Back to Basics: Lanthanide and Heavy Actinide Chelation at High pH by Niobium Polyoxometalates

Niobium polyoxometalates (Nb-POMs) form in alkaline media, which limits their use as ligands for acidic cations, particularly lanthanides and actinides. Metal-Nb-POM moieties have the potential for emergent and enhanced properties, based on strong complexation behavior and high stability of the resultant materials. Here, we probe interactions of lanthanides (Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺) and actinides (Am³⁺, Cm³⁺) with a Nb-POM [Nb₆O₁₉]⁸⁻ (Nb₆) in alkaline media. Nb₆, the most charge-dense Nb-POM, enhances f-element luminescence emission by up to × 10⁶, via Nb-POM-mediated sensitization. Lengthened emission lifetimes correlate with the release of the metal-cation hydration sphere, replaced by multidentate Nb-POMs. The Nb₆-Ln(An) complexes resist carbonate and phosphate displacement, and Nb₆-Ln(An) complexation is retained upon isolation of the solids from solution. Electrospray ionization mass spectrometry (ESI-MS) and Raman spectroscopy both indicate the formation of the unprecedented Peacock-Weakley Nb-POM ([LnIII(Nb₅O₁₈)₂]¹⁹⁻) in addition to simple Nb₆-Ln coordination complexes. Luminescence emission spectra support the presence of simple Nb₆-Ln coordination complexes. Small-angle X-ray scattering (SAXS) evidence the formation of Ln-Nb-POM aggregates. This foundational investigation highlights the potential of Nb-POMs as metal-ligands at basic pH, with value-added properties including scaffolding extended materials and controlling light absorption and emission.