Synthesis of new oxamide-based ligand and its coordination behavior towards copper(II) ion: Spectral and electrochemical studies

Article abstract of DOI:10.1016/j.saa.2007.06.006

A new ligand N,N′-bis{3-(2-formyl-4-methyl-phenol)-6-iminopropyl}oxamide (L) and its mono- and binuclear copper(II) complexes have been synthesized and characterized. The ligand shows absorption maxima at 249 and 360 with a weak transition at 455 nm. The ligand was found to be fluorescent and shows an emission maximum at 516 nm on excitation at 360 nm. The electronic spectra of the mono- and binuclear Cu(II) complexes exhibited a d-d transition in the region 520-560 nm characteristic of square planar geometry around Cu(II) ion. The ESR spectrum of the mononuclear complex showed four lines with nuclear hyperfine splitting. The binuclear complex showed a broad ESR spectrum with g = 2.10 due to antiferromagnetic interaction between the two Cu(II) ions. The room-temperature magnetic moment values (μ_eff) for the mono- and binuclear Cu(II) complexes are found to be 1.70μ_B and 1.45μ_B, respectively. The electrochemical studies of the mononuclear Cu(II) complex showed a single irreversible one-electron wave at -0.70 V (E_pc) and the binuclear Cu(II) complex showed two irreversible one-electron reduction waves at -0.75 V (E_pc¹) and -1.27 V (E_pc²) in the cathodic region.

Full text of DOI:10.1016/j.saa.2007.06.006

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 1077–1081
Synthesis of new oxamide-based ligand and its coordination behavior
towards copper(II) ion: Spectral and electrochemical studies
K.R. Krishnapriya, D. Saravanakumar, P. Arunkumar, M. Kandaswamy∗
Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600025, India
Received 2 February 2007; accepted 2 June 2007
Abstract
A new ligand N,Nꢀ-bis{3-(2-formyl-4-methyl-phenol)-6-iminopropyl}oxamide (L) and its mono- and binuclear copper(II) complexes have been
synthesized and characterized. The ligand shows absorption maxima at 249 and 360 with a weak transition at 455 nm. The ligand was found
to be fluorescent and shows an emission maximum at 516 nm on excitation at 360 nm. The electronic spectra of the mono- and binuclear Cu(II)
complexes exhibited a d–d transition in the region 520–560 nm characteristic of square planar geometry around Cu(II) ion. The ESR spectrum of the
mononuclear complex showed four lines with nuclear hyperfine splitting. The binuclear complex showed a broad ESR spectrum with g = 2.10 due
to antiferromagnetic interaction between the two Cu(II) ions. The room-temperature magnetic moment values (μeff) for the mono- and binuclear
Cu(II) complexes are found to be 1.70μB and 1.45μB, respectively. The electrochemical studies of the mononuclear Cu(II) complex showed a
single irreversible one-electron wave at −0.70 V (Epc) and the binuclear Cu(II) complex showed two irreversible one-electron reduction waves at
−0.75 V (Ep1c) and −1.27 V (Ep2c) in the cathodic region.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Oxamide ligand; Fluorescence; Mono- and binuclear Cu(II) complexes; Magnetic studies; Electrochemical studies
1. Introduction
ing its conformation from cis to trans form. Further, Schiff bases
have photochromic properties, many of them have non-linear
optical property [16], and the complexes with tetradentate Schiff
base ligands having the salen-type skeleton are found to gen-
erate molecular oxygen [17,18] in the absence of p-quinone,
and all of these have been given great attention now [19]. With
these facts in mind and in continuation of our recent work on
oxamide-based compounds [20–23], we have designed and syn-
thesizedaligandandstudieditscomplexationbehaviourtowards
copper(II) ions. This paper reports absorbance and fluorescence
spectra of the ligand, and spectral, electrochemical and magnetic
studies of the mono- and binuclear copper(II) complexes of the
ligand.
The current interest for inorganic chemists is to design
and synthesize [1] bi- and polydentate ligands and their tran-
sition metal complexes because of their applications such
as new types of magnetic materials [2,3], catalytic reagents
[4,5], models of metalloenzymes [6], sensors [7] and DNA-
cleavage agents [8]. Generally two main strategies followed
for the synthesis of bi and polymetallic complexes are (i)
the use of polynucleating ligands and (ii) the use of ‘metal
complexes as ligands’ [9,10]. In this respect, oxamide-bridged
compounds have gained increasing interest [11], since they
can form a large number of supramolecules in simple routes
by following both the strategies. Oxamide-based ligands such
as N,Nꢀ-bis(aminoalkylsubstituted)oxamide [12] have been of
great interest due to their ability to form variety of complexes
from mono to poly [13]. The monomeric cis-oxamidato metal
complexes of N,Nꢀ-bis(aminoalkylsubstituted)oxamide easily
yields binuclear [14] and polynuclear [15] complexes by chang-
2. Experimental
2.1. Physical measurements
Elemental analysis was carried out on a Carlo Erba model
1106 elemental analyzer. 1H NMR spectrum was recorded using
a model FX-80-Q Fourier transition NMR spectrometer. IR
spectra were recorded on a Hitachi model 270-50 spectropho-
∗
Corresponding author. Tel.: +91 44 22300488; fax: +91 44 22300488.
E-mail address: mkands@yahoo.com (M. Kandaswamy).
tometer on KBr disc in the wavenumber range 4000–250 cm−1
.
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.06.006

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K.R. Krishnapriya et al. / Spectrochimica Acta Part A 69 (2008) 1077–1081
The EI–mass spectrum of the ligand was recorded on a JEOL
DX-303 mass spectrometer. Electronic spectral studies were
carried out on a Hitachi model 320 spectrophotometer in the
wavelength range of 250–800 nm. Molar conductivity was mea-
sured with an Elico model SX 80 conductivity-bridge using a
freshly prepared solution of the complex in DMF. The cyclic
voltammograms of complexes (10−3 M DMF solution) were
recorded on CHI600A electrochemical analyzer. The measure-
ments were carried out under oxygen-free conditions by purging
nitrogen gas using a three-electrode cell in which glassy carbon
electrode was the working electrode, saturated Ag/AgCl elec-
trode was reference electrode and platinum wire was used as
the auxiliary electrode. For coulometric studies the platinum
foil (1 cm2) is used as a working electrode instead of glassy
carbon electrode. Ferrocene/ferrocenium (1+) couple was used
as an internal standard and E1/2 of the ferrocene/ferrocenium
(Fc/Fc+) couple, under the experimental condition, is 470 mV
in dimethylformamide (DMF) and the ꢀEp value for Fc/Fc+ is
70 mV. Tetra(n-butyl) ammonium perchlorate was used as the
supporting electrolyte. Room-temperature magnetic moments
for Cu(II) complexes were measured on a PAR Model-155
vibrating sample magnetometer. The apparatus was calibrated
(s, CH N, 2H), 10.2 (s, Ar–OH, 2H), 10.5 (s, CHO, 2H).
Selected IR data (KBr, ν cm−1): 1585, 1635, 1667, 3271,
3340. UV–vis (λmax, nm) (ε, M−1 cm1) in DMF: 455 (81), 360
(58,000), 249 (208,000).
2.4. Synthesis of complexes
2.4.1. Synthesis of mononuclear copper(II) complex [CuL]
A solution of Cu(CH3COO)2·H2O (0.40 g, 2 mmol) dis-
solved in methanol (60 ml) was added in dropwise to a stirred
solution of the ligand (1.0 g, 2 mmol) in MeOH (60 ml). After
the addition, the stirring was continued for one more hour. A
green colour compound precipitated, that was filtered off and
washed with diethyl ether. The resultant complex was recrys-
tallized from acetonitrile. Yield: 1.12 g (75%). Anal. Calc. for
C26H28N4O6Cu: C, 56.16; H, 5.08; N, 10.08; Cu, 11.43. Found:
C, 56.14; H, 5.08; N, 10.06; Cu, 11.42%. Selected IR data (KBr,
␯ cm−1): 1627, 1668, 3410. Conductance in DMF was given as
Λm = 14 −1 mol−1 cm2. UV–vis (λmax, nm) (ε, M−1 cm1) in
DMF: 520 (235), 372 (21,400), 267 (35,300). μeff = 1.70μB, g
⊥
at 2.11, g = 2.34 and A = 133.
ꢁ
ꢁ
using Ni. The effective magnetic moment was calculated using
2.4.2. Synthesis of binuclear copper(II) complex [Cu2L]
A methanolic solution (60 ml) of Cu2(CH3COO)2·H2O
(1.0 g, 5 mmol) was added dropwise to the stirred solution con-
taining the ligand (1.26 g, 2.5 mmol) dissolved in MeOH (60 ml)
and stirring was continued for further one more hour. The resul-
tant pale green precipitate was filtered off and washed with
diethyl ether. The crude complex was then recrystallized from
acetonitrile. Yield: 1.64 g (80%). Anal. Calc. C26H26N4O6Cu2:
C, 50.56; H, 4.24; N, 9.07; Cu, 20.58. Found: C, 50.54; H, 4.24;
N, 9.06; Cu, 20.57%. Selected IR data (KBr, ␯ cm−1): 1624,
1669, 2930. Conductance in DMF (Λm) = 13 −1 mol−1 cm2.
UV–vis (λmax, nm) (ε, M−1 cm−1) in DMF: 560 (470), 365
(27,200), 266 (38,400). μeff = 1.45μB, g = 2.10.
√
the relation μCu = 2.83 χCuT, where χCu is the molar mag-
netic susceptibility per Cu. X-band ESR spectra was recorded
on a Varian EPR-E 112 spectrometer using diphenylpicrylhy-
drazine (DPPH) as the reference. The emission spectrum of the
ligand was recorded in HPLC grade chloroform on Perkin-Elmer
LS-5B spectrophotometer.
2.2. Materials
2,6-Diformyl-4-methyl-phenol [24] and N,Nꢀ-bis(3-amino-
propyl)oxamide [25] were prepared by following the liter-
ature methods. Tetra(n-butyl)ammonium perchlorate (TBAP)
was used as the supporting electrolyte in electrochemical mea-
surements (Caution! TBAP is potentially explosive; hence, care
should be taken in the handling of the compound). All the other
chemicals and solvents were of analytical grade and used as
received.
3. Results and discussion
A new oxamide-based ligand (L) was synthesized by Schiff’s
base condensation between 2,6-diformyl-4-methyl-phenol and
N,Nꢀ-bis(3-amino propyl)oxamide as shown in Scheme 1. The
mono- and binuclear Cu(II) complexes of this ligand were
prepared by using the ligand and copper(II) acetate mono-
hydrate in 1:1 and 1:2 mole ratios, respectively as shown in
Scheme 2.
2.3. Synthesis of the ligand N,Nꢀ-bis{3-(2-formyl-
4-methyl-phenol)-6-iminopropyl} oxamide (L)
The ligand L was prepared by the dropwise addition of
ethanolic solution (30 ml) of N,Nꢀ-bis(3-aminopropyl)oxamide
(0.6 g, 3 mmol) to a stirred solution of 2,6-diformyl-4-methyl-
phenol (1 g, 6 mmol) in ethanol (30 ml). After the addition
was completed, the stirring was continued for further 1 h.
The resulting yellow precipitate was filtered off, and then
washed with diethyl ether and air-dried. The crude product
was recrystallized from chloroform. Yield: 0.85 g (60%). m.p.
188–190 ◦C. EI–mass spectrum: m/z = 496 (M + 1)+. Anal. Calc.
for C26H30N4O6: C, 63.15; H, 6.11; N, 11.33. Found: C, 63.15;
3.1. Spectral studies of ligand
The EI–mass spectrum of the ligand L is shown in Fig. 1. The
ligand R showed the molecular ion peak at m/z = 495, which is
assignable to the (M + 1)+ fragment. The IR spectrum of the
ligand showed a ␯(C O) of aldehyde peak at 1695 cm−1 and at
1667 cm−1 for ␯(C O) of oxamide carbonyl group. The peak
appeared at 1644 cm−1 is due to ␯(C N). A band responsible
for OH group ν(OH) was observed at 3270 cm−1. The bands at
3271 and 1585 cm−1 appeared due to –NH stretching and –NH
bending, respectively in the IR spectrum of the ligand, and these
1
H, 6.10; N, 11.32. H NMR (CDCl3, δ ppm): 1.90–1.96 (m,
NCH2CH2CH2N, 4H), 2.0 (t, CH2NCH, 4H), 2.32 (s, CH3,
6H), 3.65 (t, CH2NHCO, 4H), 7.01–7.12 (m, Ar–H, 4H), 7.33

K.R. Krishnapriya et al. / Spectrochimica Acta Part A 69 (2008) 1077–1081
1079
Scheme 1. Synthesis of the ligand R.
Scheme 2. Synthesis of [CuH2L] and [Cu2L] complexes.
bands disappeared on complexation supporting the coordination
with the metal ion in complexes as in Scheme 2.
The ligand was found to be exhibiting fluorescence and the
electronic absorption and emission spectra of the ligand were
recorded in CHCl3. The absorption and emission spectra of the
ligand are shown in Figs. 2 and 3, respectively. The absorption
spectrum (5 × 10−5 M) showed bands at 249 and 360 nm with
a weak band at about 455 nm. The first band (249 nm) could
Fig. 1. EI–mass spectrum of the ligand L.

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K.R. Krishnapriya et al. / Spectrochimica Acta Part A 69 (2008) 1077–1081
Fig. 2. Absorption spectrum of the ligand L.
be assigned to the excitation of the ␲ electrons of the aromatic
system. The second band (around 360 nm) is due to transition
between the ␲ orbital localized on the band of azomethine group
(C N). The third band in the region (320–400 nm) which can be
effect of an intramolecular charge-transfer transitions within the
whole of the Schiff base molecule [26]. The ligand displayed an
emission maximum at 516 nm on excitation at 360 nm.
Fig. 4. ESR spectra of the copper(II) complexes (a) [CuH2L] and (b) [Cu2L].
3.3. Magnetic studies
The room-temperature (25 ◦C) solid-state X-band ESR spec-
tra of the mono- and binuclear copper (II) complex are shown in
Fig. 4. The mononuclear complex showed four lines [30] due to
nuclear hyperfine splitting (spin 3/2) with the g value of 2.11,
ꢁ
3.2. Spectral studies of complexes
g = 2.34 and A = 133. The ESR spectrum of the binuclear cop-
⊥
ꢁ
per(II) complex showed a broad band centered at g = 2.10 and
this broad ESR spectrum indicated the presence of antiferro-
magnetic interaction [31] between two copper(II) ions in the
complex.
The IR spectra of the mono- and binuclear Cu(II) complexes
showed ␯(C N) bands at 1627 and 1624 cm−1, respectively. A
band due to ␯(OH) appeared at 3340 and 3410 cm−1 for the free
ligand and the mononuclear Cu(II) complex, respectively. The
absence of this band in the binuclear complex is an indication
of deprotonation of phenolic-OH group.
Theelectronicspectraofthemono-andbinuclearCu(II)com-
plexesshowedthreemaintransitions. Themononuclearcomplex
CuL exhibited a d–d transition (T2g → Eg) at 520 nm consistent
with the tetracoordinated Cu in CuN4 in a nearly square planar
geometry [27,28]. Binuclear complex displayed a d–d band at
560 nm. The λmax value of the binuclear complexes is higher
than that of the mononuclear species; such a red shift implies
that the conformation of ligand is altered from cis to trans so
as to change the environment of Cu(II) ions from N4 to N2O2.
This is consistent with a previously reported observation [29].
Besidesd–dtransition, amoderateintensivebandobservedinthe
region 360–370 nm due to ligand to metal charge-transfer tran-
sition, and the strong band observed around 260–270 nm due to
intraligand charge-transfer transition [28] for these complexes.
The room-temperature magnetic studies of mononuclear
Cu(II) complex resulted in magnetic moment value of 1.70μB,
which is nearer to the spin-only value of 1.73μB. This indicates
the presence of one Cu(II) ion with a single unpaired electron
sited in the dx2−y2 orbital [32]. The observed room-temperature
magnetic moment value for binuclear Cu(II) complex is found
to be 1.45μB per metal ion. This also suggested the presence of
strong antiferromagnetic exchange interaction between the two
Cu(II) paramagnetic centers in the same environment through
the orbital overlap of the bridging atoms to an adjacent metal
ion [22,33]. The orbitals are located in same plane and hence,
they overlap on each side of the bridge, favouring strong anti-
ferromagnetic coupling the oxamide bridge.
3.4. Electrochemical studies
Conductivity measurements [34] of both mono- and binu-
clear Cu(II) complexes in DMF resulted in Λm values in the
range of 10–15 −1 mol−1 cm2 indicated that the complexes are
neutral. The electrochemical properties of both the complexes
reported in the present work were studied by cyclic voltamme-
try in DMF containing 10−1 M TBAP as supporting electrolyte.
Cyclic voltammograms for the complexes were recorded in the
potential range −0.2 to −1.5 V. The cyclic voltammograms of
the complexes are shown in Fig. 5.
The mononuclear Cu(II)complex showed reduction potential
at −0.70 V (Epc) and the reduction process is found to be irre-
versible [35] in nature. Coulometric studies performed for this
complex at 100 mV more negative relative to the reduction wave
consumed approximately one electron (n = 0.93), which indi-
Fig. 3. Emission spectrum of the ligand L.

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1081
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A new oxamide-based ligand L has been designed and syn-
thesized in a simple route. The ligand shows fluorescence at
516 nm. Depending upon the metal ion concentration, the ligand
forms either mononuclear or binuclear complexes. In mononu-
clear complex, four nitrogens were coordinated to the metal ion
in cis manner and, two nitrogens and two oxygen atoms of the
polydentate ligand were coordinated to the metal ions, and the
binuclear complex is existed in the trans configuration. This
fluorescent compound can be useful as a fluorescent probe for
cations and anions. We are currently studying the recognition
behaviour of this receptor towards various halide anions and
transition metal cations to find the sensitivity and selectivity of
this receptor.
Acknowledgement
The authors acknowledge Department of Science and Tech-
nology, Government of India and Science City (Government of
Tamil Nadu), Chennai for financial assistance.
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