Synthesis and characterization of cobalt(II), nickel(II), and zinc(II) complexes with N,N′-bis(2-nitrocinnamaldehyde)-1,2-diiminoethane ligand. Crystal structure of Zn(nca2en)Br2

Article abstract of DOI:10.1080/15533170601172427

A series of complexes of the type M(nca2en)X2, where nca2en=/N,N′- bis(2-nitrocinnamaldehyde)-1,2-diiminoethane, M(II) =Co, Ni or Zn and X=Cl, Br, I have been synthesized and characterized. The crystal and molecular structure of Zn(nca2en)Br2 8 was determined by X-ray crystallography from single-crystal data. The complex 8 crystallizes in monoclinic system, having Zn ion in distorted tetrahedral geometry, Br(1)-Zn(1)- Br(2), 117.37(4)°; N(3)-Zn(1)-N(2), 82.57(18)°. This structure consists of intermolecular hydrogen bond of the type C-H-X. The formation of the C-H-M weak intramolecular hydrogen bonds due to the trapping of C-H bonds in the vicinity of the metal atoms is reported for 8. The 1H NMR study of Zn complexes gives further evidence for the presence of such interactions. The spectral properties of the above complexes are also discussed. Copyright

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Synthesis and Characterization of Cobalt(II),
Nickel(II), and Zinc(II) Complexes with
N,N′‐bis(2‐nitrocinnamaldehyde)‐1,2‐diiminoethane
Ligand. Crystal Structure of Zn(nca2en)Br2
Saeed Dehghanpour a & Ali Mahmoudi b
a Department of Chemistry, Alzahra University , Tehran, Iran
b Department of Chemistry, Islamic Azad University , Karaj Branch, Karaj, Iran
Published online: 30 Mar 2007.
To cite this article: Saeed Dehghanpour & Ali Mahmoudi (2007) Synthesis and Characterization of Cobalt(II), Nickel(II), and
Zinc(II) Complexes with N,N′‐bis(2‐nitrocinnamaldehyde)‐1,2‐diiminoethane Ligand. Crystal Structure of Zn(nca2en)Br2 ,
Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 37:1, 35-40
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 37:35–40, 2007
Copyright # 2007 Taylor & Francis Group, LLC
ISSN: 1553-3174 print/1553-3182 online
DOI: 10.1080/15533170601172427
Synthesis and Characterization of Cobalt(II), Nickel(II), and
Zinc(II) Complexes with N,N0-bis(2-nitrocinnamaldehyde)-
1,2-diiminoethane Ligand. Crystal Structure
of Zn(nca2en)Br2
Saeed Dehghanpour
Department of Chemistry, Alzahra University, Tehran, Iran
Ali Mahmoudi
Department of Chemistry, Islamic Azad University, Karaj Branch, Karaj, Iran
of transition metal centers.[7] Many MLX2 complexes of
A series of complexes of the type M(nca2en)X2, where Co(II), Ni(II), and Zn(II) with L ¼ diimine have been investi-
M(II) 5Co, Ni or Zn and X 5 Cl, Br, I have been synthesized and
nca2en 5 N,N0-bis(2-nitrocinnamaldehyde)-1,2-diiminoethane,
gated. In spite of the diversity in the coordination environment
and the structure of these complexes, which depend on the type
characterized. The crystal and molecular structure of Zn(nca2-
en)Br2 8 was determined by X-ray crystallography from single-
crystal data. The complex 8 crystallizes in monoclinic system,
of Schiff base and the anion, bulky ligands and/or anions will
force tetrahedral coordination,[8–13] here, we have synthesized
having Zn ion in distorted tetrahedral geometry, Br(1)-Zn(1)- and characterized a series of M(nca2en)X2 complexes with a
new bidentate ligand nca2en (N,N0-bis(2-nitrocinnamalde-
hyde)-1,2-diiminoethane) and X ¼ halides (Fig. 1). The
crystal structure of Zn(nca2en)Br2 complex is presented here.
Br(2), 117.37(4)8; N(3)-Zn(1)-N(2), 82.57(18)8. This structure
consists of intermolecular hydrogen bond of the type C-H-X.
The formation of the C-H-M weak intramolecular hydrogen
bonds due to the trapping of C-H bonds in the vicinity of the
metal atoms is reported for 8. The 1H NMR study of Zn complexes
gives further evidence for the presence of such interactions. The
spectral properties of the above complexes are also discussed.
EXPERIMENTAL
All reagents were used as supplied by Aldrich and Fluka
without further purification. Solvents used for the reactions
were purified and dried by conventional methods.[14] NMR
spectra were obtained on a Bruker Avance DRX500
(500 MHz) spectrometer. Proton chemical shifts are reported
in part per million (ppm) relative to an internal standard of
Me4Si, and the J values are reported in Hertz. The UV-
Vis spectra were recorded on a Perkin-Elmer Lambda 9
spectrophotometer with quartz cells (1 cm path length).
Elemental analyses were performed by using Heraeus CHN-
O RAPID elemental analyzer. IR spectra were recorded as
KBr pellets on a Shimadzu 435 spectrophotometer. The
ligand N,N0-bis(2-nitrocinnamaldehyde)-1,2-diiminoethane,
nca2en, Co(nca2en)Cl2 and Ni(nca2en)Br2 was prepared
according to previously reported method.[12,13]
Keywords Schiff base complexes, cobalt(II), nickel(II), zinc(II), tet-
rahedral, crystal structures, nitrocinnamaldehyde
INTRODUCTION
The coordination chemistry of transition metal containing
the Schiff base ligands have been of interest for many
years.[1,2] These complexes play an important role in the devel-
oping of coordination chemistry related to catalysis and enzy-
matic reactions, magnetism and molecular architectures.[3–6]
Bidentate ligands containing imine groups have also been
used as the modulators of structural and electronic properties
General Procedures for Synthesis of Cobalt Complexes,
[Co(nca2en)X2]
Received 30 August 2006; accepted 13 November 2006.
S.D. would like to acknowledge the Islamic Azad University
Research Council for partial support of this work.
Address correspondence to Saeed Dehghanpour, Department
of Chemistry, Alzahra University, Tehran, Iran. E-mail:
dehganpour_farasha@yahoo.com
To a solution of nca2en 1 mmol in 10 ml CH2Cl2 was added
1 mmol of anhydrous CoX2, and stirred at room temperature
(r.t.) for 3 h under nitrogen atmosphere. The reaction mixture
was filtered off. The volume of the solvent was reduced
35

36
S. DEHGHANPOUR AND A. MAHMOUDI
˚
(l ¼ 0.71073 A). The structure was solved by direct
methods (SHELXS-97).[15] The structure refinement was
performed by a full-matrix least-squares method against F2
(SHELXL-97).[16] All non-H-atoms were refined aniso-
tropically, all H-atoms were inserted in calculated positions.
Crystallographic data (excluding structure factors) for the
structure 8 reported in this paper have been deposited with
the Cambridge Crystallographic Data Center, CCDC No.
616198. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: þ44 1223 336033; e-mail: deposit@ccdc.
cam.ac.uk or khttp:// www.ccdc.cam.ac.ukl).
RESULTS AND DISCUSSION
N,N0-bis(2-nitrocinnamaldehyde)-1,2-diiminoethane, nca2en
was prepared by the condensation of 2-nitrocinnamaldhyde
with ethylenediamine. The product, as a white solid material,
was characterized by IR, UV-Vis and 1H, 13C-NMR spec-
troscopy. The cobalt complexes were prepared by reacting
equimolar quantities of the anhydrous cobalt halides salts and
nca2en in dichloromethane. These complexes are stable in air
in the solid state. The stability of cobalt complexes in
solution depends on the solvent used. The compounds are
stable in dichloromethane for at least 24 h, while they are
much less stable in methanol. A methanolic solution in open
air slowly turns brown in 6 h.
FIG. 1. Chemical formula of the complexes 1–9.
under vacuum to about 1 ml. The diffusion of diethyl ether
vapor into the concentrated solution gave needle like blue
crystals suitable for X-ray studies. The crystals were filtered
off and washed with a mixture of diethylether-dichloromethane
(9:1 v/v), and dried in vacuo. The spectral data and elemental
analysis of complexes are summarized in Table 1.
To avoid any oxidation of the cobalt complexes, the syn-
thesis of them were carried out under nitrogen atmosphere.
The nickel complexes were prepared by reacting equimolar
quantities of the anhydrous nickel halides salts and nca2en in
boiling acetonitrile. The zinc complexes were prepared by
reacting equimolar quantities of anhydrous zinc salts and
nca2en in dichloromethane. The zinc complexes are colorless
solids dissolving in chloroform, methanol, ethanol and
acetone and they are stable at ambient conditions.
General Procedures for Synthesis of Nickel Complexes,
[Ni(nca2en)X2]
To a stirring solution of 1 mmol of anhydrous NiX2 in 20 ml
acetonitrile was added dropwise a solution of 1 mmol nca2en in
10 ml acetonitrile, and refluxed for 30 min. The solvent was
evaporated on a rotatory evaporator at 408C under reduced
pressure. The solid residue was dissolved in 20 ml acetonitrile
and filtered off. The filtrate was left overnight under diffusion
of diethyl ether to give pink-red crystals. The spectral data and
elemental analysis of complexes are summarized in Table 1.
IR Spectra
All complexes show similar IR spectral features, exhibiting
a strong band between 1629 and 1639 cm21 corresponding to
n(C55N). This band is shifted to the lower frequencies by
6–20 cm21 relative to the free ligand upon the coordination
of the nitrogen atoms.[17] The observation of a strong bands
at 1350 and 1550 cm21 in the IR spectrum of complexes
assigned to the NO2 group of ligand in these complexes.[18]
General Procedures for Synthesis of Zinc Complexes,
[Zn(nca2en)X2]
To a solution of 1 mmol ZnX2 in 10 ml ethanol was added a
solution of 1 mmol nca2en in 5 ml methanol and stirred at room
temperature for 3 h. The product precipitated as a white micro-
crystalline powder and collected by filtration. The crude
product was recrystallized from acetone to give white needle-
shape crystals. The crystals were collected by filtration and
dried in vacuo. The spectral data and elemental analysis of
complexes are summarized in Table 1.
1H-NMR Spectra
The 1H-NMR spectral data of Zn complexes are summarized
in Table 1. The 1H-NMR data of the Zn complexes suggest that
the ligand has a symmetrical structure with H2 and H3 protons
(Fig. 1) in trans positions (J ¼ 9.5 Hz). The four methylene
Crystal Structure Determination
Suitable single crystal of [Zn(nca2en)Br2], was obtained protons appear as a singlet at about 4 ppm. The two vinyl CH
by slow evaporation of an acetone solution of 6 at r.t. protons (H3) are observed as a quartet centered at about
Diffraction data for 8 were collected on a STOE-IPDSII dif- 7.38 ppm, and the multiplet centered at 7.9 ppm is assigned to
fractometer using graphite-monochromated MoKa radiation the phenyl and H4 protons. The two H-C-N protons (Hb)

TABLE 1
˚
Selected bond lengths [A] and bond angles [8] for 1b, 1c, and 2a
Co
Br
Ni
Zn
Br
[M(nca2en)X2], X
IR spectra
Cl
I
Cl
Br
I
Cl
I
C55N (cm21
NO2 (cm21
UV-Vis spectra
)
1629
1633
1636
1635
1631
1639
1633
1632
1637
)
645(2.78)
638(2.80)
585(3.10)
321(3.10)
650(3.00)
605(3.01)
577(3.20)
325(4.51)
689(2.32)
633(2.225)
590(3.10)
328(4.91)
510(2.22)
320(4.65)
513(2.30)
323(4.68)
547(2.69)
325(3.84)
323(4.243)
325(3.29)
329(4.73)
CHN analysis
%C, Calcd (found) 47.27(47.28) 40.23(40.26) 34.76(34.75) 47.29(47.28) 40.24(40.25) 34.77(37.75)
46.67(46.69)
10.89(10.88)
3.53(3.54)
39.80(30.81)
9.28(9.28)
3.01(3.02)
34.44(34.43)
8.03(8.04)
2.60(2.07)
%N, Calcd (found) 11.02(11.00)
9.38(9.37)
3.04(3.05)
8.11(8.10) 11.03
2.63(2.63) 3.57
9.39(9.38)
3.04(3.04)
8.11(8.12)
2.63(2.64)
%H, Calcd (found)
1H-NMR spectra
H1
3.57(3.56)
4.00(s, 4H)
8.37(d, 2H)
7.36(q, 2H)
4.03(s, 4H)
8.37(d, 2H)
7.38(q, 2H)
4.04(s, 4H)
8.38(d, 2H)
7.41(q, 2H)
H2
H3
Ar-H, H4
7.55–8.10(m, 10H) 7.58-8.9 (m, 10H) 7.59-8.13(m, 10H)
13C-NMR spectra
N13CH123CH2N
C55N
61.59
163.83
62.30
164.25
62.26
164.14
Other carbons
124.83, 127.13,
128.87, 129.07,
131.29, 132.60,
136.59, 147.90
124.90, 127.25,
128.88, 129.15,
131.32, 132.65,
136.63, 147.98
124.89, 127.68,
128.63, 129.28,
131.21, 132.69,
136.82, 148.10

38
S. DEHGHANPOUR AND A. MAHMOUDI
appear as a doublet at about 8.37 ppm. In the 1H-NMR spectra of
compounds 7–9, the hydrogen of the imine CH groups (Ha)
exhibits a downfield shift relative to the free ligand and shows
no significant difference by changing the anion.
In the 1H NMR spectra of compounds 7–9, the hydrogen 3
groups (H18 in Fig. 2) exhibits a downfield shift relative to the
free ligand and shows no significant difference by changing the
anion. Nevertheless, the signals due to H3 protons appear at
7.36, 7.38, and 7.41 ppm for ZnLCl2, ZnLBr2, and ZnLI2
respectively (Table 1). In search for an explanation for this
behavior, we have found no interaction between H3 and the
chloride atoms in ZnLCl2.
TABLE 2
Crystal data and structure refinement for 8
Empirical formula: C20H18Br2N4O4Zn
Formula weight ¼ 603.57
Crystal structure: monoclinic
Space group: P21/n
Z ¼ 4
˚
a ¼ 7.8386(12) A
˚
b ¼ 16.381(3) A
˚
c ¼ 18.226(3) A
a ¼ 908
b ¼ 100.054(12)8
However, the observed variation in the H3 chemical shifts
could be attributed to an intramolecular interaction with metal
...
atom in the form of C-H M weak hydrogen bonds (see more
g ¼ 908
3
˚
V ¼ 2304.3(7) A
Calculated density ¼ 1.740 Mg/m3
Absorption coefficient/mm21 ¼ 4.567
Temperature ¼ 293(2) K
details in the crystal structures). Such interactions have also
been reported by others.[19,20] The extent of interaction
depends on the steric restrictions imposed by the coordinated
anion and the electron density of metal atom. The shielding
effect resulting from this intramolecular interaction is strongest
for Cl followed by Br and I coordinated to the metal atom.
The 13C-NMR spectrum of the free ligand exhibits 10
signals due to the phenyl rings and the ethylenic protons. In
the 13C-NMR spectra of the complexes 6-9, the carbon atoms
adjacent to the donor nitrogen atoms show downfield shift in
their positions as compared with the free ligand, clearly indi-
cating the coordination of the ligand and retention of the struc-
ture in the chloroform solutions.[23]
F(000) ¼ 1192
Crystal size ¼ 0.45 ꢁ 0.25 ꢁ 0.15 mm3
No. of reflection used ¼ 4394
No. of parameters ¼ 280
˚
l(Mo-Ka) ¼ 0.71073 A
umax ¼ 25.728
R1 ¼ 0.0585[I ꢂ 2s(I)]
wR2 ¼ 0.1273
Goodness-of-fit on F2 ¼ 1.067
(D/s)max ¼ 0.001
23
23
˚
(D/r)max ¼ 0.857 eA
˚
(D/r)min ¼ 20.720 eA
UV-Vis Spectra
The UV-Vis spectral data of M(nca2en)X2 complexes are
presented in Table 1. The Co(nca2en)Cl2 show three closely
spaced weak bands in the visible region and a very intense
band in the UV region. In the regular tetrahedral and near-tetra-
hedral Co(II) complexes only one d-d transition
closely spaced transitions in the spectrum of Co(nca2en)X2
complexes arise from the distortion in the tetrahedral
symmetry around the metal center. This splitting originates
from the reduction of the orbital degeneracy due to the differ-
ence in the ligand field strength of imine and halides donor
atoms and the restricted bite angle of the N(1)-M-N(2) chelat-
ing ligand.[11,21,23–25]
4
[4A2(F) ! T1(P), assigned as n3] is observed in the visible
region. This transition has been reported for the tetrahedral
[Co(NCS)4]22 at 615 nm and for the pseudotetrahedral
[Co(morpholine)2(NCS)2] at 616 nm.[11,21–23] The three
The intensity of the UV-band is consistent with its being a
ligand-centered p ! pꢀ transition or/and a charge-transfer
3
transition.
The
3T1(F) ! T1(P)
transition[26,27]
in
TABLE 3
Selected bond length and bond angle of complex 8
Zn(1)-N(3)
Zn(1)-N(2)
Zn(1)-Br(1)
Zn(1)-Br(2)
2.036(4)
2.071(5)
C(6)-C(7)
C(7)-C(8)
2.3328(11) C(8)-C(9)
1.517(9)
1.440(8)
1.333(8)
1.469(8)
2.3652(12) C(10)-C(11)
N(3)-Zn(1)-N(2)
N(3)-Zn(1)-Br(1)
N(2)-Zn(1)-Br(1)
82.57(18)
116.69(14)
115.96(15)
N(3)-Zn(1)-Br(2) 112.20(15)
N(2)-Zn(1)-Br(2) 106.73(15)
Br(1)-Zn(1)- Br(2) 117.37(4)
FIG. 2. ORTEP view of the crystal structure of [Zn(nca2en)Br2], 8 showing
the atom labeling scheme. The thermal ellipsoids enclose 50% of the electronic
density.

SYNTHESIS OF ZINC COMPLEXES
39
TABLE 4
Torsion angles for chelating rings
2a
1b
N1-C1-C2-N2
1c
I
II
0.0(3)
178.3(2)
3.6(3)
N2-C1-C2-N1
N2-C1-C2-C3
Cu1-N1-C2-C1
C1-C2-C3-C4
Cu1-N2-C1-C2
2 12.2(9) N1-C8-C9-C10 174.4(5) N3-C57-C58-N4
169.2(6) N1-C8-C9-C10 174.4(5) N3-C57-C58-C59
4.2(9)
2171.4(6)
0.1(7)
N1-C1-C2-C3
Cu-N1-C1-C2
3.9(7) Cu1-N1-C8-C9
2179.0(6) C1-C2-C3-C4
13.3(7) Cu1-N2-C9-C8
4.1(6) Cu2-N3-C57-C58
179.8(6) C57-C58-C59-C60
2.7(6) Cu2-N4-C58-C59
C1-C2-C3-C4 2177.0(2)
Cu-N2-C2-C1
N1-Cu-N2-C2
175.9(7)
169.7(6)
0.1(7)
23.4(2)
4.01(14) N2-Cu1-N1-C2
2.1(4) N1-Cu1-N2-C9 20.5(4) Cu2-N3-C57-C58
Ni(nca2en)X2 complexes (4, 5), appears as a low intensity band Due to the geometrical restrictions in 8 the H13 atom, lie in
at 510 nm which is similar to that reported for the vicinity of the metal atoms making intramolecular inter-
Ni(Me4propylenediamine)2Cl2 (512 nm).[21] A strong band at actions possible in the form of weak C-H M hydrogen bond.
...
320 nm is assigned to a ligand-centered p ! pꢀ transition This type of interaction is somewhat similar to the agnostic
or/and a charge-transfer transition. The electronic spectral interactions found in many organometallic complexes,[29,30]
...
data conform to the structures determined by X-ray crystallo- although a C-H M charge-assisted hydrogen bonding could
graphy for cobalt and nickel complexes.[12,13] The electronic be encountered here. As a matter of fact, the involvement of
absorption spectra of the Zn(nca2en)X2 complexes (7–9) in the metal centers in hydrogen bonding is now well established
chloroform, are dominated by the broad band in the region and known as nontraditional hydrogen bonds.[31,32] A recent
325 nm corresponding to the intra-ligand transition of nca2en survey by Desiraju and Steiner,[33] gives the range 2.5–3.2 A
˚
...
or/and a charge-transfer transition. No d-d transition are for the weak C-H M hydrogen bonds. Our structural data
expected for d10 Zn(II) complexes.
reveals that the C-H M interactions is about 3 A (C(8)-
˚
...
˚
˚
...
...
H(8) Zn(1), 2.98 A and C(13)-H(13) Zn(1), 3.031 A) in 8
would meet the above criteria and can be regarded as intramo-
lecular weak hydrogen bonds.
Crystal Structure of Zn(nca2en)Br2, 8
Consequently, the vinilic protons are influenced by the
change in the electron density on the central metal atom and
The crystallographic and refinement data are summarized in
Table 2 and the selected bond distances and angles are given in
Table 3. The molecular structure of 8 is illustrated in Fig. 2,
where metal atom is coordinated by the bidentate Schiff base
ligand and two halogen atoms. While a tetrahedral geometry
might be expected for a fourcoordinated Zn(II) center, the
geometry about Zn(II) in 2 is distorted by the restricting bite
of chelating ligand (Table 4). For 8 (see Fig. 2), the N(3)-
Zn(1)-N(2) angle is only 82.57(18)8 being in the range of 82-
898 found for ethylenediamine chelated complexes.[23,28] The
Br(1)-Zn(1)-Br(2) angle is 117.37(4)8 but the Br(1)-Zn(1)-
N(3) angle, 116.69(14)8, is larger than the tetrahedral values.
1
this can be followed easily by H NMR spectroscopy. Our
1
results from H NMR study of Zn(nca2en)X2 complexes, in
which X ¼ Cl, Br, I, clearly give further evidence to the exist-
...
ence of such C-H M intramolecular interactions, although
being weak but can be significant. This type of interaction
has also been reported by others[19,23,26,34,35] for closely
related structures.
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