Temperature-controlled synthesis of metal-organic coordination polymers: crystal structure, supramolecular isomerism, and porous property

Abstract

Five new supramolecular metal-organic coordination polymers (MOCPs), {[Ni(bipy) (H₂O)₄](2,6-nds)·4H₂O} (1), {[Ni(bipy)(H₂O)₄](2,6-nds)·2H₂O} (2), {[Ni (bipy)(H₂O)₄](2,6-nds)} (3), {[Ni (bipy)(H₂O)₄](2,6-nds)} (4), {[Cu (bipy)(H₂O)₄](2,6-nds)} (5) (bipy = 4,4′-bipyridyl; 2,6-nds = 2,6-naphthalenedisulphonate) have been synthesized and structurally characterized. Compounds 1 and 5 were synthesized at room temperature in H₂O/EtOH medium, whereas 2−4 were isolated under hydrothermal conditions. Compounds 1−4 were synthesized maintaining the same stoichiometric ratio of metal and ligand under different reaction temperatures, and the different structures of the compounds indicate that the temperature plays a significant role in the construction of the coordination polymers. Structural characterization reveals that the one-dimensional [M(bipy)(H₂O)₄]²⁺ cationic chain is a basic building unit for all of the MOCPs, while 2,6-nds remains as a counteranion. In all cases, 2,6-nds counteranions interact with water and bipy molecules through strong hydrogen-bonding and Π−Π interactions to afford three-dimensional supramolecular structures. Compounds 1−4 have the same building unit with different network superstructures and are related as supramolecular isomers. Supramolecular isomerism in 3 and 4 is very interesting since they have the same molecular formula, {[Ni(bipy)(H₂O)₄](2,6-nds)}, and are polymorphs. Compounds 4 and 5 are isomorphous. The thermogravimetric study suggests that the dehydrated compounds are stable up to 300 °C. Furthermore, sorption studies suggest that dehydrated compounds of 1 and 2 are permanently porous.