Neutral bis(benzimidazole) Λ-shaped anion receptor

Article abstract of DOI:10.1016/j.tetlet.2015.09.075

A neutral receptor, integrating a series of -N(H) and -C(H) donor groups organized in a Λ-shape motif was designed and prepared. This compound can form stable 1:1 complexes in acetonitrile solution with halides (F^-, Cl^-, Br^-, and I^-) and nitrate (NO₃^-) anions. The main binding site in the receptor is on the cleft and recognition occurs through a series of co-operative hydrogen bonding interactions including intramolecular hydrogen bonds which stabilizes the co-planar conformation of the host-guest complex. For halides, complex stability is determined by anion basicity, but for anions having other geometries, size and shape complementarity play a major role.

Full text of DOI:10.1016/j.tetlet.2015.09.075

Tetrahedron Letters 56 (2015) 6177–6182
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Neutral bis(benzimidazole)
K
-shaped anion receptor
⇑
Claudia Montoya, Ruy Cervantes, Jorge Tiburcio
Departamento de Química, Centro de Investigación y de Estudios Avanzados (Cinvestav), Avenida IPN 2508, Zacatenco, 07360 México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
A neutral receptor, integrating a series of –N(H) and –C(H) donor groups organized in a
K-shape motif
Received 15 September 2015
Accepted 17 September 2015
Available online 25 September 2015
was designed and prepared. This compound can form stable 1:1 complexes in acetonitrile solution with
halides (Fꢀ, Clꢀ, Brꢀ, and Iꢀ) and nitrate (NOꢀ3 ) anions. The main binding site in the receptor is on the cleft
and recognition occurs through a series of co-operative hydrogen bonding interactions including
intramolecular hydrogen bonds which stabilizes the co-planar conformation of the host–guest complex.
For halides, complex stability is determined by anion basicity, but for anions having other geometries,
size and shape complementarity play a major role.
Keywords:
Anion receptor
Host–guest complexes
Benzimidazoles
Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
As part of our ongoing research on the self-assembly of
supramolecular complexes based on benzimidazolium deriva-
The detection and removal of certain anions, such as fluoride
and nitrate, are issues of relevance in public health and environ-
mental quality. For this purpose, it is important to properly design
and synthesize versatile receptors. In synthetic anion receptor
chemistry, the molecular design of the host must take into consid-
eration: (i) basicity of the anionic guests, (ii) nature of the non-co-
valent interactions involved, (iii) solvent, and (iv) host–guest
geometric complementarity.1 According to their electrostatic
charge, two different kinds of anion receptors have been devel-
oped: cationic and neutral. For the latter, the most common motifs
incorporated are: ureas,2 thioureas,3 amides,4 squaramides,5
pyrroles,6 indoles,7and lately, –C(H) hydrogen bond donors.8
Particularly, imidazole- and benzimidazole-based receptors
interact with anions predominantly through –N(H)ꢁ ꢁ ꢁXꢀ hydrogen
bonding. Baitalik9 and Ye10 reported that ruthenium(II) complexes
of 4,5-bis(benzimidazol-2-yl)imidazole and 2,20-biimidazole form
hydrogen bonded adducts with a variety of anions in acetonitrile
with association constants ranging from 104 to 106 Mꢀ1; especially,
the interaction with fluoride and acetate develops intense colors,
naked eye visible, due to a stepwise deprotonation of the –N(H)
fragments. On the other hand, Gale and co-workers11 have shown
that tautomeric switching in benzimidazole-based receptors in
DMSO-aqueous solution can modulate their affinity for certain
anionic guests via hydrogen bond interactions, making them selec-
tive for a specific anion; however, for the monoatomic anionic
guests only weak binding is observed.
tives,12 we have designed and synthesized a neutral 1,3-bis(benz-
imidazole)benzene anion receptor, BBB. The 1,3-substitution
pattern would provide two strong –N(H) hydrogen bonds donors
and an aryl –C(H) bond, in a
K-shaped arrangement, allowing
the interaction of this receptor with diverse anions, as it has previ-
ously been observed for similar-shaped complexes.4c,13
The expected most-stable molecular arrangement of BBB would
be a non-planar structure, with the central aromatic ring on a dif-
ferent plane respect to the benzimidazole rings. In the presence of
anions, a co-planar conformation is anticipated containing three
hydrogen bond donors converging into a cleft and two intramolec-
ular hydrogen bonds assisting the complexation (Scheme 1). This
host–guest geometrical arrangement, which can establish the
maximum number of non-covalent interactions and displays a
suitable complementary, is likely to be the most stable.
In this Letter we report the synthesis and characterization of a
neutral bis(benzimidazole)benzene receptor with –N(H) and –C
(H) donor motifs and its association in acetonitrile with diverse
anions such as halides, nitrate, and perchlorate.
Results and discussion
Compound 1,3-bis(1H-benzo[d]imidazol-2-yl)benzene, BBB,
was synthesized by a condensation reaction between isophthalic
acid and two equivalents of 1,2-diaminobenzene under high tem-
perature and acidic conditions (Scheme 2). This compound was
isolated in a 57% yield and is slightly soluble in acetonitrile and sol-
uble in dimethylsulfoxide; it was fully characterized by 1H and 13
NMR spectroscopy as well as HR-MS (ESI-TOF).
C
⇑
Corresponding author. Tel.: +52 55 57 47 3721; fax: +52 55 57 47 3389.
E-mail address: jtiburcio@cinvestav.mx (J. Tiburcio).
http://dx.doi.org/10.1016/j.tetlet.2015.09.075
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

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C. Montoya et al. / Tetrahedron Letters 56 (2015) 6177–6182
Scheme 1. Binding motif for a hypothetical anion (Xꢀ) in the cleft of a neutral receptor BBB.
Upon saturation of the corresponding halide (Clꢀ, Brꢀ, and Iꢀ),
protons Hf and Hd of receptor as well as the –N(Hc) protons, are
shifted toward higher frequencies, characteristic of hydrogen
bonding interactions: [BBBꢁClꢀ] (ppm) Hf
Dd = +0.95, Hd Dd =
D
+0.16; [BBBꢁBrꢀ] (ppm): Hf
D
d = +0.83, Hd
d = +0.17; [BBBꢁIꢀ]
(ppm): Hf Dd = +0.40, Hd Dd = +0.16. As a representative example,
Figure 2 shows a stacked plot of the titration of BBB with TBABr.
There is a direct correlation between the maximum chemical
shift of proton Hf and the halide basicity; it is known that halides
show a similar trend in their basicity in acetonitrile with respect
to their values in aqueous solutions.14 This behavior is indicative
of a direct interaction between this proton and the anion. Further-
more, proton Hd preserves the same chemical shift regardless of
the halide used in the titration experiments; this result is probably
due to an intramolecular hydrogen bond between proton Hd and
the lone pair of the benzimidazole nitrogen atoms. This particular
interaction would be a consequence of a co-planar arrangement of
the aromatic rings in the receptor, induced by the presence of the
anion inside the cleft.
In order to support the proposed host–guest geometrical
arrangement, a series of DFT calculations were performed using
the Spartan ’08 software package15 (see Supplementary data).
Equilibrium geometries for neutral receptor BBB and the 1:1 com-
plex with chloride [BBBꢁClꢀ] are shown in Figure 3. Geometrical
optimization analysis for BBB renders a semi-planar molecular
arrangement with both benzimidazole rings in an anti-conforma-
tion (the four dihedral angles concerning the central aromatic rings
and the two benzimidazole substituents span from 4° to 6°),
whereas for complex [BBBꢁClꢀ] the anion occupies the cleft formed
by two –N(H) (3.26 Å; 178.6°) and one aryl –C(H) (3.49 Å; 178.6°)
hydrogen bonds; with the rings in a co-planar conformation
Scheme 2. Synthetic procedure for the preparation of receptor BBB and labeling
scheme. Reagents and conditions: (i) polyphosphoric acid, 190 °C, 24 h; (ii) NaOH
(aq), pH = 7.
The 1H NMR spectrum of compound BBB in DMSO-d6 showed
the expected resonances for the proposed molecular structure: (i)
a single high frequency signal (13.1 ppm) attributed to the protons
–N(H) of the imidazole rings, (ii) an AB2C pattern for the aromatic
protons of the 1,3-disubstituted benzene, and (iii) an AA0BB0 pat-
tern for the aromatic protons of the benzimidazole rings. In addi-
tion, the 13C NMR spectrum displayed the expected eleven
signals in the aromatic region. The identity of all signals was con-
firmed by NMR 2D experiments (see Supplementary data). Exper-
imental high-resolution mass spectra (ESI-TOF) showed a peak at
m/z = 311.1294, which corresponds to the expected molecular
ion, displaying an isotopic pattern consistent with the proposed
formula and with a relative error in the molecular weight of
1 ppm (see Supplementary data).
The neutral BBB bis(benzimidazole)benzene compound should
act as an anion receptor by combining two strong –N(H) donor
groups with a weaker aryl –C(H) donor motif. The orientation of
the three hydrogen bonds in a convergent form should optimize
the interactions with specific anions. The selected anions have
diverse shapes and basicity, and include (i) halides, with a spheri-
cal shape, (ii) trigonal planar nitrate, and (iii) perchlorate as a tetra-
hedral oxoanion.
We performed a series of 1H NMR titrations in acetonitrile
between neutral receptor BBB with tetrabutylammonium (TBA)
salts of several halides (Fꢀ, Clꢀ, Brꢀ, and Iꢀ). Experimental results
of the interaction with fluoride will be discussed in the following
section due to its particular behavior. The consecutive addition of
a concentrated acetonitrile solution, 0.1 M, of TBA halide salts to
a 2 ꢂ 10ꢀ3 M solution of neutral receptor BBB in acetonitrile-d3
led to gradual changes in the chemical shifts and widths of the
receptor proton resonances (Fig. 1).
Figure 1. Change in chemical shift of proton Hf for the 1H NMR titration of neutral
compound BBB (2 ꢂ 10ꢀ3 M) with the stepwise addition of tetrabutylammonium
salts (0.1 M) of chloride (d), bromide (j) and iodide (N) in an acetonitrile-d3
solution.

C. Montoya et al. / Tetrahedron Letters 56 (2015) 6177–6182
6179
Figure 2. 1H NMR titration stacked plot of receptor BBB (2 ꢂ 10ꢀ3 M) on stepwise addition of TBABr (0.1 M) (300 MHz, acetonitrile-d3).
Figure 3. DFT (B3LYP/6-31G⁄) equilibrium geometry calculated for (A) BBB and (B) [BBBꢁClꢀ].
(dihedral angles are less than 1°) allowing the formation of hydrogen
bond interactions between the lone pair of the benzimidazole
nitrogen atoms and the Hd atoms of the central aromatic ring.
A similar calculation was performed for the chloride complex
with 1,3-bis(1H-benzo[d]indole-2-yl)benzene, an isoelectronic
compound of BBB. This particular indole cannot form the
intramolecular hydrogen bonds, as in the case of the imidazole
complex; therefore the resulting equilibrium geometry displays
well as the proposed co-planar arrangement for the supramolecu-
lar complexes [BBBꢁX] (X = Clꢀ, Brꢀ, Iꢀ) in acetonitrile solution.
Association constants were derived from 1H NMR titration data
at 25 °C, in acetonitrile-d3, by a least-square fitting analysis of the
chemical shifts of protons Hf and Hd.16 Experimental data were fit-
ted by using a 1:1 receptor to anion stoichiometry model; the use
of any other model rendered poor correlation (see Supplementary
data). There is no significant variation in the magnitude of the
association constants regardless the proton used in the analysis,
in agreement with the existence of a single binding mode in
a
non-planar arrangement (dihedral angles over 13°) (see
Supplementary data). These results support the experimental
chemical shifts for protons Hf and Hd in 1H NMR titration data as
solution. Association constants values span from 1.1 ꢂ 103 Mꢀ1
,

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C. Montoya et al. / Tetrahedron Letters 56 (2015) 6177–6182
for the most basic anion chloride, to 4.6 ꢂ 101 Mꢀ1, for the least
basic anion iodide; indicating a lower stability of the complex as
the basicity of the halide decreases. A summary of the binding con-
stants is shown in Table 1.
Fluoride ion is the strongest base among the halides, it is well
established that compounds containing –N(H) or –O(H) hydrogen
bond groups can undergo a deprotonation process upon addition
of fluoride.17 In our case, 1H NMR titration of a 2 ꢂ 10ꢀ3 M solution
of neutral compound BBB with a concentrated acetonitrile solu-
tion, 0.1 M, of TBA fluoride salt in acetonitrile-d3 suggests that
the interaction of the receptor with fluoride takes place in two
steps. Upon addition of the first equivalent of fluoride, a shift
toward higher frequency of protons Hf and Hd is observed (Hf
D
d = +0.27, Hd Dd = +0.19) indicative of hydrogen bonding between
receptor BBB and the fluoride anion, similar to the other halides.
Further addition of fluoride, induces a trend change of the chemical
shift to lower frequencies (Hf Dd = ꢀ0.12, Hd Dd = ꢀ0.13) probably
due to deprotonation process of benzimidazole –N(H) groups.
Figure 4 shows the behavior of proton Hf during titration with TBAF.
We propose that the first added equivalent of fluoride associ-
ates with the neutral receptor BBB by hydrogen bonding interac-
tions, shifting proton Hf toward higher frequency; it is reasonable
to assume that the association constant of the 1:1 complex should
be at least ꢃ104 Mꢀ1, which is the maximum value that can be reli-
ably measured by 1H NMR spectroscopic titration.18 Subsequent
addition of fluoride deprotonates the benzimidazole –N(H) groups
with the formation of monoanionic [BBBꢀ] and dianionic [BBB2ꢀ
]
species and the very stable [HFꢀ2 ] self-complex.19 The observed
low frequency shifts for protons Hf and Hd, after one equivalent
of fluoride, could be due to an increase in the chemical shielding
as a consequence of the presence of formal negative charges in
the anionic receptor-based species.
High resolution mass spectrum (negative ESI-TOF) of an ace-
tonitrile solution of neutral receptor BBB and two equivalents of
TBAF showed two peaks corresponding to: [BBB–H+]ꢀ (calcd, m/z
309.1145; found, m/z 309.1146; error = 0.1 ppm) and [BBB–2H+
+ TBA]ꢀ (calcd, m/z 550.3915; found, m/z 550.3898;
error = 3.0 ppm). This result supports the evidence of the formation
of the mono- and di-anionic species observed in solution by NMR.
We also undertook 1H NMR titrations of neutral receptor BBB
with nitrate and perchlorate TBA salts. Stepwise addition of a con-
centrated acetonitrile solution, 0.1 M, of nitrate TBA salt to a
2 ꢂ 10ꢀ3 M solution of neutral receptor BBB in acetonitrile-d3, pro-
duced progressive changes in the chemical shifts and widths of the
receptor proton resonances. Upon saturation with nitrate TBA salt,
aromatic protons Hf and Hd of receptor BBB were shifted toward
higher frequencies in a similar manner to those observed with
halide anions: [BBBꢁNO3] (ppm) Hf Dd = +0.12, Hd Dd = +0.14 (see
Supplementary data). This behavior is indicative of a phenyl –C
(H)ꢁ ꢁ ꢁNOꢀ3 hydrogen bond interaction and the intramolecular
Figure 4. Change in chemical shift of proton Hf during the 1H NMR titration of
neutral compound BBB (2 ꢂ 10ꢀ3 M) with the addition of TBA fluoride (0.1 M) in
acetonitrile-d3 solution and the proposed stepwise deprotonation of BBB.
of a related complex, vide infra. An association constant was
derived from the experimental data and is included in Table 1.
The measured association constant, under the same experimental
conditions, for the nitrate complex is two orders of magnitude
weaker than the one determined for the chloride analog. This
observation indicates a lower complex stability with nitrate com-
pared to chloride, despite its higher basicity. We attribute this
result to a poorer size and shape complementarity between recep-
tor BBB with nitrate anion, respect to that observed for halides.
Based on these results, no interaction was expected between
neutral receptor BBB and tetrahedral perchlorate anion in acetoni-
trile solution. Experimentally, no changes of the chemical shifts of
protons Hf and Hd of receptor BBB were observed in 1H NMR titra-
tion experiments, even in the presence of 80 equivalents of TBA
perchlorate salt, indicating an extremely low association or no
association at all.
hydrogen bonds, suggesting
a co-planar arrangement of the
host–guest complex, as it would be shown by the crystal structure
Table 1
Association constants (Ka) and associated free energies (DG°) of neutral receptor BBB
with different anionic guests determined by 1H NMR titration in an acetonitrile-d3
solution
Anion
Ka (Mꢀ1
a
)
D )
G°b (kJ molꢀ1
Fꢀ
>104
<ꢀ23
Clꢀ
Brꢀ
Iꢀ
(1.0 0.1) ꢂ 103
(4.6 0.1) ꢂ 102
(4.6 0.1) ꢂ 101
(8.1 0.2) ꢂ 101
<1
ꢀ17.5 0.2
ꢀ15.1 0.2
ꢀ9.5 0.3
ꢀ10.9 0.3
ꢃ0
All our efforts to obtain single-crystals of a complex, containing
the neutral receptor with any of the different anions employed,
were unsuccessful; so we decided to try with the protonated ver-
sion of the receptor. Addition of nitric acid to an acetonitrile solu-
tion of BBB led to the isolation of suitable single-crystals of
[H2BBB][NO3]2 for an X-ray diffraction study.20 Analyzed crystal
NOꢀ3
ClOꢀ4
a
The data were fitted to a 1:1 receptor to anion stoichiometry model using the
least-squares method.
b
D
G° = ꢀRTlnK (T = 298 K).

C. Montoya et al. / Tetrahedron Letters 56 (2015) 6177–6182
6181
forms stable 1:1 complexes in acetonitrile solution with halides
(Fꢀ, Clꢀ, Brꢀ, and Iꢀ) and nitrate (NOꢀ3 ) anions with association con-
stants up to 104 Mꢀ1, these values are slightly lower relative to
those observed in analogous receptors. The binding occurs through
cooperative hydrogen bonding with the anion inside the cleft
including intramolecular hydrogen bonds which stabilizes the co-
planar conformation of the host–guest complex. For halides,
complex stability is determined by anion basicity, as follows: fluo-
ride > chloride > bromide > iodide; however, in the presence of
more than one equivalent of fluoride, the receptor undergoes
deprotonation to generate mono- and di-anionic species. For
anions having different geometries,
a diverse behavior was
observed, trigonal planar nitrate (NOꢀ3 ) forms a rather weak com-
plex and no interaction could be determined with tetrahedral per-
chlorate oxoanion (ClOꢀ4 ).
Acknowledgments
This work was financially supported by CONACYT – México pro-
ject number 128411. We thank Marco Leyva and Geiser Cuellar for
their assistance obtaining X-ray data and mass spectra.
Supplementary data
Supplementary data (1H and 13C NMR spectra, HR-MS (ESI-TOF),
titration experiments with anions as well as details of theoretical
calculations) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2015.09.075.
Figure 5. Ball-and-stick and space-filling representations of the molecular struc-
ture of [H2BBB][NO3]2 determined by single-crystal X-ray diffraction. N = blue,
C = black, O = red, H = white.
References and notes
1. (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486–516; (b) Alcalde,
E.; Dinares, I.; Mesquida, N. In Anion Recognition in Supramolecular Chemistry;
Gale, P. A., Dehaen, W., Eds.; ; Springer: Berlin, 2010; Vol. 24, pp 267–300.
2. (a) Jia, C.; Wu, B.; Li, S.; Yang, Z.; Zhao, Q.; Liang, J.; Li, Q.-S.; Yang, X.-J. Chem.
Commun. 2010, 5376–5378; (b) Custelcean, R.; Remy, P.; Bonnesen, P. V.; Jiang,
D.-E.; Moyer, B. A. Angew. Chem., Int. Ed. 2008, 47, 1866–1870; (c) Carroll, C. N.;
Coombs, B. A.; McClintock, S. P.; Johnson, C. A., II; Berryman, O. B.; Johnson, D.
W.; Haley, M. M. Chem. Commun. 2011, 5539–5541; (d) Kim, M. J.; Lee, H. W.;
Moon, D.; Jeong, K. S. Org. Lett. 2012, 14, 5042–5045; (e) Zhou, Y. P.; Zhang, M.;
Li, Y. H.; Guan, Q. R.; Wang, F.; Lin, Z. J.; Lam, C. K.; Feng, X. L.; Chao, H. Y. Inorg.
Chem. 2012, 51, 5099–5109.
3. (a) Pérez-Casas, C.; Höpfl, H.; Yatsimirsky, A. J. Inclusion Phenom. Macrocyclic
Chem. 2010, 68, 387–398; (b) Duke, R. M.; McCabe, T.; Schmitt, W.;
Gunnlaugsson, T. J. Org. Chem. 2012, 77, 3115–3126; (c) Krishnamurthi, J.;
Ono, T.; Amemori, S.; Komatsu, H.; Shinkai, S.; Sada, K. Chem. Commun. 2011,
1571–1573.
belongs to the monoclinic space group P21/n. The asymmetric unit
consists of a complete protonated receptor and two nitrate anions
as shown in Figure 5. One anion is situated in the cleft of receptor
BBB forming two strong hydrogen bonds with the acidic protons of
the imidazolium ring (ONOꢁ ꢁ ꢁN(+)(H) 2.83 Å; ONOꢁ ꢁ ꢁH–N(+) 173.1°
av) and a weaker one with the hydrogen atom Hf of the central aro-
matic ring (ONOꢁ ꢁ ꢁC(Hf) 2.93 Å; ONOꢁ ꢁ ꢁHf–C 126.4°).21 In order to
maximize the three interactions, the receptor converges the hydro-
gen bonds to one vertex of the nitrate adopting a pseudo-planar
geometry with a torsion angle between the central aromatic ring
and both benzimidazolium substituents of 22.5° av. The other
nitrate anion interacts with one of its edges (two different oxygen
atoms) with the external part of the receptor by forming two
hydrogen bonds; one with the acidic proton of one benzimida-
zolium ring (ONOꢁ ꢁ ꢁN(+)(H) 2.82 Å; ONOꢁ ꢁ ꢁH–N(+) 148.2°) and
the other with hydrogen atom Hd of the central benzene
(ONO3ꢁ ꢁ ꢁC(Hd) 3.26 Å; ONO3ꢁ ꢁ ꢁHd–C 131.7°). The 2-D propagation
of the crystal structure occurs throughout weak hydrogen bonds
between the non-coordinated oxygen atoms of the nitrates and
–C(H) or –N(+)(H) donors of different protonated receptors.
4. (a) Gross, D. E.; Mikkilineni, V.; Lynch, V. M.; Sessler, J. L. Supramol. Chem. 2010,
22, 135–141; (b) Ozturk, G.; Colak, M.; Togrul, M. J. Inclusion Phenom.
Macrocyclic Chem. 2010, 68, 49–54; (c) Hossain, M. A.; Kang, S. A.; Kut, J. A.;
Day, V. W.; Bowman-James, K. Inorg. Chem. 2012, 51, 4833–4840; (d) Serpell, C.
J.; Cookson, J.; Ozkaya, D.; Beer, P. D. Nat. Chem. 2011, 3, 478–483; (e) Beeren, S.
R.; Sanders, J. K. M. J. Am. Chem. Soc. 2011, 133, 3804–3807.
5. (a) Amendola, V.; Bergamaschi, G.; Boiocchi, M.; Fabbrizzi, L.; Milani, M. Chem.
Eur. J. 2010, 16, 4368–4380; (b) Rostami, A.; Wei, C. J.; Guerin, G.; Taylor, M. S.
Angew. Chem., Int. Ed. 2011, 50, 2059–2062; (c) Jin, C.; Zhang, M.; Wu, L.; Guan,
Y.; Pan, Y.; Jiang, J.; Lin, C.; Wang, L. Chem. Commun. 2013, 2025–2027.
6. (a) Mani, G.; Guchhait, T.; Kumar, R.; Kumar, S. Org. Lett. 2010, 12, 3910–3913;
(b) Yoon, D.-W.; Gross, D. E.; Lynch, V. M.; Sessler, J. L.; Hay, B. P.; Lee, C.-H.
Angew. Chem., Int. Ed. 2008, 47, 5038–5042; (c) Sokkalingam, P.; Kim, D. S.;
Hwang, H.; Sessler, J. L.; Lee, C.-H. Chem. Sci. 2012, 3, 1819–1824.
7. (a) Makuc, D.; Albrecht, M.; Plavec, J. Supramol. Chem. 2010, 22, 603–611; (b)
Suk, J.-M.; Naidu, V. R.; Liu, X.; Lah, M. S.; Jeong, K.-S. J. Am. Chem. Soc. 2011,
133, 13938–13941; (c) Gale, P. A. Chem. Commun. 2008, 4525–4540.
8. (a) Cao, Q.-Y.; Pradhan, T.; Kim, S.; Kim, J. S. Org. Lett. 2011, 13, 4386–4389; (b)
McDonald, K. P.; Ramabhadran, R. O.; Lee, S.; Raghavachari, K.; Flood, A. H. Org.
Lett. 2011, 13, 6260–6263; (c) Ahmed, N.; Shirinfar, B.; Geronimo, I.; Kim, K. S.
Org. Lett. 2011, 13, 5476–5479; (d) McDonald, K. P.; Hua, Y.; Lee, S.; Flood, A. H.
Chem. Commun. 2012, 5065–5075.
The molecular structure of this complex confirms the
K-shaped
binding motif with the anions located inside the cleft of receptor
BBB in a co-planar conformation stabilized by three hydrogen
bonds formed with the two –N(H) and one aryl –C(H) donors. It
is worth noticing that 1H NMR titration experiments of the proto-
nated receptor [H2BBB]2+ with the previously used TBA salts were
performed. However, in all cases, the formation of a precipitate in
early stages of the titration was observed, thus no reliable associa-
tion constants could be determined.
9. Saha, D.; Das, S.; Bhaumik, C.; Dutta, S.; Baitalik, S. Inorg. Chem. 2010, 49, 2334–
2348.
10. Cui, Y.; Mo, H.-J.; Chen, J.-C.; Niu, Y.-L.; Zhong, Y.-R.; Zheng, K.-C.; Ye, B.-H.
Inorg. Chem. 2007, 46, 6427–6436.
Conclusions
11. (a) Gale, P. A.; Hiscock, J. R.; Lalaoui, N.; Light, M. E.; Wells, N. J.; Wenzel, M.
Org. Biomol. Chem. 2012, 10, 5909–5915; (b) Hiscock, J. R.; Gale, P. A.; Lalaoui,
N.; Light, M. E.; Wells, N. J. Org. Biomol. Chem. 2012, 10, 7780–7788.
A neutral
K-shaped bis(benzimidazole) receptor incorporating
–N(H) and –C(H) donor groups was prepared. This compound

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12. (a) Castillo, D.; Astudillo, P.; Mares, J.; González, F. J.; Vela, A.; Tiburcio, J. Org.
Biomol. Chem. 2007, 5, 2252–2256; (b) Moreno-Olivares, S. I.; Cervantes, R.;
Tiburcio, J. J. Org. Chem. 2013, 78, 10724–10731; (c) Castillo, D.; Cervantes, R.;
Frontana, C.; González, F. J.; Tiburcio, J. J. Phys. Org. Chem. 2014, 27, 701–706.
13. (a) Yoon, D. W.; Gross, D. E.; Lynch, V. M.; Sessler, J. L.; Hay, B. P.; Lee, C.-H.
Angew. Chem., Int. Ed. 2008, 47, 5038–5042; (b) Wisner, J. A.; Beer, P. D.; Drew,
M. G. B. Angew. Chem., Int. Ed. 2001, 40, 3606–3609.
14. (a) Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.; Kaljurand, I.;
Koppel, I. A.; Garlyauskayte, R. Y.; Yagupolskii, Y. L.; Yagupolskii, L. M.;
Bernhardt, E.; Willner, H.; Leito, I. J. Org. Chem. 2011, 76, 391–395; (b) Raamat,
E.; Kaupmees, K.; Ovsjannikov, G.; Trummal, A.; Kütt, A.; Saame, J.; Koppel, I.;
Kaljurand, I.; Lipping, L.; Rodima, T.; Pihl, V.; Koppel, I. A.; Leito, I. J. Phys. Org.
Chem. 2013, 26, 162–170.
16. Hynes, M. J. J. Chem. Soc., Dalton Trans. 1993, 311–312.
17. (a) Shenderovich, I. G.; Tolstoy, P. M.; Golubev, N. S.; Denisov, G. S.; Limbach,
H.-H. J. Am. Chem. Soc. 2003, 125, 11710–11720; (b) Camiolo, S.; Gale, P. A.;
Hursthouse, M. B.; Light, M. E.; Shi, A. J. Chem. Commun. 2002, 758–759.
18. Sessler, J. L.; Gross, D. E.; Cho, W.-S.; Lynch, V. M.; Schmidtchen, F. P.; Bates, G.
W.; Light, M. E.; Gale, P. A. J. Am. Chem. Soc. 2006, 128, 12281–12288.
19. Esteban-Gómez, D.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Org. Biomol. Chem.
2005, 3, 1495–1500.
20. CCDC 1024679 contains the supplementary crystallographic data for this
Letter. These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.
21. (a) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565–573; (b) Desiraju, G. R. Chem.
Commun. 2005, 2995–3001; Sheldrick, G. M. SHELXTL 6.14 Program Library;
Brüker Analytical Instrument Division: Madison, Wisconsin, USA, 2003.
15. Spartan Software, Wavefunction Inc., Irvine, California, USA, Version 08.