Zinc selective chemosensors based on the flexible dipicolylamine and quinoline

Article abstract of DOI:10.1016/j.ica.2012.09.009

Zinc sensor molecules containing quinoline have been synthesized, which show fluorescence in the presence of Zn²⁺. The nitrogen in quinoline is critical to fluorescence and fluorescence enhancement is promoted by deprotonating the sensor's amide. One of the sensors is highly selective for Zn²⁺ over Cd²⁺ and other cations such as Hg²⁺, Fe²⁺, Mn²⁺ and Ca²⁺. This selectivity can be attributed to the increased absorption of the sensor in the presence of Zn ²⁺ and the strong binding of Zn²⁺. Structural studies, including X-ray and NMR, show the ability of dipicolylamine (DPA) to bind in facial and meridional manners to Zn²⁺. Crystal structures of different compounds show Zn²⁺ coordinating to three, four, and five nitrogens from the compounds. They also indicate that the selectivity of DPA containing compounds towards Zn²⁺ may originate from Zn²⁺ being stable in different coordination environments.

Full text of DOI:10.1016/j.ica.2012.09.009

Inorganica Chimica Acta 394 (2013) 542–551
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Zinc selective chemosensors based on the flexible dipicolylamine and quinoline
a
a
a,b
a
c
c
Hong Gyu Lee , Ju Hoon Lee , Seung Pyo Jang , In Hong Hwang , Sung-Jin Kim , Youngmee Kim ,
a,
⇑
b,
⇑
Cheal Kim , Roger G. Harrison
a
Department of Fine Chemistry, Seoul National University of Science & Technology, Seoul 139-743, Republic of Korea
Department of Chemistry and Biochemistry, Brigham Young University, Provo, UT, USA
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2012
Received in revised form 14 September
Zinc sensor molecules containing quinoline have been synthesized, which show fluorescence in the pres-
2+
ence of Zn . The nitrogen in quinoline is critical to fluorescence and fluorescence enhancement is pro-
2+
2+
moted by deprotonating the sensor’s amide. One of the sensors is highly selective for Zn over Cd
2
012
2+
2+
2+
2+
and other cations such as Hg , Fe , Mn and Ca . This selectivity can be attributed to the increased
Accepted 17 September 2012
Available online 27 September 2012
2
+
2+
absorption of the sensor in the presence of Zn and the strong binding of Zn . Structural studies, includ-
ing X-ray and NMR, show the ability of dipicolylamine (DPA) to bind in facial and meridional manners to
Zn . Crystal structures of different compounds show Zn coordinating to three, four, and five nitrogens
from the compounds. They also indicate that the selectivity of DPA containing compounds towards Zn
may originate from Zn being stable in different coordination environments.
2
+
2+
Keywords:
Zinc sensor
Dipicolylamine
Fluorescence enhancement
2+
2
+
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
These properties make it coordinate more strongly in different
2
+
geometries than larger divalent cations such as Ca and neighbor-
2
+
Over the last several years research has steadily progressed on
ing transition metal cations, such as Fe , that prefer a certain li-
2
+
the synthesis of zinc sensors. Zinc ions are colorless in aqueous
solutions, do not have unpaired electrons, and remain in the plus
two oxidation state. It is thus difficult to image zinc ions and know
their concentration in biological environments. However, it has
been discovered that some unique molecules in the presence of
Zn fluoresce, which fluorescence can be monitored and quanti-
fied [1]. In fact, Zn can now be imaged in cells and living organ-
isms [2].
Zinc sensors often have two parts, a chelating group that binds
zinc and an aromatic group that fluoresces when zinc is bound [3].
Dipicolylamine (DPA) is one molecule that has been used for the
zinc binding group, it having three nitrogens for zinc chelation
gand geometry. This ability of Zn to accommodate different
geometries may even make it detectable in the presence of Cu2+
.
DPA has the flexibility to coordinate in meridional and facial
arrangements, as well as others in between. Thus, DPA can accom-
2
+
modate Zn in different geometries such as octahedral and trigo-
nal bipyramidal.
2
+
2
+
Quinoline is still an important fluorophore for Zn2+ sensors and
has been combined with DPA. The quinoline group was one of the
2
+
first molecules used as a fluorophores for Zn sensing [6]. Re-
2
+
cently, Zn has been shown to enhance fluorescence of quinoline
molecules that contain amide amine ethers [7], hydroxyl quino-
lines [8], hydroxyl quinolines with amino rings [9], and spiropy-
rans [10]. Combining DPA with quinoline has produced sensors
with an array of properties such as femtomolar zinc detection
[11], large Stokes shifts [12], fluorescence by two-photon [13],
ratiometric detection of zinc [14], and multiple zinc binding sites
[15].
[
4]. DPA has been bonded to many different fluorophores, includ-
ing ones with amide linkages [5]. An important property of DPA
which makes it especially good for binding Zn is its flexibility.
2+
2
+
Zn has 10 valence electrons in d orbitals, which provide no ligand
field stabilization energy to be in an octahedral, tetrahedral, or
other configuration. However, Zn2+ has a plus two charge and a rel-
atively small radii for its nuclear charge. Taking these properties
together means Zn is a fairly strong Lewis acid with preference
to bind nitrogen ligands and have stability in various geometries.
We report the synthesis of the zinc sensor QP, which has a DPA
2
+
subunit for Zn coordination and a quinoline group for fluores-
cence (Scheme 1). We have intentionally increased the carbon
chain length by one carbon between the fluorophore and zinc-
binding domain in an attempt to change the fluorescence response
to zinc. We also investigate zinc-binding molecules QA and NA,
which are closely related to QP, but vary by a carbon atom between
the binding and fluorescence domains (QA) or a nitrogen atom in
2
+
⇑
Corresponding authors. Tel.: +82 2 970 6693; fax: +82 2 973 9149 (C. Kim), tel.:
+
1 801 422 8096; fax: +1 801 422 0153 (R.G. Harrison).
E-mail addresses: chealkim@seoultech.ac.kr (C. Kim), roger_harrison@byu.edu
(
R.G. Harrison).
0
020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.09.009

H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
543
Energy calculations were done using SPARTAN 08 version 1.2.1 by
Wavefunction, Inc.
2.2. Preparation of 3-chloro-N-(quinolin-8-yl)propanamide
3
-Chloropropionyl chloride (2.00 mL, 20.9 mmol) was dissolved
in chloroform (10 mL), it was then added dropwise to a cooled and
stirred solution of 8-aminoquinoline (0.740 g, 5.13 mmol) and pyr-
idine (0.43 mL, 5.2 mmol) in chloroform (10 mL) over a period of
Scheme 1. Structures of Zn2+ binding compounds, QP, QA and NA.
1
h. After the reaction solution was allowed to warm to room tem-
perature, it was stirred for 2 h and the solvent was removed under
reduced pressure to obtain an ivory powder, which was washed
the naphthyl ring (NA). Crystal structures of the molecules coordi-
2
+
with dichloromethane and diethyl ether. Yield: 1.00 g (75.3%), 1
nated to Zn show DPA coordinating in meridional and facial
arrangements. Also, proton NMR data indicate the different DPA
binding modes. We found that the quinoline-based sensors have
intense fluorescence in the presence of zinc and fluoresce selec-
tively for zinc over main group and transition metal ions.
H
NMR (DMSO-d , 400 MHz): d 10.32 (NHCO, s, 1H), 8.94 (d,
6
J = 4.0 Hz, 1H), 8.66 (d, J = 4.0 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H),
7.66 (t, J = 8.0 Hz, 1H), 7.62 (t, J = 8.0 Hz, 1H), 7.58 (d, J = 8.0 Hz,
1H) 3.94 (–CH –, t, J = 8.0 Hz, 2H) 3.14 (–CH –, t, J = 8.0 Hz, 2H).
2
2
1
3
C NMR (DMSO-d
6
, 300 MHz): d 170.99, 159.01, 150.71, 149.77,
1
3
1
40.84, 137.32, 123.59, 122.99, 109.89, 60.48, 58.80. IR(KBr):
334 (N–H), 1688 (C@O), 1529, 1484, 1424, 1386, 1345, 1324,
2
. Experimental
ꢀ1
261, 1236, 1186, 1163, 951, 828, 796, 756, 745, 696, 588 cm
.
+
FAB MS m/z (M ): calcd, 259.06; found, 259.27.
2.1. Materials and instruments
Reagents and solvents were purchased from commercial suppli-
2.3. Preparation of 3-dipicolylamine-N-(quinoline-8-yl)propanamide
(QP)
ers and used as received. QA was synthesized similar to a pub-
lished procedure [16]. Caution: perchlorate salts are potentially
explosive with organic ligands, handle them carefully. Absorption
spectra were recorded at 25 °C using a Perkin Elmer model Lambda
3-Chloro-N-(quinolin-8-yl)propanamide (1.17 g, 4.52 mmol),
0
2,2 -dipicolyamine (0.93 mL, 4.9 mmol), N,N-diisopropylethyl-
2
S UV–Vis spectrometer. A BIO RAD FTS 135 spectrometer was
amine (0.91 mL, 5.1 mmol) and potassium iodide (20 mg,
0.12 mmol) were dissolved in acetonitrile (30 mL), stirred and re-
fluxed for 10 h under a nitrogen atmosphere. The mixture was
cooled to room temperature and the solvent removed under re-
duced pressure to obtain a yellow oil, which was purified by silica
gel column chromatography using chloroform/methanol (30:1, v/
v) as eluent to afford product. Yield: 1.61 g (89.6%). 1H NMR
(DMSO-d , 400 MHz): d 10.53 (NHCO, s, 1H), 8.86 (d, J = 4.0 Hz,
6
1H), 8.67 (d, J = 4.0 Hz, 1H), 8.45 (d, J = 4.0 Hz, 2H), 8.40 (d,
J = 4.0 Hz, 1H), 7.66 (t, J = 8.0 Hz, 1H), 7.63 (t, J = 8.0 Hz, 1H), 7.55
1
used for IR KBr pellets. H NMR measurements were performed
on a Varian 400 MHz spectrometer and chemical shifts are re-
corded in ppm. Fluorescence measurements were performed on a
Perkin Elmer model LS45 Fluorescence spectrometer. X-ray diffrac-
tion data were collected on a Bruker SMART AXS diffractometer.
Electrospray ionization mass spectra (ESI-MS) were collected on
a Thermo Finnigan (San Jose, CA, USA) LCQ™ Advantage MAX
quadrupole ion trap instrument. Elemental analysis for carbon,
nitrogen, and hydrogen was carried out by using a vario MACRO
(
Elemental Analysensysteme, Germany) in the Laboratory Center
(m, 5H) 7.16 (t, J = 8.0 Hz, 2H), 3.85 (–CH
2 2
–, s, 4H) 2.91 (–CH –, t,
1
3
of Seoul National University of Science and Technology, Korea.
J = 8.0 Hz, 2H) 2.83 (–CH –, t, J = 8.0 Hz, 2H) ppm. C NMR (CDCl ,
2
3
Table 1
Crystallographic data for NA–Zn(NO
3
)
2
, QA–ZnClO
4
, and QP–ZnCl
2
.
NA–Zn(NO
3
)
2
QA–ZnClO
22ClN
565.28
293(2)
0.71073
4
QP–ZnCl
27Cl
565.79
170(2)293(2)
0.71073
2
Empirical formula
Formula weight
T (K)
C
24
H
22
6
N O
7
Zn
C
23
H
5
O
6
Zn
C
25
H
2 5 2
N O Zn
571.85
170(2)
0.71073
k (Å)
Space group
P1ꢀ
1
P2 /n
P1ꢀ
a (Å)
b (Å)
c (Å)
9.0792(12)
12.3072(18)
13.6258(19)
14.514(2)
90.00
14.514(2)
90.00
2322.9(6)
4
1.616
7.5246(7)
9.0423(8)
20.3129(18)
84.1570(10)
87.076(2)
65.5400(10)
1251.4(2)
2
10.5728(15)
13.6730(18)
98.742(3)
103.224(2)
104.426(2)
1206.6(3)
2
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc. (Mg/m3)
1.574
1.077
1.501
1.228
Absorption coefficient (mmꢀ1)
Crystal size (mm)
1.224
0.15 ꢁ 0.10 ꢁ 0.03
0.15 ꢁ 0.10 ꢁ 0.03
0.23 ꢁ 0.18 ꢁ 0.08
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
6698
11861
6984
4600 (Rint = 0.0243)
4600/0/343
1.008
4420 (Rint = 0.1320)
4420/5/329
0.925
4794 (Rint = 0.0458)
4794/1/322
0.932
Final R indices [I > 2
r
(I)]
R
R
1
= 0.0422, wR
= 0.0618, wR
2
= 0.0919
= 0.0974
R
R
1
= 0.0793, wR
= 0.1337, wR
2
= 0.1985
= 0.2464
R
R
1
= 0.0361, wR
= 0.0468, wR
2
= 0.0833
= 0.0856
R indices (all data)
1
2
1
2
1
2
Largest difference in peak and hole (e Åꢀ3)
0.545 and ꢀ0.555
0.900 and ꢀ1.578
0.514 and ꢀ0.432

5
44
H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
Scheme 2. Synthesis of QP.
3 2
Fig. 2. Crystal structure of NA–Zn(NO ) . Zn is five-coordinate and along with a
nitrate group, binds nitrogens from DPA and an oxygen from the amide. Atom color
code: Zn2 green, N blue, O red, C grey, and hydrogen white. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
+
Fig. 1. Crystal structure of QP–ZnCl
geometry and does not bind the amide nitrogen or oxygen. Atom color code: Zn
green, N blue, O red, C grey, hydrogen white, and chloride pink. (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of this article.)
2
. Zn is five-coordinate, has trigonal bipyramidal
2+
Table 2
Selected bond lengths (Å) and angles (°) for NA–Zn(NO
3
)
2
, QA–ZnClO
4
2
, and QP–ZnCl .
NA–Zn(NO
3 2
)
Zn(1)–O(51)
Zn(1)–O(7)
Zn(1)–N(2)
2.007(2)
2.0639(19)
2.211(2)
Zn(1)–N(3)
Zn(1)–N(1)
2.045(2)
2.094(3)
O(51)–Zn(1)–N(3)
N(3)–Zn(1)–O(7)
N(3)–Zn(1)–N(1)
O(51)–Zn(1)–N(2)
O(7)–Zn(1)–N(2)
105.47(9)
112.64(9)
120.55(9)
170.22(8)
78.07(8)
O(51)–Zn(1)–O(7)
O(51)–Zn(1)–N(1)
O(7)–Zn(1)–N(1)
N(3)–Zn(1)–N(2)
N(1)–Zn(1)–N(2)
92.21(8)
106.09(10)
114.98(9)
79.62(9)
77.55(9)
QA–ZnClO
4
N(1)–Zn(1)
N(3)–Zn(1)
N(5)–Zn(1)
2.156(6)
2.172(5)
2.120(5)
79.1(2)
95.4(2)
107.7(2)
90.8(2)
N(2)–Zn(1)
N(4)–Zn(1)
O(2)–Zn(1)
N(4)–Zn(1)–O(2)
N(4)–Zn(1)–N(1)
O(2)–Zn(1)–N(1)
N(5)–Zn(1)–N(3)
N(1)–Zn(1)–N(3)
N(5)–Zn(1)–N(2)
N(1)–Zn(1)–N(2)
2.227(5)
2.080(5)
2.121(5)
174.4(2)
96.5(2)
86.4(2)
98.9(2)
153.3(2)
159.3(2)
77.4(2)
N(4)–Zn(1)–N(5)
N(5)–Zn(1)–O(2)
N(5)–Zn(1)–N(1)
N(4)–Zn(1)–N(3)
O(2)–Zn(1)–N(3)
N(4)–Zn(1)–N(2)
O(2)–Zn(1)–N(2)
N(3)–Zn(1)–N(2)
2
+
Fig. 3. Crystal structure of QA–Zn(ClO )(H O) complex. Zn coordinates to all five
4
+
2
nitrogens of QA. Atom color code: Zn2 light blue, N blue, O red, C grey, hydrogen
white, and chlorine green. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
88.8(2)
80.30(19)
105.1(2)
78.60(19)
QP–ZnCl
2
Zn(1)–N(3)
Zn(1)–N(2)
Zn(1)–Cl(2)
N(3)–Zn(1)–N(1)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–Cl(1)
N(3)–Zn(1)–Cl(2)
N(2)–Zn(1)–Cl(2)
2.161(2)
2.233(2)
2.2744(7)
151.62(8)
75.38(7)
96.28(6)
98.47(6)
120.71(6)
Zn(1)–N(1)
Zn(1)–Cl(1)
2.184(2)
2.2675(8)
1
5
386.4, 1345.8, 1324.7, 1263.8, 951.3, 796.4, 756.1, 745.8, 696.1,
ꢀ1
+
88.0 cm . FAB MS m/z (M ): calcd, 397.47; found, 397.40. Anal.
O (397.47): C, 72.52; H, 5.83; N, 17.62. Found:
C, 72.50; H, 5.85; N, 17.66%.
23 5
Calc. for C24H N
N(3)–Zn(1)–N(2)
N(3)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(2)
Cl(1)–Zn(1)–Cl(2)
76.24(8)
98.67(6)
121.47(6)
95.69(6)
117.73(3)
2.4. Preparation of 2-chloro-N-(naphthalen-4-yl)acetamide
2
-Chloroacethyl chloride (2.00 mL, 20.9 mmol) was dissolved in
1
1
4
00 MHz): d 170.82, 148.55, 138.94, 136.30, 133.29, 128.10,
27.33, 121.82, 121.56, 116.70, 72.32, 70.70, 61.82, 53.77,
chloroform (10 mL) and added dropwise to a cooled, and stirred
solution of naphthylamine (0.720 g, 5.03 mmol) and pyridine
(0.43 mL, 5.2 mmol) in chloroform (10 mL) within 1 h. After the
9.43 ppm. FT-IR (KBr):
mmax 3334.7, 1688.1, 1529.8, 1424.3,

H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
545
0
0
0
0
0
.5
.4
.3
.2
.1
0
(a)
700
(
a)
6
5
4
3
2
1
00
00
00
00
00
00
0
2
70
320
370
420
Wavelength (nm)
420
520
620
Wavelength (nm)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(b)
350
(
b)
3
00
250
2
1
1
00
50
00
2
30
290
350
410
5
0
0
Wavelength (nm)
3
50
400
450
500
550
600
(c) 0.4
Wavelength (nm)
0.3
0.2
0.1
0
200
150
(
c)
1
00
5
0
2
00
250
300
350
0
Wavelength (nm)
3
70
420
470
520
570
620
Fig. 4. UV–Vis spectra of receptors QP (a), QA (b), and NA (c) (10
l
M) upon the
Wavelength (nm)
addition of Zn2 (1, 2, 3, 4, 5, 6, 7, 8, 9 and 10
+
lM) in Bis–tris buffer (10 mM,
Fig. 5. Fluorescence spectra of receptor QP (a), QA (b), and NA (c) (5
l
M) upon the
3
CH CN:water = 5:5 (v/v), pH = 6.5).
addition of Zn2 (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7.0
+
l
M)
in a Bis–tris buffer (10 mM, CH
30 nm, NA 366 nm).
3
CN:water = 5:5 (v/v), pH = 6.5, kex: QP 365 nm, QA
3
reaction solution was stirred for 2 h at room temperature, the sol-
vent was removed under reduced pressure and the white powder
was washed with dichloromethane and diethyl ether. Yield:
temperature and the solvent was removed under reduced pressure
to obtain a yellow oil, which was purified by silica gel column
0
.950 g (86.0%). Spectral characteristics matched those in the liter-
chromatography using chloroform/methanol (30:1, v/v) as eluent
1
ature [17]. H NMR (DMSO-d
H, J = 9.2 Hz), 7.97 (d, 1H, J = 6.8 Hz), 7.82 (d, 1H, J = 8.0 Hz), 7.69
d, 1H, J = 6.8 Hz), 7.60 (m, 3H), 4.45 (s, 2H). 13C-NMR (DMSO-d
6
, 400 MHz): d 10.27 (s, 1H), 8.07 (d,
1
to afford product. Yield: 1.38 g (73.6%). H NMR (DMSO-d
6
,
1
(
4
00 MHz): d 10.82 (s, 1H), 8.47 (d, 2H, J = 7.6 Hz), 8.36 (d, 1H,
J = 4.0 Hz), 7.95 (t, 2H, J = 6.4 Hz), 7.77 (t, 3H, J = 7.6 Hz), 7.63 (t,
6
,
1
1
1
00 MHz): d 165.4, 140.8, 134.3, 128.6, 126.0, 125.1, 124.7, 121.0,
09.4, 42.8 ppm. FT-IR (KBr): max 3253, 1663, 1548, 1505, 1392,
348, 1318, 1203, 1169, 1017, 960, 789 cm
13
2
(
1
H, J = 8.0 Hz),7.51 (t, 3H, J = 8.0 Hz), 3.56 (s, 2, H) ppm. C NMR
DMSO-d , 100 MHz): d 168.5, 156.2, 148.7, 140.8, 136.2, 134.2,
28.6, 126.6, 126.0, 124.1, 121.0, 119.0, 59.6, 56.6 ppm. FT-IR
max 3258.38(s), 1667.28(s), 1557.79(s), 1505.58(w),
398.9(m), 1349.43(m), 1249.6(m), 1207.6(m), 814.15(w),
m
6
ꢀ1
.
ESI-MS m/z
+
(
M+Na ): calcd, 242.04; found, 242.07.
(
KBr): m
1
ꢀ1
2.5. Preparation of 2-dipicolylamine-N-(naphthalene-4-yl)acetamide
20 4
793.36(m), 770.45(m) cm . Anal. Calc. for C24H N O (382.46):
(
NA)
C, 75.37; H, 5.80; N, 14.65. Found: C, 75.36; H, 5.87; N, 14.74%.
2.6. X-ray crystallography
2
-Chloro-N-(naphthalen-4-yl)acetamide (1.10 g, 5.01 mmol),
0
2
,2 -dipicolyamine (0.93 mL, 4.9 mmol), N,N-diisopropylethyl-
amine (0.91 mL, 5.1 mmol) and potassium iodide (20 mg) were
dissolved in acetonitrile (30 mL), stirred and refluxed for 10 h
under nitrogen atmosphere. The mixture was cooled to room
The diffraction data for compounds were collected on a Bruker
SMART AXS diffractometer using Mo K
a (k = 0.71073 Å). The crys-
tals were mounted on a glass fiber under epoxy. The CCD data were

5
46
H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
6
5
4
3
2
1
0
0
0
0
0
0
0
fluorophores, thus a five-membered metal–ligand ring will form
2
+
when Zn binds to the amine nitrogen of DPA and the amide oxy-
gen or nitrogen. A third molecule, NA, was also synthesized in a
similar manner to QA, but with the use of amino naphthalene in
place of amino quinoline. NA is similar to QA, having only one
2
CH group between DPA and the amide, but it is also different, it
having a naphthalene group instead of a quinoline group. It thus
does not have a nitrogen in the fluorophore. These three com-
pounds allow for testing the effect of an extra methylene group be-
tween DPA and the fluorophore and a nitrogen in the fluorophore.
2
+
Coordinating these three compounds to Zn shows the ability
of DPA to coordinate in different modes and the versatile coordina-
2
+
tion geometries of Zn to sensor molecules. Crystals of QP coordi-
nated to ZnCl show planar (meridional) DPA and five coordinated
Zn in a trigonal bipyramidal geometry (Fig. 1). The DPA of the
QP–ZnCl complex has a Npy–Zn–Npy bond angle of 151° and
py–Zn–Namine bond angles less than 90° (76° and 75°, Table 2).
1
2
3
4
5
6
7
8
9 10 11 12
pH
2
2
+
Fig. 6. Effect of the pH on the fluorescence intensity of receptor QP (5
lM) in the
2
presence of Zn2 (10
lM).
+
N
The Cl–Zn–Cl bond angle is 118° and close to 120°. Surprisingly,
neither the amide oxygen or nitrogen nor the nitrogen of quinoline
are bound to the zinc. Thus the anions are competitive binders for
Zn in polar organic environments, such as methanol from which
the crystals were grown. The Zn–Npy bond lengths are 2.16 and
4
3
3
2
2
1
1
00
50
00
50
00
50
00
NA+Zn
NA+base+Zn
QA+Zn
2
.18 Å and the Zn–Namine is a little longer at 2.23 Å. These Zn–N
bond lengths are very similar to other meridional DPA–Zn com-
plexes [18]. The Zn–Cl bond lengths are not much longer than
the Zn–N bond lengths and are 2.27 Å.
QA+base+Zn
QP+Zn
QP+base+Zn
2
+
When NA coordinates to Zn , it binds through the DPA nitro-
2+
gens and the amide oxygen (Fig. 2). The five-coordinate Zn also
binds to an oxygen atom of a nitrate anion and resides in a trigonal
bipyramidal geometry, with the amine N and nitrate O at the
apexes. The Npy–Zn–Npy bond angle is 120° and the Npy–Zn–Oamide
bond angles are slightly less at 115° and 113°. Although the Npy
–
5
0
Zn–Namine bond angles are 80° and 78°, the Npy–Zn–Npy bond angle
is 121°. Thus the DPA is clearly not in a meridional configuration,
but has a facial orientation. NA bonding through the oxygen im-
plies that it is more favorable for oxygen to bind than it is for nitro-
gen in methanol solvent, from which the Zn–NA complex was
crystallized. Bonding through oxygen might be due to the oxygen
being more basic than the nitrogen or the naphthalene group hav-
ing more entropy when the oxygen is bound. The DPA is in a nearly
facial arrangement on the Zn. Computer energy minimization stud-
ies show that the energy of the Zn–NA complex bonded through
the amide oxygen is very similar to the Zn–NA complex bonded
through the amide nitrogen. Except for the Zn–Nam, which is
0
4
00
450
500
550
600
650
700
Wavelength (nm)
Fig. 7. Fluorescence spectra of receptors NA, QA and QP (5
in the absence and presence of added NH OH.
l
M) with Zn2+ (10
lM)
4
integrated and scaled using a Bruker SAINT, and the structures were
solved and refined using SHELXTL. Hydrogen atoms were located in
the calculated positions. The crystallographic data is in Table 1.
2
.21 Å, the bond lengths are shorter than they were for the Zn–
3
. Results and discussion
QP complex. The Zn–Npy bond lengths are 2.05 and 2.09 Å, almost
a tenth of an angstrom shorter. The shorter distance is probably an
artifact of DPA not being in a meridional conformation, and thus
the N’s are allowed to approach the Zn more closely. The Zn–Oam
bond length is 2.06 Å and close to the Zn–Npy bond lengths. The
Zn–Onitrate is the shortest bond at 2.01 Å.
3.1. Synthesis and crystal structures
The QP compound was synthesized by adding chloropropionyl
chloride to aminoquinoline in the presence of pyridine as base
Scheme 2). After the amide bond is formed, DPA is attached to
(
The crystal structure of QA bound to Zn shows a nearly octahe-
dral complex with Zn bonded to all of the nitrogens of QA and a
water molecule (Fig. 3). This complex was prepared by adding base
during the synthesis, resulting in the amide N being deprotonated.
As seen by the crystal structure, zinc is in a pseudooctahedral envi-
ronment with water being the sixth ligand. The Npy–Zn–Npy bond
angle is 153° and the Npy–Zn–Namine bond angles are 77° and 79°,
creating a planar (meridional) DPA. The Namine–Zn–Nquin bond an-
gle is also less than 180°(159°) and the Namin–Zn–Namide is 80° and
the Nquin–Zn–Namide is 79°. Chelation of five atoms from the sensor
produces a very stable complex. The structure also shows coordi-
nation through the amide nitrogen and not the oxygen. This is fa-
the ethyl group and QP is complete. The DPA has three zinc binding
sites and the nitrogen or oxygen of the amide can act as another
binding site, along with the nitrogen of the quinoline. There is a
five atom distance between the amine nitrogen of DPA and the
nearest coordinating atom of the fluorophore, the nitrogen or oxy-
gen of the amide. A six-membered ring will be created when zinc
coordinates to both the amine nitrogen and either the amide oxy-
gen or nitrogen. For zinc, six membered rings are possible, but they
are not as favorable as five-membered rings. Thus we propose that
the QP sensor will have different binding abilities to transition me-
tal ions than QA.
2
+
A second sensor, QA, is synthesized in a similar manner to QP,
except chloroacetyl chloride is substituted for chloropropioinyl
chloride [16]. QA has one less CH
vored due to the bonding of Zn to the very basic anionic amide
nitrogen. Also, coordination through the oxygen would create a se-
2
+
2
group between DPA and the
ven-member ring containing Zn , amide oxygen and the quinoline

H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
547
Fig. 8. 1H NMR spectra of QP with different amounts of Zn(ClO CN. Equivalents of Zn2+: (I) 0; (II) 0.2; (III) 0.4; (IV) 0.6; (V) 0.8; (VI) 1.0.
) in CD
4 2 3
nitrogen. The Zn–Namine bond length is 2.23 Å and close to what is
was in the QP–Zn and NA–Zn complexes. The Zn–Npy bond lengths
are 2.16 and 2.17 Å and close to those in QP–Zn. The Zn–Namide
bond length is the shortest at 2.08 Å. The Zn–Owater bond length
is 2.12 Å and similar to the Zn–Npy bond lengths.
2
+
These Zn complexes of QP, NA, and QA show the versatility of
Zn coordination, it showing stable five and six coordinate com-
plexes and octahedral and trigonal bipyramidal geometries. Sev-
eral zinc salts were used to grow single crystals due to solubility
and crystal growth. These crystal structures show how anions
can also affect the coordination, where halides bind to zinc even
in the presence of nitrogen ligands. These structures show the flex-
ibility of the DPA ligand, which takes on meridional and facial
structures as well as intermediate structures between these two
extremes.
2+
Fig. 9. Possible structures of QP binding to Zn . Note the 7 and 6 membered rings
when Zn2 binds to the amide O and the 5 and 6 membered rings when Zn binds
to the amide N. Atom color code: zinc green, nitrogen blue, oxygen red, carbon grey,
and hydrogen white. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
+
2+
3.2. UV–Vis changes
Zinc binding to QP results in the growth of one absorbance band
and the reduction of another. QP has an absorbance in the ultravi-
2+
olet and one at around 310 nm. When Zn is added to QP, a band
at 370 nm grows in and the absorbance at 310 nm decreases
very little change in its UV–Vis signals when Zn2+ is added to it
(
Fig. 4).
3.3. Fluorescence enhancement
The fluorescence of QP is greatly enhanced by Zn and is pH
(
Fig. 4). The intensity of the 370 nm band increases up to one
2
+
equivalent of Zn and results in QP beginning to absorb in the vis-
ible light region. This increase in absorbance was seen with differ-
ent solvents such as acetonitrile, water, and
a mixture of
2+
acetonitrile and water. The red shift of the absorbance band of
QP has also been noted in other zinc sensors [5]. The absorption
2
+
dependent. As Zn is added to QP, the fluorescence at 525 nm in-
creases by 10 times (Fig. 5). The percent quantum yield in acetoni-
trile goes from 0.29 for QP to 5.7 for the QP–Zn complex. The
2
+
2+
enhancement at 370 nm by Zn is also seen for Co , but remark-
2+
ably is not observed for Cd . QA also has a peak that grows in
when Zn is added to it, but it is at 360 nm (Fig. 4). NA displays
2
+
2+
fluorescence increases up to one equivalent of Zn and then levels

5
48
H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
Fig. 10. 1H NMR spectra of QA with different amounts of Zn(ClO = 5:5 (v/v). Equivalents of Zn2+: (I) 0; (II) 0.2; (III) 0.4; (IV) 0.6; (V) 0.8; (VI) 1.0.
) in CD CN:DMSO-d
4 2 3 6
off. As the equivalents of Zn2+ increase, the fluorescence band blue
shifts by 10 nm and a shoulder at 475 nm grows in. As with the QP
of the amide N to Zn2+ due to its anionic nature and thus rendering
the electrons of the N less able to be involved in photo-induced
electron transfer (PET). It could also be due to the lowering of
the energy of the amide N orbital which contains the lone pair of
electrons and thus making it more energetically unfavorable for
them to participate in PET.
2
+
sensor, Zn enhances the fluorescence of QA (Fig. 5) [16]. When
2
+
Zn is added to NA, there is very little change in fluorescence ob-
served (Fig. 5). Thus, the nitrogen in the quinoline ring is important
to fluorescence enhancement.
2
+
To gain the Zn detection limit with QP, the equilibrium con-
2
+
stant for QP and Zn was found. To calculate the equilibrium con-
3.4. NMR characterization
2
+
stant for QP–Zn , the method of Nagano et al. was used, which
2+
It is evident by the proton chemical shift changes that Zn2+
binds to QP. The methylene protons of DPA move downfield and
entailed adding QP to a HEPES buffer solution of Zn and nitrilotri-
acetic acid [19]. Fluorescence changes were monitored under these
conditions and used to calculate an apparent dissociation constant
of 0.024 nM. Thus QP binds Zn2 very strongly, as expected due to
its DPA and amide groups and Zn2 detection limits for QP could be
in the sub nanomolar range. The dissociation constant for QP is
similar to QA and those of other sensors [4,11,16].
2
+
split into doublets as soon as Zn is added, whether the solvent
is acetonitrile, DMSO, methanol, or acetonitrile/water (Fig. 8). The
doublet formation is characteristic of facial DPA coordination to
+
+
2
+
Zn . In DMSO, the N–H proton moves slightly downfield and in
2
+
acetonitrile it moves slightly upfield, implying Zn coordination
through the amide N. In acetonitrile, however, the N–H proton
moves upfield by 0.5 ppm before returning to a chemical shift close
to its original position. Since the UV–Vis data show a nice isos-
bestic point, we propose that the proton shifts are for one or more
intermediates that have the same UV–Vis absorbance. There is a
great energy difference to bind through the amide N instead of
the O. Binding through the amide N results in ligand–metal coordi-
nation rings of five- and six-membered, both of which are common
for coordination compounds (Fig. 9). In contrast, binding through
the amide O creates seven- and six-membered rings, which seven
membered rings are not favorable. A computer calculation shows
that binding through the N is over one hundred kJ/mol lower in en-
ergy than binding through the O.
The enhanced fluorescence of QP by Zn2 is small at low pH,
raises from 2 to 6, and reaches a maximum at 6, where it plateaus
until the pH is above 10 (Fig. 6). This pH range is good for imaging
+
2
+
2+
+
Zn at around pH 7 and in biological media. At low pH, Zn and H
2+
are competing for the nitrogen sites of QP and Zn does not bind
+
strongly. As the pH increases and the H concentration decreases,
2
+
Zn binds more strongly to QP and enhances fluorescence.
While using QA, we also noted that the Zn2 enhanced fluores-
cence of QA can be doubled if the amide proton of QA is removed
+
ꢀ
+
(
Fig. 7). When base is added to QA and the [QA ꢀZn] complex is
formed ( = 0.15), the fluorescence is much greater than it is for
QA ( = 0.002), QA ( (U = 0.078). This
remarkable fluorescence enhancement due to deprotonation is
not observed to such a great extent for QP (QP
U
-
2+
U
U
= 0.023) and [QAꢀZn]
ꢀ
U
= 0.003, QP
= 0.061). The en-
When the ZnCl salt is used in place of Zn(ClO ) , Zn binding oc-
2
4 2
2
+
ꢀ
+
2+
U
= 0.004, [QPꢀZn]
U
= 0.057, [QP ꢀZn]
U
curs in a different manner. After one equivalent of Zn has been
added, the methylene protons of DPA are still a singlet, implying
hanced fluorescence of QA can be explained by stronger binding

H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
549
Fig. 11. 1H NMR spectra of QA with different amounts of Zn(ClO
.6; (V) 0.8.
)
in the presence of NH
= 95:5 (v/v). Equivalents of Zn2+: (I) 0; (II) 0.2; (III) 0.4; (IV)
4
2
4
OH in CD
3
CN:DMSO-d
6
0
meridional coordination of DPA as seen in the crystal structure. It
requires over 2 equivalents of Zn2 before the methylene protons
split into doublets and for the DPA to bind facially. Whether in
DMSO or acetonitrile, the amide N–H proton shifts upfield by 0.4
and 0.8 ppm, respectively. This implies Zn2 binds through the car-
bonyl oxygen. The quinoline protons do not shift downfield, unlike
the aromatic DPA protons, thus, it looks like the nitrogen in quin-
oline does not bind to Zn2 , as seen in the crystal structure.
rings, instead of five- and seven-membered rings. As with QP, com-
puter calculations show that Zn binding to N creates a structure
+
2+
that is over 100 kJ/mol lower in energy.
When base is present with QA and Zn is added, the N–H pro-
ton signal decreases upon addition of Zn and the proton is re-
2
+
+
2+
moved by the base (Fig. 11). The DPA methylene protons 5 and 6
move downfield due the cationic and electron withdrawing nature
+
2+
of Zn . Protons 5 are a singlet, meaning DPA is meridional as seen
2+
NMR proton chemical shifts show QA binds Zn . Whether the
solvent is acetonitrile, DMSO, or methanol the methylene protons
of DPA split into doublets of doublets, implying facial coordination
of DPA (Fig. 10). The DPA aromatic protons move slightly down-
in the crystal structure. Some of the aromatic protons on DPA also
move downfield. Aromatic protons of the quinoline also move
downfield, specifically, proton 8 on the ortho carbon next to the
quinoline N moves 0.3 ppm downfield and the proton on the para
carbon to the N of the quinoline moves 0.3 ppm downfield. Indeed,
2
+
field as more Zn is added. Again in all of the solvents listed above,
the proton on the ortho carbon next to the N of quinoline moves
1
ꢀ
+
the H NMR of the [QA ꢀZn] complex in DMSO has the proton 8
2
+
downfield as Zn is added and then goes back upfield to a position
close to where it started. This implies that the quinoline is binding
to Zn2+ and as more Zn is added, its proton is being less deshil-
ded. The amide N–H proton moves a large distance upfield when
QA is in DMSO or acetonitrile, 0.6 and 0.9 ppm, respectively. In
other compounds that have DPA linked to amides, a large change
signal 0.5 ppm downfield. This substantial movement downfield
2
+
implies the N of the quinoline ring is binding to Zn , as seen in
the crystal structure.
2+
NA also binds Zn2+as shown by changes in the proton NMR sig-
nals. In acetonitrile and methanol, the methylene protons of DPA
shift and remain a broad singlet until the last partial equivalent
2
+
2+
in chemical shift of the amide N–H hydrogen implies that Zn is
binding to O [5]. We don’t think this is the case with QA. The in-
of Zn is added, at which point they form a doublet of doublets
(Fig. 12). This is different for DMSO, where the DPA methylene pro-
tons shift downfield and begin to form a doublet of doublets after
2
+
crease in absorption and fluorescence of QA due to Zn , which
has a quinoline N, is much greater than what is observed for NA,
2
+
the initial Zn is added. This implies that in acetonitrile and meth-
anol, DPA binds meridionally and then switches to facial, while in
DMSO it binds facially at the beginning. The N–H shifts slightly
downfield in DMSO and slightly upfield in acetonitrile, potentially
2+
which doesn’t have a quinoline N. Thus, Zn is binding to the quin-
oline N. If this is the case, then Zn2 binding also through the amide
N, creates a much more stable structure than if it were to bind
through the amide O. It creates two five-membered ligand–metal
+
2
+
indicating Zn binds to the amide N. This is in contrast to what is

5
50
H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
Fig. 12. 1H NMR spectra of NA with different amounts of Zn(ClO
)
in CD
CN. Equivalents of Zn2+: (I) 0; (II) 0.2; (III) 0.4; (IV) 0.6; (V) 0.8; (VI) 1.0.
4
2
3
seen in the crystal structure of the Zn–NA complex, which shows
Zn bonded through the amide O. The crystals were grown in meth-
anol, which might be why binding through the amide O is favored.
We cannot track the N–H proton by NMR in methanol due to deu-
terium exchange, so we do not know whether it moves upfield
significantly.
700
2
+
6
5
4
3
2
00
00
00
00
00
Zn
2+
2+
2+
2+
Cd 2+, Cu , Ni , Co ,
2+
2+
2+
,
,
,
Mn , Cr Hg +Mg
2+
+
+
3.5. Selectivity
Ca , Li , Na , K
2+
Fluorescence selectivity is critical for a Zn sensor and QP
2
+
2+
shows high Zn selectivity. Unlike when Zn is added to QP and
enormous fluorescence enhancement is seen, when metal ions
100
0
+
+
+
2+
2+
ranging from alkali and alkaline earth (Li , Na , K , Mg , Ca ) to
first row transition metals (Cr2 , Mn , Fe , Co , Ni , Cu2+) to sec-
+
2+
2+
2+
2+
400
450
500
550
600
650
700
2+
+
2+
Wavelength (nm)
ond and third row transition metals (Cd , Ag , Hg ) are added to
QP in acetonitrile it does not fluoresce (Fig. 13). When a mixture of
acetonitrile/water or pure water is used, Zn2 still causes the most
fluorescence enhancement of QP. Remarkably, this selectivity by
QP to not fluoresce in the presence of Cd is not seen for QA, which
fluoresces when Cd is present [16]. As was noted by the absorp-
tion studies, Cd did not cause changes to the absorption of QP.
Cd most likely doesn’t bind to the amide nitrogen or oxygen
and thus doesn’t stop the PET process.
Fig. 13. Fluorescence spectra of receptor QP (5
metal ions (2 equiv) in Bis–tris buffer [10 mM, CH CN:water = 5:5 (v/v)].
Fluorescence of Zn with QP is shown in pink. The fluorescence as a result of the
lM) upon the addition of different
+
a
3
2
+
2
+
2+
2+
2+
2+
2+
2+
2+
+
2+
2+
other metal ions including Cd , Cu , Ni , Co , Fe , Mn , Cr , Hg , Ag , Mg ,
2+
+
+
+
2
+
Ca , Li , Na , and K are the other colors. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
2
+
2
+
rivals the Lewis acidity of Zn2+, is competitive for binding to QP.
Also, the ability of Zn to be stable in various geometries such
as trigonal bipyramidal and octahedral, renders it a strong binding
ion.
2
+
2
+
The fluorescence of QP is enhanced by Zn even when other
metal ions are present. In acetonitrile, Zn2+ enhanced fluorescence
is seen when other metal ions are present, except for high concen-
2
+
2+
2+
trations of Cu , Fe , and Co (Fig. 14). In a mixture of acetonitrile
and water or pure water, the fluorescence of QP is greatly en-
4. Conclusion
2+
2+
hanced, except again when high equivalents of Cu or Co are
2+
present. This notable selectivity for Zn is most likely due to the
The new sensor QP shows enhanced fluorescence in the pres-
2
+
2+
2+
strong binding of Zn to DPA and the amide group. Cu , which
ence of Zn . The nitrogen in the quinoline ring is important to

H.G. Lee et al. / Inorganica Chimica Acta 394 (2013) 542–551
551
Fig. 14. Competitive selectivity of receptor QP (5
l
M) toward Zn2+ ions (5
l
M) in the presence of other metal ions (5
l
M Mn+ blue, 10
l
M Mn+ red, 25
l
M Mn+ yellow, and
n+
1
00 lM M green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
fluorescence enhancement. QP shows selective fluorescence for
(e) Y. Mei, C.J. Frederickson, L.J. Giblin, J.H. Weiss, Y. Medvedeva, P.A. Bentley,
Chem. Commun. 47 (2011) 7107.
3] A.P. de Silva, H.Q.N. Gunaratne, T. Gunnlaugsson, A.J.M. Huxley, C.P. McCoy, J.T.
Rademacher, T.E. Rice, Chem. Rev. 97 (1997) 1515.
2
+
Zn and not for other transition metal ions. The DPA group of zinc
sensors is able to conform to different structures from meridional
[
2+
to facial. The fluorescence enhancement of Zn binding to QA is
[4] (a) C.R. Goldsmith, S.J. Lippard, Inorg. Chem. 45 (2006) 555;
1
(b) K. Kiyose, H. Kojima, Y. Urano, T. Nagano, J. Am. Chem. Soc. 128 (2006)
greatly enhanced by adding base. The H NMR chemical shift
6
(
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(2011) 7563;
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changes of QP, QA, and NA when Zn2 binds to them show the type
+
of DPA binding.
(
1
Acknowledgments
(
(
(
e) P. Das, S. Bhattacharya, S. Mishra, A. Das, Chem. Commun. 47 (2011) 8118;
f) A.E. Lee, M.R. Grace, A.G. Meyer, K.L. Tuck, Tetrahedron Lett. 51 (2010) 1161;
g) Z. Liu, C. Zhang, Y. Li, Z. Wu, F. Qian, X. Yang, W. He, X. Gao, Z. Guo, Org. Lett.
Financial support from Korean Science & Engineering Founda-
tion (2009-0074066), Converging Research Center Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (2009-
11 (2009) 795.
[
[
5] (a) Z. Xu, K.-H. Baek, H.N. Kim, J. Cui, X. Qian, D.R. Spring, I. Shin, J. Yoon, J. Am.
Chem. Soc. 132 (2010) 601;
(
b) H.G. Lee, J.H. Lee, S.P. Jang, H.M. Park, S.-J. Kim, Y. Kim, C. Kim, R.G. Harrison,
Tetrahedron 67 (2011) 8073.
6] C.J. Fahrni, T.V. O’Halloran, J. Am. Chem. Soc. 121 (1999) 11448.
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0
082832), and Brigham Young University is gratefully
acknowledged.
[
[
8] X. Zhou, B. Yu, Y. Guo, X. Tang, H. Zhang, W. Liu, Inorg. Chem. 49 (2010) 4002.
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Appendix A. Supplementary material
(
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Lippolis, B. Valtancoli, Inorg. Chem. 48 (2009) 9236;
CCDC 867079, 867080 and 867081 contain the supplementary
crystallographic data for complexes NA–Zn(NO
3 2 4
) , QA–ZnClO
(
c) J. Jiang, T. Xiaoliang, L. Yang, W. Dou, W. Liu, R. Fang, W. Liu, Dalton Trans.
40 (2011) 6367;
d) I. Ravikumar, P. Ghosh, Inorg. Chem. 50 (2011) 4229.
and QP–ZnCl , respectively. These data can be obtained free of
2
(
charge from The Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.09.009.
[
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