Chromotropism of a new mixed-chelate copper(II) complex

Article abstract of DOI:10.1016/j.molstruc.2009.02.023

A new mixed-chelate copper(II) complex with the general formula [Cu(Cl-acac)(diamine)]ClO₄, where Cl-acac is 3-chloroacetylacetonate ion and diamine is N,N-dimethyl,N′-benzyl-1,2-diaminoethane, was prepared and characterized on the basis of ele

Full text of DOI:10.1016/j.molstruc.2009.02.023

Journal of Molecular Structure 927 (2009) 91–95
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Chromotropism of a new mixed-chelate copper(II) complex
*
Hamid Golchoubian , Ehsan Rezaee
Department of Chemistry, University of Mazandaran, P.O. Box 453, Babol-sar, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new mixed-chelate copper(II) complex with the general formula [Cu(Cl-acac)(diamine)]ClO4, where Cl-
acac is 3-chloroacetylacetonate ion and diamine is N,N-dimethyl,N0-benzyl-1,2-diaminoethane, was pre-
pared and characterized on the basis of elemental analysis, spectroscopic and conductance measure-
ments. The mixed-chelate complex is soluble in various solvents and presents an interesting
combination of solvato- and thermochromism. Its reversible chromotropism behavior is discussed in
detail.
Received 14 January 2009
Received in revised form 26 February 2009
Accepted 26 February 2009
Available online 10 March 2009
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
Mixed-chelate
Copper(II) complex
Solvatochromism
Thermochromism
Diamine ligand
1. Introduction
functions in stabilization of the mixed-chelate complexes [24].
Thus fine tuning of electronic and steric hindrance in the chelates
During the past decade, particular interest has been directed to-
wards the investigation of chromotropism phenomenon due to its
application in thermosensitive [1], imaging [2–5], photo-switching
[6–8], and sensor materials [9]. Chromotropism defines as a revers-
ible variation of the spectral properties of compound under differ-
ing physical or chemical conditions such as temperature
(thermochromism) and solvent (solvatochromism). Whereas there
are a large number of solvatochromic compounds and complexes
[10–12], the mixed-chelate complexes, especially those containing
copper(II) exhibited the most attention due to existence of simple
and regular changes in their electronic spectra according to the
strength of interaction with solvent molecules at the axial sites
and consequently, their potential use as molecular switch
[13–16], and pollutant sensor [17,18] and Lewis-acid-base color
indicator [19]. Among ligands examined in mixed-chelate com-
plexes b-diketonato and ethylenediamine derivatives find to illus-
trate the most effective combination to form stable complexes and
demonstrate a pronounce solvatochromism [20,21]. However, the
syntheses of mixed-chelate complexes are cumbersome since the
formation of homogenous bis-chelate complexes must be sup-
pressed [22]. It was establish that diamine ligand containing bulky
substituents on the nitrogen atoms prevents the stabilization of
bis-diamine complexes if another ligand is slim such as acetylace-
tonate or its derivatives [23]. On the other hand, electronic and
structural properties of the both chelates perform important
are crucial in preparation of stable solvatochromic complexes. In
this paper which is in continuation of our studies on the solvato-
chromic behavior of mixed-chelate copper(II) complexes [25,26]
we prepared
a new mixed-chelate complex abbreviated as
[Cu(Cl-acac)(diamine)]ClO4 shown in Scheme 1 and its solvato-
chromism and thermochromism behaviors was studied.
2. Experimental
2.1. Materials and methods
[Cu(Cl-acac)2] [27] were prepared according to published pro-
cedure. All solvents were spectral-grade and all other reagents
were used as received. All the samples were dried to constant
weight under a high vacuum prior to analysis. Caution: perchlorate
salts are potentially explosive and should be handled with appropri-
ate care.
Conductance measurements were made at 25 °C with a Jenway
400 conductance meter on 1.00 ꢀ 10ꢁ3 M samples in CH3CN and
CHCl3. Infrared spectra (potassium bromide disk) were recorded
using a Bruker FT-IR instrument. The electronic absorption spec-
tra were measured using a Braic2100 model UV–Vis spectropho-
tometer. 1H NMR and 13C NMR spectra were recorded on a Bruker
500 DRX Spectrometer. Mass spectra were obtained on a VG 70E
spectrometer. Elemental analyses were performed on a LECO 600
CHN elemental analyzer. Absolute metal percentages were
determined by a Varian-spectra A-30/40 atomic absorption-flame
spectrometer.
* Corresponding author. Tel./fax: +98 1125342350.
E-mail address: h.golchobian@umz.ac.ir (H. Golchoubian).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.02.023

92
H. Golchoubian, E. Rezaee / Journal of Molecular Structure 927 (2009) 91–95
2.2. Syntheses
KBr disk): 3470 (m, O–H str.), 3261 (m, N–H str.), 1456 (m, CH2–
Ph str.), 1089 (s, Cl–O str.), 695 (w, CH2 rock.), 612 (m, Cl–O bend.).
Anal. Calcd. for C22H38N4Cl2O10Cu2: C, 36.88; H, 5.35; N, 7.82; Cu,
17.74; Found: C, 37.04; H, 5.36; N, 7.78; Cu, 17.66%.
2.2.1. Preparation of N,N-dimethyl,N0-benzyl-1,2-diaminoethane
A typical procedure is as follow: a mixture of benzaldehyde
(30 mmol) and N,N-dimethyl-1,2-diaminoethane (30 mmol) in
ethanol (60 mL) was stirred for 24 h. Then the solvent was evapo-
rated under reduced pressure. The resultant yellow oil was then
dissolved in n-hexane (40 mL), washed with a minimal volume of
water, and dried over anhydrous Na2SO4. Concentration under re-
duced pressure gave diimine product as yellow oil. To the stirred
solution of yellow oil (3.5 g) in methanol (60 mL) at room temper-
ature was gradually added NaBH4 (30 mmol) over 0.5 h. The result-
ing mixture was allowed to stand overnight, and then was added
acetic acid (10 mL). The mixture was made alkaline by NaOH
(4 M) and was extracted with dichloromethane (5 ꢀ 10 mL). The
combined CH2Cl2 fractions were dried over anhydrous Na2SO4.
Evaporation of the solvent under reduced pressure resulted in
desired product as orange oil. The typical yields were 45–55%.
3. Results and discussion
The diamine ligand was prepared by condensation of an equi-
molar of N,N-dimethyl-1,2-diaminoethane and benzaldehyde and
further reduction of diimine by sodium borohydride. The mixed-
chelate complexes of copper(II) was synthesized with mixing of
Cu(ClO4)ꢂ6H2O, 3-chloroacetylacetone, Na2CO3 anhydrous and
N,N-dimethyl,N0-benzyl-1,2-diaminoethane with molar ratios of
1:1:0.5:1, respectively, in ethanol–water mixture. The homo bis-
chelate copper(II) complexes were also prepared for comparison
proposes.
3.1. IR spectra
Selected IR data (m
/cmꢁ1 using KBr): 3370 (m, N–H str.), 2810
Several bands appear in the IR spectrum of mixed-chelate com-
plex in the region that is also observed, although with minor shifts,
in the spectra of the free ligands. The absorption band around
1040 cmꢁ1 is probably due to the stretching vibration of carbon–
nitrogen bond [28]. The strong bands at 1405 cmꢁ1 are very likely
associated with the scissoring vibration of –CH2– groups [28]. The
band at around 760 cmꢁ1 that in the spectra of free diamine ligand
appears broader and is split in two in region of 730–790 cmꢁ1 may
be due to the rocking vibration of CH2 groups [28]. The stretching
vibrations of N–CH2 groups in the free ligands, and the well-known
bands in the region 2850 100 cmꢁ1 that are associated with it, are
more important since they serve as an indication of coordination of
diamine ligand. Upon covalent bond formation, these absorption
bands apparently lose intensity, become shifted to the higher fre-
quencies and mix with other C–H absorption bands. Dependence
on coordination is also exhibited by the intense and narrow band
occurring at 3240 30 cmꢁ1 which is associated with N–H vibra-
tion and is observed at around 3370 cmꢁ1 and broader in the free
diamine ligands. As the lone pair of electrons of the donor nitrogen
atoms become involved in the metal–ligand bond, the transfer of
electron density to the metal and the subsequent polarization of
the ligands involves electron depopulation of the N–H bond, which
culminates in a shift to lower frequencies. Coordination of acetyl-
acetonate chelate can be concluded from IR spectroscopy so that
the C@O stretching vibration of the free 3-chloroacetylacetone
where observed in the 1700 cmꢁ1 was shifted to the lower wave
number in complex spectrum indicating the coordination of 3-
chloroacetylacetone to the copper ion. The presence of ClOꢁ4 group
is confirmed by two intense bands at around 1110 and 610 cmꢁ1
which are attributed to the anti-symmetric stretching and anti-
symmetric bending vibration modes, respectively [28]. Poorly
splitting of the former band is resulted from coordination of ClO4ꢁ
to the copper ion and distortion from Td symmetry. It is well
known that the degree of splitting of this band serves as a measure
of the degree and mode of the coordination of perchlorate ions to
the copper ion [29]. Infrared spectra of the bis-chelates of [Cu2(dia-
mine)2(OH)2]2+ and [Cu(Cl-acac)2] complexes as well as the mixed-
chelate complex are illustrated in Fig. 1 for comparison. These
spectra clearly indicated formation of desired mixed-chelate
complex.
(s, C–H str. aliphatic), 1475 (s, Ph–O str.), 680 (m, C–CH3 + ring
def. + Cu–O). 1H NMR (500 MHz, CDCl3), d: 2.19 (s, 6H, CH3); 2.42
(t, J = 6.0 Hz, 2H, CH2); 2.69 (t, J = 6.0 Hz, H, CH2); 3.80 (s, 2H,
CH2–Ph); 7.23–7.32 (m, 5H, Ar–H). Mass spectra for C11H18N2
(electron impact, 70 eV): m/z 58 (MW – C8H11 N), m/z 91 (MW –
C4H11N2), m/z 120 (MW – C3H8 N).
2.2.2. Preparation of mixed-chelate complexes [Cu(Cl-acac)(diamine)]
(ClO4)
To the solution of N,N-dimethyl,N0-benzyl-1,2-diaminoethane
(6 mmol, 1 g), 3-chloroacetylacetone (6 mmol, 0.7 mL), Na2CO3
(3 mmol, 0.3 g) in ethanol (30 mL) were slowly added
Cu(ClO4)2ꢂ6H2O (6 mmol, 2.2 g) in water (15 mL). After few min-
utes homo bis-chelate of [Cu(Cl-acac)2] precipitated as green solid
which was separated off by filtration. The resultant clear blue solu-
tion was allowed to stand overnight at room temperature. After
concentration at room temperature, blue solid precipitated that
was collected with filtration and washed with water and diethyl-
ether. The yield was 40%. Selected IR data (m
/cmꢁ1 KBr disk):
3320 (m, N–H str.), 1580 (s, C@O str.), 1405 (s, –CH2– bend.),
1090 (s, Cl–O str.), 620 (m, Cl–O bend.). Anal. Calcd. for
C16H24N2Cl2O6Cu: C, 40.47; H, 5.09; N, 5.90; Cu, 13.38; Found: C,
40.68; H, 4.78; N, 5.88; Cu, 13.44%. Molar conductance (Dm
X
/
ꢁ1 cm2 molꢁ1): 115.0 in ACN; 0.0 in chloroform.
2.2.3. Preparation bis-chelate complexes [Cu2(diamine)2(OH)2](ClO4)2
To the solution of desired diamine (10 mmol) in ethanol (30 mL)
were slowly added Cu(ClO4)2ꢂ6H2O (5 mmol) in ethanol (10 mL).
The resultant mixture was stirred for 2 h at room temperature.
The desired compound precipitated from the reaction mixture as
violet solid. The typical yields were 80%. Selected IR data (m
/cmꢁ1
Me
Me
O
N
Cu
Cl
NH
O
ClO4
3.2. Conductometric data
The molar conductivity values of the mixed-chelate complex in
solvents of chloroform and acetonitrile have been studied. The re-
sults show that the complex is 1:1 electrolyte in acetonitrile. How-
ever, in solvent of chloroform the value is fairly lower than that for
Scheme 1.

H. Golchoubian, E. Rezaee / Journal of Molecular Structure 927 (2009) 91–95
93
Cu2+
N
C-CH3
Cu2+
C-CH3
N
Scheme 2.
0.8
0.6
0.4
0.2
0
MeNO2
Py
acetone
MeOH
DMSO
12
14
16
18
20
22
ν/103 cm-1
Fig. 2. Absorption spectra of the mixed-chelate complex (5.0 ꢀ 10ꢁ3 mol dmꢁ3) in
selected solvents. Absorption spectra in other solvents are omitted for clarity.
donor numbers (DN). Table 1 shows mmax and
e values for the
mixed-chelate complex in some solvents.
The donor number expresses a measure of coordinating ability
of solvent on the standard of that of dichloromethane (DCM)
[33]. The mixed-chelate complex is strongly solvatochromic. The
absorption spectra of the compound shows a broad structureless
band (envelop) attributed to the promotions of the electrons in
the lower energy orbitals to the hole in dx2
orbital of the cop-
ꢁy2
per(II) ion (d9). Since the dz2 , dxz and dyz orbitals of the Cu(II) ion
in a mixed-chelate will all be lifted up by its interaction with polar
solvent molecules approaching from above and below the chelate
plane, the broad d–d transition band will be shifted to the lower
energy and demonstrates a positive solvatochromic. That means
the mmax value of this band gradually shifts to the red as the DN
of the solvent increases. An obvious exception from this general
trend is found in the mmax value in acetonitrile. The donor number
of this solvent (14.1) is logically lower than that of acetone (17.0),
so that the mmax in this solvent should be higher than that in ace-
tone. In fact, this might possibly be due to the linear shape of MeCN
molecule, and its head-on mode of coordination with Cu–N–C an-
gle of 180° which facilitates the approach of MeCN to the copper
ion with high steric hindrance. As a result, this makes MeCN an
apparently more basic species than acetone in certain case. How-
ever, it may be due to another effect, i.e. the fact that the copper
Fig. 1. Infrared spectra of [Cu2(diamine)2(OH)2](ClO4)2, [Cu(Cl-acac)(diamine)]ClO4
and [Cu(Cl-acac)2] complexes.
1:1 electrolyte [30]. Thus, it seems that an ion-pair formation (or
anion coordination) might exist in some extent in chloroform. That
means ClOꢁ4 ions are bound weakly above and below of the chelate
planes and can be driven out by high donor power solvent mole-
cules which leading to their solvatochromism. These results are
in general agreement with the expectation from the spectral data
and with our pervious results [31,32].
ion and MeCN are rather soft and MeCN can operate as a
tor ligand as shown in Scheme 2. Even small contribution from this
type of bonding makes the -bond synergetically stronger, and the
p-accep-
r
ligand field strength of MeCN remarkably higher [18].
The spectral changes of the mixed-chelate complex in some sol-
vents are illustrated in Fig. 2 as an example of its solvatochromism.
The range of kmax variation for the mixed-chelate complex in differ-
ent solvents is about 3.36 ꢀ 103 cmꢁ1 and this wide range might be
3.3. Solvatochromism
The electronic absorption spectra of the mixed-chelate complex
were measured in solution in some organic solvents with different
Table 1
Electronic spectra of the complex in various solvents: kmax/nm (
e
/Mꢁ1 cmꢁ1).
Acetone
Solvent
DN
DCM
0.0
MeNO2
2.7
ACN
14.1
THF
20.0
MeOH
23.5
DMF
26.6
DMSO
29.8
Py
33.1
HMPA
38.8
17.0
m
max/103 cmꢁ1
/Mꢁ1 cmꢁ1
17.65
(117)
18.35
(116)
17.03
(132)
17.18
(151)
17.00
(115)
16.78
(121)
16.45
(122)
16.07
(116)
14.99
(116)
15.43
(110)
(e
)

94
H. Golchoubian, E. Rezaee / Journal of Molecular Structure 927 (2009) 91–95
19
18
17
16
15
0
5
10
15
20
25
30
35
40
DN
Fig. 3. Dependence of the mmax values of mixed-chelate complex on the solvent
donor number value; R2 = 0.97, y = 18.464 ꢁ 778 ꢀ 103.
Fig. 5. Spectral changes of mixed-chelate complex (5.0 ꢀ 10ꢁ3 mol dmꢁ3) in solvent
of pyridine at 80 °C after (a) 0 min, (b) 5 min, (c) 15 min, and (d) after several days
and cooling down to the room temperature. The inset is the charge transfer region
of the complex (2.0 ꢀ 10ꢁ4 mol dmꢁ3) in the same time scale.
0.6
with the solvent donor number (DN) yields the following expres-
sion. Thus the solvatochromic behavior the complex can be studied
quantitatively.
0.4
d
mmax;Solv ¼ 18:46 ꢁ 778 ꢀ 103DNsolvent ðr2 ¼ 0:97Þ
DNsolvent ¼ 237:33 ꢁ 12:853mmax;Solv
c
0.2
The mixed-chelate complex exhibits strongly pronounced ther-
mochromism. The nitromethane solution of the complex gradually
changes color from violet to the green upon heating (Fig. 4). In the
course of these spectral changes, no distinct isosbestic point was
observed. Thus, it suggests that this phenomenon is not due to
the simple salvation equilibrium of type (A) presented in Scheme
3. Although the exact mechanism of this thermochromism is still
unknown, it seems the origin of this behavior lies in the reversible
endothermic reaction (B) given in Scheme 3.
b
a
0
12
14
16
18
20
22
/103 cm-1
ν
Fig. 4. Spectral changes of mixed-chelate complex (5.0 ꢀ 10ꢁ3 mol dmꢁ3) in solvent
of nitromethane at 80 °C after (a) 0 min, (b) 5 min, (c) 15 min, and (d) 35 min.
Increasing the temperature in solvent of nitromethane the shift
in d–d band indicates change in the coordination geometry around
the copper ion. It also indicates that the solvent molecules coordi-
nate to the copper ion replacing the diamine chelate and thus the
ligand field around the metal ion alters that resulted in the shift of
absorption maximum. However, this process is reversible and after
cooling the solution to the room temperature the diamine chelate
presence in the solution recoordinates to the copper ion and causes
the reappearance of original violet color. The reverse process is
fairly slower than the forward reaction and takes several weeks
for full development of the original color. The drop in the rate of
the reverse process could be due to the presence of a large amount
of solvent molecules in the solution which hinders the recoordina-
tion of the diamine. It is interesting to note that when a large ex-
cess of the diamine chelate (ca. an equimolar of solvent) is added
to the green solution of [Cu(x-acac)(solv)4]+ the maximum absorp-
tion changes faster so that it takes several days for reappearance of
its original color.
due to presence of a large steric hindrance in complex. It is known
that the increase of steric hindrance of the axial coordination of
incoming solvent molecules tends to make the complex more sol-
vatochromic, bringing about a large structural change with pro-
gressive solvation of the complex caused by the increase of DN.
Deviations were also observed in the solvents of dichlorometh-
ane and pyridine. This anomaly was ascribed to the formation of
ion pairs by mean of an axial coordination of ClOꢁ4 in CH2Cl2. As
the relative dielectric constant of nitromethane takes a much high-
er value (28.5 for nitromethane versus 8.9 for CH2Cl2 in room tem-
perature), this solvent facilitates the dissociation of cationic
chelate. Thus reducing the axial interactions of the ClOꢁ4 and elim-
inating the tetragonal distortion [34]. In solvent of pyridine the
absorption maximum occur at lower energy than HMPA although
the donor number of former is lower than latter. This inconsistency
in the absorption maxima may be due to planar shape of pyridine
molecule which facilitates its approach to the copper ion with high
steric impediment [35].
Interestingly the complex is also thermochromic in solvents of
pyridine, dimethyl sulfoxide and acetonitrile. In solvent of
pyridine, the original green color turns orange–yellow after
heating for few minutes (Fig. 5). Upon heating the d–d band of
the complex loses its intensity and the CT band located in the
Regression analysis of band maxima of the mixed-chelate com-
plex against donor number of solvents is shown in Fig. 3 and indi-
cates good correlation and also confirms the solvatochromic
behavior of the complex. A linear correlation of the mmax values
[Cu(Cl-acac)(diamine)]ClO4 + 2(solvent)
[Cu(Cl-acac)(diamine)]+ +4(solvent)
[Cu(Cl-acac)(diamine)(solvent) 2]ClO4 (A)
[Cu(Cl-acac)(solvent)4]+ + diamine
(B)
Scheme 3.

H. Golchoubian, E. Rezaee / Journal of Molecular Structure 927 (2009) 91–95
95
region 29–32 ꢀ 103 show bathochromic shift with an increase in
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4. Conclusion
The prepared mixed-chelate complex demonstrated high solu-
bility in a variety of organic solvents and illustrates noticeable
chromotropism. Additionally, it shows good correlation between
the d–d absorption maxima in the solution and donor strength of
the solvent used when DN lies between 3 and 30. However, incon-
sistencies appear in very weak or very strong donor solvents. As it
can be envisioned from Table 1 the kmax value of the complex de-
clines with the increase of DN in going from nitromethane to pyr-
idine, but when the basicity turns into smaller it decreases
remarkably again. Such behavior also observed in the solvents with
high basicity, as the DN of the solvent increases the mmax value in-
creases. The visualized reversible thermochromic property of the
complex seems to be on account of substitution of solvent mole-
cules with the coordinated diamine chelate and vice versa.
Acknowledgements
We are grateful for the financial support of Mazandaran Univer-
sity of the Islamic Republic of Iran.
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