Determination of Magnetic Anisotropy Tensors in Actinide Complexes Using Torque Magnetometry: A U(IV) Case Study

In this study, we employed cantilever torque magnetometry to probe the magnetic anisotropy of a single crystal of U(DOTA)(H₂O) (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), representing, to the best of our knowledge, the first application of this technique to an actinide-based molecular system. Combining cantilever torque magnetometry data with ab initio calculations allowed us to determine the magnetic anisotropy tensor of the uranium center. The resulting parameters enabled direct comparison with the isoelectronic lanthanide complex previously reported in the literature as well as with ab initio predictions for the theoretical Es⁴⁺ isostructural analogue. We find that the magnetic anisotropy axes of U(DOTA)(H₂O) (5f²) closely align with those of the Pr³⁺ (4f²) and Dy³⁺ (4f⁹) analogues and with those predicted for Es(DOTA)(H₂O) (5f⁹). The combination of CTM and electronic structure calculations was essential to describe the weakly paramagnetic, EPR-silent, non-Kramers ground state of U(DOTA)(H₂O), despite the added complexity of four noncollinear molecules in the unit cell. Overall, these results demonstrate that cantilever torque magnetometry provides a powerful route to characterize the magnetic anisotropy of actinide complexes and offers a valuable experimental benchmark for validating computational approaches to 5f-element magnetism.