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 ( 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), 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) 2 ( 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. Subsequent salt metathesis reactions with [K(Cnt)] (Cnt = C 9 H) 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.