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 Nb6O198- (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)219-) 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.