A Metal-Organic Polyhedron-to-Coordination Polymer Transition Revealed by 3D Electron Diffraction

Porous metal-organic polyhedra (MOPs) have strong covalent and coordinate bonds that define the intrinsic pore of the cage. The intermolecular interactions between cages tend to be weaker, such that they rearrange during the solvent exchange process preceding gas sorption measurements. The reduction in crystal size that this often causes limits the availability of structural data that could enable understanding of observed gas uptake. Herein, we use 3D electron diffraction (ED) to resolve this problem, and apply this technique to a MOP-based material that shows cooperative gas capture. 3D ED structure solution reveals both that the MOPs rearrange to form porous 1D polymers, and that these polymers are retained in the activated phase. Molecular simulations using these data suggest gas uptake is facilitated by rotation of functional groups appended to the backbone of the polymers in conjunction with structural expansion as gas is accommodated. Mechanical downsizing of the material leads to the loss of cooperative gas uptake, but a level of porosity is retained, attributed to the conservation of the 1D polymer structure. This work underscores the potential of 3D ED for probing structural transformations in functional supramolecular materials.