Dimensional Evolution from a Giant Molybdenum-Red Cage-like {Mo ₂₀₀ } to 1D Chains Enabling Ultrahigh Proton Conduction

High Resolution Image Download MS PowerPoint Slide 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, {Mo 8 } 0.5 @{Mo 200 } ( 1 ), featuring a host–guest architecture where a {Mo 200 } shell encapsulates a β -{Mo 8 } guest, together with its one-dimensional chain derivative, [{Mo 8 } 0.5 @{Mo 198 }{Mo 8 }] n ( 2 ), in which the β -{Mo 8 } clusters further act as linkers. The drum-shaped {Mo 200 } shell in 1 is constructed from classical {Mo 7 } and {Mo 4 } units, complemented by phenylphosphonate(L)-induced novel V-shaped {Mo 5 L} and {Mo 5 L}* 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 -{Mo 8 } linkers, two equatorial {Mo 5 L}* units in 1 each detach a {Mo 1 } tail, converting the {Mo 200 } cage into a {Mo 198 } 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.