Peroxo-Zr/Hf-Containing Undecatungstosilicates and -Germanates

A family of dimeric, peroxo-containing heteropolytungstates, [M2(O2)2(XW11O39)2]12 12 [M = Zr4+, X = Si (1), Ge (2); M = Hf4+, X = Si (3)], have been synthesized by reacting ZrCl4/HfCl4 with the respective monolacunary Keggin precursor [XW11O39]8 12 (X = Si, Ge) in an aqueous acidic medium (pH 4.8). The isostructural polyanions 1 123 are composed of two (XW11O39) Keggin units encapsulating a central diperoxo 12dimetal fragment {M2(O2)2}4+ (M = Zr4+, Hf4+). Cyclic voltammetry and exhaustive electrolysis studies indicate fast reductive release of the peroxo ligands upon reduction of 1 123. Stoichiometric oxo-transfer studies from 1 123 to the substrate l-methionine were performed, and the reactions were monitored by 1H NMR