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)center dot 2H(2)O and Eu[N(CN)(2)](3)center dot 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)center dot 2H(2)O, P2(1)/n, Z = 4, a = 7.4632(15) angstrom, b = 11.523(2) angstrom, c = 13.944(3) angstrom, beta = 94.06(3)degrees, V = 1196.2(4) angstrom(3); Eu[N(CN)(2)](3)center dot 2H(2)O, P2(1)/n, Z = 4, a = 7.4780(3) angstrom, b = 11.5429(5) angstrom, c = 13.9756(7) angstrom, beta = 93.998(4)degrees, V = 1203.41(10) angstrom(3)). Annealing of the hydrated phases of Ln[N(CN)(2)](3)center dot 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)center dot 2H(2)O) and nonhydrated (Eu[N(CN) 2] 3) europium(III) dicyanamides show red luminescence due to the dominant intensity of D-5(0)-F-7(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 lambda(max) approximate to 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)center dot 2H(2)O) and nonhydrated (Tb[N(CN)(2)](3)) terbium(III) dicyanamides show green emission at lambda(exc) = 365 nm, arising mainly from the dominant D-5(0)-F-7(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 Tb3+.