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

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 (<b>1-Ln</b>), S (<b>2-Ln</b>), or Se (<b>3-Ln</b>) donors, examining their influence on the anisotropy and relaxation behavior. The products exhibit distinct geometries, 6-coordinate <b>1-Ln</b>, 9-coordinate <b>2-Ln</b>, and <b>3-Ln</b> complexes that vary from 9-coordinate <b>3-Tb</b> to 7-coordinate <b>3-Er</b> and <b>3-Dy</b>. The geometry dominates the vibrational effects in defining the magnetic relaxation. The 7-coordinate <b>3-Dy</b>, with one short Ln-N bond, produces an axial crystal field and the best field-induced SMM behavior (<i>U</i>_eff = 72(6) cm^-1), while the high <i>D</i>_<i>3</i> symmetry in <b>2-Dy</b> suppresses slow relaxation. Low-lying excited states in <b>1-3-Er</b> promote rapid Orbach relaxation, masking Raman processes in <b>2-Er</b> and <b>3-Er</b>. In <b>1-Ln</b>, low-energy vibrations drive Raman relaxation, and contrary to expectations, <b>3-Dy</b> 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.