Influence of Pyrazolate vs N‑Heterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes

Two isostructural series of trigonal prismatic complexes, M­(Bp^Me)₃ and M­(Bc^Me)₃ (M = Y, Tb, Dy, Ho, Er, U; [Bp^Me]⁻ = dihydrobis­(methypyrazolyl)­borate; [Bc^Me]⁻ = dihydrobis­(methylimidazolyl)­borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb³⁺, Dy³⁺, and U³⁺ metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M­(Bc^Me)₃, than for the isomeric pyrazolate complexes, M­(Bp^Me)₃. Further, investigation of magnetically dilute samples containing 11–14 mol % of Tb³⁺, Dy³⁺, or U³⁺ within the corresponding Y³⁺ complex matrix reveals thermally activated relaxation is favored for the M­(Bc^Me)₃ complexes, even when dipolar interactions are largely absent. Notably, the dilute species U­(Bc^Me)₃ exhibits U_eff ≈ 33 cm^–1, representing the highest barrier yet observed for a U³⁺ molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and ¹H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers.