Structural insight into the anion-water cluster: Stabilised by alcohol and carboxylic acid containing tripodal ligand

Article abstract of DOI:10.1080/10610278.2013.844811

A new flexible N-bridged, unsymmetrical, water-soluble tripodal ligand (L) bearing alcohol and carboxylic acid groups has been synthesised and its solid-state interaction with anions has been investigated. The fully deprotonated ligand encapsulates a sodium cation in a half cryptand bowl-shaped cavity (1). The chloride complex 2, contains a cyclohexane-like water cluster incorporating ligand OH groups. However, complexes with bromide and nitrate (3 and 4) are dimeric. Tetrahedral clusters containing two water molecules and two anions were found in complexes 3 and 4. Perchlorate complex 5 forms perchlorate-methanol adducts. Complex 1 forms a hydrophilic cation-water channel and complexes 2-4 form anion-water channels between the hydrophobic layers of the naphthalene moieties. Complexes 2, 5 and 3, 4 are isostructural in nature having similar packing structures.

Full text of DOI:10.1080/10610278.2013.844811

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Structural insight into the anion–water cluster:
stabilised by alcohol and carboxylic acid containing
tripodal ligand
Md. Najbul Hoquea, Arghya Basua & Gopal Dasa
a Department of Chemistry, Indian Institute of Technology Guwahati, Assam, Guwahati 781
039, India
Published online: 18 Nov 2013.
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To cite this article: Md. Najbul Hoque, Arghya Basu & Gopal Das (2014) Structural insight into the anion–water cluster:
stabilised by alcohol and carboxylic acid containing tripodal ligand, Supramolecular Chemistry, 26:5-6, 392-402, DOI:
10.1080/10610278.2013.844811
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Supramolecular Chemistry, 2014
Vol. 26, Nos. 5–6, 392–402, http://dx.doi.org/10.1080/10610278.2013.844811
Structural insight into the anion–water cluster: stabilised by alcohol and carboxylic acid
containing tripodal ligand
Md. Najbul Hoque, Arghya Basu and Gopal Das*
Department of Chemistry, Indian Institute of Technology Guwahati, Assam, Guwahati 781 039, India
(Received 13 June 2013; accepted 6 September 2013)
A new flexible N-bridged, unsymmetrical, water-soluble tripodal ligand (L) bearing alcohol and carboxylic acid groups has
been synthesised and its solid-state interaction with anions has been investigated. The fully deprotonated ligand encapsulates
a sodium cation in a half cryptand bowl-shaped cavity (1). The chloride complex 2, contains a cyclohexane-like water cluster
incorporating ligand OH groups. However, complexes with bromide and nitrate (3 and 4) are dimeric. Tetrahedral clusters
containing two water molecules and two anions were found in complexes 3 and 4. Perchlorate complex 5 forms perchlorate–
methanol adducts. Complex 1 forms a hydrophilic cation–water channel and complexes 2–4 form anion–water channels
between the hydrophobic layers of the naphthalene moieties. Complexes 2, 5 and 3, 4 are isostructural in nature having
similar packing structures.
Keywords: water soluble tripodal ligand; sodium encapsulation; tetrahedral anion–water cluster; crystal structure; species
distribution curve
Introduction
host molecules. However, the challenges in the anion
coordination chemistry can be attributed to the various
geometries, properties of anions and high solvation
energies (10). Most recently, Bowman-James et al. reported
a tetrahedral molecule that encapsulated a tetrahedron of
water molecules which in turn holds a small guest (water or
fluoride ion), and few other reports also explain water–
water and water–anion interactions (11). In our continuing
effort (12) to explore the structural significance of anion–
water clusters, herein we demonstrate the formation of
anion–water cluster of varying geometries as well as the
encapsulation of a sodium ion in the cleft of a tripodal
ligand and the formation of sodium–water channels.
Consequently, we are able to explain the interaction
patterns of different anions such as spherical chloride and
bromide anions, planar nitrate and tetrahedral perchlorate
anion with water and tripodal host. Higher solubility in
polar solventis assisted by ZOH and ZCOOH groups in the
ligand and these also triggered the formation of water
clusters (2b, 13).
H-bonded supramolecular self-assembly, anion–water,
water–water interactions are phenomena relevant to the
structural properties of water or anion–water clusters. The
roles of these clusters in different chemical and
biochemical processes are well documented. Indeed, a
plethora of reports including theoretical (1) and exper-
imental (2) studies have been discussed to reveal the role
of water or anion–water clusters in protein folding–
unfolding, protein function and mobility of anions. Living
systems exploit the varied H-bond motifs around different
anions during their transportation between cellular
systems. The structural characterisation of water cluster
or anion–water cluster is also important to restore the
knowledge about crystallisation of proteins, interaction of
water with enzymes (3), stabilisation and functions of
biomolecules, (4) solvation chemistry, mobility of anions
in water–membrane interfaces and electrical phenomena
in ionosphere and troposphere. Hence, scientists investi-
gated hydrated anionic clusters (5) as the species play a
key role in signal transduction (6) and in atmospheric
chemistry (7), biochemistry and biology (8). Recent
advances in supramolecular chemistry have allowed the
study of self-assembled ion–water clusters through
electrostatic, p-stacking and Van der Waals interactions.
Recently, Bowman-James (9) reported the nature of
binding modes for different anions in a pragmatic way
which helps in the design of the supramolecular
architecture required for the self-assembly of anions with
Results and discussion
Crystal structure studies
[LNa2z6H2O] (1)
Structural elucidation by single crystal X-ray analysis
revealed that the sodium salt of the ligand crystallises
in monoclinic P21/c (Z ¼ 4) to yield the complex
*Corresponding author. Email: gdas@iitg.ernet.in
q 2013 Taylor & Francis

Supramolecular Chemistry
393
[LNa2z6H2O] (1). Figure 1(a) shows the encapsulation of a
sodium ion in the half crypt-like bowl-shaped cavity
formed by the tripodal ligand. Sodium ion (Na2) is
encapsulated in the bowl-shaped cavity by six coordi-
nation bonds formed by three ethereal oxygen atoms, one
water oxygen, one carboxylate oxygen and one apical
nitrogen atom. The coordinate bonds between Na2 and
different ethereal oxygen atom and apical nitrogen atom
are strong enough for stabilising Na2 in the cavity. One of
the carboxylate groups inclined towards the cavity and
interacted strongly with the encapsulated sodium ion
(Na2). The torsion angle for the arm N1ZC3ZC4ZO2
which forms ionic bond with interior sodium ion (Na2) is
little more (658) than the rest of the two arms with average
angle 538. The ethereal bond distances for O1ZO5,
˚
O2ZO5, O1ZO2 are 3.952, 3.606 and 4.560 A, respect-
ively, and they formed a half crypt-like cavity. The torsion
angle and ethereal bond distances are less than the reported
value (14), suggesting the folded conformation of the
ligand favouring the encapsulation of sodium ion in the
bowl-shaped cavity. A detailed structural study showed
that a metallacrown-like structure containing NaZOZNa
coordinate bond and O· · ·O interaction was observed and
is shown Figure 1(b). Crystal packing diagram ascribed in
Figure 1. (Colour online) (a) Perspective view of [LNa2z6H2O] (1) showing the encapsulation of sodium ion in the bowl-shaped cavity
within the tripodal platform. (b) Metallacrown-like framework constructed from NaZOwZNa coordinate bond and O· · ·O interaction. (c)
Crystal packing along the b-axis depicting sodium-water channel entrapped within hydrophobic environment. (d) Close-up view of zig-
zag cation–water channel. (e) Hexa-aqua sodium ion-assisted supramolecular self-assembled architecture.

394
M.N. Hoque et al.
Table 1 – continued
Table 1. Selected bond distances and bond angles.
˚
d(D· · ·A) A
˚
d(H· · ·A) A
˚
d(D· · ·A) A
˚
d(H· · ·A) A
D· · ·A
/(DHA)/8
D· · ·A
/(DHA)/8
O12· · ·O7
[LH3zClO4zMeOH ]
O3· · ·C4p
2.703(6)
–
–
[LNa2z6H2O]
Na1· · ·O8
Na1· · ·O9
Na1· · ·O10
Na1· · ·O11
Na1· · ·O12
Na2· · ·N1
Na2· · ·O1
Na2· · ·O2
Na2· · ·O4
Na2· · ·O5
2.323(4)
2.521(4)
2.325(4)
2.743(6)
2.533(5)
2.513(3)
2.471(4)
2.629(3)
2.225(4)
2.439(3)
2.291(4)
2.929(6)
2.862(5)
2.652(6)
3.298(6)
2.826(7)
2.830(5)
2.860(5)
2.798(6)
2.815(5)
3009(6)
2.791(5)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
3.455(6)
3.534(6)
3.475(6)
2.858(8)
3.578(8)
3.131(9)
2.740(1)
2.936(8)
2.647(5)
2.485(4)
2.594(4)
2.515(3)
2.254(6)
2.642(6)
2.438(6)
1.950(1)
2.181(7)
1.868(4)
177.1(3)
163.0(3)
170.6(3)
130.9(5)
162.4(3)
142.9(5)
162.4(4)
157.8(4)
168.3(3)
O3· · ·C16p
O7· · ·C4q
O8· · ·O1r
O8· · ·C16s
O9· · ·O1
O11· · ·O4t
O12· · ·O1r
O12· · ·O7u
Na2· · ·O13
O3· · ·O12
O4· · ·O9
ax,1/2-y,-1/2 þ z; bx,1/2-y,-1/2 þ z; cx,-1 þ y,z; dx,1.5-y,-1/2 þ z; ex,y,z;
fx,y, 2 1 þ z; gx, 2 1 þ y,z; h1 2 x,y,3/2-z; ix, 2 y, 2 1/2 þ z; jx,
1 2 y,1/2 þ z; kx,y,1 þ z; x,y,z; mx,1-y, 2 1/2 þ z; n 2 x,1 2 y, 2 z; ox,
l
–
–
2 1 þ y,z; px,3/2 2 y, 2 1/2 þ z; qx,1 þ y,z; r 2 x,1 2 y, 2 z; s 2 x,
1/2 þ y,1/2 2 z; tx,1.5 2 y,1/2 þ z; ux,1/2 2 y,1/2 þ z.
O6· · ·O8
–
2.397(4)
–
163.4(3)
O6· · ·C20a
O7· · ·O11
O7· · ·O12
–
–
–
2.075(3)
–
160.2(3)
O9· · ·O1
O10· · ·O3
O12· · ·O1
O13· · ·O8
O13· · ·O9
Figure 1(c) showed the sodium–water cluster along the
crystallographic b-axis where the sodium ion (Na1) is
pseudo-octahedrally arranged by water molecules and away
from carboxylate group. The infinite hydrophilic sodium–
water channel propagates in a zig-zag fashion containing
NaZOwZNa linker as shown Figure 1(d). The formation of
hydrophobic organic bilayer and hydrophilic hydrated
sodium layer is very similar to the ‘organic clay’ structure
reportedbyAtwoodetal. (15). Oneofthecarboxylategroups
interacts with the interior sodium ion and oxygen atom of
–
–
–
–
–
–
[LH3zClz3H2O ]
Cl1· · ·O1b
Cl1· · ·O10(w)b
O8(w)· · ·O9(w)
O8(w)· · ·O1
O9(w)· · ·O1
O9(w)· · ·O7
O9(w)· · ·C16b
O10(w)· · ·O4
C4· · ·O7c
3.072(4)
3.247(4)
1.970(1)
2.970(1)
2.846(5)
2.653(7)
3.161(6)
2.708(5)
3.482(6)
3.381(6)
3.449(5)
2.804(4)
3.763
2.271(2)
–
165.0(0)
–
–
–
–
–
–
–
–
–
2.831(3)
1.914(4)
2.515(4)
2.408(6)
2.519(4)
1.980(3)
2.892
100.9(1)
162.4(3)
174.6(2)
177.3(3)
160.8(2)
135.0(2)
156.4(3)
˚
channel water having bond lengths O3· · ·O12 ¼ 2.929(6)A
˚
and O3· · ·O10 ¼ 2.798(6) A, respectively. Other free
C4· · ·O3d
carboxylate groups linked with the adjacent moiety through
˚
C16· · ·O3d
N1· · ·O6e
C20HZOanionic and strong H-bond O7· · ·O11 ¼ 2.826(7)A
with the hydrated sodium ion which stabilises the sodium–
water channel. The axial water molecule of encapsulated
sodium (Na2) offers O13wZO9wZNa1ZO8wZO13w
nearly square geometry with the hydrated sodium ion
C7· · ·C12d
[LH3zBrz2H2O]
Br1· · ·O7f
3.234(5)
3.329(7)
3.819(5)
3.801(7)
2.936(9)
2.684(7)
2.794(8)
3.422(8)
3.425(8)
3.379(9)
2.775(7)
2.4470(7)
–
3.035(7)
3.0086(8)
–
160.9(4)
–
138.9(3)
139.7(4)
–
Br1· · ·O8(w)g
Br1· · ·C1 h
Br1· · ·C16i
O9(w)· · ·O8(w)
O9(w)· · ·O1j
O9(w) · · ·O1k
O4· · ·C17i
O6· · ·C3
˚
(Na1) of bond length O13· · ·O8 ¼ 3.009(6) A and
˚
O13· · ·O9 ¼ 2.791(6) A. The supramolecular self-assembly
that resulted from strong H-bonding among hexa-aqua
sodium water cluster and adjacent tripodal molecule is
shown in Figure 1(e). In addition, some other weak
interactions such as CZH· · ·p and CZH· · ·Oanionic also
play the key role to generate 3D self-assembled architecture.
The complete H-bonding interactions are given in Table 1.
–
–
–
–
2.471(5)
2.486(5)
2.408(6)
2.030(5)
166.7(4)
162.9(4)
179.8(4)
134.0(4)
O6· · ·C17f
N1· · ·O3f
[LH3zNO3z2H2O]
N1· · ·O6 l
O1· · ·O12
2.805(5)
2.816(7)
3.425(6)
3.440(6)
2.685(7)
2.780(1)
3.410(1)
3.340(1)
3.390(1)
2.804(9)
2.210(6)
2.078(5)
2.461(4)
2.512(4)
–
122.0(4)
149.5(3)
175.5(3)
161.0(3)
–
O3· · ·C4m
[LH3zClz3H2O] (2)
O3· · ·C16m
O4· · ·O8
As the tripodal ligand contains several sites suitable for H-
bonding and anion coordination, we moved forward to use
this features to understand anion–water and water–water
interactions. Acidification of IV with concentrated HCl
yielded colourless crystals of 2 having a monoclinic space
group P21/c and Z ¼ 4. Chloride ion forms a bridge
between water and alcoholic oxygen via sufficiently strong
O8· · ·O11(w)
O8· · ·C3 l
–
–
2.654(8)
2.580(1)
2.460(1)
–
134.8(3)
134.7(4)
160.1(4)
–
O9· · ·C1n
O10· · ·C17o
O11· · ·O12
(Continued)

Supramolecular Chemistry
395
˚
˚
˚
Cl1· · ·O1 ¼ 3.072(5) A and Cl1· · ·HO1 ¼ 2.271(2) A
distances O10· · ·Cl1 ¼ 3.247(4) A and O10· · ·O4 ¼ 2.708
˚
(5) A. The packing diagram of the complex 2 along the c-
interactions. Protonated apical nitrogen forms strong
H-bond with two ethereal oxygen and a carboxylic
axis depicted in Figure 2(b) showed the formation of
hydrophilic chloride–water channel between the hydro-
phobic layers. The weak interactions between carbonyl
oxygen (O3) and aliphatic hydrogen H18, H8 and aromatic
CH· · ·p also help to grow the molecular assembly.
˚
oxygen having distances of N1· · ·O1 ¼ 2.776(4) A,
˚
˚
N1· · ·O6 ¼ 2.862 (4) A and N1· · ·O5 ¼ 2.862 (4) A,
respectively.
The alcoholic ZOH stays away from protonated NH
and finally the tripodal unit became open and lost the
bowl-shaped conformation. The torsion angle of two
substituted arm is almost comparable with the sodium
complex 1, but the torsion angle for the non-substituted
arm is remarkably large (N1ZC1ZC2ZO1 ¼ 1578)
giving the open conformation with a greater separation
[LH3zBrz2H2O] (3)
We switched to larger spherical Br2 anion to understand the
formation of anion–water cluster in the same system. The
bromide complex [LH3zBrz2H2O] (3) crystallised in a
monoclinic space group C2/c with Z ¼ 4 with two water
moleculesandexistsasacentro-symmetricdimer. Adetailed
structural view revealed that three water molecules and two
Br· · ·OH (ZCOOH) interactions involved in the dimer
formation as shown in Figure 3(a). The interaction between
˚
between the ethereal oxygen atom O1 (,5.5 A) and the
oxygen atoms of other two arms (O2, O5). The detailed
view (Figure 2(a)) showed that the asymmetric unit forms
a centro-symmetric dimer consisting of chloride–water H-
bonding motif and chair-like cyclohexane conformation.
All four water molecules lie on the same plane in the chair
structure and offer strong water–water interaction through
H-bonding. The third water molecule interacts with
chloride ion and carboxylic oxygen atom with bond
˚
˚
O9w· · ·O4 ¼ 2.684(7) A and O9w· · ·O1 ¼ 2.794(8) A are
sufficiently strong to stabilise the dimer. Acidic group forms
˚
strong Br1· · ·O7 ¼ 3.234(5) A and Br1· · ·HZO7 ¼ 2.447
˚
(7) A interactions, but alcoholic OH does not participate in
H-bonding with Br2. Ethereal oxygen separation with non-
˚
˚
substituted arm (O1ZO2 ¼ 5.606 A, O1ZO5 ¼ 5.488 A)
and torsion angle value (N1ZC1ZC2ZO1 ¼ 153.78) again
suggests the unfolded conformation of 3 like anion complex
2. The endo-oriented H-atom of the protonated bridgehead
nitrogen offers H-bonding with ethereal oxygen atoms
˚
(average distance 2.75A) and oxygen of the acid group
˚
N1· · ·O3 ¼ 2.775(7) A. Unlike the simple linear
Cl· · ·HZOw interaction with the ligand as in complex 2,
here we observed a bromide–water tetrahedral motif
assigned as H2OzS2H2O, 2Br2 illustrated in Figure 3(b).
The central water molecule (O8) offers discrete anion
coordinating tetrahedron motif having a bond angle between
/Br1ZO8wZBr1 ¼ 108.3(3)8 and /Br1Z O8wZO9w ¼
107.0(4)8 which is little more than the ideal tetrahedral bond
angle of water molecule, whereas /O9wZO8wZ
O9w ¼ 102.8(4)8 is comparable with the ideal one. The
discrete tetrahedron motif is stabilised by relatively strong
˚
˚
O8w· · ·O9w ¼ 2.936(9) A and O8w· · ·Br1 ¼ 3.329(7) A
interactions. The tetrahedral unit gave two types of
tetrahedron edges OwZOw, OwZBr1 with certain distances
and angles given in the Table 2. Complex 3 contains a
hydrophobic chain of ligand moieties and the bromide–
water which are entrapped between the layers along c-axis.
[LH3zNO3z2H2O] (4)
At this point of study, we examined the effect of planar
ion and its interaction with water. Nitrate complex 4
crystallises in a monoclinic space group C2/c with Z ¼ 4, as
the centro-symmetric dimer contains two nitrates and three
watermoleculeslikethebromidecomplex(3). Thesalts 3and
4 are isostructural in nature and have identical packing motif.
Figure 2. (Colour online) (a) Perspective view of the dimeric
molecular structure bridged with water molecule having chair
conformation and formation of water cluster. (b) Crystal packing
of the complex 2 along c-axis showing hydrophobic layer
formation and hydrophilic chloride–water channel which were
entrapped between the organic layers.

396
M.N. Hoque et al.
Figure 3. (Colour online) (a) Perspective view depicting Br· · ·OH (acidic) bridged dimeric structure containing pentamer of water and
ZOH. (b) Discrete tetrahedral bromide–water motif H2OzS2H2O, 2Br2. (c) Crystal packing along c-axis showing inclusion of bromide–
water cluster between hydrophobic layers.
Thecrystalstructureshowedthatnitratecomplexalsoformed
a pentameric motif containing water and ZOH like complex
3, which is mainly stabilised by several H-bonding as
depicted in Figure 4(a). The structure is constructed via the
formation of several strong H-bonds such as
O8NO ZO11wZO8NO ¼ 120.3(5)8 and two water mol-
3
3
ecules /O12wZ O11wZO12w ¼ 112.2(5)8 is greater than
the bromide–water and water–water bond angle obtained
from complex 3 (Figure 4(b)). The nitrate ion of the
tetrahedral unit forms strong H-bond (O2NO· · ·OH ¼ 2.695
˚
˚
˚
O11w· · ·O12w ¼ 2.804(9) A, O1· · ·O12w ¼ 2.816(7) A
(7) A) with the acid group and the overall tetrahedron motif
˚
andO7· · ·O12w ¼ 2.703(6)A. Thecrystalstructurerevealed
leads to the building of the molecular assembly. This
assembly is stabilised by aliphatic hydrogen and nitrate ion
that one water molecule is surrounded by another two water
molecules and two nitrate ions to form anion coordinated
tetrahedral water motif of formula H2OzS2H2O, 2NO23 . The
tetrahedron motif having an angle between nitrate and water,
˚
ofbondlengthC1· · ·O9 ¼ 3.340(1) A and C3· · ·O8 ¼ 3.410
˚
(1) Awith inclusion of tetrahedral motif (Figure 4(c)). There
are several weak interactions that resulted from the carbonyl
oxygen and aliphatic hydrogen namely O3 with H9, H11 and
/O12wZO11wZ O8NO ¼ 125.1(5)8, two nitrates /
3
˚
Table 2. Dimensions (length, A; angle, 8) of the water–anion tetrahedron.
H2OzS2H2O, 2Br2(3)
H2OzS2H2O, 2NO23 (4)
O12wZO12w
˚
Distance (A)
˚
Distance (A)
O9wZO9w
O9wZBr1a
O9wZBr1a
Br1ZBr1
4.59
5.042
5.317
5.395
4.668
3.910
4.953
4.803
a
O12wZONO2a
O12wZONO2
O2NOZONO2
Angle (8)
Angle (8)
O9wZO8wZO9w
O9wZO8wZBr1a
Br1ZO8wZ O9wa
Br1ZO8wZBr1
102.81
116.00
107.01
108.26
O12wZO11wZO12w
112.16
88.92
125.06
120.30
a
O12wZO11wZONO2a
O12wZO11wZONO2
ONO2ZO11wZONO2
a Each contains two equal measurements.

Supramolecular Chemistry
397
Figure 4. (a) Structure depicting formation of H-bond-assisted pentamer containing water and ZOH. (b) Close-up view of tetrahedral
building block formulated as H2OzS2H2O, 2NO23 . (c) Nitrate ion interacts with acid group and water molecules. (d) Crystal packing along
c-axis shows the existence of nitrate–water cluster between hydrophobic layers.
CZH· · ·p interaction (C7· · ·C12g ¼ 3.777, C20· · ·Cg ¼
3.718) which also play a role in building the supramolecular
architecture. The inclusion of the nitrate–water cluster
between the hydrophobic layers of aromatic ring produced
hydrophilic nitrate–water channel along the crystallo-
graphic c-axis (Figure 4(d)), which is quite similar to the
packing structure of bromide salt 3 showing the isostructural
behaviour.
Z ¼ 4. On the basis of unit cell comparison, we found that
it is isostructural with the complex 2 with similar structural
arrangement. Instead of water, it contains methanol as
a solvent in the crystal structure. The endo-oriented
hydrogen atom on apical nitrogen forms strong NZH· · ·O
H-bonds with two ethereal O-atoms and a carbonyl oxygen
˚
of bond lengths 2.819(5), 2.804(5) and 2.785(6) A,
respectively. Methanol forms H-bonds with tripodal ligand
through alcoholic part of the complex (Figure 5(a)). The
oxygen atoms of the perchlorate ion also interact with both
ZOH and ZCOOH simultaneously. The oxygen O8 is
bifurcated, while O9 and O11 are mono-furcated giving a
four-point attachment with a tripodal ligand, whereas one
oxygen atom does not participate in any interaction. The
[LH3zClO4zMeOH] (5)
To explore the role of tetrahedral anion, we prepared the
complex (5) from IV on treatment with perchloric acid
which crystallises in a monoclinic space group P21/c with

398
M.N. Hoque et al.
Figure 5. (Colour online) (a) Depiction of the H-bonding interactions of tetrahedral perchlorate anion in the solid state. (b) Crystal
packing of complex 5 exhibiting perchlorate-methanol adducts propagating through the bilayer.
ZOH of alcohol part forms two hydrogen bonds with
the perchlorate oxygen O8 of a bond distance
4.8. Dissociation of one carboxylic group started around
at pH 4.2 and predominates upto pH 5.3. Finally, the
complete dissociation started after this pH and the
abundance of L32 increased. Deprotonation constants
˚
O1ZH· · ·O8 ¼ 2.254(6) A. In addition, the supramolecu-
lar self-assembled structure also stabilised by the
interaction between carbonyl oxygen and aliphatic
hydrogen having an average separation distance of
˚
2.554 A. The crystal packing along c-axis formed a
hydrophilic channel enriched with perchlorate-methanol
adduct and hydrophobic environment by naphthalene
moiety which is shown in Figure 5(b).
Potentiometric study
We performed potentiometric titration to get information
about the existence of several species at different pH. The
6 £ 10203 M NaOH solution was added to 1 £ 10203
M
solution of complex 2 in water–methanol (3:1) medium in
presence of 0.1 M NaCl as a supporting electrolyte
(Figure 6). This explains the stability of several protonated
and deprotonated species in the pH ranging from 3 to 6. It
is clear from the speciation distribution diagram that the
triprotonated species LH3 predominates around pH 4.2.
Upon gradual addition of base, it started to release most
acidic NH protons to form neutral LH2 and exists up to pH
Figure 6. (Colour online) Species distribution diagram of
[LH3zClz3H2O] (2).

Supramolecular Chemistry
399
a
Table 3. Overall (log bHi ) and stepwise deprotonation (log KHi ) constant of LH3 in H2O/MeOH (3:1).
Equilibrium reaction
log bHi
Equilibrium reaction
log KHi
LH3 þ OH2 ¼ LH2
LH3 þ 2OH2 ¼ LH2
LH3 þ 3OH2 ¼ L22
3.4
7.27
13.23
LH3 þ OH2 ¼ LH2
LH3 þ OH2 ¼ LH2
LH3 þ OH2 ¼ L22
3.4
3.87
5.96
a LH3 indicates as [LH3zClz3H2O], T ¼ 298 K, I ¼ 0.10 M in NaCl.
and stability constants were calculated using the HYPE-
RQUAD program (16) (Table 3).
Stepwise dissociation of the salt with pH is also confirmed
by species distribution curves.
Experimental section
Conclusion
Instrumentation
In conclusion, we have demonstrated the interaction of a
naphthoic acid-based tripodal ligand with cation or anions
and have shown the formation of water or anion–water
motifs in the solid state. The complete deprotonated ligand
at pH ,6.0 yields a sodium ion-encapsulated complex 1
where the cation resides within the bowl-shaped tripodal
cavity. By way of contrast, the protonated ligand at pH 2.0
forms complexes with various anions. In the solid state,
these complexes form different geometries of anion–water
motifs. The isostructural behaviour of the two sets of
complexes (2 and 5; 3 and 4) with different counter ions
gives identical packing. In complexes 3 and 4, discrete
anion–water tetrahedra formulated as H2OzS2H2O, 2X2
(XvBr2, NO23 ) were observed. Complex 1 forms
hydrophilic sodium–water channels and complexes 2–5
form anion–water or solvent channels adjacent to
hydrophobic layers created by a naphthalene moiety and
stabilised by CZH· · ·p and other H-bonding interactions.
The IR spectra were recorded on a Perkin Elmer–Spectrum
One FT-IR spectrometer with KBr disks in the range 4000–
500 cm21 (PerkinElmer, WA, USA). NMR spectra were
recorded on a Varian FT 2 400 MHz instrument (Agilent
Technology, CA, USA). The chemical shifts were recorded
in parts per million (ppm) on the scale using the solvent
peak as a reference.
X-ray crystallography
The intensity data were collected using a Bruker SMART
APEX-II CCD diffractometer (Bruker, Billerica, USA),
equipped with a fine focus 1.75kW sealed tube Mo Ka
˚
radiation (l ¼ 0.71073 A) at 298 K, with increasing v
(width of 0.38 per frame) at a scan speed of 6 s/frame
(Table 4). The SMART software was used for data
acquisition. Data integration and reduction were undertaken
Table 4. Crystal data and structure refinement parameters.
Code name
[LNa2z6H2O] (1) [LH3zClz3H2O] (2) [LH3zBrz2H2O] (3) [LH3zNO3z2H2O] (4) [LH3zClO4zMeOH] (5)
Empirical formula
Formula weight
Crystal system
Space group
C28H37O13NNa2 C28H34O10NCl
641.59
Monoclinic
P21/c
C56H62O17N2Br2
1194.89
Monoclinic
C2/c
41.7832(16)
9.6198(4)
14.3061(5)
90.00
107.947(2)
90.00
5470.5(4)
4
1.444
1.555
298(2)
C56H62O23N4
1159.11
Monoclinic
C2/c
41.616(2)
9.8238(6)
14.2763(8)
90.00
106.978(4)
90.00
5582.2(5)
4
1.372
C29H32O12NCl
622.01
Monoclinic
P21/c
20.5072(8)
9.8238(6)
14.7779(6)
90.00
98.448(2)
90.00
2884.0(2)
4
1.433
0.200
298(2)
580.02
Monoclinic
P21/c
˚
a (A)
17.9123(16)
23.198(2)
7.4779(9)
90.00
20.497(15)
9.714(7)
14.666(10)
90.00
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
90.138(7)
90.00
97.40(3)
90.00
3
˚
V (A )
3107.3(5)
4
1.346
2896(4)
4
1.316
Z
P (cal)g/cm3
m (Mo Ka) (mm21
T (K)
)
0.130
298(2)
0.189
298(2)
0.108
298(2)
R1; wR2 (I . 2s(I))
R1; wR2 (all)
Residual electron
density (e/A)
Goodness-of-fit
Reflection collected
0.0642; 0.2094
0.0842; 0.2449
0.584/20.381
0.0768; 0.1348
0.2051; 0.1493
0.533/20.416
0.0604; 0.1908
0.0848; 0.2118
0.833/20.793
0.1015; 0.210
0.1969; 0.25
0.687/20.445
0.0796; 0.2570
0.0967; 0.2789
0.659/20.565
˚
0.839
3724
1.088
7134
2150
1.096
2962
2142
0.984
6985
2861
1.091
3606
2679
Independent reflection 2763
CCDC No. 857847
857848
857849
857850
857851

400
M.N. Hoque et al.
with SAINT and XPREP software (17a). Multi-scan
empirical absorption corrections were applied to the data
using the program SADABS (17b). Structures were solved
by direct methods using SHELXS-97 and refined with full-
matrix least-squares on F 2 using SHELXL-97 (17c). All
non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms attached to all carbon atoms were geometrically
fixed while the hydrogen atoms connected to the tertiary
nitrogen atom were located using different Fourier maps
and the positional and temperature factors are refined
isotropically. However, we are unable to locate hydrogen
atoms of the lattice water molecule. Additionally, in
complex 2 we have one disordered water molecule.
Structural illustrations have been drawn with ORTEP-3
(17d) and MERCURY (17e) Windows.
Synthesis and characterisation
1-Hydroxy-naphthalene-2-carboxylic acid methylester II
1-Hydroxy-naphthalene-2-carboxylic acid (0.9 g, 5 mM)
was dissolved in methanol and 1 ml concentrated H2SO4
was added slowly with continuous stirring. The reaction
mixture was refluxed at 808C for 8 h. The solvent was then
evaporated under reduced pressure and a brown solid mass
was extracted with ethylacetate (3 £ 20 ml). The organic
layer was washed several times with water and dried over
anhydrous Na2SO4, and finally the solvent was removed
under reduced pressure that gave a brown solid product II;
yield 80% (Scheme 1).
Tripodal ester (III) and acid (IV)
A mixture of methyl ester II (1.5 g, 7.4 mM) and finely
powdered K2CO3 (8 g) in 30 ml acetonitrile was refluxed at
808C for half an hour. Tris(2-chloro-ethyl)amine hydro-
chloride salt (3.57 g, 14.7 mM) was added in portions over
15 min and the reaction mixture was then refluxed for
another 12 h. Then the brown coloured solution was
filtered off and the filtrate was evaporated in vacuum to
give a brown semi-solid mass III. Tripodal ester III was
dissolved in 10 ml methanol, and the methanolic solution
of sodium hydroxide (5 N, 40 ml) was added and refluxed
for 12 h at 808C to yield IV. The basic solution of IV was
then acidified cautiously with different acids.
Materials and methods
All reagents and solvents were obtained from commercial
sources and used as received. Tris(2-chloroethyl)amine
hydrochloride was prepared by the known literature
procedure (18). The tripodal ester was synthesised by
modification of the known literature procedure (19).
Ligand design
The designing of molecules to explore the anion–water and
water–water interactions in the solid state and solution
phase is a great challenge. A considerable progress has been
achieved with polyammonium-based receptors (20),
azacryptands (21) and cascade complexes (22). In this
report, we have designed a molecule that is sufficiently
soluble in water and possesses a H-bonding site for anions
and a protonation site to generate ionic environment so that
we can mimic the water–anion interaction in reality.
Characterisation of the ester II
NMR (400 MHz, CDCl3) d (ppm): 3.95 (s, 3H, Me), 8.40–
7.19 (m, 6H, Ar) 11.97 (s, 1 H, OH). 13C NMR (100 MHz,
CDCl3) d (ppm): 51.15, 106.09, 118.36, 123.84, 124.85,
125.22, 127.55, 129.27, 137.35, 161.42, 173.77. FT-IR:
1635 cm21 (ZCvO).
Scheme 1. Synthesis of the tripodal ligand and different salts.

Supramolecular Chemistry
401
Characterisation of the tripodal ester III
(400 MHz, d6-DMSO) d (ppm): 7.54–8.35 (m, Ar), 4.45
(t, OCH2), 4.09 (t, HOCH2), 3.97 (t, NCH2). FT-IR:
1997 cm21v(ZCvO).
NMR (400 MHz, CDCl3) d (ppm): 7.30–8.40 (m, Ar),
4.80 (t, OCH2), 3.80 (t, OHCH2), 4.20 (s, OMe), 3.30 (t,
NCH2). 13C NMR (100 MHz, CDCl3) d (ppm): 42.15,
52.83, 56.91, 63.64, 72.26, 105.64, 118.65, 123.92,
124.30, 125.32, 126.70, 129.45, 137.24, 160.94, 171.47.
FT-IR: 1712 cm21(ZCvO).
[LH3zClO4zMeOH] (5)
This is a similar experiment like complex 2, but the use of
concentrated HClO4 for acidification (in fume hood)
results in colourless single crystal suitable for X-ray
analysis. All crystals are stable at room temperature. Yield
55%; 1H NMR (400 MHz, d6-DMSO) d (ppm): 7.54–8.35
(m, Ar), 4.56 (t, OCH2), 4.11 (t, HOCH2), 3.97 (t, NCH2).
FT-IR: 1700 cm21vs(ZCvO).
Preparation of complex 1–5
[LNa2z6H2O] (1)
Methanolic solution of IV was neutralised to pH ,6.0
with slow addition of concentrated HCl in hood to avoid
fumes. The solution was afforded white powder after few
days. Recrystallisation of the white powder from ethanol–
water (1:1) at low temperature gave the colourless crystals
suitable for X-ray analysis. All crystals are stable at room
temperature. Yield 50%; 1H NMR (400 MHz, D2O) d
(ppm): 7.30–8.20 (m, Ar), 4.80 (t, OCH2), 3.76 (t,
HOCH2), 3.36 (t, NCH2). FT-IR: 1592 cm21 vas(ZCvO)
and 1398 cm21 vs(ZCvO).
Caution! The perchlorate salts must be handled with
care as they form potentially explosive mixtures with
organic compounds and they are also hazardous in nature.
Acknowledgements
G.D. gratefully acknowledges the Council of Scientific and
Industrial Research (01/2727/13/EMR-II) and Department of
Science and Technology (DST), New Delhi, India, for financial
support, CIF and IIT Guwahati for providing instrument facility
and DST FIST for single crystal X-ray diffraction facility. N.H.
and A.B. thank IIT Guwahati for fellowship.
[LH3zClz3H2O] (2)
However, when the pH of the solution of IV was
maintained at pH ,2.0 by adding concentrated HCl,
colourless solid sodium chloride crystals were developed
after few days and separated from the solution. Light
yellow solid was collected from the remaining solution
after removal of sodium chloride crystals, which on
recrystallisation from ethanol–water (1:1) at low tem-
perature yielded X-ray quality crystal. All crystals are
stable at room temperature. Yield 60%; 1H NMR
(400 MHz, CDCl3) d (ppm): 7.49–8.39 (m, Ar), 4.25 (t,
OCH2), 4.12 (t, HOCH2), 3.98 (t, NCH2). FT-IR:
1694 cm21 for v(ZCvO).
Supporting Information
Electronic supplementary information (ESI) available: X-ray
crystallographic file of the structures in CIF format, ORTEP
1
diagram, H NMR, 13C NMR and FT-IR Spectra. For ESI and
crystallographic data in CIF or other electronic format see DOI.
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