Guest shape-responsive fitting of porous coordination polymer with shrinkable framework

In situ synchrotron X-ray powder diffraction patterns of porous coordination polymers {[Cu₂-(pzdc)₂(bpy)]·G} have been measured (pzdc = pyrazine-2,3-dicarboxylate, bpy = 4,4′-bipyridine) (where G = H₂O for CPL-2 ⊃ H₂O, G = benzene for CPL-2 ⊃ benzene, and G = void for the apohost). The structures of apohost and CPL-2 ⊃ benzene were determined from Rietveld analysis. Adsorption of benzene in the channels induced a remarkable contraction in the crystal (b axis; 6.8%, volume; 4.9%), although the channels were occupied by the benzene molecules. This crystal transformation provides a new pore structure that is well suited for benzene molecules, and we denote it as a "shape-responsive fitting" transformation. This type of pore gives rise to a new guideline: frameworks can be composed of flexible motifs that are linked via strong bond and/or stiff motifs that are connected via weaker bonds.