Open aryl triazole receptors: Planar sheets, spheres and anion binding

Article abstract of DOI:10.1039/c0cc05685e

The morphology of the aggregates formed from the self-assembly of aryl triazole amphiphiles is disrupted upon the binding of a bromide anion due to the conformational changes experienced by these receptors.

Full text of DOI:10.1039/c0cc05685e

View Article Online / Journal Homepage / Table of Contents for this issue
This article is part of the
Supramolecular Chemistry web-
based thematic issue
celebrating the International Year of Chemistry 2011
Guest editors: Professors Philip Gale,
Jonathan Sessler and Jonathan Steed
All articles in this issue will be gathered together online at
www.rsc.org/chemcomm/supra.

Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 5016–5018
www.rsc.org/chemcomm
COMMUNICATION
Open aryl triazole receptors: planar sheets, spheres and anion bindingwz
´ ´
´
Fatima Garcıa, M. Rosario Torres, Emilio Matesanz and Luis Sanchez*
a
b
b
a
Received 20th December 2010, Accepted 4th March 2011
DOI: 10.1039/c0cc05685e
The morphology of the aggregates formed from the self-
assembly of aryl triazole amphiphiles is disrupted upon the
binding of a bromide anion due to the conformational changes
experienced by these receptors.
Biology, medicine, catalysis, or environmental sciences are dis-
ciplines in which anions play a pivotal role.1,2 The anionic nature
of many substrates engaged in natural processes prompts research
directly related to these charged species like fundamental non-
covalent binding studies,3 anion-assisted asymmetric catalysis4 or
design of new receptors.5 Most of the neutral hosts reported for
anion recognition—calixpyrroles,6 ureas7 or polypyrroles8—
utilize strong N–H groups to bind the anionic guest. However,
Fig. 1 Chemical structures of the aryl triazole receptors 1 and 2 and
schematic illustration of the supramolecular structures formed by their
self-assembly in polar acetonitrile.
receptors involving weaker neutral C–H groups are yet scarce.
1,4-Substituted 1,2,3-triazole-based macrocycles9 and foldamers10
elegantly exemplify the utilization of slightly polarized C–H
hydrogen bonding systems to bind halide anions.11 Many of these
anion receptors are endowed with hydrophilic chains that lend
them an amphiphilic character. These amphiphiles are able to
self-assemble into supramolecular structures whose shape, size
and function can be modulated by external factors.12,13
(Scheme S1, ESIw). The chemical structures of compounds 1 and
2 have been confirmed by NMR and FTIR spectroscopy and
MALDI-TOF spectrometry (see ESIw).
Single crystals suitable for X-ray diffraction were obtained by
slow evaporation of solution of 1 in acetonitrile.y The internal
dipole of the molecules is stabilized by a zig-zag ‘‘anti’’ confor-
mation of the 1,2,3-triazole moieties, these heteroaromatic rings
being coplanar to the central alkoxybenzene (Fig. 2a and Table
S1, ESIw).10c The planar geometry of 1 enhances the cofacial p–p
stacking with lateral offset (Fig. 2b). Additional intermolecular
interactions between the lateral biphenyl-triazole units with the
triethylene glycol (TEG) chains pointing outwards are also
responsible for the compact packing (Fig. 2c).
Herein, we take advantage of the conformational changes
observed in triazole-based foldamers10 upon halogen anion
complexation to disrupt the morphology of the supramolecular
structures formed by the self-assembly of aryl triazole amphiphiles
1 and 2 (Fig. 1). The chemical structure of the lateral aryl moiety
attached to the 1,2,3-triazole conditions the morphology of the
self-assembled aggregates, and flat lamellae are observed for 1
whilst compound 2 forms spheres. The bromide complexation
induces conformational changes in the aryl triazole receptors
breaking these supramolecular structures.
The self-assembly of triazoles 1 and 2 in solution and onto
surfaces has been investigated by concentration-dependent
1H NMR experiments of 1 in CDCl3 and by scanning electron
microscopy (SEM) imaging. The former studies feature slight
upfield shifts for all the aromatic resonances upon increasing
concentration which suggests the p–p stacking of the aromatic
moieties (Fig. S1, ESIw).15 Fitting the variation of the chemical
shift corresponding to Ha (see Fig. 1 for numbering) with
increasing concentration to the isodesmic or equal-K model sheds
Target aryl triazole oligomers 1 and 2 have been readily
prepared by utilizing a Cu(I)-catalyzed Huisgen 1,3-dipolar
cycloaddition of azides and alkynes14 starting from 3,5-dibromo-
phenol and [1,10-biphenyl]-4-amine or 1-naphthalen-amine in
only five synthetic steps with 25 and 89% yields, respectively
a
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas,
Universidad Complutense de Madrid, 28040 Madrid, Spain.
E-mail: lusamar@quim.ucm.es; Fax: +34 913944103
a low binding constant (Ka) of B8 M .
ꢀ1 9a,16 The scarce solubility
b
´
C. A. I. Difraccion de Rayos X, Facultad de Ciencias Quımicas,
Universidad Complutense de Madrid, 28040 Madrid, Spain
of these receptors in polar acetonitrile impeded the calculation of
the binding constant in these conditions. However, a clear
indication of the p-stacking of the reported aryl triazoles 1 and
2 in polar CD3CN can be extracted from NOE experiments
(Fig. S2 and S3, ESIw). The irradiation of the proton corres-
ponding to the 1,2,3-triazole ring (Hb in Fig. 1) in a
w Electronic supplementary information (ESI) available: Fig. S1–S11
and experimental section. CCDC 804817. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc05685e
z This article is part of a ChemComm ‘Supramolecular Chemistry’ web-
based themed issue marking the International Year of Chemistry 2011.
c
5016 Chem. Commun., 2011, 47, 5016–5018
This journal is The Royal Society of Chemistry 2011

Fig. 3 SEM images (298 K, 1 ꢁ 10ꢀ4 M in acetonitrile, glass
substrate) of flat lamellae formed from 1 (a) and spheres formed from
2 (b). The scale bar in the inset of (b) is 1 mm.
The ability of receptors 1 and 2 to bind anions has preliminary
been tested by the complexation of bromide and studied by
1H NMR titration experiments (Fig. S7 and S8, ESIw). To
eliminate the self-aggregation observed for aryl-triazoles 1 and 2,
all the titration experiments have been performed at 1 mM in
CDCl3. The addition of tetrabutylammonium bromide (TBABr)
to 1 deshields the resonances of the protons corresponding to the
1,2,3-triazoles and the central aromatic ring (Ha and Hb) and also
the proton para of the lateral biphenyl units, the resonance of
proton Hc being unaffected. These variations imply that five
C–HꢂꢂꢂBrꢀ H-bonds participate in the formation of the complex.
The changes in the chemical shift of Ha and Hb upon addition of
TBABr fit well with a 1 : 1 binding isotherm 18 affording binding
constants (Ka) of B15 Mꢀ1 (Fig. S7, ESIw). These values for Ka
are smaller to that previously reported for referable 1,2,3-triazole
receptors and can be justified by considering the lack of
preorganization.9,10c
Fig. 2 X-Ray crystal structure of 1: (a) top (left) and side (right)
views; (b) cofacial p–p stacking with lateral offset; (c) intermolecular
interactions between lateral biphenyl-triazole units (CCDC: 804817).w
concentrated solution of 1 in CDCl3 (114 mM) results
in a clear NOE effect with the two protons of the central
alkoxybenzene moiety (Ha and Hc) and one of the para
hydrogens of the biphenyl unit. These findings are diagnostic
of the zig-zag ‘‘anti’’ conformation of the 1,2,3-triazoles
determined by X-ray diffraction (Fig. S2, top, ESIw). However,
if Hb is irradiated in a more diluted sample (1 mM) in CDCl3,
the NOE effects are only visible for Hc and for the protons in
the biphenyl unit, the interaction with Ha being very weak
(Fig. S2, middle, ESIw). At this concentration, compound 1 is
molecularly dissolved and the arm with Hb pointing toward
Ha could be slightly rotated thus cancelling the NOE effect
between these two protons. The aggregation of receptor 1 in
CD3CN is clearly observed in the NOE experiment utilizing a
1 mM solution. In this experiment, irradiating Hb results again
in a NOE effect with Ha, Hc and the biphenyl moiety (Fig. S2,
bottom, ESIw). A similar behaviour has been observed in the
NOE experiments of receptor 2. Once again, for a concen-
trated solution of 2 in CDCl3 (100 mM) or diluted solution
(1 mM) in CD3CN, NOE effects are observed between protons
Hb and Ha, Hc and the naphthalene moiety, diagnostic of the
aggregation (Fig. S3, ESIw).
The supramolecular organization of receptor 1 into flat
lamellae has been visualized by SEM. SEM images of a
solution of this receptor in acetonitrile (1 ꢁ 10ꢀ4 M) on a
glass substrate exhibit stratified bidimensional lamellae that
form leaf-like structures (Fig. 3a and Fig. S4, ESIw).
The lack of planarity of the different aromatic fragments in
compound 2 impedes its self-assembly into flat lamellae, and
big spheres with diameters of B1.8 mm are observed by SEM
imaging upon deposition of 2 (1 ꢁ 10ꢀ4 M, acetonitrile) onto a
glass substrate (Fig. 3b and Fig. S5, ESIw). Although the
mechanism followed by compound 2 to self-associate into
spheres is unclear, we postulate that the formation of dimers
by CH–p H-bonds involving the lateral naphthalene units
could be a key step.17 The p–p stacking of the resulting dimers
finally would yield the spherical objects (Fig. S6, ESIw).
The spectral changes observed in the titration studies clearly
suggest the conformational switch of one of the aryl-triazole
fragments to render a ‘‘syn’’ conformation with protons Ha,
Hb, and Hp pointing to the inner cavity of the molecule and
H-bonded with the bromide anion (Fig. 4). The confor-
mational changes experienced by 1 upon bromide binding
are fairly detected by NOE experiments. Whilst in free 1, the
irradiation of the resonance corresponding to Hb (d E 8.35)
affects Hc and Hp, and Ha is only slightly affected, the addition
of TBABr changes the conformation of the receptor and a
clear connectivity of Hb with Ha is observed (Fig. 4).
The titration of 1,2,3-triazole 2 with increasing amount of
TBABr shows negligible changes in the chemical shift of any
of the protons forming the inner cavity of the receptor
(Ha, Hb, and Hb) (Fig. S8, ESIw) and would imply that
compound 2 is not able to complex the bromide anion. The
calculated geometry of 2 considering a zig-zag ‘‘anti’’ confor-
mation for the 1,2,3-triazole rings demonstrates that the
naphthyl units would be out of the plane formed by the two
1,2,3-triazoles and the central alkoxybenzene unit (Fig. S9,
ESIw). Taking into account this geometry, only three weak
C–Hꢂ ꢂ ꢂBrꢀ H-bonds, involving Ha and Hb, would participate
in the complexation of the bromide anion being insufficient to
form stable complexes. However, the complexation of the
bromide anion by receptor 2 has been detected by NOE
experiments. The irradiation of the triazole proton Hb of 2
in CDCl3 (1 mM) demonstrates the spatial connectivity of Hb
and Hc (at d E 7.63) and the lack of interaction with the
central proton of the alkoxybenzene (Ha) (Fig. S10, top,
ESIw). The addition of TBABr changes the NOE spectrum
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5016–5018 5017

reflections (Rint = 0.1344). The final R1 values were 0.0906 (I > 2s(I)).
The final wR(F2) values were 0.2322 (I > 2s(I)). The final R1 values
were 0.2415 (all data). The final wR(F2) values were 0.3279 (all data).
1 (a) Anion Receptor Chemistry, ed. J. L. Sessler, P. A. Gale and W.-S.
Cho, RSC Publishing, Cambridge, 2006; (b) Supramolecular Chemistry
´
of Anions, ed. A. Bianchi, K. Bowman-James and E. Garcıa-Espana,
Wiley-VCH, New York, 1997.
2 (a) Special Issue on Supramolecular Chemistry of Anionic Species,
Chem. Soc. Rev., 2010, 39, 3581; (b) P. A. Gale, Chem. Commun.,
2011, 47, 82.
3 (a) R. E. Dawson, A. Hennig, D. P. Weimann, D. Emery,
V. Ravikumar, J. Montenegro, T. Takeuchi, S. Gabutti,
M. Mayor, J. Mareda, C. A. Schalley and S. Matile, Nat. Chem.,
2010, 2, 533; (b) H. T. Chifotides, B. L. Schottel and K. R. Dunbar,
Angew. Chem., Int. Ed., 2010, 49, 7202.
4 (a) E. G. Klauber, C. Kanta De, T. K. Shah and D. Seidel, J. Am.
Chem. Soc., 2010, 132, 13624; (b) G. L. Hamilton, E. J. Kang,
M. Mba and F. D. Toste, Science, 2007, 317, 496.
5 (a) R. Custelcean, J. Bosano, P. V. Bonnesen, V. Kertesz and
B. P. Hay, Angew. Chem., Int. Ed., 2009, 48, 4025;
(b) C. Caltagirone and P. A. Gale, Chem. Soc. Rev., 2009, 38,
Fig. 4 Partial 1H NMR spectra (300 MHz, 298 K) (black) and NOE
experiments (red) of aryl triazole 1 in CDCl3 at 1 mM without (top)
and with TBABr (bottom). In the upper part of the spectra, the NOE
contacts are represented by curved arrows.
520; (c) D. Curiel, M. Mas-Montoya, G. Sanchez, R. A. Orenes,
´ ´
P. Molina and A. Tarraga, Org. Biomol. Chem., 2010, 8, 4811.
´
and the interaction between protons Hb and Ha is clearly
visible. This experiment also demonstrates the connectivity of
Hb with Hb (Fig. S10, bottom, ESIw) which indicates the slight
planarization of the whole molecule to generate an inner cavity
in which five C–H H-bonds complex the bromide anion
(Fig. S9, ESIw).
6 D. W. Yoon, D. E. Gross, V. M. Lynch, J. L. Sessler, B. P. Hay
and C. H. Lee, Angew. Chem., Int. Ed., 2008, 47, 5038.
7 (a) M. Boiocchi, L. Del Boca, D. E. Gomez, L. Fabbrizzi, M. Licchelli
and E. J. Monzani, J. Am. Chem. Soc., 2004, 126, 16507;
(b) S. J. Brooks, P. A. Gale and M. E. Light, Chem. Commun.,
2005, 4696; (c) X. He, F. Herranz, E. Chung-Chin Cheng, R. Vilar and
V. Wing-Wah Yam, Chem.–Eur. J., 2010, 16, 9123–9131.
The anion binding, even if weak, and the subsequent
conformational modifications experienced by receptors 1 and
2 trigger the change in the morphology of the supramolecular
structures formed upon aggregation of the resulting com-
plexes. The syn conformation adopted by the 1,2,3-triazole
moieties of both 1 and 2 upon bromide complexation in order
to maximize the number of H-bonds between the neutral C–H
groups and the bromide anion and the presence of the
tetrabutyl counterion impede the formation of organized
supramolecular structures.
8 (a) K. A. Nielsen, W. S. Cho, J. Lyskawa, E. Levillain,
V. M. Lynch, J. L. Sessler and J. O. Jeppesen, J. Am. Chem.
Soc., 2006, 128, 2444; (b) C. H. Lee, H. Miyaji, D. W. Yoon and
J. L. Sessler, Chem. Commun., 2008, 24.
9 (a) Y. Li and A. H. Flood, Angew. Chem., Int. Ed., 2008, 47, 2649;
(b) Y. Li and A. H. Flood, J. Am. Chem. Soc., 2008, 130, 12111;
(c) Y. Li, M. Pink, J. A. Karty and A. H. Flood, J. Am. Chem.
Soc., 2008, 130, 17293.
10 (a) R. M. Meudtner, M. Ostermeier, R. Goddard, C. Limberg and
S. Hecht, Chem.–Eur. J., 2007, 13, 9834; (b) H. Juwarker,
J. M. Lenhardt, D. M. Pham and S. L. Craig, Angew. Chem.,
Int. Ed., 2008, 47, 3740; (c) H. Juwarker, J. M. Lenhardt,
J. C. Castillo, E. Zhao, S. Krishnamurthy, R. M. Jamiolkowski,
K.-H. Kim and S. L. Craig, J. Org. Chem., 2009, 74, 8924;
(d) Y. Hua and A. H. Flood, J. Am. Chem. Soc., 2010, 132, 12838.
11 Y. Hua and A. H. Flood, Chem. Soc. Rev., 2010, 39, 1262.
In summary, we report on the synthesis, self-assembly and
bromide anion binding of two different amphiphilic aryl triazole
receptors. The geometry of compound 1—decorated with lateral
biphenyl moieties—is highly planar with a syn conformation of
the 1,2,3-triazole rings as demonstrated by the X-ray data and
self-assembles into flat lamellae. In contrast, compound 2
possesses lateral naphthalene units responsible for a more
distorted geometry that finally results in the formation of
spherical objects upon self-assembly. The addition of a bromide
anion to these two receptors induces the formation of slightly
polarized C–H hydrogen bonds between the aromatic moieties
and the anion. The formation of this H-bonding array implies a
U-like conformation of the receptors. The conformational
changes produced by the complexation of bromide disrupt the
apparition of organized supramolecular structures.
12 (a) G. Ferna
2008, 6567; (b) F. Garcı
Chem.–Eur. J., 2009, 15, 6740.
´
ndez, F. Garcı
´
a and L. Sa
´
nchez, Chem. Commun.,
´
ndez and L. Sanchez,
´
a, G. Ferna
´
13 (a) A. Ajayaghosh, P. Chithra and R. Varghese, Angew. Chem.,
Int. Ed., 2007, 46, 230; (b) H.-J. Kim, S.-K. Kang, Y.-K. Lee,
C. Seok, J.-K. Lee, W.-C. Zin and M. Lee, Angew. Chem., Int. Ed.,
2010, 49, 8471.
14 V. V. Rostovtsev, L. G. Green, V. V. Fokin and K. B. Sharpless,
Angew. Chem., Int. Ed., 2002, 41, 2596.
15 The behaviour has been observed for a number of aromatic
systems, see: (a) C. A. Hunter, K. R. Lawson, J. Perkins and
C. J. Urch, J. Chem. Soc., Perkin Trans. 2, 2001, 651; (b) F. Garcı
F. Aparicio, G. Fernandez and L. Sanchez, Org. Lett., 2009, 11,
2748; (c) G. Fernandez, F. Garcıa, F. Aparicio, E. Matesanz and
L. Sanchez, Chem. Commun., 2009, 7155; (d) F. Garcıa and
L. Sanchez, Chem.–Eur. J., 2010, 16, 3138.
16 (a) R. B. Martin, Chem. Rev., 1996, 96, 3043; (b) Z. Chen, A. Lohr,
C. R. Saha-Moller and F. Wurthner, Chem. Soc. Rev., 2009, 38,
´
a,
´
´
´
´
´
´
´
This work has been supported by UCM (UCM-SCH-PR34/
07-15826). F. G. is indebted to MEC for a FPU studentship.
¨
¨
564; (c) T. F. A. De Greef, M. M. J. Smulders, M. Wolffs,
A. P. H. J. Schenning, R. P. Sijbesma and E. W. Meijer, Chem.
Rev., 2009, 109, 5687.
The authors are gratefully acknowledged to Prof. N. Martı
´
n
for his generous help.
17 The interaction of referable a-substituted naphthalene derivatives
by means of weak CH–p H-bonds has been reported. For a very
recent example, see: M. Morimoto and M. Irie, J. Am. Chem. Soc.,
2010, 132, 14172.
Notes and references
y Crystal data for 1: C41H38N6O4,
M
=
678.77, monoclinic,
a = 33.436(4) A, b = 5.8453(6) A, c = 17.8776(19) A, a = 90.001,
b = 96.269(2)1, g = 90.001, V = 3473.2(6) A3, T = 293(2) K, space
group P2(1)/c, Z = 4, 25 183 reflections measured, 6101 independent
18 (a) Binding Constants: The Measurements of Molecular Complex
Stability, ed. K. A. Connors, Wiley, New York, 1987;
(b) P. Thodarson, Chem. Soc. Rev., 2011, 40, 1305–1323.
c
5018 Chem. Commun., 2011, 47, 5016–5018
This journal is The Royal Society of Chemistry 2011