Increasing the Symmetry around Lanthanide Ions: The Effect on Single-Ion Magnet Behavior and Electronic Structure

Bis-capped lanthanide compounds have dominated the single-ion magnet (SIM) field with little variation in design. Herein, we report the new synthesis of the lanthanide compounds [Ln(Tp2-py)2](BPh4) (1-Ln; Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy), which employ the use of the hexadentate hydrotris[3-(2′-pyridyl)-pyrazol-1-yl]borate (Tp2-py) ligand to encapsulate lanthanide ions in pseudo-icosahedral crystal fields. The extension of the latter lanthanide ions yields the hydrated compounds [Ln(κ4-Tp2-py)2(OH2)](BPh4) (2-Ln; Ln = Dy, Y, Ho, Er, Tm, Yb, and Lu) instead due to the decrease in the ionic radii of the lanthanide and steric demand of the ligand. Alternating current magnetometry studies on 1-Ce, 1-Tb, 1-Dy, 2-Dy, and [Dy(Tp2-py)2Cl] (3) revealed slow magnetic relaxation in applied magnetic fields for all compounds, with 1-Tb revealing an extractable Orbach regime with Ueff = 46(2) cm–1, while the remaining compounds are dominated by Raman relaxation mechanisms. The complexity and size of Tp2-py likely afford many low energy vibrational modes which would be responsible for an increase in spin-phonon coupling and the observed dominance of Raman relaxation. CASSCF calculations on the compounds showed that the ground state purity is heavily dependent on the lanthanide; however, all easy axes are aligned with the B···B axis (or pseudo B···B axis for the case of 3) except for 2-Dy, where the easy axis aligns with the Dy–OH2 bond. The resonant nature of the tris-(pyrazolyl)borate ligands results in diffusely spread charge distribution, minimizing the influence of the axially located boron atoms, resulting in the lanthanide ion being encapsulated in a “pseudosphere” of negative charge, impacting the overall crystal field splitting. The magnetic behavior and electronic structure of 1-Ln highlight the importance of low energy vibrational modes to mitigate spin-phonon coupling, charge distribution, and geometry of ligands when designing lanthanide-based SIMs.