Macrocyclic squaramides as ion pair receptors and fluorescent sensors selective towards sulfates

Article abstract of DOI:10.1039/d0dt04273k

Through the high dilution technique, we obtained macrocyclic ion pair receptorsR1andR2, an anion receptorR3, and a fluorescent sensorR4using a combination of particular members of simple libraries consisting of synthesized diamines and methyl squarates, r

Full text of DOI:10.1039/d0dt04273k

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Macrocyclic squaramides as ion pair receptors and
fluorescent sensors selective towards sulfates†
Cite this: Dalton Trans., 2021, 50,
3904
Marta Zaleskaya,
Damian Jagleniec
and Jan Romański
*
Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, an anion
receptor R3, and a fluorescent sensor R4 using a combination of particular members of simple libraries
consisting of synthesized diamines and methyl squarates, respectively. The receptors were investigated in
1
terms of anion and ion pair binding using the H NMR titration method in DMSO-d
6
. We found that the
major contribution to the anion binding comes from the interaction with the squaramide protons rather
than with the amide functions of the receptors. The receptors demonstrated the highest affinity towards
benzoates and sulfates over the anions tested, and in the case of sulfate binding more complex equilibria
in solution were observed. Unlike the anion receptor R3, the ion pair receptor R1 was found to recognize
anions in an enhanced manner with the assistance of sodium or potassium cations. Tethering of a simple
fluorophore in close proximity to the amide function of receptor R4 resulted in an optical ion pair sensor
selective towards sulfates. DFT calculations carried out for the 1 : 1 complexes of R3 with the anions
helped clarify this selectivity, showing more effective participation of tetrahedral sulfate anions in binding
with the amide function than in the case of benzoates or chlorides.
Received 16th December 2020,
Accepted 8th February 2021
DOI: 10.1039/d0dt04273k
rsc.li/dalton
5
–7,10–12
chemistry.
In the context of anion recognition, the pio-
Introduction
neering work of Costa’s team showed the excellent binding
In recent years, as the importance of anions in industrial, bio- properties of secondary squaramides, i.e., able to recognize
logical, physiological and environmental fields has been carboxylates by creating 1 : 1 complexes even in aqueous solu-
gaining increasing recognition, attention has turned to the tions. Through the suitable arrangement of binding units on
design and synthesis of receptors that are able to interact with convenient platforms, recognition of di- and tri-carboxylates
1
3
negatively charged species. Within this branch of supramole- has been achieved. Direct comparison of diarylsquaramides
cular chemistry, a special trend can be distinguished which with analogous ureas or sulfonamides was reported by
deals with more complex receptive architectures, such as Fabbrizzi’s group, which showed that the contest for anion
1
4,15
macrocyclic systems. Macrocyclic receptors, due to the preorga- recognition was won by the former compounds.
Li’s group
nization and multiplication of the binding domains in the reported on the family of sulfinyl squaramides, showing that
closed structure, have been found to be more effective and besides the acidity of the squaramide protons the additional
1
–4
selective hosts than their acyclic analogues. The squaramide C–H hydrogen bonding may tune the binding strength of the
1
6
unit, on the other hand, is one of the most promising func- receptors. Using a C-3 platform, squaramide based tripodal
tional groups and is frequently utilized in the design of anion receptors were obtained by several research groups, which
receptors.
Squaramides have been shown to interact with anions more their selective sensing and transport across lipid
then utilized them in effective binding of anions, owing to
1
7–23
effectively than other charge neutral functions, such as ureas membranes.
The reports of Gale, Sessler, Davis and co-
or thioureas, and are able to operate even in highly polar sol- workers showed the diversity of squaramide based anion
5
–9
vents and in an aqueous environment.
Besides their utiliz- receptors which can be applied as transmembrane anion trans-
ation in the areas of anion receptors, sensors and transporters, porters, with the potency to act as drugs in the treatment of
2
4–27
their superior properties have been successfully exploited in channelopathies or cancers.
The incorporation of a signal-
organocatalysis,
materials
science
and
medicinal ing unit in close proximity to the anion binding domains
resulted in the colorimetric and luminescent sensors designed
2
8,29
by Jolliffe, Elmes and co-workers,
reported by Gale, Costa, Taylor, Muthyala and co-
the fluorescent sensors
Faculty of Chemistry, University of Warsaw, Pasteura 1, PL 02-093 Warsaw, Poland.
E-mail: jarom@chem.uw.edu.pl
1
8,26,30–32
workers
and very recently, the electrochemical
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
9
d0dt04273k
sensors reported by our group. Using an indicator displace-
3904 | Dalton Trans., 2021, 50, 3904–3915
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4
7–49
ment assay, Costa and co-workers designed a squaramide its counterion (e.g., anion).
In the literature only some
based system capable of quantifying sulfate and phosphate in examples of squaramide based ion pair receptors are reported,
3
3
water. Calix[4]arene and pillar[5]arene-based anion receptors showing their superlative properties over monotopic anion
8
,50–53
have been reported by Wang and co-workers, demonstrating receptors.
Some of them are so effective that they can
that the receptors can selectively recognize fluoride, dihydro- extract extremely hydrated sulfate anions in the form of potass-
3
4,35
genphosphate and carboxylate anions.
The group of Jolliffe and Elmes has shown that amino acid salts across membranes.
based squaramides are effective anion receptors, with some However, to the best of our knowledge, multimacrocyclic,
able to operate even in aqueous solution and recognize sulfate squaramide-based ion pair receptors have so far not been
ium salts from the aqueous to the organic phase or transport
5
4,55
3
6–38
anions.
Macrocyclic compounds containing the squara- reported in the literature. By combining the features which
mide moiety have also been exemplified in the literature, arise from the macrocyclic preorganization of the squaramide
being first reported by Costa’s group, who demonstrated their function and from the presence of the cation binding domain
3
9
utility in monitoring sulfates in water. Since then, a few which should tune the anion recognition, we would like to fill
research papers have been published on macrocyclic squara- this gap in the methodology. Herein, we describe the synthesis
mides able to bind certain anions in organic solvents. For and binding properties of new multimacrocyclic ion pair
instance, the group of Jolliffe and Elmes has presented squara- receptors R1 and R2 based on a squaramide recognition
mide based macrocycles which exhibited high sulfate binding moiety appended to benzo-18-crown-6. In order to identify the
affinities and selectivities in
a
range of
H
2
O/DMSO-d
6
influence of particular functional parts of receptors on the
4
0,41
mixtures.
cyclic anion receptors that are able to efficiently extract SO4
from an aqueous Na SO solution into an organic phase, ation of a simple fluorescent naphthalene unit into the struc-
In addition, the same group has shown macro- binding properties, the analogous anion receptor R3 was also
2
−
tested. Optical salt sensing was accomplished by the incorpor-
2
4
and can transfer highly hydrophilic sulfate ions across a bulk ture of the receptor R4 (Fig. 1).
chloroform layer via an anion exchange mechanism using
nitrate ions and with the assistance of lipophilic tetraalkylam-
monium cations. Unfortunately, anion receptors are usually
tested using salts consisting of anions with the assistance of
Results and discussion
lipophilic tetraalkylammonium cations, and the presence of A typical protocol for the synthesis of receptors R1–R4 is out-
strongly coordinated cations such as alkali metals may lined in Scheme 1 and relies on the preparation of two types of
4
2
hamper or preclude their action. This drawback can be over- small libraries. Library A consists of diamines A1–A3, while
come by using ion pair receptors that can bind cation and methyl squarates E1 and E2 are members of library E. Both
1
,43–46
anion simultaneously and cooperatively.
This general libraries were prepared starting from diaminoaryls including
recognition process is often governed by the fact that the pre- commercially available o-phenylenediamine or 2,3-diamino-
bound ion can promote binding of its counterion, leading to a naphthalene and the obtained 4,5-diaminobenzo-18-crown-6.
positive allosteric effect. Broadly speaking, recognition of a The latter was prepared in a two-step protocol adopting the
5
6
first ion (e.g., a cation) facilitates the subsequent co-binding of procedure reported in the literature. Specifically, nitration of
Fig. 1 Structures of multimacrocyclic receptors R1-R4.
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3 2 2 3
Scheme 1 Reagent and conditions: (i) Et N, dry CH Cl , 12 h, r.t., 91%; (ii) HBr/AcOH (33%), r. t., 76%; (iii) 2 equiv. dimethyl squarate, Et N, MeOH,
overnight, r. t., 65%.
benzo-18-crown-6 by a two-phase reaction in chloroform and diamine A2 gave the anion receptor R3 and the reaction of
nitric acid/sulphuric acid mixture gave 4,5-dinitrobenzo-18- ester E1 with diamine A3 yielded the ion pair sensor R4
crown-6, which was converted to the diamine by means of (Scheme 2). The crude mixture of macrocycles thus obtained
reduction using hydrogen and the heterogeneous catalyst Pd/ was separated by collecting the precipitate formed, which was
C. The aromatic diamines thus obtained were coupled with then recrystallized from a DMSO/MeOH mixture. Unfortunately,
in situ generated acyl chlorides to afford the corresponding receptor R2 after crystallization was found to be insoluble
benzocrown, phenyl and naphthyl based diamides denoted as in most common solvents and without purification is not
5
7
40
D1, D2 and D3, respectively. Low solubility of the amines sufficiently pure for examination in binding studies (ESI†).
that belonged to library A precluded Cbz removal in the inert Quantitative information about the binding ability of the
medium with palladium on charcoal as the catalyst, and thus ion pair receptor R1 toward anions was obtained by H NMR
the deprotection of amine functions was achieved using a solu- titration experiments in DMSO-d . First, we tested R1 towards
1
6
tion of hydrobromic acid (33%) in AcOH. Due to this and for selected anions which were used as tetrabutylammonium
reasons of stability, library A contains amines in the form of salts. In the case of titration of R1 with nitrate anions, the
1
hydrobromide salts. A portion of the resulting diamines was small change in the position of signals in the H NMR spectra
subjected to a sequential amidation reaction with 2 equiva- suggested a very weak interaction, preventing the stability con-
lents of dimethyl squarate in the presence of triethylamine to stant from being determined. For titrations with acetate
afford the methyl squarate containing library E.
anions, on the other hand, the signals corresponding to the
High dilution techniques allowed receptors R1–R4 to be amide protons disappeared, suggesting a deprotonation event.
obtained using combinations of particular members of The addition of other TBA anion salts tested to R1 caused a
libraries A and E. Specifically, the reaction of ester E1 and substantial downfield shift of NHs belonging to the squara-
diamine A2 gave the ion pair receptor R1, ester E1 and mide function. Depending on the anions added, the change of
diamine A1 gave the ion pair receptor R2, ester E2 and resonances of squaramide NHs varied from Δδ ca. 0.1 ppm for
Scheme 2 Synthesis of the multimacrocyclic receptors R1–R4 using combinations of the appropriate diamine members of library A and methyl
squarates from library E.
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bromides and nitrites to Δδ ca. 1.5 ppm for sulfates and benzo- signal at ca. 9.6 ppm. The HSQC shows no correlation between
ates. Interestingly, a less pronounced downfield shift was also any carbon and this proton. These findings do not support the
observed for amide NHs, usually half of that of squaramide slow exchange in the NMR timescale upon the interaction of
protons. This behavior can be attributed to the unequal contri- R1 with sulfates. Furthermore, upon the titration of TBA
2 4
SO
bution of the two groups to the recognition of anions. with strong acids, the signal at ca. 9.6 ppm does not appear,
Analyzing the complexation induced shifts of the NH ruling out the possibility that its presence evidences the for-
protons and aromatic protons allowed the association con- mation of any protonated species formed from the sulfate
stants to be calculated (Table 1). The association constants anions. Taken together, this supports more complex equilibria
obtained followed the changes in the NH resonances upon in the case of interaction of R1 with sulfates and suggests that
titration, and were found to be lowest for nitrites and bro- consideration should be taken for the participation of water
mides, whereas benzoates were recognized the most strongly. molecules upon complexation, which may be manifested by
Unfortunately, due to the inconsistent shifts in the signals the signal at ca. 9.6 ppm. The literature offers several examples
corresponding to squaramide, amide and aromatic protons of receptor complexes with anions which involve water mole-
5
8
upon the addition of sulfates, we were unable to fit the col- cules, albeit in the solid state. Nevertheless, it was recently
lected data to the appropriate binding model. This suggests reported that upon the complexation of bisulfate anions by
more complex equilibria and precluded the stability constant cyanostar macrocycles more complex equilibria were evi-
from being calculated for this case. Interestingly, upon denced, with water molecules being engaged even in
5
9
addition of between two and five equivalents of sulfates to R1, solution.
an additional signal at ca. 9.6 ppm appeared. At least two scen- The affinity of receptor R1 for chloride anions in the pres-
arios might explain this behavior, taking into consideration ence of one equivalent of sodium and potassium cations was
the slow exchange in the NMR timescale upon complexation then established. In the presence of sodium cations the appar-
−
1
or the participation of water molecules in the binding event. ent stability constant was slightly enhanced to KNaCl = 78 M
In order to settle this issue, additional NMR experiments were and a 1.5 fold enhancement was observed for chloride binding
−
1
carried out. Based on the DOSY and ROESY experiments, we in the presence of potassium cations KKCl = 95 M . This
found the diffusion coefficient of receptor R1 to be equal to D enhancement corresponds well with the stability constants
0.15 × 10− m s and all signals corresponding to the NH obtained during titration of R1 with the sodium and potass-
9
2
−1
=
protons showed exchange coupling between residual water. ium cations (added as sodium perchlorate and potassium
Upon the addition of sulfate anions to the solution of R1, the hexafluorophosphate) and supports the higher affinity of R1
−
1
exchange coupling of NHs between water was not present and towards potassium rather than sodium cations (K = 366 M
K+
−9
2
−1
−1
the diffusion coefficient decreased to D = 0.14 × 10 m s
vs. KNa = 55 M ). Interestingly, further strength of chloride
+
together with the appearance of a new signal and no change in binding can be achieved by the addition of more equivalents
the integration of signals corresponding to the amide func- of potassium cations. Specifically, the titration of R1 with
tion. Interestingly, the DOSY experiments revealed that the chloride anions in the presence of three equivalents of potass-
diffusion coefficient of the new species in DMSO is equal to D ium cations produced an apparent stability constant of KKCl
=
0.91 × 10− m s and is very close to the value observed for 120 M which entails an almost double enhancement in
9
2
−1
−1
=
−
9
2
−1
−1
water in the system (D = 0.93 × 10 m s ). It is almost one relation to chloride anion binding (KTBACl = 61 M ). However,
magnitude of order higher than the diffusion coefficient of the this solution has its drawbacks, as there is a risk of the precipi-
receptor R1. A ROESY spectrum carried out in DMSO confirms tation of inorganic salt upon titration in the organic solvent.
the exchange coupling between the water molecule and the For this reason, an extension of the ion pair binding studies
a
a
Table 1 Apparent association constants (K ) for the interactions of receptors with selected anions
+
+
+
+
+
R1
R1 + 1 equiv. of K
R1 + 3 equiv. of K
R1 + 1 equiv. of Na
R3
R3 + 1 equiv. of K
R4
R4 + 1 equiv. of K
Cl−
Br−
61
95
120
78
76
71
36
72
+
R1
R1 + 1 equiv. of K
R3
41
22
77
65
−
−
NO
NO
2
44
42
b
b
b
3
−
PhCOO
c970
2 920
c1 210
−
c
CH
3
C−OO
2
d
d
d
SO
4
a 1
, temperature 298 K; anions added as TBA salts ∼0.06 M; R1 (c = 2.0 × 10 M), R3 (c = 1.6 × 10−3 M) and R4 (c = 1.7 ×
−3
H NMR, solvent DMSO-d
6
−
3
b
1
0
M); data fit to a 1 : 1 binding model; errors < 10%, see the ESI.† Low changes in the NMR spectrum upon titration to determine K
a
.
c
d
Deprotonation. The data obtained could not be fit to an appropriate binding model.
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was performed for the titration of anions in the presence of sion spectra recorded for the solution containing the
2
−
one equivalent of potassium cations, showing the ability of R1 R4 □ SO4 complex shows an increase in fluorescence inten-
to enhance anion binding by simultaneous complexation with sity together with the appearance of a new band at 420 nm
co-bound cations (Table 1, Fig. 2).
and a color change observed under UV light λ = 254 nm
In order to verify the role of the cation binding domain in (Fig. 3). The changes in the emission spectra were even more
the structure of R1, the anion receptor R3 lacking a crown pronounced when the experiments were done under excitation
ether unit was obtained and tested towards the same range of at λ = 340 nm, where the band at 440 nm substantially
anions. Inspection of the stability constants collected revealed increased. Interestingly, the emission spectra of R4 were
the same selectivity order of R3 towards anions, but in all affected by an increase in temperature. Specifically, the broad
cases stronger anion recognition than for R1 was noted. This band of R4 centered at 395 nm was narrowed at 60 °C and
is attributed to the lack of two electron donating alkoxy substi- shifted to 375 nm. This may suggest that receptor R4 is prone
tuents in the structure of anion receptor R3, which, in relation to form intramolecular hydrogen bonds at room temperature
to R1, decreases the electron density on the phenyl ring and (intermolecular bonding being excluded by dilution experi-
allows for a tighter interaction with anion. Nevertheless, the ments) which is broken at higher temperatures. On the other
lack of the cation binding domain in the structure of R3 hand, the emission spectra recorded for A3 shows a narrow
caused serious reservations in terms of ion pair binding. band centered at 375 nm (ESI†). This finding correlates well
Namely, the monotopic anion receptor R3 could not recognize with the different solubilities of these compounds, with a dis-
anions in an enhanced manner in the presence of potassium tinct predominance of library A members for which intra-
cations. In this case, a comparable recognition for chloride molecular hydrogen bonding has not been proven. Although
= 71 M− vs. there is no clear evidence, based on the H NMR titration
1
1
anion and its potassium salt was noted (K
K
KCl
TBACl = 76 M− for receptor R3).
1
experiments and UV-vis measurements, for the deprotonation
Next, we obtained sensor R4 by tethering of a simple fluoro- event of R1 upon complexation with sulfates, as is in the case
phore unit in close proximity to the amides, which contributed of interaction with acetates, we took this possibility into con-
to the interaction with anions less strongly. We envisioned sideration using sensor R4. Specifically, we examined the spec-
that by this modification selective optical sensing towards sul- tral behavior of R4 upon the addition of TBAOH. First, we
fates could be achieved; even so, our findings show that the found that the addition of this base to the solution of R4 in
benzoate anion is also effectively recognized by R1. We antici- DMSO produced a color change under UV light which is analo-
2 4
pated that the Y-shaped carboxylate anion would predomi- gous to that seen upon the addition of TBA SO . Contrary to
nantly interact with the complementary, squaramide function. the addition of sulfate anions to the solution of R4, in the
On the other hand, we estimated that in a solution of R4 pre- presence of a strong base, a larger bathochromic shift in the
treated with sulfate anions, the 1 : 1 complex also exists and maximum emission of R4 was found which is centered at λ =
the tetrahedral sulfate anion would be more involved in the 460 nm (ESI†). These findings may suggest that for the
interaction with the amides supported by the fluorophore than complex equilibria found in the interaction of receptors with
the other anions. Indeed, the addition of sulfate anions sulfate anions, partial deprotonation should not be excluded.
(
added as TBA salt to assure the solubility) to the solution of
the fluorescent sensor R4 in DMSO changed its optical pro- ium sulfate to R4, less pronounced changes were observed
perties. In particular, the addition of 15 equivalents of than that upon the addition of TBA SO , especially for emis-
TBA SO caused a slight bathochromic shift in the maximum sion experiments carried out with 340 nm excitation (ESI†).
Interestingly, upon the addition of in situ generated potass-
2
4
2
4
absorption of the receptor, from 291 nm to 296 nm. The emis- Specifically, if the significant change in emission observed at
1
Fig. 2 Representative stack plots of the partial H NMR spectra of R1 upon the addition of TBAPhCOO (left) and the selected binding isotherms for
6 6
titration of R1 (right) with TBAPhCOO alone and in the presence 1 equiv. of KPF in DMSO-d .
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Fig. 3 Top: Changes observed in the fluorescence intensity: (a) R4 (5 mM in DMSO), (b) upon the addition of TBA
2
SO
4
, (c) upon the addition of
TBA
2 4 6
SO with the assistance of KPF , (d) upon the addition of TBAPhCOO and (e) upon the addition of TBACl (under UV light, λ = 254 nm). Bottom:
−5
UV-vis absorption and emission (excitation at 300 nm) spectra of R4 (1.0 × 10 M) upon the addition of TBA salts.
In the case of modelling the structure of the R3 □ Cl−
4
40 nm for the sulfate anion may be attributed to a plausible
deprotonation event, this was not observed in the presence of complex, we observed that the chloride anion can be trapped
potassium cations. The assistance of potassium cations also inside the macrocyclic cavity by the formation of up to eight
led to another slight change of color, which still allows for the hydrogen bonds. The chloride anion is located in the central
differentiation of potassium sulfate from other salts, although part of the macrocyclic cavity and lies almost in the plane
not as clearly as in the case of sulfate anions.
formed by four squaramide nitrogen atoms and 1.8 Å above
All our attempts to obtain suitable complexes of R1–R4 for the plane formed by four nitrogen atoms of the urea group. As
RAS analyses failed; thus, to gain a better understanding of indicated by the different distances between the chloride
the mechanisms of anion binding by the receptors, density anion and the NH groups of the squaramide function, which
functional theory calculations were performed. The structure are in the range of 3.24–3.29 Å, and the NHs of the urea func-
of receptor R3 was energy minimized using molecular mech- tion which are in the range of 3.71–4.06 Å, the greatest contri-
−
2−
−
anics and Cl , SO
4
and PhCOO anions were placed into the bution to the anion binding by R3 is made by the former
center of the macrocycle and the obtained complexes opti- group. In the calculated structure, two of the amide protons
mized by density functional theory (DFT) calculations were are poorly exposed to the chloride anion, which mismatch
performed at the B3LYP/6-31G* level of theory. We realize with the directional requirements for a strong interaction. This
that this modeling of molecular docking anions does not is in line with the NMR titration experiment, which shows that
produce quantitative results for the energy of interactions; upon the addition of chloride anions to the solution of R3 the
however, it may produce a qualitative image of the possibilities signals corresponding to the squaramide protons were shifted
of placing the guest/host in the space. Receptor R3 was chosen more distinctly, with Δδ of ca. 0.52 ppm, than the signals of
for the calculations due to its ability to form the strongest amide protons, which shifted up to ca. 0.19 ppm. This may
complexes with the studied anions and for computational ease indicate a less significant contribution to the interaction for
(Fig. 4).
the latter binding site.
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Fig. 4 Minimized structures (DFT, B3LYP/6-31G*, Spartan’10, Wavefunction®) of the complexes (a) R3 □ Cl , (b) R3 □ PhCOO− and (c) R3 □ SO
−
2−.
4
Left: Top view, right: side view.
The calculation performed for the R3 □ PhCOO− complex ticipates in the binding event, albeit less effectively (Δδ ca.
indicates that the anion occupies the position above the 0.39 ppm for NH’s signals).
2−
planes formed by four nitrogen atoms of the squaramide func-
The calculated structure of the R3 □ SO
4
complex indicates
tion and is bonded mainly by these two groups. The squara- that the sulfate anion is strongly coordinated by R3, with an
mide protons are directed toward the oxygen atoms of the ben- almost equal contribution of all binding sites. Each squara-
zoate, and the hydrogen bond lengths thus formed exhibit the mide unit and each amide function pair creates two hydrogen
distances 2.847 Å, 2.855 Å, 2.874 Å and 2.861 Å, which are bonds with corners of tetrahedral sulfate anions. As a result,
shorter than in the case of chloride anion interactions. On the the distance between the oxygen atoms and the NHs of the
other hand, the amide located near the phenyl ring does not squaramide and amide functions are, on average, 2.75 Å and
directly face the benzoate anion and the distance between the 2.80 Å, respectively. The sulfate anion is slightly “nestled” in
NH group of the receptor and the O atoms of the benzoate the cavity of R3 and is exposed on the other side, creating the
anion have values well above 4 Å. Indeed, we found that possibility for other plausible interactions. In fact, this model-
among the anions tested, R3 forms strong complexes with ben- ling does not fully support the findings collected from the
zoates and upon titration the most pronounced chemical shift titration experiments which suggest unequal contributions of
was noted for the signals corresponding to the squaramide the squaramide and amide functions to the binding event.
function with Δδ ca. 1.40 ppm. The titration data, however, Specifically, the most pronounced changes induced by the
clearly indicate that in this case the amide function also par- addition of sulfate anion to R3 were observed for squaramide
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protons, with Δδ ca. 1.95 ppm, and less distinct changes were methanol. Thin-layer chromatography (TLC) was performed on
noted for the amide groups (Δδ ca. 0.55 ppm). Nevertheless, silica gel plates (Merck Kieselgel 60 F254).
1
13
the latter change that relates to the interaction of anions with
H and C NMR spectra for the characterization of pro-
amide function is substantially higher than in the case of ducts were recorded using a Bruker 300 spectrometer using a
changes in the analogous set of signals from titration of R3 residual protonated solvent as an internal standard.
1
with the other salts tested, including benzoates. This supports
The H NMR DOSY, ROESY and HSQC experiments were
the idea of introducing a fluorophore unit in close proximity conducted at 298 K using a Varian VNMRS 600 MHz instru-
to the amide function to selectively recognize sulfates. ment with a residual solvent signal as an internal standard.
However, the design principle of R4 has certain drawbacks in
The UV-vis experiments were performed using a Thermo
terms of ion pair sensing. Namely, the addition of potassium Spectronic Unicam UV 500 spectrophotometer in DMSO solu-
sulfate to the solution of R4 produced only a slight change in tion at 298 K. The fluorescence emission spectra were recorded
color, which may suggest that this salt is recognized with a using a Hitachi F-7100 Fluorescence Spectrophotometer in
higher participation of benzocrown amides rather than the DMSO solution at 298 K.
amides located on the naphthalene unit. Our future efforts
Mass spectra were recorded using a Quattro LC Micromass
will focus on synthesizing sensors containing a crown ether or Shimadzu LCMS-IT-TOF unit.
unit directly tethered into the fluorophore, allowing them to
reinforce anion recognition with the assistance of cations.
1
H NMR titration experiments
1
The H NMR titration was conducted at 298 K in DMSO-d
6
. In
each case, 500 μL of freshly prepared 2.0 mM solution of R1
1.6 mM of R3; 1.7 mM of R4) was added to a 5 mm NMR
tube. In the case of ion pair titration, the receptor was firstly
In summary, we designed and synthesized a family of macro- pretreated with one or three equivalents of KPF or NaClO
(
Conclusions
6
4
.
cyclic molecular receptors R1–R4 by reacting particular Then small aliquots of a solution of TBAX, containing the
members of small libraries consisting of diamines and methyl receptor at a constant concentration, were added and a spec-
squarates under high dilution conditions. All receptors were trum was acquired after each addition. The resulting titration
found to interact with anions by utilizing the squaramide and data were analyzed using the BindFit (v0.5) package, available
amide functions of the receptors, with unequal contribution to online at http://supramolecular.org.
the binding event coming from the two types of binding sites.
UV-Vis experiments
Receptor R1 could recognize anions moderately, in the order
−
−
−
−
2
NO , Br , Cl and PhCOO , creating 1 : 1 complexes. Ion pair To a 10 mm cuvette was added 2.5 mL of a freshly prepared
receptors were found to interact with anions in enhanced solution of R4 (c = 1.0 × 10− M) and small aliquots of TBAX
manner with the assistance of cations and the selectivity order solution containing R4 at the same concentration as in the
was retained when R1 was complexed with in situ generated cuvette were added, and a spectrum was acquired after each
potassium salts. The lack of a cation binding site in the struc- addition.
5
ture of anion receptor R3 resulted in the loss of this enhance-
Fluorescence experiments
ment. Upon titration of R1 with sulfates, more complex equili-
bria were noted and the H DOSY and 2D NMR analyses To a solution of R4 (c = 1.0 × 10−5 M) in DMSO was added
suggested water molecule involvement in the binding event. aliquots of TBAX (100 equiv.) solution containing R4 at the
Incorporation of a fluorescent signaling unit in close proximity same concentration as in the cuvette. Successive scans were
to the amide function allowed an optical sensor selective performed for measuring fluorescence (λex = 300 and 340 nm)
towards sulfates to be obtained. DFT calculations performed emission between 290 and 650 nm.
1
for complexes of R3 with anions rationalized this selectivity. It
was demonstrated that for the 1 : 1 complexes of R3 with
Synthetic details
5
6
anions, the highest participation in interactions with the
Preparation of 3,4-dinitrobenzo-18-crown-6 .
A
flask
amide function of the receptor, near which the fluorophore equipped with a stirring bar containing a solution of benzo-18-
unit in R4 was installed, was found with sulfate anions.
crown-6 (1.5 g, 4.80 mmol) in 40 ml of chloroform was placed
in an ice-water bath at 0 °C. Concentrated nitric acid (10 ml)
and 5 ml of sulfuric acid were added dropwise while keeping
the temperature under 5 °C. The ice-water bath was removed
and stirring was continued for 48 hours at room temperature.
The reaction mixture was diluted with chloroform (30 ml) and
Experimental section
General considerations
Unless specifically indicated, all other chemicals and reagents shaken twice with ice distilled water (80 ml). The organic
used in this study were purchased from commercial sources phases were collected and dried over anhydrous MgSO . The
4
and used as received. If necessary, purification of products was solvent was then removed under reduced pressure to give a
performed using column chromatography on silica gel (Merck yellow oil which was mixed with diethyl ether and kept in a
Kieselgel 60, 230–400 mesh) with mixtures of chloroform/ freezer for 2 hours. The solid was isolated by filtration, washed
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with diethyl ether and dried in vacuo, (1.75 g, 4.35 mmol, yield chromatography (5% methanol in chloroform) to give com-
9
4
4
7
1%).
HRMS (ESI): calculated for C16
25.1172, found: 425.1176.
pound D2 as a white solid (1.95 g, 3.98 mmol, yield 91%).
+
+
H
22
N
2
O
10Na, [M + Na]
26 4 6
HRMS (ESI): calculated for C26H N O Na, [M + Na] :
513.1750, found: 513.1754.
1
1
H NMR (300 MHz, CDCl ) δ 7.30 (s, 2H), 4.15–4.32 (m,
H NMR (300 MHz, DMSO-d ) δ 9.36 (s, 2H), 7.65–7.45 (m,
3
6
H), 3.88–4.00 (m, 4H), 3.58–3.78 (m, 12H).
4H), 7.44–7.25 (m, 10H), 7.22–7.12 (m, 2H), 5.07 (s, 4H), 3.86
1
3
C NMR (75 MHz, CDCl
0.5, 69.8, 69.0.
Preparation of D1. To a degassed solution of 3,4-dinitro- 128.2, 127.5, 125.1, 124.5, 66.1, 44.1.
benzo-18-crown 6-ether (1.75 g, 4.35 mmol) in 30 ml of a THF/ Preparation of D3. A flask equipped with a stirring bar con-
methanol mixture (1 : 4) was added 20 mg of 10% Pd/C. The taining solution of N-carbobenzyloxyglycine (0.67 g,
reaction mixture was kept under a H atmosphere (balloon 3.20 mmol) in 8 ml of anhydrous dichloromethane was placed
3
) δ 152.0, 136.9, 108.7, 71.0, 70.7, (d, 4H).
1
3
C NMR (75 MHz, DMSO-d ) δ 168.4, 156.4, 136.5, 130.1,
6
a
2
pressure) at room temperature overnight. Then, the catalyst in an ice bath at 0 °C under an argon atmosphere and oxalyl
was removed by filtration through a pad of Celite and washed chloride (0.40 g, 3.15 mmol) and dimethylformamide (14 mg,
with methanol. The filtrate was concentrated under reduced 0.19 mmol) were added. After being stirred for 90 min a solu-
pressure to give the crude product in quantitative yield (1.49 g, tion of 2,3-diaminonaphthalene (0.25 g, 1.58 mmol) and tri-
4
.35 mmol). The obtained 3,4-diaminobenzo-18-crown 6-ether ethylamine (0.58 g, 5.7 mmol) in anhydrous dichloromethane
was used in the next step without further purification. (8 ml) was added dropwise. The mixture was further stirred for
A flask equipped with a stirring bar containing a solution 12 hours under an argon atmosphere at room temperature and
of N-carbobenzyloxyglycine (1.83 g, 8.75 mmol) in 20 ml of then treated with 2 M hydrochloric acid (25 ml). The organic
anhydrous dichloromethane was placed in an ice bath at 0 °C phase was separated and washed with distilled water (25 ml),
under an argon atmosphere and oxalyl chloride (1.16 g, dried over anhydrous MgSO
4
, and concentrated and purified
9
.13 mmol) and dimethylformamide (41 mg, 0.56 mmol) were by silica gel column chromatography (2% methanol in chloro-
added. After being stirred at room temperature for 90 min, a form) to give compound D3 as a beige solid (0.59 g,
solution of diaminobenzo-18-crown 6-ether (1.49 g, 1.09 mmol, yield 69%).
+
4
.35 mmol) and triethylamine (1.67 g, 16.5 mmol) in anhy-
HRMS (ESI): calculated for C H N O Na, [M + Na] :
3
0
28 4 6
drous dichloromethane (20 ml) was added dropwise. The 563.1907, found: 563.1910.
1
mixture was further stirred for 12 hours under an argon atmo-
6
H NMR (300 MHz, DMSO-d ) δ 9.52 (s, 2H), 8.16 (s, 2H),
sphere at room temperature and then treated with 2 M hydro- 7.85 (m, 2H), 7.80–7.64 (m, 2H), 7.60–6.88 (m, 13H), 5.10 (s,
chloric acid (60 ml). The organic phase was separated and 4H), 3.96 (s, 4H).
1
3
washed with distilled water (60 ml), dried over anhydrous
6
C NMR (75 MHz, DMSO-d ) δ 169.3, 257.2, 137.4, 131.0,
MgSO , and concentrated and purified by silica gel column 129.9, 128.8, 128.3, 128.3, 127.6, 126.2, 122.4, 66.1, 44.8.
4
chromatography (10% methanol in chloroform) to give com-
General procedure 1 for the preparation of diamines A1–A3
pound D1 as a white solid (2.72 g, 3.75 mmol, yield 86%).
+
HRMS (ESI): calculated for C H N O Na, [M + Na] : The N-Cbz protected diamine was treated with a solution of
3
6
44 4 12
7
7
47.2853, found: 747.2860.
HBr/AcOH (33%) and the mixture was stirred at room tempera-
) δ 9.24 (s, 2H), 7.57 (s, 2H), ture until CO evolution ceased. To the solid formed, diethyl
1
H NMR (300 MHz, DMSO-d
6
2
.37 (s, 10H), 7.14 (s, 2H), 5.07 (s, 4H), 4.03 (s, 4H), 3.95–3.70 ether was added and the precipitate was filtered, washed
(
m, 8H), 3.65–3.50 (m, 12H).
several times with diethyl ether, and then vacuum dried
) δ 168.6, 157.1, 145.6, 137.4, to obtain diamines in the form of hydrobromide salts as a
1
3
C NMR (75 MHz, DMSO-d
6
1
4
28.8, 128.3, 128.2, 123.8, 110.4, 70.4, 70.3, 69.1, 68.9, 66.1, solid.
4.6. Preparation of A1 (dihydrobromide salt). Compound D1
Preparation of D2. A flask equipped with a stirring bar con- (2.72 g, 3.75 mmol) was treated with a solution (3 ml) of HBr/
taining solution of N-carbobenzyloxyglycine (1.86 g, AcOH (33%) according to procedure 1 to yield A1 as a beige
.89 mmol) in 20 ml of anhydrous dichloromethane was solid (1.87 g, 3.02 mmol, yield 81%).
placed in an ice bath at 0 °C under an argon atmosphere and HRMS (ESI): calculated for C20
oxalyl chloride (1.17 g, 9.24 mmol) and dimethylformamide 479.2118, found: 479.2133.
a
8
+
32 4 8
H N O Na, [M + Na] :
1
(41 mg, 0.56 mmol) were added. After being stirred at room
6
H NMR (300 MHz, DMSO-d ) δ 9.79 (s, 2H), 8.16 (s, 6H),
temperature for 90 min, a solution of o-phenylenediamine 7.18 (s, 2H), 4.03 (s, 4H), 3.87 (s, 4H), 3.76 (m, 4H), 3.59 (m,
(
0.47 g, 4.35 mmol) and triethylamine (1.69 g, 16.7 mmol) in 4H), 3.57–3.40 (m, 12H).
1
3
anhydrous dichloromethane (20 ml) was added dropwise. The
6
C NMR (75 MHz, DMSO-d ) δ 165.5, 145.6, 123.1, 110.0,
mixture was further stirred for 12 hours under an argon atmo- 70.3, 70.2, 69.0, 68.6, 41.5.
sphere at room temperature and then treated with 2 M hydro- Preparation of A2 (dihydrobromide salt). Compound D2
chloric acid (60 ml). The organic phase was separated and (1.95 g, 3.98 mmol) was treated with a solution (3.5 ml) of
washed with distilled water (60 ml), dried over anhydrous HBr/AcOH (33%) according to procedure 1 to yield A2 as a
MgSO , and concentrated and purified by silica gel column light gray solid (1.16 g, 3.02 mmol, yield 76%).
4
3912 | Dalton Trans., 2021, 50, 3904–3915
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+
HRMS (ESI): calculated for C10
H
15
N
4
O
2
, [M + H]: 223.1195, (0.14 g, 0.37 mmol) according to general procedure 3 to yield
found: 223.1191.
R1 as a beige solid (0.24 g, 0.29 mmol, yield 78%).
1
+
H NMR (300 MHz, DMSO-d
6
) δ 9.82 (s, 2H), 8.14 (s, 6H),
HRMS (ESI): calculated for C38
H
42
N
8
O
14Na, [M + Na] :
7
.62 (s, 2H), 7.26 (s, 2H), 3.87 (s, 4H).
857.2718, found: 857.2701.
13
1
C NMR (75 MHz, DMSO-d ) δ 165.6, 130.0, 126.0, 125.1, 41.6.
H NMR (300 MHz, DMSO-d ) δ 9.62 (s, 2H), 9.48 (s, 2H),
6
6
Preparation of A3 (dihydrobromide salt). Compound D3 7.88 (s, 4H), 7.46 (s, 2H), 7.27 (s, 2H), 7.01 (m, 2H), 4.43 (s,
0.59 g, 1.09 mmol) was treated with a solution (1.5 ml) of 8H), 4.04 (s, 4H), 3.76 (s, 4H), 3.78–3.48 (m, 12H).
(
1
3
HBr/AcOH (33%) according to procedure 1 to yield A3 as a
C NMR (75 MHz, DMSO-d ) δ 183.7, 168.3, 168.2, 167.9,
6
beige solid (0.40 g, 0.89 mmol, yield 82%).
164.0, 161.8, 146.2, 130.9, 126.3, 125.8, 123.7, 110.6, 70.3, 70.3,
+
HRMS (ESI): calculated for C14
H
17
N
4
O
2
, [M + H]: 273.1352, 70.2, 69.1, 68.9, 47.2.
found: 273.1344.
Preparation of receptor R2. Disquarate ester E1 (0.25 g,
6
) δ 10.10 (s, 2H), 8.30–8.10 (m, 0.37 mmol) was reacted with diamine dihydrobromide A1
1
H NMR (300 MHz, DMSO-d
8
H), 7.86 (m, 2H), 7.49 (m, 2H), 4.00 (s, 4H).
(0.23 g, 0.37 mmol) according to general procedure 3 to yield
1
3
C NMR (75 MHz, DMSO-d ) δ 166.0, 131.0, 129.1, 127.7, crude R2 as a brown solid, which was then recrystallized from
6
1
26.6, 122.7, 41.8.
a DMSO/MeOH mixture. R2 was found to be insoluble in most
common solvents and without purification is not pure enough
General procedure 2 for the preparation of esters E1–E2
(
0.25 g, 0.23 mmol, yield 62%).
1
A solution of appropriate diamine dihydrobromide salt (1
6
H NMR (300 MHz, DMSO-d ) δ 9.48 (s, 4H), 7.87 (s, 4H),
equiv.) and 3,4-dimethoxy-3-cyclobutene-1,2-dione (2 equiv.) in 7.02 (s, 4H), 4.41 (s, 8H), 4.05 (s, 8H), 3.76 (s, 8H), 3.77–3.46
MeOH was stirred at room temperature overnight in the pres- (m, 24H).
ence of 3 equiv. of Et
3
N. Then the reaction mixture was filtered
Preparation of receptor R3. Disquarate ester E2 (0.16 g,
and the collected solid material was washed with MeOH. The 0.36 mmol) was reacted with diamine dihydrobromide A2
obtained white solid was dried in vacuo to give the desired (0.14 g, 0.36 mmol) according to general procedure 3 to yield
methyl squarate E.
Preparation of E1. Diamine dihydrobromide A1 (1.18 g,
R3 as a off white solid. (0.17 g, 0.28 mmol, yield 78%).
+
HRMS (ESI): calculated for C28
H
23
N
8
O
8
[M + H] 599.1639,
1
3
.90 mmol) was reacted with dimethyl squarate (0.54 g, found: 599.1656.
1
.81 mmol) according to general procedure 2 to yield E1 as a
H NMR (300 MHz, DMSO-d ) δ 9.63 (s, 4H), 7.89 (s, 4H),
6
beige solid (0.76 g, 1.12 mmol, yield 59%).
HRMS (ESI): calculated for C H N O Na, [M + Na] :
7.45 (s, 4H), 7.27 (s, 4H), 4.43 (s, 8H).
+
13
Recording of the C NMR spectrum is not possible due to
3
0
36 4 14
6
99.2126, found: 699.2145.
the very poor solubility of the compound R3 in DMSO-d
6
.
1
H NMR (300 MHz, DMSO-d
6
, 80 °C) δ 9.20 (s, 2H), 8.62 (s,
Anal. Calcd for C28 : C, 56.00; H, 4.03; N, 18.66; O,
24 8 8
H N O
2
H), 7.16 (s, 2H), 4.34 (s, 6H), 4.24 (s, 4H), 4.07 (m, 4H), 3.79 21.31. Found: C, 56.04; H, 4.05; N, 18.62; O, 21.28.
(
m, 4H), 3.78–3.45 (m, 12H).
Preparation of receptor R4. Disquarate ester E1 (0.25 g,
, 80 °C) δ 189.8, 183.4, 178.4, 0.37 mmol) was reacted with diamine dihydrobromide A3
13
C NMR (75 MHz, DMSO-d
6
1
73.7, 167.4, 146.2, 123.9, 111.4, 70.5, 70.4, 69.4, 69.4, 60.6, 46.9.
Preparation of E2. Diamine dihydrobromide A2 (0.26 g, R3 as a beige solid (0.18 g, 0.20 mmol, yield 54%).
.68 mmol) was reacted with dimethyl squarate (0.19 g, HRMS (ESI): calculated for C42 14Na, [M + Na] :
.36 mmol) according to procedure 2 to yield E2 as a beige 907.2875, found: 907.2843.
(0.16 g, 0.37 mmol) according to general procedure 3 to yield
+
0
1
44 8
H N O
1
solid (0.19 g, 0.44 mmol, yield 65%).
HRMS (ESI): calculated for C20
6
H NMR (300 MHz, DMSO-d ) δ 9.77 (s, 2H), 7.48 (s, 2H),
Na, [M + Na] : 8.10–8.80 (m, 8H), 7.50 (s, 2H), 7.01 (s, 2H), 4.52–4.35 (d, 8H),
+
H
18
N
4
O
8
4
2
1
65.1022, found: 465.1034.
4.05 (s, 4H), 3.75 (s, 4H), 3.70–3.45 (m, 12H).
Recording of the C NMR spectrum was not possible due
to the very poor solubility of the compound R4 in DMSO-d
1
13
H NMR (300 MHz, DMSO-d
6
, 80 °C) δ 9.33 (s, 2H), 8.67 (s,
H), 7.59 (s, 2H), 7.22 (m, 2H), 4.31 (s, 6H), 4.30 (s, 4H).
6
.
1
3
C NMR (75 MHz, DMSO-d , 80 °C) δ 189.9, 183.5, 178.4,
Anal. Calcd for C H N O : C, 57.01; H, 5.01; N, 12.66; O,
6
42 44 8 14
73.7, 167.6, 130.7, 125.8, 125.4, 60.6, 47.0.
25.31. Found: C, 56.94; H, 5.07; N, 12.60; O, 25.34.
General procedure 3 for the preparation of receptors R1–R4
A solution of the disquarate ester (1 equiv.) in MeOH (150 ml)
was heated to 70 °C. Then, Et N (5 equiv.) was added and the
3
Conflicts of interest
appropriate diamine (1 equiv.) in MeOH (20 ml) was added There are no conflicts to declare.
−1
dropwise via a syringe pump (at 2 ml h ). The mixture was
stirred at reflux for 72 hours. The resulting crude precipitate
was isolated by filtration and finally recrystallized from a
DMSO/MeOH mixture.
Acknowledgements
Preparation of receptor R1. Disquarate ester E1 (0.25 g, This work was supported by Grant No. 2018/30/E/ST5/00841
.37 mmol) was reacted with diamine dihydrobromide A2 from the National Science Centre, Poland.
0
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2
7 N. Busschaert, S. H. Park, K. H. Baek, Y. P. Choi, J. Park,
E. N. W. Howe, J. R. Hiscock, L. E. Karagiannidis,
I. Marques, V. Félix, W. Namkung, J. L. Sessler, P. A. Gale
and I. Shin, Nat. Chem., 2017, 9, 667–675.
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