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

Four air-stable mononuclear Co(II) complexes, with the formulas [Co(dapbh)(HO)(CHOH)] (1), [Co(Hdapbh)(N)(CHOH)]·(CHOH) (2), [Co(Haapbh)(CHOH)]·(NO) (3) and [Co(Hbapbh)(HO)(NO)]·(NO) (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 HL with two protonable hydrogens adopt a heptacoordinated pentagonal-bipyramidal geometry. The combined analyses of magnetic data and calculations unveil large easy-plane magnetic anisotropies for these complexes ( = +37.338, +37.273, +41.138 and +41.139 cm 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 values. Magnetic investigations demonstrate the field-induced single-ion magnetic behavior in complexes 1, 2 and 4 with diverse energy barriers () 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 symmetry in 4 is responsible for the highest barrier.