Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation

Abstract

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.