posted on 2023-11-28, 18:14authored byMay Nyman, Tasnim Rahman, Ian Colliard
ConspectusPolyoxometalates (POMs, metals = V4/5+, Nb5+, Ta5+, Mo5/6+, and W5/6+) 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.