Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation†
Tetrahedral [MoO4]2− readily binds CO2 at room temperature to produce a robust monocarbonate complex, [MoO3(κ2-CO3)]2−, that does not release CO2 even at modestly elevated temperatures (up to 56 °C in solution and 70 °C in the solid state). In the presence of excess carbon dioxide, a second molecule of CO2 binds to afford a pseudo-octahedral dioxo dicarbonate complex, [MoO2(κ2-CO3)2]2−, the first structurally characterized transition-metal dicarbonate complex derived from CO2. The monocarbonate [MoO3(κ2-CO3)]2− reacts with triethylsilane in acetonitrile under an atmosphere of CO2 to produce formate (69% isolated yield) together with silylated molybdate (quantitative conversion to [MoO3(OSiEt3)]−, 50% isolated yield) after 22 hours at 85 °C. This system thus illustrates both the reversible binding of CO2 by a simple transition-metal oxoanion and the ability of the latter molecular metal oxide to facilitate chemical CO2 reduction.