POLYOXOMETALATE STABILIZED MANGANESE DIOXIDE NANOCUBES

We recently discovered that metal-oxide cluster anions (polyoxometalates, or POMs) could serve as covalently attached ligands for metal-oxide nanocrystals, giving new structures uniquely positioned between POM macroanions and traditional colloidal metal-oxide nanocrystals [1]. We now report the synthesis of water soluble manganese-dioxide based (MnO₂) nanocubes via reduction of permanganate (MnO₄^-) by a Keggin type POM containing a chromium(III) metallic center. During reaction, Cr(III) is oxidized to Cr(V) as Mn(VII) is reduced to Mn(IV). The process results in the formation of ca. 2-3 nm MnO₂ nanospheres (definitive confirmation is in progress) which self-assemble into ca. 100 nm nanocubes (according to DLS and cryo-TEM) which are stable in water with highly negative zeta-potential values. Electron diffraction data obtained after drying the material is being used to determine the structure of the MnO₂ component of the assemblies. The highly negative zeta-potential value of the nanocubes suggests that the POM cluster-anions are structurally integral. This was confirmed by electrospray ionization mass spectroscopy (ESI-MS), after using ascorbic acid to reduce the MnO₂ components to Mn(II), and by FT-IR, UV-vis, EPR and Raman spectroscopy, and differential pulse voltammetry. Additional analytical and reactivity studies are in progress.

[1] M. Raula, G. Gan Or, M. Saganovich, O. Zeiri, Y. Wang, M. R. Chierotti, R. Gobetto, I. A. Weinstock, Angew. Chem. Int. Ed. 2015, 54, 12416 –12421 (“Hot paper”).