[SiNb₁₂O₄₀]¹⁶⁻ and [GeNb₁₂O₄₀]¹⁶⁻: Highly Charged Keggin Ions with Sticky Surfaces

Abstract

Heteropolyanions are negatively charged clusters of cornersharing and edge-sharing early transition-metal MO 6 octahedra and heteroatom XO 4 tetrahedra, in which the tetrahedra are usually located in the interior of the cluster.[1] The geometry, composition, and charge of these clusters are varied through synthesis parameters, and cluster properties are highly tunable as a function of these characteristics. Heteropolyanions have been employed in a range of applications that include virus-binding inorganic drugs, [2] homogeneous and heterogeneous catalysts, [3,4] electro-optic and electrochromic materials, [5,6] metal and protein binding, [7] and as building blocks for nanostructuring of materials.[8] The aKeggin geometry, which was first structurally characterized in 1933 by J. F. Keggin [9] for the phosphotungstic acid (H 3 PW 12 O 40 ) is one of the most widely recognized and thoroughly studied heteropolyanion geometries.[10] Following Keggin's initial discovery, solid-state and solution chemistries of the heteropolytungstates and heteropolymolybdates (i.e., H 4 SiMo 12 O 40 ) with the Keggin geometry have been well developed. These Keggin ions have been synthesized with a variety of atoms in the XO 4 tetrahedral site, numerous addenda atom substitutions in the MO 6 octahedral sites, as acids, and as salt compounds with a range of counter cations. [11,12] Given the apparent ubiquity of the Keggin geometry, it is not surprising that the first-observed heteropolyniobate (reported Nyman et al. in 2002 [13] ) was a Keggin ion. However, we first identified [13] O 2 are chemically quite different from the related heteropolymolybdate and heteropolytungstate Keggin ions in their synthetic approach, their pH stability, and their charge. The Nb Keggin ions are synthesized and stable in basic solutions (pH 7-12.5) and decompose in acidic solutions, whereas the Mo and W Keggin ions are synthesized and stable in acidic solutions (pH % 1-3) and decompose in more basic solutions. The Nb Keggin ions are synthesized hydrothermally at 190 8C from amorphous, hydrous niobium oxide and alkoxides of germanium or silicon, whereas the Mo and W Keggin ions are generally precipitated at room temperature from tungstic or molybdic acid or alkali precursors. Use of sodium silicate or sodium germanate as precursors rather than the alkoxides results in mixtures that contain the heteropolyniobate only as a minor phase, with poorly crystalline sodium niobates [14] as the major phase. The general differences between synthesis of heteropolyniobates and synthesis of heteropolymolybdates and heteropolytungstates have been discussed in detail previously. [13] The hydrothermal formation of Na 16

Visible photoluminescence of polyoxoniobates in aqueous solution and their high electrocatalytic activities for water oxidation

“…The synthesis of the four PONbs was according to the literature methods [7][8][9][10][11][12][13][14][15][16]20 …”

“…Some other elements including Si, Ge, P and Ti were introduced to produce heteropolyoxoniobates in order to tailor the properties of niobate family [11][12][13][14][15][16]. The incorporation of transition metals into PONbs is able to control their solubility in alkaline solution.…”

Two Members of the {X4Nb16O56} Family (X = Ge, Si) Based on [(GeOH)2Ge2Nb16H2O54]12– and [K(GeOH)2Ge2Nb16H3O54]10–

“…Subsequently, the Nyman and Cronin groups have reported many heteropolyoxoniobate compounds, such as K 12 2 ]}·17H 2 O (en = ethylenediamine). [6,[9][10][11][12][13][14][15][16][17] Our group has also recently made a deeper study on the reactivity of polyoxoniobates in acidic solutions and obtained some mixed-addendum heteropolyniobates. [18] Furthermore, Ge IV -containing polyoxometalates have been known for several decades, and most of the published work was based on the Keggin-type germanotungstates [GeW 12 O 40 ] 4-and its lacunary derivatives.…”

Electronic and relativistic contributions to ion-pairing in polyoxometalate model systems

“…4 Aqueous solubility and ion-pairing is extremely complex because many phenomena are involved including the lattice energy of the crystallized or precipitated solid, the hydration sphere of both the cation and anion, the sum of ions present in solution, and pH effects if the cation and/or anion is a polyatomic oxo-ion. [5][6][7] Alkali salts of polyoxometalates (POMs), the early d 0 Group V/VI metal-oxo clusters, are ideal for probing both ion-association [8][9][10][11][12] and crystal growth mechanisms, [13][14][15][16][17][18] and these two phenomena are intimately related. The large ion size and heavy metals of the POMs scatter X-rays strongly.…”

“…8,10,[22][23][24] For instance, the Cs + salt of the hexaniobate POM ([Nb 6 O 19 ] 8-; {Nb 6 }, Fig. 1) is soluble up to 1.5 M, whereas the lithium and sodium salts are only sparingly soluble.…”