Mixed-Ligated Cobalt Phosphonates Showing Slow Magnetic Relaxation and Spin-Flop Behavior

Metal-organic frameworks or coordination polymers showing interesting magnetic properties have attracted much attention. By using 4,4′-bipyridine (4,4′-bpy) as auxiliary ligand, three new cobalt phosphonates based on 1-naphthalene phosphonic acid (1-napH2) were assembled under hydrothermal conditions at different pH, namely, Co(1-napH)2(4,4′-bpy)2(H2O)2 (1), Co2(1-nap)2(4,4′-bpy) (2), and Co2(1-nap)2(4,4′-bpy)(H2O)·2H2O (3). Compound 1 has a 0D mononuclear structure, where the Co(II) ion has a distorted octahedral geometry and 4,4′-bpy serves as an unusual terminal ligand. A 2D layer structure is found in compound 2, where the ladder-like chains made up of corner-sharing CoO3N and PO3C tetrahedra are connected by 4,4′-bpy. Interestingly, compound 3 with similar chemical composition to 2 except additional water molecules shows a 3D open-framework structure, where undulating chains of corner-sharing CoO4N trigonal bipyramids and PO3C tetrahedra are cross-linked by 4,4′-bpy. Magnetic studies reveal that compound 1 exhibits field-induced slow magnetization relaxation, whereas compounds 2 and 3 show field-induced spin-flop behavior and paramagnetism, respectively.