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)]_n (1) and {[Co(pdms)(tpb)]­·H₂O·tpb}_n (2) (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 1 and 2 display a distorted tetrahedral geometry with quasi C_2v symmetry and are linked into a one-dimensional (1D) zig-zag chain via the ditopic bridging ligand of bpe in 1 while a ribbon chain via the tetradentate linker of tpb in 2. Magnetic studies revealed the easy-axis magnetic anisotropy of the Co^II ions with different zero-field splitting D of −19 cm^–1 (1) and −33 cm^–1 (2), likely due to the distinct changes in the N_py–Co–N_py bite angles (100.20° (1) vs. 93.90° (2)). Moreover, slow magnetic relaxation proceeded via different relaxation mechanisms under applied dc fields was observed, giving an effective energy barrier (U_eff) of 69.6 K for 1 and 76.6 K for 2, respectively. The ab initio 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.