Hindawi Publishing Corporation
Bioinorganic Chemistry and Applications
Volume 2010, Article ID 932569, 7 pages
EleniE.Moushi,1ChristosKizas,1Vassilios Nastopoulos,2and AnastasiosJ. Tasiopoulos1
1Department of Chemistry, University of Cyprus, 1678 Nicosia, Cyprus
2Department of Chemistry, University of Patras, 26500 Patras, Greece
Correspondence should be addressed to Anastasios J. Tasiopoulos, firstname.lastname@example.org
Received 16 March 2010; Accepted 25 March 2010
Academic Editor: Spyros Perlepes
Copyright © 2010 Eleni E. Moushi et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
Two new compounds that consist of the linear trinuclear manganese(II) cation [Mn3(O2CMe)4(py)8]2+cocrystallizing with
different counteranions (I3
with I2in MeCO2H/py, whereas complex 2 was isolated from the reaction of [Mn3O(O2CMe)6(py)3]·py with [Mn(ClO4)2]·6H2O
in MeCN/py. The crystal structures of both compounds were determined by single crystal X-ray crystallography. Magnetic
susceptibility studies that were performed in microcrystalline powder of 1 in the 2–300K range revealed the presence of
antiferromagnetic exchange interactions that resulted in an S = 5/2 ground spin state.
−, ; ClO4
−, ) are reported. Complex 1 was prepared from the reaction of [Mn(O2CMe)2]·4H2O
Oligonunuclear Mn carboxylate clusters have attracted sig-
nificant interest since they have been located in the active
site of metalloenzymes  and also often have interesting
and sometimes novel magnetic properties . Undoubtedly,
the most well-known oligonuclear cluster that appears in
biological systems is the tetranuclear Mn complex that is
present in the active site of photosystem II and is respon-
sible for the light driven oxidation of water to molecular
dioxygen [3–7]. Other Mn compounds observed in the
active sites of metalloenzymes involve mononuclear (e.g.,
in Mn-superoxide dismutases)  and dinuclear (e.g., in
Mn-catalases) complexes [8, 9]. In all those compounds,
the ligation of the Mn ions is provided mainly by O-
and N-donor atoms from the various aminoacid residues
and structural models of the Mn compounds that are
present in metalloenzymes, efforts have been centered on
the synthesis and study of manganese carboxylate complexes
with various chelating N-donor ligands, such as 2,2?-
bipyridine (bpy) [10–13], 1,10-phenanthroline (phen) [10,
14–16], and 2-(2-pyridyl)benzimidazole . As a result a
plethora of dinuclear, trinuclear and tetranuclear manganese
compounds containing carboxylato groups or/and nitrogen-
donor ligands have been prepared and characterized [4, 7,
10–30]. Such complexes are of significant interest not only
as potential functional and structural models of the metal
clusters present in Mn-containing metalloenzymes but also
as precursors for the isolation of new model compounds. In
particular, trinuclear Mn compounds have attracted signif-
icant attention since they appear as discrete metal clusters
with various topologies including linear [10–26], triangular
, V-shaped , and so forth, clusters and also as
building blocks in multidimensional coordination polymers
. Linear trinuclear manganese (II) clusters with various
molecular formulas such as [Mn3(O2CR)6(L)2] [10–20]
and [Mn3(O2CR)4(L?)2] [21–23] have been prepared with
several types of carboxylates, bidentate (L), and tridentate or
tetradentate (L?) chelates and also terminal ligands.
Herein, we report the synthesis, structural characteriza-
tion, and magnetic properties of a new linear manganese(II)
cation, [Mn3(O2CMe)4(py)8]2+which cocrystallizes with
two different counteranions (I3−,  and ClO4
cation of 1 and 2 represents the first linear trinuclear MnII
unit that contains only carboxylate and pyridine ligands and
−, ). The
2 Bioinorganic Chemistry and Applications
Table 1: Crystallographic data for complexes 1 and 2.
Refl. collected/unique (Rint)
Obs. refl. [I > 2σ(I)].
Goodness of fit on F2
Largest diff. peak/hole/e−/˚ A−3
(a)Including counteranions.(b)Graphite monochromator.(c)R1 = Σ?Fo| − |Fc?/Σ|Fo|.(d)wR2 = [Σ[w(Fo2−Fc2)2]/Σ[wFo2)2]]1/2, w = 1/[σ2(Fo2) +
(m · p)2+n · p], p = [max(Fo2,0)+2Fc2]/3, and m and n are constants.
a rare example of a linear MnII3 cluster that is stabilized
with carboxylate and terminal ligands without containing
any polydentate chelates .
2.1. Materials. All manipulations were performed under
aerobic conditions using materials (reagent grade) and
solvents as received; water was distilled in-house. [Mn3O
(O2CMe)6(py)3]·py was prepared as described elsewhere
. Warning: Although we encountered no problems, appro-
priate care should be taken in the use of the potentially
explosives perchlorate anion.
2.2. Syntheses of Compounds
2.2.1. [Mn3(O2CMe)4(py)8](I3)2 . Solid I2 (2.07g, 8.16
mmol) was added to the yellowish solution of [Mn
(O2CMe)2]·4H2O (2.00g, 8.16mmol) in MeCOOH/py
(10/20mL). The resulting red-brown solution was left under
magnetic stirring for ∼45 minutes, filtered off and the
filtrate was left undisturbed at room temperature. After
a few weeks, dark brown crystals of 1 suitable for X-ray
crystallography were formed. The crystals were collected by
filtration, washed with MeCOOH/py (5/10mL) and dried in
vacuum. The yield was ∼60% based on total Mn content. A
sample for crystallography was maintained in contact with
the mother liquor to prevent the loss of interstitial solvent.
Anal. Calc. for C48H52Mn3N8O8I6: C, 32.11; H, 2.92; N,
6.24. Found: C 31.89; H 2.79; N 6.10%. IR data (KBr pellet,
cm−1): ? ν = 3435(m), 3059(m), 1599(s), 1580(s), 1564(s,
1067(m), 1038(m), 1005(m), 752(m), 700(s), 683(m),
br), 1483(m), 1441(s, br), 1350(m), 1215(m), 1151(m),
Method A. To a solution of [Mn3O(O2CMe)6(py)3]·py
(0.294g, 0.345mmol) in MeCN/py (10/2mL) was added
Mn(ClO4)2·6H2O (0.125g, 0.345mmol) and pdH2 (0.10
mL, 0.105g, 1.38mmol) and the mixture was left under
magnetic stirring for ∼30 minutes. The resulting dark red-
brown slurry was filtered off and the dark red-brown filtrate
was left undisturbed at room temperature. After few weeks
yellow crystals appeared, suitable for X-ray structural deter-
mination. The crystals were isolated by filtration, washed
with a copious amount of MeCN/py, and dried in vacuum;
yield, ∼20% based on total ClO4
crystallography was maintained in contact with the mother
C48H52Mn3N8O16Cl2: C, 46.77; H, 4.25; N, 9.09. Found:
C 46.63; H 4.09; N 8.95%.
−content. A sample for
Method B. Method A was repeated in a mixture of MeCN/py
(10/4mL) without using H2pd. The yield was ∼9% based on
on total ClO4
2.3. X-Ray Crystallography. Data were collected on an
Oxford-Diffraction Xcalibur diffractometer, equipped with a
CCD area detector and a graphite monochromator utilizing
Bioinorganic Chemistry and Applications3
Figure 1: A partially labeled plot of the cation of 1. Color code: Mn, purple; O, red; N, green; C, grey. H atoms are omitted for clarity.
Mo-Kα radiation (λ = 0.71073˚ A). Suitable crystals were
attached to glass fibers using paratone-N oil and transferred
to a goniostat where they were cooled for data collection.
Unit cell dimensions were determined and refined by using
12271 (3.07 ≤ θ ≤ 30.27◦) and 5746 (3.06 ≤ θ ≤ 30.29◦)
reflections for 1 and 2, respectively. Empirical absorption
corrections (multiscan based on symmetry-related measure-
ments) were applied using CrysAlis RED software .
The structures were solved by direct methods using SIR92
, and refined on F2using full-matrix least squares with
SHELXL97 . Software packages used: CrysAlis CCD 
for data collection, CrysAlis RED  for cell refinement
and data reduction, WINGX for geometric calculations ,
and DIAMOND  and MERCURY  for molecular
graphics. The non-H atoms were treated anisotropically,
whereas the hydrogen atoms were placed in calculated, ideal
positions and refined as riding on their respective carbon
atoms. Unit cell data and structure refinement details are
listed in Table 1.
2.4. Physical Measurements. Elemental analyses were per-
formed by the in-house facilities of the Chemistry Depart-
ment, University of Cyprus. IR spectra were recorded on
KBr pellets in the 4000–400cm−1range using a Shimadzu
Prestige-21 spectrometer. Variable-temperature DC mag-
netic susceptibility data down to 1.80K were collected
on a Quantum Design MPMS-XL SQUID magnetometer
equipped with a 70kG (7T) DC magnet. Diamagnetic cor-
rections were applied to the observed paramagnetic suscep-
tibilities using Pascal’s constants. Samples were embedded in
solid eicosane, unless otherwise stated, to prevent torquing.
3.1.Syntheses. Bothcomplexeswereprepared serendipitous-
ly during our investigations on two different synthetic meth-
ods. The first one involved the use of iodine as an oxidizing
agent in various reactions of [Mn(O2CMe)2]·4H2O, while
the second one included the employment of 1,3-propanediol
(pdH2) in reactions with [Mn3O(O2CMe)6(py)3]·py.
One of the most successful strategies to polynuclear
Mn clusters has been the oxidation of a Mn2+starting
material with the use of various oxidizing agents, often in
the presence of a chelating ligand. Several oxidants have
been employed for this purpose such as MnO4−, CeIV,
peroxides, bromate, and iodine to form high-oxidation
state Mn species [4, 7, 28]. Although the use of iodine
as oxidant in Mn cluster chemistry has been reported
in the past [4, 7], the oxidation of Mn2+salts from
iodine under various conditions is a rather unexplored
synthetic method. Compound 1 was prepared during our
investigations on reactions of Mn(O2CMe)2·4H2O with
iodine in MeCOOH/pyridine. A large amount of MeCOOH
was used in order to avoid the formation of various Mn
oxides/hydroxides that precipitate at basic conditions. Thus,
the reaction of [Mn(O2CMe)2]·4H2O with solid I2in a 1:1
ratio in MeCOOH/py (10/20mL) resulted in the formation
of dark brown crystals of 1 in ∼60% yield. The formation of
1 is summarized in (1):
Despite the presence of an oxidant (I2) in the reaction
ions. We believe that species that contain Mn ions in higher
oxidation states are also formed but are quite soluble and
thus do not precipitate from the reaction solution.
Another synthetic method to new polynuclear Mn
clusters employed recently by our group involves the use
of aliphatic diols such as pdH2 in Mn cluster chemistry.
These studies have resulted in a number of new polynu-
clear clusters and coordination polymers with coordinated
4Bioinorganic Chemistry and Applications
Table 2: Selected interatomic distances (˚ A) and angles for complex 1.
Bond Distances (˚ A)
Bond Angles (◦)
Table 3: Bond valence sum (BVS)(a,b)calculations for complexes 1 and 2.
(a)The underlined value is the one closest to the charge for which it was calculated.(b)The oxidation state is the nearest whole number to the underlined value.
pdH2 ligands [37–40]. Many of these compounds were
isolated from reactions that were involving the use of
[Mn3O(O2CMe)6(py)3]·py as a starting material [37, 38].
These studies, apart from compounds that contain coor-
dinated pdH2 ligands, have also resulted in complexes
that do not include the diol in their asymmetric unit,
with 2 being one of the members of this family. Thus,
compound 2 was initially prepared from the reaction of
[Mn3O(O2CMe)6(py)3]·py with Mn(ClO4)2·6H2O in the
presence of pdH2 in a 1:1:4 ratio in MeCN/py (10/2mL)
in 20% yield. When the identity of 2 was established and
known that it contained neither coordinated nor lattice
pdH2/pd2−ligands, the reaction resulted in the formation
of 2 was repeated without including pdH2 in the reaction
mixture. This reaction gave a few crystals of 2. Various
modifications were applied in this reaction in order to
optimize its yield. Finally, the larger yield (achieved when no
pdH2was included in the reaction mixture) was ∼9% and
added to the reaction solution. The exact role of pdH2in the
assembly of 2 and how its use results in larger reaction yield
still remain unidentified.
3.2. Description of the Structures. The molecular structure of
complex 1 is presented in Figure 1 and selected interatomic
distances and angles for 1 are listed in Table 2. Bond valence
in Table 3. The crystal structures of 1 and 2 present a striking
similarity with the main difference between them being
their counter-ions and thus only that of 1 will be described
Compound 1 crystallizes in the monoclinic P21/n space
group and comprises the [Mn3(O2CMe)4(py)8]2+cation and
two I3−counteranions. The cation of 1 (Figure 1) consists
of a linear array of three MnIIions coordinated by four
acetate groups and eight terminal pyridine molecules. The
oxidation states of the Mn ions were determined by BVS
calculations (Table 3), charge considerations, and inspection
Bioinorganic Chemistry and Applications5
O, red; C, grey. H atoms are omitted for clarity.
of metric parameters. The central metal ion of the trinuclear
unit (Mn1), which is located on a crystallographic inversion
center, is ligated by four oxygen atoms from four different
acetate ligands and two molecules of pyridine adopting a
distorted octahedral coordination geometry. All four acetate
ligands bridge two Mn ions with two of them operating in
the common syn-syn-η1:η1:μ2 fashion, whereas the other
two function in the less common monoatomically bridging
η2:η1:μ2mode. The above mentioned carboxylate bridging
modes have also been observed in several other linear
trinuclear manganese (II) complexes [10–23]. However, in
most linear MnII3complexes each pair of MnIIions is held
together by at least three bridging ligands, whereas in 1 the
neighboring Mn ions are connected through two bridging
[Mn3(O2CMe)6(H2O)(phen)2] where one pair of Mn ions
is linked through two acetate ligands, whereas the second
one is held together by three bridging MeCOO−ligands
. The consequence of the presence of less bridging
ligands in 1 is the larger Mn···Mn separation (3.799 (2)˚ A)
compared to the values observed in other linear trinuclear
MnIIcomplexes which are within the range of 3.2–3.7˚ A [10–
23]. The observed separation of 3.799˚ A is slightly smaller
than that (3.868 (4)˚ A) between the Mn ions bridged by two
acetate ligands in [Mn3(O2CMe)6(H2O)(phen)2]. However,
the Mn···Mn distance in the other pair of Mn ions of the
latter is significantly shorter (3.489˚ A) and thus the average
Mn···Mn separation falls within the range observed for the
other linear trinuclear MnIIcomplexes.
The distorted octahedral coordination environment
around each terminal metal ion (Mn2) is completed by
three pyridine molecules. The Mn2N3O3 octahedron is
significantly distorted, with the main distortion arising from
the acute O3–Mn2–O4 angle (58.24 (7)◦). The Mn1N2O4
octahedron is almost perfect. All Mn–N and Mn–O bond
lengths of the two crystallographically independent man-
ganese ions are within the expected range for octahedral
0 50 100150 200 250 300
Figure 3: Plot of χMT versus T for 1. The solid line is the fit of the
experimental data; see the text for the fit parameters.
A close examination of the packing of 1revealed that the
trinuclear molecules are nearly perpendicular to each other
(Figure 2) and there are no significant hydrogen bonding
interactions between neighboring units of 1.
3.3. Magnetic Properties. Solid-state dc magnetic susceptibil-
ity studies were performed on a powdered crystalline sample
of 1 in a 0.1T field and in the 5.0–300K temperature range.
The obtained data are plotted as χMT versus T in Figure 3.
The χMT product at 300 K for 1 is 12.98cm3mol−1K,
slightly smaller than the value expected for three MnII
(S = 5/2) noninteracting ions (13.125cm3mol−1K, g =
2) indicating the existence of antiferromagnetic exchange
of χMT upon cooling down to 10.63cm3mol−1K at ∼50K.
Below that temperature, the decrease is more abrupt, with
χMT reaching a value of 4.49cm3mol−1K at 5K. The 5K
χMT value is very close to the spin - only (g = 2) value
of 4.375cm3mol−1K for a spin ground state S = 5/2.
These results are indicative of antiferromagnetic exchange
interactions between the Mn ions of 1 that lead to a spin
ground state of S = 5/2.
The magnetic susceptibility was simulated taking into
account only one isotropic intracluster magnetic interaction,
J, between Mn1 and Mn2 centers since the exchange
interaction between the terminal Mn ions of 1 and also of
most of the known linear MnII3complexes is negligible (J?=
0) [10, 11, 15, 16] because of the large Mn···Mn separation
(for 1 Mn2···Mn2?= 7.598(1)˚ A). Application of the van
Vleck equation  to the Kambe’s vector coupling scheme
 allows the determination of a theoretical χM versus T
expression for 1 from the following Hamiltonian:
using the numbering scheme of Figure 1, where S1 = S2 =
S2? = 5/2. This expression was used to fit the experimental
data giving J = −1.50K and g = 2.00 (solid line, Figure 3).
H = −2J
? S1? S2+? S1? S2?
6Bioinorganic Chemistry and Applications
A temperature-independent paramagnetism (TIP) term was
held constant at 600 ×10−6cm3mol−1K.
The obtained J value is smaller than values reported
in the literature for other linear MnII3 clusters with
three bridging ligands per manganese pair which in most
cases range from ∼−2.5 to ∼−7K . This behaviour
could be rationalized on the basis of the existence of
only two bridging ligands per manganese pair and larger
Mn···Mn separations in 1 as was discussed in detail
above (description of the structures). There are, however,
examples of linear MnII3 clusters with J values compa-
rable to that of 1, such as [Mn3(L1)2(μ-O2CMe)4]·2Et2O
methylimidazol-2-yl)methyl)amine) (J = −1.7K) .
A new linear trinuclear manganese(II) complex [Mn3
(O2CMe)4(py)8]2+cocrystallizing with I3− and ClO4
 has been synthesized serendipitously. Compound 1 was
prepared in an attempt to oxidize [Mn(O2CMe)2]·4H2O
with I2 in MeCOOH/py, whereas compound 2 was ini-
tially isolated during our investigations on reactions of
[Mn3O(O2CMe)6(py)3]·py with Mn(ClO4)2·6H2O in the
presence of pdH2 in MeCN/py and was resynthesized in
lower yield without adding pdH2 in the reaction mixture.
Although several linear trinuclear MnIIcomplexes have been
prepared and studied, the cation of 1 and 2 has several novel
structural features including: (i) different type of ligation
since 1 and 2 are the first examples of linear trinuclear
Mn clusters with only acetate and pyridine ligands and (ii)
different number of bridging ligands between each pair of
MnIIions, since 1 and 2 are rare examples of linear MnII3
clusters with only two bridging ligands linking each pair of
MnIIions. Variable temperature dc magnetic susceptibility
studies revealed the existence of antiferomagnetic interac-
tions between the Mn ions of 1 resulting in an ST= 5/2 spin
The authors thank the Cyprus Research Promotion Foun-
dation (Grant: TEXNO/0506/06), for financial support of
this research. This paper is dedicated to Professor Nick
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