Balance between Crystal Field and Phonon Optimization in Defining the Magnetic Properties of Lanthanide...

Single-molecule magnets (SMMs) offer promise for high-density data storage, but suppressing fast magnetization relaxation remains a key challenge in synthetic design. Changing coordinating atoms across a group offers an avenue to control the phononic spectrum and crystal field. We report dysprosium(III) and erbium(III) tris(tetraphenyl-dichalcogenoimidodiphosphinate) complexes with O (1-Ln), S (2-Ln), or Se (3-Ln) donors, examining their influence on the anisotropy and relaxation behavior. The products exhibit distinct geometries, 6-coordinate 1-Ln, 9-coordinate 2-Ln, and 3-Ln complexes that vary from 9-coordinate 3-Tb to 7-coordinate 3-Er and 3-Dy. The geometry dominates the vibrational effects in defining the magnetic relaxation. The 7-coordinate 3-Dy, with one short Ln-N bond, produces an axial crystal field and the best field-induced SMM behavior (U_eff = 72(6) cm^-1), while the high D₃ symmetry in 2-Dy suppresses slow relaxation. Low-lying excited states in 1-3-Er promote rapid Orbach relaxation, masking Raman processes in 2-Er and 3-Er. In 1-Ln, low-energy vibrations drive Raman relaxation, and contrary to expectations, 3-Dy exhibits higher vibrational energy contributing to this process. This indicates that heavy chalcogen atoms in the first coordination sphere are a valuable resource, as they can drive crystal field changes that effectively counterbalance the onset of low-energy phonons.