Molecular Models of Early Transition Metal Oxides: Polyoxoanions as Organic Functional Groups

Abstract

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-, CH₂Mo₄O₁₅H^3-, [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 Nb₂W₄0₁₉R^3-, R = CH₃, CH₃CH₂, (CH₃)₂CH, and (CH₃)₃C, have been prepared by reacting the appropriate alcohol with either the niobotungstic acid Nb₂W₄O₁₉H^3- or its anhydride (Nb₂W₄O₁₈)₂O^6-. 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.