Structural and chromotropism properties of copper(II) complexes containing a tridentate ligand

Article abstract of DOI:10.1007/s11696-020-01228-9

Abstract: Two dinuclear complexes of [LCu(Cl)(μ-Cl)CuCl₃][LCu(Cl)(μ-Cl)CuL(H₂O)]Cl, 1 and [LCu(Br)(μ-Br)]₂Br, 2, with a tridentate ligand of 3-((pyridin-2-ylmethyl)amino)propanamide, L, were prepared and characterized by physicochemical (elemental analyses, molar conductance measurements, thermogravimetry) and spectroscopic (IR, UV–vis) data. The crystal structures of compounds have been identified by single-crystal X-ray diffraction analyses and showed that the tridentate ligand L functions as an N₂O-donor via the nitrogen atoms of the secondary amine and pyridyl moieties together with the oxygen atom of the amide group. The structural geometry about the copper(II) ions is a distorted square pyramid. The compounds are chromotropic and their reversible chromotropism was investigated by utilizing spectral analysis. The halochromism was due to structural change and followed by ionization of the coordinated water molecules and deprotonation of the secondary amine moiety. It was discovered that the solvatochromism of the compounds arisen from the structural change followed by the solvation of the vacant sites of the complexes. The compounds demonstrated ionochromism and sensitivity and selectivity towards CN^? and N₃^? anions in the presence of other pseudo-halide anions. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11696-020-01228-9

Chemical Papers
https://doi.org/10.1007/s11696-020-01228-9
ORIGINAL PAPER
Structural and chromotropism properties of copper(II) complexes
containing a tridentate ligand
Motahareh Motalebi · Hamid Golchoubian1 · Ehsan Rezaee2
1
Received: 23 December 2019 / Accepted: 26 May 2020
©
Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
Two dinuclear complexes of [LCu(Cl)(μ-Cl)CuCl ][LCu(Cl)(μ-Cl)CuL(H O)]Cl, 1 and [LCu(Br)(μ-Br)] Br, 2, with a triden-
3
2
2
tate ligand of 3-((pyridin-2-ylmethyl)amino)propanamide, L, were prepared and characterized by physicochemical (elemental
analyses, molar conductance measurements, thermogravimetry) and spectroscopic (IR, UV–vis) data. The crystal structures
of compounds have been identiꢀed by single-crystal X-ray diꢁraction analyses and showed that the tridentate ligand L func-
tions as an N O-donor via the nitrogen atoms of the secondary amine and pyridyl moieties together with the oxygen atom
2
of the amide group. The structural geometry about the copper(II) ions is a distorted square pyramid. The compounds are
chromotropic and their reversible chromotropism was investigated by utilizing spectral analysis. The halochromism was
due to structural change and followed by ionization of the coordinated water molecules and deprotonation of the secondary
amine moiety. It was discovered that the solvatochromism of the compounds arisen from the structural change followed by
the solvation of the vacant sites of the complexes. The compounds demonstrated ionochromism and sensitivity and selectiv-
−
−
ity towards CN and N anions in the presence of other pseudo-halide anions.
3
Graphic abstract
Ionochromism
Solvatochromism
Thermochromism
Halochromism
Keywords Copper(ii) complex · Chromotropism · Tridentate ligand · Dinuclear complex
Introduction
et al. 2010), biological systems (Liu et al. 2014), catalysis
Liu et al. 2018) and magnetism (Pardasani et al. 2017). One
(
Copper(II) complexes comprising multi-dentate ligands have
received substantial interest because of their potential appli-
cations in the ꢀeld of structural chemistry (Venegas-Yazigi
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01228-9) contains
supplementary material, which is available to authorized users.
of the most interesting aspects of the chemistry of copper(II)
with multi-dentate donor ligands is the phenomenon of chro-
motropism (Fukuda 2007). Chromotropism is the revers-
ible color change in a substance that is attributable to the
physical and chemical properties of its environment, such as
solvent, temperature and pH, light, as well as the presence
of ions and electrons. The most intriguing features of these
type of copper complexes demonstrating chromotropism
are due to structural change imposed with the environment.
The development of an experimental correlation between
the structural properties of metal complexes and their chro-
motropic properties are been seriously followed in many
*
Hamid Golchoubian
h.golchobian@umz.ac.ir
1
2
Department of Chemistry, University of Mazandaran,
4
7416-95447 Babol-sar, Iran
Department of Chemistry, Southern University of Science
and Technology, Shenzhen, China
Vol.:(0
1
123456789)
3

Chemical Papers
laboratories (Fukuda 2007; Linert et al. 2001; Bamꢀeld
002; Kryatova et al. 2012; Lumb et al. 2013; Golchoubian
on ꢀve-coordinate metal complexes (Golchoubian et al.
2014, 2019; Shirvan et al. 2019). Herein, the synthesis of
copper(II) complexes containing bridging halo ligand as
illustrated in Fig. 1 are reported and their halochromism is
investigated.
2
et al. 2015). Investigation of this type has yielded signiꢀ-
cant results in many complexes (Piromchom et al. 2014;
Abou-Hussen and Linert 2009; Baho and Zargarian 2007).
Copper(II) ion together with chelate ligands are known as
the most promising candidates for practical applications of
chromotropism owing to their high-level of thermodynamic
stabilities, unsaturated coordination number as well as the
presence of straightforward and steady shifts in their elec-
tronic spectra according to the power of the stimuli intruded
into the system.
Experimental
Materials
The ligand was synthesized according to our published pro-
cedures (Rostami and Golchoubian 2017). All reagents and
solvents were purchased from Merck and Aldrich and used
without further puriꢀcation. Microanalysis was performed
with a Perkin Elmer Model 2400 elemental analyzer. Con-
ductivities were measured in methanol, water and dimeth-
ylformamide solution at 25 °C with a Jenway 400 conduct-
On the other hand, By this time a number of papers have
been reported about dinuclear copper(II) complexes contain-
ing small anions such as hydroxide and halides (Venegas-
Yazigi et al. 2010; Choubey et al. 2015; Thakurta 2009;
Skorda et al. 2005; Xiang et al. 2006; Zhang et al 2012;
Qian et al. 2013; Kapoor et al. 2004; Lumb et al 2013; Lee
et al. 2005; Nethaji et al 2013; Du et al 2002; Schuitema
et al 2002; Bernalte-Garcia et al. 2006; Yraola et al. 2008;
Gungor and Kara et al. 2012). The study of this class of
dinuclear complexes is complicated, partly due to their
diverse structural features. The dinuclear complexes type
–
4
–4
ance meter on concentrations of 10.0 × 10 , 6.00 × 10 ,
–4
–4
4.00×10 and 2.00×10 M of samples. Then for each sol-
vent, a curve was plotted by drawing the molar conductance
versus concentration of sample. The curve was then extrap-
olated to an inꢀnitive dilute solution to obtain the molar
conductance value. IR spectra were recorded with a Bruker
−
−
[
LCu(μ-X) CuL] where X is Cl or Br , and L is a bi- and
2
−1
tridentate ligand are rich of structural diversity with a range
of Cu–X distances and Cu–X–Cu angles depending on the
nature of the chelating ligands (Kapoor et al. 2010; Lumb
et al. 2013, Nethaji et al. 2013; Bu et al. 2001, 2002; Schu-
itema et al. 2002; Bernalte-Garcia et al. 2006; Lee et al.
FT-IR spectrophotometer in the range 4000–500 cm using
KBr pellets. Electronic absorption spectra were recorded
with a Braic 2100 model UV–Vis spectrophotometer with
1 cm quartz cells in the range 200–800 nm. The pKa values
of the complexes were determined spectrophotometrically by
using Patterson method (Patterson 1999). The buꢁer solution
of pH = 8.5 for ionochromism study was prepared by dis-
solving 54.4 g of sodium acetate in 50 ml of water, heating
to 35 °C. After cooling, anhydrous acetic acid (10 mL) was
slowly added and diluted to 100 mL with water.
2
008; Yraola et al. 2008; Višnjevac et al. 2001; Gungor and
Kara et al. 2012; Grove et al. 2001). Until now, a variety of
bi- and tridentate ligands have been eꢁectively developed
to design the coordination sphere of the copper(II) center
in bis(μ-halo)-bridged complexes (Bu et al. 2001; Bernalte-
Garcia et al. 2006). However, the number of halo-bridged
complexes including tridentate pyridine derivative ligands is
very limited in the literature (Lee et al. 2005, 2008; Kapoor
et al. 2010; Bu et al. 2001; Choubey et al. 2015; van Albada
et al. 2004). We have currently been focusing our attention
X‑ray crystal structure analysis
The data collection for compounds was carried
with a Bruker APEXII CCD diffractometer, using
Fig.1 The structures of com-
plexes under study
1
3

Chemical Papers
graphite-monochromatized MoKα (λ=0.71073°A) radiation
green reaction mixture was stirred for 3 h at room tem-
perature. The dark green compound precipitated from the
solution, which was puriꢀed by vapor diꢁusion of diethyl
ether into the methanol solution. The crystals were suitable
for X-ray crystallography. Yielded 1.936 g, 96% as green
crystals (Scheme 1). Anal. Calcd. for C H Cu N O Cl
at 100 K. The data were integrated with SAINT V8.38A
(
Bruker 2005). The structure was solved by SHELXS-2015.
Refinement by full-matrix least-squares methods based
on F2 values against all reflections was performed by
SHELXL-2015 (Sheldrick 2015), including anisotropic dis-
placement parameters for all non-H atoms. Hydrogen atoms
were placed in their geometrically idealized positions and
constrained to ride on the atoms to which they are attached.
The molecular graphics were drawn with Olex2 (Dolomanov
et al. 2009). Crystallographic data and details of the data
collection and structure reꢀnement are listed in Table 1
2
7
41
4
9
4
8
−1
(MW=1093.48 g mol ): C, 29.66; H, 3.78; N, 11.53; found
−
1
C, 29.85; H, 3.78; N, 11.12. Select IR data (ῡ/cm using
KBr disk): 3546 (br. O–H Str.), 3382 (m, NH str.), 3248
2
(w), 3167 (m, N–H str.), 2924 (m, C–H str. aliphatic), 1651
(s, C=O str.), 1590 (s, C–N str.), 1456 (s, NH bend.), 1323
(m), 1033 (m), 775 (m, Cu–N str.).
Chemicals
Preparation [LCu(Br)(μ‑Br)] Br, 2
2
Preparation [LCu(Cl)(μ‑Cl)CuCl ][LCu(Cl)(μ‑Cl)CuL(H O)]Cl, 1
This compound was produced using the same method
3
2
described for complex 1 except that 1.268 g of CuBr ·2H O
2
2
To a solution of ligand, L (1.314 g, 7.331 mmol) in meth-
anol (5 mL) was slowly added a solution of CuCl ·2H O
(5.68 mmol) and 1.018 g (5.68 mmol) ligand L were used.
Yielded 1.067 g, 44% as dark blue crystals (Scheme 1). Anal.
2
2
−
1
(
1.294 g, 7.331 mmol) in methanol (15 ml). The resultant
Calcd. for C H Br Cu N O (MW=805.15 g mol ): C,
1
8
26
4
2
6
2
Table 1 Crystal data and structure reꢀnement of complexes 1 and 2
Empirical formula
C H Cu N O Cl , 1
C H Cu Br N O , 2
2
7
41
4
9
4
8
18 26
2
4
6
2
Formula weight
Temperature (K)
1093.45
99.97(2)
0.71073
0.25×0.2×0.2
Triclinic, P-1
805.16
99.91
0.71073
0.5×0.4×0.3
Monoclinic, P2 /n
Wavelength
Crystal size/mm3
Crystal system, space group
1
Color
Block, clear dark green
Blue
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
8.3038(5)
14.2243(8)
16.9195(10)
96.025(2)
94.990(2)
92.992(2)
1976.1(2)
2, 1.838
7.4550(3)
23.5164(9)
7.4813(3)
90
110.1140(10)
90
1231.59(8)
4, 2.171
8.244
3
Volume ( Å )
3
Z, Calculated density (Mg/m )
Absorption coeꢂcient
F(000)
2.711 mm−
1100.0
1
780.0
Theta range for data collection
Index ranges
Reꢃections collected / unique
Reꢀnement method
Data / restraints / parameters
Goodness-of-ꢀt on F2
Final R indices [I>2Sigma(I)]
R indices (all data)
2.431–26.44 °
− 10≤h ≤10, − 17≤k≤17, − 21≤l≤21
50,084 / 8129 [R(int)=0.1686]
Full-matrix least-squares on F
8129/2/477
1.041
R1=0.0371, wR2=0.0918
R1=0.0458, wR2=0.0974
0.54 and − 0.82 e.Å−
3.026
− 9≤h≤9, − 30≤k≤30, − 9≤l≤9
23,896, 2837 [R(int)=0.0905]
Full-matrix least-squares on F2
2837/0/145
2
1.094
R =0.0406, wR =0.0960
1
2
R =0.0440, wR =0.0978
1
2
3
Largest diꢁ. peak and hole
1.36 and − 1.32
P=(F 2 +2F )/3; S=∑[w(F −F ) /(N −Nparam)]1/2
2
2
2 2
o
c
o
c
obs
2
2
2
w=1/[σ (F )+(0.0001P) +15.8676P]
o
1
3

Chemical Papers
Scheme 1. Reaction of synthe-
sis of complexes 1 and 2
2
6.85; H, 3.26; N, 10.44; Cu, 15.78; found C, 26.93; H, 3.33;
of the electron density of the donor nitrogen atoms to the
metal atoms in the complexes and the subsequent polari-
zation of the ligand involves electron depopulation of the
N–H bond, which culminates in a shift to lower frequencies
(Golchoubian et al. 2011). The stretching band of C–N of the
−
1
N, 10.50. Select IR data (ῡ/cm using KBr disk) 3454 (br,
OH str.), 3374 (m,, NH str.), 3123(s, N–H str.), 3054 (s,
2
C–H str. aromatic), 2895 (m, C–H str. aliphatic), 1642 (s,
C=O str.), 1585 (s, C–N str.), 1456 (s, NH bend.), 1590 (s,
C–N str.), 1297 (w), 1043 (w), 783(m), 696 (m, Cu–N str.),
−
1
pyridyl groups appeared at around 1585 cm in the studied
complexes, which observed at a similar region in the related
complexes (Golchoubian and Rezaee 2009).
6
74 (m, Cu–O Str.).
Results and discussion
X‑ray structure
Synthesis and characterization
Crystallographic analysis of compound 1 indicates that there
are two structurally diꢁerent dinuclear complexes in the unit
cell designated as units 1a and 1b. A perspective view of the
The preparation of the compounds was performed by com-
bining CuX ·2H O and the ligand L in the solvent of metha-
crystal structure of [LCu(Cl)(μ-Cl)CuCl ][LCu(Cl)(μ-Cl)
2
2
3
nol. The IR spectra of compounds 1and 2 show (Fig. S1)
CuL(H O)]Cl, 1 and the atom labelling scheme are shown
2
−
1
a sharp band at 1651 and 1649 cm , respectively which
are attributed to ν(C=O) vibration that appeared in the free
in Fig. 2. The selected bond distances and angles are given
in Table 2. In both structural units, the tridentate ligand L
is coordinated to the copper(II) in a meridional type and the
copper atoms are linked by a chloro ligand. The geometry
around the copper(II) ions in unit 1a is diꢁerent, Cu(1) is
considered as a square pyramidal environment, with two
nitrogen atoms of the pyridyl and amine, an oxygen atoms
of the amide group and a chloro ligand in equatorial plane,
−
1
ligand at higher energy (1670 cm ). The band shifts in the
complexes are associated with the coordination of the amide
moiety of the ligand. The absorption band with medium
−
1
intensity at 3382 and 3379 cm in 1 and 2, respectively is
associated with the NH stretching vibration of the amide
2
−
1
group. The presence of a narrow band at around 3150 cm
in both complexes can be attributed to the NH stretching
vibration of the amide group, which was observed to be
broader in the free ligand and at the higher frequency. As
the lone pair of the electrons of the donor nitrogen atoms
become involved in the metal–ligand bond, the transfer
CuN OCl, and another chloro ligand in axial position that is
2
linked to Cu(2) ion that is tetra-coordinated. The distortion
factor τ value for the penta-coordinated copper ion is 0.15
5
that reveals a distorted square-pyramidal shape of the coordi-
nation polyhedron (the ideal τ value for a square-pyramidal
5
1
3

Chemical Papers
Table 2 Selected bond lengths and bond angles of complex 1
Bond Length (Ǻ)
Cu(1)-O(1)
Cu(1)-Cl(1)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-Cl(2)
Cu(3)-O(2)
Cu(3)-Cl(6)
Cu(3)-N(4)
1.973(2)
2.2342(8)
2.003(2)
2.013(3)
2.7617(9)
2.011(2)
2.2533(8)
1.999(2)
1.987(2)
2.7203(8)
Cu(2)-Cl(2)
Cu(2)-Cl(3)
Cu(2)-Cl(4)
Cu(2)-Cl(5)
Cu(2)-O(8)
Cu(4)-O(3)
Cu(4)-O(4)
Cu(4)-N(7)
Cu(4)-N(8)
Cu(4)-Cl(7)
2.2595(8)
2.2741(8)
2.2615(8)
2.2111(8)
1.996(2)
2.381(2)
2.017(2)
1.995(3)
2.2663(8)
Cu(3)-N(5)
Cu(3)-Cl(7)
Bond angles (°)
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-Cl(1)
O(1)-Cu(1)-Cl(2)
N(1)-Cu(1)-N(2)
O(3)-Cu(1)-Cl(1)
N(1)-Cu(1)-Cl(2)
173.59(10) Cl(6)-Cu(3)-Cl(7)
95.50(3)
89.05(6)
95.91(6)
173.87(8)
87.31(7)
92.43(9)
173.85(9)
82.29(10)
90.27(8)
95.88(7)
96.96(3)
135.84(3)
135.38(3)
100.92(3)
98.49(3)
96.37(3)
90.21(6)
90.89(7)
92.84(9)
95.51(8)
90.89(7)
O(2)-Cu(3)-Cl(6)
O(2)-Cu(3)-Cl(7)
N(5)-Cu(3)-Cl(6)
N(4)-Cu(3)-Cl(7)
N(5)-Cu(3)-O(2)
N(4)-Cu(3)-O(2)
N(5)-Cu(3)-N(4)
N(5)-Cu(3)-Cl(7)
N(5)-Cu(3)-Cl(6)
Cl(2)-Cu(2)-Cl(3)
Cl(2)-Cu(2)-Cl(4)
Cl(3)-Cu(2)-Cl(5)
Cl(1)-Cu(1)-Cl(2) 100.62(3)
N(1)-Cu(1)-N(2)
N(2)-Cu(1)-Cl(1)
N(2)-Cu(1)-Cl(2)
O(4)-Cu(4)-Cl(7)
O(3)-Cu(4)-Cl(7)
O(3)-Cu(4)-O(4)
O(3)-Cu(4)-N(7)
N(7)-Cu(4)-Cl(7)
N(7)-Cu(4)-O(4)
N(8)-Cu(4)-Cl(7)
N(8)-Cu(4)-O(3)
N(8)-Cu(4)-O(4)
N(8)-Cu(4)-N(7)
80.88(10)
164.64(8)
94.39(8)
86.34(6)
90.55(6)
93.00(9)
Fig. 2 ORTEP views of units 1a (top) and 1b (bottom)
173.44(10) Cl(4)-Cu(2)-Cl(5)
95.65(8)
85.26(9)
172.21(8)
93.40(9)
100.14(10)
80.71(10)
Cl(2)-Cu(2)-Cl(5)
Cl(3)-Cu(2)-Cl(4)
Cu(1)-Cl(2)-Cu(2) 124.15(3)
Cu(3)-Cl(7)-Cu(4) 92.78(3)
situation is 0 and for a trigonal bipyramidal situation is 1.0)
(
Addison et al. 1984). The apical Cu(1)–Cl(2) bond distance,
2
.7617(9)Å, is normal as the Jahn–Teller elongated bond.
The basal atoms are nearly coplanar; the deviations from
the least-squares plane through the CuN OCl atoms are N(1)
2
0
0
8
.122, N(2) − 0.131, O(1) 0.115, Cl(1) − 0.106 and Cu(1)
.157 Å. The bite angles of the ꢀve-membered chelate rings,
0.88(10)° is almost the same as those found for a chelate
ring of 2-(methylamino)pyridine complexes (Shirvan et al.
geometry around each copper(II) ions is considered as a
square pyramidal environment, with two nitrogen atoms of
the secondary amine and pyridyl group, an oxygen atom of
the amide moiety of the ligand L and a chloro ligand in the
equatorial plane, and a Cl atom. The apical site of Cu(3) is
occupied by a bridging chloride, which is in the equatorial
plane of the neighboring copper(II) complex (Fig. 3). The
axial position of Cu(4) is occupied by the oxygen atom of a
water molecule. Each copper(II) in the dinuclear complex
2
9
019). The bite angle of the six-membered chelate ring,
2.84(9)°, is almost the same as those found for a chelate
ring of amide complexes (Chao et al. 2008 and 2011). Cu(2)
atom adopts a four-coordinated CuCl environment, giving a
4
tetrahedral geometry that one of the chloride, Cl(2) bridges
to Cu(1). The geometry index for four-coordinate copper ion,
τ is 0.63 (τ index is the sum of angles α and β, the two larg-
4
4
est angles in four-coordinate species, subtracted from 360°,
all divided by 141°). The τ values are zero for a perfect
is in an ideal square-pyramid polyhedron reꢃected by the τ
4
5
square planar geometry and unity for a regular tetrahedral
one (Yang et al. 2007). As a result, the coordination envi-
ronment about the Cu(2) is seesaw. The Cu–Cl–Cu bridging
angle is 124.15(3)° and the non-bonding Cu···Cu distance
within the complexes in the unit a is 4.443(15)Å.
values of zero and 0.02 for Cu(3) and Cu(4), respectively.
The deviation of Cu(3) from the basal plane deꢀned by N(4)-
N(5)-O(2)-Cl(6) is only 0.085 Å towards the apical Cl(7).
The deviation of Cu(4) from the basal plane deꢀned by N(7)-
N(8)-O(3)-Cl(7) is only 0.041 Å towards the apical O(4).
The axial Cu(3)-Cl(7) bond length is signiꢀcantly longer
than that of Cu-Cl bonds in the basal planes (Table 2). The
Unit 1b consists of a dinuclear penta-coordinated, in
which the copper atoms are linked by a chloro ligand. The
1
3

Chemical Papers
Fig. 4 ORTEP views of 2
pyramidal geometry. The two nitrogen atoms and an oxygen
atom of the ligand L and bromine one deꢀnes a distorted
square plane while the apical position of the pyramid is
occupied by a second bromine atom belonging to the other
half of the dimer. The coordination polyhedron around the
copper(II) ion could be best described as near-ideal square
pyramid, with τ=0.08. As it was pointed out, the two Cu(II)
centers are bridged by two bromine atoms in such a fash-
ion that the two square pyramids share one base-to-apex
edge while having their basal planes parallel (Fig. 5). the
Cu(1)⋯Cu(1A) intra-dimeric distance is 3.716(4) Å. the
axial Cu–Br bond 2.7800(6) Å is considerably longer than
the corresponding basal Cu–Br bonds 2.4538(6) Å due to
the Jahn–Teller eꢁect. The angle of the ꢀve-membered che-
late ring, 82.22(13)° is nearly the same as those found for
the chelate rings of 2-(methylamino)pyridine complexes
Fig. 3 The square pyramid arrangement of the coordinated atoms
around the copper(II) center in units 1a (top) and 1b (below)
Cu–Cl–Cu bridging angle is 92.78(3) and the intramolecular
Cu···Cu separation is 3.624(3) Å in the unit 1b. The equa-
torial least-squares planes of the two copper atoms are not
parallel and form an angle of 174.93(7)°.
There are multiple inter- and intra-molecular hydrogen
bonds in the compound 1 so that an extensive hydrogen-
bonded three dimensional network is formed due to the par-
ticipation of oxygen atoms of the amide group with hydro-
gen atoms of the amide groups of adjacent molecules and
chloride atoms of the tetra coordination site with its own
amide hydrogen (Table S1). A portion of such a hydrogen
bonding network is shown in Fig. S2.
Table 3 Selected bond lengths (Å) and angles (°) for complex 2
Bond distances (Å)
Cu(1)-O(1)
Cu(1)-Br(1)
Cu(1)-N(1)
1.961(3)
2.4538(6) Cu(2)-N(2)
1.988(3) Cu(1)-Cu(1A)
Cu(1A)-Br(1)a
2.7800(6)
2.012(3)
3.716(4)
The molecular structure of 2 is displayed in Fig. 4. The
selected interatomic distances and angles are listed in
Table 2. The structure consists of centrosymmetric dimeric
o
Bond angles ( )
[
LCu(μ-Br)] Br units that two bromine atoms bridge the
2 2
Cu(1)- Br(1)-Cu(1A) 90.238(19) N(1)-Cu(1)-Br(1)a 94.17(10)
a
copper atoms forming a four-membered ring. The Cu(1)-
a
○
Br(1)-Cu(1)-Br(1)
89.762(19) N(1)-Cu(1)-Br(1) 95.68(10)
Br-Cu(1A) bridging angle is 90.238(19) , and the Br(1)-Cu-
a
O(1)-Cu(1)-Br(1)
93.19(9)
88.76(8)
N(1)-Cu(1)-N(2) 82.22(13)
N(2)-Cu(1)-Br(1) 103.46(10)
○
Br(1A) angle is 89.762(19) . The bridging is to an appreci-
a
O(1)-Cu(1)-Br(1)
O1(1)-Cu(1)-N(1)
O(1)-Cu(1)-N(2)
able extent unsymmetrical in nature with diꢁerent Cu–Br
bond distances [Cu(1)–Br(1) 2.4538(6)Å, Cu(1)–Br(1A)
171.42(13) N(2)-Cu(1)-Br(1) 166.71(10)
91.77(12)
2
.7800(6)Å] (Table 3). The structure of the complex dem-
a1-X,1-Y,1-Z
onstrates penta-coordinated Cu(II) ions forming a square
1
3

Chemical Papers
100
20
1
1
5
0
5
80
60
40
20
0
0
-
-
5
10
-15
0
100
200
300
400
o
500
600
Temperature ( C)
100
30
8
6
4
2
0
0
0
0
0
20
10
0
Fig. 5 Two square pyramidal coordination environments of the
copper(II) centers 2 sharing one base-to-apex edge
(
Shirvan et al. 2019). The bond angles of the six-membered
-10
-20
chelate ring are 91.77(12)° that is very similar to those found
for the chelate rings of the amide complexes (Chao et al.
0
100
200
300
400
500
600
2
011). The ꢀve-membered chelate ring in the complex is
o
Temperature C
puckered so that the torsion angles of N(1)-C(5)-C(6)-N(2)
o
is 21.92(10) .
Fig. 6 TGA and DTA curves for 1 (top) and 2 (below)
The crystal structure is stabilized by a two-dimensional
hydrogen-bonding network with intramolecular interactions
of hydrogen bonds between the N–H groups of the amine
moieties and the non-bridging bromine and intermolecu-
lar interactions between the bridging bromine neighboring
dinuclear units, respectively (Fig. S3 and Table S2).
corresponds to the presence of two molecules CuBr (calc.
2
55.5%).
Chromotropism study
Both compounds are chromotropic in solution and display
well-deꢀned color change. The color variations are because
of a change in absorption maxima of the d-d transition of
Thermal analyses of compounds 1 and 2
9
The thermograms of the compounds including simultaneous
TG and DTA curves are shown in Fig. 6. TGA-DTA of both
copper(II) with d electron conꢀguration. It should be noted
that in the aqueous solution, both compounds seem to be
single-core complexes with approximately similar cationic
complexes due to dissociation of the halide bridges and con-
sequently substitution with water molecules. These struc-
tural changes are conꢀrmed by an increase in molar con-
ductance of compounds and the similarity of their UV–Vis
spectra as shown in Fig. S4 in the supplementary contents.
compounds was investigated in an argon atmosphere from
○
2
5–600 °C at a heating rate of 10 C/min. DTA curve com-
pound 1 shows an endothermic process without any weight
○
loss from 135 to 175 C due to the breakage of weak Cu–Cl
bonds. Next, the compound loses weight equal to 30.2% in
the temperature range of 200–330 °C. This could correspond
to the loss of 3Py-CH -NH and a coordinated water mol-
2
ecule (calc. 29.9%). The next endothermic process occurs
Halochromism
○
at 330–00 C associated with fragmentation and thermal
degradation of the organic moiety and formation of four
Both compounds are halochromic in aqueous solution so
that, they show color change with respect to pH. This phe-
nomenon is totally reversible. The visible electronic spectra
of the solution were studied in pH ranges of 1.3–12.0. It
seems that in aqueous solution the weaken Cu-halide bonds
located in the bridge in the dinuclear complexes are broken
and substituted by water molecules as shown in Schemes
2 and 3. This was further conꢀrmed by an increase in the
molar conductance of compound 2 to the standard value
of 2:2 electrolytes. Upon raising the pH of solutions by
molecules of CuCl with a mass loss of 49.3% (calc. 49.2%).
2
Compound 2 exhibits decomposition behavior nearly like
compound 1 except the temperature ranges. DTA curve of
compound 2 shows an endothermic process devoid of weight
○
loss from 175 to 205 C owing to breakage of weak Cu-Br
in [LCu(μ-Br)] Br and formation of 2[LCuBr]Br. Then,
2
2
○
the endothermic process occurs up to 600 C associated
with fragmentation and thermal degradation of the ligand.
The ꢀnal remnant of decomposition is equal to 55.4% that
1
3

Chemical Papers
Scheme 2. Interconversion of
compound 1 in alkaline aqueous
solution (pH=5.14–11.30)
Scheme 3. Interconversion of
compound 2 in alkaline aqueous
solution (pH=5.48–11.21)
NaOH (0.1 M), the d–d band in both complexes shifts to
higher energy with the appearance of two isosbestic points.
In compound 1 the ꢀrst isosbestic point appears at 657 nm
at a pH range of 5.14–8.84 and the second at 621 nm at a
pH range of 8.14–11.3 (Fig. 7). The ꢀrst and the second
isosbestic points in compound 2 appear at 649 nm (at pH
range 5.84–8.86) and 604 nm (at pH range 8.86–11.2),
respectively. As it was illustrated in schemes 2 and 3 the
ꢀrst isosbestic point is due to ionization of the coordinated
water molecules with pK values of 5.41 and 8.40 in 1 and
a
5.68 and 8.57 in 2 and the second isosbestic is associated
with deprotonation of the secondary amine moiety with pKa
1
3

Chemical Papers
Fig. 7 pH titration of compound
100
50
0
150
100
0
.1
6
57 nm
800
1
(6 mM in H O) with NaOH
2
621 nm
800
(
0.10 M). The inset graphs (left
and middle) show isosbestic
points and the right shows the
increase of 500 nm absorbance
on titration of NaOH (0.1 M)
0.05
0
50
0
0
1
2
3
4
5
6
-
4
00
600
1000
400
600
1000
Eq[OH ]
1
1
20
00
compound 1
8
0
0
0
0
6
4
2
0
4
00
500
600
700
Wavelenght (nm)
800
900
1000
values of 10.74 in 1 and 11.14 in 2 that is assisted with
solutions changed to colorless and green respectively, but
with the addition of the other anions, the wavelength of the
maximum absorbance was unchanged as shown in Fig. 8.
copper(II) ion. These values are similar to the pK values
a
2
+
5
.5 for [LCu(H O) ] (L=2,9-Diamino-o-phenanthroline)
2
2
2
+
−
(
Wall et al. 1999) and [Cu(Me tren)(H O)] (Thaler et al.
However, upon addition of SCN the initial intensity of
6
2
−
1
−1
1
998). Spectrophotometric titration of the complexes with
the d–d band (ε = 69 Lit.cm .mol ) increases (ε = 234
−
1
−1
NaOH (0.1 M) exhibited the release of three protons for
the compounds (Insert Figs. 7 and S5). The compounds are
practically stable in aqueous acidic solution, so that their
absorption maxima (at λ =680 nm in 1 and λ =678 nm
Lit.cm .mol ) and show hyperchromicity. Although the
identity of the compound was not identiꢀed, it seems there
−
is an interaction between the cationic complex and SCN .
The dependence of absorption spectra of complex 2 in buꢁer
max
max
−
in 2) are almost unchanged upon addition of perchloric acid
solution on the concentration of CN was investigated by
(
0.1 M). (Fig. S6).
the absorption spectroscopy titration method. The color
−
variations of complex 2 upon the addition of CN salt in
Ionochromism
2
1.8
.6
.4
1.2
1
.8
.6
0.4
0.2
0
The ionochropism of compound 1 was not taken into con-
Original
2
−
NaN3
sideration due to the presence of [CuCl ] species that
4
1
1
NaNO2
NaCN
react with pseudo-halides and interferes with the d-d band
+
of [LCu(Cl)(H O) ] and makes complicated visible spec-
2
2
NaSCN
trum in aqueous solution and problematic to be resolved. As
a result, complex 2 was taken into account in ionochromism
study. The interaction of the complex with some pseudo-
0
0
−
−
−
−
halide anions (CN , NO , SCN and N ) was explored
2
3
by the visible absorption spectroscopy in buꢁer solution
400
500
600
700
800
900
(
pH=5.47). The solution of the complex was at a concen-
Wavlength (nm)
–
3
tration of 5.0 × 10 M. Upon addition of sodium cyanide
and sodium azide, the absorption spectra show the evidently
ionochromic behavior, so that the original blue color of the
Fig. 8 The absorption spectra of compound 2 (5 mM) upon addition
of NaCN, NaSCN, NaN and NaNO in buꢁer solution
3
2
1
3

Chemical Papers
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
rapidly into copper(I) cyanide and cyanogen (Brotherton
and Lynn 1959). These results indicated that complex has
Original
4
5
6
Equ NaCN
Equ. NaCN
Equ. NaCN
−
high-sensitive and selective toward CN . Upon the addition
−
of N , the absorption band of the complex at UV region
3
tailed to the visible region while the d-d band was almost no
evidently changed. Furthermore, an anion competition was
−
−−
also carried out by mixing anions NaX (X=CN , NO
,
2
−
−
SCN and N ) (5 equivalents) in buꢁer solution (pH=5.47)
3
of the complex at room temperature. The color of the mix-
400
500
600
700
800
900
ture became colorless indicating that none of the anions are
Wavelength(nm)
−
capable to compete eꢁectively with CN ion. In another
experiment, the same competition reaction was carried out
by excluding cyanide ion. It was found that the color of the
original blue solution turned to green within a few seconds.
These results indicate that complex 2 is high-sensitive and
Fig. 9 The absorbance spectra compound 2 upon addition of 4.0, 5.0,
.0 equiv NaCN
6
−
−
the buꢁer solution is shown in Fig. 9. With the addition of
selective towards CN and N anions.
3
−
CN , the absorption wavelength at 680 nm shifted to higher
energy (blue shift) along with a decline in absorbance inten-
Solvatochromism
−
sity. When the molar proportion of the complex and CN is
arrived at 1:5 ratios, the absorbance intensity of complex
at 680 nm approached almost to zero and the solution turns
colorless. This is because of the strong coordination power
of the cyanide anion that destroys the original complex by
formation of copper(II) cyanide complex that decomposes
The compounds are soluble in a polar solvent and solva-
tochromic. Their solvatochromism was investigated in
methanol, water, dimethylformamide, and dimethyl sulfox-
ide. Their visible spectra in the aforementioned solvents
are shown in Fig. 10 and their shift in wavelength maxima
Fig. 10 Absorption spectra of
the compounds 1 and 2 in some
solvents
1.4
1.2
1
Compound 2
0
0
0
0
.8
.6
.4
.2
0
DMF
DMSO
H2O
MeOH
4
00 450 500 550 600 650 700 750 800 850 900 950 1000
Wavelength (nm)
1
Compound 1
0
0
0
0
.8
.6
.4
.2
0
DMF
DMSO
H2O
MeOH
4
50 500 550 600 650 700 750 800 850 900 950 1000
Wavelength (nm)
1
3

Chemical Papers
Table 4 The solvent parameter values and electronic spectra of com-
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