Leveraging anion selection to modulate crystallographic symmetry in Yb(III) single-molecule magnets

Distortions in the local symmetry around Ln(III) ions in SMMs significantly impacts slow magnetic relaxation by introducing transverse crystal field parameters that enhance quantum tunnelling of the magnetisation (QTM). Minimising these distortions, often using macrocyclic or sterically hindered ligands, or by tuning intermolecular interactions, is essential for suppressing QTM. A less-explored strategy involves aligning the molecular symmetry elements within the crystal lattice to generate a high-symmetry crystal lattice with symmetry enforced bond angles and lengths. Here, we demonstrate that aligning the S₄ axes of [YbCl₂(Ph₃AsO)₄]⁺ and [BPh₄]^- in [YbCl₂(Ph₃AsO)₄]BPh₄ (1) enforces tetragonal symmetry and a strict 180° Cl-Yb-Cl angle. In contrast, [YbCl₂(Ph₃AsO)₄]PF₆ (2) does not possess aligned molecular rotational axes and therefore lacks the crystallographically enforced symmetry leading to a smaller Cl-Yb-Cl angle. While both compounds exhibit similar slow relaxation of the magnetisation, due to efficient Raman relaxation processes, 2 shows a significant decrease in the ab initio calculated collinearity of the anisotropy axes of the excited Kramers doublets, alongside a change in the ground m_J = ±7/2 state composition, with an increased admixture of the m_J = ±½ states. These findings highlight the potential of crystallographically enforced symmetry in designing high-performance Yb(III)-based SMMs.