The Relevance of Non-Axiality and Low-Lying Excited States for Slow Magnetic Relaxation in Pentagonal-Bipyramidal Erbium(III) Complexes Probed by High-frequency EPR.

High-frequency/high-field electron paramagnetic resonance studies on a series of seven-coordinate pentagonal-bipyramidal (PBP) erbium(III) complexes Er(DAPMBH/HDAPS)X (HDAPMBH = 2,6-diacetylpyridine bis-4-methoxy benzoylhydrazone, HDAPS = 2,6-diacetylpyridine bis-(salicylhydrazone)) demonstrate the effects of different apical ligands (X = (HO)Cl (1), (CHOH)N (2), Cl (3)) on the local magnetic anisotropy of the central Er ions. In particular, we report direct experimental determination of the effective g-values and zero field splittings of the energetically low-lying Kramers doublets. Our quantitative determination of the magnetic anisotropy highlights the relevance of an axial g-tensor for SMM behavior and suggests that fast magnetic relaxation is mainly driven by a thermally assisted quantum tunnelling process via low-lying excited states.