Download Giant Polyoxometalate Clusters Toward Applications

Giant Polyoxometalate Clusters Toward Applications
Sabbani Supriya, D. S. Kothari fellow at
School of Chemistry, University of Hyderabad, Hyderabad 500046
Zeolite-like materials having nano-sized holes and channels can act as filters and traps or hosts
for molecular guests. This type of porous compounds play an important role in many areas of
chemistry and materials science as they can be used for different tasks, for example, for
separation and storage purposes. My research interest deals with molecular metal oxide based
porous nano objects, which can encapsulate a large number of different guests. In this regard,
metal-oxide based polyoxometalate (POM) clusters, having an unparalleled range of physical
and chemical properties can be reliably utilized in the formation of new porous materials. Today,
POM chemistry is an emerging area that promises to allow the development molecule-based
materials and devices that exploit developments in instrumentation, nanoscale science, and
material fabrication methods. POM clusters related to molybdenum blue species were first
reported by Scheele in 1783. Their composition was largely unknown until Mueller and his coworkers reported the synthesis and structural characterization in 1995 of a very high nuclearity
cluster {Mo154}, which has a ring topology; this on changing the pH and increasing the amount
of reducing agent along with incorporation of acetate ligands is converted to a {Mo132}
spherical ball-like cluster. This class of highly reduced POM clusters is one of the most exciting
developments in POM chemistry with many potential applications in nanoscience.
A series of giant polyoxomolybdate (POM) molecules were separated from various
molybdenum blue solutions, including some wheel-shaped and hollow, spherical “Keplerate”
molecules, (scheme 1.) were synthesized by partially reducing MoVI to MoV via simple inorganic
synthesis
approaches.
The
spherical=
“Keplerate”
POM
(NH4)42[Mo72VIMo60V
O372(CH3COO)30(H2O)72] ca.300H20 ca.10CH3COONH4 ({Mo132}) has a very similar structure
to that of C60. Moreover, the 60 MoV can be replaced by 30 FeIII ions, leading to the formation
of giant molecules (Mo72VI Fe30
O252L102.ca.180H2O with L = H2O/CH3COO–/Mo2O8/9 n-,
III
{Mo72Fe30}) with novel magnetic properties, because it has 30 high spin FeIII centers (150
unpaired electrons) as shown in Scheme 2.
Scheme 1
These robust gaint keplerate skeleton exhibits 20 pores and 20 channels ending in the nanosized cavity (scheme 2). Majority of reports based on these giant keplerate mainly focuses
on novel topologies rather than functionality.
Scheme 2
{Mo132}
{Mo72Fe30}
The novelty of this type of inorganic molecules not only lies in their molecular structure but also
in their intriguing behavior in solution. The aim of the present proposal would be the exploration the application aspects of these giant POM molecules in the various contemporary areas, e.g., catalysis, host‐guest properties, red‐ox properties etc. Different accepts of this proposal are
discussed below in detail point by point:
Catalysis:
The {Mo132} system is a unique POM cluster anion in the sense that it has 60 electrons per
cluster anion. Thus it can be used as a reduction catalyst in organic transformation of reduction.
This can also be used in water reduction to hydrogen generation.
The {Mo72Fe30} cluster has molybdenum and iron ions in their highest oxidation states. This
nano object can be utilized as an oxidation catalyst in diverse organic transformation of industrial
importance, for example, oxidation of alkanes or arenes to corresponding alcohols, epoxidation
of alkenes etc, using molecular oxygen / hydrogen peroxide as an oxidant.
Biomimetic approach:
In many biological electron transfer reactions, iron containing enzymes / proteins is involved in
which enzyme / protein catalyzes oxidation reactions, oxidized by molecular oxygen. One such
example is the enzymatic function of sulfite oxidase. This enzyme catalyzes the oxidation of
sulfite to sulfate when {MoVIO2} center of the active site of the enzyme gets reduced to
{MoIVO}; to maintain the catalytic cycle, {MoIVO} center has to be oxidized back to {MoVIO2}.
Ferricytochrome C does this job of oxidation from {MoIVO} to {MoVIO2}. {Mo72Fe30} POM
cluster has 30 Fe(III) centers; so this cluster can be used to oxidize {MoIVO} model complexes to
corresponding {MoVIO2} model complexes to mimic this half reaction.
Surface modification / cavity modification
The {Mo-132} spherical cluster has been characterized by the presence of 132 molybdenum
atoms, each of which has at least one terminal oxo group (Mo=O) at external surface of the cage.
The terminal oxo group having lone pair of electrons can be used as potential ligands to
coordinate to other metal ions (alkali, alkaline earth, transition metal, lanthanide etc.) to get
metal coordination complexes attached to the external surface. This {Mo-132} spherical cluster
supported metal coordination complexes can be used a catalyst for organic transformations of
industrial importance. The same POM cluster has 30 acetate anions attached to the inner surface
directing toward the center of the cavity. These acetate anions can be replaced by other bidentate
mono anionic ligands and this way internal cavity surface can be modified. To the end part of the
ligand at the internal cavity surface, some N/S/O donors can be put which can form coordination
complexes in the cavity region with different metal ions. {Mo72Fe30} POM cluster has 30 Fe(III)
centers can also be used similarly.
Host Guest Chemistry
The synthesis and the detailed analyses of a supramolecular host guest type polyoxometalate cluster of the
type [SiMo12O40 MoVI 70MoV2FeIII30] have been demonstrated. The goal was to find an optimal preparation
method in order to obtain the supramolecular entities in high yield and high purity. The encapsulation of
the Keggin anion inside the cavity of the host type cluster was proved with infrared, electronic and Raman
spectroscopy, Mössbauer,
31P-NMR
spectroscopy and single crystal X-ray structure analysis. In the
present proposal, the encapsulation of lacunary Keggins will be attempted and then some transition metal
ion will be attempted to put at the lacunary position in solid-liquid interface (heterogeneous) reactions.
Methods
Synthetic:
The route to produce new POM cluster containing compounds are often very simple synthetic
manipulations requiring a small number of steps, or even just one step (one-pot syntheses). The
acidification, for example, of a solution of sodium molybdate will give rise to metal oxide
fragments (mainly as metal oxo anions), which increase in nuclearity as the pH of the solution
decreases. This gives rise to self assembly resulting in the formation / isolation of POM cluster
anion containing compounds. This traditional approach of aqueous synthesis of the POM cluster
can be extended to organic cations, and the solvent system can be extended to an
aqueous/organic solvent mixture; for example, water/CH3CN. The synthetic variations are: pH
of the reaction mixture / solution,reducing agent, concentration/type of metal oxide anion, ionic
strength, heteroatom type/concentration, presence of additional ligands, and temperature of
reaction and finally processing (e.g. microwave, hydrothermal, refluxing).
Selected Literatures Relevant to the Proposal:
A New Type of Supramolecular Compound: Molybdenum-Oxide-Based Composites consisting
of Magnetic Nanocapsules With Encapsulated Keggin-Ion Electron Reservoirs Cross-Linked to a
Two-Dimensional Network, A. Müller, Samar K. Das, Paul Kogerler, H. Bogge, M.
Schimidtmann, Alfred X. Trautwein, V. Schunemann, E. Krickemeyer, W. Preetz, Angew.
Chem. Int. Ed. 2000, 39, 3414-3417.
Polyoxometalates: Building Blocks for Functional Nanoscale Systems, D. L. Long, R.
Tsunashima, and L. Cronin, Angew. Chem. Int. Ed. 2010, 49, 1736 – 1758.
Cross-linking nanostructured spherical capsules as building units by crystal engineering: related
chemistry, A. Müller, Samar K. Das, E. Krickemeyer, Paul Kogerler, H. Bogge, M.
Schimidtmann, Solid State Sciences 2, 2000, 847–854.
Linking Icosahedral Strong Molecular Magnets {MoVI72FeIII30} to layers-A solid- State Reaction
at Room Temperature, A. Müller, E. Krickemeyer, Samar K. Das, Paul Kogerler, Sabyasachi
Sarkar , H. Bogge, M. Schimidtmann, Shatarupa Sarkar, Angew. Chem. Int. Ed. 2000, 39, 16121614.
A. Müller, E. Krickemeyer, H. Bogge, M. Schimidtmann, F. Peters, Angew. Chem. 1998, 110,
3567 – 3571; Angew. Chem. Int. Ed. 1998, 37, 3359 – 3363.
Open and Shut for Guests in Molybdenum Oxide-Based Giant Spheres, Baskets, and Rings
Containing the Pentagon as a Common Structural Element, Achim Müller, Sebastian Polarz,
Samar K. Das, Erich Krickemeyer, Hartmut Bögge, Marc Schmidtmann, and Björn Hauptfleisch,
Angew. Chem. Int. Ed. 1999, 38, 3241-3245
Catalytic properties of the macromolecular polyoxomolybdate cluster in selective oxidation of
sulfides, N. V. Izarova,a O. A. Kholdeeva,b M. N. Sokolov,a,c and V. P. Fedin, Russian Chemical
Bulletin, International Edition, 2009, 58, 134—137. Archimedean Synthesis and Magic Numbers: Sizing Giant Molybdenum-Oxide-Based Molecular
Spheres of the Keplerate Type, Achim Müller, Sabyasachi Sarkar, Syed Qaiser Nazir Shah,
Hartmut Bögge, Marc Schmidtmann, Shatarupa Sarkar, Paul Kögerler, Björn Hauptfleisch,
Alfred X. Trautwein, and Volker Schünemann, Angew. Chem. Int. Ed. 1999, 38, 3238-3241.