Synthesis and Breathing Behaviour of a New Member of the Fluoride-substituted Gallium MIL-53 Metal-Organic Framework Solid-Solution Series

A new member of the fluoride-substituted gallium MIL-53 metal-organic framework [Ga(OH)₁₋ₓFₓ(bdc)] (bdc²⁻ = 1,4-benzenedicarboxylate) solid-solution series, [Ga(OH)₀.₈₀F₀.₂₀(bdc)] (1), is reported that extends the series to [Ga(OH)₁₋ₓFₓ(bdc)] (x ≤ 0.20). The framework fluoride content is 33% greater than the previously reported highest framework fluoride-containing gallium MIL-53 [Ga(OH)₀.₈₅F₀.₁₅(bdc)] and is achieved using a novel HF-free route. The effects of the additional μ₂-F⁻ bridging anions in the framework on the breathing behaviour of 1 are profound compared to those previously reported for [Ga(OH)(bdc)] and [Ga(OH)₀.₈₅F₀.₁₅(bdc)]. Large pore-1 (lp-1) does not transform to narrow pore-1 (np-1) over the entire 500 K–100 K–500 K temperature range investigated and np-1 undergoes transformation to lp-1 on heating from 400 K to 500 K at a suitably fast heating rate. Both results indicate the greater stability of lp-1 relative to np-1. The combined use of heating and guest water desorption/adsorption is sufficient to allow 1 to exhibit a full breathing cycle. Overall, this work demonstrates the ability to tune the breathing properties of a MIL-53 MOF through the method of fluoride incorporation into the μ₂-bridging anion framework site and should be a methodology applicable to engineer the breathing and other behaviours of many MOFs in a controlled manner.