Structural Characterization of a Desolvated Flexible Porous Coordination Polymer Using MicroED

Flexible porous coordination polymers (PCPs) and metal-organic frameworks (MOFs) that undergo structural changes upon guest removal/uptake have a wide range of potential applications. However, detailed characterization of the guest removal/uptake promoted changes of these materials is frequently limited by the occurrence of crystal fragmentation which produces nanocrystals that are unsuitable for single-crystal X-ray diffraction analysis. An example of this limitation was encountered in our recent study of the flexible PCP [Cu(CF₃SO₃)₂(bpp)₂] (bpp = 1,3-bis(4-pyridyl)propane), which we demonstrated can exist in two PCP polymorphs: the one-dimensional form 1 obtained by removal of guest acetone molecules from the one-dimensional acetone-inclusion form [Cu(CF₃SO₃)(bpp)₂]·CF₃SO₃·2acetone (1·2acetone) and the two-dimensional form 2 obtained by removal of guest water molecules from the two-dimensional water-inclusion form [Cu(CF₃SO₃)₂(bpp)₂]·H₂O (2·H₂O). Although we speculated that adsorption/desorption of H₂O by 2 occurs in conjunction with a transient interlayer expansion process, confirmatory evidence could not be obtained because the crystal structure of the desolvated form 2 could not be determined and, therefore, a comparison of the structures in the presence and absence of guests was not possible. In the current study, we employed the microcrystal electron diffraction (MicroED) technique to elucidate the local and overall structural changes associated with guest H₂O removal/uptake by 2. Comparative analysis of these structures shows that H₂O guests in 2·H₂O are trapped in isolated pores and that all bpp ligands in the H₂O incorporated form exist in the TT (T = trans) conformation. On the other hand, the dehydrated form 2 possesses a more densely packed structure with smaller discrete cavities and that conformations of the bpp ligands in 2 are partially converted from TT to TG (G = gauche). The results of the structural analysis and theoretical calculations show that a drastic transient structural change involving interlayer expansion/shrinkage and/or continuous partial deformation of the framework of 2 takes place during H₂O adsorption/desorption. In addition, observations made in temperature-dependent powder X-ray diffraction and H₂O adsorption/desorption experiments reveal that H₂O guests present in 2·H₂O interact with the CF₃SO₃ anions in the framework via moderate hydrogen bonds.