Inorganica Chimica Acta 511 (2020) 119729

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Inorganica Chimica Acta

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Research paper

Reversible redox activity of {Mo72Fe30} nano-polyoxometalate cluster in T

three crystalline forms
Kesar Tandekar, Pragya Naulakha, Sabbani Supriya
⁎

School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India

ARTICLE INFO ABSTRACT

Keywords: The {Mo72Fe30} cluster is the basic unit of three crystalline materials rhombohedral
Polyoxometalate [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91]·150H2O (1), orthorhombic
Redox [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3(H2O)87]·80H2O (2) and host guest system
Solid state [HxPMo12O40 ⊂ H4MoVI III
72Fe30(CH3COO)15-O254(H2O)98]·ca.60H2O (3). The {Mo72Fe30} Keplerate cluster in all
Reversible
the three crystalline compounds actively undergoes reversible redox cycles without any significant structural
Keplerate
disintegration. Hydrogen peroxide/molecular oxygen and ascorbic acid/sodium dithionite are found to be sui-
table oxidizing and reducing agents, respectively. Compound 1 (reasonably soluble in water) shows reversible
electron transfer reactions in a homogeneous aqueous medium, whereas, compounds 2 and 3 (having extended
structures, thereby insoluble in water) undergo reversible electron transfer reactions in heterogeneous manner in
solid-liquid interface reactions.

1. Introduction and hetero-polyanions. The ability of POMs to exhibit reversible redox

activity by switching between two electronic states without disin-
Polyoxometalates (POMs) are mostly anionic clusters composed of tegration makes them potential candidates for diverse applications in-
early transition metal ions in their highest oxidation states, such as, W cluding catalysis, sensing etc. [28–34]. Though the redox activity of
(VI), Mo (VI), V(V), etc [1,2]. The characteristic properties like high Keggin and Dawson type of POMs’ have been explored immensely
charge, nanoscale dimensions, tuneable electrochemical and photo- [35–38], the redox properties of Keplerates, the spherical giant metal-
chromic properties make them attractive candidates in the field of oxide clusters are hardly investigated. The Keplerate compounds con-
materials science [2–7] and medicinal chemistry [8–16]. Keplerates, taining {Mo72Fe30}, the basic structure of which is made up of 12
discovered by Müller and co-workers are nano-sized icosahedral poly- pentagonal [{MoVI)Mo5VIO21(H2O)6]6− units held together by 30 {FeIII
oxometalate clusters, made up of 12 pentagonal units and 30 linkers linkers and further stabilized by acetate ligands [39], are reported in
[17–18]. Different combinations of linkers and pentagonal units lead to two different yellow crystalline forms, namely, rhombohedral com-
the isolation of a variety of Keplerates [2,7]. The basic pentagonal unit pound [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}
of the Keplerate is represented as [(MVI)M5VIO21]6−. Keplerates are (H2O)91]·150H2O (1) [39] and orthorhombic compound
mainly of the type {M72M'30} (M = MoVI, M' = VIV, CrIII, FeIII, MoV) and [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3
{M72Mo60} (M = Mo, W) [2]. {Mo72Fe30} and its parent cluster (H2O)87]·80H2O (2) [40,41]. In the year 2000, another very prominent
{Mo132} which differ on the type of linker connecting the pentagonal supramolecular host-guest Keplerate PMo12O40 ⊂ {Mo72Fe30} ≡
units belong to the class of Keplerates. The class of nanoscale molecular [HxPMo12O40 ⊂ H4MoVI III
72Fe30(CH3COO)15O254(H2O)98].ca·60H2O
metal-oxide based Keplerates have been expanding due to their syn- (x = 1,2) (3) [42] was reported by Müller’s group, wherein a tetra-
thetic strategies as well as their unique properties. Some of the prop- hedral guest, Keggin is encapsulated within an icosahedral host,
erties include nano-encapsulation, separation and transport along with {Mo72Fe30} held together by noncovalent interactions. Various spec-
surface supramolecular chemistry [2,19–20]. Though the magnetic troscopic studies were carried out for a molybdosilicate based reduced
properties of icosahedral Keplerates of type {M72M'30} (M = MoVI, M' host-guest system SiMo12O40 ⊂ {Mo72Fe30} similar to 3 [43]. These
= VIV, CrIII, FeIII) have been studied extensively [21–27], Keplerates are studies established that the overall reduced host-guest hybrid system,
still one of the lesser explored classes of polyoxometalates unlike iso such as, the one under study and its analogues consist of reduced shell

⁎
Corresponding author.
E-mail address: ssabbani@mail.jnu.ac.in (S. Supriya).

https://doi.org/10.1016/j.ica.2020.119729
Received 18 December 2019; Received in revised form 6 April 2020; Accepted 30 April 2020
Available online 04 May 2020
0020-1693/ © 2020 Elsevier B.V. All rights reserved.
K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

{Mo72Fe30} and a non-reduced guest. The electrons remain delocalized in a one-pot synthesis in presence of air [42]; green plate shaped
on the periphery of the {Mo72Fe30} Keplerate as this arrangement is crystals were obtained after one week. The single crystal X-ray struc-
energetically more favourable. While rhombohedral {Mo72Fe30} com- tures, reported for crystals of compounds 2 and 3 mounted from mother
pound (1) is present as discrete clusters, the orthorhombic {Mo72Fe30} liquor (namely, wet crystals), show packing of discrete spherical
compound (2) and supramolecular host-guest Keplerate PMo12O40 ⊂ {Mo72Fe30} and HxPMo12O40 ⊂ {Mo72Fe30} clusters respectively.
{Mo72Fe30} (3) are layered (extended) structure arising due to the Therefore, freshly synthesized crystals of compounds 2 and 3 are so-
linking of each cluster to four other surrounding clusters via Fe-O-Fe luble in water. The crystal structures of dried crystals of compounds 2
bonds formed in a solid state condensation process. As expected, and 3, on the other hand, show formation of sheet structure by linking
compound 1 (remain as discrete clusters in the relevant crystals) is of respective individual {Mo72Fe30} and HxPMo12O40⊂{Mo72Fe30}
moderately soluble in water and compounds 2 and 3, having extended clusters by Fe-O-Fe bonds arising due to the condensation reaction
linked structures, are insoluble in water. between the H2O ligands of the FeIII linkers. Due to the formation of this
Herein we present the reversible redox properties of {Mo72Fe30} extended structure, the crystals of 2 and 3 become insoluble (Scheme
compounds (orthorhombic as well as rhombohedral crystals) and the 2). Though the synthesis of completely oxidized form of {Mo72Fe30}
relevant host guest Keplerate compound 3. Compounds 1 and 2, which cluster has been achieved in the case of compounds 1 and 2, the
are completely oxidized, are formed by Mo(VI) and Fe(III) metal centers synthesis of empty {Mo72Fe30} cluster containing compounds in its
and isolated as yellow crystalline materials. On the other hand, com- reduced form by a direct method is tedious/complicated using inert
pound 3 is obtained as a green crystalline substance due to the existent atmosphere [43,44]. The reversible homogenous redox cycles of com-
reduced {Mo72Fe30} shell. The reversible redox properties of compound pound 1 were carried out at ambient conditions. The yellow aqueous
1 are studied in aqueous solution with sodium dithionite as reducing solution of compound 1 converted into a blue solution after treatment
agent (homogeneous) and molecular oxygen as an oxidant. Reversible with sodium dithionite. Here sodium dithionite acts as the reducing
redox behaviour of compounds 2 and 3 have been examined in the solid agent.
state as their extended structure renders them to be insoluble in As shown in Table 1, the formation of the blue colour solution
common solvents. The solid state redox properties of compound 2 are clearly indicates the reduction of the {Mo72Fe30} cluster in compound
studied by using ascorbic acid as a reducing agent and molecular 1. On standing at room temperature, the blue coloration of the solution
oxygen as an oxidant. Furthermore the solid state redox cycles of fades with regeneration of yellow colour solution of 1ox due to its slow
compound 3 can be brought about by hydrogen peroxide as an oxi- oxidation by molecular oxygen from air. However, when ascorbic acid
dizing agent and ascorbic acid as a reductant (Scheme 1). The redox is utilized as a reducing agent, although a blue colour solution is ob-
behaviours of compounds 1, 2 and 3 are presented concisely in Table 1. tained, it does not revert back to the original oxidized compound —
Multiple redox cycles have been carried out for all the three compounds demonstrating an irreversible conversion (yellow colour of the solution
without any evident structural disintegration of the system. Compound is not revived). This redox reaction of {Mo72Fe30} in compound 1 can
1, regenerated after the several redox cycles and recovered post solvent also be extended to insoluble {Mo72Fe30} compound 2. The solid state
evaporation, has been characterized by routine spectral analysis in- redox cycles of dried yellow crystals of compound 2 were carried out in
cluding IR, PXRD studies etc. The reduced and oxidized forms of com- a solid liquid interface reaction under ambient conditions. When an
pounds 2 and 3 have also been characterized by various spectroscopic aqueous suspension of yellow crystalline compound 2 is treated with
techniques. ascorbic acid solution, it transforms into green crystalline solid without
dissolution. The reduced compound 2red is separated from the reaction
mixture and washed with water. Compound 2red, on standing at room
2. Result and discussion
temperature for two hours slowly converts back to oxidized yellow
colour compound 2ox. Apart from molecular oxygen from air, 1red and
Compounds 1, 2 and 3 were synthesized following reported proce-
2red can also be oxidized using hydrogen peroxide. The various spec-
dures [39–40,42]. The hot acidified reaction mixture consisting of FeIII
troscopic techniques, presented here, have been performed on sodium
and acetate ions along with {Mo132} Keplerate after one week afforded
dithionite reduced 1red, ascorbic acid reduced 2red and molecular
yellow rhombhohedral shaped crystals of 1 [39]. The plate shaped
oxygen oxidized 1ox and 2ox. Therefore, the reversible redox conversion
crystals of compound 2 can be obtained from an acidified aqueous so-
cycles conducted for {Mo72Fe30} containing compounds 1 and 2 suc-
lution of 1 or from hot acidified reaction mixture of ferric chloride,
cessfully demonstrate the reactive nature of {Mo72Fe30} cluster. Ad-
sodium acetate and {Mo132} [40]. Compound 3 was synthesized from
ditionally, these reversible solid state redox cycles can be achieved by
an acidified mixture of FeII ions, molybdate, acetic acid and phosphate

Scheme 1. Reversible redox transformation of the {Mo72Fe30} unit in presence of appropriate oxidizing agent (H2O2 or atmospheric O2) and reducing agent
(Na2S2O4 or ascorbic acid). The {Mo72Fe30} unit is depicted in polyhedral representation.

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K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

Table 1
Redox behaviour of compounds 1, 2 and 3. The Table shows the products obtained after the respective oxidation and reduction cycle.
Compound Reaction medium Oxidizing Agent Reducing Agent Reduced form Oxidized form

Solution (aqueous medium) Molecular O2 from air Sodium Dithionite

Compound 1red Compound 1ox

Compound 1
Solid liquid interface Molecular O2 from air Ascorbic Acid

Compound 2 Compound 2red Compound 2ox

Solid liquid interface H2O2 Ascorbic Acid

Compound 3 Compound 3red Compound 3ox

Scheme 2. The schematic representation of formation of extended structure in compounds 2 and 3.

employing ascorbic acid/sodium dithionite as reducing and molecular for the solid-liquid interface oxidation of 3 to 3ox, only hydrogen per-
oxygen from air as oxidizing agents (Table 1). oxide resulted in the expected complete conversion (Table 1). The
The redox activity of {Mo72Fe30}, observed for compounds 1 and 2, peroxide-oxidized 3ox and ascorbic acid reduced 3red compounds have
can also be envisaged for the host guest compound 3, wherein the been characterized by various spectroscopic studies.
cluster cage/shell is already reduced and can be oxidized with hydrogen The regenerated compound 1ox (obtained by exposing 1red to at-
peroxide in a solid-liquid interface reaction without dissolution. The mospheric O2 followed by water evaporation) and parent compound 1
reduced compound is oxidized by addition of hydrogen peroxide to an display identical IR spectra ((Fig. 1, I a-b) showing characteristic peaks
aqueous suspension of 3. The visual colour change from green to yellow at ¯ (ATR/cm−1) = 1617 (m, δ (H2O)), 1526 (m, νas(COO)), 1415 (s-m,
is observed, indicating complete conversion to its oxidized (3ox) form νs(COO)), 950 (m, ν(Mo = O)), 850 (m), 758 (s), 626 (m), 556 (s), 425
[43]. The oxidized compound was separated from reaction mixture, (m)) and PXRD spectra (Fig. 1I). The comparison of the IR spectra of
washed with water and dried at room temperature. The dried oxidized compounds 2, 2red and regenerated compound 2ox show the structural
compound (3ox) is stable at ambient conditions and shows no tendency stability of the {Mo72Fe30} entity during the redox conversions (Fig. 1,
to convert back to its original reduced form as in compound 3. Com- IIa-c; ¯ (ATR/cm−1) = 1617 (m, δ (H2O)), 1536 (m, νas(COO)), 1415 (s-
pound 3ox though can be synthesized from a mixture of FeIII ions, m, νs(COO)), 960 (m), 948 (m, ν (Mo = O)), 849 (sh), 759 (s), 626 (w-
Keggin, acetic acid and NaOH [45], it can also be easily obtained from m), 556). The IR spectra of 3ox and 3red are similar to that of compound
an alternate method of post synthetic chemical oxidation of 3 (in the 3 indicated by presence of characteristic peaks at ¯ = 1616 (m, δ
present work). The reduced compound 3red can then be regenerated by (H2O)), 1528 (m, νas(COO)), 1420 (s-m, νs(COO)), (1062 (m, νas(PO4)),
addition of ascorbic acid to the yellow dispersion of 3ox which shows 952 (m, ν(Mo = Ot)) cm−1 (Fig. 1 IIIa-c) signifying the robustness of
the gradual transition from yellow to green form (3red). The compound the host–guest complex under redox conditions.
3 undergoes multiple cycles of reversible chemical switching between The PXRD patterns of parent yellow compound 1 and the yellow
the reduced and oxidized forms without any loss of redox activity. solid, obtained from the dithionite-reduced green solution followed by
Apart from ascorbic acid, other reducing agents, for example, sodium its aerial oxidation and evaporation of the resulting yellow solution, are
dithionite and phenyl hydrazine have been used to reduce the peroxide- shown in Fig. 2 (Ia) and Fig. 2 (Ib) respectively. The identical features
oxidized 3ox. Both ascorbic acid and sodium dithionite are able to bring of these plots establish the reversibility between compound 1ox and
about the reduction of 3ox where as phenyl hydrazine does not show the compound 1red.
desired conversion of 3ox to 3red. Though other oxidizing agents such as Interestingly, the crystallinity of the compounds 2 and 3 is main-
benzoquinone and m-chloroperoxybenzoic acid (m-CPBA) were utilized tained even after their respective reductive and oxidative cycles. The

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K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

Fig. 1. Comparative IR spectra (I) a) Compound 1, b) Compound 1ox recovered post water evaporation after first redox cycle of 1; (II) a) Compound 2, b) Ascorbic
acid reduced 2red, c) Atmospheric O2 oxidized 2ox; III) a) Compound 3, b) Peroxide oxidized 3ox, c) Ascorbic acid reduced 3red.

powder X-ray diffraction (PXRD) patterns recorded for compound 2 and O40 ⊂ H4MoVI V III
68Mo4 Fe30(CH3COO)16O254(H2O)96]·ca.60H2O[43]. This
its reduced counterpart 2red (Fig. 2 IIa-c) and compound 3 and its clearly indicates that the title compounds do not disintegrate during the
oxidized counterpart 3ox (Fig. 2 IIIa-c) are found to be identical. Also, red-ox cycles in solid liquid interface reactions.
the compounds 2ox and 3red retain their crystallinity and show similar The multiple cycles of reversible redox transformations for com-
PXRD patterns (Fig. 2 IIc and IIIc) as their respective parent compounds pound 1, compound 2 and compound 3 were monitored by the
2 and 3. UV–visible spectral studies. The solution UV–visible spectra for com-
The PXRD patterns obtained for these microcrystalline compounds pound 1red shows a single broad absorption peak at 570 nm (Fig. 4
clearly illustrate the structural stability of compounds 2 and 3 and re- inset) while the solid state electronic spectra recorded for compounds
inforce the fact that their extended structure remains intact during their 2red and 3red show three broad absorption peaks at 570, 850 and
redox transformations. The unit cell parameters of compound 3ox and 1085 nm (Fig. 4). For all the three compounds, the reduced compounds
regenerated compound 3red, which are identical, re-establish the re- are differentiated from their respective oxidized counterparts with ap-
versibility of the redox cycle and the non-disintegration of compound 3 pearance of these peaks in visible and near IR region. In all the three
under redox conditions. The little shift in the PXRD peak positions is crystalline systems, the basic molecule is {Mo72Fe30} cluster cage, in
due to the loosely held lattice water molecules, present in the crystal which there are two types of metal centres: Mo(VI) and Fe(III) that can
lattice of such giant clusters. But still the powder patterns match almost be reduced. The formation of green colored reduced forms indicate the
with the original patterns, as shown in Fig. 2. reduction of Mo(VI) centres to Mo(V), which is consistent with the
We have recorded the Raman signals of the resulting compounds appearance of inter-valence charge transfer (IVCT) bands in the UV–-
after treatment of compound [HxPMo12O40 Visible spectra, corresponding to the transitions between Mo(V) to Mo
⊂ H4MoVI III
72Fe30(CH3COO)15O254(H2O)98]·ca.60H2O (3) (which is al- (VI). These IVCT peaks are not present in the oxidized forms of icosa-
ways obtained as reduced form, from as such synthesis) with H2O2 hedral {Mo72Fe30} clusters due the presence of only Mo (VI) centres.
(oxidant) and compound [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)} The five redox cycles for compounds 1, 2 and 3 have been mon-
{H2Mo2O8(H2O)}3 (H2O)87] · 80H2O (2) (which is always obtained as itored by appearance and disappearance of the IVCT peaks, that are
oxidized form, from its as such synthesis) with ascorbic acid (reducing correlated to the reduction and oxidation of corresponding compounds
agent) and the Raman plots are shown in Fig. 3. The main two Raman (Section S3, SI). The analysis of five redox cycles of compound 1 re-
signals at around 970 cm−1 {ν(Mo = O)} and at around 830 cm−1 {νs corded in aqueous solutions shows the red-ox stability of the system.
(Mo–O–Mo)} are characteristics of {Mo72Fe30} cage. These Raman The IR and PXRD analyses (Section S1 and S2, SI) of compound 1ox
spectra are almost identical to the Raman spectrum of recovered after five redox cycles also substantiate for the stability of
Na6[SiMo12O40 ⊂ H4MoVI III
72Fe30(CH3COO)16O254(H2O)96]·ca.60H2O compound 1. The robustness of crystalline compounds 2 and 3 during
[40–43]. The other small features of the Raman spectra five redox cycles can be monitored by UV–visible spectral studies. The
(Fig. 3) are identical to those of Na6[SiMo12 oxidized and reduced forms of compounds 2 and 3, isolated after five

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K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

Fig. 2. Comparative PXRD spectra (I) a) Compound 1, b) Compound 1ox recovered post water evaporation after first redox cycle of 1; (II) a) Compound 2, b) Ascorbic
acid reduced 2red, c) Atmospheric O2 oxidized 2ox; (III) a) Compound 3, b) Peroxide oxidized 3ox, c) Ascorbic acid reduced 3red.

redox cycles, were also further characterized by IR and PXRD studies as follows. The reaction mixture of 1, reduced by dithionite, consists of
(Section S1 and S2, SI). It has been observed that compounds 2 and 3 oxidized products of sulphite along with unreacted dithionite.
remain stable retaining their sheet structure and show no considerable Furthermore, it is well known that dithionite gradually undergoes de-
disintegration even after five redox cycles (see Fig. 4). composition in presence of oxygen which further complicates the ti-
Attempts to quantify the extent of reduction of the compounds 1 tration process. Consequently, the reaction mixture which needs to be
and 2 by redox titration could not be carried out due to several reasons analyzed consists of various by-products of the dithionite which also

Fig. 3. Raman spectra: —○— plot, compound obtained by the treatment of compound 3 with H2O2; —Δ— plot, compound obtained by the treatment of compound 2
with ascorbic acid.

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K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

[PMoVI12O40]
3−
is a red-ox active POM, meaning that, its Mo(VI) centers
can be reduced to corresponding Mo(V) centers. This is why, its CV
(Fig. 5c, —○—plot) shows several other redox features besides the
common MoVI/MoV couple, found in the CVs of compounds 1 and 2.
Compound 3 is already isolated as its reduced form (it has been proven
by Mueller group [43] that cluster cage is reduced by 4 electrons, but
not the encapsulated Keggin anion). So the as-synthesized compound 3
can be represented as 3red. This can be oxidized by H2O2 in a solid
liquid interface reaction to 3ox, which shows the CV (Fig. 5c, —■—
plot). As shown in Fig. 5c, the CV features of 3ox ( —■— plot) are
identical to those (—○— plot) of parent 3 or 3red. This unequivocally
establishes that the oxidation of parent 3 or 3red to 3ox by H2O2 and
Fig. 4. Comparative diffuse reflectance spectra for compound 1 and 1ox; com- reduction back of 3ox by ascorbic acid are reversible and can be cycled
pound 2, 2red and 2ox and compound 3, 3ox and 3red. The inset shows the multiple number of times.
solution UV–Visible spectra of compound 1; 1red and 1ox.
3. Conclusions
undergo oxidation along with the oxidation of blue reduced form of
compound 1. Hence an attempt to titrate the blue reduced reaction In the past, studies related to crystalline compounds 1, 2 and 3,
mixture of compound 1 would result in an erroneous value. The in- which contain the icosahedral nano-dimensional basic unit {Mo72Fe30}
solubility of compound 2 and instability at high pH pose problems for cluster, have been limited to their synthesis, characterization and ap-
the quantification of the amount of reduction of compound 2. plication as templates for the construction of nano-materials and mixed
Therefore, in the present work, the focus has been on the qualitative metal oxides. The reversible redox conversions, mediated by these
redox activation of {Mo72Fe30} cluster. compounds have not been delved into until now. During the analysis of
We have performed the electrochemical studies of these three model the redox behaviour of these three model compounds, we have suc-
crystalline compounds of {Mo72Fe30}. Compound cessfully demonstrated their reversible redox conversions, brought
[Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)} about by diverse reducing agents. Interestingly, the oxidation of these
(H2O)91]·150H2O (1) is soluble in water. Cyclic voltammogram (CV) of compounds can be brought about by the freely available atmospheric
this compound was performed in an aqueous solution using sodium oxygen or by the cheap and environment friendly hydrogen peroxide.
sulfate as a supporting electrolyte. This shows a redox feature at The fact that these compounds undergo no kind of structural disin-
+0.12 V vs NHE (the reductive response at around 0 V vs NHE) as tegration while exhibiting the redox properties make them potential
shown in Fig. 5(a), black plot. This couple can be attributed to MoVI/ candidates for homogeneous (in case of compound 1) and hetero-
MoV couple. This indicates that this system can be easily reduced either geneous (in case of compounds 2 and 3) catalysts (see supplementary
chemically or electrochemically. When this yellow solution is reduced materials, section S4). We have effectively employed compounds 2 and
with sodium dithionite, it turns into green solution, that shows the CV 3 as oxidation catalysts in the synthesis of sulfoxides. The reduced
feature, shown in Fig. 5(a). This shows the MoVI/MoV couple at a little {Mo72Fe30} form can also be exploited as reducing agent for the
anodic shifted position with a big current height increase around synthesis of metal nano particles resulting in the generation of hybrid
+1.0 V vs NHE. This may be due to the electrocatalytic oxidation of materials of the type nanoparticle decorated Keplerates which can be
SO2, formed during the chemical oxidation of dithionite ion. The di- employed in catalysis reactions of various organic transformations.
thionite reduced green-colored solution reversibly and chemically Therefore, it would not be wrong to say that the reversible redox ac-
comes back to yellow colored solution by aerial oxygen, which shows tivation of {Mo72Fe30} cluster depicted in these representative com-
the CV, identical to that of parent 1 aqueous solution. Compound pounds opens up a new dimension in Keplerate type polyoxometalate
[H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3 chemistry.
(H2O)87]·80H2O (2), two-dimensional coordination polymer, formed
from the building unit {Mo72Fe30} cluster by Fe–O–Fe linking, is in- 4. Experimental section
soluble in water. This shows a CV (Fig. 5b), which was obtained in a
heterogeneous manner from an aqueous solution (0.1 M Na2SO4 solu- 4.1. General
tion, supporting electrolyte). As shown in Fig. 5(b), compound 2 shows
the similar MoVI/MoV couple, (but at around 0.37 V vs NHE), as ob- All the chemicals (ammonium heptamolybdate tetrahydrate
served in the case of compound 1 (at +0.12 V vs NHE), along with ((NH4)6Mo7O24·4H2O), ammonium acetate (CH3COONH4), hydrazine
other irreversible reductive responses, that can be due to the reduction sulphate (N2H4·H2SO4), ferric chloride hexahydrate (FeCl3·6H2O), so-
of iron centers of the {Mo72Fe30} cluster. So, the chemical reduction of dium acetate trihydrate (CH3COONa·3H2O), ferrous chloride tetra-
compound 2 by ascorbic acid in a solid liquid interface reaction is not hydrate (FeCl2·4H2O), sodium molybdate dihydrate (Na2MoO4·2H2O),
surprising. The —■— plot in Fig. 5(b) is the CV of 2red, obtained by sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O), sodium
reduction of compound 2 by ascorbic acid (aqueous) solution (com- chloride (NaCl), glacial acetic acid (CH3COOH), conc. hydrochloric acid
pound 2 is insoluble in water; so this reduction occurs in a solid liquid (HCl), ascorbic acid, sodium dithionite (Na2S2O4), hydrogen peroxide
interface reaction). The features of both CVs (Fig. 5b) are almost (H2O2; 30% w/v), acetonitrile (CH3CN), methyl phenyl sulfide
identical. This clearly shows that the reduction of yellow coloured (MeSPh)) were of reagent grade, received from commercial sources and
compound 2 (by ascorbic acid) to green crystalline solid 2red and oxi- used as received. Distilled water was used throughout the synthesis.
dation back of 2red (green) to yellow coloured compound 2 by aerial Fourier transform infrared spectra (FT–IR) of the solid samples were
oxygen or H2O2, are reversible and can be cycled multiple recorded in the ATR mode at room temperature in the range of
number of times. Compound [HxPMo12O40 400–4000 cm−1 on a Shimadzu FT–IR spectrophotometer. The powder
⊂ H4MoVI III
72Fe30(CH3COO)15O254(H2O)98]·ca.60H2O (3) (a host-guest
X-Ray diffraction (PXRD) studies were carried on a Rigaku X-ray
system) is also a coordination polymer (CP), like compound 2, but Powder Diffractometer Miniflex-600 using CuKα1 radiation (1.54 Å) in
encapsulates a Keggin type cluster anion in the cavity of the {Mo72Fe30} the range 5°–60° at the rate of 4° min−1. The unit cell for single crystals
cluster, the building unit of the CP. This particular Keggin anion, was measured on a Bruker Quest D8 using MoKα1 radiation (0.7 Å).
The room temperature NMR was recorded on a Bruker Advance III

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K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

Fig. 5. Cyclic voltammograms: (a) compound [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91] ·150H2O (1) and 1red green solution obtained by
reduction of solution of 1 with sodium dithionite. Cyclic voltammogram (CV) of compound 1 and 1red have been taken in homogeneous manner in aqueous solutions
(compound 1 is soluble in water). 0.1 (M) Na2SO4 was used as supporting electrolyte. (b) [H4Mo72Fe30O254(CH3COO)10{Mo2O7(H2O)}{H2Mo2O8(H2O)}3(H2O)87] ·
80H2O (2), 2red, the reduced compound of 2, obtained by reducing compound 2 with ascorbic acid. (c)
[HxPMo12O40 ⊂ H4MoVI III
72Fe30(CH3COO)15O254(H2O)98]·ca.60H2O (3), 3ox, the oxidized product of 3, obtained by oxidizing compound 3 with H2O2. 0.1 (M) Na2SO4
was used as supporting electrolyte. CVs of compounds 2, and 2red, 3 and 3ox (these are insoluble in water) were taken in heterogeneous manner. Compounds were
quoted on glassy carbon electrode. Scan rate for all CVs is 100 mV/sec.

(500 MHz). The diffused reflectance spectroscopy (DRS) experiments electrode was dried under the IR lamp (temperature ∼70 °C). The
were conducted on a Shimadzu UV 2600 UV–vis spectrophotometer in Raman measurements are carried out by taking a small amount of the
the wavelength range of 220–1400 nm. The solution state UV–visible solid sample pressed between two glass slides so as to form a thin layer
spectroscopy experiments were performed on a Shimadzu UV 2600 of the sample on one of the glass slides. A WITec Confocal Raman
UV–Vis spectrophotometer in the wavelength range 200–800 nm. The spectrometer was used to record the Raman spectra for the solid sam-
catalysis product was analyzed by gas chromatography on a Shimadzu ples deposited on the glass slide. A 532 nm laser was used as the ex-
GC-2014 A equipment. The NMR techniques were available at citation source.
Advanced Instrumentation Research Facility (AIRF), JNU. All electro-
chemical experiments were performed using a Zahner Zanium electro- 4.2. Synthesis of compounds 1, 1red and 1ox
chemical work station operated with Thales software. All electro-
chemical experiments were conducted using a three-electrode Compound 1. 1 was synthesized using the procedure, as reported in
electrochemical cell consisting of glassy carbon (GC) as working elec- literature [39].
trode, Ag/AgCl (1 M) as the reference electrode, and Pt-flag as counter Compound 1red. An aqueous solution of 1 was prepared by dissolving
electrode in 0.1 M Na2SO4 aqueous solution as electrolyte. For the in- 0.1 g (5.55 µmol) in 20 mL H2O. To 10 mL of this yellow color solution
soluble compounds 2 and 3, electrochemical measurements were car- was added 0.5 mL of aqueous sodium dithionite solution (0.16 mmol,
ried out by coating an ink suspension made from the sample on the 0.029 g in 10 mL H2O) drop wise while stirring, forming a green color
working electrode surface in a heterogeneous manner. Generally, for solution.
the preparation of sample-modified electrode, 8 mg of compound (in- Compound 1ox. Standing the green color solution of 1red in air for 2 h
soluble) and 2 mg of acetylene black carbon were taken in 2 mL of yields yellow solution of 1ox.
EtOH/H2O (3:2) mixture, and to it 20 μL of 5 wt% Nafion (aq) was
added. The resultant mixture was then sonicated for 30 min to give a 4.3. Synthesis of compounds 2, 2red and 2ox
homogeneous suspension. Ten microliters of homogeneous ink was
coated on glassy carbon electrode (geometrical area = 0.0706 cm2) by Compound 2. 2 was synthesized according to the reported procedure
drop casting resulting in a coating of 40 μg sample on the electrode [40].
surface. The similar loading was maintained for all electrochemical Compound 2red. 0.3 mL aqueous solution of reducing agent, ascorbic
experiments unless stated otherwise. The sample ink, coated on the acid (0.18 mmol, 0.03 g in 10 mL H2O) or sodium dithionite

7
K. Tandekar, et al. Inorganica Chimica Acta 511 (2020) 119729

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(SB/EMEQ-090/2014) for the financial support. The financial grant A 349 (2008) 222–228.
from UPE-II (project No. 58), DST-PURSE and DST-FIST are gratefully [38] D.R. Park, S. Park, Y. Bang, I.K. Song, Appl. Catal. A 373 (2010) 201–207.
[39] A. Müller, S. Sarkar, S.Q. Shah, H. Bögge, M. Schmidtmann, S. Sarkar, P. Kögerler,
acknowledged. KT thanks CSIR for fellowship. PN thanks UGC-BSR for B. Hauptfleisch, A.X. Trautwein, V. Schünemann, Angew. Chem. Int. Ed. 38 (1999)
fellowship. We thank AIRF an instrumentation facility at JNU. 3238–3241.
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Appendix A. Supplementary data
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