Tuning Lanthanide Reactivity Towards Small Molecules with Electron-Rich Siloxide Ligands

The synthesis, structure, and reactivity of stable homoleptic heterometallic LnL(4)K(2) complexes of divalent lanthanide ions with electron-rich tris(tert-butoxy)siloxide ligands are reported. The [Ln(OSi(OtBu)(3))(4)K-2] complexes (Ln=Eu, Yb) are stable at room temperature, but they promote the reduction of azobenzene to yield the KPhNNPh radical anion as well as the reductive cleavage of CS2 to yield CS32- as the major product. The Eu-III complex of the radical anion PhNNPh is structurally characterized. Moreover, [Yb(OSi(OtBu)(3))(4)K-2] can reduce CO2 at room temperature. Release of the reduction products in D2O shows the quantitative formation of both oxalate and carbonate in a 1:2.2 ratio. The bulky siloxide ligands enforce the labile binding of the reduction products providing the opportunity to establish a closed synthetic cycle for the Yb-II-mediated CO2 reduction. These studies show that the presence of four electron-rich siloxide ligands renders their Eu-II and Yb-II complexes highly reactive.