Chromotropism in halo-bridged dimers. Structural characterization of bis(μ-halo)bis(halo N-(pyridin-2-ylmethyl)cyclohexanamine copper(II))

Article abstract of DOI:10.1080/00958972.2016.1227972

Two dinuclear copper(II) complexes, [LCu(μ-Cl)Cl]₂ (DMF) (1) and [LCu(μ-Br)Br]₂ (2), with the bidentate ligand N-(pyridin-2-ylmethyl)cyclohexanamine, L, were synthesized and characterized by physicochemical and spectroscopic (IR, UV-vis) data. The crystal structural analysis of 1 shows that both copper(II) ions are in a distorted square pyramidal N₂Cl₃ environment with the apical position of the copper(II) being occupied by the bridging chloride anion which is equatorial to the other copper ion, forming a dimeric copper(II) complex. The chromotropic properties of both complexes, including solvato-, thermo-, and halochromism, were investigated. The complexes show reversible thermochromism in solution which is irreversible in the solid state. It was found that the solvatochromism is due to structural change followed by solvation of the vacant sites of the complexes. Their halochromic properties were studied in pH range of 1–11 by visible absorption spectroscopy. The color changes from blue to green and to colorless are due to deprotonation and protonation of the ligands.

Full text of DOI:10.1080/00958972.2016.1227972

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Chromotropism in halo-bridged dimers. Structural
characterization of bis(µ-halo)bis(halo N-(pyridin-2-
ylmethyl)cyclohexanamine copper(II))
Hamid Golchoubian & Samira Nateghi
To cite this article: Hamid Golchoubian & Samira Nateghi (2016): Chromotropism in
halo-bridged dimers. Structural characterization of bis(µ-halo)bis(halo N-(pyridin-2-
ylmethyl)cyclohexanamine copper(II)), Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1227972
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1227972
View supplementary material
Accepted author version posted online: 26
Aug 2016.
Published online: 05 Sep 2016.
Submit your article to this journal
Article views: 9
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=gcoo20
Download by: [University of Alabama]
Date: 04 October 2016, At: 10:31

Journal of Coordination Chemistry, 2016
http://dx.doi.org/10.1080/00958972.2016.1227972
Chromotropism in halo-bridged dimers. Structural
characterization of bis(μ-halo)bis(halo N-(pyridin-2-ylmethyl)
cyclohexanamine copper(II))
Hamid Golchoubian and Samira Nateghi
department of Chemistry, university of mazandaran, Babol-sar, iran
ABSTRACT
ARTICLE HISTORY
received 8 april 2016
accepted 27 June 2016
Two dinuclear copper(II) complexes, [LCu(μ-Cl)Cl]2 (DMF) (1) and [LCu(μ-Br)
Br]2 (2), with the bidentate ligand N-(pyridin-2-ylmethyl)cyclohexanamine, L,
were synthesized and characterized by physicochemical and spectroscopic
(IR, UV–vis) data. The crystal structural analysis of 1 shows that both copper(II)
ions are in a distorted square pyramidal N2Cl3 environment with the apical
position of the copper(II) being occupied by the bridging chloride anion which
is equatorial to the other copper ion, forming a dimeric copper(II) complex.
The chromotropic properties of both complexes, including solvato-, thermo-,
and halochromism, were investigated. The complexes show reversible
thermochromism in solution which is irreversible in the solid state. It was
found that the solvatochromism is due to structural change followed by
solvation of the vacant sites of the complexes. Their halochromic properties
were studied in pH range of 1–11 by visible absorption spectroscopy. The
color changes from blue to green and to colorless are due to deprotonation
and protonation of the ligands.
KEYWORDS
dinuclear copper(ii);
bis(μ-chloro)-bridge;
chromotropism; pyridine
derivative; crystal structure
CONTACT hamid Golchoubian
h.golchobian@umz.ac.ir
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1227972.
© 2016 informa uK limited, trading as taylor & francis Group

2
H. GoLCHoUBIaN aND S. NaTeGHI
1. Introduction
Chromotropism is the reversible color transformation of materials due to surrounding physical and
chemical conditions such as temperature (thermochromism), solvent (solvatochromism), ion (ionochr-
omism), pH (halochromism), pressure (piezochromism), light (photochromism), and electro-redox reac-
tion (electrochromism) [1]. The development of an empirical relationship that correlates the structural
characteristics of transition metal complexes to their chromotropism properties has been vigorously
pursued [1–6]. Studies of this kind have provided significant results in a variety of complexes [7–11].
among the transition metal complexes, copper(II) ions with chelate ligands have been recognized as
the most encouraging candidates for practical applications of chromotropism due to their high ther-
modynamic stabilities, unsaturated coordination, and also the existence of simple and regular changes
in their electronic spectra in accordance with the power of the stimuli imposed to the system.
Dinuclear copper(II) compounds with small anions (such as halide or hydroxide) as bridging groups
are the subject of numerous articles in contemporary literature [12–42]. The study of these dinuclear
complexes is complicated, partly due to their diverse structural properties. The [Cu(μ-X)2Cu] dimeric
motif, where X is Cl− or Br−, exhibits structures with a variety of Cu–X lengths and Cu–X–Cu angles
depending upon the nature of the co-ligands and also on the counter ions [23, 43]. So far, various
types of bidentate organic co-ligands have been utilized to design the coordination sphere of the
copper(II) center in bis(μ-halo)-bridged complexes [34, 35, 38, 44–46]. However, the number of halo-
bridged complexes involving bidentate pyridine derivative co-ligands is very limited [13, 47–49]. We
have recently been focusing our attention on five-coordinate dimeric species, and here, also, there is a
range of geometries from tetragonal-pyramidal to trigonal-bipyramidal at copper. In view of our interest
in chromotropism of copper(II) compounds, we wished to extend our work to dinuclear copper(II) halide
complexes in combination with a bidentate pyridine derivative, N-(pyridin-2-ylmethyl)cyclohexanamine
(L; scheme 1). We have isolated and characterized neutral dinuclear copper(II) chloro and bromo com-
plexes, [LCu(μ-X)X]2 (X = Cl, Br). Details of the synthesis, crystal structure, and chromotropism behaviors
of these compounds are described below.
2. Experimental
2.1. Materials and methods
CuCl2·2H2o and CuBr2 were purchased from Merck India Ltd and Fluka Chemie Switzerland, respectively,
and 2-pyridinecarboxaldehyde and cyclohexylamine were purchased from Sigma–aldrich, USa. all
chemicals and solvents employed for the syntheses were of analytical grade and used as received. all sol-
vents used for chromotropism studies were spectral-grade and all other reagents were used as received.
elemental analyses were performed on a LeCo CHN-600 elemental analyzer. absolute metal percent-
ages were determined by a Varian-spectra a-30/40 atomic absorption flame spectrometer. all samples
were dried to constant weight under a high vacuum prior to analysis. Conductance measurements were
X
X
N
N
H
N
Cu
NH
Cu
X
X
X= Cl, Br
Scheme 1. dinuclear complexes under study.

JoURNaL oF CooRDINaTIoN CHeMISTRy
3
made at 25 °C with a Jenway 400 conductance meter on concentrations of 10.0 × 10−4, 6.00 × 10−4
,
4.00 × 10−4 and 2.00 × 10−4 M of samples in selected solvents. Then for each solvent, a curve was plotted
by plotting the molar conductance versus concentration of sample. The curve was then extrapolated
to an infinitively dilute solution to obtain the molar conductance value. Infrared spectra were recorded
using potassium bromide disks and a Bruker FT-IR instrument. NMR spectra were measured with a Bruker
400 DRX Fourier Transform Spectrometer at room temperature. The electronic absorption spectra were
measured with a Braic2100 UV–vis spectrophotometer. The following solvents were used in the solva-
tochromism study: nitromethane (NM), nitrobenzene (NB), acetone (aC), acetonitrile (aN), propionitrile
(PN), methanol (MeoH), ethanol (etoH), water (H2o), dimethylformamide (DMF), dimethylsulfoxide
(DMSo), and hexamethylphosphorictriamide (HMPa).
2.2. Synthesis
2.2.1. Preparation of N-(pyridin-2-ylmethyl)cyclohexanamine (L)
a mixture of pyridine-2-carbaldehyde (2.9 mL, 30 mmol), cyclohexylamine (3.4 mL, 30 mmol), and a
small amount of para-toluenesulfonic acid in methanol (35 mL) was prepared and stirred for one day
at room temperature. NaBH4 (1.7 g, 45 mmol) was then added gradually to the resulting solution over
30 min. The resulting mixture was allowed to stand overnight. after heating the solution near the boiling
point, HCl (10 mL, 1.7 M) was added to it after placing the solution in an ice bath and was then heated
again to the boiling temperature. The mixture was then made alkaline by addition of NaoH (4 M). The
sodium borate precipitate was removed by filtration. evaporation of the solvent from the filtrate resulted
in a yellow oil which was subsequently extracted with dichloromethane (3 × 15 mL). The combined
CH2Cl2 fractions were dried over anhydrous Na2So4. evaporation of the solvent under reduced pressure
resulted in the desired product as a brown oil. The yield was 3.71 g (64%). Selected IR data (v/cm−1
using KBr disk): 3307 (m, N–H str.), 2927, 2853 (s, C–H str. aliphatic), 1593 (s, C=N str.), 1438 (s, C=C str.
aromatic), 1227 (s, N–C str. aliphatic). 1H NMR (400 MHz, CDCl3), δ: 1.17 (m, 5H, (cyclohexyl–); 1.57 (m, 2H,
(cyclohexyl–)); 1.70 (m, 2H, (cyclohexyl–)); 1.90 (m, 1H, (cyclohexyl–)); 2.45 (m, 1H, (cyclohexyl–)); 2.57
(br. H, –NH–); 3.89 (s, 2H, –CH2–Py); 7.11 (d,d, J = 5.6, 1.6 Hz, H, Py); 7.27 (s, H, Py); 7.58 (t,d, J = 7.6, 2.0 Hz,
H, Py); 8.50 (d, J = 4.4 Hz, H, Py). When one drop of D2o was added to the 1H NMR sample, the broad
signal at 2.57 ppm disappeared. 13C NMR (100 MHz in CDCl3), δ: 24.97, 26.19,33.42, 56.55 (cyclohexyl–);
52.27 (–NH–CH2–Py); 121.83; 122.36.136.42, 149.17, 159.97 (Py–C).
2.2.2. Preparation of bis((μ-chloro)chloro, N-(pyridin-2-ylmethyl)cyclohexanamine copper(II))
dimethylformamide, [LCu(μ-Cl)Cl]2 DMF (1)
To a solution of N-(pyridin-2-ylmethyl)cyclohexanamine, L (0.38 g, 2 mmol) in ethanol (12 mL) was
slowly added a solution of CuCl2·2H2o (0.34 g, 2 mmol) in ethanol (8 mL). The resulting green mixture
was stirred for 2 h at room temperature and was then allowed to concentrate at room temperature. The
desired compound precipitated from the solution as a greenish blue solid. The compound was recrys-
tallized by diffusion of diethyl ether into a DMF solution. yield 0.57 g, 78% as blue-green crystals. The
crystals were suitable for X-ray crystallography. anal. Calcd for C27H43N5Cu2Cl4o (MW = 722.57 g mol−1):
C, 44.88; H, 6.00; N, 9.69; Cu, 17.59%; Found: C, 44.91; H, 6.09; N, 9.77; Cu, 17.41%. Selected IR data
(v/cm−1 using KBr disk): 3427 (m), 3196 (m, N–H str.), 3072 (w), 2931, 2852 (s C–H str.), 1608 (s, C=N str.),
1448 (m, N–H bend.), 1287 (w), 771(m), 651 (w).
2.2.3. Preparation of bis((μ-bromo)bromo, N-(pyridin-2-ylmethyl)cyclohexanamine copper(II))
[LCu(μ-Br)Br]2 (2)
This complex was prepared by a similar method to that used for [LCu(μ-Cl)Cl]2, except that CuBr2 was
used (0.45 g) in place of CuCl2·2H2o. The compound was obtained as a dark green solid with typical
yield of 64% (0.53 g based on 2 mmol CuBr2 used). anal. Calcd for C24H36N4Cu2Br4 (MW = 827.28 g mol−1):
C, 34.84; H, 4.39; N, 6.77; Cu, 15.36%; Found: C, 34.89; H, 4.20; N, 6.74; Cu, 15.52 %. Selected IR data

4
H. GoLCHoUBIaN aND S. NaTeGHI
(v/cm−1 using KBr disk): 3203 (m, N–H str.), 3062 (w), 2928, 2853 (s C–H str.), 1592 (s, C=N str.), 1438
(m, N–H bend.), 1301 (w), 754(m), 663 (w).
2.3. X-ray crystallographic study
X-ray data for 1 were collected on a SToe IPDS-II diffractometer with graphite monochromated Mo-Kα
radiation. a green crystal of the complex was chosen using a polarizing microscope and was mounted
on a glass fiber which was used for data collection. Cell constants and an orientation matrix for data
collection were obtained by least squares refinement of diffraction data from 7160 unique reflections.
Data were collected at 298(2) °C in a series of ω scans in 1° oscillations and integrated using the Stöe
X-aRea [50] software package. a numerical absorption correction was applied using XReD [51] and
X-SHaPe [52] software. The data were corrected for Lorentz and polarizing effects. The structure was
solved by direct methods using SIR2004 [53]. The non-hydrogen atoms were refined anisotropically
by full matrix least squares on F2 using SHeLXL [54]. all hydrogens were added at ideal positions and
constrained to ride on their parent atoms. Crystallographic data are listed in table 1.
3. Results and discussion
3.1. Preparation
The ligand was prepared by condensation of an equimolar mixture of cyclohexylamine and pyri-
dine-2-carbaldehyde and further reduction of the resultant diimine by sodium borohydride. The dinu-
clear copper(II) complex was synthesized by mixing an equimolar quantity of CuCl2·2H2o or CuBr2 and
the ligand in ethanol in moderate yields (scheme 2).
Table 1. Crystal data and structure refinement for 1.
empirical formula
formula weight
shape, color
temperature
Wavelength
Crystal system
space group
unit cell dimensions
a
C27h43Cl4Cu2n5o
722.54
Prism, green
298(2) K
0.71073 Å
monoclinic
P21/c
12.491(3) Å
13.338(3) Å
b
c
19.756(4) Å
β
92.85(3)°
Volume
Z
3287.3(13) Å3
4
Calculated density
absorption coefficient
F (0 0 0)
Crystal size
θ range for data collection
index ranges
1.460 g cm−3
1.648 mm−1
1496
0.35 × 0.25 × 0.15 mm3
2.57°–27.00°
−15 ≤ h ≤ 15
−17 ≤ k ≤ 15
−25 ≤ l ≤ 25
μ (mm−1)
1.648
reflections collected/unique
Completeness to 2θ = 27.00
refinement method
data/restraints/parameters
final R indicesa [I > 2σ(I)]b
Goodness-of-fit on F2c
R indices (all data)
22,454/7160 [R(int) = 0.0348]
99.9%
full-matrix least-squares on F2
7160/0/362
R1 = 0.0332, wR2 = 0.0850
0.978
R1 = 0.0466, wR2 = 0.0887
largest diff. peak and hole
0.212 and −0.249 e Å−3
aR = ∑||Fo| − |Fc||/∑|Fo|.
2 2
2 2 ½
bwR = [(Σ[Fo2 − Fc ) ]/Σ[w(Fo ) ] .
2 2
½
cS = ∑[w(Fo2 − Fc ) /(Nobs − Nparam)] .

JoURNaL oF CooRDINaTIoN CHeMISTRy
5
NH2
N
N
O
NaBH4
EtOH
(L)
+
NH
L
[LCu(µ-X)X]2
+
CuX2 .YH2O
X= Cl-, Y=2
X= Br-, Y=0
Scheme 2. synthetic outline for preparation of ligands and complexes.
3.2. Characterization
The free ligand has characteristic IR bands with minor shifts in the dinuclear complexes. a medium
intensity and narrow band at 3307 cm−1 is associated with the N–H stretch of the amine group. Upon
formation of the chelate rings in the complexes, this band moves to lower energy (3196 and 3203 cm−1
in 1 and 2, respectively). as the lone pair of electrons of the donor nitrogen becomes involved in the
metal-ligand bond, the transfer of electron density to the metal and the subsequent polarization of the
ligand involves electron depopulation of the N–H bond, which culminates in a shift to lower energy
[55]. at lower frequency, the complexes also exhibited a band near 470 cm−1 which is attributed to the
ν(Cu–N) vibration mode [56].
3.3. X-ray structure
X-ray single crystal diffraction analysis reveals that 1 crystallizes in the monoclinic space group P21/c.
an oRTeP view of the compound is shown in figure 1 together with the numbering scheme. Selected
bond lengths and angles are given in table 2.
The asymmetric unit of the complex is made up of a [LCu(μ-Cl)Cl]2 dimeric unit with a lattice DMF.
In the complex, N-(pyridin-2-ylmethyl)cyclohexanamine (L) is a neutral bidentate ligand which coor-
dinates to copper(II) through its amine and pyridyl nitrogens. In addition, a terminal and a bridging
Figure 1. orteP view of [lCu(μ-Cl)Cl]2 dmf (1). thermal ellipsoids are drawn at the 50% probability level.

6
H. GoLCHoUBIaN aND S. NaTeGHI
Table 2. selected bond lengths (Å) and angles (°) for 1.
Cu(1)–n(1)
Cu(1)–Cl(1)
Cu(2)–n(3)
Cu(2)–Cl(2)
Cu(2)–Cl(3)
Cu(1)–Cu(1)
2.009(2)
2.2536(8)
2.012(2)
2.2682(8)
2.7413(8)
3.3625(9)
Cu(1)–n(2)
Cu(1)–Cl(3)
Cu(2)–n(4)
Cu(2)–Cl(4)
Cu(1)–Cl(4)
Cl(3)–Cl(4)
2.0471(18)
2.2905(7)
2.0416(18)
2.2860(8)
2.8048(8)
3.8132(10)
Bond angles (°)
n(1)–Cu(1)–n(2)
n(1)–Cu(1)–Cl(4)
n(2)–Cu(1)–Cl(4)
n(2)–Cu(1)–Cl(3)
Cl(1)–Cu(1)–Cl(4)
n(3)–Cu(2)–n(4)
n(3)–Cu(2)–Cl(4)
n(4)–Cu(2)–Cl(4)
n(3)–Cu(2)–Cl(2)
Cl(2)–Cu(2)–Cl(4)
Cu(1)–Cl(3)–Cu(2)
82.34(8)
93.91(6)
85.09(6)
89.69(6)
95.30(6)
82.34(8)
166.57(6)
89.91(6)
94.96(6)
92.64(3)
83.35(3)
n(1)–Cu(1)–Cl(1)
n(1)–Cu(1)–Cl(3)
n(2)–Cu(1)–Cl(1)
Cl(1)–Cu(1)–Cl(3)
Cl(3)–Cu(1)–Cl(4)
n(3)–Cu(2)–Cl(2)
n(3)–Cu(2)–Cl(3)
n(4)–Cu(2)–Cl(3)
Cl(2)–Cu(2)–Cl(3)
Cl(3)–Cu(2)–Cl(4)
Cu(1)–Cl(4)–Cu(2)
95.42(6)
166.41(5)
177.75(5)
92.47(3)
96.37(5)
94.96(6)
92.15(6)
92.15(6)
94.69(3)
85.97(7)
81.98(3)
Torsion angles (°)
n(1)–C(8)–C(7)–n(2)
−24.4(3)
n(3)–C(20)–C(19)–n(4)
20.3(3)
chloride coordinate to copper(II). The complex consists of double chloride-bridged dimers, in which
the bridging Cu2Cl2 unit is strictly planar due to the presence of the crystallographic inversion center
in the middle of the dimer. The copper(II) is five-coordinate, being bonded to the bidentate ligand,
and two bridging and one terminal chlorides. The bridging is unsymmetrical in nature with different
Cu–Cl bond distances [Cu(1)–Cl(3) 2.2905(7) Å, Cu(1)–Cl(4) 2.8048(8) Å; Cu(2)–Cl(3) 2.7413(8) Å and
Cu(2)–Cl(4) 2.2860(8) Å, table 2]. The fifth coordination site around each copper(II) is occupied by a
terminal Cl(1) and Cl(2) ion with Cu(1)–Cl(1) and Cu(2)–Cl(2) distances of 2.2536(8) and 2.2682(8) Å,
respectively. according to addison et al. [57], the distortion of the square pyramidal geometry towards
trigonal bipyramidal can be described by the geometrical parameter τ = |β − α|/60, where β and α are
the bond angles involving the trans donors in the basal plane. The τ values for the coordination around
the copper ions are 0.189 for Cu(1) Å and 0.178 for Cu(2) Å, confirming their distorted square pyram-
idal geometry. The bridging chlorides are in apical positions in each unit. The basal atoms are nearly
coplanar; the deviations from the least squares plane through the CuN2o2 atoms are N(1) −0.106, N(2)
0.109, Cl(1) 0.086, Cl(3) −0.089, Cu(1) 0.107 Å and N(3) −0.103, N(4) 0.106, Cl(2) 0.083, Cl(4) −0.085, Cu(2)
0.110 Å. The two planes are almost parallel with an angle of 178.29°. The mean Cu–N(amine) distance of
2.03 Å and the bite angles N(1)–Cu(1)–N(2) and N(3)–Cu(2)–N(4) of 82.34(8)° are smaller than 90°, due
to the strain caused by the three-bond ligand bite and are close to the corresponding average values
of the copper(II) complexes with ethylenediamine [7]. The cyclohexyl groups are directed away from
the N2CuCl2CuN2 plane and to opposite sides of the terminal chlorides to minimize the steric hindrance
around copper(II). Both five-membered chelate rings in the complex are puckered so that the torsion
angles of N(1)–C(8)–C(7)–N(2) and N(3)–C(20)–C(19)–N(4) are 24.34(10)° and 20.35(10)°, respectively.
Several dinuclear [N2Cu(μ-Cl)]2 complexes are known in which two chloride ions are doubly bridging
between the copper ions. Many of these compounds also have a distorted square pyramidal geometry
about the copper ions, in which the apical position of one copper is occupied by chloride, which is
equatorial to the other copper [29, 30, 37]. The reported Cu–N distances vary from 1.98 to 2.12 Å, the
in-plane Cu–Cl distances differ from 2.19 to 2.31 Å, the axial Cu–Cl distances vary from 2.63 to 2.95 Å
and the Cu⋯Cu distances differ from 3.42 to 3.74 Å. The Cu⋯Cu distance in 1 is 3.463(10) Å which lies
between these values, but the Cu–Cl–Cu angles (81.98(3)° and 85.97(7)°) in [LCu(μ-Cl)Cl]2 are rather
small compared with the Cu–Cl–Cu angles in the related compounds, which vary from 87.3° to 94.88°.
The geometry of the complex consists of two square pyramids sharing one base-to-apex edge
(SP-I in scheme 3) [42]. Similar dinuclear copper(II) complexes having a Cu(μ-Cl)2Cu core with square

JoURNaL oF CooRDINaTIoN CHeMISTRy
7
Cl
Cl
Cu
Cl
Cu
Cu
Cu
Cl
Cu
Cu
Cl
Cl
(SP-I)
(SP-III)
(SP-II)
L
L
L
L
L
Cu
L
Cl
L
Cl
Cl
L
Cu
Cu
Cu
L
Cu
L
L
L
L
Cl
L
Cl
L
Cl
Cu
L
L
L
Scheme 3. different conformations of dichloride-bridged, square-pyramidal dinuclear copper(ii) complexes.
Figure 2. arrangement of the two square pyramids of the two copper(ii) centers showing the sP-i configuration in 1.
pyramidal copper(II) centers and with nearly parallel basal planes are reported [13–17, 29–37, 39, 40,
58]. The cyclohexyl moieties are in chair conformations.
The crystal packing (figure 2) of the dimeric [Cu(L)Cl]2 complex shows that the structure is stabilized
by an intermolecular hydrogen bonding through DMF molecules in the lattice between the two dimeric
[LCu(μ-Cl)Cl]2 complexes.
3.4. Electronic spectra
The visible electronic absorption spectra for 1 and 2 in dichloromethane solution show maxima at 672
and 686 nm, respectively, that are in agreement with those of five-coordinate copper(II) complexes with
analogous geometries. Compound 2 shows a higher λmax than 1, signifying more distortion toward a
trigonal bipyramid [59]. This spectral feature is characteristic of five-coordinate copper(II) complexes
with distorted square pyramidal geometry that normally display a band in the 550–660 nm range (dxz,
dyz → dx2-y2) [60, 61]. Moreover, 2 shows one strong ligand-to-metal charge transfer (LMCT) band at
475 nm [62]. The electronic spectra of both complexes show typical absorptions at 200–300 nm allo-
cated to ligand π → π* transitions.

8
H. GoLCHoUBIaN aND S. NaTeGHI
Figure 3. tGa and dta curves for 1.
3.5. Thermal analysis
Thermal stability and thermal behavior of the complexes were investigated by thermogravimetric anal-
ysis (TGa-DTa) in aerated atmosphere from 25 to 600 °C at a heating rate of 10 °C min−1. Correlations
between decomposition steps of the complexes with the corresponding weight losses are almost the
same for 1 and 2. The DTa curve of 1 (figure 3) indicates an endothermic process without any weight
loss from 180 to 210 °C due to breakage of weak Cu–Cl in [LCu(μ-Cl)Cl]2 and formation of 2[LCuCl2]. The
next endothermic process occurs at 230–470 °C associated with fragmentation and thermal degradation
of the organic moiety and formation of CuCl2 with mass loss of 38% (Calcd 38.6%).
3.6. Thermochromism
The compounds show irreversible thermochromism in the solid state which is reversible in solution. The
original blue-green color of 1 turns to dark green upon heating to 130 °C in the solid state. The visible
spectrum of 1 was studied over the temperature range of 25–170 °C in DMSo (figure 4). The complex
Figure 4. temperature dependence of the visible absorbance of a dmso solution of 1.

JoURNaL oF CooRDINaTIoN CHeMISTRy
9
demonstrates reversible thermochromism so that the original green color of the solution gradually turns
to yellow-green up to 90 °C and to brown by increasing the temperature to 170 °C. The color change was
attributed to the substitution of DMSo molecules with the coordinated diamine and chloride ligands.
The observed brown color is due to the presence of free diamine ligand and [Cu(dmso)n]Cl2. This was
confirmed by the comparison of the visible spectrum of the diamine ligand that was added to a heated
solution of CuCl2 in DMSo. This phenomenon is a reversible process so that after cooling the solution
to room temperature the free diamine ligand and chloride ions that exist in the solution re-coordinate
to the copper ions with reappearance of the original blue-green color. The reverse process is slower
than the forward reaction and takes almost a day for full development of the original color due to tight
bond formation between DMSo and Cu(II) and presence of a significant amount of solvent molecules
around the copper centers. The same phenomenon was also observed in DMF solution. The reversi-
ble thermochromism was observed in 2 in the solvents DMSo and DMF with the same color change.
However, in this case the reverse reaction takes only a few hours for the original color to reappear.
3.7. Solvatochromism
The complexes are soluble in a wide range of organic solvents and solvatochromic. The visible spec-
tral variations of the complexes in various solvents are demonstrated in figure 5. The location of the
absorption maxima of the complexes accompanied by their molar absorptivity values are compiled in
table 3. The structures of the complexes are susceptible to changes in different solvents due to break-
ing of the weak Cu–Cl bond. In low polarity solvents, the dimeric structure of the compounds could
dissociate into two mononuclear neutral complexes along with coordination of the solvent molecules
to the vacant sites of the copper center (Route 1 in scheme 4), although, the solvation of the dimeric
complexes cannot be ruled out. However, in solvents with medium polarity such as NB, MeoH, DMF,
DMSo, and etoH, not only do the dimeric complexes dissociate into two mononuclear complexes but
also the weakly coordinated halide anion in the axial coordination site is displaced by solvent molecule
due to Jahn-Teller effect of copper(II) ion with d9 configuration (Route 2 in scheme 3). In aqueous solu-
tion, all coordinated halide ions are dissociated and replaced by water molecules (Route 3 in scheme 3).
Therefore, both complexes demonstrated identical spectra in aqueous solution as shown in figure S2 in
the Supplementary Material. The proposed mechanism is confirmed by changes in molar conductance
values of the compounds in different solvents as shown in table 4. as a result, the color variations might
be attributed to structural change followed by solvation of the resulting mononuclear complexes.
Table 3. the electronic absorption maxima of 1 and 2 in different solvents.
Compound 1
Compound 2
λmax (nm)
εa
εa
Solvent
λmax (nm)
1,2-dCe
dCm
nm
nB
an
Pn
ac
h2o
thf
etoh
meoh
dmf
dmso
Py
672
712
710
719
726
693
670
672
752
730
725
788
742
776
758
445
164
209
283
316
335
370
120
294
285
219
253
203
384
214
686
728
690
693
702
717
665
695
747
736
731
779
754
871
774
490
472
286
392
330
458
680
130
277
295
311
295
372
479
402
hmPa
aε (l cm−1 mol−1).

10
H. GoLCHoUBIaN aND S. NaTeGHI
[LCu( -Cl)Cl2], 1
Wavelength (nm)
[LCu( -Br)Br]2, 2
Wavelength (nm)
Figure 5. absorption spectra of 1 and 2 in selected solvents. absorption spectra in other solvents are omitted for clarity.
X
X
+
P
N
N
Solv
X
LP Route 1
SolvLP
NH
N
Route 2
Solv
Solv
N
Cu
Cu
Cu
X
Solv
P
X
LP
2
Cu
NH
NH
2
+ 2X-
X
NH
X
P
Solv
Solv
H2O
Route 3
N
2+
H2O
H2O
H2O
Cu
X= Cl, Br
-
2
+
4X2
LP
NH
Solv = low polar solvent
P
Solv = polar solvent
H2O
Scheme 4. structural change of the complexes caused by solvents with different polarities.

JoURNaL oF CooRDINaTIoN CHeMISTRy
Table 4. molar conductivity data (Λm) of 1 and 2 (Ω−1 cm2 mol−1) at 25 °C in different solvents.
11
Complexes
NM
NB
aC
aCN
eToH
MeoH
DMF
H2o
Complex 1
Complex 2
6
8
5
11
168
182
230
244
69
71
184
214
171
153
420
428
1 : 1 electrolytes
1 : 2 electrolytes
1 : 3 electrolytes
75–95
150–180
220–260
20–30
50–60
70–80
100–140
160–200
120–160
220–300
340–420
35–45
70–90
–
80–115
160–220
65–90
130–170
200–240
118–131
235–273
408–435
a
a
a
–
–
athe standard value is not reported.
3.8. Halochromism
In aqueous solution, the structure of the original dinuclear complexes collapses into two mononuclear
complexes followed by hydration of the coordinated halide ions as shown in scheme 5. This structural
change is accompanied with the change in the color of the complex from bluish green to blue in 1 and
dark green to blue in 2. This hydration causes an increase in the conductivity of the solution up to a
2:1 electrolyte. This phenomenon occurs in both complexes as evidenced by their identical electronic
spectra in aqueous solution. The electronic absorption spectra of the complexes in aqueous solution
are sensitive to pH, so that the original green color of the aqueous solution turns to blue upon addition
of a base (NaoH, 0.1 M). This occurrence is totally reversible. To explore the color changes, the visible
spectra of the complexes were studied from pH 5.7 (original pH) to 11. These results are shown in figure
6. There is an isosbestic point at 656 nm, which is visible in different pH scales. as the pH of the solution
increased with addition of the base (NaoH, 0.1 M), the absorption bands shifted to lower wavelength
due to deprotonation of the coordinated water molecules followed by deprotonation of amine group
as shown in scheme 5. Spectrophotometric titration of the complex with NaoH showed release of six
protons around pH 11 based on original [LCu(μ-X)X]2 complexes (figure 6). Interestingly, when the pH
of the solution was decreased to 2.0 by addition of perchloric acid (0.1 M) the original blue color of the
solution faded. The color lightening was accompanied by a red shift and decrease in intensity of the
d–d band. The original blue color of the copper(II) complexes redeveloped upon increasing the pH of
the solution to around 5 with addition of base.
Spectrophotometric titration of the original [LCu(μ-X)X]2 complexes with perchloric acid at
λmax = 706 nm (figure 7) demonstrated that the color lightening happened with consumption of four
equivalents of proton. It is possibly because of protonation of the amine and pyridyl groups and for-
mation of protonated ligand and formation of a fully hydrated copper(II) complex. In a complementary
experiment, an aqueous solution of copper(II) perchlorate was acidified and the ligand L was then added
X
N
X
NH
X= Cl, Br
Cu
Cu
X
X
NH
N
H2O
-
-X2
2+
NH+
N
OH
OH
H2O
H2O
N
6OH-
6H+
4H+
Cu
Cu
2CuX (aq) +
2
2
2
2
4OH-
+
NH2
N
NH
Scheme 5. interconversion of complexes triggered by acid and base (ph 3–11.0) in aqueous solution.

12
H. GoLCHoUBIaN aND S. NaTeGHI
Figure 6. ph titration of [lCu(μ-Cl)Cl]2, 1, and (4 mm in h2o) with naoh (0.10 m). the inset graph shows the decrease of 700 nm
absorbance on titration of 1, naoh (0.10 m).
Figure 7. the ph-dependent visible spectrum of 1 in aqueous solution at 25 °C. the inset graph shows the decrease of 706 nm
absorbance on titration of 1 (4 mm in h2o) with hClo4.
to the resultant solution. The blue color of the solution abruptly decolorized and the same spectrum
as acidified [LCu(μ-X)X]2 was generated.
4. Conclusion
Dinuclear dichloro- and dibromo-bridged Cu(II) complexes with an N2 chelating ligand were synthesized
and characterized. The structure around copper(II) with N2Cl3 cores is distorted square pyramidal. The
weakly coordinated halide ions can be replaced by donor solvents. The complexes are halochromic

JoURNaL oF CooRDINaTIoN CHeMISTRy
13
with reversible color change over the pH range 1.0–11.0, so that they can act as pH-induced off–on–off
absorption switches through protonation and deprotonation of the coordinated ligand in aqueous
solution. The compounds also demonstrate reversible thermochromism with distinct color changes from
25 to 170 °C in DMSo and DMF. The mechanism for this reversible color change is likely from removal
and addition of the coordinated halides by solvent molecules. The complexes are soluble in a large
number of organic solvents and solvatochromic. Their solvatochromism behaviors are due to structural
change followed by solvation of the complexes. This work will be of value in terms of complexes which
find their application as potential candidates for sensor materials and molecular switches.
Supplementary material
CCDC 1417858 contains the supplementary crystallographic data for this article. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1eZ, UK; Fax: (+44) 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk. Supporting Information.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the University of Mazandaran of the Islamic Republic of Iran.
References
[1] y. Fukuda. Inorganic Chromotropism, Springer, Berlin (2007).
[2] W. Linert, y. Fukuda, a. Camard. Coord. Chem. Rev., 218, 113 (2001).
[3] P. Bamfield. Chromic Phenomena: Technological Applications of Color Chemistry, Springer Verlag, Berlin (2002), p. 8.
[4] M.S. Kryatova, o.V. Makhlynets, a.y. Nazarenko, e.V. Rybak-akimova. Inorg. Chim. Acta, 387, 74 (2012).
[5] I. Lumb, M.S. Hundal, M. Corbella, V. Gómez, G. Hundal. Eur. J. Inorg. Chem., 4799 (2013).
[6] H. Golchoubian, G. Moayyedi, N. Reisi. Spectrochim. Acta, Part A, 138, 913 (2015).
[7] H. Golchoubian, M. Tarahomi, e. Rezaee, G. Bruno. Polyhedron, 85, 635 (2015).
[8] H. Golchoubian, a. Heidarain, e. Rezaee, F. Nicolò. Dyes Pigm., 104, 175 (2014).
[9] J. Piromchom, N. Wannarit, J. Boonmak, C. Pakawatchai, S. youngme. Inorg. Chem. Commun., 40, 59 (2014).
[10] a.a. abou-Hussen, W. Linert. Synth. React. Inorg. Met.-Org. Chem., 39, 570 (2009).
[11] N. Baho, D. Zargarian. Inorg. Chem., 46, 7621 (2007).
[12] D. Venegas-yazigi, D. aravena, e. Spodine, e. Ruiz, S. alvarez. Coord. Chem. Rev., 254, 2086 (2010).
[13] S. Choubey, S. Roy, S. Chattopadhayay, K. Bhar, J. Ribas, M. Monfort, B.K. Ghosh. Polyhedron, 89, 39 (2015).
[14] S. Thakurta, P. Roy, G. Rosair, C.J. Gómez-García, e. Garribba, S. Mitra. Polyhedron, 28, 695 (2009).
[15] K. Skorda, T.C. Stamatatos, a.P. Vafiadis, a.T. Lithoxoidou, a. Terzis, S.P. Perlepes, J. Mrozinski, C.P. Raptopoulou,
J.C. Plakatouras, e.G. Bakalbassis. Inorg. Chim. Acta, 358, 565 (2005).
[16] T. Rojo, M.I. arriortua, J. Ruiz, J. Darriet, G. Villeneuve, D. Beltran-Porter. J. Chem. Soc., Dalton Trans., 285 (1987).
[17] I. Banerjee, P.N. Samanta, K.K. Das, R. ababei, M. Kalisz, a. Girard, C. Mathoniere, e.D. estes, W.e. estes, W.e. Hatfield,
D.J. Hodgson. Inorg. Chem., 14, 106 (1975).
[18] J. Xiang, Q. Li, P. Mei. Acta Crystallogr., Sect. E, 62, m2348 (2006).
[19] S. Zhang, W. Chen, B. Hu, y. Chen, L. Zheng, y. Li, W. Li. J. Coord. Chem., 65, 4147 (2012).
[20] S.-S. Qian, y. Lu, Z.-L. you, H.-L. Zhu. Synth. React. Inorg. Met. Org. Nano-Met. Chem., 43, 107 (2013).
[21] R. Kapoor, a. Kataria, P. Venugopalan, P. Kapoor, M. Corbella, M. Rodríguez, I. Romero, a. Llobet. Inorg. Chem., 43,
6699 (2004).
[22] y.-M. Lee, H.-W. Lee, y.-I. Kim. Polyhedron, 24, 377 (2005).
[23] M. Rodríguez, a. Llobet, M. Corbella, a.e. Martell. Inorg. Chem., 38, 2328 (1999).
[24] a. Colombo, L. Menabue, a. Motori, G.C. Pellacani, W. Porzio, F. Sandrolini, R.D. Willett. Inorg. Chem., 24, 2900 (1985).
[25] S.G.N. Roundhill, D.M. Roundhill, D.R. Bloomquist, C. Landee, R.D. Willett, D.M. Dooley, H.B. Gray. Inorg. Chem., 18,
831 (1979).
[26] B. Scott, U. Geiser, R.D. Willett, B. Patyal, C.P. Landee, R.e. Greeney, T. Manfredini, G.C. Pellacani, a.B. Bonamartini Corradi,
L.P. Battaglia. Inorg. Chem., 27, 2454 (1988).
[27] P. Kapoor, a.P.S. Pannu, M. Sharma, M.S. Hundal, R. Kapoor, M. Corbella, N. aliaga-alcalde. J. Mol. Struct., 981, 40 (2010).

14
H. GoLCHoUBIaN aND S. NaTeGHI
[28] I. Lumb, M.S. Hundal, M. Corbella, V. Gomez, G. Hundal. Eur. J. Inorg. Chem., 4799 (2013).
[29] D.W. Phelps, W.H. Goodman, D.J. Hodgson. Inorg. Chem., 15, 2266 (1976).
[30] D.D. Swank, G.F. Needham, R.D. Willett. Inorg. Chem., 18, 761 (1979).
[31] M. Nethaji, R. Clerac, M. ali. Dalton Trans., 42, 1879 (2013).
[32] S.J. Brown, X. Tao, T.a. Wark, D.W. Stephan, P.K. Mascharak. Inorg. Chem., 27, 1581 (1988).
[33] F. Tuna, L. Patron, y. Journaux, M. andruh, W. Plass, J.C. Trombe. J. Chem. Soc., Dalton Trans., 539 (1999).
[34] X.H. Bu, M. Du, Z.L. Shang, L. Zhang, Q.H. Zhao, R.H. Zhang, M. Shionoya. Eur. J. Inorg. Chem., 1551 (2001).
[35] X.H. Bu, M. Du, L. Zhang, Z.L. Shang, R.H. Zhang, M. Shionoya. J. Chem. Soc., Dalton Trans., 729 (2001).
[36] M. Du, y.M. Guo, X.H. Bu, J. Ribas, M. Monfort. New J. Chem., 26, 645 (2002).
[37] a.M. Schuitema, a.F. Stassen, W.L. Driessen. Inorg. Chim. Acta, 337, 48 (2002).
[38] a. Bernalte-García, a.M. Lozano-Vila, F. Luna-Giles, R. Pedrero-Marín. Polyhedron, 25, 1399 (2006).
[39] H.W. Lee, N. Sengottuvelan, H.-J. Seo, J.S. Choi, S.K. Kang, y.-I. Kim. Bull. Korean Chem. Soc., 29, 1711 (2008).
[40] F. yraola, F. albericio, M. Corbella, M. Royo. Inorg. Chim. Acta, 361, 2455 (2008).
[41] a. Višnjevac, L. Tušek-Bozic, M. Majeric-elenkov, V. Šunjic, B. Kojic-Prodic. Eur. J. Inorg. Chem., 2647 (2001).
[42] e. Gungor, H. Kara. Inorg. Chim. Acta, 384, 137 (2012).
[43] H. Grove, J. Sletten, M. Julve, F. Lloret. J. Chem. Soc., Dalton Trans., 1029 (2001).
[44] J.a. Carrabine, M. Sundaralingam. J. Am. Chem. Soc., 92, 369 (1970).
[45] R.F. Drake, V.H. Crawford, N.W. Laney, W.e. Hatfield. Inorg. Chem., 13, 1246 (1974).
[46] D.J. Hodgson, e. Pedersen. Acta Chem. Scand., Ser. A, 36a, 281 (1982).
[47] W.e. Marsh, K.C. Patel, W.e. Hatfield, D.J. Hodgson. Inorg. Chem., 22, 511 (1983).
[48] a. Nielsen, S. Veltzé, a.D. Bond, C.J. McKenzie. Polyhedron, 26, 1649 (2007).
[49] G.a. van albada, o. Roubeau, P. Gamez, H. Kooijman, a.L. Spek. Inorg. Chim. Acta, 357, 4522 (2004).
[50] X-aRea. Version 1.30, Program for the Acquisition and Analysis of Data, Stoe & Cie GmbH, Darmstadt, Germany (2005).
[51] X-SHaPe. Version 2.05, Program for Crystal Optimization for Numerical Absorption Correction, Stoe & Cie GmbH,
Darmstadt, Germany (2004).
[52] X-ReD. Version 1.28b, Program for Data Reduction and Absorption Correction, Stoe & Cie GmbH, Darmstadt, Germany
(2005).
[53] M.C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G.L. Cascarano, L. De Caro, C. Giacovazzo, G. Polidori, R. Spagna.
J. Appl. Crystallogr., 38, 381 (2005).
[54] G.M. Sheldrick. Acta Cryst., 64, 112 (2008).
[55] C. Tsiamis, M. Themeli. Inorg. Chim. Acta, 206, 105 (1993).
[56] N. Raman, S. esthar, C. Thangaraja. J. Chem. Sci., 116, 209 (2004).
[57] a.W. addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor. J. Chem. Soc., Dalton Trans., 1349 (1984).
[58] H. Liu, F. Gao, D. Niu. Inorg. Chim. Acta, 362, 4179 (2009).
[59] S.J. Brudenell, L. Spiccia, e.R.T. Tiekink. Inorg. Chem., 35, 1974 (1996).
[60] G.a. McLachlan, G.D. Fallon, R.e. Martin, L. Spiccia. Inorg. Chem., 34, 254 (1995).
[61] M. yonemura, K. arimura, K. Inoue, N. Usuki, M. ohba, H. Ōkawa. Inorg. Chem., 41, 582 (2002).
[62] T.M. Donlevy, L.R. Gahan, T.W. Hambley, G.R. Hanson, K.L. McMahon, R. Stranger. Inorg. Chem., 33, 5131 (1994).