Greetings.
During experiments and reading about MOFs, I found something that is strange to me.
For instance, in MIL-100/101, the secondary building units (SBUs) are [M3(μ3-O)(O2CR)6L3]n+/m−, while some reports claim that their as-synthesized MOFs are constructed by [M3(μ3-OH)(O2CR)6L3]n+/m−.
Both of these SBUs are 6-connected, topologically they are the same. But from the aspect of chemistry, O-center or OH-center makes a lot differences especially in the valances of related metal ions (for instance, Co2+ or Co3+, Fe2+ or Fe3+, etc.) and in the counter ions in pores/L sites (like dimethylammonium ions, coordinated fluoride ions).
I am wondering, is there any way that we could determine the actual form of this 'center' ?
Locating a single proton from e density difference maps would be tricky to say the least. Structurally, μ3-O2- will be planar, whereas μ3-OH- will be pyramidal.
There are physical techniques like magnetometry and EPR that may help you assess the spin of the metal ions if they are magnetic.
I am sorry for replying this answer too late.
Thank you for your answer. Athanassios K. Boudalis
Hi, Since both the possibilities are likely, i.e. μ3-Oxo and μ3-hydroxo, please take both IR spectrum and 1H NMR spectrum for your MOF. From IR, you should be able to diagnose the presence of O-H from its stretching frequency. If your compound is soluble in organic solvents, 1H NMR should tell you about the presence of O-H and also, if the signal disappears after keeping the solution either in D2O or CDCl3 for 24 h due to exchange, further confirming the presence of OH.
From normal X-ray diffraction, identification is difficult.
Good luck,
Balakrishna
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