Mechanochemistry-Accessed MOF Polymorphism Revealed by an Integrated 3DED/DFT-Assisted Rietveld Analysis

Abstract

A combined 3DED, plane-wave DFT, and Rietveld refinement workflow uncovers the first example of interlayer-twisting polymorphism in a pillared MOF. While the parent material, PUM198, a Zn-based mixed-ligand MOF, was previously synthesized under solvothermal conditions, mechanochemistry selectively affords a new polymorph, PUM198-m. Although the two phases share identical composition and connectivity, they differ in symmetry, layer arrangement, pore architecture, and topology. Accessible only as a microcrystalline powder, PUM198-m is unsuitable for conventional single-crystal X-ray diffraction, and its flexibility and solvent-filled pores preclude structure solution from the XRPD data. The crystal structure was therefore derived from a multitechnique blueprint in which the starting model was derived from 3DED data and subsequently refined through iterative DFT-assisted Rietveld analysis to describe the as-synthesized, guest-loaded material. This integrated workflow provides a broadly applicable structural elucidation strategy for flexible MOFs that are mechanochemically synthesized, for which conventional crystallographic analysis fails.