Coordination Chemistry and Spectroscopic Properties of Eu(II), Sm(II), and Yb(II) with 12-Crown‑4

The coordinative properties of 12-crown-4 (12c4) with Sm²⁺, Eu²⁺, and Yb²⁺ have been examined using nonaqueous and inert atmosphere conditions and led to the isolation of five complexes: Ln(12c4)(THF)₂I₂ (Ln = Sm 1, Eu 2), [Ln(12c4)₂(CH₃CN)][Ph₄B]₂ (Ln = Sm 3, Eu 4), and [Yb(12c4)₂][Ph₄B]₂ (5). Most complexes were prepared via the salt metathesis of LnI₂ with tetrabutylammonium tetraphenylborate ([TBA][Ph₄B]) and 12-crown-4 in acetonitrile, while some were crystallized from THF. The half-sandwich compounds 1 and 2 crystallize with trans iodide orientation and exhibit mixed d–f and f–f and d–f photoluminescence when excited with 546 and 365 nm light, respectively. The full sandwich compounds 3 and 4 feature [Ln(12c4)₂(CH₃CN)]²⁺ complex cations, where two 12c4 molecules are not sufficiently large to coordinatively saturate these larger cations without further ligation, while the smaller Yb²⁺ cation is fully encapsulated in two 12c4 molecules (5). Solution UV–vis–NIR studies show that when 12c4 is added to acetonitrile solutions of LnI₂, the f–d transitions shift to higher energies, suggesting destabilization of the lower lying d orbitals and increased stability of the divalent state by complexation to 12c4 in solution.