A new type of two-dimensional metal coordination systems: Hydrothermal synthesis and properties of the first oxalate-bpy mixed-ligand framework (2)(infinity)[M(ox)(bpy)] (M = Fe(II), Co(II), Ni(II), Zn(II); ox = C2O42-; bpy=4,4 '-bipyridine)

A new family of two-dimensional metal coordination polymers with the genera l formula (2)(infinity)[M(ox)(bpy)] (M = Fe(II), Co(II), Ni(II), Zn(II); ox = C2O42-; bpy = 4,4'-bipyridine) have been designed and synthesized via hy drothermal routes. Quantitative yields were obtained in all cases. The titl e compounds (2)(infinity)[Fe(ox)(bpy)] (I), (2)(infinity)[Co(ox)(bpy)] (II) , (2)(infinity)[Ni(ox)(bpy)] (III), and (2)(infinity)[Zn(ox)(bpy)] (IV) rep resent the first coordination network structure constructed by bridging oxa late and 4,4'-bpy mixed ligands. I-IV are isostructural and belong to the o rthorhombic crystal system, space group Immm (No. 71). The noninterpenetrat ed structure contains octahedral metal centers coordinated by two oxalate a nd two bpy ligands to form a-two-dimensional layered network. Within each l ayer, relatively large rectangular voids are formed by four adjacent metal centers and the ligands bonded to them. The approximate dimensions of these cavities are 4 x 7 x 10 Angstrom in the composite three-dimensional struct ures. Magnetic susceptibility and field-dependent magnetization measurement s revealed spontaneous magnetic ordering with transition temperatures of 12 , 13, and 26 K for I, II, and III, respectively. The types of ordering are interpreted as the antiferromagnetic kind with certain canting structures. The mu(eff) values yielded from the fitting of chi(T) indicate the high spi n states of M ions in all three compounds. The origin of the magnetic order ing in these compounds may be attributed to the strong exchange interaction s between the M ions at the octahedral sites through the bridging oxalate m olecules along the a-axis. Possible interchain coupling dong the [011] dire ction is also considered. Thermogravimetric analysis studies indicate that all four compounds are thermally stable up to 290 degrees C (the on-set tem perature of weight losses for I, 290 degrees C; II, 340 degrees C; III, 350 degrees C; and IV, 290 degrees C).