A family of lanthanide complexes with a bis-tridentate nitronyl nitroxide radical: syntheses, structures and magnetic properties

Abstract

Eleven new lanthanide complexes based on a bis-tridentate nitronyl nitroxide radical NIT-Pm2Py (2-(4,6-di(pyridin-2-yl)pyrimidin-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy-3-oxide), namely (NIT-Pm2Py)Ln(hfac)₃ (Ln = Gd (1_Gd), Tb (2_Tb), Dy (3_Dy), Ho (4_Ho), Er (5_Er), Yb (6_Yb)), [(NIT-Pm2Py)Ln₂(hfac)₆]·xH₂O (Ln = Gd (7_Gd), Tb (8_Tb), Ho (10_Ho), x = 0.5 for 7_Gd and 1 for 8_Tb and 10_Ho) and (NIT-Pm2Py)Ln₂(hfac)₆ (Ln = Dy (9_Dy), Er (11_Er)) were prepared and characterized. These complexes can be selectively prepared by controlling the reaction ratio of Ln(hfac)₃·2H₂O to the radical ligand NIT-Pm2Py. Single crystal X-ray crystallographic analyses confirmed that 1_Gd–6_Yb are isostructural 2p–4f Ln^III–radical complexes, in which the NIT-Pm2Py radical acts as a terminal tridentate ligand chelating to one Ln^III ion. On the other hand, 7_Gd–11_Er are isostructural 4f–2p–4f Ln^III–radical–Ln^III complexes with the NIT-Pm2Py acting as a bridging ligand between two Ln^III ions. 7_Gd–11_Er represent a rare family of complexes showing the NIT bridged 4f–2p–4f three-spin motif. Alternating-current (ac) magnetic susceptibility investigations revealed that complex 6_Yb exhibits field-induced frequency dependence, suggesting a possible field-induced single-molecule magnet behavior. Ab initio calculations were performed on all these complexes. The fitting of the magnetic susceptibilities of these complexes indicates weak antiferromagnetic coupling between the Ln^III and NIT radical.