Weak intermolecular anion-π interactions in pentafluorobenzyl- substituted ammonium betaines

Article abstract of DOI:10.1002/ejic.201200184

A series of ammonium-carboxylate and ammonium-sulfonate betaines was synthesized and studied by single-crystal X-ray diffraction analysis to investigate the weak intermolecular interactions as well as the intramolecular interactions in the solid state. None of the expected intramolecular anion-π interactions could be observed, probably because of the steric demands and the reduced nucleophilicity of the anionic part of the betaines. Nevertheless, a weak intermolecular anion-π interaction between the anionic part of the betaine and the pentafluorophenyl unit is present in the structure of 5a. Copyright

Full text of DOI:10.1002/ejic.201200184

Job/Unit: I20184
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Date: 02-05-12 16:54:45
Pages: 6
FULL PAPER
DOI: 10.1002/ejic.201200184
Weak Intermolecular Anion–π Interactions in Pentafluorobenzyl-Substituted
Ammonium Betaines
Michael Giese,[a] Markus Albrecht,*[a] Katharina Wiemer,[a] Grzegorz Kubik,[a]
Arto Valkonen,[b] and Kari Rissanen[b]
Keywords: Anion–π interactions / Betaines / X-ray structures / Supramolecular chemistry / Noncovalent interactions
A series of ammonium–carboxylate and ammonium–sulfon-
ate betaines was synthesized and studied by single-crystal
X-ray diffraction analysis to investigate the weak intermo-
lecular interactions as well as the intramolecular interactions
in the solid state. None of the expected intramolecular anion–
π interactions could be observed, probably because of the
steric demands and the reduced nucleophilicity of the an-
ionic part of the betaines. Nevertheless, a weak intermo-
lecular anion–π interaction between the anionic part of the
betaine and the pentafluorophenyl unit is present in the
structure of 5a.
into a repulsive force.[8] Additionally, the effect of the anion
geometry on the interaction of pentafluorophenyl ammo-
nium cations with halides, nitrate, tetrafluoroborate, and
hexafluorophosphate were investigated. However, no de-
pendence of the anion–π interaction on the geometry of the
studied anions could be found in the solid state.[9] Attempts
to synthesize and crystallize an ammonium hydroxide salt
within this series, by substituting the para fluorine atom
with a hydroxy group, led to the corresponding phenolate
betaine 1 (Figure 1).[9]
Introduction
Supramolecular chemistry is commonly defined as the
chemistry of the noncovalent bond.[1] Noncovalent interac-
tions can vary dramatically in their strength from strong
electrostatic to weak dispersive attraction. They can even
be repulsive. Because of their biological and chemical rele-
vance, noncovalent interactions involving aromatic systems,
such as π–π stacking or cation–π interactions, play a crucial
role.[2] Recently, theoretical as well as crystallographic stud-
ies showed that the interaction between anions and elec-
tron-deficient arenes is attractive.[3] This led to intense stud-
ies in the field of anion recognition aimed at using anion–
π interactions for the development of superior anion recep-
tors.[4] While the existence of anion–π interactions in solid
phase is broadly accepted, the relevance of anion–π interac-
tions in solution[4] or in the gas phase[5] remains an open
question.
We started our work on anion–π interactions in 2008.[6]
First results showed that the position of an anion above the
electron-deficient pentafluorophenyl unit in phosphonium
and ammonium salts is flexible and can be controlled by
directing substituents.[7] Moreover, we were able to prove
that anion–π interactions depend on the fluorination degree
of the phenyl group. When the number of fluorine atoms
at the arene is successively reduced, its electron density is
enhanced. Thereby, the attractive anion–π interaction turns
Figure 1. Crystal structure of tetrafluorophenolate betaine 1. The
cocrystallized methanol molecules were omitted for clarity. C
(black), H (white), F (green), N (blue), O (red).[9]
This observation raises the following question: Could be-
taine structures manifest intramolecular anion–π interac-
tions in the solid state, or are other interactions, such as
electrostatics, C–H···O hydrogen bonds, π–π stacking, or C–
H···π interactions, overruling the weaker anion–π interac-
tions?
[a] Institut für Organische Chemie, RWTH Aachen University,
Landoltweg 1, 52074 Aachen, Germany
E-mail: markus.albrecht@oc.rwth-aachen.de
[b] Department of Chemistry, Nanoscience Center, University of
Jyväskylä,
Results and Discussion
To study intramolecular anion–π interactions of penta-
fluorobenzyl ammonium betaines in the solid state, a series
P. O. Box 35, 40014 University of Jyväskylä, Finland
E-mail: kari.t.rissanen@jyu.fi
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200184.
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M. Giese, M. Albrecht, K. Wiemer, G. Kubik, A. Valkonen, K. Rissanen
FULL PAPER
of promising betaine structures was synthesized. Therefore, anion is fixed by CH–anion interactions above the π system,
the dimethylamine carboxylesters 2a and 2b were treated because the carboxyl end does not seem to be involved in
with pentafluorobenzyl bromide (3) in diethyl ether to ob- anion–π contacts [4a: d(CH2···Br) = 2.85 Å and d(CH3···Br)
tain the corresponding pentafluorobenzyl dimethylammo- = 2.86 Å; 4bЈ: d(CH3···Br) = 2.96 and 3.06 Å]. The CAr···Br
nium bromides 4a and 4b (Scheme 1). After treatment with distances are in the range of the van der Waals (vdW) radii
silver(I) oxide, the desired betaine 5a could be isolated. Be- [4a: d(CAr···Br) = 3.64–3.99 Å; 4bЈ: d(CAr···Br) = 3.58–
taine 5b could not be obtained in pure form. Attempts to 3.82 Å] of the involved atoms, and because of the short dis-
purify the resulting waxy solid failed.
tances from the anion to the centre of the aromatic moiety
(3.57 Å for 4a and 3.44 Å for 4bЈ) the interaction can be
described as a η6 anion–π interaction. In contrast to 4bЈ,
where all pentafluorophenyl units are parallel oriented, in
4a the electron-deficient arenes interact with each other in
a T-shape fashion. The angle between the planes of the in-
teracting arenes is 45.2°, and the closest distances
d(F···CentreAr) between the fluorine atoms F4 and F5 of
one arene and the centre of another arene are 3.28 and
3.71 Å (Figure 2).
Scheme 1. Synthesis of the betaines 5a and 5b.
A more rigid betaine was synthesized by starting from 2-
amino-5-methyl-phenyl sulfonic acid (6). After the reaction
of 6 with pentafluorobenzyl bromide (3) and a following
treatment with perchloric acid, the betaine 8 was obtained
(Scheme 2).
Figure 2. Two views of the ion pairs in the crystals of 4a [side view
(a) and top view (b)] and of 4bЈ [(c) and (d)] showing the methyl
ester bromide. The corresponding acid in the crystal structure of
4bЈ is not shown. C (black), H (white), F (green), N (blue), O (red),
Br (dark red).
Pentafluorobenzyl Dimethylammonium Carboxy Betaines
5a and 5b
After the removal of the alkyl groups from the esters,
the resulting betaines were crystallized. Betaines are able
to interact both inter- and intramolecularly in the crystal
structure. In which fashion they interact depends on a com-
plex interplay of different noncovalent interactions as well
as steric and electronic effects. Scheme 3 exemplifies the dif-
ferent orientations of the betaine units in the solid phase.
In the crystal structure of 5a no intramolecular anion–π
interaction can be observed. One oxygen atom of the
carboxyl group is fixed by CH–anion interactions
[d(CH···O) = 2.29 and 2.56 Å] above the pentafluorophenyl
Scheme 2. Synthesis of betaine 8.
A series of crystals (4a, 4bЈ, 5a, 8) suitable for x-ray dif-
fraction analyses could be obtained and was analyzed with
respect to anion–π interactions in the solid state.
Pentafluorobenzyl Dimethylammonium Carboxyesters 4a
and 4b
Bromide salts 4a and 4b show structural similarities. moiety of another betaine unit (Figure 3a). The distance to
Since compound 4b was crystallized from wet methanol, a the centre of the C6F5 unit is 3.15 Å, which is quite short.
1:1 mixture 4bЈ of a methyl ester bromide (Figures 2c and Because of the short carbon–oxygen distances of three car-
d) and a bromide of the corresponding acid methanol solv- bon atoms [C4–C6, d(C···O–R) = 3.06, 3.25, and 3.27 Å],
ate was obtained. However, both in 4a and 4bЈ, the bromide the interaction can be described as a η3 anion–π contact.
2
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Weak Intermolecular Anion–π Interactions in Ammonium Betaines
Scheme 3. Possible anion–π interactions in the solid state: intra- vs.
intermolecular interactions.
An examination of the molecular packing of the betaine
units reveals a π–π stacking of the fluorinated arenes with
a plane-to-plane distance of 3.24 Å (Figure 3b).
Figure 4. Part of the crystal structure of the potassium salt 7. C
(black), H (white), F (green), N (blue), O (red), S (yellow), K (vi-
olet).
Table 1. Hydrogen bonds in 7 and 8.
Structure D–H···A
d(D···A) [Å]
Є(D-H···A) [°]
7
7
7
8
8
8
8
N1–H1···O1
N1A–H1A···O1A 2.820(4)
N1B–H1B···O1B 2.985(4)
N1–H1A···O2
N1–H1B···O1
N1–H1B···O1
C7–H7A···O2
2.984(4)
123(3)
137(3)
125(3)
172(2)[a]
139(2)
131(2)[b]
141.7
2.718(3)
2.747(3)
2.874(3)
3.369(3)
Figure 3. A top and a side view of neighbouring betaine molecules
in the crystal structure of 5a, showing (a) the anion–π as well as
(b) the π–π stacking interactions. C (black), H (white), F (green),
N (blue), O (red).
[a] Symmetry operations x, y + 1, z. [b] Symmetry operations x +
1, y + 1, z + 1.
Further attempts to crystallize compound 8 finally re-
sulted in the solid-state structure of the desired betaine
(Figure 5). Unfortunately, the structure shows no intramo-
lecular anion–π interactions. The crystal packing reveals
that two adjacent molecules are oriented parallel to each
other, whereby the sulfonate group of one molecule inter-
acts with the C6F5 unit of the other [d(C···OSO2) = 2.98–
4.29 Å]. Thereby, two oxygen atoms of each sulfonate unit
are within the range of the vdW radii of the involved atoms.
However, these anion–π contacts are induced by much
stronger NH···O hydrogen-bonding interactions between
ammonium and sulfonate groups (see Table 1 and
Figure°5b). Anion–π interactions are not required to stabi-
lize this structure, but they cannot be ruled out. There is
also a weak intramolecular CH···O contact.
In contrast to our expectation, an intramolecular anion–
π interaction was not observed in the crystal structure of
5a. This might be due to the insufficient length of the alkyl
chain between the pentafluorophenyl and the carboxyl
group. Therefore, the derivative 5b was synthesized with a
longer alkyl spacer. The resulting product could not be
purified, and because of its highly hygroscopic nature, no
crystals could be obtained for 5b.
2-(Pentafluorobenzyl)ammonium-5-methylphenyl Sulfonate
Betaine 8
A more rigid and better preorganized betaine for an in-
tramolecular anion–π interaction is 8. First attempts to iso-
late the betaine by the treatment of the potassium salt 7
with perchloric acid and the subsequent crystallization from
a mixture of DMF and ethyl ether led to crystals of the
potassium salt 7. The crystal structure of 7, containing
three organic anions, three potassium cations, and three
DMF molecules in an asymmetric unit, did not reveal an
anion–π interaction between the sulfonate group and the
pentafluorophenyl unit (Figure°4). Because of the interac-
tions between the sulfonate groups and the potassium cat-
ions [d(K···OSO2–R) = 2.69–2.86 Å], they are turned away
from the electron-deficient moiety. Two molecules of DMF
were found in the coordination sphere of each potassium
ion [d(K···ODMF) = 2.56–2.76 Å]. Between the fluorinated
arenes, a shifted face-to-face contact with a closest
CAr···CAr distance of 3.42 Å was observed. Intramolecular
N–H···O interactions were also found (see Table 1).
Figure 5. Top and side view of two adjacent betaine molecules in
the solid-state structure of 8. C (black), H (white), F (green), N
(blue), O (red).
The absence of intramolecular interactions of the anionic
part of the betaine with the electron-deficient arene can be
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M. Giese, M. Albrecht, K. Wiemer, G. Kubik, A. Valkonen, K. Rissanen
FULL PAPER
4b: Colourless solid (182°mg, 0.5 mmol, 59% yield). M.p. 52 °C.
1H NMR (CDCl3, 400 MHz): δ = 5.13 (s, 2 H, Hbenzyl), 4.16 (q, J
= 7.1 Hz, 2 H, CH2), 4.10 (t, J = 7.1 Hz, 2 H, CH2), 3.50 (s, 6 H,
CH3), 3.10 (t, J = 7.1 Hz, 2 H, CH2), 1.26 (t, J = 7.1 Hz, 3 H,
CH3) ppm. 19F NMR (CDCl3, 400 MHz): δ = –134.13 (m, 2 F,
Fortho), –145.33 (m, 1 F, Fpara), –157.83 (m, 2 F, Fmeta) ppm. MS
explained by the influence of the crystal packing. For an
intramolecular interaction, the betaine molecule has to
bend into a spherical shape. However, to achieve a closest
packing of the molecules, the linear shape with its intermo-
lecular anion–π interaction seems to be favoured. More-
over, because of the close packing of the betaine units in
the solid state, the difference between an intra- and an inter-
molecular anion–π interaction is small. In solution, the in-
tramolecular interaction should be entropically favoured
(ESI): m/z (%) = 326.0 (100) [M]+, C14H17F5NO2+. IR (KBr): ν =
˜
3425 (w), 2965 (m), 2660 (w), 2462 (w), 2083 (w), 1945 (w), 1728
(vs), 1659 (m), 1507 (vs), 1427 (w), 1376 (m), 1334 (m), 1309 (m),
1200 (s), 1133 (s), 1093 (w), 1028 (s), 973 (s), 891 (m), 854 (w), 804
over the intermolecular one. Attempts to investigate the in- (m), 737 (w), 681 (m) cm–1. C14H17BrF5NO2·1/3H2O (406.19):
calcd. C 40.79, H 4.32, N 3.40; found C 40.71, H 4.00, N 3.48.
terplay of inter- and intramolecular anion–π interactions in
solution failed.
Synthesis of Pentafluorobenzyl Ammonium Betaines 5a: The penta-
fluorobenzyl ammonium bromide 4a (1.00 g, 2.65 mmol) was dis-
solved in water (10°mL), and 1.0 equiv. of silver(I) oxide (373 mg,
2.65 mmol) was added. The suspension was stirred for 12 h in the
dark. The precipitate was filtered off. The solvent was removed,
and the remaining white solid was dried under vacuum.
Conclusions
In conclusion, the present study shows that betaines exhi-
bit anion–π interactions in the solid state. The expected in-
tramolecular interaction could not be observed. The intra-
molecular interaction prevents an efficient close packing,
and therefore the intermolecular anion–π interaction might
be favoured. Further investigations with structurally opti-
mized betaines will have to be performed to determine
whether an internal anion–π interaction is possible and will
elucidate the interplay of intra- and intermolecular anion–
π interactions in solution.
5a: Colourless solid (720°mg, 2.5 mmol, 96% yield). M.p. 225 °C.
1H NMR (CD3OD, 300 MHz): δ = 5.07 (s, 2 H, CH2), 3.96 (s, 2
H, CH2), 3.32 (s, 6 H, CH3) ppm. 19F NMR (CD3OD, 300 MHz):
δ = –137.84 (m, 2 F, Fortho), –151.85 (m, 2 F, Fpara), –163.12 (m, 2
F, Fmeta
) ppm. MS (ESI): m/z (%) =
283.9 (100) [MH]+,
C11H10F5NO2+. IR (KBr): ν = 3355 (w), 3024 (w), 2996 (w), 2963
˜
(w), 2198 (w), 2066 (w), 1993 (w), 1626 (vs), 1523 (s), 1503 (vs),
1428 (m), 1387 (s), 1370 (m), 1326 (s), 1213 (w), 1156 (w), 1133 (s),
1058 (m), 1014 (m), 961 (s), 919 (m), 896 (s), 883 (s), 774 (w), 708
(m), 682 (m) cm–1. C11H10F5NO2 (283.19): calcd. C 45.21, H 3.79,
N 4.79; found C 45.42, H 3.56, N 4.92.
Experimental Section
Synthesis of Potassium Pentafluorobenzyl-2-amino-5-methylphenyl
Sulfonate (7): To a solution of 2-amino-5-methylphenyl sulfonic
acid (144 mg, 0.82 mmol) in DMF (10°mL) were added penta-
fluorobenzyl bromide (200 mg, 0.82 mmol) and potassium carbon-
ate (424 mg, 3.11 mmol). The mixture was stirred for 12 h at room
temperature. After evaporation of the solvent, the remaining white
solid was redissolved in chloroform. The insoluble salts were fil-
tered off, and the solvent was removed. The obtained solid was
dried under vacuum. Colourless solid (78°mg, 0.2 mmol, 24%
All commercially available reagents were used as received. Solvents
were distilled and used without further purification. 1H (300 MHz)
and 19F (300 and 400 MHz) NMR spectra were obtained with a
Varian Mercury 300 or Inova 400 spectrometer in deuterated sol-
vents. The mass spectrometric data were recorded with a Finnigan
SSQ 7000 and a Thermo Deca XP system by using EI (70 eV) or
ESI, and the infrared spectra were measured with a PerkinElmer
FTIR spectrometer (Spectrum 100). The samples were measured in
KBr (4000–650 cm–1). Elemental analyses were performed with a
CHN-O-Rapid Vario EL system from Heraeus. The melting points
were measured with a Büchi B-540 system and were not corrected.
X-ray diffraction analyses are described at the end of the Experi-
mental Section.
1
yield). M.p. 138 °C. H NMR (CD3OD, 400 MHz): δ = 7.96 (s, 2
H, Hbenzyl), 7.51 (s, 1 H, Haryl), 7.07 (d, J = 8.6 Hz, 1 H, Haryl),
6.79 (d, J = 8.6 Hz, 1 H, Haryl), 4.51 (s, 3 H, CH3) ppm. 19F NMR
(CD3OD, 400 MHz): δ = –145.18 (m, 2 F, Fortho), –159.20 (m, 1 F,
Fpara), –165.54 (m, 2 F, Fmeta) ppm. MS (ESI): m/z (%) = 365.9 (100)
[M]–, C H F NO S–. IR (KBr): ν = 3368 (w), 3033 (w), 2929 (w),
˜
14
9
5
3
Synthesis of Pentafluorobenzyl Ammonium Bromides 4a and 4b:
Equimolar amounts of pentafluorobenzyl bromide and the corre-
sponding dimethyl carboxyl esters (N,N-dimethylglycine methyl es-
ter or N,N-dimethyl-β-alanine ethyl ester) were dissolved in hexane
(10°mL). After the evaporation of the solvent, the obtained solid
was dried under vacuum.
2736 (w), 2443 (w), 2215 (w), 2164 (w), 2118 (w), 2076 (w), 2044
(w), 2011 (w), 1983 (w), 1931 (w), 1844 (w), 1659 (s), 1617 (m),
1501 (vs), 1407 (m), 1366 (m), 1321 (m), 1261 (m), 1184 (vs), 1102
(s), 1073 (m), 1027 (vs), 938 (s), 887 (w), 810 (s), 701 (s), 663 (m)
cm–1. C14H9F5KNO3S·1/2DMF·3/2H2O (405.38): calcd. C 39.70,
H 3.33, N 4.48; found C 39.80, H 3.24, N 4.40.
4a: Colourless solid (2.41°g, 6.3 mmol, 82% yield). M.p. 197 °C.
1H NMR (CD3OD, 300 MHz): δ = 5.02 (s, 2 H, CH2), 4.60 (s, 2
H, CH2), 3.89 (s, 3 H, CH3), 3.32 (s, 6 H, CH3) ppm. 19F NMR
(CD3OD, 300 MHz): δ = –137.95 (m, 2 F, Fortho), –150.77 (m, 1 F,
Fpara), –162.63 (m, 2 F, Fmeta) ppm. MS (ESI): m/z (%) = 298.5
Synthesis
of
Pentafluorobenzyl-2-ammonium-5-methylphenyl
Sulfonate (8): Potassium pentafluorobenzyl-2-amino-5-meth-
ylphenyl sulfonate (200 mg, 0.5 mmol) was dissolved in methanol
(6°mL), and perchloric acid (70%, 70 mg, 0.49 mmol, 1.0 equiv.)
was added. The precipitating white solid was filtered off, and crys-
tals were directly grown by slow evaporation of the solvent.
(100) [M]+, C12H13F5NO2+. IR (KBr): ν = 3056 (w), 3012 (m), 2949
˜
(m), 2444 (w), 2043 (w), 2000 (w), 1750 (vs), 1659 (m), 1588 (w),
1556 (w), 1508 (vs), 1431 (m), 1397 (m), 1347 (w), 1312 (m), 1254
Single-Crystal X-ray Analyses
(m), 1231 (m), 1203 (vs), 1133 (vs), 1061 (m), 1013 (s), 983 (m), Single-crystal X-ray diffraction data were collected at 123(2)°K
960 (vs), 944 (s), 914 (m), 872 (s), 778 (w), 745 (m), 718 (w), 678
(m) cm–1. C12H13BrF5NO2 (377.00): calcd. C 38.12, H 3.47, N 3.70;
found C 38.08, H 3.45, N 3.72.
using a Bruker-Nonius KappaCCD diffractometer with an APEX-
II detector and graphite monochromated Mo-Kα (λ = 0.71073 Å)
radiation. COLLECT[10a] software was used for the data collection
4
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Weak Intermolecular Anion–π Interactions in Ammonium Betaines
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absorption effects were corrected with
a multi-scan method
(SADABS software[10f]), except for 5a. All C–H hydrogen positions
were calculated and refined by using a riding-atom model, except
for undisordered formyl H atoms in DMF molecules in 7. Hydro-
gen atoms bonded to N, O, or C atoms (including the mentioned
formyl H atoms) were found by using Fourier maps and fixed (by
using the command DFIX, s = 0.02) to a distance of 0.91 Å from
the N atoms, 0.84 Å from the O atoms, and 0.95 Å from the C
atoms. The thermal parameters of noncalculated H atoms were set
to values 1.2 times those of the C or N atoms they are bound to,
and they were set to values 1.5 times those of the O atoms they
were bound to. Disorder in geometry was restrained by using the
commands SADI (for 4b, s = 0.02) or SAME (for 7, s1 = 0.02),
and disorder in anisotropic parameters was restrained by using the
commands EADP (for the N atom of disordered DMF in 7),
SIMU (for 4a and 7, s = 0.01), DELU (for 7, s1 = 0.01), and ISOR
(for 4b and 7, s = 0.01) in final refinement cycles.
CCDC-868496 (for 4a), -868497 (for 4bЈ), -868498 (for 5a), -868499
(for 7), and -868500 (for 8) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Summary of crystallographic data and parameters.
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Acknowledgments
This work was supported by the Fonds der Chemischen Industrie
(M. A.) and the Academy of Finland (K. R., proj. no. 130629,
122350, 140718).
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Published Online: ᭿
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5

Job/Unit: I20184
/KAP1
Date: 02-05-12 16:54:45
Pages: 6
M. Giese, M. Albrecht, K. Wiemer, G. Kubik, A. Valkonen, K. Rissanen
FULL PAPER
Anion–π Interactions
A series of betaines bearing pentafluoro-
benzyl groups was synthesized and investi-
gated with respect to intra- and intermo-
lecular interactions in the solid state. The
carboxylic end of betaine 5a interacts with
the electron-deficient arene of a neighbour-
ing molecule in the solid phase.
M. Giese, M. Albrecht,* K. Wiemer,
G. Kubik, A. Valkonen,
K. Rissanen ....................................... 1–6
Weak Intermolecular Anion–π Interactions
in Pentafluorobenzyl-Substituted Ammo-
nium Betaines
Keywords: Anion–π interactions / Betaines /
X-ray structures / Supramolecular chemis-
try / Noncovalent interactions
6
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0