A New Type of Two-Dimensional Metal Coordination Systems: Hydrothermal Synthesis and Properties of the First Oxalate−bpy Mixed-Ligand Framework [M(ox)(bpy)] (M = Fe(II), Co(II), Ni(II), Zn(II); ox = C₂O₄^2-; bpy = 4,4‘-bipyridine)

Abstract

A new family of two-dimensional metal coordination polymers with the general formula [M(ox)(bpy)] (M = Fe(II), Co(II), Ni(II), Zn(II); ox = C2O42-; bpy = 4,4‘-bipyridine) have been designed and synthesized via hydrothermal routes. Quantitative yields were obtained in all cases. The title compounds [Fe(ox)(bpy)] (I), [Co(ox)(bpy)] (II), [Ni(ox)(bpy)] (III), and [Zn(ox)(bpy)] (IV) represent the first coordination network structure constructed by bridging oxalate and 4,4‘-bpy mixed ligands. I−IV are isostructural and belong to the orthorhombic crystal system, space group Immm (No. 71). The noninterpenetrated structure contains octahedral metal centers coordinated by two oxalate and two bpy ligands to form a two-dimensional layered network. Within each layer, 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 × 7 × 10 Å in the composite three-dimensional structures. Magnetic susceptibility and field-dependent magnetization measurements 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 μeff values yielded from the fitting of χ(T) indicate the high spin states of M ions in all three compounds. The origin of the magnetic ordering in these compounds may be attributed to the strong exchange interactions between the M ions at the octahedral sites through the bridging oxalate molecules along the a-axis. Possible interchain coupling along the [011] direction is also considered. Thermogravimetric analysis studies indicate that all four compounds are thermally stable up to 290 °C (the on-set temperature of weight losses for I, 290 °C; II, 340 °C; III, 350 °C; and IV, 290 °C).