Deliberate Construction of Polyoxoniobates Exploiting the Carbonate Ligand

Polyoxometalates (POMs, metals=V-V, Nb-V, Ta-V, Mo-VI, W-VI) are molecular metal oxides that can be isolated without capping ligands. The high negative charge of polyoxoniobates (PONb) provides strong interactions with heterocations, advantageous for electrostatic assembly of modular materials. In four single-crystal X-ray structures, we demonstrate that carbonate combined with the very reactive decaniobate [Nb10O28](6-) reassembles into a new decaniobate, [Nb10O25(CO3)(6)](12-), featuring three carbonate-ligated Nb-polyhedra. These Nb-sites can be replaced by heterometals (lanthanides), and the tridentate carbonate can serve as an anchor point to build niobate-frameworks. Small-angle X-ray scattering and two additional X-ray structures reveal that the reaction pathway proceeds through a Nb-24-PONb intermediate, and the obtained PONb (with or without carbonate) is counterion, temperature, and solvent-dependent (water or mixed water-methanol). This provides an uncommon level of control for PONb chemistry.