Terminal Atom Effects on Magnetic Relaxation Rates in Uranium(V) "Single-Molecule Magnets"

The success of lanthanide-based single-molecule magnets (SMMs) has not yet been matched by their actinide counterparts, despite large calculated barriers to magnetic relaxation. Existing studies often conclude that standard multiconfigurational methods cannot accurately predict the magnetic relaxation barrier, but these models represent only a static picture of magnetic relaxation. Here, we perform the first ab initio calculations of spin-phonon coupling and magnetic relaxation in actinide SMMs, studying [UV(TrenTIPS)(E)] (TrenTIPS = N(CH2CH2NSiPr3i)3), with the terminal atom systematically varied across E = N, O, F, P, S, Cl, Br. We find that changing E strongly modifies U-E bonding, crystal field splitting, and wave function composition; however, these static descriptors do not predict low-temperature magnetic relaxation. All complexes relax through the two-phonon Raman mechanism, with low-energy phonon pathways revealed through the Raman rate integrand. The [UV(TrenTIPS)(F)] complex relaxes faster despite a large excited state energy gap, while [UV(TrenTIPS)(P)] shows that the overall relaxation rate reflects the combined contributions from the Raman rate integrand. Our results demonstrate that Ueff is not an appropriate metric for assessing these systems and that spin-phonon coupling and phonon availability, rather than static electronic structure descriptors alone, are central to understanding magnetic relaxation in this family of actinide complexes.