Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligandsElectronic supplementary information (ESI) available: Full computational and experimental details, NMR spectra, and detailed X-ray crystallographic data in CIF format. CCDC 1895385-1895388. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt00554dDedicated to Prof. Geoffrey Cloke, an inspiring scientist and a lovely man

Two dinuclear homoleptic complexes of lanthanides( ii ) supported by the polydentate tris(tertbutoxy) siloxide ligand ([Yb 2 L 4 ], 1-Yb and [Sm 2 L 4 ], 1-Sm , (L = (O t Bu) 3 SiO − )) 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 [Yb 2 L 4 (μ-OMe) 2 (DME) 2 ], 2 . The 1-Yb and 1-Sm complexes effect the reduction of CO 2 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 CO 2 . Crystals of the tetranuclear mixed-valence oxalate intermediate [Yb 4 L 8 (C 2 O 4 )], 3 were isolated from hexane and the presence of a C 2 O 4 2− ligand bridging two [Yb II L 2 Yb III L 2 ] dinuclear moieties was shown. Crystals of the tetranuclear di-carbonate product [Sm 4 L 8 (μ 3 -CO 3 -κ 4 -O,O′,O′′) 2 ], 4 were isolated from hexane. The structures of 3 and 4 suggest that the CO 2 activation in non-polar solvents involves the interaction of two dimers with CO 2 molecules at least to some extent. Such a cooperative interaction results in both oxalate and carbonate formation. Low-coordinate dinuclear lanthanide complexes supported by siloxides effect the reduction of carbon dioxide to both carbonate and oxalate, but the cooperative binding of CO2 to the two Ln( ii ) cations in the dimer favours oxalate formation.