Direct Observation of Contact Ion‐Pair Formation in Aqueous Solution

Abstract

The formation of ion pairs in aqueous solution orchestrates vital processes in nature and engineered systems, including electron transfer [1][2][3] and other chemical reactions, [4,5] and crystallization/precipitation of synthetic and natural matter. [6][7][8][9] Polyoxometalates (POMs) have received special attention with regard to ion pairing.[10] POMs are highly charged anions with ideal topologies for binding and associating metal cations, as their assembly, [11][12][13] redox, [2] and catalytic properties [14] in aqueous solution are influenced by ion pairing. Methods for determining ion pairing [15] are largely considered to be indirect in that they observe physical changes in a solution that results from ion association. Spectroscopic methods are the exception, but these are prone to insufficient resolution, and are unable to detect solvent-separated ion pairing. Herein we show that smallangle X-ray scattering (SAXS) of the Lindqvist [Nb 6 O 19 ] 8À POM [16,17] provides a model example of direct measurement of the formation of ion pairs in aqueous solution, and reveals remarkable similarity between solution and solid-state ion contact. This study provides evidence that the structures of Lindqvist hexaniobate salts described with extreme accuracy in the solid state can be used to model the ion-pairing behavior in solution.The effectively spherical [Nb 6 O 19 ] 8À isopolyoxoanion with 25 atoms (Figure 1 a) has the greatest surface charge of all POMs.[18] High surface charge is a property noted to enhance ion pairing, both in solution [2] and in the resulting precipitates.[ . Thus, compared to tungstate, molybdate, and vanadate POMs, niobate POMs are not electrochemically reducible, they are assembled and stabilized in alkaline rather than acidic solutions, [20] they protonate rather than deprotonate in water, [21] and tend to form insoluble linked frameworks with alkali atoms [22,23] rather than soluble salts. The [Nb 6 O 19 ] 8À Lindqvist ion has been crystallized in the solid state with every alkali metal counterion, [24][25][26] and its ion 8À ion is related to its physical radius R S (blue sphere radius) by the relationship R S = ffi ffi 5 3 q R g . All the R S values presented in Table 2 were obtained directly from fitting the SAXS data with a spherical form factor model. The R SS (spherical shell radius; represented by the outermost white sphere) is the sum of the physical radius of the bare anion (fixed at 3.9 ) and the best-fit thickness of the spherical shell around the hexaniobate core.

Solubility of niobium(V) and tantalum(V) under mild alkaline conditions

“…This indicates that potassium ions stabilize the hexaniobate ions HNb 6 O 19 7- (aq) and that the solubility of Na 7 HNb 6 O 19 ·15H 2 O (s) is no more controlled by the common ion effect. The formation of the ion-pairs K 8 Nb 6 O 19(aq) and K 10 Nb 6 O 19 2+ (aq) was observed by Antonio et al (2009) in 3 mol.L -1 KOH based on small-angle X-ray scattering. Hence, ion-pairs formation between K + (aq) and HNb 6 O 19 7-(aq) could explain the increase of the solubility of Na 7 HNb 6 O 19 ·15H 2 O (s) in K + -containing media.…”

“…Hexaniobate ions are known to form ion-pairs with Cs + (aq) , Rb + (aq) and K + (aq) ions (Antonio et al, 2009;Kinnan et al, 2014). It was recently reported that ion-pairs formation decreases along the series Cs > Rb > K > Na > Li and that Nb 6 O 19 8-(aq) ion does not interact with Li + (aq) ions (Antonio et al, 2009;Kinnan et al, 2014;Fullmer et al, 2014). The sodium case has not been treated in the literature and, consequently, we tried different models to fit our solubility measurements.…”

Lindqvist Polyoxoniobate Ion-Assisted Electrodeposition of Cobalt and Nickel Water Oxidation Catalysts

“…Cobalt nitrate (Co(NO 3 43 Different ratios of Co or Ni to Nb (the initial concentrations of Co 2+ or Ni 2+ were kept constant) were also tested for electrodeposition of hydr(oxide), but a 1:1 ratio provided the best deposition efficiency as defined by the largest current per cycle of potential. …”

Extraction of Nb(v) by quaternary ammonium-based solvents: toward organic hexaniobate systems

“…16,19,[35][36][37] The species NbO 3 − is a hypothetical species which was mentioned in few solubility studies 38,39 Equilibria involving hexaniobates and other minor species which would be more extractible cannot be excluded, but given the current knowledge on polyoxoniobate chemistry, the extraction of hexaniobates seems the most likely mechanism at a pH higher than ∼10. Therefore, reaction (5) can be considered to be a rational basis to describe the extraction of Nb(V) with methyltrioctylammonium chloride from alkaline media.…”

“…Such a conclusion is in accordance with the stability of the alkali-hexaniobate ion-pairs observed in the aqueous systems, which follows the following trend: Li + < Na + < K + < Rb + < Cs + . 31,37,43 The influence of alkali ions on the extraction of hexaniobate ions was further investigated in lithium-, potassium-and cesium-based media, taking the sodium hexaniobate system as a reference. Fig.…”