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 (HBWO, PWO, SiWO, GeWO, and GaWO) were successfully crystallized with six different cations (Na, Sr, Ba, La, Ce, and Th) 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[M(XWO)]·HO. 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(XW) complex and directing complex-complex interactions in the lattice. We also define certain structural limits for this type of complex, with the large Ba 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 Ra, Ac, Bk, Cf, etc.