Polyoxometalate-based Hybrids as Catalysts for C-C Coupling

CARRARO, M.; BERARDI, S.; MARTINIS, E.M.; FIORANI, G; SARTOREL, A.; SCORRANO, G; BONCHIO, M

Encuentro; Fifth Joint Meeting of Working Groups of EU-COST; 2011

Polyanionic metal oxides clusters are known as polyoxometalates (POMs), with general formula [XxMmOy]q-, where M is the main metallic component (MoVI, WVI, VV) and X is an heteroatom (such as PV or SiIV). These complexes offer a unique opportunity because of their prevalent inorganic, robust nature and high versatility in terms of structure, chemical composition, electron density and polyanionic charge. Indeed, a great variety of structures belongs to the family of POMs and they can be obtained by tuning specific parameters like concentration, stoichiometric ratios of reactants, temperature and pH. Noteworthy, isostructural polyoxometalates can easily be obtained, displaying different properties depending on the composition. The choice of a suitable associated counterion allows to solubilize the POMs in a wide range of solvents, from apolar ones (toluene, dichloromethane), by using lipophilic cations such as tetraoctylammonium, to water, with alkaline counterions or protons. Furthermore, since they are constituted by metal atoms in their highest oxidation states, polyoxometalates result more stable towards the oxidative degradation than generic organic compounds. Molecular hybrids are of particular interest in catalysis, due to the interplay of joint organic-inorganic domains with very diverse functional environments. The hybrid strategy is expected to allow for fine-tuning of the stereo-electronic properties of the catalyst through a tailored choice of the interacting organic-inorganic moieties. Covalent POM hybrids are characterized by discrete, nanosized, multi-metal-oxides as polyanionic scaffolds. They present high stability and they can be used under harsh conditions, including the irradiation with Microwaves. The anchored organic pendant arms can also promote an increased affinity towards Ionic Liquids. As an example, robust hybrid polyoxometalates like [ã-SiW10O36(PhPO)2]4- have been used in hydrophobic ILs, under MW assisted conditions, to promote the oxidation of internal and terminal double bonds, in the presence of hydrogen peroxide, as an environmental friendly oxidant, with outstanding catalytic performance and selectivity >99%.xMmOy]q-, where M is the main metallic component (MoVI, WVI, VV) and X is an heteroatom (such as PV or SiIV). These complexes offer a unique opportunity because of their prevalent inorganic, robust nature and high versatility in terms of structure, chemical composition, electron density and polyanionic charge. Indeed, a great variety of structures belongs to the family of POMs and they can be obtained by tuning specific parameters like concentration, stoichiometric ratios of reactants, temperature and pH. Noteworthy, isostructural polyoxometalates can easily be obtained, displaying different properties depending on the composition. The choice of a suitable associated counterion allows to solubilize the POMs in a wide range of solvents, from apolar ones (toluene, dichloromethane), by using lipophilic cations such as tetraoctylammonium, to water, with alkaline counterions or protons. Furthermore, since they are constituted by metal atoms in their highest oxidation states, polyoxometalates result more stable towards the oxidative degradation than generic organic compounds. Molecular hybrids are of particular interest in catalysis, due to the interplay of joint organic-inorganic domains with very diverse functional environments. The hybrid strategy is expected to allow for fine-tuning of the stereo-electronic properties of the catalyst through a tailored choice of the interacting organic-inorganic moieties. Covalent POM hybrids are characterized by discrete, nanosized, multi-metal-oxides as polyanionic scaffolds. They present high stability and they can be used under harsh conditions, including the irradiation with Microwaves. The anchored organic pendant arms can also promote an increased affinity towards Ionic Liquids. As an example, robust hybrid polyoxometalates like [ã-SiW10O36(PhPO)2]4- have been used in hydrophobic ILs, under MW assisted conditions, to promote the oxidation of internal and terminal double bonds, in the presence of hydrogen peroxide, as an environmental friendly oxidant, with outstanding catalytic performance and selectivity >99%.