Synthesis, density functional theory calculations and luminescence of lanthanide complexes with 2,6-bis[(3-methoxybenzylidene)hydrazinocarbonyl] pyridine Schiff base ligand

Article abstract of DOI:10.1002/bio.3375

A pyridine-diacylhydrazone Schiff base ligand, L?=?2,6-bis[(3-methoxy benzylidene)hydrazinocarbonyl]pyridine was prepared and characterized by single crystal X-ray diffraction. Lanthanide complexes, Ln–L, {[LnL(NO₃)₂]NO₃.xH₂O (Ln?=?La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er)} were prepared and characterized by elemental analysis, molar conductance, thermal analysis (TGA/DTGA), mass spectrometry (MS), Fourier transform infra-red (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. Ln–L complexes are isostructural with four binding sites provided by two nitro groups along with four coordination sites for L. Density functional theory (DFT) calculations on L and its cationic [LnL(NO₃)₂]⁺ complexes were carried out at the B3LYP/6–31G(d) level of theory. The FT-IR vibrational wavenumbers were computed and compared with the experimentally values. The luminescence investigations of L and Ln–L indicated that Tb–L and Eu–L complexes showed the characteristic luminescence of Tb(III) and Eu(III) ions. Ln–L complexes show higher antioxidant activity than the parent L ligand.

Full text of DOI:10.1002/bio.3375

Received: 9 March 2017
DOI: 10.1002/bio.3375
Revised: 10 May 2017
Accepted: 7 June 2017
R E S E A R C H A R T I C L E
Synthesis, density functional theory calculations and
luminescence of lanthanide complexes with 2,6‐bis
[(3‐methoxybenzylidene)hydrazinocarbonyl] pyridine
Schiff base ligand
Ziyad A. Taha1
Taher S. Ababneh2 Ahmed K. Hijazi1 Qutaiba Abu‐Salem3
|
|
|
|
Abdulaziz M. Ajlouni1 Shroq Ebwany1
|
1 Department of Applied Chemical Sciences,
Abstract
Faculty of Arts and Sciences, Jordan University
of Science and Technology, Irbid, Jordan
A
pyridine‐diacylhydrazone Schiff base ligand,
L
=
2,6‐bis[(3‐methoxy benzylidene)
2 Department of Chemistry, Tafila Technical
hydrazinocarbonyl]pyridine was prepared and characterized by single crystal X‐ray diffraction.
Lanthanide complexes, Ln–L, {[LnL(NO3)2]NO3.xH2O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and
Er)} were prepared and characterized by elemental analysis, molar conductance, thermal analysis
(TGA/DTGA), mass spectrometry (MS), Fourier transform infra‐red (FT‐IR) and nuclear magnetic
resonance (NMR) spectroscopy. Ln–L complexes are isostructural with four binding sites pro-
vided by two nitro groups along with four coordination sites for L. Density functional theory
(DFT) calculations on L and its cationic [LnL(NO3)2]+ complexes were carried out at the B3LYP/
6–31G(d) level of theory. The FT‐IR vibrational wavenumbers were computed and compared with
the experimentally values. The luminescence investigations of L and Ln–L indicated that Tb–L and
Eu–L complexes showed the characteristic luminescence of Tb(III) and Eu(III) ions. Ln–L com-
plexes show higher antioxidant activity than the parent L ligand.
University, Al‐Tafila, Jordan
3 Department of Chemistry, Faculty of Science,
University of Al al‐Bayt, Al‐Mafraq, Jordan
Correspondence
Ziyad A. Taha, Department of Applied
Chemical Sciences, Faculty of Arts and
Sciences, Jordan University of Science and
Technology, Irbid 22110, Jordan.
Email: tahaz33@just.edu.jo
Funding information
Deanship of Research‐JUST, Grant/Award
Number: grant no. 16/2014
KEYWORDS
DFT, lanthanide complexes, luminescence, Schiff Base, X‐ray
|
1
INTRODUCTION
two alkoxybenzaldehyde moieties on each side connected by a
carbohydrazide linkage have attracted much attention not only for
Considering the photophysical and magnetic significance of lanthanide
ions, there is overriding interest in the development of their com-
plexes.[1] Lanthanide complexes exhibit potential applications in
biomedical diagnostics, catalysis, tunable photonic devices, magnetic
resonance imaging, luminescence, metalloorganic chemical vapor
deposition and sol–gel technology, organometallic and medicinal
chemistry.[2–5]
their aesthetically fascinating structures but also for their broad range
of biological activities such as antioxidant, antimicrobial, anticancer,
antiinflammatory,
analgesic,
hypotensive,
agonistic,
and
hallucinogenic.[6–11] Furthermore, pyridine‐diacylhydrazones are
important multidentate ligands for the construction of coordination
complexes with metal centers.[12–16] Their metal complexes have
attracted intense interest in many areas such as multimetallic enzymes,
homogeneous and heterogeneous catalysis.[16–18]
Acylhydrazones are an important class of Schiff base compounds
with a keto‐type structure (O = C–N–N = CH–). In the recent past, pyr-
idine‐diacylhydrazones ligands possessing a central pyridine ring and
Following our recently published work towards the complexation
of lanthanide(III) nitrate salts with dimethyl pyridine‐2,6‐
dicarboxylate, we were interested in the synthesis of a 2,6‐bis[(3‐
Abbreviations used: DFT, density functional theory; DMF, dimethylformamide;
DMSO, dimethylsulphoxide; DTGA, differential thermogravimetric analysis;
HAT, hydrogen atom transfer; IR, infra‐red; NMR, nuclear magnetic resonance;
TCSPC, time‐correlated single photon counting; TMS, tetramethylsilane; UV,
ultraviolet.
methoxybenzylidene)hydrazine carbonyl]pyridine
L ligand derived
from dimethyl pyridine‐2,6‐dicarboxylate ligand.[19] In comparison
with dimethyl pyridine‐2,6‐dicarboxylate ligand, the L ligand pos-
sesses more coordination sites that can bind and stabilize different
Luminescence. 2017;1–10.
wileyonlinelibrary.com/journal/bio
Copyright © 2017 John Wiley & Sons, Ltd.
1

2
TAHA ET AL.
lanthanide metal ions with high coordination numbers. Moreover it
has flexible groups that can accommodate and protect lanthanide
metal atoms from solvent molecules, which avoid non‐radiative deac-
tivation processes.[20] Moreover, L is efficient in the displacement of
water molecules present in the coordination sphere of lanthanide
ions that quenches the luminescence by absorption of the emitted
radiation in the vibrational excitation processes via the vibrational
levels of the O–H of water molecules.[21]
single photon counting technique (TCSPC) using an Edinburgh Instru-
ments unit model 199. Photoluminescence quantum yields (Φ) of the
complexes were obtained using sulfuric acid solution (0. 1 M) of qui-
nine sulfate (0.1 μg ml−1) as a standard reference at 25°C. Single crystal
X‐ray diffraction data of the L were collected on a Bruker APEX 2 DUO
diffractometer at 100
K with monochromatic Cu Kα radiation
(λ = 1.54178 Å) and crystal size 0.20 × 0.10 × 0.10 mm3. The data were
integrated, scaled and corrected for absorption with SADABS using
multi‐scan method.[24] The structure was solved by direct methods
SHELXS‐97 and refined with SHELXL‐2013.[25,26] H atoms were
refined at idealized positions riding on the parent C or N atoms with
isotropic displacement parameters Uiso (H) = 1.2 Ueq (C/N) or 1.5
Ueq (−CH3) and C–H 0.95–0.98 Å and N–H 0.88 Å. The antioxidant
activity of the different compounds was estimated by the DPPH•
(diphenyl picryl hydrazyl) radical scavenging method reported
previously.[19]
This work reports the synthesis of the
L ligand by a
condensation reaction between 2‐methoxybenzaldehyde and 2,6‐
bis(hydrazinocarbonyl)pyridine in ethanol and describes its crystal
structure. Additionally the coordinating behavior of L with lantha-
nide metals (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er) has been
investigated by elemental analysis, conductivity measurements,
spectral analysis (IR, NMR, UV–Vis and mass) and thermal studies.
The antioxidant activity and photoluminescence properties of L
and Ln–L have been investigated.
|
2.3
Synthesis of the ligand L
|
2
EXPERIMENTAL
Materials
To a hot suspension of 2,6‐bis(hydrazinocarbonyl)pyridine (0.195 g,
mmol) in 20 ml of absolute ethanol, solution of 2‐
1
a
|
2.1
methoxybenzaldehyde (0.272 g, 2.0 mmol) in 40 ml of ethanol was
added with stirring and refluxed for 3 h. The white precipitate obtained
was collected by filtration, washed with ethanol and air dried. The
product was further re‐crystallized from ethanol and the filtrate was
kept at room temperature. After about 2 weeks, a colorless needle sin-
gle crystal suitable for X‐ray diffraction was obtained and separated by
filtration. Yield: 91%; M. Pt. = 274.7–275.5°C; 1H NMR (400 MHz,
DMSO‐d6, ppm) δ: 3.93 (6H, s, −OCH3), 7.05–7.07 (2H, m, Ar), 7.16–
7.18 (2H, d, Ar), 7.45–7.47 (2H, m, Ar), 7.94–7.96 (2H, d, Ar), 8.27–
8.29 (1H, m, py), 8.34–8.36 (2H, dd, py), 9.09 (2H, s, −N = CH), 12.35
(2H, s, −NH). 13C NMR (100 MHz, DMSO‐d6, ppm) δ: 162.1, 160.7,
151.07, 147.9, 142.6, 134.7, 128.4, 128.2, 124.8, 123.6, 114.7, 58.5.
IR (KBR, cm−1) 3170, 1677, 1659, 1600, 1572, 1248, 1159, 1080. Anal.
Calcd. for C23H21N5O4: C, 64.03; H, 4.91; N, 16.23. Found; C, 64.17;
H, 4.85; N, 16.07. ESI‐MS (positive mode, m/z) calculated for
All materials used were purchased from commercial suppliers as AR
grade and used without further purification. 2,6‐Bis(hydra-
zinocarbonyl)pyridine was synthesized following a previously reported
procedure.[22]
|
2.2
Measurements
Elemental analyzes were measured on a Euro EA elemental CHN ana-
lyzer 3000. The Ln–L complexes were analyzed for their lanthanide
contents by complexometric titration with EDTA using xylenol orange
as an indicator after decomposition with HCl.[23] Mass spectra were
measured using an API‐3200 mass spectrometer equipped with an
ESI (electrospray ionization) source and the detection was performed
on positive mode ionization. The infrared spectra were recorded as
KBr pellets on a JASCO FT‐IR model 470 spectrophotometer in the
C
23H21N5O4 [L + H]+ = 431.2, found 432.2.
region 4000–400 cm−1 1H and 13C NMR spectra were recorded on a
.
Bruker Avance spectrophotometer (400 MHz) in D6‐DMSO using
tetramethylsilane (TMS) as an internal standard reference. Conduc-
tance measurements were recorded at room temperature in
dimethylformamide (DMF) (1.0 × 10−3 M) using a WTW LF 318 con-
ductivity meter with cell type WTW Tetracon 325 and cell constant
0.97. TGA thermograms were collected in the temperature range
300–1200 K using a Con PCT‐2A thermobalance analyzer with a
heating rate of 10 K min−1. Electronic absorption spectra were mea-
sured on a UV‐2401PC UV–Visible spectrophotometer (Schimadzu
Corporation) in the range 200–700 nm in dimethylformamide (DMF)
solutions at room temperature. Emission spectra were recorded in
DMF (1.0 × 10−6 M) at constant room temperature on an Edinburgh
instrument model FS900SDT spectrophotometer equipped with
1.0 cm quartz cuvettes in wavelength range 200 to 700 nm with a
spectral resolution of 2.0 nm. In addition, its light source and detectors
were a 450 W Xe lamp and an R955 photomultiplier tube, respectively.
Luminescence decay curves were measured using a time‐correlated
|
2.4
Synthesis of the complexes, Ln–L
All complexes were obtained following the same general procedure. To
a solution of the L ligand (1.0 mmol, 0.432 g) dissolved in 10 ml chloro-
form, 1.0 mmol of lanthanide nitrate {Ln(NO3)3.xH2O, x = 6 for Ln = La,
Pr, Nd, Sm, Gd, Tb, Dy; x = 5 for Ln = Eu and Er) dissolved in 10 ml
ethylacetate was slowly added at room temperature. The mixture
was stirred at room temperature for 3 h. The precipitate formed was
filtered off, washed several times with 1:1 V:V ratio of ethyl acetate:
chloroform mixture and dried under vacuum. All attempts to grow a
single crystal suitable for single X‐ray diffraction were unsuccessful.
[LaL(NO3)2]2(NO3)2.4H2O: Yield: 67%; 1H NMR (400 MHz,
DMSO‐d6, ppm) δ: 3.94 (6H, s, −OCH3), 3.85 (6H, s, −OCH3), 7.05–
8.35 (22H, Ar), 9.04 (2H, s, −N = CH), 8.96 (2H, s, −N = CH), 12.46
(2H, s, −NH), 12.28 (2H, s, −NH).. 13C NMR (100 MHz, DMSO‐d6,
ppm) δ: 159.44, 158.95, 148.34, 147.8, 142.9, 134.7, 128.1, 128.2,
124.6, 123.7, 114.9, 56.3.

TAHA ET AL.
3
|
seen together with a broad absorption band extending from 3700 to
3000 cm−1, Figure 1(ii, inset), due to the intermolecular hydrogen bond-
ing [N–H···O] and [C–H···O] interactions.[13] The 1H NMR spectrum of L
shows two singlets at 9.09 and 12.35 ppm corresponding to the reso-
nances of HC = N and NH protons. The pyridine and aromatic protons
are resonated in the regions of 8.27–8.36 (integrating for three pro-
tons) and 7.05–7.96 ppm (integrating for eight protons), respectively.
A sharp singlet at 3.85 ppm is attributed to six protons of methoxy
groups. 13C NMR spectrum has shown four signals for the C = O,
Ar–O, HC = N and OCH3 carbons at 162.14, 160.65, 151.07 and
58.45 ppm respectively. The δ (ppm) at 147.90, 142.59, 134.69,
128.38, 128.15, 124.83, 123.59 and 114.71 are due the carbon atoms
of pyridine and aromatic rings.
2.5
Computational method
All DFT calculations were performed with the Spartan 16 software
package at the B3LYP/6–31G(d) level of theory which uses Becke's
three‐parameter hybrid function B3[27,28] with the non‐local correla-
tion of Lee‐Yang‐Parr LYP[29] and the polarized 6–31G(d)[30,31] basis
set in the gas phase. Geometry of the L ligand and the cationic
lanthanide complexes [LnL(NO3)2]1+ were fully optimized without any
specific geometrical constraints. The absence of imaginary frequencies
in the vibrational analysis indicated a minimal energy structure.[27–31]
|
3
RESULTS AND DISCUSSION
Characterization of the L ligand
|
3.1
|
3.2
X‐ray structure of L ligand
The L ligand was synthesized in high yield (91%) by a simple
condensation reaction of 2‐methoxybenzaldehyde with 2,6‐
bis(hydrazinocarbonyl)pyridine in ethanol (Scheme 1).
The molecular structure of L is shown in Figure 2, crystal data are pre-
sented in Table 1, and selected bond lengths and angles are listed in
Table 2. L has C15/C19/N20/N21, C13/C12/N11/N10, C1/C9/
N10/N11 and C23/C22/N21/N20 torsion angles of 175.4(2)°,
174.2(2)°, 177.3(2)° and 178.0(2)°, respectively. The dihedral angles
of the pyridine ring plane with the aromatic planes II [O30, C24, C23,
C28, C27, C26 and C25] as well as III [O7, C2, C1, C6, C5, C4 and
C3] are 51.5(2)° and 38.3(2)°, respectively. Planes II and III (aromatic
rings) are almost orthogonal with a dihedral angle 84.8(2)°. The C = N
bond distance [1.283(2) Ǻ] is in good agreement with that reported
The CHN analysis is in agreement with the formula of the L ligand.
The L ligand was soluble in DMSO, DMF and chloroform. The IR spec-
trum of L displayed in Figure 1 shows several bands at 3170, 1677,
1659, 1600, 1572, 1248, 1159 and 1080 cm−1 assigned to N–H,
C = O, C = N (azomethine), C = C, C = N (pyridine), C–O (phenolic), C–
N and N–N groups, respectively, Figure 1(i, a). In addition, a group of
medium and weak absorption bands in the region 3100–2800 cm−1
due to C–H stretching vibrations of aromatic and methoxy groups are
SCHEME 1 Synthesis route for 2,6‐bis[(3‐
methoxybenzylidene)hydrazinocarbonyl]
pyridine Schiff base ligand, L
FIGURE 1 Infrared spectra for (a) l and (b) eu–l complex in the range of: 1800–1000 (i) and 1000–400 (inset 4000–2500) (ii)

4
TAHA ET AL.
FIGURE 2 Molecular structure of L with
anisotropic displacement ellipsoids drawn at
the 50% probability level
TABLE 1 Crystallographic data for the L ligand
TABLE 2 Selected bond lengths (Å) and angles (°) for the L
Empirical formula
C23H21N5O4
Bond lengths
Bond angles
Formula mass
431.45
100(2)
Triclinic
P–1
O18–C12
O29–C19
N10–C9
1.227 (2)
1.225 (2)
1.283 (2)
1.283 (2)
1.381 (2)
1.380 (2)
1.351 (2)
1.353 (2)
O18–C12–C13
O29–C19–C15
O18–C12–N11
O29–C19–N20
C9–N10–N11
C22–N21–N20
121.01 (12)
120.68 (12)
124.88 (12)
124.68 (12)
114.31 (11)
115.95 (11)
Temperature (K)
Crystal system
Space group
N21–C22
N10–N11
N20–N21
N11–C12
N20–C19
Unit cell dimension (Å, °)
a
7.9385 (1)
b
11.5587 (2)
12.5666 (2)
90.532 (1)
c
α
β
95.347 (1)
TABLE 3 Hydrogen bond geometry (Å, °)
γ
107.948 (1)
1091.33 (3), 2
1.313
D–H···A
D–H
H···A
D···A
D–H···A
Volume, Å3, Z
Calculated density, (mg/ m3)
μ (mm−1
N11–H11···O29i
N20–H20···O18ii
C9–H9···O29i
0.88
0.88
0.95
0.95
2.07
2.00
2.32
2.55
2.895 (1)
2.855 (1)
3.141 (2)
3.324 (2)
154.6
164.4
144.9
139.3
)
0.76
F(000)
Crystal size (mm3)
452
C22–H22···O18ii
0.20 × 0.10 × 0.10
3.5–66.1
Range for data collection (°)
Index ranges (h, k, l)
[a] Symmetry codes: (i) − x, −y + 1, −z + 1; (ii) − x + 1, −y + 1, −z + 1
−9/9, −13/13, −14/14
24225
Reflections collected
Independent reflections
Absorption correction
Refinement method
3665 (Rint = 0.069)
Multi‐scan
Full‐matrix least‐squares on F2
3665/0/291
1.06
as well between C9/C22–H and oxygen atoms (Table 3). The supramo-
lecular dimer structures are connected via two types of intermolecular
N–H···O and C–H···O [N20–H20···O18 2.00 Ǻ; N20–H20–O18
164.4°, C22–H22···O18 2.55 Ǻ; C22–H22–O18 139.3°] hydrogen
bonding interactions. The dimeric supramolecules are then connected
to the next one through the carbonyl oxygen O29 atom via also N–
H···O and C–H···O hydrogen bonding interactions [N11–H11···O29
2.07 Ǻ; N11–H11–O29 154.6°, C9–H9···O9 2.32 Ǻ; C9–H9–O9
144.9°]. These hydrogen bonding interactions result in supramolecular
arrays of endless chains of dimeric units running along the a‐axis
(Figure 3).
Data/ restraints/ parameters
Goodness‐of‐fit on F2
Final R indices [I > 2σ (I)]
Final R (all data)
R
1 = 0.043, wR2 = 0.112
1 = 0.044, wR2 = 0.110
R
Δρmax/min (e·Å −3
)
0.45/−0.45
in the literature.[32] Also the carbonyl C19–O29 and C12–O18 reveals
one‐dimensional network. The building unit of this network composed
of supramolecular dimer structures assembles through short hydrogen
bonding interactions between the amide H and the carbonyl oxygens
O hydrogen bonding interactions running along the a‐axis. Hydro-
gen bonds are shown as dashed red lines with some labeled atoms
(other C–H atoms were omitted for clarity).

TAHA ET AL.
5
FIGURE 3 Partial crystal packing diagram,
viewed down the b crystallographic axis,
showing N–H•••O and CH•••
|
evident that H2O is present in the structure of these complexes.
The TGA thermograms showed a gradual weight loss step starting
from 300 K to 401 K with a major loss at 342 K for Sm–L and from
303 K to 371 K with a major loss at 342 K for Eu–L. The observed
losses of 4.78% (calcd. 4.47%) and 2.28% (calcd. 2.03%) may be
attributable to the giving away of two and one uncoordinated H2O
molecules from Sm–L and Eu–L complexes, respectively. Further
weight losses were observed in the range of 401–580 K and 371–
574 K with peak temperatures at 539 K and 537 K for Sm–L and
Eu–L complexes, respectively. These can be apparently associated
with the release of three nitrates groups from Sm (weight loss
22.4%, calcd 23.1%) and Eu (weight loss 25.65%, calcd. 24.61%)
structures. Furthermore, the thermograms showed a loss starting
from 580 K to 873 K with a couple of major losses at 614 K
(11.43%,) and 745 K (20.31%,) for Sm–L and from 574 K to 861 K
with major losses at 605 K (10.58%,) and 727 K (20.88%,) for Eu–L.
The latter stage seems to be the final step of thermolysis and may
be due to the collapse of the L ligand. Thereafter, weight decreased
gradually with a rate of about 5% per 100 K and reduced at
1100 K to 20.1% (calc. 21.61) and 22.89% (calcd. 22.35%) of the ini-
tial values for Sm–L and Eu–L complexes, respectively. The residual
weight of the complexes basically agrees with the calculated values
and may correspond to the formation of Sm2O3 and Eu2O3.[34]
3.3
Properties of the complexes
Direct mixing of L with the nitrates of Ln(III) ions at room temperature
in a molar ratio 1:1, formed complexes with the general formula
LnL(NO3)3.XH2O as is evident from the elemental analysis. The com-
plexes were stable, non‐hygroscopic and yellow in color (except for
La, which was white). They were soluble in DMF and DMSO, insoluble
in H2O, CHCl3, THF, CH3CN and C6H6. In DMF, Ln‐L molar conductiv-
ity (Λm) values lie in the range of 69.7–79.8 Ω−1 cm−1 mol−1 and are
consistent with that reported for 1:1 electrolytes.[33] The mass spec-
trum of the Sm–L complex showed peaks at 707.24, 645.44 and
586.52 m/z that are attributed to Sm mono‐ligand species
[SmL(NO3)2]+, [SmL(NO3)]+2 and [SmL]+3, respectively. In addition, sev-
eral atomic ions were presented in the low mass range of the spectrum,
−
including Sm (m/z = 151.3) and NO3 (m/z = 62.47). Mass spectra of
other Ln–L complexes showed fragmentation patterns qualitatively
similar to each other and similar to the Sm–L complex, exhibiting a
mono‐nuclear peak [LnL(NO3)2]+ (Table 4).
Thermograms of the free L revealed a remarkable single‐step
mass loss centered at 624 K and corresponding to complete degrada-
tion of L (Figure 4). Thermograms of the Ln–L complexes were
approximately similar, indicating isostructural complexes. As general
examples, Sm–L and Eu–L thermograms are shown in Figure 4. It is
TABLE 4 Yield, elemental analytical and molar conductance (Λm) data of the L ligand and its complexes
Found (calculated)%
Complex
Abbreviation
Yield (%)
C
H
N
Ln
Λma (Ω−1 cm−1 mol−1)
C23H21N5O4
L
91
67
87
95
83
92
89
94
85
91
64.17 (64.03)
35.28 (34.86)
35.23 (34.77)
35.76 (35.43)
34.12 (34.37)
34.68 (35.08)
34.73 (34.85)
33.73 (34.00)
33.63 (33.85)
34.29 (34.41)
4.85 (4.91)
3.20 (3.18)
3.13 (3.17)
2.79 (2.97)
3.15 (3.13)
3.30 (2.94)
2.71 (2.92)
3.55 (3.16)
3.43 (3.09)
3.58 (2.89)
16.07 (16.23)
13.79 (14.14)
14.15 (14.11)
14.62 (14.37)
13.48 (13.94)
14.48 (14.23)
14.41 (14.14)
13.37 (13.79)
13.29 (13.73)
13.70 (13.96)
–
0.89
72.3
[LaL(NO3)2]NO3.2H2O
[PrL(NO3)2]NO3.2H2O
[NdL(NO3)2]NO3.H2O
[SmL(NO3)2]NO3.2H2O
[EuL(NO3)2]NO3.H2O
[GdL(NO3)2]NO3.H2O
[TbL(NO3)2]NO3.2H2O
[DyL(NO3)2]NO3.2H2O
[ErL(NO3)2]NO3.H2O
La–L
Pr–L
Nd–L
Sm–L
Eu–L
Gd–L
Tb–L
Dy–L
Er–L
17.0 (17.5)
19.1 (18.7)
18.0 (18.5)
18.2 (18.7)
19.3 (19.3)
19.7 (19.8)
19.8 (19.6)
19.6 (19.9)
20.6 (20.8)
69.7
76.7
74.0
77.8
76.0
79.8
74.8
73.4
[a] 1.0 × 10−3 M at 25°C in DMF.

6
TAHA ET AL.
diformyl‐4‐chlorophenol and hydrazides.[35] The coordination in the
keto form has been further supported by the appearance of the C–N
and N–N bands at 1164 and 1079 cm−1 nearly in the same positions
as in the free L ligand.[36] The observed large shift in the azomethine
υ(C = N) at 1545 cm−1 is again in agreement with Tamboura et al.
report and is evidence of C = N nitrogen atom coordination with Eu(III)
ion, Figure 1(i).[35] Conversely, the υ(C = N) mode of the pyridine ring at
1572 cm−1 and its other two vibrations at 1465 and 1487 cm−1 are
nearly in the same positions as in the free L, Figure 1(i).[14] This result
confirmed that the pyridine N atom is not involved in the coordination.
Further evidence for the coordination through both N and O atoms
comes from the appearance of two new non‐ligand bands at 430 and
477 cm−1, which were tentatively assigned to ν(Eu–O) and ν(Eu–N),
respectively, Figure 1(ii).[37] Moreover, the presence of a broad band
extending from 3700 to 3250 cm−1 with two maxima at 3420 and
3265 cm−1 could mean that uncoordinated water molecules are pres-
ent in the complex, as is evident from the TGA study. The water mole-
cules may be hydrogen bonded with each other or with the oxygen of
the nitrates.[17,35] The Eu–L spectrum also exhibited nitrate absorption
bands at 1495, 1322, 1295, 1021 and 818 cm−1 (Figure 1). The sepa-
ration between the 1495 and 1295 cm−1 bands, which were assigned
to ν5 and ν2 stretching modes of the coordinated nitrate groups (C2v),
is 200 cm−1 and consistent with the general trend observed for nitrate
ions coordinated bi‐dentately to metals in complexes.[38] The absorp-
tion band that appeared at 1385 cm−1 was attributed to D3h of free
ionic nitrate[38] (Figure 1i).
FIGURE
4
Thermogravimetric analysis (TGA) and differential
thermogravimetric analysis (DTGA) of: L, Eu–L and Sm–L complexes
Complexation affects the NMR spectrum by making the HC = N,
O–CH3 and NH proton resonances non‐equivalent. This effect shows
up most clearly in the 1H NMR spectrum of the La–L complex in which
the resonances appear as pairs, within about 0.2 ppm of one
another.[13] The chemical shift for the N = CH proton is observed as
a pair at δ 9.04 and 8.96 ppm. The signals at δ 12.46 and 12.28 ppm
corresponded to NH protons. The signals in the δ 7.05–8.35 ppm
region integrating for 11 protons were assigned to the protons of the
aromatic and pyridine rings. The two singlets at δ 3.85 and 3.94 ppm
integrating for 6 H correspond to the protons of the –CH3OH groups.
The 13C NMR spectrum of La–L exhibits 12 resonances as in the free L
ligand. The four signals at δ 159.44, 158.95, 148.34 and 56.3 ppm
were due to C atoms of C = O, C–O, CH = N and OCH3 groups, respec-
tively. The eight resonances located between δ 145.00–112.02 ppm
were attributable to the aromatic and pyridine carbon atoms.
As all attempts performed to get a single crystal suitable for X‐ray
diffraction were unsuccessful for any one of the isolated Ln–L com-
plexes, we have proposed a mono‐nuclear structure based on the ana-
lytical and spectral studies discussed above. The Ln(III) in the proposed
structure was coordinated with two C = O oxygen atoms and two
C = N nitrogen atoms of L. The coordination sphere of Ln(III) was com-
pleted by two bi‐dentate nitrate ions for a total coordination number
of eight (Figure 5).
The FT‐IR spectral data of L and its Ln–L complexes are listed in
Table 5. The data of Ln–L complexes were almost similar, reflecting
the similarity in the coordination environment around Ln ions in the
complexes. Comparing the IR spectrum of Eu–L complex with the free
L ligand, we observed the presence of an absorption band attributable
to υ(N–H) at 3176 cm−1 and a shift of the υ(C = O) band to a lower fre-
quency of 1645 cm−1 (Figure 1i). This may indicate that the coordina-
tion to Eu(III) ion occurs through the C = O oxygen atoms in keto
form. Tamboura et al. report a similar observation for La and Er
complexes with polyhydrazone Schiff base ligands derived from 2,6‐
|
3.4
Computational
DFT calculations were carried out to investigate the structure, energy
and properties of the title L ligand and its cationic monoligated lantha-
nide species [LnL(NO3)2]+ at the B3LYP/6–31G(d) level of theory. The
TABLE 5 Infrared spectral data of the L ligand and its Ln–L complexes
−
ν(NO3
)
ν(M–N)
ν(M–O)
ν1– ν4
Complex
ν(N–H)
ν(C = O)
ν(C = N)
ν(C–N)
ν(C = N)pyr
ν(N–N)
ν1
ν4
ν0
v(M‐N)
v(M‐O)
azo
L
3170
3162
3165
3164
3164
3176
3169
3174
3172
3163
1677
1645
1642
1644
1645
1645
1642
1641
1643
1677
1659
1545
1549
1547
1543
1546
1547
1545
1549
1546
1159
1166
1164
1162
1162
1164
1165
1163
1161
1159
1572
1573
1572
1573
1575
1574
1571
1573
1572
1573
1078
1080
1074
1079
1075
1079
1078
1076
1079
1075
–
–
–
–
–
–
La–L
Pr–L
Nd–L
Sm–L
Eu–L
Gd–L
Tb–L
Dy–L
Er–L
1495
1489
1492
1498
1495
1495
1493
1490
1493
1284
1287
1888
1285
1295
1289
1288
1285
1887
211
202
204
213
200
206
205
205
206
1384
1386
1384
1385
1385
1384
1385
1384
1384
476
477
475
477
477
475
479
477
477
425
429
424
428
430
428
431
427
426

TAHA ET AL.
7
FIGURE 5 The optimized ground state
geometry for the [LnL(NO3)2]1+ complexes at
the B3LYP/631G(d) level of theory showing th
atom‐numbering scheme around the
lanthanide ion
optimized ground state geometries of L and its Ln(III) complexes at the
B3LYP/6–31G(d) level of theory are shown in Figures 6 and 5, respec-
tively. Selected structural parameters for the optimized L ligand along
with the experimentally determined values are listed in Table 2. Con-
sidering the noticeably small percent differences Δ% (0.06–1.8%)
between the observed and calculated structural parameters, it can be
appropriately noted that there is a good agreement between the X‐
ray crystal structure and the theoretically predicted geometry.
azomethine, C = C of phenyl, C = N of pyridine, phenolic C–O, C–N
and N–N groups, respectively. The medium bands observed in the
3100–2800 cm−1 region in the IR spectrum and assigned to C–H
stretching vibrations of aromatic and methoxy groups were theoreti-
cally predicted in the 3000–3229 cm−1 range. The computed FT‐IR
vibrational frequency analysis of Ln–L complexes supported the coor-
dination of Ln to two C = O oxygen atoms of the L ligand. This result
was well predicted by the shift in the υ(C = O) to a lower frequency cal-
culated in the range 1683–1691 cm−1 for complexes.
The outcome for the geometrical optimization supports
octadentate coordination of the L and nitro groups to the lanthanide
ion. This proposal is feasible through four binding sites provided by
the two nitro groups along with four coordination sites for the L ligand
depicted in two Ln–O(carbonyl) and two Ln–N(imine) bonds. Selected
calculated bond lengths (Å) for the [LnL(NO3)2]1+ complexes at the
B3LYP/631G(d) level of theory are listed in Table 6. The computed
FT‐IR vibrational analysis of the L ligand revealed several bands at
3483, 1760, 1677, 1647 1620, 1288, 1139 and 1110 cm−1. These
bands are associated with N–H and C = O of amide, C = N of
Additionally, the calculated υ(C = N) frequency of the pyridine ring
at 1620 cm−1 in the free ligand was not much changed upon complex-
ation (1616–1614 cm−1). This result matches well with the experimen-
tal IR data and confirms that the pyridine N atom was not involved in
the coordination. The two non‐ligand absorption bands attributed to
υ(Ln–N) and υ(Ln–O) in the calculated IR spectrum appeared at 489–
491 cm−1 and 421–427 cm−1, respectively. This result further supports
the coordination of Ln(III) to L through the N and O atoms. Moreover,
the calculated absorption bands in the 1674–1682 cm−1, 1041–
FIGURE 6 The optimized ground state
geometry for the L ligand at the B3LYP/
631G(d) level of theory

8
TAHA ET AL.
TABLE 6 Calculated bond lengths (Å) for the [SmL(NO3)2]1+ complex
at the B3LYP/631G(d) level of theory
Bond
Length (Å)
Bond
Length (Å)
Sm–O1(nitro)
Sm–O2(nitro)
Sm–O3(nitro)
Sm–O4(nitro)
2.347
2.368
2.368
2.347
Sm–O5(carbonyl)
Sm–O6(carbonyl)
Sm–N1(imine)
2.653
2.653
2.751
2.751
Sm–N2(imine)
1044 cm−1, 248–251 cm−1 and 1275–1304 cm−1 ranges are assigned
to the stretching modes of the two coordinated nitrate groups, which
is consistent with the obtained IR data for the complexes.
|
FIGURE 8 UV–Vis absorption spectra of L ligand (top) and Sm–L
complex (bottom) attained theoretically at the B3LYP/6–31G() level
of theory
3.5
Photophysical properties
The absorption spectrum of L in DMF exhibited three main absorption
bands at 328 nm (ε = 2.42 × 105 M−1 cm−1), 306 nm (ε = 1.50 × 105 M−1
cm−1) and 284 nm (ε = 1.47 × 105 M−1 cm−1), Figure 7. These bands may
be attributed to n ! π* and π ! π* intra‐ligand transitions. The molar
absorptivity, ε, increased significantly upon complexation with none
or minimum shifts in the maximum wavelengths of the two higher‐
energy absorption bands. The low‐energy absorption band was red
shifted by 10–12 nm (Figure 7). These changes implied coordination
of L‐to‐Ln(III) ions. All complexes in DMF displayed almost identical
absorption bands with a different increase in ε depending on the kind
of Ln(III) ion. The electronic spectrum of L was attained theoretically
at the B3LYP/6–31G(d) level of theory predicting a main absorption
band at 333 nm and is in good correlation with the experimentally
determined λmax from the ligand spectrum. Additionally, all Ln(III)–L
complexes exhibited similar calculated spectra with a main absorption
band around 343 nm with minimal shifts in the absorption band across
all complexes (Figure 8).
FIGURE
9
Emission spectrum for Tb–L complex, λex = 281 nm
recorded in DMF at room temperature. Inset refers to the
emissionspectra of (a) L, (b) Sm–L, (c) Dy–L, (d) Tb–L and (e) Eu–L
complexes
Upon UV irradiation (λex = 277 nm), L in DMF emitted a strong
luminescence with λmax = 436 nm assigned to π ! π* intra‐ligand tran-
sitions, Figure 9(inset). Excitation of Tb–L and Eu–L complexes in DMF
at 283 and 281 nm revealed, in addition to the expected transitions of
Tb(III) and Eu(III) ions, very weak broad emission bands peaking at 448
and 457 nm, respectively, due to the ligand centered luminance
(Figures 9 and 10. The emission spectrum of Eu–L complex showed
five peaks at 582, 597, (608, 619 and 626), 663, and 698 nm assigned
to the transition of electrons from 5D0 to 7FJ (J = 0, 1, 2, 3, 4) (Figure 9).
It is worth noting that the number of Stark components observed for
the electric‐dipole transition 5D0
nent intensity is the highest.[39] In addition, the high intensity ratio of
5D0 7F2/5D0 7F1 and the presence of a single line for non‐
!
7F2 is three and the first compo-
!
!
FIGURE 7 UV–Vis absorption spectra of (a) the L ligand and (b) the
Sm–L complex recorded in DMF at room temperature
FIGURE 10 Emission spectrum for Eu–L complex, λex = 283 nm
recorded in DMF at room temperature

TAHA ET AL.
9
FIGURE 11 The lifetime decay curves for (a) Tb–L, and (b) Eu–L complexes in DMF solution
degenerated 5D0
!
7F2 transition indicated that Ln(III) ions occupied
at 582 nm the decay is also multi exponantial fit, Figure 11(b), with
the lifetimes τ1 0.48 μsec (22.3%), τ2 2.27 μs (36.5%),
sites with low symmetry and with no inversion center.[40] In the emis-
sion spectrum of theTb–L complex, the following 5D4 ! FJ transitions
could be detected: J = 6 (491 nm), 5 (547 nm), 4 (596 nm), 3 (623 nm)
=
=
7
τ3 = 12.856 μs (41.2%) (χ2 = 1.15) and τ = 11.2 μsec. Luminescence
quantum yields equal to 9.5% and 7.3% were determined for Tb–L
and Eu–L complexes, respectively. The relatively low yields obtained
may be due to non‐radiative deactivation pathways through har-
monics of O–H vibrations of H2O molecules present in the coordina-
tion sphere. The higher quantum yield of the Tb–L complex compared
with the Eu–L complex may be due to the effective match between
the triplet state of L and the emitting level of Tb(III).
and 0 (678 nm) (Figure 9). The 5D4
!
7F5 had the highest intensity.
In contrast, no emission lnes characteristic of lanthanide ions could
be detected in the emission spectra of Sm–L and Dy–L complexes in
DMF. This finding may be interpreted as a proof of disabling any
ligand‐to‐metal energy‐transfer process that is supported by the
appearance of large broad bands peaking at 466 and 465 nm for
Sm–L and Dy–L complexes, respectively, and attributed to π ! π*
intra‐ligand transitions (Figure 9, inset). These data clearly demon-
strated that the L‐to‐Ln energy‐transfer process takes place more effi-
ciently in the Eu–L and Tb–L complexes. This finding is most likely
related to the energy gap between the lowest triplet energy level of
L and the resonant energy level of Ln(III). The gap may be optimum
for Eu–L and Tb–L complexes and not for Dy–L and Sm–L complexes
and lead to less efficient energy transfer.[41]
|
3.6
Antioxidant activity
The DPH• assay was used to investigate the ability of the L ligand
and Ln–L complexes to quench the stable DPPH• radical. DPPH•
can be converted by antioxidant to 2,2‐diphenyl‐1‐picryl hydrazine
either by radical quenching via hydrogen atom transfer (HAT) or by
direct reduction through electron transfer (EA) mechanisms.[43]
Figure 12 clearly shows that Ln–L complexes are more effective than
L in quenching DPPH. This enhancement in the activity can be attrib-
uted to the formation of lanthanide–ligand complexes that enhances
the HAT process.[44] At the concentration of 312.5 μM, the Dy–L
complex with activity of 34% was the strongest quencher and La–L
complex with activity of 11% was the poorest effective quencher
of all complexes. Notably, the antioxidant potential of the complexes
could be seen even at lower concentrations (250, 187.5, 125, and
62.5 μM). The effectiveness of the Dy–L complex over other com-
plexes showed the significance of metal ion types in enhancing the
antioxidant capability.
Luminescence decay and fit curves for the 5D4 level of Tb
obtained by monitoring the emission at 547 nm attributed to the
hypersensitive 5D4
!
7F5 transition with exciting at 281 nm are
shown in Figure 11(a). The decay curve is best fit to multiple
exponantial and the experimental lifetimes are obtained to be
τ1 = 1.221 μsec (15.6%), τ2 = 5.341 μsec (38.8%), τ3 = 18.45 μsec
(45.6%) (χ2 = 1.019) and τ = 14.17 μsec [τ = (A1τ21 + A2τ2 + A3τ2)/
2
3
(A1τ1 + A2τ2 + A3τ3)].[19,42] When the 5D0 state of Eu(III) is exited
|
4
CONCLUSION
An acyl‐dihydazone pyridine ligand was synthesized and characterized
by single X‐ray diffraction. The ligand was explored for the first time as
a coordinating ligand and sensitizer for Ln(III) luminescence. Ln–L com-
plexes were prepared successfully, and their structures were deter-
mined by the means of elemental analysis, molar conductance, UV,
MS and FT‐IR spectra as well as TGA studies. Evidence was presented
for the existence of mono‐nuclear complexes with a 1:1 ligand‐to‐
metal ion stoichiometry. The optimized ground state geometries and
FT‐IR spectral data for L and its [LnL(NO3)2]+ complexes were reported
FIGURE 12 DPPH• radical scavenging activity of
complexes measured in DMF
L and Ln–L

10
TAHA ET AL.
[19] Z. A. Taha, A. M. Ajlouni, A. K. Hijazi, N. A. Al‐Rawashdeh, K. A. Al‐
Hassan, Y. A. Al‐Haj, M. A. Ebqaai, A. Y. Altalafha, J. Lumin. 2015,
161, 229.
using DFT calculations at the B3LYP/6–31G(d) level of theory and the
obtained results were in good agreement with the experimental data.
The luminescence properties of L and Ln–L indicated that only Eu–L
and Tb–L complexes showed corresponding metal luminescence, while
the remaining complexes showed only the luminescence of the ligand.
The scavenging activities of Ln–L complexes on the DPPH• were more
effective than that of L, and the Dy–L complex was the most effective.
[20] V. G. Shterev, V. B. Delchev, Spectrochim. Acta A 2014, 125, 384.
[21] A. Monguzzi, A. Milani, A. Mech, L. Brambilla, R. Tubino, C. Castellano,
F. Demartin, F. Meinardi, C. Castiglioni, Synthetic Met. 2012, 161,
2693.
[22] X. Y. Chen, S. Z. Zhan, Q. J. Meng, Transition Met. Chem. 1996, 21, 345.
[23] A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, 3rd ed.,
ELBS 1961.
ACKNOWLEDGEMENTS
[24] G. M. Sheldrick, SADABS, University of Göttingen, Göttingen,
This work was supported financially by the Deanship of Research
(grant no. 16/2014), Jordan University of Science and Technology,
Jordan. We are very grateful to Dr Naba Kamal (NIT Meghalaya) for
X‐ray measurement and structural analysis. We are also very grateful
to Prof. Khader A. Al‐Hassan (Yarmouk University) for luminescence
measurements.
Germany, 1996.
[25] G. M. Sheldrick, Acta Crystallogr. A 2008, 64, 112.
[26] G. M. Sheldrick, SHELXL2013. University of Göttingen, Göttingen,
Germany, 2013.
[27] A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
[28] A. D. Becke, J. Chem. Phys. 1996, 104, 1040.
[29] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B. 1988, 37, 785.
[30] G. A. Petersson, A. Bennett, T. G. Tensfeldt, M. A. Al‐Laham, W. A.
Shirley, J. A. Mantzaris, J. Chem. Phys. 1988, 89, 2193.
REFERENCES
[1] J.‐C. G. Bünzli, A.‐S. Chauvin, in Handbook on the Physics and Chemistry
of Rare Earths, (Eds: J.‐C. G. Bünzli, V. K. Pecharsky) Vol. 44, Ch. 261,
Elsevier Science, B.V., Amsterdam 2014 169.
[31] G. A. Petersson, T. G. Tensfeldt, J. A. Montgomery Jr., J. Chem. Phys.
1991, 94, 6091.
[32] A. K. Hijazi, Q. Abu‐Salem, A. Ajlouni, Z. A. Taha, Z. Kristallogr. NCS
2016, 231, 129.
[2] C. Huang, Rare Earth Coordination Chemistry, Fundamentals and Appli-
cations, John Wiley & Sons (Asia), Singapore 2010.
[33] W. J. Geary, Coord. Chem. Rev. 1971, 7, 81.
[3] J.‐C. G. Bünzli, J. Coord, Chem. 2014, 67(23–24), 3706.
[34] Z. A. Taha, A. M. Ajlouni, J. Al‐Mustafa, Chemical Papers 2013, 67, 194.
[4] W. Al Momani, Z. A. Taha, A. M. Ajlouni, Q. Shaqra, M. Zouby, Asian
Pac. J. Trop. Biomed. 2013, 3, 367.
[35] F. B. Tamboura, O. Diouf, A. H. Barry, M. Gaye, A. S. Sall, Polyhedron
2012, 43, 97.
[5] V. Chandrasekhar, S. Das, A. Dey, S. Hossain, J.‐P. Sutter, Inorg. Chem.
2013, 52, 11956.
[36] S. Riaz, I. U. Khan, M. Yar, M. Ashraf, T. Ur Rehman, A. Shaukat, S. B.
Jamal, V. C. M. Duarte, M. J. Alves, Bioorg. Chem. 2014, 57, 148.
[6] S. A. S. Ghozlan, M. A. Al‐Omar, A. E. E. Amr, K. A. Ali, A. A. Abd
El‐Wahab, J. Heterocyclic, Chem. 2011, 48, 1103.
[37] A. M. Ajlouni, Z. A. Taha, W. Al Momani, A. K. Hijazi, M. Ebqa'ai, Inorg.
Chim. Acta 2012, 388, 120.
[7] S. V. Bhandari, K. G. Bothara, M. K. Raut, A. A. Patil, A. P. Sarkate, V. J.
[38] A. M. Ajlouni, Z. A. Taha, K. A. Al‐Hassan, A. M. Abu Anzeh, J. Lumin.
2012, 132, 1357.
Mokale, Bioorgan. Med. Chem. 2008, 16, 1822.
[8] M. Maja, C. Milan, Z. S. Zec, Lett. Org. Chem. 2012, 9, 401.
[39] L. N. Puntus, J. Phys. Chem. B 2009, 113, 9265.
[40] J. Zhou, Coord. Chem. Rev. 2016, 315, 112.
[9] D. Kumar, S. Sundaree, E. O. Johnson, K. Shah, Bioorg. Med. Chem. Lett.
2009, 19, 4492.
[41] M. Latva, H. Takalo, V.‐M. Mukkala, C. Matachescu, J. C. Rodriguez‐
Ubis, J. Kankare, J. Lumin. 1997, 75, 149.
[10] X. Q. Wang, Z. Zhao, X. Liu, W. Li, J. Chem. Res. 2010, 34, 307.
[11] J. M. Figueiredo, C. A. Camara, E. G. Amarante, A. L. P. Miranda, F. M.
Santos, C. R. Rodrigues, C. A. M. Fraga, E. J. Barreiro, Bioorgan. Med.
Chem. 2000, 8, 2243.
[42] J. Zhang, W.‐X. Li, B.‐Y. Ao, S.‐Y. Feng, X.‐D. Xin, Spectrochim, Acta
Part A 2014, 118, 972.
[43] A. Jiménez, A. Selga, J. L. Torres, L. Julià, Org. Lett. 2004, 6, 4583.
[12] A. Bacchi, L. P. Battaglia, M. Carcelli, C. Pelizzi, G. Pelizzi, C. Solinas,
[44] Z. A. Taha, A. K. Hijazi, T. S. Ababneh, I. Mhaidat, A. M. Ajlouni, K. A.
Al‐Hassan, C. Mitzithra, S. Hamilakis, F. Danladi, A. Y. Altalafha,
J. Lumin. 2017, 181, 230.
M. A. Zoruddu, J. Chem, Soc. Dalton Trans. 1993, 5, 775.
[13] R. S. Vadavi, R. V. Shenoy, D. S. Badiger, K. B. Gudasi, L. G. Devi, M.
Nethaji, Spectrochim. Acta A 2011, 79, 348.
[14] K. B. Gudasi, R. V. Shenoy, R. S. Vadavi, M. S. Patil, S. A. Patil, J. Incl.
Phenom. Macr. 2006, 55, 93.
How to cite this article: Taha ZA, AbabnehTS, Hijazi AK, Abu‐
Salem Q, Ajlouni AM, Ebwany S. Synthesis, density functional
theory calculations and luminescence of lanthanide complexes
with 2,6‐bis[(3‐methoxybenzylidene)hydrazinocarbonyl] pyri-
dine Schiff base ligand. Luminescence. 2017;1–10. https://doi.
org/10.1002/bio.3375
[15] A. Adhikary, S. Goswami, J. A. Sheikh, S. Konar, Eur. J. Inorg. Chem.
2014, 963, 963.
[16] S. Qin, H. Huang, Z. Chen, F. Liang, Struct. Chem. 2011, 22, 559.
[17] S.‐N. Qin, H.‐M. Huang, Z.‐L. Chen, F.‐P. Liang, Transit. Met. Chem.
2011, 36, 653.
[18] Y. Bai, D. B. Dang, C. Y. Duan, Y. Song, Q. J. Meng, Inorg. Chem. 2005,
44, 5972.