Self-sorting self-complementary assemblies of π-conjugated acyclic anion receptors

Article abstract of DOI:10.1039/c1cc12120k

Pyrrole-based π-conjugated anion-responsive molecules bearing an anionic moiety form self-complementary dimers, which exhibit self-sorting behaviours depending on the substituted positions of anionic sites.

Full text of DOI:10.1039/c1cc12120k

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COMMUNICATION
Self-sorting self-complementary assemblies of p-conjugated acyclic anion
receptorsw
Hiromitsu Maeda,* Kota Kinoshita, Kazumasa Naritani and Yuya Bando
Received 13th April 2011, Accepted 6th June 2011
DOI: 10.1039/c1cc12120k
Pyrrole-based p-conjugated anion-responsive molecules bearing
an anionic moiety form self-complementary dimers, which
exhibit self-sorting behaviours depending on the substituted
positions of anionic sites.
Self-complementary assembly is an essential process for
constructing biotic and artificial functional nanoscale
architectures.1 Among all association protocols, anion binding
Fig. 1 (a) Acyclic anion receptors 1–3 and (b) anion-binding mode of 1b.
is effective for achieving the formation of molecular assemblies
Therefore, receptors bearing an anionic site would form anion-
binding self-assemblies, which would provide useful oligo-
meric and macromolecular structures. This communication
describes the formation of anion-binding self-assembled
dimers that exhibit narcissistic self-sorting behaviours.7
by anion-driven electrostatic hydrogen bonding, and multiple
building units can be connected in a fairly compact manner
with the help of a small number of noncovalent bonds.2
Covalent linkages of anionic species with appropriate electro-
nically neutral receptor molecules can facilitate the formation
of complementary stimuli-responsive self-assembled dimers
and oligomers. Such assemblies possess multiple negative
charges, which may develop into various organized structures
by combining with appropriate counter cations. However,
very few examples of anion-mediated self-assemblies based
on electroneutral receptor units have been reported thus far.
One example reported by Sessler et al. was the formation of
self-assembled dimers comprising the calix[4]pyrrole bearing a
carboxylate moiety in solution and in the solid state.3 It is
more difficult to prepare self-assemblies of electroneutral
receptor units possessing anionic sites than those of
zwitterions,4 presumably because of the electrostatic repulsion
exhibited by the former self-assemblies and the absence
of suitable anion receptors. As compared to cyclic anion
receptors, appropriately designed open-chain structures seem
more capable of recognizing themselves as compared to the
other units, mainly owing to the geometrical restrictions of
cyclic structures. As acyclic p-conjugated anion-responsive
molecules, dipyrrolyldiketone boron complexes (e.g., 1a–c,
Fig. 1a), which form planar [1+1]-type receptor–anion
complexes (Fig. 1b), serve as essential building blocks of
anion-responsive supramolecular assemblies and oligomers.5,6
We have reported the efficient binding behaviours of a series of
dipyrrolyldiketone boron complexes with acetate as one of the
representative carboxylate anions.5,6 The binding constants for
acetate are higher than those for Clꢀ and Brꢀ, presumably
because of the basicity of the anion and the multiple hydrogen
bonding interactions between the pyrrole NH and acetate
oxygen. The binding modes of anion receptors for carboxylates
have been investigated in detail by 1H NMR and theoretical
studies. As a result, we can now elucidate the exact structure of a
carboxylate-binding complex by introducing a benzoate instead
of an acetate. Single-crystal X-ray analysis of 1aꢁbenzoate as a
tetrapropylammonium (TPA) salt shows that it provides a
[1+1]-type binding mode (Fig. 2),z which is formed through
N–HꢁꢁꢁO and C–HꢁꢁꢁO hydrogen bonding: the N(–H)ꢁꢁꢁO and
C(–H)ꢁꢁꢁO distances are 2.704/2.733 and 3.329 A, respectively.
The benzoate is associated with 1a in an unsymmetrical fashion,
as observed from 163.091 of bridging-C–carboxylate-C–ipso-C;
this is a substantial aberration from 1801 that we expect in a
College of Pharmaceutical Sciences, Institute of Science and
Engineering, Ritsumeikan University, Kusatsu 525–8577, Japan.
E-mail: maedahir@ph.ritsumei.ac.jp; Fax: +81-77-561-2659
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data, self-assembled behaviours and X-ray
structural analysis of 1aꢁTPA benzoate. CCDC 809340. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc12120k
Fig. 2 Single-crystal X-ray structure of 1aꢁbenzoate as a TPA salt:
(a) top and (b) side view. The TPA cation is omitted in (a). Atom
colour code: brown, pink, yellow, blue and red indicate carbon,
hydrogen, boron, nitrogen and oxygen, respectively.
c
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Chem. Commun., 2011, 47, 8241–8243 8241

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symmetrical binding arrangement. Further, the dihedral angle
between the receptor plane consisting of 16 atoms and the
benzoate plane including the carboxylate unit is estimated to
be 30.831, suggesting that the two p-planes are not arranged in a
coplanar fashion.
On the basis of the above observation, meta- and para-
carboxylic-acid-substituted receptors 2m and 2p (Fig. 1a) were
synthesized from the mixture of two arylpyrroles according to the
procedures described in the reference literature.6a During
the preparation of carboxylate-appended anion receptors, the
treatment of 2m and 2p with TBAOH afforded the corresponding
TBA salts of the carboxylates 2mꢀ and 2pꢀ after appropriate
purification.
Dipyrrolyldiketone boron complexes efficiently bind carboxylate
in solution. The binding constants (Ka) of reference molecules—
carboxylic-acid-free 1d and ester-substituted 2me and 2pe
(Fig. 1a)—for acetate as a tetrabutylammonium (TBA) salt in
CH2Cl2 were estimated to be 47 000 (ꢂ10 000), 2 600 000
(ꢂ1 200 000) and 520 000 (ꢂ42 000) Mꢀ1, respectively, on the basis
of UV/vis absorption spectral changes. The difference between the
Ka values of 2me and 2pe may be derived from the electron-
withdrawing effects depending on the substituted positions.
The formation of self-assembled dimers in solution was initially
revealed by negative-mode ESI-TOF-MS in CH3CN, showing
the peaks of the dianion species as dimers 2mꢀ and 2pꢀ and
2
2
excluding the formation of higher discrete oligomers. Self-
assembled dimers 2mꢀ and 2pꢀ as TBA salts (5 ꢃ 10ꢀ4 M as
2
2
monomers; the following values of concentrations were also for
monomers) were also observed in the 1H NMR spectra in CDCl3
at 20–50 1C, wherein monomeric 2mꢀ and 2pꢀ were not observed
(Fig. 3). The fairly low solubilities of 2mꢀꢁTBA and 2pꢀꢁTBA
below 10 1C interfered with the measurements recorded at low
temperatures. For example, at 20 1C, the signals of NH, bridging
CH and o-CH of 2pꢀ were observed at 14.4/12.5, 8.9 and
2
7.3 ppm, respectively, which were in the downfield region as
compared to those of 2p (9.7/9.6, 6.5 and 6.8 ppm, respectively)
owing to the hydrogen bonding with a carboxylate unit (Fig. 3i
and ii). In the diluted solution of 2mꢀ2 in CDCl3 (5 ꢃ 10ꢀ5 M) at
30 1C, equilibrium was observed between the monomer and the
dimer, and the dimerization constant (Kdim) was estimated to be
about 104 Mꢀ1. On the other hand, under the same conditions, no
monomeric state 2pꢀ was observed owing to a much higher
Fig. 3 (a) 1H NMR spectra of (i) 2m in CDCl3 at 20 1C, (ii) 2mꢀ2 in
CDCl3 at 20 1C, with the scheme showing the equilibrium between
2mꢀ and 2mꢀ2, (iii) 2mꢀ in CD3OD at 50 1C and (iv) 2mꢀ and 2mꢀ2 in
equilibrium in 2% CD3OD/CDCl3 at 20 1C; (b) 1H NMR spectra of (i)
K
dim value of 4109 Mꢀ1 for 2pꢀ
.
2
2p in CDCl3 at 20 1C, (ii) 2pꢀ in CDCl3 at 20 1C, with the scheme
1H NMR DOSY (1 ꢃ 10ꢀ3 M, 20 1C) was used to obtain the
2
showing the equilibrium between 2pꢀ and 2pꢀ2, (iii) 2pꢀ in CD3OD at
diffusion constants (10ꢀ10 m2 sꢀ1) of 3.3, 4.4 and 4.4 for 2pꢀ2, 2pe
and 1d, respectively; these values were consistent with those
observed during dimer formation in the anionic state. From the
UV/vis absorption spectra for 2pꢀ2 at 1 ꢃ 10ꢀ5 M, we speculated
that the self-assembly existed as a major species. The fluorescence
emission for 2pꢀ2 excited at 513 nm as an absorption maxima was
observed at 558 nm. DFT calculations of 2mꢀ and 2pꢀ
50 1C and (iv) 2pꢀ and 2pꢀ in equilibrium in 2% CD3OD/CDCl3 at
2
20 1C. The concentration of monomers in each case was 5 ꢃ 10ꢀ4 M.
In (a(iv)) and (b(iv)), the signals of hydrogen-bonding NH in the down-
field region disappear on account of the exchanges between H and D.
dimer of 2pꢀ2, the dimer 2mꢀ showed a distorted butterfly-like
2
structure. The DFT-based relative stability of 2pꢀ2, as compared
to 2mꢀ2, was 2.36 kcal molꢀ1, which was consistent with the value
estimated from the dimerization constants, as discussed above.
Further, in acetone-d6 (5 ꢃ 10ꢀ4 M) at 20 1C, only the
dimers of 2mꢀ and 2pꢀ were stabilized, whereas, in CD3OD
2
2
(as alkoxy-free forms) at the B3LYP/6-31G(d,p) level8 indicated
the formation of complementary self-assembled dimer structures
(Fig. 4), wherein the planes of the carboxylate moieties of 2mꢀ
2
and 2pꢀ were tilted at angles of 42.81 and 26.71 with respect to
2
2
2
the core receptor plane, respectively. These two values were
respectively larger and slightly smaller than the X-ray-based
dihedral angle of 31.441 between the receptor 1a and the benzoate
carboxylate moiety. As compared to the fairly planar plate-like
(5 ꢃ 10ꢀ4 M) at 20–50 1C, only monomeric 2mꢀ and 2pꢀ were
observed (Fig. 3iii). These results suggest that the protic solvents
interfered with dimerization because of their hydrogen-bonding
solvation at the carboxylate unit of 2mꢀ and 2pꢀ and that they
c
8242 Chem. Commun., 2011, 47, 8241–8243
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more appropriate hydrogen-bonding direction and less sterical
hindrance in 2pꢀ2, as estimated by DFT calculations for alkoxy-
free forms. In other words, the self-sorting phenomenon discussed
in this report occurred mainly because of the difference in the self-
dimerization ability of the anion-appended receptor molecules.
In summary, we have found carboxylate-appended pyrrole-
based p-conjugated anion-responsive molecules that form self-
assembled dimers and exhibit self-sorting behaviours depending
on the substituted positions of anionic sites. Such assemblies are
constructed on the basis of the binding abilities of the anion
receptor moieties, even though efficient anion binding is restricted
in less polar solvents. Anion-binding self-assembly can be applied
to supramolecular polymers and various macromolecular systems
such as thermotropic liquid crystals, which were preliminarily
observed. Relevant investigations are currently underway.
Fig. 4 Optimized structures (top and side views) of (a) 2mꢀ and
2
(b) 2pꢀ at the B3LYP/6-31G(d,p) level. Hexadecyloxy substituents
2
are omitted for concise calculations. Atom colour code: gray, white,
pink, yellow-green, blue and red represent carbon, hydrogen, boron,
fluorine, nitrogen and oxygen, respectively.
can be used as chemical stimuli to induce the dissociation of self-
assembled dimers. In 2% CD3OD/CDCl3 (1 ꢃ 10ꢀ3 M), the
This work was supported by PRESTO/JST (2007–2011),
Grants-in-Aid for Young Scientists (B) (No. 21750155) and (A)
(No. 23685032) from MEXT and the Ritsumeikan R-GIRO
project (2008–2013). The authors thank Prof. Atsuhiro Osuka
and Mr Tomohiro Higashino, Kyoto University, for the single-
crystal X-ray analysis, Prof. Hiroshi Shinokubo and Dr Satoru
Hiroto, Nagoya University, and Dr Naoki Aratani, Kyoto
University, for their help with ESI-MS and Prof. Hitoshi
Tamiaki, Ritsumeikan University, for various measurements.
equilibrium between 2mꢀ and 2mꢀ and that between 2pꢀ and
2
2pꢀ2 were observed in the 1H NMR spectra at 20–40 1C (Fig. 3iv).
Van’t Hoff plots were obtained using the following Kdim values
(103 Mꢀ1) of 2mꢀ and 2pꢀ: 0.53 and 1.7 (at 20 1C), 3.2 and 13 (at
30 1C) and 55 and 210 (at 40 1C), respectively. The plots provided
the thermodynamic parameters DH0 = 17 and 17 kJ molꢀ1 and
DS0 = 0.65 and 0.68 kJ molꢀ1
K
ꢀ1, respectively. For both the
receptors, the positive DH0 values suggest the occurrence of an
endothermic process, owing to the more favourable solvation of
methanol for carboxylate moieties than for hydrogen bonding
between the carboxylate and the receptor binding sites. However,
the positive DS0 values were derived from the release of CD3OD
molecules that were entropically predominant over the self-
dimerization.
Notes and references
z Crystal data for 1aꢁbenzoateꢁTPA (from EtOAc/i-propyl ether):
C30H42N3O4BF2, Mw = 557.48, monoclinic, P21/a (no. 14), a =
18.0991(3), b = 10.0405(2), c = 18.2608(3) A, b = 113.7402(10)1,
V = 3037.62(9) A3, T = 93(2) K, Z = 4, Dc = 1.219 g cmꢀ3
,
m(Cu-Ka) = 0.724 mmꢀ1, 29 033 reflections measured, 5521 unique
(Rint = 0.0658), R1 = 0.0720, wR2 = 0.1867, GOF = 1.219 (I 4 2s(I)
for R1, wR2 and GOF). CCDC 809340.
Fascinatingly, the narcissistic self-sorting behaviour7 of the self-
assembled dimers was observed in CDCl3 by mixing equivalent
amounts of 2mꢀ2 and 2pꢀ2 (Fig. 5). There were no signals derived
1 Selected books for supramolecular assemblies: (a) Supra-
molecular Polymers, ed. A. Ciferri, Marcel Dekker, New York, Basel,
2000; (b) G. A. Ozin and A. C. Arsenault, Nanochemistry: A Chemical
Approach to Nanomaterials, RSC, Cambridge, 2005; (c) Supramolecular
from 2mꢀꢁ2pꢀ except the independent signals of 2mꢀ2 and 2pꢀ
.
2
Dodecyloxy-substituted 3pꢀ was used with a negative-mode
ESI-TOF-MS in CH3CN, which elucidated the major peaks of
2mꢀ and 3pꢀ along with a smaller peak corresponding to
Dye Chemistry, Topics in Current Chemistry, ed. F. Wurthner, Springer-
Verlag, Berlin, 2005, vol. 258, p. 324.
¨
2
2
2 Selected books for anion binding: (a) Anion Sensing, ed. I. Stibor,
Topics in Current Chemistry, Springer-Verlag, Berlin, 2005, vol. 255,
pp. 238; (b) J. L. Sessler, P. A. Gale and W.-S. Cho, Anion Receptor
Chemistry, RSC, Cambridge, 2006; (c) in Recognition of Anions,
ed. R. Vilar, Structure and Bonding, Springer-Verlag, Berlin, 2008.
3 J. L. Sessler, A. Andrievsky, P. A. Gale and V. Lynch, Angew.
Chem., Int. Ed. Engl., 1996, 35, 2782.
2mꢀꢁ3pꢀ. These results suggest that each carboxylate-substituted
anion receptor seemed to bind itself instead of the other receptor,
depending on the positions of anionic moieties. This is because of
the preferential formation of the relatively stable 2pꢀ2 over 2mꢀꢁ
2pꢀ (+2.26 kcal molꢀ1) and 2mꢀ2, presumably owing to a
4 Self-assemblies using ion-pair-induced anion binding were reported:
T. H. Rehm and C. Schmuck, Chem. Soc. Rev., 2010, 39, 3597.
5 As a selected review: H. Maeda, in Anion Complexation in Supra-
molecular Chemistry, Topics in Heterocyclic Chemistry, ed. P. A. Gale
and W. Dehaen, Springer-Verlag, Berlin, 2010, vol. 24, pp. 103.
6 (a) H. Maeda, Y. Haketa and T. Nakanishi, J. Am. Chem. Soc., 2007,
129, 13661; (b) H. Maeda, Y. Terashima, Y. Haketa, A. Asano,
Y. Honsho, S. Seki, M. Shimizu, H. Mukai and K. Ohta, Chem.
Commun., 2010, 46, 4559; (c) Y. Haketa, S. Sasaki, N. Ohta,
H. Masunaga, H. Ogawa, N. Mizuno, F. Araoka, H. Takezoe and
H. Maeda, Angew. Chem., Int. Ed., 2010, 49, 10079; (d) Y. Haketa and
H. Maeda, Chem.–Eur. J., 2011, 17, 1485.
7 As some representative examples of narcissistic self-sorting:
(a) K. Roland, J.-M. Lehn and A. Marquis-Rigault, Proc. Natl. Acad.
Sci. U. S. A., 1993, 90, 5394; (b) D. L. Caulder and K. N. Raymond,
Angew. Chem., Int. Ed. Engl., 1997, 36, 1439; (c) L. Avram and
Y. Cohen, J. Am. Chem. Soc., 2004, 126, 11556; (d) E. S. Barrett,
T. J. Dale and J. Rebek, Jr., J. Am. Chem. Soc., 2008, 130, 2344.
8 M. J. Frisch, et al., Gaussian 03, Revision C.01, Gaussian, Inc,
Wallingford CT, 2004.
Fig. 5 1H NMR spectra of 2mꢀ2 (red, top), 2pꢀ (blue, bottom) and
2
the mixture of equivalent amounts of 2mꢀ and 2pꢀ (middle) in
2
2
CDCl3 (1 ꢃ 10ꢀ3 M for each species, 20 1C).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8241–8243 8243