Tuning Adsorption-Induced Responsiveness of a Flexible Metal–Organic Framework JUK‑8 by Linker Halogenation

Flexible stimuli-responsive metal–organic frameworks have become promising candidates for numerous applications in gas-related technologies; however, the methods of fine tuning their responses are still limited and sought after. In this work, we demonstrate control over the adsorption properties of a flexible platform by incorporating halogen substituents (X = F, Cl, Br, I) into an eightfold interpenetrated isoreticular series [Zn­(oba)­(X-pip)]_<i>n</i> (<b>JUK</b>-<b>8X</b>; <i>X-pip</i> = 4-pyridyl-functionalized benzene-1,3-dicarbo-5-halogenohydrazide; oba^2– = 4,4′-oxydibenzoic carboxylate). The introduced halogen atoms allow for precise tuning of CO₂ gate-opening pressures from <i>p</i>/<i>p</i>₀ = 0.08 for the parental <b>JUK</b>-<b>8</b> to 0.78 for the chlorine-functionalized <b>JUK</b>-<b>8Cl</b>. The presence of fluorine or chlorine substituent in the X-pip linker practically does not influence the maximum molar CO₂ uptake as compared to <b>JUK</b>-<b>8</b>, whereas larger bromine or iodine atoms increase this uptake by 59 and 48%, respectively. Utilizing in situ powder X-ray diffraction (PXRD) during CO₂ adsorption for a model <b>JUK</b>-<b>8F</b>, we propose a detailed mechanism of phase transitions including positions of the adsorbed gas molecules for the two loaded phases. Density functional theory calculations supported by in situ PXRD measurements at a saturation pressure shed light on the unusual CO₂ adsorption properties of <b>JUK</b>-<b>8Br</b> and <b>JUK</b>-<b>8I</b>. Overall, our report demonstrates the use of halogen interactions for the control of a gas-responsive system and provides insightful guidance for the further development of flexible, adaptable materials.