Structures and optical properties of cyclic-ether complexes of europium/ytterbium made via a redox approach

Abstract

To obtain cyclic-ether complexes of europium/ytterbium with generally less preferable cyclic ethers (i.e., those with large/small ring openings, cage-type cryptands, and N-containing crown ethers), a novel redox approach is used. Accordingly, 24-crown-8 (24c8), benzo-15-crown-5 (benzo-15c5), N-phenylaza-15-crown-5 (pac), [2.2.2]cryptand (c222), and kryptofix21 (k21) are made to react with europium and ytterbium metal nanoparticles, 2–4 nm in size. Europium and ytterbium are selected as both lanthanides are flexible in their oxidation state (II/III), and cover a typical range of cation radii (r(Eu²⁺): 139 pm to r(Yb³⁺): 113 pm) in the 4f-row (radii for CN = 8). As a result, [Yb(24c8)]₄[AlCl₄]₈·THF (1), [Eu(benzo-15c5)₂]I₂ (2), [Yb(benzo-15c5)₂]I₂ (3), [BMIm][Eu₄Cl₄(pac)₄][AlCl₄]₅ (4), [{Yb(pac)}₂(μ-Cl)₂][AlCl₄]₂ (5), [EuCl(c222)][AlCl₄] (6), [EuI(c222)]I (7), [{Yb(c222)}₂(μ-Cl)]₂[AlCl₄]₆·1.5THF (8), [BMIm][Yb(c222)]I₃ (9), [Eu(k21)₂]I₂·naph (10), and [BMIm][{Yb(k21)}₂(μ-Cl)₃]I₂ (11) are obtained as new compounds. The title compounds show various coordination scenarios with mono- to tetranuclear building units. All are luminescent with a narrow-line blue emission of 4 and the strong second harmonic generation (SHG) effect of 6 being specifically interesting. As the redox approach used here leads to the coordination of the lanthanide cations with all applied cyclic ethers – although exhibiting less preferred size and/or coordinative binding with the lanthanide cation – it may present a novel synthesis strategy to generally prepare new metal complexes with cyclic ethers with interesting optical, magnetic or catalytic properties.