Tuning Magnetic Anisotropy of Four-Coordinated Cobalt(II) in Chain Complexes via Varying Organic Linkers

Tuning magnetic anisotropy via perturbing the coordination environment of metal ions is of great importance for developing high-performance single-ion magnets (SIMs). We herein reported the syntheses and characterization of four one-dimensional cobalt(II) coordination polymers featuring distorted tetrahedral metal center, namely {[Co(pdms)(bpy)]·bpy}n (Cobpy), {[Co(pdms)(bpen)]}n (Cobpen), {[Co(pdms)(bpdoz)]·H2O}n (Cobpdoz), and {[Co(pdms)(bimp)]·2DMF·3H2O}n (Cobimp) (bpy = 4,4′-bipyridine, bpen = 1,2-di(4-pyridyl)ethylene, bpdoz = 2,5- di(pyridin-4-yl)-1,3,4-oxadiazole, bimp = 1,4-bis[(1H-imidazol-1-yl)methyl]benzene). By the assembly of [Co(pdms)2]2+ anions and different organic bridging ligands, the chains with highly anisotropic four-coordinate Co(II) ions exhibit a significant modification of the structures, especially the tetrahedral coordination sphere. Notably, the two ∠N–Co–N bite angles, which mainly determine the magnetic anisotropy of tetrahedral mononuclear cobalt complexes, were significantly changed due to the use of different organic linkers. Magnetic measurements reveal that the four complexes possess uniaxial magnetic anisotropy with the zero-field splitting (ZFS) parameters D being −39.7, −32.1, −23.8, and −43.4 cm–1 for Cobpy, Cobpen, Cobpdoz, and Cobimp, respectively. This observation and their electronic structures were further unveiled by theoretical calculations. Dynamic alternative current magnetic susceptibility measurements indicate that both the chains exhibit slow magnetic relaxation under an external field, indicating field-induced slow magnetic relaxation of the coordination polymers (1D SIM-CPs). Notably, the relaxation behaviors are also modified due to the structural differences induced by distinct linkers. The foregoing results not only enrich the family of 1D SIM-CPs showing low-coordinated metal centers but also offer a coordination chain platform for tuning magnetic anisotropy of four-coordinate Co(II) ions in 1D chains.