Uncovering the true mechanism of optical detection of HSO4 - in water by Schiff-base receptors - Hydrolysis vs. hydrogen bonding

Article abstract of DOI:10.1039/c2cc33130f

The mechanism of optical detection of HSO₄^- in aqueous medium by Schiff-base receptors has previously been proposed to depend on selective hydrogen-bond interactions. Here, we clearly demonstrate for the first time that the acidic na

Full text of DOI:10.1039/c2cc33130f

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COMMUNICATION
ꢀ
Uncovering the true mechanism of optical detection of HSO in water
4
by Schiff-base receptors – hydrolysis vs. hydrogen bondingw
Virendra Kumar,z Ajit Kumar,z Uzra Diwan and K. K. Upadhyay*
Received 1st May 2012, Accepted 8th August 2012
DOI: 10.1039/c2cc33130f
ꢀ
The mechanism of optical detection of HSO4 in aqueous
medium by Schiff-base receptors has previously been proposed
to depend on selective hydrogen-bond interactions. Here, we
clearly demonstrate for the first time that the acidic nature of
this anion gives rise to hydrolysis of the Schiff base, which leads
to the optical changes observed in this family of receptors.
observed in UV-visible spectra of most of the above Schiff-
ꢀ
base receptors upon their binding with HSO4
.
Hence we went on to re-examine the mechanistic aspect of
3
these Schiff-base receptors already reported in literature for
ꢀ
their HSO
4
sensing. In this context first of all we were drawn
to one of the recently reported fluorescent Schiff-base recep-
3
tors reported by Kim et al. According to this report a Schiff
a
4ꢀ
base containing a coumarin (receptor 1; lmax = 355 nm), gave
The detection and sensing of HSO ion is of current interest
1
due to its Janus-faced properties in various fields. For this
highly selective and sensitive turn-on fluorogenic response
ꢀ
towards HSO4 in aqueous solution (lmax = 370 nm, lem
=
purpose a number of chemoreceptors have been designed,
synthesized and evaluated by various workers in last few
4
85 nm). Two analogues (receptors 2 and 3; Fig. 1) were also
2–4
prepared by the same workers and their fluorescent properties
years. Most of them are optical sensors and give fluorimetric/
2
colorimetric responses. These receptors can be classified into
,3
were also examined in the same fashion. Upon addition of
ꢀ
2
two classes viz., non-Schiff bases and Schiff bases. The
3
HSO
4
to 2 the observed fluorescence changes were marginal
ꢀ
4ꢀ
and non-selective. Receptor 3 (lmax = 375 nm) showed weak
former involve real hydrogen bonding with HSO
4
. Their
ꢀ
fluorescence which increased in the presence of HSO
4
(lem =
tailored design enable them to interact with HSO as well
ꢀ
B 430 nm), as noted for receptor 1.
as a few other tetrahedral analytes such as HClO
ꢀ
4
, ClO
4
,
1
2ꢀ
2
etc. through hydrogen bonding. One of the
According to Kim et al., H NMR studies and DFT
calculations revealed that hydrogen bonding between the
phenolic –OH and imine nitrogen of receptor 1 played a
H
2
PO
4
, SO
4
major bottle-necks of these receptors is their water intolerance
which restricts their use for real sample analysis in aqueous
solutions.
ꢀ
crucial role in its high selectivity towards HSO4 . The rupture
of this intramolecular hydrogen bonding and formation of
However, the Schiff-base type receptors reported by a
intermolecular hydrogen bonding in receptors 1 and 3 upon
ꢀ
number of workers in the last four to five years have been
ꢀ
3
aqueous solutions. The mechanism proposed is similar to that
their interactions with HSO4 were proposed to be the plau-
quite successful for HSO detection even in aqueous/semi-
4
sible reason for its fluorescent detection. It should be noted,
1
however, that the H NMR studies were performed in CD
CN
for the non-Schiff-base type receptors i.e., hydrogen bonding
ꢀ
3
while UV-visible and fluorescence investigations were performed
between the HSO
4
and the corresponding receptor. As we
have also been working in the field of design and synthesis of
5
optical receptors we went through these papers. In this
context reports by Kim et al. in 2009 involved fluorescent
3
receptors while all the remaining reports were of colorimetric
a
3b–e
type.
In these research papers two fundamental questions
perturbed us. The first one was how in aqueous/semi aqueous
ꢀ
+
media the HSO4 acted as a hydrogen (H ) acceptor in spite
2
of its low pK value of 1.99 and its competing nature with
h
a
6
water. The second question was why only blue shifts were
Department of Chemistry (Centre for Advanced Study),
Faculty of Science, Banaras Hindu University, Varanasi-221005,
India. E-mail: drkaushalbhu@yahoo.co.in; Tel: +91 542 670 2488
w Electronic supplementary information (ESI) available: Experimental
details and discussion; various spectroscopic characterization data for
the receptors. CCDC 873622. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc33130f
z These authors contributed equally to this work.
4ꢀ
Fig. 1 Fluorescent receptors (1–3) for HSO synthesized by Kim
et al. and by us (receptor 4).
9
540 Chem. Commun., 2012, 48, 9540–9542
This journal is c The Royal Society of Chemistry 2012

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in CH
3
CN–H
2
O (1 : 1, v/v) mixture. This variation of solvents
Table 1 Observed and reported labs and lem of starting materials and
of receptors 1, 3 and 4 with various analytes
a
in the above studies led Kim and co-workers to assign the
sensing effect as being due to hydrogen bonding without
exploring other plausible possibilities. In 2007, Kim and
Entry
labs/nm
lem/nm
b
b
ꢀ
7-AMC
1
370
355 /358
486
Almost NF
co-workers had suggested that the acidity of the HSO4 anion
3a
b
b
in water may be responsible for its different behaviour towards
bis(indolyl)calix[4]crown-6 as compared to basic anions such
4ꢀ
4ꢀ
ꢀ
3a
370 /371
3a
1
4
4
+ HSO
485
Almost NF
b
450, 476
369
b
b
ꢀ
ꢀ
+ HSO
487
430
as F , CH
pK value of HSO
3
COO etc. Indeed, as stated above, the literature
ꢀ
b
b
2h,7
1-AMP
3
3 + HSO4 /Hg
340
375 /380
340
a
4
is 1.99 (in aqueous medium)
+
and it
3a
8
Almost NF
B430
2+
3a,8
3a,8
must behave as a hydrogen (H ) donor in aqueous/semi-
+
aqueous medium instead of accepting H as for basic anions
ꢀ
a
UV-visible data were measured at 50 mM while fluorescence data
CN–H O (1 : 1, v/v) solution; NF =
7-amino-4-trifluoromethylcoumarin;
ꢀ
such as F , CH COO etc.
3
were obtained at 3 mM in CH
non-fluorescent; 7-AMC
3
2
Hence, we resynthesized receptor 1 (Fig. 1) as described by
1
Kim et al. and investigated its H NMR spectral behavior in
=
b
1-AMP = 1-aminopyrene. This work.
ꢀ
CD CN–D O (1 : 1, v/v) in the presence of HSO . Surpris-
3
2
4
ingly, we observed hydrolysis of receptor 1, as we observed the
peaks for the corresponding aldehyde and amine (see Fig. S1w;
due to presence of D O the signal for –OH of salicylaldehyde
absorbs at lmax = 370 nm while it emits at lem = 485 nm in
CH CN–H O (1 : 1, v/v) (Fig. S9 and S10w).
When we carried out H NMR studies upon addition of
3
2
1
2
ꢀ
was not observed). We thus performed a further experiment in
ꢀ
HSO
4
3 2
to receptor 4 in CD CN–D O we observed peaks for
which we added HSO
4
to receptor 1 solution in CH
3
CN–
the individual amine and aldehyde similarly to above for 1 +
ꢀ
1
H
2
O (1 : 1, v/v) and left to hydrolyse for 3–4 h. After extrac-
HSO
4
(Fig. S11w). Moreover the H NMR (Fig. S12w) and
tion with diethyl ether and evaporation we characterized the
1
recovered product through H NMR as the amine counterpart
ESI mass spectrum of extracted products as above in the case
of receptor 1, clearly supports the hydrolysis of receptor 4 into
its constituents (Fig. S13w). The changes in absorption and
emission wavelengths of receptors (1, 3 and 4) before and after
of the receptor 1 i.e., 7-amino-4-trifluoromethylcoumarin
(
Fig. S2w).
Hence the above studies showed evidence for hydrolysis of
ꢀ
2+
binding with HSO4 /Hg are summarized in Table 1.
We tried to co-relate the above observations with one yet
8
another piece of work by Kim et al. published in 2010. In this
the Schiff base instead of any hydrogen bonding adduct as was
reported by Kim et al. due to non-consideration of water in
1
their H NMR studies. The hydrolysis of the Schiff base in the
work they proposed a chemodosimetric approach (hydrolysis
ꢀ
2+
presence of HSO
4
was further supported by mass spectral
ꢀ
of Schiff base) for the sensing of Hg
through the same
CN–H
(9 : 1, v/v). The corresponding fluorescence changes in the
studies for receptor 1 + HSO
4
. This showed a molecular ion
Schiff-base receptor 3 as mentioned above in CH
3
2
O
peak for the corresponding aldehyde and amine (Fig. S3w).
The mass spectrum showed no peak for either receptor 1 or its
2
+
receptor 3 by Hg matched excellently well with the case of
ꢀ
ꢀ
HSO
4
complex.
3 + HSO
4
(lmax = 341 nm lem = 430 nm; see Fig. S14w).
The amine counterpart of the receptor 3 i.e., 1-aminopyrene
To further establish the above hydrolysis mechanism we
synthesized a new Schiff-base receptor 4 (see Fig. 1) having
the same amine i.e., 7-amino-4-trifluoromethylcoumarin
8
absorbs at 340 nm and fluoresces at 430 nm (see Fig. S15w)
ꢀ
2+ 3a,8
.
which matched well with that of 3 + HSO4 and 3 + Hg
(
(
fluorescent constituent) but different aldehyde counterpart
2-hydroxynapthaldehyde instead of salicylaldehyde). Receptor
Two different mechanistic pathways by Kim et al. for fluoro-
2
genic sensing of Hg and HSO4 by the same receptor 3 are
+
ꢀ
4
was characterized through various spectroscopic methods
very difficult to understand. Hence our proposal of hydrolytic
ꢀ
along with single-crystal X-ray diffraction study (Fig. S4–S8;
mechanism for the sensing of HSO
4
by Schiff-base receptors in
Table S1w). Indeed, preliminary investigations of visual
aqueous medium was further strengthened. In the light of above
ꢀ
responses of 4 towards HSO
4
were similar to that of 1. We
discussions the fate of receptor 1 and 3 in the presence of
ꢀ
then performed more detailed UV-visible and fluorescence
ꢀ
HSO
4
and other analytes can be summarized conveniently
titrations of HSO
4
and receptor 4 in CH CN–H O (1 : 1, v/v).
3 2
through Schemes 1 and 2 in ESIw.
Kim et al. extended their similar research work later on with
Interestingly the UV-visible absorption and fluorescence emis-
sion bands were observed at almost the same wavelengths
a few other Schiff-base receptors for the detection of a variety
9
2
+
3+
(
lmax = 369, lem = 487 nm; Table 1) as was found for
of metal ions viz., Cu and Fe etc. The lability of 4CQN
towards hydrolysis under the influence of above metal ions
ꢀ
receptor 1 by Kim et al. The l
for the complex 1 + HSO4
max
ꢀ
and 4 + HSO4 were at 370 and 369 nm while lem were
observed at 485 and 487 nm respectively (Fig. S9 and S10w).
This similarity of observations in terms of lmax and lem even
after changing the aldehyde constituent of receptor 4 also
suggested the hydrolytic mechanism of sensing proposed by us
rather than by hydrogen bonding as suggested previously.
Since receptors 1 and 4 both possess the same amine counter-
part i.e., 7-amino-4-trifluoromethylcoumarin, hence in both
cases we observed lmax and lem at almost the same wave-
lengths, around 370 and 485 nm, respectively. The same amine
was made the basis for sensing. In analogy to Kim’s work with
ꢀ
metal ions, HSO
4
can also have a similar hydrolytic effect due
+
to its ability to release H , a factor which is often overlooked.
3+
and
+
2+
In fact the smaller charge/size of H than Cu , Fe
2
+
Hg , provides it good acidic character. Since the hydrolytic
cleavage of Schiff bases were observed by a number of metal
ions we also performed similar studies on receptors 1 and 4.
3
+
Out of the various metal ions only Al
was found to be
effective (Fig. S16w). The hydrolysis of receptor 3 was also
observed in the presence of Al , which was not considered by
3
+
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 9540–9542 9541

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4ꢀ
Table 2 Reported labs of starting materials and of receptors 5 and
a–d with various analytes
HSO in aqueous medium is responsible. The present study
further establishes that the Schiff-base type receptors may
prove to be good candidates for selective and sensitive detec-
6
Entry
-PASA
Receptor 6a/6b
l
abs/nm
ꢀ
tion of HSO4 in aqueous medium.
12
5
341
3
c
Authors are thankful to Dr P. K. Roychowdhury, Associate
Director, Chembiotek Research International, Kolkata for
enabling the facility of 400 MHz NMR and mass spectrometer.
A. K. acknowledges UGC, New Delhi for DSK-Postdoctoral
Fellowship [F.4-2/2006(BSR)/13-398/2011(BSR)] while V. K.
and U. D. are thankful to UGC, New Delhi for University
Research Fellowships.
6a = B350 (s), B480 (w)
b = B330 (s), B480 (w)
B340
6
4ꢀ
6
5
5
5
a/6b + HSO
-PNSA
3b
376
376, 480
370
6c = B380 (m), B540 (s)
d = 412 (w), 530 (s)
B374
3
b
4ꢀ
+ HSO
3
c
Receptor 6c/6d
6
4ꢀ
6c/6d + HSO
5-PASA = 5-(phenylazo)salicylaldehyde; 5-PNSA = 5-(p-nitro-
phenylazo)salicylaldehyde, w = weak, m = medium, s = strong.
Notes and references
1
(a) E. A Katayev, Y. A. Ustynyuk and J. L. Sessler, Coord. Chem.
Rev., 2006, 250, 3004; (b) A. Bianchi, E. Garcia-Espana and
K. Bowman-James, in Supramolecular Chemistry of Anions,
Wiley-VCH, New York, 1997.
Kim et al. in their studies. Here it should be noted that not all
the Schiff bases undergo this type of hydrolysis with every
+
3
2
(a) J. V. Ros-Lis, R. Martinez-Manez, A. Benito and J. Soto,
Polyhedron, 2006, 25, 1585; (b) M. Alfonso, A. Tarraga and
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X.-H. Zhang, S.-K. Wu and S.-T. Lee, Org. Lett., 2004, 6, 1071;
metal ion. Our own report showed Al
enhanced fluores-
10
5
cence of a Schiff-base receptor. The work of Lehn’s et al.
a
1
1
and others have presented an excellent account in this
context.
On the similar line to the above fluorescent Schiff-base
3
b–e
receptors we went through a number of reports
ꢀ
involving
colorimetric Schiff-base receptors for HSO
4
detection. Such
(
g) A. Mallick, T. Katayama, Y. Ishibasi, M. Yasuda and
receptors are different from the fluorescent ones in terms of
not having any fluorescent constituents i.e., aldehyde or
amine, rather they behaved as strong intramolecular charge
transfer (ICT) probes and absorbed towards higher wave-
H. Miyasaka, Analyst, 2011, 136, 275; (h) X. He, S. Hu, K. Liu,
Y. Guo, J. Xu and S. Shao, Org. Lett., 2006, 8, 333; (i) R. Shen,
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3
(a) H. J. Kim, S. Bhuniya, R. K. Mahajan, R. Puri, H. Liu,
K. C. Ko, J. Y. Lee and J. S. Kim, Chem. Commun., 2009, 7128;
lengths. All these receptors were reported to undergo bleaching
ꢀ
with HSO
4
and thus exhibited blue shifting in their UV-visible
(
b) P. Li, Y.-M. Zhang, Q. Lina, J.-Q. Li and T.-B. Wei, Spectro-
spectra. However bathochromic shifting has been reported for
genuine hydrogen bonded cases of anions with receptors. This
differing observation led us to think once again in terms of a
hydrolytic mechanism as we have discussed above. The mecha-
nistic details of interactions of colorimetric Schiff-base recep-
tors (receptors 5 and 6; reported by Wei et al. and Zhang et al.,
chim. Acta, Part A, 2012, 90, 152; (c) T.-B. Wei, J. Wang and
Y.-M. Zhang, Sci. China, Ser. B: Chem., 2008, 51, 1051; (d) W. Lu,
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Z. Shen, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 590.
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ꢀ
Fig. S17w) with HSO4 are discussed in detail (see ESIw;
Discussion section). The corresponding absorption changes
5
ꢀ
of these receptors before and after binding with HSO4 are
given in Table 2.
Hence, our above discussion and experimental evidences
clearly proved that the hydrolysis of Schiff-base receptors
ꢀ
6 S. Kubik, R. Kirchner, D. Nolting and J. Seidel, J. Am. Chem.
Soc., 2002, 124, 12752.
through HSO
4
in aqueous media was the key step towards
7
8
9
J. W. Lee, S. Y. Park, B.-K. Cho and J. S. Kim, Tetrahedron Lett.,
007, 48, 2541.
J. H. Kim, H. J. Kim, C. W. Bae, J. W. Park, J. H. Lee and
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Chem. Commun., 2011, 47, 5142.
the sensing rather than hydrogen bonding as claimed pre-
ꢀ
2
viously. The overall effect of HSO
4
on Schiff-base type
receptors is highly dependent on the solvent in which the
experimental studies are carried out as it plays the key role
in deciding the acidic/basic character of an amphiphilic ion
such as HSO4ꢀ
. So the role of solvents cannot be ignored in
the above type of sensing phenomenon. It is clearly necessary to
maintain the same solvent medium throughout entire experi-
mental studies while establishing a mechanism for its action.
10 C. Godoy-Alcantar, A. K. Yatsimirsky and J.-M. Lehn, J. Phys.
Org. Chem., 2005, 18, 979 and references therein.
11 (a) A. C. Dash, B. Dash and S. Praharaj, J. Chem. Soc., Dalton
In conclusion, we were successful in establishing that hydrogen
ꢀ
¨
Trans., 1981, 2063; (b) V. Saggiomo and U. Luning, Tetrahedron
Lett., 2009, 50, 4663; (c) I. A. Muller, F. Kratz, M. Jung and
bonding between HSO
4
and Schiff-base receptor was not the
¨
A. Warnecke, Tetrahedron Lett., 2010, 51, 4371; (d) S. Zameo,
B. Vauzeilles and J.-M. Beau, Eur. J. Org. Chem., 2006, 5441.
2 M. Odabasoglu, C. Albayrak, R. Ozkanca, F. Z. Aykan and
P. Lonecke, J. Mol. Struct., 2007, 840, 71.
determining factor in the sensing mechanism in aqueous
medium as reported previously. Rather, hydrolysis of the
Schiff-base receptor by a sufficiently strong acid such as
1
9
542 Chem. Commun., 2012, 48, 9540–9542
This journal is c The Royal Society of Chemistry 2012