Synthesis, structural and solvent influence studies on solvatochromic mixed-ligand copper(II) complexes with the rigid nitrogen ligand: bis[N-(2,4,6-trimethylphenyl)imino]acenaphthene

Article abstract of DOI:10.1016/j.molstruc.2004.01.040

Three mixed-ligand copper (II) complexes containing the rigid bidentate nitrogen ligand bis[N-(2,4,6-trimethylphenyl)imino]acenaphthene (abbr. 2,4,6-Me₃C₆H₂-BIAN) and β-diketonate (dike) ligands are reported and characterized. These complexes namely, [Cu(dike)(2,4,6-Me₃C₆H₂-BIAN)]ClO₄ {where dike=acac (acetylacetonate), bzac (benzoylacetonate) or (dibm) dibenzoylmethanate}have been synthesized and characterized by elemental analysis, spectroscopic, magnetic and molar conductance measurements. In addition to their high solubility in various organic solvents, these three complexes show a color change on going from one solvent to another, that is, strong solvatochromism of their solutions. The observed solvatochromism is mainly due to the solute-solvent interaction between the chelate cation and the solvent molecules. Reported also are the X-ray crystal structures of the free 2,4,6-Me₃C₆H₂-BIAN ligand and the [Cu(acac)(2,4,6-Me₃C₆H₂-BIAN)]ClO₄ complex.

Full text of DOI:10.1016/j.molstruc.2004.01.040

Journal of Molecular Structure 692 (2004) 209–216
www.elsevier.com/locate/molstruc
Synthesis, structural and solvent influence studies on solvatochromic
mixed-ligand copper(II) complexes with the rigid nitrogen ligand:
bis[N-(2,4,6-trimethylphenyl)imino]acenaphthene
b
c
b
Usama El-Ayaana,b, , Fumiko Murata , Soheir El-Derby , Yutaka Fukuda
*
aDepartment of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
bDepartment of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan
cOrganisch-Chemisches Institut, Universitaet Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Received 16 July 2003; accepted 22 January 2004
Abstract
Three mixed-ligand copper (II) complexes containing the rigid bidentate nitrogen ligand bis[N-(2,4,6-trimethylphenyl)imino]ace-
naphthene (abbr. 2,4,6-Me3C6H2-BIAN) and b-diketonate (dike) ligands are reported and characterized.
These complexes namely, [Cu(dike)(2,4,6-Me3C6H2-BIAN)]ClO4 {where dike ¼ acac (acetylacetonate), bzac (benzoylacetonate) or
(dibm) dibenzoylmethanate}have been synthesized and characterized by elemental analysis, spectroscopic, magnetic and molar conductance
measurements.
In addition to their high solubility in various organic solvents, these three complexes show a color change on going from one solvent to
another, that is, strong solvatochromism of their solutions. The observed solvatochromism is mainly due to the solute–solvent interaction
between the chelate cation and the solvent molecules.
Reported also are the X-ray crystal structures of the free 2,4,6-Me3C6H2-BIAN ligand and the [Cu(acac)(2,4,6-Me3C6H2-BIAN)]ClO4
complex.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Mixed-ligand copper(II) complexes; X-ray structure; Solvatochromism
1. Introduction
Fukuda et al. [4,5] have extensively studied the
solvatochromic behavior of mixed-ligand copper(II) com-
plexes of the general formula [Cu(dike)(diam)]X, {where
dike ¼ b-diketonates, diam ¼ N-alkylated diamines and
X ¼ ClO24 (counter ion) or halides (coordinated anions).
Interestingly these copper chelates are easily soluble in
various organic solvents and characteristic solvatochro-
mism (color change caused by the change in solvent
polarity) is observed in their solutions.
Solvatochromic behavior of mixed-ligand copper(II)
complexes has attracted a considerable interest as a Lewis
acid–base color indicator [1]. It provides a quantitative
approach to recognize the solvent behavior in different
solvents and the role of the solvent in physicochemical
studies [2].
Developing environmental sensor materials have
found a great important because of the accelerating
demands for monitoring pollutant levels in the environ-
ment. Such materials are chromotropic and exhibit
color change when exposed to solvent or pollutant
molecules [3].
In the present work, we study the syntheses and
solvatochromic behavior of some perchlorate mixed-ligand
copper(II) complexes of the general formula,
[Cu(dike)(2,4,6-Me3C6H2-BIAN)]ClO4 complexes, where
dike ¼ acac (acetylacetonate), bzac (benzoylacetonate) or
dibm (dibenzoylmethanate). In these complexes, the
diimine ligand, bis [N-(2,4,6-trimethylphenyl)imino] ace-
naphthene (2,4,6-Me3C6H2-BIAN) acting as a bidentate
ligand via the two imine nitrogen atoms. The study also
covers the X-ray crystal structure of two novel
*
Corresponding author. Address: Department of Chemistry, Faculty of
Science, Mansoura University, Mansoura 35516, Egypt. Fax: þ20-2-05-
023-55-871.
E-mail address: usama@mans.edu.eg (U. El-Ayaan).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2004.01.040

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U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
compounds, the free ligand, 2,4,6-Me3C6H2-BIAN (1)
and the complex, [Cu(acac)(2,4,6-Me3C6H2-BIAN)]ClO4
(2) and will be compared with those previously
reported.
shifts are given in parts per million relative to internal TMS
(tetramethylsilane).
2.2. X-ray data collection and structure refinement
of 2,4,6-Me3C6H2-BIAN ligand (1) and the complex
[Cu(acac) 2,4,6-Me3C6H2-BIAN]ClO4 (2)
2. Experimental
Crystallographic data and conditions used for the data
collection refinement of 1 and 2 are summarized in Table 1.
In 1, prismatic crystal of 0.29 £ 0.27 £ 0.2 mm3 was
mounted on a SMART/RA CCD diffractometer using
graphite monochromator Mo Ka radiation at 100 K. In 2,
data were collected with MacScience MXC03K diffract-
ometer using graphite monochromator Mo Ka radiation at
298 K (crystal of 0.2 £ 0.3 £ 0.2 mm3 was selected for the
study).
2.1. Materials and instrumentation
All starting materials were purchased from Wako Pure
Chemical Industries Ltd, and used without further
purification.
Elemental analyses (C, H, N) were performed on a
Perkin–Elmer 2400 Series II CHNSIO Analyzer. Electronic
spectra were recorded on a UV-3100PC Shimadzu spectro-
photometer using 10 mm quartz cells at room temperature.
Powder reflectance spectra were obtained using the same
instrument equipped with an integrating sphere and using
BaSO4 as a reference. Infrared spectra were recorded on a
Perkin–Elmer FT-IR Spectrometer Spectrum 2000 as KBr
pellets and as Nujol mulls in the 4000–370 cm21 spectral
Cell constants and orientation matrix for data collection
were obtained from least-squares refinement, using the
setting angles of 22 reflections in the range 1:73 , u ,
28:358 for 1, and in the range 1:55 , u , 27:588 for 2. A
total 2312 reflections for 1 and 3666 for 2 were observed
with I . 2sðIÞ:
1
range. H- and 13C-NMR measurements at room tempera-
The initial structure was solved by SHELXS-97 [6] for 1
and SIR92 for 2 [7]. In both cases, the structure was refined
on F2 using SHELXL-97 [8]. The non-hydrogen atoms were
ture were run on a JEOL JNM LA 300 WB spectrometer at
400 MHz, using a 5 mm probe head in CDCl3. Chemical
Table 1
Crystal data, data collection and structure refinement for 1 and 2
1
2
Crystal data
Empirical formula
Formula weight
Crystal system
Space group
C30H28N2
416.56
C35H35ClCuN2O6
678.67
Monoclinic
C2=c
Orthorhombic
Pnma
˚
a (A)
˚
b (A)
˚
c (A)
23.872(3)
12.2194(14)
7.9405(9)
90.00
14.401(9)
17.266(13)
13.101(9)
90.00
a; g (8)
b (8)
98.811(2)
2288.9(5)
4
90.00
3
˚
V (A )
3258 (4)
4
Z
Crystal size (mm3)
0.29 £ 0.27 £ 0.2
1.209
0.20 £ 0.30 £ 0.20
1.384
Dcalcd: (g cm23
)
Fð000Þ
888
1412
m (mm21
)
0.07
0.8
Data collection
Temperature (K)
u range (8)
100(2)
298
1.73–28.35
1.55–27.58
˚
Mo Ka (Mon), 0.71073 A
˚
Radiation
Mo Ka (Mon), 0.71073 A
F and v
Scan mode
68 (min)
218 # h # 0; 0 # k # 22; 20 # l ¼ 17
Index ranges
231 # h # 30; 215 # k # 16; 210 # l ¼ 9
Reflections collected
Independent reflections
7739
4158
2843 ½RðintÞ ¼ 0:084Š
3507 ½RðintÞ ¼ 0:1017Š
Refinement
Refinement method
Full-matrix least-squares on F2
2312/0/202
Full-matrix least-squares on F2
3666/0/215
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices ðI . 2:00sðIÞÞ
1.104
1.949
R1 ¼ 0:0486; wR2 ¼ 0:1375
R1 ¼ 0:1017; wR2 ¼ 0:3724

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
211
refined anisotropically by full-matrix least squares method.
Hydrogen atoms were included, but their positions were not
refined. The refinement gave the final R and Rw values of
0.0486 and 0.1375 for 1, and 0.1017 and 0.3724 for 2,
respectively.
13C NMR (CDCl3, 400 MHz, 24.5 8C): d ¼ 17:60 (o-Me),
20.91 ( p-Me), 129.53 (C1), 122.30 (C2), 126.03 (C3),
128.58 (C4), 130.81 (C5), 140.34 (C6), 160.84 (C7), 146.57
(C8), 124.41 (C9, C13), 128.74 (C10, C12), 132.6 (C11).
3.2. Complexes
3. Synthesis
3.2.1. [Cu(acac)(2,4,6-Me3C6H2-BIAN)]ClO4 (2)
2,4,6-Me3C6H2-BIAN (0.208 g, 0.50 mmol) ligand was
dissolved in 20 ml EtOH and then (0.05 ml, 0.50 mmol) of
acac was added to it. To this mixture ethanolic solution
(10 ml) (0.185 g, 0.5 mmol) of Cu(ClO4)2·6H2O was added.
After 2 h stirring, the mixture was filtered off and the green
solid product was washed with ethanol and dried in air.
Crystals suitable for X-ray measurements were obtained by
slow diffusion of diethyl ether into dichloromethane
solution of the chelate. Yield 0.25 g (74.0%) of CuC35H35-
N2O6Cl (678.661): Calcd. C 61.94, H 5.20, N 4.13%; Found
C 61.89, H 5.10, N 4.08%. meff: 1.73 BM (24 8C).
3.1. bis [N-(2,4,6-trimethylphenyl)imino]acenaphthene,
(2,4,6-Me3C6H2-BIAN) (1)
Preparation of the ligand was carried out in two steps
(Fig. 1) as follow:
(A) Preparation of (2,4,6-Me3C6H2-BIAN)ZnCl2
A mixture of 6.0 g acenaphthenequinone (33.0 mmol),
5.13 g anhydrous ZnCl2 (37.5 mmol) and 10.5 ml 2,4,6-
trimethylaniline (75.0 mmol) in 100 ml acetic acid was
heated under reflux (80 8C) for 1 h. Then, the mixture
was cooled to room temperature and the solid product was
filtered off to give an orange solid that was washed with
acetic acid followed by diethyl ether and air-dried, yield
16.5 g of (2,4,6-Me3C6H2-BIAN)ZnCl2 (90%).
3.2.2. [Cu(bzac)(2,4,6-Me3C6H2-BIAN)]ClO4 (3)
2,4,6-Me3C6H2-BIAN (0.208 g, 0.50 mmol) ligand was
dissolved in 20 ml EtOH and then (0.081 g, 0.50 mmol) of
bzac was added to it. To this mixture ethanolic solution
(10 ml) (0.185 g, 0.5 mmol) of Cu(ClO4)2·6H2O was added.
After 2 h stirring, the resulting clear green solution was
stirred for 2 h and left to stand in air for possible
crystallization. After several days green needle crystals
were formed. Yield 0.29 g (78.0%) of CuC40H38N2O6Cl
(741.74): Calcd. C 64.77, H 5.16, N 3.78%; Found C 63.72,
H 5.13, N 3.27%. meff : 1.75 BM (24 8C).
(B) Removal of ZnCl2
Twelve grams of (2,4,6-Me3C6H2-BIAN)ZnCl2 were
added to a solution of 100 g K2CO3 in 250 ml water and the
mixture was heated under reflux with continuous stirring.
After 3 h the mixture was cooled to room temperature and
the solid product filtered off and washed repeatedly with
water. The product was dissolved in boiling ethanol while
the solid zinc carbonate was removed by filtration. The
ethanolic solution of the ligand was evaporated to the
quarter and set aside. After 24 h the product was filtered and
dried in vacuo. Crystals suitable for X-ray measurements
were obtained by slow evaporation of (hexane: dichlor-
omethane) mixture (5:1). Yield: 7.00 g (77%). Found: C,
86.0; H, 6.37; N, 6.45. Calc. for C30H28N2(416.56): C,
3.2.3. [Cu(dibm)(2,4,6-Me3C6H2-BIAN)]ClO4 (4)
2,4,6-Me3C6H2-BIAN (0.208 g, 0.50 mmol) ligand was
dissolved in 20 ml EtOH and then (0.112 g, 0.50 mmol) of
dibm was added to it. To this mixture ethanolic solution
(10 ml) (0.185 g, 0.5 mmol) of Cu(ClO4)2·6H2O was added.
Immediately a green solid product was formed. After 1 h
stirring, the mixture was filtered off and the green solid
product was washed with ethanol and dried in air. Yield
0.32 g (80.0%) of CuC45H40N2O6Cl (803.808): Calcd. C
67.24, H 5.02, N 3.49%; Found C 66.50, H 5.41, N 3.32%.
meff : 1.74 BM (24 8C).
1
86.50; H, 6.78; N, 6.73%. H-NMR (CDCl3, recorded at
400 MHz at 21.9 8C) d ¼ 2:11 (s, o-Me), 2.39 (s, p-Me),
6.78 (d, H2), 6.98 (s, H12), 7.39 (pst, H3), 7.88 (d, H4).
Fig. 1. Preparation of 2,4,6-Me3C6H2-BIAN ligand.

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U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
4. Results and discussion
4.1. Crystal structure analysis of 2,4,6-Me3C6H2-BIAN
ligand
The ORTEP plot with the atomic-numbering scheme of
2,4,6-Me3C6H2-BIAN is depicted in Fig. 2. Unit cell view is
shown in Fig. 3. Selected bond lengths, angles and torsion
angles are compiled in Table 2.
The X-ray structure of (2,4,6-Me3C6H2-BIAN) confirms
a slight deviation of bis(imino)acenaphthene skeleton from
planarity as evidenced by torsion angles [N(1)–C(7)–C(7)–
N(1) of 26.28 and C(1)–C(7)–C(7)–C(1) of 24.38.
Comparison to the analogous bis[N-(2,6-diisopropylphenyl)
imino]acenaphtene (o,o0-i Pr2C6H3-BIAN) [9] with the
corresponding torsion angles of 20.79(0.29)8 and of
21.70(0.21)8 shows a less planar arrangements of the
bis(imino)acenaphthene skeleton for the former. In the
latter, (o,o0-i Pr2C6H3-BIAN), nearly perfect planar
arrangement is confirmed [9].
Fig. 2. Ortep view of 2,4,6-Me3C6H2-BIAN ligand.
˚
The imine CyN bond of 1.2662(16) A is near to the
˚
corresponding value of 1.275(6) A (mean value) in
Fig. 3. Unit cell view of 2,4,6-Me3C6H2-BIAN ligand.
Table 2
˚
Selected bond lengths (A), angles (8), and torsion angles (8) for 1 and 2
1
2
N(1)–C(7)
1.2662(16)
1.4205(15)
1.3761(13)
1.4178(17)
1.3755(19)
1.4192(14)
1.406(2)
Cu(1)–N(4)
2.026(3)
1.882(3)
1.277(2)
1.443(5)
1.539(7)
1.469(6)
1.413(5)
1.385(7)
1.403(6)
1.395(6)
1.426(6)
1.372(8)
1.368(9)
82.70(18)
95.7(2)
N(1)–C(8)
Cu(1)–O(3)
C(1)–C(2)
N(4)–C(5)
C(2)–C(3)
N(4)–C(8)
C(3)–C(4)
C(5)–C(5)
C(4)–C(5)
C(5)–C(6)
C(5)–C(6)
C(6)–C(9)
C(7)–C(7)
1.528(2)
C(7)–C(15)
C(8)–C(13)
1.3986(18)
1.4004(19)
1.3940(17)
1.392(2)
C(7)–C(8)
C(8)–C(9)
C(8)–C(12)
C(9)–C(10)
C(12)–C(16)
C(11)–C(12)
C(15)–C(20)
C(12)–C(13)
1.3952(17)
121.96(10)
119.37(11)
133.07(11)
118.25(11)
122.44(11)
120.56(12)
116.19(8)
123.16(7)
121.10(11)
118.55(12)
121.83(12)
118.17(11)
122.06(12)
118.27(12)
26.2
C(16)–C(20)
C(7)–N(1)–C(8)
C(2)–C(1)–C(6)
N(1)–C(7)–C(1)
C(1)–C(2)–C(3)
C(4)–C(3)–C(2)
C(3)–C(4)–C(5)
C(6)–C(5)–C(4)
C(5)–C(6)–C(1)
C(13)–C(8)–C(9)
C(10)–C(9)–C(8)
C(9)–C(10)–C(11)
C(10)–C(11)–C(12)
C(11)–C(12)–C(13)
C(12)–C(13)–C(8)
N(1)–C(7)–C(7)–N(1)
C(1)–C(7)–C(7)–C(1)
C(7)–N(1)–C(8)–C(9)
C(7)–N(1)–C(8)–C(13)
C(1)–C(7)–C(7)–N(1)
N(4)–Cu(1)–N(4)
O(3)–Cu(1)–O(3)
N(4)–Cu(1)–O(3)
Cu(1)–O(3)–C(11)
Cu(1)–N(4)–C(5)
Cu(1)–N(4)–C(8)
C(5)–N(4)–C(8)
N(4)–C(5)–C(6)
N(4)–C(5)–C(5)
N(4)–C(8)–C(12)
N(4)–C(8)–C(7)
90.35(15)
125.5(3)
111.0(3)
128.5(2)
120.4(3)
136.0(4)
116.5(2)
120.3(4)
117.5(4)
C(7)–C(8)–N(4)–C(5)
C(12)–C(8)–N(4)–C(5)
Cu(1)–N(4)–C(8)–C(7)
C(5)–C(6)–C(9)–C(6)
Cu(1)–O(3)–C(11)–C(13)
2101.7(5)
77.6(5)
75.9(5)
20.1(7)
0.1(7)
24.3
2102.1
85.4
174.8

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
213
Fig. 4. Ortep view of [Cu(acac)2,4,6-Me3C6H2-BIAN]ClO4.
the free (o,o0-i Pr2C6H3-BIAN) ligand [9] and is shorter
in free 2,4,6-Me3C6H2-BIAN ligand. This is simply because
of coordination to the copper(II) center. Angles between the
planes of the naphthalene and aromatic N substituents
remain unchanged (788) as in case of free 2,4,6-Me3C6H2-
BIAN ligand. Comparison with the corresponding angle in a
similar complex namely, [Cu(acac)(AcOH)(o,o0-i Pr2C6H3-
BIAN)](ClO4) [14] indicates a more perpendicular angle of
(878) in the latter. In [Cu(acac)(AcOH)(o,o0-i Pr2C6H3-
BIAN)](ClO4) complex the diisopropylphenyl groups are
bent more toward the naphthalene backbone away from the
copper center with the result of nearly perpendicular angle
between the planes of naphthalene and the aromatic N
substituents.
0
˚
than the C–N bonds in 2,2 -bipyridine [1.35 A] [10] and
0
˚
2,2 -biquinoline [1.323(2) A] [11], that are part of a
heteroaromatic ring system. The bond lengths N(1)yC(7)
˚
˚
of 1.2662(16)A and C(1)–C(7) of 1.482(6)A are
very similar to the standard Nðsp2Þ ¼ Cðsp2Þ and Cðsp2Þ 2
2
˚
Cðsp Þ double and single bonds [1.27 and 1.48 A,
respectively,] [12] which implies that the structure is
regarded as a diimine bridged by a naphthalene to keep the
imine groups in a fixed cis orientation and not as a
conjugated 14-electron p-system.
Aromatic N substituents make an angle of (788) with the
plane of the naphthalene backbone, close to the correspond-
ing angle of (768) in case of (o,o0-i Pr2C6H3-BIAN)
and larger than the corresponding angle of (618) in case of
( p-Tol-BIAN) [13].
The Cu–O(acac) distances, namely Cu(1)–O(3) of
˚
1.882(3) A, and the bite angle O(3)–Cu(1)–O(3) of
95.7(2)8 are similar to the corresponding distances and
Table 3
4.2. Crystal structure analysis of [Cu(acac)
(2,4,6-Me3BIAN)]ClO4 (2)
Infrared band positions and band assignments of complexes 2–4
Band position (cm21
)
Assignment
The ORTEP plot with the atomic-numbering scheme of
[Cu(acac)(2,4,6-Me3BIAN)]ClO4 is given in Fig. 4.
Selected bond lengths, angles and torsion angles are
compiled in Table 2.
Complex 2
Complex 3
Complex 4
432
577
684
410
555
684
430
574
689
n (Cu–O) þ n (C–CH3)
Ring def. þ n (Cu–O)
n (C–CH3) þ ring def. þ n
(Cu–O)
This complex contains a square–planar copper(II)
moiety with two imine nitrogen atoms of 2,4,6-Me3C6H2-
BIAN and two oxygen atoms of acac (acetylacetonate)
occupying the basal plane. The copper atom is deviated
˚
from the square plane by 0.1227 A. X-ray analysis indicates
˚
that the imine CyN bond N(4)–C(5) of 1.277(2) A is
930
930
1277
1516
929
1290
1516
n (CyC) þ n (CyO)
n (C–CH3) þ n (CyC)
n (CyC) þ n (CyO)
combination
1277
1516
1575
1562
1586
˚
slightly longer than the corresponding bond of 1.2662(16) A

214
U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
Fig. 5. Infrared spectra of free 2,4,6-Me3C6H2-BIAN ligand and complex 2.
bite angles observed in other mixed ligand copper(II)
complexes comprising (acac) ligand [15–17].
the starting diketones in the 1700–1800 cm21 region.
Bands assigned to nðCyNÞ are shifted to lower wavenum-
bers in complex spectra indicating the coordination of both
diimine nitrogen atoms of 2,4,6-Me3C6H2-BIAN ligand to
the copper ion.
4.3. IR spectra of ligand and the complexes
Strong band observed at 1116 cm21 (antisymmetric
stretch) and the sharp band at 623 cm21 (antisymmetric
bend), suggest uncoordinated perchlorate anions [18] in
complexes 2–4.
Formation of the free 2,4,6-Me3C6H2-BIAN ligand can
be concluded from the IR spectroscopy where only CyN
stretching vibrations were observed in the 1635–
1668 cm21 range and no CyO stretching vibrations of
Fig. 6. Electronic absorption spectra of 1023 mol l21 [Cu(acac)(2,4,6-Me3C6H2-BIAN)]ClO4 in DCE, DCM, AN, AC and DMF at 25 8C.

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
215
Table 4
Absorption maxima lmax (nm) of perchlorate complexes, 2–4, in different
solvents
Solvent
DNa
lmax (1 l mol21cm21 b
)
2
3
4
CH2Cl2 (DCM)
CH3NO2 (NM)
0
566(286)
n.d.
560(191)
n.d.
555(379)
559(240)
n.d.
2.7
14.1
17
CH3CN (AN)
613(220)
629(231)
653(141)
610(201)
620(218)
648(124)
CH3COCH3 (AC)
HCON(CH3)2 (DMF)
613(221)
637(211)
26.6
Fig. 7. Solvation free energy relationship (DN vs nmax=103).
n.d., not detected.
a
the copper center. In other words, the coordination sphere
around the metal ion becomes electron-poor, what make it
easier for a solvent molecule to approach the axial centers
leading to more solvatochromic effect.
Donor number of solvents, from Ref. [20].
b
The extinction coefficient values are given in parenthesis.
Infrared spectroscopic data (Table 3) suggest the
coordination of (acac) ligand to the copper(II) centre in
agreement with the X-ray structure of 2. Similarly, band
position and band assignments listed in Table 3 suggest
the coordination of (bzac and dibm) ligands in complexes 3
and 4, respectively [19]. Infrared spectra of the free 2,4,6-
Me3C6H2-BIAN ligand and complex 2 are shown in Fig. 5.
From conductivity data (Table 5) of these perchlorate
complexes it can be seen that ClO24 acts only as a counter ion
and do not interact with any solute (complex cation) [20].
Therefore, the complex structure depends only on the donor
properties of the solvent used (Gutmann’s donor number
DN) [21], and only solute–solvent interactions can be
considered.
4.4. Electronic spectra of complexes in various solvents
This solvatochromic behavior can be studied quantitat-
ively by applying the linear solvation free energy relation-
ship [22], nmax=103 ¼ n8 þ a (DN); where nmax; the
measured d–d absorption frequency; n8; the extrapolated
frequency and a; the slope, represents the sensitivity of the
complex toward solvent. Linearity of the nmax vs. DN,
(Fig. 7) confirms the solvatochromic behavior in perchlorate
complexes. The slope value ðaÞ in the order (acac , bzac ,
dibm) shows increasing solvatochromic effect in the same
order.
The characteristic properties of the obtained perchlorate
complexes(2–4) are (1) their high solubility in various
solvents, and (2) the change in the color of the solution
observed in going from one solvent to another, that is, strong
solvatochromism of their solutions. In Fig. 6 the spectral
changes of [Cu(acac)(2,4,6-Me3C6H2-BIAN)]ClO4 in var-
ious solvents are shown as an example of this
solvatochromism.
The absorption spectra show one broad band attributed to
the promotions of the electron in the lower energy orbitals to
the hole in dx 2y orbital of the copper(II) ion ðd9Þ: The
position of this band is shifted to longer wavelength (red
shift) as the Lewis basicity of solvent increases (Table 4).
This red shift is attributed to the strong repulsion of the
electrons in dz2 orbital by the lone pair electrons of the
solvent that is axially coordinated to the copper center,
decreasing the energy required to transfer the electrons to
dx22y2 orbital.
2
2
5. Conclusion
In addition to their high solubility in various organic
solvents, these mixed-ligand copper complexes showed a
good correlation between their d-d absorption maxima in
solution and the donor strength of the solvent used (positive
solvatochromism). This behavior is further confirmed by
applying the linear solvation free energy relationship.
The X-ray crystal structure of complex 2 confirms a
square–planar geometry, which makes it possible for
solvent molecules to attack the axial positions through
solute–solvent interactions, thus leading to structural
changes in the first coordination sphere.
Introducing electron-withdrawing substituents on dike
decrease its equatorial ligand field strength and conse-
quently, favor the axial coordination of the solvent to
Table 5
Electrical conductivity data, Lm (V21cm2mol21), of complexes 2–4 in
DCE and AC
Complex
DCE
AC
6. Supplementary material
2
3
4
18
20
22
105
123
112
Crystallographic data for 1 and 2 have been deposited at
the Cambridge Crystallographic Data Centre as supplemen-
tary publication numbers CCDC 211374 for 1 and CCDC
211612 for 2. Copies of the data can be obtained free of
Standard values for a 1:1 electrolyte type in DCE (dichloroethane) and
AC (acetone) are 20 and 100 (V21cm2mol21), respectively. From Ref. [19].

216
U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216
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Acknowledgements
This work was partly supported by a Grant-in-Aid for
Scientific Research (Project No. 11304046) from the
Ministry of Education, Science, Culture and Sports of
Japan. Dr Usama El-Ayaan thanks JSPS (Japan Society for
the Promotion of Science) for supporting his stay in Japan.
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