Engineering Mononuclear Ln(III) Complexes with a Pseudo-Macrocyclic Hexadentate N₄O₂ Schiff Base Ligand Exhibiting Slow Magnetic Relaxation

We report here the synthesis of a series of nine coordinated mononuclear LnIII complexes [LnL1Cl2(DMF)]Cl·2.5DMF and [LnL1(L2)2]Cl·4CH3OH (LnIII = GdIII, DyIII, ErIII and YbIII, HL2 = 9-anthracenecarboxylic acid), where L1 is a hexadentate N4O2 Schiff base ligand prepared from the condensation of 1,10-phenanthroline-2,9-dicarbaldehyde and semicarbazone. The X-ray crystal structures of these complexes show the LnIII ions to possess LnN4O2Cl2 and LnN4O4 coordination spheres, which can be considered to be derived from a hexagonal bipyramidal geometry, with the ligand in the equatorial plane and the anions (chloride or 9-antracenecarboxylate) in axial positions, which undergo distortion after coordination of either a molecule of DMF or a bidentate coordination of the 9-anthracenecarboxxylate ligand. All these compounds exhibit field-induced slow magnetization relaxation (SMR). The absence of SMR at zero field due to QTM, as well as the processes involved in the magnetic relaxation under a field of 0.1 T, have been justified on the basis of theoretical calculations and the distortion of the respective coordination spheres. The severe discrepancy between the calculated and experimental thermal energy barriers for the DyIII complexes seems to indicate that the relaxation occurs with the contribution of spin–vibrational coupling, which is favored by the flexibility of the ligand.