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Reactive Zr IV and Hf IV Butterfly Peroxides on Polyoxometalate Surfaces: Bridging the Gap between Homogeneous and Heterogeneous Catalysis
Author(s) -
Carraro Mauro,
Nsouli Nadeen,
Oelrich Holger,
Sartorel Andrea,
Sorarù Antonio,
Mal Sib Sankar,
Scorrano Gianfranco,
Walder Lorenz,
Kortz Ulrich,
Bonchio Marcella
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201003103
Subject(s) - polyoxometalate , catalysis , reactivity (psychology) , chemistry , protonation , stoichiometry , electron transfer , redox , electrochemistry , photochemistry , crystallography , inorganic chemistry , organic chemistry , medicine , alternative medicine , pathology , ion , electrode
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 .