Chemodosimeter for fluoride ions based on F--triggered Si-O cleavage followed by the deprotonation/autoxidation of secondary nitrile C-H group

Article abstract of DOI:10.1039/c4ra07870e

A completely new strategy for a chemodosimeter for fluoride based on F^--triggered Si-O bond cleavage, followed by the deprotonation/autoxidation of secondary nitrile C-H group was developed for the first time. The chemodosimeter exhibited high selectivity and sensitivity, dramatic color turn-on , and fluorescence turn-off for the detection for fluoride.

Full text of DOI:10.1039/c4ra07870e

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Chemodosimeter for fluoride ions based on Fꢀ-
triggered Si–O cleavage followed by the
deprotonation/autoxidation of secondary nitrile
C–H group†
Cite this: RSC Adv., 2014, 4, 46016
Received 31st July 2014
Accepted 15th September 2014
*
Baiyun Li, Chuanxiu Zhang, Chuanxiang Liu, Jinju Chen, Xinyu Wang, Zhenjiang Liu
and Fengping Yi
DOI: 10.1039/c4ra07870e
www.rsc.org/advances
A completely new strategy for a chemodosimeter for fluoride based ethers, we planned to investigate a dual reaction-based site
on Fꢀ-triggered Si–O bond cleavage, followed by the deprotonation/ utilized by the Fꢀ-triggered Si–O bond cleavage, followed by the
autoxidation of secondary nitrile C–H group was developed for the deprotonation/autoxidation of the secondary nitrile group. To
first time. The chemodosimeter exhibited high selectivity and sensi- the best of our knowledge, this is a completely new sensing
tivity, dramatic color “turn-on”, and fluorescence “turn-off” for the mechanism for Fꢀ compared to only Fꢀ-triggered Si–O bond
detection for fluoride.
cleavage. Based on the mechanism, we designed dosimeter 5
based on a 1,8-naphthalimide system containing a tert-butyl-
diphenylsilyl (TBDPS) moiety combined with a secondary nitrile
CH group, together with the reference compounds 6 and 7 for
comparison (Scheme 1). First, 2-(4-(tert-butyldiphenylsilyloxy)
phenyl)acetonitrile (2, Fig. S1†) was prepared in 98% yield using
Fluoride (Fꢀ) ions have attracted much interest because of their
important roles in biological and chemical sciences.1 The over-
intake of uoride may cause osteoporosis and apoptosis in
mammalian cells.2 Therefore, the design and development of
articial receptor-based sensors for the selective detection of Fꢀ
is a challenging task in supramolecular chemistry.3 Colori-
metric and uorescent sensors with advantages such as no
equipment requirement, simple detection by naked eye, high
sensitivity, and low detection limits are more promising.4 Apart
from the conventional hydrogen bonding and the deprotona-
tion of labile protons, one of the emerging strategies is the
development of reaction-based chemodosimeters for Fꢀ.5 In
particular, the most intriguing approaches, which were mainly
based on the Si–O bond cleavage using the great affinity of Fꢀ
for silicon,6 have been investigated in details. These methods
usually show specic selectivity for Fꢀ, and some of them are
applicable in polar solvent or aqueous environments.7 However,
the sensors with small absorption shi sometimes lack sensi-
tivity as most of them require several minutes to even hours to
generate a signal response. Therefore, the development of more
effective chemodosimeters for Fꢀ ions overcoming these limi-
tations is still challenging.
a
classical hydroxyl-protected procedure,10 in which the
hydroxyl moiety of the corresponding 2-(4-hydroxyphenyl)
acetonitrile 1 was transformed to the corresponding silyl ether
using tert-butyldiphenylsilyl chloride (TBDPSCl). Further, the
NaH-catalyzed
condensation
of
N-butyl-4-bromo-1,8-
naphthalimide 4 with intermediate 2 and 2-(4-methoxyphenyl)
acetonitrile 3 afforded dosimeter 5 and reference compound 6
in 51% and 92% yields, respectively.9 Dosimeter 5 was desily-
lated and oxidatively decyanated with tetrabutylammonium
uoride (TABF) to afford compound
7
for comparison.
In our endeavor to develop highly sensitive chemo-
dosimeters for Fꢀ,8 a mechanism involving the deprotonation/
autoxidation of diaryl secondary nitriles was developed.9 To
improve the sensitivity of the conventional hydrolysis of silyl
School of Chemical and Environmental Engineering, Shanghai Institute of Technology,
201418 Shanghai, P. R. China. E-mail: cxliu@sit.edu.cn
Scheme
1
Synthesis of chemodosimeter
5
and the reference
compounds 6, 7. (a) tert-Butyldiphenylsilyl chloride (TBDPSCl), imid-
azole, CH2Cl2, r.t., yield, 98%; (b) NaH, THF, N2, r.t., yield: 5 (51%), 6
(92%); (c) TBAF, CH2Cl2, r.t., yield, 78%.
† Electronic supplementary information (ESI) available: Synthesis procedures,
additional spectra and experimental details. See DOI: 10.1039/c4ra07870e
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1
Compounds 5–7 were characterized by H-NMR, 13C-NMR, IR,
and HRMS-ESI analyses (Fig. S2–S13†).
First, the UV-visible spectra of receptors 5 and 6 with
increasing concentrations of Fꢀ (as its TBA salt) were investi-
gated in acetonitrile (Fig. 1). Upon the addition of 0.0–2.0 equiv.
of Fꢀ, two new bands centered at 370 and 625 nm appeared in
the UV-visible spectra of dosimeter 5 (Fig. 1a). The appearance
of a blue color and a largerꢀred shi (z275 nm) indicate the
major formation of [C–H/F ] complexes.9 On further addition
of Fꢀ, the intensity of the band at 625 nm decreased and slightly
red-shied to 660 nm (the color of the solution turned light
brown), accompanied by a strong increasing band centered at
367 nm with isosbestic points at 390 and 500 nm (Fig. 1b). The
strong peak at 367 nm was further veried by the comparison of
the UV-visible spectra of reference compound 7 with increasing
concentrations of Fꢀ (Fig. S14†), indicating the formation of
phenolate anions because of the cleavage of the TBDPS group,
followed by the oxidation of the C–H group to a rigid carbonyl
group. Compared to compound 6 without Si–O group, a highly
similar spectroscopic response was observed, as shown in
Fig. 1c; however, as shown in Fig. 1d, the peak at 625 nm
signicantly decreased without red shi. Moreover, the satu-
rated amounts of Fꢀ at 625 nm were different for dosimeters 5
and 6, in which dosimeter 5 containing a dual binding site was
saturated by adding only 2.0 equiv. of Fꢀ. Thus, the sensitivity of
5 was signicantly higher than that of the dosimeter containing
only a C–H group (3.0 equiv. for 6). Furthermore, when more Fꢀ
was added (2.0–9.0 equiv. Fꢀ), the slightly red-shied peak at
660 nm for dosimeter 5 was ascribed to the formation of a
phenolate anion with a strong negatively charged donor oxygen
atom, to further enhance the potential intramolecular charge-
transfer state of the [C–H/Fꢀ] complexes.11
Fig. 2 (a) UV-visible spectral changes of 20 mM solution of 5 in CH3CN
with various anions (inset: from left to right: AcOꢀ, Iꢀ, Brꢀ, BF4ꢀ, Clꢀ,
NO3ꢀ, ClO4ꢀ, H2PO4ꢀ, HSO4ꢀ, Fꢀ, 5 only); (b) fluorescence spectral
changes of 5 (lex ¼ 350 nm) upon the titration with TABF (0 to 4.0
equiv.); (c) fluorescence emission intensity of 5 (20 mM) in CH3CN–
H2O (95 : 5, v/v) after 10 min for various anions (40.0 equiv.); and (d)
interference experiments of dosimeter 5 (20 mM) in CH3CN–H2O
(95 : 5, v/v) for Fꢀ in the presence of other anions. The gray bars
represent the emission at 500 nm of 5 in the presence of 40.0 equiv. of
the anion of interest (from 1 to 9: AcOꢀ, Iꢀ, Brꢀ, BF4ꢀ, Clꢀ, NO3
,
ꢀ
ClO4ꢀ, H2PO4ꢀ, HSO4ꢀ). The red bars indicate the change that occurs
upon subsequent addition of 40.0 equiv. of Fꢀ to the solution con-
taining 5 and the anion of interest.
equiv.) was added to a solution of 5, two new peaks at 625 and
370 nm were observed in the UV-visible spectrum of dosim-
eter 5 accompanied by an instantaneous color change from
colorless to blue (Fig. 2a, inset, naked-eye detection), whereas
no considerable change was observed in the absorption
spectra by adding other anions. These results indicate that
dosimeter 5 shows excellent selectivity for Fꢀ over other
anions (for the interference experiments by UV-visible spec-
troscopy, see: Fig. S15†). Moreover, a specic emission
response to Fꢀ in acetonitrile was observed (Fig. 2b). Upon
the addition of Fꢀ ions to the solution of 5, a signicant
decrease in the uorescence intensity at 500 nm (lex ¼ 350
nm) was observed, indicating that the photoinduced electron
transfer (PET) was enhanced by an increased negative charge
on the phenolate oxygen atom and a potential conjugate
effect induced by the oxidation of diaryl secondary nitriles
C–H group. Importantly, a linear uorescence response to Fꢀ
Next, the selectivity of dosimeter 5 for Fꢀ was investigated
by UV-visible spectroscopy by adding various anions (AcOꢀ,
Iꢀ, Brꢀ, BF4ꢀ, Clꢀ, NO3ꢀ, ClO4ꢀ, H2PO4ꢀ, HSO4ꢀ and Fꢀ ions,
as the TBA salts) to 5 in acetonitrile (Fig. 2a). When TBAF (2.0
ions concentration in the range 0–40 mM (Fig. S16,† R2
¼
0.9925) was observed, indicating that dosimeter 5 has great
potential for the quantitative determination of Fꢀ ions. The
corresponding detection limits in the uorescence mode was
determined to be 0.105 mM (Fig. S17†). Subsequently, AcOꢀ,
Iꢀ, Brꢀ, BF4ꢀ, Clꢀ, NO3ꢀ, ClO4ꢀ, H2PO4ꢀ, HSO4ꢀ and Fꢀ ions
(as their TBA salts) were used to measure the selectivity of
dosimeter 5 in a mixture of acetonitrile–water (95 : 5, v/v),
and the uorescence spectra were recorded aer 10 min
upon addition of 40.0 equiv. of each of these anions (Fig. 2c,
in acetonitrile, see: Fig. S18†). Compared to the other anions
Fig. 1 UV-visible absorption spectral changes of 20 mM solution of (a)
5, (0.0–2.0 equiv. Fꢀ) (b) 5, (2.0–9.0 equiv. Fꢀ) (c) 6, (0.0–3.0 equiv. Fꢀ)
(d) 6, (3.0–8.0 equiv. Fꢀ) upon titration with TBAF (5000 mM) in CH3CN
(inset: plot of absorption changes at 625 nm versus TBAF
concentration).
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investigated, only Fꢀ exhibited a remarkable uorescence
quenching at 530 nm, and the corresponding uorescence
titration of dosimeter 5 with Fꢀ ions in acetonitrile–water
(95 : 5, v/v) was also recorded, as shown in Fig. S19.† This
excellent selectivity was further highlighted by the interfer-
ence experiments (Fig. 2d), in which a consistent turn-off
uorescent response was observed upon the addition of
40.0 equiv. of uoride ions to the solutions of 5 containing
equal concentrations of potentially competing anions (in
acetonitrile, see: Fig. S20†). Dosimeter 5 might be selective
based on anion basicity, and this could also be further
conrmed by the UV-visible titration of dosimeter 5 as well as
the reference compound 6 with the OHꢀ ions (Fig. S21–S22†).
To investigate the interactions between dosimeter 5 and Fꢀ,
1H-NMR titration experiments were performed in DMSO-d6. The
addition of increasing concentration of Fꢀ ions results in the
most remarkable upeld shi of the phenyl proton peaks “Ha0
and Hb0”. These results are in agreement with the formation of
phenolate anion by Fꢀ-triggered silyl ether hydrolysis. At a
higher Fꢀ ions concentration, the “Ha0 and Hb0” peaks were
assigned to the broad peak at d 6.0 ppm (red, #, Fig. 3), and a set
of peaks (red, #) was highly overlapped with the peaks observed
(*, blue) in nal titration of reference 7 with Fꢀ ions. Mean-
while, the LC-MS analysis was further conducted to conrm the
mechanism (Fig. S23†), and which clearly indicates the forma-
tion of compound phenolate 7 (calcd for 372 for C23H18NO4).
Therefore, the mechanism base on Fꢀ-triggered Si–O cleavage
followed by the deprotonation/autoxidation of a secondary
nitrile C–H group is nally established.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant no. 21202099, 21102093) and the Opening Fund of
Shanghai Key Laboratory of Chemical Biology (no. SKLCB-2013-
05) for nancial support.
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Fꢀ based on a dual reaction-based binding site including Fꢀ-
triggered Si–O cleavage, followed by the deprotonation/
autoxidation of a secondary nitrile C–H group. The dosimeter
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