Synthesis and spectral characterization of zinc(II) and cadmium(II) complexes of acetone-N(4)-phenylsemicarbazone: Crystal structures of acetone-N(4)-phenylsemicarbazone and a cadmium(II) complex

Article abstract of DOI:10.1016/j.poly.2010.03.011

Seven Zn(II) and Cd(II) complexes of ON donor acetone-N(4)- phenylsemicarbazone (HL) have been synthesized and physico-chemically characterized by partial elemental analyses, molar conductance measurements, infrared, electronic and ¹H NMR spect

Full text of DOI:10.1016/j.poly.2010.03.011

Polyhedron 29 (2010) 2035–2040
Contents lists available at ScienceDirect
Polyhedron
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Synthesis and spectral characterization of zinc(II) and cadmium(II) complexes
of acetone-N(4)-phenylsemicarbazone: Crystal structures of
acetone-N(4)-phenylsemicarbazone and a cadmium(II) complex
b
V.L. Siji a, M.R. Sudarsanakumar a, , S. Suma
*
a Department of Chemistry, M.G. College, Thiruvananthapuram 695 004, Kerala, India
b Department of Chemistry, S.N. College, Chempazhanthy, Thiruvananthapuram 695 587, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven Zn(II) and Cd(II) complexes of ON donor acetone-N(4)-phenylsemicarbazone (HL) have been syn-
thesized and physico-chemically characterized by partial elemental analyses, molar conductance mea-
surements, infrared, electronic and 1H NMR spectral studies. The semicarbazone binds the metal as a
neutral bidentate ligand in all the complexes. The crystal structures of acetone-N(4)-phenylsemicarbaz-
one and [Cd(HL)2Cl2] have been determined by X-ray diffraction studies. The coordination geometry
around cadmium(II) in the complex [Cd(HL)2Cl2] is distorted octahedral.
Received 26 January 2010
Accepted 12 March 2010
Available online 10 April 2010
Keywords:
Semicarbazone
Zn(II) complex
Cd(II) complex
IR spectra
Ó 2010 Elsevier Ltd. All rights reserved.
X-ray diffraction
NMR spectra
1. Introduction
pounds. The Zn(II) ion has been found to be of catalytic importance
in enzymatic reactions [8]. A review on 200 crystal structures of
There has been less attention devoted to the synthesis and bio-
logical properties of the structurally analogous semicarbazones
and their metal complexes, despite the fact that, for some of these
compounds, considerable anticonvulsant [1,2], antimicrobial [3]
and antitumor [4] activities have been found, and a great advan-
tage of semicarbazone derivatives over their thiosemicarbazone
analogues seems to be their lower toxicity. Semicarbazones are
compounds having the formula R2C@N–NH–(CO)–NH2 formally
derived by condenzation of aldehyde or ketone with semicarbaz-
ide. A number of arylsemicarbazones were devoid of sedative-hyp-
notic activity and exhibited anticonvulsant activity with less
neurotoxicity [5].
Cd(II) complexes showed that cadmium(II) has coordination num-
bers of 4, 5 and 6 in about 19%, 18% and 56%, respectively [9]. Here
we report the synthesis and spectral characterization of the semi-
carbazone ligand, acetone-N(4)-phenylsemicarbazone (Scheme 1)
and its Zn(II) and Cd(II) complexes. It also describes the single crys-
tal X-ray diffraction studies of ligand acetone-N(4)-phenylsemicar-
bazone and one of its Cd(II) complexes. To the best of our
knowledge, there exist only one previous structural report on cad-
mium(II) complex of semicarbazone [10].
2. Experimental
Semicarbazones are compounds with versatile structural fea-
tures and can coordinate to the metal either as a neutral ligand
or as a deprotonated anion. Semicarbazones exist in two tauto-
meric forms, keto and enol. The coordination possibilities in the
semicarbazones are increased if the substituents of the aldehyde
2.1. Materials
N(4)-phenylsemicarbazide (Sigma–Aldrich) and acetone
(Merck) were of analar grade and used as received. The metal salts
of Zn(II) and Cd(II) were used as supplied. Solvent used was meth-
anol (Merck) and used without further purification.
or ketone include additional donor atoms. The
p-delocalization
and the configurational flexibility of their molecular chain can give
rise to a variety of coordination modes [6].
Cadmium [7] and zinc form complexes with chloride, acetate,
sulfate and carboxy-ligands to yield both charged and neutral com-
2.2. Synthesis of acetone-N(4)-phenylsemicarbazone (HL)
N(4)-phenylsemicarbazide (0.151 g, 1 mmol) in methanol was
refluxed with acetone (1 mmol) in presence of four drops of dilute
acetic acid for 3 h. On slow evaporation, colorless crystalline
* Corresponding author. Tel.: +91 471 2592617.
E-mail address: sudarsanmr@gmail.com (M.R. Sudarsanakumar).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.03.011

2036
V.L. Siji et al. / Polyhedron 29 (2010) 2035–2040
Table 1
CH3
1
4
Crystal data and structure refinement parameters for HL and [Cd(HL)2Cl2].
NH
N
C
2
Parameters
HL
[Cd(HL)2Cl2]
CH3
C
O
NH
Empirical formula
Formula weight (M)
Temperature (T) (K)
C10H13N3O
191.23
293(2)
C20H26CdCl2N6O2
565.77
293(2)
Scheme 1. Acetone-N(4)-phenylsemicarbazone.
Wavelength (Mo K
Crystal system
Space group
Lattice constants
a (Å)
a) (Å)
0.71073
monoclinic
P21/n
0.71073
monoclinic
P21/c
compound formed was filtered, washed with ether and recrystal-
lized from methanol and dried over P4O10 in vacuo. The structure
has been determined by single crystal X-ray diffraction studies.
Melting point = 152 °C.
13.3696(7)
5.4201(3)
15.8946(8)
90.00
10.9664(5)
12.4218(5)
9.4043(4)
90.00
b (Å)
c (Å)
a
(°)
HL: Elemental Anal. Calc.: C, 62.80; H, 6.87; N, 21.98. Found: C,
62.76; H, 6.56; N, 21.95%.
b (°)
114.651(2)
90.00
1046.83(10)
4
1.213
0.082
110.3120(10)
90.00
1201.41(9)
2
1.564
1.159
c
(°)
Volume V (Å3)
Z
Dcalc
Absorption coefficient,
(mmꢀ1
F (0 0 0)
Crystal size (mm)
Colour, nature
h Range for data collection 1.68–27.53
(°)
Limiting indices
(q
) (mg/m3)
2.3. Synthesis of complexes
l
)
The complexes were prepared by refluxing the methanolic solu-
tion of HL with a methanolic solution of appropriate salts for 6 h.
The complexes separated were filtered, washed with methanol
and finally with ether and dried over P4O10 in vacuo. X-ray quality
single crystal of the complex [Cd(HL)2Cl2] was obtained by slow
evaporation of its solution in methanol over a period of one week.
408
572
0.30 ꢁ 0.20 ꢁ 0.20
0.30 ꢁ 0.20 ꢁ 0.20
White, block
2.57–32.95
Colorless, block
ꢀ17 6 h 6 17
ꢀ5 6 k 6 7
ꢀ20 6 l 6 20
20274
ꢀ16 6 h 6 16
ꢀ17 6 k 6 19
ꢀ13 6 l 6 14
18000
2.4. Physical measurements
Reflections collected
Independent reflections
2395(0.0229)
4476(0.0229)
(Rint
Observed reflections
[(I > 2 (I)]
Completeness to h
Absorption correction
)
Elemental analyses were carried out using a Vario EL-III CHN
analyzer at SAIF, Cochin University of Science and Technology, In-
dia. Infrared spectra were recorded on a Thermo Nicolet, Avator
370 spectrometer in the range 4000–400 cmꢀ1 using KBr pellets.
Electronic spectra were recorded in methanol on a Shimadzu UV-
2450 UV–Vis spectrophotometer. The 1H and 13C NMR spectra
were recorded using Bruker DRX-500 MHz NMR spectrometer,
with CDCl3 as solvent and TMS as the standard at the SAIF, Indian
Institute of Technology, Madras, India. The molar conductances of
the complexes in methanol solution (10ꢀ3 M) at room temperature
were measured using a Systronics direct reading conductivity
meter.
1598
3381
r
27.53° (99.6%)
Semi-empirical from
equivalents
25° (99.9%)
Semi-empirical from
equivalents
0.85 and 0.68
Maximum and minimum
transmission
Refinement method
0.936 and 0.901
Full-matrix least
Full-matrix least
squares on F2
4476/0/152
1.032
R1 = 0.0262
wR2 = 0.0665
R1 = 0.0401
squares on F2
Data/restraints/parameters 2395/0/138
Goodness-of-fit on F2
1.025
Final R indices [(I > 2
r(I)]
R1 = 0.0454
wR2 = 0.1151
R1 = 0.0761
wR2 = 0.1477
R indices (all data)
wR2 = 0.0744
0.379 and ꢀ0.300
Largest difference peak and 0.139 and ꢀ0.177
2.5. X-ray crystallography
hole (e Åꢀ3
)
w(Fo ꢀ Fc2)2/
R .
w(Fo2)2]1/2
R|Fo|.
2
wR2 = [
R
Single crystals of compounds HL and [Cd(HL)2Cl2] (5) of X-ray
diffraction quality were grown from their methanol solutions by
slow evaporation at room temperature in air and were found to
be monoclinic. The crystallographic data and structure refinement
parameters are given in Table 1. The data were collected using a
Bruker AXS Kappa Apex2 CCD diffractometer, with graphite-mono-
R1
=
R
||Fo| ꢀ |Fc||/
two since it has an amide –NH–C@(O) function. However, the IR
spectrum of HL indicates that in the solid state it remains in the
keto form. The semicarbazone HL acts as a neutral bidentate ligand
in all the complexes.
The colors and elemental analyses of HL and its complexes are
presented in Table 2. The complexes are soluble in CH3OH, CHCl3,
DMF and DMSO. The conductivity measurements were made in
methanol solutions and the values (Table 2) show that the com-
plexes are non-electrolytes.
chromated Mo K
sions and intensity data were recorded at 293 K. The program
XPREP was used for data reduction and APEX2/SAINT for cell refine-
a (k = 0.71073 Å) radiation. The unit cell dimen-
SAINT
/
ment [11]. The structure was solved using SIR92 [12] and refine-
ment was carried out by full-matrix least squares on F2 using
SHELXL-97 [13]. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. All hydrogen atoms with the exception
of those on nitrogen atoms were geometrically fixed and refined
using a riding model. The hydrogen atoms on nitrogen atoms were
located from the difference Fourier maps and refined isotropically.
Molecular graphics employed were DIAMOND Version 3.1f [14], MER-
CURY [15] and PLATON [16].
3.1. Crystal structure of HL
The molecular structure of HL along with atom numbering
scheme is given in Fig. 1. Selected bond lengths and bond angles
of HL are given in Table 3. The compound crystallized into a mono-
clinic space group P21/n. The molecule exists in the Z conformation
with respect to the C(2)–N(1) bond. However, the O(1) and N(1)
atoms are in E conformation with respect to the N(2)–C(4) bond
and hence the semicarbazone moiety as a whole exists in the ZE
conformation [17]. A torsion angle of ꢀ179.43(18)° corresponding
to the O(1)–C(4)–N(2)–N(1) moiety implies a trans alignment of
3. Results and discussion
The elemental analyses data obtained is in good agreement with
the stoichiometry of acetone-N(4)-phenylsemicarbazone. HL can
exist in the keto or enol form or an equilibrium mixture of the

V.L. Siji et al. / Polyhedron 29 (2010) 2035–2040
2037
Table 2
Colors and elemental analyses of HL and its Zn(II) and Cd(II) complexes.
a
Compound
Color
Calc. (found) (%)
C
km
H
N
HL
Colorless
White
Yellow
Pale yellow
Pale yellow
White
62.80(62.76)
46.30(46.23)
50.93(51.01)
42.01(41.98)
44.16(44.11)
42.46(42.40)
47.03(46.91)
38.82(38.87)
6.87(6.56)
5.06(5.15)
5.71(5.84)
4.59(4.65)
4.83(4.73)
4.64(4.71)
5.27(5.39)
4.24(4.34)
21.98(21.95)
16.20(16.07)
14.85(14.89)
19.60(19.67)
15.46(15.49)
14.86(14.75)
13.71(13.77)
18.11(18.31)
[Zn(HL)2Cl2] (1)
10.3
10.2
8.3
12.3
8.2
[Zn(HL)2(CH3COO)2] (2)
[Zn(HL)2(NO3)2] (3)
[Zn(HL)2(SO4)] (4)
[Cd(HL)2Cl2] (5)
[Cd(HL)2(CH3COO)2] (6)
[Cd(HL)2(NO3)2] (7)
Yellow
White
11.3
9.1
a
Molar conductivity, 10ꢀ3 M methanol at 298 K.
Table 3
Selected bond lengths (Å) and bond angles (°) of HL and [Cd(HL)2Cl2].
HL
[Cd(HL)2Cl2]
Bond lengths
C(2)–N(1)
C(4)–O(1)
C(4)–N(3)
C(4)–N(2)
C(5)–N(3)
N(1)–N(2)
N(1)–Cd(1)
O(1)–Cd(1)
Cl(1)–Cd(1)
1.270(2)
1.220(2)
1.354(2)
1.354(2)
1.402(2)
1.380(2)
1.2765(19)
1.2292(17)
1.3480(19)
1.3665(19)
1.407(2)
1.3770(2)
2.3995(12)
2.3234(11)
2.5418(5)
Bond angles
N(1)–C(2)–C(1)
N(1)–C(2)–C(3)
O(1)–C(4)–N(3)
O(1)–C(4)–N(2)
N(3)–C(4)–N(2)
C(6)–C(5)–N(3)
C(10)–C(5)–N(3)
C(2)–N(1)–N(2)
C(4)–N(2)–N(1)
C(4)–N(3)–C(5)
C(2)–N(1)–Cd(1)
N(2)–N(1)–Cd(1)
C(4)–O(1)–Cd(1)
O(1)–Cd(1)–N(1)
O(1)–Cd(1)–O(10)
O(10)–Cd(1)–N(1)
N(1)–Cd(1)–N(10)
O(1)–Cd(1)–Cl(10)
O(10)–Cd(1)–Cl(10)
N(1)–Cd(1)–Cl(10)
N(10)–Cd(1)–Cl(10)
Cl(10)–Cd(1)–Cl(1)
116.90(16)
125.59(16)
123.80(16)
121.18(15)
115.02(15)
116.88(15)
123.95(15)
118.58(14)
120.17(14)
128.26(15)
117.99(14)
124.56(15)
124.42(14)
121.15(15)
112.58(13)
124.79(17)
115.64(16)
117.91(12)
119.54(12)
129.25(14)
131.41(10)
110.31(9)
116.57(10)
70.36(4)
180
109.64(4)
180
88.36(4)
91.64(4)
91.04(4)
88.96(4)
180
Fig. 1. The molecular structure of HL, showing the atom numbering scheme.
Ellipsoids are drawn with 50% probability. Hydrogen atoms are omitted for clarity.
the keto carbonyl O(1) atom in HL. The azomethine bond, C(2)–
N(1) 1.270(2) Å is in conformity with a formal C@N double bond
(1.28 Å) and the C(4)–O(1) bond distances of 1.220(2) Å, is very
close to a formal C@O bond length 1.21 Å. It confirms the existence
of the semicarbazone in the keto form in the solid state. The N(1)–
N(2) (1.380(2) Å) and N(2)–C(4) (1.354(2) Å) bond distances in HL
are intermediate between ideal values of corresponding single [N–
N, 1.45 Å; C–N, 1.47 Å] and double bonds [N@N, 1.25 Å, C@N,
1.28 Å], giving evidence for an extended
the semicarbazone chain [18].
p-delocalization along
The packing of the molecule in a unit cell along the b-axis is gi-
ven in Fig. 2. The assemblage of molecules in the respective man-
ner in the unit cell is resulted by the H-bonding and C–H–
p
interactions as depicted in Table 4. The intramolecular hydrogen
bonding interaction N(3)–H(3)ꢂ ꢂ ꢂN(1), leads to the formation of
one five membered ring comprising of atoms N(1), N(2), C(4),
N(3), H(3) and one intermolecular hydrogen bonding interaction
O(1)–H(2)ꢂ ꢂ ꢂN(2) is also observed in Fig. 2. A weak intramolecular
hydrogen bonding interaction is observed between C(10)–H(10A)
ter. The Cd(II) center is coordinated by azomethine nitrogen, car-
bonyl oxygen from the semicarbazone and two chloro ligands.
The chloro and semicarbazone ligands serve as monodentate and
bidentate ligands. The semicarbazone ligand is coordinated to
cadmium as a neutral bidentate N, O-donor forming two five
membered chelate rings. The bond angles of O(1)–Cd(1)–O(10),
N(1)–Cd(1)–N(10) and Cl(1)–Cd(1)–Cl(10) are 180° so the complex
is centrosymmetric with the Cd atom situated on an inversion
centre. The distortion from an ideal octahedral stereochemistry is
suggested by the values for the orthogonal bite angles N(1)–Cd(1)–
O(1) and N(10)–Cd(1)–O(10) (70.36(4)°). The two semicarbazones
and chloro ligands have occupied trans position. The Cd(II) ion in
the monomer exhibit distorted octahedral geometry with O(1),
N(1), O(l0), N(10) atoms defining the equatorial plane and the termi-
nal chlorine atoms takes the axial position. It is interesting to note
that the azomethine and carbonyl bond distances are slightly
increased in the complex, when compared to the corresponding
values in the free ligand HL, and this supports coordination.
and O(1) with an angle of 122°. The C–H–p interactions C(1)–
H(1C) ? Cg(1) (atom numbering according to Fig. 1) at a distance
of 2.886 Å contribute stability to the unit cell packing.
3.2. Crystal structure of [Cd(HL)2Cl2]
The molecular structure of the complex [Cd(HL)2Cl2] along with
the atom numbering scheme is represented in Fig. 3 and selected
bond lengths and bond angles are summarized in Table 3. The com-
pound crystallizes in a monoclinic space group P21/c. The molecu-
lar structure of Cd(II) semicarbazone complex obtained from single
crystal X-ray diffraction studies showed a six coordinate Cd(II) cen-

2038
V.L. Siji et al. / Polyhedron 29 (2010) 2035–2040
Fig. 2. Unit cell packing diagram of HL viewed along b-axis.
length (1.47 Å). The Cd–O, Cd–N and Cd–Cl bond distances are
Table 4
2.3234(11) Å, 2.3995(12) Å and 2.5418(5) Å. The bond angle,
O(1)–Cd(1)–N(1) is 70.36(4)° indicating that the compound has
distortion from octahedral geometry. The azomethine bond,
C(2)–N(1) 1.2765(19) Å is very close to an ideal C@N double bond
(1.28 Å). The Cd–N azomethine bond length is comparable with the
previously reported Cd-semicarbazone complex [10]. The N(1)–
N(2) 1.3770(17) Å and N(2)–C(4) 1.3665(19) Å bond distances in
the complex are intermediate between ideal values of correspond-
Interaction parameters of the compound HL.
D–H–A
D–H (Å)
H–A (Å)
D–A (Å)
D–H–A (°)
H bonding
N(2)–H(2)–O(1)a
N(3)–H(3)–N(1)
0.86
0.86
2.03
2.20
2.870(2)
2.620(2)
164
110
Equivalent position code: a = ꢀx, 2 ꢀ y, 2 ꢀ z
X–H(I)–Cg(J)
CH– interactions
C(1)–H(1C))–Cg(1)b
Hꢂ ꢂ ꢂCg (Å)
X–Hꢂ ꢂ ꢂCg (°)
Xꢂ ꢂ ꢂCg (Å)
p
ing single and double bonds, giving evidence for an extended
delocalization along the semicarbazone chain.
p-
2.886
135.59
3.634(2)
Equivalent position code: b = ½ + x, 3/2 ꢀ y, 1/2 + z
The packing of the molecule is stabilized by intermolecular
hydrogen bonding interactions are given in Table 5. Two intermo-
lecular hydrogen bonding interactions [N(3)–H(3)ꢂ ꢂ ꢂCl(1)] and
[N(2)–H(2)ꢂ ꢂ ꢂCl(1)] [symmetry code: x, 1/2 ꢀ y, z ꢀ 1/2; with a
H(3)ꢂ ꢂ ꢂC1(1) and H(2)ꢂ ꢂ ꢂCl(1) distance 2.40(2) Å and 2.53(2) Å]
are observed in Fig. 4. The two chloro ligands attached to the metal
ion are intermolecularly hydrogen bonded to the hydrogens of N(2)
and N(3) of the N(4)-phenylsemicarbazone moiety.
Cg(1) = C(5), C(6), C(7), C(8), C(9), C(10)
D = donor, A = acceptor, Cg = centroid.
3.3. IR spectra
The tentative assignments for the IR spectral bands useful for
determining the ligand’s mode of coordination are listed in Table 6.
The strong band at 3375 cmꢀ1 in the spectrum of HL has been as-
signed to
m
(4NH), shift to lower energies in all the complexes [20].
The medium band at 3196 cmꢀ1 in the ligand due to
m
(2NH) vibra-
tion indicates that the ligand remains in the keto form in the solid
state [21]. In all the complexes, the presence of a band corresponds
Fig. 3. Structure of [Cd(HL)2Cl2] showing the atom numbering scheme. Ellipsoids
are drawn with 50% probability. Hydrogen atoms are omitted for clarity.
Table 5
H-bonding interactions of the compound [Cd(HL)2Cl2].
D–H–A
D–H (Å)
H–A (Å)
D–A (Å)
D–H–A (°)
In the case of this Cd(II) complex the coordination of carbonyl
oxygen from the semicarbazone ligand occurs through ketonic
form. It was confirmed by the C(4)–O(1) bond length
1.2292(17) Å, which is slightly longer than the standard C@O bond
length (1.21 Å) [19] and the N(2)–C(4) bond length 1.3665(19) Å.
which is considerably shorter than the standard N–C single bond
H bonding
N(2)–H(2)–Cl(1)
0.81(2)
0.80(2)
2.53(2)
2.40(2)
3.2875(15)
3.1843(15)
155.5(19)
166.2(18)
N(3)–H(3)–Cl(1)a
Equivalent position code: a = x, 1/2 ꢀ y, ꢀ1/2 + z
D = donor, A = acceptor.

V.L. Siji et al. / Polyhedron 29 (2010) 2035–2040
2039
plex 4, the spectral data indicates the bidentate nature of the sul-
phate anion.
The distorted octahedral structure of the complex [Cd(HL)2Cl2]
was established by single crystal X-ray diffraction studies. For
the other complexes the coordination geometry was arrived at by
IR spectral and conductance data. The conductance data of the
complexes support the non-electrolytic nature of the complexes.
The IR spectral data show that the other complexes are also hexa
coordinate with the semicarbazone acting as a neutral bidentate li-
gand satisfying the four coordination positions and the ions satis-
fying the other two coordination sites.
3.4. Electronic spectra
The electronic absorption bands of the Zn(II) and Cd(II) com-
plexes, recorded in methanol solution, are given in Table 7. The
semicarbazone HL has a ring
p ? p
* band at 41,150 cmꢀ1 and a
band at 33,960 cmꢀ1 due to n ?
p* transition associated with the
azomethine linkage. These bands in the complexes have shown a
bathochromic shift due to the donation of a lone pair of electrons
to the metal and hence the coordination of azomethine [29]. The
moderately intense broad bands for the complexes in the region
24,000–28,900 cmꢀ1 are assigned to Zn(II) ? O and Cd(II) ? O me-
tal to ligand charge transfer transition (MLCT). The complexes
show no appreciable absorption in the region below 22,000 cmꢀ1
in methanol solution, in accordance with the d10 electronic config-
uration of the Zn(II) and Cd(II) ions.
Fig. 4. Intermolecular bonding interactions of [Cd(HL)2Cl2] (hydrogen atoms other
than involved in hydrogen bonds are removed for clarity).
to
m
(2NH) vibration indicates that the semicarbazone is coordi-
nated in the neutral form. A sharp band at 1682 cmꢀ1 in the ligand
has significant contribution from C@O stretching vibration and the
presence of band in the region 1662–1642 cmꢀ1 supports the keto
form of the ligand in these complexes [10].
3.5. 1H NMR spectra
The 1H NMR signals of the ligand HL and one each of the Zn(II)
and Cd(II) complexes recorded in CDCl3 are listed in Table 8. The
1H NMR spectrum of HL, shows a singlet, integrating as one hydro-
gen, at d = 8.21 ppm which is assigned to the proton attached to the
N(2)H. The N(4)H is observed as a singlet at d = 8.04 ppm. The low
field position of N(4)H can be attributable to the deshielding caused
by phenyl group. This downfield shift also explains the intramolec-
ular H-bonding interaction with nitrogen atom N(1). This is con-
firmed from the crystal structure. Hydrogen bonding decreases
the electron density around the proton and thus moves the proton
absorption to a lower field. Absence of any coupling interaction by
N(2)H and N(4)H protons due to lack of availability of protons on
neighboring atoms render singlet peaks for the imine protons.
The ligand does not show any peak attributable to –OH proton indi-
cating that it exists in the keto form. IR spectral data are also in con-
A sharp band at 1591 cmꢀ1 in the semicarbazone can be attrib-
uted to the characteristic >C@N group. On coordination of the azo-
methine nitrogen, the IR stretching frequency
m(C@N) at
1591 cmꢀ1 shows an upward shift and is observed in the region
1598–1620 cmꢀ1 for complexes [22–24]. It is further supported
by the appearance of bands in the region 408–416 cmꢀ1 and
445–456 cmꢀ1 which correspond to
[26]. The increase in the
m(Zn–N) [25] and
m(Cd–N)
m
(N–N) value in the spectra of the com-
plexes is due to the increase in double bond character offsetting
the loss of electron density via donation to the metal and it is a
confirmation of the coordination of the ligand through the azome-
thine nitrogen.
The acetato complexes 2 and 6 show asymmetric stretching
bands at 1571 and 1565 cmꢀ1 and symmetric stretching bands at
1368 and 1372 cmꢀ1 suggest the presence of unidentate type of
acetato group [27,28]. The nitrato complexes 3 and 7 have two
Table 7
strong bands at 1331 and 1448 cmꢀ1
, with a separation of
Electronic spectral assignments, k(cmꢀ1) for HL and its Zn(II) and Cd(II) complexes.
117 cmꢀ1 corresponding to m1 and m4 and a medium band at
1031 cmꢀ1 corresponding to m2 of the nitrato group indicating
the presence of terminal monodentate nitrato group [28]. In com-
Compound
p
?
p
*
n ?
p*
MLCT
HL
41,150
43,010
46,940
46,940
45,770
45,450
44,840
45,350
33,960
34,130
34,430
40,980
40,810
40,840
34,130
40,660
–
[Zn(HL)2Cl2] (1)
28,940
25,920
24,090
26,040
25,740
28,940
28,940
[Zn(HL)2(CH3COO)2] (2)
[Zn(HL)2(NO3)2] (3)
[Zn(HL)2(SO4)] (4)
[Cd(HL)2Cl2] (5)
[Cd(HL)2(CH3COO)2] (6)
[Cd(HL)2(NO3)2] (7)
Table 6
IR spectral assignments for HL and its Zn(II) and Cd(II) complexes.
Compound
m(C@O) m(C@N) m m m(N–N) m(M–N)
(2NH) (4NH)
HL
1682
1643
1591
1601
1599
3196
3113
3154
3375
3276
3249
1129
1148
1150
–
416
408
[Zn(HL)2Cl2] (1)
[Zn(HL)2(CH3COO)2] 1655
(2)
Table 8
1H NMR (CDCl3) assignments of HL and its Zn(II) and Cd(II) complexes (d in ppm).
[Zn(HL)2(NO3)2] (3) 1657
1600
1598
1620
1603
3158
3110
3148
3150
3268
3276
3249
3255
1143
1177
1142
1153
415
416
445
456
[Zn(HL)2(SO4)] (4)
[Cd(HL)2Cl2] (5)
1648
1662
Compound
–2NH
–4NH
–1CH3
–3CH3
Aromatic protons
[Cd(HL)2(CH3COO)2] 1661
(6)
[Cd(HL)2(NO3)2] (7) 1652
HL
8.21
8.45
8.48
8.04
8.30
8.31
2.03
2.04
2.04
1.90
1.91
1.91
7.02–7.50
7.03–7.52
7.04–7.52
[Zn(HL)2Cl2] (1)
[Cd(HL)2Cl2] (5)
1599
3157
3298
1139
453

2040
V.L. Siji et al. / Polyhedron 29 (2010) 2035–2040
Table 9
13C NMR (CDCl3) assignments of HL (d in ppm).
Compound
HL
C(4)
C(2)
C(5)
C(7) and C(9)
128.90
C(8)
C(6) and C(10)
119.36
C(3)
C(1)
153.69
147.72
138.25
123.10
25.28
16.52
firmity with these observations. Two singlets at d values 2.03 and
1.90 ppm are attributed to the methyl protons which are chemi-
cally and magnetically equivalent. The resonances for the phenyl
group appear as triplet at 7.02 and 7.33 ppm for para and meta pro-
tons and as doublet at 7.53 ppm for ortho phenyl protons.
In the 1H NMR spectra of the complexes, a signal observed as
singlet at d value 8.45 and 8.48 ppm is assigned to the proton at-
tached to the nitrogen atom N(2). The presence of signal due to
N(2)H in the complexes indicates that there is no enolization of li-
gand in the complexes. Another singlet observed at d value 8.30
and 8.31 ppm is assigned to the N(4) proton. The N(4) phenyl pro-
tons observed within the range 7.04–7.52 ppm in both the com-
plexes. Also two singlets observed at d values 2.04 and 1.91 ppm
are assigned to methyl protons.
Appendix A. Supplementary data
CCDC 750911 and 751638 contain the supplementary crystallo-
graphic data for compounds HL and [Cd(HL)2Cl2]. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.poly.2010.03.011.
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The 13C NMR spectrum provides direct information about the
carbon skeleton of the molecule is given in Table 9. There are 8 un-
ique carbon atoms in the molecule, which give a total of 8 different
peaks in the 13C NMR spectrum. The C(4) carbon atom resonance is
observed farthest downfield of 153.69 ppm, resultant of the conju-
gative effect of the –N(1)–N(2)–C(O)–N(4)– semicarbazone skele-
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in the spectrum (d 147.72 ppm). The methyl carbon atoms are
observed at d 25.28 and 16.52 ppm. The three different types of
aromatic carbons on the substituted phenyl ring are clearly distin-
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counterparts. The N(4) phenyl resonances are: C(6) and C(10),
119.36 ppm; C(7) and C(9), 128.90 ppm; C(8), 123.10 ppm; C(5),
138.25.
Acknowledgements
V.L. Siji thanks the Council of Scientific and Industrial Research,
New Delhi, India for financial support in the form of Junior Re-
search Fellowship. We are thankful to Dr. M.R.P. Kurup, Depart-
ment of Applied Chemistry, Cochin University of Science and
Technology for using the software DIAMOND version 3.1f. We are
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