Superior perrhenate anion recognition in water by a halogen bonding acyclic receptor

Article abstract of DOI:10.1039/c4cc10130h

A halogen bonding bis-iodotriazolium water-soluble acyclic receptor exhibits enhanced perrhenate anion recognition in water compared to the hydrogen bonding analogue, and is shown to be capable of sensing the oxoanion via a fluorescence response. This jou

Full text of DOI:10.1039/c4cc10130h

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Superior perrhenate anion recognition in water by
a halogen bonding acyclic receptor†
Cite this:DOI: 10.1039/c4cc10130h
Jason Y. C. Lim and Paul D. Beer*
Received 18th December 2014,
Accepted 26th January 2015
DOI: 10.1039/c4cc10130h
www.rsc.org/chemcomm
A halogen bonding bis-iodotriazolium water-soluble acyclic receptor bis-iodotriazolium acyclic receptor over a HB analogue in pure
exhibits enhanced perrhenate anion recognition in water compared water and demonstrate its capability to detect the oxoanion via
to the hydrogen bonding analogue, and is shown to be capable of a fluorescence response.
sensing the oxoanion via a fluorescence response.
Inspired by the plethora of water-soluble hydrophilic systems
26,27
used in drug delivery,
we employed a tris-tetra(ethylene glycol)
As a result of anthropogenic activities, oxoanions of many d-block (TEG)-based unit to confer water solubility onto the lipophilic HB
1
metals have increasingly found their way into the environment.
(1a) and XB (1b) host systems (Fig. 1). The rigid phenyl spacer unit
9
9
À
Among which, radioactive pertechnetate ( TcO
4
), a by-product between the triazolium motifs preorganises the binding site for
of the nuclear industry, is an especially hazardous environmental convergent XB and HB interactions with a potential anion guest,
2
pollutant due to its long half-life and stability, physiological while the dicationic nature of the hosts serves to electrostatically
3
4
toxicity, and high mobility through the biosphere. However, enhance the binding strength in water. In addition, the conju-
9
9
À
direct studies on TcO
4
have been thwarted by its radioactive gated nature of the host molecules facilitates the opportunity of
À
nature, and hence, the perrhenate anion (ReO
4
) has been anion sensing by optical spectroscopic techniques.
The water-soluble terminal units were synthesised by copper(I)-
in solution due to their comparable physical properties such as mediated Ullmann amination of the TEG-functionalised aryl
commonly used as a model for studying pertechnetate binding
5–7
hydration energies and size. Nevertheless, perrhenate detection in bromide to form the aniline intermediate, followed by Sandmeyer-
aqueous media is also of scientific interest due to the oxoanion type substitution to afford the aryl-azide 2 in excellent overall yield
being a precursor to medical beta-emitting radioimmunotherapeutic (Scheme 1). Copper(I)-catalysed azide–alkyne cycloaddition (CuAAC)
agents arising from the radioisotopes Re and Re, and as of two equivalents of 2 with 1,3-diethynylbenzene produced the
a convenient starting material for rhenium-based industrial bis-prototriazole compound 3a (Scheme 2). Surprisingly, one-pot
1
86
188
8,9
1
0
28,29
catalysts. In spite of this, there are very few synthetic receptors protocols
to synthesise the corresponding bis-iodotriazole 3b
capable of recognising perrhenate in water, with the majority from 2 and the bis-alkyne gave no product. Hence, 3b was prepared
30
operating in organic solvents utilising convergent hydrogen utilising a two-pot methodology by first synthesising 1,3-bis-
5
–7
11
bonding interactions and metal-coordination.
(iodoethynyl)benzene, followed by CuAAC with two equivalents of
In recent years, halogen bonding (XB) has emerged as a powerful
complement to hydrogen bonding (HB) for anion recognition in
1
2–16
solution,
exhibiting the highly-directional intermolecular
XB interaction between a Lewis-acidic halogen atom on the
1
7–19
host receptor and the Lewis-basic guest anion.
However, its
exploitation in aqueous-phase anion recognition remains in its
2
0–24
infancy.
Recently, we reported a water-soluble XB rotaxane,
which displayed a remarkable enhancement in iodide binding
2
5
in pure water compared to HB rotaxane analogues. Herein, we
report the superior perrhenate anion binding properties of a XB
Chemistry Research Laboratory, Department of Chemistry, University of Oxford,
Mansfield Road, Oxford, OX1 3TA, UK. E-mail: paul.beer@chem.ox.ac.uk
Fig. 1 Structures of the target dicationic water-soluble acyclic HB and XB
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc10130h
2 2 4
host molecules. OTEG = (OCH CH ) OMe.
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À1
a
Table 1 Anion association constants (K /M ) in water
a
À1
1a
1b
DGhyd/kJ mol
À
32
Cl
11
14
14
10
12
22
44
51
À340
4À
32
ClO
À205
À
32
Br
À315
4À
b
5
ReO
—
29
À330
À
32
I
À275
2
À
b
b
32
SO
4
—
—
À1295
a
1
1
: 1 stoichiometric association constants were calculated from H NMR
3
1
2
titrations in D O at 298 K using the WinEQNMR2 software by monitoring
H (see Scheme 2 for assignment); anions were added as their sodium salts
a
b
(
see ESI for further details). Errors were found to be less than 15%. No
binding observed.
2 2 4
Scheme 1 Synthesis of aryl azide 2. OTEG = (OCH CH ) OMe.
energies to perrhenate, the association constant for perrhenate
binding with XB receptor 1b is approximately four times larger
in magnitude. Interestingly, despite being dianionic, sulfate did
not show any evidence of binding to either receptor, presumably
due to its very high hydration energy.
In order to determine the thermodynamic driving force for
perrhenate binding in water by the XB receptor 1b, van’t Hoff
1
analysis was undertaken via variable-temperature (VT) H NMR
titration experiments (Fig. 2). It is noteworthy that perrhenate
binding is driven by a favourable enthalpic contribution
À1
(
DH = À12 Æ 1 kJ mol ) and is disfavoured entropically (DS =
À1
À1
À9 Æ 3 J K mol ). This is a clear indication that the classical
Scheme 2 Synthesis of the nitrate salts of water-soluble receptors 1a and
hydrophobic effect, well-known to be driven largely by an entropic
1
b. OTEG = (OCH
2
CH
)
2 4
OMe.
33
increase arising from desolvation, may not be the dominant
factor accounting for binding in this case. The enthalpic gain may
2
in 89% yield. The triazole moieties of 3a and 3b were then arise from the iodotriazolium moieties of 1b forming strong XB
methylated using trimethyloxonium tetrafluoroborate followed interactions with the perrhenate anion. Indeed, iodide binding
by anion exchange to the weakly coordinating nitrate anion‡ by a XB rotaxane host system in water was also shown to be
s
25
using an Amberlite column, to produce the water soluble host enthalpically favoured and opposed by entropy.
systems, 1a and 1b (Scheme 2).
The conjugated nature of receptors 1a and 1b also enabled
The anion recognition properties of receptors 1a and 1b were perrhenate binding to be investigated by preliminary fluorescence
1
investigated using H NMR titration experiments in D O at 298 K, spectroscopic titration experiments in an aqueous solution (10 mM
2
À
À
À
by adding increasing quantities of the sodium salts of Cl , Br , I , HEPES, pH 7.4).8 Upon titration of sodium perrhenate with receptor
2À
À
À
SO
addition of anions caused downfield perturbations§ of the signal consistent with the results obtained from H NMR titrations
arising from the ortho-aromatic protons (H in Scheme 2) of the
4
, ClO
4
and ReO
4
to the respective host solution. The 1a, no significant spectral changes were observed (Fig. 3A), which is
1
a
tris-TEG units, as well as from the phenyl spacer proton in between
the triazolium units. This indicated that anion binding was taking
place within the acyclic receptors’ cleft flanked by the triazolium
31
units. WinEQNMR2 analysis of the titration data determined 1 : 1
stoichiometric association constants shown in Table 1.¶
Both receptors 1a and 1b showed Hofmeister bias for halide
binding in water, with iodide being bound the strongest. The
XB receptor 1b was found to bind bromide and iodide more
strongly than the HB receptor 1a which is consistent with
2
5
previous observations.
Most importantly, the XB receptor 1b was found to bind
perrhenate almost as strongly as iodide. In stark contrast, the
analogous HB receptor 1a, although capable of binding iodide,
did not show any clear evidence of perrhenate binding in water.
This observation clearly demonstrates the superiority of XB over
HB bonding for perrhenate recognition in water. Furthermore,
Fig. 2 Van’t Hoff plot showing the variations of association constants (K
a
)
with temperature (T), determined from VT NMR titrations (500 MHz, D O,
2
in spite of perchlorate’s lower and chloride’s similar hydration T = 298 K, 308 K, 318 K, 338 K). Errors in parentheses.
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1
¶
The H NMR signal arising from proton of the phenyl spacer in
between the triazoliums was not used to probe anion binding as
coalescence of the signal was observed with those arising from the
other phenyl protons during the titration experiments. Instead, the
ortho-aromatic tris-TEG proton signals for 1a and 1b were monitored for
consistency.
1
8
A H NMR titration was also performed on 1b using 10 mM HEPES
solution in D
constant obtained (K
from the titrations carried out in pure D
2
O (pD = 7.4) with perrhenate at 298 K. The association
À1
a
= 45 Æ 4 M ) was consistent with those obtained
2
O (see ESI†).
1
2
3
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Fig. 3 Luminescence spectra of (A) HB receptor 1a and (B) XB receptor 1b
À
upon titration of increasing quantities of ReO
4
up to 500 equivalents ([host] =
10 mM in 10 mM aqueous HEPES buffer, pH = 7.4, lex = 320 nm, T = 293 K).
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1
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In summary, a water-soluble XB bis-iodotriazolium acyclic
receptor has been shown to exhibit superior perrhenate anion
recognition behaviour in water compared to the hydrogen bonding
analogue. Thermodynamic analysis has revealed that the halogen
bond-driven perrhenate binding is favoured enthalpically and
disfavoured entropically. Furthermore, the XB receptor is capable
of sensing perrhenate in water via a fluorescent response. To the
best of our knowledge, this is the first example of fluorescent
perrhenate sensing utilising halogen bonding interactions. The
design and construction of XB receptors for anion recognition and
sensing applications in water is continuing in our laboratories.
J.Y.C.L acknowledges the Agency for Science, Technology and
Research (A*STAR), Singapore, for a postgraduate scholarship.
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Notes and references
1
‡
A control H NMR titration experiment was performed using 1a.2PF
6
with sodium nitrate in water (see ESI†), which showed no evidence of
nitrate binding even at a nitrate: 1a.2PF mole ratio of 120 : 1.
6
§
All anions gave downfield perturbations of the tris-TEG aromatic ortho- 32 B. A. Moyer and P. V. Bonnesen, in Supramolecular Chemistry of
À
proton signal for receptor 1a except ClO
shift (see ESI†). All anions gave downfield perturbations of this signal
for receptor 1b, including ClO
4
, which showed an upfield
Anions, ed. A. Bianchi, K. Bowman-James and E. Garcia-Espana,
Wiley-VCH, Inc., New York, 1979, pp. 1–44.
À
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.
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Chem. Commun.