Terbium(III) and Yttrium(III) Complexes with Pyridine-Substituted Nitronyl Nitroxide Radical and Different β‑Diketonate Ligands. Crystal Structures and Magnetic and Luminescence Properties

A terbium­(III) complex of nitronyl nitroxide free radical 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro1H-imidazolyl-1-oxy-3-oxide (NIT2Py), [Tb­(acac)­3NIT2Py]·0.5H₂O (3) (acac = acetylacetonate), was synthesized for comparison with the previously reported [Tb­(hfac)₃NIT2Py]·0.5C₇H₁₆ (1) (hfac = hexafluoroacetylacetonate), together with their yttrium analogues [Y­(hfac)₃NIT2Py]·0.5C₇H₁₆ (2) and [Y­(acac)₃NIT2Py]·0.5H₂O (4). The crystal structures show that in all complexes the nitronyl nitroxide radical acts as a chelating ligand. Magnetic studies show that 3 like 1 exhibits slow relaxation of magnetization at low temperature, suggesting single-molecule magnet behavior. The luminescence spectra show resolved vibronic structure with the main interval decreasing from 1600 cm^–1 to 1400 cm^–1 between 80 and 300 K. This effect is analyzed quantitatively using experimental Raman frequencies.