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

Abstract

Two mononuclear trigonal prismatic Co(II) complexes [Co(tppm*)][BPh 4 ] 2 (1) and [Co(hpy)][BPh 4 ] 2 •3CH 2 Cl 2(2) (tppm* = 6,6′,6″-(methoxymethanetriyl)tris(2-(1H-pyrazol-1yl)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 (D 3 local symmetry) for 1 and a more distorted configuration for 2 (C 3 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.

Single Molecule Magnetism in Linear Fe(I) Complexes with Aufbau and non-Aufbau ground-state

“…18 However, the complexes with high axial symmetry, show signs of unquenched angular momentum and hence moderate magnetic anisotropy, but the molecules with high local symmetry are quite scarce. 19 The breaking of symmetry significantly lowers the magnitude of magnetic anisotropy as elegantly reported by Feng et al where the Fe(III) complex i.e. [(PMe 3 ) 2 FeCl 3 ] with appropriate local symmetry results in D = -50 cm −1 which lowers to -17 cm −1 in [(PMe 2 Ph) 2 FeCl 3 ] complex with the broken symmetry.…”

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…Very importantly, the U eff values of 192 cm −1 (at zero dc field) in the case of complex 16 set a record for Co II SIMs and is second only to the linear two‐coordinate Co II complexes (see also Table 1). [31] The U eff for complexes 17 and 18 are 20 cm −1 and 52.8 cm −1 (in dc field), respectively, and are much smaller compared with complex 16 . This can be attributed to the higher twist angle between the trigonal planes or rhombic anisotropy.…”

“…This complex also showed a U eff of 71 cm −1 at zero applied dc magnetic field (152 cm −1 for Orbach only) and 101 cm −1 at 1.5 kOe applied dc field. Following this path, very recently, Dunbar and co‐workers investigated three TP Co II SIMs, namely, [Co(tppm*)][BPh 4 ] 2 ( 16 ), [Co(hpy)][BPh 4 ] 2 ( 17 ) and [CoTp py ]PF 6 ( 18 ) (where tppm*=6,6′,6′′‐(methoxymethanetriyl)tris(2‐(1 H ‐pyrazol‐1‐yl)pyridine; hpy=tris(2,2′‐bipyrid‐6‐yl)methanol); Tp py =tri(3‐pyridylpyrazolyl)borate), which are the three best TP Co II SIMs reported so far [27, 31] . These three complexes exhibit D (or B 2 0 ) values of −97 (−97) cm −1 , −113.6 (−108) cm −1 and −151 (−157) cm −1 , respectively, for complexes 16 , 17 and 18 as obtained from NEVPT2 (PHI fitting of dc data) calculations (see Figure 3 g).…”

Experimental and theoretical insights into Co–Ln magnetic exchange and the rare slow-magnetic relaxation behavior of [CoII2Pr]2+ in a series of linear [CoII2Ln]2+ complexes

“…The large negative value of the ZFS is consistent with the literature for cobalt in a highly distorted octahedral geometry. 64–66…”