Crown-Ether Coordination Compounds of Europium and 24-Crown-8

Crown-ether coordination compounds of europium(II/III) and the crown ether (C₂H₄O)₈ (24-crown-8, 24c8) are prepared, aiming at novel compounds, structures, and coordination modes as well as potential luminescence properties. By reacting EuCl₂, EuI₂, or EuCl₃ with 24c8 or its derivatives in ionic liquids, the novel compounds [Bu₃MeN]₂[Eu(II)(NTf₂)₄] (<b>1</b>), [BMIm]₆[Eu₃I₁₂] (<b>2</b>), [EuCl₂(dibenzo-18c6)] (<b>3</b>), [EuI₂(dibenzo-24c8)] (<b>4</b>), [(Eu(III)Cl₃)₂(C₁₄H₃₀O₈)](24c8) (<b>5</b>), and [Eu(III)Cl(24c8)]I₂ (<b>6</b>) are obtained (BMIm: 1-butyl-3-methylimidazolium; EMIm: 1-ethyl-3-methylimidazolium). Based on different reaction conditions, different coordinative modes including the absence of the crown ether in the product (<b>1</b>, <b>2</b>), splitting of the crown ether (<b>5</b>), and coordination of 24c8 via six of eight oxygen atoms (<b>4</b>) and, finally, via all oxygen atoms (<b>6</b>) are observed. Crystallization of the title compounds is generally difficult, which can be attributed to the flexibility of the crown-ether molecule that can be rotated around all 24 tetrahedral (C) and pseudo-tetrahedral (O) centers. Besides structural characterization via single-crystal structure analysis and X-ray powder diffraction with Rietveld refinement, compounds <b>1</b>-<b>6</b> are examined by infrared spectroscopy and thermal analysis. The title compounds show blue to red emission, and the influence of structure and coordinative mode on the luminescent properties is analyzed.