Synthesis, characterization and quantum-chemical investigations of new fluorescent heterocyclic Schiff-base ligands and their cobalt(II) complexes

Article abstract of DOI:10.1016/j.ica.2018.09.050

Two new fluorescent heterocyclic Schiff base ligands were synthesized and characterized by elemental analyses, IR, mass, and NMR spectra. Results from NOESY experiment confirmed the E configuration of Schiff-bases. Coordination of the bidentate ligands wi

Full text of DOI:10.1016/j.ica.2018.09.050

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Research paper
Synthesis, characterization and quantum-chemical investigations of new fluo-
rescent heterocyclic Schiff-base ligands and their cobalt(II) complexes
Shirin Ramezani, Mehdi Pordel, Abolghasem Davoodnia
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S0020-1693(17)31110-6
https://doi.org/10.1016/j.ica.2018.09.050
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Please cite this article as: S. Ramezani, M. Pordel, A. Davoodnia, Synthesis, characterization and quantum-chemical
investigations of new fluorescent heterocyclic Schiff-base ligands and their cobalt(II) complexes, Inorganica
Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.09.050
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Synthesis, characterization and quantum-chemical investigations of new
fluorescent heterocyclic Schiff-base ligands and their cobalt(II) complexes
Shirin Ramezani, Mehdi Pordel* and Abolghasem Davoodnia
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
*Corresponding author. Tel.: ‏+98 513 8414182; fax: +98 513 8424020. E-mail:
mehdipordel58@mshdiau.ac.ir
Abstract– Two new fluorescent heterocyclic Schiff base ligands were synthesized and
characterized by elemental analyses, IR, mass, and NMR spectra. Results from NOESY
experiment confirmed the E configuration of Schiff-bases. Coordination of the bidentate ligands
with Co(II) cation led to the formation of deep green complexes. The structures of the complexes
have been confirmed by spectral, analytical data and Job’s method. Schiff-base ligands and
cobalt complexes were spectrally characterized by UV-Vis and fluorescence spectroscopy. An
efficient charge transfer of electron from p-orbital on ligand to Co(II) d-orbital can be proposed
as the main reason for the color of the complexes. In addition, the DFT methods were employed
to achieve deeper insight into geometry, spectral properties and energy difference between the
HOMO and LUMO frontier orbitals of the synthesized compounds. The DFT-calculated spectral
properties are in good agreement with the experimental values, confirming suitability of the
optimized geometries for Co(II) complexes.
Keywords: Fluorescent Schiff base; Co(II) complex; UV-Vis and fluorescence spectroscopy;
Density functional theory.
1

1. Introduction
Cobalt has many strategic and irreplaceable industrial uses but it is as the central component of
Vitamin B12 that it is vital. In hence, the cobalt complexes have received considerable attention
owing to their effective biological importance such as antibacterial [1–3] antifungal [4,5]
antivirus [6] antiproliferative [7,8] and anticancer activity [9–12]. They are also used as catalysts
for a diverse range of organic reactions such as electrochemical reactions [13], cross-coupling
reactions [14], polymerization [15], hydrogenation [16,17], Lewis acid catalysts in organic
synthesis [18], etc.
Benzo[1,2-c]isoxazoles are an important class of heterocyclic bioactive and pharmaceuticals
compounds prescribed as antipsychotic risperidone [19] and anti-HIV drugs [20] and play a key
role in many organic reactions [21]. Isoxazole-metal complexes are often postulated as
intermediates in reactions of considerable synthetic utility, for example the reductive ring
opening of isoxazoles. Several isoxazole-metal complexes have been reported and well
characterized. In a review of the literature of isoxazole-metal complexes [22], the binding
characteristics of the isoxazoles in the complexes have been examined, and some tentative
conclusions regarding the regularity of isoxazole complexation behavior have been discussed.
On the other hand, Schiff bases are the most versatile studied ligands in coordination chemistry
because of their structural varieties and very exclusive characteristics. Schiff bases have number
of applications such as preparative use, identification, detection and determination of aldehydes
or ketones, purification of carbonyl or amino compounds, or protection of these groups during
complex or sensitive reactions. They also form basic units in certain dyes. Transition metal
complexes derived from the Schiff base ligands with biological activity have been widely studied
2

[23–25]. Based on these results, we have synthesized and characterized two new fluorescent
heterocyclic Schiff-base bidentate ligands derived from imidazo[4',5':3,4]benzo [1,2-c]isoxazole
and their Co(II) complexes. The optical properties of the compounds were also investigated by
UV-Vis and fluorescence spectroscopy. Furthermore, density functional theory (DFT)
calculations are performed to provide the optimized geometries, structural parameters,
vibrational frequencies and energy difference between the HOMO and LUMO frontier orbitals of
the investigated compounds.
2. Experimental
Equipment and Materials
Melting points were measured on an Electrothermaltype-9100 melting-point apparatus. The FT-
IR spectra were recorded as potassium bromide pellets using a Tensor 27 spectrometer and only
noteworthy absorptions are listed. The 13C NMR (75 MHz), 1H NMR (300 MHz) and NOESY
spectra were obtained on a Bruker Avance DRX-300 spectrometer. Chemical shifts are reported
in ppm downfield from TMS as internal standard; coupling constant J is given in Hz. The mass
spectrum was recorded on a Varian Mat, CH-7 at 70 eV and ESI mass spectrum was measured
using a Waters Micromass ZQ spectrometer. Elemental analysis was performed on a Thermo
Finnigan Flash EA microanalyzer. Absorption and fluorescence spectra were recorded on Varian
50-bio UV-Visible spectrophotometer and Varian Cary Eclipse spectrofluorophotometer. UV–
Vis and fluorescence scans were recorded from 200 to 1000 nm. Percentage of the Co(II) was
obtained by using a Hitachi 2-2000 atomic absorption spectrophotometer.
3

All solvents were dried according to standard procedures. Compounds 1[26], 3[27], 4 [28] and
5a,b [29] were obtained according to the published methods. Other reagents were commercially
available.
Computational methods
All of the calculations have been performed using the DFT method with the B3LYP functional
[30] as implemented in the Gaussian 03 program package [31]. The 6-311+G(d,p) basis sets were
employed except for the Co(II) where the LANL2DZ basis sets were used with considering its
effective core potential. Geometry of the Co(II) complex was fully optimized, which was
confirmed to have no imaginary frequency of the Hessian. Geometry optimization and frequency
calculation simulate the properties in the gas/solution phases. Natural Bond Orbitals (NBO)
analysis was used for electron population analysis in Gaussian.
The fully-optimized geometries were confirmed to have no imaginary frequency of the Hessian.
The solute-solvent interactions have been investigated using one of the self-consistent reaction
field methods, i.e., the sophisticated Polarizable Continuum Model (PCM) [32].
General procedure for the synthesis of 7a,b from 5a,b
To a solution of compound 5a,b (0.34 g, 1 mmol) in EtOH (15 mL), aldehyde 6a,b (1 mmol)
was added. The reaction mixture was heated under reflux for 5 hours. The solvent was removed
under reduced pressure and the yellow product was filtered and washed with cold EtOH to give
Schiff base (7a,b), which was purified in hot acetone.
4

((E)-N-(4-chlorobenzylidene)-8-(4-chlorophenyl)-3-propyl-3H-imidazo[4',5':3,4]benzo[1,2-c]
isoxazol-5-amine (7a, L1, Scheme 1) was obtained as a yellow powder. m.p.: 178–181 ºC; yield:
82%.
(E)-4-(((3-butyl-8-(4-chlorophenyl)-3H-imidazo[4',5':3,4]benzo[1,2-c]isoxazol-5-yl)imino)
methyl) phenol (7b, L2, Scheme 1) was obtained as a yellow powder. m.p.: 188-190 ºC. yield:
75%.
General procedure for the synthesis of the complexes 8a,b from ligands 7a,b
Cobalt(II) nitrate hexahydrate (0.29 gr, 1 mmol) was added to the yellow solution of ligand 7a,b
(2 mmol) in aqueous metanolic solution (15 mL, MeOH, H2O, 90:10), resulting in color change
to deep green. The reaction was carried out for another 12 h at rt. The complex was isolated by
evaporation of the solvent and washed with cold MeOH and then H2O.
[Co(L1)2] N2O6.2(H2O) (8a, Scheme 1): was obtained as a dark green powder. m.p. > 300 ºC
(decomp).
[Co(L2)2] N2O6.2(H2O) (8b, Scheme 1): was obtained as a dark green powder. mp > 300 ºC
(decomp).
3. Results and discussion
Synthesis and structure of the new ligands 7a,b and complexes 8a,b
Two new heterocyclic Schiff-base ligands were prepared in five steps explained in the following
manners. Initial reaction of 5-nitro-1H-benzimidazole with 1-bromopropane and 1-bromobutane
in KOH and DMF led to the formation of 1-alkyl-5-nitro-1H-benzimidazoles (1a,b) [27]. The
5

reaction of 1-alkyl-5-nitro-1H-benzimidazoles 1a,b with (4-chlorophenyl)acetonitrile (2) in basic
MeOH solution gave 3-alkyl-8-(4-chlorophenyl)-3H-imidazo[4',5':3,4]benzo[1,2-c]isoxazoles
(3a,b) [28]. Regioselective nitration of 3a,b using a mixture of sulfuric acid and potassium
nitrate produced 3-alkyl-8-(4-chlorophenyl)-5-nitro-3H-imidazo[4',5':3,4]benzo[1,2-c]isoxazoles
4a,b in good yield [29].
8-(4-
chlorophenyl)-3-alkyl-3H-imidazo[4',5':3,4]benzo[1,2-c]isoxazol-5-amines (5a,b) in high yields.
The new heterocyclic Schiff-bases 7a,b were finally obtained by the reaction of amines 5a,b
with p-chlorobenzaldehyde 6a and p-hydroxybenzaldehyde 6b in high yields (Scheme 1).
Scheme 1. Synthesis of the new ligands 7a,b
The unambiguous assignment of Schiff-bases 7a,b was carried out by elemental analyses, IR,
mass, 1H- and 13C-NMR spectroscopy and two-dimensional NMR techniques. For example, in
the 1H NMR spectrum of compound 7b there is an exchangeable peak at δ 10.45 ppm attributed
to OH group proton. Also, there are four doublet signals (δ = 6.95, 7.71, 7.85, 8.93 ppm) and two
singlet signals (δ = 7.66 and 8.30 ppm) assignable to ten protons of aromatic rings and a singlet
6

signal (δ = 9.07) attributed to the imine CH proton. Also, 21 different carbon atom signals are
observed in the 13C NMR spectrum of compound 7b. Furthermore, the IR spectrum of compound
7b in KBr showed a broad absorption band at 3345 cm-1 assignable to OH group. The results of
mass spectroscopy (m/z 446 [M(37Cl)]+) and elemental analyses support the structure of Schiff-
base 7b. In addition, the data from NOESY experiment of compound 7b revealed a cross-peak
between the H-4 proton (δH 7.71, s, CH-benzene) and the imine CH proton (δH 9.07), confirming
the E configuration of Schiff-base 7b (Scheme 2).
Scheme 2. NOESY spectrum of Schiff-base 7b.
The coordination of Schiff-bases 7a,b with Co(II) gave green complexes 8a,b in aqueous
metanolic solution. The stoichiometry of the complexes 8a,b obtained by Job’s method (Figures
S1 and S2, Supplementary Data: ML2) [33] together with the elemental analysis results
7

(Experimental section) proposed the formula [Co(L)2]N2O6.2(H2O) for the complexes 8a,b
(Scheme 3). Moreover, molecular ion peak at m/z 955 ([Co(L1)2]2+) and m/z 947 ([Co(L2)2]2+)
strongly support the structure of the new complexes 8a and 8b, respectively.
Scheme 3. The structure of the Co(II) complexes 8a,b.
3.1.Photophysical properties of the new ligands and complexes.
New fluorescent heterocyclic Schiff base ligands 7a,b, and Cobalt(II) complexes 8a,b were
characterized by UV-Vis and fluorescence spectroscopy in the wavelength range of 200–1000
nm. The absorption and fluorescence emission spectra of Schiff base 7a,b and Cobalt(II)
complexes 8a,b are depicted in Figures 1 and 2, respectively. Numerical spectral data are also
presented in Table 1. Values of extinction coefficient (ε) were calculated as the slope of the plot
of absorbance vs concentration. As seen in Figure 1, the spectra of the complexes 8a,b have an
absorption maximum at 690 nm at which the ligands have no absorbance. An efficient charge
transfer of electron from p-orbital on ligand to Co(II) d-orbital can be considered as the main
reason for the color of the complexes described as Ligand to-Metal charge transfer (LMCT) [34].
Moreover Schiff-base ligands 7a,b, and Cobalt complexes 8a,b produced fluorescence in dilute
8

solution of MeOH (Figure 2). Also, using fluorescein as a standard sample in 0.1 M NaOH and
MeOH solution, the fluorescence quantum yields of the compounds were determined via
comparison methods [35]. The used value of the fluorescein emission quantum yield is 0.79 and
the obtained emission quantum yields of the new compounds are around 0.21 – 0.61. As can be
seen from Table 1, extinction coefficient (ε) in Schiff-base 7a, fluorescence intensity and the
emission quantum yield in Schiff-base 7b were the biggest values.
1.6
1.4
1.2
1
0.8
7a
0.6
7b
0.4
0.2
0
8a
8b
300
500
700
900
Wavelength (nm)
Figure 1. The absorption spectra of Schiff-base 7a,b and Co(II) complexes 8a,b in MeOH
solution (1 × 10-4 M)
9

350
300
250
200
150
100
50
7a
7b
8a
8b
0
400
500
600
700
800
Wavelength (nm)
Figure 2. The fluorescence emission spectra of 7a,b and Co(II) complexes 8a,b in MeOH
solution (1 × 10-5 M)
Table 1. Spectroscopic data for 7a,b and 8a,b at 298 K.
Dye
7a
7b
8a
8b
λabs (nm)a
370
435
690
5.9
555
690
7.9
555
ε × 10 -3 [(mol L-1 )-1cm-1] b
λflu (nm)c
10.3 9.9
550 550
d
ΦF
0.36 0.61 0.21 0.27
a Wavelengths of maximum absorbance (λabs)
b Extinction coefficient
c Wavelengths of fluorescence emission (λflu) with excitation at 370 nm
d Fluorescence quantum yield
10

3.2.DFT Calculations
Based on our experimental results and reported literatures [36, 37], an octahedral geometry was
proposed for Co(II) complexes 8a,b. To get a deeper insight into the geometries, structural
parameters and role of HOMO and LUMO frontier orbitals in the UV-visible absorption spectra
of ligands 7a,b and Cobalt complexes 8a,b, DFT calculations at the B3LYP/6-311+G(d,p) level
were performed and the optimized geometries and HOMO and LUMO frontier orbitals of
fluorescent ligands 7a,b and Co(II) complex 8b were obtained. Geometry of Co(II) complex 8b
was optimized in both of the gas phase and MeOH as the solvent (PCM model). Figure 3 shows
the optimized geometry of the ligands 7a,b. The optimized geometry of the complex 8b in two
different views with labeling of its atoms is also shown in Figure 4. Some of the calculated
structural parameters of the Co(II) complex are listed in Table 2. Selected structural parameters
of the ligands 7a,b can be found in Supplementary Data (Tables S1 and S2)
Figure 3. The optimized geometry of the fluorescent ligands 7a (left) and 7b (right).
11

In the optimized geometry of the complex 8b, the ligand 7b coordinates to the Co(II) via
nitrogen atom of the imine group (–N=CH) and nitrogen atom of the isoxazole ring and acts as a
bidentate ligand.
Figure 4. The optimized geometry of the complex 8b.
The aromatic rings of the ligand are approximately in a same plane. Two other positions of the
complex are filled by two H2O molecules, which are Trans to each other. The H2O ligands are
perpendicular to the square plane of the complex. The Co-O and Co-N Lengths bonds are
collected in Table 2.
12

Table 2. Selected calculated structural parameters of Co(II) complex 8b.
Bond
Bond length (A0)
2.059
Angle
(°)
Dihedral angle
N1-N4-N5-N8
N1-N4-N5-Co
O1-N1-Co-O6
O3-N5-Co-O6
(°)
Co-N1
Co-N4
Co-N5
Co-N8
Co-O6
Co-O5
N5-O3
O3-C31
N1-Co-N5
N1-Co-N4
N5-Co-N8
N1-Co-N8
N4-Co-N5
N1-Co-O5
N5-Co-O3
N1-Co-O6
N5-Co-O5
O5-Co-O6
Co-N5-C27
N5-C27-C28
C3-N1-Co
Co- N1-O1
C3-C4-N4
Co-O5-N1
C3- C2-C1
N1-O1-C7
C1-C6-N3
C1-N2-C8
161.26
74.228
77.596
97.249
-18.504
-13.405
-110.484
-101.793
-15.932
116.874
-76.825
-20.405
-25.540
-60.302
-0.589
2.936
2.495
2.095
2.297
2.143
113.342 O1-N1-Co-N8
2.252
83.004
17.282
C27-N5-Co-O6
C27-N5-Co-O5
1.396
1.357
104.171 C27-C28-N8-O6
80.140 C28-N8-Co-N5
C31-C26 1.400
C26-C27 1.423
C27-C28 1.429
C31-C36 1.447
C36-C38 1.410
C28 –N8 1.420
165.245 C28-N8-N1-Co
109.901 O1-C7-C2-C3
123.210 O1-C7-C12-C13
117.662 C7-C2-C1-N2
133.125 C6-C1-N2-C8
115.388 N2-C8- N3-C6
-1.463
-0.010
-0.538
2.941
N8-C42
C27-N5
C25-N6
N6-C32
C32-N7
N7-C33
1.307
1.337
1.365
1.321
1.366
1.469
45.598
C2-C7-C12-C14
116.723 N5-C27-C26-C31
110.464 C1-C6-N3-C8
104.816 C3-C4-C5-C6
104.669 N3-C9-C10-C11
-0.590
0.477
1.430
-179.894
13

The DFT computed vibrational modes of Co(II) complex 8b are listed in Table 3 together with
the experimental values for comparison. The atoms are numbered as in Figure 4. As seen in
Table 3, there is good agreement between the experimental and DFT-calculated frequencies of
the complex 8b, confirming validity of the optimized geometry as a proper structure for the
complex 8b.
14

Table 4. Selected experimental and calculated IR vibrational frequencies (cm-1) of Co(II)
complex 8b.
Calculated
Experimental
frequencies
IR
Frequency Intensity
(km.mol-1)
Vibrational assignment
476 (w)
699 (w)
1024 (m)
1095 (vs)
497
627
1027
1105
63
191
67
υsym ( Co-N)
υasym( Co-N)
δop(C-H) aromatic
υ(C-Cl)
294
1262 (vs)
1459 (m)
1237
1427
1457
1482
82
υ(C1-N2, C9-N3, C25-N6, C33-N7)
υasym (C1-C6-N3) + υasym (C25-C30-N7)
υ(C=C, C=N) of the aromatic rings
105
279
1256
υ(C7-C12)+ υ(C31-C36)
υ(C=N) of the aromatic rings + υ(C9-N3,
C33-N7)
υ(C=N) of the aromatic rings + υ(C4-N4,
C28-N8)
1549 (s)
1626 (s)
1485
1506
119
178
1588,
1620
85, 67
157
υ(C=C) + υ(C=N) of the aromatic rings
1647
δ sci(H-O-H) of the H2O ligands
υ(C18-N4)+ υ(C42-N8) of the azomethine
C=N groups
1656
954
1666
1696
779
38
υ(C=C) of the benzene rings
υ(C=C) of 1 moiety
3059-3081 9-38
3152 33
υsym (C-H) aliphatic
υ(C18-H13)+ υ(C42-H30)
2964 (m)
3151-3139 13-67
υasym (C-H) aliphatic
3197-3213 5-76
υ(C-H) aromatic
3240
3736
3782
3865
3910
8
υ(C8-H2) ) + υ(C32-H19)
υsym (O-H) of the H2O ligands
υasym (O-H) of the H2O ligands
υsym (O-H) of the aromatic rings
υsym (O-H) of the aromatic rings
49
127
52
93
3430 (vs,br)
Abbreviation: op, out-of-plane; ip, in-plane; w, weak; m, medium; s, strong; vs, very strong; br, broad; sh, shoulder.
15

The 3D-distribution map for the highest-occupied-molecular orbital (HOMO) and the lowest-
unoccupied-molecular orbital (LUMO) of the ligands 7a,b and the complex 8b are illustrated in
Figure 5. As seen, the HOMO orbital of the ligands is localized on the benzimidazole and
isoxazole rings. On the other hand, the LUMO orbital is mainly localized on the benzene ring
and its substitutions. Since, electron transition from the HOMO orbital to the LUMO orbital is
π → π* transition in the ligands 7a,b. In contrast, the HOMO and LUMO frontier orbitals of the
complex 8b species are mainly localized on the isoxazole ring and Co atom, respectively. It
implies that electron transition from the HOMO orbital to the LUMO orbital is Ligand to-Metal
charge transfer (LMCT) [34].
The energy difference between the HOMO and LUMO frontier orbitals is one of the important
characteristics of molecules, which has a determining role in such cases as electric properties,
electronic spectra and photochemical reactions. Energy separation between the HOMO and
LUMO (Δε = εLUMO – εHOMO) of 7a, 7b and 8b is 3.28 eV (378 nm), 3.21 eV (386 nm) and 1.91
eV (649 nm), compared with the experimental values of 370, 435 and 690 nm respectively.
Also, a quantitative molecular orbital diagram along with a table containing the energies and the
composition of all calculated orbitals can be found in Supporting Information, Table S3 and
Figure S3.
The quenching fluorescence properties of Co (II) complex in comparison to the ligands can also
be deduced from the 3D-distribution maps for HOMO and LUMO of the ligands 7a,b and the
complex 8b. An efficient intramolecular charge transfer (ICT) states from the donor sites
(endocyclic N and O of benzimidazole and isoxazole rings) to the acceptor moiety (imine group)
can be considered as the main reason for the fluorescence intensity in ligands 7a,b. As shown in
16

Figure 5, intramolecular charge transfer can be influenced by coordinating Co (II) to the ligands
and leads to the quenching fluorescence intensity of cobalt(II) complex compared to the ligands
(see also Figure 2 and Table 1).
Figure 5. The HOMO (down) and LUMO (up) frontier orbitals of 7a,b and Co(II) complex 8b.
Conclusion
Two new fluorescent heterocyclic Schiff base ligands were synthesized from the reaction of 8-
(4-chlorophenyl)-3-alkyl-3H-imidazo[4',5':3,4]benzo[1,2-c]isoxazol-5-amines with aldehydes.
NOESY experiment of the ligands confirmed the E configuration of Schiff-bases. Coordination
of the ligands with Co(II) cation produce deep green complexes. The structures of the complexes
have been confirmed by spectral, analytical data and Job’s method and an octahedral geometry
was proposed for the complexes. Fluorescent Schiff-base ligands and cobalt complexes were
17

spectrally characterized by UV-Vis and fluorescence spectroscopy. Results revealed that Schiff-
base ligands and Co(II) complexes generate fluorescence in dilute solution of MeOH. The DFT
methods were also employed to gain a deeper insight into geometry and spectral properties of the
synthesized compounds. The DFT-calculated vibrational modes of Co(II) complex are in good
agreement with the experimental values, confirming suitability of the optimized geometries for
Co(II) complexes. Moreover, Energy separation between the HOMO and LUMO of ligands and
Co(II) complexes is comparable with the experimental values.
Acknowledgment
We would like to express our sincere gratitude to Research Office, Mashhad Branch, Islamic
Azad University, Mashhad-Iran, for financial support of this work.
18

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Highlights
 New Co(II) complexes were synthesized by reaction of Co(II) with Schiff base ligands.
 The structures of the ligands and complexes were characterized by spectral data.
 Their optical properties and quantum chemical calculations were studied.
 Their energy gap between the HOMO and LUMO frontier orbitals was obtained.
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Graphical abstract
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