Electronic substitution effects on anion coordination of a tripodal thiourea receptor: Evidences of deprotonation of oxy-anions in solid and solution

Article abstract of DOI:10.1080/10610278.2013.808343

Two simple tris(2-aminoethyl)amine (tren)-based electron-donating p-methyl (L₁) and p-methoxy (L₂)-substituted tristhioureas have been extensively studied with various anions. The X-ray diffraction studies reveal that in the case of both receptors L₁ and L₂, halide anions (F^-, Cl^- and Br^-) are encapsulated into the tripodal cavity of the receptors with 1:1 stoichiometry and anions such as SO^2-₄ and HPO^2-₄ form dimeric capsular assemblies of the receptors with 1:2 stoichiometry. The solution-state binding and encapsulation of anions have also been confirmed by ¹H NMR titration and 2D NOESY NMR experiments. The ¹H NMR titration experiments show that receptor L₁ is able to deprotonate HSO^-₄, HCO^-₃ and H ₂PO^-₄ in solution, which was further verified with the addition of tetrabutylammonium hydroxide. On the other hand, receptor L₂ failed to deprotonate the hydrogenated oxy-anions in solution.

Full text of DOI:10.1080/10610278.2013.808343

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Electronic substitution effects on anion coordination
of a tripodal thiourea receptor: evidences of
deprotonation of oxy-anions in solid and solution
Abhijit Gogoia & Gopal Dasa
a Department of Chemistry, Indian Institute of Technology, GuwahatiAssam781039, India
Published online: 21 Jun 2013.
To cite this article: Abhijit Gogoi & Gopal Das (2013) Electronic substitution effects on anion coordination of a tripodal
thiourea receptor: evidences of deprotonation of oxy-anions in solid and solution, Supramolecular Chemistry, 25:12, 819-830,
DOI: 10.1080/10610278.2013.808343
To link to this article: http://dx.doi.org/10.1080/10610278.2013.808343
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Supramolecular Chemistry, 2013
Vol. 25, No. 12, 819–830, http://dx.doi.org/10.1080/10610278.2013.808343
Electronic substitution effects on anion coordination of a tripodal thiourea receptor: evidences of
deprotonation of oxy-anions in solid and solution
Abhijit Gogoi and Gopal Das*
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781039, India
(Received 9 March 2013; final version received 21 May 2013)
Two simple tris(2-aminoethyl)amine (tren)-based electron-donating p-methyl (L1) and p-methoxy (L2)-substituted tris-
thioureas have been extensively studied with various anions. The X-ray diffraction studies reveal that in the case of both
receptors L1 and L2, halide anions (F2, Cl2 and Br2) are encapsulated into the tripodal cavity of the receptors with 1:1
stoichiometry and anions such as SO242 and HPO422 form dimeric capsular assemblies of the receptors with 1:2
1
stoichiometry. The solution-state binding and encapsulation of anions have also been confirmed by H NMR titration and
1
2D NOESY NMR experiments. The H NMR titration experiments show that receptor L1 is able to deprotonate HSO24 ,
HCO23 and H2PO24 in solution, which was further verified with the addition of tetrabutylammonium hydroxide. On the other
hand, receptor L2 failed to deprotonate the hydrogenated oxy-anions in solution.
Keywords: tripodal receptor; thiourea; anion encapsulation; crystal structure; deprotonation of oxy-anions
Introduction
array of 12 H-bonds in which the carboxylate from an
aspartate residue (Asp-56) is specifically coordinated to
orthophosphate (10), whereas the sulphate-binding protein
from Salmonella typhimurium uses only neutral amino acid
residues to encapsulate the anionvia seven H-bonds (11). In
both cases, the binding site is located in a deep hydrophobic
pocket far from the protein surface where the anions can be
bound in a fully dehydrated form, efficiently shielded from
the hydrophilic environment.
Anions are ubiquitous in nature, and owing to their
fundamental importance in many chemical and biological
processes, they have significant relevance in medicine, on
the environment and industry (1). However, selective
recognition of anions and their extraction and separation
by synthetic receptors with high binding selectivity
rendered through H-bonds are still challenging, hence an
active area of research in supramolecular chemistry (2).
Although the field began with a focus on polyammonium
hosts, a plethora of organic scaffolds and functional groups
containing hydrogen-bond donors have been utilised for
anion recognition, such as hydrazones, amides, sulphona-
mides, thioamides, pyrroles, imidazolium cations, ureas,
thioureas, guanidinium and amidinium cations (3–6).
Given their vital role in many biochemical processes,
sulphate (7), inorganic phosphates (H2PO24 , HPO422 and
PO342) and phosphate-containing metabolites such as
AMP22, ADP32 and ATP42 are among the most targeted
anionic species (8). Sulphate is also known as an inorganic
pollutant in the environment and because of its large energy
of hydration (21080 kJ/mol), extraction of sulphate ions
from an aqueous to an organic phase is a challenging task
(9). However, there are interesting examples in nature,
wherein certain proteins can effectively and selectively
bind anions through weak intermolecular forces which help
in their transportation. For instance, in the active site of the
phosphate-binding protein from Escherichia coli, the anion
is encapsulated between the two lobes of protein with an
The observations in natural systems have inspired
researchers to develop numerous neutral receptors that use
hydrogen bonds to recognise anions. Among the synthetic
receptors, acyclic tripodal urea and thiourea scaffolds have
been widely used for anion binding as they can coordinate
with both halides and oxy-anions through the formation of
monomeric and dimeric capsular assemblies. One of the
most fascinating features of hydrogen-bonded capsules is
their ability to isolate the encapsulated guests from the bulk,
which thereby leads to a phenomenon such as molecular
sorting (12). Raudu et al. and Ghosh et al. have demonstrated
effective sensing and separation of oxy-anions using acyclic
tripodal urea and thiourea receptors (13). Very recently, Gale
et al. have reported a series of fluorinated tren-based ureas
and thioureas which facilitate transmembrane transport of
anions in vitro and have also shown the unexpected
decreasing trend of binding constant with the addition
of increasing electron-withdrawing substituents (14).
Motivated by the aforementioned studies, we have also
synthesised two tren-based tripodal receptors with electron-
*Corresponding author. Email: gdas@iitg.ernet.in
q 2013 Taylor & Francis

820
A. Gogoi and G. Das
functions. A lattice DMSO offers two CZH· · ·p int-
eractions to two neighbouring L1 . Cl complexes
˚
(C48ZH48C· · ·Cg ¼ 3.316 A, C47ZH47A· · ·Cg ¼ 3.456 -
˚
A) extending the monomeric capsular assembly along a-axis
(Figure 1(d)). Again the CZH· · ·p interaction from the
counter tetrabutylammonium (TBA) cation to the receptor
˚
(C35ZH35B · · ·Cg ¼ 3.168 A) facilitates the TBA cation to
come closer and partially seals the capsule from bottom. In
addition to this, there are two CZH· · ·O interactions of
˚
˚
average distance 2.673 A (C36ZH36B· · ·O1 ¼ 3.168 A,
˚
C40ZH40BZO1 ¼ 2.643 A) in between the DMSO and
the TBA cation which partially sealed the capsule.
Scheme 1. Molecular structures of p-methyl and p-methoxyphenyl
appended tris(thiourea) receptors, L and L .
1
2
Bromide complex 2, TBA[L1·(Br)]·DMSO. The asym-
metric unit of complex 2 contains one bromide-
encapsulated receptor molecule along with counter
tetrabutyl cation and solvent DMSO as the solvent of
crystallisation. The guest bromide anion was encapsulated
donating methyl and methoxy functionality (Scheme 1) and
compared their solution and solid-state behaviour with our
reported nitro (electron withdrawing) functionalised recep-
tor (15). Surprisingly, solution studies revealed the
selectivity of receptor L1 in deprotonating the oxy-anions
(HSO24 , HCO23 and H2PO42) as new peaks emerged in the
˚
inside the tripodal cleft at a distance of 4.180 A from the
1
apical nitrogen through six NH· · ·Br interactions.
However, the respective bond distances were weaker
than those of complex 1 (average dN· · ·Br ¼ 3.599 and
downfield region of the H NMR spectra. Titration with
tetrabutylammonium hydroxide (TBAOH) confirmed this
deprotonation and also suggested different degrees of
deprotonation of H2PO24 due to the presence of less acidic
NH protons when compared with the reported tripodal nitro
receptor. Again in the solid state, both L1 and L2 form 2:1
dimeric sulphate capsule, whereas with the earlier receptor
1:1 solvent sealed monomeric capsule was reported.
Solution-phase encapsulation of the deprotonated oxy-
anions was further confirmed from the analysis of Nuclear
Overhauser Effect Spectroscopy (NOESY) spectra. Among
halideanions, F2 interactsstronglywithboth the receptorsin
spite of less acidic NH protons due to the electron-donating
nature of methyl and methoxy groups, and the chemical
shifts were even more than that of the reported nitro receptor.
However, analysis of the NOESY showed weak interactions
of the halides with the receptors and competition with the
water present in DMSO-d6 for the binding site.
˚
3.397 A for ZNHa and ZNHb, respectively, Table S1 of
the Supplementary Information, available online). The
˚
TBA cation with one CZH· · ·p interaction (3.630 A) and
the crystallised DMSO molecule through another CZH· · ·
˚
p interaction (2.293 A) approaches the receptor from two
side and thereby partially sealed the bromide anion in the
monomeric capsule (Figure S39 of the Supplementary
Information, available online). However, change in the
coordination behaviour of the DMSO and other supporting
˚
weak interactions (average CZH· · ·O ¼ 2.6255 A, Table
S1 of the Supplementary Information, available online)
leads to a 2D decoration of the monomeric bromide
capsules along b-axis in up-down fashion (Figure 1(e)).
Though receptor L1 was also crystallised with excess
tetrabutylammonium fluoride (TBAF) from DMSO
solution, the structure could not be determined due to the
poor crystal quality.
Results and discussion
Crystal structure analysis
Fluoride complex 4. The fluoride ion was very tightly
encapsulated in the centrosymmetric P31c cavity of
receptor L2 (Z ¼ 4 and Z0 ¼ 2) which was internally
functionalised with six NZH· · ·F interactions of average
Structural analysis of the halide complexes
Chloride complex 1, TBA[(L1·Cl)]·DMSO. The chloride
anion crystallises to give 1:1 monomeric complex by six
internally functionalised NZH· · ·Cl interactions with more
involvement of NHb protons than the NHa protons. In the C3v
symmetric cavity of complex 1, the spherical chloride anion
˚
N· · ·F distance 2.831 A, which was comparable with our
reported nitro functionalised tripodal thiourea receptor
˚
(average N· · ·F ¼ 2.830 A) (15). The torsion angles
˚
was encapsulated at a distance of 4.036 A from the apical
involving N1apicalCCNamine are 244.48 and 54.57 for
conformers C1 and C2, respectively. In both the
conformers, encapsulated F2 anion was bound more
strongly to more acidic ZNHb than the ZNHa proton.
nitrogen through six average NZH· · ·Cl (average
˚
dN· · ·Cl ¼ 3.434 and 3.28 A for ZNHa and ZNHb,
respectively, Table S1 of the Supplementary Information,
available online) hydrogen bonds from the three thiourea
˚
Moreover, the Napical· · ·F (3.586 A) distance appeared to

Supramolecular Chemistry
821
Figure 1. A comparative presentation of (a) chloride capsule (complex 1), (b) bromide capsule (complex 2) and (c) fluoride capsule
(complex 4). The crystal packing figures of (d) complex 1 and (e) complex 2 with the specific interactions are indicated in red.
˚
be the shortest Napical· · ·X (X ¼ F, Cl, Br) distance among
the discussed halogenated capsules. Squeezed technique
had been applied to remove the distorted counter cations
from the crystal structure.
(dN1· · ·N1 ¼ 9.54 A) with one receptor interacting axially
and the other facially creating a 2:1 caged supramolecular
structure (Figure 2). This dimeric molecular capsule was
internally functionalised by 18 hydrogen bonds from six
functionalities thiourea, out of which 15 were NZH· · ·O
interactions and rest three were weak CZH· · ·O inter-
actions (Figure 2(b)). Again, 11 of these NZH· · ·O
hydrogen bonds can be classified as strong hydrogen bonds
Structural analysis of the oxy-anion complexes
Sulphate complex 3, 2TBA[2L1·(SO24)]. Excess treatment
of tetrabutylammonium hydrogen sulphate (TBAHSO4)
with a DMSO solution of L1 gave block-shaped crystals of
complex 3. Structural aspects of complex 3 revealed that
anion was deprotonated and encapsulated by two
inversion-symmetric units of L1. Multiple hydrogen
bonding interactions in the solid state lead to the formation
of stable fully deprotonated anion encapsulation within the
ligand cavity, which in turn shifts the deprotonation
equilibrium towards the formation of sulphate-encapsu-
lated complex. However, 1:1 solvent sealed capsule was
reported with the nitro receptor where the CZH· · ·O
interactions from the TBA cations to terminal nitro groups
and to the sulphate anion contribute to seal the capsule
(15). In complex 3, the distorted sulphate anion was
encapsulated by two receptor units flipped inwards
˚
with N· · ·O distances ,3.2 A and /DZH· · ·A . 1408
˚
(N· · ·O distances from 2.877(9) to 3.188(4) A) (Table S1 of
the Supplementary Information, available online). Out of
four oxygens of sulphate, O1 accepts six hydrogen bonds
and the other three act as four hydrogen bond acceptors.
In the dimeric capsule, the orientation of the thiourea
groups deviats from the ideal tetrahedral arrangement.
Instead, the two L1 ligands are symmetry related by an
inversion centre, resulting in distorted hydrogen-bonding
geometries for three of the thiourea groups (lower half in
Figure 1(a)), with two of them chelating O vertices rather
than OZSZO edges from sulphate. Another consequence
of this centrosymmetric arrangement was that sulphate,
which lacks an inversion centre, is rotationally disordered
over two positions to emulate the symmetry of the capsule.
The molecular capsule was further stabilised and extended
towards each other in
a
face-to-face fashion

822
A. Gogoi and G. Das
Figure 2. X-ray structures of the sulphate complexes with (a) receptor L1 and (c) methoxy-substituted receptor L2. (b) The 18 hydrogen
bonds encapsulating the sulphate in the dimeric cleft and (d) the interacting 12 thiourea NZH along with three aromatic ortho-CZH from
receptor L2 encapsulating the sulphate anion. TBA counter cations and other non-interacting hydrogens were deleted for clarity.
in three dimensions by 3 NZH· · ·S (average DZA distance
˚
3.813 A) as shown in Figure S38 of the Supplementary
groups, these dimeric sulphate capsules were inter-
connected through CZH· · ·O (C30ZH30C· · ·O1 ¼
˚
˚
Information, available online. However in this complex,
the bonds are comparatively longer and weaker than the
respective bonds in the nitro functionalised receptor (15).
2.360 A, C10ZH10B· · ·O3 ¼ 2.600 A) interaction from
each of the two arms of receptor L2 giving a spiral-shaped
structure, and are extended through other types of
interactions to give a sulphate-encapsulated 2D sheet (Figure
S39 of the Supplementary Information, available online). In
addition, each of such dimeric capsules was shielded by a
hydrophobic environment of six TBA cations and through
six CZH· · ·S and four CZH· · ·p interactions (Table S1 of
the Supplementary Information, available online).
Sulphate complex 5, 2TBA[2L2·(SO24)]. Likewise, in
complex 5, the guest sulphate anion was embedded in the
distorted C3v symmetric dimeric capsular assembly of
receptor L2 where the two inversion-symmetric receptor
molecules are flipped inwards towards each other in a face-
˚
to-face fashion with a distance of 9.929(5)A between the
apical nitrogens. The cleft was internally functionalised with
18 hydrogen bonds from six thiourea NH and three aromatic
CH protons (Figure 2(c)), out of which 15 are NZH· · ·O
interactions and rest three are aromatic CZH· · ·O inter-
actions (Figure 2(d)). Eleven of these NZH· · ·O hydrogen
bonds can be classified as moderate with N· · ·O distances
Hydrogen phosphate complex 6, 2TBA[2L2·(HPO24)]. Slow
evaporation of a DMSO solution of receptor L2 with
TBAH2PO4 resulted in the formation of HPO242 complex,
which was also obtained by adding potassium hydrogen
phosphate and tetrabutylammonium iodide to a DMSO
solution of L2. The guest anion (HPO242) was embedded
˚
,3.2 A (N· · ·O distances from 2.925(8) to 3.175(5)A),
whereas three of them are weaker (N· · ·O distances from
˚
3.264(9) to 3.408(5)A, Table S1 of the Supplementary
˚
˚
inside the dimeric cleft (dN1 · · ·N1 ¼ 10.024A) of two
inverse symmetric receptor units through 18 hydrogen bonds
from six thiourea functionality to two tetra-furcated and two
other penta-furcated oxygens of hydrogen phosphate
(Figure 3(a)). Twelve of these NZH· · ·O hydrogen bonds
Information, available online). Again, O6 accepts six
hydrogen bonds, whereas the other three were four hydrogen
bond acceptors (Figure 2(d)). Due to the terminal OMe
˚
can be classified as strong with N· · ·O distances ,3.2 A

Supramolecular Chemistry
823
receptor L1 with TBAF yielded multiple peaks, and thus,
interpretation of the spectra was obfuscated.
The guest fluoride anion strongly interacts with the
thiourea NH with different stoichiometries as addition of
0.6 equiv. of TBAF leads to emergence of new peaks at
higher downfield region with broad aromatic CH peaks.
The incremental increase in the intensity of the newly
emerged peak along with the broadening of the CH peaks
was continued with further addition of TBAF. Finally, in
presence of 3 equiv. of TBAF, the broad aromatic peaks
also split up to new signals, which clearly suggest the
presence of multiple species in equilibrium (Figure 4 and
Figure S7 of the Supplementary Information, available
online). This may be due to the encapsulation of fluoride
internally and externally by the receptor or because of the
presence of different equilibriums in the solution.
However, the spectra recorded after keeping the same
solution for couple of days resembled to the spectra of
complex 4, which strongly suggest the dominance of
fluoride-encapsulated L1 complex. Moreover, again due to
the presence of multiple species in equilibrium, the data
could not be fitted to any of the model. Similarly, from
Job’s plot analysis, a 1:1 host–guest model may not be
appropriate for the titration of TBAF with L2. With 1 equiv.
of TBAF to L2, the resonance values 1.536 and 1.207 ppm
of NHb and NHa protons, respectively, were higher than
that of the free receptor (Figure 4(b)), suggesting that the
guest fluoride anion was bound more strongly by NHb
protons of the receptor in solution as well in solid state.
Further addition of the anion solution makes the NH
protons to resonate at higher downfield region. Interest-
ingly, the chemical shift values are comparable with those
of the previously reported urea/thiourea receptors, and for
NHa proton the chemical shift was even more than that of
the previously reported nitro receptor (15). Again, the
aromatic CHd resonance (0.095 ppm) was higher, whereas
CHc resonance (0.126 ppm) was lower than the free
receptor with 1 equiv. of TBAF (Figure 4).
Figure 3. (a) X-ray structure of the hydrogen phosphate
complex with space filled hydrogen phosphate in the centre of the
dimeric distorted C3v cleft of the receptor L2. (b) Interacting
portion of dimeric capsule with 18 hydrogen bonds.
˚
(N· · ·O distances from 2.81(1) to 3.17(1) A, Table S1 of the
Supplementary Information, available online). O4 interacts
with the axial receptor through four NZH· · ·O hydrogen
bonds, O5 andO6throughfourNZH· · ·O andone CZH· · ·O
hydrogen bonds each from both axial and facial receptors
and O7 accepts four NZH bonds from both the axial and
facial receptors (Figure 3(b)). Similar to the earlier sulphate
complex (complex 5), the dimeric capsules are extended via
CZH· · ·O (C30ZH30A· · ·O2) interactions, and are shielded
by a hydrophobic environment of six TBA cations (Figure
S40 of the Supplementary Information, available online)
For the methyl-substituted receptor L1, NHb and NHa
protons deshielded to resonance with Dd (ppm) ¼ 0.138
and 0.130, respectively, which are higher (w.r.t. L1)
with 1 equiv. of tetrabutylammonium chloride (TBACl)
solution and continued to move downfield with further
addition of anion solution (Figure 4(a) and Figure S9 of the
Supplementary Information, available online). Similar
results were also obtained from methoxy substituent
receptor L2, where NHb and NHa deshielded to 0.106 and
0.133 ppm, respectively, with 1 equiv. of the same anion
(Figure 4(b) and Figure S10 of the Supplementary
Information, available online). With further addition of
anion solution, the NH protons experienced deshielding
and slowly continued to move to downfield. Bromide anion
interacted very weakly with both the receptors and revealed
only a marginal downfield shift (Dd (ppm) ¼ 0.063 and
0.079 in case of L1 and Dd (ppm) ¼ 0.063 and 0.079 in case
˚
through six CZH· · ·S and two CZH· · ·p (3.206 A)
interactions.
Anion binding in solution
Solution-phase complexation of anions and binding
stoichiometry (1:1/2:1 host–guest) with the receptors
were monitored using 1H NMR titration, 31P and 2D
NOESY NMR experiments in DMSO-d6.
Halide anions binding in solution
Among the halide ions, the highly basic fluoride anion
strongly interacts with the thiourea NH protons of both the
1
receptors. However, the H NMR titration experiment of

824
A. Gogoi and G. Das
Figure 4. A comparison of 1H NMR spectra of the receptors in DMSO-d6 at 298 K with tetrabutylammonium halide (TBAX) (X ¼ F,
Cl, Br); (i) free receptor; (ii) receptor with TBAF; (iii) receptor with TBACl and (iv) receptor with TBABr, where set (a) is for L1 and set
(b) for L2.
of L2) for NHb and NHa protons with 1 equiv. of
tetrabutylammonium bromide (TBABr) solution (Figure 4
and Figures S13, S14 of the Supplementary Information,
available online).
With L1, deprotonation of HSO24 starts after 0.6 equiv.
addition of TBAHSO4 solution as evidenced by emergence
of new peaks at higher downfield region. With further
addition of anion, the new peaks (Figure S15 of the
Supplementary Information, available online) intensified,
whereas the other peaks shifted downfield, suggesting
slow solution-phase transformation of the HSO24 complex
to the SO242 complex. After the addition of 1 equiv. of
TBAHSO4, the NHb and NHa protons show a downfield
shift of 0.531 and 0.785 ppm for the HSO24 complex.
The magnitude of the downfield shift clearly indicates
more involvement of the less acidic NHa protons than the
NHb protons. Similarly, for the SO242 complex, the NHb
and NHa protons show a downfield shift of 0.811 and
1.205 ppm. To prove this deprotonation of HSO24 , an
aliquot of TBAOH was added to the solution mixture of L1
and TBAHSO4. The addition of TBAOH enhanced the
intensity of the newly emerged peaks, which was due to
the further deprotonation of the bisulphate anion and the
formation of L1 2 SO242 complex (Figure 5(c)). Simul-
Oxy-anions binding in solution
Although both the receptors are functionalised with
electron-donating groups, receptor L1 with more acidic
NH protons and thus with stronger multiple hydrogen
bonding interactions induces proton transfer between the
free and bound anions. Gale et al. had reported such
deprotonation of HCO23 and H2PO42 with a tripodal-
fluorinated thiourea system (14) and acyclic indole system
in the solution (16). Our group had also reported the
deprotonation and encapsulation of PO342 by a tripodal
nitro-functionalised thiourea receptor in solution (15).
However, the solution-phase deprotonation of HSO42
anion by a thiourea receptor functionalised with electron-
donating substituent was not reported earlier.

Supramolecular Chemistry
825
1
Figure 5. Stack plot of partial H NMR titration of receptor L1 in DMSO-d6; (a) the free receptor, (b) L1 þ 1 equiv. TBAOH; (c) L1
þ TBAHSO4 þ TBAOH; (d) L1 þ TBAHCO3 þ TBAOH; (e) L1 þ TBAH2PO4 þ TBAOH; (i) with excess TBAX (X ¼ HSO24 , HCO32,
H2PO24 ) and (ii) with 1 equiv. of TBAX and 1 equiv. TBAOH.
taneously, a control experiment was also carried out with
the addition of TBAOH to receptor L1 alone (in the
absence of guest anion), which revealed complete
deprotonation of the thiourea protons (NHa and NHb) of
L1. Again, the CH(meta) was shifted nearly to 0.1 ppm
downfield but CH(ortho) shifted marginally upfield which
may be due to the conformational change in the receptor
with 1 equiv. TBAHSO4 anion. However, on keeping the
solution for days, the new downfield peaks related to
sulphate complex are dominated. Again, due to the
presence of multiple species, the binding constant could
not be calculated. On the contrary, for receptor L2 with less
acidic protons (NH protons) than L1 only shifting and
broadening of NH and aromatic CH peaks to higher ppm
values were observed with the added TBAHSO4 solution
(Figure S16 of the Supplementary Information, available
online). However, in the solid state, HSO24 anion was
deprotonated to give the dimeric sulphate-encapsulated
complex. With the addition of 1 equiv. of the same anion, a
downfield shift of 0.092 and 0.168 ppm was observed for
the more acidic NHb and NHa protons, respectively.
However, these values are quite small compared with the
earlier sulphate complexes, and the binding constant is
reported in Table S2 of the Supplementary Information,
available online.
Similarly, new sets of signals were also observed
1
during the H NMR titration of L1 with TBAHCO3 in
DMSO-d6, and were confirmed as the peaks for in situ-
generated L1 . CO232 complex (Figure 5(d)). During the
titration, new peaks were generated after the addition of
0.4 equiv. of anion solution besides aromatic CH and NH
peaks (Figure S19 of the Supplementary Information,
available online) and intensified with the further addition
of TBAHCO3 solution. After 1 equiv. HCO23 solution, the
resonance values 0.115 and 0.104 ppm of NHa and NHb
protons were higher than that of the free ligand L1 and
continued to move towards the newly generated NH peaks
showing slow transformation of the L1-encapsulated
HCO23 to the CO232-encapsulated complex. However
after saturation, the resonance NHa and NHbwith values of
0.992 and 1.203 ppm of NHa and NHb protons were higher
(w.r.t. L1) for the carbonate complex. The binding constant
could not be calculated due to the presence of multiple
species in equilibrium and again Job’s plot failed to fit to
any model. However, for the OMe-substituted L2, simple
spectra with NH peak broadening were observed when

826
A. Gogoi and G. Das
titrated with TBAHCO3 under similar experimental
condition (Figure S20 of the Supplementary Information,
available online). This peak broadening was an indication
of multiple interactions with the anion and also with the
solvent DMSO and was thus inconvenient to determine the
binding constant. After the addition of 1 equiv. of
TBAHCO3 solution, a considerable upfield shift of
0.141 ppm for CH(ortho) was observed.
Analysis of the NOESY spectra of the complex 1 showed
that the important NOE interactions such as Ha· · ·Hb and Ha/
Hb· · ·Hc/Hd were absent. However, the spectra showed weak
space interaction of the acidic Ha/Hb proton with the H2O
present in the solvent DMSO. This suggests that the chloride
anion was encapsulatedinside the capsule butcould not expel
the water molecule originally present in the receptor cavity.
Analysis of the NOESY spectra of L with bromide anion
1
1
Data analysis of the H NMR titration of receptor L1
(complex 2) suggests that the anion was even more loosely
bound within the receptor than the chloride complex as all
important NOE interactions were present (Figure S28 of the
Supplementary Information, available online). However, in
solution complex 4 showed space interaction of NH protons
with the water present in DMSO, whereas all the other
important couplings such as Ha· · ·Hb and Hb· · ·Hc were
absent, suggesting encapsulation of both fluoride and water
molecule in the receptor cavity (Figure S29 of the
Supplementary Information, available online). For the
sulphate complex 3, space coupling through Ha· · ·Hd and
Hb· · ·Hd was negligible whereas the other interactions were
absent, suggesting solution-state encapsulation of SO422
anion (Figure 6(b)) with no water molecule inside the cavity.
Again solution-phase SO422 encapsulation with 2:1 host–
guest stoichiometry can be claimed for the other sulphate
complex (complex 5), as the NOE interactions such as
Ha· · ·Hb, Hb· · ·Hc, Hb· · ·Hd and Ha/Hb· · ·H2O were absent
(Figure 6(c)). Similarly, HPO242 was also encapsulated by
with TBAH2PO4 was greatly hindered by peak broadening,
indicating proton transfer between the ligand and H2PO42
in solution. Multiple hydrogen bonds from receptor L1 also
deprotonate TBAH2PO4 in solution, and new peaks were
1
seen with 0.8 equiv. of the anion in the H NMR spectra
(Figure S21 of the Supplementary Information, available
online). In the presence of 1 equiv. of H2PO24 , NHb and
NHa protons shifted to 0.903 and 1.635 ppm downfield,
respectively. The new emergent peaks were because of the
interaction of L1 with deprotonated H2PO24 and continued
to shift towards downfield region with further addition
of anion (dDmax(NHa) . dDmax(NHb)). However, the
addition of aliquots of strong base TBAOH to L1 or to
the solution mixture of TBAH2PO4 and L1 did not yield the
same spectra as did the excess addition of H2PO24 to L1
(Figures 5(e) and Figure S21 of the Supplementary
Information, available online). The addition of aliquot
TBAOH to the solution mixture of L1 and 1 equiv.
TBAH2PO4 revealed development of new peaks with
higher ppm values near the NH resonance region along
with new peaks in the aromatic region of the spectra, which
suggest further deprotonation of the anion and presence of
multiple species in equilibrium.
receptorL in 2:1 host–guest fashion as the NOE interactions
2
between the NH protons, between NH and aromatic CH
protonsandbetweenNHandwaterwereabsent(Figure6(d)).
Again up to 1 equiv. of TBAH2PO4, NHa and NHb of
L2 resonance with values 0.986 and 1.24 ppm, which were
higher than the free receptor and continued to shift
downfield with further addition of the anion solution.
The aromatic CHd(meta) protons shifted to 0.177 ppm
downfield, whereas the aromatic CHc(ortho) experienced
shielding to 0.107 ppm compared with the free receptor
(Figure S22 of the Supplementary Information, available
online). However, the data could either not be fitted to any
model or the 1:1 fit gave larger errors.
Conclusions
In summary, we have compared the effect of electron-
donating functionality on anion coordination chemistry of a
tripodal thiourea receptor with reported electron-with-
drawing functionality on the same tripodal framework. The
spherical halide anions were engulfed in 1:1 binding
stoichiometry with the expanding receptors cavity size in the
1
solid state from fluoride to bigger bromide anion. The H
NMR titration results especially with fluoride were
interesting due to the strong participation of NH protons
eventhoughtheywereweaklyacidicandtheshiftswereeven
larger than our reported nitro receptor. However, NOESY
experiment showed weak interactions of the halides and
competition with the water present in DMSO-d6 for binding
with the host, and thus, a 1:1 binding stoichiometry may not
be suitable. Again, receptorL1 broadly deprotonates the oxy-
anions such as HSO42, HCO32 and H2PO24 in the solution
which was further confirmed from the experiment with
TBAOH under similar condition. X-ray crystal structure
analysis showed a variation of SO242-encapsulated capsules
from 1:1 solvent sealed to 2:1 host–guest complexes on
changing the nature of the substituent groups from electron-
In order to characterise the solution structures of the
complexes, i.e. encapsulation and binding stoichiometry of
the anions with receptors L1 and L2, 2D NOESY NMR
experiments were carried out in DMSO-d6. Interpretation
of NOESY spectra of the free receptors showed strong
nuclear Overhauser effect (NOE) coupling between the
thiourea protons and water (Ha· · ·HZOZH, Hb-
HZOZH) present in highly moisture sensitive DMSO-d6
and it depicts the formation of water capsule in solution
(Figure S27 of the Supplementary Information, available
online) (15). However, water molecule was probably
loosely bound to receptor L2 as the spectra also show an
important NOE interaction such as Ha· · ·Hb and Hc· · ·Hd.

Supramolecular Chemistry
827
Figure 6. 2D NOESY NMR spectra of (a) complex 1, (b) complex 3, (c) complex 5 and (d) complex 6 in DMSO-d6 at room temperature.
withdrawing nitro to electron-donating methyl and methoxy
groups. Again a distinct change was also observed in the
crystal structures on moving from methyl to methoxy
functionality. NOESY also proved this solution-phase total
encapsulation of the deprotonated oxy-anions by the
receptors. However, in the solid state, single proton transfer
from H2PO24 leads to the formation of dimeric HPO242
capsule with receptor L2 and the same can be anticipated
from the 1H NOSEY experiment.
1H and 31P/19F NMR spectra were recorded on a Varian
FT-400 MHz instrument, and chemical shifts were recorded
in ppm on the scale using tetramethylsilane or residual
solvent peak as a reference and 13C spectra were obtained at
100 MHz at 298 K. The electrospray ionisation mass
spectrometry spectra of all the complexes were recorded in
a Waters Q-Tof Premier (Waters Corporation, USA) liquid
chromatography-MS system in acetonitrile. The IR spectra
were recorded on a Perkin-Elmer Spectrum One FT-IR
spectrometerwithKBrdiscsintherangeof4000–450 cm21
.
Experimental
Materials and methods
Synthesis and characterisation
All reagents and solvents were obtained from commercial
sources and used as received. Tris-(2-aminoethyl)amine
(tren), 4-tolylphenylisothiocyanate, 4-methoxyisothiocya-
nate and tetraalkyl ammonium salts were purchased from
Sigma-Aldrich (Bangalore, India) and used as received.
Solvents for synthesis and crystallisation experiments
were purchased from Merck (Mumbai, India) and used
without further purification.
Tris(thiourea) receptors were synthesised by known
literature procedure where the reaction of tris(2-amino-
ethyl)amine, (tren) with 4-methylphenylisothiocyanate and
4-methoxyphenyl isothiocyanate in a 1:3 molar ratio in dry
tetrahydrofuran at room temperature yielded L1 and L2,
respectively. Yield of L1: 82%, 1H NMR (400 MHz,
DMSO-d6) d ¼ 2.253 (s, 9H, CH3), 2.663 (s, 6H, NCH2),
7.106–7.125 (d, 6H, Ar(m)H), 7.203–7.223 (d, 6H, Ar(o)

828
A. Gogoi and G. Das
H), 7.495 (s, 3H, CH2 NH(NHa)), 9.512 (s, 3H, ArZNH
(NHb)); 13C NMR (100 MHz, DMSO-d6): d 20.6 ( £ 3C,
CH3), 41.9 ( £ 3C, NCH2), 52.3 ( £ 3C, NCH2CH2), 123.7
( £ 6C, ArCH), 129.2 ( £ 6C, ArCH), 133.7 ( £ 3C, ArH),
136.4 ( £ 3C, ArH), 180.3 ( £ 3C, C ¼ S). mp ¼ 1308C. m/
z ¼ 594.1981 (calcd m/z ¼ 593.24 in positive mode). FT-
IR: KBr (y cm21) 3300, 2928, 1510, 1389, 818.
Complex 3
Slow evaporation of a solution of an excess (10 equiv.)
TBAHSO4 charged to 50 mg of L1 dissolved in 5 ml of
DMSO give suitable block-shaped single crystals of
complex 3. Isolated yield of 3 is 72%, and has been
characterised by 1H NMR and FT-IR spectroscopy.
1H NMR (400 MHz, DMSO-d6) d ¼ 1.077–1.105
(t, CH3, TBA), 2.460 (s, 6H, NCH2), 2.605 (s, 6H,
NCH2CH2), 3.100–3.154 (q, NþZCH2, TBA), 7.642–
7.697 (t, 12H, ArH), 11.750 (s, 3H, CH2NH), 12.845
(s, 3H, ArZNH). FT-IR: KBr (y cm21) 3282, 2960, 1539,
1338, 818.
Characterisation of L2
Yield of L2: 75%, 1H NMR (400 MHz, DMSO-d6)
d ¼ 2.643 (s, 6H, NCH2), 3.726 (s, 9H, OCH3), 6.884–
6.906 (d, 6H, Ar(m)H), 7.202–7.179 (d, 6H, Ar(o)H),
7.346 (s, 3H, CH2NH(NHa)), 9.407 (s, 3H, ArZNH
(NHb)); 13C NMR (100 MHz, DMSO-d6): d 42.730
( £ 3C, CH3), 52.9 ( £ 3C, NCH2), 55.9 ( £ 3C,
NCH2CH2), 114.7 ( £ 6C, ArCH), 126.8 ( £ 6C,
ArCH), 132.1 ( £ 3C, ArH), 157.3 ( £ 3C, ArH), 181.2
( £ 3C, C ¼ S). mp ¼ 1708C. m/z ¼ 642.2333 (Calcd m/
Complex 4
Complex 4 was obtained as single crystals suitable for the
XRD analysis by charging an excess (10 equiv.) of TBAF
to 50 mg of L2 dissolved in 5 ml of DMSO. The solution
was stirred for about 15 min and filtered in a test tube for
crystallisation. Slow evaporation of the filtrate at room
temperature yielded complex 4. Isolated yield of 4 is 78%,
and has been characterised by 1H NMR and FT-IR
spectroscopy. 1H NMR (400 MHz, DMSO-d6) d ¼ 1.077–
1.105 (t, CH3, TBA), 2.460 (s, 6H, NCH2), 2.605 (s, 6H,
NCH2CH2), 3.100–3.154 (q, NþZCH2, TBA), 7.642–
7.697 (t, 12H, ArH), 11.750 (s, 3H, CH2NH), 12.845 (s,
3H, ArZNH). FT-IR: KBr (y cm21) 3284, 2960, 1510,
1233, 1035, 828.
z ¼ 641.23 in positive mode). FT-IR: KBr (y cm21
)
3312, 2968, 1510, 1289, 978.
Complex 1
Complex 1 was obtained as single crystals suitable for
the X-ray diffraction (XRD) analysis by charging an
excess (10 equiv.) of TBACl to 50 mg of L1 dissolved in
5 ml of DMSO. The solution was stirred for about 15 min
and filtered in a test tube for crystallisation. Slow
evaporation of the filtrate at room temperature yielded
complex 1. Isolated yield of 1 is 78%, and has been
characterised by 1H NMR and FT-IR spectroscopy.
1H NMR (400 MHz, DMSO-d6) d ¼ 1.135–1.166
(t, CH3, TBA), 2.238 (s, 6H, NCH2), 2.605 (s, 6H,
NCH2CH2), 3.159–3.213 (q, NþZCH2, TBA), 7.060–
7.097 (d, 6H, ArH), 7.301–7.321(d, 6H, ArH), 7.893
(s, 3H, ZCH2NH), 9.916 (s, 3H, ArZNH). FT-IR: KBr
(y cm21) 3284, 2962, 1535, 723.
Complex 5
Complex 5 was obtained as single crystals suitable for the
XRD analysis by charging an excess (10 equiv.) of
TBAHSO4 to 50 mg of L2 dissolved in 5 ml of DMSO.
The solution was stirred for about 15 min and filtered in a
test tube. Slow evaporation of the filtrate at room
temperature yielded complex 5. Isolated yield of 5 is
70%, and has been characterised by 1H NMR and FT-IR
spectroscopy. 1H NMR (400 MHz, DMSO-d6) d ¼ 0.910–
0.946 (t, CH3, TBA), 1.258–1.329 (q, 6H, NCH2CH2CH2),
1.557 (b, NCH2CH2), 2.544 (s, 6H, NCH2), 3.129–3.170
(t, NþZCH2, TBA), 3.710 (s, OMe), 6.797–6.818 (d, 6H,
ArH), 7.307–7.326 (d, ArCH, 6H), 11.750 (s, 3H,
Complex 2
Complex 2 was obtained as single crystals suitable for
the XRD analysis by charging an excess (10 equiv.) of
TBABr to 50 mg of L1 dissolved in 5 ml of DMSO. The
solution was stirred for about 15 min and filtered in a test
tube for crystallisation. Slow evaporation of the filtrate at
room temperature yielded complex 2. Isolated yield of 2
is 78%, and has been characterised by 1H NMR and
FT-IR spectroscopy. 1H NMR (400 MHz, DMSO-d6)
d ¼ 1.077–1.105 (t, CH3, TBA), 2.460 (s, 6H, NCH2),
2.605 (s, 6H, NCH2CH2), 3.100–3.154 (q, NþZCH2,
TBA), 7.642–7.697 (t, 12H, ArH), 11.750 (s, 3H,
CH2NH), 12.845 (s, 3H, ArZNH). FT-IR: KBr (y cm21
3268, 2876, 1539, 1318, 1118.
)
Complex 6
Block-shaped phosphate-encapsulated crystals were
obtained from DMSO solution of TBAH2PO4 and L2.
Isolated yield of 6 is 70%, and has been characterised by
1H NMR and FT-IR spectroscopy. H NMR (400 MHz,
1
CH2NH), 12.845 (s, 3H, ArZNH). FT-IR: KBr (y cm21
)
DMSO-d6) d ¼ 0.907–0.943 (t, CH3, TBA), 1.274–1.347
3284, 2962, 1535, 723.
(q, NCH2CH2CH2), 1.556 (b), 2.568 (s, 6H, NCH2),

Supramolecular Chemistry
829
3.128–3.169 (t, 6H, NþCH2), 3.699 (s, OMe), 6.756–
6.779 (d, 6H, ArH), 7.372–7.395 (d, 6H, ArH), 8.746 (s,
3H, CH2NH), 10.487 (s, 3H, ArZNH). FT-IR: KBr
(y cm21) 3368, 2896, 1539, 1318, 1087.
unique and 5759 observed reflections (Rint ¼ 0.1688),
R1(F) ¼ 0.0841 (I . 2s(I), wR2(F 2) ¼ 0.2212 (all data),
GOF(F 2) ¼ 1.048).
Complex 4: C30H39FN7O3S3, CCDC 892712,
Mw ¼ 660.89, T ¼ 298(2) K, hexagonal, space group P31c,
a ¼ 13.1553(9), b ¼ 13.1553(9), c ¼ 36.442(4), a ¼ 908,
3
Single-crystal X-ray crystallography
˚
b ¼ 908,
g ¼ 1208,
V ¼ 5461.7(7) A ,
Z ¼ 4,
In each case, a crystal of suitable size was selected from
the mother liquor and immersed in silicone oil, and it was
mounted on the tip of a glass fibre and cemented using
epoxy resin. The intensity data were collected using a
Bruker SMART APEX-II CCD diffractometer, equipped
with a fine focus 1.75 kW sealed tube Mo Ka radiation
m ¼ 0.165 mm21, Dx ¼ 0.804 g/cm3, 3131 unique and 3413
observed reflections (Rint ¼ 0.0914), R1(F) ¼ 0.0493 (I . 2s
(I), wR2(F 2) ¼ 0.1434 (all data), GOF(F 2) ¼ 0.972).
Complex 5: C92H150N16O10S7, CCDC 892713,
Mw ¼ 1864.77, T ¼ 298(2) K, monoclinic, space group P21/
c, a ¼ 19.0182(10), b ¼ 14.3509(8), c ¼ 27.0319(12),
3
˚
˚
a ¼ 908, b ¼ 133.935(3)8, g ¼ 908, V ¼ 5312.9(5) A ,
(l ¼ 0.71073 A) at 298(3) K, with increasing v (width of
0.38/frame) at a scan speed of 5 s/frame. The SMART
software was used for data acquisition. Data integration
and reduction were undertaken with SAINT and XPREP
(17) software. Multi-scan empirical absorption corrections
were applied to the data using the program SADABS (18).
Structures were solved by direct methods using SHELXS-
97 (19) and refined with full-matrix least-squares on F 2
using SHELXL-97 (20). Squeezed technique was applied
to remove the distorted counter cations in complex 4. All
non-hydrogen atoms were refined anisotropically, hydro-
gen atoms attached to all carbon atoms were geometrically
fixed and the positional and temperature factors are refined
isotropically. Hydrogen atoms attached with the urea
nitrogen atoms were located from electron Fourier map
and refined isotropically. Usually, temperature factors of H
atoms attached to carbon atoms are refined by restraints
21.2 or 21.5 Uiso (C), although the isotropic-free
refinement is also acceptable. Structural illustrations have
been drawn with MERCURY-2.3.1 for Windows. Complex
6 exhibits the weakest diffraction. Again, solvent variation
for crystallisation or repeated data collection did not
improve the crystal quality of complex 6.
Z ¼ 2, m ¼ 0.208 mm21, Dx ¼ 1.166 g/cm3, 13,388 unique
and 5186 observed reflections (Rint ¼ 0.1736),
R1(F) ¼ 0.0844 (I . 2s(I), wR2(F 2) ¼ 0.225 (all data),
GOF(F 2) ¼ 1.099).
Complex 6: C92H151N16O10PS6, CCDC 893126,
Mw ¼ 1864.68, T ¼ 298(2) K, monoclinic, space group P21/
c, a ¼ 18.941(2), b ¼ 14.4469(16), c ¼ 26.856(3), a ¼ 908,
3
˚
b ¼ 133.744(6)8, g ¼ 908, V ¼ 5309.1(10) A , Z ¼ 2,
m ¼ 0.203 mm21, Dx ¼ 1.166 g/cm3, 13,650 unique and
2602 observed reflections (Rint ¼ 0.3214), R1(F) ¼ 0.0723
(I . 2s(I), wR2(F 2) ¼ 0.2014 (all data), GOF(F 2) ¼ 0.991).
Supporting Information Available
Crystallographic files in CIF format, additional crystal-
lographic data, characterisation data, 1H NMR spectra, 13
C
NMR spectra, 31P spectra, NOESY spectra and IR spectra
of all the complexes are provided in the supporting
information.
Acknowledgements
Crystaldata, complex 1:C48H80ClN8OS4, CCDC892709,
Mw ¼ 948.93, T ¼ 298(2) K, orthorhombic, space group
P212121, a ¼ 15.3667(8), b ¼ 16.1344(8), c ¼ 22.3991(11),
G.D. acknowledges the Council of Scientific and Industrial
Research (EMR/01/3664/12) and 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.G acknowl-
edges IIT Guwahati, India, for research fellowship.
3
˚
a ¼ 908, b ¼ 908, g ¼ 908, V ¼ 5553.5(5) A , Z ¼ 4,
m ¼ 0.259 mm21, Dx ¼ 1.136 g/cm3, 7732 unique and 4944
observed reflections (Rint ¼ 0.2029), R1(F) ¼ 0.0777 (I . 2s
(I), wR2(F 2) ¼ 0.2454 (all data), GOF(F 2) ¼ 0.920).
Complex 2: C48 H81BrN8OS4, CCDC 892710,
Mw ¼ 994.39, T ¼ 298(2) K, orthorhombic, space group
P212121, a ¼ 15.3526(6), b ¼ 16.1712(7), c ¼ 22.4668(9),
Note
1. Mercury 2.3, Supplied with Cambridge Structural Database;
CCDC, Cambridge, 2007. Lied with Cambridge Structural
Database, CCDC, Cambridge, UK, 2007.
3
˚
a ¼ 908, b ¼ 908, g ¼ 908, V ¼ 5577.8(4) A , Z ¼ 4,
m ¼ 0.927 mm21, Dx ¼ 1.184 g/cm3, 7564 unique and 4662
observed reflections (Rint ¼ 0.2041), R1(F) ¼ 0.0676
(I . 2s(I), wR2(F 2) ¼ 0.1780 (all data), GOF(F 2) ¼ 0.927).
Complex 3: C92H150N16O4S7, CCDC 892711,
Mw ¼ 1768.77, T ¼ 298(2) K, monoclinic, space group
P21/c, a ¼ 19.206(2), b ¼ 14.1388(14), c ¼ 26.243(2),
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