Substituent effect in the γ-position of acetylacetonate on the solvatochromic property of mixed-chelate copper(II) complexes consisting of diamine and acetylacetonate ligands

Article abstract of DOI:10.1016/j.molstruc.2009.04.019

The infrared and electronic absorption spectra of a series of new mixed-chelate copper(II) complexes that encompass N,N-dibenzylethylenediamine (diamine) and 3-substituted derivatives of acetylacetone (x-acacH) were studied. The IR and electronic absorpti

Full text of DOI:10.1016/j.molstruc.2009.04.019

Journal of Molecular Structure 929 (2009) 154–158
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Substituent effect in the c-position of acetylacetonate on the solvatochromic
property of mixed-chelate copper(II) complexes consisting of diamine
and acetylacetonate ligands
*
Hamid Golchoubian , Ehsan Rezaee
Department of Chemistry, University of Mazandaran, P.O. Box 453, 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:
The infrared and electronic absorption spectra of a series of new mixed-chelate copper(II) complexes that
encompass N,N-dibenzylethylenediamine (diamine) and 3-substituted derivatives of acetylacetone (x-
acacH) were studied. The IR and electronic absorption spectra and the molar conductivity of the newly
prepared complexes are presented and discussed. The molar conductivity in dichloromethane reveals a
predominance of electrostatic interactions between [Cu(diamine)(x-acac)]+ entity and perchlorate anions
that counterbalance the positive charge. The resulting complexes with local symmetry of CuO2N2 attains
a square-coplanar structure and exhibits the tendency for axial ligation, which is enhanced when an elec-
Received 8 January 2009
Received in revised form 5 April 2009
Accepted 9 April 2009
Available online 21 April 2009
Keywords:
Mixed-chelate
Copper(II) complexes
Solvatochromism
N-ligand
Acetylacetonate
Substituent effect
tron-attracting substituent is attached to the c-position of acetylacetonate moiety. The tendency for axial
ligation is particularly fulfilled when suitable nucleophiles (solvents) with deferent donor abilities exist,
leading to solvatochromism. The solute–solvent interactions are revealed by shifts in the ligand field
absorption spectra that are enhanced as the donor power of the solvent increases. Linear dependence
of the ligand field absorption maximum on solvent donor number is generally observed.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
mechanism of salvation. However, the synthesis of mixed-chelate
complexes is not a trivial procedure since the formation of homo
Solvatochromic behaviors of inorganic compounds have been
recently reviewed by many researchers [1–3]. There are a number
of empirical scales for solvent polarity such as Dimroth and Reic-
bis-chelate complexes must be suppressed [12]. It was established
that combination of diamine ligand containing bulky substituents
on the nitrogen atoms and a slim chelate such as acetylacetonate
or its derivatives desired hetero-bis chelate complex forms with
characteristic properties such as chromotropic and catalytic prop-
erties [13]. We recently reported two classes of mixed-chelate cop-
per complexes that show reversible solvatochromism in various
solvents [14,15]. It was suggested that the steric hindrance and
electronic effects exert significant role in solvatochromic behavior
of mixed-chelate complexes.
Fukuda and co-workers extensively studied on the effect of
substituent groups on the diamine and acac chelates on the chro-
motropic behavior of mixed-chelate complexes [16]. Their studies
mainly focused on the electronic effect of substituents in the b-po-
sition of the acac chelate.
hardt’s ET, Kosower’s Z, Kamlet and Taft’s
stant and Gutman’s donor DN and acceptor numbers AN [4–9].
a, b, p
*, dielectric con-
e
Among these, only a few have gained widespread use, ET mainly
for organic compounds [10] and DN and AN for transition metal
complexes [1]. The methods using solvatochromic behavior of
transition metal complexes will serve, as visualized model of sol-
ute–solvent interactions. However, the position and intensity of
d–d bands also have been correlated with some success with dif-
ferent solvent parameters. There exist a number of examples of
solvatochromism metal complexes and origins of color changes
are quite diverse. A certain class of complexes such as mixed-che-
late complexes of copper(II) show gradual color change over a wide
range of solvent donor number (DN). The donor number expresses
a measure of coordinating ability of solvent on the standard of that
of 1,2-dichloromethane (DCM) or dichloroethane (DCE) [11]. This
phenomenon is not only interesting in its own right but also pro-
vides information on various factors determining coordination
geometries of transition metal ions in solution and also on the
In this paper which is a continuation of our studies on the sol-
vatochromic behavior of the mixed-chelate copper(II) complexes
we intend to study the electronic property imposed by a substitu-
ent in the
c-position of the acac chelate on the copper ion. Based on
to our knowledge such investigation has not been carried out on
the mixed-chelate complexes. Thus a series of mixed-chelate cop-
per(II) complexes, that shown in Scheme 1 where x is substituent
groups with different electronic properties were synthesized and
their solvatochromic behavior were investigated.
* 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.2009.04.019

H. Golchoubian, E. Rezaee / Journal of Molecular Structure 929 (2009) 154–158
155
Table 2
Infrared band positions and band assignments of complexes.
Complex
mN–H (cmꢁ1
)
mC@O
,
mC@C (cmꢁ1
)
m )
ClOꢁ4 (cmꢁ1
[Cu(diamine)(H-acac)](ClO4)a
[Cu(diamine)(Cl-acac)](ClO4)
[Cu(diamine)(CH3-acac)](ClO4) 3235
[Cu(H-acac)2]
[Cu(Cl-acac)2]
3231
3223
1583
1607
1580
1580
1592
1585
–
1110, 623
1110, 627
1110, 624
–
–
–
O
O
NH
NH
–
–
–
3230
Cu
X
[Cu(CH3-acac)2]
[Cu(diamine)2](ClO4)2
1110, 623
ClO4
a
Results taken from Ref. [14].
was also observed that compound III in solvent of high donor
power decomposes in ambient temperature over few days and its
decomposition enhances in higher temperature. However, this
phenomenon was not detected for compounds I and II. In conjunc-
tion with this proposal, Fukuda and co-workers reported impossi-
bility of preparation of mixed-chelate complex of [Cu(teen)(hfa)]+
and [Cu(teen)(hfa)2] was obtained instead (‘‘teen” and ‘‘hfa” stand
for N,N,N0,N0-tetraethylenediamine and hexafluoroacetylacetonate,
respectively) [19] (Table 2).
X= H, CH3, Cl
Scheme 1.
2. Experimental
2.1. Materials and methods
3.1. IR spectra
The complexes of [Cu(diamine)2](ClO4)2 [14], [Cu(x-acac)2] [17]
and [Cu(diamine)(x-acac)](ClO4) [14] were prepared according to
published procedures. 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
appropriate care.
Conductance measurements were made at 25 °C with a Jenway
400 conductance meter on 1.00 ꢀ 10ꢁ3 M samples in CH3CN and
CH2Cl2. 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 spectrophotome-
ter. The elemental analyses were performed on a LECO 600 CHN
elemental analyzer. Absolute metal percentages were determined
Certain vibrational bands that are characteristics of the [Cu(dia-
mine)2](ClO4)2 and [Cu(x-acac)2] complexes are presented in the IR
spectra of the mixed-chelate complexes (see Fig. 1). In the spectra
the disappearance of other bands in conjunction with the merger
of absorptions in specific regions of the spectra are indicative of
the band formed and the entities present in the newly obtained
chelate complexes (Table 3). The bands are distinguished into
those emanating form the ligands and those associated with the
counter ion, ClOꢁ4 .
Insertion of substituents in the
c-position of x-acacH culmi-
nates in a merge of the bands emanating from the perturbed dou-
ble bands. Replacement of one acetylacetonate chelate ring by that
of the diamine and formation of mixed-chelate complex dimin-
ishes the extent of conjunction and the repercussions of the pres-
ence of diamine on the aforementioned perturbed double bonds
become evident when the wave number of the vibrational modes
are compared with the corresponding ones in the [Cu(x-acac)2].
For instance, while the absorption due to the perturbed C@O in
[Cu(x-acac)2] peaks at around 1585 cmꢁ1, upon formation of the
by
a
Varian-spectra A-30/40 atomic absorption-flame
spectrometer.
3. Results and discussion
The reaction of copper(II) perchlorate hexahydrate, N,N-dibezy-
lethylenediamine (diamine), 3-substituted derivative of acetylace-
tone (x-acacH) and sodium carbonate with molar ratios of
1:1:1:0.5 resulted in the formation of mixed-chelate complexes.
The color, typical yields and analytical date of the new mixed che-
late complexes are presented in Table 1. The obtained results indi-
cate the formation of the desired complexes.
mixed-chelate species the band is shifted to 1595 cmꢁ1
.
Corroboration of the variable nature of the interactions of the
bulky perchlorate group with the copper(II) complex cation, im-
plied by the molar conductivity values, is obtained from the IR
spectra. The presence of perchlorate group is confirmed by two
bands around 1110 cmꢁ1 and 620 cmꢁ1 which correspond to the
triply degenerate asymmetric stretching and asymmetric bending
vibration modes of T2 of the tetrahedral ClOꢁ4 group, respectively
[20]. The former band at 1110 cmꢁ1 of compounds I–III, not with-
standing 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 this band
serves as a measure of the degree and mode of the coordination of
perchlorate ions to the copper ion [21]. These bands disappear in
the spectrum of [Cu(x-acac)2].
The yield of compound III is much lower than compounds I and
II (74%) [14]. It seems that insertion of the Cl as a electron-attract-
ing group in the
c-position of acetylacetonate chelate alter the
electron distribution around the copper(II) ion and in some extent
destabilizes the formation of the mixed-chelate complex [18]. It
Table 1
Color, yields and analytical data of the new complexes obtained.
No.
Compound (color/yield)
Analysisa
C%
3.2. Conductometric data
H%
N%
Cu%
I
[Cu(diamine)(Cl-acac)]ClO4
(green/45%)
[Cu(diamine)(CH3-acac)]ClO4
(blue/71%)
46.74
(46.98)
51.11
4.45
(4.88)
5.60
4.80
(5.22)
5.12
11.75
(11.84)
11.14
The molar conductivity values of the mixed-chelate complexes
I–III in solvents of dichloromethane and acetonitrile are illustrated
in Table 3. The solutions of the mixed-chelate complexes in aceto-
nitrile are electrically conductive with molar conductivity values
indicative of 1:1 electrolytes [22]. However, significant drop in
III
(51.16)
(5.66)
(5.42)
(11.20)
a
Calculated values are in parentheses.

156
H. Golchoubian, E. Rezaee / Journal of Molecular Structure 929 (2009) 154–158
the molar conductivities are observed in solvent of dichlorometh-
ane. These results suggest that an ion-pair formation or covalent
interactions of perchlorate anions might exist to some extent in
CH2Cl2 mainly, in [Cu(diamine)(Cl-acac)]ClO4 in which exhibits
non-conductive behavior. That means ClOꢁ4 ions are bound weakly
in compounds I and III and moderately in compound II in above
and below of the chelate planes and can be removed by high donor
power solvent molecules which leads to the solvatochromism.
These results are in general agreement with the expectation from
the spectral data and with our pervious results taken from X-ray
analyses of type [Cu(diamine)(H-acac)]ClO4 [23,24]. Those results
demonstrated that two ClOꢁ4 ions lie above and below the plane
of chelate rings and are weakly coordinate to the copper ion. The
forces holding ClOꢁ4 ions in those complexes are relatively weak
so that the ions will be easily ejected from the coordination sphere
by solvent molecules in solution.
3.3. Solvatochromism
The electronic absorption spectra of the mixed-chelate com-
plexes are characterized by a broad structureless band in the visi-
ble region attributed to the promotion of the electron in the low
energy orbitals to the hole in dx2
orbital of the copper(II) ion
ꢁy2
(d9). The position of this band is shifted to longer wavelengths as
the donor number of solvent increases (Table 4). This red shift is
ascribed to the strong repulsion of the electrons in d2z orbital by
lone pair electrons of the solvent molecules that are axially coordi-
nated to the copper center, decreasing the energy required to
transfer the electrons to the dx2
orbital.
ꢁy2
The spectral changes of the complexes in solvents with various
donor numbers (DN) are illustrated in Fig. 2. Values of DN of the sol-
vents utilized are as follows: DCM = 0.0; MeNO2 = 2.7; MeCN = 14.1;
acetone = 17.0; THF = 20.0; MeOH = 23.5; DMF = 26.6; DMSO = 29.8
pyridine = 33.1; HMPA = 38.8.
In the series the shifts induced by certain solvent depend upon
the electronic properties of substituent group attached in the c-po-
sition of the acac chelate which controls the magnitude of the 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(diami-
ne)(x-acac)]+. Introducing an electron-attracting (x = Cl) or elec-
tron-releasing (x = CH3) substituent on the acac chelate alters the
Lewis acidity of CuN2O2 chromophore and consequently, its equato-
rial ligand field strength. In other words, presence of Cl-acac chelate
makes the coordination sphere around the metal ion electron-poor,
which makes it easier for solvent molecules to approach the axial
center and accordingly, leading to a greater solvatochromic effect.
As evident from Fig. 3 and Table 4 the kmax values in one particular
solvent decreases in the following order of acac chelate.
Fig. 1. Infrared spectra of [Cu(x-acac)2], [Cu(diamine)(x-acac)]ClO4 and [Cu(diami-
ne)2](ClO4)2, complexes.
Table 3
Molar conductivities data
(
Km
)
of the complexes
(X
ꢁ1 cm2 molꢁ1
,
at 25 °C)a in
dichloromethane and acetonitrile.
Complexes
I
IIb
III
CH2Cl2
CH3CN
1.1
161.1
4.2
155.0
2.5
113.6
Cl-acac > H-acac > CH3-acac
a
Concentration: ca. 1 ꢀ 10ꢁ3 M. Standard values for a 1:1 electrolyte type in
CH2Cl2 and CH3CN are in the range of 10–20 and 120–160 (X
ꢁ1 cm2 molꢁ1, at
It is clear from Fig. 2 that the spectra in coordinating solvents
such as pyridine and DMSO do not follow a single-Gaussian profile
while non-coordinating solvents, at least, follow such a profile.
25 °C), respectively, from Ref. [22].
b
Results taken from Ref. [14].
Table 4
Electronic spectra of the complexes in various solvents in order of DN values.
Complexes
kmax/nm (e )
/Mꢁ1 cmꢁ1
DCM (0.0)b
MeNO2 (2.7)
ACN (14.1)
Acetone (17.0)
THF (20.0)
MeOH (23.5)
DMF (26.6)
DMSO (29.8)
Py (33.1)
HMPA (38.8)
I
569 (104)
572 (96)
551 (93)
558 (104)
554 (111)
526 (101)
631 (96)
584 (110)
571 (105)
590 (110)
585 (102)
573 (94)
595 (107)
594 (103)
573 (98)
602 (115)
594 (111)
581 (101)
615 (113)
610 (107)
599 (95)
625 (119)
616 (113)
605 (104)
656 (102)
646 (117)
633 (119)
635 (112)
–
639 (90)
IIa
III
a
Results taken from Ref. [14].
Donor number of solvents.
b

H. Golchoubian, E. Rezaee / Journal of Molecular Structure 929 (2009) 154–158
157
Fig. 3. kmax values of complexes I–III in various solvents in order of DN values. The
symbols ‘‘a” to ‘‘j” represent: a = CH2Cl2, b = MeNO2, c = CH3CN, d = acetone, e = THF,
f = MeOH, g = DMF, h = DMSO, I = py, j = HMPA.
650
600
♦ I
ΙΙ
550
ΙΙΙ
500
0
10
20
30
40
DN
Fig. 4. Dependence of the kmax values of compounds I–III on the solvent donor
number values.
Another deviation also observed in solvent of dichloromethane.
The kmax of the complexes in CH2Cl2 occur at longer wavelength
than in nitromethane, although the donor number of CH2Cl2 (=0)
is lower than that of nitromethane (=2.7). This anomaly is ascribed
to the formation of ion pairs by mean of an axial coordination of
ClOꢁ4 in CH2Cl2. As the relative dielectric constant of nitromethane
is much higher value (28.5 for nitromethane versus 8.9 for CH2Cl2
in room temperature), this solvent facilitates the dissociation of
cationic chelate [25].
4. Conclusion
Fig. 2. Absorption spectra of complexes in selected solvents. Absorption spectra in
other solvents are omitted for clarity.
In conclusion, the mixed-chelate copper(II) complexes are
highly soluble in various organic solvents and represent solvato-
chromism. The complexes are almost non-conductive in dichloro-
methane indicating predominance of covalent interaction
between copper(II) center and perchlorate anions, while the elec-
tronic absorption spectra suggest the likelihood of coordination
taking place along an axis perpendicular to the CuN2O2 plane. In
high donor power solvents donor–acceptor interactions induce
tetragonal distortion of the CuN2O2 chromophore and the ligand
field band maxima correlate linearly with DN of the solvents em-
ployed (positive solvatochromism). Solvatochromism studies of
these complexes showed that presence of an electron-withdrawing
Thus, it can be reasonably assumed that an octahedral coordination
geometry contributes to the solution molecular structure, making
the copper(II), d9, complexes susceptible to the Jahn-Teller effect
as manifested by splitting of the ligand-field transition.
Regression analyses of the band maxima of compounds I–III
against the donor number of the solvents is represented in Fig. 4
and demonstrate good correlation and also proved the solvatochro-
mic behavior of these complexes. The slope represents the sensitiv-
ity of the compounds and linearity of the kmax values against donor
number confirms the solvatochromic behavior of complexes. Devi-
ations were also observed in this correlation indicative of the com-
plex nature of the solute–solvent interaction. An obvious deviation
is found in the solvent of pyridine and HMPA. As stated before, in
strongly coordinating solvents the electronic structure becomes
more similar to an octahedral instead of square planar structure.
However, this deviation may be due to another factor, i.e. the field
strength of the ligating solvents, the trend of which does not nec-
essarily correlate with the DN trend for the different solvents.
group on the c-position of the acetylacetonate chelate makes the
copper ion electron-poor and favors the approach of solvent mole-
cules along the z axis. In contrast, an electron-releasing group pro-
ceeds in reverse trend.
Acknowledgement
We are grateful for the financial support of Mazandaran Univer-
sity of the Islamic Republic of Iran.

158
H. Golchoubian, E. Rezaee / Journal of Molecular Structure 929 (2009) 154–158
[13] A. Paulovincova, U. Al-Ayaan, Y. Fukuda, Inorg. Chim. Acta 321 (2001)
References
56.
[14] H. Asadi, H. Golchoubian, R. Welter, J. Mol. Struct. 779 (2005) 30.
[15] E. Movahedi, H. Golchoubian, J. Mol. Struct. 787 (2006) 167.
[16] U. El-Ayann, F. Murata, Y. Fukuda, Monatsh. Chem. 132 (2001) 1279. and
references cited therein.
[17] D.P. Graggon, J. Inorg. Nucl. Chem. 14 (1960) 161.
[18] D.E. Fenton, R.S. Nyholm, M.R. Truter, J. Chem. Soc. A (1971) 1577.
[19] N. Shintani, J. Kotaki, K. Sone, Y. Fukuda, Bull. Chem. Soc. Jpn. 66 (1993) 784.
[20] C. Tsiamis, M. Themeli, Inorg. Chim. Acta 206 (1993) 105.
[21] Y. Fukuda, A. Shimura, M. Mukaida, E. Fujita, K. Sone, J. Inorg. Nucl. Chem. 36
(1974) 1265.
[1] W. Linert, Y. Fukuda, A. Camard, Coord. Chem. Rev. 218 (2001) 113.
[2] K. Sone, Y. Fukuda, Rev. Inorg. Chem. 11 (1991) 123.
[3] W. Linert, Chem. Soc. Rev. 23 (1994) 429.
[4] V. Gutmann, Coordination Chemistry in Non-aqueous Solutions, Springer,
Vienna, 1968.
[5] V. Gutmann, The Donor–Acceptor Approach to Molecular Interactions, Plenum
Press, New York, 1978.
[6] W. Linert, V. Gutmann, Coord. Chem. Rev. 117 (1992) 159.
[7] W. Mizerski, M.K. Kalinowski, Monatsh. Chem. 123 (1992) 675.
[8] Y. Marcus, Chem. Soc. Rev. 22 (1993) 409.
[22] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[23] H. Golchoubian, Anal. Sci. 24 (2008) x169.
[24] H. Golchoubian, Anal. Sci. 24 (2008) x195.
[25] R.W. Soukup, K. Sone, Bull. Chem. Soc. Jpn. 60 (1987) 2286.
[9] Y. Marcus, The Properties of Solvents, Wiley, Chichester, 1998.
[10] C. Reichardt, Chem. Rev. 94 (1994) 2319.
[11] V. Gutmann, Coord. Chem. Rev. 18 (1976) 225.
[12] W. Linert, A. Taha, J. Coord. Chem. 29 (1993) 265.