Making the ‘crystalline sponge’ more user-friendly

In 2013, when University of Tokyo’s Makoto Fujita reported a way to determine a complex molecule’s X-ray crystal structure without having to actually crystallize the molecule in question, the technique was hailed as a breakthrough for structural determination. But the technique, known as the “crystalline sponge” method because guest molecules soak into a metal-organic framework crystal where they orient themselves in a way that their structure can be determined via X-ray analysis, has yet to be adopted as a routine characterization tool. The technique, as originally reported, takes a significant amount of trial-and-error. Only about 5% of the crystals prepared are suitable for soaking, and it can take up to 16 days to create a single guest-soaked crystalline sponge. Seeking a way to shorten and improve this process, University of Illinois, Chicago, researchers Bernard D. Santarsiero, Neal P. Mankad, and Greyson W. Waldhart made some tweaks to existing protocols (Org. Lett. 2016, DOI: 10.1021/acs.orglett.6b03119). Rather than grow crystalline sponges in a batch process as the original protocol indicates, the UIC researchers used multiwell microplates and layered a drop of ZnI₂ in methanol with a drop of the sponge ligand 2,4,6-tris(4-pyridyl)-1,3,5-triazene in nitrobenzene. This step produces crystals, 90% of which are suitable sponges, in only 10 hours. Because it takes seven days for this step under the original protocol, this improvement dramatically cuts down on experiment time.