Research advances of polyoxotantalates

Abstract

Polyoxometalates (POMs), as early transition metal-oxo clusters of V, Mo, W, Nb, and Ta, have been attracting extensive interest owing to their structural variety associated with interesting structural characteristics such as high negative charge, oxygen-rich surface, and rich redox properties. In the past two centuries, the main success was achieved in POM chemistry of V, Mo, and W. In contrast, the advancement of polyoxotantalate (POTa) chemistry falls far behind that of POM chemistry of V, Mo, and W. The reasons lie in the narrow working pH region, the slight solubility, and the low reaction activity of tantalum species. Despite these hurdles, as an important subclass of POMs, POTas have received more and more intense interest and attentions because of their promising applications in photocatalysis and the base-catalyzed reactions. In recent years, considerable advancements in POTa chemistry have been made. During the past two decades, POM chemistry of polyoxovanadates, polyoxoniobates, polyoxomolybdates, and polyoxotungstates has been reviewed extensively. While, no review about POTas has been known to our knowledge. We are thus very interested in making a summary on the research advances of POTa chemistry with attempt to provide some insight into the syntheses of new POTa clusters, in particular brand-new and high-nuclearity POTas, which is a subject of current intense investigation. In this review, we systematically summarized the research progress in structural types, synthetic methods, and related applications of POTas, which were achieved in recent two decades. This review aims to be not only a snapshot of the recent developments of design, architecture, and application of POTas, but also to be employed as a guide to understand the different cluster types of POTas. The main content of the review includes the following four parts: (1) Isopolyoxotantalate species with different counter cations (Li, Na, K, Cs, and organic amines) and different aggregations (hexamer and decamer); (2) Tantalum-based polyoxometalates, including Ta-substituted polyoxotungstates derived from different structures such as Keggin-, Linqvist- and Dawson-type configurations; (3) Heteropolyoxotantalate species, including Ti- and P-containing POTas; and (4) Organic-inorganic hybrid polyoxotantalates constructed from lindqvist-type Ta6 decorated by various organic species (e.g., organometallic complexes and metal-organoamine complexes). According to the above four aspects, we not only summarized the different classifications of POTas but also their corresponding synthetic strategies. Especially, the synthetic methods of POTas has been summarized from the traditional hydrothermal method to a variety of synthetic strategies such as solvothermal, diffusion, and steam-heat. Further, on the issue of how to improve the aggregation degree of Ta atoms, we analyze the difficulties in constructing high-nuclearity POTas and the key factors affecting the construction of high-nuclearity POTas. Additionally, the review also represents that POTas have a variety of excellent properties and exciting application prospects in photocatalytic hydrogen production and degradation of organic pollutant. In summary, this review provides a personal view on the research in the area of POTa chemistry. From the review, it can be found that, even after more than decades of research, known POTas species are really very limited compared to POMs of V, Mo, W, and even Nb on the one hand. And on the other hand, alomost all known POTas are in the small nuclearity range ( 20). Obviously, the design and synthesis of POTas are of great challenge. Nevertheless, it is believed that the current state of POTas will continue to evolve, leading to a variety of expected and unexpected new species with fascinating structures and properties. It could be expected that the review may initiate more extensive research on the discovery of novel POTa materials and open up new perspectives for the development of POTa chemistry.