Tuning lanthanide reactivity towards small molecules with electron-rich siloxide ligands.

The synthesis, structure, and reactivity of stable homoleptic heterometallic LnL4K2 complexes of divalent lanthanide ions with electron-rich tris(tert-butoxy)siloxide ligands are reported. The [Ln(OSi(OtBu)3)4K2] 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 CS3(2-) as the major product. The Eu(III) complex of the radical anion PhNNPh is structurally characterized. Moreover, [Yb(OSi(OtBu)3)4K2] 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.