An azo-azomethine ligand and its copper(II) complex: Synthesis, X-ray crystal structure, spectral, thermal, electrochemical and photoluminescence properties

Article abstract of DOI:10.1016/j.ica.2015.03.010

Abstract In this study, a new azo-azomethine ligand, 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-(propan-2-yl)phenyl]imino}methyl]phenol (HL) and its Cu(II) complex, [CuL₂] were synthesized and characterized by the analytical and spectroscopic methods su

Full text of DOI:10.1016/j.ica.2015.03.010

Inorganica Chimica Acta 430 (2015) 268–279
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
An azo-azomethine ligand and its copper(II) complex: Synthesis, X-ray
crystal structure, spectral, thermal, electrochemical and
photoluminescence properties
Tugba Eren a, Muhammet Kose a, Nurcan Kurtoglu b, Gokhan Ceyhan a, Vickie McKee c,d
,
Mukerrem Kurtoglu a,
⇑
a Department of Chemistry, Kahramanmaras Sutcu Imam University, Kahramanmaras 46050, Turkey
b Department of Textile Engineering, Kahramanmaras Sutcu Imam University, Kahramanmaras 46050, Turkey
c Department of Chemistry, Loughborough University, Leicestershire LE11 3TU, UK
d School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a new azo-azomethine ligand, 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-(propan-2-yl)phenyl]
imino}methyl]phenol (HL) and its Cu(II) complex, [CuL2] were synthesized and characterized by the
analytical and spectroscopic methods such as elemental analyses, FT-IR, 1H, 13C NMR and mass spectra.
Molecular structures of the ligand and its Cu(II) complex were determined by single crystal X-ray
diffraction studies. In the structure of the ligand, there is an intramolecular phenol-imine hydrogen bond
(O1ꢀ ꢀ ꢀN1) with a distance of 2.580(3) Å. There are also intermolecular hydrogen bond type interactions
CHꢀ ꢀ ꢀO and CHꢀ ꢀ ꢀN@N stabilizing the structure. The crystal structure of the Cu(II) was solved in the
Received 17 November 2014
Received in revised form 10 February 2015
Accepted 5 March 2015
Available online 26 March 2015
Keywords:
Azo-azomethine
Copper complex
Crystal structure
Spectroscopic studies
Electrochemistry
Photoluminescence
ꢀ
triclinic space group P1. In the structure of the complex, the Cu(II) ion is four coordinate bonded to two
phenolate oxygens and two imine nitrogens of two ligand molecules in an approximate square-planar
geometry. The Cu–O bonds are in trans configuration. Thermogravimetric data revealed that the Cu(II)
complex is thermally more stable than the azo-azomethine compound (HL). Upon coordination with
Cu(II) ion, both excitation and emission intensities of the ligand shifted to lower values.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
In the last few decades many copper complexes have been
reported [13,14]. The growing interest on the coordination com-
Azo compounds, containing two phenyl rings separated by an
azo (–N@N–) bond, are versatile molecules and have received
much attention in both fundamental and applied research areas.
Azo dyes have been widely used in many practical applications
such as coloring fibers [1], photoelectronics [2], printing systems
[3], optical storage technology [4,5], textile dyes [6,7] as well as
in many biological reactions [8,9] and in analytical chemistry
[10,11]. Both Schiff bases and azo compounds are also important
structures in medicinal and pharmaceutical fields and it has been
suggested that the azomethine linkage might be responsible for
the biological activities displayed by Schiff bases [12]. These
compounds have also received special attention because of their
mixed soft–hard donor characters, versatile coordination behavior,
optical and pharmacological properties and thermal properties.
pounds of copper with various N-donor ligands, comes mainly
from their capability of combining characteristic structural flexibil-
ity [13,14], mimicking of protein active sites [15–17], ease of pre-
paration [18,19], and stabilization of both oxidation states of the
metal usual in biological systems [20,21]. Considerable interest
in various N and O donor ligands especially Schiff bases and their
transition metal complexes have also grown in the areas of
chemistry and biology due to biological activities, such as antiviral
[22,23], antitumor [24,25], bactericidal [26,27], fungicidal [18] and
nonlinear optical properties [28,29]. This type of the compounds
have been used for metal analyses, for device applications related
to telecommunications, optical computing [30,31], storage
[32,33], and information processing [34,35]. The redox potential
of copper complexes with various N and O-donor ligands has
deserved a strong interest since it seems to be directly related to
some of the biologically important properties of this type of
compounds. Additionally, many of Schiff bases exhibit tautomeric
rearrangements because of intramoleculer proton transfer
⇑
Corresponding author. Tel.: +90 344 280 1450; fax: +90 344 280 1352.
E-mail addresses: kurtoglu01@gmail.com, mkurtoglu@ksu.edu.tr (M. Kurtoglu).
http://dx.doi.org/10.1016/j.ica.2015.03.010
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
269
between the enolimine (OH) and the ketonamine (NH) forms pos-
sessing different electronic absorption spectra. X-ray single-crystal
diffraction is a very powerful tool to investigate tautomerisms
because it provides detailed information on molecular con-
formation and supromolecular interactions in the solid state. The
tautomerism of azomethines received considerable interest due
to application of improved experimental and theoretical methods.
Recently, some azo-azomethines and their metal chelates have
been reported [36–39]. In our previous work, 4-[(E)-phenyl-
diazenyl]-2-[(E)-(phenylimino)methyl]phenol and 2-{(E)-[(2-hy-
droxy-5-methylphenyl)imino]methyl}-4-[(E)-phenyldiazenyl]phenol
compounds were synthesized and structurally characterized
[40,41].
[CuL2] were collected on a Perkin Elmer LS55 luminescence spec-
trometer. All samples were prepared in spectrophotometric grade
solvents and analyzed in a 1 cm optical path quartz cuvette.
Data for the ligand and its copper(II) complex were collected at
150(2) K on a Bruker ApexII CCD diffractometer using Mo K
a
radiation (k = 0.71073 Å). Data reduction was performed using
Bruker SAINT [42]. SHELXTL was used to solve and refine the structures
[43].
2.3. Synthesis of 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde (a)
The azo-aldehyde compound 2-hydroxy-5-[(E)-phenyldiazenyl]
benzaldehyde (a) was prepared according to the published papers
[40,41].
Because of the importance of azo-azomethine compounds and
in continuance of our interest in the synthesis of azo-azomethine
2.4. Synthesis of 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-(propan-2-
yl)phenyl]imino}methyl] phenol, (HL)
compounds and their transition metal complexes,
a novel
azo-azomethine ligand, 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-(pro-
pan-2-yl)phenyl]imino}methyl]phenol and its Cu(II) complex were
prepared and characterized by spectroscopic and analytic methods.
Molecular structures of the ligand and its Cu(II) complex were also
determined by single crystal X-ray diffraction studies. Thermal
studies of the compounds were performed under nitrogen atmo-
sphere in the temperature range of 20–1000 °C. Electrochemical
and photoluminescence properties of the synthesised compounds
were also investigated.
The azo-azomethine ligand (HL) was prepared according to the
known condensation method. 4-Isopropylaniline (0.135 g,
10 mmol) and 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde (a)
(0.226 g, 10 mmol) were dissolved in 30 mL MeOH. The solution
was refluxed for 2 h and then cooled to room temperature. Upon
cooling, the azo-azomethine ligand was obtained as red-orange
microcrystals which were filtered off, washed with cold MeOH
and dried in air. X-ray quality crystals of the ligand were obtained
by slow evaporation of the methanol solution.
2. Experimental
Yield: 0.146 g (42.5%). m.p.: 139–140 °C. Anal. Calc. for
C
22H21N3O: C, 76.94; H, 6.16; N, 12.24. Found: C, 76.28; H, 5.97;
N, 11.96%. IR (KBr, cmꢁ1): 3320 (OH), 3050 (aromatic), 2957
(C–H), 1616 (C@N). 1H NMR (CDCl3 as solvent,
in ppm); 1.32
2.1. Chemicals
c
(d, CH13,2, J = 6.93 Hz), 2.99 (st, CH3, J = 6.93 Hz), 7.17 (d, aromatic
CH13, J = 6.13 Hz), 7.31 (d, aromatic CH14, J = 6.13 Hz), 7.33 (d, aro-
matic CH5,9, J = 8.45 Hz), 7.35 (s, aromatic CH16), 7.49 (t, aromatic
CH20, J = 6.76 Hz), 7.56 (t, aromatic CH19,21, J = 7.37 Hz), 8.04 (d,
aromatic CH6,8, J = 8.45 Hz), 8.07 (d, aromatic CH18,22, J = 7.37 Hz),
8.78 (s, 1H, –CH10@N–), 14.06 (b, 1H, –OH). 13C NMR (CDCl3 as sol-
All reagents and solvents for the synthesis and analysis were
purchased from commercial sources and used as received unless
otherwise noted.
2.2. Physical measurements
vent,
c
in ppm): 24.02 (C1, C2), 33.83 (C3), 118.17 (C13), 119.00
NMR spectra were performed using a Bruker Advance 400 MHz.
Spectrometer. Mass spectra were recorded on a Thermo Fisher
Exactive + Triversa Nanomate mass spectrometer. The FT-IR
spectra were obtained (4000–400 cmꢁ1) using a Perkin Elmer
spectrum 100 spectrophotometer. Carbon, hydrogen and nitrogen
(C11), 121.15 (C6, C8), 122.61 (C16), 127.36 (C14), 127.53 (C5, C9),
127.78 (C19, C21), 130.56 (C20), 145.53 (C15), 148.47 (C4), 152.63
(C7), 161.19 (C17) 164.18 (C12), 164.23(C10). ESI mass (m/z): 239
(MꢁC6H9), 263 [MꢁC6H9]+, 344 [M+H]+, 366 [M+Na]+, 686 [2M]+,
709 [2M+Na]+.
elemental analyses were performed with
a model CE-440
elemental analyzer. Thermogravimetric analyses were done on a
Perkin Elmer Diamond TG/DTA (Technology by SII) under nitrogen
atmosphere with a heating rate of 20 °C/min. Melting points were
obtained with a Electrothermal LDT 9200 Apparatus in open
capillaries. Cyclic voltammograms were recorded on a Iviumstat
Electrochemical workstation equipped with a low current module
(BAS PA–1) recorder. The electrochemical cell was equipped with a
BAS glassy carbon working electrode (area 4.6 mm2), a platinum
coil auxiliary electrode and a Ag/AgCl reference electrode filled
with tetrabutylammonium tetrafluoroborate (0.1 M) in DMF and
CH3CN solution and adjusted to 0.00 V versus SCE. Cyclic voltam-
metric measurements were performed at room temperature in
an undivided cell (BAS model C–3 cell stand) with a platinum
counter electrode and an Ag/AgCl reference electrode (BAS). All
potentials are reported with respect to Ag/AgCl. The solutions were
deoxygenated by passing dry nitrogen through the solution for
30 min prior to the experiments, and during the experiments the
flow was maintained over the solution. Digital simulations
were performed using DigiSim 3.0 for windows (BAS, Inc.).
Experimental cyclic voltammograms used for the fitting process
had the background subtracted and were corrected electronically
for ohmic drop. The single-photon fluorescence spectra of the
azo-aldehyde (a), azo-azomethine ligand HL and its Cu (II) complex
2.5. Synthesis of the Cu(II) complex [CuL2]
The complex [CuL2] was synthesized by a general method. To a
hot solution of HL ligand (0.250 g, 0.73 mmol) in 35 mL CHCl3 was
added 1/2 equivalent amount of copper(II) acetate (0.073 g,
0.37 mmol) in 25 mL CHCl3 and 20 mL EtOH. The mixture was
refluxed for 3 h and then cooled. A brown-red precipitate formed
was collected by filtration, washed with cold water and MeOH
and dried under vacuum. The brown single crystals of the complex
were obtained by slow evaporation of the chloroform solution.
Yield: 0.250 g. (91%). m.p.: 267–268 °C. Anal. Calc. for C44H40
CuN6O2: C, 70.62; H, 5.39; N, 11.23. Found: C, 70.13; H, 5.33; N,
10.54%. IR (KBr, cmꢁ1): 3065 (C–H aromatic), 2956, 2921 (C–H iso-
propyl), 1611 (C@N), 1595 (C@C), 1471 (N@N), 1379 (C–N), 1341
(C–O), 589 (Cu–O), 462 (Cu–N). ESI mass (m/z): 770 [C44H40
CuN6O2+Na]+, 538 [C32H31CuN2O2ꢁ1]+, 468 [C27H21CuN2O2ꢁ1]+,
344 [C17H17CuN2O2]+, 308 [C23H17CuN2O2+1]+.
2.6. X-ray structure solution and refinement
The structures were solved by direct methods and refined on F2
using all the reflections [43]. All the non-hydrogen atoms were

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T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
benzaldehyde obtained by treating a diazonium salt solution of
Table 1
Crystallographic data of the ligand and its Cu(II) complex.
aniline with salicylaldehyde in MeOH media with an aqueous solu-
tion of NaNO2 at ꢁ5 °C. The general preparation scheme, involving
aniline diazonium salt solution, diazo coupling and condensation,
is shown in Scheme 1. The level of impurity in the product was
checked by t.l.c. The synthesized ligand (HL) was subsequently
characterized by its melting point, elemental analysis, FT-IR, 1H
and 13C NMR spectroscopic studies and CHN microanalysis. The
results of these analyses are presented in experimental section
and the mass, 1H and 13C NMR spectra are displayed in Figs. 1–3,
respectively. The elemental analysis results of the azo-azomethine
ligand HL are in good agreement with the theoretical values.
The azo-azomethine ligand, 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-
(propan-2-yl)phenyl]imino}methyl]phenol, formed a mononuclear
complex with Cu(II) as following Eq. (1):
Identification code
Empirical formula
Formula weight
Crystal size (mm3)
Crystal color
Crystal system
Space group
Unit cell
HL
[CuL2]
C44H40CuN6O2
748.36
C22H21N3O
343.42
0.64 ꢃ 0.13 ꢃ 0.07
0.40 ꢃ 0.26 ꢃ 0.10
orange
brown
orthorhombic
Pna21
triclinic
ꢀ
P1
a (Å)
b (Å)
c (Å)
29.466(4)
10.1273(12)
6.1388(8)
90
12.0937(12)
14.1101(14)
14.2416(14)
60.4500(10)
65.7310(10)
64.9300(10)
1850.9(3)
2
0.637
21919
7585 [0.0360]
0.0364, 0.0833
0.0517, 0.0902
902594
a
(°)
b (°)
90
c
(°)
90
1831.9(4)
4
0.08
18061
V (Å3)
Z
Absorption coefficient (mmꢁ1
Reflections collected
)
CHCl3—EtOH
2HL þ CuðCH3COOÞ2
½CuL ꢂ þ 2CH COOH
ð1Þ
ƒƒƒƒƒ!
2
3
Reflux
Independent reflections [Rint
]
2491 [0.0394]
0.0414, 0.0965
0.0533, 0.1030
841748
The structure of the Cu(II) complex was also elucidated using a
number of analytical methods and spectroscopic techniques. The
level of impurity in the product was also checked with t.l.c. The
metal complex, [CuL2], is stable in air as solid without decom-
position, and soluble in chloroform, dichloromethane, dimethylsul-
foxide and dimethylformamide and insoluble in n-hexane and
water. Single crystals of the complex suitable for X-ray diffraction
study were obtained from the CHCl3 solution of the complex by
slow evaporation.
R1, wR2 [I > 2 (I)]
r
R1, wR2 (all data)
CCDC number
refined using anisotropic atomic displacement parameters and
hydrogen atoms bonded to carbon atoms were inserted at calcu-
lated positions using a riding model. The phenolic hydrogen atom
in the ligand (HL) was located from difference maps and not further
refined. Details of the crystal data and refinement are given in
Table 1.
The results of the elemental analyses of the compound are in
good agreement with the theoretical values. The analyses of the
[CuL2] coordination compound indicate that the metal-ligand ratio
is 1:2 in the Cu(II) complex.
3. Results and discussion
3.1. Synthesis
3.2. 1H- and 13C NMR spectra
The azo-azomethine ligand, 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-
(propan-2-yl)phenyl]imino}methyl] phenol was prepared by react-
ing 4-isopropylaniline with 2-hydroxy-5-[(E)-phenyldiazenyl]
The 1H and 13C NMR spectral data of the bidentate azo-azome-
thine ligand recorded in CDCl3 are given in experimental section.
The NMR spectral analysis was carried out for the azo-azomethine
O
NH2
N
N
i
HO
ii
O
NH
N
N
N
N
N
OH
azo-imine form
azo-enamine form
O
N
N
NH
hydrozone-imine form
i: (1) NaNO2, HCl, 0 oC, (2) salicylaldehyde; ii) 4-isopropylaniline, MeOH and reflux
Scheme 1. Preparation reaction of 4-[(E)-phenyldiazenyl]-2-[(E)-{[4-(propan-2-yl)phenyl]imino}methyl]phenol, (HL) dye and its possible tautomeric equilibriums.

T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
271
Fig. 1. 1H NMR spectrum of the ligand HL.
Fig. 2. 13C NMR spectrum of the ligand HL.

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T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
Fig. 3. Mass spectrum (top) and isotope distribution (bottom) of the ligand HL.
ligand synthesized which provided further evidence for the struc-
tural characteristics of the ligand. The spectra of HL ligand are
shown in Figs. 1 and 2, respectively. The 1H NMR spectrum of
the ligand shows two peaks at 14.06 [44] and 8.78 ppm character-
istic of the hydroxyl (O–H) and azomethine (–CH@N–) protons,
respectively [40]. This indicates that the azo-imine form is more
stable than other forms in CDCl3. The signals associated with
different aromatic and aliphatic protons can be observed in the
J = 6.93 Hz) are due to CH13,2 and methylene CH3 protons according
to the chemical shift.
The 13C NMR spectrum of the ligand exhibited the signals due to
the presence of aromatic and aliphatic carbons. The presence of
azomethine group is evident from the characteristic signal at the
farthest downfield (ꢄ164.23 ppm) corresponding to the carbon of
azomethine group (C10). The peak at 164.18 ppm was assigned to
the aromatic carbon linked hydroxyl group (C12). The aliphatic
region of the 13C NMR spectrum of HL showed two peaks as seen
in Fig. 2. The medium intense peak CH3 (24.02 ppm, C1, C2) was
conveniently assigned to the methyl carbon of the isopropyl group.
The peak –CH– (33.83 ppm, C3) is due to tersiyer carbon according
to the chemical shift. In the aromatic region of the 13C NMR spec-
trum of compound HL, the peaks at 119.00 (C11), 121.15 (C6, C8),
spectrum. The peaks at 7.17 (d, CH13, J = 8.27 Hz), 7.31 (d, CH14
,
J = 6.13 Hz), 7.33 (d, CH5,9 J = 7.66 Hz), 7.35 (s, CH16), 7.49
,
(t, CH20, J = 6.76 Hz), 7.56 (t, CH19,21, J = 7.37 Hz), 8.04 (d, CH6,8
,
J = 8.45 Hz), and 8.07 ppm (d, CH18,22, J = 9.78 Hz) were assigned
to the aromatic protons. As expected, the isopropyl group showed
two signals at 1.32 (d, CH13,2, J = 6.92 Hz) and 2.99 (st, CH3,

T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
273
122.61 (C16), 127.36 (C14), 127.53 (C5, C9), 127.78 (C19, C21), 130.56
(C20), 145.53 (C15), 148.47 (C4), 152.63 (C7) and 161.19 (C17) ppm
were assigned to the aromatic carbons. The peaks were then tenta-
tively assigned by matching the observed and calculated chemical
shifts. The spectroscopic data obtained in this work are agreed well
with previous works [40].
3320 cmꢁ1 and comparatively weak band at 1616 cmꢁ1 were
assigned to phenolic OH [45] and azomethine C@N stretching
vibrations of the azomethine ligand containing –N@N– chro-
mophore, respectively [46,47]. The absence of band due to phenolic
OH group in the spectrum of the metal complex suggests the coor-
dination of ligand to the metal via deprotonation. A strong band
observed in the IR spectrum of the ligand in the region of 3400–
3200 cmꢁ1 is due to hydrogen bonded
m(OH) in the azo tautomer.
3.3. FT-IR spectra
In the copper(II) complex, the band at 1616 cmꢁ1 shifted toward
lower frequency because of the coordination of the nitrogen to
the copper ion. This fact can be explained by the withdrawing of
electrons from nitrogen atom to the metal ion due to coordination.
The aromatic and aliphatic (isopropyl) moieties in the ligand can
be identified by the bands at 3050 cmꢁ1 (aromatic C–H stretching)
and 2957 cmꢁ1 (aliphatic C–H stretching). The ligand exhibits a
broad medium intensity band at 2700 cmꢁ1, which is assigned to
the intramolecular hydrogen-bonding O–Hꢀ ꢀ ꢀN vibration. This sit-
uation is common for aromatic azomethine compounds containing
o-OH groups, and in the complex this band disappears completely.
The complex [CuL2] shows a band at 1471 cmꢁ1 could be
assigned to the –N@N– azo group. The new band at 589 cmꢁ1 for
the Cu(II) complex is assigned to (Cu–O) [48]. The band observed
at 462 cmꢁ1 can be assigned to (Cu–N), indicating the participation
of azomethine nitrogen in the coordination to copper ion.
Accordingly, the azo-azomethine ligand acts as monobasic biden-
tate chelating agent coordinated to copper(II) via the azome-
thine–nitrogen and phenol–oxygen atoms forming the more
stable six-membered chelate ring. A medium intensity band in
the region 2921–2956 cmꢁ1 is due to the aliphatic (C–H) stretch-
ing. The weak absorption peak observed at 3065 cmꢁ1 is attributed
to the (C–H) of the phenyl rings. A comparison between infrared
spectra of HL and the [CuL2] complex also show that a band, char-
The important infrared characteristic absorption bands of the
ligand and its copper(II) complex, along with their proposed
assignments, are summarized in experimental section. The
functional groups of the azo-azomethine ligand and its copper(II)
chelate have been identified from their infrared spectra. Schiff
bases are capable of forming coordinate bonds with many metal
ions through both azomethine group and phenolic group or via
its azomethine or phenolic groups. A strong absorption band at
HO
+
N
N
N
+Na
-Na
H
N
N
N
m/z=709
HO
•
C44H42N6O2
m/z=686.329
acteristic of m
(C–O) at 1315 cmꢁ1, shifted to 1345–1325 cmꢁ1, due
+
+
to C–O–Cu bond formation.
3.4. Mass spectra
N
N
N
N
N
N
The mass spectral studies the azo-azomethine ligand (HL) and
its metal complex [CuL2] were performed using ESI method and
obtained data are given in experimental section. The mass
spectrum and isotope distribution of the ligand (HL) is shown in
+H
+Na
HO
HO
Table 2
UV–Visible absorption spectra of HL and [CuL2] in various solvents.
Compounds
Solvent
kmax (nm, L molꢁ1 cmꢁ1 ꢃ 103)
HL
Dichloromethane
Chloroform
Dimethylformamide
Dimethylsulfoxide
235 (30.43), 335 (10.20)
230 (30.88), 335 (10.02)
240 (30.27), 330 (10.89)
235 (33.43), 335 (7.30)
m/z=366
m/z=344
[CuL2]
Dichloromethane
Chloroform
Dimethylformamide
Dimethylsulfoxide
230 (34.12), 305 (11.92), 385 (21.09)
235(30.56), 300 (10.90), 385(19.52)
245(30.88), 295 (16.32), 375(20.09)
240(33.48), 301 (7.20), 390 (10.98)
•
C6H9
m/z=81
C16H12N3O•
m/z=262
+H
N
N
C14H13N3O••
m/z=239
N
N
N
•
C2H2
m/z=26
HO
HO
N
Scheme 2. Mass fragmentation pattern of the azo-azomethine dye (HL).
Fig. 4. Absorption spectra of the azo-azomethine dye in various solvents.

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T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
Table 4
Hydrogen-bond geometry (Å, °) for the ligand (HL).
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
C20–H20ꢀ ꢀ ꢀN2i
C10–H10ꢀ ꢀ ꢀO1ii
C13–H13ꢀ ꢀ ꢀC8iii
C13–H13ꢀ ꢀ ꢀC7iii
O1–H1ꢀ ꢀ ꢀN1
0.95
0.95
0.95
0.95
0.94
2.83
2.82
2.85
2.84
1.75
3.509 (3)
3.585 (3)
3.606 (3)
3.359 (3)
2.580 (3)
129.6
138.6
137.0
115.6
145.3
Symmetry codes: (i) ꢁx + 1/2, y ꢁ 1/2, z + 1/2; (ii) x, y, z + 1; (iii) ꢁx + 1, ꢁy + 2, z ꢁ 1/2.
Fig. 5. Absorption spectra of the Cu(II) complex in various solvents.
distribution for [M+1]+ is well agreed with the theoretical isotope
distribution (Fig. 3). The mass spectrum of the ligand shows two
fragmentation peaks at m/z 262 and m/z 239. These peaks can be
attributed to the [C16H12N3O+1]+ and [C14H13N3O]+ ions, respec-
tively. On the other hand, a peak at m/z 686 can be assigned to
[2M]+ dimeric structure ion. The higher mass peak at m/z 709
(60%) can be attributed to [2M+Na]+. The mass spectrum of the
copper(II) complex shows molecular ion signal at m/z 770 assigned
to [M+Naꢁ1]+. Moreover, the fragmentation peaks at m/z 537 and
m/z 468 can be attributed to the [C32H31CuN2O2ꢁ1]+ and
[C27H21CuN2O2ꢁ1]+ ions, respectively.
Fig. 6. Perspective view of the ligand HL with atom numbering; thermal ellipsoid
50% probability, the hydrogen bond is shown as a dashed line.
3.5. Electronic spectra
Table 3
The UV–Visible spectra of the HL azo-azomethine ligand and its
copper(II) metal chelate, [CuL2] in various solvents with different
polarities such as chloroform, dichloromethane, dimethylfor-
mamide and dimethylsulfoxide between 200 and 800 nm were
taken and their wave lengths of absorption maximum are reported
in Table 2. The spectra of the HL and [CuL2] compounds in various
solutions are shown in Figs. 4 and 5, respectively. The spectro-
scopic data obtained in this work are agreed well with the previous
work [40]. Electronic spectrum of the HL ligand in these solvents
showed broad absorption bands in the range of 230–240 nm.
Selected bond lengths and angles [Å and °] for the ligand (HL).
N(1)–C(10)
C(7)–N(1)
C(12)–O(1)
C(15)–N(2)
N(2)–N(3)
N(3)–C(17)
1.280(3)
1.421(3)
1.346(2)
1.420(3)
1.259(2)
1.434(3)
C(10)–N(1)–C(7)
O(1)–C(12)–C(13)
O(1)–C(12)–C(11)
C(14)–C(15)–N(2)
C(16)–C(15)–N(2)
C(22)–C(17)–N(3)
N(2)–N(3)–C(17)
C(18)–C(17)–N(3)
N(3)–N(2)–C(15)
122.1(2)
118.79(19)
121.16(19)
124.98(18)
115.92(19)
123.94(18)
113.57(17)
115.89(18)
114.45(17)
C(8)–C(7)–N(1)
C(6)–C(7)–N(1)
N(1)–C(10)–C(11)
116.7(2)
124.7(2)
121.4(2)
These absorption bands can be attributed to
p ?
p⁄ transitions
[40,41]. The
p
?
p⁄ transitions are low energy transitions observed
in organic molecules and are used in structural analysis. Since the
Fig. 3 and the fragmentation pattern for the ligand is presented in
Scheme 2. The mass spectrum of the azo-azomethine ligand
showed signals at m/z 344 (40%) and 366 (100%) assigned to
[M+1]+ and [M+Na]+, respectively. The experimental isotope
azo-azomethine ligand contains an imine (–HC@N–) and an azo
(–N@N–) chromophore groups, the
p ?
p⁄ transition of the ligand
observed in different wavelengths in different solvents, which can
be attributed to the polarity of the used solvent. In general, the
Fig. 7. Packing plot of the ligand HL viewing down c axis, hydrogen bonds are shown as dashed lines.

T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
275
Fig. 8. Perspective view of the complex [CuL2].
Table 5
to antibonding orbital of the ligand and a spin-allowed transition
of the ligand. However, no absorption band attributed to the d–d
transitions in the metal center was observed. It may be lost in
the low-energy tail of the intense transition.
Selected bond lengths and angles [Å and °] for the complex [CuL2].
Cu(1)–O(1)
Cu(1)–O(2)
1.8701(13)
1.8787(14)
Cu(1)–N(1)
Cu(1)–N(4)
2.0102(16)
2.0175(16)
O(1)–Cu(1)–O(2)
O(1)–Cu(1)–N(1)
O(2)–Cu(1)–N(1)
172.24(6)
92.23(6)
87.66(6)
O(1)–Cu(1)–N(4)
O(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(4)
88.84(6)
91.98(6)
174.65(6)
3.6. Crystal structure of the azo-azomethine ligand (HL)
The ligand can be easily isolated as crystalline material with a
sharp melting point therefore its molecular structure can also be
characterized by single crystal X-ray analysis, the most straight-
forward technique to confirm the identity. The title compound
crystallizes in the non-centric orthorhombic space group Pna21.
A perspective view of the molecule with atom numbering is
shown in Fig. 6. Selected bond lengths and angles are given in
Table 3, all bond lengths and angles are within the normal
ranges. The diazenyl (–N2@N3) and azomethine (–C10@N1–)
linkages are 1.259(2) and 1.280(3) Å, respectively; similar to
those observed for related azo-azomethine compounds [29,30].
In the structure, both outer rings slightly twisted with respect
to the central benzene ring. The mean planes of C4–C9 and
C17–C22 are at 10.30(14) and 12.23(12)° to the central (C11–
C16) ring, respectively. Aromatic rings (C4–C9) and (C11–C16)
adopt the trans configuration with regard to the azo double bond
(–N@N–) with the torsion angle C(15)–N(2)–N(3)–C(17) of
178.18(17)° which is in good agreement with published values
[29]. There is an intramolecular phenol-imine hydrogen bond
p
?
p⁄ transitions shift to longer wavelengths as the polarity of the
solvent increases. Due to the dipole moment of the solvent, a new
dipole moment on the ligand is formed. This solvent effect is
greater on the p⁄ orbital than the
p
orbital. Therefore, the energy
p⁄ transitions decreases as the energy level of the p⁄
orbital decreases and, the
p⁄ transition absorption red shifts.
However, in the UV–Visible spectrum of the ligand, weaker and
broader (compare to the
p⁄ transitions) absorption bands in
of the
p ?
p
?
p
?
the range of 300–400 nm were observed. These transitions bands
could be attributed to the n ? p⁄ transitions due to the presence
of n electrons on the –CH@N– or –N@N– chromophore groups.
Thus, the n ? p⁄ transition bands shifted to shorter wavelengths
in more polar solvents. Similar absorption bands were observed
for the [CuL2] metal chelate in the same region. An additional band
was observed at around 375–390 nm assigned to the coordination
of azomethine nitrogen atom to the metal center. These bands
were assigned to both a charge transfer transition from the metal
Fig. 9.
p–p stacking interactions in [CuL2].

276
T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
Fig. 10. Packing diagram of [CuL2].
Fig. 11. Cyclic voltammograms of the synthesized compounds in the presence of 0.1 M NBu4BF4-DMF solution.

T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
277
(O1ꢀ ꢀ ꢀN1) with a distance of 2.580(3) Å forming a S(6) hydrogen
3.8. Thermal analysis
bonding motif. There are also weak intermolecular hydrogen
bond type interactions C10–Hꢀ ꢀ ꢀO and C20–Hꢀ ꢀ ꢀN(2)@N(3) (with
Dꢀ ꢀ ꢀA distances of 3.585(3) and 3.509(3) Å, respectively) stabiliz-
ing the structure (Fig. 7). Hydrogen bond parameters are listed
in Table 4.
Thermogravimetric analysis of the azo-azomethine ligand HL
and its Cu(II) complex were examined under N2 atmosphere and
the TG/DTA curves are shown in Figs. S1 and S2. The ligand HL
shows a weight loss of ꢅ0.4% in the temperature range 103–
183 °C accompanied by an endothermic peak with Tmax = 145 °C
on the DTA curve. The loss may be due to small amount of moisture
in the sample. The main framework of the ligand is thermally
stable up to 183 °C. The ꢅ54% of the sample decomposes in the
temperature range 183–404 °C with an exothermic peak
(Tmax = 331 °C). The rest of the organic moiety gradually decom-
poses up to higher temperatures.
Thermogravimetric data shows that the Cu(II) complex
thermally more stable than the azo-azomethine compound (HL).
The Cu(II) complex [CuL2] exhibits a weight loss of ꢅ1.3% in the
temperature range 103–183 °C and this weight loss was attributed
to small amount of moisture in the sample. The loss in the tem-
perature range 103–183 °C follow an endothermic peak with
Tmax = 190 °C. The ꢅ47.6% of the sample decomposed in the tem-
perature range 226–458 °C with an endothermic (274 °C) and an
exothermic (330 °C) peaks. Rest of the organic moiety slowly
decomposes up to higher temperatures (up to 1000 °C) leaving
CuO as final residue.
3.7. Crystal structure of the complex [CuL2]
ꢀ
The structure was solved in the triclinic space group P1. The
molecular structure is depicted in Fig. 8. Selected bond lengths
and angles are listed in Table 5. All bond lengths and angles
are within the normal ranges. The asymmetric unit contains
two independent bi-dentate azo-azomethine ligands and one
Cu(II) center. Azo-benzene moiety (N5–C44) of the one of the
ligands is disordered and this was modelled over two positions
with 60:40 occupancy (N5–C44: N50–C440). The Cu(II) ion is
coordinated to two phenolate oxygen atoms and two
imine nitrogen atoms of two azo-azomethine molecules with
approximate square planar geometry. The Cu–O bonds are in
trans configuration and Cu–O distances are shorter than Cu–N
distances.
The iso-propylbenzene ring in both independent ligands signifi-
cantly twisted with respect to the central phenolate ring with the
dihedral angles of 66.93(7)° for rings C4–C9 and C11–C16 and
64.59(6)° for C26–C31 and C33–C38 this is probably due to mini-
mize the steric hindrance to bind metal center.
3.9. Electrochemistry
Electrochemical properties of the synthesized azo-aldehyde (a),
azo-azomethine (HL) and its Cu(II) complex [CuL2] were studied in
DMF – 0.1 M NBu4BF4 as supporting electrolyte at room tempera-
ture. Electrochemical behaviors of the compounds were investi-
gated in two different concentrations (1 ꢃ 10ꢁ3 and 1 ꢃ 10ꢁ4 M)
and five different scan rates (in the 100–1000 mV/s range) against
The principal interactions between complex molecules are p–p
stacking interactions. There are two sets of the structure
illustrated in Fig. 9. Crystal packing of the complex molecules is
determined by
p–p stacking interactions. Packing diagram is
shown in Fig. 10.
Table 6
The electrochemical data of the synthesized compounds.
Compounds
(a)
Conc. (M)
Scan-rate (mV/s)
Epa (V)
Epc (V)
E1/2 (V)
Ipa/Ipc
DEp (V)
1 ꢃ 10ꢁ3
100
250
500
750
1000
100
250
500
750
1000
ꢁ0.30, 0.94
ꢁ0.33, 0.86
ꢁ0.40, 0.85
ꢁ0.45, 0.80
ꢁ0.54, 0.77
ꢁ0.50, 0.63
ꢁ0.53, 0.56
ꢁ0.56, 0.53
ꢁ0.60, 0.46
ꢁ0.66, 0.40
0.94, 0.09, ꢁ1.13
0.86, 0.12, 1.17
0.85, 0.15, ꢁ1.23
0.80, 0.21, ꢁ1.26
0.77, 0.24, ꢁ1.33
1.00, ꢁ1.13
0.94
0.86
0.85
0.80
0.77
–
–
–
–
–
1.00
1.00
1.00
1.00
1.00
0.63
0.58
0.43
0.51*
0.45
0.83
0.84
0.83
0.81
0.79
0.63
0.67
0.67
0.70
0.67
1 ꢃ 10ꢁ4
0.95, ꢁ1.20
0.93, ꢁ1.23
0.90, ꢁ1.30
0.87, ꢁ1.33
HL
1 ꢃ 10ꢁ3
100
250
500
750
1000
100
250
500
750
1000
ꢁ0.67, 0.30, 1.33
ꢁ0.64, 0.33, 1.36
ꢁ0.61, 0.39, 1.43
ꢁ0.54, 0.42, 1.47
ꢁ0.45, 0.48, 1.50
ꢁ0.46, 0.34
1.33, 0.30, ꢁ1.30
1.36, 0.33, ꢁ1.33
1.43, 0.39, ꢁ1.35
1.47, 0.42, ꢁ1.43
1.50, 0.48, ꢁ1.49
0.79, ꢁ1.30
0.30
0.33
0.39
0.42
0.48
–
–
–
–
–
1.00
1.00
1.00
1.00
1.00
0.37
0.41
0.47
0.48
0.50
0.63
0.69
0.74
0.89
1.04
0.84
0.90
1.00
1.05
1.13
1 ꢃ 10ꢁ4
ꢁ0.43, 0.37
0.81, ꢁ1.33
ꢁ0.40, 0.40
0.84, ꢁ1.40
ꢁ0.38, 0.42
0.87, ꢁ1.43
ꢁ0.34, 0.45
0.90, ꢁ1.47
[CuL2]
1 ꢃ 10ꢁ3
100
250
500
750
1000
100
250
500
750
1000
0.09, 1.06
0.10, 1.12
0.12, 1.21
0.14, 1.27
0.15, 1.33
ꢁ0.45, 0.85
ꢁ0.42, 0.80
ꢁ0.36, 0.74
ꢁ0.30, 0.71
ꢁ0.27, 0.65
1.06, ꢁ0.06, ꢁ1.33
1.12, ꢁ0.09, ꢁ1.30
1.21, ꢁ0.12, ꢁ1.23
1.27, ꢁ0.15, ꢁ1.16
1.33, ꢁ0.21, ꢁ1.13
0.85, ꢁ1.16
1.06
1.12
1.21
1.27
1.33
0.85
0.80
0.74
0.71
0.65
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.15
0.19
0.24
0.29
0.36
0.71
0.78
0.90
1.00
1.09
1 ꢃ 10ꢁ4
0.80, ꢁ1.20
0.74, ꢁ1.26
0.71, ꢁ1,30
0.65, ꢁ1.36
*
Epa and Epc are anodic and cathodic potentials, respectively. E1/2 = 0.5 ꢃ (Epa + Epc),
D
Ep = Epa ꢁ Epc
.

278
T. Eren et al. / Inorganica Chimica Acta 430 (2015) 268–279
Table 7
The photoluminescence data for the prepared compounds.
Compounds Conc.(M) Excitation
Emission
k (nm) Intensity (a.u) k (nm) Intensity (a.u)
(a)
1 ꢃ 10ꢁ3
1 ꢃ 10ꢁ4
1 ꢃ 10ꢁ5
1 ꢃ 10ꢁ6
1 ꢃ 10ꢁ7
385
387
392
416
439
134
98
78
45
32
629
624
618
602
595
144
138
97
74
69
HL
1 ꢃ 10ꢁ3
1 ꢃ 10ꢁ4
1 ꢃ 10ꢁ5
1 ꢃ 10ꢁ6
1 ꢃ 10ꢁ7
394
396
398
400
402
124
96
72
53
38
625
621
618
615
611
140
128
92
65
51
[CuL2]
1 ꢃ 10ꢁ3
1 ꢃ 10ꢁ4
1 ꢃ 10ꢁ5
1 ꢃ 10ꢁ6
1 ꢃ 10ꢁ7
408
410
412
414
420
127
87
62
36
18
678
670
664
658
650
130
109
84
68
36
peak potentials observed in 1 ꢃ 10ꢁ3
M disappeared and all
reduction and oxidation processes are irreversible in 1 ꢃ 10ꢁ4
M
concentration. The cyclic voltammograms for the Cu(II) complex
in the 100–1000 mv/s range exhibits three anodic and three peak
potentials in 1 ꢃ 10ꢁ3 M. However, in 1 ꢃ 10ꢁ4 M, there are two
anodic and two cathodic peak potentials. All the oxidation and
reduction processes of the complex at both concentrations are
reversible. As the scan rate increases, the cathodic and anodic peak
potentials shifted to more negative or positive regions. In the
reversible electrochemical processes of the Cu(II) complex may
be proposed as shown Eq. (2).
CuðIIÞ þ eꢁ ꢀ CuðIIÞ þ eꢁ ꢀ Cuð0Þ
ð2Þ
ꢁeꢁ
ꢁeꢁ
3.10. Photoluminescence
The effect of different concentration on the photoluminescence
properties of the azo-aldehyde (a), azo-azomethine ligand HL and
its Cu (II) complex [CuL2] was investigated in the 1.0 ꢃ 10ꢁ3 to
1.0 ꢃ 10ꢁ7 M range in DMF solution. The emission and excitation
spectra of the synthesized compounds containing an azo chro-
mophore group (–N@N–) are shown in Fig. 12a–c and the data
are given in Table 7. The azo-aldehyde (a) and the azo-azomethine
ligand HL exhibit only one emission maxima in the 595–629 nm
range upon excitation in the 385–439 nm range. Both emission
and excitation intensities increased with the increase in
concentration. When concentration increased emission maxima
for the azo-aldehyde (a) and ligand (HL) shifted to higher wave-
length values (lower energy regions).
Fig. 12. The fluorescence emission and excitation spectra of the compound (a) azo-
aldehyde (a), (b) HL, (c) [CuL2] in various concentrations (1.0 ꢃ 10ꢁ3 to
1.0 ꢃ 10ꢁ7 M, DMF).
The complex [CuL2] exhibits similar excitation and emission
spectra to the azo-azomethine ligand (Fig. 12c). However, upon
coordination with Cu(II) ion, both excitation and emission intensi-
ties shifted to lower values. The reason why both excitation and
emission intensities reduced can be due to complex formation with
the Cu(II) ion. Therefore, the energy transfer from the excited state
of the ligand (HL) to the Cu(II) ion might be possible, hence non-
radiated transition of the ligand excited state increases and decli-
nes the emission intensity [49].
ferrocence–ferrocenium standard. The electrochemical curves of
synthesized compounds are shown in Fig. 11. The data are given
in Table 6. The azo-aldehyde in the 1 ꢃ 10ꢁ3 M shows two reversi-
ble anodic peak potentials (0.09 and 1.33 V) in 100–1000 mv/s scan
rate range and three cathodic peak potentials in the ꢁ1.33 to
1.17 V range. In the 1 ꢃ 10ꢁ4 M and scan rates range studied, the
azo-aldehyde (a), there are two anodic and cathodic peak poten-
tials are observed. The oxidation and reduction processes of the
compound in this concentration (1 ꢃ 10ꢁ4 M) are irreversible.
The azo (–N@N–) chromophore group containing Schiff base ligand
(HL) in the 1 ꢃ 10ꢁ3 M and 100–1000 mv/s range shows the two
reversible anodic peak potentials in the ꢁ0.54 to 0.94 V range. In
this scan rate range, the ligand shows three cathodic peak poten-
tials in the ꢁ1.33 to 0.94 V range. However, one of the cathodic
4. Conclusion
In summary, azo-azomethine ligand 4-[(E)-phenyldiazenyl]-2-
[(E)-{[4-(propan-2-yl)phenyl]imino}methyl]phenol (HL) derived
from 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde (a) was syn-
thesized and characterized. Elemental analyses confirms the

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mass, 1H- and 13C NMR spectroscopy confirms the functional
groups, particularly –N@N– and –HC@N imine groups, of the
ligand. The metal-to-ligand ratio of the copper(II) complex was
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Acknowledgments
The authors thank Research Found of Kahramanmaras Sutcu
Imam University for financial support (Project No: 2011/8-5 YLS),
Kahramanmaras, Turkey. The authors are grateful to the
Department of Chemistry, Loughborough University, Leicester, UK
for providing laboratory and analytical facilities.
Appendix A. Supplementary material
CCDC 841748 and 902594 contains the supplementary crys-
tallographic data for the ligand and its Cu(II) complex. These data
can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.ica.
2015.03.010.
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