Trigonal antiprismatic Co^II complexes: why don't they work like single-molecule magnets in a zero-field?

This paper describes the synthesis, crystal structures, detailed dc and ac magnetic properties, far-infrared magnetic spectroscopy (FIRMS), and theoretical calculations of two further examples of the scarce trigonal antiprismatic Co^II complexes (octahedral complexes distorted along the C₃ axis). One has been prepared from the scorpionate ligand hydridotris(1,2,4-triazolyl)borate (Ttz^-) and the other from the tridentate bridging ligand 1-methylimidazole-2-carboxamide oxime (HL). The first complex has the formula [Co^II(Ttz)₂]·5.5H₂O (1), whereas the second, [Co₃(L)₆](NO₃)₂·6H₂O (2), is a mixed-valence Co^III-Co^II-Co^III trinuclear complex. In 1, the pseudo-C₃ axis of the trigonal antiprismatic high spin Co^IIN₆ coordination sphere lies along the B⋯Co⋯B direction, whereas in 2 the C₃ axis of the Co^IIO₆ unit lies along the Co^IIICo^IICo^III direction. Although both complexes exhibit very strong easy-axis magnetic anisotropy, they do not show slow magnetic relaxation under zero field but rather behave like field-induced single-ion magnets (SIMs). Experimental and theoretical magneto-structural correlations carried out on all the trigonal antiprismatic Co^II complexes reported to date, as well as on related model complexes, confirm, on the one hand, that the axiality g_xy/g_z increases when the s/h ratio decreases, that is to say when the compounds are more elongated along the C₃ or pseudo C₃ axis, and on the other hand, that all of them exhibit easy-axis magnetic anisotropy but with significant intrinsic transversal anisotropy (g_xy/g_z > 0.1), which prevents SIM behaviour in a zero-field. Furthermore, the results of the theoretical studies suggest that the use of neutral ligands based on scorpionate-tris(pyrazole)methane with bulky substituents could cause sufficient elongation along the C₃ axis, due to steric hindrance, to reduce the quantum tunneling of magnetization (QTM) and increase the relaxation time to such an extent that SIM behavior would be observed at zero field.