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

Usama El-Ayaana,b,*, Fumiko Muratab, Soheir El-Derbyc, Yutaka Fukudab

aDepartment of Chemistry, Faculty of Science, Mansoura University, Mansoura 35516, EgyptbDepartment 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

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].

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 ClO42(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.

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

0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2004.01.040

Journal of Molecular Structure 692 (2004) 209–216

www.elsevier.com/locate/molstruc

* 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).

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.

2. Experimental

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

range. 1H- and 13C-NMR measurements at room tempera-

ture were run on a JEOL JNM LA 300 WB spectrometer at

400 MHz, using a 5 mm probe head in CDCl3. Chemical

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)

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).

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:

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

Table 1

Crystal data, data collection and structure refinement for 1 and 2

1 2

Crystal data

Empirical formula C30H28N2 C35H35ClCuN2O6

Formula weight 416.56 678.67

Crystal system Monoclinic Orthorhombic

Space group C2=c Pnma

a (A) 23.872(3) 14.401(9)

b (A) 12.2194(14) 17.266(13)

c (A) 7.9405(9) 13.101(9)

a; g (8) 90.00 90.00

b (8) 98.811(2) 90.00

V (A3) 2288.9(5) 3258 (4)

Z 4 4

Crystal size (mm3) 0.29 £ 0.27 £ 0.2 0.20 £ 0.30 £ 0.20

Dcalcd: (g cm23) 1.209 1.384

Fð000Þ 888 1412

m (mm21) 0.07 0.8

Data collection

Temperature (K) 100(2) 298

u range (8) 1.73–28.35 1.55–27.58

Radiation Mo Ka (Mon), 0.71073 A Mo Ka (Mon), 0.71073 A

Scan mode F and v 68 (min)

Index ranges 231 # h # 30; 215 # k # 16; 210 # l ¼ 9 218 # h # 0; 0 # k # 22; 20 # l ¼ 17

Reflections collected 7739 4158

Independent reflections 2843 ½RðintÞ ¼ 0:084� 3507 ½RðintÞ ¼ 0:1017�

Refinement

Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2

Data/restraints/parameters 2312/0/202 3666/0/215

Goodness-of-fit on F2 1.104 1.949

Final R indices ðI . 2:00sðIÞÞ 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) 209–216210

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.

3. Synthesis

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%).

(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,

86.50; H, 6.78; N, 6.73%. 1H-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).

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

3.2.1. [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).

3.2.2. [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).

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).

Fig. 1. Preparation of 2,4,6-Me3C6H2-BIAN ligand.

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216 211

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].

The imine CyN bond of 1.2662(16) A is near to the

corresponding value of 1.275(6) A (mean value) inFig. 3. Unit cell view of 2,4,6-Me3C6H2-BIAN ligand.

Fig. 2. Ortep 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) Cu(1)–N(4) 2.026(3)

N(1)–C(8) 1.4205(15) Cu(1)–O(3) 1.882(3)

C(1)–C(2) 1.3761(13) N(4)–C(5) 1.277(2)

C(2)–C(3) 1.4178(17) N(4)–C(8) 1.443(5)

C(3)–C(4) 1.3755(19) C(5)–C(5) 1.539(7)

C(4)–C(5) 1.4192(14) C(5)–C(6) 1.469(6)

C(5)–C(6) 1.406(2) C(6)–C(9) 1.413(5)

C(7)–C(7) 1.528(2) C(7)–C(15) 1.385(7)

C(8)–C(13) 1.3986(18) C(7)–C(8) 1.403(6)

C(8)–C(9) 1.4004(19) C(8)–C(12) 1.395(6)

C(9)–C(10) 1.3940(17) C(12)–C(16) 1.426(6)

C(11)–C(12) 1.392(2) C(15)–C(20) 1.372(8)

C(12)–C(13) 1.3952(17) C(16)–C(20) 1.368(9)

C(7)–N(1)–C(8) 121.96(10) N(4)–Cu(1)–N(4) 82.70(18)

C(2)–C(1)–C(6) 119.37(11) O(3)–Cu(1)–O(3) 95.7(2)

N(1)–C(7)–C(1) 133.07(11) N(4)–Cu(1)–O(3) 90.35(15)

C(1)–C(2)–C(3) 118.25(11) Cu(1)–O(3)–C(11) 125.5(3)

C(4)–C(3)–C(2) 122.44(11) Cu(1)–N(4)–C(5) 111.0(3)

C(3)–C(4)–C(5) 120.56(12) Cu(1)–N(4)–C(8) 128.5(2)

C(6)–C(5)–C(4) 116.19(8) C(5)–N(4)–C(8) 120.4(3)

C(5)–C(6)–C(1) 123.16(7) N(4)–C(5)–C(6) 136.0(4)

C(13)–C(8)–C(9) 121.10(11) N(4)–C(5)–C(5) 116.5(2)

C(10)–C(9)–C(8) 118.55(12) N(4)–C(8)–C(12) 120.3(4)

C(9)–C(10)–C(11) 121.83(12) N(4)–C(8)–C(7) 117.5(4)

C(10)–C(11)–C(12) 118.17(11)

C(11)–C(12)–C(13) 122.06(12)

C(12)–C(13)–C(8) 118.27(12)

N(1)–C(7)–C(7)–N(1) 26.2 C(7)–C(8)–N(4)–C(5) 2101.7(5)

C(1)–C(7)–C(7)–C(1) 24.3 C(12)–C(8)–N(4)–C(5) 77.6(5)

C(7)–N(1)–C(8)–C(9) 2102.1 Cu(1)–N(4)–C(8)–C(7) 75.9(5)

C(7)–N(1)–C(8)–C(13) 85.4 C(5)–C(6)–C(9)–C(6) 20.1(7)

C(1)–C(7)–C(7)–N(1) 174.8 Cu(1)–O(3)–C(11)–C(13) 0.1(7)

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216212

the free (o,o0-i Pr2C6H3-BIAN) ligand [9] and is shorter

than the C–N bonds in 2,20-bipyridine [1.35 A] [10] and

2,20-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 Cðsp2Þ2

Cðsp2Þ 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].

4.2. Crystal structure analysis of [Cu(acac)

(2,4,6-Me3BIAN)]ClO4 (2)

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.

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

slightly longer than the corresponding bond of 1.2662(16) A

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.

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

Fig. 4. Ortep view of [Cu(acac)2,4,6-Me3C6H2-BIAN]ClO4.

Table 3

Infrared band positions and band assignments of complexes 2–4

Band position (cm21) Assignment

Complex 2 Complex 3 Complex 4

432 410 430 n (Cu–O) þ n (C–CH3)

577 555 574 Ring def. þ n (Cu–O)

684 684 689 n (C–CH3) þ ring def. þ n

(Cu–O)

930 930 929 n (CyC) þ n (CyO)

1277 1277 1290 n (C–CH3) þ n (CyC)

1516 1516 1516 n (CyC) þ n (CyO)

combination

1575 1562 1586

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216 213

bite angles observed in other mixed ligand copper(II)

complexes comprising (acac) ligand [15–17].

4.3. IR spectra of ligand and the complexes

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

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.

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.

Fig. 5. Infrared spectra of free 2,4,6-Me3C6H2-BIAN ligand and complex 2.

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) 209–216214

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.

4.4. Electronic spectra of complexes in various solvents

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 dx22y2 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.

Introducing electron-withdrawing substituents on dike

decrease its equatorial ligand field strength and conse-

quently, favor the axial coordination of the solvent to

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.

From conductivity data (Table 5) of these perchlorate

complexes it can be seen that ClO42acts 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.

This solvatochromic behavior can be studied quantitat-

ively by applying the linear solvation free energy relation-

ship [22], nmax=103 ¼ n8þ 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.

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.

6. Supplementary material

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

Table 5

Electrical conductivity data, Lm (V21cm2mol21), of complexes 2–4 in

DCE and AC

Complex DCE AC

2 18 105

3 20 123

4 22 112

Standard values for a 1:1 electrolyte type in DCE (dichloroethane) and

AC (acetone) are 20 and 100 (V21cm2mol21), respectively. From Ref. [19].

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) 0 566(286) 560(191) 555(379)

CH3NO2 (NM) 2.7 n.d. n.d. 559(240)

CH3CN (AN) 14.1 613(220) 610(201) n.d.

CH3COCH3 (AC) 17 629(231) 620(218) 613(221)

HCON(CH3)2 (DMF) 26.6 653(141) 648(124) 637(211)

n.d., not detected.a Donor number of solvents, from Ref. [20].b The extinction coefficient values are given in parenthesis.

Fig. 7. Solvation free energy relationship (DN vs nmax=103).

U. El-Ayaan et al. / Journal of Molecular Structure 692 (2004) 209–216 215

charge on application to CCDC, 12 Union Road, Cambridge

CB2 1EZ, UK [Fax: (internat.) þ 44-1223/336-033; E-mail:

deposit@ccdc.cam.ac.uk].

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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