Syntheses and chromotropic behavior of two halo bridged dinuclear copper(II) complexes containing pyridine-based bidentate ligand

Article abstract of DOI:10.1016/j.molstruc.2019.06.034

Two dinuclear doubly bridged copper(II) complexes [LCu(μ-Cl)Cl]₂ (1) and [LCu(μ-Br)Br]₂ (2), where L represents N-benzyl(pyridine-2-yl)methaneamine, have been synthesized to investigate their chromotropic behavior. The structures of the complexes were characterized by elemental analyses, various spectroscopic techniques (IR, UV–Vis, and EPR), molar conductance measurements and thermal analysis. The crystal structure of compound 1 indicated both copper(II) centers are located in N₂Cl₃ environments with distorted square pyramidal geometries, which sharing one base to apex edge. The complexes are chromotropic and their solvatochromism, halochromism, and thermochromism properties were investigated by visible absorption spectroscopy. The solvatochromic properties of the complexes are attributed to structural changes in various solvents. Their halochromism phenomena arise from protonation and deprotonation of coordinated ligands in the pH range of 2.0–10.5, and the reversible thermochromism in DMF and DMSO solutions is triggered by replacing the ligands by solvent molecules.

Full text of DOI:10.1016/j.molstruc.2019.06.034

Journal of Molecular Structure 1195 (2019) 769e777
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Syntheses and chromotropic behavior of two halo bridged dinuclear
copper(II) complexes containing pyridine-based bidentate ligand
,
Atie Shirvan a, Hamid Golchoubian a *, Elisabeth Bouwman b
a Department of Chemistry, University of Mazandaran, Babol-sar, 47416-95447, Iran
b Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300, RA, Leiden, the Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Two dinuclear doubly bridged copper(II) complexes [LCu(m-Cl)Cl]2 (1) and [LCu(m-Br)Br]2 (2), where L
Received 18 March 2019
Received in revised form
19 May 2019
Accepted 10 June 2019
Available online 13 June 2019
represents N-benzyl(pyridine-2-yl)methaneamine, have been synthesized to investigate their chromo-
tropic behavior. The structures of the complexes were characterized by elemental analyses, various
spectroscopic techniques (IR, UVeVis, and EPR), molar conductance measurements and thermal analysis.
The crystal structure of compound 1 indicated both copper(II) centers are located in N2Cl3 environments
with distorted square pyramidal geometries, which sharing one base to apex edge. The complexes are
chromotropic and their solvatochromism, halochromism, and thermochromism properties were inves-
tigated by visible absorption spectroscopy. The solvatochromic properties of the complexes are attributed
to structural changes in various solvents. Their halochromism phenomena arise from protonation and
deprotonation of coordinated ligands in the pH range of 2.0e10.5, and the reversible thermochromism in
DMF and DMSO solutions is triggered by replacing the ligands by solvent molecules.
Keywords:
Thermochromism
Halochromism
Solvatochromism
Copper(II)
Dinuclear
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
multi-nuclear compounds. On the other hand, our study on the
penta-coordinated copper(II) complexes showed that the sol-
During the last decades, chromotropism of coordination com-
pounds have become subject of interest due to their potential ap-
plications in the field of sensors and molecular switches materials
[1e5]. Numerous studies have been carried out on the chromo-
tropism behavior of the transition metal complexes to investigate
the correlation between these properties and the structural char-
acteristics [1,5e7]. Among the coordination compounds, copper(II)
complexes with a combination of the proper organic ligand are a
suitable candidate for practical applications of chromotropism, due
to their various geometrical structures, diverse coordination
numbers, and presence of strong JahneTeller effect. Several classes
of mixed-chelate copper(II) complexes containing unsaturated co-
ordination sphere of central ion have been synthesized and their
solvatochromism properties were investigated [8,9]. However, the
color intensity of these compounds is low since the origin of their
color variation arises from the d-d transition of Cu(II) ion. Thus, the
preparation of chromotropic probes with higher molar absorptivity
is extremely desirable. This disadvantage can be improved in di- or
vatochromism property is sensitive to the structural geometry of
the copper center [10,11]. So that a change in the structure of the
compounds from square pyramid to trigonal bipyramid the sol-
vatochromism property almost vanishes [12]. On the other hand,
the structural feature of copper complexes is influenced by the
nature of co-ligands and their steric crowding imposed around the
metal center. It was found that utilizing a chelate ligand with bulky
substituents suppresses the formation of the homoleptic mono-
nuclear complexes and bridging dinuclear halo- or hydroxo com-
plexes form [13]. However, an increase in the bulkiness of
substituents causes a deviation in the geometry of copper(II)
complex from square pyramidal to trigonal bipyramidal. Therefore,
a compromise must be taken into account in the selection of the
degree of the bulkiness of the chelate ligand and the structural
features of the copper complex. On the other hand, it is well known
that the coordination of a secondary amine moiety to a metal ion
enhances its acidity power and the resultant complex becomes
sensitive to the pH in aqueous solution and shows halochromism.
The resultant ionized amine changes the color of the solution due to
change in the ligand field strength around the metal center. In
particular, the repeatable conversion between deprotonation and
protonation of the coordinated amine ligand in response to the pH
* Corresponding author.
E-mail address: h.golchobian@umz.ac.ir (H. Golchoubian).
https://doi.org/10.1016/j.molstruc.2019.06.034
0022-2860/© 2019 Elsevier B.V. All rights reserved.

770
A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
value of the solution is accompanied by spectral change. In addi-
tion, Many mixed-ligand copper(II) chloride and bromide com-
plexes demonstrated thermochromic behavior [12,14,15] when
small differences in their electronic properties are reflected in the
structures that they prefer upon heating. Presence of JahneTeller
effect and also other weakly bounded ligand make copper(II)
complexes susceptible to exhibit chromotropism.
The present work reports on the preparation of two new dinu-
clear halo bridged copper(II) complexes with the formula [LCu(m-
X)2X]2 (X ¼ Cl, Br) where L is N-benzyl(pyridine-2-yl)methane-
amine. These complexes exhibit thermochromism, solvatochrom-
ism, and halochromism collectively. Synthesis, structure and
chromotropism behavior of the compounds have described below.
1.6 Hz, 1H, Py); 8.57 (d, J ¼ 4.0 Hz, 1H, Py). When one drop of D2O
was added to the 1H NMR sample, the broad signal at 2.58 ppm
disappeared. 13C NMR (100 MHz in CDCl3),
d:
53.39
(eNHeCH2ePy); 53.45 (eNHeCH2eph); 122.01, 122.43, 136.50,
149.29, 159.55 (PyeC); 127.04, 128.30, 128.43, 139.94 (C-Ph).
2.2.2. Bis((
m-chloro)chloro,N-benzyl(pyridine-2-yl)methaneamine
copper(II)), [LCu(
m-Cl)Cl]2 (1)
To the solution of ligand L (0.396 g, 2 mmol) in methanol (7 mL)
was slowly added a solution of CuCl2$2H2O (0.34 g, 2 mmol) in
methanol (5 mL). The resultant bluish-green mixture was stirred
for 3 h at room temperature. The desired compound precipitated
from the solution as blue solid and separated by filtration. The
obtained product was recrystallized by diffusion of diethyl ether
into a methanol solution. The typical yield was 60% (0.40 g). The
crystals were suitable for X-ray crystal structure determination.
Anal. Calcd for C26H28N4Cu2Cl4 (MW ¼ 665.43 g molꢀ1): C, 46.93; H,
2. Experimental
2.1. Materials and physical measurements
4.24; N, 8.42%; Found: C, 46.40; H, 4.17; N, 8.30%. Selected IR data (n/
cmꢀ1 using KBr disk): 3175 (s, N-H str.); 3069 (m, C-H str. aromatic);
2924, 2870 (m, C-H str. aliphatic); 1607 (s, C]N str.); 1484 (m, C]C
str.); 1450 (s, C-H bend.); 1095 (m); 1054 (m); 1021 (m); 755 (s);
705 (m).
All chemicals and reagents were purchased from commercial
sources and used as received. The solvents used in the sol-
vatochromic study were spectroscopic grade and as follows:
nitromethane (NM), nitrobenzene (NB), methanol (MeOH), ethanol
(EtOH), dichloromethane (DCM), acetonitrile (AN), benzonitrile
(BN), pyridine (Py) dimethylformamide (DMF), dimethylsulfoxide
(DMSO), and hexamethylphosphorictriamide (HMPA). The C, H, and
N elemental analyses were carried out on a PerkinElmer model
2400 elemental analyzer. The infrared spectra were recorded in a
range of 400e4000 cmꢀ1 using a Bruker FT-IR instrument in pellets
with KBr. 1H and 13C NMR spectra were acquired on a Bruker
400 MHz DRX Fourier Transform Spectrometer at room tempera-
ture. Conductance measurements were made at 25 ꢁC with a Jen-
way 400 conductance meter on 1.00 ꢂ 10ꢀ3 M samples in selected
solvents. The electronic absorption spectra of solutions were
measured with a Varian Cary 50 spectrophotometer using 1 cm
quartz cells. UV-Vis spectra of powder samples were collected using
Avantes Avaspec spectrometer with Avalight-DH-S-BAL light
source. Thermogravimetric analyses of complexes were performed
using Bahr Thermo Analyse STA 504 instrument in an argon at-
mosphere. The X-band electron paramagnetic resonance (EPR)
spectrum was recorded in a Bruker X-band spectrometer operating
at a field modulation of 100 kHz at 80 K.
2.2.3. Bis((
copper(II)), [LCu(
This compound was prepared according to the procedure used
for [LCu( -Cl)Cl]2, except that CuBr2 was used (0.45 g, 2 mmol) in
m-bromo)bromo,N-benzyl(pyridine-2-yl)methaneamine
m-Br)Br]2 (2)
m
place of CuCl2$2H2O. The complex was obtained as a green solid
with a typical yield of 53% (0.45 g). Anal. Calcd for C26H28N4Cu2Br4
(MW ¼ 843.24 g molꢀ1): C, 37.03; H, 3.35; N, 6.64%; Found: C, 37.18;
H, 3.12; N, 6.66%. Selected IR data (n
/cmꢀ1 using KBr disk): 3195 (s,
N-H str.); 3064, 3025 (m, C-H str. aromatic); 2930, 2851 (m, C-H str.
aliphatic); 1605 (s, C]N str.); 1478 (m, C]C str.); 1448 (s, C-H
bend.); 1094 (m); 1057 (m); 1009 (m); 771 (s); 743 (m).
2.3. X-ray structural analysis
Single crystals of 1 were obtained by diffusion of diethyl ether
into the methanol solution at room temperature. A suitable crystal
was selected and used for data collection. The crystal was kept at
100.01 K during data collection. Diffraction data were measured on
a Bruker PHOTO 100 area (Apex 2 CCD) detector diffractometer
2.2. Synthesis
with
mirror
optics
monochromated
Mo-K
a
radiation
(l
¼ 0.71073 Å) generated from a microfocus x-ray sealed tube
2.2.1. N-benzyl(pyridine-2-yl)methaneamine (L)
source. Data collection, cell refinement, and data reduction were
performed by Brucker software. Multi-scan absorption correction
was applied using SADABS-2014/5 software program. The structure
was solved with ShelXT [16] structure solution program using
intrinsic phasing and refined with the ShelXL [17] refinement
package using Least Squares minimization. The hydrogen atoms
were located geometrically and refined isotropically. Crystal data
and details of structure determination are reported in Table 1.
A mixture of pyridine-2-carbaldehyde (1.9 ml, 20 mmol), 1-
phenylmethaneamine (2.2 ml, 20 mmol) and a few drops of acetic
acid in methanol (30 ml) was prepared and refluxed for 10 min.
Solid NaBH4 (1.1 g, 30 mmol) was then added gradually to the
resulted solution over a period of 30 min and the mixture was
allowed to stand overnight. After heating the solution to near the
boiling point, HCl (6 ml, 17 M) was added to it dropwise, while
placing the solution in an ice bath. The mixture was then made
alkaline (pH ¼ 8) by adding NaOH (25 mL, 4 M). The sodium borate
precipitate was removed by filtration and the filtrate was concen-
trated to obtain a brown oil which was subsequently extracted with
dichloromethane (3 ꢂ 15 mL). The combined organic fractions were
dried over anhydrous Na2SO4. Evaporation of the solvent under
reduced pressure resulted in the desired product as pale brown oil.
3. Results and discussion
3.1. Preparation of samples
The diamine ligand was synthesized by condensation of an
equimolar solution of 1-phenylmethaneamine and pyridine-2-
carbaldehyde under reflux condition followed by reduction of the
resulted Schiff base with sodium borohydride. The reaction of 1:1 M
ratio of the bidentate ligand and CuX2 (X ¼ Cl or Br) in methanol led
to the formation of halo bridged dinuclear copper(II) complexes
(Scheme 1).
The yield was 3.48 g (88%). Selected IR data (n
/cmꢀ1 using KBr disk):
3313 (br, s, N-H str.); 3061, 3028 (s, C-H aromatic); 2917, 2837 (s, C-
H aliphatic); 1593 (s, C]N str.); 1435 (s, C]C str.); 1118 (m, C-N str.
aliphatic); 755(s); 700(s). 1H NMR (400 MHz, CDCl3),
d: 2.58 (br,1H,
-NH-); 3.86 (s, 2H, -CH2-Ph); 3.94 (s, 2H, -CH2-Py); 7.17 (m, 1H, Ph);
7.27 (m, 1H, Ph); 7.34 (m, 3H, Ph); 7.37 (m, 2H, Py); 7.65 (t,d, J ¼ 7.6,

A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
771
Table 1
associated with NH stretching vibration. This band appears at
3313 cmꢀ1 and broader in the free ligand [18]. Upon covalent bond
formation, these absorption bands evidently lose intensity, become
shifted to the higher energy and merge with other C-H absorption
bands. A strong band at 1645 cmꢀ1 in free ligand corresponds to the
C]N bond. This band appears in the complexes at 1607 and
1605 cmꢀ1 in 1 and 2 respectively; the decreased values by
38e40 cmꢀ1 confirm coordination of the pyridyl group of the
ligand to the metal ion [19].
Crystal data and structure refinement for 1.
Empirical formula
C26H28Cl4Cu2N4
Formula weight
Color
665.40
Blue
100(2)
0.71073
Monoclinic
P21/c
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
10.9434(5)
b (Å)
c (Å)
12.8949(5)
10.0913(4)
98.6020(10)
1408.01(10)
2
1.569
1.913
676.0
3.3. X-ray crystallographic study
b
(ꢁ)
Volume (Å3)
Compound 1 crystallized in the monoclinic P21/c space group.
The crystal structure along with coordination polyhedron of the
compound is shown in Fig. 1 and the important bond lengths and
angles are given in Table 2. Compound 1 is a dinuclear complex
contains two identical copper ions with near square pyramidal
geometry. Each copper ion is penta-coordinated by two nitrogen
atoms from amine and pyridine moieties of bidentate ligand, one
terminal chloride ion, and two bridging chloride atoms. The
Z
Calculated density (g cmꢀ3
)
m
(mmꢀ1
)
F (0 0 0)
Crystal size (mm3)
range for data collection (ꢁ)
0.5 ꢂ 0.3 ꢂ 0.2
q
2.46-27.49
Index ranges
ꢀ14 ꢃ h ꢃ 14
ꢀ16 ꢃ k ꢃ 16
ꢀ12 ꢃ l ꢃ 13
50161/3235(0.0442)
99.9%
parameter
t
[20] is calculated to be 0.22 (
t
¼ 0 for standard square
Reflections collected/unique(Rint
Completeness to
Refinement method
Data/restraints/parameters
)
pyramidal and
t
¼ 1 for trigonal bipyramidal), which indicates the
q
¼ 27.522
Full matrix least-squares on F2
3235/0/163
distorted square pyramidal geometry. The Cu-Cl bond length for
terminal Cu-Cl(1) is 2.2494(4)Å, while two bridging Cu-Cl(2) and
Cu-Cl(2a) bond distances are 2.2766(3)Å and 2.7508(4)Å, respec-
tively. The unsymmetrical nature of the bridging chlorides is due to
the geometrically difference in location of anions. One of the
bridging chloride (Cl(2a)) as well the nitrogen atoms and the ter-
minal chloride ion are located in the basal plane and the other
bridging chloride (Cl(2)) occupies the apical position of the pyramid
and has the longer band length. The geometry of the dimer contains
two pyramids which share one base to apex edge, and their basal
planes are parallel (Fig. 1b); this geometry is one of the known
classes of configurations for di-copper complexes in which two
chloride ions are doubly bridging between the metal centers
Final R indices [I > 2
s(I)]
R1 ¼ 0.0209, wR2 ¼ 0.0519
R1 ¼ 0.0225, wR2 ¼ 0.0527
1.082
0.39 and ꢀ0.63
R indices (all data)
Goodness-of-fit on F2
Largest diff. peak and hole (e Åꢀ3
)
3.2. IR spectroscopy
In the IR spectra of dinuclear complexes 1 and 2, several bands
appear in the region 700e1600 cmꢀ1 that are observed, although
with minor shifts, in the spectra of the free ligand (Fig. S1 in the
supplementary material). A comparison between IR spectra of
ligand and complexes may provide invaluable evidence for the
bonding patterns of the ligand to copper(II) ion. The band around
1050 cmꢀ1 with medium intensity in the complexes is probably
attributable to the stretching vibration of C-N bond [18]. The strong
bands at 1440-1464 cmꢀ1 are very likely related to the scissoring
vibration of eCH2- groups [18]. The band at around 750 cmꢀ1 that
appears in the spectrum of free ligand broader and is split in two in
the region of 730e790 cmꢀ1 may be owing to the rocking vibration
of CH2 groups [18]. The stretching vibration of N-CH2 moiety in the
free ligand, and the well-known bands in the region
2850 100 cmꢀ1 that are accompanying with it are more important
because they serve as an indication of coordination of the amine
group of the ligand. Another evidence for the coordination of the
amine group of the ligand to the copper(II) ion is the appearance of
strong bands at 3175 and 3195 cmꢀ1 in 1 and 2, respectively,
[21,22]. The Cu(m-Cl2)Cu core has a symmetrical rhombic geometry
with Cu-Cl-Cu angle of 83.089(11)ꢁ and Cl-Cu-Cl angle of
96.911(11)ꢁ and Cu/Cu distance of 3.3530(3) Å which are rather
smaller in compared with the reported values for related com-
pounds [23,24]. The bidentate ligand coordinates to the copper ion
through the nitrogen atoms with bite angle of 81.73(5)ꢁ and the
bond distances of 2.0104(12) Å and 2.0274(11) Å for Cu-N(pyridine)
and Cu-N(amine), respectively. The benzyl rings are directed away
from the Cu2Cl2 plane with an angle of 88.01ꢁ, and to opposite sides
of terminal chloride ions to reduce the steric hindrance.
3.4. EPR study
EPR spectra of compounds 1 and 2 were recorded at the X-band
in frozen DMF solution at 80 K (Fig. 2). The spectral pattern is
characteristic of copper(II) complexes with axial distortion and
Scheme 1. Synthetic outline for preparation of ligand and complexes.

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A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
Fig. 2. EPR spectra of complexes 1 and 2 in DMF at 80 K. The vertical bar locates
ge ¼ 2.003.
Fig. 1. (a) ORTEP view with atom labeling scheme and (b) coordination polyhedron for
complex 1.
without tetrahedral distortion in the basal plane for both of the
complexes.
anisotropic hyperfine coupling. The calculated parameters from the
EPR signals are g ¼ 2.282, and A ¼ 166 ꢂ 10ꢀ4 cmꢀ1 for 1 and
EPR spectra of dinuclear complexes with two coupled copper(II)
ions are expected to show the seven hyperfine features (2nIþ1;
I ¼ 3/2 and n ¼ 2), and a half field signal attributed to the singlet-
triplet transition. In spite of the short Cu/Cu distance in the
complexes, the spectra do not show characteristic signals of cop-
per(II) dimers. It is most likely due to dissociation of dinuclear
complexes and the formation of mononuclear species and/or ex-
istence of an equilibrium mixture with a higher percentage of
monomer [27,28]. The halo bridges are destroyed when the com-
plexes solved in DMF and solvent molecules replaced the labile
halide ligands. The additional humps that are observed in the
spectra, especially for 2, possibly arise from dinuclear molecules
exist in the equilibrium mixture.
jj
jj
jj
g ¼ 2.251, and A ¼ 171 ꢂ 10ꢀ4 cmꢀ1 for 2. In both copper(II)
jj
complexes, the trend of g ˃ g⊥ ˃ ge (2.003) suggest a square pyra-
jj
midal structure with the unpaired electron located in the dx2ꢀ y2
orbital and preclude the possibility of a trigonal bipyramidal ge-
ometry which would be expected to have g⊥> g ˃ ge (2.003). The
jj
ratio of f ¼ g /A has been suggested as an empirical index of
jj jj
distortion of donor set from planer toward tetrahedral. The value of
f varies from 105 to 135 cm for square planar complexes and that
depends on the nature of the coordinated atoms. In the complexes
with a tetrahedral distortion, this quotient increases noticeably to
about 150e250 cm [25,26]. The f values of 137 cm and 131 cm in 1
and 2, respectively, indicate the square pyramidal geometry
Table 2
Selected bond lengths (Å) and angles (ꢁ) for 1.
Bond distances
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-Cl(2a)
Cu(1)-Cu(1a)
2.0104(12)
2.0274(11)
2.7508(4)
3.3530(3)
Cu(1)-Cl(1)
Cu(1)-Cl(2)
Cl(2)-Cl(2a)
2.2494(4)
2.2766(3)
3.7759(4)
Bond angles
N(1)-Cu(1)-N(2)
N(1)-Cu(1)-Cl(1)
N(2)-Cu(1)-Cl(1)
N(2)-Cu(1)-Cl(2)
Cl(1)-Cu(1)-Cl(2a)
Cu(1)-Cl(2)-Cu(1a)
81.73(5)
96.10(3)
177.14(3)
89.18(3)
92.216(13)
83.089(11)
N(1)-Cu(1)-Cl(2)
N(1)-Cu(1)-Cl(2a)
N(2)-Cu(1)-Cl(2a)
Cl(1)-Cu(1)-Cl(2)
Cl(2)-Cu(1)-Cl(2a)
163.71(3)
95.94(3)
86.17(3)
93.360(13)
96.911(11)
a. 2-X,1-Y,2-Z.

A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
773
3.5. Thermal analysis
polar solvents such as dichloromethane, the electronic absorption
spectra of the compounds are very similar to the related spectra in
the solid state as shown in Fig. S2 in the supplementary material. It
could be due to the solvation of compounds with minimal changes
in the structure. Therefore, the dimeric structure of complexes is
preserved in the low polar solvents (Route 1 in Scheme 2), while
the solvation of the neutral monomers produced from cleavage of
weak Cu-X bridging bands cannot be excluded. In the solvents with
moderate polarity such as MeOH, DMSO and DMF both complexes
display almost a single unsymmetrical broad ded band in the re-
gion 700e800 nm and strong charge transfer bands with shoulders
at 330 and 380 nm for 1 and 2, respectively owing to the halide
ligand-to-metal charge-transfer (LMCT) transitions of coordinated
halide [29]. The former band is characteristic of an unsymmetrical
geometric structure due to ligand field and JahneTeller effects
around the copper(II) ions. Generally, this band is assigned to the
overlapping transitions dxz /dyz /dx2ꢀ y2 , dxy/dx2ꢀ y2 , and dz2
/dx2ꢀ y2 . This may result in a mononuclear complex [CuLX(solv)3]
X (X ¼ Cl or Br) (Route 2 in Scheme 2). This proposed mechanism is
further confirmed by increasing the molar conductance values of
the complexes in the polar solvents as shown in Table 4. In aqueous
solution, both complexes turn blue and their absorption spectra are
also similar (Fig. S3 in supplementary material). This point to that in
aqueous solution complex 1 behaves like 2. Both of the complexes
exhibit a very broad and weak band centered at 680 nm (ded band)
and strong charge transfer band at 260 nm (Route 3 in Scheme 2).
This may occur if the CueX bond is broken by water molecules and
formation of [CuL(solv)4]X2 complex. This structural change is
along with an increase in the molar conductance values of com-
plexes in the aqueous solution as shown in Table 4.
Thermal gravimetric (TG) and differential thermal analysis
(DTA) were carried out in a static argon atmosphere from 50 to
1000 ꢁC to investigate the thermal behavior of the complexes. The
DTA curve of complexes 1 and 2 (Fig. ꢁ3) exhibit an endothermic
process without loss of weight at 210 C and 184 ꢁC respectively,
attributed to cleavage of bridging Cu-X bands in [LCu(m-X)X]2
(X ¼ Cl, Br) and formation of mononuclear complexes[LCuX2]. The
complexes begin to lose weight above 250 ꢁC. Complex 1 loss 57.2%
of its weight gradually in the range of 250e560 ꢁC that is equivalent
to the loss of the diamine ligands (calcd 59.6%). The same decom-
position occurred in complex 2 in the range of 250e435 ꢁC
(observed 47.1%; calcd 48.9%). Further continues weight loss up to
1000 ꢁC may be attributed to the loss of organic moieties to leave
copper(II) chloride (observed 40.5%; calcd 40.7% for 1) and metallic
copper for 2 (observed 15.5%; calcd 15.07%).
3.6. Chromotropism
3.6.1. Solvatochromism
The complexes are soluble in a wide range of organic solvents
and exhibit noticeable solvatochromic property. The origin of the
color changes in the solutions is attributed to the shift in the d-
d transition of the copper(II) ions as a result of interaction with
solvents. The visible spectral changes in the studied solvents for
compounds 1 and 2 are shown in Fig. 4. The locations of the lmax
values of the complexes along with the molar absorptivity values
are collected in Table 3. The observed solvatochromism could be
explained by the structural changes in different solvents. In low
3.6.2. Halochromism
The electronic spectra of the aqueous solution of the compounds
Fig. 3. TG-DTA curves of complexes 1 and 2.
Fig. 4. Absorption spectra of complexes 1 and 2 in different solvents.

774
A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
Table 3
Table 4
The electronic absorption maxima of 1 and 2 in different solvents.
Molar conductivities data (Lm) of 1 and 2 (U
ꢀ1 cm2 molꢀ1) at 25 ꢁC in different
solvents.
Solvent
Compound 1
Compound 2
DCM
NB
ACN
MeOH
DMF
H2O
lmax (nm)
ε (L cmꢀ1 molꢀ1
)
lmax (nm)
ε (L cmꢀ1 molꢀ1
)
Compound 1
Compound 2
0.2
1.1
0.4
4.2
5
30
138
152
46
79
65e90
409
432
118e131
BN
Py
709
786
750
695
710
831
692
763
689
706
683
225
381
205
299
161
211
212
174
110
213
188
690
704
737
670
714
812
649
798
674
671
670
326
333
257
455
279
183
421
384
106
429
546
1:1 electrolytes 10e20 20e30 120e160 80e115
1:2 electrolytes 60e75 50e60 220e300 160e220 130e170 235e273
1:3 electrolytes
DMSO
NB
MeOH
HMPA
NM
DMF
H2O
ACN
DCM
a
a
-
70e80 340e420
-
200e240 408e435
a
The standard value is not reported.
in Scheme 3. This mechanism is evidenced by spectrophotometric
titration of the complexes with sodium hydroxide (0.1 M) which
showed consumption of two equivalents hydroxide anions in the
pH range of 5.4e10.5 (insert of Fig. 5). In the other hand, upon
addition of the acid (HClO4 0.1 M), the blue color of the solution was
gradually diminished. This process was reversible so that, the
original blue color re-developed by increasing the pH of the solu-
tion to around 5.4 with the addition of a base (NaOH 0.1 M). The
electronic absorption spectra of the compound (Fig. 6) shows a
decreasing in the intensity of d-d band accompanied with a redshift
upon lowering the pH to around 2 by adding an acid (HClO4 0.1 M).
Spectrophotometric titration of the complexes with perchloric acid
demonstrated that decolorization occurred with consumption of
four equivalents protons (inset of Fig. 6). It is probably attributed to
protonation of pyridyl and amine groups and formation of a totally
hydrated copper(II) complex (Scheme 3). This phenomenon was
confirmed by comparison of the acidified spectrum with the
spectra of the ligand and copper (II) halide in acidic solution.
1 and 2 are sensitive to pH. When the dinuclear complexes are
dissolved in water, dissociation of the halide ions and hydration of
the compound led to the mononuclear complex as shown in
Scheme 2. This phenomenon occurred in both compounds. As a
result, the aqueous solution of two compounds has identical elec-
tronic spectra. The sky-blue color of the aqueous solution turned to
purple by addition of a base (NaOH 0.1 M) that reversibly reap-
pearance to its original color by the addition of an acid (HClO4
0.1 M). The visible spectral changes for the compound upon
increasing the pH of the solution from 5.4 (the origin pH) to 10.5 are
illustrated in Fig. 5. The blue shift in the absorption maxima is along
with an isosbestic point at 662 nm which is possibly due to
deprotonation of the amine group in an alkaline medium as shown
Scheme 2. Proposed structural change of the complexes in solvents with different polarities.

A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
775
Fig. 5. The pH titration of 1 with NaOH (0.1 M) in the pH range of 5.4e10.5.
Scheme 3. Proposed interconversion of complexes caused by acid and base (pH 2e10.5) in aqueous solution.
3.6.3. Thermochromism
totally reversible. However due to the strong bond formed between
copper(II) ion and DMSO molecules and the presence of a copious
amount of solvent molecules around the metal center, the reverse
reaction is slow, and it takes about 5 days to the full development of
the original color. The same thermochromism behavior was
observed in compound 2 in the solvent of DMSO.
The compounds show reversible thermochromism in solvents
with high boiling points such as DMF and DMSO. The effect of
temperature on the visible spectra of compound 1 was studied over
a temperature range of 21e170 ꢁC in DMSO (Fig. 7). By heating the
solution up to 90 ꢁC the original blue color changed to green and
further increasing the temperature up to 170 ꢁC became brown. It
was presumably associated with gradual substitution of the chlo-
ride anions and the chelating ligand by solvent molecules. The
observed brown color is attributed to the presence of free chelating
ligand and [Cu(dmso)n]Cl2 in the solution. This phenomenon is
4. Conclusion
Two new dinuclear copper(II) complexes in which, two metal
centers connected with doubly chloride or bromide bridge were

776
A. Shirvan et al. / Journal of Molecular Structure 1195 (2019) 769e777
Fig. 6. The pH titration of 1 with HCl (0.1 M) in the pH range of 5.4e2.
exhibited solvatochromism behavior. The origin of this phenome-
non is structural changes followed by solvation in solvents with
different polarities. The complexes are halochromic and show
different colors in pH range 2e10.5 due to protonation and
deprotonation of coordinated ligands in aqueous solutions. More-
over, the compounds displayed reversible thermochromism with
obvious color changes in the solvents of DMSO and DMF through
the temperature range 21e170 ꢁC. The mechanism for this revers-
ible color change is likely dissociation of the coordinated halides at
elevated temperature and their re-coordination at room tempera-
ture. The observed color changes in both complexes are approxi-
mately in the same range. However since compound 2 is more
soluble in the studied solvents, it can be a better chromotropic
probe. This study will be of value with regard to complexes which
find their application as possible candidates for molecular switches
and sensor materials.
Acknowledgement
We are grateful for the financial support of University of
Mazandaran of the Islamic Republic of Iran.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.06.034.
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