Expanding polyoxopalladate diversity: Ce⁴⁺-containing structures and chiral lanthanide clusters

The incorporation of lanthanides into polyoxometalates (POMs) is a fascinating area of research due to their unique chemical and physical properties, as well as their potential applications. Polyoxopalladate (POP) clusters feature flexible central cavities and coordination geometries dictated by their central metal ion templates and external capping groups. Two primary structure types have been reported: {Pd₁₂} cubes and {Pd₁₅} stars, which can accommodate various central metal ions, ranging from common transition metals in the +2 oxidation state to less conventional lanthanides in the +3 oxidation state. This work explores POP clusters containing cerium ions, leading to the discovery of two new structure types: [CePd₁₃O₉(SeO₃)₉(H₂O)]⁶⁻ (1) and [Ce₃Pd₂₂O₁₆(SeO₃)₁₆]⁸⁻ (2). Cluster 1 adopts a fused “star-cube” structure, combining half {Pd₁₅} star and half {Pd₁₂} cube with Ce⁴⁺ as the central template. 2 exhibits a twisted dumbbell-shaped structure (D_2d symmetry), where two Ce⁴⁺-centred {CePd₁₁} cubes are bridged by a third Ce⁴⁺ ion at the cluster core. Parallel experiments with other lanthanides yielded two propeller-like chiral POPs [LnPd₁₂O₇(SeO₃)₈Cl(H₂O)₂]⁴⁻ (Ln = Eu³⁺ (3); Ln = Gd³⁺ (4)), both displaying C₃ symmetry. Notably, this represents one of the rare instances in POP chemistry where a capping ligand (SeO₃²⁻) directly coordinates to the central lanthanide ions (Eu³⁺ and Gd³⁺). The twisted coordination of this capping ligand induces a uniform tilt in a set of {PdO₄} planes, resulting in the clusters’ propeller-like chirality. In contrast, similar reaction conditions for Tb³⁺ produced [TbPd₁₂O₈(SeO₃)₈]⁵⁻ (5), a normal cubic structure.