New Insights\ninto the Hydrogen Bond Network in Al-MIL-53 and Ga-MIL-53

Abstract

Metal−organic\nframework-type Al-, Ga-, and Ga­(OH,F)-MIL-53 have been characterized\nby solid-state NMR and powder X-ray diffraction (PXRD). ¹H 2D double-quantum–single-quantum (DQ–SQ) magic angle\nspinning (MAS) NMR experiments unambiguously evidence two inequivalent\nwater molecules in Al-MIL-53_np_H₂O. A careful reinvestigation\nof the XRD structure of hydrated Al-MIL-53 proves, for the first time,\nthe doubling of the unit cell supporting the presence of two inequivalent\nwater molecules. One type of water molecule is located in the type\nA channel, interacting with the aromatic protons of framework organic\nmoieties, whereas the other type of water molecule is in the type\nB channel far away from aromatic protons. Assignment of the corresponding ¹H resonances to each water molecule was possible. ¹H 1D MAS NMR leads to the same conclusion (i.e., two inequivalent\nwater molecules) for the isostructural gallium-based material thanks\nto the positioning of the hydrogen atoms by Rietveld refinement. Moreover,\nwhen Ga-MIL-53 is prepared with fluorine in the synthesis medium (giving\nGa­(OH,F)-MIL-53_np_H₂O), the situation is different. 2D ¹H–¹³C heteronuclear correlation MAS NMR,\nhigh-field ¹H DQ–SQ, and radio-frequency-driven\nrecoupling MAS NMR indicate that only one type of water molecule is\nfound in this material, in agreement with the structure.