Europium (III) complexes of amino acid-derived bis-imine-substituted phenanthroline ligands for phosphate recognition

Article abstract of DOI:10.1016/j.ica.2020.119428

Phenanthroline-based chiral hexadentate ligands L1, L2 and L3, as well as their europium (III) complexes 1–3 are synthesized and characterized. The coordination compounds show strong red emission typical for europium (III) derivatives. Various anionic spe

Full text of DOI:10.1016/j.ica.2020.119428

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Research paper
Europium (III) complexes of amino acid-derived bis-imine-substituted phenan-
throline ligands for phosphate recognition
Yanfang Hu, P.S. Subramanian, Markus Albrecht
PII:
S0020-1693(19)31926-7
https://doi.org/10.1016/j.ica.2020.119428
ICA 119428
DOI:
Reference:
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Inorganica Chimica Acta
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8 January 2020
Please cite this article as: Y. Hu, P.S. Subramanian, M. Albrecht, Europium (III) complexes of amino acid-derived
bis-imine-substituted phenanthroline ligands for phosphate recognition, Inorganica Chimica Acta (2020), doi:
https://doi.org/10.1016/j.ica.2020.119428
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Europium (III) complexes of amino acid-derived
bis-imine-substituted phenanthroline ligands for phosphate
recognition
a
b
*a
Yanfang Hu , P. S. Subramanian , and Markus Albrecht
a
Institut for Organische Chemie, RWTH Aachen University, Aachen 52070,
Germany.
b
Central Salt and Marine Chemicals Research Institute(CSIR-CSMCRI), Academy of
Scientific and Innovative Research (AcSIR), Bhavnagar 364002, Gujarat India.
Abstract
Phenanthroline-based chiral hexadentate ligandsL1, L2 and L3, as well as
their europium (III) complexes 1-3 are synthesized and characterized. The
coordination compounds show strong red emission typical for europium (III)
-
-
-
-
2-
-
-
2-
derivatives. Various anionic species, F , Cl , Br , I , SO , NO , NO , HPO ,
4
3
2
4
2
-
-
-
CO , HCO and OAc , are employed to evaluate the chemo-sensor properties
3
3
of the complexes using fluorescence spectroscopy. For complex 1, only the
addition of HPO 2 leads to quenching of luminescence. For the other
-
4
–
2-
complexes 2 and 3, NO and HPO exhibit a strong response in the emission
2
4
properties. However, complexes 2 and 3 are found to be more sensitive for
HPO 2- than for NO . The corresponding binding constants (logK ) of the
–
4
2
β
adducts are determined using the Benesi-Hildebrand method. The described
studies show that the europium(III) complexes of the hexadentate ligands L1-3
can be considered as highly specific sensors for the recognition ofphosphates
2
-
with some competition by NO ions. While phosphate binding is strong in all
1

three examples, the sensitivity for nitrite increases in the order 1<2<3. The
obtained results are remarkable because the differences are due to less
sterically hindered substituents in the periphery of the ligandof 2 and 3, which
enable the binding of the small nitrite while the bigger phosphate is bound less
tightly.
*
Corresponding author, Tel: +49 241 809 4678; Fax: +49 241 809 2385;
E-mail: markus.albrecht@oc.rwth-aachen.de
Keywords: luminescence; europium (III); anion recognition; phosphate.
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2

1
. Introduction
Recently, selective recognition and sensing of anions has become one of the most
active areas of supramolecular chemistry due to the important role of anions in
biological, industrial, and environmental processes.[1-3] After long-term exploration
and unremitting efforts, many detection techniques have been established to study
molecular recognition processes involving anions.[4-6] Compared with the traditional
analytical methods, such as titration,[7, 8] more modern analytics inductively coupled
plasma-atomic emission spectroscopy, electrochemical methods,[9] ion
chromatography or fluorescent sensing have received great attention because of their
simple operation and high selectivity and sensitivity.[10, 11]
Developing effective fluorescent probes for phosphate recognition and sensing is
highly desirable because of the critical role of this anion in a range of biological
processes. Much effort has been devoted to the development of luminescent
chemosensors and with their help, phosphate can be recognized even in water or
under physiological conditions.[12] As example, in 2014, an efficient and highly
selective fluorescent sensor, a terpyridine-Zn(II) complex was successfully prepared
which shows unprecedented fluorescence response (∼500 fold increase) to
pyrophosphate in water at physiological pH.[13] Soon afterwards, a hemicryptophane
cage revealing strong fluorescence was synthesized as an efficient and selective
sensor for choline phosphate.[14] Recently, a novel fluorescent receptor was made by
Schiff base condensation of 1-pyrenemethylamine with the vitamin B6 cofactor
3

pyridoxal, and a unique change in the fluorescence behaviour of its Zn(II) complex
–
has been found upon its interacted with H PO due to demetalation.[15]
2
4
Scheme 1. Europium(III) complexs 1-3used in this study.
Among various luminescence sensors, probes based on lanthanide complexes are
exceptional standouts due to their unique photo-physical properties such as narrow
emission, large Stokes shifts and long luminescent lifetimes.[16-18] For example, the
3
+
3+
Eu and Tb complexes of DTTA ([4’-(2,4-dimethoxyphenyl)-2,2’:6’,2’’-terpyrid-
3
+
ine-6,6’’-diyl]bis(methylenenitrilo)tetrakis(acetic acid)), [Eu(DTTA)] or [Tb(DTT-
3
+
A)] , were found to be highly water soluble and strongly luminescent with
luminescence quantum yields of 10.0 or 9.9 %, and long luminescence life times of
1
.38 or 0.26 ms.[19]
Recently, the phenanthroline-based chiral ligand L1 and its luminescent europium
complex 1 have been described, which recognize ATP and lead to selective quenching
of luminescence.[20, 21] In this study, two related phenanthroline-based hexadentate
4

ligands L2/3as well as their europium complexes 2,3 are prepared in order to compare
their photophysical behavior in the presence of anions with the one of complex 1.
Hereby only the side chains of the amino acid residues are modified but the anion
binding site at the metal ion is not directly altered. To evaluate those chemo-sensors,
-
-
-
-
2-
-
-
2-
2-
-
various anionic species, F , Cl , Br , I , SO , NO , NO , HPO , CO , HCO and
4
3
2
4
3
3
-
OAc , are tested as sodium salts.
. Experimental section
.1 Material and Characterizations
All reagents were purchased from Alfa Aesar and Acros Organics, and were used
2
2
directly without further purification. NMR spectroscopy was carried out with a Varian
VNMRS 400 or 600 MHz spectrometer and the chemical shifts were expressed in
ppm. IR spectroscopy was recorded using KBr pellets (1% w/w) on a PerkinElmer
Spectrum 100 spectrometer. Mass spectrometry measurements were performed on a
LTQ Orbitrap XL (ESI). Elemental analysis was measured with a varioEL
Analyzer. UV-Vis measurements were carried out on a PerkinElmer Lambda 35
UV/Vis Spectrometer using PS/PMMA cuvettes (300-900 nm). For baseline
calibration (auto zero) the corresponding solvents were measured as standard.
Luminescence measurements at 360 nm were carried out on a PerkinElmer LS45
Fluorescence Spectrometer using quartz cuvettes. The slit sizes for emission and
excitation are adjusted as 10.0/10.0 nm.
2
.2 Preparation of 1,10-phenanthroline-2,9-dicarbaldehyde
The synthetic procedure of this compound was reported previously.[22]
5

2
.3
Prepartion
of
(2S,
2’S)-2,
2’-(((1,
10-phenanthroline-2,
9
-diyl)bis(methanylyliden)) bis(azanylyliden))-bis(3-methyl-butanamid) (L1)
1
L1 was synthesized as reported before.[20] H NMR (CDCl ,600 MHz): δ= 8.77 (s,
3
2
2
6
1
H), 8.46 (d, J=8.4 Hz,2H), 8.37(d, J=8.4 Hz 2H), 7.92 (s, 2H), 6.62 (d, J=3.8 Hz,
H),5.49 (d, J=3.8 Hz, 2H), 3.88 (d, J=4.3 Hz, 2H), 2.42 (m, 2H), 1.05 (d, J=6.8Hz,
1
3
H), 0.99 (d, J=6.8Hz, 6H) ppm. C NMR (CDCl , 150 MHz): δ= 174.7, 164.1,
3
53.9, 145.6, 137.2, 130.0, 127.7, 120.8, 79.5, 33.0, 19.6, 17.7 ppm; IR (KBr): 3310
(br), 2963 (s), 2873 (m), 1666 (vs), 1585(s), 1498 (s),1463 (m), 1363 (s), 1321 (w),
1
259 (vs), 1087 (vs), 1018(vs), 863 (vs), 796 (vs), 744(m). MS (ESI): m/z found
+
+
(
calcd): [M+H] 433.24 (433.23), [M+Na] 455.22 (455.22). Elemental analysis (%)
calcd for C H N O •3H O: C: 59.24, H: 7.04, N: 17.27; found: C: 59.77, H: 6.52, N:
2
4
28
6
2
2
1
2
2
7.04.
.4
Preparation
of
(2S,
2’S)-2,
’-(((1,10-Phenanthroline-2,9-diyl)bis(methanylyliden))
bis(azanylyliden))-bis(4-methyl-pentanamide) (L2)
,10-Phenanthroline-2,9-dicarbaldehyde (0.12 g, 0.5 mmol, 1.0 eq.) and 4 Å
molecular sieves were added into a mixture of l-leucine amide hydrochloride (0.25 g,
.5 mmol, 3.0 eq.) and NaHCO (0.50 g, 6.0mmol, 12.0 eq.) in CH Cl (50 mL). The
1
1
3
2
2
mixture was heated to reflux at 50°C for 3 days. After filtration, the resulting mixture
was extracted with a saturated solution of brine. The solvent was removed under
1
vacuum and the product was isolated (0.14 g, yield: 60%). H NMR (CDCl ,600
3
MHz): δ= 8.82 (s, 2H), 8.50 (d, J=8.4 Hz, 2H), 8.39 (d, J=8.4 Hz, 2H), 7.94 (s, 2H),
6

6
.65 (d, J=3.8 Hz, 2H), 5.46 (s, 2H), 4.16-4.14 (m, 2H), 1.89-1.86 (m, 6H), 0.98-0.94
1
3
(
m, 12H) ppm. C NMR (CDCl , 150 MHz): δ= 175.5, 163.8, 154.1, 145.5, 137.1,
3
1
2
1
30.0, 127.7, 120.6, 72.1, 43.7, 24.2, 23.4, 21.4 ppm. IR (KBr): 3313 (br), 2960 (s),
871 (m), 2160 (w), 2019 (w), 1947 (w), 1743 (w), 1667 (vs), 1586 (s), 1498 (m),
463 (m), 1365 (m), 1259 (vs), 1087 (vs), 1017 (vs), 862(s), 795 (vs), 694 (m). MS
+
+
(
ESI): m/z found (calcd): [M+H] 461.27 (461.27), [M+Na] 483.25 (483.25).
Elemental analysis (%) calcd for C H N O •HCl•2H O•2CH OH: C: 56.32, H: 7.60,
2
6
32
6
2
2
3
N: 14.07; found: C: 56.97, H: 7.05, N: 13.12.
2
.5
Preparation
of
(2S,
2
’S)-2,2’-(((1,10-Phenanthrolin-2,9-diyl)bis(methanylylide-n))
bis(azanylyliden))-bis(3-phenylpropanamide) (L3)
To a solution of 1, 10-phenanthroline-2,9-dicarbaldehyde (0.10 g, 0.4 mmol, 1.0
eq) dissolved in dichloromethane containing 4Å molecular sieves (1.5 g),
l-phenylalanine amide (0.21 g, 1.3 mmol, 3.3eq) was added. The reaction mixture was
stirred and heated at 50°C for 3 days. Subsequently, the reaction mixture was filtered
and the solvent was removed under vacuum. The product was washed by water
several times and dried (0.14 g, yield: 64%). 1 H NMR (CDCl ,600 MHz): δ=
3
8
5
.38-8.33 (m, 6H), 7.89 (s, 2H), 7.38-7.03 (m, hidden under CDCl ), 6.66 (s, 2H),
3
1
3
.70 (s, 2H), 4.22 (m, 2H), 3.44 (m, 2H), 3.15 (m, 2H) ppm. C NMR (CDCl , 150
3
MHz): δ= 174.46, 164.65, 153.96, 145.67, 137.28, 129.98, 128.50, 127.77, 126.86,
20.80, 75.32, 41.21 ppm. IR (KBr): 3345 (w), 3311 (w), 3173 (w), 3029 (w), 2963
1
(s), 2162 (w), 2044 (w), 1950 (w), 1667 (vs), 1495 (s), 1450 (m), 1403 (m), 1259 (vs),
7

1
5
6
2
084 (vs), 1016 (vs), 863 (s), 794 (vs), 696(vs). MS (ESI): m/z found (calcd): [M+H]+
29.23 (529.23). Elemental analysis (%) calcd for C H H O N •6H O: C: 60.37, H:
3
2
28
6
2
6
2
.33, N: 13.20; found: C: 60.32, H: 6.28, N: 13.19.
.6 General procedure for the preparation of metal complexes
Ligand (0.05 mmol, 1.0 eq.) was dissolved in methanol (25 mL) and europium (III)
chlorid hexahydrate (0.06 mmol, 1.1 eq.) was added gradually. The reaction mixture
was stirred over night at room temperature. During the process, a negligible amount
of precipitate formed and was removed by filtration. Finally the resulting filtrate was
evaporated under reduced pressure to obtain a solid powder.
1
([EuL1Cl ]): L1 (22mg, 0.05 mmol) and [EuCl ]•6H O (20mg,0.055 mmol) were
3 3 2
-
+
+
used. MS (ESI): [L1+Eu+2Cl ] 655.086 (calc. 655.09), [L1+Eu+Cl-H] 619.109
(calc. 619.11). UV/Vis (methanol): λmax=360 and 432. IR (KBr): 3335 (br), 2641 (w),
2
1
8
3
2
0
320 (m), 2227 (m), 2154 (w), 2113 (w), 2063 (w), 1996 (m), 1949 (w), 1626(vs),
506 (s),1463(s), 1395 (vs), 1290 (s),1203(m), 1109 (vs), 1058 (s), 931(m), 870(vs),
11(m), 779(m). Elemental analysis (%) calcd for C H H O N EuCl •10H O: C:
2
4
28
6
2
6
3
2
3.10, H: 5.55, N: 9.65; found: C: 32.16, H: 4.39, N: 9.02.
([EuL2Cl ]): L2 (23mg, 0.05 mmol) and europium (III) chlorid hexahydrate (20mg,
3
+
.055 mmol) were employed. MS (ESI): [L2+Eu+Cl-H] 647.153 (calc. 647.12).
UV/Vis (methanol): λmax=321, 347 and 360. IR (KBr): 3319 (br), 2961 (vs), 2310 (w),
195 (w), 2060 (w), 1997 (w), 1940 (w), 1626 (vs), 1504 (s), 1462 (s), 1429 (s), 1393
vs), 1308 (m), 1260 (s), 1097 (vs), 925 (w), 871 (s), 800 (s), 756 (m), 696 (w).
Elemental analysis (%) calcd for C H H O N EuCl •7H O: C: 36.93, H: 5.44, N:
2
(
2
6
32
6
2
6
3
2
8

9
.94; found: C: 36.78, H: 4.97, N: 9.69.
3
([EuL3Cl ]): L3 (26mg, 0.049 mmol) and [EuCl ]·6H O (20mg,0.055 mmol) were
3
3
2
-
+
used.MS (ESI): [L3+Eu+2Cl ] 751.087 (calc. 751.09). UV/Vis (methanol): λmax=312
and 360.IR (KBr): 3784 (m), 3212 (br), 2503 (m), 2242 (m), 2182 (s), 2023 (w), 1990
(
m), 1951(m), 1913 (w), 1855 (w), 1648 (vs), 1499 (s), 1352 (vs), 1261 (w) 1208 (m),
101(s), 918 (w), 866 (s). Elemental analysis (%) calcd for
C H H O N EuCl •15H O: C: 36.36, H: 5.53, N: 7.95; found: C: 35.31, H: 4.38, N:
1
3
2
28
6
2
6
3
2
7
.46.
3
. Results and Discussion
L1, and the europium complex 1 ([EuL1Cl ]), are prepared as reported earlier.
3
Similar approaches are used for L2, L3 and 2([EuL2Cl ]) and 3([EuL3Cl ]).[20, 21]
3
3
The characterization of the ligands and their europium (III) complexes were done
using standard analytical techniques (Experimental section, SI).
The normalized emission spectra of 1, 2 and 3 recorded in HEPES buffer at pH
7
.4areshown in Figure S2. In aqueous buffer, the complexes are highly luminescent
witha similar excitation pattern. The characteristic strong red europium (III) emission
is observed around λ=615 nm in all spectra. This emission is attributable to a J=2
5
7
transition ( D → F ).[17, 22]
0
2
Initially, 1 has been evaluated as a chemosensor for various anionic species by
fluorescence spectrometry. Therefore, a solution of 1 (50μM, HEPES, pH=7.4) has
-
-
-
-
2-
-
-
2-
been treated with the representative anions F , Cl , Br , I , SO , NO , NO , HPO ,
4
3
2
4
2
-
-
-
CO , HCO , and OAc (50 equiv). As illustrated in Figure 1 (a), 1only responded
3
3
9

2
-
significantly to HPO , resulting in luminescence quenching and indicating binding
4
2
-
of HPO .The dependence of luminescence quenching on the concentration of
4
phosphate has also been investigated by adding 0.2 to 50 equiv Na HPO , as shown in
2
4
Figure 1(b).
-5
Fig.1 a) Relative luminescence intensities for a solution of 1 at 615 nm (5•10 M)
-
-
-
-
2-
-
-
2-
2-
upon addition of the 50 eq. of F , Cl , Br , I , SO , NO , NO , HPO , CO ,
4
3
2
4
3
-
-
HCO , and OAc as sodium salts. b) Changes in the luminescence spectra obtained
3
-5
for a solution of complex 1(5×10 M) in 10 mM HEPES (pH = 7.4) upon addition of
2
-
increasing amounts of HPO (sodium salt). Excitation wavelength was set at 360 nm.
4
Based on this titration, the corresponding binding constant of 1 with HPO42- of
logK =4.28±0.01 (Table 1), has been determined from the linear fit (Figure S3 (a))
β
using the Benesi–Hildebrand equation.[24, 25]
1
0

Binding
Constants
1
2
3
2
-
log K (HPO )
4.28±0.01
-
3.30±0.02
2.62±0.001
3.03±0.03
2.62±0.002
β
4
-
log K (NO )
β
2
Table 1.The binding constants of 1, 2 and 3 with HPO42- (sodim salt) in HEPES buffer
(10 mM, pH7.4) determined by luminescence spectroscopy
Subsequently, 2 and 3 have been tested by the same methods as 1as shown in
2
-
-
Figure 2 (a) and 3 (a). Unlike 1, they respond not only to HPO but also to NO .
4
2
2
-
-
However, they are found to be more sensitive towardsHPO than toNO . For 2, as
4
2
shown in Figure 2 (a), the intensity of luminescence descends after the addition of
-
5
0equiv of NO , however, the decrease is not remarkable. Furthermore, only 10.7 %
2
–
of reduction of the signal intensity is observed when 10 equiv of NO are added. As
2
for 3, the values of the relative intensity of the luminescence are 54, 73 and 93%,
–
upon addition of 50, 10 or 5 equiv of NO , respectively.
2
1
1

-5
Fig. 2 a) Luminescence intensity changes of 2 (5•10 M) in the presence of various
test anions (50 eq.) in HEPES-buffered (pH 7.4, 10 mM solution. b) Luminescence
2
-
titrations of 2 (70 µM) upon addition of the increasing amounts of HPO in HEPES
4
(
excitation at 360 nm). b) Luminescence titration spectra (λ = 360 nm) of 2(60μM)
ex
with the increasing amounts of HPO42- (0−50 eq.) in 10 mM HEPES buffer (pH 7.4)
solution.
2
-
Titrations of 2 and 3 with HPO havealso been carried out (Figure 2 (b) and 3 (b)).
4
Corresponding binding constants (log K ) of 3.30±0.02 (2) and 3.03±0.03 (3) have
β
been determined to be lower than in case of 1 with the sterically most demanding
isopropyl substituent.
1
2

-5
Fig. 3 a) Luminescence intensity changes of 3 (5•10 M) in the presence of various
anions (50 eq.) in HEPES-buffered(pH 7.4, 10 mM solution. b) Luminescence
2
-
titrations of 3 (70 µM) upon addition of the increasing amounts of HPO in HEPES
4
(
excitation at 360 nm). b) Luminescence titration spectra (λ = 360 nm) of 3 (60μM)
ex
with increasing amounts ofHPO42- (0−50 eq.) in 10 mM HEPESbuffer (pH 7.4)
solution.
4
. Conclusions
A series of the phenanthroline-based chiral hexadentate ligands (L1, L2 and
L3) and their europium ( Ⅲ ) complexes (1-3) were synthesized and
characterized. The strong red emissions were observed in all the luminescence
spectra of these complexes and their luminescence property was studied for the
anion sensing. Except HPO42- and NO , the studied anions did not show any
-
2
significant influence on the luminescence property. For Complex 1, a strong
1
3

response was detected only for hydrogen phosphate and the intensity of
luminescence declined sharply. Complex 2 and 3 were also found sensitive to
2-
-
HPO . In the same time, NO was also found leading to a certain reduction of
4
2
the intensity of their luminescence, however, the decreases were not
dramatically. The selectivity of the quenching is not well understood but should be
due to the specific binding of the respective anions. Furthermore, the corresponding
binding constants (logK ) of these complexes, which were 4.28±0.01,
β
3
.30±0.02 and 3.03±0.03, respectively, were derived from the linear fit by
using the Benesi–Hildebrand equation. The limits of detection 1, 2 and 3, were
estimated to be 7.6 nM, 7.9 nM and 10.6 nM, respectively.
Acknowledgements
Yanfang Hu acknowledges a scholarship by the CSC.
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Author statement:
The work in this manuscript has been designed and supervised by M. Albrecht and P. S.
Subramanian. The experimental work has been done by Y. Hu. A concept of the manuscript has
been made by Y. Hu and finally the paper was written by M. Albrecht and P. S. Subramanian.
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