A polyoxometalate supported copper dimeric complex: Synthesis, structure and electrocatalysis

Article abstract of DOI:10.1016/j.ica.2020.119554

A polyoxometalate (POM) supported copper dimeric complex, [Mo₈O₂₆{Cu₂(2,2′-bpy)₂ (CH₃COO)₂H₂O}₂]?H₄Mo₈O₂₆·16H₂O (1) has been synthesized using conventional wet synthesis. The compound has been characterized using routine spectral techniques and its molecular structure has been elucidated using single crystal X-ray crystallography. The two copper(II) ions are bridged by acetate ions and each copper ion is also coordinated to 2,2′-bipyridine molecule. In each dimer, one of the copper ions is bonded to the POM anion from one side, whereas the other copper ion is bonded to a water molecule, hence acquiring a penta-coordinated square pyramidal structure around each copper center. The lattice water molecules play an important role to stabilize the system, by forming a water pentamer, which binds both the polyoxometalate supported transition metal complex and the other POM anion [Mo₈O₂₆]^4–. Interestingly, compound 1 is found to function as a versatile heterogeneous electro-catalyst for water oxidation, proton reduction and hydrogen peroxide reduction.

Full text of DOI:10.1016/j.ica.2020.119554

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Research paper
A polyoxometalate supported copper dimeric complex: Synthesis, structure and
electrocatalysis
N. Tanmaya Kumar, Umashis Bhoi, Pragya Naulakha, Samar K. Das
PII:
S0020-1693(19)31797-9
https://doi.org/10.1016/j.ica.2020.119554
ICA 119554
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Inorganica Chimica Acta
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24 February 2020
Please cite this article as: N.T. Kumar, U. Bhoi, P. Naulakha, S.K. Das, A polyoxometalate supported copper dimeric
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A polyoxometalate supported copper dimeric complex: Synthesis, structure
and electrocatalysis
a
a
b
a
N. Tanmaya Kumar, Umashis Bhoi, Pragya Naulakha and Samar K. Das *
a
School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabaad, Telangana,
India 500046
b
School of Physical Sciences, Jawaharlal Nehru University, New Delhi, India 110067
Abstract. A polyoxometalate (POM) supported copper dimeric complex, [Mo O {Cu (2,2’-bpy)
2
8
26
2
(
CH COO) H O} ]٠H Mo O ·16H O (1) has been synthesized using conventional wet synthesis.
3 2 2 2 4 8 26 2
The compound has been characterized using routine spectral techniques and its molecular structure
has been elucidated using single crystal X-ray crystallography. The two copper(II) ions are bridged by
acetate ions and each copper ion is also coordinated to 2,2’-bipyridine molecule. In each dimer, one of
the copper ions is bonded to the POM anion from one side, whereas the other copper ion is bonded to
a water molecule, hence acquiring a penta-coordinated square pyramidal structure around each copper
center. The lattice water molecules play an important role to stabilize the system, by forming a water
pentamer, which binds both the polyoxometalate supported transition metal complex and the other
4
-
POM anion [Mo O ] . Interestingly, compound 1 is found to function as a versatile heterogeneous
8
26
electro-catalyst for water oxidation, proton reduction and hydrogen peroxide reduction.
Copper ions play a major role in
biological processes. Among the copper
containing metalloenzymes, hemocyanins
1
. Introduction
Polyoxometalates (POMs) are
(found in mollusks) are oxygen carriers.
interesting transition metal oxide clusters,
that have grabbed the attention of the
contemporary scientists, because of their
structural versatility and their applicability
in a vivid range of areas ranging from
catalysis, materials sciences, biology,
magnetism, luminescence to molecular
These hemocyanins contain a copper
dimer, which is bridged by the oxygen
atoms. Many modeling studies have been
done, so far, in order to mimic the
functional as well as the structural aspects
of the copper dimeric complexes in copper
4
1
metalloenzymes. Melnik and co-workers
electronics through medicine. A recent
5
in the year 1997, published a review
modification to the polyoxometalates is the
introduction of the coordination complexes
into the framework by covalently grafting
the complex through the terminal /
bridging oxygen atom(s) of the
reporting more than nine hundred types of
Cu(II) dimers, which evidences the
importance and applicability of this family
of compounds. According to this review,
the earliest quotation to copper acetate was
polyoxometalates,
taking
into
5
5
from the year 1594. The review also
generalizes the fact that, as far as the
consideration the charge compensation or
the structure direction, which are called
polyoxometalate supported transition
metal complexes
modification enhances the potential
stereochemistry
of
these
dimeric
(PSTMCs).2
compounds is concerned, they favor a
square pyramidal geometry with different
degrees of distortions about the Cu(II) ion.
In the last two decades, there has been an
extensive study on the reactivity of tetra-µ-
This
applications of these hybrid PSTMCs in
many diverse fields.3

acetatodicopper(II) towards the 2,2’-
bipyridine (bpy). In 1992, Perlepes and co-
next year, they also reported the reaction
of in situ generated copper(II)-
monosubstituted Keggin polyoxometalates
and copper(II)-phenanthroline-oxalato
workers reported the isolation of [Cu (µ-
2
O CMe) (H O) (bpy) ] (ClO ) ·H O from
2
2
2
2
2
4 2
2
the reaction of [Cu (OH) (bpy) ]ClO with
complexes in ammonium or rubidium
acetate buffers leading to the formation of
two new hybrid POMs, where the
dinuclear copper-oxalato complex is
2
2
2
4
6
an excess of MeCO H. Followed by this
2
report, Chakravarty and Meenakumari
reported the synthesis and crystal structure
of [Cu (µ-O CMe) (H O)(bipy) ](PF ) ,
sandwiched
monosubstituted
by
two
POMs
copper-
E4[Cu(phen)
2
2
2
2
2
6 2
where the two Cu(II) ions exhibited two
different coordination geometries.7
(H O) ] [Cu (phen) (H O) (ox) ]
3 0.6
2
4 2
4
4
2
4
In the present work, we have
synthesized the polyoxometalate supported
[Cu (phen) (H O) (ox)] [Cu(phen)(ox)]
2 2 2 4 0.4 0.8
[{SiW O Cu(H O)} {Cu (phen) (ox)}]
1
1
39
2
2
2
2
+
+ 12
dimeric
Mo O {Cu (2,2’-bpy) (CH COO)
2
transition
metal
complex
·20H O [E: Rb or NH ]. Ma and
2 4
[
coworkers reported a one dimensional
polyoxometalate-based chain, constructed
8
26
2
2
3
H O} ] ٠H Mo O ·16H O (1).
2
2
4
8
26
2
4
‒
Prior to this work, there are few reports
on copper dimeric complexes supported on
diverse polyoxometalates (POMs).
Gutiérrez-Zorrilla, Lezama and their co-
workers have done enormous work in this
field, i.e., POM supported dimeric copper
complex. They reported a unique dinuclear
copper complex, supported on a Keggin
from Keggin anions of [SiMo O ]
12 40
weakly connected by dinuclear [Cu(ppy)2]
groups. The two copper atoms are
connected to two terminal oxygen atoms of
two different Keggin atoms, forming a
planar rhombic copper dimer.13 Wang and
group reported reduced tungsten Keggin
supported chloro-bridged copper dimers
type
of
polyoxometalate
including
related
supramolecular
K [{Cu (bpy) (μ-ox)}
{SiW O
chemistry.14 Ma, Liu and co-workers
reported the hydrothermal synthesis of a
hybrid POM complex, [Cu (bipy) (µ -
1
4
2
2
11 39
8
Cu(H O)}] [SiW O Cu(H O)]٠~55H O.
2
2
11 39
2
2
Subseq-uently, they depicted a full account
of this work describing variable
temperature magnetic studies.9 In yet
another report, they also reported reaction
2
3
1
H O) (µ -H O)(µ OH)(H BW O )]
2
2
2
2
2
2
12 40
·4H O (bipy =4,4’-bipy) having a Cu
2
dimer connected to 4, 4’-bipyridine from
one side and the terminal oxygen of the
cluster from the other side. The two copper
ions are bridged by water molecules and
showed the existence of weak anti-
ferromagnetic interactions.15 A similar
of
a
monosubstituted
Keggin
polyoxometalate (POM) generated in situ
with copper–phenanthroline complexes,
where the two Keggin cluster anions are
connected through a terminal oxygen atom
yielding
complex was also reported by Ma and
Pang, where the POM was [PW O ] .
n‒ 16
A [Cu (ac) (phen) (H O) ][Cu (ac)
7
2
2
2
2
2
3
3
12 40
(
phen) (H O) ][Si W Cu O (H O)]·~18
Zhao and group have described
(hydroxo)(chloro) bridged copper dimer,
supported by the silicotungsten Keggin in
compound [Cu (phen) Cl(H O)(OH)] [α-
3
2
3
2
22
+
2
78
2
+ 10
H O, where A= NH or Rb . A little
2
4
later, the same groups described two
interesting POM supported copper dimeric
compounds, where the second one is the
2
2
2
2
1
7
SiW O ] ٠8H O. Ma, Pang and their
1
2
40
2
1
1
condensed dimer of the first one. In the
co-workers reported two copper dimeric-

POM
complexes
The present years have also seen an
ample interest to design new molecular
catalysts, containing earth-abundant 3d-
transition metals as the active site for
{
[Cu (en) (ox)][HPW O ]}·(en) ·2H O
2
2
12 40
2
2
and
{[Cu (en) (ox)]
[H BW O ]}
3
2
2
12 40
19
1
8
·
(en) ·2H O. Xu and co-workers, have
2 2
reported a similar PSTMC [Cu(bpy)(μ2-
electrocatalytic
hydrogen
evolution
OH)] [(H O)-(bpy) HPW Cu O ]
39 2
reactions. Nickel,22 cobalt23 and iron24
based complexes are few popular
molecular catalysts, which have been
employed in electrocatalytic hydrogen
evolution reactions (HER). In 2014, Wang
and co-workers, for the first time, reported
a Cu-complex that can electrochemically
catalyze water reduction25a with a very
high catalytic activity with 96% Faradic
efficiency and 420 mV onset overpotential
in pH 2.5 under buffer conditions. After
this report, many Cu-based complexes
were discovered, which showcased activity
in catalyzing HER electrochemically.25
Recently, our group has described two
4
2
2
11
2
·
2C H OH·10H O, where the Cu(II) ions
2 5 2
are located in the lacunary sites of the
7
‒
monolacunary anion [PW O ] forming
1
1
39
a dumbbell type structure.
Even though these impactful reports
have brought about structural aesthetics
and interesting magnetism on these
polyoxometalate supported copper dimeric
complexes, their use as electro-catalysts is
scarcely reported. Ma, Pang and co-
workers have reported the role of copper
dimer, supported on polyoxometalates as
electrocatalysts for the reductions of
hydrogen peroxide, potassium iodate and
nitrite.18 There are not many reports on
electrocatalytic water oxidation and
electrocatalytic proton reduction, catalyzed
by POM supported copper dimers, that are
enormously important today because the
water splitting (WS) process using an
inexpensive POM-based catalyst is a
powerful alternative to curb the energy
crisis on the earth surface. Water oxidation
Anderson-POM
supported
copper
complexes acting as electrocatalysts for
hydrogen evolution reaction in neutral
medium.26 The electrocatalytic proton
reduction to molecular hydrogen has
attracted immense attention because the
molecular hydrogen (H ) is a clean and
2
highly efficient fuel. Likewise, since
hydrogen peroxide is commonly used as
an oxidant in chemical mechanical
planarization slurries leading to the
metallization
microelectronics
electrocatalytic reduction of H O by
2 2
copper-containing catalyst is utmost
demanding. In this article, we have
(WO) is considered to be the bottleneck
process of water splitting because it
advances in a thermodynamically uphill
of
copper
for
the
−
manner with the involvement of 4e and
applications,
+
20
4
2
H , according to the equation:
+
−
Eo= –1.23 V
H O  4H + O + 4e
2
2
where E° is the standard Nernst potential
for water oxidation; here a negative
potential signifies the thermodynamic
uphill nature of the process. Our group has
reported a nickel coordination complex,
attached to a polyoxometalate cluster
anion, to be a robust electrocatalyst for
water oxidation process.21
demonstrated
polyoxometalate supported dimeric copper
complex [Mo O {Cu (2,2’-bpy)
CH COO) H O} ]٠H Mo O ·16H O (1)
can be used as a versatile heterogeneous
catalyst for electrochemical water
that
the
synthesized
8
26
2
2
(
3
2
2
2
4
8
26
2
oxidation, proton reduction, and hydrogen
peroxide reduction in acidic medium.

2
. Results and Discussion
The detailed synthetic procedure is
provided in the Experimental Section. The
powder X-ray diffraction (PXRD) pattern,
generated from CIF file of single-crystal
data of compound 1 (simulated PXRD
pattern), matches well with that, obtained
from the bulk PXRD measurement of the
sample (vide infra). This confirms the
purity in the bulk sample.
2
.1 Synthesis
Compound
1
was
synthesized
following a one-pot conventional wet
synthesis protocol at 70 C. The blue-
colored filtrate yielded 0.8 g (20.3% based
on Mo) blue block crystals within four
days. The relevant synthesis includes the
reaction of sodium molybdate, 2,2’-
o
2.2 Crystallography
bipyridine,
Cu(NO ) ·6H O
and
The asymmetric unit of compound 1
consists of two halves of the
octamolybdate anions, one copper acetate
dimer, and eight lattice water molecules
(Fig. 1a). Thus the full molecule contains
two POM clusters, two copper dimers, and
sixteen solvent water molecules as shown
in Fig. 1b. An octamolybdate anion,
3
2
2
CH COONa in an aqueous acidic medium.
3
The sodium molybdate solution, when
acidified with glacial acetic acid
immediately gives a white precipitate of
4
‒
[
Mo O ] , which upon addition of
8 26
Cu(NO ) ·6H O
and
CH COONa
3
3
2
2
coordinates to Cu(II) through its terminal
oxygen atom. Looking at the formula of 1,
the following reactions can be proposed
during the synthesis of compound 1:
4
‒
[Mo O ] consists of eight edge-sharing
8
26
octahedral {MoO } units. The Mo‒O bond
6
distances can be grouped into four
categories: 14 Mo‒O (O = terminal
t
t
2
‒
+
8 2
] + 12H  {Mo O O
26}4‒ + 6H
8
(
2
[MoO
eqn.1)
{Mo
4
oxygen) with the bond lengths in the range
of 1.686-1.706 Å; six µ -type bridged
Mo‒O (O = bridging oxygen) with bond
4
‒
2+
2
8
O
26} + 4Cu + 4CH
O  [Mo 26{Cu (2,2’-bpy)
]٠H Mo 26 ·16H O (1)
3
COOH + 4(2,2’-bpy)
+
H
18H
O}
2
8
O
2
2
(CH COO)
3
2
b
b
2
2
4
8
O
2
distances in the range of 1.740- 2.295 Å;
(eqn.2)
four µ -type bridged Mo‒O bonds in the
3
b
range of 1.984-2.329 Å and two µ -type
5
The
tetrahedral
molybdate
Mo‒O bonds (2.155 and 2.402 Å). The
b
[
MoO4]2‒ in an acidic aqueous solution
polyhedral representation of the POM
clusters is shown in Fig. 1c.
undergoes protonation followed by
coordination expansion around
In the crystal structure, there are
molybdenum and subsequent condensation
reaction results in the formation of the
4
‒
two independent POM cluster [Mo O ]
8
26
4
‒
anions. One of these, supports two copper
dimers via terminal Mo=O coordination
resulting in the formation of POM
supported copper dimeric complexes,
isopolyanion {Mo O } , as shown in eqn.
8
26
1
{
. Subsequently, the in-situ formed
4
‒
Mo O } acts as the ligand through its
8
26
terminal oxo donor to coordinate to one of
the copper centres of each copper dimer
(there are two such copper dimers per one
POM cluster anion, formed in situ from
the copper(II) salt, acetate anion and 2,2’-
bipyridine (eqn. 2)).

with the apical position being coordinated
to one of the terminal oxygen atoms of the
octamolybdate for one Cu(II) ion and
water to another Cu(II) ion. Overall, in the
POM supported copper dimer complex
system,
[Mo O {Cu (CH COO) (bpy)
8 26 2 3 2 2
(
H O)} ], the isopolyanion is covalently
2
2
bonded to two copper dimeric complexes
4
+
[
Cu (CH COO) (bpy) (H O)]
from
2
3
2
2
2
opposite sides. The β conformation of the
POM clusters, stabilized in compound 1, is
shown in Fig. 1d that shows octahedral
coordination of each molybdenum centre
4
‒ 27
in {Mo O } .
8
26
Fig. 1. (a) The thermal ellipsoidal plot of
compound at the 50% probability level.
Hydrogen atoms are omitted for clarity. (b) The full
molecule of excluding the lattice water
1
The detailed crystal structure analysis
of compound 1 shows that the POM anion,
coordinating copper dimers, interacts with
a crystallographically independent POM
cluster, remaining as a lattice component
1
molecules. (c) Polyhedral representation of the
POM clusters in compound 1 excluding the water
molecules. (d) Two independent cluster anions of
[Mo
8
O
26]4‒ polyoxometalate, showing their
β
as an acid salt H
supramolecular cyclic water pentamer
H O) . The water pentamer is an open
4
Mo
8
O
26
,
via a
conformation. Color code: Cyan: molybdenum;
Red: oxygen atoms of the octamolybdate cluster
anion; violet: copper; grey: carbon; orange: oxygen
atoms of acetate ions; green: nitrogen; blue: water
molecule attached to the copper dimer; brown:
lattice water molecules; white: hydrogen.
(
2
5
envelope-shaped, as determined crystallo-
graphically in compound 1. The five water
molecules, involved, are the four lattice
water molecules and the water,
coordinated to the copper atom (Fig. SI
S1). The relevant O···O separations in the
water pentamer, (H O) are: O18···O36:
[
Mo O {Cu (CH COO) (bpy) (H O)} ]
8 26 2 3 2 2 2 2
and other POM cluster remains as a lattice
component as a proton salt along with
sixteen lattice water molecules per formula
unit, thereby giving the overall formula of
the title compound as [Mo O {Cu (2,2’-
2
5
2
2
2
.731Å; O36···O37: 2.809Å; O37···O32:
.792Å; O32···O33: 2.978Å; O33···O18:
.871Å.
8
26
2
bpy) (CH COO) H O} ]H Mo O
26
2
3
2
2
2
4
8
II
·
16H O (1). Each copper (Cu ) dimeric
Besides the stabilization of a water
pentamer, there also exist the C‒H···O
2
4
‒
complex is connected to the [Mo O ]
8
26
cluster through an apical position of the
copper complex. The copper dimeric
complex comprises of two square-
pyramidal Cu(II) atoms bridged by two
acetic acid ligands in a syn-syn fashion
with a Cu‒Cu distance of 3.007 Å. The
basal plane is occupied by two bipyridine
N atoms and O atoms from the acetate ions
hydrogen bonding interactions which
contribute
to
stabilizing
the
supramolecular chainlike arrangement in
the crystal structure of compound 1, as
shown in Fig. 2. The ‒CH‒ groups of the
carbon atoms C1, C2, C3, C4, C9, C10,
C11, C12, C14, C19, C20, C22 and C24
donate their hydrogens to the surrounding

Table 1 Crystal data and structure refinement for
compound 1
available possible surface sites of the
octamolybdate cluster anion (that include
bridging and terminal oxygen atoms), (SI,
Fig. S2).
Compound 1
Empirical
formula
C H Cu Mo N O
48 44 4 16 8 78
fw
T(K), λ(Å)
Crystal system
Space group
a (Å)
3770.16
273(2), 0.71073
Triclinic
Fig. 2. The chain-like structure formed through
hydrogen bonds between the dimeric complex, the
lattice octamolybdate cluster, and the water
pentamers, (hydrogen atoms are not shown for
clarity).
P-1
12.135(3)
12.766(3)
19.016(5)
104.490(8)
98.770(9)
111.063(8)
2564.0(11)
1, 2.442
b (Å)
The π‒π stacking interactions in the
compound 1 are also studied. Since there
are four bipyridine ligands connected to
the four copper(II) centres, each bipyridine
ligand faces the other one, in a dimeric
pair. As can be seen in Fig. S3, the
centroids of each pyridine ring pair are at a
distance of 3.615 and 3.677 Å. These
pyridine rings are slip-stacked to each
other and not completely sandwiched, as
the later leads to less stable compound, on
account of high electronic repulsion of the
electrons in the π-orbitals of the pyridine
rings.
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
Z, dcalcd (g cm3)
-
1
µ (mm ),
2.808, 2139.0
F(000)
goodness-of-fit
1.407
on F2
2
.3 Spectroscopy
R1
0.0728
0.3005
2.6
The Fourier transformed infrared spectrum
of the compound 1 was recorded. (Fig. S4)
The strong peak at 938 cm-1 can be
attributed to the asymmetric stretch of the
wR2
Largest diff.
peak/hole
Mo‒ O bond. Similarly, the asymmetric
t
stretch of Mo‒O ‒Mo bond is found to be
b
1
-
3
(
e Å )
-
seen at 724 cm . The peaks at 724, 766,
-1
8
38 and 893 cm are the characteristic
peaks for Mo‒O asymmetric stretch. All
these values almost match with the
vibrational features, reported for the β-
b
isomer of the octamolybdate in the
literature.28 The vibrational features of
compound 1 in two different ranges are

given in SI (Fig.s S4 and S5), which
clearly show the characteristics of the β-
isomer of the octamolybdate. The presence
of the water molecules in the compound is
evident from the broad peak around 3000
cm . The peaks at 3520 and 3460 cm are
attributed to the symmetric stretch
corresponding to the water molecules
bonded to the Cu(II) ions. Similarly, the
broad peak at 2950 cm-1 is for the
without prior activation in a high-
temperature vacuum. With an increase in
o
o
temperature from 30 C to nearly 240 C,
as shown in Fig. S6b, SI, there is a weight
loss, which can be attributed to the loss of
lattice water molecules as well as the water
molecules coordinated to the Cu(II) ion.
Followed by this, there is a weight loss of
up to nearly 62% of the weight when the
-1
-1
o
temperature increases from 240 C to 380
symmetric stretch of the CH groups of the
oC. In this temperature range, the four
acetate ligands and four bipyridine ligands
of the copper dimers are lost leaving the
octamolybdate clusters and the copper
3
acetate ions. The bands at 1600 and 1577
-1
cm can be attributed to the asymmetric
‒
stretch of the COO group and at 1441 and
-
1
Then after 750 oC, the
1
428 cm for its symmetric stretch (Fig.
contents.
-1
S5, SI). The peak at 1636 cm is for the
C=C bond of the bipyridine ligand. Other
peaks at 1577, 1428, 1312, 1254, 1035 and
polyoxometalate clusters and the copper
content start to degrade into their oxides.
-1
1
020 cm can be attributed to the various
2.4 Electrochemical Studies
vibrational features of the bipyridine
ligands.29
The electrochemical responses of
compound 1 were studied in various
electrolytes. The responses in different
electrolytes, as expected, were different.
The electronic absorption spectrum of
compound 1 is transformed into a
Kubelka-Munk (K-M) derived plot as
shown in Fig. S6a, SI. The sharp peak at
4
‒
As we know that the [Mo O ] cluster is
8
26
stable in aqueous solution in pH less than
.5 and is transformed to paramolybdic
5
2
26 nm can be ascribed to the ligand to
anions in pH 5.5 to 8,33 hence all the
electrochemical studies were performed in
pH less than 5.5. The cyclic voltammetric
studies were done in various electrolytes
and at various pH values.
metal charge transfer from the oxygen 2p
to the molybdenum d orbital.30 The
shoulder at 271 nm can be attributed to the
intra-ligand bipyridine π‒π* transitions.
The broad peak centred around 467 nm
can be attributed to the d to π*
The cyclic voltammograms of
compound 1 in 0.1M phosphate buffer and
3
1
transitions. A diffused broad peak is also
seen at around 350 nm, which can be
attributed to the acetate to copper LMCT.32
A broad and weak peak is also seen around
0
.1M Na SO solution at different pH
2 4
values are shown in Fig. 3. The compound
shows similar redox responses at different
pH values (except at pH 4 in 0.1M
phosphate buffer) in both the electrolytes.
A quasi-reversible redox couple, that has
appeared as a major redox response in all
the cases (except at pH 4 in 0.1M
phosphate buffer), can be attributed to the
6
00 nm, which is due to d-d transitions of
9
the d Cu(II) center in the title compound.
The thermal stability of the title
compound has been
thermogravimetric
studied by
The
analysis.
II
I
compound was taken for thermal studies
Cu /Cu couple. In Na SO solution, at pH
2 4

Fig.
Bu
4.
[Mo
Cyclic
Voltammetric
plot
of
[
4
N]
4
8
O
26] in 0.1M Na
2
SO at pH 5 at a scan
4
-1
rate of 100 mVsec .
Fig. 3. Cyclic Voltammograms of compound 1 at
various pH values in (a) 0.1M phosphate buffer and
-1
(
2 4
b) 0.1M Na SO , at a scan rate of 100 mVsec .
3
, a careful observation reveals a weak
reduction feature at ‒0.4V, which can be
VI
V
attributed to the reduction of Mo to Mo
of the cluster, based on the weak redox
responses appeared for the precursor
compound [Bu N] [Mo O ] in 0.1M
4
4
8
26
Na SO at pH 5 as shown in Fig. 4. The
2
4
voltammograms at varied scan rates in
different electrolytes (at different pH
values) are given in supplementary
information (Fig. S9, SI). Table S1 (SI)
Fig. 5. (a) Cyclic voltammogram of compound 1 in
.1M KCl. (b) The water oxidation activity of
compound 1 after 10 cycles.
0

gives the E1/2 values of the compound at
different pH values
oxidized electrocatalytically. The relevant
voltammograms are represented in Fig. 6.
As can be seen from Fig. 6, there is an
increase in current in the anodic side with
the sequential addition of water, with onset
potential at around 0.9 V versus NHE.
Thus compound 1 shows a significant
electrocatalytic water oxidation response
that the current intensity at around 0.9 V
gradually increases with increasing amount
of water. This is characteristic of
electrocatalytic water oxidation. The small
features, marked with A2 and C2 (Fig. 5a),
2
.5 Electrocatalytic water oxidation
We tested the activity of compound
1
in 0.1M KCl solution and the results, so
obtained, are given in Fig. 5.
As can be seen from Fig. 5a, the
increase in the current at around 1.27 V
signifies the electrocatalytic water
oxidation process. The reversible couple
A1/C1 (E = 0.063 V; E = -0.05; E =
A1
C1
1/2
0
.0065 V vs. NHE) can be attributed to the
III
II
can be considered as Cu /Cu couple. It
can be believed that the Cu(III) of this
couple oxidizes water and comes back to
its resting state, Cu(II), after water
oxidation. The reason for the small feature
II
I
Cu /Cu couple. The activity of the
compound is almost retained up to ten
cycles as can be seen in Fig. 5b.
III
II
of this Cu /Cu couple can be explained
by the fact that Cu(II) center with
II
{
Cu N O } coordination with two soft
2
3
donors would not be stable at its Cu(III)
oxidation state, referring soft and hard
acid-base principle.
2
.6 Electrocatalytic Proton Reduction
The electrocatalytic proton reducing
activity of the complex 1 was studied in
0
.1M phosphate buffer, in 0.1M Na SO at
2 4
Fig. 6. Cyclic voltammogram of compound 1 in
THF with [Bu
4
N][BF
4
] as supporting electrolyte
various pH values, in 0.1M KPF , in 0.1M
6
and sequential addition of water. The values in the
box indicate the amount of water added to the cell
in µL.
KNO , in 0.1M H SO as well as in 0.1M
3
2
4
[
Bu N][PF ] in acetonitrile, with acetic
4 6
acid addition, in the last case. As can be
seen in Fig. 7a, in 0.1M phosphate buffer,
at pH 2, we get an onset at around ‒0.5V
for electrocatalytic proton reduction and
the onset goes farther towards ‒1V, in pH
5. As shown in Fig. 7a, the current height
of electrocatalytic proton reduction
increases enormously, as we go from pH 5
to pH 2. This proves that more is the
number of protons (lesser the pH), more is
the production of molecular hydrogen
In order to assess the electrocatalytic
water oxidation activity of the title
compound, cyclic voltammograms of
compound 1
coated glassy carbon
electrode were obtained in THF, a non-
aqueous solvent, with [Bu N][BF ] as the
4
4
supporting electrolyte. This is because; we
can add water sequentially (in the scale of
micro-liters) to the non-aqueous reaction
mixture in the electrochemical cell to
investigate, whether the added water gets
(H ). The formation of hydrogen
2

Fig. 7. Proton reduction in (a) 0.1M phosphate buffer at different pH (b) 0.1M Na
KNO (e) 0.1M H SO . In Fig. 7e the inset shows a closer view of the redox behaviour of compound 1 (f) 0.1M
Bu N][PF ] in acetonitrile with sequential addition of acetic acid in µL.
2 4 6
SO (c) 0.1M KPF (d) 0.1M
3
2
4
[
4
6
molecules was also evident from the visual
observation of gas bubbles, found at the
surface of the (working) glassy carbon
electrode. In the case of 0.1M Na SO
the cell, observed visually. Cyclic
voltammograms of compound 1 in 0.1M
H SO at different scan rates (in mV/sec)
2
4
are given in SI Fig. S10.
2
4
(Fig. 7b), a similar trend is observed, i.e.,
The behavior of electrocatalytic proton
reduction, catalyzed by compound 1 was also
performed in dry acetonitrile with continuous
with the decrease of pH, there is an
increase in the amount of H production.
2
II
I
nitrogen gas purging with 0.1M [Bu
as the supporting electrolyte (shown in Fig.
f). In this case, we found that with the
4 6
N][PF ]
Notably, in this case, the Cu /Cu couple is
visible (Fig. 7b), unlike the case, where we
have used phosphate buffer. Similarly, in
7
addition of acetic acid to the cell, the current
intensity of the reduction at ‒0.6V increased
and the height of this reduction peak
continuously increased as acetic acid
concentration was increased in the cell through
sequential addition from 40 µL to 200 µL.
This continuous growth of the reduction peak
is characteristic of the electrocatalytic
reduction of acetic acid protons to hydrogen.
0
.1M KPF aqueous solution, along with
6
the distinct A1/C1 couple (which can be
II
I
assigned to the Cu /Cu couple), we see an
onset at ‒0.75V (Fig. 7c), which can be
attributed to the electrocatalytic proton
reduction
to
molecular
hydrogen,
catalyzed by compound 1. A similar surge
of current is also seen when the
electrochemical reaction in carried out in
2
.7 Electrocatalytic H O reduction
2 2
0
.1M KNO (Fig. 7d). The huge reduction
3
The electrocatalytic reduction of hydrogen
peroxide was also assessed for compound 1.
As we know that hydrogen peroxide undergoes
thermal decomposition at room temperature,
hence the electrocatalytic experiments were
performed keeping the cell in an ice bath. The
current can be attributed to the same fact
that the concerned experiment of
electrocatalytic proton reduction has been
performed at pH = 1.3. This is also evident
from the hydrogen bubbles that evolved in

Fig. 8. The electrocatalytic hydrogen peroxide reduction (a) without and (b) with compound 1 as a catalyst in
.1M Na SO at pH 5.
0
2
4
voltammograms
corresponding
to
the
copper dimer complex. In the present
article, we, not only, have demonstrated
reduction of hydrogen peroxide in 0.1M
Na SO at pH 5 without and with the catalyst
2
4
the
successful
synthesis
and
are shown in Fig. 8a and Fig. 8b respectively.
As we can see, in both cases, the reduction
peak increases with an increase in the
concentration of H O . But, the role of the
catalyst is clear if we see the current produced
in both cases. There is a substantial increase in
the reduction current in the presence of
compound 1, which proves that the compound
plays the role of an electro-catalyst, which
readily catalyzes the reduction of hydrogen
peroxide.
characterization of a copper dimeric
complex, supported on a Keggin POM
cluster anion, but also we have shown the
electrocatalytic water oxidation to
molecular oxygen, proton reduction to
molecular hydrogen and electrocatalytic
reduction of hydrogen peroxide using the
synthesized POM supported copper
dimeric complex.
2
2
4
. Experimental Section
3
. Conclusion
Polyoxometalate (POM) supported
4
.1 General Materials and Methods
All the starting materials were
transition metal coordination complexes
are a huge group of heterogeneous
compounds in the POM family. And POM
supported copper dimers belong to a very
small class of compounds in this family;
very few members of this class exhibit
electrocatalytic applications. To our
knowledge, there is no report, so far, on
electrocatalytic water oxidation and proton
reduction, catalyzed by a POM supported
purchased as analytical grade and were
used as received. IR spectrum of the
compound was obtained on a JASCO-5300
FTIR spectrophotometer. UV-visible DRS
electronic absorption spectrum was
recorded
using
a
Shimadzu-2600
spectrophotometer. TGA was carried out
on an STA 409 PC Analyzer. PXRD plots
were recorded on a Bruker D8-Advance
diffractometer
using
graphite-

monochromated Cu Kα (1.5406 Å) and
geometries
were
determined
in
1
3
6
Kα (1.55439 Å) radiation.
PLATON. Crystal parameters are given
in Table 1 and hydrogen bond distances
are shown in Table S3 (SI). Selected bond
lengths and bond angles of compound 1
are summarized in Table S2 (ESI).
Crystallographic cif files (CCDC No.
2
4
.2 Synthesis of 1:
.1037g (0.64 mmol) of 2, 2’-
0
bipyridine was dissolved in 2 mL
methanol followed by the addition of 50
mL water. To this solution, 15 mL glacial
acetic acid was added and was stirred for 1
minute. To the stirred mixture, 1.4g (5.85
mmol) sodium molybdate was added and
the resulting solution was kept for stirring.
After 5 minutes, 0.5g (2.06 mmol)
Cu(NO ) ·6H O was added followed by
1
960325)
are
available
at
www.ccdc.cam.ac.uk/data or as part of the
Supporting Information.
4
.4 Electrochemical Studies
All electrochemical experiments
were conducted using a Zahner Zannium
electrochemical workstation operated by
Thales software. Complete electrochemical
experiments were accomplished using a
three-electrode electrochemical cell with
compound 1 modified glassy carbon as the
working electrode, homemade Ag/AgCl (1
M) as the reference electrode and a Pt flag
as the counter electrode in acidic pH in an
aqueous medium. For the preparation of a
1-modified electrode, 4 mg of 1 and 1 mg
of acetylene carbon black were placed in a
1 mL of 3:2 mixture of ethanol/water and
to it, was added 10 µL of 5 wt % Nafion
3
2
2
the addition of 1g (12.97 mmol)
ammonium acetate. The resulting solution
o
was then stirred for 1 hour at 70 C. The
reaction mixture was then filtered and the
blue color filtrate was kept at room
temperature to yield blue block crystals
after four days. Yield: 0.8 g (20.3% based
-1
on Mo). FT-IR (cm ): 650, 700, 838, 895,
9
2
40, 1032, 1250, 1314, 1423, 1495, 1571,
804, 3080, 3207, 3510. Elemental
analysis (in %): Calculated: C: 15.29; H:
1
2
.18; N: 2.97; Found: C: 14.89; H 1.2; N:
.68.
(
3
aqueous). The mixture was sonicated for
0 minutes to make it a homogeneous
4
.3 Crystal Data Collection
suspension. Then 5 µL of this mixture was
coated on a 3-mm‒diameter glassy carbon
electrode (geometrical area = 0.0706 cm2),
resulting in 10 µg of the sample
X-ray reflections were collected on
Bruker D8 QUEST, CCD diffractometer
equipped with a graphite monochromator
and Mo−Kα fine-focus sealed tube (λ =
.71073 Å), and the reduction was
performed using APEX-II Software.34
(compound 1) in each coating on the
0
glassy carbon electrode. The same amount
on the electrode surface was maintained
for all electrochemical experiments. The
coating mixture on the electrode was dried
under an IR lamp (temperature approx. 70
oC) prior to the experiment. All the
experiments were conducted under
ambient temperature. Electrode potentials
were converted to the normal hydrogen
Intensities were corrected for absorption
using SADABS, and the structure was
3
5
solved and refined using SHELX97. All
non-hydrogen atoms were refined
anisotropically. Hydrogen atoms on atoms
were located from difference electron-
density maps, and all C−H H atoms were
fixed
geometrically.
Hydrogen-bond

electrode (NHE) scale using the
relationship E(NHE) = E(Ag/AgCl) +
Rev., 36, (2007) 105. (e) C. Y. Sun,
S. X. Liu, D. D. Liang, K. Z. Shao,
Y. H. Ren, Z. M. Su, J. Am. Chem.
Soc. 131 (2009) 1883. (f) D. L.
Long, R. Tsunashima, L. Cronin,
Angew. Chem., Int. Ed., 49 (2010)
1736.
0
.1263 V as Ag/AgCl was used as the
reference electrode. CV scans were
initiated at the open-circuit potential and
the anodic side was scanned first, followed
by the cathodic side scan. Five cycles were
taken consecutively for each set of CV
measurements in the solution. Cyclic
voltammograms were also recorded at
various scan rates. For experiments in a
non-aqueous medium, nitrogen gas was
purged into the cell throughout the
experiment.
2. (a) C. Singh, S. Mukhopadhyay, S.
K. Das, Inorg. Chem. 57 (2018)
6479. (b) X.-M. Li, Y.-G. Chen, T.
Shi, Solid State Sci., 52 (2016) 112.
(c) M.-T. Li, J.-Q. Sha, X.-M. Zong,
J.-W. Sun, P.-F. Yan, L. Li, X.-N.
Yang, Cryst. Growth Des. 14 (2014)
2
794. (d) C.-M. Liu, Y.-H. Huang,
Acknowledgments This work is supported
D.-Q. Zhang, F.-C. Jiang, D.-B.
Zhu, J. Coord. Chem. 56 (2003)
by
DST-SERB
(project
number:
EMR/2017/002971) and the UGC-BSR
project (project No. 19-232/2019(BSR)).
We also acknowledge DST-FIST, UGC-
SAP, and UPE-II for facilities. NTK thanks
CSIR for fellowship. NTK thanks Ms.
Chandani Singh and Mr. Subhabrata
Mukhopadhyay for their help and valuable
suggestions.
9
53. (e) H. Wang, C, Cheng, Y.
Tian, Chin. J. Chem. 27 (2009)
099. (f) Y.-F Li, D. G. Hubble, R.
1
G. Miller, H.-Y. Zhao, W.-P. Pan,
S. Parkin, B. Yan, Polyhedron 29
(2010) 3324. (g) Z. Yi, X. Zhang,
X. Yu, L. Liu, X. Xu, X. Cui, J. Xu,
J. Coord. Chem. 66 (2013) 1876.
3
. (a) Y. Xu, J.-Q. Xu, K.-L. Zhang,
Y. Zhang, X. -Y. You, Chem.
Commun. (2000) 153. (b) Y. Wang,
B. Zou, L.-N. Xiao, N. Jin, Y. Peng,
F.-Q. Wu, H. Ding, T.-G. Wang, Z.-
M. Gao, D.-F. Zheng, X.-B. Cui, J.-
Q. Xu, J. Solid State Chem. 184
Keywords:
reduction,
reduction, Electrocatalytic water oxidation,
Polyoxometalates,
interactions.
Electrocatalytic
Electrocatalytic
H O
2 2
proton
Supramolecular
(
2011) 557. (c) J.-W. Cui, X.-B.
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A Polyoxometalate Supported
Copper Dimeric Complex:
Synthesis, Structure and
Electrocatalysis
N. Tanmaya Kumar, Umashish Bhoi and
Samar K. Das*
Art work for TOC

CRediT author statement
for the manuscript ICA_2019_1650
N. Tanmaya Kumar: Methodology,
Visualization, Writing- Original draft
preparation, Investigation. Umashis Bhoi:
Investigation, Methodology. Pragya
Naulakha: Software, Validation. Samar K. Das:
Conceptualization, Visualization, Supervision,
Reviewing and Editing.

Highlights.
►
Polyoxometalate
supported
copper dimeric complexes. ►
Crystal
structure analysis. ► Spectral
analysis. ► Electrocatalytic
water oxidation and
reduction.
►
Electrocatalytic
hydrogen peroxide reduction.