Capturing the Precise Structure of Liquids: The Crystalline Sponge Method for an Undergraduate Laboratory Course

The crystalline sponge method allows for direct and precise molecular structure determination of liquid and gaseous targets and thus has been recognized as a revolutionary breakthrough in crystallography. To expose undergraduates to this cutting-edge technique, we have developed a comprehensive laboratory experiment with reaction conditions and characterizations systematically tailored for students to perform in a mild and accessible way. In this experiment, students investigate the preparation of networked complexes {[(ZnI2)3(TPT)2]·x(solvate)}n as crystalline sponges with benzonitrile, methyl salicylate, and (trifluoromethyoxy)benzene as solvent, respectively. Crystalline sponges obtained in benzonitrile were exposed to solvent exchange in cyclohexane to afford {[(ZnI2)3(TPT)2]·x(cyclohexane)}n, and the progress was monitored by IR and GC-MS. All four crystals were evaluated under a microscope and subjected to single crystal X-ray diffraction (SC-XRD) analysis. The students are provided with the opportunity to learn about scientific software, such as SHELX, Olex2, and Mercury, and carry out structure analysis and visual representation of the sponges and liquid molecules. Moreover, hierarchical experiments have been designed to provide flexibility to students and best fit their individual needs and resources. The experiment has been carried out for three semesters in our school. It may refresh students' understanding of crystallography and help them excel in future endeavors especially like synthetic chemistry, pharmaceutical R&D, etc.