Comparative study of bis-chelate M(II) complexes (M?=?Ni, Cu, Zn) as new heterogeneous photocatalysts for degradation of methylene blue under visible light

Article abstract of DOI:10.1007/s11164-018-3484-z

Three tetrahedral coordination compounds formed from Ni(II), Cu(II), and Zn(II) transition-metal ions with ON-donor, (2-aminophenol) ligand (L), were synthesized and characterized by Fourier-transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV–

Full text of DOI:10.1007/s11164-018-3484-z

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3484-z
Comparative study of bis-chelate M(II) complexes
(M 5 Ni, Cu, Zn) as new heterogeneous photocatalysts
for degradation of methylene blue under visible light
Hamid Reza Mardani1 Alireza Esmaeili2 Azim Malekzadeh2
•
•
Received: 1 January 2018 / Accepted: 22 May 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Three tetrahedral coordination compounds formed from Ni(II), Cu(II),
and Zn(II) transition-metal ions with ON-donor, (2-aminophenol) ligand (L), were
synthesized and characterized by Fourier-transform infrared (FTIR) spectroscopy,
ultraviolet–visible (UV–Vis) absorption spectroscopy, elemental analysis, and
magnetic susceptibility measurements. The magnetic properties revealed that the
NiL2 and CuL2 complexes were paramagnetic while the ZnL2 complex was dia-
magnetic. Also, photodegradation of methylene blue as model organic pollutant by
the synthesized complexes was studied based on an oxidation process under visible-
light irradiation. The results showed that the photocatalytic activity of the Cu
complex was greater compared with the Ni and Zn complexes, with degradation
efficiency of 100, 85, and 60 %, respectively, after 30 min of irradiation in this
condition. Thus, the Cu (bis-chelate) complex is more efficient, produces higher
yield, is easily produced, and represents a more stable heterogeneous photocatalyst
for degradation of organic dyes such as methylene blue.
Keywords Visible light Á Methylene blue Á Heterogeneous catalyst Á
Photodegradation Á Bis-chelate complexes Á NO-donor ligand
&
Hamid Reza Mardani
hamidreza.inorg@yahoo.com; hmardani@mpnu.ac.ir
1
2
Department of Chemistry, Payame Noor University (PNU), PO BOX 19395-3697, Tehran, Iran
Faculty of Chemistry, Damghan University, Damghan, Iran
123

H. R. Mardani et al.
Introduction
Organic dyes are one of the most important classes of organic pollutant found in
wastewater. Elimination of organic pollution is a significant challenge [1]. Different
methods have been developed to remove pollutants from wastewater [2]. In recent
decades, degradation of organic pollutants by photochemical processes has become
a very interesting research topic as it represents an effective, green, and easy
approach for removal of organic contaminants from water [3–6].
Generally, semiconductor compounds have been used for this purpose. TiO2,
ZnS, and ZnO have been widely applied as semiconductor photocatalysts [7–11].
However, to date, few coordination compounds have been used in this context.
Complexes such as [Mn(III)–SALEN–Schiff base] [12], ferric and ferrous oxalate
[13], iron(III)–phenolate [14], [(Ph3Sn)4Fe(CN)6] [15], copper(II) porphyrin–TiO2
[16], and complexes of Ni(II) and Co(II) with NO-donor Schiff-base ligand [17]
have been reported as photocatalysts for degradation of organic dyes in recent years.
In most previous works, the photocatalytic process was performed under
ultraviolet radiation, which is a major disadvantage of such photooxidation systems,
because ultraviolet radiation is dangerous and not economically affordable.
Therefore, in a bid to develop a photooxidative system that is cheaper, safer, more
efficient, and compatible with green chemistry concepts, researchers are now
applying special attention to this area.
In this work, three [M(II) (bis-chelate)] complexes formed by complexation of
Cu(II), Ni(II), and Zn(II) transition-metal ions with an NO-donor ligand were
synthesized and characterized, and their catalytic activity compared for pho-
todegradation of methylene blue as model organic pollutant with H2O2 under
visible-light irradiation.
Experimental
General
All materials were of commercial reagent grade, purchased from Merck and Fluka
chemical companies, and used without any purification.
Synthesis of complexes
To a solution of 2-aminophenol as ligand (2 mmol) in EtOH (10 mL) was added
Et3N (2 mmol) under reflux conditions for 1 h. This solution was then added to a
solution of M(CH3COO)2 salt where M is Cu, Ni or Zn (1 mmol) in EtOH (10 mL).
The reaction mixture was stirred to reaction completion under reflux conditions for
24 h. After completion of complex formation, a colored substance was obtained.
The solid product was filtered off and washed with EtOH and Et2O. The solid
product was put into a vacuum oven at 40 °C for 24 h (yield = 85–95 %). The
123

Comparative study of bis-chelate M(II) complexes…
products were characterized by FTIR spectroscopy, UV–Vis spectroscopy, and
elemental (CHN) analysis.
Characterization techniques
IR spectra were recorded from pressed KBr discs using a PerkinElmer RXI FTIR
instrument. Electronic absorption spectra were recorded on an Specord 205 UV–Vis
spectrophotometer (Analytik Jena) in the range of 200–900 nm.
Photocatalytic activity
Photocatalytic experiments were carried out using a homemade photoreactor with
400-W metal-halide lamps for visible irradiation, two computer fans to cool the
photoreactor cabinet, and magnetic stirring with circulating water to cool the
reaction mixture. The distance between the solution surface and light source was
about 25 cm [10].
In a typical experiment (Fig. 1), 10 mL aqueous MB at initial concentration of
1 9 10-5 M was placed in a 25-mL round-bottomed flask. Photocatalyst (0.01 g)
was added with 1 mL H2O2 (30 %), and the suspension was stirred for 30 min in the
dark at room temperature to ensure establishment of adsorption–desorption
equilibrium. The lamp was then turned on while the suspension was magnetically
stirred. At fixed intervals of time, 4 mL of sample was withdrawn and centrifuged,
and the supernatant transferred to a spectrophotometer cell for measurement of the
absorbance of MB at 900 to 500 nm.
Results and discussion
Synthesis of bis-chelate complexes
As shown in Scheme 1, one equivalent of bis-chelate metal complex was
synthesized by complexation of one equivalent of metal ion with two equivalents
hv
Cat. (0.01 g)
Cat. (0.01 g)
MB (10 mL)
After (30 min)
MB (10 mL)
H2O2 (1 mL)
adsorption / desorption equilibrium
H2O2 (1 mL)
Turn Off Vis-Light
(Dark Condition)
Turn On Vis-light
(Start of Photodegradation Process)
Fig. 1 Photodegradation process
123

H. R. Mardani et al.
NH2
M
OH
O
+
M2
2
+
O
H2N
NH2
+
+
+
+
2
M2
:
N
u2
C
n2
Z
i
,
,
Scheme 1 Synthesis root for bis-chelate complexes
of NO-donor ligand in alcoholic solution under basic condition. Each ligand has one
negative charge in the coordination site as a result of removal of a phenolic
hydrogen by an organic base. Therefore, the complex was precipitated by
neutralizing the negative charge of the two ligands and the 2? charge of the metal
ion.
IR spectra
The IR spectra of all the complexes and ligand were similar, as shown in Fig. 2;
some important vibration bonds are presented in Table 1. However, they showed a
weak split band at around 3300 cm-1, which can probably be attributed to –NH2
stretching of the amine group of the ligand. All complexes showed the expected
phenyl C=C (around 1590 cm-1), phenolate (Ph–O) (about 1290 cm-1), and C–H
ZnL2
NiL2
CuL2
3900
3400
2900
2400
1900
1400
900
400
Wavenumber (cm-1)
Fig. 2 FTIR spectra of all complexes
123

Comparative study of bis-chelate M(II) complexes…
Table 1 Some stretching and bending vibrations of complexes and ligand
Compound
–NH2
C=C(Arom)
Ph–O
C–H
M–N
M–O
Ligand (LH) [18]
Cu(L2)
3306
3254
3268
3300
1612
1548
1596
1598
1288
1290
1288
1294
752
742
750
750
–
–
592
650
618
530
438
532
Ni(L2)
Zn(L2)
(around 740 cm-1) stretching bands in their spectrum. The positions of these bands
were very similar in all complexes and similar to that of the ligand [18]. The band
found at around 600 and 500 cm-1 for all complexes can be attributed to metal–N
and metal–O bond sequences, providing strong evidence that the metal ions were
loaded at the NO-coordination site.
Elemental analysis
The results of elemental (CHN) analysis of the complexes are presented in Table 2.
The experimental data are in accordance with calculated data for the bis-chelate
metal complexes.
Electronic absorption spectra
The electronic absorption spectra of the three complexes and the ligand were
measured in acetonitrile solution (Fig. 3).
The data are summarized in Table 3. The electronic absorption spectra of all the
complexes and ligand showed an intense band at around 400 nm, attributed to
charge transfer of metal ion to ligand (MLCT), while the bands at around 200 and
300 nm are attributed to p ? p* transition of p system of ligand. The Cu and Ni
complexes, in contrast to the Zn complex, exhibited absorption bands in the
550–1050 nm region, which can be attributed to d–d transitions of copper(II) and
nickel(II) ions in Td environment [19].
Table 2 Results of elemental (CHN) analysis of complexes (found)
Complex
CuL2
Chemical formula (color)
FW (g mol-1
)
%C
%H
%N
C/N
C12H12CuN2O2
(green)
279.78
51.51
4.32
10.01
(9.89)
10.19
(10.09)
9.95
5.15
(51.21)
52.42
(4.11)
4.40
(5.18)
5.14
NiL2
ZnL2
C12H12NiN2O2
(brown)
274.93
281.62
(52.30)
51.18
(4.48)
4.29
(5.18)
5.14
C12H12ZnN2O2
(yellow)
(51.02)
(4.06)
(9.86)
(5.17)
123

H. R. Mardani et al.
Cu
Ni
Zn
L
200
250
300
350
400
450
500
Wavelength (nm)
Fig. 3 UV–Vis spectra of ligand and the complexes in CH3CN
Table 3 Electronic spectral
Compound kmax (nm)
data for complexes in
n ? p*, p ? p* Charge transfer d–d transition
acetonitrile solution
CuL2
NiL2
208, 238, 270
206, 237, 274
212, 234, 288
212, 234, 286
435
420
450
–
565, 645
590, 820, 1045
ZnL2
Ligand
–
–
Magnetic behavior
The magnetic properties based on spin only for all complexes are presented in
Table 4. The results reveal that the NiL2 and CuL2 complexes were paramagnetic
while the ZnL2 complex was diamagnetic.
Table 4 Magnetic moment results
Complex
l
S(experimental), BM
lS(theoretical), BM
na(experimental)
n(theoretical)
Td
SP
CuL2
NiL2
ZnL2
1.629
2.951
0.46
1.73
2.83
0
0.91 & 1.0
2.1 & 2.0
0.1 & 0.0
1
2
0
1
0
0
an is the number of single electrons
123

Comparative study of bis-chelate M(II) complexes…
Cu2+
Ni2+
Zn2+
Td
n=1e
n=0e
n=2e
SP
n=1e
n=0e
n=0e
Fig. 4 Electronic configuration of three complexes in two proposed structures, viz. tetrahedral (Td) and
square planer (SP)
Figure 4 shows the electronic configuration of the three complexes for two
proposed structures, viz. tetrahedral (Td) and square planer (SP). Each complex has
1, 0, and 0 single electrons in the SP structure, but in the Td structure, each complex
has 1, 2, and 0 single electrons for Cu(II), Ni(II), and Zn(II) ion, respectively. The
experimental magnetic moment results show that all the complexes had Td or
distorted-Td structure, because the number of single electrons of the Cu, Ni, and Zn
complexes was 0.91, 2.1, and 0.1, respectively. The magnetic moment of the Zn
complex was not significant, so we can ignore it.
Photodegradation
Adsorption–desorption equilibrium
The absorption peak of an organic dye will be decreased due to some adsorption
onto the surface of the photocatalyst solid phase. Thus, the initial mixture for the
photoreaction must be allowed to reach adsorption–desorption equilibrium in dark
condition before measurements. The electronic absorption spectrum of the organic
dye is useful for detection of this equilibrium. Therefore, the variation of the
absorption of MB at kmax (around 670 nm) was recorded at 5, 10, 20, 30, 40, and
50 min in dark condition as shown in Fig. 5. The absorption peak decreased from
123

H. R. Mardani et al.
0.795
0.79
0.7931
0.785
0.78
0.7811
0.775
0.77
0.765
0.76
0.7623
0.7582
0.7559
0.7555
0.7552
50
0.755
0.75
0
10
20
30
40
Time (min)
Fig. 5 Variation of absorption of MB at kmax (around 670 nm) recorded at 5 to 50 min in dark condition
0 to 30 min, but after 30 min remained almost fixed. Thus, after 30 min, the
concentration of MB was at equilibrium.
Photocatalytic activity
First, the reaction mixture was allowed to equilibrate in the dark for 30 min before
the visible irradiation lamp was switched on. When the solution had reached
adsorption–desorption equilibrium (denoted 0 min in spectrum), the 400-W metal-
halide lamp was switched on. For decoloration studies, the decrease in absorption
peak at 500–800 nm was monitored.
Initially, experiments were carried out in presence of either visible light, complex
or H2O2 alone and complex/visible light or complex/H2O2 together; neither of these
treatments had an effect on the absorption maximum of the dye solution and caused
no decoloration. However, decoloration of the dye occurred when subjected to
visible light in presence of H2O2, probably due to production of hydroxyl radicals,
which could in turn react with the dye and lead to dye degradation [20].
Figures 6, 7, and 8 show the decrease of the absorption peak at 500–800 nm in
the photodegradation process. According to these figures, the Cu complex degraded
the dye in 30 min, while the Zn and Ni complexes decolorized the solution in 45 and
105 min. Thus, the copper complex is an effective compound for photodegradation
of methylene blue under visible-light irradiation.
The degradation efficiency (% DE) was calculated as the percentage photodegra-
dation of MB using
%DE ¼ ð1 À C=C0Þ Â 100%
where C and C0 are the equilibrium concentration of methylene blue after and
before visible-light irradiation.
123

Comparative study of bis-chelate M(II) complexes…
M. B.
0 min
[Cu L2] Complex
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
15min
30min
500
550
600
650
700
750
800
850
900
wavelength (nm)
Fig. 6 Time-dependent decrease of absorption pick of MB by [CuL2] complex under visible light/H2O2
within 30 min
0.8
M. B.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 min
15min
30min
45min
60min
75min
[NiL2] Complex
450
550
650
750
850
950
wavelength (nm)
Fig. 7 Time-dependent decrease of absorption pick of MB by [NiL2] complex under visible light/H2O2
within 105 min
0.8
M. B.
0 min
0.7
15min
[Zn L2] Complex
30min
0.6
45min
0.5
0.4
0.3
0.2
0.1
0
500
550
600
650
700
750
800
850
900
wavelength (nm)
Fig. 8 Time-dependent decrease of absorption pick of MB by [ZnL2] complex under visible light/H2O2
within 45 min
123

H. R. Mardani et al.
In previous study, it was reported that the %DE of MB was less than 10 after
100 min of visible-light irradiation only [10].
Figure 9 shows a plot of the change of %DE as a function of irradiation time for
MB dye in presence of each photocatalyst, revealing percentage degradation
efficiency after 30 min of about 60, 85, and 100 % for the Ni(II), Zn(II), and Cu(II)
complex, respectively. The photocatalytic activity of the complexes lay in the
following order: Cu(II) [ Zn(II) [ Ni(II). These results demonstrate that the MB
dye was degraded much more quickly in presence of the Cu(II) complex compared
with the other complexes. This result can be attributed to the influence of the nature
of the metal cation used, with Cu being a more active metal cation than Zn, and Zn
more than Ni [21–23].
The same result regarding the different activities of copper and nickel complexes
was observed previously [24]. This different reactivity of the metal complexes is
illustrated by the tentative mechanism shown in Scheme 2. First, the metal complex
system is excited under visible-light irradiation, and hydroxyl radicals are produced
100
90
Cu-comp.
Zn-comp.
80
70
60
50
40
30
20
10
0
Ni-comp.
0
20
40
60
80
100
Time/min
Fig. 9 Degradation efficiency of ML2 (M = Cu, Ni, Zn) complexes versus reaction time
O
N
N
O
M
OH
LUMO
e
Recombination
hν
M.B.
h+
HOMO
∗
ML2
CO2, H2O
H2O2, H2O
Scheme 2 Tentative photodegradation mechanism of metal complex
123

Comparative study of bis-chelate M(II) complexes…
X
O
N
O
N
O
N
In Reaction
M
M
N
O
X
X= H2O and H2O2
S.P. / Td structure
Oh structure
Cu2+(S.P.)
Z-Out deviation
Not deviation
Ni2+ (S.P.)
Zn2+ (Td)
Not deviation
Fig. 10 Structural change of metal complexes in reaction condition
by interaction between the excited complex and hydrogen peroxide and water,
during the photocatalytic reaction. The radicals are strong oxidizing agents that
react with the dye and cause its decolorization. The metal complexes have different
activity due to differences in electron–hole (e-/h?) separation and recombination.
Also, it seems that the photocatalytic activity can be attributed to the geometrical
and electronic structures of the complexes (Fig. 10). In the basic geometrical
structure (before reaction), Cu(II) has a single electron, whereas nickel and zinc
have no single electrons. This difference in the number of single electrons can
influence the photocatalytic activity.
The reaction mixture includes more species, including solvent (H2O), H2O2, and
methylene blue. Also, the square-planar metal complexes have two coordination
sites, above and below the molecular plane. Therefore, the reaction components can
coordinate to these sites and a structural change occurs from the square planar to
123

H. R. Mardani et al.
3
2.5
2
Zn-comp.
Cu-
1.5
1
Ni-comp.
0.5
0
0
20
40
60
80
100
120
Time/min
Fig. 11 Relationship between -ln(C/C0) and irradiation (visible light) time for kinetic data versus
degradation rate of MB by complexes
octahedral geometry. In the structure with octahedral geometry, Cu(II) has d9
electron configuration and tetragonal deviation. The ninth electron is far from the
nucleus in the tetragonal deviation, thus having potential for excitation, whereas
Zn2? has no single electrons while Ni2? has two single electrons. In general,
according to the explanation above, the role of electrons and the geometric structure
of the catalysts could be important factors in the reactivity of these complexes.
Kinetics of photocatalytic degradation of MB
The dependence of the photocatalytic reaction rate on the concentration of organic
pollutant can be well described by the Langmuir–Hinshelwood kinetic model [10]:
r ¼ dC=dt ¼ kKC=ð1 þ KCÞ:
This equation can be simplified to a pseudo-first-order equation:
LnðC0=CÞ ¼ kKt ¼ k0t;
where r is the oxidation rate of dye (mg/L h-1), C0 is the initial concentration of dye
(mg/L), C is the concentration of dye (mg/L) at irradiation time t, k is the reaction
rate constant (h-1), and K is the adsorption coefficient of dye onto catalyst particles
(L/mg).
To determine whether the reaction rate was consistent with such a first-order
reaction under different conditions, ln(C/C0) is plotted versus reaction time for the
different photocatalysts in Fig. 11. As shown, a good linear relationship was found
between ln(C/C0) and t in the initial reaction step. This indicates that the latter
equation indeed describes the photocatalytic rate of the dye.
123

Comparative study of bis-chelate M(II) complexes…
Conclusions
Cu, Ni, and Zn metal-ion complexes with NO-bidentate chelate ligands were
prepared and used as photocatalysts for degradation of aqueous solution of
methylene blue. The complexes were stable under visible-light irradiation and more
efficient as photocatalysts. The Cu bis-chelate complex was more efficient than the
Ni or Zn complex for removal of the organic pollutant from water under visible-
light irradiation. This degradation system represents an efficient method because it
is environmentally friendly, economical, and a chemical process for removal of
organic pollution.
Acknowledgements The authors are grateful for financial support from Damghan University and
Payame Noor University (PNU) of Iran.
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