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.

本研究报道了三种新型银基金属有机框架（metal–organic frameworks）材料，它们分别以3-膦酰基苯甲酸或4-膦酰基苯甲酸与硝酸银为原料合成。这类新型杂化材料采用水热法制备，通过添加不同用量的尿素调控反应介质的pH值，从而得到不同拓扑结构的产物。化合物1（化学式Ag₃(4-PO₃-C₆H₄-COO)）在尿素存在的条件下合成，其具有致密的三维网状结构，膦酰基与羧基官能团均与银基无机骨架相连。化合物2（化学式Ag(4-PO₃H-C₆H₄-COOH)）在低pH环境下（无需添加尿素）合成，呈现层状结构，仅4-膦酰基苯甲酸配体的膦酰基官能团与银基无机骨架相连，羧基官能团则通过氢键相互作用结合。最后，材料3（化学式Ag(3-PO₃H-C₆H₄-COOH)）以3-膦酰基苯甲酸与硝酸银为原料，在不添加尿素的条件下合成，同样具备层状结构，其层间空间的羧基官能团通过氢键相互连接。在对上述三种材料进行全面表征（包括单晶X射线衍射结构分析、红外光谱、热重分析）后，研究团队测定了它们在水环境中的银盐释放能力。采用阴极溶出伏安法对水溶液中的银释放量进行定量检测，结果显示，这些材料的银释放能力与其杂化结构的拓扑结构密切相关。结构更为致密的化合物1在水中具有极高的稳定性，仅能检测到痕量银离子。与之相反，仅膦酰基官能团与无机骨架相连的化合物2和3，可向水溶液中释放出更多的银离子。将上述结果与已报道的化合物4（化学式Ag₆(3-PO₃-C₆H₄-COO)₂）的银释放性能进行对比。实验结果清晰表明，银基金属有机框架的银释放能力可通过调控其拓扑结构进行调节；而本研究中，拓扑结构主要由有机配体的区域异构体以及杂化材料的合成条件决定。