Structural and photophysical properties of lanthanide complexes with N'- (2- methoxybenzylidene) - 2-pyridinecarbohydrazide Schiff base ligand: Catalyzed oxidation of anilines with hydrogen peroxide

Article abstract of DOI:10.1016/j.molstruc.2021.130451

Several lanthanide (III) complexes {Ln: La, 1; Sm, 2; Eu, 3; Gd, 4; Tb, 5; Dy, 6} with the Schiff base ligand, N'-?(2-?methoxybenzylidene)?-?2-pyridinecarbohydrazide, L, have been synthesized. These complexes having general formula LaL(NO₃)₃.xH₂O and LnL₂(NO₃)₃.xH₂O, and the ligand L have been characterized by elemental analysis, spectral analysis (IR, ¹H and ¹³C NMR), molar conductivity and thermogravimetric analysis. In the proposed structure of complex 1, La(III) ion is ten coordinate. Six coordination sites are satisfied by three bi-dentate nitrate ions, one site is occupied by H₂O and the remaining three positions are occupied by ONO belonging to one neutral L. In contrast the lanthanide ion in the proposed structures of complexes 2 - 6, is twelve coordinated by two three bi-dentate ONO L and three bi-dentate nitrate anions. The photophysical properties of L and 1 – 6 in DMF solution have been investigated and reported. Complexes 1 - 6 catalyze the oxidation of aniline 1a and its derivatives 1b - 1 g using H₂O₂ as an oxygen source at an ambient conditions. Aniline is selectively converted to nitroso-benzene in the presence of complexes 2 and 3. Complexes 4 – 6 gave mixtures of nitrozo- and azoxybenzenes. With complex 1 a mixture of nitroso-, azoxy- and azobenzenes were obtained. Among the substrates tested, the best result was obtained in the case of 1e with 78.6% conversion and 100% selectivity for the nitroso-benzene in the presence of 4 as a catalyst.

Full text of DOI:10.1016/j.molstruc.2021.130451

Journal of Molecular Structure 1238 (2021) 130451
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Structural and photophysical properties of lanthanide complexes with
N’- (2- methoxybenzylidene) - 2-pyridinecarbohydrazide Schiff base
ligand: Catalyzed oxidation of anilines with hydrogen peroxide
Ziyad A. Taha∗, Ahmed K. Hijazi
Department of Applied Chemical Sciences, Faculty of Arts and Sciences, Jordan University of Science and Technology, Irbid 22110, Jordan
a r t i c l e i n f o
a b s t r a c t
Article history:
Several lanthanide (III) complexes {Ln: La, 1; Sm, 2; Eu, 3; Gd, 4; Tb, 5; Dy, 6} with the Schiff base
ligand, N’- (2- methoxybenzylidene) - 2-pyridinecarbohydrazide, L, have been synthesized. These com-
plexes having general formula LaL(NO3)3.xH2O and LnL2(NO3)3.xH2O, and the ligand L have been char-
acterized by elemental analysis, spectral analysis (IR, 1H and 13 C NMR), molar conductivity and thermo-
gravimetric analysis. In the proposed structure of complex 1, La(III) ion is ten coordinate. Six coordination
sites are satisfied by three bi-dentate nitrate ions, one site is occupied by H2O and the remaining three
positions are occupied by ONO belonging to one neutral L. In contrast the lanthanide ion in the proposed
structures of complexes 2 - 6, is twelve coordinated by two three bi-dentate ONO L and three bi-dentate
nitrate anions. The photophysical properties of L and 1 – 6 in DMF solution have been investigated and
reported. Complexes 1 - 6 catalyze the oxidation of aniline 1a and its derivatives 1b - 1 g using H2O2
as an oxygen source at an ambient conditions. Aniline is selectively converted to nitroso-benzene in the
presence of complexes 2 and 3. Complexes 4 – 6 gave mixtures of nitrozo- and azoxybenzenes. With
complex 1 a mixture of nitroso-, azoxy- and azobenzenes were obtained. Among the substrates tested,
the best result was obtained in the case of 1e with 78.6% conversion and 100% selectivity for the nitroso-
benzene in the presence of 4 as a catalyst.
Received 30 December 2020
Revised 29 March 2021
Accepted 7 April 2021
Available online 13 April 2021
Keywords:
Lanthanide complex
Absorption spectra
Emission spectra
Catalytic activity
Oxidation of aniline
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
ing biological, clinical, analytical and industrial applications in ad-
dition to their important roles in homogeneous and heterogeneous
Over the past decade the interest in lanthanide complexes con-
taining Schiff base ligands has grown considerably on the basis
of their specific physical and chemical properties [1,2]. The po-
tential of lanthanide complexes, especially Sm(III), Tb(III), Eu(III)
and Dy(III) complexes, relies on their good optical properties such
as large Richardson shifts, narrow emission bandwidths, relatively
long-lived luminescence, high emission quantum yield and ligand-
dependent luminescence sensitization. Promising applications may
be envisaged, such as biomedical actuators and sensors, functional
materials such as lasers materials, structural and analytical lumi-
nescent probes, light emitting diodes and bioanalytical assays such
as immunoassays, nucleic acid hybridizations and enzyme assays
[3,4].
catalysis [5-7]. Acylhydrazones belong to Schiff base family with
keto-type structure (-O=C-N-N=C-) contain π-bonds and a variety
of heteroatoms such as nitrogen and oxygen are able to form stable
complexes with metal. Such complexes have been extensively stud-
ied because of their excellent biological activity. Acylhydrazones
have been reported to possess, antitumoral, antimicrobial, antitu-
bercular, anticonvulsant, analgesic, anti-inflammatory, antiplatelet,
antifungal, antiviral, antibacterial and antimalarial activities [8].
Catalytic oxidation of aniline to their corresponding oxygen
containing derivatives; azobenzene, azoxybenzene, nitrobenzene
and nitrosobenzene by H2O2 is very attractive from the view point
of industrial technology and synthetic purposes [9-11]. The prod-
uct compositions in the oxidation of aniline depends on the oxi-
dants, catalysts, and reaction conditions employed. To the best of
our knowledge, only few examples on the use of lanthanide com-
plexes for the oxidation of anilines have been reported to date
[10,12].
Schiff bases have been extensively studied with especial inter-
est in their utility in coordination chemistry. Schiff bases and their
metal complexes are attractive for a variety of applications, includ-
Therefore, in continuation to our interest in Schiff base lig-
ands and their lanthanide chelates, this work deals with the syn-
thesis and characterization of N’- (2- methoxybenzylidene) - 2-
∗
Corresponding author.
E-mail address: tahaz33@just.edu.jo (Z.A. Taha).
https://doi.org/10.1016/j.molstruc.2021.130451
0022-2860/© 2021 Elsevier B.V. All rights reserved.

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
pyridinecarbohydrazide Schiff base ligand L and it lanthanide (III)
complexes {La, 1; Sm, 2; Eu, 3; Gd, 4; Tb, 5; Dy, 6} [12-18]. The co-
ordination behavior of L towards lanthanide ions is investigated via
elemental analysis, spectroscopic analysis (UV-Visible, FTIR and 1H
and 13C NMR), molar conductance and thermal analysis. The pho-
tophysical properties of L and 1 - 6 complexes are investigated in
DMF solution. The catalytic oxidation of aniline and aniline deriva-
tives into their corresponding oxygen containing derivatives have
been investigated in the presence of complexes 1 - 6 using H2O2
as an oxygen source.
Edinburgh instrument model FS900SDT spectrophotometer. Spectra
were determined in the wavelength range 200 – 700 nm in DMF
(1.0 × 10−6 M) at room temperature in 1.0 cm quartz cuvettes. In
addition, its light source and detectors are 450 W xenon lamp and
R955 photomultiplier tube, respectively.
2.2. Syntheses of the Schiff base Ligand, L
The synthesis of L was accomplished using literature proce-
dure with some modifications [20]. A mixture of methyl picol-
inate [C7H7NO2], (4.11 g 30 mmol), hydrazine [N2H4], (2.88 g,
90 mmol) and ethanol (15 mL) were refluxed for overnight and
then cooled to room temperature. A white solid compound was
collected under vacuum. The product was recrystallized in ethanol
to obtain picolinohydrazide in a yield of 75%, Scheme 1. In pres-
ence of few drops of acetic acid, a 20 mL ethanolic solution of 2-
Methoxybenzaldehyde (0.496 g, 3.65 mmol) was added in a drop
wise manner to a 20 mL of ethanolic solution of picolinohydrazide
(0.50 g, 3.65 mmol). Then the reaction mixture was refluxed for
two hours. After completion of the reaction, the solvent volume
was reduced and cooled to room temperature. The resulting white
precipitate was filtered off, washed with cold ethanol and dried at
room temperature (yield 75%).
1H NMR [DMSO-d6, 400 MHz; δ (ppm)]: 12.21 (s, 1H, NH), 8.98
(s, 1H, CH=N), 8.70 (s, 1H, H3), 8.13–8.11 (d, 1H, H10), 8.07 (t, 1H,
H11), 7.89 (d, 1H, H5), 7.66 (t, 1H, H12), 7.45 (t, 1H, H3), 7.12 (d,
1H, H2), 7.03 (t, 1H, H4), 3.86 (s, 3H, O-CH3). 13C NMR [400 MHz,
DMSO-d6, δ (ppm)]: 160.2 (C2), 157.7 (C7‘), 149.5 (C1‘), 148.2 (C3‘),
144.5 (C8‘), 137.7 (C15‘), 131.4 (C4), 126.6 (C6), 125.5 (C4‘), 122.4
(C6‘), 122.3 (C5), 120.5 (C‘), 111.7 (C3), 55.5 (C9‘).
2. Experimental
2.1. Materials and physical measurements
All chemicals used in the present work were purchased from
Sigma Aldrich Chemical Company and used without any further
purification. The metal nitrates were used as hydrated salts. El-
emental analyses C, H, and N were performed by using an Euro
EA elemental analyzer. Lanthanide content was measured by titra-
tion with EDTA using xylenol orange as an indicator and acetic
acid as a buffer [19]. The thermal gravimetric analyses (TGA) were
recorded on a PCT-2A thermobalance analyzer. Samples weight was
in the 2.5–25 mg range, and the thermogravimetric analyzer was
scanning a temperature domain from 300 to 1200 K at a heating
rate of 10 K/min. Infrared spectra were recorded on a JASCO FT-
IR model 470 spectrometer. 1H and 13C NMR spectra were mea-
sured on Bruker Avance 400 MHz spectrometer using DMSO-d6 as
solvent with TMS as internal standard. Molar conductance of com-
plexes 1 - 6 (10−3 mol/L) was measured in DMF at room tempera-
ture using a WTW LF 318 conductivity meter equipped with WTW
Tetracon 325 conductivity cell. UV–Vis spectra were recorded us-
ing a Schimadzu UV–Vis spectrophotometer. Excitation and emis-
sion spectra with a resolution of 2 cm−1 were recorded on an
2.3. Preparation of lanthanide complexes(1 - 6)
A
solution of La(NO3)3•6H2O (1.0 mmol) in ethylacetate
(20 mL) was added to a stirred solution of L (2.0 mmol) in chloro-
Scheme 1. General scheme for the synthesis of L and its 1 - 6 complexes.
2

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
form (20 mL) at room temperature in small portions over 20 min-
utes, Scheme 1. The reaction mixture was kept for 3 hours under
stirring. Solid yellow precipitate was collected by filtration, washed
repeatedly with equivolume chloroform-ethylacetate mixture, and
dried at room temperature.
der the test conditions. No consumption of aniline was observed in
all cases, indicating the necessity of the catalyst for the process.
3. Results and Discussion
1H NMR [DMSO-d6, 400 MHz; δ (ppm)]: 12.18 (s, 1H, NH), 8.94
(s, 1H, CH=N), 8.68 (s, 1H, H3), 8.07 (d, 1H, H10), 8.02 (t, 1H, H11),
7.85 (d, 1H, H5), 7.64 (t, 1H, H12), 7.41 (t, 1H, H3), 7.09 (d, 1H, H2),
7.00 (t, 1H, H4), 3.83 (s, 3H, O-CH3). 13C NMR [400 MHz, DMSO-
d6, δ (ppm)]: 160.6 (C2), 158.0 (C7‘), 149.7 (C1‘), 148.6 (C3‘), 144.9
(C8‘), 138.0 (C5‘), 131.7 (C4), 127.0(C6), 125.8 (C4‘), 122.7 (C6‘),
122.5 (C5), 120.8 (C1), 112.0 (C3), 55.8 (C9‘),
The Schiff base ligand L, N’- (2- methoxybenzylidene) - 2-
pyridinecarbohydrazide, was obtained from the condensation of
picolinohydrazide with 2-Methoxy-benzaldehyde, Scheme 1. The
lanthanide complexes were prepared by the reaction of equimo-
lar amounts of L with different lanthanide salts Ln(NO3)3.xH2O in
equivolume of ethylacetate-chloroform solvent, Scheme 1. The iso-
lated yellow colored solid lanthanide complexes 1 – 6 {Ln: La, 1;
Sm, 2; Eu, 3; Gd, 4; Tb, 5; Dy, 6} are stable in air and insolu-
ble in water, ethanol but readily soluble in DMF and DMSO. The
complexes obtained have an elemental compositions correspond-
ing to LnL(NO3)3•3H2O for 1, Ln(L)2(NO3)3.H2O for 2, 3, 5 and
Ln(L)2(NO3)3•2H2O for 4, 6, Table 1. All complexes exhibit low mo-
lar conductance values in DMF solution, in the range of 9 to 17
ꢀ−1 cm−1 mol−1 which are comparable to that of the free ligand 7
ꢀ−1 cm−1 mol−1, . These values indicate that 1 - 6 complexes are
nonelectrolytes with no ionic nitrate groups, Table 1, [21]. All three
nitrates must be bonded to the lanthanide ion as mono-dentate,
bi-dentate, and/or bridging ligands.
Complexes
SmL2(NO3)3.H2O
(2),
EuL2(NO3)3.H2O
(3),
GdL2(NO3)3•2H2O (4), TbL2(NO3)3.H2O (5) and DyL2(NO3)3•2H2O
(6) were prepared in a similar manner to 1. Despite of exten-
sive efforts, single crystals of complexes were not obtained.
Scheme 1 shows the proposed structures of 1 - 6 complexes.
2.4. Oxidation of aniline
The catalytic reaction for oxidation of aniline was conducted in
a liquid phase using 0.015 mmol of complexes 1 - 6 as catalysts.
To a round-bottomed flask were added (0.745 g, 8.0 mmol) aniline,
propionitrile (7.0 ml), 6 catalyst (0.009 g, 0.015 mmol) and H2O2
(0.34 g, 30%(wt/wt), 3.0 mmol. The reaction mixture was stirred
magnetically for 1.0 hr at 25 °C. Sample was taken and injected
into GC–MS for analysis. GC–MS analyses were performed using
electronic impact ionization mode on a Varian Saturn 2000 ion
trap spectrometer, interfaced with a Varian GC CP-3800 apparatus.
Aniline conversion was quantified relative to a standard sample
which was prepared by mixing of (0.745 g, 8.0 mmol) aniline, pro-
pionitrile (7.0 ml), and H2O2 (0.34 g, 30%(wt/wt), 3.0 mmol) with-
out catalyst. Conversion of aniline was calculated by eq. (1) [12]:
3.1. Thermal studies
The thermal gravimetric analyses (TGA) were performed on
1 − 6 to examine their thermal stability and to quantify the wa-
ter content in their structures, Fig. 1, Table 3. The TGA curve of
L shows 100% weight loss in one step, and DTGA displays a sin-
gle peak at 347 °C. The TGA curve of 1 underwent a mass loss of
5.40% (calcd 5.68%) in the first step (30−110 °C), which is consis-
tent with the calculated loss of two uncoordinated H2O molecules.
The second mass loss occurred at 110−160 °C with a mass loss
of 2.37%, which is also agree well with the calculated loss of one
coordinated H2O molecule (2.84%). The TGA profiles of 2 - 6 are
observed to be very similar to each other, suggesting that they
are structurally similar to each other, similar physical behavior is
also observed, Fig. 1i. The first weight loss of 2.61% for 2, 2.60%
for 3 and 2.21% for 5 at an onset temperatures of 67 °C, 51 °C,
76 °C, corresponds to the removal of one equivalent of uncoordi-
nated H2O molecule per formula unit (calcd: 2.07% for 1, 2.08%
for 2, and 2.06% for 3) in the temperature range of 30−115 °C.
moles of aniline reacted
Conversion %
=
x 100
(1)
( )
initial moles of aniline used
Selectivity of products was determined using the following
eq. (2) [12]:
Selectivity =
Yield product
x 100
(2)
Conversion aniline
A similar procedure was followed to determine the catalytic ac-
tivity for 1 - 5 catalysts. Control experiments were conducted un-
Fig. 1. TG curves for L and its 1 - 6 complexes (i); TGA and DTG curves for L and complex 3. Heating rate 10 K/minute.
3

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
Table 1
Yield, elemental analytical and molar conductance (ꢁm) data of L and its 1–6 complexes.
ꢁm
a(ꢀ−1cm−1mol−1
)
Complex
Yield (%)
Found (Calculated) %
H
C
N
Ln
L
1
2
3
4
5
6
72
51
50
52
60
64
73
65.48 (65.87)
26.23 (26.51)
38.47 (38.88)
38.42 (38.81)
37.41(37.79)
38.99 (38.50)
37.45 (37.57)
5.52 (5.13)
3.40 (3.02)
3.68 (3.26)
3.71 (3.26)
3.47(3.40)
3.32 (3.23)
3.85 (3.38)
16.50 (16.46)
13.10 (13.25)
14.34 (14.57)
14.96 (14.55)
13.66 (14.17)
14.37 (14.43)
14.66 (14.08)
-
-
21.7 (21.9)
18.0 (17.4)
17.8 (17.5)
17.4 (17.7)
18.5 (18.2)
17.7 (18.2)
6.69
17.33
15.84
9.16
11.91
14.08
a
1.0 × 10−3 M at 25 °C in DMF.
absorptions of the nitrate ions, listed in Table 2, provide some
useful structural information. The nitrates may have ionic or even
more complicated structures (mono- and bi-dentate chelating ni-
trates, even both may be present) [26]. IR spectra do not generally
give a clear-cut distinction between mono- and bi-dentate chelat-
ing nitrates, but the lack of an absorption band at 1384 cm−1 con-
firmed the absence of free nitrate (ionic nitrates) [27]. The nitrate
bands appear as a broad absorption with a maximum at around
1485 cm−1 assigned as ν5, a sharper band due to ν1 at 1290
cm−1, and a sharp medium intensity peak from ν2 at 1035 cm−1
clearly identify that complex as containing only coordinated nitrate
groups, Fig. 2 [28]. IR spectra do not generally give a clear-cut dis-
tinction between mono- and bi-dentate chelating nitrates, but the
separation between the two strongest absorptions of the nitrate
stretching at 1485 and 1290 cm−1 is indicative of a strong interac-
tion of the oxygens of the nitrate with the lanthanide ion and typ-
ical of bi-dentate bonding [29]. More significantly the separations
between the two very weak combination bands observed at 1731
and 1774 cm−1 in complex 2 is 43 cm−1 which is in the 20–66
cm−1 range found for known bi-dentate nitrate complexes rather
than the 5–26 cm−1 range found for known mono-dentate nitrate
complexes [30], Fig. 3i. In compounds where crystals are unobtain-
able, these criterions are often used to determine the nitrate bind-
ing mode, especially in lanthanide nitrate complexes where the
higher coordination requirements of the metals favor bidentate lig-
ation of these anions. The coordination of L to Sm(III) ion results
in the birth of new bands in FT-IR spectrum Fig. 3ii. The peaks ob-
served at 546 and 413 cm−1 vibrations have been designated as
N→Sm and O→Sm stretching vibrations coming up with the coor-
dination of the ligand to Sm(III) ion [31]. The IR spectra of 2 - 6
show the expected bands with OH stretches between 3500−3300
cm−1 for complexes with lanthanide lattice water.
Fig. 2. FTIR spectra of L and complex 2.
For 4 and 6wt losses of 5.03% and 4.10% is observed at an on-
set temperature of 61.7 °C and 61.6 °C, respectively, which corre-
sponds to the removal of two uncoordinated H2O molecules (calcd:
4.04% for 4 and 4.02% for 6). The major mass loss for 2 - 6 oc-
curs at 193 °C (࢞m = 45.30 %), 208 °C (࢞m = 49.2%), 183 °C
(࢞m = 51.2%), 216 °C (࢞m = 45.7%) and 176 °C (࢞m = 46.7%),
respectively. The remaining framework is stable up to 392, 385,
410, 413, 470 and 419 °C for 1 - 6, respectively, beyond this tem-
perature, the framework decomposes completely. The observed fi-
nal mass residue could be ascribed to the residue of corresponding
Ln2O3, [22].
The IR spectra of 3 - 6 complexes are closely resemble that
of complex 2. However, the IR spectrum of complex 1 is similar
but not identical with those of 2 - 6 complexes. In complex 1,
the ργ (H2O) and ρω(H2O) bands at 853 and 642 cm−1, respec-
tively, are found, indicating that there is coordinated water [32].
The spectrum of 1 shows the expected bands with OH stretches
between 3500−3300 cm−1 for complexes with lanthanide coordi-
nated and lattice water.
3.2. Infrared spectroscopy
The IR spectrum of L exhibits the amide carbonyl stretching
frequency at 1665 cm−1, Fig. 2 [12]. In complex 2, this band has
shifted to 1630 cm−1 Fig. 2. This red-shift in the C-O frequency is
likely due to metal-CO back-donation evidencing that the Sm(III)
is coordinated through the carbonyl oxygen atom [23]. Bands at
1604 and 1568 cm−1 observed in the spectrum of the free L are
attributed to υaz(C=N) of azomethine group and υpy(C=N) of pyri-
dine ring, respectively [18,22,24], Table 2. In 2, the υaz(C=N) band
is shifted to lower frequency by 7 cm−1 but the υpy(C=N) band is
unchanged, which suggests that not pyridine nitrogen but azome-
thine nitrogen atom participate in coordination. The spectrum of
L shows two absorptions at 1256 and 1150 cm−1 arising from
the C-O stretching modes of phenoxy (O-Ph) and methoxy (O-
CH3) groups, respectively, [25]. In complex 2, these bands shifted
to 1240 and 1163 cm−1, respectively. This can be explained by a
strong interaction between the Sm(III) and the oxygen atom. The
Based on the above results, we suspect that the chemical struc-
ture of [LaL(NO3)3.H2O] (1) and [LnL2(NO3)3] {Ln: Sm (2), Gd (3),
Eu (4). Tb (5), Dy (6)} complexes may be as shown in Scheme 1. On
one hand La(III) in complex 1 coordinates the two oxygen atoms of
–OCH3 and C=O groups and the nitrogen atom of –C=N group of
−
L, three bi-dentate NO3 and the coordination sphere is completed
to a coordination number of ten by oxygen atom of one coordi-
nated H2O molecule. On the other hand Ln(III) in 2 - 6 complexes
is coordinated to the two oxygen atoms of –OCH3 and C=O groups
and the nitrogen atom of –C=N group of two molecules of L and
4

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
Table 2
Thermal analysis data for L and its 1–6 complexes.
Complexes
Stage
Onset temperature ( °C))
Mass loss %
Mass residual %
L
I
I
347
61.6
100
5.40
00.0
21.3
1
II
III
IV
I
124.0
174.8
392.1
67.1
2.37
31.49
39.46
2.61
2
3
4
5
6
24.45
20.59
29.31
28.44
31.5
II
III
I
193.2
384.9
51.2
45.30
27.64
2.6
II
III
I
208.5
409.8
61.7
49.2
27.61
5.03
II
III
I
183.4
413.2
75.6
51.22
14.44
2.21
II
III
I
216
45.72
23.63
4.10
469.7
61.6
II
III
175.6
418.8
46.66
17.74
Fig. 3. FTIR spectra of L and complex 2 (i) 1800 – 1700 cm−1 (ii) 500 – 400 cm−1
.
−
three bi-dentate NO3 thus giving rise to a coordination number
of twelve.
the absorption intensities for complexes 2 - 6 are approximately
two and three times higher than these for complex 1 and L, re-
spectively. Such a trend is may be due to the presence of two lig-
ands in each complex of 2 - 6 and one ligand in complex 1.
3.3. Photophysical properties
For the free L, excitations at 290 nm gives one broad emission
band centered on λmax = 492 nm and is tentatively assigned to
the π-π∗ transition, Fig. 5. Upon excitation at 292 nm, the emis-
sion spectra of 1 - 6 complexes display a broad emission bands
similar to that of the free L, Fig. 5. In the case of 1, the broad
band observed in the electronic spectrum of L is only slightly red-
shifted to λmax = 529 nm compared with the free L. In contrast,
in 2 - 6 complexes, the broad emission band is blue shifted to
λmax = 468 nm for 2, 462 nm for 3, 470 nm for 4, 460 nm for
5 and 452 nm for 6, Fig. 5. The emission bands of all complexes
is believed to occur from a predominantly ligand based π∗-π ex-
cited state. All the lanthanide(III) complexes showed an enhance-
ment in emission intensity than the ligand. The enhancement of
luminescence intensity depends on the nature of lanthanide ion.
The electronic absorption and emission spectra of L and its 1 -
6 complexes were recorded at room temperature in DMF and are
displayed in Figs. 4 and 5. The absorption spectrum of 1 features
one main band located around λmax = 331 nm and two additional
bands with λmax = 305 and λmax = 292 nm, assigned to π → π∗
and n → π∗ transitions, similar to those of the free L, Fig. 4. How-
ever, a weak bathochromic shift (~3 nm) that discernible in the ab-
sorption maxima of the complex is attributed to the perturbation
induced by metal coordination. Replacing La3+ in 1 with other lan-
thanide ions Sm3+ (2), Gd3+ (2), Eu3+ (2), Tb3+ (2) and Dy3+ (2)
has a very slight influence on the absorption, indicating that the
nature of the metal ions plays only a minor role in the electronic
structures of the complexes, Fig. 4. In general, the magnitudes of
5

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
Fig. 6. Absorption (dashed line); Excitation (blue solid line); and emission (red
solid line) spectra of complex 2 in DMF (1 × 10−6 M) at room temperature.
Fig. 4. Uv-Vis absorption spectra of Schiff base ligand (L) and its 1–6 complexes in
DMF (1.0 × 10−4 M) at room temperature.
Enhancement of luminescence intensity might be attributed to co-
ordination of the ligand to the lanthanide(III) ion, which effectively
increases the rigidity of the complexes and reduced the loss of en-
ergy by radiationless decay [33]. These changes in the position of
the π-π∗ state between the free and complexed ligand and the
emission intensity enhancments confirm the formation of the com-
plexes.
The excitation spectra obtained by monitoring the emission
wavelengths at 492, 529, 468, 462, 470, 460 and 452 nm, for L
and 1–6 complexes, respectively, matches the corresponding ab-
sorption profiles throughout the UV spectral region, Fig. 6 [34].
However, the typical narrow emission bands of the Sm3+, Eu3+,
Tb3+ and Dy3+ ions cannot be detected upon excitation of the
ligand-centered absorption bands. This is may be due to the en-
ergy gap between the lowest triplet energy level of the organic lig-
and and the resonant energy level of the lanthanide ion which is
probably very small. Accordingly, thermally activated back-energy-
transfer between the excited metal-centered of Tb3+ (5D4), Eu3+
(5D0), Sm3+ (4G5/2) and Dy3+ (4F9/2) levels and the triplet energy
level of the ligand (3ππ∗) can easily occur, thus depressing the lu-
minescence output of the of Sm3+, Eu3+ Dy3+ and Tb3+ ions [35].
,
3.4. Catalytic activity study
The catalytic performances of 1 - 6 complexes were monitored
using the aniline oxidation as a probe reaction. Initially the influ-
ence of reaction time in the aniline conversion was studied. Using
8.0 mmol aniline, 7.0 mL propionitrile, 0.015 mmol catalyst and
0.34 g, 30%(wt/wt) H2O2 significant aniline conversion were ob-
served at the first one hour of catalytic reaction and did not vary
significantly with time. The results revealed that aniline conver-
sion and product selectivity’s were influenced by the nature of the
lanthanide ion present, Table 4. Nitroso and azoxy benzene were
detected as major products. Moderate aniline conversions 45.6%
Fig. 5. Emission spectra of L and its 1–6 complexes in DMF (1 × 10−6 M) at room
temperature. λex = 290, 292, 294, 293, 293, 294 and 292 nm for L and 1–6 com-
plexes, respectively.
6

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
Table 3
Infrared Spectral data of L and its 1–6 complexes.
Bi-dentate nitrate
Complex
ν(C=O) ν(C=N) ν(O-Ph)
ν(O-CH3)
ν(M-N)
ν(M-O)
ν5 + ν2
ꢂ
ν1 + ν4
ꢂ
L
1
2
3
4
5
6
1664
1645
1631
1644
1632
1644
1648
1587
1591
1594
1593
1594
1594
1591
1256
1250
1251
1253
1251
1253
1251
1150
1163
1165
1163
1165
1163
1163
-
-
-
-
-
-
-
-
1464
1466
1465
1466
1464
1465
1277
1291
1299
1295
1298
1280
187
175
166
171
166
185
1772
1766
1774
1770
1774
1773
1733
1731
1737
1733
1737
1733
39
35
37
37
37
40
542
546
532
545
533
544
441, 416
448, 413
444, 416
448, 414
443, 415
446, 417
Table 4
Oxidation of aniline catalyzed by complexes 1 - 6 in propionitrile at 25 °C.
Aniline
Yields of products (%)
Azo-
Selectivity of products (%)
Azo- Azoxy-
Catalyst
conversion (%)
Nitroso-
Azoxy-
Nitroso-
1
2
3
4
5
6
12.1
45.6
44.2
51.7
45.6
44.2
5.22
45.6
44.2
13.4
26.5
29.3
4.61
0.0
0.0
0.0
0.0
0.0
2.26
0.0
43.2
100
100
25.9
58.1
66.4
38.1
0.0
0.0
0.0
0.0
0.0
18.7
0.0
0.0
0.0
38.3
19.1
14.9
74.1
41.9
33.6
and 44.2% were obtained by 2 and 3 catalysis with 100% selectiv-
ity for nitrosobenzene. In the case of 5 and 6 catalysts, the con-
version was 45.6% and 44.2%, respectively, however the selectiv-
ity for the nitroso product decreased to 58.1% and 66.4%, respec-
tively, Table 1. When catalyst 4 was tested the highest conver-
sion 51.7% was obtained. It is interesting to observe that catalyst 4
had an opposite trend presenting a poor selectivity to nitrosoben-
zene 25.9% and higher selectivity for azoxybenzene (74.1%). Oxida-
tion by 1 catalyst leads to formation of a mixture of azo, azoxy,
and nitrosobenzene products with low conversion 12.15%. More-
over, nitrobenzene, another possible product, was not detected in
any of the catalysis [10, 12]. For comparison, the catalytic activity
of the pure lanthanide nitrates in the oxidation aniline reactions
was carried out under similar reaction conditions and found to be
inactive.
(1b), 4-(trifluoromethyl)aniline (1c), 4-(methylthio)aniline (1d),
4-chloro-3-(trifluoromethyl)aniline (1e), 3-nitro-4-(trifluoro-
methyl)aniline (1f) and 4-(4-chlorophenoxy)aniline (1 g), Table 5.
Among the substrates tested, the best result was obtained in the
case of 1e with 78.6% conversion and 100% selectivity for the
nitrosobenzene. Haloginated anilines, 1c, 1e, 1f and 1 g were
oxidized successfully with a high selectivity for nitrosobenzene.
In the case of 1b, a good conversion was obtained 55.8% with
100% selectivity for azoxybenzene. It is interesting to note that
the conversion drop from 51.7% for aniline to 10.0% for nitro
aniline derivative 1f. Similar reduction in yield from 77% for
aniline to 10% for p-nitroaniline derivative was reported when
[Mo(O2)(O)(H2O)(hmpa)] was used as a catalyst. However, the
differences in the reactivity of various substituted anilines are
dependent on the synergetic effect of the molecular parameters
such as mesomeric, inductive and the steric effect of substituent’s
[36].
Considering the highest conversion and higher selectivity for
azoxybenzene product, catalyst 4 was tested for the oxidation
of a variety of substituted aniline such as -(2-aminoethyl)aniline
7

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
Table 5
Oxidation of aniline and substituted anilines catalyzed by complex 6 in propionitrile at 25 °C.
4. Conclusion
Declaration of Competing Interest
Lanthanide complexes 1 - 6 containing N’- (2- methoxybenzy-
lidene) - 2-pyridinecarbohydrazide Schiff base ligand L were syn-
thesized and characterized. In the proposed structure of com-
plex 1 the lanthanide ion is ten coordinate. Six coordination sites
are satisfied by three nitrate ions, one site is occupied by one
H2O molecule and the remaining three positions are occupied
by ONO belonging to one neutral L. In contrast the lanthanide
ion in the proposed structures of complexes 2 - 6, is twelve co-
ordinated by two three bi-dentate ONO Schiff base ligand and
three bi-dentate nitrate anions. Complexes 1–6 exhibit reason-
able catalytic activity for oxidation of aniline and aniline deriva-
tives into their corresponding oxygen containing derivatives us-
ing H2O2 as an oxygen source. Complex 2 and 3 were found
to show higher catalytic activity for oxidation of aniline with
100% selectivity to nitroso-benzene than other complexes un-
der similar conditions. Among the substrates tested, the best re-
sult was obtained in the case of 1e with 78.6% conversion and
100% selectivity for the nitrosobenzene in the presence of 4 as a
catalyst.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
Ziyad A. Taha: Conceptualization, Data curation, Formal anal-
ysis, Funding acquisition, Investigation, Methodology, Project ad-
ministration, Resources, Software, Supervision, Validation, Visual-
ization, Writing - original draft. Ahmed K. Hijazi: Methodology,
Writing - review & editing.
Acknowledgmnts
The authors are thankful to Jordan University of Science and
Technology (Project no: 20180137) for providing necessary assis-
tance and financial support for carrying out this work. Prof. Khader
A. Al-Hassan at Yarmouk University is gratefully acknowledged for
performing the luminescence measurements.
8

Z.A. Taha and A.K. Hijazi
Journal of Molecular Structure 1238 (2021) 130451
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