A new family of homo- and heterometallic manganese complexes at high oxidation states derived from the oxidation of MnII with Ce IV: Syntheses, structures, and magnetic properties

Article abstract of DOI:10.1002/ejic.201100719

A series of homo- and heterometallic manganese(III or IV) clusters, [Mn₂O₂(pybim)₄](NO₃) ₄·11H₂O [1; pybim = 2-(2-pyridyl)benzimidazole], [Mn₃O₄(bipy)₄(H₂O) ₂][Ce(NO₃)₅(H₂O)][NO ₃]₂·2H₂O (2; bipy = 2,2′- bipyridine), [Mn₂Ce₃O₆(pta)₆(NO ₃)₂(dmf)₄]·H₂O (3; Hpta = p-toluic acid), and [Mn₈CeO₈(pta)₁₂(dmf) ₄] (4), were prepared by the reaction of simple Mn²⁺ salts with (NH₄)₂[Ce(NO₃)₆] under different experimental conditions. The single-crystal X-ray diffraction analyses established that these complexes possess inorganic cores that have various structural topologies, which range from the [Mn₂O₂] ⁴⁺ rhomb, a Mn₃ triangle within [Mn₃O ₄]⁴⁺, and the nonplanar [Mn₈O₈] ⁸⁺ loop that centrally traps a Ce⁴⁺ ion to the [Mn ₂Ce₃O₆]⁸⁺ cage that contains a Mn₂Ce₃ trigonal bipyramid. The solid-state magnetic susceptibility studies revealed that these four complexes possess distinctly different magnetic properties, namely, both 1 and 2 show strong antiferromagnetic interactions between the Mn^IV centers, whereas 3 is weakly antiferromagnetic, and 4, however, has predominantly ferromagnetic interactions. Four Mn or mixed Mn/Ce clusters at high oxidation states were prepared by the reaction of Mn²⁺ salts where (NH₄) ₂[Ce(NO₃)₆] was used as an oxidizing agent. These complexes possess inorganic cores that have topologies that range from the rhomb, the triangle, and the nonplanar loop to the trigonal bipyramid. The four complexes possess distinctly different magnetic properties.

Full text of DOI:10.1002/ejic.201100719

FULL PAPER
DOI: 10.1002/ejic.201100719
A New Family of Homo- and Heterometallic Manganese Complexes at High
Oxidation States Derived from the Oxidation of MnII with CeIV: Syntheses,
Structures, and Magnetic Properties
Cheng-Bing Ma,*[a] Ming-Qiang Hu,[a] Hui Chen,[a] Chang-Neng Chen,*[a] and
Qiu-Tian Liu[a]
Keywords: Manganese / Cerium / Cluster compounds / Magnetic properties
A series of homo- and heterometallic manganese(III or IV)
clusters, [Mn2O2(pybim)4](NO3)4·11H2O [1; pybim = 2-(2-
from the [Mn2O2]4+ rhomb, a Mn3 triangle within [Mn3O4]4+
,
and the nonplanar [Mn8O8]8+ loop that centrally traps a Ce4+
ion to the [Mn2Ce3O6]8+ cage that contains a Mn2Ce3 trigonal
bipyramid. The solid-state magnetic susceptibility studies re-
vealed that these four complexes possess distinctly different
magnetic properties, namely, both 1 and 2 show strong anti-
ferromagnetic interactions between the MnIV centers,
whereas 3 is weakly antiferromagnetic, and 4, however, has
predominantly ferromagnetic interactions.
pyridyl)benzimidazole],
(H2O)][NO3]2·2H2O (2; bipy = 2,2Ј-bipyridine), [Mn2Ce3O6-
(pta)6(NO3)2(dmf)4]·H2O (3; Hpta p-toluic acid), and
[Mn3O4(bipy)4(H2O)2][Ce(NO3)5-
=
[Mn8CeO8(pta)12(dmf)4] (4), were prepared by the reaction
of simple Mn2+ salts with (NH4)2[Ce(NO3)6] under different
experimental conditions. The single-crystal X-ray diffraction
analyses established that these complexes possess inorganic
cores that have various structural topologies, which range
als;[6] and (iii) the high oxidation state Mn oxides and Mn/
Ce composite oxides constitute a good catalytic wet-air oxi-
dation system with high activities for the treatment of waste
water that contains toxic organic and inorganic pol-
lutants.[7] Thus, as molecular analogues to these oxides, the
high-valent Mn or mixed Ce/Mn molecular clusters should
have similar oxidative/catalytic properties.
Several pathways have been developed to elevate the oxi-
dation state of the Mn ion during the formation of the man-
ganese complexes, for example, the aerobic oxidation of the
Mn2+ salts under weakly basic conditions and the compro-
portionation reaction between the permanganate and the
Mn2+ salts. Recently Christou et al. employed (NH4)2-
[Ce(NO3)6] as an oxidizing agent to react with various Mn-
containing precursors and they obtained a series of high
oxidation state Mn/Ce clusters, which included MnIV–CeIV
(MnIV2Ce, MnIV6CeIV, and MnIV2CeIV3), MnIV–CeIII
(MnIV2CeIII2), MnIII–CeIV (MnIII8CeIV), and MnIII–CeIII/IV
(MnIII10CeIV2CeIII2) compounds.[8] We have also prepared
several high-valent high nuclearity Mn/Ce complexes with
either discrete (MnIII8CeIV and MnIV6CeIV2) or polymeric
([MnIV12CeIV18CeIII4]n) structures.[9] It is worth noting that
this approach often introduces the oxophilic metal Ce ion
Introduction
Discrete, high-valent oxomanganese complexes that con-
tain Mn ions in the +3, +4, or mixed +3/+4 oxidation states
have attracted extensive attention over the past few dec-
ades.[1–6] Besides the aesthetically pleasing structures they
usually possess, the reasons for this attention mainly involve
the following aspects: (i) the oxygen-evolving complex
(OEC) in the Photosystem II (PSII) is believed to contain
a high-valent oxo-bridged tetranuclear Mn cluster that is
responsible for the photo-induced oxidation of H2O to pro-
duce O2, which supports all aerobic life on earth.[2] The
structural and the spectroscopic properties, as well as the
function, of the OEC are expected to be replicated from the
synthetic lower-nuclearity Mn complexes;[3] (ii) the high-
valent high nuclearity Mn clusters often have large, and
sometimes abnormally large, ground state spin (S) values.[4]
If these are combined with a large and negative zero-field
splitting parameter (D) they would exhibit highly unusual
magnetic phenomena as has been discovered from the sin-
gle-molecule magnets (SMMs).[5] This makes these Mn
clusters highly attractive as precursors to magnetic materi-
[a] State Key Laboratory of Structural Chemistry, Fujian Institute into the coordination system, which leads to the richer
structural chemistry of the Mn clusters.[8–11]
of Research on the Structure of Matter, Chinese Academy of
Sciences, Fuzhou, Fujian 350002, P. R. China
In the present work we have explored the reaction of sim-
ple Mn2+ salts and (NH4)2[Ce(NO3)6], which was used as
an oxidant, under a variety of experimental conditions that
included a variation in the Ce4+/Mn2+ ratios, the acidity of
Fax: +86-591-83792395
E-mail: mcb@fjirsm.ac.cn
ccn@fjirsm.ac.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100719.
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FULL PAPER
the medium, and the solvents that were used. A new family (dmf)4]·H2O (3) and [Mn8CeO8(pta)12(dmf)4] (4). The com-
of homo- and heterometallic Mn clusters, with various inor- mon dmf solvent was initially used in order to ensure that
ganic core topologies that range from the [Mn2O2]4+ rhomb the reactant, Mn(pta)2·4H2O, would dissolve but it was
(1), the Mn3 triangle within [Mn3O4]4+ (2), and the non- found that it also acted as a terminal ligand to both the Mn
planar [Mn8O8]8+ loop that centrally traps a Ce4+ ion (3) and Ce centers. The Mn/Ce reagent stoichiometry was not
to the [Mn2Ce3O6]8+ cage that contains a Mn2Ce3 trigonal a key factor for the formation of the products, as mentioned
bipyramid (4), have been obtained. Their syntheses and above, however, a nearly neutral medium was important for
structures, as well as their magnetic properties, are dis- the isolation of 3 and 4 with the CeIV incorporated. As is
cussed.
the case with almost all of the Mn cluster chemistry, the
products 1 to 4 are unlikely to be the only species present.
In other words, it is likely that there are several species in
each of the reaction equilibria and that the subsequent
crystallization that gives the respective single-crystals is
strongly dependent on the solubility of the species in a given
Results and Discussion
Syntheses
The complexes described in this work were prepared by solution system, which is associated with the solvents used,
the oxidation of the MnII reagent with CeIV in the presence the acidity of the medium and so on. This means that the
or absence of the N-donor chelating coligands under vari- successful isolation of 1 to 4 is due to the fact that they
ous experimental conditions. All of the experiments were crystallize out before the other species in solution get to
found to give high-valent Mn (III or IV) products. CeIV is the saturation point. Interestingly, a closer inspection of the
an unusually strong, one-electron oxidant, and it is fully preparation reactions (especially for 3 and 4) showed that
capable of oxidizing MnII to MnIII or MnIV since the redox atmospheric oxygen may partly participate in the oxidation
pair CeIV/CeIII possesses a standard electrode potential of of MnII to MnIII, or even to MnIV, although the atmo-
1.72 V that is higher than that of the MnIV/MnII pair spheric oxidation of MnII to MnIV is not expected under
(1.22 V vs. the standard hydrogen electrode). In fact, some normal conditions. Thus, the oxide (O2–) ligands in 1 to
high-valent Mn complexes have been derived previously 4 may originate from multiple sources, namely, from the
from the oxidation of the Mn2+ salts by CeIV.[12] The high-, atmospheric O2, the water molecule, or their conjunction.
mixed-valent manganese clusters are primarily stabilized Generally, under neutral or weakly basic conditions, atmo-
with the help of the oxide (O2–) ligands, where the harder spheric O2 has a stronger trend of oxidizing MnII to the
oxide ions help to bridge the Mn centers. The presence of higher-valent species (MnIII or MnIV). Equations (1), (2),
the hard O2– ions also favors the incorporation of the hard, (3), and (4) tentatively describe the formation of complexes
high oxidation state Ce4+ ion. This facilitates the formation 1 to 4, respectively.
of the mixed-metal Mn/Ce complexes, although the general-
–
2Mn2+ + 4(1–x)Ce4+ + xO2 + (13–2x)H2O + 4pybim + 4NO3
[Mn2O2(pybim)4](NO3)4·11H2O + 4(1–x)H+ + 4(1–x)Ce3+
Ǟ
(1)
ity of this as a route to the Mn/Ce products, as well as the
structural types of the products, and the Mn/Ce ratio in the
products, still needs to be investigated further. Some di- and
polyimines, for example bipyridyl and terpyridyl, that have
been used as ancillary chelating ligands have been demon-
strated to play important roles in the formation and stabili-
zation of oxomanganese clusters that contain Mn ions in
the +3 and +4 oxidation states.[12,13] The reaction of
Mn(pta)2·2H2O with (NH4)2[Ce(NO3)6] in the presence of
pybim [pybim = 2-(2-pyridyl)-benzimidazole] or bipy (bipy
= 2,2Ј-bipyridine) gave [Mn2O2(pybim)4](NO3)4·11H2O (1)
3Mn2+ + (6–4x)Ce4+ + xO2 + (9–2x)H2O + 4bipy + 7NO3
Ǟ
–
[Mn3O4(bpy)4(H2O)2][Ce(NO3)5(H2O)][NO3]2·2H2O + (5–4x)Ce3+
+ 4(2–x)H+
(2)
2Mn2+ + (7–4x)Ce4+ + xO2 + 4dmf + 6pta– + (7–2x)H2O + 2NO3– Ǟ
[Mn2Ce3O6(pta)6(NO3)2(dmf)4]·H2O + 4(1–x)Ce3+ + 4(3–x)H+
(3)
8Mn2+ + (9–4x)Ce4+ + xO2 + 4dmf + 12pta– + 2(4–x)H2O Ǟ
[Mn8CeO8(pta)12(dmf)4] + 4(4–x)H+ + 4(2–x)Ce3+
(4)
and
[Mn3O4(bpy)4(H2O)2][Ce(NO3)5(H2O)][NO3]2·2H2O
O2 is used to balance Equations (1) to (4), which reflects
the participation of O2 in the oxidizing of Mn(II) to
Mn(III) or Mn(IV), and the coefficient x varies between 0
and 1.
(2), respectively. The lower nuclearity of both 1 and 2 is
consistent with the presence of the N-donor coligands that
function as effective chelates but not as bridging groups.
The low pH conditions (the presence of inorganic or or-
ganic acid) in both of the reactions excluded the incorpora-
tion of Ce4+ into the Mn4+ coordination sphere (the Ce3+
monomer in 2 exists as a counterion outside the Mn clus-
ter). The Mn/Ce ratio of the reactants was not found to
Description of the Structures of the Complexes 1 to 4
The structure of complex 1 consists of one discrete
strictly affect the formation of the products, but an appro- [Mn2O2(pybim)4]4+ cation, four nitrate anions, and eleven
priate increase in the amount of (NH4)2[Ce(NO3)6] can im- water molecules of crystallization. The bond lengths and
prove the product yields. Under nearly neutral medium con- bond angles that pertain to the inner coordination of Mn
ditions, the reaction between Mn(pta)2·4H2O (Hpta = p- are listed in Tables 1 and S1, respectively, and the structural
toluic acid) and (NH4)2[Ce(NO3)6], without an ancillary N- drawing of the cation is shown in Figure 1. The cation is
donor chelating coligand, afforded [Mn2Ce3O6(pta)6(NO3)2- situated on a twofold axis of symmetry, which passes
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Homo- and Heterometallic Manganese Complexes
through the Mn1–Mn2 bond, and hence half of the cation the nitrate/water O atoms, the water molecules and the ni-
is crystallographically independent. The two manganese trate anions trigger a complicated hydrogen bond network
centers in the cationic cluster are bridged by two O2– ions to where the O···O distances are in the range of 2.814(12) to
yield a planar Mn2O2 rhombic core in which the Mn···Mn 2.909(10) Å. The cationic clusters are associated together by
distance is 2.7161(16) Å, which is similar to that found in means of these hydrogen bonding interactions to yield an
the [Mn2O2]4+ cores of the phenanthroline or picolinic acid ordered 3D supramolecular array (Figure S1).
complexes,[14] and therefore indicates no significant metal–
Complex 2 crystallizes in the monoclinic, non-centric
metal bond. The two chelating pybim ligands complete the space group C/c with the absolute structure Flack param-
roughly octahedral coordination geometry around the Mn eter refined to 0.061(15). The structure consists of one tri-
center with cis angles that range from 78.97(12) to nuclear cationic cluster [Mn3O4(bipy)4(H2O)2]4+ with one
100.90(11)°. The average Mnl–N distance [2.023(3) Å] is monomeric [Ce(NO3)5(H2O)]2– anion and two nitrates as
very close to the average Mn2–N distance [2.030(3) Å], the counterions, and two water molecules of crystallization.
which suggests that the coordination geometries of the two All of the components are in general crystallographic posi-
Mn atoms are almost identical. The lack of evidence for the tions. As shown in Figure 2, the cluster cation contains a
Jahn–Teller elongation, as well as the bond valence sum core of [Mn3O4], in which three Mn atoms reside on the
calculations (Table S2), confirm that both of the Mn atoms vertices of a nearly isosceles triangle. One of the three Mn
are in the +4 oxidation state. It is worth noting that the atoms (Mn3) is linked to the other two Mn atoms (Mn1
Mn···Mn span of 2.7161(16) Å may be used to support the and Mn2) by means of two single oxo-bridges (O3 and O4).
existence of a [Mn2(μ-O)2] fragment in the OEC of PSII The Mn1···Mn3 and Mn2···Mn3 distances are nearly equal
since the crystal structure data on this enzyme at 1.9 Å reso- [3.2590(14) and 3.2531(14) Å, respectively]. Mn1 and Mn2
lution has indicated that a short (ca. 2.8–3.0 Å) Mn···Mn are linked to each other by the di-μ-oxo bridge (O1 and
separation is present.[2f]
O2), which affords a slightly shorter Mn···Mn distance
[2.6704(14) Å] than that found in 1 [2.7161(16) Å]. The two
single μ-oxo bridges (O3 and O4) are nearly coplanar with
the three Mn atoms, while the doubly oxo-bridged [Mn2O2]
segment exhibits a slight fold along the Mn1–Mn2 bond
with a small dihedral angle of 16.2°. The two motifs (Mn1/
Mn2/Mn3/O3/O4 and Mn1/Mn2/O1/O2) are nearly perpen-
dicular to each other. The Mn coordinations are octahe-
drally completed either by four bipy N atoms (for Mn3) or
by two bipy N atoms and one water O atom (for both Mn1
and Mn2).
Table 1. The selected bond lengths [Å] for 1.
Mn1–O1
Mn1–N2
Mn1–N1
Mn1···Mn2
1.794(2)
1.963(3)
2.083(3)
2.7161(16)
Mn2–O1
Mn2–N5
Mn2–N4
1.803(2)
1.974(3)
2.086(3)
Figure 2. A structural view of [Mn3O4(bipy)4(H2O)2][Ce(NO3)5-
(H2O)] in 2 with the selected atom labeling scheme ellipsoids at
30% probability. The hydrogen atoms, except for those on the coor-
dinated water molecules, have been omitted for clarity.
Figure 1. A structural view of the cluster cation in 1 with the se-
lected atom labeling scheme ellipsoids at 30% probability. All of
the hydrogen atoms have been omitted for clarity.
The average Mn–N and Mn–Ooxo distances [2.057(6) and
1.803(5) Å, respectively], as well as the bond valence sum
calculations (Table S2, Supporting Information), confirm
Abundant hydrogen bonds are present within the crystal that the cationic cluster in 2 contains three MnIV ions. As
lattice of 1 (Table S3). Besides the two short N–H···O hy- listed in Tables 2 and S4, the structural parameters that re-
drogen bonds [N···O 2.680(4) and 2.659(5) Å] that formed late to the [Mn3O4] core are comparable to the analogous
between the imidazole N atoms of the pybim ligands and complex cations, namely [Mn3O4(bipy)4Cl2]2+ and
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C.-B. Ma, M.-Q. Hu, H. Chen, C.-N. Chen, Q.-T. Liu
However, the Mn–Oaqua Table 3. The selected bond lengths [Å] for 3.
FULL PAPER
[Mn3O4(phen)4(H2O)2]4+ [12b,15]
.
distances [1.986(6) and 2.039(6) Å] are shorter than those
that have been reported previously for any of the ana-
logs,[13a,15] even though the coordinated water molecules in
the present cation extensively engage in hydrogen bonding
Ce1–O18
Ce1–O16
Ce1–O15
Ce1–O17
Ce1–O4
2.290(3)
2.301(3)
2.320(3)
2.336(3)
2.382(3)
2.397(3)
2.401(3)
2.500(4)
2.631(4)
2.293(3)
2.308(3)
2.316(3)
2.316(3)
2.399(3)
2.418(4)
2.432(3)
2.489(4)
2.633(4)
2.264(3)
Ce3–O17
Ce3–O18
Ce3–O13
Ce3–O12
Ce3–O26
Ce3–O1
Ce3–O28
Mn1–O17
Mn1–O15
Mn1–O13
Mn1–O5
Mn1–O3
Mn1–O2
Mn2–O16
Mn2–O14
Mn2–O18
Mn2–O8
Mn2–O10
Mn2–O11
2.271(3)
2.296(3)
2.305(3)
2.375(3)
2.394(4)
2.405(3)
2.444(4)
1.827(3)
1.829(3)
1.833(3)
1.976(3)
1.993(3)
1.998(3)
1.827(3)
1.833(3)
1.836(3)
1.966(3)
1.970(3)
1.988(3)
with the lattice water molecules and nitrates [Oaqua···Oaqua
/
Ce1–O7
nitrate distances are in the range of 2.563(2)–2.878(2) Å]. It
has been postulated that the water that is directly bound to
a high-valent MnIV center with strong hydrogen bonds is
the source of the O2 that evolves during photosynthe-
sis[2f,3e,16] since the hydrogen bonds render the oxygen of
the water molecule more electron-rich.[17]
Ce1–O25
Ce1–O20
Ce1–O19
Ce2–O13
Ce2–O16
Ce2–O15
Ce2–O14
Ce2–O9
Ce2–O27
Ce2–O6
Ce2–O22
Ce2–O23
Ce3–O14
Table 2. The selected bond lengths [Å] for 2.
Mn1–O1
Mn1–O2
Mn1–O3
Mn1–O5
Mn1–N1
Mn1–N2
Mn2–O1
Mn2–O2
Mn2–O4
Mn2–O6
Mn2–N3
1.806(5)
1.807(5)
1.827(5)
1.986(6)
2.071(6)
2.076(6)
1.803(5)
1.811(5)
1.835(5)
2.039(6)
2.058(6)
Mn2–N4
Mn3–O3
Mn3–O4
Mn3–N6
Mn3–N8
Mn3–N7
Mn3–N5
Mn1···Mn2
Mn1···Mn3
Mn2···Mn3
2.070(6)
1.758(5)
1.779(5)
2.006(6)
2.011(6)
2.077(6)
2.092(6)
2.6704(14)
3.2590(14)
3.2531(14)
On the basis of the electron-neutrality consideration for
the whole complex 2, the Ce atom in the [Ce(NO3)5-
(H2O)]2– counterion is in the +3 oxidation state. The CeIII
center, which is in a distorted monocapped pentagonal pris-
matic geometry, is eleven-coordinated by five bidentate che-
lating nitrato ligands and one water molecule and has Ce–
O bond lengths in the range of 2.494(9) to 2.731(8) Å
(Table S5). The [Ce(NO3)5(H2O)]2– anion is associated with
the complex cation through extensive hydrogen bonding in-
teractions, which appear to involve all of the components
since too many donor/acceptor groups are present in the
structure (Table S6).
Complex 3 is a neutral discrete pentanuclear heterometal
cluster. There was no evidence of Jahn–Teller elongation
and the charge considerations, as well as the bond valence
Figure 3. A structural view of 3 with the selected atomic labeling
scheme ellipsoids at 30% probability. For clarity, all of the carbon
atoms are shown as small spheres of reduced radii and the uncoor-
dinated O/N atoms of both dmf and NO3 are shown as small
spheres of arbitrary radii.
sum calculations (Table S2), established that a MnIV2CeIV
3
situation exists in complex 3. The selected bond lengths and
bond angles are listed in Tables 3 and S7, respectively. The
whole molecule lies on a general crystallographic position.
As shown in Figures 3 and 4, the structure of the cluster
contains a cage-like core of [Mn2Ce3O6], in which five metal
atoms display a trigonal bipyramidal arrangement with
three Ce atoms that define the triangle equatorial plane
[Ce···Ce 3.694(3)–3.737(3) Å]. The Ce3 triangle is attached
to two Mn atoms [Mn···Mn 4.798(3) Å] above and below
the triangle plane by means of six μ3-O ions. The six μ3-O
ions are each located above the six triangle faces of the tri-
–
gonal bipyramid, which thus completes
a cage-like
[Mn2Ce3O6] core. Each of the Mn/Ce ion pairs is addition-
ally bridged by one pta– ligand [Mn···Ce 3.1857(9)–
3.2458(9) Å]. The two Mn atoms are thus normally six-co-
ordinate. The peripheral ligands around the three Ce atoms
also contain four terminal dmf molecules and two chelating
nitrate anions, which affords two kinds of coordination en-
Figure 4. The [Mn2Ce3O6]8+ core in 3, which shows the cage-like
structure.
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Homo- and Heterometallic Manganese Complexes
vironments around the three Ce atoms. One of the Ce atoms torted tricapped trigonal prism or heptagonal bipyramid 9-
(Ce3) is eight-coordinate with a distorted dodecahedral ge- coordination geometries, where each Ce is peripherally lig-
ometry that is completed by two terminal dmf molecules, ated by one dmf molecule and one chelating nitrate anion.
whereas the other two Ce atoms (Ce1 and Ce2) exhibit dis-
Complex4ismadeupofdiscreteneutral[Mn8CeO8(pta)12-
(dmf)4] clusters and crystallizes in the tetragonal space
group P42/n. The selected bond lengths and bond angles
are listed in Tables 4 and S8, respectively. The whole mole-
cule lies on a crystallographic fourfold axis of symmetry
that passes through the central Ce atom with the asymmet-
ric unit thus containing a quarter of the cluster. As shown
in Figures 5 and 6, the structure contains a cluster core of
[Mn8CeO8], in which the eight Mn atoms and the eight O
atoms form a nonplanar [Mn8O8] loop with the Mn and O
atoms arranged alternately. The adjacent Mn···Mn dis-
tances of the loop are 3.0259(12) and 3.2376(12) Å. The
[Mn8O8] ring is further attached to the central Ce atom by
means of eight O2– ions, which each act as a μ3-linker. The
peripheral eight μ1,3- and four μ1,1,3-bridging pta– ligands
and the four terminal dmf molecules complete the near-
octahedral coordination around each of the Mn atoms. The
charge consideration and the bond valence sum calculations
(Table S2), as well as the Jahn–Teller effect established that
all of the Mn atoms are in the +3 oxidation state and that
Table 4. The selected bond lengths [Å] for 4.
Ce1–O8
Ce1–O7
Mn1–O8
Mn1–O7
Mn1–O5
Mn1–O1
Mn1–O4A[a]
2.305(3)
2.381(4)
1.853(4)
1.869(4)
1.959(4)
1.961(4)
2.145(4)
Mn1–O3
Mn2–O8B
Mn2–O7
Mn2–O6
Mn2–O2B
Mn2–O9
Mn2–O3
2.345(4)
1.854(4)
1.867(4)
1.983(4)
1.979(4)
2.220(5)
2.332(4)
[a] Symmetry codes, A: –y + 3/2, x, –z + 1/2; B: y, –x + 3/2, –z +
1/2.
Figure 5. A structural view of 4 with the selected atomic labeling
scheme ellipsoids at 30% probability. All of the carbon atoms are
shown as small spheres of reduced radii for clarity.
Figure 6. The core of [Mn8CeO8]12+ in 4, which shows the Ce-
trapped Mn8O8 loop structure.
Figure 7. A view of the π–π-interaction between a pair of clusters in 4.
Eur. J. Inorg. Chem. 2011, 5043–5053
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FULL PAPER
the central Ce atom is in the +4 oxidation state. The eight maining constant down to 2 K. This is behavior is typical
MnIII atoms exhibit the expected Jahn–Teller distortion for of a strong antiferromagnetic couple between the paramag-
a high-spin MnIII (d4) in an octahedral geometry with six- netic centers. The experimental data were fitted by em-
teen oxygen atoms (O3, O4, and O9, and their symmetry- ploying Equation (5) that was derived from an isotropic
related equivalents) from the μ1,1,3-pta– ligands and the dmf spin-exchange Hamiltonian, H = –2JS1 S2 (S1 = S2 = 3/2),
molecules, which are located at the elongated axial positions and the Van Vleck equation.[19] The best least-squares fit
[Mn–O 2.145(4)–2.332(4) Å]. Presumably it is this axial results were obtained in the full temperature range with pa-
elongation, which is associated with the MnIII (d4) Jahn– rameters J = –143.38 cm–1, TIP = 400ϫ10–6 cm3 Kmol–1
Teller effect, that makes the occasionally strict orthogonal- (fixed), g = 2.00 (fixed), P = 0.047, and R = 7.58ϫ10–4,
ity of the magnetic orbitals of the interacting MnIII ions,[18] where TIP is the temperature-independent paramagnetism,
ˆ ˆ
ˆ
which thus leads to the ferromagnetically coupled MnIII
–
P is the percentage of a paramagnetic impurity that was
MnIII species (vide infra). The central CeIV atom, which is assumed to be a mononuclear MnIV species, and R is the
in a distorted trigonal dodecahedral geometry, is eightfold agreement factor. The relatively large coupling constant
coordinated to the O2– ions and the Ce–O bond lengths (–143.38 cm–1), which falls in the range of –79 to
range from 2.305(3) to 2.381(4) Å. Significant stacking in- –188 cm–1,[20] suggested a strong magnetic exchange inter-
teractions that involve the pta– ligands from the inversion- action between the two MnIV centers that are closely linked
related clusters, which have a centroid–centroid distance of (ca. 2.7 Å) through the di-μ-oxo bridge.
3.891 Å between the benzene rings (C17–C22) at (x, y, z)
As shown in Figure 9, the product χMT of complex 2 at
and (1 – x, 2 – y, –z), were observed (Figure 7). Each cluster room temperature is equal to 2.69 cm3 Kmol–1, which is
unit thus functions as a 4-connected node to π–π-interact much lower than the expected spin-only value
with four other adjacent units, which results in a three-di- (5.63 cm3 Kmol–1) for the sum of three isolated MnIV ions
mensional supramolecular pack mode for the whole solid and one CeIII ion, and decreases gradually to
structure (Figure S2).
0.64 cm3 Kmol–1 at 2 K. This behavior is characteristic of
antiferromagnetic interactions between the paramagnetic
centers. On the basis of the structure of 2, the experimental
data were fitted to a simplified isotropic exchange model
that is based on three MnIV (S1 = S2 = S3 = 3/2) centers
that are arranged in an isosceles triangle where the assump-
tion is that Mn1 and Mn2 are equidistant from Mn3 and
where an isolated outer CeIII ion is included. The spin
Magnetic Properties of Complexes 1 to 4
The temperature (T) dependence of the molar magnetic
susceptibility (χM) complex 1 is shown in Figure 8 in the
form of a χMT versus T plot. The product χMT of
0.79 cm3 Kmol–1 at room temperature is much smaller than
the spin-only value (3.75 cm3 Kmol–1) that is expected for
two noninteracting MnIV centers, and it monotonically de-
clines to 0.19 cm3 Kmol–1 at approximately 50 K before re-
ˆ ˆ
ˆ ˆ
ˆ
Hamiltonian is thus written as H = –2J1S1 S2 – 2J2(S1 S3 +
ˆ ˆ
ˆ
S2 S3) + SCe, where the two distinct coupling parameters J1
and J2 represent the interactions across the di-μ-oxo bridge
and across the mono-μ-oxo bridges, respectively, and the
third term on the right-hand side accounts for the isolated
CeIII center. Equation (6) describes the molar magnetic
susceptibility (χM) as a function of temperature and was
derived from a combination of a Van Vleck equation for
the isosceles triangular MnIV trimer and a Curie–Weiss for-
mula for the single CeIII ion.[19,21]
(5)
Figure 8. Plots of the product (χMT) vs. temperature (T) for 1; χM
is the molar magnetic susceptibility and the solid line represents
the calculated value.
Figure 9. Plots of the product (χMT) vs. temperature (T) for 2; χM
is the molar magnetic susceptibility and the solid line represents
the calculated value.
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Homo- and Heterometallic Manganese Complexes
(6)
Fairly good least-squares fit results were obtained in the gand field splitting of the ground state 2F5/2 into three Kra-
full temperature range with the parameters J1 mers doublets. Both of the fittings gave almost coincident
=
–69.21 cm–1, J2 = –36.16 cm–1, gMn = 2.04, gCe = 2.06, the coupling parameters, J1 and J2, within the [Mn3O4] core,
Weiss constant θ = –1.91 K, and R = 1.87ϫ10–3. In prac- which unequivocally shows that the exchange interaction of
tice, the magnetic properties of the rare-earth-containing J1 across the di-μ-oxo bridge is larger than that of J2 across
complexes may be rather complicated because they are af- the mono-μ-oxo bridges. However, the J1 value is much
fected by numerous factors that include the contribution of smaller than that which was found in 1 (–143.38 cm–1)
the orbital momentum, the spin-orbit coupling effect, the where the exchange interaction of J1 was similarly mediated
thermal population of the excited states, and so on, thus, by the di-μ-oxo bridge. This may be due to the difference
the precise interpretation of the experimental data remains in the overlap of the magnetic orbitals of two interacting
difficult in most cases, which is in contrast to the 3d transi- metal centers as a result of different crystal symmetries.
2
tion metal species. The F ground term of the CeIII ion is
For complex 3, as shown in Figure 10, the χMT value of
split by the spin-orbit coupling into two states, namely, the 3.59 cm3 Kmol–1 at room temperature is slightly lower than
ground state 2F5/2 and the excited state 2F7/2. The two states the spin-only value (3.75 cm3 Kmol–1) that is expected for
are well separated in energy (above 2200 cm–1) from each two isolated MnIV ions (the CeIV ion, f0 configuration is
other, such that only the ground state is thermally popu- diamagnetic) and it remains essentially constant before a
lated at room temperature and below.[21] In the free-ion small drop that occurs at approximately 10 K. This behav-
approximation, the molar magnetic susceptibility for the ior suggests that there are overall weak antiferromagnetic
CeIII monomer is thus expressed in the form of a Curie-like interactions within the complex. The experimental data
formula [Equation (7)] as
were fitted by employing the same isotropic spin Hamilto-
ˆ ˆ
ˆ
nian H = –2JS1 S2 (S1 = S2 = 3/2) as was used for 1. The
best least-squares fit results were obtained in the full tem-
perature range with parameters J = –0.46 cm–1, TIP =
(7)
where J is the quantum number that is associated with the
total magnetic momentum J defined as J = L + S, and gJ
is the Zeeman factor.[21,22] In the combination of Equa-
tions (6) and (7) an alternative fit was carried out where the
contribution of the orbital momentum of the CeIII ion was
considered. The least-squares fit results were obtained with
J1 = –66.35 cm–1, J2 = –35.13 cm–1, gCe = 0.64, the Weiss
constant θ = –2.39 K, and R = 3.35ϫ10–3, with gMn fixed
at 2.00, in the full temperature range. Although the latter
fit was also mathematically fairly good, the gCe value of
0.64 is smaller than the expected value (gJ = 6/7) for the
ground state CeIII ion (2F5/2). The attempts to elevate the
gCe value during the data fitting resulted in an increase in
the agreement factor, R, which indicates worse fitting re-
Figure 10. Plots of the product (χMT) vs. temperature (T) for 3; χM
is the molar magnetic susceptibility and the solid line represents
sults. The deviation is probably the consequence of the li- the calculated value.
Eur. J. Inorg. Chem. 2011, 5043–5053
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C.-B. Ma, M.-Q. Hu, H. Chen, C.-N. Chen, Q.-T. Liu
FULL PAPER
500ϫ10–6 cm3 Kmol–1 (fixed), g = 1.93, P = 0.039, and R with Anisofit2.0,[23] by using S = 7 by diagonalization of
= 4.21ϫ10–4. The small J value indicates a very weak ex- the spin Hamiltonian matrix including axial zero-field split-
2
ˆ
change coupling in 3, which is expected for the two ting (DSz ) and Zeeman interactions. The fitting results (the
4.798(2) Å-separated MnIV ions that interact through the solid lines in Figure 12) were obtained with parameters D
long four-bond superexchange pathways.
= –0.0058 cm–1 and g = 1.87. An alternative fit with S = 8
For complex 4, as shown in Figure 11, the χMT value of or 6 has been done but no improved results were obtained.
22.65 cm3 Kmol–1 at room temperature is close to the value The ground state of S = 7 is significantly lower than that
of 24 cm3 Kmol–1 for eight noninteracting MnIII centers found in [Mn8CeO8(O2CMe)12(H2O)4] (S = 16) but is
(CeIV is diamagnetic) and increases steadily to larger than that found in the previously reported
28.91 cm3 Kmol–1 at approximately 50 K before rapidly [Mn8CeO8(O2CMe)12(py)4] (S = 4 or 5),[24] which is proba-
decreasing to 6.22 cm3 Kmol–1 at 2 K. This behavior sug- bly as a result of the differences in the crystal symmetry.
gests that predominantly ferromagnetic interactions exist
within the complex and a relatively large spin ground state
Conclusions
is present as judged from the maximum value of
28.91 cm3 Kmol–1. The χMT drop in the low-temperature
region was attributed to the Zeeman effects, zero-field split-
ting, and/or weak intermolecular interactions. The eight
MnIII centers will give total S values in the range of 0 to
16. It was not easy to calculate the various Mn2 pairwise
exchange parameters within 4 owing to the size and low
symmetry of the molecule. In order to determine the nature
of the ground state, the magnetization (M) data were col-
lected in the 2.0 to 10.0 K and 5 to 40 kG ranges and plots
of M/NμB versus H/T are shown in Figure 12. By assuming
that only the ground state is populated, the data were fitted
The reaction of the Mn2+ salts with the oxidizing agent
(NH4)2[Ce(NO3)6] was explored in order to investigate the
effect of a variety of conditions, which included the Mn2+
/
Ce4+ ratio, the acidity of the medium, the solvents used,
and the addition of chelating coligands, on the formation
of the high oxidation state Mn complexes. Four homo- and
heterometallic complexes with various inorganic core top-
ologies, which included the [Mn2O2]4+ rhomb, the Mn3 tri-
angle within [Mn3O4]4+, the nonplanar [Mn8O8]8+ loop
with the centrally trapped Ce4+ ion, and the [Mn2Ce3O6]
cage that contains a Mn2Ce3 trigonal bipyramid, were pre-
pared. It became apparent that CeIV is a suitable oxidant
for oxidizing MnII to MnIII or MnIV. There is a strong ten-
dency for both metals to be incorporated into one coordi-
nation environment to yield mixed-metal clusters at high
oxidation states since the hard O2– ion favors both bridging/
binding to and stabilizing of the hard metal ions, such as
MnIII, MnIV, and CeIV. The magnetic studies established
that the coupling interactions between the Mn atoms in
complexes 1 to 3 are all antiferromagnetic, as is expected
for the MnIV spin carriers, and that the differences in the
interaction magnitudes can be rationalized on the basis of
the structural parameters. Complex 4 displays ferromag-
netic interactions among the eight MnIII atoms, which re-
lates to the high-spin MnIII (d4) configuration with a Jahn–
Teller distortion. This work further demonstrates a suitable
approach to the high-valent oxomanganese complexes by
means of the CeIV oxidation of the MnII salts. The resultant
heterometallic Ce/Mn products are a rich source of new
structural types and are an important addition to the Mn
chemistry, although the general rules still need to be investi-
gated further in future studies.
Figure 11. Plot of the product (χMT) vs. temperature (T) for 4; χM
is the molar magnetic susceptibility.
Experimental Section
General Remarks: All of the manipulations were performed under
aerobic conditions and all of the chemicals were of reagent grade
and were used as received. Mn(pta)2·2H2O was prepared from
manganese(II) chloride tetrahydrate, p-toluic acid, and sodium hy-
droxide in a 1:2:2 molar ratio in water and the composition was
confirmed by elemental analysis. The elemental analyses were per-
formed with a Vario EL-III element analyzer. The IR spectra (KBr
pellets, 4000–400 cm–1) were recorded with a Magna-75-FT-IR
spectrophotometer. The variable-temperature magnetic suscep-
Figure 12. Plots of reduced magnetization (M/NμB) vs. H/T for 4
at 40 (᭞), 30 (᭛), 20 (ᮀ), 10 (᭺), and 5 (᭝) kG. The solid lines
represent the fits to the data as was calculated by using Anisofit2.0.
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Eur. J. Inorg. Chem. 2011, 5043–5053

Homo- and Heterometallic Manganese Complexes
tibilities were measured over a temperature range of 2 to 300 K on
crystalline samples with a Quantum Design MPMS-XL SQUID
magnetometer in an applied field of 1 kG (for complex 1) or 10 kG
(for complexes 2–4). Background corrections for the sample holder
and diamagnetic corrections that were estimated with Pascal’s con-
stants for all of the complexes were applied.
(s), 1161 (m), 1124 (w), 1108 (m), 1073 (w), 1057 (w), 1032 (s), 1022
(s), 893 (w), 818 (m), 765 (s), 724 (s), 691 (s), 665 (m), 651 (m), 612
(m), 568 (w), 503 (m), 469 (w), 451 (w), 418 (w) cm–1.
[Mn2Ce3O6(pta)6(NO3)2(dmf)4]·H2O (3): Solid (NH4)2[Ce(NO3)6]
(1.64 g, 3.0 mmol) was added in batches to a stirred red solution
of Mn(pta)2·2H2O (0.72 g, 2 mmol) in EtOH/dmf (35 mL, v/v 6:1)
over a period of 0.5 h, which resulted in a clear brown solution.
The solution was stirred at room temperature for 4 h and then fil-
tered. The filtrate was allowed to stand undisturbed for several
weeks at room temperature, during which time red-brown amorph-
ous crystalline materials formed. The X-ray quality crystals of 3
were obtained by recrystallizing the amorphous materials with
MeCN; yield 0.41 g (22% based on Mn). C60H72Ce3Mn2N6O29
(1871.48): calcd. C 38.51, H 3.88, N 4.49; found C 38.62, H 3.83,
[Mn2O2(pybim)4](NO3)4·11H2O (1): Solid 2-(2-pyridyl)benzimid-
azole (0.78 g, 4 mmol) was added to a stirred red solution of
Mn(pta)2·2H2O (0.72 g, 2 mmol) in EtOH/dmf(20 mL, v/v 3:1).
The solution was stirred for 0.5 h and yielded a pale yellow slurry.
H2O/MeCOOH (20 mL, v/v 3:1) was added to the mixture, which
gave a clear yellow solution. Solid (NH4)2[Ce(NO3)6] (2.74 g,
5 mmol) was then added in small batches to the above solution
over a period of 0.5 h and resulted in a clear dark solution. The
solution was stirred for an additional 6 h, then filtered and the
filtrate was left undisturbed for several weeks at room temperature,
during which time the black hollow column crystals of 1 were de-
posited, collected by filtration, washed with a small amount of
EtOH, and dried in air; yield 0.32 g (23% based on Mn).
C48H58Mn2N16O25 (1368.98): calcd. C 42.11, H 4.27, N 16.37;
N 4.40. Selected IR data (KBr): ν = 3433 (s, br), 1706 (s), 1648 (s),
˜
1610 (s), 1597 (s), 1556 (s), 1524 (s), 1413 (s), 1387 (s), 1292 (m),
1180 (s), 1113 (m), 1021 (m), 961 (w), 847 (m), 764 (s), 693 (m),
623 (s), 567 (s), 503 (w), 471 (m), 408 (w) cm–1.
[Mn8CeO8(pta)12(dmf)4] (4): 4,4Ј-Bipyridine (0.16 g, 1 mmol) was
added to a stirred red solution of Mn(pta)2·2H2O (1.45 g, 4 mmol)
in EtOH/dmf (35 mL, v/v 6:1) and then solid (NH4)2[Ce(NO3)6]
(0.55 g, 1 mmol) was added in small portions over a period of 0.5 h.
The resulting mixture was stirred at room temperature overnight
to yield a dark solution in which a large amount of brown material
was suspended. The mixture was filtered and the filtrate was left
undisturbed to produce several black crystals that were crystallo-
graphically confirmed to be 3. The solid brown residue was dis-
solved in a mixture of dmf/dmso/CHCl3 (30 mL, v/v/v 1:1:1) and
the insoluble substance was discarded by filtration. The greenish-
brown hexagonal column crystals of 4 were formed within one
week following natural evaporation of the dark filtrate. The crystals
were collected by filtration and dried in air; yield 0.40 g (30% based
on Mn). C108H112CeMn8N4O36 (2621.66): calcd. C 49.48, H 4.31,
found C 42.24, H 4.33, N 16.26. Selected IR data (KBr): ν = 3434
˜
(s, br), 1609 (s), 1481 (s), 1457 (s), 1385 (vs), 1324 (s), 1303 (s),
1150 (m), 1116 (m), 1061 (w), 1019 (w), 1007 (w), 992 (m), 975 (m),
908 (w), 820 (s), 790 (m), 752 (s), 693 (s), 655 (m), 582 (m), 501
(w), 435 (m), 417 (w) cm–1.
[Mn3O4(bipy)4(H2O)2][Ce(NO3)5(H2O)][NO3]2·2H2O (2): Solid
2,2Ј-bipyridine (0.62 g, 4 mmol) was added to a stirred red solution
of Mn(pta)2·2H2O (1.08 g, 3 mmol) in EtOH/dmf(35 mL, v/v 6:1),
which resulted in a yellow suspension. Concentrated nitric acid
(63%, 5 mL) was then added and resulted in a clear solution. Solid
(NH4)2[Ce(NO3)6] (5.48 g, 10 mmol) was added in portions over a
period of 1 h, which resulted in a clear dark solution. The solution
was stirred overnight at ambient temperature and then filtered. The
filtrate was left undisturbed for several days, during which time a
large amount of X-ray quality block crystals of 2 were deposited.
The crystals were collected by filtration, washed with water, and
dried in air; yield 1.02 g (67% based on Mn). C40H42CeMn3N15O30
(1517.83): calcd. C 31.65, H 2.79, N 13.84; found C 31.74, H 2.87,
N 2.14; found C 49.59, H 4.34, N 2.09. Selected IR data (KBr): ν
˜
= 3466 (s, br), 1763 (w), 1648 (s), 1599 (s), 1557 (m), 1529 (s), 1511
(s), 1293 (w), 1256 (w), 1211 (w), 1180 (s), 1150 (w), 1110 (m), 1065
(w), 1040 (w), 1021 (m), 848 (m), 808 (w), 765 (s), 694 (m), 676
(m), 622 (s), 573 (s), 513 (w), 467 (s), 415 (m) cm–1.
N 13.75. Selected IR data (KBr): ν = 3401 (s, br), 1763 (w), 1634
X-ray Crystallography: Data for complexes 1 to 4 were collected at
˜
(s), 1603 (s), 1568 (m), 1497 (s), 1448 (s), 138 2 (s), 1306 (s), 1249 293 K with a Rigaku Mercury CCD area-detector diffractometer
Table 5. The selected crystallographic data and refinement details for complexes 1 to 4.
1
2
3
4
Empirical formula
Formula weight
Space group
C48H58Mn2N16O25
1368.98
P2/c
C40H42CeMn3N15O30
1517.83
Cc
C60H72Ce3Mn2N6O29
1871.48
P1
C108H112CeMn8N4O36
2621.66
P42/n
¯
a [Å]
b [Å]
c [Å]
α [°]
13.198(7)
12.370(6)
18.439(10)
90
105.821(9)
90
2896(3)
2
0.535
1416
24.816(2)
15.2835(10)
17.4941(16)
90
122.636(3)
90
5587.4(8)
4
1.569
13.610(3)
17.429(3)
18.111(4)
68.235(7)
71.562(7)
70.043(9)
3660.6(13)
2
20.6466(6)
20.6466(6)
17.6530(10)
90
90
90
7525.2(5)
2
1.005
β [°]
γ [°]
V [Å3]
Z
Absorption coefficient [mm–1]
F(000)
2.247
1860
3040
2668
θ range for data collection [°]
Reflections collected/unique (Rint
Data/restraints/parameters
Goodness-of-fit on F2
R1/wR2 [IϾ2σ(I)]
R1/wR2 [all data]
(Δσ)max./min. [eÅ–3]
2.30–27.50
21245/6610 (0.0460)
6610/9/414
1.007
0.0714/0.2114
0.0896/0.2358
0.737/–0.546
2.37–27.48
21434/9628 (0.0471)
9628/9/820
1.005
0.0513/0.1240
0.0650/0.1367
0.573/–0.656
2.33–27.48
28467/16524 (0.0225)
16524/1/892
1.002
0.0420/0.1053
0.0554/0.1151
1.202/–1.307
2.29–27.48
56985/8615 (0.0362)
8615/0/354
1.002
0.0677/0.1616
0.0747/0.1641
0.952/–1.166
)
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C.-B. Ma, M.-Q. Hu, H. Chen, C.-N. Chen, Q.-T. Liu
FULL PAPER
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with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
The data reduction and cell refinement were performed with the
CrystalClear program,[25] and the absorption correction was ap-
plied by using the SADABS program.[26] The structure was solved
by direct methods and subsequent difference Fourier syntheses and
was refined on F2 by full-matrix least-squares methods with the
SHELXTL-97 program package.[27] All of the non-hydrogen atoms
were refined anisotropically. The aromatic/alkyl hydrogen atoms
were introduced in the calculated positions and were treated as ri-
ding atoms, while the hydrogen atoms that were bonded on the
water molecules in complexes 1 and 2 were located from the differ-
ence Fourier syntheses. The water O atoms (O11, O12, and O13)
in 1 displayed some disorder, so no effort was made to add H atoms
to them. One of the water molecules of crystallization in 3 was
disordered and could not be modelled properly, thus the
SQUEEZE instruction of the program PLATON was carried out
in order to remove its contribution to the overall intensity data.[28]
The crystallographic data for 1–4 are outlined in Table 5.
[4]
[5]
CCDC-833032 (for 1), -833033 (for 2), -833034 (for 3), and -833035
(for 4) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): The bond valence sum calculations for the Mn atoms in 1–4,
two packing diagrams for 1 and 4, two tables containing the hydro-
gen bonds for 1 and 2, and five tables containing the selected bond
angles and/or bond lengths for 1–4 are contained in the Supporting
Information.
[6]
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant numbers 20973172 and 21071145).
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Received: July 12, 2011
Published Online: October 10, 2011
Eur. J. Inorg. Chem. 2011, 5043–5053
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5053