Cation mediated self-assembly of inorganic cluster anion building blocks.

Polyoxometalate (POM) cluster anions form highly organized monolayers on planar surfaces, stabilize metal nanoparticles in solution, and serve as structural components of hollow, single-walled vesicles. Until recently, each of these classes of superstructures was viewed as fundamentally distinct. Now, however, new data show that metal nanoparticles serve as templates for the assembly of spherical, "metal-core"-supported POM monolayers. As such, POM-protected metal nanoparticles are pivotal members of a continuum that ranges from planar arrays to hollow-spheres. Moreover, it is now apparent that, in all three classes of superstructures, similar electrostatic forces between POMs and their counter-cations are intimately involved in self-assembly, structure and stability. This common role for counter-cations is the theme of this Perspective article. In it, we highlight the role of cation-anion interactions in the formation and structure of newly documented POM monolayers on metal nanoparticles, and establish a unifying principle for better understanding the self-assembly of diverse supramolecular structures from highly charged molecular-ion building blocks.