Encapsulation of a discrete cyclic halide water tetramer [X 2(H2O)2]2-, X = Cl -/Br- within a dimeric capsular assembly of a tripodal amide receptor

Article abstract of DOI:10.1039/c3cc40955d

A conformationally flexible C_3v symmetric N-bridged tripodal amide receptor encapsulates a tetrameric halide water cluster [X ₂(H₂O)₂]^2- (X = Cl ^-/Br^-) within its dimeric capsular assembly and forms a non-capsular 1D polymeric assembly with higher homologous iodide anions upon protonation.

Full text of DOI:10.1039/c3cc40955d

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Encapsulation of a discrete cyclic halide water tetramer
[X2(H2O)2]2À, X = ClÀ/BrÀ within a dimeric capsular
assembly of a tripodal amide receptor†
Cite this: Chem. Commun., 2013,
49, 3997
Received 4th February 2013,
Accepted 26th March 2013
Arghya Basu and Gopal Das*
DOI: 10.1039/c3cc40955d
www.rsc.org/chemcomm
A conformationally flexible C3v symmetric N-bridged tripodal
Acyclic receptors with a multi-armed hydrogen bonding
amide receptor encapsulates a tetrameric halide water cluster functionality have often been found to coordinate with targeted
[X2(H2O)2]2À (X = ClÀ/BrÀ) within its dimeric capsular assembly anionic species via formation of capsular assemblies.9 The most
and forms a non-capsular 1D polymeric assembly with higher interesting feature of molecular capsules is their unique ability
homologous iodide anions upon protonation.
to create a guest specific micro-cavity that isolates the encapsu-
lated guest from the bulk of the solvent media.10 Therefore,
Recently, recognition of hydrated anions has attracted a great carefully designed multi-armed acyclic receptors having a con-
deal of attention due to their occurrence in natural and biological vergent hydrogen bonding functionality can be utilized for the
environments such as marine water and various ecosystems. In recognition of hydrated anions via self-assembly processes.8
particular, recognition of discrete ordered anion–water clusters is
In a previous report, we have shown the formation of a cyclic
more challenging, because anions in their hydrated form are chloride–water pentamer [Cl3(H2O)4]3À within a benzimidazole
highly random and display a wide range of complicated inter- based tripodal receptor.11 Herein, we structurally demonstrate the
actions due to spontaneous formation of strong stable hydrogen encapsulation of a discrete cyclic halide–water tetrameric cluster
bonds in an aqueous environment. The formation of ordered [X2(H2O)2]2À (where X = ClÀ/BrÀ) inside the dimeric capsular
anion–water clusters within chemical receptors provides a unique assembly of a newly synthesized conformationally flexible, C3v
opportunity to understand the detailed molecular interactions in symmetric tripodal amide receptor, L (Scheme 1). However, in the
these restricted environments.1 A great deal of attention has case of higher homologous iodide anions the receptor L prefers to
especially been paid to understanding the water clusters in adopt a non-capsular polymeric aggregation, which confirms the
hydrophobic environments2 due to their vast importance in adaptability of flexible L as an anion receptor.
chemical and biological interfaces.3 The structural characteri-
The tripodal receptor L was synthesized in good yield by the
zation of anion–water clusters is important from the viewpoint reaction of triamine (ESI†) with three equivalents of 4-nitro-
of understanding the solvation mechanism, ion-mobilities in benzoyl chloride in the presence of triethylamine in DCM. The
the bulk, ion translocation in water–membrane interfaces, and deliberate placement of the anion binding amide functions far
electrical phenomena.4,5
apart from the bridgehead nitrogen (protonation site) on a
Among the various anions, aqueous solvation of halides has highly organized tripodal scaffold is appropriate for encapsula-
drawn much research attention due to their high abundance in tion of anionic guests via hydrogen bonding coordination.
nature. Thus, numerous theoretical calculations have predicted
the existence of several hydrated halides X(H2O)n in the recent
past.6 Although several hydrated anions have been characterized
previously within lattices of organic hosts, or metal complexes,7
examples of encapsulated discrete anion–water clusters by a
dimeric assembly of synthetic acyclic receptors are rare.8
Department of Chemistry, Indian Institute of Technology Guwahati,
Guwahati 781039, Assam, India. E-mail: gdas@iitg.ernet.in;
Fax: +91 0361 2582349; Tel: +91 361 258231
† Electronic supplementary information (ESI) available: Detailed synthesis pro-
cedure and characterization data; figures, tables and 1H NMR titration spectra.
CCDC 922581 (1), 922582 (2) and 922583 (3). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3cc40955d
Scheme 1 Conformationally flexible tripodal amide receptor L and the encap-
sulated halide water cluster [X2(H2O)2]2À (where X=ClÀ/BrÀ).
c
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Chem. Commun., 2013, 49, 3997--3999 3997

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Single crystals of the chloride complex [(LH)+ÁClÀÁH2O] (1) were
obtained by slow evaporation of a DMF solution of L in the
presence of excess hydrochloric acid (HCl). X-ray structure determi-
nation of 1 revealed that the protonated receptor molecule encap-
sulates a monohydrated chloride anion in its symmetric tripodal
cleft, where all the three arms of LH+ are projected in one direction.
Each etheric O-atom of a LH+ unit is hydrogen bonded to the
proton attached to the bridgehead N-atom that actually organizes
the flexible arms in one direction. Two units of monohydrated
chloride encapsulated LH+ are held together by two ClÀÁÁÁOH2
hydrogen bonds to form a dimeric capsular assembly (dN1ÁÁÁN1 =
20.621(3) Å) that encapsulates the [Cl2(H2O)2]2À cluster in its centre
via hydrogen bonding interactions and the two receptors are
assembled by weak pÁÁÁp interactions of the nitro-phenyl rings
(C1gÁÁÁC3g = 3.717 Å). Probably the hydrogen bonding amidic
functions of the receptor are too widely placed to offer an appro-
priate binding pocket for a single chloride anion, resulting in the
formation of the interesting [Cl2(H2O)2]2À cluster encapsulated
within a dimeric capsular assembly of a protonated receptor LH+
(Fig. 1). Fig. 1b and c show the hydrogen bonding pattern of the
[Cl2(H2O)2]2À cluster. The cluster is formed via strong chloride–
water interactions and possesses a planar cyclic tetrameric arrange-
ment where each chloride anion is hydrogen bonded with the two
water molecules having hydrogen bonding distances of 3.127(2) Å
and 3.277(2) Å. Close inspections of the distance and angle
measurement values of the terameric water cluster confirmed it
to be a perfect parallelogram (Fig. 1c). The [Cl2(H2O)2]2Àcluster is
completely enclosed by two protonated receptor units and stabi-
lized by two N–HÁÁÁClÀ, two N–HÁÁÁOH2, four C–HÁÁÁClÀ and four
C–HÁÁÁOH2 hydrogen bonds. These hydrogen bonds are the main
driving forces in the formation of the dimeric capsular assembly. In
this assembly, each chloride anion is stabilized by six hydrogen
bonds in a octahedral geometry involving one –NH proton, two
Fig. 1 (a) Space-filling representation depicting complete encapsulation of the
[Cl2(H2O)2]2À cluster inside the dimeric capsule-like assembly of LH+. (b) The
hydrogen bonding interactions of the [Cl2(H2O)2]2À cluster with two LH+.
(c) Close up view of the chloride water tetramer and the side view showing
the perfect planar nature of [Cl2(H2O)2]2À. (d) Crystal packing of complex 1 as
viewed down along the crystallographic a axis.
However, single crystal X-ray structure analysis of the iodide
–OH protons (lattice water), and three aromatic –CH protons. The complex 3 revealed a non-capsular 1D polymeric assembly of the
water molecules are also strongly held to the protonated receptors protonated receptors when obtained from DMSO solution. In
via N–HÁÁÁO and C–HÁÁÁO interactions (Table S2, ESI†). Interest- complex 3 there is a significant change in conformation of the
ingly, two out of six amidic –NH protons of two LH+ moieties are receptor observed, where one arm of the protonated tripodal
not involved in hydrogen bonding interactions with [Cl2(H2O)2]2À receptor is directed opposite to the other two arms, in contrast to
in the capsular assembly. Instead, they form hydrogen bonds with complexes 1 and 2 (where all three arms are unidirectional).
the oxygen atom of the amide function from a neighboring Probably the larger size and lower charge density of the iodide
capsular assembly and subsequently form a 1D chain of capsular ion is responsible for this conformational change in the receptor.
assembly along the crystallographic b-axis. Two such 1D arrays of This change in conformation of LH+ indeed helps the formation
capsular assemblies are further interconnected with one another of the 1D polymeric structure upon iodide coordination in the
via hydrogen bonds with the nitro group (C–HÁÁÁO), and generate crystalline solid state (Fig. 2 and ESI†). The iodide anion in
hexagonal networks of [Cl2(H2O)2]2Àencapsulated dimeric cages complex 3 is stabilized by five hydrogen bonds involving three
around each capsular unit along the a-axis (Fig. 1d).
amide –NH protons and two aromatic –CH protons (ESI†). It is
To understand the assembly process with higher homologous important to mention here that efforts were also made to crystal-
spherical anions, complexation with bromide and iodide has lize the fluoride complex of the receptor L but in spite of several
been carried out by protonating L in the presence of HBr and HI, attempts we were unable to grow good quality single crystals.
respectively. Interestingly, X-ray crystal structure analysis of the
The presence of hydrogen bonded hydrated halides (ClÀ/BrÀ) in
bromide complex 2 established it to be isostructural with the complexes 1 and 2 have also been confirmed by solid-state FT-IR
chloride complex 1, and the receptor interacts with the bromide analysis. Both the complexes showed a distinct band at around
anion in an exactly identical fashion to that of the chloride anion B3250 cmÀ1 that is assigned to the O–H stretching vibration of
in 1 (Fig. S20, ESI†). The capsular size (20.747 Å) and hydrogen water in the complexes. Additionally the significant shift (B58 cmÀ1
)
bond distances of the bromide complex are somewhat greater in the stretching frequency of the amidic –NH in the complexes in
than those of the chloride complex (Table S2, ESI†) due to the comparison to that of L (3326 cmÀ1) further supports the existence
larger ionic radii of the bromide ion compared to chloride ion. of strong N–HÁ Á ÁXÀ hydrogen bonds between the receptor and
c
3998 Chem. Commun., 2013, 49, 3997--3999
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G.D. acknowledges the DST, New Delhi, India, for financial
support, CIF, IIT Guwahati and DST-FIST for providing instru-
ment facilities. A. B. thanks IIT Guwahati for fellowship.
Notes and references
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Fig. 2 The molecular structure of complex 3 and polymeric aggregation upon
iodide coordination.
halide anions in all three complexes (ESI†). The thermal
stability of the halide–water cluster in complexes 1 and 2 has
further been studied by thermo-gravimetric analysis (ESI†).
Complex 1 exhibits a first weight loss of 1.9% at a temperature
of 193 1C, which corresponds to the water molecule, whereas
complex 2 loses water (1.8%) at 181 1C (Fig. S18 and S19, ESI†).
These temperatures are well above 100 1C, which indicate the
strong hydrogen bonding between the anion–water cluster and
the host molecules for both complexes 1 and 2.
The binding properties of LH+ÁClO4 with halides in solution
À
are investigated by performing 1H NMR experiments in DMSO-d6
by using (n-Bu)4N+ salts of different halides. Upon gradual addi-
tion of FÀ, no downfield shift is observed for the amide NH peak
until the addition of 1 equivalent of fluoride anion. However, the
aliphatic –CH protons are shifted upfield and remain almost
unchanged after further addition of fluoride anions. This observa-
tion suggests that the centrally bridged N-atom is deprotonated
after addition of 1 equivalent of fluoride anion and thus, in
solution fluoride complexation is not possible with the protonated
receptor. In the case of (n-Bu)4N+ClÀ a significant downfield shift
0.1 ppm of amide NH proton resonance is observed and then the
binding constant with chloride anions is determined from titra-
tion experiments and was found to be 118 MÀ1. However, addition
of (n-Bu)4N+BrÀ produces a very slight downfield shift (0.02 ppm)
of the amide –NH proton resonance of the protonated receptor,
while no notable change in the chemical shift of the amide NH
proton resonance is observed in the case of (n-Bu)4N+IÀ.
The solution state receptor-hydrated halide (ClÀ and BrÀ)
interaction in complexes 1 and 2 has further been confirmed
using 2D-NOESY NMR experiments. The NOESY spectra of both
the complexes show significantly strong NOE coupling between
the amide NH protons and water (H–O–H) (Fig. S8 and S11,
ESI†), whereas, no such type (N–HÁ Á ÁH–O–H) of interaction is
observed in the case of the free receptor and complex 3 (see
ESI†). These results imply that for complexes 1 and 2 the receptor
(LH+) encapsulates hydrated halide anion(s) (ClÀ or BrÀ) via
hydrogen bonding interactions inside its tripodal cavity.
In conclusion, we have shown the encapsulation of a planar
cyclic tetrameric halide water cluster [X2(H2O)2]2À X = ClÀ/BrÀ
within the dimeric capsular assembly of a conformationally
flexible tripodal amide receptor, where [X2(H2O)2]2À X = ClÀ/BrÀ
has acted as a template in the formation of the dimeric capsular
complexes. However, in the case of higher homologous iodide
anions the receptor changes its conformation and adopts non-
capsular polymeric aggregation upon iodide coordination.
11 M. N. Hoque, A. Basu and G. Das, Cryst. Growth Des., 2012, 12, 2153.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3997--3999 3999