Encapsulation of [F4(H2O)10]4- in a dimeric assembly of an unidirectional arene based hexapodal amide receptor

Article abstract of DOI:10.1039/c1cc10742a

An unidirectional orientation of all six arms of an arene based hexapodal amide receptor shows dimeric capsular assembly templated by a [F ₄(H₂O)₁₀]^4- cluster.

Full text of DOI:10.1039/c1cc10742a

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COMMUNICATION
Encapsulation of [F4(H2O)10]4ꢀ in a dimeric assembly of an unidirectional
arene based hexapodal amide receptorw
M. Arunachalam and Pradyut Ghosh*
Received 8th February 2011, Accepted 9th February 2011
DOI: 10.1039/c1cc10742a
An unidirectional orientation of all six arms of an arene based
hexapodal amide receptor shows dimeric capsular assembly
templated by a [F4(H2O)10]4ꢀ cluster.
The hexasubstituted amide receptor, L was synthesized
from hexakis(aminomethyl)benzene9 in good yield and the
anion binding ability of
L in solution was validated
by 1H NMR experiments in DMSO-d6 at 2 1C in the presence
of tetrabutylammoniun (TBA) salts of different aꢀnions. In
Recognition of anions has attracted a great deal of attention in
the recent past due to their diverse functions in health and the
environment.1 In particular, recognition and sensing of fluoride
is an area of immense research interest to the supramolecular
chemist due to its role in chemistry, industry, food and
toxicity.2 Fluoride exists as various types of fluoride-water
clusters in water or even in the presence of moisture and its
recognition is a challenging task owing to its small size, high
electronegativity, and high hydration enthalpy. Thus, recent
studies indicate that the receptor for anions such as fluoride
should target the hydrated ions rather than the naked ions.3
Numerous theoretical calculations have predicted the existence
of a range of fluoride-water clusters.4 On the other hand, self-
assembled supramolecular capsules or dimeric assemblies that
provide an isolated nano-cavity have attracted much attention
in recent years for unusual guest encapsulation.5 One of the
most fascinating features of these assemblies is their ability to
isolate the encapsulated guests from the bulk.6 The hydrogen
bond motif has been applied successfully for the construction
of supramolecular assembly capable of engulfing suitable
guests. Within the area of self-assembly driven by hydrogen
bonding, Rebek, Atwood, and Steed et al. have independently
constructed molecular capsules by the self-assembly of various
subunits.7 The anion or hydrated anion assisted formation of
capsular assembly are important in the area of molecular
recognition and self-assembly.3b–c,8 Herein, we demonstrate
[F4(H2O)10]4ꢀ cluster encapsulation inside the dimeric capsular
assembly of a newly synthesized arene based hexapodal amide
receptor, L (Scheme 1), where all six arms are projected in one
direction (an unfavorable conformation) along with the binding
of other anions in solution and solid states.
1
qualitative H NMR studies, Fꢀ, AcOꢀ and H2PO4 showed
huge changes in the NMR spectral pattern. (Fig. S9, ESIw).
These studies indicated the solution state binding of Fꢀ, AcOꢀ
ꢀ
and H2PO4 with L. For detailed solution state binding of
1
these ions, we carried out H-NMR titration experiments in
DMSO-d6 at 25 1C which showed the existence of multiple
equilibriums in the solution state (See ESIw). In the case of Fꢀ,
the aryl–CH (singlet) signal is monitored for Job’s plot
analysis. The Job’s plot for L with Fꢀ showed host–guest
stoichiometry 1 : 2 (Fig. 1b). In addition to the parent signals,
a set of new signals are generated after the addition of two
equivalents of Fꢀ (Fig. 1).
The fluoride complex, 1, is isolated by adding TBAF,
[(CH3(CH2)3)4NFꢁxH2O], to the dioxane solution of L,
whereas the acetate complex 2 is obtained from acetone
solution upon charging TBA-OAc, [(CH3(CH2)3)4NꢁOAc].
The single-crystal X-ray structure of complexes 1 and 2
confirm the versatility of L as an anion receptor.zThe solid
state structure of 1 revealed the encapsulation of an unusual
fluoride-water cluster, [F4(H2O)10]4ꢀ inside the dimer of L
with the highly unfavourable aaaaaa conformation of the
hexapodal bowl (Fig. 2). Hexapodal molecules generally adopt
a conformation in which the arms alternate in positions above
and below the plane of the central benzene ring.10
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: Synthetic
procedures, Copies of the 1H NMR, 13C NMR, HRMS (ESI-MS)
characterization data and 1H NMR titration data. CCDC
812289–812290. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc10742a
Scheme 1 (a) Molecular structure of L and (b) pictorial representation
showing the encapsulation of [F4(H2O)10]
4ꢀ inside the dimeric assembly
of L.
c
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hydrogen atom of O5 are not located in the electron fourier
map. The cluster contains four, five and six membered rings,
constructed by hydrogen bonding interactions. Only two of
the five water molecules (O1 and O2) are directly hydrogen
bonded to the amide protons of L whereas other three water
molecules (O3, O4 and O5) in the asymmetric unit form a
symmetry induced cyclic hexameric water cluster with a chair
conformation. The symmetry related fused cyclic hexameric
unit in the [F4(H2O)10]4ꢀ cluster are strongly held together by
O–Hꢁ ꢁ ꢁO interactions, with Oꢁ ꢁ ꢁO distances ranging from
2.737 to 2.902 A (average: 2.823 A). A fluoride-water dimer
is held by strong N–Hꢁ ꢁ ꢁF and N–Hꢁ ꢁ ꢁO interactions with the
half capsule. The fluoride ion, F1 is hydrogen bonded with the
amide N–H protons of L designated as N8 and N78 and
the fluoride ion F2 is hydrogen bonded with the amide N–H
protons of N36 and N50 via N–Hꢁ ꢁ ꢁF interactions (Fig. 2c). In
addition, F1 and F2 are strongly hydrogen bonded with O1
and O2 via O–Hꢁ ꢁ ꢁF interactions with average Oꢁ ꢁ ꢁF bond
distances of 2.683 A ranging from 2.628 A to 2.728 A. Apart
from the hydrogen bonding interactions with F1 and F2, the
water molecules O1 and O2 are held strongly with the receptor
via N–Hꢁ ꢁ ꢁO interactions with the amide hydrogen atoms
(N22 and N64) with Nꢁ ꢁ ꢁO bond distances 2.868 A and
2.883 A respectively. Thus all of the six amide protons are
engaged in hydrogen bonding with the encapsulated
[F4(H2O)10]4ꢀ guest (four for fluoride ions and two for two
water molecules). Apart from the interactions with the receptor
molecule the encapsulated fluoride ions are directly hydrogen
bonded to the hexameric water cluster via O–Hꢁ ꢁ ꢁF and
O–Hꢁ ꢁ ꢁO interactions. F1 is held with water hexamer by
O–HꢁꢁꢁF (O3) hydrogen bonding interaction with OꢁꢁꢁF distance
2.685 A and +O–Hꢁ ꢁ ꢁF = 168.721 and F2 with Oꢁ ꢁ ꢁF (O5)
distance 2.783 A and O–Hꢁ ꢁ ꢁF = 167.321. In addition to the
classical hydrogen bonding interactions, the encapsulated
[F4(H2O)10]4ꢀ cluster is also stabilized by C–Hꢁ ꢁ ꢁF and
C–Hꢁ ꢁ ꢁO interactions with the aryl C–H protons of the
receptor bowl. Thus, the encapsulated [F4(H2O)10]4ꢀ cluster
is constructed and stabilized by various hydrogen bonding
interactions like N–Hꢁ ꢁ ꢁF, N–Hꢁ ꢁ ꢁO, O–Hꢁ ꢁ ꢁF, O–Hꢁ ꢁ ꢁO,
C–Hꢁ ꢁ ꢁF and C–Hꢁ ꢁ ꢁO interactions. In 1, all arms of L are
projected unidirectionally to engulf the fluoride-water cluster
Fig. 1 (a) 1H-NMR spectral changes of L with added TBA-F ([L]
is varied from 5.18 mM to 3.84 mM by the addition of aliquots of
100.03 mM TBAF). Ratio of concentration [L]/[Fꢀ]: (i) 0, (ii) 0.62,
(iii) 1.04, (iv) 1.45, (v) 2.08, (vi) 2.49, (vii) 3.12, (viii) 3.53, (ix) 3.95, and
(x) 6.02. (b) Job’s plot for L with TBAF.
inside the dimeric assembly of the receptor. Vogtle and Weber
¨
reported the unusual cation complexing tendency of an octopus-
like hexapodal ligand and its spatial crowding impeded
conformational mobility.11 Recently, DFT calculations on a
benzene platform based hexanonanamide receptor predicted
the existence of a bowl shaped hexapodal receptor with all six
substituents intramolecularly H-bonded together on one side
of the benzene plane.9 A crystal structure of a self-assembled
Fig. 2 (a) Space-filled view of [F4(H2O)10
]
4ꢀ cluster inside the dimeric
4ꢀ cluster and
capsule-like assembly of 1, (b) inset showing [F4(H2O)10
]
(c) partial structure showing the hydrogen bonding interactions of
4ꢀ
[F4(H2O)10
]
cluster with L. L has been represented in purple and
orange for clarity. (Color code: C—purple and orange; N—Blue;
O—Red; F—Yellow; H—Green) TBA counter cations, lattice dioxane
and non-bonding hydrogen atoms are omitted for clarity.
supramolecular complex stabilized by
a water decamer
Complex 1 possesses [L.(TBA-F)2ꢁ5H2Oꢁ0.5dioxane] in an
asymmetric unit. The source of water in the crystal could be
from the hydrated TBAF salt and atmospheric moisture.
Fig. 2b shows the hydrogen bonding pattern of the
[F4(H2O)10]4ꢀ cluster. The template cluster, [F4(H2O)10]4ꢀ is
formed via strong fluoride-water interactions. Five water
molecules (O1, O2, O3, O4 and O5), two fluorides (F1 and
F2) and half a molecule of disordered dioxane are present in
the asymmetric unit. Both hydrogen atoms of O4 and one
(H2O)10 has been reported by Atwood and co-workers.12
Whilst our recent report showed the encapsulation of
[F2(H2O)6]2ꢀ inside the dimeric assembly of a benzene based
1,3,5-trisubstituted tripodal triamide receptors.3b–c However,
no report on molecular capsule formation by dimeric association
of unidirectional hexapodal receptor templated by anion-water
cluster has been reported or predicted in the literature.
In the case of the planar acetate ion, colorless crystals of
complex 2 are obtained as single crystals suitable for single
c
6270 Chem. Commun., 2011, 47, 6269–6271
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13 536 were unique, 1211 parameters were refined; Goodness of fit
1.041, R1 = 0.0750, wR2 = 0.1985; max/min. residual electron density
0.770/0.513 e Aꢀ3
.
Crystallographic parameters of Complex 2: C98H136N14O26, M =
1926.21, Crystal dimension 0.18 ꢂ 0.10 ꢂ 0.08 mm3; T = 100(2) K;
Monoclinic, space group P21/n; a = 13.115(3), b = 27.983(6),
c = 13.542(3) A, a = 901, b = 93.325(5)1, g = 901, V = 4961.8(18) A3,
Z = 2, rc = 1.289 g cmꢀ3, m = 0.094 mmꢀ1, F(000) = 2060, 25 345
reflections collected of which 8700 were unique, 627 parameters were
refined; Goodness of fit 1.009, R1 = 0.0656, wR2 = 0.1634; max/min.
residual electron density 0.456/0.396 e Aꢀ3
.
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molecule above and below the benzene scaffold of L. (Color code:
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mixture (Fig. 3). Solid state structural analysis revealed 1 : 2
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anions encapsulated per receptor molecule. The acetate anion
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via strong N–Hꢁ ꢁ ꢁO interactions. Detailed hydrogen bonding
interactions are as follows: for N7–H7ꢁ ꢁ ꢁO1, d N7ꢁ ꢁ ꢁO1 =
2.755(4) A and +N7–H7ꢁ ꢁ ꢁO1 = 153.01, N21–H21ꢁ ꢁ ꢁO1,
d N21ꢁ ꢁ ꢁO1 = 2.838(4) A and +N21–H21ꢁ ꢁ ꢁO1 = 168.01
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N35–H35ꢁ ꢁ ꢁO3 = 168.51.
d
N35ꢁ ꢁ ꢁO3
= 2.886(4) A.
In conclusion, the arene capped hexapodal amide receptor
showed an encapsulation of a fluoride-water cluster containing
four fluoride and ten water molecules in the cavity of its
dimeric capsular type assembly where all six arms of the
receptor are unidirectional. The aryl terminals (m-nitro phenyl)
might have played an important role for its stabilization of the
unusual conformation to capture the relatively huge guest,
[F4(H2O)10]4ꢀ whereas pentafluorophenyl terminals with a
similar system showed the most favorable conformation upon
nitrate ion binding.13 Our approach of recognizing different
fluoride-water clusters would be useful to develop a new
generation of fluoride receptors in their hydrated form.
PG thanks Department of Science and Technology (DST),
India, for financial support through a Swarnajayanti Fellowship.
M.A. would like to acknowledge CSIR, India, for SRF. 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 parameters of Complex 1: C88H128F2N14O24
1804.04, Crystal dimension 0.22 0.18
0.16 mm3;
T = 120(2) K; Triclinic, space group P1; a = 15.7347(17), b =
,
11 F. Vogtle and E. Weber, Angew. Chem., Int. Ed. Engl., 1974, 13,
¨
814–816.
M
=
ꢂ
ꢂ
ꢀ
12 L. J. Barbour, G. W. Orr and J. L. Atwood, Nature, 1998, 393,
671–673.
13 M. Arunachalam and P. Ghosh, Org. Lett., 2010, 12, 328–331.
17.1841(18),
g = 99.502(3)1, V = 4732.7(9) A3, Z = 2, rc = 1.266 g cmꢀ3, m =
0.095 mmꢀ1, F(000) = 1928, 38 293 reflections collected of which
c = 19.617(2) A, a = 101.4281, b = 109.8351,
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6269–6271 6271