Salicylimine-based fluorescent chemosensor for aluminum ions and application to bioimaging

Article abstract of DOI:10.1021/ic2024583

In this study, an assay to quantify the presence of aluminum ions using a salicylimine-based receptor was developed utilizing turn-on fluorescence enhancement. Upon treatment with aluminum ions, the fluorescence of the sensor was enhanced at 510 nm due to formation of a 1:1 complex between the chemosensor and the aluminum ions at room temperature. As the concentration of Al ³⁺ was increased, the fluorescence gradually increased. Other metal ions, such as Na⁺, Ag⁺, K⁺, Ca²⁺, Mg²⁺, Hg²⁺, Mn²⁺, Co²⁺, Ni ²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb ²⁺, Cr³⁺, Fe³⁺, and In³⁺, had no such significant effect on the fluorescence. In addition, we show that the probe could be used to map intracellular Al³⁺ distribution in live cells by confocal microscopy.

Full text of DOI:10.1021/ic2024583

Article
pubs.acs.org/IC
Salicylimine-Based Fluorescent Chemosensor for Aluminum Ions and
Application to Bioimaging
Soojin Kim,† Jin Young Noh,‡ Ka Young Kim,† Jin Hoon Kim,‡ Hee Kyung Kang,† Seong-Won Nam,†
So Hyun Kim,† Sungsu Park,*,†,§ Cheal Kim,*,‡ and Jinheung Kim*,†
†Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 120-750, Korea
‡Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul, Korea
§Mechanobiology Institute, National University of Singapore, Singapore 117411
S
* Supporting Information
ABSTRACT: In this study, an assay to quantify the presence
of aluminum ions using a salicylimine-based receptor was
developed utilizing turn-on fluorescence enhancement. Upon
treatment with aluminum ions, the fluorescence of the sensor
was enhanced at 510 nm due to formation of a 1:1 complex
between the chemosensor and the aluminum ions at room
temperature. As the concentration of Al3+ was increased, the
fluorescence gradually increased. Other metal ions, such as Na+, Ag+, K+, Ca2+, Mg2+, Hg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+,
Pb2+, Cr3+, Fe3+, and In3+, had no such significant effect on the fluorescence. In addition, we show that the probe could be used to
map intracellular Al3+ distribution in live cells by confocal microscopy.
probe molecules that consist of a photon interaction site as a
fluorophore and a metal binding site. In the presence of specific
metal ions, the fluorophore−receptor communication gets
turned on as a result of the binding of the metal ions at the
receptor site. Until recently, only a few fluorescent chemo-
sensors have been developed for detection of Al3+.4−9 Most
fluorescent sensors for Al3+ have good selectivity, but this
approach has several disadvantages including complicated
synthetic procedures and poor water solubility. Meanwhile,
some Schiff base compounds coordinated to metal ions were
reported to have antitumor and antioxidative activities.10,11
Although many Schiff base derivatives incorporating a
fluorescent moiety have been used to detect various metal
ions, Schiff base-type Al3+ chemosensors are very rare and no
examples of water-soluble devices as well as sensors that can
be used for cell imaging have been developed.4 Here, we examine
the metal-binding properties of the π-conjugated Schiff base
receptor, o-phenolsalicylimine (PSI), and report the tridentate
ligand PSI as a chemosensor, which exhibits enhanced
fluorescence upon binding to Al3+ with high selectivity. Possible
utilization of PSI as intracellular sensors of Al3+ was also
examined by confocal fluorescence microscopy.
INTRODUCTION
■
Chemosensors for selective detection of various biologically
and environmentally relevant metal ions have recently attracted
great attention. The widespread use of aluminum in food
additives, aluminum-based pharmaceuticals, and storage/cook-
ing utensils often exposes people to aluminum ions. In addition,
frequent use of aluminum foil, vessels, and trays for
convenience results in moderate increases in the Al3+
concentration in food. After absorption, aluminum ions
would be distributed to all tissues in humans and animals and
eventually accumulate in the bone. The iron binding protein is
known to be the main carrier of Al3+ in plasma, and Al3+ can
enter the brain and reach the placenta and fetus. Aluminum
ions may stay for a very long time in various organs and tissues
before being excreted through the urine. In addition, aluminum
ions have been implicated as a causative factor of Alzheimer’s
disease and associated with damage to the central nervous
system in humans.1−3 Although trace amounts of aluminum
ions are present in the drinking water, low-dose chronic
exposure to the ions may cause Alzheimer’s disease possibly
due to accumulation of oxidative damage induced by the ions.2
Sensitive bioimaging of Al3+ in the cell is a prerequisite for
understanding the underlying mechanism about how aluminum
ions cause aluminum-induced human diseases including
Alzheimer’s disease.3 Thus, detection of Al3+ is important to
control the concentration levels in the biosphere and minimize
direct affects on human health.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All solvents and reagents
(analytical grade and spectroscopic grade) were obtained from
Sigma-Aldrich and used as received. Solutions of metal ions were
prepared with metal perchlorate salts. 1H NMR spectra were recorded
on a Varian 400 spectrometer. Chemical shifts (δ) are reported in
In recent years, fluorescent chemosensors have attracted
significant interest because of their potential use in medicinal
and environmental research. The most commonly employed
method used for chemosensor detection is the development of
Received: November 15, 2011
Published: March 2, 2012
© 2012 American Chemical Society
3597
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602

Inorganic Chemistry
Article
ppm, relative to tetramethylsilane Si(CH3)4. Absorption spectra were
recorded at 25 °C using a Perkin-Elmer model Lambda 2S UV/vis
spectrometer. Emission spectra were recorded on a Perkin−Elmer
LS45 fluorescence spectrometer. Elemental analysis for carbon,
nitrogen, and hydrogen was carried out using a vario MACRO
(Elemental Analysensysteme, Germany) in the Laboratory Center of
the Seoul National University of Science and Technology, Korea.
Electrospray ionization mass spectra were collected on a Thermo
Finnigan (San Jose, CA, USA) LCQ@ Advantage MAX quadrupole
ion trap instrument by infusing samples directly into the source using a
manual method. Spray voltage was set at 4.2 kV, and the capillary
temperature was at 80 °C.
Synthesis of o-Phenolsalicylimine (PSI). A solution of 2-
hydroxyaniline (1.42 g, 13 mmol) in absolute ethanol was added to a
solution containing 2-hydroxyl-benzaldehyde (1.59 g, 13 mmol) in
ethanol. The mixture was refluxed for 1 h under nitrogen. The solution
was then cooled to room temperature, and the solvent was evaporated.
The orange product was recrystallized from ethanol. The yield of PSI
was 91%. 1H NMR (methanol-d4, 400 MHz) δ: 8.88 (s, 1H), 7.49 (d,
1H), 7.36 (t, 1H), 7.31 (d, 1H), 7.12 (t, 1H), 6.92 (m, 4H). 13C NMR
(DMSO-d6, 100 MHz): δ 161.73, 160.77, 151.18, 134.95, 132.89,
132.37, 128.14, 119.64, 119.60, 119.54, 118.79, 116.74, 116.54
(Supporting Information, Figure S1). FAB MS m/z (M+): calcd,
213.23; found, 213.22. Anal. Calcd for C13H11NO2 (213.23): C, 73.23;
H, 5.20; N, 6.57. Found: C, 73.22; H, 5.22; N, 6.66.
Figure 1. Chemical structures of the receptor PSI and a 1:1 complex of
PSI and Al3+ in CH3OH:H2O.
First, the fluorescence response behavior of PSI was
examined upon treatment with various metal ions in
methanol−water (1:1, v/v) (Figure 2a). PSI alone showed no
Optical Detection of Al3+ Using PSI. The receptor (1.0 μM) was
mixed with different concentrations of metal ions in CH3OH:H2O
(1:1, v/v) in a 1 cm cell. Solutions of metal ions were prepared using
nitrate salts. After equilibrium at ambient temperature for 1 min,
absorption and fluorescence spectra of the mixtures were measured.
Fluorescence spectra were measured at an excitation wavelength of
411 nm.
Methods for Cell Imaging. HeLa cell line (CCL-2, ATCC,
Manassas, VA) was cultured in DMEM (Dulbecco’s Modified Eagle
Medium, Invitrogen, Carlsbad, CA) supplemented with 100 units/mL
penicillin (SIGMA), 100 mg/mL streptomycin (SIGMA), and 10%
fetal bovine serum (SIGMA) at 37 °C in a humidified incubator. Cells
were seeded onto an 18 mm × 8 mm cover glass (Marienfeld, Lauda-
Koenigshofen, Germany) at a density of 2 × 105 cells. Cells were then
incubated with various concentrations (0.1−100 μM) of Al(NO3)3 at
37 °C for 4 h. After washing with PBS three times to remove the
remaining Al(NO3)3, the cells were then incubated with 10 μM PSI for
30 min at room temperature. The incubated cells were washed with
PBS and mounted onto a glass slide with ClearMount aqueous mount-
ing medium (Invitrogen). Fluorescent images of the mounted cells
were obtained using a confocal laser scanning microscope (CLSM
LSM510, Carl Zeiss) with 480 nm excitation and 520 nm emission
filters at various magnifications (from 200× to 400×).
To fluorescently visualize apoptosis in cells previously exposed to
Al(NO3)3, a TACS 2 TdT-Fluor in situ apoptosis detection kit
(Trevigen, Gaithersburg, MD) was used.12 Cells were incubated and
exposed to Al(NO3)3 as described above. Cells were immersed in TdT
buffer (30 mM Trizma base, pH 7.2, 140 mM sodium cacodylate,
1 mM cobalt chloride). TdT labeling solution was then added to cells
as suggested by the manufacturer. Cells were then incubated at 37 °C
for 60 min. The reaction was terminated by transferring cells to TdT
stop buffer (300 mM sodium chloride, 30 mM sodium citrate) for
5 min at room temperature. Cells were rinsed with distilled deionized
water (DDW) and incubated with Strep-Fluor solution for 20 min at
room temperature. To fluorescently visualize necrosis in cells, the cells
previously exposed to Al(NO3)3 were rinsed with DDW and 1.5 μM
propidium iodide (Invitrogen) was added for 15 min. After rinsing
with PBS buffer and DDW, cells were prepared for imaging.
Figure 2. (a) Fluorescence spectra of PSI (1 μM) before and after
addition of metal salts (12 μM) of Ag+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+,
Fe3+, Ga3+, Hg2+, In3+, K+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+ in
CH3OH:H2O (1:1). (b) Bar graph shows the relative emission
intensity of PSI at 510 nm upon treatment with various metal ions.
significant emission after excitation at 411 nm. However, upon
addition of Al3+, the fluorescence intensity of PSI increased by a
factor of 1000 at a wavelength of 510 nm. In contrast, addition
of other relevant metal ions, such as Na+, Ag+, K+, Ca2+, Mg2+,
Hg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Cr3+, Fe3+, and
In3+, caused almost no fluorescence increase. Ga3+, which
belongs to the same group on the periodic table, also generated
a similar fluorescence enhancement centered at 545 nm, but the
intensity was significantly lower than with Al3+ under the same
conditions. It seems that the fluorescence enhancement was
derived from the more widespread formation of the π-conjuga-
tion system in PSI upon metal binding, and the fluorescence
was sensitive to specific metal ions. The selectivity for Al3+ with
RESULTS AND DISCUSSION
■
The Schiff base probe PSI was synthesized by coupling of
2-hydroxylbenzaldehyde and 2-aminophenol with 91% yield in
ethanol (Figure 1 and details in the Experimental Section).
3598
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602

Inorganic Chemistry
Article
1 μM PSI was plotted as a bar graph in Figure 2b. Only Al3+
resulted in a pronounced fluorescence enhancement relative to
the control ions.
When the PSI sensor was titrated with Al3+, the fluorescence
intensity increased up to 13 equiv and then no changes were
observed up to 27 equiv (Figure 3). A binding constant (log Ka)
Figure 4. (a) UV−vis spectral changes of 50 μM PSI after addition of
aluminum ions (2, 3, 4, 5, 15, and 25 equiv) in CH3OH−H2O (1:1).
(b) Job’s plot for the binding of PSI with Al3+. Absorbance at 415 nm
was plotted as a function of the molar ratio [Al3+]/([PSI] + [Al3+]).
Figure 3. (a) Fluorescence spectra of 1 μM PSI (λex = 411 nm) after
addition of increasing amounts of Al3+ ions (0.5, 1.0, 1.5, 2.0, 2.5, 3.0,
5.0, 7.0, 9.0, 11, 13, 15, 17, and 27 μM) at room temperature.
(b) Graph of the fluorescence intensity at 510 nm as a function of
Al3+ concentration.
of 4.5 was obtained from a nonlinear least-squares fit according
to a 1:1 binding stoichiometry (Figure 3b). The close agree-
ment of the experimental data to the theoretical fit indicates
that this reaction involves a 1:1 complexation stoichiometry.
The log Ka of PSI was within the range 2.9−8.7 of those
reported for Al3+-binding sensors.4−9
1
Figure 5. H NMR spectra of PSI with and without Al(ClO4)3 in
methanol-d4: (a) PSI (bottom); (b) PSI with 1 equiv of Al3+ (top).
We then evaluated changes in the absorbance of PSI upon
treatment with Al3+ ions. Upon addition of Al3+, the intensity of
the absorption bands of PSI at 260 and 345 nm decreased with
a concomitant increase in new low-energy peaks at 305 and
415 nm (Figure 4a). Three well-defined isosbestic points at ca.
310, 365, and 460 nm were observed, indicative of a clean con-
version of PSI into the PSI−Al3+ complex. In addition, the Job’s
plot, which was based on the changes in absorbance at 415 nm, con-
firmed formation of a 1:1 complex of Al3+ with PSI (Figure 4b).
The absorption spectrum of the complex matched the excita-
tion spectrum for formation of the PSI−Al3+ complex,
suggesting that the observed absorbance and fluorescence changes
resulted from complex formation between PSI and Al3+.
The sensitivity of the aluminum fluorescent probe PSI
toward variations in the sample pH was examined by absorp-
tion and fluorescence measurements (Supporting Information,
Figures S2 and S3). At pH 5.0, 6.0, and 7.0, addition of Al3+ to
PSI resulted in similar changes in the absorption spectra of the
probe. Although the fluorescence intensity of PSI was very
weak at low and high pH, PSI showed a meaningful response
between pH 4 and 8, while the biologically relevant pH range is
3599
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602

Inorganic Chemistry
Article
The binding mode of PSI with Al3+ was examined by 1H NMR
in methanol-d4. Spectra of PSI before and after treatment with 1
equiv of Al3+ are shown in Figure 5. Complexation of PSI with Al3+
shifted the aromatic proton peaks of phenol to low magnetic field
due to the reduction of electron density upon coordination to the
metal ion. The Ha, Hb, and Hc protons of the phenol moiety
underwent overall large downfield shifts of 1.00, 0.43, and 0.02 ppm,
respectively, upon addition of 1.0 equiv of Al3+ salt. Similarly, the
peak corresponding to the other phenol ring (Hf) was downfield
shifted by 0.74 ppm. The imine proton peak (Hd) at 8.82 ppm was
also downfield shifted by ca. 0.08 ppm and broadened upon
complexation. NMR data demonstrated that adduct formation
between Al3+ and PSI resulted in these chemical shifts. There were
no significant changes with the proton signals upon addition of 2
and 4 equiv of Al3+ to PSI (Supporting Information, Figure S4).
Electrospray ionization mass spectral analysis also indicated the 1:1
binding stoichiometry of PSI with Al3+. The positive-ion mass
spectrum of PSI upon addition of 1 equiv of Al3+ exhibited one
Figure 6. Positive-ion electrospray ionization mass spectrum of PSI
upon addition of Al(NO3)3 (1 equiv) in CH3OH.
intense peak at m/z = 302, corresponding to the ion [Al3+
+
PSI2− + 2CH3OH]+ (Figure 6). Upon addition of 2 equiv of
Al3+ to a solution of PSI, no significant changes were observed
(Supporting Information, Figure S5).
To examine the selectivity for Al3+ in a complex background
of potentially competing species, the fluorescence enhancement
of PSI with Al3+ was investigated in the presence of other metal
ions (Figure 7). Except for Fe3+, Cu2+, Ga3+, and In3+, a
background of most competing metal ions did not interfere
with detection of Al3+ by PSI in methanol−H2O. On the basis
of the fluorescence intensities shown in Figure 7, Fe3+, Cu2+,
and In3+ quenched about 75−80% of the fluorescence obtained
with Al3+ alone. Interestingly, Ga3+ quenched about 40% under
the same conditions. These data demonstrate that Fe3+, Cu2+,
and In3+ interfere with the binding of Al3+ to PSI, but the
interference from Ga3+ was relatively weak.
Subsequent experiments were conducted to test if PSI can be
used to fluorescently visualize intracellular Al3+. For this
purpose, HeLa cells were first incubated with various con-
centrations (from 0.1 μM to 1 mM) of Al(NO3)3 for 4 h and
then treated with 10 μM PSI for 30 min before imaging. No
fluorescence was observed in cells (Figure 8e) that were not
previously exposed to Al(NO3)3, while strong fluorescence was
Figure 7. Relative fluorescence of PSI and its complexation with Al3+
in the presence of various metal ions. Response of PSI was included as
controls. (Left to right) PSI alone (L corresponds to PSI), PSI + Al3+,
PSI + Al3+ + Ag+, PSI + Al3+ + Ca2+, etc. Conditions: 1.0 μM of PSI, 12
equiv of Al3+ in the absence and presence of 12 (gray) and 24 equiv
(black) of metal ions with (λex = 411 nm).
6.0−7.6. These differences may be attributed to decomposition
of PSI at a high pH and protonation of the phenol moieties at a
low pH.
Figure 8. Representative fluorescence images of HeLa cells exposed to various concentrations of Al(NO3)3 with 10 μM PSI: (a and e) incubated
with PSI only; (b and f) incubated with 1 μM Al(NO3)3 for 4 h and exposed with PSI; (c and g) incubated with 10 μM Al(NO3)3 for 4 h and
exposed with PSI; (d and h) 100 μM Al(NO3)3 for 4 h and stained with PSI. (a−d) DIC images and (e−h) fluorescent images (excitation = 480 nm,
emission = 520 nm LP). Details are found in the Experimental Section. Scale bar represents 50 μm.
3600
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602

Inorganic Chemistry
Article
Figure 9. Representative fluorescence images of HeLa cells exposed to various concentrations of Al3+ after tunnel assay using a TACS 2 TdT-Fluor
in situ apoptosis detection kit and incubation with 1.5 μM propidium iodide: (a and e) cells were not incubated with Al(NO3)3; (b) and f) cells
incubated with 1 μM Al(NO3)3 for 4 h; (c and g) 10 μM Al(NO3)3 for 4 h; (d and h) 100 μM Al(NO3)3 for 4 h. (a−d) Fluorescent images of
apoptotic cells (excitation = 480 nm, emission = 520 nm long pass) with a TACS 2 TdT-Fluor in situ apoptosis detection kit, and (e−h) fluorescent
images of necrotic cells (excitation = 540 nm, emission = 590 nm long pass) with 1.5 μM propidium iodide. Details are found in the Experimental
Section. Scale bar represents 50 μm.
observed in cells (Figure 8f−h) exposed to Al(NO3)3 at
different concentrations (1−100 μM). Cells that were exposed
to 0.1 μM Al(NO3)3 displayed negligible fluorescence (data not
shown). These results demonstrate that the probe is permeable
to cells, binds to intracellular Al3+, and emits fluorescent light
upon binding to the metal ion. PSI is highly suitable for
determining the exposure level of cells to Al(NO3)3.
spectrum. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Phone: +82-2-3277-4453. Fax: +82-2-3277-3419. E-mail:
jinheung@ewha.ac.kr (J.K.); nanopark@ewha.ac.kr (S.P.);
chealkim@snut.ac.kr (C.K.).
The probe was further used to determine the concentration of
Al(NO3)3 causing cytotoxicity. For this application, a TACS 2
TdT-Fluor in situ apoptosis detection kit and propidium iodide
were used to visualize apoptosis and necrosis in cells, respectively.
Neither apoptosis nor necrosis fluorescence was observed in the
cells that were not exposed to Al(NO3)3 as shown in Figure 9a
and 9e. Cells at 1 μM Al(NO3)3 (Figure 9b) displayed the
strongest apoptosis fluorescence, and apoptosis fluorescence in
cells diminished as the concentration of Al(NO3)3 was
increased to 10 and 100 μM, as shown in Figure 9c and 9d,
respectively. In contrast with the apoptosis staining results
(Figure 9b−d), cells at 1 μM Al(NO3)3 (Figure 9f) displayed
the weakest necrosis fluorescence, and necrosis fluorescence
in cells became stronger as the concentration of Al(NO3)3 was
increased to 10 and 100 μM, as shown in Figure 9g and 9h,
respectively. On the basis of these results, it is suggested that
cytotoxic damages depend on the intracellular level of Al3+.
In conclusion, we developed a turn-on fluorescence method to
detect Al3+ using a simple salicylimine-based chemosensor with
high selectivity in methanol−water. In our assay, Al3+ could
selectively participate in complex formation with the receptor,
which resulted in a color change and fluorescence enhancement.
We have also shown that this PSI chemosensor can be applied to
living cell imaging to detect and measure the distribution of
intracellular Al3+.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea Grant funded by the Korea Government (2010-
0009525), Korea Science & Engineering Foundation (2009-
0074066), Converging Research Center Program through the
National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology
(2011K000675), and BK21 (to S.L., K.Y.K., and H.K.K.).
REFERENCES
■
(1) Cronan, C. S.; Walker, W. J.; Bloom, P. R. Nature 1986, 324,
140−143.
(2) Walton, J. R. J. Inorg. Biochem. 2007, 101, 1275−1284.
(3) (a) Nayak, P. Environ. Res. 2002, 89, 101−115. (b) Fasman, G. D.
Coord. Chem. Rev. 1996, 149, 125−165.
(4) Park, H. M.; Oh, B. N.; Kim, J. H.; Qiong, W.; Hwang, I. H.;
Jung, K.-D.; Kim, C.; Kim, J. Tetrahedron Lett. 2011, 52, 5581−5584.
(5) (a) Maity, D.; Govindaraju, T. Chem. Commun. 2010, 4499−
4501. (b) Maity, D.; Govindaraju, T. Inorg. Chem. 2010, 49, 7229−
7231.
(6) (a) Lohani, C. R.; Kim, J.-M.; Chung, S.-Y.; Yoon, J.; Lee, K.-H.
Analyst. 2010, 135, 2079−2084. (b) Jang, H. O.; Nakamura, K.; Yi,
S.-S.; Kim, J. S.; Go, J. R.; Yoon, J. J. Inclusion Phenom. Macrocycl.
Chem. 2001, 40, 313−316.
ASSOCIATED CONTENT
■
(7) Ren, J. L.; Zhang, J.; Luo, J. Q.; Pei, X. K.; Jiang, Z. X. Analyst
2001, 126, 698−702.
(8) Wang, Y.-W.; Yu, M.-X.; Yu, Y.-H.; Bai, Z.-P.; Shen, Z.; Li, F.-Y.;
You, X.-Z. Tetrahedron Lett. 2009, 50, 6169−6172.
S
* Supporting Information
13C NMR spectrum of PSI, additional UV−visible and
1
fluorescence spectra, H NMR spectra of PSI, and ESI mass
3601
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602

Inorganic Chemistry
Article
(9) Kim, S. H.; Choi, H. S.; Kim, J.; Lee, S. J.; Quang, D. T.; Kim,
J. S. Org. Lett. 2010, 12, 560−563.
(10) Padhye, S.; Kauffman, G. B. Coord. Chem. Rev. 1985, 63, 4823−
4831.
(11) de Silveira, V. C.; Luz, J. S.; Oliveira, C. C.; Graziani, I.; Ciriolo,
M. R.; Ferreira, A. M. D. C. J. Inorg. Biochem. 2008, 102, 1090−1103.
(12) Nam, S. W.; Chen, X.; Lim, J.; Kim, S. H.; Kim, S. T.; Cho,
Y. H.; Yoon, J.; Park, S. PLos One 2011, 6, e21387.
3602
dx.doi.org/10.1021/ic2024583 | Inorg. Chem. 2012, 51, 3597−3602