Syntheses, structures, and magnetic properties of three two-dimensional cobalt(ii) single-ion magnets with a CoIIN4X2 octahedral geometry

Abstract

Three two-dimensional Co^II complexes, namely, [Co(dps)₂Cl₂]_n (1), [Co(dps)₂Br₂]_n (2), and [Co(dps)₂(H₂O)₂·I₂·(H₂O)₄]_n (3) (where dps = 4,4′-dipyridyl sulfide), have been synthesized using a rationally designed synthetic approach and characterized structurally and magnetically. Although they crystallize in different space groups, these compounds all have similar two-dimensional (4,4) layer structures where the Co²⁺ centers are bridged by the bis(monodentate) ligand dps. In addition, the coordination geometry of the Co²⁺ centers is of a general CoN₄X₂ octahedral environment formed by four equatorial N atoms from four dps ligands and two X atoms (X = Cl, Br, and O atoms for 1–3, respectively) in the axial positions. Ab initio calculations revealed that due to the strong spin–orbit coupling, the spin Hamiltonian containing the zero-field splitting parameters D and E cannot be used to describe their magnetic anisotropy. According to the calculation, the g_z values for the spin–orbit ground doublets of 1–3 are larger than the g_x and g_y values, which indicates the easy-axis magnetic anisotropy of these compounds. Due to the existence of the magnetic anisotropy, field-induced single-ion magnet behavior can be observed in all these complexes. This work illustrated that care should be taken to interpret the magnetic data of octahedral Co^II compounds. In addition, extended frameworks with long and flexible organic spacers are good candidates for the construction of SIMs with different axial ligands and different magnetic properties.