N,N′-dipyridoxyl Schiff bases: Synthesis, experimental and theoretical characterization

Article abstract of DOI:10.1016/j.saa.2011.08.067

Three N,N′-dipyridoxyl Schiff bases (L1, L2 and L3) have been newly synthesized and characterized by IR, ¹H NMR, mass spectrometry and elemental analysis. Their optimized geometries together with the theoretical assignment of the vibrational fr

Full text of DOI:10.1016/j.saa.2011.08.067

Spectrochimica Acta Part A 83 (2011) 467–471
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
N,Nꢀ-dipyridoxyl Schiff bases: Synthesis, experimental and theoretical
characterization
S. Ali Beyramabadia,∗, Ali Morsalia, Malihe Javan Khoshkholghb, Abbas Ali Esmaeilib
a Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
b Department of Chemistry, Faculty of Science, Birjand University, Birjand, Iran
a r t i c l e i n f o
a b s t r a c t
Three N,Nꢀ-dipyridoxyl Schiff bases (L1, L2 and L3) have been newly synthesized and characterized by
IR, 1H NMR, mass spectrometry and elemental analysis. Their optimized geometries together with the
theoretical assignment of the vibrational frequencies and the 1H NMR chemical shifts of them have been
computed by using density functional theory (DFT) method. In the optimized structures of the Schiff
bases, two pyridine rings are not in a same plane; however the substitutions are essentially in the same
plane with the pyridine rings. Also, the benzene ring(s) in the bridge region is (are) not in the same
plane with the pyridine rings and azomethine moieties. In all the species, engagement in intramolecular-
hydrogen bonds causes to weakness of the phenolic O–H bonds. Consistency between the theoretical
results and experimental evidence confirms suitability of the optimized geometries for the synthesized
Schiff bases.
Article history:
Received 3 June 2011
Received in revised form 13 August 2011
Accepted 29 August 2011
Keywords:
DFT
IR assignment
NMR
Schiff base
Dipyridoxyl
Theoretical
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
computational chemistry, such as kinetics and mechanism studies,
spectroscopic assignments and so on [12–19].
Due to their structural varieties and very unique characteristics
Schiff bases are the most versatile studied ligands in coordination
chemistry [1,2]. Also, because of a variety of applications, including
biological [3–5], analytical [6] and industrial use as catalysts [7], the
Schiff-base ligands and their complexes are of great importance.
From biological point of view, study on the Schiff bases of
pyridoxal and their metal complexes are attractive from different
aspects, the significance of which are: the pyridoxal is a close ana-
logue of pyridoxine (Vitamin B6) [8], the manganese dipyridoxyl
diphosphate is a contrast agent for magnetic resonance imaging
of the liver [9,10], use of a copper(II) complex of pyridoxal in the
treatment of diabetic complications [11], and so on.
In continuation of our studies [12,13], here we report the
synthesis and characterization of three dipyridoxyl Schiff bases
by IR, 1H NMR, mass spectrometry and elemental analysis.
The newly synthesized Schiff bases are N,Nꢀ-dipyridoxyl (1,2-
diphenyldiamine) = L1, N,Nꢀ-dipyridoxyl (2,4-toluendiamine) = L2
and N,Nꢀ-dipyridoxyl (4,4ꢀ-diaminodiphenylether) = L3. Also,
geometry optimizations and theoretical assignment of the IR and
1H NMR spectra of them have been performed using DFT method,
which is widely used as a remarkable method in many areas of the
2. Experimental
2.1. Materials and methods
All the chemicals and solvents were purchased from Merck
except for pyridoxal hydrochloride which obtained from Fluka.
They were used as recieved. The IR spectra were recorded on
a Perkin Elemer 783 infrared spectrophotometer. Melting points
were determinated by using an electrothermal 9100 melting point
apparatus. Elemental analysis (C, H, N) was performed on a Her-
aeus elemental analyzer CHN-O-Rapid. Mass spectra were scanned
on a Shimadzu-GC-Mass-Qp 1100 Ex. The 1H NMR spectra were
recorded on a Bruker Drx-500 Avance spectrometer (500.13 MHz),
with (CD3)2CO as a solvent.
2.2. Synthesis of the Schiff bases
Pyridoxal hydrochloride (611 mg, 3mmol) was dissolved in 5 mL
of methanol, and then was added to a methanolic solution of
Et3N (303 mg, 3 mmol in 5 mL). This mixture was stirred for a
few minutes. Then, a methanolic solution of diamine (1.5 mmol
in 5 mL) was slowly added to the mixture, where diamine
is benzene-1,2-diamine, 4-methylbenzene-1,3-diamine and 4-(4-
aminophenoxy)benzenamine for the L1, L2 and L3 Schiff bases,
respectively. The mixture was stirred for 5 h. The solid was filtered,
∗
Corresponding author. Tel.: +98 511 8414182; fax: +98 511 8414182.
E-mail addresses: beiramabadi6285@mshiau.ac.ir, beiramabadi@yahoo.com
(S.A. Beyramabadi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.067

468
S.A. Beyramabadi et al. / Spectrochimica Acta Part A 83 (2011) 467–471
washed with cold methanol and dried in air (for L1, L2 and L3:
Yield: 77, 73 and 89%, respectively, and Decomp. P.: 199, 208.5 and
197.5 ◦C, respectively).
In comparison with the species L1, other notable changes
are observed in the shape of the L2, where the planes of two
pyridine rings are perpendicular to each other. The calculated
C2–C4–C10–C12 and C8–N1–N2–C16 dihedral angles are 98.7 and
89.7◦, respectively.
2.3. Computational details
In the species L3, longer bridge causes more deformation in
its molecular structure. The pyridine rings make a dihedral angle
of about 100◦ to each other. The calculated C1–O1–O2–C9 and
C8–N1–N2–C16 dihedral angles are −102.9 and 98.7◦, respectively.
The benzene rings of the L3 species are twisted relative to each
other, so that they are not in the same plane. For example, the calcu-
lated C17–C19–C24–C18 dihedral angle is 65.5◦. Also, the benzene
rings are not in the same plane with the pyridine rings.
The dihedral angle between C N azomethine and benzene ring
is about 35◦. In the bridge region, an oxygen atom binds two
benzene rings, where the C–O bond length and C–O–C angle are
138.0 pm and 121◦, respectively.
All the present calculations have been performed by using the
gradient-corrected DFT method with the B3LYP functional [20],
where the 6–311+G(d, p) basis set was employed. The Gaussian
98 program package [21] was used with its default procedures.
First, geometries of the Schiff bases were fully optimized in
the gas phase. The optimized geometries were confirmed to have
no imaginary frequency of the Hessian. Then, the gas phase opti-
mized geometries were used to compute theoretical vibrational
frequencies of them. The use of finite basis sets and incomplete
treatment of electron correlation result in the higher DFT vibra-
tional wavenumbers in comparison with the experimental ones.
This can be corrected by applying the scaling of wavenumbers.
Here, the scale factor of 0.9614 was used for calculated wavenum-
bers [22]. Also, their 1H NMR chemical shifts were predicted with
respect to tetramethylsilane. The GIAO method was used for pre-
diction of the DFT nuclear shielding [23].
In the structure of each of three Schiff bases: the pyridine
rings are essentially planar, where the bond lengths of C
C
(141.2–141.7 pm) and C N (128.9 pm) bonds are in the expected
range [28]. The pyridine–C bond lengths for the –CH2OH and
–CH3 substitutions are 152.1 and 150.2 pm, respectively, which
are appropriate sizes for the pyridine-carbon bond. The substi-
tuted groups are essentially in the same plane with the pyridine
rings. For example, the calculated C6–C2–C1–C5, C7–C4–C5–C1 and
H11–C11–N4–C10 dihedral angles are 179.9◦, 179.8◦ and 179.2◦,
respectively. Also, the azomethine moieties are essentially in the
same plane with the pyridine rings, but make a dihedral angle about
40◦ with the benzene ring. The C8–N1–C17–C19 dihedral angle is
42◦.
3. Results and discussion
3.1. Chemistry
The newly synthesized Schiff bases (L1, L2 and L3) were char-
acterized by the elemental and spectroscopic analysis (IR, 1H NMR
and Mass).
In all of the three species, the phenolic –OH groups are
essentially in the same plane with the corresponding pyridine
rings (Table 2). The phenolic hydrogens are engaged in the
intramolecular-hydrogen-bond interactions with the azomethine-
nitrogen atoms, which elongates the phenolic O–H bonds. The
calculated OH· · ·N distances of the species L1–L3 are about
171.7 pm, showing a strong hydrogen bonds. Due to this interac-
tion, the phenolic O–H bonds are longer than the alcoholic ones
(96.3 pm). The calculated parameters for the investigated com-
pounds are in agreement with the previously reported values for
the similar compounds [10,11,24–31].
In the metallic complexes, the dipyridoxyl Schiff-bases act usu-
ally as a dianionic tetra dentate ligand in N,N,O−,O− manner, via
phenolic oxygens and azomethine nitrogens. According to the
optimized geometries, complexation tendencies of the species is
expected to be of order of L1 > L2 > L3. Since, the steric hindrance
around the coordinating nitrogens increases from the L1 to L3
species.
Theoretical investigation of the IR and NMR spectra can be
employed as an important tool for identification of chemical
species, especially analyzing of proposed geometries for com-
pounds with undetermined X-ray structure [12–16,22]. Here, we
used this approach for identification of the newly synthesized com-
pounds.
3.2. Elemental analysis and mass spectroscopy
The elemental analysis results of the L1–L3 species are given in
Table 1, serving as a basis for the determination of their empirical
formulas. Also, the mass losses of the compounds were found in
consistent with the proposed formulas, which may be taken as extra
evidence for the correctness of these formulas. The molecular ion
peaks, m/z (M+), observed in the mass spectra of the synthesized
Schiff bases are given in Table 1.
3.3. Geometry optimization
No single crystal has been obtained for the investigated com-
pounds. Hence, their structural parameters have been determined
theoretically. The optimized structures of the L1–L3 species with
labeling of their atoms are shown in Fig. 1. From structural point of
view, the most difference between them is the –N–R–N– region,
which bridges two pyridoxal moieties. The selected structural
parameters of the Schiff bases are gathered in Table 2, which are in
agreement with the structural data reported for the similar com-
pounds [10,11,24–31].
As seen, the optimized structure of the L1 molecule is V shape.
Each of the benzene ring and two pyridine rings are in a separate
plane, where the pyridine rings make approximately a 20◦ dihedral
angle to each other. For example, the calculated N3–C5–C13–N4
and C1–O1–O2–C9 dihedral angles are 22.4 and −170.5◦, respec-
tively. In the bridge region, the azomethine nitrogens are bonded
to the ortho positions of the benzene ring.
3.4. 1H NMR spectra
The experimental and theoretical 1H NMR chemical shifts (ı) of
the L1, L2 and L3 Schiff bases are given in Table 3, where the atom
positions are numbered as in Fig. 1. The appearance of a signal at
about 14 ppm is attributed to the phenolic protons (H1, H2), where
their engagement in the intramolecular hydrogen bond interaction
(O–H· · ·N), shifts their signals upfield [12,13,32].
The calculated chemical shifts are in good agreement with
the experimental values, confirming suitability of the optimized
geometries for the species L1–L3. The only exception is the chem-
ical shift of the H9 and H17 atoms, where the calculated chemical
shifts are significantly lower than the experimental ones. These
alcoholic protons may be engaged in intermolecular hydrogen
bonds. On the other hand, the experimental data are from (CD3)2CO
solutions but the calculations correspond to the isolated molecule

S.A. Beyramabadi et al. / Spectrochimica Acta Part A 83 (2011) 467–471
469
Table 1
The CHN analysis and the molecular ion peak of the species L1–L3.
Compound
Calculation
C
Found
C
m/z (M+)
H
N
H
N
L1 = C22H22N4O4
L2 = C23H24N4O4
L3 = C28H26N4O5
65.00
65.69
67.45
5.47
5.76
5.27
13.79
13.33
11.24
64.78
65.13
67.18
5.34
5.76
5.24
13.43
13.27
11.37
406
420
498
Fig. 1. The B3LYP optimized geometries of the L1, L2 and L3 N,Nꢀ-dipyridoxyl Schiff bases.
in the gas phase. Obviously, the solvent molecules interact with the
alcoholic protons.
In the infrared spectra of the investigated species, a strong dou-
blet band is appeared at 1100–1020 cm−1, deconvolution of which
is given in Table 4. The very intensive band in the 1660–1500 cm−1
region is diagnostic of the IR spectra of Schiff bases [12,13,24–27].
For the L1 to L3 Schiff bases, this band is appeared at 1612, 1623 and
1631 cm−1, respectively, which is related to the stretching modes
of the azomethine C N bonds.
3.5. Vibrational spectroscopy
The selected vibrational frequencies of the species L1–L3
together with their intensities are gathered in Table 4. The vibra-
tional modes were analyzed by the results of DFT calculations and
the previously reported data [12,13,24–30]. As seen, the calculated
results have good consistency with the experimental ones.
In the 3600–2000 cm−1 region of the IR spectra, overlapping
of stretching vibrations of the O–H and C–H bonds leads to band
broadening [30,33], the deconvolution of which is given in Table 4.
The most intensive bands are related to the stretching vibrations

470
S.A. Beyramabadi et al. / Spectrochimica Acta Part A 83 (2011) 467–471
Table 2
Calculated structural parameters for the species L1–L3.
L1
L2
L3
L1
L2
L3
Bond lengths (pm)
C1–O1
C1–C2
C2–N3
C2–C6
C5–C8
C8–N1
N1–C17
N1–N2
O1–O2
Dihedral angles (◦)
H1–O1–C1–C5
O1–C1–C5–C8
C1–C2–N3–C3
C3–N3–C2–C6
C1–C5–C4–C7
C2–C1–C5–C8
C1–C5–C8–N1
O1–N1–N2–O2
C1–O1–O2–C9
C8–N1–C17–C19
133.9
141.7
132.4
150.2
145.4
128.9
140.5
276.1
408.0
99.4
134.0
141.2
132.5
150.2
145.5
128.9
140.9
484.8
879.4
99.4
134.1
−1.5
−0.3
−0.7
141.6
132.5
150.2
145.5
128.9
140.7
997.0
1326.6
99.4
0.5
−0.2
−0.3
0.5
−0.3
0.2
−179.8
−179.8
−179.4
−0.1
−179.9
179.9
179.4
−0.2
180.0
180.0
−179.4
0.3
−46.5
−102.9
34.1
48.1
42.9
−170.5
−98.6
O1–H1
42.0
36.9
N1–H1
N1–O1
171.6
260.5
171.8
261.0
171.7
260.8
Angles (◦)
H1–O1–C1
O1–C1–C5
C1–C5–C8
C5–C8–N1
C1–O1–O2
C8–N1–N2
107.6
122.8
120.3
122.1
110.7
143.7
107.4
122.6
120.4
122.3
133.3
144.8
107.4
122.7
120.4
122.1
138.8
148.8
Table 3
Experimental and theoretical 1H NMR chemical shifts of the L1–L3 in (CD3)2CO solution, ı [ppm].
L1
L2
L3
Atom position
Exp.
5.32
Theo.
Atom position
Exp.
5.46
Theo.
Atom position
Exp.
5.45
Theo.
H17
0.17
0.24
2.61
5.13
7.03
7.48
8.59
8.93
13.11
13.24
H17
H9
0.12
0.18
1.9
H17
0.08
0.13
2.57
5.1
7.07
7.26
8.48
8.71
13.62
13.66
H9
5.32
2.58
4.92
6.96
7.56
8.02
9.28
13.95
13.95
5.46
2.40
2.43
2.45
4.79
4.82
7.44
7.46
7.59
8.00
8.02
9.19
9.22
13.92
13.98
H9
5.45
2.42
4.78
7.18
7.58
7.99
9.18
13.91
13.91
Ha
Hb
Ha
H (CH2)
Ha
2.57
2.62
5.07
5.12
6.88
6.98
7.37
8.51
8.52
8.61
8.74
13.60
13.82
H (CH2)
H21, H26
Hc
H3, H11
H10, H18
H2
Ho
Ha
Hm
H (C15–H)
H (C7–H)
H19
H20
H21
H3
H11
H10
H18
H2
H3, H11
H10, H18
H2
H1
H1
H1
Ha = methyl hydrogen of the pyridine-substituted rings; Hb = methyl hydrogen of the methyl-substituted benzene ring; Hc = H19–H20 and H22–H25 atoms; Hm = meta hydrogen
of benzene with respect to the N atom; Ho = ortho hydrogen of benzene with respect to the N atom.
Table 4
Selected experimental and calculated IR vibrational frequencies (cm−1) of the species L1–L3.
L1
L2
L3
Vibrational assignment
Exp.
Cal. (intensity)
Exp.
Cal. (intensity)
Exp.
Cal. (intensity)
836 (m)
813 (118)
–
1021 (94)
1058 (136)
1177 (147)
–
829 (62)
863 (32)
1024 (74)
1059 (89)
1187 (79)
–
852 (m)
835 (109)
–
ıop(O–H) phenolic
ıop(C–H19)
ı(C–H) Me + ꢀ(py–C)
846 (w)
–
1020 (s)
1033 (s)
1206 (s)
–
–
1041 (m)
1056 (m)
1185 (m)
1222 (vs)
1283 (m)
1421 (s)
1038 (m)
1056 (s)
1182 (m)
–
1275 (m)
1412 (vs)
1508 (m)
1566 (w)
1623 (s)
1023 (68)
1059 (50)
1171 (118)
1216 (919)
1271 (90)
1372 (209)
1474 (254)
1459 (428)
1596 (340)
2872 (39)
2915 (19)
2953 (50)
2960 (13)
3056–3077 (30)
3063
ꢀ
asym(py–C–O) alcoholic
ꢀ(C–C, C–N)a
asym(C–O–C) bridge
ꢀ
1262 (w)
1378 (vs)
1268 (39)
1369 (198)
1521 (39)
1550 (93)
1595 (239)
2877 (37)
2916 (16)
2952 (37)
2961 (20)
3048–3073 (35)
3078 (486)
3154 (38)
1266 (86)
1371 (199)
1521 (67)
1555 (150)
1599 (146)
2875 (41)
2913 (23)
2929 (43)
2960 (13)
3044–3066 (25)
3069 (885)
3674 (35)
ꢀ(ph–C)
ꢀ(C–O) phenolic
ꢀ(pyridine ring)
ꢀ(benzene ring)
ꢀ(C N)
1510 (vs)
1631 (s)
2775
1546 (w)
1612 (vs)
ꢀ
ꢀ
sym(CH) alcoholic
sym(CH) Me
2820
3020
2873 (br)
ꢀ(C8H10) + ꢀ(C16–H18
asym(CH) Me
)
2865
ꢀ
ꢀ(C–H) aromatic
ꢀ(O–H) phenolic
ꢀ(O–H) alcoholic
3165 (br,s)
3170 (br,s)
3189 (br,s)
3674
Abbreviations: a: except for C N azomethine; w, weak; m, medium; s, strong; vs, very strong; br, broad.

S.A. Beyramabadi et al. / Spectrochimica Acta Part A 83 (2011) 467–471
471
of the phenolic O–H bonds. These vibrations are observed at lower
energies than the alcoholic O–H ones, which is consistent with the
weaker phenolic O–H bonds with respect to the alcoholic ones. Also,
the stretching vibrations of the aromatic C–H bonds have lower
energies than the aliphatic ones.
In the IR spectrum of the L3, a strong band was appeared in
the 1550–1500 cm−1 region. This broad and doublet band arises
from the overlapping of the stretching vibrations of the aro-
matic rings (Table 4). Also, appearance of a new intensive band
at 1222 cm−1 was assigned to the asymmetric stretching vibration
of the C21–O5–C26 bonds. As expected, no band is observed in this
region of the IR spectra of the species L1 and L3.
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dipyridoxyl Schiff-bases (L1–L3) have been newly synthesized and
experimentally characterized. The geometries of the Schiff bases
were optimized using the B3LYP level and 6–311 + G(d, p) basis sets.
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the species have been calculated at the same computational level.
The obtained results are in good agreement with the experimen-
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the species L1–L3. The DFT results can be used for analysis of the
similar compounds, too.
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