The first crystalline sponges developed by Makoto Fujita is [(Co(NCS))(TPT)], which is an infinitely extensive framework of Co octahedral complex. Each octahedral complex is composed of six Co(NCS) vertexes and four 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) ligands. Besides the basic octahedral cavity (ML), there are also two different cuboctahedral cavities (ML and ML) which can accommodate larger molecules, such as C, C.
In 2013, the Fujita and his team discovered that the [(ZnI)(TPT)] organometallic network can also act as a crystalline sponge. Because Zn crystalline sponges are less symmetrical (C) than Co crystalline sponges, the X-ray diffraction analysis of its guest molecules is easier to be elucidated.
The Zn-based sponges also have following advantages: (1) Pores sizes are suitable for accommodating general organic compounds (5 × 8Å), (2) The distribution of TPT ligand is flat, providing better stacking opportunity of aromatic guest compounds or C—H π interaction opportunity with aliphatic guest compounds, (3) The iodine atoms and the pyridinium protons can be hydrogen bond acceptors or donors, respectively. They would enhance the interaction between the substrate and the crystalline sponges, (4) The framework of Zn-based crystalline sponge is flexible to some extent. The molecule which is a little bit larger than the sponge cavity can still be accommodated via framework expansion to adjust the pore size.