1D-Polyoxometalate-Based Composite Compounds - Design, Synthesis, Crystal Structures, and Properties of [{Ln(NMP)6}(PMo12O 40)]n (Ln = La, Ce, Pr; NMP = N-methyl-2-pyrrolidone)

Article abstract of DOI:10.1002/ejic.200300048

The structures of three 1:1 composite compounds prepared with the polyoxometalate (POM) [PMo₁₂O₄₀]^3- and the cations [Ln(NMP)₆]³⁺ [Ln = La (1), Ce (3), Pr (2); NMP = N-methyl-2-pyrrolidone] exhibit two types of zig-zag chains with alternating cations and anions through Mo-O_t-Ln-O_t-Mo links in the crystal. The compounds were characterized by IR, UV, and ESR spectroscopy, single-crystal X-ray structural analysis, and by a study of their thermal properties. In all the compounds, the La³⁺, Pr³⁺, and Ce³⁺ centers are eight-coordinate with the oxygen atoms in bicapped trigonal-prismatic geometries. The variation of the average Ln-O separations along the La, Ce, and Pr series is consistent with the effects of the lanthanide contraction. The results of the single-crystal X-ray diffraction analyses and the IR spectroscopic studies are in agreement and both show the metal cation units are coordinatively bonded to the Keggin clusters. The UV spectra of the title compounds suggest that their structures are entirely disrupted in dilute solution. Low-temperature ESR spectra indicate that thermal electron delocalization occurs among the Mo atoms in the three compounds. The results of cyclic voltammetry (CV) show that compounds 1, 2, and 3 all undergo five two-electron reversible reductions and that the [PMo₁₂O ₄₀]^3- anions are the active centers for electrochemical redox reactions in solution, while the corresponding cations have only a small effect on the electrochemical properties. Studies of magnetic properties show that 1 is diamagnetic, while 2 and 3 exhibit antiferromagnetic Pr-Pr or Ce-Ce exchange interactions. In addition to the results of CV, the average bond lengths and the magnetic properties of compound 3 indicate that the oxidation state of cerium is iii in the compound of formula [{Ce(NMP) ₆}(PMo₁₂O₄₀)]_n. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200300048

FULL PAPER
1D-Polyoxometalate-Based Composite Compounds ؊ Design, Synthesis,
Crystal Structures, and Properties of [{Ln(NMP)6}(PMo12O40)]n
(Ln ؍ La, Ce, Pr; NMP ؍ N-methyl-2-pyrrolidone)
Jing-Yang Niu,*[a] Mei-Lin Wei,[a] Jing-Ping Wang,*[a] and Dong-Bin Dang[b]
Keywords: Polyoxometalates / Oxo ligands / Lanthanides / Structure elucidation / Zig-zag chain
The structures of three 1:1 composite compounds prepared
their structures are entirely disrupted in dilute solution. Low-
temperature ESR spectra indicate that thermal electron de-
3−
with the polyoxometalate (POM) [PMo12O40
]
and the cat-
ions [Ln(NMP)6]3+ [Ln = La (1), Ce (3), Pr (2); NMP = N- localization occurs among the Mo atoms in the three com-
methyl-2-pyrrolidone] exhibit two types of zig-zag chains
with alternating cations and anions through
pounds. The results of cyclic voltammetry (CV) show that
compounds 1, 2, and 3 all undergo five two-electron revers-
3−
Mo−Ot−Ln−Ot−Mo links in the crystal. The compounds were
characterized by IR, UV, and ESR spectroscopy, single-crystal
X-ray structural analysis, and by a study of their thermal
properties. In all the compounds, the La3+, Pr3+, and Ce3+
centers are eight-coordinate with the oxygen atoms in bi-
capped trigonal-prismatic geometries. The variation of the
average Ln−O separations along the La, Ce, and Pr series is
consistent with the effects of the lanthanide contraction. The
results of the single-crystal X-ray diffraction analyses and the
IR spectroscopic studies are in agreement and both show the
metal cation units are coordinatively bonded to the Keggin
clusters. The UV spectra of the title compounds suggest that
ible reductions and that the [PMo12O40
]
anions are the act-
ive centers for electrochemical redox reactions in solution,
while the corresponding cations have only a small effect on
the electrochemical properties. Studies of magnetic proper-
ties show that 1 is diamagnetic, while 2and 3 exhibit antifer-
romagnetic Pr−Pr or Ce−Ce exchange interactions. In addi-
tion to the results of CV, the average bond lengths and the
magnetic properties of compound 3 indicate that the oxida-
tion state of cerium is iii in the compound of formula
[{Ce(NMP)6}(PMo12O40)]n.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
others enhance electric or magnetic properties.[13Ϫ18] The
interactions existing in the structures of organic substrates
and heteropoly acids or salts are mainly facilitated
by hydrogen bonds, van der Waals forces, and electro-
static forces. However, infinitely extended chains based
on Keggin- or Dawson-type anions and joined
together by W(Mo)ϪOtϪM[19] links are relatively rare.
Structurally characterized notable examples include
[ET]8[PMnW11O39]·2H2O [ET ϭ bis(ethylenedithio)tetra-
thiafulvalene],[20] [NEt3H]5[XCoW11O39]ϫ3H2O (X ϭ P or
As),[21] [Cu(en)2(OH2)]2[H2en][{Cu(en)2}-P2CuW17O61]ϫ
As a rich and diverse class of inorganic cluster systems,
the polyoxometalate ions formed by the early transition me-
tals are noted for their fascinating structural, electrochemi-
cal, catalytic, magnetic, medicinal, and photophysical
properties.[1] In the field of designing and assembling mo-
lecular-based materials[2Ϫ10] with structures based on the
so-called anion-cation salts or host-guest solids, polyoxo-
metalates have been found to be extremely versatile inor-
ganic building blocks due to their ability to accept elec-
trons.[11] So far, many intermolecular complexes based on
polyoxometalates have been synthesized and reported.[11Ϫ23]
Through electron transfer between organic units and poly-
anions, some are able to enhance the SHG (second har-
monic generation) nonlinear optical response[12] while
5H2O (en ϭ ethylenediamine), [Cu(en)2(OH2)]2[Cu(en)]0.5
-
[H2en]0.5[{Cu(en)2}P2CuW17O61]ϫ5H2O,[22] [{Ba(DMSO)5-
(H2O)}2(SiMo12O40)], [{Ba(DMSO)3(H2O)3}{Ba(DMSO)5-
(H2O)GeMo12O40}],[11] and [{Ca(DMF)5}2SiMo12O40]n.[23]
In polyoxometalate chemistry, one of the major chal-
lenges is the ‘‘rational’’ synthesis of new compounds. The
development of rational methods for the modification and
functionalization of polyoxometalate systems may provide
the means to fully exploit the desirable attributes mentioned
above. From the standpoint of molecular design, we at-
tempted to use traditional synthesis to realize such molecu-
lar assemblies. Firstly, we introduced metal ions into the
framework formed by polyanions and organic units, so that
[a]
School of Chemistry and Chemical Engineering,
Henan University,
Kaifeng 475001, P. R. China
E-mail: jyniu@henu.edu.cn.
[b]
State Key Laboratory of Coordination Chemistry,
Nanjing University,
Nanjing 210093, P. R. China
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
160
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DOI: 10.1002/ejic.200300048
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1D-Polyoxometalate-Based Composite Compounds
FULL PAPER
the combination of polyanions and organic units through pounds. Stable crystals of compounds 1, 2, and 3 were ob-
metal ions could be achieved. Here, we chose lanthanide() tained from acetonitrile/water mixtures at room tempera-
metal ions, which have potential uses in bioinorganic chem- ture. Compounds 1, 2, and 3 are all soluble in CH3CN and
istry and materials science.[24Ϫ26] Secondly, we chose NMP its mixtures with water. When 1, 2, and 3 were dissolved in
molecules as electron donors, since polyoxometalate anions pure CH3CN, irregular crystals were obtained because of
are good electron acceptors, thus, they can interact leading rapid loss of solvent, therefore necessitating control of the
to electron transfer. In this paper, we report the synthesis concentration rate of the solution. This problem was over-
and single-crystal X-ray structural analyses of three infi- come by simply adding water to the acetonitrile solutions.
nitely extended polyoxometalate-based composites: Using this method, crystals suitable for X-ray diffraction
[{Ln(NMP)6}(PMo12O40)]n (Ln ϭ La, Ce, Pr; NMP ϭ N- were eventually obtained. In addition, exposure of the solu-
methyl-2-pyrrolidone). In the structures of these com- tions to sunlight was avoided as a result of the photosensi-
posites, the rare-earth ions are firstly coordinated by six tivity of the crystals, the reasons for which have been men-
NMP and polyanionic ligands to form a structural unit. tioned above.
These structural units are then connected through
The structures of the three composite compounds con-
MoϪOtϪLnϪOtϪMo[19] links to form one-dimensional
structed from [PMo12O40]3Ϫ and the cations [Ln(NMP)6]3ϩ
chain-like structures. To the best of our knowledge, the
types of structure exhibited by these compounds have not
been previously reported. This paper reports studies of the
magnetic and electrochemical properties as well as solid-
(Ln ϭ La, Ce, Pr) exhibit two types of zig-zag chains with
alternating cations and anions through MoϪOtϪLnϪ
OtϪMo links in the crystal. The novelty of these composite
compounds is based on the fact that the alternating chains
state structural aspects of three organic-inorganic com-
now contain only one cation per anion instead of two[11,23]
plexes with novel structures, in which infinite stacking as-
(which is the usual case observed in other POM-cation
semblies between polyanions and cationic units have been
chains). [{La(NMP)6}(PMo12O40)]n and [{Pr(NMP)6}-
achieved.
(PMo12O40)]n show the same zig-zag chains (Figure 1). Rep-
resentative molecular structural units and the coordination
environment of compound 1 are shown in Figures 2 and
Results and Discussion
3(a). However, [{Ce(NMP)6}(PMo12O40)]n forms different
Experiments demonstrated that selection of a suitable zig-zag chains (Figure 4) from 1 and 2, with molecular
solvent was crucial for growing crystals of the title com- structural units and the coordination environments shown
Figure 1. View of the one-dimensional chains in 1 and 2; hydrogen atoms are omitted for clarity
Figure 2. Molecular structural unit with atomic labeling for compound 1; ellipsoids are shown at the 30% probability level; hydrogen
atoms are omitted for clarity
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J.-Y. Niu, M.-L. Wei, J.-P. Wang, D.-B. Dang
FULL PAPER
the title compounds. In addition, the O(10)#1ϪLa(1)ϪO(8)
bond angle is 124.27(15)°, the O(10)#1ϪPr(1)ϪO(8) bond
angle is 124.8 (3)°, and the O(6)#1ϪCe(1)ϪO(6) bond an-
gle is 122.4(3)° resulting in the formation of zig-zag chains.
Comparing 3 with 1 and 2, the striking difference lies in
the type of zig-zag chains observed. In 1 and 2, for each
[PMo12O40]3Ϫ unit, there are two terminal coordinating
oxygen atoms, O(10) and O(8) are almost at 90° in the Keg-
gin-type cage while in 3, O(6) and O(6)#1 are almost in a
para position.
The LaϪO, PrϪO, and CeϪO bond lengths range from
˚
˚
2.397 to 2.660 A, from 2.347 to 2.592 A, and from 2.388 to
˚
2.649 A, respectively. The average bond length for all LaϪO
˚
˚
bonds is 2.474 A, which is 0.028 A longer than the
LaϪO1A bond length (2.446 A) in [LaIII(NIT-
˚
BzImH)4](ClO4)3ϫ2THFϫ2H2O (NITBzImH ϭ nitronyl
nitroxide radicals),[27] in which the coordination number is
8 and the coordination polyhedron around the La3ϩ ion
is distorted from dodecahedral toward a square-anti-
˚
prismatic geometry. This bond is 0.006 A shorter than
the LaϪO1A bond (2.48 A) in [LaIII(NITMeBz-
˚
ImH)4](ClO4)3ϫ2THFϫ1H2O (NITMeBzImH ϭ nitronyl
nitroxide radicals),[27] in which the coordination number of
the La3ϩ ion is 8 and the coordination polyhedron around
the La3ϩ ion is a distorted cube. The average bond length
˚
˚
for all PrϪO bonds is 2.429 A, which is 0.099 A longer than
˚
the mean Pr(1)ϪO bond length (2.393 A) of [Pr2(bbpen)2-
(NO3)(H2O)]NO3ϫCH3OH,[28] in which the coordination
number of the Pr3ϩ ion is 8 and the coordination poly-
hedron around the Pr3ϩ ion is a distorted cube. The average
˚
bond length for all CeϪO bonds is 2.47 A, which is 0.064
˚
˚
A shorter than the mean Ce(1)ϪO bond length of 2.53 A
in [(THF)3Ce(SC6F5)3]2,[29] in which the coordination num-
ber of the Ce3ϩ(1) ion is 8. The variation of the LnϪO
average bond separations along the La, Ce, and Pr series
(LaϪO Ͼ CeϪO Ͼ PrϪO) is consistent with the effects of
Figure 3. (a; top) Coordination environment of La3ϩ in 1, hydrogen
atoms are omitted for clarity; (b; bottom) coordination polyhedron
around La3ϩ in 1
in Figures 5 and 6. The coordination numbers of the La3ϩ
,
Pr3ϩ and Ce3ϩ ions are all 8 (six O atoms from the NMP the lanthanide contraction (ionic radius: La3ϩ Ͼ Ce3ϩ
Ͼ
ligands, and the remaining two O atoms from two adjacent Pr3ϩ). The fact that the mean LaϪO(NMOP),
heteropolyanions). The coordination environments of the PrϪO(NMP), and CeϪO(NMP) bond lengths of 2.361,
La3ϩ, Pr3ϩ, and Ce3ϩ ions can be described as bicapped 2.387, and 2.405 A, respectively, are 0.257, 0.169, and 0.244
˚
trigonal-prismatic, an example being that of La3ϩ shown in A shorter than the corresponding LaϪOt, PrϪOt, and
˚
˚
Figure 3(b). This is one of the common coordination geo- CeϪOt distances of 2.618, 2.556, and 2.649 A indicates that
metries for complexes with a coordination number of 8.[27] the LnϪO(NMP) bonds are more stable than the LnϪOt
In the three compounds each La3ϩ, Pr3ϩ, and Ce3ϩ ion has bonds. This is probably because the charge density at the
six O atoms from six NMP molecules forming two trigonal oxygen atom of CϭO is larger than that of Ot, resulting
planes, and two terminal oxygen atoms from the polyanions from the electron-donating effect of the methyl groups
occupying the caps due to the steric hindrance of the poly- through the N atoms in the NMP molecules and the elec-
anions. These factors all contribute towards the stability of tron-withdrawing effect of the polyanions.
Figure 4. View of the one-dimensional chain in 3; hydrogen atoms are omitted for clarity
162
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1D-Polyoxometalate-Based Composite Compounds
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Figure 5. Molecular structural unit with atomic labeling of 3; ellipsoids are shown at the 30% probability level; hydrogen atoms are
omitted for clarity
units to a larger extent than do the La3ϩ and Pr3ϩ ions.
Moreover, the MoϪOtϪLnϪOtϪMo (Ln ϭ La, Ce, Pr)
links are important, firstly because polyoxometalates as li-
gands are electron acceptors which, in some cases, can be
reduced by one or more electrons, giving rise to mixed-val-
ence clusters. Secondly, because rare-earth atoms with mag-
netic properties have been introduced into the structures of
compounds 1, 2, and 3, they may have the potential to be-
come novel solid-state materials in which delocalized elec-
trons coexist with localized magnetic moments, thus afford-
ing the opportunity for the investigation of molecular sys-
tems combining magnetic and conducting properties.
There are four characteristic asymmetric vibrations re-
sulting from heteropolyanions with the Keggin structure,
namely νas(MoϭOt), νas(MoϪOb), νas(MoϪOc), and
νas(PϪOa). Upon comparison of the IR spectra of the title
compounds with those of α-H3PMo12O40ϫnH2O[30] the vi-
Figure 6. Coordination environment of Ce3ϩ in 3; hydrogen atoms
brational bands of the MoϭOt bonds exhibit red shifts
from 975 to 961 cmϪ1 in 1, from 975 to 959 cmϪ1 in 2 and
from 975 to 961 cmϪ1 in 3. The peaks due to the MoϪOc
are omitted for clarity
The PϪO bond lengths range from 1.521 to 1.546 A for bonds show red shifts from 810 to 804 cmϪ1 in 1, from 810
˚
1, from 1.523 to 1.550 A for 2, and from 1.438 to 1.604 A to 801 cmϪ1 in 2 and from 810 to 803 cmϪ1 in 3. Lastly,
for 3, respectively, with the OϪPϪO bond angles ranging the PϪOa bond vibrations show red shifts from 1067 to
from 108.4 to 109.9° for 1, from 108.3 to 110.5° for 2, and 1063 cmϪ1 for 1, 2, and 3, while the vibrational bands of the
from 103.7 to 116.5° for 3, respectively, indicating that the MoϪOb bonds exhibit blue shifts from 870 to 881 cmϪ1in 1
PO4 tetrahedrons are distorted to a different extent in each and 3, and from 870 to 878 cmϪ1 in 2. These results indicate
case. The MoϪOt bond lengths range from 1.663 to 1.689 that the polyanions in the title compounds still retain the
˚
˚
˚
˚
˚
˚
A (av. 1.676 A) for 1, from 1.651 to 1.697 A (av. 1.674 A) basic Keggin structure, but are distorted due to the effects
for 2, and from 1.633 to 1.664 A (av. 1.650 A) for 3 while of coordination. This is in agreement with the results of the
˚
˚
[19]
˚
the MoϪOa
bond lengths range from 2.380 to 2.452 A single-crystal X-ray diffraction studies. In addition, there
˚
˚
˚
(av. 2.416 A) for 1, from 2.394 to 2.462 A (av. 2.428 A) for are three characteristic asymmetric vibrations resulting
2, and from 2.384 to 2.483 A (av. 2.427A) for 3. Finally, the from NMP molecules, namely νas(CϭO) (1666.1 cmϪ1),
˚
˚
[19]
MoϪOb,c bond lengths range from 1.804 to 2.023 A for νas(CϪN) (1506 cmϪ1), and νas(NϪCH3) (1114 cmϪ1).[31]
˚
˚
˚
1, from 1.776 to 2.041 A for 2, and from 1.688 to 2.178 A Upon comparison of the IR spectra of the title compounds
for 3. These observations show that the MoO6 octahedra of with those of NMP molecules,[31] νas(CϭO) is lower by 25.3
the polyanions in the title compounds are severely distorted cmϪ1 (from 1666.1 to 1640.8 cmϪ1) in 1, 28.2 cmϪ1 (from
because of the influence of the outer coordinating cations. 1666.1 to 1637.9 cmϪ1) in 2, and 27.1 cmϪ1 (from 1666.1
Clearly, although the La3ϩ, Pr3ϩ, and Ce3ϩ ions in the title to 1639 cmϪ1) in 3, confirming that the NMP ligands are
compounds have the same coordination number and coor- coordinated to the rare-earth ions by means of their oxygen
dination polyhedra, the Ce3ϩ ions distort the [PMo12O40]3Ϫ atoms. The νas(CϪN) and νas(NϪCH3) frequencies in the
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J.-Y. Niu, M.-L. Wei, J.-P. Wang, D.-B. Dang
FULL PAPER
three compounds rise slightly by 8 cmϪ1 and 2Ϫ6 cmϪ1
,
difference potentials for α-H3PMo12O40ϫnH2O and com-
respectively. This can be explained by the fact that the pounds 1, 2, and 3 (in a mixture of 50% 1,4-dioxane and
charge density at the oxygen and carbon centers is de- 50% H2O containing 0.5 mol/L H2SO4 as the supporting
creased due to the O atoms of the CϭO bonds being coor- electrolyte, sensitivity 10 µA, v ϭ 50 mV·sϪ1, SCE as the
dinated to the rare-earth ions, leading to an increase in the reference electrode). Figure 7 shows the CVs for α-
electron-donating effect of the methyl groups through the H3PMo12O40ϫnH2O and compounds 1, 2, and 3 carried
N atoms. The IR spectroscopic studies show that there are out at a sensitivity of 10 µA. For phospho-polyoxomolybd-
strong interactions between the polyanions and the organic ate it is difficult to obtain well-defined redox waves in cyclic
groups in the solid state.
voltammetry in aqueous electrolytes due to the ease of hy-
The UV spectra of compounds 1, 2, and 3 measured in drolysis of [PMo12O40]3Ϫ. However, it can be stabilized by
aqueous solution are similar to that of α- addition of comparatively large amounts of organic sol-
H3PMo12O40ϫnH2O in the same solution. There are ab- vents. Thus, cyclic voltammetry was measured in organic/
sorption peaks at approximately 205 nm, which can be as- aqueous solvent mixtures (50% 1,4-dioxane and 50% redis-
signed to the OtǞMo charge-transfer bands which are also tilled H2O) containing 0.5 mol/L H2SO4 as the supporting
strong due to the absorption of NMP molecules at the same electrolyte. α-H3PMo12O40ϫnH2O as well as compounds 1,
wavelength.[31] This suggests that although the compounds 2, and 3 all undergo five two-electron reversible reductions.
exist in polymeric form in the solid state, the polymeric The reversibility criterion used was ∆Ep ϭ Eap Ϫ Ecp
chains are entirely disrupted in dilute solution. This has 59/n mV (n ϭ number of electrons). The small difference in
been observed in a previous study.[12]
E1/2 [V] of the three compounds shows that the
ϭ
The three complexes are strongly photosensitive towards [PMo12O40]3Ϫ anions are the active centers for electro-
irradiation with sunlight, resulting in charge-transfer by chemical redox activity in the solutions, while the corre-
oxidation of the NMP moieties and reduction of the sponding cations have only a small effect on the electro-
[PMo12O40]3Ϫ groups. ESR spectra of powders of the title chemical redox behavior. Upon comparison of the results
compounds after exposure to sunshine are typical of of the CVs of the three compounds, it was found that the
molybdenum().[32Ϫ33] At 110 K, g ϭ 1.949 for 1, g ϭ 1.947 effect of [Ce(NMP)6]3ϩ on the electrochemical redox be-
for 2, and g ϭ 1.948 for 3. Observation of an intense, rela- havior of [PMo12O40]3Ϫ is stronger than that of
tively narrow ESR signal from about 3460 to 3500 G (40 [La(NMP)6]3ϩ or [Pr(NMP)6]3ϩ
G peak to peak) for 1, 2, and 3 at 110 K indicates that In aqueous solution, cerium has two accessible oxidation
intramolecular electron transfer is relatively slow on the states namely  and . Only strong oxidants such as per-
.
ESR timescale, and that the electron is delocalized.
manganate and peroxodisulfate can oxidize CeIII. In gen-
eral, the determination of the oxidation state of cerium is
frequently a difficult problem. Previous work has shown
that selected heteropolyoxotungstates can significantly in-
fluence the behavior of complex f-ions. In 1971, Peacock
and Weakley reported that heteropolyoxotungstates could
stabilize tetravalent cerium.[34] This stabilization was appar-
ent from the ease of oxidation of CeIV in [Ce(PW11O39)2],
[Ce(SiW11O39)2] and [Ce(P2W17O61)2]. In aqueous solution,
these oxidations were found to occur at ϩ0.6 to ϩ0.9 V vs.
Ag/AgCl, which is less than the standard reduction poten-
tial of CeIV. More recently, Haraguchi et al.[35] have con-
firmed the shifts of the CeIII/CeIV redox potentials in these
heteropolyoxotungstates. The implication of their work is
that the oxidation of CeIII is possible with the use of only
mild oxidants.[36] In contrast, the shifts of the CeIII/CeIV
redox potentials in heteropolymolybdates have not been re-
ported and consequently we have examined the shift of the
CeIII/CeIV redox potential in [{Ce(NMP)6}(PMo12O40)]n.
Figure 8 (A and B) shows the CVs of α-
Table 1. Cathodic peak potentials (Ecp) and anodic (Eap) and po-
tential differences for α-H3PMo12O40ϫnH2O and compounds 1
and 2 (in a mixture of 50% 1,4-dioxane and 50% H2O containing
0.5 mol/L H2SO4 as the supporting electrolyte, sensitivity 10 µA,
v ϭ 50 mV·sϪ1, SCE as the reference electrode)
Compound
Ecp [V] Eap [V] ∆Ep [mV] E1/2 [V]
α-H3PMo12O40ϫnH2O
0.308
0.179
0.339 31
0.208 29
0.324
0.194
Ϫ0.037
Ϫ0.186
Ϫ0.263
0.326
Ϫ0.052 Ϫ0.022 30
Ϫ0.201 Ϫ0.17 31
Ϫ0.282 Ϫ0.244 38
1
2
3
0.311
0.181
0.341 30
0.212 31
0.197
Ϫ0.050 Ϫ0.019 31
Ϫ0.208 Ϫ0.176 32
Ϫ0.286 Ϫ0.247 39
Ϫ0.035
Ϫ0.192
Ϫ0.267
0.324
0.308
0.181
0.340 32
0.212 31
0.197
Ϫ0.053 Ϫ0.022 31
Ϫ0.208 Ϫ0.179 29
Ϫ0.288 Ϫ0.249 39
Ϫ0.038
Ϫ0.194
Ϫ0.269
0.321
H3PMo12O40ϫnH2O (1 mmol/L) and compound
3
(1 mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O
containing 0.5 mol/L H2SO4 as the supporting electrolyte
with Ag/AgCl as the reference electrode. Figure 8 (C) de-
picts the CV of compound 3 (1 mmol/L) in a mixture of
50% 1,4-dioxane and 50% H2O containing 1.0 mol/L
HClO4 as the supporting electrolyte with Ag/AgCl as the
0.304
0.177
0.338 34
0.209 32
0.193
Ϫ0.058 Ϫ0.021 37
Ϫ0.210 Ϫ0.174 36
Ϫ0.303 Ϫ0.262 41
Ϫ0.040
Ϫ0.192
Ϫ0.283
Table 1 summarizes the cathodic and anionic peak poten- reference electrode. In contrast, it was found that CeIII in
tials [V], the half-wave potentials E1/2 [V], as well as the [{Ce(NMP)6}(PMo12O40)]n cannot be oxidized to CeIV even
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1D-Polyoxometalate-Based Composite Compounds
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Figure 7. (A) CV of α-H3PMo12O40ϫnH2O(1 mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/L H2SO4 as
the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV·sϪ1, SCE as the reference electrode; (B) CV of compound 1 (1 mmol/L) in a
mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/L H2SO4 as the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV·sϪ1
,
SCE as the reference electrode; (C) CV of compound 2 (1 mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/
L H2SO4 as the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV sϪ1, SCE as the reference electrode; (D) CV of compound 3 (1
mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/L H2SO4 as the supporting electrolyte, sensitivity 10 µA,
ν ϭ 50 mV sϪ1, SCE as the reference electrode
under rigorous conditions (ϩ1.5 V vs. Ag/AgCl; electrolyte terial can be observed. Filtration of the mixture to recover
1 mol/L HClO4 purged with O2). These conditions are suf- the solid, which is soluble in 5 mL of dilute HCl (0.5 mol/
ficient to oxidize the noncomplexed CeIII aqua ion. The L), followed by the addition of two drops of concentrated
CeIII ion in compound 3 is not oxidized at potentials higher phosphoric acid to the resultant solution produces a white
than would be required for its oxidation in noncomplexing precipitate which is soluble in 4 mol/L HNO3.
media, indicating a stabilization of CeIII in this case. In ad-
Finally, the average CeIIIϪO distance in compound 3 is
˚
dition, the solution chemistry of cerium() is similar to that 2.47 A, which is essentially the same as the sum of the Ce
of Zr, Hf and, particularly, tetravalent actinides. Thus, CeIII and O ionic radii of 2.49 A for eight-coordinate Ce and
˚
gives phosphates insoluble in 4 mol/L HNO3 and iodates two-coordinate O centers.[36]
insoluble in 6 mol/L HNO3, as well as an insoluble oxalate.
Thus, based on the results of CV, the solution chemistry,
Upon addition of 5 mL of 4 mol/L NaOH to 20 mL of an the average bond lengths and the following magnetic sus-
aqueous solution of 1 mmol of [{Ce(NMP)6}(PMo12O40)]n, ceptibilities, we can conclude that [{Ce(NMP)6}-
which dissociates into small molecules, white deposited ma- (PMo12O40)]n contains a tervalent cerium center.
Eur. J. Inorg. Chem. 2004, 160Ϫ170
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165

J.-Y. Niu, M.-L. Wei, J.-P. Wang, D.-B. Dang
FULL PAPER
Figure 8. (A) CV of α-H3PMo12O40ϫnH2O (1 mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/L H2SO4 as
the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV·sϪ1, Ag/AgCl as the reference electrode; (B) CV of compound 3 (1 mmol/L) in
a mixture of 50% 1,4-dioxane and 50% H2O containing 0.5 mol/L H2SO4 as the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV·sϪ1
,
Ag/AgCl as the reference electrode; (C) CV of compound 3 (1 mmol/L) in a mixture of 50% 1,4-dioxane and 50% H2O containing 1.0
mol/L HClO4 as the supporting electrolyte, sensitivity 10 µA, v ϭ 50 mV sϪ1, Ag/AgCl as the reference electrode
The variable-temperature magnetic susceptibilities of po- χM ϭ Ng2βs (s ϩ 1)/3k(T Ϫ θ) (sCe ϭ 1/2; sPr ϭ 1). The
lycrystalline powders of the three compounds were meas- Weiss constants θPr ϭ Ϫ58.88 and θCe ϭ Ϫ108.78 show
ured with a Cahn-2000 magnetometer between 75 and that there are strong antiferromagnetic PrϪPr or CeϪCe
300 K. The studies of magnetic properties show that com- exchange interactions in 2 and 3.
pound 1 is diamagnetic, while compounds 2and 3 exhibit
antiferromagnetic PrϪPr or CeϪCe exchange interactions.
The results are displayed in the form of 1/χM (the molar
magnetic susceptibility per formula unit) vs. absolute tem-
perature in Figures 9 and 10 for compounds 2 and 3. The
Conclusions
Three one-dimensional composite compounds have
mean molar magnetic susceptibility of [{La(NMP)6}- been prepared based on the polyoxometalate anion
(PMo12O40)]n (1) (Ϫ0.00018 cm3·molϪ1) was used as a dia- [PMo12O40]3Ϫ and the cations [LnNMP]6]3ϩ (Ln ϭ La, Ce,
magnetic correction for the molar magnetic susceptibilities Pr). Single-crystal X-ray diffraction studies showed that the
of 2 and 3. The magnetic data for compounds 2 and 3 have three compounds exhibit two types of zig-zag chains with
been fitted to the CurieϪWeiss law χM ϭ C/(T Ϫ θ) and alternating cations and anions through MoϪOtϪLnϪ
166
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1D-Polyoxometalate-Based Composite Compounds
FULL PAPER
Figure 10. Plot of molar susceptibility vs. absolute temperature for
compound 3 (black squares); the solid line represents the best fit
of the data with value in the text
Figure 9. Plot of inverse molar susceptibility vs. absolute tempera-
ture for compound 2 (black squares); the solid line represents the
best fit of the data with value in the text
OtϪMo links in the crystal. The variation of the LnϪO
average bond separations along the La, Ce, and Pr series is
consistent with the effects of the lanthanide contraction
Syntheses
[{La(NMP)6}(PMo12O40)]n (1), [{Pr(NMP)6}(PMo12O40)]n (2), and
(ionic radius: La3ϩ Ͼ Ce3ϩ Ͼ Pr3ϩ) and the polyanions in [{Ce(NMP)6}(PMo12O40)]n (3): The formation of heteropolyacid
lanthanide salts was accomplished by neutralization of the acids.
α-H3PMo12O40 ϫ nH2O (3.8 g, 2 mmol) and LnCl3 (2 mmol) were
dissolved in water (15 mL) and the solution was heated to dryness
at 80 °C in a water bath. NMP (2 mL) was then added to the
resultant dry solid (2 g) with stirring until the mixture became a
paste. After standing for ca. 15 min, the paste was dissolved in a
minimum of an acetonitrile/water mixture (3:2, v/v). Finally, the
solution was filtered and left standing to concentrated at room tem-
perature. After 1Ϫ2 d, yellow crystals suitable for X-ray diffraction
were obtained; yield based on α-H3PMo12O40 ϫ nH2O ca. 80%.
C30H54LaMo12N6O46P (1) (2555.95): calcd. C 14.08, H 2.11, N
3.29; found C 13.97, H 2.01, N 3.39. C30H54Mo12N6O46PPr (2)
(2557.95): calcd. C 14.07, H 2.11, N 3.28; found C 14.07, H 2.06,
N 3.39. C30H54CeMo12N6O46P (3) (2557.16): calcd. C 4.08, H 2.11,
the title compounds are severely distorted as a result of the
influence of the outer coordinating cations. The polymeric
structures of the compounds are entirely disrupted in dilute
solution, and the cyclic voltammograms measured show
that the three compounds all undergo five two-electron re-
versible reductions. The studies of magnetic properties indi-
cate that 1 is diamagnetic while 2 and 3 exhibit antiferro-
magnetic PrϪPr or CeϪCe exchange interactions.
Experimental Section
Materials: All organic solvents and materials used for synthesis
were of reagent grade and used without further purification. α-
H3PMo12O40ϫnH2O was prepared according to
a literature N 3.28; found C 14.15, H 2.13, N 3.35. TG/DTA data: 1: Weight
method.[30] The metal chlorides LnCl3 (Ln ϭ La, Ce, Pr) were pre-
loss of 23.5% below 506.3 °C without an obvious endothermic peak
and with two exothermic peaks at 285 °C (where the LaϪO bonds
pared by addition of concentrated HCl to Ln2O3 (99.9%).
Table 2. Crystal data for 1, 2, and 3
1
2
3
Empirical formula
Formula mass
Crystal size [mm]
Crystal system
Space group
Z
C30H54Mo12N6O46PLa
2555.95
0.39ϫ0.33ϫ0.29
monoclinic
Pn
C30H54Mo12N6O46PPr
2557.95
0.61ϫ0.48ϫ0.46
monoclinic
Pn
C30H54Mo12N6O46PCe
2557.16
0.30ϫ0.26ϫ0.18
monoclinic
C2/c
2
2
4
˚
a [A]
12.954(3)
13.417(3)
18.810(4)
99.67(3)
3222.6(11)
3.029
12.947(3)
13.294(3)
18.977(4)
100.77(3)
3208.7(11)
3.135
25.524(5)
12.678(3)
22.723(5)
112.10(3)
6813(2)
˚
b [A]
˚
c [A]
β [°]
3
˚
V [A ]
µ [mmϪ1
]
3.407
Total no.of reflections
Independent reflections; Rint
R1, wR2 [I Ͼ 2σ(I)]
10938
10929; 0.0145
0.0278, 0.0686
10472
10472; 0.0000
0.0591, 0.1531
10788
5877; 0.0164
0.0488, 0.1365
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J.-Y. Niu, M.-L. Wei, J.-P. Wang, D.-B. Dang
FULL PAPER
˚
Table 3. Selected bond lengths [A] and angles [°] for 1
Mo(1)ϪO(14)
Mo(1)ϪO(37)
Mo(2)ϪO(2)
Mo(21)ϪO(36)
Mo(2)ϪO(13)
Mo(2)ϪO(17)
Mo(2)ϪO(31)
Mo(2)ϪO(37)
Mo(3)ϪO(3)
Mo(3)ϪO(16)
Mo(3)ϪO(14)
Mo(3)ϪO(13)
Mo(3)ϪO(15)
Mo(3)ϪO(37)
Mo(4)ϪO(4)
Mo(4)ϪO(30)
2.016(5)
Mo(4)ϪO(16)
Mo(4)ϪO(19)
Mo(4)ϪO(38)
Mo(5)ϪO(5)
Mo(5)ϪO(24)
Mo(5)ϪO(20)
Mo(5)ϪO(38)
Mo(6)ϪO(6)
Mo(6)ϪO(18)
Mo(6)ϪO(19)
Mo(6)ϪO(20)
Mo(6)ϪO(21)
Mo(6)ϪO(38)
Mo(8)ϪO(8)
Mo(10)ϪO(10)
O(10)ϪLa(1)#2[a]
O(45)ϪLa(1)ϪO(10)#1
O(46)ϪLa(1)ϪO(10)#1
O(44)ϪLa(1)ϪO(10)#1
O(43)ϪLa(1)ϪO(10)#1
1.989(5)
1.994(5)
2.434(4)
1.682(5)
1.821(5)
1.841(5)
2.438(5)
1.678(5)
1.816(5)
1.846(5)
1.997(5)
2.018(5)
2.452(5)
1.687(4)
1.689(5)
2.576(5)
148.16(18)
72.47(16)
129.8(2)
68.62(17)
2.461(5)
1.669(5)
1.820(5)
1.846(5)
2.001(5)
2.006(5)
2.425(5)
1.663(5)
1.820(5)
1.840(5)
2.005(5)
2.013(5)
2.420(4)
1.665(5)
1.826(5)
73.27(16)
77.15(18)
150.11(16)
124.27(15)
O(41)ϪLa(1)ϪO(10)#1[a]
O(45)ϪLa(1)ϪO(8)
O(41)ϪLa(1)ϪO(8)
O(10)#1ϪLa(1)ϪO(8)
[a]
Symmetry transformation used to generate equivalent atoms: #1: x, y ϩ 1, z; #2: x, y Ϫ 1, z.
break with the oxidation of NMP molecules) and 474 °C (where
the heteropolyanionic cages decompose to the lower oxides of the
metals like MoO3); calcd. 23.27% for six NMP units. 2: Weight loss
of 22.8% below 525 °C without an obvious endothermic peak and
with two exothermic peaks at 287 °C (where the PrϪO bonds break
with two exothermic peaks at 282 °C (where the CeϪO bonds
break with the oxidation of NMP molecules) and 465 °C (where
the heteropoly anionic cages decompose to the lower oxides of the
metals like MoO3); calcd. 23.26% for six NMP units.
with the oxidation of NMP molecules) and 493 °C (where the Methods: C, H, and N elemental analyses were performed with a
heteropolyanionic cages decompose to the lower oxides of the met- PerkinϪElmer 240C elemental analyzer. IR spectra of samples
als like MoO3); calcd. 23.27% for six NMP units. 3: Weight loss of were recorded in KBr pellets with a Nicolet 170 SXFT-IR spec-
23.6% below 530 °C without an obvious endothermic peak and
trometer in the range of 4000Ϫ500cmϪ1. UV spectra were recorded
˚
Table 4. Selected bond lengths [A] and angles [°] for 2
Pr(1)ϪO(3A)
Pr(1)ϪO(1A)
Pr(1)ϪO(4A)
Pr(1)ϪO(2A)
Pr(1)ϪO(10)#1[a]
Pr(1)ϪO(8)
2.401(12)
2.404(9)
2.409(10)
2.416(10)
2.519(10)
2.592(11)
1.667(10)
1.802(10)
1.834(9)
2.012(11)
2.023(10)
2.441(9)
1.651(10)
1.814(10)
1.853(11)
1.989(9)
2.006(9)
2.409(9)
1.666(11)
1.819(10)
1.822(10)
1.997(11)
2.041(10)
2.432(9)
1.696(11)
1.829(10)
1.830(9)
1.986(9)
124.8(3)
Mo(4)ϪO(16)
Mo(4)ϪO(38)
Mo(5)ϪO(5)
Mo(5)ϪO(24)
Mo(5)ϪO(20)
Mo(5)ϪO(26)
Mo(5)ϪO(22)
Mo(5)ϪO(38)
Mo(6)ϪO(6)
Mo(6)ϪO(18)
Mo(6)ϪO(19)
Mo(6)ϪO(21)
Mo(6)ϪO(20)
Mo(6)ϪO(38)
Mo(8)ϪO(8)
Mo(10)ϪO(10)
1.998(9)
2.416(9)
1.670(9)
1.810(9)
1.844(10)
1.999(9)
1.999(10)
2.423(8)
1.658(11)
1.802(10)
1.857(9)
1.990(10)
2.000(11)
2.462(9)
1.697(11)
1.695(10)
2.519(10)
Mo(1)ϪO(1)
Mo(1)ϪO(23)
Mo(1)ϪO(17)
Mo(1)ϪO(18)
Mo(1)ϪO(14)
Mo(1)ϪO(37)
Mo(2)ϪO(2)
Mo(2)ϪO(36)
Mo(2)ϪO(13)
Mo(2)ϪO(31)
Mo(2)ϪO(17)
Mo(2)ϪO(37)
Mo(3)ϪO(3)
Mo(3)ϪO(14)
Mo(3)ϪO(16)
Mo(3)ϪO(13)
Mo(3)ϪO(15)
Mo(3)ϪO(37)
Mo(4)ϪO(4)
Mo(4)ϪO(30)
Mo(4)ϪO(22)
Mo(4)ϪO(19)
O(10)#1ϪPr(1)ϪO(8)
O(10)ϪPr(1)#2[a]
O(5A)ϪPr(1)ϪO(10)#1
O(6A)ϪPr(1)ϪO(10)#1
O(3A)ϪPr(1)ϪO(10)#1
O(1A)ϪPr(1)ϪO(10)#1
O(4A)ϪPr(1)ϪO(10)#1
O(2A)ϪPr(1)ϪO(10)#1
O(5A)ϪPr(1)ϪO(8)
O(6A)ϪPr(1)ϪO(8)
O(3A)ϪPr(1)ϪO(8)
O(1A)ϪPr(1)ϪO(8)
O(4A)ϪPr(1)ϪO(8)
O(2A)ϪPr(1)ϪO(8)
145.3(4)
72.0(3)
69.2(3)
74.7(3)
136.9(4)
71.4(4)
76.9(4)
66.8(3)
75.8(4)
148.1(3)
71.0(3)
130.8(4)
[a]
Symmetry transformation used to generate equivalent atoms: #1: x, y ϩ 1, z; #2: x, y Ϫ 1, z.
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1D-Polyoxometalate-Based Composite Compounds
FULL PAPER
˚
Table 5. Selected bond lengths [A] and angles [°] for 3
Ce(1)ϪO(1A)
Ce(1)ϪO(1A)#1[a]
Ce(1)ϪO(2A)#1
Ce(1)ϪO(2A)
Ce(1)ϪO(3A)
Ce(1)ϪO(3A)#1
Ce(1)ϪO(6)
2.388(7)
2.388(7)
2.409(7)
2.409(7)
2.418(6)
2.418(6)
2.649(6)
2.649(6)
1.633(8)
1.809(7)
1.830(7)
1.990(7)
2.000(7)
2.408(11)
2.464(11)
1.662(7)
1.807(8)
1.818(12)
1.888(13)
1.967(13)
1.990(6)
2.113(13)
2.396(10)
1.642(7)
1.688(13)
1.807(7)
1.975(9)
2.034(13)
2.066(12)
2.428(10)
1.639(7)
1.803(9)
1.808(7)
2.001(8)
2.041(12)
2.384(11)
1.657(7)
1.795(9)
Mo(5)ϪO(7)
1.824(7)
1.985(7)
1.995(9)
2.483(11)
2.516(10)
1.657(12)
1.664(6)
1.705(12)
1.866(13)
119.4(4)
144.8(3)
83.3(3)
83.3(3)
144.8(3)
92.1(4)
76.0(3)
74.8(3)
85.9(3)
139.9(3)
74.8(3)
76.0(3)
139.9(3)
85.9(3)
120.1(4)
137.1(3)
75.7(2)
71.1(2)
69.8(2)
Mo(5)ϪO(17)
Mo(5)ϪO(8)#2[a]
Mo(5)ϪO(21)#2
Mo(5)ϪO(22)
Mo(6)ϪO(9)#2
Mo(6)ϪO(6)
Mo(6)ϪO(10)#2
Mo(6)ϪO(11)
Ce(1)ϪO(6)#1
Mo(1)ϪO(1)
Mo(1)ϪO(16)
Mo(1)ϪO(14)
Mo(1)ϪO(15)
Mo(1)ϪO(13)
Mo(1)ϪO(22)
Mo(1)ϪO(20)
Mo(2)ϪO(2)
Mo(2)ϪO(15)
Mo(2)ϪO(11)
Mo(2)ϪO(11Ј)
Mo(2)ϪO(12)
Mo(2)ϪO(18)
Mo(2)ϪO(12Ј)
Mo(2)ϪO(19)
Mo(3)ϪO(3)
Mo(3)ϪO(12)#2
Mo(3)ϪO(17)
Mo(3)ϪO(14)
Mo(3)ϪO(10)
Mo(3)ϪO(10Ј)
Mo(3)ϪO(19)#2
Mo(4)ϪO(4)
O(1A)ϪCe(1)ϪO(1A)#1
O(1A)ϪCe(1)ϪO(2A)#1
O(1A)#1ϪCe(1)ϪO(2A)#1
O(1A)ϪCe(1)ϪO(2A)
O(1A)#1ϪCe(1)ϪO(2A)
O(2A)#1ϪCe(1)ϪO(2A)
O(1A)ϪCe(1)ϪO(3A)
O(1A)#1ϪCe(1)ϪO(3A)
O(2A)#1ϪCe(1)ϪO(3A)
O(2A)ϪCe(1)ϪO(3A)
O(1A)ϪCe(1)ϪO(3A)#1
O(1A)#1ϪCe(1)ϪO(3A)#1
O(2A)#1ϪCe(1)ϪO(3A)#1
O(2A)ϪCe(1)ϪO(3A)#1
O(3A)ϪCe(1)ϪO(3A)#1
O(1A)ϪCe(1)ϪO(6)
O(1A)#1ϪCe(1)ϪO(6)
O(2A)#1ϪCe(1)ϪO(6)
O(2A)ϪCe(1)ϪO(6)
O(3A)ϪCe(1)ϪO(6)
O(3A)#1ϪCe(1)ϪO(6)
O(1A)ϪCe(1)ϪO(6)#1
O(1A)#1ϪCe(1)ϪO(6)#1
O(2A)#1ϪCe(1)ϪO(6)#1
O(2A)ϪCe(1)ϪO(6)#1
O(3A)ϪCe(1)ϪO(6)#1
O(3A)#1ϪCe(1)ϪO(6)#1
O(6)ϪCe(1)ϪO(6)#1
Mo(6)ϪO(6)ϪCe(1)
144.3(3)
70.7(2)
75.7(2)
137.1(3)
69.8(2)
71.1(2)
Mo(4)ϪO(8)
Mo(4)ϪO(18)
Mo(4)ϪO(16)
Mo(4)ϪO(9)
Mo(4)ϪO(21)
Mo(5)ϪO(5)
70.7(2)
144.3(3)
122.4(3)
176.3(4)
Mo(5)ϪO(13)
[a]
Symmetry transformation used to generate equivalent atoms: #1: Ϫ x, y, Ϫz ϩ 1/2; #2: Ϫx Ϫ 1/2, Ϫy Ϫ 1/2, Ϫz.
in aqueous solution with a UV-250 spectrometer in the range of
190Ϫ400 nm. Thermogravimetric analyses (TG/DTA) were per-
tropic displacement parameters. All calculations were performed
using the SHELXTL-97 program.[37] A summary of the X-ray crys-
formed in air with a PerkinϪElmer-7 instrument with a heating tal data for the three compounds is shown in Table 2. Selected bond
rate of 10 °C/min from room temperature to 600 °C. ESR spectra
of powdered samples of the title compounds after exposure to sun-
shine were recorded with a Bruker ER-200-D-SRC spectrometer in
the X-band frequency at 110 K. Cyclic voltammograms were ob-
tained with an LK98BII electrochemical analyzer at 25 °C. A glass-
carbon (diameter ϭ 3 mm) electrode was employed as the working
lengths and bond angles are listed in Tables 3, 4, and 5. CCDC-
201822 (1), -201823 (2), and -201821 (3) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223/336-
electrode. The electrode was subjected to ultrasonic washing for 033; E-mail: deposit@ccdc.cam.ac.uk].
5 min followed by washing with water and acetone prior to every
measurement. A platinum rod (diameter ϭ 2 mm) was used as the
counter electrode. The reference electrode was an SCE or an Ag/
AgCl electrode. A Cahn-2000 magnetometer was used to measure
Acknowledgments
This work was supported by the Natural Science Foundation of
Henan Province and the Outstanding Youth Foundation of
Henan Province.
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