A family of lanthanide compounds with reduced nitronyl nitroxide diradical: syntheses, structures and magnetic properties

Abstract

A novel nitronyl nitroxide diradical based on the pyrazine ring, BNITPz (1, BNITPz = 2,2′-(3,6-dimethylpyrazine-2,5-diyl)bis(1-hydroxy-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide)) and six new dinuclear compounds based on the reduced form of 1, namely [Ln(hfac)₃]₂(BHIMPz)(H₂O)₂ (Ln = Gd (2_Gd), Dy (3_Dy), Ho (4_Ho), Er (5_Er), Tm (6_Tm), Yb (7_Yb); BHIMPz = 2,2′-(3,6-dimethylpyrazine-2,5-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide); hfac = hexafluoroacetylacetonate) were prepared and characterized. Single crystal X-ray crystallographic analyses confirmed the diradical structure of 1, and revealed that complexes 2_Gd–7_Yb are isostructural and crystallize in the triclinic space group P. The reduced form BHIMPz was confirmed by its longer N–O bond lengths and the characteristic N–H stretching absorption at 3500–3300 cm⁻¹ of the IR spectroscopy of 2_Gd–7_Yb. In 2_Gd–7_Yb, the lanthanide ions are eight-coordinated in a square antiprism environment (D_4d). Two symmetric related Ln(III) ions are bridged by the reduced BHIMPz ligand to form a dinuclear structure. Magnetic measurements revealed the antiferromagnetic exchange interaction between the two radicals of the diradical ligand 1 through the pyrazine ring. For complexes 2_Gd to 7_Yb with the reduced non-radical ligand, their magnetic properties are consistent with the isolated Ln^III centers. Alternating-current (ac) magnetic susceptibility investigations indicate that complexes 3_Dy and 7_Yb exhibit field-induced frequency dependence, which indicates the presence of slow relaxation of the magnetization. Ab initio calculations were performed on these compounds to understand their magnetic anisotropy and their different magnetic dynamics.