Catechol-containing schiff bases on thiacalixarene: Synthesis, copper (ii) recognition, and formation of organic-inorganic copper-based materials

Article abstract of DOI:10.3390/molecules26082334

For the first time, a series of catechol-containing Schiff bases, tetrasubstituted at the lower rim thiacalix[4]arene derivatives in three stereoisomeric forms, cone, partial cone, and 1,3-alternate, were synthesized. The structure of the obtained compoun

Full text of DOI:10.3390/molecules26082334

molecules
Article
Catechol-Containing Schiff Bases on Thiacalixarene: Synthesis,
Copper (II) Recognition, and Formation of Organic-Inorganic
Copper-Based Materials
1
,
1
2
2
1
Pavel Padnya * , Ksenia Shibaeva , Maxim Arsenyev
, Svetlana Baryshnikova , Olga Terenteva ,
1
1
1
, Alexander Gerasimov 1 , Denis Grishaev ,
3
Igor Shiabiev , Artur Khannanov , Artur Boldyrev
Yurii Shtyrlin 3 and Ivan Stoikov *
1,
1
A.M. Butlerov’ Chemistry Institute, Kazan Federal University, 18 Kremlevskaya Street, 420008 Kazan, Russia
;
alleoks@mail.ru (K.S.); olga-potrekeeva@mail.ru (O.T.); shiabiev.ig@yandex.ru (I.S.);
arthann@gmail.com (A.K.); boldyrev25@gmail.com (A.B.); alexander.gerasimov@kpfu.ru (A.G.)
G.A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 49 Tropinin Street,
2
3
603137 Nizhny Novgorod, Russia; mars@iomc.ras.ru (M.A.); baryshnikova@iomc.ras.ru (S.B.)
Scientific and Educational Center of Pharmaceutics, Kazan Federal University, 420008 Kazan, Russia;
dionis.grishaev@yandex.ru (D.G.); yurii.shtyrlin@kpfu.ru (Y.S.)
*
Correspondence: padnya.ksu@gmail.com (P.P.); ivan.stoikov@mail.ru (I.S.); Tel.: +7-843-233-7241 (I.S.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Abstract: For the first time, a series of catechol-containing Schiff bases, tetrasubstituted at the lower
rim thiacalix[4]arene derivatives in three stereoisomeric forms, cone, partial cone, and 1,3-alternate,
were synthesized. The structure of the obtained compounds was proved by modern physical methods,
ꢀ
ꢁꢂꢃꢄꢅꢆ
Citation: Padnya, P.; Shibaeva, K.;
Arsenyev, M.; Baryshnikova, S.;
Terenteva, O.; Shiabiev, I.; Khannanov,
A.; Boldyrev, A.; Gerasimov, A.;
Grishaev, D.; et al. Catechol-
such as NMR, IR spectroscopy, and HRMS. Selective recognition (K difference by three orders of
b
magnitude) of copper (II) cation in the series of d-metal cations (Cu2+, Ni2+, Co2+, Zn2+) was shown
by UV-vis spectroscopy. Copper (II) ions are coordinated at the nitrogen atom of the imine group and
the nearest oxygen atom of the catechol fragment in the thiacalixarene derivatives. High thermal
stable organic-inorganic copper-based materials were obtained on the base of 1,3-alternate + Cu (II)
complexes.
Containing Schiff Bases on
Thiacalixarene: Synthesis, Copper (II)
Recognition, and Formation of
Organic-Inorganic Copper-Based
Materials. Molecules 2021, 26, 2334.
https://doi.org/10.3390/molecules
Keywords: thiacalix[4]arenes; Schiff base; synthesis; complexing properties; copper cation; organic-
inorganic materials
2
6082334
Academic Editor: Eugene L Kolychev
1
. Introduction
The design of new hybrid materials with unusual and unique properties is a promising
challenge due to the development of modern industry and the possibilities of their use in
advanced technologies [ ]. Metals and their ions, such as silver, gold, and copper, are
often applied in the composition of organic-inorganic materials, such as an inorganic com-
ponent [ ]. The development of hybrid organic-inorganic materials based on copper ions
is promising due to the combination of their unique properties and low cost [9–11]. Such
materials have high thermal and electrical conductivity, biological activity (for example,
antifungal, antibacterial), and are also used as catalysts and sensors [12–15].
Received: 31 March 2021
Accepted: 15 April 2021
Published: 17 April 2021
1–7
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8
Schiff bases are one of the most convenient and effective complexing compounds [16–18]
.
They are interesting due to a number of reasons, e.g., (1) easy synthesis by condensation
of aldehydes or ketones to primary amines under mild conditions; (2) the possibility of
designing and changing the properties of target structures by varying the substituents
in the starting amine and aldehyde (ketone). There are a large number of examples of
the creation of promising materials based on the Schiff bases [19–23]. However, their low
selectivity is a main problem of such complexing and chelating agents.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
Synthetic macrocyclic compounds, such as calixarenes, thiacalixarenes, resorcinarenes,
pillararenes, and others, have excellent complexing properties and are often selective for a
4
.0/).
certain substrate [24–32]. The introduction of fragments of Schiff bases into the structure
Molecules 2021, 26, 2334. https://doi.org/10.3390/molecules26082334
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2334
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of (thia)calixarenes makes it possible to increase both the efficiency and selectivity with
respect to metal cations.
At the moment, there are many examples of the synthesis of Schiff bases based on
(
thia)calixarene derivatives, as well as the study of complexing properties, with respect to a
series of metal cations, including copper (II) cations [33 37]. However, most of the obtained
macrocyclic Schiff bases do not possess selectivity toward d-element cations, such as Cu ,
–
2
+
2+
2+
2+
Ni , Co , and Zn . The introduction of amide groups capable of spatial pre-organization
of the receptor fragments via additional hydrogen bonds into the macrocycle structure can
lead to a change in the complexing properties and improvement in the selectivity of the
resulting molecules.
In this work, Schiff bases with catechol fragments on thiacalix[4]arenes substituted at
the lower rim in three conformations (cone, partial cone, and 1,3-alternate) were synthesized
for the first time. Selective recognition of copper (II) cations in the series of d-element
cations (copper (II), nickel (II), cobalt (II), and zinc (II)) was shown by UV-vis spectroscopy.
Organic-inorganic copper-based materials were obtained on the base of thiacalixarene-
copper (II) complexes, and their properties were studied.
2. Results and Discussion
2.1. Synthesis of thiacalix[4]arene-Based Schiff Bases
Previously, in the group of Professor Hosseini, catechol-containing Schiff bases, tetra-
substituted at the lower rim calix[4]arene and thiacalix[4]arene (L) in 1,3-alternate confor-
mation, were synthesized [37]. These compounds were capable of complexation of metal
(II) ions (Cu (II), Co (II), Ni (II), and Zn (II)) with low selectivity. The authors have shown
the formation of complexes with the composition M/L = 2/1 in the CH OH/CHCl (1:1)
3
3
solution and in the crystal in case of the macrocycle-Cu (II) complexes.
It is known that the arrangement of substituents of (thia)calixarenes strongly affects
complexing properties of the resulting receptors in relation to various substrates (metal
cations, anions, small organic molecules, proteins, nucleic acids) [38–43]. The macrocycle
acts as a platform capable of rigid spatial arrangement of receptor fragments. Therefore, we
proposed to synthesize new Schiff bases on thiacalix[4]arene derivatives in three different
stereoisomeric forms, cone, partial cone, and 1,3-alternate, and also to study their complexing
properties with metal cations to obtain new organic-inorganic nanomaterials. Easily ob-
tainable tetraester derivatives 2–4 were proposed as precursors of desired stereoisomers.
Spatial fixation of ester groups in space, without the possibility of conversion of stereoiso-
meric forms into each other without breaking chemical bonds, was the advantage of these
compounds. Further, tetrasubstituted thiacalix[4]arenes 5–7, containing both amide and
primary amine groups separated by a hexamethylene spacer, were synthesized in three
conformations, cone, partial cone, and 1,3-alternate, according to the method developed in
our research group [44] (Scheme 1).
Further reaction of the compounds
in methanol at room temperature gave Schiff bases
,6-Di-tert-butyl-1,2-dihydroxybenzaldehyde is a convenient synthetic block for the synthe-
5–7
with 4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde
8–10 (Figure 1) in high yields (91–96%).
4
sis of oligomeric Schiff bases with catechol or o-quinone fragments, redox-active ligands
for coordination chemistry. Due to the presence of tert-butyl groups, it is possible to isolate
corresponding o-benzoquinones and register the oxidized forms of catecholate complexes.
This aldehyde also has the potential to act as both an O,O-chelating moiety and an O,N-
chelating moiety [45–47]. Unlike vanillin derivatives, it can participate in the formation of
dimeric structures via intermolecular hydrogen bonds. Monomeric compound 11 (Figure 1)
was synthesized with a yield of 90% in a similar synthetic way to compare complexing
properties of the obtained macrocyclic compounds. The structure and composition of the
1
13
obtained compounds
NMR, IR spectroscopy, high-resolution mass spectrometry (HRMS), and elemental analysis
Figures S1–S16).
8
–
11 were confirmed by modern physical methods, e.g., H,
C
(

Molecules 2021, 26, 2334
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Scheme 1. Reagents and conditions: (i) M CO (M = Na, K, Cs for cone, partial cone, 1,3-alternate stereoisomers, corre-
2
3
spondingly), BrCH C(O)OEt, acetone, reflux; (ii) H N(CH ) NH , methanol/toluene, reflux [44]; (iii) 4,6-di-tert-butyl-1,2-
2
2
2 6
2
dihydroxybenzaldehyde, CH OH, r.t.
3
Figure 1. Structures and cartoon images of the compounds 8–11.

Molecules 2021, 26, 2334
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Two-dimensional NMR spectroscopy is a common technique for establishing the
structure and conformation of macromolecules. However, it is known that different con-
1
formations of thiacalixarene derivatives can be unequivocally confirmed by H NMR
spectroscopy data of specific proton signals. The conformational differentiation of cone
and 1,3-alternate stereoisomers of amide derivatives of p-tert-butylthiacalix[4]arenes tetra-
substituted at the lower rim can be performed by chemical shifts of the tert-butyl group,
1
aromatic ring, oxymethylene, amide, and amidomethylene protons in the H NMR spectra
(
Table S1). In the case of compound 10 (1,3-alternate), the protons of the OCH , NHCH ,
2 2
and amide groups were located in the zone shielded by neighboring aromatic rings of
1
the macrocycle. Their signals were recorded at stronger fields in the H NMR spectrum
(4.06, 3.24, and 7.75 ppm, respectively) than those of the protons of the macrocycle 8 in
cone conformation (4.80, 3.33, and 7.93 ppm, respectively). Chemical shifts of the aromatic
protons depended less on the conformation of the macrocyclic cavity. They drifted upfield
by only 0.19 ppm from cone
8 (7.33 ppm) to 1,3-alternate 10 (7.52 ppm) stereoisomers. This
was evidence of the shielding effect of neighboring aryl fragments in cone stereoisomer
on the aryl protons of macrocycle ring. The signals of the tert-butyl group protons of cone
8
were found at a stronger field (1.09 ppm) against corresponding proton signals of the
1
,3-alternate stereoisomer 10 (1.18 ppm). This was probably due to the spatial location of
the tert-butyl groups of the 1,3-alternate stereoisomer shielded by neighboring fragments of
the macrocycle.
However, the spectrum of compound
9 in partial cone conformation was complicated
by the decrease of the structure symmetry. The signals of the tert-butyl protons appeared as
three singlets (1.02, 1.26, 1.29 ppm), with an integral intensity ratio of 2:1:1. Signals of the
amide protons appeared as three broad triplets (7.53, 7.88, and 8.64 ppm), with an integral
intensity ratio of 1:2:1. Meanwhile, the signals of the oxymethylene and aromatic protons
formed two singlets and an AB-system, respectively (Table S1).
It is interesting to note that chemical shifts of the signals of the remaining protons
were almost independent of the macrocycle conformation. They were close to the chemical
shifts of the signals of the same protons in the monomer 11 (Table S1). It is likely that their
effect did not play a significant role at a certain distance from the thiacalixarene platform.
The structure of the obtained Schiff bases
8–11 was also studied by FT-IR spectroscopy
(
Figures S9–S12). One of the main significant absorption bands of the C=N bond was in the
−
1
−1
range of 1616–1618 cm in the case of the macrocyclic compounds 8–10 and 1623 cm in
the case of the monomeric compound 11.
The obtained compounds
8–11 were also characterized by HRMS (Figures S13–S16).
Mass spectra of the macrocyclic compounds
ions with two, three, and four protons ([M + 2H] , [M + 3H] , and [M + 4H] ). In the case
8
–
10 showed peaks corresponding to molecular
2+
3+
4+
of the monomeric compound 11, a peak (m/z = 539.3849) corresponding to the protonated
+
molecular ion [M + H] was recorded.
Thus, new Schiff bases, tetrasubstituted at the lower rim thiacalixarenes containing
catechol fragments in the structure, were obtained in high yields. The structure of the
complexes obtained was proven by modern physical methods.
2
.2. The Study of Complexing Properties of the Obtained thiacalix[4]arene-Based Schiff Bases
Complexing ability of the obtained Schiff bases 11 with divalent d-metal cations
copper (II), zinc (II), cobalt (II), and nickel (II)) was studied by UV-vis spectroscopy in the
CH OH-CHCl mixture (1:1) at room temperature. Figure 2 and Figures S17–S20 show the
8–
(
3
3
UV-vis spectra of the mixtures of thiacalixarenes
8–10 and the monomer 11 with a 10-fold
excess of all the studied metal cations in the mixture CH OH-CHCl (1:1). UV-vis spectra
3
3
of the macrocyclic compounds
There are four main bands in the UV spectra of pure macrocyclic compounds
8
–
10 are typical for catecholaldimines in polar solvents [48].
10. Two
8–
bands at 250 and 285 nm are assigned to π → π *– transitions of the macrocyclic main
chains, a band at 320 nm is assigned to n → π *– transitions of the C=N imine groups, while
a broad band at 430 nm corresponds to intramolecular charge transfer.

Molecules 2021, 26, 2334
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−
5
Figure 2. UV-vis spectra of compound 10 (1
(
×
10 M) without/with 10-fold excess of d-metal
II) cations (Zn2+, Co , Ni , Cu ) (CH OH-CHCl (1:1), t = 25 C). The insert contains photos of
2+
2+
2+
◦
3 3
real samples.
Changes in the electronic spectra of the thiacalix[4]arenes 8–10 were found only in
the case of the addition of the copper (II) solution. In this case, the absorption band in the
region of 320 nm disappeared and a hypsochromic (blue) shift of the absorption band at
430 nm to the region of 400 nm was observed. The appearance of new bands corresponded
to the formation of new coordination bonds between the copper (II) cations and the ligands.
An increase in absorption in the region of 520 nm was also observed, so that color of the
2+
solution changed from yellow-orange to orange-red when adding the Cu ions (Figure 2).
Complexing properties of the macrocyclic compounds 10 were compared with
those of the monomeric compound 11. Figure S20 shows UV-vis spectra of the compound
without/with a 10-fold excess of all the studied metal cations in the CH OH-CHCl
8–
11
3
3
mixture (1:1). One can see that compound 11 interacted with all the cations (copper (II),
zinc (II), cobalt (II), and nickel (II)) and did not exhibit any selectivity.
Further, binding constants and stoichiometry of the complexes of the copper (II) cation
with the compounds
Figure 3B and Figures S21–S24 show the results of complexation of the compounds
In the case of the macrocyclic compounds 10, maximum value of the molar fraction was
.5, corresponding to the 1:1 stoichiometry of the macrocycle-Cu (II). In the case of the
monomeric compound 11, maximum molar fraction corresponded to the value close to
.33, which corresponded to the 2:1 stoichiometry of compound 11/Cu (II).
Values of the binding constants of the compounds 11 with the copper (II) ions were
measured by the UV-vis titration (Figure 3A, Figures S25–S28, Table 1). Using spectroscopic
data obtained for the complexes of the compounds 11 with the copper (II) cations, binding
constants (K ) were calculated for all four complexes using the Bindfit software [49]. The
8–11 were determined. Stoichiometry was determined by Job’s plot.
8–11.
8
–
0
0
8–
8
–
b
stoichiometry of the complexes of the macrocycles
(2:1) was additionally confirmed by Bindfit (Figures S25–S28.). The calculated values
of the logK of the macrocyclic compounds 10 were close to each other in the range of
.04–5.46. Such low binding constants of the macrocyclic compounds 10 in comparison
8–10 (1:1) and the monomer compound
11
8–
b
5
8–
with the monomeric compound 11 (logK’ = 8.52) can be explained by steric preorganization
b
of the complexing fragments relative to the thiacalixarene platform. This lead to less
flexibility in the macrocyclic structures. However, this preorganization lead to the selectivity
of recognition of the copper (II) cation by thiacalixarenes
8–10 in the series of d-metal cations
in comparison with the monomeric compound 11, which bound all the studied cations.

Molecules 2021, 26, 2334
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−
5
Figure 3.
(
A
) UV-vis titration of compound 10 (1,3-alternate) (1
×
10 M) with the copper (II) cation
◦
) Job’s plot of thiacalix[4]arene 10 (1,3-alternate) and Cu2+ in
(
CH OH-CHCl (1:1), t = 25 C); (
B
3
3
CH OH:CHCl (1:1).
3
3
Table 1. Binding constants (log K ) for copper (II) complexes of the compounds 8–11 in the
b
CH OH/CHCl (1:1) mixture at room temperature.
3
3
Ligand
M/L
8 (cone)
1/1
9 (partial cone)
1/1
10 (1,3-alternate)
1/1
11 (Monomer)
1/2
Log Kb
5.04 ± 0.11
5.36 ± 0.62
5.46 ± 0.51
8.52 ± 0.20
It is known that changes in the UV-vis spectra may not be observed at low binding
constants (Kb 100) [50]. Based on the data, we asserted that the selectivity of the
recognition of the copper (II) ions by the obtained macrocyclic compounds 10 was at
least 1000 (K difference by three orders of magnitude).
≤
8
–
b
It was shown earlier by Hosseini [37] that the coordination of the copper (II) ions
occurs with two fragments of the (thia)calix[4]arene substituents. In this case, coordination
occurred at the nitrogen atom of the C=N bond and nearest the oxygen atom of the catechol
fragment. In our case, binding constants for the complexes of the macrocycles 8–10 with
the copper (II) cations (and the stoichiometry is 1:1) were close (log K = 5.04–5.46), while
b
the stoichiometry of the complex of the monomeric compound with Cu (II) was 2:1. Such
results can be explained by the fact that two substituents located oppositely each other were
coordinated by the copper (II) ions, while the other two substituents did not participate in
the complexation. Inability of free substituents to complexation can be explained by the
allosteric effect. Change of the conformation of the macrocyclic platform occurred upon
the complexation of one copper ion, and the coordination of the second copper (II) cation
became impossible. Figure 4 schematically shows the complexation of the macrocycles
8–10 and the monomer 11 with the copper (II) cation.
However, our results are fundamentally different from the previous work of professor
Hosseini [37]. Here, introduction of the imine groups with the catechol fragments into
the macrocyclic platform of thiacalix[4]arene led to the creation of selective receptors (Kb
difference by three orders of magnitude) for the copper (II) cations in a series of d-metal
cations (Cu2 , Co , Ni , and Zn ). Complexes with the stoichiometry of 1:1 were formed
+
2+
2+
2+
in our case.

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Figure 4. Cartoon images of complexation of thiacalix[4]arenes 8–10 and monomer 11 with the copper (II) cation.
It is known that the copper (II) cation is paramagnetic [51]. Therefore, the study
of complexation by the NMR spectroscopy can often be difficult. We decided to study
the interaction of compounds
coordination of the cations. Figure 5 and Figures S29–S32 show H NMR spectra of
8
–
11 with the deficiency of the copper ions to confirm
1
compounds
the copper (II) cations. Chemical shifts of the proton signals at 8.70 ppm (N=CH), 6.44 ppm
ArHcatechol), and 3.51 ppm (CH N=C) shifted downfield by 0.15–0.20 ppm (Figure 5). Such
8–11 (10 mM, CD OD:CDCl = 1:1), with the addition of different amounts of
3 3
(
2
changes unambiguously confirmed the coordination of the copper (II) ions at the nitrogen
atom of the C=N group and the nearest oxygen atom of the catechol fragment. Small
changes in the chemical shifts of the protons of the aromatic fragments and the tert-butyl
groups of the macrocyclic platform (0.02–0.04 ppm) confirmed hypothesis of the changes
in the conformation of the macrocycles upon complexation.
Figure 5. 1H NMR spectra of compound 10 (10 mM) in CD OD:CDCl (1:1) upon addition of 0 equiv
3
3
2
+
(
blue), 0.5 equiv (green), and 1 equiv of Cu (red).

Molecules 2021, 26, 2334
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Thus, the studied macrocyclic compounds
8–
10 were selective receptors for the copper
(II) cation among the cations of other divalent d-metals. The main reason for this selectivity
is associated with steric preorganization of coordination groups (C=N groups and catechol
fragments) by the macrocyclic thiacalixarene platform. In this case, the logarithms of the
binding constants turned out to be close.
2.3. Synthesis and Characterization of Thiacalixarene-Copper (II) Materials
Design of thermally stable materials is a promising and popular direction for the
creation of new optical, photoelectric, and electrochemically active devices. The use
of copper-based organic-inorganic hybrid materials is convenient due to good target
properties and low cost of copper against commonly used metals, such as silver and
gold [9–12]. Due to high selectivity of the studied compounds 8–10 to the copper (II) cation
in the series of d-metal cations, we proposed to obtain organic-inorganic materials based
on them and to study their properties.
Thiacalixarene 10 in 1,3-alternate conformation with the highest binding constant was
used as an organic component to create organic-inorganic materials. Solution of the copper
(
II) chloride (1 mM, CH OH-CHCl (1:1)) was added to the yellow solution (1 mM) of the
3 3
thiacalixarene 10 in the mixture CH OH:CHCl (1:1) at room temperature. Concentration
3
3
and evaporation of the solution gave orange-red amorphous powder (10 + Cu complex),
which was dried in vacuum for 48 h. Unfortunately, we failed to obtain the crystalline
sample. Starting thiacalixarene 10 and copper (II) chloride were isolated under the same
conditions. Mechanical mixture of the thiacalixarene-CuCl2 (10 + Cu mixture) was also
prepared to compare the properties of the obtained organic-inorganic material.
Initially, FT-IR spectroscopy was used to determine the complexation behavior of
compound 10 with the copper (II) cation (Figure 6 and Figure S33). In the FT-IR spectrum
−
1
of compound 10, the C=N bond band with the wavenumber of 1618 cm and the amide
−
1
group band at 1668 cm were characteristic. In the obtained material 10 + Cu complex,
−
bifurcation of the ν(C=N) stretching band (1627 and 1595 cm ) was observed, as well
1
−
1
as the shift of the absorption band of the amide group (up to 1655 cm ). The latter can
be explained by the allosteric effect of the interaction of compound 10 with the copper
(II) ions described in Section 2.2. There was convergence of the catechol fragments of
the Schiff bases on one side of the macrocyclic platform as a result of complexation with
the copper ion. Meanwhile, the distance between the substituents increased on the other
side. Therefore, two bands of the C=N bond appeared to correspond to the C=N fragment
−
1
(
1595 cm ) bound to the copper (II) ion, as well as free substituents with Schiff base
−
1
fragments (1627 cm ). Schematic representation of the coordination is shown in Figure 4.
It should be noted that the spectrum of the mechanical mixture thiacalixarene-CuCl2 (10
+
Cu mixture) corresponded to the sum of the spectra of initial thiacalixarene 10 and CuCl2.
Shift of the absorption bands and the appearance of new bands were also not observed in
this case.
Further, the obtained samples were studied by the PXRD method. Figure S34 shows
PXRD spectra of compound 10, CuCl , their mechanical mixture (10 + Cu mixture),
2
and our material (10 + Cu complex). Since the studied samples were isolated from the
CH OH:CHCl mixture, we observed peaks corresponding to a mixture of CuCl hydrate
3
3
2
and methanolate structures in the case of a copper salt. It is worth noting the lack of
crystallinity of the macrocycle 10, as well as the resulting organic-inorganic material (10
+
Cu complex). One can see that the PXRD spectrum of the obtained material differed from
that of the mechanical mixture (10 + Cu mixture). This confirmed the complexation of the
copper (II) cations with the organic ligand.
The obtained samples were also studied using the TG method (Figure 7 and Figure S35).
It turned out that compound 10 in its pure form was thermally stable (decomposition began
◦
at a temperature of 314 C). The resulting organic-inorganic material 10 + Cu complex was
◦
more thermally stable (up to 326 C) against compound 10.

Molecules 2021, 26, 2334
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Figure 6. Fragments of FT-IR spectra of compound 10 (red), 10 + Cu mixture (blue), and 10 + Cu
complex (purple).
Figure 7. TG curves of compound 10 (red) and 10 + Cu complex (green) in the dynamic argon
◦
atmosphere 75 mL/min in the temperature range from 40 to 550 C.
Thus, as a result of this work, we synthesized Schiff bases with catechol fragments
based on thiacalix[4]arenes, substituted at the lower rim in three conformations, cone, partial
cone, and 1,3-alternate. Selective recognition (K difference by three orders of magnitude)
b
of the copper (II) cations in the series of d-element cations (copper (II), nickel (II), cobalt
(II), and zinc (II)) was shown by UV-vis spectroscopy. It was demonstrated by a number of
physical methods that the copper (II) ions coordinated at the nitrogen atom of the imine
group and the nearest oxygen atom of the catechol fragment in the thiacalixarene deriva-
tives. Thermally highly stable organic-inorganic copper-based materials were obtained on
the base of thiacalixarene-copper (II) complexes.

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3. Materials and Methods
3.1. General
All chemicals were purchased from Acros (Fair Lawn, NJ, USA), and most of them
were used as received without additional purification. Organic solvents were purified by
1
13
standard procedures. H NMR and C NMR spectra were obtained on a Bruker Avance-
4
13
1
1
00 spectrometer (Bruker Corp., Billerica, MA, USA) ( C{ H} 100 MHz and H 400 MHz).
Chemical shifts were determined against the signals of residual protons of deuterated
solvent (CDCl ). The concentrations of the compounds were equal to 3–5% in all the
3
records. The FTIR ATR spectra were recorded on the Spectrum 400 FT-IR spectrometer
(
Perkin Elmer, Seer Green, Lantrisant, UK) with a Diamond KRS-5 attenuated total internal
−
1
reflectance attachment (resolution 0.5 cm , accumulation of 64 scans, recording time 16 s in
−
1
the wavelength range 400–4000 cm ). HRMS mass spectra were obtained on a quadrupole
time-of-flight (t, qTOF) AB Sciex Triple TOF 5600 mass spectrometer (AB SCIEX PTE. Ltd.,
Singapore) using a turbo-ion spray source (nebulizer gas nitrogen, a positive ionization
polarity, needle voltage 5500 V). Recording of the spectra was performed in “TOF MS” mode
with collision energy 10 eV, declustering potentially 100 eV and with a resolution of more
than 30,000 full-width half-maximum. Samples with the analyte concentration of 5 µmol/L
were prepared by dissolving the test compounds in the mixture of methanol (HPLC-
UV Grade, LabScan). Elemental analysis was performed on Perkin–Elmer 2400 Series II
instruments (Perkin Elmer, Waltham, MA, USA). Melting points were determined using
Boetius Block apparatus (VEB Kombinat Nagema, Radebeul, Germany). Thermogravimetry
(
TG) analysis of solid samples were performed using the thermoanalyzer STA 449F1
◦
Jupiter (Netzsch, Germany) at the temperature range of 40–550 C. In each experiment,
the temperature scanning rate was 10 C/min, and an argon with a total flow rate of
◦
7
5 mL/min was used. Powder X-ray diffraction (PXRD) studies were performed using a
MiniFlex 600 diffractometer (Rigaku, Japan) equipped with a D/teX Ultra detector. Cu
radiation (40 kV, 15 mA) was used for all the measurements, which were collected at
K
α
◦ ◦
room temperature in the range of 2θ from 3 to 100 with 0.02 steps and 0.24 s exposure
time at each point without sample rotation. UV-vis spectra were recorded on a Shimadzu
UV-3600 spectrometer (Kyoto, Japan). Quartz cuvettes with an optical path length of 10 mm
were used. A mixture of CH OH:CHCl (1:1) was used for preparation of the solutions.
3
3
◦
Absorption spectra of mixtures were recorded after 1 hour of incubation at 20 C.
Thiacalix[4]arenes were synthesized according to the literature procedure [44] and
5–7
4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde according to [52].
3.2. General Procedure for the Synthesis of the Compounds 8–10
A mixture of 4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde (148.8 mg, 0.1486 mmol)
and thiacalix[4]arene-based amines
stirred at room temperature. The precipitate formed was then filtrated. The residue was
washed with methanol (3 15 mL), isolated as orange crystalline powders, and dried
under reduced pressure over phosphorus pentoxide.
5–7 (200 mg, 0.1486 mmol) in methanol (5 mL) was
×
3
.2.1. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[N-(6-(4,6-di-tert-butyl-2,3-
dihydroxybenzylideneamino)hexyl)carbamoylmethoxy]-2,8,14,20-tetrathiacalix[4]arene
in cone Conformation (8)
◦
1
Yellow-orange powder, yield: 314 mg (92%). M.p. 110 C. H NMR (CDCl ,
δ
, ppm,
3
J/Hz): 1.09 (s, 36H, (CH ) C), 1.38 (s, 36H, (CH ) C), 1.39 (s, 36H, (CH ) C), 1.35–1.42
3
3
3 3
3 3
(
(
m, 16H, NH(CH ) (CH ) (CH ) N=CH), 1.59 (m, 8H, NHCH2 CH2(CH ) N=CH), 1.70
2 2 2 2 2 2 2 4
m, 8H, NH(CH2)4CH2CH N=CH), 3.33 (m, 8H, NHCH2(CH ) N=CH), 3.56 (m, 8H,
2
2 5
NH(CH2)5CH2N=CH), 4.80 (s, 8H, OCH CO), 6.52 (s, 4H, ArH(C H )), 7.33 (s, 8H, ArH),
2
6
1
13
7
.92 (br.t, 4H, NH), 8.80 (s, 4H, CH=N). C NMR (CDCl3,
δ, ppm): 168.4, 164.1, 157.8, 147.6,
1
2
43.9, 135.0, 128.1, 115.3, 113.3, 110.0, 74.6, 54.0, 39.2, 35.6, 34.4, 33.8, 33.4, 32.5, 31.2, 30.3,
9.4, 29.2, 29.0, 26.4, 26.3. FTIR ATR (ν, cm ): 3326 (NH), 3061 (NH), 1671 (C(O)NH, amide
−
1
I), 1618 (C=N), 1532 (C(O)NH, amide II), 1092 (CArOCH ). Elemental analysis. Calculated
2

Molecules 2021, 26, 2334
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for C132H192N O S C, 66.74; H, 8.44; N, 6.77; S, 7.75; Found: C, 66.68; H, 8.16; N, 6.71; S,
8
16 4
2
+
3+
4+
4+
7
.39. HRMS: calculated [M + 2H] m/z = 1137.6743, [M + 3H] m/z = 758.7852, [M + 4H]
m/z = 569.3408, found [M + 2H] m/z = 1137.6723, [M + 3H] m/z = 758.7846, [M + 4H]
m/z = 569.3409.
2+
3+
3
.2.2. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[N-(6-(4,6-di-tert-butyl-2,3-
dihydroxybenzylideneamino)hexyl)carbamoylmethoxy]-2,8,14,20-tetrathiacalix[4]arene
in partial cone Conformation (9)
◦
1
Yellow-orange powder, yield: 308 mg (91%). M.p. 117 C. H NMR (CDCl , δ,
3
ppm, J/Hz): 1.02 (s, 18H, (CH ) C), 1.26 (s, 9H, (CH ) C), 1.29 (s, 9H, (CH ) C), 1.37 (s,
3
3
3 3
3 3
3
6H, (CH ) C), 1.41 (s, 36H, (CH ) C), 1.42–1.48 (m, 16H, NH(CH ) (CH ) (CH ) N=CH),
3 3 3 3 2 2 2 2 2 2
1
.59–1.67 (m, 8H, NHCH2 CH2(CH ) N=CH), 1.72–1.76 (m, 8H, NH(CH ) CH2CH N=CH),
2 4 2 4 2
3
.27–3.42 (m, 8H, NHCH (CH ) N=CH), 3.61 (m, 8H, NH(CH ) CH2N=CH), 4.26 (d of AB
2
2 5
2 5
2
2
system, 2H, OCH CO, J = 14.4), 4.86 (d of AB system, 2H, OCH CO, J = 14.4), 4.44
2
HH
2
HH
(
s, 2H, OCH CO), 4.96 (s, 2H, OCH CO), 6.53 (s, 4H, ArH(C H )), 7.05 (d of AB system, 2H,
2 2 6 1
4
4
ArH, J = 2.4), 7.45 (d of AB system, 2H, ArH, J = 2.4), 7.53 (br.t, 1H, NH), 7.61 (C, 2H,
HH
HH
1
3
ArH), 7.76 (C, 2H, ArH), 7.88 (br.t, 2H, NH), 8.64 (br.t, 1H, NH), 8.83 (s, 4H, CH=N).
NMR (CDCl3, , ppm): 168.8, 168.5, 168.1, 163.9, 159.4, 158.0, 155.9, 147.6, 146.6, 143.8, 139.9,
39.8, 139.8, 136.4, 135.2, 135.1, 133.4, 127.9, 127.4, 125.9, 125.8, 112.5, 109.7, 74.3, 73.8, 70.2,
C
δ
1
5
2
4.5, 54.3, 54.3, 39.3, 39.2, 38.9, 35.5, 35.3, 33.3, 31.4, 31.2, 31.1, 30.6, 30.5, 29.7, 29.6, 29.2, 26.9,
−
1
6.6, 26.5, 26.5, 26.4. FTIR ATR (
616 (C=N), 1532 (C(O)NH, amide II), 1088 (CArOCH ). Elemental analysis. Calculated for
ν, cm ): 3312 (NH), 3062 (NH), 1673 (C(O)NH, amide I),
1
2
C132H192N O S C, 66.74; H, 8.44; N, 6.77; S, 7.75; Found: C, 66.72; H, 8.54; N, 6.43; S, 7.79.
8
16 4
2+
3+
4+
4+
HRMS: calculated [M + 2H] m/z = 1137.6743, [M + 3H] m/z = 758.7852, [M + 4H]
m/z = 569.3408, found [M + 2H]2+ m/z = 1137.6745, [M + 3H] m/z = 758.7857, [M + 4H]
m/z = 569.3418.
3+
3
.2.3. 5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[N-(6-(4,6-di-tert-butyl-2,3-
dihydroxybenzylideneamino)hexyl)carbamoylmethoxy]-2,8,14,20-tetrathiacalix[4]arene
in 1,3-alterate Conformation (10)
◦
1
Yellow-orange powder, yield: 324 mg (96%). M.p. 115 C. H NMR (CDCl ,
δ
, ppm,
3
J/Hz): 1.18 (s, 36H, (CH ) C), 1.38 (s, 36H, (CH ) C), 1.41 (s, 36H, (CH ) C), 1.35–1.43
3
3
3 3
3 3
(
m, 16H, NH(CH ) (CH ) (CH ) N=CH), 1.60 (m, 8H, NHCH2 CH2(CH ) N=CH), 1.74
2 2 2 2 2 2 2 4
(
m, 8H, NH(CH2)4CH2CH N=CH), 3.24 (m, 8H, NHCH2(CH ) N=CH), 3.61 (m, 8H,
2
2 5
NH(CH2)5CH2N=CH), 4.06 (s, 8H, OCH CO), 6.60 (s, 4H, ArH(C H )), 7.52 (s, 8H, ArH),
2
6
1
13
7
.75 (br.t, 4H, NH), 8.84 (s, 4H, CH=N). C NMR (CDCl3,
δ, ppm): 168.2, 164.0, 156.9, 147.5,
1
2
43.8, 133.3, 127.2, 115.3, 112.8, 110.1, 71.2, 54.6, 39.4, 35.6, 35.3, 34.4, 33.8, 33.4, 31.2, 30.6,
9.6, 29.2, 29.0, 26.8, 26.5. FTIR ATR (ν, cm ): 3319 (NH), 3065 (NH), 1668 (C(O)NH, amide
−
1
I), 1618 (C=N), 1531 (C(O)NH, amide II), 1085 (CArOCH ). Elemental analysis. Calculated
2
for C132H192N O S C, 66.74; H, 8.44; N, 6.77; S, 7.75; found: C, 66.52; H, 8.64; N, 6.63; S,
8
16 4
2
+
3+
4+
4+
7
.74. HRMS: calculated [M + 2H] m/z = 1137.6743, [M + 3H] m/z = 758.7852, [M + 4H]
m/z = 569.3408, found [M + 2H] m/z = 1138.1733, [M + 3H] m/z = 758.7826, [M + 4H]
m/z = 569.3399.
2+
3+
3.3. Procedure for the Synthesis of Compound 11 (Monomer)
A mixture of 4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde (163.4 mg, 0.6527 mmol)
and N-(6-aminohexyl)-2-(4-tert-butylphenoxy)acetamide [44] (200 mg, 0.6527 mmol) in
methanol (5 mL) was stirred at room temperature. The precipitate formed was then filtrated.
The residue was washed with methanol (3
and dried under reduced pressure over phosphorus pentoxide.
×
15 mL), isolated as orange crystalline powders,
2
-(4-tert-Butylphenoxy)-N-(6-(4,6-di-tert-butyl-2,3-
dihydroxybenzylideneamino)hexyl)acetamide (11)
◦
1
Yellow-orange powder, yield: 316 mg (90%). M.p. 55 C. H NMR (CDCl , δ,
3
ppm, J/Hz): 1.29 (s, 9H, (CH ) C), 1.40 (s, 9H, (CH ) C), 1.42 (s, 9H, (CH ) C), 1.33–1.48
3
3
3 3
3 3

Molecules 2021, 26, 2334
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(
m, 4H, NHCH CH (CH ) (CH ) N=CH), 1.56 (m, 2H, NHCH2CH2(CH ) N=CH), 1.72
2 2 2 2 2 2 2 4
(
m, 2H, NH(CH2)4CH2CH N=CH), 3.34 (m, 2H, NHCH2(CH ) N=CH), 3.58 (m, 2H,
2
2 5
NH(CH2)5CH2N=CH), 4.46 (s, 2H, OCH CO), 6.56 (s, 1H, ArH(C H )), 6.63 (br.t, 1H,
2
6
1
13
NH), 6.85(m, 2H, ArH), 7.33 (m, 2H, ArH), 8.82 (s, 1H, CH=N). C NMR (CDCl3,
δ
, ppm):
1
3
3
68.5, 163.8, 157.8, 155.0, 148.1, 143.7, 136.2, 126.4, 114.2, 112.3, 109.8, 67.5, 54.2, 38.8, 35.5,
−
1
5.2, 34.2, 33.8, 33.3, 31.5, 30.5, 29.5, 29.2, 29.1, 26.4, 26.3. FTIR ATR (
068 (NH), 1666 (C(O)NH, amide I), 1623 (C=N), 1511 (C(O)NH, amide II), 1058 (CArOCH2).
ν, cm ): 3298 (NH),
Elemental analysis. Calculated for C H N O C, 73.57; H, 9.35; N, 5.20; Found: C, 73.42;
3
3
50
2
4
+
+
H, 9.25; N, 5.43. HRMS: calculated [M + H] m/z = 539.3843, found [M + H] m/z =
539.3849.
3.4. Job’s Plots
A series of the solutions of compounds
8
–
11 and CuCl2
×
2H O were prepared in
2
CH OH:CHCl (1:1). The volume ratio of the host and guest solutions varied from 0.3:2.7
3
3
to 2.7:0.3, respectively, with the total concentration of the host (H) and guest (G) being
−
5
constant and equal to 3
×
10 M. The solutions were used without further stirring. The
absorbance A of the complexation systems was measured at the maximum absorbance
i
wavelength of the complex. The absorbance values were used to plot a diagram from which
maximum the structures of the complexes were deduced. Three independent experiments
were carried out for each system.
3
.5. Determination of the Stability Constant and Stoichiometry of the Complex by
Spectrophotometric Titration
−
5
10 M in the case of 11) solution of the Cu2+ (300, 600, 900,
−5
The 1
×
10 M (3
×
1
200, 1500, 1800, 2100, 2400, and 2700
µ
L) in CHCl :MeOH (1:1) was added to 0.3 mL of
3
−
4
−4
the solution of
8
–
10 (1
×
10 M) or 11 (3
×
10 M) in CHCl :MeOH (1:1) and diluted
3
to the final volume of 3 mL with CHCl -MeOH (1:1). The UV-vis spectra of the solutions
3
were then recorded. The stability constants of complexes were calculated by Bindfit. Three
independent experiments were carried out for each series.
3.6. Procedure for the Synthesis of Thiacalixarene 10—Copper (II) Materials
A solution of copper (II) chloride (1 mM, CH OH:CHCl (1:1)) was added to a yellow
3
3
solution (1 mM) of thiacalixarene 10 in a mixture CH OH:CHCl (1:1) at room tempera-
3
3
ture. Concentration and evaporation of the solution gave amorphous orange-red powder
10 + Cu complex) with yield 95%. This powder was dried in vacuum for 48 h.
(
4. Conclusions
For the first time, a series of catechol-containing Schiff bases, tetrasubstituted on
the lower rim thiacalix[4]arene derivatives in three stereoisomeric forms, cone, partial
cone and 1,3-alternate, were synthesized with high yields. The structure of the obtained
compounds was proved using a number of modern physical methods, such as NMR, IR,
UV-vis spectroscopy, and high-resolution mass spectrometry. Selective recognition (Kb
difference by three orders of magnitude) of the copper (II) cations in the series of d-metal
cations (copper (II), nickel (II), cobalt (II), and zinc (II)) was shown by UV-vis spectroscopy.
The logarithms of binding constants were found to be similar for all three stereoisomers
(cone + Cu (II): 5.04; partial cone + Cu (II): 5.36; 1,3-alternate + Cu (II): 5.46). It was shown by
a series of physical methods, that the copper (II) ions were coordinated at the nitrogen atom
of the imine group and the nearest oxygen atom of the catechol fragment in thiacalixarene
derivatives. Thermally stable organic-inorganic copper-based materials were obtained on
the base of 1,3-alternate + Cu (II) complexes. The obtained materials can find application as
antifungal and antibacterial coatings and can also be used as catalysts or in the assembly of
chemical and electrochemical sensors.

Molecules 2021, 26, 2334
13 of 15
Supplementary Materials: The following are available online, Table S1: Chemical shifts of the pro-
1
1
tons of compounds
Figures S5–S8: 13C NMR spectra of compounds
11; Figures S13–S16: HRMS spectra of compounds
pounds 11 without/with d-metal cations; Figure S21–S24: Job’s plots for the determination of
the stoichiometry; Figures S25–S28: UV-vis spectra of compounds
8
–
11 in H NMR spectra; Figures S1–S4: H NMR spectra of compounds
11; Figures S9–S12: FT-IR spectra of compounds
11; Figures S17–S20: UV-vis spectra of com-
8–11;
8–
8–
8–
8
–
2
+
8–
11 with Cu cations; Figures
11 without/with Cu cations; Figure S33: FT-IR spectra
10 + Cu mixture, 10 + Cu complex, and CuCl ; Figure S34: Powder X-ray diffrac-
1
2+
S29–S32: H NMR spectra of compounds
of compound 10
8–
,
2
tograms of compound 10, CuCl2, 10 + Cu mixture, and 10 + Cu complex; Figure S35: TG curves of
the compound 10 and 10 + Cu complex.
Author Contributions: Conceptualization, writing—review and editing, supervision, I.S. (Ivan
Stoikov); investigation, writing—original draft preparation, funding acquisition, and visualization,
P.P.; investigation and data curation, K.S., M.A., S.B., O.T., and I.S. (Igor Shiabiev); investigation and
software, A.K., A.B., A.G., D.G., and Y.S. All authors have read and agreed to the published version
of the manuscript.
Funding: The work was supported by the Russian Science Foundation (Grant No. 19-73-10134).
Data Availability Statement: The data presented in this study are available in supplementary
material.
Acknowledgments: The investigation of spatial structure of the compounds by NMR spectroscopy
was carried out within the framework of the grants of the President of the Russian Federation for
state support of young Russian scientists—candidates of sciences and leading scientific schools of
Russian Federation (MK-12.2020.3, NSh-2499.2020.3). The synthesis of the monomeric compound
was carried out within the framework of the scholarship of the President of the Russian Federation
(SP-2139.2021.1).
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of all obtained compounds are available from the authors.
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