Europium and terbium thiocyanates: Syntheses, crystal structures, luminescence and magnetic properties

Abstract Thiocyanates [M(H2O)5(NCS)3]·H2O (M(III) = Eu, Tb) and Eu3(H2O)12(EtOH)3(NCS)9 have been synthesized. These compounds react with 18-crown-6 to give moisture-stable polymorphous modifications of [M(18-crown-6)(NCS)3]. On transition from aqua complexes to complexes with the macrocycle, the coordination number of the complex-former increases from 8 to 9. Thermolysis of [M(H2O)5(NCS)3]·H2O in the temperature range 200–300 °C revealed a principal difference in the behavior of isostructural complexes of europium and terbium. Europium thiocyanate was found to undergo a redox process. The [M(18-crown-6)(NCS)3] ensembles maintain thermal stability up to 300 °C. The magnetic behavior of the [Eu(H2O)5(NCS)3]·H2O complex is determined by crystal field effects, while that of the Eu3(H2O)12(EtOH)3(NCS)9 ensemble and of the compounds containing the macrocycle is mostly governed by the properties of the free Eu(III) ion. The [M(H2O)5(NCS)3]·H2O complexes manifest photoluminescence under direct excitation that is typical of Eu and Tb ions. Incorporation of the 18-crown-6 macrocycle in the complexes quenches the luminescence of REE ions completely.