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Silver-Based Hybrid Materials from meta- or para-Phosphonobenzoic Acid: Influence of the Topology on Silver Release in Water

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journal contribution
posted on 02.03.2015 by Jean-Michel Rueff, Olivier Perez, Vincent Caignaert, Gary Hix, Mathieu Berchel, François Quentel, Paul-Alain Jaffrès
Three novel silver-based metal–organic frameworks materials, which were synthesized from either 3-phosphono or 4-phosphonobenzoic acid and silver nitrate, are reported. The novel hybrids were synthesized under hydrothermal conditions; the pH of the reaction media was controlled by adding different quantities of urea thereby producing different topologies. Compound 1 (Ag3(4-PO3-C6H4-COO)), synthesized in the presence of urea, exhibits a compact 3D structure in which both phosphonic acid and carboxylic acid functional groups are linked to the silver-based inorganic network. Compound 2 (Ag­(4-PO3H-C6H4-COOH)), which was synthesized at lower pH (without urea), has a layered structure in which only the phosphonic acid functional groups from 4-phosphonobenzoic acid moieties are linked to the silver inorganic network; the carboxylic acid groups being engaged in hydrogen bonds. Finally, material 3 (Ag­(3-PO3H-C6H4-COOH)) was synthesized from 3-phosphonobenzoic acid and silver nitrate without urea. This material 3 features a layered structure exhibiting carboxylic acid functional groups linked via hydrogen bonds in the interlayer space. After the full characterization of these materials (single X-ray structure, IR, TGA), their ability to release silver salts in aqueous environment was measured. Silver release, determined in aqueous solution by cathodic stripping voltammetry, shows that the silver release capacity of these materials is dependent on the topology of the hybrids. The more compact structure 1 is extremely stable in water with only trace levels of silver ions being detected. On the other hand, compounds 2 and 3, in which only the phosphonic acid functional groups were bonded to the inorganic network, released larger quantities of silver ions into aqueous solution. These results which were compared with the silver release of the previously described compound 4 (Ag6(3-PO3-C6H4-COO)2). The results clearly show that the release capacity of silver-based metal organic framework can be tuned by modifying their topology which, in the present study, is governed by the regio-isomer of the organic precursor and the synthetic conditions under which the hybrids are prepared.