Trigonal Prismatic Cobalt(II) Single-Ion Magnets: Manipulating the Magnetic Relaxation Through Symmetry Control

Two mononuclear trigonal prismatic Co(II) complexes [Co(tppm*)][BPh₄]₂ (1) and [Co(hpy)][BPh₄]₂·3CH₂Cl₂ (2) (tppm* = 6,6',6″-(methoxymethanetriyl)tris(2-(1H-pyrazol-1-yl)pyridine; hpy = tris(2,2'-bipyrid-6-yl)methanol) were synthesized by incorporating the Co(II) ions in two pocketing tripodal hexadentate ligands. Magnetic studies indicate similar uniaxial magnetic anisotropy while having distinct dynamic magnetic properties for two complexes, of which 1 exhibits clear hysteresis loops and Orbach process governed magnetic relaxation with an effective energy barrier (U_eff) of 192 cm^-1, among the best examples in transition metallic SIMs, about 10 times larger than that of 2 (U_eff = 20 cm^-1, extracted by fitting the data to an Orbach relaxation process but there is no real state at this energy). Such pronounced difference is ascribed to the dominant Raman process and quantum tunneling of magnetization (QTM) in 2 owing to the structural distortion and symmetry breaking, indicated by a nearly perfect trigonal prismatic geometry (D₃ local symmetry) for 1 and a more distorted configuration for 2 (C₃ local symmetry). Ab initio calculations predict strong axial anisotropy for 1 with minimal QTM probability, with the transverse component of anisotropy being estimated to be much higher for 2 than 1, leading to a 10-fold lower U_eff value than 1.