Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion

Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [<i>Ln</i>{C₂F₅B­(CN)₃}₃(OH₂)_<i>n</i>] (<i>Ln</i> = La, Eu, Ho; <i>n</i> = 0, 3; [<i>Ln</i><b>1</b>₃(OH₂)_<i>n</i>]) were synthesized using the oxonium salt (H₃O)­[C₂F₅B­(CN)₃] ((H₃O)<b>1</b>) and lanthanide chlorides <i>Ln</i>Cl₃·<i>n</i>H₂O as starting compounds. Single-crystals of _∞³[La­{C₂F₅B­(CN)₃}₃] (_∞³[La<b>1</b>₃]) are obtained from the room temperature ionic liquid (RTIL) [EMIm]<b>1</b> using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La<b>1</b>₃] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [<i>Ln</i><b>1</b>₃(OH₂)₃] (<i>Ln</i> = Eu, Ho) to give microcrystalline _∞³[<i>Ln</i><b>1</b>₃] is achieved in a vacuum at elevated temperatures. All compounds were characterized by vibrational and NMR spectroscopy, thermogravimetry, and elemental analysis. The structures of the three-dimensional coordination polymers _∞³[Eu<b>1</b>₃(OH₂)₃] and _∞³[La<b>1</b>₃] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous _∞³[<i>Ln</i><b>1</b>₃] (<i>Ln</i> = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu³⁺ compounds, [Eu<b>1</b>₃] and [Eu<b>1</b>₃(OH₂)₃], are strongly luminescent, the emission of the anhydrous framework being significantly more intense than the one of the hydrate. The Eu-compounds benefit from a sensitizer effect of the anion. In contrast, the Ho-containing framework _∞³[Ho<b>1</b>₃] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho³⁺ ions