![]() ![]() From this point of view, our system based only in light-atoms, could offer an alternative to MOF-based crystalline sponges. 15 These atoms contribute heavily to the overall diffraction pattern masking the information available for the guest. However, the crystalline sponges reported so far are based on metal–organic frameworks (MOFs) that include electron-rich atoms, such as iodine, bromine, chlorine as well as transition metals in their structures. 14 In this method, accurate molecular structures of samples can be obtained by analyzing the diffraction patterns of single crystals without requiring the crystallization of the samples. More specifically, we were interested in knowing if a crystalline porous material based only on dispersive forces, could be used as the crystalline solid support in X-ray diffraction studies by applying the “crystalline sponge method” developed by Fujita. The resulting porous material displays 1-D channels that are large enough for liquid–solid sorption applications. 12 These interactions are established between the phenyl and cyclohexylidene residues, the “sticky groups” of 3. 11 The hierarchical self-assembly of the macrocycle 3 into a porous material arises from the combination of multiple van der Waals and aromatic interactions. Previously, we have described the synthesis of the 1,3-phenylene-bis-propargylic diamine 1 used in this work as rigid spacer to avoid the collapse of the macrocycle 3. Scheme 1 Synthesis of macrocyclic tetraimine 3. Our report constitutes a paradigmatic example of the so-called Gulliver's principle 10 applied to the design of porous organic materials. A key aspect of our design is the use of multiple weak dispersive interactions as the glue element to organize and maintain together the components of the stable porous molecular material. In this work, we present a new organic material featuring permanent porosity based on the macrocyclic tetraimine 3 ( Scheme 1). In this example, the macrocyclic ureas aligned into columnar 1-D assemblies and maintained its porous structure even upon guest removal. 2d–f,7 Using this strategy, Shimizu and colleagues reported a microporous crystalline bis-urea macrocyclic host that self-assembled by hydrogen bonding and aromatic stacking interactions. ![]() 8 A logical development of this approach that produced more stable porous molecular materials relied on the use of molecules interacting through hydrogen bonding and aromatic interactions, either separately or in combination. These authors showed that interstitial van der Waals interactions could be employed as the main component for the design of porous host–guest assemblies. In this regard, Atwood and Barbour's contributions are particularly significant. 7 Related materials exclusively stabilized by weak dispersive interactions are scarce. Hence, molecular materials based on non-directional intermolecular forces are metastable and tend to collapse upon removal of the template agents used to stabilize the voids of their porous crystalline structures. 6 The molecular components of porous organic molecular crystals (POMC) lacking an extended directional covalent or coordination bonding are held together only by weak intermolecular forces. This approach takes advantage of the structural versatility provided by organic synthesis and the possibility to obtain porous solid materials simply by crystallization. 4 In this context, the synthesis of porous materials constructed from discrete molecules 5 is particularly attractive. Introduction Porous organic materials 1 are currently of growing interest owing to their potential applications in diverse fields such as storage and separation of gases, 2 molecular sorting 3 and catalysis. Theoretical studies demonstrated the vital role played by the dispersive forces in the overall stabilization of the crystal packing. Remarkably, this crystalline material enabled the structure determination by X-ray diffraction of the included molecules. Being a true molecular sponge, the channel framework of this material allowed the inclusion of a variety of molecular sample guests without compromising its crystalline nature. Upon removal of the template, the structure of the empty solid exhibited permanent microporosity ( S BET = 342 m 2 g −1). The multiple dispersive forces established among the aliphatic residues glutted the 1-D channels and provided thermal stability to the material at temperatures below 160 ☌. The 1-D nature of the material is intrinsic to the conformationally rigid structure of a macrocyclic sub-unit bearing four cyclohexylidene residues. ![]() ![]() A crystalline porous material showing one-dimensional (1-D) rectangular micropores (12 × 9 Å 2) has been assembled from a semirigid macrocyclic tetraimine and EtOAc as the templating agent. ![]()
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