Copper (II) halide complexes with NNO tridentate ligand as chromotropic probes; synthesis, structural characterization and spectroscopic properties

Article abstract of DOI:10.1016/j.molstruc.2021.130930

This study reports chromotropism of two newly synthesized copper(II) complexes of formula [CuLCl₂] and [CuLBr₂]?MeOH where L = N-(2-pyridylmethyl)-1-hydroxypropylamine. The structure of the complexes was investigated by infrared spectroscopy, electronic absorption spectroscopy, elemental analysis, molar conductance measurements, and thermal analysis. Single Crystal X-ray structure determination reveals both complexes to adopt a distorted square-pyramidal geometry. Their chromotropic properties were investigated using electronic absorption spectroscopy. The complexes are solvatochromic in solvents with different polarities. The aqueous solutions of the complexes exhibit a variety of colors in the pH range of 1.8 to 11.4 showing that the compounds are halochromic. The complexes are also thermochromic and show reversible color changes in different temperatures in dimethyl sulfoxide solutions. The solution study reveals that the observed chromotropism originates from structural changes of the complexes in different conditions.

Full text of DOI:10.1016/j.molstruc.2021.130930

Journal of Molecular Structure 1243 (2021) 130930
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Copper (II) halide complexes with NNO tridentate ligand as
chromotropic probes; synthesis, structural characterization and
spectroscopic properties
Atie Shirvana,∗, Hamid Golchoubiana,∗, Maxime A. Sieglerb, Elisabeth Bouwmanc
a Department of Chemistry, University of Mazandaran, Babol-sar, 47416-95447, Iran
b Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, United States
c Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
This study reports chromotropism of two newly synthesized copper(II) complexes of formula [CuLCl2] and
[CuLBr2]•MeOH where L = N-(2-pyridylmethyl)-1-hydroxypropylamine. The structure of the complexes
was investigated by infrared spectroscopy, electronic absorption spectroscopy, elemental analysis, molar
conductance measurements, and thermal analysis. Single Crystal X-ray structure determination reveals
both complexes to adopt a distorted square-pyramidal geometry. Their chromotropic properties were in-
vestigated using electronic absorption spectroscopy. The complexes are solvatochromic in solvents with
different polarities. The aqueous solutions of the complexes exhibit a variety of colors in the pH range of
1.8 to 11.4 showing that the compounds are halochromic. The complexes are also thermochromic and
show reversible color changes in different temperatures in dimethyl sulfoxide solutions. The solution
study reveals that the observed chromotropism originates from structural changes of the complexes in
different conditions.
Received 26 January 2021
Revised 21 May 2021
Accepted 15 June 2021
Available online 19 June 2021
Keywords:
Solvatochromism
Thermochromism
Halochromism
Copper
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
color changes [7-9]. Copper(II) complexes with strong Jahn-Teller
effects form a family of chromotropic metal complexes whose color
The design and preparation of novel materials with chro-
motropic properties have gained increasing importance due to
their potential applications in various areas. Chromotropic mate-
rials are able to exhibit discernible color changes upon exposure
to various external stimuli such as temperature, pressure, differ-
ent ions, pH, electric field, irradiation and/or polarity of the sol-
vents [1,2]. Due to the optical response which lies within the vis-
ible region, the chromotropic compounds can generate simple and
immediate visual signals and thus can be utilized efficiently as
sensors and indicators [3-5]. Furthermore, investigation of chro-
motropism behavior of the compounds in solution and scrutiniz-
ing the mechanism of their color change, may lead to understand-
ing of their possible solute-solution interactions in the various sur-
rounding media. Consequently, this could provide valuable data for
scientists in chemistry and biochemistry to choose the best reac-
tion conditions such as solvent, temperatures and pH range [6].
Examples of chromotropic organic, organometallic and coordina-
tion compounds have been reported with diverse origins of their
change arises from changes in their d-d transitions. Most of these
complexes contain four- or five-coordinate copper(II) centers stabi-
lized with diamine and/or diketone ligands. Spectroscopic studies
on these complexes demonstrated that the interaction of the sol-
vent molecules with the axial positions of the complexes cause a
shift in the d-d transitions and thus a substantial solvatochromism
is observed in solvents with different polarities [10-12]. The de-
sign of complexes with two or more chromotropic features can be
highly beneficial. The presence of labile metal-ligand bonds in the
coordination sphere is a feasible approach to increase the sensitiv-
ity of the complexes to environmental changes [13]. Earlier stud-
ies show that the structure and spectroscopic properties of copper
complexes containing chloride and/or bromide ions change upon
heating or increasing the polarity in the solution [14]. Therefore,
copper halide complexes with the proper combination of chelat-
ing ligands are expected to exhibit thermo and solvatochromic be-
havior [15,16]. Tridentate chelating ligands containing NNO donat-
ing atoms are of special interest because of the potential biological
activities [17]. Besides, the multidentate ligands have the advan-
tage of enhancing the stability of the resulting complexes due to
the chelating effect. In this study, a tridentate NNO ligand contain-
∗
Corresponding authors.
E-mail address: h.golchobian@umz.ac.ir (H. Golchoubian).
https://doi.org/10.1016/j.molstruc.2021.130930
0022-2860/© 2021 Elsevier B.V. All rights reserved.

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Scheme 1. Complexes under study.
ing hydroxyl and secondary amine moieties was used in connec-
tion with their ability to undergo deprotonation-protonation pro-
cesses, which could potentially cause the formation of the com-
plexes with pH-induced color change i.e. halochromism. In this
regard, the two mononuclear copper(II) halide complexes shown
in Scheme 1 were synthesized and characterized, with the ligand
N-(2-pyridylmethyl)−1-hydroxypropylamine. Their structural fea-
tures, as well as their spectroscopic sensitivity to external stimuli
were evaluated, and the complexes were shown to exhibit ther-
mochromic, solvatochromic, and halochromic properties.
Fig. 1. Displacement ellipsoid plot (50% probability) of [CuLCl2], 1 at 110(2) K.
2. Experimental
2.1. General procedures
The ligand N-(2-pyridylmethyl)1-hydroxypropylamine (L) was
synthesized according to the procedure reported by Gentschev
et al. [18]. All chemicals were purchased from commercial sources
and used without further purification. The solvents employed for
chromotropism studies were of spectroscopic grade.
IR spectra were recorded on a PerkinElmer UATR spectrum
equipped with a single reflection diamond. UV-Vis spectra were
acquired with JASCO V-750 double beam spectrophotometer with
1 cm quartz cells. Conductance measurements were performed
at room temperature using a Jenway 400 conductivity meter on
1.00×10−3 M samples in selected solvents. Elemental analysis were
performed by a PerkinElmer model 2400 elemental analyzer. Ther-
mal analysis was carried out on a Bahr Thermo analyse STA 504
instrument in an argon atmosphere.
Fig. 2. Displacement ellipsoid plot (50% probability) of [CuLBr2], 2 at 110(2) K.
2.2. Synthesis
2.2.1. Preparation of complex [CuLCl2], 1
A solution of CuCl2 (0.536 g, 4 mmol) in methanol (8 mL) was
added to a solution of L (0.66 g, 4 mmol) in methanol (10 mL). The
resulted greenish-blue solution was stirred for 4 h at room tem-
perature. After one day, the obtained blue crystals were collected
by filtration and recrystallized by diffusion of diethyl ether into a
methanol solution. Yield: 65% (0.78 g). Selected IR data (ν/cm−1):
3147 (s); 3074 (m); 2946 (m); 1610 (s); 1573 (m); 1479 (m); 1463
(m); 1447 (s); 1421 (s); 1327 (s); 1284 (s); 1067(s); 1047 (s); 957
(s); 770 (s); 541 (m); 420 (m). Anal. Calcd for C9H14 Cl2CuN2O
(MW = 300.67 g mol−1): C, 35.95; H, 4.69; N, 9.32%; Found: C,
35.97; H, 4.46; N, 9.29%.
2.2.2. Preparation of complex [CuLBr2]•MeOH, 2
This compound was synthesized by the same reaction proce-
dure described for [CuLCl2], except that CuBr2 (0.90 g, 4 mmol)
was used in place of CuCl2. The desired complex was obtained
as blue crystals with a typical yield of 43% (0.66 g). Selected IR
data (ν/cm−1): 3163 (s,); 3079 (s); 2862 (m); 1607 (s); 1569 (m);
1475 (m); 1446 (m); 1432 (s); 1422 (s); 1284 (s); 1053(s); 1028 (s);
951 (s); 778 (s); 541 (m); 418 (s). Anal. Calcd for C9H14 Br2CuN2O
(MW = 389.57 g mol−1): C, 27.75; H, 3.62; N, 7.19%; Found: C,
27.33; H, 3.20; N, 6.55%.
2

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Table 1
Crystal data and structure refinement for 1 and 2.
1
2
•
Empirical formula
Formula weight
Color, morphology
Temperature (K)
C9H14Cl2CuN2O
300.66
C9H14Br2CuN2O CH4O
421.62
Light blue, small block
110(2)
Blue, small block
110(2)
˚
Wavelength (A)
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Unit cell dimensions
˚
a (A)
7.95176 (19)
10.9791 (2)
7.41692 (15)
12.7280 (2)
˚
b (A)
˚
c (A)
13.2995 (3)
15.1141 (3)
β (°)
97.926 (2)
93.4959 (18)
1424.15 (5)
3
˚
Volume (A )
1150.00 (4)
Z
4
4
Calculated density (g cm−3
)
1.737
1.966
μ (mm−1
)
2.34
7.14
F (0 0 0)
612
828
Crystal size (mm3)
0.17 × 0.15 × 0.06
3.19–27.67
0.20 × 0.17 × 0.11
3.12–27.67
θ range for data collection (°)
Index ranges
−10 ≤ h ≤ 10
−14 ≤ k ≤ 14
−17 ≤ l ≤ 17
17,148/2644(0.029)
100%
−9 ≤ h ≤ 9
−16 ≤ k ≤ 16
−19 ≤ l ≤ 19
21,215/3275(0.029)
99.9%
Reflections collected/unique(Rint
Data completeness
)
Refinement method
Full matrix least-squares on F2
Full matrix least-squares on F2
Data/restraints/parameters
Final R indices [I > 2σ(I)]
R indices (all data)
2644/2/142
3275/3/165
R1 = 0.0210, wR2 = 0.0491
R1 = 0.0237, wR2 = 0.0506
1.04
R1 = 0.0159, wR2 = 0.0374
R1 = 0.0181, wR2 = 0.0381
1.06
Goodness-of-fit on F2
−3
˚
Largest diff. peak and hole (e A
)
0.66 and −0.33
0.39 and −0.32
Table 4
Table 2
Selected bond lengths (A) and angles (°) for 1 and 2.
Molar conductivity values ( m) of 1 and 2 (ꢁ−1 cm2 mol−1) at 25 ◦C in dif-
˚
a
ferent solvents
.
Complex 1
Bond distances
Complex 2
Bond distances
NB
EtOH
MeOH
DMF
H2O
Compound 1
Compound 2
1
21
90
61
262
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-Cl(1)
Cu(1)-Cl(2)
Cu(1)-O(1)
1.9995(14)
2.0389(14)
2.4901(4)
2.2794(4)
2.0071(13)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-Br(1)
Cu(1)-Br(2)
Cu(1)-O(1)
2.0127(14)
2.0287(14)
2.7251(3)
2.3947(3)
1.9887(12)
4
27
104
72
254
1:1 electrolytes
1:2 electrolytes
1:3 electrolytes
20–30
50–60
70–80
35–45
70–90
80–115
160–220
65–90
130–170
200–240
118–131
235–273
408–435
b
b
-
-
a
Bond angles
Bond angles
Standard values are given from Ref. [29].
The standard value is not reported.
b
Cl(1)-Cu(1)-O(1)
Cl(1)-Cu(1)-N(1)
Cl(1)-Cu(1)-N(2)
Cl(1)-Cu(1)-Cl(2)
Cl(2)-Cu(1)-N(1)
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-O(1)
O(1)-Cu(1)-Cl(2)
N(2)-Cu(1)-Cl(2)
N(1)-Cu(1)-O(1)
94.05(4)
95.17(4)
96.37(4)
109.073(17)
96.55(4)
81.73(6)
91.40(6)
85.97(4)
154.53(4)
169.06(5)
Br(1)-Cu(1)-O(1)
Br(1)-Cu(1)-N(1)
Br(1)-Cu(1)-N(2)
Br(1)-Cu(1)-Br(2)
Br(2)-Cu(1)-N(1)
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-O(1)
O(1)-Cu(1)-Br(2)
N(2)-Cu(1)-Br(2)
N(1)-Cu(1)-O(1)
98.77(4)
97.50(4)
92.41(4)
102.395(9)
96.71(4)
82.00(5)
88.16(5)
88.74(4)
165.17(4)
161.31(5)
2.3. Single crystal X-ray crystallography
All reflection intensities were measured at 110 K for 1 and
2 using a SuperNova diffractometer (equipped with Atlas detec-
˚
tor) with Mo Kα radiation (λ = 0.71073 A) under the program
CrysAlisPro (Version CrysAlisPro 1.171.39.29c, Rigaku OD, 2017).
The same program was used to refine the cell dimensions and
for data reduction. The two structures were solved with the pro-
gram SHELXS-2018/3 (Sheldrick, 2018) and was refined on F2 with
SHELXL-2018/3 (Sheldrick, 2018) [19]. Numerical absorption cor-
rection based on Gaussian integration over a multifaceted crystal
model was performed using CrysAlisPro. The temperature of the
data collection was controlled using the system Cryojet (manufac-
tured by Oxford Instruments). The H atoms were placed at calcu-
lated positions (unless otherwise specified) using the instructions
AFIX 23, AFIX 43 or AFIX 137 with isotropic displacement param-
eters having values 1.2 or 1.5 Ueq of the attached C atoms. The
H atoms attached to N2, O1 and O1S (only for 2) have been lo-
cated via difference Fourier maps, and their coordinates were re-
fined pseudo-freely using the DFIX instructions in order to keep
the N − H and O − H bond distances within acceptable ranges.
Both structures are ordered. Detailed information on structure de-
Table 3
The electronic absorption maxima of complexes 1 and 2 in selected solvents at
room temperature.
complex 1
Complex 2
solvent
λmax (nm)
ε (L cm−1 mol−1
)
λmax (nm)
ε (L cm−1 mol−1
)
NB
749
698
757
776
708
682
731
784
741
102
106
162
107
180
130
107
159
197
147
711
703
749
774
714
681
765
704
734
93
111
127
102
218
188
102
150
165
188
MeOH
NM
DMF
EtOH
H2O
Ac
Py
DMSO
ꢀλ
3

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Fig. 3. TGA and DTA curves of 1 and 2.
termination and crystal data for complexes 1 and 2 are presented
halide salts with N-(2-pyridylmethyl)−1-hydroxypropylamine in
a molar ratio of 1:1 in methanol at ambient temperature. The
proposed formulation of the complexes has been confirmed by
spectroscopic and analytical data. The infrared spectra of the com-
pounds 1 and 2 exhibit comparable signals due to their similar
structures as was confirmed by single crystal X-ray crystallography
(Fig. S1 in supplementary content). The stretching vibrations of
in Table 1.
3. Results and discussion
3.1. Synthesis and characterization
The
mononuclear
copper
complexes
[CuLCl2]
and
–
N-H and O H groups appear as overlapping bands at 3074–3147
[CuLBr2]•MeOH were synthesized by the reaction of copper(II)
4

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Fig. 4. Electronic absorption spectra of 1 and 2 in different solvents.
cm−1 and 3079–3163 cm−1 in the spectra of complexes 1 and 2,
respectively. In the IR spectrum of the free ligand these bands
are broader and appear at higher frequencies (3283 cm−1). This
noticeable shift confirms that the secondary amine and alcohol
groups of the ligand are coordinated to the copper center. The
bands at 1610 cm−1 and 1573 cm−1 in complex 1 and 1607 cm−1
and 1569 cm−1 in complex 2 are attributed to C=N and C=C
vibration modes of the pyridine ring [20]. Coordination of the
chelating ligand to the copper(II) ion can be also substantiated
of L along with one halide ion are located in the basal plane of
the square pyramid and the other halide ligand occupies the apical
position. The geometrical parameter τ = ǀα-βǀ/60 where β and α
are the two largest Ll-M-L bond angles in the coordination sphere,
can be used to describe the distortion of square-pyramidal geome-
try (in which τ = 0) towards a trigonal bipyramid (in which τ = 1)
[23]. The τ values of 0.24 for 1 and 0.06 for 2 confirm the distorted
square-pyramidal geometries around the copper centers. The devi-
ations from the least-square plane through the CuON2X atoms are
by the appearance of the bands around 450 cm−1 and 540 cm−1
,
˚
N(1) 0.085, N(2) −0.231, O(1) 0.086, Cl(2) −0.199, Cu(1) 0.259 A
in complex 1, and N(1) −0.066, N(2) −0.051, O(1) −0.065, Br(2)
which are attributed to Cu-N and Cu-O vibrations, respectively
[21,22].
˚
−0.050, Cu(1) 0.232 A in complex 2. The tridentate ligand offers
an N2O donor set chelated to the copper ion, thereby forming two
fused metallocyclic (5,6)-membered rings with N,N and N,O co-
ordination angles of 81.73(6)° and 91.40(5)° in 1, and 82.00(6)°
and 88.16(5)° in 2, respectively. The five-membered chelate rings
in both complexes are puckered, so that the N(1)-C(5)-C(6)-N(2)
torsion angles are 18.4(2)° and 28.4(2)° in 1 and 2, respectively.
The six-membered chelate rings have chair conformations. The Cu-
3.2. Description of structures
Both complexes crystallized in the monoclinic space group
P21/n. A projection of the crystal structures of 1 and 2 along with
the atom numbering scheme are shown in Figs. 1 and 2 and se-
lected bond distances and angles are provided in Table 2. The cop-
per metal centers in both mononuclear complexes adopt a five co-
ordinated N2OX2 (X=Cl, Br) environment. The three donor atoms
˚
O bond lengths of 2.0071(13) and 1.9887(12) A in 1 and 2 are
in the normal range of copper-alcohol bond distances in equato-
5

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Scheme 2. Proposed structural changes of 1 and 2 in different solvents.
Fig. 5. The pH titration of 1 with HClO4 (100 mM) in the pH range of 5.4–1.8.
6

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Scheme 3. Interconversion of complexes 1 and 2 triggered by base and acid (pH=1.8–11.4) in aqueous solution.
rial positions [24]. In both complexes, the halide ion located in
the apical site has longer Cu-X bond length than the equatorial
halide ligand. The Cu-Cl(1) and Cu-Cl(2) bond lengths in 1 are
and 350◦C in complex 1 and between 250 and 375 ◦C in complex
2, as shown by the mass loss of 30.98% (calculated 31.64%) and
28.34% (calculated 23.68%) in 1 and 2, respectively. This step is suc-
ceeded by the removal of the remaining aminopropanol moiety of
the ligand with a mass loss of 22.87% (calculated 24.9%) and 17.25%
(calculated 19.27%) in 1 and 2, respectively. The residual mass at
600 ◦C is attributed to copper(II) chloride (observed 46.15%; calcu-
lated 44.71%) and copper(II) bromide (observed 54.41%; calculated
57.33%) in 1 and 2, respectively.
˚
2.4901(4) and 2.2794(4) A, respectively, whereas the corresponding
˚
Cu-Br bond lengths in 2 are 2.7251(3) and 2.3947(3) A. The Cu-N
˚
bond lengths are in the range of 1.9995(14) to 2.0389(14) A and
are comparable to literature values [18,25,26]. In the crystal lattice
of compound 1, hydrogen bonding occurs between the hydroxyl
group of the ligand and the equatorial chloride ion of a neigh-
˚
boring molecule (O(1)H(1A)•••Cl(2) = 3.1517(13) A), by which a
dimeric entity forms that is further linked to neighboring ones
through hydrogen bonding between the secondary amine hydro-
3.4. Chromotropism study
˚
gen and apical chloride ions (N(2)H(2A)•••Cl(1) = 3.2430(15) A).
3.4.1. Solvatochromism
In compound 2, the crystal lattice is stabilized by the hydrogen
bonding between the secondary amine and the apical bromide
ion of a neighboring molecule (N(2)H(2A)•••Br(1) = 3.3866(14)
The complexes are solvatochromic and show a range of color
changes in solvents with different polarity including dimethylfor-
mamide (DMF), pyridine (Py), dimethylsulfoxide (DMSO), acetone
(Ac), methanol (MeOH), ethanol (EtOH), nitromethane (NM), ni-
trobenzene (NB) and water. The λmax values of the visible ab-
sorption bands in different solvents are collected in Table 3 and
the solvent-dependent electronic absorption spectra of 1 and 2
are shown in Fig 4. The observed solvatochromism arises from a
shift in the d-d transition, which can be attributed to the struc-
tural changes of the complexes in different solvents. The d-d ad-
sorptions in all solvents appear as unsymmetrical broad bands at
700–800 nm with low intensities as is characteristic of octahedral
complexes [27]. The crystallographic data showed that the com-
plexes are square pyramidal in the solid state. However, in solution
it is possible for solvent molecules to coordinate to the unoccupied
apical position of the copper(II) ions, thus changing the structure
to octahedral [1,28]. On other hand, the Jahn-Teller elongation in
the Cu-X (X=Cl or Br) bonds make the apical halide ligands sus-
ceptible to dissociation and to be substituted by solvent molecules
[14]. However, the probability of this phenomenon depends on the
polarity and coordinating ability of the solvent molecule. In or-
der to investigate this hypothesis, the molar conductance of the
compounds in different solvents was determined and compared
with the standard values [29] as presented in Table 4. In more
˚
A), and furthermore by hydrogen bonding between the methanol
molecule and the apical bromide as well as the hydroxyl
˚
3.2715(13) A;
group of the ligand (O(1S)H(1O1)•••Br(1)
˚
=
O(1)H(1A)•••O(1S) = 2.6542(18) A). The crystal packing of com-
pounds 1 and 2 are depicted in Figs. S2 and S3 in the supplemen-
tary content, respectively.
3.3. Thermal analysis
The thermal stabilities of the complexes were studied using
thermal gravimetry (TG) and differential thermal analysis (DTA) in
a temperature range of 25 to 600 ◦C under static argon atmo-
sphere. The complexes have the same pattern of thermal decom-
position due to their similar molecular formula and structures, as
shown in Fig. 3. The endothermic signals which appeared in the
DTA curves of 1 and 2 at 175 and 181 ◦C, respectively, are proba-
bly due to dissociation of the apical halogen ions which are coor-
dinated loosely to the copper(II) ions. In both complexes decompo-
sition of the organic ligand starts above 250 ◦C in two distinguish-
able stages. The first step is associated with the elimination of the
methylpyridine moiety of ligand, which takes place between 250
7

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Fig. 6. The pH titration of 1 with NaOH (100 mM) in the pH range of 5.4–11.4 (a), spectroscopic titration of complex at 710 nm with NaOH (0.1 M) at pH range of 5.4–8.4
and presentation of isosbestic point at 688 nm (b), spectroscopic titration of complex at 460 nm with NaOH (0.1 M) at pH range of 8.4–11.4 and presentation of isosbestic
point at 476 nm (c). Inset graphs of (b) and (c) show decrease in molar absorption values at 710 nm and 460 nm, respectively, during the titration.
polar solvents, the possibility of breaking of the Cu-X (X= Cl or
Br) bond increases and accordingly the molar conductance values
of the solutions are enhanced. In solvents of low polarity the Cu-
X (X=Cl or Br) bonds remain intact and solvatochromism is asso-
ciated with coordination of the solvent at the vacant site of the
compounds (route 1 in Scheme 2) without considerable change
in molar conductance value of the solution. In more polar sol-
vents such as MeOH and DMF, the axial halide ligands are also
replaced by solvent molecules as shown in route 2 of Scheme 2,
as indicated by the increase of the molar conductance of the so-
lutions to the standard value of a 1:1 electrolyte. In aqueous so-
lution both halide ligands dissociate and are replaced with wa-
ter molecules [15]. Since this reaction leads to the formation of
the same cationic complex in both aqueous solutions of 1 and 2,
their color and electronic absorption spectra become the same (Fig.
S4 in supplementary content). Meanwhile, the molar conductiv-
ity of the aqueous solution of 1 and 2 increase to the standard
range for 1:2 electrolytes, which is in harmony with the proposed
mechanism.
3.4.2. Halochromism
The aqueous solutions of the complexes exhibit reversible color
changes in response to pH changes. As discussed above, in aqueous
solution the halogen ions dissociate and are substituted by water
molecules, resulting in the formation of the same cationic com-
plex [Cu(L)(H2O)x]2+. Although halochromism of both compounds
was carried out, the study of halochromism compound 1 is re-
ported due to their similar identities and their halochromism be-
havior (see Figs. S5, S6). Upon decreasing pH of the solution from
5.4 (original pH) to 1.8 by the addition of an acid (HClO4 100 mM),
the blue color of the solution fades. This color change is reflected
in the electronic absorption spectra as a reduction in the inten-
sity of the absorption band along with a redshift in the absorp-
tion maxima (Fig 5). This phenomenon is probably due to proto-
8

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
Fig. 7. Spectroscopic changes of DMSO solution of complexes 1 and 2 at different temperatures.
nation of the coordinated donor atoms, resulting in dissociation of
the tridentate ligand and creation of the hydrated Cu(II) complex
as shown in Scheme 3. Spectrophotometric titration of the com-
plex with HClO4 shows that this process is accompanied with con-
sumption of three equivalent of protons (inset Fig 5), which agrees
with the proposed mechanism.
3.4.3. Thermochromism
The thermochromic behavior of the complexes was studied in
DMSO solution. The complexes exhibit continuous color change
from blueish green to pale yellow upon heating the solution from
room temperature to the boiling point of the solvent. The spec-
troscopic changes of complexes 1 and 2 in DMSO solution in the
temperature range of 25–180 ◦C are shown in Fig 7. The ob-
served thermochromism may be attributed to the gradual substi-
tution of halide anions and chelating ligands by solvent molecules
and the formation of solvated copper(II) ions. The molar conduc-
tance of the heated solutions increases to higher values due to
the dissociation of anionic ligands. Due to the centrosymmetric
structure of the resulting [Cu(DMSO)6]2+in the solution [30], the
d-d transitions are Laporte forbidden and consequently the in-
tensity of the d-d adsorption bands decrease and the color of
the solutions fade upon heating. This phenomenon is totally re-
versible and the original color of solutions reappears by cooling
the solutions to room temperature. However, due to the pres-
ence of a significant amount of solvent molecules around the cop-
Alkalization of the solution using a base (NaOH 100 mM) causes
a change in the color of the solution from blue to purple. The
changes in the spectra upon addition of the base in the pH range
of 5.4–11.4 (Fig 6a) show a blue shift in the absorption maxima
from 682 to 613 nm along with the development of two isosbestic
points at 688 and 476 nm (Figs 6b and 6c). As shown in Figs 6b
and 6c, spectroscopic titration of the solution with the base indi-
cates consumption of two equivalents of hydroxide ions (the first
in pH range of 5.4–8.4 and the second in the pH range of 8.4 to
11.4). These observations indicate that the observed halochromism
caused by the addition of base arises from the deprotonation of the
OH and NH groups of the chelating ligand with two subsequent
equilibria that are shown in Scheme 3.
9

A. Shirvan, H. Golchoubian, M.A. Siegler et al.
Journal of Molecular Structure 1243 (2021) 130930
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Declaration of Competing Interest
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The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
sor derived from 2-methoxybenzylamine and 2,3-dihydroxybenzaldehyde
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synthesis, spectral characterization and electrochemistry of its bis-bidentates
Schiff bases metal complexes, Spectrochim. Acta, Part A 184 (2017) 299–307,
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