Spectral, electrochemical and molecular orbital studies on solvatochromic mixed ligand copper(II) complexes of malonate and diamine derivatives

Article abstract of DOI:10.1016/S1386-1425(02)00337-2

Solvatochromic mixed ligand complexes of copper(II) with malonate and diamine derivatives, Cu_n(RMal)(diam)_nX_m (where n = 1 or 2, m = 1-4, RMal, malonic acid (H₂Mal), diethylmalonate (HDEtMal) or diethylethoxyethylenemalonate (DEtEMal), and diam, ethylenediamine (en), 1,3-propylenediamine (1,3-pn), N,N,N′-trimethylethylenediamine (Me₃en), N,N,N′-triethylethylenediamine (Et₃en), N,N,N′,N′-tetramethylethylenediamine (Me₄en), N,N,N′,N′-tetramethylpropylenediamine (Me₄pn), or N-methyl-1,4-diazacycloheptane (medach); and X = ClO₄^- or Cl^-), has been synthesized and characterized by spectroscopic, magnetic, molar conductance and electrochemical measurements. The mass spectra along with the analytical data of the complexes show peaks with m/e corresponding to a bridged binuclear structure for the chloride complexes, while perchlorate complexes showed either mononuclear structure for DEtMal and DEtEMal or bridged binuclear structure for Mal complexes. These results correspond to IR spectral data, which indicated that the modes of ester and carboxylato coordination sites are mono- and/or bidentate. The d-d absorption bands in weak donor solvents suggest square-planar and distorted square pyramidal-trigonal bipyramid geometries for the perchlorate and chloride complexes; respectively. On the other hand, an octahedral structure is identified for complexes in strong donor solvents. Perchlorate complexes show a drastic color change from violet to green as the donation ability of solvent increases, whereas chloride complexes are highly affected by the acceptor properties of the solvent. Cyclic voltammetric measurements on the complexes, proposed a quasi-reversible or irreversible and mainly diffusion controlled reduction process. Such behavior has been explained according to the ECE mechanism. A linear correlation has been found between the Cu(II) reduction potential and the spectral data. Molecular orbital calculations were performed for the ligands on the bases of PM3 level and the results corresponded to the experimental data. The data are discussed in terms of chromotropic concept and its applications as a Lewis acid-base color indicator.

Full text of DOI:10.1016/S1386-1425(02)00337-2

Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
www.elsevier.com/locate/saa
Spectral, electrochemical and molecular orbital studies on
solvatochromic mixed ligand copper(II) complexes of malonate
and diamine derivatives
Ali Taha *
Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt
Received 16 April 2002; received in revised form 4 October 2002; accepted 4 October 2002
Abstract
Solvatochromic mixed ligand complexes of copper(II) with malonate and diamine derivatives, Cun(RMal)(diam)nXm
(where nꢀ
/
1 or 2, mꢀ
/
1ꢁ4, RMal, malonic acid (H2Mal), diethylmalonate (HDEtMal) or diethylethoxyethylenema-
/
lonate (DEtEMal), and diam, ethylenediamine (en), 1,3-propylenediamine (1,3-pn), N,N,N?-trimethylethylenediamine
(Me3en), N,N,N?-triethylethylenediamine (Et3en), N,N,N?,N?-tetramethylethylenediamine (Me4en), N,N,N?,N?-tetra-
methylpropylenediamine (Me4pn), or N-methyl-1,4-diazacycloheptane (medach); and Xꢀ
ClO4ꢂ or Clꢂ), has been
/
synthesized and characterized by spectroscopic, magnetic, molar conductance and electrochemical measurements. The
mass spectra along with the analytical data of the complexes show peaks with m/e corresponding to a bridged binuclear
structure for the chloride complexes, while perchlorate complexes showed either mononuclear structure for DEtMal
and DEtEMal or bridged binuclear structure for Mal complexes. These results correspond to IR spectral data, which
indicated that the modes of ester and carboxylato coordination sites are mono- and/or bidentate. The dꢁd absorption
/
bands in weak donor solvents suggest square-planar and distorted square pyramidal-trigonal bipyramid geometries for
the perchlorate and chloride complexes; respectively. On the other hand, an octahedral structure is identified for
complexes in strong donor solvents. Perchlorate complexes show a drastic color change from violet to green as the
donation ability of solvent increases, whereas chloride complexes are highly affected by the acceptor properties of the
solvent. Cyclic voltammetric measurements on the complexes, proposed a quasi-reversible or irreversible and mainly
diffusion controlled reduction process. Such behavior has been explained according to the ECE mechanism. A linear
correlation has been found between the Cu(II) reduction potential and the spectral data. Molecular orbital calculations
were performed for the ligands on the bases of PM3 level and the results corresponded to the experimental data. The
data are discussed in terms of chromotropic concept and its applications as a Lewis acidꢁ
/base color indicator.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Solvatochromic; Copper(II); Mixed ligand; Bridged binuclear complexes; Lewis acidꢁ
/
base indicators; Electrochemistry
* Present address: Faculty of Science, Chemistry Department, UAE University, P.O. Box 17551, Al-Ain, United Arab Emirates.
Tel.: ꢃ971-3-7064384; fax: ꢃ971-3-7671291.
E-mail address: a.taha@uaeu.ac.ae (A. Taha).
/
/
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(02)00337-2

1374
A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
1. Introduction
One of the decisive experimental techniques for
investigating soluteꢁsolvent interactions is based
/
on the dependence of the absorption spectra on the
nature of the solvent [1,2]. This is because the
variations in color are direct measures of the
specific interactions between the solute and solvent
molecules [3ꢁ6]. Developing environmental sensor
/
materials has been substantially important due to
the accelerating demands for monitoring pollutant
levels in the environment. Such materials are
chromotropic as a result of exhibiting color change
when exposed to solvent or environmental pollu-
tant molecules [7].
Recently, our research efforts have been focused
on the synthesis and characterization of mixed
ligand complexes of the type [M(Oꢁ
/
O)(Nꢁ
/
N]ꢃ
Scheme 1. (a) Position of the substituents at the diamine
ligands (diam) in the investigated Cun (RMal)(diam)n Xm com-
plexes.(b) Position of the substituents in the RMal ligands:
[8ꢁ10], which are likely to demonstrate sensitivity
/
to solvation or anation. Such behavior can be
monitored by absorption spectra, electrochemical
H2Mal; R1ꢀ
ever, the two hydrogen atoms of the methylene group are
replaced by: ꢀCOC2H5 in DEtEMal ligand.
/
R2: OH and HDEtMal: R1ꢀR2: OC2H5, how-
/
and quantum mechanical studies [11ꢁ15]. In this
/
/
direction, there has been no systematic studies
reported in literature on the solvatochromicity of
the ternary Cu(II) complexes of malonate (RMal)
and diamine derivatives.
the ligands and the data are correlated with the
experimental results.
Herein, we report newly synthesized solvato-
chromic complexes with the general formula,
2. Experimental
Cun(RMal)(diam)nXm, where nꢀ
1ꢁ4, RMal, malonato (Mal), diethylmalonato
(DEtMal) or diethylethoxyethylenemalonate
/
1 or 2, mꢀ
/
/
2.1. Materials
(DEtEMal), diam, ethylenediamine (en), propyle-
nediamine (1,3-pn), N,N,N?,N?-tetramethylethyle-
nediamine (Me4en), N,N,N?-trimethylethylene-
diamine (Me3en), N,N,N?-triethylethylenediamine
(Et3en), N,N,N?,N?-tetramethylpropylenediamine
(Me4pn) or N-methyl-1,4-diazacycloheptane (me-
All chemicals used were of the analytical reagent
grade and obtained from either Merck or Aldrich
and were used without further purification. Sol-
vents used for spectral and cyclic voltammetric
studies, nitromethane (MeNO2), acetonitrile
(MeCN), acetone (Me2CO), methanol (MeOH),
dach); and Xꢀ
/
ClO4ꢂ or Clꢂ (Scheme 1aꢁ
b). The
/
formamide
(FA),
N,N-dimethylformamide
effect of substituents at both diamine and RMal
ligands, and also changing the anion on the
structure and sensitivity of the complex toward
solvents have been investigated using spectral and
electrochemical techniques. One goal of this study
is to examine the applicability of these complexes
(DMF) and dimethylsulfoxide (DMSO), were
‘Spectro-grade’ and further purified using stan-
dard methods [16].
2.2. Physical measurements
as a Lewis acidꢁbase color indicator. As further
support to our experimental work, molecular
orbital calculations have been accomplished for
/
The mass spectra were recorded on AX 500 Jeel
GC-mass spectrometer using fast atom bombard-
ment (fast/ms) method. The infrared spectra in

A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ
/1386
1375
KBr (400ꢁ
/
4000 cmꢂ1) and in polyethene (50ꢁ
/
650
ings, are supported by the molar conductance (L/
S cm2 molꢂ1) values for aqueous solutions of the
complexes. These values are in the range expected
to 1:1 for the perchlorate of DEtMal complexes,
and 1:2 for the rest perchlorate complexes [18].
Nevertheless, aqueous solutions of the chloride
complexes showed molar conductance values
higher than expected. This result might be attrib-
uted, at least in part, to the effect of ion-pair
cmꢂ1) were recorded using Shimadzu FTIR 8101
and Nicolet 20F Far-IR spectrometers. Electronic
spectra were obtained on UV-2101 pc w/full
spectrophotometer using 10 mm quartz cells
thermostated at 25 8C. Magnetic moments were
obtained using a MSB-AUTO magnetic suscept-
ibility balance by the Gouy method. The molar
conductance was measured with Metrohm 660
conductivity bridge in water and DMF solutions
at 25 8C. Cyclic voltammetric measurements were
performed as reported elsewhere [11,17].
dissociation [19ꢁ21]. This suggestion is confirmed
/
by the white precipitate that appears when AgNO3
solution is added to aqueous solutions of these
complexes. This phenomenon, is more pronounced
in the case of the chloride complex of DEtMal, 15.
It exhibited much higher molar conductance value
than expected for 1:1 electrolyte. This implies
dissociation of the bridged binuclear (as suggested
for the complex in solid state) into mononuclear
species in aqueous solution. This interpretation
could be due to the higher acceptor properties of
2.3. Syntheses of Cun(RMal)(diam)nXm
complexes
These complexes were prepared by adding a
mixture of either 5 or 10 mmol of the malonate
derivative (RMal) in 25 ml absolute EtOH and
solid anhydrous Na2CO3 (20 mmol) to an etha-
nolic solution of 10 mmol CuX2×
/
nH2O. The
water (ANꢀ54.8) [22], which will strongly solvate
/
mixture was continuously stirred for about 30
min resulting in a green solution, which was then
filtered. Then a solution of 10 mmol diamine
derivative (diam) in 10 ml EtOH was added
dropwise to the filtrate with a continuous stirring
for an additional 25 min. After that, the resulting
solution was filtered and left to stand overnight.
The complexes obtained were re-crystallized from
dichloromethane.
the chloride anion [23,24], and also to the weak
preferences of the ester group to act as a bridged
bidentate ligand. Measuring the molar conduc-
tance of the chloride complexes 13 and 15 in DMF
solution, which has weak acceptor properties
(ANꢀ16.0) further supports this suggestion. The
/
obtained values: 163 and 65 S cm2 molꢂ1 corre-
sponding to 1:2 and 1:1 electrolyte, respectively
[18].
Preparation of mixed ligand complexes of
The magnetic moments data of the current
Cu(II)ꢁ
/
DEtMal and Cu(II)ꢁ/DEtEMal with non-
complexes are found in the range 1.57ꢁ2.20 B.M
/
alkylated diamine ligands (en and 1,3-pn) was not
possible due to the formation of template schiff-
base complexes.
(Table 1). These values are in consistent with those
obtained in the cases of four- and five-coordinated
Cu(II)-complexes [4,5,10,11].
2.4. Results and discussion
2.5. Mass spectra
Perchlorate complexes (1ꢁ
/
11) were successfully
Mass spectroscopy was performed for the se-
lected representative complexes to determine their
molecular weights and fragmentation patterns.
The mass spectra of the investigated compounds
are relatively complex and exhibit a large number
of peaks that extend to m/e value above 700 a.m.u.
synthesized. The color of complexes varied from
bluish for 4 and 7 and reddish-violet for the other
perchlorate complexes, while a greenish crystals
were isolated for all chloride complexes, 12ꢁ15.
The elemental analyses data suggest the formation
of binuclear and mononuclear structures for
/
(Scheme 2aꢁd). The most significant features in
/
perchlorate complexes 1ꢁ
On the other hand binuclear are established for the
chloride complexes, 12ꢁ15 (Table 1). These find-
/
5 and 6ꢁ
/
11, respectively.
the spectra of all investigated complexes is the
most abundant small molecular ion peak due to
the loss of the diamine ligand fragment
/

Table 1
Colors, analytical data, molar conductance in aqueous solution, and magnetic moments (per one Cu2ꢃ) of Cun (RMal)(diam)n Xm complexes
No. Complex/empirical formula
Formula
weight
Color
Yield,
%
Anal, found (Calc)
C% H%
L/
meff
S cm2 molꢂ1
(B.M)
N%
(1) [Cu2(Mal)(en)2](ClO4)2C7H18N4O12Cl2Cu2
(2) [Cu2(Mal)(1,3-pn)2](ClO4)2C9H22N4O12Cl2Cu2
(3) [Cu2(Mal)(Et3en)2](ClO4)2C19H42N4O12Cl2Cu2
(4) [Cu2(Mal)(Me4pn)2](ClO4)2C17H38N4O12Cl2Cu2
(5) [Cu2(Mal)(Me4en)2](ClO4)2C15H34N4O12Cl2Cu2
(6) [Cu(DEtEMal)(Me4en)](ClO4)2C16H32N2O13Cl2Cu
548.0
Reddish-vio- 46.4
let
14.96(15.33) 3.08
(3.29)
19.04 (18.74) 3.79
(3.85)
32.14 (31.83) 5.67
(5.91)
29.38 (29.64) 5.81
(5.56)
26.92 (27.26) 5.06
(5.19)
32.49 (32.28) 5.53
(5.42)
33.29 (33.51) 5.35
(5.63)
35.26 (35.62) 5.98
(6.21)
33.83 (34.04) 5.79
(5.96)
39.05 (38.71) 6.56
(6.72)
36.21 (35.86) 5.58
(5.79)
33.56 (33.82) 6.50
(6.44)
37.21 (36.81) 6.92
(6.69)
38.29 (38.65) 6.81
(7.03)
36.14 (36.49) 6.78
(6.94)
9.89 (10.22) 269.9
9.65 (9.72) 247.3
8.07 (7.82) 223.4
8.21 (8.14) 193.2
8.65 (8.48) 187.9
5.03 (4.71) 190.4
4.85 (4.60) 178.5
6.17 (6.39) 107.5
6.47 (6.62) 90.4
6.23 (6.02) 87.6
6.27 (6.44) 103.2
1.68
1.73
1.81
1.66
1.62
1.57
1.67
2.03
1.72
1.68
2.14
2.06
576.18
716.34
688.30
660.27
594.73
Reddish-vio- 37.3
let
Reddish-vio- 43.4
let
Bluish
54.2
Reddish-vio- 63.5
let
Dark-violet
74.1
63.2
(7) [Cu(DEtEMal)(Me4pn)](ClO4)2C17H34N2O13Cl2Cu 608.73
Bluish
(8) [Cu(DEtMal)(Me4en)]ClO4C13H27N2O8ClCu
(9) [Cu(DEtMal)(Me3en)]ClO4C12H25N2O8ClCu
(10) [Cu(DEtMal)(Et3en)]ClO4C15H31N2O8ClCu
(11) [Cu(DEtMal)(medach)]ClO4C13H25N2O8ClCu
(12) [Cu2(Mal)(Me4en)2Cl2]C15H34N4O12Cl2Cu2
438.0
423.0
465.0
435.0
532.27
Reddish-vio- 57.8
let
Reddish-vio- 44.5
let
Reddish-vio- 59.6
let
Violet
Green
Green
Green
Green
67.4
58.5
62.5
54.8
63.7
10.68
(10.52)
123.9
(13) [Cu2(DEtEMal)(Me4en)2Cl2]Cl2C22H48N4O5Cl4Cu2 717.23
(14) [Cu2(DEtEMal)(Me4pn)22Cl2]Cl2C24H52N4O5Cl4Cu2 745.23
7.67 (7.81) 243.1 (163.3)a 1.98
7.23 (7.52) 248.7
1.98
2.12
(15) Cu2(DEtMal)(Me4en)2Cl2]ClC19H43N4O4Cl3Cu2
624.84
9.17 (8.96) 311.2 (65.3)a
a
Molar conductance measured in DMF solution.

A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ
/1386
1377
Scheme 2. (a) Proposed structures of the Cu2(Mal)(diam)2(ClO4)2 complexes 1ꢁ/5, and mass spectral fragmentation pattern of
complex, 5. (b) Proposed structures of Cu(DEtMal)(diam)ClO4 complexes 8ꢁ11, and mass spectral fragmentation pattern of
/
Cu(DEtMal)(Me4en)ClO4 complex, 8. (c) Proposed structures of [Cu2(DEtEMal)(Me4en)2Cl2]Cl2 complex, 13, and its mass spectral
fragmentation pattern. (d) Proposed structures of [Cu2(DEtMal)(Me4en)2Cl2]Cl complex, 15, and its mass spectral fragmentation
pattern.

1378
A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
(CH3)2NC2H4N(CH3)CH2ꢃ (m/eꢀ
all Me4en complexes, and the other more abun-
dant peak due to Cu(Me4en)ꢃ (m/eꢀ
179 a.m.u).
/
115 a.m.u) in
nCuꢁCl vibrations, which is confirmed by the
reported bands for [(Me4N)2CuCl4] [28ꢁ30]. How-
ever, complexes 13 and 15, show bands at 332, 287,
/
/
A less abundance peak at different m/e positions
depending on the type of RMal ligand and the
anions employed were observed. The extent of
fragmentation of the molecular ions is inversely
proportional to the relative stability of the com-
plex, which depends on the basicity of RMal
ligand. Consequently, Mal-complexes show less
fragmentation than the corresponding DEtEMal-
complexes. This is due to the fact that Mal is
dibasic but DEtEMal ligand is neutral. According
to the highest mass peak in the spectrum a bridged
binuclear structure could be suggested for Mal-
256 and 198, 165 cmꢂ1; 310, 291, 255 and 198, 173
cmꢂ1; respectively, indicating a terminal nCuꢁCl
.
The vibration bands observed at 165 and 173
cmꢂ1 for complexes 13 and 15; respectively could
be assigned as a bridging nCuꢁCl vibration [25,31].
The formation of five coordinate Cu(II)-com-
plexes containing terminal and/or bridging chlo-
ride anion(s) is supported by the X-ray
crystallography for similar complexes, such as
[Cu(bipy)2Cl]ꢃ and Cu2(m-Cl)(L1)2](ClO4)3; L1ꢀ
1,4-bis(pyridin-2-ylmethyl)-1,4-diazacycloheptane
[32ꢁ34].
/
/
complexes, 1ꢁ
complexes, 13ꢁ
proposed for the rest of complexes, 6ꢁ
shown in Scheme 2aꢁd.
/
5, and 12 as well as for chloride
15. However; a mononuclear is
11, as
Furthermore, perchlorate and chloride of Mal-
complexes exhibited two partially resolved bands
at 1650 and 1575 cmꢂ1, which assigned to nas
(COO) and a single band at about 1470 cmꢂ1 for
ns (COO) [25]. However, the rest of perchlorate
complexes showed only single intense bands in the
/
/
/
2.6. Infrared spectra
regions 1700ꢁ
splitting observed for the Mal-complexes, 1ꢁ
ns) values in the range of 102ꢁ
/
1600 and 1470 cmꢂ1. The strong
5 and
185
Selected IR absorption frequencies for the
investigated complexes are shown in Table 2. The
observed bands are assigned based on comparison
with related Cu(II)-complexes [4,5,10,11,25,26].
The main characteristic IR absorption bands for
the current complexes are observed in the finger
print region. The broad band observed in the
/
12 with Dn (nasꢂ
/
/
cmꢂ1, indicates that, the carboxylate group acts as
a bidentate ligand, which is due to the difference in
the binding strength with the metal ion. Therefore
a bridged binuclear structure is proposed for
Cu(II)-Mal complexes [27]. Similar conclusions
were elucidated from the crystal structures of
resembling complexes [35,36]. The presence of
range 3225ꢁ
3320 cmꢂ1 are assigned to nNH of the
unsymmetrical alkylated and non-alkylated dia-
mine complexes [25].
/
ꢂ
nClO vibrations in the IR spectra of the Mal
4
The coordination modes of perchlorate and
chloride anions have been inferred from the IR
data. Two intense stretching vibrational bands at
1100 and 630 cmꢂ1 are observed for the perchlo-
complexes 1ꢁ5, which have a diprotic ligand
/
(Mal2ꢂ), suggest that malonato act as a tetra-
dentate ligand, because the other possibility (bi-
dentate) should lead to a neutral complex with
Cu(II) ion. This agrees with the molar conduc-
tance data of 1:2 electrolyte as discussed above.
rate complexes, 1ꢁ
complexes 12ꢁ15, show the following bands: at
about 340, 290, 250 and 170 cmꢂ1 which might
correspond to the stretching frequencies of CuꢁCl
[14].
/
11 [25,26]. While, chloride
/
The perchlorate of DEtEMal-complexes 6ꢁ
/
7,
228
/
display Dn (nasꢂ
/
ns) values in the range 205ꢁ
/
cmꢂ1, suggesting that COOEt group behaves as
mono-dentate, while the splitting values for the
perchlorate of DEtMal-complexes, 8 11 are in the
The partial splitting observed for the broad
band at 1100 cmꢂ1 indicates a mixture modes
for perchlorate anions, properly one being par-
tially contributing to the coordination sphere of
Cu(II) as bidentate, and the other being free [27].
Complex 12, shows a stretching vibration band at
343, 296 and 234 cmꢂ1, attributed to a terminal
/
ꢁ
range 135ꢁ
/
186 cmꢂ1. This may seem at first, to be
due to a bidentate behavior for COOEt group
[37,38], but it is clearly due to the conjugation of
nCꢀO with nCꢀC upon chelation, which leads to
lower nCꢀO and Dn (nasꢂ
/
ns) values [25]. This

Table 2
Wavenumbers (cmꢂ1) of the main absorption bands in the infrared spectra of Cun (RMal)(diam)n Xm complexes
No. Complex
n(NH)
n(COO)
Dn (nasꢂ
/
ns)
n(ClO4ꢂ
)
n(Cuꢁ/O)
n(Cuꢁ
/
N)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
[Cu2(Mal)(en)2](ClO4)2
3308
3250
3315
1640
1636
1645
1653
1620
1673
1698
1655
1605
1620
1637
1647
1673
1694
1653
1582
1590
1580
1577
1576
1455
1470
1474
1472
1474
1468
1470
1474
1470
1477
1475
1472
1468
1472
1470
127, 185
120, 166
106, 171
105, 181
102, 146
205
1144
1146
1145
1146
1146
1142
1142
1146
1145
1146
1149
1090
1090
1090
1090
1090
1090
1089
1090
1090
1089
1090
1046
1026
1020
999
627 525
478
[Cu2(Mal)(1,3-pn)2](ClO4)2
[Cu2(Mal)(Et3en)2](ClO4)2
[Cu2(Mal)(Me4pn)2](ClO4)2
[Cu2(Mal)(Me4en)2](ClO4)2
[Cu(DEtEMal)(Me4en)](ClO4)2
[Cu(DEtEMal)(Me4pn)](ClO4)2
[Cu(DEtMal)(Me4en)]ClO4
[Cu(DEtMal)(Me3en)]ClO4
627 494
629 507
627 500
625 517
625 513
629 475
629 521
ꢁ/
455
472
482
480
ꢁ/
ꢁ/
ꢁ/
ꢁ/
1054
1045
1024
1043
1028
1053
1073
228
181
ꢁ/
ꢁ/
482
475
481
453
474
489
475
484
3225
3320
3290
135
143
623
626 520
ꢁ
/
(10) [Cu(DEtMal)(Et3en)]ClO4
(11) [Cu(DEtMal)(medach)]ClO4
(12) [Cu2(Mal)(Me4en)2Cl2]
162
625 570
ꢁ/
1601
1607
1611
1559
129, 175
139, 205
139, 222
89, 183
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ
ꢁ/
ꢁ/
ꢁ/
/
512
511
513
506
(13) [Cu2(DEtEMal)(Me4en)2Cl2]Cl2
(14) [Cu2(DEtEMal)(Me4pn)2Cl2]Cl2
(15) [Cu2(DEtMal)(Me4en)2Cl2]Cl
ꢁ/
ꢁ/

1380
A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
suggest a mono-dentate behavior for the ester
group of HDEtMal ligand, yielding a mononuc-
lear chelate, whereas, the splitting observed in the
case of chloride complexes of DEtEMal and
DEtMal ligands is due to a bidentate behavior
for the ester group (Table 2). This, conclusion
agrees with the elemental analyses and mass
spectral data.
17 500 cmꢂ1 (except, at about 16 500 and 15 150
cmꢂ1 for 4 and 7; respectively), in non- or very
weak-coordinating solvent (MeNO2), suggests
square planar geometry for these complexes. The
exceptions might be explained by the presence of
steric 1,3-diamine ligand (Me4pn) in these com-
plexes, that forms also a less favored six-members
chelate ring. This leads to a highly distorted square
planar expected for these complexes. So, when the
ligand field strength of the equatorial ligands is
weak, the positive charge on Cu2ꢃ cannot be
efficiently obscured by their coordinating electrons
[7]. Consequently, the Cu(II) ion will strongly
attract the perchlorate anion to compensate the
electron deficiency around it. This explanation is
further supported by the splitting observed for the
broad band of the perchlorate anion at 1100
cmꢂ1, indicating a bidentate chelation of perchlo-
rate anion toward the central Cu(II) ion. This
leads to a highly distorted octahedral structure
that has a bluish color in solid state [25].
2.7. Electronic spectra
Fig. 1 shows the visible absorption spectra of the
Cu2(Mal)(Me4en)2(ClO4)2 complex, 5 in different
organic solvents. There is only one broad band
that could be observed in the visible region, which
is due to the promotion of an electron to the hole
in dx ꢂy orbital of the Cu(II) ion (d9) [7]. The
2
2
/
position of the dꢁd transition band exhibits a red
/
shift as the Lewis basicity of solvent increases [22],
which indicates that these complexes are positively
solvatochromic [1,3,7]. This red shift could be
attributed to the strong repulsion of the electrons
A number of significant points can be extracted
from the data in Table 3 are with discussing. (a)
Non-alkylated diamine complexes, 1 and 2, exhibit
a less pronounced color change in comparison
2
in dz orbital by the lone pair of solvent that is
axially coordinated to the central copper ion,
therefore a less energy will be required to transfer
with those of alkylated diamine, 3ꢁ5. (b) Solutions
/
2
the electrons to dx ꢂy
2
.
/
of complexes 2, 4 and 7 in different solvents
display higher lmax values than the corresponding
1, 5 and 6. This can be explained by the fact that
the former type has 1,3-diamine ligands (1,3-pn
The dꢁ
present complexes in various solvents are given in
Table 3. The position of the dꢁd band, of the
perchlorate complexes in the region of 19 000ꢁ
/
d visible absorption spectral data of the
/
/
Fig. 1. Electronic absorption spectra of 4.0ꢄ
/
10ꢂ3 mol lꢂ1 Cu2(Mal)(Me4en)2(ClO4)2 complex, 5 solutions in MeNO2, MeCN,
Me2CO, DMF and DMSO solvents at 25 8C.

A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ
/1386
1381
Table 3
Absorption maxima bands, lmax (nm) (molar extinction coefficients are in the range 100ꢁ
Cun (RMal)(diam)n Xm complexes solutions in various organic solvents at 25 8C
/
120 l molꢂ1 cmꢂ1
)
for the
No.
Complex
DMSO
DMF
MeOH
Me2CO
MeCN
MeNO2
(1)
(2)
[Cu2(Mal)(en)2](ClO4)2
552
573
670
694
669
635
733
677
645
666
666
696
705
737
733
755
555
575
657
677
654
629
727
661
628
649
649
698
719
788
726
750
544
562
641
665
629
616
697
644
626
625
625
650
649
542
544
598
641
597
596
677
594
592
565
565
667
723
797
700
717
553
566
616
635
617
609
694
626
600
560
563
660
702
764
695
718
532
558
536
614
560
562
660
572
574
541
541
669
705
774
694
700
[Cu2(Mal)(1,3-pn)2](ClO4)2
[Cu2(Mal)(Et3en)2](ClO4)2
[Cu2(Mal)(Me4pn)2](ClO4)2
[Cu2(Mal)(Me4en)2](ClO4)2
[Cu(DetEMal)(Me4en)](ClO4)2
[Cu(DetEMal)(Me4pn)](ClO4)2
[Cu(DetMal)(Me4en)]ClO4
[Cu(DetMal)(Me3en)]ClO4
[Cu(DetMal)(Et3en)]ClO4
[Cu(DetMal)(medach)]ClO4
[Cu2(Mal)(Me4en)2Cl2]
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
[Cu2(DetEMal)(Me4en)2Cl2]Cl2
[Cu2(DetEMal)(Me4pn)2Cl2]Cl2
[Cu2(DetMal)(Me4en)2Cl2]Cl
Cu(DEM)(medach)Cl
ꢁ/
684
682
(15)
a
a
Chloride complex is formed in MeNO2 solution by mixing Bu4NCl with the perchlorate complex, 11 in a ratio (5:1).
and Me4pn), that form six-members chelate rings
and, the later has 1,2-diamines (en and Me4en)
which form five-members chelate rings; respec-
tively. This behavior indicates that the five-mem-
bers ring is more favorable than the six-members
The solvatochromic behavior of the present
complexes is conceivably characterized quantita-
tively using the linear solvation free energy rela-
tionship. The correlation’s data is summarized in
Table 4. nmax/103 cmꢂ1ꢀ
/
n0ꢃ
/
a (DN); where nmax,
d absorption frequency; n0, the
the measured dꢁ
/
ring. (c) The dꢁd visible absorption band of
/
extrapolated frequency (measured the relative
stability of un-solvated species); and a, the slope,
represent the sensitivity of the complex toward
solvent (solvatochromicity). The linearity of the
nmax vs. DN, predicts, stabilization of the reactive
perchlorate complexes toward Lewis bases (Lewis
complexes 5 and 12 in most solvents show a less
noticeable red shift than those of the complexes, 8
and 15. This could be due to the increase in the l.f.s
of the equatorial ligand Mal (dibasic) as compared
to the monobasic DEtMal ligand. Consequently, a
stronger repulsion between the equatorial ligand,
acidꢁbase interactions) [5].
/
2
Mal with the electron of dx ꢂy (highest occupied
2
/
According to the solvatochromic parameters
data, n0 and a-values, for the complexes 1, 5 and
6 in comparison with 2, 4 and 7 (Table 4), one can
deduce that, the five-members chelate ring is
thermodynamically favored and offers a greater
solvatochromic response than six-members ring.
Where, the former complexes contain 1,2-diamine,
while the later have 1,3-diamine ligands.
molecular orbital) shift it to a higher energy than
the case of DEtMal ligand. Hence, a greater dꢁd
/
gap and blue shift will be remarkable in the former
case than the later. This behavior is converse to
that discussed above for the effect of the axially
coordinated solvent. (d) Finally, although donor
strength (DN) [22], of acetone is higher than that
of acetonitrile, the solutions of the present per-
chlorate complexes in acetone show a slightly
lower lmax values than in acetonitrile. This dis-
crepancy was recognized to the effect of steric and
p-bonding in acetone and acetonitrile, since they
are acting in opposite directions [11].
Additive rule is holding to deduce the effect of
subsituents at both diamine and RMal ligands on
the solvatochromic parameters of the perchlorate
complexes. Linear relationships of both, n0 and a-
values vs. s-Hammett constants were found as
follows: a-valuesꢀ
/
ꢂ
/
91.34ꢁ161.2 s-Hammett,
/

1382
A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
Table 4
Solvatochromic parameters of the perchlorate complexes, Cun (RMal)(diam)n (ClO4)m (nꢀ
/
1 or 2, mꢀ1 or 2), according to the
/
equation, nc/103ꢀn0ꢃ
/
/a (DN)
No.
Complex
n0
ꢂ
/
a
r
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
[Cu2(Mal)(en)2](ClO4)2
18.87
17.97
19.02
16.49
18.02
17.73
15.36
17.77
17.62
18.01
18.84
17.70
17.74
28.0
19.20
140.40
65.4
0.95
0.95
0.99
0.98
0.97
0.99
0.99
0.99
0.99
0.99
0.99
0.98
0.99
[Cu2(Mal)(1,3-pn)2](ClO4)2
[Cu2(Mal)(Et3en)2](ClO4)2
[Cu2(Mal)(Me4pn)2](ClO4)2
[Cu2(Mal)(Me4en)2](ClO4)2
[Cu(DetEMal)(Me4en)](ClO4)2
[Cu(DetEMal)(Me4pn)](ClO4)2
[Cu(DEtMal)(Me4en)]ClO4
[Cu(DEtMal)(Me3en)]ClO4
[Cu(DEtMal)(Et3en)]ClO4
[Cu(DEtMal)(medach)]ClO4
Cu(EtAA)(Me4en)ClO4
103
66.30
57.4
98.30
67.85
108.00
126.50
70.58
69.87
(11)
a
a
Cu(EtBA)(Me4en)ClO4
r, Correlation coefficient.
a
Data taken from Ref. [11].
rꢀ
/
0.98, and n0/103ꢀ
/
14.36ꢁ
/
3.16 s-Hammett rꢀ
/
parison with the analogous perchlorate complexes.
The position of the dꢁd absorption bands of these
0.94. The negative slopes in both cases, indicates
an increase in the solvatochromicity and relative
stability of the square planar species as the
electron donating properties of the subsituents at
both RMal and diamine ligands increase. Accord-
ingly, increasing the relative stability of square
planar species will make the environment of Cu(II)
more planar in which the coordinated solvent
molecule comfortably attacks the axial position
in the same order [11]. Once again, the solvato-
chromicity of the current perchlorate complexes
are mainly attributed to the axial solvation extent
at the square planar species, and the l.f.s. of the
equatorial ligands.
All phenomena discussed above could be clar-
ified by considering the dissociation of the per-
chlorate complex solution into the square planar
complex cation and ClO4ꢂ anion. The complex
cation then converts into a series of structures with
various degrees of elongation (tetragonalities)
from a regular octahedron, depending upon the
coordination ability of solvent molecules. This
enhances the possibility of utilizing these com-
plexes as a color indicator for Lewis basicity [1,11].
/
complexes in the region of 14 000 cmꢂ1, suggests a
square-based pyramidal distorted trigonal bipyr-
amide (SBRDTBP) geometry [10,39,40]. This in-
dicates that, the chloride anion is strongly
coordinated to the central metal ion, since the
coordination ability of chloride anion is strong as
expected from its anion’s donor number (DNX,
ꢀ/34.1) [13]. The formation of the five-
MeNO2
coordinated structure [7,14,30,41ꢁ44], explains
/
the terminal coordination of the chloride anion
in case of complex, 12 and a mixture of terminal
and bridging coordination in complexes, 13ꢁ
discussed above in the IR section.
The effect of solvents on the chloride complexes
12ꢁ15, is rather complicated as it depends upon
/15, as
/
both the donor and acceptor properties of the
solvent. To provide an independent interpretation
of their nmax results in different solvents (Table 3),
the linear solvation energy relationship method,
based on multi-parametric Kamletꢁ
[45], has been used to analyze the spectral data,
where nmax/103 cmꢂ1ꢀ
aDNꢃbANꢃc. Regres-
/Taft equation
/
/
/
sion coefficients and constants were determined
by multi-linear regression analysis, using
standard software, and the results are listed in
Table 5.
The dꢁ
the chloride complexes 12ꢁ
extremely shifted to a longer wavelength in com-
/d visible absorption maximum bands of
/
15 in most solvents is

A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ
/1386
1383
Table 5
Solvatochromicity parameters of the chloride complexes, Cun (RMal)(diam)n Clm according to the equation, nc/103ꢀ
b(AN)
/
n0ꢃ
/
a(DN)ꢃ
/
Complex
n0
a
b
r
Relative contribution, %
AN
DN
(12)
(13)
(14)
[Cu2(Mal)(Me4en)2Cl2]
14.12
12.90
8.20
ꢂ
/
18.8
1.7
43.1
62.1
0.99
0.99
0.97
0.98
0.99
30.4
2.7
69.6
97.3
87.6
60.0
62.2
[Cu2(DEtEMal)(Me4en)2Cl2]Cl2
[Cu2(DEtEMal)(Me4pn)2Cl2]Cl2
[Cu2(DEtMal)(Me4en)2Cl2]Cl
[Cu(DEM)(medach)Cl]
32.5
24.0
33.4
229.0
35.9
55.0
12.4
40.0
37.8
(15)
a
13.75
13.26
ꢂ
ꢂ/
/
r, Correlation coefficient.
a
Prepared in solution by mixing Bu4NCl with MeNO2 solution of the perchlorate complex, 11 in a ratio 5:1.
The term that quantifies the Lewis acidity
parameter (AN) of the solvent is relatively im-
values [46ꢁ48]. The reduction process of the
/
investigated complexes in all solvents exhibited a
quasi-reversible or irreversible, redox associated
with a two-electron reduction, and mainly diffu-
sion controlled the process as indicated from the
linear dependence of the current peak on the
square root of the scan rate [49].
Fig. 2 shows the cyclic voltammogram of
[Cu2(DEtMal)(Me4en)2Cl2]Cl complex, 15 in
DMF solution. This voltammogram indicates a
quasi-reversible two-electron reduction in a single
step for either (a) the dicopper(II) complex, or (b)
dissociation of the dicopper(II) into a mononuc-
lear complex in solution first, followed by a
reduction process for the product; according to
the following sequences:
portant for chloride complexes 12ꢁ15, as reflected
/
from the magnitude of the coefficient b and its
higher contribution percentage (Table 5). The
positive sign of this coefficient indicates blue shift
as the solvent’s Lewis acidity increases. However;
the effect of solvent’s Lewis basicity term, DN, is
less significant as indicated from the low contribu-
tion percentage and/or the negative sign of the
coefficient a. This leads to a red shift as the DN of
solvent increases. The former case might arises
because, when a solvent exhibits strong acceptor
properties, it will compete for the coordination of
anion (preferentially solvating the anion), i.e.
weaken the interaction between the chloride anion
and the central copper(II). Consequently, the dꢁd
/
absorption exhibits a blue shift as the solvent’s
acceptor property increases. Whereas, in the later
case when the DN of the solvent increased the
solvent preferably coordinated to the central metal
ion forming an octahedral geometry, resulting in a
CuIICuII ?CuICuI
CuII ?Cu0
(1)
(2)
red shift for the dꢁd band.
/
2.8. Electrochemical studies
Electrochemistry of selected complexes 8, 11 and
15, has been investigated, by means of cyclic
voltammetry using a platinum microelectrode,
for 1.0 mM lꢂ1 of the complex solutions in
different solvents and 0.1 M lꢂ1 Bu4ClO4 as
supporting electrolyte at scanning rate of 100
Fig. 2. Cyclic voltammogram recorded with a platinum micro-
electrode for the DMF solution of 1.0ꢄ
10ꢂ3 mol lꢂ
[Cu2(DEtMal)(Me4en)2Cl2]Cl complex, 15 in presence of 0.1
mol lꢂ1 Bu4NCO4 as supporting electrolyte and scan rate of
100 mV sꢂ1
/
1
mV sꢂ1
.
Ferrocine/ferrocinium (Fc/Fcꢃ) and
bis(biphenyl)chromium (I)/(0) were used as inter-
nal standards for assignment of the potential

1384
A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ1386
/
The sequence (Eq. (1)), is quieting unexpected;
solvents are shown in Table 6. The Epc potentials
for the mononuclear perchlorate and binuclear
chloride Cu(II) complexes depend on the solvent’s
properties. The trend of data also agrees well with
the visible absorption spectral results in various
even if the two-electron process does not proceed
stepwise, one electron changes at different poten-
tials due to the different environments around the
two Cu(II) ions, the reduction of Cuꢁ
/
N2ꢁ
/
Oꢁ/Cl2 is
more facile than CuꢁN2ꢁO2ꢁCl (Scheme 2d)
/
/
/
solvents where, Epc/Vꢀ
0.99, and Epcꢀ
0.048 nmax/103ꢃ
for complexes, 8 and 11, respectively.
/
0.175 nmax/103ꢂ
1.45, rꢀ
/
2.06, rꢀ
/
[50,51]. In electrochemical terms this should in-
volve an ECEC, or EEC mechanism (C, chemical
complication following the charge transfer E),
rather than a simple EE mechanism. It is more
likely that, according to the Marcus theory, the
activation barrier to electron transfer is increased,
slowing down the rate of heterogeneous charge
transfer, i.e. causing distinct deviation from the
pure reversible character of the charge transfer
[52]. The other sequence (Eq. (1)) could be
excluded according to the molar conductance
data of complex 15 which supported the bridged
binuclear structure in DMF solution.
/
ꢂ
/
/
/0.99,
Once again, the dependence of Epc values of the
bridged binuclear chloride complex 15, on the
solvent parameters is rather complicated, so the
multi-parametric equation has been applied yield-
ing, Epc/Vꢀ
/
0.048 ANꢃ
/
0.02 DNꢂ
/
0.62, rꢀ0.99.
/
The relative contribution percentage of the solvent
donor properties is only 29.4%, i.e., the reduction
process of the chloride complex is extremely
controlled by the acceptor properties of the solvent
(60.6%). This result agrees with that obtained from
the spectral data (Table 5). In conclusion, the
combination of the electrochemical and spectro-
scopic data emphasizes the axial ligation of the
perchlorate complexes and well explains the
The character of the reduction pathway for the
investigated mononuclear perchlorate complexes 8
and 11 showed a stepwise two-electron reduction
according to the following sequence,
soluteꢁsolvent interactions.
/
CuII ?CuI ?Cu0
2.9. Molecular orbital calculations
In fact, the voltammetric picture for the present
mononuclear complexes corresponds well to that
found for similar copper(II) complexes, which has
been also explained according to the ECE mechan-
The structural parameters data of the present
ligands as calculated by means of a semi-empirical
molecular orbital calculations at the PM3 level
provided by the Alchemy 2000 program are shown
in Table 7. The calculated energies of the lowest
unoccupied and highest occupied molecular orbi-
ism [11,51,53ꢁ55].
/
The potential values of the redox peaks (Epc and
Epa) of the investigated complexes in different
Table 6
Electrochemical data for the redox processes of Cu(DEtMal)(diam)X complexes in various solvents, potentials E/V vs. bis(biphenyl)-
chromium(I)/(0) reference electrode
Solvent
[Cu(DEtMal)(Me4en)]ClO4
Epa2
[Cu(DEtMal)(medach)]ClO4
Epa2
[Cu2(DEtMal)(Me4en)2Cl2]Cl
Epa2
Epc
Epa1
Epc
Epa1
Epc
Epa1
MeNO2
MeCN
0.99
0.72
0.12
0.55
0.07
ꢂ
/
0.20
0.86
0.60
0.26
0.38
ꢂ
0.19
/
0.13
0.42
0.48
0.60
0.66
ꢂ0.94
/
ꢂ
/
0.11
FA
1.02
0.62
0.50
0.57
ꢂ
/
0.31
0.69
0.51
0.26
0.11
ꢂ
/
0.85
ꢁ
/
ꢁ
/
ꢁ
/
DMF
0.87
0.19
0.29
0.81
0.70
0.89
0.74
ꢁ/
0.67
0.73
0.52
0.56
ꢁ/
DMSO
0.75
0.49
0.42
0.18
ꢂ
/
0.67
0.30
ꢂ
/
0.35
ꢂ
/
0.31

A. Taha / Spectrochimica Acta Part A 59 (2003) 1373ꢁ
/1386
1385
Table 7
3
2
˚
˚
Atomic charges, Q (e), HOMO (eV), LUMO (eV), volume of the molecule, V (A ) surface area, S (A ) and dipole moment, m (Debye)
from the Alchemy 2000 at (PM3) level for the ligands
3
2
˚
V, A
˚
S, A
Ligand
EHOMO, eV
ELUMO, eV
m, Debye
ꢂ
/
QS, e (N)
en
1,3pn
69.2
86.2
101.8
125.0
204.0
179.1
165.8
224.9
165.6
117.19
272.54
204.80
169.47
228.0
140.38
ꢂ
/
9.43
9.36
8.95
8.82
9.03
8.97
8.97
2.97
2.88
2.67
2.63
2.58
2.44
2.41
0.47
0.16
1.70
1.48
0.75
1.49
1.57
2.06
1.72
4.39
4.30
3.63
2.80
0.42
ꢂ
/
0.382
0.383
0.324
0.323
ꢂ
/
ꢂ
/
Me4pn
Me4en
Me3en
Et3en
154.7
ꢂ
/
ꢂ
/
137.23
121.03
170.82
126.65
82.76
ꢂ
/
ꢂ
/
ꢂ
/
ꢂ
/
0.323 (ꢂ
0.316 (ꢂ
0.319 (ꢂ
/
0.350)a
0.347)a
0.346)a
ꢂ
/
ꢂ
/
/
medach
H2Mal
DetEMal
HDEtMal
HetAA
HetBA
Hacac
ꢂ
/
ꢂ/
/
ꢂ
/
11.67
10.34
11.32
11.13
10.12
10.95
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
203.49
150.75
124.77
179.10
100.34
ꢂ
/
ꢂ
/
0.46
0.61
0.32
0.63
0.34
ꢂ
/
ꢂ
/
ꢂ
/
ꢂ
/
ꢂ/
a
Charge at NH of the unsymmetrical alkylated diamine ligands given in parenthesis.
tals (ELUMO and EHOMO) of the ligands are
correlated with the solvatochromic parameters of
complexes given in Table 4. The following correla-
References
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values)ꢂ9.64, rꢀ0.975; EHOMO
values)ꢂ7.66, rꢀ0.974, and ELUMO
values)ꢂ2.72, rꢀ
(1ꢁ5), and [Cu(Oꢁ
O)(Me4en)]ꢃcomplexes, respec-
tively. The OꢁO ligand is, Mal, DEtMal, EtAA
ꢀ
/
ꢂ0.0085(a-
/
/
/
ꢀ
/
ꢂ
/
0.038(a-
/
/
ꢀ
/
ꢂ
/
0.032(a-
/
/
0.97 for [Cu2(Mal)(diam)2]2ꢃ
[5] H. Miyamae, H. Kudo, G. Hihara, K. Sone, Bull. Chem.
Soc. Jpn 71 (1998) 2621.
/
/
/
[6] R.W. Soukup, R. Schmid, J. Chem. Educ. 62 (1985) 459.
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/
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Unfortunately, X-ray single crystal structure
was not possible due to the lack of facilities. Based
on the physicochemical studies discussed above, a
general tentative picture of the structures was
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