Anisotropic Polyoxometalate Cages Assembled via Layers of Heteroanion Templates

The
synthesis of anisotropic redox-active polyoxometalates (POMs)
that can switch between multiple states is critical for understanding
the mechanism of assembly of structures with a high aspect ratio,
as well as for their application in electronic devices. However, a
synthetic methodology for the controlled growth of such clusters is
lacking. Here we describe a strategy, using the heteroanion-directed
assembly, to produce a family of 10 multi-layered, anisotropic POM
cages templated by redox-active pyramidal heteroanions with the composition
[W16Mo2O54(XO3)]n−, [W21Mo3O75/76(XO3)2]m−, and [W26Mo4O93(XO3)3]o− for the double, triple,
and quadruple layered clusters, respectively. It was found that the
introduction of reduced molybdate is essential for self-assembly and
results in mixed-metal (W/Mo) and mixed-valence (WVI/MoV) POM cages, as confirmed by an array of analytical techniques.
To probe the archetype in detail, a tetrabutyl ammonium (TBA) salt
derivative of a fully oxidized two-layered cage is produced as a model
structure to confirm that all the cages are a statistical mixture
of isostructures with variable ratios of W/Mo. Finally, it was found
that multilayered POM cages exhibit dipolar relaxations due to the
presence of the mixed valence WVI/MoV metal
centers, demonstrating their potential use for electronic materials.