Synthesis, structure and DNA binding studies of mononuclear copper(II) complexes with mixed donor macroacyclic ligands, 2,6-bis({N-[2&3-(phenylselenato)alkyl]}benzimidoyl)-4-methylphenol

Article abstract of DOI:10.1016/j.poly.2009.05.062

Two new phenol based macroacyclic Schiff base ligands, 2,6-bis({N-[2-(phenylselenato)ethyl]}benzimidoyl)-4-methylphenol (bpebmpH, 1) and 2,6-bis({N-[3-(phenylselenato)propyl]}benzimidoyl)-4-methylphenol (bppbmpH, 2) of the Se₂N₂O typ

Full text of DOI:10.1016/j.poly.2009.05.062

Polyhedron 28 (2009) 2591–2598
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structure and DNA binding studies of mononuclear copper(II)
complexes with mixed donor macroacyclic ligands,
2,6-bis({N-[2&3-(phenylselenato)alkyl]}benzimidoyl)-4-methylphenol
a
a
b
c
Vinay K. Verma a, , Ashish K. Asatkar , Tripti A. Jain , Santosh K. Tripathi , Rajendra Singh ,
*
Peter B. Hitchcock d, Siddhi Nigam e, Sushil K. Gupta e
a Disha Institute of Management and Technology, Satya Vihar, Raipur, Chhattisgargh (C.G.) 492101, India
b Defence Materials and Stores Research & Development Establishment, Kanpur, India
c Defence Research and Development Organization, DRDO Bhawan, New Delhi, India
d Department of Chemistry, School of Life Sciences, University of Sussex, Brighton, UK
e School of Studies in Chemistry, Jiwaji University, Gwalior, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new phenol based macroacyclic Schiff base ligands, 2,6-bis({N-[2-(phenylselenato)ethyl]}-
benzimidoyl)-4-methylphenol (bpebmpH, 1) and 2,6-bis({N-[3-(phenylselenato)propyl]}benzimidoyl)-
4-methylphenol (bppbmpH, 2) of the Se2N2O type have been prepared by the condensation of
4-methyl-2,6-dibenzoylphenol (mdbpH) with the appropriate (for specific reactions) phenylselena-
to(alkyl)amine. These ligands with Cu(II) acetate monohydrate in a 2:1 molar ratio in methanol
form complexes of the composition [(C6H2(O)(CH3){(C6H5)C@N(CH2)nSe(C6H5)}{(C6H5)C@O}2Cu]
(3 (n = 2), 4 (n = 3)) with the loss of phenylselenato(alkyl)amine and acetic acid. In both these
complexes, one arm of the ligand molecule undergoes hydrolysis, and links with Cu(II) in a bidentate
(NO) fashion, as confirmed by single crystal X-ray crystallography of complex 3. The selenium
atoms do not form part of the copper(II) distorted square planar coordination sphere which has a
trans-CuN2O2 core. The average Cu–N and Cu–O distances are, respectively, 1.973(3) and 1.898(2) Å.
The N–Cu–N and O–Cu–O angles are, respectively, 167.4(11)° and 164.5(12)°. The compounds 1–4 have
been characterized by elemental analysis, conductivity measurements, mass spectrometry, IR,
electronic, 1H and 77Se{1H} NMR spectroscopy and cyclic voltammetry. The interaction of complex 3
with calf thymus DNA has been investigated by a spectrophotometric method and cyclic voltam-
metry.
Received 2 December 2008
Accepted 18 May 2009
Available online 6 June 2009
Keywords:
Selenium containing hybrid ligands
Copper(II) complexes
NMR
X-ray structure
DNA binding
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
between the ‘hard’ and ‘soft’ donors towards the metal center
[28,29] and also stabilize low as well as high oxidation states of
Ligands bearing variable donor functionalities such as O, S, N
and P in their structural framework have been studied for several
years and continue to be an area of active research for under-
standing the properties of donors that offer the opportunity to
design metal complexes for specific purposes and applications
[1–27]. The coordination chemistry of organochalcogen ligands
containing ‘hard’ donor atoms (such as N and O) along with a ‘soft’
donor (Se or Te) is interesting because such a ligand framework
can provide insight into competitive coordination behaviour
a metal atom. Such molecular systems may be important in tran-
sition metal catalyzed asymmetric synthesis [30,31] and as single
source precursors in MOCVD processes [32–34]. Recently Khan-
delwal et al. have reported some pyridine based acyclic [35,36]
and phenol based cyclic [37] Schiff bases. In fact, a number of
polydentate ligands bearing N3N2, N3O2, N3S2 and NS2P2 are well
documented and among them, a few of the systems have shown
promising catalytic, biological, environmental and material prop-
erties [38–41]. We report herein the design and synthesis of a
new (Se2N2O) molecular system (bpebmpH, 1 and bppbmpH, 2),
having both ‘hard’ and ‘soft’ donors in the ligand framework,
and its reactivity towards Cu(II) ions. We also report our findings
on a DNA binding study with one of the synthesized Cu(II)
complexes.
* Corresponding author. Tel.: +91 771 4200292; fax: +91 771 4200110.
E-mail address: verma_vinay2002@rediffmail.com (V.K. Verma).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.062

2592
V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
over a period of 7–8 h. The progress of the reaction was monitored
CH3
CH3
by TLC. It took about 60 h for completion of the reaction. The
organic solvent was removed under reduced pressure to obtain a
viscous oily crude product. The product was purified by column
chromatography (silica gel, 60–120 mesh, second fraction) using
a solvent mixture of hexane:ethylacetate in 85:15 or 70:30 ratio.
The characteristics of these compounds are given below.
OH
OH
N
N
N
N
2.2.1. 2,6-Bis({N-[2-(phenylselenato)ethyl]}benzimidoyl)-4-
methylphenol(bpebmpH, 1) [2,6-{PhSe(CH2)2N@CPh}2C6H2
(4-CH3)(OH)]
Se
Se
Se
Se
Colour and state: yellow viscous liquid; yield: 2.48 g (73%). Anal.
Calc. for C37H34N2OSe2: C, 65.29; H, 5.03; N, 4.11. Found: C, 65.23;
H, 4.38; N, 4.08%. Positive ESꢀMS: m/z 683 [MH+], 523, 500, 399,
bpebmpH, 1
bppbmpH, 2
158. FT IR (selected, cmꢀ1):
m(OH) 3450,
m(C@N) 1597,
m
(CꢀO)
1252. UV–Vis kmax/nm (e
/Mꢀ1 cmꢀ1) (CH3CN): 391 (PhOꢀ ? Ph).
1H NMR (CDCl3, d ppm): 15.90 (s, 1H, OH), 7.90–7.20 (m, 22H,
C6H5 and C6H2), 3.47 (t, 4H, N–CH2), 2.99 (t, 4H, SeꢀCH2), 2.03
(s, 3H, CH3). 77Se{1H} NMR (CDCl3, d ppm): 288.
2. Experimental
All the chemicals used were of reagent grade. Solvents were
purified by standard methods [42] and freshly distilled prior to
use. The precursors 4-methyl-2,6-dibenzoylphenol (mdbpH)
[43,44] and phenylselenato(alkyl) amine [45] were synthesized
following the reported methods. Calf thymus DNA was obtained
from Sigma and used as received. The sodium salt of CT-DNA
was stored at 4 °C.
2.2.2. 2,6-Bis({N-[3-(phenylselenato)propyl]}benzimidoyl)-4-
methylphenol(bppbmpH, 2) [2,6-{PhSe(CH2)3N@CPh}2C6H2
(4-CH3)(OH)]
Colour and state: yellow viscous liquid; yield: 3.29 g (93%). Anal.
Calc. for C39H38N2OSe2: C, 65.74; H, 5.37; N, 3.93. Found: C, 65.63;
H, 5.31; N, 3.96%. Positive ESꢀMS: m/z 711 [MH+], 553, 539, 514,
353, 158. FTIR (selected, cmꢀ1):
m
(OH) 3416,
/Mꢀ1 cmꢀ1
ppm): 15.90 (s, 1H, OH),
m
(C@N) 1582,
m
(CꢀO) 1253. UV–Vis kmax/nm
(e
) (CH3CN): 390
2.1. Physical measurements
(PhOꢀ ? Ph). 1H NMR (CDCl3,
d
7.90ꢀ7.20 (m, 22H, C6H5 and C6H2), 3.40 (t, 4H, NꢀCH2), 2.85
(t, 4H, SeꢀCH2), 2.19 (q, 4 H, mid CH2 (CH2 between N–CH2 and
Se–CH2)), 2.15 (s, 3H, CH3). 77Se{1H} NMR (CDCl3, d ppm): 288.
Melting points of the compounds in the capillary tubes were re-
corded and are uncorrected. Elemental analyses were carried out
on a Carlo-Erba Model DP 200 analyzer. The electrospray ion mass
spectra (ESIMS) were recorded on a MICROMASS QUATTRO II triple
quadrupole mass spectrometer. The solution was introduced into
2.3. Reactions of Schiff bases (1) and (2) with Cu(II)
the ESI source through a syringe pump at the rate of 5 lL/min.
A degassed solution of Cu(OCOCH3)2.H2O (0.10 g, 0.5 mM) in
freshly distilled methanol (5 mL) was added to the degassed solu-
tion of the ligand bpebmpH (0.68 g, 1.0 mL)/bppbmpH (0.71 g,
1.0 mM) in dry methanol (15 mL) under an argon atmosphere.
The reaction was carried out at room temperature with vigorous
stirring. Precipitation of a green solid started almost immediately
after mixing the reactants. After stirring the reaction mixture for
12 h, the precipitate thus obtained was filtered, washed with cold
methanol and diethylether, and dried under vacuum. The charac-
teristics of these complexes are given below.
The ESI capillary was set at 3.5 kV and the cone voltage was
40 V. The spectral print outs are the average of 6–8 scans. The 1H
NMR spectra were recorded on a Brucker AMX-400 FT NMR spec-
trophotometer in CDCl3; the chemical shifts are recorded, relative
to SiMe4. The 77Se{1H} NMR spectra were recorded on the same
instrument using Ph2Se2 as an external reference. Infrared spectra
were recorded in the range 4000–400 cmꢀ1 on a Nicolet Megna
750 FTIR spectrophotometer on a KBr disc. Electronic spectra were
obtained with a Shimadzu UV-160A recording spectrophotometer.
2.1.1. Electrochemical measurements
2.3.1. Bis (2-[N-{2-(phenylselenato)ethyl}benzimidoyl]-6-benzoyl-4-
methylphenolato)copper(II) (3) [(C6H2(O)
(4-CH3){PhC@N(CH2)2SePh}PhCO)2.Cu]
Cyclic voltammetric measurements were carried out with an
advanced electrochemical system, PARSTAT 2253 instrument
equipped with a three-electrode system. The micro-cell model
KO264 consisted of a glassy carbon working electrode, Pt wire as
auxiliary electrode and a non-aqueous Ag/Ag+ reference electrode
with 0.1 M AgNO3 in acetonitrile as the filling solution. Tetrabutyl-
ammonium perchlorate (0.1 M solution in CH3CN) was used as the
supporting electrolyte. Cyclic voltammograms with scan speeds of
50–350 mV/s were run in 10ꢀ4 M CH3CN solution in a nitrogen
atmosphere. Under these conditions the ferrocenium/ferrocene
(fc+/fc) couple shows a peak separation of 84 mV.
Colour and state: dark green crystalline solid; yield: 0.42 g
(79%). M.p.165 °C. Anal. Calc. for C58H48N2O4Se2Cu: C, 65.81; H,
4.57; N, 2.64. Found: C, 65.12; H, 4.68; N, 2.51%. Positive ESꢀMS:
m/z 1060 [MH+]. FTIR (selected, cmꢀ1):
m
(C@O) 1656,
(Cu–O) 445. UV–Vis (nm)
e
/Mꢀ1cmꢀ1)(CH3CN): 610 (167), 380 (10,080). 1H NMR (CDCl3, d
m(C@N)
1531,
m(C–O) 1244, m(Cu–N) 578, m
(
ppm): 7.90–7.20 (m, 34H, C6H5 and C6H2), 3.56 (t, 4H, N–CH2),
3.05 (t, 4H, Se–CH2), 2.07 (s, 3H, CH3). KM (10ꢀ3 M, CH3CN,
298 K): 15 S cm2 molꢀ1
.
2.2. Synthesis of Schiff bases 1 and 2
2.3.2. Bis (2-[N-{3-(phenylselenato)propyl}benzimidoyl]-6-benzoyl-4-
methylphenolato)copper(II) (4) [(C6H2(O)(4-CH3)
A solution of 4-methyl-2,6-dibenzoylphenol (mdbpH) (1.58 g,
5.0 mM) in dry acetonitrile (200 mL) was added drop-wise to a
solution of the appropriate phenylselenato(ethyl)amine (2.0 g,
10.0 mM)/phenylselenato(propyl)amine (2.14 g, 10.0 mM) in ace-
tonitrile (800 mL) in a 1:2 molar ratio respectively with stirring
{PhC@N(CH2)3SePh}PhCO)2.Cu)]
Colour and state: light green crystalline solid; yield: 0.29 g,
(54%). M.p.176 °C. Anal. Calc. for C60H52N2O4Se2Cu: C, 66.32; H,
4.82; N, 2.57. Found: C, 66.42; H, 4.61; N, 2.48%. Positive ESꢀMS:
m/z 1088 [MH+]. FTIR (selected, cmꢀ1):
m(C@O) 1657, m(C@N)

V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
2593
Table 1
1532,
M
m
(CꢀO) 1244,
m(Cu–N) 569, m(Cu–O) 445. UV–Vis (nm) (e/
ꢀ1 cmꢀ1) (CH3CN): 615 (170), 389 (10,100). 1H NMR (CDCl3, d
Crystal data and structure refinement for the copper complex 3.
ppm): 7.90–7.20 (m, 34H, C6H5 and C6H2), 3.56 (t, 4H, N–CH2),
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
C58H48CuN2O4Se2
1058.44
173(2)
0.71073
monoclinic
P21/c (No. 14)
2.85 (t, 4H, Se–CH2), 2.19 (q, 4 H, mid-CH2), 2.14 (s, 3H, CH3). KM
(10ꢀ3 M, CH3CN, 298 K): 70 S cm2 molꢀ1
.
2.4. DNA binding studies
Unit cell dimensions
The experiments involving the interaction of the complex with
CT-DNA were carried out in doubly distilled water buffer containing
5.0 mM Tris [Tris(hydroxymethyl)-aminomethane] and 50 mM
NaCl and adjusted to pH 7.2 with hydrochloric acid. Solutions of
CT-DNA gave ratios of absorbance at 260 and 280 nm of about
1.8–1.9, indicating that the DNA was free of protein contamination
[46]. The DNA concentration per nucleotide was determined spec-
trophotometrically by employing a molar absorption coefficient of
6600 Mꢀ1 cmꢀ1 at 260 nm after 1:100 dilution [47]. The complex
was dissolved in 1% DMSO and 99% Tris–HCl buffer (5.0 mM Tris–
HCl, 50 mM NaCl, pH 7.2) at a concentration of 3.0 ꢁ 10ꢀ5 M. An
a (Å)
b (Å)
c (Å)
9.4574(1)
32.4848(7)
15.7356(3)
90
5.842(1)
90
4809.21(15)
4
1.46
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
2.02
2156
Crystal size (mm3)
0.25 ꢁ 0.20 ꢁ 0.05
h Range for data collection (°)
Index ranges
3.45–26.02.
absorption titration was performed on 30 l
mol Lꢀ1 compound by
ꢀ11 6 h 6 11, ꢀ36 6 k 6 40,
ꢀ19 6 l 6 18
32 720
9413 [Rint = 0.060]
6496
varying the concentration of nucleic acid. While measuring the
absorption spectra, an equal amount of CT-DNA was added to both
the compound solution and the reference solution to eliminate the
absorbance of CT-DNA itself. Titration curves were constructed
from the fractional change in absorption intensity as a function of
DNA concentration. The intrinsic binding constant, Kb of the com-
plex with CT-DNA was determined according to the following equa-
Reflections collected
Independent reflections
Reflections with I > 2r(I)
Completeness to h = 26.02°
Tmax. and Tmin.
99.0%
0.7705 and 0.6207
full-matrix least-squares on F2
9413/0/606
Refinement method
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
1.009
Final R indices [I > 2
r(I)]
R1 = 0.050, wR2 = 0.098
R1 = 0.088, wR2 = 0.112
0.99 and ꢀ1.09
tion [48] through a plot of [DNA]/(ea
½DNAꢂ=ðea ef Þ ¼ ½DNAꢂ=ðeb ef Þ þ 1=Kbðeb
where [DNA] is the concentration of DNA in base pairs, the apparent
absorption coefficients ea ef and eb correspond to Aobsd/[Cu], the
ꢀ
e
f) versus [DNA]/(eb
ꢀ
ef)
R indices (all data)
Largest difference peak and hole (e Åꢀ3
)
ꢀ
ꢀ
ꢀ
ef Þ;
ð1Þ
,
extinction coefficient for the free copper complex and the extinction
Table 2
coefficient for the bound copper complex, respectively. In plots of
Selected bond lengths (Å) and bond angles (°) for compound 3.
[DNA]/(ea
the slope to the intercept.
ꢀ
e
f) versus [DNA]/(eb
ꢀ
ef), Kb is given by the ratio of
Bond lengths
Cu–O(3)
1.884(2)
For the cyclic voltammetry, the supporting electrolyte was
50 mM NaCl, 5 mM Tris, pH 7.2 and an aqueous Ag/AgCl reference
electrode with 3 M NaCl in saturated AgCl as the filling solution
was used.
Cu–O(1)
Cu–N(1)
Cu–N(2)
1.913(2)
1.973(3)
1.974(3)
1.925(4)
1.938(4)
1.913(4)
1.949(4)
1.311(4)
1.217(4)
1.315(4)
1.220(4)
1.298(4)
1.479(4)
1.298(4)
1.485(4)
Se(1)–C(24)
Se(1)–C(23)
Se(2)–C(53)
Se(2)–C(52)
O(1)–C(1)
O(2)–C(7)
O(3)–C(30)
O(4)–C(36)
N(1)–C(15)
N(1)–C(22)
N(2)–C(44)
N(2)–C(51)
2.5. X-ray structure determination
Crystals of the complex 3 were grown as dark green needles by
slow evaporation from the chloroform solution. Data were col-
lected on a Nonius-Kappa CCD diffractometer and the structure
solution and refinements were made by SHELXL-97 [49], as incorpo-
rated in the package WINGX [50]. Absorption corrections were made
by MULTISCAN [51]. Non-hydrogen atoms were anisotropic and
hydrogen atom positions were included in the riding mode. The
ORTEP-3 program [52] was used to prepare the molecular drawings.
Data collection parameters are given in Table 1. Selected bond
lengths and angles are listed in Table 2.
Bond angles
O(3)–Cu–O(1)
O(3)–Cu–N(1)
O(1)–Cu–N(1)
O(3)–Cu–N(2)
O(1)–Cu–N(2)
N(1)–Cu–N(2)
C(22)–N(1)–Cu
C(51)–N(2)–Cu
167.38(11)
88.23(10)
89.35(11)
92.27(11)
93.35(11)
164.50(12)
116.8(2)
3. Results and discussion
112.6(2)
3.1. Ligand syntheses and characterization
The monoprotic hybrid macroacyclic ligands 1 and 2 were syn-
thesized via a one step dipodal condensation of 4-methyl-2,6-dib-
enzoylphenol with the appropriate phenylselenato(alkyl)amine in
a 1:2 molar ratio in acetonitrile (Scheme 1).
The condensation product is dependent on the reaction condi-
tions. In a concentrated solution of reactants, only one carbonyl
group of phenol condenses with the amine group of the phenylsel-
enato(alkyl)amine to give rise to open-chain end-off unsymmetri-
cal (SeNO2) ligands, whereas in dilute solution both arms condense
to yield symmetrical (Se2N2O) ligands in high yield along with
some one arm condensed products. The open-chain end-off
unsymmetrical (SeNO2) type ligands are less soluble in the solvent,
and therefore separate out first as a yellow solid powder. However
following the experimental conditions, open-chain end-off

2594
V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
1:2
[2,6-{PhSe(CH2)2N=CPh}2C6H2(4-Me)(OH)]
H2N(CH2)2SePh
BpebmpH, 1
(2,6-PhCO)2(4-Me)C6H2OH
1:2
[2,6-{PhSe(CH2)3N=CPh}2C6H2(4-Me)(OH)]
H2N(CH2)3SePh
BppbmpH, 2
Scheme 1. Synthesis of ligands 1 and 2.
symmetrical (Se2N2O) macroacyclic ligands (1) and (2) were ob-
tained in the pure form. Since the ligands (1) and (2) are obtained
in good yield, after chromatographic workup, only these have been
used for further characterization and complexation reactions.
The elemental analyses agreed well with their formulation. The
ESI mass spectra of the ligands show the molecular ion peak along
with typical isotopic patterns for selenium containing peaks. The
molecular ion peak of these ligands corresponds to the condensa-
tion product of the precursors, 4-methyl-2,6-dibenzoylphenol with
phenylselenato(ethyl)amine and phenylselenato(propyl)amine in a
1:2 molar ratio, respectively. The IR spectra of both ligands (1) and
(2) show no N–H stretching frequency, suggesting that both the
show a molecular ion peak with the typical isotopic patterns for
selenium containing peaks.
In the infrared spectra of the Cu(II) complexes the absence of a
m
(OH) absorption band suggests the combination of the phenolic
proton with the CH3COOꢀ ion of Cu(OCOCH3)2H2O, and the linkage
of the phenolic O with the Cu(II) ion. The combination of the phe-
nolic proton of the ligand with the acetate group of Cu(OCOCH3)2-
H2O resulted in the release of water and the formation of
CH3COOH. The acid thus liberated probably catalyzes the hydroly-
sis of one arm of the ligand at the C=N position. Further, the m(C@N)
vibration is shifted to a lower frequency by ꢃ50 cmꢀ1, suggesting
the coordination of the azomethine N with Cu(II). However, the
nitrogens are tertiary. The bands at 1597 cmꢀ1 in
1581 cmꢀ1 in 2 are attributable to
(C@N). In addition, another
band of low intensity appears in both the ligands at 1662 cmꢀ1
which may be due to (C@O). The appearance of this band may
1
and
m(C@O) band is found to be at almost the same position as in the
m
precursor 4-methyl-2,6-dibenzoylphenol. This has been attributed
to the non-involvement of the benzoyl O in coordination with the
Cu(II) ion. Medium intensity bands, one in the 569–578 cmꢀ1 re-
gion and one at 445 cmꢀ1, for both complexes can be assigned to
,
m
be due to presence of ethylacetate in traces, which was used as
an eluent during the purification of the ligands by column chroma-
tography. Further, a broad band appearing around 3450 in 1 and
m
(M–N) and
The paramagnetic square planar Cu(II) complexes are expected
to show three allowed absorption bands in the electronic spectra
corresponding to the transitions: eg ? b1g (dxz, dyz ? dx2 y2),
m(M–O), respectively [53,54].
3416 cmꢀ1 in 2 due to
intramolecular hydrogen bonding between the phenolic hydrogen
and azomethine nitrogen.
m
(OꢀH) strongly suggests the presence of
ꢀ
a1g ? b1g (dz2 ? dx2ꢀy2), b2g ? b1g (dxy ? dx2ꢀy2) [55]. In the pres-
Intramolecular hydrogen bonding is also confirmed by signifi-
cant deshielding of the phenolic (OH) proton signal by the order
of 3.7 ppm as compared to 4-methyl-2,6-dibenzoylphenol (appears
at d 12.2 ppm) in their 1H NMR spectra. The triplets due to N–CH2
and Se–CH2 protons in both ligands are appreciably shifted
downfield as compared to the precursors 2- phenylselena-
to(ethyl)amine (d 3.1 (t, 4H, N–CH2), 2.9 (t, 4H, Se–CH2)) and
phenylselenato(propyl)amine (d 2.9 (t, N–CH2), 2.71 (t, Se–CH2)).
The appreciable deshielding in these proton signals along with
the intensity ratios, as well as the disappearance of the –NH2 signal
[d = 1.55 ppm {phenylselenato(ethyl)amine}, d = 1.05 ppm {phe-
nylselenato(propyl)amine}] suggest the condensation of the
benzoyl oxygen of 4-methyl-2,6-dibenzoylphenol with the –NH2
group of phenylselenato(alkyl)amine in a 1:2 molar ratio.
Structurally, this ligand framework is interesting as the pres-
ence of hetero (N, O, Se) donor sites could make it practical to
design molecular complexes with a variety of coordination modes,
depending on the nature of the central metal atom.
ent investigation the Cu(II) complexes in CH3CN show only one
band in the 610–615 nm region before the charge transfer band
takes over at higher energies. This band seems to have a major con-
tribution from the eg ? b1g transition, characteristic of a distorted
square planar geometry [56].
The 1H NMR spectra of the Cu(II) complexes do not show any
phenolic proton signal, suggesting its displacement and coordina-
tion of the phenolic oxygen with Cu(II). Thus, based on the linkages
in 3 and 4, the partially hydrolyzed ligand molecules behave in a
bidentate fashion and two molecules coordinate through the azo-
methine N and phenolic O atoms to one Cu(II) ion to give a N2O2
coordination sphere and forming two stable six membered rings
around the Cu(II) ion. These binding modes are also confirmed by
the X-ray crystal structure of complex 3.
CH
3
3.2. Synthesis and properties of complexes 3 and 4
O
O
N
The reactivity of the selenium bearing ligands towards
Cu(OCOCH3)2H2O has been studied by reacting them in a 2:1
molar ratio. The products obtained are found to have the composi-
tion [(C6H2(O)(CH3){(C6H5)C@N(CH2)nSe(C6H5)}{(C6H5)C@O})2Cu];
n = 2, 3. The analytical data surprisingly suggests that one arm of
the ligand is hydrolyzed at the C@N position, resulting in the re-
lease of the PhSe(CH2)nNH2 moiety, whilst the other arm remains
intact. The Cu(II) compounds are soluble in polar as well as non-
polar solvents and are non-electrolytes.
Se
N
Cu
Se
O
O
The mass spectra of the Cu(II) complexes exhibit a molecular
ion peak corresponding to a mass of double the partially hydro-
lyzed ligand plus one Cu(II) atom, suggesting the formation of
complexes bearing a 2:1 ligand to metal stoichiometry. The ESIMS
CH
3
4

V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
2595
3.3. Crystal structure of complex 3
3.4. Cyclic voltammetry
The molecular structure of complex 3 is shown in Fig. 1. The two
monoanionic ligands are bidentate, coordinating through the
imino nitrogen and phenolic oxygen atoms, with both the nitrogen
donors and the phenolic oxygen donors being trans. The copper
coordination sphere is almost square planar, slightly distorted to-
wards tetrahedral [O–Cu–O 167(11)°, N–Cu–N 164(12)°]. The Se
atoms are not coordinated to Cu [Cu–Se 4.05, 4.82 Å]; they are
on one side of the molecule and the phenol fragments are on the
other side. There is no symmetry in the molecule as a whole. Both
cis and trans complexes with the CuO2N2 core are known. The
configuration is probably determined mainly by intraligand
constraints e.g. chains linking N atoms or by steric interactions be-
tween the ligands. The latter effect probably accounts for the trans
configuration here. The phenolic C–O distances in the complex
[1.311(4) and 1.315(4) Å] are significantly shorter than that in
mdbpH [1.360(4) Å] [43]. The average Cu–O and Cu–N bond
lengths [1.898(2) and 1.973(3) Å, respectively] are similar to those
in other trans-Schiff base complexes [57,58]. It is possible that the
bond lengths in the cis-complexes are slightly longer [54] [Cu–N
1.964(2)–1.974(2) Å, Cu–O 1.926(2)–1.938(2) Å]. The Cu–N bonds
in imino complexes, or where the rings are saturated, are longer
[59–62]. One chelate ring, containing NO, is almost planar, with
Cu slightly above the mean plane; the other ring is in a distorted
boat configuration. The benzoyl O does not coordinate to the cop-
per because the large Cu–N–CH2CH2SePh angle directs the ligand
away from it [Cu–N1–C22 116.8(2)° and Cu–N2–C51 112.6(2)°,
both wider than tetrahedral angles].
The electrochemical properties of the ligands and complexes
were investigated by cyclic voltammograms (CV) in 0.1 M [NBu4][-
ClO4] in CH3CN solution with 100–500 mV/s scan rates. All CV data
were collected under a nitrogen atmosphere and potentials are re-
ported with reference to Ag/0.1 M Ag+. The results are summarized
in Tables 3 and 4. The cyclic voltammograms of the ligands
bpebmpH 1 and bppbmpH 2 (Fig. 2) exhibit an oxidation peak at
Table 4
CV data of a 0.1 mM solution of the complexes 3 and 4 in CH3CN/0.1 M NBu4ClO4 at a
platinum electrode vs. Ag/0.1 M AgNO3 at different scan rates.
Complex
Scan Rate
Epa(V)
ipa
(
lA)
Epc(V)
ipc (lA)
Complex 3
100
200
300
400
500
100
200
300
400
500
ꢀ0.962
ꢀ0.374
ꢀ0.386
ꢀ0.374
ꢀ0.374
ꢀ0.872
ꢀ0.326
ꢀ0.296
ꢀ0.278
ꢀ0.254
5.585
15.27
22.49
30.56
42.78
5.109
20.54
29.10
40.23
51.40
ꢀ1.382
ꢀ1.382
ꢀ1.412
ꢀ1.466
ꢀ1.466
ꢀ1.400
ꢀ1.562
ꢀ1.574
ꢀ1.598
ꢀ1.616
ꢀ26.20
ꢀ49.54
ꢀ73.99
ꢀ99.47
ꢀ119.70
ꢀ18.98
ꢀ62.19
ꢀ79.46
ꢀ89.64
ꢀ108.10
Complex 4
5.0E-05
4.0E-05
3.0E-05
2.0E-05
1.0E-05
0.0E+00
-1.0E-05
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Potential (V)
(a)
1.4E-05
1.2E-05
1.0E-05
8.0E-06
6.0E-06
4.0E-06
2.0E-06
0.0E+00
-2.0E-06
-4.0E-06
Fig. 1. Molecular structure of complex 3.
Table 3
CV data of a 0.1 mM solution of the Schiff bases, bpebmpH 1 and bppbmpH 2 in
CH3CN/0.1 M NBu4ClO4 at a platinum electrode vs. Ag/0.1 M AgNO3 at different scan
rates.
Compound
Scan Rate
Epa (V)
ipa (lA)
bpebmpH 1
100
200
300
400
500
100
200
300
400
500
0.760
0.769
0.784
0.787
0.793
0.559
0.454
0.589
0.601
0.634
28.40
40.61
49.45
56.81
62.83
1.570
5.344
6.399
7.681
9.366
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Potential (V)
bppbmpH 2
(b)
Fig. 2. Cyclic voltammograms of 0.1 mM solutions of the Schiff base bpebmpH 1 (a)
and bppbmpH 2 (b) in CH3CN/0.1 M NBu4ClO4 at a platinum electrode vs. Ag/0.1 M
Ag+ at 100 mV/s scan rate.

2596
V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
2.0E-05
1.0E-05
0.0E+00
-1.0E-05
-2.0E-05
-3.0E-05
-4.0E-05
-5.0E-05
-3
-2
-1
0
1
2
Potential (V)
(a)
3.0E-05
2.0E-05
1.0E-05
0.0E+00
-1.0E-05
-2.0E-05
-3.0E-05
-4.0E-05
-5.0E-05
-6.0E-05
-7.0E-05
-8.0E-05
Fig. 5. Cyclic voltammograms of complex 3 (30
l
M) in the absence (A) and the
presence (B) of CT-DNA (30
100 mV/s.
lM) in 50 mM NaCl, 5 mM Tris, pH 7.2. Scan rate,
Epa 0.778 and 0.567 V, respectively. The oxidation of 1 and 2 is
electrochemically irreversible in spite of the strong intramolecular
O–H. . .N hydrogen bond involving the phenol O and the imine N
atoms. In the cyclic voltammograms of complexes 3 and 4
(Fig. 3), there is one oxidation peak (Epa ꢀ0.494 (3), ꢀ0.405 V (4))
and one reduction peak (Epc ꢀ1.422 (3), ꢀ1.550 V (4)). The reduc-
tion peaks for the complexes are due to CuII changing to CuI. The
peak separation and current ratio between the related cathodic
and anodic waves change with scan rate, indicating a quasi-revers-
ible process. Free Cu(CH3COO)2ꢄH2O shows a different reduction
-3
-2
-1
0
1
2
Potential (V)
(b)
Fig. 3. Cyclic voltammograms of a 0.1 mM solution of complex 3 (a) and complex 4
(b) in CH3CN/0.1 M NBu4ClO4 at
200 mV/s scan rate.
a platinum electrode vs. Ag/0.1 M AgNO3 at
Fig. 4. Absorption spectra of complex 3 (30
of DNA with the Cu(II) complex 3.
lM) at 7.2 pH in the presence of increasing amounts of CT-DNA (0ꢀ80 lM). Inset: plot of [DNA]/(ea ꢀ ef) vs. [DNA] for the titration

V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
2597
Table 5
Cyclic voltammetric behaviour of complex 3 in the presence of CT-DNA.
Scan rate (mV/s)
100
R
Epa/V (ipa)/
l
A
Epc/V (ipc)/
l
A
D
Ep (mV)
E°0 (V)
ipa/ipc
0
0.192 (13.75)
0.224 (27.95)
0.192 (27.66)
0.196 (32.21)
0.196 (32.31)
0.200 (24.52)
0.240 (71.87)
0.236 (62.85)
0.228 (56.41)
0.256 (88.62)
ꢀ0.068 (ꢀ22.34)
ꢀ0.060 (ꢀ46.10)
ꢀ0.032 (ꢀ42.63)
ꢀ0.028 (ꢀ44.81)
ꢀ0.032 (ꢀ45.03)
ꢀ0.076 (ꢀ33.31)
ꢀ0.056 (ꢀ56.65)
ꢀ0.064 (ꢀ67.44)
ꢀ0.056 (ꢀ67.50)
ꢀ0.052 (ꢀ65.38)
260
284
224
224
228
276
296
300
284
308
0.062
0.082
0.080
0.084
0.082
0.062
0.092
0.086
0.086
0.102
0.61
0.61
0.64
0.71
0.71
0.73
1.26
0.93
0.84
1.35
30
60
90
120
0
30
60
90
120
200
D
Ep = (Epa ꢀ Epc); E°0 = 1/2(Epa + Epc); complex 30
l
M; R = [DNA]/[complex].
Supporting electrolyte: 50 mM NaCl + 5 mM Tris, pH 7.2; working electrode: Pt; reference electrode: Ag/AgCl.
peak at ꢀ0.707 V under similar conditions. The anodic peak poten-
tials of the complexes are shifted to more negative values than that
observed in the case of the ligand, which is in further conformity
with the formation of a complex between the ligand and the Cu
metal ion [63].
was maintained or even improved under these conditions. The
apparent E°0 shifted to more positive potentials by 20 mV in the
presence of DNA. The peak current for reduction of the Cu(II) spe-
cies, ipc, decreased to 51% of that in the absence of DNA, for a solu-
tion with R = 30 (100 mV/s). The peak current for oxidation did not
decrease under similar conditions. The ratio ipa/ipc was 0.61–0.71
with increasing R. Among the three kinds of binding modes for
small molecules to DNA, Bard and coworkers [67] reported that if
E°0 shifted to a more negative value when small molecules inter-
acted with DNA, the interaction mode was electrostatic binding.
On the contrary, if E°0 shifted to a more positive value, the interac-
tion mode was intercalative binding. Thus, the observed results
suggest that complex 3 interacts with DNA through an intercala-
tive mode that involves a stacking interaction between the aro-
matic chromophore of the complex and the base pair of DNA.
3.5. DNA binding properties of compound 3
3.5.1. Absorption spectral studies
The application of electronic absorption spectroscopy is one of
the most useful techniques for DNA binding studies [64]. Complex
binding with DNA through intercalation usually results in
hypochromism and bathochromism, due to the intercalative mode
involving a strong stacking interaction between an aromatic chro-
mophore and the DNA base pairs. The extent of the hypochromism
commonly parallels the intercalative binding strength. The elec-
tronic absorption spectrum of complex 3 in 5 mM Tris–HCl/
50 mM NaCl buffer exhibits bands around 408 and 264 nm. The
band at 408 nm, arising from ligand-based transition, was used
for the absorption spectral titration with CT-DNA. On incremental
addition of DNA, the band shows only a small (6.0%) decrease in
absorption accompanied by a small blue shift of 4 nm (Fig. 4).
The binding constant Kb for the complex is found to be
1.46 ꢁ 104 Mꢀ1. These features are equivalent to those observed
for the square planar Cu(II) complex Cu(o-VANAHE)2 (o-VANA-
HE = 2(o-vanillinamine)-1-hydroxy-ethane; a bidentate Schiff base
with a trans-CuN2O2 core) (Kb = 7.3 ꢁ 104 Mꢀ1) [65]. The results
suggest an association of the compound with DNA and it is also
likely that this complex binds to the helix by intercalation [66].
4. Conclusions
In summary, two phenol based ligands 1 and 2 have been syn-
thesized by dipodal condensation of 4-methyl-2,6-dibenzoylphe-
nol with phenylselenato(alkyl)amine in acetonitrile in
a 1:2
molar ratio respectively. The reaction of these ligands with Cu(II)
metal ion revealed the cleavage of one arm of these ligands due
to hydrolysis at the C@N position, leading to the release of the
PhSe(CH2)nNH2 (where n = 2, 3) moiety and the formation of a ben-
zoyl group at one end. The partially hydrolyzed moiety of the li-
gands then forms complexes 3 and 4, bearing a 2:1 ligand to
metal stoichiometry in which two monoanionic ligands are biden-
tate coordinating through the azomethine N and phenolic O to one
Cu(II) to give a N2O2 coordination sphere. Thus the phenolic proton
of the ligand molecules is found to combine with the CH3COOꢀ
group of Cu(OCOCH3)H2O, liberating CH3COOH. The structure of
complex 3 revealed that the N2O2-donor sets of the two ligand
fragments form a trans-distorted square planar configuration.
3.5.2. Electrochemical studies
The cyclic voltammetric technique has been employed to study
the interaction of the redox active Cu(II) complex with DNA in or-
der to confirm the DNA bonding modes, suggested by the spectral
studies. Typical CV behaviour of the copper complex in the absence
(curve A) and presence (curve B) of CT-DNA is shown in Fig. 5.
Summaries of the voltammetric results are given in Table 5. The
Complex
3 shows binding with the helix of CT-DNA by
intercalation.
cyclic voltammogram of complex 3 (m = 100 mV/s) in the absence
5. Supplementary data
of DNA featured the copper reduction of +2 to +1 at a cathodic peak
potential, Epc, of ꢀ0.068 V versus Ag/AgCl. Reoxidation of the +1
state occurred upon scan reversal at 0.192 V. The separation of
CCDC 706265 contains the supplementary crystallographic data
for complex 3. 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.
the anodic and the cathodic peak potentials,
DEp, 260 mV (Table
5), indicates a quasi-reversible 1ꢀeꢀ redox process. The formal po-
tential, E°0 (or voltammetric E½), taken as the average of Epc and
Epa, was 0.062 V, in the absence of DNA. In presence of 30 lM
DNA, at the same concentration of the complex, Epc = ꢀ0.060 V
and Epa = 0.224 V. Thus, both the anodic and cathodic peak poten-
tials shifted to more positive values versus a solution without DNA
Acknowledgements
We are grateful to the DRDO, New Delhi for financial assistance
and for the award of a fellowship to A.A. We thank Dr. J. D. Smith,
University of Sussex, UK and Prof. B.L. Khandelwal of DIMAT,
(E°0 = 0.082 V). The value of
D
Ep in the presence of DNA was ca.
240 mV, showing that reversibility of the electron transfer process

2598
V.K. Verma et al. / Polyhedron 28 (2009) 2591–2598
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