Deciphering the Role of Symmetry and Ligand Field in Designing Three-Coordinate Uranium and Plutonium Single-Molecule Magnets

Actinide single-molecule magnets (SMMs) have gained paramount interest in molecular magnetism as they offer a larger barrier height of magnetization (U_eff) reversal compared to the lanthanide analogue, thanks to their greater metal-ligand covalency. However, the reported actinide SMMs to date yield a relatively smaller U_eff as there is no established design principle to enhance U_eff values. To address this issue, we have employed ab initio CASSCF/CASPT2/NEVPT2 calculations to study a series of three-coordinate U³⁺ and Pu³⁺ SMMs. To begin with, we have studied two experimentally characterized U³⁺ ion-field-induced SMMs, namely, planar [U{N(SiMe₂^tBu)₂}₃] (1) and pyramidal [U{N(SiMe₃)₂}₃] (2) complexes reported earlier. Both the complexes were found to stabilize m_J = |±1/2⟩ as the ground state with a very strong quantum tunneling of magnetization (QTM), rendering them unsuitable for SMMs. Our calculations reveal that in the pyramidal geometry (such as in 2), the energy of the 5f²6d¹ state is lowered compared to the planar geometry (as in 1), resulting in a slightly better SMM characteristic in the former. To unravel the effect of symmetry in magnetic properties, ab initio calculations were performed on two reported T-shaped complexes [U(NSiⁱPr₂)₂(I)] (3) and [U(NHAr^iPr6)₂I] (4, Ar^iPr6 = 2,6-(2,4,6-iPr₃C₆H₂)₂C₆H₃). Quite interestingly, m_J = |±9/2⟩ is found to be the ground state for both the complexes with a blocking barrier exceeding 900 cm^-1. Furthermore, to decipher the effect of the transuranic element in magnetic anisotropy, ab initio calculations were extended to the Pu analogue of 2, [Pu{N(SiMe₃)₂}₃] (5), which yields a record-breaking blocking barrier of ∼1933 cm^-1. Among the three-coordinate geometries studied, the pyramidal geometry was found to offer substantial magnetic anisotropy for Pu³⁺ ions, while a T-shaped geometry is best suited for U³⁺ ions. While the chosen theoretical protocols' overestimation of barrier height cannot be avoided, these values are still several orders of magnitude larger than the U_eff values reported for any actinide SMMs and unveil a design principle for superior three-coordinate actinide-based SMMs.