Ion pair receptors based on anion-π interaction

Article abstract of DOI:10.1039/c1cc12075a

A novel type of ditopic ion pair receptors based on anion-π interaction is reported. Oxacalix[2]arene[2]triazine azacrowns were synthesized efficiently from a one-pot reaction between 2,4-dichloro-4-methoxytriazine and 3,5-dihydroxybenzaldehyde followed b

Full text of DOI:10.1039/c1cc12075a

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Ion pair receptors based on anion–p interactionw
Yin Chen,a De-Xian Wang,*a Zhi-Tang Huanga and Mei-Xiang Wang*ab
Received 12th April 2011, Accepted 3rd June 2011
DOI: 10.1039/c1cc12075a
A novel type of ditopic ion pair receptors based on anion–p
interaction is reported. Oxacalix[2]arene[2]triazine azacrowns
were synthesized efficiently from a one-pot reaction between
2,4-dichloro-4-methoxytriazine and 3,5-dihydroxybenzaldehyde
followed by condensation with a diamine and reduction of bisimine;
they acted as selective ditopic receptors to recognize ion pairs.
in anion–p interactions lead us to undertake the current study.
We envisioned that the fabrication of oxacalix[2]arene[2]triazine
parent macrocyclic molecules with cation binding sites would
give rise to a novel type of ditopic ion pair receptors in
which anion recognition is wholly based on anion–p interactions.
We report herein the expedient and efficient synthesis of
oxacalix[2]arene[2]triazine azacrown molecules. Fluorescence
1
and H NMR titration studies have shown that the calixaza-
Ion pair recognition is one of the central themes in supra-
molecular chemistry.1 The sophistication of the design and
synthesis of novel ditopic ion pair receptors based on non-
covalent bond interactions enables the selective interaction of
various ion pairs.2–7 Among the ion pair receptors reported,
crown ether moieties,2 multidentate N- or O-ligands,3 and
electron-rich aromatic rings4 are usually utilized as the cation
binding sites while the anion binding sites are mainly com-
posed of hydrogen bond donors,2a,b,3b,4a,5 Lewis acids2d–f,4b,6
or halogen bonding elements.7
crowns synthesized are novel ditopic ion pair receptors able to
selectively interact with ion pairs.
Scheme 1 illustrates the synthesis of oxacalix[2]arene[2]-
triazine azacrowns 5 and 6. In the presence of K2CO3,
aromatic nucleophilic substitution reaction of 3,5-dihydroxy-
benzaldehyde 1 with 2,4-dichloro-4-methoxytriazine 2 proceeded
rapidly in refluxing acetone to afford the desired oxacalix[2]-
arene[2]triazine 3 in 75% yield. The efficient formation of
macrocyclic product 3 in a one-pot reaction fashion is probably
due to a thermodynamic controlled reaction process.11 Taking
the advantage of dynamic formation of an imine bond from the
reaction of an aldehyde and an amine, the pre-functionalized
oxacalix[2]arene[2]triazine 3 was treated with diamine 4 directly
in solution. As we expected, the reaction at room temperature in
As a new supramolecular motif, anion–p interaction has
attracted growing interest in recent years.8,9 However, experi-
mental evidence to support ‘‘pure’’ anion–p interactions
between an anion and a charge-neutral electron-deficient
arene is still very rare.10 We10b have reported recently that
oxacalix[2]arene[2]triazine, a conformation and cavity tunable
macrocyclic host molecule, using its electron-deficient V-shaped
cleft of two triazine rings, recognizes halides in solution and
forms ternary complexes with halide and water in the solid state
due to the formation of typical anion–p interaction between
chloride, bromide and one triazine ring, and lone-pair electrons–
p (lpe–p) interaction between the included water molecule and
the other triazine ring. Very recently, we10e have also observed
weak s-interaction between chloride and the triazine ring that is
incorporated into a conformationally rigid bis(oxacalix[2]arene-
[2]triazine) cage molecule. Our continuous interests in the supra-
molecular chemistry of heteroatom-bridged calixaromatics and
a Beijing National Laboratory for Molecular Sciences,
CAS Key Laboratory of Molecular Recognition and Function,
Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: dxwang@iccas.ac.cn,
wangmx@mail.tsinghua.edu.cn; Fax: +86-10-6279-6761;
Tel: +86-10-6279-6761
b The Key Laboratory of Bioorganic Phosphorus Chemistry &
Chemical Biology (Ministry of Education), Department of
Chemistry, Tsinghua University, Beijing 100084, China
w Electronic supplementary information (ESI) available: Experimental
details, 1H and 13C NMR spectra of products, fluorescence titration
data. CCDC 816915. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc12075a
Scheme 1 Synthesis of oxacalix[2]arene[2]triazine azacrowns 5 and 6.
c
8112 Chem. Commun., 2011, 47, 8112–8114
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acetonitrile took place efficiently to produce bisimine compound
5 in high yield. The formation of a macrocyclic bisimine ring did
not necessarily require a highly diluted solution, as all reactions
performed in the substrate concentration of 5–25 mmol Lꢀ1
either in acetonitrile or in chloroform were found to afford
product 5 in comparably high chemical yields (90–94%). To
have the azacrown ether moiety that may bind selectively for
alkali and alkaline-earth metals, the imine bonds of compound 5
were reduced. Surprisingly, under various conditions, the CQN
bond of 5 was not reduced (Table S1 in ESIw). Finally, we were
pleased to find that bisimine 5 was reduced efficiently to calix-
azacrown ether 6 by NaHB(OCOMe)3 at room temperature
when dichloromethane was used as solvent.
(Ka = 1.54 ꢁ 105 Mꢀ1) association constants were calculated
(Fig. S5 and Table S3, ESIw).
After revealing the binding properties of 5 and 6 towards
anions and metal cations, we then turned our attention to ion
pair recognition. Since fluoride was able to form an anion–p
complex with both 5 and 6 as aforementioned, it was first
chosen as an anion probe to examine ion pair interaction of
ditopic receptors. Upon the addition of fluoride to a solution
of equimolar of 5 and ions such as Fe2+, Co2+, Cu2+ and
Hg2+, the fluorescence emission resulted from the 5–metal ion
complex at about 425–475 nm was quenched (Fig. S6, ESIw). It
indicated that fluoride might compete with the azacrown
moiety of 5 to bind metal ions, leading to the dissociation of
the complexes [5ꢂM]n+. Interestingly, in the case of complexes
[5ꢂZn]2+ and [5ꢂPb]2+, however, titration with fluoride gave
rise to further enhancement of intensity of the fluorescence
band at 425 nm (Fig. 1 and Fig. S6, ESIw). The association
constants for the complexes between fluoride and [5ꢂZn]2+ and
[5ꢂPb]2+ were 1.53 ꢁ 105 Mꢀ1 and 3.71 ꢁ 103 Mꢀ1, respec-
tively. While the binding of [5ꢂPb]2+ with fluoride was
comparable with that of parent 5 (Ka = 6.59 ꢁ 103 Mꢀ1), a
more than 23-fold increase of the binding constant from
6.59 ꢁ 103 Mꢀ1 [5ꢂF]ꢀ to 1.53 ꢁ 105 Mꢀ1 [Znꢂ5ꢂF]+ was
significant. Remarkably, the [5ꢂZn]2+ complex was also able
to interact with some other anions that were not recognized by
the parent host 5 its own. For example, chloride, bromide, and
nitrate were bonded by [5ꢂZn]2+, giving the association con-
The structures of oxacalix[2]arene[2]triazine bisimine 5 and
azacrown 6 were established on the basis of spectroscopic data
and microanalyses (ESIw). It was noteworthy that the two
ditopic receptors show a single set of proton and carbon
signals in their 1H and 13C NMR spectra, respectively,
indicating the symmetric structure in solution (Fig. S17–S23,
ESIw). The calixcrown structure was further proved unambi-
guously by the X-ray single crystal structure of 6.z As shown
in Fig. S1 (ESIw), the oxacalix[2]arene[2]triazine moiety of 6
adopts a 1,3-alternate conformation, similar to most of the
heteracalix[2]arene[2]triazines.12 Two triazine rings, which are
conjugated with the bridging oxygen atoms, form a V-shaped
electron-deficient cleft. On the upper-rim positions of two
benzene rings, there is a macrocyclic azacrown ether loop.
Having had synthetic ditopic receptors in hand, we first
examined their ability to interact with anions by means of
fluorescence titration. As illustrated in Fig. S3 and Table S2
(ESIw), the fluorescence spectrum of 5 was changed upon the
titration of tetrabutylammonium fluoride, cyanide and acetate,
and the association constants, which were calculated based on
fluorescence titration curves using a Hyperquad 2003 soft-
ware,13 were 6.59 ꢁ 103 Mꢀ1 (Fꢀ), 4.16 ꢁ 103 Mꢀ1 (CNꢀ)
and 4.52 ꢁ 103 Mꢀ1 (CH3CO2ꢀ), respectively. Selective binding
of 6 towards fluoride (Ka = 1.11 ꢁ 103 Mꢀ1) and cyanide (Ka =
5.87 ꢁ 103 Mꢀ1) was also observed. No interactions of 5 and 6
with other anions were evident (Fig. S3 and S5, ESIw). The
interaction between 5, 6 and anions was further investigated
using 1H NMR spectroscopy. As demonstrated in Fig. S15 and
S16 (ESIw), no variation at all in the 1H NMR spectra of 5 and
6 occurs with the addition of anions. It is consistent with our
early reports that anion–p interaction is responsible for the
change of fluorescence spectrum of the host molecules.10b
Being azacrown entities, macrocycles 5 and 6 exhibited
expectedly selective interactions with metal ions. For example,
titration of bisimine 5 with perchlorate salts of varied transi-
tion and heavy metal ions caused the emergence and the
enhancement of a new fluorescence emission band at about
425–475 nm, whereas addition of alkali and alkaline-earth
metals did not affect the fluorescence spectrum of 5 at all
(Fig. S4, ESIw). The association constants for the complexes of
5 with metal ions ranged from 7.23 ꢁ 103 Mꢀ1 to 9.21 ꢁ
104 Mꢀ1 (Table S2, ESIw). Calixazacrown 6, in contrast, was
found to be able to complex with alkali metals in addition to
transition and heavy metals. Based on the changes of fluores-
cence emission bands that were blue-shifted to about 320 nm
stant of 7.39 ꢁ 103 Mꢀ1, 1.59 ꢁ 103 Mꢀ1 and 4.25 ꢁ 103 Mꢀ1
,
respectively. Addition of cyanide into the [5ꢂZn]2+ complex,
however, did not influence the fluorescence spectrum (Fig. S7,
Table S4, ESIw). Interestingly, while most of the anions
examined did not interact with complexes between 6 and
metals, cyanide was found to bind [6ꢂM] (M = K+, Cs+,
Ca2+, Sr2+) with an association constant ranging from 3.46 ꢁ
103 Mꢀ1 to 8.36 ꢁ 103 Mꢀ1 (Fig. S8 and S9, Table S5, ESIw).
To shed light on the mechanism of ditopic interaction of 5
with Zn2+ and anion species, following experiments were
conducted. To exclude the possibility that the fluorescence
enhancement resulted from the binding of anions with zinc ion
of the complex [5ꢂZn]2+, a model bisimine compound 7 was
prepared (Fig. 2) and its fluorescence titration with ion pairs
was studied. Although compound 7 formed indeed a complex
Fig. 1 Left: fluorescence titration of 5 (3.99 ꢁ 10ꢀ4 M in 2 ml aceto-
nitrile) with Fꢀ (up, 0–3.41 ꢁ 10ꢀ4 M) and Zn2+ (down, 0–9.65 ꢁ 10ꢀ4 M).
Right: fluorescence titration of 5 (3.99 ꢁ 10ꢀ4 M in 2 ml acetonitrile)
upon titration, moderate (Ka = 9.86 ꢁ 102
M
ꢀ1) to strong
with Fꢀ (0–4.18 ꢁ 10ꢀ4 M) in the presence of 1 eq. Zn2+
.
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Chem. Commun., 2011, 47, 8112–8114 8113

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Notes and references
z Crystallographic data of 6: C57H62Cl2N16O16, M = 1298.13, mono-
clinic, P21/c, a = 15.195(6) A, b = 25.537(9) A, c = 17.451(7) A,
a = 90.001, b = 114.634(4)1, g = 90.001, V = 6155(4) A3, Z = 4,
m(Mo-Ka) = 0.71073 mmꢀ1, T = 173(2) K. The structure was solved
and refined by SHELXL-97 in the WinGX package. Final residuals
(871 parameters) R1 = 0.1380 for 9377 reflections with I > 2s (I), and
R1 = 0.1588, wR2 = 0.2746. GoF = 1.353 for all 11 127 data. CCDC
816915.
Fig. 2 Structure of 7.
with Zn2+, however, addition of anions into the solution of
[7ꢂZn]2+ complex did not generate a similar red-shifted emission
band as that observed for 5 (Fig. S10, ESIw). We also carefully
1
recorded H NMR spectra of 5 with the addition of zinc ion
1 For a very recent overview of ion pair receptors, see: S. K. Kim and
J. L. Sessler, Chem. Soc. Rev., 2010, 39, 3784 and references cited
therein.
followed by the addition of Clꢀ. As illustrated in Fig. 3,
interaction of 5 with one equivalent of Zn2+ produced
complex [5ꢂZn]2+ almost quantitatively. Addition of Clꢀ up
to one equivalent did not affect the 1H NMR spectrum of
complex [5ꢂZn]2+. These results indicated clearly that the
fluorescence enhancement upon the addition of anions such
as halides and nitrate to the complex [5ꢂZn]2+ (Fig. 1) is due to
the interaction of anions with the electron-deficient triazine
ring such as [anion–5–Zn]+ rather than the interaction
between anions and zinc ion [5–Zn–anion]+.
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We thank NNSFC (20875094, 21072197, 20972161), MOST
(2011CB932501, 2007CB808005) and CAS for financial support.
Fig. 3 Left: 1H NMR titrations of 5 upon the addition of Zn(ClO4)2.
Right: 1H NMR titrations of [5ꢂZn2+](ClO4)2 upon the addition of
tetrabutylammonium chloride.
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Fig. 4 Illustration of interactions of 5 with Zn2+ and anion species.
8114 Chem. Commun., 2011, 47, 8112–8114
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This journal is The Royal Society of Chemistry 2011