Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.