A pentagonal bipyramidal Co(II) single-ion magnet based on an asymmetric tetradentate ligand with easy plane anisotropy

High-coordinate 3d single-ion magnets(SIMs) especially with pentagonal bipyramidal geometry as an emerging class of representative molecular magnets have received considerable concern. Whereas, it is not easy to construct and obtain 3d SIMs with high coordination numbers (CN = 7 or 8). Herein, a mononuclear Co(II) complex ([Co(pypzbeyz)(NCS)2(DMF)], pypzbeyz = N-((6-(1H-pyrazol-1-yl)pyridin-2-yl)methylene)benzohy-drazide) based on a tetradentate ligand has been synthesized and structurally and magnetically characterized. Single-crystal X-ray diffraction analysis reveals that the Co(II) complex adopts a seven-coordinate distorted pentagonal bipyramidal geometry. Magnetic investigations show that the Co(II) complex has large easy plane anisotropy with a positive D value (D = +36.8 cm-1) and a small E value, and exhibits slow magnetic relaxation behavior under a dc field. Theoretical calculations show a positive zero-field splitting of 33.5 cm-1, which is in excellent agreement with the experiment. The results support that the 3d high-coordinate SIMs could be effec-tively achieved by using a suitable multi-dentate ligand with a certain ligand field.