Jung-Ho Son

A Soluble Phosphorus‐Centered Keggin Polyoxoniobate with Bicapping Vanadyl Groups

A water-soluble tetramethylammonium (TMA) salt of a novel Keggin-type polyoxoniobate has been isolated as TMA9[PV2Nb12O42]·19H2O (1). This species contains a central phosphorus site and two capping vanadyl sites. Previously only a single example of a phosphorus-containing polyoxoniobate, [(PO2)3PNb9O34](15-), was known, which is a lacunary Keggin ion decorated with three PO2 units. However, that cluster was isolated as an insoluble structure consisting of chains linked by sodium counterions. In contrast, the [PV2Nb12O42](9-) cluster in 1 is stable over a wide pH range, as evident by (31)P and (51)V NMR, UV/Vis spectroscopy, and ESI-MS spectrometry. The ease of substitution of phosphate into the central tetrahedral position suggests that other oxoanions can be similarly substituted, promising a richer set of structures in this class.

Hafnium Sulfate Prenucleation Clusters and the Hf18 Polyoxometalate Red Herring

In prior studies, aqueous Hf sulfate-peroxide solutions were spin-coated, dehydrated, patterned by electron-beam lithography, ion-exchanged (OH(-) for SO4(2-)), and finally converted to HfO2 hard masks via annealing. The atomic-level details of the underlying aqueous chemistries of these processes are complex and yet to be understood. Yet a thorough understanding of this specific chemical system will inspire development of design rules for other aqueous-precursor-to-solid-state metal oxide systems. Often-observed crystallization of the Hf18 polyoxometalate from aqueous Hf sulfate-peroxide precursor solutions has led us to believe that Hf18 may represent an important intermediate step in this process. However, via detailed solution studies described here (small-angle X-ray scattering, electrospray ionization mass spectrometry, and Raman spectroscopy), we ascertained that Hf18 is in fact not a prenucleation cluster of Hf sulfate coatings. Rather, the Hf tetramers, pentamers, and hexamers that are the core building blocks of Hf18 are robustly persistent over variable compositions and aging time of precursor solutions, and therefore they are likely the rudimentary building blocks of the deposited thin-film materials. These Hf clusters are capped and linked by sulfate and peroxide anions in solution, which probably prevents crystallization of Hf18 during the rapid dehydration process of spin-coating. In fact, crystallization of Hf18 from the amorphous gel coating would be detrimental to formation of a high-density conformal coating that we obtain from precursor solutions. Therefore, this study revealed that the well-known Hf18 polyoxometalate is not likely to be an important intermediate in the thin-film process. However, its subunits are, confirming the universal importance of deriving information from the solid state, albeit judiciously and critically, to understand the solution state.

Highly soluble iron- and nickel-substituted decaniobates with tetramethylammonium countercations

Iron- and nickel-substituted decaniobates, [H2Fe(III)Nb9O28](6-) and [H3Ni(II)Nb9O(28)](6-) were hydrothermally synthesized as tetramethylammonium salts and the structures were determined by X-ray crystallography. The highly soluble title compounds were characterized by ESI-MS, FT-IR and UV-Vis titration.

Acid-stable peroxoniobophosphate clusters to make patterned films.

Two new peroxoniobophosphate clusters were isolated as tetramethylammonium (TMA) salts having the stoichiometries: TMA5[HNb4P2O14(O2)4]⋅9 H2O and TMA3[H7Nb6P4O24(O2)6]⋅7 H2O. The former is stable over the pH range: 3<pH<12 and the latter is stable only below pH 3. These two molecules interconvert as a function of solution pH. The [H7Nb6P4O24(O2)6](3-) cluster can be used to fabricate patterned niobium phosphate films by electron-beam lithography after solution deposition.

Titanium-Substituted Polyoxotantalate Clusters Exhibiting Wide pH Stabilities: [Ti2 Ta8 O28 ]8− and [Ti12 Ta6 O44 ]10−

Two new substituted polyoxotantalate clusters, [Ti2 Ta8 O28 ](8-) and [Ti12 Ta6 O44 ](10-) , considerably expand the pH range where tantalates persist in aqueous solution. The structures of [Ti2 Ta8 O28 ](8-) and [Ti12 Ta6 O44 ](10-) are reported as tetramethylammonium salts after synthesis at hydrothermal conditions in aqueous solution. These Ti-substituted polyoxotantalate clusters have analogues among recently discovered niobates, but are slightly larger and more persistent in solution. Most importantly, they exhibit a much wider range of pH stability than the familiar hexatantalate cluster, which is the only other tantalate known to be stable at highly basic pH conditions. These molecules are kinetically stable to near-neutral pH, making them excellent synthons for further development into materials and catalysts, and an significant advance in adapting tantalates for use in aqueous solutions.