Switching of easy-axis to easy-plane anisotropy in cobalt(II) complexes

A tetranuclear cubane-type complex [Co4(ntfa)4(CH3O)4(CH3OH)4] (1) with a {Co4O4} core, and a mononuclear complex [Co(ntfa)2(CH3OH)2] (2) have been rationally obtained by adjusting the ratio of the β-diketonate and Co(II) ions, being the synthetic processes monitored by in-situ microcalorimetry. Then, following synthetic conditions to obtain 2, but using three distinct N-donor coligands –2,2'-bipyridyl (bpy), 6,6'-dimethyl-2,2'-bipyridyl (6,6-(CH3)2-bpy) and 5,5'-dimethyl-2,2'-bipyridyl (5,5-(CH3)2-bpy)– three novel mononuclear complexes have been obtained, [Co(ntfa)2(bpy)2] (3), [Co(ntfa)2(6,6-(CH3)2-bpy)2] (4) and [Co(ntfa)2(5,5-(CH3)2-bpy)2] (5). The introduction of different capping coligands –as single-crystal X-ray crystallography ascertain– fine-tune the structures, with changes in both the distortion degree of the coordination geometry and the intermolecular interactions, which have a direct impact on magnetic properties of these complexes. Magnetic investigations reveal field-induced single-ion magnet behavior in all complexes with distinct energy barriers (Ueff) –39.06 (1), 36.65 (2), 36.32 (3), 28.26 (4) and 15.85 K (5). Magnetic experiments together with HF-EPR measurements and theoretical calculations demonstrate that 2 features easy-axis magnetic anisotropy (D = -60.48 cm-1), whereas 3-5 show easy-plane magnetic anisotropies–D = +70.77 cm-1 for 3, +35.71 cm-1 for 4, +51.28 cm-1 for 5. To our knowledge, such reversal of anisotropic nature driven by coligands is unprecedented.