A new sandwich-type polyoxometalates containing mixed transition metals, [Cd2Zn2(H2O)2(P2W15O56)2]16-: Syntheses, spectrochemical characterization and its electrocatalytical activity comparison with [M4(H2O)2(P2W15O56)2]16- (M = Cd2+ and Zn2+) and [P2W15O56]12-

Article abstract of DOI:10.1016/j.ica.2016.01.030

The mixed-metal sandwich-type polyoxometalate [Cd₂Zn₂(H₂O)₂(P₂W₁₅O₅₆)₂]^16- (abbreviated Cd₂Zn₂P₄W₃₀) has been synthesized and characterized by FT-IR, ³¹P NMR, ¹¹³Cd NMR spectroscopies, cyclic voltammetry (CV) and elemental analysis. The electrochemical behavior of this sandwich-type complex in aqueous solution has been investigated, and also systematic comparisons of that have been made by [M₄(H₂O)₂(P₂W₁₅O₅₆)₂]^16- (M = Cd²⁺ and Zn²⁺) (abbreviated M₄P₄W₃₀) and [P₂W₁₅O₅₆]^12- (abbreviated P₂W₁₅). All these compounds show successive reduction processes of the tungsten centers in a negative potential range, but in some cases, the electrochemistry involves redox reactions originating from cadmium ions. These sandwich-type complexes have efficient electrocatalytic activities towards the reduction of nitrite. However, there is a significant decrease in efficiency as the number of cadmium centers in the complex increases.

Full text of DOI:10.1016/j.ica.2016.01.030

Inorganica Chimica Acta 444 (2016) 87–94
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A new sandwich-type polyoxometalates containing mixed transition
16ꢀ
metals, [Cd Zn (H O) (P W O ) ] : Syntheses, spectrochemical
2
2
2
2
2
15 56 2
characterization and its electrocatalytical activity comparison with
1
6ꢀ
2+
2+
12ꢀ
[
M (H O) (P W O ) ]
(M = Cd and Zn ) and [P W O ]
4
2
2
2
15 56 2
2
15 56
⇑
Farrokhzad Mohammadi Zonoz , Fatemeh Aliabadi
Department of Chemistry, Hakim Sabzevari University, Sabzevar 96179-76487, Iran
a r t i c l e i n f o
a b s t r a c t
1
6ꢀ
Article history:
Received 27 August 2015
Received in revised form 8 January 2016
Accepted 16 January 2016
Available online 23 January 2016
The mixed-metal sandwich-type polyoxometalate [Cd Zn (H O) (P W
Cd Zn P W30) has been synthesized and characterized by FT-IR, P NMR,
2 2 4
O
) ]
(abbreviated
2
2
2
2
2
15 56 2
113
31
Cd NMR spectroscopies,
cyclic voltammetry (CV) and elemental analysis. The electrochemical behavior of this sandwich-type
complex in aqueous solution has been investigated, and also systematic comparisons of that have been
1
6ꢀ
2+
2+
12ꢀ
56
made by [M O)
4
(H
2
2
(P
2 15
W O
)
56 2
]
4 4 2 15
(M = Cd and Zn ) (abbreviated M P W30) and [P W O ]
(
abbreviated P W15). All these compounds show successive reduction processes of the tungsten centers
2
Keywords:
in a negative potential range, but in some cases, the electrochemistry involves redox reactions originating
from cadmium ions. These sandwich-type complexes have efficient electrocatalytic activities towards the
reduction of nitrite. However, there is a significant decrease in efficiency as the number of cadmium
centers in the complex increases.
Polyoxometalate
Sandwich complexes
Cadmium
Electrochemistry
Ó 2016 Elsevier B.V. All rights reserved.
1
. Introduction
arsenic (V) analogues of the Wells–Dawson type, [M
4
(OH
2 2
)
1
6ꢀ
II
II
II
II
II
II
2 15 56 2
(As W O ) ]
(M = Mn , Co , Ni , Cu , Zn , Cd ) was reported by
Polyoxometalates (POMs) and their transition metal substituted
Bi et al. [25]. The first sample of a sandwich-type compounds with
less than four transition metals was published by Hill et al. in 2001
derivatives (TMSPs) are a multifaceted and huge group of metal–
oxygen anionic clusters with applications in catalysis, medicine,
materials science and molecular magnetism [1–5]. Among known
POM structural types, sandwich complexes are the largest class
of TMSPs due to the number (2, 3, 4 or 6) and the nature of the
metal centers that could be included between 2 or 3 multivacant
POM fragments [6–17]. The structural properties of sandwich com-
[26]. They reported that [Fe
2 2 2
O14(NaOH ) ] unit was sandwiched
12ꢀ
between two Wells–Dawson [P
2 15 56
W O ]
fragments and formed
16ꢀ
2 2 2 2 15 56 2
[(NaOH ) Fe (P W O ) ]
mixed-metal polyoxometalate. In this
III
compound, the two similar internal locations are occupied by Fe
centers, while two similar external positions are occupied by sodium
ions (Fig. 1B). In 2001, Hill et al. was reported two asymmetric diferric
complexes of sandwich-type mixed-metal polyoxoanions as
12ꢀ
pounds derived from B-
a-[P W
2 15
O
56
]
are among the most well
II III
II
16ꢀ
known because they are more amenable to NMR studies (such as
[TM Fe
2
(P
2
W
15
O
56)(P
2
TM
2
W
13
O
52)] , TM = Cu or Co. These com-
3
1
nꢀ
16ꢀ
2 2 15 56 2
(P W O ) ]
P NMR) than are the analogous Keggin (i.e. B-
a
-[XW
16 unit encap-
(X = PV or As ) trivacant POMs
9
O
34
]
) spe-
pounds were formed by the reaction of [(NaOH
2
)
2
Fe
2+
2+
cies. These complexes are composed of a central M
4
O
unit with Cu or Co ions in aqueous solution [27].
12ꢀ
V
sulated by two B-
a
-[X W
2 15
O
56
]
From then till now, a lot of reports have been published based on
mixed-metal sandwich-type polyoxometalates. For instance, the
(
Fig. 1) [18–22].
In 1983, the first Wells–Dawson derivatives [M
1
6ꢀ
4 2
(OH )
(P W
2 2 15
mixed-metal complexes [(MOH
)
2 2
Fe
2
(P W
2 15
O
56
)
2
]
,
where
1
6ꢀ
2+
2+
2+
II
II
II
II
O
56 2
) ]
(M = Co , Cu , Zn ) was informed by Finke et al. [23].
M = Ni , Mn , Co orZn canbesynthesizedbytheadditionofseveral
first row transition metal cations on [(NaOH ) Fe (P W O ) ]
2 2 2 2 15 56 2
16ꢀ
Kirby and Baker, in 1995, published the first article about cad-
mium-containing Wells–Dawson sandwich-type polyoxometalate
[19,28,29]. These mixed-metal compounds have valuable
significance because of their properties can be easily adjustable by
changing the number and/or nature of metals ‘‘sandwiched” in the
tetra nuclear cluster. The preparations of new POMs probably play
a main role in catalytic and electrocatalytic reactions [30]. Actually,
16ꢀ
as [Cd
4
2
(OH )
2
(P
W
2 15
O
56
)
2
]
[24]. After few years, the first
⇑
Corresponding author.
E-mail addresses: f.zonoz@hsu.ac.ir, fzmohamadi@yahoo.com (F.M. Zonoz).
http://dx.doi.org/10.1016/j.ica.2016.01.030
020-1693/Ó 2016 Elsevier B.V. All rights reserved.
0

8
8
F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
56]1 . (B) Polyhedral representation of [M
2ꢀ
(H
W O ]
2 15 56
O)
(P
W
O
16ꢀ
. (C) Wells–Dawson-based sandwich-type structure,
Fig. 1. (A) Polyhedral representation of B-
a-[P
W
2 15
O
4
2
2
2
15
2ꢀ
56
)
2
]
1
with the cluster off our metal atoms and two water molecules (ball-and-stick) between two [P
referred to as external and internal positions, respectively. (Color online.)
fragments (polyhedra). Typically, green and blue spheres are
it was found that the nature, number and location of the d metal centers
in the POM framework may affect the interactions between redox-
2.2. Synthesis
activecenterswithinthemolecule. In2004, Nadjo etal. reportedthat
2.2.1. Synthesis of Na16[Cd
A variation on the method described by Finke et al. was used to
prepare this complex [23]. A 2.50 g (0.625 mmol) of P 15 was
dissolved in 25 ml of 1 M NaCl solution. To this suspension at
50–60 °C was added mixture of 0.187 g (0.625 mmol) of
Cd(NO and 0.185 g (0.625 mmol) of Zn(NO
2
Zn
2
(H
2
O)
2
(P
2
W
15
O
56
)
2
]ꢁ50H
2
O
nꢀ
the electroreduction of dioxygen by [Fe
(X W
y 2 15
O
56
)
2
]
(where
X = As or P and y = 2, 3 or 4) improved when the number of Fe atoms
2
W
was increased. It is well established that mixed-metal sandwich-
ꢀ
type POMs catalyze the reduction of NO
2
[22,32,33].
a
In the present article, we report the synthesis of a new cad-
mium-containing Wells–Dawson sandwich-type polyoxometalate.
Several attempts to prepare single crystal of Cd Zn P W30 were
2 2 4
3
)
2
2
ꢁ4H O
3
)
2
2
ꢁ6H O
respectively. After heating and stirring for approximately 30 min,
the colorless solution was allowed to stand overnight in a
unsuccessful. Therefore, this compound has been characterized
by elemental analysis, cyclic voltammetry and spectrochemical
3
1
113
methods (FT-IR, P and
Cd NMR). The electrochemical proper-
ties of this compound and electrocatalytic reductions of nitrite
have been investigated in detail by using cyclic voltammetry and
1
6ꢀ
its results have been compared with [M
4
(H
2 2 2 15 56 2
O) (P W O ) ]
2
+
2+
12ꢀ
56
(
M = Cd and Zn ) and [P
. Experimental
.1. Materials and general methods
2 15
W O ]
complexes.
2
2
All reagents were of analytical grade and were used as received
from commercial sources without further purification. The
1
6ꢀ
2+
2+
12ꢀ
56
[
M
4
(H O)
2 2
(P W
2 15
O
56
)
2
]
2 15
(M = Cd and Zn ) and [P W O ]
compounds were prepared according to the published methods
and characterized by elemental analysis and FT-IR spectroscopy
[
4
23,24,34]. FT-IR spectra were recorded in the range of 4000–
00 cm on Shimadzu 8400S FT-IR spectrophotometer using a
ꢀ1
KBr pellet. The NMR spectrum was recorded on a Bruker BRX-
3
1
3
and
00 AVANCE Spectrometer. The resonance frequencies for the
P
1
13
Cd nuclei are 202.46 and 110.92 MHz, respectively. Chemi-
3
1
113
cal shifts for P and
relative to 85% H PO
Cd NMR spectra were externally referenced
4
and 0.1 M cadmium perchlorate, respec-
3
tively. Elemental analyses were carried in an Integra XL Inductively
Coupled Plasma Spectrometer. Water used for all electrochemical
measurements was obtained by passing through a Millipore Q
water purification set. The solutions were deaerated thoroughly
2 2
for at least 10 min by bubbling pure N and kept under N atmo-
sphere during the whole experiment. The electrochemical set-up
was a Metrohm 797VA computrace polarographic analyzer. A con-
ventional three-electrode system was used. The working electrode
was a glassy carbon (GC) electrode. A platinum electrode was used
as counter electrode and an Ag/AgCl (3 M KCl) electrode was used
as reference electrode. All potentials were measured and reported
versus the Ag/AgCl electrode. All voltammetric experiments were
carried out at room temperature.
Fig. 2. Comparison of the IR spectra of the hydrated sandwich heteropolyanions
P W15, Cd P W30, Cd Zn P W30, Zn P W30 and P W .
2 4 4 2 2 4 4 4 2 18

F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
89
Fig. 3. 31P NMR of Cd
P W30 (A) and Cd Zn P W30 (B) polyoxometalates.
4 4 2 2 4
refrigerator at 5 °C. After 1 day, the white powder which formed
As seen in Fig. 2, it is notable that only two bands of (P-O)
ꢀ1
was collected, recrystallized by 0.5 M NaCl aqueous solution and
stretching vibration are observed at 1090 and 1022 cm in the
-P 18 heteropolyanion (assigned to PW ), whereas three bands
are observed at 1130, 1087 and 1010 cm
heteropolyanion. The additional band at 1130 cm can be referred
to the PW subunit [20]. The IR spectra of the Cd 30, Zn
and Cd Zn 30 are almost identical, which indicates a structural
similarity. Moreover, the stretching bands between 950 and
ꢀ1
dried in air. Yield: 1.23 g (0.135 mmol, 43.3%). IR (KBr, cm ):
081, 1045 (P-O stretching bonds), 925 (W-Oterm stretching
bonds), 881 (W-O stretching bonds), 773, 720 (W-O stretching
a
2
W
9
ꢀ1
1
2 15
in the a-P W
ꢀ1
b
c
3
1
113
bonds). P NMR (D
2
O), (d: ppm): ꢀ4.47, ꢀ14.59.
O), (d: ppm): 42.38. Elemental Anal. Calc. for Na16[Cd
(P O: Requires Na, 4.04; Cd, 2.47; Zn, 1.43; P,
]ꢁ50H
O, 9.88. Found: Na, 4.14; Cd, 2.58; Zn, 1.30; P,
O, 10.04%.
Cd NMR
6
4 4
P W
4 4 30
P W
(
O)
1
1
D
2
2
Zn (H
2
2
-
2
2 4
P W
2
2
W
15
O
56
)
2
2
ꢀ1
.36; W, 60.6; H
.45; W, 61.3; H
2
720 cm are attributed to terminal W-O and bridging W-O-W
bands which are at lower wavenumbers than for the parent satu-
2
2
rated a-P W18 anion.
3
1
3
. Results and discussion
3.2. P NMR characterization
3
1
3.1. IR characterization
P NMR is a well-suited technique to characterize the POMs
and it is suitable to evaluate the purities of POM species. The 31
NMR spectra of Cd P W and Cd Zn P W are reported in Fig. 3
P
The IR spectrum of Cd
2
Zn
2
P
,
4
W
30 is shown in Fig. 2 and is com-
4
4
30
2
2
4
30
pared to Cd
4
P
4
W
30, Zn
4
P
4
W
30
a-P 15 and -P 18 polyoxomet-
2
W
a
2
W
and their data as well as some results of phosphotungstate Daw-
son-type POMs are given in Table 1. Two remarkably different
alates. These compounds show the spectral patterns characteristic
of the Dawson POM framework (Fig. 2) [35].
3
1
types of P NMR are observed in the symmetrical Wells–Dawson
sandwich-type polyoxometalate (C2h symmetry) [19].
Besides the W-O and W-O-W bands appeared at lower than
ꢀ
1
1
000 cm
,
the IR spectra of phosphorus centered polyoxo-
In Table 1, P(1) refers to the resonance attributed to the P atom
closer to the M O metal centers (PW M subunits) and P(2) refers
tungstates have been defined by well-separated P-O stretching
4
16
6
3
ꢀ1
vibrations between 1200 and 1000 cm [36]. By considering to
the local symmetry of the PO unit, the triply-degenerate mode
(for T symmetry) may split into two or three absorption bands
4 16
P atom far away from the M O metal centers (PW9 subunits)
3
1
4
(Fig. 1C). The P NMR spectrum of the diamagnetic compound
F
2
d
by decreasing symmetry (the splitting depends on the deviation
from ideal symmetry). In phosphotungstate Keggin-type lacunary
anions and for the substituted metallophosphate of first row tran-
Table 1
31
P
NMR chemical shift patterns for some phosphotungstate Dawson-type
sition metals it was presented that the splitting
the transition metal interacts more strongly with the oxygen atom
of the PO tetrahedron [37]. For lacunary or substituted unsym-
D
m
declines when
polyoxometalates.
dP (1), ppm
dP (2), ppm
Refs.
4
a
a
-P
-P
2
W
W
15
+0.5
+0.1
ꢀ13.8
[20]
[34]
[20]
[23]
[20]
This work
This work
metrical Wells–Dawson POMs, this situation is more complicated
because of the overlap of vibrational absorption bands deriving
2
15
ꢀ13.3
Zn
Zn
4
P
P
4
W
W
W
W
30
30
30
30
ꢀ3.92
ꢀ4.31
ꢀ3.60
ꢀ4.20
ꢀ4.47
ꢀ13.95
ꢀ14.30
ꢀ14.36
ꢀ14.90
ꢀ14.59
4
from two different PO tetrahedra; although, in desirable instance,
4
4
it is feasible to distinguish between the bands deriving from the
Cd
4
Cd
4
Cd
2
P
P
4
unperturbed subunit PW
ment [38].
9
x y
and those deriving from the PW M seg-
4
2 4 30
Zn P W

9
0
F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
Cd
isomer (Fig. 3A). The most shielded line, at d = ꢀ14.9 ppm, is
related to P(2) of the PW unit. The deshielded line, at
d = ꢀ4.20 ppm, corresponds to P(1) in the PW unit; its inte-
grated intensity appears smaller than that of P(2) due to a slower
relaxation rate (large T
). The 31P NMR spectrum of mixed-metal
Cd Zn
30 shows two peaks (P(1): d = ꢀ4.47 ppm and P(2):
d = ꢀ14.59 ppm) which has remarkable agreement with
30 (M = Cd and Zn) Dawson sandwich complexes (Fig. 3B).
This pattern suggests C2h symmetry for Cd Zn 30 in which var-
ious metallic centers (Cd and Zn) occupy identical positions in
16 metal center framework [28]. The little difference of chem-
ical shifts between Cd Zn 30 with M 30 (M = Cd and Zn)
can be attributed to the different size and effective nuclear charge
4
P
4
W
30 shows two lines, consistent with the presence of a single
probably due to the quadrupolar moment of the chloro ligand.
Thus, they assigned this signal to the external cadmium ions in
1
2ꢀ
9
4
[Cd Cl
2
(B-
9
a-AsW O
34
)
2
]
whereas the other, sharp signal
6
M
3
corresponds to the internal cadmium centers. And also, they
assigned the upfield signal at 11.1 ppm to the external cadmium
ions and the downfield signal at 57.0 ppm to the internal cadmium
1
1
0ꢀ
2
2
P
4
W
ions of [Cd
the
4
(H
Cd NMR of Cd
analogy to the works of Kortz et al., in 2005, the upfield signal at
38.14 ppm can be assigned to the external Cd ions in Cd
while the downfield signal at 70.21 ppm can be referred to the
2
O)
2
(B-
a
4 4
-AsW
9
O
34
)
2
]
[40]. By considering Table 2,
1
13
a
P W30 shows two singlet peaks (Fig. 4A). In
4 4
M P W
2
P
2 4
W
4 4 30
P W ,
M
4
O
internal Cd ions in Cd
4
P
4
W
30. These results are also in good agree-
1
13
10ꢀ
2
2
4
P W
4
4
P W
ment with the
Cd NMR study of [Cd (H O) (B-a-PW O ) ]
4 2 2 9 34 2
by Alizadeh et al. in which they observed two singlet peaks with
different intensities [39]. By attention to the mentioned result,
2
+
2+
of Cd and Zn ions.
1
13
one singlet peak in
attributed to the cadmium ions which are located in external posi-
tion at M 16 metal centers in Dawson-type sandwich POMs.
These results are also in proper agreement with
2 2 4
Cd NMR of Cd Zn P W30 (Fig. 4B) can be
1
13
3.3.
Cd NMR characterization
4
O
1
13
Due to the diamagnetic nature of cadmium, Cd NMR (113Cd and
Cd NMR) is a practical technique for characterizing cadmium-
Cd NMR of
1
11
mixed-metal Keggin-type sandwich POMs in the report of Shojaie
et al. [41].
containing POMs. The Cd NMR data for some cadmium-containing
POMs are given in Table 2. According to the results of Kortz et al.
1
11
12ꢀ
2
the
Cd NMR spectrum of [Cd
4
Cl
2
(B-a
-AsW
9
O
34
)
]
shows
3
3
.4. Electrochemistry
two singlet peaks that the upfield signal is very broad which is
.4.1. Electrochemical behavior
In 2002, Ruhlmann et al. investigated the electrochemical
1
6ꢀ
II
II
II
II
II
behavior of [M
4
(H
2
O)
2
(P
2
W
15
O
56
)
2
]
(M = Mn , Co , Ni , Cu , Zn ,
Table 2
1
13Cd NMR chemical shift patterns for some cadmium-containing POMs.
II
or Cd ) in aqueous solution by cyclic voltammetry at different pH
values [20]. And also, redox behavior and electrocatalytic proper-
was studied by Nadjo et al. in
003 [42]. Moreover, in 2007, Herring and his group reported the
application of H12[(P W O ) Fe (H O) ] as electrocatalyst materi-
2 15 56 2 4 2 2
als for fuel cells [43]. In addition, Ruhlmann and his coworkers inves-
tigated electrochemical behavior of mixed-metal Dawson sandwich
dCdina, ppm
dCdexb, ppm
Refs.
1
6ꢀ
ties of [Cu
(H
4 2
O)
2
(P
2
W
15
O
56
)
2
]
c
Cd
4
Cd
4
Cd
4
P
P
As
4
2
W
W
30
ꢀ49
52.76
57
–
–
ꢀ79
[24]
[39]
[40]
[41]
[41]
This work
This work
2
18
26.25
11.1
III
d
2
W
18
18
18
CdZn
3
P
2
W
30.52
29.16
38.14
42.38
Cd
2
Cd
4
Cd
2
Zn
P
Zn P W
2 4 30
2
P
2
W
4
W
30
70.21
–
II
complexes {Na16[M Co
3
(H
2
O)
2
(P
2
W
15
O
)
56 2
] (M = Mn, Co, Ni, Zn
and Cd)} in aqueous solution (pH = 3.5) [44]. Recently, Mbomekalle
et al. reported the electrochemical behavior of mixed d metal-iron
containing Wells–Dawson sandwich-type complexes: [(FeOH )
2 2
a
b
c
Internal Cd in M
External Cd in M
4
O
16 metal centers.
4
O
16 metal centers.
ꢁ2.5H O was used as an external standard.
Acidified 2 M CdCl
2
2
nꢀ
nꢀ
III
III
d
111
2
M (P
W
2 15
O
56
)
2
]
and [(MOH
2
)
2
Fe
2
(P
2
W
15
O
56
)
2
]
(M = Cr , Mn ,
Cd NMR.
Fig. 4. 113Cd NMR of Cd
P W30 (A) and Cd Zn P W30 (B) heteropolyanions.
4 4 2 2 4

F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
91
Fig. 5. (A) Cyclic voltammograms of Cd
4
P
4
W30, Cd
2
Zn
2
P
4
W30 and Zn
4
P
4 4 4 2 2 4
W30. (B) The comparison of cyclic voltammograms of Cd P W30 and Cd Zn P W30 compounds.
Table 3
Electrochemical data of the Wells–Dawson and sandwich complexes obtained from cyclic voltammetry (scan rate = 50 mVs 1) in 0.5 mol Lꢀ1 acetate buffer (pH = 4.7). All redox
ꢀ
potentials E°, approximated by (Eap + Ecp)/2 for the reversible steps, are given in V vs. Ag/AgCl.
W(1)a
W(2)a
W(3)a
W(4)a
W(5)a
Refs.
b
b
a
a
Cd
Cd
Zn
-P
-P
2
W
W
18
+0.02
ꢀ0.52
ꢀ0.49
ꢀ0.48
ꢀ0.44
ꢀ0.15
ꢀ0.78
ꢀ0.61
ꢀ0.62
ꢀ0.67
ꢀ0.52
–
ꢀ0.79
ꢀ0.84
ꢀ0.83
ꢀ0.67
ꢀ0.90
[34]
[34]
This work
This work
This work
2
15
–
–
–
–
–
c
c
4
P
4
W
30
–
–
2
Zn
2
P
4
W
30
4
P
4
W
30
ꢀ0.98
a
b
c
= |Ea
c
In parentheses: number of exchanged electron(s) n and
Redox potentials are given V vs. SCE.
D
E
p
p
ꢀ E
p
|.
These waves correspond to the redox potential of W and Cd centers.
II
II
II
II
II
Mn , Co , Ni , Cu , Zn and n = 12 or 14) in 1.0 M LiCH
CH COOH (pH = 6.0) and 0.5 M Li SO + H SO (pH = 3.0) [22].
In this work, the electrochemical behavior of Cd
30 and Cd Zn 30 in aqueous solution (0.5 M acetate
buffer, pH = 4.7) have been compared by cyclic voltammetry. The
CVs of 0.5 mM Cd 30, Zn 30 and Cd Zn 30 in 0.5 M
3
COO +
replacing of internal cadmium ions by zinc in the equatorial metal
cluster of Cd 30 compound [15,31].
Fig. 6 shows the cyclic voltammograms of the P
30 and Cd Zn 30 at different scan rates in the 0.5 M
acetate buffer solution. When the scan rate varied from 25 to
750 mV/s, the cathodic peak potentials shifted to the negative
direction and the corresponding anodic peak potentials shifted to
the positive direction with increasing scan rate. The plots of peak
(II) current versus scan rates are shown in the insets of Fig. 6. As
shown in this Figure, at scan rates lower than 100 mV/s, the anodic
3
2
4
2
4
4 4
P W
4
P
4
W
30
,
2 4 4 30
W15, Cd P W ,
Zn
4
P
4
W
2
2
P
4
W
Zn P
4 4
W
2
2 4
P W
4
P
4
W
4
P
4
W
2
2 4
P W
acetate buffer are presented in Fig. 5. All electrochemical data are
collected in Table 3. By considering these data and cyclic voltam-
mograms in Fig. 5A, it is clear that the waves in the range of
ꢀ
0.4 to ꢀ0.6 V in Cd
4 4 2 2 4
P W30 and Cd Zn P W30 corresponds to
tungsten and cadmium centers and the wave at about ꢀ0.8 V can
2 4 4 2 2 4
currents of P W15, Cd P W30 and Cd Zn P W30 compounds are
be attributed to tungsten centers [33,45]. The current intensity of
proportional to the square root of the scan rate, which indicates
that the redox process is diffusion-controlled; however, at scan
rates higher than 100 mV/s, the anodic currents of these com-
pounds are proportional to the scan rate, suggesting that the redox
process is surface-confined. On the other hand, by considering to
Fig. 6D at scan rates lower than 100 mV/s, the anodic currents of
2 2 4 4 4
wave II in Cd Zn P W30 is the half of that in Cd P W30. This result
has a proper agreement with the number of cadmium atoms in
these compounds. The cyclic voltammograms of the compounds
Cd
cathodic potential shift of wave I in Cd
tials and the decreasing of current intensity in Cd
major differences in this pattern, which can be attributed to the
4
P
4
W
30, and Cd
2
Zn
2
P
4
W
30 are compared in Fig. 5B. The slight
Zn 30 to positive poten-
Zn 30 are the
2
2 4
P W
2
P
2 4
W
4 4
Zn P W30 are proportional to the scan rate, suggesting that the
redox process is surface-confined; but, at scan rates higher than

9
2
F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
Fig. 6. The cyclic voltammograms of P
2
W
15 (A), Cd
4 4 2 2 4 4 4
P W30 (B), Cd Zn P W30 (C) and Zn P W30 (D) in 0.5 M acetate buffer solutions at different scan rates (from inner to outer:
ꢀ
1
2
5–750 mV s ). The inset shows plots of the anodic peak currents of wave II against scan rates.
1
00 mV/s, the anodic currents of this compound are proportional
The polyoxometalates
Zn 30 are tested at pH = 4.7 for their activity in the reduc-
tion of nitrite ion (Fig. 7). Clearly, with the addition of NaNO
P
2
W
15
,
Cd
4
P
4
W
30
,
4 4
Zn P W30 and
to the square root of the scan rate, which indicates that the redox
process is diffusion-controlled [46].
Cd
2
2 4
P W
2
to
is
the solution, even for small values of the excess parameter
c (c
0
0
the excess parameter defined here as
c
¼ C
=C
), cathodic
ðNO
x
Þ
ðPOMÞ
ꢀ
3.4.2. Electrocatalysis of NO
2
reduction
current of all waves rose and the corresponding anodic current
declined indicated the existence of an electrocatalytic process in
the examined potential area. The insets of Fig. 7 present three or
four straight lines over a wide range of concentrations with different
slopes, indicating that wave III or IV (the last tungsten wave) has
higher catalytic activity than other waves.
As is known, POMs have been applied extensively in electrocat-
alytic reductions [2]. Direct electroreduction of nitrite ion needs a
large overpotential at most electrode surfaces, but the reduction
can be catalyzed in aqueous solution by various transition metal
complexes [47–50]. In these catalysts, the transition metals show
considerable roles as active sites, and are coordinated to NO form-
ing transition metal nitrosyl complexes as principal intermediates
followed by multi-electron reduction processes [51]. The real
catalytic process yielding ammonia occurs by reduction of this
metal-nitrosyl derivative at more negative potentials where the
necessary number of electrons is provided by the W framework.
By comparison to previous works, the catalytic efficiency (CAT)
of NaMn P W
3
4
30,
NaCo P W
3
4
30
and Co P W
4
4
30
decreases with
increasing pH. For example, in NaCo P W compound the cat-
3
4
30
alytic efficiency, CAT, was 300%, 220%, 88%, 28%, and 0% at
pH = 0.59, 1.77, 2.52, 3.00, and 3.50, respectively; while, no cat-
alytic effect was seen at pHs P 3.5 [32,33]. Moreover, for Cu P W
30
4
4
It is well-known that HNO
2
can disproportionate to form NO and
the catalytic efficiency in the reduction of nitrite ion was observed
in pH = 5 (in the high quantity value of NO2 concentration) which
ꢀ
ꢀ
NO
3
,
can be related to the transition metal’s nature [42]. On the other
hand, the maximum CATs (in the low quantity value of NO2
ꢀ
! NOꢀ þ 2NO þ Hþ þ H
To ensure that the effect of this reaction is minimal, experimen-
3
HNO
2
2
O
3
concentration) for the last tungsten wave of P
2 4 4 30
W15, Cd P W ,
Cd Zn P W and Zn P W in the same at pH = 4.7 are 37.2%,
2
2
4
30
4
4
30
c
tal data relevant to nitrite catalysis have been always obtained
from freshly prepared solutions, and also our experiments have
been carried out at pH = 4.7 in order to avoid complications associ-
14.2%, 36.6% and 174.7% respectively. The graphs of CATs vs.
c
30
and CATs vs. cadmium numbers of Cd P W30, Cd Zn P W
4
4
2
2
4
and Zn P W are shown in Fig. 8. According to this figure, by
4
4
30
ated with the possible disproportionate of HNO
efficiency is defined as CAT = 100 ꢂ {[I (POM, NaNO
(POM)}, where I (POM) and I (POM, NaNO ) are the cathodic
2
[52]. Catalytic
reducing the number of cadmium ions in the central tetranuclear
cluster of sandwich-type POMs, catalytic efficiency increases.
These results have a suitable agreement with the electrochemical
behavior of cadmium-containing monolacunary Wells–Dawson
polyoxometalates [53].
p
2
) ꢀ I (POM)]/
p
I
p
p
p
2
peak currents in the absence and in the presence of NaNO
respectively.
2
,

F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
93
Fig. 7. Cyclic voltammograms (scan rate: 75 mV s 1) for the electrocatalytic reduction of NOꢀ
solution: (A) P 15 (from up to bottom = 0, 1, 2, 4), (B) Cd 30 (from up to bottom = 0, 1, 2, 5), (C) Cd
up to bottom = 0, 1, 2, 3, 4). The excess parameter defined as
ꢀ
with a 5 ꢂ 10 M solution of polyoxometalates in a 0.5 M acetate buffer
ꢀ4
2
2
W
c
c
4
P
4
W
c
2
Zn
2
P
4
W
30 (from up to bottom
c
= 0, 1, 2, 5) and (D) Zn
4
P
4
W30 (from
0
ꢀ
0
ꢀ
c
= C (NO
2
)/C (POM). Inset: the relationship between catalytic current and NO
2
concentration.
Fig. 8. (A) The CAT quantities comparison of Zn
Cd Zn 30 and Cd 30 in = 2.
4 4 2 2 4 4 4 4 4 30
P W30, Cd Zn P W30 and Cd P W30 versus c number. (B) The plot of CAT quantities versus Cd number for Zn P W ,
2
2
P
4
W
4
P
4
W
c
analysis. The 31P NMR pattern of [Cd
suggests C2h symmetry for Cd Zn 30 in which various metallic
centers (Cd and Zn) occupy identical positions in M 16 metal cen-
Cd NMR result of this compound demon-
strates that the cadmium ions are located at the external position
of M 16 metal centers in Dowson-type sandwich POMs.
The electrochemical behavior of the Cd 30, Zn
Zn 30 compounds has been investigated systematically
in aqueous solution (0.5 M acetate buffer, pH = 4.7) by cyclic
Zn (H O) (P W O ) ]
16ꢀ
4
. Conclusion
Mixed-metal sandwich-type compounds based on Wells–Daw-
2 2 2 2 2 15 56 2
2
2 4
P W
4
O
12ꢀ
113
son fragments [P
has been discovered very recently. Only very few compounds of
this type have been described and fully characterized. This work
presents the compound [Cd
been synthesized and characterized by FT-IR, P NMR,
NMR spectroscopy, cyclic voltammetry (CV) and elemental
W
2 15
O
56
]
constitute a novel class of POMs that
ter framework and the
4
O
16ꢀ
Zn
2 2
(H O)
2 2
(P W
2 15
O
56
)
2
]
, which has
4
4
P W
4 4
P W30 and
3
1
113
Cd
Cd
2
2 4
P W

9
4
F.M. Zonoz, F. Aliabadi / Inorganica Chimica Acta 444 (2016) 87–94
voltammetry. The electrochemistry of Cd
steps redox waves attributed to the tungsten-oxo and cadmium-
oxo redox processes.
2
Zn P
2 4
W
30 exhibits three
[22] F. Doungmene, P.A. Aparicio, J. Ntienoue, C.S.A. Mezui, P. Oliveira, X. López, I.M.
Mbomekallé, Electrochim. Acta 125 (2014) 674.
[
23] R.G. Finke, M.W. Droege, Inorg. Chem. 22 (1983) 1006.
24] J.F. Kirby, L.C.W. Baker, J. Am. Chem. Soc. 117 (1995) 10010.
[
The compounds studied in this article exhibit remarkable elec-
trocatalytic activities for the reduction of nitrite ion. By reducing
the number of cadmium ions in the central tetranuclear cluster
of sandwich-type POMs, catalytic efficiency increases.
[25] L.H. Bi, E.-B. Wang, J. Peng, R.D. Huang, L. Xu, C.W. Hu, Inorg. Chem. 39 (2000)
71.
6
[
[
26] X. Zhang, T.M. Anderson, Q. Chen, C.L. Hill, Inorg. Chem. 40 (2001) 418.
27] T.M. Anderson, K.I. Hardcastle, N. Okun, C.L. Hill, Inorg. Chem. 40 (2001) 6418.
[28] L. Ruhlmann, J. Canny, J. Vaissermann, R. Thouvenot, Dalton Trans. (2004) 794.
[
29] I.M. Mbomekalle, R. Cao, K.I. Hardcastle, C.L. Hill, M. Ammam, B. Keita, L. Nadjo,
T.M. Anderson, C. R. Chim. 8 (2005) 1077.
References
[
[
30] B. Keita, L. Nadjo, J. Mol. Catal. A 262 (2007) 190.
31] B. Keita, I.M. Mbomekalle, Y.W. Lu, L. Nadjo, P. Berthet, T.M. Anderson, C.L. Hill,
Eur. J. Inorg. Chem. (2004) 3462.
[
[
1] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983.
2] M.T. Pope, A. Müller, Polyoxometalates: From Platonic Solids to Anti-Retroviral
Activity, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994.
3] M.T. Pope, A. Müller, Polyoxometalate Chemistry: From Topology via Self-
Assembly to Applications, Kluwer Academic Publishers, Dordrecht, The
Netherlands, 2001.
4] T. Yamase, M.T. Pope, Polyoxometalate Chemistry for Nano-Composite Design,
Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.
5] D.-L. Long, E. Burkholder, L. Cronin, Chem. Soc. Rev. 36 (2007) 105.
6] T.J.R. Weakley, H.T. Evans Jr., J.S. Showell, G.F. Tourné, C.M. Tourné, J. Chem.
Soc., Chem. Commun. 4 (1973) 139.
[32] L. Ruhlmann, J. Canny, R. Contant, R. Thouvenot, Inorg. Chem. 41 (2002) 3811.
[33] L. Ruhlmann, C. Costa-Coquelard, J. Canny, R. Thouvenot, J. Electroanal. Chem.
603 (2007) 260.
[34] R. Contant, Inorg. Synth. 27 (1990) 104.
[35] M. Rusu, Gh. Marcu, D. Rusu, C. Rosu, A.-R. Tomsa, J. Radioanal. Nucl. Chem.
242 (1999) 467.
[36] C. Rocchiccioli-Deltcheff, R. Thouvenot, R. Franck, Spectrochim. Acta 22A
(1976) 587.
[37] C. Rocchiccioli-Deltcheff, R. Thouvenot, J. Chem. Res. (1977) (S) 46–47, (M)
549–571.
[38] C. Rocchiccioli-Deltcheff, R. Thouvenot, Spectrosc. Lett. 12 (1979) 127.
[39] M.H. Alizadeh, H. Razavi, F. Mohammadi Zonoz, Polyhedron 22 (2003) 933.
[40] F. Hussain, L.-H. Bi, U. Rauwald, M. Reicke, U. Kortz, Polyhedron 24 (2005) 847.
[41] M.A. Rezvani, A.F. Shojaie, M.H. Loghmani, Catal. Commun. 25 (2012) 36.
[42] B. Keita, I.-M. Mbomekalle, L. Nadjo, Electrochem. Commun. 5 (2003) 830.
[43] M.-C. Kuo, R.J. Stanis, J.R. Ferrell III, J.A. Turner, Electrochim. Acta 52 (2007)
2051.
[
[
[
[
[
[
[
7] H.T. Evans, C.M. Tourné, G.F. Tourné, T.J.R. Weakley, J. Chem. Soc., Dalton Trans.
(
1986) 2699.
8] S.H. Wasfi, A.L. Rheingold, G.F. Kokoszka, A.S. Goldstein, Inorg. Chem. 16 (1987)
934.
9] C.M. Tourné, G.F. Tourné, F. Zonnevijlle, J. Chem. Soc., Dalton Trans. (1991) 143.
2
[
10] C.J. Gómez-García, E. Coronado, P. Gómez-Romero, N. Casañ-Pastor, Inorg.
Chem. 32 (2007) 3378.
[
[
[
11] T.J.R. Weakley, R.G. Finke, Inorg. Chem. 29 (1990) 1235.
12] A.M. Khenkin, C.L. Hill, Mendeleev Commun. (1993) 140.
13] X. Zhang, Q. Chen, D.C. Duncan, R.J. Lachicotte, C.L. Hill, Inorg. Chem. 36 (1997)
[44] L. Ruhlmann, C. Costa-Coquelard, J. Canny, R. Thouvenot, Eur. J. Inorg. Chem.
(2007) 1493.
[45] D.S. Skoog, D.M. West, Fundamentals of Analytical Chemistry, fourth ed., Holt-
Saunders International, 1982.
[46] F.M. Zonoz, I.M. Zonoz, A. Jamshidi, M.H. Alizadeh, Solid State Sci. 32 (2014) 13.
[47] J. Hayon, A. Raveh, A. Bettelheim, J. Electroanal. Chem. 359 (1993) 209.
[48] K. Ogura, H. lshikawa, Faraday Trans. I (80) (1984) 2243.
[49] I. Taniguchi, N. Nakashima, K. Matsushita, K. Yasakouchi, J. Electroanal. Chem.
224 (1987) 199.
[50] J.E. Toth, F.C. Anson, J. Am. Chem. Soc. 111 (1989) 2444.
[51] M.H. Batlley, M.R. Rhodes, T.J. Meyer, Inorg. Chem. 26 (1987) 1746.
[52] W.L. Jolly, Inorganic Chemistry of Nitrogen, W. A. Benjamin, New York, 1964.
78.
4
381.
[
[
14] U. Kortz, S. Isber, M.H. Dickman, D. Ravot, Inorg. Chem. 39 (2000) 2915.
15] I.M. Mbomekalle, B. Keita, L. Nadjo, P. Berthet, K.I. Hardcastle, C.L. Hill, T.M.
Anderson, Inorg. Chem. 42 (2003) 1163.
[
16] F. Hussain, M. Reicke, V. Janowski, S. de Silva, J. Futuwi, U. Kortz, C. R. Chim. 8
(
2005) 1045.
[
[
[
[
17] B. Botar, P. Koegerler, Dalton Trans. (2008) 3150.
18] R.G. Finke, M.W. Droege, P.J. Domaille, Inorg. Chem. 26 (1987) 3886.
19] T.M. Anderson, X. Zhang, K.I. Hardcastle, C.L. Hill, Inorg. Chem. 41 (2002) 2477.
20] L. Ruhlmann, L. Nadjo, J. Canny, R. Contant, R. Thouvenot, Eur. J. Inorg. Chem.
(
2002) 975.
[53] F.M. Zonoz, A. Jamshidi, F. Hajizadeh, K. Yousefi, J. Serb. Chem. Soc. 79 (2014)
1379.
[
21] D. Gabb, C.P. Pradeep, H.N. Miras, S.G. Mitchell, D. Long, L. Cronin, Dalton
Trans. 41 (2012) 10000.