From +I to +IV, Alkalis to Actinides: Capturing Cations across the Periodic Table with Keggin Polyoxometalate Ligands

Coordination chemistry trends across the periodic table are often difficult to probe experimentally due to limitations in finding a versatile but consistent chelating platform that can accommodate various elements without changing its coordination mode. Herein, we present new metal/ligand systems covering a wide range of ionic radii, charges, and elements. Five different ligands derived from the Keggin structure (HBW₁₁O₃₉^8-, PW₁₁O₃₉^7-, SiW₁₁O₃₉^8-, GeW₁₁O₃₉^8-, and GaW₁₁O₃₉^9-) were successfully crystallized with six different cations (Na⁺, Sr²⁺, Ba²⁺, La³⁺, Ce⁴⁺, and Th⁴⁺) and characterized by single-crystal X-ray diffraction. Twenty-five new compounds were obtained by using Cs⁺ as the counterion, yielding a consistent base formula of Cs_<i>x</i>[M(XW₁₁O₃₉)₂]·<i>n</i>H₂O. Despite having a similar first-coordination sphere geometry (i.e., 8-coordinated), the nature of the central cation was found to impact the long-range geometry of the complexes. This unique crystallographic data set shows that, despite the traditional consensus, the local geometry of the cation (i.e., metal-oxygen bond distance) is not enough to depict the full impact of the complexed metal ion. The bending and twisting of the complexes, as well as ligand-ligand distances, were all impacted by the nature of the central cation. We also observed that counterions play a critical role by stabilizing the geometry of the M(XW₁₁)₂ complex and directing complex-complex interactions in the lattice. We also define certain structural limits for this type of complex, with the large Ba²⁺ ion seemingly approaching those limits. This study thus lays the foundation for capturing the coordination chemistry of other rarer elements across the periodic table such as Ra²⁺, Ac³⁺, Bk⁴⁺, Cf³⁺, etc.