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‚2H2O and Eu[N(CN)2]3‚2H2O, 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 LnN6O2 polyhedra connected to each other by three crystallographically independent dicyanamide ([N(CN)2]3) ions (Tb[N(CN)2]3‚2H2O, P21/n, Z ) 4, a ) 7.4632(15) Å, b ) 11.523(2) Å, c ) 13.944(3) Å, â ) 94.06(3)°, V ) 1196.2(4) Å3; Eu[N(CN)2]3‚2H2O, P21/n, Z ) 4, a ) 7.4780(3) Å, b ) 11.5429(5) Å, c ) 13.9756(7) Å, â ) 93.998(4)°, V ) 1203.41(10) Å3). Annealing of the hydrated phases of Ln[N(CN)2]3‚2H2O (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‚2H2O) and nonhydrated (Eu[N(CN)2]3) europium(III) dicyanamides show red luminescence due to the dominant intensity of D0FJ (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 λmax ≈ 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‚2H2O) and nonhydrated (Tb[N(CN)2]3) terbium(III) dicyanamides show green emission at λexc ) 365 nm, arising mainly from the dominant D0-F4 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 4f8-4f75d1 transition of Tb3+.