Layered Cobalt(II) and Nickel(II) Diphosphonates Showing Canted Antiferromagnetism and Slow Relaxation Behavior

Reactions of 2-(1-imidazole)-1-hydroxyl-1,1‘-ethylidenediphosphonic acid (ImhedpH₄) and cobalt or nickel salts under hydrothermal conditions lead to four new metal phosphonates with two types of structures:  M₃(ImhedpH)₂(H₂O)₄·2H₂O [M = Co(II) (1) and Ni(II) (2)] and M(ImhedpH₃)₂(H₂O)₂ [M = Co(II) (3) and Ni(II) (4)]. Compounds 1 and 2 are isostructural and show a layered structure made up of M₃(ImhedpH)₂(H₂O)₄ trimer units. These trimers are connected by edge-sharing of the {Co(2)O₆} octahedra, forming an undulating chain. The adjacent chains are fused through the coordination of the phosphonate oxygen atom from one chain to the Co(2) atom of the other, thus generating a two-dimensional layer containing 4-, 8-, and 16-membered rings. The imidazole groups are grafted on the two sides of the layer. Compounds 3 and 4 are also isostructural and show a mononuclear structure with extensive hydrogen bond interactions. Magnetic measurements reveal that both 1 and 2 exhibit canted antiferromagnetism at 2.6 and 5.0 K, respectively. Below these temperatures, slow relaxation is observed from the ac susceptibility measurements corresponding to the spin-glass-like behaviors.