The curious case of salicylidene-based fluoride sensors: Chemosensors or chemodosimeters or none of them

Article abstract of DOI:10.1039/d0ra02293d

Fluoride-ion induced hydrolysis of imine (CN) bonds has not been documented in the literature, in spite of the numerous salicylidene-based fluoride sensors studied over the years. Herein, we have proposed a mechanism for fluoride-ion induced hydrolysis of salicylidene compounds (SL and CL1-3) based on ¹H-NMR, ¹⁹F-NMR, LC-MS and UV-vis spectroscopy experiments.

Full text of DOI:10.1039/d0ra02293d

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The curious case of salicylidene-based fluoride
sensors: chemosensors or chemodosimeters or
none of them†
Cite this: RSC Adv., 2020, 10, 14689
Sandeep Kumar Dey* and Christoph Janiak *b
a
Received 11th March 2020
Accepted 3rd April 2020
Fluoride-ion induced hydrolysis of imine (C]N) bonds has not been documented in the literature, in spite of
the numerous salicylidene-based fluoride sensors studied over the years. Herein, we have proposed
a mechanism for fluoride-ion induced hydrolysis of salicylidene compounds (SL and CL1–3) based on
DOI: 10.1039/d0ra02293d
1
19
rsc.li/rsc-advances
H-NMR, F-NMR, LC-MS and UV-vis spectroscopy experiments.
curious to understand, why is it difficult to obtain crystals of
salicylidene receptor–uoride complex or deprotonated salicy-
Introduction
Chromogenic and uorogenic anion sensing has witnessed lidene receptor. In order to address this question, we have
extensive research interest over the past two decades due to analyzed the mechanism of uoride sensing/binding by a new
instant detection of anions of biological and environmental salicylidene based tripodal amide receptor (SL) and three sali-
relevance by visual colour change, or highly sensitive spectro- cylidene control compounds (CL1–3) based on commercially
1
photometric techniques. Several salicylidene Schiff-base available aromatic amines (Scheme 1).
compounds have been synthesized for anion sensing over the
It is important to mention that the objective here is not to
years due to their facile synthetic procedures and their anion- explore the uoride sensing properties of salicylidene Schiff-
2
binding constants have been determined. It is interesting to bases but to address the limitations of their applicability as
note that all salicylidene-based anion sensors have been re- uoride sensors which have not been addressed before. Our
ported to sense uoride and oen acetate in aprotic organic nding of a uoride-induced imine-bond hydrolysis strongly
2
,3
media, with visible colour changes.
The mechanism of suggests that regenerated salicylaldehyde can also behave as
ꢀ
sensing has been attributed to either –OH/F hydrogen a uoride-ion sensor even aer the salicylidene Schiff-base has
bonding or –OH deprotonation by uoride to give HF/HF in hydrolyzed in solution. This implies that the optical properties
solution. However, it is surprising to note that no crystal
ꢀ
2
3
structures of hydrogen-bonded salicylidene receptor-uoride
complexes or deprotonated salicylidene receptor (with

uoride-supplied counter-cations) has been reported in any of
the published literature, to the best of our knowledge. In
contrast, anion receptors with –NH hydrogen bond donor
groups (such as amide, urea, pyrrole, indole and even amines)
are oen reported with crystal structures of receptor–anion
complexes or deprotonated receptor alongside their solution
4
state anion binding properties. Given the numerous reports on
2,3
salicylidene based anion sensors, it is hard to assume that no
one has ever tried crystallization of a salicylidene receptor with

uoride but might have failed in their attempts. We have been
a
School of Chemical Sciences, Goa University, Taleigao Plateau, Goa 403206, India.
E-mail: sandeepdey@unigoa.ac.in; Tel: +91-7387633550
b
Institute for Inorganic and Structural Chemistry, Heinrich-Heine University
D u¨ sseldorf, 40225 D u¨ sseldorf, Germany. E-mail: janiak@uni-duesseldorf.de; Tel:
+
49-2118112286
†
Electronic supplementary information (ESI) available: Synthesis details with
Scheme 1 Molecular structures of tripodal salicylidene receptor SL
synthesized from tris(4-amino-N-ethylbenzamide)amine AL and sali-
cylaldehyde SA, and control receptors CL1–3 (synthesis details are
provided in the ESI†).
1
19
NMR and IR spectra of Schiff bases, H and F-NMR spectra of reaction with
uoride and UV-vis spectra of SL and CL1–3 in presence of TBAF. See DOI:
0.1039/d0ra02293d

1
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shown by the regenerated salicylaldehyde in presence of uo- the new peak at 10.1 ppm (–CHO) was a possible indication
ride could be mistaken as uoride sensing by salicylidene towards the formation of salicylaldehyde by imine bond
1
Schiff-bases.
cleavage in SL. H-NMR of the solution mixture (SL and TBAF)
when recorded on the next day (aer 24 h) showed further
upeld shi and broadening of the –OH signal (D2, Fig. 1),
indicating gradual deprotonation of the –OH protons. Disap-
pearance of the salicylidene –OH proton upon addition of TBAF
has also been observed for the reported salicylidene-based
anion sensors. Further, the intensity of the salicylaldehyde
–CHO peak at 10.1 ppm strengthens along with the other new
aromatic peaks notably the –NH2 peak of regenerated AL at
Results and discussions
1
H-NMR spectra of SL or CL1–3 recorded in the presence of

uoride salt(s) showed interesting results which was either not
2,3
addressed or observed in earlier reports on salicylidene-based
1

uoride sensors. Our H-NMR experimental results showed
initial deprotonation of salicylidene –OH in SL followed by slow
hydrolysis of imine bonds in the presence of uoride salts (as
TBAF/CsF with TBA ¼ tetra-n-butylammonium). Similar to SL,
5.6 ppm, validating the slow hydrolysis of the SL imine bonds. It
was expected that the hydrolysis of imine bonds would nally
1
lead to the disappearance of the imine –N]CH proton signal at
H-NMR results showed hydrolysis of imine bond(s) in control
1
8
.9 ppm and thus, we continued to record the H-NMR spectra
compounds CL1–3 in the presence of a uoride salt both in
of the solution mixture till complete hydrolysis resulted.
Complete hydrolysis of SL was observed aer 9 days (D10,
Fig. 1), where the –N]CH proton signal has disappeared almost
completely and the aromatic proton signals corresponding to
regenerated AL and deprotonated salicylaldehyde could be
observed distinctly. Integration of the –CHO and –N]CH peaks
of the spectra showed 23%, 44%, 64%, 83% and 93% hydrolysis
on day2, day3, day4, day7 and day9 respectively (Fig. S20, ESI†).
It is noted that upon complete hydrolysis, the amide –NH and
an aromatic –CH proton (ortho to the amide group) signals in
dimethyl sulfoxide-d6 (DMSO-d ) and chloroform-d (CDCl3).
This raises the fundamental question if salicylidene-based
6
1
anion receptors are applicable as uoride sensors to begin with.
1
The H-NMR spectrum of SL in DMSO-d
6
(0.6 mL) showed
the –OH signal at 12.8 ppm, imine –N]CH at 8.9 ppm and
amide –NH at 8.4 ppm (Fig. 1). Addition of TBAF (10 equiv.)
showed a large upeld shi of the –OH signal (Dd ¼ 2.1 ppm)
ꢀ
with concomitant broadening indicative of O–H/F hydrogen
bonds, and also showed the emergence of a new peak at
10.1 ppm together with several low intensity peaks appearing in
1
regenerated AL was downeld shied with respect to the H-
the aromatic region (D1, Fig. 1) within an hour. Appearance of
NMR spectrum of AL due to hydrogen bonding interactions
with uoride present in the solution mixture (D10 and AL,
Fig. 1). Downeld shi of amide –NH and ortho-CH protons in
the presence of TBAF has previously been observed for Tren-
5
19
based tripodal amide receptors.
F-NMR of the solution
showed two peaks occurring at ꢀ69 and ꢀ138 ppm for uoride
ꢀ
and hydrogen diuoride (HF
2
) anions respectively (Fig. S25,
ꢀ
ESI†). The in situ formation of the HF2 anion is discussed later
in the plausible mechanism of imine-bond hydrolysis by uo-
ride (Scheme 2). The liquid chromatography-mass spectrum
(
LC-MS) of the NMR solution mixture (diluted with acetonitrile)
+
recorded aer 10 days showed m/z at 504.27 (AL + H ), and no
peak at m/z 816.34 (SL + H ) was observed, thereby supporting
+
1
Fig. 1 H-NMR spectra (DMSO-d
6
) of SL in the presence of TBAF for
1
0 days (D1–D10). Signals labelled with red, black, green and blue
circles indicate amide –NH, ortho-CH, meta-CH and –NH protons of
2
AL respectively. Signals labelled with red, and blue stars indicate –CHO
and –OH protons of salicylaldehyde and SL respectively. (A larger sized
version of the spectra with also additional spectra recorded on D3 and Scheme 2 Plausible mechanism for the fluoride ion induced imine
D9 is provided in Fig. S20, ESI†).
bond hydrolysis in salicylidene based Schiff bases SL and CL1–3.
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the suggested complete hydrolysis of SL to AL and salicylalde-
hyde (Fig. S21 and S22, ESI†).
In another experiment, addition of CsF (10 equiv.) to
a DMSO-d solution (0.6 mL) of SL showed immediate disap-
6
pearance of the –OH signal, the downeld shi of the amide
–
NH signal (Dd ¼ 0.9 ppm) with concomitant broadening, and
the distinct resolution of six different types of aromatic protons
with upeld or downeld shis (Fig. S23, ESI†). H-NMR of the
1
solution mixture (SL and CsF) recorded on the next day (aer 24
h) showed a feeble appearance of the –CHO peak at 10.1 ppm
which subsequently intensied in the spectra recorded on
successive days. In contrast to the complete hydrolysis of SL
observed with TBAF, only 50% hydrolysis of SL was observed to
1
Fig. 2 H-NMR spectra (DMSO-d
2
6
) of CL1 in the presence of TBAF for
consecutive days (D1–D2). Signals labelled with red and blue circles
indicate –CHO and –NH protons of regenerated salicylaldehyde and
2
be completed with CsF in 10 days, possibly due to the weaker 4-aminobenzonitrile, respectively.
dissociation of CsF in an aprotic medium compared to TBAF
(further hydrolysis could be observed on successive days). No
observable shi of the imine –N]CH signal was noticed, hydrolysis was observed (D3 in Fig. S30, ESI†). Addition of 3
however the signal gradually split into two peaks (separated by more equivalents of TBAF to the solution mixture showed
0.03 Hz) possibly due to the presence of both keto and enol further hydrolysis of imine bonds revealing 50% hydrolysis of
forms of deprotonated SL in the presence of uoride salt, which CL2 beyond which again no further hydrolysis was observed (D4
has also been observed in the spectra showing hydrolysis of SL and D5, Fig. S30, ESI†). The LC-MS of the NMR solution mixture
by TBAF. Imine bond cleavage of SL has also been observed in (diluted with acetonitrile) recorded aer 6 days showed m/z at
the presence of KF (Fig. S24, ESI†).
213.10, which suggests partial hydrolysis of CL2 with the loss of
1
9
To understand the origin of hydrolysis of SL in the presence only one salicylaldehyde molecule (Fig. S32, ESI†). F-NMR of
1
of TBAF/CsF, we have recorded H-NMR spectra of SL in the the solution showed two peaks at ꢀ69 and ꢀ138 ppm for uo-
ꢀ
presence of Cs CO . Carbonate being a stronger base than ride and hydrogen diuoride (HF ) anions, respectively
2
3
2

uoride would result in a faster base-catalyzed hydrolysis of SL. (Fig. S33, ESI†), similar to that observed for SL added with TBAF.
However, the spectral changes were largely the same as Hydrolysis of CL2 in the presence of TBAF has also been
observed with CsF. A DMSO-d solution (0.6 mL) of SL and observed in CDCl (Fig. S31, ESI†).
Cs CO (3 equiv.) showed the appearance of the salicylaldehyde
CHO signal at 10.1 ppm with 37% hydrolysis completed in 24
6
3
2
3
–
hours. The solution mixture when recorded on the subsequent
days showed up to 70% hydrolysis beyond which no further
hydrolysis was observed (Fig. S26, ESI†). Due to solubility issue,
addition of more than 3 equiv. of Cs CO into the NMR solution
2
3
was not achieved.
In order to further validate the uoride-ion induced hydro-
lysis of imine bonds, we have synthesized three salicylidene-
based control compounds CL1–3 (Scheme 1). Structurally
similar Schiff bases have previously been reported as uoride
2,3
1
sensors in aprotic media. H-NMR experiments revealed that,
addition of 2 equiv. of TBAF to a solution of CL1 in DMSO-d6
showed 50% hydrolysis in one hour and complete hydrolysis
within 24 hours (Fig. 2), also conrmed by LC-MS (Fig. S29,
ESI†). However, hydrolysis was observed to be much faster in
3
CDCl , with 90% hydrolysis of CL1 in one hour in the presence
of 2 equiv. of TBAF (Fig. S28, ESI†). Thus, it is understood that
the nature of solvents signicantly affects the rate of imine
bond hydrolysis, where the solubility of the salicylidene
compound, solvent polarity and extent of dissociation of the

uoride salt into free ions might play a crucial role.
Both CL2 and CL3 (positional isomers) have two imine
1
Fig. 3 H-NMR spectra (DMSO-d
consecutive days (D1–D5). Signals labelled with (e) and (k) indicate
the enol and keto forms of CL3, respectively. Signals labelled with red
and blue circles indicate –CHO and –NH protons of salicylaldehyde
and regenerated amine, respectively. (Additional spectra recorded for
aer 24 hours (D2 in Fig. S30, ESI†), beyond which no further D3, D7 and D10 are provided in Fig. S34, ESI†).
6
) of CL3 in the presence of TBAF for
linkages, and addition of excess TBAF (5 equiv.) resulted in the
5
hydrolysis of only one imine bond which was conrmed by LC-
1
MS and H-NMR experiments. Addition of 2 equiv. of TBAF to
2
6
a solution of CL2 in DMSO-d showed about 40% hydrolysis
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2ꢀ
being present in excess) to give HF and the deprotonated
receptor in the second step. The deprotonated enol form is in
1
equilibrium with the keto form as observed in the H-NMR
spectra of SL and CLs which are mixed with uoride salt,
showing the splitting of the imine –N]CH proton signals. In
the third step, a nucleophilic attack of the in situ generated
ꢀ
ꢀ
ꢀ
2
hydroxide ion (2F + H O $ OH + HF ) on the imine carbon
2
followed by protonation of the imine nitrogen in the solution
forms hydroxy(phenylamino)methyl phenolate. Finally, forma-
tion of the C]O double bond by the loss of the O–H proton
results in the cleavage of C–N bond to give salicylaldehyde and
respective aromatic amine.
To further understand the uoride-induced hydrolysis of
salicylidene compounds, we have carried out UV-vis spectro-
photometric studies of SL and CL1–3. The UV-vis spectrum of SL
ꢀ
5
ꢀ1
Fig. 4 (a) UV-vis spectra of SL (1 ꢁ 10 mol L ) in presence of TBAF
(
(
10 equiv.) recorded over a period of one-week (b) UV-vis spectra of AL
ꢀ5
ꢀ1
1 ꢁ 10 mol L ) and salicylaldehyde (SA) in presence of TBAF and/or
CsF (day-1: spectrum was recorded immediately after addition of TBAF
to a SL solution, day-2: the solution mixture of SL and TBAF of day-1
was recorded after 24 hours, and so on).
ꢀ
5
ꢀ1
in DMSO (1 ꢁ 10 mol L ) showed two absorption bands at
Similar experiments have been carried out with CL3 which 280 nm and 345 nm. Addition of uoride salt (10 equiv.)
showed about 38% hydrolysis in the presence of 2 equivalents of resulted in the development of a new band at 410 nm respon-
TBAF in DMSO-d (Fig. 3). Further addition of TBAF (3 equiv.) sible for the visible change in colour from colourless to yellow
6
revealed 55% hydrolysis in 5 days beyond which no further (Fig. 4a). No signicant visible spectral changes of the solution
ꢀ
hydrolysis observed (Fig. 3 and S34, ESI†). LC-MS of the NMR mixture (SL + F ) were observed over a period of one week
solution mixture (diluted with acetonitrile) showed m/z at (Fig. 4a), although we were expecting the absorbance at 410 nm
2
13.10 (Fig. S35, ESI†), similar to CL2. Interestingly, upon to be signicantly reduced due to hydrolysis of imine bonds
addition of TBAF to CL3 solution, the singlet peak of –N]CH (loss of conjugation). Similar spectral behaviours have also been
9.0 ppm) splits into two distinct peaks occurring at 8.9 and observed for a DMSO solution mixture of CL added with TBAF
.0 ppm further conrming the presence of both keto and enol (Fig. S38–S40, ESI†). To understand this, we have carried out
forms of salicylidene compounds in presence of uoride control experiments with DMSO solutions of salicylaldehyde
(
9
(
Fig. 3), as discussed earlier for SL. Splitting of the –N]CH peak and AL. No spectral changes have been observed upon addition
ꢀ
5
ꢀ1
has also been observed for CL2 in presence of TBAF with of TBAF to a solution of AL (1 ꢁ 10
mol L ) showing
a separation of 0.01 Hz, similar to those observed for SL (Fig. 1). absorption at 280 nm. However, UV-vis spectral analysis of
ꢀ
5
ꢀ1
Thus, due to the apparently unavoidable imine-bond salicylaldehyde (1 ꢁ 10 mol L ) in the presence of CsF or
hydrolysis it would not seem possible to obtain crystal struc- TBAF showed the emergence of a new peak at 405 nm (Fig. 4b),
tures of hydrogen-bonded salicylidene receptor–uoride with concomitant change in colour from colourless to yellow
complex or deprotonated salicylidene receptor. We note that the (Fig. S41, ESI†). Thus, it is conrmed that salicylaldehyde alone
presence of water is hard to avoid as the uoride-containing can behave as a colorimetric sensor for uoride. This is the
medium is hygroscopic (TBAF and CsF salts are hygroscopic). reason why imine-bond hydrolysis did not show any effect on
1
Based on the H-NMR experimental results, we have deduced the absorption of a uoride-containing SL or CL solution.
a mechanism for uoride-ion induced imine-bond hydrolysis as
Thus, deprotonated salicylaldehyde formed by uoride-ion
depicted in Scheme 2. Before discussing the mechanism, it is induced imine bond hydrolysis could be mistaken as uoride
important to learn that some urea-based anion receptors when sensing by salicylidene Schiff-bases.
crystallized in the presence of excess TBAF formed crystals of
hydrogen bonded urea-bicarbonate (or urea-carbonate)
complex. It has been proposed that two uoride ions react
Conclusion
6
ꢀ
ꢀ
ꢀ
with a water molecule to generate OH and HF anions (2F
+
In conclusion, we have demonstrated a uoride-ion induced
H O $ OH + HF ). Hydroxide formed then reacts with CO at imine-bond hydrolysis in salicylidene-based receptors and
2
ꢀ
ꢀ
2
2
2
1
19
the air–solvent interface to form bicarbonate. Along this line, it proposed a mechanism based on H-NMR, F-NMR, LC-MS
can be argued that the uoride induced in situ generation of and UV-vis experiments. However, one question that needs to
hydroxide ions is quite feasible in the solutions containing be addressed in this context is that “if the salicylidene Schiff-base
salicylidene compounds mixed with uoride salt.
sensor molecule which hydrolyses in the presence of a uoride salt,
The rst two steps in the proposed mechanism (Scheme 2) can it be called a uoride sensor”? The answer to this is arguable,
are based on the mechanisms proposed for uoride sensing by because if the receptor and the analyte react to produce a new
3
the reported salicylidene based sensors, and further supported compound (in situ) resulting in changes in absorbance/
by our experimental observations. In the rst step, a uoride ion uorescence/NMR peaks, it can be called a reactive sensor or
7
forms a transient hydrogen-bonded complex with the –OH chemodosimeter, but not when the original starting materials,
group of salicylidene unit followed by deprotonation of –OH here salicylaldehyde, are regenerated. Finally, the fact that sal-
(
–
due to the basicity of uoride and acidic nature of phenolic icylaldehyde can sense uoride by distinctive colour changes
ꢀ
ꢀ
OH) releasing HF which combines with another F ion (F
(colourless to yellow) due to the emergence of a peak above
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4
00 nm is concerned with the applicability of several salicyli-
(f) L. Zang, D. Wei, S. Wang and S. Jiang, Tetrahedron, 2012,
68, 636–641.
dene Schiff-base sensors showing similar uoride induced
colour changes due to absorption at 400–450 nm responsible for 3 (a) R. Sivakumar, V. Reena, N. Ananthi, M. Babu, S. Anandan
transmittance of yellow colour.
and S. Velmathi, Spectrochim. Acta, Part A, 2010, 75, 1146–
1151; (b) Q. Li, Y. Guoa, J. Xua and S. Shao, Sens. Actuators,
B, 2011, 158, 427–431; (c) K. Liu, X. Zhao, Q. Liu, J. Huo,
H. Fu and Y. Wan, J. Photochem. Photobiol., B, 2014, 138,
75–79; (d) S. Dalapati, M. A. Alam, S. Jana and N. Guchhait,
J. Fluorine Chem., 2011, 132, 536–540; (e) X. Huang, Y. He,
Z. Chen and C. Hu, Chin. J. Chem., 2009, 27, 1526–1530; (f)
X. Bao, J. Yu and Y. Zhou, Sens. Actuators, B, 2009, 140, 467–
Thus, the overall results discussed here concerned the
applicability of salicylidene Schiff bases as uoride sensors in
aprotic media which we suggest rather as a regenerated salicy-
laldehyde uoride-ion sensing.
Conflicts of interest
472.
There are no conicts to declare.
4
(a) P. A. Gale, N. Busschaert, C. J. E. Haynes,
L. E. Karagiannidis and I. L. Kirby, Chem. Soc. Rev., 2014,
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(DST) India, for providing nancial support through INSPIRE
Faculty award (DST/INSPIRE/04/2016/001867). We thank the
Alexander von Humboldt Foundation, Germany for providing
the opportunity for research collaboration.
(
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