Copper(II) complexes of hydroxyflavone derivatives as potential bioactive molecule to combat antioxidants: Synthesis, characterization and pharmacological activities

Article abstract of DOI:10.1002/aoc.1831

A variety of novel copper complexes were synthesized and characterized of the formulae [Cu(L¹)(OAc)], [CuL²(H₂O)], [CuL³(H₂O)], [CuL⁴(OAc)], [CuL ⁵(H₂O)] [CuL⁶

Full text of DOI:10.1002/aoc.1831

Full Paper
Received: 1 April 2011
Revised: 24 June 2011
Accepted: 25 June 2011
Published online in Wiley Online Library: 10 August 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1831
Copper(II) complexes of hydroxyflavone
derivatives as potential bioactive molecule
to combat antioxidants: synthesis, characteri-
zation and pharmacological activities
K. Nagashria, J. Josepha∗ and C. Justin Dhanarajb
A variety of novel copper complexes were synthesized and characterized of the formulae [Cu(L1)(OAc)], [CuL2(H2O)],
[CuL3(H2O)], [CuL4(OAc)], [CuL5(H2O)] [CuL6], [CuL7], [CuL8](OAc) and [CuL9], where L1 –L9 represents Schiff base ligands
[derived by the condensation of 5-hydroxyflavone with 4-aminoantipyrine (L1), o-aminophenol (L2), o-aminobenzoic acid (L3),
o-aminothiazole (L4), thiosemicarbazide (L5), 4-aminoantipyrine-o-aminophenol (L6), 4-aminoantipyrine-o-aminobenzoic acid
(L7), 4-aminoantipyrine-o-aminothiazole(L8) and 4-aminoantipyrine-thiosemicarbazide (L9)]. The spectral and magnetic results
of the Cu(II) complexes exhibit square planar geometry. The DNA binding properties of copper complexes were studied by
using electronic absorption spectra, viscosity and thermal denaturation experiments. The results show that the complexes were
interacting with calf thymus (CT DNA). The in vitro antimicrobial activities of the investigated compounds were tested against
the bacterial species and fungal species. Superoxide dismutase and antioxidant activities of the copper complexes have also
c
been studied. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: screening; antioxidant; inhibition; superoxide dismutase; thermal denaturation
Introduction
of allergies, hypertension, inflammation and for bacterial and
HIV infections. Likewise, the unusual antitumour activity of 2-
(4-aminophenyl) benzothiazole was originally discovered in a
programme of screening for tyrosine kinase inhibitors. Since this
discovery, analogues based on the aminothiazole scaffold have
been synthesized that display superior growth inhibitory prop-
erties. The structures of these compounds show remarkable and
intriguing pharmacological properties and their biological profile
is unlike that of any known biological agent. Flavones constitute
one of the major classes of naturally occurring products. Synthesis
of flavones and their derivatives has attracted considerable atten-
tion owing to their significant biocidal,[13] pharmaceutical[14] and
antioxidant[15] activities. It has been observed that the presence of
hydroxyl groups at position 5 or 7 is frequently required for higher
biological activities.[16] On the other hand, aminoflavones have
been studied as tyrosine kinase inhibitors[17] and as antimitotic
agents.[18]
Much research indicates that metal complexes have the ability
to bind and nick double-stranded deoxyribonucleic acid (ds-DNA)
underphysiologicalconditions.[1–3] Inaddition,DNAistheprimary
target molecule for most anticancer and antiviral therapies. Thus,
investigations of the interactions of DNA with metal complexes are
basic for designing new types of pharmaceutical molecules.[4,5]
The Schiff bases have been the subject of great interest for a
number of years because of their various chemical and structural
characteristics and also their proved applications as biologically
active molecules.[6] Their complexes are known to be biologically
important and to act as models to understand the structure
of biomolecules and metalloproteins. They also have a variety
of applications, including biological, clinical, analytical and also
industrial purposes.[7] Interest in the study of Schiff bases and
their complexes containing oxygen and nitrogen donor atoms
arises from their significant antifungal activity.[8] Inhibition of
tumour growth was recently demonstrated for some Schiff base
complexes.[9]
Schiff bases of 4-aminoantipyrine and its complexes are
known for their variety of applications in the areas of catalysis,
clinical applications and pharmacology.[10] Antipyrine and its
derivatives possess antibacterial and antitumour activities.[11] New
kinds of chemo-therapeutic agents containing Schiff bases have
gained significant attention among biochemists and, of those,
aminopyrines are commonly administered intravenously to detect
liver disease in clinical treatment.[12]
Coordination of organic compounds with metal, i.e. chelation,
causesdrasticchangeinthebiologicalpropertiesoftheligandand
also the metal moiety. In particular, copper is an important metal
and is extensively used for industrial, agricultural and domestic
∗
Correspondence to: J. Joseph, Department of Chemistry, Noorul Islam Centre
for Higher Education, Kumaracoil-629 180, India.
E-mail: chem joseph@yahoo.co.in
a
Department of Chemistry, Noorul Islam Centre for Higher Education,
Kumaracoil-629 180, India
2-Aminothiazoles are a remarkably versatile group of com-
pounds that have found applications in drug development. For
instance, these compounds have been used for the treatment
b
Department of Chemistry, Anna University Tirunelveli, University VOC College
of Engineering, Thoothukudi, India
c
Appl. Organometal. Chem. 2011, 25, 704–717
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

Copper(II) complexes of hydroxyflavone derivatives
purposes owing to its properties of high electrical conductivity,
chemical stability, plasticity and capacity to form alloys with
many metals. Copper is widespread in the environment, where
its determination is necessary because it is known to be toxic
at higher concentrations and causes dyslexia, hypoglycemia,
gastrointestinal problems and Wilson’s disease.[19,20] Cu(II) is also
involved in the causation and cure of cancer.[21]
To design improved drugs that target the cellular DNA and
to understand the mechanism of action at the molecular level,
we therefore decided to examine the synthesis and biological
properties of Schiff base copper complexes derived from 4-
aminoantipyrine/o-aminothiazoleandotherligands.Furthermore,
we have tested the antimicrobial activity of the synthesized
ligands and their complexes using bacteria such as Staphylococcus
aureus, Escherichia coli, Klebsiella pneumaniae, Proteus vulgaris
and Pseudomonas aeruginosa and fungi such as Aspergillus niger,
Rhizopus stolonifer, Aspergillus flavus, Rhizoctonia bataicola and
Candida albicans. Superoxide dismutase and antioxidant activities
of copper complexes have also been measured and compared.
molar absorption coefficient was taken as 6600 M−1 cm−1. Stock
solutions were kept at 4 ◦C and used after not more than 4 days.
CT DNA was purchased from Himedia Chemicals Co. Ltd.
Preparation of Ligands L1 –L5
5-Hydroxyflavone was synthesized from 2,6-dihydroxyaceto-
phenoneaccordingtothemethodofLookeret al.[22] andidentified
by elemental analysis, melting point, IR and UV spectrum.
Equimolaramountsof5-hydroxyflavoneand4-aminoantipyrine
(L1), o-aminophenol (L2), o-aminobenzoic acid (L3), o-amino-
thiazole (L4) and thiosemicarbazide (L5) were dissolved in ethanol
(40 ml). Aceticacid(1.0 ml)wasaddedtothissolution. Thesolution
was stirred for 3 h and precipitates formed. The precipitate was
filtered and washed with water and ethanol.
L1
Yield: 76%. Anal. calcd for C26H21N3O3: C, 73.74; H, 4.99; N, 9.90.
Found: C, 73.68; H, 4.91; N, 9.82. FAB mass spectrometry (FAB-MS):
m/z 424 [M + 1]. 1H-NMR (400 MHz, CDCl3, δ, ppm): 3.0 (1H, s, 3-H),
6.4–7.5 (13H, m, Ar–H), 1.5 (3H, s, H3C–C), 1.8 (s, 3H, H3C–N) and
11.2 (1H, s, O–H, D2O exchangeable). 13C-NMR (400 MHz, CDCl3,
ppm): 10.1 (H3C–C), 18.9 (H3C–N), 142.2 (H3C–C), 164.2 (C O),
152.8 ( C–N), 122.5 (C-2), 116.8 (C-3), 153.9 (C-4), 117.7 (C-5),
145.8 (C-6), 122.5 (C-7), 125.8 (C-8), 156.2 (C-9), 120.1 (C-10), 132.5
(C-1ꢁ), 126.3 (C-2ꢁ, 6ꢁ), 140.6 (C-3ꢁ, 5ꢁ), 132.5 (C-4ꢁ), 125.4 (C-1ꢁꢁ),
118.2 (C-2ꢁꢁ, 6ꢁꢁ), 117.5 (C-3ꢁꢁ, 5ꢁꢁ) and 120.4 (C-4ꢁꢁ).
Experimental
Materials and Methods
The chemicals used were of AnalaR grade. Copper(II) acetate
was obtained from Merck. Micro analytical data (Carlo Erba
1108, accuracy 5%) of the compounds were recorded at the
Regional Sophisticated Instrumentation Center, Central Drug
Research Institute, Lucknow. The amount of copper present in
the copper complexes was estimated using ammonium oxalate
method. The NMR spectra of the ligands were recorded using
Tetramethylsilane (TMS) as internal standard. Chemical shifts (δ)
are expressed in units of parts per million relative to TMS. The
Fast Atom Bombardment (FAB) mass spectra of the ligands and
their complexes were recorded on a Jeol SX 102/DA-6000 mass
spectrometer/data system using argon/xenon (6 kV, 10 mA) as
the FAB gas. The accelerating voltage was 10 kV and the spectra
were recorded at room temperature using m-nitrobenzylalcohol
(NBA) as the matrix. Molar conductance of the copper complexes
was measured in Dimethylsulphoxide (DMSO) solution using a
coronation digital conductivity meter. The IR spectra of the ligands
and their copper complexes were recorded on a Perkin–Elmer
783 spectrophotometer in 4000–200 cm−1 range using a KBr disc.
Electronic spectra were recorded with a Systronics 2201 double
beam UV–vis spectrophotometer in the 200–1100 nm region. The
magnetic susceptibility values were calculated using the relation
µeff = 2.83 (χm.T)1/2. The diamagnetic corrections were made by
Pascal’s constant and Hg[Co(SCN)4] was used as a calibrant. The
ESR spectra of the copper complexes were recorded at 300 and
77 K on a Varian E112 X-band spectrometer. Cyclic voltammetric
measurements were performed using a glassy carbon working
electrode, Pt wire auxiliary electrode and an Ag–AgCl reference
electrode. Tetrabutylammoniumperchlorate was used as the sup-
porting electrolyte. All solutions were purged with N2 for 30 min
prior to each set of experiments. The X-ray diffractometer system
Jeol JDX 8030 was used to record powder data for the copper
complexes, at the Central Elelectrochemical Research Institute,
Karaikudi. Solutions of CT DNA in 50 mM NaCl/5 mM Tris–HCl (pH
= 7.0) gave a ratio of UV absorbance at 260 and 280 nm, A260/A280
of ca. 1.8–1.9, indicating that the DNA was sufficiently free of
protein contamination. The DNA concentration was determined
by the UV absorbance at 260 nm after 1 : 100 dilutions. The
L2
Yield: 60%. Anal. calcd for C21H15NO3: C, 76.58; H, 4.59; N, 4.25.
Found: C, 76.50; H, 4.52; N, 4.18. FAB mass spectrometry (FAB-MS),
m/z330[M+1].1H-NMR(400 MHz,CDCl3)δ:3.1(1H,s,3-H),6.6–7.8
(12H,m,Ar–H)and11.2and10.8(2H,s,O–H,D2Oexchangeable,5-
hydroxyflavone and o-aminophenol moities). 13C-NMR (400 MHz,
CDCl3, ppm): 150.6 (C-2), 102.4 (C-3), 154.6 (C-4), 142.8 (C-5), 144.6
(C-6), 121.4 (C-7), 124.9 (C-8), 154.6 (C-9), 118.8 (C-10), 133.6 (C-1ꢁ),
124.3 (C-2ꢁ, 6ꢁ), 126.5 (C-3ꢁ, 5ꢁ), 126.4 (C-4ꢁ), 130.6 (C-1ꢁꢁ), 115.2
(C-2ꢁꢁ), 120.6 (C-3ꢁꢁ), 119.2 (C-4ꢁꢁ), 126.2 (C-5ꢁꢁ) and 140.8 (C-6ꢁꢁ).
L3
Yield: 65%. Anal. calcd for C22H15NO4: C, 73.94; H, 4.23; N, 3.91.
Found: C, 73.88; H, 4.16; N, 3.88. FAB mass spectrometry (FAB-MS),
m/z358[M+1].1H-NMR(400 MHz,CDCl3)δ:2.9(1H,s,3-H),6.7–7.9
(12H, m, Ar–H), 11.6 and 10.4 (2H, s, O–H, D2O exchangeable,
5-hydroxyflavone and o-aminobenzoic acid moities). 13C-NMR
(400 MHz, CDCl3, ppm): 149.4 (C-2), 110.2 (C-3), 154.5 (C N), 115.6
(C-5), 146.4 (C-6), 122.8 (C-7), 126.4 (C-8), 152.6 (C-9), 119.9 (C-10),
132.5(C-1ꢁ), 125.4(C-2ꢁ, 6ꢁ), 127.6(C-3ꢁ, 5ꢁ), 126.5(C-4ꢁ), 148.6(C-1ꢁꢁ),
113.4 (C-2ꢁꢁ), 132.6 (C-3ꢁꢁ), 116.5 (C-4ꢁꢁ), 128.9 (C-5ꢁꢁ), 140.8 (C-6ꢁꢁ)
and 168.2 (COOH).
L4
Yield: 62%. Anal. calcd for C18H12N2O2S: C, 67.49; H, 3.78; N, 8.75.
Found: C, 67.42; H, 3.72; N, 8.69. Fast atom bombardment mass
spectrometry(FAB-MS), m/z 322[M+ 1]. 1H-NMR(400 MHz, CDCl3)
δ: 2.7 (1H, s, 3-H), 5.1(1H, dd, J, 10.5 Hz, -CH CH-) and 5.4 (1H,
dd, J, 10.5 Hz, -CH CH-), 5.9–7.9 (8H, m, Ar–H), 12.9 (1H, s, O–H,
D2O exchangeable). 13C-NMR (400 MHz, CDCl3): 149.4 (C-2), 110.2
(C-3), 154.5 (C N), 115.6 (C-5), 147.4 (C-6), 122.6 (C-7), 126.9 (C-8),
154.6 (C-9), 121.9 (C-10), 132.5 (C-1ꢁ), 125.4 (C-2ꢁ, 6ꢁ), 127.6 (C-3ꢁ,
5ꢁ), 126.5 (C-4ꢁ), 158.6 (C-11), 103.4 (C-12) and 148.9 (C-13).
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K. Nagashri, J. Joseph and C. J. Dhanaraj
L5
Yield: 70%. Anal. calcd for C16H13N3O2S: C, 61.72; H, 4.20; N, 13.49.
Found: C, 61.65; H, 4.15; N, 13.40. Fast atom bombardment mass
spectrometry(FAB-MS), m/z 311[M+ 1]. 1H-NMR(400 MHz, CDCl3)
δ: 3.3 (1H, s, 3-H), 6.9–7.8 (8H, m, Ar–H), 11.4 (1H, s, -NH), 8.0 (1H,
s, br, HN of -NH2), 7.5 (1H, s, br, HN of -NH2), 11.5 (1H, s, O–H, D2O
exchangeable). 13C-NMR (400 MHz, CDCl3): 168.2 (C O), 149.4
(C-2), 110.2 (C-3), 154.5 (C N), 115.6 (C-5), 146.9 (C-6), 123.5 (C-7),
124.8 (C-8), 152.6 (C-9), 119.5 (C-10), 132.5 (C-1ꢁ), 125.4 (C-2ꢁ, 6ꢁ),
127.6 (C-3ꢁ, 5ꢁ), 126.5 (C-4ꢁ) and 176.9 (C S).
O
O
N
N
+
NH2
O
OH
O
Preparation of Ligands L6 –L9
Equimolar volumes of 5-hydroxyflavone and 4-aminoantipyrine-
o-aminophenol (L6), o-aminobenzoic acid (L7), o-aminothiazole
(L8) and thiosemicarbazide (L9) were dissolved in ethanol (40 ml).
Acetic acid (1.0 ml) was added to this solution. The solution was
refluxed for 6 h with stirring and a yellow precipitate formed
(Scheme 1). The precipitate was filtered and washed with water
and ethanol.
OH
N
N
O
N
L6
H2N
Yield: 65%. Anal. calcd for C32H26N4O3: C, 74.69; H, 5.09; N, 10.88.
Found: C, 74.62; H, 5.01; N, 10.82. FAB mass spectrometry (FAB-MS):
m/z 515 [M + 1]. 1H-NMR (400 MHz, CDCl3, δ, ppm): 2.9 (1H, s, 3-H),
6.6–7.7 (17H, m, Ar–H), 1.4 (3H, s, H3C–C), 1.7 (3H, s, H3C–N) and
11.0 and 11.4 (2H, s, O–H, D2O exchangeable, 5-hydroxyflavone
and o-aminophenol moieties). 13C-NMR (400 MHz, CDCl3, ppm):
10.1 (H3C–C), 18.9 (H3C–N), 142.1 (H3C–C), 151.6 ( C–N), 154.5
(C N), 122.3 (C-2), 115.8 (C-3), 153.7 (C-4), 116.7 (C-5), 144.7 (C-6),
122.86 (C-7), 125.8 (C-8), 156.1 (C-9), 118.1 (C-10), 132.4 (C-1ꢁ),
127.1 (C-2ꢁ, 6ꢁ), 140.4 (C-3ꢁ, 5ꢁ), 132.5 (C-4ꢁ), 125.2 (C-1ꢁꢁ), 118.0
(C-2ꢁꢁ, 6ꢁꢁ), 117.3 (C-3ꢁꢁ,5ꢁꢁ), 120.0 (C-4ꢁꢁ), 129.6 (C-1ꢁꢁꢁ), 115.2 (C-2ꢁꢁꢁ),
120.6 (C-3ꢁꢁꢁ), 119.1 (C-4ꢁꢁꢁ), 126.1 (C-5ꢁꢁꢁ) and 139.2 (C-6ꢁꢁꢁ).
HO
O
N
OH
N
N
N
HO
L7
Yield: 69%. Anal. calcd for C33H26N4O4: C, 73.04; H, 4.83; N, 10.32.
Found: C, 72.95; H, 4.75; N, 10.26. FAB mass spectrometry (FAB-MS),
m/z543[M+1].1H-NMR(400 MHz,CDCl3)δ:3.1(1H,s,3-H),6.8–7.9
(17H, m, Ar–H), 1.2 (3H, s, H3C–C), 1.8 (s, 3H, H3C–N) and 11.2
and 10.9 (2H, s, O–H, D2O exchangeable, 5-hydroxyflavone and
o-aminobenzoic acid moities). 13C-NMR (400 MHz, CDCl3, ppm):
10.5 (H3C–C), 18.8 (H3C–N), 142.0 (H3C–C), 153.1 ( C–N), 154.5
(C N of o-aminobenzoic acid moiety), 122.5 (C-2), 116.7 (C-3),
153.8 (C-4), 117.8 (C-5), 145.9 (C-6), 122.4 (C-7), 125.7 (C-8), 156.2
(C-9), 120.2 (C-10), 132.4 (C-1ꢁ), 126.2 (C-2ꢁ, 6ꢁ), 140.5 (C-3ꢁ, 5ꢁ), 132.5
(C-4ꢁ), 125.3 (C-1ꢁꢁ), 118.2 (C-2ꢁꢁ, 6ꢁꢁ), 117.4 (C-3ꢁꢁ, 5ꢁꢁ), 120.4 (C-4ꢁꢁ),
147.2 (C-1ꢁꢁꢁ), 113.4 (C-2ꢁꢁꢁ), 132.5 (C-3ꢁꢁꢁ), 116.6 (C-4ꢁꢁꢁ), 128.8 (C-5ꢁꢁꢁ),
139.4 (C-6ꢁꢁꢁ) and 166.4 (COOH).
Scheme 1. Synthesis of Schiff base ligand L1.
(C-5), 146.0 (C-6), 123.4 (C-7), 126.6 (C-8), 156.0 (C-9), 120.4 (C-10),
132.6(C-1ꢁ), 126.8(C-2ꢁ, 6ꢁ), 140.9(C-3ꢁ, 5ꢁ), 132.8(C-4ꢁ), 125.6(C-1ꢁꢁ),
119.1 (C-2ꢁꢁ, 6ꢁꢁ), 118.2 (C-3ꢁꢁ, 5ꢁꢁ), 121.5 (C-4ꢁꢁ), 157.5 (C-11), 102.2
(C-12) and 148.1 (C-13).
L9
Yield: 56%. Anal. calcd for C27H24N6O2S: C, 65.30; H, 4.87; N, 16.92.
Found: C, 65.24; H, 4.83; N, 16.86. Fast atom bombardment mass
spectrometry (FAB-MS), m/z 497 [M + 1]. 1H-NMR (400 MHz, CDCl3,
ppm) δ: 3.3 (1H, s, 3-H), 6.3–7.8 (13H, m, Ar–H), 1.1 (s, 3H, H3C–C),
1.6 (s, 3H, H3C–N) and 12.2 (1H, s, O–H, D2O exchangeable),
(2H, s, -NH2). 13C-NMR (400 MHz, CDCl3, ppm): 10.5 (H3C–C), 18.9
(H3C–N), 142.6 (H3C–C), 151.9 ( C–N), 122.9 (C-2), 117.4 (C-3),
154.2 (C-4), 118.1 (C-5), 146.4 (C-6), 122.9 (C-7), 126.2 (C-8), 156.9
(C-9), 120.8 (C-10), 132.9 (C-1ꢁ), 127.1 (C-2ꢁ, 6ꢁ), 141.5 (C-3ꢁ, 5ꢁ), 133.2
(C-4ꢁ), 125.9 (C-1ꢁꢁ), 118.8 (C-2ꢁꢁ, 6ꢁꢁ), 118.4 (C-3ꢁꢁ, 5ꢁꢁ), 121.6 (C-4ꢁꢁ)
and 169.9 (C S). The proposed structures of ligands are given in
Fig. 1.
L8
Yield: 60%. Anal. calcd for C29H23N5O2S: C, 68.89; H, 4.58; N, 13.85.
Found: C, 68.82; H, 4.52; N, 13.78. Fast atom bombardment mass
spectrometry(FAB-MS), m/z 507[M+ 1]. 1H-NMR(400 MHz, CDCl3)
δ: 3.4 (1H, d, J, 10.5 Hz, -CH CH-), 3.6 (1H, d, J, 10.5 Hz, -CH CH-),
6.2–7.9 (13H, m, Ar–H), 1.5 (3H, s, H3C–C), 2.1 (s, 3H, H3C–N) and
12.9 (1H, s, O–H, D2O exchangeable). 13C-NMR (400 MHz, CDCl3,
ppm): 10.3 (H3C–C), 19.1 (H3C–N), 142.4 (H3C–C), 151.1 ( C–N),
168.2 (-N C–N ), 122.8 (C-2), 116.9 (C-3), 153.9 (C N), 118.1
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Appl. Organometal. Chem. 2011, 25, 704–717

Copper(II) complexes of hydroxyflavone derivatives
Figure 1. The proposed structures of ligands L1 –L5.
Preparation of Copper Complexes of Ligands (L1 –L9)
spectrometry (FAB-MS), m/z 411 [M + 1]. µeff (BM) = 1.92; ꢀm
(mho cm2 mol−1) = 24.
Theligands(0.05 mM)andcopperacetate(0.05 mM)weredissolved
in acetone (20 ml). Under stirring, triethylamine (0.075 mM) was
then dropped to the mixture with caution. After stirring for 4 h
at room temperature, the precipitate was separated by suction
filtration, purified by washing several times with acetone and dried
in vaccum.
Complex of L3
Yield: 79%. Anal. calcd for CuC22H17NO5: C, 60.18; H, 3.91; N, 3.19,
Cu, 14.49. Found: C, 60.14; H, 3.86; N, 3.15; Cu, 14.43. FAB mass
spectrometry (FAB-MS), m/z [M + 1]. µeff (BM) = 2.06; ꢀm (mho
cm2 mol−1) = 22.
Complex of L1
Complex of L4
Yield: 62%. Anal. calcd for CuC28H24N3O5: C, 61.57; H, 4.43; N, 7.70,
Cu, 11.64. Found: C, 61.52; H, 4.37; N, 7.65; Cu, 11.61. FAB mass
spectrometry (FAB-MS), m/z 545 [M + 1]. µeff (BM) = 2.06; ꢀm
(mho cm2 mol−1) = 18.
Yield: 76%. Anal. calcd for CuC20H15N2O4S: C, 54.22; H, 3.42; N,
6.33; Cu, 14.36. Found: C, 74.62; H, 3.35; N, 6.29; Cu, 14.31. FAB
mass spectrometry (FAB-MS), m/z 443 [M + 1]. µeff (BM) = 1.98;
ꢀm (mho cm2 mol−1) = 16.
Complex of L2
Complex of L5
Yield: 74%. Anal. calcd for CuC21H17NO4: C, 61.36; H, 4.17; N, 3.41,
Cu, 15.47. Found: C, 61.31; H,4.15; N, 3.36; Cu, 15.43. FAB mass
Yield: 60%. Anal. calcd for CuC16H15N3O3S: C, 48.90; H, 3.85; N,
10.70; Cu, 16.18. Found: C, 74.62; H, 3.81; N, 10.63; Cu, 16.14. FAB
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K. Nagashri, J. Joseph and C. J. Dhanaraj
mass spectrometry (FAB-MS), m/z 393 [M + 1]. µeff (BM) = 2.06;
ꢀm (mho cm2 mol−1) = 29.
IR Spectra
The IR spectra of the ligands show a ν(C N) peak in the region
1645–1632 cm−1. The IR spectra of all complexes show ν(C N)
bands at 1629–1590 cm−1 [27]
and the ν(C N) bands in the
Complex of L6
,
Yield: 66%. Anal. calcd for CuC32H26N4O3: C, 66.47; H, 4.54; N, 9.69;
Cu, 11.00. Found: C, 66.43; H, 4.51; N, 9.64; Cu, 10.93. FAB mass
spectrometry (FAB-MS), m/z 579 [M + 1]. µeff (BM) = 1.99; ꢀm
(mho cm2 mol−1) = 14.
complexes are shifted to lower energy regions compared with
the free ligands. The shift of this band towards the energy side is
probably caused by an increase in the C N bond order owing to
the coordination of the nitrogen with the copper atom.
However, the spectra of complexes show two characteris-
tic bands at 1630–1602 and 1404–1344 cm−1, attributed to
υasy(COO−) and υsy.(COO−), respectively, indicating the partici-
pation of the carboxylate oxygen atom in the complexes. The
mode of coordination of carboxylate group has often been
deduced from the magnitude of the observed separation be-
tween the υasy(COO−) and υsy.(COO−). The separation value (A)
between υasy(COO−) and υsy.(COO−) in copper complexes was
more than 200 cm−1 (260–216 cm−1), suggesting the coordina-
tion of carboxylate group in copper complexes of the ligands
in a monodentate fashion.[28] The Schiff base ligands display a
band around 844 cm−1, ascribed to υ(C–S–C),[29] which shifts to
lower frequencies in their spectra of copper complexes in the
region 838–832 cm−1, suggesting the coordination of copper
ion through the sulfur atom of thioazole moiety. The band at
3466 cm−1 for υ(OH) in the free ligand disappeared on complexa-
tion, indicating coordination through a deprotonated oxygen.
In the IR spectra of the ligands, two sharp bands at ca. 3436
and 3352 cm−1, probably owing to asymmetric and symmetric
vibrations of the NH2 group, do not undergo any change in
the spectra of the complexes, indicating the noninvolvement
of the NH2 group on coordination. In the spectrum of copper
complexes, the coordination of azomethine nitrogen atom is
furthersubstantiatedbytheobservedpositiveshiftof34–48 cm−1
in the N–N stretching mode in the complexes. Monodentate
coordination of the N–N moiety is reported to increase its
stretching frequency by this amount. The absence of thioamide
bandν(HN–C S)atca.782–813 cm−1 andtheappearanceofnew
band at ca. 610–631 cm−1 confirmed the conversion of ν(C S)
into the ν(C–S) band. The reduction of the thioamide band
ν(N C–SH) observed at ca. 982 cm−1 suggests that coordination
occurs through the S atom. The IR spectra of complexes show new
band owing to the stretching vibrations of > C N–N C < bonds
Complex of L7
Yield: 58%. Anal. calcd for CuC33H26N4O4: C, 65.37; H, 4.33; N, 9.25;
Cu, 10.49. Found: C, 65.32; H, 4.28; N, 9.21; Cu, 10.43. FAB mass
spectrometry (FAB-MS), m/z 606 [M + 1]. µeff (BM) = 1.88; ꢀm
(mho cm2 mol−1) = 20.
Complex of L8
Yield: 76%. Anal. calcd for CuC31H26N5O4S: C, 62.74; H, 4.18; N,
10.10; Cu, 11.46. Found: C, 62.70; H, 4.14; N, 10.03; Cu, 11.41. FAB
mass spectrometry (FAB-MS), m/z 556 [M + 1]. µeff (BM) = 1.98;
ꢀm (mho cm2 mol−1) = 58.
Complex of L9
Yield: 80%. Anal. calcd for CuC27H24N6O2S: C, 57.89; H, 4.32; N,
15.01; Cu, 11.35. Found: C, 74.62; H, 4.27; N, 14.96; Cu, 11.29. FAB
mass spectrometry (FAB-MS), m/z 561 [M + 1]. µeff (BM) = 1.90; ꢀm
(mho cm2 mol−1) = 10. The Schiff base was prepared according to
the following scheme (Scheme 1).
Results and Discussion
All the copper complexes are stable at room temperature,
insoluble in water but soluble in DMSO and MeCN. The isolated
solid complexes of Cu(II) ion with the ligands were subjected
to elemental analyses (C, H and N), IR, magnetic moments,
1
molar conductance, H NMR and ESR to identify their tentative
formulae in a trial to elucidate their molecular structures of copper
complexes. Analytical data of the ligands and their complexes
served as a basis for the determination of their empirical formula.
The synthesized ligands were checked by comparing the TLC with
thestarting materials, whichresultedinasinglespotdifferentfrom
the starting materials. All complexes gave satisfactory elemental
analyses results (as shown in the Experimental section) within the
limits of experimental error. All complexes decomposed above
280 ◦C, indicating their thermal stability.[23–25]
at 1568–1560 cm−1
The band observed in the region 1534–1526 cm−1 is due
to the νC stretching of the aromatic ring system. In all the
.
C
copper–Schiff base complexes, most of the band shifts observed
in the wave number region 1142–980 cm−1 are in agreement with
the structural changes observed in the molecular carbon skeleton
after complexation, which cause some changes in (C–C) bond
lengths. Conclusive evidence of the bonding is also shown by the
observation that new bands in the spectra of all copper complexes
appear in the low frequency regions at 550–516, 504–498 and
486–448 cm−1, characteristic of υ(M–O), υ(M–N) and υ(M–S)
stretching vibrations, respectively, that are not observed in the
spectra of both free ligands.[30] The IR bands at 810–854 and
784–799 cm−1, υ(H2O) of coordinated water, are an indication of
the binding of the water molecule to the copper ion.
Molar Conductance
The molar conductance data for the copper complexes measured
in DMSO solution for the 0.001 M solutions are given in
the Experimental section. The values of complexes, except
[CuL8](OAc), fall in the range of 10–29 ꢁ−1 cm2 mol−1, which
is within the expected range of 1–35 ꢁ−1 cm2 mol−1 for the
complexes to behave as nonelectrolytes.[26] Thus, the complexes
have a nonelectrolytic nature as evidenced by the involvement
of the acetate group in coordination. This result was confirmed
from the chemical analysis of the CH3COO− ion not precipitated
by addition of FeCl3. The [CuL8](OAc) complex shows a 1 : 1
electrolytic nature, which was confirmed by the chemical analysis
of CH3COO− ion precipitated by the addition of FeCl3. This shows
that the complex has a 1 : 1 electrolytic nature as evidenced by the
noninvolvement of the acetate group in coordination.
Electronic Spectra
The electronic absorption spectra of the Schiff base ligands and
their copper complexes in DMSO were recorded at room tem-
perature and the band positions of the absorption maxima;
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 704–717

Copper(II) complexes of hydroxyflavone derivatives
Table 1. Electronic spectral data of ligands and their copper complexes
Sample no.
1
Compound
L1
Solvent
DMSO
Absorption (nm)
Band assignment
Geometry
–
215
312
232
336
248
342
215
308
232
336
248
342
215
308
232
336
248
342
228
342
578
248
342
530
254
348
572
228
342
544
248
342
538
254
348
569
228
342
556
248
342
538
254
348
562
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
INCT
2
3
L2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
–
–
–
–
–
–
–
–
L3
4
L1
5
L2
6
L3
7
L1
8
L2
L3
9
10
[CuL1(OAc)]
2B1g
2B1g
2B1g
2B1g
2B1g
2B1g
2B1g
2B1g
2B1g
→
2A1g
2A1g
2A1g
2A1g
2A1g
2A1g
2A1g
2A1g
2A1g
Square planar
Square planar
Square planar
Square planar
Square planar
Square planar
Square planar
Square planar
Square planar
11
12
13
14
15
16
17
18
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
INCT
INCT
→
INCT
INCT
→
INCT
INCT
→
INCT
INCT
→
INCT
INCT
→
[CuL7]
INCT
INCT
→
[CuL8](OAc)
[CuL9]
INCT
INCT
→
INCT
INCT
→
band assignments and the proposed geometry are listed in
Table 1. The electronic spectra of the ligands and their com-
plexes were recorded in DMSO as a solvent. The absorption
spectrum for L1 shows bands at 225 and 312 nm. These bands
can be attributed to n–π∗ and π –π∗ transitions within the
Schiff base molecule. The electronic spectrum of the corre-
The electronic spectra of all the complexes exhibit bands in the
regions 200–225, 272–332 and 362–390 nm, which may be due
to the π –π∗ transition of the benzenoid/or n–π∗ (COO−), π –π∗
transition of the > C N- chromophore and n–π∗ transition of
the > C N- chromophore, coupled with the secondary band of
the benzene ring, respectively. Further, there were a few sharp
bands observed in the region 233–257 nm in the spectra of the
complexes, which could be assigned as charge transfer bands.
The magnetic susceptibility measurements in the solid state
show that the copper complexes were paramagnetic in nature
at room temperature. The observed magnetic moments of these
sponding [CuL1(OAc)] complex in DMSO reveals a broad band
2
at 558 nm assignable to B1g
→
2A1g transition,[31,32] which is
characteristic of square planar environment around the cop-
per(II) ion. Similar spectral features were assigned for other
complexes.
c
Appl. Organometal. Chem. 2011, 25, 704–717
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

K. Nagashri, J. Joseph and C. J. Dhanaraj
Figure 2. FAB mass spectrum of ligand L1.
complexes are quite close to the values expected for copper(II)
complexes without any metal–metal interaction. The magnetic
susceptibility study of the [CuL1(OAc)] complex gives a magnetic
moment value (µeff) of 2.06 BM at room temperature,[33] which
is normal for mononuclear complexes of magnetically diluted d9
systems with S = 1/2 spin state having square planar structure,
and there is no metal–metal interaction along the axial position
in the complex. A similar magnetic behaviour was observed for
other copper complexes.
nature. This fact was also evident from the absence of the half-field
signal, observed in the spectrum at 1600 G owing to the ms =
2
transitions, ruling out any Cu–Cu interaction.[36] The ESR spectral
data are given in Table 2.
The value of g < 2.3 in the present complex gives a clear
||
indication of the covalent character of the metal–ligand bond
and delocalization of the unpaired electron into the ligand. The
trend of g (2.25) > g (2.04) > ge (2.0036) describes the axial
||
⊥
2
2
symmetry with the unpaired electron residing in the dx
–
y
orbital. For the present Cu(II) complex, the observed g values
are g (2.25) > g (2.04) > ge (2.0036), which suggest that the
1H-NMR Spectra
||
⊥
2
unpairedelectronliesinthedx
–
2 orbital. TheA andA valuesin
y
||
⊥
1
The H and 13C-NMR spectra of ligands were recorded in CDCl3
the order: A (155) > A (38.5) also indicate that the complex has
||
⊥
square planar geometry and the system is axially symmetric.[37]
and are given in the Experimental section. All the protons were
found to be in their expected region.[34] The conclusions drawn
from these studies lend further support to the mode of bonding
discussed in their IR spectra. The number of protons calculated
from the integration curves and those obtained from the values of
the expected CHN analyses agree with each other.
The g and A values for square planar CuN4 chromophore
||
||
are around 2.2 and 200 cm−1 respectively. On replacing the
nitrogen donors by oxygen donors, the g value increases and
||
the A value decreases and thus the g and A values for a CuO4
||
||
||
chromophore are around 2.42 and 145 cm−1, respectively. Several
CuN2O2 chromophores have been shown to possess g and A
||
||
values around 2.25 and 160 cm−1. Thus, all the present complexes
FAB Mass Spectra
exhibit g (2.24–2.26) and A (148–158) values consistent with a
CuN2O2 chromophore.
||
||
Mass spectra provide a vital clue for elucidating the structure of
compounds. The mass spectra of the ligand (L1) and its copper
complex [CuL1(OAc)] were recorded and their stoichiometric com-
positions were compared, as shown in Figs 1 and 2. The intensity
of these peaks reflects the stability and abundance of the ions.[35]
The molecular ion peak for the ligand (L1) is observed at 424 m/z,
whereas its copper complex shows a molecular ion peak at 515
m/z, which confirms the stoichiometry of the copper complexes to
be [CuL1(OAc)] type. Elemental analysis values are in close agree-
mentwiththevaluescalculatedfrommolecularformulaeassigned
to these complexes, which is further supported by the FAB-mass
studies of respective complexes. Similar mass spectral features
were assigned for other ligands and their copper complexes.
From above EPR data, the f values for copper complexes were
determined to be in the range of 142–158 cm.[38] Therefore,
our synthesized copper complexes exhibiting appreciable square
planar distortion are expected to show high superoxide dismutase
(SOD)-like activity.
Molecular orbital coefficients α2 (in-plane σ-bonding), β2 (in-
plane π-bonding) and γ 2 (out-plane π-bonding) were calculated
using eqns (1)–(3).
α2 = −(A /0.036) + (g − 2.0036)
||
||
+ 3/7 (g⊥ − 2.0036) + 0.04
(1)
(2)
(3)
β2 = (g − 2.0036)E/ − 8λα2
||
γ 2 = (g⊥ − 2.0036)E/−2λα2
ESR Spectra
The ESR spectrum of the [CuL1(OAc)] complex was recorded in
DMSO at 300 and 77 K. The spectrum at 300 K shows one intense
absorption band at high field, which is isotropic owing to the
tumbling motion of the molecules. However, this complex in the
frozen state shows four well-resolved peaks with low intensities
in the low-field region and one intense peak in the high-field
region. The magnetic susceptibility value reveals that the copper
complex has a magnetic moment of 2.06 BM corresponding to one
unpaired electron, indicating that the complex is mononuclear in
The α2 value of 0.5 indicates complete covalent bonding, while
that of 1.0 suggests complete ionic bonding. The observed value
of 0.73 for the present complex indicates that the copper complex
has some covalent character. The observed β2 and γ 2 values of
1.24 and 0.74 indicate that there is interaction in the out-of-plane
π-bonding, whereas the in-plane π-bonding is predominantly
ionic. Significant information about the nature of bonding in the
Cu(II) complex can be derived from the relative magnitudes of K
||
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 704–717

Copper(II) complexes of hydroxyflavone derivatives
Table 2. ESR spectral data of the copper complexes
Complex
g
g
giso
A
||
A
⊥
K
||
K
⊥
α2
β2
γ 2
f(g /A )
|| ||
||
⊥
[CuL1(OAc)] at 300 K
[CuL1(OAc)] at 77 K
[CuL2(H2O)] at 300 K
[CuL2(H2O)] at 77 K
[CuL3(H2O)] at 300 K
[CuL3(H2O)] at 77 K
[CuL4(OAc)] at 300 K
[CuL4(OAc)] at 77 K
[CuL5(H2O)] at 300 K
[CuL5(H2O)] at 77 K
[CuL6] at 300 K
2.05
–
2.25
2.24
2.26
2.26
2.26
2.26
2.26
2.26
2.26
2.04
2.06
2.05
2.05
2.05
2.05
2.05
2.05
2.05
155
148
158
158
158
158
158
158
158
39
44
52
52
52
52
52
52
52
0.92
0.86
0.74
0.74
0.74
0.74
0.74
0.74
0.74
0.54
0.43
0.49
0.49
0.49
0.49
0.49
0.49
0.49
0.73
0.76
0.81
0.81
0.81
0.81
0.81
0.81
0.81
1.2
1.4
1.3
1.3
1.3
1.3
1.3
1.3
1.3
0.74
0.70
0.82
0.82
0.82
0.82
0.82
0.82
0.82
145
151
143
143
143
143
143
143
143
2.10
–
2.12
–
2.12
–
2.12
–
2.12
–
[CuL6] at 77 K
[CuL7] at 300 K
[CuL7] at 77 K
2.12
–
[CuL8](OAc) at 300 K
[CuL8](OAc) at 77 K
[CuL9] at 300 K
2.12
–
2.12
–
[CuL9] at 77 K
electrolyte (scan rate 100 mV s−1) at 300 K in the potential
range of +0.8 to −0.8 V) was examined. Table 3 summarizes
the potentials and their assignments, which mainly depend on
the geometry and environment around the copper ion (i.e. ligand
core). The electrochemical data are given in Table 3. Reduction at
negative potential is the usual trend observed for phenoxo copper
complexes because of the electronegativity and hard nature of
the phenoxide atoms in the ligand. The cyclic voltammogram of
[CuL1(OAc)] complex in DMSO solution at 300 K in the potential
range +0.6 to −0.8 V at scan rate 0.1 V s−1 was recorded. It shows
a well-defined redox process corresponding to the formation
of the quasi-reversible Cu(II)/Cu(I) couple. The anodic peak at
Epa = −0.658 mV vs Ag–AgCl and the associated cathodic peak
at Epc = −0.766 mV correspond to the Cu(II)/Cu(I) couple. The
[CuL1(OAc)] complex exhibits a quasi-reversible behaviour. It also
shows one irreversible peak at 0.432 mV which was assigned to
Cu(II)/Cu(III). Similar electrochemical behaviour was observed and
assigned for other complexes.
On comparing the cyclic voltammograms, we observed that
the variation in oxidation and reduction potential may be due to
distortion in the geometry of the complexes, which arises owing
to different donor atoms coordinated to the copper ion. It is
concluded that the present ligand systems stabilize the unusual
oxidation states of copper ion during electrolysis. It is essential
for pharmacological activities and plays a crucial role in curing or
prevention of untreatable diseases.
Table 3. Cyclic voltammetric data of copper complexes
Potential
Complex
Epa
Epc
ꢂEp
assignment
[CuL1(OAc)]
−0.658
0.432
−0.766
–
108
–
Cu(II)/Cu(I)
Cu(II)/Cu(III)
Cu(II)/Cu(I)
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
−0.588
1.10
−0.742
–
154
Ligand oxidation
Cu(II)/Cu(I)
−0.628
0.464
−0.768
–
140
–
Cu(II)/Cu(III)
Cu(II)/Cu(I)
−0.600
0.432
−0.766
−0.98
−0.762
−0.92
−0.742
–
166
–
Cu(I)/Cu(0)
−0.650
0.432
112
Cu(II)/Cu(I)
Cu(I)/Cu(0)
−0.632
0.364
110
–
Cu(II)/Cu(I)
Cu(II)/Cu(III)
Cu(II)/Cu(I)
[CuL7]
−0.532
0.282
−0.702
–
170
–
Cu(II)/Cu(III)
Cu(II)/Cu(I)
[CuL8](OAc)
[CuL9]
−0.526
0.486
−0.686
–
160
–
Cu(II)/Cu(III)
Cu(II)/Cu(I)
−0.582
−1.20
−0.658
–
132
–
Ligand oxidation
and K .
⊥
K = α2β2
⊥
(4)
(5)
||
K
= α2γ 2
Antimicrobial Activity
Thecompoundssynthesizedwereevaluatedfortheirantibacterial,
antifungal, DNA binding and antioxidant studies. The antibacterial
and antifungal tests were carried out using the serial dilution
method.
The in vitro antimicrobial activities of the investigated com-
pounds were tested against the bacterial species Staphylococcus
aureus, Escherichia coli, Klebsiella pneumaniae, Proteus vulgaris
and Pseudomonas aeruginosa and fungal species Aspergillus niger,
Rhizopus stolonifer, Aspergillus flavus, Rhizoctonia bataicola and
For the present complex, the observed order K (0.92) >
||
K
(0.54) implies a greater contribution from out-of-plane
π-bonding than from in-plane π-bonding in metal–ligand π-
⊥
bonding.
Cyclic Voltammetry
The electrochemical behaviour of the Schiff base Cu(II) complexes
in DMSO (0.1 M of tetrabutylammoniumperchlorate as supporting
c
Appl. Organometal. Chem. 2011, 25, 704–717
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

K. Nagashri, J. Joseph and C. J. Dhanaraj
Table 4. Minimum inhibitory of concentration of the synthesized compounds against growth of bacteria (µg ml−1
)
Escherichia
coli
Klebsiella
pneumoniae
Proteus
vulgaris
Pseudomonas
aeruginosa
Staphylococcus
aureus
Sample no.
Compound
1
2
L1
L2
60
24
28
60
24
28
60
24
28
34
26
52
34
26
52
34
26
52
10
12
06
08
64
26
36
64
26
36
64
26
36
38
28
54
38
28
54
38
28
54
15
10
14
10
66
20
26
66
20
26
66
20
26
32
30
58
32
30
58
32
30
58
6
66
16
32
66
16
32
66
16
32
28
26
60
28
26
60
28
26
60
12
04
10
06
72
28
30
72
28
30
72
28
30
42
48
63
42
48
63
42
48
63
4
3
L3
4
L4
5
L5
6
L6
7
L7
8
L8
9
L9
10
11
12
13
14
15
16
17
18
19
20
21
22
[CuL1(OAc)]
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
[CuL7]
[CuL8](OAc)
[CuL9]
PenicillinG
Ampicillin
Vancomycin
Ofloxacin
08
12
04
06
08
14
Candida albicans. The minimum inhibitory concentration (MIC)
values of the compounds are summarized in Tables 4 and 5. A
comparative study of the ligands and their complexes (MIC values)
indicates that complexes exhibit higher antimicrobial activity than
the free ligands. In this study, the antimicrobial activity of the
ligands may be due to the heteroaromatic residues. Compounds
containing > C N group have enhanced antimicrobial activity
than > C C < group. The growth of certain microorganisms takes
place even in the absence of O2. Hence, compounds containing
> C C < group though capable of absorbing O2 are not related
with the growth of microorganisms.
The enhanced activity of the complexes can be explained on the
basis of Overtone’s concept[39] and Tweedy’s chelation theory.[40]
According to Overtone’s concept of cell permeability, the lipid
membrane that surrounds the cell favours the passage of only the
lipid-soluble materials, which makes liposolubility an important
factor that controls the antimicrobial activity. On chelation, the
polarity of the metal ion will be reduced to a greater extent
owing to the overlap of the ligand orbital and partial sharing of
the positive charge of the metal ion with donor groups. Further,
it increases the delocalization of π-electrons over the whole
chelate ring and enhances the lipophilicity of the complexes. This
increased lipophilicity enhances the permeation of the complexes
into lipid membranes and blocking of the metal binding sites in
the enzymes of microorganisms. These complexes also disturb the
respiration process of the cell and thus block the synthesis of the
proteins, which restricts further growth of the organism and as a
result microorganisms die.
with the normal cell process.[41] Another mechanism of toxicity of
thesecomplexestomicroorganismsmaybeduetotheinhibitionof
energy production or ATP production,[42] by inhibiting respiration
or by the uncoupling of oxidative phosphorylation. The biological
activity involves inhibition of DNA synthesis[43] by creating lesions
in DNA strands by oxidative rupture and by binding the nitrogen
bases or DNA or RNA, hindering or blocking base replication. The
inhibition growth may be due to the effect on the biosynthesis of
phospholipids in cell membrane and proteins.
The observed variation in the activity of the copper complexes
acrossthevariousclassesoforganismsstudiedmaybeattributable
to differences in cell wall and/or membrane construction (Gram-
positive bacteria, peptidoglycan and teichoic acid; Gram-negative
bacteria, peptidoglycan and liposaccharide). It is expected that
the more extensive heteroaromatic ring system of antipyrinyl and
the presence of the lipophilic group C N would confer greater
lipophilicity on the copper complexes and enable it to penetrate
the cell wall and promote adverse intracellular interactions.
Among the studied compounds, [CuL1(OAc)] (MIC 6 µg ml−1
)
complex presented good activity against Candida albicans. Since
this fungi is very harmful to humans, we consider this result of
major importance. This confirms that antibacterial and antifungal
activities are dependent on the molecular structure of the ligands
and the type of the complex formed. It appears from the above
results that Cu(II) Schiff-base complex may be able to maintain
good antibacterial and antifungal activities and be an effective
antibacterial broad-spectrum drug that may be able to solve some
problems of antibacterial resistance.
The increased activity of the complexes can also be explained
on the basis of their high solubility, fitness of the particles, size of
the metal ion and the presence of the bulkier organic moieties.
Formation of a hydrogen bond through the azomethine group
with the active centre of cell constituents resultd in interference
Interaction of Copper Complexes with Microorganisms
Copper complexes having more biological activitys due to redox
processes could be involved in the observed biological activity.
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Copper(II) complexes of hydroxyflavone derivatives
Table 5. Minimum inhibition of concentration of the synthesized compounds against growth of fungi (µg ml−1
)
Aspergillus
niger
Rhizopus
stolonifer
Aspergillus
flavus
Rhizoctonia
bataicola
Candida
albicans
Sample no.
Compound
1
2
L1
L2
60
72
85
69
88
94
70
84
78
28
32
52
28
32
52
28
32
52
10
12
08
14
66
84
76
79
90
102
81
72
65
30
26
55
30
26
55
30
26
55
16
08
06
10
72
63
69
88
96
110
70
66
76
34
46
68
34
46
68
34
46
68
8
80
77
64
82
70
64
60
56
90
38
36
80
38
36
80
38
36
80
14
06
10
06
50
65
102
86
78
94
99
76
80
32
38
50
32
38
50
32
38
50
12
12
04
12
3
L3
4
L4
5
L5
6
L6
7
L7
8
L8
9
L9
10
11
12
13
14
15
16
17
18
19
20
21
22
[CuL1(OAc)]
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
[CuL7]
[CuL8](OAc)
[CuL9]
Nystatin
Ketoconazole
Clotrimazole
Bavstatin
16
12
08
[CuL1(OAc)] in DMSO was mixed into the culture media with
different microorganisms (1 mM as final concentration) and their
UV–vis spectra were recorded at different times of incubation.
Mostoftheobtainedspectradidnothavethebestqualityowingto
interference of some of the components of the culture media. The
bandof[CuL1(OAc)]inculturemediaatabout410 nmdisappeared
after interacting with the different microorganisms. In the case
of Proteus vulgaris and Pseudomonas aeruginosa, against which
[CuL1(OAc)] was active, this band was still observed at 180 min of
incubation but completely disappeared after 24 h. It is important
topointoutthat[CuL1(OAc)]presentsabandat332 nm, indicating
that the first transformation of the complex occurred immediately
(shift to 410 nm) with a second change at 24 h. The disappearance
of the characteristic band of a Cu(I) complex at 332 nm indicates
that a redox process took place.
Table 6. Superoxide dismutase activity of some copper(II) complexes
Sample no.
Complex
IC50 (mol dm−1
)
1
2
3
4
5
6
7
8
9
[CuL1(OAc)]
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
86
98
90
74
65
72
84
70
69
[CuL7]
[CuL8](OAc)
[CuL9]
From this observation, each organism takes a different in-
cubation time. During that period, a compound interacts with
microorganisms (turbidity was observed) and some redox change
occurs [the complex interacts with the microorganism; the elec-
tronic absorption band is shifted to a lower wavelength and
completely disappears after 24 h (bacteria) and 48 h (fungi)]. The
activity may also be due to the compounds containing lipophilic
groups (azomethine linkage or heteroaromatic nucleus) crossing
the cell membrane of the microorganism and interfering with their
processes, causing growth to be inhibited.
proteins, DNA and other biomolecules. Further, the potential
reduction of the Cu(II)/Cu(I) process is related to the potential
SOD mimetic activity. The synthesized copper complexes also
have reduction potentials greater than −0.6 V, so it is possible
for electron uptake to occur in the biological milieu, followed by
donation to an acceptor.
SOD Activity
The SOD mimetic activities of the copper(II) complexes were
determined and have been compiled in Table 6. In the present
complexes, the higher SOD activity was [CuL7] owing to the
presence of electron-withdrawing substituents compared with
other complexes. A greater interaction between superoxide ion
and Cu(II) complex was induced owing to the stronger axial bond,
resultinginanincreasedcatalyticactivity.Inaddition,thedesigned
ligands containing electron-withdrawing substitutent stabilized
the Cu(I) complex formed during superoxide dismutation reaction,
Electron Transfer as a Possible Mode of Action
In the present study, the observed cyclic voltammetric behaviour
of copper complexes showed that the redox cycle may also be
contribute to their inherent toxicity. For example, redox cycling
between Cu(II) and Cu(I) can catalyse the production of highly
reactive hydroxyl radicals, which can subsequently damage lipids,
c
Appl. Organometal. Chem. 2011, 25, 704–717
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K. Nagashri, J. Joseph and C. J. Dhanaraj
which further reacted with superoxide ion to give hydrogen
peroxide. The distorted geometry of these complexes may favour
the geometrical change, which is essential for catalysis as the
geometry of copper in the SOD enzyme also changes from
distorted square planar geometry. The difference in reactivities of
the synthesized complexes may be attributed to the coordination
environment and the redox potential of the couple Cu+/Cu2+
in copper(II) complexes during the catalytic cycle. It has been
reported that the redox potential of copper (II) ions is affected by
the coordination structure of copper(II) complexes. The nature of
substituent plays a key role in stabilizing the Cu+ oxidation state
during the catalytic cycle.[44,45]
Table 7. DNA binding constant and melting temperature data
Complex
Kb (M−1
)
Tm (◦C)
σT (◦C)
[CuL1(Oac)]
[CuL2(H2O)]
[CuL3(H2O)]
[CuL4(OAc)]
[CuL5(H2O)]
[CuL6]
1.2 × 106
2.4 × 106
1.8 × 106
2.5 × 105
3.6 × 105
1.2 × 105
4.2 × 105
1.4 × 106
2.1 × 106
60
58
69
64
66
72
66
62
56
26
30
29
25
23
31
24
20
18
[CuL7]
[CuL8](OAc)
[CuL9]
DNA Binding Studies
DNA binding studies are important for the rational design
and construction of new and more efficient drugs targeted
to DNA.[46–48] A variety of small molecules interact reversibly
with double-stranded DNA, primarily through three modes:
(i) electrostatic interactions with the negative charged nucleic
sugar–phosphate structure, which are along the external DNA
doublehelixanddonotpossessselectivity;(ii)bindinginteractions
with two grooves of the DNA double helix; and (iii) intercalation
betweenthestackedbasepairsofnativeDNA.Toexplorethemode
of the Cu(II) complex binding to DNA, the following experiments
were carried out.
The absorption spectra of the four complexes in the absence
and presence of DNA are shown in Fig. 3. In the UV region, the
Cu(II) complex exhibits two absorption bands: one at ca. 354 nm
and another at ca. 297 nm. With increasing DNA concentration,
the absorption bands of the complexes were affected, resulting
in a tendency to hypochromism and slight shifts to longer
wavelengths, which indicates that the Cu(II) complex can interact
with DNA. The observed hypochromism and bathochromism for
the Cu(II) complex are large compared with those observed for
potential intercalators. The intrinsic binding constant (Kb) was
obtained by monitoring the change in absorbance with increasing
concentrations of DNA for the Cu(II) complexes (Table 7).
The finding of hypochromism and bathochromism through
spectroscopic titration for metal complexes with planar ligands
has been previously taken as evidence of intercalation,[51]
but such data alone are certainly insufficient to rule out
alternative mechanisms. Thus it becomes necessary to carry out
hydrodynamic measurements such as viscosity. Such experiments
have frequently been used to evaluate structural changes in the
DNA helix by intercalative interaction.[52] This increase in the
viscosity can be therefore be attributed to the enlargement of
the separation between the base pairs, which are pushed apart to
accommodate the intercalating molecule.
Electronic absorption spectroscopy
Electronic absorption spectroscopy is universally employed to
determine the binding characteristics of metal complexes with
DNA. The intercalative mode of binding usually results in
hypochromism and red shift owing to the strong stacking
interaction between an aromatic chromophore and the base
pairs of DNA. The extent of red shift and hypochromism is
commonly found to correlate with the binding strength, but
metal complexes that bind nonintercalatively or electrostatically
with DNA may result in hyperchromism or hypochromism.[49]
Hyperchromic effect and hypochromic effect are the spectral
features of DNA concerning its double-helix structure.[50] This
spectral change process reflects the corresponding changes of
DNA in its conformation and structures after the drug has bound
to DNA. Hypochromism results from the contraction of DNA in the
helix axis, as well as from the change in conformation on DNA,
while hyperchromism results from damage to the DNA double
helix structure.
Viscosity measurements
Hydrodynamic methods, such as determination of viscosity, which
is exquisitely sensitive to the change of length of DNA, may be the
most effective means studying the binding mode of complexes
to DNA in the absence of X-ray crystallographic or NMR structural
data.[53] To further confirm the interaction mode of the Cu(II)
Figure 3. FAB mass spectrum of [CuL1(OAc)2] complex.
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Appl. Organometal. Chem. 2011, 25, 704–717

Copper(II) complexes of hydroxyflavone derivatives
Figure 5. Plot of relative viscosity versus [complex]/[DNA] effect of copper
complex on the viscosity of CT DNA at 25 0.1 ◦C. Copper complex =
0–100 µM. [DNA] = 50 µM.
Figure 4. UV–vis, spectra of copper complex in the absence and in the
presence of DNA.
complex with DNA, a viscosity study was carried out (Fig. 4). The
viscosity measurement is based on the flow rate of a DNA solution
through a capillary viscometer. The specific viscosity contribution
(g) owing to the DNA in the presence of a binding agent was
obtained. The results indicate that the absence and presence of
the metal complex have a marked effect on the viscosity of the
DNA. The specific viscosity of the DNA sample increases obviously
with the addition of the complex. The viscosity studies provide a
strong argument for intercalation. The viscosity increase of DNA
is ascribed to the intercalative binding mode of the drug because
this could cause the effective length of the DNA to increase.[54]
In essence, the length of the linear piece of B-form DNA is given
by the thickness of the base pairs that are stacked along the helix
axis in Van der Waals contact with each other. Introducing another
aromatic molecule into the stack therefore increases the length.
Therefore, the viscosity increase of the DNA caused by the addition
of the complex can provide further support for the intercalative
mode of the Cu(II) complex.
Figure 6. Melting curves of CT DNA in the absence and presence of copper
complex.
Thermal denaturation studies
DNA thermal melting is a measure of the stability of the DNA
double helix with temperature; an increase in the thermal melting
temperature (Tm) indicates an interaction between DNA and the
metal complex. In the present case, thermal melting studies were
carried out at DNA to complex concentration ratios of 25 and
Tm and σT (the temperature range between which 10 and 90%
of the absorption increase occurred) values were determined by
monitoring the absorbance of DNA at 260 nm as a function of
temperature. As shown in Fig. 5, the Tm of DNA in the absence of
any added drug was found to be 60 1 ◦C, under our experimental
conditions. Under the same set of conditions, the presence of
complexes increased the Tm by 4 and 2 ◦C, respectively, and the
values are given in Table 7.
safranin, and so decreased the absorbance of the reaction mixture,
indicating a decrease in hydroxyl radical scavenging ability. The
scavenging ratio for OH· was calculated from the following
expression:
Scavenging ratio (%) = [(Ai − Ao)/(Ac − Ao)] × 100
(6)
where Ai is the absorbance in the presence of the tested
compound; Ao is the absorbance of the tested compound; and
Ac is the absorbance in the absence of the tested compound,
EDTA-Fe(II) and H2O2.
Figure 6 depicts the inhibitory effect of the complexes on
OH•. The inhibitory effect of the complexes is marked and the
suppression ratio increases with increasing concentration in the
range of tested concentration. The order of the suppression ratio
Hydroxyl radial scavenging activity
The hydroxyl radicals (OH•) in aqueous media were generated
through the Fenton system. The hydroxyl radical bleached the
c
Appl. Organometal. Chem. 2011, 25, 704–717
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K. Nagashri, J. Joseph and C. J. Dhanaraj
yieldhighlypotentSODmimics.Theobservedcorrelationbetween
the SOD activity and the redox potential of the Cu+/Cu2+
emphasizestherolesplayedbyelectronicaswellasstereochemical
factors in the biological activities of these complexes. Further
work to investigate this role of copper ions by determining
the lethal dose of the complex in biological systems and their
pharmacological screening is in progress and will be reported in
due course.
Acknowledgement
We express our sincere thanks to the Chancellor, Noorul Islam
Centre for Higher Education, Kumaracoil for providing research
facilities. This work was financially supported by DST, New Delhi
under INSPIRE fellowship (IF 10544).
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Figure 7. Scavenging effect of copper complexes and mannitol on
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