Anion, cation and ion-pair recognition by bis-urea based receptors containing a polyether bridge

Article abstract of DOI:10.1080/10610278.2019.1578884

Two bis-urea type receptors were synthesized containing a polyether bridge and two 4-nitrophenyl groups as chromogenic units, R1 and R2. Molecular recognition studies of receptors towards different tetraalkylammonium and alkali metal salts were carried out in DMSO by UV-Vis and ¹H-NMR spectroscopy. The receptors were found to have high affinity for diverse anions and ion-pairs, showing the highest affinities towards the tetramethylammonium and sodium salts. The cation binding ability of the receptors was evidenced by means ¹H NMR, mass spectrometric ESI⁺ studies and the crystal structures of some precursors. Additionally, the molecular modelling at the DFT level of the tetramethylammonium acetate complexes illustrates the potential ion-pair binding ability of the receptors: the anion is recognized through strong hydrogen bonds from the NH─ groups from the two urea sites, while the cation is bound by a combination of cation─π, C-H···O and C-H···π interactions.

Full text of DOI:10.1080/10610278.2019.1578884

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: https://www.tandfonline.com/loi/gsch20
Anion, cation and ion-pair recognition by bis-urea
based receptors containing a polyether bridge
Jancarlo Gómez-Vega, Ramón Alfonso Moreno-Corral, Hisila Santacruz
Ortega, David Octavio Corona-Martínez, Herbert Höpfl, Rogerio R. Sotelo-
Mundo, Adrián Ochoa-Terán, Raymundo Enrique Escobar-Picos, José
Zeferino Ramírez-Ramírez, Octavio Juárez-Sánchez & Karen Ochoa Lara
To cite this article: Jancarlo Gómez-Vega, Ramón Alfonso Moreno-Corral, Hisila Santacruz
Ortega, David Octavio Corona-Martínez, Herbert Höpfl, Rogerio R. Sotelo-Mundo, Adrián Ochoa-
Terán, Raymundo Enrique Escobar-Picos, José Zeferino Ramírez-Ramírez, Octavio Juárez-
Sánchez & Karen Ochoa Lara (2019): Anion, cation and ion-pair recognition by bis-urea based
receptors containing a polyether bridge, Supramolecular Chemistry
To link to this article: https://doi.org/10.1080/10610278.2019.1578884
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SUPRAMOLECULAR CHEMISTRY
https://doi.org/10.1080/10610278.2019.1578884
ARTICLE
Anion, cation and ion-pair recognition by bis-urea based receptors containing a
polyether bridge
a
a
a
Jancarlo Gómez-Vega , Ramón Alfonso Moreno-Corral , Hisila Santacruz Ortega , David Octavio Corona-
a,b
c
d
e
Martínez
, Herbert Höpfl , Rogerio R. Sotelo-Mundo , Adrián Ochoa-Terán , Raymundo Enrique Escobar-
a
a
f
a
Picos , José Zeferino Ramírez-Ramírez , Octavio Juárez-Sánchez and Karen Ochoa Lara
a
b
Departamento de Investigación en Polímeros y Materiales, Universidad de Sonora, Hermosillo, México; Departamento de Ciencias de la
c
Salud, Universidad de Sonora, Campus Cajeme, Cd. Obregón, México; Centro de Investigaciones Químicas, Instituto de Investigación en
Ciencias Básicas y Aplicadas, Cuernavaca, México; Laboratorio de Estructura Biomolecular, Centro de Investigación en Alimentación y
d
e
Desarrollo, Hermosillo, México; Centro de Graduados e Investigación en Química. Tecnológico Nacional de México, Tijuana, México;
f
Departamento de Investigación en Física, Universidad de Sonora, Hermosillo, México
ABSTRACT
ARTICLE HISTORY
Two bis-urea type receptors were synthesized containing a polyether bridge and two 4-nitrophenyl
groups as chromogenic units, R1 and R2. Molecular recognition studies of receptors towards different
tetraalkylammonium and alkali metal salts were carried out in DMSO by UV-Vis and H-NMR spectro-
Received 15 April 2018
Accepted 16 December 2018
1
KEYWORDS
scopy. The receptors were found to have high affinity for diverse anions and ion-pairs, showing the
highest affinities towards the tetramethylammonium and sodium salts. The cation binding ability of the
Urea; polyether bridge;
heteroditopic receptor;
anion recognition; ion-pair
recognition
1
+
receptors was evidenced by means H NMR, mass spectrometric ESI studies and the crystal structures
of some precursors. Additionally, the molecular modelling at the DFT level of the tetramethylammo-
nium acetate complexes illustrates the potential ion-pair binding ability of the receptors: the anion is
recognized through strong hydrogen bonds from the NH─ groups from the two urea sites, while the
cation is bound by a combination of cation─π, C-H···O and C-H···π interactions.
CONTACT Karen Ochoa Lara
karenol@polimeros.uson.mx
Departamento de Investigación en Polímeros y Materiales, Universidad de Sonora, Rosales y
Encinas s/n, Col. Centro CP 83000, Hermosillo, Sonora, México
Supplemental data for this article can be accessed here.
©
2019 Informa UK Limited, trading as Taylor & Francis Group

2
J. GÓMEZ-VEGA ET AL.
1
. Introduction
Anion recognition has emerged as an important topic in
modern supramolecular chemistry as a consequence of the
key role that anions play in biological processes, agriculture,
food technology and environmental chemistry (1–8).
Because of this, many research groups have focused their
investigations on anion detection and quantification (1, 5,
π→π* and n→π* transitions. In this work, we report on
the synthesis of two new acyclic heteroditopic receptors
and their binding properties towards a series of tetraalk-
ylammonium and alkali metal salts by means of UV-Vis and
1
H-NMR spectroscopy in DMSO. For selected samples, the
study was accomplished by mass spectrometric technique,
single-crystal X-ray diffraction analysis and molecular mod-
elling at the DFT level. The receptors showed the highest
affinities towards the tetramethylammonium and sodium
salts studied herein, which could be attributed to the
possible simultaneous binding of both species of the ion-
pair. The detection and quantification of quaternary
ammonium salts is important, because they are ingredients
in many household chemicals and are employed as phase
transfer catalysts in organic chemistry (24).
9–12). Within this context several classes of neutral and
charged synthetic and natural, acyclic and macrocyclic
receptors have been studied. The most common neutral
receptors for anion recognition include diverse chemical
functions such as urea, thiourea, amide, sulfonamide, pyr-
role and imidazole (4, 10, 12–16). Of these, urea-based
monotopic receptors have received particular attention
for anion complexation, because they have several interest-
ing features and advantages, such as providing two acidic
NH fragments, which can establish strong and directional
hydrogen bonds with carboxylate groups or the oxygen
atoms of other di- or poly-oxoanions (4, 13–15). Besides, the
acidity of the NH fragments strongly depends upon the
nature of the substituent at the nitrogen atoms (17). The
synthetic methods for the preparation of urea based com-
pounds typically involve few simple steps, affording recep-
tors usually in high yield and purity and at low cost (13, 14).
There has been a considerable effort to design
monotopic receptors for anion and cation recognition.
However, these receptors have limited applications in
the presence of strongly ion-paired species, since
the energetic cost for the ion-ion separation decreases
the affinity towards the receptor (18). For this reason,
the design and the synthesis of heteroditopic receptors,
i.e. frameworks including binding sites for both anionic
and cationic species, have become an emergent field of
research during the last years (13, 18, 19). Many exam-
ples of heteroditopic receptors combine urea groups
within calixarenes, crown ethers, polyether and ester
functionalities for ion-pair binding (13, 18–23).
However, this type of host molecules are frequently
difficult to synthesize and require expensive reagents.
Considering all the above mentioned, we designed two
new simple heteroditopic bis-urea based receptors con-
taining a polyether bridge and 4-nitrophenyl groups as
chromogenic units, which can be synthesized by a simple
synthetic route. According to their structural properties,
these receptors are envisioned good candidates for anion,
cation and ion-pair recognition. The urea sites were incor-
porated to establish strong hydrogen-bond interactions
with anions, while the polyether bridge attached to two
electron-rich aromatic rings were included to facilitate
cation binding through ion-dipole and cation─π interac-
tions. The nitrophenyl substituent generates UV-Vis
absorption bands, which are well-separated from other
2. Experimental section
2.1. General procedures and materials
All reagents and solvents employed for synthesis and
molecular recognition studies were purchased from com-
mercial suppliers and used without further purification.
2
.1.1. Instrumental
NMR studies were carried out with a Bruker Avance III 400
1
spectrometer. Standard references were used: TMS (δ H
13
(
400 MHz) = 0 and δ C (100 MHz) = 0). IR spectra were
recorded on a Perkin Elmer FTIR/FIR Spectrometer model
FRONTIER using Attenuated Total Reflection (ATR) and KBr
pellets techniques. Mass spectrometry was conducted on
+
an Agilent 6100 LC/MS using the ESI mode. UV-Vis mea-
surements were performed on an Agilent 8435 (Agilent
Technologies) diode array with a quartz cells 1 cm optical
path. Melting points were recorded on a model Büchi
Melting point B-545 apparatus.
2.1.2. Solution studies
As part of the preliminary studies, electronic absorption
spectra were obtained at different concentrations in the
−6
−5
order of 10 to 10 M in DMSO. The spectral character-
istics of the compounds studied herein, such as the
absorption maxima and molar extinction coefficients (ε),
were determined in DMSO and DMSO:water 95:5 (v/v).
1
Complexation studies were carried out by UV-Vis and H
NMR in DMSO in order to evaluate the ability of the recep-
tors to recognize different tetraalkylammonium and alkali
−
−
−
−
−
−
−
metal salts of F , Cl , Br , I , NO , CH COO , HSO and
3
3
4
−
H PO . The typical titration procedure started from a solu-
tion with a fixed receptor concentration (10 M for UV-Vis
and 10 for H NMR), to which were added successively
2
4
−
5
−3
1

SUPRAMOLECULAR CHEMISTRY
3
aliquots of a concentrated salt solution. Each titration was
performed in triplicate. The binding constants of the com-
plexes were determined by nonlinear fitting analyses of the
titration curves according to the host–guest complex equa-
tions reported by Thordarson (25). The t-student test was
performed to the binding constants with a confidence
interval of 95%, according to the procedure reported by
Hirose (26).
(d, J = 9.0, 4H, H-6), 4.02 (t, J = 4.8, 4H, H-4), 3.71
(t, J = 4.5, 4H, H-3), 3.56 (dt, J = 4.8, 2.6, 8H, H-1 and H-2),
13
2.00 (s, 6H, H-11). C NMR (101 MHz, DMSO-d , δ, ppm):
6
168.2 (C-10), 154.6 (C-5), 133.1 (C-8), 121.0 (C-7), 114.9
(C-6), 70.3 (C-2), 70.2 (C-1), 69.4 (C-3), 67.7 (C-4), 29.3
+
(C-11). ESI-MS: m/z (positive ion mode) 461.0 [M + H]
+
+
(5.61%), 499.9 [M + K] (25.97%), 958.9 [2M+K] (2.56%).
Elem. Anal. Calcd for C H IKN O .H O: C, 44.72; H, 5.32;
24
32
2
7
2
N, 4.35; O, 19.86. Found: C, 44.56; H, 5.06; N, 4.29.
2
.2. Preparative part
2
.2.1. Synthesis of diamide precursor P4
The synthesis of precursor P4 was realized by a Williamson
ether procedure. To a solution of 1.50 g (9.92 mmol) of 4-
(acetylamino)phenol (1) in anhydrous DMF (30 mL) and
inert N atmosphere, were added 10.0 equivalents of anhy-
2
drous K CO , whereupon the mixture was stirred over
2
3
30 minutes at 120°C. Subsequently were added 755 µl
(4.90 mmol) of 1,2-bis(2-chloroethoxy)ethane (P2) and 1.0
equivalent of KI. The mixture was maintained with stirring
over a twelve-hour period at 120°C. Finally, the mixture was
filtered in order to remove the salt, then the solvent was
evaporated in vacuum and acetone was added. A precipi-
tate was obtained, which was filtered, washed with acet-
one and then with water and vacuum dried to give a white
solid. Yield: 1.83 g (90%). Melting point: 160–161°C. IR (KBr,
2.2.3. Synthesis of diamine precursor P6
Diamine P6 was obtained according to the method
reported by Gangadhara et al. (27): 6.0 g of precursor P4
(14.4 mmol) were added in a three-necked flask, containing
25 ml of a 50:50 (v/v) solution of ethanol and concentrated
hydrochloric acid. The mixture was kept under magnetic
stirring and refluxed over 15 hours. Then, the mixture
reaction was cooled and diluted until 100 ml with deio-
nized water, and neutralized by adding KOH pellets (special
care must be taken in this step). A brown solid precipitated,
which was filtered and washed several times with deio-
nized water and then dried with vacuum. The product was
−1
1
ν, cm ): 3306, 1659, 1594, 1530, 1243. H NMR (400 MHz,
DMSO-d , δ, ppm, J, Hz): 9.84 (s, 2H, H-8), 7.45 (d, J = 9.0, 4H,
6
H-6), 6.84 (d, J = 9.0, 4H, H-5), 4.02 (t, J = 4.7, 4H, H-3), 3.71 (t,
13
J = 4.7, 4H, H-2), 3.60 (s, 4H, H-1), 1.99 (s, 6H, H-10). C NMR
101 MHz, DMSO-d , δ, ppm): 167.7 (C-9), 154.2 (C-4), 132.6
(
6
(
2
C-7), 120.5 (C-6), 114.4 (C-5), 69.9 (C-1), 69.0 (C-2), 67.2 (C-3),
+
3.8 (C-10). ESI-MS: m/z (positive ion mode) 416.9 [M + H]
isolated as a light brown solid. Yield 4.52 g (94%). Melting
+
+
1
(28.55%), 454.9 [M + K] (82.30%), 871.9 [2M+K] (23.92%).
point: 85–86.7°C. H NMR (400 MHz, CDCl , δ, ppm, J, Hz):
3
Elem. Anal. Calcd for C H N O .H O: C, 60.82; H, 6.96; N,
6.78 (d, J = 8.8, 4H, H-6), 6.64 (d, J = 8.8, 4H, H-5), 4.08 (t,
J = 4.7, 4H, H-3), 3.84 (t, J = 4.7, 4H, H-2), 3.76 (s, 4H, H-1),
22
28
2
6
2
6.45; O, 25.78. Found: C, 60.9; H, 6.37; N, 6.23.
13
2
1
7
.90–3.60 (br, 4H, H-8). C NMR (101 MHz, CDCl , δ, ppm):
3
51.9 (C-4), 140.0 (C-7), 116.4 (C-6), 115.9 (C-5), 70.8 (C-1),
0.0 (C-2), 68.1 (C-3). ESI-MS: m/z (positive ion mode) 333.1
+
+
[M + H] (21.87%), 665.1 [2M+H] (19.82%). Elem. Anal.
Calcd for C H N O .0.2H O: C, 64.34; H, 7.32; N, 8.34; O,
18
24
2
4
2
20.0. Found: C, 64.28; H, 7.03; N, 7.97.
2
.2.2. Synthesis of diamide precursor P5
Diamide P5 was prepared in an analogous manner to that
described for P4, using 1.5 g (9.92 mmol) of N-(4-hydro-
xyphenyl)acetamide (1) and 971 µl (4.9 mmol) of bis[2-(2-
chloroethoxy)-ethyl]ether (P3). Yield: 2.09 g (93%).
−1
Melting point: 158–159°C. IR (KBr, ν, cm ): 3354, 1667,
1
1
603, 1527, 1236. H NMR (400 MHz, DMSO-d , δ, ppm, J,
6
Hz): 9.81 (s, 2H, H-9), 7.47 (d, J = 9.0, 4H, H-7), 6.85

4
J. GÓMEZ-VEGA ET AL.
2
.2.4. Synthesis of diamine precursor P7
Diamine P7 was obtained by the same method as
described for compound P6, using 6.0 g (13.02 mmol) of
precursor P5. In this case there was not obtained a pre-
cipitate, therefore a diethyl ether-water extraction was
performed and after evaporation of the organic phase,
the product was isolated as a brown-colored viscous
1
liquid. Purity and yield ≥ 90%. H NMR (400 MHz, CDCl ,
3
δ, ppm, J, Hz): 6.77 (d, J = 8.9, 4H, H-7), 6.64 (d, J = 8.9, 4H,
H-6), 4.07 (t, J = 4.8, 4H, H-4), 3.83 (t, J = 4.8, 4H, H-3), 3.72
1
3
(
(
7
td, J = 5.3, 2.6, 8H, H-1 and H-2), 3.44 (s, 4H, H-9). C NMR
101 MHz, CDCl , δ, ppm): 152.2 (C-5), 139.4 (C-8), 116.7 (C-
2
.2.6. Synthesis of receptor R2
3
Receptor R2 was prepared in an analogous manner to that
described for R1, using 0.043 g (0.11 mmol) of P7 and
), 115.8 (C-6), 70.7 (C-2), 70.6 (C-1), 69.9 (C-3), 68.0 (C-4).
ESI-MS: m/z (positive ion mode) 377.0 [M + H] (32.86%),
+
0
0
3
.0371 g (0.22 mmol) of 4-nitrophenyl isocyanate. Yield:
.079 g (99%). Melting point: 85.8–86.5°C. IR (ATR, ν, cm ):
311, 2924, 1717, 1674, 1597, 1492, 1413, 1216. UV absorp-
+
+
−1
3
99.0 [M+ Na] (71.45%), 753.0 [2M+H] (6.42%).
3
−1
−1
tions: λmax (DMSO)/360 nm (ε/dm mol cm = 39,470),
3
−1
λ
(DMSO:water, 95:5 v/v)/360 nm (ε/dm mol cm-
max
−
1
1
=
39,200). H (400 MHz, DMSO-d , δ, ppm, J, Hz): (s, 2H,
6
H-11), 8.68 (s, 2H, H-9), 8.17 (d, J = 9.2, 4H, H-14), 7.67 (d,
J = 9.2, 4H, H-13), 7.37 (d, J = 9.0, 4H, H-7), 6.90 (d, J = 9.0, 4H,
H-6), 4.05 (t, J = 4.6, 4H, H-4), 3.74 (t, J = 4.6, 4H, H-3), 3.58
13
(
td, J = 4.9, 2.5, 8H, H-1, H-2). C NMR (101 MHz, DMSO-d6,
δ, ppm): 154.1 (C-10), 152.1 (C-5), 146.6 (C-15), 140.8 (C-12),
1
6
32.1 (C-8), 125.2 (C-14), 120.5 (C-7), 117.3 (C-13), 114.7 (C-
), 69.92 (C-2), 70.0 (C-1), 69.0 (C-3), 67.3 (C-4). ESI-MS: m/z
+
+
(positive ion mode) 705.3 [M + H] (100%), 727.3 [M+ Na]
(
5
78%). Elem. Anal. Calcd for C H N O .H O: C, 56.51; H,
.30; N, 11.63; O, 26.57; Found: C, 56.72; H, 4.95; N, 11.74.
34
36
6
11
2
2
.2.5. Synthesis of receptor R1
Receptor R1 was prepared according to the following
procedure. To a solution of 0.040 g (0.120 mmol) of P6 in
anhydrous acetonitrile and inert N atmosphere were
2
added 0.040 g (0.24 mmol) of 4-nitrophenyl isocyanate.
A brown solid was precipitated after stirring the mixture
over 3 days at room temperature. The solid was filtered
and washed with chloroform, acetone and acetonitrile
and vacuum dried. Yield: 0.078 g (97.5%). Melting point:
−
1
1
1
3
9
64.7–166.6°C. IR (ATR, ν, cm ): 3284, 2910, 1738, 1639,
604, 1498, 1407, 1216. UV absorptions: λ (DMSO)/
max
3
−1
−1
60 nm (ε/dm mol cm = 42,590): λ
(DMSO:water,
max
3
−1
−1
1
5:5 v/v)/360 nm (ε/dm mol cm = 38,260). H NMR
(
400 MHz, DMSO-d , δ, ppm, J, Hz): 9.49 (s, 2H, H-10), 8.83
6
(
s, 2H, H-8), 8.18 (d, J = 9.2, 4H, H-13), 7.67 (d, J = 9.2, 4H,
2.3. X-ray crystallography
H-12), 7.37 (d, J = 8.9, 4H, H-6), 6.90 (d, J = 8.9, 4H, H-5),
4
4
9
1
.05 (t, J = 4.7, 4H, H-3), 3.74 (t, J = 4.7, 4H, H-2), 3.62 (s,
Single-crystal X-ray diffraction studies were carried out on
an Agilent Technologies SuperNova diffractometer
equipped with a CCD area detector (EosS2) using Mο-Kα
radiation (λ = 0.71073 Å) from a microfocus X-ray source
and an Oxford Instruments Cryojet cooler. Frames were
collected at T = 100 K. The measured intensities were
1
3
H, H-1). C NMR (101 MHz, DMSO-d , δ, ppm): 154.1 (C-
6
), 152.2 (C-4), 146.7 (C-14), 140.8 (C-11), 132.1 (C-7),
25.2 (C 13), 120.4 (C-6), 117.3 (C-12), 114.7 (C-5), 70.0
(C-1), 69.1 (C-2), 67.3 (C-3). ESI-MS: m/z (positive ion
+
+
mode) 661.2 [M + H] (100%), 683.3 [M+ Na] (48%).
Elem. Anal. Calcd for C H N O .H O: C, 56.63; H, 5.05;
2
32
32
6
10
2
reduced to F and corrected for absorption using spherical
N, 12.38; O, 25.93. Found: C, 56.23; H, 4.71; N, 12.42.
harmonics (CryAlisPro) (28). Intensities were corrected for

SUPRAMOLECULAR CHEMISTRY
5
Lorentz and polarization effects. Structure solution, refine-
2.4. Molecular modelling
ment, and data output were performed with the OLEX2
The geometries of the complexes formed between tetra-
methylammonium acetate and receptors R1 and R2 were
constructed with a molecular editor, whereupon the struc-
tures were fully optimized by using the B3LYP-gCP-D3/6-
(29) program package using SHELXL-2014 (30) for the
refinement. Non-hydrogen atoms were refined anisotro-
pically. All hydrogen atoms were placed in geometrically
calculated positions using the riding model.
For compound P4, only weakly diffracting crystals
could be grown. The asymmetric unit of compound
31G* level of theory (32). The calculations were carried out
with the Gaussian 09 (version B.01) software package (33).
The molecular structures were visualized with the
Avogadro (version 1.2.0) and Chemcraft programs (34, 35).
{
[P5K]I} ⋅ 1.41 H O contains two crystallographically
2
2
independent molecules. One of the iodide counterions
(I2) exhibits positional disorder over two positions with
occupancies of 40.7% for I2A and 59.3% for I2B. Of these,
the iodide ion I2A is accompanied by a water molecule
3. Results and discussion
(O51), which was refined with the same occupancy. Due
3.1. Synthesis and structural characterization
to the variation in the intermolecular hydrogen pattern
with I2A/O51 and I2B, the aminophen fragment with
labels C17 > C23 and part of the crown ether attached
to K1 exhibit disorder over two positions. Additionally,
also part of the polyether fragment embedding K2 is
disordered over two positions. For the refinement of
this disorder, the DFIX, SAME and SIMU instructions
implemented in SHELXL have been employed.
Crystal data collection, solution, and structure refine-
ment parameters for compounds P4-P6 are listed in
Table 1. Figures were illustrated with Diamond (31).
Crystallographic data for the structures reported in this
paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publica-
tions no. CCDC 1527532–1527534. Copies of the data can
be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-
The compounds described herein were synthesized follow-
ing the synthetic protocol outlined in Scheme 1. Treatment
of N-(4-hydroxyphenyl)acetamide (1) with the correspond-
ing dichlorides in a 2:1 molar ratio gave the corresponding
polyether precursors P4 and P5, which were then depro-
tected to afford the corresponding diamine derivatives P6
and P7. Compound P6 has been reported previously in the
literature (27), while diamide P7 and diamines P4 and P5
are novel compounds. The receptors were prepared in
anhydrous dichloromethane by reaction of diamines P6
or P7 with the corresponding isocyanate. Following this
procedure, the bis-urea compounds R1 and R2 were
obtained in quantitative yields. The products were charac-
1
13
terized by IR and NMR ( H and C) spectroscopy using
standard one-dimensional and two-dimensional techni-
ques, and mass spectrometry. In all cases, the spectroscopic
data were consistent with the proposed structures (see
Experimental Section).
033; e-mail: deposit@ccdc.cam.ac.uk, www: http://www.
ccdc.cam.ac.uk).
Table 1. Crystallographic data for compounds P4, P6 and {[(P5)K]I} .1.41 H O.
2
2
a
a
b
Crystal data
P4
P6
{[(P5)K]I}
2(C24 32IKN
1278.39
P2 /n
2
.1.41 H
2
O
Formula
MW (g mol
C
22
H
28
N
2
O
6
C
18
H
24
N
2
O
4
H
2
O
7
)
2
, 1.41 H O
2
−1
)
416.46
332.39
Space group
Temp (K)
a (Å)
b (Å)
c (Å)
Pbcn
Pbca
99.99(14)
5.16540(7)
13.34338(18)
24.5876(3)
90
1
100.00(16)
13.1909(10)
9.5812(10)
16.9019(17)
90
100.0(2)
11.10479(16)
31.6310(4)
16.2751(3)
90
α (°)
β (°)
90
90
2136.2(3)
4
0.780
90
90
104.8499(18)
90
5525.79(15)
4
1.356
1.537
0.0405
0.0819
γ (°)3
V (Å )
Z
1694.68(4)
4
0.754
−1
µ (mm
)
−3
ρ
R
calcd (g cm
)
1.295
1.303
c,d
1
0.0778
0.2218
1.105
0.0324
0.0852
1.084
e,f
wR
GOF
2
1.190
a
λCuKα ¼ 1:54184 Å
b
c
λMoKα ¼ 0:71073 Å
F0 > 4σðF0Þ
P
P
d
R ¼ jjF0jꢀjFCjj= jF0j
e
All data
P ꢀ
ꢁ2 P ꢀ ꢁ2 1
f
2
0
2
2
=
RW ¼ ½ w F ꢀ F
=
w F ꢁ 2
C
C

6
J. GÓMEZ-VEGA ET AL.
Scheme 1. Synthesis of R1 and R2: a) K CO , KI, anhydrous DMF, N , 120°C, 12 h, 90% (P4), 93% (P5.KI); b) HCl:EtOH 50:50 (v/v),
2
3
2
reflux, 15 h, 94% (P6), ≥90% (P7); c) 4-nitrophenyl isocyanate, anhydrous MeCN, N , rt, 72 h, 98% (R1), 99% (R2).
2
The formation of the bis-urea receptors was demon-
strated first by the H NMR spectra that gave signals for
3.2. Molecular recognition studies
.2.1. UV-Vis Studies
Despite the apparent structural simplicity of the receptors
Scheme 2), it can be expected that these flexible bis-urea
1
3
the urea ─NH hydrogens at δ = 9.49 and 8.83 ppm for
R1 and at δ = 9.31 and 8.68 ppm for R2. According to
these chemical shifts values both of the NH hydrogens
are more acidic in R1 than in R2. Regarding to this
latter, such differences must be attributed to the addi-
tional oxygen atom in the polyether chain of R2.
Besides, for both receptors the ureidic N-H hydrogens
adjacent to the electron withdrawing 4-nitrophenyl
groups are more acidic than those connected to the
aromatic ring of the phenyl ether fragment.
(
derivatives are able to bind anions, cations and certain ion-
pairs in a cooperative binding mode. The urea sites can
establish strong hydrogen bonding interactions with
anions and the polyether bridge with adjacent electron-
rich aromatic rings should enable the linkage of cationic
species, mainly through a combination of ion-dipole and
cation─π interactions. Other weaker hydrogen-bonding
interactions such as C-H···O and C-H···π could contribute
further to the complex stability. Nevertheless, the efficiency
of the ion-pair recognition of compounds R1 and R2
depends strongly on the medium. In a very competitive
solvent such as DMSO, undoubtedly strong hydrogen-
bonding will be a very important driving force for the
binding of the anion of the respective ion-pair, thus
enabling ion-pair separation.
Further evidence of urea formation was observed in
1
3
the C NMR spectra, where the signal corresponding to
the carbonyl group was observed at δ = 154.13 and
54.14 ppm for R1 and R2, respectively. The ESI mass
1
spectra in the positive ion mode gave peaks at m/
+
z = 661.2 and 705.3 that correspond to the [M + H] ion
of compounds R1 and R2, respectively. Moreover, the
formation of sodium complexes of both receptors was
also evidenced by mass spectrometry.
Attempts to crystallize R1 and R2 were not successful,
but the structures of the precursor compounds P4-P6
could be examined by single-crystal X-ray diffraction ana-
lysis. Crystallographic data are given in Table 1 and the
molecular structures of P4-P6 are shown in Fig. S1-S7,
respectively (Supporting Information).
In order to investigate the binding properties of R1
and R2, complexation studies towards several tetraalk-
ylammonium and alkali metal salts were performed by
1
means of UV/Vis and H NMR spectroscopy in DMSO. In
this regard, first preliminary UV/Vis experiments were
carried out, which consisted in recording the spectra of
the free receptors and different receptor-anion mixtures;
Scheme 2. Chemical structures of receptors R1 and R2.

SUPRAMOLECULAR CHEMISTRY
7
the anions were studied as their tetramethylammonium
TMA), tetraethylammonium (TEA) or tetrabutylammo-
delocalization of the negative charge into the 4-nitro-
phenyl group. This phenomenon is well documented in
the literature (13, 14). An additional, more direct evi-
dence for the receptor deprotonation will be given in
(
nium (TBA) salts. Figure 1 shows the spectral changes
of R1 and R2 induced by the presence of the studied
anions. Both receptors showed a similar behavior, having
a maximum absorption band at about 360 nm in DMSO.
Significant spectral changes were observed in the pre-
sence of the basic anions acetate (TMAA), bisulfate
1
the H NMR section.
Taking into account these preliminary results, the
anion complexation process was analyzed in detail by
UV/Vis titration experiments for receptors R1 and R2 in
the presence of TMAA, TMABS, TMAF and TBAP in DMSO.
Figure 2(a) shows the family of spectra obtained on titra-
tion of R1 with TMAA (for the remaining guests see Fig.
S9-S12 Supporting Information). Upon addition of TMAA
(TMABS) and dihydrogenphosphate (TBAP), exhibiting
either hyperchromic or bathochromic shifts of the max-
imum absorption band of about 20 nm. On the contrary,
−
−
nitrate (TMAN) and the halides Cl (TMAC), Br (TMAB)
−
−5
and I (TMAI), gave no significant spectral changes. A
to a solution of R1 (1 × 10 M), the intensity of the
particular behavior was observed in the presence of
fluoride (TMAF). In this case a new red-shifted absorption
band appeared at 487 nm, accompanied by a ‘naked-
eye’ color change from colorless (free receptor) to
orange (mixture), see Fig. S8. Due to the relatively high
concentration of fluoride, this observation should be
attributed to the deprotonation of acidic urea hydrogens
from both sites of the receptors with subsequent
absorption band at 360 nm decreased, while a new red-
shifted absorption band at around 380 nm appeared,
which increases with increasing concentration of TMAA.
Moreover, an isosbestic point was observed at approxi-
mately 365 nm. Similar results were obtained for the
remaining experiments realized in this section. It is impor-
tant to mention that all titrations gave significant changes
in the absorbance, which allowed the reproduction of the
−
5
Figure 1. (colour online) Absorption spectra of solutions of free receptor and mixtures of receptors (1 × 10 M) and different salts
−
2
(
1 × 10 M), in DMSO: (a) for receptor R1 and (b) for receptor R2.
−
5
Figure 2. (colour online) (a) Variation of the absorption spectra for receptor R1 (1 × 10 M) upon addition of TMAA (0–
−
4
−5
2
.94 × 10 M), in DMSO. (b) Plot of the absorbance of R1 (1 × 10 M) at 380 nm vs [TMAA].

8
J. GÓMEZ-VEGA ET AL.
experiments with accuracy. Plotting the absorbance of
the free receptors and the corresponding complexes at
affinities obtained for some of the tetraalkylammonium
salts could be explained in part in terms of the ion-pair
recognition ability of R1 and R2. Once the anion is bound
through the two urea sites both R1 and R2 might develop
a pseudo-cycle cavity suitable for cation recognition. In
this context, there are some works on heteroditopic
receptors for tetraalkylammonium salts (38–44). For
example, Cametti et al. have analyzed the ion-pair recog-
nition properties of a series of acyclic uranyl-salophen
based heteroditopic receptors for quaternary ammonium
halides and alkali metal halides. The ion-pair recognition
by these receptors was very efficient as the result of a
combination of Lewis acid-base and cation─π interac-
tions (38).
3
80 nm versus the guest concentration, a hyperbolic
curve was observed in all cases, see Figure 2(b). The
binding constants of the various complexes are summar-
ized in Table 2, showing large values even though a
competitive solvent medium was employed, particularly
in the presence of acetate which gave binding constants
4
4
−1
of 2.53 × 10 and 3.34 × 10 M for R1 and R2, respec-
tively. According to the literature, complexes formed in
DMSO between monotopic bis-urea based compounds
and anions such as acetate, phosphate and bisulfate,
2
typically have binding constants in the order of 10 –
3
−1
1
0 M (13, 14, 22, 36, 37). The increased affinities of R1
and R2 could attributed to their potential ability to exhibit
ion-pair recognition. Moreover, particularly high affinities
were found for R2 despite the lower acidity of the NH
hydrogens adjacent to the polyether chain (vide supra).
This behaviour of R2 could be attributed to its larger
conformational flexibility and therefore, depending on
the salt type, it could allow a better complementarity
with both components of the ion-pair.
On the other hand, when comparing the binding con-
stants of the receptors for the different anions as their
tetramethylammonium salts, higher affinities were
obtained for the acetate complexes due to the higher
basicity of this anion. Additionally, when comparing the
affinity of the receptors for different tetraalkylammonium
acetates, including the tetraethylammonium (TEAA) and
tetrabutylammonium (TBAA) salts, an influence of the
cation on the binding constant was observed, being
more pronounced for the case of R2 with the affinity
order: TMAA >TEAA >TBAA. The binding constant for
R2-TMAA is ca. 2.8 times larger than that for R2-TBAA,
thus indicating that R2 is suitable for selective ion-pair
recognition in the presence of TMA cations. Another inter-
esting finding is the preference of receptor R1 for TEAA.
Fig. S13 and S14 show the family of spectra obtained on
titrations of R1 and R2 with TEAA and TBAA. The high
To test the ion pair recognition ability of R1 and R2,
titrations of both receptors were performed with TBAA in
+
+
+
the presence of one equivalent of Li , Na , or K , as their
perchlorate salts. Fig. S15-S17 show the family of spectra
obtained from these titrations and Table 3 summarizes
the binding constants of the various salt complexes. By
simple inspection of the data, it can be observed that R2
showed a higher affinity for the acetate in the presence of
the alkali metals with a high cooperative factor for the
sodium cation. More interestingly, receptor R1 was appar-
ently insensitive to the alkali metal salt type, with binding
constants similar to those found for the tetraalkylammo-
nium acetate salts (see Table 2).
1
3.2.2. H NMR studies
In order to gain a better insight into the complexes, H
1
NMR spectroscopic titrations of both R1 and R2 were
performed in DMSO. As shown in Figure 3(a), upon
addition of TMAA the signals of the ureidic NH protons
H-9 and H-11 of receptor R2 were progressively and
markedly downfield shifted until saturation. These
results indicate that the acetate anion is bound through
hydrogen bonding via the NH─hydrogens from both
urea groups. Additionally, hydrogens H-7 and H-13,
which are adjacent to the ureidic hydrogens, are also
shifted downfield. Less significant changes occurred in
the region of the polyether hydrogens. Table 4 resumes
Table 2. Logarithmic binding constants (Log K) between recep-
1
tors and selected anions by UV/Vis and H NMR, in DMSO at
Table 3. Binding constants (K) between receptors (R) and
acetate in the presence of one equivalent of alkali perchlorate
salt by UV/Vis, in DMSO at 298 K.
298 K.
R1
R2
R1
R2
a
1
a
Guest
UV-Vis
H NMR
R1
R2
b
b
TMABS
TMAF
TMAA
TEAA
TBAA
TBAP
3.87
3.96
-
-
-
-
a
b
a
b
c
c
c
c
R + Alkali cation
KAC
KAC/KTBAA
KAC
KAC/KTBAA
-
-
d
d
d
+
4
4
4
4
4
4
4.4
4.52
4.3
4.08
4.43
4.64
4.21
>6.00 d
R + Li
2.80 × 10
2.82 × 10
2.56 × 10
1.09
1.10
0.99
1.63 × 10
2.44 × 10
1.88 × 10
1.35
2.02
1.55
+
4.57
4.41
4.32
4.51
R + Na
+
3.73
R + K
c
c
4.37
-
-
a
Errors < 10%
Cooperative factor = Kobs, ion par/K anion, free
a
b
Errors < 10%
Not determined
Urea deprotonation.
Logarithmic cumulative binding constant (Log β)
b
c
KAC = Binding constant for the complex receptor-acetate in the pre-
sence of one equivalent of an alkali cation
TBAA = Binding constant for the complex receptor-TBAA (Table 2)
d
K

SUPRAMOLECULAR CHEMISTRY
9
1
−3
Figure 3. (a) H NMR spectra of R2 (2.84 × 10 M) with increasing amounts of TMAA (0–0.0289 M) in DMSO-d . (b) Plot of the
6
chemical shift of proton H11 of R2 (ppm) vs the molar ratio [TMAA]/[R2].
Table 4. Experimental chemical shifts (Δδ) for different protons
of R1 and R2 induced on saturation by addition of TMAA.
As already mentioned, a very different behavior
was observed in the case of the system R2-TMAF. In
this titration, signal broadening and a downfield
shift of the ureidic NH- hydrogens were observed
until 12.3 equivalents of TMAF were added. Beyond
this point, these signals vanished and a new signal
appeared at approximately 16.2 ppm, which is
R1
R2
Proton
Δδ (ppm)
Proton
Δδ (ppm)
H1
H2
H3
H5
0.003
−0.003
−0.015
−0.055
0.156
H1-H2
H3
H4
H6
H7
0.002
−0.005
−0.014
−0.055
0.156
H6
−
attributed to the formation of the FHF species, see
H8
3.376
H9
3.458
H10
H12
H13
3.500
0.164
−0.054
H11
H13
H14
3.581
0.165
−0.052
Figure 4. An analogous behavior was observed for
the R1-TMAF system (Fig. S19). Titration of receptors
R1 and R2 with TBAP gave a similar behavior (Fig.
S20 and S21).
the experimental chemical shifts (Δδ) induced on
saturation for different protons of R2 by addition of
TMAA. According to the above mentioned, the N-
Additionally, in order to validate the findings
obtained by the UV/Vis solution studies, titrations of
the receptors with TEAA and TBAA salts (Fig. S22-S25)
were performed. Data fitting of the plots of the chemi-
cal shifts of hydrogens of the receptors against the salt
concentration allowed to calculate the binding con-
stants for the complexes between the receptors and
−
H···OAc hydrogen bonds are postulated as the main
interaction responsible for the complex stability.
However, as discussed before, weaker interactions of
+
the N ─π, C-H···π and C-H···O type could be expected
1
for the binding of the tetramethylammonium cation,
thus allowing for ion-pair recognition resulting in a
high complex stability of the R2-TMAA system.
tetraalkylammonium acetate salts. A typical H NMR
titration plot for the complex R2-TMAA is shown in
Figure 3(b), whereas the logarithmic binding constants
are resumed in Table 2. In general, the results found by
Chemical shift changes of aromatic signals in some
systems provide experimental evidence of the cation─π
interaction (45–47). At this respect, Pletneva et al. stu-
1
the H NMR technique are in agreement with those
found by UV/Vis: a) the high affinities of the complexes
between the receptors and acetate tetralkylammonium
salts; the influence of the cation on the binding con-
stant b) with an affinity order TMAA >TEAA >TBAA for
R2 and c) with a preference for TEAA in the case of R1.
In an attempt to study the cation binding recogni-
tion further, the receptors were titrated with lithium,
sodium and potassium perchlorate salts in DMSO-d6.
However, there were no significant changes in the che-
mical shifts for any of protons of either receptor, even
at considerable concentrations of the cations, therefore
it was not possible to quantify the binding in this
competitive medium. Taking into account these
+
died the N ─π interaction in amino acid derivatives,
and found that the direction of the chemical shift was
dependent on the solvent. The aromatic hydrogen
shifts moved upfield in water, while they moved down-
field in DMSO (47). Similar results have been reported
for other cation─π systems (45, 46). According to these
reports, the chemical shift changes of H-6 and H-7
during the titration of R2 with TMAA may be attributed
also to effects of the tetrametylammonium cation on
the electron-rich aromatic rings connected to the poly-
ether bridge. A similar behavior was observed for the
R1-TMAA complex system, see Fig. S18.

1
0
J. GÓMEZ-VEGA ET AL.
1
−3
Figure 4. H NMR spectra of R2 (2.84 × 10 M) with increasing amounts of TMAF (0–0.0457 M) in DMSO-d6.
findings, titration experiments were performed with a
TBAA in DMSO-d . In particular, the results for the stu-
6
mixture of the respective receptor (3.0 mM) and acetate
dies of R2 are in agreement with the cooperative factor
(
as TBAA 10.3 mM) to guarantee having the acetate
tendencies found by the ion-pair binding solution stu-
dies by UV/Vis (see Table 3). Similar findings were
reported for complexes of macrocyclic heteroditopic
receptors and ammonium and metal alkali salts (43).
Finally, ion-pair recognition studies similar to those
previously described for the UV/Vis titrations were per-
formed, but in these cases an equimolar mixture solution
of receptor and metal alkali perchlorate (3.0 × 10 M)
was titrated with increasing amounts of TBAA, in DMSO-
d6. Fig. S32-S37 show all the spectra of these titrations
while the Table 6 collects the binding constants (K), and
the cooperative factors, between receptors and acetate
in the presence of one equivalent of alkali perchlorate
cobound in R1 and R2, thus allowing to pre-organized
the receptors (as pseudo-crown ethers). All the spectra
of these studies are resumed in Fig. S26-S31.
Interestingly, very significant upfield shifts were
observed particularly for the hydrogen bonded ureidic
protons. In Figure 5 are shown the chemical shift
changes for ureidic protons H10 and H11 of R1 and
R2 respectively, in the presence of TBAA, with increas-
ing amounts of the three metal alkali perchlorates stu-
died; the most significant changes were observed in the
presence of sodium and lithium for both R1 and R2.
The Table 5 shows the binding constants (K ) between
−3
11
receptors and alkaline perchlorates in the presence of
salt, in DMSO-d . Only for R2 the general trend is similar
6
−
3
Figure 5. (colour online) a) Chemical shift changes for the proton H10 of R1 (3.03 × 10 M) in the presence of TBAA
−
3
(
(
10.35 × 10 M) with increasing amounts of alkali perchlorates in DMSO-d . b) Chemical shift changes for the proton H11 of R2
6
−
3
−3
2.84 × 10 M) in the presence of TBAA (10.35 × 10 M) with increasing amounts of alkali perchlorates in DMSO-d6.

SUPRAMOLECULAR CHEMISTRY
11
−
1
Table 5. Binding constants (K , M ) between receptors and
(100% of abundance) corresponds to the sodium complex
and the peak at m/z 499.0 (20% of abundance) is attributed
11
1
alkaline perchlorates in the presence of TBAA by H NMR, in
DMSO-d at 298 K.
System
6
to the potassium complex. Besides, the dimeric sodium
+
+
+
Li
Na
K
+
complex [(P5) Na] was detected at m/z 943.9. The different
2
a
a
a
R1
-
-
-
+
+
abundance of [P5Na] and [(P5) Na] may be attributed to
R1∙TBAA
R2
R2∙TBAA
119
-
174.2
15.1
-
399.7
42.1
-
55.1
2
a
a
a
the higher affinity of diamide P5 for sodium over potas-
sium, which is in agreement with the findings of the solu-
tion studies for R2 and with the behaviour of some acyclic
polyethers reported in literature (50). Furthermore, these
results help to understand the slightly higher affinity of R2
for acetate in the presence of sodium and affirms once
more the conclusion that this receptor is suitable for ion-
pair recognition.
1
a. Insufficient changes in the H NMR spectra to determine the binding
constants.
Table 6. Binding constants (K) between receptors (R) and
acetate in the presence of one equivalent of alkali perchlorate
salt by H NMR, in DMSO-d at 298 K.
1
6
R1
R2
a
b
a
b
The observations made by the mass spectrometric ana-
lyses are somewhat confirmed by the solid state structure
of [P5K]I, which was analyzed by single-crystal X-ray diffrac-
tion analysis (Table 1). In [P5K]I, each potassium cation is
coordinated by all five oxygens of the polyether chain of
one P5 molecule, introducing the expected folding in the
structure (Fig. S6-S7), resulting in a pseudo-crown ether.
Two additional oxygen atoms from the amide groups of
neighboring complex molecules complete the coordina-
tion sphere to give overall seven-coordinate potassium
ions, which are thus embedded in distorted pentagonal-
bipyramidal environments.
R + Alkali cation
K
AC
K
AC/KTBAA
K
AC
K
AC/KTBAA
+
4
3
R + Li
1.10 × 104
1.34 × 103
2.33 × 10
0.68
0.83
0.14
9.60 × 104
1.31 × 103
5.40 × 10
1.78
2.42
0.19
+
R + Na
+
R + K
a
Errors < 10%
b
Cooperative factor = Kobs, ion par/K anion, free
AC = Binding constant for the complex receptor-acetate in the pre-
sence of one equivalent of an alkali cation
K
K
TBAA = Binding constant for the complex receptor-TBAA (Table 2)
to that observed by UV/Vis, with a high cooperative
factor in the presence of sodium; which is in agreement
also with the results of Table 5. The differences found
between both techniques may be attributed to the dif-
ferent concentrations employed in each one; as docu-
mented in the literature (40).
3.3. Molecular modelling
The geometry-optimized structure of the complex formed
between TMAA and R1 is shown in Figure 6. The interaction
of the acetate anion and the urea groups of receptor R1 is
indicated by hydrogen bonds given in form of dashed lines
with N-H···O distances in the range from 1.86 to 1.99 Å. Both
urea groups interact with the bridging anion, but not in the
3
.2.3. Mass spectrometry complexation studies
Considering that R2 is a better monotopic receptor for
ammonium and sodium cations compared to R1, some
mass spectrometric experiments using the ESI technique
were carried out to explore the cation-binding properties of
R2 and its diamide precursor P5. The experiments were
realized in methanol (with 0.3% formic acid). Chloride salt
+
+
same manner. The TMA counterion interacts by means of
+
+
solutions of TMA and TBA were mixed in 1:1 stoichiome-
try with R2 and P5, which allowed to confirm the formation
+
+
of R2:TMA and P5:TMA by ESI mass spectrometry in the
positive ion mode, giving peaks at m/z 778.8 and 535.0 m/z
+
respectively, whereas the corresponding TBA complexes
were not detected. These results resemble to those for the
complexes between diverse crown ethers and tetraalkyla-
monium salts reported earlier by Bartoli et al. (48) and
Fransky et al. (49). In this context, it is important to mention
that monomeric and dimeric potassium complexes of com-
+
+
positions [(P4/P5)K] and [(P4/P5) K] were observed for
2
diamides P4 and P5 by this technique (see Experimental
+
−
Section). An equimolar mixture of [P5K] .I and NaCl in
methanol containing 5% of H O (v/v) was also analyzed,
thus evidencing the formation of both sodium and potas-
sium complexes of the diamide. The peak at m/z 483.0
2
Figure 6. (colour online) A perspective view of the calculated
molecular structure of R1-TMAA by the B3LYP-gCP-D3/6-31G*
level theory.

1
2
J. GÓMEZ-VEGA ET AL.
cation and ion-pair recognition towards tetramethylam-
monium and alkali metal salts. According to the molecular
modelling, the ion-pair recognition of tetramethylammo-
nium salts could be the result of a combination of strong
N-H···O hydrogen bonds as well as cation···π, C-H···O and
C-H···π interactions.
In summary, this work demonstrates the viability of
employing simple acyclic compounds as heteroditopic
receptors. Via the fine-tuning of their structural properties
it is possible to enhance the ion-pair recognition of the
desired target salt and these compounds may be used for
different applications. We are currently working on the
synthesis of further analogues and the synthesis of het-
eroditopic receptors supported on polymeric resins.
Figure 7. (colour online) A perspective view of the calculated
molecular structure of R2-TMAA by the B3LYP-gCP-D3/6-31G*
level theory.
Acknowledgments
This work was supported by the “Consejo Nacional de Ciencia y
Tecnología (CONACyT)” under a graduate Grant (for PJGV
and REEP), under a sabbatical Grant (number 121614), under
Grants CB-239581 and 281251 (“Red Temática de Química
Supramolecular”) and under an equipment grant INFR-2014-01-
C-H···O and C-H···π interactions with two of the oxygen
atoms in the polyether bridge of the receptor, with the
electron-rich aromatic spacer and with the acetate anion.
The distances of the C-H···O interactions are in the range
from 2.30 to 2.85 Å, while the distances of the C-H···π
interactions are in the range from 2.64 to 3.01 Å. The short-
est distances of the C-H···O interactions correspond to those
formed with the acetate anion and are, thus, driven by the
225455 (for the single crystal X-ray diffractometer). Finally, access
to the X-ray facilities of the Chemical Research Center at Morelos
State University (LANEM) is gratefully acknowledged.
Disclosure statement
−
+
electrostatic attraction of the OAc and TMA ions.
The geometry-optimized structure of the complex
formed between TMAA and R2 is shown in Figure 7. In
this case the interaction of the acetate anion and the urea
sites of the receptor exhibit the same kind of hydrogen
bonds as for R1-TMAA with distances being in general
slightly shorter, which are in the range of 1.86 and 1.94 Å.
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Consejo Nacional de Ciencia y
Tecnología [CB-239581,Red Temática de Química Supramolecular
281251,Sabbatical grant 121614].
+
The TMA cation interact with four oxygen atoms of the
polyether bridge of the receptor and with the electron-rich
aromatic units by C-H···O and C-H···π hydrogen bonds. The
distances of the C-H···O interactions are in the range from
ORCID
Jancarlo Gómez-Vega
72X
Ramón Alfonso Moreno-Corral
627-5400
http://orcid.org/0000-0001-5641-
2.31 to 2.60 Å. In this case, the anion and cation are slightly
8
separated upon recognition by the receptor R2. Regarding
to this last aspect, stronger binding could be expected in
the case of R1, which is not in agreement with the results
observed from the solution studies. In contrast, the dis-
tances for the different strong and weak hydrogen bonds
are shorter for the complex of R2 together with the better
possibility of electrostatic interactions related with the big-
ger amount of oxygen atoms of the polyether chain, as well
as their orientation, which may explain the better results
found for this last receptor, among other aspects.
http://orcid.org/0000-0003-
2
Hisila Santacruz Ortega
8791
http://orcid.org/0000-0002-7123-
David Octavio Corona-Martínez
002-4699-7948
http://orcid.org/0000-
0
Herbert Höpfl
Rogerio R. Sotelo-Mundo
http://orcid.org/0000-0002-4027-0131
http://orcid.org/0000-0001-5543-
6
889
Adrián Ochoa-Terán
Raymundo Enrique Escobar-Picos
http://orcid.org/0000-0002-3746-3960
http://orcid.org/0000-
0
001-5528-1901
José Zeferino Ramírez-Ramírez
3616-9137
http://orcid.org/0000-0002-
4
. Conclusions
Octavio Juárez-Sánchez
http://orcid.org/0000-0003-2078-
The molecular recognition studies described herein have
4
626
shown that receptors R1 and R2 are capable of anion,
Karen Ochoa Lara
http://orcid.org/0000-0002-5271-2718

SUPRAMOLECULAR CHEMISTRY
13
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