Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry

ConspectusPolyoxometalates (POMs, metals = V^4/5+, Nb⁵⁺, Ta⁵⁺, Mo^5/6+, and W^5/6+) can be described as molecular metal oxides. The V, Mo, and W-POMs (classic POMs) exhibit rich structural diversity with interesting redox properties, acid catalysis, inorganic ligands, and colorimetric properties and behavior. Nb and Ta POMs, while structurally similar, are generally stable only in base and redox behavior is rare, and they are synthetically far less accessible. The V, Mo, and W-POMs have been studied for well over a century, Nb-POM chemistry has emerged in the last 20 years, and Ta-POM chemistry is yet to see consistent and significant advances. Early and current success in Nb-POM chemistry is owed mainly to hydrothermal synthesis, which is wholly unsatisfying, given the black box nature of this technique.For the last 5 years and as summarized in this Account, we have exploited decaniobate, [Nb₁₀O₂₈]^6- (Nb₁₀), as a foundation to perform room-temperature, nearly pH-neutral manipulations of Nb-POM solutions. Nb₁₀, with a rare neutral self-buffering pH, responds to any interactions with electrolytes (specifically oxoanions and metal cations) by undergoing transformations, leading to new topologies. The ease of Nb₁₀ transformation yielding new generations of Nb-POMs, akin to an inorganic analogue of biological model organisms such as the fruit fly, inspired the title of this Account. The common building unit born from the disassembly of Nb₁₀ is [Nb₇O₂₀(OH, H₂O)₂]^(5-7)-, and the hydroxyl/aqua ligands provide reactivity for linking via condensation reactions, ligand exchange, heterometals, or oxoanions. We can coax these newly assembled Nb-POMs (detected by small-angle X-ray scattering, SAXS) to crystallize via the usual methods of vapor diffusion, salting out, and reduced temperature, and the single-crystal X-ray diffraction structures are valuable for understanding reaction mechanisms to fine-tune control and yield a landscape of topologies and compositions. Beyond providing an opportunity to comprehend and diversify POM chemistry, the reactivity of Nb₁₀ yields highly soluble (i.e., >2 M Nb), nearly neutral aqueous solutions of niobium, ideal for the solution-phase deposition of thin films, demonstrated with LiNbO₃, (Na,K)NbO₃, Nb₂O₅, and heterometal-doped Nb₂O₅. The obtained films are cohesive and smooth, enabled by the tendency of these solutions to gel if simply evaporated quickly.Per our current endeavors, this gelation behavior provides an opportunity to develop new soft, flexible materials including inorganic networks, organic-inorganic networks, and porous solids and explore their material properties including base catalysis and sorption (i.e., CO₂). Nb-POM (and Ta-POM) discovery and implementation of properties is far from complete. While heterometal (d and f element) substitution is easy with classic POMs, imparting a whole host of functions (tuned luminescence, catalysis, electroactivity, etc.), it remains a challenge with Nb-POMs due to pH incompatibility with most heterometals. This grand challenge that defies fundamental aqueous behavior of metal cations requires the creation of liquid mixtures that include polymer and/or ionic liquid components, and the creation of such reaction media can impact synthesis beyond POM chemistry. The goal of this Account is to describe the recent advances in Nb-POM chemistry, afforded by the Nb₁₀ "fruit fly", and to also provide insight into the next large steps needed to advance Nb-POM chemistry.