| Abstract
| - The layered compound Co−pyromellitate displays four magnetic ground states: paragmagnetic (T> TN = 16 K), collinear AF (16 K > T> 13 K), canted AF (T< 13 K, H< 2 kOe), and ferromagnetic (T> 13 K, H> 2 kOe), depending on the applied magnetic field.
- 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)), aredescribed. 1 consists of one-dimensional chains of edge-sharing CoO6 octahedra that are connected into layers viaO−C−O bridges. The layers are held together by the pyromellitate (pm4-) backbone to give a three-dimensionalstructure, each ligand participating in an unprecedented 12 coordination bonds (Co−O) to 10 cobalt atoms. 2consists 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 sixtrans-Co(H2O)2. 3 possesses a hydrogen-bonded sandwich structure associating layers of [Co(H2O)6]2+ and planarH2pm2-. The IR spectra, reflecting the different coordination modes and charges of the pyromellitate, are presentedand discussed. The magnetic properties of 1 indicate complex behavior with three ground states (collinear andcanted antiferromagnetism and field-induced ferromagnetism). Above the Néel temperature (TN) of 16 K it displaysparamagnetism with short-range ferromagnetic interactions (ϑ = +16.4 K, μeff = 4.90 μB per Co). Below TN aweak 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 aboutzero field and the second about the metamagnetic critical field. The critical field and the hysteresis width increaseas the temperature is lowered. The heat capacity data suggest that 1 has a 2D or 3D magnetic lattice, and thederived magnetic entropy data confirm an anisotropic seff = 1/2 for the cobalt(II) ion. Magnetic susceptibility dataindicate that 2 and 3 are paramagnets.
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