Copper(I) hydrazone complexes: Synthesis, structure, DNA binding, radical scavenging and computational studies

Article abstract of DOI:10.1016/j.inoche.2011.05.004

Potential bidentate hydrazone ligands, HL¹ (1) and HL ² (2) prepared by the condensation of benzaldehyde or furfuraldehyde with benzhydrazide upon reaction with [CuCl₂(PPh₃) ₂] yielded corresponding m

Full text of DOI:10.1016/j.inoche.2011.05.004

Inorganic Chemistry Communications 14 (2011) 1318–1322
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Inorganic Chemistry Communications
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Copper(I) hydrazone complexes: Synthesis, structure, DNA binding, radical
scavenging and computational studies
P. Krishnamoorthy a, P. Sathyadevi a, K. Senthilkumar b, P. Thomas Muthiah c, R. Ramesh c, N. Dharmaraj a,
⁎
a
Department of Chemistry, Bharathiar University, Coimbatore-641 046, India
Department of Physics, Bharathiar University, Coimbatore-641 046, India
b
c
School of Chemistry, Bharathidasan University, Tiruchirappalli-620 024, India
a r t i c l e i n f o
a b s t r a c t
Potential bidentate hydrazone ligands, HL1 (1) and HL2 (2) prepared by the condensation of benzaldehyde or
furfuraldehyde with benzhydrazide upon reaction with [CuCl2(PPh3)2] yielded corresponding mononuclear
complexes of the compositions [Cu(L1)(PPh3)2] (3) and [Cu(L2)(PPh3)2] (4). The exact nature of coordination
of the hydrazones to the metal ion and the structure of the complexes were confirmed by spectral and single
crystal X-ray diffraction studies. Interestingly, the reactions of 1 and 2 with [CuCl2(PPh3)2] resulted in the
formation of the first structurally characterized copper(I) hydrazone complexes 3 and 4 along with the
previously reported complex [CuCl(PPh3)3] 5 as a minor product in both the reactions. The metal complexes 3
and 4 showed significant binding towards calf thymus DNA (CT-DNA) via groove binding mode with binding
constants in the magnitude 104–105 M−1. In addition, the antioxidant activities of the complexes were also
investigated through scavenging effect on DPPH•, NO˙ and OH˙ radicals. The density functional theory
calculations of complex 4 also supported the structure and stability of the reduced complex.
© 2011 Elsevier B.V. All rights reserved.
Article history:
Received 8 March 2011
Accepted 12 May 2011
Available online 20 May 2011
Keywords:
Copper(I) hydrazones
DNA binding
DFT studies
Since the success of cisplatin and related platinum complexes as
anticancer agents, developing other active transition metal complexes
with better anticancer efficiency has attracted the attention of many
research groups and became a central research theme in bioinorganic
chemistry. In the search toward new metallic species with biological
applications, copper compounds have proved to be excellent
candidates [1–7]. The specific roles of ligands analogous to the
molecules that bind to or modulate the function of biological
receptors make them good candidates for drug development. In
recent years, the design and synthesis of new ligands with combined
biological, structural and coordination properties have burgeoned.
Interest in these studies stems not only from the different coordina-
tion modes but also from their chemical reactivity. Hydrazine
derivatives containing N-heterocycles and their complexes exhibit
strong antitumor and antivirus activities [8,9]. Metal complexes
constitute an important subset of DNA-binding compounds, often
DNA-intercalators and groove binders [10–12], the latter character-
ized by non-covalent sequence specific interactions [13].
three parameter hybrid exchange functional (B3) [14] combined with
correlation functional of Vosko, Wilk and Nusair (VWN) [15] and Lee,
Yang and Parr (LYP) [16], together called as B3LYP. The effective core
potential (ECP) basis set SRSC [17] for Cu atom and 6-311G(d,p) basis
set for all other atoms have been used. The vibrational frequency
calculation has been performed at the above level of theory and the
results confirm that the optimized geometry is at a stationary point of
the potential energy surface without any imaginary frequency.
Theoretical calculations have been performed by using Q-Chem 3.0
program package [18].
The ligands 1 and 2 were synthesized according to the reported
procedure [19] as shown in Scheme 1. The new copper(I) complexes
[Cu(L1)(PPh3)2] (3) and [Cu(L2)(PPh3)2] (4) were prepared from
equimolar amounts of the ligands 1 or 2 with [CuCl2(PPh3)2] [20] as
the starting precursor. Elemental analyses revealed that complexes 3
and 4 possess 1:1 metal-to-ligand stoichiometry (see Supplementary
material for experimental section).
The Cu(I) complex 4 crystallized in the monoclinic lattice with P21/
c space group (X-ray diffraction data shown in Table S1). The crystal
structure presented in Fig. 1 showed the composition of complex 4 as
[Cu(L2)(PPh3)2], in which the central metal ion was found to be in the
1+ oxidation state instead of the 2+ oxidation state as in the case of
the starting precursor. The hydrazone ligand 2 was found to be
coordinated to the copper ion in complex 4 in a bidentate fashion
through the azomethine nitrogen and enolic oxygen atoms, forming a
stable five-member chelate ring with [P(1)–Cu(1)–P(2)], [P(1)–Cu
(1)–O(2)], [O(2)–Cu(1)–N(1)] and [N(1)–Cu(1)–P(2)] bite angles of
Herein, we report the synthesis, structural investigation of
hydrazone-based ligands and their new copper(I) complexes along
with studies on the DNA binding and antioxidant activities. Further,
quantum chemical investigations on complex 4 have been carried out
by applying the density functional theory (DFT) employing Becke's
⁎
Corresponding author. Tel.: +91 422 2428316; fax: +91 422 2422387.
E-mail address: dharmaraj@buc.edu.in (N. Dharmaraj).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.004

P. Krishnamoorthy et al. / Inorganic Chemistry Communications 14 (2011) 1318–1322
1319
H
copper ion gets reduced to a monovalent species in which the
hydrazone acted as a good reducing agent. Fig. S2 represents the crystal
structure of complex 5. The crystal structure, unit cell parameters, and
bond lengths of 5were found to be in good agreement with the earlier
report on the same complex from a different reaction [21].
O
H2N
R
O
+
N
H
The DFT calculations using the hybrid exchange correlation
functional B3LYP have been performed on the related mononuclear
species [Cu(L2)(PPh3)2] with the ground-state geometry adopted
from the truncated X-ray data. After careful comparison, we found
that most of the optimized bond lengths and angles are slightly larger
than the experimental values, which is not surprising due to the
known fact that theoretical calculation belongs to the isolated
molecule in the gaseous phase and the experimental result belongs
to the molecule in the solid state with intermolecular interactions and
crystal packing effect. The optimized structure of the stable conformer
by DFT method for complex 4 is shown in Fig. 2. Some selected single
crystal X-ray diffraction data together with the optimized geometrical
parameters at the B3LYP method are listed in Table S2.
EtOH
Reflux, 5h
H
H
H
N
N
R
N
R
N
OH
O
amido form
iminol form
MeOH/KOH
Reflux, 8h
[CuCl2(PPh3)2]
H
The electronic spectra of the ligands and their complexes were
recorded in methanol. The spectra of the ligands 1 and 2 displayed
two bands in the 231–335 nm region due to intra-ligand charge
transfer transitions. The electronic spectra of complexes 3 and 4
exhibited three bands in the range of 200–400 nm. Bands that
appeared in the region of 215, 288 and 293 nm have been assigned to
the π→π* intra-ligand charge transfer transitions and a weak band at
371 and 394 nm corresponding to the ligand to metal charge transfer
transition (LMCT) [22]. To understand the nature of electronic
transitions, the molecular orbital analysis of complex 4 has been
performed which is comparable to that of experimentally observed
absorption spectral data. Fig. 3 represents the frontier molecular
orbital's of complex 4, i.e., the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO), in
which HOMO is mainly delocalized on imine group and the LUMO is
mainly delocalized over the imine as well as in aromatic group with an
energy gap of 3.54 eV between the frontier orbitals.
N
PPh3
Cu
R
N
+
O
Cu
PPh3
Ph3P
Cl
Ph3P
PPh3
where,
R
=
C6H5 (HL1) (or) C4H3O (HL2)
Scheme 1. Synthesis of hydrazone ligands and their Cu(I) complexes.
121.99(5)°, 104.3(1)°, 77.5(1)° and 112.9(1)° respectively with the
bond distances of Cu1–P1, Cu1–P2, Cu1–O2 and Cu1–N1 as 2.235(1),
2.253(1), 2.112(3) and 2.044(4)Å, respectively. Each unit cell
contains four molecules and the corresponding unit cell packing
diagram is shown in Fig. S1. In this reaction, another minor product
(complex 5) was obtained and crystallized to get suitable crystals for
characterization using XRD. The single crystal XRD data proved that
the molecular formula of 5 is [CuCl(PPh3)3] with lack of coordinated
hydrazone ligand. Hence, it is understood that during the course of the
reaction between the starting precursor and the hydrazone ligand, the
From the elemental analyses, IR, photoluminescence, 1H NMR
(Figs. S3 and S4) and cyclic voltammetric data (given in the
Supplementary material) we understand that complexes 3 and 4 are
structurally similar to each other.
Fig. 1. Molecular structure of complex 4 showing the atom-numbering scheme with
Fig. 2. The optimized ground-state structure of complex 4 at the B3LYP/6-311G(d,p)
thermal ellipsoids at 25% probability level.
level.

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P. Krishnamoorthy et al. / Inorganic Chemistry Communications 14 (2011) 1318–1322
HOMO plot
LUMO plot
Fig. 3. HOMO–LUMO plots of complex 4.
Metal complexes are known to bind with DNA via both covalent
and non-covalent interactions. In covalent binding the labile ligand of
the complexes is replaced by a nitrogen base of DNA such as guanine
N7. On the other hand, non-covalent DNA interactions include
intercalative, electrostatic and groove (surface) binding of cationic
metal complexes along outside of DNA helix, along a major or minor
groove. Intercalation involves the partial insertion of aromatic
heterocyclic rings of the ligands between the DNA base pair [23]. In
this study, we selected complexes 3 and 4 to investigate the binding
properties with DNA which can be monitored through UV–visible and
fluorescence studies.
emission intensity increases to 6.64% and 11.35%, respectively when
[DNA]/[complex]=1.
Further, to know the interaction of complexes 3 and 4 with DNA, a
competitive binding experiment using ethidium bromide (EB) as a
probe was carried out and the fluorescence quenching of EB bound to
CT-DNA by the above complexes are shown in Fig. 4. The addition of
complexes 3 and 4 to EB-bound CT-DNA solution caused obvious
reduction in emission intensities with 3 nm blue shift (complex 3)
and 3 nm red shift (complex 4), indicating that complexes compet-
itively bound to CT-DNA and caused release of bound EB from the
hydrophobic environment into the aqueous solution or accepting an
excited state electron from EB. The emission intensity of the DNA–EB
system (λem =605 nm) decreased apparently as the concentration of
the complexes increased. For complex 4, an isobathic point appeared
at 549 nm, indicating the formation of a complex–DNA system.
According to the equation KEB [EB]=Kapp[complex] (where the
complex concentration has the value at a 50% reduction of the
fluorescence intensity of EB and KEB =1.0×107 M−1, ([EB]=10 μM))
the Kapp value was calculated to be 8.42×104 M−1, 1.16×105 M−1 for
complexes 3 and 4, respectively, from the plot of I0/I vs. [complex]
suggesting that the interaction between the above-mentioned
complexes and DNA is through moderate groove binding. The
quenching constants kq are often used to evaluate the quenching
efficiency for every compound and vary with experimental condi-
tions. kq values are given by the ratio of the slope to the intercept and
concentration of the corresponding complexes and are found to be
8.43×104 M−1 and 1.15×105 M−1, respectively. Experimental re-
sults indicated that complex 4 bound to DNA more effectively than
complex 3 which may be due to the presence of heterocyclic moiety,
i.e., furan ring.
Based on the known fact that hydrazones and their corresponding
transition metal complexes displayed significant antioxidant activity,
we undertook a systematic investigation on the antioxidant potential
of the ligands 1 and 2 and their complexes 3 and 4 on DPPH˙, OH˙ and
NO˙ at 50 μM concentrations. The results of these experiments were
shown in Fig. S7. IC50 values of ligands 1 and 2 on DPPH, OH and NO
radicals are 62.42, 60.02, 59.59, 61.14, 58.84 and 57.18 μM, respec-
tively, whereas, complexes 3 and 4 showed their IC50 values at 52.47,
46.18, 42.14, 50.77, 41.21 and 37.29 μM, respectively. From the above
results, it can be concluded that the scavenging effects of the free
ligands are less when compared to that of their corresponding
complexes which is due to the chelation of the organic molecules with
the metal ions. However, among the two investigated complexes, the
one containing furan moiety (complex 4) in its molecular architecture
showed higher antioxidant activity on DPPH˙, OH˙ and NO˙ than the
Generally, the binding of an intercalative molecule to DNA is always
accompanied by hypochromism and/or significant bathochromism in
the absorption spectra due to strong stacking interactions between the
aromatic chromophore of the compounds and DNA base pairs [24]. To
obtain some conclusive proof for the binding of the test complexes
with DNA, spectroscopic titrations of complex solutions with DNA
have been performed. The absorption spectra of the investigated metal
complexes in the absence and presence of DNA are given in Fig. S5. In
the absence of DNA, the absorption spectra of complex 3 displayed
three bands around 215, 288 and 371 nm. Similarly, complex 4
exhibited absorptions at 215, 293 and 394 nm. However, upon the
addition of DNA to the solution of the metal hydrazone complexes 3
and 4, the bands observed at 215 nm for both complexes exhibited
respective hypochromisms of about 5.59% and 22.54% accompanied by
a red shift of 4 and 6 nm corresponding to them. However, in the case
of complex 3, the band appearing at 288 nm showed an increase in the
absorption intensity only after the addition of the first fraction of DNA
solution, whereas further additions resulted in 9.99% hypochromism
and the band observed at 371 nm showed a hyperchromism of about
18%. Absorptions appearing at 293 and 394 nm for complex 4
exhibited a hypochromism of 6.9% with respect to the former band
and a hyperchromism of 12.91% corresponding to the later absorption.
The intrinsic binding constant (Kb), was calculated according to the
classical Eq. (1) (shown in supplementary material), and the Kb values
obtained for complexes 3 and 4 were found to be 2.05×104 M−1 and
1.80×105 M−1, respectively revealing that complex 4 exhibited more
intense hypochromism with higher Kb value than complex 3,
indicating that a stronger interaction exists between CT-DNA and
complex 4 via groove binding [25].
Complexes 3 and 4 in methanol as well as in Tris–HCl buffer upon
excitation with 288 and 293 nm wavelengths at room temperature
showed luminescence emissions at 348 and 337 nm, respectively and
the results of the emission titration for the complexes added with
DNA is represented in Fig. S6. Upon addition of calf thymus DNA, the

P. Krishnamoorthy et al. / Inorganic Chemistry Communications 14 (2011) 1318–1322
1321
Complex 3
Complex 4
1.14
1.12
1.10
1.08
1.06
1.04
1.02
1.00
1.10
1.08
600
600
1.06
1.04
500
400
300
200
100
500
1.02
1.00
0
2
4
6
8
10
0
2
4
6
8
-6
10
-6
400
300
200
100
0
[Complex] x 10
M
[Complex] x 10 M
0
550
600
650
700
750
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 4. Emission spectra of DNA–EB system (10 μM), in the presence of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 μM complexes 3 and 4. An arrow indicates the emission intensity
changes upon increasing complex concentrations. Inset: Stern–Volmer plot of the fluorescence titration data corresponding to complexes 3 and 4.
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scavenge radicals and hence the potential as drugs to eliminate the
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We thank the University Grants Commission (UGC), New Delhi,
India for the award of Fellowship in Science for Meritorious Students
(RFSMS) to one of the authors (P. Krishnamoorthy) under UGC-SAP-
DRS programme. PTM is thankful to DST-India (FIST Programme at
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CCDC 771893 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via www.
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tallographic Data Center, 12 Union Road, Cambridge CB21EZ, UK)
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http://www.ccdc.cam.ac.uk.
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