The thermal decomposition of Samarium-Thiocyanate-Based ionic liquids

New lanthanide containing ionic liquids (ILs) based on thiocyanate complexes of the type [BMIM]_x-3[Ln(NCS)_x(H₂O)_y] (BMIM = 1-Butyl-3-Methylimidazolium; x = 6,7,8; y = 0–2, x + y < 10; Ln = Y, La-Yb) have been reported in recent studies. These complexes are highly efficient in dissolving lanthanide complexes within ILs. The present work includes a thermal decomposition study of the anhydrous ionic liquids of this type with Samarium cation as the lanthanide. The thermal decomposition of [BMIM][NCS], [BMIM]₃[Sm(NCS)₆], [BMIM]₄[Sm(NCS)₇] and [BMIM]₅[Sm(NCS)₈] was investigated by employing a combination of thermo-gravimetric analysis (TGA), and temperature programmed desorption (TPD) coupled with mass spectrometry. The decomposition temperature of the samarium-containing ILs were found to be higher than that measured for [BMIM][NCS], suggesting stabilization of the ILs by complexation to the samarium cation. Thermal gravimetric analysis of [BMIM]_x-3[Sm(NCS)_x] confirms that the thermal decomposition occurs in a stepwise fashion to give Sm(NCS)₃ via a [BMIM]₃[Sm(NCS)₆] intermediate, whereby [BMIM][NCS] units are decomposed to give gaseous products. Additional experimental evidence obtained with TPD-MS supporting this decomposition route and reveal that each [BMIM][NCS] unit decomposes by the rapid nucleophilic attack of the thiocyanate anion at the C-N bond of the methyl- and butyl- imidazole groups, and their proposed reaction mechanism are introduced here.