Slow Magnetic Relaxation and Luminescence in Mononuclear Ln Complexes with Pseudo-mono-axial Ligand Field: Modulation of Magnetic Properties through Variation of an Equatorial Donor Atom

Two pairs of isostructural mononuclear Ln(III) complexes sharing the same competent ligand tris{[2-{(imidazol-4-yl)methylidene}amino]ethyl}amine (H3L) and the alternative auxiliary ligands 8-hydroxyquinoline (H8-HQ) and salicylaldehyde (Hsal) were synthesized, namely, [Ln(H3L)(8-HQ)][BPh4]2 (Ln = Dy), 1; Eu, 3 and [Ln(H3L)(sal)][BPh4]2 (Ln = Dy, 2; Eu, 4). In all four complexes, the Ln(III) ions are nine-coordinated and exhibit an spherical-capped square antiprismatic coordination polyhedron. It is noteworthy that the neutral N-containing ligand H3L combining with the negatively charged 8-HQ–/sal– ligand affords a pseudo-mono-axial ligand field in all complexes. The Dy(III) derivatives 1 and 2 were found to exhibit slow magnetic relaxation behavior with Ueff of 82(2) and 38.3(5) K, respectively. Ab initio calculations suggest that the distinctly different magnetic properties between 1 and 2 can be attributed to the change of one of the equatorial donor atoms from N to O. The luminescence properties of Eu(III)-based complexes 3 and 4 were investigated; these complexes show a brilliant emission of characteristic red light.