Two complexes of Sm(ii) with crown ethers—electrochemical synthesis, structure and spectroscopy

Abstract

Two complexes of divalent samarium have been synthesized by electrochemical reduction in methanol–tetrahydrofuran solutions: [Sm(18-crown-6)(ClO₄)₂] and [Sm(15-crown-5)₂](ClO₄)₂. In [Sm(18-crown-6)(ClO₄)₂] the metal cation is ten-coordinate and its coordination sphere comprises six oxygen atoms of the crown ligand and four oxygen atoms from two perchlorate anions. [Sm(15-crown-5)₂](ClO₄)₂ shows a sandwich structure with decacoordinate samarium located between two 15-crown-5 molecules. At 77 K both compounds show f–f luminescence originating from the ⁵D₀ level, and also the 15-crown-5 complex shows a weak luminescence in the range 20000–25000 cm⁻¹, which has been tentatively interpreted as originating from ³P₀ and ⁵H₃ levels. At room temperature the emission of [Sm(15-crown-5)₂](ClO₄)₂ is dominated by broad f–d bands. In the excitation spectra some Fano resonances have been observed. The 18-crown-6 compound is unstable, but the 15-crown-5 compound is fairly stable in air.