Modulation of the magnetic anisotropy of octahedral cobalt(ii) single-ion magnets by fine-tuning the axial coordination microenvironment

Abstract

Two mononuclear cobalt(II) complexes, with the formulas [Co(2,6-dfba)₂(bpp)₂(H₂O)₂]_n (1) and [Co(2,6-dfba)₂(bpe)₂(H₂O)₂]_n (2) (2,6-Hdfba = 2,6-difluorobenzoic acid, bpp = 1,3-bis(4-pyridyl)propane, bpe = 1,2-bis(4-pyridyl)ethylene), have been synthesized by combining Co(II) ions with benzoate derivatives and two homogeneous N-donor ligands, respectively. Constrained by the analogous CoN₂O₄ coordination spheres, the discretely hexa-coordinated Co(II) cores in both complexes display stretched octahedral geometries. The equatorial environments in both complexes are identical, whereas the axial sites are finely modulated by the different chemical natures of the terminal N-donor ligands. The combined analyses of the magnetic data, the high-frequency electron paramagnetic resonance (HF-EPR) and the ab initio calculations unveil large easy-plane magnetic anisotropies for both complexes (D = +53.19 and +65.67 cm⁻¹ for 1 and 2, respectively), which function as field-induced single-ion magnets (SIMs) with effective barriers (U_eff) of 45.34 (1) and 57.97 K (2). This work demonstrates how fine-tuning the coordination microenvironment of metal ions results in a non-negligible manipulation of their magnetic anisotropy.