Synthesis, characterization and computational studies of zinc(II)-halide complexes with a bidentate schiff base ligand (2,5-MeO-ba)2En: The crystal structure of (2,5-MeO-ba)2En

Article abstract of DOI:10.1134/S0022476613040161

In this study four coordinated complexes of zinc(II) halides with a new symmetrical bidentate Schiff base ligand (2,5-MeO-ba)₂En are synthesized and characterized. The metal to ligand ratio of the complexes is found to be 1:1 with the formula of Zn((2,5-MeO-ba)₂En)X₂ (X = Cl (1), Br (2), I (3)). The crystal structure of the Schiff base ligand (2,5-MeO-ba)₂En is determined by X-ray crystallography from single crystal data. Also, the optimized geometries of the Schiff base ligand (2,5-MeO-ba)₂En and its zinc(II) complexes are calculated using the density functional theory method (B3LYP/6-31G). The obtained structural parameters of (2,5-MeO-ba)₂En are in agreement with the experimental data.

Full text of DOI:10.1134/S0022476613040161

Journal of Structural Chemistry. Vol. 54, No. 4, pp. 766-773, 2013
,
Original Russian Text Copyright © 2013 by A. D. Khalaji, H. Mighani, M. Gholinejad, G. Grivani, S. Jalali Akerdi, K. Fejfarova, M. Dusek
SYNTHESIS, CHARACTERIZATION AND COMPUTATIONAL
STUDIES OF ZINC(II)-HALIDE COMPLEXES WITH A
BIDENTATE SCHIFF BASE LIGAND (2,5-MeO-ba) En:
2
THE CRYSTAL STRUCTURE OF (2,5-MeO-ba) En
2
1
© A. D. Khalaji, H. Mighani, M. Gholinejad, G. Grivani,
1
1
2
UDC 541.6:547.13:546.47
2
3
S. Jalali Akerdi, K. Fejfarova, and M. Dusek
3
In this study four coordinated complexes of zinc(II) halides with a new symmetrical bidentate Schiff base
ligand (2,5-MeO-ba)2En are synthesized and characterized. The metal to ligand ratio of the complexes is
found to be 1:1 with the formula of Zn((2,5-MeO-ba)2En)X2 (X = Cl (1), Br (2), I (3)). The crystal structure
of the Schiff base ligand (2,5-MeO-ba)2En is determined by X-ray crystallography from single crystal data.
Also, the optimized geometries of the Schiff base ligand (2,5-MeO-ba)2En and its zinc(II) complexes are
calculated using the density functional theory method (B3LYP/6-31G). The obtained structural parameters
of (2,5-MeO-ba)2En are in agreement with the experimental data.
DOI: 10.1134/S0022476613040161
Keywords: zinc(II) halides, Schiff base, crystal structure, density functional theory.
Azomethines (known as Schiff bases) formed by the condensation of a primary amine and an aldehyde [1, 2] developed
as a new class of organic compound in 1864 by Hugo Schiff [3] have been widely used as ligands in the preparation of transition
metal complexes [4, 5] The coordination behavior of Schiff base ligands depends on the metal ion, the reaction conditions, and
the nature of the anion used [4-12]. Schiff base compounds have played an important role in the development of coordination
chemistry [4-12]; they exhibit photochromism and thermochromism [13], anti-microbial activity [16-18] and are used in non-
linear optics [14, 15], anion sensors [19] and other applications. Zinc(II) complexes with Schiff base ligands have been
investigated particularly for their catalytic activities [20] and as potential luminescent materials [21]. As an additional
contribution to the synthesis, characterization and crystal structures of transition metal Schiff base complexes and in the course
of our ongoing studies of these kinds of materials [22-24], we describe here the preparation, characterization and computational
studies of a new symmetrical bidentate Schiff base ligand (2,5-MeO-ba)2En and its Zn(II) complexes (Fig. 1).
EXPERIMENTAL
All reagents and solvents for the synthesis and analysis were commercially available and used as received without
further purifications. Infrared spectra were recorded using KBr disks on a FT–IR Perkin–Elmer spectrophotometer. Elemental
1
analyses were carried out using a Heraeus CHN-O-Rapid analyzer. H-NMR spectra were measured on a BRUKER
1
Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran; alidkhalaji@yahoo.com;
2
3
ad.khalaji@gu.ac.ir. School of Chemistry, Damghan University, Damghan, Iran. Institute of Physics, Academy of Sciences,
Prague, Czech Republic. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 54, No. 4,
pp. 721-727, July-August, 2013. Original article submitted March 4, 2012.
766
0022-4766/13/5404-0766

Fig. 1. Chemical structure of the Schiff base ligand (2,5-MeO-ba)2En
and its Zn(II) complexes. X = Cl (1), Br (2), and I (3).
DRX-500 AVANCE spectrometer at 500 MHz for the Schiff base ligand and at 400 MHz for the zinc(II) complexes. All
chemical shifts are reported in δ units downfield from TMS.
Preparation of (2,5-MeO-ba) En. A solution of 2,5-dimethoxybenzaldehyde (0.02 mol) in 25 ml of methanol was
2
heated for 25 min at 50°C and then stirred for about 25 min. To this stirring solution, a solution of ethylenediamine
(0.01 mol) in 20 ml of methanol was added dropwise under constant stirring. The mixture was heated at about 50°C for 1.5 h
and then allowed to cool overnight to room temperature. After evaporation of the solvent the resulting crude solid was
collected by filtration and dried at room temperature. Colorless crystals were grown by the slow evaporation technique at
room temperature in 25 ml of a methanol-chloroform (1:4 v/v) solution for 5 days. At the period of super saturation, tiny
crystals were nucleated. They were allowed to grow to a maximum possible size, then filtered off from the mother liquor and
were finally dried at room temperature. The yield was 91%. Anal. Calc.,%: C, 67.39; H, 6.79; N, 7.86. Found, %: C, 67.24; H,
–1
1
6.73; N, 7.32. FT–IR (KBr pellet, cm ): 2834 (–HC=N), 2880-2970 (C–H aliphatic), 3012 (C–H, aromatic), 1635 (s, C=N). H
NMR (CDCl3, δ(ppm)): 3.75 (s, 6H), 3.78 (s, 6H), 3.86 (s, 4H), 6.80 (d, 2H), 6.91 (dd, 2H), 7.46 (d, 2H), 8.67 (s, 2H).
General procedure for the zinc(II) complexes. The Zn(II) complexes were synthesized by the addition of the (2,5-
MeO-ba)2En ligand (0.2 mmol, in 5 ml of chloroform) to a solution of anhydrous metal salts (ZnX2, 0.2 mmol, 10 ml
methanol). The resulted solution was stirred in air at room temperature for about 2 h and then left at 273 K for several days
without disturbance, yielding single crystals of the complexes which subsequently were filtered off and washed with Et2O.
Zn((2,5-MeO-ba) En)Cl (1): Bright yellow crystals. The yield was 86%. Anal. Calc., %: C, 48.75; H, 4.91; N,
2
2
–1
5.66. Found, %: C, 46.14; H, 4.99; N, 5.45. FT–IR (KBr pellet, cm ): 2837 (–HC=N), 2900-2980 (C–H aliphatic), 3001 (C–
1
H, aromatic), 1628 (s, C=N). H NMR (CDCl3, δ(ppm)): 3.66 (s, 6H), 3.70 (s, 6H), 3.81 (s, 4H), 6.96 (s, 4H), 7.29 (s, 2H),
8.54 (s, 2H).
Zn((2,5-MeO-ba) En)Br (2): Bright yellow crystals. The yield was 83%. Anal. Calc., %: C, 41.30; H, 4.16; N,
2 2
–1
4.82. Found, %: C, 41.12; H, 4.04; N, 4.88. FT–IR (KBr pellet, cm ): 2834 (–HC=N), 2900-2970 (C–H aliphatic), 3001 (C–
1
H, aromatic), 1618 (s, C=N). H NMR (CDCl3, δ(ppm)): 3.67 (s, 6H), 3.70 (s, 6H), 3.83 (s, 4H), 6.97 (s, 4H), 7.30 (s, 2H),
8.55 (s, 2H).
Zn((2,5-MeO-ba) En)I (3): Bright yellow crystals. The yield was 85%. Anal. Calc., %: C, 35.55; H, 3.58; N, 4.15.
2 2
–1
Found, %: C, 35.75; H, 3.44; N, 4.17. FT–IR (KBr pellet, cm ): 2832 (–HC=N), 2900-2965 (C–H aliphatic), 2994 (C–H,
1
aromatic), 1617 (s, C=N). H NMR (CDCl3, δ(ppm)): 3.67 (s, 6H), 3.70 (s, 6H), 3.81 (s, 4H), 6.98 (s, 4H), 7.29 (s, 2H), 8.54
(s, 2H).
Single crystal X-ray analysis. A single crystal of the Schiff base ligand (2,5-MeO-ba)2En with the dimensions
0.24×0.11×0.09 mm was chosen for the X-ray diffraction study. Crystallographic measurements were conducted at room
767

TABLE 1. Crystal Data and Summary of Experimental Details for the (2,5-MeO-ba)2En Ligand
Empirical formula
Formula weight
Crystal system
Space group
T, K
C20H24N2O4
356.4
Monoclinic
P21/c
120
a, b, c, Å
4.51540(10), 14.3707(3), 14.3946(3)
92.6738(18)
933.04(3)
2
β, deg
3
V, Å
Z
–1
0.72
μ, mm
Tmin, Tmax
0.860, 0.934
4263/1465
1277
Measured/Independent reflections
Reflection with I > 3σ(I )
Rint
S
0.013
1.76
2 2
R [F > 2σ(F )]
0.031
2
wR (F )
0.090
Parameters
119
–3
0.13/–0.13
Δρmax/Δρmin, e/Å
Crystal size, mm
0.24×0.11×0.09
temperature with a four circle Gemini CCD diffractometer of Oxford Diffraction, Ltd., using CuK radiation (λ = 1.54184 Å).
α
The crystal structures were solved by direct methods with the SIR2002 program [25] and refined with the Jana2006 program
2
package [26] by the full-matrix least-squares technique on F . The molecular structure plots were prepared by ORTEP III
[27] for Windows. Hydrogen atoms were mostly discernible in difference Fourier maps and could be refined to reasonable
geometry. According to common practice they were nevertheless kept in ideal positions during the refinement. The isotropic
atomic displacement parameters of hydrogen atoms were evaluated as 1.2Ueq of the parent atom. Crystallographic data and
details of the data collection and structure refinements are listed in Table 1.
Computational details. The geometry of the Schiff base ligand (2,5-MeO-ba)2En and its zinc(II) complexes 1-3
have been optimized using the HF model with the B3LYP/6-31g basis set. All HF calculations were performed using the
Gaussian 98 R-A.9 package [28].
RESULTS AND DISCUSSION
The analytical and physical data of the Schiff base ligand (2,5-MeO-ba)2En and its Zn(II) complexes are given in the
EXPERIMENTAL section. The analytical data show that the metal to ligand ratio is 1:1 in all the complexes. The Schiff base
1
ligand (2,5-MeO-ba)2En was synthesized in a one-step process and characterized by elemental analyses, infrared and H
NMR spectroscopy, and single crystal X-ray crystallography. Zinc(II) complexes Zn(2,5-MeO-ba)2EnX2 (X = Cl (1), Br (2), I
1
(2)) were characterized by elemental analyses, infrared and H NMR spectroscopy.
The Schiff base ligand (3,4-MeO-ba)2En is soluble in common solvents such as CHCl3 and CH2Cl2, while the
respective Zn complexes are soluble in DMF and DMSO, but insoluble in EtOH, MeOH, CHCl3, and CH2Cl2. All complexes
synthesized in this work are stable at room temperature in the solid state. The stability of dissolved complexes depends on the
nature of the solvent. They are stable in coordinating solvents such as DMF and DMSO at room temperature for about 10 days
and at 273 K for about 20 days. The Schiff base ligand (2,5-MeO-ba)2En was obtained as colorless prism crystals with 93%
768

Fig. 2. Molecular structure of the Schiff base ligand
(2,5-MeO-ba)2En, showing 50% probability
displacement ellipsoids and atom numbering.
yield, and this yield was higher than the yield of the complexes. The Schiff base ligand (2,5-MeO-ba)2En coordinated to metal
ions as a chelating ligand to form mononuclear four coordinated zinc(II) complexes with suggested structures is shown in Fig. 1.
The IR spectra of the free (2,5-MeO-ba)2En ligand exhibit the characteristic band of the imine group (–C=N–),
–1
which appears at 1635 cm [22]. This band is shifted in the complexes toward lower frequencies because of the coordination
–1
of the nitrogen atom to the metal ion [22], and appears at 1628 cm (1), at 1618 cm (2), and at 1617 cm (3). The observed
–1
–1
red shift of the –C=N– vibration with an additional coordination to the Zn atom can be attributed to the metal-to-ligand π-
–1
back bonding. The ligand stretching vibrations at about 2850-3010 cm assigned to C–H iminic, aromatic, and aliphatic
groups are shifted to lower energy after coordination to metal ions. In the infrared spectra of the complexes, the bands in the
–1
range 440-500 cm that are absent in the spectrum of the pure ligand may be due to the vibrational modes of Zn–N bonds,
supporting the coordination of two iminic nitrogen atoms [29].
1
The H NMR spectra of the (2,5-MeO-ba)2En ligand and its complexes were recorded using CDCl3 and DMSO-d
6
as the solvent respectively, and their data were summarized in the EXPERIMENTAL section. The Schiff base ligand has a
symmetric nature, therefore the hydrogen atoms in the two halves of the –CH2–CH2– are equivalent [30, 31]. For example,
1
two hydrogen atoms of the azomethine groups (–CH=N–) have one signal in the H NMR spectrum. The azomethine groups
(–CH=N–) were shown at δ = 8.67 ppm as a singlet. In the metal complexes, these groups shifted to the lower regions due to
the coordination of the nitrogen atom of the –CH=N– group to metal ions [30-33]. Aromatic ring protons of (2,5-MeO-ba)2En
1
are shown in the range of δ = 6.80 ppm to δ = 6.91 ppm. In the H NMR spectrum of the complexes, the aromatic ring
protons are shifted to lower regions. In the ligand, hydrogen atoms of the methoxy groups are observed at δ = 3.75 and
δ = 3.78, and the signals of these protons are shifted to weak fields after coordination [30-33].
Crystal structure of the Schiff base ligand (2,5-MeO-ba) En. The Schiff-base ligand (2,5-MeO-ba)2En
2
crystallizes in the centrosymmetric space group P21/c. The C20H24N2O4 molecule has a centre of symmetry with one half-
molecule in the asymmetric unit. The molecular structure of (2,5-MeO-ba)2En with the atom numbering scheme is given in
Fig. 2 and the crystal packing in the unit cell is shown in Fig. 3. Selected bond distances and angles are listed in Table 2. All
bond distances and angles are normal and are in good agreement with those reported for similar Schiff-base compounds [23].
The C(7)=N(1) bond length of 1.2660(14) Å conforms to the value for a double bond, while the C(8)–N(1) bond length of
1.4544(15) Å conforms to the value for a single bond, as in similar Schiff base compounds [23]. The C(1)–C(7)–N(1) and
2
C(7)–N(1)–C(8) bond angles are 121.90(10)° and 117.30(9)° respectively, which is consistent with the sp hybrid character of
C(7) and N(1) atoms. The two 2,5-dimethoxybenzylidene groups in (2,5-MeO-ba)2En (Fig. 3) are bridged by the –CH2–CH2–
fragment via two C=N double bonds, with the C(1)…C(1i) distance between aromatic rings of ca 8.283 Å. A weak C(5)–
H(5)⋯N(1) hydrogen bond with the H–N distance of 2.6039(9) Å and the C–H–N angle 163.485(71)° exists in the structure.
This bond connects the molecules into a slab extended along the bc plane, without connection in a direction.
769

Fig. 3. Crystal packing of the Schiff base ligand (2,5-MeO-ba)2En, viewed along the a axis and showing a weak
C5–H5…N1 hydrogen bond.
TABLE 2. Selected Bond Distances (Å) and Angles (deg) for (2,5-MeO-ba)2En
Distances
Exp
B3LYP (6-31G)
Angles
Exp
B3LYP (6-31G)
O(1)–C(3)
O(1)–C(9)
N(1)–C(7)
N(1)–C(8)
O(2)–C(6)
O(2)–C(10)
C(1)–C(7)
1.3727(14)
1.4183(16)
1.2660(14)
1.5444(15)
1.3705(13)
1.4230(14)
1.4753(15)
1.5135(16)
1.3781
1.4567
1.2792
1.5785
1.3781
1.4585
1.4712
1.5451
C(3)–O(1)–C(9)
C(7)–N(1)–C(8)
C(2)–C(1)–C(6)
C(2)–C(1)–C(7)
C(6)–O(2)–C(10)
N(1)–C(7)–C(1)
117.99(9)
117.30(9)
119.06(10)
120.21(9)
117.44(9)
121.90(10)
110.21(9)
120.72(9)
117.58
117.49
118.85
120.41
118.33
121.69
108.63
119.87
i
N(1)–C(8)–C(8 )
i
C(8)–C(8 )
C(6)–C(1)–C(7)
Structural description of zinc(II) complexes from theoretical studies. As shown in Figs. 4, 5, and 6, our
theoretical results demonstrate that the zinc(II) ion is coordinated with two nitrogen imines of the Schiff base ligand and two
halide ions, forming a four-coordination tetrahedral geometry. Selected theoretical bond distances and angles of the described
coordination environment of zinc(II) complexes are given in Table 3.
Depending on the halide effects, the coordination geometry of the zinc(II) complexes containing bidentate Schiff
base ligands does not differ from tetrahedral. In the presence of the bidentate N2 Schiff base ligand, the coordination
geometry around the zinc(II) ion adopts a distorted tetrahedral geometry.
Theoretical results (Table 3) show that the Zn–X distances are longer than those of Zn–N bonds. The Zn–N bond
distances are 1.92 Å, 2.06 Å, and 2.07 Å in 1-3 respectively, while the Zn–I distances are 2.29 Å, 2.38 Å, and 2.57 Å in 1-3
respectively [31, 32]. The C=N double bond distancesare of about 1.29 Å in all complexes, while the N–C single bond
distances are of about 1.48 Å in all complexes [31, 32].
770

Fig. 4. Molecular structure of 1 from the calculation.
Fig. 5. Molecular structure of 2 from the calculation.
Fig. 6. Molecular structure of 3 from the calculation.
TABLE 3. Selected Theoretical Bond Distances (Å) and Angles (deg) for 1, 2, and 3
Angles Angles
1.488 1.474 1.474 N10Zn13N14 88.80 84.78 84.77 Zn13N10C1 128.62 126.42 126.40
Distances
1
2
3
1
2
3
1
2
3
N10–C11
N10–C1
1.289 1.292 1.292 X28Zn13X29 118.01 132.86 139.53 Zn13N10C11 107.13 107.47 107.71
1.497 1.476 1.477 X28Zn13N10 108.62 110.0 108.54 Zn13N14C12 108.23 107.01 106.26
1.293 1.293 1.295 X29Zn13N10 108.49 102.70 106.70 Zn13N14C15 133.90 132.61 133.69
N14–C12
N14–C15
N14–Zn13 1.928 2.069 2.070 X28Zn13N14 111.09 110.19 109.73 C11N10C1 124.22 125.88 125.74
N10–Zn13 1.929 2.060 2.066 X29Zn13N14 117.49 105.61 106.70 N10C1C2 129.58 129.33 129.32
X29–Zn13 2.293 2.386 2.571 C12N14C15 117.39 119.76 119.42
X28–Zn13 2.295 2.377 2.561 N14C15C16 125.00 124.68 124.84
It is worth noting the angular deviations of the coordination environment from the ideal tetrahedral geometry: NZnN
(88.88° in 1, 84.78° in 2, and 84.77° in 3) and XZnX (118.01° in 1, 132.86° in 2, and 139.53° in 3). However, the XZnX
angles have opened up due to the steric effect from the bulky X groups [31, 32]. These deviations reflect the chelating ring
strain in the place of imine nitrogen atoms. The four NZnX bond angles are found to be in the range 108-111° in 1, 102-105°
in 2, and 106-109° in 3°, which are different from those in a regular tetrahedron (109.5°).
771

CONCLUSIONS
In summary, a new Schiff base ligand (2,5-MeO-ba)2En and its zinc(II) complexes were synthesized and
1
characterized by elemental analyses, FT–IR and H NMR spectroscopy. The crystal and molecular structure of the Schiff
base ligand (2,5-MeO-ba)2En was successfully characterized. Elemental analyses confirm the chemical composition of the
1
synthesized compounds, while FT–IR and H NMR spectroscopy confirm the functional groups, particularly –HC=N imine
groups, of the compounds. The Schiff base ligand (2,5-MeO-ba)2En has a centre of symmetry with one half-molecule in the
asymmetric unit.
We acknowledge the Golestan University (GU) and Damghan University (DU) for partial support of this work, the
institutional research plan No. AVOZ10100521 of the Institute of Physics ASCR, v.v.i. Na Slovance 2, 18221 Prague 8,
Czech Republic.
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