Syntheses, structures and gas sorption properties of two coordination polymers with a unique type of supramolecular isomerism

Abstract

The solvothermal reaction of a short bridging ligand 1H-pyrazole-4-carboxylic acid (H₂pc) and Zn(NO₃)₂ in a mixed solvent containing N,N-dimethylacetamide (DMA) at 110 °C produced two genuine supramolecular isomers [Zn₂(pc)₂(DMA)] (1) and [Zn₂(pc)₂]·DMA (2). Single-crystal X-ray diffraction studies showed that 1 is a densely packed layered structure with DMA molecules coordinated on the layered surface, and 2 is a porous three-dimensional framework structure with DMA molecules filled inside the pore without coordination to any metal ion, which represents a rare case of coordination-sphere isomerism in coordination polymers. Removal of the coordinated DMA molecules in 1 occurs at high temperatures, accompanying the irreversible formation of an unidentified nonporous phase. On the other hand, the guest DMA molecules in 2 can be readily removed under mild conditions to give a new porous phase [Zn₂(pc)₂] (2′). Gas-sorption measurements of 2′ revealed that the framework has significant flexibility and selective CO₂ adsorption at room temperature.