Reactive Zr(IV) and Hf(IV) 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)(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.

Peroxo-Zr/Hf-containing undecatungstosilicates and -germanates.

A family of dimeric, peroxo-containing heteropolytungstates, [M(2)(O(2))(2)(XW(11)O(39))(2)](12-) [M = Zr(4+), X = Si (1), Ge (2); M = Hf(4+), X = Si (3)], have been synthesized by reacting ZrCl(4)/HfCl(4) with the respective monolacunary Keggin precursor [XW(11)O(39)](8-) (X = Si, Ge) in an aqueous acidic medium (pH 4.8). The isostructural polyanions 1-3 are composed of two (XW(11)O(39)) Keggin units encapsulating a central diperoxo-dimetal fragment {M(2)(O(2))(2)}(4+) (M = Zr(4+), Hf(4+)). Cyclic voltammetry and exhaustive electrolysis studies indicate fast reductive release of the peroxo ligands upon reduction of 1-3. Stoichiometric oxo-transfer studies from 1-3 to the substrate l-methionine were performed, and the reactions were monitored by (1)H NMR.

6-Peroxo-6-zirconium crown and its hafnium analogue embedded in a triangular polyanion: [M6(O2)6(OH)6(gamma-SiW10O36)3]18- (M = Zr, Hf).

We have synthesized and structurally characterized the unprecedented peroxo-zirconium(IV) containing [Zr6(O2)6(OH)6(gamma-SiW10O36)3]18- (1). Polyanion 1 comprises a cyclic 6-peroxo-6-zirconium core stabilized by three decatungstosilicate units. We have also prepared the isostructural hafnium(IV) analogue [Hf6(O2)6(OH)6(gamma-SiW10O36)3]18- (2). We investigated the acid/base and redox properties of 1 by UV-vis spectroscopy and electrochemistry studies. Polyanion 1 represents the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units. The simple, one-pot synthesis of 1 and 2 involving dropwise addition of aqueous hydrogen peroxide could represent a general procedure for incorporating peroxo groups into a large variety of transition metal and lanthanide containing POMs.