Synthesis, properties and crystal structure of some polyoxometallates containing the tris(hydroxymethyl)-aminomethane cation

Article abstract of DOI:10.1016/S0020-1693(00)00132-8

Polyoxometallates [(CH₂OH)₃CNH₃]₃PMo₁₂O₄₀·5H₂O, [(CH₂OH)₃CNH₃]₃PW₁₂O₄₀·11H₂O, [(CH₂OH)₃CNH₃]₂H₂SiW₁₂O₄₀·10H₂O, [(CH₂OH)₃CNH₃]₄GeW₁₂O₄₀·9H₂O, [C₄H₁₂NO₃]₄SiMo₁₂O₄₀·5H₂O, [(CH₂OH)₃CNH₃]₂H₄P₂W₁₈O₆₂·3H₂O and [(CH₂OH)₃CNH₃]₃H₃As₂W₁₈O₆₂·4H₂O have been synthesized for the first time, purified, and structurally characterized by means of elemental analyses, IR spectrum, electronic spectrum, cyclic voltammogram, ³¹P NMR and X-ray diffraction. The crystal structure of [(CH₂OH)₃CNH₃]₄PMo₁₂O₄₀·5H₂O has been determined by X-ray diffraction. Hydrogen bonding between (CH₂OH)₃CNH₂ and PMo₁₂O₄₀/^3- was inferred from heavy atom distances. The solid reflectance electronic spectra and IR spectra indicate that there is interaction between the H₃PMo₁₂O₄₀ and the organic substrate. The complexes have photosensitivity under irradiation of sunlight to result in charge transfer by oxidation of (CH₂OH)₃CNH₂ and the reduction of the polyoxometallates. We also found that [(CH₂OH)₃CNH₃]₃PMo₁₂O₄₀·5H₂O and [(CH₂OH)₃CNH₃]₃ PW₁₂O₄₀·11H₂O exhibited substantial catalytic activity toward the electro-reduction of bromate and nitrite, respectively. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00132-8

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Inorganica Chimica Acta 305 (2000) 163–171
Synthesis, properties and crystal structure of some
polyoxometallates containing the tris(hydroxymethyl)-
aminomethane cation
LiHua Bi, EnBo Wang *, Lin Xu, RuDan Huang
Department of Chemistry, Northeast Normal Uni6ersity, Changchun 130024, PR China
Received 10 November 1999; accepted 28 March 2000
Abstract
Polyoxometallates [(CH2OH)3CNH3]3PMo12O40·5H2O, [(CH2OH)3CNH3]3PW12O40·11H2O, [(CH2OH)3CNH3]2H2SiW12O40·
10H2O, [(CH2OH)3CNH3]4GeW12O40·9H2O, [C4H12NO3]4SiMo12O40·5H2O, [(CH2OH)3CNH3]2H4P2W18O62·3H2O and [(CH2-
OH)3CNH3]3H3As2W18O62·4H2O have been synthesized for the first time, purified, and structurally characterized by means of
elemental analyses, IR spectrum, electronic spectrum, cyclic voltammogram, 31P NMR and X-ray diffraction. The crystal structure
of [(CH2OH)3CNH3]4PMo12O40·5H2O has been determined by X-ray diffraction. Hydrogen bonding between (CH2OH)3CNH2
3−
40
and PMo12O
was inferred from heavy atom distances. The solid reflectance electronic spectra and IR spectra indicate that there
is interaction between the H3PMo12O40 and the organic substrate. The complexes have photosensitivity under irradiation of
sunlight to result in charge transfer by oxidation of (CH2OH)3CNH2 and the reduction of the polyoxometallates. We also found
that [(CH2OH)3CNH3]3PMo12O40·5H2O and [(CH2OH)3CNH3]3 PW12O40·11H2O exhibited substantial catalytic activity toward
the electro-reduction of bromate and nitrite, respectively. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Polyoxometallate complexes; Electro-catalysis; Molybdenum complexes; Tungsten complexes; Crystal structures
1. Introduction
we know that if there is a donor and a acceptor in a
molecule, it is easy for a charge transfer to happen in
the molecule, and the molecule will probable show
good photochromism and nonlinear optical properties
[7]. We have reported the photochromism and nonlin-
ear optical properties of the charge transfer salts
formed by the H3PMo12O40 and the quinolin-8-ol and
1,10-phenanthroline [8]. Base on the previous work, we
choose tris(hydroxymethyl)aminomethane as electron
donors which combine with Keggin and Dawson to
form seven organic–inorganic salts, among which three
salts exhibit good photochromism under the sunlight to
become charge-transfer salts. And they will be the base
of the new photochromism materials. The tris(hydroxy-
methyl)aminomethane molecule is a kind of biochemi-
cal reagent and chiral molecule. The combination of
seven organic–inorganic salts will supply information
for the break of DNA and the designation of anti-HIV
drugs, antineoplastic drugs, and anti-viral drugs [9].
Although a large amount of literature has reported
the photochromism of polyoxometallates in solution
A recent development of polyoxometallate anions in
materials science deals with the use of these soluble
metal-oxide clusters as inorganic components of new
organic–inorganic hybrid molecular materials based on
electron donor molecules. Most hybrids of this kind are
based on p-electron and organic amine [1]. The donors
combined with polyoxometallate clusters of various
nuclearities, shapes, and charges have resulted in the
preparation of new types of insulating, semiconducting,
or even metallic charge transfer salts having unusual
structural and/or electronic features [2]. The charge
transfer salts have attracted much interest since they
have electrical [3], magnetic [4], optical properties [5],
and the same character as liquid crystals [6]. According
to ‘charge-transfer theory’ and ‘molecule engineering’,
* Corresponding author. Tel.: +86-431-568 5085; fax: +86-431-
568 4009.
E-mail address: wangenbo@public.cc.jl.cn (E. Wang).
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00132-8

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L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
and discussed its mechanism [10,11], the photo-
chromism of solid materials made of polyoxometallates
has been studied rarely. Here we characterized the
charge-transfer salts by IR spectra, solid electronic
spectra and ESR spectra. The results indicated that
electron transfer occurred between the organic ions and
the heteropolyanions under the sunlight; in the same
time the heteropolyanion were reduced and the organic
donors were oxidized. So we can find that not only the
substance formed by the solvent molecules and the
heteropolyanions but also the ionic salts formed by the
organic cations and the heteropolyanions can become
charge-transfer salts.
N, 1.84; P, 1.36; Mo, 50.54; H2O, 3.94. Found: C, 6.19;
N, 1.72; P, 1.34; Mo, 50.46; H2O, 4.15%.
The following polyoxometallate salts were obtained
by the same procedure as described above.
[(CH2OH)3CNH3]3·PW12O40·11H2O (2), yield 62%.
Anal. Calc. for C12H58N3O60PW12: C, 4.15; N, 1.21; P,
0.89; W, 63.59; H2O, 5.71. Found: C, 4.04; N, 1.28; P,
0.91; W, 63.64; H2O, 5.83%.
[(CH2OH)3CNH3]2·H2SiW12O40·10H2O (3), yield
58%. Anal. Calc. for C8H46N2O56SiW12: C, 2.91; N,
0.85; Si, 0.85; W, 66.85; H2O, 5.45. Found: C, 2.87; N,
0.91; Si, 0.89; W, 66.62; H2O, 5.75%.
[(CH2OH)3CNH3]4·GeW12O40·9H2O (4), yield 65%.
Anal. Calc. for C16H66GeN4O61W12: C, 5.38; N, 1.57;
Ge, 2.03; W, 61.81; H2O, 4.54. Found: C, 5.42; N, 1.63;
Ge, 2.08; W, 61.76; H2O, 4.48%.
2. Experimental
[(CH2OH)3CNH3]4·SiMo12O40·5H2O (5), yield 52%
Anal. Calc. for C16H58Mo12N4O57Si: C, 8.01; N, 2.34;
Si, 1.17; Mo, 48.01; H2O, 3.75. Found: C, 7.95; N, 2.38;
Si, 1.23; Mo, 47.95; H2O, 3.83%.
[(CH2OH)3CNH3]2·H4P2W18O62·3H2O (6), yield 43%.
Anal. Calc. for C8H34N2O71P2W18: C, 2.06; N, 0.60; P,
1.33; W, 70.93; H2O, 1.16. Found: C, 2.21; N, 0.66; P,
1.29; W, 71.08; H2O, 1.21%.
[(CH2OH)3CNH3]3·H3As2W18O62·4H2O (7), yield
46%. Anal. Calc. for As2C12H47N3O75W18: C, 2.94; As,
3.07; N, 0.86; W, 67.64; H2O, 1.47. Found: C, 2.69; As,
3.15; N, 0.88; W, 67.85; H2O, 1.43%.
2.1. Materials and methods
H3PMo12O40·nH2O, H3PW12O40·nH2O, H4SiW12O40·
nH2O, H4GeW12O40·nH2O, H4SiMo12O40·nH2O, H6P2-
W18O62·nH2O and H6As2W18O62·nH2O were prepared
by literature methods [12] and characterized by means
of elemental analyses and IR spectra. Other reagents
used were of analytical grade.
The elemental analyses were carried out by means of
an ICP-AES analyzer. The IR spectra (2% sample, in
KBr) were recorded on an Alpha Centaurt FT/IR
spectrometer. The electronic absorption spectra were
measured on a Beckman DU-8B spectrometer. The
cyclic voltammograms were obtained with a CH Instru-
ment (Model 600V) analyzer and an X–Y recorder. A
typical three-electrode cell was used with a glassy car-
bon working electrode, a platinum counter electrode,
and a silver ꢀ silver chloride reference electrode for the
voltammetry experiments. The glassy carbon electrode
was polished with 1.0 and 0.3 mm Al2O3 powders in
turn and then washed ultrasonically. 31P NMR spectra
were recorded on a Varian Unity-400NMR spectrome-
ter. The spectra were obtained with the sample in the 10
mm sample tubes put on the equipment and reported
with respect to external H3PO4 at the operating temper-
ature. TG were measured on a Perkin–Elemer TG-7
thermal analysis system.
2.3. X-ray crystallography
A crystal with approximate dimensions of 0.52×
0.42×0.38 mm was mounted on a glass fiber in an
arbitrary orientation. Data collection were performed
on a Siemens diffractometer with Mo Ka radiation
,
(u=0.71073 A). The heavy atoms were located by the
direct method and the other non-hydrogen atoms were
found from Fourier synthesis. Structure refinement was
carried out by the full-matrix least-squares method and
the crystal and refinement parameters are listed in
Table 1.
3. Results and discussion
2.2. Preparation of [(CH2OH)3CNH3]3·PMo12-
O40·5H2O (1), [(CH2OH)3CNH3=tris(hydroxymethyl)-
aminomethane]
3.1. Synthesis, chemical formula, and properties of 1
The dodecamolybdophosphoric anion forms readily
in an acidic medium and has been well characterized
both in solution and in the solid state. Mixing a solu-
tion of dodecamolybdophosphoric anions and an acidic
solution of (CH2OH)3CNH2 yielded a crystalline mate-
rial characterized as 1 (see above). The NꢀH distances
(CH2OH)3CNH2 (0.1g) was dissolved in a HCl solu-
tion (20 ml, 1.0 M), to which H3PMo12O40·nH2O (0.5 g)
was added. The solution was stirred for 1 h, and then
filtered. The filtrate was kept in a refrigerator. A large
quantity of yellow octahedral crystals was isolated after
several days. After drying in air, 0.3 g (60%) of 1 was
obtained. Anal. Calc. for C12H46Mo12N3O53P: C, 6.32;
,
(0.890 A) observed for the (CH2OH)3CNH2 are in the
range of protonated amines, suggesting that the
(CH2OH)3CNH2 existed in protonated form in 1. If the

L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
165
overall charges of (CH2OH)3CNH3+are thus considered
to be one, the most possible formula of 1 consistent
Table 1
Crystal data and structure refinement for 1
Empirical formula
Formula weight
Temperature (K)
C12H46Mo12N3O54
2278.77
293(2)
P
,
Wavelength (A)
0.71073
monoclinic
Cc
Crystal system
Space group
Unit cell dimensions
,
a (A)
15.148(3)
25.866(5)
15.067(3)
90
118.77(3)
90
5175(2)
4
2.925
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Fig. 2. Drawing of the structure 1. Hydrogen bonds between the
Z
3−
[PMo12O40
]
anion and the [(CH2OH)3CNH3]+ cations are indi-
Dcalc (Mg m−3
)
Absorption coefficient (mm−1
F(000)
)
2.968
4360
cated by dashed lines.
Crystal size (mm)
q Range for data collection (°)
0.52×0.42×0.38
1.57–28.02
Limiting indices
−15h520, −15k534,
−195l517
with the elemental analysis and mass and charge bal-
ance would be [(CH2OH)3CNH3]3PMo12O40·5H2O. This
formula is also consistent with the results of the X-ray
structure determination (see below). The IR spectrum is
similar to that reported for other trianionic molybdo-
phosphate compounds [13].
Reflections collected
7559
Independent reflections
Max. and min. transmission
Refinement method
Data/refinement/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
6930 (Rint=0.0143)
0.59644 and 0.38580
full-matrix least-squares on F2
6930/2/749
1.047
R1=0.0249, wR2=0.0649
R1=0.0262, wR2=0.0653
0.41(3)
3.2. Structure of 1
Absolute structure parameter
Extinction coefficient
0.00133(4)
Largest difference peak and hole 0.688 and −0.655
Fig. 1 shows the structure and labeling scheme of
the dodecamolybdophosphoric anion, while Fig. 2
shows the hydrogen-bonding interactions in the dodeca-
molybdophosphoric anion and (CH2OH)3CNH2 units.
The selected bond lengths and bond angles are listed in
−3
,
(e A
)
Table 2. The bond angles and distances observed for
3−
40
the PMo12O
unit indicate that the geometry is quite
similar to that found in previously reported do-
decamolybdophosphoric salt. It is of Keggin structure
[14]: the central P atom is surrounded by a tetrahedron
whose oxygen vertices are each linked to one of the
four W3O13 groups. Each W3O13 is consisted of three
MoO6 octahedra linked in a triangular arrangement by
sharing edges, and the four W3O13 are linked together
by sharing corners. The PꢀO bond lengths are in
,
the normal range of 1.530–1.538
MoꢀO(terminal) bond lengths 1.661–1.694 A. The
A
and the
,
MoꢀO(bridge) bonds in each MoO6 unit fall into two
well resolved categories: the long pair (1.923–2.057 A)
and the short pair (1.793–1.914 A) of MoꢀO bonds
,
,
which are in the cis positions. The mean bond order is
0.8 for the long ones and 1.2 for the short ones. The
MoꢀOꢀMo bond angles are in the range of 122.2–
157.4°.
3−
Fig. 1. Structure and labeling scheme for [PMo12O40
]
.

166
L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
Table 3
It is interesting to compare bond lengths and angles
3− a
,
Comparison of bond distances (A) and angles (°) in [PMo12O40
]
of 1 with those of some other phosphomolybdates [15].
As shown in Table 3, the increasing sequence of the
mean bond length of the long (l) MoꢀOb/c bonds
[MoꢀOb/c(l)] for the three compounds is C4H9-
NOꢀPMo12 B (CH2OH)3CNH2ꢀPMo12 B C10H8S8ꢀP-
Mo12,whereas the mean bond length of the short (s)
MoꢀOb/c bonds [MoꢀOb/c(s)] are in the sequence of
C4H9NOꢀPMo12 \(CH2OH)3CNH2ꢀPMo12\C10H8-
S8ꢀPMo12, i.e. the distortion of the MoO6 octahedra in
1 is in the middle position of the sequence. Probably
the extent of distortion is roughly parallel to the vol-
umes of the organic molecules [4]. The distortion of the
MoO6 octahedron has made the symmetry of the het-
eropolyanion decreased.
1
2
3
4
Mo−Ob/c(l)
MoꢀOb/c(s)
MoꢀOb/c(lꢀs)
1.923–2.057 1.929–1.974 1.97–2.01 1.96–1.97
(0.134)
[1.983]
(0.045)
[1.953]
(0.04)
[1.99]
(0.01)
[1.965]
1.793–1.914 1.842–1.879 1.81–1.82 1.85–1.87
(0.121)
[1.858]
(0.037)
[1.861]
(0.01)
[1.815]
(0.02)
[1.86]
0.125
0.092
0.175
0.105
MoꢀOb/cꢀMo 122.2–157.4 125.1–152.1
125–152
(27)
[138.5]
(35.2)
[138.08]
(27)
[138.55]
a ( ) differences; [ ] averages. [(CH2OH)3CNH3]3PMo12O40 (1) (this
work), [H3PMo12O40·6DMA·CH3CN] (2) [15], [C10H8S8]8[PMo12O40
(3) [13] and H3PMo12O40 (4) [13].
]
Table 2
,
Selected bond distances (A) and bond angles (°)
It is interesting to observe the interactions between
the three protonated (CH2OH)3CNH2 units and the five
water molecules found in a single crystal and their
interactions with the dodecamolybdophosphoric anion
too. As the three neutral organic molecules with the
Mo(2)ꢀO(40)
Mo(2)ꢀO(28)
Mo(2)ꢀO(36)
Mo(8)ꢀO(35)
Mo(8)ꢀO(34)
Mo(8)ꢀO(30)
PꢀO(14)
1.684(5) Mo(2)ꢀO(6)
1.868(5) Mo(2)ꢀO(29)
2.018(5) Mo(2)ꢀO(13)
1.690(5) Mo(8)ꢀO(8)
1.906(5) Mo(8)ꢀO(24)
2.057(5) Mo(8)ꢀO(3)
1.530(5) PꢀO(18)
1.817(5)
1.970(5)
2.403(4)
1.793(5)
1.924(5)
2.404(4)
1.533(4)
1.531(4)
1.405(11)
1.423(9)
1.426(8)
1.424(8)
1.511(9)
1.504(9)
1.507(12)
1.530(9)
1.521(9)
1.527(10)
same structure were protonated, they reacted with
3−
40
PMo12O
anion and form a polyoxometallate salt,
PꢀO(3)
1.538(5) PꢀO(13)
which displays the following characters: (i) There are
intra- and inter-molecular hydrogen-bonds among the
three organic molecules (N1···O41, 2.790; N1···O43,
2.377; N2···O45, 2.749; N2···O46, 2.833; N3···O47,
2.878; N1···O44, 2.824; N2···O48, 2.710; O44···O48,
2.966; O46···O49, 2.783; O42···OW3, 2.907; O43···OW2,
O(41)ꢀC(2)
O(43)ꢀC(4)
O(45)ꢀC(7)
O(47)ꢀC(10)
O(49)ꢀC(12)
N(2)ꢀC(5)
C(1)ꢀC(2)
C(1)ꢀC(3)
C(5)ꢀC(8)
C(9)ꢀC(10)
C(9)ꢀC(11)
1.446(13) O(42)ꢀC(3)
1.429(11) O(44)ꢀC(6)
1.426(9) O(46)ꢀC(8)
1.425(10) O(48)ꢀC(11)
1.403(10) N(1)ꢀC(1)
1.487(9) N(3)ꢀC(9)
1.511(10) C(1)ꢀC(4)
1.521(11) C(5)ꢀC(6)
1.526(10) C(5)ꢀC(7)
1.528(10) C(9)ꢀC(12)
1.541(10)
,
2.783 A). Three organic molecules exibit the action of
hydrogen-bonds to the water molecules as well:
O42···OW3, 2.907; O43···OW2, 2.783; O44···OW2,
2.704; N3···OW1, 2.825; O48···OW5, 2.652; O47···OW5,
O(40)ꢀMo(2)ꢀO(6) 103.4(2)
O(6)ꢀMo(2)ꢀO(28)
O(6)ꢀMo(2)ꢀO(29)
,
2.759; O49···OW4, 2.757 A. (ii) The inter-molecular
O(40)ꢀMo(2)ꢀO(28) 100.9(2)
O(40)ꢀMo(2)ꢀO(29) 101.2(2)
O(6)ꢀMo(2)ꢀO(36) 157.5(2)
O(29)ꢀMo(2)ꢀO(36) 82.6(2)
97.4(2)
87.3(2)
hydrogen bonds in the organic molecules are not de-
stroyed, indicating weak interaction between the or-
ganic molecules and the heteropolyanion. (iii) Among
O(40)ꢀMo(2)ꢀO(36) 98.2(2)
O(28)ꢀMo(2)ꢀO(36) 84.2(2)
O(40)ꢀMo(2)ꢀO(13) 170.0(2)
O(28)ꢀMo(2)ꢀO(13) 74.1(2)
O(36)ꢀMo(2)ꢀO(13) 72.9(2)
O(35)ꢀMo(8)ꢀO(8) 104.8(3)
O(6)ꢀMo(2)ꢀO(13)
86.0(2)
the three organic molecules, only one of them exhibits
3−
40
O(29)ꢀMo(2)ꢀO(13) 82.5(2)
O(35)ꢀMo(8)ꢀO(34) 101.3(2)
O(35)ꢀMo(8)ꢀO(24) 100.6(2)
O(34)ꢀMo(8)ꢀO(24) 154.9(2)
O(8)ꢀMo(8)ꢀO(30) 159.8(2)
O(24)ꢀMo(8)ꢀO(30) 83.7(2)
O(8)ꢀMo(8)ꢀO(3)
O(24)ꢀMo(8)ꢀO(3)
N(1)ꢀC(1)ꢀC(2)
C(2)ꢀC(1)ꢀC(4)
C(2)ꢀC(1)ꢀC(3)
O(41)ꢀC(2)ꢀC(1)
O(43)ꢀC(4)ꢀC(1)
C(12)ꢀC(9)ꢀN(3)
C(12)ꢀC(9)ꢀC(11)
weak action of hydrogen-bond to the PMo12O
anion
(O49···O40, 2.936), while the other two form much
weaker hydrogen-bonds to the heteropolyanion
(O43···O35, 3.340; O46···O36, 3.475; O46···O40, 3.663
O(8)ꢀMo(8)ꢀO(34)
O(8)ꢀMo(8)ꢀO(24)
95.8(2)
90.3(2)
,
O(35)ꢀMo(8)ꢀO(30) 95.3(2)
O(34)ꢀMo(8)ꢀO(30) 82.3(2)
O(35)ꢀMo(8)ꢀO(3) 167.1(2)
A).
Day and Klemperer found that the triple (Oc) and
87.6(2)
82.5(2)
double (Ob) bridged oxygen atoms are more basic than
terminal oxygen atoms, and that in turn Oc sites are
more basic than Ob sites [16]. Kempf et al. confirmed
that the triple bridged oxygen atoms are more basic
than the double bridged oxygen atoms by means of ab
initio and electrostatic potential calculations [17].We
have studied and reported hydrogen bonding interac-
tions between the C4H11O3N unit and terminal oxygen
atom in dodecamolybdophosphoric anion (O49···O40,
107.3(6)
111.4(7)
110.7(7)
110.4(7)
111.3(7)
109.1(6)
110.8(6)
O(34)ꢀMo(8)ꢀO(3)
O(30)ꢀMo(8)ꢀO(3)
N(1)ꢀC(1)ꢀC(4)
N(1)ꢀC(1)ꢀC(3)
N(4)ꢀC(1)ꢀC(3)
O(42)ꢀC(3)ꢀC(1)
N(2)ꢀC(5)ꢀC(6)
O(46)ꢀC(8)ꢀC(5)
C(10)ꢀC(9)ꢀN(3)
73.5(2)
72.5(2)
107.4(7)
108.2(6)
111.6(7)
113.8(7)
107.8(6)
111.2(6)
108.1(6)

L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
167
Table 4
The IR spectral data of compounds
+
(NH3)as
+
(NH3)s
Compounds
XꢀOa
MꢀOd
MꢀObꢀM
MꢀOcꢀM
H3PMo12O40
[(CH2OH)3CNH3]3
PMo12O40
H3PW12O40
[(CH2OH)3CNH3]3
PW12O40
H4SiW12O40
[(CH2OH)3CNH3]3
H2SiW12O40
H4GeW12O40
[(CH2OH)3CNH3]4GeW12O40
H4SiMo12O40
[(CH2OH)3CNH3]4SiMo12O40
H6P2W18O62
[(CH2OH)3CNH3]2 H4P2W18O62
H6As2W18O62
[(CH2OH)3CNH3]3
(CH2OH)3CNH2
1064
1060
1058
1081
1080
1079
926
923
921
827
826
910
903
1091
1088
864
964
962
959
984
982
978
980
973
972
977
972
964
955
996
980
971
968
867
882
879
889
897
898
881
886
883
894
882
862
860
916
892
828
882
788
788
783
810
812
805
785
798
795
776
770
776
785
796
780
775
770
yellow
blue
1613
1622
1460
1468
white
blue
1625
1622
1463
1471
white
blue
1620
1629
1470
1479
1625
1627
1632
1466
1490
1481
H
3As2W18O62
862
1628
1589
1485
1460
,
2.936 A). We also observed this phenomenon for the
decavanadate guanidinium salt [18].
and part of their vibrational bands have shifted, which
indicate the existence of the organic molecules in the
compounds. These changes are related to the charges of
protonated organic molecules.
3.3. 31P NMR spectrum
The IR spectra indicate that polyoxometallate anions
and the organic substrates have interactions in the solid
state.
The 31P NMR spectrum of an aqueous solution of 1
showed a resonance peak −3.58 ppm with a very slight
shift (0.3 ppm) compared with the 31P NMR spectrum
of an aqueous solution of H3PMo12O40 at −3.9 ppm
[19]. Although (CH2OH)3CNH2 is a chiral molecule,
the doubling lines as described in the literature [20] did
not appear in the 31P NMR spectrum of 1, implying
that the intermolecular interaction between [(CH2OH)3-
CNH3]+ and [PMo12O40]3− is weak.
3.5. Electronic spectra
Comparison of the UV spectra of [(CH2OH)3-
CNH3]3·PW12O40 with that of H3PW12O40 dissolved in
an aqueous solution indicates that the interactions be-
tween (CH2OH)3CNH2 and polyoxometallate are
rather weak and the complexes are almost entirely
disrupted in solution (Fig. 3), but if the solution is
concentrated (Fig. 4), we find that the low-energy tail of
the charge-transfer absorption band has a little red
3.4. IR spectra
The IR spectral data of polyoxometallate salts are
listed in Table 4.
shift, which is similar to the spectrum of the HMPA
3−
40
The title compounds possess characteristic absorp-
tion bands similar to those of HPAs by Table 4 except
that the positions of characteristic bands have shifted a
little. This shows that their primary structures haven’t
been destroyed, but each bond becomes stronger or
weaker in different extents in these organic-inorganic
salts, which agrees with the behaviors of those organic–
inorganic salts formed by HPAs and other electron
donors [20]. Comparing the IR spectra of the irradiated
samples with that of the original salts, the characteristic
vibrational bands appeared in red shifts. The results
indicate that HPAs have obtained some electrons and
formed heteropoly blues [21].
solution of PW12O
[22]. The results indicate that
Fig. 3. Comparison of the electronic spectra of H3PW12O40 and
[(CH2OH)3CNH3]3 PW12O40 in aqueous solution: (a) H3PW12O40; (b)
[(CH2OH)3CNH3]3 PW12O40. All spectra were obtained on 2×10−6
M solutions in a 1 cm cell at r.t.
The vibrational bands in the range of 1200–1700
cm−1 are similar to that of neutral organic molecules,

168
L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
ing to note that the low-energy tail of the charge-trans-
fer absorption band of [(CH2OH)3CNH3]3·PW12O40 has
quite a large red-shift, which is similar to the spectrum
of H4SiW12O40 HMPA [5]. This suggests that the inter-
action between the organic substrate and the poly-
oxometallate anion must play a role in the observed CT
effect if the distance of both is short enough (in concen-
trated solution or the solid state) [23].
The electronic spectra in the solid ulteriorly indicate
that H3PW12O40 and (CH2OH)3CNH2 have an interac-
tion in the solid state.
Fig. 4. Comparison of the electronic spectra of H3PW12O40 and
[(CH2OH)3CNH3]3 PW12O40 in aqueous solution: (a) H3PW12O40; (b)
[(CH2OH)3CNH3]3 PW12O40. All spectra were obtained on 5×10−4
M solutions in a 1 cm cell at r.t.
3.6. Photochromism
By irradiation of three pure solid sample under the
sunlight, [(CH2OH)3CNH3]3PMo12O40, [(CH2OH)3CN-
H3]3PW12O40, and [(CH2OH)3CNH3]2H2SiW12O40 im-
mediately induces intense photochromism in solid state.
The solid reflectance electronic spectra of three irradi-
ated and original samples are listed in Fig. 6. We found
that the O“W charge transfer transition bands show
no evident changes of the position, but only a little
decrease of its intensity. In addition, several new aborp-
tion bands appear in visible region, which are assigned
to the intervalence charge transfer (M5+ “M6+) band
of the heteropoly anions and are characteristic bands of
heteropoly blues [24]. This indicates that electron trans-
fer occurs between organic substrates and heteropoly
anions, resulting in the reduction of heteropoly anion
(MVIO6) into heteropoly blue (MVO5(OH)) with simul-
taneous oxidation of the organic substrate. This case
has been verified by the ESR spectra of the irradiated
sample. The paramagnetic signals were not observed in
the ESR spectrum at room temperature (r.t.), but they
were observed at 77 K. The paramagnetic signals of
sample [(CH2OH)3CNH3]3PMo12O40 and [(CH2OH)3-
CNH3]2H2SiW12O40 are very weak. The signal of sam-
ple [(CH2OH)3CNH3]3PW12O40 is shown as Fig. 7 and
g=1.826,and this is the signal of W5+. The poly-
oxometallate anions are in the section of one electron
reduction. The paramagnetic signals of sample [(CH2-
OH)3CNH3]3PMo12O40 and [(CH2OH)3CNH3]2H2Si-
W12O40 are reduced probably because their lives are too
short or they coupled with each other.
Fig. 5. Solid electronic spectra of the compounds: (a) H3PW12O40; (b)
[(CH2OH)3CNH3]3PW12O40
.
Fig. 6. Solid electronic spectra of the compounds: (a)
[(CH2OH)3CNH3]3PMo12O40; (b) [(CH2OH)3CNH3]3 PW12O40; (c)
[(CH2OH)3CNH3]2H2SiW12O40; (d) [(CH2OH)3CNH3]3 PW12O40. —:
before irradiation; ···: after irradiation.
From the above results, it turns out that under
sunlight, charge transfer between the organic substrates
and the polyoxometallate anions can take place in the
compounds.
Fig. 7. ESR spectra of the irradiated. [(CH2OH)3CNH3]3 PW12O40 at
77 K.
When the irradiated samples are placed in the air and
sheltered from the light, they turn back to their original
color, and recover again when exposed to sunlight. But
if color-changed samples are put under nitrogen atmo-
sphere and sheltered from light, their color does not
fade, indicating that the fading process is a chemical
one and that it is the oxygen in the air that has
reoxidized MV into MVI, not the self redox occurring in
there is a interaction between the organic molecules
and the polyanions and the charge-transfer of
polyoxometallate anion is obviously influenced by the
presence of (CH2OH)3CNH2. Fig. 5 compares the
reflectance electronic spectrum of [(CH2OH)3-
CNH3]3·PW12O40 with that of H3PW12O40. It is interest-

L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
169
the system. Thus it can be speculated that the whole
process may be proceed in the following way [25].
substantial catalytic activity toward the electro-reduc-
tion of bromate, but only very weak activity toward
chlorate, which is known otherwise to be more inert
than bromate.
The catalytic reactions were conducted in pH 1.0
solution. The cyclic voltammograms for the reduction
of bromate in 1 solution containing different concentra-
tion of bromate are shown in Fig. 8. From Fig. 8 it can
be seen that by the addition of an equimolar quantity
of NaBrO3 to a solution of 1, the cathodic peak current
of the second wave is enhanced while its anodic coun-
terpart is greatly diminished. It seems clear that the
reduced forms of 1 react with the added bromate. As
the bromate concentration increases, the enhancement
of the second peak current increases, so that the reduc-
tion of bromate appears to be catalyzed by the four-
electron reduction of 1.
3.7. Electrocatalytic reduction of bromate and nitrite
In order to characterize electrocatalytic properties of
compound 1 in an aqueous solution, bromate was
chosen as test reactant for catalytic reduction. The
bromate reduction does not occur at a bare GC elec-
trode, at least not, before the onset of hydrogen evolu-
tion [26]. We found that the compound 1 exhibited
According to the work of Kulesza and Faulkner [27]
and others [28], the catalytic reduction of bromate is
accompanied by the removal of oxygen from the bro-
mate, but only the initial oxygen abstraction step is
considered to be the slowest. Br2 is not a final product,
because at the potentials of the catalytic reduction of
Br2 must be reduced to Br− [29]. In order to verify the
bromide as the final catalytic product, the controlled
potential-electrolysis was performed in the presence of 1
and BrO3− at the potential where the catalytic effect
was observed. The solution subjected to the electrolysis
reacted with silver nitrate to yield the yellow precipi-
tates of silver bromide.
Thus, we expect the catalytic reaction to proceed as
follows [30]:
PMo12O430− +2e− +2H+ X H2PMo12O
3−
40
Fig. 8. Cyclic voltammograms for the electrocatalytic reduction of
bromate in 1.0 mM [(CH2OH)3CNH3]3PMo12O40, pH 1.0 solutions
containing different concentration bromate: (a) 0; (b) 0.2 mM; (c) 0.4
H2PMo12O460− +2e− +2H+ X H4PMo12O
3−
40
mM; (d) 0.6 mM; (e) 0.8 mM. Scan rate=50 mV s−1
.
H4PMo12O40 3−+BrO−3 “H2PMo12O430−+Br−+H2O
The direct electroreduction of nitrite ion to ammonia
requires a large overpotential at most electrode surfaces
but the reduction can be catalyzed in aqueous solution
by various transition-metal complexes [31]. By contrast,
the heteropoly compounds examined in this study ap-
pear to accomplish a multiple-electron reduction of
nitrite and nitric oxide to ammonia entirely, while
hydroxylamine is not. In addition, controlled-potential
electrolyses involving a large number of catalyst
turnovers showed that these anions have excellent long-
term durability [32].
With addition of an equimolar quantity of NaNO2 to
a solution of 2, no changes occur in IR and UV spectra
of the reaction between two anions. However, there are
significant changes in the voltammetry of the mixed
solution as shown in Fig. 9. At pH 2 the cathodic peak
current of the third wave is enhanced while its anodic
Fig. 9. Cyclic voltammograms for the electrocatalytic reduction of
nitrite in 1.0mM [(CH2OH)3CNH3]3PW12O40 and before (—) and
after (···) addition 5.0 mM NO−2 . pH 2.0. Scan rate=50 mV s−1
.

170
L. Bi et al. / Inorganica Chimica Acta 305 (2000) 163–171
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Seven organic–inorganic salts have been synthesized
and the crystal structure of [(CH2OH)3CNH3]3P-
Mo12O40·5H2O was determined. By comparing [(CH2-
OH)3CNH3]3[PMo12O40]·5H2O with a series of organic-
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organic–inorganic salts [(CH2OH)3CNH3]3PMo12-
O40·5H2O and [(CH2OH)3CNH3]3PW12O40·11H2O show
high catalytic activity towards the electrocatalytic re-
duction of bromate in pH 1.0 and nitrite in pH 2.0
solutions, respectively. With the increased concentra-
tion of bromate and nitrite, the cathodic peak current
increased, and the anodic peak current decreased.
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