Synthesis, characterization and electrochemistry of carboxamido Co(III) complexes: The crystal structure of [CoIII(Mebpb)(N-MeIm)2]BPh4·CH3OH

Article abstract of DOI:10.1016/j.ica.2010.01.007

Three cobalt(III) complexes of the type trans-[Co^III(Mebpb)(amine)₂]X {Mebpb^2- = N,N′-bis(pyridine-2-carboxamido)-4-methylbenzene dianion, and amine = N-methylimidazole (N-MeIm) (1), 3-methylpyridine (3-MePy) (2), 3-acetylpyridine (3-AcPy) (3), X =BPh₄^- (1), ClO₄^- (2, 3)} were synthesized and characterized by elemental analyses, IR, UV-Vis, and ¹H NMR spectroscopy. The structure of 1·CH₃OH was determined by X-ray crystallography and was found to have a distorted octahedral geometry around Co. The electrochemical behavior of these complexes with the goal of evaluating the effect of axial ligation on the redox properties is also reported. The reduction potential of Co(III), ranging from -0.63 V for (1) to -0.20 V for (3) shows a relatively good correlation with the σ-donor ability of the axial ligands.

Full text of DOI:10.1016/j.ica.2010.01.007

Inorganica Chimica Acta 363 (2010) 1587–1592
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and electrochemistry of carboxamido Co(III) complexes:
The crystal structure of [CoIII(Mebpb)(N-MeIm)2]BPh4ꢀCH3OH
b
a
a
Soraia Meghdadi a, Mehdi Amirnasr a, , Kurt Mereiter , Ahmad Amiri , Vahid Ghodsi
*
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
Three cobalt(III) complexes of the type trans-[CoIII(Mebpb)(amine)2]X {Mebpb2ꢁ = N,N0-bis(pyridine-2-
carboxamido)-4-methylbenzene dianion, and amine = N-methylimidazole (N-MeIm) (1), 3-methylpyri-
dine (3-MePy) (2), 3-acetylpyridine (3-AcPy) (3), X = BPhꢁ4 (1), ClO4ꢁ (2, 3)} were synthesized and charac-
terized by elemental analyses, IR, UV–Vis, and 1H NMR spectroscopy. The structure of 1ꢀCH3OH was
determined by X-ray crystallography and was found to have a distorted octahedral geometry around
Co. The electrochemical behavior of these complexes with the goal of evaluating the effect of axial liga-
tion on the redox properties is also reported. The reduction potential of Co(III), ranging from ꢁ0.63 V for
Article history:
Received 6 July 2009
Received in revised form 1 December 2009
Accepted 6 January 2010
Available online 13 January 2010
Keywords:
Bispyridylamide complexes
Cobalt(III)
Substituted N4-amido ligand
Crystal structure
Electrochemistry
(1) to ꢁ0.20 V for (3) shows a relatively good correlation with the
r-donor ability of the axial ligands.
Ó 2010 Elsevier B.V. All rights reserved.
p-Acceptor amines
1. Introduction
gand favoring N-coordination of the planar tetradentate ligand,
although rare examples of coordination via the oxygen atom of
the deprotonated amide group, and deviation from planarity have
been encountered [12].
In continuation of our work devoted to the synthesis and spec-
troscopic characterization of transition metal complexes utilizing
relatively inexpensive and conveniently synthesizable bis(picoli-
namide) chelating ligands [13–17], we have prepared a group of
cobalt(III) complexes of general formula trans-[Co(Mebpb)X2]+
Metal complexes of many amido ligands have been extensively
used to mimic the properties of biologically active systems. In this
context, metal complexes of a wide variety of picolinamide-based
ligands have been studied as structural and/or functional models
of several metalloproteins [1]. The number of ligands that can be
synthesized using bispyridylamide is practically enormous [2].
The convenient structural variation of the compounds by a modu-
lar approach gives access to ligands with varied structures having
different electronic and steric properties which show a broad spec-
trum of affinities for different metal ions. The simple, generic
chemistry involved in synthesizing different amido ligands renders
them a highly attractive and under-exploited class of ligands for
the development of metallamacrocycles [3], molecular devices (lo-
gic gates) [4], selective electrodes [5,6], and catalytic applications
[7]. Ligands containing one or more amide functionality have
proved to be useful in self-assembly, since they give predictable
patterns of hydrogen bonding that can add extra dimensionality
and helicity to the supramolecular structures and crystal engineer-
ing, in a biomimetic fashion [8–11].
(X = monodentate p-acceptor amine) to explore the effect of axial
ligand on the spectral properties and redox potentials. The X-ray
crystal structure of trans-[Co(Mebpb)(N-MeIm)2]BPh4ꢀCH3OH com-
plex is also reported.
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. The H2Mebpb ligand
was synthesized by a procedure reported in the literature [16]. Ele-
mental analyses were performed by using a Perkin–Elmer 2400II
CHNS–O elemental analyzer. UV–Vis spectra were recorded on a
JASCO V-570 spectrophotometer. 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 X300 (300 MHz) spectrometer. Proton chemical shifts
Complexes with the neutral ligand show a wide array of geom-
etries. The deprotonated amide nitrogen atom is a strong field li-
* Corresponding author. Tel.: +98 311 3912351; fax: +98 311 3912350.
E-mail address: amirnasr@cc.iut.ac.ir (M. Amirnasr).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.007

1588
S. Meghdadi et al. / Inorganica Chimica Acta 363 (2010) 1587–1592
are reported in parts per million (ppm) relative to an internal stan-
dard of Me4Si. Cyclic voltammograms were recorded by using a
SAMA 500 Research Analyzer. Three electrodes were utilized in
this system, a glassy carbon working electrode, a platinum disk
auxiliary electrode and Ag wire as reference electrode. The glassy
carbon working electrode (Metrohm 6.1204.110) with
tion mixture was filtered off and a solution of 0.25 mmol
(30.6 mg) of NaClO4ꢀH2O in methanol was slowly added to the fil-
trate. The resulting deep green solution was left undisturbed to
give dark green crystals. The crystals were isolated by filtration
and washed with a mixture of ethanol–ether (1:9 v/v) and dried
in vacuum. Yield 45%. Anal. Calc. for C33H28N6O8ClCo: C, 54.22; H,
3.86; N, 11.5. Found: C, 53.91; H, 3.93; N, 11.53%. FT-IR (KBr,
2.0 0.1 mm diameter was manually cleaned with 1 lm alumina
polish prior to each scan. Tetrabutylammonium hexafluorophos-
phate (TBAH) was used as supporting electrolyte. Acetonitrile
was dried over CaH2. The solutions were deoxygenated by purging
with Ar for 5 min. All electrochemical potentials were calibrated
versus internal Fc+/0 (E0 = 0.40 V versus SCE) couple under the same
conditions [18].
cmꢁ1
1119, 1090 (s, Cl–O). UV–Vis: kmax (nm) (
) mmax: 1703, 1637 (s, C@O), 1602 (s, C@C), 1567 (m, C–N),
e
, L molꢁ1 cmꢁ1) (CH3CN):
680 (136), 402 (7639). 1H NMR (CDCl3, 300 MHz): d = 2.19 (6Hi, s),
2.99 (3H, CH3, s), 7.25 (1Hf, d), 7.42 (2Hk, dd), 8.09–8.15 (4Hb, j, m),
8.25 (2Hd, dd), 8.46 (2Hc, dd), 8.55 (2Hl, d), 8.69 (2Hh, s), 9.04 (1Hg,
s), 9.08 (1He, d), 10.44 (2Ha, dd).
Caution: Although we have encountered no difficulties, perchlo-
rate salts with organic compounds are potentially explosive and
should be handled with care.
2.3. X-ray crystallography
2.2. Synthesis of complexes
Dark green crystals of 1ꢀCH3OH suitable for X-ray crystallogra-
phy were obtained by crystallization from methanol. X-ray data of
1ꢀCH3OH were collected at T = 100 K on a Bruker Smart APEX CCD
2.2.1. Synthesis of trans-[Co(Mebpb)(N-MeIm)2]BPh4ꢀCH3OH
(1ꢀCH3OH)
diffractometer
with
graphite
monochromated
Mo
Ka
To a solution of Co(CH3COO)2ꢀ4H2O (125.4 mg, 0.5 mmol) in
methanol (20 mL) was added a boiling solution of H2Mebpb
(166.2 mg, 0.5 mmol) in methanol (30 mL). To this solution was
added dropwise 9.5 mmol (0.75 mL) of N-methylimidazole and
air was bubbled through the reaction mixture for 3 h. The final
reaction mixture was filtered off and a solution of 0.5 mmol
(171.1 mg) of NaBPh4 in methanol was slowly added to the filtrate.
The resulting deep green solution was left undisturbed to give dark
green crystals, suitable for X-ray crystallography. The crystals were
isolated by filtration and washed with methanol, and dried in vac-
uum. Yield 95%. Anal. Calc. for C52H50N8O3BCo: C, 69.03; H, 5.57; N,
(k = 0.71073 Å) radiation and 0.3°
x
-scan frames. Cell refinement
and data reduction were performed with the help of program SAINT
[19]. Correction for absorption was carried out with the multi-scan
method and program SADABS [19]. The structure was solved with di-
rect methods using program SHELXS97 and structure refinement on
F2 was carried out with program SHELXL97 [20]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were inserted
in idealized positions and refined riding with the atoms to which
they were bonded. The methanol solvent molecule was refined
in population (result 0.81(1)) and its OH group with distance re-
straints. Since the methanol deficit was probably due to sample
storage prior to measurement, chemical formula and quantities
derived thereof are given for the idealized methanol content of
one molecule per formula unit. Crystal data, together with other
relevant information on structure determination, are listed in
Table 1.
12.38. Found: C, 69.1; H, 5.26; N, 12.74%. FT-IR (KBr, cmꢁ1
3606–3296 (w, br, O–H), 1633 (s, C@O), 1598 (s, C@C), 1566 (m,
) mmax:
C–N). UV–Vis: kmax (nm) (e
, L molꢁ1 cmꢁ1) (CH3CN): 577 (211),
412 (7578). 1H NMR (CDCl3, 300 MHz): d = 2.44 (3H, CH3, s), 3.07
(6Hi, s), 6.02, 6.36 (4Hj,k, s), 6.71 (2Hh, s), 6.80, 6.93, 7.51 (20HTPB),
7.01 (1Hf, d), 7.17 (2Hb, dd), 7.94 (2Hd, dd), 8.10 (2Hc, dd), 8.60
(1Hg, s), 8.62 (1He, d), 8.77 (2Ha, dd).
Table 1
2.2.2. Synthesis of trans-[Co(Mebpb)(3-MePy)2]ClO4 (2)
Crystal data and structure refinement for 1ꢀCH3OH.
To a solution of Co(CH3COO)2ꢀ4H2O (62.3 mg, 0.25 mmol) in
methanol (20 mL) was added a boiling solution of H2Mebpb
(83.1 mg, 0.25 mmol) in methanol (30 mL). To this solution was
added dropwise 7.73 mmol (0.75 mL) of 3-methylpyridine and air
was bubbled through the reaction mixture for 3 h. The final reac-
Chemical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C51H46BCoN8O2ꢀCH3OH
904.74
100(2)
triclinic
ꢀ
P1
12.4711(14)
14.2143(15)
14.8115(16)
69.210(2)
75.133(2)
65.999(2)
tion mixture was filtered off and
a solution of 0.25 mmol
b (Å)
c (Å)
(30.6 mg) of NaClO4ꢀH2O in methanol was slowly added to the fil-
trate. The resulting deep green solution was left undisturbed to
give green crystals. The crystals were isolated by filtration and
washed with a mixture of ethanol–ether (1:9 v/v) and dried in vac-
uum. Yield 40%. Anal. Calc. for C31H28N6O6ClCo: C, 55.16; H, 4.18; N,
a
(°)
b (°)
c
(°)
V (Å3)
2223.0(4)
2, 1.352
Z, calculated density (g/cm3)
Crystal size (mm)
12.45. Found: C, 54.95; H, 4.34; N, 12.44%. FT-IR (KBr, cmꢁ1
) mmax:
0.58 ꢂ 0.36 ꢂ 0.16
l
(mmꢁ1
F (0 0 0)
)
0.441
1638 (s, C@O), 1602 (s, C@C), 1566 (m, C–N), 1111, 1090 (s, Cl–O).
UV–Vis: kmax (nm) (e
, L molꢁ1 cmꢁ1) (CH3CN): 652 (164), 406
948
h Ranges (°)
Index ranges
2.48–27.00
ꢁ15 6 h 6 15, ꢁ18 6 k 6 18,
ꢁ18 6 l 6 18
multi-scan
28 709
(6659). 1H NMR (CDCl3, 300 MHz): d = 2.10 (6Hi, s), 2.52 (3H,
CH3, s), 7.02 (2Hk, dd), 7.13 (1Hf, d), 7.38 (2Hj, d), 7.83 (2Hh, s),
7.94 (2Hl, d), 8.07 (2Hb, m), 8.16 (2Hd, m), 8.38 (2Hc, m), 8.92
(1Hg, s), 8.97 (1He, d), 10.27 (2Ha, dd).
Absorption correction
Reflections collected
Independent reflections (Rint
Minimum and maximum
transmission
)
9646 (0.0281)
0.76, 0.93
2.2.3. Synthesis of trans-[Co(Mebpb)(3-AcPy)2]ClO4 (3)
To a solution of Co(CH3COO)2ꢀ4H2O (62.3 mg, 0.25 mmol) in
methanol (20 mL) was added a boiling solution of H2Mebpb
(83.1 mg, 0.25 mmol) in methanol (30 mL). To this solution was
added dropwise 6.87 mmol (0.75 mL) of 3-acetylpyridine and air
was bubbled through the reaction mixture for 3 h. The final reac-
Data/restraints/parameters
9646/2/594
1.066
R1 = 0.0408, wR2 = 0.1046
R1 = 0.0537, wR2 = 0.1146
0.65 and ꢁ0.53
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
r
(I)]
R indices (all data)
Maximum/minimum
D
q
(e Åꢁ3
)

S. Meghdadi et al. / Inorganica Chimica Acta 363 (2010) 1587–1592
1589
Fig. 1. ORTEP diagram of compound 1ꢀCH3OH with the atom labeling scheme.
The Co–Namide (Co–N2 = 1.8771(16) Å and Co–N3 = 1.8797(17)
Å) and Co–Npyridine (Co–N1 = 1.9822(17) Å and Co–N4 =
1.9802(16) Å) bond distances of [Co(Mebpb)(N-MeIm)2]+ are similar
to those in [Co(Me2bpb)(py)2]+ [21], and [Co(Mebpb)-
(Prldn)2]+ [16]. The Co–Namide bond is significantly shorter than that
to the pyridyl N atom, the former being comparable to Co–Npeptide
bonds, 1.87(1) Å [22]. The somewhat shorter Co–Naxial (Co–N5 =
1.9316(16) Å and Co–N7 = 1.9410(17) Å) bond lengths of 1ꢀCH3OH
versus those reported for the pyridine complex (1.960(4) and
1.981(4) Å) reflect the higher basicity of the N-MeIm ligand
(pKa = 6.95) versus py (pKa = 5.23) [23]. The C–O = 1.24 0.01 Å,
Cpyridine–N = 1.35 0.01 Å, and Ccarboxy–N = 1.34 0.01 Å bond dis-
tances, agree well with those reported for related complexes
[22,24–26].
3. Results and discussion
3.1. Description of the structure of (1ꢀCH3OH)
An ORTEP diagram of the complex with the atomic numbering
is presented in Fig. 1. Selected bond distances and angles are listed
in Table 2. Hydrogen-bonding data are listed in Table 3.
The cobalt(III) center resides in a distorted octahedral environ-
ment with the Mebpb2ꢁ ligand occupying the equatorial plane of
the octahedron. The remaining coordination sites are occupied by
two N-methylimidazole ligands in trans position relative to each
other.
The trans-N5–Co–N7 unit with a bond angle of 177.38(7)° is al-
most linear. The two axial ligands are perpendicular to the
Mebpb2ꢁ ligand. However, the planes of the two N-MeIm rings
form a dihedral angle of 83.0(1)° with each other.
Table 2
Selected bond lengths (Å) and angles (ꢁ) for 1ꢀCH3OH.
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)
1.9822(17)
1.8771(16)
1.8797(17)
1.9802(16)
1.9316(16)
1.9410(17)
All cis angles around the Co center deviate from 90°, indicating a
distortion from regular octahedron. The N–Co–N bond angles in
the equatorial planes consist of one angle that is distinctly larger
than 90° (N1–Co1–N4 = 109.57(7ꢁ)). The Co atom is displaced
slightly out of the mean plane of the four N atoms of the planar
Mebpb2ꢁ ligand, 0.015 Å toward the N5 atom. The dihedral angle
between the Co/N2/N3 and Co/N1/N4 planes is 4.8(1)° at the cobalt
atom. The angles between the pyridine rings and the N1/N2/N3/N4
plane are 3.2(1) and 12.9(1)°. The dihedral angle between the two
pyridine portions is 16.0(1)°. Steric congestion between the H
atoms H1 and H18 of the pyridine moieties is the major reason
for these distortions. The H1ꢀ ꢀ ꢀH18 contact distance of 2.06 Å is
less than the sum of van der Waals radii (2.20 Å) [27].
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)
82.96(7)
167.32(7)
109.57(7)
87.44(7)
91.02(7)
84.73(7)
166.85(7)
92.44(7)
89.48(7)
82.94(7)
90.09(7)
91.86(7)
92.12(7)
86.40(7)
177.38(7)
3.2. Spectral characterization
The newly prepared metal complexes are all air-stable solids
and have satisfactory elemental analyses. The octahedral geometry
of complex 1 in 1ꢀCH3OH is evident from the X-ray structure anal-
ysis (Fig. 1). An octahedral trans-structure for other complexes (2,
3) 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. Differences between the spectra
of H2Mebpb and its cobalt compounds are readily noticeable in the
Table 3
Hydrogen bond lengths (Å) and angles ꢁ() for 1ꢀCH3OH.
D–Hꢀ ꢀ ꢀA
O(3)–H(3O)ꢀ ꢀ ꢀO(2)
D–H (Å)
0.80
Hꢀ ꢀ ꢀA (Å)
1.88
Dꢀ ꢀ ꢀA (Å)
2.672(3)
D–Hꢀ ꢀ ꢀA ꢁ()
168
regions of the mNH, mCO, and mCN of the amide ligand vibrations.

1590
S. Meghdadi et al. / Inorganica Chimica Acta 363 (2010) 1587–1592
Compound 1ꢀCH3OH exhibits a broad band at 3418 cmꢁ1, which is
which exhibit a slight upfield shift upon coordination. Ha is also
the most downfield broad doublet by virtue of its proximity to
electron withdrawing pyridyl nitrogen. The next most downfield
shifted signals correspond to He,g. This is presumably due to the
magnetic anisotropy induced by the ring current.
It is interesting to note that a good correlation between the
chemical shifts of the equatorial ligand protons, the UV–Vis spec-
tral data, and electrochemistry of these complexes exists. The trend
in the chemical shifts of Ha–g and CH3 protons is inversely related
to the donorability of the axial ligands, (N-MeIm > 3-MePy > 3-
AcPy). This in turn is evident from the electronic spectra and elec-
trochemical data of these complexes (Table 4).
due to the presence of a methanol molecule, as solvent of crystal-
lization. The absence of the mN–H band and the shift of the mCO band
to a lower energy in the spectra of metal complexes confirm that
the amide nitrogens are coordinated to the Co in their deproto-
nated form [28]. All three complexes display strong carbonyl
stretching frequencies (
are red shifted compared to the mCO values of the free ligands (for
H2Mebpb,
CO = 1659, 1689 cmꢁ1). The band, generally considered
m
CO) in the range 1633–1638 cmꢁ1 which
m
to be a combination of C–N stretch and N–H band, undergoes a siz-
able shift to higher frequencies in the deprotonated complexes
(from 1520 cmꢁ1 in H2Mebpb to 1566 cmꢁ1 in the complexes). This
might be expected since on removal of the amide proton this band
becomes a pure C–N stretch. The perchlorate stretching vibrations
for the perchlorate salts 2–3 are observed in the 1090 and
1119 cmꢁ1 region [29].
3.3. Electrochemistry
The electrochemical behavior of these complexes has been
studied by cyclic voltammetry in acetonitrile with 0.1 M TBAH as
the supporting electrolyte at a glassy carbon working electrode un-
der argon atmosphere at 25 °C. Ferrocene (Fc) was used as the
internal standard, and all redox potentials are referenced to the
Fc+/0 couple. The electrochemical data are presented in Table 4
The UV–Vis data of 1–3 in MeCN are presented in Section 2. The
electronic spectra of these complexes show a low intensity ligand
field band (136 <
gand field strength, appears in the region between 577 and
680 nm. In most cases however [30], the expected 1A1 1T2 band
e < 211) which, depending on the axial amine li-
?
is masked by the intense charge-transfer bands. In addition, a
and the voltammetric behavior of
a representative complex
strong absorption with kmax in the 402–412 nm region was seen.
[Co(Mebpb)(3-MePy)2]ClO4 is shown in Fig. 2. The approximate
The high extinction coefficient (6659–7639 Mꢁ1cmꢁ1
) of this
concentrations of the compounds were 2 ꢂ 10ꢁ3–10ꢁ6 M.
absorption band suggests a CT transition [31]. All three complexes
exhibit one strong intraligand absorption band with kmax at
ꢃ320 nm [32].
3.3.1. Electrochemistry of the complexes in the absence of amines
It has been demonstrated that transition metal complexes of
dianionic bis(amide) ligands, derived from picolinic acid and 2,6-
pyridinedicarboxylic acid, are subject to electrochemical oxidation
at both the metal and the ligand to form metal- and ligand-cen-
tered oxidation products, respectively [39]. The purpose of electro-
chemical studies on these cobalt(III) complexes was to investigate
the sensitivity of the CoIII/CoII redox potentials with a change in the
donor strength of the axial ligands.
Proton NMR spectral measurements were performed in CDCl3
solution to throw light on the solution structure of these com-
plexes. Scheme. 1 shows the numbering system for 1–3. The results
are given in Section 2. On formation of the deprotonated cobalt
complexes, the benzene and pyridyl protons show chemical shifts
similar to those found for analogous complexes [13,30,33–37].
1H NMR spectral studies reveal that most tetradentate ligands
are generally coordinating in a planar manner so that the two
monodentate ligands are trans to each other [30]. The appearance
of the amine signals indicates the coordination of two amine li-
gands to the metal center [13]. The absence of the amide N–H sig-
nal in the spectrum of these complexes, clearly demonstrate
coordination of the amide ligand in its deprotonated form
[36,37]. Due to the electron withdrawing nature of the Co(III) cen-
ter, all proton resonances of the ligand shift downfield upon coor-
dination to cobalt, as compared to those of the free ligand [38]. The
only exceptions are the meta protons on the pyridine moiety, Hd,
For all redox couples, the peak-to-peak separation,
DEp, is rela-
tively independent of the scan rate, , typical of diffusion-con-
m
trolled experiments with fast simple one electron transfer at the
electrode [33]. As previously mentioned [16], the ligand is electro-
active over a range of 1.5 to ꢁ2.2 V. For the [Co(Mebpb)(amine)2]+
complexes, the first oxidation couples have virtually identical
potentials in the range 1.01–1.1 V. Thus, the first oxidation couple
is suggested to be mainly ligand-centered (Fig. 2) [26]. This is also
supported by the results reported by Che and co-workers [22].
+
h
H C
3
amine
i
H3C
Hf
N
N
Co
Hg
He
j
k
O
O
Hd
N
N
i
H3C
Co
h
N
N
Hc
N
Co
j
Ha
Hb
amine
k
l
CH3
i
Compound
Amine
O
h
N-Methylimidazole
3-Methylpyridine
3-Acetylpyridine
1
2
N
Co
j
3
k
l
Scheme 1. Structural formulas of the complexes and ligands.

S. Meghdadi et al. / Inorganica Chimica Acta 363 (2010) 1587–1592
1591
Table 4
Redox potentials of cobalt complexes 1, 2, and 3 in acetonitrile.a
No.
Compound
Epc1
Epa1
Epc2
Epa2
Epc3
Epa3
Epc4
Epa4
Epc5
Epa5
1
2
3
[Co(Mebpb)(N-MeIm)2]B(Ph)4
[Co(Mebpb)(3-MePy)2]ClO4
[Co(Mebpb)(3-AcPy)2]ClO4
0.97
1.04
1.05
1.04
1.12
1.15
ꢁ0.63
ꢁ0.31
ꢁ0.20
ꢁ0.02
0.14
0.15
ꢁ1.32
ꢁ1.30
ꢁ1.29
ꢁ1.26
ꢁ1.22
ꢁ1.24
ꢁ1.42
ꢁ1.40
ꢁ1.41
ꢁ1.36
ꢁ1.33
ꢁ1.31
ꢁ2.07
ꢁ2.06
ꢁ2.06
ꢁ1.99
ꢁ1.99
ꢁ1.99
a
Potentials are vs. Fc+/0 in 0.1 M TBAH, T = 298 K. Scan rate 100 mV/s. Approximate concentrations: 2 ꢂ 10ꢁ3–10ꢁ6 M.
Fig. 2. Cyclic voltammogram of [CoIII(Mebpb)(3-MePy)2]ClO4, 2, c = 2 ꢂ 10ꢁ3 M, in
acetonitrile at 298 K. Scan rate, 100 mV sꢁ1
.
In addition, complexes 1–3 each display an irreversible reduc-
tion wave at ꢁ0.2 to ꢁ0.63 V. As an example, the cyclic voltammo-
grams of 2 (Fig. 2) show an electrochemically irreversible CoIII/CoII
reductive response (
D
Ep = 450 mV) with Epc value of ꢁ0.31 V, and
the occurrence of a structural change due to the reduction is im-
plied [16]. The axial ligands have a profound effect on the Epc value
of the reduction process indicating that the reduction processes are
presumably metal centered. From the Epc values (Table 4) it is evi-
dent that stronger axial ligands shift the potential to more negative
values.
All complexes display an additional reduction process (Table 4).
We assign this as resulting from the combination of CoII/CoI couple
[40] and the redox couple corresponding to one of the pyridyl rings
(Fig. 2), based on our previous observations [16,17]. The reduced
sensitivity of the CoII/CoI reductive potential to the change in the
axial ligands suggests that the species being reduced at this stage
have almost the same structure for the three complexes, [CoII(-
Mebpb)(MeCN)], resulting from the solvolysis of the CoIII/CoII
reduction product [30].
The final electrochemically reversible reduction couple ob-
served in the voltammograms is attributed to the second pyridyl
ring. These reductions were also observed in the cyclic voltammo-
gram of the free ligand though only the reduction at more negative
potential was reversible [16,17]. These two reductions probably in-
volve the pyridyl rings in Mebpb ligand as no reductions, either
reversible or irreversible, were observed in the CVs of the pyrroli-
dine-based carboxamido ligands [41].
3.3.2. Electrochemistry of the complexes in the presence of amines
It is well established that in many trans-[CoIII(N2O2)(L)2]+ Schiff
base complexes, the first irreversible reduction process (CoIII/CoII)
can become quasi-reversible under controlled concentration con-
ditions [42,43]. In an attempt to examine the possibility of revers-
ible electrochemical behavior in analogous amido complexes, the
electrochemistry of [CoIII(Mebpb)(3-MePy)2]ClO4 was carried out
in the presence of excess 3-methylpyridine. Fig. 3 shows the cyclic
voltammogram of trans-[CoIII(Mebpb)(3-MePy)2]ClO4 (2) at differ-
ent 3-Mepy concentrations. In the absence of additional amine
the first reduction processes of (CoIII/CoII) is electrochemically irre-
Fig. 3. Cyclic voltammograms of [CoIII(Mebpb)(3-MePy)2]ClO4, 2, in acetonitrile
solution (c = 2 ꢂ 10ꢁ3 M) in the presence of excess 3-methylpyridine concentrations
ratios: (a) 0, (b) 20, (c) 60, (d) 100 at 298 K. Scan rate: 100 mV sꢁ1
.
tion wave of Co(II) is shifted towards more negative potentials
(Fig. 3) ( E = 220 mV), indicating the increased formation of six-
D
coordinated adducts; i.e. the electrode process is preceded by a
reversible chemical reaction (Eq. (1));
versible (DE = 450 mV). Upon addition of excess amine, the oxida-

1592
S. Meghdadi et al. / Inorganica Chimica Acta 363 (2010) 1587–1592
½CoIIðMebpbÞꢄ þ excess amine ! ½CoIIIðMebpbÞðamineÞ2ꢄþ þ eꢁ
[4] S.H. Lee, J.Y. Kim, S.K. Kim, J.H. Lee, J.S. Kim, Tetrahedron 60 (2004) 5171.
[5] J.J. Gooding, D.B. Hibbert, W. Yang, Sensors 1 (2001) 75.
[6] A.K. Singh, P. Saxena, S. Mehtab, B. Gupta, Talanta 69 (2006) 521.
[7] D.A. Conlon, N. Yasuda, Adv. Synth. Catal. 343 (2001) 137.
[8] T.J. Burchell, D.J. Eisler, M.C. Jennings, R.J. Puddephatt, Chem. Commun. 9
(2003) 2228.
[9] T.J. Burchell, D.J. Eisler, R.J. Puddephatt, Inorg. Chem. 43 (2004) 5550.
[10] T.J. Burchell, D.J. Eisler, R.J. Puddephatt, Chem. Commun. 10 (2004) 944.
[11] B.-L. Wu, P. Zhang, Y.-Y. Niu, H.-Y. Zhang, Z.-J. Li, H.-W. Hou, Inorg. Chim. Acta
361 (2008) 2203.
ð1Þ
The redox potential of the Co(II/I) couple is not significantly
changed by the addition of excess amine and can be regarded as
approximately independent.
[12] W.-H. Leung, T.S.M. Hun, K.-N. Hui, I.D. Williams, Polyhedron 15 (1996)
421.
4. Conclusion
[13] M. Amirnasr, K.J. Schenk, S. Meghdadi, Inorg. Chim. Acta 338 (2002) 19.
[14] S. Meghdadi, H.R. Khavasi, S. Nalchigar, Acta Crystallogr., Sect. E62 (2006)
o5492.
[15] S. Meghdadi, M. Amirnasr, V. Langer, A. Zamanpoor, Can. J. Chem. 84 (2006)
971.
[16] S. Meghdadi, M. Amirnasr, M.H. Habibi, A. Amiri, V. Ghodsi, A. Rohani, R.W.
Harrington, W. Clegg, Polyhedron 27 (2008) 2771.
[17] A. Amiri, M. Amirnasr, S. Meghdadi, K. Mereiter, V. Ghodsi, A. Gholami, Inorg.
Chim. Acta 362 (2009) 3934.
[18] N.G. Connelly, W.E. Geiger, Chem. Rev. 96 (1996) 877.
[19] SADABS, Version 2.10; XPREP, Version 6.14, Bruker AXS Inc., Madison, WI, USA,
2003.
[20] G.M. Sheldrick, SHELX97: Program System for Crystal Structure Determination,
University of Göttingen, Göttingen, Germany, 1997.
[21] Y.-J. Kim, C. Kim, Y. Kim, Anal. Sci. 21 (2005) x39.
[22] S.-T. Mak, W.-T. Wong, V.W.-W. Yam, T.-F. Lai, C.-M. Che, J. Chem. Soc., Dalton
Trans. (1991) 1915.
[23] S. Mukerjee, K. Skogerson, S. DeGala, J.P. Caradonna, Inorg. Chim. Acta 297
(2000) 313.
We have reported the synthesis and characterization of several
trans-[CoIII(Mebpb)(amine)2]X complexes and part of our charac-
terization included the single-crystal X-ray structure analysis of
trans-[Co(Mebpb)(N-MeIm)2]BPh4ꢀCH3OH (1ꢀCH3OH). The fused
methyl group in the structure of Mebpb, increases the
r donating
character of Mebpb. Consequently, contrary to [CoII(bpb)], which is
not reactive towards oxidative addition of -acceptor amines, the
reactivity of Co(II) in [CoII(Mebpb)] towards
-acceptor amines is
increased and acceptor ligands are suitable for addition to the ax-
p
p
p
ial positions in an oxidative process. The coordination geometry
around Co(III) center in these complexes is a distorted octahedron.
Electrochemical studies revealed that the first reduction process
Co(III/II) is irreversible and strongly influenced by the nature of
the axial amines. When the axial ligands are changed from 1 to 3
a change in the Co(III/II) cathodic peak of 450 mV is observed. This
process shows a tendency towards becoming quasi-reversible
upon the addition of excess amine.
[24] N. Azuma, T. Ozawa, S. Tsuboyama, J. Chem. Soc., Dalton Trans. (1994)
2609.
[25] A.K. Patra, R. Mukherjee, Polyhedron 18 (1999) 1317.
[26] S.K. Dutta, U. Beckmann, E. Bill, T. Weyhermüller, K. Wieghardt, Inorg. Chem.
39 (2000) 3355.
[27] R.S. Rowland, R. Taylor, J. Phys. Chem. 100 (1996) 7384.
[28] R.L. Chapman, R.S. Vagg, Inorg. Chim. Acta 33 (1979) 227.
[29] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds Part II: Application in Coordination, Organometallic and
Bioinorganic Chemistry, fifth ed., Wiley-Interscience, New York, 1997.
[30] M. Ray, R.N. Mukherjee, Polyhedron 11 (1992) 2929.
[31] A.K. Patra, M.J. Rose, K.A. Murphy, M.M. Olmstead, P.K. Mascharak, Inorg.
Chem. 43 (2004) 4487.
Acknowledgment
Partial support of this work by the Isfahan University of Tech-
nology Research Council is gratefully acknowledged.
Appendix A. Supplementary material
[32] M. Ray, R. Mukherjee, J.F. Richardson, R.M. Buchanan, J. Chem. Soc., Dalton
Trans. (1993) 2451.
[33] S.-T. Mak, V.W.-W. Yam, C.-M. Che, J. Chem. Soc., Dalton Trans. (1990) 2555.
[34] C.F. Fortney, R.E. Shepherd, Inorg. Chem. Commun. 7 (2004) 1065.
[35] C.F. Fortney, S.J. Geib, F.-T. Lin, R.E. Shepherd, Inorg. Chim. Acta 358 (2005)
2921.
[36] R.L. Chapman, F.S. Stephens, R.S. Vagg, Inorg. Chim. Acta 52 (1981) 161.
[37] D.J. Barnes, R.L. Chapman, F.S. Stephens, R.S. Vagg, Inorg. Chim. Acta 51 (1981)
155.
CCDC 737919 contains the supplementary crystallographic
data for 1ꢀCH3OH. These data can be obtained free of charge from
The Cambridge Crystallographic Data Center via www.ccdc.cam.
ac.uk/data_request/cif. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.ica.2010.01.007.
[38] S.L. Jain, P. Bhattacharyya, H.L. Milton, A.M.Z. Slawin, J.A. Crayston, J.D.
Woollins, J. Chem. Soc., Dalton Trans. (2004) 862.
[39] A.K. Patra, M. Ray, R. Mukherjee, Inorg. Chem. 39 (2000) 652.
[40] A.K. Patra, M. Ray, R. Mukherjee, J. Chem. Soc., Dalton Trans. (1999) 2461.
[41] C.L. Weeks, P. Turner, R.R. Fenton, P.A. Lay, J. Chem. Soc., Dalton Trans. (2002)
931.
[42] A. Böttcher, T. Takeuchi, K.I. Hardcastle, T.J. Meade, H.B. Gray, D. Cwikel, M.
Kapon, Z. Dori, Inorg. Chem. 36 (1997) 2498.
References
[1] C. Jubert, A. Mohamadou, C. Gerard, S. Brandes, A. Tabard, J.P. Barbier, Inorg.
Chem. Commun. 6 (2003) 900.
[2] O. Belda, C. Moberg, Coord. Chem. Rev. 249 (2005) 727.
[3] Z.-H. Ni, H.-Z. Kou, L.-F. Zhang, C. Ge, A.-L. Cui, R.-J. Wang, Y. Li, O. Sato, Angew.
Chem., Int. Ed. 44 (2005) 7742.
[43] M. Amirnasr, R. Vafazadeh, A. Mahmoudkhani, Can. J. Chem. 80 (2002) 1196.