Synthesis and chiral recognition properties of a novel colorimetric chiral sensor for carboxylic anions

Article abstract of DOI:10.1071/CH08009

Chiral colorimetric sensor 4 has been synthesized, and the structure characterized by IR and ¹H NMR spectroscopy, mass spectrometry, and elemental analysis. The chiral recognition of the receptor was studied by ¹H NMR and UV-vis spec

Full text of DOI:10.1071/CH08009

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2008, 61, 310–315
www.publish.csiro.au/journals/ajc
Synthesis and Chiral Recognition Properties of a Novel
Colorimetric Chiral Sensor for Carboxylic Anions
Zhi-Hong Chen,A Yong-Bing He,A,B Chen-Guang Hu,A
Xiao-Huan Huang,A and Ling HuA
ADepartment of Chemistry, Wuhan University, Wuhan, Hubei 430072, China.
BCorresponding author. Email: ybhe@whu.edu.cn
1
Chiral colorimetric sensor 4 has been synthesized, and the structure characterized by IR and H NMR spectroscopy,
1
mass spectrometry, and elemental analysis. The chiral recognition of the receptor was studied by H NMR and UV-vis
spectroscopy. The results of non-linear curve fitting indicate that the receptor 4 and l- or d-mandelate and alanine anions
form a 1:1 stoichiometric complex. The obvious colour change of receptor 4 can be observed by the naked eye when the
enantiomers of mandelate and alanine anions are added, which demonstrates that receptor 4 may be used as the colorimetric
sensor for the two carboxylic anions.
Manuscript received: 12 January 2008.
Final version: 2 March 2008.
Introduction
Results and Discussion
Synthesis
The study of anion recognition is one of the more active areas
in supramolecular chemistry.[1] In particular, chiral recognition
is one of the most fundamental and significant processes in liv-
ing systems.[2] Chirality is a fundamental property of nature,[3]
with many natural living systems composed of chiral biologi-
cal molecules, which exist in only one enantiomeric form for a
certain living process. The study of enantiomeric recognition of
biologically important substrates is a very important research
area because it can provide valuable information for under-
standing the mechanism of interactions in biological systems,
and for developing useful molecular devices in biochemical and
pharmaceutical studies, catalysis, and sensing.[4]
The synthesis of 4 is outlined in Scheme 1. Compound 2
was synthesized according to a literature procedure.[8] Com-
pound 2 was treated with N-Boc-l-alanine in the presence
of N,Nꢀ-carbonyldiimidazole (CDI) to afford 3. Compound 3
was deprotected by treating with trifluoroacetic acid and then
p-nitrophenyl isothiocyanate to give the target molecule 4. The
structures of these compounds were characterized by IR, 1H
NMR, and 13C NMR spectroscopy, electrospray ionization mass
spectrometry (ESI-MS), and elemental analysis.
Many artificial receptors of different kinds have been syn-
thesized for selective anion recognition, which include chiral
macrocyclic and acyclic polyamines, chiral calixarenes, and
chiral cyclodextrin derivatives.[5] For the molecular design of
chemosensors, how to achieve the specific recognition of a cer-
tain molecule and how to transfer the recognition event into a
signal are crucial points. UV-Vis-based chemosensors are very
useful for detecting and discriminating enantiomers, and have
attracted the intense interest of chemists.[4a]
Many receptors for mandelate anions have been designed and
synthesized.[6b,6c,7] Their enantioselective recognition abilities
have been tested by 1H NMR spectroscopy and fluorescence, and
thedifferentsignalswithinthe1HNMRspectrumandchangesof
fluorescence spectra to d/l-mandelate indicate that these recep-
tors have selective recognition towards the guest. However a
colorimetric chemosensor for mandelate is rare. Here we report
the synthesis of a new chiral receptor 4 that bears thiourea as the
binding site. The enantioselective recognition ability for chiral
carboxylate anions has been investigated by 1H NMR and UV-
vis spectra. Obvious and different colour changes are observed
when receptor 4 interacts with enantiomers of mandelate, which
illustrates that 4 has a good enantioselective recognition ability
for mandelate anions.
UV-Vis Spectroscopic Study
The UV-vis spectroscopic titration experiment was undertaken
to investigate the possible interaction between the host and
each enantiomer of the anions. All titrations were carried out
in dimethyl sulfoxide (DMSO) as solvent in the absence and
presence of a chiral guest.
Figure 1 shows the UV-vis spectra of the interaction between
4 and l-mandelate anions in DMSO. In the absence of anions,
the receptor 4 in DMSO (5.0 × 10−5 mol L−1) has a maximum
absorption at 356 nm, which can be assigned to intramolecular
charge–transfer absorption. With the addition of l-mandelate
anions, the absorption at 356 nm gradually decreased, and a new
absorption at 470 nm appeared. This illustrates that a complex
has been formed between the receptor and anion. Meanwhile,
an isosbestic point was observed at 388 nm, which indicates that
there is a balance between the complex and the solution.[5e] By
gradually increasing the amount of anion, the solution changed
from colourless to golden yellow, which could be observed by
the naked eye. This can be attributed to the appearance of a
long-wavelength peak.
Asimilarphenomenonwasalsoobservedonthegradualaddi-
tion of d-mandelate to the solution of receptor 4. The UV-vis
© CSIRO 2008
10.1071/CH08009
0004-9425/08/040310

Novel Colorimetric Chiral Sensors
311
N-Boc-L-alanine, CDI
Hexamethylenetetramine Conc. HCl
Et3N, CHCl3, r.t.
CHCl3, reflux 48h
Reflux 60h
NH2 HCl NH2 HCl
Br
Br
1
2
NH
H
NH
O
H
O
H3C
HN
(1) TFA/CHCl3
H3C
NH
H
NH
O
H3C
O
H3C
NH
H
S
S
(2) Et3N, O2N
NCS, r.t.
HN
NH
NHBoc
NHBoc
NO2
NO2
3
4
Scheme 1. The synthesis of receptor 4.
1.0
0.8
0.6
0.4
0.2
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.8
1.6
1.4
1.2
1.0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
ꢀ20
0
20 40 60 80 100120140160
Equiv. of anions
ꢀ20
0
20 40 60 80 100 120 140
Equiv. of anions
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
550
600
650
Wavelength [nm]
300
350
400
450
500
550
600
650
Fig. 2. UV-Vis absorption spectra of receptor
4
in DMSO
(5.0 × 10−5 mol L−1) upon addition of various amounts of d-mandelate
anion. Anion equivalence: 0 → 143. The non-linear curve fitting for the
change in absorption at 470 nm with respect to the amount of d-mandelate
anion added is shown in the inset. The correlation coefficient (R) of
the non-linear curve fitting is 0.9968.
Wavelength [nm]
Fig. 1. UV-Vis absorption spectra of receptor
4
in DMSO
(5.0 × 10−5 mol L−1) upon addition of various amounts of l-mandelate
anion. Anion equivalence: 0 → 139. The non-linear curve fitting for the
change in absorption at 470 nm with respect to the amount of l-mandelate
anion added is shown in the inset. The correlation coefficient (R) of the
non-linear curve fitting is 0.9973.
a protic solvent, such as water or methanol, was added to the
coloured solution of 4 and mandelate anion in DMSO, the mix-
ture changed to colourless. This phenomenon illustrates that the
addition of protic solvent destroys the complex formed by 4 with
the mandelate anion, which demonstrates that the interaction
between 4 and the mandelate anion was, in essence, hydrogen
bonding.
The satisfactory non-linear curve fitting (absorption intensity
at 470 nm versus the equivalent of mandelate anion; correla-
tion coefficient >0.99) confirmed that receptor 4 and l- or
d-mandelate formed a 1:1 complex (see the insets of Figs 1
and 2). The enantioselectivity of 4 towards the enantiomers of
the mandelate anion was Kass(D)/Kass(L) = 4.2, which demon-
strates that the receptor 4 has a good enantioselective recognition
ability for the mandelate anions. The different and obvious
colour change in the interaction of 4 and the enantiomers of
the mandelate anions illustrates that receptor 4 may be used as
a colorimetric sensor for mandelate anions.
spectra of 4 with addition of d-mandelate anions is shown in
Fig. 2.
With a gradual increase of the concentration of d-mandelate
anions, theintensityofabsorptionofthehostinDMSOat356 nm
decreased and a new peak at 470 nm appeared. The intensity of
the absorption at 470 nm was remarkably increased compared
with that of adding the l-enantiomer. Meanwhile, an obvious
colour change of the solution from colourless to orange-red was
observed (Fig. 3).
The absorption of the receptor at 356 nm can be ascribed
to an intramolecular charge transfer between the electron-rich
thiourea group and the electron-deficient p-nitrophenyl moi-
eties. When the receptor binds an anion, hydrogen bonds are
constructed to form stable complexes and the electron den-
sity in the supramolecular system is markedly increased. This
enhances the charge transfer interactions between the electron-
rich and electron-deficient moieties, which results in a new
absorption band at 470 nm and a visible colour change.[9] When
Receptor 4 also exhibits a similar response to the N-Boc ala-
nine anions, and also showed a good enantioselective recognition

312
Z.-H. Chen et al.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
60
80
Equiv. of anions
Fig. 3. Effect of anions (as Bu4N salts) on colour changes of recep-
tor 4 in DMSO. From left to right: receptor 4 (5 × 10−5 M), recep-
tor 4 (5 × 10−5 M) + 100 equiv. l-mandelate, receptor 4 (5 × 10−5 M) +
100 equiv. d-mandelate, receptor 4 (5 × 10−5 M) + 100 equiv. l-alanine,
receptor 4 (5 × 10−5 M) + 100 equiv. d-alanine.
300
350
400
450
500
550
600
650
Wavelength [nm]
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Fig. 5. UV-Vis absorption spectra of receptor
4
in DMSO
(5.0 × 10−5 mol L−1) upon addition of various amounts of d-alanine
anion. Anion equivalence: 0 → 80. The non-linear curve fitting for
the change in absorption at 466 nm with respect to the amount of
d-alanine anion added is shown in the inset. The correlation coefficient (R)
of the non-linear curve fitting is 0.9997.
0
20
40
60
80
Equiv. of anions
l- and d-mandelate anions at 470 nm in DMSO. When the molar
fraction of the guest is 0.50, the absorption reaches a maximum,
which demonstrates that receptor 4 forms a 1:1 complex with
l- or d-mandelate anions.[10]
The interactions of receptor 4 with other anions were also
investigated by UV-vis spectroscopy. The results are shown in
Table 1.
300
350
400
450
500
550
600
650
Wavelength [nm]
For a complex with a stoichiometry of 1:1, the association
constant Kass can be calculated from Eqn (1).[11]
Fig. 4. UV-Vis absorption spectra of receptor
4
in DMSO
(5.0 × 10−5 mol L−1) upon addition of various amounts of l-alanine
anion. Anion equivalence: 0 → 80. The non-linear curve fitting for
the change in absorption at 466 nm with respect to the amount of l-alanine
anion added is shown in the inset. The correlation coefficient (R) of the
non-linear curve fitting is 0.9913.
X = X0 + (Xlim − X0)/2c0{CH + CG + 1/Kass
− [(CH + CG + 1/Kass)2 − 4CHCG]1/2},
(1)
where X is the absorption intensity and CH and CG are the cor-
responding concentration of the host and guest anion. C0 is the
initial concentration of the host. The association constants (Kass
)
ability and similar colour changes. Figures 4 and 5 show the
absorption spectra of 4 with N-Boc-l- and d-alanine anions,
respectively.
and correlation coefficients (R) were obtained by a non-linear
least-squares analysis of X versus CH and CG, the results are
listed in the Table 1.
The data in Table 1 illustrate that 4 has a better enantioselec-
tive recognition ability for mandelate and alanine anions than
for other anions. The relatively rigid structure and the good pre-
organization property of the receptor may result in the good
enantioselective recognition of the enantiomers of mandelate
and alanine anions.
The satisfactory non-linear curve fitting (absorption intensity
at 466 nm versus equivalent of alanine anion; coefficient >0.99)
confirmed that receptor 4 and l- or d-alanine formed a 1:1 com-
plex (see the insets of Figs 4 and 5). The association constants of
the two anions with 4 are different [Kass(D) = 8.14 × 102 M−1
,
Kass(L) = 2.43 × 102 M−1]. The ratio of Kass(D)/Kass(L) is 3.3,
which demonstrates that receptor 4 has a good enantioselective
recognition ability for the N-Boc alanine anions.
1H NMR Study
Figure 6 exhibits the UV-vis absorption changes of receptor
4 with d/l-mandelate (A) or alanine (B) anions in DMSO. Dif-
ferent responses of the UV-vis absorption for chiral mandelate
and alanine anions indicate that 4 has a good enantioselective
recognition ability for both the chiral guests.
Continuous variation methods were employed to determine
the stoichiometric ratios of complexes formed between receptor
4 and l- and d-mandelate anions. The Job plots were taken at
the total concentration (10−3 M) of host and guest in DMSO,
whereas the molar fraction of the guest {[G]/([G] + [H])} was
continuously varied. Figure 7 shows the Job plots of 4 with
1H NMR experiments were undertaken to assess the chiral
recognition properties between receptor 4 and the mandelate
anion, because they can directly provide structural and dynamic
information.[12] Studies on the chiral recognition were carried
out on a 300 MHz NMR spectrometer using receptor 4 as a
chemical shift reagent.
The 1H NMR spectra of receptor 4 (10−2 M) and its complex
with equimolar amounts of racemic, d-, or l-mandelate anions
(10−2 M) in (D6)DMSO are shown in Fig. 8. Figure 8b shows the
1H NMR spectra of racemic mandelate anions. Only one singlet

Novel Colorimetric Chiral Sensors
313
(a)
(b)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ꢀ0.1
0.6
0.5
0.4
0.3
0.2
0.1
0.0
ꢀ20
0
20 40 60 80 100 120 140 160
Equiv. of anions
0
20
40
60
80
Equiv. of anions
4 ꢁ L-mandelate
4 ꢁ D-mandelate
4 ꢁ L-alanine
4 ꢁ D-alanine
Fig. 6. UV-Vis absorption changes of receptor 4 (5 × 10−5 M) (a) with d/l-mandelate, or (b) alanine anions, in DMSO.
l-mandelate. The obvious difference in the change of the
chemical shift for the enantiomers of mandelate illustrate that
receptor 4 has a stronger interaction with d-mandelate than with
l-mandelate.Theaboveresultsillustratethattheenantioselective
recognition of receptor 4 for d- or l-mandelates, except for steric
interactions, is through multiple hydrogen-bonding interactions.
The 1H NMR experiments of receptor 4 interacting with
d,l-N-Boc-alanine, glutamate, and aspartate anions were also
carried out. (The 1H NMR spectra can be found in theAccessory
Publication.)
0.30
0.25
0.20
0.15
0.10
Conclusions
In summary, the novel chiral colorimetric sensor 4 has been syn-
thesized. The enantioselective recognition of the receptor was
studied by 1H NMR spectroscopy and UV-vis absorption spec-
troscopy. Receptor 4 shows a good chiral recognition ability for
the enantiomers of d- and l-tertrabutylammonium mandelate
and alanine, and forms a 1:1 complex between the host and the
guest. The steric effects, relatively rigid structure, and good pre-
organization properties of receptor 4 may be responsible for the
enantiometric recognition of mandelate and alanine anions. The
different and obvious colour changes upon interaction of recep-
tor 4 with enantiomers of mandelate or alanine anions illustrate
that receptor 4 may be used as a colorimetric sensor for the two
anions.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
[G]/([H][G])
4 ꢁ D-mandelate anion
4 ꢁ L-mandelate anion
Fig. 7. The Job plots of 4 with l- or d-mandelate anions (at 470 nm). The
total concentration of the host and guest is 1.0 × 10−3 M in DMSO.
(δ 4.37 ppm) is exhibited for the CH proton resonance of racemic
mandelate anions. In the present of receptor 4, the CH proton sig-
nal of the racemic mandelate was broadened and shifted down-
field (Fig. 8c). The chiral CH proton resonance of d-mandelate
was shifted from 4.37 to 4.56 ppm (Fig. 8d) and the CH signal of
l-mandelate was shifted from 4.37 to 4.61 ppm (Fig. 8e) with
the separation of the two peaks being 0.05 ppm, which indi-
cates that the interactions of receptor 4 with the d- and l-forms
of mandelate are different. The CH proton signals of d- and
l-mandelate were shifted downfield ∼0.19 (Fig. 8d) and
0.24 ppm (Fig. 8e), respectively.
Experimental
Materials and Methods
CHCl3 was washed with water and dried over CaCl2. Et3N
was dried and distilled from CaH2. N,N-Dimethylformamide
(DMF) was dried with CaH2 at 50◦C, and distilled under
reduced pressure. Compound 2 was synthesized according to
the literature.[8] Other commercially available reagents were
used without further purification. Melting points were deter-
mined with a Reichert 7905 melting-point apparatus and were
uncorrected. Optical rotations were taken on a Perkin–Elmer
341 polarimeter. IR spectra were obtained on a Nicolet 670
1
In the H NMR spectra of the host–guest complex, besides
the changes of the chiral hydrogen signal of the guest,
the thiourea NH proton signal resonances of the host were
also changed. The proton signal of ArNHCS (NHa) was
broadened and underwent a remarkable downfield shift from
10.41 to 11.45 ppm (δ 1.04 ppm, in Fig. 8d) for the d-
mandelate anion and 10.89 ppm (δ 0.48 ppm, in Fig. 8e) for the
l-mandelate anion. For CSNHCH (NHb), the proton resonance
was also broadened and downfield shifted from 8.34 to 9.39 ppm
(δ 1.05 ppm) for d-mandelate and 8.80 ppm (δ 0.46 ppm) for
1
FT-IR spectrometer. H and 13C NMR spectra were recorded
in CDCl3 or (D6)DMSO on a Varian Mercury VX-300 MHz
spectrometer. Mass spectra were recorded on a Finnigan LCQ
advantage mass spectrometer. Elemental analysis was deter-
mined with a Carlo-Erba 1106 instrument. UV-spectra were

314
Z.-H. Chen et al.
Table 1. Association constants (Kass),correlation coefficients (R),enantioselectivities (Kass(D)/Kass(L)),Gibbs free energy changes (−ꢀG0),and ꢀꢀG0
calculated from ꢀG0 for the complexation of receptor 4 with d/l-guests in DMSO at 25◦C
A
Entry
Guest
Kass
R
Kass(D)/Kass(L)
−ꢀG0
ꢀꢀG0
[kJ mol−1
]
[M−1
]
[kJ mol−1
]
1
2
3
4
5
6
7
8
d-mandelate
l-mandelate
227.1 19.55
53.13 5.29
(4.4 0.4) × 103
(3.2 0.3) × 103
814 20
243 12
232 14
258 22
(9.3 0.98) × 103
(6.1 0.8) × 103
(9.0 0.4) × 103
(1.0 0.03) × 104
0.9968
0.9973
0.9931
0.9941
0.9913
0.9997
0.9988
0.9963
0.9954
0.9921
0.9988
0.9994
14.434
9.832
4.28
1.38
3.30
0.90
1.52
0.90
4.602
0.788
d-α-phenylglycine
l-α-phenylglycine
d-N-Boc-Ala
l-N-Boc-Ala
d-N-Boc-Phe
l-N-Boc-Phe
d-N-Boc-Glu
l-N-Boc-Glu
d-N-Boc-Asp
l-N-Boc-Asp
20.775
19.987
16.597
13.603
13.488
13.751
22.629
21.584
22.547
22.808
2.994
−0.263
1.045
9
10
11
12
−0.261
AData were calculated from results of UV-vis titrations in DMSO. All error values were obtained from non-linear curve fitting.
(a)
NHa
NHb
(b)
(c)
H*
H*
NHa
NHb
(d)
(e)
H*
NHa
NHb
H*
NHb
NHa
12
11
10
9
8
7
6
5
4
3
2
1
ꢀ0 ppm
Fig. 8. 1H NMR spectra of 4 and its guest complex at 25◦C ([4] = [guest] = 10−2 M) in (D6)DMSO at 300 MHz.
(a) Receptor 4, (b) racemic tertrabutylammonium mandelate, (c) receptor 4 + racemic mandelate, (d) receptor
4 + d-mandelate, and (e) receptor 4 + l-mandelate.
performed with a TU-1901 spectrophotometer. All anions were
used as their tetrabutylammonium salts.
residue was purified on a column of silica gel (CHCl3/MeOH,
20/1) to give the pure product 3 as a white foam (0.55 g, 60%),
mp 107–109◦C. [α]D20 −12.6 (c 0.048, CHCl3). νmax/cm−1 (KBr)
3445, 3345, 3066, 2978, 2932, 2875, 1680, 1655, 1522, 1449,
1367, 1251, 1166, 1073, 1054, 859, 785, 766, 702, 631. δH
(CDCl3, 300 MHz) 7.22 (s, 1H, ArH), 7.19 (s, 2H, NH), 7.14 (br
s, 3H, ArH), 5.37 (s, 2H, NHBoc), 4.41 (br s, 2H, chiral H), 4.29
(s, 4H, CH2), 1.80 (d, J 20, 6H, CH3), 1.39 (d, J 4.2, 18H, Boc-
CH3). δC (CDCl3, 75 MHz) 173.6, 155.9, 138.6, 129.0, 127.1,
80.0, 50.2, 42.5, 28.5, 19.2. (Calc. for C24H38N2O4: C 68.9,
H 9.2, N 6.7. Found: C 68.8, H 9.2, N 6.7%.)
Synthesis
Synthesis of Compound 3
CDI (1.10 g, 6.8 mmol) was added to a solution of compound
2 (0.40 g, 2 mmol) and Et3N (0.5 mL) in 20 mL of dry CHCl3
under argon and stirred for 2 h at room temperature. Boc-l-
alanine (0.80 g, 4.2 mmol) was added and the mixture was stirred
for 48 h at room temperature. The reaction mixture was poured
into 50 mL of 10% HCl and extracted with CHCl3. The organic
layer was washed with 10% NaHCO3 and water, respectively.
The organic layer was dried over anhydrous NaSO4. After fil-
tration, the solvent was evaporated under reduced pressure. The
Synthesis of Compound 4
A mixture of 3 (0.20 g, 0.42 mmol) with trifloroacetic acid
(0.23 g, 2 mmol) was stirred in dry chloroform (2 mL) for 1 h

Novel Colorimetric Chiral Sensors
315
at room temperature. The solvent and excess acid were then
evaporated under reduced pressure, to give the TFA salt as a
colourless solid, which was used directly without further purifi-
cation. The TFA salt and triethylamine (0.5 mL) were dissolved
in dry CHCl3 (20 mL), and then a solution of p-nitrophenyl
isothiocyanate (0.18 g, 1 mmol) in dry chloroform (10 mL) was
added. The mixture was stirred vigorously for 24 h at room tem-
perature. The resulting precipitate was filtered off and washed
with CHCl3 to give the pure product 4 as a light yellow solid
(0.21 g, 80%), mp 128–130◦C. [α]2D0 +47.0 (c 0.064, DMSO).
νmax/cm−1 (KBr) 3412, 3330, 3117, 3066, 2926, 1659, 1597,
1536, 1509, 1447, 1420, 1342, 1326, 1302, 1254, 1178, 1112,
848, 754, 727, 700. δH ((D6)DMSO, 300 MHz) 10.41 (s, 2H,
NH), 8.71 (m, 2H, NH), 8.34 (d, J 7.5, 2H, NH), 8.17 (d, J 8.7,
4H, ArH), 7.93 (d, J 8.7, 4H, ArH), 7.29 (m, 1H, ArH), 7.15
(m, 3H, ArH), 4.87 (t, J 6.6, 2H, chiral H), 4.30 (d, J 5.1, 4H,
CH2), 1.37 (d, J 6.6, 6H, CH3). δC ((D6)DMSO, 75 MHz) 184.4,
177.2, 151.7, 147.3, 144.6, 133.8, 131.3, 131.0, 129.9, 125.7,
58.1, 51.2, 47.5, 45.7, 24.3. m/z 637.5 (M+, 33%). (Calc. for
C28H30N8O6S2: C 52.7, H 4.7, N 17.5. Found: C 52.6, H 4.8, N
17.5%.)
[3] (a) K. Jennings, D. Diamond, Analyst 2001, 126, 1063. doi:10.1039/
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Accessory Publication
The 1H NMR spectra are available from the author or, untilApril
2013, the Australian Journal of Chemistry.
Acknowledgements
We thank the National Natural Science Foundation for financial support
(grant no. 20572080).
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