Breathing Behaviour Modification of Gallium MIL-53 Metal-Organic Frameworks Induced by the Bridging Framework Inorganic Anion

Controlling aspects of the μ₂ -X^- bridging anion in the metal-organic framework Ga-MIL-53 [GaX(bdc)] (X^- =(OH)^- or F^- , bdc=1, 4-benzenedicarboxylate) is shown to direct the temperature at which thermally induced breathing transitions of this framework occur. In situ single crystal X-ray diffraction studies reveal that substituting 20 % of (OH)^- in [Ga(OH)(bdc)] (1) for F^- to produce [Ga(OH)_0.8 F_0.2 (bdc)] (2) stabilises the large pore (lp) form relative to the narrow pore (np) form, causing a well-defined decrease in the onset of the lp to np transition at higher temperatures, and the adsorption/desorption of nitrogen at lower temperatures through np to lp to intermediate (int) pore transitions. These in situ diffraction studies have also yielded a more plausible crystal structure of the int-[GaX(bdc)] ⋅ H₂ O phases and shown that increasing the heating rate to a flash heating regime can enable the int-[GaX(bdc)] ⋅ H₂ O to lp-[GaX(bdc)] transition to occur at a lower temperature than np-[GaX(bdc)] via an unreported pathway.