Synthesis, Assembly, and Sizing of Neutral, Lanthanide Substituted Molybdenum Blue Wheels {Mo₉₀Ln₁₀}

Polyoxometalate molybdenum blue (MB) complexes typically exist as discrete multianionic clusters and are composed of repeating Mo building units. MB wheels such as {Mo₁₇₆} and {Mo₁₅₄} are made from pentagon-centered {Mo₈} building blocks joined by equal number of {Mo₁} units as loin, and {Mo₂} dimer units as skirt along the ring edge, with the ring sizes of the MB wheels modulated by the {Mo₂} units. Herein we report a new class of contracted lanthanide-doped MB structures that have replaced all the {Mo₂} units with lanthanide ions on the inner rim, giving the general formula {Mo₉₀Ln₁₀}. We show three examples of this new decameric {Mo₉₀Ln₁₀} (Ln = La, Ce, and Pr) framework synthesized by high temperature reduction and demonstrate that later Ln ions result in {Mo₉₂Ln₉} (Ln = Nd, Sm), conserving one {Mo₂} linker unit in its structure, as a consequence of the lanthanide contraction. Remarkably the {Mo₉₀Ln₁₀} compounds are the first examples of charge-neutral molybdate wheels as confirmed by BVS, solubility experiments, and redox titrations. We detail our full synthetic optimization for the isolation of these clusters and complete characterization by X-ray, TGA, UV-vis, and ICP studies. Finally, we show that this fine-tuned self-assembly process can be utilized to selectively enrich Ln-MB wheels for effective separation of lanthanides.