Magnetic Anisotropy from Main-Group Elements: Halides versus Group 14 Elements

Precise modulation of the magnetic anisotropy of a transition-metal center would affect physical properties ranging from photoluminescence to magnetism. Over the past decade, exerting nuanced control over ligand fields enabled the incorporation of significant magnetic anisotropy in a number of mononuclear transition-metal complexes. An alternate approach to increasing spin-orbit coupling relies upon using heavy diamagnetic main-group elements as sources of magnetic anisotropy. Interacting first-row transition metals with main-group elements enables the transfer of magnetic anisotropy to the paramagnetic metal center without restricting coordination geometry. We sought to study the effect of covalency on this anisotropy transfer by probing the effect of halides in comparison to early main-group elements. Toward that end, we synthesized a series of four isostructural heterobimetallic complexes, with germanium or tin covalently bound to a triplet spin Fe(II) center. These complexes are ligated by a halide (Br^- or I^-) in the apical position to yield a series of complexes with variation in the mass of the main-group elements. This series enabled us to interrogate which electronic structure factors influence the heavy-atom effect. Using a suite of approaches including magnetometry, computation, and Mössbauer spectroscopy, we probed the electronic structure and the spin-orbit coupling, as parametrized by axial zero-field splitting across the series of complexes, and found an increase in zero-field splitting from -11.8 to -17.9 cm^-1 by increasing the axial ligand mass. Through direct comparison between halides and group 14 elements, we observe a greater impact on magnetic anisotropy from the halide interaction. We attribute this counterintuitive effect to a larger spin population on the halide elements, despite greater covalency in the group 14 interactions. These results recommend modification of the intuitive design principle of increasing covalency toward a deeper focus on the interactions of the spin-bearing orbitals.