Recently, I am trying to synthesize MOFs contain different Rod SBUs, and I realized a problem ( it should have been noticed earlier!!! ).
Several different solvents were used, like DMF, DMAc, NMP, DEF, MeOH, dioxane....and the final products were dramatically different.
I have several questions that need help from you guys:
1. How do solvents make influences to MOF structures? Or more basically, topology structures?
2. How can one measure the polarity of a pore/channel of a MOF? (i think it may be relevant with polarity of solvents.)
3. Is there any empirical rules (about solvents) that can be used in the synthesis of MOFs?
Many thanks to all of you.
Kevin Xiao
Dear Kevin,
solvents do basically two things there: coordinate/solvate your building blocks/ions/molecules and assisting the assembly of your MOF. I find really difficult to provide you with general advice in this regard, as the effects of solvents are not only MOF-dependent but also temperature- and pressure- dependent, and the list may be not finished. For sure if you have a MOF with inorganic moieties developing along one specific crystallographic direction and linkers connecting these moieties in the other two directions you could expect platelets and not rod-shaped crystals by using solvents that are very good in coordinating the metals and not so good in solvating the linkers. This will result in a much higher reactivity of the organic components whereas the inorganic one's growth will be hampered for kinetic reasons. But what if you see the opposite? what if needles appear? It might mean that in your specific reaction conditions you have a linker-depleted solution because of the mentioned solvation issues, while the metal concentration is much higher because all the metal is dissolved and the inorganic part growth is favoured. For the time being I usually rationalise what happens after the reaction is over to gather information on every system with a certain solvent in specific reaction conditions. I have, however, a general suggestion: always consider in your solvent mixture also water traces if your solvent is not dry. Water can really have dramatic effects on many aspects of MOFs' assembly, polymorphism and, with some solvents like DMF, solvent's degradation.
Thank you so much, Dr. Canossa, for your inspiring answer.
Water actually is needed in my reaction system (for separately dissolving metal salts), so degradation is not avoidable :(
Because of my job here, i also found that room temperature synthesis is also solvent-dependent. Is that can also be explained by their different solvation abilities, which turn out different structures? And how about their polarity?
For sure different temperatures are associated to different exchange constants of your solvation/coordination sphere and therefore to different reactivity of your reagents, I use to play with this aspect to change the crystal size of my MOFs. Water is often important and unaviodable, for instance because many inorganic building units are oxoclusters and that water must come from water traces in the solvent or water molecules present in the salt used as starting reagent. As for polarity, some studies (both experimental and computational) do address the dielectric constant of MOFs, although it is not a very extensively studied property.Article Probing Dielectric Properties of Metal-Organic Frameworks: M...
;Article Dielectric Properties of Selected Metal–Organic Frameworks
Glad to be helpful :) I hope more comments from more expert researchers will follow. Good luck with your research!
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