Probing Magnetic Anisotropy in (NBun4)2[ReX6] (X = Cl, Br, I): Ligand Field of the ReIV-Halide Bonds in Distorted Octahedral Complexes

In six-coordinate complexes with S ≥ 1, deviations from perfect Oh geometry lead to magnetic anisotropy. We have studied (NBun4)2[ReIVX6] (Re–X; X = Cl, Br, I; S = 3/2), including zero-field splitting (ZFS) in Re–I, by advanced spectroscopies [far-IR magneto-spectroscopy (FIRMS), high-frequency and -field EPR (HFEPR), and inelastic neutron scattering (INS)]. AC susceptibility of Re–I shows field-induced SMM (single-molecule magnet) properties. Unlike Re–I with one unique [ReI6]2– center, there are four and three different [ReX6]2– centers in the crystallographic unit cells of Re–Cl and Re–Br, respectively. Ab initio calculations determine ZFS parameters and show that magnetic anisotropies are influenced by deviations from the Oh geometry and by increased covalency from Re–Cl to Re–Br to Re–I. Although Re–X bond lengths increase from Re–Cl to Re–Br to Re–I, the calculations reveal that the longer Re–I bonds are compensated by the diffuse, better-overlapping orbitals of I– ligands. The diffuseness in ligand orbitals likely contributes to the SMM performance of Re–I. The calculations also indicate that deviations from the Oh geometry in [ReX6]2– ions are the sole source of magnetic anisotropy and that there is no heavy-atom effect from the halide ligands. Spin-phonon couplings in Re–I have been probed by FIRMS using results from DFT phonon calculations.