Modulation of the magnetic dynamics of pentagonal-bipyramidal Co(ii) complexes by fine-tuning the coordination microenvironment

Abstract

Four air-stable mononuclear Co(II) complexes, with the formulas [Co(dapbh)(H₂O)(CH₃OH)] (1), [Co(Hdapbh)(N₃)(CH₃OH)]·(CH₃OH) (2), [Co(H₂aapbh)(CH₃OH)₂]·(NO₃)₂ (3) and [Co(H₂bapbh)(H₂O)(NO₃)]·(NO₃) (4), have been synthesized and structurally characterized by single crystal X-ray diffraction. In all of the complexes, the Co(II) centers constrained by the rigid pentadentate ligand H₂L^R with two protonable hydrogens adopt a heptacoordinated pentagonal-bipyramidal geometry. The combined analyses of magnetic data and ab initio calculations unveil large easy-plane magnetic anisotropies for these complexes (D = +37.338, +37.273, +41.138 and +41.139 cm⁻¹ for 1–4, respectively), which indicate that the chemical alterations of the equatorial ligand and the ligand field strength in the axial positions synergistically fine-tune the magnitude of the D values. Magnetic investigations demonstrate the field-induced single-ion magnetic behavior in complexes 1, 2 and 4 with diverse energy barriers (U_eff) of 12.25 for 1, 44.15 for 2 and 48.72 K for 4, corresponding to the geometrical distortion of the heptacoordinated Co(II) ion. That is, the greatest deviation from the ideal D_5h symmetry in 4 is responsible for the highest barrier.