Titanium‐Substituted Polyoxotantalate Clusters Exhibiting Wide pH Stabilities: [Ti₂Ta₈O₂₈]⁸⁻ and [Ti₁₂Ta₆O₄₄]¹⁰⁻

Abstract

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.

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

“…QAAs first utilized in POM chemistry are simple tetraalkyl ammoniums including tetramethyl (TMA), tetrabutyl (TBA) and tetrahexyl (THA) ammonium that can modulate solubility of POMs. TMA is a notoriously “innocent” counterion that does not promote aggregation and allows high solubility and stability in water, particularly for polyoxoniobates and tantalates . On the other hand, TBA and THA have been commonly used for decades to extract (or precipitate) POMs into nonaqueous solvents for numerous purposes including electrochemical studies and applications involving POM redox chemistry, catalytic studies in organic solvents, and to stabilize water‐ or pH‐sensitive POMs .…”

“…TMA is a notoriously "innocent" counterion that does not promote aggregation and allows high solubility and stability in water, particularly for polyoxoniobates and tantalates. [30,[109][110][111][112] On the other hand, TBA and THA have been commonly used for decades to extract (or precipitate) POMs into nonaqueous solvents for numerous purposes including electrochemical studies and applications involving POM redox chemistry, [3] catalytic studies in organic solvents, [113] and to stabilize water-or pH-sensitive POMs. [114] While TMA salts of POMs generally crystallize well for structure elucidation, the longer-chain alkylammonium salts are notoriously difficult to crystallize, due to the flexibility and lack of order of the alkyl chains, and the weak intermolecular interactions in the solid-state.…”

Jenseits von Ladungsausgleich: Gegenkationen in der Polyoxometallat‐Chemie

“…Anfänglich wurden in der POM‐Chemie meist einfache Tetraalkylammonium‐Verbindungen wie Tetramethylammonium (TMA), Tetrabutylammonium (TBA) und Tetrahexylammonium (THA) genutzt, um die POM‐Löslichkeit einzustellen. TMA ist ein bekanntes “unschuldiges” Kation, das nicht zur POM‐Aggregation führt und hohe Löslichkeit und Stabilität in wässriger Lösung ermöglicht, besonders für Niobate und Tantalate . Zudem wurden TBA und THA oft genutzt, um POMs in nichtwässrige Lösungen zu extrahieren (oder zu fällen).…”

“…TMA ist ein bekanntes "unschuldiges" Kation, das nicht zur POM-Aggregation führt und hohe Lçslichkeit und Stabilität in wässriger Lçsung ermçglicht, besonders für Niobate und Tantalate. [30,[109][110][111][112] Zudem wurden TBA und THA oft genutzt, um POMs in nichtwässrige Lçsungen zu extrahieren (oder zu fällen). Dies ermçglichte neue Anwendungen in der POM-Redoxchemie, [3] Katalyse in organischen Lçsungsmitteln [113] und die Stabilisierung wasser-oder pH-empfindlicher POMs.…”

Photocatalytic decolorization of three commercial dyes using a new heteropolyoxotantalate catalyst

“…Recently, Liu et al [26] and Huang et al [27] reported several Ta/W mixed-addendum POMs, respectively. In 2016, Son & Casey [28] communicated two Ti-substituted POTa clusters, [Ti 2 Ta 8 O 28 ] 8− and [Ti 12 Ta 6 O 44 ] 10− . Very recently, Niu and co-workers [29] reported two 6-Peroxotantalo-4-phosphate clusters, from the viewpoint of structure, which can be regarded as the first two examples of hetero-POTas (HPOTas).…”