Molecular Models of Early Transition Metal Oxides: Polyoxoanions as Organic Functional Groups
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Early transition metal oxides are known to be effective heterogeneous catalysts or catalyst precursors for numerous organic transformations, including methanol oxidation to give formaldehyde, methacrolein oxidation to give methacrylic acid, propylene oxidation to give acrolein, methane oxidation to give formaldehyde, and propylene metathesis to give ethylene and butene. Due to the difficulty of characterizing surface-bound intermediates, efforts have been made to synthesize and study the reaction chemistry of analogous polyoxoanion derivatives containing organic subunits. Several polyoxomolybdate derivatives have been structurally characterized during the past decade that incorporate simple organic moieties believed to be surface intermediates in these heterogeneous systems, including [(HCO2)2(Mo8O26)]6-, CH2Mo4O15H3-, [CH3C(CH2O)3]2(CH3CH2O)2Mo4O8, and [(CH3O)4(Mo8O)24]4-. More recently, two polyoxoanion systems have been investigated that illustrate solution pathways for incorporating organic groups into polyoxoanions and effecting organic oxidations and dehydrations. In the first, the niobotungstic acid esters Nb2W4019R3-, R = CH3, CH3CH2, (CH3)2CH, and (CH3)3C, have been prepared by reacting the appropriate alcohol with either the niobotungstic acid Nb2W4O19H3- or its anhydride (Nb2W4O18)2O6-. In the second system, molybdophosphoric acid esters such as (P3O9)MoO3CH2CH3 2- have been prepared from the appropriate alcohol and then degraded thermally or photochemically. The ethyl ester, for example, yields acetaldehyde plus ethanol thermally, and acetaldehyde, ethanol, plus ethylene photochemically.
KeywordsMolybdenum Oxide Alkoxy Group Ethoxy Group Molybdenum Trioxide Early Transition Metal
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