Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion

Abstract

Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [ Ln {C 2 F 5 B(CN) 3 } 3 (OH 2 ) n ] ( Ln = La, Eu, Ho; n = 0, 3; [ Ln 1 3 (OH 2 ) n ]) were synthesized using the oxonium salt (H 3 O)[C 2 F 5 B(CN) 3 ] ((H 3 O) 1 ) and lanthanide chlorides Ln Cl 3 · n H 2 O as starting compounds. Single-crystals of ∞ 3 [La{C 2 F 5 B(CN) 3 } 3 ] ( ∞ 3 [La 1 3 ]) are obtained from the room temperature ionic liquid (RTIL) [EMIm] 1 using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La 1 3 ] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [ Ln 1 3 (OH 2 ) 3 ] ( Ln = Eu, Ho) to give microcrystalline ∞ 3 [ Ln 1 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 1 3 (OH 2 ) 3 ] and ∞ 3 [La 1 3 ] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous ∞ 3 [ Ln 1 3 ] ( Ln = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu 3+ compounds, [Eu 1 3 ] and [Eu 1 3 (OH 2 ) 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 1 3 ] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho 3+ ions.