Solvent-directed assembly of Ln and Ln–Co complexes based on a pyridyl-triazolyl functionalized nitronyl nitroxide radical: in situ reduction from NIT to IM and magnetic properties

Abstract

A pyridine-triazole functionalized nitronyl nitroxide radical ligand NIT-4trz-3Py (NIT-4trz-3Py = 3-(4-(1-triazole)-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) was designed to construct a series of lanthanide and Ln–Co magnetic complexes through solvent-regulated assembly. In total, seven complexes were successfully synthesized and structurally characterized: three dinuclear cyclic complexes [Ln(hfac)₃(NIT-4trz-3Py)]₂ (Ln = Gd (1_Gd), Tb (2_Tb) and Dy (3_Dy), hfac = hexafluoroacetylacetonate), two one-dimensional lanthanide chains [Ln₂(hfac)₆(NIT-4trz-3Py)₂] (Ln = Dy (4_Dy) and Yb (5_Yb)), and two Ln–Co heterometallic chains [LnCo(hfac)₅(IM-4trz-3Py)] (Ln = Dy (6_DyCo), and Yb (7_YbCo), IM-4trz-3Py = 3-(4-(1-triazole)-3-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl). The cyclic and chain-like lanthanide assemblies were harvested from n-heptane/dichloromethane mixed solvent and toluene, respectively, wherein lanthanide ions are interconnected in a head-to-head mode by intact NIT radicals. An unanticipated in situ reduction of NIT to IM radical occurred during the crystallization of Ln–Co chains. Differently, Ln^III ions in heterometallic chains are linked in a head-to-tail fashion via IM radicals, and Co^II ions are chelated between pyridine and triazole nitrogen sites. Slow relaxation of the magnetization can be observed under a zero dc field for complex 4_Dy, while complexes 2_Tb, 5_Yb, 6_DyCo, and 7_YbCo reveal the field-induced slow magnetic relaxation behavior under an applied dc field. In the fluorescence spectra, complex 2_Tb was found to exhibit four characteristic emission peaks of the Tb^III ion. This work presents unambiguous solvent-dependent structural diversity of nitroxide-based complexes, providing reliable insights for the rational design and controlled fabrication of low-dimensional molecular magnetic materials.