Zr-monosubstituted Lindqvist-type polyoxometalates (Zr-POMs), (Bu 4 N) 2 [W 5 O 18 Zr(H 2 O) 3 ] (1) and (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ] (2), have been employed as molecular models to unravel the mechanism of hydrogen peroxide activation over Zr(IV) sites. Compounds 1 and 2 are hydrolytically stable and catalyze the epoxidation of CC bonds in unfunctionalized alkenes and α,β-unsaturated ketones, as well as sulfoxidation of thioethers. Monomer 1 is more active than dimer 2. Acid additives greatly accelerate the oxygenation reactions and increase oxidant utilization efficiency up to >99%. Product distributions are indicative of a heterolytic oxygen transfer mechanism that involves electrophilic oxidizing species formed upon the interaction of Zr-POM and H 2 O 2 . The interaction of 1 and 2 with H 2 O 2 and the resulting peroxo derivatives have been investigated by UV−vis, FTIR, Raman spectroscopy, HR-ESI-MS, and combined HPLC-ICPatomic emission spectroscopy techniques. The interaction between an 17 O-enriched dimer, (Bu 4 N) 6 [{W 5 O 18 Zr(μ-OCH 3 )} 2 ] (2′), and H 2 O 2 was also analyzed by 17 O NMR spectroscopy. Combining these experimental studies with DFT calculations suggested the existence of dimeric peroxo species [(μ-η 2 :η 2 -O 2 ){ZrW 5 O 18 } 2 ] 6− as well as monomeric Zr-hydroperoxo [W 5 O 18 Zr(η 2 -OOH)] 3− and Zr-peroxo [HW 5 O 18 Zr(η 2 - O 2 )] 3− species. Reactivity studies revealed that the dimeric peroxo is inert toward alkenes but is able to transfer oxygen atoms to thioethers, while the monomeric peroxo intermediate is capable of epoxidizing CC bonds. DFT analysis of the reaction mechanism identifies the monomeric Zr-hydroperoxo intermediate as the real epoxidizing species and the corresponding α-oxygen transfer to the substrate as the rate-determining step. The calculations also showed that protonation of Zr-POM significantly reduces the free-energy barrier of the key oxygen-transfer step because of the greater electrophilicity of the catalyst and that dimeric species hampers the approach of alkene substrates due to steric repulsions reducing its reactivity. The improved performance of the Zr(IV) catalyst relative to Ti(IV) and Nb(V) catalysts is respectively due to a flexible coordination environment and a low tendency to form energy deep-well and low-reactive Zr-peroxo intermediates.

Octahedral molybdenum and tungsten clusters have potential biological applications in photodynamic therapy and bioimaging. However, poor solubility and hydrolysis stability of these compounds hinder their application. The first water-soluble photoluminescent octahedral tungsten cluster [{W I }(DMSO) ](NO ) was synthesised and demonstrated to be at least one order of magnitude more stable towards hydrolysis than its molybdenum analogue. Biological studies of the compound on larynx carcinoma cells suggest that it has a significant photoinduced toxicity, while the dark toxicity increases with the increase of the degree of hydrolysis. The increase of the dark toxicity is associated with the in situ generation of nanoparticles that clog up the cisternae of rough endoplasmic reticulum.

Ti- and Nb-monosubstituted tungstates of the Lindqvist structure, (Bu4N)3[(CH3O)­TiW5O18] (TiW 5 ) and (Bu4N)2[(CH3O)­NbW5O18] (NbW 5 ), display catalytic reactivity analogous to that of heterogeneous Ti- and Nb-containing catalysts in alkene oxidation with aqueous hydrogen peroxide. In this work, we make an attempt to rationalize the differences observed in the catalytic performance of Ti and Nb single-site catalysts for alkene epoxidation with H2O2 using MW 5 (M = Ti and Nb) as tractable molecular models. In the oxidation of cyclohexene, NbW 5 reveals higher catalytic activity and heterolytic pathway selectivity than its Ti counterpart, while TiW 5 is more active for decomposition of H2O2. The heterolytic and homolytic oxidation pathways have been investigated by means of kinetic and computational tools. The kinetic trends established for MW 5 -catalyzed epoxidation, comparative spectroscopic studies (IR, Raman, UV–vis, and 1H and 17O NMR) of the reaction between MW 5 and hydrogen peroxide, and DFT calculations implemented on cyclohexene epoxidation over MW 5 strongly support a mechanism that involves interaction of either MW 5 or its hydrolyzed form “MOH” with H2O2 to afford a protonated peroxo species “HMO2” that is present in equilibrium with a hydroperoxo species “MOOH”, followed by electrophilic oxygen atom transfer from “MOOH” to the CC bond to give epoxide and “MOH”. For both Ti and Nb, the peroxo species “HMO2” is more stable than the hydroperoxo species “MOOH”, but the latter is more reactive toward alkenes. For the Ti catalyst, which has a rigid and hindered metal center, the hydroperoxo species transfers preferentially the nondistorted β-oxygen, whereas for the Nb catalyst the transference of the more electrophilic α-oxygen is favored. Moreover, upon increasing the oxidation state from Ti­(IV) to Nb­(V), the reaction accelerates and selectivity toward electrophilic products increases. Calculations showed that the Nb­(V) catalyst reduces significantly the free-energy barrier for the heterolytic oxygen transfer because of the higher electrophilicity of the metal center. The improved performance of the Nb­(V) single site is due to a combination of a flexible coordination environment with a higher metal oxidation state.

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