Spectroscopic studies on Solvatochromism of mixed-chelate copper(II) complexes using MLR technique

Article abstract of DOI:10.1016/j.saa.2011.08.042

Mixed-chelate copper(II) complexes with a general formula [Cu(acac)(diamine)]X where acac = acetylacetonate ion, diamine = N,N-dimethyl,N′-benzyl-1,2-diaminoethane and X = BPh₄ ^-, PF₆^-, ClO₄^-

Full text of DOI:10.1016/j.saa.2011.08.042

Spectrochimica Acta Part A 85 (2012) 25–30
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic studies on Solvatochromism of mixed-chelate copper(II)
complexes using MLR technique
Hamid Golchoubian∗, Golasa Moayyedi, Hakimeh Fazilati
Department of Chemistry, University of Mazandaran, Babol-sar 47416-95447, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 November 2010
Accepted 23 August 2011
Mixed-chelate copper(II) complexes with
a
general formula [Cu(acac)(diamine)]X where
acac = acetylacetonate ion, diamine = N,N-dimethyl,Nꢀ-benzyl-1,2-diaminoethane and X = BPh4
,
−
−
−
PF6−, ClO4 and BF4 have been prepared. The complexes were characterized on the basis of elemental
analysis, molar conductance, UV–vis and IR spectroscopies. The complexes are solvatochromic and
their solvatochromism were investigated by visible spectroscopy. All complexes demonstrated the
positive solvatochromism and among the complexes [Cu(acac)(diamine)]BPh4·H2O showed the highest
ꢀꢁmax value. To explore the mechanism of interaction between solvent molecules and the complexes,
different solvent parameters such as DN, AN, ˛ and ˇ using multiple linear regression (MLR) method
were employed. The statistical results suggested that the DN parameter of the solvent plays a dominate
contribution to the shift of the d–d absorption band of the complexes.
Keywords:
Mixed-chelate
Solvatochromism
MLR method
Copper(II) complexes
Counter ion
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
correlation between the spectral shifts and solvent parameters can
be examined by computational methods [18].
A great number of studies have been dedicated to solva-
tochromism of metal complexes because of their potential ability to
act as a Lewis acid–base color indicator [1], molecular switch [2–5],
pollutant sensor [6,7] and imaging device [8]. Solvatochromism is
defined as the influence of solvent on the electronic absorption and
emission spectra of molecules. Among the solvatochromic metal
complexes the mixed-chelate copper(II) complexes have received
great attention due to presence of a strong Jahn–Teller effect which
resulted in a simple and regular changes in their electronic spectra
according to the strength of interactions with solvent molecules
at the axial sites[9]. There are different solvent parameters to
describe the interaction between solvent and the metal complexes
such as Gutmann’s DN (donor number is a measure of coordinating
ability of solvents on the standard of dichloromethane) [10], Mayer
and Gutmann’s AN (the electron acceptor property of a solvent)
[11], Dimroth and Reichardt’s ET(30) (a measure of the ionization
power of a solvent) [12], Kosower’s Z (an empirical measure of
solvent polarity), Kamlet-Taft’s ˛ (hydrogen bond donation of a
solvent), ˇ (hydrogen bond acceptance of a solvent), and ꢂ* (polar-
ity/polarisability parameter of a solvent) [13–17]. In many cases
it was found that, the shift in the electronic absorption spectra of
the complexes depends on more than one solvent parameter. The
In this report, we prepared four mixed-chelate copper(II) com-
plexes shown in Scheme 1. The complexes are soluble almost in a
large number of solvents and show solvatochromism. The mecha-
nism of interaction between solvents and the complexes and effect
of counter ions on the solvatochromism of the complexes are inves-
tigated.
2. Experimental
2.1. Materials and methods
The preparation of the ligand N,N-dimethyl,Nꢀ-benzyl-1,2-
diaminoethane (diamine) was carried out as reported earlier [10].
All the chemicals used were reagent grade and solvents for spectro-
scopic and conductivity measurements were “spectro-grade” used
without further purification. Caution: perchlorate salts are poten-
tially explosive and should be handled with appropriate care.
Infrared spectra measurements were carried out using KBr
disk on a Bruker FT-IR spectrometer in the range 450–4000 cm−1
.
Conductivity measurements were performed with a Jenway 400
instrument at 25 ◦C. UV–vis spectra of the solutions were obtained
with a Braic 2100 spectrophotometer using 1 cm quartz cells. Ele-
mental analyses (C, H, and N) were performed using a LECO 600
elemental analyzer. Absolute metal percentages were determined
by a Varian-spectra A-30/40 atomic absorption-flame spectrom-
eter. The following solvents were used for solvatochromic study:
dichloromethane (DCM), nitromethane (NM), nitrobenzene (NB),
∗
Corresponding author. Tel.: +98 1125342350; fax: +98 1125342350.
E-mail address: h.golchobian@umz.ac.ir (H. Golchoubian).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.042

26
H. Golchoubian et al. / Spectrochimica Acta Part A 85 (2012) 25–30
2.2.4. Preparation of [Cu(acac)(diamine)]BF4, 4
Me
Me
The same procedure as 1 was used for preparation of compound
4 except that using methanol as solvent and a saturated solution of
NaBF4 in water instead of NaBPh4. The violet crystals were obtained
with yield of 69%. Selected IR data (ꢁ/cm−1 KBr disk): 3268 (m, N–H
str), 1588 (s, C O str.), 1520 (s, C C str.), 1083, 1038 (s, B–F str.), 740,
O
N
Cu
NH
O
710 (s, C–H bend.). Anal. calcd. for C16H25N2BF4O2Cu: C, 44.93; H,
5.89; N, 6.55; Cu, 14.89; found: C, 44.88; H, 5.93; N, 6.57; Cu, 14.86%.
X-
2.3. MLR analysis
All the absorption maxima reported were taken from experi-
mental curves of the d–d transition of the complexes. Multivariate
statistical methods have been used in the classification and selec-
tion of solvents. The empirical parameters of the solvent polarity
were used as basic data sets. These parameters can be obtained
directly from literature [19–21]. The extraction of the chemical
information contained in such a data set can be carried out by sta-
tistical method of Multiple Linear Regression analysis (MLR). In this
method, a dependent variable Y is described in terms of a series of
explanatory variables X1, . . ., Xn, as given in Eq. (1):
-
-
-
-
X = B(Ph)4 ,PF6 , ClO4 ,BF4
Scheme 1. The structure of the complexes.
benzonitrile (BN), acetonitrile (AN), propionitrile (PN), acetone
(Ac), tetrahydrofuran (THF), ethanol (EtOH), methanol (MeOH),
dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine
(Py) and hexamethylphosphorictriamide (HMPA).
Y = Y0 + a1X1 + a2X2 + · · · + anXn
(1)
It is assumed that all the explanatory variables are independent
of each other and truly additive as well as relevant to the prob-
lem under study [22,23]. Y is the value of a solvent dependent
physicochemical property (ꢁmax in this study) in a given solvent
and Y0 is the statistical quantity corresponding to the value of
this property in the gas phase or in an inert solvent. X1, X2, . . .,
Xn represent independent but complementary solvent parameters,
which account for the different solute/solvent interaction mecha-
nisms. a1, a2, . . ., an are the regression coefficients describing the
sensitivity of the property Y to the different solute/solvent inter-
action mechanisms. The postulate is that the solvent effect on a
solute property Y can be represented as a linear function of some
independent but complementary parameters describing the Lewis
acidity and basicity of a given solvent. The AN and ˛, values are
chosen as a measure of Lewis acidity. In addition, Gutmann’s donor
number DN and ˇ were selected as a measure of solvent basic-
ity [24–28]. In this approach, the Backward and Enter procedure
were used for selection of the most relevant variables. A final set of
selected equations was examined for stability and validity through
a variety of statistical methods. The choice of a suitable equation
for further consideration was made by using four criteria, namely,
multiple correlation coefficient (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, the least number of variables and high prediction
ability [29]. 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 S.E. To determine the
relative significance of the solvent parameters, the regression coef-
ficients are described in terms of percentage contribution value. Eq.
(2) could be statistically qualified into percentage contribution fac-
tor P(Xi). To attain this, the regression coefficients, which emerge
from multiple regression equations are normalized to numerical
range 0–1 [14]. Hence, the percentage contribution P(Xi) of a solvent
parameter in multiple regression is calculated [30].
2.2. Syntheses
2.2.1. Preparation of [Cu(acac)(diamine)]BPh4·H2O, 1
A solution of containing Cu(OAc)2·H2O (1.2 g, 6 mmol) and
Na2CO3 anhydrous (0.3 g, 3 mmol) in absolute ethanol (45 ml)
was added to the mixture of N,N-dimethyl,Nꢀ-benzyl-1,2-
diaminoethane (1.0 g, 6 mmol) and acetylacetone (0.62 ml,
6 mmol). The resultant suspension was stirred for 2 h, and then
was filtered to remove [Cu(acac)2] by-product. A saturated solution
of NaBPh4 in absolute ethanol was then added to the resultant
clear blue solution. After concentration of the solution at room
temperature, the blue crystals were collected with filtration
and dried in vacue. The yield was 58%. Selected IR data (ꢁ/cm−1
KBr disk): 3525 (m, O–H str.), 3217 (m, N–H str), 1578 (s, C
O
str.), 1521 (s, C C str.), 735, 706 (s, C–H bend.). Anal. calcd. for
C40H47N2BO3Cu: C, 70.84; H, 6.99; N, 4.13; Cu, 9.37; found: C,
70.80; H, 7.02; N, 4.18, Cu, 9.33%.
2.2.2. Preparation of [Cu(acac)(diamine)]PF6·H2O, 2
This complex was prepared with the same procedure as complex
1, except that using NaPF6 instead of NaBPh4 (note: this compound
is hygroscopic and should be kept in a dessicator). The yield was
32%. Selected IR data (ꢁ/cm−1 KBr disk): 3649 (m, O–H str.), 3293
(m, N–H str.), 1583 (s, C O str.), 1528 (s, C C str.), 854 (s, P–F str.),
564 (m, P–F bend.). Anal. calcd. for C16H27N2O3PF6 Cu: C, 38.14; H,
5.40; N, 5.56; Cu, 12.61 found: C, 38.64; H, 5.96; N, 5.02; Cu, 12.39%.
2.2.3. Preparation of [Cu(acac)(diamine)]ClO4, 3
To the solution of N,N-dimethyl,Nꢀ-benzyl-1,2-diaminoethane
(1 g, 6 mmol), acetylacetone (0.62 ml, 6 mmol), Na2CO3 (0.3 g,
3 mmol) in ethanol (20 ml) was slowly added Cu(ClO4)2·6H2O
(2.2 g, 6 mmol) in water (20 ml). After 15 min the by-product of
[Cu(acac)2] was separated by filtration. The midnight blue crystals
were obtained from the resultant clear blue solution after concen-
tration at room temperature. The yield was 76%. Selected IR data
(ꢁ/cm−1 KBr disk): 3247 (m, N–H str.), 1587 (s, C O str.), 1520 (s,
100|ai|
P(X ) =
(2)
ꢀ
i
n
i=1|ai|
C
C str.), 1091 (s, Cl–O str.), 625 (m, Cl–O bend.). Anal. calcd. for
A comparison of the relative importance of the solvent property
can easily be defined using P(Xi), which show a good agreement
between various system under study.
C16H25N2O6ClCu: C, 43.37; H, 5.72; N, 6.36; Cu, 14.43; found: C,
43.33; H, 5.48; N, 6.28; Cu, 14.38%.

H. Golchoubian et al. / Spectrochimica Acta Part A 85 (2012) 25–30
27
Table 1
Molar conductivity data (ꢃm) of the complexes (ꢄ−1 cm2 mol−1, at 25 ◦C) in different solvents.
Complexes
DCM
NM
NB
AN
Ac
MeOH
1
2
3
4
7.48
5.6
4.42
4.79
10–20
51.45
83.13
87.26
80.14
75–95
16.31
25
21.71
20.2
93.02
83.99
133
133
120.18
100–140
53.54
84.1
80
86.7
80–115
133.19
142.65
134.5
1:1 electrolytes
20–30
120–160
Table 2
Electronic spectra of the complexes in various solvents: ꢁmax/103 cm−1 (ε/M−1 cm−1) and their solvent parameter values.
Solvent
DN
0.0
2.7
4.4
11.9
14.1
16.1
17.0
20.0
22.9
23.3
26.6
29.8
33.1
38.8
AN
ˇ
˛
Complex 1
Complex 2
Complex 3
Complex 4
DCM
NM
NB
BN
AN
20.40
20.50
14.80
15.50
18.90
16.00
12.50
8.00
37.10
41.30
16.00
19.30
14.20
10.86
0.10
0.06
0.30
0.37
0.40
0.39
0.43
0.55
0.75
0.66
0.69
0.76
0.64
1.05
0.13
0.22
0.00
0.00
0.19
0.00
0.08
0.00
0.86
0.98
0.00
0.00
0.00
0.00
18.71(107)
18.26(107)
17.82(103)
17.36(105)
17.17(119)
17.30(120)
17.31(103)
17.19(110)
16.99(106)
16.80(112)
16.45(105)
16.09(115)
15.57(110)
15.38(117)
18.35(91)
17.83(101)
17.42(92)
17.83(101)
18.15(110)
17.95(106)
17.39(100)
17.27(115)
17.04(121)
17.15(108)
17.33(111)
16.58(111)
16.86(113)
16.26(103)
16.26(124)
15.50(106)
15.31(119)
18.08(116)
17.33(106)
18.21(123)
17.33(103)
17.15(119)
17.09(115)
16.98(113)
17.27(109)
16.64(118)
16.81(122)
16.42(110)
16.16(118)
15.48(119)
15.50(120)
16.81(108)
16.67(105)
16.84(106)
16.75(102)
17.21(99)
16.37(116)
16.36(109)
16.00(108)
15.90(111)
15.46(115)
15.36(124)
PN
Ac
THF
EtOH
MeOH
DMF
DMSO
Py
HMPA
The solvent effect on the absorption spectra of four copper(II)
complexes were recorded in the range of 12,000–25,000 cm−1
using visible spectroscopy. Thus four equations were obtained for
the copper(II) complexes 1–4 by applying multiple linear regres-
sion (MLR) method using SPSS software [31]. The role of the
solvatochromic parameters, such as ˛, ˇ, DN and AN in the above
equations have been studied to investigate the mechanism of the
interactions between complexes and solvents [18,32,33].
the same and the only differences are due to presence of different
counter ions. Two intense absorption bands in the region around
1520–1585 cm−1 in the complexes could be assigned to the C
C
and C O vibrational modes [34–37]. The N–H stretching vibration
of the diamine ligand appears in the 3200 cm−1 region. The vibra-
tional bands around 1040 cm−1 are corresponded to the stretching
vibration of C–N bond and the bands around 1450 cm−1 are asso-
ciated to the scissoring vibration of –CH2– groups [38]. The strong
bands around 1380 cm−1 are associated to the bending vibrations of
CH3 groups. The two strong bands at 735 and 706 cm−1 in complex
1 are corresponded to the phenyl group of the tetraphenylborate
ion [39]. These two bands also appear in other complexes with low
intensity due to the presence of phenyl group in the diamine ligand.
The vibrational bands at 854 and 564 cm−1 which are characteristic
of PF6− anion confirms the formation of complex 2 [35]. Complexes
1 and 2 also show sharp bands around 3550 cm−1 region that are
associated to the presenc−e of water molecule in the complexes
[40]. The presence of ClO4 ions in complex 3 is confirmed by two
intense bands at 1091 and 625 cm−1 which are related to the anti-
symmetric stretching and anti-symmetric bending vibration modes
of this group [38]. A strong broad band in the 10−83–1038 cm−1
region in complex 4 approves the presence of BF4 anion in the
complex [35].
3. Results and Discussion
The complexes 1,
2 and 4 were prepared by mixing of
Cu(OAc)2·H2O, diamine, acetylacetone and Na2CO3 with molar
ratio of 1:1:1:0.5, respectively in absolute ethanol or methanol
and then addition of a saturated solution of appropriate counter
ion salt. However, complex 3 was synthesized with direct use of
Cu(ClO4)·6H2O instead of Cu(OAc)2·H2O in an ethanol–water mix-
ture. The undesired by-product of [Cu(acac)2] was formed during
the preparation of the complexes which was isolated and then sepa-
rated simply by filtration. Analytical data of the obtained complexes
indicated the formation of the desired mixed-chelate copper(II)
complexes. The establishment of the purity of compound 2 to an
acceptable level was hindered by hygroscopic nature of compound.
3.1. IR spectra
3.2. Conductometric data
Formation of the mixed-chelate copper(II) complexes can be
concluded from IR spectroscopy. The observed bands can be cat-
egorized into those corresponded to the ligands, counter ions and
also the bonds formed between copper(II) and coordinating sites
of the ligands [1]. The similarity among the infrared spectra of the
complexes might imply that the structures of all complexes are
Table 1 shows the molar conductivity values of complexes 1–4
and the standard values for 1:1 electrolytes in different solvents
[41]. The results exhibited that the complexes are 1:1 electrolyte
in all solvents except in solvent of dichloromethane. However, the
results reveal that in solvent of dichloromethane the conductance
values for all complexes are much lower than those expected for
Table 3
Selected models of backward MLR method and related values of F, R, S.E. and ꢀꢁmax
.
Complexes
Equations
F
R
S.E.
ꢀꢁmax
1
2
3
4
ꢁmax/103 = −0.079DNsolv + 18.502
ꢁmax/103 = −0.071DNsolv + 17.979
ꢁmax/103 = −0.071DNsolv + 18.251
ꢁmax/103 = −0.066DNsolv + 18.123
231.17
115.77
141.57
89.55
0.97
0.95
0.97
0.94
0.21
0.27
0.25
0.29
3326
2988
2835
2736

28
H. Golchoubian et al. / Spectrochimica Acta Part A 85 (2012) 25–30
Table 4
P(Xi) values of complexes 1–4.
Complexes
1
2
3
4
P(DN)
P(AN)
P(ˇ)
73.12%
12.2%
9.06%
5.60%
69.72%
19.89%
10.33%
0.045%
66.14%
16.50%
13.09%
2.45%
59.83%
6.50%
31.66%
1.98%
P(˛)
1:1 electrolytes. Thus, it seems that an ion-pair formation (or anion
coordination) might exist in some extent in dichloromethane. Addi-
tionally, the conductivity values in complex 1 are less than standar−d
value for 1:1 electrolytes due to the low ionic mobility of BPh4
anion [41]. These results demonstrate the existence of weak inter-
actions between the copper centers and counter ions that are driven
out by high polar solvent molecules, which leads to 1:1 electrolytes
[42,43] as well as solvatochromism.
3.3. Solvatochromism
The complexes 1–4 are soluble in a wide range of organic sol-
vents and demonstrate solvatochromic properties. The electronic
absorption spectra of the mixed-chelate complexes are character-
ized by a broad structureless band in the visible region attributed to
the d–d transition of the copper(II) ions. As the electron configura-
tion of copper(II) ion is d9, the broad structureless absorption band
is associated to the transition of the electron from the lower energy
orbitals to the hole in dx
orbital. The visible spectral changes of
2
2
−y
these complexes in some selected solvents are illustrated in Fig. 1.
All of the complexes show the solvatochromic behavior and the
shifts induced in certain solvents depends upon the nature of the
counter ions. The position of the ꢁmax of the complexes along with
the molar absorptivity and their respective solvent parameter val-
ues are collected in Table 2.
To explore the solvent effects on the absorption spectra of the
complexes, the absorption frequencies (ꢁmax) were correlated with
the solvatochromic Eq. (1). Hence, The frequencies of the d–d
absorption transition band (ꢁmax) of each complexes in various sol-
vents with their own solvent parameters shown in Table 2 were
offered in Eq. (3) one by one to the statistical computer program,
being accepted, rejected, or exchanged until certain statistical crite-
ria are met. The solvent parameters used include Gutmann’s donor
DN and acceptor numbers AN, electron pair donating ability ˇ and
hydrogen bonding ability ˛.
vmax = vmax◦ + a DN + b AN + c ˇ + d ˛
(3)
The results obtained from the correlation of the absorption fre-
quencies with the solvent parameters are illustrated in Table 3. The
data in Table 3 show that dominate solvent effect in all complexes is
DN and contribution of the other solvent parameters were rejected
based on the statistical criteria explained in the experimental sec-
tion.
Fig. 1. Absorption spectra of the complexes in selected solvents. Absorption spectra
in other solvents are omitted for clarity.
To determine the relative contribution percentage of DN and
other parameters on the shift of the ꢁmax values an Enter MLR
method was also applied. Thus, the DN and AN parameters were
normalized first using Marcus method [14]. The resulting normal-
ized parameters values along with ˛ and ˇ parameter were then
offered in Eq. (2) [30,33]. The calculated results are presented in
Table 4.
The results again confirm the dominate contribution of DN
parameter in the solvatochromism of the complexes. It is valuable
to note that although the percentage of ˇ parameter in Table 4 is
9–32% its contribution in solvatochromism is discarded as a result
of its high Sig. value (0.026–0.768, this value should be less than
0.005 to be accepted [44]).
A cross-validation methodology was also carried out for choos-
ing prediction power of the proposed model for all complexes.
The cross-validation methodology is essential since a model with
good statistics values necessarily do not have a good prediction
potential. Three cross-validation parameters, calculated for the
proposed model [45], are presented in Table 5. The parameters
in Table 5 are defined as: PRESS, as the predictive residual error
ꢀ
sum of squares, PRESS = (Ypred − Yexp)2; SSY as the total sum of
squares of deviation of the experimental values from their means,
ꢀ
SSY = (Yexp − Ymean
)
2 and Q2 as the cross validation squared coef-
ficient, Q2 = 1 − PRESS/SSY, where Ypred is the predicted, Yexp is the
experimental, and Ymean is the mean values of the target properties
(ꢁmax).

H. Golchoubian et al. / Spectrochimica Acta Part A 85 (2012) 25–30
29
Table 5
complex 1
The quality of the obtained models by means of Q2, PRESS and PRESS/SSY and pre-
dictive correlation coefficient (Rpre).
19
17
15
Compound
Q2
PRESS
0.56
PRESS/SSY
0.049
0.950
1
ꢁmax experimental = 1.004ꢁmax predicted − 0.077
Rpre = 0.975
0.91
0.89
0.093
2
3
4
ꢁmax experimental = 0.993ꢁmax predicted + 0.136
Rpre = 0.952
15
17
19
pred/103
ν
0.92
0.75
0.078
ꢁmax experimental = 1.009ꢁmax predicted − 0.164
Rpre = 0.961
complex 2
0.88
1.01
0.117
ꢁmax experimental = 1.004ꢁmax predicted − 0.068
19
17
15
Rpre = 0.939
As the results showed in Table 5, good cross-validation Q2 values
were obtained (Q2 > 0.75; Q2 = 0.88 − 0.95). PRESS is an important
cross-validation parameter accounting for a good estimate of the
real predictive error of the model. Its value less than SSY indicate
that the model predicts better than chance and can be consid-
ered statistically significant. To have a reasonable model, PRESS/SSY
should be less than 0.4. In our case, PRESS/SSY ranges between
0.117 and 0.094, indicating that the DN model for complexes is sig-
nificant. To confirm our finding, ꢁmax values predicted by Eq. (1) are
compared with the obtained ꢁmax values. Within the experimental
error, the values agree well with donor number of solvents. The
plots obtained from ꢁmax (experimental) against ꢁmax (predicted)
values are shown in Fig. 2.
15
17
19
pred/103
ν
complex 3
19
17
15
According to the results obtained, the DN parameter of the sol-
vent has a dominate contribution in Eq. (3) for compounds 1–4
and governs in the shift of the d–d absorption band of the com-
plexes. The negative sign of the coefficient a indicates a red shift
as the donor number of solvents increases. The red shift observed
in the d–d visible absorption band originates in variation of Lewis
acid–base interaction between the chelates around the copper ions
and the respective solvents molecules. Since approaching of the
polar solvent molecules to the axial position of the complexes
causes a strong repulsion between electron in the dz2 orbital of
the copper(II) ions and the electron pair of the solvents and hence
decreasing the required energy for transferring the electrons to
15
17
pred/103
19
ν
complex 4
19
17
15
dx
orbital. Accordingly, the position of this band decreases
2
15
17
pred /103
19
2
−y
nearly linearly with the increasing of the donor numbers of the sol-
vents. The plots of the observed ꢁmax values of the complexes versus
the donor number of the solvents as shown in Fig. 3 demonstrated
good correlation for all complexes.
ν
Fig. 2. The plots of the observed ꢁmax values against the calculated ꢁmax values for
complexes 1–4.
There is an identical trend in the visible absorption spectra
of the investigated complexes. The nature of counter ion X in all
investigated complexes is generally ineffective to the ꢁmax value in
solvents with high donor power. However in low DN solvents dif-
ferences in ꢁmax is larger among the complexes. It seems that as the
approach of the solvent molecules with different donor power to
the axial vacant sites of the complexes causes a change in the geom-
etry of the copper center from square planar to octahedron. This
conclusion is in agreement with our pervious results [10,46]. How-
ever, It was found that the ꢀꢁmax(= (ꢁmax
)
DCM − (ꢁmax
)HMPA) values
depend on the counter ion, X used in the complex and are in order
of BPh4− > PF6− > ClO4− > BF4−. The best results were obtained for
complex 1 indicating this fact that the size of counter ions are effec-
tive in the solvatochromism phenomenon. The bulkier counter ions
have looser binding to the Cu(II) center and can be driven out more
easily by solvent molecules cause the higher ꢀꢁmax value.
Fig. 3. Dependence of the ꢁmax values of complexes 1–4 on the solvent DN values.

30
H. Golchoubian et al. / Spectrochimica Acta Part A 85 (2012) 25–30
4. Conclusion
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Four new mixed-chelate copper(II) complexes with solva-
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examined with different solvent parameters models using MLR
computational method. The statistical evaluation of the data (R, S.E.,
F-test, Q2 and PRESS/SSY) indicated that DN model of the solvent
plays the most important role in the solvatochromic behavior of the
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the geometry of the complexes from square planar to octahe-
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counter ion X increase the ion pair interaction between cation com-
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Acknowledgement
We are grateful for the financial support of Mazandaran Univer-
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Appendix A. Supplementary data
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