Influence of Radicals on Magnetization Relaxation Dynamics of Pseudo-Octahedral Lanthanide Iminopyridyl Complexes

Controlling quantum tunneling of magnetization (QTM) is a persistent challenge in lanthanide-based single-molecule magnets. As the exchange interaction is one of the key factors in controlling the QTM, we targeted lanthanide complexes with an increased number of radicals around the lanthanide ion. On the basis of our targeted approach, a family of pseudo-octahedral lanthanide/transition-metal complexes were isolated with the general molecular formula of [M(L^•-)₃] (M = Gd (1), Dy (2), Er (3), Y (4)) using the redox-active iminopyridyl (L^•-) ligand exclusively, which possess the highest ratio of radicals to lanthanide reported for discrete metal complexes. Direct current magnetic susceptibility studies suggest that dominant antiferromagnetic interactions exist between the radical and lanthanide ions in all of the complexes, which is strongly corroborated by magnetic data fitting using a Heisenberg-Dirac-Van Vleck (HDVV) Hamiltonian (-2 J Hamiltonian). A good agreement between the fit and the experimental magnetic data obtained using g = 2, J_rad-rad = -111.9 cm^-1 for 4 and g = 1.99, J_rad-rad = -111.9 cm^-1, J_Gd-rad = -1.85 cm^-1 for 1. Complex 2 shows frequency-dependent slow magnetization relaxation dynamics in the absence of an external magnetic field, while 3 shows field-induced frequency-dependent χ_M'' signals. An ideal octahedral geometry around the lanthanide ion is predicted to be unsuitable for the design of a single-molecule magnet (SMM); nevertheless, complex 2 exhibits slow relaxation of magnetization with a record high anisotropy barrier for a six-coordinate Dy(III) complex. A rationale for this unusual behavior is detailed and reveals the strength of the synthetic methodology developed.