Self-Assembly and Ionic-Lattice Like Secondary Structure of a Flexible Linear Polymer of Highly Charged Inorganic Building Blocks.

Among molecular building blocks, metal-oxide cluster anions and their counter cations provide multiple options for the self-assembly of functional materials. Currently, however, rational-design concepts are limited to electrostatic interactions with metal or organic countercations, or the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging -oxo linkages can be used to string together a bi-functional Keggin-anion building block, [PNb2Mo10O40]5- (1), the di-niobium(V) analog of [PV2Mo10O40]5- (2). Induction of-oxo ligation between NbV=O moieties of 1 in acetonitrile, via step-growth polymerization, gives linear polymers with entirely inorganic backbones, some of which are comprised of over 140,000 repeating units, each with a 3- charge, exceeding that of previously reported organic or inorganic polyelectrolyte. As the chain grows, its flexible -oxo linked backbone, with associated countercations, coils into compact 270-nm diameter spherical secondary structures due to electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multi-dimensional structures based on -oxo linkages between NbV=O functionalized building blocks.