A Tetrakisquinoline Analog of Calcium Indicator Quin2 for Fluorescence Detection of Cd2+

Article abstract of DOI:10.1002/ejic.201901323

Quin2 (2-{2-[bis(carboxymethyl)amino-5-methylphenoxy]methyl}-6-methoxy-8-[bis(carboxymethyl)amino]quinoline) is a well-known fluorescent Ca²⁺ indicator. In this study, we replaced its four carboxy groups with quinolines and deleted a methyl group on the benzene ring to afford TQquin2 [N,N,N',N'-tetrakis(2-quinolylmethyl)-8-amino-2-(2-aminophenoxy)methyl-6-methoxyquinoline]. In methanol solution, TQquin2 exhibited fluorescence enhancement toward Cd²⁺ with excellent selectivity.

Full text of DOI:10.1002/ejic.201901323

A Journal of
Accepted Article
Title: A Tetrakisquinoline Analog of Calcium Indicator Quin2 for
Fluorescent Detection of Cd2+
Authors: Yuji Mikata, Minori Kaneda, Marin Tanaka, Satoshi Iwatsuki,
Hideo Konno, and Arimasa Matsumoto
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901323
Link to VoR: http://dx.doi.org/10.1002/ejic.201901323

European Journal of Inorganic Chemistry
10.1002/ejic.201901323
A Tetrakisquinoline Analog of Calcium Indicator Quin2 for Fluorescent
2
+
Detection of Cd
,
[a,b,c]
[c]
[a]
[d]
Yuji Mikata,*
Minori Kaneda, Marin Tanaka, Satoshi Iwatsuki, Hideo
[
e]
[a]
Konno, and Arimasa Matsumoto
[
a]
Department of Chemistry, Biology, and Environmental Science, Faculty of Science,
Nara Women's University, Nara 630-8506, Japan
Prof. Dr. Yuji Mikata (0000-0002-9450-0908), Ms. Marin Tanaka, and Dr. Arimasa
Matsumoto
[
b]
KYOUSEI Science Center, Nara Women's University, Nara 630-8506, Japan
Prof. Dr. Yuji Mikata
[
c]
Department of Chemistry, Faculty of Science, Nara Women's University, Nara 630-
8
506, Japan
Prof. Dr. Yuji Mikata and Ms. Minori Kaneda
[
d]
Department of Chemistry, Konan University, 8-9-1 Okamoto, Higashinada, Kobe
6
58-8501, Japan
Prof. Dr. Satoshi Iwatsuki (ORCID ID: 0000-0002-6756-1908)
[
e]
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1
Higashi, Tsukuba, Ibaraki 305-8565, Japan
Dr. Hideo Konno (ORCID ID: 0000-0001-8639-7923)
,
[a]
*
To whom correspondence should be addressed. Tel./fax: +81-742-20-3095, e-mail:
mikata@cc.nara-wu.ac.jp. ORCID ID: 0000-0002-9450-0908. http://www.chem.nara-
wu.ac.jp/~mikata/index.html.
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European Journal of Inorganic Chemistry
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2
Supporting Information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201901323.
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European Journal of Inorganic Chemistry
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3
Abstract:
Quin2
(2-(2-(bis(carboxymethyl)amino-5-methylphenoxy)methyl)-6-
2
+
methoxy-8-(bis(carboxymethyl)amino)quinoline) is a well-known fluorescent Ca
indicator. In this study, we replaced its four carboxy groups with quinolines and
deleted a methyl group on the benzene ring to afford TQquin2 (N,N,N’,N’-tetrakis(2-
quinolylmethyl)-8-amino-2-(2-aminophenoxy)methyl-6-methoxyquinoline).
In
2
+
methanol solution, TQquin2 exhibited fluorescence enhancement toward Cd with
excellent selectivity.
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European Journal of Inorganic Chemistry
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4
Introduction
The development of fluorescent sensor molecules for specific metal ions is of
[
1-7]
significant interest.
Once suitable sensor molecules have been established,
fluorescence sensing becomes a convenient, fast, sensitive, specific, and low-cost
detection method. Therefore, many fluorescent probe molecules concomitant with a
novel sensing mechanism have been extensively explored to fulfill largely diverse
needs of metal ion detection and quantification.
Among such objectives for metal ion detection, the strict discrimination of
2
+
2+
[8-12]
10
Cd from Zn is still challenging.
These two group 12 metals possess a closed d
electron configuration with small (21 pm) difference in their ionic radii. Several
2
+
2+
strategies to discriminate Cd from Zn include use of quinoline or aniline nitrogen
donor atoms to serve a moderate coordination environment around the metal
[
[
13-19]
20]
center,
adjustment of chelation ring size considering the bite angle to the metal
center,
and multi-dentate strategy highlighting the difference in numbers of
[
21-22]
coordination atoms surrounding the metal center.
In spite of its high toxicity, cadmium metal is utilized in industrial products
[
23]
such as butteries and electroplating, resulting in severe global pollution.
Development of an efficient molecular design strategy for fluorescent detection
2
+
and/or quantification of Cd is urgently required.
Recently we have reported that TriMeOBAPTQ (N,N,N’,N’-tetrakis(5,6,7-
trimethoxy-2-quinolylmethyl)-1,2-bis(2-aminophenoxy)ethane, Scheme 1), which is a
tetrakisquinoline derivative of BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N’,N’-
2
+
tetraacetic acid, Scheme 1), exhibits Cd -specific fluorescence enhancement in
[
24]
methanol-HEPES buffer (9:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5). Corresponding
EGTA (ethylene glycol bis(2-aminoethyl ether)-N,N,N’,N’-tetraacetic acid) derivative,
namely TriMeOEGTQ (N,N,N’,N’-tetrakis(5,6,7-trimethoxy-2-quinolylmethyl)-1,2-
2
+
bis(2-aminoethoxy)ethane), did not show any Cd -specific fluorescent enhancement.
In this study, to improve the metal ion specificity and sensitivity, the core structure
was
changed
from
BAPTA
to
quin2
(8-amino-2-((2-amino-5-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901323
5
methylphenoxy)methyl)-6-methoxyquinoline-N,N,N’,N’-tetraacetic acid, Scheme 1),
2
+
[25-26]
which is fluorescent and conformation-restricted Ca -specific probe.
designed a new compound TQquin2, N,N,N’,N’-tetrakis(2-quinolylmethyl)-8-amino-
-(2-aminophenoxy)methyl-6-methoxyquinoline (Scheme 1), which exhibits
Thus, we
2
2
+
improved Cd -specificity compared with BAPTQ derivatives. The use of the quin2
skeleton affords exclusively ML and ML complexes, in contrast to the observation
2
that only ML and M
derivatives.
2
L complexes were observed in the BAPTA and EGTA
Scheme 1.
COOH
OCH3
H CO
OCH3
3
HOOC
N
O
O
N
COOH
COOH
N
N
BAPTA
COOH
H CO
N
3
OCH3
OCH3
O
H CO
O
3
COOH
N
OCH3
N
N
N
OCH3
H CO
3
O
N
TriMeOBAPTQ
HOOC
N
H CO
OCH3
3
COOH
quin2
OCH3
N
O
H CO
N
3
N
N
N
N
N
TQquin2
Results and discussion
Synthesis
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European Journal of Inorganic Chemistry
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6
Scheme 2 shows the synthetic scheme of TQquin2. Starting from 4-methoxy-2-
[
27-28]
nitroaniline, the five-step synthesis via 6-methoxy-2-methyl-8-nitroquinoline
afforded TQquin2 in 10% yield. In spite of the well-known structure of quin2, its
[
29]
derivatives have been scarcely reported.
TQquin2 is not only the first quin2
derivative in which four carboxylates are replaced by other metal-binding motifs,
but also the simple derivative of quin2 that lacks 5-methyl group in the aminophenol
1
13
moiety. All new compounds were characterized by H/ C NMR and elemental
analysis.
Scheme 2.
O
H CO
H CO
3
3
NaOAc, I , NaIO
H CO
6
N HCl
2
3
3
H
+
7
7%
AcOH, r.t.
I
NH2
CH3
N
CH3
N
4
9%
NO2
NO2
NO2
1
2
NO2
HO
H CO
H CO
3
3
NH
2
,
K CO
NO2
2
3
5
% Pd/C, H2
O
O
N
EtOH, r.t.
84%
N
DMF, 75 °C
9%
NO2
NH
2
4
3
4
N
O
H CO
N
3
Br
N
,
K CO
2
3
N
N
N
CH CN, reflux
3
6
4%
N
N
TQquin2
2
+
Cd -Induced Fluorescence Enhancement and Metal Ion Selectivity
Initial screening of the metal ion sensing ability of TQquin2 was conducted in
methanol-HEPES buffer (9:1, 50 mM HEPES, 0.1 M KCl, pH = 7.5), which is the
2
+
[24]
media used for Cd detection with TriMeOBAPTQ.
As shown in Figure S1,
2
+
TQquin2 poorly responded to 10 equiv. of Cd with emission enhancement around
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European Journal of Inorganic Chemistry
10.1002/ejic.201901323
7
4
30 nm in this solvent system as found for the unsubstituted BAPTQ. Since we
thought that water-containing solvent may prevent the coordination of TQquin2
with metal ions, 100% methanol was chosen as a solvent for fluorescent metal ion
detection with this molecule.
Figure 1 demonstrates the fluorescence spectral changes of TQquin2 in the
2
+
presence of 3 equiv. of metal ions in methanol solution. In the presence of Cd , the
fluorescence intensity at 408 nm was increased by a factor of 170. No other metal
2
+
ions responded to TQquin2. As shown in Figure 1b, Zn exhibits only 3.6-fold
fluorescence increase compared to free ligand and strict Zn/Cd selectivity was
established (IZn/ICd = 2%). The UV-vis spectrum changed in the presence of 3 equiv. of
2
+
3+
3+
2+
2+
2+
metal ion such as Cd , Al , Fe , Cu , Hg , and Pb (Figure S2), revealing that only
2
+
Cd induces the fluorescence enhancement upon binding with TQquin2. Similar
2
+
Cd -specific fluorescence enhancement was also observed for BAPTQ and
TriMeOBAPTQ in methanol solution (Figures S3 and S4); however, the fluorescence
2
+
enhancement factor (ICd/I ) and Cd specificity (IZn/ICd) were inferior to those of
0
TQquin2 (Table 1). Importantly, the fluorescence quantum yield (f) of cadmium
complex of TQquin2 (0.058) is similar to that of TriMeOBAPTQ (0.05) which
possesses trimethoxyquinoline chromophores.
(b)
(a)
Cd2+
I / I0
Cd2+ Na
+
K
+
+
Mg2+Ca2+Al3+ Cr3+Mn2+Fe2+Fe3+Co2+Ni2+Cu2+ Ag Zn2+Hg2+ Pb2+
Wavelength (nm)
Metal Ions
Figure 1. Fluorescence response of 34 µM TQquin2 toward 3 equiv. of various metal
ions in methanol solution at 25 °C (lex = 317 nm). (a) Fluorescence spectra. (b)
Fluorescence intensity change at 408 nm. I is the emission intensity of the free ligand.
0
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European Journal of Inorganic Chemistry
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8
2
+
Table 1. Fluorescent Properties of Cd Complexes of TQquin2, BAPTQ, and
a
TriMeOBAPTQ in Methanol
Ligand
l
ex (nm)
l
em (nm)
I
Cd/I
0
IZn/ICd
fCd
b
b
b
TQquin2
BAPTQ
317
317
408
413
468
170
2%
0.058
–
c
c
9.2
28%
12%
d
d
d
TriMeOBAPTQ 347
34
0.05
a
Conditions: 34 µM solution at 25 °C.
In the presence of 3 equiv. of metal ion.
In the presence of 10 equiv. of metal ion.
In the presence of 1 equiv. of metal ion.
b
c
d
2
+
The Cd -induced fluorescence of TQquin2 was shut down in the presence of
2
+
+
equimolar amount of Co and Ag (Figure S5). Although the fluorescence intensity
2
+
2+
2+
of TQquin2-Cd complex tolerates against Zn and Ni under the same
experimental condition, the metal ion exchange took place in the presence of excess
amount of these competitive metal ions.
Metal Binding Modes and Binding Affinity
Absorption and fluorescent titration experiments were conducted to elucidate
2
+
2+
the metal binding modes and binding affinity of TQquin2 with Cd and Zn . As
2
+
shown in Figures 2 and S6, gradual addition of Cd to the methanol solution of
TQquin2 induced absorbance spectral changes and fluorescence enhancement. The
[
30]
overall metal binding event was fitted to the binding sequence model shown in
Scheme 3. The distinct sigmoidal feature in the fluorescence intensity titration data
2
+
of Cd (Figure 2b) reveals that poorly-fluorescent ML and fluorescent ML are
2
[
30]
involved in the binding equilibrium. The calculated titration curves shown in
Figures S6b and 2b well reproduced the experimental data. The formation constants
obtained from UV-vis and fluorescence titration data matched well (logbML2
=
=
1
3.5±0.9 and logbML = 7.3±0.7 for UV-vis data and logbML2 = 13.1±0.3 and logbML
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European Journal of Inorganic Chemistry
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9
6
.9±0.2 for fluorescence data), indicating the suitability and accuracy of the analyses
using the binding model shown in Scheme 3 (Table 2).
(a)
(b)
Cd2+ (equiv.)
Wavelength (nm)
Figure 2. (a) Fluorescence spectra of 34 µM TQquin2 in methanol solution at 25 °C in
2
+
the presence of increasing concentrations of Cd (lex = 317 nm). (b) Fluorescence
2
+
intensity change at 408 nm during the Cd titration of TQquin2 shown in panel (a)
and theoretical fluorescence intensity titration curve based on the binding
equilibrium model and formation constants shown in Scheme 3 and Table 2,
respectively.
Scheme 3.
2+
2+
-1
M2+
(ML)2+
βML = [(ML) ]/([M ][L]) M
+
L
2+
2+
2
-2
M2+ + 2L
(ML2)2+
βML2 = [(ML ) ]/([M ][L] ) M
2
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European Journal of Inorganic Chemistry
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1
0
2
+
2+
a
Table 2. Formation Constants for TQquin2 with Cd and Zn in Methanol at 25 °C
Metal Ion
Analyzed
Spectra
UV-vis
FL
logbML2
logbML
logbM2L
Figure No.
2
+
Cd
13.5±0.9
7.3±0.7
6.9±0.2
5.9±0.3
6.1±0.4
–
Figure S6
Figure 2
Figure S8
Figure 3
13.1±0.3
–
2
+
Zn
UV-vis
–
–
11.0±0.5
11.0±0.4
FL
a
2+
2+
Cd and Zn titration data were analyzed by the equilibrium model shown in
Schemes 3 and 6, respectively.
Since ML
2
complex does not emit significant fluorescence, we propose the
and ML as shown in Scheme 4. Since the ortho-alkoxy substituent
structures of ML
2
of the benzene ring would enhance the electron density of the anilinic nitrogen atom,
2
+
the TQquin2 likely binds to Cd
with the benzene-containing di(2-
the 8-amino-6-methoxyquinoline
quinolylmethyl)amine moiety, leaving
2
+
uncoordinated (structure I in Scheme 4). The mononuclear Cd complex (ML)
becomes fluorescent because the nitrogen atoms at the central 8-amino-6-
methoxyquinoline core bind to the metal center (structure II in Scheme 4). ESI-MS
1
indicated the ML species only and H NMR titration did not clarify the structures of
ML
2
and ML complexes (data not shown).
The mechanism of fluorescence enhancement of TQquin2 is due to the
combination of inhibition of PeT (photo-induced electron transfer) and CHEF
[
31]
(
chelation enhanced fluorescence) of central and side-arm quinolines. In addition,
we consider that the intramolecular excimer formation with unsubstituted quinoline
[
21, 32-33]
rings may be involved.
The BAPTQ exhibits lower energy emission at 413 nm
2
+
in methanol in the presence of Cd , which suggests the intramolecular excimer
formation arising from the unsubstituted tetrakisquinoline moiety for this ligand.
2
+
The TriMeOBAPTQ-Cd complex showed lower-energy shift of the emission
wavelength, indicating the similar fluorescence mechanism with unsubstituted
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European Journal of Inorganic Chemistry
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1
1
BAPTQ. For TQquin2, there are two kinds of chromophores, namely methoxy-
substituted and unsubstituted quinolines. The emission wavelength (408 nm) is
compatible for monomer emission of 6-methoxyquinoline in the central core of the
molecule and excimer emission due to the unsubstituted bis- or tetrakisquinoline
2
+
moieties. The high quantum yield of TQquin2-Cd complex may attribute to the
dual emission mechanism of two different chromophores. Such a fluorescence
2
+
mechanism is not available with Zn and other metal complexes.
Scheme 4.
Q
N
Q
N
O
Q
N
Q
OCH3
H CO
3
Q
N
Q
Cd2+
Q
O
Q
N
H CO
Q
N
3
N
Cd2+
O
Q
N
Q– =
Q
N
N
Q
I
II
2
+
Importantly, the Cd -induced fluorescence enhancement of TQquin2 is
highly sensitive to the counteranion of metal salt. As shown in Figure S7, addition of
Cd(OAc)
2
·2H
2
O instead of Cd(ClO
4
)2
·6H O did not induce the fluorescence
2
enhancement of TQquin2 even in the presence of 3 equiv. of metal ion. This is
2
+
probably because the acetate ion binds to the metal center of TQquin2-Cd complex.
The plausible binding mode of acetate is proposed as structure III in Scheme 5, in
which no fluorescence would be expected similar to the structure I in Scheme 4.
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European Journal of Inorganic Chemistry
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1
2
Scheme 5.
AcO
OAc
Cd2+
Q
H CO
3
N
Q
Q– =
O
N
N
Q
N
Q
III
2
+
The titration data of TQquin2 with Zn shown in Figures 3 and S8 afford the
equilibrium including mononuclear (ML) and dinuclear (M L) zinc complexes
Scheme 6). The fluorescence titration curve shown in Figure 3b is significantly
2
(
2
+
different from that of Cd titration (Figure 2b) especially in low metal concentration
region. This is quite reasonable when smaller ionic radii and lower coordination
2
+
number of Zn are considered. Although the structure of dinuclear zinc complex has
2
+
not been fully elucidated, the compact structures of ML
2
and ML species for Zn
complexes would become less stable in comparison to the extended structure of
[
24, 34-36]
dinuclear zinc complexes that are often found with related ligands.
The
obtained formation constants are logbML = 5.9±0.3 and logbM2L = 11.0±0.5 (UV-vis) and
logbML = 6.1±0.4 and logbM2L = 11.0±0.4 (fluorescence), demonstrating the reliability of
the proposed equilibrium model and data analyses (Table 2). The difference in the
2
+
2+
2+
structures of Zn and Cd complexes would be the critical point for the Cd -specific
fluorescent response of TQquin2.
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European Journal of Inorganic Chemistry
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1
3
(a)
(b)
Wavelength (nm)
Zn2+ (equiv.)
Figure 3. (a) Fluorescence spectra of 34 µM TQquin2 in methanol solution at 25 °C in
2
+
the presence of increasing concentrations of Zn (lex = 317 nm). (b) Fluorescence
2
+
intensity change at 418 nm during the Zn titration of TQquin2 shown in panel (a)
and theoretical fluorescence intensity titration curve based on the binding
equilibrium model and formation constants shown in Scheme 6 and Table 2,
respectively.
Scheme 6.
2
+
2+
-1
M2+
(ML)2+
4+
βML = [(ML) ]/([M ][L]) M
+ L
4
+
2+ 2
-2
M2+ + L
βM2L = [(M L) ]/([M ] [L]) M
2
(M L)
2
2
Fluorescence lifetime and limit of detection (LOD)
2
+
The fluorescence lifetimes (t) of TQquin2 and its Cd complex were
measured in methanol solution (Figures S9 and S10). Relatively short fluorescence
lifetimes were obtained for free ligand (t = 1.3 ns (49%) and 5.4 ns (11%)). On the
2
+
other hand, Cd -bound TQquin2 exhibits significant components with elongated
2
+
lifetimes (t = 3.3 ns (67%) and 7.5 ns (33%)). The obtained long lifetime of the Cd
complex with unsubstituted quinoline chromophore suggests the involvement of
[
21, 32-33]
intramolecular excimer component as mentioned previously.
2
+
From the fluorescence titration at low Cd concentration range, the limit of
detection (LOD) were estimated to be 182 nM (Figure S11), which is insufficient
value for environmental analysis. This owes to the formation of poorly fluorescent
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European Journal of Inorganic Chemistry
10.1002/ejic.201901323
1
4
2
+
ML complex at low Cd concentration as discussed above. Future molecular design
2
based on quin2 platform should pay attention to this point.
Conclusion
A tetrakisquinoline ligand, TQquin2, exhibits highly selective fluorescence
2
+
enhancement upon binding with Cd in methanol. The possible metal complex
2
+
species in the solution include ML
2
, ML, and M L, in which ML for Cd complex
2
emits the highest fluorescence intensity at 408 nm. The introduction of fluorescent
and structurally rigid 6-methoxy-8-aminoquioline moiety in the central core of the
2
+
BAPTQ significantly enhance the fluorescence intensity and Cd -specificity of the
2
+
molecule; however, in order to detect low concentration of Cd , new molecular
design strategy to prevent the formation of less fluorescent ML species would be
2
necessary. Other drawbacks of TQquin2 include short emission/excitation
wavelength and sensitivity to water and counteranion due to the weak metal
binding ability, which could be improved by introduction of appropriate electron-
donating substituents on the side-arm quinolines which act as a metal binding site
and chromophore. The present results provide important information for future
molecular design methodology of highly fluorescent probes targeting specific metal
ions.
Experimental
General: All commercial reagents and solvents used for preparation of new
compounds were used without purification. Methanol used for spectroscopic
measurements were spectral grade (Spectrosol). Following metal salts were used for
measurements of absorption and fluorescence spectra unless otherwise indicated:
Cd(ClO
4
)
2
·6H
2
O, NaClO
4
·H
2
O, KCl, MgSO
4
·7H
2
O, CaCl
2
, Al(NO
O,
3
)
3
, CrCl
3
2
·6H
2
2
O,
O,
MnCl ·4H
2
2
O,
FeCl ·4H
2
2
O,
Fe(ClO ·6H
4
)3
2
O,
CoCl ·6H
2
2
Ni(NO )
3
·6H
1
13
Cu(ClO
4
)2
·6H
2
O, AgNO
3
, Zn(ClO
4
)2
·6H
2
O, Hg(ClO
4
)2
·6H
2
O, and Pb(NO
3
)2
. H and C
NMR spectra were recorded on a Varian GEMINI 2000 (300 and 75.5 MHz)
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European Journal of Inorganic Chemistry
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1
5
spectrometer with reference to tetramethylsilane or solvent signals. UV-vis and
fluorescence measurements were performed on a Jasco V-660 spectrophotometer
and Jasco FP-6300 spectrofluorometer, respectively. Fluorescence quantum yields
were measured on a HAMAMATSU photonics C9920-02 absolute PL quantum yield
measurement system. Fluorescence lifetime measurements were performed on a
HORIBA fluorescence lifetime system Tempro.
[
27-28]
6
-Methoxy-2-methyl-8-nitroquinoline
(1): The mixture of 4-methoxy-2-
nitroaniline (5.00 g, 29.7 mmol) and 1N HCl (35 mL) was refluxed for 15 min. To the
solution was added crotonaldehyde (3.00 mL, 29.7 mmol) in dropwise manner, and
the mixture was refluxed for 1 h. After the reaction mixture was cooled to room
temperature, the solution was washed with ether and 40% NaOHaq was added. The
organic materials were extracted with chloroform, dried, and evaporated to afford 6-
methoxy-2-methyl-8-nitroquinoline (1) as brown solid (4.98 g, 22.8 mmol) in 77%
[
27]
[28]
yield (ref. 50%, 30% ).
-Iodomethyl-6-methoxy-8-nitroquinoline
methyl-8-nitroquinoline (1) (924 mg, 4.24 mmol) in acetic acid (4 mL) was added
[
28]
2
(2): To the solution of 6-methoxy-2-
NaOAc (139 mg, 1.69 mmol), NaIO
3
(170 mg, 0.86 mmol), and I (436 mg, 1.72 mmol)
2
and stirred overnight in the dark at room temperature under the nitrogen
atmosphere. The precipitated material was collected with filtration, washed with
acetic acid, and dried to give 2-iodomethyl-6-methoxy-8-nitroquinoline (2) as white
solid (709 mg, 2.06 mmol, 49%).
1
H NMR (CDCl
3
, 300 MHz): d (ppm) 8.05 (d, J = 8.5 Hz, 1H), 7.64 (d, J = 2.4 Hz, 1H),
7
.60 (d, J = 8.9 Hz, 1H), 7.23 (d, J = 2.4 Hz, 1H), 4.63 (s, 2H), 3.97 (s, 3H).
1
3
C NMR (CDCl
3
, 75.5 MHz): d (ppm) 157.9, 156.1, 150.5, 135.5, 134.3, 128.5, 123.1,
1
16.2, 109.1, 56.3.
Anal. Calcd for C11
H9
IN
2
O3 (2): H, 2.64; C, 38.40; N, 8.14. Found: H, 2.74; C, 38.47; N,
7
.88.
-Methoxy-2-(2-nitrophenoxymethyl)-8-nitroquinoline (3): The mixture of 2-
6
nitrophenol (425 mg, 3.05 mmol), 2-iodomethyl-6-methoxy-8-nitroquinoline (2) (700
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European Journal of Inorganic Chemistry
10.1002/ejic.201901323
1
6
mg, 2.03 mmol), and potassium carbonate (571 mg, 4.13 mmol) in DMF (10 mL) was
stirred for 10 min at 75 °C. After the reaction mixture was cooled to room
temperature, 20 mL of water was added and precipitated materials were collected by
filtration. The solid was washed with saturated K
2
CO aq and chloroform to afford 6-
3
methoxy-2-(2-nitrophenoxymethyl)-8-nitroquinoline (3) as white solid (350 mg, 0.985
mmol, 49%).
1
H NMR (DMSO-d , 300 MHz): d (ppm) 8.52 (d, J = 8.5 Hz, 1H), 8.06 (s, 1H), 7.93 (d, J
6
=
7
7.9 Hz, 1H), 7.80 (d, J = 8.2 Hz, 1H), 7.73 (s, 1H), 7.65 (t, J = 8.0 Hz, 1H), 7.49 (d, J =
.9 Hz, 1H), 7.15 (t, J = 8.3 Hz, 1H), 5.50 (s, 2H), 3.95 (s, 3H).
1
3
C NMR (DMSO-d
25.0, 24.1, 121.2, 121.0, 115.7, 115.5, 114.6, 109.8, 71.5, 56.5.
(3·0.7CHCl ): H, 3.15; C, 48.44; N, 9.57. Found: H,
6
, 75.5 MHz): d (ppm) 155.9, 155.7, 150.5, 148.0, 136.4, 134.4, 128.8,
1
Anal. Calcd for C17.7
H13.7Cl2.1
N3
O6
3
2
.96; C, 48.09; N, 9.33.
-Amino-2-(2-aminophenoxymethyl)-6-methoxyquinoline (4): The mixture of 6-
8
methoxy-2-(2-nitrophenoxymethyl)-8-nitroquinoline (3) (350 mg, 0.985 mmol) and
5
0
% Pd/C (68.6 mg) in ethanol (30 mL) was hydrogenated at ambient pressure of H
2
at
°C to room temperature for overnight. After the removal of the catalyst by
filtration, solvent was removed to afford 8-amino-2-(2-aminophenoxymethyl)-6-
methoxyquinoline (4) as brown oil (245 mg, 0.831 mmol, 84%).
1
H NMR (CDCl
3
, 300 MHz): d (ppm) 7.91 (d, J = 8.5 Hz, 1H), 7.49 (d, J = 8.5 Hz, 1H),
6
.9-6.6 (m, 4H), 6.56 (d, J = 2.4 Hz, 1H), 6.45 (d, J = 2.7 Hz, 1H), 5.3 (s, 2H), 3.84 (s, 3H).
1
3
C NMR (CDCl
34.2, 128.8, 121.3, 119.8, 118.3, 115.1, 112.2, 101.6, 71.8, 55.3.
O): H, 5.96; C, 67.09; N, 13.81. Found: H, 5.63; C,
3
, 75.5 MHz): d (ppm) 158.5, 151.9, 146.0, 144.5, 136.1, 134.7, 135.3,
1
Anal. Calcd for C17
6.63; N, 13.41.
H18
N3
O
2.5 (4·0.5H
2
6
N,N,N′,N′-Tetrakis(2-quinolylmethyl)-8-amino-2-(2-aminophenoxymethyl)-6-
methoxyquinoline (TQquin2): The mixture of 2-bromomethylquinoline (991 mg,
4
0
.46 mmol), 8-amino-2-(2-aminophenoxymethyl)-6-methoxyquinoline (4) (245 mg,
.831 mmol), and potassium carbonate (1.17 g, 83.1 mmol) in dry acetonitrile (30 mL)
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European Journal of Inorganic Chemistry
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7
was refluxed for 1.5 days. After the reaction mixture was cooled to room
temperature, solvent was evaporated and organic materials were extracted with
chloroform−water. The organic layer was dried and evaporated, and the residue was
purified by silica gel column chromatography (eluent: dichloromethane:ethyl acetate
=
5:1-0:1, R = 0.06 (ethyl acetate)) to afford N,N,N′,N′-Tetrakis(2-quinolylmethyl)-8-
f
amino-2-(2-aminophenoxymethyl)-6-methoxyquinoline (TQquin2) as brown oil (458
mg, 0.532 mmol, 64%).
1
H NMR (CDCl , 300 MHz): d (ppm) 8.03 (d, J = 8.5 Hz, 2H), 7.97 (d, J = 8.5 Hz, 4H),
3
7
.89 (d, J = 8.5 Hz, 2H), 7.7-7.6 (m, 10H), 7.54 (d, J = 7.0 Hz, 2H), 7.5-7.3 (m, 6H), 7.04
dd, J = 7.3, 1.8 Hz, 1H), 6.87 (d, J = 2.4 Hz, 1H), 6.8-6.6 (m, 3H), 6.46 (d, J = 2.4 Hz,
H), 5.16 (s, 4H), 4.99 (s, 2H), 4.74 (s, 4H), 3.76 (s, 3H).
(
1
1
3
C NMR (CDCl , 75.5 MHz): d (ppm) 160.3, 160.0, 157.6, 151.3, 147.6, 147.4, 147.2,
3
1
1
1
39.1, 137.8, 136.5, 136.0, 135.8, 135.6, 129.8, 129.6, 129.1, 129.0, 128.6, 128.5, 127.3,
27.2, 127.1, 127.0, 126.1, 125.7, 125.6, 122.3, 120.8, 120.6, 120.1, 120.0, 118.1, 112.5,
09.0, 98.0, 77.2, 71.7, 64.2, 60.5, 59.6, 55.4.
+
HRMS (ESI) m/z: [M+H] calcd for C57
H46
N7
O2 860.37130; found 860.37059.
Measurements: For spectroscopic measurements, metal salts were dissolved in Milli-
Q water (Millipore) and added to the methanol solution (3 mL) of ligands as a 3 µL
portion. For spectral titration experiments, Cd(ClO
4
)2
·6H
2
O and Zn(ClO
4
)2
·6H O
2
were dissolved in methanol and added to the methanol solution (3 mL) of ligands as
µL portions, with total 100 µL (for 5 equiv. of metal ion) at maximum.
4
Corresponding Author
Tel/fax: +81-742-20-3095. E-mail: mikata@cc.nara-wu.ac.jp
*
ORCID
Yuji Mikata: 0000-0002-9450-0908
Satoshi Iwatsuki: 0000-0002-6756-1908
Hideo Konno: 0000-0001-8639-7923
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European Journal of Inorganic Chemistry
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1
8
Acknowledgments
This work was supported by the Research for Promoting Technological Seeds, JST,
Adaptable and Seamless Technology Transfer Program through Target-driven R&D,
JST, Grant-in Aid for Scientific Research provided by JSPS KAKENHI Grant Number
JP15K05454 and 19H04583, and the Nara Women's University Intramural Grant for
Project Research.
Keywords: Cadmium · N ligands · Fluorescence · Sensors
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Synopsis for
A Tetrakisquinoline Analog of Calcium Indicator Quin2 for Fluorescent
2
+
Detection of Cd
,
a,b,c
c
a
d
e
Yuji Mikata,*
Minori Kaneda, Marin Tanaka, Satoshi Iwatsuki, Hideo Konno,
a
and Arimasa Matsumoto
a
Department of Chemistry, Biology, and Environmental Science, Faculty of Science, Nara
Women's University, Nara 630-8506, Japan
b
KYOUSEI Science Center, Nara Women's University, Nara 630-8506, Japan
c
Department of Chemistry, Faculty of Science, Nara Women's University, Nara 630-8506,
Japan
d
Department of Chemistry, Konan University, 8-9-1 Okamoto, Higashinada, Kobe 658-8501,
Japan
e
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
TQquin2
with Cd2+
Q
H3CO
Q
N
Q
O
N
N
Q
Q— =
N
TQquin2
TQquin2 only,
with Zn2+, and
with other metal ions
φ = 0.058
in MeOH
Wavelength (nm)
Key Topic: Cadmium Probe
Replacement of carboxylate to quinoline in quin2 (2-(2-(bis(carboxymethyl)amino-5-
methylphenoxy)methyl)-6-methoxy-8-(bis(carboxymethyl)amino)quinoline)
2
+
structure exhibits Cd -specific fluorescence enhancement in methanol.
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