Synthesis, characterization and spectroscopic and electrochemical studies of new axially coordinated cobalt(III) salen (salen = N,N′-bis(salicylidene)-1,2-ethylenediamine) complexes. The crystal structure of [CoIII(salen)(aniline)2]ClO4

Article abstract of DOI:10.1016/j.poly.2005.12.001

Cobalt(III) complexes with some salen ligands (salen = N,N′-bis(salicylidene)-1,2-ethylenediamine) as equatorial and potentially tetradentate ligands and with different axial amine ligands (cyclohexylamine, aniline, 4-picoline and pyridine) were synthesized in situ and characterized by IR, ¹H NMR and elemental analysis. Electronic spectra and electrochemical properties of the complexes were studied in DMF solutions. Electronic structure calculations indicate that the absorption between 420 and 440 nm, which are the lowest energy transfers in the complexes, are attributed to metal-ligand charge transfer transitions. The cyclic voltammetry of the complexes show an irreversible reduction of Co(III) to Co(II) and a reversible reduction of Co(II) to Co(I). The axial ligands affect reduction potentials of Co(III) in the order of changes of axial ligands' pK_a. The crystal structure of [Co(salen)(aniline)₂]ClO₄ was determined. In the [Co(salen)(aniline)₂]ClO₄, cobalt(III) center is six coordinated surrounded by an equatorial tetradentate salen Schiff-base ligand and two aniline axial ligands and multiscripts(ClO, 4, mml:none(), mml:none(), -) acts as a counter ion.

Full text of DOI:10.1016/j.poly.2005.12.001

Polyhedron 25 (2006) 1893–1900
www.elsevier.com/locate/poly
Synthesis, characterization and spectroscopic and
electrochemical studies of new axially coordinated cobalt(III)
salen (salen = N,N0-bis(salicylidene)-1,2-ethylenediamine)
complexes. The crystal structure of [CoIII(salen)(aniline)2]ClO4
a,
a
b
a
Ali Akbar Khandar *, Behrouz Shaabani , Ferdinand Belaj , Akbar Bakhtiari
a
Department of Inorganic Chemistry, Faculty of Chemistry, Tabriz University, Daneshgah Street, Tabriz, East Azarbaijan 51664, Iran
b
Institut fu¨r Chemie/Anorganische Chemie Karl-Franzens-Universita¨t Graz, Austria
Received 30 August 2005; accepted 1 December 2005
Available online 18 January 2006
Abstract
Cobalt(III) complexes with some salen ligands (salen = N,N0-bis(salicylidene)-1,2-ethylenediamine) as equatorial and potentially tet-
radentate ligands and with different axial amine ligands (cyclohexylamine, aniline, 4-picoline and pyridine) were synthesized in situ and
characterized by IR, 1H NMR and elemental analysis. Electronic spectra and electrochemical properties of the complexes were studied in
DMF solutions. Electronic structure calculations indicate that the absorption between 420 and 440 nm, which are the lowest energy
transfers in the complexes, are attributed to metal–ligand charge transfer transitions. The cyclic voltammetry of the complexes show
an irreversible reduction of Co(III) to Co(II) and a reversible reduction of Co(II) to Co(I). The axial ligands affect reduction potentials
of Co(III) in the order of changes of axial ligands’ pKa. The crystal structure of [Co(salen)(aniline)2]ClO4 was determined. In the
[Co(salen)(aniline)2]ClO4, cobalt(IIꢀI) center is six coordinated surrounded by an equatorial tetradentate salen Schiff-base ligand and
two aniline axial ligands and ClO4 acts as a counter ion.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Cobalt(III) complexes; Schiff-base; Axial amine ligand; Crystal structure; Electrochemistry
1. Introduction
in metallo-proteins and metallo-enzymes [9–13]. The cobalt
complexes of tetradentate Schiff-base ligands have
been widely used to mimic cobalamine (B12) coenzymes
[14–18]. Cobalt(III) Schiff-base complexes are potent anti-
viral, antibacterial and antitumor agents and those con-
taining labile axial ligands exhibit higher activities and
more stability [19].
Hopping to mimic such living systems and to shed some
light on binding modes and activity in this systems, we
report here, the synthesis, characterization, spectroscopic
and electrochemical studies of some cobalt(III) complexes
of salen and its phenyl azo derivative with axial amine
ligands, where salen is N,N0-bis(salicylidene)-1,2-ethylene-
diamine. Moreover the crystal structure of [CoIII(salen)-
(aniline)2]ClO4 was determined.
Cobalt Schiff-base complexes have been used as catalysts
for the oxygenation reactions, dioxygen carriers and oxy-
gen activators [1–5] and enantioselective and asymmetric
catalysis [6–8]. Modeling of transition metal coordination
species existing in living systems and studying of intermedi-
ate species involved in reactions of such systems are impor-
tant investigation fields that attract lots of studies [8]. In
the area of bioinorganic chemistry interest in Schiff-base
complexes has centered on the role of such complexes in
providing synthetic models for the metal-containing sites
*
Corresponding author. Tel.: +98 411 3393128; fax: +98 411 3340191.
E-mail address: akhandar@yahoo.com (A.A. Khandar).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.12.001

1894
A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
2. Experimental
2.1. Materials
2.3.1. Salen (C16H16N2O2) (H2L1)
1
Yellow crystals, yield 78% (0.4186 g); m.p.: 168 ꢁC. H
NMR (CDCl3): d 13.22 (br s, 2H, OH), 8.35 (s, 2H, He),
8.29 (m, 2H, Hb), 7.22 (dd, 2H, Hd), 6.94 (d, 2H, Ha),
6.85 (m, 2H, Hc), 3.94 (s, 4H, Hf, g). FT-IR(KBr, cmꢀ1):
3300–3400 (m, OH), 2900 (aliphatic CH), 1645 (s, imine
C@N), 1200 (phenolic CO).
All of used chemicals were reagent grade. Salicylalde-
hyde and aniline were used after distillation under reduced
pressure. Ethylenediamine and cyclohexylamine were dis-
tilled before use. The other reagents were used without
further purification. 5-Phenylazo salicylaldehyde was syn-
thesized as described in the literature [20].
2.3.2. Bis(5-phenyl azo)salen (C28H24N6O2) (H2L2)
Orange crystals, yield 92% (0.8768 g); m.p.: 229 ꢁC. H
NMR (CDCl3): d 13.79 (br s, 2H, OH), 8.52 (s, 2H, He),
1
2.2. Physical measurements and methods
7.97 (dd, 2H, Hb), 7.92 (d, 2H, Hd), 7.84–7.87 (m, 4H,
H
i, m), 7.42–7.52 (m, 6H, Hj, k, l), 7.07 (d, 2H, Ha), 4.04 (s,
Infrared (FT-IR) spectra were recorded using KBr
4H, Hf, g). FT-IR(KBr, cmꢀ1): 3300–3400 (m, OH), 1635
1
discs on a Bruker Tensor 27 instrument. H NMR spectra
(s, imine C@N), 1185 (phenolic CO).
were taken on a Bruker Spectrospin Avance 400 MHz
ultrashield spectrometer. The electronic spectra in the
200–600 nm range were obtained on a Shimadzu UV-
265 FW spectrophotometer and quartz cells of light pass
length 1.00 cm, using solutions with the concentration of
1 · 10ꢀ5 in DMF. CHN elemental analyses were per-
formed using a Heraeus CHN-O RAPID elemental ana-
lyzer. Melting points were taken using an electrothermal
IA 9100 apparatus in open capillary tubes. Cyclic voltam-
mograms were recorded using an AMEL instrument
Model 2053 as potentiostat connected with AMEL Model
568 as function generator. All solutions were deoxygen-
ated by passing a stream of argon into the solution for
at least 10 min before recording the voltammogram. All
potentials reported herein were measured at room temper-
ature and referenced to the saturated Ag/AgCl electrode
with ferrocene as an internal standard. A glassy carbon
disc with a diameter of 3 mm was used as the working
electrode and a platinum wire was used as counter elec-
trode. Before each experiment the working electrode
was polished with alumina and rinsed thoroughly with
distilled water and acetone. The electrolytic medium con-
sisted of 0.1 M LiClO4 in DMF. Under these conditions
the ferrocenium–ferrocene couple shows a peak separa-
2.4. Synthesis of the complexes
The complexes shown in Scheme 1 were synthesized as
follows.
1 mmol of Co (II) acetate tetrahydrate, 1 mmol of the
appropriate Schiff-base ligand, 2 mmol of axial amine
and 4 mmol of LiClO4 were added to 100 ml of ethanol.
The obtained solution turned red rapidly upon the forma-
tion of Co(II)(salen) complex. Then air was bubbled
through the reaction mixture for oxidizing the Co(II)(salen)
and the mixture stirred and refluxed for 2 h. Then air bub-
bling and stirring was continued for 24 h. After which the
obtained brown material was washed and recrystallized
from appropriate solvent systems.
2.4.1. Preparation of [CoL1(cyclohexylamine)2]ClO4
The complex was recrystallized from hot ethanol, yield-
ing brown crystals. Yield 96.5% (0.6012 g). Anal. Calc. for
CoC28H40N4O6Cl: C, 54.0; H, 6.5; N, 9.0. Found: C, 53.3;
H, 6.4; N, 8.8%; m.p.: 199.2 ꢁC. 1H NMR (CD3CN): d 8.17
tion of 89 mV.
A Pentium IV computer with a
H
c'
H
b'
H
H
H
1200 MHz processor, 256 MB of RAM and the HYPER-
CHEM package version 7 [21] were used for quantum
chemical calculations.
b'
a'
amine
H
c'
a'
H
g
H
NH
f
H
NH
e
H
d'
H
d'
H
H
N
N
d
b
Co
2.3. Synthesis of Schiff-base ligands
R
O
O
R
H
c'
H
c'
H2C
N
H
amine
H
b'
b'
a'
H
a
Schiff-base ligands were prepared in a similar manner
according to the literature [22] as the following procedure:
To a stirring solution of 2 mmol of appropriate aldehyde
in ethanol was added a solution of 1 mmol diamine in
ethanol dropwise and then refluxed for 2 h. The precipi-
tated Schiff-base was filtered and recrystallized from hot
ethanol giving N,N0-bis(5-X-salicylidene)-1,2-ethylenedia-
mine, where X is H and N2ph (diazophenyl) that will
abbreviated here as H2L1 and H2L2, respectively. Some
analytical and physical data of the ligands are summa-
rized as follows.
N
H
H
a'
H
H
l
m
L1: R=Hc L2: R=
N
N
H
k
H
H
j
i
H2L1 (salen) complexes: amine= pyridine, picoline, aniline, cyclohexylamine
H2L2 (bis(5-phenylazo)salen) complex: amine= pyridine
Scheme 1. A schematic representation of the studied complexes.

A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
1895
(s, 2H, He), 7.34 (dd, 2H, Hd), 7.24 (m, 2H, Hb), 7.10 (d,
2H, Ha), 6.53 (q, 2H, Hc), 3.39 (s, 4H, Hf, g), 2.82 (br d,
All the measurements were performed using graphite
monochromatized Mo Ka radiation (0.71069 A) at 95 K:
˚
0
0
4H, Hd ), 2.21 (br m, 2H, Hc ), {1.37 (m, 10H) and 0.85
C28H28ClCoN4O6, Mr 610.92, monoclinic, space group
(m, 10H): Ha and Hb }. FT-IR(KBr, cmꢀ1): 3295.69 and
3245.86 (m, NH), 1629 (s, C@N), 1090 (br s, ClO4ꢀ).
P21/c, a = 11.422(3) A, b = 16.219(5) A, c = 14.216(4) A,
˚
˚
˚
0
0
3
˚
b = 101.77(2)ꢁ, V = 2578.2(13) A , Z = 4, Dcalc = 1.574 g/
cm3, absorption coefficient l = 0.822 mmꢀ1. Due to the
large organic ligand, the linear absorption coefficient (l)
is only 0.822 mmꢀ1 and the crystal used was small (max
diameter r = 0.28 mm), so the product of l · r is only
0.23 and the absorption correction was not carried out.
The other crystal data and structure refinement informa-
tion are as follows: crystal size: 0.28 · 0.18 · 0.12(mm),
F(000): 1264, diffractometer: Stoe, radiation source: fine-
focus sealed tube, scan type: x scans, standard reflections:
3 for every 100 reflections, intensity decay: 1.4%, h range
for data collection: 2.51–26.00 (ꢁ), index ranges:
ꢀ14 6 h 6 13, ꢀ1 6 k 6 20, ꢀ1 6 l 6 17, reflections col-
lected/unique: 6080/5061, significant unique reflections:
4047 with I > 2r(I), R(int)/R(sigma): 0.0319/0.0635, com-
pleteness to H = 26.0ꢁ: 99.8%, refinement method: full-
matrix least-squares on F2, data/parameters/restraints:
5061/371/2, goodness-of-fit on F2: 1.063, final R indices
[I > 2(I)]: R1 = 0.0452, wR2 = 0.0925, R indices (all data):
R1 = 0.0647, wR2 = 0.1029, extinction expression: none,
2.4.2. Preparation of [CoL1(aniline)2]ClO4
The complex was recrystallized from acetonitrile/etha-
nol (1:2 v/v) yielding brown crystals. Yield 98.2%
(0.5999 g). Anal. Calc. for CoC28H28N4O6Cl: C, 55.1; H,
4.6; N, 9.2. Found: C, 54.3; H, 4.8; N, 8.9%; m.p.:
228.6 ꢁC. 1H NMR (CD3CN): d 7.92 (s, 2H, He), 7.32
0
(m, 2H, Hd), 7.16 (dd, 4H, Ha ), 7.06 (m, 2H, Hb), 6.90–
0
0
6.94 (m, 4H, Ha;c ), 6.54–6.61 (m, 6H, Hc;b ), 4.52 (br s,
4H, Hd ), 3.80 (s, 4H, Hf, g). FT-IR(KBr, cmꢀ1): 3234 and
0
3145 (m, NH), 1631 (s, C@N), 1208 (phenolic CO), 1099
(br s, ClO4ꢀ).
2.4.3. Preparation of [CoL1(4-picoline)2]ClO4
The complex was recrystallized from acetonitrile/etha-
nol (1:2 v/v) yielding brown crystals. Yield 93.2%
(0.5694 g). Anal. Calc. for CoC28H28N4O6Cl: C, 55.1; H,
4.6; N, 9.17. Found: C, 54.1; H, 4.7; N, 9.0%; m.p.:
1
249.1 ꢁC. H NMR(CD3CN): d 8.39 (s, 2H, He), 7.89 (d,
2
4H, Ha ), 7.21–7.34 (m, 8H, Hb;d;b ), 7.10 (d, 2H, Ha),
weighting scheme: w ¼ 1=½r2ðF o2Þ þ ðaPÞ þ bPꢁ, where
0
0
P ¼ ðF 2o þ 2F 2cÞ=3, weighting scheme parameters a, b:
0.0242, 2.0103, largest D/r in last cycle: 0.001, largest differ-
0
6.52 (m, 2H, Hc), 4.12 (s, 4H, Hf, g), 2.27 (s, 6H, Hc ).
FT-IR(KBr, cmꢀ1): 1626 (s, C@N), 1210 (phenolic CO),
3
1097 (br s, ClO4ꢀ).
ence peak and hole (e/A ): 0.401 and ꢀ0.318, structure
˚
solution program: SHELXS-97 (Sheldrick, 1990), structure
refinement program: SHELXL-97 (Sheldrick, 1997).
2.4.4. Preparation of [CoL1(pyridine)2]ClO4
The complex was recrystallized from acetonitrile/etha-
nol (1:2 v/v) yielding brown crystals. Yield 94.0%
(0.5479 g). Anal. Calc. for CoC26H24N4O6Cl: C, 53.6; H,
4.2; N, 9.6. Found: C, 53.4; H, 4.2; N, 9.5%; m.p.:
3. Results and discussion
3.1. Crystal structure
1
282.5 ꢁC. H NMR (CD3CN): d 8.42 (s, 2H, He), 7.89 (d,
0
0
4H, Ha ), 7.86 (m, 2H, Hb), 7.22–7.42 (m, 8H, Hd;b ;c0 ),
7.12 (d, 2H, Ha), 6.52 (m, 2H, Hc), 4.14 (s, 4H, Hf, g).
FT-IR(KBr, cmꢀ1): 1626 (s, C@N), 1210 (phenolic CO),
1093 (br s, ClO4ꢀ).
The molecular and crystal structure of the
[Co(salen)(aniline)2]ClO4 are shown in Figs. 1 and 2,
respectively. Some selected bond angles and lengths are
listed in Table 1. The complex is composed of the
[Co(salen)(aniline)2]+ cation complex and the anion
ClO4ꢀ. Cobalt (III) center is coordinated by two pheno-
late oxygens, two imine nitrogens of the salen ligand and
by the two nitrogen atoms of aniline groups, which
occupy the axial positions of a distorted octahedral
coordination geometry. The two axial ligands are
approximately normal to the salen plane. The axial bond
angle N(3)–Co–N(4) is 175.92(10)ꢁ which is comparable
with those of corresponding angles in similar Co(III)
complexes [14,15,19]. The salen ligand has a planar
coordination geometry and the Co atom lies in the best
plane through the atoms O(1), O(2), N(1) and N(2)
2.4.5. Preparation of [CoL2(pyridine)2]ClO4
The complex was recrystallized from acetonitrile/etha-
nol (1:2 v/v) yielding brown crystals. Yield 90.0%
(0.7120 g). Anal. Calc. for CoC38H32N8O6Cl: C, 57.7; H,
4.1; N, 14.2. Found: C, 57.0; H, 4.4; N, 13.8%; m.p.:
265.1 ꢁC. 1H NMR (CD3CN): d 8.66 (s, 2H, He), 8.14
0
0
(dd, 4H, Ha ), 8.06 (d, 2H, Hd), 7.87–7.94 (m, 4H, Hb;c ),
0
7.75–7.78 (m, 4H, Hi, m), 7.55 (m, 4H, Hb ), 7.44–7.5 (m,
6H, Hj, k, l), 7.32 (d, 2H, Ha), 4.25 (s, 4H, Hf, g). FT-IR(KBr,
cmꢀ1): 1628 (s, C@N), 1117 (br s, ClO4ꢀ).
˚
2.5. X-ray crystallography
(max dev. 0.006 (11) A). The Co–O (1.894(2),
˚
˚
1.911(2) A) and Co–Nimine (1.895(3), 1.892(2) A) bond
lengths are in good agreement with those of similar
complexes [14,15,19]. Every N(4) of ion complex is
hydrogen bonded to two phenolate oxygen atoms of
other symmetrically related ion complex (Table 2 and
The red plate like single crystals of the complex
[CoL1(aniline)2]ClO4 obtained by slow evaporation of sol-
vent from acetonitrile/ethanol (1:1 v/v) solution of the
complex at room temperature.

1896
A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
Fig. 1. Stereoscopic ORTEP plot of [CoL1(aniline)2]ClO4, showing the atomic numbering scheme. The probability ellipsoids are drawn at the 50%
probability level.
Fig. 2. bc-Plane of [CoL1(aniline)2]ClO4, viewed along the a-axis. Hydrogen bonds are shown as dashed lines.

A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
1897
Table 1
Table 2
Hydrogen bonds for [CoL1(aniline)2]ClO4 (A, ꢁ)
˚
˚
Selected bond lengths (A), bond angles (ꢁ) and torsion angles (ꢁ) for
[CoL1(aniline)2]ClO4
D–Hꢂ ꢂ ꢂA
d(D–H)
d(Hꢂ ꢂ ꢂA)
d(Dꢂ ꢂ ꢂA)
\(DHA)
˚
Bond lengths (A)
N(3)–Hꢂ ꢂ ꢂO(11)
N(3)–H0ꢂ ꢂ ꢂO(12)a
N(4)–Hꢂ ꢂ ꢂO(1)b
N(4)–H0ꢂ ꢂ ꢂO(2)b
0.92
0.92
0.92
0.92
2.25
2.25
2.16
2.22
3.160(4)
3.161(3)
2.898(3)
3.032(3)
167.9
173.4
136.7
146.9
Co(1)–O(1)
Co(1)–O(2)
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–N(4)
O(1)–C(11)
C(16)–C(17)
C(17)–N(1)
N(1)–C(18)
C(18)–C(28)
O(2)–C(21)
C(26)–C(27)
C(27)–N(2)
N(2)–C(28)
N(3)–C(31)
N(4)–C(41)
1.894(2)
1.911(2)
1.895(3)
1.892(2)
2.006(2)
2.002(2)
1.322(3)
1.434(4)
1.289(4)
1.477(4)
1.500(5)
1.320(3)
1.427(4)
1.290(4)
1.466(4)
1.455(4)
1.432(4)
Symmetry transformations used to generate equivalent atoms: a1 ꢀ x,
1 ꢀ y, 1 ꢀ z; b1 ꢀ x, 1 ꢀ y, ꢀz.
3.2. Spectral characterization of the ligands and complexes
1
The IR and H NMR are in accord with the proposed
structures. The FT-IR spectra of all complexes, compared
with those of the corresponding ligands, indicate that the
m(C@N) band at 1631–1626 cmꢀ1 is shifted to lower energy
by 14–19 cmꢀ1 for salen complexes, but 7 cmꢀ1 in the case
of azosalen complex, indicating that the ligands are coordi-
nated to the metal ions through the nitrogen atoms of the
azomethine groups. On the other hand, the OH proton sig-
nals in 1H NMR spectra of free ligands (13.2 and
13.79 ppm for H2salen and H2azosalen, respectively) dis-
appears in the case of complexes indicating that the OH
group has been deporotonated and bonded to metal ions
as oxygen anion, therefore, the presence of one perchlorate
counter ion for each ion-complex of Co(III) can be
expected. The appearance of a broad strong band in the
IR spectra of the complexes in 1090–1118 cmꢀ1 is assigned
to perchlorate stretching vibrations, which are typical of
non-coordinating perchlorate [24].
Bond angles (ꢁ) and torsion angles (ꢁ)
N(2)–Co(1)–O(1)
N(1)–Co(1)–O(2)
178.91(10)
179.26(9)
175.92(10)
125.25(18)
119.5(3)
N(4)–Co(1)–N(3)
C(11)–O(1)–Co(1)
C(17)–N(1)–C(18)
C(17)–N(1)–Co(1)
C(18)–N(1)–Co(1)
C(21)–O(2)–Co(1)
C(27)–N(2)–C(28)
C(27)–N(2)–Co(1)
C(28)–N(2)–Co(1)
C(31)–N(3)–Co(1)
C(41)–N(4)–Co(1)
N(2)–Co(1)–N(1)
O(2)–Co(1)–O(1)
O(1)–Co(1)–N(1)
O(2)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(2)–Co(1)–N(3)
O(1)–Co(1)–N(3)
O(2)–Co(1)–N(3)
N(1)–Co(1)–N(4)
N(2)–Co(1)–N(4)
O(1)–Co(1)–N(4)
O(2)–Co(1)–N(4)
N(1)–C(18)–C(28)–N(2)
N(2)–Co(1)–O(2)–C(21)
N(1)–Co(1)–O(1)–C(11)
O(1)–Co(1)–N(3)–C(31)
O(1)–Co(1)–N(4)–C(41)
126.6(2)
113.81(19)
125.82(18)
119.9(2)
126.5(2)
113.37(19)
118.37(18)
123.48(19)
85.01(11)
86.67(8)
Comparing the 1H NMR spectra of the free ligands and
complexes, the imine proton resonances of the ligands (8.35
and 8.52 ppm for H2L1 and H2L2, respectively) are shifted
to lower fields (8.39, 8.42 and 8.66 ppm) in the case
of [CoL1(4-picoline)2]ClO4, [CoL1(pyridine)2]ClO4 and
[CoL2(pyridine)2]ClO4, respectively, and are shifted to high-
field (7.92 and 8.14 ppm) in the case of [CoL1(aniline)2]ClO4
and [CoL1(cyclohexylamine)2]ClO4, respectively.
Table 3 shows the electronic absorption data for ligands
and complexes. The electronic absorption spectrum
(>280 nm) of free salen ligand consists of an intense band
centered at 316.6 nm attributable to p ! p* transitions
associated with azomethine chromophore [25]. Since such
a band is seen for reduced imines too, it seems that the
charge transfer from phenolate oxygens to phenyl rings
93.95(9)
94.37(10)
91.60(10)
87.97(10)
92.39(9)
88.77(10)
92.39(10)
93.23(10)
86.49(9)
87.25(9)
ꢀ37.4(3)
11.3(2)
17.5(2)
ꢀ4.6(2)
ꢀ135.5(2)
Table 3
Fig. 2), so that two ion complexes are linked to each
other by four hydrogen bonds to form a dimerized ion
complex. Every N(3), also, connects to two perchlorate
ions by hydrogen bonds. One of the perchlorate oxygens
is hydrogen bonded to one of the N(3) of the other
dimerized ion complex, so that, the dꢀimerized ion com-
plexes are linked together via ClO4 bridging group
and form a ribbon along the c-axis. The ribbons in bc-
plane have short contacts by the distances of less than
the max sum of atomic van der walls radii.
Electronic absorption data for ligands and complexes in DMF solution
Compound
k (nm)
H2L1
H2L2
273.8
268.6
316.6
367.2 428 (sh)
390.6 420 (sh)
[CoL1(cyclohexylamine)2]- 268.2 300 (sh)
ClO4
[CoL1(aniline)2]ClO4
[CoL1(4-picoline)2]ClO4
[CoL1(pyridine)2]ClO4
[CoL2(pyridine)2]ClO4
267.6 300 (sh) 328 (sh) 392.2 420 (sh)
268.2 300 (sh) 330 (sh) 392.6 420 (sh)
268.0 300 (sh)
267.4 300 (sh)
390.0 420 (sh)
398.2 440 (sh)

1898
A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
are may be involved in this band appearance too [26]. The
similar azomethine p ! p* transitions take place at 367 nm
in the case of azosalen ligand. A new band, as a shoulder,
with a maximum at 428 nm appears in the UV–Vis spec-
trum of azosalen in contrast to salen, which is attributable
to azophenyl substitute. There is some other intense band
in high-energy region (about 270 nm) of the spectra of
the free ligands which are related to p ! p* transitions of
phenyl rings [19].
The Co(III) complexes of these ligands exhibit absorp-
tion bands that are red shifted approximately. Complexa-
tion of this ligands with Co(III) with different axial ligands
does not affect the energy of the phenyl rings p ! p* transi-
tions, whereas the azomethine p ! p* transitions in com-
plexes show red shift about 70 nm in salen complexes and
30 nm in azosalen complex. These p ! p* azomethine tran-
sition bands, show a shoulder at 420 nm in salen complexes
and at 440 nm in azosalen complex. Fig. 3 compares the
electronic spectra for complexes and free ligands.
These shoulders in salen complexes spectra are attribut-
able to metal–ligand charge transfer transitions (MLCT)
(d ! p*) transitions [14,19], while in azosalen complexes
the shoulder can be attributed to p ! p* intraligand charge
transfer with a small contribution of MLCT. These assign-
ments of shoulders are supported by electronic structure
calculation employing Zindo/S semiempirical Hamiltonian
with p–p overlap weighting factor of 0.585 for structures
optimized by Zindo/1 method [27–34]. The selected main
atomic orbitals contributions to HOMO and LUMO of
the complexes are shown in Table 4.
Azosalen spectrum bands show red shift in comparison
to salen spectra due to the extended conjugated system in
azosalen complexes. Different axially coordinated ligands
have no effect on the energies of the electronic transitions.
3.3. Electrochemical studies
By scanning in the negative direction initiated at 0.0 mV,
the first and second reduction waves correspond to
Co(III) ! Co(II) and Co(II) ! Co(I) processes appear in
the range of (ꢀ586)–(ꢀ247) and (ꢀ1107)–(ꢀ1037) mV,
respectively (Table 5). The peak separation between the
related cathodic and anodic waves indicate a quasirevers-
ible and reversible redox reactions for Co(III)/Co(II)
(min DEp = 302 mV) and Co(II)/Co(I) (max DEp = 86 mV)
systems, respectively. The quasireversible redox reaction
can be due to the structural changes say by elongation of
axial bond lengths or losing of axial amine ligands because
of the addition of one electron to ðdz2 Þꢃ orbital [19]. The
redox peak heights increase by increasing of scan rate.
Multiple scans result in nearly superposable cyclic voltam-
mograms, i.e., the three oxidation states of cobalt involved
are markedly stable.
Fig. 3. The UV/Vis spectra of the ligands and complexes: H2L1 (a),
[CoL1(cyclohexylamine)2]ClO4 (1a), [CoL1(aniline)2]ClO4 (2a), [CoL1-
(4-picoline)2]ClO4 (3a), [CoL1(pyridine)2]ClO4 (4a), H2L2 (b) and
[CoL2(pyridine)2]ClO4 (1b).
increasing pKa of the axial lewis base. The more negative
cathodic potentials for Co(III) ! Co(II) process are seen
in the order of cyclohexylamine > 4-picoline > pyri-
dine > aniline, that correlates with the pKa values order
(Table 5). Fig. 4 shows the cyclic voltamogram of
[CoL1(cyclohexylamine)2]ClO4.
Existing of a 5-phenylazo substitution on equatorial
salen Schiff-base ligand, which acts as a good electron with-
drawing group, causes a more electron affinity on cobalt
For the cathodic process of the Co(III) ! Co(II), the
observed peak potentials are strongly depend on donor
strength of the appropriate axial ligands, since the elec-
tron affinity of the Co(III) complexes, decreases by

A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
1899
Table 4
The selected main atomic orbitals contributions to the HOMO and LUMO molecular orbitals of complexes
Complex
MO
E (eV)
Symmetrya
Contribution of AO to MOb
Co
Cimine
Nimine
Cphenolic
Ophenolic
Nax
(average)
(average)
(average)
(average)
(average)
[CoL1(cyclohexylamine)2]+
[CoL1(aniline)2]+
HOMO
LUMO
ꢀ0.39034
ꢀ0.13475
p
p
0.02598
0.00575
0.00247
0.29903
0.03896
0.09717
0.07473
0.05460
0.07657
0.01048
0.00109
0.00035
HOMO
LUMO
ꢀ0.38778
p
p
0.02304
0.00574
0.00265
0.20332
0.02859
0.09629
0.04696
0.05517
0.10592
0.01052
0.00086
0.00041
ꢀ0.13589
[CoL1(4-picoline)2]+
[CoL1(pyridine)2]+
HOMO
LUMO
ꢀ0.38185
ꢀ0.12131
p
p
0.05220
0.00357
0.00075
0.19811
0.03886
0.09713
0.06401
0.05351
0.10796
0.01108
0.00068
0.00058
HOMO
LUMO
ꢀ0.38498
p
p
0.04997
0.00312
0.00078
0.19963
0.03829
0.09672
0.06624
0.05263
0.10403
0.01060
0.00039
0.00159
ꢀ0.12442
[CoL2(pyridine)2]+
HOMO
LUMO
ꢀ0.36773
ꢀ0.12343
p
p
0.00995
0.00545
0.00178
0.20040
0.01214
0.09768
0.03813
0.04764
0.04218
0.01084
0.00094
0.00119
a
Since the molecular plane acts as a nodal plane, the symmetry of the orbital is p.
Calculated as the sum of the squared coefficient of the 2s, 2px, 2py and 2pz orbitals of C, N and O atoms and 3dz2 , 3dxz, 3dyz, 3dx2ꢀy2 , 3dxy, 4s, 4px, 4py
b
and 4pz of Co atom in the linear combination that defines the MO.
Table 5
Cyclic voltammetry data (mV) for complexes in DMF solutions (scan rate 100 mV/s)a
Complex
Epc(III ! II) Epa(II ! III) DEp(III M II) Epc(II ! I) Epa(I ! II) DEp(II M I) E1/2(II M I) pKa
(axial amine)b
[CoL1(cyclohexylamine)2]ClO4 ꢀ586
27
35
24
16
55
613
340
351
335
302
ꢀ1107
ꢀ1029
ꢀ1105
ꢀ1107
ꢀ1037
ꢀ1040
ꢀ1107
ꢀ1043
ꢀ1029
ꢀ951
67
78
62
78
86
ꢀ1073
ꢀ1068
ꢀ1074
ꢀ1068
ꢀ994
10.64
4.60
6.00
5.17
5.17
[CoL1(aniline)2]ClO4
[CoL1(4-picoline)2]ClO4
[CoL1(pyridine)2]ClO4
[CoL2(pyridine)2]ClO4
a
ꢀ305
ꢀ327
ꢀ319
ꢀ247
The ferrocenium–ferrocene couple shows a peak separation of 89 mV under the experimental conditions.
b
The pKa values (in aqueous solution) of the used axial amines are reported from Ref. [23].
60
40
20
0
-20
-40
-60
-80
-2
-1.5
-1
-0.5
0
0.5
1
1.5
Potential (V)
Fig. 4. Cyclic voltammogram of [CoL1(cyclohexylamine)2]ClO4.
center and so a less negative cathodic peak for [CoL2(pyri-
dine)2]ClO4 complex in contrast with [CoL1(pyridine)2]-
ClO4.
12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk). Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2005.12.001.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 255047. Copies of this information
may be obtained free of charge from The Director, CCDC,
Acknowledgments
We are grateful to Tabriz University Research Council
for the financial support of this research. Also authors

1900
A.A. Khandar et al. / Polyhedron 25 (2006) 1893–1900
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