The Archetypal Homoleptic Lanthanide Quadruple-Decker—Synthesis, Mechanistic Studies, and Quantum Chemical Investigations

The synthesis of the first well-defined neutral and homoleptic (same metal, same ligand) all-carbon-based quadruple-decker complexes [Sm$^{III/II/III}$$_{3}$(COT$^{1,4-SiiPr3}$)$_{4}$] (COT$^{1,4-SiiPr3}$=1,4-($^{i}$Pr$_{3}$Si)$_{3}$C$_{8}$H$_{6}$)) along with other unique sandwich complexes is shown.
Reduction of [Sm$^{III}$(COT$^{1,4-SiiPr3}(BH$$_{4}$)(thf)] (COT$^{1,4-SiiPr3}$=1,4-($^{i}$Pr$_{3}$Si)$_{3}$C$_{8}$H$_{6}$) with KC$_{8}$ resulted in [Sm$^{III/II/III}$$_{3}$(COT$^{1,4-SiiPr3}$)$_{4}$] the first example of a homoleptic lanthanide quadruple-decker. As indicated by an analysis of the bond metrics in the solid-state, the inner Sm ion is present in the divalent oxidation state, while the outer ones are trivalent. This observation could be confirmed by quantum chemical calculations. Mechanistic studies revealed not only insight into possible formation pathways of [Sm$^{III/II/III}$$_{3}$(COT$^{1,4-SiiPr3}$)$_{4}$] but also resulted in the transformation to other mixed metal sandwich complexes with unique structural properties. These are the 1D-polymeric chain structured [KSm$^{III}$(COT$^{1,4-SiiPr3}$)$_{4}$]$_{n}$ and the hexametallic species [(tol)K(COT$^{1,4-SiiPr3}$)Sm$^{II}$COT$^{1,4-SiiPr3}$)K]$_{2}$ which were initially envisioned as possible building blocks as part of different retrosynthetically guided pathways that we developed.