Synthesis, spectral characterization and crystal structure of copper(II) complexes of 2-benzoylpyridine-N(4)-phenylsemicarbazone

Article abstract of DOI:10.1016/j.poly.2010.09.030

An interesting series of nine new copper(II) complexes [Cu ₂L₂(OAc)₂]·H₂O (1), [CuLNCS]·H₂O (2), [CuLNO₃]·H₂O (3), [Cu(HL)Cl₂]·H₂O (4), [Cu₂(HL)

Full text of DOI:10.1016/j.poly.2010.09.030

Polyhedron 30 (2011) 70–78
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, spectral characterization and crystal structure of copper(II) complexes
of 2-benzoylpyridine-N(4)-phenylsemicarbazone
a
a
a
a
M.R. Prathapachandra Kurup a, , Binu Varghese , M. Sithambaresan , Suja Krishnan , S.R. Sheeja ,
⇑
Eringathodi Suresh b
a Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India
b Analytical Sciences Division, Central Salt and Marine Chemicals Research Institute, Bhavanagar 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An interesting series of nine new copper(II) complexes [Cu2L2(OAc)2]ꢀH2O (1), [CuLNCS]ꢀ½H2O (2), [CuL-
NO3]ꢀ½H2O (3), [Cu(HL)Cl2]ꢀH2O (4), [Cu2(HL)2(SO4)2]ꢀ4H2O (5), [CuLClO4]ꢀ½H2O (6), [CuLBr]ꢀ2H2O (7),
[CuL2]ꢀH2O (8) and [CuLN3]ꢀCH3OH (9) of 2-benzoylpyridine-N(4)-phenyl semicarbazone (HL) have been
synthesized and physico-chemically characterized. The tridentate character of the semicarbazone is
inferred from IR spectra. Based on the EPR studies, spin Hamiltonian and bonding parameters have been
calculated. The g values, calculated for all the complexes in frozen DMF, indicate the presence of the
Received 22 April 2010
Accepted 23 September 2010
Available online 1 October 2010
Keywords:
2-Benzoylpyridine
Semicarbazone
Copper(II) complexes
EPR
unpaired electron in the dx2
orbital. The structure of the compound, [Cu2L2(OAc)2] (1a) has been
ꢁy2
resolved using single crystal X-ray diffraction studies. The crystal structure revealed monoclinic space
group P21/n. The coordination geometry about the copper(II) in 1a is distorted square pyramidal with
one pyridine nitrogen atom, the imino nitrogen, enolate oxygen and acetate oxygen in the basal plane,
an acetate oxygen form adjacent moiety occupies the apical position, serving as a bridge to form a cen-
trosymmetric dimeric structure.
X-ray crystal structure
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
and provides an aromatic nitrogen whose unshared electron pairs
are well placed to act courteously in binding metal ions. Further
Semicarbazones are molecules of great interest due to their po-
tential pharmacological properties such as antibacterial, anti-
fungal, antihypertensive, hypolipidemic, antineoplastic, hypnotic
and anticonvulsant [1–4]. A variety of semicarbazones and their
metal complexes possess anti-protozoa activity also. These proper-
ties are due to wide variation in their stereochemistry and modes
of bonding through oxygen and azomethine nitrogen atoms [1].
There has been considerable amount of interest in the studies of
semicarbazones recently due to their unusual coordination modes
when bound to metals, high pharmacological potentiality and good
chelating property.
more, semicarbazones derived from 2-benzoylpyridine have a
small negative charge resides on the nitrogen atom of the pyridine
ring which enhances its biological activity [8]. This paper deals
with the synthesis and spectral characterization of nine Cu(II) com-
plexes of 2-benzoylpyridine-N(4)-phenylsemicarbazone along
with the crystal structure of [Cu2L2(OAc)2] (1a).
2. Experimental
2.1. Materials
Semicarbazones are compounds with adaptable structural fea-
tures and can coordinate to the metal either as a neutral ligand
or as a deprotonated anion. The coordination possibilities in semi-
carbazones are increased by introducing different substituted alde-
hydes or ketones witch include additional donor atoms [5,6].
In present study, we have chosen the studies of Cu(II) com-
plexes of 2-benzoylpyridine-N(4)-phenylsemicarbazone. It is
mainly due to the fact that the heteroaromatic moiety of 2-benzo-
ylpyridine can provide a further binding site for metal cations [7]
4-Phenylsemicarbazide (Fluka), 2-benzoylpyridine (CDH),
potassium thiocyanate (CDH), sodium azide (CDH), copper acetate,
copper chloride, copper nitrate, copper bromide, copper perchlo-
rate and copper sulfate (BDH) were used without further purifica-
tion. All solvents obtained commercially were distilled before use.
2.2. Synthesis of the ligand HL and its metal complexes
The semicarbazone ligand was synthesized by the following
method. 4-Phenyl semicarbazide (0.151 g, 1 mmol) dissolved in
hot methanol (50 ml) was added to a solution of 2-benzoylpyridine
(0.183 g, 1 mmol) in the same solvent. Two drops of glacial acetic
⇑
Corresponding author. Tel.: +91 484 2862423; fax: +91 484 2575804.
E-mail addresses: mrp@cusat.ac.in, mrp_k@yahoo.com (M.R.P. Kurup).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.09.030

M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
71
AVATAR 370 DTGS model FT-IR Spectrophotometer with KBr pel-
lets and ATR technique at SAIF, Kochi, India. A Cary 5000, version
1.09 UV–Vis-NIR Spectrophotometer was used for recording elec-
tronic spectra in acetonitrile and DMF as solvents. Magnetic sus-
ceptibility measurements were carried out in the polycrystalline
state on a PAR model 155 Vibrating Sample Magnetometer at 5 k
Oersted field strength. EPR spectra of the complexes in the poly-
crystalline and solution state at 298 K and in frozen DMF at 77 K
were recorded in the X-band frequency, using 100 kHz field mod-
ulation on a Varian E-112 X-band EPR spectrometer using TCNE
as a standard (g = 2.00277) at SAIF, IIT, Bombay, India.
N
N
O
NH
NH
2.4. X-ray crystallography
Scheme 1. Structure of 2-benzoylpyridine-N(4)-phenylsemicarbazone.
Single crystals of compound [Cu2L2(OAc)2] suitable for X-ray
diffraction studies were grown from its solution in a mixture of
dimethylformamide and methanol (1:1 v/v). A green crystal of
approximate dimensions 0.20 ꢂ 0.14 ꢂ 0.06 mm3 was selected
and mounted on a BRUKER SMART CCD diffractometer, equipped
with a graphite crystal, incident-beam monochromator, and a fine
acid were added to the reaction mixture. Then the reaction mixture
was refluxed for 6 h on a water bath and cooled to room tempera-
ture. The product formed was filtered off and washed with metha-
nol to obtain a pale yellow crystalline compound. The structural
formula of the ligand (HL) is represented in Scheme 1.
focus sealed tube, Mo Ka (k = 0.71073 Å) X-ray source. The crystal-
The complex [Cu2L2(OAc)2]ꢀH2O was prepared by refluxing a hot
solution of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot fil-
tered methanolic solution of Cu(OAc)2ꢀH2O (0.398 g, 2 mmol) for
4 h. The complex [CuLNO3]ꢀ½H2O was prepared by refluxing a hot
solution of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot fil-
tered methanolic solution of CuNO3ꢀ3H2O (0.483 g, 2 mmol) for 3 h.
The complex [Cu(HL)Cl2]ꢀH2O was prepared by stirring a hot solu-
tion of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot filtered
methanolic solution of CuCl2ꢀ2H2O (0.340 g, 2 mmol) for 3 h. The
complex [Cu2(HL)2(SO4)2]ꢀ4H2O was prepared by refluxing a hot
solution of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot fil-
tered methanolic solution of CuSO4ꢀ5H2O (0.499 g, 2 mmol) for 5 h.
The complex [CuLClO4]ꢀ½H2O was prepared by stirring a hot solu-
tion of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot filtered
methanolic solution of copper perchlorate (0.371 g, 2 mmol) for 3 h.
The complex [CuLBr]ꢀ2H2O was synthesized by refluxing a hot solu-
tion of HL (0.633 g, 2 mmol) in methanol (20 cm3) with a hot filtered
methanolic solution of copper bromide (0.446 g, 2 mmol) for 3 h.
These microcrystals obtained were separated and filtered. They
were thoroughly washed with water, methanol and then ether and
finally dried over P4O10 in vacuo.
lographic data along with details of structure solution refinements
are given in Table 1. The unit cell dimensions were measured and
the data collection was performed at 273 K. Bruker SMART software
was used for data acquisition and Bruker SAINT Software for data
integration [9]. Absorption corrections were carried out using SAD-
ABS based on Laue symmetry using equivalent reflections [10].
The structure was solved by direct methods and refined by full-
matrix least-squares calculations with the SHELXTL-PLUS software
package (version 5.1) [11]. The non-hydrogen atoms were made
Table 1
Crystal refinement parameters of [Cu2L2(OAc)2].
Empirical formula
Formula weight
Color
C42H38Cu2N10O6
875.86
Green
T (K)
273(2)
k (Mo K
a
) (Å)
0.7107
Crystal system
Space group
monoclinic
P21/n
Lattice constants
0
8.6851(7)
a (AÅ)
0
16.5515(14)
13.6252(12)
The complexes [CuLNCS]ꢀ½H2O and [CuLN3]ꢀCH3OH were syn-
thesized by the following method. A hot solution of HL (0.633 g,
2 mmol) in methanol (20 cm3) was mixed with a hot filtered meth-
anolic solution of potassium thiocyanate (0.194 g, 2 mmol) or so-
b (AÅ)
0
c (AÅ)
a
(°)
90
b (°)
91.680(2)
c
(°)
90
1957.8(3)
dium azide (0.130 g, 2 mmol).
A hot filtered solution of
V (Å3)
Cu(OAc)2ꢀH2O (0.398 g, 2 mmol) was added with constant stirring.
The mixture was then refluxed for 3 h. The complexes separated as
microcrystals were filtered and thoroughly washed with water,
methanol and ether and finally dried over P4O10 in vacuo.
The complex CuL2ꢀH2O was obtained when a hot solution of HL
(0.633 g, 2 mmol) in methanol (20 cm3) was mixed a hot filtered
methanolic solution of Cu(OAc)2ꢀH2O (0.199 g, 1 mmol) with con-
stant stirring and then the mixture was refluxed for 3 h. The complex
obtained as microcrystals was filtered and thoroughly washed with
water, methanol and ether and finally dried over P4O10 in vacuo.
Single crystals of [Cu2L2(OAc)2] (1a) suitable for X-ray diffrac-
tion were obtained from solution of 1 in a mixture of dimethyl-
formamide and methanol (1:1).
Z
4
DCalc.
(
q
) (Mg mꢁ3
)
1.496
1.151
900
Absorption coefficient,
F(0 0 0)
l
(mmꢁ1
)
Crystal size (mm3)
0.20 ꢂ 0.14 ꢂ 0.06
h Range for data collection
Limiting indices
1.94–28.28
ꢁ9 6 h 6 11, ꢁ21 6 k 6 21,
ꢁ17 6 l 6 15
Reflections collected
7862
Unique reflections (Rint
Completeness to 2h
Absorption correction
Refinement method
)
3556 [R(int) = 0.087]
28.28 70.8 %
SADABS
Full-matrix least-squares on F2
3556/0/266
0.859
R1 = 0.0741, wR2 = 0.1575
R1 = 0.1128, wR2 = 0.1695
0.826 and ꢁ0.421
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
Largest difference peak and hole
r
(I)]
2.3. Physical methods
(e Åꢁ3
)
P
P
R1
=
||Fo| ꢁ |Fc||/ |Fo|.
A Vario EL III CHNS analyzer at SAIF, Kochi, India was used for
elemental analyses. IR spectra were recorded on a Thermo Nicolet
P
P
2
2 1=2
wR2 ¼ ½ wðF2o ꢁ Fc2Þ = wðF2oÞ ꢃ
.

72
M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
anisotropic. Hydrogen atoms were inserted at calculated positions
and rode on the atoms to which they are attached. The hydrogen
atom attached to nitrogen atoms were located from difference Fou-
rier maps and refined with isotropic displacement parameters. The
molecular graphics employed was DIAMOND version 3.2d [12].
be less than 20 Oꢁ1 molꢁ1 cm2. This shows that all complexes are
found to be non-electrolytes [13], which indicates that the anion
and the ligand are coordinated to the copper(II).
The magnetic susceptibilities of the complexes at room temper-
ature in the polycrystalline state except complexes 1 and 5 fall in
the range of 1.60–2.00 B.M., which are very close to the spin-only
value of 1.73 B.M. for a typical copper(II) system. The low value of
complexes 1 and 5 is an evidence for their behavior as dimers.
3. Results and discussion
The semicarbazone ligand (HL) was synthesized by the direct
condensation of 2-benzoylpyridine with N(4)-phenylsemicarbaz-
ide. Semicarbazone can exist in two tautomeric forms, keto and enol
or an equilibrium mixture of the two since it has an amide –NH–
C@(O) function. However, the IR spectrum of HL indicates that it re-
mains in keto form in solid state. However, the IR spectra of com-
plexes except two complexes do not show any intense absorption
band at 1698 cmꢁ1, due to the carbonyl stretching of the semicarba-
zone moiety. This shows that it tautomerises to the enol form and
coordinates to the metal in the enolate form in solution.
3.1. Crystal structure of [Cu2L2(OAc)2]
The single crystal X-ray diffraction study of the compound [Cu2-
L2(OAc)2] shows that the compound exists as an oxygen bridged di-
mer. The molecular structure of the compound along with atom
numbering scheme is given in Fig. 1. The asymmetric unit is
formed by one half of the molecule and the other half is related
by a centre of inversion. The lattice nature is monoclinic with space
group P21/n. The structure contains two copper centers where each
centre is pentacoordinated with azomethine nitrogen (N2), pyridyl
nitrogen (N1) and enolate oxygen (O1) of semicarbazone moiety
and oxygens (O2, O2A) from acetato group. The compound exhibits
a distorted square pyramidal geometry with the basal plane occu-
pied by semicarbazone ligand and the acetate oxygen (O2). The rel-
evant bond lengths and angles are presented in Table 3. The oxygen
(O2A) from acetato group of the adjacent monomer plugs in to the
axial position resulting in a dimer with a Cuꢀ ꢀ ꢀCu separation of
approximately 3.422 Å. The comparison of bond distances Cu(1)–
O(2) (1.945(3) Å), Cu(1)–O(2A) (2.344(4) Å), Cu(1A)–O(2)
(2.346 Å) and Cu(1A)–O(2A) (1.944 Å) confirms the possibility of
a bridging dinuclear structure. The Cu–N and Cu–O bond lengths
are smaller ꢄ1.936 and 2.009 Å indicating the domination of semi-
carbazone moiety in the bonding. It is found that copper atom is
closer to semicarbazone moiety than the acetato group. The Cu–
Npyridyl bonds are 0.09 Å larger than Cu–Nimine bonds show the
The complexes [CuLNCS]ꢀ½H2O and [CuLN3]ꢀCH3OH were read-
ily formed by the reaction of the ligand and heterocyclic base,
potassium thiocyanate or sodium azide with copper acetate. The
complexes [Cu2L2(OAc)2]ꢀH2O, [CuLNO3]ꢀ½H2O, [Cu(HL)Cl2]ꢀH2O,
[Cu2(HL)2(SO4)2]ꢀ4H2O, [CuLClO4]ꢀ½H2O, [CuLBr]ꢀ2H2O, were
formed by the reaction of the ligand with corresponding copper
salts. The complex CuL2ꢀH2O was obtained when a hot solution of
HL was reacted with
a hot filtered methanolic solution of
Cu(OAc)2ꢀH2O. The ligand HL undergoes deprotonation to L- and
chelates in enolate form as evidenced by the IR spectra. The ele-
mental analyses suggest the general formula as (CuLX)ꢀxH2O for
complexes 1–8 but for the complex 9, CH3OH takes the place of
water. The complexes are appreciably soluble in alcohol and DMF.
The colors, elemental analyses, stoichiometries of HL and its
complexes are presented in Table 2. The conductivity measure-
ments were made in DMF solutions and the values were found to
Table 2
Colors, elemental analyses and magnetic susceptibilities of 2-benzoylpyridine-N(4)-phenyl semicarbazone and its copper(II) complexes.
Compound
Colour
Found (Calc.) %
C
l
(B.M.)
H
N
HL
Pale yellow
Green
Green
Pale green
Dark green
Pale green
Green
Pale green
Green
72.47(72.13)
56.20(56.43)
53.72(54.97)
50.97(50.72)
49.25(48.68)
44.11(44.66)
46.65(46.83)
46.12(45.83)
63.71(64.08)
52.71(53.03)
5.20(5.10)
4.08(4.28)
3.12(3.62)
3.64(3.58)
3.84(3.87)
4.42(3.75)
2.96(3.31)
3.38(3.87)
4.02(4.53)
4.07(4.23)
18.14(17.71)
12.67(12.54)
15.31(15.70)
15.72(15.57)
11.94(11.95)
10.86(10.96)
11.35(11.50)
10.97(11.32)
15.28(15.73)
21.85(21.65)
–
[Cu2L2(OAc)2]ꢀH2O (1)
[CuLNCS]ꢀ½H2O (2)
[CuLNO3]ꢀ½H2O (3)
[Cu(HL)Cl2]ꢀH2O (4)
[Cu2(HL)2(SO4)2]ꢀ4H2O (5)
[CuLClO4]ꢀ½H2O (6)
[CuLBr]ꢀ2H2O (7)
[CuL2]ꢀH2O (8)
1.06
1.75
1.87
1.93
1.15
1.71
1.76
1.89
1.83
[CuLN3]ꢀCH3OH (9)
Dark green
Fig. 1. The molecular structure of [Cu2L2(OAc)2] (1a) along with the atom numbering scheme. Symmetry code: 1 ꢁ x, ꢁy, 1 ꢁ z. The hydrogen atoms are omitted for clarity.

M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
73
Table 3
The IR spectral bands of HL and its copper(II) complexes and its
tentative assignments for determining the ligand’s mode of coordi-
Selected bond lengths (Å) and bond angles (°) of
[Cu2L2(OAc)2].
nation are presented in Table 6. The m(C@N) band of the semicarba-
zone at 1600 cmꢁ1 undergoes a negative shift of wavenumber in
the complexes indicating coordination via azomethine nitrogen.
This has been confirmed by the bands in the 410–440 cmꢁ1 range
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–O(2A)
C(13)–O(1)
C(6)–N(2)
C(13)–N(3)
Cu1ꢀ ꢀ ꢀCu1A
2.024(4)
1.938(4)
2.010(4)
1.945(3)
2.344(4)
1.261(7)
1.306(6)
1.363(6)
3.422
and it is assignable to the m(Cu–N) band [22,23]. A strong band
found at 1132 cmꢁ1 is assigned to the
semicarbazone.
m
(N–N) band of the
(N–N) band in
The positive shift of the frequency assigned to
m
the spectra of complexes is owing to the increase in double bond
character, counterbalancing the loss of electron density via dona-
tion to the metal. This confirms the coordination of the ligand to
the metal via the azomethine nitrogen. The presence of bands in
the 1510–1560 cmꢁ1 range in complexes except in complexes 4
and 5 due to asymmetric stretching vibration of the newly formed
C@N bond demonstrates the enolization of the ligand. A band at
1698 cmꢁ1 in the ligand has significant contribution from C@O
stretching vibration and absence of such a band in all the com-
plexes except 4 and 5 supports the enolate form of the ligand in
these complexes [24]. For the 4 and 5 this band is shifted to
1641 and 1627 cmꢁ1, respectively showing the coordination in
the keto form [25].
N(1)–Cu(1)–N(2)
N(2)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(2)
N(1)–Cu(1)–O(2A)
N(2)–Cu(1)–O(2)
80.86(18)
79.05(16)
159.91(17)
99.82(17)
92.03(16)
171.06(17)
strength of azomethine nitrogen coordination. The four coplanar
base atoms show a significant distortion from a square geometry
as indicated by the N1–Cu1–O1 bond angle of 159.91(17)°. The
deviation of the central copper atom from the basal plane in the
direction of the axial oxygen is evident from the bond angles of
N(2)–Cu1–O(2), 171.06(17)° and N1–Cu1–O2, 99.82(17)°. Most of
the angles involving the central copper atoms are widely different
from 90° and 180°, indicating significant distortion from the square
pyramidal geometry.
The unit cell-packing diagram of the compound 1 viewed along
the a-axis is given in Fig. 2. It can be observed that the molecules
are packed in a two-dimensional manner with the parallel arrange-
ment of the rings. The adjacent units are interconnected through H-
bonding interactions involving the oxygen atom of the acetate group
and N(4) hydrogen of the phenyl group (Fig. 2). The H-bonding, C–
The m(N–H) band of semicarbazone disappears in complexes ex-
cept in [Cu2(HL)2(SO4)2]ꢀ4H2O and [Cu(HL)Cl2]ꢀH2O also adding
more evidences for the enolization and deprotonation of HL fol-
lowed by coordination to Cu and it is supported to a great extent
by the appearance of a new peak in the 500–540 cmꢁ1 range due
to m(Cu–O) stretching frequency. Based on the above spectral evi-
dences, it is confirmed that the ligand HL is tridentate, coordinating
to copper(II) via the azomethine nitrogen, the pyridyl nitrogen and
keto or enolate oxygen.
In the thiocyanato complex, a very strong band at 2119, a med-
Hꢀ ꢀ ꢀ
ing interaction is observed with donor-acceptor distance of 2.852 Å.
The aromatic stacking interactions between pyridine rings with
p and ring metal interactions are shown in Table 4. The H-bond-
ium band at 902 and a strong band at 453 cmꢁ1 are assigned to
m(CN), m(CS) and d(NCS) modes of the NCS group, respectively.
p–p
The intensity and position of these bands indicate the unidentate
coordination of the thiocyanate group through the nitrogen
[26,27]. In the nitrato complex, the three bands observed at
Cg–Cg distance of 3.6755 Å (Sym. code: 2 ꢁ x, ꢁy, 1 ꢁ z) reinforces
crystal structure cohesion in molecular packing in the crystal lattice.
The trigonality index
= (b ꢁ )/60 [14] (for perfect square pyramidal and trigonal bipy-
ramidal geometries the values of are zero and unity, respec-
tively). The value of for molecule is 0.185, which shows
s is calculated using the equation
1448, 1354 and 1130 cmꢁ1 indicate the m4
, m1 and m2 modes of
s
a
the nitrato group. The fact that the nitrato group is terminally
bonded is understood from the separation of 94 cmꢁ1 between
its two highest frequency bands mentioned above [28].
s
s
distorted square pyramidal geometry.
In the azido complex, the strong band observed at 2054 cmꢁ1 is
assigned to the azido group indicative of azide coordination [29].
The perchlorate complex showed single broad bands at 1121 and
1033 cmꢁ1, a strong band at 623 cmꢁ1 and a weak band at
923 cmꢁ1 indicating the presence of coordinated perchlorate [23].
3.2. Electronic and IR spectra
The electronic spectral bands of the ligand and the complexes
recorded in acetonitrile and DMF solution are presented in Table 5.
The electronic spectrum of HL consists of a broad band at
The coordination is confirmed by a
The bands at 1121 and 623 cmꢁ1 are assignable to
4(ClO4), respectively. For the sulfato complex, strong bands at
1142 and 1055 cmꢁ1 are assignable to m3 of the monocoordinated
sulfato group. Medium bands at 976 cmꢁ1 1) and 650 cmꢁ1
confirm the unidentate behaviour of sulfato group [23].
m
(Cu–O) band at 534 cmꢁ1
3(ClO4) and
.
m
32 890 cmꢁ1 which is due to n ?
p
* band of the pyridine ring. An
absorption band observed at 37 410 cmꢁ1 in the spectrum of the li-
gand may be due to * transition. The electronic spectra of
Cu(II) complexes consist of bands in the region 27 000–
33 000 cmꢁ1 which are assigned to n ?
* band of the pyridine
m
p
?
p
(m
(m4)
p
Thus it is seen that the semicarbazone (HL) acts as a monoan-
ionic tridentate ligand in complexes [Cu2L2(OAc)2]ꢀH2O, [CuL-
NCS]ꢀ½H2O, [CuLNO3]ꢀ½H2O, [CuLClO4]ꢀ½H2O, [CuLBr]ꢀ2H2O,
[CuL2]ꢀH2O and [CuLN3]ꢀCH3OH and neutral one in complexes
[Cu(HL)Cl2]ꢀH2O and [Cu2(HL)2 (SO4)2]ꢀ4H2O.
ring. The semicarbazone and copper(II) complexes have two bands;
one centered around 37 000 cmꢁ1 and another around
32 000 cmꢁ1. These bands are assigned to
p ? p* and n ? p* tran-
sitions of phenyl rings and semicarbazide moiety, respectively
[15]. The charge transfer bands were observed in the 23 000–
25 000 cmꢁ1 range, and its broadness can be explained as due to
the combination of O ? Cu and N ? Cu LMCT transitions [16].
For Cu(II) complexes, there are three spin allowed transitions,
A1g B1g, B2g B1g and Eg B1g. But it is very difficult to resolve
them into separate bands due to the very low energy difference be-
tween these bands. All the complexes gave d–d bands in the
14 000–17 500 cmꢁ1 range [17–21].
3.3. Magnetic and EPR studies
Variable temperature magnetic susceptibility measurements of
complexes [Cu2L2(OAc)2]ꢀH2O and [Cu2(HL)2(SO4)2]ꢀ4H2O were car-
ried out in the temperature range 80–298 K. The magnetic moment
leff of Cu2L2(OAc)2]ꢀH2O decreases from a value 1.06 at 298 K to
0.54 lB at 80 K indicating a moderate antiferromagnetic exchange

74
M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
Fig. 2. Packing diagram of compound 1a showing H bonding.
Table 4
Hydrogen bonding.
D–Hꢀ ꢀ ꢀA
D–H (Å)
0.76(6)
Hꢀ ꢀ ꢀA (Å)
2.10(6)
Dꢀ ꢀ ꢀA (Å)
2.852(7)
\D–Hꢀ ꢀ ꢀA (°)
169(6)
N(4)–H4ꢀ ꢀ ꢀO(3)a
Equivalent position code: a = 2 ꢁ x, ꢁy, 1 ꢁ z.
[
pꢀ ꢀ ꢀp interactions]
Cg(I)ꢀ ꢀ ꢀCg(J)
Cg–Cg (Å)
3.6755(25)
a
(°)
b (°)
c (°)
Cg(2)ꢀ ꢀ ꢀCg(2)b
0.02
18.68
18.68
Equivalent position code: b = 2 ꢁ x, ꢁy, 1 ꢁ z.
Cg (1) = Cu(1), O(2), Cu(1)A, O(2)A
Cg (2) = Cu(1), O(1), C(13), N(3), N(2)
C–Hꢀ ꢀ ꢀp interactions
X–H(I)ꢀ ꢀ ꢀCg(J)
Hꢀ ꢀ ꢀCg (Å)
Xꢀ ꢀ ꢀCg (Å)
\X–H–Cg (°)
138
C(21)–H(21B)ꢀ ꢀ ꢀCg(2)c
2.69
4.17
Equivalent position code: c = 1 ꢁ x, ꢁy, 1 ꢁ z.
D, donor; A, acceptor; Cg, centroid;
a, dihedral angles between planes I and J; b, angle between Cgꢀ ꢀ ꢀCg and Cg(J) perp.; c, angle between Cgꢀ ꢀ ꢀCg and Cg(I) perp.
Table 5
interaction between the two Cu(II) ions in one molecule, namely,
parallel arrangement with a total spin S = 1 and the ground state
S = 0 [30]. A similar trend is shown by [Cu2(HL)2(SO4)2]ꢀ4H2O in
which leff value decreases from 1.15 at 298 K to 0.43 lB at 80 K.
The antiferromagnetic behavior of the binuclear complexes is
attributed to spin–spin interaction via the super exchange path-
way provided by the bridging groups rather than a direct metal–
metal interaction [31]. The copper–copper distance in complex 1
is observed to be 3.422 Å from X-ray diffraction studies. This sep-
aration generally rules out any significant amount of direct cop-
per–copper interaction. The magnetic moment values for other
complexes were calculated at room temperature and found to be
Electronic spectral assignments, k (cmꢁ1) for HL and its copper(II) complexes.
Compound
State
d ? d
LMCT
–
n ?
p
*
p ? p*
HL
Acetonitrile
–
32 890 37 410
[Cu2L2OAc2]ꢀH2O ((1)
Acetonitrile 14 330 23 970 31 750 37 590
DMF 16 040 23 970 31 610 37 450
Acetonitrile 14 390 23 870 31 850 37 590
DMF 14 850 23 970 31 860 37 450
Acetonitrile 17 210 23 950 32 000 37 170
DMF 17 410 23 830 32 110 37 450
Acetonitrile 17 430 24 700 31 060 36 630
DMF 17 370 24 700 31 130 36 770
[CuLNCS]ꢀ½H2O((2)
[CuLNO3]ꢀ½H2O(3)
[Cu(HL)Cl2]ꢀH2O (4)
[Cu2(HL)2(SO4)2]ꢀ4H2O(5) Acetonitrile 14 440 24 040 32 000 37 340
DMF 16 210 23 900 32 070 35 070
Acetonitrile 15 580 23 870 31 750 37 340
DMF 15 580 23 910 31 680 37 540
Acetonitrile 16 130 23 980 31 970 34 480
DMF 16 130 23 890 32 070 36 080
Acetonitrile 15 690 23 910 30 860 35 740
DMF 15 690 23 910 31 040 35 740
Acetonitrile 16 060 23 920 31 350 37 590
DMF 16 400 23 830 31 370 37 450
very close to the spin only value 1.73
clear copper(II) systems.
lB, expected for the mononu-
[CuLClO4]ꢀ½H2O (6)
[CuLBr]ꢀ2H2O (7)
[CuL2]ꢀH2O (8)
The EPR parameters of the copper(II) complexes are presented
in Table 7. The EPR spectra of the complexes recorded in polycrys-
talline state at room temperature provide information about the
coordination environment around copper(II) in these complexes.
The EPR spectra of [CuLNCS]ꢀ½H2O, [CuLNO3]ꢀ½H2O and [CuL-
ClO4]ꢀ½H2O in the polycrystalline state at 298 K showed only one
[CuLN3]ꢀCH3OH (9)

M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
75
Table 6
Infrared spectroscopic assignments (cmꢁ1) for the 2-benzoylpyridine-N(4)-phenylsemicarbazone and its copper(II) complexes.
Compound
m(C@N)
m
(N–N)
m(C@O)
m(Cu–N)
m(C@N)
m(NH)
m(Cu–O)
HL
1600
1568
1595
1599
1597
1595
1599
1598
1568
1568
1132
1145
1158
1213
1213
1213
1212
1211
1146
1213
1698
–
–
–
1641
1627
–
–
–
–
–
–
3375
–
–
–
3068
3199
–
–
–
–
–
[Cu2L2OAc2]ꢀH2O (1)
[CuLNCS]ꢀ½H2O (2)
[CuLNO3]ꢀ½H2O (3)
[Cu(HL)Cl2]ꢀH2O (4)
[Cu2(HL)2(SO4)2]ꢀ4H2O (5)
[CuLClO4]ꢀ½H2O (6)
[CuLBr]ꢀ2H2O (7)
[CuL2]ꢀH2O (8)
416
420
418
415
415
410
436
412
420
1520
1554
1546
1568
1575
1538
1536
1516
1517
508
510
529
503
507
534
508
510
506
[CuLN3]ꢀCH3OH (9)
Table 7
Spin Hamiltonian and bonding parameters of copper(II) complexes of 2-benzoylpyridine-N(4)-phenylsemicarbazone.
1
2
3
4
5
6
7
8
9
Polycrystalline (298 K)
g||
g\
2.206
2.084
2.124
2.452
2.204
2.074
2.116
2.702
2.216
2.062
2.064
2.625
2.262
2.134
2.165
2.084
2.148
3.256
2.047
2.076
3.110
2.045
2.085
3.672
g
iso/gav
2.141
2.076
2.120
G
DMF (77 K)
g||
g\
2.224
2.062
2.248
2.054
2.285
2.273
2.051
2.212
2.289
2.055
2.287
2.298
2.058
2.257
2.057
2.005(g1)
2.087(g2)
2.113
2.074 (g1)
2.084 (g2)
2.075
2.016 (g1)
2.097 (g2)
2.133
157.0
1.91
0.9200
0.9042
0.7828
0.8319
0.7200
gav
A||
A\
a2
b2
c2
K||
2.116
199.35
2.118
194.67
2.125
167.0
2.133
161.0
2.138
148.0
2.123
172.0
170.68
193.3
0.9926
0.7373
0.7630
0.7319
0.7574
0.9796
0.7568
0.6919
0.7414
0.6777
0.9335
0.9225
0.7221
0.8612
0.6741
0.9188
0.9161
0.7740
0.8418
0.7112
0.8077
0.8815
0.9263
0.7128
0.7485
0.9301
0.8821
0.7535
0.8205
0.7009
0.9120
0.9168
0.7930
0.8362
0.7233
0.9346
0.8488
0.7840
0.7933
0.7328
K\
A values in 10ꢁ4 cmꢁ1
.
broad signal at giso = 2.076, 2.141 and 2.012, respectively. Such iso-
tropic spectra, consisting of only one broad signal and hence only
one g value is due to dipolar broadening and enhanced spin lattice
relaxation. These types of spectra unfortunately give no informa-
tion on the electronic ground state of the Cu(II) ion present in
the complexes. However, the consistency of these giso values with
the gav values of the spectra in frozen solution show the existence
of the same geometry in both states. The spectra of other com-
pounds are typically axial with well defined g|| and g\ values.
The spectra are often broad because of the fast spin lattice relaxa-
tion and exchange coupling. All complexes having g|| values less
than 2.3 indicate a fair degree of covalent character in Cu–L bond-
ing [28]. The geometric parameter G is the measure of the ex-
change interaction between the copper centres in polycrystalline
state and this could be calculated using the equation:
G = (g|| ꢁ 2.0023)/(g\ ꢁ 2.0023) for axial spectra and for rhombic
spectra, g\ = (g1 + g2)/2 [32]. The value of G is less than 4 indicates
considerable exchange interaction in the crystalline compound and
if G > 4, the exchange interaction is negligible [30]. Calculated G
valued of the complexes 1 and 5 are found to be less than 4 and re-
veals with considerable exchange interaction. In all the polycrys-
talline complexes, g|| > g\ > 2.0023 which is consistent with a
The EPR spectra of complexes [CuLBr]ꢀ2H2O (Fig. 3), [Cu2(HL)2(-
SO4)2]ꢀ4H2O and [CuLNO3]ꢀ½H2O showed five nitrogen superhy-
perfine lines in the perpendicular region, which arise from the
coupling of the electron spin with the nuclear spin of the two coor-
dinating nitrogen atoms while the EPR spectra of the complexes
[CuLNCS]ꢀ½H2O [Cu(HL)Cl2]ꢀH2O and [CuLN3]ꢀCH3OH (Fig. 4)
showed four hyperfine splitting only in the parallel region without
any superhyperfine splitting.
The complex [Cu2(HL)2(SO4)2]ꢀ4H2O gave a rhombic spectrum
in DMF solution at 77 K. The appearance of four line pattern in
solution may be due to the decomposition of the dimer into mono-
mers or due to slight loss of magnetic coupling between the two
copper ions. In addition it exhibited a half field signal (g = 4.162)
with well resolved seven line hyperfine structure with an average
hyperfine splitting of A = 83.3 G. The half field signal was retained,
but not well resolved, when it is recorded in DMSO at 77 K and
again confirms the existence of binuclear geometry. It also shows
antiferromagnetic interaction (leff = 1.15 lB at 298 K) between
the two copper nuclei. This situation could arise when two equiv-
alent copper(II) ions are coupled via an exchange interaction, in a
binuclear complex.
The solution EPR spectrum of [Cu2L2(OAc)2]ꢀH2O at 77 K is axi-
ally symmetric (g|| = 2.224 and g\ = 2.062) having a quartet hyper-
fine structure (Fig. 5) on the parallel component arising from the
interaction of an unpaired electron with a single copper nucleus.
This signal is typical of copper mononuclear complex. Apart from
this, a weak signal observed at lower field (near half of the reso-
nance field) suggests a dimeric structure for the complex. This half
field signal arises from the coupling of the two copper ions and cor-
dx2
ground state in a square planar or square pyramidal geome-
ꢁy2
try [32,33]. Although the compounds 3, 5 and 7 show axial nature
in polycrystalline state, they show rhombic symmetry in the solu-
tion at 77 K.
All the EPR spectra of the samples in DMF solution at 77 K were
simulated using EasySpin package [34]. Four well resolved hyper-
fine lines corresponding to coupling of the electron spin with the
nuclear spin (63,65Cu, I = 3/2) are obtained in the parallel region.
responds to the forbidden transition
DMs = 2. However, a seven

76
M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
Fig. 3. EPR spectrum of [CuLBr]ꢀ2H2O in DMF at 77 K.
Fig. 4. EPR spectrum of [CuLN3]ꢀCH3OH in DMF at 77 K.
line hyperfine structure is absent in this half field region. A half
field signal was found in DMSO in frozen state and it strengthens
the existence of binuclear geometry of the complex. It has been
known that if the separation between the two metal centers is
more than 3.5 Å, an interaction is not expected. From the X-ray
structural analysis, the distance between two copper ions is
3.422 Å in Cu2L2(OAc)2]ꢀH2O. Therefore, only a small interaction
is expected and the seven line hyperfine splitting is not observed.
In addition, the dihedral angle between the two copper atoms
and the connecting ligand atoms also plays an important role in
deciding the extent of interaction.
The g|| values are nearly the same for all the complexes indicat-
ing that the bonding is dominated by the semicarbazone moiety.
The g|| > g\ values accounts to the distorted square pyramidal
structure and rules out the possibility of a trigonal bipyramidal
structure which would be expected to have g\ > g|| [35,36].
The EPR parameters g||, g\, A|| (Cu) and the energies of d–d tran-
sitions were used to evaluate the bonding parameters
2, b2 and c2
which may be regarded as measures of the covalency in the in-
plane -bonds, in-plane -bonds and out-of-plane -bonds. The
-bonding parameter a2 estimated from the
a
r
p
p
value of in-plane
r
expression [36],
a2 ¼ ꢁA =0:036 þ ðg ꢁ 2:00277Þ þ 3=7ðg? ꢁ 2:00277Þ þ 0:04
k
k
The following simplified parameters were used to evaluate the
bonding parameters [37],
K2k ¼ ðg ꢁ 2:00277ÞEdꢁd=8ko
k
K2? ¼ ðg? ꢁ 2:00277ÞEdꢁd=2ko
where K2k
¼
a
2b2 and K2?
¼
a2c2, K|| and K\ are orbital reduction fac-
tors and ko represents the one electron spin orbit coupling constant
which equals ꢁ828 cmꢁ1
.

M.R.P. Kurup et al. / Polyhedron 30 (2011) 70–78
77
Fig. 5. EPR spectrum of [Cu2L2OAc2]ꢀH2O in DMF at 77 K (the half field signal is also given at the bottom left corner).
Hathaway [38] has pointed out that, for pure sigma bonding,
K|| ꢅ K\ ꢅ 0.77, and for in plane -bonding, K|| < K\, while for
out-of-plane -bonding K\ < K||. In all complexes, except the com-
pounds 1 and 5, it is observed that K|| > K\ which indicates the
presence of significant out-of-plane bonding while the com-
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orbital. The X-ray single crystal data and
ꢁy2
the EPR spectra show that the compounds 1 and 5 have distorted
square pyramidal arrangement whereas the compounds 2, 3, 4, 6,
7 and 9 reflect a square planar environment around the metal atom
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distorted atmosphere around the central metal atom by the two
ligands.
Acknowledgments
M. Sithambaresan thanks the Indian Council for Cultural Rela-
tions (ICCR) for financial support. S.R. Sheeja gratefully acknowl-
edges CSIR for the award of Senior Research Fellowship. The
authors are thankful to the Sophisticated Analytical Instrumenta-
tion Facility, Cochin University of Science and Technology, Kochi
22, India for elemental analyses and IIT, Bombay, India for EPR
spectra.
Appendix A. Supplementary data
Crystallographic data for the structural analysis has been depos-
ited with the Cambridge Crystallographic Data Center CCDC refer-
ence number is 667699 for [Cu2L2(OAc)2]. Copies of this
information may be obtained free of charge via www.ccdc.cam.a-
c.uk/conts/retrieving.html or from the Director, CCDC, 12 Union
Road, Cambridge, CB2, IEZ, UK (fax: +44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk).

78
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