Theoretical Investigations\non the Molecular Magnetic\nBehavior of Actinide Molecules [AnPc<sub>2</sub>]<sup>0/–</sup> (An = U, Cf): Prediction of the High Magnetic Blocking Barrier and\nMagnetic Blocking Temperature in [CfPc&

Searching for single-molecule magnets (SMM) with large\neffective\nblocking barriers, long relaxation times, and high magnetic blocking\ntemperatures is vitally important not only for the fundamental research\nof magnetism at the molecular level but also for the realization of\nnew-generation magnetic memory unit. Actinides (An) atoms possess\nextremely strong spin–orbit coupling (SOC) due to their 5<i>f</i> orbitals, and their ground multiplets are largely split\ninto several sublevels because of the strong interplay between the\nSOC of An atoms and the crystal field (CF) formed by ligand atoms.\nCompared to TM-based SMMs, more dispersed energy level widths of An-based\nSMMs will give a larger total zero field splitting (ZFS) and thus\nprovide a necessary condition to derive a higher <i>U</i><sub>eff</sub>. In combination of the density functional theory (DFT)\nas well as the CF model Hamiltonian and <i>ab initio</i> calculation, we have investigated the structural stability and electronic\nstructures as well as the magnetodynamic behavior of [AnPc<sub>2</sub>]<sup>0/–</sup> (An = U, Cf) molecules. We find that An atoms\ncan strongly interact with its ligand N atoms in forming An-N ionic\nbonds, and 5<i>f</i> electrons are more localized in the\nCf atom than in the U atom, giving U<sup>4+</sup>(5f<sup>2</sup>)\nand Cf<sup>3+</sup>(5f<sup>9</sup>) valence states. Although the UPc<sub>2</sub> molecule has a modest value of <i>U</i><sub>eff</sub> = 514 cm<sup>–1</sup>, it is not a good SMM due to the easy\noccurrence of quantum tunneling of magnetization (QTM). Based on the\nconsistent results of CF Hamiltonian and <i>ab initio</i> calculations on the [CfPc<sub>2</sub>]<sup>−</sup> molecule,\nwe propose that almost prohibited QTM within the Kramers doublets\n(KDs) as well as very low transition probabilities between different\nstates via hindered spin-flip transitions would result in a high <i>U</i><sub>eff</sub> = 1401 cm<sup>–1</sup>. The estimated\nhigh magnetic blocking temperature (<i>T</i><sub>B</sub>) of 58 K renders [CfPc<sub>2</sub>]<sup>−</sup> an excellent\nSMM candidate, implying that magnetic hysteresis could be observed\nin future experiments.