On the development of selective chelators for cadmium: Synthesis, structure and chelating properties of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo [d]isothiazole 1,1-dioxide, a novel thiadiazolyl saccharinate

Article abstract of DOI:10.3390/molecules26061501

Aquatic contamination by heavy metals is a major concern for the serious negative con-sequences it has for plants, animals, and humans. Among the most toxic metals, Cd(II) stands out since selective and truly efficient methodologies for its removal are not known. We report a novel multidentate chelating agent comprising the heterocycles thiadiazole and benzisothiazole. 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) was syn-thesized from cheap saccharin and characterized by different techniques, including single crystal X-ray crystallography. Our studies revealed the efficiency and selectivity of AL14 for the chelation of dissolved Cd(II) (as compared to Cu(II) and Fe(II)). Different spectral changes were observed upon the addition of Cd(II) and Cu(II) during UV-Vis titrations, suggesting different complexation interactions with both metals.

Full text of DOI:10.3390/molecules26061501

molecules
Communication
On the Development of Selective Chelators for Cadmium:
Synthesis, Structure and Chelating Properties of
3
-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo
[
d]isothiazole 1,1-dioxide, a Novel Thiadiazolyl Saccharinate
Joana F. Leal 1
,2
, Bruno Guerreiro 1,2 , Patrícia S. M. Amado
1,2
, André L. Fernandes 1,2, Luísa Barreira 1,2
,
José A. Paixão 3 and Maria L. S. Cristiano
1,2,
*
1
Center of Marine Sciences, CCMAR, Gambelas Campus, University of Algarve, UAlg,
005-139 Faro, Portugal; jfleal@ualg.pt (J.F.L.); brunoecguerreiro@gmail.com (B.G.);
patricia.s.amado@gmail.com (P.S.M.A.); alofernandes@ualg.pt (A.L.F.); lbarreir@ualg.pt (L.B.)
Department of Chemistry and Pharmacy, Faculty of Sciences and Technology, FCT, Gambelas Campus,
University of Algarve, UAlg, 8005-139 Faro, Portugal
8
2
3
CFisUC, Department of Physics, University of Coimbra, 3004-516 Coimbra, Portugal; jap@fis.uc.pt
Correspondence: mcristi@ualg.pt; Tel.: +351-289-800-953
*
Abstract: Aquatic contamination by heavy metals is a major concern for the serious negative con-
sequences it has for plants, animals, and humans. Among the most toxic metals, Cd(II) stands
out since selective and truly efficient methodologies for its removal are not known. We report a
novel multidentate chelating agent comprising the heterocycles thiadiazole and benzisothiazole.
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Leal, J.F.; Guerreiro, B.;
Amado, P.S.M.; Fernandes, A.L.;
Barreira, L.; Paixão, J.A.; Cristiano,
M.L.S. On the Development of
3
-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) was syn-
thesized from cheap saccharin and characterized by different techniques, including single crystal
X-ray crystallography. Our studies revealed the efficiency and selectivity of AL14 for the chelation
of dissolved Cd(II) (as compared to Cu(II) and Fe(II)). Different spectral changes were observed
upon the addition of Cd(II) and Cu(II) during UV-Vis titrations, suggesting different complexation
interactions with both metals.
Selective Chelators for Cadmium:
Synthesis, Structure and Chelating
Properties of 3-((5-(trifluoromethyl)-
1
,3,4-thiadiazol-2-yl)amino)benzo
d]isothiazole 1,1-dioxide, a Novel
Thiadiazolyl Saccharinate. Molecules
021, 26, 1501. https://doi.org/
0.3390/molecules26061501
[
Keywords: cadmium chelation; aquatic remediation; saccharin-based ligands; benzisothiazoles;
thiadiazoles; selectivity; molecular structure; π-π stacking
2
1
Academic Editor: Luana Bagnoli
1
. Introduction
Received: 21 February 2021
Accepted: 6 March 2021
Published: 10 March 2021
Contamination of aquatic systems with heavy metals is a long-standing problem
that mostly affects rivers and estuaries where effluents are discharged or to where debris
is leached. Several studies point to excessive amounts of metals in the aquatic systems
worldwide [
crustaceans, algae [
propagation through the food chain.
In this context, the development of effective tools for the selective sequestration of
toxic metals is a major challenge. Cadmium is among the toxic metals frequently de-
tected in aquatic systems, mostly arising from the intense industrial, agricultural, and
1
,
2
], with harmful consequences for aquatic species (e.g., fishes, bivalves,
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
3,
4
]) and for human health [ ] as a result of their bioaccumulation and
5,6
mining activity [7]. Previous studies [8–16] focused on adsorption and bioremediation
yielded methods that enabled the sequestration of cadmium, but proved unspecific, as
other metals were found to be preferentially adsorbed. Thus, the development of specific
and effective tools for solving the problem of environmental contamination by cadmium
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
remains a top priority. In recent years, we have designed tetrazole-saccharinates [17–20]
and demonstrated their effective and selective chelation capabilities for Cu(II) [17]. It
was also demonstrated that the ligands are non-toxic while their corresponding copper
complexes proved highly cytotoxic toward several cancer cell lines, suggesting their ap-
plication in chelant-based antineoplastic chemotherapy. Although useful as tools for
4
.0/).
Molecules 2021, 26, 1501. https://doi.org/10.3390/molecules26061501
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 1501
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selective sequestration of Cu(II), with possible impact in medicine and environmental
remediation, tetrazole-saccharinates proved photolabile due to the photoreactivity of the
tetrazole moiety [18,21,22]. Optimization of the saccharinate-based conjugates, involving
replacement of the partner heterocycle, yielded thiadiazolyl-saccharinates with remarkable
properties [19]. In this work, we disclose the synthesis, crystal structure, and chelating prop-
erties of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14;
Scheme 1), a novel thiadiazolyl-saccharinate that proved to selectively chelate cadmium
from aqueous solution.
Scheme 1. Preparation of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo-[d]isothiazole 1,1-dioxide (AL14).
2. Results and Discussion
2.1. Synthesis and Structure Analysis
The preparation of the novel thiadiazolyl saccharinate 3-((5-(trifluoromethyl)-1,3,4-
thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14; Scheme 1) involved a two-
steps synthesis, starting from saccharin ( ), where the coupling reaction linking the saccha-
1
ryl (2) and thiadiazolyl (3) building blocks occurred in moderate yield (Scheme 1).
The one-dimensional (1D) and two-dimensional (2D) NMR (Nuclear Magnetic Res-
onance) data (Supplementary Information, Figures S1–S6) proved the structure of AL14:
1
the four protons of the benzisothiazole moiety can be undoubtedly confirmed by H NMR
(Supplementary Information, Figure S1), following verification by its TOCSY (Total Corre-
lation Spectroscopy) correlations and HSQC (Heteronuclear Single Quantum Coherence)
spectra (Supplementary Information, Figures S3 and S4). Additionally, the HMBC (Het-
eronuclear Multiple Bond Correlation) spectra allowed one to thoroughly identify the
carbons of saccharin moiety (Supplementary Information, Figures S5 and S6), enabling
better identification of the carbons adjacent to the trifluoromethyl group (Figure 1, Table 1).
It is noteworthy that the data reveal the presence of the thiadiazolyl saccharinate in its
amino-linked tautomeric form, as also observed in the crystal phase.
Figure 1. Main HMBC ( J) correlations and assignments of signals (ppm) in the H and 13C NMR spectra of compound AL14
3
1
.
Table 1. Correlations found in the TOCSY, HSQC, and HMBC spectra of compound AL14.
1H, δ (ppm)
TOCSY
13C, δ (ppm)
1H, δ (ppm)
HSQC
HMBC
7
8
8
.86
.05
.33
8.05; 8.33
7.86; 8.33
7.86; 8.05
134.50
122.37
124.86
122.23; 124.92; 129.31; 134.44; 142.26
129.31; 134.20; 142.26
129.07; 134.69; 142,26; 159.60

Molecules 2021, 26, 1501
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2.2. X-ray Crystal Analysis
A single-crystal X-ray crystallography study unambiguously showed that the tau-
tomeric form present in the crystalline state is that depicted in the ORTEP shown in
Figure 2 that also depicts the atom labelling scheme. Indeed, all H-atoms could clearly
be located from the difference electron density maps. The compound crystallizes in the
centrosymmetric triclinic space group P−1.
Figure 2. ORTEP (Oak Ridge Thermal Ellipsoid Plot) drawing of the molecule of AL14. The
anisotropic displacement ellipsoids are drawn at the 50% probability level.
In the crystal, the five- and six-membered rings of the fused benzothiazole group are
◦
coplanar, the angle between the least-squares planes of the two rings being 0.57(3) . The
least squares planes of the five-membered thiadiazol-ring and the fused nine-membered
◦
ring are closely but not strictly coplanar, the dihedral angle being 7.10(3) . The molecule
has conformational flexibility around the C3—N12 and N12—C13 bonds, but the S14—
C13—N12—C3 and C13—N12—C3—N2 torsion angles observed in the crystal are small,
◦ ◦
being
−
5.4(2) and 3.7(2) , respectively. Overall, the molecule skeleton is closer to planarity
as compared to the sulfanyl-bridged thiadiazolyl-saccharinate reported in [19]. The CF3
group is disordered over two alternate positions.
Bond lengths and valency angles in the molecule are within typical average values.
The N16—C15 (1.288(2) Å), N17—C13 (1.305(2) Å) and N2—C3 (1.298(2) Å) are significantly
shorter than N12—C13 (1.372(3) Å) and N12—C3 (1.349(2) Å), showing that the former
three bonds have considerable double-bond character. The two S—O bonds have equal
lengths (1.422(1) Å). There is a short intermolecular contact between atoms S14 and N2
as the S14—N2 distance (2.651(2) Å) is considerably shorter than the sum of the atoms
van der Walls radii (3.35 Å) and even shorter than a similar short contact observed in the
crystallographic study reported for the closely related sulfanyl-bridged compound [19].
The main synthon occurring in the crystal is a pair of molecules, related by an inversion
i
centre, hydrogen-bonded via the interaction N12—H····N17 (i = 1
−
x,
−
y, 1
−
z; D—H:
◦
0
.80(2) Å; H····A: 2.23(2) Å; D····A: 3.024(2) Å;
∠
D—H—A: 169(2) ) (Figure 3). In addition
to this hydrogen bond, two additional intermolecular short contacts could be considered as
i
weak hydrogen bonds, C5—H5····N16 (i = 1
−
x,
−
y,1
−
z; D····A: 3.276(2) Å;
∠
D—H—A:
◦
ii
◦
1
69 ) and C7—H7····F19B (ii =
−
1+x,1+y,1+z; D····A: 3.341(1) Å;
∠
D—H—A: 144 ). The
latter connects hydrogen-bonded dimers in parallel layers that are stacked in the structure
Figure 3). Cohesion of the crystal structure likely has a major energetic contribution
from interactions between the electron clouds of the rings, as the parallel stacking of
(
π-π
layers of molecules positions the rings in a favourable geometry for such interactions. The
packing is compact and there are no sizeable voids in the crystal structure accessible to
solvent molecules.

Molecules 2021, 26, 1501
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Figure 3. Synthon consisting of a pair of molecules related by an inversion centre strongly hydrogen-bonded via an
N—H····N interaction (left panel) and crystal packing as a stack of parallel layers of molecules (right panel).
2.3. Metal Chelating Activities
Driven by the chelating properties evidenced by previously developed saccharin-
based conjugates, the chelating activity of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)
amino)benzo[d]isothiazole 1,1-dioxide (AL14) was first evaluated for Cd(II), Cu(II), and
Fe(II), dissolved in water, at different concentrations (1, 5, 10 mM). The results presented
in Table 2 reveal a considerably higher chelating activity of AL14 for Cd(II) than for
Cu(II) and undetected activity for Fe(II). The results are very important, since most of the
studies reporting methodologies to remove Cd(II) from aquatic system are non-selective or
preferentially remove other metal ions over cadmium [
focusing on the specific sequestration of Cd(II) from water is very small [23
8
,
10
,
12
–
16]. The number of studies
24] and a direct
,
comparison with our results is difficult since different experimental conditions were used.
Table 2. Chelating activity (%) of AL14 for Cd(II), Cu(II), and Fe(II).
Concentration (mM).
Cd(II)
Cu(II)
Fe(II)
1
5
14.7 ± 6.7
39.8 ± 3.1
58.7 ± 4.2
64.0 ± 4.2
69.0 ± 3.3
NA
NA
NA
NA
-
17.7 ± 6.2
1
1
2
0
5
0
29.2 ± 5.8
-
-
-
NA—no significant activity.
These results clearly show that the novel compound AL14 may be able to selectively
chelate Cd(II), compared to Cu(II) and Fe(II). This evidence is reinforced by the fact that
the ratio between the ligand and the available metal is lower for Cd(II) than for Cu(II) and
Fe(II). A higher ratio of ligand to metal has been associated with greater efficiency of the
ligand, as a result of the higher number of binding sites available [14].
Since the ligand revealed a greater chelating activity for Cd(II) compared to Cu(II)
and Fe(II), additional experiments were developed, now considering higher concentrations
(15 and 20 mM) of ligand (AL14). The results show an increase in chelating activity with
increasing concentrations, but this increase appears to be slower than that observed in the
first experiments at lower concentrations (1–10 mM), suggesting that the equilibrium is
being reached (Figure 4). For this set of experiments, the highest chelating activity achieved
was 69.0 ± 3.3% for [ligand] = 20 mM and [Cd] = 9 mM.

Molecules 2021, 26, 1501
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Figure 4. Chelating activity for Cd(II) at different concentrations (1–20 mM).
In order to enable future comparison with the chelating capacity of other compounds,
the IC50 value for this novel compound was determined. In this context, the IC50 corre-
sponds to the concentration of compound (ligand) capable of removing 50% of the metal
concentration in solution. The lower the concentration required to remove 50% of the
metal, the greater the chelating capacity of the compound. For this new ligand, the IC50 is
7.857 mM, with a confidence interval (95%) ranging between 7.585 and 8.139.
2.4. UV-vis Titrations
Aiming to evaluate the spectral changes upon metal addition, experiments were per-
formed by adding 2 to 40
µL of aqueous Cd(II) solution (2 mM) to the ligand solution
(
50 M, from 1 mM stock solution), in a quartz cuvette. Similar experiments were con-
µ
ducted with Cu(II), for comparative purposes. The spectrum of AL14 (Figure 5) reveals
three main bands between 200 and 400 nm. The first and most intense band lies between 200
4
−1
−1
and 230 nm (at these conditions, its maximum is at ca. 215 nm, ε ≈ 2.06 × 10 M cm
)
and it is mainly attributed to saccharin, in keeping with the spectra of other saccha-
rin derivatives also synthesized by our team (data not yet published), and data avail-
able in the literature [25,26]. The second and third main bands appear between 240
3
−1
−1
and 300 nm (εmax ≈ 9.14 × 10 M cm , at ca. 270 nm), and between 300 and 380 nm
4
−1
−1
(
εmax ≈ 1.26 × 10 M cm , at ca. 339 nm), respectively.
In general, the addition of increasing amounts of Cd(II) promotes an increase in band
intensity between 313 and 370 nm, accompanied by a decrease in intensity for wavelengths
between 250 and 313 nm, making it possible to identify an isosbestic point at 313 nm
(Figure 5). In turn, the addition of the same increasing amounts of Cu(II) causes an increase
in the intensity of the third band similar to that observed for Cd(II), which could suggest
the same binding pattern [27] for Cd(II) and Cu(II). However, a bathochromic shift of the
first and second bands, a decrease in the intensity of the 1st band and an increase in the
intensity of the second band are also observed with the addition of Cu(II) (Figure 5), which
seems to indicate that the complexation interactions could differ for both metals. Note that
for these UV-Vis titration experiments, the molar concentrations of both metals in solution
are the same, and the molar concentration of ligand is less than or approximately equal to
that of the metals. Thus, under these conditions, only 2:1 (metal: ligand) or 1:1 complexes
are expected. Further studies will be conducted to elucidate the structure of the complexes.

Molecules 2021, 26, 1501
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Figure 5. UV-Vis spectra of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14), in
ethanol, with different volumes of Cd(II) or Cu(II) addition.
3. Materials and Methods
3.1. Chemicals
All reagents for synthesis were purchased from commercial sources and were used
without further purification. Analytical thin layer chromatography (TLC) was carried out
using Merck (Darmstadt, Germany) TLC Silica gel 60 F254 aluminium sheets and visualized
under UV or by appropriate stain. P-anisaldehyde and potassium permanganate were the
most used. Column chromatography was carried out using Sigma Aldrich (Darmstadt,
Germany) technical grade silica gel (pore size 60 Å, 230–400 mesh particle size, 40–63
particle size). In the experiments to determine the chelating ability of the compound,
aqueous solutions of buffers, metals, pyrocatechol violet (C H O S, Sigma Aldrich,
µm
19
14
7
Darmstadt, Germany), and ferrozine (C H N NaO S , 97.0%, Sigma-Aldrich, Darmstadt,
20
13
4
6 2
®
Germany) were prepared using ultrapure water (Simplicity Water Purification System,
Merck, Darmstadt, Germany, 18.2 M .cm). 3CdSO .8H O (98%), CuSO .5H O (p.a, 99%),
and FeSO .7H O (p.a, 99%) were purchased from Alfa Aesar, Riedel-de-Haën and Merck
Ω
4
2
4
2
4
2
(
(
Darmstadt, Germany), respectively. Ligand solution was prepared in dimethyl sulfoxide
99.5%, LAB-SCAN Ltd., Poland). Absolute ethanol (99.8%, Honeywell, Charlotte, NC,
USA) was used to prepare the solution of 5,7-Dibromo-8-hydroxyquinoline (97%, Alfa
Aesar, Karlsruhe, Germany).
3
.2. Equipment
1H and 13C Nuclear Magnetic Resonance (NMR) spectra were recorded using a
00 MHz Bruker (Billerica, MA, USA) instrument or using a 500 MHz JEOL system
4
(Peabody, MA, USA) equipped with a Royal HFX probe in the deuterated solvents de-
scribed in each experimental procedure. The chemical shifts ( ) are described in parts per
δ
million (ppm) downfield from an internal standard of tetramethylsilane (TMS). Melting
◦
points ( C) were obtained on an SMP30 melting point apparatus and are uncorrected.
High Resolution Mass Spectrometry (HRMS) was recorded using the analytical service
within the Centre of Marine Sciences (CCMAR, Algarve, Portugal) and was conducted on
a Thermo Scientific High Resolution Mass Spectrometer (HRMS) (Waltham, MA, USA),
model Orbitrap Elite, capable of MSn, n up to 10. X-ray diffraction data were collected on

Molecules 2021, 26, 1501
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a small single-crystal (0.27
×
0.22
×
0.19 mm) at room temperature (at 292
±
2 K) using
a Bruker Apex II diffractometer (Billeriaca, MA, USA) equipped with a 4K CCD detector
using graphite monochromated MoK = 0.71073 Å) radiation. For determination of
metal chelating activities, 96-well microplates and a Biotek Synergy 4 Microplate Reader
Winooski, VT, USA) were used. UV-Vis absorption spectra were obtained on a Varian
α (λ
(
CARY 50 Bio UV-visible spectrophotometer (Varian Inc., Mulgrave, Australia) using quartz
cells (1 cm).
3.3. Synthesis Procedure
The synthesis of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-
dioxide (AL14) was conducted by adapting the methodology developed by Ismael et al. [17,28]
Scheme 1). AL14 was synthesized by coupling 2-amino-5-trifluoromethyl-1,3,4-thiadiazole
with pseudo-saccharyl chloride , which was previously prepared from commercially
available saccharin . Recrystallization from ethyl acetate gave the novel thiadiazolyl
(
3
2
1
1
13
saccharinate. Its complete structure elucidation was afforded by H, C, and 2D NMR,
X-ray crystallography, and HRMS. The standard procedures for its synthesis and the
corresponding characterization details are described in the Supporting Information (SI).
3.4. X-ray Crystallography
The structure was solved by direct methods using SHELXT-2014/5 [29]. Refine-
2
ments were carried out with SHELXL-2018/3 [30] by full-matrix least-squares on F , with
anisotropic displacement parameters for all non-hydrogen atoms (CCDC 2063726 for de-
tails regarding the crystallographic analysis procedures and also for detailed crystal data).
All hydrogen atoms could be located on a difference Fourier synthesis; their positions were
refined as riding on parent atoms with an isotropic temperature constrained to those of
their parent atoms using SHELXL-2018/3 defaults [30], except that of the amino group that
is involved in hydrogen bonding, which had its position freely refined with an isotropic
displacement factor constrained to 1.2
disordered over two positions with occupancies close to 50%. The final quality factors
of the refinement were R (I > 2 ) = 0.0317, wRall = 0.0921 and GOF = 1.067 for 3097 for
independent reflections and 221 refined parameters.
×
that of the parent N atom. The CF group is
3
σ
1
3
.5. Metal Chelating Activity
-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide
AL14) was tested for its Cd(II) chelating activity, at different concentrations (1–20 mM).
3
(
The chelating activity of AL14 was also assessed in separate experiments towards other
potentially competing metal cations (Cu(II) and Fe(II)). The metal chelating activities
were evaluated by spectrophotometry, using selective chelating indicators that produce
coloured complexes, following methods described in the literature for Cu(II) and Fe(II) [31
]
and adopting the method described by Ahmed and Chowdhury [32] for Cd(II), with
some adjustments for 96-well plates. Thus, the metal ion chelating activities were deter-
mined by measuring the colour changes due to the Cd(II)/5,7-dibromo-8-hydroxyquinoline
(DBHQ) complex (at 396 nm) [32], Cu(II)/pyrocatechol violet (PV) complex (at 632 nm), and
Fe(II)/ferrozine complex (at 562 nm) [17]. The capacity of the tested compound to chelate
the metals should be reflected in a decrease in absorbance at each corresponding wave-
length. The chelating activity was calculated using the Equation (1), where Ablank and Asample
correspond to the absorption of the solution without and with the ligand, respectively.
Ablank − A
sample
Chelating activity (%) =
× 100
(1)
A
blank
For Cd(II), ligand solution (1, 5, 10, 15, 20 mM) was added to aqueous CdSO (9 mM),
4
followed by DBHQ (3 mM) and H SO (0.5 mM). For Cu(II), ligand solution (1, 5, 10 mM)
2
4
was added to sodium acetate buffer (50 mM), followed by aqueous CuSO (0.2 mM) and PV
4
(
4 mM). For Fe(II), ligand solution (1, 5, 10 mM) was added to H O, followed by aqueous
2

Molecules 2021, 26, 1501
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FeSO (0.36 mM) and ferrozine (40 mM). In all experiments, negative controls (blanks) and
4
colour controls (without chelating indicators) were performed. At least three independent
experiments (six replicates/each) were conducted, for each experimental condition.
3
.6. Software
Chemical structures were designed using ChemDraw Ultra 12.0. Graphics and sta-
tistical analysis were performed using Microsoft Excel and GraphPad Prism 7.0. Bruker
APEXIII [33] software package including SAINT V8.38A [34] and SADABS-2016/2 [35
]
was used for data reduction of the X-ray crystallography work. SHELXT-2014/5 [29] and
SHELXL-2018/3 [30] and PLATON (v. 2010420) [36] were used for the structure solution,
refinement and structure analysis and drawing of crystallographic figures, respectively.
4. Conclusions
In short, this work unveils a new compound derived from saccharin capable of
selectively chelating Cd(II), compared to Cu(II) and Fe(II). 3-((5-(trifluoromethyl)-1,3,4-
thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) was analyzed in detail and its
1
13
structure confirmed by 1D NMR ( H and C), 2D NMR (TOCSY, HSQC, and HMBC), X-ray
+
crystallography, HRMS-ESI and UV-Vis. In these experiments, the maximum chelating
activity for Cd(II), 69.0
without previous contact time. The concentration of ligand capable of removing 50% of
Cd(II) concentration in solution (IC ) is 7.857 mM. Reduced or no activity for Cu(II) and
±
3.3%, was achieved for [AL14] = 20 mM and [Cd] = 9 mM,
50
Fe(II) was observed, respectively. These are the first studies revealing the potential of this
compound for selective Cd(II) chelation. Additional studies will be carried out, considering
other environmentally relevant metal ions to assess their possible interference with the
chelating capacity of this new and relatively low-cost ligand.
1
Supplementary Materials: Figure S1: H NMR spectra of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-
1
3
yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) in DMSO-d , Figure S2: C NMR spectra of 3-((5-
6
(
trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) in DMSO-d6,
Figure S3: TOCSY spectra of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole
,1-dioxide (AL14) in DMSO-d , Figure S4: HSQC spectra of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-
1
6
2
-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) in DMSO-d , Figure S5: HMBC spectra of 3-((5-
6
(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide (AL14) in DMSO-d6,
Figure S6: HMBC (expanded) spectra of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]
isothiazole 1,1-dioxide (AL14) in DMSO-d , Figure S7: Electrospray ionization mass spectrum
6
+
in positive-ion mode (HRMS-ESI ) of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]
isothiazole 1,1-dioxide (AL14), X-ray crystallography data, detailed synthesis procedure, Scheme S1
:
Preparation of 3-((5-(trifluoromethyl)-1,3,4-thiadiazol-2-yl)amino)benzo[d]isothiazole 1,1-dioxide
(AL14).
Author Contributions: Conceptualization, J.F.L., L.B. and M.L.S.C.; methodology, all authors; val-
idation, J.F.L., L.B., J.A.P. and M.L.S.C.; formal analysis, J.F.L. and L.B.; investigation, all authors;
resources, L.B., J.A.P. and M.L.S.C.; writing—original draft preparation, J.F.L., B.G. and P.S.M.A.;
writing—review and editing, J.F.L., L.B. and M.L.S.C.; visualization, J.F.L., P.S.M.A., J.A.P.; supervi-
sion, M.L.S.C.; project administration, J.F.L., A.L.F. and M.L.S.C.; funding acquisition, M.L.S.C. All
authors have read and agreed to the published version of the manuscript.
Funding: This study received Portuguese national funds from FCT—the Foundation for Science
and Technology, through projects UIDB/04326/2020 (CCMar) UIDB/04564/2020 (CFisUC) and
UIDP/04564/2020 (CFisUC), and from the operational programmes CRESC Algarve 2020 and COM-
PETE 2020 through project EMBRC.PT ALG-01-0145-FEDER-022121. J.F.L. and A.L.F. also acknowl-
edge CCMAR for fellowships. P.S.M.A thanks FCT for the doctoral grant SFRH/BD/130407/2017.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: A CIF file containing supplementary crystallographic data was de-
posited at the Cambridge Crystallographic Data Centre with reference CCDC 2063726.

Molecules 2021, 26, 1501
9 of 10
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: The samples are available from the authors.
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