In reactions between the hexamolybdate [Mo6O19I2-and 1,4-diaminobenzener terminal oxides are replaced by 4-aminophenylimido ligands and the first structurally characterised polyoxometallate to contain functionalised organoimido ligands, ( B U ~~N ) ~[ M O ~O ~~( N C ~H ~N H ~) ~I 5, is also the first example of a trans-bis(imido) derivative; further condensation also occurs to yield species in which hexamolybdate units are linked by Ir4-phenylenediimido bridges.

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.

No abstract

Oxygen-1 7 N M R studies indicated that the hexametalates [MW,O,,]"-or their derivatives [(MeO)MW,-0 18 1'" ' ) ( M = Ti, Zr, V, Nb, Ta, Mo or W) can be obtained by hydrolysis of the appropriate mixture of

Polyoxometalates (POMs) are discrete, molecular metal oxides with dimensions ranging from about one to tens of ngstroms, a wide variety of topologies and compositions, and an extensive range of chemical and electronic properties which, together with their thermal and oxidative stabilities, are leading to applications in catalysis, electrooptics, magnetics, medicine, and biology. [1][2][3] Consequently, POMs are attractive as functional components of active materials. In recent years, systematic methods for POM synthesis and derivatization have been developed, providing an expanding range of robust "designer" components for "bottom-up" materials synthesis, and a major challenge now facing those engaged in POM research is to devise generic methods for constructing functional nanoscale architectures from these versatile building blocks.The self-assembly of organic monolayers on surfaces has developed over the last two decades into a powerful strategy for the construction of hierarchical structures from molecular components and, although self-assembled inorganic monolayers have received much less attention, several groups have investigated the incorporation of POMs into surface-confined structures. Ordered monolayers of POMs have been obtained on silver and gold by adsorption from solution, [4,5] and evaporative solution deposition has been used to produce catalytically active POM layers on highly oriented pyrolytic graphite (HOPG). [6][7][8] Hybrid organic-POM multilayered magnetic structures have been produced by LangmuirBlodgett techniques, [9][10][11] while electrostatic layer-by-layer assembly with cationic polyelectrolytes has produced more robust hybrid structures.[12] These approaches rely upon either electrostatic interactions or ill-defined chemisorption for the assembly of POM monolayers or multilayers and, to our knowledge, the only example of well defined covalent attachment to a surface is that of a thiol-derivatized POM on gold nanoparticles.[13] Our previous work has shown that monofunctional alkoxide POM derivatives [(RO)MW 5 O 18 ] nÀ (M = Ti, Zr, Nb) are accessible through hydrolytic aggregation reactions involving metal alkoxides.

Bu'aPtCHzlsPBu'i reacts with RhCl3-3H20 in ethanol to give a mixture of the binuclear 16-atom ring complex [RhaHaCUIBukPCHzCI-nCHsCI-hCl-hPBukizL which exists in solution as a mixture of two rotamers: a cyclometalated complex [RhHCljBukfrc^C^CHCl-hCI-nPBukl] of configuration 6 and another species, probably an olefin complex [RfiCl{Bu'2PCH2CH2CH=CHCH2PBu'2|] (8). A similar reaction in refluxing tetrahydrofuran gave pure 6. Treatment of the binuclear hydrido complex [Rt^^CUlBukPC^CThCTHC^C^PBukia] with 2-methylpyridine promoted cyclometalation to give 6. This cyclometalated hydride ( 6) is fluxional and at ambient temperature rapid C-H fission occurs, rendering

No abstract

No abstract