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)(H 2 O)(CH 3 OH)] (1), [Co(Hdapbh)(N 3 )(CH 3 OH)]·(CH 3 OH) (2), [Co(H 2 aapbh)(CH 3 OH) 2 ]·(NO 3 ) 2 (3) and [Co(H 2 bapbh)(H 2 O)(NO 3 )]·(NO 3 ) (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 H 2 L 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.