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

Abstract

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, with the synthetic processes being 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 ascertains – fine-tunes the structures, with changes in both the distortion degree of the coordination geometry and the intermolecular interactions, which have a direct impact on the magnetic properties of these complexes. Magnetic investigations reveal field-induced single-ion magnet behavior in all complexes with distinct energy barriers (U_eff) −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, and +51.28 cm⁻¹ for 5. To our knowledge, such reversal of anisotropic nature driven by coligands is unprecedented.