Copper(II) complexes with bioactive carboxyamide: Synthesis, characterization and biological activity

Article abstract of DOI:10.1016/j.saa.2007.09.008

Complexes of Cu(II) with bioactive carboxyamide ligands N′,N″-bis(3-carboxy-1-oxoprop-2-enyl)2-amino-N-arylbenzamidine, N′,N″-bis(3-carboxy-1-oxopropanyl)2-amino-N-arylbenzamidine and N′,N″-bis(3-carboxy-1-oxophenelenyl)2-amino-N-arylbenzamidine have been synthesized and characterized by various physico-chemical techniques. Mass spectrum explains the successive degradation of the molecular species in solution and justifies ML complexes. Vibrational spectra indicate coordination of amide and carboxylate oxygen of the ligands along with water molecules. Electronic spectra and magnetic susceptibility measurements reveal octahedral geometry for Cu(II) complexes. The EPR of the reported complex show g_∥ > g_⊥ > 2.0023 and G value within the range 2.08-4.49 are consistent with d_{x2 - y2} ground state in an octahedral geometry. The voltammogram of the copper(II) complex shows a quasi-reversible redox process and a simple one electron process assignable to the Cu(II)/Cu(I) couple. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first order kinetics. The bio-efficacy of the ligands and its copper complexes have been examined against the growth of bacteria and pathogenic fungi in vitro to evaluate their antimicrobial potential. The results indicate that the ligand and its metal complexes possess notable antimicrobial properties.

Full text of DOI:10.1016/j.saa.2007.09.008

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Spectrochimica Acta Part A 70 (2008) 758–765
Copper(II) complexes with bioactive carboxyamide:
Synthesis, characterization and biological activity
Bibhesh K. Singha,∗, Narendar Bhojakb, Parashuram Mishraa, Bhagwan S. Garga
a Department of Chemistry, University of Delhi, Delhi 110007, India
b Department of Chemistry, Dunger College, University of Bikaner, Bikaner, India
Received 12 June 2007; received in revised form 3 September 2007; accepted 17 September 2007
Abstract
Complexes of Cu(II) with bioactive carboxyamide ligands Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxoprop-2-enyl)2-amino-N-arylbenzamidine, Nꢀ,Nꢀꢀ-
bis(3-carboxy-1-oxopropanyl)2-amino-N-arylbenzamidine and Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)2-amino-N-arylbenzamidine have been
synthesized and characterized by various physico-chemical techniques. Mass spectrum explains the successive degradation of the molecular
species in solution and justifies ML complexes. Vibrational spectra indicate coordination of amide and carboxylate oxygen of the ligands along
with water molecules. Electronic spectra and magnetic susceptibility measurements reveal octahedral geometry for Cu(II) complexes. The EPR
of the reported complex show g > g > 2.0023 and G value within the range 2.08–4.49 are consistent with d
2
2
ground state in an octahedral
ꢁ
⊥
x −y
geometry. The voltammogram of the copper(II) complex shows a quasi-reversible redox process and a simple one electron process assignable to the
Cu(II)/Cu(I) couple. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm
first order kinetics. The bio-efficacy of the ligands and its copper complexes have been examined against the growth of bacteria and pathogenic
fungi in vitro to evaluate their antimicrobial potential. The results indicate that the ligand and its metal complexes possess notable antimicrobial
properties.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Bioactivity; Carboxyamide; Copper(II) complexes; Electro-chemical studies; Spectra; Thermal
1. Introduction
bioactive peptoid exhibit selective, potent antibacterial activ-
ity against both Gram-positive and Gram-negative bacteria [4].
These readily synthesized, protease-resistant peptoids repre-
sents an important advance in peptide biomimicry, namely the
development of nonpeptide foldameric analogues of anti bacte-
rial peptides [4]. The present paper is mostly devoted to the study
of the complexation behaviour of copper(II) ion with newly
synthesized amide ligands containing carboxylic anchors con-
densed with amidines depicted in Fig. 1a–c. The amidine unit is
the key functional group in a wide range of biologically active
molecules [9], including a variety of serine protease inhibitors
[10,11] and antimicrobial agents [12]. The aim of antimicrobial
activity was to assess the effect on microbal activities and to
contribute a better understanding of the antimicrobial activity of
the copper complexes (Fig. 2).
The amide group containing ligand acts as potential model for
naturally occurring and biologically important complexes such
as metal carrier proteins, metalloenzyme or antibiotic peptides
and as an angiotensin I/II receptor antagonists [1–3]. The selec-
tivity of amide moieties in the binding of metal ions strongly
depends on the ease of deprotonation of the amidic group
to form stable chelated compounds. The role of substituents
of the amidic chain appears essential for the geometry and
stability of the complexes formed. There are numerous exam-
ples of the in vivo interactions of metal ions with the amide
group(s), and these interactions can be of biological impor-
tance [4–6]. Copper(II) complexes are also known to play a
significant role either in naturally occurring biological systems
such as transport, storage and activation of molecular oxy-
gen or as pharmacological agents[7,8]. Magainin-2-amide, the
The coordination behaviour was ascertained on the basis of
spectroscopic as well as thermal studies. There is a continuous
interest in investigating the relationship between the redox
reactivity and geometry of polydentate carboxyamide copper
complexes in solid states, which could be the result of steric
effects and electronic effects. It was also thought of considerable
∗
Corresponding author. Tel.: +91 9868359781.
E-mail address: bibheshksingh@yahoo.co.in (B.K. Singh).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.09.008

B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
759
Fig. 1. Structure of ligands (a) (L1), (b) (L2) and (c) (L3).
interest to evaluate their electro-chemical properties. The free
ligands and its copper complexes have been tested in vitro
against Rhizopus sp. and Aspergillus niger fungi and Escherchia
coli or S. aureus bacteria with different concentration, in order
to assess their antimicrobial potential.
porting electrolyte and glassy carbon as the working electrode.
Magnetic susceptibility measurements were carried out at room
temperature in powder form on a vibrating sample magnetome-
ter PAR 155 with 5000G-field strength, using Co [Hg(SCN)4] as
the calibrant (magnetic susceptibility ≈1.644 × 10−5 cm3 g−1).
Rigaku model 8150 thermoanalyser (Thermaflex) was used for
simultaneous recording of TG–DTA curves at a heating rate
of 10 min−1. For TG, the instrument was calibrated using cal-
cium oxalate while for DTA, calibration was done using indium
metal, both of which were supplied along with the instrument.
A flat bed type aluminium crucible was used with ␣-alumina
(99% pure) as the reference material for DTA. The number
of decomposition steps was identified using TG. The activa-
tion energy and Arrhenius constant of the degradation process
was obtained by Coats and Redfern method [13]. The pow-
der diffraction pattern were recorded on a vertical type Philips
1130/00 diffractometer, operated at 40 kV and 50 Ma genera-
2. Experimental
2.1. Material and methodology
All the chemicals used were of analytical grade and used as
procured. Solvents used were of analytical grade and were puri-
fied by standard procedures. The stoichiometric analyse (C, H
and N) of the complexes were performed using Elementar vario
EL III (Germany) model. Metal contents were estimated on an
AA-640-13Shimadzuflameatomicabsorptionspectrophotome-
ter in solution prepared by decomposing the respective complex
in hot concentrated HNO3. Their IR spectra were recorded on
Perkin-Elmer FTIR spectrophotometer in KBr and polyethylene
pellets. The UV–visible spectra were recorded in water on Beck-
man DU-64 spectrophotometer with quartz cells of 1 cm path
length and mass spectra (TOF-MS) were recorded on Waters
(USA) KC-455 model with ES+ mode in D2O. X-band EPR
spectra were recorded on a Varian E-112 spectrometer with a
variable temperature liquid nitrogen cryostat at IIT, Madras,
India (the error in g value is 0.001) and g factors were quoted
relative to the standard marker TCNE (g = 2.00277). Cyclic
voltammetric measurements were carried out on a BAS CV
50 W electrochemical analyzing system (accuracy 1.0 mV).
Cyclic voltammograms of all the complexes were recorded in
1:9 water–acetonitrile solutions, with a 0.1 M TBAP as sup-
˚
tor using the Cu K␣ line at 1.54056 A as the radiation sources.
Sample was scanned between 10◦ and 70◦ (2θ) at 25 ◦C. The
crystallographic data were analyzed by using the CRYSFIRE-
2000 powder indexing software package and the space group
was found by the GSAS program. Debye–Scherer relation with
the help of 100% peak width, determined the particle size.
The density was determined using conventional Archimedes
method.
2.2. Biological activity
2.2.1. Antibacterial screening
In vitro antibacterial activity of the compounds against E. coli
and S. aureus were carried out using Muller–Hinton Agar media

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B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
2.2.2. Antifungal screening
The antifungal activity of the complexes was checked by dry
weight method for the Rhizopus sp. and A. niger fungi. The
complexes were directly added to the growth medium in varying
concentration (0.10–0.40%, w/v). The actively growing mycelia
(of the test fungi) were placed on the medium with the help of
an inoculum needle and incubated at 27 1 ◦C for 7 days. The
medium with the test solutions served as ‘treated’ while without
them as ‘control’ or ‘check’. The resulting mycelia mats in each
set were carefully removed, washed, dried and then weighed
separately. The fungal growth was calculated from the following
relation
Cg − Tg
Fungal growth inhibition (%) = 100 ×
Cg
where Cg is the average growth in the ‘control’ or ‘check’ set
and Tg is the average growth in the treated set.
2.3. Synthesis of complexes
To a methanolic solution of copper acetate (0.5 mmol) was
added a methanolic solution of the newly synthesized car-
boxyamide ligands [14] (0.5 mmol) in a separate flask. The
resulting solution was stirred for 3 h, after which the volume
was reduced to half on a warm water bath. The product obtained
was washed with a small amount of methanol and air-dried. The
above product was redissolved in an excess of warm methanol,
and the clear solution was left undisturbed for weeks to give
beautiful colored crystals of the complexes. Various attempts to
obtain the single crystals have so far been unsuccessful.
Copper(II)-Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxoprop-2-enyl)2-
amino-N-arylbenzamidine complex [complex I].
Bluish green crystal; yield: 80%; anal. calcd. for C21H19N3
O8Cu requires (%): C, 49.95; H, 3.77; N, 8.33; Cu, 12.59. Found:
C, 49.90; H, 3.81; N, 8.40, Cu, 12.55 (mol. wt. 504.5); FTIR
(KBr, cm−1): ν(N–H)Amide 3465(s), ν(C O)Amide I 1624(w),
[ν(C–N) + δ(N–H)]Amide II 1445(w), ν[(N–H) + δ(C–H)]Amide III
1319, νas(CO2−) 1555(s), νs(CO2−) 1410(s), ν(Cu–O) 382(m);
EPR (g1, g2, g3, gav, G) values: 2.0531, 2.1374, 2.2168, 2.1358,
Fig. 2. Proposed structure of (a) the complex I, (b) the complex II and (c) the
complex III.
2
2.3. Electronic spectral data (cm−1): 13661(2Eg → T2g),
30120(␲ → ␲*, N-Amide); μeff(BM): 2.00; TG[step I, step II,
step III] E* (J mol−1): 25.86, 24.73, 25.48; A (×104 s−1): 3.77,
2.67, 2.41; ꢀS* (J K−1 mol−1): −158.87, −163.84, −166.03;
ꢀH* (J mol−1): 96.58, 78.07, 158.66 and ꢀG* (kJ mol−1):
59.16, 78.54, 93.69]; DTA[step I, step II, step III]: endo, endo,
exo.
(Himedia). Theactivitywascarriedoutusingpaperdiscmethod.
Base plates were prepared by pouring 10 ml of autoclaved
Muller–Hinton agar into sterilized Petri dishes (9 mm diameter)
and allowing them to settle. Molten autoclaved Muller–Hinton
that had been kept at 48 ◦C was incubated with a broth culture of
the E. coli or S. aureus and then poured over the base plate. The
discs were air dried and placed on the top of agar layer. The plates
were incubated for 24–30 h and the inhibition zone (mm) were
measured around each disc. As the organism grows, it forms
a turbid layer, except in the region where the concentration of
antibacterial agent is above the minimum inhibitory concentra-
tion, and a zone of inhibition is seen. The size of the inhibition
zone depends upon the culture medium, incubation conditions,
rate of diffusion and the concentration of the antibacterial agent.
The solutions of all compounds were prepared in double distilled
water and chloramphenicol was used as a reference.
Copper(II)-Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxopropanyl)2-amino-
N-arylbenzamidine complex [complex II].
Green crystal; yield: 84%; anal. calcd. for C21H23N3O8Cu
requires (%): C, 49.56; H, 4.52; N, 8.26; Cu, 12.49. Found:
C, 49.52; H, 4.50; N, 8.24; Cu, 12.51 (mol. wt. 508.5); FTIR
(KBr, cm−1): ν(N–H)Amide 3429(s), ν(C O)Amide I 1610(w),
[ν(C–N) + δ(N–H)]Amide II 1431(w), ν[(N–H) + δ(C–H)]Amide III
1320, νas(CO2−) 1570(s), νs(CO2−) 1407(s), ν(Cu–O) 372(m);
EPR g value: g: 2.0339. Electronic spectral data (cm−1):
13378(2Eg → T2g), 30488(␲ → ␲*, N-Amide); μeff(BM):
2
1.86; TG[step I, step II, step III] E* (J mol−1): 10.99, 13.49,

B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
761
13.76; A (×103 s−1): 1.61, 1.93, 1.61; ꢀS* (J K−1 mol−1):
−186.28, −186.29, −190.03; ꢀH* (J mol−1): 60.10, 37.80,
21.54 and ꢀG* (kJ mol−1): 80.39, 101.50, 128.77]; DTA[step
I, step II, step III]: endo, endo, exo.
respectively). The vibrational bands due to rocking and wagging
modes of water are observed in the 800–650 cm−1 region for all
the complexes of all the ligands and may be attributed to coor-
dinated water in all the complexes [20]. This can be confirmed
with the help of thermograms. A new band in the 395–360 cm−1
regions in the spectra of the complexes is assignable to ν(Cu–O)
[22].
Copper(II)-Nꢀ,Nꢀꢀ-bis(3-carboxy-1-oxophenelenyl)2-amino-
N-arylbenzamidine complex [complex III].
Bluish green crystal; yield: 86%; anal. calcd. for C29H23N3
O8Cu requires (%): C, 57.57; H, 3.80; N, 6.95; Cu, 10.50. Found:
C, 57.60; H, 3.77; N, 6.93; Cu, 10.45 (mol. wt. 604.5); FTIR
(KBr, cm−1): ν(N–H)Amide 3461(s), ν(C O)Amide I 1614(w),
[ν(C–N) + δ(N–H)]Amide II 1488(m), ν[(N–H) + δ(C–H)]Amide III
1398, νas(CO2−) 1575(s), νs(CO2−) 1442(s), ν(Cu–O) 390(m);
EPR (g1, g2, gav) value: 2.0564, 2.1200, 2.0882. Electronic
3.2. TOF-MS spectra
Mass spectrometry has been successfully used to investigate
molecular species in solution [23–25]. The pattern of the mass
spectrum gives an impression of the successive degradation of
the target compound with the series of peaks corresponding to
the various fragments. The ligands have similar tendency of
initial loss of two molecules of [HC O]+ with 100% m/z val-
ues. Ligands (L1 and L2) seem to be almost of similar trend
of degradation and finally forms two molecules of [aniline]+,
whereas ligand (L3) finally degrades to [amidine]+. In the TOF-
mass spectra of copper complexes initial fragmentation pattern
is again similar (loss of two water molecules), a mononuclear
nature for these complexes [M(L)]+ can be deduced. The last two
fragments appears in nearly all the complexes at positions (m/z
values) 236 (24% complex I, 50% complex II and 100% com-
plex III) and 222 (100% complex I, 34% complex II and 47%
complex III) corresponds to [C14H10N3O]+ and [C14H10N2O]+,
respectively, which could be the result of demetallation and sub-
sequently a partial intramolecular hydrogen bonding. Another
prominent peak in the middle of the degradation pathway at
position (m/z) 268 (42% complex II, 20% complex III) and
324 (45%, complex III) [C15H15N3O2]+ and [C20H11N3O2]+,
respectively (Fig. 3a–c, TOF-MS spectrum of complexes with
specific fragments). Each complex has a peak at position (m/z
value) 180 (12% complex I, 100% complex II, 17% complex
III) corresponds to the formation of [C10H10N]+. Likewise
peaks attributable to unstable monomeric species as [M(L)]+ and
[H2L]+ are usually present in the mass spectra of these systems
[26,27].
2
spectral data (cm−1): 13822(2Eg → T2g), 31446(␲ → ␲*, N-
Amide); μeff(BM): 1.92; TG[step I, step II, step III] E*
(J mol−1): 27.80, 28.25, 27.47; A (×104 s−1): 5.76, 4.48,
3.31; ꢀS* (J K−1 mol−1): −156.67, −161.11, −165.66; ꢀH*
(J mol−1): 85.12, 10.28, 28.23 and ꢀG* (kJ mol−1): 68.33,
93.13, 122.62]; DTA[step I, step II, step III]: endo, endo,
exo.
3. Results and discussion
Satisfactory results of elemental analysis and spectral studies
revealed that the complexes were of good purity. The complexes
were soluble in polar solvents.
3.1. IR spectra
An ambidentate nature of the amide can coordinate with
metal ion either through nitrogen of the –NH or the oxy-
gen of the C O group [15,16]. The bands due to the amide
ν(N–H) mode at 3200–3400 cm−1 for the free ligands are
shifted towards higher frequencies, while the ν(C O) (amide
I) frequency (1630–1650 cm−1) undergoes shift towards lower
frequency (1600–1640 cm−1) in the copper complexes indicat-
ing non-involvement of the amide nitrogen and involvement
of the carbonyl oxygen atom of amide group in coordina-
tion with the copper(II) ion [17]. This result is explained by
the lengthening of the C O bond and shortening of the C–N
bond which cause the decrease and increase of the frequencies,
respectively [18,19]. Amide II (1410–1480 cm−1 in ligands) and
amide III (1275–1305 cm−1 in ligands) bands arising out of
ν(C–N) as well as δ(N–H) modes (coupled to one another), shift
3.3. Electronic spectra and magnetic susceptibility
measurements
The ligand and their copper complexes show characteristic
UV–vis spectra for the amide and other associated groups. In
the uv region, the shoulder band observed at 35842 cm−1 may
be assigned to n → n* transition within the C O group of the
amide moiety in the free ligands. This band is disappeared in all
the complexes, revealing the involvement of the C O of amide
group or carboxylate oxygen in chelate formation. The band
observed at 30120, 30488 and 31446 cm−1 in ligands L1, L2
towards higher frequencies 1430–1500 and 1295–1325 cm−1
,
respectively, in complexes further confirming the coordination
through carbonyl oxygen. The ν(C O) and ν(C–O) stretch-
ing frequencies in the 1540–1590 and 1420–1400 cm−1 region
−
in the uncomplexed ligands have been assigned to νas(CO2
)
*
and νs(CO2−) modes of the –COOH group and these bands
shows considerable shift in all the complexes. A positive and
negative shift in complexes indicates a ꢀν enhancement upon
complexation and suggests coordination of –COO− groups in
all the complexes of the three ligands in a monodentate fashion
[20,21]. Here ꢀν implies the difference between νas(CO2−) and
νs(CO2−) (i.e., asymmetric and symmetric stretching modes,
and L3, respectively, may be assigned to the ␲ → ␲ transition
of the N-amino group of the amide ligand, remains at the same
position in all the respective copper complexes, indicating they
are not affected by metal chelation [28].
In the visible region copper complexes show a prominent
broad band (Fig. 4) in the range 13,300–15,400cm−1, ascribed
2
to the d–d transition (2Eg → T2g). These results suggesting a

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B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
Fig. 4. Electronic spectra of the complexes.
ms = 1/2 leading to a doubly degenerate spin state in the
absence of a magnetic field. In a magnetic field the degeneracy
is lifted between these states and the energy difference between
them is given by E = hν = gβH, where h is the Planck’s constant,
ν is the microwave frequency for transition from ms = +1/2 to
ms = −1/2, g is the Lande splitting factor (equal to 2.0023 for
a free electron), β is the Bohr magneton and H is the magnetic
field. In the case of 3d9 copper(II) ion, the appropriate spin
Hamiltonian assuming a B1g ground state is given by
ˆ
H = β[g HzSz + g (HxSx + HySy)
ꢁ
⊥
+A IzSz + A (IxSx + IySy)]
ꢁ
⊥
The EPR spectrum of polycrystalline samples show similar
features at room temperature (RT) and at liquid nitrogen
temperature (LNT). Basic spectral features at both temperatures
are same with slightly better resolution at LNT. The EPR
spectrum of the complex I gives three g values, viz. g1, g2, g3
(g3 > g2 > g1) which indicate rhombic distortion is very small
(Fig. 5a). The geometric parameter G, which is a measure of
the exchange interaction between the copper centers in the
polycrystalline compound, is calculated using the equation:
Fig. 3. TOF-MS spectrum of (a) complex I, (b) complex II and (c) complex III.
G = (g − 2.0023)/(g − 2.0023) for axial spectra and for rhom-
ꢁ
⊥
bic spectra G = (g3 − 2)/(g − 2) and g = (g1 + g2)/2. If G > 4,
⊥
⊥
distorted octahedral geometry [29]. The complexes also exhibit
a band at 25,000 cm−1, which can be assigned to charge transfer.
The complexes are paramagnetic with μeff values correspond-
ing to the presence of one unpaired electron (1.80–2.02 BM).
The relatively higher values for the amide complexes can be
rationalized in terms of decreased 3d–4p orbital mixing.
exchange interaction is negligible if it is less than 4; considerable
exchange interaction is indicated in the solid complex [30]. The
EPR spectrum of the complex II in the polycrystalline state show
only one broad signal at 2.0339 due to dipolar and enhanced spin
lattice relaxation whereas the spectrum of complex III (Fig. 5b)
show two sharp signals at 2.0564 and 2.1200, respectively. It was
not possible to calculate G values for complex II and III because
we obtained only one and two spectral peak(s), respectively. In
the transition metal complexes, we can obtain very important
information about the structure of the complex with the help
of g values. In complex I and III the lowest g > 2.04, such a
3.4. EPR investigation
The copper(II) ion, with d9 configuration, has an effective
spin of S = 1/2 and is associated with a spin angular momentum,

B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
763
Fig. 6. Cyclic voltammogram of the complexes (I and II).
[(Epa + Epc)/2] are equal to −108.3 and −22 mV in complex I
and II, respectively. The neutral uncomplexed ligands are proved
not be electroactive over the range −0.6 to +0.6 V.
3.6. Kinetics of thermal decomposition
Thermogravimetric (TG) and differential thermogravimetric
analyses (DTA) were carried out for copper(II)–amide com-
plexes in ambient conditions (Fig. 7). In the complexes, loss of
water molecules takes place in the temperature range 368–440 K
followed by the decomposition of the organic moiety with the
formation of CuO as residue [25]. This has been again confirmed
by comparing the observed/estimated and the calculated mass
of the pyrolysis product. On the basis of thermal decomposi-
tion, thethermodynamicactivationparameterssuchasactivation
energy (E*), enthalpy of activation (ꢀH*), entropy of activation
(ꢀS*), free energy change of decomposition (ꢀG*) were evalu-
ated by employing the Coats–Redfern relation [13]. The Coats
and Redfern linearization plots (Fig. 7 (inset)), confirms the
Fig. 5. EPR spectrum of (a) the complex I and (b) the complex III.
spectrum can be observed for a copper(II) ion in elongated
rhombic symmetry with all the axes aligned parallel and would
be consistent with elongated rhombic-octahedral complexes
[30].In the complex I, g > g > 2.0023 and G value within
ꢁ
⊥
the range 2.08–4.49 are consistent with dx2−y2 ground state
[31].
It has been shown that g is a moderately sensitive function
ꢁ
for indicating covalency [32]. Relatively speaking g > 2.3 is
ꢁ
characteristic of an ionic environment and g < 2.3 of a cova-
lent environment in M–L bonding. In complex III, g < 2.3 was
observed and which indicates a fair degree of covalent character
in the Cu–L bonding.
ꢁ
ꢁ
3.5. Electrochemical studies
The electrochemical properties of the copper(II) car-
boxyamide complexes have been studied by cyclic voltammetry
(CV). The voltammogram of the complexes are shown in Fig. 6.
The voltammogram of the copper(II) complex shows only a
single reduction peak Cu(II)/Cu(I) at −0.312 and −0.222 V
in complex I and II, respectively, during the cathodic poten-
tial scan. During the return anodic potential scan, just after
the reduction peak, an anodic peak Cu(I)/Cu(II) is observed
at 0.095 and 0.178 V in complex I and II, respectively. The
separation between the cathodic and anodic peak potentials
ꢀEP (ꢀEP = Epa − Epc) of 407 and 400 mV indicates a quasi-
reversible redox process and the ratio of anodic to cathodic peak
currents (IPc/IPa ≈ 1) correspondingto asimpleoneelectronpro-
cess [33] assignable to the Cu(II)/Cu(I) couple. The value of E1/2
Fig. 7. Thermogravimetric curve and Coats–Redfern plot (inset) of the com-
plexes.

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B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
Table 2a
Antifungal activity of the synthesized compounds
Serial
Compound Fungal growth inhibition Fungal growth inhibition
number
(%) with Rhizopus sp.
(%) with A. niger
1
2
3
4
5
6
Ligand (L1) 25
Complex I 45
Ligand (L2) 20
Complex II 54
Ligand (L3) 20
Complex III 40
20
50
35
65
30
60
has been calculated. The crystallographic data of the copper(II)
complexes are shown in Table 1.
Fig. 8. X-ray powder diffraction of the complex III.
first order kinetics for the decomposition process. Among metal
complexes, activation energy increases as complex II < complex
I < complex III, same trends happens with thermal stability of
metal complexes. The trends can be explained on the basis
of increase in delocalisation of electron in the ligand (L1
and L3) molecules. The negative value of entropy indicates a
more ordered activated state that may be possible through the
chemisorptions of oxygen and other decomposition products.
The negative values of the entropies of activation are compen-
sated by the values of the enthalpies of activation, leading to
almost the same values for the free energy of activation [34].
3.8. Antimicrobial activities
The free ligands and its copper complexes were screened
against Rhizopus sp. and A. niger fungi and E. coli and S. aureus
bacteria to asses their potential antimicrobial activity. The results
are quite promising. It is clear from the antifungal screening data
(Table 2a) that the copper complexes are more fungitoxic than
the chelating agent itself. The ligands and its copper complexes
showed the maximum activity against A. niger fungi. Among
ligands and its copper complexes, ligand (L2) and its copper
complexes (complex II) showed better activity against A. niger
fungi than others. The bacterial screening results (Table 2b)
reveal that the free ligands (L2 and L3) and its complexes (II
and III) showed maximum activity against S. aureus bacteria
whereas ligand (L1) and its copper complex (complex I) com-
plex showed the activity against E. coli bacteria. Again complex
III is better antibacterial than complex I and II. The antimicro-
bial data reveal that the complexes are more bioactive than the
free ligands. The enhanced activity of the metal complexes may
be ascribed to the increased lipophilic nature of the complexes
arising due to chelation. It is probably due to faster diffusion of
3.7. X-ray powder diffraction studies
In absence of single crystal, X-ray powder data is especially
useful to deduce accurate cell parameters. The diffraction pat-
tern reveals the crystalline nature of the complex. The indexing
procedures were performed using (CCP4, UK) CRYSFIRE [35]
giving triclinic crystal system with P1 space group for all the
copper complexes (Fig. 8, XRPD spectra of complex III). The
merit of fitness and particle size of the copper(II) complexes
Table 1
Crystallographic data of the copper complexes
Complexes
Complex I
504.5
Complex II
508.5
Complex III
604.5
Formula weight
Unit cell dimensions
a = 6.397563
a = 9.707080
a = 7.685788
˚
(A)
b = 6.460189
c = 15.43264
α = 60.61603◦
β = 59.06222◦
γ = 93.20528◦
b = 11.043350
c = 10.450080
α = 83.2756◦
β = 69.756120◦
γ = 136.055400◦
b = 10.24293
c = 19.409050
α = 36.85360◦
β = 44.25097◦
γ = 38.73720◦
3
˚
Volume (A )
434.29
603.07
555.91
Limiting indices
−1 ≤ h ≤ 3
−2 ≤ k ≤ 3
0 ≤ l ≤ 8
1.928
1
−3 ≤ h ≤ 3
−3 ≤ k ≤ 3
0 ≤ l ≤ 6
1.399
1
−2 ≤ h ≤ 2
−2 ≤ k ≤ 3
0 ≤ l ≤ 6
1.805
1
Density (g/cm3)
Z
Avs. eps
0.0000769
0.0000460
0.0000730
Merit of fitness
Particle size (nm)
M(20) = 7, F(20) = 5
18.25
M(18) = 13, F(18) = 7
8.25
M(22) = 11, F(22) = 7
4.097

B.K. Singh et al. / Spectrochimica Acta Part A 70 (2008) 758–765
765
Table 2b
Antibacterial activity of the synthesized compounds
Serial number
Compound
Diameter of zone of inhibition
Diameter of zone of inhibition
in mm for E. coli
in mm for S. aureus
50 ppm
100 ppm
50 ppm
100 ppm
1
2
3
4
5
6
7
Ligand (L1)
Complex I
Ligand (L2)
Complex II
Ligand (L3)
Complex III
Chloramphenicol (reference)
–
–
–
04
04
08
11
02
04
02
08
06
08
22
–
–
–
–
03
08
04
06
10
04
12
10
14
20
the chelates as a whole through the cell membrane or due to the
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4. Conclusion
Copper(II) complexes were found to be monomer and
involved coordination through carboxylate oxygen of the amide
ligands along with two water molecules. Magnetic susceptibility
measurements supports spectroscopic data has been suggested
octahedral coordination geometry for the complexes. Kinetic
parametershowsthatthedecompositionfollowsfirstorderkinet-
ics and proceeds in three-step decomposition. Thermal stability
of the complexes has been compared. The all three complexes
crystallize in the triclinic crystal system. The ligands and its
copper complex exhibit noble antimicrobial activity against all
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University of Delhi, Delhi, is also thankfully acknowledged for
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