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Chemists in Japan have synthesized a new porous material that acts as a microscopic solid-state reaction vessel. Chemical changes taking place in each pore can be tracked using X-ray crystallography the team explains. Monitoring solid-state reactions using X-ray crystallography can provide direct information on how a reaction is proceeding as well as the structures of the final products, However, it requires single crystals from start to finish and so has been limited to reactions in which any structural changes are relatively small. Now, Makoto Fujita, Takehide Kawamichi, and Masaki Kawano of the Department of Applied Chemistry, School of Engineering, at The University of Tokyo, working with Tomoki Kodama of the Higashifuji Technical Center, at the Toyota Motor Corporation in Shizuoka, Japan, have developed a solid-state reaction vessel that can circumvent that problem. The team has synthesized their porous material using the reaction of a triazine ligand together with zinc iodide in the presence of a 2-aminotriphenylene to create what they term a "single crystalline molecular reaction flask". They explain that their material crystallizes into a robust network. The network structure has two distinct, large pores separated by stacked pillars of the triazine ligand and triphenylene. These large pores can accommodate even rather bulky chemicals which enter by diffusion, swapping places with solvent molecules, and then becoming lodged within. "The pore interior is a pseudo-solution state in which chemical reactions may proceed as in a solution, yet can be directly analyzed by crystallography," the researchers explain. By arranging for strategic reactive groups, such as amino groups, to be present in the final porous product and protruding into the spaces within they can carry out specific reactions with various compounds. By dipping the material into a solution containing such reactants the porous material brings them all together within the pores where they are essentially held in the solid state once solvent is excluded. Reaction can then proceed with crystal structure changes being monitored along the route by X-ray diffraction. The team has now demonstrated proof of principle by reacting the amino groups lining the pores of their material with two common, yet large, reagents acetic anhydride in cyclohexane or aniline. They explain that within their material, the reactivity of the reagents they used and the course of the reaction are no different than if the reactants were simply meeting each other in the mix of a freely floating solution in a conventional reaction flask. Importantly, although the crystal changed colour little by little as the reaction proceeded, changing from red to yellow within 3 hours for the acetic anhydride reaction, its overall crystallinity remained intact despite the reaction taking place within. "The robust nature of the crystal network tolerates the rapid diffusion of acetic anhydride into the pores and the complete conversion of starting material into product," the researchers add. The crystallinity was again confirmed using XRD and the findings supported by Fourier-transform infrared (FTIR) spectroscopic data. "Taking advantage of the network's robust crystallinity, we succeeded in the acylation and ureidation of aromatic amines, and imine formation from aromatic aldehydes within a single crystal," the team explains. The researchers add that they have published preliminary results elsewhere that show they can embed aldehydes within the material's spaces and so generate a porous network of Schiff bases. These compounds contain a carbon-nitrogen double bond, with the nitrogen connected to an alkyl or aryl chemical group ripe for converting into and so are useful intermediates in the manufacture of numerous organic compounds. "Our results overcome a fatal problem in solid-state reactions, namely the preservation of crystallinity, and provide a new application of porous coordination complexes," the researchers conclude, "In situ preparation and determination of reaction intermediates or labile molecules can be achieved by utilizing these ?single-crystalline molecular flasks." Related links:
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