Cobalt(II), nickel(II) and copper(II) complexes of a Schiff base ligand: synthesis, structural characterization and luminescence properties

Article abstract of DOI:10.1007/s11243-017-0195-z

A Schiff base ligand (HL), 2,4-dimethoxy-N-(5-chloro-2-hydroxybenzylidene)-benzenamine, derived from 5-chloro-2-hydroxybenzaldehyde and 2,4-dimethoxyaniline, and its metal complexes [Co(L)₂]·CH₃OH (1), [Ni(L)₂] (2), [Cu(L)₂] (3) have been synthesized. The compounds were characterized by analytical and spectroscopic methods. In addition, the structures of the Schiff base HL and its Co(II) complex were determined by single-crystal X-ray analysis. The Co(II) center is six-coordinate, being coordinated to two imine nitrogen, two phenolate oxygen and two methoxy oxygen atoms of two crystallographically independent Schiff base ligands. Luminescence properties of HL and its complexes were investigated both in solution and in the solid state.

Full text of DOI:10.1007/s11243-017-0195-z

Transition Metal Chemistry
https://doi.org/10.1007/s11243-017-0195-z
Cobalt(II), nickel(II) and copper(II) complexes of a Schif base ligand:
synthesis, structural characterization and luminescence properties
İlyas Gönül1 · Ender Fakı1 · Burak Ay1 · Muhammet Köse2 · Selahattin Serin1
Received: 6 September 2017 / Accepted: 29 November 2017
© Springer International Publishing AG, part of Springer Nature 2017
Abstract
A Schif base ligand (HL), 2,4-dimethoxy-N-(5-chloro-2-hydroxybenzylidene)-benzenamine, derived from 5-chloro-2-hy-
droxybenzaldehyde and 2,4-dimethoxyaniline, and its metal complexes [Co(L)2]·CH3OH (1), [Ni(L)2] (2), [Cu(L)2] (3) have
been synthesized. The compounds were characterized by analytical and spectroscopic methods. In addition, the structures
of the Schif base HL and its Co(II) complex were determined by single-crystal X-ray analysis. The Co(II) center is six-
coordinate, being coordinated to two imine nitrogen, two phenolate oxygen and two methoxy oxygen atoms of two crystal-
lographically independent Schif base ligands. Luminescence properties of HL and its complexes were investigated both in
solution and in the solid state.
Introduction
ethyl groups, ethylene and aromatic rings, involving meso-
meric and inductive efects) [6, 7].
Schif bases have an important role as ligands in coordi-
nation chemistry, being readily obtained by condensation
of aromatic aldehydes or ketones with amines. Transition
metal complexes of Schif bases are of interest for a wide
variety of applications and structural aspects. It has been
suggested that the imine bond is responsible for the biologi-
cal activities of Schif bases which can include antitumor,
anticancer, antioxidant and other biological properties [1,
2]. On the other hand, they are also utilized in organic syn-
thesis, dyes, pigments, polymer stabilizers, corrosion inhibi-
tors, fungicidal, agrochemical ion exchange, nonlinear optics
and industrial catalysis [3–5]. Additionally, Schif base com-
pounds can show photoluminescence properties depending
on the electron-donating groups and π-electrons in diferent
parts of the structure (such as imine bonds, methoxy and
In the present study, a new Schif base (HL), namely
2,4-dimethoxy-N-(5-chloro-2-hydroxybenzylidene)-ben-
zenamine, and its cobalt(II), nickel(II) and copper(II) com-
plexes were synthesized. The compounds were character-
ized by elemental analysis, 1H and 13C NMR, FTIR, TGA,
ICP, Q-TOF LC/MS, magnetic susceptibility, melting point
and single-crystal X-ray difraction techniques (for HL and
[Co(L)2]·CH3OH). The luminescence and thermal properties
of the compounds were also investigated.
Experimental
Materials and methods
All chemicals were purchased from commercial sources
and used without further purification. Co(OAc)2·4H2O,
Ni(OAc)2·4H2O, Cu(OAc)2, 2,4-methoxy aniline and
5-chloro-2-hydroxybenzaldehyde were purchased from
Sigma-Aldrich. Infrared spectra were obtained on a Perki-
nElmer RX-1 (KBr disk 4000–400 cm−1) FTIR spectrometer.
Thermogravimetric analysis (TGA) was performed with a
PerkinElmer Pyris Diamond TG/DTA N2 (50–900 °C range)
at a heating rate of 20 °C/min. A PerkinElmer Lambda 25
spectrometer was used for the UV–Vis spectrophotometric
studies. 1H and 13C NMR spectra were recorded on a Bruker
600 MHz instrument using TMS as internal standard and
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-017-0195-z) contains
supplementary material, which is available to authorized users.
* İlyas Gönül
ilyasgonul01@gmail.com
1
Department of Chemistry, Arts and Science Faculty,
Çukurova University, 01330 Adana, Turkey
2
Department of Chemistry, Kahramanmaraş Sütçü Imam
University, 46100 Kahramanmaraş, Turkey
Vol.:(0123456789)
1 3

Transition Metal Chemistry
DMSO-d6 as solvent. A Bruker APEX2 CCD difractometer
was used for the X-ray difraction analysis of HL and the
Co(II) complex. Data reduction was performed using Bruker
SAINT. SHELXS97 was used to solve and SHELXL2014/6
to refne the structures [8]. A PerkinElmer LS55 lumines-
cence spectrometer was used for the luminescence studies.
All samples were prepared both in the solid state and in dif-
ferent solvents at constant concentration (1.10−5 M, DMSO,
ethanol, methanol and chloroform solutions) and analyzed
in a 1 cm optical path quartz cuvette. Mass analysis was per-
formed on an Agilent 6545 Accurate Mass Q-TOF LC/MS.
crystals of [Co(L)2]·CH3OH were obtained by slow evapo-
ration of a methanol solution. The other compounds were
obtained as powders (Scheme 2).
Complex (1): Yield: 95%, Red solid. Anal. Calcd.: For
C31H30N2O7Cl2Co: C, 56.27; H, 4,09; N, 4.37%; Found: C,
55.64; H, 4.22; N, 3.98%. FTIR (KBr pellet, cm−1): 3393
methanol or water, 3008–2836 υ(C–H) aliphatic, 1613–1591
υ(CH=N), 1505 υ(C–C) aromatic, 1317–1266 υ(C–O) phe-
nolic, 510 υ(Co–O), 455 υ(Co–N). MS: m/z 641.05 [M+].
Complex (2): Yield: 94%, Dark yellow solid. Anal.
Calcd.: For C30H26N2O6Cl2Ni: C, 56.29; H, 4.09; N, 4.38%;
Found: C, 56.04; H, 4.18; N, 4.10%. FTIR (KBr pellet,
cm−1): 2943, 2837 υ(C–H aliphatic, 1617 υ(CH=N), 1597
υ(C–C) aromatic, 1268 υ(C–O) phenolic, 512 υ(Ni–O), 457
υ(Ni–N). MS: m/z 641.06 [M+].
Synthesis of HL
HL was prepared by refuxing 2,4-dimethoxy aniline (0.77 g,
5.0 mmol) with 5-chloro-2-hydroxybenzaldehyde (0.78 g,
5.0 mmol) in ethanol (20 mL) for 3 h. The dark orange solu-
tion was fltered, and the solvent was removed by rotary
evaporation. Single crystals of HL suitable for X-ray dif-
fraction were obtained from slow evaporation of a solution
in methanol (Scheme 1) [9].
Complex (3): Yield: 96%, Brown solid. Anal. Calcd.:
For C30H26N2O6Cl2Cu: C, 55.87; H, 4.06; N, 4.34%; Found:
C, 55.80; H, 3.93; N, 4.61%. FTIR (KBr pellet, cm−1):
2967–2836 υ(C–H) aliphatic, 1619 and 1597 υ(CH=N),
1284 υ(C–O)phenolic, 510 υ(Cu–O), 463 υ(Cu–N). MS:
m/z 646.05 [M+].
HL: Yield: 96%, Orange solid. Anal. Calcd.: For
C15H14NO3Cl: C, 61.76; H, 4.84; N,4.80%. Found: C, 61.71;
H, 4.78; N, 4.64%. FTIR data (KBr pellet, cm−1): 3851–3734
υ(O–H) aromatic or water, 2998–2836 υ(C–H) aliphatic,
1602 υ(CH=N), 1480 υ(C–C) aromatic, 1303–1205 υ(C–O)
phenolic. 1H NMR (600 MHz, DMSO-d6) δ ppm: 14.21 (s,
–NH–, 1H), 8.95 (s, =CH–N, 1H), 7.65 (d, J = 3 Hz, 1H),
7.43 (d, J1 = 9 Hz, J2 = 1.2 Hz, 1H), 7.37 (dd, J1 = 12 Hz,
J2 = 3 Hz, 1H), 6.95 (d, J = 8.4 Hz, 1H), 6.70 (d, J = 2.4 Hz,
1H), 6.63 (dd, J1=11.4 Hz, J2=2.4 Hz, 1H), 3.86–3.82 (s,
OCH3, 6H). 13C NMR(150 MHz, DMSO-d6) δ ppm: 160.64
(C=O), 159.87 (C=N–), 158.78–99.75 (Ar–C), 56.39–55.99
(OCH3). MS: m/z 292.08 [M+].
X‑ray structure determination
X-ray crystallographic data for both HL and
[Co(L)2]·CH3OH were collected at room temperature on
a Bruker ApexII CCD difractometer (Mo–Kα radiation,
λ = 0.71073 Å). Bruker SAINT was used for data reduc-
tion [10]. The structures were solved by direct methods and
refned on F2 using all the refections. All the nonhydrogen
atoms were refned using anisotropic atomic displacement
parameters, and hydrogen atoms bonded to carbon and
oxygen were inserted at calculated positions using a riding
model and refned with temperature factors. The crystal and
refnement data for the compounds are given in Table 1.
Synthesis of the complexes
HL (0.58 g, 2.0 mmol) was dissolved in CH3OH (20 mL),
followed by addition of the required metal salt (0.25 g,
1.0 mmol) for Co(OAc)2·4H2O; (0.25 g, 1.0 mmol) for
Ni(OAc)2·4H2O; (0.18 g, 1.0 mmol) for Cu(OAc)2; in
CH3OH (20 mL). The solution was refuxed for about 3 h.
After cooling to room temperature, dark brown single
Results and discussion
The Schif base (HL) and its metal complexes are stable in
air and soluble in common organic solvents, as reported in
Table S1. The compounds were characterized by elemen-
tal analysis, FTIR spectra, ICP, Q-TOF LC/MS, UV–Vis
Scheme 1 Synthesis of
2,4-dimethoxy-N-(5-chloro-
2-hydroxybenzylidene)-ben-
zenamine (HL)
1 3

Transition Metal Chemistry
Scheme 2 Synthesis of complexes of HL
Table 1 Crystal data and structure refnement for the HL and [Co(L)2]·CH3OH
Identifcation code
HL
[Co(L)2]·CH3OH
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
C15H14ClNO3
291.72
293 (2) K
0.71073 Å
Monoclinic
C2/c
C31H30Cl2CoN2O7
672.40
293 (2) K
0.71073 Å
Triclinic
P-1
Space group
Unit cell dimensions a= 39.513 (9) Å
b = 3.9544 (9) Å
c = 18.126 (4) Å
Volume (Å3)
α = 90°
β = 107.090 (7)°
γ = 90°
2707.1 (11)
8
1.432
a = 11.4715 (9), α = 115.650 (2)°
b = 11.6019 (8), β = 93.245 (3)°
c = 13.0508 (10), γ = 103.566 (3)°
1497.1 (2)
Z
2
Density (calculated) (Mg/m3)
1.492
0.803
0.16 × 0.12 × 0.11
Absorption coefcient (mm−1
Crystal size (mm)
)
0.289
0.09 × 0.06 × 0.05
Index ranges
Refections collected
Independent refections
Completeness to theta = 25.242°
Final R indices [I > 2sigma(I)]
R indices (all data)
− 48 ≤ h ≤ 48, − 4 ≤ k ≤ 4, − 22 ≤ l ≤ 22 − 15 ≤ h ≤ 15, − 15 ≤ k ≤ 15, − 17 ≤ l ≤ 17
12,324
2713 [R(int)= 0.1813]
99.8%
83,056
7446 [R(int) = 0.0686]
99.8%
R1 = 0.0738, wR2 = 0.1199
R1 = 0.2069, wR2 = 0.1532
R1 = 0.0388, wR2 = 0.0827
R1 = 0.0616, wR2 = 0.0909
absorption studies and X-ray difraction studies. The charac-
terization data are in good agreement with theoretical values.
The physical and analytical data of the obtained compounds
are shown in Table S2.
1 3

Transition Metal Chemistry
Fig. 1 Molecular structure of HL with thermal ellipsoids at 50%
probability. The intramolecular hydrogen bonds are shown as dashed
lines
Fig. 2 π–π interactions in HL
X‑ray crystal structure of HL
The molecular structure of the Schif base ligand was deter-
mined by single-crystal X-ray difraction. The structure was
solved in monoclinic crystal system, C2/c space group, and
is shown in Fig. 1. All bond lengths and angles are within
the normal ranges and comparable with those of similar
compounds in the literature [11]. The values are given in
Table S3.
The azomethine (N1–C7) bond length is 1.269 (5) Å,
which is in the range characteristic of C=N values. There
is an expected phenol–imine intramolecular hydrogen bond
in the structure (O1⋯N1). Hydrogen bond parameters are
listed in Table S4. The aromatic rings (C1/C6 and C8/C13)
are slightly twisted, and the dihedral angle between the two
is 8.07 (20)°. Crystal packing of HL is stabilized by π–π
interactions, CH⋯π and CH⋯Cl weak hydrogen bonding
interactions. The phenol (C1/C6) and dimethoxy benzene
(C8/C13) rings are stacked with the same section of an adja-
cent molecule. The centroid–centroid distance (C1/C6⋯C1/
C6 and C8/C13⋯C8/C13) is 3.954 Å (Fig. 2), and a packing
diagram of the compound is shown in Figure S1.
Fig. 3 Molecular structure of [Co(L)2]·CH3OH with thermal ellip-
soids at the 50% probability level, hydrogen atoms are omitted for
clarity
Table 2 Hydrogen bonds for [Co(L)2]·MeOH [Å and °]
X‑ray structure of [Co(L)2]·CH3OH
D–H⋯A
d(D–H)
0.82
d(H⋯A)
d(D⋯A)
< (DHA)
The Co(II) complex of HL was crystallized from slow evap-
oration of a methanol solution. Its molecular structure is
shown in Fig. 3. The asymmetric unit contains one com-
plex molecule and a methanol solvate. The Co(II) center is
six-coordinate, being coordinated by two imine nitrogens,
two phenolate oxygens and two methoxy oxygens from two
crystallographically independent Schif base ligands. The
phenolic group of each ligand is deprotonated such that
the two anionic Schif base ligands are charge-balanced by
Co(II). As expected, the Co–O (methoxy) bond distances
are longer than the Co-O and Co-N bond distances. The
imine bond distances (N1–C7 and N2–C22) are 1.290 (2)
and 1.292 (2) Å, respectively, which are within the normal
O(7)–H(7A)⋯O(1)
1.95
2.767 (2) 170.2
range of C=N double-bond distances. Selected bond lengths
and angles are listed in Table S5.
The methanol solvate is involved in hydrogen bonding
with one of the phenolate oxygen atoms. Hydrogen bond
details are given in Table 2. The aromatic rings in the
Schif base ligands are slightly twisted. The dihedral angles
between the aromatic rings in each ligand (C1/C6–C8/C13
and C16/C21–C23/C28) are 5.32 (11)° and 15.97 (12)°,
respectively. The complex molecules are linked by π–π
1 3

Transition Metal Chemistry
coordination of azomethine nitrogen to the metal centers
[12]. These results are further supported by the presence
of new bands in the regions of 510–512 and 430–457 cm−1
assigned to ν(M-O) and ν(M–N) stretching frequencies,
respectively. The band arising from the ν(Ar–OH) vibra-
tion was observed at 3851–3734 cm−1 in the spectrum of
free HL. A band assigned to ν(O–H) of methanol present
in the crystal lattice of the Co(II) complex was observed
at 3393 cm−1. The IR spectra of HL and its complexes are
given in Figs. S7–S10 [13, 14].
1H and 13C NMR spectra
The 1H NMR spectrum of free HL was recorded in DMSO-
d6 using TMS as internal standard. The aromatic protons
of HL were observed at δ 6.63–7.65 (Ar–H, 6H). A peak
at 8.95 (s, 1H) ppm is assigned to the azomethine pro-
ton (=CH–N) of HL and a peak at 14.21 (s, 1H) ppm to
the N–H group [18]; signals at 3.86 (s, 3H)–3.82 (s, 3H)
ppm are assigned to the methoxy protons of HL [15–17].
The 13C NMR spectrum of HL showed signals between
99.75 and 158.78 ppm from the aromatic carbons. The
azomethine carbon resonated at 159.87 ppm. A peak at
160.64 ppm can be assigned to the carbonyl double bonds
(C=O) (see Scheme 3b) [18] arising from the tautomeric
form. Signals at 56.39–55.99 ppm are assigned to the
Fig. 4 Packing diagram of [Co(L)2]·CH3OH, hydrogen atoms are
omitted for clarity
interactions as shown in Fig. 4. The chlorobenzene ring
(C16/C21) of one ligand stacks with the same section of
an adjacent molecule, with a centroid–centroid distance of
3.641 Å. There are also edge to edge π–π interactions, fur-
ther stabilizing the crystal structure.
It is informative to compare the crystal structure of
[Co(L)2]·CH3OH with the Co(II) complex [Co(HA)2] of
an analogous Schif base ligand (HA) prepared from 5-bro-
mosalicylaldehyde and 2-methoxyaniline. The [Co(HA)2]
complex was synthesized and structurally characterized by
Galini et al. [11]. In both [Co(L)2]·CH3OH and [Co(HA)2]
(Fig. S2), the Co(II) centers are six-coordinate with simi-
lar Co-ligand distances. The coordination geometry around
Co(II) centers in both complexes is a slightly distorted
octahedron.
1
methoxy carbon [19]. The H and 13C NMR spectra data
of HL are given in Figs. S11–S12. Free HL may exist in
two tautomeric forms, an OH form (A) and an NH form
(B) as shown in Scheme 3. From the NMR data, we have
determined that HL exists as the NH tautomer in DMSO
solution. Hence, the polar DMSO solvent causes reversible
proton transfer from the OH to NH form in solution [20].
Mass spectra of the compounds
Both HL and its complexes were characterized by Q-TOF
LC/MS analysis (Figs. S3–S6). The mass spectrum of HL
indicated a molecular weight of 291.50 (m/z: 292.08 MH+
ion) which is consistent with the molecular weight of the
compound. The mass spectra of the complexes indicated
molecular weights of 640.14 for 1 (m/z: 641.05 MH+ ion),
640.38 for 2 (m/z: 641.06 MH+ ion), 644.99 for 3 (m/z:
646.05 MH+ ion), which are consistent with the formula
weights.
FTIR spectra
The FTIR spectrum of free HL showed a strong band at
1602 cm−1 which is ascribed to the azomethine ν(HC=N)
group. The equivalent ν(C=N) stretching frequencies for
the complexes were shifted to 1613–1619 cm−1, indicating
Scheme 3 Enol(A)–Keto(B) tautomerism of the Schif base HL in
solution
1 3

Transition Metal Chemistry
Electronic spectra
Magnetic susceptibilities
The electronic spectra of free HL and its complexes were
recorded in ethanol and are shown in Fig. 5. The spec-
trum of the HL Schif base showed two main bands. An
intense peak at 258 nm (38,759 cm−1) can be assigned
to the π → π* transitions of the aromatic rings. The sec-
ond band at 372 nm (26,882 cm−1) is assigned to n → π*
transition of the nitrogen lone pair to the aromatic ring.
Three intense bands were observed in the electronic spec-
tra of each of the complexes. They were observed around
279–434 nm (35,842–23,041 cm−1) for Co(II), 277–450 nm
(36,101–22,222 cm−1) for Ni(II) and 276–423 nm
(36,232–23,641 cm−1) for Cu(II). In all three complexes,
the π → π* transitions of the azomethine groups were
redshifted by about 20 nm, consistent with coordination
through the nitrogen atoms of the C=N groups. The n → π*
transitions shifted to lower wavelengths, confrming coor-
dination of the oxygen atoms of HL to the metal centers
[21, 22]. In the electronic spectra of the complexes, charge
transfer (CT) bands were also observed. Peaks assigned to
metal → ligand charge transfers (MLCT) were observed at
434 nm (23,041 cm−1) for Co(II), 450 nm (22,222 cm−1) for
Ni(II) and 423 nm (23,641 cm−1) for Cu(II) [23, 24].
The magnetic susceptibilities of the complexes were deter-
mined using an Evans balance at room temperature in the
solid state. Diamagnetic corrections for each complex were
calculated using Pascal’s constants. The magnetic moment
data are shown in Table 3. The experimental μef values for
the complexes are within the ranges found for mononuclear
complexes for [Co(L)2]·CH3OH (d7); [Ni(L)2] (d8); [25] and
[Cu(L)2] (d9). Also, the magnetic moments (µef: 4.12, 2.93
and 1.58 B.M., respectively) provide additional evidence as
reported in the literature of an octahedral geometry for the
Co(II) and Ni(II) complexes and square planar geometry for
the Cu(II) complex [26, 27].
Thermal analysis
Thermal analyses of all three complexes revealed that
the water or methanol molecules are stepwise eliminated,
namely the crystallization solvent up to 140 °C and coor-
dinated water between 140 and 200 °C [28]. The TG
curves show distinct mass losses; the curves are shown
in Figs. S13–S16. The Schif base HL decomposed in a
single step, continuous up to 700 °C with complete mass
Fig. 5 UV–Vis spectra of the Schif base and its complexes
1 3

Transition Metal Chemistry
Table 3 Selected FTIR bands
(cm−1) and magnetic moment
values for the complexes
Compound
υ (OH)
υ (C=N)
υ (M–O)
υ (M–N)
µef (B.M.) Exp./Lit.
HL
3851–3734
3393
–
–
1602
1613
1617
1619
–
–
–
[Co(L)2]·CH3OH
[Ni(L)2]
[Cu(L)2]
510
512
510
455
457
430
4.12/4.25 [25]
2.93/3.20 [26]
1.58/1.73 [25]
Exp. experimental, Lit. literature
loss (Found: 99.25%, Calcd.: 100%). The decomposi-
tion of [Co(L)2]·CH3OH occurs in two steps. The frst
corresponds to the loss of a lattice methanol molecule
in the temperature range 50–110 °C, with a mass loss
of 3.87% (Calcd.: 4.75%). The second step is assigned
to decomposition of the organic content, with a mass
loss of 83.67% (Calcd.: 82.3%) in the temperature range
50–899 °C. The remaining weight of 12.47% is attributed
to the fnal product of Co2O3 (Calcd.: 12.95%). The ther-
mogram of [Ni(L)2] showed two weight loss steps. This
complex is thermally stable up to 300 °C. Decomposition
of the organic residue with a mass loss of 88.68% (Calcd.:
88.33%) occurs in the temperature range 300–900 °C. The
remaining residue (11.32%) is attributed to NiO (Calcd.:
11.67%) [19]. [Cu(L)2] is thermally stable up to 290 °C.
Decomposition of the organic components occurs with a
mass loss of 87.38% (Calcd.: 87.67%) in the temperature
range 290–900 °C. The residue is assigned to CuO, Found
12.62% (Calcd.: 12.33%) [29].
Luminescence properties
To determine the luminescence properties of the synthe-
sized compounds and the efect of solvents, we performed
luminescence measurements in the solid state and also in
diferent solvents. The resulting UV–Vis spectra and maxi-
mum excitation–emission wavelengths are summarized in
Table 4. As shown in Fig. 6, sharp and single emission bands
were obtained in the solid state. The free Schif base showed
an emission band with a maximum at 615 nm upon excita-
tion at 307 nm which is attributed to π → π* transitions
[30]. Compared to the free ligand, the maximum emission
of the Co(II) complex showed a blueshift of about 45 nm.
The emission spectrum of the Ni(II) complex was similarly
64 nm blueshifted compared to free HL, while the Cu(II)
complex showed a blueshift of 50 nm. These blueshifts may
arise from ligand to metal charge transfer (LMCT) [31].
The photoluminescence spectra of the Schif base and
its metal complexes were also recorded in various solvents
Table 4 Photophysical data of
Compounds
HL
π →π*
n → π*
CT
–
λex
307s
λem
615s
Stoke shift
the HL ligand and its complexes
258
372
308s
241a
131b
223c
132d
224s
137a
108b
113c
182d
179s
159a
160b
59c
287a
398b
309c
403d
346s
349a
377b
372c
347d
372s
367a
368b
311c
372d
360s
366a
367b
346c
363d
528a
529b
532c
535d
570s
486a
485b
485c
529d
551s
526a
528b
370c
519d
565s
521a
518b
514c
471d
[CoL2]·CH3OH
[NiL2]
279
277
276
339
342
331
434
450
423
147d
205s
155a
151b
168c
108d
[CuL2]
s: solid state, a: CH3OH, b: C2H5OH, c: CHCl3, d: DMSO
1 3

Transition Metal Chemistry
when excited at 367 and 366 nm, respectively. In ethanol,
free HL gave two emission peaks. The complexes showed
maxima at 529 nm for Ni(II) and 518 nm for Cu(II) upon
excitation at 398, 377, 368 and 367 nm, respectively. These
emission bands may be assigned to ligand-based π → π*
transitions [32, 33]. In DMSO solution, the complexes
showed broad emission bands at 535 nm (HL), 529 nm (1),
519 nm (2) and 471 nm (3) when excited to their correspond-
ing absorption bands (403, 347, 372 and 363 nm, respec-
tively). In chloroform solution, HL and its complexes exhib-
ited emissions with λmax at 508–532 nm upon irradiation at
309–372 nm (see Table 4 and Fig. 7). The full version of
photophysical data of the free ligand HL and its complexes
is given in Table S6.
Fig. 6 Solid-state emission spectra of the compounds at room tem-
These results confrm an emission predominantly arising
not only from π → π* transitions, but also due to chelation-
enhanced fuorescence by coordination of imine group to
the metals [34]. It is interesting to note that the copper(II)
complex gave very sharp and single emission bands in all
solvents except DMSO. The λem of the compounds in difer-
ent solvents were almost identical, showing that the solvent
polarity has very little infuence on the emission bands of
these complexes. Signifcant Stoke shifts were observed for
perature
at room temperature (Fig. 7). In methanol solution, the free
Schif base showed a maximum emission centered at 528 nm
upon excitation at 287 nm. The Co(II) complex showed an
emission band at 486 nm upon excitation at 349 nm, while
the Ni(II) (λem: 526 nm) and Cu(II) (λem: 521 nm) complexes
showed similar emission maxima with higher intensities
Fig. 7 Emission spectra of the compounds in diferent solvents: chloroform, dimethyl sulfoxide, ethanol and methanol with a concentration of
1.0 × 10−5
M
1 3

Transition Metal Chemistry
5. Shabbir M, Akhter Z, Ahmad I, Ahmed S, Ismail H, Mirza B,
McKee V, Bolte M (2016) J Mol Struct 1116:84–92
6. Pal MK, Kushwah N, Wadawale AP, Dey S, Sudarsan V, Jain VK
(2016) J Organomet Chem 808:128–133
the compounds, as can be seen from the diference in the
wavelengths of the excitation and emission peak maxima
(Table 4). These large Stokes shifts confrm the existence of
excited state intramolecular proton transfer, involving inter-
conversion of the enol and keto forms of the ligand [35].
7. Tümer M, Güngör SA, Çiftaslan AR (2016) J Lumin 170:108–120
8. Bruker (1998). APEX2 and SAINT Bruker AXS Inc
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Conclusions
11. Galini M, Salehi M, Kubicki M, Amiri A, Khaleghian A (2017)
Inorg Chim Acta 461:167–173
12. Kokare DG, Kamat V, Naik K, Nevrekar A, Kotian A, Revankar
VK (2017) J Mol Struct 1127:289–295
In this study, we have synthesized a new Schif base and
its metal complexes. All compounds were characterized by
spectroscopic and analytical methods, and the molecular
structures of HL and [Co(L)2]·CH3OH were determined by
single-crystal X-ray difraction studies. Studies of the pho-
toluminescence properties showed that the solvent has very
little infuence on the emission bands of these complexes.
Signifcant Stokes shifts were observed in all three cases,
confrming the existence of excited state intramolecular pro-
ton transfer between the enol and keto forms of the ligand.
The luminescence studies suggest that these compounds
may have potential applications as luminescent blue light-
emitting materials.
13. Rad M, Dehghanpour S, Fatehfard S, Gholamrezazadeh C,
Mahmoudi A (2016) Polyhedron 106:10–17
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Morgan M, Abou-Dobara MI, Nozha SG (2016) J Mol Liq
215:423–442
15. Jing C, Wang C, Yan K, Zhao K, Sheng G, Qua D, Niu F, Zhu H,
You Z (2016) Bioorg Med Chem 24:270–276
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B 154:67–76
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18. Matijević-Sosa J, Vinković M, Vikić-Topić D (2006) Croat Chem
Acta 79:489–495
19. Abd-Elzaher MM, Labib AA, Mousa HA, Moustafa SA, Ali MM,
El-Rashedy AA (2016) Beni-Seuf Univ J Appl Sci 5:85–96
20. Salehi M, Ghasemi F, Kubicki M, Asadi A, Behzad M, Ghasemi
MH, Gholizadeh A (2016) Inorg Chim Acta 453:238–246
21. Grivani G, Vakili M, Khalaji AD, Bruno G, Rudbari HA, Taghavi
M (2016) J Mol Struct 1116:333–339
Appendix: Supplementary material
22. Rudbari HA, Iravani MR, Moazam V, Askari B, Khorshidifard M,
Habibi N, Bruno G (2016) J Mol Struct 1125:113–120
23. Sallam SA, Abbas AM (2013) J Lumin 136:212–220
24. Back DF, Oliveira GM, Fontana LA, Ramao BF, Roman D, Igle-
sias BA (2015) J Lumin 1100:264–271
CCDC 1509986 and 1509987 contain the supplementary
crystallographic data for the Schif base HL and its com-
plex [Co(L)2]·MeOH, respectively. The data can be obtained
free of charge via www.ccdc.cam.ac.uk/datarequest/cif, by
e-mailing data request@ccdc.cam.ac.uk or by contacting
The Cambridge Crystallographic Data Centre 12 Union
Road Cambridge CB2 1EZ, UK Fax: +44(0)1223-336033.
25. Bhattacharyya A, Harms K, Chattopadhyay S (2014) Inorg Chem
Commun 48:12–17
26. Manjunath M, Kulkarni AD, Bagihalli GB, Malladi S, Patil SA
(2017) J Mol Struct 1127:314–321
27. Fekri A, Zaky R (2016) J Org Chem 818:15–27
28. Abdel-Kader NS, El-Ansary AL, El-Tayeb TA, Elnagdi MMF
(2016) J Photochem Photobiol A 321:223–237
29. Abdel-Rahman LH, Abu-Dief AM, Newair EF, Hamdan SK
(2016) J Photochem Photobiol B 160:18–31
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