Recent studies on the reaction of [X6O19]8− (X = Nb, Ta) polyanions under acidic conditions

Abstract

Polyoxoniobates (PONbs) and polyoxotantalates (POTas) are anionic metal–oxide clusters composed of Nb^V and Ta^V, which have higher charge–size ratios than polyoxotungstates and polyoxomolybdates. The synthesis of PONbs and POTas usually involves the use of [X₆O₁₉]⁸⁻ (X = Nb, Ta) as precursors, which confine the pH of aqueous solutions to be consistently confined to the alkaline region of the pH scale, rendering them incompatible with the reactivity of most metal cations. Due to the lack of negative analogues to MO₄²⁻ (M = Mo/W) that can be polymerized through acidification, the progress in the study of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions remains in its nascent stages. The methods reported to date for synthesizing PONb and POTa clusters can be categorized into two broad strategies: ligand protection under alkaline conditions and hydrogen peroxide (H₂O₂) utilization under acidic conditions. In the latter strategy, H₂O₂ is used to prevent the formation of Nb₂O₅ or Ta₂O₅ in an acidic solution by grafting peroxo groups onto the Nb or Ta atoms. The subsequent cleavage of the peroxo O–O bond enhances the reactivity of oxo intermediates, facilitating the prediction and rational design of novel reactions. In this review, we provide an overview of recent developments from the recent studies on the reaction of [X₆O₁₉]⁸⁻ (X = Nb, Ta) polyanions under acidic conditions, spanning from the synthesis to the structural elucidation of PONb and POTa clusters. These developments fall into two main categories: mixed-addendum niobotungstates/tantalotungstates and hetero-peroxo-PONb/Ta clusters. Furthermore, we briefly introduce the properties and applications of some compounds. The concluding section is dedicated to discussing the prospects and essential guidance in this field.