Co₃(2-OOCC₆H₄PO₃)₂(H₂O)₃·H₂O: a layered metal phosphonate showing reversible dehydration-rehydration behavior and ferrimagnetism.

Two isostructural metal phosphonates M₃(2-cpp)₂(H₂O)₃·H₂O [M(II) = Co (1), Zn (2), 2-cppH₃ = 2-carboxyphenylphosphonic acid] are synthesized and structurally characterized. Both exhibit layer structures in which -Co-O- "columns" are connected by the {PO₃C} linkages. The "column" consists of triangular shaped {M₃O₃} trimers, inter-linked through either corner- or edge-sharing of the {MO₆} octahedra. The phenyl groups are grafted on the two sides of the inorganic layer. Thermal analyses suggest that the layer structures of 1 and 2 are stable after removal of the lattice and coordination water. The dehydrated sample can be rehydrated reversibly in the case of compound 1. Magnetic studies reveal that antiferromagnetic interactions dominate in both 1 and 1-de, resulting in ferrimagnetic layers in both cases. The large inter-layer distance in 1 favors a ferromagnetic interaction between the layers. Hence ferrimagnetism is observed in both cases at low temperature. For 1-de, slow magnetization relaxation is also observed below ca. 2.8 K.