Uptake of one and two molecules of CO[subscript 2] by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation

Abstract

Tetrahedral [MoO4][superscript 2−] readily binds CO[subscript 2] at room temperature to produce a robust monocarbonate complex, [MoO[subscript 3](κ[superscript 2]-CO[subscript 3])][superscript 2−], that does not release CO[subscript 2] 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 CO[subscript 2] binds to afford a pseudo-octahedral dioxo dicarbonate complex, [MoO[subscript 2](κ[superscript 2]-CO[subscript 3])[subscript 2][superscript 2−], the first structurally characterized transition-metal dicarbonate complex derived from CO[subscript 2]. The monocarbonate [MoO[subscript 3](κ[superscript 2]-CO[subscript 3])][superscript 2−] reacts with triethylsilane in acetonitrile under an atmosphere of CO[subscript 2] to produce formate (69% isolated yield) together with silylated molybdate (quantitative conversion to [MoO[subscript 3](OSiEt[subscript 3])][superscript −], 50% isolated yield) after 22 hours at 85 °C. This system thus illustrates both the reversible binding of CO[subscript 2] by a simple transition-metal oxoanion and the ability of the latter molecular metal oxide to facilitate chemical CO[subscript 2] reduction.