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

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.