Application of thiouronium derivatives as anion ionophores for ion selective electrodes

Article abstract of DOI:10.1246/cl.2001.382

Novel anion ionophores based on thiouronium derivatives were synthesized, and their application to ion selective electrodes (ISEs) was examined. The electrodes clearly exhibited an anti-Hofmeister selectivity pattern with enhanced selectivity for the iodi

Full text of DOI:10.1246/cl.2001.382

382
Chemistry Letters 2001
Application of Thiouronium Derivatives as Anion Ionophores for Ion Selective Electrodes
Shin-ichi Sasaki,† Akio Hashizume,† Satoru Ozawa,† Daniel Citterio,†† Naoko Iwasawa,†† and Koji Suzuki*†,††
†Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522
††Kanagawa Academy of Science and Technology (KAST), KSP West-614, 3-2-1 Sakato, Kawasaki 213-0012
(Received February 6, 2001; CL-010103)
salts of the ionophores were also examined,10 but none of them
gave clear PVC membranes due to their limited solubility.
Thus, three ionophores 1a, 1b, and 2b were successfully incor-
porated in PVC electrode membranes and examined for their
potentiometric response to several representative anions. The
membrane composition was 1% (by weight) ionophore, 66%
membrane solvent o-NPOE and 33% PVC in all cases. The
electrode potential (emf) measurements were performed
according to the reported procedure at 25 ± 0.5 °C using the
following cell assembly;11
Novel anion ionophores based on thiouronium derivatives
were synthesized, and their application to ion selective elec-
trodes (ISEs) was examined. The electrodes clearly exhibited
an anti-Hofmeister selectivity pattern with enhanced selectivity
for the iodide anion.
The design and synthesis of new anion receptors is of cur-
rent interest due to their possible application to ion selective
electrodes (ISEs) or optodes.1,2 For example, several receptors
based on hydrogen bonding with urea or thiourea groups have
been synthesized,3 and applied to anion sensing as chromore-
ceptors in solution4 or as ionophores for ISEs.5 In contrast to
these neutral receptors, it has been known that thiouronium
compounds, which correspond to the charged derivative of
thiourea, can form complexes with anions. Their binding is
stronger than in the case of thiourea because of the enhanced
acidity of the NH groups.6 However, the number of thiouroni-
um compounds used for anion sensing is quite limited except
for a few examples.7 In fact, to our knowledge, there appears to
be no reports on ISEs using thiournium derivative as an anion
ionophore, despite the fact that several charged ionophores hav-
ing hydrogen bonding sites such as the guanidinium derivatives
have been successfully used for anion selective detection with
ISEs.8 Here we report the characteristics of the thiouronium
derivatives as anion ionophores for ISEs.
All sample solutions were made from sodium salts and
adjusted to pH 7.0 using 0.1 M HEPES-NaOH buffer solution.
The selectivity coefficients were calculated from the response
potentials using the separate solution method, based on the
IUPAC recommendation.12
The ISEs based on these charged ionophores responded to
several anions without the addition of a cationic additive. The
ISE based on ionophore 1a exhibited a sub-Nernstian response
to I– (slope = –44 mV/decade) as shown in Figure 1. A weak
Ionophores 1a and 2a, which have N-butyl side chains and
S-methyl groups, were prepared by the reaction of the corre-
sponding thiourea9 with methyl iodide. In order to increase the
solubility of the charged ionophores in the membrane solvent,
ionophores 1b and 2b were also synthesized. Thiourea com-
pounds having dodecyl groups were prepared by the reaction of
benzylamine or m-xylylenediamine with dodecyl isothio-
cyanate, and then converted into thiouronium iodide salts by the
reaction with butyl iodide in methanol in quantitative yield.
Among the synthesized ionophores, it turned out that the
doubly charged ionophore 2a having short alkyl chains did not
show enough solubility in the membrane solvent. The chloride
–
anionic response to lipophilic anions like ClO4 or salicylate
was observed at relatively high anion concentrations. The ISEs
based on 1b and 2b exhibited essentially the same response
characteristics as the ISE based on 1a (the response to I– was
–45 and –44 mV/decade, respectively), and no structural effects
such as the number of anion binding sites or the length of the
alkyl chains could be observed.
The selectivity coefficients are summarized in Figure 2.
Although the theoretical response slope could not be observed,
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
383
and E. Pretsch, Chem. Rev., 97, 3083 (1997). b) P.
Bülmann, E. Pretsch, and E. Bakker, Chem. Rev., 98, 1593
(1998).
3
4
a) S. Sasaki, M. Mizuno, K. Naemura, and Y. Tobe, J. Org.
Chem., 65, 275 (2000). b) B. H. M. S.-Ruël, M. G.
Antonisse, J. F. J. Engbersen, P. Timmerman, and D. N.
Reinhoudt, Eur. J. Org. Chem., 2000, 165 and references
cited therein.
a) S. Nishizawa and N. Teramae, Anal. Sci., 13, 485
(1997). b) S. Nishizawa, R. Kato, T. Hayashita, and N.
Teramae, Anal. Sci., 14, 595 (1998). c) S. Nishizawa, H.
Kaneda, T. Uchida, and N. Teramae, J. Chem. Soc., Perkin
Trans 2, 1998, 2325. d) H. Xie, S, Yi, and S. Wu, J. Chem.
Soc., Perkin Trans 2, 1999, 2751.
5
a) K. P. Xiao, P. Bülmann, S. Nishizawa, S. Amemiya, and
Y. Umezawa, Anal. Chem., 69, 1038 (1997). b) S.
Nishizawa, P. Bülmann, K. P. Xiao, and Y. Umezawa,
Anal. Chim. Acta, 358, 35 (1998). c) S. Amemiya, P.
Bülmann, Y. Umezawa, R. C. Jagessar, and D. H. Burns,
Anal. Chem., 71, 1049 (1999).
every electrode exhibited a high selectivity to I–, which is dif-
ferent from the well known Hofmeister series (TDDMACl, col-
umn 1). Moreover, it should be noted that the interference from
other anions is quite limited since only a weak response to high-
ly lipophilic anions is observed. This behavior is comparable to
those of the ISEs based on other iodide selective ionophores
represented by metal complexes or quaternary ammonium
salts.13 Although it is reported that thiouronium derivatives
effectively form complexes with oxoanions such as phosphate
or acetate in organic solution,6,7b ISEs based on 1a, 1b, and 2b
showed almost no response to phosphate which is located in the
last part of the Hofmeister sequence. Since the positive charge
in the thiouronium group is delocalized, the interaction of this
functional group with anions seems rather weak compared to
that of the quaternary ammonium salts such as TDDMACl.
Therefore, it is postulated that the high selectivity to iodide ions
is the result of the lipophilicity.
6
7
W.-S. Yeo and J.-I. Hong, Tetrahedron Lett., 39, 8137
(1998).
a) W.-S. Yeo and J.-I. Hong, Tetrahedron Lett., 39, 3769
(1998). b) Y. Kubo, M. Tsukahara, S. Ishihara, and S.
Tokita, Chem. Commun., 2000, 653.
a) R. S. Hutchins, P. Molina, M. Alajarín, A. Vidal, and L.
G. Bachas, Anal. Chem., 66, 3188 (1994). b) R. S.
Hutchins, P. Bansal, P. Molina, M. Alajarín, A. Vidal, and
L. G. Bachas, Anal. Chem., 69, 1273 (1997). c) M.
Fibbioli, M. Berger, F. P. Schmidtchen, and E. Pretsch,
Anal. Chem., 72, 156 (2000).
a) L. E. Weller, C. D. Ball, and H. M. Sell, J. Am. Chem.
Soc., 74, 1104 (1952). b) P. Bülmann, S. Nishizawa, K. P.
Xiao, and Y. Umezawa, Tetrahedron, 53, 1647 (1997).
8
9
In summary, we have demonstrated that the nature of the
thiouronium group is promising for the development of new
anion ionophores for ISEs. Further attempts for improving the
selectivity caused by factors other than the ionophore, for
example, the addition of a cationic additive, are now in
progress.
10 The chloroform solutions of the iodide salts of the
ionophores were shaken several times with saturated aque-
ous NaCl. The complete exchange of the counter anion
could be confirmed by TLC monitoring.
11 K. Suzuki, K. Tohda, H. Aruga, M. Matsuzoe, H. Inoue,
and T. Shirai, Anal. Chem., 60, 1714 (1988).
12 a) IUPAC Recommendations for Nomenclature of Ion-
Selective Electrodes. Pure Appl. Chem., 66, 2527 (1994).
b) IUPAC Selectivity Coefficients for Ion-Selective
D.C. gratefully acknowledges a post-doctoral fellowship
granted by the Science and Technology Agency of Japan
(STA).
pot
Electrodes: Recommended Methods for Reporting KA,B
Values. Pure Appl. Chem., 67, 507 (1995).
References and Notes
13 a) S. Daunert and L. G. Bachas, Anal. Chem., 61, 499
(1989). b) V. J. Wotring, D. M. Johnson, and L. G. Bachas,
Anal. Chem., 62, 1506 (1990). c) R. Yuan, Y.-Q. Chai, D.
Liu, D. Gao, J.-Z. Li, and R.-Q. Yu, Anal. Chem., 65, 2572
(1993). d) F. Z. E. Aamrani, A. Sastre, M. Aguilar, L.
Beyer, and A. Floride, Anal. Chim. Acta, 329, 247 (1996).
1
Reviews for anion receptors, a) J. Scheerder, J. F. J.
Engbersen, and D. N. Reinhoudt, Recl. Trav. Chim. Pays-
Bas, 115, 307 (1996). b) F. P. Schmidtchen and M.
Berger, Chem. Rev., 97, 1609 (1997).
2
Reviews for ISEs and optodes, a) E. Bakker, P. Bülmann,