Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple-Decker Arene-Bridged Cerium(II) Sandwich

Organometallic multi-decker sandwich complexes containing f-elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the Ce-III siloxide complex, [KCeL4] (1; L = OSi(OtBu)(3)), with excess potassium in a THF/toluene mixture afforded a quadruple-decker arene-bridged complex, K(2.2.2-crypt)[{(KL3Ce)(mu-eta(6):eta(6)-C7H8)}(2)Ce] (3). The structure of 3 features a [Ce(C7H8)(2)] sandwich capped by [KL3Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of Ce-II ions involved in delta bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi-decker complex and the first containing non-classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf = O3SCF3) yielded the Ce-IV complex, [CeL4] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.