Syntheses, structures and magnetic properties of a series of lanthanide complexes with reduced nitronyl nitroxide radical ligands

Abstract

A series of new lanthanide complexes [Ln(hfac)3](HIM-6-X-Py)2 [HIM-6-X-Py = 2-(6-X-2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide; hfac = hexafluoroacetylacetonate; X = Br, Ln = Tb (Br-Tb), Dy (Br-Dy), Ho (Br-Ho), Er (Br-Er), Yb (Br-Yb); X = Cl, Ln = Dy (Cl-Dy); X = F, Ln = Dy (F-Dy)] have been prepared and studied. During the preparation of the lanthanide complexes, the nitronyl nitroxide radical ligands NIT-6-X-Py (2-(6-X-2-pyridyl)-4,4,5,5-tetramethylimidazolyl-1-oxyl-3-oxide; X = F, Cl, Br) were reduced to the non-radical ligands HIM-6-X-Py, forming mononuclear lanthanide complexes with two HIM-6-X-Py ligands chelated to one lanthanide(III) ion. The reduced ligands were evidenced by the color of the lanthanide complexes, the characteristic IR band of N–H stretching absorption, the residue electron density near the N atom of the ligand, and the N–O bond lengths in the final metal complexes. Except for compound F-Dy which crystallized in the triclinic space group P1¯ with two isolated DyIII centers in an asymmetricunit, all other complexes crystallized in the monoclinic space group P21/n. Due to the reduced non-radical ligands, the magnetic properties of all these complexes are consistent with mononuclear LnIII compounds. Alternating-current (ac) magnetic susceptibility investigations revealed that complexes Br-Dy, Br-Er, Br-Yb, Cl-Dy and F-Dy exhibit field-induced frequency dependence which indicates the slow magnetic relaxation of these complexes.