Resolving the Cu(bdc) Conundrum: Identifying Non‐Porous Packing of Prototypical Coordination‐Network Thin Films Combining Advanced Diffraction Techniques and Computational Modelling

The functional properties of metal-organic framework (MOF) thin films crucially depend on their structure, which is often difficult to determine. A widely investigated material is Cu(bdc) (bdc = benzene-1,4-dicarboxylic acid) grown from solution, for which various thin-film structures have been suggested. Unfortunately, none of them represents a (meta)stable polymorph and is simultaneously consistent with the material's ferromagnetic nature, the difficulty of loading guest molecules, and the available diffraction data. This conundrum is resolved by combining evidence from rotating grazing-incidence X-ray diffraction, X-ray reflectivity, infrared absorption, and spin-polarized simulations. The experimental data are collected for thin films grown by layer-by-layer and ceramic-to-MOF conversion techniques. It is unambiguously shown that both samples consist of a non-porous coordination network with Cu$_2$(OH)$_2$(bdc) stoichiometry with densely packed Cu$^{2+}$/OH$^−$ layers connected by bdc linkers. X-ray reflectivity data confirm the dense nature of the films, and infrared spectroscopy supports the presence of additional OH$^−$ groups. A hitherto unknown polymorph is identified as the only structure consistent with all data. For this structure, spin-polarized calculations predict a ferromagnetic ground state, in line with previous experiments. This shows that for identifying the structure of a MOF-type thin film, one has to think outside the box, combining data from complementary approaches.