Structure control and sorption properties of porous coordination polymers prepared from M(NO3)2 and 4,4′-bipyridine (M = Co2+, Ni2+)

The inclusion of labile non-bridging ligands in coordination polymers can be used to modify their topology and provide a route to the preparation of materials with vacant metal coordination sites. The reaction of Co(NO3)2 and 4,4′-bipyridine (bpy) in ethanol is known to produce a 2D coordination polymer [Co2(bpy)3(NO3)4]n, 1. The inclusion of non-bridging aqua ligands produces new coordination polymer structures, 1D chains of [Co(bpy)(H2O)2(NO3)2]n, 2, and [Ni(bpy)(H2O)2(NO3)2]n, 3. The structures of the 2 and 3 were determined by single-crystal X-ray diffraction, which showed that they had the same packing motif, layers of parallel chains stacked in an ABAB pattern along the c axis. The crystals had 4 Å channels running parallel to the c axis. The disordered guest molecules in the channels of 2 were removed by heating at 90 °C, along with the two aqua ligands. This produced a coordination polymer with vacant metal coordination sites that had a high selectivity for the adsorption of water and its coordination to the Co. Aqua ligands also bound to the Co when 1 adsorbed water and, but the process was much slower due to the larger structural changes this required in the 2D framework and the lack of vacant coordination sites.