Reactive ZrIV and HfIV Butterfly Peroxides on Polyoxometalate Surfaces: Bridging the Gap between Homogeneous and Heterogeneous Catalysis

At variance with previously known coordination compounds, the polyoxometalate (POM)-embedded Zr-IV and Hf-IV peroxides with formula: [M-2(O-2)(2)(alpha-XW11O39)(2)](12-) (M = Zr-IV, X = Si (1), Ge (2); M = Hf-IV, X = Si (3)) and [M-6(O-2)(6)(OH)(6)(gamma-SiW10O36)(3)](18-) (M = Zr-IV (4) or Hf-IV (5)) are capable of oxygen transfer to suitable acceptors including sulfides and sulfoxides in water. Combined H-1 NMR and electrochemical studies allow monitoring of the reaction under both stoichiometric and catalytic conditions. The reactivity of peroxo-POMs 1-5 is compared on the basis of substrate conversion and kinetic. The results show that the reactivity of POMs 1-3 outperforms that of the trimeric derivatives 4 and 5 by two orders of magnitude. Reversible peroxidation of 1-3 occurs by H2O2 addition to the spent catalysts, restoring oxidation rates and performance of the pristine system. The stability of 1-3 under catalytic regime has been confirmed by FT-IR, UV/Vis, and resonance Raman spectroscopy. The reaction scope has been extended to alcohols, leading to the corresponding carbonyl compounds with yields up to 99% under microwave (MW) irradiation. DFT calculations revealed that polyanions 1-3 have high-energy peroxo HOMOs, and a remarkable electron density localized on the peroxo sites as indicated by the calculated map of the electrostatic potential (MEP). This evidence suggests that the overall description of the oxygen-transfer mechanism should include possible protonation equilibria in water, favored for peroxo-POMs 1-3.