Synthesis and redox behavior of dicobalt complexes having flexible and rigid linkers

Article abstract of DOI:10.1246/bcsj.78.1040

New dicobalt complexes, [Co^II₂L], with Schiff-base ligands (L1-L4) having two discrete metal chelating sites, which are connected to each other by flexible and rigid linkers, have been synthesized. The complexes were characterized by

Full text of DOI:10.1246/bcsj.78.1040

1
040 Bull. Chem. Soc. Jpn., 78, 1040–1046 (2005)
ꢀ 2005 The Chemical Society of Japan
Synthesis and Redox Behavior of Dicobalt Complexes
Having Flexible and Rigid Linkers
1
1
Hisashi Shimakoshi, Satoshi Hirose, Masaaki Ohba, Takuya Shiga,
1
ꢀ
¯
Hisashi Okawa, and Yoshio Hisaeda
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Hakozaki, Higashi-ku, Fukuoka 812-8581
1
Department of Chemistry, Graduate School of Science, Kyushu University,
Hakozaki, Higashi-ku, Fukuoka 812-8581
Received October 18, 2004; E-mail: yhisatcm@mbox.nc.kyushu-u.ac.jp
II
New dicobalt complexes, [Co 2L], with Schiff-base ligands (L1–L4) having two discrete metal chelating sites,
which are connected to each other by flexible and rigid linkers, have been synthesized. The complexes were character-
ized by IR, UV–vis, and mass spectroscopy as well as elemental analyses. A magnetic study indicates an antiferromag-
netic interaction in 4. The redox behavior of the complexes was examined by means of cyclic voltammetry compared
III
II
II
I
with those of the corresponding mononuclear complexes. Redox waves, identified to Co /Co and Co /Co ,
I
2ꢁ
were clearly observed. Electro-generated [Co 2L] reacts with alkyl halide at each metal center to give an organocobalt
complex.
The recent growth of studies on multi-metallic compounds
involves the areas of homogeneous catalysis, magnetic ex-
change between paramagnetic centers, and bioinorganic chem-
R
N
Co
N
Co
O
O
O
N
N
1
–7
istry. Molecular systems having two (or more) redox active
centers in close proximity, capable of cooperative interactions,
are of interest in relation to their potential as catalysts for sub-
O
N
N
N
O
O
O
Co
Co
O
N
8
strate reduction or oxidation. Considerable effort has thus
R = -CH - : 1
2
been directed towards the synthesis of a ligand capable of
holding two separated metal ions, either the same or different,
which are subject to control by an appropriate modification of
the molecular topology.9 Recently, we synthesized a dinu-
cleating ligand in which two H2salen units, H2salen is a
-CH2CH2- : 2
-O- : 3
4
,10
N
N
O
0
N,N -ethylenebis(salicylideneamine), are covalently linked to-
gether so as to bring two bound metal centers into close prox-
Co
O
R
R
1
1,12
R
R
imity.
The corresponding dicobalt complex of this ligand
formed a cobalt–carbon bond at each metal center, and cleav-
age of this bond afforded a coupling product via radical cou-
pling. In this ligand, the connecting linker is limited to meth-
ylene, and the distance and geometry of two metal centers are
limited. Thus, in this paper, we report on the synthesis of new
R = t-Bu : 4'
II
H : [Co (saloph)]
Chart 1.
stored in a refrigerator under a nitrogen atmosphere in the pres-
ence of activated molecular sieves 4A. Care was taken to remove
basic impurities that coordinate to the CoIII species and cause ad-
II
dicobalt complexes that have a [Co (saloph)] unit connected
to each other with various linkers, as shown in Chart 1. The
redox behavior of dicobalt complexes and the reactivities of
ditional reduction peaks in the voltammograms.13 Tetrabutylam-
monium perchlorate (n-Bu4NClO4) was purchased from Nakalai
tesque (special grade) and dried at room temperature under vacu-
I
I
the electrogenerated Co Co species toward alkyl halide were
investigated.
0
0
0
um before use. 3,3 ,4,4 -Tetraaminodiphenylmethane, 1,2-(3,3 ,
,4 -tetraaminodiphenyl)ethane, and 3,3 ,4,4 -tetraaminodiphenyl-
Experimental
0
0
0
4
ether were synthesized by a reported method. [Co (saloph)], sal-
14
II
Materials. All solvents and chemicals used in the synthesis
were of reagent grade, and were used without further purification.
For electrochemical studies, N,N-dimetylformamide (DMF) was
stirred for one day in the presence of BaO under a nitrogen atmo-
sphere, and distilled under reduced pressure. The distillation was
performed with the exclusion of light, and thus-purified DMF was
0
oph, which is a dianion of N,N -(o-phenylenediamine)bis(salicyli-
deneamine), was synthesized by a reported method.15
General Analyses and Measurements. Elemental analyses
were obtained from the Service Center of Elementary Analysis
of Organic Compounds at Kyushu University. The H, C, and
1
13
Published on the web June 6, 2005; DOI 10.1246/bcsj.78.1040

H. Shimakoshi et al.
-D COSY-NMR spectra were recorded on a Bruker Avance 500
spectrometer installed at the Center of Advanced Instrumental
Analysis in Kyushu University, and the chemical shifts (in ppm)
were referenced relative to the residual protic solvent peak. The
UV–vis absorption spectra were measured on a Hitachi U-3300
spectrophotometer at room temperature. The FAB-mass spectra
were obtained on a JEOL JMS-HX110A using m-nitrobenzylalco-
hol as a matrix. The IR spectra were recorded on a JASCO IR-810
spectrophotometer using KBr discs.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1041
ꢁ1
ꢁ1
2
UV–vis (in DMF): [ꢄmax/nm ("/M cm )], 307 (49300), 341
(33700), 390 (44000).
Synthesis of L2. L2 was synthesized in the same manner as
0
0
for L1, except for use of 3,3 ,4,4 -tetraaminodiphenylethane in
0
0
place of 3,3 ,4,4 -tetraaminodiphenylmethane. Yield: 84%. Anal.
found: C, 76.51; H, 5.24; N, 8.45%. calcd for C42H34N4O4: C,
ꢁ
1
76.58; H, 5.20; N, 8.51%. IR [KBr; ꢂ/cm ]: 3450 (br, OH),
2950 (s, CH), 1620 (s, C=N), and 1175 (s, C–O); 1H NMR
(DMSO-d6, 500 MHz): ꢃ 3.05 (4H, s, –CH2CH2–), 6.95 (8H, m,
Ph), 7.30 (2H, dd, Ph), 7.39 (8H, m, Ph), 7.63 (4H, dd, Ph)
8.88 (2H, s, N=CH), 8.94 (2H, s, N=CH), 12.93 (2H, s, OH),
13.06 (2H, s, OH); 13C NMR (DMSO-d6, 125 MHz): ꢃ 36.5
(–CH2CH2–), 116.6, 116.7, 119.0, 119.1, 119.4, 119.5, 119.6,
119.8, 127.9, 132.3, 132.4, 133.3, 133.4, 140.0, 141.5, 142.3
(Ph), 160.4, 160.5 (N=CH), 163.3, 163.9 (PhO). HRMS (FAB,
Solid State Susceptibility Measurement. The magnetic sus-
ceptibilities of powdered samples were measured on a Quantum
Design MPMS XL SQUID susceptometer in the temperature
range of 2–300 K; the apparatus was calibrated with [Ni(en)3]-
[
(
S2O3] (en = ethylenediamine).16 Effective magnetic moments
1=2
ꢀ ) were calculated by the equation ꢀeff ¼ 2:828ðꢁ TÞ
,
eff
M
þ
þ
where ꢁM is the molar magnetic susceptibility per molecule.
The data were corrected for the sample holder contribution and
for the diamagnetic contribution estimated through Pascal’s con-
stant.17
m=z): Calcd for C42H35N4O4: [MH] , 659.2658. Found: [MH] ,
659.2662.
Synthesis of 2. 2 was synthesized in the same manner as for 1,
except for use of L2 in place of L1. Yield: 94%. Anal. found: C,
Cyclic Voltammetry. Cyclic voltammograms were obtained
using a BAS CV 50 W electrochemical analyzer. A three-elec-
trode cell equipped with a 1.6-mm diameter platinum wire as
the working and counter electrodes were used, respectively. An
62.87; H, 3.94; N, 6.92%. calcd for C42H30N4O4Co2 2H2O: C,
62.39; H, 4.24; N, 6.93%. IR [KBr; ꢂ/cm ]: 2950 (s, CH),
1580 (s, C=N), and 1180 (s, C–O). HRMS (FAB, m=z): Calcd
ꢃ
ꢁ1
þ
þ
for C42H30N4O4Co2: [M] , 772.0931. Found: [M] , 772.0955.
ꢁ
3
ꢁ1
ꢁ1
Ag–AgCl (3.0 mol dm NaCl) electrode served as a reference.
Nonaqueous DMF solutions containing
UV–vis (in DMF): [ꢄmax/nm ("/M cm )], 307 (42800), 337
(32600), 391 (39800).
Synthesis of L3. L3 was synthesized in the same manner as
a cobalt complex
)
were deaerated prior to each measurement, and the inside of the
cell was maintained under an argon atmosphere throughout each
measurement. All measurements were carried out at 298 K. The
(
5:0 ꢂ 10ꢁ mol dm ) and n-Bu4NClO4 (1:0 ꢂ 10 mol dm
4
ꢁ3
ꢁ1
ꢁ3
0 0
for L1, except for use of 3,3 ,4,4 -tetraaminodiphenylether in
0
0
place of 3,3 ,4,4 -tetraaminodiphenylmethane. Yield: 91%. Anal.
found: C, 74.02; H, 4.67; N, 8.50%. calcd for C40H30N4O5: C,
ꢁ
1
ꢁ1
scan rate was varied over the range from 10 through 500 mV s
þ
.
74.29; H, 4.68; N, 8.66%. IR [KBr; ꢂ/cm ]: 3450 (br, OH),
The E1=2 value of ferrocene/ferrocenium (Fc/Fc ) was þ0:56 V
1615 (s, C=N), and 1180 (s, C–O). 1H NMR (DMSO-d6, 500
MHz): ꢃ 6.95 (8H, m, Ph), 7.12 (2H, dd, Ph), 7.29 (2H, d, Ph),
7.40 (4H, q, Ph), 7.56 (2H, d, Ph), 7.66 (4H, t, Ph), 8.94 (2H, s,
N=CH), 8.95 (2H, s, N=CH), 12.75 (2H, s, OH), 12.98 (2H, s,
OH); 13C NMR (DMSO-d6, 125 MHz): ꢃ 110.2, 116.6, 116.7,
117.6, 119.1, 119.2, 119.5, 119.6, 120.9, 132.4, 132.5, 133.3,
133.6, 138.0, 144.0, 156.0 (Ph), 160.3, 160.4 (N=CH), 163.2,
164.7 (PhO). HRMS (FAB, m=z): Calcd for C40H31N4O5:
vs Ag–AgCl with this setup.
0
0
Synthesis of L1. To a solution of 3,3 ,4,4 -tetraaminodiphen-
ylmethane (46 mg, 0.2 mmol) in 2 mL of ethanol and 1 mL of
chloroform was slowly added a salicylaldehyde (488 mg, 4 mmol)
in 1 mL of ethanol; a yellow precipitate appeared during stirring
for 3 h, which was allowed to stand in a refrigerator for 1 h.
The precipitates were collected by filtration, washed by cold etha-
nol and dried in vacuo. Yield: 88%. Anal. found: C, 76.11; H,
þ
þ
[MH] , 647.2294. Found: [MH] , 647.2313.
Synthesis of 3. 3 was synthesized in the same manner as for 1,
except for use of L3 in place of L1. Yield: 88%. Anal. found: C,
5
8
.00; N, 8.55%. calcd for C41H32N4O4: C, 76.38; H, 5.00; N,
ꢁ
1
.69%. IR [KBr; ꢂ/cm ]: 3450 (br, OH), 2950 (s, CH), 1620
1
(
4
(
s, C=N), and 1175 (s, C–O). H NMR (CDCl3, 500 MHz): ꢃ
.12 (2H, s, CH2), 6.91 (4H, m, Ph), 7.04 (4H, dd, Ph), 7.08
2H, s, Ph), 7.21 (4H, s, Ph), 7.37 (8H, m, Ph), 8.61 (2H, s,
N=CH), 8.64 (2H, s, N=CH), 13.02 (2H, s, OH), 13.07 (2H, s,
62.11; H, 3.41; N, 7.23%; calcd for C40H26N4O5Co2 1H2O: C,
ꢃ
ꢁ1
61.71; H, 3.62; N, 7.20%. IR [KBr; ꢂ/cm ]: 2950 (s, CH),
1580 (s, C=N), and 1180 (s, C–O). HRMS (FAB, m=z): Calcd
þ
þ
for C40H26N4O5Co2: [M] , 760.0567. Found: [M] , 760.0568.
13
ꢁ1
ꢁ1
OH); C NMR (CDCl3, 125 MHz): ꢃ 41.1 (–CH2–), 117.6,
UV–vis (in DMF): [ꢄmax/nm ("/M cm )], 307 (45500), 338
(35000), 393 (44500).
Synthesis of L4. To a suspension of 1,2,4,5-tetraaminoben-
1
1
(
17.7, 119.0, 119.1, 119.2, 119.3, 119.9, 120.4, 128.1, 132.3,
32.4, 133.3, 133.5, 140.4, 140.8, 142.9 (Ph), 161.3, 161.4
N=CH), 163.4, 164.0 (PhO). HRMS (FAB, m=z): Calcd for
zene 4HCl (224 mg, 0.8 mmol) in 20 mL of pyridine was slowly
ꢃ
þ
þ
C41H33N4O4: [MH] , 645.2502. Found: [MH] , 645.2494.
Synthesis of 1. All procedures were carried out using a stan-
dard Schlenk apparatus to avoid oxidation by atmospheric dioxy-
gen. To a solution of L1 (64 mg, 0.1 mmol) in 10 mL of chloro-
added 3,5-di-tert-butylsalicylaldehyde (1.48 g, 6.3 mmol) in 10
mL of ethanol; the reaction mixture was stirred for 1 h. The pre-
cipitates were collected by filtration, washed by cold ethanol and
dried in vacuo to give a yellow solid. Yield: 66%. Anal. found: C,
78.90; H, 9.08; N, 5.56%. calcd for C66H90N4O4: C, 79.00; H,
form was dropwise added Co(OAc)2 4H2O (55 mg, 0.22 mmol)
ꢃ
ꢁ
1
in 10 mL of ethanol. A brown solid was precipitated immediately
ꢄ
9.04; N, 5.58%. IR [KBr; ꢂ/cm ]: 3450 (br, OH), 2950 (s,
1
and the mixture was stirred at 70 C. After 3 h, the solid was col-
lected by filtration, washed by ethanol and a small amount of
chloroform, and dried in vacuo. Yield: 89%. Anal. found: C,
CH), 1610 (s, C=N), and 1170 (s, C–O). H NMR (CD2Cl2,
500 MHz): ꢃ 1.34(36H, s, t-butyl), 1.45 (36H, s, t-butyl), 7.26
(2H, s, Ph), 7.32 (4H, d, Ph), 7.49 (4H, d, Ph), 8.83 (4H, s,
13
6
6
1
1.59; H, 4.03; N, 7.01%. calcd for C41H28N4O4Co2 2H2O: C,
N=CH), 13.54 (4H, s, OH); C NMR (CD2Cl2, 125 MHz):
ꢃ 29.7 (–C(CH3)), 31.7 (–C(CH3)), 34.6 (–C(CH3)), 35.5
(–C(CH3)), 111.4, 119.0, 127.6, 129.1, 137.6, 141.3, 142.2 (Ph),
159.0 (N=CH), 165.3 (PhO). HRMS (FAB, m=z): Calcd for
ꢃ
ꢁ
1
1.98; H, 4.06; N, 7.05%. IR [KBr; ꢂ/cm ]: 2950 (s, CH),
580 (s, C=N), and 1180 (s, C–O); HRMS (FAB, m=z): Calcd
þ
þ
for C41H28N4O4Co2: [M] , 758.0775. Found: [M] , 758.0787;

1
042 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Dicobalt Complexes Having Various Linkers
H N
NH2
NH2
CHO
OH
R
2
N
OH
N
H N
R
+
in CHCl - EtOH
HO
2
3
N
N
HO
OH
R = -CH2-
: L1
-
CH CH - : L2
2 2
-O- : L3
Co(OAc) •4H O
R
2
2
N
Co
N
Co
O
in CHCl -EtOH
O
3
O
N
N
under N2
O
R = -CH2-
: 1
-
CH CH - : 2
2
2
-O- : 3
Scheme 1.
þ
þ
C66H91N4O4: [MH] , 1003.7040. Found: [MH] , 1003.7026.
Synthesis of 4. All procedures were carried out using a stan-
dard Schlenk apparatus to avoid oxidation by atmospheric dioxy-
gen. To a solution of L4 (150 mg, 0.15 mmol) in 20 mL of chloro-
H, 7.73; N, 4.68%. calcd for C36H46N2O2Co1: C, 72.34; H, 7.76;
ꢁ
1
N, 4.69%. IR [KBr; ꢂ/cm ]: 2950 (s, CH), 1580 (s, C=N), and
þ
1180 (s, C–O). MS (FAB, m=z): [M] , 597.3. UV–vis (in CHCl3):
ꢁ
1
ꢁ1
[ꢄmax/nm ("/M cm )], 312 (23700), 411 (18900), 457 (sh,
14200), 542 (5800), 587 (3400).
form was added dropwise Co(OAc)2 4H2O (92 mg, 0.37 mmol)
ꢃ
in 10 mL of ethanol. A brown solid was precipitated immediately,
and the mixture was stirred at room temperature. After 1 h, the
solid was collected by filtration, washed by a small amount of cold
methanol, chloroform, and dried in vacuo. A microcrystal was
obtained by recrystallization from diethylether/acetonitrile. Yield:
4%. Anal. found: C, 70.73; H, 7.63; N, 4.99%. calcd for
C66H86N4O4Co2: C, 70.95; H, 7.76; N, 5.01%. IR [KBr;
ꢂ/cm ]: 2950 (s, CH), 1580 (s, C=N), and 1180 (s, C–O).
HRMS (FAB, m=z): Calcd for C66H86N4O4Co2: [M] ,
Results and Discussion
Syntheses of Ligands and Dicobalt Complexes. Dinu-
cleating ligands, (L1, L2, and L3) were effectively synthesized
by similar methods reported previously, as shown in
Scheme 1.18 The dinucleating ligand L4 was also synthesized
by the reaction of commercial 1,2,4,5-tetraaminobenzene and
3,5-di-tert-butylsalicylaldehyde in pyridine, as shown in
Scheme 2.19 The free-base form of 1,2,4,5-tetraaminobenzene
was very sensitive to air, and was used as a hydrochloride salt.
A desirable product was obtained under aerobic condition
within 1 h when pyridine was used as a solvent. These ligands
afforded a stable dicobalt complex in the reaction of cobalt
acetate under a nitrogen atmosphere at high yields. Satisfacto-
ry elemental analyses and high-resolution FAB-mass spectra
were obtained for these complexes.
8
ꢁ1
þ
þ
1
[
(
116.5313. Found: [M] , 1116.5311. UV–vis (in CHCl3):
ꢁ1
ꢁ
1
ꢄ
70700), 481 (69900), 573 (sh, 15300).
max/nm ("/M cm )], 337 (46200), 348 (44700), 464
0
Synthesis of L4 . To a solution of o-phenylenediamine (246
mg, 1.05 mmol) in 5 mL of ethanol was slowly added 3,5-di-
tert-butylsalicylaldehyde (54 mg, 0.5 mmol) in 5 mL of ethanol;
a yellow precipitate appeared during stirring for 1 day, which
was then allowed to stand in a refrigerator for 6 h. The precipitates
were collected by filtration, washed by cold ethanol and dried
in vacuo. Yield: 50%. Anal. found: C, 79.79; H, 8.86; N,
Magnetic Properties of Dicobalt Complexes. The mag-
netic susceptibilities of complexes 2 and 4 were determined
over the temperature range of 2 K–300 K. Plots of the molar
5
[
.22%. calcd for C36H48N2O2: C, 79.96; H, 8.95; N, 5.18%. IR
ꢁ
1
KBr; ꢂ/cm ]: 3450 (br, OH), 2950 (s, CH), 1620 (s, C=N),
susceptibility (ꢁ ) and the effective magnetic moment
1
M
and 1175 (s, C–O). H NMR (CD2Cl2, 500 MHz): ꢃ 1.34 (36H,
s, t-butyl), 1.45 (36H, s, t-butyl), 7.26 (2H, s, Ph), 7.32 (4H,
d, Ph), 7.49 (4H, d, Ph), 8.83 (4H, s, N=CH), 13.54 (4H, s,
(
ꢀ ), per Co atom, as a function of the temperature for 2
eff
and 4 are shown in Figs. 1a and 1b, respectively. The dicobalt
complex 2, which contains a flexible ethylene linker, has a
magnetic moment of 2.85 mB at 300 K. This value is in the
13
OH); C NMR(CD2Cl2, 125 MHz): ꢃ 29.7 (–C(CH3)3), 31.7
–C(CH3)3), 34.6 (–C(CH3)3), 35.5 (–C(CH3)3), 111.4, 119.0,
(
1
(
II
range of values expected for a non-interacted low spin Co
27.6, 129.1, 137.6, 141.3, 142.2 (Ph), 159.0 (N=CH), 165.3
þ
ion. The moment monotonously decreased with decreasing
temperature to 2.46 m at 2 K, due to temperature-independent
paramagnetism (Nꢅ) associated with the Co ion. In the case
PhO). MS (FAB, m=z): [MH] , 541.7.
0
Synthesis of 4 . All procedures were carried out using a stan-
dard Schlenk apparatus to avoid oxidation by atmospheric dioxy-
B
II
0
of 4, the susceptibility curves showed rounded maxima at 18
K, whereas those of ꢀeff exhibited a continuous decrease upon
cooling from ꢀeff ¼ 2:28 mB/Co at 300 K, as shown in Fig. 1b.
The occurrence of maxima in ꢁM is indicative of the existence
of antiferromagnetic coupling between the paramagnetic co-
balt centers.20 The variable temperature data was thus fitted
gen. To a solution of L4 (43 mg, 0.08 mmol) in 10 mL of chloro-
form was added dropwise Co(OAc)2 4H2O (22 mg, 0.09 mmol)
in 10 mL of ethanol. A brown solid was precipitated immediately,
and the mixture was stirred at room temperature. After 3 h, the
precipitates were collected by filtration, washed by a small amount
of ethanol and dried in vacuo. Yield: 76%. Anal. found: C, 72.40;
ꢃ

H. Shimakoshi et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1043
OH
H N
NH2
2
CHO
OH N
OH N
N
N
OH
OH
+
excess
H N
NH2
in pyridine - EtOH
at room temp.
2
•
4HCl
L4
Co(OAc) •4H O
2
2
O
N
N
N
O
O
Co
Co
in CHCl - EtOH
O
N
3
4
Scheme 2.
ference between the singlet and triplet states, ꢁM is expressed
per mole of cobalt atoms, N is Avogadro’s number, ꢆ is the
Bohr magneton, k is the Boltzmann constant, T is the absolute
temperature, Nꢅ is the temperature-independent paramagnet-
ism, and ꢇ is the fraction of paramagnetic impurity.
(
a)
0
.4
3.0
2
.5
.0
0
0
0
.3
.2
.1
ꢀ
ꢁ
2
2
2
2
Ng ꢆ
1 ꢁ ꢇ
3 þ expðꢁ2J=kTÞ
0:45ꢇ
ꢁM
¼
þ
þ Nꢅ: ð1Þ
kT
T
1.5
This procedure treats a complex as a ground-state singlet
with a low-lying triplet state. A simple curve-fitting routine
was used to determine the parameters g and ꢁ2J. The best data
1
0
.0
.5
ꢁ1
ꢁ6
fit to Eq. 1 gave g ¼ 2:50, ꢁ2J ¼ 20 cm , Nꢅ ¼ 100 ꢂ 10
3
ꢁ1
cm mol , ꢇ ¼ 0:030, and a least-squares error (R) of 2:20 ꢂ
0
0
ꢁ2
1
interaction in complex 4.
0
. These data indicate an intramolecular antiferromagnetic
0
50 100 150 200 250
T / K
Redox Behavior of Dicobalt Complexes. Cyclic voltam-
mograms of dicobalt complexes 1, 2, and 3 in DMF are shown
in Fig. 2, which also display a cyclic voltammogram of the
(
b)
0
0
.018
.014
2.5
2
II
monocobalt complex [Co (saloph)] under the same conditions.
III
II
II
I
The Co /Co and Co /Co redox couples (E1=2) for 1 were
observed at þ0:14 vs Ag–AgCl and ꢁ1:06 V vs Ag–AgCl,
respectively. The redox potentials were very similar to those
1
1
0
.5
.0
.5
II
0
.01
of the corresponding mononuclear complex [Co (saloph)], as
shown in Table 1. The ratios between the anodic and cathodic
peak currents, ipa=ipc, were almost unity and independent of
0
0
.006
ꢁ1
the scan rate (from 10 to 500 mV s ) for the two redox cou-
1=2
ples in DMF. Plots of ip (¼ ipa þ ipc) vs v (v is the scan rate,
ꢁ1
.002
0
mV s ) were linear, and the potential separation between the
anodic and cathodic peaks varied from 100 to 80 mV for the
two redox couples, while the E1=2 values remained constant
within an accuracy of 5% regardless of the scan-rate variation.
The present electrochemical redox reactions in DMF are con-
sistent with reversible one-electron transfer processes.22 The
redox behaviors of 2 and 3 were almost similar to those of
0
0
50
100 150 200 250
T / K
Fig. 1. Temperature dependences of the magnetic suscepti-
bility ( ) and effective magnetic moment ( ) of (a) 2 and
(
b) 4 under 500 G.
III
II
II
I
1
. Both Co /Co and Co /Co redox couples appeared at al-
to the Bleaney–Bowers equation21 (Eq. 1), using the Heisen-
most the same potentials, and no significant change was ob-
served due to the type of linkers. In this way, electron transfers
of 1, 2, and 3 occur independently at each cobalt center with no
berg (isotropic) exchange Hamiltonian (H ¼ ꢁ2JS1 S2) for
ꢃ
two interacting S ¼ 1=2 centers, where ꢁ2J is the energy dif-

1
044 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Dicobalt Complexes Having Various Linkers
(a)
(a)
Co
Co
Co
Co
2
µA
5
µA
(
b)
(b)
(c)
(d)
Co
Co
Co
2
µA
5
µA
O
−2.0
−1.5
−1.0
−0.5
E / V vs. Ag-AgCl
Co
Co
0.0
+0.5
+1.0
2
µA
ꢁ
4
Fig. 3. Cyclic voltammograms of (a)
4
(5 ꢂ 10
mol dm 3), (b) 4 (1 ꢂ 10 mol dm ) in tetrahydrofuran
THF) containing 0.1 M n-Bu4NClO4. Scan rate is 100
ꢁ
0
ꢁ3
ꢁ3
(
mV s 1.
ꢁ
Co
e−
2
µA
CoI
CoI
CoII CoI
−
1.41 V
vs Ag-AgCl
ð2Þ
CoIII CoIII
e−
2e−
CoII CoII
−1.5
−1.0
−0.5
0
+0.5
−1.22 V
+0.37 V
4
E / V vs. Ag-AgCl
vs Ag-AgCl
vs Ag-AgCl
4
Fig. 2. Cyclic voltammograms of (a)
ꢁ3
1
(5 ꢂ 10ꢁ
In order to provide an unambiguous assignment of these redox
couples, coulometry studies were carried out. The charge
passed corresponded to one electron at ꢁ1:30 V and ꢁ1:50
V, and the charge passed corresponded to two electrons at
þ0:60 V vs Ag–AgCl. In this way, the dicobalt complex 4
shows two sequential reduction, E1 and E2, separated by an
amount E ¼ E1 ꢁ E2 ¼ 0:190 V. The thermodynamic signifi-
cance of E can be illustrated by the comproportionation equi-
librium (Kcom) shown in Eqs. 3 and 4.
ꢁ
3
ꢁ4
ꢁ4
mol dm ), (b) 2 (5 ꢂ 10 mol dm ), (c) 3 (5 ꢂ 10
ꢁ
3
II
ꢁ3
ꢁ3
mol dm ), and (d) [Co (saloph)] (1 ꢂ 10 mol dm
)
ꢁ3
in DMF containing 0.1 mol dm n-Bu4NClO4. Scan rate
is 100 mV s 1
ꢁ
.
Table 1. Redox Potentials for the Various Cobalt Com-
plexes in DMFa)
E1=2/V vs Ag–AgCl
Complex
III
II
II
I
Co /Co
Co /Co
Kcom
CoII CoII
CoI CoI
CoII CoI
+
2
1
2
3
4
þ0:14
þ0:12
þ0:13
þ0:37
ꢁ1:06
ꢁ1:06
ꢁ1:06
ꢁ1:22
ð3Þ
ð4Þ
b)
E ¼ RT=nF ln Kcom:
ꢁ
1:41
ꢁ1:22
0
b),c)
The value is Kcom ¼ 1630, indicating that the present mixed
4
Co(saloph)]
þ0:52
þ0:14
II I
c)
valence state (Co Co ) was somewhat stabilized.
[
ꢁ1:05
I
I
Reactivity of Co Species. Square–planar Co complexes
are strong nucleophiles that react with various organic halides
to give organocobalt complexes.23 As a consequence, the addi-
tion of alkyl halides to cobalt complexes generally leads to
changes in the cyclic voltammetric pattern of the complex.
In the case of 2, the addition of excess benzyl bromide gives
rise to a new irreversible redox wave at the more cathodic po-
a) Working and counter electrode: Pt, ½complexꢅ ¼ 5:0 ꢂ
ꢁ
4
1
room temperature. Scan rate, 100 mV/s. b) Solvent, THF. c)
0
M, ½n-Bu4NClO4ꢅ ¼ 0:1 M, under Ar atomosphere at
3
½
complexꢅ ¼ 1:0 ꢂ 10ꢁ
M
interaction between two metal centers.
In the case of 4, redox pairs, corresponding to the
III
III
II
II
II
II
II
I
II
I
I
I
II
I
Co Co /Co Co , Co Co /Co Co , and Co Co /Co Co
couples, were observed at þ0:37 V, ꢁ1:22 V (E1), and
1:41 V (E2) vs Ag–AgCl, respectively, as shown in Fig. 3a.
tential for the Co /Co reduction peak, as shown in Fig. 4. The
II
I
Co /Co redox wave became irreversible, and shifted to the
more anodic side. The disappearance of the oxidation peak
ꢁ

H. Shimakoshi et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1045
1
.0
5
µA
Abs.
0.5
(a)
+ 2e-
I
I
II
II
Ph Ph
Co Co
Co Co
CH2CH2
2
+
2e-
III
III
2
PhCH2Br
Co Co
2
(
c)
Ph Ph
CH2CH2
I
I
Co Co + 2PhCH2
Co Co
unstable
(
b)
0
.0
−1.7
−1.5
−1.3
−1.1
−0.9
−0.7
−0.5
300
400
500
600
700
E / V vs. Ag-AgCl
λ / nm
Fig. 4. Cyclic voltammogram of 2 in the presence of benzyl
ꢁ
Fig. 5. Electronic spectral changes observed during the
controlled potential electrolysis of a DMF solution con-
4
bromide in DMF. Initial concentrations: 2, 5:0 ꢂ 10
ꢁ
3
ꢁ2
ꢁ3
mol dm ; benzyl bromide, 1:0 ꢂ 10
mol dm ; n-
ꢁ1
ꢁ4
ꢁ3
taining
2
(5:0 ꢂ 10
mol dm ), benzyl bromide
ꢁ1
ꢁ3
Bu4NClO4, 1:0 ꢂ 10 mol dm . Scan rate: 100 mV s
.
ꢁ2
ꢁ3
ꢁ1
(
1:0 ꢂ 10
mol dm ), and n-Bu4NClO4 (1:0 ꢂ 10
mol dm ) at ꢁ1:15 V vs Ag-AgCl; (a) before electrolysis,
b) after electrolysis and (c) after irradiation with visible
ꢁ
3
I
II
I
2ꢁ
for Co /Co suggests that the reaction of [Co 2L2] with ben-
III
(
zyl bromide gives [(benzyl–Co )2L2] with a cobalt–carbon
bond on both metal centers, as shown in Eq. 5.
light.
CoI
In conclusion, all of the dicobalt complexes synthesized in
CoI
+ 2 PhCH Br
2
III
II
II
I
this study showed the reversible Co /Co and Co /Co redox
couple. Cyclic voltammograms of the complexes in the pres-
ence of alkyl halide exhibited the formation of a cobalt–carbon
bond at both cobalt centers. Further work on the reactivity of
dialkylated cobalt complexes with various linkers is currently
in progress in our laboratory.
I
2−
[
Co L2]
2
CH Ph
CH Ph
2
2
CoIII
CoIII
III
[(benzyl-Co ) L2]
2
We thank Prof. K. Sakata, Kyushu Insitute of Technology,
for measurements of the FAB-MS. The present work was sup-
ported by a Grant-in-Aid for Scientific Research on Priority
Areas (417) from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan, Grant-in-Aid for
Scientific Research (16750121 and 16350076) from the Japan
Society for Promotion of Science (JSPS).
ð5Þ
Indeed, a controlled potential electrolysis of 2 at ꢁ1:15 V vs
Ag–AgCl in the presence of benzyl bromide was carried out
and followed by electronic spectroscopy. The starting spec-
trum of 2 was changed to a new spectrum having absorption
maxima at 387, 568, and 613 nm, as shown in Fig. 5(b). The
spectrum then changed to Fig. 5(c) by irradiation with visible
III
light. The spectrum (c) is ascribed to Co species for auto-oxi-
II
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ð6Þ
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1
046 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
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9