Periodic Trends in Radical-Mediated Exchange Coupling and Magnetic Anisotropy in Ln2N2 Complexes

Large crystal-field anisotropy and strong exchange interactions are key ingredients in the design of next-generation molecule-based magnetic materials. Radical-bridged lanthanide complexes are particularly promising in this regard owing to their highly anisotropic ground states and strong exchange interactions mediated by the radical bridge. Here, we present a comprehensive ab initio investigation of the magnetic and exchange anisotropy in the {[((Me3Si)2N)2(THF)Ln]2(N2)}− complexes, where Ln = Gd3+, Tb3+, Dy3+, Ho3+, and Er3+. For the isotropic gadolinium (Ln = Gd3+) analogue, the microscopic origins of the exchange interaction are disentangled using density functional theory calculations, revealing significant contributions from both direct and kinetic exchange mechanisms, with the latter dominating due to efficient virtual Gd-to-radical electron transfer involving out-of-plane Gd 4f orbitals and the radical π* orbital. For the anisotropic complexes (Ln = Tb3+, Dy3+, Ho3+ and Er3+), crystal-field and exchange interactions are described using irreducible tensor-operator model Hamiltonians parametrized from multiconfigurational ab initio calculations. The resulting framework employs a reduced set of physically transparent parameters, facilitating systematic comparison across the lanthanide series. While the crystal field potential is largely constant across the series, the exchange interactions vary significantly.