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Variety of Oxidation States of
Manganese Ions in Compounds with
Tripod-Like Tetradentate Ligands
Y uzo N ishida*, M iyuki N asu, an d
T adashi T okii+
D ep artm en t o f C hem istry, F acu lty o f Science,
Y am ag ata U niversity, Y am ag ata 990, Ja p a n , an d
+ D ep artm en t o f C hem istry, F acu lty o f Science
and Engineering, Saga U niversity, Saga 840,
Jap an
Z. N aturforsch. 45b, 1 0 9 3 -1096(1990);
received Jan u ary 2, 1990
M anganese
C om pou n d s
w ith
T rip o d -L ik e
L igands, O xidation State o f M anganese Ion
F ro m the reaction m ixture o f M n (III) acetate
and several tripod-like ligands, a M n (II) com plex,
a binuclear M n(III) com plex w ith (ju-oxo)(//-acetato ) core, and a M n (III)-M n (IV ) com plex w ith
a di-/i-oxo bridge were obtained. T his d em o n ­
strates th a t the oxidatio n state o f th e m anganese
ion in these com pound s is drastically affected by
sm all changes in ligand character.
1. Introduction
M anganese is know n to p articipate in a variety
o f biological reactions related to the m etabolism
and evolution o f m olecular oxygen. Evidence for
this conclusion derives from the fact th a t m a n ­
ganese is required for activity in enzym es such as
pseudocatalase [ 1 ], superoxide dism utase [2 ], and
the oxygen-evolving com plex in photo sy stem II
[3]. This should be due to the facile change o f
the oxidation state ( + 2 —» +4) o f m anganese und er
the usual experim ental conditions. In th e case o f
iron ion, it is know n th a t various o x id atio n states
(+ 2 —►+4) also occur in the biological systems
[4, 5]. T hen new questions arise in this respect; for
exam ple w hy can the iron atom n o t replace the
function o f the m anganese atom regarding evolu­
tion o f the oxygen m olecule. The above discussion
implies th a t it is very im p o rtan t to clarify the dif­
ferences betw een the chem ical features o f iron and
m anganese com plexes in order to elucidate the
reaction m echanism in the enzym es co ntaining
iron and m anganese ions. Thus, we have started to
investigate the difference between iro n an d m a n ­
ganese com pounds w ith the same ligands, and re­
ported several results. F o r exam ple, we have p re ­
pared the m anganese(III) com plex w ith H 5 (L) (il* R eprint requests to D r. Y. N ishida.
Verlag der Zeitschrift für Naturforschung, D-7400 Tübingen
0932-0776/90/0700-1093/$ 01.00/0
HOOcTp^
H00C
0H N::^(;ooh
H5(L )
cooh
lu strated below), [M n 2 (L )(C H 3C O O )2]“; the an a lo ­
gous binuclear iron(III) com plex has been
ch aracterized by Que et al. [6 ]. The iron(III) com ­
plex w ith H 5(L) is very stable at room tem perature
in aqueous and in organic solvents. H ow ever, the
m anganese(III) com plex is unstable in w ater, de­
com posing to a M n(II) com plex [7], In the reaction
w ith H 20 2, the iron(III) com plex form s an adduct
[6 ], b ut the M n(III) com plex exhibits high catalase
activity to w ard H 20 2 [7],
In the previous paper [8 ], we have reported the
p rep ara tio n and properties o f binuclear iron(III)
com plexes w ith tripod-like ligands as illustrated
below. These com plexes are obtained with
iron (III) from the reaction m ixture o f the ligand
and [Fe 30 ( C H 3C 0 0 ) 6 (H 20 ) 3]+. In this article we
have found th a t the oxidation state o f the m a n ­
ganese ion in com pounds obtained from above li­
gands and M n (III) acetate depend on the ligand
character, an d we discuss the origin o f the differ­
ence o f chem ical properties between iron and m an ­
ganese com pounds.
2. M aterials and Method
The ligands, L 1, L 2, and H L 3, were obtained ac­
cording to published m ethods [8-10], The p rep a ­
ratio n m ethods o f the m anganese com pounds are
as follows;
M n (L !)(C H 3C 0 0 ) (C 1 0 4) • 2 H 20 (1): T o a m eth­
anol solution (20 ml) o f M n(C H 3C 0 0 ) 3 -2 H 70
(0.002 m ol) and L 1 (0.002 mol) N aC 10 4 (500 mg)
was added, and the resulting solution was kept to
stand for one day. The precipitated pale yellow
crystals were filtered.
Analysis fo r Ci0H28N 7OsMn Cl
C alcd "C 47.54 ' H 4.30
F o u n d C 47.05 H 4 .1 7
N 14.92,
N 14.50.
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The corresponding h ex afluorophosphate salt,
M n (L ')(C H 3C O O )(P F 6) • 2 H 20 (2) was also o b ­
tained as cream yellow crystals by the sam e m e th ­
od as described above by using N H 4 P F 6 (300 mg)
instead o f N aC 1 0 4.
Analysis fo r C-,6H ^ N 70 4MnPF6
Calcd "C 44.46 H 4.02
F o u n d C 44.29 H 3.98
N 13.96,
N 13.84.
M n 20 2(L 2)2(P F 6) 2 5(C H 3C O O ) 0 5 • 2 H 20 (3): T o
a
m ethanol
solution
( 1 0 ml)
containing
M n (C H 3C O O ) 3 ■2 H 20 (0.002 m ol), L 2 (0.002 m ol)
and N H 4 P F 6 (300 mg), w ater (10 ml) was added,
and the resulting solution was kept to stand for
one day. D eposited deep greenish crystals were fil­
tered.
Analysis fo r C4SH4^5N r O^Mn^P^ 5F /5
' C alcd C 41.36" H 3.51 N 12.86 M n 8.41,
F o u n d C 41.52 H 3.74 N 12.49 M n 8.54.
M n 20 ( C H 3C 0 0 ) ( L 3)2(C104) •2 H 20 (4): T o a
m eth an o l solution (20 ml) containing H L 3
(0.002 m ol)
and
M n (C H 3C 0 0 ) 3 -2 H 20
(0.002 m ol) N aC 1 0 4 (300 mg) was added, and the
resulting solution was kept to stand for one day.
The precipitated brow n crystals were filtered.
Analysis fo r C40H 4IN 8O n Mn->Cl
Calcd C 47.14 H 4.06 N 10.99 M n 10.78,
F o u n d C 46.80 H 3.96 N 10.86 M n 10.5.
E SR spectra were obtained with a JE O L ESR
a p p a ra tu s m odel JE S -F E -3 X at liquid nitrogen
tem p eratu re using the X -band. M agnetic suscepti­
bility (j) were m easured by the F a ra d a y m ethod at
Saga U niversity in the tem perature range
8 1 -2 9 0 K. M agnetic m om en ts were calculated by
the equatio n ^ eff = 2.878 V / T .
M agne tic
fie ld / mT
Fig. 1. E S R spectra o f the co m p o u n d s (in D M F , 77 K,
X -b an d ). A: c o m p o u n d 1; B: com p o u n d 3.
ganese(III) com plexes with N -alkyl-N ,N -bis(benzim idazol- 2 -ylm ethyl)am ine (illustrated be­
low; their chem ical features are very sim ilar to th at
H
H
3. Results and Discussion
Tw o com pounds, 1 and 2, obtained from m ang an ese(lll) acetate and L 1, are pale yellow, im ply­
ing th a t these com plexes con tain a M n(II) ion.
This was su pported by the m agnetic m easure­
m ents; the m agnetic m om ents are 6.08 and 6.15 //B
at 292.4 and 81.1 K, respectively, for com p ound 2,
and its m agnetic behaviour obeys the Curie law in
the tem p eratu re range (8 1 -2 9 0 K). The ESR spec­
tru m o f 1 is also consistent w ith the above conclu­
sion (cf. Fig. 1, trace A). This indicates th a t the
M n (III) in the starting m aterial is reduced to
M n(II) in the reaction course.
T he greenish color and low m agnetic m om ents
(1.88 //B at 292.8 K ) o f com pound 3 suggest th at
this is not a M n (III) com plex, since the m an-
o f L 2) are all brow n and exhibit m agnetic m om ents
in the range 4.9 —> 5.3 /uBat room tem perature [11].
The tem p eratu re dependence o f the m agnetic sus­
ceptibility o f this com pound is show n in Fig. 2.
The m agnetic beh av io u r can be rationalized by the
assum ption o f a M n (III)-M n (IV ) mixed valence
com p o u n d with J - —149.2 c m '1; J was evaluated
by fitting the % V5 T d a ta to the expression derived
from the spin exchange H am iltonian, x =
- 2 / S , • S2 for S, = 2 and S2 - 3/2 [12]. In Table I,
the - J values o f the know n M n (III)-M n (IV )
m ixed-valence com plexes are sum m arized; these
are in the range 1 3 4 -1 5 0 cm “1, and thus it seems
reasonable to assum e th a t the present com plex 3 is
also a M n (III)-M n (IV ) mixed-valence com plex
w ith a di-^-oxo bridge. The ESR spectrum o f this
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T able I. - / v a l u e s in di-//-oxo M n (III)-M n (IV ) m ixedvalence com pounds.
Ligand
- J cm
Bipyridyl
P h en an th ro lin e
150
134
148
146
T ren
Fig. 2. V ariation w ith tem peratu re o f m o lar susceptibili­
ty (per M n) o f com pound 3. ♦ ♦ ♦ experim ental value;
----------calculated curve based on the isotropic H eisen­
berg m odel where g, J, and N a were assum ed to be 2.0,
- 149.2 cm -1, and 0, respectively.
Fig. 3. V ariation w ith tem peratu re o f m o lar susceptibili­
ty (per M n) o f com pound 4. + + + experim ental value;
----------calculated curve based on the iso tro p ic H eisen­
berg m odel where g, J, and N a were assum ed to be 2.0,
-2 .0 7 cm -1, and 0, respectively.
1
Ref.
13
13
14
15
co m p o u n d (cf. Fig. 1, trace B) is consistent with
this assum ption [13-15], The above results dem ­
o n stra te th at M n(III) in the starting m aterial is ox­
idized in the reaction course with ligand L2. In the
p re p a ratio n o f com pound 3, the addition o f w ater
to the reaction m ixture gave a higher yield o f this
com plex (cf. experim ental section), although the
sam e com pound was also obtained w ithout ad d i­
tion o f w ater. C om pound 3 was n ot obtained from
the reaction m ixture o f ligand L 2 and M n(II) ace­
tate u n d er the sam e experim ental conditions.
The tem peratur dependence o f the m agnetic sus­
ceptibility o f com pound 4 is illustrated in Fig. 3.
T he m agnetic m om ents are 4.70 and 4.53 //B at
288.0 and 81.6 K, respectively, suggesting th at this
is a M n(III) com plex. In fact, the m agnetic behav­
io ur can be understood on the assum ption th at
there is w eak antiferrom agnetic interaction ( - J =
2.07 c m “1) between two M n(III) ions. Based on the
analytical d ata and the fact th at only w eak ferro­
m agnetic or very weak antiferrom agnetic in terac­
tion is operating in M n(III) com pounds w ith a
(//-oxo)bis(//-acetato) core [ 1 1 , 16], it is reasonable
to suggest th at the present com pound 4 has a
(//-oxo)(//-acetato) core.
O u r present results clearly indicate th a t the oxi­
d atio n state o f the m anganese ion in these com ­
p o u n d s is drastically affected by small changes in
the ligand character. A t first we will consider the
difference observed for the oxidation state o f the
p ro d u cts between iron and m anganese complexes.
In the com parison o f iron and m anganese com ­
po unds, it seems natu ral to anticipate th a t the oxi­
d atio n o f M (II) to M (III) occurs m ore sm oothly in
iron com pounds th an in m anganese com pounds,
because M n(III) has a (3 d ) 4 electronic structure,
w hich is subject o f the Jahn-T eller effect, i.e ., the
o x idation o f M n(II) to M n(III) requires some
stru ctu ral change in the reaction course. In the
case o f iron com pounds, no structural change is
necessary for the oxidation step from Fe(II) to
F e(III). This m ay be one o f the reason for the a p ­
p earance o f a M n(II) com plex o f L 1, in addition to
the recognition th at tripod-like ligands containing
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benzim idazole groups favour the lower oxidation
state o f m etal ions [11]. Similarly, the oxidation
from Fe(III) to Fe(IV ) is unfavorable for the same
reason as in the case o f M n(II) to M n(III). In the
case o f m anganese the oxidation from M n(III) to
M n(IV ) m ay proceed sm oothly if the ligand field
strength is sufficient to stabilize the M n(IV ) state.
Based on the present results and the above dis­
cussion it is clear th a t the oxidation state o f the
m anganese ion in the com pounds is m ore sensitive
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[3] G . C. D ism ukes, Photochem . P hotobiol. 43, 99
(1986).
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th a n iron ion tow ard the change o f ligand field
strength aro u n d the m etal ion, and each state (II,
III, and IV) o f m anganese ion is discretely stabi­
lized. O n the o th er h an d the oxidation state o f III
in the iron com pounds is m uch m ore stabilized u n ­
der aerobic conditions. This m ay be an im portant
criterion to select a certain m etal (M n or Fe) for
the m etalloenzym e cofactors in order to perform
their functions in the biological systems.
[9] Y. N ishida, H. Shim o, an d S. K ida, J. Chem . Soc.
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T. T okii, C hem . Lett. 1989, 321.
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m ukes, J. Am. Chem . Soc. I l l , 3221 (1989).
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