Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases

Article abstract of DOI:10.1007/s10973-017-6563-2

The complexes of N₂O₂-, N₂O₃- and N₂O₅-donors Schiff bases with Cu^II ions in the methanol solution have been synthesized. They were characterized by elemental analysis, X-ray cry

Full text of DOI:10.1007/s10973-017-6563-2

J Therm Anal Calorim
DOI 10.1007/s10973-017-6563-2
Synthesis, thermal behaviour and some properties of CuII
complexes with N,O-donor Schiff bases
Agata Bartyzel1
Received: 24 February 2017 / Accepted: 29 June 2017
Ó The Author(s) 2017. This article is an open access publication
Abstract The complexes of N2O2-, N2O3- and N2O5-
donors Schiff bases with CuII ions in the methanol solution
have been synthesized. They were characterized by ele-
mental analysis, X-ray crystallographic techniques, spec-
troscopic (UV–Vis, IR) and thermal (TG, TG-FTIR)
methods. The catalytic activity of the prepared complexes
for the hydrogen peroxide-assisted degradation of methy-
lene blue (MB) as the model compound in water was also
investigated. The complexes were obtained as crystalline
solids. Depending on the Schiff bases and the molar ratio of
CuII:ligand used in the synthesis, they can form monomeric
or dimeric structures. The coordination environment
around the metal centre in both solution and solid states is a
slightly distorted square-planar. The values of magnetic
moments determined at room temperature show the exis-
tence of antiferromagnetic interactions. The complexes are
stable at ambient temperature. After heating, at first sol-
vates lose solvent molecules; after that, an organic part
undergoes gradual defragmentation and combustion.
Degradation efficiency of MB in the presence of complexes
was found to be 68.18–97.47%. A tentative mechanism
involving HOÁ radical as an oxidant for degradation of MB
was proposed.
Keywords Schiff base Á Copper(II) complexes Á Thermal
studies Á Spectroscopic studies Á X-ray diffraction analysis Á
Catalytic properties
Introduction
The coordination chemistry of transition metals with
Schiff base has been studied extensively due to their
structural diversities, versatile properties and applications
[1–3]. Schiff base transition metal complexes have sig-
nificant contribution to the development of coordination
chemistry related to magnetism, catalysis and enzymatic
reactions, molecular architectures and optical materials
[4–7]. These complexes can also exhibit biological
activity as antifungal, antibacterial, antimalarial, antitu-
mor, antiproliferative, antiinflammatory, antiviral,
antioxidant agents [8, 9]. Copper complexes attract spe-
cial attention, namely due to their ability to exhibit such
phenomena as molecular magnetism [10] and magnetic
refrigeration [11, 12]. Several copper complexes con-
taining Schiff base ligands have been also studied towards
biological and pharmaceutical activities such as DNA
binding and cleavage, antimicrobial, anticancer and
antioxidant behaviour [13–15]. Furthermore, copper is
one of the crucial transition metals which is involved in
many biological processes in living organisms. The cop-
per is required as a catalytic cofactor (e.g. cytochrome c
oxidase and Cu/Zn superoxide dismutase) or as a struc-
tural component for proteins which participate in cellular
reactions such as antioxidant defense, respiration, neuro-
transmitter biosynthesis and pigment formation. This is
due to the fact that copper is highly redox active and has
the ability to oxidize/reduce (switch between CuI and
CuII) which allows Cu-containing proteins to play an
Electronic supplementary material The online version of this
article (doi:10.1007/s10973-017-6563-2) contains supplementary
material, which is available to authorized users.
& Agata Bartyzel
agata.bartyzel@poczta.umcs.lublin.pl
1
Department of General and Coordination Chemistry, Maria
Curie-Skłodowska University, Maria Curie-Skłodowska Sq.
2, 20-031 Lublin, Poland
123

A. Bartyzel
Preparation of Schiff bases
important role as electron carriers and redox catalysts in
living organisms. Copper can also have toxic effects in
biological systems if not properly regulated, e.g. Menkes
disease, Wilson’s disease, oxidative stress or neurode-
generative diseases [16–20].
The Schiff base ligands were synthesized (Scheme 1) using
a similar method described in previous papers [1, 25–27].
Mixtures of 10 mmol of appropriate ketone and 5 mmol of
diamine in 40 mL of methanol were refluxed for 2 h. The
excess of the solvent (ca. 30 mL) was then evaporated.
After cooling to 4 °C, yellow solids were formed. The
products were collected by filtration, washed with cold
methanol and dried in air.
The main aim of the paper was synthesis and char-
acterization of the CuII complexes with different N,O-
donor Schiff bases as well as exploration of their cat-
alytic potential in chemical oxidation of dyes in water
with hydrogen peroxide. The synthesized copper(II)
complexes were characterized based on spectroscopic
(IR, UV–Vis), X-ray crystallographic techniques and
thermal (TG, TG-FTIR) methods as well as elemental
analysis. In order to explore their catalytic properties,
methylene blue (MB) was selected as a model dye
because it is extensively used for dyeing and printing
cotton, silk, paper, etc. [21, 22]. MB is also widely
applied in different fields of medicine, e.g. as drug for
tropical treatment and photodynamic therapy; as a main
component of many staining solution, in antimicrobial
chemotherapy or cancer research [23, 24].
H2L1 (where L1 = C29H24N2O2): yellow powder; yield:
(62.0%); Elem. anal. for C29H26N2O2 (FW 434.53 g mol-1
)
(%): calcd C 80.16, H 6.03, N 6.45; found C 79.96, H 5.89,
N 6.26 [25].
H2L2 (where L2 = C31H28N2O2): yellow powder; yield:
(74.5%); Elem. anal. for C31H30N2O2 (FW 462.58 g mol-1
)
(%): calcd C 80.49, H 6.54, N 6.06; found C 80.41, H 6.63;
N, 5.98 [26].
H3L3 (where L3 = C29H23N2O3): yellow powder; yield:
(86.0%); Elem. anal. for C29H26N2O3 (FW 450.53 g mol-1
)
(%): calcd C 77.31, H 5.82, N 6.22; found C 76.97, H 5.75,
N 6.48.
H5L4ÁH2O (where L4 = C29H21N2O5): yellow powder;
yield: (86.4%); Elem. anal. for C29H28N2O6 (FW
500.54 g mol-1) (%): calcd C 69.59, H 5.64, N 5.60; found
C 69.26, H 5.70, N 5.68.
Experimental
Materials
Synthesis of CuII complexes
Amines (1,3-propanediamine, 2,2-dimethyl-1,3-propanedi-
amine and 1,3-diamino-2-propanol) and ketones (2-hy-
droxybenzophenone and 2,20-dihydroxybenzophenone)
(Scheme 1) used for the synthesis of Schiff bases, cop-
per(II) acetate (Cu(CH3COO)2ÁH2O) and solvents (MeOH,
EtOH, ACN, DMF and THF) were commercially available
from Sigma-Aldrich Co., Alfa Aesar Co. and Polish
Chemical Reagents. All solvents and chemicals were
reagent grade and used without further purification.
The synthesis of complexes was performed in the methanol
solution with the ratio of metal acetate to the Schiff base
1:1 and 2:1. An appropriate amount of copper(II) acetate
dissolved in methanol (20 mL) was added to a stirred hot
solution of appropriate Schiff base (0.2 g) in MeOH
(30 mL). The obtained mixtures were refluxed for 1 h. The
formed solid products were filtered off and washed several
Scheme 1 Synthetic route of
Schiff bases synthesis
″
′
R
R
R
R
N
R
N
O
MeOH
T = 65 °C, t = 2 h
OH
HO
OH
′
″
R
R
H
R
H
H2L1
H2L2
H3L3
H
H
H
CH3 CH3
H2N
NH2
OH
H
H
′
″
R
R
H5L4 OH OH
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
times with methanol. The yields of the purified complexes
and some of their physicochemical properties are listed as
follows:
4.10; FTIR bands (m/cm-1): 3611w, 3412vw, 3050w,
3018w, 2939w, 2858w, 2710w, 2588w, 1568m, 1537m,
1504w, 1465w, 1451s, 1434vs, 1385m, 1343m, 1321s,
1289m, 1233s, 1214s, 1170w, 1153s, 1130w, 1119m,
1099w, 1042m, 1032m, 1007w, 978w, 945w, 927w, 897w,
850s, 829m, 754vs, 728m, 707m, 688m, 668m, 644m,
629m, 614m, 562m, 548w.
CuL1 (1): olive green powder; yield, 85.27%; leff
(298 K): 1.54 lB; Elem. anal. for CuC29H24N2O2 (FW
496,06 g mol-1) (%): calcd C 70.22, H 4.88, N 5.65; found
C 69.93, H 4.84, N 5.75; FTIR bands (m/cm-1): 3054w,
3022vw, 2963vw, 2950w, 2938w,2882vw, 2867vw,
1595m, 1578s, 1525s, 1489w, 1463m, 1454s, 1442m,
1431s, 1360w, 1340s, 1275w, 1257m, 1241s, 1180vw,
1169w, 1143s, 1119m, 1089m, 1069m, 1030m, 999w,
984w, 971w, 950m, 927vw, 908m, 871vw, 852m, 836s,
800m, 783w, 756vs, 724m, 716s, 704s, 696vs, 658m,
638m, 620w, 605vw, 595s, 586m, 544w.
CuL2 (2): dark green powder; yield, 94.10%; leff
(298 K): 1.55 lB; Elem. anal. for CuC31H28N2O2 (FW
524.11 gmol-1) (%): calcd C 71.04, H 5.38, N 6.00; found
C 70.78, H 5.34, N 6.01; FTIR bands (m/cm-1) 3076vw,
3046vw, 2993vw, 2974w, 2964w, 2926vw, 2864vw,
1594m, 1569m, 1526m, 1521m, 1489w, 1467m, 1460w,
1455m, 1440m, 1429m, 1391w, 1368vw, 1355w, 1339m,
1332s, 1289w, 1273w, 1259m, 1235s, 1213w, 1180w,
1164w, 1141m, 1117w, 1073w, 1048w, 1039w, 1024w,
1000w, 977w, 969w, 952w, 925vw, 910w, 894w, 857w,
838m, 783s, 763vs, 722s, 705s, 692m, 655m, 629w, 611w,
603m, 574w, 566w, 544w.
CuHL3ÁH2O (3): beige powder; yield, 67.94%; leff
(298 K): 1.59 lB; Elem. anal. for CuC29H26N2O4 (FW
530.07 gmol-1) (%): calcd C 65.71, H 4.94, N 5.28; found
C 65.45, H 4.89, N 5.41; FTIR bands (m/cm-1): 3647vw,
3489vw, 3053vw, 2948vw, 2922vw, 2898vw, 2834vw,
1631w, 1597s, 1582s, 1540m, 1527m, 1492w, 1464m,
1443s, 1342s, 1326w, 1314m, 1285w, 1260s, 1255s, 1247s,
1226w, 1171w, 1152m, 1119w, 1092w, 1072w, 1039vw,
1030w, 1007vw, 1001vw, 971w, 945w, 915m, 866w,
859w, 837m, 817w, 784m, 765m, 756vs, 735w, 709vs,
666w, 635m, 616m, 592m, 587m, 569vw, 548w, 541vw.
Cu2L3(CH3COO) (4): dark green powder; yield,
69.57%; leff (298 K): 2.13 lB; Elem. anal. for Cu2C31
H26N2O5 (FW 633.64 gmol-1) (%): calcd C 58.76, H 4.14,
N 4.42; found C 58.75, H 3.98, N 4.52; FTIR bands (m/
cm-1): 3056vw, 3020vw, 2976vw, 2948vw, 2908vw,
2852vw, 1598m, 1579m, 1563s, 1558s, 1526s, 1490m,
1457m, 1434vs, 1410s, 1363w, 1341vs, 1289m, 1241s,
1202m, 1169w, 1147s, 1134m, 1121m, 1112m, 1087w,
1073w, 1059w, 1047w, 1029m, 1000w, 972w, 942w,
911m, 893m, 856m, 842s, 787m, 769m, 756vs, 732w,
701vs, 691s, 667s, 649m, 640s, 618m, 610m, 585w, 560w,
557w, 545w, 541w.
Methods and physical measurements
Determination of C/H/N was done using a PerkinElmer
CHN 2400 analyser. Magnetic susceptibilities for the
powdered samples were measured at 298 K using a mag-
netic susceptibility balance MSB-MKI, Sherwood Scien-
tific Ltd., Cambridge. The data were corrected for
diamagnetic susceptibilities [28, 29]. The effective mag-
netic moment is calculated from Eq. (1):
1=2
leff ¼ 2:828ðvcorTÞ
ð1Þ
An Empyrean powder diffractometer (PANalytical, the
Netherlands) was used for powder X-ray diffraction (XRD)
measurements. The studies were carried out on using CuKa
˚
radiation (k = 1.5405 A) in the 2h range from 4° to 80°
with a step of 0.026°. The X-ray powder patterns of all
complexes are given in Supplementary material (Figs. S1–
S5). The analytical software package WinPLOTR [30] was
used to establish the positions of the peaks and to deter-
mine their intensities. Determination of the unit cell and
indexing of the X-ray diffraction pattern were performed
with the computer programs DICVOL06 [31]. The first
twenty lines were used for the calculations, and addition-
ally, the absolute error on each observed line was fixed at
0.03–0.06°. Reliability of the obtained unit cell was esti-
mated by the figure of merit MN [32] and FN [33, 34]. The
unit cell parameters of complexes calculated from the
X-ray powder diffraction data are given in Table 1.
The FTIR spectra measurements were performed on a
Nicolet 6700 FTIR using the ATR accessory on the dia-
mond crystal for the pressed powder samples. The spectra
were collected between 4000 and 540 cm-1, with a reso-
lution of 4 cm-1 for 16 scans (Figs. S6–S10). The most
important bands are given in Table 2. The UV–Vis spectra
of complexes were recorded in different solvents for the
solutions of compounds at a concentration 2.5 9 10-5 M;
see Table 3. The measurements were performed in the
10 9 10 mm quartz cell over the range 200–1100 nm using
a GENESYS 10S UV–Vis spectrophotometer. Thermal
behaviour of the analysed complexes under flowing air
atmosphere was studied on a Setaram Setsys 16/18 deri-
vatograph, recording TG, DTG and DSC curves (Fig. 1;
Table 4). The samples (5.2–7.6 mg) were put in a ceramic
crucible and heated (10 °C min-1) in the temperature range
Cu2H2L4(CH3COO)ÁH2OÁ0.5CH3OH (5): dark tur-
quoise powder; yield, 79.76%; leff (298 K): 2.18 lB; Elem.
anal. for Cu2C31.5H30N2O8.5 (FW 699.69 gmol-1) (%):
calcd C 54.070 H 4.32, N 4.00; found C 53.82, H 4.17, N
123

A. Bartyzel
Table 1 Unit cell parameters of copper(II) complexes calculated from the X-ray powder diffraction data
3
V (A )
˚
a/A
˚
b/A
˚
c/A
˚
Complex
a/°
b/°
c/°
FOM M20; F20; FN
1
8.999(6)
14.942(6)
9.727(4)
15.538(5)
25.715(2)
17.084(1)
12.376(3)
15.513(5)
106.46(5)
90.23(6)
102.23(5)
1951.81
2432.84
1716.40
1334.37
1524.81
M20 = 11.1; F20 = 29.5(0.0119, 57);
F26 = 7.7(0.0142, 283)
2
3
4
5
9.727(4)
8.731(5)
10.168(2)
7.978(5)
90.00
90.00
90.00
M20 = 120.8; F20 = 265.4(0.0022, 34);
F59 = 95.3(0.0033, 186)
12.418(1)
11.686(2)
12.589(7)
77.23(7)
96.76(2)
89.95(4)
79.63(1)
93.18(2)
80.02(7)
73.26(5)
113.16(2)
83.57(1)
M20 = 9.1; F20 = 24.0(0.0161, 52);
F35 = 8.9(0.0151, 280)
M20 = 48.7; F20 = 144.8(0.0042, 33);
F53 = 21.4(0.0073, 339)
M20 = 10.7; F20 = 22.4(0.0151, 59);
F29 = 8.7(0.0156, 214)
N number of indexed peaks
Table 2 Selected infrared absorption bands of the ligands and complexes 1–5 (cm-1
)
Compound
m(O–H)
m(C = N)
mas(COO-)
mas(COO-)
d(O–H)a
1302
m(C–O)
H2L1
1
H2L2
2561a
–
1606
1595
1596
1594
1598
1597
–
–
–
–
–
–
–
–
–
–
–
–
1257a
1257a, 1241a
1258a
1259a, 1234a
1270a, 1023b
1260a, 1255a, 1030b
–
–
1300
–
2
–
H3L3
3
3218b
3647b
3489c
–
3612b
3218c
2548a
3611d
3412c
1297
–
4
1598
1589
1563
–
1432
–
–
1241a, 1029b
1258a, 1024b
H5L4
1336
5
1568
1568
1434
1344
1233a, 1214a, 1042b, 1032d
a
Phenol/phenoxide
b
Alcohol/alkoxide
c
H2O
d
Methanol
˚
of 30–700 °C. TG-infrared spectrometry (TG-FTIR) of the
title compounds was performed using a TG Q5000 analyser
interfaced to the Nicolet 6700 FTIR spectrophotometer. The
appropriate complex (about 11.42–20.35 mg) was put in an
open platinum crucible and heated from an ambient
temperature (*23–26 °C) to 700 °C (heating rate of
20 °C min-1). The analysis was carried out under flowing
nitrogen atmosphere (flow rate of 25 mL min-1). To reduce
the possibility of gases condensing along the transfer line,
the temperatures of the gas cell and transfer line were set to
250 and 240 °C. The FTIR spectra were recorded in the
range of 4000–600 cm-1 with a resolution of 4 cm-1 and
6 scans per spectrum.
graphite-monochromated MoKa radiation (k = 0.71073 A).
Data sets were collected at 293 K using the x scan tech-
nique, with an angular scan width of 1.0°. For data col-
lection, cell refinement and data reduction the programs
CrysAlis CCD [35] and CrysAlis Red [35] were used. A
multi-scan absorption correction was applied. The struc-
tures were solved by direct methods using SHELXS-2013
and refined by the full-matrix least squares on F2 using
SHELXL-97 [36]; both programs are implemented in
WinGX [37]. All non-H atoms were refined with the ani-
sotropic displacement parameters. Hydrogen atoms resid-
ing on carbon atoms were positioned geometrically and
refined applying the riding model [with Uiso(H) = 1.2 or
1.5 Ueq(C)]. The hydroxyl hydrogen did not refine satis-
factorily although definitely located in a difference map,
The crystallographic analysis was carried out using an
Oxford Diffraction Xcalibur CCD diffractometer with the
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
Table 3 Electronic spectra data of complexes 1–5
Solvent
k
max/nm (e 9 104/L mol-1 cm-1
)
1
2
3
4
5
MeOH
212(5.26)
240(4.72)
278(2.95)
363(1.14)
581(0.01)a
241(3.57)
279(2.29)
367(0.90)
248(4.64)
278(3.10)
366(1.35)
588(0.02)
229(14.39)
278(2.70)
363(1.06)
584(0.02)
223(13.18)
272(2.88)
367(1.21)
631(0.05)
230(15.59)
275(3.14)
363(1.22)
620(0.03)
EtOH
ACN
248(4.38)
280(2.96)
370(1.31)
578(0.02)
253(3.86)
277(2.37)
374(1.36)
602(0.02)
268(2.80)
368(1.42)
612(0.03)
258(3.42)
381(0.84)
599(0.02)
230(13.76)
279(2.62)
367(1.04)
579(0.03)
230(15.50)
277(2.39)sh
371(1.17)
591(0.03)
269(2.48)
373(1.17)
599(0.03)
237(8.29)
276(2.50)sh
379(1.18)
601(0.03)
227(12.12)
272(2.64)
368(1.21)
615(0.06)
224(13.63)
271(2.67)
371(1.23)
624(0.04)
269(2.98)
375(1.45)
631(0.05)
230(10.55)
272(3.22)
379(1.43)
626(0.05)
230(14.96)
276(2.84)
363(1.20)
627(0.02)
230(16.24)
273(2.90)
370(1.23)
617(0.04)
269(2.75)
371(1.27)
631(0.04)
244(11.03)
273(3.21)
376(1.32)
625(0.04)
223(4.75)
243(4.48)
276(2.41)
371(1.20)
267(2.78)
373(1.21)
DMF
THF
214(5.00)
223(4.80)
243(4.11)
275(2.62)
379(1.25)
possibly due to unresolved disorder. Consequently, it was
included in a calculated position (SHELXL HFIX 147
command) and as a riding contribution. The crystal data,
data collection and structure refinement details are sum-
marized in Table 5. The selected bond distances and angles
are presented in Table S1 (Supplementary material).
Molecular plots were drawn with ORTEP-3 for Windows
[38, 39] and Mercury [40]. Geometrical calculations were
performed using the PLATON program [41]. In the case of
2, the Flack parameter was refined to -0.07(5), and for
further confirmation of the absolute configuration, deter-
mination using the Bayesian statistics on the Bijvoet [42]
differences as implemented in the program PLATON [41]
was made. The CIF file for 2 and 4 refinements can be
retrieved from the Cambridge Crystallographic Data Cen-
tre (CCDC) (deposition numbers CCDC 1534459 for 2 and
1534460 for 4).
poured into solution kept continuous stirring. The reaction
progress of methylene blue degradation was monitored
with UV–Vis spectroscopy. The control catalysis experi-
ment (B) was also performed under the same condition
without addition of any complex.
Results and discussion
In their structure, the Schiff bases H2L1 and H2L2 have
four possible coordination sites, two nitrogen atoms of the
azomethine groups and four oxygen atoms of the phenolic
groups. Therefore, the synthesis of metal complexes was
conducted through the reaction between the metal ions and
the ligand at 1:1 CuII:ligand ratio in methanol solutions. In
the case of Schiff bases H3L3 and H5L4, the hydroxyl
group present in the structure of amine used for the syn-
thesis of ligands provides an additional donor atom which
is capable of coordination with the copper ions. For this
reason, the synthesis of complexes was carried out in the
stoichiometric ratio CuII:ligand 1:1 and 2:1. For the ligand
H3L3, change in the ratio of the Schiff base and copper(II)
ions leads to formation of two different compounds,
monomer (3) and dimer (4). The analysis of the powder
diffraction patterns of complexes reveals that they are
The catalytic properties of complexes towards the
methylene blue dye degradation in the presence of hydro-
gen peroxide were determined according to the following
procedure. 100 mL of MB (10 mg L-1) and 50 mg of
appropriate complex were added to the conical flask and
the temperature was maintained at 40 2 °C. The mix-
tures were magnetically stirred for half hour to ensure the
equilibrium was reached. Then 10 mL of 30% H2O2 was
123

A. Bartyzel
(a)
(b)
TG
TG
(2)
(1)
100
80
Exo
100
80
Exo
20
15
10
5
30
25
20
15
10
5
DTG
DTG
60
60
40
40
20
20
DSC
DSC
0
0
0
200
400
600
0
200
400
600
Temperature/°C
Temperature/°C
(c)
(d)
TG
DTG
TG
(4)
(3)
Exo
100
100
80
Exo
20
15
10
5
25
20
15
10
DTG
80
60
60
40
40
5
0
20
20
DSC
DSC
0
0
200
400
600
0
200
400
600
Temperature/°C
Temperature/°C
(e)100
(f)
TG
100
80
(5)
Exo
1
DTG
30
25
20
15
10
5
2
3
4
5
80
60
40
20
60
40
20
DSC
0
0
200
400
600
0
200
400
600
Temperature/°C
Temperature/°C
Fig. 1 a–e TG, DTG and DSC curves of 1–5 in air; f TG curves of 1–5 in nitrogen
crystalline compounds (see Figs S1–S5 in Supplementary
material). The unit cell parameters for the studied com-
plexes are given in Table 1. In the case of complexes 2 and 4,
during the recrystallization process single crystals were
obtained which allowed to perform a single-crystal X-ray
analysis and verify the research results obtained by other
measurements.
given in ‘‘Experimental’’ section and shown in Figs. S6–S10,
Supplementary material. A sharp and intense peak with the
maximum at 1606–1589 cm-1 on the FTIR spectra of
ligands can be assigned to m(C=N) vibration. In the com-
plexes, this band is more intense and occurs in the same or
shifted to lower frequencies indicating participation of
nitrogen atoms in coordination of CuII. The metal ions are
also bound by phenoxide oxygen. The O–H in-plane
bending vibrations of phenolic groups are observed in the
FTIR spectra of Schiff bases as the intense peaks at
1344–1297 cm-1. The lack of these bands in the spectra of
the complexes can be associated with the deprotonation of
the phenol groups and binding to the copper(II) ions by the
Infrared spectroscopy
The most characteristic bands and their assignments for the
Schiff bases and their copper(II) complexes are reported in
Table 2. The FTIR spectra data of CuII complexes are
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
Table 4 Thermal analysis data for complexes (1–5)
Table 5 Crystal data and structure refinement for 2 and 4
Complex
Atm
Temperature
range/°C
Tpeak/°C
Mass
loss/%
Compound
2
4
Empirical formula
Temperature K
Crystal system
Space group
C31H28CuN2O2
293(2)
C32H30Cu2N2O6
293(2)
1
Air
283–398
437–612
322–473
293–398
436–570
295–453
44–120
321
540
347
337
485; 514
372
78
50.30
33.29
72.28
59.53
25.65
80.64
3.14
Tetragonal
P41212
Triclinic
N2
ꢀ
P1
2
3
Air
˚
a/A
9.7852(8)
9.7852(8)
25.820(5)
90.00
9.3687(4)
10.6522(5)
14.6721(6)
88.085(4)
85.236(4)
85.323(4)
1453.8(1)
2
˚
b/A
N2
˚
c/A
Air
A/°
b/°
c/°
218–383
385–550
46–104
298
483
67
55.95
26.49
3.12
90.00
90.00
3
˚
N2
Volume/A
2472.2(5)
4
243–477
293–365
380–550
304–531
43–102
325
317
464
323
75
70.68
46.88
28.18
64.53
2.61
Z
4
5
Air
Calculated density/
g cm-3
l/mm-1
1.408
1.521
N2
0.915
1.511
Air
Absorption correction Multi-scan
Multi-scan
684
110–194
258–311
311–367
396–550
42–88
142
281
325
451
61
2.06
F(000)
1092
32.74
5.07
Crystal size/mm
h range/°
Reflections collected/
unique
0.20 9 0.15 9 0.08 0.50 9 0.15 9 0.10
2.61 to 26.37
5588/2458
2.69 to 27.48
11,676/6653
35.10
2.56
N2
Rint
0.1643
0.0449
102–191
309–407
581–
130
323
–
2.12
Data/
restraints/parameters
GooF on F2
2458/1/165
6653/0/392
49.79
–
0.915
0.839
Atm atmosphere of analysis, Tpeak DTG peak temperature (maximum
change in mass)
Final R indices
[I [ 2r(I)]
R indices (all data)
R1 = 0.0831,
wR2 = 0.1094
R1 = 0.0469,
wR2 = 0.0942
R1 = 0.2131,
wR2 = 0.1549
R1 = 0.0932,
wR2 = 0.1026
oxygen atoms [26, 43, 44]. Moreover, the stretching
vibration m(C–O) occurred at 1270–1257 cm-1 in the
Schiff bases spectra as a single peak; after complexation, it
appears as a doublet for 1–3 and 5 [26, 43, 44] or shifts to
the lower wavenumber [1, 2] which confirms coordination
of metal centre by oxygen atoms of phenoxy groups. In the
case of complexes 4 and 5, the bands characteristic of
deprotonated carboxylic group, asymmetric and symmetric
stretching vibration COO- are observed at 1563–1568 and
1432–1434 cm-1, respectively. It is noteworthy that for
complex 5 the band derived from mas(COO) is coupled with
m(C=N) vibration resulting in a significant shift of the
azomethine vibration towards a lower frequency compared
to the free ligand.
Largest diff.
peak/hole, e A
0.926/-0.465
0.845/-0.397
-3
˚
are more intense and split in comparison with the free
ligands as a result of extensive conjugation of p electrons
in the complexes indicating anionic character of the Schiff
bases and the chelation of the ligands to CuII ions [3, 45].
The next band observed at 363–381 nm has a much lower
molar extinction coefficient than the previous ones and
generally can be assigned to the charge transfer (CT)
transitions combined with p ? p* transitions of the imine
group [44, 46]. These CT transition bands exhibit a weak
negative solvatochromism, i.e. a blue shift with the
increasing solvent relative polarity. This can indicate that
the ground-state molecules are better stabilized by solva-
tion than the molecules in the excited state [25, 47, 48].
The electronic spectra of 2–5 show also bands associated
with d–d transitions (2B1g?2A1g) in the range
578–631 nm. In the case of 1, the bands characteristic of
the d–d transitions are not observed on the spectra at the
Electronic absorption spectra and magnetic
moments
The electronic spectra of complexes in protic and aprotic
solvents are summarized in Table 3. The vibrations
occurring below 280 nm are probably due to p ? p*
transitions associated with the benzene ring. These bands
123

A. Bartyzel
applied concentration due to their low intensity. The low-
intensity band assigned to the d–d transition appears on the
spectrum of 1 recorded in more concentrated methanol
solution (10-3 mol L-1) at 581 nm. Appearance of d–d
transitions at such wavelengths is consistent with the
square-planar geometry of CuII complexes as it was
observed for other related compounds [1, 44, 49–51].
The leff values at room temperature (298 K) are
1.54–1.59 lB for monomers 1–3 and 2.13–2.18 lB for
dimers 4 and 5. The values for complexes 1 and 2 are a bit
smaller than for the expected spin-only value for uncoupled
copper(II) ions (1.73 lB) which can suggest that some
coupling of copper atoms can occur. The values of mag-
netic moments for 4 and 5 are also smaller than the
expected spin-only value for two noninteracting copper(II)
ions (2.45 lB) and can be indicative of the presence of
antiferromagnetic intradimer interactions [1, 52].
750–600 cm-1), phenol (3649 cm-1 (m(O–H)); 3066 cm-1
(m(C–H)); 1607 and 1507 cm-1 (m(Car = Car); 1339 cm-1
(d(O–H); 1248 cm-1 (m(C–O)); 1195 and 1177 cm-1 (d(C–
H)); 747 cm-1 (c(C–H)); 687 cm-1 (ring)), H2O
(4000–3450 and 1950–1300 cm-1) and benzonitrile (in the
range 3100–3000 (m(C–H)), 2275–2200 (m(C:N)) and
825–650 cm-1(d(Car = Car), c(CC:N)) [1, 26, 53–55].
With the increasing temperature, the bands related to ben-
zonitrile (411 °C) were not observed in the FTIR spectra,
whereas several bands characteristic of toluene were
recorded. The most characteristic, qualitative bands
assigned to toluene occur in the ranges 3200–2800 and
800–600 cm-1 [1]. At the end of this stage in the mixture of
released gases, the molecules of CO2, H2O, phenol, toluene
and NH3 are found. Further heating of 1 causes slow
decomposition and thermal degradation of organic part of
the compound. This process is not clearly marked on the
DTG curve, but the difference in mass loss recorded at
473 °C and in the final temperature (700 °C) is 2.49%.
During this slow pyrolysis at 573 °C, the new gaseous
products CO (double peaks at 2275–2050 cm-1 [55]) and
HNCO and its methyl derivatives (3508 and
2300–2180 cm-1 [1, 53]) appear. At the end of analysis
(665 °C) in the FTIR spectra of evolved gas phase, only the
bands characteristic of CO, CO2, H2O, NH3, CH4, HNCO
and its methyl derivatives are recorded. Probably, methane
was emitted at a lower temperature, but its vibrations
(several peaks at 3175–2850 cm-1 with a characteristic
maximum at 3016 cm-1) were covered by the bands of
phenol and toluene.
Thermal analysis
Thermal behaviour of complexes (1–5) was studied using
the TG-DSC (air) and TG-FTIR (nitrogen) techniques. The
TG-DSC curves providing information about thermal
behaviour of the complexes are shown in Fig. 1.
Complex 1
As shown in Fig. 1a, complex 1 is characterized by high
thermal stability. In air atmosphere, the decomposition
process of 1 proceeds in two steps and starts at 283 °C. The
first step is characterized by a fast mass loss rate (50.30%)
and is connected with partial degradation and combustion of
the Schiff base anion. The most probable product, which is
formed during this stage, can be copper(II) phenolate. The
theoretical mass loss value due to removal of –C17H14N2
fragment and formation of Cu(C6H5O)2 residue is 49.59%.
The second stage starts at 437 °C and corresponds to
degradation and combustion of the remaining part of the
ligand. The final product formed at 612 °C was identified as
CuO (found/calcd. total mass losses: 83.59/83.96%). The
thermal decomposition of complex was also studied under
nitrogen atmosphere (see Fig. 1f) together with the infrared
spectroscopy analysis of gaseous decomposition products.
The pyrolysis of 1 proceeds in one step, and the decom-
position process is not completed under measurement
conditions. The degradation process under N2 stream starts
at higher temperature than it was observed in air (above
322 °C) by breaking azomethine bonds as evidenced by the
presence of several bands of NH3 molecule with the char-
acteristic maxima at 3333, 966 and 931 cm-1 at 330 °C
[1, 26, 53]. Around 345 °C in the FTIR spectra of evolved
gases (Fig. S11, Supplementary material), there also appear
bands characteristic of CO2 (2450–2300 cm-1 and
Complex 2
Complex 2 shows similar thermal stability as 1. The
decomposition process of complex starts above 293 °C and
proceeds in two stages. Within the first step of 2 decom-
position almost 60% of initial mass loss is observed
(Table 4) and an intermediate product is formed. The
second stage starts above 436 °C and corresponds to
combustion of the remaining part of organic ligand. The
final product, CuO, is formed at 570 °C (found/calcd.
overall mass loss: 85.18/84.82%). The thermal decompo-
sition of 2 under nitrogen atmosphere undergoes also in
one stage as it was observed for 1. In contrast to the pre-
vious complex, the pyrolysis starts at a similar temperature
to that of the combustion process and a larger part of the
ligand is degraded compared to complex 1 (more than
80%). At the beginning of pyrolysis (310 °C), the bands of
CO2, H2O and aromatic compounds (probably toluene)
were found in the FTIR spectra indicating that degradation
of the Schiff base anion takes place in a slightly different
way than observed for compound 1 (Fig. S12, Supple-
mentary material). Breaking of the azomethine bonds starts
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
around 346 °C; then, the vibrations of ammonia, HNCO
and its methyl derivatives are recorded in FTIR spectra.
With the increasing temperature (*365 °C), the bands
related to benzonitrile and methane also appeared in the
FTIR spectra. At the end of this stage in the FTIR spectra,
only the bands characteristic of CO2, H2O, NH3, HNCO/
CH3NCO, toluene and CH4 were recorded. Heating above
453 °C leads to further slow pyrolysis. Although this pro-
cess is not clearly marked on the DTG curve, the difference
in the mass loss at 453 °C and the final temperature of
analysis (700 °C) is almost 2%. During this step, the
remaining part of the unburnt fragment of organic com-
pounds undergoes pyrolysis resulting in evolution of the
following gaseous products: CO, CO2, H2O, HNCO/
CH3NCO and traces amount of CH4 and toluene.
heating above the final temperature of the second stage
leads to slow pyrolysis of residue; the mass loss in the
range of 477–700 °C is 1.5%. This part of pyrolysis is
mainly related to the emission the similar gases as it was
observed at the end of the second stage. Only at the end of
the analysis at *620 °C, the bands associated with phenol
were not observed in the FTIR spectra of evolved volatile
pyrolysis products, whereas the bands characteristic of
NH3, CH4 and CO were recorded.
Complex 4
Complex 4 shows thermal stability comparable to that of 1
and 2. The combustion process takes place in two steps.
The first change in mass accompanied by a sharp
endothermic effect on the DSC curve was recorded at
293 °C. This interval corresponds to the removal and
decomposition of acetic ions as well as partial combustion
and transformation of Schiff base anion. Although this step
is characterized by a fast mass loss rate as it was observed
for the previous complexes, here a lower part of the
compound is combusted; mass loss is less than 47%. The
more energetic combustion process starts above 380 °C
and is associated with complete degradation and oxidation
of the remaining part of the organic matrix. The final
product CuO is formed at 531 °C (found/calcd. overall
mass loss: 75.06/74.89%).The pyrolysis process of 4 pro-
ceeds in one major step wherein the loss of about 64.53%
of the initial mass is observed. At the beginning at 310 °C
in the FTIR spectra, the bands characteristic of CO2, H2O,
HNCO/CH3NCO and acetic acid are only recorded
(Fig. S13, Supplementary material). Further heating leads
to emission of ammonia (340 °C), phenol and benzonitrile
(360 °C) while acetic acid disappears at 360 °C. The gas-
eous products, which are released and recorded on the
FTIR spectra at the end of the fast stage of pyrolysis, i.e.
CO2, H2O, NH3, HNCO/CH3NCO and phenol, are also
observed above 531 °C where slow pyrolysis of organic
matrix takes place. Additionally, around 620 °C a new
gaseous product, CO, appears.
Complex 3
The TG, DTG and DSC curves corresponding to complex 3
show that decomposition under air atmosphere proceeds in
three steps (Fig. 1c). The first stage of compound thermal
transformation occurs at 44–120 °C and is associated with
an endothermic peak which indicates that it is a desolvation
process. Based on the TG-FTIR analysis, it was found that
in this stage water molecules are released. A calculated
mass loss due to removal of water molecule is 3.39%,
while the measured mass loss is 3.14%. The enthalpy of
dehydration process is equal to 29.30 kJ mol-1. In contrast
to the previously described compounds, complex 3 is
characterized by the lowest thermal stability, i.e. degrada-
tion and combustion of the organic part of complex begin
at 218 °C. The product formed in this stage is unstable and
is almost immediately further decomposed and combusted
to CuO (found/calcd. total mass losses: 85.58/84.99%). The
pyrolysis process of 3 occurs in two steps. The first change
in the mass on the TG curve recorded in nitrogen is found
to be above 44 °C; similar to the thermal decomposition of
3 in air, it is associated with the dehydration process which
was confirmed by the absence of any other bands in the
FTIR spectra except those related to water molecules; see
Fig. 2. Further decomposition of 3 in nitrogen, linked to
pyrolysis of organic ligand, is observed at a lower tem-
perature (243 °C) compared to that of remaining com-
plexes in this atmosphere (295–322 °C). On the basis of the
FTIR spectra analysis of the evolved gas phase, it was
found that water, carbon dioxide, HNCO and its methyl
derivatives are released at the beginning of this process. At
a slightly higher temperature (290 °C) in a mixture of the
emitted gases, there are additional compounds such as
ammonium and phenol. At the end of this step (above
477 °C) in the FTIR spectra, the bands characteristic of
CO2, H2O, HNCO/CH3NCO and phenol were found.
Similar to the previously described complexes, further
Complex 5
As shown in Fig. 1e, decomposition under air atmosphere
of 5 follows five steps. The first two intervals are accom-
panied by endothermic effects suggesting that those are
related to the desolvation process. During the first interval
in the range 43–102 °C, the observed mass loss is 2.61%.
This step corresponds to removal of water molecule (cald.
2.57%). The enthalpy of dehydration process is equal to
29.30 kJ mol-1. The second step of desolvation starts with
an initial well-separated mass loss of 2.06%, between 110
and 194 °C and corresponding to the loss of half molecule
123

A. Bartyzel
Fig. 2 FTIR spectra of volatile
products of thermal
decomposition of 3 recorded at
different temperatures
0.10
0.05
T = 680 °C
T = 577 °C
T = 477 °C
0.00
0.06
0.04
0.02
0.00
0.04
0.02
0.00
T = 387 °C
0.02
0.01
0.00
T = 293 °C
0.03
0.02
0.01
0.00
T = 255 °C
0.01
0.00
0.01
T = 100 °C
T = 55 °C
0.00
0.01
0.00
0.01
T = 30 °C
0.00
4000
3500
3000
2500
2000
1500
1000
Wavenumbers/cm–1
of methanol (calcd. 2.28%). Similar processes are found on
the TG curves recorded in nitrogen atmosphere. The
analysis of the FTIR spectra of the evolved gas phase
(Fig. 3) confirms that water molecule is a product released
during the first stage, whereas during the second stage the
bands characteristic of methanol and products of its partial
pyrolysis (CO2 and H2O) are observed. The complex
deprived of the solvents in air atmosphere is stable up to
258 °C. Heated above this temperature, it undergoes
further degradation while gradual combustion of organic
ligands is observed. The first stage of removal of the
organic matrix is connected with removal and decompo-
sition of acetic ions as well as partial degradation and
transformation of the Schiff base anion. The formed
products are unstable and immediately undergo further
decomposition. In the second step of combustion, the
organic part of complexes is characterized by a significant
reduction in the mass loss rate (5.07%). The remaining of
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
Fig. 3 FTIR spectra of volatile
products of thermal
decomposition of 5 recorded at
different temperatures
T = 625 °C
0.10
0.05
0.00
0.06
0.04
T = 520 °C
0.02
0.00
T = 406 °C
0.04
0.02
0.00
0.06
T = 345 °C
0.04
0.02
0.00
0.06
T = 318 °C
0.04
0.02
0.00
0.04
T = 315 °C
0.02
0.00
T = 125 °C
0.02
0.00
0.05
T = 50 °C
0.00
4000
3500
3000
2500
2000
1500
1000
Wavenumbers/cm–1
the Schiff base is oxidative eliminated in the last step with
CuO formation; the found and calculated total mass losses
are 77.58 and 77.26%, respectively. In nitrogen atmosphere
after the desolvation process, further thermal decomposi-
tion of 5 proceeds in two steps and it can be observed from
the TG curve that the decomposition process was not
completed under the measurement condition (Fig. 1f). The
first step (309–407 °C) is assigned to removal of acetate
ion as well as destruction and partial pyrolysis of the Schiff
base. This is confirmed by the FTIR spectra analysis of the
evolved gas phase. At 315 °C, the bands of acetic acid,
CO2, H2O and HNCO/CH3NCO are observed on the
spectra of emitted gases. The bands characteristic of
ammonia, benzonitrile and phenol appear around 325, 340
and 345 °C, respectively, confirming further degradation of
the Schiff base. Before the beginning of the second stage of
decomposition process, slow pyrolysis, which is not clearly
marked on the DTG curve, is observed. The mass loss
123

A. Bartyzel
2
C11
4
C21
C22
C20
C19
C10
C9
C12
C11i
C12i
C13i
C10
C9
C8
C16
C15
C10i
C9i
C11
C16i
C13
C23
C16
C18
C8
C12
C13
C14
C14i
C17
C24
C14
C7
N1
C8i
C29
C7
C1
C7i
C1i
C15a
C28
C6
N2
C1
C15b
N1i
N1
C6i
C6
C27
C26
C5
Cu2
Cu1
O2
Cu1
C2
O6
C2
C3
C25
C5
C5i
O3
C2
C2i
C4
O1i
O1
O1
O5
C4
C4i
O4
C3
C3i
C30
C31
C32
Fig. 4 ORTEP view with the atom numbering scheme of 2 and 4. Displacement ellipsoids are drawn at the 50% probability level
between 407 and 581 °C is *3.7%. The main gases during
this interval are CO2, H2O, NH3 and HNCO/CH3NCO. The
last stage of pyrolysis is associated with emission of gases
similar to those mentioned above as well as CO and CH4.
parameters which are slightly different from that calculated
for the solvent-free compound (Table 1). Complex 4
crystallizes in the centrosymmetric triclinic space group
ꢀ
P1 as a dimer. As shown in Fig. 4, the deprotonated ligand,
L3-, acts as a hexadentate one. The complex is dinuclear
having metal centres doubly bridged by the l-alkoxo
oxygen and the l-acetate anion giving rise to a Cu2O3
Structure of 2 and 4 complexes
˚
During recrystallization of complexes 2 and 4 from
methanol solutions, the single crystals were obtained which
made it possible to carry out a single-crystal X-ray analysis
and confirm the results of other studies. The molecular
structures of complexes 2 and 4 are given in Fig. 4. The
crystallographic and refinement data are summarized in
Table 5. The selected bond distances and angles are given
in Table S1, and the hydrogen bond parameters are listed in
Table S2 (see Supplementary material). The similar
structure of complex 2 but obtained from the ethanol
solution was reported earlier by Kargar et al. [56]. For this
reason, it is not discussed here in detail. The results of the
measurements are given in order to confirm the correctness
of determination of the unit cell parameters calculated
based on the data obtained from the powder diffraction
analysis as well as the coordination around the metal ion.
CuII ion is bound by oxygen atoms of the deprotonated
phenolic groups as well as two imine nitrogen atoms. The
structural index parameter s4 is 0.45 which can indicate
that the coordination environment around CuII ion is a very
distorted square-planar geometry; the values of s4 range
from zero (ideal square-planar geometry) to 1.00 (perfect
tetrahedral geometry) [57].
metallacycle with the Cu1–Cu2 separation of 3.468(1) A
and the Cu1–O2–Cu2 angle equals 132.05(13)°. This dis-
tance is in the range of similar dinuclear copper(II) com-
plexes [58–60]. According to the literature [58–62], the l-
oxo bridged CuII complexes are usually antiferromagnetic
with the Cu–O–Cu angle being greater than 120–135° and
the larger angles should lead to stronger antiferromagnetic
coupling. The existence of some intramolecular antiferro-
magnetic interactions in the studied complex was con-
firmed by the magnetic susceptibility measurement at
298 K. In addition, each CuII ion is coordinated to one
imine nitrogen and one phenoxo oxygen originating from
ligand. The square-planar NO3 coordination geometry
around both CuII ions is slightly tetrahedrally distorted as
indicated by structural index parameters s4 which are 0.05
for Cu1 and 0.11 for Cu2. The higher tetrahedral distortion
of geometry around Cu2 ion is also apparent from the
dihedral angles (h = 11.00°) between two N–Cu–Ophenoxo
and Ocarboxylate–Cu–Oalkoxo planes, whereas for Cu1 it
equals 5.27°. The NO3À core around Cu1 is nearly planar
˚
where the largest deviation is 0.026(1) A for N1 atom; the
Cu1II ion lies in the mentioned plane with the deviation
˚
being -0.056(1) A. The RMS deviation of the four coor-
dinating atoms (NO3À core) of Cu2II ion from their best
In the case of complex 4, during the recrystallization
process the methanol molecule is incorporated into the
structure of the compound and affected the unit cell
II
plane is 0.129 A; the Cu2 ion lies almost in the mentioned
˚
˚
plane with the deviation being only 0.031(1) A. The
123

Synthesis, thermal behaviour and some properties of CuII complexes with N,O-donor Schiff bases
c
a
b
Fig. 5 Part of the crystal structure of 4 showing formation of 1D polymer along the a-axis
dihedral angle between the two coordination planes is
The discoloration results are shown in Fig. 6. Significant
15.63(8)° indicating that the bridging moiety differs
catalytic properties towards degradation of MB are exhibited
by the complexes of Schiff bases synthesized from 1,3-di-
amino-2-propanol. As shown in Fig. 6, complex 3 acts very
fast. After half an hour, more than 15% of MB undergoes
degradation. This can be due to the complex form, i.e. fine
crystalline powder, which is dispersed while stirring in the
whole volume of solution forming a suspension. Compound 4
shows the most effective catalytic properties. This complex
did not react so quickly with hydrogen peroxide as 3, but after
4 h more than 80% of MB was decomposed. The good cat-
alytic activity is also observed in the case of complex 5. After
7 h, more than 97% of dye was oxidized. In the case of 4 and
5, effective catalytic properties can be associated with their
structure, i.e. the fact that these are the dimers. Moreover,
from the FTIR spectra (Fig. S15, Supplementary material) it
can be clearly seen there are no changes in characteristic
peaks of complexes and discoloration process is due to cat-
alytic oxidation of MB than adsorption on the complexes
surface. Complexes 1 and 2 show a lower degradation ability
of dye than the remaining compounds; degradation efficiency
of MB is around 70% after 7 h. Catalytic properties of the
studied complexes towards degradation of MB with H2O2 can
be explained by a mechanism similar to those reported by dos
Santos et al. [64] and Shan et al. [65]. CuII ions can
decompose hydrogen peroxide, reducing to CuI and generat-
ing the hydroperoxyl radical (HOO•) and the superoxide
anion (•OÀ2 ):
˚
slightly from planarity. The Cu–O (1.879(2)–1.944(2) A)
˚
and Cu–N (1.951(3), 1.941(3) A) bond lengths are similar
to the corresponding distances observed in the other Schiff
base CuII complexes of square-planar coordination
[44, 60, 63].
Methanol molecule is linked to the complex via the
intermolecular three-centre bifurcated intramolecular
hydrogen bond with two oxygen atoms of phenoxy and
carboxylate groups. The structure is also stabilized by C–
HÁÁÁO hydrogen bonds and C–HÁÁÁp interactions. The
presence of weak C–HÁÁÁO hydrogen bond results in the
formation of a one-dimensional network; see Fig. 5. Such
1D arrays were further packed in the parallel fashion sta-
bilized by the C–HÁÁÁCg interactions leading to formation
of a 2D network (Figure S14, Supplementary material).
Catalytic properties
Copper(II) is an established Fenton catalyst that can react
with oxygen or peroxide to generate highly reactive oxygen
species (ROS). In order to study the catalytic ability of
synthesized complexes for the degradation of organic dyes,
MB was used as a model pollutant in the aqueous media due
to its highly stable structure and being easy to monitor. The
degradation of methylene blue with H2O2 in the presence of
complexes was monitored by UV–Vis spectroscopy at
665 nm through the discoloration of the dye solution. The
concentration of MB solution was determined quantitatively
through the calibration graph constructed from the solutions
of MB at various concentrations (R2 = 0.9988). The dis-
coloration ratio (g) was calculated from Eq. (2):
Cu2þ þ H2O2 ꢀ Cuþ þ HOOꢀ þ Hþ
Cu2þ þ HOOꢀ ꢀ Cuþ þꢀOÀ2 þ Hþ
ð3Þ
ð4Þ
Then, the cuprous ions can react with H2O2 and generate
the most reactive ROS, i.e. the hydroxyl radical which can
be responsible for direct oxidation and degradation MB:
ꢀ
ꢁ
co À ct
gð%Þ ¼
 100%
ð2Þ
co
Cuþ þ H2O2 ꢀ Cu2þ þ HOꢀ þ OHÀ
MB þ HOꢀ ! degradation products
ð5Þ
ð6Þ
where ct and co represent the time-dependent and initial
concentrations of dye (mg L-1), respectively.
123

A. Bartyzel
Fig. 6 Discoloration plot of
methylene blue oxidation in the
presence of H2O2 and CuII
complexes as catalysts
100
B
3
1
4
2
5
80
60
40
20
0
0
1
2
3
4
5
6
7
Time/h
in evolution of: CO2, CO, water, methane, toluene/phenol
and benzonitrile molecules as volatile products of organic
part thermal decomposition. The complexes exhibit cat-
alytic properties towards oxidation of MB in water with
hydrogen peroxide. A tentative mechanism involving the
HOÁ radical as an oxidant for degradation of MB was
reported.
In the blank solution, a decrease in the concentration of
MB is also observed. The discoloration can be explained by
formation of •OH radical during homolysis of hydrogen
peroxides induced by light as well as heat [66, 67]. In
comparison with the solutions containing the complexes
addition, this process is slower; a significant change in the
concentration of MB (\2%) can be seen only after an hour.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
Conclusions
The complexes of N2O2-, N2O3- and N2O5-donors Schiff
bases with CuII ions in the methanol solution were syn-
thesized and characterized using different physiochemical
methods. Depending on the Schiff bases used in the syn-
thesis and the molar ratio of CuII:ligand, they can form
monomeric or dimeric structures. The coordination envi-
ronment around the metal centre in both solution and solid
states is a slightly distorted square planar. The complexes
are stable at room temperature. In the case of complexes 3
and 5, thermal decomposition of compounds is preceded by
the desolvation process. The TG-FTIR analysis confirms
the presence of water (for 3 and 5) and methanol (for 5)
molecules in their structures. Then, in the case of 4 and 5,
the acetate ions are removed from the structures of com-
plexes, which is confirmed by the presence of acetic acid
bands in the FTIR spectra of gaseous products. The thermal
degradation of the Schiff bases occur in consecutive reac-
tions where in almost all complexes (except 2) azomethine
double bonds firstly undergo the destruction which is
confirmed by the TG-FTIR analysis; the ammonia and
HNCO/CH3NCO bands appear as the first on the FTIR
spectra of evolved gases. Heating in nitrogen also resulted
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