Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

Abstract

Two dinuclear homoleptic complexes of lanthanides(II) supported by the polydentate tris(tertbutoxy) siloxide ligand ([Yb₂L₄], 1-Yb and [Sm₂L₄], 1-Sm, (L = (O^tBu)₃SiO⁻)) were synthesized in 70–80% yield and 1-Sm was crystallographically characterized. 1-Yb and 1-Sm are stable in solution at −40 °C but cleave the DME C–O bond over time at room temperature affording the crystal of [Yb₂L₄(μ-OMe)₂(DME)₂], 2. The 1-Yb and 1-Sm complexes effect the reduction of CO₂ under ambient conditions leading to carbonate and oxalate formation. The selectivity of the reduction towards oxalate or carbonate changes depend on the solvent polarity and on the nature of the ion. For both the lanthanides, carbonate formation is favoured but oxalate formation increases if a non-polar solvent is used. Computational studies suggest that the formation of oxalate is favoured with respect to carbonate formation in the reaction of the dimeric lanthanide complexes with CO₂. Crystals of the tetranuclear mixed-valence oxalate intermediate [Yb₄L₈(C₂O₄)], 3 were isolated from hexane and the presence of a C₂O₄²⁻ ligand bridging two [Yb^IIL₂Yb^IIIL₂] dinuclear moieties was shown. Crystals of the tetranuclear di-carbonate product [Sm₄L₈(μ₃-CO₃-κ⁴-O,O′,O′′)₂], 4 were isolated from hexane. The structures of 3 and 4 suggest that the CO₂ activation in non-polar solvents involves the interaction of two dimers with CO₂ molecules at least to some extent. Such a cooperative interaction results in both oxalate and carbonate formation.