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 {LnII(η8-CotTIPS)} (CotTIPS = 1,4-(iPr3Si)2C8H62-) into a classical divalent sandwich complex [LnII(η9-Cnt)2] (Ln = Sm, Eu, Yb; Cnt = C9H9-), the unprecedented triple-decker sandwich complexes of the type [(η9‑Cnt)LnII(μ‑η8:η8‑CotTIPS)LnII(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 [LnIII(thf)2(μ‑η8:η8‑CotTIPS)LnIII(thf)2] (Ln = Sm, Eu, thf = tetrahydrofuran) and [YbII(BH4)(thf)2(μ‑η8:η8‑CotTIPS)YbII(BH4)(thf)2] consisting of a cyclooctatetraene middle deck sandwiched between two divalent lanthanides as precursors. Subsequent salt metathesis reactions with [K(Cnt)] (Cnt = C9H9-) gave rise to the title compounds [(η9‑Cnt)LnII(μ‑η8:η8‑CotTIPS)LnII(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.