Recognition and complexation of hydrated fluoride anion: F 2(H2O)62- templated formation of a dimeric capsule of a tripodal amide

Article abstract of DOI:10.1039/b910014h

An arene based tripodal amide receptor shows selective dimeric capsule formation templated by fluoride-water cluster, [F₂(H ₂O)₆]^2-, and non-capsular aggregation for chloride, nitrate and acetate anions. The Roy

Full text of DOI:10.1039/b910014h

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2ꢀ
Recognition and complexation of hydrated fluoride anion: F2(H2O)6
templated formation of a dimeric capsule of a tripodal amidew
M. Arunachalam and Pradyut Ghosh*
Received (in Cambridge, UK) 20th May 2009, Accepted 7th August 2009
First published as an Advance Article on the web 14th August 2009
DOI: 10.1039/b910014h
An arene based tripodal amide receptor shows selective dimeric
capsule formation templated by fluoride–water cluster,
[F2(H2O)6]2ꢀ, and non-capsular aggregation for chloride, nitrate
and acetate anions.
chloride in presence of triethylamine in chloroform. The
reaction mixture is filtered and washed with plenty of water
to remove the triethylammonium chloride and dried under
vacuum to yield the white solid of L. Further, L is treated with
an eꢀxcess of tetrabutylammonium (TBA) salts of Fꢀ, Clꢀ,
NO3 and AcOꢀ in dioxane to get the corresponding anion
complexes as crystals suitable for single crystal X-ray studies.z
Upon complexation with fluoride in dioxane, the receptor
Molecular capsules are an intriguing class of compound
achieved by both covalent binding and self-assembly through
noncovalent interactions.1 One of the most fascinating
features of molecular capsules is their ability to isolate the
encapsulated guests from the bulk. This may even allow
stabilization of the reactive intermediates and intermediates
in catalytic reactions.2 Self-complementary units of a molecu-
lar capsule are generally based on conformationally restricted
systems such as calixarenes,3a,b resorcinarenes,3c glycoluril3d
and tripodal3e,f derivatives. Among tripodal systems the
trialkylbenzene core provides some degree of preorganization
into a conical conformation with all three binding arms
projected in one direction.4 In recent times anions have
been explored in the area of supramolecular syntheses and
assemblies such as molecular capsules.5
L
yielded complex 1 which possesses two units of
[LꢁTBAFꢁ3H2Oꢁdioxane] in an asymmetric unit. The X-ray
crystal structure of 1 revealed the dimeric capsular assembly of
the receptor
L
with encapsulated hydrated fluoride,
[F2(H2O)6]2ꢀ, as a guest in the capsule (Fig. 1). The source
of water in the crystal could be from the hydrated TBAF salt
and atmospheric moisture. ꢀIt is worth mentioning that
selective binding of Cl(H2O)4 in a metal–organic framework
has also been demonstrated previously.7
In our case, probably the [F2(H2O)6]2ꢀ cluster has acted as a
template in the formation of the dimeric capsule, where all the
three arms of L projected in one direction to form a bowl
shaped cavity and two such bowls intercalate to form the
dimeric assembly. Fig. 1b and 2 show the hydrogen bonding
pattern of the [F2(H2O)6]2ꢀ cluster. This template is formed via
strong fluoride–water interactions. The cluster possesses a
tricyclic arrangement where two fluoride ions are in the apex
of the tricycle and are bridged by two water dimers and two
molecules of water (Fig. 1b). Therefore, each fluoride is
hydrogen bonded with four water molecules with an average
hydrogen bonding distance of 2.748 A ranging from 2.655 A to
2.912 A (Fig. 2). Two fluoride ions (F1) and four water oxygen
atoms (designated by O4 and O7) of [F2(H2O)6]2ꢀ are in two
N–Hꢁ ꢁ ꢁF and four N–Hꢁ ꢁ ꢁO interactions respectively with
amide centers of L as shown in Fig. 2 and Table 1.
Fluoride recognition is an area of immense research interest
to the scientific community due to its role in chemical,
industrial, food and toxicity.6 Fluoride is one of the most
challenging targets for anion recognition because of its high
hydration enthalpy. In a recent feature article on fluoride
binding, Cametti and Rissanen rightly mentioned—‘‘in
principle, a partially hydrated fluoride could be thought of
as the target species instead of a naked fluoride anion’’.6a
Herein, we structurally demonstrate selective hydrated fluoride
anion, F2(H2O)62ꢀ, cluster templated dimeric capsule formation
of a newly synthesized arene based tripodal amide receptor, L.
Further, anions like nitrate, acetate and chloride prefer
non-capsular aggregation of L, whereas exchange of these
anions with fluoride, resulted in the hydrated fluoride
encapsulated complex.
In addition to the conventional hydrogen bonding inter-
actions, fluoride ion (F1) and two waters (designated by O7)
of [F2(H2O)6]2ꢀ are in interactions with aryl-C–H protons,
resulting in two C–Hꢁ ꢁ ꢁF and two C–Hꢁ ꢁ ꢁO interactions.
Nitro group substitution in the aryl terminals increases the
aryl C–H acidity which facilitate these C–Hꢁ ꢁ ꢁF and C–Hꢁ ꢁ ꢁO
interactions between the two bowls in present study. Probably
The tripodal receptor
L (Chart 1) is based on a
1,3,5-methyl substituted arene having amide functionality
with nitro substituted (electron deficient) aryl terminals. L is
synthesized by the reaction of 1,3,5-tris(aminomethyl)-2,4,6-
trimethylbenzene4d with three equivalents of 2-nitrobenzoyl
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science, 2A & 2B Raja S. C. Mullick Road,
Kolkata 700 032, India. E-mail: icpg@iacs.res.in;
Fax: (+91) 33-2473-2805
w Electronic supplementary information (ESI) available: Experimental,
characterization data for L, 1, 2, 3 and 4 and crystallographic
refinement details for 1, 2, 3 and 4. CCDC 733118–733121. For ESI
and crystallographic data in CIF or other electronic format, see DOI:
10.1039/b910014h
Chart 1 Synthesis of triamide L.
ꢂc
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Chem. Commun., 2009, 5389–5391 | 5389

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Fig. 3 View of the zipper-like assembly of 3 upon nitrate encapsulation.
Tetrabutylammonium counter anions and the non-bonded hydrogen
atoms are omitted for clarity.
performed. The single crystal X-ray analysis of the chloride
complex 2, [LꢁTBAClꢁH2O] revealed discrete dimeric non-capsular
assembly (Fig. S13, ESIw). To study the role of the geometry of
the anions in the self-assembly processes of L, we have
synthesized two complexes, 3 and 4, for planar anions, nitrate
and acetate respectively. The single crystal X-ray analysis of 3
and 4 revealed the encapsulation of the respective anion
having similar aggregational patterns. As a representative of
planar anion complexes, details of complex 3, [LꢁTBANO3],
are presented here. Nitrate is recognized in the bowl shaped
cleft of L via three strong N–Hꢁ ꢁ ꢁO interactions between
amide protons of L and oxygen atoms of nitrate, and also
via one intramolecular C–Hꢁ ꢁ ꢁO interaction between aryl-C–H
and one of the oxygen atoms of nitrate (Fig. 3). Two such bowl
shaped nitrate encapsulated L are held together via inter-
molecular aryl-C–Hꢁ ꢁ ꢁO interactions between two of the
nitrate oxygens of one bowl and two aryl-C–H of the other
bowl. Further, one of the oxygen atoms of the encapsulated
nitrate is in intermolecular aryl-C–Hꢁ ꢁ ꢁO interactions with the
third bowl in the lattice and leads to a infinite zipper-like
assembly where nitrate is all together seven coordinate
(Fig. 3). Details of these interactions are available in the
ESI (Table S3).w
Fig. 1 (a) Space-filled view of the [F2(H2O)6]2ꢀ cluster inside the
dimeric capsule 1, (b) inset showing the [F2(H2O)6]2ꢀ cluster. One of
the two units has been described here and TBA counter cations, lattice
dioxane and the hydrogen atoms are omitted for clarity.
In the case of acetate complex 4, [LꢁTBAOAcꢁH2O], due to
the planar shape, the acetate anion prefers to bind with three
receptor molecules and that directs the infinite zipper-like
aggregation via various NHꢁ ꢁ ꢁO and C–Hꢁ ꢁ ꢁO interactions
upon anion encapsulation (Fig. S14, ESIw). In the solid state
crystal structure, along with the acetate anion, a disordered
water molecule is also located in the cleft of L. When anions
are an integral part of supramolecular aggregates, it is
expected that if the templating anion is exchanged by other
anions it might reorient the assembly. To demonstrate the
anion dependent aggregation of L, here we have carried out
the following two proof-of-concept experiments: (a) added
tetrabutylammonium fluoride to the crystals of complex 3 in
dioxane and allowed for crystallization, and (b) charged a
mixture of fluoride and acetate (1 : 1) with L in dioxane.
Interestingly, in both cases we isolated crystals suitable for
single crystal X-ray studies and confirmed the isolated crystals
as complex 1, i.e. [F2(H2O)6]2ꢀ templated dimeric capsules of
L. These results indeed prove that fluoride is preferred
over other anions and assists capsular assembly over other
aggregation observed for planar anions.
Fig. 2 Partial structure of 1 showing the hydrogen bonding pattern of
the [F2(H2O)6]2ꢀ cluster and its interactions with L.
Table 1 Crystallographic bond parameters of the interactions of the
[F2(H2O)6]2ꢀ cluster with L
D–Hꢁ ꢁ ꢁA
D–H/A
Hꢁ ꢁ ꢁA/A
1.95
1.96
2.03
2.689
2.548
Dꢁ ꢁ ꢁA/A
+D–Hꢁ ꢁ ꢁA/1
N20–H20ꢁ ꢁ ꢁF1
N27–H27ꢁ ꢁ ꢁO4
N43–H43ꢁ ꢁ ꢁO7
C31–H31ꢁ ꢁ ꢁO7
C16–H16ꢁ ꢁ ꢁF1
0.860
0.860
0.860
0.931
0.930
2.763(4)
2.807(5)
2.884(5)
3.380
157.6
168.6
170.5
131.68
144.19
3.347
these are the driving force in the formation of the staggered
dimeric capsular assembly (Fig. S12a, ESIw). Details of all
these H-bonding interactions are listed in Table 1. It is worth
mentioning that C–Hꢁ ꢁ ꢁanion hydrogen bonds draw increas-
ing attention among various anion binding interactions.8
Further, [F2(H2O)6]2ꢀ templated capsules are connected via
C–Hꢁ ꢁ ꢁO interactions between O7 oxygen of [F2(H2O)6]2ꢀ and
the C–H proton of the aryl terminal group of another capsule
(Fig. S12b, ESIw).
Qualitative 1H-NMR experiments were performed with
tetrabutylammonium salts of Fꢀ, Clꢀ, AcOꢀ and NO3 to
ꢀ
understand the solution state behaviour of L in the presence of
To understand the assembly with a higher homologous
these anions. Disappearance of the amide –NH signal is
spherical anion, complexation with chloride has been
ꢂc
This journal is The Royal Society of Chemistry 2009
5390 | Chem. Commun., 2009, 5389–5391

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observed immediately after the addition of (n-Bu)4N+Fꢀ to
independent (Rint
[I Z 2s(I)], 641 refined parameters, R = 0.0732, wR2 = 0.1747.
4: C51H71N7O12
a = 12.9277(19), b = 14.647(2), c = 15.513(2) A, a = 62.355(2),
b = 73.693(2), g = 77.867(2)1, V = 2486.4(6) A3, Dc = 1.301 g cmꢀ3
Z = 2, l = 0.71073 A, T = 100(2) K, 23 294 reflections, 8696
independent (Rint 0.0388), and 6356 observed reflections
=
0.0963), and 4320 observed reflections
the DMSO-d6 solution of L (Fig. S15, ESIw), whereas a very
slight downfield shift (0.03 ppm) of the –NH proton is
ꢀ
, M = 974.15, triclinic, space group P1,
ꢀ
observed in the case of (n-Bu)4N+Clꢀ and (n-Bu)4N+NO3
.
,
In the case of (n-Bu)4N+CH3COOꢀ, the chemical shift
position of the amide –NH protons shifted 0.2 ppm downfield
and we have performed 1H-NMR titration experiments of
(n-Bu)4N+CH3COOꢀ with L (Fig. S16, ESIw). A Job’s plot
analysis of the titration data showed a 1 : 3 host to guest
binding in solution whereas the solid state single crystal X-ray
study showed 1 : 1 binding. A difference in binding pattern
in solid and solution states is not uncommon.9 Here, the
difference in the binding of three acetate anions in solution
versus one acetate in the solid state may be due to side clefts
binding anions in solution which could allow multiple anions
interaction with a single receptor. In the solid state, the
receptor is more organized and prefers anion encapsulation
in the cavity of the three arms of tripodal system L and the
binding of a single anion can be observed. Further, solvent
systems (dioxane vs. dimethyl sulfoxide) might have played an
important role in imposing different binding patterns in solid
and solution states.
=
[I Z 2s(I)], 650 refined parameters, R = 0.0529, wR2 = 0.1383.
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¨
In conclusion, the arene capped amide based tripodal
receptor showed encapsulation of fluoride hydrate selectively
inside the capsule whereas infinite zipper-like aggregates were
formed for chloride, nitrate and acetate. Our approach of
recognizing the partially hydrated fluoride anion could
motivate the development of a new generation of fluoride
receptors. Presently, we are working on other rigid/semi-rigid
systems using the above approach to develop fluoride
receptors in aqueous/semi-aqueous media.
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P.G. gratefully acknowledges the Department of Science
and Technology (DST), New Delhi, India for financial
support. M.A. would like to acknowledge CSIR, New Delhi,
India for Senior Research Fellowship. X-Ray Crystallography
study is performed at the DST-funded National Single Crystal
X-ray Diffraction Facility at the Department of Inorganic
Chemistry, IACS.
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Notes and references
z Crystallographic data for: 1: C106H160F2N14O28, M = 2116.48,
ꢀ
triclinic, space group P1,
a = 12.9823(7), b = 20.3519(12),
c = 23.1725(13) A, a = 105.659(2), b = 92.575(2), g = 105.648(2)1,
V = 5630.8(5) A3, Dc = 1.248 g cmꢀ3, Z = 2, l = 0.71073 A,
T = 100(2) K, 54 203 reflections, 14 944 independent (Rint = 0.0676),
and 9162 observed reflections [I Z 2s(I)], 1388 refined parameters,
R = 0.0759, wR2 = 0.2118. Data collected on several crystals of
complex 1 did not show diffraction beyond theta (max) = 22.671.
ꢀ
950.55, triclinic, space group P1,
2: C49H68ClN7O10
,
M
=
a = 12.945(2), b = 14.4605(10), c = 15.5879(12) A, a = 117.393,
b = 95.911(2), g = 101.767(2)1, V = 2470.5(5) A3, Dc = 1.278 g cmꢀ3
Z = 2, l = 0.71073 A, T = 100(2) K, 19 525 reflections, 6810
independent (Rint 0.0401), and 5194 observed reflections
,
J. L. Sessler, V. Kral and V. Lynch, J. Am. Chem. Soc., 1996,
´
118, 5140–5141; (i) B. Dietrich, J.-M. Lehn, J. Guilhem and
C. Pascard, Tetrahedron Lett., 1989, 30, 4125–4128.
=
[I Z 2s(I)], 631 refined parameters, R = 0.0415, wR2 = 0.0873.
Data collected on several crystals of complex 2 did not show
7 R. Custelcean and M. G. Gorbunova, J. Am. Chem. Soc., 2005, 127,
16362–16363.
diffraction beyond theta (max) = 22.951.
8 (a) O. B. Berryman, A. C. Sather, B. P. Hay, J. S. Meisner and
D. W. Johnson, J. Am. Chem. Soc., 2008, 130, 10895–10897;
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Chem., Int. Ed., 2007, 46, 7849–7852.
ꢀ
959.10, triclinic, space group P1,
3: C49H66N8O12
,
M
=
a = 12.922(6), b = 14.152(7), c = 15.351(7) A, a = 116.614(6),
b = 92.811(7), g = 101.864(7)1, V = 2424(2) A3, Dc = 1.314 g cmꢀ3
Z = 2, l = 0.71073 A, T = 100(2) K, 21 099 reflections, 7484
,
ꢂc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5389–5391 | 5391