A Novel Synthetic Pathway to Lanthanide Triple‐Decker Complexes: Selective Expansion of a Sandwich Complex by an Insertion Reaction

Commonly, multi-decker sandwich complexes can either be formed in a one-step reaction or in a stepwise fashion by stacking deck by deck. Herein, we showcase a new reaction pathway, in which for the first time a lanthanide half-sandwich unit inserts into an existing sandwich complex. As a result of the insertion of a cyclocene half-sandwich fragment {Ln$^{II}$(η$^8$ -Cot$^{TIPS}$)} (Cot$^{TIPS}$ = 1,4-($^i$Pr$_3$Si)$_2$C$_8$ H$_6$$^{2−}$) into a classical divalent sandwich complex [Ln$^{II}$(η$^9$ -Cnt)$_2$] (Ln = Sm, Eu, Yb; Cnt = C$_9$ H$_{9-}$ ), the unprecedented triple-decker sandwich complexes of the type [(η$^9$ -Cnt)Ln$^{II}$(μ-η$^8$:η$^8$-Cot$^{TIPS}$)Ln$^{II}$(η$^{9}$ -Cnt)] were obtained. A plausible reaction pathway was determined by quantum chemical calculations. Additionally, we synthesize the same lanthanide sandwich complexes in a traditional, stepwise fashion. For this, we initially present the novel inverse sandwich compounds [Ln$^{II}$ I(thf)$_2$ (μ-η$^8$:η$^8$ -Cot$^{TIPS}$ )Ln$^{II}$ I(thf)$_ 2$] (Ln = Sm, Eu, thf = tetrahydrofuran) and [Yb$^{II}$ (BH$_4$)(thf)$_4$ (μ-η$^8$:η$^8$ -Cot$^{TIPS}$ )Yb$^{II}$ (BH$_4$ )(thf)$_2$] consisting of a cyclooctatetraene middle deck sandwiched between two divalent lanthanides as precursors. ... mehrSubsequent salt metathesis reactions with [K(Cnt)] (Cnt = C$_9$H$_{9−}$) gave rise to the title compounds [(η$^9$ -Cnt)Ln$^{II}$ (μ-η$^8$:η$^8$ -Cot$^{TIPS}$ )Ln$^{II}$(η$^9$ -Cnt)]. The unique feature of these compounds is the combination of the two largest aromatic all-carbon rings known in coordination chemistry—the 8- and 9-membered rings—into lanthanide
triple-decker sandwich compounds.