In addition to the use of metal-oxide cluster-anions (polyoxometalates, or POMs) as ligand shells of gold-nano particles,[1] a new role of POMs as covalently coordinated inorganic ligands for individual anatase-TiO2 nanocrystals has recently been reported.[2] Meanwhile, POMs could also serve as pentadentate “capping” ligands for hematite-Fe2O3 nano-cores including oxide, oxy-hydroxide and hydroxide cores of metal cations from throughout the transition series, as well as from main group elements. Covalent attachment of the POM anions allows repeated precipitation (by added salt), and re-dissolution in water without any aggregation, and the optically clear solutions of these unique complexes are stable in water over a wide range of pH values, which spans the isoelectric point of metal-oxides. This new class of materials, reside at the interface between molecular macroanions and traditional colloidal metal-oxide nanocrystals. A brief summary of preparation and characterization of POM complexes of α-Fe2O3 will be presented, followed by kinetic data and mechanistic analyses of visible-light-driven water oxidation. The new findings show how POM ligation can harness the reactivity of metal-oxide nanocrystals within inherently stable nano-structures, whose mechanistic roles in catalysis can be investigated using solution-state methods traditionally reserved for molecular complexes.

Figure 1. Water oxidation by water-soluble complexes of hematite

References
[1] (a) M. Zhang, J. Hao, A. Neyman, Y. Wang, I. A. Weinstock, Inorg. Chem. 56 (2017) 2400-2408 (Front cover) (b) Y. Wang, M. Raula, Y. Wang, O. Zeiri, S. Chakraborty, G. Gan-Or, E. Gadot, I. A. Weinstock, Angew. Chem. Int. Ed. 56 (2017) 7083-7087 (c) Y. Wang, O. Zeiri, M. Raula, B. Le Ouay, F. Stellacci, I. A. Weinstock, Nature Nanotech. 12 (2017) 170-176 (Highlighted in Nanotechweb.org, AZO Materials, Phys.org).
[2] M. Raula, G. Gan-Or, M. Saganovich, O. Zeiri, Y. Wang, M. R. Chierotti, R. Gobetto, I. A. Weinstock, Angew. Chem.Int. Ed. 54 (2015) 12416 –12421 (“Hot paper”).
[3] B. Chakraborty, M. Raula, G. Gan Or, E. Gadot, I. A. Weinstock, (manuscript in preparation).