Photophysical, DFT and molecular docking studies of Sm(III) and Eu(III) complexes of newly synthesized coumarin ligand

Article abstract of DOI:10.1016/j.inoche.2020.108213

Herein, [M(HL)₂(H₂O)₂]NO₃·2H₂O (M = Sm (III) /or Eu (III); HL = 4-(2-hydroxy benzylidene acetohyrazide)-7-hydroxy coumarin) were synthesized and characterized using several spectroscopic methods. The complexes stoichiometry with molar ratio 2L:1M were confirmed by Job's method. Tridentate ligand (HL) was coordinated to the lanthanide ions Sm(III) or Eu(III) through azomethine nitrogen atom, phenolic oxygen of hydrazide, and ketonic oxygen of the amide group. The photophysical properties of the ligand and its complexes were studied in different organic solvents and their fluorescence quantum yields were determined as well. Strong fluorescence emissions to red shifts of europium complex were observed at 580, 593, 617, 653, 693, 704 nm which were attributed to Eu(III) emission of ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, ⁵D₀→⁷F₄, and ⁵D₀→⁷F₅, respectively. First-principles DFT calculations were performed to evaluate the optimized structure and separation energies of the HL compound, using the B3LYP/6-311++g(d,p) basis set. Molecular docking studies were carried out to predict the binding modes between the HL compound and active site of the xanthine oxidase enzyme [ECNo. (1.17.3.2) PDB ID: 1FIQ] which produced from liver patients of hepatitis C. The observed activity of the HL gave rise to the conclusion that it might exert its action through inhibition of the xanthine oxidase enzyme.

Full text of DOI:10.1016/j.inoche.2020.108213

Inorganic Chemistry Communications 121 (2020) 108213
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Photophysical, DFT and molecular docking studies of Sm(III) and Eu(III)
complexes of newly synthesized coumarin ligand
Mohamed M. Elsenety , Badr A. Elsayed , Ibrahem A. Ibrahema,b, Mahmoud A. Bedaira,c
a
a,⁎
a
b
c
Department of Chemistry, Faculty of Science, Al-Azhar University, P.O. 11823, Nasr City, Cairo, Egypt
Chemistry Department, Faculty of Science and Arts in Al- Mandaq, Al-Baha University, Al-Baha 1988, Saudi Arabia
College of Science and Arts, University of Bisha, Al-Namas 61977, P.O. Box 101, Saudi Arabia
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
A B S T R A C T
Herein, [M(HL) (H
Keywords:
2
2
O)
2
]NO
3
·2H O (M = Sm (III) /or Eu (III); HL = 4-(2-hydroxy benzylidene acetohyrazide)-7-
2
Coumarin
hydroxy coumarin) were synthesized and characterized using several spectroscopic methods. The complexes
stoichiometry with molar ratio 2L:1M were confirmed by Job's method. Tridentate ligand (HL) was coordinated
to the lanthanide ions Sm(III) or Eu(III) through azomethine nitrogen atom, phenolic oxygen of hydrazide, and
ketonic oxygen of the amide group. The photophysical properties of the ligand and its complexes were studied in
different organic solvents and their fluorescence quantum yields were determined as well. Strong fluorescence
emissions to red shifts of europium complex were observed at 580, 593, 617, 653, 693, 704 nm which were
Photophysical properties
Lanthanide complexes
Molecular docking
DFT
5
7
5
7
5
7
5
7
5
7
5
7
attributed to Eu(III) emission of
D
0
→ F
0
,
D
0
→ F
1
,
D
0
→ F
2
,
D
0
→ F3,
D
0
→ F4, and
D
0
→ F
5
, respectively.
First-principles DFT calculations were performed to evaluate the optimized structure and separation energies of
the HL compound, using the B3LYP/6-311++g(d,p) basis set. Molecular docking studies were carried out to
predict the binding modes between the HL compound and active site of the xanthine oxidase enzyme [EC#
(
1.17.3.2) PDB ID: 1FIQ] which produced from liver patients of hepatitis C. The observed activity of the HL gave
rise to the conclusion that it might exert its action through inhibition of the xanthine oxidase enzyme.
⁎
Corresponding author.
E-mail address: badrelsayed@gmail.com (B.A. Elsayed).
https://doi.org/10.1016/j.inoche.2020.108213
Received 3 July 2020; Received in revised form 25 August 2020; Accepted 30 August 2020
Available online 04 September 2020
1387-7003/ © 2020 Elsevier B.V. All rights reserved.

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
1
. Introduction
2.2. Characterization methods
Study of the chemistry of lanthanide metal ions complexes is one of
The percentage of carbon, hydrogen, and nitrogen of the prepared
organic ligand and its rare earth complexes were measured on
(Elemental analyzer -Vario EL Fab. CHNS Nr. 11042023). HACH con-
ductometer has been used for measuring the molar conductance of the
the most important fields that attract the attention of a wide sector of
researchers because of their unique photophysical properties that
qualify them to play a large and effective role in many biological,
medical, industrial and catalytic applications [1–6]. The nature of both
organic ligand and lanthanide metal ion has a remarkable role in esti-
mating the photoluminescence properties of the lanthanide complexes
through the ability of the organic compound to the strong absorption in
the ultraviolet region, as well as the effectiveness of the charge transfer
processes between the ligands and the central rare earth metal ions in
their complexes in the excited states [7–9]. In recent years, there has
been increasing interest in the photoluminescence complexes that de-
pend on both trivalent samarium or europium metal ions due to their
promising opportunities in various industrial applications such as bio-
sensing, optoelectronic and electro-luminescent devices as well as in the
laser technology [3,10–13]. Compared with their organic compounds
ligandated with the rare earth ions in their complexes, the coumarin
compounds family has a great advantage for use in many medical and
pharmaceutical applications as anti-microbial agents, anti-cancer cells
such as hepatocellular carcinoma (HCC) [14–17]. Regarding the pho-
tophysical properties, many fluorescence dyes such as coumarin deri-
vatives have been developed to label bimolecular compounds [18,19].
Fluorescent dye materials exhibit fluorescence emission at longer wa-
velengths in the red-light region plays an essential role in full-color
electroluminescence displays [20,21]. Because these dyes have ab-
sorption and emission wavelength in the UV–visible region, the biolo-
gical matrix exhibits high absorption and autofluorescence background.
Also, fluorescence coumarin dye probe is used to detect the structures of
RNA and DNA, study the remedy of DNA damaged basic group, identify
the status of an amino group and the active area of a protein molecule,
detect protein in pool scalar, distinguish nucleic acids with different
conformation and the chemical reactive activities of related drugs [22].
They are widely used as fluorescent labels and pigments, as fluorescent
probes for physiological and enzymatic measurements, as signaling
units in sensors and in sophisticated photophysical systems [23].
Therefore, presence of Sm (III) and Eu (III) as central metallic ions and
coumarin derivatives as an organic ligand improves the characteristics
of the lanthanide complexes, especially optical properties and increases
their efficiency in various applications [24–30]. In the present study, a
new coumarin derivatives and their complexes with Sm (III) and Eu (III)
as a selective lanthanide metal ions were synthesized and characterized
by different techniques such as elemental analysis, FTIR, Electronic
spectra and thermal analysis. Photo-physical properties of the prepared
compounds were examined. The effect of structural parameters and
physicochemical properties in the coumarin compound was studied by
DFT computational calculations. Moreover, Molecular docking studies
were carried out to predict the inhibition effect using coumarin ligand
for the xanthine oxidase enzyme produced from liver patients of he-
patitis C.
−4
−1
prepared organic ligand and its complexes (10
mol L ) in DMSO
solvent. Detection of the characteristics function groups of the prepared
compounds was done by (IR 200 spectrometer thermoelectric) in the
−1
range of 4000–400 cm . In the presence of dimethyl sulfoxide (DMSO)
as a solvent, the electronic spectra of the ligand and its complexes were
identified on (UV-spectrophotometer-PERKIN-ELMER) in the range of
200–800 nm. The photophysical properties were measured by using
(Thermo Fisher Scientific Spector fluorophotometer) with an emission
slit width of 5 nm in the range of 190–900 nm. For the prepared organic
ligand, the proton NMR spectrum was recorded by (Varian
Spectrometer 300 MHz) in the presence of d -DMSO solvent versus
6
internal standard tetramethylsilane (TMS). Thermal analysis data were
obtained by (PERKIN-ELMER DIAMOND), and the range of measure-
ments (room temperature to 800 °C) with the heating rate of 10 °C/min.
2.3. DFT, docking and molecular modeling calculations
The density functional theory (DFT) method using Becker’s three
parameterized Lee-Yang-Par (B3LYP) exchange functional with 6-311+
+g(d, p) basis sets were used to optimize the geometrical and elec-
tronic structure of ligand in the gas phase and DMSO as a solvent. The
Time-Dependent on Density Functional Theories (TD - DFT) has been
used for calculating the absorption and emission spectra [31,32]. The
PCM theory was used to model the solvent in the calculation that only
takes into consideration the solvent electrostatic field on the solvent. All
calculations were carried out using the software package Gaussian 09
[33].
Molecular docking studies using MOE have been carried out to
predict the binding modes between the ligand and active sites (B and C)
of the xanthine oxidase enzyme. Docking and molecular modeling
calculations were carried out on the Molecular Operating Environment
(MOE 2014.09; Chemical Computing Group, Montreal, Canada) as the
computational software. All of the minimizations were performed with
MOE until an RMSD gradient 0.05 kcal mol −1 Å −1 with MMFF94X
force field and the partial charges were automatically calculated. The
coordinates of the X-ray crystal structure of (MTE) phosphonic acid
mono-(2-amino-5,6-dimercapto- 4-oxo-3,7,8a,9,10,10a-hexahydro-4 h-
8-oxa- 1,3,9,10-tetraaza-anthracen-7-ylmethyl)ester and (FAD ) flavin-
adenine dinucleotide, were bound to chain (C) and (B) of xanthine
oxidase enzyme respectively. The structures of enzymes were checked
for missing atoms, bonds, and contacts. Hydrogen atoms were added to
the enzyme structure. Water molecules and bound ligands were
manually deleted. The ligand molecule was constructed using the
builder molecule and was energy minimized. The active sites were
generated using the MOE-alpha site finder. Dummy atoms were created
from the obtained alpha spheres. The ligand was docked within the
xanthine oxidase active sites using the MOE- dock with simulated an-
nealing used as the search protocol and an MMFF94X molecular me-
chanics force field for 8000 interactions.
2
. Experimental
2
.1. Chemical reagents
2.4. Synthesis procedures
Hydrated nitrate salts of Eu(NO
3
)
3
·5H
2
O and Sm(NO
3
)
3
·6H O
2
(
Sigma-Aldrich, purity, 99.9%). Organic solvents: absolute ethanol,
2-(7-hydroxy-2-oxo-2H-chromen-4-yl) acetic acid, ethyl 2-(7-hy-
acetonitrile, dimethylformamide (DMF), and dimethyl sulfoxide
DMSO) were of reagent grade and used without further purification.
droxy-2-oxo-2H-chromen-4-yl) acetate, and salicylaldehyde hydrazone
are the three organic compounds that termed by compounds (I, II and
III) were prepared as an introductory step to the synthesis of the organic
coumarin ligand (IV).
(
Citric acid monohydrate, Thionyl chloride (SOCl ), Resorcinol,
2
Hydrazine monohydrate (98%), Sulfuric acid, Salicylaldehyde were of
reagent grade and were used without further purification.
a) Synthesis of compound (I)
2

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
The hydrated citric acid (0.1 mol) was mixed with 28 ml of H
2
SO
4
3. Results and discussion
(
98%) under stirring for 60 min. at 250 °C, then the produced solution
is heated slowly to 75 °C with continuous quite stirring for 45 min. and
the solution turns to yellow color. This followed by quickly cooling of
the yellow solution to 0 °C, to add both (0.08 mol) resorcinol and
3.1. Properties of the complexes
Analytical data for the complexes, presented in Table 1, with the
1
1.2 ml H
2
SO
4
in three equivalent contents in the container of an ice
chemical formula [M (L)
2
(H
2
O)
2
]NO
3
·2H O M = Eu(III) or Sm(III).
2
bath, and then it is kept overnight at 0 °C. After that, the previous
product was poured into ice and the resulting crystalline white and
Both complexes are soluble in DMSO and DMF, slightly soluble in
ethanol, insoluble in water, and benzene. Conductance measurement
data for the synthesized complexes in DMSO were shown also in
Table 1, indicating that both complexes of 2:1 (L/M) molar ratio with
ionic characters [36].
treated with NaHCO (200 ml 10%, w/v). The filtrate was acidified by
3
the addition of HCl (133 ml,10%v/v) to reaching the strong acidic
medium (pH 1.43). Finally, the produced compound was purified by its
washing with deionized water and dried in a thermal oven to give the
prepared sample [34,35]. (Yield 79%and M.P 205 °C).
3.2. IR spectra
b) Synthesis of compound (II)
i Infrared spectral analysis of compounds I, III and their ligand (IV)
The FTIR technique is an effective powerful method to identify the
1
mol of compound (I) was dissolved in 100 ml of ethanol, and a few
drops of catalytic thionyl chloride (SOCl ) were added. The reaction
2
Characteristic vibrational modes of the prepared compounds. For the
compound (1) 2-(7-Hydroxy-2-oxo-2H-chromen-4-yl) acetic acid, the
mixture was refluxed with heating for 6 h. The resulting product (II)
was extracted with ethyl acetate, flowed by washing with sodium bi-
carbonate, and dried in an oven [35]. (Yield 85%, M.P 135 °C).
−
1
−1
−1
selected peaks appeared at 3498 cm , 1670 cm , 1598 cm
,
−1,
−1
1
248 cm
OH of coumarin moiety), (C]O carboxylic), (C]C), (CeO) asym-
metric. and symmetric modes, respectively and was shifted to
and 1144 cm are belonging to the stretching vibration of
(
c) Synthesis of compound (III)
−1
−1
3
425 cm , 1624 cm ) to indicate the appearance of the amide group
The salicylaldehyde hydrazone (III) was prepared according to the
previous method [35]. (Yield 81%, M.P 98 °C).
and the participation of the carboxylic group in the formation of the
−1
−1
−1
ligand (, 1541 cm , 1269 cm
the ligand. The peaks at 3126 cm
and 1128 cm
in the IR spectra of
−1
−1
and 1454 cm
corresponded to
d) Synthesis of coumarin ligand (IV)
(
COOH) and (CeO of carboxylic) are absent in the ligand spectra. The
most frequencies of the compound (III) salicylaldehyde hydrazone was
A solution of compound (III) (100 mmol, 4.996 g) in 100 ml ethanol
was added dropwise to the solution of compound (II) (100 mmol,
−1
−1
−1,
−1
presented at 3378 cm , 3283 cm , 1584 cm
and 1401 cm
are
due to ν (eOH), ν (NeH) of NH
2
, OH deformation and NH def., ν
1
2.212 g) under stirring, and the mixture was refluxed for 12 h. The
−
1
(
> C]N), and ν (C]C), respectively and shifted to 3359 cm
,
prepared solid compound was left to coagulate for overnight, then fil-
tered off and recrystallized from absolute ethanol solvent.
−
1
3
1
291 cm
(on the formation of the amide group in the ligand),
−1
−1
541 cm
and 1364 cm
in the IR spectra of the compound (IV),
coumarin ligand. The Characteristic peaks of compounds (I), (III) and
e) Synthesis of lanthanide coumarin (V) complexes
(
IV) coumarin ligand were presented in Table S1, Fig. S1–S3.
For the Eu(III) complex, a solution of the organic ligand (1 mmol,
ii Infrared spectra of the coumarin ligand and its lanthanide com-
plexes
0
.348 g in absolute ethanol) was added to the europium nitrate solution
(
0.5 mmol, 0.214 g in absolute ethanol) under constant stirring.
Triethanolamine (0.1 ml) was added to the above mixture to accelerate
the coordination process and the solution was more refluxed for 6 h.
After that, the mixture solution was concentrated until theformation of
the product.. The resulted product was washed by ethanol and left to
dry at room temperature for 12 h. Preparation of the Sm(III) complex is
done in the same order as the steps in the Eu(III) complex. All the
preparation reactions and analytical data of the prepared ligand and its
complexes are presented in Scheme 1, 2 and Table 1.
Table S2 and Figs. S4, S5 display the IR bands and their assignments
for the prepared coumarin (IV) ligand and its complexes (V, VI) with
trivalent Eu and Sm cation. Detection of the chelation process was ex-
amined by the comparison between the IR spectra of both coumarin
ligand and its rare earth complexes. For the Eu(III) complex, Lowering
the distinct peak of azomethine in the organic ligand that appears at
1
541 indicates the weakening of the imine double bond and confirms
that coordination occurs through the nitrogen atom of azomethine with
the trivalent europium metal ion [37,38]. Also, the complexation was
−
1
confirmed by the appearance of the main absorption peak at 586 cm
Scheme 1. Reaction of prepared Ligand (HL).
3

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
Scheme 2. Reaction of prepared M(III)-Complexes (where M = Eu or Sm).
Table 1
Analytical data of the Ligand and its lanthanide complexes:
Molecular Formula Molecular Weight Melting point o
C
C% (found) calc. H% (found) calc. N% (found) calc. Molar conductance (S cm2 Mol−1
)
Color
Yield %
73%
C
18
C
36
C
36
H
H
H
14
34EuN
N
2
O
5
5
5
338
893
892
182–180
≫300
(63.94) 63.90
(47.92) 48.55
(47.89) 48.64
(4.17) 4.17
(3.18) 3.57
(3.18) 3.58
(8.34) 8.28
(7.86) 7.29
(7.86) 7.27
–
yellow
O
17
17
113.50
110.10
Dark yellow 71%
Dark yellow 74%
34SmN
O
≫300
1.0
0.8
0.6
0.4
0.2
0.0
that assigned to the stretching vibration of (SmeN of azomethine).
−1
Moreover, the absence of the characteristic IR peak at 3359 cm that
attributed to the stretching vibration of aromatic hydroxyl substituent
of salicylaldehyde hydrazone moiety and appearing a new peak at
−
1
4
12 cm due to presence of (SmeO of amide group) give an additional
(
1)
2)
ligand
Sm-complex
Eu-complex
(
confirmation for the complex formation. Also, the absorption band re-
(3)
lated to ν(OeH) str. of coumarin group in of coumarin ligand at
−
1
−1
3
435 cm
was shifted to 3577 cm
in the IR spectrum of Eu(III)
complex, this confirms that the 7-OH group is not involved in the co-
ordination. Finally, Eu(III) complex showed six bands at 1446, 1304,
−1
1
4 1 2 6 3
045, 899, 751, and 650 cm which were assigned to ν ν ν ν ν and
ν
(
5
vibrations of nitrate groups, respectively [39]. The IR spectrum of Sm
III) complex gives very similar results with the obtained data of the Eu
(III) complex, especially the presence of (Sm- N) and (Sm - O) by ap-
(
2)
(
3)
(1)
−1
−1
pearing the peaks at 585 cm
.3. Electronic spectra
and 413 cm
respectively [40].
2
80 300 320 340 360 380 400 420 440 460 480 500
3
Wavelength (nm)
Fig. 1. Electronic spectra of the ligand and its lanthanide complexes in DMSO.
The UV–Vis absorption spectra of the investigated ligand (HL) and
its Ln(III) complexes were measured in DMSO solution at room tem-
perature. The observed electronic transition bands associated with the
4

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
Table 2
Thermogravimetric data of Eu-complex.
Decomposition steps
Wt. loss %
Lost species
E
A
ΔH
ΔS
ΔG
R2
−
1
−1
−1
−1
−1
kJ mol
min
kJ mol
kJ mol
kJ mol
−
−
−
−
−
6
2
3
4
6
I
3.15
Crystalline 2H
2
O
67,395
14,918
7318
5699
29
7.45 × 10
1.69 × 10
1.61 × 10
6.85 × 10
1.72 × 10
64,532
10,276
2384
83
−95
97,354
0.91
1.00
1.00
0.96
0.93
II
3.28
Coordinated 2H
Nitrate ion
2
O
−264
−285
−293
−344
158,105
171,402
197,990
285,469
III
IV
V
6.22
22.61
64.74
Coumarin moiety
Europium sesquioxide Eu
2
O
3
−7026
Fig. 2. Excitation and Emission spectra of the ligand in different solvents.
Table 3
Stoke shift and Fluorescence quantum yield of the ligand in a different solvent
at (ƛex = 340):
Solvent
ƛ
ex (nm) Absorbance (A)
ƛ
em (nm) refractive
ΔVst
quantum
index (η)
F
(nm) yield (Φ )
DMSO
Ac
331
291
331
331
0.71
1.58
1.29
0.59
461
397
404
404
1.48
1.34
1.36
1.43
130
106
73
0.35
0.03
0.12
0.10
EtOH
DMF
73
Using quinine sulfate (Φ
S
= 0.55) as a standard at room temperature, ΔVst
(
stokes shift).
ligand and it's Ln(III) complexes were collected in Table S3, as well
their spectra were shown in Fig. 1.
The spectrum of the coumarin ligand showed two main character-
istic absorption bands in the regions 265–440 nm. The absorption band
at 298 were attributed to the π → π* transition states of the azomethine
group and the broadband at ƛmax 331 nm was attributed to n → π*
transition states of the azomethine nitrogen atoms and the nonbonding
electrons on the oxygen atoms as well the intramolecular charge
transfer [40]. The absorption spectra of the Ln(III) coumarin complexes
were identical, with similar structures and showed the characteristic
bands associated with the free ligand with a red shift of the char-
acteristic band by 17 nm and 24 nm for Eu and Sm complexes respec-
tively, compared to the free ligand as shown in Table S3. It’s worth to
mention that, the shifts of the absorption bands at 331 nm of the ligand
associated with complexes confirmed the coordination of the ligand to
the metal ion, through azomethine nitrogen atoms and the nonbonding
Fig. 3. Fluorescence spectra of Sm complex in DMSO.
electrons of the oxygen atoms. It is well known that the lanthanide ions
have no absorption bands in their complexes, science the f-f transitions
are Laporte-forbidden and very weak in nature [41].
.4. 1H NMR studies
3
1
H NMR spectrum (Fig. S6) of the coumarin ligand (IV) was re-
corded in DMSO‑d in the presence of TMS as internal standards. The
6
5

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
Fig. 4. Fluorescence spectra of Eu complex in different solvents.
ligand has the following signals ϐ(ppm):3.6(2H, CH
2
group), 7.5(1H,
uncoordinated water molecules (Cal. 3.75%) with weak exothermic
(Tdta). The mass loss at (180–200 °C) of 3.28% indicated two co-
ordinated water molecules (Cal. 3.75). Moreover, one uncoordinated
nitrate group was eliminated with a mass loss of 6.22% at (220–225 °C)
as an endothermic peak. The mass loss at (280–440 °C) of 22.61%
identified the decomposition of two coumarin moieties of the complex.
Were followed by the last step, in which oxidation of Eu(III) to
CH group), 6.10(H, eCH = in C3 of coumarin moiety),7.8(1H, NH
group), 3.4 (1H, Ar-OH), 3.51(1H, OH-coumarin moiety) and
6
.2–7.5 ppm are assigned to the proton of both phenolic and coumarin
moiety.
It worth to mention, we observed such broad bands at
1
(
~7–7.5 ppm) in aromatic protons of the H NMR spectrum, which
could be attributed to coumarin moiety itself or to presumably owing to
the quick proton exchange between the proton of both coumarin and
Europium sesquioxide Eu
2
3
O .
The general thermal behavior of the Eu-complex in terms of stability
ranges, peak temperatures, and values of the kinetic parameter. The
kinetic parameters E and Z were evaluated using the following Coast-
Redfern Equation [45].
(
NHe) in hydrazone moiety, as well due to the different environments
around the protons of coumarin and phenolic moieties [42–44].
3
.5. Thermal analysis of the Eu(III)complex
ln(1
T
)
E
RT
AR
E
ln[
] =
+ ln
2
The TGA/DTA of the Eu(III)complex as an example, reveals seven
where A is constant, β is the heating rate, R is the universal gas con-
stant, α is a fraction of decomposition and E is the activation energy.
Therefore, plotting against 1/T according to equation 1 should give a
straight line whose slope is directly proportional to the activation en-
ergy − R E. The activation entropy ΔS, the activation enthalpy ΔH, and
the free energy (Gibbs function ΔG) parameters were calculated using
the following equations:
steps in the sequential decomposition of the coumarin europium com-
plex over the experimental temperature range (25–800 °C) under N
2
flow which determined for all steps in the decomposition sequence as
long as shown in TGA/DTA. The mechanism of the thermal decom-
position process of the complex could be investigated as follows: five
successive degradation steps have occurred which corresponding to the
mass loss at (40–80 °C) of 3.15% equivalent due to the evolution of two
uncoordinated water molecules (Cal. 3.75%) with weak exothermic
Ah
KT
S = 2.303(Log
)R
(
Tdta). The mass loss at (180–200 °C) of 3.28% indicated two co-
ordinated water molecules (Cal. 3.75). On the other hand, one un-
coordinated nitrate group was eliminated with mass loss 6.22% at
H = E RT
(
220–225 °C) as an endothermic peak. The mass loss at (280–440 °C) of
G =
H
T
S
2
2.61% identified the decomposition of two coumarin moieties of the
complex. Were followed by the last step, in which oxidation of Eu(III) to
where k and h are Boltzmann and Planck constants respectively, T is the
temperature involved in the calculation selected as the peak tempera-
ture of DTGA. The entropy ΔS gives information about the degree of
order of the system, the enthalpy ΔH gives information about the total
thermal motion, and Gibbs or free energy gives information about the
stability of the system. The kinetic ΔH, ΔS, and ΔG are tabulated in
Table 2.
Europium sesquioxide Eu
2 3
O .
The TGA/DTA of the Eu(III)complex reveals seven steps in the se-
quential decomposition of the coumarin europium complex over the
experimental temperature range (25–800 °C) under N flow which de-
2
termined for all steps in the decomposition sequence as long as shown
in TGA/DTA. The mechanism of the thermal decomposition process of
the complex could be investigated as follows: five successive degrada-
tion steps have occurred which corresponding to the mass lose at
Taking the main decomposition steps as a criterion, the value of the
activation entropy (−ΔS) in this complex indicate that the activated
complex has a more ordered structure than the reactants, ΔG is positive
(
40–80 °C) of 3.15% equivalent due to the evolution of two
6

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
for the reaction for which ΔH is positive and ΔS is negative [ the only
ΔH is negative for the last step of Eu
2
O ]. The reactions for ΔG is po-
3
sitive and ΔS is negative are considered as unfavorable or non-sponta-
neous. Reactions are classified as either exothermic (ΔH < 0) or en-
dothermic (ΔH > 0) [only ΔH is negative −7026 for Eu
2
O ] based on
3
whether they give off or absorb heat. Reactions can also be classified as
exergonic (ΔG < 0) or endergonic (ΔG > 0) based on whether the
free energy of the system decreases or increases during the reaction.
The correlation coefficients of the Arrhenius plots of the thermal de-
composition steps were found to lie close to the value 1.00, with a
linear function. It is clear that the thermal decomposition process of the
Eu-complex is non-spontaneous, the complex is thermally stable.
3
.6. Photophysical properties of the ligand and its Ln(III) complexes
a Study of the fluorescence spectra of the ligand in different solvents:
The fluorescence excitation and emission spectra of the ligand in the
different solvent DMSO, AC, EtOH, and DMF were shown in Fig. 2.
The emission bands associated with the ligand in different solvents
at excitation wavelength 340 nm show maximum emission bands
around 461, 397, 404 and 404 nm using DMSO, Ac, EtOH and DMF
solvents respectively, which were attributed to π* → π electronic
transition with a shift of ΔVst (73–130 nm) from their ƛex depending on
their polarity indexes.
The magnitudes of stokes shift were attributed to the charge transfer
transitions, where the large magnitudes of stokes shift indicated that
the excited-state geometry could be different from that of the ground-
state. The general observation visualized is that there is an increase in
the stokes shift values with increasing solvent polarity and an increase
in the dipole moment of excitation. The fluorescence quantum yield
−
4
(
Φ
F
) was calculated by preparing solutions of the (1 × 10
M),
measuring their absorbance, and find their area under the emission
peaks for each solvent. The absorbance and the area were compared to
a reference fluorescence quantum yield (quinine sulfate) [46,47]. The
data were collected in Table 3. From the obtained data the fluorescence
quantum yield of the ligand in different solvents was calculated using
the following equation [46,48]:
F = S(F/Fss)(As/A)( 2
/
s )
2
where: Φ
F
= fluorescence quantum yield, Φ = Quantum yield of re-
S
Fig. 5. Visualization of the HOMO-2, HOMO-1, HOMO, LUMO, LUMO+1, and
LUMO+2 atomic orbitals as well as separation energies of the investigated
compound at B3LYP/6-311++g(d,p) basis set.
ference, F = area under the fluorescence emission curve of the sample,
F
s
= area under the fluorescence emission curve of reference, A = Ab-
sorbance of the sample, A = Absorbance of reference, η = refractive
s
Fig. 6. 2D Diagram and 3D Active site of docking of the HL into the XO enzyme chine (C).
7

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
Fig. 7. 2D Diagram and 3D Active site of docking of the HL into the XO enzyme chine (B).
Scheme 3. The representative keys
for the type of interaction between
substrates and the enzyme.
index of sample, η
s
= refractive index of reference. Here Quinine Sul-
wavelengths due to the TD -DFT method failure and the basis set size
that conforms to the experimental spectrum of the first, second, third,
and fourth absorption bands. These electronic transitions are because of
the allowed excitation of HOMO to LUMO state (singlet 397.07 nm,
3.1224 eV), HOMO-1 to LUMO+1 (singlet 319.76 nm, 3.8774 eV),
HOMO to LUMO+2 (singlet 285.98 nm, 4.3355 eV) and HOMO-2 to
LUMO (singlet 251.08 nm, 4.9380 eV). The HOMO and LUMO of the
coumarin ligand compound are presented in Fig. 5.
phate was used as a reference, which has a quantum yield of 0.55 when
dissolved in (0.50 M) H
2
SO had a refractive index of 1.35.
4
b Study of the fluorescence spectra of the lanthanide complexes in
different solvents:
The effect of the solvents on the fluorescence intensities of the Eu
and Sm complexes were studied in (DMSO, AC, EtOH, and DMF) sol-
vents. The photoluminescence spectral bands associated with Sm-
complex at excitation wavelength 380 nm Fig. 3.
The HOMO was distributed on the salicylaldehyde hydrazone
moiety, so this is the most favorable site for forming complexes with
3
+
Eu
ion, whereas the LUMO was spread over the coumarin moiety.
were Obscured in Ac, EtOH, and DMF while appeared in DMSO at
4
6
4
(
561, 597, 642, 685) nm which attributed to ( G5/2 → H5/2, G5/2
→
3.8. Molecular docking
6
4
6
4
6
H
7/2
,
G
5/2
→
H
9/2,
G
5/2
→
H
11/2 electronic transitions) respec-
tively. On the other hand, the photo spectral bands of Eu-complex
The three-dimensional co-crystal structure of xanthine oxidase (XO)
Fig. 4, Fig. S7, Were appeared at (572, 588, 618, 650, 699) nm in
chains (B) and (C) were combined with the inhibitor (MTE)and (FDA)
respectively. The docking study of the inhibitor was performed using
the XO co-crystallized with MTE as a template with rmsd value (1.24)
and energy (−3.00). Docking of the ligand (HL) with the XO chain (C)
active sites revealed the presence of H-donor interactions between N
atoms in the ligand and GLN 1194 amino acid residue with rmsd value
(
(
DMSO, AC, EtOH, and DMF) solvents respectively, which attributed to
5 7 5 7 5 7 5 7 5 7
D
0
→
F
0
,
D
0
→
1
F ,
D
0
→
F
2,
D
0
→
F
3,
D
0
→
4
F electronic
transitions) respectively [48,49].
3.7. Computational study
(
0.99) and energy (−6.86) as shown in Fig. 6.
From the calculations, one can predict that the inhibitor (ligand) is
The effect of structural parameters and physicochemical properties
behaving strongly with chain (C) of XO than the reference (MTE) ac-
cording to their rmsd values.
in the coumarin compound was investigated by DFT calculations. By
optimizing, the bond length, bond angle and dihedral angle, of HL
geometric and electronic structure were calculated. The optimized
structure, Mulliken charges, and electrostatic potential surface (ESP) for
HL in the ground state are shown in Fig. S8. It is well known that the
more negative the atomic partial charges of the atom, the atom donates
its electrons to the metal ion, thus chelation easily occurs. The calcu-
lated charges for Mulliken and ESP indicate that the negative charge is
high close to two atoms of nitrogen, the C]O group, and the OeH
Also, the interaction between HL and active site bound to chain (B)
of XO gives the same rmsd value (1.49) like that of reference FAD
(
1.49), but its energy value (-3.00) was larger than FAD (-6.8563) as
shown in Fig. 7.
Scheme 3 illustrates the representative keys for the type of inter-
action between ligand as an inhibitor with the XO enzyme, which can
be used as a guide in the docking in Figs. 6 and 7.
3
+
group indicating that Eu
cation may be activated. The values of
atomic charges are −0.36, −0.23, and −0.46 for N]C, O]C, and
OeH respectively. The absorption spectrum of HL in DMSO as a solvent
has been also calculated as shown in Fig. S9. The calculated spectrum
showed three absorption bands with a shift to about 66 nm to higher
4. Conclusion
A systematic study of the structural and photoluminescence prop-
erties of Sm(III) and Eu(III) complexes with 4-(2-hyroxybenzylidene
8

M.M. Elsenety, et al.
Inorganic Chemistry Communications 121 (2020) 108213
acetol hydrazide)-7-hydroxy coumarin anion was carried out. In both of
luminescent sensing probes for multi-phosphate anions: modulating selectivity by
ligand choice, Chem. Commun. (2008) 1127–1129, https://doi.org/10.1039/
b715719c.
the two structures of [Sm(L)
2
2
(H O)
2
]NO
3
·2H
2
O and [Eu(L)
2
(H
2
O) ]
2
NO
3
·2H O the ligand molecules were coordinated as tridentate via the
2
[
[
[
[
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oxygen of phenolic-OH, Ketonic oxygen of amido group, and the ni-
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central Ln(III) which mean, coordination number eight was suggested
for the metal ion in this lanthanide(III) nitrate complexes. As well an-
other two uncoordinated water molecules were bound in the outer
coordination sphere in complexes. The fluorescence data exhibit char-
acteristic luminescence of Sm(III) and Eu(III) ions, which revealed the
role of organic moiety chelator to absorb and transfer energy to Sm(III)
and Eu(III) ions. The assessment of xanthine oxidase (XO) activity in
several serum samples from various human patients with cirrhosis of
chronic hepatitis C using a highly sensitive and spectrofluorimetric
method using the Eu(III) complex (VIII) as an optical sensor undertook
in our laboratory and will be sent for publication soon. Molecular
Docking was performed for the ligand as an inhibitor for the xanthine
oxidase (XO) chain (C) enzyme in a trial to predict mode of action in-
hibitor to be used as an antibiotic drug.
2
020.124861.
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Declaration of Competing Interest
3
89 (2020) 112221, , https://doi.org/10.1016/j.jphotochem.2019.112221.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
[
[
20] H. Iwanaga, Relationships between molecular structures of aromatic- and aliphatic-
substituted diphosphine dioxide ligands and Properties of Eu(III) complexes, Opt.
Mater. (Amst) 85 (2018) 418–424, https://doi.org/10.1016/j.optmat.2018.08.071.
21] E.T. Knittl, A.A. Abou-Hussein, W. Linert, Syntheses, characterization, and biolo-
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Appendix A. Supplementary material
Supplementary data to this article can be found online at https://
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