Some quantum chemical study on the structural properties of three unsymmetrical Schiff base ligands

Article abstract of DOI:10.4314/bcse.v28i2.10

In this studythree Schiff base ligands derived of 2-hydroxybenzaldehyde: H₂L¹, H₂L² and H₂L ³ have been subjected to theoretical studies by using density functional theory (DFT) calculation

Full text of DOI:10.4314/bcse.v28i2.10

Bull. Chem. Soc. Ethiop. 2014, 28(2), 265-270.
Printed in Ethiopia
ISSN 1011-3924
 2014 Chemical Society of Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v28i2.10
SOME QUANTUM CHEMICAL STUDY ON THE STRUCTURAL PROPERTIES OF
THREE UNSYMMETRICAL SCHIFF BASE LIGANDS
Iran Sheikhshoaie1*, VahidSaheb1 and Parisa Iranmanesh2
1Department of Chemistry, Shahid Bahonar University of Kerman, Iran
2Department of Chemistry, Payame Nour University of Kerman, Iran
(Received May 30, 2013; revised November 9, 2013)
ABSTRACT. In this studythree Schiff base ligands derived of 2-hydroxybenzaldehyde: H2L1,
H2L2 and H2L3 have been subjected to theoretical studies by using density functional theory (DFT)
calculations. The electronic properties such as highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO) energy, HOMO-LUMO gap energy, RHF energy, some of
important bond lengths and angles, dipole moment and molecular densities have been
investigated. Also the calculated IR spectra of these ligands are in good consistency with the
experimental results.
KEY WORDS: Schiff base, 2-Hydroxybenzaldehydeand, 2-Aminoethanthiol, HOMO, LUMO,
Band gap
INTRODUCTION
Schiff’s base ligands have been in the chemistry catalogue for over 150 years [1]. The literature
clearly shows that the study of this diverse ligand system is linked with many of the key
advances made in inorganic chemistry. Not only have they played a seminal role in the
development of modern coordination chemistry [2], but they can also be found at key points in
the development of structural and theoretical chemistry [3-7], inorganic biochemistry [8],
catalysis [9-11] and optical materials [12]. Also Schiff base ligands and some of their
complexes have been extensively investigated for more than a century and have been employed
in areas that include analytical and bioinorganic chemistry, non-linear optics, fluorescence
studies, catalysis and materials chemistry [13-18].
The molybdenum Schiff base complexes have continued to play the role of one of the most
important models of molybdoenzymes [19]. Complexes containing cis-dioxomolybdenum(VI)
fragment dominate coordination chemistry of molybdenum(VI) [20]. Depending on the
preparation method, molybdenum precursor and reaction conditions, the complexes synthesized
with dibasic tridentate Schiff bases are of the type: monomeric - Mo(O)2(L)(S), which is
dominant, where L = Schiff base, S = monodentate neutral ligand [21], or dimeric - [MoO(m-
O)(L)]2 with asymmetric double oxygen bridge [22]. On the other hand quantum chemistry
calculations have been shown to be useful in the description of the relationship among the
electronic structure of the molecular systems and their optical response [23].
In this paper we report some calculated structural parameters for three salen type Schiff base
ligands 2-((E)-(2-mercaptoethylimino) methyl)-4-methoxyphenol (H2L1), 2-((E)-(2-mercapto-
ethylimino) methyl) phenol (H2L2) and 2-((E)-(2-mercaptoethylimino)methyl)-4-nitrophenol
(H2L3) (Scheme 1). These compounds have been described by Syamal [24].
__________
*Corresponding author. E-mail: i_shoaie@yahoo.com

266
Iran Sheikhshoaie et al.
EXPERIMENTAL
Materials and reagents
2-Hydroxy-5-methoxybenzaldehyde, 2-htydroxybenzaldehyde, 2-hydroxy-5-nitrobenzaldehyde,
2-aminoethanthiol and ethanol were used as received. Methanol was distilled from magnesium
methoxide. All reactions and workups were carried out under air.
Instrumentation
The IR spectra were measured as KBr pellets on a Perkin–Elmer 780 spectrophotometer (4000–
400 cm-1). UV–Vis spectra were recorded as a solution (DMSO) on a 160 Shimadzu
1
spectrophotometer. H-NMR with using d6-DMSO spectra were recorded on a Brucker Avance
500 MHz instrument. Micro analyses for C, H, and N were determined on a Heracuse CHN
rapid analyzer.
Synthesis of Schiff base ligands
In this research, we prepared three salen-type Schiff base ligands. 2-((E)-(2-mercapto-
ethylimino) methyl)-4-methoxyphenol (H2L1), 2-((E)-(2-mercaptoethylimino) methyl) phenol
(H2L2) and 2-((E)-(2-mercaptoethylimino) methyl)-4-nitrophenol (H2L3) as literatures [24]. By
condensation of 2-hydroxy-5-methoxybenzaldehyde, 2-htydroxybenzaldehyde or 2-hydroxy-5-
nitrobenz- aldehyde (0.01 mol, 1.52 g, 1.22 g or 1.67 g) and 2-aminoethanthiol (0.01 mol, 0.77
g) in 40 mL methanol and the mixtures were refluxed for 4 h (Scheme 1). The three imines
prepared were filtered and washed several times with cold methanol and dried in vacuum. Then
the imines were readily purified by recrystallization from ethanol. 1H NMR, UV–Vis
spectroscopy and microanalyses were used to characterize these ligands. Some physical property
of H2L1, H2L2 and H2L3 Schiff base compounds are given in Table 1 and the relevant results of
the FT-IR measurements are given in Table 2.
Table 1. Some important properties with C, H and N analysis of H2L1 H2L2 and H2L3 Schiff base ligands.
,
Schiff base and
their complexes
H2L1
Formula
Formula weight Color Yield C, H and N analyses Exp.(Theor.)
m.p.
(οC)
(gmol-1)
211
(%)
C10H13NO2S
C9H11NOS
C9H10N2O3S
Yellow 85 56.2(56.87), 6.0(6.16), 6.4(6.63) 118-120
Yellow 91 59.5(59.66), 6.1(6.07), 7.7(7.73) 112-115
Orange 56 47.6(47.78), 4.4(4.42), 12.2(12.38) 130-133
H2L2
181
226
H2L3
Table 2. Some important absorption bands in the FT-IR spectra (400-4000 cm-1) for H2L1 H2L2 and
,
H2L3Schiff base ligands.
Schiff base and their complexes
νC=N (cm-1)
νOH (cm-1)
νC-H(imini) (cm-1)
H2L1
H2L2
H2L3
1638
1631
1612
3287
3306
3303
2835-2995
2905
2976-2926
1
The H NMR spectra (DMSO as an internal solvent): [H2L1: δ (6.8, 6.9, d, and 5.9, s, for
aromatic hydrogen, 3, 3.9 aliphatic hydrogen, 3.8 methoxy protons and 8.4, s, for SH proton)],
[H2L2: δ (7.2, 7.4, t aromatic hydrogen 5.8, s, H imine group, 3.9, t, aliphatic hydrogen, 7.0, s,
SH group)], H2L3: δ (7, 8.1, d aromatic hydrogen, 2.8, d and 3.3, t, aliphatic hydrogen, 8.3,
s,(SH group)]. The UV/Vis (DMSO), λmax, nm (log ε, L mol-1 cm-1): 2 H2L1: 486(4.81), H2L2:
449(4.72), H2L3: 437(4.37).
Bull. Chem. Soc. Ethiop. 2014, 28(2)

Quantum chemical study on structural properties of unsymmetrical Schiff base ligands 267
Theoretical study
Quantum chemical calculations (Density Functional Theory, B3LYP/6-31G (d)) were used to
purposed the electronic parameters for H2L1, H2L2 and H2L3 Schiff base ligands. All density
functional theory (Becke’s hybrid HF-DFT procedure [25] with the Lee-Yang-Parr [26]
correlation functional [B3LYP/6-31G(d)]) calculations were done by using Gaussian 03
program package [27], and all three structures of these compounds were optimized (Scheme 2).
Some theoretical calculated structural properties for H2L1, H2L2 and H2L3 Schiff base ligands are
shown in Table 3. Table 4 shows some optimized geometric parameters of H2L1 H2L2 and H2L3
,
Schiff base ligands. Some important experimentally observed picks are given in Table 5 and
compared with the most intense vibrational frequencies calculated at the B3LYP/6-31G(d) level
of theory.
14
H
H
6
S
11
5
N
8
R
C
R
CHO
+
3
10
SH
4
7
H2N
2
H
9
1
O
13
OH
12
R= OCH3 (H2 L1 ), R= H (H2 L2), R= NO2 (H2 L3 )
Scheme 1. The preparation of H2L1, H2L2andH2L3Schiff base ligands.
H2L1
H2L2
H2L3
Scheme 2. The optimized structures of H2L1, H2L2 and H2L3 Schiff base ligands.
Table 3. Some calculated electronic parameters for H2L1 H2L2 and H2L3 Schiff base ligands.
,
Compound Mulliken atomic charges
EHOMO
(eV)
ELUM
(eV)
E
(eV)
Band gap
(eV)
0.2675
0.2953
0.3360
µ(Debye)
O(1)
N(5)
S(8)
H2L1
H2L2
H2L3
-0.6802 -0.1679 0.0238 -0.3284 0.0609 3.9019 -26873.7000
-0.1961 -0.1828 0.0641 -0.3361 0.0608 2.9372 -23776.5110
-0.7709 -0.4126 -0.0819 -0.3573 0.0213 4.1491 -11895.7340
The HOMO and LUMO orbitals are very important in defining its reactivity [28]. Higher
value of HOMO of a molecule has a tendency to donate electrons to appropriate acceptor
molecule with low energy, empty molecular orbitals. An electronic system with a larger HOMO-
LUMO gap should be less reactive than having smaller gap [29]. The larger the HOMO-LUMO
orbital energy gap, the harder the molecule. The hardness has been associated with the stability
of the chemical system. In the present study, the HOMO-LUMO gap the HL3 molecule is largest
Bull. Chem. Soc. Ethiop. 2014, 28(2)

268
Iran Sheikhshoaie et al.
than HL2 and HL1, respectively as shown in Table 3, which clearly indicates that HL1 molecule
is very reactive than the other. The calculated dipole moment values show that the HL3 molecule
is highly polarin nature than the other molecules.
Table 4. Some of important optimized geometries for H2L1 H2L2 and H2L3 Schiff base ligands.
,
Bond length (Å)
C2-C12
H2L1
1.388
1.381
1.389
1.379
1.397
1.390
1.491
1.265
1.463
1.520
1.888
1.354
1.379
0.950
2.718
H2L2
1.388
1.382
1.393
1.383
1.397
1.391
1.491
1.266
1.463
1.502
1.888
1.354
1.377
0.950
2.706
H2L3
1.394
1.375
1.392
1.381
1.388
1.407
1.467
1.266
1.461
1.525
1.885
1.354
1.344
0.966
1.861
C12-C9
C9-C10
C10-C11
C11-C3
C3-C2
C3-C4
C4-N5
N5-C6
C6-C7
C7-S8
S8-H14
C2-O1
O1-H13
H13….N5
Bond angles (o)
H13-O1-C2
O1-C2-C3
C2-C3-C4
C3-C4-N5
C4-N5-C6
N5-C6-C7
C6-C7-S8
115.890
123.120
122.439
128.287
124.464
107.309
108.789
112.981
121.986
122.243
122.392
125.128
116.457
107.688
112.981
121.986
122.243
122.398
125.128
116.457
107.688
Scheme 3. Selected molecular orbital plots in various electronic excitations of H2L1 H2L2 and
,
H2L3 Schiff base ligands.
Bull. Chem. Soc. Ethiop. 2014, 28(2)

Quantum chemical study on structural properties of unsymmetrical Schiff base ligands 269
Table 5. Some of important vibrational frequencies (cm-1) of H2L1, H2L1 and H2L1 Schiff base ligand
compared with calculated.
H2L1
H2L2
H2L3
Assignment
Experimental Theoretical Experimental Theoretical Experimental Theoretical
1180
1166
3287
1638
2995
1107
1140
4026
1874
2983
1073
1047
3306
1631
2905
1135
1139
4025
1873
2988
938
1030
1033
3678
1892
2953
νC=C
νC=C
1038
3303
1612
2926
νO-H
νC=N
(aromatic)νC-H
RESULTS AND DISCUSSION
Three Schiff base ligands H2L1 H2L2 and H2L3 with NSO donor atom derived of salicylaldehyde
,
were synthesized as literatures. A comparative study of their physico-chemical properties has
1
been made through elemental analysis, IR, electronic spectra and H NMR spectra. Also in the
present study we have some theoretical calculations by using Gaussian 03 program to determine
the best molecular configurations of the title compounds and we obtained some parameters such
as the energy of the molecular orbital, EHOMO, ELUMO, dipole moment and Mulliken atomic
charge. The results are given in Table 3. The calculated HOMO-LUMO orbital energies, dipole
moments and total energy are very important physical parameters for chemical reactivity and
biological activities of the studied compounds. The energy of HOMO is often associated with
the electron-donating ability of a molecule; high values of EHOMO are likely to indicate a
tendency of the molecule to donate electrons to appropriate acceptor molecules with energy and
empty molecular orbital. Therefore, the energy of LUMO indicates the ability of the molecule to
accept electrons. Table 3 indicates O(1), N(5) and S(8) are the coordination sites for these
ligands. Following the EHOMO from Table 3 and Figure 3 looks apparently easier for the
molecular to offer electrons to unoccupied d-orbital of a metal ion. The higher HOMO energy
from H2L1 > H2L2 > H2L3 can be used as one of the parameters for the explanation of its good
coordination performance, and R group has an important role in the complex formation by these
ligands.1H NMR spectra were recorded in DMSO as an internal solvent for these ligand in our
laboratory, [H2L1: (6.8, 6.9, d, and 5.9, s, for aromatic hydrogen, 3, 3.9 aliphatic hydrogen, 3.8
methoxy protons and 8.4, s, for SH proton)], [H2L2: (7.2, 7.4, t aromatic hydrogen 5.8, s, H
imine group, 3.9, t, aliphatic hydrogen, 7.0, s, SH group)], H2L3: (7, 8.1, d aromatic hydrogen,
2.8, d and 3.3, t, aliphatic hydrogen, 8.3, s,(SH group)]. Figure 3 shows the highest molecular
orbitals (HOMO) for these ligand as have major contribution from the S atomic orbital of S-H
group and the lowest molecular orbital (LUMO) have major contribution from the salen rings.
Table 5 shows a good agreement between the experimental with theoretical data.
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