Synthesis, characterization, and crystal structure of two zinc(II) halide complexes with the symmetrical bidentate Schiff-base ligand (3,4-MeO-ba) 2en

Article abstract of DOI:10.1007/s00706-010-0296-2

In this study two zinc(II) halide complexes with the Schiff-base ligand (3,4-MeO-ba)₂en [N,N′-bis(3,4-dimethoxybenzylidene)ethane-1,2- diamine] have been synthesized and characterized by elemental analyses (CHN), single-crystal X-ray diffractio

Full text of DOI:10.1007/s00706-010-0296-2

Monatsh Chem (2010) 141:539–543
DOI 10.1007/s00706-010-0296-2
ORIGINAL PAPER
Synthesis, characterization, and crystal structure of two zinc(II)
halide complexes with the symmetrical bidentate Schiff-base
ligand (3,4-MeO-ba)2en
•
Aliakbar Dehno Khalaji Matthias Weil
•
•
Gholamhossein Grivani Samaneh Jalali Akerdi
Received: 30 December 2009 / Accepted: 15 March 2010 / Published online: 16 April 2010
Ó Springer-Verlag 2010
Abstract In this study two zinc(II) halide complexes with
the Schiff-base ligand (3,4-MeO-ba)2en [N,N0-bis(3,
4-dimethoxybenzylidene)ethane-1,2-diamine] have been
synthesized and characterized by elemental analyses
(CHN), single-crystal X-ray diffraction, Fourier-transform
infrared (FT-IR), and 1H nuclear magnetic resonance
(NMR) spectroscopy. The metal-to-ligand ratio was found
to be 1:1 within the formula ZnX2((3,4-MeO-ba)2en)
(X = Br, I). Crystal structure analysis reveals that the
coordination geometry around the zinc(II) ions in the two
isotypic complexes is distorted tetrahedral. The Schiff-base
ligand (3,4-MeO-ba)2en acts as a chelating ligand and
coordinates via two N atoms to the metal center and adopts
an (E,E) conformation. The coordination spheres of the
metal atoms are completed by the two halide atoms, which
are also involved in weak non-classical hydrogen-bonding
interactions of the type C–HꢀꢀꢀX–Zn.
Introduction
Schiff-base compounds are typically formed by condensa-
tion of a primary amine and an aldehyde [1, 2]; they are used
as substrates in the preparation of a number of transition-
metal complexes [3–5]. They have good catalysis ability
[6, 7] and are suitable for generation of supramolecular
structures [8, 9]. The coordination behavior of Schiff-base
ligands depends on the metal ion, the reaction conditions,
and the nature of the anion and the solvent used [10–15].
Transition-metal complexes with neutral bidentate Schiff-
base compounds as ligands have played an important role in
the development of coordination chemistry [16–20].
Zinc(II) complexes with Schiff-base ligands have been
investigated particularly for their catalytic activities [21]
and as potential luminescent materials [22]. Recent works
have focused on synthesis of new Schiff-base ligands
containing the diimine chelating function [23]. As an
additional contribution to the synthesis, characterization,
and crystal structures of zinc(II) Schiff-base complexes and
in the course of our ongoing studies of these kinds of
materials [24, 25], we describe herein the synthesis, char-
acterization, and crystal structures of two Zn(II) complexes
[ZnX2((3,4-MeO-ba)2en), X = Br (1), I (2)] (Scheme 1).
Keywords Zinc(II) complexes ꢀ Schiff base ꢀ
Crystallography ꢀ Tetrahedral coordination
A. D. Khalaji (&)
Department of Chemistry, Faculty of Science,
Golestan University, Gorgan, Iran
e-mail: alidkhalaji@yahoo.com
Results and discussion
Synthesis and characterization
M. Weil
Division of Structural Chemistry, Institute for Chemical
Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9/164-SC, 1060 Vienna, Austria
(3,4-MeO-ba)2en was prepared by condensation of 3,4-
dimethoxybenzaldehyde with ethylenediamine following
literature methods [26, 27]. 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
G. Grivani ꢀ S. J. Akerdi
School of Chemistry, Damghan University of Basic Science,
P.O. Box 36715-364, Damghan, Iran
123

540
A. D. Khalaji et al.
Scheme 1
O
N
N
CH3OH
2 OH2
+
2
+
NH2
H2N
OMe
MeO
OMe
MeO
OMe
MeO
ZnX2
X
compound
Br Zn((3,4-MeO-ba)2en)Br2 (1)
Zn((3,4-MeO-ba)2en)I2 (2)
N
N
I
Zn
X
X
OMe
MeO
OMe
MeO
in dimethyl formamide (DMF) and dimethyl sulfoxide
(DMSO), but insoluble in EtOH, MeOH, CHCl3, and
CH2Cl2. All compounds 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.
Infrared spectra
Fig. 1 Molecular structure of 1, showing 50% probability displace-
ment ellipsoids and atom numbering. Dashed lines indicate
nonclassical C–HꢀꢀꢀBr hydrogen bonds
The IR spectra of the free ligand (3,4-MeO-ba)2en exhibit
the characteristic band of the imine group (–C=N–), which
appears at 1636 cm-1 [26, 27]. This band is shifted in the
complexes toward lower frequencies because of the coor-
dination of the nitrogen atom to the metal ion [28], and
appears at 1625 cm-1 (1) and at 1623 cm-1 (2). The
observed red-shift of the –C=N– vibration with an addition
coordination to the Zn atom can be attributed to the metal-
to-ligand p-back bonding. The ligand’s stretching vibra-
tions at about 2820–3050 cm-1, assigned to C–H iminic,
aromatic, and aliphatic groups, are shifted to lower energy
after coordination to metal ions [28]. In the infrared spectra
of the complexes, the bands in the range 450–490 cm-1
that are absent in the spectrum of the pure ligand may be
due to vibrational modes of Zn–N bonds, supporting the
coordination of two iminic nitrogen atoms [28].
Fig. 2 Molecular structure of 2, showing 50% probability displace-
ment ellipsoids and the atom numbering. Dashed lines indicate
nonclassical C–HꢀꢀꢀI hydrogen bonds
1H NMR spectra
The Schiff-base ligand (3,4-MeO-ba)2en has symmetric
nature, therefore the hydrogen atoms in the two halves of
the –CH2–CH2– fragment are equivalent. The azomethine
groups show up at d = 8.16 ppm as a single signal [26,
27]. In the complexes, the signals related to this group are
shifted to lower regions [8.30 ppm (1) and 8.22 ppm (2)].
This shift shows that the nitrogen atom of the –CH=N–
group is coordinated to zinc(II) ions [29, 30]. Signals of
aromatic ring protons of the (3,4-MeO-ba)2en fragment
appear in the range 6.75–7.50 ppm and are also shifted to
the lower regions in the complexes. In the free ligand,
signals of hydrogen atoms of the methoxy groups and
123

Synthesis, characterization, and crystal structure of two zinc(II) halide complexes
541
Table 1 Selected bond distances and angles for 1 and 2
Table 3 Intra- and intermolecular hydrogen-bond geometry for 2
˚
˚
˚
1
2
D-HꢀꢀꢀA
D-H (A) HꢀꢀꢀA (A) DꢀꢀꢀA (A) D-HꢀꢀꢀA (°)
˚
Zn(1)-N(1) (A)
2.062 (2)
2.087 (2)
2.3550 (6)
2.3571 (8)
1.275 (3)
1.468 (3)
1.284 (3)
1.477 (3)
83.74 (9)
2.066 (5)
2.084 (5)
2.5475 (8)
2.5502 (8)
1.295 (7)
1.459 (8)
1.276 (8)
1.478 (8)
83.73 (19)
C(14)-H(14)ꢀꢀꢀI(1)
0.93
3.297
3.539
2.56
4.176 (8) 158
4.134 (8) 122
3.493 (9) 163
˚
Zn(1)-N(2) (A)
C(19)-H(19A)ꢀꢀꢀI(1) 0.96
C(19)-H(19B)ꢀꢀꢀO(1)a 0.96
˚
Zn(1)-X(1) (A)
˚
Zn(1)-X(2) (A)
a
-x, 1 - y, 1 - z
˚
N(1)-C(1) (A)
˚
N(1)-C(10) (A)
˚
N(2)-C(12) (A)
˚
N(2)-C(11) (A)
N(1)-Zn(1)-N(2) (°)
N(1)-Zn(1)-X(1) (°)
N(2)-Zn(1)-X(1) (°)
N(1)-Zn(1)-X(2) (°)
N(2)-Zn(1)-X(2) (°)
X(1)-Zn(1)-X(2) (°)
C(1)-N(1)-C(10) (°)
C(1)-N(1)-Zn(1) (°)
C(10)-N(1)-Zn(1) (°)
C(12)-N(2)-C(11) (°)
C(12)-N(2)-Zn(1) (°)
C(11)-N(2)-Zn(1) (°)
107.46 (7)
113.01 (6)
118.96 (6)
111.68 (7)
117.37 (3)
119.3 (2)
107.74 (15)
112.56 (15)
118.26 (15)
112.01 (15)
117.75 (3)
118.8 (5)
133.67 (19)
106.00 (16)
115.9 (2)
134.6 (4)
105.8 (4)
115.5 (5)
135.30 (18)
108.34 (16)
135.4 (4)
108.4 (4)
Table 2 Intra- and intermolecular hydrogen-bond geometry for 1
˚
˚
˚
D-HꢀꢀꢀA
D-H (A) HꢀꢀꢀA (A) DꢀꢀꢀA (A) D-HꢀꢀꢀA (°)
C(14)-H(14)ꢀꢀꢀBr(1)
0.93
3.122
3.358
2.56
3.999 (3) 158
3.961 (3) 123
3.521 (4) 170
3.514 (4) 168
Fig. 3 Non-classical C–HꢀꢀꢀO hydrogen bonds in complex 1 (dashed
lines)
C(19)-H(19A)ꢀꢀꢀBr(1) 0.96
C(10)-H(10B)ꢀꢀꢀO(3)a 0.97
C(19)-H(19B)ꢀꢀꢀO(1)b 0.96
2.57
a
1 ? x, 1 ? y, z
Zn(II) complexes [24, 25, 31]. The Br(1)–Zn(1)–Br(2)
angle is 117.37(3)° in 1 and I(1)–Zn(1)–I(2) is 117.75(3)°
in 2. The deviation from a regular tetrahedral angle
(109.5°) is due to the steric effects from the bulky bromo
and iodo groups. The other bond angles, N–Zn–Br in 1 and
N–Zn–I in 2, are also indicative of large deviation from the
idealized tetrahedral symmetry. These angles are similar to
those found in [ZnCl2(Phca)2en] [31], [ZnBr2(3,4,5-MeO-
ba)2en] [24], and [ZnI2(3,4,5-MeO-ba)2en] [25]. The
b
-x, 1 - y, 1 - z
ethylenediamine are observed in the 3.70–4.00 ppm region
[26, 27], and are likewise shifted to lower regions after
coordination [29, 30].
Description of the crystal structures of 1 and 2
´
´
˚
˚
average Zn–N bond lengths of 2.074 A in 1 and 2.075 A in
The molecular structures of the complexes 1 and 2,
including the atom-numbering schemes, are illustrated in
Figs. 1 and 2, respectively, and selected bond lengths and
angles are given in Table 1. The zinc(II) ions in 1 and 2 are
tetrahedrally coordinated by two nitrogen atoms from the
Schiff-base ligand (3,4-MeO-ba)2en and by two halogen
atoms. The coordination about the Zn(II) ions is consid-
erably distorted due to the restricting bite angles of the
chelating ligands. The N(1)–Zn(1)–N(2) angle is only
83.74(9)° in 1 and 83.73(19)° in 2, being in the same range
of 82(3)°–90(3)° found for ethylenediamine-chelated
´
´
˚
˚
2 and the Zn–X bond lengths of 2.356 A in 1 and 2.548 A
in 2 agree well with corresponding bond lengths in other
complexes with tetrahedrally coordinated zinc(II) atoms
[24, 25, 31]. The ligand (3,4-MeO-ba)2en adopts an (E,E)
configuration in these complexes [24, 25, 31]. The values
for the dihedral angles C(10)–N(1)–C(1)–C(2) and C(11)–
N(2)–C(12)–C(13) are 175.4(2)° and 176.1(2)° in 1 and
175.0(6)° and 173.5(6)° in 2, indicating an almost planar
configuration of this moiety for the complexes studied here.
There are similar non-classical hydrogen bonds of type
123

542
A. D. Khalaji et al.
Table 4 Crystallographic and refinement data for 1 and 2
1
2
Empirical formula
Formula weight
C20H24Br2N2O4Zn C20H24I2N2O4Zn
581.60 675.58
ꢀ
Crystal system, space group Triclinic, P 1
ꢀ
Triclinic, P 1
8.1860 (10)
10.4659 (13)
14.9302 (17)
99.725 (5)
99.572 (5)
107.444 (5)
1170.3 (2)
2
˚
a (A)
8.2334 (16)
10.014 (2)
14.920 (3)
99.84 (3)
101.35 (3)
106.54 (3)
1122.0 (4)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Dx (g/cm3)
1.722
1.917
l (mm-1
)
4.68
3.71
T
min, Tmax
0.250, 0.603
29520
0.187, 0.569
20286
Measured reflections
Independent reflections
Reflection with I [ 2r(I)
Parameters
6463
4516
4429
3457
266
266
Fig. 4 Non-classical C–HꢀꢀꢀO hydrogen bonds in complex 2 (dashed
lines)
Rint
0.038
0.046
S
1.01
1.03
R[F2 [ 2r(F2)]
wR(F2)
0.035
0.051
0.090
0.141
C(14)–H(14)ꢀꢀꢀX(1)–Zn and C(19)–H(19A)ꢀꢀꢀX(1)–Zn in
the crystal of 1 and 2 (Tables 2, 3; Figs. 1, 2). These
intramolecular hydrogen bonds are formed between aro-
matic and methyl H-atoms of the bidentate Schiff-base
ligand and the coordinated halogen atom. The [ZnX2((3,4-
MeO-ba)2en)] molecules are eventually linked together via
other non-classical C–HꢀꢀꢀO hydrogen bonds (Figs. 3, 4;
Tables 2, 3).
-3
˚
Dqmax, Dqmin (eA
)
0.78, -0.32
2.95, -0.67
General procedure for synthesis of the zinc(II)
complexes
The Zn(II) complexes were synthesized by addition of the
(3,4-MeO-ba)2en ligand (0.2 mmol, in 5 cm3 chloroform)
to a solution of the anhydrous metal salts (ZnX2, 0.2 mmol,
10 cm3 methanol, 1: X = Br, 2: X = I). The resulting
solution was stirred in air at room temperature for about
2 h and was then left at 273 K for several days without
disturbance, yielding single crystals of the complexes that
subsequently were filtered off and washed with Et2O.
Experimental
Materials and characterization
All reagents and solvents for synthesis and analysis were
commercially available and used as received without fur-
ther purifications. Infrared spectra were recorded using
KBr disks on a FT-IR PerkinElmer spectrophotometer.
Elemental analyses were carried out using a Heraeus
CHN–O-Rapid analyzer, and results agreed with calculated
Dibromo[N,N0-bis[(3,4-dimethoxyphenyl)methyl]-1,2-eth-
anediamine-jN, jN0]zinc (1, C20H24Br2N2O4Zn)
ꢀ
Light yellow crystals. Yield: 75%. IR (KBr): m ¼ 2837
(–HC=N–), 2917–2966 (C–H aliphatic), 2999 (C–H
aromatic), 1625 (s, C=N), 1472–1596 (C=C aro-
1
matic) cm-1; H NMR (DMSO-d6): d = 3.77 (s, 16H, 2
CH3–O– and –CH2–CH2–), 6.97 (d, 2H), 7.21 (dd, 2H),
1
values. H NMR spectra were measured on a BRUKER
DRX-500 AVANCE spectrometer at 500 MHz for the
Schiff-base ligand and on a BRUKER DRX-400 AVANCE
spectrometer at 400 MHz for zinc(II) complexes. All
chemical shifts are reported in d units downfield from
tetramethyl silane (TMS). (3,4-MeO-ba)2en was synthe-
sized according to Refs. [26, 27], and measured analytical
data were in agreement with values reported there.
7.40 (d, 2H), 8.30 (s, 2H, –CH=N–) ppm.
Diiodo[N,N0-bis[(3,4-dimethoxyphenyl)methyl]-1,2-eth-
anediamine-jN, jN0]zinc (2, C20H24I2N2O4Zn)
ꢀ
Yellow crystals. Yield: 79%. IR (KBr): m ¼ 2835 (–HC=
N–), 2915–2963 (C–H aliphatic), 3004 (C–H aromatic),
1623 (s, C=N), 1469–1594 (C=C aromatic) cm-1 1H
;
123

Synthesis, characterization, and crystal structure of two zinc(II) halide complexes
543
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CH3–O–), 3.78 (s, 4H, –CH2–CH2–), 6.96 (dd, 2H), 7.18
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Single crystals of 1 with dimensions 0.41 9 0.18 9
0.12 mm3 and 2 with dimensions 0.28 9 0.20 9 0.16 mm3
were chosen for X-ray diffraction studies. Intensity data
collections were performed with Mo Ka radiation
˚
(k = 0.71073 A) at room temperature on an APEXII four-
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full-matrix least-squares technique on F2. H atoms were
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˚
positioned geometrically (C–H distance = 0.93 or 0.96 A)
and were refined as riding, with Uiso(H) = 1.2Ueq(C) or
1.5Ueq(methyl C), allowing for free rotation of the methyl
groups. For structure 2 the highest remaining electron
˚
density peak and the deepest hole are located 1.04 A from
˚
atom C20 and 0.80 A from atom Zn1, respectively. Struc-
ture data and details of the data collections and structure
refinements are listed in Table 4. The molecular structure
plots were prepared by ORTEP III for Windows [34].
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Center, CCDC
Nos. 759100 (1) and 759099 (2). Copies of the data can be
obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, fax:
?44-1223-336033, e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk.
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Acknowledgments We acknowledge the Golestan University (GU)
and Damghan University of Basic Science (DUBS) for partial support
of this work.
31. Amirnasr M, Mahmoudkhani AH, Gorji A, Dehghanpour S,
Bijanzadeh H (2002) Polyhedron 21:2733
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123