Abstract
Metal-organic frameworks (MOFs) are promising functional compounds; however, they are known to be moisture sensitive. Here, we investigate the hydration and dehydration processes of the carboxylate-containing MOFs [Ni2(nic)4(H2O)]n (nic = nicotinic acid) and the mononuclear complex [Ni(nic)2(H2O)4], respectively. X-ray single crystal diffraction reveals that the hydrogen bonds between water and carboxylic oxygen play a key role in these processes. Molecular-level mechanisms of reversible hydration and dehydration are proposed, based on the competition between water and carboxylic oxygen for coordination with Ni. This study provides important information for future studies on the hydrolytic stability of MOFs in moisture.
Highlights
Owing to their excellent physical and chemical properties, metal-organic frameworks (MOFs) are regarded as promising functional materials with significant application potentials [1] [2] [3]
In the dehydrated Metal-organic frameworks (MOFs) [Ni2(nic)4(H2O)]n 1, the adsorbed water molecules were localized in the vicinity of the metal ions, due to the hydrogen bonds between the coordinated water molecules and carboxylic groups
Located in the free space within the octahedron of coordinated water molecules and carboxylic groups, the adsorbed water molecules generated another ligand field that interacted with the Ni ion and competed with the original oxygen ligands
Summary
Owing to their excellent physical and chemical properties, metal-organic frameworks (MOFs) are regarded as promising functional materials with significant application potentials [1] [2] [3]. The relatively poor stability of MOFs in moisture has severely limited their industrial applications. Water adsorption may degrade their structural ordering and stability, as hydration causes the carboxylic groups coordinated to the metallic core to detach in these materials [17] [18] [19]. For this reason, improvements in the hydrolytic stability of MOFs are urgently needed. This study of framework complex hydration at the molecular level facilitates the understanding of the corresponding mechanisms, and provides good references for improving the hydrothermal stability of framework complexes
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