Toward Understanding Prolate 4f Monomers: Numerical Predictions and Experimental Validation of Electronic Properties and Slow Relaxation in a Muffin-Shaped ErIII Complex.

We report the synthesis, crystal structure, and magnetic properties of the muffin-shaped complex [Er(PPTMP)2(H2O)][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 Δ1→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.