May Nyman

Polyoxoniobate chemistry in the 21st century

Polyoxometalate (POM) chemistry of W, Mo and V is rich and diverse; and new discoveries are frequent and abundant. The prolificacy of this POM chemistry is attributed to rich redox chemistry, an acidic nature that is compatible with addendum metal cations, and most importantly an understanding and ability to control solution behavior. In contrast, the POM chemistry of Nb and Ta (PONb, POTa) is hindered by minimal redox chemistry, alkalinity that is incompatible with the solubility of most metal cations, and a relatively poor understanding of the behavior in aqueous media. Despite these hurdles, considerable advancements in PONb chemistry (and to a much lesser extent POTa chemistry) have been made in the last decade. These include synthesis of the first heteropolyniobate Keggin derivatives, utilization of organic countercations to obtain unprecedented PONb geometries and compositions, and investigation of PONb solution behavior using advanced techniques such as nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI MS) and small-angle X-ray scattering (SAXS). This Perspective article summarizes the recent successes, continued shortcomings, and some unique and potentially exploitable features of PONb chemistry. More importantly, this annotated compilation of recent PONb literature has revealed the most logical and promising directions for the continued growth of the most challenging of polyoxometalate chemistries.

Direct observation of contact ion-pair formation in aqueous solution.

The formation of ion pairs in aqueous solution orchestrates vital processes in nature and engineered systems, including electron transfer and other chemical reactions, 5] and crystallization/precipitation of synthetic and natural matter. Polyoxometalates (POMs) have received special attention with regard to ion pairing. POMs are highly charged anions with ideal topologies for binding and associating metal cations, as their assembly, redox, and catalytic properties in aqueous solution are influenced by ion pairing. Methods for determining ion pairing 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 [Nb6O19] 8

Uranyl peroxide enhanced nuclear fuel corrosion in seawater

The Fukushima-Daiichi nuclear accident brought together compromised irradiated fuel and large amounts of seawater in a high radiation field. Based on newly acquired thermochemical data for a series of uranyl peroxide compounds containing charge-balancing alkali cations, here we show that nanoscale cage clusters containing as many as 60 uranyl ions, bonded through peroxide and hydroxide bridges, are likely to form in solution or as precipitates under such conditions. These species will enhance the corrosion of the damaged fuel and, being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances.

Aqueous formation and manipulation of the iron-oxo Keggin ion

From iron clusters to iron mineral Growing a mineral out of solution, either in the lab or in nature, requires the assembly of atoms or clusters of ions. The structure of some common iron oxides hints that tiny iron-oxygen clusters may serve as mineral building blocks, but isolating these often unstable clusters is challenging. Sadeghi et al. not only isolated but were able to control the growth and dissolution of an iron-oxygen cluster that is a likely precursor to the most common iron oxide mineral, ferrihydrite. Science, this issue p. 1359 A 13-atom iron-oxygen cluster acts as a precursor to ferrihydrite crystallization. There is emerging evidence that growth of synthetic and natural phases occurs by the aggregation of prenucleation clusters, rather than classical atom-by-atom growth. Ferrihydrite, an iron oxyhydroxide mineral, is the common form of Fe3+ in soils and is also in the ferritin protein. We isolated a 10 angstrom discrete iron-oxo cluster (known as the Keggin ion, Fe13) that has the same structural features as ferrihydrite. The stabilization and manipulation of this highly reactive polyanion in water is controlled exclusively by its counterions. Upon dissolution of Fe13 in water with precipitation of its protecting Bi3+-counterions, it rapidly aggregates to ~22 angstrom spherical ferrihydrite nanoparticles. Fe13 may therefore also be a prenucleation cluster for ferrihydrite formation in natural systems, including by microbial and cellular processes.

Highly Versatile Rare Earth Tantalate Pyrochlore Nanophosphors

Rare earth tantalate materials are of considerable interest in energy and environmentally related applications including photocatalytic H(2) generation or contaminant decomposition, ion conductivity for batteries and fuel cells, and phosphors for light-emitting diodes (LEDs). These Eu-doped rare earth tantalate pyrochlore nanoparticles, K(1-2x)LnTa(2)O(7-x):Eu(3+) (Ln = Lu, Y, Gd; x = (1)/(3) for Gd, x = 0 for Lu and Y), have quantum yields up to 78% when excited with blue light (464 nm), which is remarkable for nanoparticle forms that can suffer efficiency loss by surface effects or poor crystallinity, and are furthermore quite suitable for LED applications. The Gd analogue with its framework distortions has particularly high quantum yields. The blue excitation peak matches the emission of the GaN LED. The combination of the high quantum yield under blue excitation, low thermal quenching, and chemical stability renders these new materials promising red phosphors for blue-excitation white LEDs for solid-state lighting. In addition, the pyrochlore lattice is very accommodating to dopants and vacancies and will incorporate virtually any metal and coordination environment ranging from four-coordinate to eight-coordinate. Thus, there are virtually unlimited possibilities for tailoring and optimizing photoluminescent properties, as demonstrated by these scoping studies.

Observing assembly of complex inorganic materials from polyoxometalate building blocks.

Understanding the aqueous state of discrete metal-oxo clusters, prenucleation clusters, and even simple ions is valuable for controlling the growth of metal-oxide materials from water. Niobium polyoxometalates (Nb-POMs) are unique in the aqueous metal-oxo cluster landscape in their unusual solubility behavior: specifically, their solubility in water increases with increasing ion-pairing contact with their counterions, and thus provides a rare opportunity to observe these and related solution phenomena. Here, we isolate in the solid state the monomeric and dimeric building blocks, capped Keggin ions, of the extended Keggin chain materials that are now well-known: not only in Nb-POM chemistry, but Mo and V POM chemistry as well. Rb13[GeNb13O41]·23H2O (Rb1), Cs10.6[H2.4GeNb13O41]·27H2O (Cs1) and Cs18H6[(NbOH)SiNb12O40]2·38H2O (Cs2) were characterized by single-crystal X-ray diffraction. Small angle X-ray scattering (SAXS) of solutions of Rb1 and Cs1 in varying conditions revealed oligomerization of the monomers into chain structures: the extent of oligomerization is controlled by pH, concentration, and the counterion. We distinctly observe chains of up to six Keggin ions in solution, with the large alkali cations for charge-balance. This combined solid state and solution study reveals in great detail the growth of a complex material from discrete monomeric building blocks. The fundamentals of the processes we are able to directly observe in this study, ion-association and hydrolysis leading to condensation, universally control the self-assembly and precipitation of materials from water.

Soluble Heteropolyniobates from the Bottom of Group IA

Surprising solubility: While it is already well known that [Nb(6)O(19)](8-) salts exhibit an unusual solubility trend, that is, Cs>Rb>K>Na>Li, the heteropolyniobates of Cs and Rb had not yet been crystallized. These very soluble entities have now been obtained from solution by a simple and universal process. New polyoxoniobate geometries are thus unveiled, and the [SiNb(12)O(40)](16-) Keggin ion is characterized in solution for the first time.

Selectivity for Cs and Sr in Nb-substituted titanosilicate with sitinakite topology

Abstract The 25% niobium substituted crystalline titanosilicate with the composition Na 1.5 Nb 0.5 Ti 1.5 O 3 SiO 4 ·2H 2 O (Nb–TS) was synthesized under hydrothermal conditions. Its selectivity for radioactive 137 Cs and 89 Sr was compared with the TS, Na 2 Ti 2 O 3 SiO 4 ·2H 2 O, having sitinakite topology. The Nb–TS shows significantly higher uptake value for 137 Cs but lower for 89 Sr than the TS. To investigate the origin of selectivity, the ion exchanged Cs + and Sr 2+ forms with the composition, Cs x NaH y Nb 0.5 Ti 1.5 O 3 SiO 4 · z H 2 O ( x =0.1, 0.2 and 0.3, x + y =0.5 and z =1–2) and Sr 0.2 Na 0.6 H 0.5 Nb 0.5 Ti 1.5 O 3 SiO 4 ·H 2 O, respectively, were structurally characterized from the X-ray powder diffraction data using the Rietveld refinement technique. Simultaneously the kinetics of 137 Cs and 89 Sr uptake was investigated for the Nb V free and doped samples. While the Cs + and Sr 2+ exchanged form of Nb–TS and the Cs + exchanged form of TS retain the symmetry of the parent compound, the Sr 2+ exchanged form of TS undergoes a symmetry change. The differences in the uptake of Cs + and Sr 2+ result from the different coordination environments of cesium and strontium in the eight-ring channel, that result from various hydration sites in the tunnel. The origin of selectivity appears to arise from the higher coordination number of cesium or strontium. Other effects due to Nb V substitution are reflected in the increase of both, the a - and c -dimensions and thus the unit cell volume, and the population of water vs. Na + in the channel to charge-balance the Nb 5+ ↔Ti 4+ substitution.

A Journey inside the U28 Nanocapsule

Anionic uranyl-peroxide U(28) nanocapsules trap cations and other anions inside, whose structures cannot be resolved by X-ray diffraction, owing to crystallographic disorder. DFT calculations enabled the complete characterization of the geometry of these complex systems and also explained the origin of the disorder. The stability of the capsules was strongly influenced by the entrapped cations. Excellent agreement between experiment and theory was also obtained for the electronic character and redox properties.

Dynamics of Uranyl Peroxide Nanocapsules.

Discrete aqueous metal oxide polyionic clusters that include aluminum polycations, transition-metal polyoxometalates, and the actinyl peroxide clusters have captivated the interest of scientists in the realm of both their fundamental and applied chemistries. Yet the counterions for these polycations or polyanions are often ignored, even though they are imperative for solubility, crystallization, purification, and even templating cluster formation. The actinyl peroxide clusters have counterions not only external, but internal to the hollow peroxide capsules. In this study, we reveal the dynamic behavior of these internal alkali counterions via solid-state and liquid NMR experiments. These studies on two select cluster geometries, those containing 24 and 28 uranyl polyhedra, respectively, show that the capsules-like clusters are not rigid entities. Rather, the internal alkalis both have mobility inside the capsules, as well as exchange with species in the media in which they are dissolved. The alkali mobilities are affected by both what is inside the clusters as well as the composition of the dissolving medium.