Toward Understanding Prolate 4f Monomers: Numerical Predictions and Experimental Validation of Electronic Properties and Slow Relaxation in a Muffin-Shaped Er$^{III}$ Complex

We report the synthesis, crystal structure, and magnetic properties of the muffin-shaped complex [Er(PPTMP)$_2$(H$_2$O)][OTf]$_3$ (PPTMP = (4-(6-(1,10-phenanthrolin-2-yl)pyridin-2-yl)-1H-1,2,3-triazol-1-yl)methyl pivalate) (1). Complex 1 is shown to exhibit field-induced slow relaxation of the magnetization at B = 0.1 T via two distinct relaxation paths. Using tunable high-frequency/high-field electron paramagnetic resonance spectroscopy, we experimentally determine the effective g-factors and zero field splittings (ZFS) of the two energetically lowest Kramers doublets (KD). Our data reveal that the distorted muffin-shaped ligand field favors an m ≃ ±9/2 magnetic ground state, while the main contribution to the first excited KD at Δ$_{1→2}$ = 780(5) GHz is suggested to be m ≃ ±5/2. The ground state g-tensor has generally an axial form but hosts significant transversal components, which we conclude to be the source of single molecule magnet (SMM)-silent behavior in zero field. Our findings are backed up by ab initio spin–orbit configuration interaction calculations showing excellent agreement with the experimental data and, in particular, highlight that the counterions should be included in the numerical modeling of the crystalline structure.