Halogen ••• Halogen Interactions in the Supramolecular Assembly of 2D Coordination Polymers and the CO2 Sorption Behavior

Four new mixed-ligand divalent coordination polymers (CPs) [Cu-2(muco)(2)(4-clpy)(2)] (1), [Cu-2(muco)(2)(4-brpy)(2)] (2), [Zn-2(bdc)(2)(4-clpy)(2)] (3), and [Zn-2(bdc)(2)(4-brpy)(2)] (4) (H(2)muco = trans,trans-muconic acid or 1,3-butadiene-1,4-dicarboxylic acid; H(2)bdc = 1,4-benzene dicarboxylic acid; 4-clpy = 4-chloropyridine and 4-brpy = 4-bromopyridine) have been synthesized and well characterized by elemental analysis, infrared spectra, single-crystal X-ray diffraction techniques, powder X-ray diffraction patterns, and thermogravimetric analysis. All the compounds 1-4 have a two-dimensional (2D) coordination polymeric sheet structure. Of these, 1 and 2 are isotypical and form a 3D supramolecular aggregation based on type-I halogen-halogen interactions (Cl center dot center dot center dot Cl or Br center dot center dot center dot Br) and have an impact on CO2 sorption properties. For the first time, halogen-halogen interactions have been used as a tool in the construction of high-dimensional CPs for sorption studies. However, analogous compounds 3 and 4 are expanded to 3D supramolecular structures based on pi center dot center dot center dot pi interactions. These compounds have no halogen-halogen interactions and hence become nonporous toward CO2 sorption. It appears that the halogen-halogen interactions between the 2D sheets are desirable for the uptake of CO2 gas.