Radical-lanthanide ferromagnetic interaction in a TbIII bis-phthalocyaninato complex

Recent studies have highlighted the importance of organic ligands in the field of molecular spintronics, via which delocalized electron-spin density can mediate magnetic coupling to otherwise localized 4f moments of lanthanide ions, which show tremendous potential for single-molecule device applications. To this end, high-field/ high-frequency electron paramagnetic resonance (EPR) spectroscopy is employed to study a neutral terbium bis-phthalocyaninato metalorganic complex, [TbPc2](0), with the aim of understanding the magnetic interaction between the Ising-like moment of the lanthanide ion and the unpaired spin density on the coordinating organic radical ligand. The measurements were performed on a previously unknown[TbPc2](0) structural phase crystallizing in the Pnma space group. EPR measurements on powder samples of [TbPc2](0) reveal an anisotropic spectrum, which is attributed to the spin-1/2 radical coupled weakly to the EPR-silent Tb-III ion. Extensive double-axis rotation studies on a single crystal reveal two independent spin-1/2 signals with differently oriented (albeit identical) uniaxial g-tensors, in complete agreement with x-ray structural studies that indicate two molecular orientations within the unit cell. The easy-axis nature of the radical EPR spectra thus reflects the coupling to the Ising-like Tb-III moment. This is corroborated by studies of the isostructural [YPc2](0) analog (where Y is nonmagnetic yttrium), which gives a completely isotropic radical EPR signal. The experimental results for the terbium complex are well explained on the basis of an effective model that introduces a weak ferromagnetic Heisenberg coupling between an isotropic spin-1/2 and an anisotropic spin-orbital moment, J = 6, that mimics the known, strong easy-axis Tb . . . Pc-2-crystal-field interaction.