Synthesis and Characterization of Tb[N(CN)₂]₃·2H₂O and Eu[N(CN)₂]₃·2H₂O: Two New Luminescent Rare-Earth Dicyanamides

Two new rare-earth dicyanamides, namely, Tb[N(CN) 2 ] 3 ·2H 2 O and Eu[N(CN) 2 ] 3 ·2H 2 O, have been prepared by ion exchange in aqueous solution, followed by evaporation of the solvent at room temperature. The structures of both compounds have been solved and refined from single-crystal and powder X-ray diffraction data, respectively. The two compounds are isostructural and are built up from irregular quadratic antiprismatic LnN 6 O 2 polyhedra connected to each other by three crystallographically independent dicyanamide ([N(CN) 2 ] 3 - ) ions (Tb[N(CN) 2 ] 3 ·2H 2 O, P 2 1 / n, Z = 4, a = 7.4632(15) Å, b = 11.523(2) Å, c = 13.944(3) Å, β = 94.06(3)°, V = 1196.2(4) Å 3; Eu[N(CN) 2 ] 3 ·2H 2 O, P 2 1 / n, Z = 4, a = 7.4780(3) Å, b = 11.5429(5) Å, c = 13.9756(7) Å, β = 93.998(4)°, V = 1203.41(10) Å 3 ). Annealing of the hydrated phases of Ln[N(CN) 2 ] 3 ·2H 2 O (Ln = Eu, Tb) at 150 °C under an argon atmosphere leads to the formation of nonhydrated Ln[N(CN) 2 ] 3 (Ln = Eu, Tb). Both the hydrated (Eu[N(CN) 2 ] 3 ·2H 2 O) and nonhydrated (Eu[N(CN) 2 ] 3 ) europium(III) dicyanamides show red luminescence due to the dominant intensity of 5 D 0 − 7 F J ( J = 1, 2, 4) emission lines by excitation at 365 nm. The broad excitation band of europium(III) dicyanamide (fwhm = 8000 cm -1 ) ranging between 260 and 420 nm with λ max ≈ 30000 cm -1 is ascribed to a Eu−N charge-transfer transition, which is significantly shifted to lower energy compared to that of oxo compounds due to the nephalauxetic effect. Similarly, both the hydrated (Tb[N(CN) 2 ] 3 ·2H 2 O) and nonhydrated (Tb[N(CN) 2 ] 3 ) terbium(III) dicyanamides show green emission at λ exc = 365 nm, arising mainly from the dominant 5 D 0 − 7 F 4 transition. However, unlike europium(III) dicyanamide, the broad excitation band of terbium(III) dicyanamide ranging between 250 and 400 nm with a maximum at 33000 cm -1 can be assigned to the 4f 8 −4f 7 5d 1 transition of Tb 3+ .