 (m), 1504 (vs), 1436 (w),
1381 (m), 1309 (m), 1261 (w), 1155 (s), 1115 (s), 1060 (w), 984
(vs), 926 (vs), 886 (m), 799 (w), 749 (w), 700 (w), 664 (w).
C7H5F5NCl 0.5 H2O (233.57): C 37.48, H 2.47, N 6.24 (%);
found: C 37.09, H 2.45, N 5.99 (%).
20 min the solvent was removed and the obtained solid was
dried in vacuo. Yield: 590 mg colorless solid (2.4 mmol, quant).
Mp: 240 1C. 1H NMR (MeOD, 300 MHz): d (ppm) = 3.16 (dt,
J = 6.2/6.8 Hz, 4H, CH2). 19F NMR (MeOD, 300 MHz):
d (ppm) = À145.31 (m, 2F, Fortho), À158.72 (m, 1F, Fpara),
À165.10 (m, 2F, Fmeta). MS (EI, 70 eV): m/z (%) = 210.2 (20,
[M–H]+, C8H6F5N), 181.1 (100, C7H2F5). IR (KBr) n (cmÀ1
)
= 2953 (m), 2895 (m), 2612 (w), 2014 (w), 1659 (w), 1590 (w),
1500 (vs), 1295 (w), 1123 (s), 1043 (m), 994 (s), 966 (s), 935 (s),
857 (m), 787 (w), 718 (w), 669 (w). C8H7F5NCl (247.59):
C 38.81, H 2.85, N 5.66 (%); found: C 39.59, H 2.91, N 5.52 (%).
X-Ray structural analysis
Crystal data. 3b: Colorless plates from EtOH–EtOAc,
C7H5NF5Cl, F.W. = 233.57, crystal size 0.36 Â 0.25 Â
0.04 mm3, monoclinic, space group P21/c (no. 14), a =
2-(Pentafluorophenyl)acetonitrile 9. To
a solution of
12.1722(7),
b = 6.1586(3), c = 12.0356(6) A, b =
2,3,4,5,6-(pentafluoro)benzylbromide (500 mg, 2,4 mmol) in
7 mL of ethanol was added a solution of potassium cyanide
(157 mg, 2.4 mmol) in 3 mL of water. After stirring for 5 h at
50 1C the mixture was extracted with 10 mL of diethylether
(3 times). The organic layer was dried with magnesium sulfate.
The solvent was removed under reduced pressure and a light-
yellow liquid was obtained. Yield: 270 mg light-yellow liquid
(1.3 mmol, 68%). 1H NMR (CDCl3, 400 MHz): d (ppm) = 3.76
(s, 2H, Hbenzyl). 19F NMR (CDCl3, 400 MHz): d (ppm) = À141.09
(m, 2F, Fortho), À151.85 (m, 2F, Fpara), À160.08 (m, 2F, Fmeta). MS
(ESI): m/z (%) = 230.2 (100, [M + Na]+, C8H2F5N + Na+). IR
(KBr) n (cmÀ1) = 3944 (w), 3727 (w), 3515 (w), 3327 (w), 3209 (w),
3062 (w), 2977 (m), 2883 (w), 2794 (w), 2634 (w), 2322 (m), 2237
(m), 2144 (w), 2182 (m), 2059 (m), 1981 (m), 1922 (w), 1856 (w),
1798 (w), 1718 (vs), 1589 (m), 1522 (vs), 1441 (m), 1380 (vs), 1269
(m), 1240 (s), 1167 (m), 1071 (w), 1013 (s), 864 (m), 829 (m), 784
(m), 761 (m), 718 (vs), 675 (m). C8H2F5N 0.25 H2O (207.10): C
45.41, H 1.19, N 6.62 (%); found: C 45.78, H 1.12, N 5.91 (%).
90.762(3)1, V = 902.15(8) A3, Z = 4, Dcalc = 1.720 g cmÀ3
,
m = 0.461 mmÀ1, F(000) = 464, 4987 collected reflections
(ymax = 25.021) of which 1594 independent [Rint = 0.0386],
Tmax = 0.9818, Tmin = 0.8517, full-matrix least-squares
on F2 with 3 restraints and 136 parameters, GOF = 1.026,
R1 = 0.0366 [I 4 2s(I)], wR2 (all data) = 0.0895, largest
peak/hole = 0.290/À0.253 eÀ AÀ3
.
3c: Colorless plates from DMF–Et2O, C8H7NF5Cl, F.W. =
247.60, crystal size 0.30 Â 0.12 Â 0.08 mm3, orthorhombic,
space group Pca21 (no. 29), a = 14.8569(5), b = 5.4919(3),
c = 23.4876(10) A, V = 1916.41(15) A3, Z = 8 (Z’ = 2),
Dcalc = 1.716 g cmÀ3, m = 0.439 mmÀ1, F(000) = 992,
7988 collected reflections (ymax = 25.241) of which 3161
independent [Rint = 0.0416], Tmax = 0.9657, Tmin = 0.8796,
full-matrix least-squares on F2 with 91 restraints and 290
parameters, GOF = 1.048, R1 = 0.0462 [I 4 2s(I)], wR2
(all data) = 0.1201, absolute structure parameter (BASF) =
0.2(2), largest peak/hole = 0.459/À0.264 eÀ AÀ3
.
2-(Pentafluorophenyl)ethylamine 10. The synthesis of compound
10 was performed under nitrogen atmosphere. To a solution of
lithiumaluminium hydride (229 mg, 6.03 mmol) in 6 mL of THF a
solution of aluminium trichloride (805 mg, 6.03 mmol, in 10 mL
THF) was added. To this mixture 2-(pentafluorophenyl)acetoni-
trile (9, 500 mg, 2.41 mmol, 311 mL) was added. After stirring for
1 h the mixture was poured on ice. The aqueous layer was
extracted with 15 mL of diethylether for three times. After
evaporation of the solvent a yellow oil could be obtained. Yield:
330 mg yellow oil (1.6 mmol, 65%). 1H NMR (CDCl3, 300 MHz):
d (ppm) = 2.88 (t, J = 6.7 Hz, 2H, CH2), 2.77 (t, J = 6.7 Hz,
2H, CH2). 19F NMR (CDCl3, 300 MHz): d (ppm) = À143.77
(m, 2F, Fortho), À157.34 (m, 1F, Fpara), À162.74 (m, 2F, Fmeta).
MS (EI, 70 eV): m/z (%) = 211.1 (8, [M]+, C8H6F5N+), 195.1
(90, C8H4F5+). IR (KBr) n (cmÀ1) = 3353 (w), 2936 (w), 2641
(w), 2420 (w), 2114 (w) 1931 (w), 1623 (m), 1495 (vs), 1439 (m),
1372 (m), 1310 (m), 1286 (m), 1206 (w), 119 (s), 1073 (m), 978
(s), 942 (vs), 869 (m), 815 (m), 755 (w), 709 (m), 586 (s).
C8H6F5N 0.5 H2O (211.04): C 44.25, H 3.09, N 6.45 (%);
found: C 44.15, H 3.01, N 5.91 (%).
Notes and references
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