title: Reactive Zr(IV) and Hf(IV) butterfly peroxides on polyoxometalate surfaces: bridging the gap between homogeneous and heterogeneous catalysis.
abstract: At variance with previously known coordination compounds, the polyoxometalate (POM)-embedded Zr(IV) and Hf(IV) peroxides with formula: [M(2)(O(2))(2)(α-XW(11)O(39))(2)](12-) (M=Zr(IV), X=Si (1), Ge (2); M=Hf(IV), X=Si (3)) and [M(6)(O(2))(6)(OH)(6)(γ-SiW(10)O(36))(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 (1)H 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 H(2)O(2) 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.
authors.affiliation: Department of Chemical Sciences, ITM-CNR and University of Padova, Via Marzolo 1, 35131 Padova, Italy. mauro.carraro@unipd.it
authors.forename: Mauro
authors.initials: M
authors.lastname: Carraro
authors.forename: Nadeen
authors.initials: N
authors.lastname: Nsouli
authors.forename: Holger
authors.initials: H
authors.lastname: Oelrich
authors.forename: Andrea
authors.initials: A
authors.lastname: Sartorel
authors.forename: Antonio
authors.initials: A
authors.lastname: Sorarù
authors.forename: Sib Sankar
authors.initials: SS
authors.lastname: Mal
authors.forename: Gianfranco
authors.initials: G
authors.lastname: Scorrano
authors.forename: Lorenz
authors.initials: L
authors.lastname: Walder
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kortz
authors.forename: Marcella
authors.initials: M
authors.lastname: Bonchio