Rare-Earth-Guided Hierarchical Assembly of Teddy-Bear-Like Giant Nickel-Tungstate Polyoxometalates

The controlled assembly of giant polyoxometalates (POMs) remains a significant challenge in inorganic chemistry. Herein, we demonstrate that rare-earth ions can direct distinct structural outcomes in nickel-substituted tungstate systems. Hydrothermal reaction of the flexible precursor [B-α-SbW₉O₃₃] with KH₂PO₄, Er₂O₃, and NiCl₂ yielded a giant all-inorganic cluster {W₈₈Ni₃₉} (1), adopting an unprecedented "Teddy-Bear"-like architecture. Under identical conditions, omission of Er₂O₃ (or replacement with Dy₂O₃) afforded the smaller cluster {W₅₅Ni₁₇} (2), corresponding to the "head" of the "Teddy-Bear". Notably, Dy₂O₃ improves the crystallization yield of {W₅₅Ni₁₇} relative to the rare-earth-free reaction. Remarkably, replacing Er₂O₃ with Gd₂O₃ afforded a giant tetramer {W₁₄₀Gd₁₀Ni₄₈} (3), whose monomer is analogous to the [W₃₃Ni₂₁] "Teddy-Bear body" motif in {W₈₈Ni₃₉}. These clusters rank among the largest discrete tungsten-based POM assemblies known. Importantly, the crystallization of {W₈₈Ni₃₉} and {W₁₄₀Gd₁₀Ni₄₈} enables an efficient organic-free separation of the rare-earth oxides Er₂O₃ and Gd₂O₃ with a separation factor of 360.37, revealing a new strategy based on structure-directed hierarchical assembly for selective crystallization in rare-earth-mediated POM systems.