Neutral acyclic anion receptor with thiadiazole spacer: Halide binding study and halide-directed self-assembly in the solid state

Article abstract of DOI:10.1021/ic201656y

A halide binding study of a newly synthesized neutral acyclic receptor LH₂ with a thiadiazole spacer has been methodically performed both in solution and in the solid state. Crystal structure analysis of the halide complexes elucidate the fact that fluoride forms an unusual 1:1 hyrogen-bonded complex with monodeprotonated receptor, whereas in the case of other congeners, such as chloride and bromide, the receptor binds two halide anions along with formation of a halide-bridged 1D polymeric chain network by participation of N-H...X^- and aromatic C-H...X^- hydrogen-bonding (where X = Cl and Br) interactions. The presence of a rigid thiadiazole spacer presumably opens up enough space for capturing two halide anions by a single receptor molecule, where the coordinated -NH protons are pointed in the same direction with respect to the spacer and eventually favor formation of halide (Cl^- and Br^-) induced polymeric architecture, although no obvious chloride- or bromide-directed polymeric assembly is found in solution. A significant red shift of 243 nm in the absorption spectra of LH₂ was solely observed in the presence of excess fluoride anion, which enables LH ₂ as an efficient colorimetric sensor for optical detection of fluoride anion (yellow to blue). Furthermore, spectroscopic titration experiments with increasing equivalents of fluoride anion suggest formation of a H-bonded complex with subsequent stepwise deprotonation of two N-H groups, which can be visually monitored by a change in color from yellow to blue via pink.

Full text of DOI:10.1021/ic201656y

Article
pubs.acs.org/IC
Neutral Acyclic Anion Receptor with Thiadiazole Spacer: Halide
Binding Study and Halide-Directed Self-Assembly in the Solid State
Arghya Basu and Gopal Das*
Department of Chemistry, Indian Institute of Technology Guwahati, Assam-781039, Guwahati, India
S
* Supporting Information
ABSTRACT: A halide binding study of a newly synthesized
neutral acyclic receptor LH2 with a thiadiazole spacer has been
methodically performed both in solution and in the solid state.
Crystal structure analysis of the halide complexes elucidate the
fact that fluoride forms an unusual 1:1 hyrogen-bonded
complex with monodeprotonated receptor, whereas in the case
of other congeners, such as chloride and bromide, the receptor
binds two halide anions along with formation of a halide-
bridged 1D polymeric chain network by participation of N−
H···X− and aromatic C−H···X− hydrogen-bonding (where X =
Cl and Br) interactions. The presence of a rigid thiadiazole spacer presumably opens up enough space for capturing two halide
anions by a single receptor molecule, where the coordinated −NH protons are pointed in the same direction with respect to the
spacer and eventually favor formation of halide (Cl− and Br−) induced polymeric architecture, although no obvious chloride- or
bromide-directed polymeric assembly is found in solution. A significant red shift of 243 nm in the absorption spectra of LH2 was
solely observed in the presence of excess fluoride anion, which enables LH2 as an efficient colorimetric sensor for optical
detection of fluoride anion (yellow to blue). Furthermore, spectroscopic titration experiments with increasing equivalents of
fluoride anion suggest formation of a H-bonded complex with subsequent stepwise deprotonation of two N−H groups, which
can be visually monitored by a change in color from yellow to blue via pink.
protonated ligands9 or utilizing the metal−ligand interaction to
INTRODUCTION
The field of anion coordination chemistry continues to expand
■
assist formation of networks.10 Moreover, anion-directed metal-
free self-assembled coordination polymer formation in a neutral
with new synthetic molecules capable of recognizing anions
system is rarely highlighted in the literature.11 In this context,
which are not only within the interest of supramolecular
the structure of the receptor requires a tailored design to ensure
chemistry but also have immense environmental and
biomedical significance.1,2 Various types of neutral synthetic
that the anion-binding sites of the receptor should be well
separated by a rigid spacer. The spacer favors formation of an
receptors have been developed that employ hydrogen bonds
offered by specific binding sites as in amides,3 urea/thiourea,4
anion-assisted self-assembled architecture by adapting suitable
pyrroles,5 and indole6 to bind anions with size and shape
selectivity and are well explored in various media. However,
design and synthesis of new class of neutral anionic receptor
always remains a challenge for the researcher due to the high
directionality of H-bond donors required to obtain a stable
receptor−anion complex. As a consequence of this interest,
considerable efforts have been devoted to development of new
H-bonding receptors, capable of selective recognition of a
specific anion from a mixture of anions. Recently, Hay and co-
workers have shown their active interest toward the design of
computer-aided sophisticated receptors for targeted anions of
varying geometry.7
As compared to metal-directed coordination polymers,
anion-directed assembly of coordination polymers has also
attracted considerable attention motivated by their potential
applications in recognition, separation, guest inclusion, and
catalysis.8 Indeed, several approaches have been fruitfully made
to pursue anion-assisted variety of architectures by strengthen-
ing hydrogen bonding through electrostatic interaction with
receptor−anion hydrogen-bonding interactions.
In our continuing effort toward design and synthesis of new
anion receptors,12 herein we report the synthesis and
characterization of an anion receptor LH2 with a thiadiazole
spacer and its detailed halide binding studies both in solid and
in the solution state. This receptor selectively recognizes
fluoride among the other halides, which is manifested by a
visual color change. Except iodide, the receptor also forms
significant H-bonded complexes with other halides. Crystal
structure analysis of the chloride and bromide complexes
reveals that the receptor LH2 binds two halides along with
formation of a halide-bridged 1D chain polymeric network
structure through participation of N−H···X− and aromatic C−
H···X− H-bonding interactions (where X = Cl and Br).
However, no obvious chloride- or bromide-directed polymeric
assembly is found in solution. Therefore, such structural
Received: August 1, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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Article
features are worthwhile only in the solid state, while in the case
of the fluoride complex the receptor is present in its
monodeprotonated form with a hydrogen-bonded fluoride
anion.
Table 1. Crystal Parameters and Refinement Data
compound
formula
chloride complex
bromide complex fluoride complex
C46H84Cl2N8O5S
932.18
C46H84Br2N8O5S
1021.08
orthorhombic
Pna2(1)
31.372(7)
8.1971(19)
21.581(5)
90.00
C46H81FN8O4S
861.26
fw
cryst syst
space group
a (Å)
orthorhombic
Pna2(1)
31.492(3)
8.0970(7)
21.497(2)
90.00
orthorhombic
Pnma
RESULTS AND DISCUSSION
■
The receptor LH2 was synthesized with satisfactory yield by
refluxing the 1:2 mixture of 4-nitrophenylisothiocyanate and
hydrazine hydrate in the presence of acetic acid (catalytic
amount) in EtOH for 24 h (Scheme 1). Receptor LH2 is fully
40.264(7)
12.8688(18)
9.7766(16)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å)
Z
90.00
90.00
90.00
Scheme 1. Synthesis of the Receptor LH2
90.00
90.00
90.00
5481.5(8)
4
5550.0(2)
4
5065.7(14)
4
T (K)
298(2)
298(2)
298(2)
−1
μ (cm )
dcalcd (g cm−3)
0.204
1.545
0.115
1.130
1.222
1.131
cryst dimens
(mm3)
0.38 × 0.31 × 0.29 0.39 × 0.31 × 0.27 0.42 × 0.34 ×
0.29
no. of reflns
collected
4827
4780
4604
4548
4598
4549
no. of unique
reflns
no. of params
583
585
314
R1; wR2 (I >
2σ(I))
0.0533, 0.1082
0.0459, 0.1072
0.0667, 0.2491
characterized by NMR, ESI-MS, UV−vis, and IR spectros-
copies. The receptor LH2 consists of two acidic N−H protons
which are separated by a rigid thiadiazole spacer. The 4-nitro
phenyl rings help to increase the acidity of N−H protons and
also act as a chromophoric signaling unit during the receptor−
halide interaction process.
R(int)
GOF (F2)
0.0633
0.997
0.0857
0.992
0.1882
1.099
CCDC NO.
835051
835050
801324
X-ray Crystal Structure Analysis of Receptor Halide
Complexes. Light orange block-shaped crystals of chloride
complex [LH2(Cl)2(n-Bu4N)2]·H2O suitable for single-crystal
X-ray diffraction analysis were obtained after 20 days of slow
evaporation of an acetonitrile solution of the receptor LH2 in
the presence of excess [n-Bu4N]Cl with an isolated yield of
about 48%. The complex crystallized into the orthorhombic
space group Pna21 with Z = 4 (Table 1). Structural elucidation
of the complex reveals that the receptor is involved in a 1:2
binding stoichiometry of host to guest where each chloride
anion is bound to a single receptor molecule by a moderate N−
H···Cl− interaction together with the hydrogen-bonding
participation from adjacent TBA countercations and the lattice
water molecule. The hydrogen-bonding interactions of chloride
anions with the receptor and adjacent tetrabutylammonium
groups are shown in Figure 1 along with the atom numbering
scheme. The water molecule behaves as a hydrogen-bonded
bridge between two coordinated anions forming a [Cl2H2O]2−
triangle with a Cl−−O−Cl− angle of 114.9(1)° and Cl−−Cl−
separation distance of 5.643(2) Å (Figure 1 inset). The
presence of a rigid thiadiazole spacer between the two acidic
NH protons of LH2 resists the convergent hydrogen bonding
to a single chloride anion, whereas convergent hydrogen bonds
to an acceptor species are quite common for urea/thiourea-
based anion receptors.4 Both the N−H protons of the receptor
are directed in the same direction and participate in hydrogen-
bonding interactions with two different chloride anions. The
donor−acceptor distances (N···Cl−) fall in the range 3.10−3.14
Å, which corresponds to formation of moderate hydrogen
bonds, mainly electrostatic in nature.13 Structural analysis of the
complex reveals that Cl− (1) is stabilized by four hydrogen
bonds involving one −NH proton (N2H), one −OH proton
Figure 1. Hydrogen-bonding interactions of chloride anions with
receptor (LH2) and tetrabutylammonium groups. Only the interacting
parts of the tetrabutylammonium unit have been shown for clarity.
(Inset) Magnified view of the water chloride hydrogen-bonding
interactions. Dashed lines indicate the hydrogen-bonding interactions.
(O5H, lattice water), and two aliphatic−CH protons (C15H
and C24H) from a nearby tetrabutylammonium unit (Tables 2
and 3). On the other hand, Cl− (2) is stabilized by participation
of seven hydrogen bonds involving one −NH proton (N5H),
one aryl −CH proton (C5H), one −OH proton (O5H, lattice
water), and four aliphatic −CH protons from two neighboring
tetrabutylammonium units donating two C−H···Cl− hydrogen
bonds each. One of the nitro phenyl rings is slightly twisted out
of the plane relative to the central thiadiazole ring, where the
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The other chloride anion Cl− (1) is not involved in this
polymeric array.
Table 2. N−H···Anion and O−H···Anion Hydrogen-Bonding
Interactions in Receptor−Halide Complexes
Single crystals suitable for X-ray diffraction analysis of the
bromide complex were obtained as [LH2(Br)2(n-Bu4N)2]·H2O
with a fair yield (42%) by slow evaporation of an acetonitrile
solution of the receptor LH2 in the presence of a large excess of
[n-Bu4N]Br. The complex crystallized into the orthorhombic
space group Pna21 with Z = 4 (Table 1). Interestingly, X-ray
crystal structure analysis of the bromide complex reveals that
the receptor LH2 interacts with bromide anions in exactly the
same fashion as that of chloride anions (Supporting
Information, Figure S6). Hydrogen-bond distances of the
bromide complex are slightly greater than those of the chloride
complex (Tables 2 and 3). This is due to the larger ionic radii
and lower basicity of the bromide ion compared to chloride.
Similar to the chloride complex, here also the bromide-bridged
1D chain polymeric network structure is observed.
donor group
D···A [Å] H···A [Å] D−H···A [deg] acceptor atom
N(2)−H(2N)
N(2)−H(2N)
N(2)−H(2N)
O(5)−H(5O)
O(5)−H(6O)
N(2)−H(2N)
N(2)−H(2N)
O(5)−H(5O)
O(5)−H(6O)
2.300(9)
3.076(4)
3.113(4)
3.320(5)
3.374(6)
3.193(8)
3.255(7)
3.404(7)
3.464(6)
1.442(5)
2.216(4)
2.258(6)
2.465(6)
2.537(7)
2.300(8)
2.300(8)
2.568(7)
2.645(4)
177.0(5)
168.0(4)
173.3(5)
172.8(5)
167.6(9)
173.0(6)
166.6(4)
165.2(6)
162.0(5)
F1
Cl1
Cl2
Cl1
Cl2
Br1
Br2
Br1
Br2
Table 3. C−H···Anion Hydrogen-Bonding Interactions in
Receptor−Halide Complexes
acceptor
atom
donor group
D···A [Å] H···A [Å] D−H···A [deg]
The crystal structure of the fluoride complex revealed some
interesting features of the complex. Dark violet block-shaped
single crystals of the fluoride complex were grown from
acetonitrile as [LH(F)(n-Bu4N)2] with a satisfactory yield of
about 55% upon slow evaporation of a mixture of LH2 and 5
equiv of [n-Bu4N]F. The fluoride complex crystallizes in the
orthorhombic system with a Pnma space group with Z = 4
(Table 1). In contradiction to the chloride and bromide
complexes, fluoride forms an unusual 1:1 host−guest complex
with monodeprotonated receptor. A structural study of the
complex reveals that fluoride anion is stabilized by six hydrogen
bonds involving one −NH proton (N2H), one aryl −CH
proton (C2H) of the monodeprotonated receptor, and four
aliphatic−CH protons from the acidic −CH2 group, adjacent to
the bridgehead nitrogen of two different tetrabutylammonium
cations present in close proximity with fluoride anion. The
presence of two tetrabutylammonium cations and a hydrogen-
bonded fluoride in the crystal lattice of the fluoride complex
supports the fact that the receptor present is in its
monodeprotonated form. The significantly small N···F−
distance of 2.300(9) Å suggests that the deprotonated receptor
forms comparatively a much stronger hydrogen bond with
fluoride as compared to chloride and bromide in their
respective complexes, where the receptors are in the neutral
form. The mode of hydrogen-bonding interactions of the
fluoride anion with monodeprotonated receptor and tetrabu-
tylammonium groups is shown in Figure 3a, and the
corresponding parameters are given in Tables 2 and 3.
Comparative bond distance analysis of the complexes again
confirms that in the fluoride complex the receptor exists in its
monodeprotonated form (Supporting Information, Table S2.).
C(2)−H(2)
3.250(1)
3.061(5)
3.254(5)
3.664(6)
3.616(6)
3.689(6)
3.743(6)
3.612(9)
3.825(7)
3.802(7)
3.673(8)
3.670(7)
3.800(8)
3.844(8)
3.640(1)
3.873(8)
3.899(9)
2.668(5)
2.178(3)
2.434(4)
2.810(1)
2.717(1)
2.917(1)
2.787(1)
2.771(1)
2.913(1)
2.905(1)
2.814(9)
2.774(9)
3.006(9)
2.878(8)
2.812(9)
2.963(8)
3.011(9)
121.4(7)
150.6(3)
142.1(3)
153.3(3)
154.4(4)
137.2(4)
168.9(4)
145.6(7)
157.0(4)
154.4(4)
154.0(4)
153.8(5)
140.0(5)
172.8(5)
144.1(9)
156.6(5)
152.8(6)
F1
C(15)−H(15A)
C(23)−H(23B)
C(5)−H(5)
C(39)−H(39A)
C(43)−H(43A)
C(31)−H(31A)
C(40)−H(40B)
C(15)−H(15A)
C(24)−H(24B)
C(5)−H(5)
C(39)−H(39A)
C(43)−H(43A)
C(31)−H(31A)
C(40)−H(40B)
C(15)−H(15A)
C(24)−H(24B)
F1
F1
Cl2
Cl2
Cl2
Cl2
Cl2
Cl1
Cl1
Br2
Br2
Br2
Br2
Br2
Br1
Br1
angle between the planes of the two rings is 25.08°.
Additionally, one of the ortho aryl hydrogens (C5H) with
respect to the nitro group of the twisted phenyl ring forms a
significant C−H···Cl− hydrogen-bonding interaction with the
neighboring Cl− (2) anion. This C−H···Cl− interaction leads to
formation of an anion-bridged 1D polymeric chain network in
association with the moderate N−H···Cl− (2) H bond along
with a weak intermolecular C−H···O (nitro) interaction
(Figure 2). The chloride Cl− (2) anions in this polymeric
chain extension are alternately arranged in a zigzag array with a
distance of 11.49(2) Å and Cl−−Cl−−Cl− angle of 149.20(2)°.
Figure 2. Formation of a chloride Cl− (2) directed network obtained by self-assembly (view along the b axis). Countercations [n-Bu4N+] and solvent
molecules and the other chloride anion (Cl− (1)) are omitted for clarity. Hydrogen bonds have been drawn as dashed lines.
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any fluoride-induced polymeric assembly in the solid state.
Significantly, in all three halide complexes the aliphatic CH
protons of the tetrabutylammonium countercations actively
participate in the hydrogen-bonding interactions with halides.
This type of hydrogen-bonding interaction between anions and
nonaromatic CH groups as hydrogen-bond donors is very rare
in the literature,14 and the C−H···anion interactions involving
organic cations have also attracted considerable attention
because of their biological and chemical importance.15
Halide Binding Study by UV−Vis Spectroscopy. The
halide binding properties of LH2 have been investigated in
solution by qualitative as well as quantitative UV−vis
spectroscopic experiments in CH3CN. The absorption
spectroscopic study reveals that the receptor is capable of
selectively detecting fluoride ion by a visible color change from
yellow to blue even in the presence of other halides like Cl−,
Br−, and I− (Supporting Information, Figure S11). Free receptor
shows an absorption maximum at 385 nm in CH3CN.
However, addition of excess fluoride ion (50 equiv as TBA
salt) shows the disappearance of the peak at 385 nm with
concomitant emergence of a new peak at 628 nm (Figure 4a).
Figure 3. (a) Hydrogen-bonding interactions of fluoride anion with
monodeprotonated receptor (LH−) and two tetrabutylammonium
groups. Only the interacting parts of the tetrabutylammonium units
have been shown for clarity. Dashed lines indicate the hydrogen-
bonding interactions. (b) Crystal structure of the fluoride complex
showing the presence of F− anion in the same molecular plane.
It is interesting to observe that in the fluoride complex the
monodeprotonated receptor is present in its anti conformation
with respect to the thiadiazole spacer. This conformation is due
to minimizing the repulsion between two negatively charged
species (fluoride anion and N5 atom of monodeprotonated
receptor), while in the bromide and chloride complexes the
receptor LH2 is present in its syn conformation. Furthermore, it
is important to mention here that while interacting with the
fluoride ion the monodeprotonated receptor (LH−) adopts an
exactly planar geometry (Figure 3b) which eventually favors
formation of an aromatic C−H···F− interaction involving the
aryl proton C2H of the adjacent nitro phenyl ring due to its
directionality acquired because of the planarity, whereas such
type of hydrogen-bonding interactions have not been observed
in the chloride and bromide complexes. Close inspection of the
crystal structure elucidates that the fluoride anion is getting
sandwiched between two cationic tetrabutylammonium units
by four aliphatic C−H···F− interactions with an average donor
to acceptor distance 3.15 Å (Supporting Information, Figure
S8). Formation of a 1:1 complex between fluoride and
monodepronated receptor could be justified due to the higher
basicity of the fluoride ion. The approaches of more than two
fluoride anions to the receptor LH2 initiate the deprotonation
process, and thereby, we found in the fluoride complex the
receptor present in its monodeprotonated form with a
hydrogen-bonded fluoride anion.
The overall crystal structure analyses of the receptor−halide
complexes reveal that 1:2 chloride and bromide complexes
form a halide-bridged 1D polymeric chain network by
participation of N−H···X− and aromatic C−H···X− hydrogen-
bonding (where X = Cl and Br) interactions. Most likely the
rigid thiadiazole spacer opens up enough space to confine two
halide anions by a single receptor molecule which eventually
favors formation of halide (Cl− and Br−) induces 1D polymeric
chain, whereas no such fluoride-induced polymeric architecture
is observed in the fluoride complex. Presumably the fluoride
induces formation of a 1:1 complex between monodeproto-
nated receptor, and the fluoride anion prevents construction of
Figure 4. (a) Changes in the UV−vis spectra of LH2 in CH3CN upon
addition of tetrabutylammonium salts of different halides (50 equiv).
(b) Changes of UV−vis specctra of LH2 (4.6 × 10−6 M) in CH3CN
upon addition of different equivalents of [n-Bu4N]F solution at 298 K .
A significant red shift of 243 nm is fairly comparable to the
report by Zhao et al.,16a although the highest value of a fluoride-
induced red shift has recently been reported by Ghosh et al.16b
To evaluate the fluoride binding, quantitative UV−vis titration
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of LH2 is carried out with increasing equivalents of F−. The
titration experiment (Figures 4b and 5) reveals that a fluoride-
combination of both receptor−fluoride H-bonded complex
formation and fluoride-induced monodeprotonation of the
receptor.18 Moreover, further addition of fluoride decreases the
intensity of the band centered at 522 nm with the emergence of
a new band at 628 nm with a sharp isosbestic point at 562 nm
(Figure 5b). This process is initiated when the fluoride
concentration is more than 4 equiv and reaches its saturation
after addition of 20 equiv of fluoride. Further generation of a
new peak in the red-shifted region (628 nm) indicates the
fluoride-induced double deprotonation of the receptor in
solution.17a
The presence of the absorption band at 522 nm in the UV−
vis spectrum of isolated single crystals of the fluoride complex
supports our assumption that during titration the fluoride-
induced new band at 522 nm is a combination of both
receptor−fluoride H-bonded complex formation and fluoride-
induced monodeprotonation of the receptor (Supporting
Information, Figures S10 and S16), whereas the fluoride-
induced stepwise double deprotonation of the receptor LH2
was confirmed by titrating the receptor with standard [n-
Bu4N]OH (Supporting Information, Figure S17). The titration
spectra display consecutive development of bands at 522
(associated to monodeprotonation) and 628 nm (associated to
double deprotonation) after addition of 1 and 5 equiv of
standard [n-Bu4N]OH solution respectively.
As the spectral changes due to formation of the H-bonded
complex and the first deprotonation occur in the same
concentration range of added F−, the individual steps cannot
be analyzed in solution. Therefore, the apparent binding
constant of fluoride binding to LH2 is calculated on the basis of
the change in absorbance at 522 nm and considering a 1:1
binding isotherm; a value of log Kapp = 3.75 (error 10%) is
determined. The apparent binding constant Kapp is a combined
binding constant for both the hydrogen-bonded complex and
the first deprotonation steps. Moreover, the significantly high
fluoride-induced red shift (243 nm) of the receptor is due to its
planarity and conjugation, which help complete delocalization
of the negative charge(s) of the deprotonated receptor. The
fluoride-induced complex UV−vis spectral behavior of the
receptor LH2 also supports the unusual formation of a 1:1
fluoride complex with monodeprotonated receptor (LH−) in
the solid state.
Figure 5. Family of spectra taken over the course of the titration of a
4.6 × 10−6 M solution of LH2 upon incremental addition of [n-Bu4N]F
in CH3CN at 298 K: (a) from 0 to 4 equiv of [n-Bu4N]F; (b) from 4
to 20 equiv of [n-Bu4N]F.
induced color change of the receptor occurs in a ratiometric
fashion. Initially, at lower concentration of F ions, the light
yellow solution of LH2 turned pink at 4 equiv of F− addition
and then turned blue at higher F− equivalents (∼20 equiv)
(Figure 6). This significant change in the UV−vis spectrum of
1
Halide Binding Study by H NMR Spectroscopy. The
binding properties of the receptor LH2 with halides in solution
1
are investigated by H NMR experiments in d6-DMSO in the
presence of various halides as their [n-Bu4N]X salts (where X =
F, Cl, Br, and I) at room temperature. Figure 7 shows the
chemical shift changes found by addition of different halides to
the receptor LH2 in d6-DMSO. The most substantial changes
are observed for the −NH protons, indicating that the −NH
protons of receptor LH2 provides suitable sites of interaction
between the receptor and the halides.
Figure 6. Stepwise color change observed for the receptor LH2
(∼10−5) upon addition of different equivalents of standard [n-Bu4N]
F solution.
1H NMR titration of receptor LH2 with [n-Bu4N]F shows
that upon gradual addition of standard F− solution a small but
important change in the chemical shift of the aryl −CH protons
Δδ = 0.08 is observed, indicating fluoride induced a change in
the electronic environment of the −CH protons. In addition, a
significant disappearance of the −NH proton is observed. The
disappearance of the −NH proton is noticed even after the first
addition (0.25 equivalents) of fluoride ion (Supporting
Information, Figure S17). This is due to the binding induced
broadening of the −NH signals rather than deprotonation,
which is supported by the absence of a characteristic HF2− peak
receptor LH2 upon quantitative addition of fluoride anion
suggests that the interaction between the fluoride and LH2
takes place in three well-defined steps, similar to the sequence
previously reported.16a,17 In particular, upon gradual addition of
standard fluoride solution (1 × 10−3 M) to the solution of LH2
(4.6 × 10−6 M) the UV−vis absorption at 385 nm decreases
and a new peak at 522 nm appears with two distinct isosbestic
points at 426 and 335 nm up to 4 equiv of fluoride addition
(Figure 5a). We assume this spectral change is due to the
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Figure 9. Plot of the change in the chemical shift of the −NH proton
of LH2 with increasing amounts of [n-Bu4N]X in d6-DMSO at 298 K
(where X = Cl and Br).
Figure 7. 1H NMR spectra (400 MHz, d6-DMSO, 298 K) of LH2 and
change of NH resonance upon addition of F−, Cl−, Br−, and I− as their
tetrabutylammonium salts.
which suggests a host−guest binding in a 1:2 stoichiometry
(Supporting Information, Figure S24 and S25). The cumulative
binding constant for Cl− ion (log β = 3.92) is higher than that
of Br− (log β = 2.87), indicating that the receptor preferentially
binds chloride more than bromide. This is due to the higher
basicity and smaller ionic radii of chloride compare to bromide
anion. Detailed 1H NMR titration experiments showed that the
affinity order of different halides toward LH2 is Cl− > Br− > I−.
We kept the fluoride anion out of the halide binding affinity
order of the receptor LH2 because the spectroscopic results
demonstrate that apart from receptor−fluoride hydrogen-
bonded complex formation the fluoride anion also deproto-
nates the receptor in the same concentration range (4 equiv).
Therefore, it is difficult to get a real binding constant for
fluoride from this experiment.
at ∼16.0 ppm.16a Moreover, after addition of 4 equiv of F−, the
appearance of broad HF2− signals at 16.1 ppm suggests
deprotonation of the N−H protons of the receptor LH2
1
(Supporting Information, Figure S19). These results from H
NMR titration experiments preclude determination of a suitable
binding constant with fluoride, whereas significant downfield
shifts of the N−H proton Δδ = 0.84 for Cl− and Δδ = 0.15 in
the case of Br− are observed. There is no notable change in the
chemical shift of the NH protons with I−, indicating the
hydrogen-bonding interactions between receptor and the
1
iodide anion are energetically unfavorable. Subsequently, H
NMR titrations were carried out with Cl− and Br− in d6-DMSO
at 298 K. The 1H NMR titration spectra of the receptor LH2 in
the presence of increasing amounts of [n-Bu4N]Cl are shown in
Figure 8, whereas the titration spectra with [n-Bu4N]Br are
CONCLUSION
■
We systematically investigated the halide binding of a newly
synthesized anion receptor with a thiadiazole spacer LH2 both
in solution and in the solid state. The experimental results
corroborate that except iodide the receptor forms a hydrogen-
bonding complex with the other halides. Detailed crystal
structure analysis of the halide complexes ascertains that the
fluoride forms a 1:1 hydrogen-bonded complex with mono-
deprotonated receptor (LH−), while in the case of the other
halides (Cl− and Br−) the receptor LH2 binds two halides along
with formation of a halide-bridged 1D zigzag chain polymeric
network structure by participation of N−H···X− and aromatic
C−H···X− hydrogen-bonding interactions (where X = Cl and
Br), although no chloride- or bromide-directed polymeric
assembly are found in solution. Therefore, such structural
features are worthwhile only in the solid state. Moreover, the
presence of a thiadiazole spacer in the receptor significantly
affects the hydrogen-bonding nature between the receptor and
the halides, which in turn helps to form an anion-induced 1D
polymeric structure. This finding could motivate the develop-
ment of anion-directed coordination polymers.
1
Figure 8. Stack plot of the H NMR spectra of receptor LH2 in the
presence of increasing amounts of [n-Bu4N]Cl recorded in d6-DMSO
at 298 K.
given in the Supporting Information (Figure S22). The binding
stoichiometry between the receptor LH2 and both anions is
determined by Job plot experiments (Figure 9). The maximum
changes in the chemical shift during the titrations are obtained
when the mole fraction of both anions has reached about 0.67,
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents and tetrabutylammonium
salts were obtained from commercial sources and used as received.
Solvents were distilled freshly following standard procedures.
Instruments. IR spectra were recorded on a Perkin-Elmer-
Spectrum One FT-IR spectrometer with KBr disks in the range
887
dx.doi.org/10.1021/ic201656y | Inorg. Chem. 2012, 51, 882−889

Inorganic Chemistry
Article
4000−400 cm−1. UV−vis spectra were recorded with a Perkin-Elmer
Lambda-25 UV−visible spectrophotometer. NMR spectra were
recorded on a Varian FT-400 MHz instrument. Chemical shifts were
recorded in parts per million (ppm) on the scale using
tetramethylsilane (TMS) as a reference. ESI-MS spectra were recorded
in a WATERS LC-MS/MS system, Q-Tof Premier in the Central
Instrument Facility (CIF) of IIT Guwahati.
Synthesis of LH2. The receptor LH2 was synthesized by refluxing
the 1:2 mixture of 4-nitrophenylisothiocyanate and hydrazine hydrate
in the presence of acetic acid (catalytic amount) in EtOH for 24 h.
The precipitate was filtered and washed with 15 mL of ethanol five
times. An orange solid was obtained after drying the precipitate in a
vacuum (yield 62%).1H NMR (d6-DMSO) δ (ppm): 10.852(s, H−N),
8.216 (J = 8 Hz d, 4H), and 7.760 (J = 8 Hz, d, 4H). 13C NMR (d6-
DMSO) δ (ppm): 116.63, 125.604, 140.544, 146.668, and 156.064.
ESI mass spectrometry: calcd for 359.06. [M + H+]; found 359.06 [M
+ H+].
X-ray Crystallography. Intensity data were collected using a
Bruker SMART APEX-II CCD diffractometer, equipped with a fine
focus 1.75 kW sealed tube Mo Kα radiation (λ) 0.71073 (Å) at 298 K,
with increasing ω (width of 0.3° per frame) at a scan speed of 5 s/
frame. The SMART software was used for data acquisition. Data
integration and reduction were performed with SAINT and XPREP
software.19 Multiscan empirical absorption corrections were applied to
the data using the program SADABS.20 Structures were solved by
direct methods using SHELXS-9721a and refined with full-matrix least-
squares on F2 using the SHELXL-9721b program package. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
attached to all carbon atoms were geometrically fixed, while the
hydrogen atoms connected to the nitrogen atom were located from
the difference Fourier maps, and the positional and temperature
factors were refined isotropically. Structural illustrations have been
drawn with ORTEP-322a and MERCURY22b Windows.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic procedures, NMR, IR, LC-MS, UV−vis, and optical
micrograph images of crystals, and crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: +91-361-2582313. Fax: +91-361-2582349. E-mail:
gdas@iitg.ernet.in.
■
ACKNOWLEDGMENTS
■
We acknowledge DST (SR/S1/IC-01/2008) and CSIR (01-
2235/08/EMR-II), New Delhi, India, for financial support, CIF
and IIT Guwahati for providing the instrument facility, DST
FIST for the single-crystal X-ray diffraction facility, and S. K.
Dey for scientific discussion. A.B. thanks IIT Guwahati for a
fellowship.
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