Substituent and solvent effects in the spectra of new mixed-chelate copper (II) complexes containing N,N′-disubstituted ethylenediimine and acetylacetonate ligands

Article abstract of DOI:10.1016/j.molstruc.2005.10.047

Four mixed-chelate copper (II) complexes containing acetylacetonate ion and various diimine ligand were prepared and characterized by elemental analysis, spectroscopic and molar conductance measurements. These complexes are fairly soluble in various solve

Full text of DOI:10.1016/j.molstruc.2005.10.047

Journal of Molecular Structure 787 (2006) 167–171
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Substituent and solvent effects in the spectra of new mixed-chelate
copper (II) complexes containing N,N0-disubstituted
ethylenediimine and acetylacetonate ligands
*
Elaheh Movahedi, Hamid Golchoubian
Department of Chemistry, University of Mazandaran, P.O. Box 453, Babol-sar, Iran
Received 27 July 2005; received in revised form 24 October 2005; accepted 28 October 2005
Available online 15 December 2005
Abstract
Four mixed-chelate copper (II) complexes containing acetylacetonate ion and various diimine ligand were prepared and characterized by
elemental analysis, spectroscopic and molar conductance measurements. These complexes are fairly soluble in various solvents and show positive
solvatochromism. The examinations of the electronic properties of the groups attached to the diimine chelate on the solvatochromic behavior of
these complexes were investigated.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Mixed-chelate: copper(II) complexes; Solvatochromism; N-ligand; Diimine chelate; Acetylacetonate
1. Introduction
attached to the acac moiety in order to study the electronic
properties, that imposed by these ligands on the copper ion
[16–20]. Another focus of attention was to study the influence
of the steric hindrance to the axial salvation caused by N-alkyl
ethylenediamine derivatives [21–25].
The effect of the solvent on the spectral properties of
molecules, generally referred as solvatochromism, has been
investigated for many years, generating a copious literature [1].
Solvatochromism of metal complexes can be divided into two
types [2,3]; the first type comprises the case where the color
changes are brought about by the direct attachment of solvent
molecules onto metal center, and the second type is due to the
attachment of solvent molecules onto ligands. Among the
former whose color changes are due to those of d–d transitions,
copper(II) complexes with a strong Jahn-Teller effect can be
anticipated to show simple and regular changes in their
electronic spectra according to the strength of interactions with
solvent molecules at the axial sites. In this regard extensive
studies have been concentrated on the mixed-chelate
complexes of copper(II) with acac and diamine derivatives
(acac, stands for acetylacetonate ion) [4–8]. Some of these
complexes were synthesized in order to better understand the
role of the counter ion in [Cu(acac)(diamine)]X (X is an
univalent anion such as ClOK4 , NOK3 and etc.) [9–15]. At other
times, these complexes were made with various substituents
In this paper, which is a continuation of our studies on the
solvatochromic behavior of mixed-chelate copper(II)
complexes we intend to study the electronic properties imposed
by the N–N diimine chelate on the copper ion. Based on our
knowledge such investigation has not been carried out on the
diimine mixed-chelate complex. Thus a series of mixed-chelate
copper(II) complexes, that shown in Scheme 1 where X is
substituents with different electronic properties were syn-
thesized and their solvatochromic behavior were investigated.
This design removes the steric hindrance along the z-axis of the
molecules and let us to investigate the effect of the electronic
properties imposed by different diimine chelate on the
solvatochromism behavior of these new copper complexes.
2. Experimental
2.1. Materials and methods
*
Corresponding author. Tel./fax: C98 1125242002.
E-mail address: h.golchobian@umz.ac.ir (H. Golchoubian).
Diimine compounds [26] and [Cu(acac)2] [27] were
prepared according to published procedure. 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
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.10.047

168
E. Movahedi, H. Golchoubian / Journal of Molecular Structure 787 (2006) 167–171
The resulting mixture was stirred for 2 h at room temperature.
The obtained violet crystals were collected by filtration. The
typical yields were 80%.
+
X
X
O
O
N
N
3. Results and discussion
Cu
The mixed-ligand copper(II) chelates were obtained with
mixing of Cu(ClO4)$6H2O, acacH, and the diimine with molar
ratios of 1:1:1:, respectively, in ethanol–water mixture. The
typical yields were about 50%. The color, yields and the
analytical data of the obtained complexes are shown in Table 1
and indicated the formation of the mixed-chelate copper(II)
complexes. The obtained results show that using Na2CO3 as a
base which was used in mixed-chelate complexes [14,15]
reduces the yield of the desired mixed-chelate complexes and
increases the formation of bis-chelate complexes of [Cu(dii-
mine)]2C and [Cu(acac)2]. These complexes are hygroscopic.
X = H, CH3, Cl, Br
Scheme 1.
prior to analysis. Caution: perchlorate salts are potentially
explosive and should be handled with appropriate care.
Conductance measurements were made at 25 8C with a
Jenway 400 conductance meter on 1.00!10K3 M samples in
CH3CN and ClCH2CH2Cl. Infrared spectra (potassium
bromide disk) were recorded using a Bruker FT-IR instrument.
The electronic absorption spectra were measured using a
Braic2100 model UV–vis spectrophotometer. 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.
3.1. IR spectra
Formation of the mixed-chelate copper(II) complexes can
be concluded from IR spectroscopy so that the CaN stretching
vibrations of the free diimine compounds where observed in
the 1635–1645 range were shifted to lower wave numbers in
complexes spectra indicating the coordination of diimine
compounds to the copper ion.
Formation of a six-membered chelate ring comprising
the copper(II) and the oxygen atoms of the acac ligand is
disclosed by two intense absorption bands in the region 1520–
1590 cmK1 that can be assigned to the CaC and CaO
vibrational modes [29–32].
2.2. General procedure for preparation of copper complexes
2.2.1. Mixed-chelate complexes [Cu(acac)(Diimine)]ClO4
To the desired diimine (5 mmol) and acetylacetone (5 mmol),
in ethanol (20 mL) were slowly added Cu(ClO4)2$6H2O (5 mmol)
in water (10 mL). The resulting blue–green mixture was stirred for
2 h at room temperature. The solution was then taken to dryness
under reduced pressure. The blue–green solid was collected and
was washed with ethyl acetate (2!10 mL) to remove the impurity
of bis-chelate complexes. The resulted green solution was
concentrated to dryness and the residue was collected, washed
with Et2O (2!5 mL), and hexane (2!5 mL) and was dried under
reduced pressure. The green solid was recrystallized from ethanol:
diethyl ether (1:1). The yields were 44–67%
The presence of the ClOK4 group is declared by two intense
bands at around 1110 and 620 cmK1 which are attributed to the
anti-symmetric stretching and anti-symmetric bending
vibration modes, respectively [16]. The former band at
1110 cmK1 of compound I–V, not withstanding other groups
that also absorb in the same region, is split with a poorly
defined maximum showing the deformation from Td symmetry.
It is well known that the degree of splitting of the band at
1100 cmK1 serves as a measure of the degree and mode of the
coordination of perchlorate ions to the copper ion [18,26].
Infrared spectra of the bis-chelate ([Cu(diimine)]2C and
[Cu(acac)2]) and mixed-chelate complexes are shown in Fig. 1
for comparison. These spectra clearly indicated formation of
desired mixed-chelate complexes.
2.2.2. Bis-chelate complexes [Cu(diimine)2](ClO4)2
To the desired diimine (10 mmol), in ethanol (30 mL) were
slowly added Cu(ClO4)2$6H2O (5 mmol) in ethanol (10 mL).
Table 1
Color, yields and analytical data of the complexes obtained
No.
Formula
X
Analysisa
(Color/yield)
C%
H%
N%
Cu%
I
[Cu(acac)(DiimH)]ClO4 (green/44%)
[Cu(acac)(DiimM)]ClO4 (green/67%)
[Cu(acac)(DiimCl)]ClO4 (green/47%)
[Cu(acac)(DiimBr)]ClO4 (green/48%)
H
50.30 (50.61)
52.78 (52.47)
44.34 (44.46)
38.50 (38.44)
4.94 (4.65)
5.47 (5.17)
3.64 (3.73)
3.50 (3.23)
5.70 (5.62)
5.39 (5.32)
4.66 (4.94)
4.32 (4.27)
12.56 (12.75)
12.18 (12.07)
11.01 (11.20)
9.48 (9.68)
II
CH3
Cl
III
IV
Br
a
Calculated values are in parentheses.

E. Movahedi, H. Golchoubian / Journal of Molecular Structure 787 (2006) 167–171
169
different donor number (DN). The donor number expresses a
measure of coordinating ability of solvent on the standard of
that of dichloroethane (DCE) [28]. Values of DN of the
solvents used are as follows: DCEZ0.0; MeNO2Z2.7;
MeCNZ14.1; DioxaneZ14.8; acetoneZ17.0; MeOHZ19.0;
DMFZ26.6; DMSOZ29.8. The absorption spectra of
compounds I–IV show one broad band attributed to the
promotion of the electron in the low energy orbitals to the hole
in dx2Ky2 orbital of the copper(II) ion (d9). The position of this
band is shifter to the lower wave number as the DN of the
solvent increases. In Fig. 2 the spectral changes of
[Cu(acac)(DiimBr)]ClO4 in various solvents are illustrated as
an example of this solvatochromism. This shift in maximum
absorption depends on the nature of the substituent groups
attached in diimine moiety which controls the magnitude of in-
plane ligand field strength, the strength of the axial bonds
formed between the central copper(II) ion and solvent
molecules and finally the species (counter ion) neutralizing
the charge of the [Cu(acac)(Diimine)]C.
Assuming that the approach of the solvent occurs along the
z-axis, the solvent molecules are repelled by the two electrons
in the dz2 orbital of copper(II) and only the more basic species
can force their way in to form relatively strong bonds. Upon
coordination, they exert a z component field proportional to
their position in the spectrochemical series. Thus while
solvents of low basicity (low donor number) have less effect
in the band maxima, other that are considerably more basic
induce large red shifts.
Fig. 1. Infrared spectra of [Cu(DiimH)2](ClO4)2, (a); [Cu(acac)(DiimH)]ClO4,
(b); [Cu(acac)2], (c) complexes.
As evident from Fig. 3 and Table 3 the nmax values in one
particular solvent decreases in the following order of diimie
chelate.
3.2. Conductometric data
Table 2 illustrates the molar conductivity values of
complexes I–IV in solvents of ClCH2CH2Cl and acetonitrile.
The results show that in acetonitrile all of the four complexes
are 1:1 electrolyte [33]. However, in solvent of 1,2-
dichloroethane the values are much lower than that for 1:1
electrolyte. Thus, it seems that an ion-pair formation (or anion
coordination) might exist in some extent in ClCH2CH2Cl. That
means ClOK4 ions are bound weakly above and below of the
chelate planes and can be driven out by high donor power
solvent molecules which leading to their solvatochromism.
These results are in general agreement with the expectation
from the spectral data and with our pervious results [26].
DiimMeODiimHODiimClwDiimBr
This can be explained with the nature of the electron
properties of the substituent on the diimine. The two methyl
groups make the DiimM chelate more negative with their
electron-releasing effect. So that its ligand field strength
become higher, the coordination sphere around Cu(II) turns
out to be more electron-rich, and approach of solvents
molecule from axial directions become more difficult, as
0.6
0.5
0.4
3.3. Solvatochromism
The electronic absorption spectra of complexes I–IV were
measured in the solution state in some organic solvents with
0.3
DMSO
Table 2
Molar conductivities data (Lm) of the complexes (UK1 cm2 molK1, at 25 8C) in
1,2-dichloroethane and acetonitrile
0.2
ace tone
CH CN
3
MeNO2
0.1
Complexes
I
II
10.5
151.1
III
6.5
144.7
IV
6.1
151.2
0.0
12.00
ClCH2CH2Cl
CH3CN
8.1
157.8
14.00
16.00
18.00
20.00
22.00
ν / 103cm–1
Concentration: ca.1!10K3 M. Standard values for a 1:1 electrolyte type in
ClCH2CH2Cl and CH3CN are in the range of 10–20 and 120–160 (UK1 cm2 molK1
at 25 8C), respectively. From Ref. [33].
,
Fig. 2. Absorption spectra of [Cu(acac)(DiimBr)]ClO4 in selected solvents.
Absorption spectra in other solvents are omitted for clarity.

170
E. Movahedi, H. Golchoubian / Journal of Molecular Structure 787 (2006) 167–171
ν /103cm–1
13.4 13.7 14.0 14.3 14.6 14.9 15.2 15.5 15.8 16.1 16.4 16.7 17.0
h
g
f
e
d c
a
b
II
I
III
IV
Fig. 3. nmax values of complexes I–IV in various solvents. The symbols ‘a’ to ‘h’ represent: aZCH2Cl2, bZMeNO2, cZCH3CN, dZdioxane, eZacetone, THF, fZ
MeOH gZDMF, hZDMSO.
Table 3
Electronic spectra of the complexes in various solvents: nmax/!103 cmK1 (3/MK1 cmK1
)
Complexesa
nmax/!103 cmK1 (3max
)
DMSO
DMF
MeOH
Acetone
DIOX
CH3CN
MeNO2
DCE
I
13.87(103)
14.02(109)
13.52(121)
13.70(89)
14.35(92)
14.45(109)
14.10(103)
14.43(92)
15.44(90)
14.96(89)
15.12(104)
15.18(84)
14.62(89)
15.71(92)
15.37(89)
15.47(91)
15.92(94)
15.89(98)
15.99(98)
15.98(86)
16.01(83)
16.10(80)
15.93(86)
15.98(90)
16.93(91)
16.90(96)
16.82(109)
16.89(86)
16.52(90)
16.66(93)
16.40(106)
16.48(101)
II
III
IV
a
Cf. Table 1.
compared with DiimH as a base. This effect therefore, shifts the
nmax values toward the blue, that is, toward the direction of less
octahedral, more square planar structure [25]. The opposite case
is happened with DiimCl or DiimBr chelates, with electron-
withdrawing groups which make the coordination sphere more
electron-poor and favor the approach of solvent molecules.
Regression analysis of band maxima of compound I–IV
against donor number of solvents is shown in Fig. 4 and
indicated good correlation and also confirms the solvato-
chromic behavior of these complexes. Deviations were
observed indicative of the complex nature of the solute–
solvent interaction. For instants, the nmax of these complexes in
CH2Cl2 occur at longer wavenumber than in nitromethane,
although the donor number of CH2Cl2 (Z0) is lower than that
of nitromethane (Z2.7). This anomaly was ascribed to the
formation of ion pairs by mean of an axial coordination
of ClOK4 in CH2Cl2. As the relative dielectric constant
of nitromethane takes a much higher value (28.5 for
nitromethane versus 8.9 for CH2Cl2 in room temperature),
this solvent facilitates the dissociation of cationic chelate. Thus
reducing the axial interactions of the ClOK4 and eliminating the
tetragonal distortion [15].
4. Conclusion
It can be concluded that the prepared new mixed-chelate
copper(II) complexes exhibited high solubility in various
organic solvents. They showed a good correlation between
their d–d absorption maxima in solution and the donor number
of the solvent used (positive solvatochromism). Solvatochro-
mism studies of these complexes showed that presence of
electron-withdrawing groups on the diimine chelate make
copper ion electron-poor and favor the approach of solvent
molecules along the z-axis. On the contrary, electron-releasing
groups proceed in a reverse way.
Acknowledgements
18
I
II
III
IV
We are grateful for the financial support of Mazandaran
University of the Islamic Republic of Iran.
17
16
15
14
13
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