Construction of Hydrogen-Bonded and Coordination-Bonded Networks of Cobalt(II) with Pyromellitate: Synthesis, Structures, and Magnetic Properties

Synthesis (hydrothermal and metathesis), characterization (UV−vis, IR, TG/DTA), single-crystal X-ray structures,
and magnetic properties of three cobalt(II)−pyromellitate complexes, purple [Co2(pm)]n (1), red [Co2(pm)(H2O)4]n·2nH2O (2), and pink [Co(H2O)6](H2pm) (3) (H4pm = pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid)), are
described. 1 consists of one-dimensional chains of edge-sharing CoO6 octahedra that are connected into layers via
O−C−O bridges. The layers are held together by the pyromellitate (pm4-) backbone to give a three-dimensional
structure, each ligand participating in an unprecedented 12 coordination bonds (Co−O) to 10 cobalt atoms. 2
consists of a three-dimensional coordination network possessing cavities in which unbound water molecules reside.
This highly symmetric network comprises eight coordinate bonds (Co−O) between oxygen atoms of pm4- to six
trans-Co(H2O)2. 3 possesses a hydrogen-bonded sandwich structure associating layers of [Co(H2O)6]2+ and planar
H2pm2-. The IR spectra, reflecting the different coordination modes and charges of the pyromellitate, are presented
and discussed. The magnetic properties of 1 indicate complex behavior with three ground states (collinear and
canted antiferromagnetism and field-induced ferromagnetism). Above the Néel temperature (TN) of 16 K it displays
paramagnetism with short-range ferromagnetic interactions (ϑ = +16.4 K, μeff = 4.90 μB per Co). Below TN a
weak spontaneous magnetization is observed at 12.8 K in low applied fields (H < 100 Oe). At higher fields (H >
1000 Oe) metamagnetic behavior is observed. Two types of hysteresis loops are observed; one centered about
zero field and the second about the metamagnetic critical field. The critical field and the hysteresis width increase
as the temperature is lowered. The heat capacity data suggest that 1 has a 2D or 3D magnetic lattice, and the
derived magnetic entropy data confirm an anisotropic seff = 1/2 for the cobalt(II) ion. Magnetic susceptibility data
indicate that 2 and 3 are paramagnets.