Organophosphonate Ligation Approach for the Controlled Assembly of Gigantic Polyoxometalate Clusters

The controlled assembly of gigantic polyoxometalate (POM) clusters remains one of the most formidable challenges in molecular self-assembly, as it is highly dependent on delicate synthesis parameters that can yield a wide variety of products. In this study, we report the synthesis of a series of unprecedented wheel-shaped molybdenum-blue (MB) clusters directed by organophosphonate (L) and acetate ligands, resulting in a new range of giant MB-type POMs: {Mo₁₃₆Na₄}, {Mo₁₂₀}, {Mo₁₁₈Na₂}, {Mo₁₁₈}, and {Mo₁₅₇}. These structures, constructed from fundamental {Mo₁}, {Mo₂}, and {Mo₈} building blocks, exhibit new features of organic ligand coordination on their exterior surfaces. Notably, the {Mo₁₅₇} framework acts as a host capable of capturing the fully reduced ε-Keggin-based {Mo₁₆} guest. It represents the first pure dodecameric Mo wheel reported to date. Systematic variation of reaction parameters─including ligand type, concentration, solvent composition, and precursor identity─enabled precise control over cluster topology, revealing competitive coordination between organophosphonate and acetate ligands. Structural analyses unveiled new connection modes involving reduced edge-sharing {e-Mo₂} units and their derivative {Mo₃L₂} motifs, which reinforce the overall cage architecture. Mass spectrometry and NMR spectroscopy confirmed the structural integrity of these assemblies in solution. This work not only expands the library of gigantic MB clusters but also establishes a new strategy for their controlled construction using anchored organophosphonate ligands. The resulting clusters exhibit significantly enhanced solubility in organic solvents compared with traditional MB species, offering new opportunities for postsynthetic modification, improved interactions with biomolecules, and diverse applications.