A Unique Two-Dimensional Terephthalate-Bridged Structure with Alternate Tetra- and Penta-Coordinate Cobalt(II) Sites. Synthesis, Crystal Structure and Magnetic Properties of [{Co3(tp)2(OH)2(phen)2}n]

A two-dimensional (2D) cobalt coordination polymer [{Co3(tp)2(OH)2(phen)2}n] (1), bridged by the terephthalate (tp) ligand, was synthesized under hydrothermal conditions, and its structure was determined by single-crystal X-ray diffraction. Complex 1 crystallized in the triclinic space group P1- with a = 8.2620(11) ?, b = 10.4409(12) ?, c = 11.0840(10) ?, (alpha) = 111.332(8)°, (beta) = 91.394(10)°, (gamma) = 91.638(10)°, Z = 2, V = 889.65(18) ?3. A structure refinement showed that complex 1 is the first example of a two-dimensional (2D) framework with both carboxylate bridges and (mu)-hydroxo bridges, in which cobalt atoms adopt square-pyramidal and square-planar geometries. Complex 1 exhibited a interesting magnetic behavior: the shape of the (mu)eff curve indicates a ferrimagnetic-like behavior between 300-41.0 K, immediately followed by a strong decay of the magnetic moment. Moreover, complex 1 was characterized by IR spectroscopy, inductively coupled plasma (ICP), elemental and thermogravimetric (TGA) analyses.