Dimensional Evolution from a Giant Molybdenum-Red Cage-like {Mo₂₀₀} to 1D Chains Enabling Ultrahigh Proton Conduction

The controlled assembly of giant molybdenum-red polyoxometalates (POMs) into high-order architectures and elucidating their structure-property relationships remain a formidable challenge. We report an unprecedented 204-nuclearity hollow nanocage, {Mo₈}_0.5@{Mo₂₀₀} (1), featuring a host-guest architecture where a {Mo₂₀₀} shell encapsulates a β-{Mo₈} guest, together with its one-dimensional chain derivative, [{Mo₈}_0.5@{Mo₁₉₈}{Mo₈}]_n (2), in which the β-{Mo₈} clusters further act as linkers. The drum-shaped {Mo₂₀₀} shell in 1 is constructed from classical {Mo₇} and {Mo₄} units, complemented by phenylphosphonate(L)-induced novel V-shaped {Mo₅L} and {Mo₅L}* building blocks. Featuring highly reduced molybdenum centers and localized reducing electrons on Mo-Mo bonds, it forms a massive internal cavity (∼2.2 nm × 1.5 nm). The discovery of 1 significantly expands the structural library of rare giant molybdenum-red clusters. Intriguingly, its assembly triggers a symmetry breaking from pseudo-D_5d to C_2h, shifting the architecture further away from an ideal fullerene-like topology. To accommodate the directional assembly of β-{Mo₈} linkers, two equatorial {Mo₅L}* units in 1 each detach a {Mo₁} tail, converting the {Mo₂₀₀} cage into a {Mo₁₉₈} entity with open connection sites to ultimately form the 1D polymeric chain 2. Benefiting from dense proton-conductive sites and large open windows, 1 exhibits a high conductivity of 8.28 × 10^-2 S cm^-1 (80 °C, 98% RH). More impressively, continuous 1D pathways in 2 boost this value to an ultrahigh 1.28 × 10^-1 S cm^-1, ranking among the best POM-based proton conductors. This work demonstrates rare dimensional evolution in giant POMs and establishes a new paradigm for designing high-performance solid-state proton conductors.