A tripodal receptor bearing catechol groups for the chromogenic sensing of F- ions via frozen proton transfer

Article abstract of DOI:10.1016/j.tet.2009.08.023

A neutral tripodal Schiff base receptor (3) having catechol as end groups has been synthesized and characterized with the help of spectroscopic and single crystal X-ray crystallographic studies. The receptor behaves as a visually detectable optical sensor

Full text of DOI:10.1016/j.tet.2009.08.023

Tetrahedron 65 (2009) 8556–8562
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A tripodal receptor bearing catechol groups for the chromogenic sensing
of Fꢀ ions via frozen proton transfer
*
*
Vimal K. Bhardwaj, Maninder Singh Hundal , Geeta Hundal
Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2009
Received in revised form 31 July 2009
Accepted 4 August 2009
Available online 13 August 2009
A neutral tripodal Schiff base receptor (3) having catechol as end groups has been synthesized and
characterized with the help of spectroscopic and single crystal X-ray crystallographic studies. The re-
ceptor behaves as a visually detectable optical sensor for Fꢀ ions in DMSO, sensitive enough to recognize
the ions up to a concentration 1ꢁ10ꢀ5 M with naked eye. The chromogenic response is based upon the
deprotonation of the highly acidic catechol moieties in the presence of highly basic Fꢀ ions in a polar
solvent like DMSO.
Keywords:
Tripodal receptor
Catechols
Ó 2009 Elsevier Ltd. All rights reserved.
Chromogenic anion sensing
1. Introduction
workers used a high throughput assay10 to compare the anion
binding ability of bisamides and phenols, and they found catechol
The development of anion sensing receptors1,2 has gained im-
petus due to better understanding of their roles in biological,
environmental, and chemical processes. Especially the sensing of
Fꢀ ions is the most important due to its function in dental care,
treatment of osteoporosis,3 and partly due to its release during
hydrolysis of nerve gas Sarin.4 Among various chemosensors, the
chromogenic receptors5 are exceptionally interesting for their
recognition process is accompanied by a simple, naked eye de-
tection, which is both quantitative and qualitative in nature. Most
of the neutral chromogenic receptors for anions employ N–H/
anion H-bonding interactions for anion recognition and are based
upon amides, thioureas, and amidoureas molecules.6 These are
strong, directional H-bond donors and can easily be accommodated
in other preorganized scaffolds. The recognition phenomenon in
these examples involves either H-bonding (incipient proton
transfer) or complete deprotonation of –N–H protons (frozen pro-
ton transfer).5h On the other hand, in the biological context the
O–H/anion H-bonding interactions7 are almost as crucial as the
ubiquitous N–H/anion interactions. Although there are a few re-
ports known where such interactions have been seen to supple-
ment other kinds of H-bonding interactions,8 but there are only
a few examples known9 where the former interactions alone have
been exploited for developing anion receptors. Smith and co-
to be a stronger H-bond donor for Clꢀ ion and subsequently were
successful even in developing a colorimetric response of Fꢀ ion
with catechol, which occurred via a new mechanism.11 Working on
the same lines we have incorporated catechols into the tripodal
receptor (3) so as to study the effect of their organization on the
recognition behavior, ability, and selectivity toward anions.
Here we report the synthesis, characterization, and chromo-
genic studies on a tripodal ditopic Schiff base receptor (3) with
catechol moiety as the end groups so as to dispose six –OH groups
capable of forming H-bonding or undergoing deprotonation in the
presence of anions. There are some reports12 for the synthesis of
catechol bearing tripodal ligands but they were not used as anion
sensors. Barring an off-the-shelf compound Alizarin, which is
reported8e to sense various anions in dichloromethane, to the best
of our knowledge this is the first example of a synthetic receptor,
bearing catechol group being used as an anion sensor. The re-
ceptor acts as a highly selective, visually detectable optical
sensor for Fꢀ ions in DMSO working via frozen proton transfer.
2. Results and discussion
2.1. Synthesis
The receptor (3) was synthesized as in Scheme 1. The receptor
was prepared by Schiff base condensation reaction of reactant
1 and 2 in chloroform/methanol mixture in the presence of cata-
lytic amount of zinc perchlorate. The solution was stirred for half an
* Corresponding authors. Tel.: þ91 183 2502113; fax: þ91 183 2258820.
E-mail addresses: hundal_chem@yahoo.com (M.S. Hundal), geetahundal@
yahoo.com (G. Hundal).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.08.023

V.K. Bhardwaj et al. / Tetrahedron 65 (2009) 8556–8562
8557
H2N
S
N
CHO
NH2
HO
HO
S
S
OH
CHCl3, Zn(ClO4)2
+
OH
OH
CH3OH
OH
S
S
N
N
NH2
S
HO
OH
1
2
3
H2N
S
S
H2N
S
N
OH
OH
4
Scheme 1.
hour, orange colored solid was separated, which was filtered,
washed with methanol, and dried under vacuum. It was charac-
terized by elemental analyses, IR, 1H, 13C NMR, UV–vis absorption
spectroscopy, and X-ray crystal structure analysis. The IR bands at
1616 and 3418 cmꢀ1 show the presence of imine and H-bonded OH
groups. The same are evident by the signals at
d 8.79, 9.18, and 13.08
in the 1H NMR and at 162.88 and 182.26 in 13C NMR spectra, re-
d
spectively. The chemical shift of one of the hydroxyl protons is
typical for the resonance-assisted hydrogen-bonded (RAHB) proton
of O–H/N as found by others and us.13 In the 1H and 13C NMR
spectra there are single peaks corresponding to methyl, methylene,
imine, and hydroxyl protons and their corresponding carbons,
pointing toward a three-fold symmetry of the molecules being
retained in the solution phase unlike the solid phase (cf. the X-ray
structure given below). The CHN data are also in accordance with
the molecular formula.
2.2. X-ray crystal structure
The X-ray crystal structure of (3) is shown in Figure 1. All seven
aromatic rings are planar. One arm of the tripodal ligand is highly
unsymmetrical with respect to the other two thus, removing any
three-fold symmetry expected of the compound as found in the
solution state. The rings B (C8–C13), C (C22–C27), and D (C36–C41)
are making dihedral angles 78.7(2)ꢂ, 39.5(2)ꢂ, and 87.0(2)ꢂ with the
central ring A (C1–C6), respectively. Thus rings B and D are almost
perpendicular to the central ring A but ring C is gauche to it. Sim-
ilarly rings E (C15–C20) and G (C43–C48) are perpendicular
(dihedral angles 78.2(2)ꢂ and 80.7(2)ꢂ, respectively) but ring
F (C29–C34) is parallel (dihedral angle 4.7(2)ꢂ) with respect to ring
A. The torsion angles about S1–C8 and S3–C36 are anti, being
ꢀ170.1(6)ꢂ and 171.7(7)ꢂ, respectively, but gauche ꢀ94.9(8)ꢂ,
around S2–C22. This conformation analysis shows that the tripod is
not in its fully extended form but one of its arms has folded up to
make a loop having ring F parallel to ring A. This loop is stabilized
by various intramolecular interactions (Table S1, Supplementary
Figure 1. ORTEP diagram of the receptor (3) showing partial labeling scheme. The
aromatic rings are labeled as in text and hydrogens have been removed for clarity.
A and F; C–H/p interaction between methylene carbon C35 and
ring F and methyl carbon C51 and ring C; H-bonding between
methyl carbon C50 and S2; H-bonding between phenylene carbon
C41 and O4; and H-bonding between phenylene carbon C29 and
O3. There are intra- and intermolecular H-bonding interactions
found in the unit cell. The hydroxyl oxygens O1 and O6 are acting as
double H-bond donors to imine nitrogens N1, N3 and thioether
sulfurs S1 and S3, respectively, in the two extended arms. In the
folded arm of the tripod however, O3 is having such intramolecular
H-bonding interactions only with imine nitrogen N2 but not to S2.
Intermolecular H-bonding between symmetry related O2 groups
produces H-bonded dimers. The packing of the molecule in the unit
cell has stacking down the b axis and shows weak intermolecular
C–H/O and C–H/N, and C–H/S type of H-bonding interactions.
data) such as edge to edge p/p interactions between the two rings

8558
V.K. Bhardwaj et al. / Tetrahedron 65 (2009) 8556–8562
Figure 2. Showing changes in the UV–vis spectrum of (3) in DMSO (10 mM) on addition of (100 m
M) various anions, inset shows the visual color change on addition of Fꢀ ion.
2.3. Anion sensing
12 equiv of Fꢀ. The appearance of a single isobestic point indicates
the presence of only two species, L and Lꢀ in the solution, Figure 4
shows the gradual decrease and increase in the concentration of
these two species with increase in equivalents of TBAF, respectively.
To investigate the binding sites of the receptor for the ions, NMR
titrations were carried out in DMSO-d6, with increasing amounts of
tetrabutylammonium fluoride as shown in Figure 5. NMR spectra
show that both signals for the –OH protons, which are observed at
9.18 and 13.08 ppm disappear completely much before the com-
plete addition of 1 equiv of TBAF, indicating that the interaction
with the Fꢀ ions is not through H-bonding but through proton
transfer (acid–base) process. Such a process has been found to be
important in anion binding specially for basic anions such as Fꢀ or
H2PO4ꢀ and moderately acidic groups like N–H15,16 and has also
been seen earlier in the case of nitrocatechol for the chromogenic
recognition of HSOꢀ4 and H2PO4ꢀ anions.11 This is however, different
than the two-step mechanism supposed to be working for the
recognition of halide ions by neutral H-bonding receptors having
urea/thiourea as binding groups.6d,17 It has been shown there that
the mechanism involves occurrence of two consecutive equilibria,
i.e., formation of a genuine H-bonded complex followed by
a deprotonation step. Fluoride ion sensors based upon the color
change accompanied by deprotonation are well known and some of
them have shown two different lmax in UV–vis spectra
The anion binding affinity of receptor (3) was determined by the
changes in absorption spectra of receptor (3) upon addition of
various anions such as Fꢀ, Clꢀ, Brꢀ, Iꢀ, NOꢀ3 , CNꢀ, ClO4ꢀ, AcOꢀ, HSOꢀ4 ,
and H2PO4ꢀ (Fig. 2). These experiments were performed with
10 mM solution of receptor (3) in DMSO by 100 mM tetrabuty-
lammonium salts of the anions. In the absence of anions, the
spectrum of receptor (3) in DMSO showed a band at lmax 274 nm
(
emax 44,600 Mꢀ1 cmꢀ1) and two shoulders at lmax 306 nm (emax
30,400 Mꢀ1 cmꢀ1) and lmax 353 nm (emax 19,200 Mꢀ1 cmꢀ1). The
latter has been designated as an intraligand or internal charge
transfer (ICT) band involving imine and hydroxyl group.13e,14 Ad-
dition of Fꢀ ion brings significant changes in the spectrum, on the
other hand, no change was observed with Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, CNꢀ,
ClOꢀ4 , AcOꢀ, HSO4ꢀ, and H2PO4ꢀ ions. On addition of Fꢀ ion in the
solution of (3) the highest energy band shows a slight hyp-
sochromic shift whereas the shoulder at 353 nm disappears and
a new band appears at lmax 433 nm. Figure 3 shows these changes
with gradual increase in concentration of Fꢀ ion.
Addition of 1.0 M equiv of Fꢀ to a 10
mM solution of receptor
3 causes visual changes in color from very pale yellow to bright
yellow, the color became more intense upon further addition of
Fꢀ to receptor (3) and is almost invariant after an addition of
0 eq
0.8
0.6
0.4
0.2
0
0.25 eq
0.5 eq
0.75 eq
1.0 eq
2.5 eq
5.0 eq
7.5 eq
10 eq
12 eq
25 eq
40 eq
250
350
450
nm
550
650
50 eq
60 eq
Figure 3. Changes in the absorption spectrum of (3) (10 m
M) on adding increasing amount of Fꢀ ion in it.

V.K. Bhardwaj et al. / Tetrahedron 65 (2009) 8556–8562
8559
0.5
0.4
0.3
0.2
0.1
0
analogous to those found in the case of Fꢀ ion. The negative charge
brought about by the anion induced deprotonation increases the
dipole moment and stabilizes the excited state causing a red shift of
Dl¼80 nm. We hypothesize that due to the strong acidity of the O–
H protons in the receptor (3), a genuine H-bond complex, [LH/F]ꢀ
has not been observed during the UV–vis titration, and the anion
causes deprotonation of the receptor. So the process may be de-
scribed by the proton-dissociation equilibrium only. The equilib-
rium constant (or proton-dissociation constant) K for Fꢀ ion has
been calculated to be 2.2ꢁ103 M from the Benesi–Hildebrand
plot.19 The absence of H-bonding steps may further be stressed by
comparison of UV–vis spectrometric results with that of nitro-
353 nm
433 nm
0
20
40
60
Eq. Of TBAF
catechol with H2POꢀ4 anion which also gives a shift from
to 430 nm after deprotonation.11
l 330 nm
Figure 4. Showing plot of absorption versus concentration of Fꢀ showing decrease and
The bright yellow color obtained on addition of TBAF to (3) is
reverted back on addition of traces of water to it. So the receptor,
unfortunately cannot be used in aqueous medium. We tried to see
the effect of change of polarity on the sensing ability of (3) by
changing the solvents from DMSO to CHCl3/CH3CN (1:9) since (3) is
insoluble in acetonitrile. In this solvent mixture the receptor (3)
shows a peak at lmax 316 nm and a shoulder at 366 nm (Supple-
mentary data, Fig. S1). The latter is tentatively labeled as the one
due to ICT transition as it shows a bathochromic shift of 30 nm in
the presence of 10 M equiv of TBAF. This shift is much less as in
comparison to what has been observed in DMSO and does not lead
to any visual color change. It shows that in less polar acetonitrile
the H-bonding interactions are prevalent whereas deprotonation is
being favored by DMSO, which is a very good proton acceptor. It
corroborates the fact that the receptor provides selectivity for
increase in absorption of the 353 nm and 433 nm bands, respectively.
F
ꢀ ion which is solely based upon its deprotonation and is related to
the factors (a) intrinsic acidity of the receptor (b) basicity of the
anion, and (c) polarity of the solvent. No deprotonation or colori-
metric response has been seen with anions like Clꢀ, Brꢀ, Iꢀ, NO3ꢀ,
CNꢀ, ClOꢀ4 , AcOꢀ, HSO4ꢀ, and H2PO4ꢀ ions, which are less basic than
OHꢀ (pKa¼32 in DMSO).20 Fluoride ion though is a weaker base
(pKa¼15 in DMSO) than AcOꢀ (pKa¼12.3 in DMSO)20 however the
extreme stability21 of [HF2]ꢀ is well documented and it is known to
behave as a very strong base, second to OHꢀ only and may induce
deprotonation of even lesser acidic moieties like urea protons.5m,6d
Here we maintain that the deprotonation forms the stable [HF2]ꢀ
anion. The existence of [FHF]ꢀ is unequivocally proved by the
Figure 5. Showing changes in 1H NMR (in DMSO) on addition of TBAF (0.0–0.2 mol
equiv). *Represent the peaks due to DMSO and DMSO–water.
corresponding to the H-bonded and the deprotonated
species.5h,5m,6d,15,18
The deprotonation of the receptor was confirmed by Bronsted
acid–base reaction between (3) and [n-Bu4]NOH (Fig. 6). A stepwise
increase in the concentration of the TBAOH produces results
presence of a tell–tale,1c,5a well formed triplet at w
d 16.07 ppm in
the proton NMR of (3) in the presence of 12 equiv Fꢀ ion as shown
in Figure 7. As the stability of [HX2]ꢀ increases in the order6d
X¼Fꢀ>CH3COOꢀ>H2POꢀ4 therefore the deprotonation is facilitated
Lig
1
0.8
0.6
0.4
0.2
0
Lig.+ TBAOH 1eq
Lig.+ TBAOH 2eq
Lig.+ TBAOH 3eq
Lig.+ TBAOH 4eq
Lig.+ TBAOH 5eq
Lig.+ TBAOH 6eq
Lig.+ TBAOH 7eq
Lig.+ TBAOH 8eq
Lig.+ TBAOH 9 eq
250
350
450
nm
550
650
Lig.+ TBAOH 10 eq
Figure 6. Showing changes in the absorption spectrum of (3) (10
m
M) on adding increasing amount of TBAOH (10–100 mM) in DMSO.

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V.K. Bhardwaj et al. / Tetrahedron 65 (2009) 8556–8562
3. Conclusions
In conclusion, a chromogenic tripodal receptor 3 has been
synthesized that contains recognition sites for anions. The receptor
had been designed to provide anion recognition through H-bond-
ing interactions employing –OH groups of catechol only. However
the results show that the deprotonation rather than the H-bonding
is the key factor triggering the chromogenic effect. This deproto-
nation is being facilitated by the high intrinsic acidity of catechol
groups, highly basic Fꢀ and OHꢀ ions, and a polar solvent like
DMSO. Although this frozen proton transfer5h from the receptor to
a highly basic anion pushes the recognition event out of the realm
of supramolecular chemistry but nonetheless it forms an example
of a highly selective and efficient naked eye sensor for Fꢀ ion at
a concentration of 10 mM. A comparison of the sensing ability of
tripodal receptor with monopodal receptor has showed that the
former gives a much enhanced response towards the fluoride ion.
Figure 7. Partial 1H NMR (DMSO) of (3) on addition of 12 M equiv of TBAF showing
presence of triplet corresponding to [FHF]ꢀ ion.
4. Experimental
4.1. General
in the same order. It is equally important that polarity of the solvent
and the intrinsic acidity of the receptor itself also support the
fluoride induced double deprotonation. As the catechol group is
much more acidic than urea or thiourea so can be more effectively
deprotonated in the presence of Fꢀ ion. However, unlike free cat-
echol in the presence of TBAF the deprotonation here in case of (3)
does not lead to oxidative degradation of the catechol moiety to
give muconic acid11 because the compound remains fairly stable in
the presence of higher equivalents of TBAF (cf. 1H NMR) and no blue
coloration is induced by the Fꢀ ions on prolonged standing. Thus
incorporation of catechol group in (3) has decreased its tendency of
oxidative degradation and improved its prospects as a highly se-
lective chromogenic sensor for Fꢀ ions.
All the commercially available chemicals were purchased from
Aldrich and used without further purification. All solvents were
dried by standard methods. Unless otherwise specified, chemicals
were purchased from commercial suppliers and used without fur-
ther purification. TLC was performed on glass sheets pre-coated
with silica gel. The 1H and 13C NMR spectra were performed in
DMSO with TMS as an internal reference, on a 300 MHz NMR
spectrometer. The infrared spectrum (KBr pellet) was recorded
using PYE Unicam IR spectrophotometer in the range 400–
4000 cmꢀ1. The electronic absorption spectra were recorded on
a Shimadzu Phramaspec UV-1700 UV–vis spectrophotometer.
The results obtained with the tripodal receptor (3) were also
compared with the analogous monopodal receptor (4) just con-
taining one catechol moiety. For this purpose, the receptor (4) was
prepared from the tripodal amine (2) by changing the amine to
aldehyde ratio and was characterized by various spectroscopic
methods. The anion binding affinity of receptor (4) for Fꢀ was de-
termined by tracking the changes in absorption spectra of receptor
(4) upon addition of TBAF. This experiment was also performed
4.2. Synthesis of receptors
4.2.1. Preparation of the tripodal ligand (3). Ligand 3 was prepared
by stirring tripodal amine22 (1) (531 mg, 1.0 mmol) along with 2,3-
dihydroxybenzaldehyde (2) (442 mg, 3.2 mmol) in the presence of
3–4 mg of zinc perchlorate taken in methanol/chloroform (8:2)
solvent mixture. The color of the solution changed immediately to
dark orange and precipitates separated out in quantitative yield.
These precipitates were filtered and dried. Yield 74%. Mp 238–
240 ꢂC. IR (nmax/cmꢀ1) 1616, 3418 (br), NMR data. dH (300 MHz,
DMSO, TMS): 2.58 (9H, s, –CH3), 3.98 (6H, s, –CH2), 6.74–7.05 (9H,
m, –Ar), 7.23–7.52 (12H, m, –Ar), 8.79 (3H, s, –CH]N), 9.18 (3H, s,
–OH), 13.08 (3H, s, –OH). dC (75 MHz, DMSO, Me4Si) 15.5 (–CH3),
32.20 (–CH2), 117.94 (Ar), 118.91 (Ar),119.32 (Ar),123.00 (Ar), 127.82
(Ar), 130.84 (Ar), 132.96 (Ar), 136.47 (Ar), 145.61 (Ar), 148.99 (Ar),
162.88 (CH]N), 186.26 (Ar). Found: C, 68.89; H, 5.09; N, 4.86; S,
10.87. Calcd for C51H45N3O3S3: C, 68.66; H, 5.08; N, 4.71; S, 10.78%.
with 10
of 100
ceptor (4) in DMSO showed
20,600 Mꢀ1 cmꢀ1
and shoulder at lmax 366 nm
m
M solution of receptor (4) in DMSO by stepwise addition
m
M TBAF salt. In the absence of Fꢀ ion, the spectrum of re-
a
band at lmax 310 nm
(
(
emax
emax
)
a
9600 Mꢀ1 cmꢀ1). The former corresponds to presence of an intra-
ligand CT related to amine group22 and the latter has been desig-
nated as ICT band involving imine and hydroxyl group13e,14 as for
(3) above. However the intensity of the latter in (4) is significantly
less than that in (3) due to only one chromophore being there.
Again there are significant differences between the UV–vis spectra
of (3) and (4) on addition of Fꢀ ion. Although as similar to (3), on
addition of TBAF to (4) the shoulder at 366 nm starts decreasing
and a new band starts emerging at lmax 433 nm. However, the in-
tensity of the latter is significantly less on comparing with same
number of equivalents of Fꢀ anion in (3). In fact it never takes the
shape of a well formed band unlike in (3) even on addition of
100 equiv of the anion in the solution of (4) (Fig. S2, Supplementary
data). The monopodal shows some discernable optical response
only at higher equivalents (w100 equiv) whereas (3) shows as
much for quite fewer equivalents (2.5 equiv) of added TBAF. At the
same time no naked eye detection of Fꢀ ion is possible with (4) at
these concentrations. The experiment clearly demonstrates that an
enhanced colorimetric response is obtained with the tripodal sys-
tem compared to the monopodal system.
4.2.2. Preparation of the monopodal ligand (4). Ligand 4 was pre-
pared as above but by drop wise addition of only (133.2 mg,
1.1 mmol) 2,3-dihydroxybenzaldehyde (2). The reaction mixture
was stirred for 4 h, after the completion of reaction solvent was
evaporated and product was recrystallized from methanol as red-
dish solid. Yield 72%. Mp 130 ꢂC. Rf value 0.675 in 40% ethyl acetate,
IR (nmax/cmꢀ1) 1605, 3443 (br), 3413, NMR data. dH (300 MHz,
DMSO, TMS): 2.30 (3H, s, –CH3), 2.35 (6H, s, –CH3), 3.92 (4H, s,
–CH2), 4.16 (2H, s, –CH2), 5.32 (4H, s, –NH2), 6.50 (2H, d, –Ar, J¼6.9),
6.72–6.81 (3H, m, –Ar), 6.90 (1H, d, –Ar, J¼6.6), 7.01–7.07 (3H, m,
–Ar), 7.17–7.21 (2H, m, –Ar), 7.29–7.39 (2H, m, –Ar), 7.49 (1H, d, –Ar,
J¼6.6), 7.56 (1H, d, –Ar, J¼7.2), 8.87(1H, s, –CH]N), 9.09 (1H, s,
–OH), 13.03 (1H, s, –OH). dC (75 MHz, DMSO, Me4Si) 15.79 (–CH3),
34.84 (–CH3), 79.34 (–CH2), 84.00 (–CH2), 114.78 (Ar), 116.94 (Ar),

V.K. Bhardwaj et al. / Tetrahedron 65 (2009) 8556–8562
8561
117.03 (Ar), 117.31 (Ar), 119.37 (Ar), 123.49 (Ar), 128.34 (Ar), 129.83
(Ar), 130.67 (Ar), 132.25 (Ar), 133.81 (Ar), 134.80 (Ar), 135.10 (Ar),
135.80 (Ar), 136.14 (Ar), 145.80 (Ar), 149.30 (Ar), 149.46 (Ar), 163.26
(CH]N). Found: C, 68.26; H, 5.89; N, 6.60; S, 14.83. Calcd for
C37H37N3O2S3: C, 68.17; H, 5.72; N, 6.45; S, 14.76%.
not very good and many crystals were tried before finally obtaining
this data set. It has resulted in giving a low ratio of observed/unique
reflections (18%). Nevertheless the anisotropic refinement of all the
non-hydrogen atoms with resulting estimated standard deviations
at the fourth place indicates that the overall accuracy of the
structure is not compromised. The crystallographic data and other
refinement parameters are given in Table 1.
4.3. Anion recognition studies
Acknowledgements
Anion binding ability of receptors (3) and (4) was determined by
preparing solutions containing 10 mM of receptor and 100 mM of
G.H. and V.K.B. are thankful to the CSIR, India for research grant
No. 01(2104)/07/EMR-II and research fellowship, respectively.
tetrabutylammonium salts of a particular anion in DMSO. Changes
in the electronic absorption spectra of the ligands were observed.
The anion binding ability of receptor (3) and (4) with tetrabuty-
lammonium fluoride (TBAF) was investigated using UV–vis titra-
tion experiments. The titrations were carried out in DMSO with
Supplementary data
10 mM concentration of receptor upon addition of incremental
amounts of TBAF solution.
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.08.023.
References and notes
4.4. X-ray crystal structure determination
1. (a) Sessler, J. L.; Gale, P. A.; Cho, W. S. Anion Receptor Chemistry; Royal Society of
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2006, 250, 3219; (c) Gunnlaugsson, T.; Glynn, M.; Tocci (nee Hussey), G. M.;
Kruger, P. E.; Pfeffer, F. M. Coord. Chem. Rev. 2006, 250, 3094; (d) Steed, J. W.
Chem. Commun. 2006, 2637; (e) Filby, M. H.; Steed, J. W. Coord. Chem. Rev. 2006,
250, 3200; (f) Nguyen, B. T.; Anslyn, E. V. Coord. Chem. Rev. 2006, 250, 3118; (g)
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The crystals suitable for crystallographic work were grown by
vapor diffusion method using CHCl3 as a solvent and petroleum
ether as precipitant. The intensity data were collected at 295 K with
a Siemens P4 X-ray diffractometer by using
with graphite monochromated Mo K radiation. A total of 8551
reflections were measured, of which 8090 were unique and 1416
were considered observed [Iꢃ2 (I)]. The data were corrected for
q–2q scanning mode
a
s
Lorentz and polarization effects but not for absorption correction.
The structure was solved by direct methods using SIR-200023 and
refined by full-matrix least-squares refinement techniques on F2
using SHELX-9724 in the WINGX program.25 All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were attached
geometrically with Uiso values of 1.2 times (for methylene and
phenylene carbon atoms) and 1.5 times (methyl carbon atoms) the
Uiso values of their respective carrier atoms. The crystal quality was
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´ ˜
`
05, 15, 267; (d) Martı`nez- Ma´n˜ez, R.;
Manez, R.; Sancenon, F. J. Fluoresc. 20
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Table 1
Crystal data and structure refinement for (3)
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Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C51H45N3O6S3
892.08
295(2) K
0.71069 Å
Triclinic
P ꢀ1
Unit cell dimensions
a¼11.466(3) Å
b¼12.652(5) Å
c¼16.172(4) Å
2205.0(12) Å3
2
a
b
¼108.980(5)ꢂ
¼91.240(3)ꢂ
g
¼95.450(4)ꢂ
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range
for data collection
Index ranges
1.344 mg mꢀ3
0.224 mmꢀ1
936
0.20ꢁ0.10ꢁ0.10 mm3
1.33–25.50ꢂ
0ꢄhꢄ13, ꢀ14ꢄkꢄ14,
ꢀ19ꢄlꢄ19
Reflections collected
8551
Independent reflections
Completeness to theta¼25.50ꢂ
Absorption correction
Refinement method
8090 [R(int)¼0.0843]
98.4%
`
´ ˜
´
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None
Full-matrix least-squares
on F2
8090/0/484
Data/restraints/parameters
Goodness-of-fit on F2
0.703
´
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Final R indices [I>2
R indices (all data)
Largest diff.
peak and hole
CCDC number
s
(I)]
R1¼0.0737, wR2¼0.1631
R1¼0.3495, wR2¼0.2798
0.285 and ꢀ0.272 e.Åꢀ3
721,608

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