How reproducible is the synthesis of Zr--porphyrin metal--organic frameworks? An interlaboratory study

Metal--organic frameworks (MOFs) are a rapidly growing class of materials that offer great promise in various applications. However, the synthesis remains challenging: for example, a range of crystal structures can often be accessed from the same building blocks, which complicates the phase selectivity. Likewise, the high sensitivity to slight changes in synthesis conditions may cause reproducibility issues. This is crucial, as it hampers the research and commercialisation of affected MOFs. Here, we present the first-ever interlaboratory study of the synthetic reproducibility of two Zr--porphyrin MOFs, PCN-222 and PCN-224, to investigate the scope of this problem. For PCN-222, only one sample out of ten was phase pure and of the correct symmetry, while for PCN-224, three were phase pure, although none of these show the spatial linker order characteristic of PCN-224. Instead, these samples resemble dPCN-224 (disordered PCN-224), which was recently reported by us. The variability in thermal behaviour, defect content, and BET surface area of the synthesised samples are also studied. Our results have important ramifications for field of metal--organic frameworks and their crystallisation, by highlighting the synthetic challenges associated with a multi-variable synthesis space and flat energy landscapes characteristic of MOFs.