Syntheses, characterizations, X-ray crystal structures, and antibacterial activities of Co(II), Ni(II), and Zn(II) complexes of the Schiff base derived from 5-nitro-2-hydroxybenzaldehyde and benzylamine

Article abstract of DOI:10.1080/00958972.2014.996144

Three new complexes, [Co^IIL₂(bzln)₂] (1), [Ni^IIL₂(DMF)₂] (2), and [Zn^IIL₂] (3), with the bidentate (NO) Schiff base HL = 2-((E)-(benzylimino)methyl)-4-nitrophenol have

Full text of DOI:10.1080/00958972.2014.996144

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Syntheses, characterizations, X-ray
crystal structures, and antibacterial
activities of Co(II), Ni(II), and Zn(II)
complexes of the Schiff base derived
from 5-nitro-2-hydroxybenzaldehyde
and benzylamine
Mehdi Amirnasra, Roghayeh Sadeghi Eramia, Kurt Mereiterb, Kurt
Schenk Joßc, Soraia Meghdadia & Soheila Abbasid
a Department of Chemistry, Isfahan University of Technology,
Isfahan, Iran
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b Faculty of Chemistry, Vienna University of Technology, Vienna,
Austria
c CCC-IPSB, École Polytechnique Fédérale de Lausanne, Lausanne,
Switzerland
d Department of Biological Sciences, University of Isfahan,
Isfahan, Iran
Accepted author version posted online: 13 Dec 2014.Published
online: 06 Jan 2015.
To cite this article: Mehdi Amirnasr, Roghayeh Sadeghi Erami, Kurt Mereiter, Kurt Schenk Joß,
Soraia Meghdadi & Soheila Abbasi (2015): Syntheses, characterizations, X-ray crystal structures,
and antibacterial activities of Co(II), Ni(II), and Zn(II) complexes of the Schiff base derived
from 5-nitro-2-hydroxybenzaldehyde and benzylamine, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2014.996144
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Journal of Coordination Chemistry, 2014
http://dx.doi.org/10.1080/00958972.2014.996144
Syntheses, characterizations, X-ray crystal structures, and
antibacterial activities of Co(II), Ni(II), and Zn(II) complexes
of the Schiff base derived from 5-nitro-2-
hydroxybenzaldehyde and benzylamine
MEHDI AMIRNASR*†, ROGHAYEH SADEGHI ERAMI†, KURT MEREITER‡,
KURT SCHENK JOߧ, SORAIA MEGHDADI† and SOHEILA ABBASI¶
†Department of Chemistry, Isfahan University of Technology, Isfahan, Iran
‡Faculty of Chemistry, Vienna University of Technology, Vienna, Austria
§CCC-IPSB, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
¶Department of Biological Sciences, University of Isfahan, Isfahan, Iran
(Received 28 June 2014; accepted 10 November 2014)
Three new complexes, [CoIIL2(bzln)2] (1), [NiIIL2(DMF)2] (2), and [ZnIIL2] (3), with the bidentate
(NO) Schiff base HL = 2-((E)-(benzylimino)methyl)-4-nitrophenol have been synthesized and char-
acterized. [CoIIL2(bzln)2] (1) was initially formed from the reaction of H2L1 (N,N′-bis(5-nitrobenzal-
dehyde)-2-aminobenzylamine = H2L1), with CoII(CH3COO)2·4H2O in the presence of benzylamine.
The X-ray crystal structure of 1 revealed that HL was formed from the hydrolysis of H2L1 and the
recondensation of benzylamine with 2-hydroxy-5-nitrobenzaldehyde produced in H2L1 hydrolysis.
The Schiff base HL was then prepared by direct reaction of 2-hydroxy-5-nitrobenzaldehyde and ben-
zylamine. Complexes 1–3 were also directly synthesized and their crystal and molecular structures
determined by X-ray crystallography. The ligand and its metal complexes were also screened for
in vitro antibacterial activity.
Keywords: Co(II), Ni(II), and Zn(II) complexes; Unsymmetrical Schiff base; Crystal structure;
Antibacterial activity
*Corresponding author. Email: amirnasr@cc.iut.ac.ir
© 2014 Taylor & Francis

2
M. Amirnasr et al.
1. Introduction
Schiff base ligands, defined by Jacobsen as “privileged ligands” [1], are fascinating due to
their varying coordination sites, different shapes, functionalities, flexibility, and symmetry.
The structural diversity of transition metal complexes of Schiff base ligands and their struc-
ture–function relationships have been a focus of attention in recent years [2–8]. A great deal
of effort has been directed toward the design and synthesis of functional materials with a
variety of applications such as production of highly efficient catalysts [9–13], redox-active
sensors [14], ionic ferroelectrics [15], and development of supramolecular architectures with
interesting properties [2]. Moreover, they possess a wide variety of biological activity
against bacteria, fungi, and certain types of tumors [10–12]. Recently, there has been inter-
est in the synthesis of unsymmetrical Schiff base ligands and their Co(II), Ni(II), and Zn(II)
complexes, and investigation of their biological activities [16–21].
In continuation to our recent studies on unsymmetrical Schiff base ligands and their metal
complexes [22–24], herein, we report the preparation of the tetradentate unsymmetrical
Schiff base H2nitrosalabza, H2L1 [25], and its reaction with cobalt(II) acetate in the
presence of benzylamine leading to the octahedral complex [CoIIL2(bzln)2] (1), in which L–
is a bidentate monoanionic ligand produced in the hydrolysis–recondensation process
(scheme 1). The direct synthesis and spectroscopic characterization of [CoIIL2(bzln)2] (1)
and two other complexes of HL, [NiL2(DMF)2] (2), and [ZnL2] (3), are also reported
(scheme 2). The X-ray crystal structures of 1, 2, and 3 and preliminary antibacterial activity
of HL ligand and its complexes are also discussed.
2. Experimental
2.1. Materials and general methods
All solvents and chemicals were of commercial reagent grade and used as received from
Aldrich and Merck. Elemental analyses were performed by using a Perkin–Elmer 2400II
CHNS–O elemental analyzer. Infrared spectra (KBr pellets) were recorded on a FT-IR
1
JASCO 680 instrument. H NMR spectra were obtained on a Bruker AVANCE III 400
spectrometer. Proton chemical shifts are reported in ppm relative to an internal standard of
Me4Si. Electronic spectra were obtained on a JASCO V-570 spectrophotometer. The molar
conductivity of freshly prepared 1.4 × 10−4 M DMSO solution of the Co(II) complex was
measured using a Digital-Konduktometer CG 855 instrument.
2.2. Synthesis
2.2.1. Synthesis of ligands. H2L1 and HL were prepared as reported in the literature
[25, 26]. These ligands were characterized by elemental analysis and spectroscopic meth-
ods. The melting points of the ligands are 248–250 °C for H2L1 and 148–150 °C for HL.
FT-IR: (KBr pellet, cm−1): 1650 s (υ C=N). UV–Vis: λmax (nm) (ε, M−1 cm−1) (DMF): 426
1
(14,633), 364 (11,193). H NMR (DMSO-d6, 400 MHz), δ (ppm): 14.52 (1H, OH), 8.91
(1H, CH=N), 6.70–8.48 (8H, ArH), 3.33 (2H, CH2 benzylic).

Co(II), Ni(II), Zn(II) Schiff base complexes
3
Scheme 1. Formation of HL and its Co(II) complex in the hydrolysis–recondensation reaction of H2L1.
Scheme 2. Direct synthesis of the Co(II), Ni(II), and Zn(II) complexes from HL.

4
M. Amirnasr et al.
2.2.2. Synthesis of [CoIIL2(bzln)2] (1). To a stirring solution of Co(CH3COO)2·4H2O
(24.9 mg, 0.1 mmol) in 10 mL ethanol was added a solution of L (51.2 mg, 0.2 mmol) in
15 mL CHCl3 at room temperature. To this solution, 3.0 mmol of benzylamine was added
dropwise within 3 h. The final reaction mixture was filtered off and the filtrate was left undis-
turbed at room temperature for three days. The resulting yellowish brown crystals of 1
suitable for X-ray crystallography were filtered off and washed with cold ethanol and dried
under vacuum. Yield 73%. Anal. Calcd for C42H40N6O6Co (%): C, 64.36; H, 5.14; N, 10.72.
Found: C, 64.14; H, 5.06; N, 10.42. FT-IR (KBr pellet, cm−1): 3283 and 3336 m (υ NH2).
1632 s (υ C=N), UV–Vis: λmax (nm) (ε, M−1cm−1) (DMF): 577 (232), 390 (23,686).
2.2.3. Synthesis of [NiL2(DMF)2] (2). A solution of the ligand (51.2 mg, 0.2 mmol) in
15 mL CHCl3 was added to a stirring solution of Ni(CH3COO)2·4H2O (24.8 mg, 0.1 mmol)
in 15 mL ethanol. The solution turned green immediately and the product precipitated as
microcrystalline solid. The precipitate was filtered off and washed with cold ethanol. Dark
green crystals of 2 suitable for X-ray analysis were obtained by recrystallization of the
product in DMF after seven days. The crystals were filtered off and washed with cold ethanol.
Yield 79%. Anal. Calcd for C34H36N6O8Ni (%): C, 57.08; H, 5.07; N, 11.75. Found: C,
57.21; H, 5.14; N, 11.42. FT-IR (KBr pellet, cm−1): 1615 s (υ C=ODMF), 1604 s (υ C=N),
UV–Vis: λmax (nm) (ε, M−1cm−1) (DMF): 594(136), 420 (1920), 309 (22,600).
2.2.4. Synthesis of [ZnL2] (3). To a stirring solution of Zn(CH3COO)2·2H2O (10.95 mg,
0.05 mmol) in 15 mL ethanol was added a solution of the ligand (25.7 mg, 0.1 mmol) in
15 mL CHCl3. The solution turned yellow after 4 h. Yellow crystals of 3 were obtained
during a week by slow evaporation of the solvent in open air. The crystals were filtered off
and washed with cold ethanol. Yield 75%. Anal. Calcd for C28H22N4O6Zn (%): C, 58.40; H,
3.85; N, 9.73. Found: C, 58.83; H, 3.90; N, 9.73. FT-IR (KBr pellet, cm−1): 1631s (υ C=N).
1
UV–Vis: λmax (nm) (ε, M−1cm−1) (DMF): 364 (3252). H NMR (DMSO-d6, 400 MHz), δ
(ppm): 8.39 (2H, CH=N), 6.49–8.20 (16H, ArH), 4.59 (4H, CH2 benzylic).
2.3. Crystal structure determination
2.3.1. X-ray crystallography for 1 and 3. Yellowish brown crystals of 1 and yellow crys-
tals of 3 grown as described above were used for X-ray crystallography. Diffraction data of
1 and 3 were collected at T = 100 and 297 K, respectively, on a Bruker Smart APEX CCD
diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). After cell
refinement and data reduction with SAINT, a multi-scan absorption correction was applied
to the data with SADABS [27]. The structures were solved with direct methods (SHELXS
[28]) and refined on F2 using SHELXL-97 [28]. Hydrogen was placed at geometrically
calculated positions and refined using a riding model with fixed C–H and N–H distances.
Details of crystallographic data and refinement of the complexes are summarized in table 1,
while selected bond lengths and angles are presented in table 2. A list of hydrogen bonds is
given in table 3.
2.3.2. X-ray crystallography for 2. Green crystals of 2 suitable for X-ray crystallography
were grown by slow evaporation of a DMF solution of the complex at room temperature.

Co(II), Ni(II), Zn(II) Schiff base complexes
5

6
M. Amirnasr et al.
Table 2. Selected bond lengths (Å) and angles (°) for 1, 2, and 3.
[CoIIL2(bzln)2] (1)
[NiL2 (DMF)2] (2)
[ZnL2] (3)
Bond lengths
Co1–O1
2.0457(11)
2.0457(11)
2.1428(13)
2.1428(13)
2.1878(13)
2.1878(13)
Ni1–O1
Ni1–O1ia
Ni1–N1
Ni1–N1i
Ni1–O4
Ni1–O4i
2.025(5)
2.025(5)
2.105(5)
2.105(5)
2.101(4)
2.101(4)
Zn1–O4
Zn1–O1
Zn1–N1
Zn1–N3
1.9278(16)
1.9286(16)
1.996(2)
Co1–O1ia
Co1–N1
Co1–N1i
Co1–N3
1.996(2)
Co1–N3i
O1–C2
O2–N2
O3–N2
C1–C7
N1–C7
N1–C8
1.2866(18)
1.2402(19)
1.2405(19)
1.452(2)
1.282(2)
1.479(2)
1.268(7)
1.230(12)
1.222(12)
1.444(11)
1.287(9)
1.477(10)
1.293(3)/1.297(3)b
1.239(3)/1.232(3)
1.226(3)/1.231(3)
1.445(3)/1.458(3)
1.280(3)/1.277(3)
1.473(3)/1.479(3)
Bond angles
O1–Co1–O1i
O1–Co1–N1
O1–Co1–N1i
O1–Co1–N3
O1–Co1–N3i
N1–Co1–N1i
N1–Co1–N3
N1–Co1–N3i
N3–Co1–N3i
180.00
O1–Ni1–O1i
O1–Ni1–N1
O1–Ni1–N1i
O1–Ni1–O4
O1–Ni1–O4i
N1–Ni1–N1i
N1–Ni1–O4
N1–Ni1–O4i
N3–Ni1–O4i
180.0
O1–Zn1–O4
O1–Zn1–N1
O1–Zn1–N3
O4–Zn1–N1
O4–Zn1–N3
N1–Zn1–N3
128.46(8)
95.76(8)
107.37(8)
105.78(8)
96.55(7)
126.32(9)
88.27(5)
91.73(5)
92.34(5)
87.66(5)
180.00
90.46(5)
89.54(5)
180.00
89.0(2)
91.0(2)
90.0(2)
90.0(2)
180.0
93.2(2)
86.8(2)
180.00
C2–O1–Zn1
C16–O4–Zn1
124.04(15)
122.94(14)
aSymmetry code: (i) = −x, −y, −z.
bSecond value reports equivalent bond lengths for the second ligand.
Table 3. Hydrogen bond lengths (Å) and angles (°) for 1, 2, and 3.
D–H⋯A
d(D–H)
d(H⋯A)
d(D⋯A)
D–H⋯A
[CoIIL2(bzln)2] (1)
N3–H3A⋯O2ia
N3–H3A⋯O3ia
N3–H3B⋯C9b
N3–H3B⋯C10b
0.92
0.92
0.92
0.92
2.51
2.53
2.74
2.80
3.425(2)
3.192(2)
3.459(2)
3.477(2)
175
129
135
131
[NiIIL2(DMF)2] (2)
C16–H16C⋯O2ii
0.96
2.49
3.358(12)
150
[ZnIIL2] (3)
C3–H3⋯O5iii
C11–H11⋯O6iv
C17–H17⋯O2v
C21–H21⋯O3vi
0.93
0.93
0.93
0.93
2.59
2.49
2.55
2.44
3.439(3)
3.286(4)
3.398(3)
3.354(3)
152
143
152
166
Note: Symmetry codes: (i) x, y, 1 + z; (ii) −x, −y, 1 − z; (iii) −x, −y, −z; (iv) −x, 1 − y, −z; (v) 1 − x, −y,
1 − z; (vi) x − 1, y, z − 1.
aBifurcated N–H⋯O, O bond.
bC–H⋯π bond.
Bragg intensities from 2 were collected at T = 293 K on a Stoe IPSD II image plate system
with graphite monochromated MoKα radiation (λ = 0.71073 Å). Cell refinement and data
reduction were performed with the X-AREA program (1.62) [29]. Absorption factors were
obtained by numerical integration in XRED 32 (1.31) [30]. The structure was solved with
the help of DIRDIF [31] and was refined like 1 and 3 on F2 using SHELXL-97 [28].

Co(II), Ni(II), Zn(II) Schiff base complexes
7
Crystal data and details of the refinement are summarized in table 1, while selected bond
lengths and angles are presented in tables 2 and 3.
2.4. Antibacterial activity
Antibacterial activities of the ligand and its complexes were tested by the well-known diffu-
sion method using Sabouraud dextrose agar and Müller Hinton agar [32]. Stock solutions
of HL and 1–3 were prepared in DMSO. The zone of inhibition was recorded on the com-
pletion of the incubation as the mean diameter (MZI) for each complex at 650 μg mL−1
(~1 mM). The MIZ produced by the compounds were compared with the standard antibiotic
Penicillin of 1 mM concentration. Each test was carried out three times to minimize the
error. In order to clarify any effect of DMSO in the biological screening, blank studies were
carried out, and no activity was observed against any bacterial strains in pure DMSO.
3. Results and discussion
3.1. Synthesis
The trans-[CoIIL2(bzln)2] complex was obtained unexpectedly as the product of an
attempted synthesis of [CoIII(L1)(bzln)2]+ from the reaction between cobalt(II) acetate and
the tetradentate Schiff base ligand H2L1 in the presence of benzylamine (scheme 1). Liter-
ature reports [33] show that metal complexes of Schiff base ligands may undergo hydroly-
sis resulting in new products of rearranged Schiff base ligands. Factors such as metal
ions, ligand systems, solvents, and anions can have a marked influence on the reaction
path. We have recently demonstrated the effect of counteranions of the metal salts on the
hydrolysis of a tridentate unsymmetrical Schiff base HBacabza = 3-(2-aminobenzylimino)-
1-phenylbutan-1-one in the presence of Cu(ClO4)2·6H2O in methanol and recondensation
to its isomer 3-(2-aminomethylphenyleneimino)-1-phenylbutan-1-one [23]. The results of
the aforementioned transformation of the tetradentate H2L1 to the bidentate ligand HL dur-
ing trans-[CoIIL2(bzln)2] formation inspired us to synthesize and fully characterize Co(II),
Ni(II), and Zn(II) complexes of the bidentate Schiff base ligand HL, derived directly from
5-nitro-2-hydroxybenzaldehyde and benzylamine. These complexes were obtained in good
yields of 73% for 1, 79% for 2, and 75% for 3. The complexes were characterized using
different spectroscopic methods and X-ray crystallography. In DMSO, the cobalt complex
is a non-electrolyte as reflected in its low ΛM value (11.5 Ω−1 cm2 M−1) [34, 35].
3.2. Spectral characterization
The newly prepared metal complexes are all air-stable solids and have good elemental anal-
yses. The octahedral geometry of Co and Ni in 1 and 2, and also tetrahedral geometry of
Zn in 3, are evident from the X-ray structure analysis (figures 1, 3 and 4). FT-IR data of the
complexes are listed in Section 2. Significant information regarding the bonding sites of
ligand can be obtained by comparing the IR spectra of the metal complexes with the free
ligand. A strong absorption at 1632 cm−1 for 1, 1604 cm−1 for 2, and 1631 cm−1 for 3 is
assigned to ν(C=N) of the coordinated imine. These bands are at lower frequencies relative
to that of the free Schiff base, HL (1650 cm−1), indicating a decrease in C=N bond order

8
M. Amirnasr et al.
Figure 1. The molecular structure of [CoIIL2(bzln)2] (1) with atom labeling scheme. Displacement ellipsoids are
drawn at 50% probability. The complex is centrosymmetric with Co in x, y, z = 0, 0, 0. The thin blue lines indicate
intramolecular N–H⋯π bonding (N3⋯C9 = 3.459(2), N3⋯C10 = 3.477(2) Å) (see http://dx.doi.org/10.1080/
00958972.2014.996144 for color version).
due to coordination of the imine nitrogen to the metal and back bonding from the metal to
the π* orbitals of the azomethine groups. A band at 1615 cm−1 in the spectrum of 2 is
assigned to ν(C=O) of coordinated DMF. Free DMF exhibits a band at 1680 cm−1 due to
ν(C=O), which shifts to lower frequencies in spectra of O-bonded DMF metal complexes,
indicating amide oxygen coordination [36]. These results conform to the structural data
obtained by X-ray crystallography.
The UV–Vis data are presented in the experimental section. The electronic absorption
spectrum of HL and complexes were recorded using DMF as the solvent. The electronic
spectrum of HL in DMF shows a band at 426 nm (ε = 14,633 M−1 cm−1) and a band in the
UV region at 364 nm (ε = 11,193 M−1 cm−1) corresponding to the intramolecular π–π* and
n–π* transitions. Characteristic bands for the complexes were also detected as follows. The
electronic absorption spectrum of 1 shows an IL-CT band at 390 nm (ε = 23,686 M−1 cm−1)
and a band in the visible region at 577 nm (ε = 232 M−1 cm−1) corresponding to the ligand
field transition of the cobalt(II) complex. The electronic spectrum of 2 shows one band at
309 nm (ε = 22,600 M−1 cm−1) and two bands at 420 nm (ε = 1920 M−1 cm−1) and 594 nm
(ε = 136 M−1 cm−1), assigned to the charge-transfer and the ligand field [3A2 g → 3T1 g(F)]
transitions, respectively. Complex 3 shows one band at 364 nm (ε = 3252 M−1 cm−1), which
may be attributed to the intramolecular or charge-transfer transitions.

Co(II), Ni(II), Zn(II) Schiff base complexes
9
Figure 2. Packing and intermolecular interactions in the crystal structure of 1 viewed along [1 0 0].
Figure 3. The molecular structure of 2 with atom labeling scheme. Displacement ellipsoids are drawn at 20%
probability. The complex is centrosymmetric with Ni in x, y, z = 0, 0, 0. The distance between N3 and the centroid
of ring C9–C14 is 3.50 Å.
1H NMR spectral measurements were performed in DMSO-d6 for L and its Zn(II)
complex and the spectroscopic data are given in the experimental section. As expected, the
OH of the ligand is at 14.42 ppm as a broad singlet due to intramolecular hydrogen

10
M. Amirnasr et al.
bonding. The singlet at 8.91 ppm corresponds to the methylidene proton. The signals
corresponding to the eight aromatic protons resolve in the range 6.70–8.48 ppm, and the
benzylic (CH2) protons appear at 3.33 ppm. The spectrum of the zinc complex features a
singlet at 8.39 ppm for the methylidene protons. The signals at 6.49–8.20 ppm for 3 are
attributed to the sixteen aromatic protons of the two monoanionic bidentate (L–) ligands,
coordinated to the zinc center. The benzylic (CH2) protons appear as a singlet at 4.59 ppm.
3.3. Crystal structure of [CoIIL2(bzln)2] (1)
The molecular structure and atom numbering scheme of 1 is presented in figure 1. Selected
bond distances and angles are given in tables 2 and 3. This complex crystallizes in triclinic
space group Pī with Co located on a center of inversion and coordinated in an octahedral
fashion. As expected, HL is a bidentate mono-anionic ligand and binds to the cobalt ion via
phenolate O1 and imine N1 so that O1, O1i, N, and N1i are in equatorial positions. The
two axial sites are occupied by the amine nitrogens N3 and N3i of two benzylamine mole-
cules. The bond lengths Co–N1=Co–N1i=2.1428(13) Å and Co–O1=Co–O1i=2.0457(11) Å
are comparable to the average bond distances Co–N (2.121(3) Å) and Co–O (2.097(3) Å)
observed in a related Co(II) complex [37]. The two axial bond distances Co1–N3=Co–
N3i = 2.1878(13) Å are longer than the equatorial bond distances due to different donor/
back donation properties of the axial and equatorial ligands, and a weak Jahn–Teller effect.
A comparison of the observed Co–N, O bond lengths in 1 with those of related complexes
in the Cambridge Crystallographic Database indicates that cobalt is d7 high-spin with three
unpaired electrons. Similar Co–N bond distances (2.1272(15) and 2.0671(19) Å) in high
spin cobalt(II) complex [Co(pbba)2(NCS)2], pbba = N-((pyridine-yl-)benzylidene)benzyl-
amine, are reported [38]. Molar conductivity measurements gave additional support to the
molecular and non-electrolyte nature of [CoIIL2(bzln)2] complex (vide supra).
All three trans angles about Co are ideal 180°. The six-membered chelate ring formed by
phenolate-O and imine-N has a bite angle of O1–Co1–N2 = 88.27(5)°. The bond angles
between equatorial and axial ligands deviate only by 0.46(5)° and 2.34(5)° from 90°.
The 2-oxy-5-nitrobenzylidenimine moiety is nearly flat showing a rms deviation from pla-
narity of 0.034 Å with Co deviating by 0.371(1) Å from this plane. It encloses interplanar
angles of 87.8(4)° and 87.7(4)° with the phenyl rings C9–C14 and C16–C21, respectively,
the latter two being almost parallel to each other with an interplanar angle of only 4.7(1)°.
The shortest intramolecular approach between these two phenyl rings is C11⋯C21 = 3.559
(3) Å (figure 1) but their ring–ring overlap is too small to consider this a significant π–π
interaction. However, there is a stabilizing intramolecular N–H⋯π type hydrogen bond
interaction between NH2 and the phenyl ring C9–C14, with N3⋯C9 = 3.459(2) Å and
N3⋯C10 = 3.477(2) Å (thin blue lines in figure 1 and table 3). In the crystal lattice, the
complexes are aligned with their phenyl rings C9–C14 and C16–C21 in layers parallel to
(222), and with their 2-oxy-5-nitrobenzylidenimine moieties oriented parallel to (1-3-1), i.e.
approximately perpendicular to (2 2 2). These layers repeat in both directions every 3.3 Å
and enable intermolecular slipped π–π interactions between centrosymmetric pairs of nitro-
phenyl moieties with an interplanar distance of 3.179 Å (shown as double arrows in figure
2), and between the phenyl rings C9–C14 at x, y, z and C16–C21 at 1−x, −y, −z with a
corresponding distance of 3.490 Å, effective in view of the direction to which these rings
are inclined by ca. 30° (figure 2). Further coherence of the crystal lattice is provided by an
intermolecular bifurcated N–H⋯O, O hydrogen bond between the amino group and the two
nitro oxygens O2 and O3 as acceptors (red dashed lines in figure 2 and table 3).

Co(II), Ni(II), Zn(II) Schiff base complexes
3.4. Crystal structure of [NiL2(DMF)2] (2)
11
The molecular structure and atom numbering scheme of 2 is presented in figure 3. The crys-
tallographic data and structure analysis for this complex are summarized in table 1. Selected
bond distances and angles are given in table 2. This complex crystallizes in triclinic space
group Pī and its metal ion is in an octahedral environment containing two N1 and two O1
donors of two L– ligand in the equatorial positions and two oxygens (O4) of DMF in the
axial positions. Like 1, 2 is also centrosymmetric and, therefore, trans bond distances are
identical and trans bond angles are 180°. The bond distances Ni1–N1 = 2.110(6) Å, Ni1–
O1 = 2.025(5) Å, and Ni1–O4 = 2.101(5) Å are comparable to the average bond distances
Ni–N (2.012(5) Å) and Ni–O (1.998(4) Å) observed in a related Ni(II) complex [39].
The chelate bite angle formed by phenolate-O and the imine-N, O1–Ni1–N1 = O1i–Ni1–
N1i = 89.0(2)°, is similar to 1. The cis bond angles to DMF O4 are 90.0(2)° (O1) and 90
3.2(2)° (N1). The DMF is near parallel to the adjacent phenyl ring C9–C14 (interplanar
angle 5.9°) and both are approximately perpendicular to the nitrophenyl moiety (89.7° and
84.1°, respectively). In the crystal lattice, DMF ligands and phenyl rings C9–C14 form con-
tinuous stacks along the a axis with weak C–H⋯π interactions not included in table 3
because of their excessive length (C⋯C > 3.6 Å). Further coherence of the complexes in
the crystal lattice is provided by slipped π–π interactions between pairs of nitrophenyl moie-
ties (perpendicular distance 3.403(3) Å) and a C–H⋯O (nitro) hydrogen bond (table 3).
The crystal of 2 shows some similarities to that of 1.
3.5. Crystal structure of [ZnL2] (3)
The molecular structure and atom numbering scheme of 3 is presented in figure 4. The crys-
tallographic data for this complex are summarized in table 1. Selected bond distances and
angles are given in table 2. Like 1 and 2, 3 crystallizes in the triclinic space group Pī, but
unlike 1 and 2, the Zn complex 3 is not centrosymmetric and there are two complexes
instead of one in the unit cell. Zn(II) is coordinated by four N, O-donors: two nitrogens
(N1, N3) from imino groups and two oxygens (O1, O4) from phenolates of L–, respectively.
The coordination environment around Zn(II) is a distorted tetrahedron. All Zn–N and Zn–O
bond distances are within the normal range observed in other four-coordinate zinc(II) com-
plexes. The bonds Zn–N1 and Zn–N3 (1.996(2) Å) are equal and compare well with the
average Zn–N bond lengths in [Zn(L1)2] (2.006 Å) [40]. The bond distances Zn–O1
(1.9286(16) Å) and Zn–O4 (1.9278(16) Å) are very close and are also in agreement with
the Zn–O bond distance (1.942(6) Å) in [Zn(L1)2] [40].
The two chelate angles O1–Zn1–N1 (95.76(8)°) and O4–Zn1–N3 (96.55(7)°) formed by
the phenolate-O and the imine-N are similar and close to those reported for the related
[Zn(L1)2]. The remaining bond angles in the ZnN2O2 tetrahedron are either near the ideal
tetrahedral angle 109.47° for O1–Zn–N3 and O4–Zn–N1, or very big for O1–Zn–O4
(128.46(8)°) and N1–Zn1–N3 (126.32(9)°) showing that the tetrahedron is considerably
compressed along the vertical axis (figure 4). While the ZnN2O2 tetrahedron and the two
independent 2-oxy-5-nitrobenzylidenimine moieties (C1–C7, N1–N2, O1–O3, and
C15–C21, N3–N4, O4–O6) define a nearly C2-symmetric arrangement (C2 axis through
the centroid of N1 and N3 and the centroid of O1 and O4, the compression axis of the
ZnN2O2 tetrahedron), the two benzyl residues C8–C14 and C22–C28 adopt an asymmetric
arrangement where ring C9–C14 is near parallel to ring C15–C20 (interplanar angle 12.5
(2)°) and ring C23–C28 is steeply inclined to ring C1–C6 (interplanar angle 49.2(2)°).

12
M. Amirnasr et al.
Figure 4. The molecular structure of 3 with atom labeling scheme. Displacement ellipsoids are drawn at 20%
probability.
The two 2-oxy-5-nitrobenzylidenimine moieties have rms deviations from planarity of
0.076 and 0.045 Å with Zn by 0.477(2) and 0.472(2) Å displaced from these planes. They
subtend an interplanar angle of 76.43(4)°.
Intermolecular interactions in the crystal lattice of 3 comprise slipped π–π stacking
between centrosymmetric pairs of 2-oxy-5-nitrobenzylidenimine moieties with C⋯C,O dis-
tances of 3.54–3.61 Å, two interesting sides on contacts between each NO2 group and a phe-
nolate oxygen with N⋯O = 3.0–3.11 Å, and two C–H⋯O interactions to each nitro group.
These interactions, shown in figure 5, generate infinite molecular chains that extend parallel
to [1 0 1] through the crystal lattice. Contact distances are (Å): C1–C2ii 3.539(4), C7–C3ii
3.538(4), C6–O1ii 3.605(3), N2–O4ii 3.114(3), O2–C17ii 3.398(3) (ii = 1 − x, −y, 1 − z),
C15–C16i 3.574(3), C21–C17i 3.536(3), C20–O4i 3.612(3), N4–O1i 3.089(3), O5–C3i 3.439
(3) (i = −x, −y, −z), where O2–C17ii and O5–C3i represent C–H⋯O bonds. The cross-linking
of these chains perpendicular to [1 0 1] is less organized and involves, apart from unremark-
able van der Waals contacts, only two further C–H⋯O bonds from the benzyl groups to nitro
oxygens, C11⋯O6 (−x, 1 − y, −z) = 3.286(4) Å and C11⋯O6 (x − 1, y, z − 1) = 3.354(3) Å.
The π–π stacking between centrosymmetric pairs of nitrophenyl fragments is common to 1,
2, and 3. However, the mutual overlap of the fragments is notably bigger in 3 than in 1 and 2,
with the effect that it includes here the phenolate O and the imine carbon.
3.6. Antibacterial activity
The ligand and its metal complexes were tested for antibacterial activity against Bacillus
cereus and Staphylococcus aureus as Gram +ve and Salmonella typhi, Klebsiella pneumo-
niae, and Escherichia coli as Gram –ve species. The screening results (minimum zone of
inhibition, MZI) are summarized in table 4. The data revealed that, though no significant
activity is observed against the Gram –ve species, the activity of the ligand is enhanced
against the Gram +ve bacteria upon coordination. Both complexes, 1 (MZI = 30 mm) and 2

Co(II), Ni(II), Zn(II) Schiff base complexes
13
Figure 5. Three molecules of 3 and their interactions, mainly π–π stacking type, to form an infinite chain extend-
ing parallel to [101]. Short intermolecular C–C and C–O contacts are indicated by thin green lines, N–O contacts
by orange lines, and C–H⋯O bonds by thin blue lines. Only hydrogens involved in these interactions are shown
(see http://dx.doi.org/10.1080/00958972.2014.996144 for color version).
Table 4. MZI (mm) for the antimicrobial activity of HL, 1, 2, and 3.
Bacillus
cereus
Staphylococcus
aureus
Salmonella
typhi
Klebsiella
pneumoniae
Escherichia
coli
Compound
HL
17
13
15
None
20
8
30
28
18
10
8
7
8
[CoIIL2(bzln)2] (1)
[NiIIL2(DMF)2] (2)
[ZnIIL2] (3)
Penicillin
None
None
13
None
None
None
25
None
15
20
33
18
DMSO
None
None
None
None
None
(MZI = 28 mm), exhibit antibacterial activity against Staphylococcus aureus comparable to
that observed for penicillin, MZI = 33 mm (figures 6 and 7; table 4). Moreover, the antibac-
terial effect of these two complexes on Staphylococcus aureus is higher than that reported
for related complexes (MZI = 14.0 mm for CoIIL2 [41], 17 mm for CoIIAFL, and 18 mm
for NiIIAFL [42]).
In the metal complex, the polarity of the metal ion is presumably reduced due to overlap
of the ligand orbital and partial sharing of the positive charge of the metal ion with donor
groups. Furthermore, the increased delocalization of π electrons over the whole chelate ring

14
M. Amirnasr et al.
Figure 6. Antibacterial effect of [CoIIL2(bzln)2] (1) on S. aureus (MZI = 30 mm).
Figure 7. Antibacterial effect of penicillin on S. aureus (MZI = 33 mm).
enhances the lipophilicity of the complex. This, in turn, enhances the penetration of
complexes into the lipid membrane and blocking the metal bonding sites on enzymes of
microorganisms, leading to increase in biological activity [43].
4. Conclusion
In two consecutive reactions, we prepared new Co(II), Ni(II), and Zn(II) complexes with
the bidentate Schiff base HL = 2-((E)-(benzylimino)methyl)-4-nitrophenol. The complexes
obtained, [CoIIL2(bzln)2] (1), [NiL2(DMF)2] (2), and [ZnL2] (3), are mononuclear with Co

Co(II), Ni(II), Zn(II) Schiff base complexes
15
and Ni in octahedral coordination by two bidentate chelating ligands L and two coligands,
all in trans disposition, while Zn has a distorted tetrahedral coordination by two bidentate
chelating L. A common feature of the three crystal structures is that they all are stabilized
by π–π interactions between centrosymmetric pairs of nitrophenyl fragments generating
chain-like arrangements. Antimicrobial tests showed that the synthesized compounds
possess biological activity and are effective on Gram + ve but have no significant effect on
Gram −ve bacterial species. However, the exact mechanism is not known and further
biological studies are necessary to get a clear picture of this behavior.
Supplementary material
X-ray crystallographic data for 1, 2, and 3 have been deposited with the Cambridge Crystal-
lographic Data Center as supplementary publication CCDC No. 1004450 (for 1), No.
1004451 (for 2), and No. 1004452 (for 3). Copies of the data can be obtained free of charge
on request at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Center (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; Fax:+44(0)1223
336033; Email: deposit@ccdc.cam.ac.uk).
Acknowledgments
Partial support of this work by the Isfahan University of Technology Research Council is
gratefully acknowledged. The X-ray center of the Vienna University of Technology is
acknowledged for providing access to the single-crystal diffractometer.
Supplemental data
Supplemental data for this article can be accessed here [http://dx.doi.org/10.1080/00958972.2014.996144].
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