A novel phosphate chemosensor utilizing anion-induced fluorescence change

Article abstract of DOI:10.1021/ol0171564

(figure presented) The neutral receptor N,N′-bis{3,5-di[(1-pyrenylmethyl)carbamoyl]benzyl} pyridine-2,6-dicarbamide (2) provides a pseudo-tetrahedron cleft and multiple hydrogen bondings to form a 1:1 complex with phosphate ion in a highly selective manne

Full text of DOI:10.1021/ol0171564

ORGANIC
LETTERS
2002
Vol. 4, No. 4
561-564
A Novel Phosphate Chemosensor
Utilizing Anion-Induced Fluorescence
Change
Jen-Hai Liao, Chao-Tsen Chen, and Jim-Min Fang*
Department of Chemistry, National Taiwan UniVersity,
Taipei, Taiwan 106, Republic of China
jmfang@ccms.ntu.edu.tw
Received November 30, 2001
ABSTRACT
The neutral receptor N,N′-bis{3,5-di[(1-pyrenylmethyl)carbamoyl]benzyl} pyridine-2,6-dicarbamide (2) provides a pseudo-tetrahedron cleft and
multiple hydrogen bondings to form a 1:1 complex with phosphate ion in a highly selective manner, by comparison with other anions (F-, Cl-,
Br-, SCN-, AcO-, NO -, ClO -, and HSO -). The binding strength can be inferred from the emission intensity ratio of the pyrene monomer
3
4
4
(λmax 377 nm) to the excimer (λmax 477 nm). Fluorescence titration, X-ray analysis, and NMR studies support a proposed complexation model.
As a result of the biological and environmental significance
of phosphate ions, selective recognition and sensing of
phosphates are of great importance in the modern host-guest
chemistry.1 However, the inherent tetrahedral structure of
phosphate ion also disposes a challenging problem for the
design of effective receptors.2 Several synthetic receptors
bearing polyaza,1c,d,3 thiourea,4a-c urea,4d,e amide1a,5 and
quanidinium1b,6 moieties have been demonstrated to bind or
transport phosphates.7 The neutral receptors of thiourea and
amide types are particularly interesting, because the transport
of phosphates through cell membrane is also regulated by
neutral binding proteins. We report herein two artificial
neutral phosphate receptors 1 and 2 with multiple amide
(4) (a) Nishizawa, S.; Bu¨hlmann, P.; Iwao, M.; Umezawa, Y. Tetrahedron
Lett. 1995, 36, 6483-6486. (b) Bu¨hlmann, P.; Nishizawa, S.; Xiao, K. P.;
Umezawa, Y. Tetrahedron 1997, 53, 1647-1654. (c) Sasaki, S.; Mizuno,
M.; Naemura, K.; Tobe, Y. J. Org. Chem. 2000, 65, 275-283. (d) Xie, H.;
Yi, S.; Yang, X.; Wu, S. New J. Chem. 1999, 23, 1105-1110. (e) Snellink-
Rue¨l, B. H. M.; Antonisse, M. M. G.; Engbersen, J. F. J.; Timmerman, P.;
Reinhoudt, D. N. Eur. J. Org. Chem. 2000, 165-170.
(5) (a) Raposo, C.; Pe´rez, N.; Almaraz, M.; Mussons, M. L.; Caballero,
M. C.; Mora´n, J. R. Tetrahedron Lett. 1995, 36, 3255-3258. (b) Beer, P.
D.; Graydon, A. R.; Sutton, L. R. Polyhedron 1996, 15, 2457-2461. (c)
Szemes, F.; Hesek, D.; Chen, Z.; Dent, S. W.; Drew, M. G. B.; Goulden,
A. J.; Graydon, A. R.; Grieve, A.; Mortimer, R. J.; Wear, T.; Weightman,
J. S.; Beer, P. D. Inorg. Chem. 1996, 35, 5868-5879. (d) Beer, P. D.;
Szemes, F.; Balzani, V.; Sala`, C. M.; Drew, M. G. B.; Dent, S. W.; Maestri,
M. J. Am. Chem. Soc. 1997, 119, 11864-11875. (e) Beer, P. D.;
Timoshenko, V.; Maestri, M.; Passaniti, P.; Balzani, V. Chem. Commun.
1999, 1755-1756. (f) Cooper, J. B.; Drew, M. G. B.; Beer, P. D. J. Chem.
Soc., Dalton Trans. 2000, 2721-2728. (g) Hossain, Md. A.; Llinares, J.
M.; Powell, D.; Bowman-James, K. Inorg. Chem. 2001, 40, 2936-2937.
(6) (a) Gala´n, A.; de Mendoza, J.; Toiron, C.; Bruix, M.; Deslongchamps,
G.; Rebek, J., Jr. J. Am. Chem. Soc. 1991, 113, 9424-9425. (b) Ariga, K.;
Anslyn, E. V. J. Org. Chem. 1992, 57, 417-419. (c) Mu¨ller, G.; Du¨rner,
G.; Bats, J. W.; Go¨bel, M. W. Liebigs Ann. Chem. 1994, 1075-1092.
(1) (a) Beer, P. D. Acc. Chem. Res. 1998, 31, 71-80. (b) Watanabe, S.;
Onogawa, O.; Komatsu, Y.; Yoshida, K. J. Am. Chem. Soc. 1998, 120,
229-230. (c) Beer, P. D.; Cadman, J.; Lloris, J. M.; Mart´ınez-Ma´n˜ez, R.;
Padilla, M. E.; Pardo, T.; Smith, D. K.; Soto, J. J. Chem. Soc., Dalton
Trans. 1999, 127-133. (d) Beer, P. D.; Cadman, J. New J. Chem. 1999,
23, 347-349. (e) Anzenbacher, P., Jr.; Jurs´ıkova´, K.; Sessler, J. L. J. Am.
Chem. Soc. 2000, 122, 9350-9351. (f) Miyaji, H.; Sessler, J. L. Angew.
Chem., Int. Ed. 2001, 40, 154-157. (g) Sanceno´n, F.; Descalzo, A. B.;
Mart´ınez-Ma´n˜ez, R.; Miranda, M. A.; Soto, J. Angew. Chem., Int. Ed. 2001,
40, 2640-2643.
(2) Schmidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609-1646.
(3) Marecek, J. F.; Fischer, P. A.; Burrows, C. J. Tetrahedron Lett. 1988,
29, 6231-6234.
10.1021/ol0171564 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/24/2002

tetrahedral cleft to hold phosphates. As shown in the scheme,
5-(azidomethyl)isophthalic acid12 was elaborated to N,N′-
dicyclohexyl-5-(aminomethyl)benzene-1,3-dicarboxamide,
which coupled with pyridine-2,6-dicarboxyl dichloride to
give the host molecule 1 in 75% overall yield. Compound 2
was similarly prepared in 66% yield by replacing cyclo-
hexylamine with (1-pyrenyl)methylamine.
Scheme 1. Synthesis of Compounds 1-3a
The X-ray analysis of 1 revealed that the conformation of
pyridine 2,6-biscarboxamide was indeed confined by in-
tramolecular hydrogen bondings with N(1)‚‚‚H-N(2,5)
distances of 2.3 Å. The two phenyl rings were 6.73 Å apart
and almost in a parallel disposition, with a dihedral angle of
5.6° (Figure 1). The pyridyl ring was perpendicular to the
phenyl rings. This confined conformation thus constructed
a preorganized structural motif for accommodation of the
incoming phosphate ion.
a (i) SOCl2, CH2Cl2, rt; (ii) cyclohexylamine (for 1) or (1-
pyrenyl)methylamine hydrochloride (for 2), Et3N, CH2Cl2, rt; (iii)
H2, Pd/C, MeOH (for 1); PPh3/THF, then reflux in THF/H2O (for
2 and 3); (iv) pyridine-2,6-dicarboxyl dichloride, Et3N, CH2Cl2, rt;
overall yield: 1, 75%; 2, 66%; (v) pyridine-2-carboxyl chloride,
Et3N, CH2Cl2, rt; overall yield: 3, 72%.
scaffolds (Scheme 1). These compounds provide a delicate
structural cleft with complementary amido groups that
Figure 1. The crystal structure of compound 1 (side view).
Incorporation of MeOH molecules and partial intercalation among
molecules 1 was shown in the crystal packing (see Supporting
Information).
3-
-
selectively hydrogen bonded with PO4 and H2PO4 ions.
Compound 2 bearing the pyrene fluorophores also exhibits
a unique sensing property when it binds with phosphate ion.8
The structural cleft is established by pyridine 2,6-biscar-
boxamide, which has a rigid conformation through the
intramolecular hydrogen bondings.9 It is known that the two
amido protons can serve as hydrogen-bond donors for
acetate,10 nitrate,10 and fluoride11 ions. Thus, modification
of pyridine-2,6-biscarboxamide by incorporating four ad-
ditional amido groups would likely provide a pseudo-
The binding properties of 1 with a variety of anions were
1
evaluated by the H NMR studies in DMSO-d6 solutions.
Pertinent protons of 1 showed significant chemical-shift
changes upon addition of tetrabutylammonium dihydrogen
phosphate, for example, a 0.98 ppm downfield shift of N2,N5-
H, 0.27 ppm downfield shift of N3,N4,N6,N7-H, and 0.16 ppm
downfield shift of phenyl protons Ho were observed in the
-
presence of 4 equiv of (Bu4N)+H2PO4 . The 1:1 complex-
(7) (a) Rudkevich, D. M.; Verboom, W.; Brzozka, Z.; Palys, M. J.;
Stauthamer, W. P. R. V.; van Hummel, G. J.; Franken, S. M.; Harkema,
S.; Engbersen, J. F. J.; Reinhoudt, D. N. J. Am. Chem. Soc. 1994, 116,
4341-4351. (b) Antonisse, M. M. G.; Reinhoudt, D. N. Chem. Commun.
1998, 443-448. (c) Chrisstoffels, L. A. J.; de Jong, F.; Reinhoudt, D. N.
Chem. Eur. J. 2000, 6, 1376-1385.
ation stoichiometry of receptor 1 with H2PO4- was confirmed
by a continuous variation method (Job plot).13 The similar
NMR changes also occurred when 1 was treated with tris-
(tetrabutylammonium)phosphate (Bu4N+)3PO43-. These ex-
(8) Pyrene has been used as fluorosence probes. For sensing acetate ion
according to the mechanism of photoinduced electron transfer, see: (a)
Nishizawa, S.; Kaneda, H.; Uchida, T.; Terame, N. J. Chem. Soc., Perkin
Trans. 2 1998, 2325-2327. For sensing pyrophosphate-induced self-
assembly of (guanidinium)methylpyrene, see: (b) Nishizawa, S.; Kato, Y.;
-
periments indicated that the complexes 1‚H2PO4 and
3-
1‚PO4 might be formed via hydrogen bondings with the
six amido protons. Compound 1 also formed a 1:1 complex
with CH3CO2- in a relatively weak binding. The association
constants Ka and free energy data for acetate and phosphate
ions were collected in Table 1.
Teramae, N. J. Am. Chem. Soc. 1999, 121, 9463-9464. For sensing Ca2+
,
Cd2+, and Cu2+ ions, see: (c) Yang, J.-S.; Lin, C.-S.; Hwang, C.-Y. Org.
Lett. 2001, 3, 889-892.
(9) Yu, Q.; Baroni, T. E.; Liable-Sands, L.; Rheingold, A. L.; Borovik,
A. S. Tetrahedron Lett. 1998, 39, 6831-6834.
(10) Bisson, A. P.; Lynch, V. M.; Monahan, M.-K. C.; Anslyn, E. V.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2340-2342.
(11) Kavallieratos, K.; Bertao, C. M.; Crabtree, R. H. J. Org. Chem.
1999, 64, 1675-1683.
(12) Dimick, S. M.; Powell, S. C.; McMahon, S. A.; Moothoo, D. N.;
Naismith, J. H.; Toone, E. J. J. Am. Chem. Soc. 1999, 121, 10286-10296.
(13) Connors, K. A. Binding Constants; Wiley: New York, 1987.
562
Org. Lett., Vol. 4, No. 4, 2002

3-
Figure 2. The proposed models for fluorescence change before and after complexation of compound 2 with PO4
.
Unlike the previous report10 on the binding of pyridine
emission appeared at λmax ) 377 and 397 nm, whereas the
excimer emission appeared at λmax ) 477 nm. The excimer
emission was resulted from the intramolecular excimer, rather
than intermolecularly, as indicated by the dilution experi-
ments at different concentrations (5 × 10-,6 1 × 10-,6 5 ×
10-7, and 1 × 10-7 M), in which the intensities of
fluorescence of monomer and excimer decreased simulta-
neously on dilution.
To discern which pair of pyrene rings were responsible
for the excimer emission, the one-arm analogue 3 was
prepared as a control molecule by condensation of pyridine-
2-carboxyl chloride with N,N′-di(1-pyrenylmethyl)-5-ami-
nomethyl-1,3-benzenedicarboxamide (Scheme 1). Compound
3 in THF solution at 1 × 10-6 M concentration showed only
monomer emissions but no excimer emission. Upon addition
of H2PO4- to 3, the chemical-shift change of N3,N4,N6,N7-H
was much more prominent than that of N2,N5-H, indicating
-
2,6-biscarboxamide with NO3 , compound 1 did not bind
-
effectively with this anion. Neither Br-, I-, SCN-, ClO4 ,
-
nor HSO4 ions showed appreciable binding with 1. The
neutral amido groups of 1 might not interact effectively with
-
the anions of low basicity such as Br-, I-, SCN-, NO3 ,
-
-
ClO4 , and HSO4 . The structural complementarity was
another important factor to account for the effective hydrogen
bondings. Thus, molecule 1 with a pseudo-tetrahedron cleft
would bind preferably with H2PO4- and PO43- of tetrahedral
geometry but not with spherical (e.g., Br-), linear (e.g.,
-
SCN-), or planar (e.g., NO3 ) ions.
By replacing the cyclohexyl groups in 1 with 1-pyrenyl-
methyl groups, compound 2 served as a selective chemosen-
sor for phosphates.14 The pyrene rings were annexed to the
terminals to avoid changing the cleft shape. The association
-
constant of 2‚H2PO4 was determined to be 1374 M-1 in
DMSO-d6 by 1H NMR titration method (Table 1). This value
-
was even higher than that for 1‚H2PO4 (Ka ) 549 M-1).
-
-
Receptor 2 also had a better H2PO4 /CH3CO2 selectivity
-
-
[Ka (H2PO4 )/Ka (CH3CO2 ) ) 5.75] by comparison with
-
-
that for receptor 1 [Ka (H2PO4 )/Ka (CH3CO2 ) ) 3.45].
The UV-vis spectrum of 2 in anhydrous THF showed
absorptions at λmax ) 277 and 345 nm at a concentration of
1 × 10-6 M. In the fluorescence spectrum, the monomer
Table 1. Association Constants Ka (M-1) of Receptors 1 and 2
with Anions in DMSO-d6
receptor 1
receptor 2
Ka
(M-1
-∆G300
Ka
(M-1
-∆G300
b
b
aniona
AcO-
)
(kJ /mol)c
)
(kJ /mol)c
159
549
12.6
15.7
239
1374
13.7
18.0
-
H2PO4
Figure 3. Fluorescence titration of 2 with various anions in THF.
The binding strength can be inferred from the intensity ratio of the
pyrene monomer to the excimer emission.
a Bu4N+ as the counterion. b The average value of two independent
titrations at 300 K. c -∆G300 ) RT lnKa.
Org. Lett., Vol. 4, No. 4, 2002
563

-
3-
Figure 4. Fluorescence spectra of 2 with Bu4N+H2PO4 (left) and (Bu4N+)3PO4 (right). [R0] ) 1 × 10-6 M in THF (2 mL).
-
the 3‚H3PO4 complex was formed with a binding mode
different from that of 1‚H2PO4- or 2‚H2PO4- complexes. By
1H NMR titration, the association constant of 3‚H2PO4- was
determined to be 198 M-1 in DMSO-d6, much smaller than
intramolecular excimer of 2 requires an orientation of pyrene
rings in close proximity. The pyrene rings would be pushed
apart on complexation with phosphate ion to disfavor the
excimer formation. The N3/N6 and N4/N7 pyrene pairs were
thus segregated to reduce the excimer emission, accompanied
by increase of the monomer emissions.16
-
that of 2‚H2PO4 (Ka ) 1374 M-1).
Thus, the excimer emission of compound 2 could be
attributable to the excimer formed by the pair of N3- and
N6-CH2-pyrenes (or N4- and N7-CH2-pyrenes), rather than
the pyrene rings on the same sidearm. This uncommon
organized conformation of 2 might be partly due to the π-π
stacking of pyrene rings (Figure 2).
In summary, we have designed a new type of phosphate
receptor with unique recognition and sensing properties.
Receptors 1 and 2 have well-defined structures for efficient
and selective complexation with phosphates in 1:1 binding
stoichiometry. Although the binding of 1 and 2 with
phosphates is not extremely strong, the binding selectivity
is very high by comparison with other anions (e.g., halide,
thiocyanate, acetate, nitrate, perchlorate, and sulfate ions).
Addition of phosphate causes a simultaneous change of the
monomer and excimer emissions of 2. This fluorescence
change can possibly be utilized in diagnostic devices for the
measurement of phosphate concentrations, even in the
presence of other anions. Indeed, our preliminary experiments
indicated that receptor 2, in the presence of excess F- or
The sensing properties of compound 2 with various anions
-
-
-
-
(F-, Cl-, Br-, SCN-, AcO-, NO3 , ClO4 , HSO4 , H2PO4 ,
and PO43-) were investigated by fluorescence titration studies
(Figure 3). It appeared that the conformation of 2 only
changed substantially on binding with phosphate ions.
A typical fluorescence titration curve of sensor 2 (in THF)
with phosphates (Figure 4) was derived from the experiments
-
3-
with portionwise additions of H2PO4 or PO4 [1-37.5
equiv of Bu4NH2PO4 or (Bu4N)3PO4 in THF/CHCl3 (1:1)].
In agreement to our prediction, the ratio of excimer emission
to monomer emissions decreased as the amounts of anion
increased. An isoemissive point occurring at 430 nm that
supported only one type of complexation (2‚phosphate)
involved. The association constants in THF were deduced
to be 186300 ( 16200 and 328400 ( 19200 M-1 for
2‚H2PO4- and 2‚PO43-, respectively. The sensing mechanism
appeared to have phosphate ion encapsulated into the core
of the cleft via six hydrogen bonds (Figure 2).15 The binding
AcO- ions (50 equiv), showed a significant fluorescence
3-
change upon addition of (Bu4N+)3PO4
.
Acknowledgment. We thank the National Science Coun-
cil for financial support and the Instrumentation Center
(National Taiwan University) for X-ray analysis.
Supporting Information Available: The detailed ex-
perimental procedures, spectral data, NMR spectra of selected
compounds, and NMR and fluorescence titration curves, as
well as the crystal data, bond distances and bond angles of
compound 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
-
of one-arm compound 3 with H2PO4 is relatively weak as
it cannot provide six hydrogen bondings. Formation of the
(14) For application of anion recognition in sensors, see: (a) Snowden,
T. S.; Anslyn, E. V. Curr. Opin. Chem. Biol. 1999, 3, 740-746. (b) Beer,
P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486-516. For use of
pyrene as the fluorophore in cation sensing, see: (c) Jin, T.; Ichikawa, K.;
Koyama, T. Chem. Commun. 1992, 499-501. (d) Takeshita, M.; Shinkai,
S. Chem. Lett. 1994, 125-128. (e) Diamond, D.; McKervey, M. A. Chem.
Soc. ReV. 1996, 15-24. (f) Jin, T. Chem. Commun. 1999, 2491-2492. For
use of pyrene as the fluorophore in barbital sensing, see: (g) Aoki, I.;
Harada, T.; Sakaki, T.; Kawahara, Y.; Shinkai, S. Chem. Commun. 1992,
1341-1345. (h) Aoki, I.; Kawahara, Y.; Sakaki, T.; Harada, T.; Shinkai,
S. Bull. Chem. Soc. Jpn. 1993, 66, 927-933.
OL0171564
(15) A molecule containing two 2,6-diaminopyridine units linked through
an isophthalate spacer forms six hydrogen bonds with barbiturte substrates
as shown by the NMR and X-ray analyses. See: Chang, S. K.; Van Engen,
D.; Fan, E.; Hamilton, A. D. J. Am. Chem. Soc. 1991, 113, 7640.
(16) For a general review of guest molecule induced fluorescence change,
see: de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J.
M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515-1566.
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Org. Lett., Vol. 4, No. 4, 2002