Synthesis, characterization, and redox behavior of new dicobalt complexes having monoanionic imine/oxime-type ligands

Article abstract of DOI:10.1246/bcsj.75.1553

New dicobalt complexes, [Co^III₂LBr₄], with monoanionic ligands (L) have been synthesized by the reaction of a tetraamine and oximes, followed by an aerobic reaction with CoBr₂·6H₂O. The complexes were characterized by elemental analyses, IR, ESR, and NMR as well as mass spectroscopies. The redox behavior of the complexes was examined in DMF by means of cyclic voltammetry in comparison with that of the corresponding mononuclear complex. Redox waves identified to Co^III/Co^II and Co^II/Co^I for [Co^III₂L] were observed at -0.19 V and -0.69 V vs Ag/AgCl, respectively. These potentials are quite similar to those for the corresponding mononuclear complex. An electrogenerated [Co¹₂L] species reacts with methyl p-toluenesulfonate to give an organocobalt complex. Two-electron reduction of the dinuclear organocobalt compound yields an unstable intermediate that undergoes rapid decomposition by cleavage of the cobalt-carbon bond. The dimethylated complex, which has cobalt-carbon bonds at one axial site of each cobalt, was disproportionated to a tetramethylated complex, involving two cobalt-carbon bonds at both axial sites, and a dicobalt(I) species by two-electron reduction. The tetramethylated cobalt complex was inactive for an electrochemical reduction, but transformed into the dimethylated complex via cleavage of the cobalt-carbon bonds upon electrochemical oxidation.

Full text of DOI:10.1246/bcsj.75.1553

Bull. Chem.
Soc. Jpn.
75
7
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1553–1558 (2002) 1553
The Chemical
Society of Ja-
pan
The Chemical
Society of Ja-
pan
AI
Synthesis, Characterization, and Redox Behavior of New Dicobalt
Complexes Having Monoanionic Imine/Oxime-Type Ligands
Dicobalt Complexes Having Monoanionic Ligands
H. Shimakoshi et al.
7
15
Hisashi Shimakoshi, Masaomi Koga, and Yoshio Hisaeda*
2002
1553
1558
BCSJA8
0009-2673
02029
2
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Hakozaki, Higashi-ku, Fukuoka 812-8581
(Received February 6, 2002)
Ⅲ
6
New dicobalt complexes, [Co 2LBr4], with monoanionic ligands (L) have been synthesized by the reaction of a tet-
2002
4
8
raamine and oximes, followed by an aerobic reaction with CoBr2•6H2O. The complexes were characterized by elemental
analyses, IR, ESR, and NMR as well as mass spectroscopies. The redox behavior of the complexes was examined in
DMF by means of cyclic voltammetry in comparison with that of the corresponding mononuclear complex. Redox
Ⅲ
Ⅱ
Ⅱ
Ⅰ
Ⅲ
waves identified to Co /Co and Co /Co for [Co 2L] were observed at −0.19 V and −0.69 V vs Ag/AgCl, respectively.
2002
Ⅰ
These potentials are quite similar to those for the corresponding mononuclear complex. An electrogenerated [Co 2L]
species reacts with methyl p-toluenesulfonate to give an organocobalt complex. Two-electron reduction of the dinuclear
organocobalt compound yields an unstable intermediate that undergoes rapid decomposition by cleavage of the cobalt–
carbon bond. The dimethylated complex, which has cobalt–carbon bonds at one axial site of each cobalt, was dispropor-
tionated to a tetramethylated complex, involving two cobalt–carbon bonds at both axial sites, and a dicobalt(Ⅰ) species by
two-electron reduction. The tetramethylated cobalt complex was inactive for an electrochemical reduction, but trans-
formed into the dimethylated complex via cleavage of the cobalt–carbon bonds upon electrochemical oxidation.
2.2
2.4.3
Much interest has been focused on the synthesis of a dinu-
Experimental
cleating ligand which can coordinate two metal ions at a suit-
able distance and geometry, aiming at the cooperative interac-
Materials. Methyl p-toluenesulfonate was purified by distil-
lation under reduced pressure. 1,3-Phenylenebis(methylene)bis-
tion of two metallic sites in metal complexes.1–4 In such a
ligand, the coordination sites of the two metal ions are highly
designed and regulated by the coordinating group and ligand
framework. p-, m- , and o-Xylene units are available bridging
moieties because of their variable conformation.5,6 As a pen-
dant donor and a coordinating group, cyclam or related tet-
raazamacrocyclic ligands have been extensively developed in
view of their remarkable stability and specificity for metal
ions.7,8 We now consider an imine/oxime skeleton, a so-called
Costa-type monoanionic ligand, such as (DO)(DOH)pn. Al-
though (DO)(DOH)pn, which is N2, N2ꢀ-propanediylbis(2,3-
butanedione 2-imine 3-oxime), has been extensively studied in
organometallic chemistry, especially in the field of vitamin B12
chemistry,9 little work has been made to extend the use of this
ligand toward a dinucleating system. The Costa-type cobalt
complex can reproduce the E1/2 value for Co /Co in base-off
aquacobalamin,10,11 and permits one of the best model com-
plexes for B12-dependent enzymic rearrangements.12,13 The
most remarkable property of this complex is to form a stable
cobalt–carbon bond. What happens when a dicobalt complex,
bearing two cobalt–carbon bonds, is irradiated with visible
light? It would be possible to effect a simultaneous, or near-
synchronous, homolytic cleavage of both cobalt–carbon bonds.
Therefore, it is expected to activate a pair of substrates via a di-
organocobalt complex as an intermediate when we use the di-
cobalt complex.14,15 In this paper, we report on the synthesis,
characterization, and redox behavior of new dicobalt complex-
es with monoanionic Costa-type ligands.
{2-(1,3-propanediamine)} and 2-benzyl-1,3-diaminopropane were
prepared by the same method, reported in the literature.16 3-Hy-
droxyimino-4-pentanone was prepared by a reaction between 2-
pentanone and methyl nitrate.17 [Co(salen)], salen, a dianion of
disalicylidene ethylenediamine, and [Co(Hdmg)2], Hdmg, a
monoanion of dimethylglyoxime, were synthesized by a reported
method.18,19 All of the solvents and chemicals used in the synthe-
ses were of reagent grade. For electrochemical studies, N, N-dim-
ethylformamide (DMF) was stirred for one day in the presence of
BaO under a nitrogen atmosphere, and distilled under reduced
pressure. Tetra-n-butylammonium perchlorate (TBAP) and tetra-
n-butylammonium bromide (TBABr) were purchased from Na-
kalai Chemicals (special grade) and dried at 60 °C under a vacuum
before use. Dicobalt complexes, 1a and 1b, and the reference
mononuclear cobalt complexes, 2a and 2b, were prepared as fol-
low (refer to Scheme 1).
Synthesis of 1a. To a methanol solution (20 mL) of 1,3-phen-
ylenebis(methylene)bis{2-(1,3-propanediamine)} (249 mg, 1.0
mmol) was added 2,3-butanedione monoxime (1.62 g, 16.0
mmol); the reaction mixture was refluxed for 30 min. After the re-
action mixture was cooled to room temperature, CoBr2•6H2O (719
mg, 2.2 mmol) was added while air was being bubbled through the
solution. The reaction mixture was stirred at room temperature
for 8 h to give a greenish-brown powder. The powder was collect-
ed by suction filtration, and then washed by water and diethyl
ether. After being dried in vacuo, the powder was recrystallized
from methanol/water to give a green powder. Yield: 86 mg (17%).
Found: C, 34.12; H, 4.76; N, 10.14%. Calcd for C30H44N8Br4Co2-
Ⅱ
Ⅰ

1554 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
Dicobalt Complexes Having Monoanionic Ligands
Scheme 1.
O4•2H2O: C, 34.18; H, 4.59; N, 10.63%. IR [KBr; ν/cm−1] 1520
(CwN). 1H NMR [CDCl3] δ 2.50 (s, 12H, -CH3), 2.56 (s, 12H,
-CH3), 2.91(d, 4H, -CH2PhCH2-), 3.28 (m, 2H, methine), 3.73 (d-
d, 4H, -CH2N), 4.17 (d-d, 4H, -CH2N), 7.17 (d, 2H, Ph), 7.19 (s,
1H, Ph), 7.36 (t, 1H, Ph), 19.25 (s, 2H, OH). 13C NMR [CDCl3] δ
13.92 (-CH3), 17.90 (-CH3), 38.38 (CH2N), 39.56 (-CH2PhCH2),
55.26 (methine), 127.9, 129.3, 129.5, 138.7 (Ph), 156.1 (oxime),
174.4 (imine). ESI MS m/z [M−Br]+, 937; [M−2Br]+, 858;
[M−3Br]+, 777; [M−4Br]+, 697.
Synthesis of 1b. Another dicobalt complex 1b was synthe-
sized in the same manner as for 1a, except for using 3-hydroxy-
imino-4-pentanone in place of 2,3-butanedione monoxime. The
product was recrystallized from acetone/water to give a green
powder. Yield: 26%. Found: C, 37.59; H, 5.22; N, 10.43%. Calcd
for C34H52N8Br4Co2O4•H2O: C, 37.39; H, 4.98; N, 10.26%. IR
[KBr; ν/cm−1] 1510 (CwN). 1H NMR [CDCl3] δ 1.15 (t, 12H,
-CH2CH3), 2.49 (s, 12H, -CH3), 2.91 (d, 4H, PhCH2), 2.97 (m, 8H,
-CH2CH3), 3.26 (m, 2H, methine), 3.73 (d-d, 4H, -CH2N), 4.18 (d-
d, 4H, -CH2N), 7.17 (d, 2H, Ph), 7.19 (s, 1H, Ph), 7.36 (t, 1H, Ph),
19.23 (s, 2H, OH). 13C NMR [CDCl3] δ 10.2 (-CH2CH3), 17.6
(-CH3), 20.9 (-CH2CH3), 38.5 (CH2N), 39.6 (-CH2PhCH2-), 55.4
m/z (methine), 127.9, 129.3, 129.6, 138.8 (Ph), 161.5 (oxime),
174.2 (imine). ESI MS m/z [M−Br]+, 993; [M−2Br]+, 914;
[M−3Br]+, 833; [M−4Br]+, 753.
Synthesis of 2a. A dry isopropyl ether solution (20 mL) of
2,3-butanedione monoxime (1.3 g, 12.86 mmol) and 2-benzyl-1,3-
diaminopropane (520 mg, 3.17 mmol) was heated for 4 h under re-
flux until 0.1 mL of water was collected in a Dean–Stark trap. The
solution was evaporated to give a yellow oil as the target ligand.
An acetone solution (20 mL) of the ligand was added dropwise to
an aqueous solution (20 mL) of CoBr2•6H2O (1.04 g, 3.18 mmol)
while air was bubbled through the aqueous solution. After the re-
action mixture was stirred for 4 h, precipitates were recovered by
suction filtration and washed with water and diethyl ether. The re-
sulting cobalt complex was recrystallized from acetone/water to
afford light-green needles. Yield: 370 mg(44%). Found: C, 39.70;
H, 4.64; N, 10.27%. Calcd for C18H25N4Br2CoO2: C, 39.44; H,
4.60; N, 10.22%. IR [KBr; ν/cm−1] 1520 (CwN). 1H NMR
[CDCl3] δ 2.51 (s, 6H, -CH3), 2.57 (s, 6H, -CH3), 2.91 (d, 2H,
PhCH2-), 3.28 (m, 1H, methine), 3.75 (d-d, 2H, -CH2N), 4.17 (d-
d, 2H, -CH2N), 7.26 (m, 3H, Ph), 7.37 (t, 2H, Ph), 19.28 (s, 1H,
OH). 13C NMR [CDCl3] δ 13.9 (-CH3), 17.7 (-CH3), 38.3 (CH2N),
39.5 (PhCH2-), 55.3 (methine), 127.0, 128.9, 129.0, 137.7 (Ph),
156.0 (oxime), 174.3 (imine). ESI MS m/z [M−Br]+, 467;
[M−2Br]+, 388.
Synthesis of 2b. Another mononuclear cobalt complex 2b
was synthesized in the same manner as for 2a, except for using 3-
hydroxyimino-4-pentanone in place of 2,3-butanedione mon-
oxime. The cobalt complex was recrystallized from acetone/water
to give dark-green needles. Yield: 34%. Found: C, 41.87; H, 5.14;
N, 9.65%. Calcd for C20H29N4Br2CoO2: C, 41.69; H, 5.07; N,
9.72%. IR [KBr; ν/cm−1]: 1510 (CwN). 1H NMR [CDCl3] δ 1.18
(t, 6H, -CH2CH3), 2.50 (s, 6H, -CH3), 2.91 (d, 2H, PhCH2-), 3.01
(m, 4H, -CH2CH3), 3.30 (m, 1H, methine), 3.74 (d-d, 2H, -CH2N),
4.17 (d-d, 2H, -CH2N), 7.28 (m, 3H, Ph), 7.37 (t, 2H, Ph), 19.26
(s, 2H, OH). 13C NMR [CDCl3] δ 10.2 (-CH2CH3), 17.4 (-CH3),
20.9 (-CH2CH3), 38.4 (CH2N), 39.5 (PhCH2-), 55.3 (methine),
127.0, 129.0, 129.1, 137.8 (Ph), 161.5 (oxime), 174.0 (imine).
ESI MS m/z [M−Br]+, 495; [M−2Br]+, 416.
Measurements. Elemental analyses were obtained from the
Service Center of Elementary Analysis of Organic Compounds at
1
Kyushu University. The H, 13C and 2-D COSY-NMR spectra
were recorded on a Bruker AMX 500 spectrometer installed at the
Center of Advanced Instrumental Analysis in Kyushu University,
and the chemical shifts (in ppm) were referenced relative to SiMe4
as the internal standard. The IR spectra were recorded on a
JASCO IR-810 spectrophotometer. Electrospray ionization (ESI)
mass spectra were obtained on a PE SCIEX API Ⅲ. The ESR
spectra were obtained on a JEOL JES-FE1G X-band spectrometer
equipped with an Advantest TR-5213 microwave counter and an
Echo Electronics EFM-200 NMR field meter. Cyclic voltammo-
grams were obtained using a BAS CV 50W electrochemical ana-
lyzer. A three-electrode cell equipped with a 1.6-mm diameter
platinum wire as the working and counter electrodes was used,

H. Shimakoshi et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1555
and an Ag/AgCl (3.0 M NaCl) electrode served as a reference.
DMF solutions containing a cobalt complex (1.0 × 10−3 M),
TBAP (1.0 × 10−1 M), and TBABr (2.0 × 10−2 M) were deaerat-
ed prior to each measurement, and the inside of the cell was main-
tained under an argon atmosphere throughout each measurement.
All of the measurements were carried out at room temperature.
The scan rate was varied over the range from 10 through 500 mV
s−1. The E1/2 value of ferrocene/ferricinium (Fc/Fc+) was 0.54 V
vs Ag/AgCl with this setup.
Measurements of ESR Spectra. The ESR spectra of diva-
lent cobalt complexes were measured as follows. After complex
1a or 2a (1 mM) was dissolved in methanol–benzene (9:1 v/v) and
completely degassed by freeze-pump-thaw cycles, 5 equivalent
moles of NaBH4 were added to reduce the cobalt complex. ESR
spectra of in-situ formed divalent cobalt complexes were obtained
at 77 K.
Results and Discussion
Preparation and Characterization of Cobalt Complexes.
New dicobalt complexes having monoanionic imine/oxime-
type ligands have been synthesized by a one-pot reaction.
After a mixture of 2,3-butanedione monoxime and tetraamine
was stirred for 30 min in hot MeOH, cobalt(Ⅱ) bromide was
added while air was bubbled through the solution to yield a
hexacoordinated cobalt(Ⅲ) complex. Complexes 1a and 1b,
isolated as a green solid, gave various satisfactory spectro-
scopic analyses. Mononuclear analogues, 2a and 2b, were pre-
pared in the same manner to give a green crystal, which gives
good analytical data. In the IR spectrum, a broad band at 2900
cm−1 (-NH2) in the precursor tetraamine disappeared, and new
bands assignable to CwN vibration of coordinated azomethine
groups appeared at 1520 and 1510 cm−1 for 1a and 1b, respec-
tively. Diamagnetic cobalt(Ⅲ) complexes show well-defined
proton NMR spectra. The dicobalt complex 1b has C2v sym-
metry, and all signals for the mononuclear unit appeared in the
same region as shown in Fig. 1. A signal at 19.2 ppm is as-
signed to the protons in intramolecular hydrogen bonds (see
the signal (h) in Fig. 1). The unequivalent methylene hydro-
gens of the propanediimine chelate ring appear at 3.73 and
4.18 ppm, respectively, and split by each other (Jgeminal = 15.0
Hz) and by the methine proton (Jvicinal = 13.5, 2.5 Hz) (see the
signals (f) and (g) in Fig. 1). The proton NMR spectrum and
the detailed spectral assignments by 2-D NMR measurements
provide conclusive evidence for the structure of 1b. The for-
mation of 1b has also been confirmed by ESI-MS. The intro-
duction of a methanol solution of 1b into an ESI mass spec-
trometer afforded a positive-ion mass spectrum with the most
prominent peak clusters at m/z 993, 914, 833 and 753, which
have mass values and isotope patterns consistent with the ions
[M-Br]+, [M-2Br]+, [M-3Br]+, [M-4Br]+, respectively, as
shown in Fig. 2. This is the first example of a Costa-type dico-
balt complex linking with a flexible spacer. The formation of
another dicobalt complex 1a and corresponding mononuclear
cobalt complexes, 2a and 2b, was also supported by similar
spectroscopic data.
Fig. 1. 1H NMR (500 MHz) spectra of the complex 1b in
CDCl3. The assignments are shown in the figure.
type ESR spectrum (dZ2 ground state, g = 2.28, g = 2.01,
⊥
ꢀ
A
Co = 109 G),20 and is almost similar to that for a correspond-
ꢀ
ing mononuclear complex which was prepared by the reduc-
Co
tion of 2a (g = 2.30, g = 2.02, A = 112 G). Therefore,
⊥
ꢀ
ꢀ
we concluded that no metal-metal interaction exists in the di-
cobalt complex. The following electrochemical measurements
also supported that there was no interaction between the two
cobalt centers.
Electrochemistry of Cobalt Complexes. The cyclic vol-
tammogram of the dinuclear cobalt complex 1b in DMF is
shown in Fig. 4a, which also displays a cyclic voltammogram
of the mononuclear cobalt complex 2b under the same condi-
Ⅱ
Ⅰ
Ⅲ
Ⅱ
tions. The Co /Co and Co /Co redox couples (E1/2) for 1b
were observed at −0.69 V and −0.19 V vs Ag/AgCl, respec-
tively. The redox behavior for 1b was very similar to that for
the corresponding mononuclear complex 2b, as shown in Fig.
4b. In order to provide an unambiguous assignment of the re-
dox couples, coulometry studies were carried out at −1.0 V
and −0.5 V vs Ag/AgCl. The charge passed corresponded to
two electrons and one electron at each potential for 1b and 2b,
respectively. The redox potentials for complex 1a are listed in
Table 1. Modifications of the peripheral moieties provided no
ESR Study of Cobalt(ꢀ) Complexes. To elucidate the
existence of a metal–metal interaction in the molecule, ESR
spectra were measured in its divalent state. The ESR spectrum
of a dicobalt(Ⅱ) complex, which was prepared by the reduction
of 1a, is shown in Fig. 3. The complex shows a typical axial-

1556 Bull. Chem. Soc. Jpn., 75, No. 7 (2002)
Dicobalt Complexes Having Monoanionic Ligands
Fig. 2. (a) ESI-mass spectrum of complex 1b. (b) Calculat-
ed isotope pattern is represented by bars under a peak
cluster of the parent ion.
Fig. 4. Cyclic voltammograms of (a) 1b (5.0 × 10−4 M) and
(b) 2b (1.0 × 10−3 M) in DMF containing 0.1 M TBAP
and 0.02 M TBABr. Scan rate, 100 mV sec−1
.
Table 1. Redox Potentials for the Various Cobalt Complexes
in DMFa)
E1/2/V vs Ag/AgCl
Complex
Ⅲ
Ⅱ
Ⅱ
Ⅰ
Co /Co
Co /Co
1ab)
1bb)
2ab)
2bb)
−0.19
−0.19
−0.21
−0.20
+0.10
−0.71
−0.69
−0.74
−0.71
−1.21
−1.02
−0.74
−0.61
Fig. 3. ESR spectrum of dicobalt(Ⅱ) complex, which was
prepared by the reduction of 1a, in methanol–benzene (9:1
v/v) at 77 K.
Ⅱ
[Co (salen)]
Ⅱ
[Co (Hdmg)2]
+0.12
c)
Vitamin B12
Hydrophobic B12
ca. +0.3
ca. +0.4
remarkable change in the redox potentials in comparison to 1a
with 1b, in which the oxime methyl groups of 1a are replaced
by ethyl groups. The ratios between the anodic and cathodic
peak currents of 1a and 1b, ipa/ipc, were almost unity and inde-
pendent of the scan rate (from 10 to 500 mV s−1) for the two
redox couples in DMF. The plots of ip (= ipa + ipc) vs ν1/2(ν is
the scan rate, mV s−1) were linear, and the potential separation
between the anodic and cathodic peaks varied from 160 to 60
mV for the two redox couples, while the E1/2 values remained
constant within an accuracy of 5%, regardless of the scan rate
variation. Therefore, the present redox reactions in DMF are
d)
a) Working and counter electrodes: Pt, [1a] = [1b] = 5.0
× 10−4 M, [2a] = [2b] =1.0 × 10−3 M, [TBAP] = 0.1 M,
[TBABr] = 2.0 × 10−2 M, under Ar atmosphere at room
temperature. Scan rate 100 mV/s. b) In the presence of 2.0
× 10−2 M TBABr. c) Ref. 24. d) Ref. 25.
consistent with reversible one-electron transfer processes for
each metal.21
Comparisons with the redox potentials for other cobalt com-

H. Shimakoshi et al.
Bull. Chem. Soc. Jpn., 75, No. 7 (2002) 1557
The two-electron reduction intermediate of 3 was unstable
and disproportionated to a dicobalt(Ⅰ) species and the tetrame-
thylated cobalt complex 4 at the above potential, as shown in
Eq. 3. Another possibility is that it intramolecularly dispropor-
tionates to form 5. The electrochemical behavior of a Costa-
type mononuclear cobalt complex with a cobalt–carbon bond
was previously proved by us.12,27 We have also prepared a
Costa-type cobalt complex bearing two methyl groups at both
axial sites, which is inactive for electrochemical reduction, and
oxidized at +0.73 V vs Ag/AgCl to form a methylated com-
plex and a methyl radical.12 Although complex 4 was inactive
for electrochemical reduction, an irreversible oxidation peak
was observed at +0.80 V vs Ag/AgCl, when scanned to the an-
odic direction. The similarity of the cyclic voltammogram to
that of the mononuclear complex suggests that the cobalt–
carbon bond of complex 4 is cleaved to afford the dimethylated
complex 3 and methyl radicals by the electrochemical oxida-
tion, as shown in Eq. 4.
Fig. 5. Cyclic voltammogram of 1b in the presence of meth-
yl p-toluenesulfonate in DMF. Initial concentration: 1b,
1.0 × 10−3 M; methyl p-toluenesulfonate, 3.0 × 10−2 M;
TBAP, 0.1 M. Scan rate, 100 mV sec−1
.
Ⅱ
Ⅱ
plexes with dianionic ligands, [Co (salen)] and [Co (Hdmg)2]
Ⅱ
Ⅰ
are also listed in Table 1. The Co /Co redox couples for
[Co(salen)] and [Co(Hdmg)2] in DMF were observed at −1.21
Ⅱ
Ⅰ
V and −1.02 V vs Ag/AgCl, respectively. The Co /Co couple
is sensitive primarily to the basicity of the equatorial
ligand.22,23 The complexes synthesized here have less negative
Co /Co redox couples because they have monoanionic
ligands. A natural vitamin B12 and a modified vitamin B12 hav-
ing a monoanoinic corrin ligand afford similar redox poten-
tials, as shown in Table 1.24,25
(3)
(4)
Ⅱ
Ⅰ
Costa-type dicobalt complexes are easily reduced to the cor-
responding cobalt(Ⅰ) species, as shown in Eq. 1. Square-planar
cobalt(Ⅰ) complexes are strong nucleophiles and react with var-
ious electrophiles, such as organic halide or alkyl p-toluene-
sulfonate, to give organocobalt complexes.26 The addition of
methyl p-toluenesulfonate as an electrophile significantly
changed the voltammetric pattern of 1b, as shown in Fig. 5. In
the presence of an excess methyl p-toluenesulfonate, a new ir-
reversible peak appeared at ca. −1.50 V vs Ag/AgCl and the
In conclusion, new dicobalt complexes having monoanionic
imine/oxime-type ligands have been synthesized and charac-
terized by various spectroscopic methods as well as elemental
analyses. The metal sites of the complexes, which are linked
by a m-xylene spacer, are completely discrete. Such dicobalt
complexes could activate two molecules at the same time to
form a pair of active species, such as radicals, which are ex-
pected to selectively form dimerized compounds.15 If we used
a dicobalt complex having monoanionic ligands, it is expected
to work as a catalyst under very mild reduction conditions.
Further work on the reactivities of the complexes with multi-
reaction sites is in progress in our laboratory.
Ⅱ
Ⅰ
Co /Co redox wave became somewhat irreversible. A methy-
lated Costa-type complex was reduced at ca. −1.50 V vs Ag/
AgCl in DMF, as we previously reported.27 The decrease on
Ⅰ
Ⅱ
the oxidation peak for Co /Co suggests that the cobalt(Ⅰ) spe-
cies reacts with methyl p-toluenesulfonate to give [(CH3-
Ⅲ
Co )2L], 3, with a cobalt–carbon bond on both metal centers,
We wish to thank Prof. Y. Naruta and Dr. Y. Tachi, Institute
for Fundamental Research in Organic Chemistry, Kyushu Uni-
versity, for measurements of the ESI-MS spectra. We also
thank Mr. H. Horiuchi, a glassworker in our department, for
his skill in preparing specific ESR cells. The present work was
supported by a Grant–in–Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture.
as shown in Eq. 2, where L represents the corresponding
ligand. The new peak at ca. −1.50 V vs Ag/AgCl is assigned
to the reduction of 3.
(1)
(2)
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