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

Abstract

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(η⁸-Cot^TIPS)} (Cot^TIPS = 1,4-(ⁱPr₃Si)₂C₈H₆²⁻) into a classical divalent sandwich complex [Ln^II(η⁹-Cnt)₂] (Ln = Sm, Eu, Yb; Cnt = C₉H$$), the unprecedented triple-decker sandwich complexes of the type [(η⁹-Cnt)Ln^II(μ-η⁸:η⁸-Cot^TIPS)Ln^II(η⁹-Cnt)] were obtained. A plausible reaction pathway was determined by quantum chemical calculations. Additionally, we synthesize the same lanthanide sandwich complexes in a traditional, stepwise fashion. For this, we initially present the novel inverse sandwich compounds [Ln^III(thf)₂(μ-η⁸:η⁸-Cot^TIPS)Ln^III(thf)₂] (Ln = Sm, Eu, thf = tetrahydrofuran) and [Yb^II(BH₄)(thf)₂(μ-η⁸:η⁸-Cot^TIPS)Yb^II(BH₄)(thf)₂] consisting of a cyclooctatetraene middle deck sandwiched between two divalent lanthanides as precursors. Subsequent salt metathesis reactions with [K(Cnt)] (Cnt = C₉H$$) gave rise to the title compounds [(η⁹-Cnt)Ln^II(μ-η⁸:η⁸-Cot^TIPS)Ln^II(η⁹-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.