Syntheses of dinuclear copper(II) complexes containing of tertiary diamine and amido groups with hydroxo bridges; Chromotropic properties

Article abstract of DOI:10.1016/j.dyepig.2014.01.002

Two new symmetric dinuclear complexes type [Cu(L)(μ-OH)] ₂(ClO₄)₂, where either L = N,N-dimethyl, N′-3-propylamide-ethylenediamine or N,N-diethyl,N′-3-propylamide- ethylenediamine were synthesized and characterized on the basis of elemental analysis, conductance measurement, spectroscopic techniques and X-ray crystal analysis. The complexes show halochromism in aqueous solution. The pH effects on the visible absorption spectra of the two complexes were studied which act as pH-induced off-on-off switches through protonation and deprotonation in aqueous solution at room temperature. The dinuclear complexes were also observed to exhibit thermochromism, solvatochromism and ionochromism as a result of hemilability of amide groups. The solvatochromism of the complexes was investigated with different solvent parameter models using stepwise multiple linear regression method. The results suggested that the donor number parameter of the solvent has a dominant contribution to the shift of the d-d absorption band of the complexes.

Full text of DOI:10.1016/j.dyepig.2014.01.002

Dyes and Pigments 104 (2014) 175e184
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journal homepage: www.elsevier.com/locate/dyepig
Syntheses of dinuclear copper(II) complexes containing of tertiary
diamine and amido groups with hydroxo bridges; chromotropic
properties
,
a
a,
Hamid Golchoubian a , Asieh Heidarain , Ehsan Rezaee b, Francesco Nicolò c
*
a Department of Chemistry, University of Mazandaran, Babol-sar 47416-95447, Iran
b Department of Chemistry, Mohammad Reza Hariri Science Foundation, Babol 47146-38474, Iran
c Dipartimento di Scienze Chimiche, Università di Messina, Viale F. D’alcontres 31, 98166 Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new symmetric dinuclear complexes type [Cu(L)(
m
-OH)]2(ClO4)2, where either L ¼ N,N-dimethyl,N0-
3-propylamide-ethylenediamine or N,N-diethyl,N0-3-propylamide-ethylenediamine were synthesized
and characterized on the basis of elemental analysis, conductance measurement, spectroscopic tech-
niques and X-ray crystal analysis. The complexes show halochromism in aqueous solution. The pH effects
on the visible absorption spectra of the two complexes were studied which act as pH-induced “offeon
eoff” switches through protonation and deprotonation in aqueous solution at room temperature. The
dinuclear complexes were also observed to exhibit thermochromism, solvatochromism and ion-
ochromism as a result of hemilability of amide groups. The solvatochromism of the complexes was
investigated with different solvent parameter models using stepwise multiple linear regression method.
The results suggested that the donor number parameter of the solvent has a dominant contribution to
the shift of the ded absorption band of the complexes.
Received 29 October 2013
Received in revised form
1 January 2014
Accepted 4 January 2014
Available online 18 January 2014
Keywords:
Chromotropism
Tertiary diamine
Copper(II) complex
Diamine ligand
Dinuclear complex
Amide ligand
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
electronic spectra according to the strength of the stress imposed to
the system. In our previous study, a series of new homoleptic
Chromotropism may be defined as a reversible color change of
the materials caused by the surrounding chemical or physical
environment; the environment could be solvent (solvatochrom-
ism), temperature (thermochromism), pressure (piezochromism),
light (photochromism), pH (halochromism), ion (ionochromism) or
electrons (electrochromism) [1]. Chromotropism has attracted
much attention because of the wide variety of potential applications
as thermosensitive materials [2,3], imaging [4], photo-switching
materials [5], sensor materials [6] molecular switches [7e10],
pollutant sensors [11,12] and Lewis-acidebase color indicators [13].
A combination of metal-ion recognition moieties with appropriate
ligands has been reported to afford metal-ion responsive chromo-
tropic molecules [1]. Among chromotropic metal complexes, copper
(II) ions with a combination of chelate ligands have been recognized
as the most promising candidates for practical applications due to
their high thermodynamic stabilities, accessibility of other oxida-
tion states and also existence of simple and regular changes in their
dinuclear copper(II) complexes with hydroxo bridges as shown A in
Scheme 1 was prepared that demonstrated distinctive solvato- and
theromochromism properties [14]. Our investigation also showed
that in these homoleptic complexes the copper(II) centers are five-
coordinated with square pyramidal structures that a water molecule
or a counter ion coordinated on the apexof each copper(II) ions [15e
17]. On the other hand, Chao and coworkers reported nickel(II) and
copper(II) complexes containing amide groups that displayed
spectral changes in the aqueous solution due to a pH dependent
isomerization of the amide ligand [18]. This result motivated us to
design a series of five-coordinated copper(II) complexes including
chelate ligand with a hemilabile amide group attached to the amine
nitrogen of the ethylenediamine derivatives in order to obtain metal
complexes with versatile chromotropism properties. It should be
pointed out that transfer of protons can be regarded as one of the
simplest external stimulations and can induce the switching prop-
erties such as visible absorption for pH sensors. pH sensors are
especially attractive since various living organism like enzyme
function within a narrow pH gap that their actions or behavior can
be explained as being “oneoff” switching as a function of pH [19].
Halochromic polythiophene derivative in particular was used for
* Corresponding author. Tel./fax: þ98 112 534 2350.
E-mail
addresses:
h.golchobian@umz.ac.ir,
golchoubian@yahoo.com
(H. Golchoubian), fnicolo@unime.it (F. Nicolò).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.002

176
H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
Scheme 1. The complexes previously prepared (A) and the complexes under study (B).
the visual detection of CO2 gas [20] or halochromic dyes was utilized
in preparation of smart and interactive textiles [21]. However, the
majority of pH sensors is based on organic molecules and less
attention has been focused on metal complexes [22]. In this report,
we present the design and synthesis of dinuclear copper complexes
shown as B in Scheme 1. Additionally, an investigation was carried
out in order to ascertain the effect of pH, temperature, solvent and
also influence of pseudo halide anions on the electronic absorption
spectra of the complexes. Based on our knowledge, this is the first
time that a compound shows all these properties collectively. To
function as chemical sensor devices these compounds can be
encapsulated into zeolite cages [23] or by anchoring to the polymer
backbones [24].
3 h. The resulting solution was filtered to remove unreacted
acrylamide. The solvent was then removed under reduced pressure.
The pale yellow viscous oil was purified by vacuum distillation.
The desired compound was obtained as pale yellow oil (7.40 g,
93%). Selected IR data (n
/cmꢂ1): 3360 (br. m, NeH str.), 2953 (m, Ce
H str. aliphatic), 2829 (m, NeCH3 str.), 1668 (s, C]O str.), 1464
(m, CeN str.), 1411 (m, NeH bend.), 772 (m). 1H NMR (400 MHz,
CHCl3),
HNCH2CH2C(O)NH2), 2.21(s, 1H, NH), 2.66 (t, 2H, J ¼ 5.6 Hz,
HNCH2CH2NMe2), 2.84 (t, 2H, 6.4 Hz, eHNCH2CH2C(O)
NH2), 6.28 (s, 1H, eNH2), 7.69 (s, 1H, eNH2). 13C NMR (100 MHz,
CHCl3), (ppm): 33.79 (HNCH2CH2C (O) NH2), 35.10 (e
d (ppm): 2.15 (s, 6H, CH3), 2.34 (m, 4H, Me2NCH2e and
J
¼
d
HNCH2CH2C(O)NH2), 45.33 (CH3), 46.55 (eHNCH2CH2Ne),
58.40(HNCH2CH2NMe2), 175.48 (eC(O)NH2).
2. Materials and methods
2.2.2. Preparation of N,N-diethyl-N0-3-propylamide-
ethylenediamine (L2)
2.1. Materials and measurements
The same procedure as L1 was used for the preparation of ligand
L2, except that using N,N-diethyl-ethylenediamine instead of N,N-
dimethylethylenediamine. The yield was 72% as pale yellow oil.
[Cu(m
-OH)LPh]2(ClO4)2 was prepared according to our earlier
reported procedure [14]. All solvents were spectral-grade and all
other reagents were used as received. Caution! Perchlorate salts are
potentially explosive and should be handled with appropriate care.
Elemental analyseswereperformed byLECOCHN-600 Elemental
Analyzer. Absolute metal percentages were determined by a Varian-
spectra A-30/40 atomic absorption-flame spectrometer. All the
samples were dried to constant weight under a high vacuum prior to
analysis. Conductance measurements were made at 25 ꢀC with a
Selected IR data (
aliphatic), 2820 (m, NeEt str.), 1673 (s, C]O str.), 1449 (m, CeN str.),
1405 (m, NH bend.), 772 (m). 1H NMR (400 MHz, CHCl3),
(ppm):
n
/cmꢂ1): 3339 (br. m, NeH str.), 2970 (m, CeH str.
d
0.96 (t, 6H, J ¼ 7.2 Hz, CH3CH2e), 2.34 (t, J ¼ 6.4 Hz, Et2NCH2e), 2.47
(q, 4H, J ¼ 7.2 Hz, CH3CH2e), 2.49 (t, 2H, J ¼ 6.4 Hz, eHNCH2CH2NEt2),
2.50 (s, H, NH), 2.63 (t, 2H, J ¼ 6.0 Hz, eHNCH2CH2C(O)NH2), 2.84 (t,
2H, J ¼ 6.0 Hz, HNCH2CH2C(O)NH2), 6.02 (s, 1H, eNH2), 7.73 (s, 1H, e
Jenway 400 conductance meter on concentrations of 10.0 ꢁ 10ꢂ4
,
NH2). 13C NMR (100 MHz, CHCl3),
d (ppm): 11.59 (CH3CH2e), 35.38
6.00 ꢁ 10ꢂ4, 4.00 ꢁ 10ꢂ4 and 2.00 ꢁ 10ꢂ4 M of samples in selected
solvents. Then for each solvent a curve was plotted by drawing the
molar conductance versus concentration of sample. The curve was
then extrapolated to infinitive dilute solution to obtain the molar
conductancevalue. Infrared spectra (potassium bromide disks)were
recorded using a Bruker FT-IR instrument. 1H NMR spectra were
measured with a Bruker 400 DRX Fourier Transform Spectrometer at
room temperature. The electronic absorption spectra were
measured using Braic2100 model and Speckol 2000 UVeVis spec-
trophotometers. The buffer solution of pH ¼ 8.5 for ionochromism
study was prepared by mixing 100 mL of ammonium chloride
(0.3 M) and 100 mL of aqueous ammonia solution (0.05 M).
(HNCH2CH2C(O)NH2), 45.41(Et2NCH2CH2Ne), 46.91 (CH3CH2e),
47.31 (HNCH2CH2C(O)NH2), 52.28 (Et2NCH2CH2Ne), 175.71(eC(O)
NH2).
2.2.3. Preparation of [Cu(L1)(
m-OH)]2(ClO4)2, 1
To the solution of diamine ligand L1 (0.955 g, 6 mmol) in
methanol (24 mL) were slowly added Cu(ClO4)2$6H2O (1.11 g,
3 mmol) in methanol (6 mL). The resultant deep blue mixture was
stirred for 30 min at room temperature. The desired compound
precipitated from the solution as blue solid. The compound was
recrystallized by diffusion of toluene into acetonitrile solution. The
typical yield was 25%. The crystals were suitable for X-ray crystal-
lography. Anal. calcd for C14H36N6Cu2Cl2O12 (MW ¼ 678.48 g
molꢂ1): C, 24.78; H, 5.35; N, 12.39; Cu, 17.73; Found: C, 24.92; H,
2.2. Synthesis
5.25; N, 12.38; Cu, 17.63%. Selected IR data (n
/cmꢂ1 using KBr disk):
2.2.1. Preparation of N,N-dimethyl-N0-3-propylamide-
ethylenediamine (L1)
Acrylamide (3.91 g, 55 mmol) and N,N-dimethyl-ethylenedi-
amine (5.44 mL, 50 mmol) in methanol (25 mL) was refluxed for
3606 (w), 3531 (m, OeH str.), 3419 and 3282 (m doublet, NH2 str),
3163 (m, NeH str.), 2905 (w CeH str.), 1664 (s, C]O str.), 1467 (s,
NeH bend.), 1118 (s, OeClO3 str.), 1087 (s, OeClO3 str.), 631 (s, Oe
ClO3 bend.), 565 (w, CueO str.), 507 (w, CueN str.).

H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
177
Table 1
techniques and refined using full-matrix weighted least-squares
procedure on F2 [28] with anisotropic thermal parameters for all
non-hydrogen atoms. Several hydrogen atoms were located on
the final difference map, the ligand H atoms were included in
the refinement via the “riding model” method with the XeH
bond geometry and the H isotropic displacement parameter
depending on the parent atom X. The position and isotropic
displacement parameter of the OH-bridge hydrogen have been
free refined with only the OeH bond distance restrained to the
Crystal data and structure refinement of [Cu(L1)(
m
-OH)]2(ClO4)2.
Empirical formula
Formula weight
Color
Temperature (K)
Wavelength (A)
Crystal system
Space group
Crystal size (mm)
C14H36Cu2N6O4$2ClO4
678.48
Needle, blue
293 (2)
0.71073
Triclinic
ꢀ
P1
0.20 ꢁ 0.11 ꢁ 0.09
ꢀ
usual value 0.9 A by literature. The chlorine displacement el-
Unit cell dimensions
ꢀ
a (A)
6.7993(2)
9.8288(3)
11.3800(4)
70.360(1)
74.535(1)
74.613(1)
677.28(4)
1
lipsoids are very large due to a significant rotational disorder of
the perchlorate anion but each attempt to treat it by the suitable
overlap of two rotated conformations was unsuccessful. The last
difference Fourier maps has shown the largest electron density
residuals around the disordered perchlorate anion. A summary
of crystal data and structure determination is reported in
Table 1.
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
3
ꢀ
V (A )
Z
Calculated density (Mg/m3)
1.663
1.832
m
(mmꢂ1
)
F (0 0 0)
q
350
2.4. MLR analysis in solvatochromism study
range for data collection (ꢀ)
3.17e26.88
ꢂ8 ꢃ h ꢃ 8
ꢂ12 ꢃ k ꢃ 12
ꢂ14 ꢃ I ꢃ 14
23,397/3110 [R(int) ¼ 0.029]
99.8%
Index ranges
All the absorption maxima reported were taken from experi-
mental curves of ded transition of the complexes. Multivariate
statistical methods have been used in the classification and selec-
tion of solvents. Empirical parameters of solvent polarity were used
as basic data sets. These parameters can be obtained directly from
the literature [29e31]. The extraction of chemical information
contained in such a data set can be carried out by statistical method
of Multiple Linear Regression analysis (MLR) using the SPSS/PC
software package [32]. In this approach, the stepwise procedure
(SMLR) was used for selection of the most relevant solvent pa-
rameters. This method combines the forward and backward se-
lection procedures. Due to the complication of inter-correlations,
the variance explained by certain variables will change when new
variables enter to the equation. Sometimes a variable that is qual-
ified to enter losses some of its predictive validity when other
variables are entered. If this occurs, the stepwise method will
remove the weakened variable. A final set of selected equations was
tested for stability and validity through a variety of statistical
methods [33]. The choice of equation suitable for further consid-
eration was made by using four criteria, namely, multiple correla-
tion coefficients (R), standard error (S.E.), F-statistic and the
number of variables (n) in the model. The best multiple linear
regression model is one that has high R and F-values, low standard
error, least number of variables and high prediction ability. In
parameter selection, variables with small variance t (not significant
at the 5% level) were then removed. “t” value is the solvent-
independent coefficients divided by SE. The solvent parameters
considered in this study for interpreting soluteesolvent in-
Reflections collected/unique
Completeness to
q
¼ 27.50
Absorption correction
Refinement method
Data/restraints/parameters
R indicesa [2806 with I > 2
Goodness-of-fitc on F2
R indices (all data)
Multi-scan
Full-matrix least-squarea on F2
3110/1/167
R1 ¼ 0.0363, wR2 ¼ 0.1038
1.045
s
(I)]b
R1 ¼ 0.0408, wR2 ¼ 0.1078
ꢀꢂ3
Largest diff. peak and hole, e A
0.656 and ꢂ0.586
P
P
a
b
c
R ¼ (jjFoj ꢂ jFcjj)/j jFo.
P
P
wR ¼ {[( [F2o ꢂ F2c)2]/ w[(F2o)2]}1/2
.
P
S ¼ [w(F2o ꢂ Fc2)2/(Nobs ꢂ Nparam).
2.2.4. Preparation of [Cu(L2)(
m-OH)]2(ClO4)2$H2O, 2
This complex was prepared by a similar method used for
[Cu(L1)( -OH)]2(ClO4)2 except that L2 was used in place of L1. The
m
compound was obtained as blue crystals with typical yield of 32%.
Anal. calcd for C18H44N6Cu2Cl2O12 (MW ¼ 734.57 g molꢂ1): C,
29.43; H, 6.04; N,11.44; Cu,16.89; Found: C, 29.82; H, 5.80; N,11.68;
Cu, 16.52%. Selected IR data (n
/cmꢂ1 using KBr disk): 3626 (m, OeH
str.), 3414 and 3284 (m, doublet, NH2 str), 3223 (m, NeH str), 2972
(m CeH str.), 2924 (m CeH str.), 1665 (s, C]O str.), 1444 (m, NeH
bend.) 1387 (s), 1117 (s, OeClO3 str.), 1089 (s, OeClO3 str.) 628 (m,
OeClO3 bend.), 567 (w, CueO str.), 507 (w, CueN str.).
2.3. X-ray structure determination
The X-ray measurements of [Cu(L)(
m
-OH)]2(ClO4)2 were made
teractions are hydrogen bonding ability “a”, electron pair donating
on a Bruker Apex-II CCD single crystal diffractometer at room
temperature by using graphite-monochromated Mo-Ka radia-
tion. Data were collected and reduced by SMART and SAINT
software [25] in the Bruker packages. The structure was solved
by direct method [26,27] and subsequent Fourier difference
ability “ ”, polarity/polarizability parameter “p
b
*”, Gutmann’s donor
number “DN”, Mayer and Gutmann’s acceptor number “AN” as well
as Dimorth and Richardt’s “ET30”. The following solvents were used
for solvatochromism study: nitromethane (NM), acetonitrile (AN),
water (H2O), dimethylformamide (DMF), dimethylsulfoxide
Scheme 2. Synthetic outline for preparation of ligands and complexes.

178
H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
(DMSO), pyridine (Py) and hexamethylphosphorictriamide
(HMPA).
3. Results and discussion
3.1. Synthesis
The ligands were prepared in good yield by Michael addition of
N,N-dialkylethylenediamine to acrylamide in methanol under
reflux. Consequently, the treatment of Cu(ClO4)2$6H2O with the
appropriate ligand in mole ratios of 1:2 in methanol at room
temperature led to the formation of desired complexes in moderate
yields (Scheme 2).
3.2. Characterization
3.2.1. IR spectra
The free ligands have indicator bands which are apparent with
minor shift in the dinuclear complexes. Medium intense and nar-
row bands between 3400 and 3150 cmꢂ1 are associated to the NeH
and NH2 stretching vibrations of the amide and amine groups
respectively, those are broader in the free ligands. Additionally,
upon formation of the chelate rings in the complexes, the C]O
vibration of the amide moiety, which appeared as a strong band
around 1670 cmꢂ1 in free ligands, moved to around 1660 cmꢂ1. The
IR spectra of the free ligands showed bands with strong to medium
intensities between 2820 and 2760 cmꢂ1, which are characteristic
of the CeH stretching vibrations of the NeCH3 (or C2H5) group [34],
provided that the lone-pair of electrons on the tertiary nitrogen
atom is not involved in a bond but upon coordination, these bands
disappeared. The presence of a sharp signal at around 3600 cmꢂ1 in
both complexes can be assigned to the OeH stretching frequency of
the hydroxo bridges [35]. At lower frequency the complexes also
exhibited bands around 565 and 507 cmꢂ1 which are attributed to
Fig. 1. ORTEP view of [Cu(L1)(
m-OH)]2(ClO4)2, 1, placed on a crystallographic center of
symmetry (the asymmetric unit is half complex)a.
unit is constituted by half complex which therefore shows a perfect
Ci symmetry at the solid state. The copper(II) ion has a square py-
ramidal geometry in which the basal plane is composed of two
nitrogen atoms of tertiary (N1) and secondary (N2) amines and two
oxygen donor of hydroxyl groups (O2, O20). Among them two ox-
ygen atoms are forms of hydroxo bridging anions. On the other
hand, the fifth coordination site of the square pyramid in the
complex is occupied by the oxygen atom (O1) of amide moiety of
the ligand L1. The basal atoms are nearly coplanar; the deviations
from the least-squares plane through the CuN2O2 atoms are
the
the larger dipole moment change for the CueO band compared to
the CueN band, the (CueO) band usually appears at higher fre-
quency than the
(CueN) band [37]. The presence of the ClOꢂ4 group
n(CueO) and n(CueN) vibration modes, respectively [36]. Due to
n
n
was declared by an intense band at around 1100 cmꢂ1 and a me-
dium band at 630 cmꢂ1 which are attributed to the anti-symmetric
stretching and anti-symmetric bending vibration modes, respec-
tively [38].
0
ꢀ
N(1) ꢂ0.060, N(2) 0.060, O(2) 0.069, O (2) ꢂ0.068, Cu(1) 0.165 A.
According to Addison and Rao [40] the distortion of the square
pyramidal geometry toward trigonal bipyramidal can be described
by geometrical parameter
largest L-M-L angles of coordination sphere. The
for a regular square pyramidal geometry and unity for a regular
trigonal bipyramidal one. The value for the coordination around
the each copper atoms is 0.122, confirming the square pyramidal
geometry of the copper sites. The Cu sites are bridged by two eCu-
s
¼ j
a
-
b
j/60, where
a
and
b are the two
s
values are zero
3.2.2. Conductometric data
Molar conductance of the dinuclear complexes measured in
some solvents with different polarities is presented in Table 2. The
standard values of 1:2 electrolytes in the respective solvents are
shown in the same table [39]. The conductivity data show that the
complexes are almost consistent with 1:2 electrolytes.
s
m
OH through basal coordination linkages. The CueO2eCu angle is
found to be 101.53(4)ꢀ. The bond distance of CueO2 and CueO20 are
3.2.3. X-ray structure
Table 3
A perspective view of the complex 1 with the atom numbering
scheme is shown in Fig. 1. Selected bond lengths and angles are
gathered in Table 3. The compound crystallized in the space group
ꢀ
ꢀ
Selected bond lengths (A) and angles ( ) for complex 1.
Bond distances
P1. The structure of [Cu(L1)(
m
-OH)]2(ClO4)2 consists of a centro-
CueO(2)a
CueO(2)
CueN(1)
O(1)eC(7)
1.9206(18)
1.9268(19
2.027(2)
CueN(2)
CueO(1)
CueCua
2.022(2)
2.352(2)
2.9815(5)
1.310(4)
symmetric Cu(II) dimer. Therefore, the crystallographic asymmetric
1.244(4)
N(3)eC(7)
Table 2
Bond angles
The molar conductivity values (lM) of the complexes (
some solvents.
U
ꢂ1 cm2 molꢂ1, at 25 ꢀC) in
O(2)eCueO(2)a
O(2)eCu(1)eN(2)
O(2)aeCu(1)eN(2)
O(2)aeCueN(1)
O(2)eCueN(1)
O(1)eC(6)eC(7)
78.40(8)
97.53(9)
173.27(9)
96.31(9)
165.87(10)
121.6(3)
N(1)eCueN(2)
O(2)aeCueO(1)
O(2)eCueO(1)
N(2)eCueO(1)
N(1)eCueO(1)
O(1)eC(7)eN(3)
86.39(11)
96.71(8)
91.25(9)
88.69(9)
102.44(10)
121.8(3)
Compound
H2O
AN
DMF
1
2
267
255
253
265
161
155
1:2 electrolyte
235e273
220e300
130e170
a
1 ꢂ X, 1 ꢂ Y, 2 ꢂ Z.

H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
179
Table 4
their donor powers, leading to the observed solvatochromism. The
closest interdimer Cu/Cu separation is 6.799(12) A. Details about
Hydrogen bonds for [Cu(L1)(
m-OH)]2(ClO4)2 (D, donor atom; A, acceptor atom).
ꢀ
ꢀ
ꢀ
ꢀ
hydrogen bonding are shown in Table 4. Hydrogen bonding inter-
action between the hydrogen atoms of the bridged hydroxo groups
(O(2)eH(2)) with perchlorate anions (O(4)) and hydrogen atoms of
the amide (N(3)-H(3A)) and oxygen atom of amide group (O(1))
stabilizes the dimeric structure (dotted lines in Fig. 2) and form 1D
chain that extend along the crystallographic ‘a’ axis. Molecular
packing is mainly determined by several weak hydrogen bonds
involving perchlorate oxygen atoms, secondary and tertiary amine
as well as other heteroatoms which are responsible for the
observed packing (Fig 3).
DeH/A
d (DeH) A
d (H/A) A
d (D/A) A
DeH/A (ꢀ)
N(3)eH(3A)/O(1)a
N(3)eH(3B)/O(5)b
N(2)eH(2A)/O(6)
O(2)eH(2)/O(4)
0.86
0.86
0.91
0.845(18)
2.20
2.25
2.30
2.095(19)
3.023(4)
3.095(5)
3.134(5)
2.934(4)
161.3
168.7
152.2
172(4)
Symmetry transformations used to generate equivalent atoms.
a
ꢂX, 1 ꢂ Y, 2 ꢂ Z.
b
ꢂ1 þ X, þY, þZ.
ꢀ
ꢀ
1.9268(19) A and 1.9206(18) A, respectively. The N2 nitrogen is
chiral and therefore the dimer complex is a meso-form having
opposite chirality in the two centrosymmetric moiety. Intradimer
3.3. Halochromism
ꢀ
Cu/Cu separation is 2.9815(5) A. The bond length of CueO(1) is
ꢀ
ꢀ
2.353(2) A which is within the range of 2.2e2.9 A, known as the
axial CueO bond length, and hence the amide groups may weakly
coordinate in the complex. The distance shows that the forces
holding amide group in the complex are relatively weak and can be
driven out from the coordination sphere by solvent molecules in
solution, which will be more sensible bonded with the increase in
The electronic absorption spectra of dinuclear complexes 1 and
2 in aqueous solution show the ded transitions at 624 and 632 nm,
respectively, which are consistent with the square pyramidal ge-
ometry of Cu(II) ion [41]. The electronic absorption spectra of the
complexes are sensitive to the pH of the solution, so that the
original blue color of the solution turns to purple upon addition of a
Fig. 2. Hydrogen bonding of [Cu(L1)(
m-OH)]2(ClO4)2, 1 along the ‘a’ axis.
Fig. 3. Crystal packing of [Cu(L1)(
m-OH)]2(ClO4)2, 1 along the ‘a’ axis.

180
H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
Fig. 5. pH titration of complexes [Cu(L1)(
OH)LPh]2(ClO4)2 (4 mM in H2O) with NaOH (0.10 M).
m-OH)]2(ClO4)2, and dinuclear complex [Cu(m-
Fig. 4. Visible absorption spectra of complex 1 (4.0 ꢁ 10ꢂ3 mol dmꢂ3) in water at 25 ꢀC.
The pH values are denoted on the figure.
with perchloric acid at lmax ¼ 624 nm (Fig. 6) displayed that
decolorization occurred with consumption of eight equivalents
protons. It is perhaps due to protonation of all of the amine, amide
and bridging OHꢂ groups and formation of protonated ligand and
base (NaOH, 0.1 M). This phenomenon is totally reversible. To
investigate the color changes, the visible spectra of the complexes
were studied in pH ranges 8 to 11. These results are shown in Fig. 4.
There are two isosbestic points at 595 and 610 nm in complex 1 and
578 and 597 cmꢂ1 in complex 2 which became visible in different
pH scales. As the pH of the solution increased with addition of the
base, the absorption bands shifted toward lower wavelength due to
ionization of the amide protons and following the CueO to CueN
bond rearrangement. It seems that the first isosbestic point is due
to ionization of the amide protons and the second isosbestic point
belongs to the reversible switching of the coordination modes Cue
O to CueN as shown in Scheme 3. The blue shift in the absorption
maxima of the complexes with increasing the pH of the solution
shows that the nitrogen atom of the deprotonated amide moiety is
a stronger donor than neutral amide. Such phenomena were
observed by Choa in nickel and copper complexes and amide
derived ligands [42,43]. This phenomenon was also observed by
using weaker base such as ligand L or triethylamine. Titration of the
mononuclear copper(II) complexes. Such
observed for the related dinuclear complex [Cu(
a
phenomenon is
m
-OH)LPh 2þ
]
2
. In an
complementary experiment an aqueous solution of copper(II)
perchlorate was acidified and the ligand L, Lph or ethylenediamine
was then added to the resultant solution. The blue color of the
solution abruptly decolorized regardless of the nature of diamine
ligands. However, decolorization was not observed in absence of
diamine ligands.
3.4. Solvatochromism
The complexes 1 and 2 are soluble in solvents with high
dielectric constant and demonstrate distinctive solvatochromism.
The presence of a strong JahneTeller effect on the Cu(II) ions made
them good solvatochromic probes. The origin of the color changes
in the compounds is attributed to the shift in the ded transition of
the copper(II) ions as a result of solventesolute interactions. The
visible spectral changes of the complexes in the selected solvents
are shown in Fig. 7. The observed lmax values of the complexes
along with the molar absorption coefficients (ε) and their respec-
tive solvent parameter values are illustrated in Table 5. Based on the
results illustrated in the table, the effect of the substituent attached
to the tertiary amine on the ded transition of the complexes is
insignificant. The solvatochromism property of the complexes is
possibly due to the dissociation of the weak amide-copper bonds
from the coordination sphere that facilitates the approach of the
solvent molecules above and below the chelate plane to the copper
(II) centers. The relationship between the energy of the ded tran-
sitions of the complexes and the solvent parameters has been
complexes with NaOH demonstrated that [Cu(
two protons around pH ¼ 10.3 (Fig. 5). In contrast, for the related
complex ([Cu( ) no proton release is observed at pH
-OH)LPh 2þ
m
-OH)L]22þ releases
m
]
2
below 11. Thus, the acidity of the complexes cannot be explained by
deprotonation of hydroxo bridges and/or the secondary amine
since the coordinated hydroxo group and secondary amine should
have similar pKa values in the structurally related complexes [Cu(
m-
OH)LPh 2þ
]
shown in Scheme 1A. Interestingly, when the pH of the
2
solution was decreased to 2.0 by addition of perchloric acid (0.1 M)
the original blue color of the solution diminished. The decoloriza-
tion was along with red shift and decrease in intensity of the de
d band. The original blue color of the copper (II) complexes rede-
veloped by increasing the pH of the solution to around 8 with
addition of a base. Spectrophotometric titration of [Cu(m
-OH)L]22þ
Scheme 3. Interconversion of complexes triggered by acid and base (pH ¼ 8.5e11.0) in aqueous solution.

H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
181
Table 5
The solvent parameter valuesa and electronic spectra of the complexes in various
solvents: lmax/nm (ε/Mꢂ1 cmꢂ1).
Solvent DN
AN
ET(30)
b
a
p
*
Complex 1 Complex 2
NM
AN
H2O
DMF
DMSO
Py
2.7 20.5 46.3
14.1 18.9 45.6
19.5 54.8 63.1
26.6 16.0 43.8
29.8 19.3 45.1
33.1 14.2 40.5
38.8 10.6 40.9
0.0
0.13 0.85 635 (208)
657 (228)
610 (208)
626 (188)
628 (179)
632 (180)
648 (163)
653 (200)
0.40 0.19 0.75 595 (208)
0.47 1.17 1.09 620 (177)
0.69 0.00 0.88 619 (184)
0.76 0.00 1.00 621 (194)
0.64 0.00 0.87 637 (185)
1.05 0.00 0.87 660 (208)
HMPA
a
These values are taken from Refs. [29e31].
certain statistical criteria are met and the best model is obtained. In
both complexes the results confirm the dominant contribution of
the DN (donor number) parameter in the solvatochromism of the
complexes (Table 6). The positive sign of the coefficient indicates a
red shift as the donor number of the solvents increases. Although
the emptiness of one of the axial sites of the complexes and also
facile removal of the coordinated amide group on the other site of
the chelate plane lead to approach of the solvents molecules to the
metal center with no difficulty, but each solvent has its own basicity
strength due to their electron donating power. An increase in DN of
the solvents means more power and more ability of the solvent
molecule to coordinate to the copper(II) centers. Due to approach of
the polar solvent molecules to the axial position of the complexes, a
strong repulsion occurs between electron in the dz2 orbital of the
copper(II) ions and the electron pair of the solvents that causes the
required energy for transferring the electrons to dx2ꢂy2 orbital de-
clines. Accordingly, the position of this band decreases nearly lin-
early with the increase of the electron pair donating power of the
solvents. Regression analysis of the band maxima of the dinuclear
complexes against the electron pair donating power of the solvents
is shown in Fig. 8. An obvious exception from this general trend is
found in the lmax values of nitromethane that is far away from
linearity trend obtained in Fig. 8. This inconsistency in the ab-
sorption maxima can be ascribed to the low donor number of
nitromethane (2.7) which hinders the amide groups to be replaced
by solvent molecules. As a result, the complexes are not sol-
vatochromic in nitromethane. However, the high dielectric con-
stant of nitromethane (28.5) resulted in solvation of the complexes
in this solvent. This was supported by the similarity of the solid-
state spectra of the complexes with those in nitromethane that
display almost similar lmax values (632 and 658 nm for complexes 1
and 2, respectively). As a result, the data in nitromethane were
eliminated in the SMLR investigation. Although the basicity nature
of the solvents such as pyridine can give rise to deprotonation of the
amide groups and could be responsible in color change of solution,
this effect on the solvatochromism of the complexes is negligible
because, the deprotonated amide groups and/or NH-coordinated
amide groups are substituted by solvent molecules. This is also
confirmed by lack of contribution of other solvent parameter than
DN in the model obtained by SMLR method (see Table 6).
Fig. 6. The pH dependent visible spectrum of 1 in aqueous solution at 25 ꢀC. The inset
graph shows the decrease of 627 nm absorbance on titration of 1 (4 mM in H2O) with
HClO4.
investigated by SMLR technique. As a result, Eq. (1) has been
considered as the base equation, in which lmax values are taken
from Table 5 and lomax is the value of the absorption energy in an
inert solvent (1,2-dichloroethane) and a, b, c, d, e and f values are
the regression coefficients describing the sensitivity of the property
lmax to the different solvent parameters like DN, AN, ET30,
*, and solute/solvent interaction mechanisms.
b
,
a
and
p
lmax
¼
lomax þ a DN þ b AN þ c ET30 þ d
b
þ e
a
þ f
p
*
(1)
By using the SMLR method, this equation would be changed and
the solvent parameters will be added or removed by program until
Table 6
The equations resulted from the linear correlation of the lmax values with DN of the
solvents.a
Complexes
1
Equations
F
R
S.E.
lmax ¼ 2.21
23
0.92
9.31
(ꢄ0.46)DNsolv þ 586.55)(ꢄ13.03)
lmax ¼ 1.66
2
42
0.96
5.16
(ꢄ0.26)DNsolv þ 588.03(ꢄ7.22)
a
Fig. 7. Absorption spectra of complexes 1 and 2 in selected solvents.
The number of variables, n ¼ 6.

182
H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
Fig. 9. Temperature dependence of the UVevis absorbance of a DMSO solution of 1.
The factor which causes color change in ionochromic substances is
ions. A flow of ions through an ionochromic material results in a
reaction/color change from the material. Ionochromic substances
are suitable for detection of charged particles. These compounds can
be used as pM indicators for complexometric titrations [44].
Interaction between the complexes and different halides and
pseudo halides were investigated by visible spectroscopy at a
concentration of 2.0 ꢁ 10ꢂ3 mol Lꢂ1 of complexes in the buffer
solution (pH ¼ 8.5). Variation of absorption spectra of the com-
plexes upon addition of different anions including Clꢂ, Brꢂ, Iꢂ, CNꢂ,
OCNꢂ, SCNꢂ, N3ꢂ and NOꢂ2 are shown in Fig. 10. The spectra show no
obvious change in the absorption maxima upon the addition of the
anions Clꢂ, Brꢂ and NOꢂ2 , while a visible spectral change have been
observed by addition of other anions and the variation in the color
changes are significant. The color change might be due to substi-
tution of the weakly coordinated amide groups by the anions which
causes change in ligand field strength around the copper(II) ions.
However, in case where CNꢂ was used the color of the solution
gradually turns to colorless. The dependence of the absorption
spectra of complex 1 in aqueous solution on the concentration of
cyanide anion was investigated by the absorption spectroscopy
titration method (Fig. 11). Upon the addition of CNꢂ salt, the de
d band gradually underwent a hypochromic shift (decrease in
Fig. 8. Dependence of the lmax values of complexes 1 and 2 on the solvent DN values.
3.5. Thermochromism
UVevis absorption spectra of the complexes in DMSO were
recorded at temperatures in the range 25e130 ꢀC (Fig. 9). At room
temperature, two bands are observed at 260 and 627 nm. The
former band is accompanied with a shoulder at 315 nm. The
shoulder is assigned to a metal-to-ligand charge transfer (MLCT)
involving the amide chelate ligand transition, and the second one is
due to a ded transition of the copper(II) ion. Upon heating, the
original blue color of the solution turns to green and the band at
627 nm and the shoulder underwent red shifts while band at
260 nm remained constant. However, at 130 ꢀC the shoulder shifted
to 325 nm and tails into visible region while the ded band moved to
560 nm. We attributed the observed red shift to the substitution of
DMSO molecules with weakly coordinated amide moieties. The
hemilability of the amide groups along with JahneTeller effect that
is imposed by the copper(II) ion (with d9 electron configuration)
made the amide groups susceptible for bond rupture at high tem-
perature and further substitution by solvent molecules (DMSO).
This phenomenon is an irreversible process due to tight bond for-
mation between DMSO and Cu(II) and the presence of significant
amount of solvent molecules around the copper centers. This
phenomenon was also observed in other polar solvents.
3.6. Ionochromism
Fig. 10. The absorption spectra of complex 1 (2.0 ꢁ 10ꢂ3 mol Lꢂ1) upon addition of
Ionochromic materials, similar to photochromic, thermochromic
and other chromic materials, alter color in the presence of a factor.
NaCl, NaBr, NaI, NaCN, NaOCN, NaSCN, NaN3 and NaNO2 in aqueous solution.

H. Golchoubian et al. / Dyes and Pigments 104 (2014) 175e184
183
models using the SMLR computational method. The obtained re-
sults suggested that the donor number parameter of the solvent has
the dominant contribution to the shift of the ded absorption band
of the complexes. The ded visible absorption band exhibits a red
shift with increasing the donor number (DN) of the solvents. As a
result, the complexes respond to combinations of stimuli (heat,
solvent, pH and anion) mainly due to presence of hemilabile amide
groups in the axial position of the complexes.
Acknowledgment
We are grateful for the financial support of university of
Mazandaran of the Islamic Republic of Iran.
Appendix A. Supplementary data
CCDC 965074 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: (þ44) 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
porting information: assigned 1H NMR, 13C NMR and IR spectra for
ligands and complexes, 1H NMR, 13C NMR and IR spectra of com-
pound 1 in dilute D2SO4 and visible absorption spectra of complex 2
in basic aqueous solution.
Fig. 11. The color changes of complex 2 (2.0 ꢁ 10ꢂ3 mol Lꢂ1) upon addition of NaCN
salt in aqueous solution. The NaCN concentration is (1) 0.0, (2) 1.0, (3) 2.0, (4) 3.0, (5)
4.0, (6) 5.0 and (7) 6.0 equiv, respectively.
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cyanide anion that enables it to destroy the original complex by
formation of copper(II) cyanide complex that rapidly decomposes
into copper(I) cyanide and cyanogen [45]. The hypsochromic shift
of the ded band is attributed to the increasing concentration of
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