¹⁷O NMR and Computational Study of a Tetrasiliconiobate Ion, [H_2+xSi₄Nb₁₆O₅₆]^(14−x)−

Abstract

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H(2+x)Si(4)Nb(16)O(56)]((14-x)-) to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 10(4) across the molecule at 6<pH<13 but with similar pH dependencies. There is NMR evidence of an intra- or intermolecular reaction at pH∼7, where new peaks began to slowly form without losing the (17)O isotopic tag, and at pH ≤ 6 these new peaks formed rapidly. The oxygen atoms bonded to silicon atoms began to isotopically exchange at pH 9 and below. The (17)O NMR peak positions also vary considerably with pH for some, but not all, nonequivalent oxygen sites. This variation could be only partly accounted by electronic calculations, which indicate that oxygen atoms should shift similarly upon protonation. Instead, we see that some sites change enormously with pH, whereas other, similarly coordinated oxygen atoms are less affected, suggesting that either some protons are exchanging so rapidly that the oxygen sites are seeing an averaged charge, or that counterions are modulating the effect of the coordinated protons.

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.…”

17 O NMR as a Tool in Discrete Metal Oxide Cluster Chemistry

“…The polyoxoniobates are a class of polyoxometalates which were long ignored in favour of the polyoxovanadates, -tungstates and -molybdates. In contrast with 51 V, 183 W and 95/97 Mo, 93 Nb is not a practicably useful nucleus for NMR experiments, so 17 [109]. This study was part of a greater investigation of rates of oxygen exchange kinetics of structural oxygen sites by 17 O NMR in polyoxoniobates [2,72,110,111], but the number of distinct oxygen sites and the low symmetry presented particular problems with assigning observed 17 O NMR signals to oxygen sites.…”

“…This functional has seen limited use in the ensuing decade and a half, perhaps owing to it only being implemented as a standard functional in a single piece of computational software, PQS, where it is supplied as the 'WAH' functional [171]. Nonetheless, a recent application of the WAH functional was in the computation at WAH/def2-svp of 17 [109]. Importantly, the authors used the signal assignments to suggest the protonation state, as well as explain in part the observed pH dependency of the signals of the oxygen sites in the molecule.…”

Reaction Dynamics and Solution Chemistry of Polyoxometalates by Electrospray Ionization Mass Spectrometry

“…While V, Mo, and W mainly exist as monomeric oxide ions at pH > 9, several examples of polyoxoniobates and -tantalates which are stable to pH > 13 are known. [3][4][5] Polyoxometalates possess several attractive features, as they resemble small fragments of minerals in terms of structural features. [6] The high oxidation state of the metals in the oxide framework has also suggested their use either as catalysts or ligands [7] in oxidative chemistry, such as water oxidation.…”