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(k(2)-CO3)](2-), that does not release CO2 even at modestly elevated temperatures (up to 56 degrees C in solution and 70 degrees 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(k(2)-CO3)(2)](2-), the first structurally characterized transition-metal dicarbonate complex derived from CO2. The monocarbonate [MoO3(k(2)-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 degrees 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.