Discovery and Evolution of Polyoxopalladates

Noble metal catalysts, in particular palladium-containing materials, are of prime commercial interest, because of their role as oxidation catalysts in automobile emission-control systems and reforming catalysts for the production of high-octane gasoline. However, despite almost two centuries of research, the precise structure of such materials is still ill-defined on the subnanometer scale, which severely limits the understanding of the underlying catalytic mechanisms. As a burgeoning class of structurally well-defined noble metal oxide nanoclusters, polyoxopalladates (POPs) have been highly rated as ideal models to fully decipher the molecular mechanism of noble metal-based catalysis. Being at the frontier of polyoxometalates (POMs), the chemistry of POPs, which are based exclusively on Pd-II centers as addenda is currently progressing rapidly, owing to their structural and compositional novelty, high solution stability, combined with promising applications especially as noble metal-based catalysts. Controlled hydrolysis-condensation processes of square-planar (PdO4)-O-II units in the presence of external oxyacid heterogroups (e.g., AsO43-, PO43-, and SeO32-) drive the self-assembly of such discrete, polynuclear Pd-II-oxo nanoclusters in facile one-pot reactions using aqueous solvents. By now, more than 70 POPs have been discovered, encompassing a large structural variety, including cube, star, bowl, dumbbell, wheel, and open-shell archetypes. Moreover, the POP cages can serve as adaptable molecular containers for encapsulation/interaction with a range of metallic elements across the s, p, d, and f blocks of the periodic table, resulting in a library of host-guest assemblies of varying shapes and sizes. Besides a delicate balance of experimental variables, the fine-tuning of POP structure, composition, and properties is possible by systematic replacement of the metal ion guest and/or the capping heterogroups. Besides, nearly all POPs obtained so far could be perfectly rationalized by theoretical calculations, and even prediction of the design and synthesis of new POP structures is possible. The excellent stability of POPs in the solid state and in solution (both aqueous and organic media) and gas phase allows for applications mainly in homo- and heterogeneous catalysis or as molecular precursors for monodisperse nanoparticles via an ingenious bottom-up route for functional nanotechnology. Apart from catalysis, owing to the unique structural features of POPs, other areas of interest exist, for example, in magnetism as molecular spin qubits and in biology as aqueous-phase macromolecular models. Overall, as a distinct subclass of POMs, POPs not only integrate the advantages of tunable shape, size, composition, solution stability, redox activity, and facile synthetic procedures, but drive immense potential for achieving an atom-to-atom fabrication and modulation of nanostructures as well, thereby providing models for unveiling mechanistic insight of noble metal-based catalysis at the molecular level, which will, in turn, guide the programmed assembly of nanomaterials with improved performance in a controllable manner. This Account is directed to cover the main structural types of POPs and to discuss the structure-directing template effects induced by the guest ions on the resultant host-guest assemblies.