Synthesis, spectral, photolysis and electrochemical studies of mononuclear copper(II) complex with a new asymmetric tetradentate ligand: Application as copper nanoparticle precursor

Article abstract of DOI:10.1016/j.saa.2012.05.081

A copper(II) complex with asymmetric tetradentate Schiff base ligand, obtained by the single condensation of 1,2-diaminopropane with 2-hydroxy-5-methoxy benzaldehyde was prepared. The ligand and complex were characterized by their IR, UV-Vis, FT-IR, NMR s

Full text of DOI:10.1016/j.saa.2012.05.081

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 501–505
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral, photolysis and electrochemical studies of mononuclear
copper(II) complex with a new asymmetric tetradentate ligand: Application
as copper nanoparticle precursor
⇑
Mohammad Hossein Habibi , Maryam Mikhak
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Photochemical and electrochemical characterization of a copper(II) complex with a new Schiff base was
studied. The copper(II) complex was applied as new precursor for synthesis of copper nanoparticle.
"
A new asymmetric tetradentate
Schiff base derived from 1,2-
diaminopropane is reported.
Copper(II) complex was applied as
copper nanoparticle precursor.
The redox properties of complex
exhibit pseudo-reversible Cu(II)/
Cu(I) reduction and Cu(I)/Cu(II)
oxidation.
"
"
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2012
Received in revised form 21 May 2012
Accepted 29 May 2012
Available online 6 June 2012
A copper(II) complex with asymmetric tetradentate Schiff base ligand, obtained by the single condensa-
tion of 1,2-diaminopropane with 2-hydroxy-5-methoxy benzaldehyde was prepared. The ligand and
complex were characterized by their IR, UV–Vis, FT-IR, NMR spectra and CV. Crystal structures of the
mononuclear copper complex have been obtained by X-ray diffraction studies which revealed to be dis-
torted square planner coordination geometry. The spectral data confirm coordination of ligand to copper
ion center. The redox properties of complex at different scan rates exhibit grossly similar features consist-
ing of an electrochemically pseudo-reversible Cu(II)/Cu(I) reduction at ca. À0.97 V and pseudo-reversible
Cu(I)/Cu(II) oxidation at ca. À0.81 V. The copper nanoparticles with average size of 73 nm were formed by
thermal reduction of copper complex in the presence of triphenylphosphine.
Keywords:
Schiff base
Complex
Spectral
NMR
Ó 2012 Elsevier B.V. All rights reserved.
Voltammetry
Nanoparticle precursor
Introduction
[1,2], reversible oxygen transport [3,4] various aspects of bioinor-
ganic chemistry [5–10], biological activities [11–14], clinical [15]
Multidentate Schiff base ligands have played an important role
in the development of coordination chemistry due to their prepara-
tive accessibility and spectral properties. Schiff bases and their com-
plexes embrace very wide and diversified subjects such as, catalysis
and analytical fields [16–18]. Schiff base complexes of copper are
amongst the most versatile catalysts known for oxygenation reac-
tions. The copper ions with the active sites have stimulated efforts
to design and characterize copper complexes as models for biolog-
ical systems [19,20]. Synthesis and characterization of nano-struc-
tures are important for scientific and industrial applications [21].
Nanoparticles have attracted great interest in recent years because
⇑
Corresponding author. Tel.: +98 311 7932707; fax: +98 311 6689732.
E-mail addresses: habibi@chem.ui.ac.ir, habibi284@gmail.com (M.H. Habibi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.05.081

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M.H. Habibi, M. Mikhak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 501–505
of their unique chemical and physical properties, which are differ-
ent from those of either the bulk materials or single atoms. Nano-
structure materials have potential applications in ceramics,
optoelectronics and catalysis [22]. Copper nanoparticle has received
enormous attention owing to its important properties and wide-
spread applications [23]. Research on the synthesis of nanomateri-
als using metal complexes as precursors has been less reported.
Application of copper complexes as precursors may be helpful to
control the physical properties of copper oxide nanoparticles [24].
In continuation of our earlier work [25,26] and to widen the
scope of investigation on the coordination behavior of Schiff base
ligands, herein we report the synthesis and spectroscopic charac-
terization of copper(II) complex of new Schiff base ligand (Scheme
1). The new copper complex is applied for the preparation of cop-
per nanoparticles as a precursor. The results of our studies on the
preparation of copper nanoparticles using a new copper complex
as a precursor are also discussed.
Fig. 1. Cyclic voltammogram of [(N,N-bis(2-hydroxy-5-methoxybenzylidene)pro-
pane-1,2-diamine]copper(II) in acetonitrile at 298 K in different scan rates (from
100 mV sÀ1 to 600 mV sÀ1), c = 4 Â 10À3 M.
Experimental
(5 mL) of Cu(CH3COO)2ÁH2O (1.0 mmol, 199 mg) was then added.
The mixture was stirred for further 30 min and then filtered. After
keeping the filtrate in air for 3 days, green crystals were formed at
the bottom of the vessel on slow evaporation of the solvent. Yield:
82%, mp: 183 °C, IR (KBr, cmÀ1): 1605 (C@N), 2868 (C–H sp3).
Materials and methods
All solvents and chemicals were used as received, except for the
amines which were distilled under reduced pressure prior to use.
2-Hydroxy-3 or 5-methoxy benzaldehyde, 1,2-propanediamine,
copper acetate and other chemicals were purchased from Aldrich
or Merck. UV–Vis spectra were recorded on a Varian Cary 500 Scan
spectrophotometer. FT/IR spectra of compounds were recorded on
JASCO-FT/IR6300 spectrophotometer in the range of the 4000–
400 cmÀ1 as KBr pellets. Electronic spectra were obtained in CH3CN
solutions by use of a JASCO-V570 spectrophotometer. A Brucker H
NMR spectrometer at 400 MHz was used for recording of 1H spec-
tra in CDCl3. Philips XL-30 scanning electron microscopy (SEM)
was used to image the size and morphology of the copper nanopar-
ticle. Cyclic voltammograms of compounds have been recorded on
a BHP 2063 Potentiostat Galvanostat instrument.
Cyclic voltammetry
All cyclic voltammograms were recorded in a cell containing
three electrodes: glassy carbon as working, platinum disk as sup-
porting and silver wire as reference electrodes at room temperature.
The potentials were scanned between 0.7 and À2.05 V at a scan rate
of 100 mV sÀ1. For recording of cyclic voltammograms, 10À3 M of
ligand and its copper complex in dry acetonitrile as well as tetrabu-
tylammonium hexafluorophosphate (TBAHFP) as supporting elec-
trolyte were used. All solutions were deoxygenated by passing a
stream of pre-purified N2 into the solution for at least 10 min prior
to recording the voltammograms. All chemical potentials were mod-
ified using ferrocene/ferrocenium as internal standard electrode.
Synthesis of ligand (L), [N,N-bis(2-hydroxy-5-
methoxybenzylidene)propane-1,2-diamine], (1)
Preparation of copper nanoparticle
The title compound (1), was prepared by reaction of 2-hydroxy-
5-methoxy benzaldehyde (0.4 mmol, 60 mg) and (1,2-diaminopro-
pane) (0.2 mmol, 15 mg) which were dissolved in methanol
(10 mL). The mixture was stirred for 30 min. The 1H NMR data of
ligand are as following: 1H NMR (400 MHz, CDCl3): 1.39–1.43 (d,
3H, CH3), 3.88 (s, 6H, OCH3), 6.74–6.90 (m, 6H, HAr), 8.28–8.32
(s, 2H, N@CH), 13.72 (s, 2H, OH), IR (KBr, cmÀ1): 1620 (C@N),
2872 (C–H sp3).
[N,N-bis(2-Hydroxy-5-methoxybenzylidene)propane-1,2-diamine]
copper(II) was added to Triton X-100 as surfactant in the presence
of triphenylphosphine to create a homogenous solution, then re-
fluxed for 1 h at 140 °C. The supernatant was removed, and the
nanoparticle sediment was washed, dried (62% yield). Triton X-
100 plays a dual role, i.e. of an extractant and of a surface-capping
agent that protects the outer surface of the newly formed particles
thus avoiding the possibility of further growth.
Synthesis of copper complex, [N,N-bis(2-hydroxy-5-
Results and discussion
methoxybenzylidene)propane-1,2-diamine] copper(II), (2)
Electrochemical studies
N,N-bis(2-Hydroxy-5-methoxybenzylidene)propane-1,2-dia-
mine (1.0 mmol, 342 mg) was dissolved in methanol (5 mL) with
stirring for 30 min at room temperature. A methanol solution
The cyclic voltammogram of [N,N-bis(2-hydroxy-5-methoxy-
benzylidene)propane-1,2-diamine]copper(II) was conducted at
Scheme 1. Synthesis of copper(II) complex of new Schiff base ligand.

M.H. Habibi, M. Mikhak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 501–505
503
25 °C under an argon atmosphere using acetonitrile solutions con-
taining 0.05 mol dmÀ3 TBAHFP as supporting electrolyte and com-
plex concentrations of about 3 Â 10À3 M. The ligand is
electroinactive over a range of +0.7 to À2.0 V. The redox properties
of complex at different scan rates exhibit grossly similar features
consisting of an electrochemically pseudo-reversible Cu(II)/Cu(I)
reduction at ca. À0.97 V and pseudo-reversible Cu(I)/Cu(II) oxida-
tion at ca. À0.81 V (Fig. 1). Copper(II) complex exist as five or six
coordinate species (tetragonal) in solution while Cu(I) complex fa-
vors four (tetrahedral) coordination numbers. Reduction leads to
loss of coordinated solvent from copper.
Spectroscopic properties
FT-IR spectral data of the copper complex was compared with
the ligand. The ligand, N,N-bis(2-hydroxy-3-methoxybenzyli-
dene)propane-1,2-diamine shows a broad band characteristic of
the OH group in the 3395–3585 cmÀ1 region (SI. 1). The disappear-
ance of this band in the FT-IR spectra of the complexes is indicative
of the fact that the tetradentate N2O2 ligands are coordinated as dia-
nions. Deporotonation of OH group in free ligand forms a corre-
sponding negative phenolate ion during complex formation. The
(C@N) band appearing at 1620 cmÀ1 in the ligands is shifted to low-
er frequencies by 15 cmÀ1 in the copper complex, indicating that
the ligands are coordinated to the metal ions through the N2O2
groups (SI. 2). The UV–Vis spectrum of the free ligand showed major
Fig. 2. Spectral changes during the photolysis of 0.3 Â 10À4 M N,N-bis(3-methoxy-
benzylidene)-1,2-diaminopropane in CHCl3 at room temperature at 5 min (a),
78 min (b), 130 min (c) and 170 min (d).
peaks at 270, 340 (
characteristic
p
?
p⁄) and 300, 380 nm (n ? p⁄), identified as
p
?
p⁄ and n ? p⁄ absorption bands (Fig. 2). The
UV–Vis spectrum of the complex showed major peak at 280 related
to a ligand centered transition of the ligand and another at 400 nm
which is identified as MLCT (Figs. 3 and 4). The 1H NMR spectra of
Schiff base shows one singlet at 13.72 ppm corresponding to pheno-
lic–OH proton and one singlet at 8.27 ppm corresponding to azome-
thine proton (HC@N). Also, the 1H NMR spectra of Schiff base ligand
revealed a multiplet at 6.74–6.90 ppm corresponding to aromatic
protons [27–30]. Crystal structures of the monouclear copper com-
Fig. 3. Spectra changes during the photolysis of 0.5 Â 10À4 M of copper(II) complex
in CHCl3 at 0, 10, 20, 30, 40 and 50 min irradiation times.
Fig. 4. Photolysis set up: (a) oxygen cylinder; (b) power supply; (c) High pressure mercury lamps; (d) photoreactor with aluminum foil as reflector for a full irradiate of
catalyst; (e) fan; (f) magnetic stirrer; (g) 2 l Pyrex beaker; (h) photolysis cell; (i) water thermostat Haake model F-122.

504
M.H. Habibi, M. Mikhak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 501–505
Fig. 5. ORTEP of copper(II) complex.
Table 1
Selected bond lengths [Å] and angles [°] for complex.
N(1)–Cu(1)
N(2)–Cu(1)
O(1)–Cu(1)
O(2)–Cu(1)
1.938(6)
1.926(6)
1.906(5)
1.910(5)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(2)
O(2)–Cu(1)–N(2)
O(2)–Cu(1)–N(1)
89.2(2)
176.7(2)
92.6(2)
176.3(2)
plex have been obtained by X-ray diffraction studies which revealed
to be distorted square planner coordination geometry (Fig. 5). Table
1 shows selected bond lengths and angles for complex.
Photochemical properties
Irradiating the chloroform solution of ligand with a high pres-
sure mercury lamp, the absorption maximum at 270 and 340 nm
decreased while 300 and 380 nm increased with isobestic points
at 280, 320 and 350 nm (Fig. 2). The isobestic points indicates
the keto-enol phototutomerization of the C@N bond of the coordi-
nated ligand in solution (Scheme 2). The absorption maximum of
complex at 295 nm decreased and 379 nm increased with an iso-
bestic point at 280 and 370 nm (Fig. 3). The shift indicates the
transformation from syn to anti configuration of the C@N bond of
the coordinated ligand (Scheme 2). It is suggested that the photo-
isomerization is initiated through the rotation around the C@N
bond in both singlet and triplet states.
Fig. 6. Scanning electron microscope (SEM) image of the copper nanoparticles
synthesized by thermal reduction.
nanoparticle. Triton X-100 plays a dual role, i.e. of an extractant
and of a surface-capping agent that protects the outer surface of
the newly formed particles thus avoiding the possibility of further
growth. The SEM images of the copper nanoparticle prepared using
[N,N-bis(2-hydroxy-5-methoxybenzylidene)propane-1,2-diamine]
copper(II) as precursor and Triton X-100 showed that the average
size 73 nm for copper nanoparticle (Fig. 6). This is attributed to
the homogeneous nucleation process produced by the reduction
of Cu2+ to Cu.
Characterization of copper nanoparticle
The reaction of copper complex with Triton X-100 as surfactant
in the presence of triphenylphosphine afforded brown copper
Scheme 2. Imine-Enamine phototutomerization of the ligand in methanol solution.

M.H. Habibi, M. Mikhak / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 501–505
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absorption spectra of new unsymmetrical tetradentate ligands
Schiff base ligands and its Cu(II) complex. The synthetic procedure
in this work resulted in the formation of complex in the molar ratio
(1:1) (M:L). The newly synthesized Schiff base acts as dibasic tetra-
dentate ligand. The copper ion is coordinated through the azome-
thine nitrogens and phenolic oxygen atoms via deprotonation
forming stable. The present study revealed square planar geometry
for Cu(II) complexes. Based on IR, electronic and 1H NMR and single
crystal X-ray structural studies a square planar geometry has been
proposed for the for Cu(II) complex. Photolysis of the ligand shows
isobestic points indicate that the ligand is presumably undergoing
keto-enol tautomerization. The new copper complex was applied
as a new precursor in preparation of copper nanoparticles using
thermal reduction method. The present study illustrates simple
and significant methods for the synthesis of copper complex and
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