Structural studies of mixed-ligands copper(II) and copper(I)

complexes with the rigid nitrogen ligand:

bis[N-(2,6-diisopropylphenyl)imino]acenaphthene

Usama El-Ayaan*, Adriana Paulovicova, Yutaka Fukuda

Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan

Received 24 September 2002; revised 21 October 2002; accepted 21 October 2002

Abstract

The crystal structure of the rigid bidentate nitrogen ligand bis[N-(2,6-diisopropylphenyl)imino]acenaphtene (abbr. o,o0-

i Pr2C6H3-BIAN) and three mixed- ligand copper complexes, namely [CuII(AcOH)(o,o0-i Pr2C6H3-BIAN)Cl2] (1),

[CuII(acac)(AcOH)(o,o0-i Pr2C6H3-BIAN)]·(ClO4) (2) (where AcOH ¼ acetic acid and acac ¼ acetylacetonate) and

[CuIBr(o,o0-i Pr2-p-BrC6H2-BIAN)]2 (3) are reported.

In complexes 1 and 2, the coordination around the copper(II) centre is distorted square pyramidal with acetic acid molecule in

the apical position. Two imine nitrogen atoms of o,o0-i Pr2C6H3-BIAN are occupying the basal plane together with two chloride

atoms in case of 1 and two oxygen atoms (from acac ligand) in case of 2. In complex 3 the copper(I) ion is tetrahedrally

surrounded by two imine nitrogen atoms of o,o0-i Pr2-p-BrC6H2-BIAN, and by two bridging bromide atoms. Complex formation

is followed by certain structural changes inside the rigid N–N ligand. Such changes led to an increased planarity of the o,o0-

i Pr2C6H3-BIAN backbone, and produced a nearly perpendicular angle between the naphthalene and the aromatic imine planes.

q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Mixed-ligand complexes; N-ligands; Copper; X-ray crystal structure

1. Introduction

Rigid bidentate ligands such as bis(N-arylimi-

no)acenaphthene Ar-BIAN and their complexation

with late transition metals has increased use in

catalysis [1–5]. These Ar-BIAN ligands contain

two conjugated imine functions, but they are

markedly different from other diimine ligands such

as 2,20-bipyridine (bpy) and 1,10-phenanthroline

(phen). Firstly, the presence of two exocyclic

imines which are not part of a heteroaromatic

ring system, is expected to lead to better s-

donating and better p-accepting properties as

compared to phen and bpy [6–8]. This property

allows Ar-BIAN ligands to stabilize both the higher

and lower oxidation states of the metal ions.

Secondly, the rigid naphthalene backbone prevents

rotation around the imine carbon–carbon bond and

forces the imine N atoms to remain in a fixed cis

orientation favoring chelating coordination to a

metal centre.

0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.

PII: S0 02 2 -2 86 0 (0 2) 00 5 62 -8

Journal of Molecular Structure 645 (2003) 205–212

www.elsevier.com/locate/molstruc

* Corresponding author. Fax: þ81-359-78-5291.

E-mail address: usama@cc.ocha.ac.jp (U. El-Ayaan).

Copper complexes containing chelating diimine

ligands are finding increasing use, for example, as

catalysts for a wide range of synthetic organic

reactions [9–11] or as suitable systems for molecular

solar-energy conversion and molecular sensing

[12–15]. Another interesting quality inherent to

copper coordination compounds with diimines is the

structural difference between the Cu(I) and Cu(II)

oxidation states [16]. Although copper(I) prefers to be

four-coordinate with a nearly tetrahedral geometry,

crystal structure studies of many CuI(NN)2þ systems

show a considerable deviation from the tetrahedral

geometry [17 – 20]. Most commonly, the angle

between the two diimine ligands is less than the

idealized 908. This flattening of the ligands (towards a

square-planar geometry) lowers the symmetry from

D2d to D2.

Here we describe the isolation and

full characterization including X-ray structures

of o,o0-i Pr2C6H3-BIAN ligand and three

mixed-ligand copper complexes, namely

[CuII(AcOH)(o,o0-i Pr2C6H3-BIAN)Cl2] (1),

[CuII(acac)(AcOH)(o,o0-i Pr2C6H3-BIAN)]·(ClO4) (2)

and [CuIBr(o,o0-i Pr2-p-BrC6H2-BIAN)]2 (3).

2. Experimental

2.1. Materials and instrumentation

All chemicals were purchased from Wako Pure

Chemical Industries Ltd., and used without further

purification.

Elemental analyses (C, H, N) were performed on a

PE 2400 Series II CHNSIO Analyzer. Electronic

spectra were recorded on a UV-3100PC Shimadzu

spectrophotometer 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 in a KBr pellet

and as a Nujol mull in the 4000–370 cm21 spectral

ranges. Magnetic susceptibilities were measured at

room temperature on a Shimadzu MB-100 Torsion

Magnetometer. Diamagnetic corrections were con-

sidered by using Pascal’s constants [21]. Thermo-

gravimetric measurements were performed on

a DTG-50 Shimadzu instrument. 1H and 13C NMR

spectra were run on a JEOL JNM LA 300WB

spectrometer at 300.40 and 75.45 MHz, respectively,

using the 5 mm probe head. CD2Cl2 and CDCl3 were

used as a solvent. Chemical shifts are given in ppm

relative to internal TMS standard. The typical pulse

width was 6.25 ms for 1H and 4.25 ms for 13C

measurements. Assignment of 13C signals based on

Ref. [6] was confirmed by off-resonance decoupled

and DEPT-135 measurements.

2.2. X-ray data collection and structure refinement

Crystallographic data, data collection and struc-

ture refinement for o,o0-i Pr2C6H3-BIAN ligand and

complexes 1–3 are summarized in Table 1.

Crystal data for o,o0-i Pr2C6H3-BIAN ligand were

collected with a Rigaku AFC7R diffractometer

using graphite monochromator Cu Ka radiation.

For complexes 2 and 3 the crystal data were

collected with a SMART/RA CCD diffractometer

(Mo Ka radiation, graphite monochromator), while

for complex 1, a Mac Science DIP2030K diffract-

ometer was used. The structure of o,o0-i Pr2C6H3-

BIAN ligand was solved by direct methods

SHELXS-86 [22] and refined on F 2 using SHELXS-

93 [23]. The non-hydrogen atoms were refined

anisotropically by full-matrix least squares method.

Hydrogen atoms were included, but their positions

were not refined.

The initial structure of complexes 1–3 was

solved by SIR92 [24] and refined by least-squares

methods based on F 2 using SHELXL-97 [25].

Thermal ellipsoids (30% probability) and a mol-

ecular plot were drawn using the ORTEP program

for 2 and 3 and the maXus (MacScience) program

for 1. The multi-scan empirical absorption correc-

tions were applied by (Tmin/Tmax of 0.829/1.0000)

for 2 and by (Tmin/Tmax of 0.691/0.740) and (Tmin/

Tmax of 0.5575/1.0000) for 1 and 3, respectively.

All non-hydrogen atoms were refined anisotropi-

cally. Hydrogen atoms were added at calculated

positions and refined geometrically on their respective

carrier atoms using three common isotropic thermal

motion parameters (constrained for 1, and mixed

treatment for 2 and 3).

U. El-Ayaan et al. / Journal of Molecular Structure 645 (2003) 205–212206

3. Synthesis

3.1. [N-(2,6-diisopropylphenyl)imino]acenaphthene

ligand

The o,o0-i Pr2C6H3-BIAN ligand was synthesized

from acenaphthenequinone and 2,6-diisopropylani-

line as follow: 1.35 g (7.4 mmol) of acenaphthene-

quinone in 65 ml of acetonitrile was refluxed (80 8C)

for 30 min. Then 12 ml of acetic acid was added

and heating was continued until the acenaphthene-

quinone had completely dissolved. To this hot

solution, 3 ml (16 mmol) of 2,6-diisopropylaniline

was added directly and the solution was heated

under reflux for further 1.5 h. It was then cooled to

room temperature and the solid filtered off to give a

yellow product that was washed with hexane and

air-dried. Crystals suitable for X-ray measurements

were obtained by recrystallization from hot hexane

solution. Yield: 3.15 g (85%). Found: C, 85.85; H,

8.03; N, 5.3. Calc. for C36H40N2: C, 86.35; H, 8.05;

N, 5.60%. 1H NMR (CDCl3, recorded at

300.40 MHz at 24 8C) d ¼ 0.97 (d, H23), 1.23 (d,

H24), 3.03 (sept, H22), 6.63 (d, H2), 7.26 (s, H15,

H16, H17), 7.36 (pst, H3), 7.88 (d, H4). 13C NMR

(CD2Cl2, 300.40 MHz, 24 8C): d ¼ 23.1 (C23), 23.2

(C24), 29.1 (C22), 123.5 (C16), 123.9 (C15, C17),

124.6 (C2), 128.3 (C3), 129.2 (C4), 130.0 (C1),

131.6 (C5), 135.5 (C14, C18), 141.2 (C10), 148.0

(C13), 161.1(C11).

Table 1

Crystal data, data collection and structure refinement for o,o0-i Pr2C6H3-BIAN ligand and complexes 1–3

o,o0-i Pr2C6H3-BIAN 1 2 3

Crystal data

Formula C36H40N2 C38H44Cl2CuN2O2 C43H51ClCuN2O8 C36H38Br3CuN2

Molecular mass 500.7 695.23 822.84 801.95

Crystal system Monoclinic Orthorhombic Orthorhombic Monoclinic

Space group Cc P21 21 21 Pba2 P21/n

a (A) 15.587(2) 12.1400(6) 21.7710(2) 13.9939(2)

b (A) 8.856(2) 13.2140(6) 21.7780(3) 15.0192(2)

c (A) 21.780(7) 23.1620(5) 18.66700(10) 17.1426(2)

a; x (deg) 90.00 90.00 90.00

b (deg) 93.85(2) 90.00 90.00 78.47(2)

F(000) 1080 1460 3464 1608

V (A3] 2999.8(12) 3715.6(3) 8850.56(15) 3530.24(8)

Z 4 4 8 4

Dcalcd. (g cm23) 1.109 1.326 1.363 1.51

m (cm21) 0.481 13.19 6.12 4.066

Crystal size (mm) 0.5 £ 0.3 £ 0.1 0.35 £ 0.25 £ 0.13 0.50 £ 0.45 £ 0.25 0.5 £ 0.2 £ 0.2

Data collection

Temperature (K) 293(2) 298 293 293(2)

u range (deg) 4.07–67.97 1.08–27.98 1.09–27.46 1.72–27.45

Radiation Cu Ka (Mon),

1.54178 A

Mo Ka (Mon),

0.71073 A

Mo Ka (Mon),

0.71069 A

Mo Ka (Mon),

0.71069 A

Scan mode v–2u Image plate v v

Dataset 218 # h # 12;

0 # k # 10;

226 # l # 26

215 # h # 16;

2 17 # k # 17;

229 # l # 29

228 # h # 19;

228 # k # 27;

224 # l # 23

218 # h # 17;

219 # k # 18;

220 # l # 22

Total data 3363 8650 59497 23383

Unique data 3363 8630 19854 8010

Observed data 3345 ½I . 2:00sðIÞ� 7836 ½I . 2:00sðIÞ� 13207 ½I . 2:00sðIÞ� 5157 ½I . 2:00sðIÞ�

Refinement

Refined parameters 344 407 1014 387

Final R, wR 0.0495, 0.1550 0.0476, 0.1365 0.0608, 0.1552 0.0685, 0.1954

U. El-Ayaan et al. / Journal of Molecular Structure 645 (2003) 205–212 207

3.2. Complexes

3.2.1. [Cu(AcOH)(o,o 0-i Pr2C6H3-BIAN)Cl2] (1)

bis[N-(2,6-diisopropylphenyl)-imino]ace-

naphthene, o,o0-i Pr2C6H3-BIAN, (0.35 g, 0.7 mmol)

was dissolved in a small amount of chloroform (99%).

Then a mixture of chloroform/acetic acid (15 ml, the

2:1 ratio) was added and the solution was heated for

10 min at 60 8C. After that the methanol solution

(7 ml) of CuCl2·2H2O (0.12 g, 0.7 mmol) was added

to the o,o0-i Pr2C6H3-BIAN solution slowly, drop by

drop. This mixture was stirred for 2 h at room

temperature. After 2 days of slow evaporating in

open air, dark-green prism-shaped crystals suitable for

the single X-ray measurement appeared. Yield: 0.39 g

(80%). –C38H44Cl2N2O2Cu (695.202): Calcd. C

65.65, H 6.38, N 4.03%; Found. C 66.30, H 6.35, N

3.95%. meff: 2.0 BM (19 8C).

3.2.2. [Cu(acac)(AcOH)(o,o 0-i Pr2C6H3-

BIAN)]·(ClO4) (2)

0.07 ml (0.7 mmol) of acetylacetonate was added

directly to a methanolic solution of (0.26 g, 0.7 mmol)

of Cu(ClO4)2·6H2O. This mixture was stirred (while

heating) for 10 min. Then a hot 2:1 chloroform/acetic

acid solution (10/5 ml) of (0.35 g, 0.7 mmol) of o,o0-

i Pr2C6H3-BIAN was added. Obtained green solution

with no solid product was stirred for 1 h and left

standing freely in open air. After 3 days, needle-

shaped and prism-shaped green crystals appeared both

suitable for the X-ray measurement. Data were

collected from the green needle-shaped single crystal.

Yield 0.48 g (82%) of C43H51ClN2O8Cu (822.858):

Calcd. C 62.76, H 6.25, N 3.41%; Found C 62.55, H

6.33, N 3.58%. meff: 1.7 BM (24 8C).

3.2.3. Synthesis of [CuBr(o,o0-i Pr2-p-BrC6H2-

BIAN)]2 (3)

After dissolving o,o0-i Pr2C6H3-BIAN (0.30 g,

0.6 mmol) in a small amount of chloroform (99%),

18 ml of a chloroform/acetic acid solution (3:1) was

added. This reaction mixture was heated to 60 8C and

mixed with a methanol solution (5 ml) of CuBr2

(0.07 g, 0.3 mmol). After heating to reflux, another

5 ml of methanol CuBr2 solution (0.07 g, 0.3 mmol)

was added and the mixture was stirred for 2 h at room

temperature. The product was isolated as a brown

powder; yield: 69%, and purified by chromatography

on silica gel using dichloromethane as eluent. Crystals

suitable for the X-ray measurement were obtained

directly from the mother liquor by free evaporation in

air. C36H38Br3N2Cu (801.9): Calcd. C 53.92, H 4.78,

N 3.49%; Found. C 53.91, H 5.11, N 3.40%. UV/Vis

(lmax, nm; DCE): 480, 568 nm. meff (24 8C): diamag-

netic. 1H NMR (CD2Cl2, 300.40 MHz, 24 8C):

d ¼ 1.05 (d, H23), 1.28 (d, H24), 2.97 (sept, H22),

6.80 (d, H2), 7.50 (pst, H3), 8.10 (d, H4). 13C NMR

spectrum was not recorded due to low solubility.

4. Results and discussion

4.1. Crystal structure analysis

An ORTEP plot showing the atomic-numbering

scheme of o,o0-i Pr2C6H3-BIAN ligand and the three

mixed-ligand copper complexes 1, 2 and 3 (30%

thermal ellipsoids) is given in Fig. 1. Selected bond

lengths, angles and torsion angles are compiled in

Table 2.

Complexes 1 and 2 contain distorted square

pyramidal copper(II) moiety with two imine nitrogen

atoms of o,o0-i Pr2C6H3-BIAN occupying the basal

plane together with the two chloride atoms or the two

oxygen atoms of acac in 1 and 2, respectively. The

monodentate acetic acid is accommodated in the

apical position with relatively longer bond length of

2.467(3) A in 1 and 2.2721(13) A in 2 than those of

the basal plane. In complex 3 and during an attempt to

synthesize the bromide analogue of complex 1, an

additional factor suddenly came into play. Bromina-

tion of the phenyl rings of o,o0-i Pr2C6H3-BIAN took

place in the para position, as a result of the reduction

of Cu(II) to Cu(I), which evidenced by the X-ray

analysis. In 3 the copper(I) is tetrahedrally surrounded

by two imine nitrogen atoms of o,o0-i Pr2-p-BrC6H2-

BIAN, and by two bridging bromide atoms.

Comparison of the bond lengths and angles in 1, 2

and 3 with those of free o,o0-i Pr2C6H3-BIAN ligand

shows some differences that must result from the

coordination to the copper. These differences concern

the imine CyN bonds, N(1)–C(11) and N(2)–C(12)

of 1.250(6) and 1.295(6) A, respectively in the free

ligand. Because of their coordination to the copper

centre, these bonds become longer as follow; 1.284(3)

U. El-Ayaan et al. / Journal of Molecular Structure 645 (2003) 205–212208

and 1.292(3) A in 1; 1.359(2) and 1.286(2) A in 2 and

1.287(6) and 1.283(6) A in 3, respectively.

Upon coordination, a lowering of the strain in the

naphthalene backbone occurs, which is indicated by

the C(11)–C(12) bond shortening from 1.526(3) A in

case of the free ligand to 1.517(6) A in 3, 1.500(3) A

in 1 and 1.433(2) A in 2. Torsion angels N(1)–C(11)–

C(12)–N(2) of 20.79(0.29)8 and C(1)–C(11)–

C(12)–C(9) of 21.70(0.21)8 reflect the planarity of

bis(imino)acenaphthene moiety in the free o,o0-

i Pr2C6H3-BIAN ligand. Upon coordination to the

copper centre, a more planar arrangement was

confirmed by the smaller torsion angels (see Table 1).

In agreement with the increasing length of the

average Cu–N distance in the series 2 (2.0454 A), 1

(2.0702 A), and 3 (2.1114 A), the bite angle N–Cu–N

decreases from 81.81(5) via 80.36(8) to 80.17(15)8

(Table 1). This decrease in the N–Cu–N angle is

accompanied by bending of the aromatic groups on

the imine N atoms more towards the copper center and

away from the naphthalene backbone leading to more

perpendicular angles between the planes of naphtha-

lene and the aromatic imine. The rigid naphthalene

moiety and the aromatic groups with o- and p-

substituents show no anomalies and the bond

distances and angles are within the expected ranges

[26].

The nearly perfect perpendicular arrangement of

the o,o0-diisopropylphenyl groups in complex 2

shields the copper centre more effectively above and

below the coordination plane. Such an extra shielding

makes it difficult to accommodate the (acac) ligand

within the basal plane. This problem is perfectly

solved by the appropriate structural changes inside the

o,o0-i Pr2C6H3-BIAN ligand, namely by the

elongation in N(1)–C(11) of 1.359(2) A, larger bite

Fig. 1. An ORTEP views (30% ellipsoid probability) of o,o0-i Pr2C6H3-BIAN ligand and complexes [CuII(AcOH)(o,o0-i Pr2C6H3-BIAN)Cl2] (1),

[CuII(acac)(AcOH)(o,o0-i Pr2C6H3-BIAN)]·(ClO4) (2) and [CuIBr(o,o0-i Pr2-p-BrC6H2-BIAN)]2 (3).

U. El-Ayaan et al. / Journal of Molecular Structure 645 (2003) 205–212 209

angle N(1)–Cu–N(2) of 828, and also by the C(11)–

C(12) bond shortening of 1.433(2) A. Consequently,

all these changes within the o,o0-i Pr2C6H3-BIAN

ligand led to one relatively longer Cu–N(1) distance

of 2.0755(13) A than the other [Cu – N(2) of

2.0153(14) A]. Complex 2 with the structural index

parameter t of the 2.6% value shows more regular

square-pyramidal geometry than complex 1 with the

same structural parameter of 4.5%. According

Addison et al. [27], t should have a value of 0% for

a perfect square-pyramid and 100% for a perfect

trigonal pyramidal structure, t ¼ {ðu2 wÞ=60} £ 100;

where u and w are the largest and the second largest

basal angles, respectively.

4.2. IR and UV/Vis spectroscopy

Infrared spectral data for the free o,o0-i Pr2C6H3-

BIAN ligand and complexes 1–3 are listed in Table 3.

Formation of the free o,o0-i Pr2C6H3-BIAN ligand can

be concluded from the IR spectroscopy where only

CyN stretching vibrations were observed in the

1642–1671 cm21 range and no CyO stretching

vibrations of the starting diketones in the 1700–

1800 cm21 region. Bands assigned to n(CyN) are

shifted to lower wavenumbers in complex spectra

indicating the coordination of both diimine nitrogen

atoms of o,o0-i Pr2C6H3-BIAN ligand to the copper

ion. Strong bands observed in the 1695 and

Table 2

Selected bond lengths (A), angles (deg), and torsion angles (deg) for o,o0-i Pr2C6H3-BIA and complexes 1–3

o,o0-i Pr2C6H3-BIAN 1 2 3

N(1)–C(11) 1.250(6) 1.284(3) 1.359(2) 1.287(6)

N(1)–C(13) 1.418(6) 1.442(3) 1.442(2) 1.452(6)

C(1)–C(2) 1.37(7) 1.381(5) 1.343(3) 1.383(7)

C(1)–C(10) 1.402(7) 1.404(5) 1.395(3) 1.420(7)

C(1)–C(11) 1.505(5) 1.455(3) 1.506(2) 1.462(7)

C(2)–C(3) 1.422(8) 1.414(5) 1.358(3) 1.406(9)

C(11)–C(12) 1.526(3) 1.500(3) 1.433(2) 1.517(6)

C(13)–C(14) 1.417(8) 1.391(4) 1.339(3) 1.397(7)

C(13)–C(18) 1.397(7) 1.390(5) 1.395(3) 1.395(7)

C(14)–C(15) 1.393(8) 1.402(5) 1.418(3) 1.414(7)

C(15)–C(16) 1.384(11) 1.393(7) 1.305(3) 1.376(8)

C(16)–C(17) 1.346(12) 1.351(6) 1.327(4) 1.368(8)

C(17)–C(18) 1.387(8) 1.417(5) 1.401(3) 1.403(8)

Cu–N(1) 2.0561(19) 2.0153(14) 2.110(4)

Cu–N(2) 2.084(2) 2.0755(13) 2.122(4)

Cu–Cl(1) 2.2213(8)

Cu–Cl(2) 2.2453(7)

Cu–O (acetic acid) 2.467(3) 2.2721(13)

N(1)–Cu–N(2) 80.36(8) 81.81(5) 80.17(15)

C(11)–N(1)–C(13) 123.3(4) 117.66(19) 124.60(14) 119.4(4)

C(12)–N(2)–C(25) 122.8(4) 119.8(2) 115.78(14) 120.1(4)

C(2)–C(1)–C(10) 120.3(5) 119.4(3) 118.62(17) 118.4(5)

N(1)–C(11)–C(1) 133.8(4) 134.8(3) 126.13(16) 135.0(4)

N(1)–C(11)–C(12) 122.2(4) 117.6(2) 121.15(15) 117.8(4)

C(1)–C(11)–C(12) 104.0(4) 107.3(2) 112.41(15) 106.9(4)

N(1)–C(11)–C(12)–N(2) 20.79(0.29) 20.3(3) 0.1(2) 20.2(7)

C(1)–C(11)–C(12)–C(9) 21.70(0.21) 20.4(3) 0.3(2) 20.7(5)

C(14)–C(13)–N(1)–C(11) 2104.46(0.60) 287.8(4) 92.6(2) 299.8(6)

C(18)–C(13)–N(1)–C(11) 83.88(0.61) 92.1(4) 287.0(2) 84.7(6)

C(30)–C(25)–N(2)–C(12) 2105.25(0.64) 298.5(4) 86.7(2) 278.6(6)

C(26)–C(25)–N(2)–C(12) 82.64(0.61) 83.8(4) 294.60(19) 105.8(6)

U. El-Ayaan et al. / Journal of Molecular Structure 645 (2003) 205–212210

1705 cm21 regions for complex 1 and 2, respectively,

indicate the coordination of acetic acid molecule.

IR spectra of complex 2 show two peaks at 1570

and 1519 cm21 that can be assigned to n(CyO) and

n(CyC) of the acac ligand, respectively, in a very

good agreement with those observed for [Cu(acac)2]

[28]. Strong band observed at 1110 cm21 (antisym-

metric stretch) and the sharp band at 622 cm21

(antisymmetric bend), suggest the discrete perchlorate

anions [29] in agreement with the X-ray structure of 2.

Solid-state room temperature electronic spectra of

1 and 2 exhibit a broad d–d band at 700 and 625 nm,

respectively, typical for the square-pyramidal cop-

per(II) complexes [30–32]. Another band appeared in

the visible spectral region (558 and 545 nm) of

complexes 1 and 2, respectively, assigned to the

MLCT transition from copper(II) to the pp orbitals of

o,o0-i Pr2C6H3-BIAN. This shift to much longer

wavelength than those of other diimine ligands can

be explained on the basis that o,o0-i Pr2C6H3-BIAN

ligand provides pp orbitals at rather low energies [33,

34] what results in the increased overlap with the

metal (d) orbitals and consequently, in the stronger p-

bonding interaction accompanied by a red-shift of the

MLCT transition. Electrochemical studies [35] on the

p-acceptor properties of the bis(arylimino)ace-

naphthene reveal much higher positive potential

than those of other diimine ligands confirming higher

p-acceptor strength of bis(arylimino)acenaphthene.

Absorption spectra of complexes show various

bands below 420 nm that can be assigned to

intraligand transitions of the free diimine ligand

which absorbs in the same spectral region. Electronic

spectra were recorded also in solution using a variety

of solvents (DCE, THF, toluene and chloroform). All

solution spectra for complexes are very similar to the

corresponding solid-state spectra.

5. Supplementary material

Crystallographic data for all structures reported in

this paper have been deposited at the Cambridge

Crystallographic Data Centre as supplementary pub-

lication numbers CCDC-183689 for o,o0-i Pr2C6H3-

BIAN ligand, CCDC-158181 for 1, CCDC-158307

for 2, CCDC-162037 for 3. Copies of the data can be

obtained free of charge on application to CCDC, 12

Union Road, Cambridge CB2 1EZ, UK [Fax:

(internat.) þ44-1223/336-033; e-mail: depos-

it@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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Table 3

IR spectra (cm21) of o,o0-i Pr2C6H3-BIAN ligand and complexes 1–

3

Compound n(CyN) n(ClO42) n(AcOH)

o,o0-i Pr2C6H3-BIAN 1671, 1652, 1642 – –

Complex 1 1661, 1635 – 1695

Complex 2 1668, 1640 1110, 622 1705

Complex 3 1654, 1635 – –

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