Dinuclear manganese(II) complexes of an acyclic phenol-based dinucleating ligand with four methoxyethyl chelating arms: Synthesis, structure, magnetism and electrochemistry

Article abstract of DOI:10.1016/S0020-1693(00)00277-2

Dinuclear manganese(II) complexes [Mn₂(bomp)(PhCO₂)₂]BPh₄ (1), [Mn₂(bomp)(MeCO₂)₂]BPh₄ (2), and [Mn₂(bomp)-(PhCO₂)₂]PF₆ (3) were synthesized with a dinucleating ligand 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)]. Dinuclear zinc complex [Zn₂(bomp)(PhCO₂)₂]PF₆ (4) was also synthesized for the purpose of comparison X-ray analysis revealed that the complex 1·CHCl₃ contains two manganese ions bridged by the phenolic oxygen and two benzoate groups, forming a μ-phenoxo-bis(μ-benzoato)dimanganese(II) core. Magnetic susceptibility measurements of 1-3 over the temperature range 1.8-300 K indicated antiferromagnetic interaction (J = -4 to -6 cm ^-1). Cyclic voltammograms of 3 showed a quasi-reversible oxidation process at +0.9 V versus a saturated sodium chloride calomel reference electrode, assigned to Mn^IIMn^II/Mn^IIMn^III.

Full text of DOI:10.1016/S0020-1693(00)00277-2

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 310 (2000) 163–168
Dinuclear manganese(II) complexes of an acyclic phenol-based
dinucleating ligand with four methoxyethyl chelating arms:
synthesis, structure, magnetism and electrochemistry
Hiroshi Sakiyama a,*, Akihiko Sugawara a, Masatomi Sakamoto a, Kei Unoura a,
Katsuya Inoue b, Mikio Yamasaki c
a Department of Material and Biological Chemistry, Faculty of Science, Yamagata Uni6ersity, Kojirakawa, Yamagata 990-8560, Japan
b Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
c X-Ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara, Akishima, Tokyo 196-8666, Japan
Received 11 May 2000; accepted 7 August 2000
Abstract
Dinuclear manganese(II) complexes [Mn2(bomp)(PhCO2)2]BPh4 (1), [Mn2(bomp)(MeCO2)2]BPh4 (2), and [Mn2(bomp)-
(PhCO2)2]PF6 (3) were synthesized with a dinucleating ligand 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol [H(bomp)].
Dinuclear zinc complex [Zn2(bomp)(PhCO2)2]PF6 (4) was also synthesized for the purpose of comparison. X-ray analysis revealed
that the complex 1·CHCl3 contains two manganese ions bridged by the phenolic oxygen and two benzoate groups, forming a
m-phenoxo-bis(m-benzoato)dimanganese(II) core. Magnetic susceptibility measurements of 1–3 over the temperature range
1.8–300 K indicated antiferromagnetic interaction (J= −4 to −6 cm−1). Cyclic voltammograms of 3 showed a quasi-reversible
oxidation process at +0.9 V versus a saturated sodium chloride calomel reference electrode, assigned to MnIIMnII/MnIIMnIII
.
© 2000 Elsevier Science B.V. All rights reserved.
Keywords: Dinucleating ligand; Dinuclear manganese(II) complexes; Crystal structures; Magnetism; Electrochemistry
the disproportionation of H2O2 to O2 and H2O, and
each subunit contains a dinuclear manganese core as its
catalytic center. The dimanganese core works as a
two-electron mediator between an {Mn(III)}2 form and
a reduced {Mn(II)}2 form in H2O2 disproportionation.
A great number of dinuclear manganese complexes
have been synthesized and characterized as the model
compounds of manganese enzymes. Among them,
many complexes using macrocyclic or acyclic dinucleat-
ing ligands have been synthesized in order to stabilize
the dimanganese core even in a reaction solution. 2,6-
Bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol
[H(bpmp)] is one of the most well-known acyclic dinu-
cleating ligands, and syntheses of dimanganese(II,II)
complexes [Mn2bpmp(RCO2)2]ClO4 (R=Me, Ph) and
dimanganese(II,III) complexes [Mn2bpmp(RCO2)2]-
(ClO4)2 (R=Me, Ph) have been reported [11] as fron-
tier work in this research area.
1. Introduction
Dinuclear manganese cores are often seen in biologi-
cal systems, such as manganese catalases [1–4], man-
ganese ribonucleotide reductase [5], arginase [6],
thiosulfate oxidase [7], xylose isomerase [8], RNAase [9]
and proline aminopeptidase [10]. The dimanganese
cores exist as the catalytic centers of these enzymes.
Some hydrolyze substrates by the ‘two-metal ion mech-
anism’, and some work as electron mediators to cata-
lyze redox reactions. Manganese catalase is
a
well-studied redox enzyme that decomposes hydrogen
peroxide. Manganese catalases have been isolated from
four origins: Lactobacillus plantarum [1], Ther-
moleophilum album [2], Thermus thermophilus HB8 [3]
and Thermus species strain YS 8-13 [4]. They catalyze
In this study, the acyclic dinucleating ligand 2,6-
bis[bis(2 - methoxyethyl)aminomethyl] - 4 - methylphenol
* Corresponding author. Fax: +81-23-628 4591.
E-mail address: saki@sci.kj.yamagata-u.ac.jp (H. Sakiyama).
0020-1693/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(00)00277-2

164
H. Sakiyama et al. / Inorganica Chimica Acta 310 (2000) 163–168
[H(bomp)] is used to synthesize dimanganese com-
plexes. This ligand is expected to stabilize dinuclear
metal cores, and dizinc(II) complex [Zn2bpmp-
(MeCO2)2]BPh4 has been reported as a functional
model of dizinc aminopeptidases [12]. Dinucleating lig-
ands bpmp− and bomp− will produce topologically the
same chelating structures, but they are different in some
aspects in regard to the nature of pyridyl and methoxy
groups in their chelating arms. It is of interest to
compare the properties of dimanganese complexes of
bpmp− and bomp−. Here we report the synthesis,
structure, magnetism and electrochemical properties of
dimanganese(II) complexes of bomp− (Scheme 1).
calomel electrode (SSCE) as reference. Tetra-n-
butylammonium tetrafluoroborate was used as the sup-
porting electrolyte. Thin-layer coulometry was accom-
plished using an HECS-978 Coulometer (Huso Co.)
with a thermostatically controlled thin-layer electro-
chemical cell.
2.2. Materials
Na(bomp) was prepared by the method reported
previously [12]. All chemicals were commercial prod-
ucts and were used as supplied.
2.3. Preparations
2. Experimental
2.3.1. [Mn2(bomp)(PhCO2)2]BPh4 (1)
2.1. Measurements
To a methanolic solution (10 ml) of manganese(II)
benzoate tetrahydrate (0.74 g, 2.0 mmol) was added a
methanolic solution (5 ml) of Na(bomp) (0.42 g,
1.0 mmol), and the resulting solution was refluxed for
1 h to give a yellow solution. The addition of a
methanolic solution (5 ml) of sodium tetraphenylborate
(0.34 g, 1.0 mmol) resulted in the precipitation of white
microcrystals. Yield 0.34 g (32%) (Found: C, 66.35; H,
6.40; Mn, 11.0; N, 2.60. Calc. for C59H67BMn2N2O9: C,
66.30; H, 6.30; Mn, 10.3; N, 2.60%). Selected IR data
[w/cm−1] using KBr disk: 3050–2950, 1600, 1560, 1470,
1400, 1320, 1270, 1080, 1020–1000, 700. FAB mass
spectrum; m/z 749 [Mn2(bomp)(PhCO2)2]+. Molar con-
ductance [\M/S cm2 mol−1] in dmf: 38.
Elemental analyses were obtained at the Elemental
Analysis Service Centre of Kyushu University. Infrared
(IR) spectra were recorded on a Hitachi 270-50 spec-
trometer, fast atom bombardment (FAB) mass spectra
on a Fisons ZabSpec Q mass spectrometer in N,N-
dimethylformamide (dmf) with a glycerol matrix, molar
conductances on a DKK AOL-10 conductivity meter at
room temperature, and electronic spectra on a Shi-
madzu UV-240 spectrometer. Magnetic susceptibility
was measured between 1.8 and 300 K by using a Quan-
tum Design SQUID magnetometer MPMS-5S in fields
up to 5 T. Polycrystalline samples (ꢀ30 mg) were
placed in a Japanese pharmacopoeia c5 gel capsule.
The background data of the capsule were measured
separately and subtracted from the sample-in-cell data.
Effective magnetic moments were calculated using the
equation veff=(3kTA/Ni2)1/2, where the symbols have
their usual meanings. Voltammetric measurements were
performed using an HECS-312B polarographic ana-
lyzer coupled with an HECS-321B function generator
from Huso Co. Measurements were carried out in
acetonitrile using a three-electrode cell consisting of a
glassy carbon disk working electrode, platinum coil
auxiliary electrode and a saturated sodium chloride
2.3.2. [Mn2(bomp)(MeCO2)2]BPh4 (2)
This was prepared as white microcrystals by a
method similar to that of 1 using manganese(II) acetate
tetrahydrate instead of manganese(II) benzoate tetrahy-
drate. Yield 0.31 g (33%) (Found: C, 62.30; H, 6.70;
Mn, 11.6; N, 2.95. Calc. for C49H63BMn2N2O9: C,
62.30; H, 6.70; Mn, 11.5; N, 2.95%). Selected IR data
[w/cm−1] using KBr disk: 3050–2950, 1590, 1470, 1420,
1320, 1270, 1080, 1020–1000, 730, 700. FAB mass
spectrum; m/z 625 [Mn2(bomp)(MeCO2)2]+. Molar
conductance [\M/S cm2 mol−1] in dmf: 41.
2.3.3. [Mn2(bomp)(PhCO2)2]PF6 (3)
This was prepared as white microcrystals by a
method similar to that of 1 using ammonium hex-
afluorophosphate instead of sodium tetraphenylborate.
Yield 0.42 g (48%) (Found: C, 47.00; H, 5.30; N, 3.20.
Calc. for C35H47F6Mn2N2O9P: C, 47.00; H, 5.30; N,
3.15%). Selected IR data [w/cm−1] using KBr disk:
2950–2900, 1610, 1560, 1480, 1400, 1320, 1280, 1090,
830, 720. FAB mass spectrum; m/z 749
[Mn2(bomp)(PhCO2)2]+. Molar conductance [\M/
S cm2 mol−1] in dmf: 63.
Scheme 1.

H. Sakiyama et al. / Inorganica Chimica Acta 310 (2000) 163–168
165
2.3.4. [Zn2(bomp)(PhCO2)2]PF6 (4)
benzoate groups. The two manganese ions are bridged
by the one phenolic oxygen of the dinucleating ligand
and the two benzoate ions, forming a m-phenoxo-bis(m-
This was prepared as white microcrystals by a
method similar to that of 3 using zinc(II) benzoate
instead of manganese(II) benzoate tetrahydrate. Yield
0.09 g (8%) (Found: C, 45.95; H, 5.15; N, 3.10. Calc.
for C35H47F6N2O9PZn2: C, 45.90; H, 5.15; N, 3.05%).
Selected IR data [w/cm−1] using KBr disk: 2950–2850,
benzoato)dimanganese(II)
core
structure.
The
,
Mn(1)···Mn(2) separation is 3.3892(7) A, which is com-
mon for the Mn···Mn distance of m-phenoxo-bis(m-car-
,
(3.2–3.6 A)
boxylato)dimanganese(II)
[11,18–21].
complexes
1
1610, 1570, 1480, 1400, 1320, 1270, 1090, 840, 720. H
NMR [l/ppm] in CDCl3: 2.22 (s, 3H), 2.8–4.2 (br,
32H), 6.94 (s, 2H), 7.40 (t, 4H), 7.48 (t, 2H), 8.03 (d,
4H). 13C NMR [l/ppm] in CDCl3: 20.28, 59.54 (br),
61.98, 67.80 (br), 123.37, 126.56, 128.11, 129.70, 131.69,
132.09, 134.94, 159.93, 172.96. FAB mass spectrum;
m/z 767, 769, 771 [Zn2(bomp)(PhCO2)2]+. Molar con-
ductance [\M/S cm2 mol−1] in dmf: 64.
Atom Mn(1) has a six-co-ordinate distorted octahe-
dral geometry with O(1), O(2), O(3) and N(1) of
bomp− and O(6) and O(8) of the two benzoate groups.
Table 1
Crystallographic data for 1·CHCl3
Formula
Formula weight
Space group
C60H68BCl3Mn2N2O9
1188.25
2.4. Single crystal X-ray analysis of complex 1·CHCl3
P21/a (no. 14)
27.1889(3)
21.5748(3)
10.2173(1)
90
96.3749(4)
90
5956.4(1)
8
,
a (A)
,
b (A)
Single crystals of complex 1·CHCl3 were obtained by
the slow diffusion of propan-2-ol into a chloroform
solution of 1. A colorless platelet crystal having ap-
proximate dimensions 0.45×0.20×0.20 mm3 was
mounted in a glass capillary. All measurements were
made on a Rigaku RAXIS–RAPID Imaging Plate
diffractometer with graphite-monochromated Mo Ka
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
rcalc (g cm−3
)
1.325
Crystal size (mm)
0.45×0.20×0.20
6.14
2480.00
0.049
0.067
v(Mo Ka) (cm−1
)
,
radiation (u=0.710 69 A). The data were collected at
2391°C to a maximum 2q value of 55.0°. Of the
F(000)
Ra
54 071 reflections collected, 13 708 were unique (Rint
=
R%b
0.034). The data were corrected for Lorentz and polar-
ization effects.
a R=S ꢁꢁFoꢁ−ꢁFcꢁꢁ/S ꢁFoꢁ.
2 1/2
b R%=[S w(ꢁFoꢁ−ꢁFcꢁ)2/S wꢁFoꢁ ]
.
The structure was solved by direct methods [13] and
expanded using Fourier techniques [14]. Hydrogen
atoms were included but not refined. Refinement was
carried out by the full-matrix least-squares method,
where the function minimized was ꢀw(ꢁFoꢁ−ꢁFcꢁ)2. The
final cycle of the refinement was based on 6583 ob-
served reflections [I\3.00|(I)], and the final values of
R and R% were 0.049 and 0.067 respectively. Neutral
atom scattering factors were taken from Cromer and
Waber [15]. All calculations were performed using
TEXSAN [16]. (See Table 1.)
3. Results and discussion
3.1. X-ray crystal structure of complex 1·CHCl3
The crystal structure consists of [Mn2(bomp)-
(PhCO2)2]+ complex cations (Fig. 1), tetraphenylborate
anions and chloroform molecules in a 1:1:1 molar ratio.
Selected distances and angles with their estimated stan-
dard deviations are listed in Table 2. The structure of
the complex cation is very similar to that of
[Zn2(bomp)(MeCO2)2]+, which has been reported previ-
ously [12]. The complex cation consists of one dinucle-
ating ligand bomp−, two manganese(II) ions and two
Fig. 1. An ORTEP [17] view of the complex cation [Mn2(bomp)-
(PhCO2)2]+ with the atom numbering scheme.

166
H. Sakiyama et al. / Inorganica Chimica Acta 310 (2000) 163–168
The magnetic moments decrease with decreasing tem-
perature, suggesting an antiferromagnetic interaction
within a pair of Mn(II) ions. The temperature depen-
The co-ordination geometry around Mn(2) is also dis-
torted octahedral with O(1), O(4), O(5) and N(2) of
bomp− and O(7) and O(9) of the two benzoate groups.
The geometries around Mn(1) and Mn(2) are very
similar to each other, and the complex cation has a
pseudo-C2 axis along C(14), C(4), C(1) and O(1). The
least-squares plane of the aromatic ring of bomp− and
the plane defined by Mn(1), Mn(2) and O(1) are twisted
with a dihedral angle of 44.7°.
dencies of  and veff per Mn for 1 are shown in Fig.
A
2. The magnetic susceptibility equation for the dinu-
clear manganese(II) system based on the isotropic
Heisenberg model H= −2JS1·S2 (S1=S2=5/2) is
given by Eq. (1):
Ng2i2
kT
x28+5x24+14x18+30x10+55
30+3x28+5x24+7x18+9x10+11
The dinucleating ligand bomp− has approximately
C2 symmetry, and it has a rigid phenolate head unit,
two flexible amine shoulders and four flexible
methoxyethyl chelating arms. Owing to the nature of
ether oxygen and saturated chelating moieties, two
co-ordination sites of bomp− are larger than those of
related dinucleating ligands containing Schiff bases or
aromatic rings in their structures. Although the co-ordi-
nation sites around phenolate and amines of bomp−
are slightly too large for divalent transition metal ions,
the phenol ring is wound around the pseudo-C2 axis to
embrace the chelating arms, and the two metal ions are
incorporated. This phenomenon is significant in the
A=
·
+Nh
x
(1)
where x=exp(−J/kT), and the other symbols have
their usual meanings. The cryomagnetic properties of 1
are simulated well by Eq. (1) using the magnetic
parameters g=2.00, J= −4.5 cm−1 and Nh=0. The
reliability factor, defined as R(v)=ꢀ(vobs
−
vcal)2/ꢀ(vobs)2, is 2.5×10−5. Similarly, good magnetic
simulations have been attained for 2 and 3. The best-
fitting parameters obtained for these complexes are
dimanganese(II)
complex
[Mn2L(PhCO2)2(NCS)-
Table 2
(MeOH)], where L− is 2,6-bis{[N-(dimethylamino-
ethyl)-N-methyl]aminomethyl}-4-methylphenolate(1−)
[20]. The twisting dihedral angle for it is 55°.
,
Selected distances (A) and angles (°) for complex 1·CHCl3
Mn(1)ꢀO(1)
Mn(1)ꢀO(2)
Mn(1)ꢀO(3)
Mn(1)ꢀO(6)
Mn(1)ꢀO(8)
Mn(1)ꢀN(1)
Mn(1)···Mn(2)
2.103(2)
2.290(3)
2.298(3)
2.085(3)
2.130(3)
2.271(3)
3.3892(7)
Mn(2)ꢀO(1)
Mn(2)ꢀO(4)
Mn(2)ꢀO(5)
Mn(2)ꢀO(7)
Mn(2)ꢀO(9)
Mn(2)ꢀN(2)
2.106(2)
2.324(3)
2.275(3)
2.124(3)
2.095(3)
2.286(3)
In the case of the zinc complex [Zn2(bomp)-
(MeCO2)2]BPh4, the bond distances between zinc atoms
and axial ether oxygen atoms [2.362(4)–2.440(3) A] are
longer than those between zinc atoms and equatorial
ether oxygen atoms [2.197(3)–2.214(5) A] [12]. How-
ever, in the case of complex 1, the bond distances
between manganese atoms and axial ether oxygen
atoms [2.275(3)–2.298(3) A] are almost the same as
those between manganese atoms and equatorial ether
,
,
O(1)ꢀMn(1)ꢀO(2)
O(1)ꢀMn(1)ꢀO(3)
O(1)ꢀMn(1)ꢀO(6)
O(1)ꢀMn(1)ꢀO(8)
O(1)ꢀMn(1)ꢀN(1)
O(2)ꢀMn(1)ꢀO(3)
O(2)ꢀMn(1)ꢀO(6)
O(2)ꢀMn(1)ꢀO(8)
O(2)ꢀMn(1)ꢀN(1)
O(3)ꢀMn(1)ꢀO(6)
O(3)ꢀMn(1)ꢀO(8)
O(3)ꢀMn(1)ꢀN(1)
O(6)ꢀMn(1)ꢀO(8)
O(6)ꢀMn(1)ꢀN(1)
O(8)ꢀMn(1)ꢀN(1)
159.6(1)
105.3(1)
100.4(1)
89.8(1)
87.4(1)
81.2(1)
99.1(1)
81.4(1)
75.5(1)
88.5(1)
162.0(1)
74.4(1)
98.6(1)
162.6(1)
96.9(1)
O(1)ꢀMn(2)ꢀO(4)
O(1)ꢀMn(2)ꢀO(5)
O(1)ꢀMn(2)ꢀO(7)
O(1)ꢀMn(2)ꢀO(9)
O(1)ꢀMn(2)ꢀN(2)
O(4)ꢀMn(2)ꢀO(5)
O(4)ꢀMn(2)ꢀO(7)
O(4)ꢀMn(2)ꢀO(9)
O(4)ꢀMn(2)ꢀN(2)
O(5)ꢀMn(2)ꢀO(7)
O(5)ꢀMn(2)ꢀO(9)
O(5)ꢀMn(2)ꢀN(2)
O(7)ꢀMn(2)ꢀO(9)
O(7)ꢀMn(2)ꢀN(2)
O(9)ꢀMn(2)ꢀN(2)
94.6(1)
159.5(1)
91.4(1)
105.2(1)
87.4(1)
87.7(1)
173.0(1)
87.0(1)
73.7(1)
85.4(1)
95.3(1)
73.7(1)
95.0(1)
102.9(1)
157.9(1)
,
,
oxygen atoms [2.290(3)–2.324(3) A].
The chelating structure of the complex cation of 1,
[Mn2(bomp)(PhCO2)2]+, is topologically the same as
those of the dimanganese complexes of bpmp− [11].
Although the crystal structure of the dimangane-
se(II,III) complex of bpmp−, [Mn2bpmp(PhCO2)2]-
(ClO4)2·H2O, has been determined, the structure of the
corresponding dimanganese(II,II) complex has not been
obtained. As far as we know, complex 1 is the first
crystallographically characterized dimanganese(II,II)
complex with this chelating structure topology.
Mn(1)ꢀO(1)ꢀMn(2) 107.3(1)
Table 3
Magnetic data for complexes 1–3a
3.2. Magnetic susceptibility of complexes 1–3
Complex
veff (mB)b
J (cm−1
)
g
R(v)×105
Magnetic susceptibility measurements for manganese
complexes 1–3 were made on polycrystalline samples in
the temperature range 1.8–300 K. The veff values per
Mn(II) of the complexes at room temperature are in the
range 5.45–5.50 mB (Table 3), which is slightly lower
than the spin-only value of high-spin Mn(II) (5.92 mB).
1
2
3
5.50
5.48
5.46
−4.5
−5.5
−5.3
2.00
2.00
2.00
2.5
4.6
12.8
a Nh assumed to be zero.
b At room temperature.

H. Sakiyama et al. / Inorganica Chimica Acta 310 (2000) 163–168
167
(PhCO2)2]+ (C35H47Mn2N2O9). In the case of 2, the
signal of [Mn2(bomp)(MeCO2)2]+ (C25H43Mn2N2O9)
appeared at m/z 749 as the main peak. In the spectrum
of the zinc complex 4, the most predominant ions
appeared at m/z 767, 769 and 771. The isotope pattern
around m/z 771 corresponds to [Zn2(bomp)(PhCO2)2]+
(C35H47N2O9Zn2). These results indicate that the coun-
ter anions dissociate in dmf, but the structures of the
1
complex cations are maintained in dmf. In the H and
13C NMR of 4, signals of the four chelating arms are
broad due to the interconversion between axial and
equatorial chelating arms. This indicates the co-ordina-
tion of methoxyethyl chelating arms to Mn(II) centers.
3.5. Electrochemistry
Fig. 2. Temperature dependences of  (ꢀ) and veff (ꢁ) of complex
A
Typical cyclic voltammograms of 3 and 4 in CH3CN
at 25°C are shown in Fig. 3. Electrochemical data for
the complexes are given in Table 4. The voltam-
1. Solid curves are based on Eq. (1), using J= −4.5 cm−1, g=2.00
and Nh=0 cm3 mol−1
.
summarized in Table 3. The J values are negative in
sign and comparable to those of other m-phenoxo-bis-
(m-carboxylato)dimanganese(II) complexes (−2 to
−5 cm−1) [11,18–21].
3.3. Other physicochemical properties in the solid state
The IR spectra of complexes 1–4 show the antisym-
metric and symmetric w(CO2) vibrations of the carboxy-
late group at 1560–1590 and 1400–1420 cm−1
,
respectively. The Dw values [Dw=was(CO2)−ws(CO2)]
are smaller than 200 cm−1 (1400–1420 cm−1), and they
are typical of the bridging carboxylate group [22]. This
is consistent with the fact that 1 has a m-phenoxo-bis(m-
benzoato)dimanganese(II) core structure shown using
X-ray crystallography. Complexes 1–4 are white and
have no absorption bands in the visible region, in
accordance with the high-spin d5 electronic configura-
tion of the Mn(II) ion for 1–3 and with the d10
electronic configuration of the Zn(II) ion for 4. Judging
from the crystal structure and the other physicochemi-
cal properties in the solid state, the manganese com-
plexes 1–3 and the zinc complex 4 have similar
carboxylato bridging systems to form the m-phenoxo-
bis(m-carboxylato)dimanganese(II) complexes and the
m-phenoxo-bis(m-carboxylato)dizinc(II) complex respec-
tively.
Fig. 3. Cyclic voltammograms of the complexes (a) 3 and (b) 4 in
MeCN at a scan rate of 100 mV s−1
.
Table 4
Electrochemical data for complexes 3 and 4
d
Complex
E°% (V) versus DE (mV)b
Ipc (U)c
Ipa/Ipc
SSCEa
(A) Reduction
3
4
−2.39
−2.38
irrev.
irrev.
1790
2280
irrev.
irrev.
3.4. Properties and structure in solution
(B) Oxidation
3
4
+0.92
+1.28
100
102
983
1340
0.77
0.78
The molar conductances of complexes 1–4 measured
in dmf indicate that they act as 1:1 electrolytes in dmf,
suggesting the dissociation of counter anions (BPh4−
for 1 and 2, PF6− for 3 and 4). In the FAB mass
spectra of 1 and 3 in dmf, the most predominant ion
appeared at m/z 625, corresponding to [Mn2(bomp)-
a E°%=(Epc+Epa)/2.
b Scan rate: 500 mV s−1
.
c Peak current parameters at 100 mV s−1. U=mAs1/2 V−1/2 cm−2
mM−1
.
d Peak current ratios at 500 mV s−1
.

168
H. Sakiyama et al. / Inorganica Chimica Acta 310 (2000) 163–168
mograms of the manganese complex 3 reveal a reduc-
tion process at −2.4 V and an oxidation process at
+0.9 V versus SSCE (Fig. 3(a)). In contrast to the
voltammogram of the corresponding zinc complex 4,
whose reduction and oxidation processes should be
ascribed to the ligand-centered electron transfer of
bomp in the complex cation, the reduction process of 3
at −2.4 V versus SSCE is caused by the reduction of
bomp in the complex cation. On the other hand, the
oxidation process at +0.9 V corresponds to the oxida-
tion of Mn(II). This is tentatively assigned to
MnIIMnII/MnIIMnIII, although coulometry has not
been successful. The oxidation process of 3 is irre-
versible at a scan rate of 100 mV s−1, but it becomes
References
[1] (a) Y. Kono, I. Fridovich, J. Biol. Chem. 258 (1983) 6015. (b)
Y. Kono, I. Fridovich, J. Biol. Chem. 258 (1983) 13 646.
[2] G.S. Allgood, J.J. Perry, J. Bacteriol. 168 (1986) 563.
[3] (a) V.V. Barynin, A.I. Grebenko, Dokl. Akad. Nauk SSSR
286 (1986) 461. (b) S.V. Khangulov, M.G. Goldfeld, V.V.
Gerasimenko, N.E. Andreeva, V.V. Barynin, A.I. Grebenko, J.
Inorg. Biochem. 40 (1990) 279.
[4] M. Kagawa, N. Murakoshi, Y. Nishikawa, G. Matsumoto, Y.
Kurata, T. Mizobata, Y. Kawata, J. Nagai, Arch. Biochem.
Biophys. 362 (1990) 346.
[5] A. Willing, H. Follmann, G. Auling, Eur. J. Biochem. 170
(1988) 603.
[6] R.S. Reczkowski, D.E. Ash, J. Am. Chem. Soc. 114 (1992)
10 992.
[7] R. Cammack, A. Chapmann, W.-P. Lu, A.A. Karagouni, D.P.
Kelly, FEBS Lett. 252 (1989) 239.
quasi-reversible at a faster scan rate of 500 mV s−1
.
This indicates that the one-electron oxidation product
[MnIIMnIII(bomp)(PhCO2)2]2+ is not stable and a sub-
sequent homogeneous chemical reaction occurs. The
formal reduction potential (E°’) for the oxidation of the
complex cation [Mn2(bomp)(PhCO2)2]+ corresponding
to MnIIMnII/MnIIMnIII couple is 0.92 V. This value is
higher than that of [Mn2bpmp(PhCO2)2](ClO4)2·H2O
(ꢀ0.5 V) and than those of other m-phenoxo-bis(m-car-
boxylato)dimanganese(II) complexes of acyclic dinucle-
ating ligands (0.45–0.65 V) [11,19,20]. This is because
the co-ordination site of bomp− is not suitable for
incorporating smaller Mn(III) ions.
[8] (a) H.L. Carrel, J.P. Glusker, V. Burger, F. Manfre, D.
Tritsch, J.-F. Biellmann, Proc. Natl. Acad. Sci. U. S. A. 86
(1989) 4440. (b) M. Whitlow, A.J. Howard, B.C. Finzel, T.L.
Poulos, E. Winborne, G.L. Gilliland, Proteins 9 (1991) 153. (c)
C.A. Collyer, K. Henrick, D.M. Blow, J. Mol. Biol. 212
(1990) 211.
[9] J.F. Davies, Z. II. Hostomka, Z. Hostomsky, S.R. Jordan,
D.A. Matthews, Science 252 (1991) 88.
[10] M.C. Wilce, C.S. Bond, N.E. Dixon, H.C. Freeman, J.M.
Guss, P.E. Lilley, J.A. Wilce, Proc. Natl. Acad. Sci. U. S. A.
95 (1998) 3472.
[11] M. Suzuki, M. Mikuriya, S. Murata, A. Uehara, H. Oshio, S.
Kida, K. Saito, Bull. Chem. Soc. Jpn. 60 (1987) 4305.
[12] H. Sakiyama, R. Mochizuki, A. Sugawara, M. Sakamoto, Y.
Nishida, M. Yamasaki, J. Chem. Soc., Dalton Trans. (1999)
997.
[13] A. Altomare, M.C. Burla, M. Camalli, M. Cascarano, C. Gia-
covazzo, A. Guagliardi, G. Polidori, J. Appl. Crystallogr. 27
(1994) 435.
[14] P.T. Beurskens, G. Admiraal, G. Beurskenes, W.P. Bosman,
R. de Geder, R. Israel, J.M.M. Smits, The DIRDIF-94 program
system, Technical Report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands, 1994.
[15] D.T. Cromer, J.T. Waber, International Tables for X-Ray
Crystallography, vol. IV, Kynoch Press, Birmingham, UK,
1974 Table 2.2 A.
[16] TEXSAN, Crystal Structure Analysis Package, Molecular Struc-
ture Corporation, Houston, TX, 1985 and 1992.
[17] C.K. Johnson, ORTEP, Report ORNL 5138, Oak Ridge Na-
tional Laboratory, Oak Ridge, TN, 1976.
4. Conclusion
In this study, three new dinuclear manganese(II)
complexes are reported. The dinucleating ligand
bomp− was shown to incorporate a m-phenoxo-bis(m-
carboxylato)dimanganese(II) core. The Mn(II) ions in
the complex cation were much more difficult to oxidize
than the other related dimanganese(II) complexes due
to the large co-ordination sites containing the methoxy
groups and saturated chelate chains.
[18] H. Sakiyama, H. Tamaki, M. Kodera, N. Matsumoto, H.
Acknowledgements
(
Okawa, J. Chem. Soc., Dalton Trans. (1993) 591.
(
[19] C. Higuchi, H. Sakiyama, H. Okawa, R. Isobe, D.E. Fenton,
J. Chem. Soc., Dalton Trans. (1994) 1097.
We are grateful to Miss Tomoko Koide of the Insti-
tute for Life Support Technology, Yamagata Techno-
polis Foundation, for the help in FAB mass spectro-
metrical measurements, and Miss Rie Itoh for the help
in molar conductance measurements.
(
[20] C. Higuchi, H. Sakiyama, H. Okawa, D.E. Fenton, J. Chem.
Soc., Dalton Trans. (1995) 4015.
[21] M. Mikuriya, T. Fujii, S. Kamisawa, Y. Kawasaki, T. Tokii,
H. Oshio, Chem. Lett. (1990) 1181.
[22] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 32 (1980) 227.
.