New P-bridgehead urea-based tripodal anion receptors for H 2PO4- recognition

Article abstract of DOI:10.1016/j.tetlet.2012.05.018

A series of new P-bridgehead tripodal urea-based anion receptors 1-3 bearing phosphine oxide (PO) were synthesized and characterized. Their anion-binding ability was examined by UV-vis, ¹H NMR, ³¹P NMR, and ESI-MS. The results revealed that receptors 1-3 showed good sensitivity, selectivity, and binding affinity for H₂PO ₄^- over NO₃^- and Br^-, and among them receptor 2 showed the best binding affinity for H₂PO ₄^- over F^-, Br^-, CH ₃COO^-, HSO₄^-, and NO ₃^-. The Job plot experiments indicated that receptors 1-3 formed 1:1 stoichiometric complex with H₂PO₄^-.

Full text of DOI:10.1016/j.tetlet.2012.05.018

Tetrahedron Letters 53 (2012) 3637–3641
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
ꢀ
New P-bridgehead urea-based tripodal anion receptors for H2PO4 recognition
⇑
Lin Wu, Li Liu, Ru Fang, Chao Deng, Jun Han, Hongwen Hu, Chen Lin , Leyong Wang
Key Laboratory of Mesoscopic Chemistry of Ministry of Education, Center for Molecular Organic Chemistry, School of Chemistry and Chemical Engineering,
Nanjing University, 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new P-bridgehead tripodal urea-based anion receptors 1–3 bearing phosphine oxide (P@O)
were synthesized and characterized. Their anion-binding ability was examined by UV–vis, 1H NMR, 31P
NMR, and ESI-MS. The results revealed that receptors 1–3 showed good sensitivity, selectivity, and bind-
ing affinity for H2PO4ꢀ over NO3ꢀ and Brꢀ, and among them receptor 2 showed the best binding affinity
for H2PO4ꢀ over Fꢀ, Brꢀ, CH3COOꢀ, HSO4ꢀ, and NO3ꢀꢀ. The Job plot experiments indicated that receptors 1–
Received 17 January 2012
Revised 12 April 2012
Accepted 4 May 2012
Available online 11 May 2012
3 formed 1:1 stoichiometric complex with H2PO4
.
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Anion receptor
P-bridgehead
Urea-based
Dihydrogenphosphate
Tripod
It is well known that anion recognition plays an important role
in biology, medicine, catalysis, and environment.1 Meanwhile, an-
ion recognition as an important research subject of supramolecular
chemistry, has drawn extensive attention in the recent 40 years.2
So far, a number of anion receptors with various functional groups
as anion binding units have been designed and prepared. In gen-
eral, there are three types of anion receptors: (i) positively charged
anion receptors with positively charged ligands as anion binding
units (guanidinium groups,3 quaternary ammonium,4a porphyrin-
based ligands,4b pyridinium,4c and imidazolium ions4d); (ii)
metal- coordinating anion receptors with metal complexes as an-
ion binding units;5 (iii) neutral anion receptors with neutral li-
gands to provide hydrogen bonding donors as anion binding
units (ureas, thioureas, calix[4]pyrroles, pyrrole subunit, and
amides et al.).6 Neutral anion receptors have been extensively
studied for the reason that in the recognition process counterion
interferences can be ignored and hydrogen bonding exerts direc-
tional interactions with certain anions.6i However, their binding
ability is weaker than the positively charged anion receptors and
is sensitive to the polar solvents. One of the strategies for designing
anion receptors with good binding affinity and selectivity for anion
recognition is to increase binding dimensions and the number of
binding sites of their structures in order to launch more binding
units into appropriate sites in three dimensions for specific anion
recognition. In the recent literature, tripodal anion receptors have
shown highly effective anion-binding ability reported by several
groups,7,8 because the tripodal conformation has good ability to
form spheroidal cavities and strong binding sites. Groups of Custel-
cean,8a,b Ghosh,8c,d Wu,8e Das8f and Yang8g have developed a series
of N-bridgehead tripodal urea-based receptors based on tris(2-
aminoethyl)-amine (TREN) units which encapsulate sulfate and
phosphate ions via numerous hydrogen bonds. Very recently, Ha-
ley and Johnson9 have reported a tripodal ditopic receptor with
hydrogen bonding donors and a known tris(aminomethyl)phos-
phine oxide10 as a core scaffold to recognize halide ions in the pres-
ence of lithium. In this phosphine oxide-based receptor, its
hydrogen bonding donors toward halide ions are based on carba-
mate proton of Boc-protected glycine motif in each of three carba-
mate subunits, and the endohedral P@O as
a P-bridgehead
provides enhanced halide binding affinity in the presence of
lithium.
In 2010, our group reported11 that tripod molecules with trini-
trobenzylphosphine oxide motif built molecular networks and
open-dimer capsules. Herein we report a new class of tripodal
anion receptors designed with three urea/thiourea subunits as
hydrogen donors linked to a tribenzylphosphine oxide scaffold as
ꢀ
a P-bridgehead for H2PO4 recognition (Scheme 1). We hope that
the binding ability and selectivity of these new designed tripodal
anion receptors could be dramatically enhanced by the tripodal
conformation with known strong hydrogen bonding donors,
urea-based ligands, and by inducing a Lewis basic phosphine oxide
as a possible endohedral hydrogen bond acceptor for tetrahedral
dihydrogen phosphate ion recognition.
The tripodal receptors 1–3 were prepared smoothly by reac-
tions of triamine 412 with corresponding commercially available
isocyanates in dry CH2Cl2 in 83–90% yields in Scheme 1. The tripo-
dal receptors 1–3 have one methylene group between phosphine
⇑
Corresponding author. Tel./fax: +86 25 83597090.
E-mail address: linchen@nju.edu.cn (C. Lin).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.018

3638
L. Wu et al. / Tetrahedron Letters 53 (2012) 3637–3641
O(n-Bu)
O(n-Bu)
O(n-Bu)
O(n-Bu)
P
O
P
O
O(n-Bu)
O(n-Bu)
+
Ar NCX
HaN
NH2
X
Ha
Hb
N
NH2
HbN
Ar
X
N
Ar
NHa
NH2
X
NHb
Ar
4
1: X=O, Ar = p-tolyl, 90%
2: X=O, Ar = p-nitrophenyl, 88%
3: X=S, Ar = 3,5-di(trifluoromethyl)phenyl, 83%
Scheme 1. Synthesis of tripodal receptors 1–3 based on tribenzylphosphine oxide.
oxide and each phenyl group to increase the flexibility of the ligand
so as to obtain the suitable preorganized conformation for anion
recognition. Receptors 1–3 were all characterized by IR, 1H, 13C
NMR spectroscopies and ESI-MS.
NO3ꢀ, small bathochromic shifꢀts (
D
a
k ꢂ 1 nm) are observed, but
upon the addition of H2PO4
,
bigger bathochromic shift
(
D
k ꢂ 4 nm), and strong hyperchromic effect is observed. With
these results in hand, it could be concluded that the UV–vis detec-
Although the effort on crystal growth of 1–3 failed, the single
crystal structure of triamine 4, the precursor of 1–3, from a mixed
CH2Cl2/CH3CH2OH solution was successfully obtained (Fig. 1). As
shown in Figure 1, the structure of triamine 4 displays an endohe-
dral phosphine oxide with resembling a three-bladed propeller to
create an expected appropriate binding pocket capable of binding
anions, in which the P@O is in the same direction with three NH2
groups. With the confirmed structure 4, it could be forecasted that
this binding pocket conformation in triamine 4 could remain as the
substructure of receptors 1–3. Therefore, it supports that the endo-
hedral P@O conformation can potentially provide another anion
binding subunit at the ‘bridgehead’ site as a hydrogen bond accep-
tor in addition to three hydrogen donor urea-based ligands in the
anion recognition process.
Initially, the binding affinity of 1–3 toward selected anions of
spherical (Brꢀ), triangular (NO3ꢀ) and tetrahedral (H2PO4ꢀ) confor-
mation was investigated by the UV–vis spectra in single solvent
(CHCl3 for receptors 1 and 3, and DMF for receptor 2 due to its sol-
ubility problem in CHCl3). In each case, the counter cation was tet-
rabutylammonium. The changes in the UV–vis spectra of tripodal
receptꢀor 1 upon the addition of 6 equiv of anions Brꢀ, NO3,ꢀ and
H2PO4 in CHCl3 (1 ꢁ 10ꢀ5 M) respectively are presented in Fig-
ure 2. In the absence of anions, tripodal receptor 1 shows an
absorption maximum at 259 nm. Upon the addition of Brꢀ or
tion of the interaction between receptor 1 and H2PO4ꢀ is more sen-
sitive than NO3 or Brꢀ. Upon the gradual increase of the
ꢀ
concentration of H2PO4ꢀ, the intensity of the absorption of receptor
1 is increased and an obvious bathochromic shift is observed. The
corresponding Job plot documenꢀts the 1:1 stoichiometry of the
complex of receptor 1 and H2PO4 (Fig. 2b).
In tripodal receptor 2, compared to tripodal receptor 1, three 4-
methylphenyl groups are replaced by stronger electron-withdraw-
ing groups (4-nitrophenyl groups) in each urea subunit. This
difference between the structures of receptors 1 and 2 could
change the hydrogen bond donating capability of the ꢀNH groups
of the urea subunits. So it is expected that receptor 2 presents
stronger binding ability than receptor 1. Unfortunately, due to
the solubility problem in CHCl3, the UV–vis detection of receptor
2 was performed in DMF that could possibly affect the anion bind-
ing measurement because of the strong solvent polarity. As shown
in Figure 3a, receptor 2 shows a UV–vis absorption maximum at
352 nm in the absence of anions. A notable bathochromic shift
ꢀ
(D
k ꢂ 12 nm) is observed while 6 equiv H2PO4 is added, and the
color of the solution is changed from colorless into yellow visible
to the naked eye (Fig. S1). However, upon the addition of 6 equiv
Brꢀ or NO3ꢀ, there is nearly no change for the corresponding
adsorption curves and color of the solution, which could be be-
cause in the DMF Brꢀ or NO3ꢀ is less competitive than DMF to bind
receptor 2. On the otheꢀr hand, it suggests that the binding ability of
receptor 2 with H2PO4 is stronger than Brꢀ or NO3 in DMF. The
ꢀ
corresponding Job plot documenꢀts the 1:1 stoichiometry of the
complex of receptor 2 and H2PO4 (Fig. 3b).
In the case of tripodal receptor 3, strong electron-withdrawing
groups (two trifluoromethyls) are attached to thiourea moiety in
each thiourea subunit. An absorption maximum in the UV–vis
spectrum appears at 277 nm in the absence of anions. Upon the
addition of 6 equiv of Brꢀ, NO3,ꢀ and H2PO4ꢀ, the absorption peaks
at 277 nm are strengthened and move to longer wavelength at 282,
281, and 286 nm (Fig. 4a), respectively. The receptor 3 is similar
–
with receptor 1 showing more binding sensitivity for H2PO4 than
NO3 or Brꢀ on the UV–vis detection. Also the corresponding Job
ꢀ
plot documeꢀnts the 1:1 stoichiometry of the complex of receptor
3 and H2PO4 (Fig. 4b).
Based on all the UV–vis titration experiment results (see
Figs. 2b, 3b, 4b and the Supplementary data), the association con-
stant of receptors 1–3 for each anion, H2PO4ꢀ, NO3,ꢀ and Brꢀ could
be obtained by the following equation.13
Alim ꢀ A0
2
A ¼ A0 þ
fCH þ CG þ 1=Kass ꢀ ½ðCH þ CG þ 1=KassÞ
2C0
1=2
ꢀ 4CHCGꢃ
g
Figure 1. The crystal structure of triamine 4, the precursor of 1–3.

L. Wu et al. / Tetrahedron Letters 53 (2012) 3637–3641
3639
Figure 2. (a) UV–vis spectra of tripodal receptor 1 (1 ꢁ 10ꢀ5 M) in the absence of anions (black) and in the presence of 6 equiv of Brꢀ (red), NO3ꢀ (green) and H2PO4ꢀ (blue) in
CHCl3 at 298 K. (b) UV–vis absorption spectra of receptor 1 (CHCl3, 1 ꢁ 10ꢀ5 M) upon the addition of various amounts of H2PO4ꢀ in CHCl3. Equivalent of H2PO4ꢀ: 0, 1, 2, 3, 4, 5,
6, 10, 20, 30, 50, 80, and 100. Inset: The Job plot was constructed from absorbance changes at 267 nm using the sum of concentrations 2 ꢁ 10ꢀ5 M.
Figure 3. (a) UV–vis spectra of tripod receptor 2 (1 ꢁ 10ꢀ5 M) in the absence of anions (black) and in the presence of 6 equiv of Brꢀ (red), NO3ꢀ (green) and H2PO4ꢀ (blue) in
DMF at 298 K. (b) UV–vis absorption spectra of receptor 2 (DMF, 1 ꢁ 10ꢀ5 M) upon the addition of various amounts of H2PO4ꢀ in DMF. Equivalent of H2PO4ꢀ: 0, 1, 2, 3, 4, 5, 6,
10, 20, 30, 50, 80, and 100. Inset: The Job plot was constructed from absorbance changes at 374 nm using the sum of concentrations 1 ꢁ 10ꢀ5 M.
Figure 4. (a) UV–vis spectra of tripod receptor 3 (1.5 ꢁ 10ꢀ5 M) in the absence of anions (black) and in the presence of 6 equiv of Brꢀ (red), NO3ꢀ (green) and H2PO4ꢀ (blue) in
DMF at 298 K. (b) UV–vis absorption spectra of receptor 3 (CHCl3, 1 ꢁ 10ꢀ5 M) upon the addition of various amounts of H2PO4ꢀ in CHCl3. Equivalent of H2PO4ꢀ: 0, 1, 2, 3, 4, 5,
6, 10, 20, 50, and 100. Inset: The Job plot was constructed from absorbance changes at 293 nm using the sum of concentrations 1 ꢁ 10ꢀ5 M.

3640
L. Wu et al. / Tetrahedron Letters 53 (2012) 3637–3641
of the peak at 352 nm is observed while 6 equiv Fꢀ and CH3COOꢀ is
added, and the association constant obtained is 9380 2427 and
4432 1876 fꢀor Fꢀ and CH3COOꢀ, respectively, much lower than
that of H2PO4 binding to receptor 2.
Table 1
Association constants Kass(Mꢀ1) of receptors 1–3 with Brꢀ, NO3 and H2PO4
ꢀ
ꢀ
Anion
Kass
1
2
3
ESI-MS results (Fig. S8) also support the formation of a complex
between the dihydrogenphosphate and the urea groups of the
receptor 1. The peak (m/z = 1119.83) corresponding to a 1:1 com-
plex agrees with the isotope patterns very well, ruling out the pos-
sibility of acid–base type interactions. As for receptors 2 and 3, the
molecular ion peaks are 1212.25 and 1532.83, respectively, which
also match the formed binding complexes (Figs. S9 and S10).
The 31P NMR is applied for the investigation of the role of P@O
in the binding of receptors 1–3 with the H2PO4ꢀ. The addition of
Brꢀ
NO3
H2PO4
NDa
NDb
NDb
NDa
ꢀ
(6.26 5.17) ꢁ 103
(7.38 5.08) ꢁ 103
ꢀ
(1.25 0.25) ꢁ 104
(9.05 1.53) ꢁ 104
(4.12 1.02) ꢁ 104
a
Cannot be determined due to the failure of curve fit.
Cannot be determined due to unchanged UV–vis spectra in DMF.
b
where A represents the ultraviolet absorbance, A0 represents the
absorbance of single host, and CH and CG are the corresponding con-
centrations of host and anion guest. The association constant results
are given in Table 1.
anion H2PO4ꢀ to a solution of 1 in DMSO-d6 shows a significant up-
ꢀ
field shift (
D
d = 1.59 ppm) in the 31P signal of H2PO4 compared
with the anion in the absence of the receptor 1, and the corre-
From the UV–vis titration experiments, where the spectrꢀa
changes of receptors 1 and 3 with the addition of Brꢀ and NO3
are small, the association constant obtained might not be very
accurate. From the data in Table 1, it suggests that receptor 1
shows slightly better binding affinity for H2PO4ꢀ than NO3ꢀ; recep-
sponding 31P signal of receptor 1 has a slightly downfield shift
(
D
d = 0.1ꢀppm). Similarly, In the case of receptor 3, the 31P signals
of H2PO4 also shows a significant upfield shift (1.41 ppm), but
the corresponding 31P signal of receptor 3 has nearly no chemical
shift. However, in the case of 2, the corresponding 31P signal of
receptor 2 shows much more upfield shift (1.33 ppm) in this pro-
tor 2 has better binding ability and selectivity of H2PO4 than Brꢀ
ꢀ
and NO3ꢀ; receptor 3 shows about fivefold better binding affinity
for H2PO4ꢀ than NO3ꢀ. What is more, it also indicates that receptor
2 has the largest association constant [(9.05 1.53) ꢁ 104] for
cess (Fig. S11), although its 31P signal of the H2PO4 shows a less
ꢀ
upfield shift (1.1 ppm) than the case of receptors 1 and 3. This sug-
gests that the P@O group plays an active role in the H2PO4ꢀ recog-
nition process. Especially, efforts are presently underway in our
laboratory to ꢀgrow these single crystal substrates of receptors 1–
3 with H2PO4 to give the direct evidence for the role of P@O in
ꢀ
binding H2PO4 among receptors 1–3.
1H NMR and ESI-MS techniques were employed to further con-
firm the recognition interactions between receptors 1 and 3 and
H2PO4ꢀ, NO3ꢀ, and Brꢀ, respectively. The binding properties of
receptors 1–3 with H2PO4ꢀ, NO3ꢀ, and Brꢀ in solution state were
investigated by 1H NMR experiments6,8d in DMSO-d6 with the n-
Bu4N+ as a cation (Fig. S2). It shows the urea/thiourea proton
ꢀ
H2PO4 recognition.
The possible binding model8c,14 for the complex formed ꢀbe-
tween P-bridgehead urea-based tripodal receptor and H2PO4 is
shown in Figure 5. In this proposed binding model, two ꢀNH of
urea/thiourea group act as hydrogen bond donors to recognize an-
ionic oxygen atom and the oxygen atom of hydroxyl group of
H2PO4ꢀ, and the P@O group of tripodal receptors at the bridgehead
ꢀ
chemical shift changes observed upon the addition of H2PO4 to
the receptors 1–3 in DMSO-d6. The most substantial changes are
observed for the urea/thiourea protons of tripodal receptors in 1H
NMR spectra, indicating that urea/thiourea groups provide the
binding sites between the receptor and anion H2PO4ꢀ. Interest-
ingly, it is noticed that these receptors have different behavior
site could act as hydrogen bond acceptor to recognize the other hy-
ꢀ
droxyl proton of H2PO4
.
In conclusion, tripodal receptors 1–3 containing urea/thiourea
subunits were synthesized in good yields (83–90%). The results
of UV–vis and 1H and 31P NMR experiments show that the recep-
tors 1–3 have a better binding affinity, sensitivity, and selectivity
ꢀ
while H2PO4 is added. As for receptor 1, both the ꢀNH peaks be-
come broad8d and are somewhat downfield shifted. In the case of
receptor 3, the ꢀNH peaks become broad and the ꢀNHa resonance
is upfield shifted by 0.407 ppm while the ꢀNHb resonance shows a
0.317 ppm downfield shift. However, the ꢀNHa and ꢀNHb peaks of
receptor 2 become broad and both show the largest downfield
(0.957 ppm and 0.831 ppm) shifꢀts, which indicates that the inter-
action of receptor 2 with H2PO4 is stronger than either of recep-
for H2PO4 than Brꢀ and NO3ꢀ, and among 1–3 receptor 2 has
ꢀ
shown the best binding affinity for H2PO4 recognition over Fꢀ,
ꢀ
Brꢀ, CH3COOꢀ, HSO4ꢀ, and NO3ꢀ. Receptors 1–3 can form 1:1 com-
ꢀ
plexes with H2PO4 by multiple hydrogen bonding interactions
based on the Job plot experiments, which is further confirmed by
ESI-MS technique. Moreover, during the course of the recognition
tors
1 and 3, agreeing well with the association constant
ꢀ
obtained from the UV–vis titration experiments above. However,
the urea/thiourea protons of receptors 1–3 all remain unchanged
in the same 1H NMR experimental conditions (in DMSO-d6) upon
the addition of NO3ꢀ and Brꢀ, respectively. So these 1H NMR exper-
imental results suggest that the binding abilitꢀy of receptors 1–3
of 2 with H2PO4 an observable color change could be achieved
by the naked eye, whichꢀis promising for receptor 2 as an optical
chemosensor for H2PO4 anion. A new class of P-bridgehead
urea-based tripodal receptors represents a new kind of neutral sys-
tem for anion recognition studies.
with H2PO4 is stronger than with Brꢀ or NO3 in DMSO, which,
ꢀ
in particular for receptor 2, agrees well with the UV–vis titration
experiment results (Fig. 3a).
Since receptor 2 has shown the best binding ability and selectiv-
OR
OR
ity for H2PO4 over Brꢀ and NO3ꢀ, its binding ability is further
ꢀ
P
O
inveꢀstigated toward HSO4ꢀ, F,ꢀ and CH3COOꢀ besides Brꢀ and
OR
NO3 in DMSO-d6 by 1H NMR (Fig. S7). In 1H NMR, the NH peaks
H
are all downfield shifted while receptor 2 binds to 1 equiv of Fꢀ
,
O
P
H
HN
HN
N
H
CH3COOꢀ, and H2PO4ꢀ, respectively, and there is no change for
NH peaks while it binds to 1 equiv. of Brꢀ, NO3,ꢀ and HSO4ꢀ, which
could be due to the polar solvent, DMSO-d6, used in the measure-
ment. Then receptor 2 binding to Fꢀ and CH3COOꢀ in DMF is stud-
ied by the UV–vis titration experiments (Fig. S6), from which a
X
X
O
O
HN
N
H
O
X
R'
R'
HN
R'
X=O or S
ꢀ
small bathochromic shift (
D
k ꢂ 8, 10 nm) and the weak decreasing
Figure 5. The proposed binding model of receptors with H2PO4 .

L. Wu et al. / Tetrahedron Letters 53 (2012) 3637–3641
3641
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Acknowledgments
We gratefully thank the financial support of National Natural
Science Foundation of China (No. 20932004, 21072093), National
Basic Research Program of China (2011CB808600), the Doctoral
Fund of Ministry of Education of China (20090091110017), Natural
Science Foundation of Jiangsu (BK2011055, BK2011551).
Supplementary data
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Supplementary data (general experimental methods, synthetic
procedures, UV–vis titration, ES-MS, 1H NMR, 31P NMR spectra
and crystal data) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.tetlet.2012.05.018.
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