Synthesis and characterization of a novel unsymmetrical tetradentate schiff base complex of zinc(II) derived from N,N′-bis (5-bromosalicylidene) 2,3-diaminopyridine (H2l): Crystal structure of [ZN(ii)l]pyridine

Article abstract of DOI:10.1007/s10870-010-9749-z

Yellow crystals of [Zn(N,N′-bis (5-bromosalicylidene) 2,3-diaminopyridine)] Pyridine, [Zn(C₁₉H₁₁Br ₂N₃O₂)]C₅H₅N, have been obtained and characterized by elemental analysis, FT-IR

Full text of DOI:10.1007/s10870-010-9749-z

J Chem Crystallogr (2010) 40:831–836
DOI 10.1007/s10870-010-9749-z
ORIGINAL PAPER
Synthesis and Characterization of a Novel Unsymmetrical
Tetradentate Schiff Base Complex of Zinc(II) Derived
from N,N0-bis (5-Bromosalicylidene) 2,3-Diaminopyridine (H2L):
Crystal Structure of [Zn(II)L]Pyridine
•
Kamel Ouari Ali Ourari Jean Weiss
•
Received: 8 April 2008 / Accepted: 16 April 2010 / Published online: 5 May 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Yellow crystals of [Zn(N,N0-bis (5-bromosalicy-
lidene) 2,3-diaminopyridine)] Pyridine, [Zn(C19H11Br2N3O2)]
C5H5N, have been obtained and characterized by elemental
analysis, FT-IR, UV–Vis, 1H NMR spectroscopy and
MALDI-TOF mass spectrometry. The structure has been
determined by single crystal X-ray crystallography. The
complex crystallizes in the triclinic space group P-1 with
two molecules in the asymmetric unit and with unit cell
two nitrogen donor atoms (NNOO) have attracted consid-
erable attention during the last decades owing to the ease of
their synthesis and their structural versatility associated to
their diverse applications. This class of complexes obtained
from transition and non-transition metals with Schiff base
ligands has emerged as of very promising materials for
their use as efficient catalysts [1–3], materials [4, 5], bio-
logical sensors [6–20], antibacterial, antifungal [21] and
also as anticancer drugs [22, 23]. For example, in catalysis
and electrocatalysis, the Mn(III)- or Ni(II)-Schiff base
complexes are currently used as catalysts in the epoxida-
tion or oxidation reactions [24–30]. However, these com-
pounds namely those of zinc seem to be efficient materials
in the optoelectronic uses due to their outstanding photo-
(PL) and electroluminescence (EL) properties particularly
those of zinc [31–38] since, the first work reported by
Vanslyke et al. [39] on the organic light-emitting devices
(OLEDs) materials. Taking into account the existing
extensive investigation of symmetrical tetradentate Schiff
base complexes, our efforts have been focused on non-
symmetrical species that have drawn relatively less atten-
tion [40–44]. More recently, we have described some tet-
radentate unsymmetrical Schiff base complexes [45, 46]
where the bridging moiety positioning the two salicylal-
dehyde derivatives was 2,3-diaminophenol whereas in the
present case, the chosen spacer is 2,3-diaminopyridine.
Pyridine was introduced as a general potential pH
sensitive group, and also due to the nitrogen affinity for
zinc(II). On the long term, this study is also motivated by
further synthesis of new Schiff base structures containing
electropolymerizable units as pyrrole, thiophene or aniline
in order to use them as monomers to elaborate modified
electrodes or electronic devices using quaternization of the
pyridine moiety as a functionalization tool. In this work,
the d10 electronic configuration of zinc(II) allows the
˚
dimensions a = 8.0103(3) A, b = 12.8340(5) A, c =
˚
˚
22.3682(11) A, a = 91.715(2)°, b = 93.918(2)°, c =
3
˚
103.708(2)°. V = 2226.30(16) A , Z = 4, R1 = 0.0780 and
wR2 = 0.1192. X-ray structure determination revealed that
the complex consists of a [Zn(C19H11Br2N3O2)] moiety
with a coordinated pyridine molecule on the metallic
centre. The zinc atom adopts a distorted square pyramidal
geometry as it is shown in the crystal structure.
Keywords Unsymmetrical Schiff base ꢀ
Zinc(II) complex ꢀ Crystal structure ꢀ NMR-ROESY
Introduction
The synthesis of nonporphyrinic ligands as Schiff base
complexes containing salen chelates with two oxygen and
K. Ouari (&) ꢀ A. Ourari
´
´
´
Laboratoire d’Electrochimie, d’Ingenierie Moleculaire et de
Catalyse Redox (LEIMCR), Faculte des Sciences de l’Ingenieur,
´
´
´
Universite Ferhat Abbas, 19000 Setif, Algeria
e-mail: k_ouari@yahoo.fr
´
J. Weiss
`
ˆ ´
Laboratoire de Chimie des Ligands a Architecture Controlee
(CLAC), Institut de Chimie, UMR 7177 CNRS-ULP, BP 1032,
67070 Strasbourg, France
123

832
J Chem Crystallogr (2010) 40:831–836
formation of coordination complexes exhibiting a wide
range of symmetries and various coordination numbers 4, 5
and 6 [47–49]. Thus, the ligand (H2L) built on a pyridine
spacer afforded a five coordinated complex, which was
identified and characterized by spectroscopic methods such
calculated for C19H13O2N3Br2: C, 48.03; H, 2.76; N,
8.84%; found: C, 47.70; H, 2.92; N, 8.86%. Selected IR
data (KBr pellets, t cm-1): 3445 (O–H), 1625 (C=N), 1607
(C=C), 1271 (C–O); UV–Vis: DMF, k nm, [e 9 10-4
M-1 cm-1]: 267 [3.114] 346 [1.978], 482 [0.973];
1H-NMR: (DMSO-d6, d ppm): 13.38 (s, OH), 12.79 (s,
OH), 9.47 (s, CH=N), 8.57 (s, CH=N), 6.94–7.63 (m, ArH);
MS (MALDI-TOF, ditranol): M? = 475.94.
1
as UV–Vis, IR, H NMR and X-rays studies. The crystal
structure revealed that the four in-plane coordination sites
are occupied by the salen moiety, whereas an apical fifth
position is occupied by a pyridine ligand.
Synthesis of the Complex Zn(II)L
9.17 mg (0.5 mmol) of Zn(OAc)2 were dissolved in MeOH
(10 mL). This solution was added dropwise to a stirred
methanol solution (5 mL) containing 237 mg (0.5 mmol) of
Schiff base ligand H2L. The solution turned yellow and a
solid precipitated almost immediately. The mixture was
refluxed and stirred for 1 h under nitrogen atmosphere. The
precipitate was collected by filtration, and washed succes-
sively with methanol (3 9 5 mL) and diethyl ether
(3 9 5 mL), the product was then recrystallized from
DMSO-heptane and dried under vacuum overnight. Yield:
71%; mp [ 300 °C. Analysis calculated for C19H11O2N3
Br2Zn. DMSO: C, 40.90; H, 2.78; N, 6.81%; found: C,
40.19; H, 2.72; N, 7.57%; IR (KBr pellets, t cm-1): 3439
(O–H), 1610 (C=N), 1563 (C=C), 1301 (C–O); UV–Vis:
DMF, k nm, [e 9 10-4 M-1 cm-1]: 268 [2.594], 309
Experimental
Materials and Measurements
All preparations were carried out in reagent grade solvents
and all commercially available reagents (Aldrich or Merck)
were used without any further purification. The manipu-
lations were carried out under inert atmosphere (N2) fol-
lowing conventional procedures.
The melting points for the ligand H2L and its complex
Zn(II)L were determined with an Electro-thermal 9100
digital apparatus and are uncorrected. Infrared spectra were
recorded using KBr pellets on a Perkin Elmer FT-IR Par-
agon 1000 spectrophotometer. Electronic spectra were
performed on a Unicam UV-300 spectrophotometer using
DMF as solvent. Elemental analyses were carried out on an
Elementar-Vario EL III CHNSO analyser (Institut de
Chimie de Strasbourg—France). 1H-NMR spectra were
recorded on a Bruker Avance 500 MHz spectrometer using
DMSO-d6 as solvent and tetramethylsilane (TMS) as
internal standard. Mass spectra of ligand H2L and its
complex Zn(II)L were obtained on a Bruker Daltonics
Flex Analysis spectrometer with MALDI-TOF procedure
using ditranol as matrix, at the ‘‘Institut de Chimie de
Strasbourg—France’’.
1
[1.930], 421 [2.562], 470 [1.412]; H-NMR: (DMSO-d6, d
ppm): 9.43 (s, H11), 9.10 (s, H10), 6.71–8.50 (m, ArH1-9);
MS (MALDI-TOF, ditranol): M? = 537.83.
X-ray Crystallography
Single crystals of Zn(II)L complex were grown by slow
layer diffusion of pyridine into a MeOH solution at room
temperature. A yellow plate single crystal of dimensions
0.20 mm 9 0.15 mm 9 0.10 mm suitable for X-ray anal-
ysis was used for data collection at 173(2) K on a Nonius
Kappa-CCD diffractometer equipped with an Oxford
Cryosystem liquid N2 device, using graphite-monochro-
Synthesis of the Schiff Base Ligand H2L
˚
mated Mo Ka (k = 0.71073 A) radiation. A semi-empirical
absorption correction was applied using the MULscanABS
The unsymmetrical [N2O2] tetradentate Schiff base, 2,3-
bis(5-bromosalicylidene) diaminopyridine was prepared
following literature methods [45]. To a MeOH solution
(15 mL) of 5-bromosalicylaldehyde (0.244 g, 2 mmol) was
slowly added a MeOH solution (5 mL) of 2,3-diamino-
pyridine (0.109 g, 1 mmol). The mixture was refluxed with
constant stirring under nitrogen atmosphere for 2 h to yield
an abundant orange precipitate that was collected by
filtration. The product was washed with methanol
(3 9 5 mL) then with diethyl ether (3 9 5 mL) and dried
under vacuum for 4 h. Yield: 73%, mp: 255 °C. Analysis
routine in PLATON [50]. Transmission factors: Tmin
/
Tmax = 0.45355/0.55121. Atomic scattering factors and
anomalous dispersion constants were taken from the
International Tables for Crystallography, Volume C
[51].The structure was solved by direct methods using the
SHELXS-97 and SHELXL-97 programs and refined on F2
by the full-matrix least-squares methods. All of the non-
hydrogen atoms were refined anisotropically and the
hydrogen atoms were included in geometric positions but
not refined.
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J Chem Crystallogr (2010) 40:831–836
Results and Discussion
833
˚
Table 2 Selected bond lengths (A) and angles (°) for the complex
Bond distances
Crystal Structure Description of the Complex
N2–Zn1
2.125(5)
2.091(5)
2.094(5)
1.967(4)
1.986(4)
N6–Zn2
N7–Zn2
N8–Zn2
O3–Zn2
O4–Zn2
2.127(5)
2.090(5)
2.085(5)
1.971(4)
1.983(4)
N3–Zn1
The main crystal parameters are reported in Table 1,
selected bond distances and angles are given in Table 2 and
geometrical hydrogen bonds are given in Table 3. An
ORTEP view of the molecule with the atom- numbering
scheme is shown in Fig. 1.
N4–Zn1
O1–Zn1
O2–Zn1
Bond angles
O1–Zn1–O2
O1–Zn1–N3
O2–Zn1–N3
O1–Zn1–N4
O2–Zn1–N4
N3–Zn1–N4
O1–Zn1–N2
O2–Zn1–N2
N4–Zn1–N2
N3–Zn1–N2
96.10(17)
151.39(18)
89.59(18)
101.53(19)
98.94(19)
105.2(2)
O3–Zn2–O4
O3–Zn2–N7
O4–Zn2–N7
O3–Zn2–N8
O4–Zn2–N8
N7–Zn2–N8
O3–Zn2–N6
O4–Zn2–N6
N8–Zn2–N6
N7–Zn2–N6
97.21(17)
153.42(19)
89.11(18)
97.31(19)
100.9(2)
The asymmetric unit comprises two independent
mononuclear zinc(II) complexes that exhibit very small
differences. The complex crystallizes in the triclinic space
group P-1. Zn atom is coordinated by the pairs of N and O
atoms of the Schiff base and by N atom of a pyridine
molecule. The molecule structure features the zinc atom in
a five-coordinate pyramidal geometry forming two–six-
106.85(19)
88.50(18)
159.65(18)
97.7(2)
87.97(18)
161.17(19)
98.2(2)
Table 1 Crystal data and refinement parameters for the complex
78.46(19)
77.59(19)
Molecular formula moiety
Molecular weight
Temperature (K)
Radiation k
C24H16Br2N4O2Zn
617.60
173(2)
Table 3 Geometrical parameters for hydrogen bonds
˚
Mo Ka (0.71073 A)
˚
˚
D–HꢀꢀꢀA
d(HꢀꢀꢀA) (A) d(DꢀꢀꢀA) (A) \(D–HꢀꢀꢀA) (°)
Crystal system
Triclinic
P-1
C(27)–H(27)ꢀꢀꢀO(1) 2.401
C(26)–H(26)ꢀꢀꢀO(2) 2.674
3.336
3.450
168.01
139.46
Space group
˚
a/A
8.0103(3)
12.8340(5)
22.3682(11)
91.715(2)
93.918(2)
103.708(2)
2226.30(16)
4
˚
b/A
˚
c/A
a°
b°
c°
membered chelate rings. The bond angles subtended at the
Zn atom, ranged from 98.2(2)° to 105.2(2)° for Zn1 atom,
indicate a distorted square pyramidal geometry. All angles
that do not involve the Zn atom are equal to 120° (with
minor variations) showing sp2 hybridization of the corre-
sponding carbon atoms.
3
˚
V/A
Z
Dcalc (g cm-3
)
1.841
˚
˚
The Zn1 center lies 0.385 A (Zn2: 0.373 A) above the
N2O2 coordination plane, the atoms N2, N3, O1 and O2
define the basal plane and the pyridine molecule occupies
the apical position. The bond lengths related to the Zn atom
are comparable to the values observed in other similar
Schiff base zinc(II) complexes [52, 53]. The Zn1–
Crystal size (mm)
0.20 9 0.15 9 0.10
Plate
Crystal description
Crystal colour
Absorption coefficient (mm-1
Yellow
)
4.723
Absorption correction Tmin and Tmax 0.45355 and 0.55121
F(000)
1216
˚
N4(pyridine) distance (2.094(5) A) is also comparable to
Reflections collected/unique
Range/indices (h, k, l)
h limit
23533/10101 [Rint = 0.0816]
those of Zn1–N2(ligand) and Zn1–N3(ligand) (2.125(5)
˚
and 2.091(5) A) of the analogous coordination patterns [54,
-10, 10; -16, 16; -28, 24
1.64–27.43
6725
55]. Neighboring molecules in the asymmetric unit are
linked to each other with short C–HꢀꢀꢀO interactions to
form a dimer complex (Table 3).
No. of observed data, I [ 2r (I)
No. of variables
595
No. of restraints
Goodness of fit on F2
0
The unit cell holds two pairs of molecules (two dimers)
as it is reported for similar iron complex [56] with dibro-
minated ligand [57]. The packing along the a axis (Fig. 2)
shows that the molecules adopt a zigzag form and the
neighboring molecules lie in opposite directions regarding
to the apical pyridine molecules in order to minimize the
steric effects.
1.121
-3
)
˚
Largest diff. peak and hole (e A
R1, wR2 [I C 2r (I)]a
R1, wR2 (all data)a
1.006 and -1.365
0.0780, 0.1192
0.1309, 0.1331
P
P
P
P
R1 = ||F0| - |Fc||/ |F0|, wR2 = [ w(F20 - F2c)2/ w(F02)2]1/2
,
a
w = 1/[r2(F02) ? (0.0254P)2 ? 6.7173P], P = (F02 ? 2F2c)/3
123

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J Chem Crystallogr (2010) 40:831–836
Fig. 2 Molecular packing of the complex along the a axis with the
unit cell and the short contacts. Hydrogen bonds are not shown
Spectral Characterisation of the Ligand
and the Complex
Infrared Spectra
The IR spectra in KBr discs of H2L and Zn(II)L, recorded
in the region of 4,000–600 cm-1, contain all the charac-
teristic bands of the coordinated tetradentate ligand. The IR
spectrum of the complex, compared to its ligand, shows
that the t(C=N) band appearing at 1,625 cm-1 is shifted to
lower energy by 15 cm-1, indicating that the ligand is
coordinated to the zinc metal through the nitrogen atoms of
the azomethine groups. In addition, the stretching vibration
of C–O, t(C–O/phenolate), appears at 1,301 cm-1 in the
ligand form whereas in the complex of Zn(II)L it shifted to
higher energy by 30 cm-1 suggesting that, in this case, the
metallic centre causes a drastic increase of the electronic
density in the vicinity of the oxygen atoms. This behavior
can be interpreted as a result of coordination of both
deprotonated oxygens of phenolic groups to the zinc metal
center giving an N2O2 tetracoordinated complex [59, 60].
Fig. 1 The molecular structure of the complex (two molecules in the
asymmetric unit), showing the atomic numbering scheme. Thermal
ellipsoids are drawn at 50% probability. Hydrogen bonds are omitted
for clarity
Molecular Formula
Both the complex and the ligand are stable in air. They are
much more soluble in polar organic solvents, such as
DMSO, DMF and pyridine, than in apolar or water. The
elemental analysis of the synthesized compounds were in
good agreement with the proposed chemical formulae for
H2L and Zn(II)L. However, the complex shows a ligand–
metal stoichiometric ratio of 1:1 affording mononuclear
compound.
Electronic Spectra
The electronic spectra of the ligand and the complex car-
ried out in DMF and recorded in the region of 800–200 nm,
show only intraligand or charge transfer bands. The elec-
tronic absorption spectrum of the free Schiff base ligand
exhibits intense bands centred at 267, 346 and 482 nm. The
intense band in high-energy region of the spectrum
(267 nm) is related to p ? p* transitions of phenyl rings
[61]. The two other bands can be assigned respectively to
p ? p* transitions associated with azomethine (N=C)
chromophore [62] and to the azopyridine moiety (N-Pyr.).
MALDI-TOF spectra of the Schiff base and its corre-
sponding complex show the molecular ion peaks at 475.94
and 537.83 respectively and the expected bromine isotope
patterns supported by typical distribution of dibrominated
compounds [58].
123

J Chem Crystallogr (2010) 40:831–836
835
The corresponding Zn(II)L complex exhibits mainly the
absorption bands of the ligand which are slightly red
shifted. The absorption band appearing at 268 nm coin-
cides with that observed in the free ligand whereas the
other bands are rather shifted to higher energy when
compared to those of H2L. A shoulder appearing at 470 nm
is attributable to metal–ligand d ? p* charge transfer
transitions (MLCT).
1H-NMR
1
1H-NMR and 2D H–1H ROESY spectra were recorded
and analyzed to support the structure of the zinc complex
shown in Fig. 1. The data collected are given in the
‘‘Experimental’’ section. 1H-NMR spectra in DMSO-d6
showed the aromatic protons as multiplet in the range
6.94–7.63 ppm. The intrinsically unsymmetrical tetraden-
tate Schiff base possess two phenolic and two azomethine
groups, both azomethine protons (CH=N) and OH protons
of the phenolic groups appearing as sets of sharp singlets at
respectively 9.47 and 8.57 ppm, and 13.38 and 12.79 ppm
[45, 60]. The two phenolic proton signals disappear in the
case of the corresponding unsymmetrical complex
Zn(II)L, indicating that the OH groups have been depro-
tonated and bonded to metal ions as oxygen anions. The
azomethine proton resonances of the complex form, com-
pared to those of its ligand, are slightly shifted to higher
fields for H11, 9.43 ppm and to lower fields for H10,
9.12 ppm (Fig. 1). Thus, NMR data favour a {ZnN2O2}
coordination for the complex in solution (Fig. 3).
Fig. 4 2D ROESY spectrum of Zn(II)L complex in DMSO-d6 at
500 MHz
were found for pairs of resonances at d 9.43/7.70, 9.10/8.33
and 9.10/7.60 corresponding to space proximity of protons
H11/H4, H10/H3 and H10/H7 respectively. Thus, H10
proton is in space interaction with two protons, H3 and H7,
while H11 proton is only with one proton, H4, as is shown
in Fig. 1. The 2D ROESY spectrum is shown in Fig. 2. To
our knowledge, these azomethine proton assignments for
similar unsymmetrical complexes were not reported in the
literature [59, 63]. Consequently, these attributions allowed
us to deduce the chemical shifts of all the signals of the
aromatic protons not yet assigned. The Table 4 lists the
chemical shift attributions. Moreover, the relative areas of
the resonances are in perfect agreement with the corre-
sponding protons (Fig. 4).
1
In order to assign the H-NMR spectrum of Zn(II)L
complex, cross-peaks in 2D ROESY spectrum due to a
through-space magnetic exchange mechanism were
obtained for neighboring protons and, cross-peaks from
through bond couplings were obtained from 1H–1H COSY.
The most important cross-peaks in the 2D ROESY spectra
Conclusion
2
1
N
3
The solid state structure of unsymmetrical zinc-Schiff base
crystal was obtained and showed a five coordinate complex
involving a pyridine molecule as an apical base. In solution
the zinc complex appears as a typical four coordinate
zinc(II)salen complex. Amplification of the unsymmetrical
character using quaternization or weak interactions
involving the pyridine nitrogen atom is under investigation.
H11
10 H
N
C
C
N
7
4
Zn
O
Br
Br
O
6
5
8
9
Fig. 3 Structural representation of Zn(II)L showing the labels
1
employed in H-NMR and 2D ROESY assignments
Table 4 Proton chemical shifts (ppm) of Zn(II)L in DMSO-d6 at 500 MHz
Hi
H11
H10
H1
H3
H4
H7
H2
H5,8
H6,9
d(ppm)
9.43(s)
9.10(s)
8.44(dd)
8.33(dd)
7.70(d)
7.60(d)
7.49(dd)
7.35(dd)
6.70(dd)
s Singlet, d doublet
123

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J Chem Crystallogr (2010) 40:831–836
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Supplementary Material
CCDC-684010 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif by
e-mailing: data_request@ccdc.cam.ac.uk; or by contacting:
The Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: ?44(0)1223-
336033.
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