Design and characterization of Cu(II) complexes from 2-benzoylpyridine benzhydrazone: Crystallographic evidence for coordination versatility

Article abstract of DOI:10.1016/j.saa.2009.11.040

The syntheses and characterization of six copper(II) complexes of 2-benzoylpyridine benzhydrazone in the form of [Cu(BPB)₂], [Cu(BPB)Cl]·H₂O, [Cu(BPB)Br], [Cu₂(BPB)₂](ClO₄)₂·4H₂O, [Cu(BPB)N₃]·H₂O, and [Cu(BPB)NCS]·H₂O·CH₃OH are reported. The analytical methods used for the characterization of complexes include partial elemental analyses, IR, electronic and EPR spectra, conductivity measurements, magnetic susceptibility measurements and single crystal X-ray diffraction. From the crystal structure, it is clear that the hydrazone adopts the E conformation about the azo bond to attach to the metal through the N_py-N_azo-O chelating system. In the EPR spectra of complexes in DMF at 77 K four hyperfine quartets in the parallel region could be resolved and a half field signal is observed at 1500 G for complex [Cu₂(BPB)₂](ClO₄)₂·4H₂O in polycrystalline state at 298 K which gives evidence for its binuclear nature indicating a weak interaction between the two Cu(II) ions.

Full text of DOI:10.1016/j.saa.2009.11.040

Spectrochimica Acta Part A 75 (2010) 686–692
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Design and characterization of Cu(II) complexes from 2-benzoylpyridine
benzhydrazone: Crystallographic evidence for coordination versatility
Neema Ani Mangalama, Sarika Sivakumara, M.R. Prathapachandra Kurupa,∗, Eringathodi Sureshb
a Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, Kerala 682 022, India
b Analytical Science Discipline, Central Salt and Marine, Chemicals Research Institute, Bhavnagar 364002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 September 2009
Accepted 17 November 2009
The syntheses and characterization of six copper(II) complexes of 2-benzoylpyridine benzhydrazone in
the form of [Cu(BPB)2], [Cu(BPB)Cl]·H2O, [Cu(BPB)Br], [Cu2(BPB)2](ClO4)2·4H2O, [Cu(BPB)N3]·H2O, and
[Cu(BPB)NCS]·H2O·CH3OH are reported. The analytical methods used for the characterization of com-
plexes include partial elemental analyses, IR, electronic and EPR spectra, conductivity measurements,
magnetic susceptibility measurements and single crystal X-ray diffraction. From the crystal structure, it
is clear that the hydrazone adopts the E conformation about the azo bond to attach to the metal through
the Npy–Nazo–O chelating system. In the EPR spectra of complexes in DMF at 77 K four hyperfine quar-
tets in the parallel region could be resolved and a half field signal is observed at 1500 G for complex
[Cu2(BPB)2](ClO4)2·4H2O in polycrystalline state at 298 K which gives evidence for its binuclear nature
indicating a weak interaction between the two Cu(II) ions.
Keywords:
Hydrazones
Crystal structure
Copper(II) complexes
Spectral studies
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
of metal ions, aromatic hydrazones like di-2-pyridyl ketone ben-
zhydrazone can be used as a sensitive analytical reagent [10] and
Over the years much effort has gone into the investigation of
complexes of a wide range of hydrazone ligands, in particular into
pyridine containing systems. Hydrazone ligands are very promis-
ing compounds from the view of point of coordination chemistry
because of their ability towards complexation and involvement in
a wide range of biological and non-biological properties [1,2]. The
coordination chemistry of transition metals with ligands from the
hydrazine family has been of interest due to their different bonding
modes with both electron-rich and electron-poor metals [3,4]. The
structural characterization of pyridine containing hydrazones may
reveal some interesting facts, such as their tendency and potency to
coordinate through different sites along with their tridentate char-
acter, and coordination in similar type of thiosemicarbazones are
reported by our group [5]. In addition to this, as the substituents
present in the hydrazone strongly influence the reactivity of prox-
imal C–H bonds, a CNO mode of bonding of aryl hydrazones has
also been reported provided suitable substituents are located on
the aryl ring [6]. With flexible ligands the competition between
bridging and chelating coordination modes is an important factor
in producing mono-, di- and polynuclear metal complexes [7,8].
Anomalous magnetic interaction is reported in pyridine contain-
ing binuclear Cu(II) complexes which arise due to the bridging
ligands and ␲–␲ stacking interaction between adjacent pyridine
rings [9]. Moreover, for the optical determination of trace amounts
the optosensing behavior of their metal complexes in non-aqueous
media has been studied [11]. In addition to this, metal complexes
of hydrazones proved to have potential applications as catalysts
[12], luminescent probes [13] and recently hydrazones like pyri-
doxal isonicotinoyl hydrazone, di-2-pyridyl ketone benzhydrazone
are found to be effective iron chelators for the treatment of iron-
overload diseases [14].
Keeping in view the above points and with the aim of elucidating
the coordination geometry of the complexes of the present lig-
and (2-benzoylpyridine benzhydrazone), its Cu(II) complexes were
synthesized and characterized. Through this paper we could high-
light some of the interesting features in the EPR spectra and the
crystal structure of one of the complexes could be established. The
dimeric nature of the complexes and the coordination mode of the
ligand could be explained. In one of the spectra, splitting in the
perpendicular region is observed which yield information on the
number of nitrogen ligands coordinated to the Cu center. More-
over in one of the solution state EPR spectra of a complex at 298 K
five superhyperfine signals could be resolved which are normally
rare.
2. Experimental
2.1. Materials
∗
2-Benzoylpyridine (Aldrich), benzhydrazide (Aldrich), cop-
per(II) chloride dihydrate (E-Merck), copper(II) acetate mono-
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).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.11.040

N.A. Mangalam et al. / Spectrochimica Acta Part A 75 (2010) 686–692
687
Scheme 1. Synthesis of 2-benzoylpyridine benzoyl hydrazone.
hydrate (E-Merck), sodium azide (Reidel-De Haen), potas-
sium thiocyanate (Merck), copper(II) bromide (Aldrich), cop-
per(II) perchlorate hexahydrate (Aldrich) were used as sup-
plied and solvents were purified by standard procedures
before use.
Green precipitate obtained was filtered, washed with methanol and
dried over P4O10 in vacuo.
[Cu(BPB)N3]·H2O (5): Yield: 79%, Color: green, M.P.: 243–245 ◦C,
ꢁm (DMF): 5 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.66, Elemental Anal.
Found (Calcd.) (%): C, 53.86 (53.83); H, 3.34 (3.80); N, 19.96 (19.82).
2.2. Synthesis of 2-benzoylpyridine benzhydrazone (HBPB)
2.3.4. Synthesis of [Cu(BPB)NCS]·H2O·CH3OH (6)
Complex 6 was prepared by stirring methanolic solutions
of HBPB (1 mmol, 0.301 g) and aqueous solution of KSCN
(1 mmol, 0.097 g) for half an hour. Then an aqueous solution of
Cu(CH3COO)2·H2O (1 mmol, 0.199 g) was added to it and refluxed
for 2 h. Green precipitate obtained was filtered, washed with
methanol and dried over P4O10 in vacuo.
[Cu(BPB)NCS]·H2O·CH3OH (6): Yield: 71%, Color: green, M.P.:
190–192 ◦C, ꢁm (DMF): 26 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.77, Ele-
mental Anal. Found (Calcd.) (%): C, 53.76 (53.44); H, 3.89 (4.27); N,
12.21 (11.87).
The ligand 2-benzoylpyridine benzhydrazone (HBPB) was pre-
pared (Scheme 1) by adapting a reported procedure [15]. Yield: 79%,
Color: colorless, M.P.: 128–130 ◦C, Elemental Anal. Found (Calcd.)
(%): C, 75.46 (75.73); H, 5.23 (5.02); N, 13.98 (13.94).
Selected IR (cm−1) bands: ꢀ(N–H), 3063; ꢀ(C O), 1678; ꢀ(C N),
1571; ꢀ(N–N), 1071.
Electronic absorption bands (DMF) ꢁmax (nm): 232, 271 and 322.
2.3. Syntheses of complexes
Caution! Although no problems were encountered during this
research, azide and perchlorate salts of metal complexes with
organic ligands are potentially explosive. So they should be pre-
pared in small quantities and handled with care.
2.3.1. Synthesis of [Cu(BPB)2] (1)
Complex 1 was prepared by refluxing a methanolic solution of
HBPB (1 mmol, 0.301 g) and Cu(CH3COO)2·H2O (1 mmol, 0.199 g)
for 5 h. The resulting solution was allowed to stand at room tem-
perature and after slow evaporation, green crystalline compound
was separated, filtered and washed with ether and dried over P4O10
in vacuo. Green block shaped crystals suitable for single crystal X-
ray diffraction studies were obtained by the slow evaporation of its
solution in methanol.
2.4. Physical measurements
Elemental analyses of the ligand and the complexes were done
on a Vario EL III CHNS analyzer at SAIF, Kochi, India. The IR spectra
were recorded on a JASCO FT/IR-4100 Fourier Transform Infrared
spectrometer using KBr pellets in the range 400–4000 cm−1. Elec-
tronic spectra in DMF solution were recorded on a Spectro UV-vis
Double Beam UVD-3500 spectrometer in the 200–900 nm range.
The molar conductance of the complexes in DMF (10−3) solution
was measured at 298 K with a Systronic model 303 direct-reading
conductivity bridge. Magnetic susceptibility measurements at
room temperature were made using a Vibrating Sample Mag-
netometer at the Indian Institute of Technology, Roorkee, India.
The EPR spectra of the complexes were recorded in the X-band
frequency, using 100 kHz field modulation, on a Varian E-112 spec-
trometer using TCNE as the standard at SAIF, IIT, Bombay, India.
[Cu(BPB)2] (1): Yield: 88%, Color: green, M.P.: 268–270 ◦C, ꢁm
(DMF): 6 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.63, Elemental Anal. Found
(Calcd.) (%): C, 69.32 (68.71); H, 4.28 (4.25); N, 12.05 (12.65).
2.3.2. Synthesis of [Cu(BPB)Cl]·H2O (2), [Cu(BPB)Br] (3),
[Cu2(BPB)2](ClO4)2·4H2O (4)
Complexes 2, 3 and 4 were prepared by refluxing a methanolic
solution of HBPB with corresponding methanolic metal salt solu-
tions in equimolar ratios. The resulting solution was allowed to
stand at room temperature and after slow evaporation, the green
products were separated, filtered and washed with ether and dried
over P4O10 in vacuo.
[Cu(BPB)Cl]·H2O (2): Yield: 73%, Color: green, M.P.: 270–272 ◦C,
ꢁm (DMF): 21 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.46, Elemental Anal.
Found (Calcd.) (%): C, 55.36 (54.68); H, 3.38 (3.86); N, 10.31 (10.07).
[Cu(BPB)Br] (3): Yield: 68%, Color: green, M.P.: 275–277 ◦C, ꢁm
(DMF): 8 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.59, Elemental Anal. Found
(Calcd.) (%): C, 52.01 (51.42); H, 3.62 (3.18); N, 9.50 (9.47).
[Cu2(BPB)2](ClO4)2·4H2O (4): Yield: 89%, Color: green, M.P.:
206–208 ◦C, ꢁm (DMF): 148 ohm−1 cm2 mol−1, ꢂeff (B.M.): 1.09, Ele-
mental Anal. Found (Calcd.) (%): C, 45.80 (45.70); H, 2.99 (3.63); N,
8.48 (8.41).
2.5. X-ray crystallography
Single crystal X-ray diffraction experiments were performed
at 273(2) K using a Bruker SMART CCD area diffractometer with
graphite monochromated MoK␣ radiation (ꢁ = 0.71073 Å). The
Bruker SAINT software was used for data reduction and Bruker
SMART for cell refinement. The trial structure was solved using
SHELXTL-97 by direct methods and refinement was carried out by
full-matrix least-squares procedures on F2 using SHELXTL-97 pro-
grams [16]. Anisotropic displacement parameters were defined for
all non-hydrogen atoms. Positions of hydrogen atoms were derived
from Fourier difference maps which were placed geometrically and
refined with a riding model. The structures of the compound were
plotted using the program DIAMOND Version 3.1f [17]. Pertinent
crystallographic data and structure refinement parameters for the
structure are given in Table 1.
2.3.3. Synthesis of [Cu(BPB)N3]·H2O (5)
Complex 5 was prepared by stirring methanolic solution of HBPB
(1 mmol, 0.301 g) and aqueous solution of NaN3 (1 mmol, 0.065 g)
for half an hour. Then an aqueous solution of Cu(CH3COO)2·H2O
(1 mmol, 0.199 g) was added to the mixture and stirred for 2 h.

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N.A. Mangalam et al. / Spectrochimica Acta Part A 75 (2010) 686–692
Table 1
Crystal data and structure refinement parameters for complex 1.
Parameters
[Cu(BPB)2] (1)
Empirical formula
Formula weight (M)
Temperature (T) K
Wavelength (Mo K␣) (Å)
Crystal system
C38H28CuN6O2
664.21
273(2)
0.71073
Triclinic
P-1
Space group
Lattice constants
a (Å)
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢃ (◦)
10.6493(13)
12.4467(15)
12.9613(15)
67.641(2)
83.979(2)
83.907(2)
Volume (V) (Å3)
1576.0(3)
2
1.400
0.738
Z
Calculated density (ꢄ) (Mg m−3
)
Absorption coefficient (ꢂ) (mm−1
)
Fig. 1. The molecular structure of [Cu(BPB)2] (1) along with the atom numbering
scheme.
−13 ≤ h ≤ 13
−15 ≤ k ≤ 15
−15 ≤ l ≤ 15
Limiting indices
Reflections collected
Unique Reflections
12,215
6189 [R(int) = 0.0441]
Full-matrix least-squares on F2
6189/0/424
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2␴ (I)]
R indices (all data)
1.239
R1 = 0.0968, wR2 = 0.1859
R1 = 0.1194, wR2 = 0.1960
wR2 = [˙w(Fo2 − Fc2)2/˙w(Fo2)2]1/2. R1 = (˙||Fo| − |Fc||)/˙|Fo|.
3. Results and discussion
Herein, we demonstrate the coordination mode of 2-
benzoylpyridine benzhydrazone to generate Cu(II) monomeric and
dimeric complexes. As expected these complexes are EPR active
due to the presence of an unpaired electron. The Cu(II) com-
plexes generally exhibit room temperature magnetic susceptibility
as expected for an isolated d9 transition metal center. The effec-
tive magnetic moment (ꢂeff) values found for the complexes are
very close to the spin only value for d9 Cu(II) complex. The per-
chlorate complex shows substantial low magnetic moment of 1.09
B.M. which may be due to the coupling of two magnetic centers
suggesting the dimeric nature of the complex [18]. The molar con-
ductance values of all the complexes except 4 in DMF lie in the
(5–26 ohm−1 cm2 mol−1) range. These low values indicate their
non-electrolytic nature while complex 4 is found to be a 2:1 elec-
Fig. 2. Packing diagram of complex 1 with octahedral polyhedra around the Cu(II)
centers.
Table 2
Selected bond lengths (Å) and bond angles (◦) for [Cu(BPB)2].
Bond lengths
Bond angles
Cu1–N5
Cu1–N2
Cu1–O1
Cu1–O2
Cu1–N3
Cu1–N6
N2–N1
N5–N4
N2–C8
N5–C27
O1–C7
O2–C26
N1–C7
1.953(4)
2.012(4)
2.202(4)
2.048(4)
2.290(5)
2.083(4)
1.366(6)
1.378(6)
1.298(6)
1.290(6)
1.254(6)
1.253(6)
1.346(7)
1.349(7)
1.506(7)
1.503(7)
N5–Cu1–N2
N2–Cu1–O1
N2–Cu1–N3
N5–Cu1–O2
N5–Cu1–N6
O1–Cu1–N3
O2–Cu1–N6
N5–Cu1–N3
N2–Cu1–N6
N6–Cu1–N3
O1–Cu1–N3
N6–Cu1–O1
N2–Cu1–O2
O2–Cu1–N3
O2–Cu1–O1
C8–N2–N1
172.06(19)
75.25(16)
75.61(17)
78.21(17)
79.37(18)
149.91(15)
157.36(16)
100.19(17)
94.47(17)
98.51(17)
149.91(15)
91.07(16)
108.14(16)
88.56(16)
93.45(15)
119.2(4)
N4–C26
C7–C6
C26–C25
C27–N5–N4
122.4(4)

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689
Fig. 3. Electronic spectra of the copper(II) complexes.
trolyte [19]. The structure of one of the complexes, 1 has been
confirmed by single crystal X-ray study and was found to be in dis-
torted octahedral geometry around the Cu(II) atom coordinating
through the pyridine and azomethine nitrogens and enolic oxygen.
remaining four. From the bond angles O1–Cu1–N2, 75.25(16)◦;
N2–Cu1–N3, 75.61(17)◦; N5–Cu1–N6, 79.37(18)◦; N5–Cu1–O2,
78.21(17)◦; it is observed that the coordination geometry is quite
far from a perfect octahedron [21]. The Cu(II) center is shared by
four-fused five-membered chelate rings and the bicyclic chelate
system{Cu1,O1,C7,N1,N2,C8,C9,N3} making a dihedral angle of
86.47◦ with its counterpart {Cu1,O2,C26,N4,N5,C27,C28,N6} is
approximately planar with a maximum mean plane deviation
of 0.1517 Å for N3 and 0.0933 Å for C28 [22]. The Cu–N(py),
Cu–N(imine) and Cu–O bond lengths observed here are within the
range reported for other similar complexes of divalent metal ions.
In general, the shortest coordinate bonds are to the central imine
N donors, while those to the distal pyridyl and carbonyl groups are
significantly weaker. This can be attributed to greater ␲-back bond-
ing in the Cu–N(imine) bond than in the Cu–N(py) bond and also
to the steric requirements of the meridionally coordinated ligand
[23].
3.1. Crystal structure of [Cu(BPB)2] (1)
Molecular structure of [Cu(BPB)2] together with the atom label-
ing scheme is depicted in Fig. 1. Selected bond lengths and bond
angles are listed in Table 2. Compound crystallizes in triclinic lat-
tice with space group P-1 and the Cu(II) center exhibits a distorted
octahedral geometry comprising of two equivalent monoanionic
ligands coordinated in a meridional fashion using cis pyridyl, trans
azomethine nitrogen and cis enolate oxygen atoms positioned very
nearly perpendicular to each other. The ligand has undergone keto-
enol tautomerism and the torsional angle C9–C8–N2–N1, 176.34◦
observed supports the E conformation of the ligand on coordination
[20]. The Cu(II) complex bears a tetragonally elongated structure
with trans pair of bonds, Cu1–N3 and Cu1–O1, being longer than the
remaining four bond lengths (Cu1–N6, Cu1–N5, Cu1–O2, Cu1–N2).
This is a consequence of the Jahn-Teller effect that operates on the
d9 electronic ground state of the six coordinate complexes which
elongates one trans pair of coordinate bonds while shortening the
The packing of the complex is shown in Fig. 2. In the unit
cell the molecules are packed in the lattice in an ordered man-
ner along the a axis. The ␲–␲, C–H· · ·␲ and hydrogen bonding
interactions (Table 3) contribute to the stability in the unit cell
packing. The metal chelate ring Cg(6) is involved in ␲–␲ interac-
tion with Cg(6) of the neighboring unit at a distance of 3.807(4) Å.
Table 3
H-bonding, ␲–␲ and C–H· · ·␲ interaction parameters of [Cu(BPB)2].
H-bonding
Donor· · ·H· · ·A (Å)
C15–H(15)· · ·N(1)a
C30–H(30)· · ·O(1)b
Equivalent position code
a = 1 − x, −y, −z
D–H
0.93
0.93
H· · ·A
2.60
2.55
D· · ·A
D–H· · ·A
136
124
3.3318
3.1618
b = −x, 1 − y, −z
␲–␲ interactions
Cg(I)–Res(1)· · ·Cg(J)
Cg(6) [1]· · ·Cg(6)c
Cg–Cg (Å)
3.8074
˛ (◦)
21.44
ˇ (◦)
21.44
Equivalent position code
c = −x, 1 − y, −z
Cg(6) = N(6), C(28), C(29), C(30), C(31), C(32);
C–H· · ·␲ interaction
X–H(I)· · ·Cg(J)
H· · ·Cg (Å)
XH· · ·Cg (◦)
X· · ·Cg (◦)
C(16)–H(16) [1] ≥ Cg(1)d
2.76
142
3.5348
Equivalent position code
d = 1 − x, −y, −z
Cg(1) = Cu (1), O(1), C(7), N(1), N(2)
D, donor; A, acceptor; Cg, centroid; ˛, dihedral angles between planes I and J; ˇ, angle Cg(1)–Cg(J).

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N.A. Mangalam et al. / Spectrochimica Acta Part A 75 (2010) 686–692
Table 4
Infrared spectral assignments (cm−1) of the copper(II) complexes.
Compound
␯(C N)
␯(C N)a
␯(C–O)
␯(N–N)
␯(Cu–Nazo
)
␯(Cu–O)
[Cu(BPB)2] (1)
1530
1550
1547
1558
1526
1526
1585
1597
1594
1591
1597
1596
1297
1293
1296
1265
1283
1273
1079
1082
1083
1077
1100
1106
471
461
459
466
462
470
433
432
425
445
440
430
[Cu(BPB)Cl]·H2O (2)
[Cu(BPB)Br] (3)
[Cu2(BPB)2](ClO4)2·4H2O (4)
[Cu(BPB)N3]·H2O (5)
[Cu(BPB)NCS]·H2O·CH3OH (6)
a
Newly formed C N.
Table 5
In addition to the ␲–␲ stacking, significant C–H· · ·␲ interactions
of the phenyl hydrogens with metal chelate rings of the neighbor-
ing molecules contributes to the stability of the unit cell packing.
Two weak hydrogen bonding interactions are observed between
C(15)–H(15) and N(1) and between C(30)–H(30) and O(1) with an
angle of 136 and 124◦, respectively. No classic hydrogen bonds
are found in this complex. Ring puckering analysis shows that the
five-membered ring, Cg(1) {Cu1–O1–C7–N1–N2} is puckered and
adopts an envelope conformation with N2 forming the flap of the
envelope {Q = 0.100(4) Å; ϕ = 146(3)◦} [24,25].
Electronic spectral assignments of the copper(II) complexes.
Compound
UV–vis ꢁmax/nm
[Cu(BPB)2] (1)
261, 391, 703
272, 398, 709
267, 396, 705
272, 399, 652
265, 323, 393, 671
268, 324, 400, 672
[Cu(BPB)Cl]·H2O (2)
[Cu(BPB)Br] (3)
[Cu2(BPB)2](ClO4)2·4H2O (4)
[Cu(BPB)N3]·H2O (5)
[Cu(BPB)NCS]·H2O·CH3OH (6)
and through the imine nitrogen atom. On complexation a new band
due to ␯(N C) is often resolved, which confirms the coordination of
the ligand through imine nitrogen [29]. The out-of-plane bending
modes of vibrations of the free ligands at 622 cm−1 are found to be
shifted to higher energies in the spectra of complexes indicating
the coordination via pyridine nitrogen [30].
We have obtained three bands for compound 6, a sharp band at
2079 cm−1, a medium band at 782 and another at 484 cm−1 cor-
responding to ␯(CN) and ␯(CS), respectively which are typical for
N-bonded thiocyanate complex. The intensity and band positions
indicate the unidentate coordination of the thiocyanate through
the nitrogen atom [31]. The azide complex shows a sharp band
at 2061 cm−1 [31] and the perchlorate complex shows a band at
1120 cm−1 assignable to ␯3(ClO4) and a strong band at 624 cm−1
assignable to ␯4(ClO4), indicating the presence of ionic perchlorate
[32].
3.2. Infrared spectra
The important IR frequencies of the ligand are given in Sec-
tion 2. Exhaustive comparisons of the selected vibrational bands of
the free ligand with its copper complexes give information about
the ligating mode of the ligand upon complexation. The impor-
tant infrared spectral bands of the complexes and their tentative
assignments are given in Table 4. The characteristic spectral bands
for ␯(NH) and ␯(C O) stretches of the hydrazone ligand appears at
3063 and 1678 cm−1, respectively. Interestingly these bands disap-
pear on complexation, which supports the coordination via enolate
form instead of keto after the tautomerisation process [26]. Instead
a new band appears at 1284 cm−1 due to the coordination of the
␯(C–O) enolic mode. The strong absorption band at 1571 cm−1
ascribed to the imine stretching frequency of the uncoordinated
ligand, shifts towards lower frequency on complexation with the
metal, suggesting coordination to the metal through imine nitro-
gen [27]. A characteristic band of the ligand at 1071 cm−1 due
to ␯(N–N) stretching undergoes a shift to higher wavenumber in
complexes [28] due to the increase in the double bond character
of N–N, offsetting the loss of electron density via donation to the
metal ion which gives further evidence of coordination of the lig-
3.3. Electronic spectra
The electronic absorption bands of the complexes (Fig. 3) and
their assignments are summarized in Table 5. Assignments of the
absorption bands are made on the basis of optical transitions of
other previously reported Cu(II) complexes with similar types of
Table 6
EPR spectral assignments of copper(II) complexes in polycrystalline state, in solution state at 298 K and in frozen DMF at 77 K.
1
2
3
4
5
6
Polycrystalline (298 K)
g
g
2.352
2.092
2.178
2.025
2.210
2.079
2.122
2.707
–
–
2.209
2.066
2.113
3.244
2.202
2.061
2.108
3.402
–
–
||
⊥
giso/gav
G
2.139
–
2.102
–
DMF solution (298 K)
giso
Aiso
2.108
72.81
2.112
75.60
2.090
78.84
2.123
79.00
2.084
79.00
2.105
72.13
a
DMF (77 K)
g
g
gav
2.184
2.092
2.122
2.199
2.056
2.103
2.205
2.050
2.101
2.245
2.056
2.119
2.207
2.063
2.111
2.185
2.053
2.097
||
⊥
A
˛
ˇ2
ꢃ2
K
K
173.33
0.7482
0.8334
1.1698
0.6236
0.8753
191.66
0.803
0.804
0.837
0.646
0.673
191.66
0.809
0.812
0.785
0.657
0.635
170.00
0.799
0.936
0.878
0.748
0.701
190.00
0.811
0.835
0.906
0.677
0.736
193.33
0.790
0.808
0.849
0.639
0.671
||
2
||
⊥
a
Expressed in units of cm−1 multiplied by a factor of 10−4
.

N.A. Mangalam et al. / Spectrochimica Acta Part A 75 (2010) 686–692
691
Fig. 5. EPR spectrum of [Cu2(BPB)2](ClO4)2·4H2O in polycrystalline state at 298 K.
3.4. EPR spectra
EPR spectra of the complexes in the polycrystalline and solu-
tion state at 298 K and in frozen DMF at 77 K were recorded and
various magnetic induction parameters are summarized in Table 6.
The copper(II) ion having a d9 configuration with an effective spin
of S = 1/2 couples with nuclear spin of 63Cu (I = 3/2) and give rise to
four (2nI + 1 = 4) hyperfine lines. In polycrystalline state, since it is
magnetically concentrated the anisotropy may be lost. Dilution of
the solid isolates the electron spin of the given complex from that of
another paramagnetic molecule. The solution state EPR spectra at
298 K for all the complexes reveal the four copper hyperfine signals
and in addition to this in complex 4, five superhyperfine signals are
also observed (Fig. 4), which arise from the coupling of the electron
spin with the nuclear spin of the nitrogen atoms. This splitting is
due to the bonding of the azomethine and pyridine nitrogens.
In polynuclear Cu(II) complexes the most important applica-
tion of the measurement of the ESR spectra is in the identification
of Cu–Cu dipolar interaction. Additional transitions arise asso-
Fig. 4. EPR spectrum of [Cu2(BPB)2](ClO4)2·4H2O in solution state at 298 K.
ligand. The energy bands at 232, 271 and 322 nm may be assigned
to the intraligand ␲–␲* and n–␲* transitions. The strong absorp-
tions found for the complexes ∼396 nm is associated to the phenoxy
O → Cu(II) charge transfer bands [33]. In addition a copper(II) com-
plex with d9 configuration is expected to experience Jahn-Teller
distortion which leads to further splitting of the 2Eg and T2g
2
2
2
2
levels and give rise to the B1g → A1g (␯1), B2g (␯2), 2Eg (␯3)
transitions which is expected to be close in energy and generally
appears as a broad band. Therefore the broad band at 684 nm is
2
2
assigned to the envelope of B1g → A1g
,
2B2g and 2Eg transitions
[34].
Fig. 6. EPR spectra of [Cu(BPB)N3]·H2O and [Cu2(BPB)2](ClO4)2·4H2O in DMF at 77 K.

692
N.A. Mangalam et al. / Spectrochimica Acta Part A 75 (2010) 686–692
Supplementary information
Crystallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data Center, CCDC
729895 for compound [Cu(BPB)2]. The data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or
from Cambridge Crystallographic Data Centre (CCDC), 12 Union
Road, Cambridge CB21EZ, UK; fax: +44 1223 336 033; e-mail:
deposit@ccdc.cam.ac.uk].
Acknowledgements
M.R.P. Kurup is thankful to the KSCSTE, Thiruvanthapuram,
India for financial assistance. The authors are thankful to the SAIF,
Cochin University of Science and Technology, Kochi, Kerala, India
for elemental and IR analyses. We are thankful to IIT, Roorkee, IIT,
Bombay, India for magnetic susceptibility and EPR measurements
respectively.
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values in mononuclear complexes. In the X-band spectra, ꢅMs =
1
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transitions are associated with fields of ca. 3000 G, while the
ꢅMs = 2 generate an absorption at the half field value of ca.
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DMF at 77 K with g > g > 2.0023 relationship, consistent with a
||
⊥
dx
ground state [35] in a square planar or square pyramidal
2
2
−y
geometry. Representative EPR spectra are shown in Fig. 6. In com-
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||
siderable exchange interaction is indicated in the solid complex
[36]. The EPR parameters g , g A (Cu) and energies of d–d tran-
||
⊥, ||
sitions were used to evaluate the bonding parameters ˛(, ˇ( and
ꢃ(, which may be regarded as a measure of covalency of the
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respectively.
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Hathaway [37] pointed out that, for pure ␴-bonding,
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||
||
⊥
out-of-plane ␲-bonding, K < K . The values of the bonding param-
⊥
||
eters ˛(, ˇ( and ꢃ( < 1.0 indicate significant in-plane ␴-bonding and
in-plane ␲-bonding.