Trigonal antiprismatic CoII 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 CoII 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 [CoII(Ttz)2]·5.5H2O (1), whereas the second, [Co3(L)6](NO3)2·6H2O (2), is a mixed-valence CoIII-CoII-CoIII trinuclear complex. In 1, the pseudo-C3 axis of the trigonal antiprismatic high spin CoIIN6 coordination sphere lies along the B⋯Co⋯B direction, whereas in 2 the C3 axis of the CoIIO6 unit lies along the CoIIICoIICoIII 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 CoII complexes reported to date, as well as on related model complexes, confirm, on the one hand, that the axiality gxy/gz 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 (gxy/gz > 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.