A novel 1D polyoxometallate-based coordination polymer [{Pr(DMSO) 6(H2O)}(PMo12O40)·CH 3CN]n: Properties, crystal structure and characterization

Article abstract of DOI:10.1016/j.molstruc.2003.11.004

A novel compound, [{Pr(DMSO)₆(H₂O)}(PMo ₁₂O₄₀)·CH₃CN]_n, has been synthesized and characterized by IR and single crystal X-ray structural analysis. It forms an one-dimensional zigzag chain built from alternating polyanions and cationic units through Mo-O_t-Pr-O_t-Mo links in the crystal. The results of the single crystal X-ray diffraction analyses and IR are agreement and both show the metal cation units are coordinately bonded to the Keggin cluster. The low temperature ESR spectrum indicates thermal electron delocalization occurs among the Mo atoms in the compound. The result of CV shows that the title compound undergoes five two-electron reversible reductions and that [PMo₁₂O ₄₀]^3- anions are active centre for electrochemical redox in the solutions, while matched cations have small effect on electrochemical redox.

Full text of DOI:10.1016/j.molstruc.2003.11.004

Journal of Molecular Structure 689 (2004) 147–152
www.elsevier.com/locate/molstruc
A novel 1D polyoxometallate-based coordination polymer
[{Pr(DMSO)6(H2O)}(PMo12O40)·CH3CN]n: properties,
crystal structure and characterization
*
Jing-Yang Niu , Mei-Lin Wei, Jing-Ping Wang
Department of Chemistry, School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475001, China
Received 23 September 2003; revised 4 November 2003; accepted 4 November 2003
Abstract
A novel compound, [{Pr(DMSO)6(H2O)}(PMo12O40)·CH3CN]n, has been synthesized and characterized by IR and single crystal X-ray
structural analysis. It forms an one-dimensional zigzag chain built from alternating polyanions and cationic units through Mo–Ot–Pr–Ot–
Mo links in the crystal. The results of the single crystal X-ray diffraction analyses and IR are agreement and both show the metal cation units
are coordinately bonded to the Keggin cluster. The low temperature ESR spectrum indicates thermal electron delocalization occurs among
the Mo atoms in the compound. The result of CV shows that the title compound undergoes five two-electron reversible reductions and that
[PMo12O40]32 anions are active centre for electrochemical redox in the solutions, while matched cations have small effect on electrochemical
redox.
q 2004 Elsevier B.V. All rights reserved.
Keywords: Polyoxometalates; O-ligands; Structure elucidation; Zigzag chain
1. Introduction
configuration make them attractive precursors for the
preparation of charge-transfer hybrid salts of organic–
inorganic character. From the standpoint of molecular
design, we introduced metal ions into the framework
formed by heteropolyanions with the a-Keggin structure
and organic units, so that the one-dimensional (1D), two-
dimensional (2D) or three-dimensional (3D) combination
of polyanions and organic units through metal ions can
be realized. We have reported the crystal structures of 1D
[{Ba (DMSO)5(H2O)}2·(SiMo12O40)], 2D [{Ba(DMSO)3
(H2O)3}{Ba(DMSO)5(H2O)·GeMo12O40}] [18], 1D [{Ca
(DMF)5}2·SiMo12O40]n [19], [{La(DMSO)6(H2O)}(PMo12-
O40)·H2O]n [20] and [{Gd (NMP)6}(PMo12O40)]n [21].
Here, we report the studies of properties and X-ray single
crystal structure analysis of an infinitely extended poly-
oxometallate-based composite, [{Pr(DMSO)6(H2O)}
(PMo12O40)·CH3CN]n. In its structure, the rare-earth ions
are firstly coordinated by six DMSO, one H2O and
polyanion ligands to form a structure unit, then these
structure units are connected through Mo–Ot–Pr–Ot–Mo
[22] links to construct 1D chain-like structure.
The polyoxometallates, characterized by fascinating
structural, electrochemical, catalytic, magnetic, medicinal
and photophysical properties, have been found to be
extremely versatile inorganic building blocks due to their
ability to accept electrons in the fields of designing and
assembling molecule-based materials [1–10] with struc-
tures constructed by so-called anion–cation salts or host–
guest solids. Through electron transfer between organic
units and polyanions, some intermolecular complexes based
on polyoxometallates can enhance SHG non-linear optical
response [11], some enhance electric or magnetic properties
[12–17].
Some of the remarkable molecular features of hetero-
polyanions with the a-Keggin structure, such as the size,
shape, multi-stage redox states and an acentric molecular
*
Corresponding author. Tel.: þ86-378-219-2443; fax: þ86-378-285-
3650.
E-mail address: jyniu@henu.edu.cn (J.-Y. Niu).
0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.11.004

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J.-Y. Niu et al. / Journal of Molecular Structure 689 (2004) 147–152
2. Experimental section
Table 1
Crystal data for the title compound
2.1. Preparation of the title compound
Molecule formula
fw
C14H41Mo12NO47PPrS6
2491.00
All organic solvents and materials used for synthesis
were of reagent grade and used without further purification.
a-H3PMo12O40·n H2O were prepared according to the
literature method [23]. PrCl3 was prepared through adding
concentrated HCl to Pr2O3 (99.9%).
Crystal size (mm3)
Crystal system
Space group
Z
˚
a (A)
˚
b (A)
0.30 £ 0.10 £ 0.08
Monoclinic
P21=c
4
14.033
17.146
˚
c (A)
[{Pr(DMSO)6(H2O)}(PMo12O40)·CH3CN]n. The for-
mation of heteropolyacid praseodymium salts was realized
by replacement of the acids. a-H3PMo12O40·nH2O (3.8 g,
2 mmol) and PrCl3 (2 mmol) were dissolved in 15 ml water,
and the solution was heated at a 80 8C water bath until being
dry. Dried powder (2.0 g) from the first stage was dissolved
in 15 ml acetonitrile–water mixture (5:2, v/v) and 1 ml
DMSO liquid was added at 70 8C with stirring, then the
mixture was allowed to react for 10 min. After cooling, the
solution was filtered-off and left to evaporate at room
temperature. Several days later, yellow crystals were
obtained: yield based on a-H3PMo12O40·n H2O: about
80%. Elemental analysis, found (%) C, 6.99; H, 1.92;
calculated for [{Pr(DMSO)6(H2O)}(PMo12O40)·CH3CN]n
(%); C, 6.74; H, 1.65. The thermal analysis of the title
compound gives a total weight loss of 20.07% below
508 8C, which agrees with the calculated weight loss of
20.35% for one free CH3CN molecule, six DMSO
molecules and one coordinated H2O.
24.907
b (8)
3
V (A )
98.99
˚
5919
dcalc (M g/m3)
2.795
m/mm21
3.597
Total no. of reflections
Independent reflections
R1; wR2 ½I . 2sðIÞꢀ
13577
9581ðRint ¼ 0:0939Þ
0.0695, 0.1594a
0.994, 21.925
23
˚
Largest difference peak and hole, e A
a
w21 ¼ ½s2ðFoÞ þ ð0:0994PÞ ꢀ; ðP ¼ ðFo2 þ 2Fc2Þ=3Þ:
2
2
solution and refinement are listed in Table 1. The structure
was solved by direct methods and refined using full-matrix
least-squares calculations with anisotropic displacement
parameters for all non-hydrogen atoms. All hydrogen atoms
were geometrically fixed to allow riding on the parent atoms
to which they are attached and refined with individual
isotropic displacement parameters. All calculations were
performed using the SHELXTL-97 program [24]. Selected
bond lengths and bond angles are listed in Tables 2 and 3,
respectively.
2.2. Physical measurements
C, H and N elemental analyses were performed on a
Perkin–Elmer 240C elemental analyzer. IR spectra of
samples were recorded in KBr pellets with a Nicolet 170
Table 2
˚
Selected bond lengths (A) for the title compound
SXFT-IR spectrometer in the range of 4000–500 cm21
.
Thermogravimetric analyses (TG/DTA) were performed in
air on a Perkin–Elmer-7 instrument with a heating rate of
10 8C/min from room temperature to 800 8C. The ESR
spectrum of powder of the title compound was recorded on a
Bruker ER-200-D-SRC spectrometer at X-band at
110 K. Cyclic voltammograms were obtained on a
LK98BII electrochemical analyzer at 25 8C. A glass-carbon
(diameter, 3 mm) electrode was employed as the working
electrode. The electrode underwent ultrasonic washing for
5 min, and was washed with water and acetone prior to
every measurement. A platinum rod (diameter, 2 mm)
was used as the counter electrode. The reference electrode
was SCE.
Pr(1)–O(3A)
Pr(1)–O(6A)
Pr(1)–O(5A)
Pr(1)–O(1A)
Pr(1)–O(4A)
Pr(1)–O(2A)
Pr(1)–O(1W)
Pr(1)–O(5)
2.376(11)
2.393(10)
2.403(10)
2.435(13)
2.457(11)
2.507(11)
2.560(12)
2.823(11)
2.841(10)
1.630(11)
1.858(10)
1.891(10)
1.954(11)
1.976(10)
2.459(9)
Mo(3)–O(25)
Mo(3)–O(16)
Mo(3)–O(22)
Mo(3)–O(23)
Mo(3)–O(40)
Mo(4)–O(4)
Mo(4)–O(26)
Mo(4)–O(18)
Mo(4)–O(33)
Mo(4)–O(25)
Mo(4)–O(37)
Mo(5)–O(5)
Mo(5)–O(13)
Mo(5)–O(20)
Mo(5)–O(34)
Mo(5)–O(26)
Mo(5)–O(37)
Mo(6)–O(6)
Mo(6)–O(21)
Mo(6)–O(33)
Mo(6)–O(30)
Mo(6)–O(20)
1.850(10)
1.886(10)
1.935(11)
1.966(10)
2.415(9)
1.683(10)
1.852(10)
1.874(11)
1.949(10)
1.982(11)
2.443(10)
1.712(11)
1.849(10)
1.878(9)
Pr(1)–O(10)
Mo(1)–O(1)
Mo(1)–O(23)
Mo(1)–O(35)
Mo(1)–O(14)
Mo(1)–O(24)
Mo(1)–O(40)
Mo(2)–O(2)
Mo(2)–O(24)
Mo(2)–O(31)
Mo(2)–O(16)
Mo(2)–O(21)
Mo(2)–O(40)
Mo(3)–O(3)
1.968(10)
2.009(10)
2.433(10)
1.669(10)
1.839(10)
1.881(10)
1.965(9)
1.684(10)
1.863(10)
1.886(10)
1.960(11)
1.985(10)
2.439(9)
2.3. Crystal structure analysis
The crystal structure of the title compound was
determined from single crystal X-ray diffraction data. The
intensity data were collected on a Rigaku RAXIS-IV image
1.684(11)
1.993(9)
˚
plate area detector with Mo-Ka ðl ¼ 0:71073 A) at
Symmetry transformation used to generate equivalent atoms: #1 2 x;
y þ 1=2; 2z þ 1=2; #2 2 x; y 2 1=2; 2z þ 1=2:
248(2) K. The crystal parameters and details of the structure

J.-Y. Niu et al. / Journal of Molecular Structure 689 (2004) 147–152
149
Table 3
Selected bond angles (8) for the title compound
O(3A)–Pr(1)–O(6A)
O(3A)–Pr(1)–O(5A)
O(6A)–Pr(1)–O(1A)
O(5A)–Pr(1)–O(1A)
O(5A)–Pr(1)–O(4A)
O(1A)–Pr(1)–O(4A)
O(3A)–Pr(1)–O(2A)
O(6A)–Pr(1)–O(2A)
O(5A)–Pr(1)–O(2A)
O(1A)–Pr(1)–O(2A)
O(3A)–Pr(1)–O(1W)
O(6A)–Pr(1)–O(1W)
81.0(4)
94.2(4)
87.0(4)
80.6(4)
72.9(4)
79.6(4)
74.0(4)
72.9(4)
75.7(4)
73.9(5)
76.6(4)
75.3(4)
O(4A)–Pr(1)–O(1W)
O(6A)–Pr(1)–O(5)
O(1A)–Pr(1)–O(5)
O(4A)–Pr(1)–O(5)
O(1W)–Pr(1)–O(5)
O(3A)–Pr(1)–O(10)
O(5A)–Pr(1)–O(10)
O(4A)–Pr(1)–O(10)
O(1W)–Pr(1)–O(10)
Mo(5)–O(5)–Pr(1)
Mo(10)#2–O(10)–Pr(1)
79.2(4)
64.6(4)
64.9(4)
69.2(3)
64.2(4)
64.2(3)
69.9(3)
65.6(3)
65.9(3)
170.2(6)
154.1(5)
Symmetry transformation used to generate equivalent atoms: #1 2 x;
y þ 1=2; 2z þ 1=2; #2 2 x; y 2 1=2; 2z þ 1=2:
3. Results and discussion
Experiments proved that selection of suitable solvent
mixture was crucial for growing crystals of the title
compounds. Stable crystals of the title compound were
obtained from acetonitrile–water mixture at room tempera-
ture. In addition, exposure of the crystallization solutions to
sunlight should be avoided, since the crystals are sensitive to
sunlight, the reasons for which have been discussed above.
Molecular structure unit and the zigzag chain unit of the
title compound are shown in Figs. 1 and 2. The title
Fig. 2. Chain structure unit of the title compound.
compound consists of 1D infinite zigzag chains built from
alternate polyanions and [Pr (DMSO)6(H2O)]3þ units in the
crystal (Fig. 3). The coordination environment of Pr3þ is
shown in Fig. 4(a). The coordination numbers of Pr3þ ions
are nine, six O atoms are from the DMSO molecules, one O
atom is from the H2O and the remaining two O atoms are
from two adjacent heteropolyanions. The coordination
polyhedra around the Pr3þ ions can be described as a
tricapped trigonal prism (Fig. 4(b)), which has five DMSO
molecules and one coordinated H2O forming two trigonal
planes, and two terminal oxygen atoms of the polyanions
occupying the caps due to the steric hindering of the
polyanions; all these are benefit for the stability of the title
compound. For Pr3þ ions in the title compound, two trigonal
planes are composed by O(1W), O(3A), O(6A), and O(1A),
O(4A), O(5A), the distances between Pr3þ and two trigonal
˚
planes are 1.6775 and 1.6735 A, respectively; coplanar
O(1A), O(3A), O(5A), O(6A) form one square face (the
˚
average plane deviation is 0.007 A), with O(2A) as a cap;
coplanar O(1W), O(1A), O(4A), O(6A) form one square
˚
face (the average plane deviation is 0.0626 A), with O(5) as
a cap; and coplanar O(1W), O(3A), O(4A), O(5A) form one
˚
square face (the average plane deviation is 0.0433 A), with
O(10) as a cap.
In addition, O(5) #1–Pr(1)–O(10) bond angle is
116.8(3)8, near 1208, Mo(5)–O(5)–Pr(1) bond angle is
170.28 and Mo(10)#2–O(10)–Pr(1) bond angle is 154.18,
all resulting in the forming of zigzag chains. The fact that
Fig. 1. Molecular structure unit with atom labeling of the title compound
and ellipsoids are at 30% probability. The hydrogen atoms are omitted for
clarity.

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J.-Y. Niu et al. / Journal of Molecular Structure 689 (2004) 147–152
Fig. 3. View of the one-dimensional chain in the title compound. The hydrogen atoms are omitted for clarity.
two terminal oxygen atoms as coordination atoms, O(5) and
O(10), are in the place of almost 908 in the Keggin-type cage
is better for the smallest steric hindering between the
cations, between the polyanions, and between the cations
and the polyanions, resulting in the closed packing in chain.
1063 cm21; while the vibration band of Mo–Ob bonds
blue-shifted from 870 to 875 cm21. These results indicate
that the polyanions in the title compound still retain the
basic Keggin structure, but are distorted due to the effects
of coordination. This is in agreement with the results of the
single crystal X-ray diffraction analysis. Comparing the IR
spectra with that of free DMSO [18], the nas(SyO)
˚
The Pr–O bond distances range from 2.376 to 2.841 A and
˚
the average bond distance for all Pr–O bonds is 2.607 A.
decreases approximately 50 cm21
, from 1055 to
˚
The fact of the mean Pr–O(DMSO) bond distance, 2.429 A,
˚
is shorter than the Pr–O(H2O) bond distance, 2.560 A, and
˚
the average Pr–Ot bond distance, 2.832 A, indicates that the
1006 cm21; this result verified that the DMSO as ligands
were coordinated with metal ions by means of their
Pr–O(DMSO) bonds are more stable than the Pr–O(H2O)
and Pr–Ot bonds, that is probably because the charge
density over the oxygen of SyO is bigger than that of the
oxygen of H2O and Ot, resulting from the electron-donating
effect of the methyl groups in DMSO molecules and the
electron-withdrawing effect of the polyanions. Based on the
bond-valence parameters [25], we can obtain the bond
orders are 1.86 for O(5) and 2.01 for O(10), which are both
in the normal range, the Pr (1)–O(5) and Pr (1)–O(10)
bond lengths are longer than normal coordinated bond
distance is probably because the MoyOt bond is double
bond.
The P–O bond distances range from 1.531(10) to
˚
1.548(9) A for the title compound, and the O–P–O bond
angles range from 102.7(5)8 to 111.1(5)8. These results
show that the PO4 tetrahedrons are distorted to small extent.
The Mo–Ot bond distances range from 1.630(11) to
˚
1.712(11) A, mean 1.675 A; the Mo–Oa [22] bond distances
˚
˚
˚
range from 2.410(9) to 2.470(9) A, mean 2.437 A; the Mo–
Ob,c [22] bond distances range from 1.839(10) to
˚
2.009(10) A. These results show that the MoO6 octahedra
of the polyanions in the title compounds are severely
distorted because of the influence of the outer coordination
cations.
There are four characteristic asymmetric vibrations
resulting from heteropolyanions with Keggin structure,
namely, nas(MoyOt), nas(Mo–Ob), nas(Mo–Oc) and
nas(P–Oa). Comparing the IR spectra of the title
compound with those of a-H3PMo12O40·nH2O [23] the
vibration band of the MoyOt bond red-shifted from 975 to
956 cm21 for the title compound; the peak duo to the
Mo–Oc bonds red-shifted from 810 to 799 cm21; the
P–Oa bond vibrations are red-shifted from 1067 to
Fig. 4. (a) Coordination environment of Pr3þ in the title compound. The
hydrogen atoms are omitted for clarity and (b) coordination polyhedron
around Pr3þ in the title compound.

J.-Y. Niu et al. / Journal of Molecular Structure 689 (2004) 147–152
151
Fig. 5. The ESR spectrum of Pr3þ ion in the title compound (at 110 K).
oxygen atoms [18]. It can be explained that the decreasing
of the charges density over the oxygen and sulfur atoms
due to O atoms of the SyO bonds coordinated with metal
ions, leads to the electron-donating effect of methyl
increasing. The IR spectra studies show that there is
strong interaction between the polyanions and organic
groups in solid state.
The complex is strongly photosensitive under irradiation
with sunlight, resulting in its colour changing from yellow
into blue due to charge-transfer by oxidation of the DMSO
and reduction of the anion [PMo12O40]32. The ESR
spectrum of powder of the title compound after being
exposed to sunshine was recorded on a Bruker ER-200-D-
SRC spectrometer at X-band at 110 K. The spectrum are
typical of molybdenum (V) [26] at 110 K, g ¼ 1:95: (Fig. 5)
Table 4 summarizes the cathodic and anionic peak
potential (V), the half-wave potential E1=2(V), as well as the
difference potential for a-H3PMo12O40·nH2O and the title
compound. Fig. 6 shows the CVs for a-H3PMo12O40·nH2O
and the title compound carried out at a sensitivity of 10 mA.
Fig. 6. (A) CV of a-H3PMo12O40·nH2O(1 mmol/l) in mixture of 50% 1,4-
dioxane and 50% H2O containing 0.5 mol/l H2SO4 as the supporting
electrolyte, sensitivity 10 mA, V ¼ 50 mV s21 and (B) CV of the title
compound (1 mmol/l) in mixture of 50% 1,4-dioxane and 50% H2O
containing 0.5 mol/l H2SO4 as the supporting electrolyte, sensitivity 10 mA,
V ¼ 50 mV s21
.
For phosphopolyoxomolybdate, it is difficult to obtain well-
defined redox waves in cyclic voltammetry in aqueous
Table 4
Cathodic peak potential ðEcpÞ and anodic ðEapÞ and potential difference for
a-H3PMo12O40·nH2O and the title compound
electrolyes due to the easy hydrolysis of [PMo12O40]32
.
However, it is stabilized by addition of comparatively large
amounts of organic solvents. Thus, cyclic voltammetry is
measured in organic-aqueous solvents (50% 1,4-dioxane
and 50% H2O (secondary distilled water)) containing
0.5 mol/l H2SO4 as the supporting electrolyte. Both a-
H3PMo12O40·nH2O and the title compound undergo five
two-electron reversible reductions. The reversibility criteria
used was DEp ¼ ðEapÞ–ðEcpÞ ¼ 59=n mV (n; number of
electrons). The small difference of E1=2 (V) of two
compounds shows that [PMo12O40]32 anions are active
centre for electrochemical redox in the solutions, while
matched cations have small effect on electrochemical redox.
Compound
Ecp(V)
Eap(V)
DEp(mV)
E1=2(V)
a-H3PMo12O40·nH2O
0.308
0.179
0.339
0.208
31
29
30
31
38
0.324
0.194
20.052
20.201
20.282
20.022
20.17
20.244
20.037
20.186
20.263
The title compound
0.306
0.181
0.341
0.214
35
33
33
32
38
0.326
0.197
20.050
20.208
20.289
20.017
20.176
20.251
20.035
20.192
20.267

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J.-Y. Niu et al. / Journal of Molecular Structure 689 (2004) 147–152
[10] J.-H. Son, H. Choi, Y.-U. Kwon, J. Am. Chem. Soc. 122 (2000)
4. Supplementary material
7432–7433.
[11] (a) J.-Y. Niu, X.-Z. You, C.-Y. Duan, H.-K. Fun, Z.-Y. Zhou, Inorg.
Chem. 35 (1996) 4221.
Crystallographic data for structural analysis have been
deposited with the Cambridge Crystallographic Data Center
with the deposited numbers CCDC 220161. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cridambge, CB2
1EZ, UK (Fax: þ 44-1223-336033; e-mail: deposit@ccdc.
cam.ac.uk)
(b) J.-Y. Niu, J.-P. Wang, W. Chen, C.-H.-L. Kennard, K.-A. Byriel,
J. Coord. Chem. 53 (2001) 153.
(c) J.-Y. Niu, X.-Z. You, Polyhedron (1996).
(d) J.-Y. Niu, B.-Z. Shan, X.-Z. You, H.-K. Fan, B.-C. Yip, Transition
Met. Chem. 24 (1999) 108.
(e) J.-Y. Niu, J.-P. Wang, Y. Bo, D.-B. Dang, Z.-Z. Yuan, J. Chem.
Crystallogr. 30 (2000) 43.
(f) H.-F. Fun, K. Chinnakali, B.-C. Yip, J.-Y. Niu, J.-P. Wang, X.-Z.
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Acknowledgements
This work was supported by the Natural Science
Foundation of Henan Province and the Outstanding Youth
Foundation of Henan Provience.
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