Chemical diversity and versatility of polyoxometalate ligands: homologous series of twenty-five new structures with polyoxometalates binding to alkali, alkaline earth, lanthanide, and actinide ions

Coordination chemistry trends across the periodic table are often difficult to probe experimentally due to limitations in finding a versatile but consistent chelating platform able to 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 (HBW11O398-, PW11O397-, SiW11O399-, GeW11O399-, and GaW11O399-) were successfully crystalized with six different cations (Na+, Sr2+, Ba2+, La3+, Ce4+, Th4+) and characterized by single crystal XRD. Twenty-five new compounds were obtained by using Cs+ as counterion, yielding a consistent base formula of Csx[M(XW11O39)2]·nH2O. 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 dataset 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 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 ground for capturing the coordination chemistry of other, rarer, elements across the periodic table such as Ra2+, Ac3+, Bk4+, Cf3+, etc.