Novel calix[4]arene-based receptors with bis-squaramide moieties for colorimetric sensing of anions via two different interaction modes

Article abstract of DOI:10.1016/j.tetlet.2012.11.117

Novel calix[4]arene-based anion receptors 1-3 with bis-squaramide moieties were designed and synthesized. Their sensing properties with various anions were investigated by UV-vis, ESI-MS, and ¹H NMR spectra. The experimental results revealed that there were two types of interaction modes where receptor 1 bound biologically important H₂PO₄^-, F ^-, and AcO^- ions selectively over other anions in the hydrogen bonding interaction mode, while receptors 2 and 3 displayed deprotonation behavior with basic anions (F^-, AcO^-, and H₂PO₄^-) via acid-base interaction mode. Receptors 2 and 3 were proved to be efficient 'naked-eye' sensors for F ^- and AcO^- with marked color changes.

Full text of DOI:10.1016/j.tetlet.2012.11.117

Tetrahedron Letters 54 (2013) 796–801
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel calix[4]arene-based receptors with bis-squaramide moieties for
colorimetric sensing of anions via two different interaction modes
Can Jin a, Man Zhang a, Chao Deng a, Yangfan Guan a, Jun Gong b, Dunru Zhu b, Yi Pan a, Juli Jiang a,
,
⇑
Leyong Wang a
a Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Center for Multimolecular Chemistry, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China
b State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel calix[4]arene-based anion receptors 1–3 with bis-squaramide moieties were designed and synthe-
sized. Their sensing properties with various anions were investigated by UV–vis, ESI-MS, and 1H NMR
spectra. The experimental results revealed that there were two types of interaction modes where recep-
tor 1 bound biologically important H2PO4ꢀ, Fꢀ, and AcOꢀ ions selectively over other anions in the hydro-
gen bonding interaction mode, while receptors 2 and 3 displayed deprotonation behavior with basic
anions (Fꢀ, AcOꢀ, and H2PO4ꢀ) via acid–base interaction mode. Receptors 2 and 3 were proved to be effi-
cient ‘naked-eye’ sensors for Fꢀ and AcOꢀ with marked color changes.
Received 4 September 2012
Revised 7 November 2012
Accepted 13 November 2012
Available online 1 December 2012
Keywords:
Anion recognition
Anion receptor
Calix[4]arene
Squaramide
Ó 2012 Elsevier Ltd. All rights reserved.
Interaction mode
Anion plays an important role in a wide range of chemical, bio-
logical, and environmental science.1 Thus, anion recognition as a
significant branch of supramolecular chemistry has become an ac-
tive research field in recent decades.2 Numerous charge neutral
receptors containing polarized N–H fragments such as amides,3 ur-
eas,4 thioureas,5 and pyrroles6 as excellent hydrogen bond donors
have been widely employed in the anion recognition. However,
although a number of anion receptors bearing above N–H groups
have been studied, there is still a need for developing novel recep-
tors based on N–H binding units to enhance the binding affinity
and selectivity for various anions. Thanks to the pioneering work
of Costa’s7 and Fabbrizzi’s8 groups, aromatic squaramide moieties
containing polarized N–H binding units, whose binding affinity is
better than their counterparts of urea binding moieties, have be-
come of great significance in the anion recognition for their strong
hydrogen bond donor ability in recent years, and the superiority of
squaramide moieties over urea groups as H-bond donors was also
supported by further theoretical studies.7d,e What is more, recently,
it has been reported that squaramide binding units could be ap-
plied for potent transmembrane anion transporters, performing
much better than thiourea and urea analogues.9 As strong H-bond
donors, squaramide moieties can form stable hydrogen-bond com-
plex with anions; on the other hand, when squaramide moieties
with electron-withdrawing groups interact with strong basic an-
ions, such as fluoride (Fꢀ), acetate (AcOꢀ), or dihydrogenphosphate
(H2PO4ꢀ), they could be deprotonated via acid–base process.8
Calix[4]arene has been employed as a popular building platform
due to its well preorganized molecular architecture for ionic and
molecular recognition.10 Compared to monotopic calix[4]arene
receptors, 1,3-disubstituted ditopic calix[4]arene designed by the
introduction of two squaramide moieties could provide better pre-
organized binding pocket for anion binding so as to possibly
achieve better binding affinity and selectivity in anion recogni-
tion.2e Herein we designed and synthesized bis-squaramide ca-
lix[4]arene-based receptors 1–3, where squaramide groups were
incorporated on the calix[4]arene to make strong multiple interac-
tions through N–H moieties as well as preorganized binding pock-
et. The anion binding ability of receptor 1 was expected to be
enhanced by the introduction of two squaramide moieties com-
pared to their (thio)urea counterparts. Besides, receptors 2 and 3
with nitrophenyl group attached to the squaramide moiety were
designed as a chromogenic unit (typically with electron-withdraw-
ing groups) to achieve colorimetric sensing of anions by ‘naked-
eye’ detection. As noted above, receptor 1 with squaramide moie-
ties could form a stable hydrogen-bond complex with anions,
while receptors 2–3 with squaramide moieties could be deproto-
nated by basic anions via acid–base process.
⇑
Corresponding author. Tel.: +86 25 83597090; fax: +86 25 83317761.
E-mail address: jjl@nju.edu.cn (J. Jiang).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.117

C. Jin et al. / Tetrahedron Letters 54 (2013) 796–801
797
Receptors 1–3 based on a lower-rim 1,3-disubstituted calix[4]-
arene scaffold were synthesized11 as shown in Scheme 1. Zn(OTf)2
was chosen as an effective catalyst for the preparation of 1–3 from
4 in 73–80% yield developed by Taylor and co-workers.12 The
structures of 1–3 were all characterized by 1H NMR, 13C NMR,
and ESI-MS analyses (see Supplementary data). As an example,
the 1H NMR spectra (in DMSO-d6) of 1 showed two sets of N–H
protons (9.76 and 7.99 ppm) assigned to squaramide protons,
respectively, and ESI-MS data gave a peak for the [M+Na]+ ion at
1099.75 (m/z).
rium model.4f These values showed an order of 1 binding with
H2PO4ꢀ (4.72) > Fꢀ (4.66) > AcOꢀ (4.34). The best selectivity of 1 to-
ꢀ
ward H2PO4 indicated that the preorganized calix[4]arene scaf-
ꢀ
fold might be more complementary for tetrahedron H2PO4 ion.
In order to confirm thꢀe hydrogen binding mode of receptor 1
with AcOꢀ, Fꢀ, and H2PO4 anions and compare with the reported
urea or thiourea derivatives in anion recognition, 1H NMR titration
experiments of receptor 1 with AcOꢀ, Fꢀ, and H2PO4 in DMSO-d6
ꢀ
solution were also carried out, respectively. From the titration
experiment of receptor 1, instant and similar downfield shifts were
observed with the above three anions. As presented in Figure 3,
both squaramide N–H protons peaks at 9.76 and 7.83 ppm of
receptor 1 showed largeꢀdownfield shifts to 11.75 and 9.75 ppm
upon addition of H2PO4 ion to host solution, and it indicated
clearly that both squaramide N–H participated in the binding in
the hydrogen bond interaction mode.15 In addition, ortho-position
C–H in the aromatic ring of squaramide at 7.31 ppm and O–H at
7.99 ppm were downfield shifted to 7.58 and 8.32ꢀppm, respec-
tively, indicating that the complex of 1 and H2PO4 was formed
via multiple hydrogen bonds. Besides the association constant ob-
tained from UV–vis titration experiment, the association constants
logKa based on 1H NMR titration were calculated using EQNMR
software 16 and these values were presented in Table 1. The results
demonstrated that receptor 1 formed a stronger hydrogen bond
complex with anions (logKa in DMSO-d6, Fꢀ: 3.35, AcOꢀ: 3.04,
and 3.73 for H2PO4ꢀ) than urea (logKa in CDCl3, Fꢀ: 2.86, AcOꢀ:
2.69, and 2.43 for H2PO4ꢀ) and thiourea (logKa in CDCl3, Fꢀ: 2.68,
AcOꢀ: 1.68, and 2.00 for H2PO4ꢀ) derivatives reported previously
The single crystal structure of receptor 3 was successfully ob-
tained by slow evaporation of a mixed DMSO/H2O solution in three
weeks. As depicted in Figure 1a, the crystal structure of 3ꢁH2O
shows monoclinic space groups P21/c, and the asymmetric unit of
3ꢁH2O contains one vase-like calix[4]arene moiety skeleton, two
N-(2-nitrophenyl)-squaramide arms, and one lattice water mole-
cule. The overall conformation of 3ꢁH2O is similar to the reported
1,3-bridged calix[4]arene derivatives,13 in which two bridged aro-
matic rings become more ‘vertical’ than the other two aromatic
rings, and tend to adopt the favored anti–anti conformation and
head-to-tail packing of squaramide sheets.14 In receptor 3,
although N–H bonds point to the same direction, the squaramide
moieties do not pack in a linear formation as forecasted (Fig. 1b).
Intra-molecular interactions (Table S3) are formed between two
squaramide moieties through N–Hꢁ ꢁ ꢁO hydrogen bonds with the
donor–acceptor distance of 2.773(3) Å, while N(5) and N(2) form
another two intense intra-molecular bonds (2.634(4) Å for N(5)–
H(5)ꢁ ꢁ ꢁO(9) and 2.657(3) Å for N(2)–H(2)ꢁ ꢁ ꢁO(11)) with the nitro
groups of the ortho-nitrobenzene. The squaramide moieties con-
nect to the adjacent molecules through the intermolecular N4–
H4ꢁ ꢁ ꢁO4i and N5–H5ꢁ ꢁ ꢁO4i (i = x, 1 + y, z) hydrogen bonds, resulting
in the formation of a zigzag chain arrangement in the crystals.
The anion binding affinity of receptor 1 with Fꢀ, Clꢀ, Brꢀ, Iꢀ,
15
where logKa was obtained from 1H NMR titration, which could
be due to the more excellent H-bond donor properties of squara-
mide moieties even in polar solvent than urea or thiourea moieties.
Sequentially, the anion binding affinity of ꢀreceptors 2–3 with
Fꢀ, Clꢀ, Brꢀ, Iꢀ, H2PO4ꢀ, HSO4ꢀ, AcOꢀ, and NO3 (tetrabutylammo-
nium as the counter cation) were investigated by UV–vis experi-
ments in DMSO (Fig. 4) as receptor 1 was. In both cases, spectral
changes were observed only upon addition of Fꢀ, AcOꢀ, and
H2PO4ꢀ, in which the addition of H2PO4ꢀ caused the smallest spec-
tral change. In receptors 2–3, the nitrophenyl subunit, as a chromo-
phore and electron withdrawing group, caused different results in
the anion recognition mode and the colorimetric proprieties of
receptors 2–3 compared to receptor 1 due to the fact that the elec-
tron-withdrawing effect of the nitrophenyl subunits increased the
acidity of squaramide N–H protons. The absorbance spectrum of
only 2 in DMSO has two bands at 282 and 406 nm ascribed to
the highest-energy and the lowest-energy charge transfer transi-
H2PO4ꢀ, HSO4ꢀ, AcOꢀ, and NO3 (tetrabutylammonium as the
ꢀ
counter cation) were initially investigated by UV–vis spectra in
DMSO due to the solubility issue. For receptor 1 (Fig. 2a), slight
bathochromic shifts (8–ꢀ10 nm) were observed only upon addition
of AcOꢀ, Fꢀ, and H2PO4 ions, respectively, while the addition of
other anions (Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, and NO3ꢀ) resulted in negligible
changes of 1 in UV–vis spectra. Therefore, UV–vis titration experi-
ments of AcOꢀ, Fꢀ, and H2PO4 in receptor 1 were conductedꢀfur-
ꢀ
ther, respectively. The UV–vis titration spectra of H2PO4 in
receptor 1 were illustrated in Figure 2b, showing that the peak at
329 nm decreased while the peak at 339 nm increased with slight
ꢀ
bathochromic shift. Job’s plot of receptor 1 with H2PO4 (Fig. 2b)
8
was carried out to give the 1:1 stoichiometry which was also con-
firmed by ESI-MS experiment (Fig. S20). Based on UV–vis titration
spectra of receptor 1 upon addition of anions Fꢀ (Fig. S10), AcOꢀ
(Fig. S11), and H2PO4ꢀ, the association constants logKa were ob-
tained using non-linear fitting (Table 1) in a 1:1 binding equilib-
tion (Fig. 4a). There are similar two charge transfer transition
bands to only 3 at 276 and 431 nm from absorbance spectrum
(Fig. 4b). Furthermore, the titrations of Fꢀ, AcOꢀ, and H2PO4 into
ꢀ
receptors 2–3 were investigated by UV–vis titration experiments,
respectively. Titration of receptor 2 with AcOꢀ (Fig. 5a) or Fꢀ
O
O
R2
5 7
-
N
H
OEt
O
OH
R1
O
OH
O
O
OH
OH
5 7
-
O
4
O
NH2
H2N
NH
HN
1 5
,
R1 = H; R2 = H
1-3
O
2 6
O
,
R
1 = H; R2 = NO2
NH
R1
HN
R1
3, 7 R1 = NO2; R2 = H
R2
R2
Scheme 1. Synthesis of receptors 1–3 based on squaramide derivatives 5–7.

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C. Jin et al. / Tetrahedron Letters 54 (2013) 796–801
Figure 1. (a) ORTEP view (30% thermal ellipsoids) of the crystal structure of 3, H2O. (b) The hydrogen bonds in 3, H2O involving the squaramide moieties along b axis (H atoms
which are not involved in hydrogen bonding and the lattice water molecules are omitted for clarity).
Figure 2. (a) Absorption spectra of 1 (1.0 ꢂ 10ꢀ5 M) in the presence of various anions (1.0 ꢂ 10ꢀ4 M) in DMSO. (b) Absorption spectra of receptor 1 (2.0 ꢂ 10ꢀ5 M) upon
ꢀ
addition of H2PO4 (0–2.4 ꢂ 10ꢀ4 M) in DMSO. Inset: job’s plot was constructed based on absorbance changes at 339 nm in the total concentration of 2.0 ꢂ 10ꢀ5 M.
(Fig. 5b) resulted in notable bathochromic shift from the band at
406 nm to the new band at 536 nm due to the internal charge
transfer (ICT) transition.17 This new ICT band at 536 nm matched
Table 1
well with the absorbance band obtained when 2 was reacted with
strong base tetrabutylammonium hydroxide (TBAOH) (Fig. S13), so
it was reasonable to propose the acid–base process to give the
deprotonated receptor 2 upon addition of Fꢀ or AcOꢀ. Moreover,
Association constants logKa and equilibrium constants logKD for receptors 1–3 with
anionsa
Anionb
1
2
3
c
d
c
c
logKa
logKa
logKD
logKD
ꢀ
the intensity of new ICT band by the addition of H2PO4 ion
Fꢀ
4.66
4.34
4.72
3.35
3.04
3.73
9.08
8.77
7.85
9.01
8.69
7.36
(Fig. S12) increased much more slightly than Fꢀ or AcOꢀ, which
was consistent with its weaker basicity in solution.18 ESI-MS tech-
nique was employed to support this acid–base interactiꢀon mode
for receptor 2 with the addition of Fꢀ, AcOꢀ, or H2PO4 anions,
where no corresponding binding complex peak was found. Com-
pared with receptor 2, very similar experimental results of
AcOꢀ
ꢀ
H2PO4
a
b
c
All errors were <15%.
Anions were used as tetrabutylammonium salts.
Determined by UV–vis titration.
d
Obtained by 1H NMR titration using EQNMR software.

C. Jin et al. / Tetrahedron Letters 54 (2013) 796–801
799
Figure 3. Partial 1H NMR spectra (300 MHz, DMSO-d6) of receptor 1 (2.0 ꢂ 10ꢀ3 M) upon addition of H2PO4
.
ꢀ
Figure 4. UV–vis absorption spectra of receptors (1.0 ꢂ 10ꢀ5 M) (a) 2 and (b) 3 with the selected anions (1.0 ꢂ 10ꢀ4 M) in DMSO.
UV–vis titration were also obtained for receptor 3 upon addition of
mental results to confirm that receptor 2 underwent the direct
deprotonatꢀion of N–H adjacent to nitrophenyl group by Fꢀ, AcOꢀ,
and H2PO4 ions in DMSO-d6 solution (see Supplementary data).
Aromatic proton signals of nitrophenyl group in receptor 2 were
shifted downfield or upfield, which also supported the typical
acid–base interaction mode between receptor and anions. The
changes of aromatic protons of nitrophenyl rings linked to squara-
mide moiety in NMR spectra could be explained by (1) the through-
bond propagation with the increasing electron density of nitro-
phenyl rings which caused a shielding effect leading to an upfield
shift and (2) the polarization of the C–H bonds within the electro-
static effect causing a downfield shift reported by Fabbrizzi.17 No
effect on the change of O–H protons in this case indicated that
O–H groups of calix[4]arene did not participate in the acid–base
process (Fig. S23–25).
OHꢀ (Fig. S17), AcOꢀ (Fig. 6a), Fꢀ (Fig. 6b), and H2PO4 (Fig. S16).
ꢀ
These above-mentioned results strongly indicated a typical Bron-
sted acid–base reaction for receptors 2–3 upon addition of basic
anions (Fꢀ, AcOꢀ, or H2PO4ꢀ).17 Fabbrizzi et al reported that
squaramide N–H adjacent to nitrophenyl group, which is exactly
the same with our case, might be directly deprotonated by basic
anions and the resonance formula of the deprotonated squaramide
form was given in Scheme 2.8
To further investigate the interaction mode of 2 upon addition
of Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, NO3ꢀ, Fꢀ, AcOꢀ, and H2PO4 anions, espe-
ꢀ
cially to confirm the acid–base interaction mode with Fꢀ, AcOꢀ,
ꢀ
and H2PO4
,
1H NMR experiments were also carried out. The re-
sults showed that it was in accordance with UV–vis experimental
results, where no noticeable 1H NMR spectral changes of receptor
2 were observed upon addition of Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, and NO3ꢀ an-
In general, the deprotonation process of receptor H2L by basic
anions Aꢀ can be associated with a common equilibrium of the fol-
lowing type (Eq. 1):19
ions, but the addition of Fꢀ, AcOꢀ, and H2PO4 caused obvious 1H
ꢀ
NMR spectral changes which agree well with the above experi-

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C. Jin et al. / Tetrahedron Letters 54 (2013) 796–801
Figure 5. UV–vis absorption spectra of receptor 2 (2.0 ꢂ 10ꢀ5 M) upon addition of increasing amount (0–2.4 ꢂ 10ꢀ4 M) of (a) AcOꢀ and (b) Fꢀ in DMSO.
Figure 6. UV–vis absorption spectra of receptor 3 (2.0 ꢂ 10ꢀ5 M) upon addition of increasing amount (0–2.4 ꢂ 10ꢀ4 M) of (a) AcOꢀ and (b) Fꢀ in DMSO.
H2L þ 4Aꢀ ꢀ L2ꢀ þ 2HAꢀ2
ð3Þ
O
O
N
O
O
N
The equilibrium constants logKD of 2 and 3 with Fꢀ, AcOꢀ, and
H2PO4 ions were calculated20 from UV–vis titration experiments
ꢀ
R
R
R
R
N
H
N
H
depicted in Table 1, and 2 or 3 showed higher affinity for Fꢀ ion.
b
a
Different from the hydrogen binding process, it seemed that
acid–base process might be solely determined by the anion basi-
Scheme 2. Resonance formulae of the deprotonated squaramide-based receptor
given by Fabbrizzi.
ꢀ 5c
city (Fꢀ > AcOꢀ > H2PO4
)
instead of anion recognition. Besides,
in the light of these data, intramolecular H-bonds existing in 3
H2L þ 2Aꢀ ꢀ L2ꢀ þ 2HAðKDÞ
Moreover, considering the stable dimers formatted by excess
Aꢀ,19a the following dimer formation equilibrium (Eq. 2) should
be applied in the subsequent step:
ð1Þ
might prevent the deprotonation process to certain extent of the
N–H hydrogen adjacent to the nitrobenzene moiety.21
The interaction of 1–3 with anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, H2PO4
,
ꢀ
HSO4ꢀ, AcOꢀ, and NO3ꢀ, tetrabutylammonium as the counter cat-
ion) was further monitored through visual observations in DMSO.
Receptor 1 did not give any visual response with anions. The recep-
HA þ Aꢀ ꢀ þ HAꢀ2
ð2Þ
tor 2 (20 lM) exhibited a unique and instant color change from
yellow to purple upon addition of 10 equiv Fꢀ or AcOꢀ ions,
This two-step equilibrium can be applied to the N–H deproto-
nation processes promoted by Fꢀ, AcOꢀ, and H2PO4 ions which
ꢀ
whereas there was no obvious color change upon addition of
can form relatively stable [HA2]ꢀ dimers.18 Moreover, UV–vis titra-
tion spectra of 2 and 3 revealed that the process reached a satura-
tion point on addition of 4 equiv of Fꢀ and AcOꢀ ions (see
Supplementary data, Fig. S14, S15, S18 and S19). These observa-
tions supported the acid–base process induced by basic anions in
an overall 1:4 stoichiometry for H2L-anion interactions (Eq. 3) ob-
tained by combining (Eqs. 1) and (2):
10 equiv other anions (Clꢀ, Brꢀ, Iꢀ, H2PO4ꢀ, HSO4ꢀ, and NO3
)
ꢀ
(Fig. 7). Similarly, receptor 3 also gave a marked color change only
upon addition of 10 equiv Fꢀ or AcOꢀ ions from yellow to light pink
under the same conditions. The above ‘naked-eye’ color changes
can be attributed to the deprotonation of the N–H on the
squaramide moiety of 2 and 3 induced by the relative stronger ba-
sic anions, Fꢀ or AcOꢀ,17 compared with a weaker basic anion

C. Jin et al. / Tetrahedron Letters 54 (2013) 796–801
801
Figure 7. The color changes of receptors 2 (above) and 3 (below) (20 lM) when treated with 10 equiv of various anions in DMSO solutions.
ꢀ
L.; Liu, L.; Fang, R.; Deng, C.; Han, J.; Hu, H.; Lin, C.; Wang, L. Tetrahedron Lett.
2012, 53, 3637.
H2PO4 which are in agreement with the results of the UV–vis
spectra changes.
5. (a) Boyle, E. M.; McCabe, T.; Gunnlaugsson, T. Supramol. Chem. 2010, 22, 586;
(b) Li, A.-F.; Wang, J.-H.; Wang, F.; Jiang, Y.-B. Chem. Soc. Rev. 2010, 39, 3729; (c)
Gomez, D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E. Org. Biomol. Chem. 2005, 3,
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Org. Biomol. Chem. 2004, 2, 1582.
In this Letter, we synthesized and discussed the sensing proper-
ties of three new calix[4]arene-based anion receptors 1–3 with bis-
squaramide moieties. The receptor 1 selectively binds biologically
important H2PO4ꢀ, Fꢀ, and AcOꢀ ions via stronger hydrogen bond
interaction than (thio)urea counterparts whereas 2 and 3 are
deprotonated by basic anions via acid–base interaction mode.
Receptors 2 and 3 exhibit prominent ‘naked-eye’ color changes
with basic anions such as Fꢀ and AcOꢀ through efficient deprotona-
tion. These results pave a way for the development of designing
and studying colorimetric receptors for anions with squaramide
moieties. We are currently working on developing novel receptors
based on calixarene platform using squaramide moieties to im-
prove the selectivity of anion recognition.
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We gratefully thank the financial support of National Natural
Science Foundation of China (No. 21102073, 21072093), the Doc-
toral Fund of Ministry of Education of China (20090091110017),
and the Natural Science Foundation of Jiangsu (BK2011055).
Supplementary data
Supplementary data (general experimental methods, synthetic
procedures, 1H NMR, 13C NMR, ESI-MS, UV–vis titration, NMR titra-
tion and crystal data) associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
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