Tuning the binding properties of a new heteroditopic salt receptor through embedding in a polymeric system

Article abstract of DOI:10.1039/c2cc36607j

In this communication, we describe a polymerizable, heteroditopic salt receptor, based on an amino acid scaffold, which is able to bind sodium salts of chloride, acetate and nitrate. A poly(butyl methacrylate) derivative 2 containing receptor 1 was then prepared. In contrast to receptor 1 copolymer 2 is capable of extracting sodium nitrate from aqueous media.

Full text of DOI:10.1039/c2cc36607j

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COMMUNICATION
Tuning the binding properties of a new heteroditopic salt receptor
through embedding in a polymeric systemw
Jan Romanski* and Piotr Pia˛tek*
Received 11th September 2012, Accepted 4th October 2012
DOI: 10.1039/c2cc36607j
In this communication, we describe a polymerizable, heteroditopic salt
receptor, based on an amino acid scaffold, which is able to bind
sodium salts of chloride, acetate and nitrate. A poly(butyl methacry-
late) derivative 2 containing receptor 1 was then prepared. In contrast
to receptor 1 copolymer 2 is capable of extracting sodium nitrate
from aqueous media.
Another possible way to bind salts is to use a binary mixture
of anion and cation receptors.5 A major advantage of this
so-called ‘‘dual receptor’’ strategy is that relatively simple
monotopic receptors may be used. Sessler et al. recently
applied the dual receptor approach to prepare a functional
polymer that is able to extract potassium fluoride and chloride
salts from aqueous media.6,7 These authors showed that only a
proper combination of both cation (benzo-15-crown-5) and
anion (calix[4]pyrrole) receptors and polymeric matrix (methyl
methacrylate) leads to a material capable of extracting hydro-
philic ion pairs from aqueous solution.
The development of supramolecular chemistry is mainly stimulated
by the interest in designing synthetic monotopic receptors for ionic
species.1,2 As these receptors may effectively associate only with
cations or anions there is a competitive process of receptor binding,
related to the ion-pairing of a given ion and its counterion.
Thus a receptor must compete with a counterion in binding a
The extraction of anions from the water to the organic
phase is a difficult task because of the high hydration energy
of most anions.8 Therefore we envisioned that the use of
polymeric materials containing a salt receptor and a hydro-
phobic comonomer could substantially increase the binding
affinity and selectivity of ion pair receptors towards larger,
diffused charge anions.9
target ion.3 This process is minimized in the laboratory by
À
applying soft, ‘‘non-coordinating’’ counterions such as ClO4
,
À
PF6 for studying cation binding or tetra-n-butylammonium
(TBA+) for studies of anion binding. This strategy cannot be
used in real-life applications because the complexed ions are
usually accompanied by hard counterions, such as Na+, K+
or ClÀ, SO42À, for anion and cation binding, respectively.
Therefore, to diminish the counterion-induced inhibition of
receptor ion binding, systems capable of binding both cations
and anions are needed. One such system is a heteroditopic
receptor that combines a cation and an anion binding domain
in a single molecule.4 One advantage gained by covalently
linking the anion and cation binding sites is the possibility
of binding cooperativity. The binding of the first ion may
increase receptor affinity to the second ion. This positive
cooperative effect is especially important for anions which
normally coordinate poorly with monotopic molecular receptors
(i.e. BrÀ, NO3À, ClO4À). To maximize cooperative effects the
binding sites have to be incorporated into a suitably preorga-
nized molecular platform that holds them in close proximity.
Designing effective heteroditopic molecular receptors is there-
fore a challenging task.
As none of the known ion pair receptors can be easily function-
alized with a polymerizable group, we designed, synthesized and
characterized a new heteroditopic receptor 1 that can be used
as a comonomer (Fig. 1). A polymeric material 2, containing 1
as a pendant group, was synthesized and its ability to extract
sodium salts from aqueous media was investigated.
Receptor 1 was prepared in five steps starting from
commercially available Na-Boc-Nd-Cbz-L-ornithine by the
sequential decoration of amino acid functional groups with
aza-18-crown-6, methacrylate and nitrophenyl-thiourea moieties
(Scheme 1).
Quantitative evaluation of the anion, cation and salt
binding ability of 1 was investigated by the 1H NMR titration
method in CD3CN. The addition of TBA salts of various
Department of Chemistry, University of Warsaw, Pasteura 1,
02-093 Warsaw, Poland. E-mail: jarom@chem.uw.eu.pl,
ppiatek@chem.uw.edu.pl; Fax: +48 22 822 5996;
Tel: +48 22 822 0211
w Electronic supplementary information (ESI) available: Synthesis
and characterization data for receptor 1 and polymer 2. General
procedures used for 1H NMR titration and extraction experiments.
Binding isotherms, Job plots and NMR spectra. See DOI: 10.1039/
c2cc36607j
Fig. 1 The structures of receptor 1 and polymer 2.
c
11346 Chem. Commun., 2012, 48, 11346–11348
This journal is The Royal Society of Chemistry 2012

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Fig. 2 Chemical shift of ArNH of 1 as a function of increasing
amounts of TBANO3 in the absence and presence of NaPF6.
Scheme 1 Synthesis of receptor 1. Reagents and conditions: (i) DCC,
1-aza-18-crown-6, CH2Cl2, 1C to rt, 92%; (ii) H2, Pd/C,
MeOH–THF, rt, quantitative; (iii) methacryloyl chloride, Et3N,
CH2Cl2, 0 1C to rt, 50%; (iv) TFA–CH2Cl2 (1 : 1), rt, quantitative;
(v) 4-nitrophenyl isothiocyanate, Et3N, THF, reflux, 72%.
The Ka of complexes of that analog with TBANO3 is 135 MÀ1
whereas in the presence of Na+ it decreases to 95 MÀ1 (see
ESIw). It is noteworthy that the selectivity of 1 towards anions
in the presence of sodium cations is different than for TBA
0
salts (ClÀ > AcOÀ > NO3 vs. AcOÀ > ClÀ > NO3À). In
À
anions to 2.7 mM solution of 1 caused a considerable down-
field shift of thiourea protons next to the aromatic ring (Dd:
1.3–3.2 ppm). The association constants calculated by the
nonlinear regression analysis of the binding isotherms are
listed in Table 1. Receptor 1 binds the acetate anion with
association constants that are too high to be accurately
determined by the 1H NMR titration method; chloride associates
less strongly whereas nitrate is bound only weakly. This is in
agreement with the general trend observed for thiourea and urea
based anion receptors.2c
solid/liquid extraction experiments the preference of receptor 1
towards NaNO3 is even more pronounced because this salt is
nearly 100% extracted, compared to 80% and 30% extraction
efficiency for sodium acetate and chloride, respectively.
To further tune the selectivity of 1 towards nitrate salts
we prepared polymeric material containing receptor 1 as a
pendant group. The copolymerization reaction of n-butyl
methacrylate and receptor 1 (9 : 1 molar ratio) in the presence
of 2% AIBN leads to copolymer 2 in 72% yield. The incorpora-
tion of 1 into the polymer was determined to be 8% by 1H NMR
and combustion analysis. Using gel permeation chromatography,
polymer 2 was found to possess a Mn of 96 100 Da and a
polydispersity index of 2.8.
The cation-binding properties of 1 were probed by adding
À
increasing amounts of sodium and potassium PF6 salts to
1
2.7 mM solution of the receptor. The variation of H NMR
chemical shift of the signal corresponding to crown ether
ÀO–CH2 was used to determine the association constants.
These studies revealed that 1 coordinates to the sodium cation
(Ka = 6460 MÀ1) over 30 times more strongly than to the
potassium cation (Ka = 195 MÀ1).10 The salt binding experiments
were therefore conducted in the presence of sodium cations.
Titrations of 1 with acetate or chloride in the presence
of one equivalent of hard sodium cations gave association
constants drastically lower than in the absence of Na+. This
observation can be rationalized in terms of ion pair formation
in solution, outside the receptor. In contrast, the association
constant for nitrate anions is remarkably higher in the
presence of sodium cations (Fig. 2). This positive cooperative
effect resulted from solvent or ligand separated ion pair
formation – a conclusion validated by analogous experiments
conducted with an analog of 1 lacking the crown ether moiety.
The ability of receptor 1 and polymer 2 to extract NaNO3
from the water to the chloroform phase was then tested. The
1.5 mM solution of NaNO3 was extracted with 12.5 mM
(effective concentration) CHCl3 solution of 1 or 2, respectively.
Organic phases were back-extracted to distilled water and the
content of nitrate anions was determined by the colorimetric
nitrite–nitrate method. While no extraction was seen in the case
of receptor 1, polymer 2 was able to transfer nitrate anions into
chloroform with an extraction efficiency of 32%.11
To confirm the coextraction of sodium cations in the above
experiments atomic emission spectroscopy was used. The
content of sodium in the organic phase obtained after NaNO3
extraction was determined to be negligible for 1 and 44% for 2.
Analogous extraction experiments for NaCl showed that 2
could extract 19% of sodium cations (Table 2). Thus by
embedding receptor 1 in a hydrophobic polymeric matrix the
selectivity of 1 towards NaCl has been shifted towards NaNO3.
This is in accord with standard hydration energy of chloride and
nitrate anions (DGh = À347 kJ molÀ1 for ClÀ vs. À306 kJ molÀ1
for NO3À) and therefore the more hydrophobic anion is
extracted more effectively into hydrophobic media.8
Table 1 Association constants (Ka) for interactions of 1 with anions
in the absence or presence of 1 equivalent of sodium cations
TBA+
Na+
AcOÀ
ClÀ
NO3
>50 000
13 990
160
1940
4520
410
In conclusion, we have prepared a polymerizable hetero-
ditopic salt receptor that is able to associate with sodium salts
of chloride, acetate and nitrate. We then prepared copolymer 2,
which we believe to be the first example of a polymer containing
À
À
PF6
n.b.a
6460
a
No binding.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11346–11348 11347

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Table 2 Summary of extraction dataa
ed. J. L. Atwood and J. W. Steed, Pergamon, New York, 2004,
pp. 326–333.
AIb
c [mg lÀ1
]
Extraction efficiency [%]
2 (a) in Top. Heterocycl. Chem., Anion Recognition in Supramolecular
Chemistry, ed. P. A. Gale and W. Dehaen, Springer, 2010, vol. 24;
(b) M. Wenzel, J. R. Hiscock and P. A. Gale, Chem. Soc. Rev.,
2012, 41, 480–520; (c) V. Amendola, L. Fabbrizzi and L. Mosca,
Chem. Soc. Rev., 2010, 39, 3889–3915.
3 J. W. Steed, D. R. Turner and K. J. Wallace, in Core Concepts in
Supramolecular Chemistry and Nanochemistry, Wiley, Chichester,
UK, 2007, pp. 73–78.
Extracted ion
Polymer 2
NO3
ClÀ
0.487
0.219
12.63
5.60
44
19
32c
––
À
ReceÀptor 1
ClÀ
NO3
0.032
0.025
0.832
0.637
2.9
2.2
1.3c
—
a
No extraction of ClÀ and NO3À into pure chloroform is obseÀrved (see
4 (a) A. J. McConnell and P. D. Beer, Angew. Chem., Int. Ed., 2012,
51, 5052–5061; (b) S. K. Kim and J. L. Sessler, Chem. Soc. Rev.,
2010, 39, 3784–3809.
5 (a) S. J. Moore, M. G. Fisher, M. Yano and P. A. Gale, Chem.
Commun., 2011, 47, 689–691; (b) G. Cafeo, G. Gattuso,
F. H. Kohnke, A. Notti, S. Occhipinti, S. Pappalardo and
M. F. Parisi, Angew. Chem., Int. Ed., 2002, 41, 2122–2126;
(c) K. Kavallieratos, A. Danby, G. J. Van Berkel, M. A. Kelly,
R. A. Sachleben, B. A. Moyer and K. Bowman-James, Anal.
Chem., 2000, 72, 5258–5264.
b
c
ESI). Absorption intensity. Determined by the NO2À/NO3 colori-
metric test (Roche Applied Science) after back-extraction into H2O.
a heteroditopic salt receptor. We demonstrated that only
copolymer 2 is able to efficiently extract NaNO3 from aqueous
media. This shows that the proper combination of a salt
receptor and a polymeric matrix is needed for preparation of
an effective salt extractant.
6 A. Aydogan, D. J. Coady, S. K. Kim, A. Akar, C. W. Bielawski,
M. Marquez and J. L. Sessler, Angew. Chem., Int. Ed., 2008, 47,
9648–9652.
This work was supported by the Ministry of Science and
Higher Education (501/86-DSM-102400) and by the Faculty
of Chemistry of Warsaw (501-64BST-163220). We are grateful
to Prof. Janusz Jurczak for his help and encouragement.
7 For selective Cs+ cation extraction using a cation receptor incor-
porated in a polymeric system see: B. M. Rambo, S. K. Kim,
J. S. Kim, C. W. Bielawski and J. L. Sessler, Chem. Sci., 2010, 1,
716–722.
8 R. Custelcean and B. A. Moyer, Eur. J. Inorg. Chem., 2007,
1321–1340.
9 B. M. Rambo, E. S. Silver, C. W. Bielawski and J. L. Sessler,
Top. Heterocycl. Chem., 2010, 25, 1–37.
10 L. Maton, D. Taziaux, J.-P. Soumillion and J.-L. H. Jiwan,
J. Mater. Chem., 2005, 15, 2928–2937.
11 Polymer 2 (but not receptor 1) is also able to extract nitrate anions
from aqueous KNO3 with an extraction efficiency of 27%.
Notes and references
1 (a) G. W. Gokel, M. W. Leevy and M. E. Weber, Chem. Rev.,
2004, 104, 2723–2750; (b) A. W. Coleman, A. W. Lazar and
E. Da Silva, in Encyclopedia of Supramolecular Chemistry, ed. J. L.
Atwood and J. W. Steed, Pergamon, New York, 2004, 137–144;
(c) G. W. Gokel, in Encyclopedia of Supramolecular Chemistry,
c
11348 Chem. Commun., 2012, 48, 11346–11348
This journal is The Royal Society of Chemistry 2012