Sulfate-guided assembly of a fully closed {Mo192} molybdenum blue capsule with accessible Mo sites for selective phenol oxidation

Fully closed molybdenum blue cages remain difficult to prepare because long-range shell closure needs to be balanced with the stability of a mixed-valence Mo^VI/Mo^V framework. Retaining accessible surface metal sites adds a further challenge when such cages are considered for catalysis. Here we report a sulfate-guided fragment/anion co-assembly strategy for the synthesis of a capsule-shaped {Mo192} molybdenum blue cage, formulated as K₆Na₂₀[H₄₄Mo^VI₁₃₀Mo^V₆₂O₅₄₈(H₂O)₁₀₂(OH)₄₄(SO₄)₁₀]·238H₂O. The cluster is obtained from a Waugh-type K₆MnMo₉O₃₂·6H₂O precursor and molybdate under acidic, reducing, and hydrothermal conditions. Single-crystal X-ray diffraction reveals two fused {Mo96} half-shells, inward sulfate-bound {S-Mo6} units, equatorial {Mo2}-based connections, and continuous windows. The inward-facing sulfate ligands help support shell curvature without blocking the outer surface of the cage. Redox titration, bond-valence-sum analysis, and XPS indicate a mixed Mo^VI/Mo^V framework with an overall reduction degree of ca. 32%. The accessible mixed-valence cage was further evaluated in the tert-butyl hydroperoxide (TBHP) oxidation of 2-tert-butylphenol, giving 99% conversion and 99% selectivity for 2-tert-butyl-p-benzoquinone at 70 °C in 6 h. Hot-filtration, ICP-OES, recycling, FT-IR, and PXRD results support {Mo192} as the major recoverable catalytic phase. These results suggest that internal anion support is a useful way to assist the closure of giant POM cages while retaining surface accessibility for catalytic function.