Cooperative Transport and Selective Extraction of Sulfates by a Squaramide-Based Ion Pair Receptor: A Case of Adaptable Selectivity

Article abstract of DOI:10.1021/acs.inorgchem.0c02114

The use of a squaramide-based ion pair receptor offers a solution to the very challenging problem of extraction and transport of extremely hydrated sulfate salt. Herein we demonstrate for the first time that a neutral receptor is able not only to selectively extract but also to transport sulfates in the form of an alkali metal salt across membranes and to do so in a cooperative manner while overcoming the Hofmeister bias. This was made possible by an enhancement in anion binding promoted by cation assistance and by diversifying the stoichiometry of receptor complexes with sulfates and other ions. The existence of a peculiar 4:1 complex of receptor 2 with sulfates in solution was confirmed by UV-vis and 1H NMR titration experiments, DOSY and DLS measurements, and supported by solid-state X-ray measurements. By varying the separation technique and experimental conditions, it was possible to switch the depletion of the aqueous layer into extremely hydrophilic or less lipophilic salts, thus obtaining the desired selectivity.

Full text of DOI:10.1021/acs.inorgchem.0c02114

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Article
Cooperative Transport and Selective Extraction of Sulfates by a
Squaramide-Based Ion Pair Receptor: A Case of Adaptable
Selectivity
́
Marta Zaleskaya, Marcin Karbarz, Marcin Wilczek, Łukasz Dobrzycki, and Jan Romanski*
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ABSTRACT: The use of a squaramide-based ion pair receptor offers a
solution to the very challenging problem of extraction and transport of
extremely hydrated sulfate salt. Herein we demonstrate for the first time that
a neutral receptor is able not only to selectively extract but also to transport
sulfates in the form of an alkali metal salt across membranes and to do so in
a cooperative manner while overcoming the Hofmeister bias. This was made
possible by an enhancement in anion binding promoted by cation assistance
and by diversifying the stoichiometry of receptor complexes with sulfates
and other ions. The existence of a peculiar 4:1 complex of receptor 2 with
1
sulfates in solution was confirmed by UV−vis and H NMR titration
experiments, DOSY and DLS measurements, and supported by solid-state
X-ray measurements. By varying the separation technique and experimental
conditions, it was possible to switch the depletion of the aqueous layer into
extremely hydrophilic or less lipophilic salts, thus obtaining the desired selectivity.
that they can be used in liquid−liquid extraction (LLE) of salts
from the aqueous layer to the organic phase, including the
selective extraction of extremely hydrophilic sulfate salts (ΔGh°
= −1080 kJ/mol).6,7 Selective sulfate removal is a nontrivial
task and involves defeating the hydration energy of anions and
overcoming the Hofmeister series to preferentially extract
these ions to the organic phase even in the presence of less
hydrated anions (e.g., nitrates ΔGh° = −306 kJ/mol).7,8 This
task is highly desired, e.g., in the context of the storage of
radioactive waste after nuclear processes containing sulfates.9
Sulfates cause corrosion of glass melters and the constituent
electrodes, and due to their low solubility in borosilicate glass,
they are responsible for problems with vitrification, resulting in
reduced durability of glass logs and thus entailing a safety
hazard upon storage.
To date, however, few examples of receptors capable of
overcoming the Hofmeister series and allowing the selective
extraction (LLE) of sulfates from aqueous solutions have been
developed. The group of these compounds mainly includes
anion receptors: macrocyclic calix[n]pyrroles, TREN-based
hexaurea, diiminoguanidine, and more recently macrocyclic
INTRODUCTION
■
Ion recognition, extraction, and transport are among the
particular and still challenging tasks of modern supramolecular
chemistry. They are extremely important because they are
closely related to real problems including the regulation of ion
transport in organisms, their detection, water treatment, and
waste disposal.1 Research has shown that one solution to these
problems may involve the use of monotopic anion receptors
possessing binding domains capable of interacting with
negatively charged species. This group of compounds
comprises squaramides, which are a special class of monotopic
anion receptors able to interact with anions more effectively
than their urea or thiourea analogues.2 However, monotopic
receptors are capable of interacting with one type of ion
(anions), which in the presence of strongly coordinating
counterions (cations) may lose their effectivity in ion binding
due to the formation of ion pairs out of the receptor’s binding
site.3 To eliminate this dysfunction, ion pair receptors,
compounds capable of simultaneously binding anions and
cations, have been proposed and recently intensively
investigated.4 Cooperative binding by heteroditopic receptors
can be achieved by properly oriented binding domains within a
receptor platform.
Received: July 16, 2020
Unexpectedly, within this group of compounds, only a few
examples of ditopic ion pair receptors possessing a squaramide
unit as an anion binding domain can be distinguished.5 Our
contribution in this field has involved the design of
squaramide-based ion pair receptors which are so effective
https://dx.doi.org/10.1021/acs.inorgchem.0c02114
© XXXX American Chemical Society
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Article
squaramide derivatives.10 Some of these receptors were found
to recognize sulfate salts in unusual manner via contact ion pair
interactions.11 However, this approach requires the assistance
of lipophilic counterions during the extraction or transport
processes. The solution proposed by our group, however, relies
on using squaramide-based ion pair receptors and the
formation of 4:1 complexes with sulfates, rather than 1:1
complexes with other salts tested.6 This facilitated the selective
extraction of sulfates from aqueous to organic phase even in
the presence of alkali potassium cations. However, due to the
insufficient solubility of such a receptor in chloroform, in
extraction experiments its suspension was used. We found that
complete dissolution and phase separation was observed only
after such a suspension was contacted with an aqueous solution
of potassium sulfate, which was not the case for the other salts
tested. This characteristic was interpreted as a component that
causes the selectivity of sulfate extraction in the presence of
other salts whose complexes were not soluble in the organic
layer. In order to verify whether more complex equilibria (like
that in the case of the interaction of sulfates with squaramides)
are the main driving force for the selective extraction process,
in the present study we eliminated the solubility drawback and
developed and tested squaramide-based ion pair receptors
equipped with a pentafluorophenyl unit, soluble in chlor-
oformic phase. Ion pair receptor bearing pentafluorophenyl
unit was recently found to be selective in solid−liquid
extraction of potassium bromide.12 We proved, however, that
replacing urea with a squaramide function in the ion pair
receptor structures opens up its potential to act under liquid−
liquid conditions.6 By comparing ditopic receptors to a
monotopic one, the impact of the presence of a cation binding
domain and the ability to simultaneously bind cations and
anions on the extraction process were also established. The
ability of neutral ion pair receptor 2 not only to selectively
extract but also to transport them in a selective and adaptable
manner across membranes without the assistance of lipophilic
cation was successfully developed for the first time.
quantitative yield (0.368 g). The obtained 4-aminobenzo-18-crown 5-
ether was used in the next step without further purification. To the
solution of 4-aminobenzo-18-crown 5-ether (0.368 g, 1.3 mmol) in
methanol (10 mL) was added compound M1 (0.381 g, 1.3 mmol) at
room temperature. After being stirred for 48 h, the reaction mixture
was concentrated and residue was purified by silica gel column
chromatography (2% methanol in chloroform) to give receptor 1 as a
1
yellow solid (0.582 g, 1.07 mmol, 82% yield). H NMR (300 MHz,
DMSO-d6) δ 9.92 (s, 2H), 7.06 (s, 1H), 7.00−6.85 (m, 1H), 6.85−
6.72 (m, 1H), 4.12−3.95 (m, 4H), 3.85−3.70 (m, 4H), 3.68−3.55
(m, 8H). 13C NMR (75 MHz, DMSO-d6) δ 184.2, 183.4, 167.0,
166.8, 149.1, 145.3, 141.5−140.5 (m), 140.5−135.2 (m), 132.9,
117.0−115.5 (m), 115.1, 111.8, 106.6, 70.7, 70.7, 70.1, 70.0, 69.4,
69.3, 69.0, 68.6. HRMS (ESI): calcd for C24H21F5N2O7Na [M +
Na]+: 567.1167. Found: 567.1171.
Preparation of Receptor 2. 4-Aminobenzo-18-crown 6-ether
(0.622 g, 1.9 mmol) was reacted with compound M1 (0.557 g, 1.9
mmol) according to procedure described for receptor 1 to yield
1
receptor 2 as a light beige solid (0.823 g, 1.4 mmol, 74% yield). H
NMR (300 MHz, DMSO-d6) δ 10.65 (s, 2H), 7.36 (s, 1H), 7.25−
6.85 (m, 2H), 4.17−3.95 (m, 4H), 3.90−3.70 (m, 4H), 3.65−3.45
(m, 12H). 13C NMR (75 MHz, CDCl3) δ 183.2, 182.2, 166.5, 166.0,
148.8, 144.4, 144.4−140.5 (m), 140.5−135.0 (m), 132.8, 115.5−
114.3 (m), 113.6, 110.4, 104.8, 70.0, 69.1, 68.9, 68.4, 67.9. HRMS
(ESI): calcd for C26H25F5N2O8Na [M + Na]+: 611.1429. Found:
611.1422.
Preparation of Receptor 3. To a solution of compound M1
(0.337 g, 1.15 mmol) in MeOH (5 mL) was added aniline (0.11 mL,
1.2 mmol), and the mixture was stirred 24 h at room temperature.
Then the reaction mixture was filtered, and the collected solid
material was washed with MeOH. The obtained white solid was dried
in vacuo to give the desired compound (0.321 g, 0.91 mmol, 79%
1
yield). H NMR (300 MHz, DMSO-d6) δ 10.00 (s, 2H), 7.45−7.28
(m, 4H), 7.18−7.00 (m, 1H). 13C NMR (75 MHz, DMSO) δ 184.5,
183.1, 166.9, 166.8, 144.5−139.2 (m), 138.9, 137.8−135.2 (m),
129.7, 124.2, 119.7, 115.5−113.0 (m). HRMS (ESI): calcd for
C16H7F5N2O2Na [M + Na]+: 377.0325. Found: 377.0329.
UV−Vis Titration Experiments. UV−vis analyses were per-
formed using Thermo Spectronic Unicam UV500 Spectrophotometer
in CH3CN solution at 298 K. To 10 mm cuvette was added 2.5 mL of
freshly prepared solution of studied receptor (receptor 1, 2.85 × 10−5
M; receptor 2, 2.63 × 10−5 M; receptor 3, 2.16 × 10−5 M), and in case
of ion pair binding studies 1 mol equiv of cation (KPF6 or NaClO4)
was added prior to titrations. Small aliquots of ca. 1.5 × 10−3 M of
solution of anions were added (added as TBA salts: TBAX;
containing receptor 1, 2, or 3 at the same concentration as that in
the cuvette), and a spectrum was acquired after each addition. The
resulting titration data were analyzed using BindFit (v0.5) package,
available online at http://supramolecular.org.
EXPERIMENTAL SECTION
■
All reagents and chemicals were reagent-grade and purchased
1
commercially. H and 13C NMR spectra were recorded on a Bruker
1
300 MHz spectrometer. H NMR chemical shifts δ are reported in
parts per million referenced to residual solvent signal (deuterated
dimethyl sulfoxide (DMSO-d6 or CDCl3). Mass spectra were
measured on Quattro LC Micromass or Shimadzu LCMS-IT-TOF
unit.
1H NMR Titration Experiments. The 1H NMR titration was
carried out on a Bruker AVANCE III HD 300 MHz spectrometer, at
298 K in CD3CN. In each case, 500 μL of freshly prepared ca. 3 mM
solution of receptor was added to a 5 mm NMR tube. In the case of
ion pair titration receptor was first pretreated with 1 equiv of NaClO4
or KPF6 (referenced to receptor). Then, small aliquots of a solution of
TBAX (containing the receptor at constant concentration) were
added, and a spectrum was acquired after each addition. The resulting
titration data were analyzed using BindFit (v0.5) package, available
online at http://supramolecular.org.
Preparation of Compound M1. Compound M1 was synthesized
according to the literature procedure with small modifications.13
2,3,4,5,6-Pentafluoroaniline (2.01 g, 11.3 mmol) was added to a
solution of dimethyl squarate (1.63 g, 11.5 mmol) in MeOH (20 mL).
After being stirred for 48 h at room temperature, the reaction mixture
was concentrated and purified by silica gel column chromatography
(5% methanol in chloroform) to give compound M1 as a white solid
1
(2.65 g, 9.01 mmol, 80% yield). H NMR (300 MHz, DMSO-d6) δ
11.11 (s, 1H), 4.32 (s, 3H). 13C NMR (75 MHz, DMSO-d6) δ 190.5,
187.5, 184.9, 183.1, 180.0, 170.8, 167.4, 142.9−140.7 (m), 138.6−
135.8 (m), 113.5−112.7, 60.8. HRMS (ESI): m/z calcd for
C11H4O3NF5Na [M + Na]+: 316.0009. Found 316.0003.
1H NMR DOSY experiments were conducted at 298 K on a Bruker
AVANCE III HD 500 MHz spectrometer with a residual solvent
signal as an internal standard.
Atomic emission measurements were carried out using PerkinElm-
er AAnalyst 300 spectometer.
Ion chromatography data were recorded on a Metrohm ion
chromatograph model 930 Compact IC Flex equipped with
conductivity detector and Metrosep A Supp 5−250/4.0 column.
Dynamic light scattering (DLS) analyses were carried out using a
Malvern Zetasizer instrument (Nano ZS, UK) equipped with a 4 mW
helium−neon laser of light wavelength 632.8 nm was used. The
Preparation of Receptor 1. To a degassed solution of 4-
nitrobenzo-15-crown 5-ether (0.407 g, 1.3 mmol) in 10 mL of a
THF/MeOH mixture (1/4) was added 10 mg of 10% Pd/C. The
reaction mixture was kept under a H2 atmosphere (balloon pressure)
at room temperature overnight. Then, the catalyst was removed by
filtration through a pad of Celite and washed with MeOH. The filtrate
was concentrated under reduced pressure to give the crude product in
B
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Table 1. Crystal Data and Structure Refinement Parameters for Obtained Complexes of 2 with Various Salts
2 + KNO3
2 + KCl/NaCl
2 + Na2SO4
a
C56H60F10K2N6O23
2 × 2 + 2 × KNO3 + Et2O
1453.30
C52H50Cl2F10K1.31N4Na0.69O16
2 × 2 + 1.31 × KCl + 0.69 × NaCl
1315.02
C457.20H460F80N52Na8O147.20S4
formula
a
16 × 2 + 4 × Na2SO4 + 20 × MeCN + 1.2 × MeOH + 2 × H2O
Mx/g mol−1
T/K
10870.50
130.0(5)
100.0(5)
100.0(5)
λ/Å
1.54178
1.54178
0.71073
crystal size/mm3
space group
unit cell dimensions
0.038 × 0.241 × 0.294
P21/c
0.024 × 0.168 × 0.295
P21/c
0.148 × 0.201 × 0.357
C2/c
a = 19.5699 (6) Å
b = 7.9685 (3) Å
c = 20.9742 (7) Å
β = 104.0030 (13)°
3173.58 (19), 2
1.521
a = 18.1253 (5) Å
b = 7.8928 (2) Å
c = 19.8048 (5) Å
β = 90.0610 (12)°
2833.26 (13), 2
1.541
a = 12.0787 (9) Å
b = 43.414 (3) Å
c = 23.9907 (18) Å
β = 95.624 (3)°
12519.8 (16), 1
1.442
V/Å3, Z
Dx/g cm3
μ/mm−1
2.324
2.885
0.148
F(000)
1500
1349
5617
diffractometer
radiation source
θmin, θmax
Bruker D8 Venture
Incoatec IμS tube
2.33, 66.50°
Bruker D8 Venture
Incoatec IμS tube
2.44, 66.48°
Bruker D8 Venture
Fine focus sealed tube
2.20, 25.05°
index ranges
−23 ≤ h ≤ 23
−9 ≤ k ≤ 9
−24 ≤ l ≤ 24
−21 ≤ h ≤ 21
−9 ≤ k ≤ 9
−23 ≤ l ≤ 22
−14 ≤ h ≤ 14
−51 ≤ k ≤ 51
−28 ≤ l ≤ 28
reflections collected/
independent
42667/5591 (Rint = 0.0380)
30223/4996 (Rint = 0.0323)
119249/11108 (Rint = 0.0364)
completeness
99.9%
99.8%
99.9%
absorption correction
multi-scan
multi-scan
multi-scan
T
max, Tmin
0.917, 0.548
full-matrix LSQ on F2
0.934, 0.483
full-matrix LSQ on F2
0.978, 0.949
full-matrix LSQ on F2
refinement method
data/restraints/
parameters
goodness of fit on F2
5591/132/523
4996/51/455
11108/72/933
1.033
1.032
1.099
final R indices
5166 data; I > 2σ(I)
R1 = 0.0482, wR2 = 0.1410
all data
4685 data; I > 2σ(I)
R1 = 0.0363, wR2 = 0.0930
all data
9555 data; I > 2σ(I)
R1 = 0.0484, wR2 = 0.1047
all data
R1 = 0.0506, wR2 = 0.1442
R1 = 0.0384, wR2 = 0.0944
R1 = 0.0576, wR2 = 0.1099
0.00026(3)
0.721, −0.302 e Å−3
extinction coefficient
Δρmax, Δρmin
0.308, −0.318 e Å−3
0.284, −0.208 e Å−3
a
Hydrogen atoms of partial occupancy disordered water and methanol moieties not assigned.
scattering angle was set to 173° at 25 °C. The hydrodynamic diameter
distributions were obtained by volume using the software package of
the apparatus. Each curve represents the average of 3 measurements
(16 runs each). Prior to analyses, all solutions were filtered and
degassed.
Crystallization and Single-Crystal X-ray Diffraction. The
phases suitable for single-crystal X-ray diffraction experiment were
prepared by slow diffusion of diethyl ether to acetonitrile or
acetonitrile/methanol solution containing 2 and the appropriate
inorganic salt. The samples were measured at lowered temperatures
on Bruker D8 Venture single-crystal diffractometer equipped with
PhotonII detector. Data were collected, integrated, and scaled with
the help of Bruker software,14−16 then solved and refined in SHELX
software package.17−19 Data collection and refinement parameters for
the measured samples are grouped in a suitable part of the Supporting
Information together with figures displaying atomic displacement
parameters and packing diagrams. These figures were prepared in
Mercury 4.1 software.20 CCDC 1988974−1988976 contain the
supplementary crystallographic data for this paper. The data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/structures. The crystal data and final
structure refinement parameters are collected in Table 1.
Figure 1. Structure of receptors 1−3.
synthesized in a modular fashion as outlined in Scheme 1.
Briefly, pentafluoroaniline and dimethyl squarate were coupled
to afford monoester module. This module was reacted further
with a second equivalent of amine (amino benzocrown ethers
or aniline) to furnish receptors 1−3.
Binding Studies. Initial evidence that receptors 1 and 2
could bind ion pairs with significantly enhanced affinity relative
to the monotopic anion receptor 3 came from UV−vis
spectroscopic titration experiments carried out in acetonitrile.
To verify this assumption, we carried out selected titrations of
receptors 1−3 with chloride anions (as tetrabutylammonium
salt) in the presence and absence of cations (added as NaClO4
or KPF6). Indeed, while all receptors were able to bind the
chloride anion with a similar stability constant (KTBACl = 1.88
RESULTS AND DISCUSSION
■
Receptor Design and Synthesis. Ion pair receptors 1 and
2 as well as monotopic anion receptor 3 (Figure 1) were
C
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a
Interestingly, the high affinity of receptors toward chlorides
allowed for interaction with these salts even in the presence of
5% water in acetonitrile, conditions where sodium or
potassium chloride can be directly used instead of in situ
generated salts. In such competitive media, receptor 2 was able
to interact much more strongly with sodium or potassium
Scheme 1. Synthesis of Receptors 1−3
chloride than with tetrabutylammonium chloride (KTBACl
=
1.53 × 103 M−1, KNaCl = 4.33 × 103 M−1, and KKCl = 5.94 × 103
M−1). However, when we carried out titration experiments in
acetonitrile with receptors 1−3 and sulfate anions, we could
not fit the obtained data to the appropriate binding model,
suggesting that more complex equilibria are present under
1
experimental conditions. Thus, H NMR titrations in CD3CN
were carried out to support the aforementioned findings. We
first tested halide salts. The isotherms obtained from the
titration experiments carried out with bromide salts (bromides
were chosen due to expected range of values of Ka’s suitable for
NMR measurements) could be simply fitted to the 1:1 binding
model, producing comparable apparent stability constants like
those in the case of experiments under UV−vis control (KTBABr
= 1.18 × 104 M−1 and KNaBr = 2.49 × 104 M−1 for receptor 1
and KTBABr = 1.15× 104 M−1 and KKBr = 3.43 × 104 M−1 for
receptor 2). In contrast, upon addition of incremental
equivalents of sulfate anions into the receptor solutions,
inconsistent perturbations in signals were noticed, with signals
corresponding to the aromatic protons as well as to the crown
ethers moving downfield or upfield, back and forth. To confirm
that this was a manifestation of the formation of complexes
with higher stoichiometry which should significantly affect the
diffusion coefficient, we performed comparative studies using
1H NMR DOSY experiments. Specifically, we compared the
change in the diffusion coefficients of receptors 1−3 in
CD3CN after the addition of 1 equiv of bromide or sulfate
anions (added as TBA salts). We found that only the addition
of the latter anions causes a significant drop in diffusion
coefficients, in the range of 30.3−34.3%, suggesting that a large
complex is formed. In contrast, the presence of bromide anions
affects the change in the diffusion coefficients of the receptors
only weakly, which may evidence the formation of smaller
complexes of receptors with bromides, most likely with 1:1
stoichiometry. The obtained data are in good agreement with
approximate diffusion coefficients for 1:1 R × Br− complexes
a
Reagents and conditions: (i) H2, Pd/C, MeOH/THF, 12 h, room
temperature, quantitative; (ii) methanol, 48 h, room temperature,
80%; (iii) methanol, 48 h, room temperature, 82 and 74% for 1 and 2,
respectively; (iv) aniline, methanol, 24 h, room temperature, 79%.
× 105, 2.00 × 105, and 2.01 × 105 M−1 for interaction of
chloride anions with 1, 2 and 3, respectively), only in the case
of receptors 1 and 2 equipped with the cation binding domain
was stronger binding observed in the presence of cations
(KNaCl = 3.03 × 105 M−1 for interaction of 1; KNaCl = 3.58 ×
105 M−1 and KKCl = 4.92 × 105 M−1 for interaction of 2 with in
situ generated salts). The same trend was observed for the
other salts tested, with the exception of acetate and benzoate
anions, which induce deprotonation of receptors (Table 2).
The increase in stability constants correlates well with the
order of increased affinity of cations to the macrocyclic cavity
of esters: Receptor 2 equipped with a benzo-18-crown unit
binds chloride anion more strongly in the presence of sodium
cations than does receptor 1, while the highest enhancement in
ion pair binding was noted for the interaction of 2 with in situ
generated potassium chloride. This is supported by the
increased stability constants determined for complexes of
receptors 1 and 2 with cations. Specifically, we found that
+
receptor 1 binds sodium cations with stability constants of KNa
= 1.77 × 104 M−1, while receptor 2 recognized this cation more
strongly with KNa = 3.73 × 104 M−1. The highest interaction
+
was observed for complexes of 2 with potassium cation, which
4
−1
+
produces stability constants of KK = 4.71 × 10 M . However,
the lack of a cation-binding domain in the structure of receptor
3 resulted in a decrease in the apparent stability constants for
anions when titrations were performed in the presence of
sodium or potassium cations (KNaCl = 1.65 × 105 M−1 and KKCl
= 1.66 × 105 M−1 for receptor 3).
D1
D2
M2
(calculated according to the equation
=
for rod-like
M1
2−
molecules) and 4:1 4R × SO4 complexes (calculated
D1
D2
M2
3
according to the equation
=
for spherical molecules)
M1
Table 2. Association Constants for Interactions of Receptors 1 and 2 with Selected Anions and Apparent Association
a
Constants for Interactions of 1 and 2 with Anions
1
1 + 1 equiv of NaClO4
2
2 + 1 equiv of NaClO4
2 + 1 equiv of KPF6
Cl−
Br−
I−
NO2
NO3
1.88 × 105
1.25 × 104
2.13 × 103
7.39 × 104
1.60 × 103
6.80 × 103
3.03 × 105
2.52 × 104
3.35 × 103
1.15 × 105
2.40 × 103
7.75 × 103
2.00 × 105
1.21 × 104
2.51 × 103
7.51 × 104
1.53 × 103
7.12 × 103
3.58 × 105
3.96 × 104
3.02 × 103
1.10 × 105
2.21 × 103
7.64 × 103
4.92 × 105
4.05 × 104
4.67 × 103
1.23 × 105
2.53 × 103
8.05 × 103
−
−
−
HSO4
a
In the presence of 1 equiv of sodium perchlorate or potassium hexafluorophosphate UV−vis, solvent CH3CN, temperature 293 K, [1] = 2.85 ×
10−5 M, [2] = 2.63 × 10−5 M, [3] = 2.16 × 10−5 M, anions added as TBA salts [TBAX] ∼ 1.5 × 10−3 M. Estimated errors < 10% (Supporting
Information). Stability constants for receptor 3: KTBACl = 2.01 × 105 M−1 for Cl− and KNaCl = 1.65 × 105 M−1 for Cl− in the presence of 1 equiv of
Na+ and KKCl = 1.66 × 105 M−1 for Cl− in the presence of 1 equiv of K+.
D
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a
Table 3. Summarized Results of DOSY NMR Experiments for Receptors 1−3
1
2
3
D × 10−9 m2 s−1
Δ (%)
D × 10−9 m2 s−1
Δ (%)
D × 10−9 m2 s−1
Δ (%)
R
1.17
1.13
1.09
0.81
0.72
1.12
1.08
1.05
0.73
0.70
1.53
1.44
1.39
1.01
0.95
R + 1 equiv of TBABr
calculated
R + 1 equiv of TBA2SO4
calculated
2.8
3.7
6.2
30.3
32.0
34.3
a
In CD3CN and for their in situ generated complexes with bromide and sulfate anions.
Figure 2. Molecular aggregates and their arrangements in the crystals of 2 × KNO3 (a), 2 × KCl/NaCl (b), and 2 × K2SO4 (c). Hydrogen atoms
are omitted for clarity.
(Table 3).21 This confirms the presence of large complexes of
squaramides with sulfate anions in solution.
two positions. Also, nitrate anions occupy two alternative but
closely located sites. The K+ moiety bound by the crown ether
ring is coordinated from one side by the disordered nitrate
anion and by the O(7) carbonyl part of the squaramide unit
from the opposite side. The distances between the K+ cation
and the O atoms in the crown ether part range from 2.63 to
2.82 Å. In the case of nitrate the distances for highly occupied
(93%) fragment yields 2.74 and 2.83 Å, whereas for the
carbonyl group the distance is equal to 2.76 Å. Such strongly
coordinated moieties form 1D helical polymers with 2-fold
Crystallization and Single-Crystal X-ray Diffraction.
Our findings were further corroborated by analyses of receptor
2 complexes with ion pairs in the solid state by means of X-ray
measurements. In the presence of potassium nitrate, crystals
containing 1:1 ratio of inorganic salt and receptor 2 were
formed. Such behavior could be expected based on the
complexation experiments in the liquid phase. In the structure,
the fluorinated phenyl ring of the receptor is disordered over
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symmetry. These polymers, due to strong hydrogen bonds
between NO3− ions and amide groups of the anion binding site
(N···O distances equal to 2.68 and 2.82 Å), are grouped
together forming molecular layers parallel to the (100) lattice
plane. Between these layers, in channels walled by fluorinated
phenyl ring, disordered diethyl ether molecules are located.
Both the helical polymer of 2 with NO3− and molecular layers
are presented in Figure 2a. Surprisingly, the formation of
helical structures in the crystals of 2 and KNO3 is similar to the
case of 1:1 KCl salt complex with a squaramide-based, same-
sized crown ether receptor containing a ferrocene signaling
unit instead of the fluorinated phenyl moiety.5e
Å. Sulfate complexation by 2, giving a 1:4 ratio of anion and
ionic pair receptor both in solution and in the solid state, was
also observed for a similar squaramide-based system, which
seems to be promising in competitive extraction of sulfates
from aqueous media.6a The structure of 2 and K2SO4 contains
K+ ion complexed by the crown ether ring disordered over
three alternative positions. The central cation is additionally
coordinated from two sides by acetonitrile molecules. In the
crystal disordered methanol, additional CH3CN and water
molecules are also present. The tetramer of receptor 2 and
sulfate anion is significantly flattened with acetonitrile CH3
groups weakly coordinated by empty crown ether moieties.
Similar coordination of the acetonitrile by empty crown ethers
of receptors in neighboring tetramers results in formation of
Crystallization of 2 and KCl instead of the planned complex
serendipitously gave a nonstoichiometric mixed KCl/NaCl salt
complex but still with a 1:1 receptor/anion ratio. As only pure
reagents were used during all the steps, one of the reasonable
explanations for such behavior is slow Na+ diffusion from the
glassware, especially given that the crystal growth process took
quite a long time, combined with the energetically preferred
formation of mixed instead of simple crystals. Because of mixed
cationic composition the structure is disordered and contains
0.655(2):0.345(2) K+/Na+ ions ratio. Both these ions are
located in the crown ether part. Because sodium is slightly too
small for the 18-crown-6 unit, it is nonsymmetrically
coordinated by five O ether atoms, resulting in disorder of
part of the noncoordinating crown ether moiety. Both cations
are additionally coordinated from two sides by a Cl− anion and
O(8) atom of the carbonyl group; thus, the pattern of the
interaction is almost identical as in the 2 × KNO3 complex.
However, due to differences of the ionic radii of Na+ and K+
the chloride and carbonyl ligands are disordered as well. The
distances to potassium ion and oxygen in the crown ether ring
are in the range of 2.71−2.86 Å, whereas for sodium the
corresponding distances are 2.45−2.78 Å. An understandable
discrepancy is also visible for distances to the carbonyl O atom
and chloride anion, which are 2.46 and 2.84 Å for sodium and
2.67 and 3.05 Å for potassium, respectively. Unlike in the case
of complex 2 with KNO3, in the structure containing 2 and
KCl/NaCl, organic molecules form centrosymmetric dimers
which further interact via Cl−···H−N hydrogen bonds to
develop molecular layers parallel to the (100) lattice plane.
Distances between anions and amide nitrogen atoms of the
squaramide moiety are in range of 3.05 −3.13 Å. Molecular
diagrams presenting ion complexation, the formation of
centrosymmetric dimers, and molecular layers in the structure
of 2 with KCl/NaCl are given in Figure 2b. A similar
centrosymmetric dimer formation can be observed for an ion
pair receptor complex analogous to 2 (but containing a 15-
crown-5 ether unit and 3,5-bis(trifluoromethyl)phenyl moiety)
with a mixed NaNO3/NaCl salt.5f However, in this Na+ case
due to the lack of additional coordination of the central ion by
the carbonyl oxygen fragment, there is no further hydrogen-
bond aggregation of the dimers into molecular layers.
stacks of tetramers parallel to the [100] direction plane. Views
2−
of the tetramer coordinating SO4
anion and weak
intermolecular interactions in the structure of 2 × K2SO4 are
presented in Figure 2c.
Extraction Studies. Taking into account the different
stoichiometry of the complexes formed with selected salts and
the good solubility of salt receptors in chloroform, we
conducted qualitative extraction tests to verify whether the
ion pair receptors can operate under interfacial conditions and
are able to extract salts from aqueous solution into organic
phase. Specifically, we have treated the solution of receptors 1
and 2 in chloroform or deuterated chloroform with aqueous
−
solution of sodium or potassium salts of Br−, NO3 , Cl−, and
2−
SO4 and tracked the formation of complexes with salts in
1
organic phase using UV−vis or H NMR analyses (Figures 3
and 4).
Figure 3. UV−vis spectra of receptor 2 at 3 × 10−5 M (a) in wet
CHCl3; (b) after KBr (50 mM) extraction from the aqueous phase;
(c) after K2SO4 (50 mM) extraction from the aqueous phase.
We noticed a considerable bathochromic shift in the UV−vis
spectrum after contacting a wet chloroformic solution of 2 with
all aqueous solutions of potassium salts, including extremely
hydrophilic sulfates, as well as for the extraction of
chloroformic solution of 2 with aqueous solution of NaCl.
This demonstrates that receptor 2 is able to form complexes in
organic phase extracting ions from aqueous solution. Further
However, the same receptor 2 with K2SO4 salt gives a
structure distinctly different from the examples described
above. The crystal lattice contains one sulfate anion
coordinated by four receptor molecules, with every second
organic molecule complexing potassium in the crown ether
unit. Such a supramolecular complex is located on a 2-fold
symmetry axis with the anion equally disordered over two
possible orientations. The sulfate anion is coordinated by the
squaramide N−H binding domains, resulting in hydrogen
bond formation with N···O distances ranging from 2.66 to 3.02
1
support came from H NMR measurements showing that the
signals in the NMR spectrum corresponding to the squaramide
protons as well to the aromatic ones were shifted downfield
after the solution of 2 in wet CDCl3 was contacted with
aqueous solution of salts, indicating interaction with anions.
Specifically, the signals corresponding to the squaramide
protons, which initially resonated at 8.95 and 9.32 ppm,
were shifted downfield after extraction with aqueous solution
of KBr or K2SO4 to 9.92 and 10.33 ppm or 10.00 and 10.27
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and K2SO4. The obtained results clearly demonstrate that large
species are formed only after contacting of receptor 2 with
potassium sulfate. The drop in diffusion coefficient value in this
case was calculated to be ΔD = 21%, from D = 0.53 × 10−9 m2
s−1 for 2 in wet CDCl3 to D = 0.43 × 10−9 m2 s−1 after contact
with aqueous solution of K2SO4. In contrast, after the
extraction of chloroformic solution of 2 with an aqueous
solution of KBr the change was less pronounced, and the
diffusion coefficient after extraction was found to be D = 0.52
× 10−9 m2 s−1 (ΔD ca. 2%). The changes in the size of
receptor 2 and supramolecular complexes formed in wet
chloroform after extraction were also measured by means of
DLS measurements. The value of the solvodynamic diameter
was found to be d = 1.2 nm for receptor 2 in wet chloroform,
and after extraction with aqueous solution of KBr or K2SO4, it
increased to 1.4 or 2.0 nm, respectively (see the Supporting
Information). This is in accordance with DOSY experiments
and provides confirmation that the largest assembly is formed
in organic phase after contacting with aqueous K2SO4 solution.
Next, we estimated the selectivity of receptor 2 under
interfacial conditions, tracking the loss of particular anions in
the aqueous source phase after extraction. Specifically, we
carried out extraction experiments using a 5 mM aqueous
solution of K2SO4, an aqueous binary mixture consisting of
KNO3 and K2SO4 (5 mM each), an aqueous mixture of more
complicated potassium salts (KCl, KBr, KNO3, KNO2,
KH2PO4, and K2SO4, 5 mM each), and a 20 mM solution of
2 in chloroform. The drop in anion concentrations in aqueous
phase after extraction was monitored using the ion
chromatography technique. The obtained results demonstrated
the aforementioned findings that receptor 2 is able to extract
potassium sulfate from aqueous into organic phase and do so
even from diluted samples. Under such conditions, the sulfate
concentration decrease in the aqueous phase was 45%, which
assuming a 4:1 (receptor/sulfate) complex stoichiometry and 4
times higher receptor 2 concentration in chloroform than
potassium sulfate in aqueous phase, corresponds to the same
extraction yield. More importantly, competitive extraction
experiments showed that by using receptor 2, it is possible to
overcome the Hofmeister bias and extract extremely hydro-
philic sulfate salts in the presence of lipophilic nitrates: After
extraction of binary mixtures of KNO3 and K2SO4 with 2 in
chloroform, the drop in concentration was calculated to be 12
and 44% for nitrates and for sulfates, respectively. Taking into
account the stoichiometry of complexes formed (1:1 for
nitrates and 4:1 for sulfates), the extraction yield was calculated
to be 3 and 44% for nitrates and sulfates, respectively (see the
Supporting Information). Furthermore, the selectivity and
effectivity of receptor 2 toward sulfate was maintained even
when the spectrum of potassium salts in the aqueous mixture
was extended to nitrites, bromides, chlorides, and dihydrogen
phosphates (Figure 5). This demonstrates that the main
driving force for selective sulfate extraction is the formation of
higher stoichiometry complexes of 2 with sulfates, rather than
1:1 complexes for other monovalent anions. This enables the
formation of an inorganic−organic (sulfate-receptor) core−
shell-like assembly, thereby selectively smuggling the extremely
hydrophilic sulfate anion into the organic phase. However,
basic anions such as acetate, benzoate, or hydrogen phosphate
promote deprotonation of 2, which causes no phase separation
during the extraction experiments.
1
Figure 4. Partial H NMR spectra of receptor 2 at 3 mM (a) in wet
CDCl3; (b) after KBr (50 mM) extraction from the aqueous phase;
(c) after K2SO4 (50 mM) extraction from the aqueous phase; (d)
after back extraction of solution to distilled water.
ppm, respectively. Less pronounced, yet observable changes
(Δppm in the range 0.03−0.07 ppm) were found for signals
assigned to the crown ether protons, suggesting that potassium
cation also occupies the crown ether cavity. After back-
extraction experiments, the signals returned to the initial
position, which confirms salt release and suggests that receptor
2 may act as a suitable salt transporter. We found that receptor
1 was ineffective under interfacial conditions, and no evident
spectral changes were noted after the solution of 1 in wet
chloroform was treated with aqueous solutions of salts.
Interestingly, anion receptor 3 is not soluble in chloroform,
which implies that the presence of the cation binding domain
in receptors 1 and 2 is not only responsible for the
enhancement in anion biding in the presence of cations but
also facilitates solubility of ion pair receptors in lipophilic
media.
In order to establish the extraction efficiency, which was
defined as the occupancy ratio of receptor 2 with cation, we
applied atomic emission spectroscopy (AES) and quantified
the potassium content in organic phase. Specifically, we carried
out extractions of 0.5 M aqueous solution of selected
potassium salts with 1 mM solution of 2 in chloroform. The
obtained results clearly demonstrate the presence of potassium
cations in the chloroformic phase after each extraction
experiment. We found that increased extraction efficiency in
the case of potassium halides follows decreased hydration
energy of extracted anions. Specifically, after extraction of
potassium chloride, bromide, and iodide the content of
receptor 2 molecule occupied with potassium cation was
calculated to be 51, 58, and 61%, respectively. The lowest
content of potassium cation in the organic layer (extraction
efficiency equal to 45%) was noted after extraction of aqueous
solution of potassium sulfate with 2 in chloroform. However,
taking into consideration the 4:1 stoichiometry for complexes
of 2 with sulfates, where only two of four ligands are occupied
by cation, the yield of extraction can be calculated at 90%.
To verify this assumption and to evidence the disparity in
the stoichiometry of complexes formed in the organic layer
1
after extraction, H NMR DOSY analyses were applied once
again. We measured the diffusion coefficients of receptor 2 in
wet CDCl3 and after extraction with aqueous solutions of KBr
Nevertheless, the system has some disadvantages because
the replacement of the potassium cation with a sodium ion in
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layer to half the concentration in the initial source phase) after
7 days was calculated to be 22%. Unprecedently, the analogous
experiment with potassium sulfate revealed the ability of
neutral receptor 2 to transport sulfate in the form of an alkali
metal salt across a membrane. Moreover, in this case the yield
of transport was remarkably higher than that for TBA2SO4
transport, and after 7 days, it was found to be 38.5%. This
clearly demonstrates the cooperativity in ion pair transport
across membranes and enables not only the transport of K2SO4
by receptor 2 but also enhances sulfate anion transport with
the assistance of potassium cation (Figure 6). When we
Figure 5. Chromatograms obtained during extraction experiments
after 10-fold dilution: (a) source phase, (b) after extraction with 20
mM of 2 in CHCl3.
extraction experiments significantly reduced the receptor’s
efficiency toward sulfates. When the source of sulfates was
changed to sodium salt (5 mM Na2SO4 in water), the sulfate
concentration in aqueous phase after extraction with solution
of 20 mM receptor 2 in chloroform decreased only by 1%. This
suggests that apart from the binding ability toward cations, the
hydration energy of the cation may affect the receptor behavior
under interfacial conditions. Verification of this hypothesis
came from extraction experiments carried out for aqueous
solution of 5 mM of sulfates added as a mixture of sodium and
tetrabutylammonium salts. Specifically, in the presence of the
lipophilic TBA cation, receptor 2 was able to extract sulfates
from aqueous to organic phase more efficiently, and the drop
in sulfate concentration was calculated to be 12%. Further
enhancement in sulfate extraction was achieved when
TBA2SO4 was used as a sulfate source (22%). In no case,
however, was the efficiency achieved as for the extraction of
potassium sulfate, showing the cooperation in extraction using
this salt.
Transport across Membrane. Finally, taking into
consideration the ability of receptor 2 not only to extract but
also to release sulfate anion, we carried out transport
experiments using the U-tube technique. Very recently, Jolliffe
and co-workers have successfully demonstrated the ability of
neutral, cyclic squaramide to transport sulfate ion across a bulk
chloroform layer via an anion exchange mechanism with
nitrate.10h However, because it uses an anion receptor as a
transporter, this process and all those reported so far in the
literature required the assistance of lipophilic counterion.10 We
envisioned that by using receptor 2 it would be possible to
eliminate this drawback, and for the first time, transport
sulfates in the form of alkali metal salts across a membrane. For
comparative purposes, we investigated the U-tube transport
experiments with both scenarios where the aqueous source
phase contained a 50 mM solution of TBA2SO4 or K2SO4 and
the bulk chloroform phase contained 5 mM receptor 2. The
sulfate concentration in the receiving phase was monitored by
conductometry. We found that receptor 2 is able to transport
tetrabutylammonium sulfate, and the yield of this process
(defined as the ratio of sulfate concentration in the receiving
Figure 6. Sulfate transport by 2 across a bulk chloroform membrane
determined by the sulfate concentration in the receiving phase. Source
phase: 50 mM solution of TBA2SO4, K2SO4, or KCl in water; organic
phase: 5 mM 2 in CHCl3; receiving phase: water.
extended the time for the potassium sulfate transport
experiment to 14 days, the yield was even higher, reaching
63%. Interestingly, receptor 2 is also an efficient chloride
transporter, which also makes it an excellent candidate in the
treatment of channelopathy.22 The transport experiments with
potassium chloride in the source phase proved to be an even
more efficient process than in the case of K2SO4 transport.
After 7 and 14 days, the yield of the process was calculated to
be 62 and 80%, respectively (Figure 6).
Comparison of the data from extraction and transport
experiments implies that the ability of receptor 2 to separate
salts can be achieved in a tailored manner by using the
appropriate technique. Due to the relatively stable complexes
of receptor 2 with sulfates, the extraction process favors
selective separation of these salts. For the same reason
(stability), the transport of sulfates across the membrane,
which requires the release of the ion from the complex, should
be hampered, and other salts which form less stable complexes
with receptor 2 should be transported more effectively. To
conclusively confirm this assumption, competitive U-tube
transport experiments were carried out under ion chromatog-
raphy control. Indeed, when the source phase contained binary
mixtures (50 mM each) of KNO3/K2SO4 or KCl/K2SO4, the
transport of chloride or nitrate salt across the membrane was
more efficient than that of potassium sulfate (Figure 7C,D).
However, from the point of view of nuclear waste disposal,
selective sulfate extraction is necessary, preferably in a
continuous process. Thus, we modified transport experiments
and instead of using water as a receiving phase, aqueous
solutions of specific salts were used. For the competitive
KNO3/K2SO4 or KCl/K2SO4 transport experiments, an
aqueous solution of KNO3 or KCl was used as a receiving
phase, respectively. As a consequence, the transport of
lipophilic salts was “frozen” by reaching equilibrium and
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Figure 7. Schematic illustration of the ability of receptor 2 to remove specific salts from aqueous solutions by varying the separation techniques and
experimental conditions. (A) Extraction experiment: top and bottom chromatograms refer to initial aqueous phase and after extraction with the
solution of 2 in chloroform, respectively. (B−F) Transport experiments: the concentration of specific salts on the charts refers to the receiving
phase.
facilitated the selective transport of sulfates in the presence of
more lipophilic salts in a continuous process (Figure 7E,F).
The capabilities of receptor 2 are schematically depicted on
Figure 7, and the drop of salt concentration in the source phase
by using appropriate technique is presented on Figure 8. This
experiments, DOSY, DLS, and solid-state X-ray measurements.
The formation of 4:1 complexes of receptor 2 with sulfates
combined with enhancement in anion binding promoted by
cation assistance facilitated the selective extraction of
potassium sulfate from aqueous into organic phase, over-
coming the Hofmeister bias. The ability of receptor 2 not only
to uptake but also to release salts was utilized, and U-tube
transport experiments were conducted. We demonstrated for
the first time that neutral receptor 2 is capable of transporting
sulfates in the form of alkali metal salt without the presence of
a bulky counterion. Both techniques (extraction and transport)
were shown to be cooperative, and potassium sulfate was
extracted or transported more effectively than tetrabutylam-
monium salt. By changing the extraction or transport
technique or the composition of the receiving phase, it was
possible to alter the selectivity and switch the depletion of the
aqueous layer into extremely hydrophilic or less lipophilic salts.
Figure 8. Drop-in salt concentration in the source phase after 7 days
of transport experiments illustrated in Figure 7: C, competitive
KNO3/K2SO4 transport; D, competitive KCl/K2SO4 transport; E,
supported competitive KNO3/K2SO4 transport; and F, supported
competitive KCl/K2SO4 transport.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02114.
■
sı
1
General information, synthetic details, H NMR and
UV−vis titration data, extraction experimental details,
clearly indicates the unique properties of receptor 2 and the
possibility of its use for the selective transport of selected salts,
even those which are extremely hydrophilic, in an adaptable
manner by varying the separation technique and experimental
conditions.
DOSY and DLS measurements (PDF)
Accession Codes
CCDC 1988974−1988976 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSION
■
Squaramide-based ion pair receptors 1 and 2 and anion
receptor 3, possessing a pentafluorophenyl unit conferring
lipophilicity and assuring acidity of the anion binding domain,
were synthesized in modular fashion. Contrary to monotopic
receptor 3, ditopic receptors 1 and 2 were able to bind anions
more strongly in the assistance of alkali metal cations. The high
affinity of these receptors for ions was successfully used in salt
recognition even in highly competitive aqueous media.
Squaramides 1−3 were found to form 4:1 complexes with
sulfates, rather than 1:1 complexes as with other monovalent
anions. The difference in stoichiometry of the complexes was
confirmed by means of UV−vis and 1H NMR titration
AUTHOR INFORMATION
Corresponding Author
■
́
Jan Romanski − Faculty of Chemistry, University of Warsaw, PL
02-093 Warsaw, Poland; orcid.org/0000-0002-7675-
885X; Email: jarom@chem.uw.edu.pl
Authors
Marta Zaleskaya − Faculty of Chemistry, University of Warsaw,
PL 02-093 Warsaw, Poland
I
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́
Romanski, J. An Ion Pair Receptor Facilitating the Extraction of
Chloride Salt from the Aqueous to the Organic Phase. New J. Chem.
2016, 40, 7190−7196. (c) Frontera, A.; Orell, M.; Garau, C.;
Marcin Karbarz − Faculty of Chemistry, University of Warsaw,
PL 02-093 Warsaw, Poland; orcid.org/0000-0002-7813-
0513
Marcin Wilczek − Faculty of Chemistry, University of Warsaw,
PL 02-093 Warsaw, Poland
Łukasz Dobrzycki − Faculty of Chemistry, University of
Warsaw, PL 02-093 Warsaw, Poland; orcid.org/0000-
0002-4426-963X
Quinonero, D.; Molins, E.; Mata, I.; Morey, J. Preparation, Solid-State
̃
characterization, and Computational Study of a Crown Ether
Attached to a Squaramide. Org. Lett. 2005, 7, 1437−1440. (d) Yu,
X. H.; Cai, X. J.; Hong, X. Q.; Tam, K. Y.; Zhang, K.; Chen, W. H.
Syntheisis and biological evaluation of aza-crown ether-squaramide
conjugates as anion/cation symporters. Future Med. Chem. 2019, 11
(10), 1091−1106. (e) Zaleskaya, M.; Jagleniec, D.; Karbarz, M.;
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02114
́
Dobrzycki, Ł.; Romanski, J. Squaramide based ion pair receptors
possessing ferrocene as a signalling unit. Inorg. Chem. Front. 2020, 7,
972−983. (f) Jagleniec, D.; Siennicka, S.; Dobrzycki, Ł.; Karbarz, M.;
Notes
́
Romanski, J. Recognition and extraction of sodium chloride by a
The authors declare no competing financial interest.
squaramide-based ion pair receptor. Inorg. Chem. 2018, 57, 12941−
12952.
ACKNOWLEDGMENTS
This work was supported by Grant no. 2018/30/E/ST5/
00841 from the National Science Centre, Poland.
■
́
(6) (a) Jagleniec, D.; Dobrzycki, Ł.; Karbarz, M.; Romanski, J. Ion-
pair induced supramolecular assembly formation for elective
extraction and sensing of potassium sulfate. Chem. Sci. 2019, 10,
9542−9547. (b) Jagleniec, D.; Karbarz, M.; Romanski, J. Polish
Patent Application PL429164, 2019.
́
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