[Ln(6)O(8)] Cluster‐Encapsulating Polyplumbites as New Polyoxometalate Members and Record Inorganic Anion‐Exchange Materials for ReO(4) (−) Sequestration

Various types of polyoxometalates (POMs) have been synthesized since the 19th century, but their assortment has been mostly limited to Groups 5 and 6 metals. Herein, a new family of POMs composed of a carbon group element as the addenda atoms with two distinct phases, LnPbOClO(4)‐1 (Ln = Sm to Ho, Y) and LnPbOClO(4)‐2 (Ln = Er and Tm) is reported. Both structures are built from [Ln(6)O(8)] rare‐earth metal hexamers being incorporated in [Pb(18)O(32)]/[Pb(12)O(24)] polyplumbites, and unbound perchlorates as charge‐balancing anions. Impressively, YPbOClO(4)‐1 and ErPbOClO(4)‐2 exhibit exceptional uptake capacities (434.7 and 427.7 mg g(−1)) toward ReO(4) (−), a chemical surrogate for the key radioactive fission product in the nuclear fuel cycle (99)TcO(4) (−), which are the highest values among all inorganic anion‐exchange materials reported until now. The sorption mechanism is clearly elucidated and visualized by single‐crystal‐to‐single‐crystal structural transformation from ErPbOClO(4)‐2 to a perrhenate‐containing complex ErPbOReO(4), revealing a unique ReO(4) (−) uptake selectivity driven by specific interaction within Pb···O‐ReO(3) (−) bonds.