Synthesis, structure and electrochemistry of Co(III) complexes with an unsymmetrical Schiff base ligand derived from 2-aminobenzylamine and pyrrole-2-carboxaldehyde

Article abstract of DOI:10.1016/j.poly.2011.03.041

A series of cobalt(III) complexes with a new unsymmetrical tetradentate Schiff base ligand N,N'-bis(pyrrol-2-ylmethylene)-2-aminobenzylamine, H ₂pyrabza, formed by the condensation of 2-pyrrole carboxaldehyde and 2-aminobenzylamine, has been sy

Full text of DOI:10.1016/j.poly.2011.03.041

Polyhedron 30 (2011) 1651–1656
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Synthesis, structure and electrochemistry of Co(III) complexes with
an unsymmetrical Schiff base ligand derived from 2-aminobenzylamine
and pyrrole-2-carboxaldehyde
b
a
a
Soraia Meghdadi a, Mehdi Amirnasr a, , Kurt Mereiter , Hajar Molaee , Ahmad Amiri
⇑
a Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060 Vienna, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of cobalt(III) complexes with a new unsymmetrical tetradentate Schiff base ligand N,N’-bis(pyr-
rol-2-ylmethylene)-2-aminobenzylamine, H2pyrabza, formed by the condensation of 2-pyrrole carboxal-
dehyde and 2-aminobenzylamine, has been synthesized and characterized by elemental analyses, IR,
UV–Vis and 1H NMR spectroscopy. These complexes have the general formula trans-[CoIII(L)(amine)2]X
{L2ꢀ = pyrabza, and amine = N-methylimidazole (N-MeIm) (1), 3-methylpyridine (3-Mepy) (2), 4-methyl-
pyridine (4-Mepy) (3), pyridine (py) (4), X = BPh4ꢀ}. The structure of compound 1 has been determined by
X-ray crystallography. The geometry around the central cobalt ion is distorted octahedral. The electro-
chemical behavior of these complexes was also investigated. The reduction potential of Co(III), ranging
Received 8 February 2011
Accepted 17 March 2011
Available online 1 April 2011
Keywords:
Unsymmetrical N4 Schiff base
Co(III) complexes
Crystal structure
Electrochemistry
from ꢀ0.79 V for 1 to ꢀ0.44 V for 4, shows a relatively good correlation with the
r-donor ability of
the axial ligands.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
two amines in axial positions have also been used as antiviral and
antibacterial agents [32]. A few reports are sited on the unsymmet-
rical N4 tetradentate Schiff base complexes of cobalt. In continua-
tion of our work on the synthesis and structural studies of Co(III)
complexes containing Schiff base ligands [33–35], we report here
the synthesis, spectroscopic characterization and electrochemistry
of four trans-[CoIII(L)(amine)2]BPh4 complexes with a new unsym-
metrical Schiff base ligand. The crystal structure of complex 1
with amine = N-methylimidazole (N-MeIm) is also reported and
discussed.
For decades, Schiff bases have played a key role as chelating li-
gands in main group and transition metal coordination chemistry,
due to their ease of synthesis, stability under a variety of oxidative
and reductive conditions, and their structural versatility associated
with their diverse applications [1–6]. Schiff base complexes of
transition metals have been used as drugs and they possess a wide
variety of biological activity against bacteria, fungi and certain
types of tumors [7–13]. Numerous research groups have devoted
their effort to the design, synthesis and characterization of transi-
tion metal complexes with unsymmetrical ligands. Most of these
unsymmetrical ligands are Schiff bases obtained by stepwise con-
densation of the appropriate diamine with two different carbonyl
compounds [14–17]. These compounds may serve as models of rel-
evance for biologically important species [18–20] or as catalysts for
various organic transformations [21–25], and are promising mate-
rials for optoelectronic applications [26–29] and the design of bio-
sensors [30].
2. Experimental
2.1. Materials and general methods
All solvents and chemicals were of commercial reagent grade
and used as received from Aldrich and Merck. Elemental analyses
were performed using a Perkin–Elmer 2400II CHNS-O elemental
analyzer. UV–Vis spectra were recorded on a JASCO V-570 spectro-
photometer. Infrared spectra (KBr pellets) were obtained on a FT-IR
JASCO 680 plus spectrophotometer. The 1H NMR spectra of the
complexes were obtained on a BRUKER AVANCE DR (500 MHz)
spectrometer. Proton chemical shifts are reported in parts per mil-
lion (ppm) relative to an internal standard of Me4Si. Cyclic voltam-
mograms were recorded using a SAMA 500 Research Analyzer.
Three electrodes were utilized in this system, a glassy carbon
Recently, the discovery of the role played by some cobalt(III)
Schiff-base complexes as potent antitumor agents opened up
new avenues for the investigation of Co(III) interactions with pro-
teins and nucleic acids [31]. Cobalt(III) Schiff base complexes with
⇑
Corresponding author. Tel.: +98 311 391 2351; fax: +98 311 391 2350.
E-mail address: amirnasr@cc.iut.ac.ir (M. Amirnasr).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.03.041

1652
S. Meghdadi et al. / Polyhedron 30 (2011) 1651–1656
working electrode, a platinum disk auxiliary electrode and Ag wire
as a reference electrode. The glassy carbon working electrode
(Metrohm 6.1204.110) with 2.0 0.1 mm diameter was manually
give red green crystals. The crystals were isolated by filtration,
washed with cold methanol and dried under vacuum. Yield
58.4 mg, 70%. Anal. Calc. for C53H48N6BCo: C, 75.90; H, 5.77; N,
cleaned with 1
lm alumina polish prior to each scan. Tetrabutyl-
10.02. Found: C, 75.63; H, 5.64; N, 10.08%. FT-IR (KBr, cmꢀ1
) mmax:
ammonium hexafluorophosphate (TBAH) was used as a supporting
electrolyte. Acetonitrile was dried over CaH2. The solutions were
deoxygenated by purging with Ar for 5 min. All electrochemical
potentials were calibrated vs. an internal Fc+/o (E° = 0.40 V vs.
SCE) couple under the same conditions [36].
1560, 1586 (s, C@N), 3053–2980 (m, C–H). UV–Vis: kmax (nm) (e,
L molꢀ1 cmꢀ1) (CH2Cl2): 592 (266), 422 (23 280), 357 (11 150),
269 (24 720), 230 (65 600). 1H NMR (CDCl3, 500 MHz) d: 1.55 (s,
6H, CH3), 1.86 (s, 2Hbenzylic), 6.4–7.61 (18HAr, 20HBPh4), 7.76 (s, 1Hi-
mine), 8.09 (s, 1Himine).
2.2. Synthesis
2.2.4. Synthesis of trans-[Co(L)(4-Mepy)2]BPh4 (3)
2.2.1. Preparation of the asymmetric Schiff base ligand, H2pyrabza
The asymmetric ligand, H2pyrabza, was prepared by condensa-
tion of pyrrole-2-carbaldehyde with 2-aminobenzylamine accord-
ing to the following procedure (Scheme 1). To a stirring solution
of 2-pyrrolecarboxaldehyde 0.950 g (10 mmol) in 20 mL of ethanol
in a round bottom flask was added 0.620 g (5 mmol) of 2-amino-
benzylamine. The mixture was refluxed for 3 h to give a yellow
solution. The final reaction mixture was filtered off and the filtrate
was kept in a refrigerator for 12 h. The resulting yellow crystals
were filtered off, washed with cold ethanol and dried under vac-
uum. Yield: 1.12 g, 81%. Anal. Calc. for C17H16N4: C, 73.89; H,
5.84; N, 20.27. Found: C, 73.70; H, 5.72; N, 20.19%. FT-IR (KBr,
This complex was prepared by the same method as for 1, except
that 4-Mepy was used instead of N-MeIm. The resulting deep green
solution was left undisturbed at room temperature for few hours to
give dark green crystals. The crystals were isolated by filtration,
washed with cold methanol and dried under vacuum. Yield
54.4 mg, 65%. Anal. Calc. for C53H48N6BCo: C, 75.90; H, 5.77; N,
10.02. Found: C, 75.58; H, 5.63; N, 10.03%. FT-IR (KBr, cmꢀ1
) mmax:
1562, 1583 (s, C@N), 3049–2984 (m, C@C). UV–Vis: kmax (nm) (e,
L molꢀ1 cmꢀ1) (CH2Cl2): 593 (281), 420 (22 350), 351 (16 690),
260 (21 440), 231 (66 900). 1H NMR (CDCl3, 500 MHz) d: 2.16 (s,
6H, CH3), 3.77 (s, 2Hbenzylic), 6.43–7.52 (18HAr, 20HBPh4), 7.79 (s,
1Himine), 8.1 (s, 1Himine).
cmꢀ1
(nm) (
) mmax: 1616, 1636 (s, C@N), 3155 (b, N–H). UV–Vis: kmax
e
, L molꢀ1 cmꢀ1) (CH2Cl2): 330 (28 548), 286 (21 725), 218
(129 150). 1H NMR (CDCl3, 500 MHz) d: 4.84 (s, 2H, CH2), 6.15–
6.69 (6H, Hpyrrole), 7–7.32 (4H, ArH), 7.99 (s, 1Himine), 8.09 (s, 1Hi-
mine), 8.4–11.5 (br, 2H, N–H pyrrole).
2.2.5. Synthesis of trans-[Co(L)(py)2]BPh4 (4)
This complex was prepared by the same method as for 1 except
that py was used instead of N-MeIm. The resulting deep green
solution was left undisturbed at room temperature for a few hours
to give dark green crystals. The crystals were isolated by filtration,
washed with cold methanol and dried under vacuum. Yield 49 mg,
60%. Anal. Calc. for C51H44N6BCo: C, 75.56; H, 5.43; N, 10.37. Found:
2.2.2. Synthesis of trans-[Co(L)(N-MeIm)2]BPh4 (1)
To a stirring solution of Co(CH3COO)2ꢁ4H2O, 25 mg (0.1 mmol)
in methanol (40 mL) was added an equimolar of H2pyrabza
28 mg (0.1 mmol) in methanol (40 mL). The pink solution turned
brown immediately upon the formation of the cobalt complex.
To this solution was added dropwise 25 mmol (2.0 mL) of N-meth-
ylimidazole over a period of 45 min and the reaction mixture was
stirred at room temperature for an additional 15 min. The final
reaction mixture was filtered off and a solution of 34.2 mg
(0.1 mmol) of NaBPh4 in methanol (20 mL) was slowly added to
the filtrate. The final solution was left undisturbed at room temper-
ature. Brown crystals of the complex (suitable for X-ray crystallog-
raphy) were obtained after a few hours. The crystals were filtered,
washed with cold methanol and dried under vacuum. Yield
53.6 mg, 66%. Anal. Calc. for C49H46N8BCo: C, 72.06; H, 5.68; N,
C, 75.43; H, 5.34; N, 10.40%. FT-IR (KBr, cmꢀ1
C@N), 3053–2982 (m, C–H). UV–Vis: kmax (nm) (
)
m
max: 1560, 1585 (s,
e
, L molꢀ1 cmꢀ1
)
(CH2Cl2): 598 (277), 421 (24 810), 354 (18 035), 292 (28 045),
231 (75 220). 1H NMR (CDCl3, 500 MHz): d: 3.81 (s, 2Hbenzylic),
6.44–7.55 (20HAr, 20HBPh4), 7.80 (s, 1Himine), 8.09 (s, 1Himine).
2.3. X-ray crystallography
Brown crystals of 1 suitable for X-ray crystallography were ob-
tained from methanol. X-ray data of 1 were collected at T = 100 K
on a Bruker Kappa APEX-2 CCD diffractometer with graphite
13.72. Found: C, 71.85; H, 5.61; N, 13.58%. FT-IR (KBr, cmꢀ1
) mmax:
monochromated Mo K
a (k = 0.71073 Å) radiation and 0.5° x- and
1561, 1582 (s, C@N), 3049–2982 (m, C–H). UV–Vis: kmax (nm) (e,
u
-scan frames. Cell refinement and data reduction were performed
L molꢀ1 cmꢀ1) (CH2Cl2): 566 (307), 418 (32 075), 341 (24 520),
273 (52 230), 230 (75 620). 1H NMR (CDCl3, 500 MHz) d: 2.55 (s,
6H, CH3), 4.22 (s, 2Hbenzylic), 5.85–7.55 (16HAr, 20 HBPh4), 7.74 (s,
1Himine), 8.04 (s, 1Himine).
with the help of the program SAINT [37]. Correction for absorption
was carried out with the multi-scan method and the program SAD-
ABS [37]. The structure was solved with direct methods using the
program SHELXS97 and structure refinement on F2 was carried out
with the program SHELXL97 [38].
2.2.3. Synthesis of trans-[Co(L)(3-Mepy)2]BPh4 (2)
All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were inserted in calculated positions and treated as riding
on their parent atoms. Crystal data, together with other relevant
information on the structure determination, are listed in Table 1.
This complex was prepared by the same method as for 1, except
that 3-Mepy was used instead of N-MeIm. The resulting red green
solution was left undisturbed at room temperature for few hours to
H
N
O
C
N
N
Reflux in EtOH
+
2
H
NH2
HN
NH
NH2
Scheme 1. Synthesis of the asymmetric H2pyrabza ligand.

S. Meghdadi et al. / Polyhedron 30 (2011) 1651–1656
1653
Table 2
Selected bond lengths (Å) and angles (°) for 1.
Bond lengths
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–N(4)
Co(1)–N(5)
Co(1)–N(7)
1.9282(11)
1.9607(10)
1.9125(10)
1.9217(11)
1.9617(11)
1.9384(11)
Bond angles
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(4)
N(1)–Co(1)–N(5)
N(1)–Co(1)–N(7)
N(2)–Co(1)–N(3)
N(2)–Co(1)–N(4)
N(2)–Co(1)–N(5)
N(2)–Co(1)–N(7)
N(3)–Co(1)–N(4)
N(3)–Co(1)–N(5)
N(3)–Co(1)–N(7)
N(4)–Co(1)–N(5)
N(4)–Co(1)–N(7)
N(5)–Co(1)–N(7)
83.87(5)
172.83(5)
100.54(5)
87.38(4)
93.00(4)
92.40(4)
174.66(4)
93.16(4)
89.13(4)
83.54(5)
86.72(4)
93.06(4)
90.07(4)
87.64(4)
177.71(4)
mixture was stirred for an additional 15 min while in contact with
air. Suitable crystals of these complexes were obtained in good
yield (about 70%).
Fig. 1. ORTEP drawing of complex 1 with the atom numbering scheme.
3.2. Description of the structure of 1
The molecular structure and atom numbering scheme of 1 is
presented in Fig. 1. The crystallographic data and structure analysis
for complex 1 is summarized in Table 1. Selected bond distances,
and bond angles are given in Table 2.
Table 1
Crystal data and structure refinement for 1.
Chemical formula
Formula weight
Temperature (K)
Crystal system, space group
Unit cell dimensions
a (Å)
C25H26CoN8C24H20B
816.68
100(2)
As expected, the ligand (pyrabza2-) in compound 1 binds to the
trivalent metal ion (CoIII) via two pyrrole N atoms and two imine N
atoms in the equatorial plane of the octahedron. Two N-methylim-
idazole molecules occupy the axial positions. The pyrabza ligand is
not flat but shows a characteristic saddle-like deformation, which
is attributable mainly to the benzylic CH2 group.
monoclinic, P2(1)/n
10.4816(11)
22.534(2)
b (Å)
c (Å)
17.3479(19)
98.770(2)
4049.5(8)
b (°)
V (Å3)
The angles around the Co center deviate significantly from 90°.
The N2–Co–N3 and N1–Co–N4 angles have opened up to 92.40(4)°
and 100.53(5)° respectively, and the N1–Co–N2 and N3–Co–N4 an-
gles have reduced to 83.87(5)° and 83.54(5)°, indicating distortion
Z, Dcalcy (g/cm3)
Crystal size (mm)
4, 1.340
0.60 ꢂ 0.18 ꢂ 0.12
l
(mmꢀ1
)
0.47
F(0 0 0)
1712
from a regular octahedron.
h Range (°)
Index ranges
2.3–30.0
0
ꢀ14 6 h 6 14, ꢀ31 6 k 6 31,
ꢀ24 6 l 6 24
59 873
multi-scan
0.84, 0.95
The Co–Nimine bond distances, Co–N2 = 1.9607(10) ÅA and Co–
0
0
N3 = 1.9125(10) ÅA, show a difference of ca. 0.05 ÅA, which reflects
the chemical inequivalence of the two imine nitrogen atoms (aryl
and benzyl bonded nitrogen). On average, these distances may be
Reflections collected
Absorption correction
Minimum and maximum
transmission
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
R indices (all data)
Maximum/minimum
compared with those reported for [Co{(pyrrol)2dien}(4-Me-
0
)
11 743(0.0333)
11 743/0/534
1.024
R1 = 0.0334, wR2 = 0.0785
R1 = 0.0462, wR2 = 0.0855
0.37 and ꢀ0.65
py)]BPh4
(Co–Nimine
:
Co–N2 = 1.9128(9) ÅA
and
Co–N4 =
0
1.9020(8) ÅA) [39], and [Co{(BA)2pn}(py)2]ClO4, {(BA)2pn = N,N0-
bis(benzoylacetone)-1,3-propylenediimine dianion} (1.939(4) and
0
r
(I)]
1.962(3) ÅA) [35]. The somewhat shor0ter Co–Naxial (Co–
0
D
q
(e Åꢀ3
)
N5 = 1.9617(11) ÅA and Co–N7 = 1.9384(11) ÅA) bond lengths of 1
relative to those reported for the [Co{(pyrrol)2dien}(4-Mepy)]BPh4
0
complex (1.9765(8) ÅA) reflect the higher basicity of the N-MeIm li-
gand (pKa = 7.06) vs. 4-Mepy (pKa = 5.25) [40]. The trans N5–Co–N7
unit, with a bond angle of 177.71(4)°, is almost linear and the
planes of the two N-MeIm rings form a dihedral angle of 74.71°
with each other.
3. Results and discussion
3.1. Synthesis of complexes
The Co atom is displaced slightly out of the mean plane of the
four N atoms of the planar Schiff base ligand by 0.023 Å toward
the N5 atom. The dihedral angle between the Co/N2/N3 and Co/
N1/N4 planes is 6.55(1)° at the cobalt atom. The angles between
the two pyrrole rings and the N1/N2/N3/N4 plane are 8.76(1)
All the cobalt(III) complexes have been prepared by the direct
reaction of the corresponding ligand, H2pyrabza, and Co(CH3-
COO)2ꢁ4H2O in the presence of the appropriate amine at room tem-
perature. In the preparation of complexes 1–4, the final reaction

1654
S. Meghdadi et al. / Polyhedron 30 (2011) 1651–1656
and 6.72(1)°. The dihedral angle between the two pyrrole portions
is 15.44°.
3.3. Spectral characterization
The newly prepared metal complexes are all air-stable solids
and have good elemental analyses. The octahedral geometry of
complex 1 is evident from the X-ray structure analysis (Fig. 1).
An octahedral trans-structure for other complexes (2, 3 and 4)
can be inferred based on the similarity of the spectral data of these
complexes with those of complex 1.
The FT-IR data of the complexes are listed in Section 2. All the
complexes show similar IR spectral features. The IR spectrum of
the free ligand exhibits the characteristic bands of two different
imine C@N, which appear at 1636 and 1616 cmꢀ1 [34]. Similar fea-
tures are observed in the IR spectra of the cobalt complexes, with
the C@N stretching vibrations appearing in the 1560–1583 cmꢀ1
region. The m(CN) band is generally shifted to lower frequencies
relative to the free ligand, indicating a decrease in the C@N bond
order due to the coordination of the imine nitrogen to the metal
and back bonding from the Co(III) metal to the p⁄ orbital of the
azomethine group [41]. Moreover, the absence of a broad band at
3155 cmꢀ1, characteristic of the N–Hpyrrole group, in the IR spectra
of the complexes is indicative of the fact that the equatorial ligand
is coordinated in its deprotonated form.
The UV–Vis data of H2pyrabza and complexes 1–4 in CH2Cl2 are
presented in the Experimental Section. The electronic absorption
spectrum of pyrabza in dichloromethane consists of two relatively
intense bands centered at 218 and 288 nm, assigned to the
p–
p⁄
transitions of the aromatic rings and the azomethine group, and
a third band at 330 nm, corresponding to a n–p⁄ transition
[34,42]. All the four cobalt complexes show 4 bands at kmax 213–
423 nm, assigned to intraligand
matic rings and a CT transition with some ligand field admixture.
The first d-d transition appears as a shoulder in the range 566–
p–
p⁄ transitions within the aro-
Fig. 3. Cyclic voltammogram of trans-[Co(pyrabza)(N-MeIm)2]BPh4 (1) in acetoni-
trile at 298 K, c = 2.1 ꢂ 10ꢀ4 M, scan rate = 100 mV sꢀ1
.
598 nm (e = 266–307) and is in agreement with a similar transition
in related complexes [43]. Since the equatorial positions in all four
complexes are occupied by the tetradentate ligand pyrabza2ꢀ, the
energy of the ligand field transitions is directly related to the pKa
of the axial ligands. It is therefore expected to see a difference in
the color of these complexes, notably for the N-MeIm complex
with pKa = 7.4 (brown) and the py complex with pKa = 5.21 (dark
green). In addition, the size and types of crystals that are obtained
for these compounds may lead to some difference in their appear-
ance and color. As in related low spin d6-metal complexes, the ex-
pected 1A1g ? 1T2g band is apparently masked by intense charge-
transfer bands [44].
The 1H NMR spectral measurements were performed in CDCl3
solution to throw light on the solution structure of these com-
plexes. The chemical formula of 1–4 are presented in Scheme 2.
The main parameters of 1H NMR spectroscopic data of the Schiff
base ligand pyrabza and the four cobalt complexes 1–4 are given
in Section 2. The absence of the pyrrole N–H signal at 11.5–
8.4 ppm in the spectrum of these complexes clearly demonstrates
the coordination of the Schiff base ligand in its deprotonated form
[35,45]. The signal appearing as a singlet at 4.2 ppm for 1,
1.98 ppm for 2, 3.77 ppm for 3 and 3.81 ppm for 4 in the 1H
NMR spectra of these complexes is assigned to the benzylic CH2
group of pyrabza. The two signals appearing as singlets at 8.1–
7.7 ppm are assigned to the two different = CHimine groups. These
+
amine
amine
N-Methylimid
N
N
Co
d
3-Methylpyri
4-Methylpyrid
pyridine
N
N
amine
Fig. 2. Cyclic voltammogram of the H2pyrabza ligand in acetonitrile at 298 K,
c = 1 ꢂ 10ꢀ3 M, scan rate = 100 mV sꢀ1
.
Scheme 2. Chemical formula of the cobalt complexes.

S. Meghdadi et al. / Polyhedron 30 (2011) 1651–1656
1655
features, that are common between the 1H NMR spectra of com-
plexes 1, 2, 3, and 4, indicate that, as in 1, the equatorial coordina-
tion sites in complexes 2, 3 and 4 are occupied by the pyrabza
ligand. The aromatic protons of the coordinated Schiff base and
the axial amine ligands, and those of the BPh4 counter-ion, appear
in the appropriate region (5.8–7.9 ppm) [34,43]. The signals due to
the aliphatic protons of the methyl substituent on the axial ligands
appear in the range 1.55–2.55 ppm.
3.4. Electrochemistry
The electrochemical behavior of the H2pyrabza ligand and the
corresponding complexes 1–4 was studied at 25 °C using acetoni-
trile solutions with 0.1 M TBAH as the supporting electrolyte at a
glassy carbon working electrode under an argon atmosphere.
Ferrocene (Fc) was used as the internal standard and all redox
potentials are referenced to the Fc+/0 couple. The approximate con-
centrations of the compounds were 10ꢀ3 to 10ꢀ5 M.
It is evident from the cyclic voltammogram of H2pyrabza, dis-
played in Fig. 2, that the ligand is electroactive over a range from
ꢀ1.5 to ꢀ2.3 V in acetonitrile solvent. Two ligand-centered reduc-
tions observed at ꢀ1.9 and ꢀ2.21 V are attributed to the pyrrole
rings.
Fig. 4. Cyclic voltammograms of trans-[CoIII(Pyrabza)(N-MeIm)2]BPh4 (1) at differ-
ent N-MeIm concentrations ratios (0,
blue), (20,
green), (60, red), (100,
black) in acetonitrile at 298 K. Scan rate 100 mV sꢀ1, c = 5.3 ꢂ 10ꢀ4 M.
CoII/CoI reduction potential to the change in the axial ligands sug-
gests that the species being reduced at this stage have almost the
same structure for all of complexes 1–4, namely [CoII(pyra-
bza)(MeCN)], which results from the solvolysis of the CoIII/CoII
reduction product [47–50].
3.4.1. Electrochemistry of the complexes
The cyclic voltammogram of the trans-[Co(pyrabza)(N-
MeIm)2]BPh4 complex is shown in Fig. 3 as a representative exam-
ple for the cobalt complexes. Three main features are observed in
these cyclic voltammograms. (i) The counter ion oxidation process.
For all the complexes 1–4, the first electrochemically irreversible
oxidation wave in the potential range 0.80–0.89 V is attributed to
the oxidation of the benzene rings of BPh4 [46]
The nature of the CoII/I reduction process in CH3CN is dependent
on the used scanning potential window and is irreversible when
the measurements is carried out in the potential range 1.5 to
ꢀ2.2 V. However, when the scanning potential window is nar-
rowed to 0.6 to ꢀ1.6 V, this process becomes quasireversible
(D
E = 118 mV, ipa/ipc = 0.77) (Fig. 3). The irreversibility of CoII/I is
apparently due to the instability of the reduced product in the
more negative potentials. These observations are in accord with
the results obtained for the related Co(I) complexes of tetradentate
Schiff base ligands with steric strain on the structural framework
[34].
(iii) The ligand centered redox processes. The final electrochem-
ically irreversible reduction process observed in the voltammo-
grams is mostly ligand centered and is due to the reduction of
the pyrrole rings. The corresponding reduction potential remains
close to that of the free ligand. This is in accord with expectation,
since the pyrrrole moieties are now coordinated in their deproto-
nated form to Co(II).
BPhꢀ4 ! BPh4 ꢃ þ eꢀ
(ii) The metal centered redox processes. An irreversible wave in the
range ꢀ0.44 to ꢀ0.79 V is observed, due to the electrochemically
irreversible CoIII/CoII reductive response. As an example, an electro-
chemically irreversible CoIII/CoII reduction process (
DEp= 725 mV)
with an Epc value of ꢀ0.79 V appears in the cyclic voltammograms
of 1 (Fig. 3). This is presumably due to the occurrence of a structural
change during the reduction process. Accordingly we assume that
the lack of reversibility observed is most likely due to the loss of ax-
ial ligands from the cobalt(II) centre, because the electron is added
to the antibonding dz2 orbital of the metal ion [34,47–50].
trans-½CoIIIðpyrabzaÞðN-MeImÞ2ꢄþ þ eꢀ
! ½CoIIðpyrabzaÞꢄ þ 2N-MeIm
3.4.2. Electrochemistry of the complexes in the presence of amines
In many trans-[CoIII(N4)(L)2]+ Schiff base and carboxamide com-
plexes, the first irreversible reduction process (CoIII/CoII) can be-
come quasi-reversible under controlled concentrations of axial
amine ligands [47–50]. In an attempt to investigate the possibility
of reversible electrochemical behavior in cobalt pyrabza com-
plexes, the electrochemistry of [CoIII(pyrabza)(N-MeIm)2]BPh4
was studied in the presence of excess N-MeIm. Fig. 4 shows the
cyclic voltammograms of trans-[CoIII(pyrabza)(N-MeIm)2]BPh4
(1), at different concentrations of N-MeIm. As already pointed
out, the first reduction process of (CoIII/CoII) is electrochemically
The Epc value of this reduction process is greatly influenced by
the nature of the axial ligands, indicating that this process is pre-
sumably metal centered. From the Epc values (Table 3) it is evident
that stronger axial ligands shift the potential to more negative
values.
The next reduction process observed at ꢀ1.53 to ꢀ1.59 V is due
to the reduction of the CoII/CoI center. The low sensitivity of the
Table 3
Redox potentials of complexes 1–4 in acetonitrile.a
No.
Compound
Epa1
Epc2
Epa2
Epc3
Epa3
Epc4
Epa4
1
2
3
4
[Co(pyrabza)(N-MeIm)2]BPh4
[Co(pyrabza)(3-MePy)2]BPh4
[Co(pyrabza)(4-MePy)2]BPh4
[Co(pyrabza)(Py)2]BPh4
0.86
0.89
0.80
0.84
ꢀ0.79
ꢀ0.50
ꢀ0.45
ꢀ0.44
ꢀ0.07
ꢀ0.02
ꢀ0.08
ꢀ0.05
ꢀ1.59
ꢀ1.55
ꢀ1.50
ꢀ1.53
ꢀ1.48
ꢀ1.43
ꢀ1.41
ꢀ1.42
ꢀ2.23
ꢀ2.25
ꢀ2.19
ꢀ2.22
ꢀ2.05
ꢀ2.05
ꢀ2.04
ꢀ2.03
a
Potentials are vs. Fc+/0 in 0.1 M TBAH, T = 298 K. Scan rate 100 mV/s. Approximate concentrations: 1 ꢂ 10ꢀ3–10ꢀ5ꢀM.

1656
S. Meghdadi et al. / Polyhedron 30 (2011) 1651–1656
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½CoIIðpyrabzaÞꢄ Excess amine
¢
½CoIIðpyrabzaÞðamineÞ2ꢄ
! ½CoIIIðpyrabzaÞðamineÞ2ꢄþ þ eꢀ
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Electrochemical studies revealed that the first reduction process,
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CCDC 796474 contains the supplementary crystallographic
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Partial support of this work by the Isfahan University of Tech-
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