Highly selective fluorescence turn-on chemosensor based on naphthalimide derivatives for detection of copper(II) ions

Article abstract of DOI:10.1016/j.saa.2012.12.005

A new fluorescent probe, DN-SB, was synthesized. DN-SB was based on the naphthalimide derivatives and exhibited high selectivity and sensitivity for Cu²⁺ ions. As a Cu²⁺-amplified fluorescent probe, its fluorescence spectrum showed 4

Full text of DOI:10.1016/j.saa.2012.12.005

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 57–61
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Highly selective fluorescence turn-on chemosensor based
on naphthalimide derivatives for detection of copper(II) ions
a
a
a
a
a
Zhijun Chen a, Limin Wang a,b, , Gang Zou , Jun Tang , Xiaofei Cai , Mingshuang Teng , Li Chen
⇑
a Shanghai Key Laboratory of Functional Materials Chemistry and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237,
PR China
b College of Chemistry & Materials Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
It was a turn-on sensor for copper(II)
ions based on naphthalimide
derivatives.
"
"
It had a good selectivity for
copper(II) ions.
The sensor had a low detection limit
(0.15 lM).
a r t i c l e i n f o
a b s t r a c t
Article history:
A new fluorescent probe, DN-SB, was synthesized. DN-SB was based on the naphthalimide derivatives
and exhibited high selectivity and sensitivity for Cu2+ ions. As a Cu2+-amplified fluorescent probe, its fluo-
rescence spectrum showed 4.5-fold enhancement in the intensity of the signal at 519 nm on binding with
the Cu2+. Based on the fluorescence titration spectra and job’s-plot, binding mode of DN-SB with Cu2+ was
Received 17 September 2012
Received in revised form 30 November 2012
Accepted 5 December 2012
Available online 14 December 2012
proposed. Fluorescence intensity was linear with concentration of Cu2+ cation in a range from 4
l
M to
7
l
M. DN-SB was also sensitive for copper ions. The detection limit was calculated to be 0.15
lM which
Keywords:
Copper(II) ions
Fluorescent probe
Fluorescence enhancement
Naphthalimide derivatives
High selectivity
indicated DN-SB was sensitive to Cu2+
.
Ó 2012 Elsevier B.V. All rights reserved.
Schiff base
Introduction
and a wide variety of neuro-logical problems [5–8]. Currently,
chemosensors are widely employed as the signal output due to its
high sensitivity and the simplicity of its equipment requirement
[9–14]. So, the design and construction of sensory molecules of cop-
per ions is essential because of their important roles in biological,
industrial, and environmental processes. Recently, the detection
and estimation of Cu2+ always constitutes an active area of research
[15–24]. Schiff base is always used as a typical acceptor for copper
ions [25–27] and a number of luminescent probes for copper ions
have been reported, many of them which show enhancement in
the intensity of the signal are based on the coupling of two
Copper is an essential soft transition metal ion that plays a piv-
otal role in environmental, biological, and chemical systems [1–4].
In addition, excessive levels of copper could do harm to liver. As re-
ported, copper deficiency may lead to hematological manifestations
⇑
Corresponding author at: Shanghai Key Laboratory of Functional Materials
Chemistry and Institute of Fine Chemicals, East China University of Science and
Technology, 130 Meilong Road, Shanghai 200237, PR China. Tel./fax: +86 21
64253881.
E-mail address: wanglimin@ecust.edu.cn (L. Wang).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.12.005

58
Z. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 57–61
O
N
O
O
O
O
O
N
O
O
N
O
NH2-NH2
H2N
CH3COOH, reflux
CHO-CHO
CH3OCH2CH2OH, reflux
C2H5OH, reflux
Br
Br
1
NH
HN
H2N
N
4
O
3
2
NH2
OH
O
N
O
N
C2H5OH, reflux
HN
OH
N
DN-SB
Scheme 1. The synthetic path of DN-SB.
structural subunits: rhodamine derivatives (used for signal trans-
duction) and Schiff base (used for the selective recognition of cop-
per ions) [28–31]. Few of chemosensors for copper ions based on
the other fluorophores can show fluorescence enhancement. The
‘‘off–on’’ type signal is superior compared to the ‘‘on–off’’ type
due to their better overall signal processing in biosystems and spec-
ificity of mode of action [32]. With this in mind, we designed and
synthesized a new fluorescent sensor for detection of copper ions
based on naphthalimide derivatives. The synthesis of DN-SB started
from 4-bromine naphthalene anhydride 1, via four steps, DN-SB
was obtained, as shown in Scheme 1.
gen atmosphere for 6 h, After the completion of reaction, the reac-
tion mixture was then poured into a mixture of cracked ice and
water and filtered to get a pale yellow solid. The crude product
was recrystallized from chlorobenzene to give 4.6 g light gray solid
in a yield of 85%. 1H NMR, (CDCl3): 8.64 (d, 1H, J = 7.2 Hz), 8.53–
8.55 (m, 1H), 8.39 (d, 1H, J = 7.6 Hz), 8.02 (d, 1H, J = 8.0 Hz),
7.81–7.85 (m, 1H), 4.17 (t, 2H, J = 7.6 Hz), 1.67–1.75 (m, 2H),
1.40–1.49 (m, 2H), 0.98 (t, 3H, J = 7.2 Hz).
Synthesis of 4-hydrazine hydrate-N–butyl-1,8-naphthalic anhydride
(3)
Compound 2 (5.4 g, 16.3 mmol) and 5 mL hydrazine hydrate
(85%, w/w) was added into 50 mL of ethylene glycol monomethyl
ether and refluxed for 6 h. After cooled to room temperature, the
mixture was poured into 150 mL water, the precipitated solids
were filtered, dried in vacuum to give a red solid. Yield: 80%. 1H
NMR, (DMSO-d6): 8.60 (d, 1H, J = 8.4 Hz), 8.41 (d, 1H, J = 7.2 Hz),
8.28 (d, 1H, J = 8.4 Hz), 7.63–7.65 (m, 1H), 7.24 (d, 1H, J = 8.4 Hz),
4.60–4.67 (m, 2H), 4.01 (t, 2H, J = 7.2 Hz), 3.24–3.25 (m, 1H),
1.54–1.62 (m, 2H), 1.30–1.36 (m, 2H), 0.92 (t, 3H, J = 7.6 Hz).
Materials and methods
Experimental
General
Reagents and solvents were reagent grade, used as received un-
less otherwise noted. All solvents used in water-sensitive reactions
were freshly distilled. Reactions were monitored by TLC. Yields
were of purified product and were not optimized. 1H NMR were
recorded at 400 MHz. 13C NMR were recorded at 100 MHz.
Fluorescence spectra was measured on a Thermo Fluorescence
spectrophotometer.
Synthesis of 4-hydrazine-N–butyl-1,8-naphthalic anhydride (4)
Naphthalimide hydrazine 3 (283.0 mg, 1 mmol) was dissolved
in 20 mL ethanol, then 3.0 mL aqueous glyoxal (30%, w/w) was
then added to the solution. The mixture was stirred at room tem-
perature for 5 h. The precipitated crystals were filtered and washed
with ethanol to give the orange solid (217 mg, yield: 66%). 1H NMR,
(DMSO-d6): 12.19 (s, 1H), 9.62 (s, 1H), 8.37–8.48 (m, 2H), 7.72–
7.83 (m, 3H), 6.57–6.79 (m, 1H), 4.01 (t, 2H, J = 7.2 Hz), 1.58–1.61
(m, 2H), 1.34–1.36 (m, 2H), 0.93 (t, 3H, J = 7.2 Hz).
Synthesis
Synthesis of synthesis of 4-bromo-N-butyl-1,8-naphthalic anhydride
(2)
A mixture of compound 1 (5 g, 18 mmol), 1-aminobutane (1.7 g,
23.3 mmol) in 40 mL acetic acid was stirred under reflux in a nitro-

Z. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 57–61
59
Synthesis of DN-SB
3000
Compound 4 (0.1 g, 0.3 mmol) and o-aminophenol (0.03 g,
0.3 mmol) was added into 15 mL ethanol and refluxed for 6 h. After
the mixture cooled to room temperature, the precipitated crystals
were filtered and washed with ethanol to give the red solid. yield:
93%.1H NMR, (DMSO-d6): 11.87 (s, 1H), 9.28 (s, 1H), 8.77 (d, 1H,
J = 7.6 Hz), 8.48–8.53 (m, 2H), 8.27–8.40 (m, 2H), 7.81–7.84
(m, 1H), 7.71 (d, 1H, J = 8.4 Hz), 7.23 (d, 1H, J = 7.6 Hz), 7.09–7.13
(m, 1H), 6.82–6.93 (m, 1H), 6.51–6.64 (m, 1H), 4.02 (t, 2H,
J = 7.2 Hz), 1.56–1.64 (m, 2H), 1.32–1.37 (m, 2H), 0.93 (t, 3H,
J = 7.2 Hz). 13C NMR (DMSO-d6): d:163.52, 162.90, 157.13, 151.26,
145.13, 143.92, 137.28, 133.03, 130.94, 128.15, 128.10, 123.90,
122.05, 119.96, 119.55, 119.44, 119.05, 116.24, 114.38, 114.29,
29.70, 19.79, 18.50, 13.70. MS: m/z 414.1692[ Calcd 414.1689].
2500
2000
1500
1000
500
0
0.0
0.2
0.4
0.6
0.8
1.0
The metal salts used
[Cu2+]/[Cu2++DN-SB]
Fe2(SO4)3Á9H2O, ZnSO4Á6H2O, Co(ClO4)2Á6H2O, NiSO4Á6H2O,
Ca(ClO4)2, SnCl2Á6H2O, Pb(ClO4)2Á3H2O, Cr(ClO4)3Á6H2O, CuSO4Á
Fig. 3. Job’s plot for determining the stoichiometry of DN-SB ([DN-SB] +
[ Cu2+] = 40 M) and Cu2+ in acetonitrile–water (70:30, v/v) with a buffer solution
of 3-(N-morpholino) propanesulfonic acid (MOPS, 10 mM, pH = 7.0). kex = 455 nm.
l
Cu2+/equiv
4000
4000
3500
10
3000
3500
5
2500
1
2000
3000
0.9
1500
1000
0.8
3000
Cu
500
2500
2000
1500
1000
500
0.7
0.6
0.5
.
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
3
5
10
2+
C/Cu (equiv)
2500
2000
1500
.
0.1
0
Probe, other cations
1000
500
0
0
500
550
600
650
Wavelength(nm)
500
550
600
650
Wavelength(nm)
Fig. 1. Fluorescence spectra of DN-SB (1 Â 10À5 M) in acetonitrile–water (70:30,
v/v) with
a buffer solution of 3-(N-morpholino) propanesulfonic acid (MOPS,
Fig. 4. Fluorescent response of DN-SB (1 Â 10À5 M) to various metal ions at 10
equiv concentration in acetonitrile–water (70:30, v/v) with a buffer solution of
3-(N-morpholino) propanesulfonic acid (MOPS, 10 mM, pH = 7.0).
10 mM, pH = 7.0). in the presence of different concentrations of Cu2+ (0–10equiv),
kex = 455 nm. Inset: fluorescence intensity at 519 nm of DN-SB as a function of Cu2+
concentration.
5H2O, Mn(ClO4)2Á6H2O, Na2SO4Á10H2O, K2SO4, AgClO4ÁH2O,
Hg(ClO4)2Á3H2O, Cd(ClO4)2Á6H2O.
2000
1800
1600
1400
1200
1000
Result and discussion
The interaction of sensor DN-SB with the cations was investi-
gated through fluorescence spectra. All the fluorescence behavior
of sensor DN-SB was studied in acetonitrile–water (70:30, v/v)
with a buffer solution of 3-(N-morpholino) propanesulfonic acid
(MOPS, 10 mM, pH = 7.0). Fluorescence titrations of DN-SB with
Cu2+ were performed and the responding spectra are shown in
Fig. 1. Free DN-SB exhibited very slight fluorescence response in
the range from 495 to 650 nm, which was due to efficient photoin-
duced electron transfer (PET) process from the electron-rich recep-
tor to the excited DN-SB fluorophore. Upon addition of Cu2+, the
PET mechanism was quenched. Thus, the emission band peaked
at 519 nm significantly increased in intensity, and a continuous in-
crease of fluorescence intensity could be observed with Cu2+ con-
centration increasing (Fig. 1).
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Cu2+/uM
Fig. 2. Curve of fluorescence intensity at 519 nm of DN-SB (1 Â 10À5 M) versus
This fact meant that DN-SB could act as an off–on fluorescent
increasing concentrations of Cu2+ (4–7
lM).
probe for Cu2+. The fluorescence peak no longer changed when

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Z. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 57–61
at 519 nm measured at a given Cu2+ concentration and x repre-
sented the concentration (10À5mol/L) of Cu2+ added. According to
IUPAC, the detection limit was determined from three times the
4
3
2
1
0
standard deviation of the blank signal (3s) as 0.15
lM. The result
indicated that DN-SB was sensitive to Cu2+
.
In order to further understand the coordination of DN-SB with
Cu2+, Job’s-plot analysis was carried out (Fig. 3). The results also
indicated that DN-SB formed a 1:1 stoichiometrical complex with
Cu2+, and the association constant between DN-SB and Cu2+ was
determined to be 1.1 Â 1010 MÀ1 in acetonitrile by using nonlinear
least-square analysis.
To evaluate the selectivity of DN-SB in acetonitrile–water
(70:30, v/v) with a buffer solution of 3-(N-morpholino) propane-
sulfonic acid (MOPS, 10 mM, pH = 7.0). Various metal ions such
as Na+, Ag+, Ca2+, Mg2+, Zn2+, Pb2+, Hg2+, Ni2+, Mn2+, Co2+, Cd2+, K+,
Fe3+ and Cr3+ were used (Fig. 4). As expected, DN-SB showed a
weak fluorescence. Upon addition of Cu2+ (10 equiv) to a solution
of DN-SB, significant enhancement of fluorescence with an emis-
sion maximum at 519 nm was observed and the fluorescence in-
creased by about 4.5-fold, illustrated in Fig. 5. However, other
metal ions were added, the fluorescence behavior of DN-SB showed
almost negligible effects. In addition, the fluorescence increase
took place immediately after Cu2+ addition (within 10 s), indicating
Probe K Na Ca Mg Ni Cr Mn Fe Hg Cu Co Cd Zn Pb Ag
Fig. 5. Fluorescent emission changes of DN-SB (1 Â 10À5 M) upon addition of 10
equiv of each relevant analyte (Cu2+, Mg2+, Hg2+, Na+, K+, Zn2+, Fe2+, Cr3+, Co2+, Ni2+
,
Ca2+, Pb2+, Cd2+, Sn2+, Mn2+) in acetonitrile–water (70:30, v/v) with a buffer solution
of 3-(N-morpholino) propanesulfonic acid (MOPS, 10 mM, pH = 7.0). kex = 455 nm.
that DN-SB enables rapid detection of Cu2+
.
To examine whether sensor DN-SB could still retain the sensing
response to Cu2+ under the potential competition of relevant ana-
lytes, the sensor was treated with Cu2+ in the presence of other me-
tal ions. As displayed in Fig. 6, all the relevant analytes tested had
virtually no influence on the detection of Cu2+. Thus, sensor DN-SB
seemed to be useful for selectively sensing Cu2+, even with these
relevant analytes.
The above-mentioned findings clearly indicated that DN-SB be-
haved as a highly selective turn-on fluorescent probe for Cu2+. As
for the highly selectivity for Cu2+, we thought that the following
two factors might play critical role for high selectivity of DN-SB.
On one hand, the avoidance of using sulfur atom in our scaffold
would avoid interference from other metal ions especially mer-
cury. On the other hand, the receptor of DN-SB was relatively rigid
and had a cavity-like, which probably fitted Cu2+ best compared to
other metal ions.
4
3
2
1
0
Probe K Na Ca Mg Ni Cr Mn Fe Hg Cu Co Cd Zn Pb Ag
Fig. 6. The fluorescent intensity at 533 nm of DN-SB (1 Â 10À5 M) with 10 equiv of
Cu2+ and 10 equiv of various metal ions in acetonitrile–water (70:30, v/v) with a
buffer solution of 3-(N-morpholino) propanesulfonic acid (MOPS, 10 mM, pH = 7.0).
kex = 455 nm.
Based on the fluorescence titration spectra, although Cu2+ could
induce an increasing fluorescence, it could not cause shift in emis-
sion maximum. This phenomenon usually occurred when the
nitrogen, which was also the donor of push–pull system, did not
chelates to metal ions. Therefore, we believed that the nitrogen
that linked to the naphthalimide ring did not involve in Cu2+ che-
lation. In addition to that, some reports indicated that Cu2+ ions
were coordinated to Shift base nitrogen atoms and the phenol oxy-
gen atom preferably [33,34]. According to these factors and Job’s-
plot, we proposed a plausible binding mode of the probe DN-SB
with Cu2+ as shown in Scheme 2.
the concentration of Cu2+ increased from 1 to 10 equiv (Fig. 1, in-
set), implicating a 1:1 complexation of DN-SB with the Cu2+
.
Fig. 2 also showed the good linearity between the emission at
519 nm and concentrations of Cu2+ in the range from 4
lM to
7 lM, indicating that sensor DN-SB can detect quantitatively rele-
vant concentrations of Cu2+. The linear equation was found to be
y = 292.6 Â 105x À 804.5 (R = 0.9986), where y was the emission
OH
N
OH
2-
SO4
Cu2+
N
N
HN
OH
N
N
O
N
O
HN
CuSO4
O
PET
PET
Scheme 2. Proposed binding mode of DN-SB with Cu2+
.

Z. Chen et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 57–61
[9] R. Martínez-Máñez, F. Sancenon, Chem. Rev. 103 (2003) 4419–4476.
61
Conclusion
[10] Y.M. Zhou, H. Zhou, J.L. Zhang, L. Zhang, J.Y. Niu, Spectrochim. Acta Part A 98
(2012) 14–17.
[11] Z.J. Chen, L.M. Wang, G. Zou, L. Zhang, G.J. Zhang, X.F. Cai, M.S. Teng, Dyes Pigm.
94 (2012) 410–415.
Naphthalimid-based fluorescent probe DN-SB was successfully
developed. DN-SB exhibited ‘‘turn-on’’ fluorescence, which dis-
played a fluorescence enhancement response to Cu2+ via a 1:1
binding mode. Fluorescence intensity was linear with concentra-
[12] A.F. Li, J.H. Wang, F. Wang, Y.B. Jiang, Chem. Soc. Rev. 39 (2010) 3729–3745.
[13] W. Liu, K.M. Pin, D. Lee, J. Am. Chem. Soc. 131 (2009) 8703–8707.
[14] J. Shao, D. Yu, H. Lin, J. Mol. Recognit. 21 (2008) 425–430.
[15] A.P. de Silva, H.Q.N. Gunaratne, T.H. Gunnlaugsson, M.J. Allen, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515–1566.
[16] Q. Wu, E.V. Anslyn, J. Am. Chem. Soc. 126 (2004) 14682–14683.
[17] S.H. Kim, J.S. Kim, S.M. Park, S.K. Chang, Org. Lett. 8 (2006) 371–374.
[18] H.S. Jung, P.S. Kwon, J.W. Lee, J.I. Kim, C.S. Hong, J.W. Kim, S. Yan, J.Y. Lee, J.H.
Lee, T. Joo, J.S. Kim, J. Am. Chem. Soc. 131 (2009) 2008–2012.
[19] M. Beltramello, M. Gatos, F. Mancin, P. Tecilla, U. Tonellato, Tetrahedron Lett.
42 (2001) 9143–9146.
tion of Cu2+ cation in a range from 4
lM to 7 lM. Its selectivity to-
ward Cu2+ was very high and little interference was observed for
other commonly coexistent metal ions. As a chemosensor, DN-SB
was also sensitive for copper ions. The detection limit was calcu-
lated to be 0.15 lM.
Acknowledgements
[20] H.X. Wang, L. Yang, W.B. Zhang, Y. Zhou, B. Zhao, X.Y. Li, Inorg. Chim. Acta 138
(2012) 111–116.
[21] S. Dalapati, S. Jana, M.A. Alam, N. Guchhait, Sens. Actuat. B: Chem. 160 (2011)
1106–1111.
[22] Q. Zou, X. Li, J.J. Zhang, J. Zhou, B.B. Sun, H. Tian, Chem. Commun. 48 (2012)
2095–2097.
[23] X.M. Huang, Z.Q. Guo, W.H. Zhu, Y.S. Xie, H. Tian, Chem. Commun. 41 (2008)
5143–5145.
[24] N. Shao, G.X. Pang, X.R. Wang, R.J. Wu, Y. Cheng, Tetrahedron 66 (2010) 7302–
7308.
The authors gratefully acknowledge the support from the Fun-
damental Research Funds for the Central Universities, the Opening
Foundation of Zhejiang Provincial Top Key Discipline, the National
Nature Science Foundation of China (20672035), and Baihehua
Group.
[25] N. Aksuner, E. Henden, I. Yilmaz, A. Cukurovali, Dyes Pigm. 83 (2009) 211–217.
[26] N.R. Chereddy, S. Thennarasu, Dyes Pigm. 91 (2011) 378–382.
[27] S.T. Chen, X.L. Zhang, Q.H. Zhang, X.M. Hou, Q. Zhou, J.L. Yan, W.H. Tan, J.
Lumin. 131 (2011) 947–951.
[28] C.W. Yu, J. Zhang, J.H. Li, P. Liu, P.H. Wei, L.X. Chen, Microchim. Acta 174 (2011)
247–255.
[29] Y. Xiang, A.J. Tong, P.Y. Jin, Y. Ju, Org. Lett. 8 (2006) 2863–2866.
[30] M. Kumar, N. Kumar, V. Bhalla, P.R. Sharma, T. Kaur, Org. Lett. 14 (2012) 406–
409.
[31] X.Q. Chen, M.J. Jou, H.Y. Lee, S.Z. Kou, J.S. Lim, S.W. Nam, S. Park, K.M. Kim, J.Y.
Yoon, Sens. Actuat. B: Chem. 137 (2009) 597–602.
References
[1] T. Elisa, M.N. Elizabeth, J. Jacek, J.L. Stephen, J. Am. Chem. Soc. 130 (2008)
15776–15777.
[2] B. Tang, H. Huang, K.H. Xu, L.L. Tong, G.W. Yang, X. Liu, L.G. An, Chem.
Commun. 34 (2006) 3609–3611.
[3] M.N. Elizabeth, J.L. Stephen, J. Am. Chem. Soc. 129 (2007) 5910–5918.
[4] P. Li, X. Duan, Z.Z. Chen, Y. Liu, T. Xie, L.B. Fang, X.R. Li, M. Yin, B. Tang, Chem.
Commun. 47 (2011) 7755–7757.
[5] T.R. Halfdanarson, N. Kumar, C.Y. Li, R.L. Phyliky, W.J. Hogan, Eur. J. Haematol.
80 (2008) 523–531.
[6] D.J. Waggoner, B.T. Bartnikas, J.D. Gitlin, Neurobiol. Dis. 6 (1999) 221–230.
[7] C. Vulpe, B. Levinson, S. Whitney, S. Packman, J. Gitschier, Nat. Genet. 3 (1993)
7–13.
[8] L.I. Bruijn, T.M. Miller, D.W. Cleveland, Annu. Rev. Neurosci. 27 (2004) 723–
749.
[32] Z.P. Liu, C.L. Zhang, X.Q. Wang, W.J. He, Z.J. Guo, Org. Lett. 14 (2012) 4378–
4381.
[33] P.S. Zhao, J. Song, X.J. Sun, L. Du, F.F. Jian, Chinese J. Struct. Chem. 30 (2011)
346–353.
[34] K.R. Edelman, B.J. Holliday, Inorg. Chem. 49 (2010) 6787–6789.