Cation exchange at the secondary building units of metal–organic frameworks

Abstract

Cation exchange is an emerging synthetic route for modifying the secondary building units (SBUs) of metal-organic frameworks (MOFs). This technique has been used extensively to enhance the properties of nanocrystals and molecules, but the extent of its applications for MOFs is still expanding. To harness cation exchange as a rational tool, we need to elucidate its governing factors. Not nearly enough experimental observations exist for drawing these conclusions, so we provide a conceptual framework for approaching this task. We address which SBUs undergo exchange, why certain ions replace others, how the framework influences the process, the role of the solvent, and current applications. Using these guidelines, certain trends emerge from the available data and missing experiments become obvious. If future studies follow this framework, then a more comprehensive body of observations will furnish a deeper understanding of cation exchange and inspire future applications.

Crystalline central-metal transformation in metal-organic frameworks

“…In most cases, Cu II ion exchange products are the most stable and they are rarely transformed [95], which is usually explained by the Jahn-Teller effect [94,116]. However, in an early study by our group [117], Cu II ions in the double-helical octanuclear Cu II wheel complex [Cu 8 L 16 ] (29; HL = 4 -[4-methyl-6-(1-methyl-1H-benzimidazolyl-2-group)-2-n-propyl-1H-benzimidazolyl methyl]) were partially replaced by Zn II and Co II ions.…”

“…If the coordination ability of the incoming metal ions is weak or similar to that of the centralmetal ions in MOFs, CME cannot proceed successfully. However, the exchange process is also influenced by many other factors, such as the concentration of the incoming metal ion solution, the identity of the metal ion, preferential coordination geometry, the pore size of the framework, and the solvent employed [23,94,95]. CME usually requires a long reaction time, ranging from a few days to several weeks.…”

“…Despite its comparatively late emergence, CME has become a hot research topic in the eyes of many chemists and material scientists. [41,95]. CME reactions involve breaking and regenerating chemical bonds between metal ions and organic ligands.…”

Synthesis and Crystal Structure of a Zn(II)-Based MOF Bearing Neutral N-Donor Linker and SiF62− Anion

“…MOFs can be readily functionalized using different organic building blocks thus imparting flexibility to tune the properties [8]. Metal-organic frameworks can also be tuned by using varied metals/metal clusters along with counter ions with different coordinating tendency [9][10][11][12][13]. MOFs have been classified as neutral and ionic MOFs (iMOFs) based on the framework charge [14].…”

Analytical STEM Investigation of the Post-Synthetic Modification (PMS) of Metal-Organic Frameworks (MOFs): Metal- and Ligand-Exchange in UiO-66

“…Recently, post-synthetic exchange (PSE) of metal ions in the secondary-building units (SBUs) of MOFs has been demonstrated in the UiO-66 system, whereby Hf or Ti ions are exchanged for the initial Zr ions in the SBUs, while appearing to retain structural integrity and porosity [2]. Metal ion PSM (transmetalation) has been observed for both solvent-mediated solid-solid reactions and solid-liquid conditions in "robust, inert" MOFs, such as the UiO-66 system [1,3]. This has been proposed as a route to achieve stable MOF structures with specific metal nodes yet to be directly produced using conventional synthesis (e.g.…”