Encapsulation of divalent tetrahedral oxyanions of sulfur within the rigidified dimeric capsular assembly of a tripodal receptor: First crystallographic evidence of thiosulfate encapsulation within neutral receptor capsule

Article abstract of DOI:10.1039/c2dt30999h

A simple tris(2-aminoethyl)amine based meta-chloro substituted tripodal thiourea receptor L has been extensively studied with two divalent oxyanions of sulfur, such as sulfate and thiosulfate, with identical dimensionality. The solid state crystal structure of the anion complexes with L reveal that the anions are encapsulated within the dimeric rigid capsular assembly of the receptor via N-H...anion interactions. To the best of our knowledge this is the first report on the encapsulation of thiosulfate within dimeric capsular assembly of a neutral receptor. The tight capsular sizes for both anion complexes are quite comparable, whereas the coordination mode of the anions and the hydrogen bonding parameters are significantly varied. The three dimensional solid state structural orientations of the capsular complexes are mainly governed by the Cl...Cl (for thiosulfate complex) and Cl...S (for sulfate complex) halogen bonding interactions. The solution-state binding and encapsulation of oxyanions by N-H...anion hydrogen bonding has also been confirmed by quantitative ¹H NMR titration and 2D NOESY NMR experiments. Both the experiments confirm that in contradiction of 2:1 solid state binding, in solution the studied anions are bound within the pseudocavity of the receptor with 1:1 binding stoichiometry. Moreover, the change in chemical shifts of thiourea -NH protons and the binding constant values suggest the receptor-sulfate interaction is more energetically favorable compared to the receptor thiosulfate interaction.

Full text of DOI:10.1039/c2dt30999h

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PAPER
Encapsulation of divalent tetrahedral oxyanions of sulfur within the rigidified
dimeric capsular assembly of a tripodal receptor: first crystallographic
evidence of thiosulfate encapsulation within neutral receptor capsule†
Arghya Basu and Gopal Das*
Received 8th May 2012, Accepted 22nd June 2012
DOI: 10.1039/c2dt30999h
A simple tris(2-aminoethyl)amine based meta-chloro substituted tripodal thiourea receptor L has been
extensively studied with two divalent oxyanions of sulfur, such as sulfate and thiosulfate, with identical
dimensionality. The solid state crystal structure of the anion complexes with L reveal that the anions are
encapsulated within the dimeric rigid capsular assembly of the receptor via N–H⋯anion interactions. To
the best of our knowledge this is the first report on the encapsulation of thiosulfate within dimeric capsular
assembly of a neutral receptor. The tight capsular sizes for both anion complexes are quite comparable,
whereas the coordination mode of the anions and the hydrogen bonding parameters are significantly
varied. The three dimensional solid state structural orientations of the capsular complexes are mainly
governed by the Cl⋯Cl (for thiosulfate complex) and Cl⋯S (for sulfate complex) halogen bonding
interactions. The solution-state binding and encapsulation of oxyanions by N–H⋯anion hydrogen
bonding has also been confirmed by quantitative 1H NMR titration and 2D NOESY NMR experiments.
Both the experiments confirm that in contradiction of 2 : 1 solid state binding, in solution the studied
anions are bound within the pseudocavity of the receptor with 1 : 1 binding stoichiometry. Moreover, the
change in chemical shifts of thiourea –NH protons and the binding constant values suggest the receptor–
sulfate interaction is more energetically favorable compared to the receptor thiosulfate interaction.
solvent media and thereby leads to the molecular sorting
Introduction
phenomena, when there is a possibility of formation different
capsular assembly in the same solution.8 The high solvation
The field of anion coordination chemistry continues to expand
with new synthetic molecules capable of recognizing anions
which are not only within the interest of supramolecular chem-
istry but also have vast significance in environmental and clinical
applications.1,2 The observations in natural anion binding
systems have motivated researchers to develop several neutral
receptors that employ hydrogen bonds offered by specific
binding sites from amides,3 urea/thiourea,4 pyrroles,5 and
indole6 functionalities for the recognition and binding of size-
and shape-selective anionic guests on appropriate frameworks in
various media. Acyclic receptors with multi-armed hydrogen-
bonding functionality have been frequently found to coordinate
with targeted anionic species via the formation of monomeric
and dimeric capsular assemblies.7 The most interesting features
of molecular capsules is their ability to create an anion specific
cavity that isolates the encapsulated guest from the bulk of the
energies of anions must be compensated for by the host for
effective anion recognition in competitive media.9 In this
context, the structure of the receptor requires a tailored design
where the receptor satisfies the higher coordination numbers
required for the binding of anions and hydrated anions. Tris-
(2-aminoethyl)amine-based (tren-based) urea/thiourea functiona-
lized tripodal scaffolds offer a flexible and structurally preorga-
nized cavity, which has been widely employed in the binding
and recognition of anions because of their favorable confor-
mation for multiple hydrogen bonds that favor the formation of a
stable host–guest complex.10
Among the various oxyanions, the harmful effect of sulfate
(tetrahedral oxyanion) has been recognized as a prominent
species of concern in cleanup processes of nuclear waste and
hard water; e.g., contamination of nuclear waste sites by this
anion has been a matter of increased concern, hampering the
vitrification process.11 Because of its large standard Gibb’s
energy of hydration (−1080 kJ mol−1), extraction of sulfate ions
from an aqueous to an organic phase presents a particularly chal-
lenging task.10b To defeat this problem the receptor must have
both exceptional affinity and selectivity for the sulfate ion. In
this regard the most promising approach can be obtained from
Nature’s sulfate-binding protein, where sulfate isolation from the
Department of Chemistry, Indian Institute of Technology, Guwahati,
India. E-mail: gdas@iitg.ernet.in; Fax: +91–361–2582349;
Tel: +91-361-2582313
†Electronic supplementary information (ESI) available: Additional crys-
tallographic data; characterization data; PXRD of complexes; 1H NMR
titration spectra. CCDC reference numbers 879915 (1) and 879916 (2).
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt30999h
10792 | Dalton Trans., 2012, 41, 10792–10802
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surrounding solvent is achieved by encapsulation of the anion
inside hydrophobic cavities functionalized with suitable binding
groups. The crystal structure of SBP reveals that an individual
sulfate anion is completely encapsulated within the core of the
protein (8 Å below the surface), between the two lobes of SBP
through seven hydrogen bonds involving five from peptide –NH
groups, one from serine –OH group, and the last from the trypto-
phan –NH group. In recent years the tren-based tripodal trisurea/
thiourea backbone has also been found to encapsulate the sulfate
ion in a 1 : 1 or 2 : 1 (host–guest) ratio, and some of them have
been efficiently employed in sulfate-ion separation based on
liquid–liquid anion-exchange technology or competitive crystal-
lization techniques.10a,e,m In recent review, Ghosh and co-worker
give a nice account of sulfate recognition and extraction by
various synthetic receptors.10l
capsular assemblies for both anions are quite comparable,
whereas the coordination mode of the anions and the hydrogen
bonding parameters are noticeably varied. The three dimensional
solid state structural orientations of the capsular complexes are
mainly governed by the Cl⋯Cl and Cl⋯S halogen bonding
interaction, which is directional in nature and has two types of
preferred geometries, as nicely generalized by Desiraju and Guru
Row.14 Moreover, the receptor anion solution state interactions
are also studied in detail by NMR experiments in DMSO-d6 at
RT. Interestingly, in contradiction of 2 : 1 solid state binding, the
results from solution state NMR experiments confirm that both
the anions are bound within the pseudocavity of the receptor L
with 1 : 1 binding stoichiometry.
Another tetrahedral sulfur-containing oxyanion is thiosulfate,
widely used (as the sodium salt) in different fields, for instance,
the photographic industry, analytical chemistry (iodometric titra-
tion), paper making, gold extraction and also useful in a surpris-
ingly broad range of clinical situations.12 Moreover, sodium
thiosulfate has been safely used as a therapeutic agent for a long
time (almost 100 years). Nowadays it is widely used as an anti-
dote for the treatment of cyanide poisoning,12b by converting
cyanide to thiocyanate (excreted in the urine), catalyzed by the
enzyme rhodanase and also found useful in prevention of the
toxicity of cisplatin in cancer therapy. It reacts with free radicals
(oxygen) to form a sodium sulfate compound which prevents the
radicals from destroying or attacking the living cells. Addition-
ally, it has also been used as potential remedy of renal dis-
eases12c and anti-fungal (tinea versicolor) infections. These
versatile applications of the thiosulfate anion make it an impor-
tant target analyte for recognition. In this regard the slow release
or transport of thiosulfate anion in the specific target site via
encapsulation would be a promising approach in its clinical
application.
Experimental
Materials and methods
All reagents were obtained from commercial sources and used as
received. Solvents were distilled freshly following standard
procedures. Tris(2-aminoethyl)amine (tren), 3-chlorophenyl iso-
cyanate, and tetraalkylammonium salts were purchased from
Sigma-Aldrich and used as received. Solvents for synthesis and
crystallization experiments were purchased from Merck, India,
and used as received.
Instruments
IR spectra were recorded on a Perkin-Elmer-Spectrum One
FT-IR spectrometer with KBr disks in the range 4000–400 cm−1
.
NMR spectra were recorded on a Varian FT-400 MHz instru-
ment. Chemical shifts were recorded in parts per million (ppm)
on the scale solvent peak as 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.
In our ongoing effort in the field of anion receptor chemistry,13
herein, we report the solid and solution state binding of two
tetrahedral sulfur containing oxyanions (sulfate and thiosulfate)
of a chloro-substituted tris(thiourea) receptor, L (Scheme 1), in
DMSO. The solid state crystal structure of both the anions with
L reveals that the anions are encapsulated within the dimeric
capsular assembly of the receptor with optimal N–H⋯O and
N–H⋯S hydrogen bonding coordination. The sizes of tight
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 per frame. The
SMART software was used for data acquisition. Data integration
and reduction were performed with SAINT and XPREP soft-
ware.15 Multiscan empirical absorption corrections were applied
to the data using the program SADABS.16 Structures were
solved by direct methods using SHELXS-9717a and refined with
full-matrix least squares on F2 using the SHELXL-9717b
program package. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms attached to all carbon and nitrogen
atoms were geometrically fixed, and the positional and tempera-
ture factors were refined isotropically. Structural illustrations
have been drawn with MERCURY18 for Windows. A summary
of the crystal data and relevant refinement parameters are
given in Table 1. CCDC 879915 (1) and 879916 (2) are
contained the supplementary crystallographic data for this
paper.†
Scheme 1 Molecular structure of a tris(thiourea) receptor, L.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 10792–10802 | 10793

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Table 1 Crystal parameters and refinement data
NMR studies
1H NMR titration studies were done to determine the binding
constants of L for sulfate and thiosulfate in DMSO-d6 at room
temperature. Initial concentrations were [ligand]0 = 5 mM, and
Compound
Complex 1
Complex 2
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
γ/°
C86·H132·Cl6·N16·O4·S7 C70·H100·Cl6·N16·O3·S8
1891.27
Triclinic
ˉ
P1
1682.92
Monoclinic
C2/c
13.7854(14)
25.836(3)
48.851(5)
90.00
95.818(6)
90.00
17309(3)
8
[anion]0
= 50 mM. Each titration was performed by
10–12 measurements at room temperature. The association con-
stant K was calculated by fitting two NH signals with a 1 : 1
binding model, using the equation Δδ = ([A]0 + [L]0 + 1/K −
(([A]0 + [L]0 + 1/K)2 − 4[L]0[A]0)1/2)Δδmax/2[L]0 (where L is
the ligand and A is the anion).19 The error limit in K was less
than 10%.
13.8087(7)
14.3169(7)
26.6346(14)
93.887(3)
98.290(3)
97.087(3)
5151.1(5)
2
V/Å
Z
T (K)
298(2)
0.361
1.130
0.32 × 0.028 × 0.26
25 686
298(2)
0.444
1.292
0.30 × 0.29 × 0.25
21 045
μ (cm−1
)
Synthesis and characterization
dcal/g cm−3
Cryst dimens/mm3
No. of reflns
collected
Receptors L. Tripodal receptor L was synthesized by slight
modification of a reported literature procedure13e where the reac-
tion of tris(2-aminoethyl)amine (tren) with 3-chlorophenyl iso-
thiocyanate in a 1 : 3 molar ratio at room temperature yielded the
receptor in quantitative yield. A total of 1.98 g (3 mmol) of
3-chlorophenyl isothiocyanate was dissolved in 30 mL of dry
tetrahydrofuran (THF) in a 100 mL round-bottomed flask and
0.146 ml (1 mmol) of tren dissolved in 10 mL of dry THF were
added dropwise over a period of 15 min with constant stirring at
No. of unique reflns 25 645
20 954
846
0.1951, 0.3113
0.0945
1.062
No. of params
R1; wR2 (I > 2σ(I))
R(int)
1092
0.2527, 0.2990
0.0984
GOF (F2)
1.173
CCDC no.
879915
879916
2−
Fig. 1 (a) Space-filling representation depicting full encapsulation of the SO4 anion. (b) Sulfate encapsulation by the crystalline self-assembled
capsules L, Two molecules of L, shown as stick models and sulfate are shown as a space-filling model. (c) Ball-and-stick presentation depicting the
15 hydrogen-bonding contacts on SO42− within the dimeric capsule of L. (d) Magnified view showing coordination of SO42− with the 12 –NH groups
of the dimeric capsule. TBA countercations are omitted for clarity of presentation.
10794 | Dalton Trans., 2012, 41, 10792–10802
This journal is © The Royal Society of Chemistry 2012

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2−
2−
Table 2 Hydrogen-bonding contacts on SO4 and S2O3 anions
within the dimeric cage of L in complexes 1 and 2
room temperature. The resulting solution mixture was stirred
12 hours at room temperature. Then, the volume of the solvent
(THF) was reduced in vacuo by using rotary evaporator, and the
obtained solid product was filtered off and washed with 10 mL
of dichloromethane a couple of times to remove the unreacted
starting materials and impurities. The colorless precipitate was
collected and dried in air and characterized by NMR, FT-IR,
ESI-MS. Yield 88%.
D–H⋯A
H⋯A
D⋯A
D–H⋯A/°
Complex 1
N3H⋯O1
N4H⋯O1
N5H⋯O1
N6H⋯O1
N2H⋯O2
N5H⋯O2
N6H⋯O2
N7H⋯O2
N2H⋯O3
N3H⋯O3
N4H⋯O3
N7H⋯O3
N2H⋯O4
N4H⋯O4
N6H⋯O4
N9H⋯O5
N10H⋯O5
N11H⋯O5
N13H⋯O5
N10H⋯O6
N12H⋯O6
N13H⋯O6
C36H⋯O6
N9H⋯O7
N10H⋯O7
N11H⋯O7
N12H⋯O7
N14H⋯O7
N10H⋯O8
N11H⋯O8
N14H⋯O8
Complex 2
N4–H⋯O1
N5H⋯O1
N7H⋯O1
N12H⋯O1
N3H⋯O2
N5H⋯O2
N2H⋯O2
N14H⋯O2
C18H⋯O2
N9H⋯O3
N11H⋯O3
N12H⋯O3
N13H⋯O3
N14H⋯O3
N3H⋯S8
N6H⋯S8
N7H⋯S8
N10H⋯S8
2.313
2.209
2.63
3.016(9)
3.007(8)
2.977(9)
3.09(1)
3.343(9)
3.010(9)
3.388(9)
2.92(1)
3.275(9)
2.92(1)
3.21(1)
3.041(9)
2.923(9)
3.05(1)
2.89(1)
2.82(1)
3.39(1)
2.833(9)
2.881
2.89(1)
3.13(1)
3.131(9)
3.16(1)
2.918(9)
3.40(1)
3.330(9)
2.92(1)
3.142(9)
3.10(1)
2.97(1)
3.00(1)
139.0(3)
154.1(4)
157.9(4)
139.9(4)
141.4(3)
141.9(4)
144.4(3)
169.3(4)
142.4(3)
170.8(4)
140.7(4)
139.9(4)
177.7(4)
163.2(4)
179.1(4)
140.7(4)
138.1(4)
146.4(5)
144.2(5)
143.2(4)
151.0(5)
171.7(5)
140.3(5)
171.9(4)
150.3(4)
136.2(4)
160.0(5)
162.9(6)
163.2(4)
164.1(5)
151.0(6)
2.384
2.628
2.286
2.651
2.071
2.551
2.068
2.501
2.333
2.023
2.22
1
Melting point: 183 °C, H NMR (DMSO-d6) δ (ppm): 9.73
(s, H–N), 7.81 (s, H–N) 7.66 (s, 1H), 7.31 (m, 2H), 7.128
(t, 1H), 3.50 (d, 2H) 2.74 (d, 2H), 13C NMR (DMSO-d6)
δ (ppm): 41.91, 52.00, 121.11, 122.23, 123.65, 130.18, 132.77
140.95 and 180.30. FT-IR (ν, cm−1): 1589 (CvC), 1556 (CvS
sym), 766 (CvS, asym) 3227 (N–H), 3326 (N–H), ESI(+ve)
mass spectrometry: 656.28.
2.03
2.11
2.70
Synthesis and characterization of complex 1 2TBA[2L·(SO42−)].
Sulfate-encapsulated complex 1 was obtained by mixing L and
(n-TBA)2SO4/(n-TBA)HSO4. In both the cases, 50 mg of L was
dissolved in 5 mL of DMSO in a 25 mL round-bottomed flask.
In the case of the (n-TBA)2SO4 salt, 3 mL of (n-TBA)2SO4
(50 wt% in water) was added at once to the 5 mL of L solution
whereas, in other case 25 mg of n-TBAHSO4 was added at a
time to the 5 mL of L. Then in both cases, the mixtures were
stirred for 1 hour at room temperature and allowed to crystallize
at room temperature in open test tubes. From both the solutions,
colorless crystals of the sulfate complex of L, [2L(SO42−)]·
2TBA (1), suitable for X-ray diffraction were obtained after
seven–ten days. Isolated yield of 1 is (31 mg) 88%
2.076
2.140
2.154
2.357
2.277
2.386
2.065
2.625
2.655
2.098
2.310
2.263
2.131
2.214
Melting point: 184 °C, 1H NMR (DMSO-d6) δ (ppm):
10.36 (s, H–N), 8.69 (s, H–N) 7.82 (s, br, 3H), 7.38 (s, br, 3H),
7.17 (s, br, 3H), 7.026 (s, br, 3H) 3.50 (s, br, 6H), 3.13
(s, br, 8H) 2.59 (s, br, 6H), 1.53 (s, br, 8H) 1.28 (s, br, 8H)
0.90 (s, br, 12H) 13C NMR (DMSO-d6) δ (ppm): 13.49,
19.22, 23.08, 41.91, 53.29, 57.598, 120.90, 122.07, 122.91,
129.37, 132.21 141.69 and 180.24 FT-IR (ν, cm−1): 1085
(SO42−), 1604 (CvC), 1538 (CvS sym) 784 (CvS, asym),
3277 (N–H, br).
2.358
2.208
2.312
2.102
2.112
2.694
2.580
2.239
2.679
2.228
2.192
2.705
2.249
2.645
2.938
2.457
2.890
2.486
3.16(1)
3.05(1)
3.03(2)
2.94 (1)
2.97(7)
3.36(1)
3.34(1)
3.02(1)
3.51(2)
3.00(1)
2.96(1)
3.40(1)
3.04(1)
3.41
154.4(6)
164.3(7)
141(1)
162.8(7)
172.4(7)
134.6(6)
147.3(7)
151.6(7)
149(1)
149.8(7)
148.9(7)
138.4(7)
153.4(6)
148.4(7)
121.2(7)
163(1)
Synthesis and characterization of complex 2 2TEA[2L·(S2O32−)].
Thiosulfate-encapsulated complex 2 was obtained by charging
previously prepared 5 ml aqueous solution containing equimolar
mixture of (n-TEA)Cl and Na2S2O3 (10 equivalent each) into a
8 mL DMSO solution of L (50 mg). Then the mixture was
stirred for 15 min at room temperature and warmed at 60 °C for
5 min. After cooling to room temperature, the resulting solution
was filtered using a filter paper. Filtrate was collected in 20 mL
test tube and allowed to crystallize at room temperature. Color-
less crystals suitable for single-crystal X-ray crystallographic
analysis of [2L(S2O32−)]·2TEA are obtained after ten days. Iso-
lated Yield of 2 is (41 mg) 65%.
3.46(1)
3.29(1)
3.67(2)
3.30(1)
152(1)
156.9(7)
Results and discussion
For a receptor to bind with the anionic guests, it should, in prin-
ciple, possess preorganized directional H-bond donors tailored
on a suitable platform/framework. Receptor L possesses a highly
organized tripodal scaffold with three hydrogen-bonding
thiourea functions appropriate for anion encapsulation via their
optimal hydrogen bonding coordination. Efforts are made to find
out the binding similarities and dissimilarities of receptor L
towards two divalent tetrahedral oxyanions of sulfur (sulfate and
thiosulfate) both in the solid and solution state. From the view-
point of anion coordination chemistry, crystallization has
1
Melting point: 134 °C, H NMR (DMSO-d6) δ (ppm): 10.30
(s, H–N), 8.53 (s, H–N), 7.83 (s, 3H) 7.45 (d, 3H), 7.22 (t, 3H),
7.05 (d, 3H), 3.53 (s, 6H), 3.16 (q, 8H) 2.64 (d, 6H), 1.18 (t,
12H) 13C NMR (DMSO-d6) δ (ppm): 7.10, 42.05, 51.47, 53.35,
121.12, 122.12, 123.04, 129.61, 132.30, 141.65 and 180.15.
FT-IR (ν, cm−1): 1095 (S2O32−), 1590 (CvC), 1531 (CvS
sym) 778 (CvS, asym), 3277 (N–H, br).
This journal is © The Royal Society of Chemistry 2012
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traditionally been a route to understand the structural details of
the anion complexes formed, primarily by single-crystal XRD
analysis. Fortunately, we were able to isolate single crystals of
both sulfate and thiosulfate complexes of the receptor L, suitable
for X-ray crystallographic analysis from individual solutions of
L in presence of the respective anions. It is interesting to observe
that both the anions are entrapped within the rigid supramolecu-
lar dimeric capsular assembly of the receptor L via N–H⋯anion
interactions.
the strong hydrogen bonding interaction region of dH⋯O < 2.5 Å
and dN⋯O ≤ 3.2 Å there are thirteen contacts (Fig. 5a). The
second capsular unit present in the crystal lattice is quite similar
to the first one, only the receptor anion hydrogen bonding para-
meters are slightly varied (Table 2). A similar type of sulfate
encapsulation in a metal free system by tris thiourea based recep-
tors was previously reported by Gale and co-workers.10i Interest-
ingly, the sulfate-encapsulated dimeric cages are interlinked with
one another through halogen-bonding interactions between the
sulfur atom of the thiourea group and the meta-substituted
chloride atom of the phenyl ring, with a separation distance of
3.482 (3) Å (x,y,z), and which subsequently form a 1D chain
capsular assembly along the crystallographic a axis. Two such
1D arrays of capsular assemblies are further interconnected with
one another via TBA cations by C–H⋯S interactions, and gen-
erate hexagonal networks of sulfate-encapsulated dimeric cages
around each capsular unit along the b axis (Fig. 2). These added
interactions contribute to the high stability of 1 (MP = 184).
Moreover the presence of hydrogen bonded sulfate anions in
complex 1 has also been confirmed by solid-state FT-IR analysis.
Structural description of complex 1
We attempted to grow single crystal of complexes of L with both
HSO4− and SO42− by charging their excess n-TBA salt. Interest-
ingly in both cases the colorless crystals of the sulfate complex
of L crystallize with good yield. They crystallize in the triclinic
ˉ
space group P1. Two identical symmetric molecules of L flipped
inward toward each other in a face-to-face fashion (dN1⋯N1 =
9.334 (6) Å) form a micro-cavity that encapsulates a sulfate
anion (disordered, eight half occupied oxygen atoms) in its
centre via hydrogen bonding to the six thiourea groups (Fig. 1).
The asymmetric unit of complex 1 contains two symmetry-inde-
pendent capsular units, exhibiting conformational isomorphism,
their occurrence generally controlled by kinetic and thermo-
dynamic crystal stability because these factors are mostly con-
sidered to be the consequences of interrupted crystallization.
Both the capsular units are almost identical. In the first capsular
unit the encapsulated sulfate anion is stabilized by a total of
fifteen hydrogen bonding interactions between the twelve NH
groups of two L moieties and four O atoms of SO42−. Three out
of the four oxygen atoms O1, O2 and O3 accept four hydrogen
bonds each, while O4 accepts three hydrogen bonds in a trifur-
cated fashion. Apart from these hydrogen bonding interactions,
there are also some weak CPhenyl–H⋯O interactions present,
which give extra stabilization to the encapsulated sulfate anion.
Moreover, a close inspection of the hydrogen-bond parameters,
especially the N–H⋯O angle vs. H⋯O distances, reveal that in
The stretching frequency of –NH in complex 1 (ν 3277 cm−1
)
shows a notable shift of 65 cm−1 with subsequent broadening
of the peak in comparison to that of L (ν 3341 cm−1), supporting
the existence of strong N–H⋯O hydrogen bonds between L and
the SO42− anion. Furthermore, the presence of a moderate signal
at 2873 cm−1 and a strong signal 1085 cm−1 in complex 1 can
be attributed to the C–H stretching frequencies of the TBA
groups and symmetric stretching frequency of the sulfate anion
(Fig. S8, ESI†). Characteristically, the broad intense symmetric
absorption band at ∼1085 cm−1 is generally used to identify the
presence of sulfate in individual complexes. The powder X-ray
diffraction studies on bulk crystals obtained from both in the
presence of SO4− and HSO4− matches closely with the simulated
diffraction pattern obtained from the single-crystal structure of
complex 1 suggesting that, even in the presence of HSO4−, the
sulfate complex of the receptor L crystallizes (Fig. S13, ESI†).
Moreover, the formation of the sulfate complex even in pres-
−
ence of HSO4 anions can be attributed to the coordination
Fig. 2 (a) The Cl⋯S halogen bonding interaction between two capsular units in complex 1. (b) Halogen bonding directed 1D capsular assembly
along crystallographic a axis. (c) Crystal packing in complex 1, as viewed down the crystallographic b axis.
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2−
Fig. 3 (a) Space-filling representation depicting full encapsulation of the S2O3 anion. (b) Thiosulfate encapsulation by the crystalline self-
assembled capsules L, Two molecules of L, shown as stick models, and thiosulfate are shown as a space-filling model. (c) Ball and stick presentation
depicting the 14 hydrogen-bonding contacts on S2O32− within the dimeric capsule of L. (d) Magnified view showing coordination of S2O32− with the
12 –NH groups of the dimeric capsule. TEA countercations are omitted for clarity of presentation.
Fig. 4 (a) The Cl⋯Cl halogen bonding interaction between two capsular units in complex 2. (b) Halogen bonding directed 1D capsular assembly
along the crystallographic a axis (c) Crystal packing in complex 2, as viewed down the crystallographic bc plane.
−
induced proton transfer between the free and bound anions; such
solution-state deprotonations of the protonated state of an anion,
viz., H2PO4−, HCO3−, and HSO4 are quite well known in the
literature.10d Essentially the formation of several hydrogen-
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bonding interactions with the receptor significantly lowers the
pKa of the bound guest, which is eventually deprotonated by the
free guest species present in solution.
Structural description of complex 2
The complex 2 was synthesized from the reaction between
DMSO solutions of L and previously prepared aqueous mixture
of TEACl and Na2S2O3. Interestingly, the thiosulfate complex of
L with a TEA counter cation was crystallized from slow evapor-
ation of reaction mixture, suggesting geometric and electrostatic
2−
complementarity between S2O3 and L that prefers the for-
mation of a thiosulfate complex rather than a chloride complex.
It is worth mentioning here that efforts were also made to crystal-
2−
lize an S2O3 complex in presence of only Na2S2O3, but these
were not fruitful presumably because the thiosulfate encapsu-
lated receptor segment does not prefer hydrophilic Na+ or the
hydrated Na+ cation. This is also supported by the presence of a
TEA counter cation instead of an Na+ cation or hydrated Na+
cation in the crystal lattice of complex 2, while, it was crystal-
lized from the mixture of both the Na+ and TEA cations. The
complex 2 crystallizes in the monoclinic system with centrosym-
metric space group C2/c. Structural elucidation reveals two
inversion-symmetric molecules of L are flipped inward toward
each other in a face to face fashion [d(N1⋯N8) = 10.12 (1) Å;
(Fig. 3), with one ligand coordinating in the axial mode and the
other in the facial mode, and thereby creating a micro cage
supramolecular structure that encapsulates a thiosulfate anion in
its centre via N–H⋯O and N–H⋯S hydrogen bonds by the six
thiourea groups and the two receptors are assembled by π⋯π
interactions of the chloro-phenyl rings (C1g⋯C6g = 3.755 Å
and C2g⋯C5g = 3.996 Å). The capsular size of complex 2
(10.12 Å) is slightly larger (∼0.8 Å) compared to complex 1
(9.33 Å), which may attributed to the larger size and lower
charge density of the thiosulfate anion compared to sulfate. In
addition to this, close inspection of complex 2 reveals that the
N–H atoms of the thiourea functionalities are more directed
towards three oxygen atoms of the encapsulated thiosulfate
anion, resulting in the outer S8 atom of the thiosulfate anion
being slightly out of the capsular cage, probably due to the
Fig. 5 (a) The scatter plot of the N–H⋯A angle vs. H⋯A distance of
the hydrogen bonds for: (a) complex 1; and (b) complex 2. A = Acceptor
(O\S); circled points indicate N–H⋯S interactions.
Fig. 6 Partial 1H NMR spectra (400 MHz, DMSO-d6) of L and maximum observable shifts of thiourea –NH resonances upon the addition of excess
(5 equiv.) SO42− and S2O32− anions.
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larger S7–S8 bond distance and the lower charge density over
the S8 atom compared to the three oxygen atoms. The TEA
counter cations of complex 2 are disordered, and one TEA
counter cation equally shares two positions in the asymmetric
unit. There are thirteen N–H⋯O and four N–H⋯S hydrogen
bonds of six thiourea groups that are responsible for stabilizing
the encapsulated thiosulfate anion (Fig. 4). Two out of three
oxygen atoms O1, and O2 accepts four hydrogen bonds each,
while O3 accepts five hydrogen bonds. A correlation of the
N–H⋯O angle vs. H⋯O distance (Fig. 5a) shows that in the
strong hydrogen bonding interaction region of dH⋯O < 2.5 Å and
dN⋯O ≤ 3.2 Å, there are nine contacts stabilizing three oxygen
atoms of the encapsulated thiosulfate anion. When compared to
N–H⋯O interactions, the N–H⋯S interactions are relatively
weak. The average values of the N⋯S distance and N–H⋯S
angle are 3.43 Å and 148.2°, respectively. To the best of our
knowledge this is the first report on the full encapsulation of
thiosulfate anion within dimeric capsular assembly of a neutral
receptor, although the first encapsulated thiosulphate complex
within protonated cryptant host was previously reported by
Nelson and co-workers.20
The thiosulfate-encapsulated dimeric cages are interlinked
with one another through Cl⋯Cl (3.31(1) Å, 1/2 + x,1/2 + y,z)
halogen bonding interactions, forming a 1D chain polymeric
structure along the crystallographic a axis. Two such 1D arrays
of capsular assemblies are further interconnected with one
another by weak C–H⋯S interactions, forming a 2D layer struc-
ture along the crystallographic b axis (Fig. 4). Moreover, the
presence of a hydrogen bonded thiosulfate anion in complex 2
has also been confirmed by solid-state FT-IR analysis. Analo-
gous to complex 1 here also a significant shift (55 cm−1) with
subsequent broadening in the –NH signal compared to that of L
is observed, indicating the presence of strong N–H⋯anion inter-
action between the thiourea functionalities of L and the thiosul-
fate anion. Additionally, the presence of a moderate signal at
1075 cm−1 can be attributed to the stretching frequency corres-
ponding to the thiosulfate anion (Fig. S11, ESI†). As the
complex 2 crystallizes in presence of more than one anion, there-
fore, it is required to verify the homogeneity of the isolated bulk
crystal. The powder X-ray diffraction studies on isolated bulk
crystals match perfectly with the simulated diffraction pattern
obtained from the single-crystal structure of complex 2, indicat-
ing the homogeneity of the isolated crystals of the thiosulfate
capsules (Fig. S14, ESI†).
Fig. 7 1H NMR titration curves of L with (a) SO4 and (b) S2O3
anions in DMSO-d6 at RT. Net changes in the chemical shifts of –NH
2−
2−
are shown against the increasing amount of anion (50 mM). Ha
CH2NHCS and Hb = CSNHAr.
=
that the –NH functions of the thiourea groups provide suitable
Anion binding study by 1H NMR spectroscopy
sites of interaction between the receptor and anions in solution.
2−
It is now well known in the field of supramolecular chemistry
that the behavior of molecules/receptors in dilute or very dilute
solution is really quite dissimilar from their behavior in molecu-
lar capsules in the solid state. Therefore, to find the mode of
receptor–anion interaction in solution we have carried out quali-
tative as well as quantitative 1H NMR titration and NOESY
experiments in DMSO-d6 at RT. Fig. 6 illustrates the qualitative
test of both the anions, which shows the changes in chemical
shift observed upon addition of 5 equivalents of each anion at
once to the tris(thiourea) receptor L in DMSO-d6 at RT. This pre-
liminary study reveals the most substantial shifts have been
observed for the thiourea protons (–NHa and –NHb), indicating
1H NMR titration of receptor L with [n-TBA]SO4 shows
2−
that upon gradual addition of standard SO4 solution a large
downfield shift of thiourea –NH resonances (Δδ –NHa
=
1.666 ppm; Δδ –NHb = 1.267 ppm) and a small but important
change in the chemical shift with subsequent splitting of the aryl
–CH protons Δδ = 0.227, could be explained by the fact that the
sulfate anion also significantly interacts with the C–H protons in
solution (Fig. S6, ESI†). The considerably larger shift of –NHa
(Δδ = 1.66 ppm) relative to –NHb signals (Δδ = 1.267 ppm) indi-
cates there is an obvious discrepancy between the –NHa⋯sulfate
and –NHb⋯sulfate binding modes, whereas, no such binding
discrepancy of –NH protons was found in the solid state structure
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of complex 1. The change in the chemical shift of the –NH
resonances of the NMR spectra, as recorded with an increasing
amount of sulfate anion in solution at room temperature, gave
the best fit for a 1 : 1 binding model (log K = 4.54), which are
well consistent with the report of Gale for fluoride-substituted
tris(thiourea) receptors.10i The 1 : 1 binding stoichiometry in
DMSO-d6 were further verified by the Job’s plot analysis. The
maximum change in the chemical shift during titrations for L is
obtained when the mole fraction of sulfate anion has reached
about 0.5, which suggests a host–guest binding in a 1 : 1
stoichiometry (Fig. S7, ESI†). Despite this, the crystal structure
obtained for complex 1 revealed 2 : 1 complex stoichiometry.
Furthermore, 1H NMR titration of L with standard sodium
thiosulfate solution also occurs downfield shift of thiourea –NH
resonances (Δδ –NHa = 1.18 ppm; Δδ –NHb = 0.97 ppm). Inter-
estingly the degree of downfield shift of both the –NH reson-
ances is lesser compared to the sulfate anion (Fig. 6). Here also
there is a considerably larger shift of –NHa (Δδ = 1.18 ppm) rela-
tive to the –NHb signals (Δδ = 0.97 ppm), indicating
–NHa⋯anion interactions are more energetically favorable than
NHb⋯anion interactions (Fig. S10, ESI†). However, the solid
state crystal structure of complex 2 shows equal participation
from both the thiourea protons toward the thiosulfate anion
within the rigid dimeric capsular cage of L. Moreover, the sig-
nificant shift with concomitant splitting of the aryl –CH protons
indicates that thiosulfate induces a change of the electronic
environment of the phenyl rings of the receptor L. The associa-
2−
tion constants (log K) of L with S2O3 were calculated from
Fig. 8 2D NOESY NMR experiments of: (a) free receptor L; (b) L in presence of 1 equiv. of sulfate anion; and (c) L in presence of 1 equiv. of thio-
sulfate anion. All the spectra are taken in DMSO-d6 at 298K.
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Fig. 9 Proposed binding mode of the receptor L for sulfate and thiosulfate anion in solution.
quantitative titration experiment and found to be log K = 3.35 by
considering the 1 : 1 binding model, because no data fit for the
Job plot experiment was obtained due to complex precipitation
during the titration experiment. The comparatively large Δδ
values for thiourea protons and the higher binding constant
value of the sulfate anion can be attributed to the smaller size
and higher charge density of sulfate anion compared to thio-
sulfate (Fig. 7).
Cl⋯Cl (for the thiosulfate complex) and Cl⋯S (for the sulfate
complex) halogen bonding interactions, giving added stabiliz-
ation to the anion complexes. The solution-state binding and
encapsulation of oxyanions by N–H⋯anion hydrogen bonding
1
has also been confirmed by quantitative H NMR titration and
2D NOESY NMR experiments. The change in the chemical
shifts of the thiourea –NH protons and the binding constant
values suggests the receptor binds more strongly to sulfate
anions compare to thiosulfate. Furthermore, the 2D NOESY
NMR and Job’s plot experiments suggest that in solution the
anions SO42− and S2O32− are encapsulated in the pseudocapsular
cavity of L with 1 : 1 binding stoichiometry, indicating an
obvious disagreement of the binding mode from that observed in
their solid state crystal structures.
The mode of receptor–anion interactions and the anion-
induced electronic and conformational changes of the receptor
can be illustrated by the 2D NOESY NMR experiment, which is
a traditional and useful tool in the field of supramolecular host–
guest chemistry. Therefore, for improved understanding of the
solution state binding nature of the receptor toward the anions,
we have carried out 2D NOESY NMR experiments both for free
receptor (L) and anion complexes in DMSO-d6 at RT (Fig. 8).
The free receptor molecule shows a significantly strong NOESY
signal between the thiourea protons –NHa and –NHb. Interest-
ingly, the through-space NOE signals are considerably weakened
in both complexes, and completely absent upon addition of one
equivalent of the respective anions (sulfate and thiosulfate), indi-
cating a conformational change of L due to encapsulation of the
anions in a 1 : 1 binding stoichiometry. A similar type of anion-
encapsulation-induced change in NOESY spectra was previously
reported by Hossain and Schneider for tren-based (urea) recep-
tors.10k, 19 In contradiction of the 2 : 1 solid state binding, the
results from NMR experiments confirm that in solution the
studied anions are bound to the pseudocapsular cavity of L with
1 : 1 binding stoichiometry (Fig. 9).
Acknowledgements
G. D. acknowledges the Department of Science and Technology
(DST), New Delhi, India for financial support, CIF and IIT
Guwahati for providing instrument facilities and DST FIST for
single crystal X-ray diffraction facility. A. B. thanks IIT
Guwahati for fellowship.
Notes and references
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Conclusions
In conclusion, we have demonstrated the detailed solid and solu-
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sulfur with similar dimensionality (sulfate and thiosulfate) with a
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