The reductive disproportionation of CO2 using a magnesium(i) complex: analogies with low valent f-block chemistry

Abstract

The dimeric magnesium(I) complex, [{(^DipNacnac)Mg}₂] (^DipNacnac = [(DipNCMe)₂CH]⁻, Dip = C₆H₃Prⁱ₂-2,6), reacts with 2 equivalents of CO₂ to give a high yield of the Mg^II carbonate complex, [{(^DipNacnac)Mg}₂(μ-κ²:κ²-CO₃)], and CO via a reductive disproportionation process. The Mg^II oxalate, [{(^DipNacnac)Mg}₂(μ-C₂O₄)], is a very low yield by-product of the reaction. Reducing the carbonate complex with elemental potassium regenerates the Mg^I starting material. The carbonate complex is shown to form via a stepwise process involving the oxo-bridged intermediate, [{(^DipNacnac)Mg}₂(μ-O)], which rapidly reacts with stoichiometric CO₂ to give [{(^DipNacnac)Mg}₂(μ-κ²:κ²-CO₃)]. The oxo-bridged intermediate has been rationally synthesised via the reaction of [{(^DipNacnac)Mg}₂] with N₂O, and treated with THF to give the adduct, [{(THF)(^DipNacnac)Mg}₂(μ-O)]. The complex, [{(^DipNacnac)Mg}₂(μ-O)], reacts with the CO₂ isoelectronic analogues, CS₂ and CyNCNCy (Cy = cyclohexyl) to give the complexes, [{(^DipNacnac)Mg}₂(μ-κ²:κ²-CS₂O)] and [{(^DipNacnac)Mg}₂{μ-κ²:κ²-C(NCy)₂O}]. Rearrangement of the dithiocarbonate coordination mode of the former occurs upon treatment with diethyl ether, giving the unsymmetrical complex, [{(^DipNacnac)Mg}(μ-κ²(S,S′):κ¹(O)-CS₂O){Mg(^DipNacnac)(OEt₂)}]. The majority of the complexes prepared in this study were crystallographically characterised. Taken as a whole, this study demonstrates that magnesium(I) dimers can display very similar reactivity, with respect to small molecules activations, as do Sm^II and U^III compounds. Accordingly, magnesium(I) compounds hold considerable potential as cheaper, less toxic, non-radioactive and diamagnetic alternatives to low valent f-block metal complexes in this realm.