Synthesis, crystal structures and electrochemical properties of Co III complexes with hexadentate ligand containing thioether- amidopyridyl donor set

Article abstract of DOI:10.1007/s13738-012-0156-6

Two new cobalt(III) complexes of the hexadentate ligand [1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane] (H ₂bpctb) with N₄S₂ donor set atoms have been synthesized. A reaction of Co(CH₃COO)₂

Full text of DOI:10.1007/s13738-012-0156-6

J IRAN CHEM SOC (2013) 10:275–282
DOI 10.1007/s13738-012-0156-6
ORIGINAL PAPER
Synthesis, crystal structures and electrochemical properties
of CoIII complexes with hexadentate ligand containing
thioether-amidopyridyl donor set
•
Mehdi Amirnasr Mahdieh Rasouli
•
Kurt Mereiter
Received: 25 June 2012 / Accepted: 8 August 2012 / Published online: 28 August 2012
Ó Iranian Chemical Society 2012
Abstract Two new cobalt(III) complexes of the hexa-
dentate ligand [1,4-bis[o-(pyridine-2-carboxamidophenyl)]-
1,4-dithiobutane] (H2bpctb) with N4S2 donor set atoms have
been synthesized. A reaction of Co(CH3COO)2ꢀ4H2O with
(H2bpctb) leads to the formation of [CoIII(bpctb)]PF6 (1)
having a CoN2(pyridine)N02(amide)S2(thioether) coordina-
tion by symmetric bpctb2- ligand. A similar reaction under
slightly different conditions, however, gives [CoIII(La)(Lb)]
(2), resulting from a C–S bond cleavage reaction triggered
by an acetate ion as a base, having CoN2(pyridine)
N02(amide)S(thioether)S0(thiolate) coordination. These two
Co(III) complexes have been characterized by elemental
analyses and spectroscopic methods, and the crystal and
molecular structures of [CoIII(bpctb)]PF6 (1) in the form of
the solvate (1ꢀMeOHꢀH2O) and of [CoIII(La)(Lb)] (2) have
been determined by X-ray crystallography. The Co atoms of
both complexes exhibit distorted octahedral geometry. The
electrochemical investigation of [Co(bpctb)]PF6ꢀMeOHꢀ
H2O (1ꢀMeOHꢀH2O) and [CoIII(La)(Lb)] (2) by cyclic vol-
tammetry reveals a reversible CoIII–CoII redox process at
E1/2 = -0.32 V (DEp = 80 mV); for 1, and E1/2 = -0.
87 V (DEp = 70 mV) for 2.
Keywords Amide-thioether ꢀ Cobalt(III) complexes ꢀ
C–S bond cleavage ꢀ X-ray crystal structure ꢀ
Cyclic voltammetry
Introduction
The past two decades have seen considerable growth of
interest in the development of the coordination chemistry
of pyridine carboxamide ligands due to their application in
asymmetric catalysis [1], molecular receptors and dendri-
mer synthesis [2–4], drug design [5–7] and preparation of
complexes with diverse supramolecular network structures
[8–11]. In view of investigating the coordination chemistry
of deprotonated pyridine amide ligands we [12–16] and
others [17–20] have synthesized, structurally characterized,
and studied the properties of a large variety of interesting
transition metal complexes. The presence of deprotonated
carboxamido group in bleomycin [21, 22], iron and cobalt
centers in the cluster of nitrogenase [23] and nitrile
hydratase [24] has increased the interest in the synthesis of
carboxamido complexes for bioinorganic model studies
[25–27]. Among the important features of pyridine car-
boxamide ligands are the good r-donor ability of the
deprotonated amide nitrogen, the capacity of stabilizing
metal ions in high oxidation states [28], and the ease of
synthesis of the free ligands [16].
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-012-0156-6) contains supplementary
material, which is available to authorized users.
Amide-thioether ligands have also received much
attention in biomimetic research [29, 30]. Non-cyclic
hexadentate ligands of this type with N4S2 and N2O2S2
binding cores are well suited for the coordination to various
transition metal ions and are of importance in the devel-
opment of coordination chemistry [19, 27, 31].
M. Amirnasr (&) ꢀ M. Rasouli
Department of Chemistry, Isfahan University of Technology,
84156-83111 Isfahan, Iran
e-mail: amirnasr@cc.iut.ac.ir
K. Mereiter
Faculty of Chemistry, Vienna University of Technology,
Getreidemarkt 9/164SC, 1060 Vienna, Austria
The C–S bond cleavage is a primary step in removing
the organo-sulfur compounds from petroleum-based
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feedstock in the hydrodesulfurization (HDS) process. This
can be achieved by the activation of the C–S bond using
transition metal ions. The great practical importance of this
reaction has stimulated extensive investigation of thio-
containing transition metal complexes relevant to HDS
process and capable of being used as heterogeneous cata-
lysts [32, 33].
performed by using a Elementar Vario EL III 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. 1H
NMR spectra were obtained on a BRUKER AVANCE DR
X500 (500 MHz) and Bruker AVANCE III 400 MHz
spectrometer. Proton chemical shifts are reported in part per
million (ppm) relative to an internal standard of Me4Si. The
redox properties of the complexes were studied by cyclic
voltammetry. Cyclic voltammograms were recorded by
using a SAMA Research Analyzer M-500. Three electrodes
were utilized in this system, a glassy carbon working elec-
trode, a platinum disk auxiliary electrode and Ag wire as
reference electrode. The glassy carbon working electrode
(Metrohm 6.1204.110) with 2.0 0.1 mm diameter was
manually cleaned with 1 lm alumina polish prior to each scan.
Tetrabutylammonium hexafluorophosphate (TBAH) was used
as supporting electrolyte. 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 [35].
Herein we wish to report the synthesis and character-
ization of two new low-spin cobalt(III) complexes,
[CoIII(bpctb)]PF6 (1) and [CoIII((La)(Lb)] (2), prepared by
the reaction of cobalt(II) acetate with 1,4-bis[o-(pyridine-2-
carboxamidophenyl)]-1,4-dithiobutane in its deprotonated
form under different experimental conditions. Compound
(1) is a low-spin Co(III) complex coordinated by two
pyridine N, two deprotonated amide N and two thioether S
donor atoms. Complex (2) reveals a C–S bond cleavage
reaction [27, 34], and contains a low-spin Co(III) center
that is coordinated by two dissimilar tridentate ligands, one
containing the pyridine amide vinyl thioether unit La (1–)
and the other a pyridine amide thiolate unit Lb (2–)
(Scheme 1).
Synthesis
Experimental
Synthesis of H2bpctb ligand
Materials and general methods
The 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dith-
iobutane (H2bpctb) was synthesized using the benign
method of Meghdadi et al. [36].
All solvents and chemicals were obtained from Fluka and
Aldrich and used as received. Elemental analyses were
Scheme 1 Synthesis of
cobalt(III) complexes of
hexadentate amide-thioether
ligand
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277
Synthesis of [CoIII(bpctb)]PF6 (1)
temperature (r.t.). The diffraction data for compound (1)
and (2) were collected on a Bruker CCD platform dif-
fractometer and a Bruker APEX-II CCD diffractometer
employing graphite-monochromated Mo Ka (k = 0.71073
To a stirring solution of cobalt(II) acetate tetrahydrate
(0.125 g, 0.5 mmol) in methanol (15 mL) was added two
drops of H2O2. The pink solution turned green immediately
and to this solution was added a solution of equimolar
H2bpctb (0.243 g, 0.5 mmol) in dichloromethane (15 mL).
The green solution turned red immediately upon the for-
mation of [CoIII(bpctb)] complex. Air was then bubbled
through the reaction mixture for about 3 h. The final
solution was filtered off and a solution of NH4PF6 (0.082 g,
0.5 mmol) in 5 mL methanol was added dropwise to the
filtrate. Slow evaporation of this solution afforded red
crystals suitable for X-ray crystallography. The crystals
were filtered off and washed with diethyl ether and dried in
vacuum. Yield: 86 %. Anal. Calc. for C26H20CoF6N4O2PS2
(688.49): C, 45.36; H, 2.93; N, 8.14. Found: C, 45.34; H,
2.89; N, 8.22 %. FT-IR (KBr, cm-1) tmax: 1,639 (s,
C = O), 1,601 (s, C = C), 1,579 (C = N).UV–vis (ace-
tone) kmax (nm) (e, L mol-1 cm-1): 507 (550), 359
(12,145), 260 (35,930). 1H NMR (400 MHz, CD3CN,
assigned as in Scheme 1): d = 2.72 (d, 2H, Ha), 3.32 (d,
2H, Hb), 7.29–7.65 (8H, HAr, Hc,d,e,f), 8.00–8.20 (6H,
˚
A) radiation at T = 150 K and T = 100 K, respectively.
Cell refinement and data reduction were performed with
the help of program SAINT [37]. Correction for absorption
was carried out with the multi-scan method and program
SADABS [37]. The structures were solved with direct
methods and program SHELXS97, and were refined on F2
by SHELXL97 [38]. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were inserted in idealized
positions and were refined riding with the atoms to which
they were bonded, except for the water molecule in
(1ꢀMeOHꢀH2O) for which hydrogen positions could not be
located. A comparatively large cavity in the crystal struc-
ture of (2) was found unoccupied (infinite channels along
3
˚
[001] with a solvent accessible volume of 129 A per unit
cell and channel; solvent content possibly lost upon pro-
longed storage of the crystals under dry conditions). The
crystallographic and refinement data are summarized in
Table 1. Selected bond distances and angles are listed in
Table 2.
H
i,j,h), 9.50 (d, 2H, Hg).
Preparation of [CoIII(La)(Lb)] (2)
Results and discussion
To a stirring solution of cobalt(II) acetate tetrahydrate
(0.125 g, 0.5 mmol) in methanol (15 mL) was added a
solution of equimolar H2bpctb (0.243 g, 0.5 mmol) in
dichloromethane (15 mL). Air was then bubbled through
the reaction mixture. The resulting brown solution was
stirred for 3 h. The final solution was filtered and the fil-
trate was left in the hood undisturbed. Brown crystals of
complex (2) suitable for X-ray crystallography were
obtained after 3 days by slow evaporation of the solvent.
The crystals were filtered off and washed with diethyl ether
and dried in vacuum. Yield: 84 %. Anal. Calc. for
C26H19CoN4O2S2 (542.50): C, 57.56; H, 3.53; N, 10.33.
Synthesis of Co(III) complexes
The hexadentate ligand, H2L, used in the present work was
prepared by the reaction of pyridinecarboxylic acid with
1,2-di(o-aminophenylthio)ethane (dapte) using tetrabutyl-
ammonium bromide (TBAB) as the reaction medium [36].
The two cobalt(III) complexes were synthesized as out-
lined in Scheme 1. The synthesis of [CoIII(L)]PF6 (1)
involved the reaction of H2L with Co(CH3COO)2ꢀ4H2O in
the presence of H2O2 and NH4PF6 in MeOH and vigorous
stream of air. The diamagnetic complex [CoIII(La)(Lb)] (2),
resulting from a base-assisted C–S bond cleavage reaction,
was synthesized under slightly different conditions. For the
synthesis of (2), no hydrogen peroxide was added and the
ligand L was presumably cleaved by an intramolecular
electron transfer reaction. Crystals of these complexes were
obtained in good yield, 86 % for (1) and 84 % for (2).
Found: C, 57.07; H, 3.56; N, 10.32. %. FT-IR (KBr, cm-1
)
t
max: 1,621 (s, C = O), 1,595 (s, C = C). UV–vis (ace-
tone) kmax (nm) (e, L mol-1 cm-1): 756 (85), 457 (1,465),
356 (6,410), 292 (146,630). H NMR (400 MHz, CDCl3,
1
assigned as in Scheme 1): d = 5.05 (dd, 1H, Hc), 5.28 (d,
1H, Ha), 5.52 (d, 1H, Hb), 6.95–8.26 (m, 14H, Haromatic),
0
9.31 (d, 1H, Hg), 9.57 (d, 1H, Hg ).
Description of structures
Crystal structure of [CoIII(bpctb)]PF6ꢀMeOHꢀH2O
(1ꢀMeOHꢀH2O)
X-ray crystallography for (1) and (2)
Red crystals of (1) in the form of the solvate
(1ꢀMeOHꢀH2O) and brown crystals of (2) suitable for X-ray
crystallography were grown by slow evaporation of a
methanolic solution mixed with dichloromethane at room
The molecular structure of [CoIII(bpctb)]PF6 (1) in
(1ꢀMeOHꢀH2O) with atom numbering is presented in
Fig. 1, and selected bond angles and distances are listed
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J IRAN CHEM SOC (2013) 10:275–282
(2)
Table 1 Crystal data and
structure refinement for
(1ꢀMeOHꢀH2O) and (2)
Compound
(1ꢀMeOHꢀH2O)
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C27H26CoF6N4O4PS2
738.54
C26H19CoN4O2S2
542.50
150(2)
100(2)
Triclinic
Tetragonal
I4(1)/a
P¯ı
˚
a (A)
10.240 (2)
11.493 (2)
12.701 (3)
85.203 (2)
86.924 (3)
87.537 (3)
1,486.2 (5)
2
32.9082 (3)
32.9082 (3)
8.7865 (2)
90
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
90
90
3
˚
V (A )
9,515.3 (2)
16
Z
Dcalc (Mg/m3)
1.650
1.515
l (mm-1
)
0.85
0.93
Crystal size (mm)
F (000)
0.3 9 0.25 9 0.1
752
0.35 9 0.15 9 0.14
4,448
h Ranges (°)
Index ranges
2.3–25.7
2.5–30.0
-12 B h B 11
-13 B k B 14
-15 B l B 15
Multi-scan, SADABS
11,317
-39 B h B 46
-46 B k B 46
-12 B l B 12
Multi-scan, SADABS
73,331
Absorption correction
Reflections collected
Independent reflections (Rint
)
5,543 (0.025)
0.83, 1.00
5,543/0/408
1.96
6,934 (0.064)
0.74, 0.88
6,934/0/316
1.13
Maximum and minimum transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [F2 [ 2r(F2)]
R1 = 0.057, wR2 = 0.141
R1 = 0.059, wR2 = 0.139
in Table 2. The structure contains a [Co(bpctb)]3? cation,
with hexafluorophosphate anion, a methanol molecule, and
a water molecule on general sites. The coordination sphere
of cobalt in complex (1) is pseudo octahedral with the
cobalt atom coordinated by two aromatic nitrogen atoms
[N1 and N4], two amido nitrogen atoms [N2 and N3] and
two thioether sulfur atoms [S1 and S2] of this hexadentate
ligand. Though several isomers can be obtained with this
hexadentate ligand, the one observed here has the two
pyridyl groups cis to each other, with the aromatic nitrogen
atoms in trans positions to the thioether sulfur atoms. The
angles between trans atoms at the metal center are in the
range 171.76(10)°–178.13(12)° and the cis angles span a
wide range, 84.06(12)°–97.27(13)° and deviate signifi-
cantly from 90, indicating a distortion from regular octa-
hedron. Average Co–Npy, Co–Namide and Co–Sthioether
˚
Table 2 Selected bond lengths (A) and angles (°) for (1) in
(1ꢀMeOHꢀH2O)
Complex (1)
Bond lengths
Co–N2
1.923 (3)
1.920 (3)
1.946 (3)
1.949 (3)
2.2201 (11)
2.2227 (11)
Co–N3
Co–N1
Co–N4
Co–S1
Co–S2
Bond angles
N1–Co–N2
N1–Co–N3
N1–Co–N4
N1–Co–S1
N1–Co–S2
N2–Co–N3
N2–Co–N4
N2–Co–S1
84.22 (13)
97.27 (13)
91.72 (12)
171.76 (10)
89.16 (9)
N2–Co–S2
N3–Co–N4
N3–Co–S1
N3–Co–S2
N4–Co–S1
N4–Co–S2
S1–Co–S2
93.31 (9)
84.06 (12)
90.97 (9)
87.84 (9)
89.30 (9)
171.90 (10)
90.99 (4)
˚
distances are 1.9475(3), 1.936(3) and 2.2214(11) A,
respectively. The average Co–Namide bond is shorter, by
˚
0.0115 A, than that of the average Co–Npy bond length, as
178.13 (12)
94.79 (13)
87.54 (10)
expected from the stronger r-donor ability of the depro-
tonated amido nitrogen. The average CoIII–Sthioether bond
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279
Fig. 1 View of the molecular structure of [CoIII(bpctb)]PF6 (1) in
(1ꢀMeOHꢀH2O) showing 50 % ellipsoids. H-atoms and solvent
molecules omitted for clarity
Fig. 2 View of the molecular structure of [CoIII(La)(Lb)] (2)
showing 50 % ellipsoids. H-atoms omitted for clarity
length in 1 is longer than in [CoIII(L2)](ClO4) (2.2165(2)
of the average Co–Npy bond length. The CoIII–Sthiolate and
CoIII–Sthioether distances in (2) are comparable to similar
[CoIII(L2)(L3)] complex obtained due to C–S bond cleav-
age reaction [40].
-2
˚
˚
A) and [Co(pycdadtH )](ClO4)ꢀH2O (2.211(4) A) repor-
ted in the literature [27, 39], indicating that the thioether
sulfur is more loosely bound in the present compound. The
˚
Co–S bond lengths (2.2201(11) and 2.2227(11) A in 1) are
longer than Co–Npy bond lengths in agreement with the
size and the poor coordinating ability of the thioether sulfur
donor atoms.
Spectral studies
The IR spectrum of H2bpctb exhibits a band characteristic
of amidic NH at 3,278 cm-1. The lack of N–H stretching
bands in the IR spectra of these complexes confirms that
the ligand is coordinated in its deprotonated form [15]. A
further indication of the formation of deprotonated com-
plexes is the rather large decrease in the m(CO) exhibited by
the ligand upon complex formation. The sharp C = O
stretching vibration band at 1,689 cm-1 for H2bpctb is
shifted to lower frequencies and appears at 1,639 and
1,621 cm-1 in (1) and (2), respectively, which is in
accordance with the data reported for the related complexes
[13, 16]. This is presumably due to the resonance
enhancement in the deprotonated amide which in turn leads
to the weakening of C = O bond [13].
Crystal structure of [CoIII(La)(Lb)] (2)
The molecular structure of [CoIII(La)(Lb)] (2) with atom
numbering is presented in Fig. 2, and selected bond angles
and distances are listed in Table 3. As evident from the
structural analysis of (2) the symmetrical hexadentate
ligand L is cleaved into two unsymmetrical tridentate parts
La (monoanionic) and Lb (dianionic), each providing an
N2S donor set. In (2), the two dissimilar tridentate ligands
are coordinated in a meridional fashion, one with a pyri-
dine amide vinyl thioether unit La (1–) and the other
including a pyridine amide thiolate unit Lb (2–). The
reaction of L(2–) with cobalt(II) acetate apparently leads to
the oxidative cleavage of the C–S bond [27], affording
distorted CoIIIN2(pyridine)N02(deprotonated amide)S(thio-
ether)S0(thiolate) coordination. The two deprotonated car-
boxamido nitrogens are trans to each other. The thiolate
and thioether sulfurs occupy positions that are trans to
pyridine nitrogens, and are cis to each other and to car-
The electronic absorption spectrum of H2bpctb in
chloroform consists of one intense band centered at
291 nm (e = 28,400 L mol-1 cm-1), assigned to p ? p*
transitions of the phenyl groups. The electronic spectra of
the cobalt (III) complexes (1) and (2) show, in addition to
the intraligand transitions, charge-transfer bands [41]
at 359 nm (e = 12,150 L mol-1 cm-1) in (1) and 356 nm
(e = 6,400 L mol-1 cm-1
)
and 458 nm (e = 1,465
˚
boxamido nitrogens. The average Co–Namide (1.914(2) A)
bond distance is in agreement with values reported for
deprotonated amide complexes of Co(III) [13, 27], and the
˚
average Co–Npy distance is 1.947 (2) A. As in (1), the
L mol-1 cm-1) in (2). In the visible region, the d–d
absorption band of complexes (1) and (2) is characteristic
of low-spin octahedral cobalt(III) with spin-allowed
absorption band. This band, arising from 1A1g ? T1g
1
˚
average Co–Namide bond is shorter (by 0.034 A) than that
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J IRAN CHEM SOC (2013) 10:275–282
˚
pyridine rings in (1) appear as a doublet at 9.50 ppm while
that of (2) is assignable as two doublets at 9.31 ppm and
9.57 ppm. This indicates that contrary to compound (1) the
Hg protons in compound (2) are not chemically equivalent,
a consequence of unsymmetrical cleavage of the hexa-
dentate ligand into two different tridentate ligands. The
vinyl protons in complex (2) appear as multiplets in the
5.02–5.54 ppm (Fig. 3), resulting from coupling to in-
equivalent adjacent vinyl protons [27, 40].
Table 3 Selected bond lengths (A) and angles (°) for (2)
Complex (2)
Bond lengths
Co1–N1
Co1–N3
1.908 (2)
1.919 (2)
1.936 (2)
1.958 (2)
2.2223 (7)
2.2184 (7)
Co1–N4
Co1–N2
Co1–S1
Co1–S2
Electrochemistry
Bond angles
N1–Co1–N2
N1–Co1–N3
N1–Co1–N4
N1–Co1–S1
N1–Co1–S2
N2–Co1–N3
N2–Co1–N4
N2–Co1–S1
84.02 (9)
179.19 (10)
96.07 (10)
88.63 (7)
91.63 (7)
96.56 (9)
88.73 (9)
172.60 (7)
N2–Co1–S2
N3–Co1–N4
N3–Co1–S1
N3–Co1–S2
N4–Co1–S1
N4–Co1–S2
S1–Co1–S2
94.66 (7)
83.39 (10)
90.80 (7)
88.89 (7)
92.89 (7)
171.88 (7)
84.68 (3)
The cyclic voltammograms of the complexes were
obtained at 25 C under an argon atmosphere using acetone
solutions containing 0.1 M TBAH as supporting electrolyte
and complex concentrations of about 4 9 10-3 M at a
glassy carbon working electrode. Ferrocene (Fc) was used
as the internal standard, and all redox potentials are ref-
erenced to the Fc?/0 couple. Figure 4 shows the cyclic
voltammograms of [CoIII(bpctb)]PF6 (1) and [CoIII(La)
(Lb)] (2) in acetone. The CoIII complexes (1) and (2) dis-
play electrochemically reversible responses corresponding
to a Co(III)/Co(II) redox process at E1/2 = -0.32 V
(DEp = 80 mV) and E1/2 = -0. 87 V (DEp = 70 mV),
respectively. The second reversible reduction response
(Fig. 4a) observed at E1/2 = -1.66 V (DEp = 90 mV) in
(1) is attributed to Co(II)/Co(I) redox process that is in
agreement with similar reported Co(III) complexes [27].
The Co(II)/Co(I) reduction process was not observed in (2)
transition, is observed at 507 nm (e = 550 L mol-1 cm-1
)
for (1) and 756 nm (e = 85 L mol-1 cm-1) for (2) [27].
1
The H NMR spectral data of the compounds are given
in the ‘‘Experimental’’ section. Two doublets at 2.72 and
3.32 ppm in (1) are assigned to the CH2–CH2 protons. The
aromatic protons of the pyridine and phenyl rings, as
complex multiplets, appear between 7.29 to 9.51 ppm in
(1) and 6.95 to 9.58 ppm in (2). Two Hg protons of two
Fig. 3 1H NMR spectra
(400 MHz) of selected regions
of [CoIII(La)(Lb)] (2) in CDCl3
at 298 K
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J IRAN CHEM SOC (2013) 10:275–282
281
shifted to more negative values in (2) relative to (1) pre-
sumably due to the higher negative charge transfer from the
ligands La and Lb to the metal center in (2).
As evident from CV results, Co(III) is stabilized better
in (2) than in (1). During the facile oxidation of CoII to
CoIII state, a base-assisted activation of the weakly acidic
C–H bond of the thioether chelate ring by acetate anion
occurs. The migration of the anionic charge to a sulfur site
leads to the cleavage of the coordinated ligand L(2–) into
two unsymmetrical ligand components via scission of the
C–S bond, affording the Co(III) complex (2) (Scheme 2)
[27]. This complex has a CoIII(N2S)(N2S0) coordination
sphere involving both thioether (S) and thiolate (S0) sulfur
with the Co(III) center surrounded by two tridentate
ligands bound in a meridional fashion.
Conclusion
This paper describes the synthesis of two new mononuclear
trivalent cobalt complexes having distorted octahedral
CoN4S2 coordination spheres prepared from a hexadentate
acyclic pyridine amide-thioether ligand and cobalt(II)
acetate. The complex [CoIII(bpctb)]PF6 (1) is formed in the
presence of hydrogen peroxide and has a CoIII(N4S2)
coordination sphere with equivalent thioether sulfur atoms.
In the absence of hydrogen peroxide, however, the ligand
L (2–) forms a five-membered chelate ring with CoII that
undergoes a facile oxidation to CoIII in air, followed by
cleavage of a C–S bond affording the complex [CoIII(La)
(Lb)] (2). This complex has a CoIII(N2S)(N2S0) coordina-
tion sphere involving both thioether (S) and thiolate (S0)
sulfurs, with two tridentate ligands arranged in a meridio-
nal fashion. In acetone solutions, the Co(III) complexes are
capable of undergoing two 1e- redox metal-centered pro-
cesses. The redox potentials of the metal center are influ-
enced by the inductive effect from charge delocalization by
C–S bond cleavage in the coordinated ligand.
Fig. 4 Cyclic voltammograms of
a
[CoIII(bpctb)]PF6 (1) and
b [CoIII(La)(Lb)] (2) c * 4 9 10-3 M in acetone at 293 K, scan
rate, 100 mVs-1
due to the limitations in the potential scanning in acetone
solvent.
The electrochemical results of the two complexes show
that the reduction potential of Co(III)/Co(II) center are
Scheme 2 Reaction pathway
for the C–S bond cleavage
H
H
H
S
S
S
H
H
H
-
II
Co
CH3COO
III
Co
Co
H
-H+
H
H
H
S
S
S
123

282
J IRAN CHEM SOC (2013) 10:275–282
Acknowledgments Partial support of this work by the Isfahan
University of Technology Research Council is gratefully acknowl-
edged. Also acknowledged is partial support from the US National
Science Foundation Grant (NSF-CHE-0749524) to PCF.
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