Incorporating Highly Anisotropic Four-Coordinate Co(II) Ions within One-Dimensional Coordination Chains

Low-coordinate metallic ions have been well recognized for constructing good-performance single ion magnets (SIMs) due to their enhanced magnetic anisotropy; however, the incorporation of such specific ions into coordination polymers is still challenging. Here, we reported two new Co^II coordination polymers, namely [Co(pdms)(bpe)]_<i>n</i> (<b>1</b>) and {[Co(pdms)(tpb)]­·H₂O·tpb}_<i>n</i> (<b>2</b>) (H₂pdms = 1,2-bis(methanesulfonamido)benzene, bpe = 1,2-di(4-pyridyl)ethane, tpb = 1,2,4,5-tetra(4-pyridyl)benzene). Single crystal X-ray diffraction experiments indicated that the Co^II centers in both <b>1</b> and <b>2</b> display a distorted tetrahedral geometry with quasi <i>C</i>_2<i>v</i> symmetry and are linked into a one-dimensional (1D) <i>zig</i>-<i>zag</i> chain via the ditopic bridging ligand of bpe in <b>1</b> while a ribbon chain via the tetradentate linker of tpb in <b>2</b>. Magnetic studies revealed the easy-axis magnetic anisotropy of the Co^II ions with different zero-field splitting <i>D</i> of −19 cm^–1 (<b>1</b>) and −33 cm^–1 (<b>2</b>), likely due to the distinct changes in the N_py–Co–N_py bite angles (100.20° (<b>1</b>) vs. 93.90° (<b>2</b>)). Moreover, slow magnetic relaxation proceeded via different relaxation mechanisms under applied dc fields was observed, giving an effective energy barrier (<i>U</i>_eff) of 69.6 K for <b>1</b> and 76.6 K for <b>2</b>, respectively. The <i>ab initio</i> calculations on both the polymers further confirmed the sign and magnitude of the ZFS parameters and nicely reproduced the experimental results. Our study demonstrated a great potential for applying the well-studied and highly anisotropic 4-coordinate metal ions within a coordination polymer, opening a viable means to tuning magnetic anisotropy via topological control.